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• VSV is a useful virus – one that is used to insert therapeutic genes into cells, and that can selectively kill cancer cells. • Scientists had assumed that VSV uses the LDL (“bad” cholesterol) receptor to break into cells, but it manages to get into cells without the receptor, as well. • Experiments show that this virus can enter though a number of related receptors in the LDL family. |
A master trickster virus called VSV has baffled scientists for quite some time. Because VSV is a true expert at sneaking into cells, it is widely used in gene therapy and might, in the future, help to treat cancer. Yet even though it’s been employed in research and in pre-clinical studies for over 30 years, how it gets into cells has remained a mystery. Weizmann Institute scientists have now solved this riddle, as reported in the Proceedings of the National Academy of Sciences (PNAS).
VSV is particularly suitable for research and therapeutic use because, although it causes disease in farm animals, it rarely does so in humans. Yet the virus is so good at penetrating human cells that scientists commonly use it in experimental gene therapy as a vehicle to deliver desired genes to cells. In addition, VSV holds promise for use in cancer therapy because it selectively kills cancer cells.
Some 20 years ago, Prof. Menachem Rubinstein and his colleagues in the Weizmann Institute’s Molecular Genetics Department discovered that virus-infected cells secrete a soluble protein that prevents further VSV infection. They then found that this soluble protein is identical to the extra-cellular portion of the receptor for LDL, the “bad” cholesterol. This observation led them to assume that VSV penetrates cells by binding to the cells’ surface receptor of LDL. But when they conducted experiments with cells that lacked LDL receptors, VSV was still able to get in.
In the new study, Rubinstein’s team – research student Danit Finkelshtein, working together with Dr. Ariel Werman, Dr. Daniela Novick and Sara Barak – revealed that VSV can indeed cheat its way into cells through the LDL receptor. The virus can do that because it is coated with a decoy molecule mimicking LDL: This decoy acts as a key that opens the LDL receptor “lock” on the cell’s surface.
But how does the virus infect cells lacking the LDL receptor? The researchers explored the hypothesis that VSV lets itself in through more than one receptor – that is, not only through LDL but also through alternative, structurally similar receptors that must all be members of what is known as the LDL receptor family. To test this hypothesis, they conducted experiments with a versatile protein called RAP, which blocks all family members of the LDL receptor (but, oddly, not the LDL receptor itself).
Indeed, when they pretreated cells lacking LDL receptors with the RAP protein, VSV was no longer able to penetrate these cells. In other words, the experiments bore out the hypothesis: VSV gets into cells mainly through the LDL receptor but also through other members of the LDL receptor family.
This new understanding may be of potential importance for the development of VSV-based cancer therapies. In particular, colon cancer cells have high levels of the LDL receptor on their surface, which suggests they could be selectively killed by VSV.
The new findings might also help improve gene therapy, by increasing the number of LDL receptors on the outer membranes of targeted cells – to facilitate the entry of viruses that carry a desired gene. This goal could, for example, be achieved by the anti-cholesterol drugs statins, which cause cells to display more LDL receptors on their surfaces.
Prof. Menachem Rubinstein’s research is supported by the Bernard and Audrey Jaffe Foundation; the Adelis Foundation; and the Florence Blau Charitable Trust. Prof. Rubinstein is the incumbent of the Maurice and Edna Weiss Professorial Chair of Cytokines Research.
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