The tight regulation of Ca2+ is essential for inner ear function, and yet the role of Ca2+ binding proteins (CaBPs) remains elusive. Using immunofluorescence and RT-PCR, we investigated the expression of oncomodulin (Ocm), a member of the parvalbumin family, relative to other EF-hand CaBPs in cochlear and vestibular organs in the mouse. In the mouse cochlea, Ocm is found only in outer hair cells and is localized preferentially to the basolateral outer hair cell membrane and to the base of the hair bundle. Developmentally, Ocm immunoreactivity begins as early as postnatal day (P) 2 and shows preferential localization to the basolateral wall and hair bundle after P8. Unlike the cochlea, Ocm expression is substantially reduced in vestibular tissues at older adult ages. In vestibular organs, Ocm is found in type I striolar or central hair cells, and has a more diffuse subcellular localization throughout the hair cell body. Additionally, Ocm immunoreactivity in vestibular hair cells is present as early as E18 and is not obviously affected by mutations that cause a disruption of hair bundle polarity. We also find Ocm expression in striolar hair cells across mammalian species. These data suggest that Ocm may have distinct functional roles in cochlear and vestibular hair cells.

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Ca2+ signaling is a major contributor to sensory hair cell function in the cochlea. Oncomodulin (OCM) is a Ca2+ binding protein (CaBP) preferentially expressed in outer hair cells (OHCs) of the cochlea and few other specialized cell types. Here, we expand on our previous reports and show that OCM delays hearing loss in mice of two different genetic backgrounds: CBA/CaJ and C57Bl/6J. In both backgrounds, genetic disruption of Ocm leads to early progressive hearing loss as measured by auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE). In both strains, loss of Ocm reduced hearing across lifetime (hearing span) by more than 50% relative to wild type (WT). Even though the two WT strains have very different hearing spans, OCM plays a considerable and similar role within their genetic environment to regulate hearing function. The accelerated age-related hearing loss (ARHL) of the Ocm KO illustrates the importance of Ca2+ signaling in maintaining hearing health. Manipulation of OCM and Ca2+ signaling may reveal important clues to the systems of function/dysfunction that lead to ARHL.

Cochlear outer hair cells (OHCs) are responsible for the exquisite frequency selectivity and sensitivity of mammalian hearing. During development, the maturation of OHC afferent connectivity is refined by coordinated spontaneous Ca2+activity in both sensory and non-sensory cells. Calcium signaling in neonatal OHCs can be modulated by Oncomodulin (OCM, β-parvalbumin), an EF-hand calcium-binding protein. Here, we investigated whether OCM regulates OHC spontaneous Ca2+activity and afferent connectivity during development. Using a genetically encoded Ca2+sensor (GCaMP6s) expressed in OHCs in wild-type (Ocm+/+) and Ocm knockout (Ocm-/-) littermates, we found increased spontaneous Ca2+activity and upregulation of purinergic receptors in OHCs from GCaMP6s Ocm-/-cochlea immediately following birth. The afferent synaptic maturation of OHCs was delayed in the absence of OCM, leading to an increased number of ribbon synapses and afferent fibers on GCaMP6s Ocm-/-OHCs before hearing onset. We propose that OCM regulates the spontaneous Ca2+signaling in the developing cochlea and the maturation of OHC afferent innervation.

In cochlear outer hair cells (OHCs), a network of Ca2+ channels, pumps and Ca2+-binding proteins (CaBPs) regulates the localization, spread, and magnitude of free Ca2+ ions. During early postnatal development, OHCs express three prominent mobile EF-hand CaBPs: oncomodulin (OCM), α-parvalbumin (APV) and sorcin. We have previously shown that deletion of Ocm (Ocm-/-) gives rise to progressive cochlear dysfunction in young adult mice. Here, we show that changes in Ca2+ signaling begin early in postnatal development of Ocm-/- mice. While mutant OHCs exhibit normal electrophysiological profiles compared to controls, their intracellular Ca2+ signaling is altered. The onset of OCM expression at postnatal day 3 (P3) causes a developmental change in KCl-induced Ca2+ transients in OHCs and leads to slower KCl-induced Ca2+ transients than those elicited in cells from Ocm-/- littermates. We compared OCM buffering kinetics with other CaBPs in animal models and cultured cells. In a double knockout of Ocm and Apv (Ocm-/-;Apv-/-), mutant OHCs show even faster Ca2+ kinetics, suggesting that APV may also contribute to early postnatal Ca2+ signaling. In transfected HEK293T cells, OCM slows Ca2+ kinetics more so than either APV or sorcin. We conclude that OCM controls the intracellular Ca2+ environment by lowering the amount of freely available [Ca2+]i in OHCs and in transfected HEK293T cells. We propose that OCM plays an important role in shaping the development of early OHC Ca2+ signals through its inimitable Ca2+ buffering capacity.