The Role of the Microenvironment on the Localization of Hair and Skin Nonuniform Pigmented Spots in Mice with a Targeted Mutation in the Endothelin B Receptor

In mammals, severe mutations in genes encoding factors critical for developing melanocytes often lead to a total loss of melanin-dependent skin and hair pigmentation. However, even null mutations in the gene encoding the G protein-coupled Endothelin-3 receptor EDNRB are associated with locally restricted pigmented spots at the head and tail on an otherwise totally white coat. Here, we investigate the developmental mechanism of these nonuniform pigmented spots in Ednrb-deficient mice and show that the local microenvironment, particularly the presence of enriched KIT ligand (KITL), dictates the appearance of these spots during development. Using a variety of transcriptomic profiling analyses and genetic and biochemical manipulations both in vivo and in vitro, we show that in E11.5 Ednrb-deficient embryos, a high expression level of KITL surrounds melanoblasts in the caudal region but not in the trunk region. The genetic reduction of KITL/KIT signaling in Ednrb-deficient miceleads to a complete loss of these nonuniform pigmented spots in the head and caudal regions. Moreover, the intradermal injection of anti-KIT antibody in the head region of adult Ednrb-deficient mice significantly reduces hair pigmentation in the secondary hair cycle. Furthermore, the exogenous addition of KITL rescues Ednrb-deficient melanoblasts from the neural crest cells in the trunk region, likely by stimulating MITF expression. Hence, these findings underscore the significance of the microenvironment for the appearance and localization of pigment spots and highlight the role of KITL as a major determinant of this microenvironment.