MMP9 and major depressive disorder: Serval lines of evidence have shown that astrocytes can release inflammatory mediators such as tumor necrosis factor-alpha (TNFα), interleukin-1 beta (IL-1β), and matrix metalloproteinase-9 (MMP-9) [3] to participate in pathological processes of brain insults, such as Alzheimer′s disease and other age-associated dementias [4], multiple sclerosis [5], major depressive disorder [6], and Parkinson′s disease [7].