Neurotropism as a Mechanism of the Damaging Action of Coronavirus

Abstract—

Clinical evidence suggests that COVID-19 is accompanied by many symptoms of damage to the central and peripheral nervous system. This article outlines new aspects of pathogenesis that consider the principle of neurotropism as the leading cause of SARS-CoV-2 infection and central nervous system dysfunction. New data demonstrate additional mechanisms for coronavirus transfection. The description of some transmembrane proteins (neuropilin, etc.) serve as an additional argument for SARS-CoV-2 neurotropism, these molecules act as cofactors for virus transfection in the tissues of the lungs, brain, and other organs. The study of the damaging effect of SARS-CoV-2 at the level of an individual neuron is formulated as a task of neurotropism investigation. The use of the organoid methodology as a new approach in biomedical analysis for modeling the relationship between the host and the pathogen is described. Numerous data on the pathogenesis of COVID-19 indicate that astrocytes and microglia are targets of SARS-CoV-2. Neuroinflammation is considered as an inverse manifestation of neurotropism and a consequence of the neural and mental complications of pathogenesis.

INTRODUCTION

The outbreak of the COVID-19 pandemic led to large-scale studies of the pathogenesis of this disease, which is a tricky complex of concomitant negative processes and consequences. An analysis of the clinical experience shows that pathogenesis of the respiratory distress syndrome caused by the SARS-CoV-2 virus exhibits a huge range of manifestations. These include clinical disorders of whole systems, individual organs, tissues, and biochemical reactions. COVID-19 represents a disturbance of cellular and molecular processes that gives reasons to identify pathogenic links. Diffuse alveolar lung injury with pronounced microangiopathy in the form of bilateral pneumonia is a typical clinical manifestation of COVID-19. Systemic infection is accompanied by a rapid increase in circulating chemokines and interleukins in the blood, which cross the blood-brain barrier (BBB) to enter the brain. Clinical materials indicate a variety of symptoms related to immediate or long-term neurodegenerative and mental disorders.

Data on the neuroinvasive potential of SARS-CoV-2 confirm damage to the structures of the brain and peripheral nervous system. A detailed understanding of the pathogenesis, and identification of cellular and biochemical targets of SARS-CoV-2 are important in order to elaborate a therapeutic anti-COVID strategy. This paper takes into account aspects of COVID-19 pathogenesis that allow us to analyze the cellular and biochemical mechanisms of viral invasion leading to various forms of neurodegenerative and mental complications.

Neurotropism is considered a leading mechanism involved in the neurodestructive effect of SARS-CoV-2. Experimental data are a basis for interpretation of the mechanisms associated with cellular tropism of coronaviruses. In addition to the traditional consideration of ACE2 (angiotensin-converting enzyme 2) as the main transporter in coronavirus entry, we assess the involvement of other molecules (neuropilins and other proteins), which facilitate transfection and contribute to SARS-CoV-2 neurotropism. The virus entry into the brain tissue is associated with a processes wherein disturbance of the immune defense plays a leading role. Neuroinflammation with an altered phenotype of microglial cells and astrocytes results in damage to brain systems. Clinical studies indicate that astrocytes and microglia are targets of neurotropic viruses, including SARS-CoV-2. The neurotropism of SARS-CoV-2 frequently extends to the postcovid period, leaving a trace in the form of neurodegenerative manifestations that can even develop into mental disorders.

COVID AND NEURAL DAMAGE

Clinical experience has generated significant material indicating the negative effects of coronaviruses on brain function, manifestations of neurotropism and neuroinvasion, which cause neurodegenerative and mental disorders in COVID-19. The initial processes of pathogenesis, i.e., expression of cyto- and chemokines, endothelial dysfunction, neuroinflammation, hypercoagulation, and immunothrombosis, determine the severity of multiple organ failure (Correia et al., 2020; Tsivgoulis et al., 2020; Morgado et al., 2021). An analysis of clinical histories made it possible to identify brain cells as the second most important pathogenetic target in COVID-19, which needs early protection from neuroinvasion (Li, Z. et al., 2020). Comorbidities, primarily age-related, neurodegenerative, and mental conditions, are associated with higher COVID-19 susceptibility. Postcovid complications have become a separate area of study (Costas-Carrera et al., 2022).

