Vascular endothelial growth factor promotes transdifferentiation of astrocytes into neurons via activation of the MAPK/Erk-Pax6 signal pathway.

Reactive astrocytes can be transformed into new neurons. Vascular endothelial growth factor (VEGF) promotes the transformation of reactive astrocytes into neurons in ischemic brain. Therefore, in this study, the molecular mechanism of VEGF's effect on ischemia/hypoxia-induced astrocyte to neuron transformation was investigated in the models of rat middle cerebral artery occlusion (MCAO) and in astrocyte culture with oxygen and glucose deprivation (OGD). We found that VEGF enhanced ischemia-induced Pax6, a neurogenic fate determinant, expression and Erk phosphorylation in reactive astrocytes and reduced infarct volume of rat brain at 3 days after MCAO, which effects could be blocked by administration of U0126, a MAPK/Erk inhibitor. In cultured astrocytes, VEGF also enhanced OGD-induced Erk phosphorylation and Pax6 expression, which was blocked by U0126, but not wortmannin, a PI3K/Akt inhibitor, or SB203580, a MAPK/p38 inhibitor, suggesting VEGF enhanced Pax6 expression via activation of MAPK/Erk pathway. OGD induced the increase of miR365 and VEGF inhibited the increase of OGD-induced miR365 expression. However, miR365 agonists blocked VEGF-enhanced Pax6 expression in hypoxic astrocytes, but did not block VEGF-enhanced Erk phosphorylation. We further found that VEGF promoted OGD-induced astrocyte-converted to neuron. Interestingly, both U0126 and Pax6 RNAi significantly reduced enhancement of VEGF on astrocytes-to-neurons transformation, as indicated Dcx and MAP2 immunopositive signals in reactive astrocytes. Moreover, those transformed neurons become mature and functional. We concluded that VEGF enhanced astrocytic neurogenesis via the MAPK/Erk-miR-365-Pax6 signal axis. The results also indicated that astrocytes play important roles in the reconstruction of neurovascular units in brain after stroke.

MicroRNA-365 Knockdown Prevents Ischemic Neuronal Injury by Activating Oxidation Resistance 1-Mediated Antioxidant Signals.

MicroRNA-365 (miR-365) is upregulated in the ischemic brain and is involved in oxidative damage in the diabetic rat. However, it is unclear whether miR-365 regulates oxidative stress (OS)-mediated neuronal damage after ischemia. Here, we used a transient middle cerebral artery occlusion model in rats and the hydrogen peroxide-induced OS model in primary cultured neurons to assess the roles of miR-365 in neuronal damage. We found that miR-365 exacerbated ischemic brain injury and OS-induced neuronal damage and was associated with a reduced expression of OXR1 (Oxidation Resistance 1). In contrast, miR-365 antagomir alleviated both the brain injury and OXR1 reduction. Luciferase assays indicated that miR-365 inhibited OXR1 expression by directly targeting the 3'-untranslated region of Oxr1. Furthermore, knockdown of OXR1 abolished the neuroprotective and antioxidant effects of the miR-365 antagomir. Our results suggest that miR-365 upregulation increases oxidative injury by inhibiting OXR1 expression, while its downregulation protects neurons from oxidative death by enhancing OXR1-mediated antioxidant signals.

Vascular endothelial growth factor increases the function of calcium-impermeable AMPA receptor GluA2 subunit in astrocytes via activation of protein kinase C signaling pathway.

