A theoretical study of calcium microdomains in turtle hair cells

Quantification of the calcium signaling deficit in muscles devoid of triadin

Triadin, a protein of the sarcoplasmic reticulum (SR) of striated muscles, anchors the calcium-storing protein calsequestrin to calcium release RyR channels at the junction with t-tubules, and modulates these channels by conformational effects. Triadin ablation induces structural SR changes and alters the expression of other proteins. Here we quantify alterations of calcium signaling in single skeletal myofibers of constitutive triadin-null mice. We find higher resting cytosolic and lower SR-luminal [Ca²⁺], 40% lower calsequestrin expression, and more Ca_V1.1, RyR1 and SERCA1. Despite the increased Ca_V1.1, the mobile intramembrane charge was reduced by ~20% in Triadin-null fibers. The initial peak of calcium release flux by pulse depolarization was minimally altered in the null fibers (revealing an increase in peak calcium permeability). The “hump” phase that followed, attributable to calcium detaching from calsequestrin, was 25% lower, a smaller change than expected from the reduced calsequestrin content and calcium saturation. The exponential decay rate of calcium transients was 25% higher, consistent with the higher SERCA1 content. Recovery of calcium flux after a depleting depolarization was faster in triadin-null myofibers, consistent with the increased uptake rate and lower SR calsequestrin content. In sum, the triadin knockout determines an increased RyR1 channel openness, which depletes the SR, a substantial loss of calsequestrin and gains in other couplon proteins. Powerful functional compensations ensue: activation of SOCE that increases [Ca²⁺]_cyto; increased SERCA1 activity, which limits the decrease in [Ca²⁺]_SR and a restoration of SR calcium storage of unknown substrate. Together, they effectively limit the functional loss in skeletal muscles.

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.

PMCA2 pump mutations and hereditary deafness

Hair cells of the inner ear detect sound stimuli, inertial or gravitational forces by deflection of their apical stereocilia. A small number of stereociliary cation-selective mechanotransduction (MET) channels admit K⁺ and Ca²⁺ ions into the cytoplasm promoting hair cell membrane depolarization and, consequently, neurotransmitter release at the cell basolateral pole. Ca²⁺ influx into the stereocilia compartment is counteracted by the unusual w/a splicing variant of plasma-membrane calcium-pump isoform 2 (PMCA2) which, unlike other PMCA2 variants, increases only marginally its activity in response to a rapid variation of the cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_c). Missense mutations of PMCA2w/a cause deafness and loss of balance in humans. Mouse models in which the pump is genetically ablated or mutated show hearing and balance impairment, which correlates with defects in homeostatic regulation of stereociliary [Ca²⁺]_c, decreased sensitivity of mechanotransduction channels to hair bundle displacement and progressive degeneration of the organ of Corti. These results highlight a critical role played by the PMCA2w/a pump in the control of hair cell function and survival, and provide mechanistic insight into the etiology of deafness and vestibular disorders.

Tonotopy in calcium homeostasis and vulnerability of cochlear hair cells

Ototoxicity, noise overstimulation, or aging, can all produce hearing loss with similar properties, in which outer hair cells (OHCs), principally those at the high-frequency base of the cochlea, are preferentially affected. We suggest that the differential vulnerability may partly arise from differences in Ca²⁺ balance among cochlear locations. Homeostasis is determined by three factors: Ca²⁺ influx mainly via mechanotransducer (MET) channels; buffering by calcium-binding proteins and organelles like mitochondria; and extrusion by the plasma membrane CaATPase pump. We review quantification of these parameters and use our experimentally-determined values to model changes in cytoplasmic and mitochondrial Ca²⁺ during Ca²⁺ influx through the MET channels. We suggest that, in OHCs, there are two distinct micro-compartments for Ca²⁺ handling, one in the hair bundle and the other in the cell soma. One conclusion of the modeling is that there is a tonotopic gradient in the ability of OHCs to handle the Ca²⁺ load, which correlates with their vulnerability to environmental challenges. High-frequency basal OHCs are the most susceptible because they have much larger MET currents and have smaller dimensions than low-frequency apical OHCs.

Ca(2+)-BK channel clusters in olfactory receptor neurons and their role in odour coding

Olfactory receptor neurons (ORNs) have high-voltage-gated Ca(2+) channels whose physiological impact has remained enigmatic since the voltage-gated conductances in this cell type were first described in the 1980s. Here we show that in ORN somata of Xenopus laevis tadpoles these channels are clustered and co-expressed with large-conductance potassium (BK) channels. We found approximately five clusters per ORN and twelve Ca(2+) channels per cluster. The action potential-triggered activation of BK channels accelerates the repolarization of action potentials and shortens interspike intervals during odour responses. This increases the sensitivity of individual ORNs to odorants. At the level of mitral cells of the olfactory bulb, odour qualities have been shown to be coded by first-spike-latency patterns. The system of Ca(2+) and BK channels in ORNs appears to be important for correct odour coding because the blockage of BK channels not only affects ORN spiking patterns but also changes the latency pattern representation of odours in the olfactory bulb.

Exocytotic machineries of vestibular type I and cochlear ribbon synapses display similar intrinsic otoferlin-dependent Ca2+ sensitivity but a different coupling to Ca2+ channels

The hair cell ribbon synapses of the mammalian auditory and vestibular systems differ greatly in their anatomical organization and firing properties. Notably, vestibular Type I hair cells (VHC-I) are surrounded by a single calyx-type afferent terminal that receives input from several ribbons, whereas cochlear inner hair cells (IHCs) are contacted by several individual afferent boutons, each facing a single ribbon. The specificity of the presynaptic molecular mechanisms regulating transmitter release at these different sensory ribbon synapses is not well understood. Here, we found that exocytosis during voltage activation of Ca(2+) channels displayed higher Ca(2+) sensitivity, 10 mV more negative half-maximum activation, and a smaller dynamic range in VHC-I than in IHCs. VHC-I had a larger number of Ca(2+) channels per ribbon (158 vs 110 in IHCs), but their Ca(2+) current density was twofold smaller because of a smaller open probability and unitary conductance. Using confocal and stimulated emission depletion immunofluorescence microscopy, we showed that VHC-I had fewer synaptic ribbons (7 vs 17 in IHCs) to which Cav1.3 channels are more tightly organized than in IHCs. Gradual intracellular Ca(2+) uncaging experiments revealed that exocytosis had a similar intrinsic Ca(2+) sensitivity in both VHC-I and IHCs (KD of 3.3 ± 0.6 μM and 4.0 ± 0.7 μM, respectively). In otoferlin-deficient mice, exocytosis was largely reduced in VHC-I and IHCs. We conclude that VHC-I and IHCs use a similar micromolar-sensitive otoferlin Ca(2+) sensor and that their sensory encoding specificity is essentially determined by a different functional organization of Ca(2+) channels at their synaptic ribbons.

