MMP9 and Stroke: Paradoxically, there is also evidence that miR-21 can contribute to BBB damage under certain conditions: Deng et al. (2013) observed that cerebral ischemia caused a significant upregulation of miR-21 along with MMP-9 in the rat brain, and administration of an antagomir-21 after stroke decreased MMP-9 levels and reduced BBB leakage and lesion size [77].