How the coronavirus affects our tissues and organs became increasingly clear in the course of the corona pandemic. However, it has so far been unclear what happens inside the cells when they are infected by Sars-CoV-2. This has now been discovered by a team of researchers using modern imaging methods. Accordingly, the infection leads to a radical remodeling of the cell structures after just a few hours. The virus hijacks cell organelles such as the endoplasmic reticulum, the Golgi apparatus and even the cytoskeleton in order to build up its “reproduction factories”.
It is now known that Sars-CoV-2 can attack many different cell types in our body. The range extends from the cells of the airway mucous membranes and lungs through heart and intestinal cells to the cells of the nervous system and brain. The coronavirus uses several proteins on the cell surface, including the ACE2 receptor, but also “auxiliary door openers” such as neuropilin-1 and cathepsin. With the binding site on its crown-like spike protein, Sars-CoV-2 docks to these cell receptors and thereby triggers processes that enable it to enter the cell interior. What happens afterwards, however, is only roughly known so far. It is clear that the coronavirus, like all viruses, causes the cell machinery to reproduce and produce its RNA and proteins instead of its own molecules. As a result of this “hostile takeover”, the infected cells usually die after 24 to 48 hours.
Membrane stealing in cell organelles
To find out what happens between the entry of Sars-CoV-2 into the cell and the death of the cell inside, Mirko Cortese from the University of Heidelberg and his colleagues used state-of-the-art imaging methods, including ion beam electron microscopy and electron tomography. These enabled them to follow the morphological changes in infected cells and to depict them in 3D reconstructions. “We provide decisive insights into virus-induced structural changes in the human cells examined,” explains senior author Ralph Bartenschlager from Heidelberg University. “In order to develop drugs that suppress viral replication and thus also the consequences of infection and virus-induced cell death, it is crucial to better understand the biological mechanisms that drive the replication cycle of the virus.”
The analyzes revealed that the coronavirus radically restructures the inside of the cells. Already around six hours after infection, membrane-coated vesicles can be seen in the cell plasma, which become larger and more numerous over time. “The interior of these vesicles contains intermediate stages of virus replication in the form of double-stranded RNA and also fully synthesized RNA strands – this proves that these bubbles are the sites of viral genetic make-up,” report Cortese and his colleagues. The virus takes the membranes for these bubble-like virus factories from the endoplasmic reticulum (ER), a system of membrane-encased tubules and cisterns that envelops the cell nucleus and permeates the inside of the cell. As the 3D reconstructions showed, the virus uses part of these membrane tubules to populate its vesicles and connect them to one another.
Protected by a filament cage
In order to produce the envelopes of its daughter viruses, the coronavirus also misappropriates a cell’s own structure, the Golgi apparatus. This cell organelle normally produces, among other things, secretions for the cell metabolism and releases them in membrane-covered vesicles. In the virus-infected cells, however, the Golgi apparatus instead forms vesicles with viral content. Together, the viral vesicles and the newly formed virus components form a zone inside the cell, which is surrounded by another original cell structure, as the researchers report. Accordingly, a kind of scaffold made of protein filaments from the cell skeleton is deposited around this virus factory zone. “Such intermediate filaments play an important role in the innate immune defense and in the antiviral response of the cells,” explain the scientists. This suggests that this protein cage induced by Sars-CoV-2 should protect the viral replication vesicles from the cellular defense system.
Taken together, these observations reveal how much the coronavirus is changing our cells. “We see how and where the virus multiplies within the cell and how it manipulates its host cells in order to be released after multiplication,” says co-author Yannick Schwab from the European Molecular Biology Laboratory (EMBL) in Heidelberg. Sars-CoV-2 therefore does not just use the cell machinery to copy its genetic material, but converts the entire cell interior into a virus factory. In order to make these important findings available to other research teams, the scientists make their data and, in particular, the collection of 3D structural information available to everyone. “I believe this is a precedent for us to share all the data we have produced with the scientific community. They are an impressive resource for scientists, ”says Schwab. “In this way we can support the global effort to study how Sars-CoV-2 interacts with its host.”
Source: Mirko Cortese (University of Heidelberg) et al., Cell Host & Microbe, doi: 10.1016 / j.chom.2020.11.003