Increased cytoplasmicTARDBPmRNA in affected spinal motor neurons in ALS caused by abnormal autoregulation of TDP-43

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disorder. In motor neurons of ALS, TAR DNA binding protein-43 (TDP-43), a nuclear protein encoded by TARDBP, is absent from the nucleus and forms cytoplasmic inclusions. TDP-43 auto-regulates the amount by regulating the TARDBP mRNA, which has three polyadenylation signals (PASs) and three additional alternative introns within the last exon. However, it is still unclear how the autoregulatory mechanism works and how the status of autoregulation in ALS motor neurons without nuclear TDP-43 is. Here we show that TDP-43 inhibits the selection of the most proximal PAS and induces splicing of multiple alternative introns in TARDBP mRNA to decrease the amount of cytoplasmic TARDBP mRNA by nonsense-mediated mRNA decay. When TDP-43 is depleted, the TARDBP mRNA uses the most proximal PAS and is increased in the cytoplasm. Finally, we have demonstrated that in ALS motor neurons—especially neurons with mislocalized TDP-43—the amount of TARDBP mRNA is increased in the cytoplasm. Our observations indicate that nuclear TDP-43 contributes to the autoregulation and suggests that the absence of nuclear TDP-43 induces an abnormal autoregulation and increases the amount of TARDBP mRNA. The vicious cycle might accelerate the disease progression of ALS.

Mice with endogenous TDP ‐43 mutations exhibit gain of splicing function and characteristics of amyotrophic lateral sclerosis

“…An abnormal upregulation of TARDBP has been shown in human TDP‐43‐ALS post‐mortem end‐stage brain, likely due to the absence of nuclear TDP‐43 protein, and a vicious cycle of increased expression has been proposed to play a role in disease (Koyama et al , 2016). Here, unexpectedly, we found an imbalance in the autoregulation and the upregulation of Tardbp levels in LCDmut , throughout life, although protein levels were not significantly increased.…”

“…TDP‐43 deposition and nuclear depletion in post‐mortem tissue support a potential role for LOF in end‐stage ALS. However, little is known about the early‐stage effects of TARDBP mutations, including the impact on RNA metabolism (Koyama et al , 2016). …”

Cleavage and polyadenylation: Ending the message expands gene regulation

“… 219 TDP-43 UG rich High levels of TDP-43 cause inhibition of pA1 site in intron 7 of its own TARDBP pre-mRNA, resulting in usage of pA2 or pA4, which both produce transcripts that are targeted by the Nonsense Mediated Decay pathway, providing a mechanism of auto-regulation. 220 THOC5 THOC5 is a member of the human transcription export complex (TREX). THOC5 knockdown activates proximal pA site usage.…”

An Intramolecular Salt Bridge Linking TDP43 RNA Binding, Protein Stability, and TDP43-Dependent Neurodegeneration

“…More than 40 different pathogenic mutations within the gene encoding TDP43 ( TARDBP ) underlie familial ALS, FTD, or both ( Barmada and Finkbeiner, 2010 ). Disease-associated TARDBP mutations elicit gain-of-function toxicity by interfering with TDP43 autoregulation ( White et al, 2018 ; Fratta et al, 2018 ; Koyama et al, 2016 ), enhancing cytoplasmic TDP43 mislocalization and deposition, and affecting TDP43 clearance ( Barmada et al, 2010 , 2014 ; Nishimura et al, 2014 ; Watanabe et al, 2013 ; Ling et al, 2010 ). Supporting the link between TDP43 turnover and neurodegeneration, toxicity is directly proportional to TDP43 abundance in individual neurons, and accelerating TDP43 turnover extends neuronal survival and mitigates disease phenotypes in disease models ( Barmada et al, 2014 ).…”