Signaling and transcriptional regulation in the neural crest-derived melanocyte lineage: interactions between KIT and MITF.

Genetic and cell culture analyses have shown that the development of melanocytes from neural crest-derived precursor cells critically depends on the tyrosine kinase receptor KIT and the basic-helix-loop-helix-leucine zipper transcription factor MITF. KIT and MITF show complex interactions in that MITF is needed for the maintenance of Kit expression in melanoblasts and KIT signaling modulates MITF activity and stability in melanocyte cell lines. Using primary neural crest cell cultures from embryos homozygous for a Kit null allele marked by an inserted LacZ gene (Kit(W-LacZ)), we show that the onset of Mitf expression in melanoblasts does not require KIT. In fact, provided that the melanocyte growth factor endothelin-3 is present, a small number of MITF/beta-Gal-positive cells can be maintained for at least 2 weeks in Kit(W-LacZ)/Kit(W-LacZ) cultures. These cells express several pigment cell-specific genes that are thought or have been shown to be activated by MITF, including dautochrome tautomerase, pMel 17/Silver and tyrosinase-related protein-1, but lack expression of the MITF target gene tyrosinase, which encodes the rate-limiting enzyme in melanin synthesis. Consequently, the cells remain unpigmented. Addition of cholera toxin, which elevates cAMP levels and mimics part of the KIT signaling pathway, increases the number of MITF-positive cells in Kit(W-LacZ)/Kit(W-LacZ) cultures, leads to tyrosinase expression, and induces the differentiation of melanoblasts into mature, pigmented melanocytes. Even when added on day 5-6 of culture, cholera toxin still rescues tyrosinase expression and differentiation. The results thus demonstrate that the presence of MITF is not sufficient for tyrosinase expression in melanoblasts and that KIT signaling influences gene expression during melanocyte development in a gene-selective manner.