Neuropilin 2 deficiency does not affect cortical neuronal viability in response to oxygen-glucose-deprivation and transient middle cerebral artery occlusion

Role of Semaphorins in Ischemic Stroke

Ischemic stroke is one of the major causes of neurological morbidity and mortality in the world. Although the management of ischemic stroke has been improved significantly, it still imposes a huge burden on the health and property. The integrity of the neurovascular unit (NVU) is closely related with the prognosis of ischemic stroke. Growing evidence has shown that semaphorins, a family of axon guidance cues, play a pivotal role in multiple pathophysiological processes in NVU after ischemia, such as regulating the immune system, angiogenesis, and neuroprotection. Modulating the NVU function via semaphorin signaling has a potential to develop a novel therapeutic strategy for ischemic stroke. We, therefore, review recent progresses on the role of semphorin family members in neurons, glial cells and vasculature after ischemic stroke.

Phaseic Acid, an Endogenous and Reversible Inhibitor of Glutamate Receptors in Mouse Brain

Phaseic acid (PA) is a phytohormone regulating important physiological functions in higher plants. Here, we show the presence of naturally occurring (-)-PA in mouse and rat brains. (-)-PA is exclusively present in the choroid plexus and the cerebral vascular endothelial cells. Purified (-)-PA has no toxicity and protects cultured cortical neurons against glutamate toxicity through reversible inhibition of glutamate receptors. Focal occlusion of the middle cerebral artery elicited a significant induction in (-)-PA expression in the cerebrospinal fluid but not in the peripheral blood. Importantly, (-)-PA induction only occurred in the penumbra area, indicting a protective role of PA in the brain. Indeed, elevating the (-)-PA level in the brain reduced ischemic brain injury, whereas reducing the (-)-PA level using a monoclonal antibody against (-)-PA increased ischemic injury. Collectively, these studies showed for the first time that (-)-PA is an endogenous neuroprotective molecule capable of reversibly inhibiting glutamate receptors during ischemic brain injury.

NRP2 transcriptionally regulates its downstream effector WDFY1

Neuropilins (NRPs) are cell surface glycoproteins that often act as co-receptors for plexins and VEGF family receptors. Neuropilin-2 (NRP2), a family member of NRPs, was shown to regulate autophagy and endocytic trafficking in cancer cells, a function distinctly different from its role as a co-receptor. WD Repeat and FYVE domain containing 1 (WDFY1)–protein acts downstream of NRP2 for this function. Our results indicated that NRP2 maintains an optimum concentration of WDFY1 by negatively regulating its expression. Since increased expression of WDFY1 reduces the endocytic activity, maintenance of WDFY1 level is crucial in metastatic cancer cells to sustain high endocytic activity, essential for promotion of oncogenic activation and cancer cell survival. Here, we have delineated the underlying molecular mechanism of WDFY1 synthesis by NRP2. Our results indicated that NRP2 inhibits WDFY1 transcription by preventing the nuclear localization of a transcription factor, Fetal ALZ50-reactive clone 1 (FAC1). Our finding is novel as transcriptional regulation of a gene by NRP2 axis has not been reported previously. Regulation of WDFY1 transcription by NRP2 axis is a critical event in maintaining metastatic phenotype in cancer cells. Thus, inhibiting NRP2 or hyper-activating WDFY1 can be an effective strategy to induce cell death in metastatic cancer.

Semaphorin3A elevates vascular permeability and contributes to cerebral ischemia-induced brain damage

Semaphorin 3A (Sema3A) increased significantly in mouse brain following cerebral ischemia. However, the role of Sema3A in stroke brain remains unknown. Our aim was to determine wether Sema3A functions as a vascular permeability factor and contributes to ischemic brain damage. Recombinant Sema3A injected intradermally to mouse skin, or stereotactically into the cerebral cortex, caused dose- and time-dependent increases in vascular permeability, with a degree comparable to that caused by injection of a known vascular permeability factor vascular endothelial growth factor receptors (VEGF). Application of Sema3A to cultured endothelial cells caused disorganization of F-actin stress fibre bundles and increased endothelial monolayer permeability, confirming Sema3A as a permeability factor. Sema3A-mediated F-actin changes in endothelial cells were through binding to the neuropilin2/VEGFR1 receptor complex, which in turn directly activates Mical2, a F-actin modulator. Down-regulation of Mical2, using specific siRNA, alleviated Sema3A-induced F-actin disorganization, cellular morphology changes and endothelial permeability. Importantly, ablation of Sema3A expression, cerebrovascular permeability and brain damage were significantly reduced in response to transient middle cerebral artery occlusion (tMCAO) and in a mouse model of cerebral ischemia/haemorrhagic transformation. Together, these studies demonstrated that Sema3A is a key mediator of cerebrovascular permeability and contributes to brain damage caused by cerebral ischemia.

