Role of tPA in Central Nervous System Pathologies: From Ischemic Stroke to Parkinson's Disease

Abstract

Neuronal cell death is a common feature observed in different central nervous system (CNS) pathologies like ischemic stroke and Parkinson’s disease. In these CNS pathologies, neuronal degeneration is ultimately associated with pathology and symptoms severity. Therefore, the identification and characterization of novel molecular pathways that contribute to neurodegeneration in the CNS are crucial for the design of effective therapeutic strategies. In this dissertation work, we identify two pathways involved in neuronal cell death regulated by tissue plasminogen activator (tPA) in mouse models of ischemic stroke and Parkinson’s disease. In addition to the identification of tPA-mediated pathways of neurodegeneration in the CNS, we also found that manipulation of norepinephrine signaling in the brain has the potential to induce neuroprotection or neurodegeneration in Parkinson’s disease pathology. tPA is best known for its role in fibrin degradation in the blood and its use in the clinic as a treatment for ischemic stroke. Beyond the fibrinolytic system, tPA expression is reported in the central nervous system (CNS), where it regulates different brain functions by mechanisms independent of fibrin degradation, including neuronal plasticity, neuroinflammation, and blood-brain barrier (BBB) integrity. We found that tPA is associated with neuronal damage in ischemic stroke and Parkinson’s diseases by two different downstream mechanisms that involve both tPA proteolytic and non-proteolytic action in the brain. In ischemic stroke, we showed that tPA proteolytic activity mediates both beneficial and harmful effects depending on tPA's location and proteolytic substrate. We found that both tPA inhibitors, plasminogen activator inhibitor -1 (PAI-1), and neuroserpin (Nsp) were involved in ischemic stroke pathology. These studies were the basis for the development of a novel combination therapy to treat ischemic stroke that blocks tPA-mediated BBB disruption and enhances endogenous fibrinolysis. In contrast, in Parkinson’s disease neither of the two main inhibitors of tPA, Nsp or PAI-1 was associated with neuronal damage. However, we identified for the first time that tPA was expressed in GABAergic axons in the substantia nigra and that the presence of tPA was necessary to induce neuronal degeneration via a non-proteolytic interaction involving the N-methyl-D-aspartate receptor subunit 1 (NMDAR1). Using an antibody that blocks tPA interaction with NMDAR1, we propose a therapeutic strategy to prevent tPA-mediated dopaminergic neuron degeneration in Parkinson’s disease. Our work showcases the complex and diverse functions of tPA in the brain that transcends its well-established function in fibrin degradation. Lastly, noradrenergic neurons in the locus coeruleus (LC) are the main source of norepinephrine in the brain, and degeneration of these neurons is reported in the early stages of Parkinson’s disease. It has been suggested that degeneration of LC noradrenergic neurons may contribute to the degeneration of other neuronal populations in Parkinson’s disease. We found that stimulation of the norepinephrine receptor, β2-adrenergic receptor (AR) prevented neuronal degeneration in Parkinson’s disease whereas β1-AR stimulation promoted neuronal cell death in the substantia nigra. This suggests that a specific β2-AR agonist may be preferred for therapeutic purposes in Parkinson’s disease pathology