Calcium regulates molecular interactions of otoferlin with soluble NSF attachment protein receptor (SNARE) proteins required for hair cell exocytosis

Mutations in otoferlin, a C2 domain-containing ferlin family protein, cause non-syndromic hearing loss in humans (DFNB9 deafness). Furthermore, transmitter secretion of cochlear inner hair cells is compromised in mice lacking otoferlin. In the present study, we show that the C2F domain of otoferlin directly binds calcium (KD = 267 μM) with diminished binding in a pachanga (D1767G) C2F mouse mutation. Calcium was found to differentially regulate binding of otoferlin C2 domains to target SNARE (t-SNARE) proteins and phospholipids. C2D-F domains interact with the syntaxin-1 t-SNARE motif with maximum binding within the range of 20-50 μM Ca(2+). At 20 μM Ca(2+), the dissociation rate was substantially lower, indicating increased binding (KD = ∼10(-9)) compared with 0 μM Ca(2+) (KD = ∼10(-8)), suggesting a calcium-mediated stabilization of the C2 domain·t-SNARE complex. C2A and C2B interactions with t-SNAREs were insensitive to calcium. The C2F domain directly binds the t-SNARE SNAP-25 maximally at 100 μM and with reduction at 0 μM Ca(2+), a pattern repeated for C2F domain interactions with phosphatidylinositol 4,5-bisphosphate. In contrast, C2F did not bind the vesicle SNARE protein synaptobrevin-1 (VAMP-1). Moreover, an antibody targeting otoferlin immunoprecipitated syntaxin-1 and SNAP-25 but not synaptobrevin-1. As opposed to an increase in binding with increased calcium, interactions between otoferlin C2F domain and intramolecular C2 domains occurred in the absence of calcium, consistent with intra-C2 domain interactions forming a "closed" tertiary structure at low calcium that "opens" as calcium increases. These results suggest a direct role for otoferlin in exocytosis and modulation of calcium-dependent membrane fusion.

Light adaptation increases response latency of alpha ganglion cells via a threshold-like nonlinearity

Adaptation is an important process of sensory systems to adjust sensitivity to ensure the appropriate information encoding. Sensitivity and kinetics of retinal ganglion cell (RGC) responses have been studied extensively using a brief flash superimposed on different but steady backgrounds. However, it is still unclear if light adaptation exerts any effect on more complex response properties, such as response nonlinearity. In this study, we found that the latency of spike responses to a repeated flashing spot stimulation increased by 30 ms in the mouse ON α RGCs (An ON-type RGC is excited when a spot is turned on in the center of its receptive field). A single dimming event preceding the test flash on a steady adapting background could also produce similar effect in increasing latency of light responses. A simple computational model with a linear transformation of the light stimulus and a threshold-like nonlinearity could account for the experimental data. Moreover, the strength of the measured nonlinearity and the response latency were affected by the duration of light adaptation. The possible biological processes underlying this nonlinearity were explored. Voltage clamp recording revealed the presence of the increase in latency and threshold-like nonlinearity in the excitatory input of RGCs. However, no comparable nonlinearity was observed in the light responses of the ON cone bipolar cells. We further excluded GABAergic and glycinergic inhibition, N-methyl-D-aspartate receptor rectification and voltage-gated Na(+) channels as potential sources of this nonlinearity by pharmacological experiments. Our results indicate the bipolar cell terminals as the potential site of nonlinearity. Computational modeling constrained by experimental data supports that conclusion and suggests the voltage-sensitive Ca(++) channels and Ca(++)-dependent vesicle release in the bipolar cell terminals as mechanistic basis.

Altered Ca2+ concentration, permeability and buffering in the myofibre Ca2+ store of a mouse model of malignant hyperthermia

  Malignant hyperthermia (MH) is linked to mutations in the type 1 ryanodine receptor, RyR1, the Ca2+ channel of the sarcoplasmic reticulum (SR) of skeletal muscle. The Y522S MH mutation was studied for its complex presentation, which includes structurally and functionally altered cell 'cores'. Imaging cytosolic and intra-SR [Ca2+] in muscle cells of heterozygous YS mice we determined Ca2+ release flux activated by clamp depolarization, permeability (P) of the SR membrane (ratio of flux and [Ca2+] gradient) and SR Ca2+ buffering power (B). In YS cells resting [Ca2+]SR was 45% of the value in normal littermates (WT). P was more than doubled, so that initial flux was normal. Measuring [Ca2+]SR(t) revealed dynamic changes in B(t). The alterations were similar to those caused by cytosolic BAPTA, which promotes release by hampering Ca2+-dependent inactivation (CDI). The [Ca2+] transients showed abnormal 'breaks', decaying phases after an initial rise, traced to a collapse in flux and P. Similar breaks occurred in WT myofibres with calsequestrin reduced by siRNA; calsequestrin content, however, was normal in YS muscle. Thus, the Y522S mutation causes greater openness of the RyR1, lowers resting [Ca2+]SR and alters SR Ca2+ buffering in a way that copies the functional instability observed upon reduction of calsequestrin content. The similarities with the effects of BAPTA suggest that the mutation, occurring near the cytosolic vestibule of the channel, reduces CDI as one of its primary effects. The unstable SR buffering, mimicked by silencing of calsequestrin, may help precipitate the loss of Ca2+ control that defines a fulminant MH event.

Dynamic measurement of the calcium buffering properties of the sarcoplasmic reticulum in mouse skeletal muscle

The buffering power, B, of the sarcoplasmic reticulum (SR), ratio of the changes in total and free [Ca(2+)], was determined in fast-twitch mouse muscle cells subjected to depleting membrane depolarization. Changes in total SR [Ca(2+)] were measured integrating Ca(2+) release flux, determined with a cytosolic [Ca(2+)] monitor. Free [Ca(2+)](SR) was measured using the cameleon D4cpv-Casq1. In 34 wild-type (WT) cells average B during the depolarization (ON phase) was 157 (SEM 26), implying that of 157 ions released, 156 were bound inside the SR. B was significantly greater when BAPTA, which increases release flux, was present in the cytosol. B was greater early in the pulse - when flux was greatest - than at its end, and greater in the ON than in the OFF. In 29 Casq1-null cells, B was 40 (3.6). The difference suggests that 75% of the releasable calcium is normally bound to calsequestrin. In the nulls the difference in B between ON and OFF was less than in the WT but still significant. This difference and the associated decay in B during the ON were not artifacts of a slow SR monitor, as they were also found in the WT when [Ca(2+)](SR) was tracked with the fast dye fluo-5N. The calcium buffering power, binding capacity and non-linear binding properties of the SR measured here could be accounted for by calsequestrin at the concentration present in mammalian muscle, provided that its properties were substantially different from those found in solution. Its affinity should be higher, or K(D) lower than the conventionally accepted 1 mm; its cooperativity (n in a Hill fit) should be higher and the stoichiometry of binding should be at the higher end of the values derived in solution. The reduction in B during release might reflect changes in calsequestrin conformation upon calcium loss.

