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

Enhancing nerve regeneration in infraorbital nerve injury rat model: effects of vitamin B complex and photobiomodulation

Orofacial nerve injuries may result in temporary or long-term loss of sensory function and decreased quality of life in patients. B vitamins are required for DNA synthesis and the repair and maintenance of phospholipids. In particular, vitamins B1, B6, and B12 are essential for neuronal function. Deficiency in vitamin B complex (VBC) has been linked to increased oxidative stress, inflammation and demyelination. Photobiomodulation (PBM) has antioxidant activity and is neuroprotective. In addition, a growing literature attests to the positive effects of PBM on nerve repair. To assess the effect of PBM and VBC on regenerative process we evaluated the expression of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), myelin basic protein (MBP), laminin and neurofilaments (NFs) using Western blotting to identify regenerative pattern after chronic constriction injury of the infraorbital nerve (CCI IoN) treated by PBM, VBC or its combination. After CCI IoN, the rats were divided into six groups naive, sham, injured (CCI IoN), treated with photobiomodulation (904 nm, 6.23 J/cm2, CCI IoN + PBM), treated with VBC (containing B1, B6 and B12) 5 times, CCI IoN + VBC) and treated with PBM and VBC (CCI IoN + VBC + PBM). The treatments could revert low expression of BDNF, MBP and laminin. Also reverted the higher expression of neurofilaments and enhanced expression of NGF. PBM and VBC could accelerate injured infraorbital nerve repair in rats through reducing the expression of neurofilaments, increasing the expression of BDNF, laminin and MBP and overexpressing NGF. These data support the notion that the use of PBM and VBC may help in the treatment of nerve injuries. This finding has potential clinical applications.

Disease progression and brain atrophy in NMDAR encephalitis: Associated factor & clinical implication

OBJECTIVE

We investigated the longitudinal pattern, determining factors, and clinical implications of brain volume changes in N-methyl d-aspartate receptor-antibody (NMDAR) encephalitis.

METHODS

Baseline clinical profiles, treatment profiles, and outcome measured using the Clinical Assessment Scale in Autoimmune Encephalitis (CASE) and modified Rankin scale (mRS) were obtained from a long-term clinical database documenting an NMDAR encephalitis cohort. In serial MRI, the change in the normalized volume of different brain regions from the baseline evaluation was measured. At each MRI evaluation time point, the cumulative disease burden (CASE score × months) and the cumulative duration of status epilepticus were also evaluated.

RESULTS

Thirty-six patients were followed-up for 28.5 months (range 12-63 months). The volume ratio at last MRI to baseline was the lowest in the cerebellum (94.4 ± 5.7%, p < 0.001). Once developed, cerebellar volume reduction followed a progressive course until 2 years from disease onset. The degree of cerebellar volume reduction was positively correlated with mRS and total CASE scores (all, p < 0.001), and CASE scores in the domains of memory, language, and psychiatric problems, gait instability/ataxia, and weakness (all, p < 0.01). In linear mixed model analyses, the degree of cerebellar volume reduction was associated with cumulative disease burden up to 2 years (p < 0.001) and duration of status epilepticus (p < 0.001), and delayed removal of teratoma for ≥1 month (p = 0.006).

INTERPRETATION

In NMDAR encephalitis, cerebellar volume reduction was progressive once developed. Cerebellar volume reduction might reflect disease burden and extent of progression and be associated with poor outcomes in multiple functional domains.

The endocannabinoidome in neuropsychiatry: Opportunities and potential risks

The endocannabinoid system (ECS) comprises two cognate endocannabinoid receptors referred to as CB1R and CB2R_. ECS dysregulation is apparent in neurodegenerative/neuro-psychiatric disorders including but not limited to schizophrenia, major depressive disorder and potentially bipolar disorder. The aim of this paper is to review mechanisms whereby both receptors may interact with neuro-immune and neuro-oxidative pathways, which play a pathophysiological role in these disorders. CB1R is located in the presynaptic terminals of GABAergic, glutamatergic, cholinergic, noradrenergic and serotonergic neurons where it regulates the retrograde suppression of neurotransmission. CB1R plays a key role in long-term depression, and, to a lesser extent, long-term potentiation, thereby modulating synaptic transmission and mediating learning and memory. Optimal CB1R activity plays an essential neuroprotective role by providing a defense against the development of glutamate-mediated excitotoxicity, which is achieved, at least in part, by impeding AMPA-mediated increase in intracellular calcium overload and oxidative stress. Moreover, CB1R activity enables optimal neuron-glial communication and the function of the neurovascular unit. CB2R receptors are detected in peripheral immune cells and also in central nervous system regions including the striatum, basal ganglia, frontal cortex, hippocampus, amygdala as well as the ventral tegmental area. CB2R upregulation inhibits the presynaptic release of glutamate in several brain regions. CB2R activation also decreases neuroinflammation partly by mediating the transition from a predominantly neurotoxic "M1" microglial phenotype to a more neuroprotective "M2" phenotype. CB1R and CB2R are thus novel drug targets for the treatment of neuro-immune and neuro-oxidative disorders including schizophrenia and affective disorders.

Symptomatologic pathomechanism of N-methyl D-aspartate receptor encephalitis

N-methyl D-aspartate receptor (NMDAR) encephalitis is a well-characterized clinical syndrome. The main molecular mechanism of NMDAR encephalitis is autoantibody-mediated NMDAR hypofunction in the neuronal synapse. Several pathomechanistic hypotheses might explain how NMDAR hypofunction causes the typical symptoms and prognosis of NMDAR encephalitis. Suppression of NMDAR-dependent gamma-aminobutyric acid interneurons provokes an accelerated activation of the positive feedback loops of the dorsolateral prefrontal cortex/subiculum-nucleus accumbens circuit in the striatum, the ventral tegmental area (VTA), and the nucleus reuniens in the thalamus-hippocampus-VTA loop. Dysregulated activation of the VTA and cortex via those positive feedback loops may explain the rapid clinical deterioration at acute stages of the disease and the well-characterized syndrome that includes limbic system dysfunction, intractable seizures, dyskinesia, coma, and the characteristic extreme delta brush. Progressive cerebellar atrophy is correlated with cumulative disease burden and is associated with worse long-term outcomes, which might be explained by the NMDAR-dependent pathways required to maintain neuronal survival. Those pathomechanistic hypotheses for NMDAR encephalitis support the rationale for the early introduction of combination immunotherapy and the use of adjuvant immunotherapy in patients with persisting symptoms in chronic disease phases.

Exogenous brain-derived neurotrophic factor attenuates neuronal apoptosis and neurological deficits after subarachnoid hemorrhage in rats

Brain-derived neurotrophic factor (BDNF) is a growth factor crucial for neuronal survival, while its role in subarachnoid hemorrhage (SAH)-induced neuronal apoptosis remains unclear. The aim of the present study was to investigate whether administering exogenous BDNF can protect against neuronal apoptosis and neurological deficits following SAH in a rat model. The BDNF level was found to be significantly decreased in the basal cortex at 6, 12, 24, 48 and 72 h following SAH. Exogenous BDNF significantly decreased the expression of Bax and reduced activation of caspase-3 and caspase-9 and the number of apoptotic neurons. Moreover, exogenous BDNF treatment significantly improved the neurological deficits at 72 h and long-term behavioral deficits (day 14) following SAH in a rat model. These findings indicate that exogenous BDNF attenuated SAH-induced neuronal injury in rats.

