A Cross Talk between Neuronal Urokinase-type Plasminogen Activator (uPA) and Astrocytic uPA Receptor (uPAR) Promotes Astrocytic Activation and Synaptic Recovery in the Ischemic Brain

Urokinase-type plasminogen activator (uPA) is a serine proteinase that, upon binding to its receptor (uPAR), catalyzes the conversion of plasminogen into plasmin on the cell surface. Our previous studies indicate that uPA and uPAR expression increase in the ischemic brain during the recovery phase from an acute ischemic injury and that uPA binding to uPAR promotes neurological recovery after an acute ischemic stroke. Here, we used male mice genetically deficient on either uPA (uPA^-/-) or uPAR (uPAR^-/-) or with a four-amino acid substitution into the growth factor domain of uPA that abrogates its binding to uPAR (Plat^GFDhu/GFDhu) to investigate the mechanism whereby uPA promotes neurorepair in the ischemic brain. We found that neurons release uPA and astrocytes recruit uPAR to their plasma membrane during the recovery phase from a hypoxic injury and that binding of neuronal uPA to astrocytic uPAR induces astrocytic activation by a mechanism that does not require plasmin generation, but instead is mediated by extracellular signal-regulated kinase 1/2 (ERK1/2)-regulated phosphorylation of the signal transducer and activator of transcription 3 (STAT3). We report that uPA/uPAR binding is necessary and sufficient to induce astrocytic activation in the ischemic brain and that astrocytes activated by neuronal uPA promote synaptic recovery in neurons that have suffered an acute hypoxic injury via a mechanism mediated by astrocytic thrombospondin-1 (TSP1) and synaptic low-density lipoprotein receptor-related protein-1 (LRP1). In summary, we show that uPA/uPAR-induced astrocytic activation mediates a cross talk between astrocytes and injured neurons that promotes synaptic recovery in the ischemic brain.SIGNIFICANCE STATEMENT To date, there is no therapeutic strategy to promote synaptic recovery in the injured brain. Here, we show that neurons release urokinase-type plasminogen activator (uPA) and astrocytes recruit the uPA receptor (uPAR) to their plasma membrane during the recovery phase from a hypoxic injury. We found that binding of neuronal uPA to astrocytic uPAR promotes astrocytic activation and that astrocytes activated by uPA-uPAR binding promote synaptic recovery in neurons that have suffered a hypoxic injury by a mechanism that does not require plasmin generation, but instead is mediated by ERK1/2-regulated STAT3 phosphorylation, astrocytic thrombospondin-1 (TSP1) and synaptic low-density lipoprotein receptor-related protein-1 (LRP1). Our work unveils a new biological function for uPA-uPAR as mediator of a neuron-astrocyte cross talk that promotes synaptic recovery in the ischemic brain.