IP3 receptor in the hair cells of frog semicircular canal and its possible functional role

Developmental GAD2 Expression Reveals Progenitor-like Cells with Calcium Waves in Mammalian Crista Ampullaris

Sense of motion, spatial orientation, and balance in vertebrates relies on sensory hair cells in the inner ear vestibular system. Vestibular supporting cells can regenerate hair cells that are lost from aging, ototoxicity, and trauma, although not all factors or specific cell types are known. Here we report a population of GAD2-positive cells in the mouse crista ampullaris and trace GAD2 progenitor-like cells that express pluripotent transcription factors SOX2, PROX1, and CTBP2. GAD2 progenitor-like cells organize into rosettes around a central branched structure in the eminentia cruciatum (EC) herein named the EC plexus. GCaMP5G calcium indicator shows spontaneous and acetylcholine-evoked whole-cell calcium waves in neonatal and adult mice. We present a hypothetical model that outlines the lineage and potential regenerative capacity of GAD2 cells in the mammalian vestibular neuroepithelium.

Pre- and Postsynaptic Effects of Glutamate in the Frog Labyrinth

The role of glutamate in quantal release at the cytoneural junction was examined by measuring mEPSPs and afferent spikes at the posterior canal in the intact frog labyrinth. Release was enhanced by exogenous glutamate, or dl-TBOA, a blocker of glutamate reuptake. Conversely, drugs acting on ionotropic glutamate receptors did not affect release; the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) blocker CNQX decreased mEPSP size in a dose-dependent manner; the NMDA-R blocker d-AP5 at concentrations <200 µM did not affect mEPSP size, either in the presence or absence of Mg and glycine. In isolated hair cells, glutamate did not modify Ca currents. Instead, it systematically reduced the compound delayed potassium current, IKD, whereas the metabotropic glutamate receptor (mGluR)-II inverse agonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY341495), increased it. Given mGluR-II decrease cAMP production, these finding are consistent with the reported sensitivity of IKD to protein kinase A (PKA)-mediated phosphorylation. LY341495 also enhanced transmitter release, presumably through phosphorylation-mediated facilitation of the release machinery. The observed enhancement of release by glutamate confirms previous literature data, and can be attributed to activation of mGluR-I that promotes Ca release from intracellular stores. Glutamate-induced reduction in the repolarizing IKD may contribute to facilitation of release. Overall, glutamate exerts both a positive feedback action on mGluR-I, through activation of the phospholipase C (PLC)/IP₃ path, and the negative feedback, by interfering with substrate phosphorylation through G_i/0-coupled mGluRs-II/III. The positive feedback prevails, which may explain the increase in overall rates of release observed during mechanical stimulation (symmetrical in the excitatory and inhibitory directions). The negative feedback may protect the junction from over-activation.

Forskolin and protein kinase inhibitors differentially affect hair cell potassium currents and transmitter release at the cytoneural junction in the isolated frog labyrinth

The post-transductional elaboration of sensory input at the frog semicircular canal has been studied by correlating the effects of drugs that interfere with phosphorylation processes on: (i) potassium conductances in isolated hair cell and (ii) transmitter release at the cytoneural junction in the intact labyrinth. At hair cells, delayed potassium currents (IKD) undergo voltage- and time-dependent inactivation; inactivation removal requires ATP, is sensitive to kinase blockade, but is unaffected by exogenous application of cyclic nucleotides. We report here that forskolin, an activator of endogenous adenylyl cyclase, enhances IKD inactivation removal in isolated hair cells, but produces an overall decrease in IKD amplitude consistent with the direct blocking action of the drug on several families of K channels. In the intact labyrinth, forskolin enhances transmitter release, consistent with such depression of K conductances. Kinase blockers - H-89 and KT5823 - have been shown to reduce IKD inactivation removal and IKD amplitude at isolated hair cells. In the labyrinth, the effects of these drugs on junctional activity are quite variable, with predominant inhibition of transmitter release, rather than the enhancement expected from the impairment of K currents. The overall action of forskolin and kinase inhibitors on K conductances is similar (depression), but they have opposite effects on transmitter release: this indicates that some intermediate steps between the bioelectric control of hair cell membrane potential and transmitter release are affected in opposite ways and therefore are presumably regulated by protein phosphorylation.

