151
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Tonotopic variation in the calcium dependence of neurotransmitter release and vesicle pool replenishment at mammalian auditory ribbon synapses. J Neurosci 2008; 28:7670-8. [PMID: 18650343 DOI: 10.1523/jneurosci.0785-08.2008] [Citation(s) in RCA: 100] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
The mammalian cochlea is specialized to recognize and process complex auditory signals with remarkable acuity and temporal precision over a wide frequency range. The quality of the information relayed to the auditory afferent fibers mainly depends on the transfer characteristics of inner hair cell (IHC) ribbon synapses. To investigate the biophysical properties of the synaptic machinery, we measured changes in membrane capacitance (DeltaC(m)) in low-frequency (apical region, approximately 300 Hz) and high-frequency (basal, approximately 30 kHz) gerbil IHCs maintained in near physiological conditions (1.3 mm extracellular Ca(2+) and body temperature). With maturation, the Ca(2+) efficiency of exocytosis improved in both apical and basal IHCs and was more pronounced in the latter. Prehearing IHCs showed a similar Ca(2+) cooperativity of exocytosis despite the smaller DeltaC(m) in apical cells. After maturation, DeltaC(m) in high-frequency IHCs increased linearly with the Ca(2+) current, whereas, somewhat surprisingly, the relationship was significantly more nonlinear in low-frequency cells. This tonotopic difference seemed to be correlated with ribbon synapse morphology (spherical in apical and ellipsoid in basal IHCs) but not with the expression level of the proposed Ca(2+) sensor otoferlin or the spatial coupling between Ca(2+) channels and active zones. Repetitive stimulation of adult IHCs showed that vesicle pool refilling could become rate limiting for vesicle release, with high-frequency IHCs able to sustain greater release rates. Together, our findings provide the first evidence for a tonotopic difference in the properties of the synaptic machinery in mammalian IHCs, which could be essential for fine-tuning their receptor characteristics during sound stimulation.
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152
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Abstract
Ion channels are modulated by multiple molecular mechanisms. In this issue of Neuron, Pietrzykowski et al. expand the mechanistic repertoire by demonstrating that ethanol-induced microRNA can modulate the pattern of mRNA splice variants from which BK potassium channels are constructed. Because BK channels are important targets of ethanol, this finding has implications for mechanisms of ethanol sensitivity and tolerance.
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153
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Xie J. Control of alternative pre-mRNA splicing by Ca(++) signals. BIOCHIMICA ET BIOPHYSICA ACTA 2008; 1779:438-52. [PMID: 18258215 PMCID: PMC3500379 DOI: 10.1016/j.bbagrm.2008.01.003] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/02/2007] [Revised: 12/18/2007] [Accepted: 01/08/2008] [Indexed: 02/06/2023]
Abstract
Alternative pre-mRNA splicing is a common way of gene expression regulation in metazoans. The selective use of specific exons can be modulated in response to various manipulations that alter Ca(++) signals, particularly in neurons. A number of splicing factors have also been found to be controlled by Ca(++) signals. Moreover, pre-mRNA elements have been identified that are essential and sufficient to mediate Ca(++)-regulated splicing, providing model systems for dissecting the involved molecular components. In neurons, this regulation likely contributes to the fine-tuning of neuronal properties.
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Affiliation(s)
- Jiuyong Xie
- Department of Physiology, Faculty of Medicine, University of Manitoba, 730 William Avenue, Winnipeg, Canada MB R3E 3J7.
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154
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Soom M, Gessner G, Heuer H, Hoshi T, Heinemann SH. A mutually exclusive alternative exon of slo1 codes for a neuronal BK channel with altered function. Channels (Austin) 2008; 2:278-82. [PMID: 18719396 DOI: 10.4161/chan.2.4.6571] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
Large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels are comprised of four pore-forming -subunits (Slo1), whose mRNA is alternatively spliced in a cell-specific manner. Here we report the first case of a correctly spliced mutually exclusive exon in a mammalian (human and mouse) BK channel; an exon coding for the region from S6 to the RCK1 domain is exchanged for an alternative exon of the same length. The slo1 transcript with this novel exon is present in native brain tissues and inclusion of the alternative exon profoundly alters the channel's gating characteristics: faster activation at low Ca(2+) concentrations and greater open probability at resting membrane potential at high Ca(2+) concentrations. The novel gating features conferred by the alternative exon are dominant over those of the commonly described Slo1 variant when coexpressed. The evolutionarily preserved splicing of the Slo1 S6-RCK1 linker segment possess great potential to fine-tune neuronal excitability.
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Affiliation(s)
- Malle Soom
- Center for Molecular Biomedicine, Department of Biophysics, Friedrich Schiller University Jena, Jena, Germany
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155
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Affiliation(s)
- Jutta Engel
- Institute of Physiology II and Tuebingen Hearing Research Centre, University of Tuebingen, Tuebingen, Germany.
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156
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Lee J, Ueda A, Wu CF. Pre- and post-synaptic mechanisms of synaptic strength homeostasis revealed by slowpoke and shaker K+ channel mutations in Drosophila. Neuroscience 2008; 154:1283-96. [PMID: 18539401 DOI: 10.1016/j.neuroscience.2008.04.043] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2008] [Revised: 04/11/2008] [Accepted: 04/15/2008] [Indexed: 11/24/2022]
Abstract
We report naturally occurring, systematic variations in synaptic strength at neuromuscular junctions along the dorsal-ventral (D-V) axis of the Drosophila larval body wall. These gradual changes were correlated with differences in presynaptic neurotransmitter release regulated by nerve terminal excitability and in postsynaptic receptor composition influencing miniature excitatory junctional potential (mEJP) amplitude. Surprisingly, synaptic strength and D-V differentials at physiological Ca(2+) levels were not significantly altered in slowpoke (slo) and Shaker (Sh) mutants, despite their defects in two major repolarizing forces, Ca(2+)-activated Slo (BK) and voltage-activated Sh currents, respectively. However, lowering [Ca(2+)](o) levels revealed greatly altered synaptic mechanisms in these mutants, indicated by drastically enhanced excitatory junctional potentials (EJPs) in Sh but paradoxically reduced EJPs in slo. Removal of Sh current in slo mutants by 4-aminopyridine blockade or by combining slo with Sh mutations led to strikingly increased synaptic transmission, suggesting upregulation of presynaptic Sh current to limit excessive neurotransmitter release in the absence of Slo current. In addition, slo mutants displayed altered immunoreactivity intensity ratio between DGluRIIA and DGluRIIB receptor subunits. This modified receptor composition caused smaller mEJP amplitudes, further preventing excessive transmission in the absence of Slo current. Such compensatory regulations were prevented by rutabaga (rut) adenylyl cyclase mutations in rut slo double mutants, demonstrating a novel role of rut in homeostatic plasticity, in addition to its well-established function in learning behavior.
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Affiliation(s)
- J Lee
- Interdisciplinary Program in Neuroscience, University of Iowa, Iowa City, IA 52242, USA
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157
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Guo Z, Lv C, Yi H, Xiong Y, Wu Y, Li W, Xu T, Ding J. A residue at the cytoplasmic entrance of BK-type channels regulating single-channel opening by its hydrophobicity. Biophys J 2008; 94:3714-25. [PMID: 18400952 PMCID: PMC2292367 DOI: 10.1529/biophysj.107.120022] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2007] [Accepted: 12/20/2007] [Indexed: 01/17/2023] Open
Abstract
Single large-conductance calcium-activated K(+) (BK) channels encoded by the mSlo gene usually have synchronous gating, but a Drosophila dSlo (A2/C2/E2/G5/10) splice variant (dSlo1A) exhibits very flickery openings. To probe this difference in gating, we constructed a mutant I323T. This channel exhibits four subconductance levels similar to those of dSlo1A. Rectification of the single-channel current-voltage relation of I323T decreased as [Ca(2+) ](in) increased from 10 to 300 microM. Mutagenesis suggests that the hydrophobicity of the residue at the position is important for the wild-type gating; i.e., increasing hydrophobicity prolongs open duration. Molecular dynamics simulation suggests that four hydrophobic pore-lining residues at position 323 of mSlo act cooperatively in a "shutter-like" mechanism gating the permeation of K(+) ions. Rate-equilibrium free energy relations analysis shows that the four I323 residues in an mSlo channel have a conformation 65% similar to the closed conformation during gating. Based on these observations, we suggest that the appearance of rectification and substates of BK-type channels arise from a reduction of the cooperativity among these four residues and a lower probability of being open.
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Affiliation(s)
- Zhaohua Guo
- Key Laboratory of Molecular Biophysics (Huazhong University of Science and Technology), Ministry of Education, College of Life Science and Technology, Wuhan, Hubei 430074, China
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158
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Panek I, Höger U, French AS, Torkkeli PH. Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons. J Neurophysiol 2008; 99:1596-606. [DOI: 10.1152/jn.01267.2007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
Activation of ionotropic γ-aminobutyric acid type A (GABAA) receptors depolarizes neurons that have high intracellular [Cl−], causing inhibition or excitation in different cell types. The depolarization often leads to inactivation of voltage-gated Na channels, but additional ionic mechanisms may also be affected. Previously, a simulated model of spider VS-3 mechanosensory neurons suggested that although voltage-activated Na+current is partially inactivated during GABA-induced depolarization, a slowly activating and inactivating component remains and may contribute to the depolarization. Here, we confirmed experimentally, by blocking Na channels prior to GABA application, that Na+current contributes to GABA-induced depolarization in VS-3 neurons. Ratiometric Ca2+imaging experiments combined with intracellular recordings revealed a significant increase in intracellular [Ca2+] when GABAAreceptors were activated, synchronous with the depolarization and probably due to Ca2+influx via low-voltage–activated (LVA) Ca channels. In contrast, GABAB-receptor activation in these neurons was previously shown to inhibit LVA current. Blockade of voltage-gated K channels delayed membrane repolarization, extending GABA-induced depolarization. However, inhibition of Ca channels significantly increased the amplitude of GABA-induced depolarization, indicating that Ca2+-activated K+current has an even stronger repolarizing effect. Regulation of intracellular [Ca2+] is important for many cellular processes and Ca2+control of K+currents may be particularly important for some functions of mechanosensory neurons, such as frequency tuning. These data show that GABAA-receptor activation participates in this regulation.