Collapsin response mediator protein 3 deacetylates histone H4 to mediate nuclear condensation and neuronal death

CRMP proteins play critical regulatory roles during semaphorin-mediated neurite outgrowth, neuronal differentiation and death. Albeit having a high degree of structure and sequence resemblance to that of liver dihydropyrimidinase, purified rodent brain CRMPs do not hydrolyze dihydropyrimidinase substrates. Here we found that mouse CRMP3 has robust histone H4 deacetylase activity. During excitotoxicity-induced mouse neuronal death, calpain-cleaved, N-terminally truncated CRMP3 undergoes nuclear translocation to cause nuclear condensation through deacetylation of histone H4. CRMP3-mediated deacetylation of H4 leads to de-repression of the E2F1 gene transcription and E2F1-dependent neuronal death. These studies revealed a novel mechanism of CRMP3 in neuronal death. Together with previous well established bodies of literature that inhibition of histone deacetylase activity provides neuroprotection, we envisage that inhibition of CRMP3 may represent a novel therapeutic approach towards excitotoxicity-induced neuronal death.

Cerebral endothelial expression of Robo1 affects brain infiltration of polymorphonuclear neutrophils during mouse stroke recovery

Increased brain infiltration of polymorphonuclear neutrophils (PMNs) occurs early after stroke and is important in eliciting brain inflammatory response during stroke recovery. In order to understand the molecular mechanism of PMN entry, we investigated the expression and requirement for Slit1, a chemorepulsive guidance cue, and its cognate receptor, Robo1, in a long-term recovery mouse model of cerebral ischemia. The expression levels of Robo1 were significantly decreased bilaterally at 24h following reperfusion. Robo1 expression levels remained suppressed in the ipsilateral cortex until 28d post MCAO-reperfusion, while the levels of Robo1 in the contralateral cortex recovered to the level of sham-operated mouse by 7d reperfusion. Circulating PMNs express high levels of Slit1, but not Robo1. Influx of PMNs into the ischemic core area occurred early (24h) after cerebral ischemia, when endothelial Robo1 expression was significantly reduced in the ischemic brain, indicating that Robo1 may form a repulsive barrier to PMN entry into the brain parenchyma. Indeed, blocking Slit1 on PMNs in a transwell migration assay in combination with an antibody blocking of Robo1 on human umbilical vein endothelial cells (HUVEC) significantly increased PMN transmigration during oxygen glucose deprivation, an in vitro model of ischemia. Collectively, in the normal brain, the presence of Slit1 on PMNs, and Robo1 on cerebral endothelial cells, generated a repulsive force to prevent the infiltration of PMNs into the brain. During stroke recovery, a transient reduction in Robo1 expression on the cerebral endothelial cells allowed the uncontrolled infiltration of Slit1-expressing PMNs into the brain causing inflammatory reactions.

Transient and bilateral increase in Neuropilin-1, Fer kinase and collapsin response mediator proteins within membrane rafts following unilateral occlusion of the middle cerebral artery in mouse

Membrane rafts, rich in sphingolipids and cholesterol, are membrane microdomains important in neuronal domain-specific signaling events such as during axonal outgrowth and neuronal death. The present study seeks to determine the spatiotemporal association of several axonal guidance signaling molecules with membrane rafts. These molecules are Neuropilin-1 (NRP-1), Fer Kinase, and collapsin response mediator proteins (CRMPs), which are known to have important functions in axonal outgrowth and neuronal death caused by cerebral ischemia. Mice were subjected to sham or a 1h unilateral middle cerebral artery occlusion (MCAO) followed by a time course of reperfusion up to 24h. Brain cortices were separated and membrane rafts were extracted based on their insolubility in Triton X-100 and separation by sucrose gradient fractionation. We demonstrate the early and transient induction of NRP-1 and CRMP-2 in membrane rafts in both ipsilateral and contralateral hemispheres, in contrast to an early, but sustained elevation of Fer kinase and other CRMPs (1, 3, 4, 5) in response to unilateral MCAO. The fact that NRP1/Fer kinase/CRMP-2 co-localize in membrane rafts early during ischemic injury suggests that the membrane rafts may form a scaffold to support and initiate NRP1/Fer/CRMP-2-mediated signal transduction in neuronal damage response during ischemia-reperfusion. Further understanding of the time-specific and membrane domain-specific protein-protein interaction may lead to the identification of therapeutic targets for stroke.