Neurological symptoms in the acute period of COVID-19 include disorders of cerebral circulation and systemic cerebral disorders. The development of ischemia affects small perforating vessels and disrupts blood supply to limbic areas of the brain (Sokolova and Fedin, 2020). Such disorders of cerebral circulation are usually characterized as complications of ischemic disease, arterial hypertension, or thrombogenic changes in diabetes (Gusev et al., 2020).

Epidemiological data and postmortem brain examination suggest that viral infections, including SARS-CoV-2, may contribute to the exacerbation of Alzheimer’s disease. The results of the previous research showed that viral fragments of CoV strains were detected in brain samples together with pathogenic beta-amyloid deposits. The areas of the brain affected by the virus include the limbic system of the cortex and subcortical structures that are associated with memory and cognitive processes (Arbour et al., 2000).

A growing number of cases provide evidence of mental manifestations associated with COVID-19. Clinical protocols document a variety of characteristic symptoms: post-traumatic stress disorder, depression, anxiety, obsessive-compulsive phenomena, first-episode psychosis, neurocognitive syndrome, and others (Fedin, 2021). Viral entry into brain tissue may lead to cerebral dysfunction with cognitive impairment, primarily in compromised or elderly patients. These disorders can be caused by impaired endothelial function and neuroinflammation (Steardo et al., 2020).

Therefore, a large number of clinical reports and reviews describe a wide range of neurological symptoms that occur in COVID-19 patients. The key question remains: which clinical signs are due to the direct influence of the virus, and which are consequence of the associated pathological processes caused by the infection? This formulation of the issue makes it possible to separate neurotropism for a better understanding of the cellular and molecular components involved in COVID pathogenesis and for better identification of new pharmacologic targets.

NEURTROPISM. BIOCHEMISTRY AND CELLULAR BIOLOGY OF THE SELECTIVE AGGRESSIVENESS OF SARS-CoV-2

In modern medical literature neurotropism is defined as an upregulation of biochemical mechanisms to facilitate entry, replication, and propagation of certain viral strains in nervous tissue. The presence of complementary chemical structures of the host cell and virion ligands is a determinant of virus host tropism. Comparison with other coronavirus strains indicates that neurotropism is a common feature of infection, which is expressed to the greatest extent in SARS-CoV-2 (Desforges et al., 2014; Hu et al., 2020).

There are a number of reasons as to why SARS-CoV-2 neurotropism should be isolated among the mechanisms of COVID-19 clinical symptoms. Although new facts about the mechanisms of neuroinvasion have become clearer, it remains necessary to clarify whether SARS-CoV-2 is a truly neurotropic agent or if neurodestruction is a consequence of systemic cellular and biochemical processes elicited by COVID-19 (ElBini Dhouib, 2021). It is emphasized that the brain, being a super-complex and dynamic cellular-tissue system, may represent an attractive environment for SARS-CoV-2 replication. At the same time, cytokine storm and cellular inflammation lead to stochastic activation of components within the immune system that trigger vascular disorders in the endothelium, immunothrombosis, and injury to the parenchyma and neurons of the brain. Neurodegenerative mechanisms, i.e., phosphorylation of tau protein, synuclein aggregation, and accumulation of toxic proteins, were reported for many infected brain structures (Nath and Smith, 2021).

The idea of neurotropism is reinforced by the information about the presence of ACE2 in some areas of the brain. The results of genetic analysis showed that the enzyme has a high concentration in the substantia nigra, the ventricles of the brain, and the middle temporal gyrus. According to cell-type distribution analysis, ACE2 expression was detected in excitatory and inhibitory neurons of the temporal gyrus and the cerebral cortex (Chen et al., 2021). It is assumed that the regional presence of ACE2 in the brain as an indispensable entry receptor for coronavirus is the primary argument in favor of a neuroinvasive mechanism.

