Muscular dystrophies are inherited disorders leading to progressive muscle destruction. Duchenne's disease and another type of muscular dystrophy result from genetic defects that affect the dystrophin-glycoprotein complex (DGC). The DGC is a network of proteins that contacts the muscle cell's outer membrane and transmits the muscle's contractile force to the extracellular matrix. The DGC also appears to play an important role in cell signalling. However, the precise contribution of the DGC to muscular degeneration remains elusive. A novel gene discovered by Steven McIntire's team at the University of California in San Francisco may provide a key towards the understanding of muscular dystrophy.
Neurotransmitters are chemicals that transmit information across the synapse, a specialized junction between two nerve cells or a nerve and a muscle cell. A nerve cell normally induces muscle contraction by releasing the neurotransmitter acetylcholine into the synaptic gap between the nerve cell and a muscle cell. This chemical stimulus was thought to be switched off by an enzyme that breaks down acetylcholine. Most other neurotransmitters are removed from the synapse by specific transporters. However, enzymatic breakdown of acetylcholine was considered to be so effective that nobody seriously considered the existence of a specific transporter for its clearance. McIntire's team now provides exciting evidence suggesting that a transporter removes acetylcholine from the neuromuscular synapse.
The roundworm, Caenorhabditis elegans, is a useful genetic model for human muscular dystrophies because disruption of DGC genes causes easily observable uncoordinated movements in the worms. The US team identified 12 mutant worms showing locomotory defects, with seven of these showing disruptions of known DGC genes. However, five other mutants exhibited mutations in a previously unidentified gene, SNF-6. After cloning and sequencing SNF-6, the team found that it is strikingly similar to mammalian genes for neurotransmitter transporters. Gene expression studies suggested that the SNF-6 protein might transport acetylcholine and clear it away from the synaptic gap. The team tested whether the newly discovered protein transports acetylcholine by measuring uptake of radiolabelled acetylcholine by mammalian cells expressing SNF-6. They observed a specific uptake of acetylcholine, which was saturable and dependent on the presence of sodium, indicating that the SNF-6 protein is indeed a sodium-dependent acetylcholine transporter.
McIntire and his colleagues suspected that the DGC mutants' uncoordinated locomotion might be due to loss of the SNF-6 transporter, resulting in elevated concentrations of acetylcholine at the neuromuscular synapse. The team showed that components of the DGC preserve the SNF-6 transporter at the synapse. Furthermore, DGC mutants have no SNF-6 at their neuromuscular junctions. These findings suggest that loss of DGCfunction leads to muscle degeneration because the SNF-6 acetylcholine transporter disappears. McIntire's team confirmed that when acetylcholine transporter function is disrupted, muscle degeneration typical of muscular dystrophy results. Thus, insufficient clearing of acetylcholine from the neuromuscular synapse may contribute to the pathogenesis of this disease.
Since basic muscle components are conserved in roundworms and humans,identification of an acetylcholine transporter orthologue in mammals may only be a question of time. Doubtless, its discovery would expedite development of new therapeutics to treat muscular dystrophy.