Fully genetically encoded light-gated actuators which do not require the addition of an exogenous cofactor appear as a viable solution for controlling neuronal activity in vivo. The hunt for candidates started in the early 2000s and quite naturally first concentrated on the phototransduction machineries underlying animal vision. The light-sensitive elements in these systems are membrane-embedded photopigments called rhodopsins, each rhodopsin molecule consisting of a protein called opsin (belonging to the family of G-protein coupled receptors or GPCRs) covalently bound to a chromophore (a vitamin-A related compound called retinal or one of its derivatives). Upon illumination, the bound retinal molecule undergoes isomerization, which induces conformational changes in the opsin backbone and activates a G-protein signaling pathway.
This path was pioneered by the team of Gero Miesenböck who managed to reconstitute a minimal fly phototransduction cascade in mammalian neurons by coexpressing NinaE, a blue-sensitive rhodopsin and two of its natural partners: the αqG-protein subunit and arrestin-2, a protein required for deactivation of rhodopsin. Upon illumination, the excited rhodopsin activates an endogenous phospholipase C through the action of the G-protein, which in turn activates non-specific cation channels through the production of second messengers. The system called “chARGe” was used to optically elicit action potential firing in cultured hippocampal neurons (Zemelman, Lee et al. 2002) but was fastidious to implement and carried intrinsic limitations like slow and variable activation and deactivation kinetics (a few hundred milliseconds to several tens of seconds). Following a similar rationale, subsequent studies showed that heterologous expression of single mammalian opsins in neurons was enough to modulate endogenous conductances through specific G-protein cascades, but with comparable slow kinetics [1, 2, 3, 4].
The slow kinetics observed in these approaches is inherent to the metabotropic nature of vertebrate and invertebrate opsin signaling, which challenges their relevance as strategies for temporally precise control of neuronal firing. However in a different perspective, animal opsins were used successfully to gain optical control over specific intracellular transduction pathways (see page on Opto-XRs).
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