Slow motility in hair cells of the frog amphibian papilla: Ca2+-dependent shape changes

The Cation Channel TMEM63B Is an Osmosensor Required for Hearing

Hypotonic stress causes the activation of swelling-activated nonselective cation channels (NSCCs), which leads to Ca²⁺-dependent regulatory volume decrease (RVD) and adaptive maintenance of the cell volume; however, the molecular identities of the osmosensitive NSCCs remain unclear. Here, we identified TMEM63B as an osmosensitive NSCC activated by hypotonic stress. TMEM63B is enriched in the inner ear sensory hair cells. Genetic deletion of TMEM63B results in necroptosis of outer hair cells (OHCs) and progressive hearing loss. Mechanistically, the TMEM63B channel mediates hypo-osmolarity-induced Ca²⁺ influx, which activates Ca²⁺-dependent K⁺ channels required for the maintenance of OHC morphology. These findings demonstrate that TMEM63B is an osmosensor of the mammalian inner ear and the long-sought cation channel mediating Ca²⁺-dependent RVD.

Oncomodulin, an EF-Hand Ca2+ Buffer, Is Critical for Maintaining Cochlear Function in Mice

Oncomodulin (Ocm), a member of the parvalbumin family of calcium binding proteins, is expressed predominantly by cochlear outer hair cells in subcellular regions associated with either mechanoelectric transduction or electromotility. Targeted deletion of Ocm caused progressive cochlear dysfunction. Although sound-evoked responses are normal at 1 month, by 4 months, mutants show only minimal distortion product otoacoustic emissions and 70-80 dB threshold shifts in auditory brainstem responses. Thus, Ocm is not critical for cochlear development but does play an essential role for cochlear function in the adult mouse.

SIGNIFICANCE STATEMENT

Numerous proteins act as buffers, sensors, or pumps to control calcium levels in cochlear hair cells. In the inner ear, EF-hand calcium buffers may play a significant role in hair cell function but have been very difficult to study. Unlike other reports of genetic disruption of EF-hand calcium buffers, deletion of oncomodulin (Ocm), which is predominately found in outer hair cells, leads to a progressive hearing loss after 1 month, suggesting that Ocm critically protects hearing in the mature ear.

Evidence for water-permeable channels in auditory hair cells in the leopard frog

Auditory hair cells in the amphibian papilla (APHCs) of the leopard frog, Rana pipiens pipiens, have a significantly higher permeability to water than that observed in mammalian hair cells. The insensitivity of water permeability in frog hair cells to extracellular mercury suggests that an amphibian homologue of the water channel aquaporin-4 (AQP4) may mediate water transport in these cells. Using immunocytochemistry, we show that an AQP4-like protein is found in APHCs. Rabbit anti-AQP4 antibody was used in multiple-immunohistochemical staining experiments along with AP hair cell and hair bundle markers in leopard frog and mouse tissue. AQP4 immunoreactivity was found in the basal and apical poles of the APHCs and shows uniform immunoreactivity. This study provides the first identification and localization of an AQP4-like protein in the amphibian inner ear. We also report a more direct measure of hyperosmotically-induced volume changes in APHCs that confirms previous findings. The presence of water channels in anuran APHCs constitutes a novel physiological difference between amphibian and mammalian hair cell structure and function.

Exocytosis in the frog amphibian papilla

Using whole-cell patch-clamp recordings, we measured changes in membrane capacitance (ΔC (m)) in two subsets of hair cells from the leopard frog amphibian papilla (AP): the low-frequency (100-500 Hz), rostral hair cells and the high-frequency (500-1200 Hz), caudal hair cells, in order to investigate tonotopic differences in exocytosis. Depolarizations of both rostral and caudal hair cells evoked robust ΔC (m) responses of similar amplitude. However, the calcium dependence of release, i.e., the relationship between ΔC (m) relative to the amount of calcium influx (Q (Ca) (2+)), was found to be linear in rostral hair cells but supra-linear in caudal hair cells. In addition, the higher numbers of vesicles released at caudal hair cell active zones suggests increased temporal precision of caudal hair cell exocytosis. ΔC (m) responses were also obtained in response to sinusoidal stimuli of varying frequency, but neither rostral nor caudal hair cell ΔC (m) revealed any frequency selectivity. While all AP hair cells express both otoferlin and synaptotagmin IV (SytIV), we obtained evidence of a tonotopic distribution of the calcium buffer calretinin which may further increase temporal resolution at the level of the hair cell synapse. Our findings suggest that the low (rostral) and high (caudal) frequency hair cells apply different mechanisms for fine-tuning exocytosis.

Osmotic properties of auditory hair cells in the leopard frog: evidence for water-permeable channels

When amphibian papillar hair cells (APHCs) of the leopard frog, Rana pipiens pipiens, are osmotically challenged, they exhibit a characteristically asymmetric (rectifying) response: small decreases (5%, or less) in the extracellular solution's osmolarity do not significantly affect the cells' volume; larger decreases produce a relatively slow volume increase in APHCs, while exposure to a hyperosmotic medium leads to rapid shrinking of these cells. Furthermore, the rate of volume change appears to be a function of the rate of extracellular osmotic change. These characteristics make the application of methods devised for the estimation of the osmotic permeability coefficient (P(f)) for semipermeable membranes - i.e., those with significant permeability only to water - to APHC membrane rather futile. We have, therefore, devised a method that takes both the permeability to solutes as well as the kinetics of the osmolarity change into consideration, in order to obtain estimates of P(f) that are to a large degree independent of these factors. We have compared the new and earlier methods. Using the new method, we have estimated the P(f) of APHCs' plasma membrane to be in the 10(-2)-cm/s range, and thus significantly larger than those reported for lipid bilayers. APHC's membrane P(f) appears to be cell-size independent and insensitive to extracellular mercury. These results suggest that APHCs express water-permeable channels in their plasma membrane. Furthermore, we suggest that asymmetric and rate dependent shape changes produced by osmolarity changes in APHCs imply the presence of significant permeability to solutes. The significance of transmembrane solute transport and water channel expression in amphibian auditory hair cells is discussed.

Slow motility in hair cells of the frog amphibian papilla: myosin light chain-mediated shape change

Using video, fluorescence and confocal microscopy, quantitative analysis and modeling, we investigated intracellular processes mediating the calcium/calmodulin (Ca(2+)/CaM)-dependent slow motility in hair cells dissociated from the rostral region of amphibian papilla, one of the two auditory organs in frogs. The time course of shape changes in these hair cells during the period of pretreatment with several specific inhibitors, as well as their response to the calcium ionophore, ionomycin, were recorded and compared. These cells respond to ionomycin with a tri-phasic shape change: an initial phase of iso-volumetric length decrease; a period of concurrent shortening and swelling; and the final phase of increase in both length and volume. We found that both the myosin light chain kinase inhibitor, ML-7, and antagonists of the multifunctional Ca(2+)/CaM-dependent kinases, KN-62 and KN-93, inhibit the iso-volumetric shortening phase of the response to ionomycin. The type 1 protein phosphatase inhibitors, calyculin A and okadaic acid induce minor shortening on their own, but do not significantly alter phase 1 response. However, they appear to counter effects of the inhibitors of Ca(2+)/CaM-dependent kinases. We hypothesize that an active actomyosin-based process mediates the iso-volumetric shortening in the frog rostral amphibian papillar hair cells.