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159
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Subunit-specific effect of the voltage sensor domain on Ca2+ sensitivity of BK channels. Biophys J 2008; 94:4678-87. [PMID: 18339745 DOI: 10.1529/biophysj.107.121590] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels, composed of pore-forming alpha-subunits and auxiliary beta-subunits, play important roles in diverse physiological processes. The differences in BK channel phenotypes are primarily due to the tissue-specific expression of beta-subunits (beta1-beta4) that modulate channel function differently. Yet, the molecular basis of the subunit-specific regulation is not clear. In our study, we demonstrate that perturbation of the voltage sensor in BK channels by mutations selectively disrupts the ability of the beta1-subunit--but not that of the beta2-subunit--to enhance apparent Ca(2+) sensitivity. These mutations change the number of equivalent gating charges, the voltage dependence of voltage sensor movements, the open-close equilibrium of the channel, and the allosteric coupling between voltage sensor movements and channel opening to various degrees, indicating that they alter the conformation and movements of the voltage sensor and the activation gate. Similarly, the ability of the beta1-subunit to enhance apparent Ca(2+) sensitivity is diminished to various degrees, correlating quantitatively with the shift of voltage dependence of voltage sensor movements. In contrast, none of these mutations significantly reduces the ability of the beta2-subunit to enhance Ca(2+) sensitivity. These results suggest that the beta1-subunit enhances Ca(2+) sensitivity by altering the conformation and movements of the voltage sensor, whereas the similar function of the beta2-subunit is governed by a distinct mechanism.
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160
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Cytoplasmic BK(Ca) channel intron-containing mRNAs contribute to the intrinsic excitability of hippocampal neurons. Proc Natl Acad Sci U S A 2008; 105:1901-6. [PMID: 18250327 DOI: 10.1073/pnas.0711796105] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
High single-channel conductance K+ channels, which respond jointly to membrane depolarization and micromolar concentrations of intracellular Ca2+ ions, arise from extensive cell-specific alternative splicing of pore-forming alpha-subunit mRNAs. Here, we report the discovery of an endogenous BK(Ca) channel alpha-subunit intron-containing mRNA in the cytoplasm of hippocampal neurons. This partially processed mRNA, which comprises approximately 10% of the total BK(Ca) channel alpha-subunit mRNAs, is distributed in a gradient throughout the somatodendritic space. We selectively reduced endogenous cytoplasmic levels of this intron-containing transcript by RNA interference without altering levels of the mature splice forms of the BK(Ca) channel mRNAs. In doing so, we could demonstrate that changes in a unique BK(Ca) channel alpha-subunit intron-containing splice variant mRNA can greatly impact the distribution of the BK(Ca) channel protein to dendritic spines and intrinsic firing properties of hippocampal neurons. These data suggest a new regulatory mechanism for modulating the membrane properties and ion channel gradients of hippocampal neurons.
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161
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Curti S, Gómez L, Budelli R, Pereda AE. Subthreshold sodium current underlies essential functional specializations at primary auditory afferents. J Neurophysiol 2008; 99:1683-99. [PMID: 18234982 DOI: 10.1152/jn.01173.2007] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Primary auditory afferents are generally perceived as passive, timing-preserving lines of communication. Contrasting this view, identifiable auditory afferents to the goldfish Mauthner cell undergo potentiation of their mixed--electrical and chemical--synapses in response to high-frequency bursts of activity. This property likely represents a mechanism of input sensitization because they provide the Mauthner cell with essential information for the initiation of an escape response. Consistent with this synaptic specialization, we show here that these afferents exhibit an intrinsic ability to respond with bursts of 200-600 Hz and this property critically relies on the activation of a persistent sodium current, which is counterbalanced by the delayed activation of an A-type potassium current. Furthermore, the interaction between these conductances with the membrane passive properties supports the presence of electrical resonance, whose frequency preference is consistent with both the effective range of hearing in goldfish and the firing frequencies required for synaptic facilitation, an obligatory requisite for the induction of activity-dependent changes. Thus our data show that the presence of a persistent sodium current is functionally essential and allows these afferents to translate behaviorally relevant auditory signals into patterns of activity that match the requirements of their fast and highly modifiable synapses. The functional specializations of these neurons suggest that auditory afferents might be capable of more sophisticated contributions to auditory processing than has been generally recognized.
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Affiliation(s)
- Sebastián Curti
- Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, NY 10461, USA
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162
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Fonseca PJ, Correia T. Effects of temperature on tuning of the auditory pathway in the cicada Tettigetta josei (Hemiptera, Tibicinidae). ACTA ACUST UNITED AC 2008; 210:1834-45. [PMID: 17488947 DOI: 10.1242/jeb.001495] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The effects of temperature on hearing in the cicada Tettigetta josei were studied. The activity of the auditory nerve and the responses of auditory interneurons to stimuli of different frequencies and intensities were recorded at different temperatures ranging from 16 degrees C to 29 degrees C. Firstly, in order to investigate the temperature dependence of hearing processes, we analyzed its effects on auditory tuning, sensitivity, latency and Q(10dB). Increasing temperature led to an upward shift of the characteristic hearing frequency, to an increase in sensitivity and to a decrease in the latency of the auditory response both in the auditory nerve recordings (periphery) and in some interneurons at the metathoracic-abdominal ganglionic complex (MAC). Characteristic frequency shifts were only observed at low frequency (3-8 kHz). No changes were seen in Q(10dB). Different tuning mechanisms underlying frequency selectivity may explain the results observed. Secondly, we investigated the role of the mechanical sensory structures that participate in the transduction process. Laser vibrometry measurements revealed that the vibrations of the tympanum and tympanal apodeme are temperature independent in the biologically relevant range (18-35 degrees C). Since the above mentioned effects of temperature are present in the auditory nerve recordings, the observed shifts in frequency tuning must be performed by mechanisms intrinsic to the receptor cells. Finally, the role of potassium channels in the response of the auditory system was investigated using a specific inhibitor of these channels, tetraethylammonium (TEA). TEA caused shifts on tuning and sensitivity of the summed response of the receptors similar to the effects of temperature. Thus, potassium channels are implicated in the tuning of the receptor cells.
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Affiliation(s)
- P J Fonseca
- Departamento de Biologia Animal e Centro de Biologia Ambiental, Faculdade de Ciências da Universidade de Lisboa, Bloco C2, Campo Grande, 1749-016 Lisboa, Portugal.
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163
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The structural and functional differentiation of hair cells in a lizard's basilar papilla suggests an operational principle of amniote cochleas. J Neurosci 2007; 27:11978-85. [PMID: 17978038 PMCID: PMC2151837 DOI: 10.1523/jneurosci.3679-07.2007] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The hair cells in the mammalian cochlea are of two distinct types. Inner hair cells are responsible for transducing mechanical stimuli into electrical responses, which they forward to the brain through a copious afferent innervation. Outer hair cells, which are thought to mediate the active process that sensitizes and tunes the cochlea, possess a negligible afferent innervation. For every inner hair cell, there are approximately three outer hair cells, so only one-quarter of the hair cells directly deliver information to the CNS. Although this is a surprising feature for a sensory system, the occurrence of a similar innervation pattern in birds and crocodilians suggests that the arrangement has an adaptive value. Using a lizard with highly developed hearing, the tokay gecko, we demonstrate in the present study that the same principle operates in a third major group of terrestrial animals. We propose that the differentiation of hair cells into signaling and amplifying classes reflects incompatible strategies for the optimization of mechanoelectrical transduction and of an active process based on active hair-bundle motility.
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164
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Abstract
Alternative pre-mRNA splicing has an important role in the control of neuronal gene expression. Many neuronal proteins are structurally diversified through the differential inclusion and exclusion of sequences in the final spliced mRNA. Here, we discuss common mechanisms of splicing regulation and provide examples of how alternative splicing has important roles in neuronal development and mature neuron function. Finally, we describe regulatory proteins that control the splicing of some neuronally expressed transcripts.
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Affiliation(s)
- Qin Li
- Howard Hughes Medical Institute, University of California, Los Angeles, 6-762 MacDonald Research Laboratories, 675 Charles E. Young Drive South, Los Angeles, California 90095, USA
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165
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Kim HJ, Lim HH, Rho SH, Bao L, Lee JH, Cox DH, Kim DH, Park CS. Modulation of the conductance-voltage relationship of the BK Ca channel by mutations at the putative flexible interface between two RCK domains. Biophys J 2007; 94:446-56. [PMID: 17890381 PMCID: PMC2157237 DOI: 10.1529/biophysj.107.108738] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Calcium-dependent gating of the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel is conferred by the large cytosolic carboxyl terminus containing two domains of the regulator of K(+) conductance (RCK) and the high-affinity Ca(2+)-binding site (the Ca(2+)-bowl). In our previous study, we located the putative second RCK domain (RCK2) and demonstrated that it interacts directly with RCK1 via a hydrophobic "assembly interface". In this study, we tested the structural model of the other interface, the "flexible interface", by strategically positioning charge pairs across the putative interface. Several charge mutations on RCK2 affected the voltage-dependent activation of the channel. In particular, the Gly-to-Asp substitution at position 803 profoundly affected channel activation by stabilizing the open conformation of the channel with minimal effects on its Ca(2+) affinity and voltage sensitivity. Various mutations at Gly-803 shifted the channel's conductance-voltage curve either left or right over a 145-mV range. Since this residue is predicted to be in the first loop of RCK2 these results strongly suggest that this loop plays a critical role in determining the intrinsic equilibrium constant for channel opening, and they support the hypothesis that this loop is part of an interface that mediates conformational coupling between RCK1 and RCK2.
