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 embryos that contain both the TAEL and C120 components (Figure 1B). 
	NOTE: Alternatively, both components can be expressed transiently through microinjection of mRNA or plasmid DNA using standard methods10.</li>
	<li>Collect embryos in Petri dishes containing egg water (60 &#181;g/mL Instant Ocean sea salt dissolved in distilled water), with approximately 30 embryos per condition to be tested.</li>
	<li>Place the dishes in a lightproof box or cover with aluminum foil to minimize unintended activation by ambient light (see Table 1).</li>
</ol>
2. Global light induction
<ol>
	<li>Use a blue-light (465 nm) LED panel to deliver activating blue light to several embryos at once.</li>
	<li>Position the LED panel relative to the Petri dishes containing embryos so that the actual power of light received by the embryos is approximately 1.5 mW/cm2 as measured by a light power and energy meter (Figure 2A).</li>
	<li>Pulse the light at intervals of 1 h on/1 h off using a timer relay if illuminating for more than 3 h to reduce the risk of photodamage to the TAEL transcriptional activator5,8. 
	NOTE: The exact duration of illumination may need to be optimized for specific applications. In this protocol, examples of illumination duration of 30 min, 1 h, 3 h, and 6 h are provided.</li>
	<li>Remove Petri dish lids to minimize light scattering from condensation.</li>
	<li>Place Petri dishes containing control embryos in a lightproof box or cover with aluminum foil for dark controls.</li>
</ol>
3. Quantitative assessment of induction by qRT-PCR
<ol>
	<li>Remove embryos from illumination after the desired activation duration.</li>
	<li>Extract total RNA from 30-50 light-activated and 30-50 dark embryos using an RNA isolation kit following the kit&#39;s instructions.
	<ol>
		<li>Transfer embryos to a 1.5 mL microcentrifuge tube and remove excess egg water. Add lysis buffer and homogenize the embryos with a plastic pestle.</li>
		<li>Transfer the lysate to a kit-provided column and continue with the kit&#39;s instructions. Immediately proceed to step 3.3 or store purified RNA at -20 &#176;C to -80 &#176;C.</li>
	</ol>
	</li>
	<li>Use 1 &#181;g total RNA for cDNA synthesis using a cDNA synthesis kit following the kit&#39;s instructions.
	<ol>
		<li>Take a thin-walled 0.2 mL PCR tube, add 1 &#181;g of RNA to 4 &#181;L of 5x cDNA reaction master mix (containing optimized buffer, oligo-dT and random primers, and dNTPs), 1 &#181;L of 20x reverse transcriptase solution, and nuclease-free water to a total volume of 20 &#181;L.</li>
		<li>Place the tube in a thermocycler programmed as follows: 22 &#176;C for 10 min, 42 &#176;C for 30 min, 85 &#176;C for 5 min, and then hold at 4 &#176;C. Immediately proceed to step 3.4 or store cDNA at -20 &#176;C.</li>
	</ol>
	</li>
	<li>Prepare qPCR reactions containing SYBR green enzyme master mix, 5-fold diluted cDNA (step 3.3), and 325 nM of each primer for GFP. 
	NOTE: Primers used in this protocol as follows. GFP forward: 5&#39;-ACGACGGCAACTACAAGCACC-3&#39;; GFP reverse: 5&#39;-GTCCTCCTTGAAGTCGATGC-3&#39;; ef1a forward 5&#39;-CACGGTGACAACATGCTGGAG-3&#39;; ef1a reverse: 5&#39;-CAAGAAGAGTAGTACCGCTAGCAT-3&#39;).</li>
	<li>Carry out qPCR reactions in a real-time PCR machine. 
	NOTE: PCR program: initial activation at 95 &#176;C for 10 min, followed by 40 cycles of 30 s at 95 &#176;C, 30 s at 60 &#176;C, and 1 min at 72 &#176;C.</li>
	<li>Perform a melt curve analysis once the PCR is completed to determine the reaction specificity. Perform three technical replicates for each sample.</li>
	<li>Calculate light-activated induction as fold change relative to embryos kept in the dark using the 2-&#916;&#916;Ct method11. Statistical significance can be determined with a statistics software package.</li>
</ol>
4. Qualitative assessment of induction by fluorescence microscopy
<ol>
	<li>Remove the embryos from illumination after the desired duration of activation.</li>
	<li>Immobilize embryos for imaging in 3% methylcellulose containing 0.01% tricaine in glass depression slides.</li>
	<li>Acquire fluorescence and brightfield images on a fluorescent stereomicroscope connected to a digital camera using standard GFP filter settings. Use identical image acquisition settings for all samples.</li>
	<li>Merge brightfield and fluorescence images after the acquisition with image processing software.</li>
</ol>

