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Uptake of nicotinic acetylcholine receptor imaging agent is reduced in the pro-inflammatory macrophage
Motofumi Suzuki1, Tatsuki Katayama1, Chie Suzuki2
1Laboratory of Bioanalysis and Molecular Imaging, Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo, Hokkaido, Japan.
Introduction:
Macrophages play a vital role in the development of atherosclerotic cardiovascular disease. Macrophages are functionally and phenotypically heterogeneous immune cells and commonly exist in two distinct or polarized subsets: pro-inflammatory M1 and anti-inflammatory M2 phenotypes. Previous reports suggest that stimulation of α7 or α4β2 nicotinic acetylcholine receptors (nAChRs) in macrophages leads to an anti-inflammatory response. However, the biological link between nAChR expression on macrophages and the polarization state is unknown. Therefore, we evaluated the relationship between nAChRs and polarized macrophages in peritoneal macrophages and atherosclerotic plaques of apolipoprotein E knockout (ApoE-/-) mice.
Methods:
Peritoneal macrophages isolated from mice were polarized into M1 and M2 macrophages, and the uptake of the nAChR-imaging agents, (R)-2-[11C]methylamino-benzoic acid 1-aza-bicyclo[2.2.2]oct-3-yl ester ([11C]MeQAA) or 2-[18F]fluoro-3-(2(S)-azetidinylmethoxy) pyridine ([18F]2FA), and 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) was assessed. We also evaluated the accumulation of imaging agents in atherosclerotic plaques of ApoE-/- mice by autoradiography. After an autoradiogram was obtained, the same aortic tissue was used for immunohistochemical staining of CD68, inducible nitric oxide synthase (iNOS), and arginine-1.
Results:
In an in vitro assay, the uptake of [11C]MeQAA or [18F]2FA was lower in M1 than in M0 and M2 macrophages. In comparison, the uptake of [18F]FDG was higher in M1 macrophages. Ex vivo autoradiography showed that [11C]MeQAA was localized to the extensive plaque area. By contrast, the accumulation of [18F]2FA and [18F]FDG was heterogeneous and found only in some plaques. Moreover, the expression of CD68 and iNOS was higher in [18F]2FA non-uptake than [18F]2FA uptake plaques.
Conclusion:
Macrophage polarization was related to nAChR expression, and α4β2 nAChR expression was suppressed in the M1 macrophage. These findings suggest that nAChR imaging has the potential to identify the inflammatory status of atherosclerotic plaque.
Insights
Nicotinic acetylcholine receptors (nAChRs) expression differs between M1 and M2 macrophages. α4β2 nAChR is suppressed in M1 macrophages, suggesting nAChR imaging can reveal atherosclerotic plaque inflammatory status.
Area of Science:
- Immunology
- Cardiovascular Disease Research
- Medical Imaging
Background:
- Macrophages are key immune cells in atherosclerosis, existing as pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes.
- Stimulation of nicotinic acetylcholine receptors (nAChRs) on macrophages induces anti-inflammatory responses.
- The relationship between nAChR expression and macrophage polarization in atherosclerosis remains unclear.
Purpose of the Study:
- To investigate the link between nAChR expression and macrophage polarization in peritoneal macrophages.
- To assess nAChR imaging agents for their ability to reflect macrophage polarization in atherosclerotic plaques of apolipoprotein E knockout (ApoE-/-) mice.
Main Methods:
- In vitro polarization of mouse peritoneal macrophages into M1 and M2 phenotypes.
- Assessment of nAChR-imaging agent ([11C]MeQAA, [18F]2FA) and [18F]FDG uptake in polarized macrophages.
- Ex vivo autoradiography of atherosclerotic plaques in ApoE-/- mice using imaging agents.
- Immunohistochemical staining for CD68, iNOS, and Arginase-1 in aortic tissues.
Main Results:
- In vitro, M1 macrophages showed lower uptake of [11C]MeQAA and [18F]2FA but higher uptake of [18F]FDG compared to M0 and M2 macrophages.
- Autoradiography revealed [11C]MeQAA localization in extensive plaque areas.
- [18F]2FA and [18F]FDG accumulation was heterogeneous, with higher CD68 and iNOS expression in [18F]2FA non-uptake plaques.
Conclusions:
- Macrophage polarization is associated with nAChR expression, specifically suppressed α4β2 nAChR in M1 macrophages.
- nAChR imaging demonstrates potential for identifying the inflammatory status of atherosclerotic plaques.
