Cleavage of ß-dystroglycan occurs in sarcoglycan-deficient skeletal muscle without MMP-2 and MMP-9.

ABSTRACT

BACKGROUND: The dystroglycan complex consists of two subunits extracellular α-dystroglycan and membrane-spanning ß-dystroglycan, which provide a tight link between the extracellular matrix and the intracellular cytoskeleton. Previous studies showed that 43 kDa ß-dystroglycan is proteolytically cleaved into the 30 kDa fragment by matrix metalloproteinases (MMPs) in various non-muscle tissues, whereas it is protected from cleavage in muscles by the sarcoglycan complex which resides close to the dystroglycan complex. It is noteworthy that cleaved ß-dystroglycan is detected in muscles from patients with sarcoglycanopathy, sarcoglycan-deficient muscular dystrophy. In vitro assays using protease inhibitors suggest that both MMP-2 and MMP-9 contribute to the cleavage of ß-dystroglycan. However, this has remained uninvestigated in vivo. METHODS: We generated triple-knockout (TKO) mice targeting MMP-2, MMP-9 and γ-sarcoglycan to examine the status of ß-dystroglycan cleavage in the absence of the candidate matrix metalloproteinases in sarcoglycan-deficient muscles. RESULTS: Unexpectedly, ß-dystroglycan was cleaved in muscles from TKO mice. Muscle pathology was not ameliorated but worsened in TKO mice compared with γ-sarcoglycan single-knockout mice. The gene expression of MMP-14 was up-regulated in TKO mice as well as in γ-sarcoglycan knockout mice. In vitro assay showed MMP-14 is capable to cleave ß-dystroglycan. CONCLUSIONS: Double-targeting of MMP-2 and MMP-9 cannot prevent cleavage of ß-dystroglycan in sarcoglycanopathy. Thus, matrix metalloproteinases contributing to ß-dystroglycan cleavage are redundant, and MMP-14 could participate in the pathogenesis of sarcoglycanopathy.