Regional selective neuronal degeneration after protein phosphatase inhibition in hippocampal slice cultures: evidence for a MAP kinase-dependent mechanism

Chronic CXCL10 alters the level of activated ERK1/2 and transcriptional factors CREB and NF-kappaB in hippocampal neuronal cell culture

Signal transduction pathways may be important targets of chemokines during neuroinflammation. In the current study, Western blot analyses show that in rat hippocampal neuronal/glial cell cultures chronic CXCL10 increases the level of protein for ERK1/2 as well as for the transcriptional factors CREB and NF-kappaB. Bcl-2, an anti-apoptotic protein whose expression can be regulated by a pathway involving ERK1/2, CREB and NF-kappaB, was also increased in the CXCL10 treated cultures. These results implicate a role for ERK1/2, CREB and NF-kappaB in effects of CXCL10 on hippocampal cells and suggest that chronic CXCL10 may have a protective role during certain neuroinflammatory conditions.

Requirement for Ras/Raf/ERK pathway in naringin-induced G1-cell-cycle arrest via p21WAF1 expression

Naringin, an active flavonoid found in citrus fruit extracts, has pharmacological utility. The present study identified a novel mechanism of the anticancer effects of naringin in urinary bladder cancer cells. Naringin treatment resulted in significant dose-dependent growth inhibition together with G(1)-phase cell-cycle arrest at a dose of 100 microM (the half maximal inhibitory concentration) in 5637 cells. In addition, naringin treatment strongly induced p21WAF1 expression, independent of the p53 pathway, and downregulated expression of cyclins and cyclin dependent kinases (CDKs). Moreover, treatment with naringin induced phosphorylation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase and c-Jun N-terminal kinase. Among the pathways examined, only PD98059, an ERK-specific inhibitor, blocked naringin-dependent p21WAF1 expression. Consistently, blockade of ERK function reversed naringin-mediated inhibition of cell proliferation and decreased cell-cycle proteins. Furthermore, naringin treatment increased both Ras and Raf activation. Transfection of cells with dominant-negative Ras (RasN17) and Raf (RafS621A) mutant genes suppressed naringin-induced ERK activity and p21WAF1 expression. Finally, the naringin-induced reduction in cell proliferation and cell-cycle proteins also was abolished in the presence of RasN17 and RafS621A mutant genes. These data demonstrate that the Ras/Raf/ERK pathway participates in p21WAF1 induction, subsequently leading to a decrease in the levels of cyclin D1/CDK4 and cyclin E-CDK2 complexes and naringin-dependent inhibition of cell growth. Overall, these unexpected findings concerning the molecular mechanisms of naringin in 5637 cancer cells provide a theoretical basis for the therapeutic use of flavonoids to treat malignancies.

Role of the activated extracellular signal-regulated kinase pathway on histological and behavioral outcome after traumatic brain injury in rats

The extracellular signal-regulated kinase (ERK) pathway, which modulates the activity of many transcriptional factors leading to the proliferation of various cells, is activated in lesions in regions of selective vulnerability after traumatic brain injury (TBI). In the present study, using the ERK inhibitor U0126, we investigated the role of the ERK pathway in histopathological and behavioral outcomes after TBI. Adult male Sprague-Dawley rats, weighing 300-400 g were subjected to lateral fluid percussion brain injury. The ERK inhibitor U0126 was injected intravenously before injury at 100, 200 and 400 microg/kg. The severity of CA3 neuronal damage was evaluated by the number of surviving CA3 neurons 7 days after injury. The contusional lesion volume 72 h after injury was analysed using a computer-assisted analysis system. Three different motor skill tasks were measured on days 1-5, 7, 14 and 21 after injury. Pretreatment with U0126 significantly reduced both CA3 neuronal damage and contusional lesion volume after injury. In addition, administration of U0126 ameliorated motor function recovery on days 3, 4 and 5 after injury. Therefore, inhibition of ERK phosphorylation could be a potentially effective therapeutic target after TBI.

Activations of nPKCε and ERK1/2 Were Involved in Oxygen-Glucose Deprivation-induced Neuroprotection via NMDA Receptors in Hippocampal Slices of Mice

Accumulated reports have suggested that activation of protein kinase C (PKC) isoforms may involve the activation of extracellular signal-regulated kinases (ERKs) in the neuronal response to ischemic/hypoxic stimuli. We have previously demonstrated that the membrane translocation of novel PKC (nPKC) epsilon increased in the early phase of cerebral ischemic/hypoxic preconditioning of mice. In this study, we used Western blot analysis and propidium iodide stain to determine whether the activations of nPKCepsilon and ERKs were involved in oxygen-glucose deprivation (OGD)-induced neuroprotection via N-methyl-D-aspartate (NMDA) receptors. The hippocampal slices of mice were exposed to OGD for 10 (OGD10) or 45 minutes (OGD45) to mimic mild (causing ischemic/hypoxic preconditioning) and severe (causing severe OGD) ischemia/hypoxia, respectively. We found that OGD10-induced nPKCepslilon membrane translocation was mediated by NMDA receptors, and both OGD10 and NMDA (1 microM, 30 min) pretreatment could protect Cornu Ammonis region 1 neurons against the subsequent severe OGD45. In addition, nPKCepsilon translocation inhibitor, epsilonV1-2 (1 microM, 30 min), and ERKs upstream mitogen-activated protein/extracellular signal regulated kinase kinase inhibitor, PD-98059 (20 microM, 30 min), could significantly inhibit OGD10 and NMDA-induced neuroprotection. These results suggest that OGD10-induced neuroprotection against severe OGD45 in the Cornu Ammonis region 1 region of the hippocampal slices was mediated by the activations of NMDA receptors, nPKCepsilon, and the downstream ERKs.

IGF-1 protects oligodendrocyte progenitors against TNFα-induced damage by activation of PI3K/Akt and interruption of the mitochondrial apoptotic pathway

Proinflammatory cytokine-mediated injury to oligodendrocyte progenitor cells (OPCs) has been proposed as a cause of periventricular leukomalacia (PVL), the most common brain injury found in preterm infants. Preventing death of OPCs is a potential strategy to prevent or treat PVL. In the current study, we utilized an in vitro cell culture system to investigate the effect of insulin-like growth factor-1 (IGF-1) on tumor necrosis factor-alpha (TNFalpha)-induced OPC injury and the possible mechanisms involved. OPCs were isolated from neonatal rat optic nerves and cultured in chemically defined medium (CDM) supplemented with platelet-derived growth factor and basic fibroblast growth factor. Exposure to TNFalpha resulted in death of OPCs. IGF-1 protected OPCs from TNFalpha cytotoxicity in a dose-dependent manner as measured by the XTT and TUNEL assays. IGF-1 activates both the PI3K/Akt and the extracellular signal-regulated kinase (ERK) pathway. However, IGF-1-enhanced cell survival signals were mediated by the PI3K/Akt, but not by the ERK pathway, as evidenced by the observation that IGF-1-enhanced cell survival was partially abrogated by Akti, the Akt inhibitor, or wortmannin, the PI3K inhibitor, but not by PD98,059, the MAPK kinase/ERK kinase inhibitor. The downstream events of IGF-1-triggered survival signals included phosphorylation of BAD, blockade of TNFalpha-induced translocation of Bax from the cytosol to the mitochondrial membrane, and suppression of caspase-9 and caspase-3 activation. These observations indicate that the protection of OPCs by IGF-1 is mediated, at least partially, by interruption of the mitochondrial apoptotic pathway via activation of PI3K/Akt.

Curcumin attenuates glutamate-induced HT22 cell death by suppressing MAP kinase signaling

Glutamate induces cell death by upsetting the cellular redox homeostasis, termed oxidative glutamate toxicity, in a mouse hippocampal cell line, HT22. Extracellular signal-regulated kinases (ERK) 1/2 are known key players in this process. Here we characterized the roles of both MAP kinases and cell cycle regulators in mediating oxidative glutamate toxicity and the neuroprotective mechanisms of curcumin in HT22 cells. c-Jun N-terminal kinase (JNK) and p38 kinase were activated during the glutamate-induced HT22 cell death, but at a later stage than ERK activation. Treatment with a JNK inhibitor, SP600125, or a p38 kinase inhibitor, SB203580, partly attenuated this cell death. Curcumin, a natural inhibitor of JNK signaling, protected the HT22 cells from glutamate-induced death at nanomolar concentrations more efficiently than SP600125. These doses of curcumin affected neither the level of intracellular glutathione nor the level of reactive oxygen species, but inactivated JNK and p38 significantly. Moreover, curcumin markedly upregulated a cell-cycle inhibitory protein, p21cip1, and downregulated cyclin D1 levels, which might help the cell death prevention. Our results suggest that curcumin has a neuroprotective effect against oxidative glutamate toxicity by inhibiting MAP kinase signaling and influencing cell-cycle regulation.

