The research work reported in this publication was supported by the National Institute of General Medical Sciences of the National Institutes of Health under the Award Number R35GM124913, Purdue University PI4D incentive program, and PVM Internal Competitive Basic Research Funds Program. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agents. We thank Koichi Kawakami for the Tol2 construct, Michael Lin for the ASAP1 construct, and Leonard Zon for the ubi promoter construct through Addgene.

The zebrafish are housed in an AAALAC-approved animal facility, and all experiments were carried out according to the protocols approved by the Purdue Animal Care and Use Committee (PACUC).
1. Tol2 Transposon Plasmid Construct Preparation
NOTE: Tol2, a transposon that was discovered in medaka fish, has widely been used in the zebrafish research community8,9. It has been successfully adopted to the Gateway site...

<ol>
	<li>Levin, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+bioelectricity:+how+endogenous+voltage+potentials+control+cell+behavior+and+instruct+pattern+regulation+in+vivo.">Molecular bioelectricity: how endogenous voltage potentials control cell behavior and instruct pattern regulation in vivo.</a> Molecular Biology of the Cell. 25 (24), 3835-3850 (2014).</li><li>Storace, D., 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=Toward+Better+Genetically+Encoded+Sensors+of+Membrane+Potential.">Toward Better Genetically Encoded Sensors of Membrane Potential.</a> Trends in Neuroscience. 39 (5), 277-289 (2016).</li><li>Inagaki, S., Nagai, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current+progress+in+genetically+encoded+voltage+indicators+for+neural+activity+recording.">Current progress in genetically encoded voltage indicators for neural activity recording.</a> Current Opinion in Chemical Biology. 33, 95-100 (2016).</li><li>St-Pierre, 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=High-fidelity+optical+reporting+of+neuronal+electrical+activity+with+an+ultrafast+fluorescent+voltage+sensor.">High-fidelity optical reporting of neuronal electrical activity with an ultrafast fluorescent voltage sensor.</a> Nature Neuroscience. 17 (6), 884-889 (2014).</li><li>Lieschke, G. 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M., 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=The+Tol2kit:+a+multisite+gateway-based+construction+kit+for+Tol2+transposon+transgenesis+constructs.">The Tol2kit: a multisite gateway-based construction kit for Tol2 transposon transgenesis constructs.</a> Developmental Dynamics. 236 (11), 3088-3099 (2007).</li><li>Mosimann, 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=Ubiquitous+transgene+expression+and+Cre-based+recombination+driven+by+the+ubiquitin+promoter+in+zebrafish.">Ubiquitous transgene expression and Cre-based recombination driven by the ubiquitin promoter in zebrafish.</a> Development. 138 (1), 169-177 (2011).</li><li>Lorenz, T. 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Although the cellular and tissue level electrical activities during embryonic development and human disease were discovered a long time ago, the in vivo dynamic electrical changes and their biological roles still remain largely unknown. One of the major challenges is to visualize and quantify the electrical changes. Patch clamp technology is a break-through for tracking single cells, but its application to vertebrate embryos is limited because they are composed of many cells. The current chemical voltage dyes ar...

Bioelectricity refers to endogenous electrical signaling mediated by ion channels and pumps located on the cell membrane1. Ionic exchanges across the cellular membrane, and the coupled electrical potential and current changes, are essential for signaling processes of excitable neuronal and muscular cells. In addition, bioelectricity and ion gradients have a variety of other important biological functions including energy storage, biosynthesis, and metabolite transportation. Bioelectrical signaling was also discovered as a regulator of embryonic pattern formation, such as body axes, the cell cycle, and cell differentiation1. Thus, it is critical for understanding many human congenital diseases that result from the mis-regulation of this type of signaling. Although patch clamp has been&#160;widely used for recording single cells, it is still far from ideal for the simultaneous monitoring of multiple cells during embryonic development in vivo. Furthermore, voltage sensitive small molecules are also not ideal for in vivo applications due to their specificities, sensitivities, and toxicities.
The creation of a variety of genetically encoded fluorescent voltage indicators (GEVIs) offers a new mechanism to overcome this issue, and allows for easy application to study embryonic development, even though they were originally intended for monitoring neural cells2,3. One of the currently available GEVIs is the Accelerated Sensor of Action Potentials 1 (ASAP1)4. It is composed of an extracellular loop of a voltage-sensing domain of voltage sensitive phosphatase and a circularly permuted green fluorescent protein. Therefore, ASAP1 allows visualization of cellular electric potential changes (polarization: bright green; depolarization: dark green). ASAP1 has 2 ms on-and-off kinetics, and can track subthreshold potential change4. Thus, this genetic tool allows for a new level of efficacy in real-time bioelectric monitoring in live cells. Further understanding of the roles of bioelectricity in embryonic development and many human diseases, such as cancer, will shed new light on the underlying mechanisms, which is critical for disease treatment and prevention.
Zebrafish have been proven a powerful animal model to study developmental biology and human diseases including cancer5,6. They share 70% orthologous genes with humans, and they have similar vertebrate biology7. Zebrafish provide relatively easy care, a large clutch size of eggs, tractable genetics, easy transgenesis, and transparent external embryonic development, which make them a superior system for in vivo imaging5,6. With a large source of mutant fish lines already present and a fully sequenced genome, zebrafish will provide a relatively&#160;unlimited range of scientific discovery.
To investigate the in vivo real-time electrical activity of cells, we take advantage of the zebrafish model system and ASAP1. In this paper, we describe how to incorporate the fluorescent voltage biosensor ASAP1 into the zebrafish genome using Tol2 transposon transgenesis, and visualize cellular electrical activity during embryonic development, fish larval movement, and in live tumor.

