Homeostatic scaling is driven by a translation-dependent degradation axis that recruits miRISC remodeling.

Balakumar Srinivasan,Sarbani Samaddar,Sivaram V S Mylavarapu,Sourav Banerjee,James P Clement,Yi-Ping Hsueh

doi:10.1371/journal.pbio.3001432

Abstract

Homeostatic scaling in neurons has been attributed to the individual contribution of either translation or degradation; however, there remains limited insight toward understanding how the interplay between the two processes effectuates synaptic homeostasis. Here, we report that a codependence between protein synthesis and degradation mechanisms drives synaptic homeostasis, whereas abrogation of either prevents it. Coordination between the two processes is achieved through the formation of a tripartite complex between translation regulators, the 26S proteasome, and the miRNA-induced silencing complex (miRISC) components such as Argonaute, MOV10, and Trim32 on actively translating transcripts or polysomes. The components of this ternary complex directly interact with each other in an RNA-dependent manner. Disruption of polysomes abolishes this ternary interaction, suggesting that translating RNAs facilitate the combinatorial action of the proteasome and the translational apparatus. We identify that synaptic downscaling involves miRISC remodeling, which entails the mTORC1-dependent translation of Trim32, an E3 ligase, and the subsequent degradation of its target, MOV10 via the phosphorylation of p70 S6 kinase. We find that the E3 ligase Trim32 specifically polyubiquitinates MOV10 for its degradation during synaptic downscaling. MOV10 degradation alone is sufficient to invoke downscaling by enhancing Arc translation through its 3' UTR and causing the subsequent removal of postsynaptic AMPA receptors. Synaptic scaling was occluded when we depleted Trim32 and overexpressed MOV10 in neurons, suggesting that the Trim32-MOV10 axis is necessary for synaptic downscaling. We propose a mechanism that exploits a translation-driven protein degradation paradigm to invoke miRISC remodeling and induce homeostatic scaling during chronic network activity.

Highlights

Codependence of protein synthesis and degradation drives synaptic homeostasis To test the existence of coordination between translation and degradation in the regulation of synaptic homeostasis, we measured miniature excitatory postsynaptic currents from cultured hippocampal neurons (days in vitro (DIV) 18 to 24) after pharmacological inhibition of protein synthesis and proteasomal activity for 24 hours
We used a paradigm of chronic network hyperactivity to invoke downscaling and determined that (a) translation and degradation apparatuses directly interact with each other and are tethered together by RNA scaffolds; (b) it is the translation of Trim32 that drives the degradation of MOV10 to cause miRNA-induced silencing complex (miRISC) remodeling, the current paradigm is an example of translation preceding degradation; and (c) miRISC is a key node in the translation-degradation axis, with the mammalian Target Of Rapamycin Complex-1 (mTORC1)-p70 p70 S6 kinase (S6K) pathway being the upstream signaling component and a part of the “sensor” machinery, and Arc-induced removal of surface AMPA receptors (AMPARs) (sAMPAR) being the final effectors of downscaling
Coregulation of protein synthesis and degradation drives AMPARmediated synaptic downscaling We find that chronic perturbation of either translation or proteasomal activity occludes synaptic homeostasis, whereas homeostasis remains unperturbed when there is simultaneous inhibition of both (Fig 1)

Summary

Introduction

We. Coordination between translation regulators, the 26S proteasome and miRISC underlies synaptic scaling provide empirical evidence demonstrating that the interaction between translation and protein degradation machineries is direct and RNA dependent. Coordination between translation regulators, the 26S proteasome and miRISC underlies synaptic scaling provide empirical evidence demonstrating that the interaction between translation and protein degradation machineries is direct and RNA dependent This coordination is achieved when the two apparatuses are tethered to actively translating transcripts linked to miRISC. We find that in contexts of chronic hyperactivity, mTORC1-dependent translation of the E3 ligase Trim promotes the polyubiquitination and subsequent degradation of MOV10 by proteasome This is triggered by the mTORC1-mediated phosphorylation of its downstream effector, p70 S6 kinase (p70 S6K). The observed increase in Arc expression in the context of synaptic downscaling happens via translation and not by transcriptional mechanisms

Results

Discussion

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