Presented here is a protocol for single-cell electroporation that can deliver genes in both excitatory and inhibitory neurons across a range of in vitro hippocampal slice culture ages. Our approach provides precise and efficient expression of genes in individual cells, which can be used to examine cell-autonomous and intercellular functions.
Electroporation has established itself as a critical method for transferring specific genes into cells to understand their function. Here, we describe a single-cell electroporation technique that maximizes the efficiency (~80%) of in vitro gene transfection in excitatory and class-specific inhibitory neurons in mouse organotypic hippocampal slice culture. Using large glass electrodes, tetrodotoxin-containing artificial cerebrospinal fluid and mild electrical pulses, we delivered a gene of interest into cultured hippocampal CA1 pyramidal neurons and inhibitory interneurons. Moreover, electroporation could be carried out in cultured hippocampal slices up to 21 days in vitro with no reduction in transfection efficiency, allowing for the study of varying slice culture developmental stages. With interest growing in examining the molecular functions of genes across a diverse range of cell types, our method demonstrates a reliable and straightforward approach to in vitro gene transfection in mouse brain tissue that can be performed with existing electrophysiology equipment and techniques.
In molecular biology, one of the most important considerations to an investigator is how to deliver a gene of interest into a cell or population of cells to elucidate its function. The different methods of delivery can be categorized as either biological (e.g., a viral vector), chemical (e.g., calcium phosphate or lipid), or physical (e.g., electroporation, microinjection, or biolistics)1,2. Biological methods are highly efficient and can be cell type-specific but are limited by the development of specific genetic tools. Chemical approaches are very powerful in vitro, but transfections are generally random; further, these approaches are mostly reserved for primary cells only. Of the physical approaches, biolistics is the simplest and easiest from a technical point of view, but again produces random transfection results at a relatively low efficiency. For applications which require transfer into specific cells without the need for developing genetic tools, we look toward single-cell electroporation3,4.
Whereas electroporation used to refer only to field electroporation, over the past twenty years, multiple in vitro and in vivo single-cell electroporation protocols have been developed to improve specificity and efficiency5,6,7, demonstrating that electroporation can be used to transfer genes to individual cells and can, therefore, be extremely precise. However, the procedures are technically demanding, time-consuming, and relatively inefficient. Indeed, more recent papers have investigated the feasibility of mechanized electroporation rigs8,9, which can help to eliminate several of these barriers for investigators interested in installing such robotics. But for those looking for simpler means, the problems with electroporation, namely cell death, transfection failure, and pipette clogging, remain a concern.
We recently developed an electroporation method that uses larger-tipped glass pipettes, milder electrical pulse parameters, and a unique pressure cycling step, which generated a much higher transfection efficiency in excitatory neurons than previous methods, and enabled us for the first time to transfect genes in inhibitory interneurons, including somatostatin-expressing inhibitory interneurons in the hippocampal CA1 region of mouse organotypic slice culture10. However, the reliability of this electroporation method in different inhibitory interneuron types and neuronal developmental stages has not been addressed. Here, we demonstrated that this electroporation technique is capable of transfecting genes into both excitatory neurons and different classes of interneurons. Importantly, transfection efficiency was high regardless of days in vitro (DIV) slice culture age tested. This established and user-friendly technique is highly recommended to any investigator interested in using single-cell electroporation for different cell types in the context of in vitro mouse brain tissue.
All animal protocols were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) at the University of Massachusetts Medical School. Slice culture preparation, plasmid preparation, and electroporation are also detailed in our previously published methods and can be referred to for additional information10.
1. Slice culture preparation
2. Plasmid preparation
3. Glass pipette preparation
4. Electroporation rig setup
5. Electroporation preparation
6. Electroporate cells of interest
7. Fixation, staining and imaging of organotypic hippocampal slice cultures
Our single-cell electroporation is capable of precisely delivering genes into visually identified excitatory and inhibitory neurons. We electroporated three different neuronal cell types at three different time points. Parvalbumin (Pv) or vesicular glutamate type 3 (VGT3) expressing neurons were visualized by crossing Pvcre (JAX #008069) or VGT3cre (JAX #018147) lines with TdTomato (a variant of red fluorescent protein) reporter line (Jax #007905), respectively named Pv/TdTomato and VGT3/TdTomato lines. Organotypic slice cultures were prepared from C57BL/6J, Pv/TdTomato, and VGT3/TdTomato mice.
First, electroporation was performed in CA1 pyramidal neurons (Py) at either 7, 14, or 21 days in vitro (DIV). EGFP was transfected into 5-20 pyramidal neurons in the hippocampal CA1 area across these slice culture ages (Figure 2B-D). CA1 pyramidal neurons were identified using differential interference contrast (DIC). To demonstrate the anatomical distribution and morphological differences between pyramidal neurons and inhibitory interneurons in slice culture, CA1 pyramidal neurons were electroporated with EGFP in a DIV7 Pv/TdTomato mouse and nuclear counterstaining was performed to display the distinct location of EGFP-positive neurons in the CA1 pyramidal cell layer (Figure 2A).
