Single-channel L-type Ca2+ currents in chicken embryo semicircular canal type I and type II hair cells

Recent advances in cochlear hair cell nanophysiology: subcellular compartmentalization of electrical signaling in compact sensory cells

In recent years, genetics, physiology, and structural biology have advanced into the molecular details of the sensory physiology of auditory hair cells. Inner hair cells (IHCs) and outer hair cells (OHCs) mediate two key functions: active amplification and non-linear compression of cochlear vibrations by OHCs and sound encoding by IHCs at their afferent synapses with the spiral ganglion neurons. OHCs and IHCs share some molecular physiology, e.g. mechanotransduction at the apical hair bundles, ribbon-type presynaptic active zones, and ionic conductances in the basolateral membrane. Unique features enabling their specific function include prestin-based electromotility of OHCs and indefatigable transmitter release at the highest known rates by ribbon-type IHC active zones. Despite their compact morphology, the molecular machineries that either generate electrical signals or are driven by these signals are essentially all segregated into local subcellular structures. This review provides a brief account on recent insights into the molecular physiology of cochlear hair cells with a specific focus on organization into membrane domains.

K+ Accumulation and Clearance in the Calyx Synaptic Cleft of Type I Mouse Vestibular Hair Cells

Vestibular organs of Amniotes contain two types of sensory cells, named Type I and Type II hair cells. While Type II hair cells are contacted by several small bouton nerve terminals, Type I hair cells receive a giant terminal, called a calyx, which encloses their basolateral membrane almost completely. Both hair cell types release glutamate, which depolarizes the afferent terminal by binding to AMPA post-synaptic receptors. However, there is evidence that non-vesicular signal transmission also occurs at the Type I hair cell-calyx synapse, possibly involving direct depolarization of the calyx by K⁺ exiting the hair cell. To better investigate this aspect, we performed whole-cell patch-clamp recordings from mouse Type I hair cells or their associated calyx. We found that [K⁺] in the calyceal synaptic cleft is elevated at rest relative to the interstitial (extracellular) solution and can increase or decrease during hair cell depolarization or repolarization, respectively. The change in [K⁺] was primarily driven by G_K,L, the low-voltage-activated, non-inactivating K⁺ conductance specifically expressed by Type I hair cells. Simple diffusion of K⁺ between the cleft and the extracellular compartment appeared substantially restricted by the calyx inner membrane, with the ion channels and active transporters playing a crucial role in regulating intercellular [K⁺]. Calyx recordings were consistent with K⁺ leaving the synaptic cleft through postsynaptic voltage-gated K⁺ channels involving K_V1 and K_V7 subunits. The above scenario is consistent with direct depolarization and hyperpolarization of the calyx membrane potential by intercellular K⁺.

A Gs-coupled purinergic receptor boosts Ca2+ influx and vascular contractility during diabetic hyperglycemia

Elevated glucose increases vascular reactivity by promoting L-type Ca_V1.2 channel (LTCC) activity by protein kinase A (PKA). Yet, how glucose activates PKA is unknown. We hypothesized that a G_s-coupled P2Y receptor is an upstream activator of PKA mediating LTCC potentiation during diabetic hyperglycemia. Experiments in apyrase-treated cells suggested involvement of a P2Y receptor underlying the glucose effects on LTTCs. Using human tissue, expression for P2Y₁₁, the only G_s-coupled P2Y receptor, was detected in nanometer proximity to Ca_V1.2 and PKA. FRET-based experiments revealed that the selective P2Y₁₁ agonist NF546 and elevated glucose stimulate cAMP production resulting in enhanced PKA-dependent LTCC activity. These changes were blocked by the selective P2Y₁₁ inhibitor NF340. Comparable results were observed in mouse tissue, suggesting that a P2Y₁₁-like receptor is mediating the glucose response in these cells. These findings established a key role for P2Y₁₁ in regulating PKA-dependent LTCC function and vascular reactivity during diabetic hyperglycemia.

