Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth.

GaYoung Lee,Thomas L Schwarz

doi:10.7554/elife.19991

Abstract

Filamin is a scaffolding protein that functions in many cells as an actin-crosslinker. FLN90, an isoform of the Drosophila ortholog Filamin/cheerio that lacks the actin-binding domain, is here shown to govern the growth of postsynaptic membrane folds and the composition of glutamate receptor clusters at the larval neuromuscular junction. Genetic and biochemical analyses revealed that FLN90 is present surrounding synaptic boutons. FLN90 is required in the muscle for localization of the kinase dPak and, downstream of dPak, for localization of the GTPase Ral and the exocyst complex to this region. Consequently, Filamin is needed for growth of the subsynaptic reticulum. In addition, in the absence of filamin, type-A glutamate receptor subunits are lacking at the postsynapse, while type-B subunits cluster correctly. Receptor composition is dependent on dPak, but independent of the Ral pathway. Thus two major aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering, require postsynaptic Filamin.

Highlights

Proper postsynaptic function depends on appropriate localization of receptors and signaling molecules
Immunolocalization of Ral expressed in muscles of wild type animals reveals a distinct halo around each synaptic bouton, a distribution resembling that of the subsynaptic reticulum (Teodoro et al, 2013; Figure 1)
To determine factors responsible for Ral localization, we expressed in larval muscle RNAi directed against candidate proteins, including filamin (Ohta et al, 1999), that are reported in the literature or in proteomic databases to interact with Ral

Summary

Introduction

Proper postsynaptic function depends on appropriate localization of receptors and signaling molecules. While usually without intrinsic enzymatic activity, scaffolds recruit, assemble, and stabilize receptors and protein networks through multiple protein-protein interactions: they can bind to receptors, postsynaptic signaling complexes, and the cytoskeleton at the postsynaptic density (Sheng and Kim, 2011). We found that the SSR is a plastic structure whose growth is regulated by synaptic activity (Teodoro et al, 2013) This phenomenon may be akin to the use-dependent morphological changes, such as growth of dendritic spines, that occur postsynaptically in mammalian brain. For analyzing the number and size of Brp/GluR –positive puncta at the synapse (Figure 5), maximum intensity projections from z-stacks were used.

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Results

Conclusion