Medial prefrontal cortex exacerbates gastric dysfunction of rats upon restraint water‑immersion stress

Restraint water-immersion stress (RWIS) can induce a gastric mucosal lesions within a few hours. The medial prefrontal cortex (mPFC) is involved in the RWIS process. The present study investigated the modulatory effects and molecular mechanisms of the mPFC on gastric function under an RWIS state. Male Wistar rats were divided into four groups; namely, the control, RWIS 4 h (RWIS for 4 h only), sham-operated and bilateral-lesioned (bilateral-lesioned mPFC) groups. The gastric erosion index (EI) and gastric motility (GM) were determined, and the proteomic profiles of the mPFC were assessed by isobaric tags for relative and absolute quantitation (iTRAQ) coupled with two-dimensional liquid chromatography and tandem mass spectrometry. Additionally, iTRAQ results were verified by western blot analysis. Compared with the RWIS 4 h group and the sham-control group, the bilateral-lesioned group exhibited a significantly lower EI (P<0.01). In the bilateral-lesioned group, RWIS led to a significant decrease in EI and GM. When comparing the control and RWIS 4 h groups, 129 dysregulated proteins were identified, of which 88 were upregulated and 41 were downregulated. Gene Ontology functional analysis demonstrated that 29 dysregulated proteins, including postsynaptic density protein 95, were directly associated with axon morphology, axon growth and synaptic plasticity. Ingenuity pathway analysis revealed that the dysregulated proteins were mainly involved in neurological disease signaling pathways, including the NF-κB and ERK signaling pathways. These data indicated that the presence of the mPFC exacerbates gastric mucosal injury in awake rats during RWIS. Although the quantitative proteomic analysis elucidated the nervous system molecular targets associated with the production of gastric mucosal lesions, such as the role of PSD95. The underlying molecular mechanisms of synaptic plasticity need to be further elucidated.

Olesoxime protects embryonic cortical neurons from camptothecin intoxication by a mechanism distinct from BDNF

BACKGROUND AND PURPOSE

Olesoxime is a small cholesterol-oxime promoting rat embryonic motor neurons survival in the absence of trophic factors. Because olesoxime can substitute for neurotrophic factors in many situations, and to gain further understanding of its mechanism of action, we wondered if it could prevent neuronal death induced by camptothecin (CPT) and compared its effects with those of brain-derived neurotrophic factor (BDNF).

EXPERIMENTAL APPROACH

E17 rat embryonic cortical neurons were treated with olesoxime, BDNF or vehicle and intoxicated with CPT. Caspase-dependent and caspase-independent death pathways along with pro-survival pathways activation were explored.

KEY RESULTS

As previously reported for BDNF, olesoxime dose-dependently delayed CPT-induced cell death. Both compounds acted downstream of p53 activation preventing cytochrome c release and caspases activation. When caspase activation was blocked, both olesoxime and BDNF provided additional neuroprotective effect, potentially through the prevention of apoptosis-inducing factor release from mitochondria. While BDNF activates both the PI3K/Akt and the ERK pathway, olesoxime induced only a late activation of the ERK pathways, which did not seem to play a major role in its neuroprotection against CPT. Rather, our results favour preserved mitochondrial membrane integrity by olesoxime.

CONCLUSIONS AND IMPLICATIONS

Albeit different, olesoxime and BDNF mechanisms for neuroprotection converge to preserve mitochondrial function. These findings emphasize the importance of targeting the mitochondria in the process of neurodegeneration. Importantly olesoxime, by mimicking neurotrophin pro-survival activities without impacting PI3K/Akt and ERK signalling, may have greater therapeutic potential in many diseases where neurotrophins were considered as a therapeutic solution.

Effects of lateral ventricular transplantation of bone marrow-derived mesenchymal stem cells modified with brain-derived neurotrophic factor gene on cognition in a rat model of Alzheimer's disease

In the present study, transplantation of bone marrow-derived mesenchymal stem cells modified with brain-derived neurotrophic factor gene into the lateral ventricle of a rat model of Alzheimer's disease, resulted in significant attenuation of nerve cell damage in the hippocampal CA1 region. Furthermore, brain-derived neurotrophic factor and tyrosine kinase B mRNA and protein levels were significantly increased, and learning and memory were significantly improved. Results indicate that transplantation of bone marrow-derived mesenchymal stem cells modified with brain-derived neurotrophic factor gene can significantly improve cognitive function in a rat model of Alzheimer's disease, possibly by increasing the levels of brain-derived neurotrophic factor and tyrosine kinase B in the hippocampus.

Impact of nuclear factor-κB on restoration of neuron growth and differentiation in hippocampus of degenerative brain

The mode of action of nuclear factor-κB (NF-κB) has been extensively observed in different aspects of cell growth and proliferation. The transcription factor regulates various genes controlling inflammation and anti-inflammatory responses in different tissues. Thus, NF-κB signal gains a therapeutic prospect. The activation of NF-κB requires nuclear localization of its p65 subunit. Research also indicates an impact of phosphorylated p65 on the transcription of genes during cell growth and the immune response. Following the trends in investigations over decades, different observations suggest that NF-κB activation and phosphorylation of p65 regulate neuronal plasticity. Also, inhibition of NF-κB activation is a well-demonstrated way to attenuate inflammation. In addition to anti-inflammatory drugs, recent researches unwind a way to regulate regeneration and repair tissue damage. Thus, keeping a critical view on NF-κB signals, we propose the importance of natural or synthetic NF-κB activators for neurogenesis.

Enhanced vascular PI3K/Akt-NOX signaling underlies the peripheral NMDAR-mediated pressor response in conscious rats

The molecular mechanisms for peripheral N-methyl-D-aspartate receptor (NMDAR)-mediated vascular oxidative stress and pressor response are not known. We conducted integrative (in vivo) and ex vivo biochemical studies to test the hypothesis that reactive oxygen species (ROS)-dependent calcium influx, triggered by the activation of vascular kinases, underlies the NMDAR-mediated pressor response. Pharmacological inhibition of phosphoinositide 3-kinase (PI3K)/Akt (wortmannin, 15 μg/kg), protein kinase C (chelerythrine: 5 mg/kg, intravenous), Ca²⁺ influx (nifedipine, 0.35 or 0.75 mg/kg), or NADPH oxidase (NOX: apocynin, 5 mg/kg) attenuated the peripheral NMDAR-mediated pressor response in conscious male Sprague-Dawley rats. NMDAR activation enhanced the phosphorylation of Akt, ERK1, JNK and p38 (Western blot), and NOX activity in vascular tissues collected during the pressor response caused by NMDA infusion (180 μg·kg⁻¹·min⁻¹, 30 minutes). Furthermore, ex vivo studies showed that wortmannin, chelerythrine, or apocynin abrogated the NMDAR-mediated vascular nitric oxide (NO) and ROS generation and NOX activation in the vasculature. These findings implicate vascular PI3K/Akt-protein kinase C signaling in the peripheral NMDAR-mediated increases in vascular NO and NOX activation (ROS), which ultimately lead to calcium influx and pressor response in conscious rats.

GluN2B N-methyl-D-aspartic acid receptor subunit mediates atorvastatin-Induced neuroprotection after focal cerebral ischemia

Statins are potent cholesterol biosynthesis inhibitors that exert protective effects in humans and in experimental models of stroke. The mechanisms involved in these protective actions are not completely understood. This study evaluates whether atorvastatin (ATV) treatment affects the GluN1 and GluN2B subunits of the N-methyl-D-aspartic acid receptor in the somatosensory cerebral cortex at short and long periods following ischemia. Sham and ischemic male Wistar rats received 10 mg/kg of ATV or placebo by gavage every 24 hr for 3 consecutive days. The first dose was administered 6 hr after ischemia-reperfusion or the sham operation. ATV treatment resulted in faster recovery of neurological scores than placebo, prevented the appearance of pyknotic neurons, and restored microtubule-associated protein 2 and neuronal nuclei staining to control values in the somatosensory cerebral cortex and the hippocampus at 72 hr and 15 days postischemia. Furthermore, ATV prevented spatial learning and memory deficits caused by cerebral ischemia. Cerebral ischemia reduced the number of GluN1/PSD-95 and GluN2B/PSD-95 colocalization clusters in cortical pyramidal neurons and reduced the levels of brain-derived neurotrophic factor (BDNF) in the cerebral cortex. These effects of the ischemic insult were prevented by ATV, which also induced GluN2B/PSD-95 colocalization in neuronal processes and an association of GluN2B with TrkB. The GluN2B pharmacological inhibitor ifenprodil prevented the increase in BDNF levels and the motor and cognitive function recovery caused by ATV in ischemic rats. These findings indicate that GluN2B is involved in the neuroprotective mechanism elicited by ATV to promote motor and cognitive recovery after focal cerebral ischemia.