A Death-Promoting Role for Extracellular Signal-Regulated Kinase

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), which are members of the mitogen-activated protein kinase superfamily, have been well characterized and are known to be involved in cell survival; however, recent evidence suggests that the activation of ERK1/2 also contributes to cell death in some cell types and organs under certain conditions. For example, ERK1/2 is activated in neuronal and renal epithelial cells upon exposure to oxidative stress and toxicants and deprivation of growth factors, and inhibition of the ERK pathway blocks apoptosis. ERK activation also occurs in animal models of ischemia- and trauma-induced brain injury and cisplatin-induced renal injury, and inactivation of ERK reduces the extent of tissue damage. In some studies, ERK has been implicated in apoptotic events upstream of mitochondrial cytochrome c release, whereas other studies have suggested the converse that ERK acts downstream of mitochondrial events and upstream of caspase-3 activation. ERK also can contribute to cell death through the suppression of the antiapoptotic signaling molecule Akt. Here we summarize the evidence and mechanism of ERK-induced apoptosis in both cell culture and in animal models.

Semaphorin 4D activates the MAPK pathway downstream of plexin-B1

Semaphorins are a large family of transmembrane and secreted proteins that signal primarily through the receptor plexin. Semaphorins have been characterized in the nervous system as axon guidance cues; however, they have also been shown to control development of other cellular systems such as the vasculature and lungs. As the role of semaphorins outside of the nervous system has broadened, so has elucidation of the intracellular signalling pathways they initiate. Previously, we and others have shown that plexin-B1 activates RhoA through the binding and activation of RhoGEF (guanine nucleotide-exchange factor)/LARG (leukaemia-associated RhoGEF) in response to semaphorin 4D stimulation. In the present study, we show that semaphorin 4D activates the MAPK (mitogen-activated protein kinase) pathway. We have found that the mechanism of activation requires the C-terminus of plexin-B1 and the activation of RhoA.

Ionotropic glutamate receptors and glutamate transporters are involved in necrotic neuronal cell death induced by oxygen-glucose deprivation of hippocampal slice cultures

Organotypic hippocampal slice cultures represent a feasible model for studies of cerebral ischemia and the role of ionotropic glutamate receptors in oxygen-glucose deprivation-induced neurodegeneration. New results and a review of existing data are presented in the first part of this paper. The role of glutamate transporters, with special reference to recent results on inhibition of glutamate transporters under normal and energy-failure (ischemia-like) conditions is reviewed in the last part of the paper. The experimental work is based on hippocampal slice cultures derived from 7 day old rats and grown for about 3 weeks. In such cultures we investigated the subfield neuronal susceptibility to oxygen-glucose deprivation, the type of induced cell death and the involvement of ionotropic glutamate receptors. Hippocampal slice cultures were also used in our studies on glutamate transporters reviewed in the last part of this paper. Neurodegeneration was monitored and/or shown by cellular uptake of propidium iodide, loss of immunocytochemical staining for microtubule-associated protein 2 and staining with Fluoro-Jade B. To distinguish between necrotic vs. apoptotic neuronal cell death we used immunocytochemical staining for active caspase-3 (apoptosis indicator) and Hoechst 33342 staining of nuclear chromatin. Our experimental studies on oxygen-glucose deprivation confirmed that CA1 pyramidal cells were the most susceptible to this ischemia-like condition. Judged by propidium iodide uptake, a selective CA1 lesion, with only minor affection on CA3, occurred in cultures exposed to oxygen-glucose deprivation for 30 min. Nuclear chromatin staining by Hoechst 33342 and staining for active caspase-3 showed that oxygen-glucose deprivation induced necrotic cell death only. Addition of 10 microM of the N-methyl-D-aspartate glutamate receptor antagonist MK-801, and 20 microM of the non-N-methyl-D-aspartate glutamate receptor antagonist 2,3-dihyroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline to the culture medium confirmed that both N-methyl-D-aspartate and non-N-methyl-D-aspartate ionotropic glutamate receptors were involved in the oxygen-glucose deprivation-induced cell death. Glutamate is normally quickly removed, from the extracellular space by sodium-dependent glutamate transporters. Effects of blocking the transporters by addition of the DL-threo-beta-benzyloxyaspartate are reviewed in the last part of the paper. Under normal conditions addition of DL-threo-beta-benzyloxyaspartate in concentrations of 25 microM or more to otherwise untreated hippocampal slice cultures induced neuronal cell death, which was prevented by addition of 2,3-dihyroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline and MK-801. In energy failure situations, like cerebral ischemia and oxygen-glucose deprivation, the transporters are believed to reverse and release glutamate to the extracellular space. Blockade of the transporters by a subtoxic (10 microM) dose of DL-threo-beta-benzyloxyaspartate during oxygen-glucose deprivation (but not during the next 48 h after oxygen-glucose deprivation) significantly reduced the oxygen-glucose deprivation-induced propidium iodide uptake, suggesting a neuroprotective inhibition of reverse transporter activity by DL-threo-beta-benzyloxyaspartate during oxygen-glucose deprivation under these conditions. Adding to this, other results from our laboratory have demonstrated that pre-treatment of the slice cultures with glial cell-line derived neurotrophic factor upregulates glutamate transporters. As a logical, but in some glial cell-line derived neurotrophic factor therapy-related conditions clearly unwanted consequence the susceptibility for oxygen-glucose deprivation-induced glutamate receptor-mediated cell death is increased after glial cell-line derived neurotrophic factor treatment. In summary, we conclude that both ionotropic glutamate receptors and glutamate transporters are involved in oxygen-glucose deprivation-induced necrotic cell death in hippocampal slice cultures, which have proven to be a feasible tool in experimental studies on this topic.

5-Bromo-2'-deoxyuridine is selectively toxic to neuronal precursors in vitro

The effect of 5-bromo-2'-deoxyuridine (BrdU) incorporation on the phenotype of progeny derived from expanded E18 rat striatal precursors was examined. BrdU was administered to cultures for 24 h prior to differentiation. Results revealed that there was selective toxicity of this compound to developing TuJ1+ neurons, but not glia, at concentrations used in most labelling studies. Therefore, a BrdU dose-response curve from 0.2 microM to 10 microM was established. The optimum dose of BrdU for labelling cells was 0.2 microM, well below the 1-10 microm recommended concentration. This dose resulted in the survival of significantly more newborn BrdU/TuJ1+ double-labelled neurons and eliminated the toxic effects of BrdU. Administration of 10 microm BrdU resulted in a significant decrease in extracellular regulated kinase (ERK) phosphorylation compared with untreated cultures, this could be completely restored by the administration of either N-methyl-D-aspartate (NMDA) receptor antagonists such as MK801 or the nitric oxide synthesis inhibitor L-methyl-arginine methyl ester (L-NAME). Our results show that high levels of BrdU are selectively toxic to neurons through a mechanism that activates classical cell death pathways. This has implications for labelling studies both in vivo and in vitro.

Extracellular signal-regulated kinase as an inducer of non-apoptotic neuronal death

Extracellular signal-regulated kinase (ERK) is a versatile protein kinase, which has been implicated in signaling numerous biological functions ranging from embryonic development to memory formation. Recent reports, including ours, indicate that ERK plays a central role in promoting neuronal degeneration in various neuronal systems including neurodegenerative diseases. Mechanisms involved in ERK-induced neuronal degeneration are beginning to emerge. In this review, we summarize evidence suggesting ERK to be a predominant inducer of a non-apoptotic mode of neuronal death. Further, we discuss the mechanisms and the putative molecular inter-players associated with ERK-mediated neuronal death.

Citicoline inhibits MAP kinase signalling pathways after focal cerebral ischaemia

The link between membrane phospholipids and different intracellular signal transduction pathways affected by cerebral ischaemia is unclear. CDP-choline, a major neuronal membrane lipid precursor and its intracellular target proteins and transcription factors were studied to further understand its role in ischaemic stroke. Cerebral ischaemia was produced by distal, permanent occlusion of the middle cerebral artery (MCAO) in the rat. Animals receiving 500 mg/kg of CDP-choline in 0.5 ml of 0.9% saline, intraperitoneally, 24 h and 1 h before MCAO and 23 h after MCAO demonstrated a notable reduction in the phosphorylation of MAP-kinase family members, ERK1/2 and MEK1/2, as well as Elk-1 transcription factor, compared with control animals treated with 0.5 ml of 0.9% saline. Immunohistochemistry showed a particular reduction in immunoreactivity in glia. The effects of CDP-choline on intracellular mechanisms of signal transduction, suggests that this molecule may play a key role in recovery after ischaemic stroke.

