Assessment of the alpha 7 nicotinic acetylcholine receptor as an imaging marker of cardiac repair-associated processes using NS14490

Background Cardiac repair and remodeling following myocardial infarction (MI) is a multifactorial process involving pro-reparative inflammation, angiogenesis and fibrosis. Noninvasive imaging using a radiotracer targeting these processes could be used to elucidate cardiac wound healing mechanisms. The alpha7 nicotinic acetylcholine receptor (alpha 7nAChR) stimulates pro-reparative macrophage activity and angiogenesis, making it a potential imaging biomarker in this context. We investigated this by assessing in vitro cellular expression of alpha 7nAChR, and by using a tritiated version of the PET radiotracer [F-18]NS14490 in tissue autoradiography studies.Results alpha 7nAChR expression in human monocyte-derived macrophages and vascular cells showed the highest relative expression was within macrophages, but only endothelial cells exhibited a proliferation and hypoxia-driven increase in expression. Using a mouse model of inflammatory angiogenesis following sponge implantation, specific binding of [H-3]NS14490 increased from 3.6 +/- 0.2 mu Ci/g at day 3 post-implantation to 4.9 +/- 0.2 mu Ci/g at day 7 (n = 4, P < 0.01), followed by a reduction at days 14 and 21. This peak matched the onset of vessel formation, macrophage infiltration and sponge fibrovascular encapsulation. In a rat MI model, specific binding of [H-3]NS14490 was low in sham and remote MI myocardium. Specific binding within the infarct increased from day 14 post-MI (33.8 +/- 14.1 Ci/g, P <= 0.01 versus sham), peaking at day 28 (48.9 +/- 5.1 mu Ci/g, P <= 0.0001 versus sham). Histological and proteomic profiling of alpha 7nAChR positive tissue revealed strong associations between alpha 7nAChR and extracellular matrix deposition, and rat cardiac fibroblasts expressed alpha 7nAChR protein under normoxic and hypoxic conditions.Conclusion alpha 7nAChR is highly expressed in human macrophages and showed proliferation and hypoxia-driven expression in human endothelial cells. While NS14490 imaging displays a pattern that coincides with vessel formation, macrophage infiltration and fibrovascular encapsulation in the sponge model, this is not the case in the MI model where the alpha 7nAChR imaging signal was strongly associated with extracellular matrix deposition which could be explained by alpha 7nAChR expression in fibroblasts. Overall, these findings support the involvement of alpha 7nAChR across several processes central to cardiac repair, with fibrosis most closely associated with alpha 7nAChR following MI.