Researchers shows how SARS-CoV-2 infects brain cells

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Researchers at University of California San Diego School of Medicine and Rady Children’s Institute for Genomic Medicine have produced a stem cell model that demonstrates a potential route of entry of SARS-CoV-2, the virus that causes COVID-19, into the human brain.
The findings are published in the July 9, 2021 online issue of Nature Medicine.
“Clinical and epidemiological observations suggest that the brain can become involved in SARS-CoV-2 infection,” said senior author Joseph Gleeson, MD, Rady Professor of Neuroscience at UC San Diego School of Medicine and director of neuroscience research at the Rady Children’s Institute for Genomic Medicine.
“The prospect of COVID19-induced brain damage has become a primary concern in cases of ‘long COVID,’ but human neurons in culture are not susceptible to infection. Prior publications suggest that the cells that make the spinal fluid could become infected with SARS-CoV-2, but other routes of entry seemed likely.”
Gleeson and colleagues, who included both neuroscientists and infectious disease specialists, confirmed that human neural cells are resistant to SARS-CoV-2 infection. However, recent studies hinted that other types of brain cells might serve as a ‘Trojan horse.’
Pericytes are specialized cells that wrap around blood vessels – and carry the SARS-CoV2 receptor. The researchers introduced pericytes into three-dimensional neural cell cultures – brain organoids – to create “assembloids,” a more sophisticated stem cell model of the human body.
These assembloids contained many types of brain cells in addition to pericytes, and showed robust infection by SARS-CoV-2.
The coronavirus was able to infect the pericytes, which served as localized factories for production of SARS-CoV-2. These locally produced SARS-CoV-2 could then spread to other cell types, leading to widespread damage. With this improved model system, they found that the supporting cells known as astrocytes were the main target of this secondary infection.
The results, said Gleeson, indicate that one potential route of SARS-CoV-2 into the brain is through the blood vessels, where SARS-CoV-2 can infect pericytes, and then SARS-CoV-2 can spread to other types of brain cells.
“Alternatively, the infected pericytes could lead to inflammation of the blood vessels, followed by clotting, stroke or hemorrhages, complications that are observed in many patients with SARS-CoV-2 who are hospitalized in intensive care units.”
Researchers now plan to focus on developing improved assembloids that contain not just pericytes, but also blood vessels capable of pumping blood to better model the intact human brain. Through these models, Gleeson said, greater insight into infectious disease and other human brain disease could emerge.
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COVID-19 and neurological symptoms
COVID-19 clinical signs are mainly associated with respiratory symptoms, but there is evidence that there are adverse neurological symptoms, including anosmia, dysgeusia, headaches, nausea and vomiting [21, 96]. These symptoms may have short- and long-term adverse effects on infected people, especially those with pre-existing health conditions. Earlier observations of COVID-19 infection found about 36% of patients had neurological manifestations [157].
This was more common in patients with severe cases and particularly for those with preconditions, such as CVD, including ischemic and haemorrhagic strokes and impaired consciousness [79]. Older patients with CVD and risk factors, such as hypertension, diabetes and higher levels of C-reactive protein compared to patients without CVD were at greater risk of severe COVID-19 symptoms and poorer prognosis [158].
Others reported that COVID-19 patients had clinical presentations of large-vessel ischemic stroke in young patients, acute necrotizing encephalopathy, encephalitis, meningitis, headaches and dizziness [81–91]. Patients with Parkinson’s disease (PD) appear also to be affected by COVID-19 by exacerbating motor symptoms, such as tremor and dyskinesias, and reducing the efficacy of dopaminergic medication [159–162].
There is emerging evidence that there are psychological and behavioural changes (mental health) associated with COVID-19 infection due, perhaps, to the mitigating efforts to minimize the viral transmission, such as the isolation. There are changes in anxiety, sleep, mood, alcohol consumption and drug abuse [163, 164], and stress from the treatment of severe cases, post-traumatic stress syndrome and depression[165]. However, it’s unclear if these clinical neurological presentations seen in COVID-19 patients are due to the virus entering brain or as a consequence of cardio-respiratory and multiorgan failure.
