The expressions of MMP-9 and MMP-2 are elevated after cerebral ischemia and able to open the BBB [50
]. In rodent models, MMP-9 responses appear to dominate in the acute phase, whereas MMP-2 elevations seem to occur in the delayed phase, days after stroke [50
]. Moreover, recent data confirm the presence of high MMP-9 levels not only in infarct tissue but also in the peri-infarct areas, suggesting that MMP-9 involve in the process of infarct growth [55
]. Furthermore, MMP-9 level seems to peak within infarcts that undergo hemorrhagic conversion, correlating with enhanced erythrocyte extravasation and neutrophil infiltration surrounding the affected capillaries together with severe collagen IV degradation in the basal lamina [53
]. Although in the brain, gelatinases have been the most intensively studied because of the ease with which they can be identified by gelatin zymography and their prominent role in injury and repair [57
], other MMP members may play important roles as well [54
]. For example, MMP-3 can be activated after ischemia-reperfusion in rat brain, causing the cleavage of the cerebral matrix agrin and contribute to BBB opening during neuroinflammation after intracerebral lipopolysaccharide (LPS) injection in mice [23
]. Whatever, in this article we just focus on MMP-9 in cerebral ischemia.
3.1. MMP-9 and Blood-Brain Barrier Opening
Blood-brain barrier (BBB) plays an important role in protecting the neuronal microenvironment. Endothelial cells of brain capillaries have tight junctions, which can restrict molecules from moving between the blood and brain. The extracellular matrix molecules constitute the basal lamina which is around the capillary with pericytes. MMP-9 can degrade a number of extracellular matrix molecules to breakdown the BBB. When the integrity of BBB is lost, inflammatory cells and fluid penetrate the brain, causing hemorrhage, vesogenic edema and neuronal cell death.
A lot of studies have shown that the expression of MMP-9 increases after permanent middle cerebral artery occlusion (pMCAO) and transient middle cerebral artery occlusion (tMCAO). Significant MMP-9 activity was observed at 12 hours and reached maximum levels by 24 hours, then persisted for 5 days at this level and returned to basal (zero) levels by 15 days using a model of pMCAO in rats [52
]. Also a finding demonstrated pro-MMP-9 expression was significantly increased in ischemic regions compared with corresponding contralateral regions after 2 hours of ischemia and remained elevated until 24 hours and activated MMP-9 was observed 4 hours after ischemia. Moreover, at the same time as the appearance of activated MMP-9, a clear increase of BBB permeability was detected. This suggested that MMP-9 may play an active role in early vasogenic edema development after stroke [18
]. The study with 50 minutes tMCAO rats showed an increase of MMP-9 from 4 hours to 4 days after reperfusion [50
]. In rats with 2 hours tMCAO, a maximally increase in MMP-9 was associated with maximal brain sucrose uptake at 48 hours of reperfusion [58
]. There is the discrepancy among these studies and this can be explained by technical differences or different animals and different testing time phase. Recently, a research showed a strong neutrophil infiltration in the infarcted and hemorrhagic areas with local high MMP-9 content closely related to basal lamina collagen IV degradation and blood-brain barrier breakdown [54
3.2. MMP-9 and Cerebral Infarction
Brain injury after middle cerebral artery occlusion (MCAO) is primarily a result of the decrease in blood flow and energy depletion. Then the loss of oxygen and glucose result in the neuronal cell death within minutes in the core zone, an area of severe ischemia. The ischemic penumbra, the hypoperfusion region associated with the focal ischemia, is variable. This area can be recovery or become infarction after reperfusion.
