GCG and obesity due to melanocortin 4 receptor deficiency: Hunger and satiety signals from adipose tissue (leptin)[159], the pancreas (insulin)[160], and the gastrointestinal tract (cholecystokinin (CCK), glucagon-like peptide-1 (GLP-1), peptide YY3-36 (PYY3-36), and ghrelin)[161] constitute key components in the gut-brain axis-mediated control of appetite, energy expenditure, and obesity.