Neuroscience Online (i,9,4)

Much of our knowledge about synapse formation comes from studies on nerve-muscle regeneration. Basal lamina patches that are allowed to contact damaged muscle tissue induce the regenerating myotubes to form postsynaptic specializations. These studies suggested that there is an AChR clustering agent in the basal lamina. Agrin was purified from basal lamina extracts as a molecule that is capable of inducing formation of clusters of AChRs on myotubes. Further study showed that agrin is present in motor neurons, is transported down motor neuron axons and is retained in the synaptic basal lamina. Antibodies against agrin inhibit AChR clustering. Agrin deficient mice have disorganized neuromuscular junctions and have decreased number, size, and density of AChRs at the neuromuscular junction.Thus, agrin appears to be a nerve-derived factor that organizes postsynaptic differentiation. Agrin may bind to several candidate receptors including alpha-distroglycan, heparan sulfate proteoglycans, and a muscle-specific tyrosine kinase (MUSK) to produce its effects.

Figure 9.20

Figure 9.21

Figure 9.22

The synaptic basal lamina possesses proteins that act in pre- and postsynaptic differentiation. A cross section of a normal muscle that contains a nerve terminal directly opposite from the AChRs on the junctional fold of the muscle is shown in Figures 9.20 - 9.22. The basal lamina (BL) ensheaths the myofiber, while the synaptic basal lamina surounds the synaptic site. The BL serves as a scaffold for regenerating myofibers. In the experiment shown, axons innervating the muscle were damaged and prevented from regenerating and the muscle was injured. New myofibers regenerate within the BL of the original myofiber in the absence of the nerve. In addition, AChRs cluster despite the absence of the nerve. This result shows that the synaptic BL contains signals for clustering AChRs (postsynaptic differentiation). If the motor axon and the muscle are damaged and the muscle is prevented from regenerating, axons regenerate to the original synaptic site in the absence of their targets. Thus, the synaptic BL contains signals for presynaptic differentiation.

The muscle plasma membrane has AChRs on its surface, which are uniformly distributed before the motor nerve growth cone arrives. Shortly after innervation, the distribution of extrajunctional receptors decreases and the density of receptors at the newly forming neuromuscular synapse increases dramatically. This effect occurs precisely at the site where the growth cone releases ACh. These changes occur by the redistribution of existing receptors and the synthesis and insertion of new receptors at synapses.

Figure 9.23

The axon of the motor neuron induces ACh receptors to cluster on muscle fibers. As the axon approaches the muscle fiber most of the AChRs are distributed diffusely on its surface. When the axon begins to establish synaptic transmission, the distribution of AChRs begins to change. AChRs begin to cluster at the newly formed synaptic contact, while extrasynaptic receptors decrease dramatically.

What factors are responsible for the changes in AChR density during nerve-muscle formation? It is NOT the release of ACh from nerve terminals. Increased AChR density during neuromuscular junction formation results from release of molecules from motor nerve terminals, including AChR inducing activity (ARIA), a molecule that promotes an increase in total AChR and receptor clusters, and Agrin, a protein that induces clustering of pre-existing AChRs and other components of the NMJ.

During nerve-muscle synapse formation, these biological properties of AChRs change after innervation: