Brain microvascular endothelial cell dysfunction during Plasmodium infection does not result from astrocyte-derived wingless-related integration site (Wnt)7a deficiency
Despite evidence that Plasmodium infection downregulates astrocyte-derived Wnt7a and that exogenous Wnt7a fails to improve survival in infected mice, the study concludes that astrocyte dysfunction is not the primary cause of blood-brain barrier breakdown and endothelial junction protein loss in cerebral malaria.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Malaria is a disease caused by a parasite that invades the blood, but its most deadly form strikes the brain. When the parasite infects red blood cells, those cells can stick to the tiny blood vessels inside the skull, causing a blockage and a breakdown of the brain's protective shield. This shield, known as the blood-brain barrier, is a tight seal that normally keeps harmful substances out of the brain while letting nutrients in. When this seal fails, fluid leaks into the brain tissue, causing swelling that can lead to seizures, coma, and death. While scientists have long known that the blood vessels themselves become damaged during this process, they have been less certain about the role of the brain's support cells. These support cells, called astrocytes, wrap around the blood vessels and normally send chemical signals to keep the seal tight. A key question has been whether the failure of these support cells to send the right signals is what causes the barrier to collapse in severe malaria.
Researchers at the University of Utah set out to investigate this specific possibility. They focused on a particular chemical signal called Wnt7a, a protein that astrocytes usually release to help maintain the integrity of the blood-brain barrier. In the context of a severe malaria infection, the team suspected that the intense inflammation might cause astrocytes to stop producing this protective protein, leaving the blood vessels vulnerable. To test this, they used a mouse model of cerebral malaria, a condition that mimics the deadly form of the disease seen in humans. They examined the brains of infected mice and found that the astrocytes were indeed reacting strongly to the infection, but not in the way they expected. Instead of simply shutting down, the astrocytes were overwhelmed by a flood of immune signals, specifically a molecule called interferon-gamma, which is part of the body's defense against infection.
The team discovered that this immune signal did suppress the production of Wnt7a in the astrocytes. In the infected mice, the astrocytes produced less of this protein, and the blood vessels showed signs of reduced activity in the pathways that Wnt7a normally controls. This initially suggested that the loss of Wnt7a might be the reason the barrier was failing. However, the researchers did not stop at this observation. They wanted to know if fixing this specific problem would actually save the mice. To find out, they used a harmless virus to deliver extra copies of the Wnt7a gene directly into the brains of mice before infecting them with malaria. They reasoned that if the lack of Wnt7a was the main cause of the barrier breakdown, then flooding the brain with this protein should restore the seal and prevent the mice from dying.
The results were surprising. Despite successfully increasing the levels of Wnt7a in the brains of the treated mice, the extra protein did not stop the disease from progressing. The mice that received the extra Wnt7a died at the same rate and with the same severity of symptoms as the mice that did not receive it. Their blood vessels still leaked, and their brains still swelled. This finding ruled out the idea that the loss of Wnt7a was the primary driver of the fatal barrier breakdown. The researchers then looked closer at what the astrocytes were actually doing. They found that while the immune signal did lower Wnt7a, it also triggered the astrocytes to release other molecules that actually helped strengthen the barrier. In laboratory experiments using human cells, the fluid from astrocytes exposed to malaria parasites and immune signals made the blood vessel walls tighter and more resistant to damage, rather than weaker.
The study concludes that the astrocytes are not failing to protect the brain; in fact, they are trying to help. The immune system's response to the malaria parasite creates a chaotic environment where the astrocytes are sending out a mix of signals. Some of these signals are protective, while others, like the reduction of Wnt7a, might be counterproductive. However, the protective efforts of the astrocytes are simply not strong enough to overcome the direct damage being done to the blood vessels by the infection and the immune cells attacking them. The barrier breaks down not because the support cells have given up, but because the force of the inflammation is too great for their natural repair mechanisms to handle. This distinction is important because it suggests that future treatments for cerebral malaria should not focus solely on boosting the signals from support cells, but rather on calming the overwhelming inflammation that is damaging the blood vessels in the first place.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.