Name: light-induced gfp expression in zebrafish embryos using the optogenetic tael/c120 system
Uploaded: 2021-08-19T14:30:00
Description: Watch this Scientific Journal Video about Light-Induced GFP Expression in Zebrafish Embryos using the Optogenetic TAEL/C120 System at JoVE.com

We thank Stefan Materna and members of the Woo and Materna labs for helpful suggestions and comments on this protocol. We thank Anna Reade, Kevin Gardner, and Laura Motta-Mena for valuable discussion and insights while developing this protocol. This work was supported by grants from the National Institutes of Health (NIH; R03 DK106358) and the University of California Cancer Research Coordinating Committee (CRN-20-636896) to S.W.

This study was performed with the approval of the Institutional Animal Care and Use Committee (IACUC) of the University of California Merced.
1. Zebrafish crossing and embryo collection
<ol>
	<li>Maintain separate transgenic zebrafish lines containing either the TAEL transcriptional activator or the C120-controlled reporter gene to minimize spurious activation.</li>
	<li>Cross 6-8 adult zebrafish from each line using standard methods9 to produce double transgenic embr...

<ol>
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This protocol describes the use of the optogenetic TAEL/C120 system to achieve blue light-inducible gene expression. This system consists of a transcriptional activator, TAEL, that dimerizes upon illumination with blue light and activates transcription of a gene of interest downstream of a C120 regulatory element. Induced expression of a GFP reporter can be detected after as little as 30 min of light exposure, suggesting that this approach possesses relatively fast and responsive kinetics.
Sev...

Inducible gene expression systems help control the amount, timing, and location of gene expression. However, achieving exact spatial and temporal control in multicellular organisms has been challenging. Temporal control is most commonly achieved by adding small-molecule compounds1 or activation of heat shock promoters2. Still, both approaches are vulnerable to issues of timing, induction strength, and off-target stress responses. Spatial control is mainly achieved by the use of tissue-specific promoters3, but this approach requires a suitable promoter or regulatory element, which are not always available, and it is not conducive to sub-tissue level induction.
In contrast to such conventional approaches, light-activated optogenetic transcriptional activators have the potential for finer spatial and temporal control of gene expression4. The blue light-responsive TAEL/C120 system was developed and optimized for use in zebrafish embryos5,6. This system is based on an endogenous light-activated transcription factor from the bacterium E. litoralis7,8. The TAEL/C120 system consists of a transcriptional activator called TAEL that contains a Kal-TA4 transactivation domain, a blue light-responsive LOV (light-oxygen-voltage sensing) domain, and a helix-turn-helix (HTH) DNA-binding domain5. When illuminated, the LOV domains undergo a conformational change that allows two TAEL molecules to dimerize, bind to a TAEL-responsive C120 promoter, and initiate transcription of a downstream gene of interest5,8. The TAEL/C120 system exhibits rapid and robust induction with minimal toxicity, and it can be activated by several different light delivery modalities. Recently, improvements to the TAEL/C120 system were made by adding a nuclear localization signal to TAEL (TAEL-N) and by coupling the C120 regulatory element to a cFos basal promoter (C120F) (Figure 1A). These modifications improved induction levels by more than 15-fold6.
In this protocol, a simple LED panel is used to activate the TAEL/C120 system and induce the ubiquitous expression of a reporter gene, GFP. Expression induction can be monitored qualitatively by observing fluorescence intensity or quantitatively by measuring transcript levels using quantitative real-time PCR (qRT-PCR). This protocol will demonstrate the TAEL/C120 system as a versatile, easy-to-use tool that enables robust regulation of gene expression in vivo.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>BioRender web-based science illustration tool</td>
			<td>BioRender</td>
			<td></td>
			<td>https://biorender.com/</td>
		</tr>
		<tr>
			<td>Color CCD digital camera</td>
			<td>Lumenara</td>
			<td>755-107</td>
			<td></td>
		</tr>
		<tr>
			<td>Compact Power and Energy Meter Console, Digital 4&#34; LCD</td>
			<td>Thorlabs</td>
			<td>PM100D</td>
			<td></td>
		</tr>
		<tr>
			<td>Excitation filter, 545 nm</td>
			<td>Olympus</td>
			<td>ET545/25x</td>
			<td></td>
		</tr>
		<tr>
			<td>illustra RNAspin Mini kit</td>
			<td>GE Healthcare</td>
			<td>95017-491</td>
			<td></td>
		</tr>
		<tr>
			<td>Instsant Ocean Sea Salt</td>
			<td>Instant Ocean</td>
			<td>SS15-10</td>
			<td></td>
		</tr>
		<tr>
			<td>MARS AQUA Dimmable 165 W LED Aquarium light (blue and white)</td>
			<td>Amazon</td>
			<td>B017GWDF7E</td>
			<td></td>
		</tr>
		<tr>
			<td>Methylcellulose</td>
			<td>Sigma-Aldrich</td>
			<td>M7140</td>
			<td></td>
		</tr>
		<tr>
			<td>NEARPOW Programmable digital timer switch</td>
			<td>Amazon</td>
			<td>B01G6O28NA</td>
			<td></td>
		</tr>
		<tr>
			<td>PerfeCTa SYBR green fast mix</td>
			<td>Quantabio</td>
			<td>101414-286</td>
			<td></td>
		</tr>
		<tr>
			<td>Photoshop image procesing software</td>
			<td>Adobe</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Prism graphing and statistics software</td>
			<td>GraphPad</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>qScript XLT cDNA SuperMix</td>
			<td>Quantabio</td>
			<td>10142-786</td>
			<td></td>
		</tr>
		<tr>
			<td>QuantStudio 3 Real-Time PCR System</td>
			<td>Applied Biosystems</td>
			<td>A28137</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>Olympus</td>
			<td>SZX16</td>
			<td></td>
		</tr>
		<tr>
			<td>Tricaine (Ethyl 3-aminobenzoate methanesulfonate)</td>
			<td>Sigma-Aldrich</td>
			<td>E10521</td>
			<td></td>
		</tr>
		<tr>
			<td>X-Cite 120 Fluorescence LED light source</td>
			<td>Excelitas</td>
			<td>010-00326R</td>
			<td>Discontinued. It has been replaced with the X-Cite mini+</td>
		</tr>
	</tbody>
</table>

