3d images

Amazing 3D images show coronavirus infecting human airway cells

A team in the Netherlands has successfully captured 3D images of human airway cells infected with SARS-CoV-2 using an extraordinary microscopic technique. The images show how the coronavirus changes the structure of cells it infects and could aid in drug development.

Researchers at Utrecht University cultured cells taken from the noses of healthy volunteers and infected some of the cells with the coronavirus. They then stained the cells with fluorescent dyes that bind to fatty membranes (the blue parts at the start of the video above), proteins (the magenta parts at the start of the video) and the coronavirus spike protein (the purple dots appearing from 0:17).

Then the cells were cut by enzymes and embedded in a gel. When water is added to the gel, it swells, enlarging the embedded structures. The technique (called expansion microscopy) was developed by other groups, but these researchers improved it, allowing them to increase samples tenfold in each dimension.

This means light microscopes can effectively see structures just 20 nanometers wide – including the SARS-CoV-2 virus, which is around 100 nanometers in diameter – whereas they normally cannot properly see objects smaller. 200 nanometers in diameter.

The resulting images show that large membrane-bound structures involved in virus formation appear inside infected airway cells. By staining a specific protein, the team identified these structures as so-called multivesicular bodies that grew abnormally large. They were also seen in electron microscope images of SARS-CoV-2 infected cells, but their identity was unclear.

The surfaces of human airway cells are covered with two types of hair-like structures. The larger ones, called cilia, beat to move mucus along the airways and keep them free of dust. Then there are the smaller microvilli that increase cell surface area to help with absorption.

The microvilli of infected cells become longer and sometimes branched. In the video, pink dots reveal the presence of spike proteins along them, often right at the tips. This suggests that new viruses are budding at the tips of microvilli in airway cells, which is known to happen with some other viruses, such as influenza viruses.

The images also show that covid-19 infection damages cilia, which are clumped and distorted in infected cells. It’s unclear why, as there’s no spike protein on them.

The team also infected cells originally taken from monkey kidneys. These cells usually have a smooth surface, but the infected cells had many protrusions called filopodia from which the viruses seem to bud. The team believe the process that causes filopodia to form is the same process that elongates microvilli in airway cells.

new scientist contacted the researchers, but they wouldn’t talk about their findings until they were published in a peer-reviewed journal.

Reference: bioXriv, DOI: 10.1101/2021.08.05.455126

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