Activation of M1 mAChRs by lesatropane rescues glutamate neurotoxicity in PC12 cells via PKC-mediated phosphorylation of ERK1/2

Lesatropane, a synthesized chiral tropane (3S, 6S-isomer of satropane), is a novel muscarinic agonist, and is being under preclinical development in China for the treatment of primary glaucoma. The reports concerning that activation of muscarinic acetylcholine receptors (mAChRs) could protect cells against apoptosis prompted us to study the neuroprotective effects of lesatropane and the mechanism. We found that lesatropane could protect PC12 cells from glutamate-induced neurotoxicity and reverse the decreased ERK1/2 activation caused by glutamate. Atropine or pirenzepine, antagonist of mAChR or M1 mAChR, antagonized the protective effects of lesatropane respectively and suppressed the lesatropane's effects on ERK1/2. Furthermore, chelerythrine, a PKC inhibitor, partially suppressed ERK1/2 activation induced by lesatropane. The results indicated that the specific M1 mAChR via PKC-ERK1/2 pathway might be involved in the neuroprotective effects of lesatropane. While M1 mAChR is a therapeutic target of Alzheimer's disease (AD), the results of this paper contribute to further information concerning the activation of M1 mAChR as a therapeutic target in AD.

Preconditioning provides neuroprotection in models of CNS disease: paradigms and clinical significance

Preconditioning is a phenomenon in which brief episodes of a sublethal insult induce robust protection against subsequent lethal injuries. Preconditioning has been observed in multiple organisms and can occur in the brain as well as other tissues. Extensive animal studies suggest that the brain can be preconditioned to resist acute injuries, such as ischemic stroke, neonatal hypoxia/ischemia, surgical brain injury, trauma, and agents that are used in models of neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. Effective preconditioning stimuli are numerous and diverse, ranging from transient ischemia, hypoxia, hyperbaric oxygen, hypothermia and hyperthermia, to exposure to neurotoxins and pharmacological agents. The phenomenon of "cross-tolerance," in which a sublethal stress protects against a different type of injury, suggests that different preconditioning stimuli may confer protection against a wide range of injuries. Research conducted over the past few decades indicates that brain preconditioning is complex, involving multiple effectors such as metabolic inhibition, activation of extra- and intracellular defense mechanisms, a shift in the neuronal excitatory/inhibitory balance, and reduction in inflammatory sequelae. An improved understanding of brain preconditioning should help us identify innovative therapeutic strategies that prevent or at least reduce neuronal damage in susceptible patients. In this review, we focus on the experimental evidence of preconditioning in the brain and systematically survey the models used to develop paradigms for neuroprotection, and then discuss the clinical potential of brain preconditioning.

Impairment of TrkB-PSD-95 signaling in Angelman syndrome

Brain-derived neurotrophic factor signaling is defective in Angelman syndrome and can be rescued by disruption of Arc/PSD95 binding.

Angelman syndrome (AS) is a neurodevelopment disorder characterized by severe cognitive impairment and a high rate of autism. AS is caused by disrupted neuronal expression of the maternally inherited Ube3A ubiquitin protein ligase, required for the proteasomal degradation of proteins implicated in synaptic plasticity, such as the activity-regulated cytoskeletal-associated protein (Arc/Arg3.1). Mice deficient in maternal Ube3A express elevated levels of Arc in response to synaptic activity, which coincides with severely impaired long-term potentiation (LTP) in the hippocampus and deficits in learning behaviors. In this study, we sought to test whether elevated levels of Arc interfere with brain-derived neurotrophic factor (BDNF) TrkB receptor signaling, which is known to be essential for both the induction and maintenance of LTP. We report that TrkB signaling in the AS mouse is defective, and show that reduction of Arc expression to control levels rescues the signaling deficits. Moreover, the association of the postsynaptic density protein PSD-95 with TrkB is critical for intact BDNF signaling, and elevated levels of Arc were found to impede PSD-95/TrkB association. In Ube3A deficient mice, the BDNF-induced recruitment of PSD-95, as well as PLCγ and Grb2-associated binder 1 (Gab1) with TrkB receptors was attenuated, resulting in reduced activation of PLCγ-α-calcium/calmodulin-dependent protein kinase II (CaMKII) and PI3K-Akt, but leaving the extracellular signal-regulated kinase (Erk) pathway intact. A bridged cyclic peptide (CN2097), shown by nuclear magnetic resonance (NMR) studies to uniquely bind the PDZ1 domain of PSD-95 with high affinity, decreased the interaction of Arc with PSD-95 to restore BDNF-induced TrkB/PSD-95 complex formation, signaling, and facilitate the induction of LTP in AS mice. We propose that the failure of TrkB receptor signaling at synapses in AS is directly linked to elevated levels of Arc associated with PSD-95 and PSD-95 PDZ-ligands may represent a promising approach to reverse cognitive dysfunction.

Angelman syndrome (AS) is a debilitating neurological disorder caused by a dysfunctional Ube3A gene. Most children with AS exhibit developmental delay, movement disorders, speech impairment, and often autistic features. The Ube3A enzyme normally regulates the degradation of the synaptic protein Arc, and in its absence the resulting elevated levels of Arc weaken synaptic contacts, making it difficult to generate long-term potentiation (LTP) and to process and store memory. In this study, we show that increased levels of Arc disrupt brain-derived neurotrophic factor (BDNF) signaling through the TrkB receptor (which is important for both the induction and maintenance of LTP). We find that the association of the postsynaptic density protein PSD-95 with TrkB is critical for intact BDNF signaling, and that the high levels of Arc in AS interfere with BDNF-induced recruitment of postsynaptic density protein-95 (PSD-95) and other effectors to TrkB. By disrupting the interaction between Arc and PSD-95 with the novel cyclic peptidomimetic compound CN2097, we were able to restore BDNF signaling and improve the induction of LTP in a mouse model of AS. We propose that the disruption of TrkB receptor signaling at synapses contributes to the cognitive dysfunction that occurs in Angelman syndrome.

Amelioration of central cardiovascular regulatory dysfunction by tropomyocin receptor kinase B in a mevinphos intoxication model of brain stem death

BACKGROUND AND PURPOSE

Little information exists on the mechanisms that precipitate brain stem death, the legal definition of death in many developed countries. We investigated the role of tropomyocin receptor kinase B (TrkB) and its downstream signalling pathways in the rostral ventrolateral medulla (RVLM) during experimental brain stem death.

EXPERIMENTAL APPROACH

An experimental model of brain stem death that employed microinjection of the organophosphate insecticide mevinphos bilaterally into the RVLM of Sprague-Dawley rats was used, in conjunction with cardiovascular, pharmacological and biochemical evaluations.

KEY RESULTS

A significant increase in TrkB protein, phosphorylation of TrkB at Tyr(516) (pTrkB(Y516) ), Shc at Tyr(317) (pShc(Y317) ) or ERK at Thr(202) /Tyr(204) , or Ras activity in RVLM occurred preferentially during the pro-life phase of experimental brain stem death. Microinjection bilaterally into RVLM of a specific TrkB inhibitor, K252a, antagonized those increases. Pretreatment with anti-pShc(Y317) antiserum, Src homology 3 binding peptide (Grb2/SOS inhibitor), farnesylthioacetic acid (Ras inhibitor), manumycin A (Ras inhibitor) or GW5074 (Raf-1 inhibitor) blunted the preferential augmentation of Ras activity or ERK phosphorylation in RVLM and blocked the up-regulated NOS I/protein kinase G (PKG) signalling, the pro-life cascade that sustains central cardiovascular regulation during experimental brain stem death.

CONCLUSIONS AND IMPLICATIONS

Activation of TrkB, followed by recruitment of Shc/Grb2/SOS adaptor proteins, leading to activation of Ras/Raf-1/ERK signalling pathway plays a crucial role in ameliorating central cardiovascular regulatory dysfunction via up-regulation of NOS I/PKG signalling cascade in the RVLM in brain stem death. These findings provide novel information for developing therapeutic strategies against this fatal eventuality.