The Main Pathways of SARS-CoV-2 Entry into the Brain

The primary task of researchers is to determine how the virus implements its neurotropic action after its entry into the brain structures? Current clinical experience indicates that SARS-CoV-2, interacting with the ACE2 protein, takes advantage of complex hematogenous transfection and/or direct neurogenic invasion into the brain.

The so-called hematogenous route involves injury to the vascular endothelium and disruption of the protective function of the BBB. Model experiments revealed how the infected cells allow the infection to enter into brain areas (Buzhdygan et al., 2020). Previous studies with various strains of SARS-CoV showed that neurons located in the centers of the medulla oblongata may be affected (Netland et al., 2008). When applying this information to the current COVID-19 situation, it is assumed (Li, Y.C. et al., 2020) that negative outcomes are often associated with neuroinvasive dysfunction of the cardiorespiratory center in the brain.

The death of endothelial cells disrupts microenvironment of the brain parenchyma and allows the virus to reach other brain regions (Alquisiras-Burgos et al., 2021). Pathoanatomical investigation revealed SARS-CoV-2 virus particles in microvascular endothelial cells in the frontal lobe of the brain (Paniz-Mondolf et al., 2020). The presence of ACE2 in the endothelium is associated with multiple organ failure due to disturbance of the vasculature (Baig et al., 2020).

Other arguments support a trans-neuronal hypothesis: SARS-CoV-2 enters the brain via the olfactory, the neural taste, and trigeminal pathways, especially at the early stages of infection (Liu et al., 2021). Direct connectivity of the olfactory bulb to the amygdala, the entorhinal cortex, and the hippocampus represents a pathway of viral infection from olfactory mucosa to reach the neural network and cause injury (Aghagoli et al., 2021).

Some authors associate loss of sense of smell, a typical symptom of COVID-19, with infection of the olfactory system by SARS-CoV-2. Introduction of infection may occur by means of axonal transport of the virus via the olfactory nerve and its entry to the olfactory cortical region (Brann et al., 2020). МRI studies revealed the presence of the virus in the epithelium of the nasal cavities and ciliated cells in patients at the early phase of the disease (Politi et al., 2020).

SARS-CoV-2 RNA is detected in autopsy studies in the brain regions involved in recognizing environmental signals. Fragments of the viral RNA were found in the olfactory bulb, amygdala, entorhinal cortex, temporal and frontal neocortex. These areas of the brain are responsible for emotional and spatial memory responses and cognitive functions (Serrano et al., 2021).

SARS-CoV-2 in Cerebrospinal Fluid

The cerebrospinal fluid regulates trophic and metabolic processes between the blood and the brain due to cerebrospinal fluid circulation and turnover. SARS-CoV-2, similarly to other coronaviruses, can use cerebrospinal fluid for its invasion. SARS-CoV-2 antibodies were detected in the cerebrospinal fluid of patients, and this may serve as an indicator of infection. Viral fragments were also revealed in the cerebrospinal fluid in meningoencephalitis associated with COVID-19 (Moriguchi et al., 2020). Electron microscopy reveals traces of coronavirus in neurons and endothelial cells in brain autopsies (Baig et al., 2020). The detected products of SARS-CoV-2 include proteins S1 and S2, fragments of the viral envelope, and nucleoproteins (Benameur et al., 2020).

Therefore, comparison of clinical and biochemical studies confirms the signs of SARS-CoV-2 neuroinvasion.

(1) When the virus was detected in the cerebrospinal fluid, the patients hardly exhibited any respiratory symptoms, but manifested signs of encephalitis or demyelinating pathology, which apparently occurred due to direct entry of the virus into the cerebrospinal fluid.

(2) Comparative analysis shows that the quantity of SARS-CoV-2 in cerebrospinal fluid is associated with the severity of neurological symptoms: the highest content was related to encephalitis and a lower quantity was observed in disorders of cerebral circulation, encephalopathy, and Guillain-Barre syndrome (Li, Y.C. et al., 2021).

(3) The presence of SARS-CoV-2 antibodies in the cerebrospinal fluid in patients with an intact BBB may indicate direct invasion of the virus into the brain. Upon entry into the cerebrospinal fluid, coronavirus invades into the main areas, including the brain stem that contains the nuclei that control cardiorespiratory functions. The invasion of SARS-CoV-2 into the brain stem may be a cause of acute respiratory failure in COVID-19 (Dey et al., 2021).