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Affiliation(s)
- Hyun-Ju Kim
- Department of Life Science, Gwangju Institute of Science and Technology, Gwangju, Korea
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166
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Petrik D, Brenner R. Regulation of STREX exon large conductance, calcium-activated potassium channels by the beta4 accessory subunit. Neuroscience 2007; 149:789-803. [PMID: 17945424 DOI: 10.1016/j.neuroscience.2007.07.066] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2007] [Revised: 06/22/2007] [Accepted: 08/07/2007] [Indexed: 12/31/2022]
Abstract
Large conductance (BK-type) calcium-activated potassium channels utilize alternative splicing and association with accessory beta subunits to tailor BK channel properties to diverse cell types. Two important modulators of BK channel gating are the neuronal-specific beta4 accessory subunit (beta4) and alternative splicing at the stress axis hormone-regulated exon (STREX). Individually, these modulators affect the gating properties of the BK channel as well as its response to phosphorylation. In this study, the combined functional consequences of STREX and the mouse beta4 subunit on mouse BK channel biophysical properties were investigated in transfected HEK 293 cells. Surprisingly, we found that the combined effects of STREX and beta4 are non-additive and even opposite for some properties. At high calcium, beta4 and the STREX individually share properties that promote BK channel opening via slowing of deactivation. However, the combined effects are a speeding of deactivation and a decreased open probability. beta4 also inhibits BK channel opening by a slowing of activation. This effect occurs across calcium concentrations in the absence of STREX, but predominates only at low calcium for STREX containing channels. BK channel responses to phosphorylation status are also altered by the combination of the beta4 subunit and STREX. beta4/STREX channels show a slowing of activation kinetics following dephosphorylation whereas beta4 channels lacking STREX do not. In contrast, beta4 confers a speeding of activation in response to cyclic AMP-dependent phosphorylation in channels lacking STREX, but not in channels containing STREX. These results indicate that the combination of the beta4 subunit and STREX confers non-additive and unique properties to BK channels. Analysis of expression in brain slices suggests that STREX and beta4 mRNA overlap expression in the dentate gyrus of the hippocampus and the cerebellar Purkinje cells, suggesting that these unique properties of BK channels may underlie BK channel gating in these cells.
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Affiliation(s)
- D Petrik
- Department of Physiology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA
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167
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Yao Q, Zeng J, Zheng Y, Latham J, Liang B, Jiang L, Zhang S. Characteristics of echolocating bats’ auditory stereocilia length, compared with other mammals. ACTA ACUST UNITED AC 2007; 50:492-6. [PMID: 17653670 DOI: 10.1007/s11427-007-0055-8] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2006] [Accepted: 03/03/2007] [Indexed: 10/23/2022]
Abstract
The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leucogaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation.
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Affiliation(s)
- Qian Yao
- Institute of Zoology, Chinese Academy of Sciences, Beijing 100080, China
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168
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Kim EY, Ridgway LD, Zou S, Chiu YH, Dryer SE. Alternatively spliced C-terminal domains regulate the surface expression of large conductance calcium-activated potassium channels. Neuroscience 2007; 146:1652-61. [PMID: 17478049 PMCID: PMC1995407 DOI: 10.1016/j.neuroscience.2007.03.038] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2007] [Revised: 03/09/2007] [Accepted: 03/13/2007] [Indexed: 01/31/2023]
Abstract
The Slo1 gene, also known as KCNMA1, encodes the pore-forming subunits of large-conductance Ca2+-activated K+ (BK(Ca)) channels. Products of this gene are widely expressed in vertebrate tissues, and occur in a large number (>or=20) of alternatively spliced variants that vary in their gating properties, susceptibility to modulation, and trafficking to the plasma membrane. Motifs in the large cytoplasmic C-terminal are especially important in determining the functional properties of BK(Ca) channels. Here we report that chick ciliary ganglion neurons express transcripts and proteins of two Slo1 splice variants that differ at the extreme C-terminal. We refer to these variants as VEDEC and QEDRL (or QEERL for the orthologous mammalian versions), after the five terminal amino acid residues in each isoform. Individual ciliary ganglion neurons preferentially express these variants in different subcellular compartments. Moreover, QEERL channels show markedly higher levels of constitutive expression on the plasma membrane than VEDEC channels in HEK293T and NG108-15 cells. However, growth factor treatment can stimulate surface expression of VEDEC channels to levels comparable to those seen with QEERL. In addition, we show that co-expression of a soluble protein composed of VEDEC C-terminal tail residues markedly increases cell surface expression of full-length VEDEC channels, suggesting that this region binds to proteins that cause retention of the these channels in intracellular stores.
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Affiliation(s)
| | | | | | | | - Stuart E. Dryer
- Author for correspondence: , +1 713-743-2697 (ph), +1 713-743-2632 (FAX)
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169
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Williamson R, Chrachri A. A model biological neural network: the cephalopod vestibular system. Philos Trans R Soc Lond B Biol Sci 2007; 362:473-81. [PMID: 17255012 PMCID: PMC2323566 DOI: 10.1098/rstb.2006.1975] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Artificial neural networks (ANNs) have become increasingly sophisticated and are widely used for the extraction of patterns or meaning from complicated or imprecise datasets. At the same time, our knowledge of the biological systems that inspired these ANNs has also progressed and a range of model systems are emerging where there is detailed information not only on the architecture and components of the system but also on their ontogeny, plasticity and the adaptive characteristics of their interconnections. We describe here a biological neural network contained in the cephalopod statocysts; the statocysts are analogous to the vertebrae vestibular system and provide the animal with sensory information on its orientation and movements in space. The statocyst network comprises only a small number of cells, made up of just three classes of neurons but, in combination with the large efferent innervation from the brain, forms an 'active' sense organs that uses feedback and feed-forward mechanisms to alter and dynamically modulate the activity within cells and how the various components are interconnected. The neurons are fully accessible to physiological investigation and the system provides an excellent model for describing the mechanisms underlying the operation of a sophisticated neural network.
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Affiliation(s)
- Roddy Williamson
- Faculty of Science, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK.
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170
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Polimeni M, Prigioni I, Russo G, Calzi D, Gioglio L. Plasma membrane Ca2+-ATPase isoforms in frog crista ampullaris: identification of PMCA1 and PMCA2 specific splice variants. Hear Res 2007; 228:11-21. [PMID: 17336006 DOI: 10.1016/j.heares.2006.12.016] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/24/2006] [Revised: 12/14/2006] [Accepted: 12/14/2006] [Indexed: 11/28/2022]
Abstract
Ca2+ ions play a pivotal role in inner ear hair cells as they are involved from the mechano-electrical transduction to the transmitter release. Most of the Ca2+ that enters into hair cells via mechano-transduction and voltage-gated channels is extruded by the plasma membrane Ca2+-ATPases (PMCAs) that operate in both apical and basal cellular compartments. Here, we determined the identity and distribution of PMCA isoforms in frog crista ampullaris: we showed that PMCA1, PMCA2 and PMCA3 are expressed, while PMCA4 appears to be negligible. We also identify PMCA1bx, PMCA2av and PMCA2bv as the major splice variants produced from PMCA1 and PMCA2 genes. PMCA2av appears to be the major Ca2+-pump operating at the apical pole of the cell, even if PMCA1b is also expressed in the stereocilia. PMCA1bx is, instead, the principal PMCA of hair cell basolateral compartment, where it is expressed together with PMCA2 (probably PMCA2bv) and PMCA3. Frog crista ampullaris hair cells lack a Na/Ca exchanger, therefore PMCAs are the only mechanism of Ca2+ extrusion. The coexpression of specific isozymes in the different cellular compartments responds to the need of a fine regulation of both basal and dynamic Ca2+ levels at the apical and basal pole of the cell.
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Affiliation(s)
- Mariarosa Polimeni
- Dipartimento di Medicina Sperimentale - Sezione di Anatomia Umana Normale, Università di Pavia, Via Forlanini 8, I-27100 Pavia, Italy.
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171
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Chang SY, Yong TF, Yu CY, Liang MC, Pletnikova O, Troncoso JC, Burgunder JM, Soong TW. Age and gender-dependent alternative splicing of P/Q-type calcium channel EF-hand. Neuroscience 2007; 145:1026-36. [PMID: 17291689 PMCID: PMC1978091 DOI: 10.1016/j.neuroscience.2006.12.054] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2006] [Revised: 12/23/2006] [Accepted: 12/28/2006] [Indexed: 11/25/2022]
Abstract
Ca(v)2.1 Ca(2+) channels (P/Q-type), which participate in various key roles in the CNS by mediating calcium influx, are extensively spliced. One of its alternatively-spliced exons is 37, which forms part of the EF hand. The expression of exon 37a (EFa form), but not exon 37b (EFb form), confers the channel an activity-dependent enhancement of channel opening known as Ca(2+)-dependent facilitation (CDF). In this study, we analyzed the trend of EF hand splice variant distributions in mouse, rat and human brain tissues. We observed a developmental switch in rodents, as well as an age and gender bias in human brain tissues, suggestive of a possible role of these EF hand splice variants in neurophysiological specialization. A parallel study performed on rodent brains showed that the data drawn from human and rodent tissues may not necessarily correlate in the process of aging.
