Ultra-structural time-course study in theC. elegansmodel for Duchenne muscular dystrophy highlights a crucial role for sarcomere-anchoring structures and sarcolemma integrity in the earliest steps of the muscle degeneration process

Abstract

Duchenne muscular dystrophy (DMD) is a genetic disease characterized by progressive muscle degeneration due to mutations in the dystrophin gene. In spite of great advances in the design of curative treatments, most patients currently receive palliative therapies with steroid molecules such as prednisone or deflazacort thought to act through their immunosuppressive properties. These molecules only slightly slow down the progression of the disease and lead to severe side effects. Fundamental research is still needed to reveal the mechanisms involved in the disease that could be exploited as therapeutic targets. By studying a Caenorhabditis elegans model for DMD, we show here that dystrophin-dependent muscle degeneration is likely to be cell autonomous and affects the muscle cells the most involved in locomotion. We demonstrate that muscle degeneration is dependent on exercise and force production. Exhaustive studies by electron microscopy allowed establishing for the first time the chronology of subcellular events occurring during the entire process of muscle degeneration. This chronology highlighted the crucial role for dystrophin in stabilizing sarcomeric anchoring structures and the sarcolemma. Our results suggest that the disruption of sarcomeric anchoring structures and sarcolemma integrity, observed at the onset of the muscle degeneration process, triggers subcellular consequences that lead to muscle cell death. An ultra-structural analysis of muscle biopsies from DMD patients suggested that the chronology of subcellular events established in C. elegans models the pathogenesis in human. Finally, we found that the loss of sarcolemma integrity was greatly reduced after prednisone treatment suggesting a role for this molecule in plasma membrane stabilization.

Development, structure, and maintenance of C. elegans body wall muscle

“…However, the amount of muscle cell death (referred hereafter as muscle degeneration) significantly increased when a dys-1 loss-of-function mutation was combined with a hypomorphic mutation in the hlh-1 gene (Gieseler et al, 2000). In addition, muscle degeneration Development, structure, and maintenance of C. elegans body wall muscle increases in both the dys-1(cx18) single mutant and the dys-1(cx18); hlh-1(cc561) double mutant with higher body bending forces on firm culture medium, supporting the hypothesis of a mechanical role for DYS-1 (Brouilly et al, 2015). This result is in agreement with studies performed in dystrophin deficient mdx mice that revealed a higher vulnerability and enhanced dystrophic progression in response to exercise (Brussee et al, 1997;De Luca et al, 2003).…”

“…The contribution of mechanical forces and movement to DYS-1 dependent muscle degeneration is further supported by a reduction of muscle degeneration in adult dys-1; hlh-1 double mutants when sarcomere contraction is inhibited by mutations in different genes affecting the myofilament lattice (Mariol et al, 2007), RNAi mediated knock down of the EGL-19 L-type voltage gated Ca 2+ channel (Mariol and Ségalat 2001), or after forced immobilization by treatment with muscimol, a GABAA agonist and anesthetic used to paralyze C. elegans (Brouilly et al, 2015). Similarly, limb immobilization or treatment with muscle relaxants prevent and reduce the occurrence of muscle degeneration in mdx mice (Mokhtarian et al, 1999).…”

“…The C. elegans dense body fulfills the function of both vertebrate Z-disks and costameres and plays a role in transducing the force generated by sarcomeres to the extracellular matrix (Figure 1A and B; Figure 5). In Y2H experiments, DYS-1 interacts with the vinculin ortholog DEB-1, which localizes at the bases of dense bodies (Brouilly et al, 2015). This suggests that DYS-1 links the DGC to the dense body.…”

“…The dys-1 gene is expressed in pharyngeal, vulval and body wall muscles of C. elegans (Bessou et al, 1998). Sub-cellular analyses using a monoclonal antibody to DYS-1 or a DYS-1::GFP expressing strain showed that DYS-1 localizes in body wall muscles similarly to its vertebrate counterparts-under the sarcolemma of striated body wall muscle, at I bands near the dense bodies, and in muscle arms (the postsynaptic elements of the NMJ in C. elegans muscle) (Dixon and Roy, 2005;Brouilly et al, 2015).…”

Trends in tissue repair and regeneration

“…Michel Labouesse (IBPS, Paris, France) showed that during embryonic elongation driven by muscle contraction, the apical extracellular matrix contributes to maintaining tissue integrity and distributing stress. He also provided insights into the degeneration of muscle cells induced by physical exercise in dystrophin mutants (Brouilly et al, 2015). Sara Wickström (MPI, Cologne, Germany) showed how extrinsic forces lead to attenuation of transcription, chromatin changes and silencing of Polycomb target genes in human epidermal progenitor cells (Le et al, 2016).…”