Cancer and many other life-threatening diseases are caused by genetic defects. These so-called mutations prevent the affected genes from fulfilling their normal functions. Gene therapies try to make use of viruses for the import of healthy genes into cells where they can replace their defective counterparts.
Newer approaches employ synthetic viruses which pose a lesser threat of pathogenic side effects. These “custom-made” molecular vehicles are designed in a way that allows them to bind almost exclusively to affected cells and leave out healthy tissue. Scientists at the Ludwig-Maximilians-Universität (LMU) Munich have now analyzed for the first time the dynamics of three different targeted synthetic viruses in living cells.
In the journal “Molecular Therapy” they report that one of these synthetic vectors recognized EGFR, the “Epidermal Growth Factor Receptor”, which is present in high amounts on the surface of many different human tumor cells. The microscopic images show that the interaction with that particular molecule accelerated and facilitated the uptake of the synthetic viruses – an important step towards efficient therapy.
For replication viruses need to invade cells where they hijack their hosts’ metabolism in order to produce a new virus generation. This ability makes them potential transport vehicles in gene therapy. A new – and safer – approach relies on artificial viruses that act with the same efficiency as their natural counterparts. A team of scientists under the lead of Professor Christoph Bräuchle, department of chemistry and biochemistry, and Professor Ernst Wagner, department of pharmacy, now analyzed the way of three different synthetic viruses into cancer cells.
“We used so-called polyplexes,” says Wagner. “These are complexes of the molecule that store genetic information, DNA, and a chemical called PEI, short for polyethylenimine.” Only one of these variants carried EGF, the “epidermal growth factor”, on its surface. This molecule plays an important role in the regulation of cellular growth and other essential processes. It enters cells by first binding to its receptor EGFR on the cells’ surfaces. This specific molecular “lock and key” combination is especially interesting because a broad range of human tumor cells display elevated levels of EGFR on the outside which makes the molecule an attractive therapeutic target. “The same goes for gene therapies,” says Dr. Manfred Ogris, corresponding author of the study. “Ideally, the molecular vehicle would be administered systemically via the blood stream, but become active only in the target tissue.”
It has been known that the uptake of synthetic viruses via EGFR is especially efficient, though the underlying mechanism remained unclear. “That is why we compared unspecific vectors with ones that carried EGF on their surface,” explains Bräuchle. These polyplexes were marked so that the team could follow them with the help of fluorescence microscopy – and in the highest resolution achieved so far.
“Our videos show that polyplexes with EGF are internalized much faster,” says Bräuchle. “The setup allowed us for the first time to observe and record the way of targeted synthetic viruses into the cells and their dynamics in the cell in detail.” In addition, the tracking revealed a three-phase sequence of events with phase I ending with internalization of the vector, which moves inside the cell during phase II and is being rapidly transported along cellular structures in the last phase. “The interaction between EGF and EGFR shortens the first phase and strongly accelerates internalization,” says Bräuchle. “The knowledge of these details is essential for the development of efficient gene therapies.”
Reference: ”Cellular Dynamics of EGF Receptor Targeted Synthetic Viruses”, Karla de Bruin, Nadia Ruthardt, Katharina von Gersdorff, Ralf Bausinger, Ernst Wagner, Manfred Ogris and Christoph Bräuchle, Molecular Therapy, 24 April 2007
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