Postsynaptic Receptor Fields Research Articles

Neto-α Controls Synapse Organization and Homeostasis at the Drosophila Neuromuscular Junction

SUMMARYGlutamate receptor auxiliary proteins control receptor distribution and function, ultimately controlling synapse assembly, maturation, and plasticity. At the Drosophila neuromuscular junction (NMJ), a synapse with both pre- and postsynaptic kainate-type glutamate receptors (KARs), we show that the auxiliary protein Neto evolved functionally distinct isoforms to modulate synapse development and homeostasis. Using genetics, cell biology, and electrophysiology, we demonstrate that Neto-α functions on both sides of the NMJ. In muscle, Neto-α limits the size of the postsynaptic receptor field. In motor neurons (MNs), Neto-α controls neurotransmitter release in a KAR-dependent manner. In addition, Neto-α is both required and sufficient for the presynaptic increase in neurotransmitter release in response to reduced postsynaptic sensitivity. This KAR-independent function of Neto-α is involved in activity-induced cytomatrix remodeling. We propose that Drosophila ensures NMJ functionality by acquiring two Neto isoforms with differential expression patterns and activities.

Hebbian Plasticity Guides Maturation of Glutamate Receptor Fields In Vivo

Synaptic plasticity shapes the development of functional neural circuits and provides a basis for cellular models of learning and memory. Hebbian plasticity describes an activity-dependent change in synaptic strength that is input-specific and depends on correlated pre- and postsynaptic activity. Although it is recognized that synaptic activity and synapse development are intimately linked, our mechanistic understanding of the coupling is far from complete. Using Channelrhodopsin-2 to evoke activity invivo, we investigated synaptic plasticity at the glutamatergic Drosophila neuromuscular junction. Remarkably, correlated pre- and postsynaptic stimulation increased postsynaptic sensitivity by promoting synapse-specific recruitment of GluR-IIA-type glutamate receptor subunits into postsynaptic receptor fields. Conversely, GluR-IIA was rapidly removed from synapses whose activity failed to evoke substantial postsynaptic depolarization. Uniting these results with developmental GluR-IIA dynamics provides a comprehensive physiological concept of how Hebbian plasticity guides synaptic maturation and sparse transmitter release controls the stabilization of the molecular composition of individual synapses.

The Immunoglobulin Super Family Protein RIG-3 Prevents Synaptic Potentiation and Regulates Wnt Signaling

Cell surface Ig superfamily proteins (IgSF) have been implicated in several aspects of neuron development and function. Here, we describe the function of aCaenorhabditis elegans IgSF protein, RIG-3. Mutants lacking RIG-3 have an exaggerated paralytic response to a cholinesterase inhibitor, aldicarb. Although RIG-3 is expressed in motor neurons, heightened drug responsiveness was caused by an aldicarb-induced increase in muscle ACR-16 acetylcholine receptor (AChR) abundance, and a corresponding potentiation of postsynaptic responses at neuromuscular junctions. Mutants lacking RIG-3 also had defects in the anteroposterior polarity of the ALM mechanosensory neurons. The effects of RIG-3 on synaptic transmission and ALM polarity were both mediated by changes in Wnt signaling, and in particular by inhibiting CAM-1, a Ror-type receptor tyrosine kinase that binds Wnt ligands. These results identify RIG-3 as a regulator of Wnt signaling, and suggest that RIG-3 has an anti-plasticity function that prevents activity-induced changes in postsynaptic receptor fields.

Developmental regulation of glutamate receptor field size by nonvesicular glutamate release.

We hypothesized that presynaptic glutamate regulates postsynaptic ionotropic glutamate receptor number during synaptogenesis. To test this idea, we genetically manipulated presynaptic glutamate levels at the glutamatergic Drosophila neuromuscular junction (NMJ), then microscopically and electrophysiologically measured postsynaptic glutamate receptor field size and function. Our data show that presynaptic glutamate is a strong negative regulator of postsynaptic receptor field size and function during development. Glutamate-triggered receptor downregulation was not affected by block of synaptic vesicle fusion, demonstrating that receptors are regulated by nonvesicular glutamate release. Our results reveal an elegant mechanism for receptor field regulation during synaptogenesis and reveal a nonpathological role for nonvesicular glutamate release at the synapse.

