Overexpression of Vascular Endothelial Growth Factor in the Germinal Matrix Induces Neurovascular Proteases and Intraventricular Hemorrhage

A transgenic mouse model implicates specific neurovascular proteases in the pathogenesis of intracranial hemorrhage in preterm infants. The Trouble with VEGF Neonatology has made numerous advances in recent decades, and premature infants now have higher chances of survival than ever before. Despite all this progress, neonatal intraventricular hemorrhage (IVH) remains a dreaded complication of premature birth and all too often leads to lifelong neurological complications or even death. It is typically attributed to a constellation of causes associated with prematurity, such as vascular fragility, hypoxia, and infections, but its mechanism is not yet understood. In this study, Yang and coauthors implicated vascular endothelial growth factor (VEGF) and neurovascular proteases in the pathogenesis of neonatal IVH. IVH in premature infants typically occurs in a vascular part of the brain called the germinal matrix. Yang and colleagues developed two mouse models that were engineered to have a tetracycline-controlled switch to regulate the expression of VEGF in the germinal matrix. One of these models continuously produces VEGF in the germinal matrix unless a tetracycline-like drug is present, whereas the other turns on VEGF production in the presence of tetracyclines. With these models, the authors showed that increased VEGF in the germinal matrix could cause cerebral hemorrhage similar to what is seen in humans. These detrimental effects of VEGF appeared to be not associated with vasculogenesis, but instead attributable to activation of neurovascular proteases. Prenatal glucocorticoid treatment interfered with the activation of these proteases and thus prevented the hemorrhage, again simulating the situation seen in human infants. The mouse models relying on tetracycline-controlled VEGF release are an elegant approach to simulating the process of IVH and appear to closely represent the pathological process. It may not be possible to determine the molecular mechanism for this disease directly in human patients or to confirm how similar it is to the mechanism seen in the mice. However, a true test of the mouse models’ usefulness will be their contribution to therapeutic development, and future work should determine whether inhibition of VEGF-stimulated neurovascular proteases is a viable route for preventing or treating IVH in premature infants. Intracranial hemorrhage in preterm neonates may result in neonatal mortality and functional disabilities, but its pathogenic mechanisms are poorly defined and better therapies are needed. We used a tetracycline-regulated transgenic system to test whether the induction of vascular endothelial growth factor (VEGF) in the germinal matrix leads to intracranial hemorrhage. This genetic strategy initially induced a dense network of loosely adjoined endothelial cells and pericytes near lateral ventricles, similar to the immature vascular rete in human fetal brains. Yet, this rich vascular network transformed into low-vasculature patches correlated with hemorrhage and caspase-3 activation near birth. Gene expression and biochemical analyses suggested that downstream mediators of VEGF in this network include transcriptional factors ETS1 and HIF2α (hypoxia-inducible factor 2α), components of the PDGFβ (platelet-derived growth factor β) and TGFβ (transforming growth factor–β) receptor signaling pathways, matrix metalloproteinase-9 (MMP-9), and cathepsins. Prenatal administration of glucocorticoids markedly reduced mortality and cerebral hemorrhage in mutant animals, as in human neonates. This protective effect was not due to blocking vasculogenesis, but was instead associated with inhibition of neurovascular proteases, notably MMP-9, cathepsin B, and caspase-3. Collectively, these results support a causative role of VEGF in perinatal cerebral hemorrhage and implicate its downstream proteases as potential therapeutic targets.