COVID-19 Can Make Brain Cells Fuse

Viruses like SARS-CoV-2 can make brain cells fuse, causing neurological symptoms such as brain fog.

While COVID-19, a disease caused by the SARS-CoV-2 virus, is primarily respiratory, increasing evidence suggests that it also causes neuropsychiatric syndromes, which may persist for months after infection, contributing to "long COVID."

To better understand SARS-CoV-2's impact on the brain, researchers at the University of Queensland infected mouse and human brain organoids, artificially grown human brain models, with viral fusogens — proteins promoting plasma membrane fusion among different cells.

They found that the virus caused fusion between neurons and between neurons and glia, non-neuron cells in the brain whose primary function is to provide support and protection to the neurons.

"After neuronal infection with SARS-CoV-2, the spike S protein becomes present in neurons, and once neurons fuse, they don't die," explains Professor Massimo Hilliard, author of the study. "They either start firing synchronously, or they stop functioning altogether."

It is generally thought that when a virus enters the brain, it causes either cell death or inflammation. The study published in Science Advances, on the other hand, suggests that neurons may fuse following the infection.

Neurons' fusion with other neurons and glial cells results in sharing of large molecules and compromised neuronal activity. While the latter can directly affect brain function and animals' behavior, sharing large molecules may lead to the spread of toxic aggregates, as observed in several neurodegenerative diseases. Additionally, sharing of molecules could result in viral spreading that eludes the immune system.

This could potentially explain why people experience neurological symptoms, such as headache, brain fog, and loss of taste and smell, after the infection. However, the impact on neuronal fusion may depend on the viral load in the brain and the specific areas infected.

A study from Israel that included over 300,000 medical records found that people aged 40 to 61 years are more likely to suffer from long COVID symptoms. However, most symptoms wain within one year.

Symptoms of long COVID may include:

• Tiredness or fatigue that interferes with daily life
• "Brain fog," which is defined as difficulty thinking or concentrating
• Headache
• Sleep problems
• Lightheadedness
• Pins-and-needles feelings
• Change in smell or taste
• Depression or anxiety
• Difficulty breathing or shortness of breath
• Chest paint

The study by researchers at the University of Queensland implies that other viral infections can also cause neuronal fusion.