Sunday, September 25, 2011

Artificial heart cells beat in response to light

Heart cells created in the lab are able to beat in synchronized fashion by externally controlling them with light. This remarkable feature was artificially built in by scientists from Stanford University, by adopting a gene found in algae. The gene, coding for the light sensitive protein channelrhodopsin-2, was inserted in the genome of human embryonic stem cells, which were consequently directed to transform into heart cells, so called cardiomyocytes. These specialized muscle cells are found in the heart, where their synchronized beating causes the contraction and relaxation needed to pump blood in and from the heart. By controlling the beating process externally with light pulses, scientists may have found a new way to generate pacemakers for artificially constructed heart tissue, that can be used in heart regeneration treatment.

The channelrhodopsin-2 that is built into the differentiated cardiomyocytes responds to pulses of blue light by opening up small gates in the cell membrane, which makes influx and efflux of molecules possible. After opening these so called ion channels, positively charged sodium ions are able to enter the cell which causes an electrical gradient over the cell membrane. This is the basis for conduction of signals in the body, either for the neurons in the nervous system or dispatching the required signal for synchronized beating by heart cells. In the heart, specifically, the electrical signal is conducted from cell to cell, through specialized openings that connect cells internally. This is needed to assure quick dispersal, which allows the required signal to arrive at the heart cells at almost the same time.

Injecting cardiomyocytes, or their precursors, into the heart is a therapy that is explored for use in patients that suffered from stroke, or other forms of heart damage. Fresh cells could replace the damaged and non-functional tissue, but they would have to cooperate with the remaining cardiomyocytes in order to achieve the required synchronized beating. Currently, integrating artificial tissue with the body's own tissue is one of the main problems. In addition, the synthetic pacemakers that are currently used to re-synchronize heart beats are mechanic instruments that are more prone to failure than endogenous pacemakers. With light pulses, the pacemaker does not have to be in contact with the heart, in contrast with defibrillators that pump an electric pulse into the body, possibly causing damage. That means that after injecting the light-sensitive cells, doctors could more safely and effectively keep them in check.

In a different study on the development of alternative pacemakers, scientist made use of gold nanowires. After wiring heart tissue, patches of cardiomyocytes were found to be beating in sync, even though the original source of electrical stimulation was relatively far away. The researchers note artificially conducting electrical signals with the nanowires could also be put to use for neuronal signalling. However, the technique has only been tested in vitro so far.

Freshly generated heart tissue with a built in tool for synchronizing their beats with existing tissue could prove to be a method to more effectively integrate regenerate heart tissue and make the cells work together. The scientists used computer models in order to assess the most effective location to inject channelrhodopsin-2 expressing cardiomyoctes. Their model also allowed them to check the optimal form of light stimulation. Of course, this therapy remains untested in humans.

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