Reverse Genetic Morpholino Approach Using Cardiac Ventricular Injection to Transfect Multiple Difficult-to-target Tissues in the Zebrafish Larva

Summary

An adaptable reverse genetic method for zebrafish to assess gene function during later stages of development and physiological homeostasis such as tissue regeneration using intraventricular injections of gene-specific morpholinos.

Abstract

The zebrafish is an important model to understand the cell and molecular biology of organ and appendage regeneration. However, molecular strategies to employ reverse genetics have not yet been adequately developed to assess gene function in regeneration or tissue homeostasis during larval stages after zebrafish embryogenesis, and several tissues within the zebrafish larva are difficult to target. Intraventricular injections of gene-specific morpholinos offer an alternative method for the current inability to genomically target zebrafish genes in a temporally controlled manner at these stages. This method allows for complete dispersion and subsequent incorporation of the morpholino into various tissues throughout the body, including structures that were formerly impossible to reach such as those in the larval caudal fin, a structure often used to noninvasively research tissue regeneration. Several genes activated during larval finfold regeneration are also present in regenerating adult vertebrate tissues, so the larva is a useful model to understand regeneration in adults. This morpholino dispersion method allows for the quick and easy identification of genes required for the regeneration of larval tissues as well as other physiological phenomena regulating tissue homeostasis after embryogenesis. Therefore, this delivery method provides a currently needed strategy for temporal control to the evaluation of gene function after embryogenesis. 

Introduction

Regeneration of organs and appendages is fundamentally important for survival and fitness; however, several vertebrates including man have limited regenerative abilities. While several animal models exist that have extensive regenerative capacity, reverse genetic techniques to assess gene function during organ and appendage regeneration remain very limited or non-existent. Therefore, new approaches are required to dissect the molecular biology of regeneration in these model organisms.

The zebrafish is a well-established model for understanding the cell and molecular biology of organ and appendage regeneration¹, not only because of its significant ability to regenerate multiple organs, tissues and appendages, but also because several transgenic fish lines exist to track cells and to overexpress gene constructs^2,3. However, gene inhibition in larval zebrafish is mainly limited to the overexpression of dominant-negative constructs, which are not available to all genes of interest or whose transgene product can acquire gain-of-function effects that do not reflect the endogenous activity of the gene. Thus, an alternative method to specifically remove gene expression by knockout or knockdown is needed to overcome these issues.

Gene-specific targeting using TALENS exists as a reverse genetic means to knockout gene function; however, this knockout strategy is very frequently limited to functional assessments during early embryogenesis, because initial requirements of the gene prevent further progression of embryonic development. Thus, studying later phenomena such as regeneration or organ homeostasis after development using TALENS is precluded^4,5. Therefore, an alternate gene removal strategy is needed that targets gene function after early development to assess gene requirements in fully formed organs and structures.

Morpholino injection has been shown to be effective in targeting genes in a few adult organs and the adult regenerating fin^6-8, but these methods require electroporation and many internal organs are difficult to electroporate either due to their location or due to their sensitivity to electrical disruption. Furthermore, some tissues in the larva are difficult to inject directly, because direct injection may disrupt their structural integrity or because their size is limiting. The caudal fin of the larva is one such structure, because direct injection into the finfold is not possible. Thus, an alternative to electroporation and direct injection was needed to target genes in tissues that are either too small to inject or can't be electroporated.

In order to target and inhibit the function of specific genes during the regeneration of the larval caudal fin, we have modified existing morpholino technologies allowing the assessment of gene function during caudal fin regeneration in late-staged larva. This method employs intraventricular delivery⁹ of fluorescein-tagged morpholinos together with Endo-Porter transfection reagent¹⁰. Once in the ventricle, the morpholino-Endo-Porter mixture quickly spreads throughout the larva via the vasculature and enters tissues that have been previously impossible to target. This injection method may be modified to target genes in specific tissues and quite possibly can be applied in other animal models that currently lack reverse genetic methods to inhibit gene function. Thus, it offers a quick and easy method with the potential for broad range use to immediately study gene function during general organ homeostasis and regeneration at larval stages.

