At the Drosophila neuromuscular junction NMJ
At the Drosophila neuromuscular junction (NMJ), the loss or pharmacological inhibition of postsynaptic glutamate receptors induces a compensatory increase in presynaptic release that precisely offsets changes in postsynaptic receptor function and maintains normal synaptic excitation in muscle [7,8]. The homeostatic modulation of neurotransmitter release is achieved by parallel increases in presynaptic calcium influx through the CaV2.1 calcium channel  and a readily releasable pool (RRP) of synaptic vesicles .
Large-scale forward genetic screening has identified several Cucurbitacin I encoding presynaptic proteins that, when mutated, block presynaptic homeostasis without altering baseline anatomical or functional NMJ development [, , , ]. In contrast, limited information is currently available on how postsynaptic mechanisms contribute to NMJ synaptic homeostasis, for example, machinery sensing synaptic perturbations and the retrograde signal sent to elicit presynaptic responses.
Dystrobrevin (Dyb), a postsynaptic protein, is a component of the large Dystrophin Glycoprotein Complex (DGC) . It was found to interact with Dystrophin (Dys) at the Torpedo electric organ postsynaptic membrane . We previously reported roles of the postsynaptic localization of DLP2, the large Dys isoform expressed throughout muscle fibers, in maintaining normal levels of neurotransmitter release at the Drosophila NMJ . The absence of Dys-DLP2 from the subsynaptic reticulum (SSR) results in a high quantal content (QC; the number of transmitter quanta released per nerve impulse), increased numbers of T-bars (presynaptic release sites), and an elevated probability of release . These effects occur in the absence of visible alterations to the localization, density, and function of postsynaptic neurotransmitter receptors. The absence of Dys also results in reduced activity or levels of the Rho-GAP cross-veinless-c (Cv-c) , which enhances the conversion of active GTP-bound Rho-GTPases to their inactive GDP-bound forms . The Rho-GTPase, Cdc42, was identified as the substrate of Cv-c at the NMJ and evidence was obtained to show that elevated Cdc42 activity is required for increased QC . However, the potential roles of DGC and the Cdc42 pathway in homeostatic responses remain unclear.
We herein demonstrated that postsynaptic DGC plays an important role in the retrograde suppression of RRP sizes and is required for the expression of presynaptic homeostatic compensation, which occurs upon the acute inhibition of the postsynaptic glutamate receptor field. We extended our previous findings by showing that the loss of Dyb from the SSR results in the abnormal enhancement of presynaptic neurotransmitter release in the Dystrophin DLP2 mutant (Dys). We confirmed that Cdc42 is a primary postsynaptic target of this pathway and demonstrated that the postsynaptic expression of constitutively-active Cdc42 alone is sufficient to increase neurotransmitter release. We further showed that Cdc42 is enriched in the SSR, at which Dyb is also localized. Our results revealed a novel postsynaptic pathway of highly conserved proteins controlling presynaptic neurotransmitter release and homeostasis at synapses. The present result may contribute to identifying the mechanisms responsible for the synaptic impairments exhibited by a significant number of DMD patients.
Materials and methods
Discussion In the present study, we described a postsynaptic function for DGC within the SSR that influences synaptic strength and homeostatic plasticity at the Drosophila NMJ. We found that Dyb was present in pre- and postsynaptic compartments at the NMJ and its localization required Dys. Dyb is localized in the larval neuropile in a pattern that is very similar to that of the CNS-specific Dys Dp186 isoform, which is required in an identified motoneuron to regulate neurotransmitter release by upstream cholinergic interneurons . Thus, Dyb may also be proved to be part of a DGC-like complex in postsynaptic motoneuron dendrites.