Reduced expression of nicotinic AChRs in myotubes from spinal muscular atrophy I patients.

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by degeneration of motoneurons and skeletal muscle atrophy. In its most severe form, it leads to death before the age of 2 years. While primary degeneration of motor neurons is well established in this disease, and this results in neurogenic atrophy of skeletal muscle, we have previously reported evidence for a primary muscle defect. In this study, we used primary cultures of embryonic human skeletal muscle cells from patients with SMA and from controls to examine the effects of muscle fiber differentiation in the absence of a nerve component. Cultured SMA skeletal muscle cells are unable to fuse correctly to form multinuclear myotubes, the precursors of the myofibers. We also show that agrin-induced aggregates of nicotinic acetylcholine receptors, one of the earliest steps of neuromuscular junction formation, cannot be visualized by confocal microscopy on cells from SMA patients. In binding experiments, we demonstrate that this lack of clustering is due to defective expression of the nicotinic acetylcholine receptors in the myotubes of SMA patients whereas the affinity of alpha-bungarotoxin for its receptor remains unchanged regardless of muscle cell type (SMA or control). These observations suggest that muscle cells from SMA patients have intrinsic abnormalities that may affect proper formation of the neuromuscular junction.