Mutant fibrillin 1 from tight skin mice increases extracellular matrix incorporation of microfibril-associated glycoprotein 2 and type I collagen.

OBJECTIVE: Skin fibrosis in the TSK mouse, a model of skin fibrosis seen in systemic sclerosis (SSc), is caused by a large in-frame duplication in the Fbn1 gene, tsk-Fbn1. We investigated whether tsk-Fbn1 might cause dermal fibrosis by affecting Fbn1 and associated extracellular matrices. We also studied whether deposition of microfibril-associated glycoprotein 2 (MAGP-2), a protein that is associated with fibrillin 1, was altered in the skin of patients with SSc.
METHODS: An in vitro model of the TSK mouse was created by conditionally expressing tsk-Fbn1 in mouse embryonic fibroblasts (MEFs). Cell cultures were examined by immunofluorescence and Western and Northern blotting to determine the effect of tsk-Fbn1 on the structure, expression, and deposition of fibrillin 1 (Fbn-1), type I collagen, and MAGP-2. The skin of TSK mice and SSc patients was analyzed by immunohistochemistry for MAGP-2 expression.
RESULTS: Expression of tsk-Fbn1 in cultured MEF cells altered the morphology of Fbn-1 fibers and increased the deposition of type I collagen into the extracellular matrix (ECM) without concomitantly changing messenger RNA expression, secretion, or processing of type I procollagen. Moreover, MEF cells expressing tsk-Fbn1 showed increased MAGP-2 matrix. MAGP-2 was increased in the dermis of TSK mice. Fibrotic SSc skin also showed higher levels of MAGP-2 in the dermis than nonfibrotic SSc skin and normal skin.
CONCLUSION: Tsk-Fbn1 altered ECM organization and caused fibrosis by affecting the deposition of MAGP-2 or other Fbn-1-associated proteins. Alterations in microfibril structure or deposition might contribute to fibrosis in SSc.