beta subunits modulate alternatively spliced, large conductance, calcium-activated potassium channels of avian hair cells

Alternative splicing in shaping the molecular landscape of the cochlea

The cochlea is a complex organ comprising diverse cell types with highly specialized morphology and function. Until now, the molecular underpinnings of its specializations have mostly been studied from a transcriptional perspective, but accumulating evidence points to post-transcriptional regulation as a major source of molecular diversity. Alternative splicing is one of the most prevalent and well-characterized post-transcriptional regulatory mechanisms. Many molecules important for hearing, such as cadherin 23 or harmonin, undergo alternative splicing to produce functionally distinct isoforms. Some isoforms are expressed specifically in the cochlea, while some show differential expression across the various cochlear cell types and anatomical regions. Clinical phenotypes that arise from mutations affecting specific splice variants testify to the functional relevance of these isoforms. All these clues point to an essential role for alternative splicing in shaping the unique molecular landscape of the cochlea. Although the regulatory mechanisms controlling alternative splicing in the cochlea are poorly characterized, there are animal models with defective splicing regulators that demonstrate the importance of RNA-binding proteins in maintaining cochlear function and cell survival. Recent technological breakthroughs offer exciting prospects for overcoming some of the long-standing hurdles that have complicated the analysis of alternative splicing in the cochlea. Efforts toward this end will help clarify how the remarkable diversity of the cochlear transcriptome is both established and maintained.

Calcium-induced calcium release in proximity to hair cell BK channels revealed by PKA activation

Large-conductance calcium-activated potassium (BK) channels play a critical role in electrical resonance, a mechanism of frequency selectivity in chicken hair cells. We determine that BK currents are dependent on inward flow of Ca2+ , and intracellular buffering of Ca2+ . Entry of Ca2+ is further amplified locally by calcium-induced Ca2+ release (CICR) in close proximity to plasma membrane BK channels. Ca2+ imaging reveals peripheral clusters of high concentrations of Ca2+ that are suprathreshold to that needed to activate BK channels. Protein kinase A (PKA) activation increases the size of BK currents likely by recruiting more BK channels due to spatial spread of high Ca2+ concentrations in turn from increasing CICR. STORM imaging confirms the presence of nanodomains with ryanodine and IP3 receptors in close proximity to the Slo subunit of BK channels. Together, these data require a rethinking of how electrical resonance is brought about and suggest effects of CICR in synaptic release. Both genders were included in this study.

Expression of the LRRC52 γ subunit (γ2) may provide Ca2+-independent activation of BK currents in mouse inner hair cells

Mammalian inner hair cells (IHCs) transduce sound into depolarization and transmitter release. Big conductance and voltage- and Ca²⁺-activated K⁺ (BK) channels are responsible for fast membrane repolarization and small time constants of mature IHCs. For unknown reasons, they activate at around -75 mV with a voltage of half-maximum activation (V_half) of -50 mV although being largely insensitive to Ca²⁺ influx. Ca²⁺-independent activation of BK channels was observed by others in heterologous expression systems if γ subunits leucine-rich repeat-containing protein (LRRC)26 (γ1) and LRRC52 (γ2) were coexpressed with the pore-forming BKα subunit, which shifted V_half by -140 and -100 mV, respectively. Using nested PCR, we consistently detected transcripts for LRRC52 but not for LRRC26 in IHCs of 3-wk-old mice. Confocal immunohistochemistry showed synchronous up-regulation of LRRC52 protein with BKα at the onset of hearing. Colocalization of LRRC52 protein and BKα at the IHC neck within ≤40 nm was specified using an in situ proximity ligation assay. Mice deficient for the voltage-gated Ca_v1.3 Ca²⁺ channel encoded by Cacna1d do not express BKα protein. LRRC52 protein was neither expressed in IHCs of BKα nor in IHCs of Ca_v1.3 knockout mice. Together, LRRC52 is a γ2 subunit of BK channel complexes and is a strong candidate for causing the Ca²⁺-independent activation of BK currents at negative membrane potentials in mouse IHCs.-Lang, I., Jung, M., Niemeyer, B. A., Ruth, P., Engel, J. Expression of the LRRC52 γ subunit (γ2) may provide Ca²⁺-independent activation of BK currents in mouse inner hair cells.

BK Channels in the Vertebrate Inner Ear

The perception of complex acoustic stimuli begins with the deconstruction of sound into its frequency components. This spectral processing occurs first and foremost in the inner ear. In vertebrates, two very different strategies of frequency analysis have evolved. In nonmammalian vertebrates, the sensory hair cells of the inner ear are intrinsically electrically tuned to a narrow band of acoustic frequencies. This electrical tuning relies on the interplay between BK channels and voltage-gated calcium channels. Systematic variations in BK channel density and kinetics establish a gradient in electrical resonance that enables the coding of a broad range of acoustic frequencies. In contrast, mammalian hair cells are extrinsically tuned by mechanical properties of the cochlear duct. Even so, mammalian hair cells also express BK channels. These BK channels play critical roles in various aspects of mammalian auditory signaling, from developmental maturation to protection against acoustic trauma. This review summarizes the anatomical localization, biophysical properties, and functional contributions of BK channels in vertebrate inner ears. Areas of future research, based on an updated understanding of the biology of both BK channels and the inner ear, are also highlighted. Investigation of BK channels in the inner ear continues to provide fertile research grounds for examining both BK channel biophysics and the molecular mechanisms underlying signal processing in the auditory periphery.

A BK (Slo1) channel journey from molecule to physiology

Calcium and voltage-activated potassium (BK) channels are key actors in cell physiology, both in neuronal and non-neuronal cells and tissues. Through negative feedback between intracellular Ca (2+) and membrane voltage, BK channels provide a damping mechanism for excitatory signals. Molecular modulation of these channels by alternative splicing, auxiliary subunits and post-translational modifications showed that these channels are subjected to many mechanisms that add diversity to the BK channel α subunit gene. This complexity of interactions modulates BK channel gating, modifying the energetic barrier of voltage sensor domain activation and channel opening. Regions for voltage as well as Ca (2+) sensitivity have been identified, and the crystal structure generated by the 2 RCK domains contained in the C-terminal of the channel has been described. The linkage of these channels to many intracellular metabolites and pathways, as well as their modulation by extracellular natural agents, has been found to be relevant in many physiological processes. This review includes the hallmarks of BK channel biophysics and its physiological impact on specific cells and tissues, highlighting its relationship with auxiliary subunit expression.