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Affiliation(s)
- Siao Yun Chang
- Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 2 Medical Drive, Singapore 117597
| | - Tan Fong Yong
- Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 2 Medical Drive, Singapore 117597
| | - Chye Yun Yu
- National Neuroscience Institute, 11 Jalan Tan Tock Seng, Singapore 308433
| | - Mui Cheng Liang
- Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 2 Medical Drive, Singapore 117597
| | - Olga Pletnikova
- Departments of Pathology and Neurology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
| | - Juan C. Troncoso
- Departments of Pathology and Neurology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
| | | | - Tuck Wah Soong
- Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, 2 Medical Drive, Singapore 117597
- National Neuroscience Institute, 11 Jalan Tan Tock Seng, Singapore 308433
- Department Biomedical Engineering, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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172
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Salkoff L, Butler A, Ferreira G, Santi C, Wei A. High-conductance potassium channels of the SLO family. Nat Rev Neurosci 2007; 7:921-31. [PMID: 17115074 DOI: 10.1038/nrn1992] [Citation(s) in RCA: 409] [Impact Index Per Article: 24.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
High-conductance, 'big' potassium (BK) channels encoded by the Slo gene family are among the largest and most complex of the extended family of potassium channels. The family of SLO channels apparently evolved from voltage-dependent potassium channels, but acquired a large conserved carboxyl extension, which allows channel gating to be altered in response to the direct sensing of several different intracellular ions, and by other second-messenger systems, such as those activated following neurotransmitter binding to G-protein-coupled receptors (GPCRs). This versatility has been exploited to serve many cellular roles, both within and outside the nervous system.
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Affiliation(s)
- Lawrence Salkoff
- Department of Anatomy and Neurobiology, Washington University School of Medicine, 660 S. Euclid Avenue, Saint Louis, Missouri 63110, USA.
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173
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Kong WJ, Guo CK, Zhang XW, Chen X, Zhang S, Li GQ, Li ZW, Van Cauwenberge P. The coupling of acetylcholine-induced BK channel and calcium channel in guinea pig saccular type II vestibular hair cells. Brain Res 2007; 1129:110-5. [PMID: 17157279 DOI: 10.1016/j.brainres.2006.10.043] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2006] [Revised: 10/06/2006] [Accepted: 10/20/2006] [Indexed: 11/22/2022]
Abstract
Molecular biological studies and electrophysiological data have demonstrated that acetylcholine (ACh) is the principal cochlear and vestibular efferent neurotransmitter among mammalians. However, the functional roles of ACh in type II vestibular hair cells (VHCs II) among mammalians are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9-containing nAChR)-activated small conductance, calcium-dependent potassium current (SK) in cochlear hair cells and frog saccular hair cells. The activation of SK current was necessary for the calcium influx through the alpha9-containing nAChR. Recently, we have demonstrated that ACh-induced big conductance, calcium-dependent potassium current (BK) was present in VHCs II of the vestibular end-organ of guinea pig. In this study, the nature of calcium influx for the activation of ACh-induced BK current in saccular VHCs II of guinea pig was investigated. Following extracellular perfusion of ACh, saccular VHCs II displayed a sustained outward current, which was sensitive to iberiotoxin (IBTX). High concentration of apamin failed to inhibit the current amplitude of ACh-induced outward current. Intracellular application of Cs(+) completely abolished the current evoked by ACh. ACh-induced current was potently inhibited by nifedipine, nimodipine, Cd(2+) and Ni(2+), respectively. The inhibition potency of these four calcium channel antagonists was nimodipine>nifedipine>cadmium>nickel. The L-type Ca(2+) channels agonist, (-)-Bay-K 8644 mimicked the effect of ACh and activated an IBTX-sensitive current. In addition, partial VHCs II displayed a biphasic waveform. In conclusion, the present data showed that in the guinea pig saccular VHCs II, ACh-induced BK channel was coupled with the calcium channel, but not the receptor. The perfusion of ACh will drive the opening of calcium channels; the influx of calcium ions will then activate the BK current.
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Affiliation(s)
- Wei-Jia Kong
- Department of Otolaryngology, Union Hospital of Tongji Medical College, Hua-Zhong University of Science and Technology, Wu Han, 430022, People's Republic of China.
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174
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Chen WC, Davis RL. Voltage-gated and two-pore-domain potassium channels in murine spiral ganglion neurons. Hear Res 2006; 222:89-99. [PMID: 17079103 DOI: 10.1016/j.heares.2006.09.002] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/23/2006] [Revised: 09/02/2006] [Accepted: 09/06/2006] [Indexed: 11/21/2022]
Abstract
The systematically varied firing features of spiral ganglion neurons provide an excellent model system for the exploration of how graded ion channel distributions can be used to organize neuronal firing across a population of neurons. Elucidating the underlying mechanisms that determine neuronal response properties requires a complete understanding of the combination of ion channels, auxiliary proteins, modulators, and second messengers that form this highly organized system in the auditory periphery. Toward this goal, we built upon previous studies of voltage-gated K+-selective ion channels (Kv), and expanded our analysis to K+-selective leak channels (KCNK), which can play a major role in setting the basic firing characteristics of spiral ganglion neurons. To begin a more comprehensive analysis of Kv and KCNK channels, a screening approach was employed. RT-PCR was utilized to examine gene expression, the major results of which were confirmed with immunocytochemistry. Initial studies validated this approach by accurately detecting voltage-dependent K+ channels that were documented previously in the spiral ganglion. Furthermore, an additional channel type within the Kv3 family, Kv3.3, was identified and further characterized. The major focus of the study, however, was to systematically examine gene expression levels of the KCNK family of K+-selective leak channels. These channel types determine the resting membrane potential which has a major impact on setting the level of neuronal excitation. TWIK-1, TASK-3, TASK-1, and TREK-1 were expressed in the spiral ganglion; TWIK-1 was specifically localized with immunocytochemistry to the neuronal somata and initial processes of spiral ganglion neurons in vitro.
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Affiliation(s)
- Wei Chun Chen
- Department of Cell Biology and Neuroscience, Nelson Laboratories, Rutgers University, 604 Allison Road, Piscataway, NJ 08854, USA
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175
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Pyott SJ, Meredith AL, Fodor AA, Vázquez AE, Yamoah EN, Aldrich RW. Cochlear function in mice lacking the BK channel alpha, beta1, or beta4 subunits. J Biol Chem 2006; 282:3312-24. [PMID: 17135251 DOI: 10.1074/jbc.m608726200] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Large conductance voltage- and calcium-activated potassium (BK) channels are important for regulating many essential cellular functions, from neuronal action potential shape and firing rate to smooth muscle contractility. In amphibians, reptiles, and birds, BK channels mediate the intrinsic frequency tuning of the cochlear hair cell by an electrical resonance mechanism. In contrast, inner hair cells of the mammalian cochlea are extrinsically tuned by accessory structures of the cochlea. Nevertheless, BK channels are present in inner hair cells and encode a fast activating outward current. To understand the role of the BK channel alpha and beta subunits in mammalian inner hair cells, we analyzed the morphology, physiology, and function of these cells from mice lacking the BK channel alpha (Slo-/-) and also the beta1 and beta4 subunits (beta1/4-/-). Beta1/4-/- mice showed normal subcellular localization, developmental acquisition, and expression of BK channels. Beta1/4-/- mice showed normal cochlear function as indicated by normal auditory brainstem responses and distortion product otoacoustic emissions. Slo-/- mice also showed normal cochlear function despite the absence of the BKalpha subunit and the absence of fast activating outward current from the inner hair cells. Moreover, microarray analyses revealed no compensatory changes in transcripts encoding ion channels or transporters in the cochlea from Slo-/- mice. Slo-/- mice did, however, show increased resistance to noise-induced hearing loss. These findings reveal the fundamentally different contribution of BK channels to nonmammalian and mammalian hearing and suggest that BK channels should be considered a target in the prevention of noise-induced hearing loss.
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Affiliation(s)
- Sonja J Pyott
- Department of Otolaryngology Head and Neck Surgery, The Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA.
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176
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Abstract
The ability of hair bundles to signal head movements and sounds depends significantly on their structure, but a quantitative picture of bundle structure has proved elusive. The problem is acute for vestibular organs because their hair bundles exhibit complex morphologies that vary with endorgan, hair cell type, and epithelial locus. Here we use autocorrelation analysis to quantify stereociliary arrays (the number, spacing, and distribution of stereocilia) on hair cells of the turtle utricle. Our first goal was to characterize zonal variation across the macula, from medial extrastriola, through striola, to lateral extrastriola. This is important because it may help explain zonal variation in response dynamics of utricular hair cells and afferents. We also use known differences in type I and II bundles to estimate array characteristics of these two hair cell types. Our second goal was to quantify variation in array orientation at single macular loci and use this to estimate directional tuning in utricular afferents. Our major findings are that, of the features measured, array width is the most distinctive feature of striolar bundles, and within the striola there are significant, negatively correlated gradients in stereocilia number and spacing that parallel gradients in bundle heights. Together with previous results on stereocilia number and bundle heights, our results support the hypothesis that striolar hair cells are specialized to signal high-frequency/acceleration head movements. Finally, there is substantial variation in bundle orientation at single macular loci that may help explain why utricular afferents respond to stimuli orthogonal to their preferred directions.
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Affiliation(s)
- M H Rowe
- Department of Biological Sciences, Ohio University, Athens, OH 45701, USA
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177
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Kim HJ, Lim HH, Rho SH, Eom SH, Park CS. Hydrophobic interface between two regulators of K+ conductance domains critical for calcium-dependent activation of large conductance Ca2+-activated K+ channels. J Biol Chem 2006; 281:38573-81. [PMID: 17040919 DOI: 10.1074/jbc.m604769200] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
It has been suggested that the large conductance Ca(2)+-activated K(+) channel contains one or more domains known as regulators of K(+) conductance (RCK) in its cytosolic C terminus. Here, we show that the second RCK domain (RCK2) is functionally important and that it forms a heterodimer with RCK1 via a hydrophobic interface. Mutant channels lacking RCK2 are nonfunctional despite their tetramerization and surface expression. The hydrophobic residues that are expected to form an interface between RCK1 and RCK2, based on the crystal structure of the bacterial MthK channel, are well conserved, and the interactions of these residues were confirmed by mutant cycle analysis. The hydrophobic interaction appears to be critical for the Ca(2+)-dependent gating of the large conductance Ca(2+)-activated K(+) channel.
