COVID-19 News: American Study Confirms That Spike Proteins Of SARS-CoV-2 Coronavirus Is Able To Disrupt The Human Blood-Brain Barrier!
: A new study led by scientist from the Lewis Katz School of Medicine at Temple University-Philadelphia along with researchers from Rowan University-New Jersey and The Shriners Hospitals Pediatric Research Center-Philadelphia shows that the spike proteins that extrude from SARS-CoV-2 promote inflammatory responses on the endothelial cells that form the blood-brain barrier. This can cause this barrier to become "leaky," potentially disrupting the delicate neural networks within the brain.
According to the study team, the study explored whether deleterious outcomes from the SARS-CoV-2 viral spike protein on primary human brain microvascular endothelial cells (hBMVECs) could be observed. The spike protein, which plays a key role in receptor recognition, is formed by the S1 subunit containing a receptor binding domain (RBD) and the S2 subunit. First, by utilizing postmortem brain tissue, the team showed that the angiotensin converting enzyme 2 or ACE2 (a known binding target for the SARS-CoV-2 spike protein), is ubiquitously expressed throughout various vessel calibers in the frontal cortex. Moreover, ACE2 expression was upregulated in cases of hypertension and dementia. ACE2 was also detectable in primary hBMVECs maintained under cell culture conditions. Analysis of cell viability revealed that neither the S1, S2 or a truncated form of the S1 containing only the RBD had minimal effects on hBMVEC viability within a 48 h exposure window.
Significantly, the introduction of spike proteins to in vitro
models of the blood-brain barrier (BBB) showed significant changes to barrier properties. Key to the findings is the demonstration that S1 promotes loss of barrier integrity in an advanced 3D microfluidic model of the human BBB, a platform that more closely resembles the physiological conditions at this CNS interface. Evidence provided suggests that the SARS-CoV-2 spike proteins trigger a pro-inflammatory response on brain endothelial cells that may contribute to an altered state of blood-brain barrier BBB function.
Together, these results are the first to show the direct impact that the SARS-CoV-2 spike protein could have on brain endothelial cells; thereby offering a plausible explanation for the neurological consequences seen in COVID-19 patients.
The research findings were published in the journal: Neurobiology of Disease. https://www.sciencedirect.com/science/article/pii/S096999612030406X?via%3Dihub
The SARS-CoV-2 virus that causes the COVID-19 disease, like a key attaches to specific molecules on the host cell surface, opening gateways into the cell interior. Viral entry into host cells triggers a prodigious immune response. Much of this battle is waged within the lungs, which explains why many patients hospitalized with COVID-19 have severe respiratory symptoms.
However respiratory symptoms are only part of the story. Increasing evidence points toward blood vessel inflammation as having a crucial impact on the severity of COVID-19.
Furthermore anywhere from 30 to 80 percent of patients experience neurological symptoms, including dizziness, headache, nausea, and loss of concentration. These symptoms suggest that
SARS-CoV-2 also affects cells of the central nervous system.
Although there is no evidence yet that the virus invades the brain, this new study shows that the spike proteins that extrude from SARS-CoV-2 promote inflammatory responses on the endothelial cells that form the blood-brain barrier.
Dr Servio H. Ramirez, Ph.D., Professor of Pathology and Laboratory Medicine at the Lewis Katz School of Medicine at Temple University and principal investigator on the new study explained to Thailand Medical News, "Previous studies have shown that SARS-CoV-2 infects host cells by using its spike proteins to bind to the angiotensin converting enzyme 2 (ACE2) on the host cell surface."
These ACE2 are expressed on endothelial cells, which form the inner lining of blood vessels, and serves a central role in mediating different functions of the cardiovascular system.
Dr Ramirez added, “Since ACE2 is a major binding target for SARS-CoV-2 in the lungs and vasculature of other organs in the body, tissues that are behind the vasculature, that receive blood from affected vessels, are at risk of damage from the virus."
However, it has been unclear whether ACE2 is also present in the brain vasculature or whether its expression changes in health conditions that worsen COVID-19, such as high blood pressure (hypertension).
In order to find out, the study team began by examining postmortem human brain tissue for vascular ACE2 expression, using tissues from individuals without underlying health conditions and from individuals in whom hypertension and dementia had been established. Analyses showed that ACE2 is in fact expressed throughout blood vessels in the frontal cortex of the brain and is significantly increased in the brain vasculature of persons with a history of hypertension or dementia.
The team then investigated the effects of the SARS-CoV-2 spike protein on brain endothelial cells in cell culture models. Introduction of the spike protein, particularly a portion designated subunit 1, produced substantial changes in endothelial barrier function that led to declines in barrier integrity.