Translocation of Ethanolamine Phosphoglyceride is Required for Initiation of Apoptotic Death in OLN-93 Oligodendroglial Cells

The possible interplay between extracellular signal-regulated protein kinase (ERK) activation and ethanolamine phosphoglycerides (PG) membrane bilayer translocation following oxidative stress (OS) (0.5 mM H2O2/0.05 mM Fe2+), was examined in oligodendroglia, OLN93, cells with altered plasma membrane PG composition. Cells supplemented with 50 microM docosahexaenoic acid (DHA, 22:6n3) to increase the number of potential double bond targets for OS in ethanolamine-PG (EPG) were compared to cells with diminished content of EPG, attained by the addition of 0.5 mM N,N-dimethylethanolamine (dEa). After 30 min OS, EPG translocation accompanied by sustained ERK activation and nuclear translocation culminating in apoptosis was found in DHA-supplemented cells in contrast to no EPG translocation, a brief ERK activation, but no nuclear translocation, and no cell death in DHA/dEa-supplemented cells. DHA/dEa-supplemented cells pretreated with the protein-tyrosine phosphatases inhibitor Na3VO4 followed by OS, although expressing a sustained ERK activation and nuclear translocation, failed to show apoptosis and lacked EPG translocation. In DHA-supplemented cells U0126, a MEK inhibitor, prevented ERK activation and EPG translocation and protected from cell death. These findings most likely indicate that ERK activation is an indispensable component for the signaling cascades leading to EPG translocation but only activation of the latter is leading to OS-induced apoptotic cell death.

Ceramide induces neuronal apoptosis through mitogen-activated protein kinases and causes release of multiple mitochondrial proteins

Ceramide accumulates in neurons during various disorders associated with acute or chronic neurodegeneration. In these studies, we investigated the mechanisms of ceramide-induced apoptosis in primary cortical neurons using exogenous C(2) ceramide as well as inducing endogenous ceramide accumulation using inhibitors of glucosylceramide synthetase. Ceramide induced the translocation of certain, but not all, pro-apoptotic mitochondrial proteins: cytochrome c, Omi, SMAC, and AIF were released from the mitochondria, whereas Endonuclease G was not. Ceramide also selectively altered the phosphorylation state of members of the MAPK superfamily, causing dephosphorylation of ERK1/2 and hyperphosphorylation of p38 MAP kinases, but not affecting the phosphorylation of JNK or ERK5. Inhibitors of the p38 MAP kinase pathway (SB-202190 or SB-203580) and an inhibitor of the ERK1/2 pathway (U0126) reduced ceramide-induced neuronal death. These p38 and ERK1/2 inhibitors appear to block ceramide-activated apoptotic signaling upstream of the mitochondria, as they attenuated mitochondrial release of cytochrome c, Omi, AIF, and SMAC, as well as reducing ceramide-induced caspase-3 activation.

Chronic fluoxetine administration inhibits extracellular signal-regulated kinase 1/2 phosphorylation in rat brain

Accumulating evidence indicates that antidepressants alter intracellular signalling mechanisms resulting in long-term synaptic alterations which probably account for the delay in clinical action of these drugs. Therefore, we investigated the effects of chronic fluoxetine administration on extracellular signal-regulated kinase (ERK) 1 and 2, a group of MAPKs that mediate signal transduction from the cell surface downstream to the nucleus. Our data demonstrate that 3-week fluoxetine treatment resulted in long-lasting reduction of phospho-ERK 1 and 2. Such an effect depends on the length of the treatment given that no changes were observed after a single drug injection or after 2 weeks of treatment and it is region specific, being observed in hippocampus and frontal cortex but not in striatum. Finally, phospho-ERK 1 and 2 were differently modulated within nucleus and cytosol in hippocampus but similarly reduced in the same compartments of the frontal cortex, highlighting the specific subcellular compartmentalization of fluoxetine. Conversely, imipramine did not reduce the hippocampal phosphorylation of both ERK subtypes whereas it selectively increased ERK 1 phosphorylation in the cytosolic compartment of frontal cortex suggesting a drug-specific effect on this intracellular target. These results point to modulation of phosphorylation, rather than altered expression, as the main target in the action of fluoxetine on this pathway. The reduction of ERK 1/2 function herein reported may be associated with the therapeutic effects of fluoxetine in the treatment of depression.

Pre-Injury Magnesium Treatment Prevents Traumatic Brain Injury–Induced Hippocampal ERK Activation, Neuronal Loss, and Cognitive Dysfunction in the Radial-Arm Maze Test

We studied the effect of pre-injury magnesium (Mg(2+)) treatment on hippocampal extracellular signal- regulated kinase (ERK) activation induced by lateral fluid-percussion (FP) brain injury, and on working and reference memory in the radial-arm maze test in rats subjected to such traumatic brain injury (TBI) (n = 56) or to sham injury (n = 12). In the ipsilateral hippocampus, an increase in the phospho-ERK level was detected at 10 min after injury in rats subjected to FP brain injury of moderate severity (1.9-2.0 atm) as compared to sham-injured controls (p < 0.01), and was maintained for at least 120 min after injury (p < 0.05). In the contralateral hippocampus, the phospho-ERK level was transiently increased at 10 min after injury but fell to nearly its basal level by 30 min. When MgCl(2) solution (150 micromol) was infused intravenously from 20 min to 5 min before injury (n = 4-5), brain injury-induced ERK activation was significantly inhibited in the ipsilateral hippocampus at 60 min but not at 10 min after injury. Mg(2+) treatment also significantly prevented injury- induced neuronal loss in the ipsilateral hippocampus (p < 0.05 vs. vehicle-treated, brain-injured controls). At 2 weeks after injury, Mg2+ treatment was found to have significantly prevented injury-induced impairments in working (p < 0.0001 vs. vehicle-treated, brain-injured controls) and reference memory (p < 0.05) in the radial-arm maze test. The present study demonstrates that pretreatment with Mg(2+) prevents post-traumatic hippocampal ERK activation and neuronal loss, and cognitive dysfunction in the radial-arm maze test.

Differential early mitogen-activated protein kinase activation in hyperglycemic ischemic brain injury in the rat

BACKGROUND

Hyperglycemia aggravates brain injury induced by focal ischemia-reperfusion. The mitogen-activated protein kinase (MAPK) members extracellular-signal regulated kinase (Erk) and c-Jun N-terminal kinase (JNK) have been proposed as mediators of ischemic brain injury, and Erk is strongly activated by combined hyperglycemia and transient global ischemia. It is unclear whether similar MAPK activation appears in focal brain ischemia with concomitant hyperglycemia.

DESIGN

Hyperglycemia was induced in rats by an intraperitoneal bolus of glucose (2 g kg(-1)). The rats were then subjected to 90 min of transient middle cerebral artery occlusion (MCAO). Erk and JNK activation were investigated with immunofluorescence and Western blot along with infarct size measurement based on tetrazolium staining and neurological score.

RESULTS

The hyperglycemic rats showed increased tissue damage and impaired neurological performance after 1 day compared with controls. The hyperglycemia was generally moderate (< 15 mM). Erk activation was increased after 30 min of reperfusion in the ischemic cortex of the hyperglycemic rats, while JNK activation was present on the contralateral side. Phospho-Erk immunofluorescence revealed marked neuronal activation of Erk in the ischemic cortex of hyperglycemic rats compared with controls.

CONCLUSION

Besides confirming the detrimental effects of hyperglycemia on focal ischemia-reperfusion, this study shows that hyperglycemia strongly activates the pathogenic mediator Erk in the ischemic brain in the early phase of reperfusion. JNK activation at this stage is present in the nonischemic hemisphere. The functional relevance of these findings needs further investigation.