light-induced gfp expression in zebrafish embryos using the optogenetic tael/c120 system

Inducible gene expression systems are an invaluable tool for studying biological processes. Optogenetic expression systems can provide precise control over gene expression timing, location, and amplitude using light as the inducing agent. In this protocol, an optogenetic expression system is used to achieve light-inducible gene expression in zebrafish embryos. This system relies on an engineered transcription factor called TAEL based on a naturally occurring light-activated transcription factor from the bacterium E. litoralis. When illuminated with blue light, TAEL dimerizes, binds to its cognate regulatory element called C120, and activates transcription. This protocol uses transgenic zebrafish embryos that express the TAEL transcription factor under the control of the ubiquitous ubb promoter. At the same time, the C120 regulatory element drives the expression of a fluorescent reporter gene (GFP). Using a simple LED panel to deliver activating blue light, induction of GFP expression can first be detected after 30 min of illumination and reaches a peak of more than 130-fold induction after 3 h of light treatment. Expression induction can be assessed by quantitative real-time PCR (qRT-PCR) and by fluorescence microscopy. This method is a versatile and easy-to-use approach for optogenetic gene expression.

For this demonstration, a C120-responsive GFP reporter line (Tg(C120F:GFP)ucm107)) was crossed with a transgenic line that expresses TAEL-N ubiquitously from the ubiquitin b (ubb) promoter (Tg(ubb:TAEL-N)ucm113)) to produce double transgenic embryos containing both elements. 24 h post-fertilization, the embryos were exposed to activating the blue light, pulsed at a frequency of 1 h on/1 h off. Induction of GFP expression was quantified by qRT-PCR at 30 min, 1 h, 3...

