Molecular and cellular mechanisms of energy homeostasis in birds

Hypothalamus and adipose tissue are essential central and peripheral sites regulating energy homeostasis. Disruption of energy homeostasis can lead to diseases like anorexia and obesity in humans and reduced productivity in animals. Therefore, integrating knowledge in hypothalamic appetite regulation and adipose tissue metabolism is essential to maintain homeostasis. The aim of this dissertation was to elucidate molecular and cellular mechanisms of energy homeostasis in birds. We determined adipose tissue physiological changes during the first two weeks post-hatch in chickens from lines selected for low (LWS) and high (HWS) body weight. LWS was more dependent on yolk and subcutaneous fat mobilization for growth from hatch to day 4 post-hatch, with hyperplasia-predominated replenishment of the reservoir. In contrast, HWS was more dependent on feed for growth and maintained depot mass through hyperplasia and hypertrophy. From day 4 to 14 post-hatch, compared to maintenance of depot weight and adipocyte size in LWS, HWS accumulated clavicular and abdominal fat with minimal lipolysis. There was greater expression of precursor and proliferation markers in LWS with more apoptotic cells in the abdominal stromal vascular fraction on day 14 post-hatch, suggesting that apoptosis contributed to lower adipogenic potential and lack of abdominal fat in LWS. Exposure to thermal and nutritional stressors at hatch impaired growth by reducing yolk utilization and lowering body weight, lean and fat masses in LWS. Stress exposure resulted in increased global DNA methylation and DNA methyltransferase activity in the arcuate nucleus of the hypothalamus in LWS. Moreover, there was decreased binding to methyl-CpG-binding domain protein 2 in the promoter of corticotropin-releasing factor (CRF) because of hypomethylation in one CpG site at its core binding site in stressed LWS, which explains the increased CRF expression in the paraventricular nucleus of the hypothalamus. We next determined effects of nutritional status on adipose tissue physiology in Japanese quail, a less-intensively selected avian species. Six-hour fasting promoted lipolysis and gene expression changes in 7-day old quail with some changes restored to original levels within 1 hour of refeeding. Overall, our results reveal novel cellular and molecular mechanisms regulating appetite and adiposity in birds early post-hatch.