APLN and Alzheimer disease: In AD patients, the serum level of apelin-13 decreased significantly (Eren et al., 2012), whereas apelin administration plays a neuroprotective role by inhibiting inflammation response, including the activation of microglia and astrocytes and the secretion of inflammatory mediators, especially TNF-α and IL-1β in animal models of brain injury (Chen et al., 2015; Xin et al., 2015).