Date Published: February 11, 2019
Publisher: Public Library of Science
Author(s): Katsuhisa Masaki, Yoshifumi Sonobe, Ghanashyam Ghadge, Peter Pytel, Raymond P. Roos, Bert L. Semler.
TDP-43, an RNA-binding protein that is primarily nuclear and important in splicing and RNA metabolism, is mislocalized from the nucleus to the cytoplasm of neural cells in amyotrophic lateral sclerosis (ALS), and contributes to disease. We sought to investigate whether TDP-43 is mislocalized in infections with the acute neuronal GDVII strain and the persistent demyelinating DA strain of Theiler’s virus murine encephalomyelitis virus (TMEV), a member of the Cardiovirus genus of Picornaviridae because: i) L protein of both strains is known to disrupt nucleocytoplasmic transport, including transport of polypyrimidine tract binding protein, an RNA-binding protein, ii) motor neurons and oligodendrocytes are targeted in both TMEV infection and ALS. TDP-43 phosphorylation, cleavage, and cytoplasmic mislocalization to an aggresome were observed in wild type TMEV-infected cultured cells, with predicted splicing abnormalities. In contrast, cells infected with DA and GDVII strains that have L deletion had rare TDP-43 mislocalization and no aggresome formation. TDP-43 mislocalization was also present in neural cells of TMEV acutely-infected mice. Of note, TDP-43 was mislocalized six weeks after DA infection to the cytoplasm of oligodendrocytes and other glial cells in demyelinating lesions of spinal white matter. A recent study showed that TDP-43 knock down in oligodendrocytes in mice led to demyelination and death of this neural cell , suggesting that TMEV infection mislocalization of TDP-43 and other RNA-binding proteins is predicted to disrupt key cellular processes and contribute to the pathogenesis of TMEV-induced diseases. Drugs that inhibit nuclear export may have a role in antiviral therapy.
Trans-activation response (TAR) DNA-binding protein of 43 kDa (TDP-43) is an RNA-binding protein (as well as DNA-binding protein) primarily present in the nucleus and important in RNA processing, mRNA transport/stability, and mRNA translation [2–4]. A variety of cellular stresses normally triggers TDP-43 to transiently shuttle into the cytoplasm and assemble into stress granules (SGs). Due to an abnormality of nucleocytoplasmic transport that is known to occur in amyotrophic lateral sclerosis (ALS), TDP-43 accumulates in insoluble aggregates in the cytoplasm of glia and degenerating neurons in the central nervous system (CNS) [5–7]. The mislocalized TDP-43 is cleaved into C-terminal fragments (CTFs), phosphorylated, and/or ubiquitinated [8–10].The importance of TDP-43 in disease pathogenesis is evidenced by the fact that mutant TDP-43 is a rare cause of familial ALS and, like wild type (wt) TDP-43, is mislocalized to the cytoplasm.
TDP-43 is a ubiquitously expressed RNA-binding protein that predominantly resides in the nucleus, but shuttles across the nuclear membrane in association with mRNAs . A hallmark of almost all cases of ALS is disruption of nucleocytoplasmic trafficking with cytoplasmic mislocalization, aggregation, cleavage, and phosphorylation of TDP-43 in neural cells [5, 7, 9]. TDP-43 mislocalization is thought to lead to abnormalities of splicing and RNA metabolism with subsequent neuronal dysfunction [4, 27, 28]. It is likely that the cytoplasmic mislocalization of other RNA-binding proteins also contributes to the abnormalities of splicing in ALS . In the present study, we demonstrate that TMEV infection leads to cytoplasmic mislocalization of TDP-43 (as well as FUS and PTB) along with cleavage into products similar in size to those found in ALS  and TDP-43 phosphorylation. Importantly, TDP-43 mislocalization was also found in neural cells following acute infections of mice, and in oligodendrocytes and other glial cells in demyelinated regions 6 weeks after DA infection.