APLN and obesity due to melanocortin 4 receptor deficiency: Apelin/APJ is upregulated in a number of (patho)physiological conditions including chronic heart failure (Földes et al., 2003), hypoxia (Geiger et al., 2011), insulin and obesity (Boucher et al., 2005; Geurts et al., 2011), exercise (Zhang et al., 2006) and stress (O’Carroll et al., 2003).