Re-engineered optogenetic switches allow direct measurement of complex cellular systems
A Harvard University chemical biology lab has re-engineered optogenetic switches, photosensitive proteins called rhodopsins inserted into mammalian cells to control electrical firing, so that the switches run backward, firing off bursts of fluorescent light that reveal newly detailed patterns of electrical activity in neural networks, beating cardiac cells and developing embryos, according to a Dec. 17 presentation at the American Society for Cell Biology Annual Meeting in San Francisco.
Adam Cohen, PhD, said that optogenetic switches were pioneered in 2005 by Stanford University neuroscientist Karl Deisseroth, MD, Phd, in mammalian neurons, using light as a remote controller to turn them on or off. Rhodopsin switches have since allowed researchers to gain optical control over the electrical state and thereby the firing of neurons of worms, fish, mice and even monkeys.
In reversing the circuitry, Dr. Cohen has made the protein an indicator light, allowing his lab to directly measure electrical activity in complex cellular systems such as heart muscle. Researchers traditionally have inserted a very fine glass capillary into the cell and used a sensitive voltimeter to calculate voltage.
"This procedure is slow and laborious. We realized that optical measurements could be conducted with much higher throughput, a possible boon for screens to identify neuronal or cardiac drugs," said Cohen, whose Harvard lab identified the rhodopsin protein Archaerhodopsin 3 (Arch), derived from the Dead Sea microorganism Halorubrum sodomense, as a fast and sensitive fluorescent indicator of membrane voltage.
Dr. Cohen and collaborators were able to express the Arch protein inside cultured rat neurons. Each time the rat neuron fired, the researchers saw and recorded a flash of fluorescence. With this technique, they created spatial maps charting the propagation of electrical impulses in neurons, providing a newly detailed look at how these impulses arise and spread. The researchers also expressed these voltage-indicating proteins in rat cardiac cells and monitored the electrical impulse associated with each heartbeat.
In human cardiac cells derived from human induced pluripotent stem cells (hiPSCs), Dr. Cohen and his team expressed Arch as a voltage indicator. Because these cells beat spontaneously in a culture dish, the researchers were able to plot the cells' responses to a wide variety of drugs by making optical measurements of the electrical activity of several thousand cells in a brief time period.
"Studies on hiPSC-derived cardiac cells are particularly exciting," said Dr. Cohen, "because they enable us to study cardiac electrophysiology in cells derived from people with genetic predispositions to a wide variety of cardiac diseases."
The Cohen lab has also expressed microbial rhodopsin proteins in living zebrafish. Because the fish are transparent, optical studies are possible without surgery. His lab has charted the electrical waves that initiate the heartbeat, the development in a zebrafish embryo of the heart from a quiescent patch of cells to a fully beating organ, and the firing of neurons in a zebrafish brain in response to external stimuli.
Arch is one of the over 5,000 known microbial rhodopsin proteins that generate colors. They share molecular kinship with the rhodopsin molecules in the human eye that enable color vision.