Watch this Scientific Journal Video about Light-Induced GFP Expression in Zebrafish Embryos using the Optogenetic TAEL/C120 System at JoVE.com

Light-Induced GFP Expression in Zebrafish Embryos using the Optogenetic TAEL/C120 System

Optogenetics is a powerful tool with wide-ranging applications. This protocol demonstrates how to achieve light-inducible gene expression in zebrafish embryos using the blue light-responsive TAEL/C120 system.

Inducible gene expression systems are an invaluable tool for studying biological processes. Optogenetic expression systems can provide precise control ...

This protocol is significant because it is an optogenetic, or light-activated method, for inducing gene expression, in an intact organism. In our case, a zebrafish embryo. The advantage of this technique, is that it uses light, as an inducing agent, which allows for very precise, spacial and temporal control of induction.This protocol would be useful for any situation where inducible gene expression would be useful. This includes things like lineage tracing, gain-of-function assays or rescue experiments. To begin with position the LED panel relative to the Petri dishes containing embryos for delivering 1.5 milliwatts per square centimeter power of light to several embryos at once.For reducing the risk of photo damage to the Tail Transcriptional Activator, post the light at intervals of one hour on-off using a timer relay, if illuminating for more than three hours. Remove Petri dish lids to minimize light scattering from condensation. Place Petri dishes containing control embryos and a light-proof box for dark controls.Remove embryos from illumination, after the desired activation duration. For total RNA extraction, use an RNA extraction kit. Transfer the embryos to a 1.5 milliliter micro centrifuge tube and remove the excess egg water.After adding lysis buffer, homogenize the embryos with the plastic pestle. Transfer the lysate to a kit provided column and follow the manufacturer's instructions to purify RNA. Store the purified RNA, at minus 20 to minus 80 degrees Celsius, until use or immediately synthesize cDNA from one microgram, total RNA using the cDNA synthesis kit, by following the manufacturer's instruction.Add one microgram of RNA into a thin wall 0.2 milliliter, PCR tube, containing four microliters, of five times cDNA reaction master mix, followed by the addition of one microliter, of 20 times reverse transcriptase enzyme solution. Next make up a final volume of 20 microliters, with nuclease-free water. Place the tube in a programmed Thermo cycler for cDNA synthesis.Store the synthesized cDNA at minus 20 degrees celsius until use or immediately use for quantitative assessment. Set up the quantitative PCR reactions containing cyber green enzyme master mix. Five fold diluted cDNA and GFP primers at 325 nanomolar concentration in triplicates and conduct the reaction in a programmed real-time PCR machine.Upon completion of the reaction, analyze the melt curve to determine the reaction specificity. Calculate light-activated induction as fold change relative to the control embryos using the double Delta CT method. And use statistical software to determine the significance.After removing the embryos from illumination, immobilize embryos for imaging in 3%methylcellulose containing 0.01%Tricaine in glass depression slides. Acquire fluorescence and brightfield images on a fluorescent stereo microscope connected to a digital camera using standard GFP filter settings. Merge brightfield and fluorescence images after the acquisition with image processing software.Quantification of GFP expression indicated induction of expression as soon as 30 minutes after the blue light exposure. The levels of GFP expression then continued to increase up to six hours, post-activation steadily. GFP induction was also qualitatively assessed by examining the fluorescence intensity at the same time points post-activation.GFP fluorescence above background levels was first observed at three hours post-activation and became noticeably brighter at six hours Post-activation. In contrast control embryos for all time points, kept in the dark, did not exhibit, any appreciable GFP fluorescence. The activating light must be in the blue range of the visible light spectrum.Both room light and sunlight contains some blue light. So take care to protect your negative controls from any ambient light sources. The TAEL C120 system, is genetically encoded and modular and can drive expression of any gene of interest.So, combined with other techniques, it can be used to answer a range of biological questions.

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