Schmidt et al. reveal that microtubules help arrange acetylcholine receptors at the neuromuscular junction (NMJ).
The NMJ is where a nerve and a muscle meet, and acetylcholine receptors cluster in the membrane on the muscle side, opposite the nerve terminal. The protein agrin, which is secreted by the nerve terminal, promotes clustering by stimulating the expression of acetylcholine receptor genes in the muscle nuclei under the synapse and by spurring construction of a scaffold to which the receptors can attach. Schmidt et al. discovered that agrin also prompts receptors to bunch by stabilizing microtubules so that they form a network that points toward the NMJ.
These microtubules spur the addition of acetylcholine receptors to the clusters, the researchers found. For example, cultured muscle myotubes clump their receptors after exposure to agrin. But when the team depolymerized the myotube microtubules with nocodazole, the clusters were 60% smaller.
Schmidt et al. teased out how agrin affects microtubule behavior. Agrin switches on phosphatidylinositol 3-kinase, which blocks another kinase, glycogen synthase kinase-3β. This permits the protein CLASP2 to attach to the side of a microtubule instead of staying at its end, allowing the protein to remain bound to microtubule segments near the cell membrane. The researchers showed that CLASP2 and its partner CLIP-170 help stabilize microtubules and fasten them to the cell membrane where agrin clusters the acetylcholine receptors.