Prototype biosensor screens drugs without killing cells

The researchers, based at the Max Planck Institute for Biochemistry, Germany, have created a device that can be used to measure the effect of potential drugs on the serotonin receptor (5-HT3A), an ion channel that plays an important role in the nervous system. The research is published in the journal Angewandte Chemie​. When serotonin binds to its receptor, its gates are opened and ions flood through the cell membrane into the cell; the receptor is activated. This stream of charged particles can be read, although current 'patch-clamp techniques', using tiny electrodes, damages the cells being analysed. However, Professor Peter Fromherz and his team have shown this doesn't have to be the case; their new technique is non-invasive and could allow pharma companies to test the efficacy of potential drugs aimed at this target, or establish a dosage-effect relationship. This is important as inhibitors of 5-HT3A are used to reduce the nausea that results from chemotherapy and for the treatment of irritable bowl syndrome. To build the biosensor, Prof. Fromherz first grew specific human embryonic kidney cells (HEK293), which produce the serotonin receptor in unusually high numbers, onto a silicon chip with a line of transistor switches. Then, a cell that covers a transistor gate is selected; it is separated from the transistor by a narrow cleft (around 50nm), which is filled with extracellular electrolyte. Also, the internal cell voltage is kept constant using a special patch-pipette electrode. This enables the team to accurately measure the effect of serotonin once it is introduced into the system. The ions flow along the narrow gap between the cell and the transistor, into the cell. The subsequent extracellular voltage is measured by the transistor. This voltage is proportional to the ion current of the activated receptor. If a potential receptor-blocking drug was then introduced into the system, the effect on transistor voltage could eb measured and the drug developer would have a quantitative measure of the compound's effectiveness. "With this coupling of a ligand-steered ion channel to a transistor at the level of an individual cell,"​ Prof. Fromherz said, "we have laid the foundation for receptor-cell-transistor biosensor technology."​ However, two important problems must still be solved by the team. First, they must find a way to avoid using the patch-pipette, although without it the current only flows for around 100 microseconds and there is not enough time to record the transistor voltage. Prof. Fromherz thinks that a potassium channel could be used to overcome this and provide efficient repolarisation. Secondly, the cells are not consistently placed over the transistors. Some of them lie completely over the gate, some have 50 per cent overlap and some as little as 10 per cent, explained Prof. Fromherz. However, it is very difficult to place the cell manually so to overcome the problem, the team, instead, want to use another piece of equipment they have developed that contains many more transistors - around 16,000 to ensure all the cells have a good overlap. Prof. Fromherz went on to explain that the device could be used to test other receptors. He particularly mentioned G-protein coupled receptors, which are the most poular group of targets in drug development.