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Affiliation(s)
- Hyun-Ju Kim
- Department of Life Science and Research Center for Biomolecular Nanotechnology, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea
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178
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Yu JY, Upadhyaya AB, Atkinson NS. Tissue-specific alternative splicing of BK channel transcripts in Drosophila. GENES BRAIN AND BEHAVIOR 2006; 5:329-39. [PMID: 16716202 DOI: 10.1111/j.1601-183x.2005.00164.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
BK-type calcium-activated potassium channels are large conductance channels that respond to changes in intracellular calcium and membrane potential. These channels are used in a wide variety of cell types and have recently been linked to drug sensitivity and tolerance. In both Drosophila and mammals, BK channels are encoded by the slowpoke gene. The Drosophila slowpoke gene includes 14 alternative exons distributed among five sites of alternative splicing. Presumably, the purpose of alternative processing is to provide transcripts tailored to the needs of the cell. The slowpoke gene is expressed in nervous, muscle and epithelial tissues. To determine whether splicing is controlled in a tissue- and/or developmental-specific manner, we built tissue- and developmental-specific cDNA libraries that preserved the relative frequency of various slowpoke splice variants. These libraries were screened by colony hybridization using alternative exon-specific DNA probes to document the frequency of individual alternative exons in different developmental stages and distinct tissue types. We demonstrate that slowpoke transcripts undergo tissue- and developmental-specific splicing in Drosophila and some exons are diagnostic for specific tissues.
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Affiliation(s)
- J Y Yu
- Section of Neurobiology, The University of Texas at Austin, Austin, TX 78712-0248, USA
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179
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Abstract
Large-conductance (BK-type) Ca2+-activated potassium channels are activated by membrane depolarization and cytoplasmic Ca2+. BK channels are expressed in a broad variety of cells and have a corresponding diversity in properties. Underlying much of the functional diversity is a family of four tissue-specific accessory subunits (β1–β4). Biophysical characterization has shown that the β4 subunit confers properties of the so-called “type II” BK channel isotypes seen in brain. These properties include slow gating kinetics and resistance to iberiotoxin and charybdotoxin blockade. In addition, the β4 subunit reduces the apparent voltage sensitivity of channel activation and has complex effects on apparent Ca2+ sensitivity. Specifically, channel activity at low Ca2+ is inhibited, while at high Ca2+, activity is enhanced. The goal of this study is to understand the mechanism underlying β4 subunit action in the context of a dual allosteric model for BK channel gating. We observed that β4's most profound effect is a decrease in Po (at least 11-fold) in the absence of calcium binding and voltage sensor activation. However, β4 promotes channel opening by increasing voltage dependence of Po-V relations at negative membrane potentials. In the context of the dual allosteric model for BK channels, we find these properties are explained by distinct and opposing actions of β4 on BK channels. β4 reduces channel opening by decreasing the intrinsic gating equilibrium (L0), and decreasing the allosteric coupling between calcium binding and voltage sensor activation (E). However, β4 has a compensatory effect on channel opening following depolarization by shifting open channel voltage sensor activation (Vho) to more negative membrane potentials. The consequence is that β4 causes a net positive shift of the G-V relationship (relative to α subunit alone) at low calcium. At higher calcium, the contribution by Vho and an increase in allosteric coupling to Ca2+ binding (C) promotes a negative G-V shift of α+β4 channels as compared to α subunits alone. This manner of modulation predicts that type II BK channels are downregulated by β4 at resting voltages through effects on L0. However, β4 confers a compensatory effect on voltage sensor activation that increases channel opening during depolarization.
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Affiliation(s)
- Bin Wang
- Department of Physiology, University of Texas Health Science Center at San Antonio, 78229, USA
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180
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Beisel KW, Rocha-Sanchez SM, Ziegenbein SJ, Morris KA, Kai C, Kawai J, Carninci P, Hayashizaki Y, Davis RL. Diversity of Ca2+-activated K+ channel transcripts in inner ear hair cells. Gene 2006; 386:11-23. [PMID: 17097837 DOI: 10.1016/j.gene.2006.07.023] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2006] [Revised: 07/13/2006] [Accepted: 07/18/2006] [Indexed: 10/24/2022]
Abstract
Hair cells express a complement of ion channels, representing shared and distinct channels that confer distinct electrophysiological signatures for each cell. This diversity is generated by the use of alternative splicing in the alpha subunit, formation of heterotetrameric channels, and combinatorial association with beta subunits. These channels are thought to play a role in the tonotopic gradient observed in the mammalian cochlea. Mouse Kcnma1 transcripts, 5' and 3' ESTs, and genomic sequences were examined for the utilization of alternative splicing in the mouse transcriptome. Comparative genomic analyses investigated the conservation of KCNMA1 splice sites. Genomes of mouse, rat, human, opossum, chicken, frog and zebrafish established that the exon-intron structure and mechanism of KCNMA1 alternative splicing were highly conserved with 6-7 splice sites being utilized. The murine Kcnma1 utilized 6 out of 7 potential splice sites. RT-PCR experiments using murine gene-specific oligonucleotide primers analyzed the scope and variety of Kcnma1 and Kcnmb1-4 expression profiles in the cochlea and inner ear hair cells. In the cochlea splice variants were present representing sites 3, 4, 6, and 7, while site 1 was insertionless and site 2 utilized only exon 10. However, site 5 was not present. Detection of KCNMA1 transcripts and protein exhibited a quantitative longitudinal gradient with a reciprocal gradient found between inner and outer hair cells. Differential expression was also observed in the usage of the long form of the carboxy-terminus tail. These results suggest that a diversity of splice variants exist in rodent cochlear hair cells and this diversity is similar to that observed for non-mammalian vertebrate hair cells, such as chicken and turtle.
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Affiliation(s)
- Kirk W Beisel
- Department of Biomedical Sciences, Creighton University, 2500 California Plaza, Omaha, Nebraska 68178, USA
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181
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Abstract
Intracellular Mg(2+) at physiological concentrations activates mSlo1 BK channels by binding to a metal-binding site in the cytosolic domain. Previous studies suggest that residues E374, Q397, and E399 are important in Mg(2+) binding. In the present study, we show that mutations of E374 or E399 to other amino acids, except for Asp, abolish Mg(2+) sensitivity. These results further support that the side chains of E374 and E399 are essential for Mg(2+) coordination. To the contrary, none of the Q397 mutations abolishes Mg(2+) sensitivity, suggesting that its side chain may not coordinate to Mg(2+). However, because Q397 is spatially close to E374 and E399, its mutations affect the Mg(2+) sensitivity of channel gating by either reducing or increasing the Mg(2+) binding affinity. The pattern of mutational effects and the effect of chemical modification of Q397C indicate that Q397 is involved in the Mg(2+)-dependent activation of BK channels and that mutations of Q397 alter Mg(2+) sensitivity by affecting the conformation of the Mg(2+) binding site as well as by electrostatic interactions with the bound Mg(2+) ion.
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Affiliation(s)
- Huanghe Yang
- Department of Biomedical Engineering and Cardiac Bioelectricity and Arrhythmia Center, Washington University, St. Louis, Missouri 63130, USA
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182
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Oliver D, Taberner AM, Thurm H, Sausbier M, Arntz C, Ruth P, Fakler B, Liberman MC. The role of BKCa channels in electrical signal encoding in the mammalian auditory periphery. J Neurosci 2006; 26:6181-9. [PMID: 16763026 PMCID: PMC1806704 DOI: 10.1523/jneurosci.1047-06.2006] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Large-conductance voltage- and Ca(2+)-activated K+ channels (BKCa) are involved in shaping spiking patterns in many neurons. Less is known about their role in mammalian inner hair cells (IHCs), mechanosensory cells with unusually large BKCa currents. These currents may be involved in shaping the receptor potential, implying crucial importance for the properties of afferent auditory signals. We addressed the function of BKCa by recording sound-induced responses of afferent auditory nerve (AN) fibers from mice with a targeted deletion of the pore-forming alpha-subunit of BKCa (BKalpha(-/-)) and comparing these with voltage responses of current-clamped IHCs. BKCa-mediated currents in IHCs were selectively abolished in BKalpha(-/-), whereas cochlear physiology was essentially normal with respect to cochlear sensitivity and frequency tuning.BKalpha(-/-) AN fibers showed deteriorated precision of spike timing, measured as an increased variance of first spike latency in response to tone bursts. This impairment could be explained by a slowed voltage response in the presynaptic IHC resulting from the reduced K+ conductance in the absence of BKCa. Maximum spike rates of AN fibers were reduced nearly twofold in BKalpha(-/-), contrasting with increased voltage responses of IHCs. In addition to presynaptic changes, which may be secondary to a modest depolarization of BKalpha(-/-) IHCs, this reduction in AN rates suggests a role of BKCa in postsynaptic AN neurons, which was supported by increased refractory periods. In summary, our results indicate an essential role of IHC BKCa channels for precise timing of high-frequency cochlear signaling as well as a function of BKCa in the primary afferent neuron.
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Affiliation(s)
- Dominik Oliver
- Physiologisches Institut, Universität Freiburg, D-79104 Freiburg, Germany.
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183
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Abstract
It is commonly held that hearing generally begins on incubation day 12 (E12) in the chicken embryo ( Gallus domesticus). However, little is known about the response properties of cochlear ganglion neurons for ages younger than E18. We studied ganglion neurons innervating the basilar papilla of embryos (E12–E18) and hatchlings (P13–P15). We asked first, when do primary afferent neurons begin to encode sounds? Second, when do afferents evidence frequency selectivity? Third, what range of characteristic frequencies (CFs) is represented in the late embryo? Finally, how does sound transfer from air to the cochlea affect responses in the embryo and hatchling? Responses to airborne sound were compared with responses to direct columella footplate stimulation of the cochlea. Cochlear ganglion neurons exhibited a profound insensitivity to sound from E12 to E16 (stages 39–42). Responses to sound and frequency selectivity emerged at about E15. Frequency selectivity matured rapidly from E16 to E18 (stages 42 and 44) to reflect a mature range of CFs (170–4,478 Hz) and response sensitivity to footplate stimulation. Limited high-frequency sound transfer from air to the cochlea restricted the response to airborne sound in the late embryo. Two periods of ontogeny are proposed. First is a prehearing period (roughly E12–E16) of endogenous cochlear signaling that provides neurotrophic support and guides normal developmental refinements in central binaural processing pathways followed by a period (roughly E16–E19) wherein the cochlea begins to detect and encode sound.
