Ischemic preconditioning-induced expression of gp130 and STAT3 in astrocytes of the rat hippocampus

Early Optic Nerve Head Glial Proliferation and Jak-Stat Pathway Activation in Chronic Experimental Glaucoma

PURPOSE

We previously reported increased expression of cell proliferation and Jak-Stat pathway-related genes in chronic experimental glaucoma model optic nerve heads (ONH) with early, mild injury. Here, we confirm these observations by localizing, identifying, and quantifying ONH cellular proliferation and Jak-Stat pathway activation in this model.

METHODS

Chronic intraocular pressure (IOP) elevation was achieved via outflow pathway sclerosis. After 5 weeks, ONH longitudinal sections were immunolabeled with proliferation and cell-type markers to determine nuclear densities in the anterior (unmyelinated) and transition (partially myelinated) ONH. Nuclear pStat3 labeling was used to detect Jak-Stat pathway activation. Nuclear density differences between control ONH (uninjected) and ONH with either early or advanced injury (determined by optic nerve injury grading) were identified by ANOVA.

RESULTS

Advanced injury ONH had twice the nuclear density (P < 0.0001) of controls and significantly greater astrocyte density in anterior (P = 0.0001) and transition (P = 0.006) ONH regions. An increased optic nerve injury grade positively correlated with increased microglia/macrophage density in anterior and transition ONH (P < 0.0001, both). Oligodendroglial density was unaffected. In glaucoma model ONH, 80% of anterior and 66% of transition region proliferating cells were astrocytes. Nuclear pStat3 labeling significantly increased in early injury anterior ONH, and 95% colocalized with astrocytes.

CONCLUSIONS

Astrocytes account for the majority of proliferating cells, contributing to a doubled nuclear density in advanced injury ONH. Jak-Stat pathway activation is apparent in the early injury glaucoma model ONH. These data confirm dramatic astrocyte cell proliferation and early Jak-Stat pathway activation in ONH injured by elevated IOP.

2-Arachidonylglycerol Protects Primary Astrocytes Exposed to Oxygen-Glucose Deprivation Through a Blockade of NDRG2 Signaling and STAT3 Phosphorylation

The human N-Myc downstream-regulated gene 2 (NDRG2) is expressed in astrocytes, and may be involved in the modulation of gliacyte function in the central nervous system. Our previous study found suppression of NDRG2 up-regulation in reactive astrocytes in cerebral ischemic tolerance. 2-Arachidonylglycerol (2-AG) can induce cerebral ischemic tolerance. However, the underlying mechanism of NDRG2 in cytoprotection induced by 2-AG in primary astrocytesis still unknown. In this study, we investigated the role of NDRG2 in cerebral ischemic tolerance induced by 2-AG after oxygen-glucose deprivation (OGD) in primary astrocytes. The results showed that primary astrocytes exposed to OGD resulted in marked increase of lactate dehydrogenase (LDH) release and decrease of methyl thiazolyl tetrazolium (MTT) reduction activity in comparison to control cultures. The levels of NDRG2 and phospho-signal transducer and activator of transcription 3 (pSTAT3) in the OGD group were comparably higher than those in the control group, and the up-regulation of NDRG2 and pSTAT3 was suppressed in NDRG2 siRNA group. The cell viability in the 2-AG group was higher than that in the OGD group, and transfecting the NDRG2 pSRL-CDH1-GFP vector reversed the protective effects of 2-AG. The levels of NDRG2 and pSTAT3 in the 2-AG group were lower than those in the OGD group. 2-AG suppressed STAT3 phosphorylation by decreased expression of NDRG2. In conclusion, 2-AG protects primary astrocytes exposed to oxygen-glucose deprivation through a blockade of NDRG2 signaling and STAT3 phosphorylation. These findings bring insight to the roles of NDRG2 in ischemic-hypoxic injury and provide novel potential targets for future potent clinical therapies on cerebral ischemia injury.

Signal transducers and activators of transcription: STATs-mediated mitochondrial neuroprotection

Cerebral ischemia is defined as little or no blood flow in cerebral circulation, characterized by low tissue oxygen and glucose levels, which promotes neuronal mitochondria dysfunction leading to cell death. A strategy to counteract cerebral ischemia-induced neuronal cell death is ischemic preconditioning (IPC). IPC results in neuroprotection, which is conferred by a mild ischemic challenge prior to a normally lethal ischemic insult. Although many IPC-induced mechanisms have been described, many cellular and subcellular mechanisms remain undefined. Some reports have suggested key signal transduction pathways of IPC, such as activation of protein kinase C epsilon, mitogen-activated protein kinase, and hypoxia-inducible factors, that are likely involved in IPC-induced mitochondria mediated-neuroprotection. Moreover, recent findings suggest that signal transducers and activators of transcription (STATs), a family of transcription factors involved in many cellular activities, may be intimately involved in IPC-induced ischemic tolerance. In this review, we explore current signal transduction pathways involved in IPC-induced mitochondria mediated-neuroprotection, STAT activation in the mitochondria as it relates to IPC, and functional significance of STATs in cerebral ischemia.