<table>
	<tbody>
		<tr>
			<td>14mL cell culture tubes</td>
			<td>VWR</td>
			<td>60818-725</td>
			<td>E.Coli culture</td>
		</tr>
		<tr>
			<td>Agarose electrophoresis tank</td>
			<td>Thermo Scientific</td>
			<td>Owl B2</td>
			<td>DNA eletrophoresis</td>
		</tr>
		<tr>
			<td>Agarose RA</td>
			<td>Amresco</td>
			<td>N605-500G</td>
			<td>For making the injection gels</td>
		</tr>
		<tr>
			<td>Attb1-ASAP1-F primer</td>
			<td>IDT DNA</td>
			<td>GGGGACAAGTTTGTACAAAAAAGCAGGCTTCACCATGGAGACGACTGTGAGGTATGAACA</td>
			<td>ASAP1 coding region amplification for subcloning</td>
		</tr>
		<tr>
			<td>Attb2-ASAP1-R primer</td>
			<td>IDT DNA</td>
			<td>GGGGACCACTTTGTACAAGAAAGCTGGGTCTTAGGTTACCACTTCAAGTTGTTTCTTCTGTGAAGCCA</td>
			<td>ASAP1 coding region amplification for subcloning</td>
		</tr>
		<tr>
			<td>Bright field dissection scope</td>
			<td>Nikon</td>
			<td>SMZ 745</td>
			<td>Dechorionation, microinjection, mounting</td>
		</tr>
		<tr>
			<td>Color camera</td>
			<td>Zeiss</td>
			<td>AxioCam MRc</td>
			<td>Fish embryo image recording</td>
		</tr>
		<tr>
			<td>Concave slide</td>
			<td>VWR</td>
			<td>48336-001</td>
			<td>For holding fish embryos during imaging process</td>
		</tr>
		<tr>
			<td>Disposable transfer pipette 3.4 ml</td>
			<td>Thermo Scientific</td>
			<td>13-711-9AM</td>
			<td>Fish embryos and water transfer</td>
		</tr>
		<tr>
			<td>Endonuclease enzyme, Not I</td>
			<td>NEB</td>
			<td>R0189L</td>
			<td>For linearizing plasmid DNA</td>
		</tr>
		<tr>
			<td>Epifuorescent compound scope</td>
			<td>Zeiss</td>
			<td>Axio Imager.A2</td>
			<td>Fish embryo imaging</td>
		</tr>
		<tr>
			<td>Epifuorescent stereo dissection scope</td>
			<td>Zeiss</td>
			<td>Stereo Discovery.V12</td>
			<td>Fish embryo imaging</td>
		</tr>
		<tr>
			<td>Fluorescent light source</td>
			<td>Lumen dynamics</td>
			<td>X-cite seris 120</td>
			<td>Light source for fluorescence microscopes</td>
		</tr>
		<tr>
			<td>Forceps #5</td>
			<td>WPI</td>
			<td>500342</td>
			<td>Dechorionation and needle breaking</td>
		</tr>
		<tr>
			<td>Gateway BP Clonase II Enzyme mix</td>
			<td>Thermo Scientific</td>
			<td>11789020</td>
			<td>Gateway BP recombination cloning</td>
		</tr>
		<tr>
			<td>Gateway LR Clonase II Plus enzyme</td>
			<td>Thermo Scientific</td>
			<td>12538120</td>
			<td>Gateway LR recombination cloning</td>
		</tr>
		<tr>
			<td>Gel DNA Recovery Kit</td>
			<td>Zymo Research</td>
			<td>D4002</td>
			<td>DNA gel purification</td>
		</tr>
		<tr>
			<td>Loading tip</td>
			<td>Eppendorf</td>
			<td>930001007</td>
			<td>For loading injection solution into capilary needles</td>
		</tr>
		<tr>
			<td>Methylcellulose (1600cPs)</td>
			<td>Alfa Aesar</td>
			<td>43146</td>
			<td>Fish embryo mounting</td>
		</tr>
		<tr>
			<td>Methylene blue</td>
			<td>Sigma-Aldrich</td>
			<td>M9140</td>
