Responses of CDKs and p53 in Delayed Ischemic Neuronal Death

Stroke is a debilitating disease that affects millions each year.While in many cases cerebral ischemic in jury can be limited by effectivw resuscitation or thrombolytic treatment,the injured neurons wither in a process known as delayed neuronal death(DND).Mounting evidence indicates that DND is not simply necrosis played out in slow motion but apoptosis is triggered.Of particular interest are two groups of signal proteins that participate in apoptosis-cyclin dependent kinases(CDKs) and p53-among a myriad of signaling events after an ischemic insult.Recent investigations have shown that CDKs,a family of enzymes initially known for their role in cell cycle regulation,are activated in injured neurons in DND.As for p53,new reports suggest that its up-regulation may represent a failed attempt to rescue in jured neurons,although its up-regulation was previously considered an indication of apoptosis.These observations thus rekindle an old quest to identify new neuroprotective targets to minimize the stroke damage.In this review,the author will examine the evidence that indicates the participation of CDKs and p53 in DND and then introduce pre-clinical data to explore CDK inhibition as a potential neuroprotective target.Finally,using CDK inhibition as an example,this paper will discuss the pertinent criteria for a viable neuroprotective strategy for ischemic in jury.     

Effects of ciliary neurotrophic factor on excitotoxicity and calcium-ionophore A23187-induced cell death in cultured embryonic striatal neurons

Ciliary neurotrophic factor (CNTF) has a protective effect on the striatum in animal models of Huntington's disease. However, the mechanism through which it exerts its effect is not clear. In this study, we show that there is a concentration-dependent direct protective effect of CNTF against N-methyl-D-aspartate-mediated excitotoxicity on striatal neurons in vitro. The CNTF has to be added more than half an hour before the insult for the effect to occur and its effect is eliminated by the presence of the protein synthesis inhibitor cycloheximide. This suggests that the protective mechanism of CNTF does not involve acute interference with the glutamate receptors, but probably requires gene/protein expression. We have also shown that the effect of CNTF against glutamate-induced excitotoxicity is dependent on the concentration of glutamate with a protective effect more evident at a low grade excitotoxic insult. Finally, we saw no effect of CNTF on calcium ionophore A23187-induced toxicity in striatal cultures, indicating that the growth factor does not promote survival by enhancing general defenses against raised intracellular levels of calcium.     

Reduced platelet glutamate uptake in Parkinson's disease

Summary. Defects in mitochondrial enzymes have been found not only in substantia nigra, but also in platelets from Parkinson's Disease (PD) patients, suggesting a systemic impairment of energy metabolism. Since platelets present an energy-dependent glutamate uptake similar to that described in central nervous system, glutamate uptake was determined in platelets from 34 PD patients and 21 age-related normal controls, as Na⁺-dependent [³H]glutamate influx; glutamate level was also analyzed by reverse-phase HPLC. A 50% reduction of glutamate uptake (p < 0.001) was observed in idiopathic PD patients, respect to controls and secondary parkinsonian syndromes. The decrease correlated with the severity of PD, measured by the UPDRS (r = −0.54; P < 0.05). Glutamate level was increased in platelets of PD patients, but was not correlated to the uptake decrease. Both phoenomena may be explained by the modifications of mitochondrial enzymes described in platelets, which could be used as a peripheral model of glutamatergic function in PD. Received October 7, 1998; accepted January 7, 1999     

Glutamate Toxicity: An Experimental and Theoretical Analysis

In slices of 8-day-old rat cerebellum, the lowest concentration of glutamate that induced toxicity (30 min exposure; 90 min recovery) was 100 microM, but the damage only occurred in the outermost regions. As the concentration was raised, the band of necrosis became progressively deeper until, at 3 mM, it was uniform across the slice thickness. At a test concentration of 300 microM, the width of the necrotic band did not change when either the exposure time or the recovery period was varied between 30 min and 3 h. These results are predicted by a theoretical model in which the diffusion of glutamate into brain tissue is countered by cellular uptake of the amino acid, and they argue against the idea that glutamate toxicity is inherently self-propagating. When slices were examined immediately after exposure (300 microM), a prominent swelling of glial cells was present at the slice surface. Swelling per se did not appear to compromise their uptake function, and the model predicts that cellular swelling, by reducing the rate of diffusion of glutamate, protects against glutamate toxicity. The damage produced by 3 mM glutamate, which was primarily exerted against granule cells, was prevented by N-methyl-d-aspartate (NMDA) receptor blockade, whereas antagonists acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors were ineffective. Under conditions of energy deprivation, the neurotoxic potency of glutamate was markedly enhanced and a normally non-toxic concentration (30 microM) became maximally toxic towards granule cells. Dark vacuolar degeneration of Purkinje cells was also present, and this could be inhibited by blocking AMPA receptors. The results and theoretical analysis suggest that intact brain tissue is remarkably resistant to glutamate toxicity, chiefly because of the formidable properties of the uptake system. However, under special circumstances, glutamate can become a potent neurotoxin and its toxicity can then involve both NMDA and AMPA receptors.     

Basic fibroblast growth factor protects auditory neurons and hair cells from noise exposure and glutamate neurotoxicity

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Neurosci,2000, 16; 6125 - 6134. ［31］Ornitz, D.M.,Xu, J., Colvin, J.S., McEwen, D.G., MacArthur,C. A., Coulier, F., Gao, G., Goldfarb, M. Receptor specificity of the fibroblast growth factor family. J. Biol. Chem, 1996, 271: 15292 -15297.     

