Brain arteriovenous malformations (bAVMs) substantially increase the risk for intracerebral hemorrhage (ICH), which is associated with significant morbidity and mortality. However, the treatment options for bAVMs are severely limited, primarily relying on invasive methods that carry their own risks for intraoperative hemorrhage or even death. Currently, there are no pharmaceutical agents shown to treat this condition, primarily due to a poor understanding of bAVM pathophysiology. For the last decade, bAVM research has made significant advances, including the identification of novel genetic mutations and relevant signaling in bAVM development. However, bAVM pathophysiology is still largely unclear. Further investigation is required to understand the detailed cellular and molecular mechanisms involved, which will enable the development of safer and more effective treatment options. Endothelial cells (ECs), the cells that line the vascular lumen, are integral to the pathogenesis of bAVMs. Understanding the fundamental role of ECs in pathological conditions is crucial to unraveling bAVM pathophysiology. This review focuses on the current knowledge of bAVM-relevant signaling pathways and dysfunctions in ECs, particularly the endothelial-to-mesenchymal transition (EndMT).

Somatic KRAS (KRAS ^G12V ) mutation in endothelial cells (EC) induces brain arteriovenous malformation (bAVM) that could lead to vascular instability and ultimately bleeding. However, the causes of bAVM instability remain unclear. Here we demonstrate that KRAS ^G12V expressing cultured ECs (KRAS-G12V-EC) have increased expression of pro-inflammatory mediators and reduced expression of blood-brain-barrier (BBB) junction constituents. The conditioned medium from KRAS-G12V-EC can activate BV2-microglia (BV2-MG) and conditioned media from this primed BV2-MG can compromise the expression of EC-junction constituents when added to wild-type ECs. In an in vitro BBB model, KRAS-G12V-EC form a dysfunctional EC barrier that is further disrupted, leading to lower transendothelial electrical resistance and increased FITC-dextran leakage when blood-derived macrophages (Mφ) are included. KRAS-G12V-EC potently stimulates BV2-MG chemotaxis. In vitro BBB leakage and BV2-MG chemotaxis were inhibited by treatment with the anti-inflammatory drug minocycline in the KRAS-G12V-EC. In our bAVM mouse model that uses AAV-BR1-KRAS ^G12V injection to produce EC expression of KRAS ^G12V in the brain, minocycline injection reduced production of inflammatory cytokines by bAVM nidus, reduced leakage of BSA-647, and restored VE-cadherin expression on malformed vessels. Our findings suggest that KRAS-G12V-EC can activate local MG/Mφ that causes vascular inflammation and instability of malformed vessels in bAVM.

Background: Flow diverter (FD) use has become a popular method for treating intracranial aneurysms, but evidence in acutely ruptured blood blister-like and dissecting morphologies is limited. Furthermore, there is little evidence regarding the role of short-term follow-up imaging to assess aneurysm stability. This study is a single-center retrospective review of dissecting aneurysms treated by FD, with an analysis of short-term follow-up angiograms to guide post-treatment management. Methods: Our single-center retrospective review included all patients from 2016 to 2024 with spontaneous subarachnoid hemorrhage from blood blister-like and dissecting aneurysms who underwent treatment with an FD. Outcomes included re-rupture, treatment failure, re-treatment, aneurysm morphological changes, and ischemic/hemorrhagic events. Short-term, inpatient angiograms were used to assess early response to treatment, and long-term follow-up angiogram, >4 months, was used to assess long-term aneurysm obliteration. Results: Twenty-eight patients underwent FD for treatment of their ruptured dissecting aneurysm. Twelve patients (43%) were treated with coiling plus FD. Treatment failure occurred in four patients (14%). Placement of additional FDs in a second procedure occurred in three patients (11%). Thromboembolic events occurred in five patients (18%). Hemorrhagic complications, not including aneurysm re-rupture, occurred in six patients (21%). Short-term aneurysm improvement occurred in 17 patients (63%). Thirteen patients had long-term follow-up, with 11 (85%) demonstrating complete occlusion. Conclusions: Flow diversion is a reasonable technique for treating ruptured dissecting aneurysms. Risks are moderate and include short-term aneurysm growth and re-rupture. Early post-treatment angiography is suggested to help identify aneurysm growth and the need for re-treatment.