Molecular heterogeneity of large-conductance calcium-activated potassium channels in canine intracardiac ganglia

Large conductance calcium-activated potassium (BK) channels are widely expressed in the nervous system. We have recently shown that principal neurons from canine intracardiac ganglia (ICG) express a paxilline- and TEA-sensitive BK current, which increases neuronal excitability. In the present work, we further explore the molecular constituents of the BK current in canine ICG. We found that the β1 and β4 regulatory subunits are expressed in ICG. Single channel voltage-dependence at different calcium concentrations suggested that association of the BKα with a particular β subunit was not enough to explain the channel activity in this tissue. Indeed, we detected the presence of several splice variants of the BKα subunit. In conclusion, BK channels in canine ICG may result from the arrangement of different BKα splice variants, plus accessory β subunits. The particular combinations expressed in canine IC neurons likely rule the excitatory role of BK current in this tissue.

CDK5 interacts with Slo and affects its surface expression and kinetics through direct phosphorylation

Large-conductance calcium-activated potassium (BK) channels are ubiquitous and play an important role in a number of diseases. In hair cells of the ear, they play a critical role in electrical tuning, a mechanism of frequency discrimination. These channels show variable kinetics and expression along the tonotopic axis. Although the molecular underpinnings to its function in hair cells are poorly understood, it is established that BK channels consist of a pore-forming α-subunit (Slo) and a number of accessory subunits. Here we identify CDK5, a member of the cyclin-dependent kinase family, as an interacting partner of Slo. We show CDK5 to be present in hair cells and expressed in high concentrations in the cuticular plate and in the circumferential zone. In human embryonic kidney cells, we show that CDK5 inhibits surface expression of Slo by direct phosphorylation of Slo. Similarly, we note that CDK5 affects Slo voltage activation and deactivation kinetics, by a direct phosphorylation of T847. Taken together with its increasing expression along the tonotopic axis, these data suggest that CDK5 likely plays a critical role in electrical tuning and surface expression of Slo in hair cells.

Shaping of action potentials by type I and type II large-conductance Ca²+-activated K+ channels

The BK channel is a Ca(2+) and voltage-gated conductance responsible for shaping action potential waveforms in many types of neurons. Type II BK channels are differentiated from type I channels by their pharmacology and slow gating kinetics. The β4 accessory subunit confers type II properties on BK α subunits. Empirically derived properties of BK channels, with and without the β4 accessory subunit, were obtained using a heterologous expression system under physiological ionic conditions. These data were then used to study how BK channels alone (type I) and with the accessory β4 subunit (type II) modulate action potential properties in biophysical neuron models. Overall, the models support the hypothesis that it is the slower kinetics provided by the β4 subunit that endows the BK channel with type II properties, which leads to broadening of action potentials and, secondarily, to greater recruitment of SK channels reducing neuronal excitability. Two regions of parameter space distinguished type II and type I effects; one where the range of BK-activating Ca(2+) was high (>20 μM) and the other where BK-activating Ca(2+) was low (∼0.4-1.2 μM). The latter required an elevated BK channel density, possibly beyond a likely physiological range. BK-mediated sharpening of the spike waveform associated with the lack of the β4 subunit was sensitive to the properties of voltage-gated Ca(2+) channels due to electrogenic effects on spike duration. We also found that depending on Ca(2+) dynamics, type II BK channels may have the ability to contribute to the medium AHP, a property not generally ascribed to BK channels, influencing the frequency-current relationship. Finally, we show how the broadening of action potentials conferred by type II BK channels can also indirectly increase the recruitment of SK-type channels decreasing the excitability of the neuron.

Gene expression gradients along the tonotopic axis of the chicken auditory epithelium

There are known differences in the properties of hair cells along the tonotopic axis of the avian auditory epithelium, the basilar papilla (BP). To determine the genetic basis of these differences, we compared gene expression between the high- (HF), middle-, and low-frequency (LF) thirds of 0-day-old chick auditory epithelia. RNA amplified from each sample was hybridized to whole-genome chicken arrays and GeneSpring software was used to identify differentially expressed genes. Two thousand six hundred sixty-three genes were found to be differentially expressed between the HF and LF segments, using a fold-change cutoff of 2 and a p value of 0.05. Many ion channel genes were differentially expressed between the HF and LF regions of the BP, an expression pattern that was previously established for some but not all of these genes. Quantitative PCR was used to verify tonotopic expression of 15 genes, including KCNMA1 (Slo) and its alternatively spliced STREX exon. Gene set enrichment analyses (GSEA) were performed on the microarray data and revealed many microRNA gene sets significantly enriched in the HF relative to the LF end, suggesting a tonotopic activity gradient. GSEA also suggested differential activity of the kinases protein kinase C and protein kinase A at the HF and LF ends, an interesting corollary to the observation that there is tonotopic expression of the STREX exon that confers on Slo sensitivity to the activity of kinases. Taken together, these results suggest mechanisms of induction and maintenance of tonotopicity and enhance our understanding of the complex nature of proximal-distal gene expression gradients in the chicken BP.

Development of tonotopy in the auditory periphery

Acoustic frequency analysis plays an essential role in sound perception, communication and behavior. The auditory systems of most vertebrates that perceive sounds in air are organized based on the separation of complex sounds into component frequencies. This process begins at the level of the auditory sensory epithelium where specific frequencies are distributed along the tonotopic axis of the mammalian cochlea or the avian/reptilian basilar papilla (BP). Mechanical and electrical mechanisms mediate this process, but the relative contribution of each mechanism differs between species. Developmentally, structural and physiological specializations related to the formation of a tonotopic axis form gradually over an extended period of time. While some aspects of tonotopy are evident at early stages of auditory development, mature frequency discrimination is typically not achieved until after the onset of hearing. Despite the importance of tonotopic organization, the factors that specify unique positional identities along the cochlea or basilar papilla are unknown. However, recent studies of developing systems, including the inner ear provide some clues regarding the signalling pathways that may be instructive for the formation of a tonotopic axis.

β4-subunit increases Slo responsiveness to physiological Ca2+ concentrations and together with β1 reduces surface expression of Slo in hair cells

Changing kinetics of large-conductance potassium (BK) channels in hair cells of nonmammalian vertebrates, including the chick, plays a critical role in electrical tuning, a mechanism used by these cells to discriminate between different frequencies of sound. BK currents are less abundant in low-frequency hair cells and show large openings in response to a rise in intracellular Ca(2+) at a hair cell's operating voltage range (spanning -40 to -60 mV). Although the molecular underpinnings of its function in hair cells are poorly understood, it is established that BK channels consist of a pore-forming α-subunit (Slo) and a number of accessory subunits. Currents from the α (Slo)-subunit alone do not show dramatic increases in response to changes in Ca(2+) concentrations at -50 mV. We have cloned the chick β(4)- and β(1)-subunits and show that these subunits are preferentially expressed in low-frequency hair cells, where they decrease Slo surface expression. The β(4)-subunit in particular is responsible for the BK channel's increased responsiveness to Ca(2+) at a hair cell's operating voltage. In contrast, however, the increases in relaxation times induced by both β-subunits suggest additional mechanisms responsible for BK channel function in hair cells.