			<td>Suppresses fungal outbreaks in Petri dishes</td>
		</tr>
		<tr>
			<td>Microinjection mold</td>
			<td>Adaptive Science Tools</td>
			<td>TU-1</td>
			<td>To prepare agaorse mold tray for holding fish embryos during injection</td>
		</tr>
		<tr>
			<td>Microinjector</td>
			<td>WPI</td>
			<td>Pneumatic Picopump PV820</td>
			<td>Microinjection injector</td>
		</tr>
		<tr>
			<td>Micro-manipulator</td>
			<td>WPI</td>
			<td>Microinjector mm33 rechts</td>
			<td>Microinjection operation</td>
		</tr>
		<tr>
			<td>Micropipette puller</td>
			<td>Sutter instrument</td>
			<td>P-1000</td>
			<td>For preparing capillary needle</td>
		</tr>
		<tr>
			<td>Mineral oil</td>
			<td>Amresco</td>
			<td>J217-500ml</td>
			<td>For calibrating injection volume</td>
		</tr>
		<tr>
			<td>mMESSAGE mMACHINE SP6 Transcription Kit</td>
			<td>Thermo Scientific</td>
			<td>AM1340</td>
			<td>mRNA in vitro transcription</td>
		</tr>
		<tr>
			<td>Monocolor camera</td>
			<td>Zeiss</td>
			<td>AxioCam MRm</td>
			<td>Fish embryo image recording</td>
		</tr>
		<tr>
			<td>Plasmid Miniprep Kit</td>
			<td>Zymo Research</td>
			<td>D4020</td>
			<td>Prepare small amount of plasmid DNA</td>
		</tr>
		<tr>
			<td>Plastic Petri dishes</td>
			<td>VWR</td>
			<td>25384-088</td>
			<td>For holding fish or fish embryos during imaging process</td>
		</tr>
		<tr>
			<td>RNA Clean &#38; Concentrator-5</td>
			<td>Zymo Research</td>
			<td>R1015</td>
			<td>mRNA cleaning after in vitro transcription</td>
		</tr>
		<tr>
			<td>Spectrophotometer</td>
			<td>Thermo Scientific</td>
			<td>NanoDrop 2000</td>
			<td>For measuring DNA and RNA concentrations</td>
		</tr>
		<tr>
			<td>Stage Micrometer</td>
			<td>Am Scope</td>
			<td>MR100</td>
			<td>Microinjection volume calibration</td>
		</tr>
		<tr>
			<td>Thermocycler</td>
			<td>Bio-Rad</td>
			<td>T100</td>
			<td>DNA amplification for gene cloning</td>
		</tr>
		<tr>
			<td>Thin wall glass capillaries</td>
			<td>WPI</td>
			<td>TW100F-4</td>
			<td>Raw glass for making cappilary needle</td>
		</tr>
		<tr>
			<td>Tol2-exL1 primer</td>
			<td>IDT DNA</td>
			<td>GCACAACACCAGAAATGCCCTC</td>
			<td>Tol2 excise assay</td>
		</tr>
		<tr>
			<td>Tol2-exR primer</td>
			<td>IDT DNA</td>
			<td>ACCCTCACTAAAGGGAACAAAAG</td>
			<td>Tol2 excise assay</td>
		</tr>
		<tr>
			<td>TOP10 Chemically Competent E. coli</td>
			<td>Thermo Scientific</td>
			<td>C404006</td>
			<td>Used for transformation during gene cloning</td>
		</tr>
		<tr>
			<td>Tricaine mesylate</td>
			<td>Sigma-Aldrich</td>
			<td>A5040</td>
			<td>For anesthetizing fish or fish embryos</td>
		</tr>
		<tr>
			<td>UV trans-illuminator 302nm</td>
			<td>UVP</td>
			<td>M-20V</td>
			<td>DNA visualization</td>
		</tr>
		<tr>
			<td>Water bath</td>
			<td>Thermo Scientific</td>
			<td>2853</td>
			<td>For transformation process of gene cloning</td>
		</tr>
	</tbody>
</table>

