Ultrastructural aspects of muscle and nerve in Werdnig-Hoffmann disease

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

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.

Differential induction of muscle atrophy pathways in two mouse models of spinal muscular atrophy

Motor neuron loss and neurogenic atrophy are hallmarks of spinal muscular atrophy (SMA), a leading genetic cause of infant deaths. Previous studies have focused on deciphering disease pathogenesis in motor neurons. However, a systematic evaluation of atrophy pathways in muscles is lacking. Here, we show that these pathways are differentially activated depending on severity of disease in two different SMA model mice. Although proteasomal degradation is induced in skeletal muscle of both models, autophagosomal degradation is present only in Smn(2B/-) mice but not in the more severe Smn(-/-); SMN2 mice. Expression of FoxO transcription factors, which regulate both proteasomal and autophagosomal degradation, is elevated in Smn(2B/-) muscle. Remarkably, administration of trichostatin A reversed all molecular changes associated with atrophy. Cardiac muscle also exhibits differential induction of atrophy between Smn(2B/-) and Smn(-/-); SMN2 mice, albeit in the opposite direction to that of skeletal muscle. Altogether, our work highlights the importance of cautious analysis of different mouse models of SMA as distinct patterns of atrophy induction are at play depending on disease severity. We also revealed that one of the beneficial impacts of trichostatin A on SMA model mice is via attenuation of muscle atrophy through reduction of FoxO expression to normal levels.

A Drosophila melanogaster model of spinal muscular atrophy reveals a function for SMN in striated muscle

Mutations in human survival motor neurons 1 (SMN1) cause spinal muscular atrophy (SMA) and are associated with defects in assembly of small nuclear ribonucleoproteins (snRNPs) in vitro. However, the etiological link between snRNPs and SMA is unclear. We have developed a Drosophila melanogaster system to model SMA in vivo. Larval-lethal Smn-null mutations show no detectable snRNP reduction, making it unlikely that these animals die from global snRNP deprivation. Hypomorphic mutations in Smn reduce dSMN protein levels in the adult thorax, causing flightlessness and acute muscular atrophy. Mutant flight muscle motoneurons display pronounced axon routing and arborization defects. Moreover, Smn mutant myofibers fail to form thin filaments and phenocopy null mutations in Act88F, which is the flight muscle-specific actin isoform. In wild-type muscles, dSMN colocalizes with sarcomeric actin and forms a complex with alpha-actinin, the thin filament crosslinker. The sarcomeric localization of Smn is conserved in mouse myofibrils. These observations suggest a muscle-specific function for SMN and underline the importance of this tissue in modulating SMA severity.

Chronic progressive and relapsing neuromyopathy with massive dilatations of endoplasmic reticulum in muscle fibers

Large intracytoplasmic inclusions arising from the endoplasmic reticulum and nuclear envelope were found in the muscle fibers of a 32-year-old individual with a life-long, chronic and progressive sensory-motor neuropathy. The morphological features of the inclusions were similar to that occasionally reported in the striated muscles in several unrelated conditions as well as to "hyaline" or "colloid" inclusions commonly seen in motor neurons of the brain stem and spinal cord. The chemical nature of the inclusions is not known. Their occurrence in the muscle fibers is probably secondary to chronic denervation.

Pi granules and related intracytoplasmic inclusions in equine Schwann cells

Suchwann cells from a variety of nerves in two adult horses and one adult pony contained perinuclear intracytoplasmic inclusion complexes consisting of lipid droplets, variably electron-dense rounded to elongated bodies and rod-shaped multilamellar structures. The latter were characteristic of pi granules of Reich. There were no significant axonal or myelin alterations associated with these inclusions. It was concluded that the inclusions are a component of normal equine Schwann cells.

Is Kugelberg-Welander spinal muscular atrophy a fetal defect?

Twelve children who had developed Kugelberg-Welander (K-W) spinal muscular atrophy (SMA) before the age of six years were investigated. Electrophysiological, histochemical, ultrastructural, and biochemical studies demonstrated features of immature muscle fibers suggesting a fetal defect as in the Werdnig-Hoffmann (W-H) form of SMA. Comparison with known patterns of human myogenesis and of experimental denervation of immature muscle suggested that the defect in K-W SMA probably takes place in fetal life at a stage later than in W-H SMA. In contrast to W-H SMA, a considerable percentage of mature fibers of normal structure and diameter were present. The immature fibers impair the normal development of muscle cells by preventing the increase in number of mature fibers and causing an overloading of the normal fibers with consequent hypertrophy and eventually destructive changes. This is the pattern in K-W disease that distinguishes the juvenile form of SMA from W-H disease.

Nemaline bodies in a progressive infantile neuromuscular disorder

Nemaline bodies are described in a case of a progressive neuromuscular disorder, which is difficult to classify. The clinical syndrome is not characteristic of a nemaline myopathy. It is argued that the finding of nemaline bodies is in itself not specific of any neuromuscular disease.