Notch1 and Notch2 receptors influence progressive hair graying in a dose-dependent manner

The Notch signaling pathway is involved in diverse biological processes such as cell fate decisions or stem cell maintenance. In this study, we assessed the role of this pathway for melanocyte development and hair pigmentation using RBP-Jkappa, Notch1, and Notch2 conditional knockout mice. Disruption of the Notch pathway by inactivating RBP-Jkappa in the melanocyte lineage using Tyr::Cre mice led to a severe coat color dilution. Similarly, hair graying was observed when Notch1 and/or Notch2 receptors were ablated in melanocytes. This phenotype was proportional to the number of floxed Notch alleles, with the most pronounced effect seen in Tyr::Cre/ degrees ; Notch1(flox/flox); Notch2(flox/flox) mice. Deletion of Notch1 and/or Notch2 in melanoblasts did not induce a congenital defect. The number of Dct-expressing cells at embryonic stages was not affected, but melanocytes located within the hair matrix progressively disappeared during the first regeneration of the hair follicle. In contrast, non-follicular melanocytes and pigmentation in the dermis and in the choroid were not affected. We suggest that both Notch1 and Notch2 receptors contribute to the maintenance of melanoblasts and melanocyte stem cells, and are essential for proper hair pigmentation.

The Notch pathway: hair graying and pigment cell homeostasis

Friedrich Beermann, Karine Schouwey

The Notch signaling pathway is an essential cell-cell interaction mechanism, which regulates processes such as cell proliferation, cell fate decisions, differentiation or stem cell maintenance. Pigmentation in mammals is provided by melanocytes, which are derived from the neural crest, and by the retinal pigment epithelium (RPE), which is part of the optic cup and hence orginates from neuroectoderm. The importance of functional Notch signaling in melanocytes has been unveiled recently. Here, the pathway is essential for the maintenance of proper hair pigmentation. Deletion of Notch1 and Notch2 or RBP-Jkappa in the melanocyte lineage resulted in a gene dosage- dependent precocious hair graying, due to the elimination of melanoblasts and melanocyte stem cells. Expression data support the idea that Notch signaling might equally be involved in development of the RPE. Furthermore, recent analyses indicate a possible role of Notch signaling in the development of melanoma. In this review, we address the essential role of Notch signaling in the regeneration of the melanocyte population during hair follicle cycles, and discuss data supporting the implication of this signaling pathway in RPE development and melanoma.

2008

Liver-specific inactivation of Notch2, but not Notch1, compromises intrahepatic bile duct development in mice

Freddy Radtke

The Notch pathway is an evolutionary conserved, intercellular signaling pathway that plays an important role in cell fate specification and the embryonic development of many organs, including the liver. In humans, mutations in the Notch receptor ligand Jagged1 gene result in defective intrahepatic bile duct (IHBD) development in Alagille syndrome. Developmental abnormalities of IHBD in mice doubly heterozygous for Jagged1 and Notch2 mutations propose that interactions of Jagged1 and its receptor Notch2 are crucial for normal IHBD development. Because different cell types in the liver are involved in IHBD development and morphogenesis, the cell-specific role of Notch signaling is not entirely understood. We investigated the effect of combined or single targeted disruption of Notch1 and Notch2 specifically in hepatoblasts and hepatoblast-derived lineage cells on liver development using AlbCre transgenic mice. Hepatocyte differentiation and homeostasis were not impaired in mice after combined deletion of Notch1 and Notch2 (N1N2(F/F)AlbCre). However, we detected irregular ductal plate structures in N1N2(F/F)AlbCre newborns, and further postnatal development of IHBD was severely impaired characterized by disorganized ductular structures accompanied by portal inflammation, portal fibrosis, and foci of hepatocyte feathery degeneration in adulthood. Further characterization of mutant mice with single deletion of Notch1 (N1(F/F)AlbCre) or Notch2 (N2(F/F)AlbCre) showed that Notch2 but not Notch1 is indispensable for normal perinatal and postnatal IHBD development. Further reduction of Notch2 gene dosage in Notch2 conditional/mutant (N2(F/LacZ)AlbCre) animals further enhanced IHBD abnormalities and concomitant liver pathology. CONCLUSION: Notch2 is required for proper IHBD development and morphogenesis

2008

The Notch signaling pathway

Karine Schouwey

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.

EPFL2009

The Notch signaling pathway

Karine Schouwey

EPFL2009