ESCRT-III Membrane Trafficking Misregulation Contributes To Fragile X Syndrome Synaptic Defects

Dominic J Vita,Kendal Broadie

doi:10.1038/s41598-017-09103-6

Abstract

The leading cause of heritable intellectual disability (ID) and autism spectrum disorders (ASD), Fragile X syndrome (FXS), is caused by loss of the mRNA-binding translational suppressor Fragile X Mental Retardation Protein (FMRP). In the Drosophila FXS disease model, we found FMRP binds shrub mRNA (human Chmp4) to repress Shrub expression, causing overexpression during the disease state early-use critical period. The FXS hallmark is synaptic overelaboration causing circuit hyperconnectivity. Testing innervation of a central brain learning/memory center, we found FMRP loss and Shrub overexpression similarly increase connectivity. The ESCRT-III core protein Shrub has a central role in endosome-to-multivesicular body membrane trafficking, with synaptic requirements resembling FMRP. Consistently, we found FMRP loss and Shrub overexpression similarly elevate endosomes and result in the arrested accumulation of enlarged intraluminal vesicles within synaptic boutons. Importantly, genetic correction of Shrub levels in the FXS model prevents synaptic membrane trafficking defects and strongly restores innervation. These results reveal a new molecular mechanism underpinning the FXS disease state.

Highlights

Studies suggest a probable link between misregulation of the ESCRT-III core protein Shrub and neural circuit synaptic refinement defects in the Fragile X syndrome (FXS) disease state
Using well-defined olfactory projection neuron (PN) innervation of the central brain mushroom body (MB) learning/memory center[46, 47], we find that Shrub gain-of-function (GOF) closely phenocopies Fragile X Mental Retardation Protein (FMRP) loss-of-function (LOF) synaptic errors with a combination of confocal and transmission electron microscopy (TEM) studies
Based on a developmental brain proteomics screen in the Drosophila FXS disease model, we identified a transient change in Shrub protein levels during the early-use critical period[21]

Summary

Introduction

Studies suggest a probable link between misregulation of the ESCRT-III core protein Shrub and neural circuit synaptic refinement defects in the FXS disease state. ESCRT-III regulates plasma membrane remodeling[44] and synaptic connectivity[24, 26,27,28] These extensive studies suggest a link between misregulation of ESCRT-III core protein Shrub mediated membrane trafficking and synaptic refinement defects in the FXS condition. We test the hypothesis that FMRP negatively regulates ESCRT-III Shrub to control both synaptic membrane trafficking and neural circuit connectivity. We strongly mitigate FMRP LOF membrane trafficking and synaptic connectivity defects through reducing Shrub levels in the FXS disease model Together, these results link ESCRT-III membrane trafficking impairments at the synapse to synaptic connectivity defects as a new causative mechanism in the FXS disease state

Methods

Results

Conclusion