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 ␤-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.brain injury ͉ neurodegeneration ͉ neurovascular ͉ stroke

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.

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Oligodendrocyte precursor cells (OPCs) in the adult brain contribute to white matter homeostasis. After white matter damage, OPCs compensate for oligodendrocyte loss by differentiating into mature oligodendrocytes. However, the underlying mechanisms remain to be fully defined. Here, we test the hypothesis that, during endogenous recovery from white matter ischemic injury, astrocytes support the maturation of OPCs by secreting brain-derived neurotrophic factor (BDNF). For in vitro experiments, cultured primary OPCs and astrocytes were prepared from postnatal day 2 rat cortex. When OPCs were subjected to chemical hypoxic stress by exposing them to sublethal CoCl 2 for 7 d, in vitro OPC differentiation into oligodendrocytes was significantly suppressed. Conditioned medium from astrocytes (astro-medium) restored the process of OPC maturation even under the stressed conditions. When astro-medium was filtered with TrkB-Fc to remove BDNF, the BDNF-deficient astro-medium no longer supported OPC maturation. For in vivo experiments, we analyzed a transgenic mouse line (GFAP cre /BDNF wt/fl ) in which BDNF expression is downregulated specifically in GFAP ϩ astrocytes. Both wild-type (GFAP wt /BDNF wt/fl mice) and transgenic mice were subjected to prolonged cerebral hypoperfusion by bilateral common carotid artery stenosis. As expected, compared with wild-type mice, the transgenic mice exhibited a lower number of newly generated oligodendrocytes and larger white matter damage. Together, these findings demonstrate that, during endogenous recovery from white matter damage, astrocytes may promote oligodendrogenesis by secreting BDNF.

Background and purpose We explored the hypothesis that injured neurons release lipocalin-2 as a help-me signal. Methods In vivo lipocalin-2 responses were assessed in rat focal cerebral ischemia and human stroke brain samples using a combination of ELISA and immunostaining. In vitro, microglia and astrocytes were exposed to lipocalin-2 and various markers and assays of glial activation were quantified. Functional relevance of neuron-to-glia lipocalin-2 signaling was examined by transferring conditioned media from lipocalin-2-activated microglia and astrocytes onto neurons to see whether activated glia could protect neurons against oxygen-glucose deprivation and promote neuroplasticity. Results In human stroke samples and rat cerebral ischemia, neuronal expression of lipocalin-2 was significantly increased. In primary cell cultures, exposing microglia and astrocytes to lipocalin-2 resulted in glial activation. In microglia, lipocalin-2 converted resting ramified shapes into a long-rod morphology with reduced branching, increased interleukin-10 release, and enhanced phagocytosis. In astrocytes, lipocalin-2 upregulated GFAP, BDNF and thrombospondin-1. Conditioned media from lipocalin-2-treated astrocytes upregulated synaptotagmin, and conditioned media from lipocalin-2-treated microglia upregulated synaptophysin and PSD95 and protected neurons against oxygen-glucose deprivation. Conclusions These findings provide proof-of-concept that lipocalin-2 is released by injured neurons as a “help-me” distress signal that activates microglia and astrocytes into potentially pro-recovery phenotypes.

Background and Purpose Pentraxin 3 (PTX3) is released upon inflammatory responses in many organs. However, roles of PTX3 in brain are still mostly unknown. Here we asked whether and how PTX3 contributes to blood-brain barrier (BBB) dysfunction during the acute phase of ischemic stroke. Methods In vivo, spontaneously hypertensive rats were subjected to focal cerebral ischemia by transient middle cerebral artery occlusion. At day 3, brains were analyzed to evaluate the cellular origin of PTX3 expression. Correlations with BBB breakdown were assessed by IgG staining. In vitro, rat primary astrocytes and rat brain endothelial RBE.4 cells were cultured to study the role of astrocyte-derived PTX3 on VEGF-mediated endothelial permeability. Results During the acute phase of stroke, reactive astrocytes in the peri-infarct area expressed PTX3. There was negative correlation between gradients of IgG leakage and PTX3-positive astrocytes. Cell culture experiments showed that astrocyte-conditioned media increased levels of tight junction proteins and reduced endothelial permeability under normal conditions. Removing PTX3 from astrocyte-conditioned media by immunoprecipitation increased endothelial permeability. PTX3 strongly bound VEGF in vitro and was able to decrease VEGF-induced endothelial permeability. Conclusions Astrocytes in peri-infarct areas upregulate PTX3 which may support BBB integrity by regulating VEGF-related mechanisms. This response in astrocytes may comprise a compensatory mechanism for maintaining BBB function after ischemic stroke.

Factors secreted by EPCs may ameliorate white matter damage in the brain by boosting oligovascular remodeling.

SUMMARY ICH is a disease with high rates of mortality and morbidity, with a substantial public health impact. Spontaneous ICH (sICH) has been extensively studied and a large body of data has been accumulated on its pathophysiology. However, the literature on traumatic ICH (tICH) is more limited, and there is a need for further investigations of this important topic. This review will highlight some of the cellular pathways in ICH with an emphasis on the mechanisms of secondary injury due to heme toxicity and to events in the coagulation process, which are common to both sICH and tICH.