The University of California, Berkeley, has discovered that "viral toxins" can cause severe COVID-19

A new study published in the journal Nature Communications reveals that viral toxins produced by the SARS-CoV-2 virus may cause severe COVID-19 infection
.

The study shows how part of the SARS-CoV-2 "spike" protein can disrupt cellular barriers inside blood vessels in body organs, such as the lungs, leading to so-called vascular leaks
.
Blocking the activity of this protein may help prevent some of the fatal symptoms of COVID-19, including pulmonary edema, which causes acute respiratory distress syndrome (ARDS
).

"Theoretically, by specifically targeting this pathway, we can block the pathogenesis that leads to vascular disease and acute respiratory distress syndrome without targeting the virus itself," said Scott B.
Biering, lead author of the study and a postdoctoral scholar at the University of California, Berkeley
.
"Given all the different variants that are emerging, and the difficulty of preventing infection with each variant individually, it may be beneficial
to focus on these pathogenic triggers in addition to stopping infection altogether.
"

While many vaccine skeptics have expressed concern about the potential dangers of the SARS-CoV-2 spike protein, a target of COVID-19 mRNA vaccines, the researchers say their work has not provided evidence that the spike protein causes symptoms
in the absence of viral infection.
Instead, their research suggests that the spike protein may work synergistically with the virus and the body's own immune response to trigger life-threatening symptoms
.

In addition, the concentration of spike protein circulating in the body after vaccination was much lower than the concentration observed in patients with severe COVID-19 and the concentration
used in the study.

Eva Harris, senior author of the study and professor of infectious diseases and vaccinology at the University of California, Berkeley, said: "The amount of spike protein contained in a vaccine will never lead to a leak
.
In addition, there is no evidence that (the spike protein) itself is pathogenic
.
The idea is that it can help and abet ongoing infections
.
”

By studying the effects of the SARS-CoV-2 spike protein on cells in human lungs and blood vessels, as well as mouse lungs, the team discovered molecular pathways
that allow the spike protein to disrupt key barriers in the body.
In addition to opening up new avenues for treating severe COVID-19, understanding how the spike protein causes vascular leakage could shed light on the pathology
behind other emerging infectious diseases.

Biering said: "We think that many viruses that cause serious disease may encode a viral toxin
.
These proteins are immune to viral infections and interact with barrier cells, causing these barriers to fail
.
This allows the virus to spread, and the amplification of the virus and leakage of blood vessels are the cause of
serious illness.
I hope we can use the principles we learned from the SARS-CoV-2 virus to find ways to block this pathogenesis so that we can be better prepared
when the next pandemic hits.
”

How spike proteins trigger vascular leakage

Vascular leakage
occurs when the cells that line the blood vessels and capillaries are destroyed, causing plasma and other fluids to leak from the blood.
In addition to the lung and heart damage observed in severe COVID-19, vascular leakage can lead to hypovolemic shock, which is the leading cause
of death from dengue.

Prior to the COVID-19 pandemic, Biering and other members of Harris' research project were studying the role
of the dengue virus protein NS1 in triggering vascular leakage and causing hypovolemic shock.
When the pandemic hit, the team wondered if similar viral toxins in SARS-CoV-2 could also contribute to acute respiratory distress syndrome
, which causes deaths in COVID-19 patients.

"People are aware of the role of bacterial toxins, but the concept of viral toxins is still a really new concept
," Harris said.
"We have determined that this protein, secreted from dengue-infected cells, is able to cause permeability of endothelial cells and disrupt the internal barrier
, even in the absence of the virus.
So, we wondered if SARS-CoV-2 proteins, such as spike, could do something similar
.
”

The spike protein covers the outer surface of SARS-CoV-2, giving the virus a multi-node appearance
.
They play a key role in helping viruses infect their hosts: The spike protein binds to a receptor on human and other mammalian cells called ACE2, acting like a key-turning lock, allowing the virus to enter the cell and hijack cell function
.
When SARS-CoV-2 virus infects cells, large amounts of spike proteins
containing receptor-binding domains (RBDs) are shed.