N-methyl-D-aspartate and TrkB receptors protect neurons against glutamate excitotoxicity through an extracellular signal-regulated kinase pathway

N-Methyl-D-aspartate (NMDA) at a subtoxic concentration (100 microM) promotes neuronal survival against glutamate-mediated excitotoxicity via a brain-derived neurotrophic factor (BDNF) autocrine loop in cultured cerebellar granule cells. The signal transduction mechanism(s) underlying NMDA neuroprotection, however, remains elusive. The mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3-K) pathways alter gene expression and are involved in synaptic plasticity and neuronal survival. This study tested whether neuroprotective activation of NMDA receptors, together with TrkB receptors, coactivated the MAPK or PI3-K pathways to protect rat cerebellar neurons. NMDA receptor activation caused a concentration- and time-dependent activation of MAPK lasting 24 hr. This activation was blocked by the NMDA receptor antagonist MK-801 but was attenuated only partially by the tyrosine kinase inhibitor k252a, suggesting that activation of both NMDA and TrkB receptors are required for maximal neuroprotection. The MAPK kinase (MEK) inhibitor U0126 (10 microM) partially blocked NMDA neuroprotection, whereas LY294002, a selective inhibitor of the PI3-K pathway, did not affect the neuroprotective activity of NMDA. Glutamate excitotoxicity decreased bcl-2, bcl-X(L), and bax mRNA levels,. NMDA increases Bcl-2 and Bcl-X(L) protein levels and decreases Bax protein levels. NMDA and TrkB receptor activation thus converge on the extracellular signal-regulated kinase (ERK) 1/2 signaling pathway to protect neurons against glutamate-mediated excitotoxicity. By increasing antiapoptotic proteins of the Bcl-2 family, NMDA receptor activation may also promote neuronal survival by preventing apoptosis.

Repeated stress and structural plasticity in the brain

Although adrenal steroid receptors are distributed widely throughout the central nervous system, specific limbic and cortical regions targeted by stress hormones play a key role in integrating behavioral and physiological responses during stress and adaptation to subsequent stressors. When the stressor is of a sufficient magnitude or prolonged, it may result in abnormal changes in brain plasticity that, paradoxically, may impair the ability of the brain to appropriately regulate and respond to subsequent stressors. Here we review how repeated stress produces alterations in brain plasticity in animal models, and discuss its relevance to behavioral changes associated with these regions. Interestingly, prolonged stress produces opposing effects on structural plasticity, notably dendritic atrophy and excitatory synapse loss in the hippocampus and prefrontal cortex, and growth of dendrites and spines in the amygdala. The granule cells of the dentate gyrus are also significantly affected through a decrease in the rate neurogenesis following prolonged stress. How functional impairments in these brain regions play a role in stress-related mental illnesses is discussed in this context. Finally, we discuss the cumulative impact of stress-induced structural plasticity in aging.

Understanding regulation of nerve cell death by mGluRs as a method for development of successful neuroprotective strategies

A common cause of nerve cell death often leading to vascular dementia is ischemic stroke. Attempts to develop clinically effective stroke treatment and prevention strategies based on pharmacological manipulations of a single mechanism have not led to clinical success. Analysis of clinical neuroprotection trials suggests that combination treatments may be more effective. To identify optimal components for such treatment, N-methyl-d-aspartate receptor (NMDAR) activation-induced cell death in organotypic hippocampal preparations was studied as a model of neurodegeneration that occurs in association with stroke or vascular dementia. Pharmacological manipulation of metabotropic glutamate receptors mGluR1 and 5 resulted in significant reduction of nerve cell susceptibility to NMDA-induced injury, suggesting that these receptors may function as physiological regulators of neuronal vulnerability. cDNA microarray analysis of over 1000 brain-related genes performed after the neuroprotective activation of group I metabotropic glutamate receptors (mGluRs) revealed a complex pattern of activation and inactivation of seemingly unrelated genes responsible for regulation of neuronal excitability, inflammation, cell death pathways, cell adhesion and transcriptional activation. Combined pharmacological targeting of these processes may provide basis for clinical trials of effective neuroprotective compounds.

Electroconvulsive seizures increase the expression of MAP kinase phosphatases in limbic regions of rat brain

The mitogen-activated protein (MAP) kinase cascades regulate a variety of cellular activities, including cell growth, proliferation, and apoptosis, and are reported to play a role in the actions of antidepressant treatment. There are a number of different classes of protein phosphatases that could influence the MAP kinase cascade. One of these, the MAP kinase phosphatase (MKP) family, is known to play a key role in dephosphorylation of activated MAP kinase. In the present study, we analyzed the expression of the MKP1, MKP2, and MKP3 isoforms in rat brain after electroconvulsive seizure (ECS), considered the most effective treatment for depression. In situ hybridization analysis demonstrates that ECS differentially regulates the expression of the MKP isoforms. Expression of MKP1 mRNA is robustly increased by acute ECS in the major cell layers of the hippocampus, including the dentate gyrus granule cell layer and the CA1 and CA3 pyramidal cell layers. In contrast, MKP2 is induced mainly in the dentate gyrus and MKP3 is preferentially increased in the CA1 and CA3 cell layers. In the prefrontal cortex, all three MKP isoforms are upregulated by acute ECS administration. Chronic ECS resulted in a similar pattern of induction for each of the MKP subtypes, demonstrating that there is little or no desensitization of the response to repeated ECS. The induction of MKP expression serves as negative feedback control for the MAP kinase cascades. Upregulation of MKP expression could dampen the actions of ECS, indicating that blockade of the MKPs could enhance the actions of antidepressant treatment.

Susceptibility to apoptosis varies with time in culture for murine neurons and astrocytes: changes in gene expression and activity

Apoptotic pathways in the brain may differ depending on cell type and developmental stage. To understand these differences, we studied several apoptotic proteins in the murine cortex and primary cultures of neurons and astrocytes of various ages in culture. We then induced apoptosis in our cultures using serum deprivation (SD) and observed changes in these apoptotic proteins. When analyzed by nuclear morphology and TUNEL staining, early cultures showed greater apoptotic injury compared with late cultures, and neuronal cultures showed greater apoptosis than astrocyte cultures. The decrease in apoptosis with development correlated best with a down-regulation of procaspase-3 and bax and decreasing caspase activation. Early culture astrocytes had higher caspase-11 levels compared with neurons. Mitogen-activated protein (MAP) kinases were also differentially expressed with activation of extracellular signal-regulated kinase (ERK) and p38 higher in early culture astrocytes and stress-activated protein kinase/C-jun N-terminal kinase (SAPK/JNK) greater in early culture neurons. However, caspase inhibitors, but not MAP kinase inhibitors reduced cell death. Our findings demonstrate that apoptosis regulatory proteins display cell type and developmentally specific expression and activation.

Neuroprotective effects of inhibiting N-methyl-d-aspartate receptors, P2X receptors and the mitogen-activated protein kinase cascade: A quantitative analysis in organotypical hippocampal slice cultures subjected to oxygen and glucose deprivation

Cell death was assessed by quantitative analysis of propidium iodide uptake in rat hippocampal slice cultures transiently exposed to oxygen and glucose deprivation, an in vitro model of brain ischemia. The hippocampal subfields CA1 and CA3, and fascia dentata were analyzed at different stages from 0 to 48 h after the insult. Cell death appeared at 3 h and increased steeply toward 12 h. Only a slight additional increase in propidium iodide uptake was seen at later intervals. The mitogen-activated protein kinases extracellular signal-regulated kinase 1 and extracellular signal-regulated kinase 2 were activated immediately after oxygen and glucose deprivation both in CA1 and in CA3/fascia dentata. Inhibition of the specific mitogen-activated protein kinase activator mitogen-activated protein kinase kinase by PD98059 or U0126 offered partial protection against oxygen and glucose deprivation-induced cell damage. The non-selective P2X receptor antagonist suramin gave neuroprotection of the same magnitude as the N-methyl-D-aspartate channel blocker MK-801 (approximately 70%). Neuroprotection was also observed with the P2 receptor blocker PPADS. Immunogold data indicated that hippocampal slice cultures (like intact hippocampi) express several isoforms of P2X receptors at the synaptic level, consistent with the idea that the effects of suramin and PPADS are mediated by P2X receptors. Virtually complete neuroprotection was obtained by combined blockade of N-methyl-D-aspartate receptors, P2X receptors, and mitogen-activated protein kinase kinase. Both P2X receptors and N-methyl-D-aspartate receptors mediate influx of calcium. Our results suggest that inhibition of P2X receptors has a neuroprotective potential similar to that of inhibition of N-methyl-D-aspartate receptors. In contrast, our comparative analysis shows that only partial protection can be achieved by inhibiting the extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase cascade, one of the downstream pathways activated by intracellular calcium overload.

Tocotrienol: The Natural Vitamin E to Defend the Nervous System?

