Enteroviral Infection: The Forgotten Link to Amyotrophic Lateral Sclerosis?

Abstract

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that primarily attacks motor neurons in the brain and spinal cord, leading to progressive paralysis and ultimately death. Currently there is no effective therapy. The majority of ALS cases are sporadic, with no known family history; unfortunately the etiology remains largely unknown. Contribution of Enteroviruses (EVs), a family of positive-stranded RNA viruses including poliovirus, coxsackievirus, echovirus, enterovirus-A71 and enterovirus-D68, to the development of ALS has been suspected as they can target motor neurons, and patients with prior poliomyelitis show a higher risk of motor neuron disease. Multiple efforts have been made to detect enteroviral genome in ALS patient tissues over the past two decades; however the clinical data are controversial and a causal relationship has not yet been established. Recent evidence from in vitro and animal studies suggests that enterovirus-induced pathology remarkably resembles the cellular and molecular phenotype of ALS, indicating a possible link between enteroviral infection and ALS pathogenesis. In this review, we summarize the nature of enteroviral infection, including route of infection, cells targeted, and viral persistence within the central nervous system (CNS). We review the molecular mechanisms underlying viral infection and highlight the similarity between viral pathogenesis and the molecular and pathological features of ALS, and finally, discuss the potential role of enteroviral infection in frontotemporal dementia (FTD), a disease that shares common clinical, genetic, and pathological features with ALS, and the significance of anti-viral therapy as an option for the treatment of ALS.

Keywords: RNA metabolism; TDP-43 pathology; amyotrophic lateral sclerosis; autophagy; enterovirus; neuroinflammation; nucleocytoplasmic trafficking.

Figure 1

Molecular and pathological similarities between amyotrophic lateral sclerosis (ALS) and enteroviral infection. Enterovirus (EV) infection impairs nucleocytoplasmic trafficking (A) via EV proteinase-mediated proteolysis of nucleoporin proteins, resulting in mislocalization of TDP-43 and heterogeneous nuclear ribonucleoproteins (hnRNPs) from the nucleus to the cytoplasm, where they are further cleaved to induce TDP-43 pathology (B) and cause RNA-processing defects (C). EV infection also results in the cleavage of several critical autophagic proteins, including SQSTM1/p62, Nbr1 and SNAP29, contributing to the disruption of the autophagic pathway (D) and consequent accumulation of misfolded proteins/damaged organelles (E). Finally, subclinical EV infection induces chronic inflammatory reaction (F) and promotes non-lytic viral spread and prion-like mechanism through extracellular microvesicles (G).