TheDrosophilaMetabotropic Glutamate Receptor DmGluRA Regulates Activity-Dependent Synaptic Facilitation and Fine Synaptic Morphology

Abstract

In vertebrates, several groups of metabotropic glutamate receptors (mGluRs) are known to modulate synaptic properties. In contrast, the Drosophila genome encodes a single functional mGluR (DmGluRA), an ortholog of vertebrate group II mGluRs, greatly expediting the functional characterization of mGluR-mediated signaling in the nervous system. We show here that DmGluRA is expressed at the glutamatergic neuromuscular junction (NMJ), localized in periactive zones of presynaptic boutons but excluded from active sites. Null DmGluRA mutants are completely viable, and all of the basal NMJ synaptic transmission properties are normal. In contrast, DmGluRA mutants display approximately a threefold increase in synaptic facilitation during short stimulus trains. Prolonged stimulus trains result in very strongly increased (ϳ10-fold) augmentation, including the appearance of asynchronous, bursting excitatory currents never observed in wild type. Both defects are rescued by expression of DmGluRA only in the neurons, indicating a specific presynaptic requirement. These phenotypes are reminiscent of hyperexcitable mutants, suggesting a role of DmGluRA signaling in the regulation of presynaptic excitability properties. The mutant phenotypes could not be replicated by acute application of mGluR antagonists, suggesting that DmGluRA regulates the development of presynaptic properties rather than directly controlling short-term modulation. DmGluRA mutants also display mild defects in NMJ architecture: a decreased number of synaptic boutons accompanied by an increase in mean bouton size. These morphological changes bidirectionally correlate with DmGluRA levels in the presynaptic terminal. These data reveal the following two roles for DmGluRA in presynaptic mechanisms: (1) modulation of presynaptic excitability properties important for the control of activity-dependent neurotransmitter release and (2) modulation of synaptic architecture.

The Metabotropic Glutamate Receptor Activates the Lipid Kinase PI3K in Drosophila Motor Neurons Through the Calcium/Calmodulin-Dependent Protein Kinase II and the Nonreceptor Tyrosine Protein Kinase DFak

“…Flies carrying the DFak CG1 deletion mutation and UAS-DFak 1 transgene (Grabbe et al 2004) were provided by Ruth Palmer (Umeå University, Umeå, Sweden). Flies carrying the DmGluRA 112b deletion mutation (Bogdanik et al 2004) were provided by Marie-Laure Parmentier (Unité Propre de Recherche Centre, Montpellier, France). All other fly stocks were provided by the Drosophila Stock Center (Bloomington, IN).…”

“…In Drosophila, the DmGluRA 112b null mutation in the single mGluR increases motor neuron excitability by preventing activation of the lipid kinase PI3K (Bogdanik et al 2004;Howlett et al 2008). Given that glutamate is the major excitatory neurotransmitter at NMJs in insects such as Drosophila (Jan and Jan 1976), it was suggested that DmGluRA in motor neurons participate in a negative feedback: glutamate released from motor nerve terminals as a consequence of neuronal activity activates DmGluRA autoreceptors in motor nerve terminals.…”

“…For example, boutons in DFak CG1 mutants often appear to be larger in size than wild type. The DmGluRA 112b mutation was similarly found to increase bouton size (Bogdanik et al 2004). These observations are consistent with the possibility that DFak activation is attenuated in the absence of DmGluRA.…”

“…The number of stimulations required to reach this LTF threshold (LTF onset rate) is decreased by increased neuronal excitability (Jan and Jan 1978;Stern and Ganetzky 1989;Stern et al 1990;Mallart et al 1991;Poulain et al 1994;Schweers et al 2002). Both the DmGluRA 112b null mutation and transgene-induced inhibition of PI3K greatly decrease the number of nerve stimulations required to reach LTF and thus greatly increase LTF onset rate (Bogdanik et al 2004;Howlett et al 2008). This increased LTF onset rate indicates increased excitability in these genotypes.…”

“…In Drosophila larval motor neurons, glutamate activation of the single mGluR, called DmGluRA, downregulates neuronal excitability (Bogdanik et al 2004); glutamate both activates PI3K and requires PI3K activity for this downregulation (Howlett et al 2008). Because glutamate is the excitatory neurotransmitter at the Drosophila neuromuscular junction (NMJ) (Jan and Jan 1976), it was hypothesized that this DmGluRA-mediated downregulation of neuronal excitability carried out a negative feedback on activity: glutamate released from motor nerve terminals would activate DmGluRA autoreceptors, which would then depress excitability.…”