alpha 3 beta 1 integrin is required for normal development of the epidermal basement membrane

Integrins alpha 3 beta 1 and alpha 6 beta 4 are abundant receptors on keratinocytes for laminin-5, a major component of the basement membrane between the epidermis and the dermis in skin. These integrins are recruited to distinct adhesion structures within keratinocytes; alpha 6 beta 4 is present in hemidesmosomes, while alpha 3 beta 1 is recruited into focal contacts in cultured cells. To determine whether differences in localization reflect distinct functions of these integrins in the epidermis, we studied skin development in alpha 3 beta 1-deficient mice. Examination of extracellular matrix by immunofluorescence microscopy and electron microscopy revealed regions of disorganized basement membrane in alpha 3 beta 1-deficient skin. Disorganized matrix was first detected by day 15.5 of embryonic development and became progressively more extensive as development proceeded. In neonatal skin, matrix disorganization was frequently accompanied by blistering at the dermal-epidermal junction. Laminin-5 and other matrix proteins remained associated with both the dermal and epidermal sides of blisters, suggesting rupture of the basement membrane itself, rather than detachment of the epidermis from the basement membrane as occurs in some blistering disorders such as epidermolysis bullosa. Consistent with this notion, primary keratinocytes from alpha 3 beta 1-deficient skin adhered to laminin-5 through alpha 6 integrins. However, alpha 3 beta 1-deficient keratinocytes spread poorly compared with wild-type cells on laminin-5, demonstrating a postattachment requirement for alpha 3 beta 1 and indicating distinct roles for alpha 3 beta 1 and alpha 6 beta 4. Our findings support a novel role for alpha 3 beta 1 in establishment andior maintenance of basement membrane integrity, while alpha 6 beta 4 is required for stable adhesion of the epidermis to the basement membrane through hemidesmosomes.