Repeated social defeat exaggerates CaCl2-induced abdominal aortic aneurysm expansion by eliminating periaortic fibrosis in tissue repair phase: possible involvement of specific subtypes of macrophages
Authors: Kubota H , Yamada H , Sugimoto T , Miyawaki D , Saburi M , Wakana N , Matoba S 
Published: November 2020

Abstract

Background and objective
Depression is an independent risk factor of cardiovascular disease (CVD) and significantly associated with the prevalence of abdominal aortic aneurysm (AAA). We have recently shown that repeated social defeat (RSD) precipitates depressive-like behaviors in apoE−/− mice and exaggerates atherosclerosis development by enhancing leukocyte activation. Here, we investigated the impact of RSD on AAA formation.

Methods and results
Eight-week-old male WT mice were exposed to RSD by housing with a larger CD-1 mouse in a shared home cage. They were subjected to vigorous physical contact daily for 10 consecutive days. Control mice were housed in the same gage without physical contact. After social interaction test to confirm depressive-like behaviors, defeated mice (28 of 48) and control mice (31 of 36) underwent application of 0.5 M calcium chloride (CaCl2) on the infrarenal aorta to induce AAA. At 1 week after application, maximum diameter and circumference of external elastic membrane were comparable between the two groups. The number of F-4/80, MMP-9, and TNF-α-positive cells in immunofluorescent images were also comparable. Further, in vitro bone marrow derived macrophages stimulation by LPS did not show any difference in mRNA expression levels of inflammatory cytokines, suggesting no discernable difference in acute inflammatory response between the two groups. In contrast, at 2 weeks after application, at the time point when MMP-9 and TNF-α-positive cells were scarcely observed, maximum diameter and circumference of external elastic membrane were significantly increased in defeated mice (0.72 mm vs. 0.90 mm, 1.59 mm vs. 2.00 mm, respectively, Control vs. Defeat, p&lt;0.01). Intriguingly, periaortic fibrotic area in aneurysmal portion was markedly decreased in defeated mice (12.5×103 μm2 vs. 3.7×103 μm2, Control vs. Defeat, p&lt;0.01). Consistently, accumulation of α-SMA-positive cells in adventitia of aneurysmal portion was much less in defeated mice than control mice (876 cells/mm2 vs. 319 cells/mm2, Control vs. Defeat, p&lt;0.05), whereas those in tunica media of non-aneurysmal portion did not show any difference between the two groups. We next focused on the segregated nucleus-containing atypical monocyte (SatM), specific subtypes of monocytes/macrophages that are involved in fibrosis in injured tissues during the healing phase. We could observe SatM fraction in AAA tissue of control mice using flow cytometry. We also found that mRNA expression level of C/EBPβ, an essential regulator for SatM differentiation, was markedly decreased by 76% in BM cells of defeated mice compared with control mice (p&lt;0.05).

Conclusions
Our findings demonstrate for the first time that RSD enhances AAA expansion by eliminating periaortic fibrosis in tissue repair phase, suggesting that the impaired resolution of acute inflammation after CaCl2 application contributes, at least in part, to the augmented expansion of AAA in defeated mice.

Funding Acknowledgement
Type of funding source: None