Altered Cell-Surface Targeting of Stem Cell Factor Causes Loss of Melanocyte Precursors inSteel17HMutant Mice

Abstract

The normal products of the murineSteel (Sl)andDominant white spotting (W)genes are essential for the development of melanocyte precursors, germ cells, and hematopoietic cells. TheSllocus encodes stem cell factor (SCF), which is the ligand of c-kit, a receptor tyrosine kinase encoded by theWlocus. One allele of theSlmutation,Sl17H,exhibits minor hematopoietic defects, sterility only in males, and a complete absence of coat pigmentation. TheSl17Hgene 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 inSl17Hmutant mice occurs. We show that in embryos homozygous forSl17Hthe 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 withSl17Hmutant SCF cDNA. Since SCF immunoreactivity normally accumulates in basolateral compartments of SCF-expressing embryonic epithelial tissues, we analyzed the localization of wildtype andSl17Hmutant SCF protein in transfected epithelial (MDCK) cellsin 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 theSl17Haltered 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 ofSl17Hmutant SCF to be targeted to the basolateral compartment of polarized epithelial keratinocytes and Sertoli cells, respectively.