Muscle: an independent contributor to the neuromuscular spinal muscular atrophy disease phenotype

Abstract

Spinal muscular atrophy (SMA) is a pediatric-onset neuromuscular disorder caused by insufficient survival motor neuron (SMN) protein. SMN restorative therapies are now approved for the treatment of SMA; however, they are not curative, likely due to a combination of imperfect treatment timing, inadequate SMN augmentation, and failure to optimally target relevant organs. Here, we consider the implications of imperfect treatment administration, focusing specifically on outcomes for skeletal muscle. We examine the evidence that muscle plays a contributing role in driving neuromuscular dysfunction in SMA. Next, we discuss how SMN might regulate the health of myofibers and their progenitors. Finally, we speculate on therapeutic outcomes of failing to raise muscle SMN to healthful levels and present strategies to restore function to this tissue to ensure better treatment results.

Figure 1. Schematic of the chief cell- and non-cell-autonomous defects arising from low SMN in skeletal muscle tissue.

The loss of SMN1 specifically in muscle results in both cell- and non-cell-autonomous effects. In muscle, reduced SMN levels leads to central nucleation of myofibers, altered regeneration, dysfunctional myogenesis, myofiber dystrophy, and impaired acetylcholine receptor (AChR) clustering. In addition, loss of SMN in muscle results in fragmented endplates, neurofilament (NF) accumulation at nerve terminals, increased circulating levels of creatine kinase (CK), altered function of neuromuscular junctions (NMJs), and nerve and muscle degeneration. Figure panels were constructed by the authors from material either generated in the laboratory or created using BioRender.com.

Figure 2. Depletion of SMN specifically in skeletal muscle is sufficient to cause pathology.

(A) Confocal transverse section image of the calf muscle of a mutant mouse selectively depleted of the SMN protein in skeletal muscle tissue. Muscle cell–autonomous pathology is observed in the form of degenerating fibers penetrated by circulating IgG (asterisks), infiltrating microglia (arrows) and numerous myofibers containing abnormal, centrally positioned nuclei (arrowheads). Muscle was dual stained with antibodies against Iba-1 and mouse IgG to visualize microglia and damaged myofibers, respectively. Scale bar: 50 μm. (B) Kaplan-Meier survival curves depicting the correlation between SMN2 copies, and thus absolute SMN levels, in muscle and life span of the SMA mutants. P < 0.0001, log-rank test, n ≥ 16 mice of each cohort. (C) Enumeration of degenerating myofibers and cells harboring central nuclei in the gastrocnemius of mutants selectively depleted of SMN in skeletal muscle. Roughly nine times as many SMA fibers were found to display central nuclei compared with those that were degenerating (IgG-positive). *P < 0.05, ** P < 0.01, t tests, n ≥ 300 fibers from n ≥ 3 mice of each cohort. Panels adapted from Kim et al. (33).