Spike proteins provide ‘plausible explanation’ for neurological complications of COVID-19

SARS-CoV-2 spike proteins triggered a proinflammatory response in brain endothelial cells that may influence a change in the function of the blood-brain barrier, according to findings published in Neurobiology of Disease.

The results showed the “direct impact” of the SARS-CoV-2 spike protein on endothelial cells in the brain, the researchers reported, and provided “a plausible explanation” for the neurological consequences observed in patients with COVID-19.

“As researchers across the globe have focused their attention on understanding SARS-CoV-2, the picture that is emerging is that of a virus that has serious effects on the vasculature in multiple organ systems including the cerebral vasculature,” the researchers wrote. “Observed effects on the central nervous system include neurological symptoms (headache, nausea, dizziness), fatal micro-clot formation and in rare cases encephalitis. However, our understanding of how the virus causes these mild to severe neurological symptoms and how the cerebral vasculature is impacted remains unclear.”

Tetyana Buzhdygan, PhD, a postdoctoral fellow at the Lewis Katz School of Medicine at Temple University, and colleagues aimed to determine whether or not the spike protein of the SARS-CoV-2 virus conversely impacted the function of the blood-brain barrier. To do so, they examined the effects of the following SARS-CoV-2 spike protein subunits on the status of the blood-brain barrier: the receptor binding subunit S1 and the fusion subunit S2, as well as receptor binding domain of the S1 subunit.

The researchers demonstrated that the angiotensin converting enzyme 2 (ACE2), a known binding target for the SARS-CoV-2 spike protein, is “ubiquitously expressed throughout various vessel calibers in the frontal cortex.” They found that ACE2 expression was upregulated in cases of hypertension and dementia and that ACE2 was also detectable in primary human brain microvascular endothelial cells preserved under cell culture conditions. Analysis of cell viability showed that neither S1, S2 and a truncated form of S1 with only the receptor binding domain had minimal effects on human brain microvascular endothelial cell viability in an exposure window of 48 hours.

Introduction of spike proteins into in vitro models of the blood-brain barrier highlighted significant changes, Buzhdygan and colleagues found. The SARS-CoV-2 spike protein demonstrated a negative effect on endothelial barrier properties and a significant impact on barrier integrity.

“Key to our findings is the demonstration that S1 promotes loss of barrier integrity in an advanced 3D microfluidic model of the human [blood-brain barrier], a platform that more closely resembles the physiological conditions at this [central nervous system] interface,” the researchers wrote.

The results could have implications for the multisystem inflammatory syndrome in children (MIS-C) that has been associated with COVID-19, according to Buzhdygan and colleagues. They specifically pointed to the finding that the SARS-CoV-2 spike protein alone “is a potent inductor of endothelial dysfunction” and that manifestations of MIS-C can be “at least partially attributed” to its action.

“Thus, the clinically observed edema results from the hyperpermeable endothelial barrier and generalized diffused hyperinflammation can be caused by the elevated secretion of pro-inflammatory cytokines in endothelium,” the researchers wrote. “These findings provide two important notions for therapeutic interventions: 1) to stabilize the endothelium in COVID-19 patients with pre-existing conditions associated with heightened vulnerability of the vascular bed and 2) productive viral infection of endothelial cells is not an absolute requirement for endothelial dysfunction, as it also can arrive as a result of viral protein shedding.”

The present study is the first, according to the researchers, that analyzed the impact of the SARS-CoV-2 spike protein on the blood-brain barrier.

“Our findings provide insight into the continued theme that this novel coronavirus triggers responses at the endothelium,” the researchers wrote. “Specifically, in regard to the brain endothelium, the SARS-CoV-2 spike protein induced destabilization of the [blood-brain barrier], 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 [blood-brain barrier], hints at the possible means in which the SARS-CoV-2 pathogen could also neuroinvade.”