Background and Purpose-Abnormal vascular remodeling triggered by hemodynamic stresses and inflammation is believed to be a key process in the pathophysiology of intracranial aneurysms. Numerous studies have shown infiltration of inflammatory cells, especially macrophages, into intracranial aneurysmal walls in humans. Using a mouse model of intracranial aneurysms, we tested whether macrophages play critical roles in the formation of intracranial aneurysms. Methods-Intracranial aneurysms were induced in adult male mice using a combination of a single injection of elastase into the cerebrospinal fluid and angiotensin II-induced hypertension. Aneurysm formation was assessed 3 weeks later. Roles of macrophages were assessed using clodronate liposome-induced macrophage depletion. In addition, the incidence of aneurysms was assessed in mice lacking monocyte chemotactic protein-1 (CCL2) and mice lacking matrix metalloproteinase-12 (macrophage elastase). Results-Intracranial aneurysms in this model showed leukocyte infiltration into the aneurysmal wall, the majority of the leukocytes being macrophages. Mice with macrophage depletion had a significantly reduced incidence of aneurysms compared with control mice (1 of 10 versus 6 of 10; PϽ0.05). Similarly, there was a reduced incidence of aneurysms in mice lacking monocyte chemotactic protein-1 compared with the incidence of aneurysms in wild-type mice (2 of 10 versus 14 of 20, PϽ0.05). There was no difference in the incidence of aneurysms between mice lacking matrix metalloproteinase-12 and wild-type mice. Conclusions-These

Abstract-Mechanisms of formation and growth of intracranial aneurysms are poorly understood. To investigate the pathophysiology of intracranial aneurysms, an animal model of intracranial aneurysm yielding a high incidence of large aneurysm formation within a short incubation period is needed. We combined two well-known clinical factors associated with human intracranial aneurysms, hypertension and the degeneration of elastic lamina, to induce intracranial aneurysm formation in mice. Roles of matrix metalloproteinases (MMPs) in this model were investigated using doxycycline, a broad-spectrum MMP inhibitor, and MMP knockout mice. Hypertension was induced by continuous infusion of angiotensin II for 2 weeks. The disruption of elastic lamina was achieved by a single stereotaxic injection of elastase into the cerebrospinal fluid at the right basal cistern. A total of 77% of the mice that received 35 milliunits of elastase and 1000 ng/kg per minute of angiotensin II developed intracranial aneurysms in 2 weeks. There were dose-dependent effects of elastase and angiotensin II on the incidence of aneurysms. Histologically, intracranial aneurysms observed in this model closely resembled human intracranial aneurysms. Doxycycline, a broad-spectrum MMP inhibitor, reduced the incidence of aneurysm to 10%. MMP-9 knockout mice, but not MMP-2 knockout mice, had reduced the incidence of intracranial aneurysms. In summary, a stereotaxic injection of elastase into the basal cistern in hypertensive mice resulted in intracranial aneurysms that closely resembled human intracranial aneurysms. The intracranial aneurysm formation in this model appeared to depend on MMP activation. Key Words: intracranial aneurysm Ⅲ subarachnoid hemorrhage Ⅲ mice Ⅲ elastase Ⅲ models Ⅲ matrix metalloproteinase I ntracranial aneurysms are considered to be common among the general population. Subarachnoid hemorrhage from ruptured intracranial aneurysms results in catastrophic consequences causing severe morbidity and high mortality. 1 Despite recent advances in diagnosis and treatment, the mechanisms for the formation, growth, and subsequent rupture of intracranial aneurysms are not yet well understood.Clinically, systemic hypertension is associated with intracranial aneurysm formation and subarachnoid hemorrhage from aneurysmal rupture. 2-4 However, a causal relationship between hypertension and intracranial aneurysm formation or subarachnoid hemorrhage has not been fully established. Histologically, degeneration and disruption of the elastic lamina are key characteristics of human intracranial aneurysm. 2,5,6 Degeneration or disruption of elastic lamina may be attributed to the normal aging process or damages caused by hemodynamic stresses. 7 Such changes in the elastic lamina have often been considered as preaneurysmal changes that eventually lead to the maturation of aneurysms. 8,9 Elastaseinduced fragmentation of elastic lamina has been successfully used to induce aneurysms in the carotid artery 10,11 and aorta 12 in animals. In these models, because of...

Background and Purpose An increasing number of unruptured intracranial aneurysms are being detected, partly due to the increased use of brain imaging techniques. Pharmacological stabilization of aneurysms for the prevention of aneurysmal rupture could potentially be an attractive alternative approach to clipping or coiling in patients with unruptured intracranial aneurysms. We have developed a mouse model of intracranial aneurysm that recapitulates key features of intracranial aneurysms. In this model, subarachnoid hemorrhage from aneurysmal rupture causes neurological symptoms that can be easily detected by a simple neurological examination. Using this model, we tested whether anti-inflammatory agents such as tetracycline derivatives, or a selective inhibitor of matrix metalloproteinases-2 and -9 (SB-3CT) can prevent the rupture of intracranial aneurysms. Methods Aneurysms were induced by a combination of induced hypertension and a single injection of elastase into the cerebrospinal fluid in mice. Treatment with minocycline, doxycycline, or SB-3CT was started six days after aneurysm induction. Aneurysmal rupture was detected by neurological symptoms and confirmed by the presence of intracranial aneurysm with subarachnoid hemorrhage. Results Minocycline and doxycycline significantly reduced rupture rates (vehicle vs. doxycycline = 80 vs. 35%, P < 0.05; vehicle vs. minocycline = 73 vs. 24%, P < 0.05) without affecting the overall incidence of aneurysms. However, SB-3CT did not affect the rupture rate (62 vs. 55%, P = 0.53). Conclusions Our data established the feasibility of using a mouse model of intracranial aneurysm to test pharmacological stabilization of aneurysms. Tetracycline derivatives could be potentially effective in preventing aneurysmal rupture.

Aortic aneurysms are common among the elderly population. 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 (1) two different methods for inducing hypertension, and (2) anti-hypertensive agents. In nine-week-old C57BL/6J male mice, hypertension was induced by angiotensin-II or deoxycorticosterone acetate (DOCA)-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 (38-50% and 30-49 %, respectively). Aneurysms were found at the two aneurysm-prone regions with site-specific morphological and histological characteristics. Treatment with anti-hypertensive agent, amlodipine, normalized blood pressure and dramatically reduced aneurysm formation in the mice that received angiotensin-II and beta-aminopropionitrile. However, a treatment with captopril, 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 was dependent on hypertension, but not on direct effects of angiotensin-II to the vascular wall.