When scientists first discovered light-absorbing molecules — in microorganisms — that could control the flow of ions into a cell, many researchers became interested in comparing how these proteins differed from their visual pigment counterparts. But Karl Deisseroth at Stanford University in California saw something else in these light-activated channels — a potential tool to dissect the function of the brain.
Deisseroth — along with Edward Boyden, his postdoctoral fellow at the time, now at the Massachusetts Institute of Technology in Cambridge, and his graduate student Feng Zhang — collaborated with microbiologists to hunt for these light-activated channels from various species. “I come from a synaptic physiology background, so we just sequentially tried these and other proteins to find the ones that worked the best,” says Deisseroth. It was a collaboration with Georg Nagel and Ernst Bamberg at the Max Planck Institute of Biophysics in Frankfurt, Germany, who supplied a clone of a gene called channelrhodopsin-2 from Chlamydomonas reinhardtii to Deisseroth that opened the floodgates.
Channelrhodopsin-2 is a gated light-sensitive cation channel that uses a molecule of all-trans retinal to absorb photons. When Deisseroth, Boyden and Zhang expressed channelrhodopsin-2 in hippocampal neurons in the mouse brain then shone blue light on the region, they found the cells with channelrhodopsin-2 responded to the light stimulation, opening the channel and initiating the flow of ions, which resulted in an action potential in those neurons6. Deisseroth says that one of the most pleasant surprises to emerge from that first series of experiments turned out to be the precision of the system. “Even though these molecules are not designed to generate action potentials and work on this time scale in neurons,” he says, “it turns out that they can.”
This result led to the start of a new field, coined 'optogenetics' by Deisseroth in 2006, where researchers are combining optics with genetics to explore the workings of neural circuits. In the years since the first description of channelrhodopsin-2, Deisseroth's lab has gone on to advance the system and develop new probes. The team identified and developed two more light-activated molecules for optogenetic control of neurons. These were an inhibitor called halorhodopsin, and a Volvox channelrhodopsin that can also initiate neuronal activity, but is more than 100 nanometres red-shifted from the peak of the Chlamydomonas channelrhodopsin, allowing separable channels of optical control. They also developed various targeting strategies to get channels expressed in specific neuronal cell types and populations, and fibreoptic/laser-diode hardware for adaptation to mammals.
The growing number of researchers interested in applying optogenetic approaches to their particular research questions has led to an unexpected effort within Deisseroth's lab. “Technology distribution and helping other researchers has been another big part of what we are doing,” says Deisseroth. His lab has supplied clones of the channels to more than 300 other labs around the globe for use in organisms ranging from mice to fruitflies. Although the channels have worked in every species tried thus far, in some cases, minor modifications have been required. Although there is sufficient all-trans retinal in the brains of mice for the channels to function properly, invertebrates must be supplemented with all-trans retinal through their diet to achieve channel activation.
As the basic use of these light-activated channels for studying brain function and circuitry gains more traction in the neuroscience community, Deisseroth is taking the technology a step further. He sees patients at Stanford Medical School and, using optogenetic approaches, his lab establishes different animal models of neuropsychiatric disease.
“We now have models of Parkinson's disease, depression and altered social behaviour relevant to autism,” notes Deisseroth. In all of these disorders, the circuit dynamics are not working well, so Deisseroth's goal is to use his optogenetic models to deconstruct the neural circuitry in an attempt to better understand which parts of the brain are functioning properly, and which are not, in the different disease states.
With the ability to work on a wide range of organisms, an expanding tool kit, and growing interest among researchers, Deisseroth sees even greater possibilities for the system. “In the end, if you can get the gene in, along with the proper light, it works very well.”
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Russian Physics Journal (2019)