The Notch signaling pathway : hair graying and pigment cell homeostasis

In animals, pigment cells are essential for coloration, as seen in feathers, fur, skin and eyes. In mammals, neural crest-derived melanocytes constitute the major population of pigment cells that is located mainly in the skin epidermis, as well as in hair follicles. A minor population of pigment cells originates from the optic cup and forms the retinal pigment epithelium (RPE), a cell monolayer lying between the neural retina and the choroid. In many cell types, the Notch signaling pathway regulates diverse biological processes such as cell fate decision, differentiation, stem cell maintenance or apoptosis. In my thesis project, the role of this pathway was investigated using Notch1, Notch2 and RBP-Jκ conditional knockout mice. Disruption of the Notch pathway by inactivation of Notch1 and Notch2, or RBP-Jκ in the melanocyte lineage using Tyr::Cre transgenic mice resulted in a gene dosage-dependent precocious hair graying, with the most pronounced effect seen in mice lacking both Notch1 and Notch2 in melanocytes. Histologically, this coat color dilution was explained by the progressive elimination of melanocytes and melanocyte stem cells, identified in skin or embryos by X-Gal staining (Dct::LacZ reporter mouse line) and immunofluorescence analyses. As in neural crest-derived melanocytes, members of the Notch signaling pathway are normally expressed in pigment cells of the RPE. The RPE-restricted deletion of RBP-Jκ using Tyrp1::Cre transgenic mice resulted in a microphthalmic eye phenotype and thus indicated a role of the Notch pathway in the development of the RPE. Further analyses demonstrated that misregulation of Notch signaling in RPE induced a reduction in the number of these pigment cells. In addition to these knockout studies, I have generated transgenic mouse lines that express the constitutively activated intracellular part of Notch (NotchIC) in melanocytes (Dct::NotchIC) and RPE cells (Tyrp1::NotchIC). Since these mice rescue the phenotypes observed in the conditional Notch receptor knockouts, they are instrumental to assess the implication of active Notch signaling in pigment cell development and tumorigenesis. Moreover, it might be determined whether constitutive activation of Notch can prevent physiological hair graying and loss of stem cells. In summary, my results underline the importance of fine-tuned Notch signaling for proper hair pigmentation and, more generally, for pigment cell development.