Optogenetics

Optogenetics is the use of light for the manipulation of light-activated proteins to control target cell populations. Optogenetics is a tool that combines optical and genetic methods to manipulate neural networks and other cells in vivo. Optogenetics is used to study behavior, physiology and pathology in animals such as rats, mice and zebrafish and to study neural circuit foundations of behavior in invertebrates such as nematode and fruit fly. In synthetic biology optogenetics is used as a traceless inducer to remotely control cellular behavior, with applications in therapeutic synthetic biology and in the control of biological circuits.

Targeting opsins, which are the optogenetic actuators, to specific neural cells is done using virus-based transfection or through the generation of transgenic animal lines that express opsins in cell specific manner. Typically light is delivered through an optical fiber inserted with stereotactic guidance.

Opsins function as light-activated ion pumps, light-gated ion channels, and G-protein coupled receptors, and are grouped into superfamilies type I (derived from microbes) or type II (derived from animals). Microbial opsin genes encode microbial rhodopsin proteins, which are functionally distinct and not related in primary sequence to the rhodopsins in the vertebrate eye that mediate phototransduction. Whereas vertebrate rhodopsins couple intracellular second-messenger cascades to indirectly influence ion channels, microbial rhodopsins directly transduce photons into electrical current. Bacteriorhodopsins, halorhodopsins and channelrhodopsins are microbial rhodopsins used in optogenetics.

Channelrhodopsin-2 (ChR-2), an algal protein from Chlamydomonas reinhardtii is a light-activated cation channel. The naturally occurring channelrhodopsins are depolarizing and excitatory as they allow positively charged ions to flow through the opsin pore. When transfected into neurons and expressed on the cell membrane, these proteins allow for optical control of the polarization of the neuron's cell membrane. For instance, neurons expressing ChR-2 will depolarize when exposed to blue light and activate synaptic events.

Inhibitory opsins, such as halorhodopsins, are a class of inward chloride pumps. By using light to induce them to pump chloride into a cells, halorhodopsins are used to hyperpolarize neurons to mediate rapid and reversible silencing of neuronal activities. Engineering of color-tuned chimeras allows different light wavelengths to be applied to the same neuronal circuit for multimodal manipulation. Bacteriorhodopsins pump protons out of the cell and halorhodopsins pump chloride ions into the cell and both are normally inhibitory in neural systems because they cause hyperpolarizing current that makes it harder for neurons to fire action potentials.

Photosensory modules in non-neural optogenetics include photoreceptors, photoswitches and photoenzymes. Photoreceptors include ion channels, natural receptors and chimeric receptors located on the plasma membrane. Upon illumination, photoreceptors trigger an endogenous signaling pathway and activated biological elements such as transcription factors can be harnessed to control transgene expression from a synthetic expression unit.

Photoswitchable proteins contain light-responsive domains which dimerize, oligomerize, monomerize or change conformation when exposed to light. Light-sensitive domains can be fused to DNA-binding domains such as dCas9, zinc finger nucleases, TALENs, transcriptional repressors, transcriptional activators or protein-binding domains such as nanobodies. This method allows light to induce or reverse assembly, dissociation, caging or clustering of complexes in order to activate or sequester a desired protein, which in turn modules biological function.

Photoenzymes convert a substrate under light irradiation. An example of a photoenzyme-based system is one that uses smartphone-controlled far-red-light-controllable cells in which the BphS photoenzyme converted Guanosine 5’-Triphosphate (GTP) to c-di-GMP. C-di-GMP triggers events which cause expression of hGLP1, a therapeutic protein that ameliorates diabetes.