Next, this protocol was also applied to TdTomato-positive Pv and VGT3 interneurons. EGFP electroporation was carried out in 1-10 fluorescently labeled interneurons. TdTomato-positive Pv (Figure 3) and VGT3 (Figure 4) neurons were successfully electroporated in the hippocampal CA1 area. Interestingly, transfection of the EGFP gene of interest in all of these inhibitory neuronal types was not significantly affected by DIV and did not differ with the transfection efficiency (~80%) observed in CA1 pyramidal neurons (Figure 5).
Figure 1: Two representative glass pipette images.
(A) Display lower resistance (6.5 MΩ) pipettes, used in this protocol, and (B) higher resistance (10.4 MΩ) pipettes typical for electroporation protocols. Please click here to view a larger version of this figure.
Figure 2: Organotypic hippocampal slice cultures were electroporated with EGFP (green) at three different time points.
(A) Representative organotypic hippocampal slice culture in a DIV7 Pv/TdTomato mouse. CA1 pyramidal neurons were electroporated with EGFP (green, white arrowheads) and showed no overlap with TdTomato (TdT)-positive Pv interneurons (red, yellow arrowheads). DAPI nuclear counterstaining (blue) was performed. DG: dentate gyrus. (B-D) CA1 pyramidal neurons were electroporated with EGFP at three different time points: (B) DIV7, (C) DIV14 and (D) DIV21. Organotypic slice cultures were fixed with 4% sucrose, 4% paraformaldehyde/ 1x PBS and imaged without further sectioning. Top left, low magnification images of the hippocampal CA1 area. Arrowheads represent individual CA1 pyramidal neurons targeted for electroporation. Transfected neurons with yellow arrowheads are zoomed in the bottom panels. White arrowheads signify additional electroporated neurons outside of the high magnification view. Top right, low magnification images of superimposed (Sup) fluorescent and Nomarski images. Scale bars: 500, 50, 100, 20 μm respectively. Please click here to view a larger version of this figure.
Figure 3: Pv/TdTomato organotypic hippocampal slice cultures were electroporated with EGFP (green) at three different time points.
(A) DIV7, (B) DIV14 and (C) DIV21. Overlap with Pv-labeled TdTomato (TdT)-positive cells (red) was observed in the hippocampal CA1 pyramidal cell layer and oriens. In the low magnification insets (top row), yellow arrowheads represent individual Pv interneurons targeted for electroporation. White arrowheads signify additional TdTomato-positive Pv interneurons electroporated outside of the high magnification view. Scale bars: 50, 20 μm. Please click here to view a larger version of this figure.
Figure 4: VGT3/TdTomato organotypic hippocampal slice cultures were electroporated with EGFP (green) at three different time points.
(A) DIV7, (B) DIV14 and (C) DIV21. Overlap with VGT3-labeled TdTomato (TdT)-positive cells (red) was observed in the hippocampal CA1 pyramidal cell layer and oriens. In the low magnification insets (top row), yellow arrowheads represent individual VGT3 interneurons targeted for electroporation. White arrowheads signify additional TdTomato-positive VGT3 interneurons electroporated outside of the high magnification view. Scale bars: 50, 20 μm. Please click here to view a larger version of this figure.
Figure 5: Comparable levels of transfection efficiency in CA1 pyramidal neurons, Pv/TdTomato, and VGT3/TdTomato interneurons at three different in vitro slice culture ages.
Summary bar graphs of three different slice culture ages: DIV7 (left), DIV14 (middle) and DIV21 (right). Each symbol represents the transfection efficiency obtained from one organotypic slice culture CA1 Py: DIV7 (12 slice cultures from 2 mice), DIV14 (8/2), DIV21 (8/2); Pv: DIV7 (5/2), DIV14 (6/2), DIV21 (6/2); VGT3: DIV7 (6/2), DIV14 (7/2), DIV21 (6/2). One-way ANOVA; n.s. (not significant). Data shown are mean ± SEM. Please click here to view a larger version of this figure.
We describe here an electroporation method that transfects both excitatory and inhibitory neurons with high efficiency and precision. Our optimized electroporation protocol has three innovative breakthroughs to achieve highly efficient gene transfection. Our first modification was to increase pipette size compared with previously published protocols3,5,6. This change enabled us to electroporate many neurons without pipette clogging. In addition, it is possible that the lower resistance pipettes allow for the use of milder electrical pulse parameters compared with previous methods while still achieving the desired result3. Next, repeated pressure cycling before electroporation markedly reduced cell death10. We often observed that with the application of a single pulse of negative pressure before electroporating, plasma membrane stuck to the inside of the pipette tip, damaging the cell. The pressure cycles helped far less plasma membrane stick to the pipette, which improved cell survival and recovery during the procedure10. Finally, the addition of TTX into aCSF greatly improved the success of electroporation in inhibitory neurons10. We consider that electroporation can cause lethal cell overexcitation which can be prevented by TTX. Above all, these critical improvements offer remarkably high success rates in electroporation to both excitatory and inhibitory neurons (Figure 5). Although we found no electrophysiological abnormalities in our neurons after transfection with this method10 in the targeted cells, it has been reported that local pH changes during electroporation reduce cell viability and the optimization of electroporation parameters can be relatively difficult compared with other gene transfection methods13,14. Therefore, it is important to consider unforeseen side effects of electroporation and to re-optimize the electrical parameters as needed to achieve a high transfection efficiency for any specific application.
