Optogenetics and pharmacogenetics: principles and applications

Abstract

Remote and selective spatiotemporal control of the activity of neurons to regulate behavior and physiological functions has been a long-sought goal in system neuroscience. Identification and subsequent bioengineering of light-sensitive ion channels (e.g., channelrhodopsins, halorhodopsin, and archaerhodopsins) from the bacteria have made it possible to use light to artificially modulate neuronal activity, namely optogenetics. Recent advance in genetics has also allowed development of novel pharmacological tools to selectively and remotely control neuronal activity using engineered G protein-coupled receptors, which can be activated by otherwise inert drug-like small molecules such as the designer receptors exclusively activated by designer drug, a form of chemogenetics. The cutting-edge optogenetics and pharmacogenetics are powerful tools in neuroscience that allow selective and bidirectional modulation of the activity of defined populations of neurons with unprecedented specificity. These novel toolboxes are enabling significant advances in deciphering how the nervous system works and its influence on various physiological processes in health and disease. Here, we discuss the fundamental elements of optogenetics and chemogenetics approaches and some of the applications that yielded significant advances in various areas of neuroscience and beyond.

Keywords: light-sensitive ion channels; modified G protein-coupled receptors; neuronal activity.

Fig. 1.

Optogenetics approach. The light-sensitive microbial opsins react rapidly to light to modulate the electrical activity of targeted neurons. A: most popular opsins include channelrhodopsin (ChR), halorhodopsin (HR), and archaerhodopsin (BR). ChR2 is a light-gated sodium channel that responds to blue light (~470 nm) to enable sodium ion influx, leading to neuronal depolarization. Conversely, HR and BR respond to yellow (540 nm) and green (566 nm) lights, respectively, to silence neurons by opening chloride ion channels and proton pumps, respectively, resulting in neuronal hyperpolarization. B: typically, the double-floxed inverse open-reading frame (DIO) contains the inverted repeat (ITR), the elongation factor-1α (EF1α) promoter, an eYFP-ChR2 transgene surrounded by loxP sites and lox2722 sites oriented inward, a woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), and a polyadenylation signal (polyA). Presence of Cre recombinase cleaves the loxP sites, resulting in the expression of the eYFP-ChR2 transgene. C: delivery of a light-sensitive opsin packaged in a viral vector in the desired brain region can be made by stereotaxic microinjection. Once infected, the targeted neurons will be equipped with opsin on the plasma membrane. An implanted fiber optic cannula delivers light causing depolarization or hyperpolarization of the targeted neurons.

Fig. 2.

Designer receptors exclusively activated by designer drugs (DREADD) approach. The modified G protein-coupled receptors (GPCRs, mutated human M₃ or M₄ muscarinic receptor) can be activated by exogenously administrated otherwise inert drugs like small-molecule clozapine N-oxide (CNO). A: the excitatory DREADD (hM3Dq) is coupled to G_q and, when activated with CNO, results in calcium influx leading to neuronal depolarization. Conversely, the inhibitory DREADD (hM4Di) is coupled to G_i, and its activation with CNO reduces cAMP levels that in turn decrease neuronal activity. B: although the delivery of DREADDs to desired brain regions still requires skilled stereotaxic microinjection, subsequent activation of infected neurons can be simply achieved by administration of CNO systemically, ip or in the drinking water. C: intravenous administration of CNO caused a rapid decrease in the sympathetic nerve activity (SNA) subserving brown adipose tissue (BAT) in mice expressing DREAAD (hM3Dq) in AgRP neurons but not in control mice (106).