Hemodynamically induced molecules regulating the initiation of intracranial aneurysms

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Project Summary Ruptured intracranial aneurysms (IAs) are the main cause of non-traumatic subarachnoid hemorrhage, the most severe form of stroke. Discovering ways to prevent IAs from forming could dramatically impact patients’ lives, but understanding of why and how IAs form is limited. This proposal aims to identify molecular signals that initiate IAs by examining arterial gene expression during IA formation. Hemodynamics play a crucial role in IAs. Arteries respond to changes in blood flow by remodeling to keep fluid shear stress at baseline levels without loss of vessel strength and integrity. But in IAs, remodeling thins and weakens the arterial wall. It is hypothesized that IAs form when flow induces endothelial cells in the intima to produce signals that cause maladaptive responses in the media. In a proof-of concept study, the investigators used RNAseq of microdissected cerebral arteries in rabbits to observe gene expression in the intima under aneurysm-inducing flow, and in the contiguous media where dystrophic remodeling occurs. The results demonstrated that this approach can reveal potential regulators of IA formation by identifying intimal genes that are uniquely expressed in nascent IAs and whose expression correlates with destructive medial responses. A comprehensive screen is proposed for genes that control dystrophic remodeling during IA formation. This will be done using a rabbit model, in which ligation of the carotid arteries increases flow in the posterior circulation, causing constructive enlargement of the basilar artery while an IA forms at the basilar terminus. Gene expression will be measured by RNAseq in intima and media that are laser microdissected from the basilar artery and terminus, and transcriptomes from ligated and unligated rabbits will be compared to identify flow-induced changes. Changes at the basilar terminus will be compared with the basilar artery to reveal genes that are unique to dystrophic IA remodeling. Correlation analysis will then be performed on intimal and medial gene pairs to detect potential signal-response relationships. Preliminary studies suggest that BMP2 is one such signal for IA formation. To test this, rabbits will be treated with JL5, an inhibitor of BMP2-receptor activity, while IAs are induced by carotid ligation. Tissues will be assessed for (a) expression of medial genes that are characteristic of aneurysmal remodeling, using RNAseq, and (b) initiation of aneurysmal damage, as determined histologically. It is predicted that disrupting the BMP2 signaling pathway will prevent dystrophic responses in the media and inhibit flow-induced IA-initiating damage. This project will identify regulatory pathways acting specifically during pathological remodeling that leads to IAs. In addition, it will provide unprecedented characterization of gene expression during trophic and dystrophic arterial remodeling. Understanding the molecular mechanisms behind vascular responses to flow will inform strategies for prevention and treatment of pathological remodeling events, and could ultimately lead to pharmacological interventions for clinical mitigation of IAs.