Calcium-induced calcium release supports recruitment of synaptic vesicles in auditory hair cells

Hair cells from auditory and vestibular systems transmit continuous sound and balance information to the central nervous system through the release of synaptic vesicles at ribbon synapses. The high activity experienced by hair cells requires a unique mechanism to sustain recruitment and replenishment of synaptic vesicles for continuous release. Using pre- and postsynaptic electrophysiological recordings, we explored the potential contribution of calcium-induced calcium release (CICR) in modulating the recruitment of vesicles to auditory hair cell ribbon synapses. Pharmacological manipulation of CICR with agents targeting endoplasmic reticulum calcium stores reduced both spontaneous postsynaptic multiunit activity and the frequency of excitatory postsynaptic currents (EPSCs). Pharmacological treatments had no effect on hair cell resting potential or activation curves for calcium and potassium channels. However, these drugs exerted a reduction in vesicle release measured by dual-sine capacitance methods. In addition, calcium substitution by barium reduced release efficacy by delaying release onset and diminishing vesicle recruitment. Together these results demonstrate a role for calcium stores in hair cell ribbon synaptic transmission and suggest a novel contribution of CICR in hair cell vesicle recruitment. We hypothesize that calcium entry via calcium channels is tightly regulated to control timing of vesicle fusion at the synapse, whereas CICR is used to maintain a tonic calcium signal to modulate vesicle trafficking.

Sensory transduction at the frog semicircular canal: how hair cell membrane potential controls junctional transmission

At the frog semicircular canals, the afferent fibers display high spontaneous activity (mEPSPs), due to transmitter release from hair cells. mEPSP and spike frequencies are modulated by stimulation that activates the hair cell receptor conductance. The relation between receptor current and transmitter release cannot be studied at the intact semicircular canal. To circumvent the problem, we combined patch-clamp recordings at the isolated hair cell and electrophysiological recordings at the cytoneural junction in the intact preparation. At isolated hair cells, the K channel blocker tetraethylammonium (TEA) is shown to block a fraction of total voltage-dependent K-conductance (IKD) that depends on TEA concentration but not on membrane potential (V_m). Considering the bioelectric properties of the hair cell, as previously characterized by this lab, a fixed fractional block of IKD is shown to induce a relatively fixed shift in V_m, provided it lies in the range −30 to −10 mV. The same concentrations of TEA were applied to the intact labyrinth while recording from single afferent fibers of the posterior canal, at rest and during mechanical stimulation. At the peak of stimulation, TEA produced increases in mEPSP rate that were linearly related to the shifts produced by the same TEA concentrations (0.1–3 mM) in hair cell V_m (0.7–5 mV), with a slope of 29.8 Hz/mV. The membrane potential of the hair cell is not linearly related to receptor conductance, so that the slope of quantal release vs. receptor conductance depends on the prevailing V_m (19.8 Hz/nS at −20 mV; 11 Hz/nS at −10 mV). Changes in mEPSP peak size were negligible at rest as well as during stimulation. Since ample spatial summation of mEPSPs occurs at the afferent terminal and threshold-governed spike firing is intrinsically nonlinear, the observed increases in mEPSP frequency, though not very large, may suffice to trigger afferent spike discharge.

Endothelin inhibits renin release from juxtaglomerular cells via endothelin receptors A and B via a transient receptor potential canonical-mediated pathway

Renin is the rate-limiting step in the production of angiotensin II: a critical element in the regulation of blood pressure and in the pathogenesis of hypertension. Renin release from the juxtaglomerular (JG) cell is stimulated by the second messenger cAMP and inhibited by increases in calcium (Ca). Endothelins (ETs) inhibit renin release in a Ca-dependent manner. JG cells contain multiple isoforms of canonical transient receptor potential (TRPC) Ca-permeable channels. The proposed hypothesis is that endothelin inhibits renin release by activating TRPC store-operated Ca channels. RT-PCR and immunofluorescence revealed expression of both ETA and ETB receptors in mouse JG cells. Incubation of primary cultures of JG cells with ET-1 (10 nmol/L) decreased renin release by 28%. Addition of either an ETA or an ETB receptor blocker completely prevented the ET inhibition of renin release. Incubation with the TRPC blocker (SKF 96365, 50 μmol/L) completely reversed the Ca-mediated inhibition of renin release by ETs. These results suggest that endothelin inhibits renin release from JG cells via both ETA and ETB receptors, which leads to the activation of TRPC store-operated Ca channels.

Role of intracellular calcium stores in hair-cell ribbon synapse

Intracellular calcium stores control many neuronal functions such as excitability, gene expression, synaptic plasticity, and synaptic release. Although the existence of calcium stores along with calcium-induced calcium release (CICR) has been demonstrated in conventional and ribbon synapses, functional significance and the cellular mechanisms underlying this role remains unclear. This review summarizes recent experimental evidence identifying contribution of CICR to synaptic transmission and synaptic plasticity in the CNS, retina and inner ear. In addition, the potential role of CICR in the recruitment of vesicles to releasable pools in hair-cell ribbon synapses will be specifically discussed.

