Protective role of Metrnl on macrophage recruitment during the acute inflammatory phase in a mouse model of myocardial infarction

Abstract Introduction Meteorin-like protein (Metrnl) is a cytokine involved in the attenuation of inflammation. Previously, a cardioprotective role of Metrnl against cardiac hypertrophy and ischemia/reperfusion has been described. After myocardial infarction (MI), macrophages recruited from the spleen, and their capacity to shift from inflammatory (M1) to reparative (M2) phases determines the fate of cardiac repair. Purpose The aim of this study was to analyse the function of Metrnl in a mouse model of permanent MI during the acute inflammatory phase. Methods Wild-type (WT), Metrnl Knock-out (Metrnl-/-) and Metrnl-reconstituted Metrnl-/- mice (AAV9-Metrnl, with myocardial-specific tropism) were subjected to MI by coronary artery ligation. After 4 days, cardiac function was assessed using echocardiography at baseline and 4 days post-MI. Monocyte populations were analysed in spleen and blood by flow cytometry, and macrophages infiltrating the infarcted myocardium by gene expression analyses. Results Echocardiographic analysis demonstrated that Metrnl deficiency led to cardiac dysfunction (FAC p=0.02) accompanied by myocardial dilation 4 days post-MI (EDV p=0.02; ESV p=0.01). On the contrary, the overexpression of Metrnl in the myocardium of AVV9-Metrnl mice restored cardiac function compared to Metrnl-/- group (FAC p=0.01, EDV p=0.04, ESV p=0.09). Furthermore, we observed a reduction in splenic monocytes (CD11b+F4/80+) in Metrnl-/- mice compared to the WT group at 4 days post-MI (p=0.06). In particular, a significant decrease in pro-inflammatory Ly6CHigh and CD86+ monocytes was noted in the spleens of Metrnl-/- mice compared to the WT group (p=0.03; p=0.04 respectively), suggesting their mobilization out of the spleen, but not in the AVV9-Metrnl group. Although the total number of Ly6CHigh and Ly6CLow monocytes in the bloodstream was similar in all groups, there was a trend to different expression in the CCR2 and CCR5 chemokine receptors (p=0.07; p=0.06 respectively). At the same time, M1 macrophage recruitment chemokines (CCL2) and inflammatory markers (TNFα) were significantly induced in the ischemic area of the infarcted myocardium compared to the remote zone in the WT and Metrnl-/- groups (CCL2 p=0.03; p=0.01 respectively; TNFα p=0.008;p=0.04 respectively). In contrast, TNFα and CCL2 genes were not induced in the ischemic area of AVV9-Metrnl mice. Regarding M2 macrophages, the Arg1 marker was significantly reduced in the ischemic zone of Metrnl-/- mice compared to the WT group (p= 0.01), while in the AVV9-Metrnl group was recovered (p= 0.04). Conclusions Our results demonstrate that the absence of Metrnl promotes the recruitment of inflammatory macrophages to the ischemic myocardium and is associated to cardiac dysfunction.