α3β1 and α6β4 integrin receptors for laminin-5 are not essential for epidermal morphogenesis and homeostasis during skin development

Continuous regeneration and homeostasis of the stratified epidermis requires coordinated regulation of cell proliferation, cell differentiation, and cell survival. Integrin-mediated cell adhesion to the extracellular matrix has important roles in regulating each of these processes. Integrins α3β1 and α6β4 are both receptors on epidermal keratinocytes for the basement membrane protein laminin5, the major ligand for epidermal adhesion in mature skin. Ablation in mice of either α3β1 or α6β4, through null mutation of the gene encoding the α3, α6, or β4 integrin subunit, results in epidermal blistering of varying severity. Our previous studies showed that, despite blistering, differentiation and stratification of the epidermis appeared essentially normal in mice that lacked either α3β1 or α6β4. However, these studies did not definitively address the specific developmental importance of each integrin, since they may have overlapping and/or compensatory functions. Given the individual importance of α3β1 or α6β4 in maintaining the dermo-epidermal junction in mature skin, we sought to determine the importance of these integrins for embryonic skin development and epidermal morphogenesis. In the current study, we analyzed skin development in mutant embryos that completely lack both integrins α3β1 and α6β4. Although α3β1/α6β4-deficient embryos displayed epidermal blistering by stage E15.5 of development, they also retained regions of extensive epidermal adhesion to the basement membrane through stage E16.5, indicating alternative adhesion mechanisms. Apoptosis was induced in detached epidermis of α3β1/α6β4-deficient embryos, exemplifying vividly the importance of epithelial attachment to the basement membrane for cell survival. However, apoptotic cells were completely absent from attached epidermis of α3β1/α6β4deficient embryos, showing that epithelial adhesion that occurred independently of α3β1 and α6β4 also protected cells from apoptosis. Remarkably, in the absence of the known laminin-5 binding integrins (α3β1, α6β4, and α6β1), keratinocytes retained the capacity to proliferate in the epidermis, and epidermal stratification and skin morphogenesis appeared normal prior to blister formation. These findings show that while α3β1 and α6β4 are both required for integrity of the dermo-epidermal junction, neither one is essential for epidermal morphogenesis during skin development. SUMMARY