Cerebral cavernous malformation (CCM) is a vascular dysplasia, mainly localized within the brain and affecting up to 0.5% of the human population. CCM lesions are formed by enlarged and irregular blood vessels that often result in cerebral haemorrhages. CCM is caused by loss-of-function mutations in one of three genes, namely CCM1 (also known as KRIT1), CCM2 (OSM) and CCM3 (PDCD10), and occurs in both sporadic and familial forms. Recent studies have investigated the cause of vascular dysplasia and fragility in CCM, but the in vivo functions of this ternary complex remain unclear. Postnatal deletion of any of the three Ccm genes in mouse endothelium results in a severe phenotype, characterized by multiple brain vascular malformations that are markedly similar to human CCM lesions. Endothelial-to-mesenchymal transition (EndMT) has been described in different pathologies, and it is defined as the acquisition of mesenchymal- and stem-cell-like characteristics by the endothelium. Here we show that endothelial-specific disruption of the Ccm1 gene in mice induces EndMT, which contributes to the development of vascular malformations. EndMT in CCM1-ablated endothelial cells is mediated by the upregulation of endogenous BMP6 that, in turn, activates the transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) signalling pathway. Inhibitors of the TGF-β and BMP pathway prevent EndMT both in vitro and in vivo and reduce the number and size of vascular lesions in CCM1-deficient mice. Thus, increased TGF-β and BMP signalling, and the consequent EndMT of CCM1-null endothelial cells, are crucial events in the onset and progression of CCM disease. These studies offer novel therapeutic opportunities for this severe, and so far incurable, pathology.

Cerebral cavernous malformations (CCMs) are vascular malformations located within the central nervous system often resulting in cerebral hemorrhage. Pharmacological treatment is needed, since current therapy is limited to neurosurgery. Familial CCM is caused by loss‐of‐function mutations in any of Ccm1, Ccm2, and Ccm3 genes. CCM cavernomas are lined by endothelial cells (ECs) undergoing endothelial‐to‐mesenchymal transition (EndMT). This switch in phenotype is due to the activation of the transforming growth factor beta/bone morphogenetic protein (TGFβ/BMP) signaling. However, the mechanism linking Ccm gene inactivation and TGFβ/BMP‐dependent EndMT remains undefined. Here, we report that Ccm1 ablation leads to the activation of a MEKK3‐MEK5‐ERK5‐MEF2 signaling axis that induces a strong increase in Kruppel‐like factor 4 (KLF4) in ECs in vivo. KLF4 transcriptional activity is responsible for the EndMT occurring in CCM1‐null ECs. KLF4 promotes TGFβ/BMP signaling through the production of BMP6. Importantly, in endothelial‐specific Ccm1 and Klf4 double knockout mice, we observe a strong reduction in the development of CCM and mouse mortality. Our data unveil KLF4 as a therapeutic target for CCM.

Cerebral cavernous malformation (CCM) is a major cerebrovascular disease affecting approximately 0.3–0.5% of the population and is characterized by enlarged and leaky capillaries that predispose to seizures, focal neurological deficits, and fatal intracerebral hemorrhages. Cerebral cavernous malformation is a genetic disease that may arise sporadically or be inherited as an autosomal dominant condition with incomplete penetrance and variable expressivity. Causative loss-of-function mutations have been identified in three genes, KRIT1 (CCM1), CCM2 (MGC4607), and PDCD10 (CCM3), which occur in both sporadic and familial forms. Autophagy is a bulk degradation process that maintains intracellular homeostasis and that plays essential quality control functions within the cell. Indeed, several studies have identified the association between dysregulated autophagy and different human diseases. Here, we show that the ablation of the KRIT1 gene strongly suppresses autophagy, leading to the aberrant accumulation of the autophagy adaptor p62/SQSTM1, defective quality control systems, and increased intracellular stress. KRIT1 loss-of-function activates the mTOR-ULK1 pathway, which is a master regulator of autophagy, and treatment with mTOR inhibitors rescues some of the mole-cular and cellular phenotypes associated with CCM. Insufficient autophagy is also evident in CCM2-silenced human endothelial cells and in both cells and tissues from an endothelial-specific CCM3-knockout mouse model, as well as in human CCM lesions. Furthermore, defective autophagy is highly correlated to endothelial-to-mesenchymal transition, a crucial event that contributes to CCM progression. Taken together, our data point to a key role for defective autophagy in CCM disease pathogenesis, thus providing a novel framework for the development of new pharmacological strategies to prevent or reverse adverse clinical outcomes of CCM lesions.

