Effects of matrix metalloproteinase-9 gene knock-out on the proteolysis ofblood-brain barrier and white matter components after cerebral ischemia

Deleterious processes of extracellular proteolysis may contribute to the pr ogression of tissue damage after acute brain injury. We recently showed tha t matrix metalloproteinase-9 (MMP-9) knock-out mice were protected against ischemic, and traumatic brain! injury. In this study, we examined the mecha nisms involved! by focusing. on relevant MMP-9 substrates in blood-brain ba rrier, matrix, and white matter. MMP-9 knock-out and wild-type mice were su bjected to transient focal ischemia. MMP-9 levels increased after ischemia in wild-type brain, with expression primarily present in vascular endotheli um. Western blots showed that the blood-brain: barrier-associated protein a nd MMP-9 substrate zonae occludens-1 was degraded after ischemia, but this was reduced in knock-out mice. There were no detectable changes in another blood-brain barrier-associated protein, occludin. Correspondingly, blood-br ain barrier disruption assessed via Evans Blue, leakage was significantly a ttenuated in MMP-9 knock-out mice compared with wild types. In white matter , ischemic degradation: of the MMP-9 substrate myelin basic protein was sig nificantly reduced in knock-out mice compared with wild types, whereas ther e was no degradation of other myelin proteins that are not MMP substrates ( proteolipid protein and DM20). There were no detectable changes in the ubiq uitous structural protein actin or the extracellular matrix protein laminin . Finally, 24 hr lesion volumes were significantly reduced in knock-out mic e compared with wild types. These data demonstrate that the protective effe cts of MMP-9 gene knock-out after transient focal ischemia may be mediated by reduced proteolytic degradation of critical blood-brain barrier and whit e matter components.