Role of GPR173 in the central control of plasma volume expansion in pregnancy

PROJECT SUMMARY Maintenance and completion of a healthy pregnancy requires profound physiological adaptations of maternal physiology. In particular, the maternal plasma volume in a singleton pregnancy must expand by ~50% by 40 weeks of gestation to ensure proper placental perfusion to accommodate and support the growing offspring. In the absence of appropriate plasma volume expansion (PVE), the pregnancy, and health of the mother, can be compromised. For example, failure of PVE in pregnancy can lead to intra-uterine growth restriction or fetuses that are small for gestational age. Central mechanisms underlying pregnancy-associated PVE include alterations to drinking behavior and neuroendocrine regulation of vascular and renal function. In particular, the osmotic thresholds for thirst and the release of arginine vasopressin (AVP) from the posterior pituitary are reduced in pregnancy. We reported the discovery of a hypothalamic peptide, phoenixin (PNX), and the identification of its receptor, GPR173, that appears to play important roles in the regulation of thirst and drinking behavior as well as AVP secretion. In pregnant rats, PNX expression increases as pregnancy progresses, and correlates significantly with AVP levels. Our preliminary data also suggest that knockdown of GPR173 in hypothalamus of rats leads to fetuses that are significantly smaller than that of controls, consistent with reduced PVE in GPR173 siRNA-treated rats. We therefore hypothesize that PNX/GPR173 signaling contributes to pregnancy-associated PVE through the regulation of AVP secretion and thirst. This hypothesis will be tested in two Specific Aims. In the first Aim, we will use our established model of siRNA-mediated knockdown of PNX or GPR173 in hypothalamus to evaluate the impact of loss of hypothalamic PNX signaling on basal and compensatory water drinking and ingestive patterning in pregnant rats. We have previously shown that PNX stimulates water drinking in non-pregnant rats via an interaction with the central angiotensin system. We therefore propose downstream activation of angiotensin signaling as a potential mechanism by which PNX impacts drinking in the setting of pregnancy. This will be assessed using pharmacological and molecular tools. In the second Aim, we will investigate how loss of PNX signaling impacts the increase in AVP secretion during normal pregnancy. In addition, using the method of Lindheimer and colleagues, we will investigate the role of hypothalamic PNX in the resetting of the osmotic threshold for AVP release in pregnant animals. Lastly, we will investigate the molecular mechanism underlying PNX’s actions in hypothalamus in the setting of pregnancy using biased (pharmacologic and immunohistochemical) and unbiased (spatial transcriptomic, i.e., spatial molecular imaging) methods. The successful completion of these proposed studies will offer insights into the basic biological mechanisms underlying changes in plasma volume associated with pregnancy. This new knowledge will serve as the basis of future experiments designed to investigate how changes in PVE in pregnancy lead to disease, as well as offer potential therapeutic targets for common pregnancy complications.