The temporal characteristics of Ca2+ entry through L-type and T-type Ca2+ channels shape exocytosis efficiency in chick auditory hair cells during development

During development, synaptic exocytosis by cochlear hair cells is first initiated by patterned spontaneous Ca(2+) spikes and, at the onset of hearing, by sound-driven graded depolarizing potentials. The molecular reorganization occurring in the hair cell synaptic machinery during this developmental transition still remains elusive. We characterized the changes in biophysical properties of voltage-gated Ca(2+) currents and exocytosis in developing auditory hair cells of a precocial animal, the domestic chick. We found that immature chick hair cells (embryonic days 10-12) use two types of Ca(2+) currents to control exocytosis: low-voltage-activating, rapidly inactivating (mibefradil sensitive) T-type Ca(2+) currents and high-voltage-activating, noninactivating (nifedipine sensitive) L-type currents. Exocytosis evoked by T-type Ca(2+) current displayed a fast release component (RRP) but lacked the slow sustained release component (SRP), suggesting an inefficient recruitment of distant synaptic vesicles by this transient Ca(2+) current. With maturation, the participation of L-type Ca(2+) currents to exocytosis largely increased, inducing a highly Ca(2+) efficient recruitment of an RRP and an SRP component. Notably, L-type-driven exocytosis in immature hair cells displayed higher Ca(2+) efficiency when triggered by prerecorded native action potentials than by voltage steps, whereas similar efficiency for both protocols was found in mature hair cells. This difference likely reflects a tighter coupling between release sites and Ca(2+) channels in mature hair cells. Overall, our results suggest that the temporal characteristics of Ca(2+) entry through T-type and L-type Ca(2+) channels greatly influence synaptic release by hair cells during cochlear development.

Measurement of RyR permeability reveals a role of calsequestrin in termination of SR Ca(2+) release in skeletal muscle

The mechanisms that terminate Ca²⁺ release from the sarcoplasmic reticulum are not fully understood. D4cpv-Casq1 (Sztretye et al. 2011. J. Gen. Physiol. doi:10.1085/jgp.201010591) was used in mouse skeletal muscle cells under voltage clamp to measure free Ca²⁺ concentration inside the sarcoplasmic reticulum (SR), [Ca²⁺]_SR, simultaneously with that in the cytosol, [Ca²⁺]_c, during the response to long-lasting depolarization of the plasma membrane. The ratio of Ca²⁺ release flux (derived from [Ca²⁺]_c(t)) over the gradient that drives it (essentially equal to [Ca²⁺]_SR) provided directly, for the first time, a dynamic measure of the permeability to Ca²⁺ of the releasing SR membrane. During maximal depolarization, flux rapidly rises to a peak and then decays. Before 0.5 s, [Ca²⁺]_SR stabilized at ∼35% of its resting level; depletion was therefore incomplete. By 0.4 s of depolarization, the measured permeability decayed to ∼10% of maximum, indicating ryanodine receptor channel closure. Inactivation of the t tubule voltage sensor was immeasurably small by this time and thus not a significant factor in channel closure. In cells of mice null for Casq1, permeability did not decrease in the same way, indicating that calsequestrin (Casq) is essential in the mechanism of channel closure and termination of Ca²⁺ release. The absence of this mechanism explains why the total amount of calcium releasable by depolarization is not greatly reduced in Casq-null muscle (Royer et al. 2010. J. Gen. Physiol. doi:10.1085/jgp.201010454). When the fast buffer BAPTA was introduced in the cytosol, release flux became more intense, and the SR emptied earlier. The consequent reduction in permeability accelerated as well, reaching comparable decay at earlier times but comparable levels of depletion. This observation indicates that [Ca²⁺]_SR, sensed by Casq and transmitted to the channels presumably via connecting proteins, is determinant to cause the closure that terminates Ca²⁺ release.

Combined computational and experimental approaches to understanding the Ca(2+) regulatory network in neurons

Ca(2+) is a ubiquitous signaling ion that regulates a variety of neuronal functions by binding to and altering the state of effector proteins. Spatial relationships and temporal dynamics of Ca(2+) elevations determine many cellular responses of neurons to chemical and electrical stimulation. There is a wealth of information regarding the properties and distribution of Ca(2+) channels, pumps, exchangers, and buffers that participate in Ca(2+) regulation. At the same time, new imaging techniques permit characterization of evoked Ca(2+) signals with increasing spatial and temporal resolution. However, understanding the mechanistic link between functional properties of Ca(2+) handling proteins and the stimulus-evoked Ca(2+) signals they orchestrate requires consideration of the way Ca(2+) handling mechanisms operate together as a system in native cells. A wide array of biophysical modeling approaches is available for studying this problem and can be used in a variety of ways. Models can be useful to explain the behavior of complex systems, to evaluate the role of individual Ca(2+) handling mechanisms, to extract valuable parameters, and to generate predictions that can be validated experimentally. In this review, we discuss recent advances in understanding the underlying mechanisms of Ca(2+) signaling in neurons via mathematical modeling. We emphasize the value of developing realistic models based on experimentally validated descriptions of Ca(2+) transport and buffering that can be tested and refined through new experiments to develop increasingly accurate biophysical descriptions of Ca(2+) signaling in neurons.

D4cpv-calsequestrin: a sensitive ratiometric biosensor accurately targeted to the calcium store of skeletal muscle

Current fluorescent monitors of free [Ca(2+)] in the sarcoplasmic reticulum (SR) of skeletal muscle cells are of limited quantitative value. They provide either a nonratio signal that is difficult to calibrate and is not specific or, in the case of Forster resonant energy transfer (FRET) biosensors, a signal of small dynamic range, which may be degraded further by imperfect targeting and interference from endogenous ligands of calsequestrin. We describe a novel tool that uses the cameleon D4cpv, which has a greater dynamic range and lower susceptibility to endogenous ligands than earlier cameleons. D4cpv was targeted to the SR by fusion with the cDNA of calsequestrin 1 or a variant that binds less Ca(2+). "D4cpv-Casq1," expressed in adult mouse at concentrations up to 22 µmole/liter of muscle cell, displayed the accurate targeting of calsequestrin and stayed inside cells after permeabilization of surface and t system membranes, which confirmed its strict targeting. FRET ratio changes of D4cpv-Casq1 were calibrated inside cells, with an effective K(D) of 222 µM and a dynamic range [(R(max) - R(min))/R(min)] of 2.5, which are improvements over comparable sensors. Both the maximal ratio, R(max), and its resting value were slightly lower in areas of high expression, a variation that was inversely correlated to distance from the sites of protein synthesis. The average [Ca(2+)](SR) in 74 viable cells at rest was 416 µM. The distribution of individual ratio values was Gaussian, but that of the calculated [Ca(2+)](SR) was skewed, with a tail of very large values, up to 6 mM. Model calculations reproduce this skewness as the consequence of quantifiably small variations in biosensor performance. Local variability, a perceived weakness of biosensors, thus becomes quantifiable. It is demonstrably small in D4cpv. D4cpv-Casq1 therefore provides substantial improvements in sensitivity, specificity, and reproducibility over existing monitors of SR free Ca(2+) concentration.

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.

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.

The diverse roles of ribbon synapses in sensory neurotransmission

Sensory synapses of the visual and auditory systems must faithfully encode a wide dynamic range of graded signals, and must be capable of sustained transmitter release over long periods of time. Functionally and morphologically, these sensory synapses are unique: their active zones are specialized in several ways for sustained, rapid vesicle exocytosis, but their most striking feature is an organelle called the synaptic ribbon, which is a proteinaceous structure that extends into the cytoplasm at the active zone and tethers a large pool of releasable vesicles. But precisely how does the ribbon function to support tonic release at these synapses? Recent genetic and biophysical advances have begun to open the 'black box' of the synaptic ribbon with some surprising findings and promise to resolve its function in vision and hearing.