visualization of cellular electrical activity in zebrafish early embryos and tumors

Bioelectricity, endogenous electrical signaling mediated by ion channels and pumps located on the cell membrane, plays important roles in signaling processes of excitable neuronal and muscular cells and many other biological processes, such as embryonic developmental patterning. However, there is a need for in vivo electrical activity monitoring in vertebrate embryogenesis. The advances of genetically encoded fluorescent voltage indicators (GEVIs) have made it possible to provide a solution for this challenge. Here, we describe how to create a transgenic voltage indicator zebrafish using the established voltage indicator, ASAP1 (Accelerated Sensor of Action Potentials 1), as an example. The Tol2 kit and a ubiquitous zebrafish promoter, ubi, were chosen in this study. We also explain&#160;the processes of Gateway site-specific cloning, Tol2 transposon-based zebrafish transgenesis, and the imaging process for early-stage fish embryos and fish tumors using regular epifluorescent microscopes. Using this fish line, we found that there are cellular electric voltage changes during zebrafish embryogenesis, and fish larval movement. Furthermore, it was observed that in a few zebrafish malignant peripheral nerve sheath tumors, the tumor cells were generally polarized compared to the surrounding normal tissues.

In a successful injection, more than 50% injected fish embryos will display some degree of green fluorescence&#160;in the somatic cells, and most of them will be positive by Tol2 transposon excise assay (Figure 2). After 2-4 generations of out-cross with wildtype fish (until the fluorescent fish reach 50%, the expected Mendelian ratio), the transgenic fish were used for the imaging experiment to track cell membrane potentials during embryonic development. Fir...

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Visualization of Cellular Electrical Activity in Zebrafish Early Embryos and Tumors

Here, we show the process of creating a cellular electric voltage reporter zebrafish line to visualize embryonic development, movement, and fish tumor cells in vivo.

Bioelectricity, endogenous electrical signaling mediated by ion channels and pumps located on the cell membrane, plays important roles in signaling ...

