Tissue Plasminogen Activator (tPA) Deficiency Exacerbates Cerebrovascular Fibrin Deposition and Brain Injury in a Murine Stroke Model

Abstract

Abstract-Although the serine protease, tissue plasminogen activator (tPA), is approved by the US Food and Drug Administration for therapy to combat focal cerebral infarction, the basic concept of thrombolytic tPA therapy for stroke was challenged by recent studies that used genetically manipulated tPA-deficient (tPAϪ/Ϫ) mice, which suggested that tPA mediates ischemic neuronal damage. However, those studies were potentially flawed because the genotypes of tPAϪ/Ϫ and wild-type control mice were not entirely clear, and ischemic neuronal injury was evaluated in isolation of tPA effects on brain thrombosis. Using mice with appropriate genetic backgrounds and a middle cerebral artery occlusion stroke model with nonsiliconized thread, which does lead to microvascular thrombus formation, in the present study we determined the risk for cerebrovascular thrombosis and neuronal injury in tPAϪ/Ϫ and genetically matched tPAϩ/ϩ mice subjected to transient focal ischemia. Cerebrovascular fibrin deposition and the infarction volume were increased by 8.2-and 6.7-fold in tPAϪ/Ϫ versus tPAϩ/ϩ mice, respectively, and these variables were correlated with reduced cerebral blood flow up to 58% (PϽ0.05) and impaired motor neurological score by 70% (PϽ0.05). Our findings indicate that tPA deficiency exacerbates ischemia-induced cerebrovascular thrombosis and that endogenous tPA protects the brain from an ischemic insult, presumably through its thrombolytic action. In addition, our study emphasizes the importance of appropriate genetic controls in murine stroke research. (Arterioscler Thromb Vasc Biol.

The Neurotoxicity of Tissue Plasminogen Activator?

“…4, 5). Deficiency of tPA augments harmful cerebrovascular fibrin deposition that correlates with enhanced brain injury, thus supporting the view that endogenous tPA protects the brain from an ischemic insult through its thrombolytic action (Tabrizi et al, 1999). Endogenous tPA activity was found to be significantly different between young and middle-aged mice brains, therefore it is possible that the absence of endogenous tPA production within the older brain may compromise brain fibrinolysis, predisposing it to larger and more disabling strokes (Ahn et al, 1999).…”

Lithium inhibits Smad3/4 transactivation via increased CREB activity induced by enhanced PKA and AKT signaling

“…Of interest, a recent report demonstrates that tPA in the amygdala is critical for stress-induced neuronal remodeling and the development of anxiety-like behavior, and is subsequently inhibited by PAI-1 . The tPA-plasminogen proteolytic cascade accelerates the clearance of fibrin and protects the brain from damage in stroke models or when the blood-brain barrier breaks down (Tabrizi et al, 1999;Akassoglou et al, 2003), and contributes to Aβ degradation and pathology , which suggest tPA's protective effect against the neurodegenerative progression of Alzheimer's disease. Because the tPA-plasminogen proteolytic cascade contributes to Aβ degradation, it may suggest the utility of tPA in blocking Aβ-induced cell death in the brain of Alzheimer's disease (reviewed in Maddika et al, 2007).…”

On Ischemic Brain Injury in Genetically Altered Mice

“…An impetus to their study is the recent report of Wang et al, 35 which purported to demonstrate that mice deficient in tPA exhibited 50% smaller cerebral infarcts than did wild-type mice. As noted by Tabrizi et al, 34 however, interpretation of that study was complicated by the fact that it compared C57BL/6 wild-type mice against mixed-strain (129/Sv and C57BL/6) tPA-deficient animals. In the present study, Tabrizi et al 34 show that C57BL/6 wild-type mice develop almost 10-fold larger infarcts than do mixed 129/Sv-C57BL/6 wild-type animals after middle cerebral artery (MCA) occlusion.…”

“…As noted by Tabrizi et al, 34 however, interpretation of that study was complicated by the fact that it compared C57BL/6 wild-type mice against mixed-strain (129/Sv and C57BL/6) tPA-deficient animals. In the present study, Tabrizi et al 34 show that C57BL/6 wild-type mice develop almost 10-fold larger infarcts than do mixed 129/Sv-C57BL/6 wild-type animals after middle cerebral artery (MCA) occlusion. By using genetically matched mixed-strain (129/Sv-C57BL/6) mice to compare tPA-deficient and wild-type animals and by the use of a nonsiliconized filament (which promotes thrombosis), Tabrizi et al 34 convincingly demonstrate that endogenous tPA deficiency in fact exacerbates the consequences of focal cerebrovascular occlusion by encouraging vascular fibrin deposition, enhancing ischemia-induced brain blood-flow decrements, and increasing the volume of the resulting brain infarct compared with wild-type animals.…”

“…The importance of rigorous control of genetic background in mutant mouse studies has been emphasized repeatedly in recent publications. 32,33 Elsewhere in this issue, Tabrizi and coworkers 34 confront this problem in a careful study designed to assess whether endogenous tissue plasminogen activator (tPA), in fact, is beneficial or detrimental in focal cerebral ischemia. An impetus to their study is the recent report of Wang et al, 35 which purported to demonstrate that mice deficient in tPA exhibited 50% smaller cerebral infarcts than did wild-type mice.…”