Nonequilibrium calcium dynamics regulate the autonomous firing pattern of rat striatal cholinergic interneurons

Striatal cholinergic interneurons discharge rhythmically in two patterns associated with different afterhyperpolarization timescales, each dictated by a different calcium-dependent potassium current. Single spiking depends on a medium-duration afterhyperpolarization (mAHP) generated by rapid SK currents that are associated with N-type calcium channels. Periodic bursting is driven by a delayed and slowly decaying afterhyperpolarization (sAHP) current associated with L-type channels. Using calcium imaging we show that the calcium transients underlying these currents exhibit two corresponding timescales throughout the somatodendritic tree. This result is not consistent with spatial compartmentalization of calcium entering through the two calcium channels and acting on the two potassium currents, or with differences in channel gating kinetics of the calcium dependent potassium currents. Instead, we show that nonequilibrium dynamics of calcium redistribution among cytoplasmic binding sites with different calcium binding kinetics can give rise to multiple timescales within the same cytoplasmic volume. The resulting independence of mAHP and sAHP currents allows cytoplasmic calcium to control two different and incompatible firing patterns (single spiking or bursting and pausing), depending on whether calcium influx is pulsatile or sustained. During irregular firing, calcium entry at both timescales can be detected, suggesting that an interaction between the medium and slow calcium-dependent afterhyperpolarizations may underlie this firing pattern.

Localization of inner hair cell mechanotransducer channels using high-speed calcium imaging

Hair cells detect vibrations of their stereociliary bundle by activation of mechanically sensitive transducer channels. Although evidence suggests the transducer channels are near the stereociliary tops and are opened by force imparted by tip links connecting contiguous stereocilia, the exact channel site remains controversial. We used fast confocal imaging of fluorescence changes reflecting calcium entry during bundle stimulation to localize the channels. Calcium signals were visible in single stereocilia of rat cochlear inner hair cells and were up to tenfold larger and faster in the second and third stereociliary rows than in the tallest first row. The number of functional stereocilia was proportional to transducer current amplitude, indicating that there were about two channels per stereocilium. Comparable results were obtained in outer hair cells. The observations, supported by theoretical simulations, suggest there are no functional mechanically sensitive transducer channels in first row stereocilia and imply the channels are present only at the bottom of the tip links.

Mechanisms contributing to synaptic Ca2+ signals and their heterogeneity in hair cells

Sound coding at hair cell ribbon synapses is tightly regulated by Ca(2+). Here, we used patch-clamp, fast confocal Ca(2+) imaging and modeling to characterize synaptic Ca(2+) signaling in cochlear inner hair cells (IHCs) of hearing mice. Submicrometer fluorescence hotspots built up and collapsed at the base of IHCs within a few milliseconds of stimulus onset and cessation. They most likely represented Ca(2+) microdomains arising from synaptic Ca(2+) influx through Ca(V)1.3 channels. Synaptic Ca(2+) microdomains varied substantially in amplitude and voltage dependence even within single IHCs. Testing putative mechanisms for the heterogeneity of Ca(2+) signaling, we found the amplitude variability unchanged when blocking mitochondrial Ca(2+) uptake or Ca(2+)-induced Ca(2+) release, buffering cytosolic Ca(2+) by millimolar concentrations of EGTA, or elevating the Ca(2+) channel open probability by the dihydropyridine agonist BayK8644. However, we observed substantial variability also for the fluorescence of immunolabeled Ca(V)1.3 Ca(2+) channel clusters. Moreover, the Ca(2+) microdomain amplitude correlated positively with the size of the corresponding synaptic ribbon. Ribbon size, previously suggested to scale with the number of synaptic Ca(2+) channels, was approximated by using fluorescent peptide labeling. We propose that IHCs adjust the number and the gating of Ca(V)1.3 channels at their active zones to diversify their transmitter release rates.

Hair cell afferent synapses

This review will cover advances in the study of hair cell afferent synaptic function occurring between 2005 and 2008. During this time, capacitance measurements of vesicular fusion have continued to be refined, optical methods have added insights regarding vesicle trafficking, and paired intracellular recordings have established the transfer function of the afferent synapse at high resolution. Further, genes have been identified with forms of deafness known as auditory neuropathy, and their role in afferent signaling explored in mouse models. With these advances, our view of the hair cell afferent synapse has continued to be refined, and surprising properties have been revealed that emphasize the unique role of this structure in neural function.

Calcium microdomains at presynaptic active zones of vertebrate hair cells unmasked by stochastic deconvolution

Signal transduction by auditory and vestibular hair cells involves an impressive ensemble of finely tuned control mechanisms, strictly dependent on the local intracellular Ca(2+) concentration ([Ca(2+)](i)). The study of Ca(2+) dynamics in hair cells typically combines Ca(2+)-sensitive fluorescent indicators (dyes), patch clamp and optical microscopy to produce images of the patterns of fluorescence of a Ca(2+) indicator following various stimulation protocols. Here we describe a novel method that combines electrophysiological recordings, fluorescence imaging and numerical simulations to effectively deconvolve Ca(2+) signals within cytoplasmic microdomains that would otherwise remain inaccessible to direct observation. The method relies on the comparison of experimental data with virtual signals derived from a Monte Carlo reaction-diffusion model based on a realistic reconstruction of the relevant cell boundaries in three dimensions. The model comprises Ca(2+) entry at individual presynaptic active zones followed by diffusion, buffering, extrusion and release of Ca(2+). Our results indicate that changes of the hair cell [Ca(2+)](i) during synaptic transmission are primarily controlled by the Ca(2+) endogenous buffers both at short (<1mu) and at long (tens of microns) distances from the active zones. We provide quantitative estimates of concentration and kinetics of the hair cell endogenous Ca(2+) buffers and Ca(2+)-ATPases. We finally show that experimental fluorescence data collected during Ca(2+) influx are not interpreted correctly if the [Ca(2+)](i) is estimated by assuming that Ca(2+) equilibrates instantly with its reactants. In our opinion, this approach is of potentially general interest as it can be easily adapted to the study of Ca(2+) dynamics in diverse biological systems.

Interactions of Recombinant Mouse Erythrocyte Transglutaminase with Membrane Skeletal Proteins

Transglutaminases (TGs) are a family of enzymes that catalyze the formation of covalent gamma-glutamyl-epsilon-lysine crosslinks between glutamine (Q) acyl-donors and lysine (K) acyl-acceptors. Here, we report the cDNA cloning of a TG from mouse reticulocytes, its 4.6-kb message size and high-yield synthesis of recombinant TG in yeast cultures. Its activity was assayed by crosslinking the amine of monodansylcadaverine (DC) onto casein and inside-out vesicles of erythrocytes. The latter contain TG substrates including the anion ion exchanger (AE1) or band 3, and the crosslinking activity was the highest at physiological [GTP] and [ATP] of erythrocytes. To study individually how TG interacts with band 3 and what role P4.2, a pseudo-TG that is normally associated with band 3, may play in their interaction, recombinant cytoplasmic domain of band 3 (cdb3) and P4.2 were also cloned by polymerase chain reaction from mouse reticulocytes, expressed and affinity-purified from Escherichia coli. Enzyme-linked immunosorbent assay and Western blot analysis revealed that increasing [CaCl(2)] enhanced TG-mediated crosslinking of DC to cdb3 but decreased TG binding to cdb3. P4.2 inhibited the TG-mediated crosslinking of cdb3 but stabilized the binding of TG to cdb3 in the presence of calcium. This in vitro study suggests a relationship among TG, cdb3 and P4.2 in erythrocyte membrane during calcium influx.

