Biglycan recruits utrophin to the sarcolemma and counters dystrophic pathology in mdx mice

Abstract

Duchenne muscular dystrophy (DMD) is caused by mutations in dystrophin and the subsequent disruption of the dystrophin-associated protein complex (DAPC). Utrophin is a dystrophin homolog expressed at high levels in developing muscle that is an attractive target for DMD therapy. Here we show that the extracellular matrix protein biglycan regulates utrophin expression in immature muscle and that recombinant human biglycan (rhBGN) increases utrophin expression in cultured myotubes. Systemically delivered rhBGN upregulates utrophin at the sarcolemma and reduces muscle pathology in the mdx mouse model of DMD. RhBGN treatment also improves muscle function as judged by reduced susceptibility to eccentric contraction-induced injury. Utrophin is required for the rhBGN therapeutic effect. Several lines of evidence indicate that biglycan acts by recruiting utrophin protein to the muscle membrane. RhBGN is well tolerated in animals dosed for as long as 3 months. We propose that rhBGN could be a therapy for DMD.biotherapeutics | protein therapeutics D uchenne muscular dystrophy (DMD) is a hereditary disease that affects ∼1:3,500 boys, the majority of whom will die by their midtwenties (1). DMD is caused by mutations in dystrophin that result in the faulty assembly and function of an ensemble of structural and signaling molecules at the muscle cell surface termed the dystrophin-associated protein complex (DAPC) (2-4). There are currently no treatments that target the primary pathology of DMD.One attractive therapeutic approach for DMD is the stabilization of the muscle cell membrane through up-regulation of utrophin, a dystrophin homolog. Transgenic overexpression of utrophin rescues dystrophic pathology and restores function in the dystrophin-deficient mdx mouse (5-7). In mature muscle, utrophin expression is restricted to the neuromuscular and myotendinous junctions. However, utrophin is expressed over the entire myofiber in developing and regenerating muscle (8-10). These observations raise the possibility that marshalling pathways that normally regulate utrophin expression in developing muscle could be a productive approach for developing DMD treatments.The extracellular matrix protein biglycan plays an important role in developing muscle. In both humans and mice, biglycan is most highly expressed in immature and regenerating muscle (11,12). Biglycan is a component of the DAPC, where it binds to α-dystroglycan (13) and α-and γ-sarcoglycan (14). Biglycan regulates the expression of the sarcoglycans as well as dystrobrevins, syntrophins, and nNOS, particularly in immature muscle. Finally, biglycan is important for timely muscle regeneration (11).Locally delivered recombinant human biglycan (rhBGN) incorporates into the extracellular matrix of bgn −/o muscle where it persists for at least 2 wk and rescues the expression of several DAPC components (15). These results raise the possibility that rhBGN might enhance function in muscle that lacks dystrophin. Here we show that utrophin is down-regulated in immature biglycan null (...

Large-scale opening of utrophin’s tandem calponin homology (CH) domains upon actin binding by an induced-fit mechanism

“…It is endogenously found in fetal or regenerating muscle but is replaced by dystrophin, the defective protein in Duchenne and Becker muscular dystrophies, as the muscle matures (2). Up-regulation of utrophin in mdx mice, which lack dystrophin, has been shown to rescue its dystrophic phenotype, improving muscle morphology and function (1,3). The full-length protein is not required to improve dystrophic pathology in mdx mice; i.e., substantial internal truncations in utrophin can be tolerated (4).…”

From Basic Research to Clinical Trials: Preclinical Trial Evaluation in Mouse Models

“…These small moleculaes included heregulin (Krag et al 2004) and biglycan (Amenta et al 2011). In general, these utrophin-based up-regulation strategies resulted in very similar benefits in the mdx mouse.…”

The Biology of Small Leucine-rich Proteoglycans in Bone Pathophysiology

“…The above studies support utilization of biglycan therapy as a credible therapeutic venue to reduce or block degenerative bone diseases. Proof of principle for the possibility of such an application derives from a recent study showing that injections of biglycan into dystrophin-deficient mdx mice can repair numerous defects that mimic muscular dystrophy, including the improvement of muscle function (94). In this regard, it can be noted that exogenously applied recombinant biglycan could "rescue" impaired LRP6 signaling caused by a mutation in the extracellular domain of this Wnt receptor (54).…”