Pharmacologically induced thoracic and abdominal aortic aneurysms in mice

Abstract

Aortic aneurysms are common among the elderly population. A large majority of aortic aneurysms are located at two distinct aneurysm-prone regions, the abdominal aorta and thoracic aorta involving the ascending aorta. In this study, we combined two factors that are associated with human aortic aneurysms, hypertension and degeneration of elastic lamina, to induce an aortic aneurysm in mice. Roles of hemodynamic conditions in the formation of aortic aneurysms were assessed using two different methods for inducing hypertension and antihypertensive agents. In 9-week-old C57BL/6J male mice, hypertension was induced by angiotensin II or deoxycorticosterone acetate-salt hypertension; degeneration of elastic lamina was induced by infusion of beta-aminopropionitrile, a lysyl oxidase inhibitor. Irrespective of the methods for inducing hypertension, mice developed thoracic and abdominal aortic aneurysms (38% to 50% and 30 to 49%, respectively). Aneurysms were found at the two aneurysm-prone regions with site-specific morphological and histological characteristics. Treatment with an antihypertensive agent, amlodipine, normalized blood pressure and dramatically reduced aneurysm formation in the mice that received angiotensin II and beta-aminopropionitrile. However, treatment with captopril, an angiotensin-converting enzyme inhibitor, did not affect blood pressure or the incidence of aortic aneurysms in the mice that received deoxycorticosterone acetate-salt and beta-aminopropionitrile. In summary, we have shown that a combination of hypertension and pharmacologically induced degeneration of elastic laminas can induce both thoracic and abdominal aortic aneurysms with site-specific characteristics. The aneurysm formation in this model depended on hypertension but not on direct effects of angiotensin II to the vascular wall.

Figure 1. Combination of angiotensin-II induced hypertension and lysyl oxidase inhibition by BAPN resulted in aortic aneurysm formation in mice

A: Incidence of aortic aneurysms. 38% and 49% of the mice that received angiotensin-II and BAPN developed thoracic and abdominal aortic aneurysms, respectively. No aneurysm formation was found in the mice that received angiotensin-II or BAPN alone. B: Representative aortic aneurysms. Macroscopically, thoracic and abdominal aortic aneurysms in this model resembled human aortic aneurysms with their site-specific morphology. Thoracic aortic aneurysms were saccular-shaped with localized dilation at the great curvature, while abdominal aortic aneurysms were fusiform-shaped aneurysms with a thick vascular wall. a: normal thoracic aorta, b-c: unruptured thoracic aortic aneurysm, d: ruptured thoracic aortic aneurysm, e: normal abdominal aorta, f-g: unruptured abdominal aortic aneurysm, h: ruptured abdominal aortic aneurysm, i: dissecting aortic aneurysm. Scale bar: 1mm. C: Systolic blood pressure in the mice that received both angiotensin-II and BAPN, and the mice that received BAPN or angiotensin-II alone. Systolic blood pressures of mice that received angiotensin-II alone or combination of angiotensin-II and BAPN are significantly higher than systolic blood pressures of mice in the control group at 3 and 6 weeks. Mean ± SD, *: P < 0.05 compared with control. TAo : thoracic aorta, AAo : abdominal aorta, BT : brachiocephalic trunk, CCA : left common carotid, artery, SA : left subclavian artery, RA : renal artery, IA : iliac arteries, Rt. : right, Lt. : left.

Figure 2. Histological assessment of thoracic and abdominal aortic aneurysms

Thoracic and abdominal aortic aneurysms in this mouse model closely resembled human thoracic and abdominal aortic aneurysms with their site-specific histological characteristics. Control aortas, thoracic and abdominal aneurysms were stained for H&E, elastic laminas, Elastica von Gieson, Trichrome, fibroblasts, smooth muscle cells, and endothelial cells. *: lumen. Scale bar: 0.1mm.

Figure 3. Inflammatory cells in thoracic and abdominal aortic aneurysms

Stainings for pan-leukocytes (CD45), macrophages (CD68), helper T-lymphocytes (CD4) and B-lymphocytes (CD19). In thoracic aneurysms, leukocytes were observed in the adventitia and media (B2, B6). In contrast, leukocytes were highly concentrated in the thick wall near the intramural thrombus in abdominal aortic aneurysms (D6). In both thoracic and abdominal aneurysms, majority of leukocytes appeared to be macrophages (B2-5, B6-9, D2-5, D6-9). *: lumen. Scale bar: 0.1mm. Arrows point to positive cells.

Figure 4. Roles of hypertension in aneurysm formation

A & B: Incidence of thoracic and abdominal aortic aneurysms. Both angiotensin-II and DOCA-salt induced hypertension were able to induce aortic aneurysms when combined with the treatment of BAPN. Anti-hypertensive agent (anti-HTN), amlodipine, dramatically reduced the incidence of aortic aneurysms. *: P < 0.05 compared to angiotensin-II and BAPN group. C: Aortic aneurysms in the mice treated with DOCA-salt and BAPN were indistinguishable from those observed in mice that were treated with angiotensin-II and BAPN. Amlodipine treatment in the mice that were receiving angiotensin-II and BAPN resulted in a complete lack of adventitial inflammation and thickening of the media and adventitia. Scale bar: 0.1mm. *: lumen. D: Systolic blood pressure. Amlodipine significantly reduced blood pressure in the mice that were treated with angiotensin-II and BAPN. There was no difference in blood pressure between mice receiving DOCA-salt and BAPN and mice receiving angiotensin-II and BAPN. Mean ± SD. *: P < 0.05 compared to control. E: There was no significant effect of amlodipine on blood flow rates.