Targeting transcriptional and translational mechanisms to enhance utrophin A expression as a therapy for Duchenne muscular dystrophy

Abstract

Duchenne Muscular Dystrophy (DMD) is a fatal, neuromuscular disorder caused by mutations/deletions in the dystrophin gene. In skeletal muscle, dystrophin is expressed along the sarcolemma, providing a mechanical link between the cytoskeleton and the extracellular matrix; loss of dystrophin results in disrupted sarcolemmal integrity and progressive muscle wasting. Utrophin is the autosomal homologue of dystrophin and is present in skeletal muscles of DMD patients, although its expression is primarily restricted to the neuromuscular junction of mature skeletal muscle fibers. Enhancing expression of utrophin in skeletal muscles of DMD animal models prevents the dystrophic pathology; thus, it is of considerable interest to identify the mechanisms controlling utrophin expression in skeletal muscle, with the ultimate goal of identifying drugs that can stimulate utrophin levels in DMD patients. Utrophin A expression is regulated by transcriptional mechanisms that are associated with the slow myogenic program. Here we uncovered that expression of utrophin A can be transcriptionally enhanced via its promoter by GW501516-mediated activation of the nuclear receptor PPARbeta/delta, a known regulator of the slow myogenic program. Treatment of dystrophin-deficient mdx mice with GW501516 stimulated utrophin A expression at the sarcolemma and mitigated the dystrophic phenotype, as evidenced by increased sarcolemmal stability and decreased susceptibility of skeletal muscles to lengthening contractions. Under multiple physiologically relevant conditions, enhancement of utrophin A protein levels in skeletal muscle are not accompanied by similar increases in mRNA levels, suggesting a role for post-transcriptional control. We identified that utrophin A expression is enhanced via Internal Ribosome Entry Site (IRES)-mediated translation in response to skeletal-muscle regeneration and glucocorticoid treatment. Using transgenic reporter mice, we found that IRES-mediated translation of utrophin A occurs exclusively in skeletal muscle fibers and no other tissues. Finally, we identified that a trans-acting factor, eEF1A2, interacts with the utrophin A 5'-UTR and is involved in regulating IRES-mediated translation of utrophin A. This work has identified novel transcriptional and translational mechanisms controlling expression of utrophin A in skeletal muscle. Our results suggest that GW501516 treatment might constitute a viable pharmacological therapy for DMD patients and IRES-mediated translational of utrophin A could serve as a new therapeutic target.