This article describes how to inject viral vectors into the mouse frontal cortex to test behavioral assays that require GPCR heteromeric formation.
The heteromeric receptor complex between 5-HT2A and mGlu2 has been implicated in some of the behavioral phenotypes in mouse models of psychosis1,2. Consequently, investigation of structural details of the interaction between 5-HT2A and mGlu2 affecting schizophrenia-related behaviors represents a powerful translational tool. As previously shown, the head-twitch response (HTR) in mice is elicited by hallucinogenic drugs and this behavioral response is absent in 5-HT2A knockout (KO) mice3,4. Additionally, by conditionally expressing the 5-HT2A receptor only in cortex, it was demonstrated that 5-HT2A receptor-dependent signaling pathways on cortical pyramidal neurons are sufficient to elicit head-twitch behavior in response to hallucinogenic drugs3. Finally, it has been shown that the head-twitch behavioral response induced by the hallucinogens DOI and lysergic acid diethylamide (LSD) is significantly decreased in mGlu2-KO mice5. These findings suggest that mGlu2 is at least in part necessary for the 5-HT2A receptor-dependent psychosis-like behavioral effects induced by LSD-like drugs. However, this does not provide evidence as to whether the 5-HT2A-mGlu2 receptor complex is necessary for this behavioral phenotype. To address this question, herpes simplex virus (HSV) constructs to express either mGlu2 or mGlu2ΔTM4N (mGlu2/mGlu3 chimeric construct that does not form the 5-HT2A-mGlu2 receptor complex) in the frontal cortex of mGlu2-KO mice were used to examine whether this GPCR heteromeric complex is needed for the behavioral effects induced by LSD-like drugs6.
Hallucinogens, such as LSD, psilocybin and mescaline cause significant changes in human consciousness, cognition and emotion7-9. Inactivation of serotonin 5-HT2A receptor signaling by either genetic or pharmacological approaches causes markedly attenuated behavioral responses to hallucinogens in both rodent models3,10 and humans11. Although hallucinogens bind other receptor subtypes8, the 5-HT2A receptor is considered as necessary for the unique behavioral activity of these chemicals.
Group II metabotropic glutamate receptors (i.e., mGlu2 and mGlu3) have been the target of considerable attention regarding the molecular mechanism of hallucinogens and their integral role underlying psychosis12. Previously, it has been demonstrated that mice with no expression of mGlu2 protein (mGlu2-KO mice) are insensitive to the cellular and behavioral effects of hallucinogens5. It has also been suggested that the 5-HT2A and the mGlu2 receptors form a specific heteromeric complex through which serotonin and glutamate ligands modulate the pattern of G protein coupling in living cells 1,2.
Structurally, transmembrane (TM) domains 4 and 5 of mGlu2 play a fundamental role in heteromeric formation with the 5-HT2A receptor5. Additionally, further investigation demonstrated that three residues located at the intracellular end of TM4 of mGlu2 are necessary to form the 5-HT2A-mGlu2 receptor heterocomplex in living cells6.
Based on these findings observed in heterologous expression systems, here we describe the use of HSV-mediated expression of wild-type mGlu2 and mGlu2/mGlu3 chimeric constructs in the frontal cortex of mGlu2-KO mice to test whether heteromeric formation between 5-HT2A and mGlu2 is necessary for the head-twitch behavior induced by hallucinogenic 5-HT2A receptor agonists.
NOTE: All procedures for animal breeding and cares were conducted according to the Institutional Animal Care and Use Committee (IACUC) regulation of Icahn School of Medicine at Mount Sinai. Be sure to use sterile gloves throughout the procedure.
1. Drug and Virus Preparation
2. Surgery
3. Head Twitch Response Experiment
Previous findings demonstrate that the head-twitch murine behavioral response is reliably and robustly elicited by hallucinogens, and it is absent in 5-HT2A-KO mice3. Furthermore, it has been shown that the head-twitch response elicited by the hallucinogenic 5-HT2A agonists DOI and LSD was significantly decreased in mGlu2-KO mice5. However, although previous findings convincingly demonstrate that 5-HT2A and mGlu2 are assembled as a heteromeric complex in vitro in transfected cells1,2,15, whether this structural arrangement behaves as such in living mice remained unsolved. To fully understand the role of the 5-HT2A-mGlu2 receptor heterocomplex in the psychoactive-like effects induced by hallucinogenic 5-HT2A receptor agonists, expression of either mGlu2 or mGlu2ΔTM4N in frontal cortex of mGlu2-KO mice to examine whether this manipulation regulates behavior.
