Notch4 Normalization Reduces Blood Vessel Size in Arteriovenous Malformations

Normalization of Notch expression restores enlarged blood vessels to microvessels through EphB4-mediated reprogramming of arterial endothelial cells. Reducing Inflation Arteriovenous malformations (AVMs) are a class of vascular abnormalities in which arteries connect directly with veins, thus bypassing the capillary beds and diverting blood flow away from tissues. In these vascular diseases, blood vessels, particularly the veins, become inflated in size and eventually rupture, resulting in hemorrhage and ischemia. AVMs, which can be found in any tissue, are particularly problematic in the brain, where surgical options are limited, and they often result in stroke or death. In a tour-de-force study, Murphy et al. now show that dialing down Notch4 receptor signaling in established AVMs in mouse brain reduces the size of enlarged blood vessels, resulting in restoration of blood flow to capillary beds and the reversal of hypoxia in mouse brain tissue. The Notch receptor is a master regulator of arteriovenous development and is up-regulated in AVMs in human brain. Overexpression of a constitutively active form of Notch4 in endothelial cells lining blood vessel walls is sufficient to induce AVMs in mice. In their new work, Murphy et al. first wanted to establish whether correction of Notch4 signaling could induce the regression of AVMs. Using their mouse brain AVM model, they obtained four-dimensional imaging data of the mouse brain vasculature viewed through a window cut into the cranium with two-photon fluorescence microscopy. When Notch4 signaling was normalized, they found regression of enlarged AVMs, which became similar in size to capillaries. This shrinkage in size enabled blood flow to return to oxygen-deprived tissues in the mouse brain. Surprisingly, the authors discovered that AVM regression was not induced by loss of endothelial cells, thrombotic occlusion, or vessel rupture. Rather, it required reprogramming of arterial endothelial cells in the enlarged AVM vessels to a venous endothelial cell specification. This reprogramming was activated by a decrease in Notch4 receptor signaling, which prompted arterial endothelial cells to start expressing the venous marker EphB4. These findings suggest that strategies to manipulate Notch receptor signaling in blood vessel endothelial cells may help to shrink AVMs and may be a new approach to treating AVMs and other vascular diseases. Abnormally enlarged blood vessels underlie many life-threatening disorders including arteriovenous (AV) malformations (AVMs). The core defect in AVMs is high-flow AV shunts, which connect arteries directly to veins, “stealing” blood from capillaries. Here, we studied mouse brain AV shunts caused by up-regulation of Notch signaling in endothelial cells (ECs) through transgenic expression of constitutively active Notch4 (Notch4*). Using four-dimensional two-photon imaging through a cranial window, we found that normalizing Notch signaling by repressing Notch4* expression converted large-caliber, high-flow AV shunts to capillary-like vessels. The structural regression of the high-flow AV shunts returned blood to capillaries, thus reversing tissue hypoxia. This regression was initiated by vessel narrowing without the loss of ECs and required restoration of EphB4 receptor expression by venous ECs. Normalization of Notch signaling resulting in regression of high-flow AV shunts, and a return to normal blood flow suggests that targeting the Notch pathway may be useful therapeutically for treating diseases such as AVMs.