"What's really interesting is that circulating spike proteins are associated
with severe cases of COVID-19 in the clinic," Biering said.
We wanted to know if this protein was also causing any of the blood vessel leaks
we saw with SARS-CoV-2.
”

Currently, scientists attribute severe COVID-19-related heart-lung injuries to an overactive immune response
known as a cytokine storm.
To test the theory that the spike protein might also work, Biering and other team members used thin layers of human endothelial and epithelial cells to mimic the lining
of blood vessels in the body.
They found that exposing these cell layers to the spike protein increased their permeability, a hallmark
of vascular leakage.

Using CRISPR-Cas9 gene editing technology, the team showed that permeability increases even in cells that do not express the ACE2 receptor, suggesting that it can occur
independently of viral infection.
In addition, they found that mice exposed to the spike protein also exhibited vascular leakage, although the mice did not express human ACE2 receptors and were not infected with SARS-CoV-2
.

Finally, with the help of RNA sequencing, the researchers found that the spike protein triggers vascular leakage
through a molecular signaling pathway involving glycans, integrins, and transforming growth factor β (TGF-β).
By blocking the activity of integrins, the team was able to reverse vascular leakage
in mice.

"We have discovered a novel pathogenic mechanism of SARS-CoV-2, in which the spike protein breaks down the barrier
of our vascular system.
The resulting increase in permeability can lead to vascular leakage, as is often observed in severe COVID-19 cases, and we can reproduce the manifestations of these diseases in our mouse models," said
study co-author Felix Pahmeier, a graduate student in Harris' lab at the UC Berkeley School of Public Health.
"It was interesting to see the similarities and differences between the spike virus protein and the dengue virus protein NS1
.
Both can disrupt the endothelial barrier, but the timeline and host pathway between the two appear to be different
.
”

While blocking integrin activity could be a promising target for the treatment of severe COVID-19, Harris said more work is needed to understand the exact role
of this pathway in disease progression.
While increased vascular permeability accelerates infection and causes internal bleeding, it can also help the body fight the virus
by giving immune mechanisms closer proximity to infected cells.

"SARS-CoV-2 evolved a spike surface protein with an enhanced
ability to interact with host cell membrane factors such as integrins by obtaining RGD motif.
This motif is a common integrin-binding factor that is used by many pathogens, including bacteria and other viruses, to infect host cells," said
Francielle Tramontini Gomes de Sousa, a former assistant project scientist in Harris' lab and co-first author of the study.
"Our study shows how peak RGD interacts with integrins, leading to TGF-β release and activation
of TGF-β signaling.
Using in vitro and in vivo models of epithelial cells, endothelial cells, and vascular permeability, we are able to better understand the cellular mechanisms underlying elevated TGF-β levels in COVID-19 patients and how spike-host cell interactions contribute to disease
.
”

The team is continuing to study the molecular mechanisms that cause blood vessel leakage and is studying possible viral toxins
in other viruses that cause severe disease in humans.

"COVID-19 has not gone away
.
We have better vaccines now, but we don't know how the virus will mutate
in the future.
Studying this process may help us develop a new drug library so that if someone experiences a leak in a blood vessel, we can target it
.
Maybe it can't stop the virus from replicating, but it can stop that person from dying
.
”

Scott B.
Biering, Francielle Tramontini Gomes de Sousa, Laurentia V.
Tjang, Felix Pahmeier, Chi Zhu, Richard Ruan, Sophie F.
Blanc, Trishna S.
Patel, Caroline M.
Worthington, Dustin R.
Glasner, Bryan Castillo-Rojas, Venice Servellita, Nicholas T.
N.
Lo, Marcus P.
Wong, Colin M.
Warnes, Daniel R.
Sandoval, Thomas Mandel Clausen, Yale A.
Santos, Douglas M.
Fox, Victoria Ortega, Anders M.
N ä ä r, Ralph S.
Baric, Sarah A.
Stanley, Hector C.
Aguilar, Jeffrey D.
Esko, Charles Y.
Chiu, John E.
Pak, P.
Robert Beatty, Eva Harris.
SARS-CoV-2 Spike triggers barrier dysfunction and vascular leak via integrins and TGF-β signaling.
Nature Communications, 2022; 13 (1) DOI: 10.
1038/s41467-022-34910-5