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	<li>Knopf, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dually+inducible+TetON+systems+for+tissue-specific+conditional+gene+expression+in+zebrafish.">Dually inducible TetON systems for tissue-specific conditional gene expression in zebrafish.</a> Proceedings of the National Academy of Sciences of the United States of America. 107 (46), 19933-19938 (2010).</li><li>Halloran, M. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser-induced+gene+expression+in+specific+cells+of+transgenic+zebrafish.">Laser-induced gene expression in specific cells of transgenic zebrafish.</a> Development. 127 (9), 1953-1960 (2000).</li><li>Hesselson, D., Anderson, R. M., Beinat, M., Stainier, D. Y. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Distinct+populations+of+quiescent+and+proliferative+pancreatic+beta-cells+identified+by+HOTcre+mediated+labeling.">Distinct populations of quiescent and proliferative pancreatic beta-cells identified by HOTcre mediated labeling.</a> Proceedings of the National Academy of Sciences of the United States of America. 106 (35), 14896-14901 (2009).</li><li>Tischer, D., Weiner, O. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Illuminating+cell+signalling+with+optogenetic+tools.">Illuminating cell signalling with optogenetic tools.</a> Nature Reviews. 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H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+natural+and+artificial+DNA+substrates+for+light-activated+LOV-HTH+transcription+factor+EL222.">Identification of natural and artificial DNA substrates for light-activated LOV-HTH transcription factor EL222.</a> Biochemistry. 51 (50), 10024-10034 (2012).</li><li>Motta-Mena, L. B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+optogenetic+gene+expression+system+with+rapid+activation+and+deactivation+kinetics.">An optogenetic gene expression system with rapid activation and deactivation kinetics.</a> Nature Chemical Biology. 10 (3), 196-202 (2014).</li><li>Avdesh, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Regular+care+and+maintenance+of+a+zebrafish+(Danio+rerio)+laboratory:+an+introduction.">Regular care and maintenance of a zebrafish (Danio rerio) laboratory: an introduction.</a> Journal of visualized experiments: JoVE. 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No conflicts of interest were declared.

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 ...

light-induced-gfp-expression-zebrafish-embryos-using-optogenetic

Research

JoVE Journal

Developmental Biology

2.8K Views.  University of California. 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.

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

Live Imaging of Innate Immune and Preneoplastic Cell Interactions Using an Inducible Gal4/UAS Expression System in Larval Zebrafish Skin

Here we describe a method to conditionally induce epithelial cell transformation by the use of the 4-Hydroxytamoxifen (4-OHT) inducible KalTA4-ERT2/UAS expression system1 in zebrafish larvae, and the subsequent live imaging of innate immune cell interaction with HRASG12V expressing skin cells. The KalTA4-ERT2/UAS system is both inducible and reversible which allows us to induce cell transformation with precise temporal/spatial resolution in vivo. This provides us with a unique opportunity to live image how individual preneoplastic cells interact with host tissues as soon as they emerge, then follow their progression as well as regression. Recent studies in zebrafish larvae have shown a trophic function of innate immunity in the earliest stages of tumorigenesis2,3. Our inducible system would allow us to live image the onset of cellular transformation and the subsequent host response, which may lead to important insights on the underlying mechanisms for the regulation of oncogenic trophic inflammatory responses. We also discuss how one might adapt our protocol to achieve temporal and spatial control of ectopic gene expression in any tissue of interest.

Studying the earliest events of preneoplastic cell progression and innate immune cell interaction is pivotal to understand and treat cancer. Here we describe a method to conditionally induce epithelial cell transformations and the subsequent live imaging of innate immune cell interaction with HRASG12V expressing skin cells in zebrafish larvae.

Here we describe a method to conditionally induce epithelial cell transformation by the use of the 4-Hydroxytamoxifen (4-OHT) inducible KalTA4-ERT2/UAS ...

