MELANOSOMAL DEFECTS IN MELANOCYTES FROM MICE LACKING EXPRESSION OF THE PINK-EYED DILUTION GENE - CORRECTION BY CULTURE IN THE PRESENCE OF EXCESS TYROSINE

Mutations in the murine pink-eyed dilution (p) gene, or its human homo logue P, result in oculocutaneous albinism. Melanocytes cultured from mice lacking p gene expression exhibit defective melanogenesis, but fo llowing culture in the presence of high concentrations of L-tyrosine, increased melanin deposition is observed. Electron microscopy and imag e analysis demonstrated that untreated p mutant melanocytes exhibited small melanosomes, largely of stages I-II. Following tyrosine treatmen t, increased proportions of stage III-IV melanosomes, almost normal in size, were observed. Levels of tyrosinase protein and to a lesser ext ent of tyrosinase-related protein-1 (TRP-1) were subnormal but rose dr amatically following stimulation by tyrosine. Levels of TRP-2 and Pmel 17/silver gene product were not altered, nor were the levels of mRNA f or tyrosinase, TRP-1, TRP-2, or the Pmel17/silver gene product. As exp ected, the 110-kDa product of the p gene was absent from both stimulat ed and unstimulated p mutant cells. In a melanoblast line derived from the same mice, excess tyrosine failed to stimulate visible melanogene sis or increase the low levels of tyrosinase. The melanosomes in these cells were smaller still than those in the mutant melanocytes even wh en cultured in the presence of excess tyrosine. Thus, absence of the p gene product affects melanosomal structure and protein composition at the posttranscriptional level. These defects are correctable at least in part by supplementation with L-tyrosine. (C) 1998 Academic Press.