Matrix metalloproteinases 2 and 9 in the cochlea: expression and activity after aminoglycoside exposition

The matrix metalloproteinases (MMPs) are a family of proteins involved in the remodelling and homeostasis of the extracellular matrix. These proteases have been well studied in the retina and the brain, marking their importance in neuronal cell survival and death [Chintala (2006) Exp Eye Res 82:5-12; Candelario-Jalil et al. (2009) Neuroscience 158:983-994]. The neuroepithelia of the eye and the inner ear share common characteristics. Therefore, we hypothesized that MMPs could play a similar role in the cochlea as described in the retina. We focused on the localization and function of MMP-2 and MMP-9 in the cochlea, by determining their expression and activity under normal conditions and after cochlear damage via aminoglycoside exposition. We examined their expression in 5-day-old Wistar rat cochleas by RT-PCR, real-time PCR, and Western blot. We used immunohistochemistry to investigate their location in the cochleas of adult C57BL/6 mice. We also determined whether or not the exposure of the organs of Corti to aminoglycosides would change MMP-2 and MMP-9 expression patterns. Western blotting identified MMP-2 and MMP-9 in neonatal spiral ganglion, stria vascularis, and to a lesser extent the organ of Corti. Neonatal mRNA expression of MMP-2 was approximately equivalent in all three tissues, while MMP-9 mRNA was highest in spiral ganglion. Immunohistochemistry showed MMP-2 primarily in adult spiral ganglion neurons and inner hair cells, while MMP-9 was found mainly in spiral ganglion neurons, inner hair cells and supporting cells. Organs of Corti treated with gentamicin for 24 h showed an upregulation of MMP-2 and MMP-9 proteins, but did not show a significant upregulation of mRNA expression 3, 6, 12, 24, and 36 h after gentamicin exposure. Inhibition of MMP activity in organs of Corti incubated with an MMP inhibitor in organotypic cultures resulted in hair cell death-suggesting that a basal level of MMP activity is required for hair cell survival.