Neurotrophic viruses

IL-6 plays an important role in cytokine release syndrome CRS , a systemic inflammatory response characterized by a sharp increase in the level of a large number of pro-inflammatory cytokines, which has been linked to severe COVID cases Zhang C. In lungs, binding of SARS-CoV-2 to alveolar epithelial cells activates the innate and adaptive immune systems, resulting in the release of a variety of cytokines, including IL-6 Zhang C.

In the CNS the physiological function of IL-6 is multifaceted; on one hand IL-6 exerts neurotrophic and on the other hand neuroprotective effects, and can also function as a mediator of inflammation, demyelination, and astrogliosis Van Wagoner and Benveniste, The predominant source of IL-6 in the CNS is reactive astrocytes, and while they retain IL-6 levels low in the normal brain, they elevate IL-6 expression during injury, stroke, inflammation, and infection Zhang C.

For example, elevated infection-evoked production of multiple cytokines and chemokines, including IL-6 has been detected after infection with flaviviruses in serum and cerebrospinal fluid of patients, brain tissue samples from mice and primary human cortical astrocyte cultures Palus et al. The notion that the immune response initiated to eradicate virus becomes pathological, causing immune-mediated damage to the host, has been documented clinically and experimentally King et al.

The current evidence implies that these cells may be considerably involved in viral replication and spread in the CNS and could play a role in inducing or further promoting neuroinflammation and neurotoxicity after infection.

Besides CoVs, viruses from other families can also be neurotropic, including flaviviruses. These are transmitted to humans mainly by arthropods, such as mosquitos and ticks, and possibly by other yet unknown vectors Mandl, As with CoVs, flavivirus transmission between humans has also been documented Mlakar et al.

In addition, the presence of several other flaviviruses e. Several flaviviruses have been recognized as neurotropic and are successful in infecting astrocytes Potokar et al. In flavivirus infections, astrocytes stand out due to their high production of the virus in comparison to other cell types in the CNS. This is highly relevant for the spread of infection through the CNS, especially because astrocytes are also more resilient to flavivirus infection.

Regardless of their resilience, TBEV, ZIKV, and JEV infections trigger substantive changes in human astrocytes, including their stress-induced reactivation, morphological changes of intracellular organelles e. All these changes enable host cells to support viral replication and spread. Human fetal astrocytes are more efficiently infected with Zika virus and release more progeny virus than neurons.

The cell boundaries of individual astrocytes and neurons are delineated. Note that the number of Zika particles is significantly higher in astrocytes compared to neurons. B The graphs represent plaque assay measurements of infectious virus particles in the supernatants at different times p. The production trend of infectious virus particles in astrocytes is higher than that in neurons for all three strains.

C A schematic representation depicting higher production and release of progeny ZIKV virus pink dots in astrocytes compared to neurons. The role of astrocytes in host immune responses is well recognized and two-sided. On the one hand, increased expression and release of pro-inflammatory cytokines from astrocytes was confirmed in cell cultures and patients and contributes to the modulation of host immune responses and flavivirus-induced neurotoxicity.

This includes BBB breakdown, which further decreases neuronal viability and consequently exacerbates the course of the disease Van Marle et al. On the other hand, astrocytes can also act protectively, as the rapid interferon IFN response after flavivirus infection restricts viral replication and spread through upregulating type I IFNs.

This consequently enhances the expression of proteins that inhibit several steps of the host cell viral cycle, thus alleviating neuropathogenesis in the CNS Schoggins and Rice, ; Lindqvist et al. These effects may influence region-specific multiple signaling mechanisms, as TBEV-, WNV-, and JEV-infected astrocytes and neurons showed better survival in certain regions of the brain Hussmann et al. Interestingly, different flavivirus strains appear to exert different effects on specific astrocyte responses.

This is perhaps most evident in the case of the WNV. On the one hand, several WNV strains were shown to be non-neuroinvasive i. On the other hand, practically all WNV strains were shown to be neurovirulent i. Of particular note, WNV strains with longer passage histories were more virulent i. As astrocytes play a key role in maintaining the functional integrity of the BBB, it is not surprising that they are considered important determinants of WNV infection.

Hussmann et al. This confirms that astrocytes play a critical role in WNV neuropathology. Upon WNV infection, astrocytes also release and activate pro-inflammatory cytokines that recruit leukocytes and matrix metalloproteases, which disrupt the BBB Van Marle et al.

