Binding of Sars-CoV-2 spike protein to ACE 2 receptors expressed on brain cells (neuronal and non-neuronal) leads to neuroinflammation resulting in encephalitis

European Commission, Join Research Centre (JRC), Ispra, Italy

Although the severe acute respiratory syndrome COVID-19 caused by the virus SARS-CoV-2 mainly manifests as an effect of acute respiratory infection (Radnis et al. 2020), recent evidence also suggests that approximately 36% of affected patients exhibit neurological sequelae (Mao et al., 2020).

Many studies have shown that SARS-CoV-2 coronavirus is neuroinvasive, neurotropic, and neurovirulent in humans. The presence of SARS-CoV-2 has been identified in the central nervous system (CNS) and cerebrospinal fluid (CSF) of patients with acute neurologic symptoms, including encephalitis (Wu et al., 2020).  Post-mortem examination of SARS-CoV-2-infected patients revealed the presence of SARS-CoV-2 viral particles in endothelial cells and pericytes of brain capillaries and neurons (Paniz-Mondolfi et al., 2020; Nath, 2020; Chigr et al. 2020) as well as in glial cells (microglia and astrocytes) (Vargas et al. 2020; Nakagaki et al., 2005; Lannes et al., 2017).

There are several routes of Sars-CoV-2 entry into the CNS. The presence of viral-like particles of SARS-CoV-2 in endothelial cells and pericytes of brain capillaries, as well as astrocytic processes, strongly supports a hematogenous endothelial neuroinvasion-based hypothesis (Paniz-Mondolfi et al., 2020). Indeed, SARS-CoV-2 induced over-activation of systemic immune cells and the release of pro-inflammatory chemokines and cytokines, provokes a cytokine storm possibly leading to the to disruption of the tight junctions and increased permeability of the blood brain barrier (BBB) (Savarin and Bergmann 2018).

In parallel, SARS-CoV-2 could also infect the endothelial cells of the blood-cerebrospinal fluid barrier, and then spread into the CNS. Olfactory pathway (through nasal epithelium), vagus and trigeminal nerves, as well as the enteric nervous system are other possible routes of probable Sars-CoV-2 entry into the CNS.

Binding of S protein to ACE2 receptors on microglia triggers their activation, which results in the release of proinflamatory cytokines/chemokines, nitric oxide, prostaglandin E2, and reactive oxygen and nitrogen species. As a consequence, activation of astrocytes occurs, which ultimately may lead to neuronal cell death (Ransohoff and Perry, 2009; Chatterjee et al., 2013; Wheeler et al., 2018). This proinflamatory factors can trigger neuronal cell death by well-known mechanisms contributing, together with brain neuroinflammation, to encephalitis.

Encephalitis has been documented in clinical observations in COVID-19 patients, characterized by acute onset and common symptoms include headache, fever, vomiting, convulsions, and consciousness disorders (Wu Y et al., 2020).  In the ongoing pneumonia epidemic, the presence of Sars-CoV-2 in the CSF of patients with COVID-19 was confirmed by genome sequencing, thereby clinically verifying viral encephalitis (Xiang et al., 2020), providing a solid basis for Sars-CoV-2 as a cause of this disease.

The present AOP describes current mechanistic understanding of causative links between binding of Spike protein to ACE2 receptors on brain cells, which leads to glia activation, neuronal inflammation and cell death, resulting in encephalitis (adverse outcome), which manifests through a variety of symptoms including headache, high fever, vomiting, convulsions, and consciousness disorders, as observed in several clinical studies in COVID-19 patients.

Two KEs of this AOP (i.e., neuroinflammation and neurodegeneration) have been described in other AOPs (i.e., neuroinflammation (in AOP 12, AOP 48, AOP 3, and AOP 17); neurodegeneration (in AOP 12, AOP 48, AOP 281); however, quantitative information of KERs is limited or non-existing.

AOP 12 Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development leads to neurodegeneration with impairment in learning and memory in aging. Available at https://aopwiki.org/aops/12

AOP 17 Binding of electrophilic chemicals to SH(thiol)-group of proteins and /or to seleno-proteins involved in protection against oxidative stress during brain development leads to impairment of learning and memory. Available at https://aopwiki.org/aops/17

AOP 281 Acetylcholinesterase Inhibition Leading to Neurodegeneration. Available at https://aopwiki.org/aops/281

AOP 3 Inhibition of the mitochondrial complex I of nigro-striatal neurons leads to parkinsonian motor deficits. Available at https://aopwiki.org/aops/3

AOP 48 Binding of agonists to ionotropic glutamate receptors in adult brain causes excitotoxicity that mediates neuronal cell death, contributing to learning and memory impairment. Available at https://aopwiki.org/aops/48

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