Neuronal Cell Death In Response Research Articles

The JNK Pathway and Neuronal Migration

The c-Jun N-terminal kinases (JNKs) are important regulators of a variety of physiological and pathological processes both in the central and in the peripheral nervous systems. JNKs are considered as crucial mediators of neuronal cell death in response to stress and injury. However, recent studies have provided substantial evidence that the JNK pathway plays an important role in neuronal migration. Here, we will give a brief introduction of the JNK signaling pathway and put more emphasis on its role in neuronal migration.

Glypican-1 as an Abeta binding HSPG in the human brain: its localization in DIG domains and possible roles in the pathogenesis of Alzheimer's disease.

Previous studies have suggested that heparan sulfate proteoglycans (HSPGs) play a role in deposition of beta-amyloid protein (Abeta) in the Alzheimer's disease (AD) brain. In the present study, we demonstrated that glypican-1 can bind fibrillar Abeta, and the binding is mainly mediated by heparan sulfate (HS) chains. Further analysis revealed that glypican-1 is the major HSPG localized in detergent-insoluble glycosphingolipid-enriched (DIG) domains where all machineries for Abeta production exist and Abeta is accumulated as monomeric and oligomeric forms. Immunohistochemical studies demonstrated that glypican-1 is localized in primitive plaques as well as classic plaques. Moreover, overexpression of glypican-1 and amyloid precursor protein in SH-SY5Y cells resulted in reduced cell viability and made cells more susceptible to thapsigargin-induced stress and Abeta toxicity. The results raise the possibility that glypican-1 interacts with oligomerized or polymerized Abeta in such a specific compartment as DIG, resulting not only in amyloid deposition in senile plaques of AD brain, but also in accelerating neuronal cell death in response to stress and Abeta.

HSP27 but not HSP70 has a potent protective effect against alpha-synuclein-induced cell death in mammalian neuronal cells.

alpha-Synuclein is a neuronally expressed protein which is mutated in familial Parkinson's disease. We have previously shown that disease-associated mutants of alpha-synuclein cause enhanced neuronal cell death in response to a variety of stimuli, whereas wild-type alpha-synuclein has a protective effect against some stimuli, whilst enhancing the death response to others. We demonstrate, for the first time, that over-expression of the heat shock protein HSP27 has a potent protective anti-apoptotic effect against the damaging effects of wild-type and particularly of mutant alpha-synuclein. In contrast, HSP70 has some protective effect against the damaging effect of the wild-type protein, but has no effect against the mutant proteins, whilst HSP56 has no protective effect in this system. Our results indicate that disease-associated mutants of alpha-synuclein enhance its death-inducing properties and lead to increased apoptosis, which can be mitigated by either the use of specific caspase inhibitors or HSP27 over-expression. This potent protective effect of HSP27 against the mutant and wild-type proteins may be of potential therapeutic importance.

Sp1 and Sp3 Are Oxidative Stress-Inducible, Antideath Transcription Factors in Cortical Neurons

Neuronal cell death in response to oxidative stress may reflect the failure of endogenous adaptive mechanisms. However, the transcriptional activators induced by oxidative stress in neurons that trigger adaptive genetic responses have yet to be fully elucidated. We report that basal DNA binding of the zinc finger transcription factors Sp1 and Sp3 is unexpectedly low in cortical neurons in vitro and is significantly induced by glutathione depletion-induced or hydrogen peroxide-induced oxidative stress in these cells. The increases in Sp1/Sp3 DNA binding reflect, in part, increased levels of Sp1 and Sp3 protein in the nuclei of cortical neurons. Similar induction of Sp1 and Sp3 protein is also observed in neurons in vivo in a chemical or a genetic model of Huntington's disease, two rodent models in which neuronal loss has been attributed to oxidative stress. Sustained high-level expression of full-length Sp1 or full-length Sp3, but not the Sp1 zinc finger DNA-binding domain alone, prevents death in response to oxidative stress, DNA damage, or both. Taken together, these results establish Sp1 and Sp3 as oxidative stress-induced transcription factors in cortical neurons that positively regulate neuronal survival.

