Maintenance of synaptic transmission at a mutant neuromuscular junction

At some synapses, such as the neuromuscular junction, the fidelity of transmission is of paramount importance. The purpose of the experiments presented here is to investigate the mechanisms by which the neuromuscular junction maximizes synaptic transmission. The model system used is the glutamatergic Drosophila larval neuromuscular junction, a synapse that exhibits homeostatic mechanisms to maintain synaptic function despite various perturbations. The starting point of the current studies is the characterization of a novel glutamate receptor gene, which we named Drosophila glutamate receptor III (DGluRIII). We found that this is an essential gene required for synaptic localization of the previously described DGluRIIA and IIB receptor subunits whereas DGluRIIA and IIB can substitute for one another and compete for access to DGluRIII. Interestingly, DGluRIIA and IIB have a tendency to preferentially localize opposite different classes of motor neurons, suggesting a capacity of the pre-synaptic neuron to specifically localize a particular subunit. We also found evidence of more than one populace of receptors at the synapse suggesting that these three subunits assemble in more than one configuration. A strong hypomorph of DGluRIII has extremely low levels of any glutamate receptors at the synapse; the post-synaptic response to glutamate is an order of magnitude smaller than normal. However, we found that the few receptors which are present are not distributed randomly; rather, certain sites have disproportionately high levels of receptor. Furthermore, we demonstrated that these enriched sites are located opposite the pre-synaptic active zones which have the highest relative probability of release, thereby maximizing synaptic function despite low receptor levels. In addition to this novel mechanism, we note that the DGluRIII mutants also upregulate pre-synaptic release to maintain synaptic function. These mechanisms are sufficient to maintain normal evoked synaptic events at low and moderate calcium levels. Surprisingly, however, at physiological calcium levels, these mechanisms are insufficient and the mutant synaptic events are significantly smaller than wild-type yet evidently still sufficient to drive the muscle to contraction (since these mutants are viable). It appears that this represents a "safety factor" in synaptic function whereby the normal synapse releases more transmitter than is actually required to drive muscle contraction.