Such a strategy is not uncommon for neuroinvasive viruses Eugenin et al. To date, three vaccines for WNV have been developed and licensed for equine use in Europe Lecollinet et al. Animal experiments have confirmed that a vaccine derived from a chimeric virus, which was constructed using the structural proteins M and E of the Kunjin WNV strain and the genome backbone of the insect-specific Binjari virus, offered efficient protection against virulent WNV strains, including the highly pathogenic WNV NY 99 strain Vet et al.

The SARS-CoV-2 G variant replicated to higher titers in nasal-wash samples early after infection and outcompeted the D variant; however, the G variant did not cause more severe symptoms than the D variant in hamsters, which corroborates current findings in humans Baric, ; Plante et al. Fortunately, the DG substitution mutation is unlikely to reduce the ability of the vaccines in clinical trials to protect against COVID Plante et al.

Similarly as with the WNV, a few amino acid alterations are also sufficient to notably change the neurovirulence, infection rate, and replication kinetics of the ZIKV Duggal et al. Higher infection rates in astrocytes compared to neurons can be attributed, in part, to the markedly different kinetics of the immune response triggered by different ZIKV strains Hamel et al.

Infection of human astrocytes with different ZIKV strains apparently leads to activation of the autophagy pathway Ojha et al. The percentages were determined by counting the number of immunolabeled cells versus the number of all DAPI-stained nuclei representing single cells. Data were collected from one experiment performed in duplicate. The percentages of ZIKV-positive astrocytes and neurons are different at both 12 h p.

Note that the rate of cell infection depends on the ZIKV strain. B In astrocytes, the average speed of endocytotic vesicles increased with longer times p. In neurons, the average vesicle speed exhibited the most prominent increase at 36 h p.

The ZIKV is depicted as pink dots. The proteins regulate numerous intracellular processes, including immunity Tzivion et al. The possible targeting of proteins by ORF3a has not been fully addressed yet; however, ORF3a mutations may affect intraviral protein—protein interactions Wu et al.

The symptoms of TBEV infection, which may range from subclinical to mild flu-like disease to sever encephalitis with a lethal outcome, may depend on the virulence of the specific strain as well as the immune status of the host Lindqvist et al.

Comparing these strains, isolated from rodents, ticks, and humans, have shown that they differ, among others, in certain amino acids of envelope E protein, which is a cell attachment protein that enables cell entry.

The E protein mediates the primary attachment of the virus to its target cell and mediates membrane fusion, thus determining, at least in part, the host-cell tropism and pathogenesis of the virus Mandl et al.

For example, among the 15 amino acids that differ between the Ljubljana I and Neudoerfl strains, the most prominent amino acid change was identified to be IV Fajs et al. Both strains exerted a similar effect on cell viability. Primary rodent cortical astrocytes exhibited extraordinary resistance to the Ljubljana I strain in terms of uncompromised cell viability after 2 weeks of infection Potokar et al.

Similarly, the Neudoerfl strain, a prototype strain of the European subtype, also did not extensively compromise the viability of primary human brain cortical astrocytes after 2 weeks of infection Palus et al. Unfortunately, the two strains were not directly compared in the same astrocyte culture in terms of their replication and effect on cell viability.

These effects were evident in neurons, while in astrocytes, no differences were noted between both strains in their host cell entry efficacy, but only in their replication rate. All of these changes correlated with increased levels of viral RNA and infiltrating T and B cells in the brain Lindqvist et al.

In addition, E protein variations between strains influence the formation of neutralizing antibodies against E protein that are important in neutralizing the virus after infection. As amino acid differences in the E protein are known to affect neutralizing antibodies, vaccine-derived antibodies are not equally efficient for different strains of TBEV and thus these E protein variations might be a contributing factor to vaccine breakthroughs Beck et al.

Different TBEV strains also show high longevity in host organs; however, the amount of viral load in particular organs appears to differ between host organisms and respective strains. In Ljubljana I-infected forest rodents Myodes glareolus and Apodemus sylvaticus , the highest viral load was measured in spleen and brain samples Knap et al. These viral loads were much higher in Hypr strain RNA-infected laboratory-bred Microtus arvalis , in which RNA was confirmed in several organs even several months after infection Achazi et al.

Mutations that increase the net positive charge of the E protein lead to lower virus pathogenicity and neuroinvasiveness due to stronger binding to glycosaminoglycans and thus a more rapid clearance of the virus from the blood, as was demonstrated in mice infected with Far-Eastern subtype Oshma strains Goto et al.

Genetic divergence of strains that circulate in the population from the vaccine strain might reduce neutralizing antibody titers and thus decrease vaccine efficacy Lindqvist et al.

