Gab1 and Mapk Signaling Are Essential in the Hair Cycle and Hair Follicle Stem Cell Quiescence

• Scaffold adaptor Gab1 controls catagen, the destructive phase of the hair cycle
• Gab1 requires Grb2 and Shp2 binding sites and acts via Mapk signaling
• Early stem cells need Gab1 to maintain quiescence at catagen entry
• Gab1 is essential in the establishment of adult stem cells of the bulge

SummaryGab1 is a scaffold protein that acts downstream of receptor tyrosine kinases. Here, we produced conditional Gab1 mutant mice (by K14- and Krox20-cre) and show that Gab1 mediates crucial signals in the control of both the hair cycle and the self-renewal of hair follicle stem cells. Remarkably, mutant hair follicles do not enter catagen, the destructive phase of the hair cycle. Instead, hair follicle stem cells lose quiescence and become exhausted, and thus no stem cell niches are established in the bulges. Moreover, conditional sustained activation of Mapk signaling by expression of a gain-of-function Mek1DD allele (by Krox20-cre) rescues hair cycle deficits and restores quiescence of the stem cells. Our data thus demonstrate an essential role of Gab1 downstream of receptor tyrosine kinases and upstream of Shp2 and Mapk in the regulation of the hair cycle and the self-renewal of hair follicle stem cells.

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