Measuring Protein Stability in Living Zebrafish Embryos Using Fluorescence Decay After Photoconversion (FDAP)

Protein stability influences many aspects of biology, and measuring the clearance kinetics of proteins can provide important insights into biological systems. In FDAP experiments, the clearance of proteins within living organisms can be measured. A protein of interest is tagged with a photoconvertible fluorescent protein, expressed in vivo and photoconverted, and the decrease in the photoconverted signal over time is monitored. The data is then fitted with an appropriate clearance model to determine the protein half-life. Importantly, the clearance kinetics of protein populations in different compartments of the organism can be examined separately by applying compartmental masks. This approach has been used to determine the intra- and extracellular half-lives of secreted signaling proteins during zebrafish development. Here, we describe a protocol for FDAP experiments in zebrafish embryos. It should be possible to use FDAP to determine the clearance kinetics of any taggable protein in any optically accessible organism.

Measuring Protein Stability in Living Zebrafish Embryos Using Fluorescence Decay After Photoconversion FDAP

Protein levels in cells and tissues are often tightly regulated by the balance of protein production and clearance. Using Fluorescence Decay After Photoconversion (FDAP), the clearance kinetics of proteins can be experimentally measured in vivo.

Protein stability influences many aspects of biology, and measuring the clearance kinetics of proteins can provide important insights into biological ...

An Optogenetic Approach for Assessing Formation of Neuronal Connections in a Co-culture System

Here we describe a protocol to generate a co-culture consisting of 2 different neuronal populations. Induced pluripotent stem cells (iPSCs) are reprogrammed from human fibroblasts using episomal vectors. Colonies of iPSCs can be observed 30 days after initiation of fibroblast reprogramming. Pluripotent colonies are manually picked and grown in neural induction medium to permit differentiation into neural progenitor cells (NPCs). iPSCs rapidly convert into neuroepithelial cells within 1 week and retain the capability to self-renew when maintained at a high culture density. Primary mouse NPCs are differentiated into astrocytes by exposure to a serum-containing medium for 7 days and form a monolayer upon which embryonic day 18 (E18) rat cortical neurons (transfected with channelrhodopsin-2 (ChR2)) are added. Human NPCs tagged with the fluorescent protein, tandem dimer Tomato (tdTomato), are then seeded onto the astrocyte/cortical neuron culture the following day and allowed to differentiate for 28 to 35 days. We demonstrate that this system forms synaptic connections between iPSC-derived neurons and cortical neurons, evident from an increase in the frequency of synaptic currents upon photostimulation of the cortical neurons. This co-culture system provides a novel platform for evaluating the ability of iPSC-derived neurons to create synaptic connections with other neuronal populations.

A protocol to generate a co-culture system consisting of neurons derived from induced pluripotent stem cells (iPSCs), primary cortical neurons and astrocytes is described. This co-culture system allows detection of the formation of synaptic contacts and circuits between new, iPSC-derived neurons and pre-existing cortical neurons expressing channelrhodopsin-2.

Here we describe a protocol to generate a co-culture consisting of 2 different neuronal populations. Induced pluripotent stem cells (iPSCs) are ...

Mechanical Vessel Injury in Zebrafish Embryos

Zebrafish (Danio rerio) embryos have proven to be a powerful model for studying a variety of developmental and disease processes. External development and optical transparency make these embryos especially amenable to microscopy, and numerous transgenic lines that label specific cell types with fluorescent proteins are available, making the zebrafish embryo an ideal system for visualizing the interaction of vascular, hematopoietic, and other cell types during injury and repair in vivo. Forward and reverse genetics in zebrafish are well developed, and pharmacological manipulation is possible. We describe a mechanical vascular injury model using micromanipulation techniques that exploits several of these features to study responses to vascular injury including hemostasis and blood vessel repair. Using a combination of video and timelapse microscopy, we demonstrate that this method of vascular injury results in measurable and reproducible responses during hemostasis and wound repair. This method provides a system for studying vascular injury and repair in detail in a whole animal model.

This article describes a method for creating a mechanical vessel injury in zebrafish embryos. This injury model provides a platform for studying hemostasis, injury-related inflammation, and wound healing in an organism ideally suited for real-time microscopy.

Zebrafish (Danio rerio) embryos have proven to be a powerful model for studying a variety of developmental and disease processes.  External development ...

