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Optical Inhibition of Larval Zebrafish Behaviour with Anion Mohamed et al. BMC Biology (2017) 15:103 DOI 10.1186/s12915-017-0430-2 METHODOLOGY ARTICLE Open Access Optical inhibition of larval zebrafish behaviour with anion channelrhodopsins Gadisti Aisha Mohamed1, Ruey-Kuang Cheng1, Joses Ho2, Seetha Krishnan3, Farhan Mohammad4, Adam Claridge-Chang2,4 and Suresh Jesuthasan1,2,4* Abstract Background: Optical silencing of activity provides a way to test the necessity of neurons in behaviour. Two light-gated anion channels, GtACR1 and GtACR2, have recently been shown to potently inhibit activity in cultured mammalian neurons and in Drosophila. Here, we test the usefulness of these channels in larval zebrafish, using spontaneous coiling behaviour as the assay. Results: When the GtACRs were expressed in spinal neurons of embryonic zebrafish and actuated with blue or green light, spontaneous movement was inhibited. In GtACR1-expressing fish, only 3 μW/mm2 of light was sufficient to have an effect; GtACR2, which is poorly trafficked, required slightly stronger illumination. No inhibition was seen in non-expressing siblings. After light offset, the movement of GtACR-expressing fish increased, which suggested that termination of light- induced neural inhibition may lead to activation. Consistent with this, two-photon imaging of spinal neurons showed that blue light inhibited spontaneous activity in spinal neurons of GtACR1-expressing fish, and that the level of intracellular calcium increased following light offset. Conclusions: These results show that GtACR1 and GtACR2 can be used to optically inhibit neurons in larval zebrafish with high efficiency. The activity elicited at light offset needs to be taken into consideration in experimental design, although this property can provide insight into the effects of transiently stimulating a circuit. Keywords: Optogenetics, Zebrafish, Chloride channel, Behaviour, Neural circuit Background function experiment, effective tools for neuronal inhib- One approach to understanding the role of specific ition are required. neurons in a given behaviour is to experimentally alter A number of different optogenetic inhibitors have their activity. A precise means of doing this is by using been developed. The first generation of these tools include light-gated channels [1–4]. Optogenetic activators such light-actuated channels like halorhodopsin [12, 13], as channelrhodopsin-2 [5], ChIEF [6] and CsChrimson which facilitates light-dependent chloride entry, and [7] can reversibly depolarize membrane potential with archaerhodopsin [14, 15], a proton pump that is also millisecond resolution and have been widely used in a hyperpolarizing. A limitation of these molecules is their range of organisms [8–11]. For numerous different low conductance and the high levels of expression and behavioural functions, these tools have enabled the illumination that are required for effective silencing. determination of neuronal sufficiency. However, estab- For example, the archaerhodopsin derivative Archer1 lishing whether neurons are normally involved requires requires 3 mW/mm2 of light to inhibit action potentials additional experiments. Optical or electrical recording in cultured mammalian neurons [16], while halorhodop- can determine which activity patterns are correlated with sin requires ~20 mW/mm2 to inhibit activity in zebrafish behaviour. However, for the crucially important loss-of- neurons [17]. A second generation of optogenetic inhibi- tors includes genetically modified channelrhodopsins * Correspondence: [email protected] such as ChloC [18] and iC1C2 [19], which hyperpolarize 1Lee Kong Chian School of Medicine, Nanyang Technological University, neurons by light-gated conductance of chloride. Both Singapore, Singapore 2Institute of Molecular and Cell Biology, Singapore, Singapore require similar levels of illumination, although an Full list of author information is available at the end of the article improved version of ChloC, iChloC, requires 10 times © Jesuthasan et al. 