Mice received intra-frontal cortical injections of bicistronic HSV expressing green fluorescent protein (GFP) and either mGlu2 or mGlu2ΔTM4N, or GFP alone. First, it was confirmed that the virus over-expresses mGlu2 or mGlu2ΔTM4N in mouse frontal cortex (Figures 1A and 1B). As previously demonstrated5, head-twitch behavior induced by DOI was absent in mGlu2-KO mice injected with the empty vector HSV-GFP. Notably, the head-twitch response induced by the hallucinogenic 5-HT2A agonist DOI was rescued in mGlu2-KO mice over-expressing mGlu2, but not mGlu2ΔTM4N, in frontal cortex as compared to that seen in animals expressing GFP (Figure 1C). Together, these findings suggest that the 5-HT2A-mGlu2 receptor complex in frontal cortex is critical for regulating psychosis-like states.
Figure 1. Expression of mGlu2 as a Receptor Heterocomplex. Expression of mGlu2 as a receptor with 5-HT2A is necessary for psychosis-like behavior induced by hallucinogenic drugs. (A) Representative image of HSV-mediated transgene expression in frontal cortex. HSV-mGlu2, which also expresses GFP, was injected into frontal cortex, and GFP expression was revealed by immunocytochemistry, scale bar, 200-um. (B) HSV-mediated transgene expression in mouse frontal cortex of mGlu2-KO mice, and anti-mGlu2 reactivity was measured by Western Blotting. Specificity of the primary antibody against the mGlu2 receptor has previously been confirmed in knockout mice6. Metabotropic glutamate receptors are GPCRs that form covalently linked homodimers. We measured immunoreactivity of mGlu2 as a monomer (100 kDa)6. (C) Viral-mediated expression of mGlu2, but not mGlu2ΔTM4N, in frontal cortex of mGlu2- KO mice significantly rescues the head-twitch response induced by the hallucinogenic 5-HT2A agonist DOI (n = 4 per group). ***p <0.001; n.s, not significant; Bonferroni's post hoc test of one-way ANOVA. Error bars represent S.E.M. Figure was modified from Moreno et al (2012)6. Please click here to view a larger version of this figure.
Supplemental Video 1. Head Twitch Response. (Right click to download). CD-1 WT mice were injected with 2.0 mg/kg DOI and placed in a cage (wall blacked out between the two cages) to elicit head-twitch response (behavior elicited after *).
Together with previous findings in mGlu2-KO mice5, the results with mGlu2 and mGlu2/mGlu3 chimeric constructs that do not form the 5-HT2A-mGlu2 receptor complex in cultured cells suggest that the 5-HT2A -mGlu2 heteromeric receptor complex in mouse frontal cortex is needed to induce head-twitch behavior by LSD-like hallucinogenic 5-HT2A receptor agonists. A limitation of this method is that it does not measure close molecular proximity at a subcellular level in native tissue. In addition, there are various critical points to be noted. Because the mice are injected with a HSV viral vector, the time frame that the experiments to be performed are 2 – 4 days after injection. The location and expression of the viral vectors should be verified with immunofluorescence staining of sectioned brain no more than 4 days after the initial injection. Mice in which the coordinates do not match or do not express the viral vector should be excluded from the experimental data as they do not express the mGlu2 or mGlu2ΔTM4N. Care after stereotactic surgery is also crucial, as improper closing of the head wound can lead to infection which can cause issues in both the in vivo experiments and the immunofluorescence staining. Lastly, after stereotactic injection any behavioral paradigm (open field, alternating t-test, etc.) can be used as long as it is within the 2 – 4 after injection of viral particles. Again, coordinates and expression should be confirmed by immunofluorescence.
The concept that GPCRs function as homo- and/or heteromers in living cells is now well established 16-18. However, despite some progress, more studies are needed to define the precise role(s) of GPCR heteromeric complexes in whole animal models 19. Approaches such as BRET and/or FRET imaging to investigate protein-protein physical proximity within deep tissues of small animal models may provide means to enhance the understanding of the functional role of GPCR heteromers in living subjects20. Using HSV-mediated expression of GPCRs either native or with modified transmembrane proteins, provide an in vivo model to evaluate the function and interaction of GPCRs.
