Apelin/APJ system: A novel promising target for neurodegenerative diseases

Neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD), are incurable diseases characterized by progressive loss of cognitive or motor function, which construct a serious threat to the life quality of aging populations and their life spans. Apelin is an endogenous ligand for the G protein-coupled receptor. Apelin is reported to be detected not only in the cardiovascular system but also in neurons of the central nervous system (CNS). In addition, alterations in the expression level of apelin appear to play a pivotal role in various physiological processes including loss of structure or function of neurons, inflammatory responses, oxidative stress, Ca²⁺ signaling, apoptosis, and autophagy. All of these processes are intimately related to the occurrence of neurodegenerative diseases. Recently, apelin is reported to improve cognitive impairment in PD by antioxidant and antiapoptotic properties. Hence, it is becoming increasingly appreciated that altering the level of apelin can change the course or dictate the outcome of neurodegenerative events such as AD, PD, and HD, suggesting that apelin could be a potential target for the treatment of neurodegenerative diseases possibly acting on a variety of signaling pathways such as suppression of inflammatory responses, inhibition of oxidative stress, reduction of Ca²⁺ signaling, induction of autophagy, and suppression of apoptosis.