Vitamin E is essential for normal neurological function. It is the major lipid-soluble, chain-breaking antioxidant in the body, protecting the integrity of membranes by inhibiting lipid peroxidation. Mostly on the basis of symptoms of primary vitamin E deficiency, it has been demonstrated that vitamin E has a central role in maintaining neurological structure and function. Orally supplemented vitamin E reaches the cerebrospinal fluid and brain. Vitamin E is a generic term for all tocopherols and their derivatives having the biological activity of RRR-alpha-tocopherol, the naturally occurring stereoisomer compounds with vitamin E activity. In nature, eight substances have been found to have vitamin E activity: alpha-, beta-, gamma- and delta-tocopherol; and alpha-, beta-, gamma- and delta-tocotrienol. Often, the term vitamin E is synonymously used with alpha-tocopherol. Tocotrienols, formerly known as zeta, , or eta-tocopherols, are similar to tocopherols except that they have an isoprenoid tail with three unsaturation points instead of a saturated phytyl tail. Although tocopherols are predominantly found in corn, soybean, and olive oils, tocotrienols are particularly rich in palm, rice bran, and barley oils. Tocotrienols possess powerful antioxidant, anticancer, and cholesterol-lowering properties. Recently, we have observed that alpha-tocotrienol is multi-fold more potent than alpha-tocopherol in protecting HT4 and primary neuronal cells against toxicity induced by glutamate as well as by a number of other toxins. At nanomolar concentration, tocotrienol, but not tocopherol, completely protected neurons by an antioxidant-independent mechanism. Our current work identifies two major targets of tocotrienol in the neuron: c-Src kinase and 12-lipoxygenase. Dietary supplementation studies have established that tocotrienol, fed orally, does reach the brain. The current findings point towards tocotrienol as a potent neuroprotective form of natural vitamin E.

Mitogen-activated protein kinases and the evolution of Alzheimer's: a revolutionary neurogenetic axis for therapeutic intervention?

Alzheimer's disease (AD) is a neurogenetic condition that affects the processes via which the brain functions. Major observable hallmarks of AD are accumulated clusters of proteins in the brain. These clusters, termed neurofibrillary tangles (NFT), resemble pairs of threads wound around each other in a helix fashion accumulating within neurons. These tangles consist of a protein called Tau, which binds to tubulin, thus forming microtubules. Unlike NFTs, deposits of amyloid precursor protein (beta-APP) gather in the spaces between nerve cells. The nearby neurons often look swollen and deformed, and the clusters of protein are usually accompanied by reactive inflammatory cells, microglia, which are part of the brain's immune system responsible for degrading and removing damaged neurons or plaques. Since phosphorylation/dephosphorylation mechanisms are crucial in the regulation of Tau and beta-APP, a superfamily of mitogen-activated protein kinases (MAPKs) has recently emerged as key regulators of the formation of plagues, eventually leading to dementia and AD. The complex molecular interactions between MAPKs and proteins (plagues) associated with the evolution of AD form a cornerstone in the knowledge of a still burgeoning field of neurodegenerative diseases and ageing. This review overviews current understanding of the molecular pathways related to MAPKs and their role in the development of AD and, possibly, dementia. MAPKs, therefore, may constitute a neurogenetic, therapeutic target for the diagnosis and evolution of a preventative medical strategy for early detection, and likely treatment, of Alzheimer's.

Bone morphogenetic proteins and neurotrophins provide complementary protection of septal cholinergic function during phosphatase inhibitor-induced stress

Cultures of embryonic rat septum were exposed for 24-48 h to 2-5 nm okadaic acid (OA), an inhibitor of pp1A and pp2A phosphatases. This stress killed approximately 75% of neurons. A neurotrophin (NT) combination (nerve growth factor and brain-derived neurotrophic factor, each 100 ng/mL) plus a bone morphogenetic protein (BMP6 or BMP7, 5 nm) reduced the death of both cholinergic and non-cholinergic neurons, and preserved choline acetyltransferase (ChAT) activity assayed 2-6 days post-stress. This NT + BMP combination preserved ChAT activity better than either NTs or BMPs alone, and was effective even if trophic factor addition was delayed until 12 h after stress onset. A general caspase inhibitor (qVD-OPH, 10 micro g/mL) also increased survival of stressed cholinergic neurons, but its protection of ChAT activity was shorter lived than that produced by the NT + BMP combination. Neither the NT + BMP combination nor the caspase inhibitor reduced the OA-induced increase in tau phosphorylation. These findings indicate that NTs and BMPs have synergistic protective effects against an OA stress, and suggest that at least some of these protective effects occur upstream of caspase activation.

Calpain mediates calcium-induced activation of the erk1,2 MAPK pathway and cytoskeletal phosphorylation in neurons: relevance to Alzheimer's disease

Aberrant phosphorylation of the neuronal cytoskeleton is an early pathological event in Alzheimer's disease (AD), but the underlying mechanisms are unclear. Here, we demonstrate in the brains of AD patients that neurofilament hyperphosphorylation in neocortical pyramidal neurons is accompanied by activation of both Erk1,2 and calpain. Using immunochemistry, Western blot analysis, and kinase activity measurements, we show in primary hippocampal and cerebellar granule (CG) neurons that calcium influx activates calpain and Erk1,2 and increases neurofilament phosphorylation on carboxy terminal polypeptide sites known to be modulated by Erk1,2 and to be altered in AD. Blocking Erk1,2 activity either with antisense oligonucleotides to Erk1,2 mRNA sequences or by specifically inhibiting its upstream activating kinase MEK1,2 markedly reduced neurofilament phosphorylation. Calpeptin, a cell-permeable calpain inhibitor, blocked both Erk1,2 activation and neurofilament hyperphosphorylation at concentrations that inhibit calpain-mediated cleavage of brain spectrin. By contrast, inhibiting Erk1,2 with U-0126, a specific inhibitor of Mek1,2, had no appreciable effect on ionomycin-induced calpain activation. These findings demonstrate that, under conditions of calcium injury in neurons, calpains are upstream activators of Erk1,2 signaling and are likely to mediate in part the hyperphosphorylation of neurofilaments and tau seen at early stages of AD as well as the neuron survival-related functions of the MAP kinase pathway.

Oxidative neuronal injury. The dark side of ERK1/2

The extracellular signal regulated protein kinases (ERK1/2) are essential for normal development and functional plasticity of the central nervous system. However, a growing number of recent studies in models of cerebral ischemia, brain trauma and neurodegenerative diseases implicate a detrimental role for ERK1/2 signaling during oxidative neuronal injury. Neurons undergoing oxidative stress-related injuries typically display a biphasic or sustained pattern of ERK1/2 activation. A variety of potential targets of reactive oxygen species and reactive nitrogen species could contribute to ERK1/2 activation. These include cell surface receptors, G proteins, upstream kinases, protein phosphatases and proteasome components, each of which could be direct or indirect targets of reactive oxygen or nitrogen species, thereby modulating the duration and magnitude of ERK1/2 activation. Neuronal oxidative stress also appears to influence the subcellular trafficking and/or localization of activated ERK1/2. Differences in compartmentalization of phosphorylated ERK1/2 have been observed in diseased or injured human neurons and in their respective animal and cell culture model systems. We propose that differential accessibility of ERK1/2 to downstream targets, which is dictated by the persistent activation of ERK1/2 within distinct subcellular compartments, underlies the neurotoxic responses that are driven by this kinase.

Paraptosis: mediation by MAP kinases and inhibition by AIP-1/Alix

Programmed cell death (pcd) may take the form of apoptotic or nonapoptotic pcd. Whereas cysteine aspartyl-specific proteases (caspases) mediate apoptosis, the mediators of nonapoptotic cell death programs are much less well characterized. Here, we report that paraptosis, an alternative, nonapoptotic cell death program that may be induced by the insulin-like growth factor I receptor (among other inducers), is mediated by mitogen-activated protein kinases (MAPKs) and inhibited by AIP-1/Alix. The inhibition by AIP-1/Alix is specific for paraptosis since apoptosis was not inhibited. Caspases were not activated in this paradigm, nor were caspase inhibitors effective in blocking cell death. However, insulin-like growth factor I receptor (IGFIR)-induced paraptosis was inhibited by MEK-2-specific inhibitors and by antisense oligonucleotides directed against c-jun N-terminal kinase-1 (JNK-1). These results suggest that IGFIR-induced paraptosis is mediated by MAPKs, and inhibited by AIP-1/Alix.

