HSV-Mediated Transgene Expression of Chimeric Constructs to Study Behavioral Function of GPCR Heteromers in Mice
July 9th, 2016
This article describes how to inject viral vectors into the mouse frontal cortex to test behavioral assays that require GPCR heteromeric formation.
The overall goal of this method is to observe how virus-mediated gene transfer affects behavioral phenotypes that require heterodimerization of G-protein coupled receptors in mouse models. This method can help answer key questions in the field of neuro and psycho pharmacology. Specifically how g protein coupled receptors interact with each other at a molecular level.
By using this method we can understand how this interaction affects behavior in models of psychiatric disorders, such as schizophrenia and depression. Using this method we are going to specifically look at seratonin 2a and metabotropic glutamate receptor 2, both of which have been shown to be implicated in the mechanism of action of drugs used to treat patients with schizophrenia. Begin by properly anesthetizing the mouse according to approved methods.
Ensure that a surgical plane of anesthesia has been obtained by performing a toe pinch and noting the absence of a response. Then apply ophthalmic gel to protect the eyes. Attach the mouse to the stereotactic frame, making sure to adjust the tilt of the head so that the skull is level and flat.
Apply povidone iodine to the exposed scalp. Using a scalpel make a sagittal incision along the midline of the skull within the exposed shaved area. Then attach pureit clamps to the skin at the incision site to make sure that the skull remains exposed.
Apply hydrogen peroxide to dissolve away the periosteum, and expose the sutures of the skull. Now that the bregma and sutures are visible, be sure to adjust the stereotactic frame to make sure that the skull is level. Align the needle tips of the syringes with bregma and record the coordinates of bregma.
To bilaterally inject the frontal cortex, add 1.6 mm to the recorded rostral caudal bregma coordinate, and 2.6 mm to the recorded medial lateral bregma coordinate. Mark the injection sites on the skull and then drill small holes at the injection sites. With a cotton tip applicator, wipe away any excess blood or bone fragments.
Bring the needle to the surface of the brain, and lower to the depth of the dorsal ventral coordinate. Then slowly inject the contents of the syringe by twisting the plunger of the needle to administer 0.1 microliters per minute for five minutes, for a 0.5 microliter injection. Allow the syringe to remain in place for five minutes.
After the allotted time, remove the animal from the stereotaxis, close the incision, and place on a warming pad. Administer post operative analgesia according to your approved animal protocol. Perform behavioral testing two to three days after stereotactic injection of viral particles.
Habituate the mice to the room for at least four hours prior to the beginning of the experiment. Begin by dissolving DOI into a 0.9 percent saline solution to two milligrams per kilogram. Prepare a home cage without any bedding and using a tripod adjust a camera so that it is directly over the home cage.
Calibrate the camera so that the entire home cage is in the field of view. Weigh the mouse and inject intraperitoneally with the appropriate dose of either 0.9 percent saline or DOI. Place the mouse back in the home cage for ten minutes.
After ten minutes, place the mouse into the center of the empty home cage, and ensure that there are no blind spots in the field of view of the camera. Press record on the camcorder and leave the room. After 30 minutes, stop the recording and place the mouse back in the original home cage.
Repeat the behavioral test on the next mouse. Have the referee review the videos blind to the experimental conditions. Manually record every head twitch throughout the video.
A head twitch is defined as a rapid shaking head movement conducted by a mouse. Process the data as outlined in the written portion of the protocol. Shown here is a representative image of HSV mediated transgene expression in the frontal cortex.
HSV mGlu2 GFP was injected into the frontal cortex and GFP expression was revealed by immunocytochemistry. This scale bar represents 200 microns. This image shows a western blot of HSV mediated transgene expression and anti-mGlu2 reactivity in the frontal cortex of mGlu2 knockout mice.
Here we measured immunoreactivity of mGlu2 as a monomer. This histogram shows that viral mediated expression of mGlu2, but not of chimeric mGlu2 in the frontal cortex of mGlu2 knockout mice significantly rescues the head twitch response induced by the hallucinogenic seratonin 2a receptor agonist DOI. The most critical steps with this experiment are the timing between the viral injections, the head twitch response, and when the mice are sacrificed.
Any delay can alter the results of the experiment or introduce ambiguity into the data. Once mastered, the mouse surgery will take approximately 30 minutes to inject the virus. The head twitch experiment should also take approximately 35 minutes per mouse.
Staggering surgeries, or operating on more than one mouse at a time is recommended. This way one can maintain a timeline that allows the head twitch experiments to be done in the three to four days after surgery which is the peak expression time of the viral construct. Thanks for watching and good luck with your experiment.
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Overview
This article presents a method to investigate the behavioral consequences of virus-mediated gene transfer targeting G-protein coupled receptor (GPCR) heterodimerization in mouse models. Specifically, it examines the interaction between serotonin 2A (5-HT2A) and metabotropic glutamate receptor 2 (mGlu2) in the context of psychosis-related behaviors, using the head-twitch response (HTR) as a behavioral readout following hallucinogenic drug administration.
