In humans, viral infections of the respiratory tract are a leading cause of morbidity and mortality worldwide. Several recognized respiratory viral agents have a neuroinvasive capacity since they can spread from the respiratory tract to the central nervous system (CNS). Once there, infection of CNS cells (neurotropism) could lead to human health problems, such as encephalitis and long-term neurological diseases. Among the various respiratory viruses, coronaviruses are important pathogens of humans and animals. Human Coronaviruses (HCoV) usually infect the upper respiratory tract, where they are mainly associated with common colds. However, in more vulnerable populations, such as newborns, infants, the elderly, and immune-compromised individuals, they can also affect the lower respiratory tract, leading to pneumonia, exacerbations of asthma, respiratory distress syndrome, or even severe acute respiratory syndrome (SARS). The respiratory involvement of HCoV has been clearly established since the 1960s. In addition, for almost three decades now, the scientific literature has also demonstrated that HCoV are neuroinvasive and neurotropic and could induce an overactivation of the immune system, in part by participating in the activation of autoreactive immune cells that could be associated with autoimmunity in susceptible individuals. Furthermore, it was shown that in the murine CNS, neurons are the main target of infection, which causes these essential cells to undergo degeneration and eventually die by some form of programmed cell death after virus infection. Moreover, it appears that the viral surface glycoprotein (S) represents an important factor in the neurodegenerative process. Given all these properties, it has been suggested that these recognized human respiratory pathogens could be associated with the triggering or the exacerbation of neurological diseases for which the etiology remains unknown or poorly understood.

The etiology of most neurodegenerative diseases of the central nervous system remains unknown and likely involves a combination of genetic susceptibility and environmental triggering factors. Given that exposure to numerous infectious pathogens occurs during childhood, and that some viral infections can lead to neurodegeneration and demyelination, it is conceivable that some viruses may act as triggering factors in neuropathogenesis. We have previously shown that the prototype OC43 strain of the common cold-associated human respiratory coronavirus has the capacity to infect human neuronal and glial cells and does persist in human brains. Moreover, it has neuroinvasive properties in susceptible BALB/c mice, where it leads to a chronic encephalitis with accompanying disabilities. Here, we show that mutations in the viral spike glycoprotein, reproducibly acquired during viral persistence in human neural cell cultures, led to a drastically modified virus-induced neuropathology in BALB/c mice, characterized by flaccid paralysis and demyelination. Even though infection by both mutated and wild-type viruses led to neuroinflammation, the modified neuropathogenesis induced by the mutated virus was associated with increased viral spread and significantly more CD4+ and CD8+ T-lymphocyte infiltration into the central nervous system, as well as significantly increased levels of the proinflammatory cytokine interleukin (IL)-6 and the chemokine CCL2 (monocyte chemoattractant protein [MCP]-1). Moreover, recombinant virus harboring the S glycoprotein mutations retained its neurotropism, productively infecting neurons. Therefore, interaction of a human respiratory coronavirus with the central nervous system may modulate virus and host factors resulting in a modified neuropathogenesis in genetically susceptible individuals.

Human coronaviruses (HCoV) are recognized respiratory pathogens, and some strains, including HCoV-OC43, can infect human neuronal and glial cells of the central nervous system (CNS) and activate neuroinflammatory mechanisms. Moreover, HCoV-OC43 is neuroinvasive, neurotropic, and neurovirulent in susceptible mice, where it induces chronic encephalitis. Herein, we show that a single point mutation in the viral spike (S) glycoprotein (Y241H), acquired during viral persistence in human neural cells, led to a hind-limb paralytic disease in infected mice. Inhibition of glutamate excitotoxicity using a 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propranoic acid (AMPA) receptor antagonist (GYKI-52466) improved clinical scores related to the paralysis and motor disabilities in S mutant virus-infected mice, as well as protected the CNS from neuronal dysfunctions, as illustrated by restoration of the phosphorylation state of neurofilaments. Expression of the glial glutamate transporter GLT-1, responsible for glutamate homeostasis, was downregulated following infection, and GYKI-52466 also significantly restored its steady-state expression level. Finally, GYKI-52466 treatment of S mutant virus-infected mice led to reduced microglial activation, which may lead to improvement in the regulation of CNS glutamate homeostasis. Taken together, our results strongly suggest an involvement of excitotoxicity in the paralysis-associated neuropathology induced by an HCoV-OC43 mutant which harbors a single point mutation in its spike protein that is acquired upon persistent virus infection.

Human coronaviruses (HCoVs) are recognized respiratory pathogens with neuroinvasive and neurotropic properties in mice and humans. HCoV strain OC43 (HCoV-OC43) can infect and persist in human neural cells and activate neuroinflammatory and neurodegenerative mechanisms, suggesting that it could be involved in neurological disease of unknown etiology in humans. Moreover, we have shown that HCoV-OC43 is neurovirulent in susceptible mice, causing encephalitis, and that a viral mutant with a single point mutation in the viral surface spike (S) protein induces a paralytic disease that involves glutamate excitotoxicity in susceptible mice. Herein, we show that glutamate recycling via the glial transporter 1 protein transporter and glutamine synthetase are central to the dysregulation of glutamate homeostasis and development of motor dysfunctions and paralytic disease in HCoV-OC43-infected mice. Moreover, memantine, an N-methyl-D-aspartate receptor antagonist widely used in the treatment of neurological diseases in humans, improved clinical scores related to paralytic disease and motor disabilities by partially restoring the physiological neurofilament phosphorylation state in virus-infected mice. Interestingly, memantine attenuated mortality rates and body weight loss and reduced HCoV-OC43 replication in the central nervous system in a dose-dependent manner. This novel action of memantine on viral replication strongly suggests that it could be used as an antiviral agent to directly limit viral replication while improving neurological symptoms in various neurological diseases with a viral involvement. IMPORTANCEMutations in the surface spike (S) protein of human respiratory coronavirus OC43 appear after persistent infection of human cells of the central nervous system, a possible viral adaptation to this environment. Furthermore, a single amino acid change in the viral S protein modulated virus-induced neuropathology in mice from an encephalitis to a neuropathology characterized by flaccid paralysis, which involves glutamate excitotoxicity. We now show that memantine, a drug that is used for alleviating symptoms associated with neuropathology, such as Alzheimer's disease, can partially restore the physiological state of infected mice by limiting both neurodegeneration and viral replication. This suggests that memantine could be used as an antiviral agent while improving neurological symptoms in various neurological diseases with a viral involvement.