Identification and H2O2 sensitivity of the major constitutive MAPK phosphatase from rat brain

The present study examined in subcellular fractions from rat brain the nature and sensitivity to hydrogen peroxide of constitutively expressed mitogen-activated protein kinase (MAPK) phosphatase activity. MAPK phosphatase activity was defined as the activity directed towards a dual-phosphorylated (pT/pY) peptide corresponding to the activation domain of the extracellular-regulated kinase (ERK) subtype of the MAPKs. The use of phosphatase inhibitors and biochemical analyses demonstrate that the MAPK phosphatase activity, which was highest in the microsomal membrane and soluble fractions, was attributable mainly, if not entirely, to protein phosphatase 2A (PP2A). Moreover, hydrogen peroxide (in the absence and presence of reduced glutathione) and glutathione disulfide inhibited the MAPK phosphatase activity by a dithiothreitol-reversible mechanism. These results provide direct support for mounting evidence that PP2A is a major regulator of MAPK phosphorylation in brain and suggest that inhibition of PP2A activity via reversible oxidation of a cysteine thiol(s) may underlie at least in part the activation of MAPKs occurring in response to hydrogen peroxide and oxidative stress.

Hyperglycemia and hypercapnia differently affect post-ischemic changes in protein kinases and protein phosphorylation in the rat cingulate cortex

Hyperglycemia and hypercapnia aggravate intra-ischemic acidosis and subsequent brain damage. However, hyperglycemia causes more extensive post-ischemic damage than hypercapnia, particularly in the cingulate cortex. We investigated the changes in the subcellular distribution of protein kinase Cgamma (PKCgamma) and the Ca2+/calmodulin-dependent protein kinase II (CaMKII), as well as changes in protein tyrosine phosphorylation during and following 10 min normoglycemic, hyperglycemic (plasma glucose approximately 20 mM) and hypercapnic (paCO2) approximately 300 mm Hg) global cerebral ischemia. During reperfusion period, the translocation to cell membranes of PKCgamma, but not CaMKII, was prolonged by intra-ischemic hyperglycemia, while it was only marginally affected by hypercapnia. The tyrosine-phosphorylation of proteins in the synaptosomal membranes, as well as the extracellular signal-regulated kinase (ERK) in the cytosol, markedly increased during reperfusion following hyperglycemic ischemia, but to a lesser degree following hypercapnic ischemia. Our data suggest that PKCgamma, tyrosine kinase and ERK systems are involved in the process of ischemic damage in the cingulate cortex, where hyperglycemia may affect these kinases through an additional mechanism other than exaggerated acidosis.

Alternative, nonapoptotic programmed cell death: mediation by arrestin 2, ERK2, and Nur77

Programmed cell death (pcd) may take the form of apoptosis or of nonapoptotic pcd. Whereas cysteine aspartyl-specific proteases (caspases) mediate apoptosis, the mediators of nonapoptotic cell death programs are much less well characterized. Here we report that alternative, nonapoptotic pcd induced by the neurokinin-1 receptor (NK(1)R) activated by its ligand Substance P, is mediated by a MAPK phosphorylation cascade recruited by the scaffold protein arrestin 2. The activation of the protein kinases Raf-1, MEK2, and ERK2 is essential for this form of nonapoptotic pcd, leading to the phosphorylation of the orphan nuclear receptor Nur77. NK(1)R-mediated cell death was inhibited by a dominant negative form of arrestin 2, Raf-1, or Nur77, by MEK1/2-specific inhibitors, and by RNA interference directed against ERK2 or MEK2 but not ERK1 or MEK1 and against Nur77. The MAPK pathway is also activated in neurons in primary culture undergoing NK(1)R-mediated death, since the MEK inhibitor PD98059 inhibited Substance P-induced death in primary striatal neurons. These results suggest that Nur77, which is regulated by a MAPK pathway activated via arrestin 2, modulates NK(1)R-mediated nonapoptotic pcd.

Altered mitogen-activated protein kinase signaling, tau hyperphosphorylation and mild spatial learning dysfunction in transgenic rats expressing the β-amyloid peptide intracellularly in hippocampal and cortical neurons

The pathological significance of intracellular Abeta accumulation in vivo is not yet fully understood. To address this, we have studied transgenic rats expressing Alzheimer's-related transgenes that accumulate Abeta intraneuronally in the cerebral and hippocampal cortices but do not develop extracellular amyloid plaques. In these rats, the presence of intraneuronal Abeta is sufficient to provoke up-regulation of the phosphorylated form of extracellular-regulated kinase (ERK) 2 and its enzymatic activity in the hippocampus while no changes were observed in the activity or phosphorylation status of other putative tau kinases such as p38, glycogen synthase kinase 3, and cycline-dependent kinase 5. The increase in active phospho-ERK2 was accompanied by increased levels of tau phosphorylation at S396 and S404 ERK2 sites and a decrease in the phosphorylation of the CREB kinase p90RSK. In a water maze paradigm, male transgenic rats displayed a mild spatial learning deficit relative to control littermates. Our results suggest that in the absence of plaques, intraneuronal accumulation of Abeta peptide correlates with the initial steps in the tau-phosphorylation cascade, alterations in ERK2 signaling and impairment of higher CNS functions in male rats.

Selective and persistent activation of extracellular signal-regulated protein kinase by nitric oxide in glial cells induces neuronal degeneration in glutathione-depleted midbrain cultures

Intracellular glutathione (GSH) levels determine whether nitric oxide (NO) is neurotrophic for dopamine neurons or triggers a cell death cascade in primary midbrain cultures. We have investigated herein the role of the extracellular-signal regulated protein kinase (ERK) 1/2 pathway in this GSH switching effect. The short-lived NO donor DEA/NO induces a transient activation of ERK-1/2 that totally disappears 2 h after NO administration. The depletion of GSH increases and the supplementation of GSH suppresses ERK-1/2 activation in response to NO treatment. More interestingly, GSH depletion changes the kinetic of phosphorylation leading to a second prolonged phase of ERK-1/2 activation from 2 to 16 h after NO addition. This change of kinetic is ultimately responsible for NO toxicity under GSH-depleted conditions, because selective blockade of the second and persistent phase of activation prevents cell death. In addition, the only transient ERK activation, induced by NO under normal GSH conditions, did not cause ERK-dependent cell death. Immunocytochemical colocalization studies demonstrate that ERK activation takes place exclusively in glial cells, mainly in astrocytes and less frequently in oligodendrocytes and glial progenitors. Furthermore, glial cell elimination or inactivation in the culture, by gliotoxic drugs, abrogates NO-induced ERK activation. Our results indicate that neurotrophism of NO switches into neurotoxicity after GSH depletion due to persistent activation of the ERK-1/2 signaling pathway in glial cells. The implication of these results in pathological conditions like Parkinson's disease, where GSH depletion and NO overproduction have been documented, are discussed.

Activation of extracellular signal-regulated kinases (ERK) during reperfusion of ischemic spinal cord

The extracellular signal-regulated kinases (ERK) participate in numerous signaling pathways and are abundantly expressed in the CNS. It has been proposed that ERK activation promotes survival in models of neuronal injury. Inhibition of MEK, the upstream kinase that activates ERK, however, leads to neuroprotection in models of cerebral ischemia and trauma, suggesting that in this context ERK activation contributes to cellular damage. The effect of ischemia and reperfusion on activity and expression of ERK was investigated using a reversible model of rabbit spinal cord ischemia. Active ERK was observed in nai;ve animals, which decreased during 15 to 60 min of ischemia. Upon reperfusion, a robust activation of ERK was observed in animals occluded for 60 min that remained permanently paraplegic. Immunohistochemical analyses revealed increased staining of phosphorylated ERK (pERK) in glial cells and faint nuclear staining in motor neurons of animals occluded for 60 min and reperfused for 18 h. In contrast ERK activity did not increase in animals occluded for 15 min that regained motor function. No evidence of increased pERK immunoreactivity in motor neurons or nuclear translocation was noted in these animals. ERK1 was demonstrated to be identical to a p46 c-Jun/ATF-2 kinase previously shown to be activated by reperfusion after a 60-min occlusion. The results suggest that activation of ERK during reperfusion of ischemic spinal cord participates in the cellular pathways leading to neuronal damage.

Mitogen and stress response kinase-1 (MSK1) mediates excitotoxic induced death of hippocampal neurones

Activation of the mitogen-activated protein kinase (MAPK/ERK) signal transduction pathway may mediate excitotoxic neuronal cell death in vitro and during ischemic brain injury in vivo. However, little is known, of the upstream regulation or downstream consequences of ERK activation under these conditions. Magnesium removal has been described to induce hyperexcitability and degeneration in cultured hippocampal neurones. Here, we show that neurotoxicity evoked by Mg2+ removal in primary hippocampal neurones stimulates ERK, but not p38, phosphorylation. Removal of Mg2+ also resulted in induction of the MAPK/ERK substrate mitogen- and stress-response kinase 1 (MSK1) and induced phosphorylation of the MSK1 substrate, the transcription factor cAMP response element binding protein (CREB). Neuronal death and phosphorylation of components in this cascade were inhibited by the Raf inhibitor SB-386023, by the MEK inhibitor U0126, or by the MSK1 inhibitors H89 and Ro318220. Importantly, this form of cell death was inhibited in hippocampal neurones cultured from MSK1-/- mice and inhibitors of Raf or MEK had no additive neuroprotective effect. Together, these data indicate that MSK1 is a physiological kinase for CREB and that this activity is an essential component of activity-dependent neuronal cell death.