EeeGAD! Glutamatergic Synapse Regulation

The Drosophila neuromuscular junction (NMJ) is a glutamatergic synapse. Featherstone et al . identified glutamic acid decarboxylase ( Dgad1 ) in a screen of mutants that are embryonic lethal with normal morphology, but that exhibited movement defects and aberrant neuromuscular synapses (detected by electrophysiological techniques). Biochemical analysis of GAD confirms that the enzyme can convert glutamate to GABA and that the mutant alleles lack enzymatic activity; however, GABA and GABA receptors are not detected at the Drosophila NMJ, suggesting that GAD is not involved in the synthesis of GABA at these synapses. GAD was detected in the presynaptic neuron of the NMJ by immunostaining. The electrophysiological defects observed in the mutants are consistent with normal clustering of glutamate receptors with normal single-channel activity, but greatly reduced channel density, suggesting that glutamate metabolism is a key regulator of postsynaptic receptor density. Furthermore, overexpression of GAD led to increased receptor activity. The authors suggest that the mechanism for GAD regulation of postsynaptic density is due to decreased glutamate metabolism, which increases intracellular glutamate concentrations leading to a decreased gradient for the glutamate transporter and, thus, increased synaptic glutamate concentrations due to nonvesicular glutamate release and concomitant down-regulation of postsynaptic receptors. Featherstone, D.E., Rushton, E.M., Hilderbrand-Chae, M., Phillips, A.M., Jackson, F.R., and Broadie, K. (2000) Presynaptic glutamic acid decarboxylase is required for induction of the postsynaptic receptor field at a glutamtergic synapse. Neuron 27 : 71-84. [Online Journal]

Presynaptic glutamic acid decarboxylase is required for induction of the postsynaptic receptor field at a glutamatergic synapse.

We have systematically screened EMS-mutagenized Drosophila for embryonic lethal strains with defects in glutamatergic synaptic transmission. Surprisingly, this screen led to the identification of several alleles with missense mutations in highly conserved regions of Dgad1. Analysis of these gad mutants reveals that they are paralyzed owing to defects in glutamatergic transmission at the neuromuscular junction. Further electrophysiological and immunohistochemical examination reveals that these mutants have greatly reduced numbers of postsynaptic glutamate receptors in an otherwise morphologically normal synapse. By overexpressing wild-type Dgad1 in selected neurons, we show that GAD is specifically required in the presynaptic neuron to induce a postsynaptic glutamate receptor field, and that the level of postsynaptic receptors is closely dependent on presynaptic GAD function. These data demonstrate that GAD plays an unexpected role in glutamatergic synaptogenesis.

Development of Electrical Properties and Synaptic Transmission at the Embryonic Neuromuscular Junction

Publisher Summary The synapse is a specialized intercellular junction devoted to communication between two electrically excitable cells. In most organisms, synapse formation, or synaptogenesis, begins during early to mid embryogenesis when the primary neural circuits are formed, but often continues postembryonically and, in specialized circumstances, can be maintained for the lifetime of the organism. This chapter defines synaptogenesis as the formation of the intercellular communication link: downstream of neuronal pathfinding and target recognition, but upstream of the established mechanism of synaptic transmission and the maintained synaptic developmental potential known as synaptic plasticity. The process of synaptogenesis defines the site of the intercellular junction and provides for the precise alignment of the presynaptic signaling apparatus and the postsynaptic receptor field. Inductive communication between synaptic partners determines both the molecular nature and the transmission strength of the assembling synapse. The chapter focuses exclusively on the development of electrical properties and synaptic transmission at the embryonic Drosophila neuromuscular junction. The development of muscle electrical properties, pre- and postsynaptic maturation of neuromuscular junction transmission characteristics, and experimental methods used to obtain electrophysiological recordings from the Drosophila embryonic preparation are also discussed.