Urokinase-type plasminogen activator (uPA) promotes ezrin-mediated reorganization of the synaptic cytoskeleton in the ischemic brain

Abstract

Synaptic repair in the ischemic brain is a complex process that requires reorganization of the actin cytoskeleton. Ezrin, radixin, and moesin (ERM) are a group of evolutionarily conserved proteins that link the plasma membrane to the actin cytoskeleton and act as scaffolds for signaling transduction. Urokinase-type plasminogen activator (uPA) is a serine proteinase that upon binding to the urokinase-type plasminogen activator receptor (uPAR) catalyzes the conversion of plasminogen into plasmin on the cell surface and activates intracellular signaling pathways. Early studies indicate that uPA and uPAR expression increase during the recovery phase from an ischemic stroke and that uPA binding to uPAR promotes neurorepair in the ischemic brain. The and studies presented here show that either the release of neuronal uPA or treatment with recombinant uPA induces the local synthesis of ezrin in the synapse and the recruitment of β3-integrin to the postsynaptic density (PSD) of cerebral cortical neurons by a plasminogen-independent mechanism. We found that β3-integrin has a double effect on ezrin, inducing its recruitment to the PSD via the intercellular adhesion molecule-5 (ICAM-5) and its subsequent activation by phosphorylation at Thr-567. Finally, our data indicate that by triggering the reorganization of the actin cytoskeleton in the postsynaptic terminal, active ezrin induces the recovery of dendritic spines and synapses that have been damaged by an acute ischemic stroke. In summary, our data show that uPA-uPAR binding promotes synaptic repair in the ischemic brain via ezrin-mediated reorganization of the actin cytoskeleton in the postsynaptic terminal.

Urokinase receptor and tissue plasminogen activator as immediate‐early genes in pentylenetetrazole‐induced seizures in the mouse brain

“…Functionally, post‐seizure uPA expression is supposed to be neuroprotective (Cho et al., ; Gur‐Wahnon et al., ; Lahtinen et al., ). The pivotal role of uPA system in brain recovery was also shown for ischemic injury (Diaz, Merino, Manrique, Cheng, & Yepes, ; Diaz et al., ; Merino, Diaz, Manrique, Cheng, & Yepes, ). Hypoxic injury resulted in prominent uPA release from neurons as well as uPAR exposure on the astrocytic plasma membranes (Diaz et al., ).…”

Urokinase-type Plasminogen Activator Induces Neurorepair in the Ischemic Brain

“…The PA system is an enzymatic cascade of proteases and their inhibitors that was initially thought to be solely involved in the conversion of plasminogen into plasmin by the activity of two serine proteases: tissue-type plasminogen activator (tPA) and uPA [ 17 ]. However, soon thereafter it was found that fibrinolysis is not the only role of tPA and uPA, and instead that both PAs are also found in the synapse where they play a central role in the development of synaptic plasticity via mechanisms that not always require plasmin generation [ 18 – 23 ]. Significantly, cerebral ischemia induces the secretion of both proteases into the synaptic cleft [ 24 , 25 ], but in contrast with tPA that is rapidly released after the onset of the ischemic injury, uPA is secreted only during the recovery phase [ 25 ].…”

“…Most of the biological effects of uPA are mediated by its binding to uPAR, a glycosylphosphatidylinositol (GPI)-anchored protein with three domains (D1, D2 and D3) with high affinity for single- and two-chain uPA and uPA’s amino terminal factor (ATF; assembled by the first two domains of uPA) [ 27 ]. Importantly, uPAR lacks a cytosolic domain, and thus needs transmembrane co-receptors, such as integrins [ 20 , 23 , 28 ] (transmembrane receptors assembled by α and β subunits that bind to different ligands in the extracellular matrix such as laminin, collagen, fibronectin, and vitronectin [ 29 ]), and the low density lipoprotein receptor-related protein 1 (LRP1; a member of the low density lipoprotein receptor gene family [ 20 , 30 ], to activate intracellular signaling pathways via plasminogen-dependent and -independent mechanisms.…”

“…A more complete understanding of the effect of uPA/uPAR binding on dendritic spine recovery in the ischemic brain was attained when a combination of proteomic and biochemical studies indicated that uPA induces the expression of ezrin in dendritic spines via a mechanism that does not require plasmin generation. This work revealed that uPA/uPAR binding induces not only the synthesis of ezrin but also its recruitment to the post-synaptic density via β3-integrin and the intercellular adhesion molecule-5 (ICAM-5), and its subsequent β3-integrin-mediated activation by phosphorylation [ 23 ]. Additionally, it was demonstrated that uPA-induced synthesis and activation of ezrin promotes the reorganization of the actin cytoskeleton in the injured dendritic spine, and that this leads not only to its recovery but also to the formation of proximal dendritic branches [ 23 ].…”

“…This work revealed that uPA/uPAR binding induces not only the synthesis of ezrin but also its recruitment to the post-synaptic density via β3-integrin and the intercellular adhesion molecule-5 (ICAM-5), and its subsequent β3-integrin-mediated activation by phosphorylation [ 23 ]. Additionally, it was demonstrated that uPA-induced synthesis and activation of ezrin promotes the reorganization of the actin cytoskeleton in the injured dendritic spine, and that this leads not only to its recovery but also to the formation of proximal dendritic branches [ 23 ]. The finding that uPA has an effect on ICAM-5 is particularly relevant because this intercellular adhesion molecule is pivotal for dendritic outgrowth and the formation and maintenance of dendritic filopodia [ 39 ].…”