Person:

Lee, GaYoung

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. DOI: http://dx.doi.org/10.7554/eLife.19991.001

Proper postsynaptic assembly is a fundamental requirement for circuit function, yet our understanding of its molecular mechanism is incomplete. The Drosophila neuromuscular junction is a powerful system for genetically dissecting synapse organization at the molecular level. Taking advantage of its features, I sought to identify novel molecular mechanisms governing synapse assembly. The Ral pathway for activity-dependent membrane addition is highly localized at the postsynapse and is essential to proper postsynaptic development, thus offering a tractable ideal starting point. Through a candidate-based genetic approach using Ral localization as the initial readout, I initially discovered that filamin is required for localizing the Ral pathway at the postsynapse.

Filamin is a scaffolding and actin-binding protein not previously known to be required for synaptic development. My findings now show that the Drosophila ortholog filamin/cheerio localizes key signaling proteins that govern morphological plasticity and the composition of glutamate receptor clusters at the neuromuscular junction (NMJ). Genetic and biochemical analysis revealed that the short FLN90 isoform of filamin, which lacks the actin-binding domain, is required in the muscle and present surrounding boutons. FLN90 is required for dPak localization and, downstream of dPak, for localization of the Ral pathway for activity-dependent membrane addition. Consequently filamin is needed for growth of the subsynaptic reticulum, the postsynaptic membrane structure at the NMJ. In addition, in the absence of filamin, GluRIIA subunits are lacking though GluRIIB subunits cluster correctly. The shift in subunit composition is likely dependent on dPak, but independent of the Ral pathway. Based on these findings, I present a model in which filamin orchestrates a pathway governing two distinct aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering.

In Chapter 1 I provide general introduction to the relevant background topics. In Chapter 2 I present and discuss the major findings of my thesis research. Based on these, I discuss some of the remaining questions and future directions in Chapters 3 and 4.