ERK1/2 activation is involved in the neuroprotective action of P2Y13 and P2X7 receptors against glutamate excitotoxicity in cerebellar granule neurons

Cerebellar granule neurons express several types of nucleotide receptors, with the metabotropic P2Y(13) and the ionotropic P2X7 being the most relevant in this model. In the present study we investigated the role of P2Y(13) and P2X7 nucleotide receptors in ERK1/2 signalling. The nucleotidic agonists 2MeSADP (2-methylthioadenosine-5'-diphosphate) for P2Y(13) and BzATP (2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate) for P2X7 receptors were coupled to ERK1/2 activation in granule neurons, being able to increase around two-fold the levels of ERK1/2 phosphorylation. These effects were sensitive to the inhibitory action of the antagonists MRS-2211 and A-438079, specific for P2Y(13) and P2X7 receptors, respectively. Although both receptor subtypes shared the same pattern of transient ERK1/2 phosphorylation, they differed in the intracellular cascades they triggered, being PI3K-dependent for P2Y(13) and calcium/calmodulin kinase II (CaMKII)-dependent for P2X7. These two different ERK-mediated pathways were involved in the neuroprotective effects displayed by both P2Y(13) and P2X7 receptors against apoptosis induced by an excitotoxic concentration of glutamate, in a similar manner to the neurotrophin, BDNF. In addition, P2Y(13) and P2X7 receptor agonists were also able to phosphorylate and activate the ERK-dependent target CREB, which could be involved in their neuroprotective effect. These results indicate that nucleotide receptors share with trophic factors the same survival routes in neurons, such as the ERK signalling route, and therefore, can contribute to the maintenance of granule neurons in conditions in which survival is being compromised.

Addition of glutamate to serum-free culture promotes recovery of electrical activity in adult hippocampal neurons in vitro

A long-term cell culture system utilizing normal adult hippocampal neurons would represent an important tool that could be useful in research on the mature brain, neurological disorders and age-related neurological diseases. Historically, in vitro neuronal systems are derived from embryonic rather than mature brain tissue, a practice predicated upon difficulties in supporting regeneration, functional recovery and long-term survival of adult neurons in vitro. A few studies have shown that neurons derived from the hippocampal tissue of adult rats can survive and regenerate in vitro under serum-free conditions. However, while the adult neurons regenerated morphologically under these conditions, both the electrical activity characteristic of in vivo neurons as well as long-term neuronal survival was not consistently recovered in vitro. In this study, we report on the development of a defined culture system with the ability to support functional recovery and long-term survival of adult rat hippocampal neurons. In this system, the cell-adhesive substrate, N-1 [3-(trimethoxysilyl) propyl]-diethylenetriamine, supported neuronal attachment, regeneration, and long-term survival of adult neurons for more than 80 days in vitro. Additionally, the excitatory neurotransmitter glutamate, applied at 25muM for 1-7 days after morphological neuronal regeneration in vitro, enabled full recovery of neuronal electrical activity. This low concentration of glutamate promoted the recovery of neuronal electrical activity but with minimal excitotoxicity. These improvements allowed electrically active adult neurons to survive in vitro for several months, providing a stable test-bed for the long-term study of regeneration in adult-derived neuronal systems, especially for traumatic brain injury (TBI).

Bone morphogenetic protein-6 promotes cerebellar granule neurons survival by activation of the MEK/ERK/CREB pathway

Bone morphogenetic proteins (BMPs) have been implicated in the generation and postnatal differentiation of cerebellar granule cells (CGCs). Here, we examined the eventual role of BMPs on the survival of these neurons. Lack of depolarization causes CGC death by apoptosis in vivo, a phenomenon that is mimicked in vitro by deprivation of high potassium in cultured CGCs. We have found that BMP-6, but not BMP-7, is able to block low potassium-mediated apoptosis in CGCs. The neuroprotective effect of BMP-6 is not accompanied by an increase of Smad translocation to the nucleus, suggesting that the canonical pathway is not involved. By contrast, activation of the MEK/ERK/CREB pathway by BMP-6 is necessary for its neuroprotective effect, which involves inhibition of caspase activity and an increase in Bcl-2 protein levels. Other pathways involved in the regulation of CGC survival, such as the c-Jun terminal kinase and the phosphatidylinositol 3-kinase (PI3K)-Akt/PKB, were not affected by BMP-6. Moreover, failure of BMP-7 to activate the MEK/ERK/CREB pathway could explain its inability to protect CGCs from low potassium-mediated apoptosis. Thus, this study demonstrates that BMP-6 acting through the noncanonical MEK/ERK/CREB pathway plays a crucial role on CGC survival.

Transient focal ischemia results in persistent and widespread neuroinflammation and loss of glutamate NMDA receptors

Stroke is accompanied by neuroinflammation in humans and animal models. To examine the temporal and anatomical profile of neuroinflammation and NMDA receptors (NMDAR) in a stroke model, rats (N=17) were subjected to a 90 min occlusion of the middle cerebral artery (MCAO) and compared to sham (N=5) and intact (N=4) controls. Striatal and parietal cortical infarction was confirmed by MRI 24h after reperfusion. Animals were killed 14 or 30-40 days later and consecutive coronal cryostat sections were processed for quantitative autoradiography with the neuroinflammation marker [(3)H]PK11195 and the NMDAR antagonist [(3)H]MK801. Significantly increased specific binding of [(3)H]PK11195 relative to non-ischemic controls was observed in the ipsilateral striatum (>3 fold, p<0.0001), substantia innominata (>2 fold) with smaller (20%-80%) but statistically significant (p=0.002-0.04) ipsilateral increases in other regions partially involved in the infarct such as the parietal and piriform cortex, and in the lateral septum, which was not involved in the infarct. Trends for increases in PBR density were also observed in the contralateral hemisphere. In the same animals, NMDAR specific binding was significantly decreased bilaterally in the septum, substantia innominata and ventral pallidum. Significant decreases were also seen in the ipsilateral striatum, accumbens, frontal and parietal cortex. The different anatomical distribution of the two phenomena suggests that neuroinflammation does not cause the observed reduction in NMDAR, though loss of NMDAR may be locally augmented in ipsilateral regions with intense neuroinflammation. Persistent, bilateral loss of NMDAR, probably reflecting receptor down regulation and internalization, may be responsible for some of the effects of stroke on cognitive function which cannot be explained by infarction alone.

Neuroprotective effects of (-)-epigallocatechin-3-gallate against quinolinic acid-induced excitotoxicity via PI3K pathway and NO inhibition

Excessive stimulation of the NMDA receptor induces neuronal cell death and is implicated in the development of several neurodegenerative diseases. While EGCG suppresses apoptosis induced by NMDA receptor-mediated excitotoxicity, the mechanisms underlying this process have yet to be completely determined. This study was designed to investigate whether (-)-epigallocatechin-3-gallate (EGCG) plays a neuroprotective role by inhibiting nitric oxide (NO) production and activating cellular signaling mechanisms including MAP kinase, PI3K, and GSK-3beta and acting on the antiapoptotic and the proapoptotic genes in N18D3 neural cells. The cells were pretreated with EGCG for 2 h and then exposed to quinolinic acid (QUIN), a NMDA receptor agonist, 30 mM for 24 h. MTT assay and DAPI staining were used to identify cell viability and apoptosis, respectively, and demonstrated that EGCG significantly increased cell viability and protected the cells from apoptotic death. In addition, EGCG had a capacity to reduce QUIN-induced excitotoxic cell death not only by blocking increase of intracellular calcium levels but also by inhibiting NO production. Gene expression analysis revealed that EGCG prevented the QUIN-induced expression of the proapoptotic gene, caspase-9, and increased that of the antiapoptotic genes, Bcl-XL, Bcl-2, and Bcl-w. Further examination about potential cell signaling candidate involved in this neuroprotective effect showed that immunoreacitivity of PI3K was significantly increased in the cells treated with EGCG. These results suggest that the neuroprotective mechanism of EGCG against QUIN-induced excitotoxic cell death includes regulation of PI3K and modulation of cell survival and death genes through decreasing of intracellular calcium levels and controlling of NO production.

