Synaptic Plasticity

Axonal sprouting and synaptic rewiring are key regulators of neuronal plasticity in the developing and adult brain. Similar to many species, modulation of synapse formation in Drosophila has been implicated in learning and memory. Synapse formation requires coordinated signaling to orchestrate pre- and post-synaptic maturation of synaptic connections.   Although communication from the presynaptic neuron via Ca²⁺ -dependent synaptic vesicle fusion has been well characterized, the molecular mechanisms that allow postsynaptic targets to transmit retrograde signals are relatively unknown. Both acute forms of synaptic plasticity such as LTP, as well as chronic homeostatic synaptic changes, have been suggested to require retrograde signals for their induction. To help elucidate the mechanisms underlying synapse formation and subsequent synaptic growth, as well as to characterize how the regulation of these events modulates plasticity and behavior, we have used forward and reverse genetic approaches to identify molecular pathways essential to the process.  As a starting point, Moto Yoshihara characterized the role of neuronal activity in synapse formation using the embryonic Drosophila glutamatergic neuromuscular junction (NMJ) as a model. Removal of postsynaptic glutamate receptors eliminates synaptic responses from the NMJ. Lack of postsynaptic activity and subsequent Ca²⁺ influx through glutamate receptors leads to a presynaptic defect in synapse formation, with the maintenance of immature growth cone-like presynaptic motor terminals.  In addition to promoting synaptic growth, postsynaptic activity and Ca²⁺ influx through glutamate receptors triggers a 100-fold induction of presynaptic miniature release following high frequency stimulation. This enhanced presynaptic function following high frequency stimulation requires retrograde signaling downstream of postsynaptic Ca²⁺ entry and activation of the presynaptic cAMP pathway.  These findings have revealed that Drosophila embryonic NMJs undergo robust stimulation-dependent synaptic plasticity that is initiated by postsynaptic Ca²⁺ influx and the release of retrograde signals that activate PKA and enhance presynaptic release. 

Ca²⁺-induced vesicle fusion in presynaptic terminals provides a temporally controlled and spatially restricted signal essential for synaptic communication. Analogous to presynaptic release, postsynaptic vesicles within dendrites have been visualized by TEM, and dendritic release of several neuromodulators has been reported.  Recent studies have also shown that postsynaptic vesicles undergo rapid Ca²⁺-triggered exocytosis that is required for LTP in the mammalian hippocampus. We postulated that postsynaptic vesicle fusion might underlie the Ca²⁺-dependent release of retrograde signals that triggers altered presynaptic function and synaptic growth at Drosophila NMJs. The synaptic vesicle protein Syt 1 functions as the major Ca²⁺ sensor for vesicle fusion at presynaptic terminals, but is not localized postsynaptically.  Recently, we have found that another isoform of the synaptotagmin family, Syt 4, is present in the postsynaptic compartment (Adolfsen et al., 2004). Mutational analysis of Syt 4 indicates it functions as a postsynaptic Ca²⁺ sensor to release retrograde signals that stimulate enhanced presynaptic function. In addition to acute changes in presynaptic function, postsynaptic Ca²⁺ influx also stimulates local synaptic differentiation and growth through Syt 4-mediated retrograde signals that activate the presynaptic cAMP pathway in a synapse specific manner. These findings indicate Ca²⁺-dependent retrograde vesicular trafficking initiates an acute change in synaptic function that is converted to synapse-specific growth, providing an important link between synaptic plasticity and activity-dependent rewiring of neuronal connections.

The finding that retrograde signaling at synapses is mediated by Ca²⁺-regulated postsynaptic vesicular fusion represents a shift in the way we think about synaptic information transfer, and we are continuing to characterize the molecular components that regulate postsynaptic vesicle fusion, as well as the retrograde signals involved.  Syt 4 expression is bi-directionally regulated by neuronal activity and modulates synaptic growth and plasticity as a function of its postsynaptic levels. Postsynaptic expression of Syt 4 promotes synaptic proliferation and enhanced activity-dependent presynaptic neurotransmitter release, and requires Ca²⁺ binding by both C2 domains. Syt 4 also contributes to activity-dependent structural plasticity at neuromuscular junctions. Our data indicate that postsynaptic Syt 4 regulates activity-dependent release of postsynaptic retrograde signals that promote synaptic plasticity, similar to the role of Syt 1 as a Ca²⁺-sensor for presynaptic vesicle fusion.