The ALS disease protein TDP-43 is actively transported in motor neuron axons and regulates axon outgrowth

Claudia Fallini1,
Gary J. Bassell1,2,* and
Wilfried Rossoll1,*

+ Author Affiliations

1Department of Cell Biology and Center for Neurodegenerative Diseases and
2Department of Neurology, Emory University School of Medicine, Atlanta, GA 30322, USA

↵*To whom correspondence should be addressed at: Department of Cell Biology, Emory University School of Medicine, Whitehead Biomedical Research Building 415, 615 Michael Street, Atlanta, GA 30322, USA. Tel: +1 4047270668; Fax: +1 4047270570; Email: wrossol@emory.edu (W.R.) or gbassel@emory.edu (G.J.B.)

Received January 23, 2012.
Revision received May 2, 2012.
Accepted May 23, 2012.

Abstract

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease specifically affecting cortical and spinal motor neurons. Cytoplasmic inclusions containing hyperphosphorylated and ubiquitinated TDP-43 are a pathological hallmark of ALS, and mutations in the gene encoding TDP-43 have been directly linked to the development of the disease. TDP-43 is a ubiquitous DNA/RNA-binding protein with a nuclear role in pre-mRNA splicing. However, the selective vulnerability and axonal degeneration of motor neurons in ALS pose the question of whether TDP-43 may have an additional role in the regulation of the cytoplasmic and axonal fate of mRNAs, processes important for neuron function. To investigate this possibility, we have characterized TDP-43 localization and dynamics in primary cultured motor neurons. Using a combination of cell imaging and biochemical techniques, we demonstrate that TDP-43 is localized and actively transported in live motor neuron axons, and that it co-localizes with well-studied axonal mRNA-binding proteins. Expression of the TDP-43 C-terminal fragment led to the formation of hyperphosphorylated and ubiquitinated inclusions in motor neuron cell bodies and neurites, and these inclusions specifically sequestered the mRNA-binding protein HuD. Additionally, we showed that overexpression of full-length or mutant TDP-43 in motor neurons caused a severe impairment in axon outgrowth, which was dependent on the C-terminal protein-interacting domain of TDP-43. Taken together, our results suggest a role of TDP-43 in the regulation of axonal growth, and suggest that impairment in the post-transcriptional regulation of mRNAs in the cytoplasm of motor neurons may be a major factor in the development of ALS.