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Mohamed et al. BMC Biology (2017) 15:103 Page 2 of 12 less light [20]. More recently, naturally evolved anion- Spontaneous movement and light stimulation conducting channels from the alga Guillardia theta were GAL4s1020t, UAS:ACR1-eYFP and GAL4s1020t, shown to silence neurons at very low light levels, i.e. in UAS:ACR2-eYFP embryos were screened with a fluores- the range of microwatts per square millimetre [21]. cence stereomicroscope at 23–24 h post-fertilization to Drosophila experiments have confirmed that these opto- identify ACR-expressing fish. Embryos, still within their genetic tools are potent inhibitors in vivo [22]. Here, we chorions, were then placed in a glass dish with 24 ask whether Guillardia theta anion channelrhodopsins concave wells on a stereomicroscope (Zeiss Stemi 2000) (GtACR1 and GtACR2) are effective inhibitors of neural with a transmitted light base. Behaviour was recorded activity in the larval zebrafish, a genetically tractable on the microscope using a Point Gray Flea2 camera vertebrate. controlled by MicroManager. Stimulating light was de- livered by LED backlights (TMS Lite), with peak inten- Methods sity at 470 nm (blue), 525 nm (green) or 630 nm (red), The experiments here were carried out in accordance placed adjacent to the glass dish. The power used for with guidelines approved by the Institutional Animal high intensity illumination was the maximum that could Care and Use Committee of Biopolis, Singapore. be delivered by these LEDs. The same voltage settings were used with the three light boxes to give high, Generation of GtACR1 and GtACR2 transgenic zebrafish medium and low intensities, but the irradiance produced lines differed. We provided 595-nm (amber) illumination Sequences encoding GtACR1 and GtACR2 fused to using LEDs from CREE (XR7090-AM-L1-0001), which eYFP [22] were placed downstream of the upstream acti- were mounted onto thermal LED holders (803122; vating sequence (UAS) using Gateway cloning (Thermo Bergquist Company). The intensity of light was mea- Fisher Scientific). The resulting construct was cloned sured using an S120VC power sensor and a PM100A into a plasmid containing Tol2 sequences to facilitate console (Thorlabs, Newton, NJ, USA). LEDs were integration into the zebrafish genome [23, 24]. The con- switched on and off using an Arduino board controlled structs (33 ng/μl) were then injected into nacre-/- eggs at by MicroManager to regulate the power supply unit. the one-cell stage, along with elavl3:Gal4 DNA (15 ng/ Embryos were recorded for a total of 45 s, with the LED μl, lacking Tol2 sequences) to induce GtACR expression, being turned on 15 s after the start of recording and and Tol2 mRNA (33 ng/μl) to facilitate genomic integra- turned off 15 s later. Each embryo was tested once for tion. Embryos were screened after 24–36 hs for eYFP each condition and tested with different intensities and expression. Healthy embryos with eYFP expression were wavelengths. grown to adulthood. At 2 months, fish were fin-clipped and PCR-screened with GtACR-specific primers Analysis of behaviour recordings (GtACR1 forward 5’-CACCGTGTTCGGCATCAC-3’, Image analysis was carried out using Fiji (RRID: GtACR1 reverse 5’-GCCACCACCATCTCGAAG-3’; SCR_002285) [28] as well as scripts written in Python. GtACR2 forward 5’-ATTACCGCTACCATCTCCCC-3’, From the raw recordings, one frame was extracted per GtACR2 reverse 5’-TGGTGAACACCACGCTGTAT-3’) second to obtain a total of 46 frames (including the first to test for the presence of transgene. This led to the gen- and last frames). Circular regions of interest (ROIs) were eration of two transgenic lines, Tg(UAS:GtACR1- manually drawn around each chorion to isolate each fish. eYFP)sq211 and Tg(UAS:GtACR2:eYFP)sq212. Each frame was then subtracted from the next frame to identify the differences between frames. The number of Zebrafish lines different pixels in each ROI was taken as a measure of Transgenic lines used in this study are elavl3:GCaMP6f movement of each embryo [22]. Any embryo that did not [25], TgBAC(gng8:GAL4)c416 [26], Et(-0.6hsp70l:Gal4- move during the entire recording was discarded from ana- VP16)s1020t [27], Et(-0.6hsp70l:Gal4-VP16)s1011t [27] lysis. Estimation statistical methods were employed to and UAS:NpHR-mCherry [17]. For brevity, the enhancer analyse mean differences between control and experimen- trap lines are referred to as GAL4s1020t and tal groups [29–31]. The 95% confidence intervals (CIs) for GAL4s1011t in the text and figures. the mean difference were calculated using bootstrap methods [32]. All CIs were bias-corrected and accelerated Confocal imaging [33], with resampling performed 5000 times. All reported Twenty-four-hour-old F1 embryos were dechorionated, P values are the results of Wilcoxon t tests. anaesthetized with 160 mg/L tricaine, and mounted in 1% low melting agarose in E3. Imaging was carried out Two-photon calcium imaging using a Zeiss LSM800 confocal microscope with a 10× Triple transgenic zebrafish embryos
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