Altered cell-surface targeting of stem cell factor causes loss of melanocyte precursors in Steel17H mutant mice

The normal products of the murine Steel (SI) and Dominant white spotting (Ur) genes are essential for the development of melanocyte precursors, germ cells, and hematopoietic cells. The S1 locus encodes stem cell factor (SCF), which is the ligand Of c-kit, a receptor tyrosine kinase encoded by the W locus. One allele of the SI mutation, S1(17H), exhibits minor hematopoietic defects, sterility only in males, and a complete absence of coat pigmentation. The S1(17H) gene encodes SCF protein which exhibits an altered cytoplasmic domain due to a splicing defect. In this paper we analyzed the mechanism by which the pigmentation phenotype in S1(17H) mutant mice occurs. We show that in embryos homozygous for S1(17H) the number of melanocyte precursors is severely reduced on the lateral neural crest migration pathway by e11.5 and can no longer be detected by e13.5 when they would enter the epidermis in wildtype embryos. The reduced number of dispersing melanocyte precursors correlates with a reduction of SCF immunoreactivity in mutant embryos in all tissues examined. Regardless of the reduced amount, functional SCF is present at the cell surface of fibroblasts transfected with S1(17H) mutant SCF cDNA. Since SCF immunoreactivity normally accumulates in basolateral compartments of SCP-expressing embryonic epithelial tissues, we analyzed the localization of wildtype and S1(17H) mutant SCE protein in transfected epithelial (MDCK) cells in vitro. As expected, wildtype forms of SCF localize to and are secreted from the basolateral compartment. In contrast, mutant forms of SCF, which either lack a membrane anchor or exhibit the S1(17H) altered cytoplasmic tail, localize to and are secreted from the apical compartment of the cultured epithelium. We suggest, therefore, that the loss of melanocyte precursors prior to epidermal invasion, and the loss of germ cells from mature testis, can be explained by the inability of S1(17H) mutant SCE to be targeted to the basolateral compartment of polarized epithelial keratinocytes and Sertoli cells, respectively. (C) 1999 Academic Press.