Control of Hearing Sensitivity by Tectorial Membrane Calcium
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Control of hearing sensitivity by tectorial membrane calcium Clark Elliott Strimbua,1,2, Sonal Prasada,1, Pierre Hakizimanaa, and Anders Fridbergera,3 aDepartment of Clinical and Experimental Medicine, Division of Neurobiology, Linköping University, SE-581 83 Linköping, Sweden Edited by Christine Petit, Institut Pasteur, College de France, INSERM, Université Pierre-et-Marie-Curie, Paris 15, France, and approved February 7, 2019 (received for review March 26, 2018) When sound stimulates the stereocilia on the sensory cells in the tation processes provide the hair cell with a high-pass filter and hearing organ, Ca2+ ions flow through mechanically gated ion the dynamic range of the transduction apparatus is extended. + channels. This Ca2 influx is thought to be important for ensuring Additionally, in both processes, as the MET channels close, they that the mechanically gated channels operate within their most exert a pulling force on the tip links that leads to a measurable sensitive response region, setting the fraction of channels open change in the position of the entire bundle of stereocilia (15, 16). at rest, and possibly for the continued maintenance of stereocilia. This mechanical correlate of adaptation results in active hair + Since the extracellular Ca2 concentration will affect the amount bundle motility that may contribute to the amplification of faint + of Ca2 entering during stimulation, it is important to determine sounds, a function that is critical for hearing (17). Much of the the level of the ion close to the sensory cells. Using fluorescence understanding of the two adaptation processes came from experi- imaging and fluorescence correlation spectroscopy, we measured ments performed on low-frequency hair cells from nonmammalian 2+ the Ca concentration near guinea pig stereocilia in situ. Surpris- vertebrates. Despite the success of the model in explaining the ingly, we found that an acellular accessory structure close to the observed MET currents in these low-frequency cells, recent work 2+ + stereocilia, the tectorial membrane, had much higher Ca than has questioned the extent to which this model of Ca2 -dependent 2+ the surrounding fluid. Loud sounds depleted Ca from the tecto- adaptation applies to mammalian hearing organs. Indeed, one re- 2+ rial membrane, and Ca manipulations had large effects on hair cent report suggests that cochlear hair cell adaptation is, in fact, + cell function. Hence, the tectorial membrane contributes to control independent of Ca2 entry (18), while others reached the opposite of hearing sensitivity by influencing the ionic environment around conclusion (19). the stereocilia. Due to its obvious physiological importance, significant effort + was expended in measuring the level of Ca2 in the fluid that hearing | temporary threshold shift | mechanoelectrical transduction | surrounds stereocilia, the endolymph. These measurements, tectorial membrane | calcium which were performed with ion-selective electrodes or by ana- lyzing aspirated endolymph samples, showed values ranging from he sense of hearing depends on mechanically sensitive ion 15 to 40 μM (20–23). With the understanding that the results are Tchannels located near the tips of the stereocilia on the sen- controversial, cochlear hair cells from mice (19) and rats (18) sory hair cells of the inner ear (1). When an acoustic stimulus tend to show little adaptation, and the number of MET channels causes these mechanoelectric transduction (MET) channels to open at rest increases substantially when stereocilia are exposed + be activated, the flow of potassium and calcium ions into the cell to solutions with micromolar [Ca2 ]. + increases (2). Recent data suggest that the Ca2 component of the receptor 2+ 2+ The Ca concentration ([Ca ]) in the vicinity of the MET current may also be important for maintaining the structural channels is functionally important. For instance, it controls the fraction of MET channels that are open at rest [i.e., high calcium Significance near stereocilia tends to close MET channels, whereas lower calcium does the opposite (3)]. Changes in the resting open A new mechanism that contributes to control of hearing sensi- probability alter the amplitude of the “silent current” (4, 5) that tivity is described here. We show that an accessory structure in continuously flows into the sensory cells, causing them to operate − the hearing organ, the tectorial membrane, affects the function at a relatively depolarized membrane potential near 40 mV (6). of inner ear sensory cells by storing calcium ions. When the The continuous depolarization is important in one class of sen- calcium store is depleted, by brief exposure to rock concert-level sory cell, the outer hair cell, where it is thought to facilitate the sounds or by the introduction of calcium chelators, the sound- detection of soft sounds by allowing amplification mediated by evoked responses of the sensory cells decrease. Upon restora- somatic electromotility (7) to operate at auditory frequencies (6). tion of tectorial