For persons interested in selective activation, these proteins are research heaven. You could insert ChR-2 into a neuron and turn it on at will with a wavelength of light. You could drop NpHR into another neuron and turn it off at will with a wavelength of light. You could even drop both genes into a single neuron—or a group of neurons—and then turn them on and off by selectively exposing them to blue or yellow light. Not only do you have an on/off switch, you have a remote control!Does it work in real life?
These manipulations worked as expected with cultured neural cells. The manipulations did not alter the cells' electrical properties or other normal functions—they just turned them on or off. In NpHR, the degree of inhibition could be manipulated like a dimmer switch; it was shown to be dependent on the intensity and duration of exposure to yellow light. The effect was so robust that short pulses of light could knock out single action potentials.
However good the work was in dishes, the results said little about real world biology. What about whole tissues? Whole organisms? A number of experiments investigated neural activity within increasingly sophisticated neurological backgrounds.
The first experiments involved transfecting neural tissues with specially engineered lentiviruses that carry these ion channel genes. The viruses are exposed to neural tissue, where they set up a productive infection cycle. This includes the insertion of the gene into the resident chromosomes and expression of their proteins. One such experiment infected the brains of mouse pups and the hippocampi of adult mice with viruses loaded with both ion channel genes. Subsequent assessment of the acute activity of cortical and hippocampal slices showed that neurological activity could be manipulated by simply flashing blue light (excitatory) or yellow light (inhibitory). The tissues turned on or off, depending on the wavelength deployed. This was the first evidence that complex neurological tissues could be regulated remotely with great precision and in a bidirectional manner.
The next experiments involved examining intact animals. The organism used was Caenorhabditis elegans, a tiny roundworm that is the subject of a great deal of genetic research (it has been particularly useful in elucidating the so-called genes of aging). It is possible to insert genes into specific tissues of this organism and assess changes in behavior. Such work was done on the roundworm using these ion channel genes. The creatures were specifically engineered to express the proteins in the body wall muscle cells that mediate the wiggling, swimming movement of the worm, or in the cholinergic neurons that control these muscles. The results were stunning. By flashing blue or yellow light onto the creature, they could control the wriggling, swimming behavior of the worm!Conclusions
This is quite a technical achievement with plenty of research and clinical applications. One of the greatest potential uses of this technology is the elucidation of neuronal pathways involved in typical behaviors. Plans are under way to trace circuits involved in tactile sensations. Using animal models of human disease, other researchers are working to identify circuits relevant to depression, Parkinson disease, and epilepsy.
There are many potentially powerful applications of this technology for humans. It is possible that these switches could be used to coax neurons in the retinas of blind patients with no functioning rods or cones to fire in the presence of blue or yellow light, perhaps restoring aspects of vision.
There may even be a psychiatric application. The suitability of deep brain stimulation for patients with severe depression has been studied by researchers such as Helen S. Mayberg and colleagues for years.1 In this experimental protocol, electrodes are inserted deep in the brain in an attempt to stimulate certain neural circuits. Working something like localized electroconvulsive therapy, the hope is that activation of these circuits could successfully treat these patients. It is very possible that such photosensitive proteins might some day replace electrodes currently in use for deep brain stimulation investigations. This might allow the clinician to target only those neurons relevant to the disease. There is a large barrier to overcome before any of this could be done in humans, of course, including issues such as the insertion of these proteins into a living brain.
Still, what biologists have in hand is no less than the wish list for the alien brain mentioned at the beginning of this column. For the first time, there is the ability to turn neural circuits on and off at will and to do so from a remote location. It is an extraordinary collaboration between the behavioral, physiological, and molecular sciences. No doubt your boss at the electronics laboratory would be proud.