Neuronal fast activating and meningeal silent modulatory BK channel splice variants cloned from rat

The big conductance calcium-activated K(+) channel (BK) is involved in regulating neuron and smooth muscle cell excitability. Functional diversity of BK is generated by alpha-subunit splice variation and co-expression with beta subunits. Here, we present six different splice combinations cloned from rat brain or cerebral vascular/meningeal tissues, of which at least three variants were previously uncharacterized (X1, X2(92), and X2(188)). An additional variant was identified by polymerase chain reaction but not cloned. Expression in Xenopus oocytes showed that the brain-specific X1 variant displays reduced current, faster activation, and less voltage sensitivity than the insert-less Zero variant. Other cloned variants Strex and Slo27,3 showed slower activation than Zero. The X1 variant contains sequence inserts in the S1-S2 extracellular loop (8 aa), between intracellular domains RCK1 and RCK2 (4 aa at SS1) and upstream of the calcium "bowl" (27 aa at SS4). Two other truncated variants, X2(92) and X2(188), lacking the intracellular C-terminal (stop downstream of S6), were cloned from cerebral vascular/meningeal tissue. They appear non-functional as no current expression was observed, but the X2(92) appeared to slow the activation of the Zero variant when co-expressed. Positive protein expression of X2(92) was observed in oocytes by immunoblotting and fluorescence using a yellow fluorescent protein-tagged construct. The functional characteristics of the X1 variant may be important for a subpopulation of BK channels in the brain, while the "silent" truncated variants X2(92) and X2(188) may play a role as modulators of other BK channel variants in a way similar to well known beta subunits.

Cloning and distribution of Ca2+-activated K+ channels in lobster Panulirus interruptus

Large conductance Ca(2+)-activated potassium (BK) channels play important roles in controlling neuronal excitability. We cloned the PISlo gene encoding BK channels from the spiny lobster, Panulirus interruptus. This gene shows 81-98% sequence identity to Slo genes previously found in other organisms. We isolated a number of splice variants of the PISlo cDNA within Panulirus interruptus nervous tissue. Sequence analysis indicated that there are at least seven alternative splice sites in PISlo, each with multiple alternative segments. Using immunohistochemistry, we found that the PISlo proteins are distributed in the synaptic neuropil, axon and soma of stomatogastric ganglion (STG) neurons.

Highly specific alternative splicing of transcripts encoding BK channels in the chicken's cochlea is a minor determinant of the tonotopic gradient

The frequency sensitivity of auditory hair cells in the inner ear varies with their longitudinal position in the sensory epithelium. Among the factors that determine the differential cellular response to sound is the resonance of a hair cell's transmembrane electrical potential, whose frequency correlates with the kinetic properties of the high-conductance Ca(2+)-activated K(+) (BK) channels encoded by a Slo (kcnma1) gene. It has been proposed that the inclusion of specific alternative axons in the Slo transcripts along the cochlea underlies the gradient of BK-channel kinetics. By analyzing the complete sequences of chicken Slo gene (cSlo) cDNAs from the chicken's cochlea, we show that most transcripts lack alternative exons. Transcripts with more than one alternative exon constitute only 10% of the total. Although the fraction of transcripts containing alternative exons increases from the cochlear base to the apex, the combination of alternative exons is not regulated. There is also a clear increase in the expression of BK transcripts with long carboxyl termini toward the apex. When long and short BK transcripts are expressed in HEK-293 cells, the kinetics of single-channel currents differ only slightly, but they are substantially slowed when the channels are coexpressed with the auxiliary beta subunit that occurs more widely at the apex. These results argue that the tonotopic gradient is not established by the selective inclusion of highly specific cSlo exons. Instead, a gradient in the expression of beta subunits slows BK channels toward the low-frequency apex of the cochlea.

Developmental expression of BK channels in chick cochlear hair cells

Background

Cochlear hair cells are high-frequency sensory receptors. At the onset of hearing, hair cells acquire fast, calcium-activated potassium (BK) currents, turning immature spiking cells into functional receptors. In non-mammalian vertebrates, the number and kinetics of BK channels are varied systematically along the frequency-axis of the cochlea giving rise to an intrinsic electrical tuning mechanism. The processes that control the appearance and heterogeneity of hair cell BK currents remain unclear.

Results

Quantitative PCR results showed a non-monotonic increase in BK α subunit expression throughout embryonic development of the chick auditory organ (i.e. basilar papilla). Expression peaked near embryonic day (E) 19 with six times the transcript level of E11 sensory epithelia. The steady increase in gene expression from E11 to E19 could not explain the sudden acquisition of currents at E18-19, implicating post-transcriptional mechanisms. Protein expression also preceded function but progressed in a sequence from diffuse cytoplasmic staining at early ages to punctate membrane-bound clusters at E18. Electrophysiology data confirmed a continued refinement of BK trafficking from E18 to E20, indicating a translocation of BK clusters from supranuclear to subnuclear domains over this critical developmental age.

Conclusions

Gene products encoding BK α subunits are detected up to 8 days before the acquisition of anti-BK clusters and functional BK currents. Therefore, post-transcriptional mechanisms seem to play a key role in the delayed emergence of calcium-sensitive currents. We suggest that regulation of translation and trafficking of functional α subunits, near voltage-gated calcium channels, leads to functional BK currents at the onset of hearing.

Tamoxifen inhibits BK channels in chick cochlea without alterations in voltage-dependent activation

Large-conductance, Ca(2+)-activated, and voltage-gated potassium channels (BK, BK(Ca), or Maxi-K) play an important role in electrical tuning in nonmammalian vertebrate hair cells. Systematic changes in tuning frequency along the tonotopic axis largely result from variations in BK channel kinetics, but the molecular changes underpinning these functional variations remain unknown. Auxiliary beta(1) have been implicated in low-frequency tuning at the cochlear apex because these subunits dramatically slow channel kinetics. Tamoxifen (Tx), a (xeno)estrogen compound known to activate BK channels through the beta-subunit, was used to test for the functional presence of beta(1). The hypotheses were that Tx would activate the majority of BK channels in hair cells from the cochlear apex due to the presence of beta(1) and that the level of activation would exhibit a tonotopic gradient following the expression profile of beta(1). Outside-out patches of BK channels were excised from tall hair cells along the apical half of the chicken basilar papilla. In low-density patches, single-channel conductance was reduced and the averaged open probability was unaffected by Tx. In high-density patches, the amplitude of ensemble-averaged BK current was inhibited, whereas half-activation potential and activation kinetics were unaffected by Tx. In both cases, no tonotopic Tx-dependent activation of channel activity was observed. Therefore, contrary to the hypotheses, electrophysiological assessment suggests that molecular mechanisms other than auxiliary beta-subunits are involved in generating a tonotopic distribution of BK channel kinetics and electric tuning in chick basilar papilla.

