Neurological Implications of COVID-19: Role of Redox Imbalance and Mitochondrial Dysfunction

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 or COVID-19 has been declared as a pandemic disease by the World Health Organization (WHO). Globally, this disease affected 159 million of the population and reported ~ 3.3 million deaths to the current date (May 2021). There is no definitive treatment strategy that has been identified, although this disease has prevailed in its current form for the past 18 months. The main challenges in the (SARS-CoV)-2 infections are in identifying the heterogeneity in viral strains and the plausible mechanisms of viral infection to human tissues. In parallel to the investigations into the patho-mechanism of SARS-CoV-2 infection, understanding the fundamental processes underlying the clinical manifestations of COVID-19 is very crucial for designing effective therapies. Since neurological symptoms are very apparent in COVID-19 infected patients, here, we tried to emphasize the involvement of redox imbalance and subsequent mitochondrial dysfunction in the progression of the COVID-19 infection. It has been articulated that mitochondrial dysfunction is very apparent and also interlinked to neurological symptoms in COVID-19 infection. Overall, this article provides an in-depth overview of redox imbalance and mitochondrial dysfunction involvement in aggravating COVID-19 infection and its probable contribution to the neurological manifestation of the disease.

The olfactory nerve is not a likely route to brain infection in COVID-19: a critical review of data from humans and animal models

One of the most frequent symptoms of COVID-19 is the loss of smell and taste. Based on the lack of expression of the virus entry proteins in olfactory receptor neurons, it was originally assumed that the new coronavirus (severe acute respiratory syndrome coronavirus 2, SARS-CoV-2) does not infect olfactory neurons. Recent studies have reported otherwise, opening the possibility that the virus can directly infect the brain by traveling along the olfactory nerve. Multiple animal models have been employed to assess mechanisms and routes of brain infection of SARS-CoV-2, often with conflicting results. We here review the current evidence for an olfactory route to brain infection and conclude that the case for infection of olfactory neurons is weak, based on animal and human studies. Consistent brain infection after SARS-CoV-2 inoculation in mouse models is only seen when the virus entry proteins are expressed abnormally, and the timeline and progression of rare neuro-invasion in these and in other animal models points to alternative routes to the brain, other than along the olfactory projections. COVID-19 patients can be assured that loss of smell does not necessarily mean that the SARS-CoV-2 virus has gained access to and has infected their brains.

Severe acute respiratory syndrome coronavirus 2 infection reaches the human nervous system: How?

Without protective and/or therapeutic agents the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection known as coronavirus disease 2019 is quickly spreading worldwide. It has surprising transmissibility potential, since it could infect all ages, gender, and human sectors. It attacks respiratory, gastrointestinal, urinary, hepatic, and endovascular systems and can reach the peripheral nervous system (PNS) and central nervous system (CNS) through known and unknown mechanisms. The reports on the neurological manifestations and complications of the SARS-CoV-2 infection are increasing exponentially. Herein, we enumerate seven candidate routes, which the mature or immature SARS-CoV-2 components could use to reach the CNS and PNS, utilizing the within-body cross talk between organs. The majority of SARS-CoV-2-infected patients suffer from some neurological manifestations (e.g., confusion, anosmia, and ageusia). It seems that although the mature virus did not reach the CNS or PNS of the majority of patients, its unassembled components and/or the accompanying immune-mediated responses may be responsible for the observed neurological symptoms. The viral particles and/or its components have been specifically documented in endothelial cells of lung, kidney, skin, and CNS. This means that the blood-endothelial barrier may be considered as the main route for SARS-CoV-2 entry into the nervous system, with the barrier disruption being more logical than barrier permeability, as evidenced by postmortem analyses.

Involvement of the Nervous System in SARS-CoV-2 Infection

As a severe and highly contagious infectious disease, coronavirus disease 2019 (COVID-19) has caused a global pandemic. Several case reports have demonstrated that the respiratory system is the main target in patients with COVID-19, but the disease is not limited to the respiratory system. Case analysis indicated that the nervous system can be invaded by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and that 36.4% of COVID-19 patients had neurological symptoms. Importantly, the involvement of the CNS may be associated with poor prognosis and disease worsening. Here, we discussed the symptoms and evidence of nervous system involvement (directly and indirectly) caused by SARS-CoV-2 infection and possible mechanisms. CNS symptoms could be a potential indicator of poor prognosis; therefore, the prevention and treatment of CNS symptoms are also crucial for the recovery of COVID-19 patients.