Neuropilin and Other Receptors for Viral Entry

The data indicate that in addition to ACE2, COVID-19 pathogenesis involves additional routes that facilitate viral entry into the host cells. The participation of transmembrane proteins as independent docking sites with virus fragments served to explain the tissue tropism of SARS-CoV-2: selective ligands of the host cell molecules act as infection cofactors, including membrane-bound serine protease TMPRSS2, furin, cathepsin L, basigin, and neuropilin-1 (NRP-1) (Coutard et al., 2020; Daly et al., 2020). One of the new players in the pathogenesis of COVID-19 is the neuropilin protein (Sarabipour and Mac Gabhann, 2021).

Neuropilins are a group of glycoproteins that have long been beyond the interest of biologists and pathophysiologists. Neuropilin was discovered in the developing brain and was identified as a neuropil, an axon-guidance receptor (Kawakami et al., 1996). The studies performed during the pre-COVID period provide a general picture of the processes involving the NRP-1 and NRP-2 neuropilins. Due to the variety of connections, neuropilins are involved in migration and invasion of various cells, membrane disorders, angiogenesis, etc. Neuropilins are connecting components involved in the control of many physiological processes (Plein et al., 2014; Kofler and Simons, 2016).

NRP-1 is both the host factor of cellular entry of SARS-CoV-2 and a component of increased contagiousness (Perez-Miller et al., 2021). In the traditional interpretation, the binding domain of the coronavirus interacts exclusively with ACE2. According to new data, the S1 and the S2 domains cleaved by furin protease mediate the fusion of virus membrane with host neuropilins. Inhibition of this interaction reduced SARS-CoV-2 infectivity in cell culture (Daly et al., 2020; Kielian, 2020). Autopsy analysis of olfactory epithelial cells in COVID-19 patients showed that NRP-1 facilitates viral entry and enhances the pathogenic effects of coronavirus (Cantuti-Castelvetri et al., 2020).

NRP-1 was investigated in individual structures of the human brain using a sequencing method. RNA expression was highest in the hippocampus when compared to the olfactory region, basal ganglia, thalamus, hypothalamus, and midbrain. This leads to the conclusion about a wide variety of possible neurological symptoms in COVID-19. An analysis of viral invasion routes in COVID-19 reveals a complex mechanism of neurological complications. The discovery of a new factor, NRP-1, provides new details related to the neurotropic mechanisms of SARS-CoV-2 (Davies et al., 2020).

Therefore, while until recently SARS-CoV-2 entry into host cells was mainly attributed to ACE2 as a unique cofactor (Hopkins et al., 2021), new data present other mechanisms that enhance coronavirus transfection in COVID-19.

The involvement of some transmembrane proteins is new evidence in favor of the tissue tropism of SARS-CoV-2, in which these endogenous compounds act as cofactors of virus transfection into the tissues of the lungs, brain, and other organs (Sarabipour and Mac Gabhann, 2021).

However, it should be noted that, using the example of neuropilins, their presence in many tissues and association with pathogenetic processes of immunothrombosis and organ injury requires control and a selective strategy of targeted therapy (Gomazkov, 2022).

ORGANOIDS—A NEW METHOD TO STUDY NEUROTROPISM

Organoids as Subcellular Model Systems

The study of the mechanisms involved in the damaging effect of SARS-CoV-2 at the neuron level is a useful task in the investigation of neurotropism. The evident difficulties of the in-life study of cellular and molecular processes encourage the use of new methodologies. A new approach using organoids has gained significant popularity in the study of cellular tropism and mechanisms of SARS-CoV-2-induced injury (Katsura et al., 2020; Ramani et al., 2020; Yang et al., 2020).