Voltage-activated ion channels and Ca(2+)-induced Ca (2+) release shape Ca (2+) signaling in Merkel cells

Ca(2+) signaling and neurotransmission modulate touch-evoked responses in Merkel cell-neurite complexes. To identify mechanisms governing these processes, we analyzed voltage-activated ion channels and Ca(2+) signaling in purified Merkel cells. Merkel cells in the intact skin were specifically labeled by antibodies against voltage-activated Ca(2+) channels (Ca(V)2.1) and voltage- and Ca(2+)-activated K(+) (BK(Ca)) channels. Voltage-clamp recordings revealed small Ca(2+) currents, which produced Ca(2+) transients that were amplified sevenfold by Ca(2+)-induced Ca(2+) release. Merkel cells' voltage-activated K(+) currents were carried predominantly by BK(Ca) channels with inactivating and non-inactivating components. Thus, Merkel cells, like hair cells, have functionally diverse BK(Ca) channels. Finally, blocking K(+) channels increased response magnitude and dramatically shortened Ca(2+) transients evoked by mechanical stimulation. Together, these results demonstrate that Ca(2+) signaling in Merkel cells is governed by the interplay of plasma membrane Ca(2+) channels, store release and K(+) channels, and they identify specific signaling mechanisms that may control touch sensitivity.

Differential distribution of Ca2+-activated potassium channel beta4 subunit in rat brain: immunolocalization in neuronal mitochondria

Large conductance Ca(2+)-activated potassium channels (BK(Ca) channels) are expressed in the plasma membrane of various cell types. Interestingly, recent studies provided evidence for the existence of BK(Ca) channels also in mitochondria. However, the molecular composition of these channels as well as their cellular and tissue distribution is still unknown. The goal of the present study was to find a candidate for the regulatory component of the mitochondrial large conductance calcium activated potassium (mitoBK(Ca)) channel in neurons. A combined approach of Western blot analysis, high-resolution immunofluorescence and immunoelectron microscopy with the use of antibodies directed against four distinct beta subunits demonstrated the presence of the BK(Ca) channel beta4 subunit (KCNMB4) in the inner membrane of neuronal mitochondria in the rat brain and cultured neurons. Within the cell, the expression of beta4 subunit was restricted to a subpopulation of mitochondria. The analysis of beta4 subunit distribution throughout the brain revealed that the highest expression levels occur in the thalamus and the brainstem. Our results suggest that beta4 subunit is a regulatory component of mitochondrial BK(Ca) channels in neurons. These findings may support the perspectives for the neuroprotective role of mitochondrial BK(Ca) channel in specific brain structures.

Alternatively spliced C-terminal domains regulate the surface expression of large conductance calcium-activated potassium channels

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.

Beta1-subunits increase surface expression of a large-conductance Ca2+-activated K+ channel isoform

Auxiliary (beta) subunits of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels regulate the gating properties of the functional channel complex. Here we show that an avian beta1-subunit also stimulates the trafficking of BK(Ca) channels to the plasma membrane in HEK293T cells and in a native population of developing vertebrate neurons. One C-terminal variant of BK(Ca) alpha-subunits, called the VEDEC isoform after its five last residues, is largely retained in intracellular compartments when it is heterologously expressed in HEK293T cells. A closely related splice variant, called QEERL, shows high levels of constitutive trafficking to the plasma membrane. Co-expression of beta1-subunits with the VEDEC isoform resulted in a large increase in surface BK(Ca) channels as assessed by cell-surface biotinylation assays, whole cell recordings of membrane current, and confocal microscopy in HEK293T cells. Co-expression of beta1-subunits slowed the gating kinetics of BK(Ca) channels, as reported previously. Consistent with this, overexpression of beta1-subunits in a native cell type that expresses intracellular VEDEC channels, embryonic day 9 chick ciliary ganglion neurons, resulted in a significant increase in macroscopic Ca(2+)-activated K(+) current. Both the cytoplasmic N- and C-terminal domains of avian beta1 are able to bind directly to VEDEC and QEERL channels. However, overexpression of the N-terminal domain by itself is sufficient to stimulate trafficking of VEDEC channels to the plasma membrane, whereas overexpression of either the cytoplasmic C-terminal domain or the extracellular loop domain did not affect surface expression of VEDEC.

Calcium-activated potassium channel of the tobacco hornworm, Manduca sexta: molecular characterization and expression analysis

Large-conductance calcium- and voltage-gated potassium channels (BK or Slowpoke) serve as dynamic integrators linking electrical signaling and intracellular activity. These channels can mediate many different Ca2+-dependent physiological processes including the regulation of neuronal and neuroendocrine cell excitability and muscle contraction. To gain insights into the function of BK channels in vivo, we isolated a full-length cDNA encoding the alpha subunit of a Slowpoke channel from the tobacco hornworm, Manduca sexta (msslo). Amino acid sequence comparison of the deduced Manduca protein revealed at least 80% identity to the insect Slo channels. The five C-terminal alternative splice regions are conserved, but the cloned cDNA fragments contained some unique combinations of exons E, G and I. Our spatial profile revealed that transcript levels were highest in skeletal muscle when compared with the central nervous system (CNS) and visceral muscle. The temporal profile suggested that msslo expression is regulated developmentally in a tissue- and regional-specific pattern. The levels of msslo transcripts remain relatively constant throughout metamorphosis in the CNS, transiently decline in the heart and are barely detectable in the gut except in adults. A dramatic upregulation of msslo transcript levels occurs in thoracic but not abdominal dorsal longitudinal body wall muscles (DLM), suggesting that the msSlo current plays an important role in the excitation or contractile properties of the phasic flight muscle. Our developmental profile of msslo expression suggests that msSlo currents may contribute to the changes in neural circuits and muscle properties that produce stage-specific functions and behaviors.

Hair Cells – Beyond the Transducer

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.

Ionic Currents Underlying Difference in Light Response Between Type A and Type B Photoreceptors

In Hermissenda crassicornis, the memory of light associated with turbulence is stored as changes in intrinsic and synaptic currents in both type A and type B photoreceptors. These photoreceptor types exhibit qualitatively different responses to light and current injection, and these differences shape the spatiotemporal firing patterns that control behavior. Thus the objective of the study was to identify the mechanisms underlying these differences. The approach was to develop a type B model that reproduced characteristics of type B photoreceptors recorded in vitro, and then to create a type A model by modifying a select number of ionic currents. Comparison of type A models with characteristics of type A photoreceptors recorded in vitro revealed that type A and type B photoreceptors have five main differences, three that have been characterized experimentally and two that constitute hypotheses to be tested with experiments in the future. The three differences between type A and type B photoreceptors previously characterized include the inward rectifier current, the fast sodium current, and conductance of calcium-dependent and transient potassium channels. Two additional changes were required to produce a type A photoreceptor model. The very fast firing frequency observed during the first second after light onset required a faster time constant of activation of the delayed rectifier. The fast spike adaptation required a fast, noninactivating calcium-dependent potassium current. Because these differences between type A and type B photoreceptors have not been confirmed in comparative experiments, they constitute hypotheses to be tested with future experiments.

