Corticosteroid receptors and Na,K-ATPase in the developing mouse cochlea
Abstract: The senses of hearing and equilibrium are highly dependent on the fluid and electrolyte homeostasis in the inner ear. The characteristic intracellular-like ionic composition of endolymph fluid in relation to the surrounding perilymph creates the endocochlear potential (EP), which is a prerequisite for inner ear function. Disturbed regulation of endolymph production is thought to be responsible for certain inner ear disorders, such as for example Ménière's disease. The stria vascularis (SV) and the spiral ligament (SL) in the cochlea and dark cell areas in the vestibular saccule, utricle and ampullae are known to be responsible for production of endolymph fluid, but the precise mechanisms remain to be explored. The ubiquitous enzyme Na,K-ATPase is known to play an important role in endolymph production. There is evidence indicating that corticosteroid hormones, including glucocorticoid and mineralocorticoid hormone, take part in the regulation of Na,K-ATPase expression and activity in the inner ear. Glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) have been demonstrated in the inner ear of the rat. The overall purpose of this work was to investigate if the expression of Na,K-ATPase in the developing and adult mouse cochlea and the maturation of the cochlea is dependent on the presence of corticosteroid receptors. Distribution of Na,K-ATPase, GR and MR in the developing mouse inner ear was investigated using an immunohistochemical method. Appearance of the complete Na,K-ATPase heterodimer in the SV and SL coincides with the rise of endolymphatic K+ concentration, the completion of the morphological maturation of the SV, and the first detectable cochlear potential. This result is concordant with established evidence concerning the role of Na,K-ATPase in the production of endolymph and the EP. The appearance of GR also seems to follow a distinct pattern from the time of early organogenesis to adulthood, paralleling Na,K-ATPase appearance and the structural and functional maturation of the cochlea, which may indicate that GR take part in the regulation of Na,K-ATPase expression in the developing cochlea. An almost adult pattern of MR immunostaining was seen as early as around birth, with no significant changes taking place thereafter, meaning that maturation of the distribution of MR in the mouse inner ear takes place much earlier than Na,K-ATPase expression. The hypothesis that MR is the major regulating factor of Na,K- ATPase expression in the cochlea cannot be confirmed. In the inner ear of both the GR-knockout mouse and the MR-knockout mouse, Na,K-ATPase content and distribution remained normal. Accordingly, the hypothesis that corticosteroid hormones via GR or MR are the major regulating factor of Na,K-ATPase expression cannot be confirmed. Finally, two mouse strains with severe hereditary inner ear disease, displaying symptoms that are typical for a combined dysfunction of hearing and equilibrium as can be seen following derangement of endolymph homeostasis, were examined. The shaker-2 mouse suffers from progressive degeneration of the SV and severely disturbed ionic composition of the endolymph. Reduced amounts of GR, MR and Na,K-ATPase are found in all tissues of the shaker-2 mouse cochlea. The mix mouse is a new strain, with deafness and circling behaviour. It seems that the cochlear and vestibular hair cells in the mix mouse are severely degenerated, the SV is atrophic, and the endolymphatic space has collapsed, indicating that there is no endolymph production. Amounts of GR, MR and Na,K-ATPase are reduced in the atrophic SV of the mix mouse, but normal in the remaining cochlea. The reduced amounts of Na,K-ATPase, in the shaker-2 mouse in the whole cochlea and in the mix mouse in the SV, may be a very important mechanism behind the disturbed endolymph production seen in both strains. The results showing that the amounts of MR and GR are diminished in the same tissues where also Na,K- ATPase abundance is reduced, however, should be handled with care and do not necessarily indicate that reduced Na,K-ATPase expression is a result of reduced amounts of corticosteroid receptors. Na,K-ATPase expression in the inner ear is regulated by mechanisms interacting in a very complex fashion. The results of this study indicate that corticosteroid hormones and their receptors are part of this regulating mechanism, but can be compensated for in case of failure.
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