The working group of the Ikerbasque researcher Xabier Contreras at Biofisika (UPV/EHU) has published a research study led by Maier Lorizate, associate lecturer in the Department of Biochemistry and Molecular Biology (to which Contreras also belongs); it focusses on how cholesterol present in the HIV membrane organises and directs the viral machinery to enable the virus to enter our e. cells and infect them.  This finding, which has recently been published in the high-impact journal 'Advanced Science', opens up a new door for the development of more direct and more efficient drugs to curb infection by the virus. 

HIV is the virus responsible for AIDS (Acquired Immunodeficiency Syndrome) because the cells it infects and destroys are immune cells. This virus has a cholesterol-rich, lipid membrane, and this lipid is in fact the subject of the study. Embedded in the lipid membrane is the fusion or envelope protein Env, which in turn comprises two subunits, gp120 and gp41. The gp120 subunit is responsible for recognising and binding to the host cell by means of cellular receptors.  Once anchored in the cell, the gp41 subunit undergoes a conformational change and inserts itself into the host cell membrane, attracting it towards the virus and bringing about the fusion of the two membranes. This fusion allows the virus to enter the cell, infect it and be replicated. 

Unlike other viruses, such as that of influenza or the various coronaviruses, HIV has very few copies of Env proteins.  In order to gain efficient access to the host cell, Env proteins have to reorganise themselves into a grouping or cluster. The work demonstrates that a region of Env protein interacts with the virus' cholesterol, and that this binding is what allows Env to reorganise itself into a cluster and infect the host cells in an effective way. 

Prior to this study published by the Biofisika group, the exact composition of the viral membrane was known, and so was the fact that the cholesterol was an important component for the stability and infectivity of the virus, because the removal or depletion of the viral cholesterol leads to the loss of its ability to enter the host cell. Env proteins were also known to rearrange themselves into a single cluster, and it was also known that this process was necessary for the virus to be infective.

However, the specific role of cholesterol in the infectivity of the virus was not known, nor whether it was involved in the formation of Env clusters, nor which region of the Env protein was responsible for interacting directly with the cholesterol.  This study addresses these questions by clarifying the molecular mechanism by which viral cholesterol and the cytoplasmic region of Env interact, allowing Env to form the clusters essential for HIV to become infective. 

Very little is known about the role of lipids in the structure and function of various proteins. This is due to a lack of tools for conducting conclusive studies and the fact that the binding of a lipid to a protein can occur extremely rapidly, in the order of milliseconds, which means that studying and clarifying such processes is a major challenge for science.  In this context, another important contribution of the study is the development and use of chemical tools in the realm of biological chemistry, which allow the study of biological processes to take place in vivo, as well as the fact that the study has been carried out on viral particles directly. 

This discovery could have immediate consequences for the development of drugs that destabilise and block the virus, thus preventing the spread of infection.  In addition, the knowledge generated by this work also has significant implications for the development of vaccines that block the virus before infection has been established. Env is the only protein on the surface of the virus and therefore the only viral protein that is accessible and susceptible to the generation of antibodies that prevent the virus from entering the host cell, the first step towards infection.  However, it has not been possible, to date, to design immunogens capable of generating neutralising antibodies that can be used in a vaccine, probably because, among other things, it has not been possible to mimic the immunogen as it occurs in an actual infection.  So, the knowledge emerging from this work about the close relationship between Env and cholesterol could help to improve the design of immunogens used in vaccine development. 

"In addition, the interaction of proteins with cholesterol could also be of great significance in SARS-CoV-2, the virus that causes COVID-19. In the case of SARS-CoV-2, the cholesterol in the patient's cells appears to play an important role in regulating the infectivity of the virus, because if a patient's normal cholesterol metabolism is altered, the strength with which SARS-CoV-2 infects his or her cells may be affected.  With the molecular approach and tools used in the work developed at Biofisika, the specific role of cholesterol in regulating SARS-CoV-2 infectivity could be studied at the molecular level, and that way the entry pathways of the virus and how to block them could be better understood," said Lorizate. 

The research process, according to Contreras, "has been multidisciplinary and almost entirely carried out in the Biofisika laboratory with tools ranging from biochemistry, molecular and cellular biology, omics (proteomics, lipidomics), biological chemistry, virology and high-resolution microscopy.  The work has taken a long time for three main reasons: the difficulty in studying complex biological processes on a nanoscale and with a very short lifespan, its multidisciplinary nature, and the fact that the resources of the group, also small, were limited.