The SARS-CoV-2 spike protein alters barrier function in 2D static and 3D microfluidic in-vitro models of the human blood–brain barrier

"Abstract: ... [T]he results presented in this report 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, using postmortem brain tissue, we show 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... Introduction of spike proteins to in vitro models of the blood-brain barrier (BBB) showed significant changes to barrier properties. Key to our 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 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...

1. Introduction

... The present study was aimed to identify whether the spike protein of the SARS-CoV-2 virus negatively affects the function of the BBB [blood brain barrier]...To test functional outcomes, two (a 2D and a 3D vessel-like) in-vitro models of the BBB using primary brain endothelial cells were used. In both models, the effects of the spike subunits of SARS-CoV-2 on barrier integrity were determined...

4. Discussion

... SARS-CoV-2 subunit S1 or S2 both caused similar effects and transient loss of electrical resistance... raising the possibility that structural reorganization rather than outright loss of tight junctional complex occurs... each subunit of SARS-CoV-2 spike protein significantly increases the rate of passive paracellular passage of small molecular tracers providing a second indicator of barrier dysfunction... subunits S1, S2 and RBD exert similar deleterious effects on the BBB function... strongly suggests that the... interaction between SARS-CoV-2 and the BBB is multifocal and involves reversible activation at more than one receptor or signaling cascade...

Endothelial activation also features increased expression of matrix metalloproteinase or MMPs, a family of enzymes involved in... tight junction protein degradation and... modifications. In this study, the spike protein increase[d] MMP3 and MMP12...MMP2 and MMP9 gene expression. MMP3 has been previously implicated in traumatic brain injury by digesting tight junctions proteins followed by the BBB opening... MMP12, on the contrary... plays role in immune cells extravasation and migration into the brain. Taking together our data... 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."