MMP9 and Alzheimer disease: These upregulated molecules further exacerbate blood-brain barrier (BBB) disruption, amplify neuroinflammation, and impair neuronal synaptic function, thereby forming a vicious cycle of “pathological stimulation-TNF signaling-NF-κB activation-MMP9 upregulation-pathology aggravation.” This cascade is involved in the core process of neurodegeneration in AD (Zhao et al., 2020; Ding et al., 2020).