Age-associated changes in endothelial transcriptome and chromatin landscape correlate with elevated risk of hemorrhage

Intracerebral hemorrhage (ICH), a devastating form of stroke, is a leading global cause of human death and disability. The major risk factors for ICH include increasing age, hypertension and cerebral amyloid angiopathy. Despite high mortality and morbidity associated with ICH, the mechanisms leading to blood-brain barrier (BBB) dysfunction with age and development of hemorrhagic stroke is poorly understood. In the vasculature of the central nervous system, endothelial cells (ECs) constitute the core component of the BBB and provide a physical barrier due to tight junctions, adherens junctions, and basement membrane layers. In this study, we show in brains of mice that incidents of intracerebral bleeding increase with advancing age. After isolation of an enriched population of cerebral ECs, we studied gene expression in ECs isolated from murine brains of increasing ages of 2, 6, 12, 18, and 24 months. The study reveals agedependent dysregulation of 1388 genes in the ECs, including many involved in the maintenance of BBB and vascular integrity. Since epigenetic mechanisms regulate gene expression, we also investigated age-dependent changes at the levels of CpG methylation and accessible chromatin in cerebral ECs. Our study reveals correlations between age-dependent changes in chromatin structure and gene expression. We find significant age-dependent downregulation of the apelin receptor (Aplnr) gene along with an age-dependent reduction in chromatin accessibility of the promoter of this gene. Aplnr is known to play a crucial role in positive regulation of vasodilation and is implicated in vascular health. Interestingly, we also observe an age-dependent reduction in the protein expression levels of the apelin receptor in the brain, potentially implicating the apelin receptor to be critical for the increased risk of intracerebral hemorrhage with ageing.