Juxtaglomerular cell CaSR stimulation decreases renin release via activation of the PLC/IP(3) pathway and the ryanodine receptor

The calcium-sensing receptor (CaSR) is a G-coupled protein expressed in renal juxtaglomerular (JG) cells. Its activation stimulates calcium-mediated decreases in cAMP content and inhibits renin release. The postreceptor pathway for the CaSR in JG cells is unknown. In parathyroids, CaSR acts through G(q) and/or G(i). Activation of G(q) stimulates phospholipase C (PLC), and inositol 1,4,5-trisphosphate (IP(3)), releasing calcium from intracellular stores. G(i) stimulation inhibits cAMP formation. In afferent arterioles, the ryanodine receptor (RyR) enhances release of stored calcium. We hypothesized JG cell CaSR activation inhibits renin via the PLC/IP(3) and also RyR activation, increasing intracellular calcium, suppressing cAMP formation, and inhibiting renin release. Renin release from primary cultures of isolated mouse JG cells (n = 10) was measured. The CaSR agonist cinacalcet decreased renin release 56 ± 7% of control (P < 0.001), while the PLC inhibitor U73122 reversed cinacalcet inhibition of renin (104 ± 11% of control). The IP(3) inhibitor 2-APB also reversed inhibition of renin from 56 ± 6 to 104 ± 11% of control (P < 0.001). JG cells were positively labeled for RyR, and blocking RyR reversed CaSR-mediated inhibition of renin from 61 ± 8 to 118 ± 22% of control (P < 0.01). Combining inhibition of IP(3) and RyR was not additive. G(i) inhibition with pertussis toxin plus cinacalcet did not reverse renin inhibition (65 ± 12 to 41 ± 8% of control, P < 0.001). We conclude stimulating JG cell CaSR activates G(q), initiating the PLC/IP(3) pathway, activating RyR, increasing intracellular calcium, and resulting in calcium-mediated renin inhibition.

Expression and localization of ryanodine receptors in the frog semicircular canal

Several experiments suggest an important role for store-released Ca²⁺ in hair cell organs: drugs targeting IP₃ and ryanodine (RyRs) receptors affect release from hair cells, and stores are thought to be involved in vesicle recycling at ribbon synapses. In this work we investigated the semicircular canal distribution of RyRs by immunofluorescence, using slice preparations of the sensory epithelium (to distinguish cell types) and flat mounts of the simpler nonsensory regions. RyRs were present in hair cells, mostly in supranuclear spots, but not in supporting cells; as regards nonsensory regions, they were also localized in dark cells and cells from the ductus. No labeling was found in nerve terminals, although nerve branches could be observed in proximity to hair cell RyR spots. The differential expression of RyR isoforms was studied by RT-PCR and immunoblotting, showing the presence of RyRα in both ampulla and canal arm and RyRβ in the ampulla only.

Exposure to reduced gravity impairs junctional transmission at the semicircular canal in the frog labyrinth

The effects of microgravity on frog semicircular canals have been studied by electrophysiological and morphological approaches. Reduced gravity (microG) was simulated by a random positioning machine (RPM), which continually and randomly modified the orientation in space of the anesthetized animal. As this procedure stimulates the semicircular canals, the effect of altered gravity was isolated by comparing microG-treatment with an identical rotary stimulation in the presence of normal gravity (normoG). Electrophysiological experiments were performed in the isolated labyrinth, extracted from the animals after the treatment, and mounted on a turntable. Junctional activity was measured by recording quantal events (mEPSPs) and spikes from the afferent fibers close to the junction, at rest and during rotational stimulation. MicroG-treated animals displayed a marked decrease in the frequency of resting and evoked mEPSP discharge, vs. both control and normoG (mean decrease ∼50%). Spike discharge was also depressed: 57% of microG-treated frogs displayed no spikes at rest and during rotation at 0.1 Hz, vs. 23–31% of control or normoG frogs. Among the firing units, during one cycle of sinusoidal rotation at 0.1 Hz microG-treated units emitted an average of 41.8 ± 8.06 spikes, vs. 77.2 ± 8.19 in controls. Patch-clamp analysis on dissociated hair cells revealed altered Ca2+ handling, after microG, consistent with and supportive of the specificity of microG effects. Marked morphological signs of cellular suffering were observed after microG, mainly in the central part of the sensory epithelium. Functional changes due to microgravity were reversible within a few days.

Expression of canonical transient receptor potential channel (TRPC) 1-7 in the mouse inner ear

CONCLUSION

It is suggested that TRPCs play a functional role in the sensory cell transduction system in the inner ear.

OBJECTIVE

To study expression of TRPC 1-7 in the mouse inner ear.

