Sarcospan selectively interfaces with sarcoglycan subunits to stabilize the sarcolemma and prevent limb-girdle muscular dystrophy

Gene duplication events provide redundancy to complex organisms, and homologous genes and their encoded proteins are potential replacements to treat loss-of-function genetic syndromes. Skeletal muscle possesses redundant molecular mechanisms that partially or fully compensate for loss of gene function. Within the dystrophin-glycoprotein complex (DGC), utrophin is known to substitute for dystrophin. However, there has been little investigation of the orthologous relationships within the sarcoglycan subcomplex of the DGC. In skeletal muscle, the sarcoglycan complex canonically consists of alpha-, beta-, gamma- and delta-subunits, with gamma- and delta-sarcoglycan showing the greatest homology but no functional redundancy. We show that sarcospan, a transmembrane scaffolding protein, mediates assembly of a compensatory DGC with gamma-sarcoglycan replacement by zeta-sarcoglycan. This alternative complex improved pathology ingamma-sarcoglycan muscular dystrophy, but not alpha- or beta-sarcoglycan muscular dystrophy where sarcospan was unable to form compensatory complexes. Three-dimensional modeling of the compensatory DGC reveals that zeta-sarcoglycan maintains specific hydrophobic interactions with sarcospan and preserves overall quaternary arrangement. This compensatory DGC protects the cell membrane from contraction-induced damage and all secondary consequences of disease. These findings demonstrate a novel mechanism stabilizing the DGC by leveraging protein redundancy, with an important role for sarcospan in assembly and scaffolding of a compensatory DGC. Quadriceps muscles form wildtype, Sgcg (gamma-sarcoglycan deficient), and SgcgTG (gamma-sarcoglycan deficient witih trangenic sarcospan expression) mice were scap frozen in liquid nitrogen (n = 1 female mouse per genotype). 7-AAD+ nuclei were isolated from snap frozen muscle by FACS and submitted for RNA sequencing using the 10x genomics platform. *************************************************************** Submitter states that missing raw files are due to file loss. ***************************************************************