The scientist also uncovered evidence that subunit 2 of the SARS-CoV-2 spike protein can directly impact blood-brain barrier function.
Postdoctoral fellow and first author on the new report Dr Tetyana P. Buzhdygan added, "This is of importance because unlike subunit 1, subunit 2 of the spike protein doesn't bind to ACE2, meaning that a breach to the blood-brain barrier could occur in a manner that is independent of ACE2."
The study team further investigated the effects of SARS-CoV-2 spike proteins on tissue-engineered microfluidic constructs designed to mimic a human brain capillary.
Dr Allison M. Andrews, Ph.D., Assistant Professor in the Department of Pathology & Laboratory Medicine at LKSOM and a co-author on the report said, "The tissue-engineered microfluidic models allow recapitulation of the 3-D cyto-architecture and mechanical forces caused by fluid movement, which the vasculature is continuously exposed to."
Research experiments showed that binding of spike protein subunit 1 increased barrier permeability in the engineered vessel-like constructs.
Dr Ramirez added, "Our findings support the implication that SARS-CoV-2, or its shed spike proteins circulating in the blood stream, could cause destabilization of the blood-brain barrier in key brain regions. Altered function of this barrier, which normally keeps harmful agents out of the brain, greatly increases the possibility of neuroinvasion by this pathogen, offering an explanation for the neurological manifestations experienced by COVID-19 patients."
Endothelial cells are an essential part of the inflammatory response since activation of the endothelium allows for recruitment and mobilization of immune cells to the tissues that are under pathogen attack. Once activated, brain endothelial cells upregulate expression of cell adhesion molecules (CAMs) and pro-inflammatory cytokines that play a key initial role in the process of neuroinflammation and transendothelial migration of immune cells in response to inflammatory challenge. Endothelial cells exposed to the each subunit of the SARS-CoV-2 spike protein showed elevated expression of the cell adhesion molecules (ICAM-1 and VCAM-1,), leukocyte chemotaxis factors (CXCL10 and CCL5 ‘RANTES’), and pro-inflammatory cytokines (IL-1β and IL-6).
In conjunction with reduced barrier tightness, the study finding that SARS-CoV-2 activates hBMVECs strongly indicates the potential for enhanced immune infiltration into the CNS.
Endothelial activation also features increased expression of matrix metalloproteinase or MMPs, a family of enzymes involved in the remodeling of extracellular matrix in both normal physiological and pathological processes. Activated by pro-inflammatory cytokines, MMPs also regulate tight junction protein degradation and post-translational modifications. In this study, we report that the spike protein increases MMP3 and MMP12, and to a lesser extent MMP2 and MMP9 gene expression. MMP3 has been previously implicated in traumatic brain injury by digesting tight junctions proteins followed by the BBB opening. These reports corroborate the study findings of decreased barrier resistance and heightened secretion of chemotactic chemokines . MMP12, on the contrary, is not involved in BBB damage, but plays role in immune cells extravasation and migration into the brain. Taking together the study data of elevated MMP3, CCL5, CXCL10 and CAMs, we can speculate that SARS-CoV-2 is a potentially neuroinvasive virus as it turns on the machinery to facilitate the migration of infected immune cells as “Trojan horses” into the brain parenchyma.
This is the first reported evaluation that examined the effects of the SARS-CoV-2 spike protein on the BBB. Our findings provide insight into the continued theme that this novel coronavirus triggers responses at the endothelium. Specifically, in regard to the brain endothelium, the SARS-CoV-2 spike protein induced destabilization of the BBB, promoted a pro-inflammatory status but did not appear to alter cell viability acutely. Dysfunction of the barrier offers a plausible explanation to the observed neurological complications seen in COVID-19. Lastly, the opening of the BBB, hints at the possible means in which the SARS-CoV-2 pathogen could also neuroinvade.
To date, the long-lasting effects of altered blood-brain barrier function in the presence of SARS-CoV-2 are unknown.
Importantly as Dr Buzhdygan explained, "the brain vasculature is extremely branched, so even a small amount of neuroinflammation can be very damaging."
Significantly, based on the team's observations of ACE2 expression in the brain, this neurological damage could be extensive in COVID-19 patients with pre-existing health conditions in which the vasculature has already suffered some amount of injury.
Also it remains unknown whether the virus can actually get inside neurons or glial cells that lie beyond the barrier.
Dr Ramirez noted, "The viral genome has not been found yet in the specific cell types of the brain. The next steps in our work are to look for genomic viral copies in different parts of the brain using autopsy material from COVID-19 cases and to investigate the pathogen's ability to neuroinvade using different cell culture and tissue-engineered constructs."
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