Brain organoids are derived from embryonic stem cells or induced pluripotent stem cells and find various applications in regenerative medicine. These systems are three-dimensional (3D) cellular structures, since they reflect tissue cytoarchitecture similar to the developing elements of the brain (Renner et al., 2017). The initial experiments with a three-dimensional (tissue) system of organoids were devoted to the phenotypic analysis of the brain. This method is used in the study of the human cerebral cortex and some forms of neuronal disorders (Lancaster et al., 2013). These studies showed the significant value of organoids as experimental platforms, including for studies related to COVID-19 pathogenesis (Lv et al., 2021). The use of organoids provides an opportunity to identify tissue-specific forms of SARS-CoV-2 interaction with host cells (Tiwari et al., 2021).

Modeling of SARS-CoV-2 Effects at the Level of an Individual Neuron

The neurotropism of SARS-CoV-2, i.e., its selective invasiveness to various brain structures, which was revealed experimentally, is considered the cause of COVID-19 neuronal dysfunction. Experiments were conducted using a hPSC-based platform of subcellular microstructures of the cerebral cortex, hippocampus, hypothalamus and midbrain exposed to convalescent serum of a patient. Immune labeling of MAP2, PU.1, and GFAP proteins confirmed that the cells obtained in the model system were identical (Ramani et al., 2021).

A range of studies revealed that the virus can cause a productive infection of cortical neurons and neural progenitors in three-dimensional systems (Mao and Jin, 2020; Zhang et al., 2020). Comparative studies of coronavirus strains demonstrated a specific ability of SARS-CoV-2 to replicate in various types of nerve cells (Chu et al., 2020). Incubation with SARS-CoV-2 led to the accumulation of viral particles, which was accompanied by an increase in the level of viral RNA (Bullen et al., 2020). Autopsies confirmed the presence of SARS-CoV-2 fragments in cortical neurons in COVID-19 patients (Song et al., 2021).

The results of subsequent experiments revealed that choroid plexus organoids were destroyed under the action of SARS-CoV-2 (Jacob et al., 2020). Infection of choroid cells causes increased viral RNA replication, and triggers the immune response, neuroinflammation, and massive apoptosis (Pellegrini et al., 2020).

Models of Organoids and Identification of Cofactors in Viral Transfection

After identification of the key role of ACE2 in SARS-CoV-2 entry into the lung tissue, this enzyme received major attention. Indeed, SARS-CoV-2 uses ACE2 as a Trojan horse to invade target cells, and ACE2 acts as a high-affinity receptor for the virus.

New cellular technologies offer more subtle arguments for the essential role of ACE2 in the tissue transfection by the virus. Experiments using human endothelial cell cultures derived from pulmonary, cardiac, brain, and kidney tissues showed that ACE2 knockout blocked the infecting capacity of SARS-CoV-2. In contrast, cultured endothelial cells transduced with recombinant ACE2 receptors were infected by SARS-CoV-2 and revealed high viral titers and endothelial cell lysis. SARS-CoV-2 infection of ACE2-expressing endothelial cells elicits the procoagulative and inflammatory responses observed in COVID-19 patients (Conde et al., 2020).

Studies using 3D-models of human neurons have shown that although the neurons of human brain organoids express low levels of ACE2, SARS-CoV-2 preferably targets the human neurons. SARS-CoV-2 exposure to the organelles of the cerebral cortex is associated with tau protein hyperphosphorylation and neuronal death (Ramani et al., 2020). In addition, experiments with human brain organoids treated with antibodies to ACE2 showed only slightly decreased viral transfection, which indirectly indicates that additional factors may contribute to SARS-CoV-2 replication (Yang et al., 2020). These new facts suggest the participation of other cofactors of viral entry and may explain why SARS-CoV-2 has such a large range of targets in various human organs. The use of model platforms of lung and brain organoids confirmed that neuropilin-1 (NRP-1), cathepsin L1 (CTSL1), furin (PACE), and basigin (BSG = CD147) are involved (Tiwari et al., 2021) in SARS-CoV-2 infection (Tiwari et al., 2021).

In summary, some conclusions illustrating the concepts of SARS-CoV-2 neurotropism are as follows.

(1) The use of organoids as a new approach for biomedical analysis allows one to model the host and pathogen interaction. The range of human tissues susceptible to virus infection includes a significant variety of organs and cellular structures.