Calcium-dependent inactivation of calcium channels in cochlear hair cells of the chicken

Voltage-gated calcium channels support both spontaneous and sound-evoked neurotransmitter release from ribbon synapses of cochlear hair cells. A variety of regulatory mechanisms must cooperate to ensure the appropriate level of activity in the restricted pool of synaptic calcium channels ( approximately 100) available to each synaptic ribbon. One potential feedback mechanism, calcium-dependent inactivation (CDI) of voltage-gated, L-type calcium channels, can be modulated by calmodulin-like calcium-binding proteins. CDI of voltage-gated calcium current was studied in hair cells of the chicken's basilar papilla (analogous to the mammalian cochlea) after blocking the predominant potassium conductances. For inactivating currents produced by 2.5 s steps to the peak of the current-voltage relation (1 mm EGTA internal calcium buffer), single exponential fits yielded an average decay time constant of 1.92 +/- 0.18 s (mean +/- s.e.m., n = 12) at 20-22 degrees C, while recovery occurred with a half-time of approximately 10 s. Inactivation produced no change in reversal potential, arguing that the observed relaxation did not result from alternative processes such as calcium accumulation or activation of residual potassium currents. Substitution of external calcium with barium greatly reduced inactivation, while inhibition of endoplasmic calcium pumps with t-benzohydroquinone (BHQ) or thapsigargin made inactivation occur faster and to a greater extent. Raising external calcium 10-fold (from 2 to 20 mm) increased peak current 3-fold, but did not alter the extent or time course of CDI. However, increasing levels of internal calcium buffer consistently reduced the rate and extent of inactivation. With 1 mm EGTA buffering and in 2 mm external calcium, the available pool of calcium channels was half-inactivated near the resting membrane potential (-50 mV). CDI may be further regulated by calmodulin-like calcium-binding proteins (CaBPs). mRNAs for several CaBPs are expressed in chicken cochlear tissue, and antibodies to CaBP4 label hair cells, but not supporting cells, equivalent to the pattern seen in mammalian cochlea. Thus, molecular mechanisms that underlie CDI appeared to be conserved across vertebrate species, may provide a means to adjust calcium channel open probability, and could serve to maintain the set-point for spontaneous release from the ribbon synapse.

Ca2+ signaling in the inner ear

The inner ear contains delicate sensory receptors that have adapted to detect the minutest mechanical disturbances. Ca(2+) ions are implicated in all steps of the transduction process, as well as in its regulation by an impressive ensemble of finely tuned feedback control mechanisms. Recent studies have unveiled some of the key players, but things do not sound quite right yet.

A functional study of plasma-membrane calcium-pump isoform 2 mutants causing digenic deafness

Ca2+ enters the stereocilia of hair cells through mechanoelectrical transduction channels opened by the deflection of the hair bundle and is exported back to endolymph by an unusual splicing isoform (w/a) of plasma-membrane calcium-pump isoform 2 (PMCA2). Ablation or missense mutations of the pump cause deafness, as described for the G283S mutation in the deafwaddler (dfw) mouse. A deafness-inducing missense mutation of PMCA2 (G293S) has been identified in a human family. The family also was screened for mutations in cadherin 23, which accentuated hearing loss in a previously described human family with a PMCA2 mutation. A T1999S substitution was detected in the cadherin 23 gene of the healthy father and affected son but not in that of the unaffected mother, who presented instead the PMCA2 mutation. The w/a isoform was overexpressed in CHO cells. At variance with the other PMCA2 isoforms, it became activated only marginally when exposed to a Ca2+ pulse. The G293S and G283S mutations delayed the dissipation of Ca2+ transients induced in CHO cells by InsP3. In organotypic cultures, Ca2+ imaging of vestibular hair cells showed that the dissipation of stereociliary Ca2+ transients induced by Ca2+ uncaging was compromised in the dfw and PMCA2 knockout mice, as was the sensitivity of the mechanoelectrical transduction channels to hair bundle displacement in cochlear hair cells.

Mechanisms of efferent-mediated responses in the turtle posterior crista

To study the cellular mechanisms of efferent actions, we recorded from vestibular-nerve afferents close to the turtle posterior crista while efferent fibers were electrically stimulated. Efferent-mediated responses were obtained from calyx-bearing (CD, calyx and dimorphic) afferents and from bouton (B) afferents distinguished by their neuroepithelial locations into BT units near the torus and BM units at intermediate sites. The spike discharge of CD units is strongly excited by efferent stimulation, whereas BT and BM units are inhibited, with BM units also showing a postinhibitory excitation. Synaptic activity was recorded intracellularly after spikes were blocked. Responses of BT/BM units to single efferent shocks consist of a brief depolarization followed by a prolonged hyperpolarization. Both components reflect variations in hair-cell quantal release rates and are eliminated by pharmacological antagonists of alpha9/alpha10 nicotinic receptors. Blocking calcium-dependent SK potassium channels converts the biphasic response into a prolonged depolarization. Results can be explained, as in other hair-cell systems, by the sequential activation of alpha9/alpha10 and SK channels. In BM units, the postinhibitory excitation is based on an increased rate of hair-cell quanta and depends on the preceding inhibition. There is, in addition, an efferent-mediated, direct depolarization of BT/BM and CD fibers. In CD units, it is the exclusive efferent response. Nicotinic antagonists have different effects on hair-cell efferent actions and on the direct depolarization of CD and BT/BM units. Ultrastructural studies, besides confirming the efferent innervation of type II hair cells and calyx endings, show that turtle efferents commonly contact afferent boutons terminating on type II hair cells.

Biophysical and pharmacological characterization of voltage-gated calcium currents in turtle auditory hair cells

Hair cell calcium channels regulate membrane excitability and control synaptic transmission. The present investigations focused on determining whether calcium channels vary between hair cells of different characteristic frequencies or if multiple channel types exist within a hair cell, each serving a different function. To this end, turtle auditory hair cells from high- (317 +/- 27 Hz) and low-frequency (115 +/- 6 Hz) positions were voltage clamped using the whole-cell recording technique, and calcium currents were characterized based on activation, inactivation and pharmacological properties. Pharmacological sensitivity to dihydropyridines (nimodipine, Bay K 8644), benzothiazepines (diltiazem) and acetonitrile derivatives (verapamil, D600) and the insensitivity to non-L-type calcium channel antagonists support the conclusion that only L-type calcium channels were present. Fast activation rise times (< 0.5 ms), hyperpolarized half-activation potentials and a relative insensitivity to nimodipine suggest the channels were of the alpha1D (CaV1.3) variety. Although no pharmacological differences were found between calcium currents obtained from high- and low-frequency cells, low-frequency cells activated slightly faster and at hyperpolarized potentials, with half-activating voltages of -43 +/- 1 mV compared to -35 +/- 1 mV. Inactivation was observed in both high- and low-frequency cells. The time course of inactivation required three time constants for a fit. Long depolarizations could result in complete inactivation. The voltage of half-inactivation was -40 +/- 2 mV for high-frequency cells and -46 +/- 2 mV for low-frequency cells. Calcium channel inactivation did not significantly alter hair cell electrical resonant properties elicited from protocols where the membrane potential was hyperpolarized or depolarized prior to characterizing the resonance. A bell-shaped voltage dependence and modest sensitivities to intracellular calcium chelators and external barium ions suggest that inactivation was calcium dependent.