An allosteric gating model recapitulates the biophysical properties of IK,L expressed in mouse vestibular type I hair cells

Key points

Vestibular type I and type II hair cells and their afferent fibres send information to the brain regarding the position and movement of the head.

The characteristic feature of type I hair cells is the expression of a low‐voltage‐activated outward rectifying K⁺ current, I_K,L, whose biophysical properties and molecular identity are still largely unknown.

In vitro, the afferent nerve calyx surrounding type I hair cells causes unstable intercellular K⁺ concentrations, altering the biophysical properties of I_K,L.

We found that in the absence of the calyx, I_K,L in type I hair cells exhibited unique biophysical activation properties, which were faithfully reproduced by an allosteric channel gating scheme.

These results form the basis for a molecular and pharmacological identification of I_K,L.

Abstract

Type I and type II hair cells are the sensory receptors of the mammalian vestibular epithelia. Type I hair cells are characterized by their basolateral membrane being enveloped in a single large afferent nerve terminal, named the calyx, and by the expression of a low‐voltage‐activated outward rectifying K⁺ current, I_K,L. The biophysical properties and molecular profile of I_K,L are still largely unknown. By using the patch‐clamp whole‐cell technique, we examined the voltage‐ and time‐dependent properties of I_K,L in type I hair cells of the mouse semicircular canal. We found that the biophysical properties of I_K,L were affected by an unstable K⁺ equilibrium potential (V_eqK⁺). Both the outward and inward K⁺ currents shifted V_eqK⁺ consistent with K⁺ accumulation or depletion, respectively, in the extracellular space, which we attributed to a residual calyx attached to the basolateral membrane of the hair cells. We therefore optimized the hair cell dissociation protocol in order to isolate mature type I hair cells without their calyx. In these cells, the uncontaminated I_K,L showed a half‐activation at –79.6 mV and a steep voltage dependence (2.8 mV). I_K,L also showed complex activation and deactivation kinetics, which we faithfully reproduced by an allosteric channel gating scheme where the channel is able to open from all (five) closed states. The ‘early’ open states substantially contribute to I_K,L activation at negative voltages. This study provides the first complete description of the ‘native’ biophysical properties of I_K,L in adult mouse vestibular type I hair cells.

Vestibular type I and type II hair cells and their afferent fibres send information to the brain regarding the position and movement of the head.

The characteristic feature of type I hair cells is the expression of a low‐voltage‐activated outward rectifying K⁺ current, I_K,L, whose biophysical properties and molecular identity are still largely unknown.

In vitro, the afferent nerve calyx surrounding type I hair cells causes unstable intercellular K⁺ concentrations, altering the biophysical properties of I_K,L.

We found that in the absence of the calyx, I_K,L in type I hair cells exhibited unique biophysical activation properties, which were faithfully reproduced by an allosteric channel gating scheme.

These results form the basis for a molecular and pharmacological identification of I_K,L.

Ser1928 phosphorylation by PKA stimulates the L-type Ca2+ channel CaV1.2 and vasoconstriction during acute hyperglycemia and diabetes