The overall goal of this procedure is to create a transgenic zebrafish line that allows the observance of cellular electrical changes during embryogenesis, larval movement, and in tumor genesis. This method could help answer key questions in the developmental biology, physiology, and cancer cell biology fields such as what are the fundamental roles of cellular electrical signaling during embryogenesis and in tumor cells? The main advantage of this technique is that it allows us to track cellular electrical signaling in vivo and in real time.After preparing Tol2 transposase mRNA and injection solution according to the text protocol, the afternoon before injection, set up four to six breeding tanks with at least two males and two females. To reduce the amount of fish waste and to induce a breeding response, avoid feeding the fish in the afternoon. The following morning, remove the prepared injection solution from the negative 80 degree Celsius freezer and place it on ice.Pull the dividers in the fish breeding tanks and allow the fish to mate. In general, fish lay eggs within 20 to 30 minutes. While waiting, using a micropipette puller with the following parameters, pull needles from capillary glass.Use a laboratory wipe to break the end of the needle and create a beveled edge. A smaller diameter is preferred for decreasing embryo mortality. Once the fish have laid eggs, collect them in a 10 centimeter diameter Petri dish.Immediately under the dissection microscope, remove all abnormal embryos and fish waste. Then pipette the fertilized embryos into a prepared 3%agarose injection mold. Remove excess water to help keep the embryos in place.Once all of the rows are filled with viable embryos, arrange them so that the single cells are all oriented at a 45 degree angle horizontally with respect to the needle. This will make injection much easier later. Next, while wearing gloves, use a 20 microliter loading pipette tip to remove five microliters of the prepared construct from the tube on ice.Carefully insert the pipette tip into the back end of the broken capillary tube to where it begins to taper to get the reagent as close to the tip as possible and expel it into the capillary. If there are still air bubbles, shake the needle making sure not to break the tip. Insert the needle straight into the micro injection needle holder and carefully tighten until the needle stays in place.Then adjust the angle to about 45 degrees. Once the needle is prepared and attached, turn on the microscope and gas pressure tank. Adjust the injection volume by using approximately 0.5 PSI for holding and 30 PSI for ejection.Check that the solution ejects from the needle when pressing the pedal. Using a stage micrometer with a drop of mineral oil, adjust the volume and flow of the solution to approximately 10 micrometers in diameter. Ensure that the back pressure allows a small amount of solution to drip out of the needle.If there is not enough back pressure, capillary action will cause liquid to enter the needle and destroy the mRNA. Once the needle is calibrated, insert the needle into the single cell of the fertilized embryos by using the edge of the gel notch to provide a backing that keeps the embryo in place and allows the needle to apply pressure without moving the embryo. Once the tip of the needle is in the single cell, press the pedal to release the desired amount of solution.It is important to inject the solution into the cell, not the yolk, for generating transgenic zebrafish. Repeat this process for all of the embryos. When completed, use a disposal 3.4 milliliter transfer pipette and fish system water to transfer the injected embryos into a labeled dish by rinsing them out of the agarose notch.Store the embryos in a 28.5 degree Celsius incubator to let them develop. Check back throughout the day to remove dead fish embryos and replace the water with 0.1%methylene blue in fish water. Around six to eight hours after injection, use 10 injected fish embryos and the hotshot method to prepare genomic DNA.The following morning, use a dissection microscope with a fluorescent light source to sort out the embryos showing GFP in the non-yolk tissues. These embryos should contain the injected construct. Perform a Tol2 excise assay to check the transposon activity as described previously.If excised plasma can be detected, keep the injected fish embryos and raise them. Otherwise, repeat the Tol2 mRNA synthesis and micro injection process until achieving positive results from the Tol2 excise assay. To image zebrafish embryos, cross-multiple F2 generation founder fish with wild type fish in individual pairs.Collect fish embryos at different desired developmental stages according to the zebrafish staging guide. For the early stage fish embryos in fish system water, under a dissection scope, use a pair of forceps to carefully peel and remove the chorions. Use a disposal transfer pipette to transfer a few embryos into a concave glass slide with 3%methylcellulose.Then using a needle underneath a dissection scope, adjust the embryos to the desired positions to view the cellular GFP activity. For embryos at the less than 12 somite stage, use an epifluorescence compound microscope with a compatible camera and software for imaging. For embryos past the 12 somite stage, use a fluorescence dissection microscope.To image tumor cell voltage, first identify fish with Malignant Peripheral Nerve Sheath Tumors or MPNST. Place the fish into a 10 centimeter diameter Petri dish and carry out whole mount imaging. Finally following imaging, dissect out the tumor before viewing the tumor cell electrical activity.In a successful injection, more than 50%of injected embryos will display some degree of GFP in the somatic cells and most of them will show a positive result from the Tol2 transposon excise assay. Here membrane potential changes were examined throughout the cell cycle during zebrafish early embryonic development. As seen in this timelapse video, the cells hyperpolarized before formation of the cleavage furrow.Moreover, different tissues showed a variety of membrane potentials in one to three-day-old fish embryos. For example, the somite and notocord were generally hyperpolarized compared to the adjacent tissues and organs. In this two-day-old fish embryo, neuromuscular electrical activities are shown during movement with color density changes corresponding to the electric signaling transduction.Bioelectrical properties of cancer cells were examined in embryos from crosses of ASAP1 reporter fish with an RPL35 mutant which is prone to spontaneous MPNST. Compared to surrounding tissues, tumor cells were more polarized. Once mastered, this micro injection technique can be done in about three hours if performed properly.While attempting this procedure, it is important to remember that the zebrafish embryos should be in the one cell stage for ideal results. With its development, this technique paves the way for researchers in the field of developmental biology and cell biology to explore in vivo electrical signaling changes in zebrafish and other model organisms. After watching this video, you should have a good understanding of how to create a transgenic zebrafish line by micro injection.

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Developmental Biology

8.6K Views.  Purdue University. The overall goal of this procedure is to create a transgenic zebrafish line that allows the observance of cellular electrical changes during embryogenesis, larval movement, and in tumor genesis. This method could help answer key questions in the developmental biology, physiology, and cancer cell biology fields such as what are the fundamental roles of cellular electrical signaling during embryogenesis and in tumor cells? The main advantage of this technique is that it allows us to track cellu...