Further studies in rodent models are also needed to examine the stability and life- time (formation and dissociation) of GPCR heteromers21-23, structural rearrangements between their components24, and potential G protein coupling after receptor internalization25,26. Given the fundamental role of GPCRs in cell signaling and function, it seems likely that this area might lead to interesting basic and translational studies.
Although further investigation is required to quantitatively characterize the ultrastructural co-localization of both receptors in human and mouse CNS, together with previous studies that convincingly demonstrate the electrophysiological, cellular, and behavioral responses induced by hallucinogens in mouse models are intrinsic to 5-HT2A receptor-expressing cortical pyramidal neurons3,27,28, the findings obtained using the HSV-mediated expression approach described here suggest that heteromeric formation between 5-HT2A and mGlu2 receptors in mouse frontal cortex is needed for the head-twitch psychosis-like behavior induced by the hallucinogenic 5-HT2A receptor agonist DOI.
HSV-mediated expression of mGlu2, but not mGlu2ΔTM4N, in frontal cortical neurons of mGlu2-KO mice rescues the head-twitch behavior induced by the hallucinogenic 5-HT2A receptor agonist DOI. This translational tool might be advantageous for preclinical studies to evaluate behavioral phenotypes of GPCR heteromers.
The authors have nothing to disclose.
NIH R01MH084894 participated in the funding of this study. We would like to thank Drs. Yasmin Hurd and Scott Russo at Mount Sinai School of Medicine for the donation of mice and the use of their surgery and behavior facilities during the filming of this work.
mGlu2 bicitronic herpes simplex virus (HSV) vector | MIT Core | mGlu2 and mGlu2DTM4N were subcloned into the bicistronic HSV-GFP virus vector p1005+ HSV expressing GFP under the control of the CMV promoter. Viral particles were produced by the Viral Core Facility at the McGovern Institute (MIT). For more information, please contact the director, Dr. Rachael Neve (rneve@mit.edu) | |
mGlu2ΔTM4N bicitronic herpes simplex virus (HSV) vector | MIT Core | mGlu2 and mGlu2DTM4N were subcloned into the bicistronic HSV-GFP virus vector p1005+ HSV expressing GFP under the control of the CMV promoter. Viral particles were produced by the Viral Core Facility at the McGovern Institute (MIT). For more information, please contact the director, Dr. Rachael Neve (rneve@mit.edu) | |
GFP bicitronic herpes simplex virus (HSV) vector | MIT Core | mGlu2 and mGlu2DTM4N were subcloned into the bicistronic HSV-GFP virus vector p1005+ HSV expressing GFP under the control of the CMV promoter. Viral particles were produced by the Viral Core Facility at the McGovern Institute (MIT). For more information, please contact the director, Dr. Rachael Neve (rneve@mit.edu) | |
xylazine | Lloyd | List no. 4811-20ml, NADA #139-236, NDC Code(s): 61311-481-10 | 1.35 mL of 100mg/ml of ketamine+.75 mL of 20mg/ml of xylazine are diluted in 12.0 mL of .9% saline solution |
ketamine | Vedco | KetaVed-10ml, NADA #200-029, NDC Code(s): 50989-161-06 | 1.35 mL of 100mg/ml of ketamine+.75 mL of 20mg/ml of xylazine are diluted in 12.0 mL of .9% saline solution |
ophthalmic gel | Fisher Scientific | NC0550805 | |
burret clips | Fisher Scientific | NC9268369 | |
Feather surgical blade | Fisher Scientific | NC9032736 | |
Hydrogen Peroxide | Fisher Scientific | 19-898-919 | |
Hamilton syringe | Fisher Scientific | 14815203 | |
Hamilton™ Small Hub Removable Needles (33 Ga) | Fisher Scientific | 14816206 | |
Cordless Micro Drill | Fisher Scientific | NC9089241 | |
Dermabond Dermal Adhesive | Fisher Scientific | NC0690470 | |
(±)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) | Sigma-Aldrich | 42203-78-1 | Dissolved in .9% saline solution to the concentration of 2.0 mg/kg |