Matrix metalloproteinase (MMP-9) expression is positively correlated with the severity of depression in patients with MDD; elevated MMP-9 activity has been observed in the hippocampi of in animal models of depression [14,15], suggesting that MMP-9 is a potential biomarker in the pathology of MDD aquaporin 4 (AQP4), a water-selective channel protein widely expressed in the central nervous system (CNS), also contributes to the pathology of MDD. Here, AQP4 is linked to major depressive disorder.