Functional Cloning Using a Xenopus Oocyte Expression System

Identification of genes responsible for embryonic induction poses a number of challenges; to name a few, secreted molecules of interest may be low in abundance, may not be secreted but tethered to the signaling cell(s), or may require the presence of binding partners or upstream regulatory molecules. Thus in a search for gene products capable of eliciting an early lens-inductive response in competent ectoderm, we utilized an expression cloning system that would allow identification of paracrine or juxtacrine factors as well as transcriptional or other regulatory proteins. Pools of mRNA were injected into Xenopus oocytes, and responding tissue placed directly on the oocytes and co-cultured. Following functional cloning of ldb1 from a neural plate stage cDNA library based on its ability to elicit the expression of the early lens placode marker foxe3 in lens-competent animal cap ectoderm, we characterized the mRNA expression pattern, and assayed developmental progression following overexpression or knockdown of ldb1. This system is suitable in a very wide variety of contexts where identification of an inducer or its upstream regulatory molecules is sought using a functional response in competent tissue.

Functional Cloning Using a Xenopus Oocyte Expression System

We describe a Xenopus oocyte and animal cap system for the expression cloning of genes capable of inducing a response in competent ectoderm, and discuss techniques for the subsequent analysis of such genes. This system is useful in the functional identification of a wide range of gene products.

Identification of genes responsible for embryonic induction poses a number of challenges; to name a few, secreted molecules of interest may be low in ...

Tracking Cells in GFP-transgenic Zebrafish Using the Photoconvertible PSmOrange System

The rapid development of transparent zebrafish embryos (Danio rerio) in combination with fluorescent labelings of cells and tissues allows visualizing developmental processes as they happen in the living animal. Cells of interest can be labeled by using a tissue specific promoter to drive the expression of a fluorescent protein (FP) for the generation of transgenic lines. Using fluorescent photoconvertible proteins for this purpose additionally allows to precisely follow defined structures within the expression domain. Illuminating the protein in the region of interest, changes its emission spectrum and highlights a particular cell or cell cluster leaving other transgenic cells in their original color. A major limitation is the lack of known promoters for a large number of tissues in the zebrafish. Conversely, gene- and enhancer trap screens have generated enormous transgenic resources discretely labeling literally all embryonic structures mostly with GFP or to a lesser extend red or yellow FPs. An approach to follow defined structures in such transgenic backgrounds would be to additionally introduce a ubiquitous photoconvertible protein, which could be converted in the cell(s) of interest. However, the photoconvertible proteins available involve a green and/or less frequently a red emission state1 and can therefore often not be used to track cells in the FP-background of existing transgenic lines. To circumvent this problem, we have established the PSmOrange system for the zebrafish2,3. Simple microinjection of synthetic mRNA encoding a nuclear form of this protein labels all cell nuclei with orange/red fluorescence. Upon targeted photoconversion of the protein, it switches its emission spectrum to far red. The quantum efficiency and stability of the protein makes PSmOrange a superb cell-tracking tool for zebrafish and possibly other teleost species.

We established the photoconvertible PSmOrange system as a powerful, straight-forward and cost inexpensive tool for in vivo cell tracking in GFP transgenic backgrounds. This protocol describes its application in the zebrafish model system.

The rapid development of transparent zebrafish embryos (Danio rerio) in combination with fluorescent labelings of cells and tissues allows visualizing ...

Multi-target Chromogenic Whole-mount In Situ Hybridization for Comparing Gene Expression Domains in Drosophila Embryos

To analyze gene regulatory networks active during embryonic development and organogenesis it is essential to precisely define how the different genes are expressed in spatial relation to each other in situ. Multi-target chromogenic whole-mount in situ hybridization (MC-WISH) greatly facilitates the instant comparison of gene expression patterns, as it allows distinctive visualization of different mRNA species in contrasting colors in the same sample specimen. This provides the possibility to relate gene expression domains topographically to each other with high accuracy and to define unique and overlapping expression sites. In the presented protocol, we describe a MC-WISH procedure for comparing mRNA expression patterns of different genes in Drosophila embryos. Up to three RNA probes, each specific for another gene and labeled by a different hapten, are simultaneously hybridized to the embryo samples and subsequently detected by alkaline phosphatase-based colorimetric immunohistochemistry. The described procedure is detailed here for Drosophila, but works equally well with zebrafish embryos.

Multi-target Chromogenic Whole-mount In Situ Hybridization for Comparing Gene Expression Domains in Drosophila Embryos

We describe a multi-target chromogenic whole-mount in situ hybridization (MC-WISH) procedure in intact Drosophila embryos allowing simultaneous and specific detection of three different mRNA distribution patterns by contrasting color precipitates.

To analyze gene regulatory networks active during embryonic development and organogenesis it is essential to precisely define how the different genes are ...