Molecular correlates of impaired prefrontal plasticity in response to chronic stress

Disturbed adaptations at the molecular and cellular levels following stress could represent compromised neural plasticity that contributes to the pathophysiology of stress-induced disorders. Evidence illustrates atrophy and cell death of stress-vulnerable neurones in the prefrontal cortex. Reduced plasticity may be realized through the destabilized function of selective proteins involved in organizing the neuronal skeleton and translating neurotrophic signals. To elucidate the mechanisms underlying these effects, rats were exposed to chronic footshock stress. Patterns of c-fos, phospho-extracellular-regulated protein kinases 1/2 (ERK1/2), calcineurin and phospho-cyclic-AMP response-element binding protein (CREB) expression were subsequently investigated. The results indicate chronic stress-induced impairments in prefrontal and cingulate signal transduction cascades underlying neuronal plasticity. The medial prefrontal cortex, demonstrated functional hyperactivity and dendritic phospho-ERK1/2 hyperphosphorylation, while reduced c-fos and calcineurin immunoreactivity occurred in the cingulate cortex. Significantly reduced phospho-CREB expression in both cortical regions, considering its implication in brain-derived neurotrophic factor (BDNF) transcription, suggests reduced synaptic plasticity. This data confirms the damaging effect of stress on cortical activity, on a molecular level. Due to the association of these markers in the regulation of BDNF signalling, these findings suggest a central role for intracellular neurotrophin transduction members in the pathways underlying cellular actions of stress in the brain.

Brain slice culture for analysis of developmental brain disorders with special reference to congenital cytomegalovirus infection

Cytomegalovirus (CMV) is the most significant infectious cause of congenital abnormalities of the central nervous system (CNS) with variation from the fatal cytomegalic inclusion disease to functional brain disorder. The phenotype and degree of the brain disorder depends on infection time during the developing stage, virulence, route of infection and the viral susceptibility of the cells. The pathogenesis of the CMV infection to the CNS seems to be strongly related to neural migration, neural death, cellular compositions and the immune system of the brain. To understand the complex mechanism of this disorder, we used organotypic brain slice cultures. In the brain slice culture system, migration of CMV-infected neuronal cells was observed, which reflects infectious dynamics in vivo. Neural progenitor cells or glial immature cells in the subventricular zone and marginal area are most susceptible to murine cytomegalovirus (MCMV) infection in this system. The susceptibility declined as the number of immature glial cells decreased with age. The immature glial cells proliferated in brain slice cultures during prolonged incubation, and the susceptibility to MCMV infection also increased in association with the proliferation of these cells. The brain slice from an immunocompromised mouse (Beige-SCID mouse) unexpectedly showed lower susceptibility than that of an immunocompetent mouse during any prolonged incubation. These results suggest that the number of immature glial cells might determine the susceptibility of CMV infection to the brain, independent of the immune system. We reviewed recent findings of CMV infection to the brain from the perspective of brain slice cultures and the possibility that this system could be a useful method to investigate mechanisms of congenital anomaly of the brain.

Neuronal injury in hippocampus with human immunodeficiency virus transactivating protein, Tat

Patients with human immunodeficiency virus infection may develop a dementing illness. Using both in vitro and in vivo models, we investigated the susceptibility of the hippocampal formation to the Tat protein of human immunodeficiency virus. We also determined the pattern of hippocampal injury in patients with human immunodeficiency virus encephalitis. Following exposure of hippocampal slices to Tat, marked susceptibility of CA3 region with relative insensitivity of the CA1/2 region was observed. Injection of Tat into different regions of the rat hippocampus produced similar neuronal loss in both CA3 region and the dentate gyrus. In animals administered Tat, lesions were dose-dependent and immunohistochemical staining showed marked gliosis and loss of microtubule associated protein-2 in the affected areas at 3 days post-injection. Interestingly, synaptophysin staining was relatively preserved. In hippocampal tissue from patients with human immunodeficiency virus encephalitis, loss of microtubule-associated protein-2 staining was reduced in the molecular layer of the dentate gyrus. The results of our experiments demonstrate a unique pattern of hippocampal injury in organotypic culture and rats exposed to Tat. Our observations that patients with human immunodeficiency virus reveal a similar pattern of damage suggests that Tat protein may be pathophysiological relevant in human immunodeficiency virus encephalitis.

Time-course phosphorylation of the mitogen activated protein (MAP) kinase group of signalling proteins and related molecules following middle cerebral artery occlusion (MCAO) in rats

Recovery from the debilitating effects of ischaemic stroke is variable and unpredictable. To maximize patient recovery, a greater understanding of the molecular mechanisms involved in regulating both apoptosis and the repair processes affecting neuronal protection, particularly in the penumbra region, is desirable. We have previously shown, in human subjects, the increased expression of several growth factors soon after stroke, together with appearance of tyrosine phosphorylated proteins, in particular mitogen activated protein (MAP) kinase (ERK1/2). In this paper, we demonstrate a relatively short-lasting (< 12 h), but substantial increase in expression of phosphorylated proteins, in particular, p-JNK (phosphorylated c-Jun N-terminal kinase) and p-ERK1/2 in both the grey matter penumbra and infarcted tissue of rats, following permanent middle cerebral artery occlusion. p-ERK1/2 was associated with neurones and endothelial cells in the vicinity of the infarct while p-JNK was mainly expressed in neurones. Expression of both p-MEK3/6 and p-p38 MAP kinase was also increased in neurones and astroglia, within 1 h of infarction, p-p38 remaining elevated and associated with neurones and in particular with astroglia in the penumbra region for > 4 days. Evidence suggests that short-term activation of these proteins may be detrimental to neuronal survival, while their transient nature makes them unlikely to support angiogenesis, revascularization and reperfusion over a period of days and weeks. On the other hand, short-medium-term up-regulation of neuronal p-JNK, p-c-Jun, p-Stat-1 and p-p38 might be a factor in the regulation of apoptosis. Therapeutic manipulation of phosphorylation/activation of these and other important signalling intermediates might form the basis of an appropriate treatment to maximize revascularization and neuronal protection after ischaemic stroke.

Role of reactive oxygen species in extracellular signal-regulated protein kinase phosphorylation and 6-hydroxydopamine cytotoxicity

A number of reports indicate the potential for redox signalling via extracellular signal-regulated protein kinases (ERK) during neuronal injury. We have previously found that sustained ERK activation contributes to toxicity elicited by 6-hydroxydopamine (6-OHDA) in the B65 neuronal cell line. To determine whether reactive oxygen species (ROS) play a role in mediating ERK activation and 6-OHDA toxicity, we examined the effects of catalase, superoxide dismutase (SOD1), and metalloporphyrin antioxidants ('SOD mimetics') on 6-OHDA-treated cells. We found that catalase and metalloporphyrin antioxidants not only conferred protection against 6-OHDA but also inhibited development of sustained ERK phosphorylation in both differentiated and undifferentiated B65 cells. However, exogenously added SOD1 and heat-inactivated catalase had no effect on either toxicity or sustained ERK phosphorylation. This correlation between antioxidant protection and inhibition of 6-OHDA-induced sustained ERK phosphorylation suggests that redox regulation of ERK signalling cascades may contribute to neuronal toxicity.

To die or not to die for neurons in ischemia, traumatic brain injury and epilepsy: a review on the stress-activated signaling pathways and apoptotic pathways

After a severe episode of ischemia, traumatic brain injury (TBI) or epilepsy, it is typical to find necrotic cell death within the injury core. In addition, a substantial number of neurons in regions surrounding the injury core have been observed to die via the programmed cell death (PCD) pathways due to secondary effects derived from the various types of insults. Apart from the cell loss in the injury core, cell death in regions surrounding the injury core may also contribute to significant losses in neurological functions. In fact, it is the injured neurons in these regions around the injury core that treatments are targeting to preserve. In this review, we present our cumulated understanding of stress-activated signaling pathways and apoptotic pathways in the research areas of ischemic injury, TBI and epilepsy and that gathered from concerted research efforts in oncology and other diseases. However, it is obvious that our understanding of these pathways in the context of acute brain injury is at its infancy stage and merits further investigation. Hopefully, this added research effort will provide a more detailed knowledge from which better therapeutic strategies can be developed to treat these acute brain injuries.