Astrocytic calcium signaling plays pivotal roles in the maintenance of neural functions and neurovascular coupling in the brain. Vascular endothelial growth factor (VEGF), an original biological substance of vessels, regulates the movement of calcium and potassium ions across neuronal membrane. In this study, we investigated whether and how VEGF regulates glutamate-induced calcium influx in astrocytes. We used cultured astrocytes combined with living cell imaging to detect the calcium influx induced by glutamate. We found that VEGF quickly inhibited the glutamate/hypoxia-induced calcium influx, which was blocked by an AMPA receptor antagonist CNQX, but not D-AP5 or UBP310, NMDA and kainate receptor antagonist, respectively. VEGF increased phosphorylation of PKCα and AMPA receptor subunit GluA2 in astrocytes, and these effects were diminished by SU1498 or calphostin C, a PKC inhibitor. With the pHluorin assay, we observed that VEGF significantly increased membrane insertion and expression of GluA2, but not GluA1, in astrocytes. Moreover, siRNA-produced knockdown of GluA2 expression in astrocytes reversed the inhibitory effect of VEGF on glutamate-induced calcium influx. Together, our results suggest that VEGF reduces glutamate-induced calcium influx in astrocytes via enhancing PKCα-mediated GluA2 phosphorylation, which in turn promotes the membrane insertion and expression of GluA2 and causes AMPA receptors to switch from calcium-permeable to calcium-impermeable receptors, thereby inhibiting astrocytic calcium influx. The present study reveals that excitatory neurotransmitter glutamate-mediated astrocytic calcium influx can be regulated by vascular biological factor via activation of AMPA receptor GluA2 subunit and uncovers a novel coupling mechanism between astrocytes and endothelial cells within the neurovascular unit.

MicroRNA-365 modulates astrocyte conversion into neuron in adult rat brain after stroke by targeting Pax6.

Reactive astrocytes induced by ischemia can transdifferentiate into mature neurons. This neurogenic potential of astrocytes may have therapeutic value for brain injury. Epigenetic modifications are widely known to involve in developmental and adult neurogenesis. PAX6, a neurogenic fate determinant, contributes to the astrocyte-to-neuron conversion. However, it is unclear whether microRNAs (miRs) modulate PAX6-mediated astrocyte-to-neuron conversion. In the present study we used bioinformatic approaches to predict miRs potentially targeting Pax6, and transient middle cerebral artery occlusion (MCAO) to model cerebral ischemic injury in adult rats. These rats were given striatal injection of glial fibrillary acidic protein targeted enhanced green fluorescence protein lentiviral vectors (Lv-GFAP-EGFP) to permit cell fate mapping for tracing astrocytes-derived neurons. We verified that miR-365 directly targets to the 3'-UTR of Pax6 by luciferase assay. We found that miR-365 expression was significantly increased in the ischemic brain. Intraventricular injection of miR-365 antagomir effectively increased astrocytic PAX6 expression and the number of new mature neurons derived from astrocytes in the ischemic striatum, and reduced neurological deficits as well as cerebral infarct volume. Conversely, miR-365 agomir reduced PAX6 expression and neurogenesis, and worsened brain injury. Moreover, exogenous overexpression of PAX6 enhanced the astrocyte-to-neuron conversion and abolished the effects of miR-365. Our results demonstrate that increase of miR-365 in the ischemic brain inhibits astrocyte-to-neuron conversion by targeting Pax6, whereas knockdown of miR-365 enhances PAX6-mediated neurogenesis from astrocytes and attenuates neuronal injury in the brain after ischemic stroke. Our findings provide a foundation for developing novel therapeutic strategies for brain injury.

Neurovascular Interaction Promotes the Morphological and Functional Maturation of Cortical Neurons.

Brain microvascular endothelial cells (BMEC) have been found to guide the migration, promote the survival and regulate the differentiation of neural cells. However, whether BMEC promote development and maturation of immature neurons is still unknown. Therefore, in this study, we used a direct endothelium-neuron co-culture system combined with patch clamp recordings and confocal imaging analysis, to investigate the effects of endothelial cells on neuronal morphology and function during development. We found that endothelial cells co-culture or BMEC-conditioned medium (B-CM) promoted neurite outgrowth and spine formation, accelerated electrophysiological development and enhanced synapse function. Moreover, B-CM treatment induced vascular endothelial growth factor (VEGF) expression and p38 phosphorylation in the cortical neurons. Through pharmacological analysis, we found that incubation with SU1498, an inhibitor of VEGF receptor, abolished B-CM-induced p-p38 upregulation and suppressed the enhancement of synapse formation and transmission. SB203580, an inhibitor of p38 MAPK also blocked B-CM-mediated synaptic regulation. Together these results clearly reveal that the endothelium-neuron interactions promote morphological and functional maturation of neurons. In addition, neurovascular interaction-mediated promotion of neural network maturation relies on activation of VEGF/Flk-1/p38 MAPK signaling. This study provides novel aspects of endothelium-neuron interactions and novel mechanism of neurovascular crosstalk.

