TDP-43 prevents retrotransposon activation in the Drosophila motor system through regulation of Dicer-2 activity

Giulia Romano,Raffaella Klima,Fabian Feiguin

doi:10.1186/s12915-020-00816-1

Abstract

BackgroundMutations in the small RNA-binding protein TDP-43 lead to the formation of insoluble cytoplasmic aggregates that have been associated with the onset and progression of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder affecting homeostasis of the motor system which is also characterized by aberrant expression of retrotransposable elements (RTEs). Although the TDP-43 function was shown to be required in the neurons and glia to maintain the organization of neuromuscular synapses and prevent denervation of the skeletal muscles, the molecular mechanisms involved in physiological dysregulation remain elusive. Here, we address this issue using a null mutation of the TDP-43 Drosophila homolog, TBPH.ResultsUsing genome-wide gene expression profiles, we detected a strong upregulation of RTE expression in TBPH-null Drosophila heads, while the genetic rescue of the TDP-43 function reverted these modifications. Furthermore, we found that TBPH modulates the small interfering RNA (siRNA) silencing machinery responsible for RTE repression. Molecularly, we observed that TBPH regulates the expression levels of Dicer-2 by direct protein-mRNA interactions in vivo. Accordingly, the genetic or pharmacological recovery of Dicer-2 activity was sufficient to repress retrotransposon activation and promote motoneuron axonal wrapping and synaptic growth in TBPH-null Drosophila.ConclusionsWe identified an upregulation of RTE expression in TBPH-null Drosophila heads and demonstrate that defects in the siRNA pathway lead to RTE upregulation and motoneuron degeneration. Our results describe a novel physiological role of endogenous TDP-43 in the prevention of RTE-induced neurological alterations through the modulation of Dicer-2 activity and the siRNA pathway.

Highlights

Mutations in the small RNA-binding protein TDP-43 lead to the formation of insoluble cytoplasmic aggregates that have been associated with the onset and progression of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder affecting homeostasis of the motor system which is characterized by aberrant expression of retrotransposable elements (RTEs)
The lack of TBPH induces the expression of retrotransposons in Drosophila We have previously indicated that the molecular function of TBPH is permanently required in Drosophila motoneurons to prevent muscle denervation, locomotive defects, and early neurodegeneration [19]
The vast majority of patients present defects in the nuclear localization of this protein with an increased accumulation of TDP-43 forming insoluble aggregates in the cytoplasm making the pathological interpretation of these alterations difficult, if not, how to distinguish between defects occasioned from the loss of the nuclear function of TDP-43 and effects due to the toxic gain of function of this protein in the cytoplasm. These possibilities are a matter of strong debate in the ALS field, and in that respect, our studies demonstrate that alterations in the nuclear role of TBPH are sufficient to provoke the pathological activation of RTEs and suggest that a tight regulation of TDP-43 activity orcellular distribution is required to prevent retrotransposon activation in the affected brains

Summary

Introduction

Mutations in the small RNA-binding protein TDP-43 lead to the formation of insoluble cytoplasmic aggregates that have been associated with the onset and progression of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder affecting homeostasis of the motor system which is characterized by aberrant expression of retrotransposable elements (RTEs). The TDP-43 function was shown to be required in the neurons and glia to maintain the organization of neuromuscular synapses and prevent denervation of the skeletal muscles, the molecular mechanisms involved in physiological dysregulation remain elusive We address this issue using a null mutation of the TDP-43 Drosophila homolog, TBPH. The formation of insoluble aggregates may disrupt the physiological function of the endogenous protein and lead to neurodegeneration through mechanisms related with the absence of TDP-43 function in the nucleus In relationship with these observations, we demonstrated that the suppression of the TDP-43 homolog protein in Drosophila (TBPH) faithfully reproduced in flies the main characteristics of the human disease alike paralysis, motoneuron degeneration, and reduced life span [17, 18].

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