Links between Schwann cells and melanocytes in development and disease.

Melanocytes are pigment-producing cells that reside in the skin, eyes, ears, heart, and central nervous system meninges of mammals. Schwann cells are glial cells, which closely associate with peripheral nerves, myelinating, and sheathing them. Melanocytes and Schwann cells both arise from the neural crest during development, and some melanocytes arise directly from Schwann cell precursors lining developing spinal nerves. In this review, we explore the connections between melanocytes and Schwann cells in development and transformation.

Genetic interactions between neurofibromin and endothelin receptor B in mice.

When mutations in two different genes produce the same mutant phenotype, it suggests that the encoded proteins either interact with each other, or act in parallel to fulfill a similar purpose. Haploinsufficiency of Neurofibromin and over-expression of Endothelin 3 both cause increased numbers of melanocytes to populate the dermis during mouse development, and thus we are interested in how these two signaling pathways might intersect. Neurofibromin is mutated in the human genetic disease, neurofibromatosis type 1, which is characterized by the development of Schwann cell based tumors and skin hyper-pigmentation. Neurofibromin is a GTPase activating protein, while the Endothelin 3 ligand activates Endothelin receptor B, a G protein coupled receptor. In order to study the genetic interactions between endothelin and neurofibromin, we defined the deletion breakpoints of the classical Ednrb piebald lethal allele (Ednrb(s-l) ) and crossed these mice to mice with a loss-of-function mutation in neurofibromin, Dark skin 9 (Dsk9). We found that Neurofibromin haploinsufficiency requires Endothelin receptor B to darken the tail dermis. In contrast, Neurofibromin haploinsufficiency increases the area of the coat that is pigmented in Endothelin receptor B null mice. We also found an oncogenic mutation in the G protein alpha subunit, GNAQ, which couples to Endothelin receptor B, in a uveal melanoma from a patient with neurofibromatosis type 1. Thus, this data suggests that there is a complex relationship between Neurofibromin and Endothelin receptor B.

Differential effects of neurofibromin gene dosage on melanocyte development.

Mutations in neurofibromin (NF1) cause the dominant genetic disorder neurofibromatosis type 1. Neurofibromatosis is characterized by Schwann cell-based tumors and skin hyperpigmentation, resulting from both haploinsufficiency and loss of heterozygosity. The fact that some pigment cells (melanocytes) arise from Schwann cell precursors suggests that neurofibromin could be required during the common precursor stage. In this study, we found a missense mutation in neurofibromin in Dark skin 9 (Dsk9) mutant mice, revealing that Nf1 mutations cause skin hyperpigmentation in mice, as they do in humans. Using tissue-specific knockouts, we found that haploinsufficiency of neurofibromin in melanocytes via Mitf-cre is insufficient to cause darker skin, whereas haploinsufficiency in bipotential Schwann cell-melanoblast precursors via Plp1-creER is sufficient. These findings suggest that there is a narrow developmental window during which Nf1 haploinsufficiency acts on pigment cells. Using fate mapping, we discovered differences in the colonization of the dermis and epidermis by melanocytes that arise from Schwann cell precursors, an unexpected complexity of melanocyte development. As homozygous knockout of Nf1 via Mitf-cre is sufficient to cause darker skin, we conclude that reduced gene dosage can act by a mechanism different from complete gene loss, even when the end result of both is very similar.