Neurovascular coupling protects neurons against hypoxic injury via inhibition of potassium currents by generation of nitric oxide in direct neuron and endothelium cocultures.

This study examined the effect of neuron-endothelial coupling on the survival of neurons after ischemia and the possible mechanism underlying that effect. Whole-cell patch-clamp experiments were performed on cortical neurons cultured alone or directly cocultured with brain microvascular endothelial cells (BMEC). Propidium iodide (PI) and NeuN staining were performed to examine neuronal death following oxygen and glucose deprivation (OGD). We found that the neuronal transient outward potassium currents (IA) decreased in the coculture system, whereas the outward delayed-rectifier potassium currents (IK) did not. Sodium nitroprusside, a NO donor, enhanced BMEC-induced IA inhibition and nitro-l-arginine methylester, a NOS inhibitor, partially prevented this inhibition. Moreover, the neurons directly cocultured with BMEC showed more resistance to OGD-induced injury compared with the neurons cultured alone, and that neuroprotective effect was abolished by treatment with NS5806, an activator of the IA. These results indicate that vascular endothelial cells assist neurons to prevent hypoxic injury via inhibiting neuronal IA by production of NO in the direct neuron-BMEC coculture system. These results further provide direct evidence of functional coupling between neurons and vascular endothelial cells. This study clearly demonstrates that vascular endothelial cells play beneficial roles in the pathophysiological processes of neurons after hypoxic injury, suggesting that the improvement of neurovascular coupling or functional remodeling may become an important therapeutic target for preventing brain injury.

Striatal astrocytes transdifferentiate into functional mature neurons following ischemic brain injury.

To determine whether reactive astrocytes stimulated by brain injury can transdifferentiate into functional new neurons, we labeled these cells by injecting a glial fibrillary acidic protein (GFAP) targeted enhanced green fluorescence protein plasmid (pGfa2-eGFP plasmid) into the striatum of adult rats immediately following a transient middle cerebral artery occlusion (MCAO) and performed immunolabeling with specific neuronal markers to trace the neural fates of eGFP-expressing (GFP(+)) reactive astrocytes. The results showed that a portion of striatal GFP(+) astrocytes could transdifferentiate into immature neurons at 1 week after MCAO and mature neurons at 2 weeks as determined by double staining GFP-expressing cells with ßIII-tubulin (GFP(+)-Tuj-1(+)) and microtubule associated protein-2 (GFP(+)-MAP-2(+)), respectively. GFP(+) neurons further expressed choline acetyltransferase, glutamic acid decarboxylase, dopamine receptor D2-like family proteins, and the N-methyl-D-aspartate receptor subunit R2, indicating that astrocyte-derived neurons could develop into cholinergic or GABAergic neurons and express dopamine and glutamate receptors on their membranes. Electron microscopy analysis indicated that GFP(+) neurons could form synapses with other neurons at 13 weeks after MCAO. Electrophysiological recordings revealed that action potentials and active postsynaptic currents could be recorded in the neuron-like GFP(+) cells but not in the astrocyte-like GFP(+) cells, demonstrating that new GFP(+) neurons possessed the capacity to fire action potentials and receive synaptic inputs. These results demonstrated that striatal astrocyte-derived new neurons participate in the rebuilding of functional neural networks, a fundamental basis for brain repair after injury. These results may lead to new therapeutic strategies for enhancing brain repair after ischemic stroke.