Luring the virus into a trap

Newswise —

Microbes such as influenza A and Ebola infiltrate human cells through various stages. Researchers from Heidelberg University and Heidelberg University Hospital explored the concluding phases of viral invasion through electron tomography and computer simulations, taking an interdisciplinary approach. They identified how the immune system combats influenza A using a small protein, and they uncovered the importance of dismantling a specific protein configuration for Ebola viruses to cause an infection. These processes are centered around fusion pores, which permit the release of viral genetic material into the host cell. Preventing their formation can obstruct the virus. The Heidelberg researchers identified previously unknown mechanisms that could lead to innovative methods for preventing infections.

Numerous human-infecting viruses possess a lipid membrane coating that contains glycoproteins capable of binding with human cells. For example, influenza A uses spike proteins to primarily attach to epithelial cells in the nose and lungs via the respiratory tract. In contrast, Ebola is extremely contagious and can infiltrate various cell types via direct contact with contaminated bodily fluids. Once inside a human cell, these viruses need to initiate a fusion pore between their membrane and the host's membrane to discharge their genetic material and spread.

In order to combat viruses, the human immune system employs a multi-stage process to hinder the formation of fusion pores. When infected cells identify the presence of foreign genetic material, they emit a signal, in the form of an interferon molecule, to as yet uninfected cells. This signal prompts the uninfected cells to produce a specific cellular protein known as interferon-induced transmembrane protein 3 (IFITM3). "This specialized protein has the ability to effectively prevent viruses, such as influenza A, SARS-CoV-2, and Ebola, from penetrating, but the underlying mechanisms were previously unknown," explains Dr. Petr Chlanda, a virologist from the BioQuant Center of Heidelberg University and the Center for Integrative Infectious Disease Research of Heidelberg University Hospital. The researchers have now demonstrated that, in the case of influenza A viruses, IFITM3 selectively organizes the lipids in the membrane locally. This prevents the formation of fusion pores, and the viruses are effectively trapped in a lipid cage. "Our research suggests that they are ultimately eradicated," adds Dr. Chlanda.

Dr. Chlanda's team utilised equipment from the Cryo-Electron Microscopy Network at Ruperto Carola to scrutinise the structural details of viruses. In an interdisciplinary effort, the groups led by Prof. Dr. Ulrich Schwarz from the BioQuant-Center and the Institute for Theoretical Physics, along with Prof. Dr. Walter Nickel from the Heidelberg University Biochemistry Center, used computer simulations to predict this process. In the context of antiviral treatment, the researchers believe that it may be possible to develop lipid-sorting peptides that insert themselves into the virus membrane, making it impossible for the viruses to fuse with the host membrane. "Such peptides could be administered as a nasal spray, for example," suggests Dr. Chlanda.

The Heidelberg researchers conducted a second study that investigated the penetration and fusion of the Ebola virus. The virus has a filamentous morphology that is determined by the VP40 matrix protein layer, which is a flexible protein envelope. "It has always been a mystery to us how this lengthy virus could penetrate the cell, fuse with the membrane, and release its genome," remarks Dr. Chlanda. Through their structural analysis of infected but inactive cells provided by collaborators from the Friedrich Loeffler Institute in Greifswald, the researchers discovered that the protein envelope of the virus disassembles at low pH, such as in an acidic environment. This step is critical for the formation of fusion pores, as demonstrated by further computer simulations conducted by Prof. Schwarz and Prof. Nickel. During this process, the VP40 matrix's electrostatic interactions with the membrane are weakened, thereby lowering the energy barrier for pore formation. The results of the Heidelberg's fundamental research indicate that blocking the disassembly of this layer would be one way to keep Ebola viruses in a state that prevents fusion pore formation. Like the influenza A virus, the Ebola virus would then be trapped and unable to escape.

The studies were part of the Collaborative Research Centre “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” as well as the EMBO Journal.