Motor nerves activate muscles via the release of acetylcholine, a neurotransmitter that binds to muscle cells' acetylcholine receptors. The occupied receptors then allow Na+ ions into the muscle cells,depolarizing them and causing them to contract. This transmission of signals across the neuromuscular junction is most efficient when the receptors are positioned close to the sites of acetylcholine release, so it is not surprising that the receptors form clusters at these locations. But how do they `know' where to congregate? A recent study of the nematode Caenorhabditis elegans by Christelle Gally and colleagues shows that a protein known as LEV-10 enables acetylcholine receptors to cluster properly,at least in this particular organism.
Gally and her co-workers at l'École Normale Supériere (Paris)and the University of Illinois at Chicago studied worms with a mutation in their lev-10 gene. The researchers initially observed that the mutants' mobility was somewhat impaired. When wild-type and mutant worms were compared in a `thrashing assay' – a common motor performance test in which worms are induced to thrash by dunking them in a solution of the neurotransmitter serotonin – the frequency of body bends per minute was about 25% lower than normal in the mutants.
Why were the lev-10 mutants' thrashing abilities impaired? Gally's group suspected a problem with a type of acetylcholine receptor that happens to be sensitive to the drug levamisole. They found that the mutants synthesize normal amounts of intact, functional levamisole-sensitive receptors; however,these receptors fail to cluster near acetylcholine release sites, dampening the muscles' responsiveness to the nervous system. Since the mutants cannot produce LEV-10, the researchers concluded that LEV-10 is required for normal clustering of these receptors.
How does LEV-10, a transmembrane protein, direct the clustering of acetylcholine receptors in wild-type worms? The mechanism must involve LEV-10's extracellular region because this region is sufficient to cause clustering, as Gally and co-workers found when they provided lev-10mutants with truncated versions of the protein. The extracellular region contains one LDLa domain and five CUB domains, which are thought to promote binding to other proteins. However, the LEV-10 protein does not appear to bind directly to acetylcholine receptors. Therefore, the researchers speculate,`LEV-10 might be indirectly involved in the recruitment of signaling molecules that, in turn, cause acetylcholine receptor clustering.' The molecules that initially draw LEV-10 to the cluster sites remain to be identified.
While the importance of LEV-10 itself may be limited to C. elegans, there are hints that analogous proteins may control the positioning of receptors within vertebrate nervous systems. Specifically, the mouse protein NETO2 has two extracellular CUB domains with amino acid sequences similar to those of LEV-10, as well as one extracellular LDLa domain. Although the functions of this and related mouse proteins are unknown,it is possible that they too promote the clustering of neurotransmitter receptors via extracellular interactions with other proteins.