(2) Experiments using the brain organoid platform confirmed the permissiveness of neurons and other cell types to SARS-CoV-2 as evidence of neurotropic capacity and neuronal replication (Fig. 1). These materials confirmed the results of autopsy and analytical in silico studies of SARS-CoV-2 in brain structures in COVID-19.

Fig. 1.

Modeling of SARS-CoV-2 infection using in vitro organoid technology (according to Trevisan et al., 2021, adapted). Using stem cell and organoid technology, the mechanisms involved in the action of SARS-CoV-2 coronavirus on the epithelium of the human lungs, central nervous system, gastrointestinal tract and cardiovascular system were investigated. The architecture and physiology of various tissues and organs can be reproduced in two-dimensional and three-dimensional cultures of differentiated human cells and organoids. These in vitro cellular systems are derived from induced pluripotent stem cells, primary cells, cell lines and ex vivo tissue biopsies. Combined cultivation with immune cells allows us to represent possible responses of the anti-inflammatory immune systems of host cells to the viral agent. The results of the studies demonstrated the potential of in vitro models for analyzing viral tropism and host cell response.

(3) The destructive activity in organoids confirms capacity of the virus to directly attack brain cells. Organoid methods illustrate a limited tropic capacity of SARS-CoV-2 toward neurons and astrocytes of several brain regions, but the virus has a significantly higher preference to injure the choroid vascular plexus of the ventricles.

(4) The results illustrate the concept of neurotropism as a mechanism of SARS-CoV-2 pathogenesis. As discussed earlier, neurotoxic effects are not caused only by the virus, but they are also caused by the induced cytoimmune toxicity, vascular inflammation, and thrombosis. Infection of organoids with SARS-CoV-2 reveals disrupted transcription regulation, impaired cellular function, and increased cell death.

The use of organoid combinations offers a new level in the pathophysiological investigation of SARS-CoV-2 neuropathology. The results provide evidence of pronounced SARS-CoV-2 neurotropism and specify the brain regions and cells susceptible to viral aggression (Song et al., 2021).

Brain Cell Organoids. Limitations and Prospects

Despite the documented evidence, the universality of organoid platforms may be limited. Authors describing the results of evidence-based research propose a number of ideas that influence interpretation (Jacob et al., 2020; Ramani et al., 2021; Trevisan et al., 2021). Since artificial models are subject to limited contact with the cytosolic medium, one may doubt the structural and functional relevance of platforms derived from transformed stem cells. The direct effect of SARS-CoV-2 in culture only reflects the general scheme of contagiousness on the organoid material. The clinical picture attributed to the influence of a huge number of biochemical components encountered in the cytosolic medium, as well as the intensity and length of exposure to the virus are apparently far from the results observed in the models.

Nevertheless, according to (Ramani et al., 2021), organoid platforms represent almost ideal model systems for studying the damaging potential of viruses that target neural cells. The use of organoid systems has shown that SARS-CoV-2 exhibits neurotropic capacity with molecular and pathochemical traces of infection. However, in COVID-19 modeling these data remain tip of the iceberg. In general, this methodology offers significant opportunities for experimental variations and demonstrates viral neurotropism and the key points in regulatory control. This approach serves as a laboratory supplement to a huge array of clinical work. The model cell screening system can also be used in the search for new targeted therapeutic agents.

NEUROGLIA AS A FACTOR OF CORONAVIRUS NEUROTROPISM

Microglia are the resident mononuclear phagocytes of the central nervous system with a characteristic cellular organization and specific gene expression. Microglial cells, as the main neuroimmune sentinels of the brain, constantly monitor changes in their environment, and recognize pathogens, toxins or cellular debris. In experiments using direct RNA sequencing to determine transcriptomes (Hickman et al., 2013), it was found that microglia synthesizes special sensory recognition proteins. At the same time, they elicit neuroprotective or neurotoxic responses as a form of reaction to foreign agents.