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Affiliation(s)
- Timothy A Jones
- Department of Communication Sciences and Disorders, School of Allied Health Sciences, East Carolina University, Greenville, NC 27858, USA.
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184
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Dreyer A, Delgutte B. Phase locking of auditory-nerve fibers to the envelopes of high-frequency sounds: implications for sound localization. J Neurophysiol 2006; 96:2327-41. [PMID: 16807349 PMCID: PMC2013745 DOI: 10.1152/jn.00326.2006] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Although listeners are sensitive to interaural time differences (ITDs) in the envelope of high-frequency sounds, both ITD discrimination performance and the extent of lateralization are poorer for high-frequency sinusoidally amplitude-modulated (SAM) tones than for low-frequency pure tones. Psychophysical studies have shown that ITD discrimination at high frequencies can be improved by using novel transposed-tone stimuli, formed by modulating a high-frequency carrier by a half-wave-rectified sinusoid. Transposed tones are designed to produce the same temporal discharge patterns in high-characteristic frequency (CF) neurons as occur in low-CF neurons for pure-tone stimuli. To directly test this hypothesis, we compared responses of auditory-nerve fibers in anesthetized cats to pure tones, SAM tones, and transposed tones. Phase locking was characterized using both the synchronization index and autocorrelograms. With both measures, phase locking was better for transposed tones than for SAM tones, consistent with the rationale for using transposed tones. However, phase locking to transposed tones and that to pure tones were comparable only when all three conditions were met: stimulus levels near thresholds, low modulation frequencies (<250 Hz), and low spontaneous discharge rates. In particular, phase locking to both SAM tones and transposed tones substantially degraded with increasing stimulus level, while remaining more stable for pure tones. These results suggest caution in assuming a close similarity between temporal patterns of peripheral activity produced by transposed tones and pure tones in both psychophysical studies and neurophysiological studies of central neurons.
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Affiliation(s)
- Anna Dreyer
- Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, 243 Charles Street, Boston, MA 02114, USA
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185
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Nickel R, Becker D, Forge A. Molecular and functional characterization of gap junctions in the avian inner ear. J Neurosci 2006; 26:6190-9. [PMID: 16763027 PMCID: PMC6675194 DOI: 10.1523/jneurosci.1116-06.2006] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2006] [Revised: 04/12/2006] [Accepted: 04/27/2006] [Indexed: 11/21/2022] Open
Abstract
To analyze the fundamental role of gap junctions in the vertebrate inner ear, we examined molecular and functional characteristics of gap junctional communication (GJC) in the auditory and vestibular system of the chicken. By screening inner ear tissues for connexin isoforms using degenerate reverse transcription-PCR, we identified, in addition to chicken Cx43 (cCx43) and the inner-ear-specific cCx30, an as yet uncharacterized connexin predicted to be the ortholog of the mammalian Cx26. In situ hybridization indicated that cCx30 and cCx26 transcripts were both widely expressed in the cochlear duct and utricle in an overlapping pattern, suggesting coexpression of these isoforms similar to that in the mammalian inner ear. Immunohistochemistry demonstrated that cCx43 was present in gap junctions connecting supporting cells of the basilar papilla, in which its immunofluorescence colocalized with that of cCx30. However, cCx43 was absent from supporting cell gap junctions of the utricular macula. This variation in the molecular composition of gap junction plaques coincided with differences in the functional properties of GJC between the auditory and vestibular sensory epithelia. Fluorescence recovery after photobleaching, adapted to examine the diffusion of calcein in inner ear explants, revealed asymmetric communication pathways among supporting cells in the basilar papilla but not in the utricular macula. This study supports the hypothesis that the coexpression of Cx26/Cx30 is unique to gap junctions in the vertebrate inner ear. Furthermore, it demonstrates asymmetric GJC within the supporting cell population of the auditory sensory epithelium, which might mediate potassium cycling and/or intercellular signaling.
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Affiliation(s)
- Regina Nickel
- Centre for Auditory Research, The Ear Institute, University College London, London WC1X 8EE, United Kingdom.
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186
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Housley GD, Marcotti W, Navaratnam D, Yamoah EN. Hair Cells – Beyond the Transducer. J Membr Biol 2006; 209:89-118. [PMID: 16773496 DOI: 10.1007/s00232-005-0835-7] [Citation(s) in RCA: 76] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2005] [Indexed: 02/06/2023]
Abstract
OVERVIEW This review considers the "tween twixt and twain" of hair cell physiology, specifically the signaling elements and membrane conductances which underpin forward and reverse transduction at the input stage of hair cell function and neurotransmitter release at the output stage. Other sections of this review series outline the advances which have been made in understanding the molecular physiology of mechanoelectrical transduction and outer hair cell electromotility. Here we outline the contributions of a considerable array of ion channels and receptor signaling pathways that define the biophysical status of the sensory hair cells, contributing to hair cell development and subsequently defining the operational condition of the hair cells across the broad dynamic range of physiological function.
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Affiliation(s)
- G D Housley
- Department of Physiology, University of Auckland, Private Bag, 92019, Auckland, New Zealand.
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187
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Dulon D, Jagger DJ, Lin X, Davis RL. Neuromodulation in the Spiral Ganglion: Shaping Signals from the Organ of Corti to the CNS. J Membr Biol 2006; 209:167-75. [PMID: 16773500 DOI: 10.1007/s00232-005-0841-9] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/04/2006] [Indexed: 11/25/2022]
Affiliation(s)
- D Dulon
- Laboratoire de Biologie Cellulaire et Moléculaire de l'Audition, Hôpital Pellegrin, INSERM et EA 3665 Université de Bordeaux 2, 33076, Bordeaux, France
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188
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Radeloff A, Smolders JWT. Brain-derived neurotrophic factor treatment does not improve functional recovery after hair cell regeneration in the pigeon. Acta Otolaryngol 2006; 126:452-9. [PMID: 16698693 DOI: 10.1080/00016480500437344] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
CONCLUSIONS Brain-derived neurotrophic factor (BDNF) supply to the inner ear does not improve the time course or the extent of functional recovery after hair cell regeneration. Specifically it does not improve the residual threshold elevation observed after the completion of spontaneous recovery. OBJECTIVE The avian inner ear is capable of hair cell regeneration and substantial functional recovery, but residual hearing deficits remain. We investigated whether functional recovery can be improved by intracochlear application of BDNF, which plays an important role in auditory ontogenesis and maintenance during adult life. METHODS Hair cells in adult pigeons were destroyed by local application of gentamicin. After 3 days either BDNF or control solution was administered to the scala tympani by implanted osmotic minipumps for 8 weeks. Auditory brain stem responses (ABR) to tone pips were used to assess recovery of hearing thresholds in both groups. RESULTS The application of gentamicin caused a frequency-dependent hearing loss that ranged from 24.8 dB SPL at low frequencies to 66.2 dB SPL at high frequencies. After day 10 substantial recovery was observed, but a significant threshold shift remained. The time course of recovery in the control and BDNF-treated groups was similar, without significant residual threshold differences in any frequency range.
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Affiliation(s)
- A Radeloff
- Physiologisches Institut II, Zentrum der Hals-Nasen-Ohrenheilkunde, Frankfurt am Main, Germany
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189
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Douglas RM, Lai JCK, Bian S, Cummins L, Moczydlowski E, Haddad GG. The calcium-sensitive large-conductance potassium channel (BK/MAXI K) is present in the inner mitochondrial membrane of rat brain. Neuroscience 2006; 139:1249-61. [PMID: 16567053 DOI: 10.1016/j.neuroscience.2006.01.061] [Citation(s) in RCA: 75] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2005] [Revised: 01/24/2006] [Accepted: 01/26/2006] [Indexed: 02/04/2023]
Abstract
Large-conductance voltage- and calcium-sensitive channels are known to be expressed in the plasmalemma of central neurons; however, recent data suggest that large-conductance voltage- and calcium-sensitive channels may also be present in mitochondrial membranes. To determine the subcellular localization and distribution of large-conductance voltage- and calcium-sensitive channels, rat brain fractions obtained by Ficoll-sucrose density gradient centrifugation were examined by Western blotting, immunocytochemistry and immuno-gold electron microscopy. Immunoblotting studies demonstrated the presence of a consistent signal for the alpha subunit of the large-conductance voltage- and calcium-sensitive channel in the mitochondrial fraction. Double-labeling immunofluorescence also demonstrated that large-conductance voltage- and calcium-sensitive channels are present in mitochondria and co-localize with mitochondrial-specific proteins such as the translocase of the inner membrane 23, adenine nucleotide translocator, cytochrome c oxidase or complex IV-subunit 1 and the inner mitochondrial membrane protein but do not co-localize with calnexin, an endoplasmic reticulum marker. Western blotting of discrete subcellular fractions demonstrated that cytochrome c oxidase or complex IV-subunit 1 was only expressed in the mitochondrial fraction whereas actin, acetylcholinesterase, cadherins, calnexin, 58 kDa Golgi protein, lactate dehydrogenase and microtubule-associated protein 1 were not, demonstrating the purity of the mitochondrial fraction. Electron microscopic examination of the mitochondrial pellet demonstrated gold particle labeling within mitochondria, indicative of the presence of large-conductance voltage- and calcium-sensitive channels in the inner mitochondrial membrane. These studies provide concrete morphological evidence for the existence of large-conductance voltage- and calcium-sensitive channels in mitochondria: our findings corroborate the recent electrophysiological evidence of mitochondrial large-conductance voltage- and calcium-sensitive channels in glioma and cardiac cells.