ERK and cell death: ERK1/2 in neuronal death

Extracellular signal-regulated kinase (ERK) is a versatile protein kinase that regulates many cellular functions. Growing evidence suggests that ERK1/2 plays a crucial role in promoting cell death in a variety of neuronal systems, including neurodegenerative diseases. It is believed that the magnitude and the duration of ERK1/2 activity determine its cellular function. In this review, we summarize recent evidence for a role of ERK1/2 in neuronal death. Furthermore, we discuss the mechanisms involved in ERK1/2 mediating neuronal death.

BDNF-mediated cerebellar granule cell development is impaired in mice null for CaMKK2 or CaMKIV

The Ca(2+)/calmodulin-activated kinases CaMKK2 and CaMKIV are highly expressed in the brain where they play important roles in activating intracellular responses to elevated Ca(2+). To address the biological functions of Ca(2+) signaling via these kinases during brain development, we have examined cerebellar development in mice null for CaMKK2 or CaMKIV. Here, we demonstrate that CaMKK2/CaMKIV-dependent phosphorylation of cAMP response element-binding protein (CREB) correlates with Bdnf transcription, which is required for normal development of cerebellar granule cell neurons. We show in vivo and in vitro that the absence of either CaMKK2 or CaMKIV disrupts the ability of developing cerebellar granule cells in the external granule cell layer to cease proliferation and begin migration to the internal granule cell layer. Furthermore, loss of CaMKK2 or CaMKIV results in decreased CREB phosphorylation (pCREB), Bdnf exon I and IV-containing mRNAs, and brain-derived neurotrophic factor (BDNF) protein in cerebellar granule cell neurons. Reexpression of CaMKK2 or CaMKIV in granule cells that lack CaMKK2 or CaMKIV, respectively, restores pCREB and BDNF to wild-type levels and addition of BDNF rescues granule cell migration in vitro. These results reveal a previously undefined role for a CaMKK2/CaMKIV cascade involved in cerebellar granule cell development and show specifically that Ca(2+)-dependent regulation of BDNF through CaMKK2/CaMKIV is required for this process.

Different mechanisms of NMDA-mediated protection against neuronal apoptosis: a stimuli-dependent effect

The mechanisms of protective effect of N-methyl-D-aspartate (NMDA) receptor stimulation on apoptosis of neurons at their early stage of development are poorly understood. In the present study, we investigated the effects of NMDA on staurosporine (St)- and low-potassium (LP)-evoked apoptotic cell death in primary cerebellar granule cell (CGC) cultures at 7 days in vitro (DIV). We found that NMDA (200 microM) attenuated the St (0.5 microM)- and LP (5 mM KCl)-induced neuronal cell death in 7 but not 12 DIV CGC as confirmed by LDH release and MTT reduction assays. Moreover, NMDA attenuated St-and LP-evoked DNA fragmentation and cytosolic apoptosis inducing factor (AIF) protein level but not caspase-3 activation induced by both pro-apoptotic factors. Neuroprotective effects of NMDA on St-induced apoptosis in CGC were attenuated by inhibitors of ERK/MAPK-signaling, PD 98059 and U0126 but not by NMDA receptor antagonists, AP-5 (100 microM) and MK-801 (1 microM) or by inhibitors of PI3-K/Akt pathway (LY 294002 and wortmannin). In contrast to staurosporine model of apoptosis, AP-5 and MK-801 but not inhibitors of PI3-K/Akt and MAPK/ERK1/2 prevented the NMDA-mediated neuroprotection in LP-induced apoptosis of CGC. In separate experiments, we observed also the anti-apoptotic action of NMDA on St (0.5 microM)- and salsolinol (250 microM)-evoked cell death in human neuroblastoma SH-SY5Y cells without its influence on caspase-3 activity, induced by these pro-apoptotic factors. These data indicate that neuroprotection evoked by NMDA in CGC strongly depends on used pro-apoptotic agent and could engage NMDA channel function or be connected with the activation of pro-survival MAPK/ERK1/2 pathway. It is also suggested that anti-apoptotic effects of NMDA is connected with inhibition of fragmentation of DNA via caspase-3-independent mechanism.

Lithium protection of phencyclidine-induced neurotoxicity in developing brain: the role of phosphatidylinositol-3 kinase/Akt and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase signaling pathways

Phencyclidine (PCP) and other N-methyl-D-aspartate (NMDA) receptor antagonists have been shown to be neurotoxic to developing brains and to result in schizophrenia-like behaviors later in development. Prevention of both effects by antischizophrenic drugs suggests the validity of PCP neurodevelopmental toxicity as a heuristic model of schizophrenia. Lithium is used for the treatment of bipolar and schizoaffective disorders and has recently been shown to have neuroprotective properties. The present study used organotypic corticostriatal slices taken from postnatal day 2 rat pups to investigate the protective effect of lithium and the role of the phosphatidylinositol-3 kinase (PI-3K)/Akt and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) pathways in PCP-induced cell death. Lithium pretreatment dose-dependently reduced PCP-induced caspase-3 activation and DNA fragmentation in layers II to IV of the cortex. PCP elicited time-dependent inhibition of the MEK/ERK and PI-3K/Akt pathways, as indicated by dephosphorylation of ERK1/2 and Akt. The proapoptotic factor glycogen synthase kinase (GSK)-3beta was also dephosphorylated at serine 9 and thus activated. Lithium prevented PCP-induced inhibition of the two pathways and activation of GSK-3beta. Furthermore, blocking either PI-3K/Akt or MEK/ERK pathway abolished the protective effect of lithium, whereas inhibiting GSK-3beta activity mimicked the protective effect of lithium. However, no cross-talk between the two pathways was found. Finally, specific GSK-3beta inhibition did not prevent PCP-induced dephosphorylation of Akt and ERK. These data strongly suggest that the protective effect of lithium against PCP-induced neuroapoptosis is mediated through independent stimulation of the PI-3K/Akt and ERK pathways and suppression of GSK-3beta activity.

Growth-compromised HSV-2 vector Delta RR protects from N-methyl-D-aspartate-induced neuronal degeneration through redundant activation of the MEK/ERK and PI3-K/Akt survival pathways, either one of which overrides apoptotic cascades

We have previously shown that intrastriatal injection of Delta RR, the growth-compromised herpes simplex virus type 2 (HSV-2) vector for the antiapoptotic protein ICP10PK, prevents apoptosis caused by the excitotoxin N-methyl-D-aspartate (NMDA) in a mouse model of glutamatergic neuronal cell death (Golembewski et al. [2007] Exp. Neurol. 203:381-393). Because apoptosis regulation is stimulus and cell type specific, our studies were designed to examine the mechanism of Delta RR-mediated neuroprotection in striatal neurons. Organotypic striatal cultures (OSC) that retain much of the synaptic circuitry of the intact striatum were infected with Delta RR or a growth-compromised HSV-2 vector that lacks ICP10PK (Delta PK) and examined for neuroprotection-associated signaling. The mutated ICP10 proteins (p175 and p95) were expressed in 70-80% of neurons from Delta RR- and Delta PK-infected cultures, respectively, as determined by double-immunofluorescent staining with antibodies to ICP10 and NeuN or GAD65. Delta RR- but not Delta PK-treated OSC were protected from NMDA-induced apoptosis, as verified by ethidium homodimer staining, TUNEL, caspase-3 activation, and poly(AD-ribose) polymerase (PARP) cleavage. Neuroprotection was through ICP10PK-mediated activation of the survival pathways MEK/ERK and PI3-K/Akt, up-regulation of the antiapoptotic proteins Bag-1 and Bcl-2, and phosphorylation (inactivation) of the proapoptotic protein Bad. It was blocked by the MEK inhibitor U0126 or the PI3-K inhibitor LY294002, suggesting that either pathway can prevent NMDA-induced apoptosis. The data indicate that Delta RR-delivered ICP10PK stimulates redundant survival pathways that override proapoptotic cascades. Delta RR is a promising gene therapy platform against glutamatergic cell death.

