The role of Alpha1-Adrenergic Receptors Promoter Methylation in Cerebral Autoregulation in Fetus

摘要：机体在应对全身血压突然升高时减少脑血流量的能力

Summary: The ability of an organism to reduce the brain blood flow in response to sudden surges in systemic blood pressure (BP) is known as cerebral autoregulation (CAR). In contrast to term neonates, preterm neonates are not able to reduce cerebral blood flow (CBF) in response to increased systemic BP. In preterm neonates, this exposes fragile cerebral vessels to a significantly increased blood flow at high pressure, leading to their rupture and brain damage. Our preliminary studies demonstrate that near-term fetuses can constrict carotid arteries and reduce CBF when systemic BP rises; however, this capability is not developed in the preterm fetus. We also observed that the constriction of carotid arteries to reduce CBF is regulated by the adrenergic nervous system, specifically by the activities of alpha-1 adrenergic receptors (α1-ARs). These receptors are expressed at a significantly lower number in preterm carotid arteries. Also, we observed that following the removal of adrenergic control in the near-term fetus by severing the superior cervical ganglion (SCG) made them lose their ability to reduce carotid blood flow (CaBF) to the brain with the rise in systemic BP. Thus, after the removal of SCG, both preterm and near-term fetuses cannot reduce CBF following an increase in systemic BP. During ex-vivo experiments on carotid segments, we observed that preterm arterial constriction in response to α1-ARs agonist was significantly lower than those from near-term lambs. Thus, we concluded that reduced activities of α1-ARs play a fundamental role in regulating CaBF with the rise in systemic BP. We also observed that the reduction in the activities of α1-ARs agonists in preterm resulted from reduced expression of α1-ARs compared to those in near-term fetal lambs. Furthermore, we present evidence that DNA hypermethylation reduces α1-ARs promoter activities by luciferase reporter assays and the involvement of histone modifications. Thus, we will test the hypothesis that promoter DNA hypermethylation and histone modifications reduce the expression and function of α1-AR subtypes (α1A-, α1B-, α1D) in the carotid arteries and play an essential role in the maturation of cerebral autoregulation from preterm to term fetus. We will also collect data from both sexes (male versus female) to identify sex-related changes. The studies will be conducted ex-vivo on isolated carotid arteries and in vivo in chronically catheterized fetal sheep. The hypothesis will be tested with two specific aims. Aim 1: From preterm to term fetus in a sex- specific manner, we will conduct an in-depth mechanistic analysis of promoter DNA methylation and histone modifications on differential expression of α1-AR subtypes in carotid arteries. Aim 2: From preterm to term fetus, in a sex-specific manner, we will determine the functional significance of differential α1-AR subtypes promoter methylation, histone modifications, and gene expression on carotid artery contractility and blood flow regulation to the brain in response to an increase in systemic pressure. The measurements will be conducted in real-time, in-vivo, with in-utero fetal maturation every week from 105 to 137 days. This will provide valuable information regarding the role of α1-AR subtypes and the epigenetic mechanisms involved in the maturation of CAR.