Topological and developmental gradients of calbindin expression in the chick's inner ear

Mobile intracellular calcium buffers play an important role in regulating calcium flux into mechanosensory hair cells and calbindin D-28k is expressed at high levels in the chick's basilar papilla. We have used RT-PCR, in situ hybridization, and immunohistology to demonstrate that calbindin expression varies systematically according to hair cell position and developmental age. RT-PCR using microdissected quarters of the posthatch basilar papilla showed that mRNA levels were lowest in the (low frequency) apex and higher in basal quadrants. In situ hybridization revealed calbindin mRNA in posthatch hair cells and supporting cells, with more intense labeling of hair cells from basal (high frequency) positions. A similar topology was obtained with calbindin antibodies. Neither calbindin riboprobe nor calbindin antibody labeled cochlear neurons. In contrast, a subset of large vestibular neurons and their calyciform endings onto Type I vestibu lar hair cells were strongly labeled by the calbindin antibody, while vestibular hair cells were negative for calbindin immunoreactivity. Likewise, calbindin in situ hybridization was negative for vestibular hair cells but positive in a subset of larger vestibular neurons. Calbindin mRNA was detected in hair cells of the basal half of the papilla at embryonic day 10 (E10) and calbindin immunoreactivity was detected at E12. Hair cells in the apical half of the papilla had equivalent calbindin expression two days later. Immunoreactivity appeared in abneural supporting cells days later than in hair cells, and not until E20 in neurally located supporting cells. These results demonstrate that calbindin message and protein levels are greater in high-frequency hair cells. This "tonotopic" gradient may result from the stabilization of a basal-to-apical developmental gradient and could be related at least in part to calcium channel expression along this axis.

Dynamics of intracellular calcium in hair cells isolated from the semicircular canal of the frog

Changes in cytosolic free Ca(2+) concentration ([Ca(2+)]i) were monitored optically in hair cells mechanically isolated from frog semicircular canals using the membrane-impermeant form of the Ca(2+)-selective dye Oregon Green 488 BAPTA-1 (OG, 100 microM). Cells stimulated by depolarization under whole-cell voltage clamp conditions revealed Ca(2+) entry at selected sites (hotspots) located mostly in the lower (synaptic) half of the cell body. [Ca(2+)]i at individual hotspots rose with a time constant tau1 approximately 70 ms and decayed with a bi-exponential time-course (tau2 approximately 160, tau3 approximately 2500 ms) following a 160 ms depolarization to -20 mV. With repeated stimulation [Ca(2+)]i underwent independent amplitude changes at distinct hotspots, suggesting that the underlying Ca(2+) channel clusters can be regulated differentially by intracellular signalling pathways. Block by nifedipine indicated that the L-type Ca(2+)channels are distributed at different densities in distinct hotspots. No diffusion barrier other than the nuclear region was found in the cytosol, so that, during a prolonged depolarization (lasting up to 1s), Ca(2+) was able to reach the cell apical ciliated pole. The effective Ca(2+) diffusion constant, measured from the progression of Ca(2+) wavefronts in the cytosol, was approximately 57 microm(2)/s. Our results indicate that in these hair cells, buffered diffusion of Ca(2+) proceeds evenly from the source point to the cell interior and is dominated by the diffusion constant of the endogenous mobile buffers.

Mechanisms underlying H(2)O(2)-mediated inhibition of synaptic transmission in rat hippocampal slices

Hydrogen peroxide (H(2)O(2)) inhibits the population spike (PS) evoked by Schaffer collateral stimulation in hippocampal slices. Proposed mechanisms underlying this effect include generation of hydroxyl radicals (.OH) and inhibition of presynaptic Ca(2+) entry. We have examined these possible mechanisms in rat hippocampal slices. Inhibition of the evoked PS by H(2)O(2) was sharply concentration-dependent: 1.2 mM H(2)O(2) had no effect, whereas 1.5 and 2.0 mM H(2)O(2) reversibly depressed PS amplitude by roughly 80%. The iron chelator, deferoxamine (1 mM), and the endogenous.OH scavenger, ascorbate (400 microM), prevented PS inhibition, confirming.OH involvement. Isoascorbate (400 microM), which unlike ascorbate is not taken up by brain cells, also prevented PS inhibition, indicating an extracellular site of.OH generation or action. We then investigated whether H(2)O(2)-induced PS depression could be overcome by prolonged stimulation, which enhances Ca(2+) entry. During 5-s, 10-Hz trains under control conditions, PS amplitude increased to over 200% during the first three-four pulses, then stabilized. In the presence of H(2)O(2), PS amplitude was initially depressed, but began to recover after 2.5 s of stimulation, finally reaching 80% of the control maximum. In companion experiments, we assessed the effect of H(2)O(2) on presynaptic Ca(2+) entry by monitoring extracellular Ca(2+) concentration ([Ca(2+)](o)) during train stimulation in the presence of postsynaptic receptor blockers. Evoked [Ca(2+)](o) shifts were apparently unaltered by H(2)O(2), suggesting a lack of effect on Ca(2+) entry. Taken together, these findings suggest new ways in which reactive oxygen species (ROS) might act as signaling agents, specifically as modulators of synaptic transmission.

Analysis of presynaptic Ca2+ influx and transmitter release kinetics during facilitation at the inhibitor of the crayfish neuromuscular junction

The inhibitory synapse of the crayfish neuromuscular junction was used to examine mechanisms underlying the F2 component of synaptic facilitation. Because previous studies have shown accelerated transmitter release during facilitation, we examined whether an activity-dependent plasticity in I(Ca) could underlie this acceleration. We established that fluorescent transients generated by Magnesium Green can resolve small differences in presynaptic Ca(2+) influx that correlate with changes in IPSC waveform. However, there was no change in Ca(2+) transients associated with the accelerated release. Analyzing the initial rise of IPSC and the duration of the presynaptic spike yielded a depolarization-release coupling plot that captures the impact of spike waveform on the initial rate of release. We conclude that accelerated release during F2 facilitation cannot be attributed to plasticity of I(Ca) or modulation of spike waveform. Kinetic analysis showed a reduction in synaptic delay during facilitation only when broad action potentials were used. In unfacilitated release, synaptic delay increased as spike duration lengthened. We propose that small single Ca(2+) channel currents during the plateau phase of broad action potentials raise local Ca(2+) concentration only enough to fill a high-affinity site. Occupation of this site in itself, or events downstream, would convert a vesicle from control to facilitated state. If the conversion were a slow process, it could explain the changes in synaptic delay reported here. This hypothesis can also account for a number of observations related to Ca(2+) cooperativity and synaptic facilitation.