Large-conductance, calcium-activated potassium channels: structural and functional implications

The large-conductance, calcium-activated potassium channels (BK, also termed BK(Ca), Slo, or MaxiK) distributed in both excitable and non-excitable cells are involved in many cellular functions such as action potential repolarization; neuronal excitability; neurotransmitter release; hormone secretion; tuning of cochlear hair cells; innate immunity; and modulation of the tone of vascular, airway, uterine, gastrointestinal, and urinary bladder smooth muscle tissues. Because of their high conductance, activation of BK channels has a strong effect on membrane potential. BK channels differ from all other potassium (K(+)) channels due to their high sensitivity to both intracellular calcium (Ca(2+)) concentrations and voltage. These features make BK channels ideal negative feedback regulators in many cell types by decreasing voltage-dependent Ca(2+) entry through membrane potential hyperpolarization. The current review aims to give a comprehensive understanding of the structure and molecular biology of BK channels and their relevance to various pathophysiological conditions. The review will also focus on the therapeutic potential and pharmacology of the various BK channel activators and blockers.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Identification of Chicken Transmembrane Channel-like (TMC) genes: Expression analysis in the cochlea

Mutations of the human gene encoding transmembrane channel-like protein (TMC)1 cause dominant and recessive nonsyndromic hearing disorders, suggesting that this protein plays an important role in the inner ear. In this study, we cloned chicken Tmc2 (GgTmc2) from a cochlear cDNA library and we annotated four additional TMC family members: GgTmc1, GgTmc3, GgTmc6, and GgTmc7. All chicken TMCs possess the defining TMC signature motif and display high conservation of their genomic structure when compared with other vertebrate TMC genes. GgTmc1 is localized on the chicken sex chromosome Z at a locus that displays conserved synteny with the loci of mammalian orthologues residing on autosomes. In contrast, the locus of GgTmc2 does not exhibit conserved synteny with its mammalian orthologues. Because murine TMC1 and TMC2 are restrictively expressed in cochlear hair cells, we determined the expression of the chicken orthologues in the basilar papilla, the avian equivalent of the organ of Corti. While GgTmc2 was present throughout the basilar papilla and in other tissues, GgTmc1 transcript was detected specifically in the basal portion of the basilar papilla and was not detectable in any other tissue or organ studied. GgTmc3 and GgTmc6 were detectable in all organs analyzed. Antibody labeling revealed that GgTmc2 is predominantly associated with the lateral membranes of hair and supporting cells. The expression of GgTmc2 by both cell types was further confirmed by RT-PCR using isolated cells. This expression and subcellular localization of GgTmc2 is in agreement with the proposed potential role of this novel class of transmembrane proteins in ion transport.

Effects of intracellular stores and extracellular Ca(2+) on Ca(2+)-activated K(+) currents in mature mouse inner hair cells

Ca(2+)-activated K(+) currents were studied in inner hair cells (IHCs) of mature mice. I(K,f), the large-conductance Ca(2+)-activated K(+) current (BK) characteristic of mature IHCs, had a fast activation time constant (0.4 ms at -25 mV at room temperature) and did not inactivate during 170 ms. Its amplitude, measured at -25 mV, and activation time constant were similar between IHCs in the apical and basal regions of the cochlea. I(K,f) was selectively blocked by 30 nm IbTx but was unaffected by superfusion of Ca(2+)-free solution, nifedipine or Bay K 8644, excluding the direct involvement of voltage-gated Ca(2+) channels in I(K,f) activation. Increasing the intracellular concentration of the Ca(2+) chelator BAPTA from 0.1 mm to 30 mm reduced the amplitude of I(K,f) at -25 mV and shifted its activation by 37 mV towards more depolarized potentials. A reduction in the size of I(K,f) and a depolarizing shift of its activation were also seen when either thapsigargin and caffeine or ryanodine were added intracellularly, suggesting that I(K,f) is modulated by voltage-dependent release from intracellular Ca(2+) stores. Mature IHCs had a small additional Ca(2+)-activated K(+) current (I(K(Ca))), activated by Ca(2+) flowing through L-type Ca(2+) channels. This current was still present during superfusion of either IbTx (60 nm) or apamin (300 nm) but was abolished in Cs(+)-based intracellular solution or during superfusion of 5 mm TEA, suggesting the presence of an additional BK-channel type. Current clamp experiments at body temperature show that I(K,f), but not I(K(Ca)), is essential for fast voltage responses of mature IHCs.

Acute modulation of adrenal chromaffin cell BK channel gating and cell excitability by glucocorticoids

Although adrenal glucocorticoids cortisol and corticosterone (CORT) have numerous "genomic" effects on adrenomedullary chromaffin cells, acute modulatory actions remain largely unknown, despite rapid stress-related changes in secretion. We report that 1 microM glucocorticoids rapidly modulate gating of chromaffin cell BK channels and action potential firing. In general, CORT, or the analog dexamethasone (DEX), increased channel activity in inside-out bovine patches, an effect not blocked by the glucocorticoid receptor (GR) antagonist RU38486. By contrast, these steroids could profoundly inhibit BK activation in many rat patches, while facilitating activation in others. We show that BK inhibition arises from a negative shift in the voltage dependence of BK inactivation paralleling that for activation. We report that rat cells characteristically exhibit greater repetitive firing ability than bovine cells in the absence of glucocorticoids. In both species, steroid application typically increased firing responses to smaller current injections, attributable to BK-enhanced repolarization and Na+ channel deinactivation. However, in rat cells, where BK inactivation is generally faster and more complete, glucocorticoids tended to dampen responses to stronger stimuli. Thus, in the context of natural variation in BK gating, glucocorticoids can either promote or limit firing responses. We suggest that steroids exploit BK gating variety to tailor catecholamine output in a species- and context-specific fashion.