Key Study Components
Area of Science
- Neuropharmacology
- Behavioral neuroscience
- Molecular neuroscience
Background
- GPCR heteromeric complexes, such as 5-HT2A-mGlu2, are implicated in psychiatric disorders like schizophrenia.
- The head-twitch response (HTR) in mice is a well-established behavioral assay for hallucinogenic drug action and is dependent on 5-HT2A receptor function.
- Previous studies show that mGlu2 knockout mice have reduced HTR to hallucinogens, suggesting a role for mGlu2 in 5-HT2A-mediated behaviors.
- The necessity of the 5-HT2A-mGlu2 receptor complex for these behaviors had not been directly tested.
Purpose of Study
- To determine whether the 5-HT2A-mGlu2 heteromeric receptor complex is required for hallucinogen-induced behavioral responses in mice.
- To assess the ability of viral-mediated expression of mGlu2 or a chimeric mGlu2 construct (unable to form the heteromeric complex) to rescue HTR in mGlu2 knockout mice.
- To provide a methodological framework for studying GPCR interactions in vivo.
Methods Used
- Stereotactic surgery for bilateral injection of herpes simplex virus (HSV) constructs into the frontal cortex of anesthetized mice.
- Expression of either wild-type mGlu2 or a chimeric mGlu2/mGlu3 construct (mGlu2ΔTM4N) via viral vectors.
- Behavioral testing using the head-twitch response (HTR) following administration of the hallucinogen DOI.
- Immunocytochemistry and western blotting to confirm transgene expression in the frontal cortex.
Main Results
- HSV-mediated expression of mGlu2 in the frontal cortex of mGlu2 knockout mice significantly rescues the DOI-induced head-twitch response.
- Expression of the chimeric mGlu2 construct, which cannot form the 5-HT2A-mGlu2 complex, does not rescue the HTR.
- Immunocytochemistry and western blotting confirm successful and specific transgene expression in the targeted brain region.
- The timing of viral injection, behavioral testing, and tissue collection is critical for experimental success.
Conclusions
- The 5-HT2A-mGlu2 heteromeric receptor complex is necessary for the full behavioral response to hallucinogenic drugs in mice.
- Virus-mediated gene transfer is an effective approach to dissect the molecular requirements of GPCR interactions in vivo.
- This method provides a valuable tool for studying the molecular basis of psychiatric disorder-related behaviors.
Frequently Asked Questions
Executive Industry Relevance
Dissecting the behavioral function of GPCR heteromers using HSV-mediated transgene expression in mice addresses a critical challenge in neuropsychiatric drug discovery: establishing mechanistic links between receptor complexes and disease-relevant phenotypes. This approach enables predictive confidence in target validation for psychiatric disorders by isolating the functional necessity of specific receptor interactions. The method supports risk-adjusted portfolio decisions at the interface of early discovery and translational research.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Enables direct interrogation of GPCR heteromer function in vivo for mechanistic de-risking.
- Clarifies the necessity of specific receptor complexes for disease-relevant behavioral phenotypes.
- Supports predictive confidence in target selection for neuropsychiatric indications.
Screening & Assay Development
- Establishes validated behavioral assays linked to molecular interventions for downstream screening.
- Provides quantitative behavioral outputs (head-twitch response) for assay standardization.
- Facilitates reproducible evaluation of genetic and pharmacological manipulations in vivo.
Translational & Preclinical Research
- Aligns preclinical behavioral models with human disease mechanisms via receptor complex interrogation.
- Enables continuity from molecular discovery to behavioral validation in animal models.
- Supports translational biomarker development by linking molecular interventions to quantifiable phenotypes.
Pipeline & Workflow Integration
This method bridges early discovery and preclinical validation by enabling hypothesis-driven testing of receptor complex function in disease-relevant behavioral systems.
- Discovery Biology: Supports null hypothesis testing for GPCR heteromer involvement in psychiatric phenotypes.
- Screening: Provides a platform for quantitative behavioral readouts following genetic or pharmacological perturbation.
- Analytics: Delivers measurable endpoints (head-twitch counts) for statistical comparison across experimental groups.
- Translational Research: Connects molecular interventions to behavioral outcomes relevant to psychiatric disorders.
- Enterprise Reuse: Offers a reusable workflow for interrogating other receptor complexes in vivo.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence in target validation and reduces mechanistic ambiguity for neuropsychiatric drug programs.
- Operational Value: Standardizes behavioral phenotyping linked to molecular interventions for reproducibility and scalability.
- Strategic Value: Informs go/no-go decisions by clarifying the necessity of receptor complexes for disease-relevant behaviors.
- Portfolio Impact: Enables risk-adjusted prioritization of targets and mechanisms for advancement in CNS pipelines.
Implementation Considerations
- Requires expertise in stereotactic surgery and behavioral phenotyping in rodents.
- Demands access to viral vector production and molecular validation infrastructure (immunocytochemistry, western blotting).
- Necessitates rigorous cross-team standardization of injection timing, behavioral testing, and data analysis.
- Adaptation to other receptor complexes or behavioral assays may require protocol optimization.
- Precise timing between viral delivery, behavioral assessment, and tissue collection is critical for data integrity.
