Through experiments on lab-grown mini-brains and mice, researchers have found that SARS-CoV-19 infection causes the accumulation of "zombie" cells, causing the premature brain aging associated with Long-COVID-19 (some call it "brain fog"). The researchers have also found a drug that can reverse this premature aging associated with the virus.
Although SARS-CoV-2 is primarily a respiratory viral pathogen, it can also produce a range of neurological complications after the acute phase. Long-COVID, often associated with cognitive impairment or brain fog, is a noteworthy complication, and there is substantial evidence that COVID-19 patients experience significant changes in brain structure.
While research supports the role of senescent or "zombie" cells - cells that have stopped dividing - in neurodegenerative diseases and the cognitive decline that occurs with aging, their contribution to brain aging associated with COVID-19 is unclear. This prompted researchers from the Australian Institute of Bioengineering and Nanotechnology (AIBN) at the University of Queensland (UQ) to study the effects of different SARS-CoV-2 variants on brain tissue and to search for drugs that might reverse the process.
"We found that COVID-19 accelerates the appearance of 'zombie' or senescent cells, which naturally accumulate in the brain as we age," said Julio Aguado, first and corresponding author of the study. "Senescent cells are known to cause tissue inflammation and degeneration, causing patients to develop cognitive impairments such as brain fog and memory loss."
The researchers hypothesized that SARS-CoV-2-induced brain aging is related to the neuroinflammatory effects of the virus during the acute phase. To test their hypothesis, the researchers analyzed the brains of patients who died from severe COVID-19 or non-infectious, non-neurological causes. They found that the number of p16 protein-positive cells increased more than sevenfold in the brains of COVID-19 patients compared with controls. Cellular senescence is often characterized by the expression of p16, and the findings suggest that SARS-CoV-2 has the potential to trigger cellular senescence, leading to cognitive decline and accelerating the neurodegenerative processes associated with Long-COVID.
The researchers then used embryonic stem cells to generate brain organoids—laboratory miniature brain models—and subjected the organoids to physiological aging for eight months before testing the efficacy of senolytics, or drugs that remove senescent cells.
"We used brain organ tissue to screen a range of therapeutic drugs, looking for drugs that can eliminate senescent cells," Aguado said.
They discovered four drugs that selectively eliminate senescent cells: Navitoclax, ABT-737, fisetin, and the combination of dasatinib and quercetin (D+Q). Navitoclax and ABT-737 inhibit Bcl-2 protein, thereby inducing apoptosis or programmed cell death in senescent cells. Fisetin and D+Q can pass through the blood-brain barrier and remove senescent cells in the brain. Senescent organ tissues were exposed to two doses (biweekly) of Navitoclax, ABT-737, or D+Q, and then subjected to bulk RNA sequencing analysis.
Compared with Navitoclax and ABT-737, D+Q has a broader scope of action and can alleviate multiple pro-inflammatory pathways unique to cellular aging. In addition to acting as a senolytic agent, D+Q restored the gene expression age of nine-month-old organ tissue to a level comparable to that of eight-month-old organ tissue. Gene expression changes induced by D+Q treatment were positively correlated with characteristics of life-extending interventions such as caloric restriction, suggesting that the drug has a health-promoting role in targeting cellular aging. In short, this therapy rejuvenates the brain tissue of the organ.
In addition to normal brain aging, the researchers infected brain organ tissue with variants of SARS-CoV-2 and found that they caused a significant increase in cellular senescence, especially the Delta variant. The expression of SARS-CoV-2 viral RNA was significantly reduced after treatment of infected organisms with senolytics.
The researchers then conducted experiments on mice infected with the SARS-CoV-2 Delta variant. Compared with the control group, the survival rate of mice was significantly improved after treatment with fisetin or D+Q, and the median life span was extended by 60%. All aging interventions significantly reduced COVID-related disease features, especially in the D+Q treatment group, including reductions in p16 and pro-inflammatory cytokines. As with the organoid experiments, the researchers found that viral gene expression was significantly reduced in mice treated with the senolytic agent compared with untreated mice, with infected mice showing reduced senolytic gene expression to levels comparable to uninfected brains.
"More research is needed to fully understand the mechanisms, but this study marks an important step in our understanding of the intricate relationship between viral infection, aging and neurological health," Aguado said. "In the long term, we can expect these drugs to be widely used to treat persistent acute post-infectious syndrome caused by viral infections such as COVID-19."
The researchers say that using brain tissue allows them to conduct ethical studies that would be practically difficult to conduct in human subjects, and that the same approach could be used to study other neurodegenerative diseases associated with aging.
Ernst Wolvetang, one of the study's co-authors, said: "Our study is a good example of how human brain models can speed up preclinical screening of therapeutics - while also moving towards animal-free testing - and could have global impacts. The same drug screening method could also help research into Alzheimer's disease, as well as a range of neurodegenerative diseases where aging is a driver."
The research was published in the journal NatureAging.