Role of signal transducer and activator of transcription 3 in neuronal survival and regeneration

Signal Transducers and Activators of Transcription (STATs) comprise a family of transcription factors that mediate a wide variety of biological functions in the central and peripheral nervous systems. Injury to neural tissue induces STAT activation, and STATs are increasingly recognized for their role in neuronal survival. In this review, we discuss the role of STAT3 during neural development and following ischemic and traumatic injury in brain, spinal cord and peripheral nerves. We focus on STAT3 because of the expanding body of literature that investigates protective and regenerative effects of growth factors, hormones and cytokines that use STAT3 to mediate their effect, in part through transcriptional upregulation of neuroprotective and neurotrophic genes. Defining the endogenous molecular mechanisms that lead to neuroprotection by STAT3 after injury might identify novel therapeutic targets against acute neural tissue damage as well as chronic neurodegenerative disorders.

Ambivalent aspects of interleukin-6 in cerebral ischemia: inflammatory versus neurotrophic aspects

Interleukin-6 (IL-6) is pleiotropic cytokine involved in many central nervous system disorders including stroke, and elevated serum IL-6 has been found in acute stroke patients. IL-6 is implicated in the inflammation, which contributes to both injury and repair process after cerebral ischemia. However, IL-6 is one of the neurotrophic cytokines sharing a common receptor subunit, gp130, with other neurotrophic cytokines, such as leukemia inhibitory factor (LIF) and ciliary neurotrophic factor. The expression of IL-6 is most prominently identified in neurons in the peri-ischemic regions, and LIF expression shows a similar pattern. The direct injection of these cytokines into the brain after ischemia can reduce ischemic brain injury. The cytokine receptors are localized on the neuron surface, suggesting that neurons are the cytokine target. The major IL-6 downstream signaling pathway is JAK-STAT, and Stat3 activation occurs mainly in neurons during postischemic reperfusion. Further investigation is necessary to clarify the exact role of Stat3 signaling in neuroprotection. Taken together, the information suggests that IL-6 plays a double role in cerebral ischemia, as an inflammatory mediator during the acute phase and as a neurotrophic mediator between the subacute and prolonged phases.

Ischemic preconditioning upregulates expression of phospholipase D2 in the rat hippocampus

To investigate the possible involvement of phospholipase D2 (PLD2) in the induction of ischemic tolerance, we analyzed the distribution and time course of PLD2 expression in the rat hippocampus after a sublethal period of ischemia. Forebrain ischemia was induced by four-vessel occlusion for 3 min. Increased PLD2 immunoreactivity after this sublethal ischemia was observed in CA1 pyramidal neurons of the rat hippocampus. In tolerance-acquired CA1 neurons, PLD2 immunoreactivity was upregulated as early as 12 h post-ischemia and was most prominent at 1-3 days, with expression sustained for at least 7 days, as shown by a time course of immunoblotting and measurement of the enzymatic activity of PLD. PLD2 expression was also increased in ischemia-resistant CA3 neurons and dentate granule cells, although weaker staining intensity was noted. Further, we showed that, in cultured SK-N-BE(2)C human neuroblastoma cells, overexpression of PLD2 inhibited cell death by chemical hypoxia induced with potassium cyanide and deoxyglucose. These data suggest that upregulation of PLD2 might be involved in the neuroprotective mechanism of ischemic tolerance in the rat hippocampus.

Ischemic preconditioning-induced activation of ERK1/2 in the rat hippocampus

We investigated the activation and cellular localization of the extracellular signal-regulated kinases ERK1/2 in a rat model of ischemic tolerance induction. Adult male Sprague-Dawley rats were subjected to 3 min of sublethal ischemic preconditioning. Activation of ERK1/2 showed the characteristic time- and cell-dependent patterns. Rapid and short-lasting activation of ERK after 3 min of cerebral ischemia was noted immediately in the dentate granule cells and mossy fibers of the hippocampus, and then occurred sequentially in CA3 and CA1 neurons and dentate hilar neurons at 10 min. Phosphorylation of ERK1/2 in hippocampal neurons returned to the basal level in an ordered manner. Basal level phosphorylation was attained first, at 30 min, by the CA1 neurons, and was then observed in CA3 and granule cells by 1 h and noted in some dentate hilar neurons at 12 h. By contrast, phosphorylation of ERK1/2 in mossy fibers and the CA1 dendritic field was sustained for at least 3 d. Transient activation of ERK1/2 was induced also in astrocytes of the dentate hilar region at 1 d post-stimulation. These data demonstrate that the short cerebral-ischemic preconditioning induced rapid and transient activation of ERK1/2 in tolerance-acquired CA1 neurons as well as in ischemia-resistant CA3 and dentate granule cells, and that the short preconditioning sustained activation in mossy fibers and neuropil areas, suggesting that ERK1/2 activation may be involved in the mechanism of ischemic tolerance in the rat hippocampus.

