“You can observe a lot just by watching,” said the accidental sage, leaving us to wonder whether he meant to celebrate the power or lament the futility of simply paying attention. Were he scientifically inclined, he might have added that mere observation is unsatisfying because many things can be learned only through experimentation—perturbing things, then seeing what happens next. So it is with cells: Once roused or stifled, cells may reveal much that would otherwise remain hidden.
The idea is to enact “what if” scenarios instead of simply watching and waiting. The trick, however, is to perturb cells without being too heavy-handed, always a risk when cells are electrically jolted or chemically lesioned. The lightest touch, it turns out, may be an actual “light touch”—the use of illumination to alter the behaviors of selected cells. This is the essence of optogenetics.
Optogenetics relies on proteins that alter their functions when they are exposed to light. These proteins, which are expressed naturally by various organisms, may be introduced to targeted cells within living organisms by means of genetic engineering. Then, by supplying illumination of the appropriate wavelength to sensitized cells, the researcher may incite or suppress selected biological processes.
It is possible to perturb the cell’s biological processes at several levels. To date, most optogenetic studies have deployed light-sensitive ion channels or pumps, mostly in neurons. By switching specific populations of neurons on and off, scientists gain the ability to disentangle neural circuits and decode the brain’s electrical signaling. Membrane-bound, light-sensitive proteins have also been used to explore the cell’s internal signaling, the biochemical cascades triggered, for example, by G-protein coupled receptors.
With optogenetics, it is also possible to “push buttons” beyond the cell membrane. Instead of using optogenetics to open or close an ion channel, researchers have created an optogenetic system that controls customized transcription machinery. This system relies on a light-sensitive protein that has been fused to a DNA-binding protein, a transcription activator-like effector (TALE), which may be customized to bind to selected regions of the genome. When illuminated, the protein-TALE complex is activated, and transcription commences. Thus, the light-sensitive protein and the TALE, used together, determine not only where, but when, transcription occurs. A similar system, one that would use CRISPR instead of TALE technology, has been proposed.
Used at yet another level, optogenetics may activate or inactivate individual proteins that have been engineered to carry a bulky, artificial, light-sensitive amino acid. This amino acid, upon illumination, sheds a side chain to become cysteine, and the protein of which it is a part alters its shape and gains activity.
One such protein was incorporated into an ion channel. The protein, at the flick of a light switch, jettisoned its artificial amino acid’s extra bulk, freeing the ion channel of an obstruction. Although this protein was used to optically open an ion channel, similar proteins could be devised to optically regulate protein modifications and protein-protein interactions.