Clearance of an immunosuppressive virus from the CNS coincides with immune reanimation and diversification

Once a virus infection establishes persistence in the central nervous system (CNS), it is especially difficult to eliminate from this specialized compartment. Therefore, it is of the utmost importance to fully understand scenarios during which a persisting virus is ultimately purged from the CNS by the adaptive immune system. Such a scenario can be found following infection of adult mice with an immunosuppressive variant of lymphocytic choriomeningitis virus (LCMV) referred to as clone 13. In this study we demonstrate that following intravenous inoculation, clone 13 rapidly infected peripheral tissues within one week, but more slowly inundated the entire brain parenchyma over the course of a month. During the establishment of persistence, we observed that genetically tagged LCMV-specific cytotoxic T lymphocytes (CTL) progressively lost function; however, the severity of this loss in the CNS was never as substantial as that observed in the periphery. One of the most impressive features of this model system is that the peripheral T cell response eventually regains functionality at ~60–80 days post-infection, and this was associated with a rapid decline in virus from the periphery. Coincident with this "reanimation phase" was a massive influx of CD4 T and B cells into the CNS and a dramatic reduction in viral distribution. In fact, olfactory bulb neurons served as the last refuge for the persisting virus, which was ultimately purged from the CNS within 200 days post-infection. These data indicate that a functionally revived immune response can prevail over a virus that establishes widespread presence both in the periphery and brain parenchyma, and that therapeutic enhancement of an existing response could serve as an effective means to thwart long term CNS persistence.

Herpes simplex virus US3 protein kinase regulates virus-induced apoptosis in olfactory and vomeronasal chemosensory neurons in vivo

A role for the US3 protein kinase of herpes simplex virus (HSV) in regulating virus-induced neuronal apoptosis was investigated in an experimental mouse system, in which wild-type HSV invades the central nervous system (CNS) via the olfactory and vomeronasal systems upon intranasal infection. Wild-type HSV-2 strain 186 infected a fraction of olfactory and vomeronasal chemosensory neurons without inducing apoptosis and was transmitted to the CNS, precipitating lethal encephalitis. In sharp contrast, an US3-disrupted mutant, L1BR1, induced neuronal apoptosis in these peripheral conduits upon infection, blocking viral transmission to the CNS and causing no signs of disease. An US3-repaired mutant, L1B(-)11, behaved similarly to the wild-type virus. Only 5 p.f.u. of L1BR1 was sufficient to compromise mice when the mutant virus was introduced directly into the olfactory bulb, a viral entry site of the CNS. These results suggest that the US3 protein kinase of HSV regulates virus-induced neuronal apoptosis in peripheral conduits and determines the neuroinvasive phenotype of HSV. Furthermore, virus-induced neuronal apoptosis of peripheral nervous system cells may be a protective host response that blocks viral transmission to the CNS.

HF10, an attenuated herpes simplex virus (HSV) type 1 clone, lacks neuroinvasiveness and protects mice against lethal challenge with HSV types 1 and 2

Herpes simplex virus (HSV), a neurotropic virus, establishes life-long and, although rare, life-threatening infection in humans, and it may precipitate substantial medical and psychosocial morbidity. Here we show that HSV-1 strain HF clone 10 (HF10) exhibits impaired neuroinvasiveness in peripheral olfactory, vomeronasal and trigeminal conduits following intranasal as well as corneal inoculation. HF10 attenuation likely arises from multiple defects of HSV genes, so that HF10 will not revert to a virulent phenotype. Intranasal vaccination of mice with HF10 conferred significant protection against lethal challenge with HSV-1 and HSV-2 via the intranasal and intravaginal routes. Thus, we propose that HF10 explicitly meets the prerequisites for a candidate live attenuated HSV vaccine.

VSV replication in neurons is inhibited by type I IFN at multiple stages of infection

Vesicular stomatitis virus (VSV) is a rhabdovirus which causes acute encephalitis in mice after intranasal infection. Because type I interferon (IFN) has been shown to be a potent inhibitor of VSV, we investigated the role of type I IFN in viral replication in neurons in culture. Pre-treatment of NB41A3 neuroblastoma cells or primary neuron cultures with IFN-beta or IFN-alpha strongly inhibits virus replication, with 1000-fold inhibition of infectious virus release occurring at 7 h post-infection, and maximum inhibition of 14,000-fold occurring at 14 h. Type I IFN inhibited both viral protein and RNA synthesis, but not enough to account for the inhibition of infectious virus yield. The influenza virus protein NS1 binds dsRNA and antagonizes induction of PKR activity, an IFN-inducible antiviral protein which phosphorylates and inactivates the elongation factor eIF-2alpha, resulting in cessation of translation. In NS1-expressing neuroblastoma cells, VSV replication was inhibited by IFN-beta as well as in control NB41A3 cells, and eIF-2alpha phosphorylation was blocked, suggesting that PKR activity was not involved in inhibition of viral protein synthesis. Similarly, inhibition of VSV by IFN-beta was not affected by addition of inhibitors of nitric oxide synthase, indicating that IFN-beta activity is not mediated by nitric oxide or superoxide. This contrasts with the essential role of NOS-1 in inhibition of VSV replication when neurons are treated with IFN-gamma. Analysis of cell culture supernatants revealed suppression of release of VSV particles from both NB41A3 cells and primary neurons treated with IFN. The inhibition of virion release closely matched the overall suppression of infectious VSV particle release, suggesting that type I IFN plays a role in inhibition of VSV assembly.

