Input-Specific Plasticity and Homeostasis at the Drosophila Larval Neuromuscular Junction

Abstract

Summary Synaptic connections undergo activity-dependent plasticity during development and learning, as well as homeostatic re-adjustment to ensure stability. Little is known about the relationship between these processes, particularly in vivo. We addressed this with novel quantal resolution imaging of transmission during locomotive behavior at glutamatergic synapses of the Drosophila larval neuromuscular junction. We find that two motor input types – Ib and Is – provide distinct forms of excitatory drive during crawling and differ in key transmission properties. While both inputs vary in transmission probability, active Is synapses are more reliable. High frequency firing “wakes up” silent Ib synapses and depresses Is synapses. Strikingly, homeostatic compensation in presynaptic strength only occurs at Ib synapses. This specialization is associated with distinct regulation of postsynaptic CaMKII. Thus, basal synaptic strength, short-term plasticity and homeostasis are determined input-specifically, generating a functional diversity that sculpts excitatory transmission and behavioral function.

Disparate Postsynaptic Induction Mechanisms Ultimately Converge to Drive the Retrograde Enhancement of Presynaptic Efficacy

“…In particular, postsynaptic overexpression of a constitutively active form of CamKII blocks PHP expression in GluRIIA mutants. More recently, a local reduction in phosphorylated (active) CamKII (pCamKII) in the postsynaptic compartment was observed in GluRIIA mutants (Newman et al, 2017). However, pCamKII levels have never been examined in the context of PhTx or Tor-OE forms of PHP signaling.…”

“…However, we found that neither mEPSP amplitude nor frequency is significantly changed when calcium is absent or calcium concentrations are elevated in the extracellular saline (Figure S1). Next, we imaged postsynaptic calcium influx using a genetically encoded calcium indictor (SynapGCaMP6f) targeted to postsynaptic glutamate receptors at the Drosophila NMJ (Newman et al, 2017). This reporter is capable of detecting calcium transients during individual mEPSP events (Newman et al, 2017), but the response during mEPSP events across varying ranges of extracellular calcium has not been determined.…”

“…Next, we imaged postsynaptic calcium influx using a genetically encoded calcium indictor (SynapGCaMP6f) targeted to postsynaptic glutamate receptors at the Drosophila NMJ (Newman et al, 2017). This reporter is capable of detecting calcium transients during individual mEPSP events (Newman et al, 2017), but the response during mEPSP events across varying ranges of extracellular calcium has not been determined. We therefore imaged the SynapGCaMP6f response during miniature activity in four extracellular calcium concentrations: 0 mM, 1.5 mM, 5 mM, and 10 mM (Figure S2).…”

“…At the Drosophila NMJ, transgenic manipulations that impact postsynaptic CamKII activity have been reported to modulate the expression of PHP in GluRIIA mutants (Haghighi et al, 2003; Newman et al, 2017). Our results indicate that pCamKII levels are reduced to similar levels in GluRIIA mutants (Figure 4; (Newman et al, 2017)) or following acute pharmacological receptor blockade, consistent with CamKII activity being capable of modulation in seconds at postsynaptic compartments (Hell, 2014).…”

“…At the Drosophila NMJ, transgenic manipulations that impact postsynaptic CamKII activity have been reported to modulate the expression of PHP in GluRIIA mutants (Haghighi et al, 2003; Newman et al, 2017). Our results indicate that pCamKII levels are reduced to similar levels in GluRIIA mutants (Figure 4; (Newman et al, 2017)) or following acute pharmacological receptor blockade, consistent with CamKII activity being capable of modulation in seconds at postsynaptic compartments (Hell, 2014). An attractive model would be that a reduction in calcium influx, either over 10 min or during chronic time scales, triggers diminished pCamKII levels and activates PHP signaling, as proposed by several labs (Frank et al, 2006; Haghighi et al, 2003; Newman et al, 2017).…”