Blood Vessels, Biomarkers, and Broken Barriers: lnvestigations of the Brain Vasculature in Models of Neonatal Brain lnjuries

Abstract

Brain injury during the perinatal period can lead to lifelong impairment in cognitive and motor function, or an early death. Term neonatal encephalopathy and preterm germinal matrix haemorrhage are two conditions that can irreversibly injure the brain, but clinical tools for diagnosing and treating these pathologies are lacking. It is known that the cerebrovasculature, i.e. the blood vessels of the brain and blood/brain barrier (BBB) plays a role during the course and recovery of injury. This thesis investigated the cerebrovascular involvement in two animal models of neonatal brain injury in an attempt to elucidate injury mechanisms, find potential new treatments, and identify biomarkers for brain vascular dysfunction. Using rodent models for hypoxic/ischemic encephalopathy (HIE) and germinal matrix‐intraventricular haemorrhage (GM‐IVH) we found raised levels of tight‐junction proteins claudin‐5 and occludin, two integral components of the BBB, in blood plasma and cerebrospinal fluid at different time points. In the HIE model, levels of tight‐junction proteins in the circulation were sex‐dependent and the amount of claudin‐5 in CSF correlated with the severity of brain injury. These proteins thus have potential as biomarkers for early detection of cerebrovascular insults. In addition, we did in‐depth assessments of the BBB function in both models and detailed the temporal and regional increases of barrier permeability after injury by measuring the extravasation of radiolabelled sucrose, visible dyes, and molecular tracers. Studies of the cerebral vasculature and angiogenesis after HI showed that the density of proliferating endothelial cells were largely unaffected after injury, but the number of growing endothelial tip cells were strongly reduced in the entire brain, accompanied by changes in the expression of angiogenesis genes. The thesis also includes the first trial of endogenous RNAse A as a neuroprotective treatment for neonatal brain injury, a treatment that has shown promise in adult models of other pathologies with cardiovascular aspects. We found significant reductions of grey and white matter tissue loss after RNAse A administration in HI animals, but no protection of BBB function or evidence for a reduced neuroinflammatory response. Taken together, this thesis contains new insights into several aspects of the vascular mechanisms involved in the pathogenesis of two major forms of injury that can occur in the neonatal brain.