Shuzhen Guo

Cell–cell Signaling in the Neurovascular Unit

Historically, the neuron has been the conceptual focus for almost all of neuroscience research. In recent years, however, the concept of the neurovascular unit has emerged as a new paradigm for investigating both physiology and pathology in the CNS. This concept proposes that a purely neurocentric focus is not sufficient, and emphasizes that all cell types in the brain including neuronal, glial and vascular components, must be examined in an integrated context. Cell–cell signaling and coupling between these different compartments form the basis for normal function. Disordered signaling and perturbed coupling form the basis for dysfunction and disease. In this mini-review, we will survey four examples of this phenomenon: hemodynamic neurovascular coupling linking blood flow to brain activity; cellular communications that evoke the blood–brain barrier phenotype; parallel systems that underlie both neurogenesis and angiogenesis in the CNS; and finally, the potential exchange of trophic factors that may link neuronal, glial and vascular homeostasis.

Fingolimod provides long‐term protection in rodent models of cerebral ischemia

The sphingosine‐1‐phosphate (S1P) receptor agonist fingolimod (FTY720), that has shown efficacy in advanced multiple sclerosis clinical trials, decreases reperfusion injury in heart, liver, and kidney. We therefore tested the therapeutic effects of fingolimod in several rodent models of focal cerebral ischemia. To assess the translational significance of these findings, we asked whether fingolimod improved long‐term behavioral outcomes, whether delayed treatment was still effective, and whether neuroprotection can be obtained in a second species.

Neuroprotection via matrix-trophic coupling between cerebral endothelial cells and neurons

The neurovascular unit is an emerging concept that emphasizes homeostatic interactions between endothelium and cerebral parenchyma. Here, we show that cerebral endothelium are not just inert tubes for delivering blood, but they also secrete trophic factors that can be directly neuroprotective. Conditioned media from cerebral endothelial cells broadly protects neurons against oxygen-glucose deprivation, oxidative damage, endoplasmic reticulum stress, hypoxia, and amyloid neurotoxicity. This phenomenon is largely mediated by endothelial-produced brain-derived neurotrophic factor (BDNF) because filtering endothelial-conditioned media with TrkB-Fc eliminates the neuroprotective effect. Endothelial production of BDNF is sustained by β-1 integrin and integrin-linked kinase (ILK) signaling. Noncytotoxic levels of oxidative stress disrupts ILK signaling and reduces endothelial levels of neuroprotective BDNF. These data suggest that cerebral endothelium provides a critical source of homeostatic support for neurons. Targeting these signals of matrix and trophic coupling between endothelium and neurons may provide new therapeutic opportunities for stroke and other CNS disorders.

Beyond NMDA and AMPA glutamate receptors: emerging mechanisms for ionic imbalance and cell death in stroke

The glutamate receptor was one of the most intensely investigated targets for neuroprotection. However, numerous clinical trials of glutamate receptor antagonists for the treatment of stroke were unsuccessful. These failures have led to pessimism in the field. But recent advances could provide hope for the future. This minireview looks beyond the traditional mechanism of glutamate-receptor-driven excitotoxicity to identify other mechanisms of ionic imbalance. These advances include findings implicating sodium-calcium exchangers, hemichannels, volume-regulated anion channels, acid-sensing channels, transient receptor potential channels, nonselective cation channels and signaling cascades that mediate crosstalk between redundant pathways of cell death. Further in vivo validation of these pathways could ultimately lead us to new therapeutic targets for stroke, trauma and neurodegeneration.

Effects of neuroglobin overexpression on mitochondrial function and oxidative stress following hypoxia/reoxygenation in cultured neurons

Neuroglobin (Ngb) is a recently discovered tissue globin with a high affinity for oxygen that is widely and specifically expressed in neurons of vertebrate central and peripheral nervous systems. Our laboratory and others have shown Ngb overexpression can protect neurons against hypoxic/ischemic insults, but the underlying mechanisms remain poorly understood. In this study, we examined the effects of Ngb overexpression on mitochondrial function, oxidative stress, and neurotoxicity in primary cortical neurons following hypoxia/reoxygenation (H/R). Ngb‐overexpressing transgenic neurons (Ngb‐Tg) were significantly protected against H/R‐induced cell death. Rates of decline in ATP levels, MTT reduction, and mitochondrial membrane potential were significantly ameliorated in Ngb‐Tg neurons. Furthermore, Ngb overexpression reduced superoxide anion generation after H/R, whereas glutathione levels were significantly improved compared with WT controls. Taken together, these data suggest that Ngb is neuroprotective against hypoxia, in part by improving mitochondria function and decreasing oxidative stress.

Dysfunctional cell-cell signaling in the neurovascular unit as a paradigm for central nervous system disease.