Analysis of presynaptic Ca2+ influx and transmitter release kinetics during facilitation at the inhibitor of the crayfish neuromuscular junction

The inhibitory synapse of the crayfish neuromuscular junction was used to examine mechanisms underlying the F2 component of synaptic facilitation. Because previous studies have shown accelerated transmitter release during facilitation, we examined whether an activity-dependent plasticity in I(Ca) could underlie this acceleration. We established that fluorescent transients generated by Magnesium Green can resolve small differences in presynaptic Ca(2+) influx that correlate with changes in IPSC waveform. However, there was no change in Ca(2+) transients associated with the accelerated release. Analyzing the initial rise of IPSC and the duration of the presynaptic spike yielded a depolarization-release coupling plot that captures the impact of spike waveform on the initial rate of release. We conclude that accelerated release during F2 facilitation cannot be attributed to plasticity of I(Ca) or modulation of spike waveform. Kinetic analysis showed a reduction in synaptic delay during facilitation only when broad action potentials were used. In unfacilitated release, synaptic delay increased as spike duration lengthened. We propose that small single Ca(2+) channel currents during the plateau phase of broad action potentials raise local Ca(2+) concentration only enough to fill a high-affinity site. Occupation of this site in itself, or events downstream, would convert a vesicle from control to facilitated state. If the conversion were a slow process, it could explain the changes in synaptic delay reported here. This hypothesis can also account for a number of observations related to Ca(2+) cooperativity and synaptic facilitation.

Calretinin modifies presynaptic calcium signaling in frog saccular hair cells

To determine whether the concentrations of calcium-binding proteins present in some neurons and sensory cells are sufficient to influence presynaptic calcium signaling, we studied the predominant calcium-binding protein in a class of sensory hair cells in the frog ear. Based on antibody affinity and molecular weight, we identified this protein as calretinin. We measured its cytoplasmic concentration to be approximately 1.2 mM, sufficient to bind approximately 6 mM Ca2+. Calcium signaling was altered when the diffusible cytoplasmic components were replaced by an intracellular solution lacking any fast calcium buffer, and was restored by the addition of 1.2 mM exogenous calretinin to the intracellular solution. We conclude that calretinin, when present at millimolar concentration, can serve as a diffusionally mobile calcium buffer/transporter capable of regulating calcium signaling over nanometer distances at presynaptic sites.

Tracking presynaptic Ca2+ dynamics during neurotransmitter release with Ca2+-activated K+ channels

Neurotransmitter release during action potentials is thought to require transient, localized [Ca2+]i as high as hundreds of micromolar near presynaptic release sites. Most experimental attempts to characterize the magnitude and time course of these Ca2+ domains involve optical methods that sample large volumes, require washout of endogenous buffers and often affect Ca2+ kinetics and transmitter release. Endogenous calcium-activated potassium (KCa) channels colocalize with presynaptic Ca2+ channels in Xenopus nerve-muscle cultures. We used these channels to quantify the rapid, dynamic changes in [Ca2+]i at active zones during synaptic activity. Confirming Ca2+-domain predictions, these KCa channels revealed [Ca2+]i over 100 microM during synaptic activity and much faster buildup and decay of Ca2+ domains than shown using other techniques.

Gating of Ca2+-activated K+ channels controls fast inhibitory synaptic transmission at auditory outer hair cells

Fast inhibitory synaptic transmission in the central nervous system is mediated by ionotropic GABA or glycine receptors. Auditory outer hair cells present a unique inhibitory synapse that uses a Ca2+-permeable excitatory acetylcholine receptor to activate a hyperpolarizing potassium current mediated by small conductance calcium-activated potassium (SK) channels. It is shown here that unitary inhibitory postsynaptic currents at this synapse are mediated by SK2 channels and occur rapidly, with rise and decay time constants of approximately 6 ms and approximately 30 ms, respectively. This time course is determined by the Ca2+ gating of SK channels rather than by the changes in intracellular Ca2+. The results demonstrate fast coupling between an excitatory ionotropic neurotransmitter receptor and an inhibitory ion channel and imply rapid, localized changes in subsynaptic calcium levels.

Two components of transducer adaptation in auditory hair cells

Mechanoelectrical transducer currents in turtle auditory hair cells adapted to maintained stimuli via a Ca(2+)-dependent mechanism characterized by two time constants of approximately 1 and 15 ms. The time course of adaptation slowed as the stimulus intensity was raised because of an increased prominence of the second component. The fast component of adaptation had a similar time constant for both positive and negative displacements and was unaffected by the myosin ATPase inhibitors, vanadate and butanedione monoxime. Adaptation was modeled by a scheme in which Ca(2+) ions, entering through open transducer channels, bind at two intracellular sites to trigger independent processes leading to channel closure. It was assumed that the second site activates a modulator with 10-fold slower kinetics than the first site. The model was implemented by computing Ca(2+) diffusion within a single stereocilium, incorporating intracellular calcium buffers and extrusion via a plasma membrane CaATPase. The theoretical results reproduced several features of the experimental responses, including sensitivity to the concentration of external Ca(2+) and intracellular calcium buffer and a dependence on the onset speed of the stimulus. The model also generated damped oscillatory transducer responses at a frequency dependent on the rate constant for the fast adaptive process. The properties of fast adaptation make it unlikely to be mediated by a myosin motor, and we suggest that it may result from Ca(2+) binding to the transducer channel or a nearby cytoskeletal element.

The endogenous calcium buffer and the time course of transducer adaptation in auditory hair cells

Mechanoelectrical transducer currents in turtle auditory hair cells adapt to maintained stimuli via a Ca2+-dependent mechanism that is sensitive to the level of internal calcium buffer. We have used the properties of transducer adaptation to compare the effects of exogenous calcium buffers in the patch electrode solution with those of the endogenous buffer assayed with perforated-patch recording. The endogenous buffer of the hair bundle was equivalent to 0.1-0.4 mM BAPTA and, in a majority of cells, supported adaptation in an external Ca2+ concentration of 70 microM similar to that in turtle endolymph. The endogenous buffer had a higher effective concentration, and the adaptation time constant was faster in cells at the high-frequency end than at the low-frequency end of the cochlea. Experiments using buffers with different Ca2+-binding rates or dissociation constants indicated that the speed of adaptation and the resting open probability of the transducer channels could be differentially regulated and imply that the endogenous buffer must be a fast, high-affinity buffer. In some hair cells, the transducer current did not decay exponentially during a sustained stimulus but displayed damped oscillations at a frequency (58-230 Hz) that depended on external Ca2+ concentration. The gradient in adaptation time constant and the tuned transducer current at physiological levels of calcium buffer and external Ca2+ suggest that transducer adaptation may contribute to hair cell frequency selectivity. The results are discussed in terms of feedback regulation of transducer channels mediated by Ca2+ binding at two intracellular sites.