Biosensing Motor Neuron Membrane Potential in Live Zebrafish Embryos

The protocols described here are designed to allow researchers to study cell communication without altering the integrity of the environment in which the cells are located. Specifically, they have been developed to analyze the electrical activity of excitable cells, such as spinal neurons. In such a scenario, it is crucial to preserve the integrity of the spinal cell, but it is also important to preserve the anatomy and physiological shape of the systems involved. Indeed, the comprehension of the manner in which the nervous system-and other complex systems-works must be based on a systemic approach. For this reason, the live zebrafish embryo was chosen as a model system, and the spinal neuron membrane voltage changes were evaluated without interfering with the physiological conditions of the embryos. 
Here, an approach combining the employment of zebrafish embryos with a FRET-based biosensor is described. Zebrafish embryos are characterized by a very simplified nervous system and are particularly suited for imaging applications thanks to their transparency, allowing for the employment of fluorescence-based voltage indicators at the plasma membrane during zebrafish development. The synergy between these two components makes it possible to analyze the electrical activity of the cells in intact living organisms, without perturbing the physiological state. Finally, this non-invasive approach can co-exist with other analyses (e.g., spontaneous movement recordings, as shown here).

Protocols described here allow for the study of the electrical properties of excitable cells in the most non-invasive physiological conditions by employing zebrafish embryos in an in vivo system together with a fluorescence resonance energy transfer (FRET)-based genetically encoded voltage indicator (GEVI) selectively expressed in the cell type of interest.

The protocols described here are designed to allow researchers to study cell communication without altering the integrity of the environment in which the ...

In Vivo Imaging of Transgenic Gene Expression in Individual Retinal Progenitors in Chimeric Zebrafish Embryos to Study Cell Nonautonomous Influences

The genetic and technical strengths have made the zebrafish vertebrate a key model organism in which the consequences of gene manipulations can be traced in vivo throughout the rapid developmental period. Multiple processes can be studied including cell proliferation, gene expression, cell migration and morphogenesis. Importantly, the generation of chimeras through transplantations can be easily performed, allowing mosaic labeling and tracking of individual cells under the influence of the host environment. For example, by combining functional gene manipulations of the host embryo (e.g., through morpholino microinjection) and live imaging, the effects of extrinsic, cell nonautonomous signals (provided by the genetically modified environment) on individual transplanted donor cells can be assessed. Here we demonstrate how this approach is used to compare the onset of fluorescent transgene expression as a proxy for the timing of cell fate determination in different genetic host environments.
In this article, we provide the protocol for microinjecting zebrafish embryos to mark donor cells and to cause gene knockdown in host embryos, a description of the transplantation technique used to generate chimeric embryos, and the protocol for preparing and running in vivo time-lapse confocal imaging of multiple embryos. In particular, performing multiposition imaging is crucial when comparing timing of events such as the onset of gene expression. This requires data collection from multiple control and experimental embryos processed simultaneously. Such an approach can easily be extended for studies of extrinsic influences in any organ or tissue of choice accessible to live imaging, provided that transplantations can be targeted easily according to established embryonic fate maps.

In Vivo Imaging of Transgenic Gene Expression in Individual Retinal Progenitors in Chimeric Zebrafish Embryos to Study Cell Nonautonomous Influences

Live tracking of individual WT retinal progenitors in distinct genetic backgrounds allows for the assessment of the contribution of cell non-autonomous signaling during neurogenesis. Here, a combination of gene knockdown, chimera generation via embryo transplantation and in vivo time-lapse confocal imaging was utilized for this purpose.

The genetic and technical strengths have made the zebrafish vertebrate a key model organism in which the consequences of gene manipulations can be traced ...

Fast and Efficient Expression of Multiple Proteins in Avian Embryos Using mRNA Electroporation

We report that mRNA electroporation permits fluorescent proteins to label cells in living quail embryos more quickly and broadly than DNA electroporation. The high transfection efficiency permits at least 4 distinct mRNAs to be co-transfected with ~87% efficiency. Most of the electroporated mRNAs are degraded during the first 2 h post-electroporation, permitting time-sensitive experiments to be carried out in the developing embryo. Finally, we describe how to dynamically image live embryos electroporated with mRNAs that encode various subcellular targeted fluorescent proteins.

We report messenger RNA (mRNA) electroporation as a method that permits fast and efficient expression of multiple proteins in the quail embryo model system. This method can be used to fluorescently label cells and record their in vivo movements by time-lapse microscopy shortly after electroporation.

We report that mRNA electroporation permits fluorescent proteins to label cells in living quail embryos more quickly and broadly than DNA electroporation. ...