Activation of Transforming Growth Factor-β/Smad Signaling Reduces Aggregate Formation of Mislocalized TAR DNA-Binding Protein-43

Abstract

Background: TAR DNA-binding protein of 43 kDa (TDP-43) is naturally located in the nucleus and has been identified as the major component of cytoplasmic ubiquitinated inclusions in patients with amyotrophic lateral sclerosis (ALS). We have reported that TDP-43 and phosphorylated Smad2 (pSmad2), an intracellular mediator protein of transforming growth factor-β (TGFβ) signaling, are co-localized within cytoplasmic inclusions in the anterior horn cells of sporadic ALS patients. Objective: To investigate the possible pathophysiological linkage between pathologic cytoplasmic inclusions containing TDP-43 and TGFβ/Smad signaling. Methods: We replicated cytoplasmic aggregates of TDP-43 in HEK293T cells by transfecting the cells with a nuclear localization signal deletion mutant of TDP-43 and inhibiting proteasome activity, and assessed the effect of TGFβ/Smad signaling on the cytoplasmic aggregate formation. Results: The aggregates contained ubiquitinated, phosphorylated, and fragmented TDP-43, consistent with the essential features of the human pathology. Moreover, the aggregates were co-localized with pSmad2 under continuous TGFβ stimulation. Overexpression of Smad2 reduced the amount of cytoplasmic aggregates in HEK293T cells, and TGFβ stimulation augmented this reduction effect in a dose-dependent manner. Conclusion: Activation of the TGFβ/Smad signaling system is protective against aggregate formation of cytoplasmically mislocalized TDP-43 and may be a potential therapeutic approach to delay progression of ALS.

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