MATERIALS AND METHODS

The localization of TRPC 1-7 in the inner ear of CBA/J mice was investigated by immunohistochemistry.

RESULTS

TRPC immunoreactivity was observed generally in the inner ear, e.g. in the lateral wall of the cochlea, organ of Corti, spiral ganglion, vestibular end organs and vestibular ganglion. The immunofluorescent reaction to TRPC 3, 4, 5, and 7 in the stria vascularis was more intense than in the spiral prominence or spiral ligament. In the organ of Corti, TRPC immunoreactivity was observed in the outer hair cells (OHCs), inner hair cells (IHCs) and some supporting cells. TRPC 1-7 were all present in the ganglion cell body, TRPC 1 and 3 showing intense fluorescence, TRPC 2 and 7 moderate fluorescence and TRPC 4, 5 and 6 weak staining in ganglion fibres. In the vestibular end organs, vestibular hair cells (VHCs) showed immunoreactivity to all TRPCs. Nerve fibres in the subepithelial tissue were stained by TRPC 1, 3, 5, 6 and 7. Immunofluorescence to TRPC 1, 3, 4, 5, 6 and 7 was observed in the dark cells. In the vestibular ganglion, TRPC 1-7 were all present in the ganglion cell body. TRPC 1-4 and 7 elicited immunofluorescence in ganglion fibres.

Effects of Phenylalanine and its Metabolites on Cytoplasmic Free Calcium in Cortical Neurons

Classic phenylketonuria (PKU) is characterized by brain lesions. However, its underlying neurotoxic mechanisms remain unknown. Based on our previous studies, we hypothesized that calcium might participate in PKU-associated neuropathy. In cultured cortical neurons, cytoplasmic free calcium concentration ([Ca(2+)](i)) decreased dramatically when treatment with phenylalanine (Phe) and phenyllactic acid, while phenylacetic acid treatment immediately increased [Ca(2+)](i), which began to decrease after 3 min. Moreover, [Ca(2+)](i) decreased dramatically after Phe treatment in the presence of EGTA suggesting that Phe might increase [Ca(2+)](i) efflux. Phe-induced [Ca(2+)](i) decrease was strongly inhibited by vanadate, a non-specific plasma membrane Ca(2+)-ATPase (PMCA) antagonist, suggesting that Phe might increase [Ca(2+)](i) efflux throught modulating PMCA. These findings were further supported by the facts that Phe could increase membrance (45)Ca-uptake capability and PMCA activity. In contrast, treatment of KBR7943 or thapsigargin, antagonists to Na/Ca Exchanger (NCX) and Sarco/Endoplasmic reticulum Ca(2+)-ATPase (SERCA), respectively, did not elicit any changes in [Ca(2+)](i). Specific siRNA against PMCA had an effect similar to vanadate. Since the brain injury induced by phenylalaninemia was thought to be a chronic process, we cultured cortical neurons in the presence of Phe for 2 weeks and measured [Ca(2+)](i), PMCA activity and (45)Ca-uptake capability at days 3, 7, 9 and 14, respectively. PMCA activity and (45)Ca-uptake capability decreased from day 9, at the same time [Ca(2+)](i) increase was observed. In conclusion, PMCA participate in regulating Phe-induced initial rapid decrease in [Ca(2+)](i) and subsequent long-term increase in [Ca(2+)](i).

Plasma membrane Ca2+-ATPase isoforms in frog crista ampullaris: identification of PMCA1 and PMCA2 specific splice variants

Ca2+ ions play a pivotal role in inner ear hair cells as they are involved from the mechano-electrical transduction to the transmitter release. Most of the Ca2+ that enters into hair cells via mechano-transduction and voltage-gated channels is extruded by the plasma membrane Ca2+-ATPases (PMCAs) that operate in both apical and basal cellular compartments. Here, we determined the identity and distribution of PMCA isoforms in frog crista ampullaris: we showed that PMCA1, PMCA2 and PMCA3 are expressed, while PMCA4 appears to be negligible. We also identify PMCA1bx, PMCA2av and PMCA2bv as the major splice variants produced from PMCA1 and PMCA2 genes. PMCA2av appears to be the major Ca2+-pump operating at the apical pole of the cell, even if PMCA1b is also expressed in the stereocilia. PMCA1bx is, instead, the principal PMCA of hair cell basolateral compartment, where it is expressed together with PMCA2 (probably PMCA2bv) and PMCA3. Frog crista ampullaris hair cells lack a Na/Ca exchanger, therefore PMCAs are the only mechanism of Ca2+ extrusion. The coexpression of specific isozymes in the different cellular compartments responds to the need of a fine regulation of both basal and dynamic Ca2+ levels at the apical and basal pole of the cell.