Urokinase-Type Plasminogen Activator Promotes Dendritic Spine Recovery and Improves Neurological Outcome Following Ischemic Stroke

Abstract

Spines are dendritic protrusions that receive most of the excitatory input in the brain. Early after the onset of cerebral ischemia dendritic spines in the peri-infarct cortex are replaced by areas of focal swelling, and their re-emergence from these varicosities is associated with neurological recovery after acute ischemic stroke (AIS). Urokinase-type plasminogen activator (uPA) is a serine proteinase that plays a central role in tissue remodeling via binding to the urokinase plasminogen activator receptor (uPAR). We report that cerebral cortical neurons release uPA during the recovery phase from ischemic stroke in vivo or hypoxia in vitro. Although uPA does not have an effect on ischemia-or hypoxia-induced neuronal death, genetic deficiency of uPA (uPA Ϫ/Ϫ ) or uPAR (uPAR) abrogates functional recovery after AIS. Treatment with recombinant uPA after ischemic stroke induces neurological recovery in wild-type and uPA Ϫ/Ϫ but not in uPAR Ϫ/Ϫ mice. Diffusion tensor imaging studies indicate that uPA Ϫ/Ϫ mice have increased water diffusivity and decreased anisotropy associated with impaired dendritic spine recovery and decreased length of distal neurites in the peri-infarct cortex. We found that the excitotoxic injury induces the clustering of uPAR in dendritic varicosities, and that the binding of uPA to uPAR promotes the reorganization of the actin cytoskeleton and re-emergence of dendritic filopodia from uPAR-enriched varicosities. This effect is independent of uPA's proteolytic properties and instead is mediated by Rac-regulated profilin expression and cofilin phosphorylation. Our data indicate that binding of uPA to uPAR promotes dendritic spine recovery and improves functional outcome following AIS.

Urokinase-type Plasminogen Activator (uPA) and its Receptor (uPAR) Promote Neurorepair in the Ischemic Brain

“…However, the expression of both, the ligand and its receptor, increases in neurons following different forms of injury. These findings are in line with observations by others indicating that the expression of uPAR increases within the first few hours of peripheral nerve [22] , spinal cord [23] and cortical neurons [6,13] injury, and has led to propose that uPAR is a marker of central nervous system damage and a potential therapeutic target to promote neurorepair.…”

“…Surprisingly, this hypothesis was proven wrong as we failed to detect a difference in the volume of the ischemic lesion between mice genetically deficient in uPA (uPA -/-) and their Wt littermate controls 24 hours after 60 minutes of MCAO. In contrast, we found that compared to Wt littermate controls, uPA -/-and uPAR -/-mice have a protracted recovery in neurological function following MCAO, and that treatment with ruPA or the release of endogenous uPA induces recovery in Wt and uPA -/-, but not in uPAR -/-mice [6,13] . In summary, these data indicate that the expression of uPA and uPAR increase in the sub-acute, recovery stages of ischemic stroke, and suggest that uPA binding to uPAR plays a central role in the process of neurorepair following an acute ischemic injury.…”

“…First we measured the release of uPA from wild-type (Wt) adult cerebral cortical neurons exposed to 60 minutes of OGD. Unexpectedly, we found that neurons do not release uPA while they are exposed to OGD but 6 -24 hours after they begin recovering from the hypoxic injury [13] . To characterize the in vivo significance of these findings we quantified the expression of uPA in the ischemic tissue of Wt mice immediately after 30 and 60 minutes of MCAO, and 1 -24 hours after 60 minutes of MCAO and successful reperfusion (recovery).…”

“…Remarkably, following axonal injury the expression of uPAR in adult neurons increases again, particularly in growth cones of injured axons, to levels comparable to those observed during the early stages of development [6] . The expression of uPA in the adult brain follows a pattern very similar to uPAR: low in the healthy brain and high following an injury [13] . In summary, the experimental evidence available to this date indicates that the expression of neuronal uPA and uPAR is high in the early stages of development and decreases to almost undetectable levels in mature cells.…”

“…The expression of uPA and uPAR in the developing central nervous system (CNS) is particularly high in neurons [6,13,14] , microglia [15] and astrocytes [16] . However, during maturation it progressively decreases to reach very low levels in the adult brain.…”