Association of beta-catenin with the alpha-subunit of neuronal large-conductance Ca2+-activated K+ channels

The association of Ca(2+)-activated K(+) channels with voltage-gated Ca(2+) channels at the presynaptic active zones of hair cells, photoreceptors, and neurons contributes to rapid repolarization of the membrane after excitation. Ca(2+) channels have been shown to bind to a large set of synaptic proteins, but the proteins interacting with Ca(2+)-activated K(+) channels remain unknown. Here, we report that the large-conductance Ca(2+)-activated K(+) channel of the chicken's cochlear hair cell interacts with beta-catenin. Yeast two-hybrid assays identified the S10 region of the K(+) channel's alpha-subunit and the ninth armadillo repeat and carboxyl terminus of beta-catenin as necessary for the interaction. An antiserum directed against the alpha-subunit specifically coprecipitated beta-catenin from brain synaptic proteins. beta-Catenin is known to associate with the synaptic protein Lin7/Velis/MALS, whose interaction partner Lin2/CASK also binds voltage-gated Ca(2+) channels. beta-Catenin may therefore provide a physical link between the two types of channels at the presynaptic active zone.

Beta1 subunits facilitate gating of BK channels by acting through the Ca2+, but not the Mg2+, activating mechanisms

The beta1 subunit of BK (large conductance Ca2+ and voltage-activated K+) channels is essential for many key physiological processes, such as controlling the contraction of smooth muscle and the tuning of hair cells in the cochlea. Although it is known that the beta1 subunit greatly increases the open probability of BK channels, little is known about its mechanism of action. We now explore this mechanism by using channels in which the Ca2+- and Mg2+-dependent activating mechanisms have been disrupted by mutating three sites to remove the Ca2+ and Mg2+ sensitivity. We find that the presence of the beta1 subunit partially restores Ca2+ sensitivity to the triply mutated channels, but not the Mg2+ sensitivity. We also find that the beta1 subunit has no effect on the Mg2+ sensitivity of WT BK channels, in contrast to its pronounced effect of increasing the apparent Ca2+ sensitivity. These observations suggest that the beta1 subunit increases open probability by working through the Ca2+-dependent, rather than Mg2+-dependent, activating mechanisms, and that the action of the beta1 subunit is not directly on the Ca2+ binding sites, but on the allosteric machinery coupling the sites to the gate. The differential effects of the beta1 subunit on the Ca2+ and Mg2+ activation of the channel suggest that these processes act separately. Finally, we show that Mgi2+ inhibits, rather than activates, BK channels in the presence of the beta1 subunit for intermediate levels of Cai2+. This Mg2+ inhibition in the presence of the beta1 subunit provides an additional regulatory mechanism of BK channel activity.

Potassium currents in vestibular type II hair cells activated by hydrostatic pressure

An elevated hydrostatic pressure in the endolymphatic space of the inner ear is discussed as pathophysiological factor in hydrops-related diseases of the inner ear. An increase in pressure by fractions of 1 cm H(2)O is sufficient to induce vertigo-like symptoms in animal models. To establish a link between hydrostatic pressure and the function of vestibular hair cells, we studied potassium currents in isolated vestibular type II hair cells from guinea-pig utricles when the hydrostatic pressure was increased by raising the height of the bath from 0.2-0.5, 0.7 or 1.0 cm. Elevated pressure enhanced K(+) currents significantly; a rise in pressure from 0.2-0.5 cm H(2)O increased the total K(+) current at +40 mV by 22+/-14% (+/-S.D.). The pressure-sensitive current I(K,p) was non-inactivating during depolarizing pulses. It was maintained when the pressure was kept elevated for several minutes and receded promptly after return to a pressure of 0.2 cm H(2)O. Voltage-gated Ca(2+) currents, in contrast, were not altered by hydrostatic pressure. A pharmacological characterization of I(K,p) revealed that tetraetylammonium (100 mM) abolished all outward currents including I(K,p). I(K,p) was partly and reversibly inhibited by 4-aminopyridine. Dihydrostreptomycin, a blocker of the transduction channel, left I(K,p) unaffected. Charybdotoxin (100 nM), a blocker of Ca(2+)-dependent K(+) channels, completely yet reversibly abolished I(K,p). We conclude that small elevations in hydrostatic pressure evoke a charybdotoxin-sensitive, probably Ca(2+)-dependent K(+) current in vestibular hair cells. This is likely to alter their frequency response and may be a relevant mechanism how hydrostatic pressure disturbs transduction.

Stimulatory action of internal protons on Slo1 BK channels

We investigated the internal pH-sensitivity of heterologously expressed hSlo1 BK channels. In the virtual absence of Ca(2+) and Mg(2+) to isolate the voltage-dependent gating transitions, low internal pH enhanced macroscopic hSlo1 currents by shifting the voltage-dependence of activation to more negative voltages. The activation time course was faster and the deactivation time course was slower with low pH. The estimated K(d) value of the stimulatory effect was approximately pH = 6.5 or 0.35 micro M. The stimulatory effect was maintained when the auxiliary subunit mouse beta1 was coexpressed. Treatment of the hSlo1 channel with the histidine modifying agent diethyl pyrocarbonate also enhanced the hSlo1 currents and greatly diminished the internal pH sensitivity, suggesting that diethyl pyrocarbonate and low pH may work on the same effector mechanism. High concentrations of Ca(2+) or Mg(2+) also masked the stimulatory effect of low internal pH. These results indicate that the acid-sensitivity of the Slo BK channel may involve the channel domain implicated in the divalent-dependent activation.

Variation in large-conductance, calcium-activated potassium channels from hair cells along the chicken basilar papilla

The mechanism for electrical tuning in non-mammalian hair cells rests within the widely diverse kinetics of functionally distinct, large-conductance potassium channels (BK), thought to result from alternative splicing of the pore-forming alpha subunit and variable co-expression with an accessory beta subunit. Inside-out patches from hair cells along the chicken basilar papilla revealed 'tonotopic' gradations in calcium sensitivity and deactivation kinetics. The resonant frequency for the hair cell from which the patch was taken was estimated from deactivation rates, and this frequency reasonably matched that predicted from the originating cell's tonotopic location. The rates of deactivation for native BK channels were much faster than rates reported for cloned chicken BK channels including both alpha and beta subunits. This result was surprising since patches were pulled from hair cells in the apical half of the papilla where beta subunits are most highly expressed. Heterogeneity in the properties of native chicken BK channels implies a high degree of molecular variation and hinders our ability to identify those molecular constituents.