The fundamental premise of neuroprotection has historically focused on the prevention of neuronal death. However, despite tremendous advances in the molecular biology of intraneuronal mechanisms and pathways, a clinically effective neuroprotectant does not yet exist. This problem is especially clear for stroke, for which a large number of neuroprotection trials have failed. The concept of the neurovascular unit emphasizes that cell-cell signaling among the various neuronal, glial, and vascular compartments underlies the homeostasis of normal brain function. Conversely, dysfunctional signaling within the neurovascular unit should contribute to disease. This minireview surveys recent data that support this basic idea, with examples drawn from experimental models broadly relevant to stroke and neurodegeneration.

Vascular Endothelial Growth Factor Regulates the Migration of Oligodendrocyte Precursor Cells

Originally identified as an angiogenic factor, vascular endothelial growth factor (VEGF-A) is now known to play multiple roles in the CNS, including the direct regulation of neuronal and astrocytic functions. Here, we ask whether VEGF-A can also have a novel role in white matter by modulating oligodendrocyte precursor cells (OPCs). OPCs were cultured from rat neonatal cortex. Expression of VEGF-receptor2/KDR/Flk-1 was confirmed with Western blot and immunostaining. VEGF-A did not affect proliferation or differentiation in OPC cultures, but VEGF-A promoted OPC migration in a concentration-dependent manner. Consistent with this migration phenotype, VEGF-A-treated OPCs showed reorganization of actin cytoskeleton in leading-edge processes. VEGF-A-induced migration and actin reorganization were inhibited by an anti-Flk-1 receptor-blocking antibody. Mechanistically, VEGF-A induced binding of focal adhesion kinase (FAK) with paxillin. The FAK inhibitor PF573228 reduced VEGF-A-induced OPC migration. VEGF-A signaling also evoked a transient rise in reactive oxygen species (ROS), and OPC migration was increased when antioxidants were removed from the culture media. Our findings demonstrate that VEGF-A can induce OPC migration via an ROS- and FAK-dependent mechanism, and suggest a novel role for VEGF-A in white-matter maintenance and homeostasis.

Crosstalk between oligodendrocytes and cerebral endothelium contributes to vascular remodeling after white matter injury

After stroke and brain injury, cortical gray matter recovery involves mechanisms of neurovascular matrix remodeling. In white matter, however, the mechanisms of recovery remain unclear. In this study, we demonstrate that oligodendrocytes secrete matrix metalloproteinase‐9 (MMP‐9), which accelerates the angiogenic response after white matter injury. In primary oligodendrocyte cultures, treatment with the proinflammatory cytokine interleukin‐1β (IL‐1β) induced an upregulation and secretion of MMP‐9. Conditioned media from IL‐1β‐stimulated oligodendrocytes significantly amplified matrigel tube formation in brain endothelial cells, indicating that MMP‐9 from oligodendrocytes can promote angiogenesis in vitro. Next, we asked whether similar signals and substrates operate after white matter injury in vivo. Focal white matter injury and demyelination was induced in mice via stereotactic injection of lysophosphatidylcholine into corpus callosum. Western blot analysis showed that IL‐1β expression was increased in damaged white matter. Immunostaining demonstrated MMP‐9 signals in myelin‐associated oligodendrocytic basic protein‐positive oligodendrocytes. Treatment with an IL‐1β‐neutralizing antibody suppressed the MMP‐9 response in oligodendrocytes. Finally, we confirmed that the broad spectrum MMP inhibitor GM6001 inhibited angiogenesis around the injury area in this white matter injury model. In gray matter, a neurovascular niche promotes cortical recovery after brain injury. Our study suggests that an analogous oligovascular niche may mediate recovery in white matter.

Mechanisms and targets for angiogenic therapy after stroke.

Stroke remains a major health problem worldwide, and is the leading cause of serious long-term disability. Recent findings now suggest that strategies to enhance angiogenesis after focal cerebral ischemia may provide unique opportunities to improve clinical outcomes during stroke recovery. In this mini-review, we survey emerging mechanisms and potential targets for angiogenic therapies in brain after stroke. Multiple elements may be involved, including growth factors, adhesion molecules and progenitor cells. Furthermore, cross talk between angiogenesis and neurogenesis may also provide additional substrates for plasticity and remodeling in the recovering brain. A better understanding of the molecular interplay between all these complex pathways may lead to novel therapeutic avenues for tackling this difficult disease.

Cellular Mechanisms of Neurovascular Damage and Repair After Stroke

The biological processes underlying stroke are complex, and patients have a narrow repertoire of therapeutic opportunities. After the National Institutes of Health (NIH) convened the Stroke Progress Review Group in 2001, stroke research shifted from having a purely neurocentric focus to adopting a more integrated view wherein dynamic interactions between all cell types contribute to function and dysfunction in the brain. This so-called “neurovascular unit” provides a conceptual framework that emphasizes cell–cell interactions between neuronal, glial, and vascular elements. Under normal conditions, signaling within the neurovascular unit helps maintain homeostasis. After stroke, cell–cell signaling is disturbed, leading to pathophysiology. More recently, emerging data now suggest that these cell–cell signaling mechanisms may also mediate parallel processes of neurovascular remodeling during stroke recovery. Because plasticity is a signature feature of the young and developing brain, these concepts may have special relevance to how the pediatric brain responds after stroke.