Hypercontractility of arterial myocytes and enhanced vascular tone during diabetes are, in part, attributed to the effects of increased glucose (hyperglycemia) on L-type Ca_V1.2 channels. In murine arterial myocytes, kinase-dependent mechanisms mediate the increase in Ca_V1.2 activity in response to increased extracellular glucose. We identified a subpopulation of the Ca_V1.2 channel pore-forming subunit (α1_C) within nanometer proximity of protein kinase A (PKA) at the sarcolemma of murine and human arterial myocytes. This arrangement depended upon scaffolding of PKA by an A-kinase anchoring protein 150 (AKAP150) in mice. Glucose-mediated increases in Ca_V1.2 channel activity were associated with PKA activity, leading to α1_C phosphorylation at Ser¹⁹²⁸ Compared to arteries from low-fat diet (LFD)-fed mice and nondiabetic patients, arteries from high-fat diet (HFD)-fed mice and from diabetic patients had increased Ser¹⁹²⁸ phosphorylation and Ca_V1.2 activity. Arterial myocytes and arteries from mice lacking AKAP150 or expressing mutant AKAP150 unable to bind PKA did not exhibit increased Ser¹⁹²⁸ phosphorylation and Ca_V1.2 current density in response to increased glucose or to HFD. Consistent with a functional role for Ser¹⁹²⁸ phosphorylation, arterial myocytes and arteries from knockin mice expressing a Ca_V1.2 with Ser¹⁹²⁸ mutated to alanine (S1928A) lacked glucose-mediated increases in Ca_V1.2 activity and vasoconstriction. Furthermore, the HFD-induced increases in Ca_V1.2 current density and myogenic tone were prevented in S1928A knockin mice. These findings reveal an essential role for α1_C phosphorylation at Ser¹⁹²⁸ in stimulating Ca_V1.2 channel activity and vasoconstriction by AKAP-targeted PKA upon exposure to increased glucose and in diabetes.

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.

Fine Tuning of CaV1.3 Ca2+ channel properties in adult inner hair cells positioned in the most sensitive region of the Gerbil Cochlea

Hearing relies on faithful signal transmission by cochlear inner hair cells (IHCs) onto auditory fibres over a wide frequency and intensity range. Exocytosis at IHC ribbon synapses is triggered by Ca²⁺ inflow through Ca_V1.3 (L-type) Ca²⁺ channels. We investigated the macroscopic (whole-cell) and elementary (cell-attached) properties of Ca²⁺ currents in IHCs positioned at the middle turn (frequency ∼2 kHz) of the adult gerbil cochlea, which is their most sensitive hearing region. Using near physiological recordings conditions (body temperature and a Na⁺ based extracellular solution), we found that the macroscopic Ca²⁺ current activates and deactivates very rapidly (time constant below 1 ms) and inactivates slowly and only partially. Single-channel recordings showed an elementary conductance of 15 pS, a sub-ms latency to first opening, and a very low steady-state open probability (P_o: 0.024 in response to 500-ms depolarizing steps at ∼−18 mV). The value of P_o was significantly larger (0.06) in the first 40 ms of membrane depolarization, which corresponds to the time when most Ca²⁺ channel openings occurred clustered in bursts (mean burst duration: 19 ms). Both the P_o and the mean burst duration were smaller than those previously reported in high-frequency basal IHCs. Finally, we found that middle turn IHCs are likely to express about 4 times more Ca²⁺ channels per ribbon than basal cells. We propose that middle-turn IHCs finely-tune Ca_V1.3 Ca²⁺ channel gating in order to provide reliable information upon timing and intensity of lower-frequency sounds.

Release from the cone ribbon synapse under bright light conditions can be controlled by the opening of only a few Ca(2+) channels

Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation. Ca(2+) channel number and single-channel current amplitude were calculated by mean-variance analysis of I(Ca). Two different comparisons-one comparing average numbers of release events to average I(Ca) amplitude and the other involving deconvolution of both EPSCs and simultaneously recorded cone I(Ca)-suggested that fewer than three Ca(2+) channel openings accompanied fusion of each vesicle at the peak of release during the first few milliseconds of stimulation. Opening fewer Ca(2+) channels did not enhance fusion efficiency, suggesting that few unnecessary channel openings occurred during strong depolarization. We simulated release at the cone synapse, using empirically determined synaptic dimensions, vesicle pool size, Ca(2+) dependence of release, Ca(2+) channel number, and Ca(2+) channel properties. The model replicated observations when a barrier was added to slow Ca(2+) diffusion. Consistent with the presence of a diffusion barrier, dialyzing cones with diffusible Ca(2+) buffers did not affect release efficiency. The tight clustering of Ca(2+) channels, along with a high-Ca(2+) affinity release mechanism and diffusion barrier, promotes a linear coupling between Ca(2+) influx and vesicle fusion. This may improve detection of small light decrements when cones are hyperpolarized by bright light.

