Tissue-type plasminogen activator in the ischemic brain: more than a thrombolytic Manuel Yepes 1 , Benoit D. Roussel 2 , Carine Ali 2 and Denis Vivien 2 1 Department of Neurology and Center for Neurodegenerative Disease, Emory University School of Medicine, Atlanta, GA 30322, USA 2 INSERM U919, Serine Proteases and Pathophysiology of the Neurovascular Unit, UMR-CNRS 6232 CINAPS, Cyceron, Universite ´ de Caen Basse-Normandie, Caen, F-14074, France Thrombolysis with tissue-type plasminogen activator (tPA) is used for the treatment of patients with acute ischemic stroke. However, a growing body of evidence indicates that, besides the unquestionable benefit from its thrombolytic activity, tPA also has a deleterious effect on the ischemic brain including cytotoxicity and increased permeability of the neurovascular unit with the development of cerebral edema. Because an increas- ing number of acute stroke patients are treated with tPA, it is important to know the mechanisms of harmful effects of tPA on the ischemic brain. Here, the best studied pathways of tPA neurotoxicity are discussed along with future directions for a safer use of tPA as a thrombolytic agent in the setting of acute ischemic stroke. Overview The World Health Organization (WHO; www.who.int) defines stroke as ‘rapidly developing clinical signs of focal (or global) disturbance of cerebral function, with symptoms lasting 24 h or longer or leading to death, with no apparent cause other than that of vascular origin’. Stroke might be classified as either ischemic or hemorrhagic. Ischemic stroke is a leading cause of disability and the third cause of mortality after heart disease and cancer (American Heart Association report, 2008, www.americanheart.org), accounting for 5.5 million deaths worldwide, two-thirds of which are in developing countries [1]. Additionally, each year millions of ischemic stroke survivors have to adapt to a life with restrictions in the activities of daily living, and many of them depend on other people’s support to survive. Three approaches have been investigated for the treat- ment of patients with acute ischemic stroke: neuroprotec- tion, thrombolysis and mechanical removal of the clot. In contrast to neuroprotection, which has been largely unsuc- cessful, thrombolysis with tissue-type plasminogen activa- tor (tPA) [2] and mechanical removal of the clot [3,4] have been approved by the Food and Drug Administration (FDA; www.fda.gov) for the treatment of patients with acute ischemic stroke. In a study performed by the National Institute of Neurological Disorders and Stroke (NINDS; www.ninds.nih.gov), treatment with tPA within 3 h of the onset of symptoms of acute ischemic stroke resulted in an 11–13% absolute increase in the number of patients with good outcome and in a reduction in the proportion of patients severely disabled or dead at 3 months [2]. Based on these results, intravenous tPA is currently the standard of care for patients with acute ischemic stroke presenting within 3 h of onset of the ischemic event. By contrast, there is a growing body of evidence indicating that tPA also has a deleterious effect in the ischemic brain. Indeed, in the NINDS study the use of tPA was associated with a 10-fold increase in the incidence of intracerebral hemorrhage, which had a 60% mortality [2]. Likewise, animal studies have demonstrated that either genetic deficiency of tPA [5,6] or inhibition of tPA with neuroserpin [7] results in a decrease in the volume of the ischemic lesion after exper- imental occlusion of the middle cerebral artery. Taken together, these studies indicate a pleiotropic role for tPA in the ischemic brain that is highly dependent of the compartment whereupon tPA acts (intravascular versus extravascular). Here, we review both the beneficial and deleterious effects of tPA in the ischemic brain and dis- cusses potential approaches to improve the treatment of patients with acute ischemic stroke. Tissue-type plasminogen activator in the central nervous system tPA is a serine protease that is found in the intravascular space, in the interface between the blood and the brain (i.e. neurovascular unit) and in the brain parenchyma (neurons, astrocytes and microglia) [8,9]. In the intravas- cular compartment, the main substrate of tPA is the inactive zymogen plasminogen and its most important role is to promote thrombolysis. Based on this property, recom- binant tPA is used for the treatment of patients with acute ischemic stroke. By contrast, in the brain tissue substrates of tPA might be multiple so that the functions of tPA extend above tPA- and plasmin(ogen)-driven extracellu- lar-matrix degradation. Indeed, there is a growing body of evidence indicating that the interaction between tPA and either the N-methyl-D-aspartate (NMDA) receptor, the low-density lipoprotein-receptor-related protein (LRP), annexin-II in glial cells and/or neurons activates cell- signaling processes that result in different outcomes including cerebral edema and cell death [9–11]. Taken Review Corresponding author: Vivien, D. (vivien@cyceron.fr). 48 0166-2236/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tins.2008.09.006 Available online 27 October 2008