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COVID-19, the disease caused by the pandemic SARS-CoV-2 coronavirus, is primarily considered a respiratory infection. Yet the virus has also become known to affect other parts of the body in a less well understood way, sometimes with longer term consequences, such as cardiac arrhythmia, fatigue, and “brain fog”.
Researchers at the University of California San Diego School of Medicine are using organoids derived from stem cells – tiny balls of human cells that look and act like mini-organs in a lab box – to study how the virus interacts with various organ systems and to develop therapies to block infection.
“We find that SARS-CoV-2 does not infect the whole body in the same way,” said Tariq Rana, PhD, professor and head of the genetics division in the pediatrics department at the Faculty of Medicine of the UC San Diego. and Moores Cancer Center. “In different types of cells, the virus triggers the expression of different genes, and we see different results.”
Rana’s team published their findings on February 11, 2021 in Stem cell reports.
Like many organs, the team’s lung and brain organoids produce ACE2 and TMPRSS2 molecules, which are found like doorknobs on the outer surfaces of cells. SARS-CoV-2 grabs these doorknobs with its spike protein as a way to enter cells and establish infection.
Rana and her team developed a pseudovirus – a non-infectious version of SARS-CoV-2 – and tagged it with a green fluorescent protein, or GFP, a bright molecule derived from jellyfish that helps researchers visualize the inner workings of cells. The fluorescent marker allowed them to quantify the binding of the virus spike protein to ACE2 receptors in human lung and brain organoids, and to assess cell responses.
The team was surprised to see about 10 times more ACE2 and TMPRSS2 receptors and therefore much higher viral infection in lung organoids, compared to brain organoids. Treatment with viral spike protein or TMPRSS2 inhibitors reduced infection levels in both organoids.
“We saw fluorescence dots in the organoids of the brain, but it was the organoids in the lungs that really lit up,” Rana said.
In addition to the differences in infectivity levels, lung and brain organoids also differed in their responses to the virus. Lung organoids infected with SARS-CoV-2 pumped molecules intended to mobilize the help of the immune system – interferons, cytokines and chemokines. Infected brain organoids, on the other hand, increased their production of other molecules, such as TLR3, a member of the toll-type receptor family that plays a fundamental role in recognizing pathogens and activating innate immunity. .
Rana explained that while it may at first seem like the brain organoid reaction is just another form of immune response, these molecules can also aid in programmed cell death. Rana’s team has previously seen a brain cell response similar to Zika virus, an infection known to delay neonatal brain development.
“The way we see brain cells react to the virus may help explain some of the neurological effects reported by patients with COVID-19,” Rana said.
Of course, organoids are not exact replicas of human organs. They lack blood vessels and immune cells, for example. But they do provide an important tool for studying disease and testing potential therapies. According to Rana, organoids more accurately mimic the actual human condition than cell lines or animal models that have been designed to overexpress human ACE2 and TMPRSS2.
“In animals overexpressing ACE2 receptors, you see everything light up with infection, even the brain, so everyone thinks that’s the real situation,” Rana said. “But we found that it probably wasn’t.”
In addition to their work with the pseudovirus, the team validated their findings by applying live, infectious SARS-CoV-2 to lung and brain organoids in a biosafety level 3 laboratory – a facility specially designed and certified to study safe germs.
Now, Rana and her collaborators are developing SARS-CoV-2 inhibitors and testing their effectiveness in organoid models derived from people of diverse racial and ethnic backgrounds who represent California’s diverse population. They recently received new funding from the California Institute for Regenerative Medicine to support the work.
Study co-authors include: Shashi Kant Tiwari, Shaobo Wang, Davey Smith, and Aaron Carlin, all at UC San Diego.
Disclosure: Tariq Rana is a founder of ViRx Pharmaceuticals and owns a stake in the company. The terms of this agreement have been reviewed and approved by the University of California, San Diego in accordance with its conflict of interest policies.
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