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Affiliation(s)
- R M Douglas
- Department of Pediatrics (Division of Respiratory Medicine), Albert Einstein College of Medicine, Bronx, NY 10461, USA
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190
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Rutherford MA, Roberts WM. Frequency selectivity of synaptic exocytosis in frog saccular hair cells. Proc Natl Acad Sci U S A 2006; 103:2898-903. [PMID: 16473940 PMCID: PMC1413814 DOI: 10.1073/pnas.0511005103] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The ability to respond selectively to particular frequency components of sensory inputs is fundamental to signal processing in the ear. The frog (Rana pipiens) sacculus, which is used for social communication and escape behaviors, is an exquisitely sensitive detector of sounds and ground-borne vibrations in the 5- to 200-Hz range, with most afferent axons having best frequencies between 40 and 60 Hz. We monitored the synaptic output of saccular sensory receptors (hair cells) by measuring the increase in membrane capacitance (deltaC(m)) that occurs when synaptic vesicles fuse with the plasmalemma. Strong stepwise depolarization evoked an exocytic burst that lasted 10 ms and corresponded to the predicted capacitance of all docked vesicles at synapses, followed by a 20-ms delay before additional vesicle fusion. Experiments using weak stimuli, within the normal physiological range for these cells, revealed a sensitivity to the temporal pattern of membrane potential changes. Interrupting a weak depolarization with a properly timed hyperpolarization increased deltaC(m). Small sinusoidal voltage oscillations (+/-5 mV centered at -60 mV) evoked a deltaC(m) that corresponded to 95 vesicles per s at each synapse at 50 Hz but only 26 vesicles per s at 5 Hz and 27 vesicles per s at 200 Hz (perforated patch recordings). This frequency selectivity was absent for larger sinusoidal oscillations (+/-10 mV centered at -55 mV) and was largest for hair cells with the smallest sinusoidal-stimuli-evoked Ca2+ currents. We conclude that frog saccular hair cells possess an intrinsic synaptic frequency selectivity that is saturated by strong stimuli.
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Affiliation(s)
| | - William M. Roberts
- Institute of Neuroscience, University of Oregon, Eugene, OR 97403
- *To whom correspondence should be addressed at:
Institute of Neuroscience, University of Oregon, 1254 University of Oregon, Eugene, OR 97403. E-mail:
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191
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Farahbakhsh NA, Narins PM. Slow motility in hair cells of the frog amphibian papilla: Ca2+-dependent shape changes. Hear Res 2006; 212:140-59. [PMID: 16426781 DOI: 10.1016/j.heares.2005.11.004] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/12/2005] [Accepted: 11/23/2005] [Indexed: 11/30/2022]
Abstract
We investigated the process of slow motility in non-mammalian auditory hair cells by recording the time course of shape change in hair cells of the frog amphibian papilla. The tall hair cells in the rostral segment of this organ, reported to be the sole recipients of efferent innervation, were found to shorten in response to an increase in the concentration of the intracellular free calcium. These shortenings are composed of two partially-overlapping phases: an initial rapid iso-volumetric contraction, followed by a slower length decrease accompanied with swelling. It is possible to unmask the iso-volumetric contraction by delaying the cell swelling with the help of K+ or Cl- channel inhibitors, quinine or furosemide. Furthermore, it appears that the longitudinal contraction in these cells is Ca2+-calmodulin-dependent: in the presence of W-7, a calmodulin inhibitor, only a slow, swelling phase could be observed. These findings suggest that amphibian rostral AP hair cells resemble their mammalian counterparts in expressing both a Ca2+-calmodulin-dependent contractile structure and an "osmotic" mechanism capable of mediating length change in response to extracellular stimuli. Such a mechanism might be utilized by the efferent neurotransmitters for adaptive modulation of mechano-electrical transduction, sensitivity enhancement, frequency selectivity, and protection against over-stimulation.
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Affiliation(s)
- Nasser A Farahbakhsh
- Department of Physiological Science, 621 Charles E. Young Drive S. University of California, Los Angeles, CA 90095-1606, USA.
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192
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King JT, Lovell PV, Rishniw M, Kotlikoff MI, Zeeman ML, McCobb DP. Beta2 and beta4 subunits of BK channels confer differential sensitivity to acute modulation by steroid hormones. J Neurophysiol 2006; 95:2878-88. [PMID: 16436475 DOI: 10.1152/jn.01352.2005] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
Membrane-associated receptors for rapid, steroidal neuromodulation remain elusive. Estradiol has been reported to facilitate activation of voltage- and Ca(2+)-dependent BK potassium channels encoded by Slo, if associated with beta1 subunits. We show here that 1) multiple members of the beta family confer sensitivity to multiple steroids on BK channels, 2) that beta subunits differentiate between steroids, and 3) that different betas have distinct relative preferences for particular steroids. Expressed in HEK 293 cells, inside-out patches with channels composed of Slo-alpha alone showed no steroid sensitivity. Cells expressing alphabeta4 exhibited potent, rapid, reversible, and dose-dependent potentiation by corticosterone (CORT; a glucocorticoid), and were potentiated to a lesser degree by other sex and stress steroids. In contrast, alphabeta2 channels were potentiated more strongly by dehydroepiandrosterone (DHEA; an enigmatic, stress-related adrenal androgen), and to a lesser extent by CORT, estradiol, testosterone, and DHEA-S. Cholesterol had no effect on any BK channel compositions tested. Conductance-voltage plots of channels composed of alpha plus beta2 or beta4 subunits were shifted in the negative direction by steroids, indicating greater activation at negative voltages. Thus our results argue that the variety of Slo-beta subunit coexpression patterns occurring in vivo expands the repertoire of Slo channel gating in yet another dimension not fully appreciated, rendering BK gating responsive to dynamic fluctuations in a multiple of steroid hormones.
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Affiliation(s)
- J T King
- Department of Neurobiology and Behavior, Cornell University, W153 Mudd Hall, Ithaca, NY 14853, USA
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193
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Xue J, Peterson EH. Hair Bundle Heights in the Utricle: Differences Between Macular Locations and Hair Cell Types. J Neurophysiol 2006; 95:171-86. [PMID: 16177175 DOI: 10.1152/jn.00800.2005] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Hair bundle structure is a major determinant of bundle mechanics and thus of a hair cell's ability to encode sound and head movement stimuli. Little quantitative information about bundle structure is available for vestibular organs. Here we characterize hair bundle heights in the utricle of a turtle, Trachemys scripta. We visualized bundles from the side using confocal images of utricular slices. We measured kinocilia and stereocilia heights and array length (distance from tall to short end of bundle), and we calculated a KS ratio (kinocilium height/height of the tallest stereocilia) and bundle slope (height fall-off from tall to short end of bundle). To ensure that our measurements reflect in vivo dimensions as closely as possible, we used fixed but undehydrated utricular slices, and we measured heights in three dimensions by tracing kinocilia and stereocilia through adjacent confocal sections. Bundle heights vary significantly with position on the utricular macula and with hair cell type. Type II hair cells are found throughout the macula. We identified four subgroups that differ in bundle structure: zone 1 (lateral extrastriola), striolar zone 2, striolar zone 3, and zone 4 (medial extrastriola). Type I hair cells are confined to striolar zone 3. They have taller stereocilia, longer arrays, lower KS ratios, and steeper slopes than do neighboring (zone 3) type II bundles. Models and experiments suggest that these location- and type-specific differences in bundle heights will yield parallel variations in bundle mechanics. Our data also raise the possibility that differences in bundle structure and mechanics will help explain location- and type-specific differences in the physiological profiles of utricular afferents, which have been reported in frogs and mammals.
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Affiliation(s)
- Jingbing Xue
- Department of Biological Sciences, Irvine Hall, Ohio University, Athens, OH 45701, USA
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194
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Hu L, Yang H, Shi J, Cui J. Effects of multiple metal binding sites on calcium and magnesium-dependent activation of BK channels. ACTA ACUST UNITED AC 2005; 127:35-49. [PMID: 16344323 PMCID: PMC2151482 DOI: 10.1085/jgp.200509317] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
BK channels are activated by physiological concentrations of intracellular Ca2+ and Mg2+ in a variety of cells. Previous studies have identified two sites important for high-affinity Ca2+ sensing between [Ca2+]i of 0.1–100 μM and a site important for Mg2+ sensing between [Mg2+]i of 0.1–10 mM. BK channels can be also activated by Ca2+ and Mg2+ at concentrations >10 mM so that the steady-state conductance and voltage (G-V) relation continuously shifts to more negative voltage ranges when [Mg2+]i increases from 0.1–100 mM. We demonstrate that a novel site is responsible for metal sensing at concentrations ≥10 mM, and all four sites affect channel activation independently. As a result, the contributions of these sites to channel activation are complex, depending on the combination of Ca2+ and Mg2+ concentrations. Here we examined the effects of each of these sites on Ca2+ and Mg2+-dependent activation and the data are consistent with the suggestion that these sites are responsible for metal binding. We provide an allosteric model for quantitative estimation of the contributions that each of these putative binding sites makes to channel activation at any [Ca2+]i and [Mg2+]i.