Protocol

1. Preparation of the Glass Needles (Figure 1)

1. Use glass capillaries with a 0.75 mm diameter to prepare injection needles (Figure 1A).
2. Place the glass capillary into a needle puller and pull the needle with the following parameter: heating cycle value: 463; pulling cycle value: 230; velocity: 150 msec; time: 150 msec (Figure 1B).
3. Break the pulled glass capillary with watchmaker tweezers to produce a 20 µm diameter needle under a stereoscope with a micrometer eyepiece.
4. Use a lathe with a wetted rubber spinning wheel to sharpen the needle and produce a 20 µm bevel (Figures 1C and 1D). Note: Sharp, beveled needles improve the ease of insertion into the ventricle and thus minimize the damage to the tissue and allow the perforated muscle to reseal after removal of the needle (Figures 1E-G).

2. Preparation of the Morpholino Solution

1. Prepare the morpholino stock by dissolving the lyophilized morpholino in 1x Phosphate-Buffered Saline (PBS) to a final concentration of 7.5 mM. [See manufacturer's instructions for details (Materials table)].
2. Prepare the morpholino injection solution by mixing 2.5 µl morpholino stock solution (7.5 mM) with 2.8 µl Endo-Porter stock solution (1 mM) (See Materials table) for a final concentration of 3.5 mM morpholino and 0.5 mM Endo-Porter.

3. Preparation of the Morpholino Injection (Figure 2)

1. Load the beveled glass needle with 5 µl of this solution using a micropipette with a 10 µl microloader pipette tip.
2. Insert the glass needle into the needle holder of the micromanipulator connected to the pneumatic pico pump (Figures 2A-C).
3. Place the needle holder next to the microscope so that the needle only needs to be moved in one planar direction to insert it into the cardiac ventricle of the larva (Figure 2A).
4. Adjust the angle for the injections at around 45°.
5. Set the microinjector values as follows: hold pressure: 20 pounds per square inch (psi); ejection pressure: 15 psi; 100 msec range of gating, period value of 1.9 (corresponds to 10.9 msec).
6. Melt agarose in 1x PBS using a standard microwave to produce 20 ml of a 1.5% (w/v) gel. Pour melted gel into a 10 cm Petri dish. Place a grooved injection form into the warm agarose so that once hardened, the agarose will have furrows in which the sedated larva will be placed¹¹ (Figures 2D and 2E).

4. Injections (Figure 3)

1. Anesthetize larva in 100 ml of aquarium water with 20 mg/L tricaine (L-Ethyl-m-amino-benzoate-methane sulfonate) until they stop responding to touch.
2. Use a plastic Pasteur pipette to carefully transfer the sedated larva into a groove of the wet agarose mold so that the ventral side is facing against the vertical agarose wall of the groove (Figure 3A).
3. Place the agarose plate under the stereomicroscope so that the ventricle is facing away from the injection needle (Figure 3A).
4. Lower the needle to insert it into the heart ventricle. Insert the needle only 1-2 µm into the ventricle taking care not to insert the needle too deeply (Figures 3B and 3C).
5. Once the needle is inserted, inject the morpholino solution into the ventricle with 4-6 pulses, each delivering 3 nl of the solution, with waiting intervals to allow clearing of the heart (Figures 3D and 3E).
6. After injection, remove the needle (parallel to the plane of insertion) and then carefully transfer the larva using a plastic Pasteur pipette filled with E3 medium back into a Petri dish containing fresh E3 medium.
7. Place the needle into a Petri dish containing 1x PBS to prevent the drying of the needle when transferring the injected larva.
8. Repeat steps 4.1-4.6 every 12-24 hr for the duration of the experiment. Note: Repeating the injections ensures the uptake and maintenance of the morpholino in cells.

