The Tight skin mouse:: Demonstration of mutant fibrillin-1 production and assembly into abnormal microfibrils

Mice carrying the Tight skin (Tsk) mutation harbor a genomic duplication within the fibrillin-1 (Fbn 1) gene that results in a larger than normal in-frame Fbn I transcript, In this study, the consequences of the Tsk mutation for fibrillin-containing microfibrils have been examined. Dermal fibroblasts from Tskl+ mice synthesized and secreted both normal fibrillin (similar to 330 kD) and the mutant oversized Tsk fibrillin-1 (similar to 450 kD) in comparable amounts, and Tsk fibrillin-1 was stably incorporated into cell layers. Immunohistochemical and ultrastructural analyses of normal and Tskl+ mouse skin highlighted differences in the gross organization and distribution of microfibrillar arrays, Rotary shadowing of high M-r preparations from Tskl+ skin demonstrated the presence of abundant beaded microfibrils. Some of these had normal morphology and periodicity, but others were distinguished by diffuse interbeads, longer periodicity, and tendency to aggregate, The presence of a structurally abnormal population of microfibrils in Tskl+ skin was unequivocally demonstrated after calcium chelation and in denaturating conditions, Scanning transmission electron microscopy highlighted the presence of more mass in Tskl+ skin microfibrils than in normal mice skin microfibrils, These data indicate that Tsk fibrillin-1 polymerizes and becomes incorporated into a discrete population of beaded microfibrils with altered molecular organization.