Cerebral cavernous malformation (CCM) is a disease of the central nervous system causing hemorrhage-prone multiple lumen vascular malformations and very severe neurological consequences. At present, the only recommended treatment of CCM is surgical. Because surgery is often not applicable, pharmacological treatment would be highly desirable. We describe here a murine model of the disease that develops after endothelial-cell-selective ablation of the CCM3 gene. We report an early, cell-autonomous, Wnt-receptor-independent stimulation of β-catenin transcription activity in CCM3-deficient endothelial cells both in vitro and in vivo and a triggering of a β-catenin-driven transcription program that leads to endothelial-tomesenchymal transition. TGF-β/BMP signaling is then required for the progression of the disease. We also found that the anti-inflammatory drugs sulindac sulfide and sulindac sulfone, which attenuate β-catenin transcription activity, reduce vascular malformations in endothelial CCM3-deficient mice. This study opens previously unidentified perspectives for an effective pharmacological therapy of intracranial vascular cavernomas.cerebral cavernous malformation | endothelial cells | β-catenin | sulindac metabolites | vascular pathology T he vascular malformations that characterize the disease known as cerebral cavernous malformation (CCM) are concentrated in the central nervous system, and they typically show multiple lumens and vascular leakage (1). These abnormalities can result in severe neurological symptoms, including hemorrhagic stroke (2), and, to date, the only possible therapy is surgery (3). In humans, loss-of-function mutations in any one of three independent genes known as cerebral cavernous malformation 1, 2, and 3 (CCM1, CCM2, and CCM3) are the cause of the genetic form of CCM (4). Similarly, in murine models, the vascular phenotype can be reproduced by endothelium-specific loss-of-function mutations of any one of these three CCM-linked genes (5-7).We have recently reported (7) that TGF-β/BMP signaling is activated after ablation of CCM1, CCM2, or CCM3 and induces endothelial-to-mesenchymal transition (EndMT) that plays a crucial role in the development of vascular malformations. Nevertheless, the sequence of signaling responses elicited by ablation of CCM genes still remains to be defined. Inhibitors of the TGF-β/BMP-signaling pathways reduce the number and size of the malformations, but not completely (7), suggesting that other signaling pathways may be implicated.The Wnt/β-catenin pathway, in synergy with TGF-β signaling (8), is responsible for the EndMT switch of endothelial cells giving rise to the heart cushion in the embryo. In addition, the knockdown of CCM1 and CCM3 expression in cultured aortic and artery endothelial cells promotes β-catenin signaling (9, 10), although no direct link with the in vivo model of the disease has been made. Activation of canonical Wnt/β-catenin signaling is critical for brain vascularization and acquisition, by the microvasculature, of blood-brain barr...

EPS8, a regulator of actin dynamics, is a novel component of the adherens junction complex of endothelial cells and acts as the key protein through which VE-cadherin controls Yap transcriptional activity both in vitro and in vivo.

Our findings support the use of common therapeutic strategies for both sporadic and genetic CCM malformations.

Although being a monolayer the vascular endothelium controls fundamental vessel functions such as permeability, leukocyte extravasation and angiogenesis. The endothelial selective transmembrane constituent of adherens junctions, Vascular Endothelial- (VE-) cadherin plays a crucial role in the regulation of such activities. The signaling pathways controlled by VE-cadherin as well as the ones that regulate VE-cadherin activity start to be elucidated. This delineates a complex network of molecular and functional interactions that can be altered in pathologies.

No abstract