Microinjection technology has also been used as a powerful approach to deliver transgenes to cells2,15,16. However, this approach generally produces a lower yield of transfection and requires a high level of skill. In contrast, our method can be used with relative ease and at minimal cost to laboratories that routinely perform whole-cell electrophysiology studies.
Recently, we showed that multiple genes can be transfected into both excitatory and somatostatin expressing inhibitory neurons with no side effects on electrophysiological properties10. In this study, we demonstrate that this electroporation method is highly efficient in CA1 pyramidal neurons (Figure 2) and Pv (Figure 3) and VGT3 (Figure 4) inhibitory interneurons. Moreover, this electroporation technique allows us to transfect genes of interest with an ~80% success rate regardless of cell type or number of days in vitro (Figure 5). Although the technique has only been tested in vitro, organotypic hippocampal slice cultures at the same DIV time points we tested have been shown to follow age-matched in vivo synaptic morphology and activity, as seen in acutely prepared hippocampal slices17. Moreover, as we have only tested this method in hippocampal neurons, it is possible that there could be unanticipated challenges in applying this technique to neurons in other brain regions. The method invites exploration of genes during organotypic slice culture development in which synaptic transmission and density, among other properties, change over time.
This protocol is an improvement over previously established ones in that it is simultaneously low-cost, less technically challenging, and more efficient in generating single-neuron gene transfection5,6,7,8,9. It also appears to be an improvement over previous methods in terms of lower rates of cell damage or death, as we observed that ~80% of cells were successfully transfected and healthy after the procedure. This method, therefore, provides a new opportunity to examine the roles of genes in multiple neuronal cell types using fluorescent mouse models. Future studies using this method can focus on protein-protein interactions between cells to examine specific molecular or physiological functions, including trans-synaptic protein interactions.
The authors have nothing to disclose.
This work was supported by National Institutes of Health Grants (R01NS085215 to K.F., T32 GM107000 and F30MH122146 to A.C.). The authors thank Ms. Naoe Watanabe for skillful technical assistance.
Plasmid preparation | |||
Plasmid Purification Kit | Qiagen | 12362 | |
Organotypic slice culture preparation | |||
6 Well Plates | GREINER BIO-ONE | 657160 | |
Dumont #5/45 Forceps | FST | #5/45 | Angled dissection forceps for organotypic slice culture preparation |
Flask Filter Unit | Millipore | SCHVU02RE | Filtration and storage of culture media |
Incubator | Binder | BD C150-UL | |
McIlwain Tissue Chopper | TED PELLA, INC. | 10180 | Tissue chopper for organotypic slice culture preparation |
Millicell Cell Culture Insert, 30 mm | Millipore | PIHP03050 | Organotypic slice culture inserts |
Osmometer | Precision Systems | OSMETTE II | |
PTFE coated spatulas | Cole-Parmer | SK-06369-11 | |
Scissors | FST | 14958-09 | |
Stereo Microscope | Olympus | SZ61 | |
Sterile Vacuum Filtration System | Millipore | SCGPT01RE | Filtration and storage of aCSF |
Electrode preparation | |||
Capillary Glasses | Warner Instruments | 640796 | |
Micropipetter Puller | Sutter Instrument | P-1000 | Puller |
Oven | Binder | BD (E2) | |
Puller Filament | Sutter Instrument | FB330B | Puller |
Single-cell electroporation and fluorescence imaging #1 | |||
3.5 mm Falcon Petri Dishes | BD Falcon | 353001 | |
Airtable | TMC | 63-7512E | |
CCD camera | Q Imaging | Retiga-2000DC | Camera |
Electroporation System | Molecular Devices | Axoporator 800A | Electroporator |
Fluorescence Illumination System | Prior | Lumen 200 | |
Manipulator | Sutter Instrument | MPC-385 | Manipulator |
Metamorph software | Molecular Devices | Image acquisition | |
Peristaltic Pump | Rainin | Dynamax, RP-2 | Perfusion pump |
Shifting Table | Luigs & Neuman | 240 XY | |
Speaker | Unknown | Speakers connected to the electroporator | |
Stereo Microscope | Olympus | SZ30 | |
Table Top Incubator | Thermo Scientific | MIDI 40 | |
Upright Microscope | Olympus | BX61WI | |
Fluorescence imaging #2 | |||
All-in-One Fluorescence Microscope | Keyence | BZ-X710 |