Differential roles of the ubiquitin proteasome system and autophagy in the clearance of soluble and aggregated TDP-43 species.

Emma L Scotter,Youn-Bok Lee,Valentina Sardone,Agnes L Nishimura,Hazel Urwin,Christopher E Shaw,Jacqueline C Mitchell,Caroline Vance,David C Rubinsztein,Han-Jou Chen,Boris Rogelj

doi:10.1242/jcs.140087

Abstract

ABSTRACTTAR DNA-binding protein (TDP-43, also known as TARDBP) is the major pathological protein in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Large TDP-43 aggregates that are decorated with degradation adaptor proteins are seen in the cytoplasm of remaining neurons in ALS and FTD patients post mortem. TDP-43 accumulation and ALS-linked mutations within degradation pathways implicate failed TDP-43 clearance as a primary disease mechanism. Here, we report the differing roles of the ubiquitin proteasome system (UPS) and autophagy in the clearance of TDP-43. We have investigated the effects of inhibitors of the UPS and autophagy on the degradation, localisation and mobility of soluble and insoluble TDP-43. We find that soluble TDP-43 is degraded primarily by the UPS, whereas the clearance of aggregated TDP-43 requires autophagy. Cellular macroaggregates, which recapitulate many of the pathological features of the aggregates in patients, are reversible when both the UPS and autophagy are functional. Their clearance involves the autophagic removal of oligomeric TDP-43. We speculate that, in addition to an age-related decline in pathway activity, a second hit in either the UPS or the autophagy pathway drives the accumulation of TDP-43 in ALS and FTD. Therapies for clearing excess TDP-43 should therefore target a combination of these pathways.

Highlights

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that are characterised by the deposition of pathological protein aggregates composed of TAR DNA-binding protein 43 (TDP-43 or TARDBP) (Neumann et al, 2006)
We have found that an equivalent enhanced GFP (EGFP)-tagged TDP43 CTF forms aggregates under ubiquitin proteasome system (UPS) inhibition but not under autophagy inhibition, similar to wild-type or deleted nuclear localisation sequence (DNLS) TDP-43
A number of genetic causes of ALS and FTD converge upon the pathways involved in TDP-43 protein degradation

Summary

Introduction

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that are characterised by the deposition of pathological protein aggregates composed of TAR DNA-binding protein 43 (TDP-43 or TARDBP) (Neumann et al, 2006). These ‘TDP-43 proteinopathies’ span a clinical spectrum, from predominantly upper- and lower-motor-neuron. In the absence of genomic mutation, wild-type TDP-43 aggregates are found in the brain and spinal cord of 90% of ALS patients and in the brain of 60% of FTD patients (Neumann et al, 2006)

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