Viruses such as influenza A and Ebola invade human cells in several steps. In an interdisciplinary approach, research teams from Heidelberg University and Heidelberg University Hospital investigated the final stages of viral penetration using electron tomography and computer simulations. In the case of influenza A, they were able to determine how the immune system fights the virus using a small protein. For Ebola viruses, they found that a specific protein structure must be taken apart for an infection to take hold. So-called fusion pores, through which the viral genome is released into the host cell, play a central role in these processes. If they can be prevented from forming, the virus is also blocked. The Heidelberg scientists describe previously unknown mechanisms, which could lead to new approaches to prevent infections.
Many viruses that infect humans are covered with a lipid membrane that has glycoproteins that can attach to human cells. In viruses such as influenza A, which enter the respiratory tract, these are the spike proteins that bind primarily to epithelial cells in the nose and lungs. By contrast, the highly infectious Ebola virus is spread through direct contact with infected body fluids and can penetrate a wide spectrum of cell types. After invading human cells, these viruses must open a fusion pore between the virus membrane and the host membrane to release their genome into the host cell and propagate.
To combat the virus, the human immune system tries to block the formation of the fusion pore in a multi-step process. Infected cells detect the presence of the foreign genome and send a signal, in the form of an interferon molecule, to cells that are not yet infected. This signal causes uninfected cells to produce a small cellular protein called interferon-induced transmembrane protein 3 (IFITM3). “This specialized protein can effectively prevent the penetration of viruses such as influenza A, SARS-CoV-2 and Ebola, but the underlying mechanisms were unknown,” says virologist Dr. Petr Chlanda, whose working group belongs to the BioQuant Center of the University of Heidelberg and the Comprehensive Research Center for Infectious Diseases of Heidelberg University Hospital. The researchers were now able to show that for influenza A viruses, IFITM3 selectively sorts lipids in the membrane locally. This prevents fusion pores from forming. “Viruses are literally captured in a lipid trap. Our research indicates that they are eventually destroyed,” explains Dr. Chlanda.
To analyze the structural details of the viruses, Dr. Chlanda and his team took advantage of equipment from the Ruperto Carola Cryoelectron Microscopy Network. In an interdisciplinary approach, the research groups led by Prof. Dr. Ulrich Schwarz from the BioQuant-Center and the Institute for Theoretical Physics together with Prof. Dr. Walter Nickel from the Center for Biochemistry at the University of Heidelberg predicted this process with the help of computer simulations. In the context of antiviral therapy, the researchers believe it is possible to develop lipid-sorting peptides that insert into the virus membrane, rendering the viruses incapable of membrane fusion. “Such peptides could be used in a nasal spray, for example,” says Petr Chlanda.
In a second study, the Heidelberg researchers investigated the penetration and fusion of the Ebola virus. The filamentous morphology of the virus is determined by a flexible protein envelope known as the VP40 matrix protein coat. “It has always puzzled us how this long virus could enter the cell, fuse with the membrane, and release its genome,” says Dr. Chlanda. Using their structural analysis of infected but quiescent cells provided by collaborators at the Friedrich Loeffler Institute in Greifswald, the researchers found that this viral protein coat disassembles at low pH, that is, in an acidic environment. This step is no less decisive for the formation of melt pores, as further computer simulations by Prof. Schwarz and Prof. Nickel have shown. During this process, the electrostatic interactions of the VP40 matrix with the membrane are weakened, which reduces the energy barrier of pore formation. The results of the Heidelberg basic research suggest that blocking the disassembly of this layer would be a way of keeping Ebola viruses in a state that does not allow the formation of fusion pores. Like the influenza A virus, the Ebola virus would be lured into a trap from which it could not escape.
The studies were part of the Collaborative Research Center “Integrative Analysis of Pathogen Replication and Spread” (CRC 1129) funded by the German Research Foundation. The research results were published in both “Cell Host & Microbe” and in the EMBO Magazine.
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