Cytokine storm is a phenomenon characterized by amplified release of pro-inflammatory cytokines, which is the main factor in the development of acute respiratory distress syndrome and multiple organ failure in COVID-19. However, clinical data show that cytokine storm and subsequent cellular inflammation frequently lead to stochastic activation of components of the immune system, provoking damage to the parenchyma and neurons of the brain. As a result, cytokines may act as direct (due to selective neurotropism) or indirect (via disruption of BBB) factors of neurotoxicity, damage, and cellular death (Vargas et al., 2020; Aghagoli et al., 2021). This information encourages one to further consider SARS-CoV-2 as the cause of neuroinvasive processes in COVID-19, since the infection triggers reactivity of resident immune and glial cells that are intended to provide protection to neurons.

Microglia as a Sensor of Virus Invasion

It is generally believed that microglia act as the main sensor of viral infections in the central nervous system (Chen et al., 2019). Equipped with a variety of molecular sensors that activate intracellular signaling cascades, microglia promote the expression of antiviral cytokines (Dantzer, 2018). Entry of the virus into the nervous system and its replication lead to direct infection of resident immune cells with SARS-CoV-2, interfere with the innate defense mechanisms, impair regulation of cyto- and chemokines, dysregulate autoimmunity, and cause neuronal dysfunction (Awogbindin et al., 2021).

A range of publications confirm that microglia use molecular recognition patterns, DAMPs/PAMPs (damage-associated molecular patterns/pathogen-associated molecular patterns) with upregulation of intracellular signaling pathways to trigger transcription and expression of protective cytokines (Furr and Marriott, 2012). Viral epitopes fuse with with the plasma membrane receptors in endosomes and in the cytoplasm of immune cells (Pichlmair and Reis e Sousa, 2007; Pedraza et al., 2010). Specialized membrane structures of monocytes and macrophages identified as Toll-like receptors recognize fragments of pathogens (Ronald and Beutler, 2010). Up-regulation of Toll-like receptors by the PAMPs and DAMPs patterns triggers a signaling cascade that culminates in the synthesis and release of proinflammatory cytokines (O’Neill et al., 2013). Clinical analysis of COVID-19 indicates that encephalopathy with multiple organ failure is associated with an elevated level of systemic inflammation markers (Cummings et al., 2020; Helms et al., 2020; Koralnik and Tyler, 2020).

Neuroinflammation. Immune Cells as Mediators of Viral Aggression

An important link in pathogenesis is the infiltration of the brain tissue with infected immune cells. Viruses can enter the brain with cells that act as transporters. Monocytes, neutrophils, and T cells enter the brain through the vasculature of the meninges and the vascular plexus, which represent a path for viral aggression (Bergmann et al., 2006; Engelhardt et al., 2017).

The features of the immune response make it possible to formulate (Tavčar et al., 2021) a general viewpoint of the intricate picture of pathophysiological processes. Systemic inflammation, which develops after infection with SARS-CoV-2, is caused by hyperactivation of the innate immune system and the release of pro-inflammatory mediators. These related reactions serve as a clinical sign of COVID-19. Neuroinflammation includes extensive activation of glial cells, release of pro-inflammatory cytokines, antioxidants, free radicals, and neurotrophic factors. In accordance with the evidence supporting that glial cells have a duality in their phenotype, neurotoxic or neuroprotective properties, these processes depend on age, infectious stimuli, and pathophysiological condition, which is especially relevant in COVID-19 (Matias et al., 2019).

Clinical materials indicate that the reactive phenotype induced by the SARS-CoV-2 virus plays a role in maintaining neuroinflammation as a leading cause of neurodegenerative and mental disorders (Merad and Martin, 2020; Murta et al., 2020; Valenza et al., 2021). Activation of microglia in the affected areas is observed in most of COVID-19 cases. The frequency of detecting coronavirus fragments is much higher in brain regions with microgliosis or lymphocytic infiltration, compared to regions exposed to hypoxic or vascular damage (Li, Y.C. et al., 2021).

In summary, the sequence of neurotropic processes in viral invasion may be as follows.

(1) Injury to endothelial cells and disruption of the protective function of the BBB triggers pro-inflammatory signals from the periphery into the brain parenchyma with upregulation of the inflammatory response in microglia and astrocytes.

(2) Glial cells, astrocytes, and microglia play a key role in the pathogenesis of inflammatory and degenerative disorders and can be regarded (Vargas et al., 2020) as neurotropic targets of SARS-CoV-2 viruses (Fig. 2).