Expression of Ca2+-activated BK channel mRNA and its splice variants in the rat cochlea

Voltage-activated K(+) channels are important for shaping the receptor potentials of cochlear hair cells. In particular, the functional maturation of inner hair cells in mice around the onset of hearing coincides with the expression of a large, fast K(+) conductance, probably mediated by Ca(2+)-activated K(+) (BK) channels. In hearing organs of lower vertebrates, frequency tuning depends on BK-type K(+) channels with different kinetics. Kinetics are varied by alternative splicing of the channels' alpha subunits and combination with modulating beta subunits. It is unclear whether similar mechanisms "fine tune" mammalian hair cells. We used various polymerase chain reaction (PCR) approaches to screen rat cochleae for splice variants of BK-type alpha subunits. We isolated mainly minimal variants and only occasionally splice variants with additional inserts. We conclude that alpha subunits with different kinetics are not substantially used in the rat cochlea. However, we isolated six variants differing in their extreme C-terminal sequences, which may be involved in the targeting of the channel protein. By using reverse transcriptase-PCR, we demonstrated also the expression of transcripts for several beta subunits. In situ hybridization experiments revealed strict coexpression of alpha with beta1 transcripts. In inner hair cells, strong labeling emerged shortly before the onset of hearing. Labeling of outer hair cells appeared later and generally weaker. Thus, our molecular data confirm electrophysiological results that suggested that BK channels underlie the large K(+) conductance in inner hair cells of mammals. Extensive splicing of BK channel transcripts, however, does not seem to be used in mammalian hair cells as is done in lower vertebrates.

Slo1 tail domains, but not the Ca2+ bowl, are required for the beta 1 subunit to increase the apparent Ca2+ sensitivity of BK channels

Functional large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels can be assembled from four alpha subunits (Slo1) alone, or together with four auxiliary beta1 subunits to greatly increase the apparent Ca(2+) sensitivity of the channel. We examined the structural features involved in this modulation with two types of experiments. In the first, the tail domain of the alpha subunit, which includes the RCK2 (regulator of K(+) conductance) domain and Ca(2+) bowl, was replaced with the tail domain of Slo3, a BK-related channel that lacks both a Ca(2+) bowl and high affinity Ca(2+) sensitivity. In the second, the Ca(2+) bowl was disrupted by mutations that greatly reduce the apparent Ca(2+) sensitivity. We found that the beta1 subunit increased the apparent Ca(2+) sensitivity of Slo1 channels, independently of whether the alpha subunits were expressed as separate cores (S0-S8) and tails (S9-S10) or full length, and this increase was still observed after the Ca(2+) bowl was mutated. In contrast, beta1 subunits no longer increased Ca(2+) sensitivity when Slo1 tails were replaced by Slo3 tails. The beta1 subunits were still functionally coupled to channels with Slo3 tails, as DHS-I and 17 beta-estradiol activated these channels in the presence of beta1 subunits, but not in their absence. These findings indicate that the increase in apparent Ca(2+) sensitivity induced by the beta1 subunit does not require either the Ca(2+) bowl or the linker between the RCK1 and RCK2 domains, and that Slo3 tails cannot substitute for Slo1 tails. The beta1 subunit also induced a decrease in voltage sensitivity that occurred with either Slo1 or Slo3 tails. In contrast, the beta1 subunit-induced increase in apparent Ca(2+) sensitivity required Slo1 tails. This suggests that the allosteric activation pathways for these two types of actions of the beta1 subunit may be different.

Characterization of large conductance Ca2+-activated K+ channels in cerebellar Purkinje neurons

We investigated the role of large conductance, calcium-activated potassium channels (BK channels) in regulation of the excitability of cerebellar Purkinje neurons. Block of BK channels by iberiotoxin reduced the afterhyperpolarization of spontaneous action potentials in Purkinje neurons in acutely prepared cerebellar slices. To establish the conditions required for activation of BK channels in Purkinje neurons, the dependence of BK channel open probability on calcium concentration and membrane voltage were investigated in excised patches from soma of acutely prepared Purkinje cells. Single channel currents were studied under conditions designed to select for potassium currents and in which voltage-activated currents were largely inactivated. Micromolar calcium concentrations activated channels with a mean single channel conductance of 266 pS. BK channels were activated by both calcium and membrane depolarization, and showed no sign of inactivation. At a given calcium concentration, depolarization over a 60-mV range increased the mean open probability (P(O)) from < 0.1 to > 0.8. Increasing the calcium concentration shifted the voltage required for half maximal activation to more hyperpolarized potentials. The apparent affinity of the channels for calcium increased with depolarization. At -60 mV the apparent affinity was approximately 35 micro m decreasing to approximately 3 micro M at +40 mV. These results suggest that BK channels are unlikely to be activated at resting membrane potentials and calcium concentrations. We tested the hypothesis that Purkinje cell BK channels may be activated by calcium entry during individual action potentials. Significant BK channel activation could be detected when brief action potential-like depolarizations were applied to patches under conditions in which the sole source of calcium was flux across the plasma membrane via the endogenous voltage-gated calcium channels. It is proposed that BK channels regulate the excitability of Purkinje cells by contributing to afterhyperpolarizations and perhaps by shaping individual action potentials.

Activity- and target-dependent regulation of large-conductance Ca2+-activated K+ channels in developing chick lumbar motoneurons

The functional expression of large-conductance (BK-type) Ca2+-activated K+ (K(Ca)) channels was examined in developing chick lumbar motoneurons (LMNs) between embryonic day 6 (E6) and E13 using patch-clamp recording techniques. The macroscopic K(Ca) current of E13 LMNs is inhibited by iberiotoxin and resistant to apamin. The average macroscopic K(Ca) density was low before E8 and increased 3.3-fold by E11, with an additional 1.8-fold increase occurring by E13. BK-type K(Ca) channels could not be detected in inside-out patches from E8 LMNs but were readily detected at E11. The density of voltage-activated Ca2+ currents did not change between E8 and E11. Surgical ablation of target tissues at E5 caused a significant reduction in average K(Ca) density in LMNs measured at E11. Conversely, chronic in ovo administration of d-tubocurarine, which causes an increase in motoneuron branching on the surface of the muscle target tissue, evoked a 1.8-fold increase in average LMN K(Ca) density measured at E11. Electrical activity also contributed to developmental regulation of LMN K(Ca) density. A significant reduction in E11 K(Ca) density was found after chronic in ovo treatment with the neuronal nicotinic antagonist mecamylamine or the GABA receptor agonist muscimol, agents that reduce activation of LMNs in ovo. Moreover, 3 d exposure to depolarizing concentrations of external K+ to LMNs cultured at E8 caused an increase in K(Ca) expression. Conversely, tetrodotoxin caused a decrease in K(Ca) expression in cultured E8 LMNs developing for 3 d in the presence of neurotrophic factors that promote neuronal survival in the absence of target tissues.

Modeling hair cell tuning by expression gradients of potassium channel beta subunits

The receptor potential of sensory hair cells arises from the gating of mechanosensitive cation channels, but its amplitude and time course also depend on the number and kinetics of voltage-gated ion channels in each cell. Prominent among these are "BK" potassium channels encoded by the slo gene that support electrical tuning in some hair cells. Hair cells tuned to low frequencies have slowly gating BK channels, whereas those of higher-frequency hair cells gate more rapidly. Alternative splicing of the slo gene mRNA that encodes the pore-forming alpha subunit can alter BK channel kinetics, and gating is dramatically slowed by coexpression with modulatory beta subunits. The effect of the beta subunit is consistent with low-frequency tuning, and beta mRNA is expressed at highest levels in the low frequency apex of the bird's auditory epithelium. How might an expression gradient of beta subunits contribute to hair cell tuning? The present work uses a computational model of hair cell-tuning based on the functional properties of BK channels expressed from hair cell alpha and beta slo cDNA. The model reveals that a limited tonotopic gradient could be achieved simply by altering the fraction of BK channels in each hair cell that are combined with beta subunits. However, complete coverage of the tuning spectrum requires kinetic variants in addition to those modeled here.