5. Analyses of Injections (Figure 4)

1. Anesthetize the fish in 100 ml of aquarium water with 20 mg/L tricaine until they stop responding to touch.
2. Place the larva laterally on a flat wet 1.5% (w/v) agarose plate covered with E3 medium.
3. Image larvae using a stereomicroscope with brightfield and fluorescence imaging. Note: As the larvae are transparent, the fluorescein-tagged morpholino should be visible as fluorescent green in the blood vessels throughout the animal directly after injection. This fluorescence will further disperse into the vascularized tissues by 15 min (Figures 4A-4C).

6. Assessment of Regenerative Outgrowth

1. Assess captured images using Fiji Image J free software (http://fiji.sc/Fiji). For investigating regeneration, use the line-tracing tool to determine the amount of regenerative growth that has occurred on control and morpholino-injected larva.
2. To calibrate images, open one of the original image files in Fiji by dragging and dropping the file into the main Fiji window.
3. To set the scale for all following images, go to "Analyze" in the main menu.
4. In the drop down menu click on "Set scale".
5. In this window, turn on the "Global" option by clicking the check box.
6. Now open the image to measure the amount of regenerative growth again by drag and drop (see step 6.1).
7. Choose the "freehand selections" tool in the tool bar.
8. Encircle the area to be measured (Figures 4J-4L).
9. Press Ctrl+M. This will open a new "Results" window showing the value of the encircled area in square pixels.

Results

The sharp, beveled injection needle is easily placed in the cardiac ventricle of the zebrafish larva when approached dorsally (Figure 3A). The heart continues pumping and blood flow is maintained despite the presence of the needle (Figures 3B and 3C). Careful injections do not disrupt the morphology of the ventricle or cardiac contractions (Figure 3D) despite injection of the morpholino into the heart (Figure 3E).

Discussion

The intraventricular injection provides a quick and reliable assessment method for testing gene function at later stages of development or of body homeostasis without affecting the gene function during embryogenesis. To ensure the success of this technique, one should be aware of four critical points: 1) needle size, 2) drying out of the needle, 3) minimizing volume, and 4) minimal exposure time in the sedation solution. Needles that are too small will clog frequently, while needles that are too large will damage th...

Materials

Name	Company	Catalog Number	Comments
Morpholino with 3´fluorescein tag	Gene Tools Inc.	Customized	More information at www.gene-tools.com
Endo-Porter	Gene Tools Inc.		More information at www.gene-tools.com
Agarose	Serva	120623	For pouring plates to place the fish during injections and imaging
Tricaine (L-Ethyl-m-amino-benzate-methane sulfonate)	Applichem		For anesthesia
E3 medium (5 mM NaCl; 0.17 mM KCl; 0.33 mM CaCl2; 0.33 mM MgSO4)			For larval husbandry
Petri Dishes	Sarstedt	821.472	For placing the fish during injections and imaging
Thin Wall Glass Capillaries	World Precision Instruments, Inc.	TW100F-3	For preparing injection needles
Microloader pipette tips	Eppendorf	5,242,956,003	For loading the needle
Pasteur Pipettes (3 ml/1 ml)	Brand	747760/ 74775	For transfering larva
Flaming/Brown needle puller	Sutter		More information at www.sutter.com
Watchmaker (Dumont) Tweezers (size 5)	World Precision Instruments, Inc.	501985	To brake the needle before sharpening
Dissecting microscope	Olympus, Leica, Zeiss	Varies with the manufacturer	Part of the whole injection setup
Fluorescence camera	Olympus, Leica, Zeiss	Varies with the manufacturer	To image the fluorescence after injeciton
PicoNozzle kit (microinjector holder)	World Precision Instruments	5430-12	For microinjections
Pneumatic PicoPump (microinjector)	World Precision Instruments	SYS-PV820	For microinjections