MMP9 and Alzheimer disease: Studies of transgenic apolipoprotein E (APOE) mice have shown that APOE4, a major genetic risk factor for AD, leads to disruption of BBB integrity by activating a pathway: proinflammatory cyclophilin-A (CypA)—nuclear factor κB (NFκB)—matrix metalloproteinase 9 (MMP-9) in pericytes [134].