APLN and pulmonary arterial hypertension: In PAH, apelin (Goetze et al., 2006; Alastalo et al., 2011; Kim et al., 2013) and ELA (Yang et al., 2017) are downregulated but receptor expression is maintained (Andersen et al., 2009; Falcão-Pires et al., 2009) therefore replacing the missing ligands could be a therapeutic strategy.