A role for protease activated receptor-1 in the neurodegenerative processes of ischemia

Even though serine proteases and their receptors are best known for their role in blood coagulation and fibrinolysis, increasing evidence suggests that serine proteases such as plasmin, tissue plasminogen activator (tPA), and thrombin as well as G-protein coupled protease activated receptors (PARs) have a role in central nervous system (CNS) function. Recent findings suggest a role for serine proteases and their receptors (PARs) in the pathological processes of ischemia, excitotoxicity, hypoxia, and other situations in which the blood brain barrier is compromised. The primary goals of the studies presented within this dissertation were to examine at the cellular and whole animal level the involvement of protease activated receptor-1 (PAR1) and one of its activators, plasmin, in the pathological processes that contribute to neurodegeneration, and to determine if PAR1 protein is expressed in human brain. The results of these studies show that the serine protease plasmin potentiates N-methyl-D-aspartate (NMDA) receptor responses over 2-fold through activation of PAR1. Increased activation of NMDA receptor responses has been implicated in several neurodegenerative situations including ischemia. Therefore plasmin activation of PAR1 and subsequent potentiation of NMDA receptor responses may be one potential mechanism for the harmful effects of plasmin in the brain. At the whole animal level, genetic removal and pharmacological inhibition of PAR1 significantly reduces cell death in a rodent transient focal cerebral ischemia model of stroke. These findings further support the idea that activation of PAR1 participates in the neurodegenerative processes of ischemia and that aberrant activation of serine protease signaling cascades may contribute to compromised cell survival. Lastly, the expression of PAR1 was examined in human brain regions to determine if the results found in animals can potentially be translated to humans. Immunohistochemical and double labeling immunofluorescence studies show that PAR1 protein is highly expressed in astrocytes of white and gray matter and moderately expressed in neurons of the cortex, hippocampus, caudate, putamen, and cerebellum in human brain. PAR1 may therefore provide a new potential therapeutic target for preventing some of the pathological effects associated with ischemia and other situations in which blood-derived serine proteases enter brain tissue.