Cochlear melanocytes and MITF signaling.

Melanocytes occur not only in the skin and eyes but in the cochlea, where they exist as intermediate cells of the stria vascularis. Intermediate cells play an important role for cochlear function: Na+K+-ATPase and potassium channels of intermediate cells are essential for production of endocochlear potential and for preparation of ionic milieu in the stria. Consistent with this notion, melanocyte deficiency due to some gene disruptions results in hearing impairment in mice and humans. Mitf/MITF is essential for development and maturation of melanocytes, including strial intermediate cells. Disruption of MITF causes deafness, heterochromia irides, and leucodermia in Waardenburg syndrome type 2 individuals, whereas that of Mitf causes phenotypes of deafness, microphthalmia, and white coat in mice. Again, all of these phenotypes may be explained by a lack of melanocytes. Many signal transduction pathways target the Mitf/MITF gene or Mitf/MITF protein, and disruption of these pathways sometimes results in the phenotype similar to that caused by Mitf/MITF disruption. If not all, certainly many roads lead to MITF in melanocytes.