Responses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, D-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (V(z)) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their V(z). nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their V(z). The decrease of inward and outward currents and the depolarization of the V(z) in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and V(z) by different AChR activation are discussed.

Elementary properties of CaV1.3 Ca(2+) channels expressed in mouse cochlear inner hair cells

Mammalian cochlear inner hair cells (IHCs) are specialized to process developmental signals during immature stages and sound stimuli in adult animals. These signals are conveyed onto auditory afferent nerve fibres. Neurotransmitter release at IHC ribbon synapses is controlled by L-type Ca_V1.3 Ca²⁺ channels, the biophysics of which are still unknown in native mammalian cells. We have investigated the localization and elementary properties of Ca²⁺ channels in immature mouse IHCs under near-physiological recording conditions. Ca_V1.3 Ca²⁺ channels at the cell pre-synaptic site co-localize with about half of the total number of ribbons present in immature IHCs. These channels activated at about −70 mV, showed a relatively short first latency and weak inactivation, which would allow IHCs to generate and accurately encode spontaneous Ca²⁺ action potential activity characteristic of these immature cells. The Ca_V1.3 Ca²⁺ channels showed a very low open probability (about 0.15 at −20 mV: near the peak of an action potential). Comparison of elementary and macroscopic Ca²⁺ currents indicated that very few Ca²⁺ channels are associated with each docked vesicle at IHC ribbon synapses. Finally, we found that the open probability of Ca²⁺ channels, but not their opening time, was voltage dependent. This finding provides a possible correlation between presynaptic Ca²⁺ channel properties and the characteristic frequency/amplitude of EPSCs in auditory afferent fibres.

Elementary properties of Kir2.1, a strong inwardly rectifying K(+) channel expressed by pigeon vestibular type II hair cells

By using the patch-clamp technique in the cell-attached configuration, we have investigated the single-channel properties of an inward rectifier potassium channel (Kir) expressed by pigeon vestibular type II hair cells in situ. In high-K(+) external solution with 2 mM Mg(2+), Kir inward current showed openings to at least four amplitude levels. The two most frequent open states (L2 and L3) had a mean slope conductance of 13 and 28 pS, respectively. L1 (7 pS) and L4 (36 pS) together accounted for less than 6% of the conductive state. Closed time distributions were fitted well using four exponential functions, of which the slowest time constant (tau(C4)) was clearly voltage-dependent. Open time distributions were fitted well with two or three exponential functions depending on voltage. The mean open probability (P(O)) decreased with hyperpolarization (0.13 at -50 mV and 0.03 at -120 mV). During pulse-voltage protocols, the Kir current-decay process (inactivation) accelerated and increased in extent with hyperpolarization. This phenomenon was associated with a progressive increase of the relative importance of tau(C4). Kir inactivation almost disappeared when Mg(2+) was omitted from the pipette solution. At the same time, P(O) increased at all membrane voltages and the relative importance of L4 increased to a mean value of 47%. The relative importance of tau(C4) decreased for all open states, while L4 only showed a significantly longer open time constant. The present work provides the first detailed quantitative description of the elementary properties of the Kir inward rectifier in pigeon vestibular type II hair cells and specifically describes the Kir gating properties and the molecule's sensitivity to extracellular Mg(2+) for all subconductance levels. The present results are consistent with the Kir2.1 protein sustaining a strong inwardly rectifying K(+) current in native hair cells, characterized by rapid activation time course and slow partial inactivation. The longest closed state (tau(C4)) appears as the main parameter involved in time- and Mg(2+)-dependent decay. Finally, in contrast to Kir2.1 results described so far for mammalian cells, external Mg(2+) had no effect on channel conductance.