Glutamate-induced c-Jun expression in neuronal PC12 cells: the effects of ketamine and propofol

Transcription factor c-Jun affects neuronal cell death and survival in mammalian brain. As general anesthetics, such as ketamine and propofol, are thought to provide some degree of neuroprotection, this study was intended to test whether the protection of injured neuronal PC12 cells by ketamine and propofol is related to the inhibition of phospho-c-Jun. Using neuronal PC12 cells from rat pheochromocytoma cells differentiated with nerve growth factor, we found that 24 hours of exposure to glutamate (1 to 100 mM) induced concentration-dependent cell death as determined by an ability to reduce the tetrazolium derivative, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) into a blue formazan salt. Neuronal PC12 cells were exposed to ketamine (0.1, 1.0 mM) or propofol (0.5, 5.0 microM) and glutamate (0, 20 mM) for 24 hours. Cell injury was assessed using MTT, in situ terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling, and c-Jun activity assay. Glutamate, 20 mM, induced about 70% of cell death as determined by MTT and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling staining. Glutamate-induced cell death was related to an increase in expression of phospho-c-Jun. Glutamate-induced cell death was reduced by ketamine (0.1, 1.0 mM) in a dose-dependent manner and also by propofol (0.5, 5.0 microM). In addition, the expression of phospho-c-Jun was substantially reduced by ketamine (0.1, 1.0 mM) and propofol (0.5, 5.0 microM), respectively, as determined by Western blot assay. These results suggest that inhibition of c-Jun activity is involved in the neuroprotective effects of ketamine and propofol on glutamate-induced injury in neuronal PC12 cells.

A quantitative method to assess extrasynaptic NMDA receptor function in the protective effect of synaptic activity against neurotoxicity

Background

Extrasynaptic NMDA receptors couple to a CREB shut-off pathway and cause cell death, whereas synaptic NMDA receptors and nuclear calcium signaling promote CREB-mediated transcription and neuronal survival. The distribution of NMDA receptors (synaptic versus extrasynaptic) may be an important parameter that determines the susceptibility of neurons to toxic insults. Changes in receptor surface expression towards more extrasynaptic NMDA receptors may lead to neurodegeneration, whereas a reduction of extrasynaptic NMDA receptors may render neurons more resistant to death. A quantitative assessment of extrasynaptic NMDA receptors in individual neurons is needed in order to investigate the role of NMDA receptor distribution in neuronal survival and death.

Results

Here we refined and verified a protocol previously used to isolate the effects of extrasynaptic NMDA receptors using the NMDA receptor open channel blocker, MK-801. Using this method we investigated the possibility that the known neuroprotective shield built up in hippocampal neurons after a period of action potential bursting and stimulation of synaptic NMDA receptors is due to signal-induced trafficking of extrasynaptic NMDA receptors or a reduction in extrasynaptic NMDA receptor function. We found that extrasynaptic NMDA receptor-mediated calcium responses and whole cell currents recorded under voltage clamp were surprisingly invariable and did not change even after prolonged (16 to 24 hours) periods of bursting and synaptic NMDA receptor activation. Averaging a large number of calcium imaging traces yielded a small (6%) reduction of extrasynaptic NMDA receptor-mediated responses in hippocampal neurons that were pretreated with prolonged bursting.

Conclusion

The slight reduction in extrasynaptic NMDA receptor function following action potential bursting and synaptic NMDA receptor stimulation could contribute to but is unlikely to fully account for activity-dependent neuroprotection. Other factors, in particular calcium signaling to the nucleus and the induction of survival promoting genes are more likely to mediate acquired neuroprotection.

Hormesis: a promising strategy to sustain endogenous neuronal survival pathways against neurodegenerative disorders

The brain developed adaptive mechanisms in the face of changing environments and stresses imposed on the nervous system. The addition of glutamate as the major excitatory amino acid neurotransmitter to the brain's complement of amino acids and peptides dictated a coordinated transcriptional and translational program to meet the demands of excitatory neurotransmission. One such program is the ability of neurons to sustain and maintain their survival given the nature of glutamate-mediated receptor activation. The unique development of endogenous neuronal pathways activated by glutamate receptors transformed neurons and allowed them to survive under conditions of high energy demands. These same endogenous survival pathways also mediate plastic responses to meet another demand of the brain, adaptation. An endogenous protein that plays a central role in glutamate receptor-mediated survival pathways is brain-derived neurotrophic factor (BDNF). Intermittent but frequent synaptic ionotropic glutamate receptor activation ensures neuronal survival through a BDNF autocrine loop. In sharp contrast, overactivation of ionotropic glutamate receptors leads to neuronal cell death. Thus, innovative strategies that induce endogenous neuronal survival pathways through low-level activation of ionotropic glutamate receptors or those that bypass receptor activation but upregulate endogenous survival pathways may not only prevent neurodegenerative disorders that involve glutamate as a final common pathway that kills neurons, but may also provide treatment alternatives critical for neurons to survive stressful conditions such as stroke, status epilepticus and hypoglycemic-induced neuronal cell death.

Superoxide dismutase/catalase mimetics but not MAP kinase inhibitors are neuroprotective against oxygen/glucose deprivation-induced neuronal death in hippocampus

Although oxygen/glucose deprivation (OGD) has been widely used as a model of ischemic brain damage, the mechanisms underlying acute neuronal death in this model are not yet well understood. We used OGD in acute hippocampal slices to investigate the roles of reactive oxygen species and of the mitogen-activated protein kinases (MAPKs) in neuronal death. In particular, we tested the neuroprotective effects of two synthetic superoxide dismutase/catalase mimetics, EUK-189 and EUK-207. Acute hippocampal slices prepared from 2-month-old or postnatal day 10 rats were exposed to oxygen and glucose deprivation for 2 h followed by 2.5 h reoxygenation. Lactate dehydrogenase (LDH) release in the medium and propidium iodide (PI) uptake were used to evaluate cell viability. EUK-189 or EUK-207 applied during the OGD and reoxygenation periods decreased LDH release and PI uptake in slices from 2-month-old rats. EUK-189 or EUK-207 also partly blocked OGD-induced ATP depletion and extracellular signal-regulated kinases 1 and 2 (ERK1/2) dephosphorylation, and completely eliminated reactive oxygen species generation. The MEK inhibitor U0126 applied together with EUK-189 or EUK-207 completely blocked ERK1/2 activation, but had no effect on their protective effects against OGD-induced LDH release. U0126 alone had no effect on OGD-induced LDH release. EUK-207 had no effect on OGD-induced p38 or c-Jun N-terminal kinase dephosphorylation, and when the p38 inhibitor SB203580 was applied together with EUK-207, it had no effect on the protective effects of EUK-207. SB203580 alone had no effect on OGD-induced LDH release either. In slices from p10 rats, OGD also induced high-LDH release that was partly reversed by EUK-207; however, neither OGD nor EUK-207 produced significant changes in ERK1/2 and p38 phosphorylation. OGD-induced spectrin degradation was not modified by EUK-189 or EUK-207 in slices from p10 or 2-month-old rats, suggesting that their protective effects was not mediated through inhibition of calpain activation. Thus, both EUK-189 and EUK-207 provide neuroprotection in acute ischemic conditions, and this effect is related to elimination of free radical formation and partial reversal of ATP depletion, but not mediated by the activation or inhibition of the MEK/ERK or p38 pathways, or inhibition of calpain activation.