神经营养因子对缺血性脑损伤的影响及其药物调节

营养因子是一类对神经系统起营养支持作用的蛋白质 ,当神经细胞受损伤时 ,可引起不同种类的神经营养因子释放 ,从而参与脑损伤的内源性保护过程。外源性施用神经营养因子可保护神经组织免受损害的实验结果也支持上述观点。因此 ,利用药物等手段诱导它们适时适量地释放 ,可能是治疗缺血性脑损伤的新靶向     

兴奋性氨基酸与青光眼

近年来随着对青光眼性视神经病变机制的深入研究,已证实兴奋性氨基酸与青光眼视网膜神经节细胞凋亡密切相关。21世纪青光眼的治疗模式正从传统的单纯降眼压向降眼压的同时从其他途径进一步加强神经保护转变,基础实验及部分临床研究证实干扰兴奋性氨基酸产生神经毒性的各环节均对神经元有保护作用,由此可以推测通过对兴奋性氨基酸的研究有望实现青光眼治疗史上的重大突破。     

Stimulation of synaptoneurosome glutamate release by monomeric and fibrillated α‐synuclein

The α‐synuclein protein exists in vivo in a variety of covalently modified and aggregated forms associated with Parkinson's disease (PD) pathology. However, the specific proteoform structures involved with neuropathological disease mechanisms are not clearly defined. Since α‐synuclein plays a role in presynaptic neurotransmitter release, an in vitro enzyme‐based assay was developed to measure glutamate release from mouse forebrain synaptoneurosomes (SNs) enriched in synaptic endings. Glutamate measurements utilizing SNs from various mouse genotypes (WT, over‐expressers, knock‐outs) suggested a concentration dependence of α‐synuclein on calcium/depolarization‐dependent presynaptic glutamate release from forebrain terminals. In vitro reconstitution experiments with recombinant human α‐synuclein proteoforms including monomers and aggregated forms (fibrils, oligomers) produced further evidence of this functional impact. Notably, brief exogenous applications of fibrillated forms of α‐synuclein enhanced SN glutamate release but monomeric forms did not, suggesting preferential membrane penetration and toxicity by the aggregated forms. However, when applied to brain tissue sections just prior to homogenization, both monomeric and fibrillated forms stimulated glutamate release. Immuno‐gold and transmission electron microscopy (TEM) detected exogenous fibrillated α‐synuclein associated with numerous SN membranous structures including synaptic terminals. Western blots and immuno‐gold TEM were consistent with SN internalization of α‐synuclein. Additional studies revealed no evidence of gross disruption of SN membrane integrity or glutamate transporter function by exogenous α‐synuclein. Overall excitotoxicity, due to enhanced glutamate release in the face of either overexpressed monomeric α‐synuclein or extrasynaptic exposure to fibrillated α‐synuclein, should be considered as a potential neuropathological pathway during the progression of PD and other synucleinopathies. © 2017 Wiley Periodicals, Inc.     

Therapeutic developments in the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease characterised by the selective death of motor neurons. The mechanisms and processes responsible for the selective loss of motor neurons are still unknown, however several hypotheses have been put forward, including oxidative damage and/or toxicity from intracellular aggregates due to mutant superoxide dismutase-1 activity, axonal strangulation from cytoskeletal abnormalities, loss of trophic factor support and glutamate-mediated excitotoxicity. These theories are based on a better understanding of the genetics of amyotrophic lateral sclerosis and on biochemical and pathological analysis of post-mortem tissue. They have led to the development of appropriate animal and cell culture models, allowing the sequence of events in motor neuronal degeneration to be unravelled and potential therapeutic agents to be screened. Unfortunately, the majority of therapeutics found to be efficacious in the animal and cell culture models have failed in human trials. Riluzole is still the only proven therapy in humans, shown to extend survival of amyotrophic lateral sclerosis patients by ～ 3 months, but it has no effect on muscle strength. Other potential therapeutic approaches are being identified, including inhibition of caspase-mediated cell death, maintenance of mitochondrial integrity and energy production, regulation of glutamate homeostasis, reduction of inflammation and control of neurofilament synthesis. Hopefully, in the near future some new agents will be found that can alter the course of this devastating and fatal disease.     

Presenilin mediates neuroprotective functions of ephrinB and brain-derived neurotrophic factor and regulates ligand-induced internalization and metabolism of EphB2 and TrkB receptors

Activation of EphB receptors by ephrinB (efnB) ligands on neuronal cell surface regulates important functions, including neurite outgrowth, axonal guidance, and synaptic plasticity. Here, we show that efnB rescues primary cortical neuronal cultures from necrotic cell death induced by glutamate excitotoxicity and that this function depends on EphB receptors. Importantly, the neuroprotective function of the efnB/EphB system depends on presenilin 1 (PS1), a protein that plays crucial roles in Alzheimer's disease (AD) neurodegeneration. Furthermore, absence of one PS1 allele results in significantly decreased neuroprotection, indicating that both PS1 alleles are necessary for full expression of the neuroprotective activity of the efnB/EphB system. We also show that the ability of brain-derived neurotrophic factor (BDNF) to protect neuronal cultures from glutamate-induced cell death depends on PS1. Neuroprotective functions of both efnB and BDNF, however, were independent of γ-secretase activity. Absence of PS1 decreases cell surface expression of neuronal TrkB and EphB2 without affecting total cellular levels of the receptors. Furthermore, PS1-knockout neurons show defective ligand-dependent internalization and decreased ligand-induced degradation of TrkB and Eph receptors. Our data show that PS1 mediates the neuroprotective activities of efnB and BDNF against excitotoxicity and regulates surface expression and ligand-induced metabolism of their cognate receptors. Together, our observations indicate that PS1 promotes neuronal survival by regulating neuroprotective functions of ligand-receptor systems.