The uPA/uPAR system in astrocytic wound healing

Abstract

The repair of injured tissue is a highly complex process that involves cell proliferation, differentiation, and migration. Cell migration requires the dismantling of intercellular contacts in the injured zone and their subsequent reconstitution in the wounded area. Urokinase-type plasminogen activator (uPA) is a serine proteinase found in multiple cell types including endothelial cells, smooth muscle cells, monocytes, and macrophages. A substantial body of experimental evidence with different cell types outside the central nervous system indicates that the binding of uPA to its receptor (uPAR) on the cell surface prompts cell migration by inducing plasmin-mediated degradation of the extracellular matrix. In contrast, although uPA and uPAR are abundantly found in astrocytes and uPA binding to uPAR triggers astrocytic activation, it is unknown if uPA also plays a role in astrocytic migration. Neuronal cadherin is a member of cell adhesion proteins pivotal for the formation of cell-cell contacts between astrocytes. More specifically, while the extracellular domain of neuronal cadherin interacts with the extracellular domain of neuronal cadherin in neighboring cells, its intracellular domain binds to β-catenin, which in turn links the complex to the actin cytoskeleton. Glycogen synthase kinase 3β is a serine-threonine kinase that prevents the cytoplasmic accumulation of β-catenin by inducing its phosphorylation at Ser33, Ser37, and Ser41, thus activating a sequence of events that lead to its proteasomal degradation. The data discussed in this perspective indicate that astrocytes release uPA following a mechanical injury, and that binding of this uPA to uPAR on the cell membrane induces the detachment of β-catenin from the intracellular domain of neuronal cadherin by triggering its extracellular signal-regulated kinase 1/2-mediated phosphorylation at Tyr650. Remarkably, this is followed by the cytoplasmic accumulation of β-catenin because uPA-induced extracellular signal-regulated kinase 1/2 activation also phosphorylates lipoprotein receptor-related protein 6 at Ser1490, which in turn, by recruiting glycogen synthase kinase 3β to its intracellular domain abrogates its effect on β-catenin. The cytoplasmic accumulation of β-catenin is followed by its nuclear translocation, where it induces the expression of uPAR, which is required for the migration of astrocytes from the injured edge into the wounded area.

Keywords: Wnt-β-catenin pathway; astrocytes; lipoprotein receptor-related protein 6; plasmin; urokinase receptor; urokinase-type plasminogen activator; wound healing; β-catenin.

Figure 1

Mechanism whereby urokinase-type plasminogen activator (uPA)/urokinase-type plasminogen activator receptor (uPAR) binding triggers the detachment of β-catenin from the intracellular domain of N-cadherin (NCAD-ICD). (A, B) A mechanical injury to a monolayer of astrocytes induces the cleavage of the extracellular domain of NCAD (NCAD-ECD), the release of uPA, and the membrane recruitment of uPAR. (C) uPA/uPAR binding triggers the interaction of uPAR with integrins, which then activate extracellular signal-regulated kinase 1/2 (ERK1/2), thus leading to ERK1/2-mediated phosphorylation of β-catenin at Tyr654. (D) Once phosphorylated at Tyr654, β-catenin detaches from NCAD-ICD.

Figure 2

Mechanism whereby urokinase-type plasminogen activator (uPA)/urokinase-type plasminogen activator receptor (uPAR) binding activates the Wnt-β-catenin pathway in astrocytes. (A) Binding of Wnt ligands to Frizzled receptors and lipoprotein receptor-related protein 6 (LRP6) on the astrocytic membrane triggers the phosphorylation of the intracellular domain of LRP6 at Ser1490, which in turn leads to Dishevelled (Dvl)-mediated recruitment of the axin inhibitory complex, thus blocking glycogen synthase kinase 3β (GSK3β)-induced proteasomal degradation of β-catenin. (B) Binding of uPA to uPAR on the astrocytic membrane prompts integrin-mediated activation of extracellular signal-regulated kinase 1/2 (ERK1/2), which in turn phosphorylates LRP6 at Ser1490, independently of Wnt ligands binding to LRP6 and Frizzled. As depicted in A, this also leads to inhibition of GSK3β-triggered proteasomal degradation of β-catenin. (C) Following its cytoplasmic accumulation, β-catenin is translocated to the nucleus where it induces the expression of uPAR, which then triggers the migration of astrocytes from the injured edge into the wounded area.