Relative neurotropism of a recombinant rhabdovirus expressing a green fluorescent envelope glycoprotein

A new recombinant vesicular stomatitis virus (rVSV) that expresses green fluorescent protein (GFP) on the cytoplasmic domain of the VSV glycoprotein (G protein) was used in the mouse as a model for studying brain infections by a member of the Mononegavirales order that can cause permanent changes in behavior. After nasal administration, virus moved down the olfactory nerve, first to periglomerular cells, then past the mitral cell layer to granule cells, and finally to the subventricular zone. Eight days postinoculation, rVSV was eliminated from the olfactory bulb. Little sign of infection could be found outside the olfactory system, suggesting that anterograde or retrograde axonal transport of rVSV was an unlikely mechanism for movement of rVSV out of the bulb. When administered intracerebrally by microinjection, rVSV spread rapidly within the brain, with strong infection at the site of injection and at some specific periventricular regions of the brain, including the dorsal raphe, locus coeruleus, and midline thalamus; the ventricular system may play a key role in rapid rVSV dispersion within the brain. Thus, the lack of VSV movement out of the olfactory system was not due to the absence of potential for infections in other brain regions. In cultures of both mouse and human central nervous system (CNS) cells, rVSV inoculations resulted in productive infection, expression of the G-GFP fusion protein in the dendritic and somatic plasma membrane, and death of all neurons and glia, as detected by ethidium homodimer nuclear staining. Although considered a neurotropic virus, rVSV also infected heart, skin, and kidney cells in dispersed cultures. rVSV showed a preference for immature neurons in vitro, as shown by enhanced viral infection in developing hippocampal cultures and in the outer granule cell layer in slices of developing cerebellum. Together, these data suggest a relative affinity of rVSV for some neuronal types in the CNS, adding to our understanding of the long-lasting changes in rodent behavior found after transient VSV infection.

Temperature-sensitive parainfluenza type 1 vaccine virus directly accesses the central nervous system by infecting olfactory neurons

Immunohistochemical investigation showed that intranasal inoculation of mice with a temperature-sensitive (ts) mutant of parainfluenza type 1 vaccine virus resulted in infection of some olfactory neurons as well as respiratory epithelial cells. It also disclosed the presence of viral antigens in glomeruli of the olfactory bulb but not in the secondary neurons (mitral and tufted cells). Polymerase chain reaction demonstrated the persistence of virus-specific nucleic acids in the olfactory bulb. These observations lead to the conclusion that parainfluenza virus, even with a ts phenotype, gains access to the central nervous system by infecting olfactory neurons.

Glycoprotein D-negative pseudorabies virus can spread transneuronally via direct neuron-to-neuron transmission in its natural host, the pig, but not after additional inactivation of gE or gI

Envelope glycoprotein D (gD) is essential for entry of pseudorabies virus (PRV) into cells but is not required for the subsequent steps in virus replication. Phenotypically complemented gD mutants can infect cells and can spread, both in vitro and in mice, by direct cell-to-cell transmission. Progeny virions released by infected cells are noninfectious because they lack gD. The aim of this study was to determine the role of gD in the neuropathogenicity of PRV in its natural host, the pig. We investigated whether gD-negative PRV can spread transneuronally via synaptically linked neurons of the olfactory and trigeminal routes. High doses of a phenotypically complemented gD mutant and gD mutants that are unable to express either gI or gI plus gE were inoculated intranasally in 3- to 5-week-old pigs. Compared with the wild-type virus, the virulence of the gD mutant was reduced. However, pigs inoculated with the gD mutant still developed fever and respiratory signs. Additional inactivation of either gI or gI plus gE further decreased virulence for pigs. Immunohistochemical examination of infected pigs showed that a PRV gD mutant could replicate and spread transneuronally into the central nervous system (CNS). Compared with the wild-type virus, the gD mutant had infected fewer neurons of the CNS on day 2. Nevertheless, on day 3, the gD-negative PRV had infected more neurons and viral antigens were present in second- and third-order neurons in the olfactory bulb, brain stem, and medulla oblongata. In contrast, gD mutants which are unable to express either gI or gI plus gE infected a limited number of first-order neurons in the olfactory epithelium and in the trigeminal ganglion and did not spread transneuronally or infect the CNS. Thus, transsynaptic spread of PRV in pigs can occur independently of gD. Possible mechanisms of transsynaptic transport of PRV are discussed.

