Abstract

Through ligation of common carotid arteries (CCA) in rabbits, we create a flow-induced intracranial aneurysm at the apex of basilar artery bifurcation (basilar terminus, BT). Correlation of image-based CFD simulation results with histology suggests that the hemodynamic microenvironment at a bifurcation apex (characterized by high shear stress and high gradient of shear stress) lead to destructive remodeling, which evolved into a saccular aneurysm at the BT. This is consistent with findings from histology-hemodynamic mapping in a created bifurcation in canine carotid arteries^1,2. However, our new results further confirmed the initiation of a clinically relevant intracranial aneurysm by increased hemodynamics in an animal model. In addition to BT aneurysm, we also observed adaptive remodeling of basilar artery (BA) trunk, which enlarged and developed tortuosity in response to increased flow. This flow-induced adaptive remodeling affords comparison, at the cellular and molecular levels, with the destructive, aneurysm-initiating remodeling at the BT and helps delineating vascular responses under the different hemodynamic stimuli. By using this in vivo intracranial aneurysm model, we may elucidate the cellular and molecular mechanisms of the flow-induced aneurysm development, thus opening the door for noninvasive pharmacological intervention.

Intracranial Aneurysm

Aneurysm is a vascular remodeling that has gone awry in response to hemodynamic, oxidative and other stressors. In contrast to healthy adaptive remodeling, aneurysm initiation results from overly aggressive, mal-adaptive remodeling of the wall. Understanding the mechanism at the molecular level will open the door for noninvasive pharmacological intervention.

      However, a clear understanding of the mechanism between hemodynamics and vascular response is hampered by the lack of a naturally occurring intracranial aneurysm animal model to allow the correlation between local flow dynamics and biological processes. The current study aims at creating a naturally occurring and cost effective intracranial aneurysm animal model, and using this model to correlate between local flow dynamics and biological processes that distinguish destructive aneurysm remodeling and healthy adaptive vascular enlargement.

Existing Aneurysm Models

●  Extracranial Models: only suitable for testing endovascular devices

1. Vein pouch³: Wrong anatomy and hemodynamics

2.  Elastase model⁴: Artificial degeneration of matrix, not reflecting biology and natural history of intracranial aneurysm

●  Intracranial Models:

1. Primate model⁵: Expensive

2. Rodent model⁶: Small vessels prevent accurate imaging

A New Intracranial Aneurysm Model

 We have generated a new intracranial aneurysm rabbit model at basilar tip (BT) by either unilateral or bilateral common carotid artery (CCA) ligation. This model is suitable for studying hemodynamics and biological mechanisms underlying spontaneous aneurysm development and growth. The advantages of this model are:

● There is no surgical manipulation of vessel of interest

● Chronically increased flow due to bilateral or unilateral CCA occlusion is a likely etiological factor in a clinical important location on the Circle of Willis.

● Rabbit vessel size is suitable for rotational angiographic imaging and computational flow dynamics.

● Simple manipulation of flow, without hypertension or pharmacological weakening of vessel wall that had been used in Primate and Rodent models, can precisely resolve the role of hemodynamic stressors.

One Model, Two Remodeling Processes

Flow-Induced Destructive Remodeling - Aneurysm at BT

At 10 weeks post unilateral CCA ligation, an aneurysm was developed at BT, a pouch with thinned wall between “a” and “b”. Van Gieson staining indicated that IEL (thick black line) disappears in the aneurysm wall. Trichrome staining indicated that medial SMC (red) was depleted in the aneurysm wall. H&E staining further indicated that medial SMC and endothelial cells are gone in the aneurysm wall. The histological findings resemble the characteristic of a human intracranial aneurysm.

Adaptive remodeling in BA - BA enlargement

Flow Increase in Rabbit Basilar Artery Post CCA ligation

Hemodynamics and Molecular Response Comparison of two remodeling processes allows finding what “ goes wrong” at BT that leads to aneurysm initiation, and how molecular responses in pathogenesis differs from those in adaptive remodeling at BA. The combination of 3D rotational angiography and computational fluid dynamics (CFD) analysis enables the quantification of hemodynamics stresses at the basilar artery and Circle of Willis.     To enable mapping and correlation of hemodynamics and resulting vascular remodeling at BT (and BA) with local cellular and molecular responses, we examined the expression and activity of matrix metalloproteinases (MMPs) that degrade the vessel wall, and nitric oxide (NO), a known regulator of vascular remodeling.     Our current data indicates that MMP activity in both BA and BT increased approximately 2 folds at 2 weeks post bilateral CCA ligation. At 4 weeks post ligation, MMP activity of BA decreased to the level similar to that prior to ligation.      References 1. Meng H, Swartz, DD, Wang, Z, Hoi, Y, Kolega, J, Metaxa, EM, Szymanski, MP, Yamamoto, J, Sauvageau, E, Levy, EI. A model system for mapping vascular responses to complex hemodynamics at arterial bifurcations in vivo. Neurosurgery. 2006; 59:1094-1101. 2. Meng H, Wang, Z, Hoi, Y, Gao, L, Metaxa, EM, Swartz, DD, Kolega, J. Complex Hemodynamics at the Apex of an Arterial Bifurcation Induces Vascular Remodeling Resembling Cerebral Aneurysm Initiation. Stroke. 2007;38:1924-1931. 3. Spetzger U, Reul J, Weis J, Bertalanffy H, Thron A, Gilsbach JM. Microsurgically produced bifurcation aneurysms in a rabbit model for endovascular coil embolization. J Neurosurg. 1996;85:488-495. 4. Miskolczi L, Guterman LR, Flaherty JD, Hopkins LN. Saccular aneurysm induction by elastase digestion of the arterial wall: A new animal   model. Neurosurgery. 1998;43:595-600; discussion 600-591. 5. Hashimoto N, Kim C, Kikuchi H, Kojima M, Kang Y, Hazama F. Experimental induction of cerebral aneurysms in monkeys. J Neurosurg. 1987;67:903-905. 6. Morimoto M, Miyamoto S, Mizoguchi A, Kume N, Kita T, Hashimoto N. Mouse model of cerebral aneurysm: Experimental induction by renal hypertension and local hemodynamic changes. Stroke. 2002;33:1911-1915.

Acknowlegements

Contributions from the imaging and clinical groups at TSRC.

● Imaging Group: Stephen Rudin, Ph.D., Kenneth Hoffmann, Ph.D., Ciprian Ionita, Ph.D.

● Clinical Group: Adnan Siddiqui, Ph.D. M.D., Elad Levy, M.D. , L. N. Hopkins, M.D. 

This research is supported by grants from NIH (1K25 NS047242, 1R01 EB002873) and Cummings Foundation.