Characterization of STAT3-expressing cells in the postnatal rat brain

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor abundantly expressed in the postnatal brain that is involved in the differentiation of cultured astrocytes. Thus far, the cellular identity and anatomical distribution of STAT3-expressing cells in the postnatal brain is poorly known. This study identifies the cell type(s), anatomical location, and temporal distribution of STAT3-expressing cells by using immunohistochemistry and confocal microscopy on postnatal day 3 (P3), 10 (P10), and 21 (P21) rat brain sections. Furthermore, the phosphorylation of STAT3 on tyrosine and serine residues was analyzed at these different stages by immunoprecipitation followed by Western blot. STAT3 immunoreactivity was observed in the cytoplasm and nucleus of many maturating astrocytes positive for nestin (at P3) or positive for GFAP (at P10) distributed throughout the white and grey matter. Moreover, robust nuclear immunoreactivity was observed in brainstem motoneurons. Phosphorylation on tyrosine and serine was observed at P3 and increased at P10, which suggests an augmented activation of STAT3 at the mid-postnatal period. At P21, STAT3 immunoreactivity dramatically decreased to remain visible only in the cytoplasm of white matter astrocytes and hypothalamic and brainstem neuronal groups. Furthermore, while the phosphorylation of tyrosine residues tended to decrease, that of serine residues further increased. In summary, our study reveals a complex regulation of STAT3 phosphorylation in the postnatal brain and provides in vivo evidence of the specific expression of STAT3 in maturating astrocytes and brainstem motoneurons.

GRP78-binding protein regulates cAMP-induced glial fibrillary acidic protein expression in rat C6 glioblastoma cells

We previously reported that a novel GRP78-binding protein (GBP) is predominantly expressed in rat brain and its expression declines through the aging process. To characterize its biological function, we established C6 glioblastoma cells that stably overexpressed GBP. Stable overexpression of GBP attenuated cAMP-induced expression of the glial fibrillary acidic protein (GFAP) gene, which was accompanied by a decrease in cAMP-induced signal transducer and activators of transcription 3 (STAT3) phosphorylation. Other distinct cAMP-induced events, including a transient reduction in extracellular signal-regulated protein kinase phosphorylation and a slowdown in cell proliferation, were hardly affected by GBP overexpression. Most importantly, treatment with siRNA against endogenous GBP markedly downregulated GBP expression in C6 glioblastoma cells, and dramatically augmented cAMP-induced GFAP mRNA expression in parallel with hyper-phosphorylation of STAT3. These results suggest a novel function of GBP in regulating GFAP gene expression via STAT3 phosphorylation.

Expression of prostaglandin E2 receptor subtypes, EP2 and EP4, in the rat hippocampus after cerebral ischemia and ischemic tolerance

We investigated the distribution and time course of expression of two subtypes of prostaglandin E(2) (PGE(2)) receptors, EP2 and EP4, in a rat model of cerebral ischemia and ischemic tolerance. Adult male Sprague-Dawley rats were subjected to either lethal global ischemia (10 min) with or without sublethal ischemic preconditioning (3 min), or ischemia only (3 min). A short 3-min cerebral ischemia and a 3-min ischemia followed by a second lethal ischemia enhanced the expression of EP2 and EP4 receptors in CA1 pyramidal neurons of the hippocampus. In tolerance-acquired CA1 neurons, the immunoreactivities of EP2 and EP4 were upregulated after 4 h and 12 h, respectively. The immunoreactivities were most prominent at 3 days and were sustained for at least 14 days, consistent with results of immunoblotting experiments. However, immunoreactivities for these PGE(2) receptors increased in reactive glial cells in the vulnerable CA1 and hilar regions of rats subjected to lethal ischemia without ischemic preconditioning. Most of the EP2 immunoreactivity occurred in microglial cells and some astrocytes, whereas increased immunoreactivity for EP4 was found only in astrocytes. These data suggest that ischemia and the induction of ischemia tolerance have different regulatory effects on the expression of EP2 and EP4 receptors. Moreover, PGE(2) may exert its unique pathophysiological functions in relation to delayed neuronal death and ischemic tolerance induction in the rat hippocampus via specific PGE(2) receptors.