Project Summary / Abstract Postnatal growth failure remains a significant morbidity in very low birth weight infants despite aggressive and modern parenteral and enteral nutrition practices. Compelling associations have been identified between in- hospital growth failure and cardiometabolic and neurodevelopmental disorders, heightening the need to further identify optimal nutritional needs of preterm infants. Studies in animals and humans demonstrate deficiencies in sodium (Na) supply or intake impair somatic growth. Very low birth weight infants (i) are at increased risk of Na depletion due to high (and often unappreciated) urine Na loss, (ii) lack osmotically-inactive Na pools that are normally accrued during late gestation and are likely mobilized after birth to maintain circulating Na pools, and (iii) demonstrate improved somatic growth when supplied with Na in amounts above that typically provided in clinical practice. Our objective is to utilize novel animal models and laboratory methodologies to address the critical lack of understanding of the links between Na homeostasis in early life and metabolic control. The PIs have generated a wealth of published and preliminary data supporting their hypothesis that insufficient Na in early life causes programmed changes in short-and long-term energy expenditure via activation of AT1AR/Gαi signaling in selected hypothalamic neurons. We will address this hypothesis using several new mouse models to (i) identify the role that osmotically-inactive Na pools and the brain RAS play in metabolic dysfunctions programmed by Na depletion in early life (Aim 1), and (ii) explore the role of AT1AR signaling within Agouti-related peptide (AgRP) neurons of the hypothalamic arcuate nucleus in mediating increased energy expenditure & subsequent growth restriction in mice with Na depletion in early life (Aim 2). We have assembled a research team with extensive experience with cutting-edge metabolic phenotyping, molecular biology and transgenic animal production that is uniquely poised to address this clinically relevant issue. Findings from these studies will greatly increase our mechanistic understanding of the role and importance of early-life Na homeostasis in growth, metabolism and energy flux and potentially result in paradigm-shifting clinical practices which address providing sufficient dietary Na to premature infants to optimize a spectrum of long-term outcomes.