NMDA-mediated activation of the tyrosine phosphatase STEP regulates the duration of ERK signaling

The intracellular mechanism(s) by which a cell determines the duration of extracellular signal-regulated kinase (ERK) activation is not well understood. We have investigated the role of STEP, a striatal-enriched tyrosine phosphatase, in the regulation of ERK activity in rat neurons. Glutamate-mediated activation of NMDA receptors leads to the rapid but transient phosphorylation of ERK in cultured neurons. Here we show that activation of NMDA receptors led to activation of STEP, which limited the duration of ERK activity as well as its translocation to the nucleus and its subsequent downstream nuclear signaling. In neurons, STEP is phosphorylated and inactive under basal conditions. NMDA-mediated influx of Ca(2+), but not increased intracellular Ca(2+) from other sources, leads to activation of the Ca(2+)-dependent phosphatase calcineurin and the dephosphorylation and activation of STEP. We have identified an important mechanism involved in the regulation of ERK activity in neurons that highlights the complex interplay between serine/threonine and tyrosine kinases and phosphatases.

Significant neuroprotection against ischemic brain injury by inhibition of the MEK1 protein kinase in mice: exploration of potential mechanism associated with apoptosis

MEK1/2 is a serine/threonine protein kinase that phosphorylates and activates extracellular signal-responsive kinase (ERK)1/2. In the present study we explored the role of MEK1/2 in ischemic brain injury using a selective MEK1/2 inhibitor, SL327, in mice. C57BL/6 mice were subjected to a 30-min occlusion of the middle cerebral artery (MCAO) followed by reperfusion. Western blot analysis demonstrated the immediate activation of MEK/ERK after reperfusion (within the first 10 min) in the ischemic brain; this activation was dose dependently blocked by SL327 (10-100 mg/kg, i.p.). A single dose of SL327 (100 mg/kg) administered 15 min before or 25 min after the onset of ischemia resulted in 63.6% (n = 18, p < 0.001) and 50.7% (n = 18, p < 0.01) reduction in infarct size, respectively, compared with vehicle-treated mice. Similarly, SL327 significantly reduced neurological deficits 1 to 3 days after reperfusion (n = 12, p < 0.01). The salutary effect of SL327-induced neuroprotection was independent of mitochondrial cytochrome c release or caspase-8-mediated apoptosis; however, SL327 markedly suppressed the levels of active caspase-3 and DNA fragmentation (as a measure of apoptosis) after ischemia/reperfusion. Our data suggest that the inhibition of MEK1/2 results in neuroprotection from reperfusion injury and that this protection may be associated with the reduction in apoptosis.

Activation of mitogen-activated protein kinases in experimental cerebral ischemia

OBJECTIVES

Mitogen-activated protein kinases (MAPK) regulate cell survival and differentiation. The aim of the present study is to investigate the activation pattern of different MAPKs [extracellular signal-regulated kinase (ERK), c-jun-N-terminal kinase (JNK) and p38] after cerebral ischemia.

MATERIAL AND METHODS

Rats were subjected to cerebral ischemia using a model for transient (2 h) and permanent middle cerebral artery occlusion (MCAO). The rats were allowed 6 h to 1 week of survival before immunohistochemical evaluation with phospho-specific antibodies, recognizing activated MAPKs.

RESULTS

ERK was activated in ipsilateral blood vessels, neurons and glia, but also in contralateral vessels. JNK activation was absent in neurons but appeared in arterial blood vessels and glia at the lesion side. Active p38 was observed in macrophages in maturing infarcts.

CONCLUSIONS

ERK and JNK may participate in the angiogenic response to cerebral ischemia. ERK, but not JNK, was activated in neurons, possibly indicating a pathophysiologic role. Active p38 might be involved in the inflammatory reaction.

Cytoplasmic aggregates of phosphorylated extracellular signal-regulated protein kinases in Lewy body diseases

A better understanding of cellular mechanisms that occur in Parkinson's disease and related Lewy body diseases is essential for development of new therapies. We previously found that 6-hydroxydopamine (6-OHDA) elicits sustained extracellular signal-regulated kinase (ERK) activation that contributes to neuronal cell death in vitro. As subcellular localization of activated kinases affect accessibility to downstream targets, we examined spatial patterns of ERK phosphorylation in 6-OHDA-treated cells and in human postmortem tissues representing the full spectrum of Lewy body diseases. All diseased human cases exhibited striking granular cytoplasmic aggregates of phospho-ERK (P-ERK) in the substantia nigra (involving 28 +/- 2% of neurons), which were largely absent in control cases (0.3 +/- 0.3%). Double-labeling studies and examination of preclinical cases suggested that these P-ERK alterations could occur relatively early in the disease process. Development of granular cytoplasmic P-ERK staining in 6-OHDA-treated cells was blocked by neuroprotective doses of catalase, supporting a role for oxidants in eliciting neurotoxic patterns of ERK activation. Evidence of nuclear translocation was not observed in degenerating neurons. Moreover, granular cytoplasmic P-ERK was associated with alterations in the distribution of downstream targets such as P-RSK1, but not of P-Elk-1, suggesting functional diversion of ERK-signaling pathways in Lewy body diseases.

Naringin-sensitive phosphorylation of plectin, a cytoskeletal cross-linking protein, in isolated rat hepatocytes

To identify phosphoproteins that might play a role in naringin-sensitive hepatocellular cytoskeletal disruption and apoptosis induced by algal toxins, hepatocyte extracts were separated by gel electrophoresis and immunostained with a phosphothreonine-directed antibody. Use of dilute (5%) polyacrylamide gels containing 6 m urea allowed the resolution of one very large (approximately 500-kDa) okadaic acid- and naringin-sensitive phosphoprotein, identified by tryptic fingerprinting, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and immunostaining as the cytolinker protein, plectin. The naringin-sensitive phosphorylation induced by okadaic acid and microcystin-LR probably reflected inhibition of a type 2A protein phosphatase, whereas the naringin-resistant phosphorylation induced by calyculin A, tautomycin, and cantharidin probably involved a type 1 phosphatase. Okadaic acid caused a collapse of the plectin-immunostaining bile canalicular sheaths and the general cytoskeletal plectin network into numerous medium-sized plectin aggregates. Inhibitors of protein kinase C, cAMP-dependent protein kinase, or Ca(2+)/calmodulin-dependent kinase II had moderate or no protective effects on plectin network disruption, whereas naringin offered 86% protection. Okadaic acid induced a naringin-sensitive phosphorylation of AMP-activated protein kinase (AMPK), the stress-activated protein kinases SEK1 and JNK, and S6 kinase. The AMPK-activating kinase (AMPKK) is likely to be the target of inhibition by naringin, the other kinases serving as downstream components of an AMPKK-initiated signaling pathway.

BK channel activity determines the extent of cell degeneration after oxygen and glucose deprivation: a study in organotypical hippocampal slice cultures

BK channels are voltage- and calcium-dependent potassium channels whose activation tends to reduce cellular excitability. In hippocampal pyramidal cells, BK channels repolarize somatic action potentials, and recent immunogold and electrophysiological analyses have revealed a presynaptic pool of BK channels that can regulate glutamate release. Agents that modulate BK channel activity would therefore be expected to affect cell excitability and neurotransmitter release also under pathological conditions. We have investigated the role of BK potassium channels in a model of ischemia-induced nerve cell degeneration. Organotypical slice cultures of rat hippocampus were exposed to oxygen and glucose deprivation (OGD), and cell death was assessed by the fluorescent dye propidium iodide. OGD induced cell death in the CA1 region and to a lesser extent in CA3. Treatment with the BK channel blockers, paxilline and iberiotoxin, during and after OGD induced increased cell death in CA1 and CA3. Both BK channel blockers also sensitized the relatively resistant granule cells in fascia dentata to OGD. The effect of paxilline and iberiotoxin was evident from 3 h after OGD, indicating a role of BK channels early in the post-ischemic phase or during OGD itself. The BK channel opener, NS1619, turned out to be gliotoxic, and this effect was not counteracted by paxilline and iberiotoxin. Our data show that blockade of BK channels aggravates OGD-induced cell damage and suggest that BK channels act as a kind of 'emergency brake' during and/or after ischemia. Accordingly, the BK channel is a potential molecular target for neuroprotective therapy in stroke.