Tissue plasminogen activator (tPA) and matrix metalloproteinases in the pathogenesis of stroke: therapeutic strategies

Abstract

Today there exists only one FDA-approved treatment for ischemic stroke; i.e., the serine protease tissue-type plasminogen activator (tPA). In the aftermath of the failed stroke clinical trials with the nitrone spin trap/radical scavenger, NXY-059, a number of articles raised the question: are we doing the right thing? Is the animal research truly translational in identifying new agents for stroke treatment? This review summarizes the current state of affairs with plasminogen activators in thrombolytic therapy. In addition to therapeutic value, potential side effects of tPA also exist that aggravate stroke injury and offset the benefits provided by reperfusion of the occluded artery. Thus, combinational options (ultrasound alone or with microspheres/nanobubbles, mechanical dissociation of clot, activated protein C (APC), plasminogen activator inhibitor-1 (PAI-1), neuroserpin and CDP-choline) that could offset tPA toxic side effects and improve efficacy are also discussed here. Desmoteplase, a plasminogen activator derived from the saliva of Desmodus rotundus vampire bat, antagonizes vascular tPA-induced neurotoxicity by competitively binding to low-density lipoprotein related-receptors (LPR) at the blood-brain barrier (BBB) interface, minimizing the tPA uptake into brain parenchyma. tPA can also activate matrix metalloproteinases (MMPs), a family of endopeptidases comprised of 24 mammalian enzymes that primarily catalyze the turnover and degradation of the extracellular matrix (ECM). MMPs have been implicated in BBB breakdown and neuronal injury in the early times after stroke, but also contribute to vascular remodeling, angiogenesis, neurogenesis and axonal regeneration during the later repair phase after stroke. tPA, directly or by activation of MMP-9, could have beneficial effects on recovery after stroke by promoting neurovascular repair through vascular endothelial growth factor (VEGF). However, any treatment regimen directed at MMPs must consider their pleiotropic nature and the likelihood of either beneficial or detrimental effects that might depend on the timing of the treatment in relation to the stage of brain injury.

Fig. 1. An odyssey: Plaque to stroke

Atherosclerosis is a major risk factor for ischemic stroke [14]. Under inflammatory conditions (OxLDL, homocysteine, cigarette smoke, shear stress and infectious agents such as Chlamydia pneumoniae) endothelia cells of the artery express adhesion molecules that allow monocytes (1) to adhere to endothelia (2). Chemoattractants such as monocyte chemoattractant protein-1 (MCP-1) draw the monocytes through the endothelium into the arterial intima. Once resident in the intima, monocytes differentiate into macrophages (3) in response to locally produced agents such as monocyte colony stimulating factor. LDL (4) under oxidative stress gets oxidized to OxLDL. The macrophages increase expression of scavenging receptors such as CD36, SR-A and SR-B. These scavenger receptors then internalize specifically oxidized LDL (OxLDL, specifically OxPC) particles such that cholesteryl esters accumulate in cytoplasmic droplets, resulting in lipid-loaded macrophages (foam cells, 5). Foam cells produce ROS, which further propagate LDL oxidation, and secrete cytokines and matrix metalloproteinases (MMPs). The MMPs contribute to degradation of the fibrous cap surrounding the plaque, resulting in its rupture and formation of a blood clot (6). If the blood clot dislodges from the plaque, arterial blood flow can carry it to the brain, where it lodges in a cerebral artery (embolism) and causes an ischemic stroke (7).

Fig. 2. Blood clot formation

Blood coagulation represents a series of sequential interactive events that lead to the repair of the vascular system following injury, traditionally distinguished in two pathways: the intrinsic and extrinsic pathways. The intrinsic pathway is defined as a cascade that utilizes only factors that are soluble in the plasma, whereas the extrinsic pathway consists of some factors that are insoluble in plasma, e.g. membrane-bound factors like Factor VII. Upon activation, individual glycoproteins serve as enzymes to convert the zymogen (inactive) form of the succeeding glycoprotein to its protease (active) form (identified by “a”). Both pathways produce Factor Xa, which then catalyses the conversion of prothrombin (factor II) to thrombin (factor IIa). Thrombin then converts fibrinogen to soluble fibrin monomers, which spontaneously aggregate. Thrombin also activates Factor XIII, which cross-links the fibrin molecules to form a stable mesh-like structure which traps blood cells, forming a clot.

Fig. 3

Pleiotropic actions of tPA.