Significance of the extracellular cleavage of BP180/type XVII collagen for skin biology and disease

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Hemidesmosomes mediate adhesion of complex and stratified epithelia to basement membranes. Bullous pemphigoid antigen 180 (BP180)/type XVII collagen is a hemidesmosomal transmembrane protein and a major component of anchoring filaments. Both an autoimmune response to BP180 and genetic defects of the protein result in subepidermal blistering emphasizing its importance for the integrity of the basement membrane zone. A 120 kDa extracellular portion of BP180 can be shed from the cell surface as a result of proteolytic cleavage. We recently found the cleavage to be mediated by a membrane-associated matrix metalloproteinase and to localize within the membraneproximal NC16A domain of BP180. This site associates with the a6 subunit of a6b4 integrin and represents the most immunogenic portion of BP180. The aim of the present proposal is to examine the physiological and pathological significance of the cleavage process. The cleavage will be reproduced by enzymatic elimination of the extracellular collagenous portion in cultured keratinocytes and its effect on the interaction between hemidesmosomal components will be studied using specific monoclonal antibodies to BP230, a6b4 integrin, and laminin 5. By sitedirectedmutagenesis, amino acid changes will be introduced within the NC16A domain near the cleavage site. Keratinocytes from patients lacking BP180 (GABEB) will be transfected with these mutant forms of BP180 to determine if the cleavage can be abolished. The mutant BP180 will also be used to study the relevance of the cleavage for cell migration and differentiation. Finally, the effect of the cleavage process on the reactivity of patients' autoantibodies to the NC16A domain of BP180 will be investigated. These studies should improve our understanding of anchoring mechanisms of basement membrane zones.