Authors: Cheng Cui, Brenda Rongish, Charles Little and Rusty Lansford
Corresponding author ([email protected])
The transfection of GFP-expressing constructs into early embryos permits key developmental events such as gastrulation to be dynamically imaged using time-lapse video-microscopy. This protocol describes the ex ovo electroporation of a DNA plasmid into avian embryos as young as stage X, nearly 24 h earlier in development than most electroporation protocols. Compared to in ovo electroporation, the ex ovo method allows easier embryo orientation (the posterior half of the embryo is darker than the anterior half). Thus, positioning of the specimen and consistency of the electroporation site between embryos is improved. Furthermore, nearly all embryos can be electroporated at the same stage using the ex ovo method: If some embryos have not reached a desired stage, it is possible to temporally stop development of those embryos already at the desired stage by keeping them at room temperature while incubating the rest at 37°C until they develop. The method described here uses relatively low voltage, and the electroporation chamber can be made easily, with no specialized equipment required.
This protocol was adapted from Cui et al. (2006) and incorporates the ex ovo culture method described by New (1955).
Construction of Electroporation Chamber
The use of a glass-bottomed chamber allows the precise placement of the specimen directly over the anode. The chamber is constructed such that the raised segment of the anode is bathed in electrolyte solution (HBSS) during electroporation. When the Millicell insert is centered over the glass window, the insert membrane touches the anode.
Figure 1. A cross-section of the glass-bottomed electroporation chamber used to electroporate pre-gastrulation stage avian embryos.
The cathode is positioned parallel to the anode during electroporation.
3.Design an anode (+) that runs along the bottom of the dish using a platinum wire (80 × 0.3 mm). Bend the wire at the midsection to create a 3-mm segment that is raised 1 mm above the center of the dish and runs parallel to the dish floor (Fig. 1, bottom electrode). Attach the lengths of wire on either side of the raised loop to the floor of the dish using clear nail polish.
4.Create a glass-bottomed chamber by boring a 20-mm hole in the bottoms of both the 60-mm dish and a 100-mm Petri dish. Align the holes, and glue the 60-mm dish inside the 100-mm dish. Center a 35-mm No. 1 glass coverslip over the hole, and glue the glass to the underside of the 100-mm dish.
DNA Plasmid Preparation
5.Isolate and purify a GFP-expression plasmid (or other DNA plasmid of interest) using an endotoxin-free DNA purification kit, e.g., the EndoFree Plasmid Maxi Kit (Qiagen). Store the purified stock in endotoxin-free H2O.
6.Prepare the plasmid DNA for electroporation by mixing a 5 μg/μL concentrated stock of endotoxin-free plasmid with buffered phenol red solution in a 1:1 ratio, such that the final DNA concentration is ~2.5 μg/μL.
Embryo Removal Using Paper Rings
This technique for isolating and securing chicken embryos on a paper ring is described by Chapman et al. (2001).
7.Gently pour the albumen and yolk (with embryo) of a fertile quail or chicken egg into a 100-mm Petri dish. Use a transfer pipette to remove the thick albumen from the poured contents. Use tissues to remove any albumen remaining on the embryo.
8.Remove the embryo from the yolk:
9.With the ventral side up, gently submerge the specimen in embryo phosphate-buffered saline (ePBS) to remove any adherent yolk.
Electroporation of Embryos
With the use of Millicell inserts (in which the volume of agar substrate and thus the distance between the embryo and anode remain constant) as well as a glass-bottomed chamber (which allows the specimen to be placed directly over the anode), a specimen can be electroporated approximately every 3 min.
10.Fill the electroporation chamber with HBSS, so that the fluid covers the anode.
11.Test the electrical continuity of the chamber:
12.Electroporate the embryo with the DNA plasmid: - i. Place the embryo ventral side down on the center of an embryonic culture (Millicell) insert. Transfer the embryo/insert assembly into the electroporation chamber. Adjust the position of the insert so that the targeted region is centered on the anode. - ii. Break the tip of a previously pulled fine-diameter glass micro-needle with forceps, to obtain a point sharp enough to penetrate the vitelline membrane. Use the technique shown in Figure 2 to push the needle tip through the vitelline membrane at an acute angle (<45°), so that the membrane is penetrated without damage to the epiblast. Position the needle tip above the desired region of the epiblast.
Figure 2. A schematic showing the movement of the needle tip used to penetrate the vitelline membrane (V.M.) without damaging the epiblast. The top layer is the vitelline membrane and the lower layer is the epiblast. (A) The needle is lowered to press on the surface of the vitelline membrane. (B) The needle tip is moved to the left until a fold of vitelline membrane is formed over the needle tip. (C) The needle tip is lifted slightly to release the pressure on the epiblast, without losing the vitelline-membrane fold. (D) The needle tip is moved to the left to penetrate through the vitelline membrane. Steps B through D may be repeated until the needle tip penetrates the vitelline membrane.
Figure 3. A schematic showing movement of the cathode for electroporation. Red indicates the injected DNA plasmid solution (with phenol red). Pink indicates the presence of electrolyte solution (HBSS). (A) After the application of one to two drops of HBSS, the cathode is lowered. (B) The cathode is lifted when tissue deformation is seen. (C) The cathode is held in position as soon as tissue deformation is no longer visible. V.M. is vitelline membrane.
Figure 4. Images of GFP-expressing embryos 3.5 h post-electroporation. (A) Bright-field image of an avian embryo. (B) Enlarged fluorescent image (GFP) of the rectangle box in A, showing that fluorescently labeled cells on the right side of the embryo are sparser than those on the left side when different voltages were used (3 V on right side and 4 V on left side). (C) A small group of cells (~100) were fluorescently labeled when a small amount of plasmid DNA was pumped into the region. Scale bars: 250 μm in A, B, and C, and 50 μm in inset picture of C.
13.Gently submerge the embryo/insert in an ePBS-filled dish. With the assembly submerged, float the embryo with attached ring away from the agar bed/insert. Transfer to an embryonic culture dish with the ventral side up.
14.Culture the specimen for a time sufficient to obtain detectable nuclear fluorescence, according to the vector or fluorescent protein used.
This technique, together with time-lapse imaging methods, allows researchers to study how these fluorescently labeled cells move or change their shapes during gastrulation. The technique can be extended when other reagents are used to study functionality of certain genes. For example, a specific gene can be over-expressed, and/or ectopically expressed, by electroporating a plasmid encoding the gene of interest. Alternatively, the functionality of a specific ligand can be down-regulated with the electroporation of a plasmid encoding the dominant-negative receptor for the ligand. A morpholino can also be used to interfere with a targeted gene pathway.
This work was funded in part by the American Heart Association Postdoctoral Fellowship (0620105Z) to C.C. and the NIH NCRR (R21HD047347-01A2) to R.L. Previous Section