membrane calcium, sensory cell function returns. Like other sensory receptors, hair cells possess an adaptation This previously unknown mechanism contributes to explaining mechanism that reduces the transduction current in response to the temporary numbness in the ear that follows from listening sustained stimuli (8). The conventional model states that this to sounds that are too loud, a phenomenon that most people adaptation is due to two distinct processes, termed fast and slow experience at some point in their lives. adaptation, which operate on different time scales (9), but which 2+ 2+ both may depend on Ca entry. In fast adaptation, or Ca - Author contributions: C.E.S. and A.F. designed research; C.E.S., S.P., and P.H. performed + mediated channel reclosure, Ca2 ions appear to interact di- research; A.F. analyzed data; and A.F. wrote the paper. rectly with the MET channel or its accessory proteins, causing The authors declare no conflict of interest the channel to rapidly close. Slow adaptation is believed to rely This article is a PNAS Direct Submission. 2+ on the Ca -dependent activation of unconventional myosins This open access article is distributed under Creative Commons Attribution-NonCommercial- (10, 11), which move the upper insertion point of the tip links NoDerivatives License 4.0 (CC BY-NC-ND). (12, 13), the tiny extracellular filaments that control the gating of 1C.E.S. and S.P. contributed equally to this work. the MET channels (14). By adjusting the tension in the tip links, 2Present address: Department of Otolaryngology, Columbia University Medical Center, slow adaptation reduces the MET current following prolonged New York, NY 10032. deflections in the positive direction, thus ensuring that the MET 3To whom correspondence should be addressed. Email: [email protected]. channels maintain their responsiveness. Together, the two adap- Published online March 5, 2019. 5756–5764 | PNAS | March 19, 2019 | vol. 116 | no. 12 www.pnas.org/cgi/doi/10.1073/pnas.1805223116 Downloaded by guest on October 1, 2021 integrity of stereocilia. When isolated rat or mouse hair cells Results + were exposed to MET channel blockers, the length of individual To measure [Ca2 ] near the hearing organ, we used temporal stereocilia and the architecture of the bundle were altered (24). bone preparations isolated from guinea pigs (26). The prepara- These changes, which were reversible upon washout of the tions retained the middle ear, allowing sound stimulation through 2+ blocker, could also be induced by increasing the internal Ca a loudspeaker positioned in the ear canal, and to ensure cellular 2+ buffering capacity of the hair cells or by decreasing external Ca viability, the preparations were continuously perfused with oxy- to levels around 100 μM. genated tissue culture medium. A small opening at the apex of the 2+ It seems clear that it is important to determine the [Ca ] cochlea allowed a glass microelectrode with 10-μm tip diameter to close to the hair bundles, particularly since a previous study (25) be positioned close to the hearing organ. The electrode was used 2+ suggested that local Ca may be elevated as a result of high to introduce the low-affinity fluorescent indicator Asante Calcium activity of plasma-membrane calcium-ATPase type 2, coupled Red. When excited at 488 nm, this dye emitted red light with + with diffusion barriers imposed by the proximity to the tectorial intensity that depended on the Ca2 level, as well as green light + membrane, an acellular accessory structure close to stereocilia. with very weak Ca2 dependence (27). By taking the ratio between 2+ However, all reported measurements of [Ca ] in the inner ear red and green fluorescence intensities, spatial and temporal vari- 2+ fluids used techniques that lack spatial resolution, so the Ca ations in dye concentration were cancelled out and a sensitive + level near the stereocilia is not known. measure of extracellular [Ca2 ] was obtained. Here, we used fluorescence imaging and spectroscopic mea- + + surements to determine Ca2 in situ. Surprisingly, these experi- High Ca2 Within the Tectorial Membrane. A pivotal finding from + ments revealed that the tectorial membrane had a much higher these experiments was that Ca2 levels were much higher in the + Ca2 concentration than the surrounding endolymph, and that tectorial membrane than in the surrounding fluid. This pattern + reductions in tectorial membrane Ca2 contributed to the acute was evident in single images (Fig. 1A), where high ratios were decrease of hearing sensitivity experienced immediately after evident in tectorial membrane regions close to the hair cells, as loud sound exposure. well as in the region close to the attachment to the spiral limbus. NEUROSCIENCE Fig. 1. High tectorial membrane Ca2+.(A) Following injection of the low-affinity ratiometric Ca2+ dye Asante Calcium Red into the endolymphatic space, high Ca2+ levels were evident in the tectorial membrane. The locations of the outer and inner hair cells are indicated by the red lines. IHC, inner hair cell. (B) + Three-dimensional reconstruction made from 100 confocal images acquired with 1-μm section spacing.