Fig. 2.

The relationships between the neurodestructive effect of SARS-CoV-2 and antiviral protection with the participation of immune mechanisms in microglia (according to Vargas et al., 2020, adapted). Microglial cells act as a mechanism of the immune response necessary to prevent viral influence and activate the systemic antiviral response. This event includes recruitment of peripheral monocytes/macrophages, an increased innate immunity response, elevated cytokine release, and activation of T cells, together, a complex of mechanisms controlling spread of the virus. In severe cases of COVID-19, excessive activation of microglial cells can contribute to negative effects by reactivating astrocytes or T-lymphocyte-mediated neurotoxicity, i.e., phenomena that contribute to synapse loss and neuron degeneration.

(3) Astrocytes are the main agents in the neuroinflammation system: fusion of the virus with ACE2 upregulates microglia and triggers the release of proinflammatory cytokines.

(4) These reactions cause transformation of the astrocyte phenotype into a reactive form and stimulate neuroinflammation along with neurodestructive processes in COVID-19.

CONCLUSIONS

The concept of neurotropism in terms of COVID-19 pathogenesis can be seen as a summary of the evidence in favor of a complex process. An analysis of recent data obtained in experiments and clinical data allowed us to find new therapeutic solutions. In conclusion, we present the main statements discussed in the article in order to advance arguments in favor of this viewpoint.

(1) A range of experimental studies and clinical data on the neuroinvasive potential of SARS-CoV-2 confirm the facts of damage to the brain and peripheral nervous system. A better understanding of pathogenesis, identification of cellular and biochemical targets are important for a therapeutic strategy. This paper outlines the basic principles that allow us to assess new aspects of neurodegenerative and mental complications in the pathogenesis of COVID-19. Herein, the idea of neurotropism as the leading cause of SARS-CoV-2 infection, as well as subsequent dysfunction and damage to the patient’s central nervous system, deserves separate consideration.

(2) The brain, being an extremely complex and dynamic cellular-tissue system, can form an attractive environment for SARS-CoV-2 replication. The results presented illustrate neurotropism as a complex mechanism of SARS-CoV-2 pathogenesis. Although new facts are constantly added constantly added to the description of neuroinvasion, it remains necessary to clarify whether SARS-CoV-2 is a direct neurotropic agent or if neurodestructive processes are a consequence of systemic cellular and biochemical changes.

(3) Analysis of viral invasion pathways in COVID-19 reveals a staged mechanism of neurological complications. Until recently, the idea of virus entry into host cells was mainly associated with the role of ACE2 as a unique mediator; new data demonstrate additional mechanisms of SARS-CoV-2 transfection. The description of some transmembrane proteins (neuropilin, etc.) served as a new argument in the concept of neurotropism, when such molecules act as cofactors of viral entry into the tissues of the lungs, brain, and other organs.

(4) The complexities of in-life study of cellular and molecular processes encourage the use of new methods. The use of organoid models as a new approach to biomedical analysis has provided opportunities for modeling host-pathogen interactions. The range of human tissues permissive to infection includes a variety of organs and cellular structures. The study of the mechanisms involved in the damaging effect of SARS-CoV-2 at the level of an individual neuron is formulated as an important task in the study of neurotropism. Organoid platforms represent a new level of research into COVID-19 neuropathology.

(5) The entry of the infecting agent into the patient’s brain and the subsequent disturbance of the immune defense are manifestations of neurotropism. Clinical materials on COVID-19 indicate that astrocytes and microglia are targets of SARS-CoV-2 viruses. Neurodestructive processes in COVID-19 may be associated with manifestations of vascular inflammation, immunothrombosis, and cytoimmune toxicity. Therefore, microglia and neuroinflammation are considered factors of neurotropism, and the neural and mental complications of COVID illness.

FUNDING

This study was supported by the Program of Basic Research in the Russian Federation for the long-term period (2021–2030, no. 122030100170-5).

COMPLIANCE WITH ETHICAL STANDARDS

The authors declare that they have no conflict of interest. This article does not contain any studies involving animals or human participants performed by any of the authors.