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Affiliation(s)
- Lei Hu
- Department of Biomedical Engineering and Cardiac Bioelectricity and Arrhythmia Center, Washington University, St. Louis, MO 63130, USA
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195
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Layton MG, Robertson D, Everett AW, Mulders WHAM, Yates GK. Cellular localization of voltage-gated calcium channels and synaptic vesicle-associated proteins in the guinea pig cochlea. J Mol Neurosci 2005; 27:225-44. [PMID: 16186634 DOI: 10.1385/jmn:27:2:225] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2004] [Accepted: 03/07/2005] [Indexed: 11/11/2022]
Abstract
The cellular localization of voltage-gated calcium channels (VGCCs) and synaptic vesicle-associated proteins, SV2, synapsin I, and vesicle-associated membrane protein (VAMP) (synaptobrevin), was investigated in the guinea pig cochlea using immunocytochemistry and confocal laser scanning microscopy. Reactivity, in guinea pig, of antibodies to the alpha1 subunits of L-type, alpha1C [Cav1.2] and alpha 1D [Cav1.3]; P/Q-type, alpha1A [Cav2.1]; and R-type, a1E [Cav2.3] high voltage-activated calcium channels, was determined by Western blotting and immunolabeling of cerebellum. In the cochlea the sensory inner hair cells of the organ of Corti displayed strong intracellular staining, predominantly localized to their basolateral poles, with an antibody directed against the alpha1C subunit. Some alpha1C labeling was also observed in the inner pillar cells, in cell bodies of afferent neurons in the spiral ganglion, and in the inferior region of the spiral ligament. The supporting pillar cells were strongly immunoreactive throughout for alpha1D, but no alpha1D labeling of the inner hair cells was seen. The alpha1A subunit showed a cytoplasmic distribution in all three rows of outer hair cells. alpha1E labeling localized to the outer hair cells, predominantly in the subcuticular plate region, and also to nerve fiber bundles beneath these hair cells. Strong immunoreactivity was consistently seen with antibodies directed against SV2 and synapsin I in neuronal structures surrounding the basolateral surfaces of both the inner and outer hair cells but was absent from the sensory cells themselves. VAMP labeling was found throughout the cytoplasm of the inner hair cells and in neuronal structures beneath the hair cells. These results reveal a differential distribution of VGCC-types in the sensory and nonsensory elements of the guinea pig cochlea, with the inner hair cells expressing alpha1C L-type channels and VAMP but not synapsin I or SV2.
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Affiliation(s)
- Maria G Layton
- The Auditory Laboratory, School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, Western Australia, 6009, Australia
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196
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Huang CS, Shi SH, Ule J, Ruggiu M, Barker LA, Darnell RB, Jan YN, Jan LY. Common molecular pathways mediate long-term potentiation of synaptic excitation and slow synaptic inhibition. Cell 2005; 123:105-18. [PMID: 16213216 DOI: 10.1016/j.cell.2005.07.033] [Citation(s) in RCA: 116] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2005] [Revised: 07/03/2005] [Accepted: 07/28/2005] [Indexed: 01/22/2023]
Abstract
Synaptic plasticity, the cellular correlate for learning and memory, involves signaling cascades in the dendritic spine. Extensive studies have shown that long-term potentiation (LTP) of the excitatory postsynaptic current (EPSC) through glutamate receptors is induced by activation of N-methyl-D-asparate receptor (NMDA-R)--the coincidence detector--and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Here we report that the same signaling pathway in the postsynaptic CA1 pyramidal neuron also causes LTP of the slow inhibitory postsynaptic current (sIPSC) mediated by metabotropic GABA(B) receptors (GABA(B)-Rs) and G protein-activated inwardly rectifying K(+) (GIRK) channels, both residing in dendritic spines as well as shafts. Indicative of intriguing differences in the regulatory mechanisms for excitatory and inhibitory synaptic plasticity, LTP of sIPSC but not EPSC was abolished in mice lacking Nova-2, a neuronal-specific RNA binding protein that is an autoimmune target in paraneoplastic opsoclonus myoclonus ataxia (POMA) patients with latent cancer, reduced inhibitory control of movements, and dementia.
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Affiliation(s)
- Cindy Shen Huang
- Howard Hughes Medical Institute and Departments of Physiology and Biochemistry, University of California, San Francisco, CA 94143, USA
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197
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Jørgensen F, Kroese ABA. Ion channel regulation of the dynamical instability of the resting membrane potential in saccular hair cells of the green frog (Rana esculenta). ACTA ACUST UNITED AC 2005; 185:271-90. [PMID: 16266369 DOI: 10.1111/j.1365-201x.2005.01495.x] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
AIMS We investigated the ion channel regulation of the resting membrane potential of hair cells with the aim to determine if the resting membrane potential is poised close to instability and thereby a potential cause of the spontaneous afferent spike activity. METHODS The ionic mechanism and the dynamic properties of the resting membrane potential were examined with the whole-cell patch clamp technique in dissociated saccular hair cells and in a mathematical model including all identified ion channels. RESULTS In hair cells showing I/V curves with a low membrane conductance flanked by large inward and outward rectifying potassium conductances, the inward rectifier (K(IR)), the delayed outward rectifier (K(V)) and the large conductance, calcium-sensitive, voltage-gated potassium channel (BK(Ca)) were all activated at rest. Under current clamp conditions, the outward current through these channels balanced the inward current through mechano-electrical transduction (MET) and Ca2+ channels. In 45% (22/49) of the cells, the membrane potential fluctuated spontaneously between two voltage levels determined by the voltage extent of the low membrane conductance range. These fluctuations were not influenced by blocking the MET channels but could be reversibly stopped by increasing [K+]o or by blocking of K(IR) channels. Blocking the BK(Ca) channels induced regular voltage oscillations. CONCLUSIONS Two intrinsic dynamical instabilities of V(m) are present in hair cells. One of these is observed as spontaneous voltage fluctuations by currents through K(IR), K(V) and h-channels in combination with a steady current through MET channels. The other instability shows as regenerative voltage changes involving Ca2+ and K(V) channels. The BK(Ca) channels prevent the spontaneous voltage fluctuations from activating the regenerative system.
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Affiliation(s)
- F Jørgensen
- IMB, Physiology & Pharmacology, University of Southern Denmark, Odense, Denmark.
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198
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Schnee ME, Lawton DM, Furness DN, Benke TA, Ricci AJ. Auditory hair cell-afferent fiber synapses are specialized to operate at their best frequencies. Neuron 2005; 47:243-54. [PMID: 16039566 DOI: 10.1016/j.neuron.2005.06.004] [Citation(s) in RCA: 94] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2004] [Revised: 05/05/2005] [Accepted: 06/01/2005] [Indexed: 11/21/2022]
Abstract
Auditory afferent fiber activity is driven by high-fidelity information transfer from the sensory hair cell. Presynaptic specializations, posited to maintain fidelity, are investigated at synapses with characteristic frequencies of 120 Hz and 320 Hz. Morphological data indicate that high-frequency cells have more synapses and higher vesicle density near dense bodies (DBs). Tracking vesicular release via capacitance changes identified three overlapping kinetic components of release corresponding to morphologically identified vesicle pools. High-frequency cells released faster; however, when normalized to release site number, low-frequency cells released faster, likely due to a greater Ca2+ load per synapse. The Ca(2+)-dependence of release was nonsaturating and independent of frequency, suggesting that release, not refilling, was rate limiting. A model of release derived from vesicle equilibration between morphologically defined pools reproduced the capacitance data, supporting a critical role in vesicle trafficking for DBs. The model suggests that presynaptic specializations enable synapses to operate most efficiently at their characteristic frequencies.
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Affiliation(s)
- M E Schnee
- Neuroscience Center and Kresge Hearing Labs, Louisiana State University, New Orleans, LA 70112, USA
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199
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Krishnamoorthy G, Shi J, Sept D, Cui J. The NH2 terminus of RCK1 domain regulates Ca2+-dependent BK(Ca) channel gating. ACTA ACUST UNITED AC 2005; 126:227-41. [PMID: 16103277 PMCID: PMC2266574 DOI: 10.1085/jgp.200509321] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Large conductance, voltage- and Ca2+-activated K+ (BKCa) channels regulate blood vessel tone, synaptic transmission, and hearing owing to dual activation by membrane depolarization and intracellular Ca2+. Similar to an archeon Ca2+-activated K+ channel, MthK, each of four α subunits of BKCa may contain two cytosolic RCK domains and eight of which may form a gating ring. The structure of the MthK channel suggests that the RCK domains reorient with one another upon Ca2+ binding to change the gating ring conformation and open the activation gate. Here we report that the conformational changes of the NH2 terminus of RCK1 (AC region) modulate BKCa gating. Such modulation depends on Ca2+ occupancy and activation states, but is not directly related to the Ca2+ binding sites. These results demonstrate that AC region is important in the allosteric coupling between Ca2+ binding and channel opening. Thus, the conformational changes of the AC region within each RCK domain is likely to be an important step in addition to the reorientation of RCK domains leading to the opening of the BKCa activation gate. Our observations are consistent with a mechanism for Ca2+-dependent activation of BKCa channels such that the AC region inhibits channel activation when the channel is at the closed state in the absence of Ca2+; Ca2+ binding and depolarization relieve this inhibition.
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Affiliation(s)
- Gayathri Krishnamoorthy
- Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
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200
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Lipscombe D. Neuronal proteins custom designed by alternative splicing. Curr Opin Neurobiol 2005; 15:358-63. [PMID: 15961039 DOI: 10.1016/j.conb.2005.04.002] [Citation(s) in RCA: 122] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2005] [Accepted: 04/19/2005] [Indexed: 11/23/2022]
Abstract
The evolution of alternative splicing in eukaryotes greatly expanded the number of functionally distinct proteins that could be produced from a finite gene pool. Extensive in the brains of higher organisms, alternative splicing might be the primary mechanism for generating the spectrum of protein activities that support complex brain functions. Alternative splicing is controlled at the level of individual neurons to custom design proteins for optimal performance. The expression profiles of splice isoforms are modified during development and as neuronal activity changes. Alternative splicing can lead to incremental, long lasting changes in ion channel and receptor activities, independent of changes in gene transcription. Recent studies of tissue-specific splicing factors are revealing how coordinated alterations in alternative splicing of RNA transcripts control synaptic function.
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Affiliation(s)
- Diane Lipscombe
- Department of Neuroscience, Brown University, Providence, RI 02912, USA.
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