Application of the red-shifted channelrhodopsin Chrimson for the Caenorhabditis elegans cGAL bipartite system

Abstract

Channelrhodopsins are light-gated ion channels that serve as photoreceptors in photosynthetic microbes and have been applied as crucial optogenetic tools in genetic model organisms. When expressed in animals, they enable light-inducible control of ionic membrane permeability, which directly manipulates the activity of neurons expressing the protein. The application of channelrhodopsin-based optogenetics is particularly powerful when used in conjugation with the cGAL (GAL4-UAS) bipartite system (Wang, 2017). The mating of neuron-specific GAL4 driver lines to new channelrhodopsin effector lines could expand the genetic toolkit to perturb and manipulate neural circuits in the organism. Blue light-gated channelrhodopsins have been widely used in C. elegans neurobiology but often have to be performed in lite-1(ce314) mutant backgrounds because short-wavelength blue light is an aversive cue in wild-type animals and directly affects C. elegans neuronal physiology. Previously, a red light-gated variant of channelrhodopsin, termed Chrimson, has been successfully applied in Drosophila and mice, and has recently been codon-optimized for use in C. elegans (Klapoetke, 2014; Schild, 2015). Here, we constructed a Chrimson (15xUAS::chrimson::gfp) cGAL effector line. We introduced the UAS::chrimson::gfp effector DNA construct as an extrachromosomal array into a previously published cGAL pan-neuronal driver line (PS6961 syIs334) and generated integrants on chromosome II (PS8023, syIs503) and chromosome V (PS8024, syIs504) (Table 1) via standard X-ray irradiation. We showed Chrimson-GFP expression in the C. elegans head and tail neurons (Fig. 1A-1D). We also showed that red light could induce a seizure-like motility phenotype in C. elegans expressing Chrimson-GFP in a pan-neuronal manner (videos), while the negative controls expressing only the effector, or without light induction showed regular motility as expected (Table 2). The body curvature maps from normal and seizure-like motilities showed distinct patterns (Fig. 1E and 1F). We report the effector construct of red-light-gated channelrhodopsin Chrimson as an addition to our cGAL toolkit, which could be widely used in future research to overcome the technical restrictions of blue light-gated channelrhodopsins in C. elegans.

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