Role of kinase suppressor of Ras-1 in neuronal survival signaling by extracellular signal-regulated kinase 1/2

Scaffolding proteins including kinase suppressor of Ras-1 (KSR1) determine specificity of signaling by extracellular signal-regulated kinase 1/2 (ERK1/2), enabling it to couple diverse extracellular stimuli to various cellular responses. The scaffolding protein(s) that contributes to ERK1/2-mediated neuronal survival has not yet been identified. In cultured rat cortical neurons, BDNF activates ERK1/2 to enhance neuronal survival by suppressing DNA damage- or trophic deprivation-induced apoptosis. Here we report that in this system, BDNF increased KSR1 association with activated ERK1/2, whereas KSR1 knockdown with a short hairpin (sh) RNA reduced BDNF-mediated activation of ERK1/2 and protection against a DNA-damaging drug, camptothecin (CPT). In contrast, BDNF suppression of trophic deprivation-induced apoptosis was unaffected by shKSR1 although blocked by shERK1/2. Also, overexpression of KSR1 enhanced BDNF protection against CPT. Therefore, KSR1 is specifically involved in antigenotoxic activation of ERK1/2 by BDNF. To test whether KSR1 contributes to ERK1/2 activation by other neuroprotective stimuli, we used a cAMP-elevating drug, forskolin. In cortical neurons, ERK1/2 activation by forskolin was protein kinase A (PKA) dependent but TrkB (receptor tyrosine kinase B) independent and was accompanied by the increased association between KSR1 and active ERK1/2. Forskolin suppressed CPT-induced apoptosis in a KSR1 and ERK1/2-dependent manner. Inhibition of PKA abolished forskolin protection, whereas selective PKA activation resulted in an ERK1/2- and KSR1-mediated decrease in apoptosis. Hence, KSR1 is critical for the antiapoptotic activation of ERK1/2 by BDNF or cAMP/PKA signaling. In addition, these novel data indicate that stimulation of cAMP signaling is a candidate neuroprotective strategy to intervene against neurotoxicity of DNA-damaging agents.

Protective up-regulation of CK2 by mutant huntingtin in cells co-expressing NMDA receptors

Huntington's disease is caused by a polyglutamine expansion in the huntingtin (htt) protein, and previous data indicate that over-activation of NMDA receptors (NMDARs) may be involved in the selective degeneration of cells expressing NR1/NR2B NMDARs. We used Kinetworkstrade mark multi-immunoblotting screens to examine expression of 76 protein kinases, 18 protein phosphatases, 25 heat shock/stress proteins, and 27 apoptosis proteins in human embryonic kidney 293 cells transfected with NR1/NR2B and htt containing 15 (htt-15Q; wild-type) or 138 (htt-138Q; mutant) glutamine repeats. Follow-up experiments revealed several proteins involved in the heat-shock response pathway to be up-regulated in the soluble fraction from cells expressing htt-138Q, including protein phosphatase 5 and cyclin-dependent kinase 5. Increased expression in the soluble fraction of htt-138Q-expressing cells was also noted for the stress- and calcium-activated protein-serine/threonine kinase casein kinase 2, a change which was confirmed in striatal tissue of yeast artificial chromosome transgenic mice expressing full-length mutant htt. Inhibition of casein kinase 2 activity in cultured striatal neurons from these mice significantly exacerbated NMDAR-mediated toxicity, as assessed by labeling of apoptotic nuclei. Our findings are consistent with up-regulation of components of the stress response pathway in the presence of polyglutamine-expanded htt and NR1/NR2B which may reflect an attempt at the cellular level to ameliorate the detrimental effects of mutant htt expression.

Subtoxic N-methyl-D-aspartate delayed neuronal death in ischemic brain injury through TrkB receptor- and calmodulin-mediated PI-3K/Akt pathway activation

Previous studies have shown that subtoxic NMDA moderated the neuronal survival in vitro and vivo. We performed this experiment to clarify the precise mechanism underlie subtoxic NMDA delayed neuronal death in ischemic brain injury. We found that pretreatment of NMDA (100 mg/kg) increased the number of the surviving CA1 pyramidal cells of hippocampus at 5 days of reperfusion. This dose of NMDA could also enhance Akt activation after ischemia/reperfusion (I/R). Here, we examined the possible mechanism that NMDA induced Akt activation. On the one hand, we found NMDA receptor-mediated Akt activation was associated with increased expression of BDNF (brain-derived neurotrophic factor) and activation of its high-affinity receptor TrkB after I/R in the hippocampus CA1 region, which could be held down by TrkB receptor antagonist K252a. On the other hand, we found that NMDA enhanced the binding of Ca2+-dependent calmodulin (CaM) to p85 (the regulation subunit of PI-3K), which led to the activation of Akt. W-13, an active CaM inhibitor, prevented the combination of CaM and p85 and subsequent Akt activation. Furthermore, NMDA receptor-mediated Akt activation was reversed by combined treatment with LY294002, the specific blockade of PI-3K. Taken together, our results suggested that subtoxic NMDA exerts the neuroprotective effect via activation of prosurvival PI-3K/Akt pathway against ischemic brain injury, and BDNF-TrkB signaling and Ca2+-dependent CaM cascade might contribute to NMDA induced activation of PI-3K/Akt pathway.

Depolarization-induced, glutamate receptor–mediated, and transactivation-dependent extracellular-signal regulated kinase phosphorylation in cultured cerebellar granule neurons

Depolarization of 7-8-day-old mouse cerebellar granule neurons in primary cultures, a glutamatergic preparation, by elevation of the extracellular potassium ion concentration ([K+]e) to 45 mM induces an increase of phosphorylation of extracellular-signal regulated kinase 1 and 2 (ERK1/2) at two time periods: 20 min and 60 min after the [K+]e increase. This effect can be mimicked by 5 min of exposure to 50 microM glutamate, suggesting that ERK1/2 phosphorylation in response to the depolarization is brought about by the resulting glutamate release. This concept is supported by the observation that the K+ -mediated stimulation of phosphorylation at both times is inhibited by MK-801, an NMDA antagonist, and by CNQX, an AMPA/kainate antagonist. These antagonists also inhibit the response to glutamate. Both increases in ERK1/2 phosphorylation are also inhibited by GM 6001 (a metalloproteinase inhibitor, preventing 'shedding' of growth factors), by AG 1478 (a receptor tyrosine kinase inhibitor, preventing epidermal growth factor [EGF] receptor activation), and also partly by heparin (inactivating heparin-binding epidermal growth factor [HB-EGF]), suggesting transactivation of epidermal growth factor receptors (EGFR). Transactivation is an intracellular/extracellular signal transduction pathway in which release from receptor- or depolarization-stimulated cells of EGFR ligand(s) (including HB-EGF), catalyzed by a metalloproteinase, stimulates receptor tyrosine kinases on the same (an autocrine effect) or adjacent (a paracrine effect) cells. The expression of HB-EGF as well as of transforming growth factor-alpha (TGF-alpha), two of the EGFR ligands, in the cells was confirmed by reverse transcription polymerase chain reaction, and the only partial inhibition by heparin suggests that both of these EGFR agonists are involved. Such a transactivation may play a major role in glutamate-mediated signaling and plasticity.

Role of brain-derived neurotrophic factor in the protective action of N-methyl-D-aspartate in the apoptotic death of cerebellar granule neurons induced by low potassium

Several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), and neurotransmitters, such as glutamate, may influence neuronal apoptotic death. Rat cerebellar granule neurons (CGN) cultured in low potassium (5 or 10 mM KCl) for more than 5 days in vitro (DIV) die apoptotically. These cells survive in the presence of high potassium (25 mM KCl, K25) or N-methyl-D-aspartate (NMDA), an agonist of glutamatergic receptors. CGN transferred from high to low potassium die apoptotically. Here, we characterized the effect of BDNF and NMDA on the apoptotic death induced by low potassium in CGN. Cell death of CGN by culturing in low potassium for 6 DIV was inhibited by BDNF and NMDA. When CGN were cultured in K25 and transferred to a low-potassium medium, 65% of neurons died after 48 hr. Under these conditions, BDNF, NMDA, or BDNF + NMDA increased CGN survival. Both BDNF and NMDA decreased caspase-9 activity and mRNA caspase-3 levels and activity induced by low potassium. CGN survival induced by BDNF is mediated by TrkB activation, whereas that induced by NMDA is mediated by NMDA receptor and TrkB activation. NMDA, but not BDNF, raised [Ca(2+)](i), which was reduced by low-potassium treatment. These results suggest that NMDA receptor stimulation induces CGN survival through the influx of extracellular Ca(2+) that may evoke the release of BDNF and the activation of TrkB. Complementary mechanisms induced by depolarization and changes in Ca(2+) levels would also contribute to the neuroprotection exerted by NMDA and potassium.