In some patents, COVID-19 is associated with thrombotic vascular events, such as strokes [166, 167]. The incidence of strokes is 7.6-fold greater with COVID-19 compared to that of influenza infections [168]. There is a similar sevenfold increase in the incidence of large vessel stroke in young people with COVID-19 compared to the previous year’s cases [169–173].
The incidence of cerebrovascular disease in COVID-19 patients is estimated at 1–6%, [169, 174], and possible mechanisms include cytokine storm, hypercoagulation, endotheliitis and endotheliopathy. Viral particles are associated with the cerebrovasculature and the endothelium of other organs [169, 174, 175]. In severe endothelium injury; there is vascular thrombosis, microangiopathy and angiogenesis [176]. The raised plasma levels of von Willebrand factor and soluble thrombomodulin may indicate vascular injury [177]. Infiltration of leukocytes and intravascular coagulation may be due to compromised vascular integrity [175].
Neurological disorders were also associated with SARS and MERS. SARS was associated with seizures, myopathy, rhabdomyolysis and viral RNA in CSF and brain tissue [178, 179]. MERS was also associated with neuropathy, delirium, acute cerebrovascular disease, confusion and seizure [180]. With the similarities between animal and human CoVs, both molecularly and symptomatically, possible mechanism for how SARS-CoV-2 behaves can be modeled. CoVs have been shown to invade, infect, and induce neurological-like disease in animal models of SARS and MERS [181–184]. It appears that neurotropism is a common feature of CoVs, and such neuroinvasion propensity of CoVs have been documented in almost all of the beta-CoVs (SARS, MERS, HCOV-229E, OC43) [103].
The impact of the SARS-CoV-2 and COVID-19 on CNS and the cerebrovasculature may be a minor and/or a secondary issue of the systemic inflammatory disease caused by the virus. The mortality of the disease is mostly due to the failure of lung, cardiovascular and kidney functions of the patients with underling diseases or those with immune dysfunction. A few cases may die from stroke as a result of thrombosis and thrombus embolization, but this could be a significant number of deaths from COVID-19-related neurological consequences, since the number of deaths from this infection is high. Currently there aren’t data that SARS-CoV-2 impacts the cerebrovasculature of mild or asymptomatic cases as there isn’t many of these cases that have been hospitalized. Further studies are needed.
How could SARS-CoV-2 enter the brain?
The exact mechanism of SARS-CoV-2 neuroinvasion is unclear. The main mode of CoV entry into the body is inhalation, and thus, the viruses access the nasal and buccal cavities. Viruses could enter the brain by retrograde transport via sensory nerve endings within these regions, such as the cranial olfactory and trigeminal, and the autonomic nervous system [98, 185].
In addition, virus that enters blood from the infected lungs may interact with the cerebrovasculature [27] and/or at the blood-CSF barrier, the choroid plexus [186]. Virus associated with leukocytes, such as monocytes, may enter the brain via receptors, such as advanced glycation endproducts (RAGE) and platelet endothelial cell adhesion molecule-1 (PECAM-1; CD31) [64, 187]. There is also the possibility of viral neuroinvasion via the gastrointestinal tract [188].
Olfactory bulb
SARS-CoV-2 may access the olfactory epithelium and penetrate the cribriform plate to enter the olfactory bulb, and from there spreads within the CNS. It can infect neurons or non-neural cells via ACE2 and/or TMPRSS2 receptors and transported along the olfactory nerve [189]. SARS-CoV-2 interaction with the olfactory mucosa may explain the anosmia or hyposmia seen in COVID-19 patients. In addition to the olfactory nerve, it is possible the virus can use other nerves, such as the trigeminal and the vagus, which innervate the buccal cavity, respiratory tract and lungs [190, 191]. SARS-CoV-1, another CoV, also shows a transneural penetration through the olfactory bulb in a mouse model and thus SARS-CoV-2 might behave similarly [183]. (Figs. (Figs.3,3, ,44).
