Conditional Ablation of β1 Integrin in Skin: Severe Defects in Epidermal Proliferation, Basement Membrane Formation, and Hair Follicle Invagination

The major epidermal integrins are α3β1 and hemidesmosome-specific α6β4; both share laminin 5 as ligand. Keratinocyte culture studies implicate both integrins in adhesion, proliferation, and stem cell maintenance and suggest unique roles for αβ1 integrins in migration and terminal differentiation. In mice, however, whereas ablation of α6 or β4 results in loss of hemidesmosomes, epidermal polarity, and basement membrane (BM) attachment, ablation of α3 only generates microblistering due to localized internal shearing of BM. Using conditional knockout technology to ablate β1 in skin epithelium, we have uncovered biological roles for αβ1 integrins not predicted from either the α3 knockout or from in vitro studies. In contrast to α3 null mice, β1 mutant mice exhibit severe skin blistering and hair defects, accompanied by massive failure of BM assembly/organization, hemidesmosome instability, and a failure of hair follicle keratinocytes to remodel BM and invaginate into the dermis. Although epidermal proliferation is impaired, a spatial and temporal program of terminal differentiation is executed. These results indicate that β1's minor partners in skin are important, and together, αβ1 integrins are required not only for extracellular matrix assembly but also for BM formation. This, in turn, is required for hemidesmosome stability, epidermal proliferation, and hair follicle morphogenesis. However, β1 downregulation does not provide the trigger to terminally differentiate.