BACKGROUND: Brain arteriovenous malformations (bAVMs) are a major risk factor for intracerebral hemorrhages in young patients. Recent clinical studies have reported that the majority of human bAVMs harbor somatic KRAS (Kirsten rat sarcoma viral oncogene homolog) mutations. In our previous study, we confirmed the causal role of endothelial KRAS ^G12V mutation in bAVM development in mice through hyperactivation of the MEK/ERK (mitogen-activated protein kinase kinase/extracellular signal–regulated kinase) pathway. However, the treatment efficacy of targeting MEK/ERK signaling in bAVMs, regarding clinical and biological outcomes, remains unclear. METHODS: In this study, we used the US Food and Drug Administration–approved MEK inhibitor, trametinib, to treat mice with bAVMs induced by brain endothelial cell (EC)–specific KRAS ^G12V overexpression (KRAS ^G12V/bEC mice). We treated KRAS ^G12V/bEC mice with either vehicle or trametinib and evaluated outcomes on mortality and the alteration of bAVMs, and the hemorrhages. We also determined the effect of trametinib on the endothelial-to-mesenchymal transition, which has recently been explored in human bAVMs and implicated in bAVM pathology, in bAVMs of KRAS ^G12V/bEC mice and cultured mouse ECs transfected with KRAS ^G12V (EC ^G12V ). RESULTS: We observed that trametinib significantly improved survivability, potentially through improved vessel integrity and reduction in the severity of bAVM ruptures. Our longitudinal observation of mice bAVMs using noninvasive magnetic resonance imaging revealed that trametinib decreased the total hemorrhagic volume at 8 weeks of treatment when administered at 1 mg/kg compared with the vehicle group, which was confirmed by histological analyses. In addition, trametinib altered endothelial-to-mesenchymal transition by reducing the expression levels of mesenchymal markers (N-Cad [N-cadherin], Slug, CD44 [cluster of differentiation 44]) and increasing an EC marker (VE-Cad [vascular endothelial cadherin]) in bAVMs of KRAS ^G12V/bEC mice. Trametinib also decreased proliferation (Ki-67) and angiogenic (CD34 and pVEGFR2 [phosphor-vascular endothelial growth factor receptor 2]) markers in KRAS ^G12V/bEC mice’s bAVMs. We also confirmed that trametinib normalized the endothelial-to-mesenchymal transition-associated marker expression and functions in EC ^G12V . CONCLUSIONS: Our data demonstrate that trametinib improves bAVM pathology by reducing hemorrhagic risk and normalizing endothelial-to-mesenchymal transition. These findings suggest that trametinib is a promising agent to treat patients with bAVM.

Mutation of KRAS in endothelial cells (KRAS-ECs) leads to intracerebral hemorrhage (ICH) in brain arteriovenous malformation (bAVM), resulting in severe disabilities or even death. However, it is unclear what causes this hemorrhagic conversion of bAVMs. Here, using a locally established, clinically relevant sporadic bAVM mouse model, created by overexpressing mutant KRAS (KRAS ^G12V ) in brain ECs, we demonstrate that KRAS-ECs act as trigger for activation of microglia (MG) and infiltration of macrophages (Mϕ). Using a 3-dimensional immunostaining approach with cleared human and mouse bAVM tissues, we demonstrate an abundance of MG/Mϕ around the bAVM nidus. The presence of MG/Mϕ was correlated to the blood-brain barrier leakage in bAVM areas. Time-lapsed intravital imaging in Cx3cr1-gfp;Ccr2-rfp reporter mice demonstrated the dynamic activation of MG and infiltration of Mϕ toward mutant KRAS ^G12V –modified dysplastic vessels. Importantly, a time-course analysis showed that these activated MG and infiltrated Mϕ are present around the bAVMs prior to hemorrhagic conversion, and controlled depletion of MG/Mϕ reduced ICH incidence in bAVMs. Inhibition of MG/Mϕ with long-term minocycline treatment attenuated the incidence of ICHs around bAVMs. Our study indicates that MG/Mϕ are involved in destabilization of KRAS ^G12V -induced bAVM, leading to hemorrhagic conversion/ICH. Thus, modulation of MG/Mϕ may reduce ICH risk in patients with bAVM.