Multiple neuroprotective mechanisms of minocycline in autoimmune CNS inflammation

Axonal destruction and neuronal loss occur early during multiple sclerosis, an autoimmune inflammatory CNS disease that frequently manifests with acute optic neuritis. Available therapies mainly target the inflammatory component of the disease but fail to prevent neurodegeneration. To investigate the effect of minocycline on the survival of retinal ganglion cells (RGCs), the neurons that form the axons of the optic nerve, we used a rat model of myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis. Optic neuritis in this model was diagnosed by recording visual evoked potentials and RGC function was monitored by measuring electroretinograms. Functional and histopathological data of RGCs and optic nerves revealed neuronal and axonal protection when minocycline treatment was started on the day of immunization. Furthermore, we demonstrate that minocycline-induced neuroprotection is related to a direct antagonism of multiple mechanisms leading to neuronal cell death such as the induction of anti-apoptotic intracellular signalling pathways and a decrease in glutamate excitotoxicity. From these observations, we conclude that minocycline exerts neuroprotective effects independent of its anti-inflammatory properties. This hypothesis was confirmed in a non-inflammatory disease model leading to degeneration of RGCs, the surgical transection of the optic nerve.

Tumor necrosis factor alpha stimulates NMDA receptor activity in mouse cortical neurons resulting in ERK-dependent death

Multiple cytokines are secreted in the brain during pro-inflammatory conditions and likely affect neuron survival. Previously, we demonstrated that glutamate and tumor necrosis factor alpha (TNFalpha) kill neurons via activation of the N-methyl-d-aspartate (NMDA) and TNFalpha receptors, respectively. This report continues characterizing the signaling cross-talk pathway initiated during this inflammation-related mechanism of death. Stimulation of mouse cortical neuron cultures with TNFalpha results in a transient increase in NMDA receptor-dependent calcium influx that is additive with NMDA stimulation and inhibited by pre-treatment with the NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid, or the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione. Pre-treatment with N-type calcium channel antagonist, omega-conotoxin, or the voltage-gated sodium channel antagonist, tetrodotoxin, also prevents the TNFalpha-stimulated calcium influx. Combined TNFalpha and NMDA stimulation results in a transient increase in activity of extracellular signal-regulated kinases (ERKs) and c-Jun N-terminal kinases (JNKs). Specific inhibition of ERKs but not JNKs is protective against TNFalpha and NMDA-dependent death. Death is mediated via the low-affinity TNFalpha receptor, TNFRII, as agonist antibodies for TNFRII but not TNFRI stimulate NMDA receptor-dependent calcium influx and death. These data demonstrate how microglial pro-inflammatory secretions including TNFalpha can acutely facilitate glutamate-dependent neuron death.

Glutamate reduces secretion of l-serine in astrocytes isolated from stroke-prone spontaneously hypertensive rats

In the CNS, l-serine (l-Ser) plays an essential role in neuronal survival by evoking a variety of biological responses in glial cells. Initially, we examined whether glutamate, hydrogen peroxide (H(2)O(2)), interleukin-1 (IL-1) beta, and sodium nitroprusside (SNP) induce the secretion of l-Ser in astrocytes isolated from Wistar Kyoto rats (WKY). The secretion of l-Ser was significantly induced with glutamate and SNP in cultured astrocytes. Next, to gain insight into the involvement of l-Ser in glutamate-induced neuroprotection, we compared the secretion of l-Ser in astrocytes isolated from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive rats, WKY. We also examined the mRNA expression of the enzyme that produces l-Ser, 3-phosphoglycerate dehydrogenase (PHGDH), and a neural amino acid transporter, ASCT1, in the cultured astrocytes. A dose-dependent study of glutamate in astrocytes of SHRSP indicated differences in the secretion of l-Ser, and gene expression of PHGDH and ASCT1, compared with levels in the WKY astrocytes. We demonstrated that both the secretion and the gene expression were significantly attenuated in glutamate-treated astrocytes from SHRSP. Cerebral ischemia in SHRSP induced a massive efflux of glutamate, causing delayed neuronal death in region CA1 of the hippocampus. The results suggest that the attenuated secretion of l-Ser in astrocytes is involved in neuronal vulnerability and survival in SHRSP during the production of glutamate, as the secretion of l-Ser, which is stimulated by glutamate, is closely related to the protective effect against glutamate-mediated neurotoxicity. We conclude that glutamate and SNP up-regulate the secretion of l-Ser in primary astrocytes. Secretion of l-Ser is regulated in astrocytes in response to glutamate and nitric oxide and may correspond to the level of l-Ser needed for neuronal survival during brain insults such as ischemic stroke in SHRSP.

NMDA receptors promote survival in somatosensory relay nuclei by inhibiting Bax-dependent developmental cell death

Naturally occurring cell death is a universal feature of developing nervous systems that plays an essential role in determining adult brain function. Yet little is known about the decisions that select a subset of CNS neurons for survival and cause others to die. We report that postnatal day 0 NMDA receptor subunit 1 (NMDAR1) knockout mice display an approximately 2-fold increase in cell death in the brainstem trigeminal complex (BSTC), including all four nuclei that receive somatosensory inputs from the face (principalis, oralis, interpolaris, and caudalis). Treatment with the NMDA receptor antagonist dizocilpine maleate (MK-801) for 24 h before birth also caused an increase in cell death that reached statistical significance in two of the four nuclei (oralis and interpolaris). The neonatal sensitivity to NMDA receptor hypofunction in the BSTC, and in its main thalamic target, the ventrobasal nucleus (VB), coincides with the peak of naturally occurring cell death and trigeminothalamic synaptogenesis. At embryonic day 17.5, before the onset of these events, NMDAR1 knockout does not affect cell survival in either the BSTC or the VB. Immunostaining for active caspase-3 and the neuronal marker Hu specifically confirms the presence of dying neurons in the BSTC and the VB of NMDAR1 knockout neonates. Finally, genetic deletion of Bax rescues these structures from the requirement for NMDA receptors to limit naturally occurring cell death. Taken together, the results indicate that NMDA receptors play a survival role for somatosensory relay neurons during synaptogenesis by inhibiting Bax-dependent developmental cell death.

Insulin-like growth factor-I enhances the biological activity of brain-derived neurotrophic factor on cerebrocortical neurons

Insulin-like growth factor (IGF)-I and brain-derived neurotrophic factor (BDNF) act within the brain to enhance neuronal survival and plasticity. We extend these findings by showing that the presence of both neurotrophins is required to depress the rise in intracellular Ca2+ caused by glutamate in primary cultures of cerebrocortical neurons. IGF-I enhanced expression of BDNF receptors (Trk-B) and increased the ability of BDNF to induce ERK1/2 phosphorylation. This IGF-I-induced increase in BDNF responsiveness describes a new interaction between these peptides in the brain.

Preconditioning and neurotrophins: a model for brain adaptation to seizures, ischemia and other stressful stimuli

The amino acid glutamate, the major excitatory neurotransmitter in the central nervous system, activates receptors coupled to calcium influx. Excessive activation of glutamate receptors in conditions such as severe epileptic seizures or stroke can kill neurons in a process called excitotoxicity. However, subtoxic levels of activation of the N-methyl-D-aspartate (NMDA) type of glutamate receptor elicit adaptive responses in neurons that enhance their ability to withstand more severe stress. A variety of stimuli induce adaptive responses to protect neurons. For example, sublethal ischemic episodes or a mild epileptic insult can protect neurons in a process referred to as tolerance. The molecular mechanisms that protect neurons by these different stressful stimuli are largely unknown but they share common features such as the transcription factor, nuclear factor kappa B (NF-kappaB), which is activated by ischemic and epileptic preconditioning as well as exposure to subtoxic NMDA concentrations. In this article, we describe stress-induced neuroprotective mechanisms highlighting the role of brain-derived neurotrophic factor (BDNF), a protein that plays a crucial role in neuronal survival and maintenance, neurogenesis and learning and memory.