New Study Reveals Single Virus, Single Receptor Binding in COVID-19 Infection

A recent study conducted using super-resolution microscopy has debunked previous theories about the COVID-19 virus and its infection mechanism. Contrary to previous assumptions, the study demonstrates that the SARS-CoV-2 virus binds to a single ACE2 receptor, rather than multiple receptors simultaneously. This finding challenges the notion of multiple virus particles attaching to numerous receptors at once. The discovery of this singular binding process could greatly aid in the development of more effective strategies for preventing and treating COVID-19.

In Europe, the SARS-CoV-2 coronavirus pandemic, which began in 2020, is now largely under control. However, the efficient spread of this virus still raises questions. To shed light on this matter, a team of researchers led by Dr. Simone Backes, Dr. Gerti Beliu, and Prof. Dr. Markus Sauer from the Julius Maximilian University of Würzburg (JMU) conducted a study published in Angewandte Chemie, revealing the need to reconsider certain previous assumptions.

One such assumption involved the virus binding with multiple surface proteins and multiple receptors on the target cell. This explanation had been put forth to account for the virus’s increased infectivity. The researchers from Würzburg now provide evidence that a single virus binds to a single receptor, enabling highly efficient infection.

Previously Speculated Mechanisms

SARS-CoV-2 carries an average of 20 to 40 spike proteins on its surface. These proteins enable the virus to bind to ACE2 receptors located in the membranes of target cells, such as those in the human nose and throat. When antibodies block these receptors, the infection of cells becomes impossible. “This indicates that the virus’s binding to the ACE2 receptor is the critical step in infection,” explains Sauer.

Until now, visualizing the ACE2 receptors and their interaction with viral spike proteins under a microscope had not been possible. As a result, much of the understanding relied on speculation, such as whether viruses could bind to multiple receptors using multiple spikes to facilitate cell entry.

It was also suggested that the receptors existed in pairs or groups of three within the membrane, enabling more efficient binding to the trimeric spike proteins. Alternatively, it was thought that the receptors formed groups only after binding to a spike protein. Both possibilities heavily depended on the density of ACE2 receptors in the membrane.

Clarity Through Super-Resolution Microscopy

To unravel this mystery, the researchers in Würzburg labeled antibodies with dyes to make the receptors visible and countable. They employed various cell lines commonly used as models for SARS-CoV infection and utilized the single-molecule sensitive super-resolution microscopy technique known as dSTORM, developed in Markus Sauer’s research group.

The results revealed that Vero cells, frequently employed as models for SARS-CoV-2 infection, possess merely one to two ACE2 receptors per square micrometer of cell membrane. This represents a very low density. “In other membrane receptors, this number is often between 30 and 80,” adds Sauer.

“The average distance between neighboring ACE2 receptors is about 500 nanometers, much larger than the size of a virus particle, which measures only 100 nanometers,” says Backes. Consequently, the idea of a virus particle with multiple spike proteins simultaneously binding to multiple receptors becomes highly unlikely.

ACE2 Receptors Exist Individually

The question then arose: Are the receptors present in pairs or groups of three within the membrane? “No, they exist only as individual receptors, and this remains the case even after a viral spike protein has bound to them,” explains Beliu, a group leader at the Rudolf Virchow Centre. For an infection to occur, it is sufficient for a single spike to bind to a single receptor.

With these results, the JMU team has refuted several original hypotheses concerning the interaction between viral particles and multiple ACE2 receptors. They have also demonstrated that host cells expressing higher levels of ACE2 are more susceptible to infection, as expected. However, factors such as the lipid composition of the membrane and other elements also influence infection efficiency.

What Lies Ahead?

The JMU research team aims to acquire as much detailed knowledge as possible about the cell entry mechanism of coronaviruses to gain a better understanding of the infection process. Ultimately, this understanding could contribute to improved prevention measures and the development of more effective drugs against COVID-19. Their next step involves analyzing the entry mechanism using high-resolution light sheet microscopy.

Reference: “Coronaviruses Use ACE2 Monomers as Entry-Receptors” by Dr. Patrick Eiring, Dr. Teresa Klein, Dr. Simone Backes, Marcel Streit, Marvin Jungblut, Dr. Sören Doose, Dr. Gerti Beliu, and Prof. Dr. Markus Sauer, 27 March 2023, Angewandte Chemie.
DOI: 10.1002/anie.202300821

The research described in the publication was funded by the European Research Council, the German Research Foundation, and the German Federal Ministry of Education and Research.

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• Study: Coronaviruses Use ACE2 Monomers as Entry-Receptors
• Journal: Angewandte Chemie