Expression of Ca(2+)-activated K(+) channel subunits and splice variants in the rat cochlea

The recently manifested important role of the Ca(2+)-activated K(+) channels, especially of the Slo gene-coded channels, for the cochlea function of the chicken raised the question of homolog expression in mammalian inner ear tissue. Molecular biological methods were used to demonstrate the expression of Ca(2+)-activated K(+) channel subunits and splice variants of the Slo gene in the rat organ of Corti. RT-PCR experiments for the detection of rat Slo alpha subunit mRNA revealed the presence of several already known splice variants including variants which appeared to be typical for the organ of Corti (+58 aa) and for the brain (+61 aa). To detect the accessory beta subunit we used Southern blot hybridization. Our data support the hypothesis that Ca(2+)-activated K(+) channel subunits (i.e. Slo variants) are also involved in the hearing of mammals in the organ of Corti.

Rapidly inactivating and non-inactivating calcium-activated potassium currents in frog saccular hair cells

1. Using a semi-intact epithelial preparation we examined the Ca(2+)-activated K(+) (K(Ca)) currents of frog (Rana pipiens) saccular hair cells. After blocking voltage-dependent K(+) (K(V)) currents with 4-aminopyridine (4-AP) an outward current containing inactivating (I(transient)) and non-inactivating (I(steady)) components remained. 2. The contribution of each varied greatly from cell to cell, with I(transient) contributing from 14 to 90 % of the total outward current. Inactivation of I(transient) was rapid (tau approximately 2-3 ms) and occurred within the physiological range of membrane potentials (V(1/2) = -63 mV). Recovery from inactivation was also rapid (tau approximately 10 ms). 3. Suppression of both I(transient) and I(steady) by depolarizations that approached the Ca(2+) equilibrium potential and by treatments that blocked Ca(2+) influx (application Ca(2+)-free saline or Cd(2+)), suggest both are Ca(2+) dependent. Both were blocked by iberiotoxin, a specific blocker of large-conductance K(Ca) channels (BK), but not by apamin, a specific blocker of small-conductance K(Ca) channels. 4. Ensemble-variance analysis showed that I(transient) and I(steady) flow through two distinct populations of channels, both of which have a large single-channel conductance (~100 pS in non-symmetrical conditions). Together, these data indicate that both I(transient) and I(steady) are carried through BK channels, one of which undergoes rapid inactivation while the other does not. 5. Inactivation of I(transient) could be removed by extracellular papain and could later be restored by intracellular application of the 'ball' domain of the auxiliary subunit (beta2) thought to mediate BK channel inactivation in rat chromaffin cells. We hypothesize that I(transient) results from the association of a similar beta subunit with some of the BK channels and that papain removes inactivation by cleaving extracellular sites required for this association.

Localization of two high-threshold potassium channel subunits in the rat central auditory system

The firing pattern of auditory neurons is determined in part by the type of voltage-sensitive potassium channels expressed. The expression patterns for two high-threshold potassium channels, Kv3.1 and Kv3.3, that differ in inactivation properties were examined in the rat auditory system. The positive activation voltage and rapid deactivation kinetics of these channels provide rapid repolarization of action potentials with little effect on action potential threshold. In situ hybridization experiments showed that Kv3.3 mRNA was highly expressed in most auditory neurons in the rat brainstem, whereas Kv3.1 was expressed in a more limited population of auditory neurons. Notably, Kv3.1 mRNA was not expressed in neurons of the medial and lateral superior olive and a subpopulation of neurons in the ventral nucleus of the lateral lemniscus. These results suggest that Kv3.3 channels may be the dominant Kv3 subfamily member expressed in brainstem auditory neurons and that, in some auditory neurons, Kv3.1 and Kv3.3 may coassemble to form functional channels. The localization of Kv3.1 protein was examined immunohistochemically. The distribution of stained somata and neuropil varied across auditory nuclei and correlated with the distribution of Kv3.1 mRNA-expressing neurons and their terminal arborizations, respectively. The intensity of Kv3.1 immunoreactivity varied across the tonotopic map in the medial nucleus of the trapezoid body with neurons responding best to high-frequency tones most intensely labeled. Thus, auditory neurons may vary the types and amount of K(+) channel expression in response to synaptic input to subtly tune their firing properties.

Maxi-K potassium channels: form, function, and modulation of a class of endogenous regulators of intracellular calcium

Large-conductance calcium-activated (maxi-K, BK) potassium channels are widely distributed in the brain. Maxi-K channels function as neuronal calcium sensors and contribute to the control of cellular excitability and the regulation of neurotransmitter release. Little is currently known of any significant role of maxi-K channels in the genesis of neurological disease. Recent advances in the molecular biology and pharmacology of these channels have revealed sources of phenotypic variability and demonstrated that they can be successfully modulated by pharmacological agents. A potential role is suggested in the treatment of conditions such as ischemic stroke and cognitive disorders.

Expression of the Kv3.1 potassium channel in the avian auditory brainstem

The Shaw-like potassium channel Kv3.1, a delayed rectifier with a high threshold of activation, is expressed in the time coding nuclei of the bird auditory brainstem. In both barn owls and chickens, Kv3.1 mRNA was expressed in the cochlear nucleus magnocellularis (NM) and the nucleus laminaris (NL). Western blot analysis showed that an antibody raised against the synthetic peptide sequence of rat Kv3.1 (rKv3.1) specifically recognized the same 92 kDa protein bands in both rat and chicken synaptosomal preparations. Immunohistochemical analyses using this anti-rKv3.1 antibody revealed a prominent gradient in Kv3.1 immunoreactivity along the tonotopic axis of the barn owl NM and NL and a less prominent gradient in the chicken. The precise localization of the Kv3.1 immunoproduct was resolved by electron microscopy. In both the owl and the chicken, Kv3.1 was targeted postsynaptically in NM and NL. The major difference in localization of Kv3.1 protein between the two birds was the expression of Kv3.1 in the NM axons and terminals in the region of the barn owl NL. This location of Kv3.1 channels supports its postulated function in reducing the width of action potentials as they invade the presynaptic terminal. The presynaptic localization may be a specialization for enabling neurons in owl NM to transmit high-frequency temporal information with little jitter.