Recently, some studies showed that the activation of MMP-9 relate to the infarct size. Pro-MMP-9 levels in plasma and activated MMP-9 levels in brain homogenates were progressively increased over the course of 24 hours after permanent middle cerebral artery occlusion in male Sprague-Dawley rats. And plasma levels of pro-MMP-9 at 24 hours were correlated with final infarct volumes [48
]. A decrease in infarct size after focal ischemic insult in rats was observed when a MMP-9 neutralizing monoclonal antibody was administered [52
]. Serial MMP-9 and MMP-2 in 39 patients with cardioembolic stroke were determined. Results suggested that higher MMP-9 levels were associated with neurological deterioration during the first 48 hours and a positive correlation was between mean MMP-9 and total infarct volume [40
]. Several inflammatory families such as proinflammatory cytokines (TNF-α, IL-6), adhesion molecules (ICAM-1), chemokines (IL-8), or matrix metalloproteinases (MMP-2, MMP-9) of 16 patients with acute middle cerebral artery stroke were studied. Among them, MMP-9 was found to be the most powerful and only predictor of the infarct volume measured as a diffusion-weighted magnetic resonance imaging lesion [43
]. The serum levels of MMP-2, MMP-3, MMP-9, MMP-13, TIMP-1, TIMP-2 and laminin in 50 patients with acute ischemic stroke were measured and results demonstrated that levels of MMP-9 and laminin varied significantly by infarct size [26
]. These studies certified the increased of MMP-9 in infracted tissue both in animal experiments and human stroke. Thus, MMP-9 can be a potential therapeutic target for the treatment of stroke.
3.3. MMP-9 and Hemorrhagic Transformation
Plasminogen activators, including tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA), are serine proteases that activate plasminogen into plasmin. Recombinant tPA (rtPA) is beneficial in ischemic stroke when thrombolytic therapy is started within 3 hours after symptom onset. On the other hand, thrombolysis is associated with the risk of hemorrhage transformation.
A lot of studies about the relationships between tPA and MMPs show that the plasminogen-plasmin system might be involved in activation of MMPs. Administration of heparin for 3 hours after MCAO increased tPA and MMP-9 activity and their mRNA expression in wild-type mice but not in tPA deficient knockout mice [72
]. Closure of the BBB with broad-spectrum MMP inhibitor BB-94 given before rtPA treatment reduced rtPA-mediated mortality in 6/12 to 33% [49
]. Recently, a study demonstrated for the first time that the injection of a MMP inhibitor for 3 hours after the ischemia in rats significantly decreased the brain edema and reduced the risk of hemorrhagic transformation after thrombolysis with rtPA [11
]. A study published in 2001 firstly showed an association between MMP-9 expression and several subtypes of hemorrhagic transformation after human cardioembolic stroke [41
]. Subsequently, a study demonstrated that high plasma levels of MMP-9 are independently associated with hemorrhagic transformation in acute ischemic stroke by studying 38 patients [8
]. Moreover, the baseline MMP-9 level was the only factor independently associated with late hemorrhagic infarction among patients they observed [41
]. Therefore, the baseline MMP-9 level predicts parenchymal hemorrhagic appearance after t-PA treatment [42
]. The findings above suggest that the endogenous tPA or rtPA treatment, through the enhancement of MMP-9 expression, play an important role in hemorrhagic transformation after cerebral ischemia. MMP inhibitors can be used before thrombolytic therapy to reduce hemorrhagic transformation.
3.4. MMP-9 and Stroke Recovery
MMP-9 promotes injury of the BBB, vasogenic edema formation, infarct size and hemorrhagic transformation in the acute phase after cerebral ischemia. But MMP-9 may have a different role during delayed phases after stroke. MMP-9 is upregulated in peri-infarct cortex at 7–14 days after stroke and is colocalized with markers of neurovascular remodeling. Treatment with MMP inhibitors at 7 days after stroke suppresses neurovascular remodeling, increases ischemic brain injury and impairs functional recovery at 14 days [74
]. MMP-9 also mediate neuroblast cells from the subventricular zone expand and migrate toward damaged tissue during the 2 week recovery period after transient focal cerebral ischemia in mice. And inhibitors of MMPs suppress neurogenic migration from subventricular zone into the striatum [31
]. These data suggest that MMP-9 are involved in endogenous mechanisms of neurovascular remodeling in peri-infarct cortex as the brain seeks to heal itself after injury.