An advanced imaging technique known as electron tomography has allowed researchers at the National Cancer Institute (NCI), part of the National Institutes of Health (NIH), to visualize an ?entry claw,? a unique structure formed between the human immunodeficiency virus (HIV) that causes AIDS and the cell it infects. The findings are in the May 4, 2007, issue of PLoS Pathogens.
?Visualizing the molecular mechanisms by which HIV and related viruses enter their host cells can potentially lead to the identification of novel drugs,? said NIH Director Elias A. Zerhouni, M.D.
Retroviruses such as HIV establish contact and enter their target cells via an interaction between proteins on the surface of the virus (called spikes) and specific host cell membrane receptors. Previous studies have suggested that several spikes and several cell receptors are involved in every virus infection event. The findings of the NCI research team, led by Sriram Subramaniam, Ph.D., Laboratory of Cell Biology at NCI?s Center for Cancer Research, demonstrate that this is true, but in a surprising way.
?This elegant research not only gives us insights into how HIV and related viruses interact with proteins on the surface of cells and then enter the host cells to integrate their DNA, it also gives us important clues as to how to design improved anti-HIV therapy,? said NCI Director John E. Niederhuber, M.D. ?Electron tomography and other new tools for imaging at the single-cell or subcellular level also have the potential to help us actually see the subcellular effects of many different diseases ? including cancer ? that we could only guess at previously.?
In this study, the scientists showed that the interaction actually takes the form of a tight cluster of five to seven rod-shaped features. This striking and unexpected arrangement was dubbed the ?entry claw? by the researchers (see figure below). They also found that the arrangement of spikes across the rest of the virus seems to largely disappear upon formation of the entry claw, suggesting a shedding event that has not previously been noted, which may have some, still uncertain relevance to infectivity.
?The discovery of the entry claw raises many fundamental questions about viral entry into host cells,? said Subramaniam. ?What are its precise molecular components? How does the viral genetic core actually transfer to the host cell? What are the other intermediate steps of the entry claw formation and can they be visualized? As we continue to improve the technology, we believe we will answer these and other related questions.?
Traditional imaging techniques based on light microscopy do not offer sufficient resolution to see how cellular molecules and small structures interact in actual life, and powerful techniques such as X-ray crystallography can only define the structure of an individual molecule or simple molecular interactions. Electron tomography and related methods for 3D electron microscopy can fill this imaging gap and reveal fine subcellular structures or virus-host interactions in great detail. The Subramaniam lab has been pioneering advances in 3D electron microscopy, and is applying the emerging technologies to understanding not only virus-host interactions, but also visualizing such things as structures inside the cell that distinguish cancer cells from normal cells.