Cerebrovasculature
Since the CNS vascular barriers are likely compromised in the severely infected patients and in the aging brain, especially in the more susceptible patient groups, SARS-CoV-2 interaction at the cerebrovasculature may potentiate its dysfunction. The leaky cerebrovascular can cause cerebral edema [192]. The local increase in interstitial pressure would decrease blood flow to the region, leading to neuronal dysfunction and cell death [105, 192]. SARS-CoV-2 induced inflammation of the meningeal cells could increase influx of fluid, which may activate resident perivascular macrophages and parenchymal microglia leading to aggravation of cerebral inflammation (Fig. 3).
SARS-CoV-2 spike proteins interact with the in vitro models of the BBB[98], and slowly cross the murine cerebrovascularture via adsorptive transcytosis that is independently of ACE2 (27; 281). Thus, further studies are needed to support significant neuroinvascion as the explanation for the neurological symptoms associated with COVID-19.
There are reports of possible SARS-CoV-2 neurotropism but further studies are needed to establish whether this is related to the degree of infection (viral load), the time this occurs in COVID-19 progression, conditions that may contribute to this and the frequency of its occurrence. Also, it is unknown whether this is due to the virus crossing the normal CNS vascular barrier to cause neuroinvasion or via the dysfunctional barriers as a consequence of pneumonia induced-hypoxia. There are reports of encephalitis, which was not due to COVID-19-induced hypoxia [193] and brain cortical hyperintensity, as seen in MRI images, which may be due to viral infection [146, 194]. The virus was detected in the cortical brain tissue, which may suggest it enters brain [195]. SARS-CoV-2 is detected in the olfactory mucosa [196].
How SARS-CoV-2 could spread within the brain
Within the brain, SARS-CoV-2 may interact with and spread through the ACE2, other facilitator receptors or by adsorptive uptake by cells. In SARS patients, SARS particles were located almost exclusively in the neurons [103]. The brainstem was heavily infected by SARS and MERS [103], and thus, all CoVs may invade the brain [103].
Neurons can then take up the virus, which then binds to intracellular ACE2 [97]. ACE2 receptor is expressed in both neurons and glia cells [195], but mainly in the cytoplasm of neurons [97]. Most studies used in vitro models to determine whether SARS-CoV-2 infect neurons or inferred that there is a potential of infection by assessing the presence of ACE2 [197, 198].
However, SARS-CoV-2 was detected in cortical neurons of infectious patients [199]. This may explain the presence of SARS in neuronal cells. Trans-synaptic transfer has been documented for other CoVs, such as HEV67 [103] and avian bronchitis virus [96].
Murine CoV can replicate and cause direct lysis of oligodendrocytes and demyelination in the CNS during the acute phase[200, 201]. In mice, infected intranasally with a large load of neuro-virulent strains of HCoV-OC43, it entered the CNS via the olfactory nerves with subsequent trans-neuronal retrograde dissemination to distant connections of the olfactory bulb, and the pyriform cortex and brainstem [201, 202].
In MERS infected mice, the presence of the virus in the CNS was associated with high mortality [103]. Neuroanatomic interconnections indicate that the death of infected animals or patients may be due to the dysfunction of the cardiorespiratory center in the brainstem [103]. It was suggested that lipids may play a role in SARS-CoV-2 in brain [203]. However, there are no data to support any mechanism for the spread of SARS-CoV-2 in brain. (Figs. (Figs.4,4, ,5,5, ,66).
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8278192/
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More information: Lu Wang et al, A human three-dimensional neural-perivascular ‘assembloid’ promotes astrocytic development and enables modeling of SARS-CoV-2 neuropathology, Nature Medicine (2021). DOI: 10.1038/s41591-021-01443-1

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