These processes may be diversely affected in different parts of the brain that express variable amounts of SARS-CoV-2 receptors. In analogy with neurotropic flavivirus strains, it is expected that different SARS-CoV-2 strains may trigger somewhat different neurological symptoms as well as variations in the response to different vaccines.

All the authors wrote the manuscript, extensively contributed to the preparation and finalization of the manuscript, and approved the manuscript for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. National Center for Biotechnology Information , U. Journal List Front Cell Neurosci v. Front Cell Neurosci. Published online Apr 9.

Author information Article notes Copyright and License information Disclaimer. Received Feb 1; Accepted Mar The use, distribution or reproduction in other forums is permitted, provided the original author s and the copyright owner s are credited and that the original publication in this journal is cited, in accordance with accepted academic practice.

No use, distribution or reproduction is permitted which does not comply with these terms. This article has been cited by other articles in PMC. Introduction Human coronaviruses CoVs were first identified in the mids and were named for the crown-like spikes on their surface Figure 1A. Open in a separate window. Neurotropic Viruses Affect the Functions of Astrocytes and Neurons A vast variety of viruses from different families are capable of invading the CNS in which they infect different cell types.

Flaviviruses Family Flaviviridae Besides CoVs, viruses from other families can also be neurotropic, including flaviviruses. Flavivirus Infection of Astrocytes Several flaviviruses have been recognized as neurotropic and are successful in infecting astrocytes Potokar et al. Flavivirus Strain-Related Effects on Astrocytes WNV Strains Interestingly, different flavivirus strains appear to exert different effects on specific astrocyte responses. TBEV Strains The symptoms of TBEV infection, which may range from subclinical to mild flu-like disease to sever encephalitis with a lethal outcome, may depend on the virulence of the specific strain as well as the immune status of the host Lindqvist et al.

Author Contributions All the authors wrote the manuscript, extensively contributed to the preparation and finalization of the manuscript, and approved the manuscript for publication.

Conflict of Interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Footnotes Funding. References Achazi K.

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Disruption of in vitro endothelial barrier integrity by Japanese encephalitis virus-Infected astrocytes. Glia 63 — Endocytosis-mediated HIV-1 entry and its significance in the elusive behavior of the virus in astrocytes. The pathogenic properties of different prion strains are considered to be enciphered in the strain-specific different conformation of PrP Sc 18 , According to the unified theory proposed by Weissmann, nucleic acid molecules have been suggested to be a factor contributing to strain-specific properties of prions Influenza-related acute neurologic complications such as seizure and encephalopathy have been reported in 1 to 4 cases per ,00 children-years after infection with seasonal IAVs H5N1 avian IAV has been also shown to cause encephalitis in a child These facts suggest that IAV infection could cause neurological complications either acutely or chronically.

Our current results may thus raise the future question of whether neurotropic virus infections, including neurotropic IAV infection, might cause or be associated with sporadic prion diseases. All experiments in this study were performed under biosafety level 2 conditions. Mouse anti-PrP monoclonal Ab clone was kindly gifted from M. Horiuchi, Hokkaido University Cells were split at to fold dilution every 4 to 5 days by dissociation in phosphate-buffered saline PBS 11,—15; Nakalai Tesque, Osaka, Japan containing 2.

The cells were then washed 3 times with PBS, permeabilized by incubation with 0. Cell viability was indicated by the number of living cells. Brain homogenates were prepared as described previously For preparation of large amount of proteins, the PK-digested reaction was terminated by addition of Protease Inhibitor Cocktail 25,—11, Nakalai Tesque and butanol:methanol solution.

The resulting pellet was resuspended in a sodium dodecyl sulfate SDS sample buffer The samples were diluted to adjust the final concentration of GdnHCl to 0. Western blotting was performed as described previously To evaluate the levels of proteins, their signals were densitometrically measured using LAS mini-chemiluminescence imaging system Fuji Film, Tokyo, Japan.

Signal intensities were determined by Image Gauge software Fuji Film. All manipulations were done at room temperature unless otherwise stated. Cells on coverslips 15 mm No. The cells were then washed three times with PBS, treated with 0. Cells approximately 7. The cells were then sonicated and passed through a 27 gauge needle several times to prepare a cell lysate inoculum. Mice were diagnosed as sick when they developed more than five of the following features: emaciation, decreased locomotion, ruffled body hair, ataxic gait, kyphosis, priapism, upright tail, crossing leg, hind leg paresis, and foreleg paresis.

Mice were also diagnosed as terminal when they became akinetic. Immunohistochemistry was performed as described previously HE staining was performed as described previously Vacuoles in 0.

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Publication types Research Support, N. Substances RNA.