Targeted Disruption Of Smad4 In Mouse Keratinocytes Results In Failure Of Hair Follicle Cycling And Formation Of Skin Tumors

Smad4 is the common mediator of the transforming growth factor-β (TGF-β) superfamily that functions in diverse developmental processes in mammals, such as regulating cell proliferation, lineage determination, motility, adhesion and apoptosis. To study the role of Smad4 in skin development, a keratinocyte-specific null mutant of Smad4 was generated in mice using the Cre-loxP system.We investigated the tissue distribution of the Cre recombinase in keratinocyte specific Cre transgenic mouse (K5-Cre) using Southern blot and the ROSA26 reproter mice. The results showed that the activity of Cre recombinase could be detected in all the tissues containing keratinocytes, indicating that the keratinocyte specific Cre transgenic mouse was a good tool for making keratinocyte specific gene knockout mice. The keratinocyte specific Smad4 knockout mice were obtained by crossing the keratinocyte specific Cre transgenic mice with the Smad4 conditional gene targeted mice. RT-PCR analysis showed that the expression of Smad4 in the epidermis of the Smad4Co/Co;K5-Cre mice was markedly decreased compared to the wild-type mouse, demonstrating that the Smad4 gene was disrupted efficiently in keratinocytes by K5-Cre mediated recombination.The Smad4Co/Co;K5-Cre mice exhibited progressive alopecia from postnatal day 20 as a result of the mutant hair follicles failing to undergo programmed regression. Histological analysis of the skin sections showed hypertrophied hair follicles (HFs) and thickened epidermis. TUNEL staining revealed that Smad4 mutants exhibited a much lower number of apoptotic cells in the hair bulb of early catagen HF. The HF cells without Smad4 showed more BrdU-positive cells at both...