HSV-1 brain infection by the olfactory nerve route and virus latency and reactivation may cause learning and behavioral deficiencies and violence in children and adults: a point of view

Two recent studies provided new evidence on the latency of HSV-1 DNA in 15.5% of olfactory bulbs and in 72.5% of trigeminal nerves from human corpses at forensic postmortems (1) and in 35% of 40 autopsied human brains (2). In the latter brains, latent HSV-1 DNA was found in the olfactory bulbs, amygdala, hippocampus, brain stem, and trigeminal ganglia. Although in these studies it is not known by which route HSV-1 entered the olfactory bulbs and brain, experimental studies in mice (3) revealed that injection of HSV-1 into the olfactory bulbs leads to virus migration into the brain amygdala and hippocampus via the olfactory nerve and locus coeruleus. If the olfactory ciliary nerve epithelium is the port of entry of HSV-1 into the olfactory bulbs and brain in humans as well, protection of the nose against HSV-1 infection may be needed to prevent virus latency in neurons in the amygdala and hippocampus (3). Infection of humans by HSV-1 was estimated to increase from 18.2% in the 0-20 year population group to 100% in persons older than 60 years (1), indicating that worldwide human populations at all ages are at risk of brain infection by the olfactory nerve route. In addition, both primary infection and reactivation of latent DNA in the brain may lead to damage of neurons in the brain involved in memory, learning, and behavior, as observed in infected, acyclovir-treated mice (3). The current introduction of a live apathogenic varicella-zoster virus (VZV) vaccine to immunize children against chickenpox (4) may suggest that the time is ripe for immunization of children and adults against HSV-1 infections, especially infections by the olfactory nerve route, to prevent potential brain damage.

The earliest events in vesicular stomatitis virus infection of the murine olfactory neuroepithelium and entry of the central nervous system

After intranasal instillation of mice with vesicular stomatitis virus (VSV), olfactory receptor neurons are infected. By 12 to 24 hr postinfection, VSV antigens are observed in adjoining supporting and basal cells and in other structures of the olfactory epithelium and lamina propria. Peripheral deafferentation of the olfactory epithelium with Triton X-100 or bilateral surgical bulbectomy does not prevent spread of VSV to the central nervous system (CNS); the route of spread differs considerably from the route taken when the olfactory nerve is intact. In contrast to rabies virus and HSV-1, VSV does not use the trigeminal nerve for entry into the brain, as the trigeminal ganglion remains virus-free following intranasal infection. These results indicate that VSV has a strong tropism for olfactory receptor cells, using them for entry into the CNS. Both retrograde and anterograde transneuronal and nonneuronal (ependymal cells and cerebrospinal fluid) pathways are utilized by VSV within the CNS.

Mechanism of neuroinvasion of Venezuelan equine encephalitis virus in the mouse

Venezuelan equine encephalitis virus (VEE) causes a biphasic disease in mice following subcutaneous inoculation in the footpad. In the initial phase, virus replicates primarily in the lymphoid tissues and induces a high titer viremia. Subsequently, the virus invades the central nervous system (CNS) from the circulation, and an encephalitis ensues. At the earliest times that VEE specific in situ hybridization signal was observed in the CNS, it was in areas of the brain involved in olfaction, leading to the hypothesis that virus may invade the brain from the circulation through the olfactory system. The results presented in this paper define the route of CNS invasion in experimental murine VEE disease initiated by subcutaneous inoculation. Virus circulating in the blood appears to seed specific areas of the peripheral nervous system during the viremic lymphoid phase of the illness. Virus replication within olfactory and dental tissues is followed by centripetal spread of virus along neural pathways. Virus enters the brain in a pattern reflecting the proximity of the peripheral invasion site to the CNS. Specifically, virus is first found in the brain within the structures of the olfactory system, followed by areas innervated by the trigeminal nerve. Virus later disseminates along fiber tracts and connected circuits within the brain, resulting in a disseminated meningoencephalitis. Surgical or chemical interruption of the olfactory system at the level of the olfactory neuroepithelium or the main olfactory bulb inhibited entry of VEE into the CNS through the olfactory nerve. However, the olfactory route is not absolutely required for CNS invasion, as virus invaded the CNS of olfactory ablated animals through the trigeminal nerve. These observations are consistent with a model of hematogenous seeding of the peripheral nervous system, followed by invasion of the CNS by direct neural spread.