The endogenous calcium buffer and the time course of transducer adaptation in auditory hair cells

Mechanoelectrical transducer currents in turtle auditory hair cells adapt to maintained stimuli via a Ca2+-dependent mechanism that is sensitive to the level of internal calcium buffer. We have used the properties of transducer adaptation to compare the effects of exogenous calcium buffers in the patch electrode solution with those of the endogenous buffer assayed with perforated-patch recording. The endogenous buffer of the hair bundle was equivalent to 0.1-0.4 mM BAPTA and, in a majority of cells, supported adaptation in an external Ca2+ concentration of 70 microM similar to that in turtle endolymph. The endogenous buffer had a higher effective concentration, and the adaptation time constant was faster in cells at the high-frequency end than at the low-frequency end of the cochlea. Experiments using buffers with different Ca2+-binding rates or dissociation constants indicated that the speed of adaptation and the resting open probability of the transducer channels could be differentially regulated and imply that the endogenous buffer must be a fast, high-affinity buffer. In some hair cells, the transducer current did not decay exponentially during a sustained stimulus but displayed damped oscillations at a frequency (58-230 Hz) that depended on external Ca2+ concentration. The gradient in adaptation time constant and the tuned transducer current at physiological levels of calcium buffer and external Ca2+ suggest that transducer adaptation may contribute to hair cell frequency selectivity. The results are discussed in terms of feedback regulation of transducer channels mediated by Ca2+ binding at two intracellular sites.

Plasma membrane Ca2+-ATPase extrudes Ca2+ from hair cell stereocilia

Mechanically sensitive hair cells of the auditory and vestibular systems use Ca2+ to control adaptation of mechanical transduction, to effect frequency tuning, to trigger neurotransmitter release, and to mediate efferent synaptic signaling. To determine the role that pumps play in regulation of Ca2+ in the hair bundle, the organelle responsible for mechanoelectrical transduction, we localized and quantified the plasma membrane Ca2+-ATPase (PMCA) of the bundle. We found that each hair bundle contains approximately 10(6) PMCA molecules or approximately 2000 per square micrometer of bundle membrane and that PMCA is the principal calmodulin binding protein of the bundle. Consistent with biochemical estimates of PMCA density, we measured with extracellular Ca2+-selective electrodes a substantial Ca2+ efflux from bundles. The number of bundle Ca2+ pumps and magnitude of resting Ca2+ efflux suggested that PMCA should generate a substantial membrane current as bundles expel Ca2+. Measurement of whole-cell currents revealed a transduction-dependent outward current that was consistent with the activity of PMCA. Finally, dialysis of hair cells with PMCA inhibitors led to a large increase in the concentration of Ca2+ in bundles, which suggests that PMCA plays a major role in regulating bundle Ca2+ concentration. Our data further indicate that PMCA could elevate the extracellular Ca2+ concentration close to hair bundles above the low level found in bulk endolymph.

Plasma membrane Ca2+-ATPase extrudes Ca2+ from hair cell stereocilia

Mechanically sensitive hair cells of the auditory and vestibular systems use Ca2+ to control adaptation of mechanical transduction, to effect frequency tuning, to trigger neurotransmitter release, and to mediate efferent synaptic signaling. To determine the role that pumps play in regulation of Ca2+ in the hair bundle, the organelle responsible for mechanoelectrical transduction, we localized and quantified the plasma membrane Ca2+-ATPase (PMCA) of the bundle. We found that each hair bundle contains approximately 10(6) PMCA molecules or approximately 2000 per square micrometer of bundle membrane and that PMCA is the principal calmodulin binding protein of the bundle. Consistent with biochemical estimates of PMCA density, we measured with extracellular Ca2+-selective electrodes a substantial Ca2+ efflux from bundles. The number of bundle Ca2+ pumps and magnitude of resting Ca2+ efflux suggested that PMCA should generate a substantial membrane current as bundles expel Ca2+. Measurement of whole-cell currents revealed a transduction-dependent outward current that was consistent with the activity of PMCA. Finally, dialysis of hair cells with PMCA inhibitors led to a large increase in the concentration of Ca2+ in bundles, which suggests that PMCA plays a major role in regulating bundle Ca2+ concentration. Our data further indicate that PMCA could elevate the extracellular Ca2+ concentration close to hair bundles above the low level found in bulk endolymph.

T-jump study of calcium binding kinetics of calcium chelators

Recent experimental studies have investigated the kinetic competition between calcium chelators and the secretion apparatus at a fast central synapse. Simultaneously, mathematical modelling studies indicate the importance of a quantitative knowledge of the binding kinetics of the chelators in studying fast physiological processes operating on a millisecond time scale. Using the temperature-jump relaxation method, I have studied the in vitro kinetics of Bis-Fura-2, Furaptra, Fluo-3, Calcium-Green-1, Calcium-Green-5N, Calcium-Orange-5N as well as EGTA, BAPTA and H-EDTA in conditions which are identical to those implemented in our patch clamp recordings, i.e. 100-140 mM CsCl, 20-40 mM Cs-HEPES, 8 mM NaCl, pH = 7.2 at 22 degrees C. The results can be summarized as follows: all fluorescent indicators have on rates in the range of 10(8)-10(9) M-1s-1. They differ significantly with respect to their off-rates from each other according to their affinities, ranging from 100 s-1 up to 26,000 s-1. BAPTA is kinetically almost indistinguishable from Fura-2. EGTA and H-EDTA have small binding rate constants for calcium in the range of 3 x 10(6) M-1s-1 since, at pH 7.20, protons need to be dissociated from the chelators before they can bind calcium ions.

Endogenous buffers limit the spread of free calcium in hair cells

Mobile Ca2+ buffers in hair cells have been postulated to play a dual role. On one hand, they carry incoming Ca2+ away from synaptic areas, allowing synapses to be rapidly reset. On the other hand, they limit the spread of free Ca2+ into the cell, preventing cross-talk between different pathways that employ Ca2+ as a second messenger. We have obtained evidence for such mobile Ca2+ buffers in hair cells by comparing the patterns of Ca2+-induced fluo-3 fluorescence under whole-cell and perforated-patch recording conditions. Fluorescent signals under perforated-patch conditions are relatively weak and are limited to the immediate vicinity of the membrane. These observations can be explained by a diffusion-reaction scheme that, in addition to Ca2+ and fluo-3, incorporates endogenous fixed and mobile Ca2+ buffers. Our experiments also suggest that the mobility of the endogenous buffer might be higher than previously thought. A high buffer mobility is expected to enhance the cell's ability to rapidly modulate transmitter release.

A general computational framework for modeling cellular structure and function

The "Virtual Cell" provides a general system for testing cell biological mechanisms and creates a framework for encapsulating the burgeoning knowledge base comprising the distribution and dynamics of intracellular biochemical processes. It approaches the problem by associating biochemical and electrophysiological data describing individual reactions with experimental microscopic image data describing their subcellular localizations. Individual processes are collected within a physical and computational infrastructure that accommodates any molecular mechanism expressible as rate equations or membrane fluxes. An illustration of the method is provided by a dynamic simulation of IP3-mediated Ca2+ release from endoplasmic reticulum in a neuronal cell. The results can be directly compared to experimental observations and provide insight into the role of experimentally inaccessible components of the overall mechanism.