Fas and Fas Ligand are associated with neuritic degeneration in the AD brain and participate in β-amyloid-induced neuronal death

It has recently been suggested that neuronal cell death in response to many brain insults may be mediated by the upregulation of tumor necrosis factor receptor (TNFR) family members and their ligands. In the present study, we investigated whether the expression of the TNFR family death domain receptor, Fas, and its ligand, FasL, is altered in association with neuropathology and activated caspase markers in Alzheimer disease (AD) brain, and Aβ-induced neuronal cell death in vitro. To evaluate this hypothesis, we examined Fas and FasL expression in AD and control brain, and Aβ-treated primary neurons, using immunocytochemistry and Western blots. Neurons in both AD brain and Aβ-treated cultures exhibited FasL upregulation and changes in immunoreactivity for Fas receptor. Further, FasL expression was remarkably elevated in senile plaques and neurofilament-positive dystrophic neurites, and in association with caspase activation and neuritic apoptosis in AD brain. Based on these and previous data regarding protection of primary neuronal cultures from Aβ 1–42-induced apoptosis by blockade of Fas-associated death domain signaling, we also tested the hypothesis that dynamic regulation of Fas and FasL may contribute to Aβ-mediated neuronal cell death. Accordingly, neuronal cultures derived from mice carrying inactivating mutations in Fas (Fas lpr) or FasL (Fas gld) exhibited protection from Aβ 1–42-induced cell death. These findings suggest that Fas–FasL interactions may contribute to mechanisms of neuronal loss and neuritic degeneration in AD.

Accolade for elegans

Accolade for elegans

Overexpression of midbrain-specific transcription factor Nurr1 modifies susceptibility of mouse neural stem cells to neurotoxins

Nurr1 is a member of the nuclear receptor superfamily of transcription factors that is highly expressed in midbrain dopaminergic (DA) neurons, the cells primarily lost in human Parkinson's disease (PD), and in Nurr1-null mice selective agenesis of midbrain DA neurons is found. To investigate possible correlation between the expression of Nurr1 gene and neurotoxin-induced cell death of DA neurons, a neural stem cell line (NSC, A3) and Nurr1-overexpressing NSC (A3.Nurr1) were exposed to DA neurotoxins 6-hydroxydopamine (6-OHDA) and methyl phenylpyridinium (MPP +). Although both neurotoxins were shown to induce cell death in A3 and A3.Nurr1 cells, patterns of cell deaths were different. A3.Nurr1 cells showed increased vulnerability to 6-OHDA cytotoxicity, but increased resistance to MPP +-induced cell death when compared to A3 cells. To investigate the differential vulnerability to neurotoxins by Nurr1 protein correlates with biochemical features that discriminate between apoptosis and necrosis, we carried out a nucleosomal DNA fragmentation assay and electron microscopy. While 6-OHDA treatment induced shrinkage of cytoplasmic membrane, condensation of nuclei and generation of apoptotic bodies in both cell lines, cells treated with MPP + showed mitochondrial swelling, indicating that 6-OHDA- but not MPP +-mediated cell death was apoptotic. These results suggest that DA neuronal cell death in response to 6-OHDA and MPP + may progress through separate signaling pathways differentially regulated by the Nurr1 protein. Our observations indicated that Nurr1 may play a role in the manifestation of DA neurotoxicity and that variations in Nurr1 expression might be a susceptibility factor for DA neurodegeneration in PD.

Does tissue transglutaminase play a role in Huntington's disease?

Tissue transglutaminase (tTG) catalyzes the incorporation of polyamines into substrates, or the formation of isopeptide bonds. tTG also binds and hydrolyzes GTP/ATP. Huntington's disease (HD) is caused by a pathological expansion of the polyglutamine domain in the protein huntingtin (htt). Because a polypeptide bound Gln residue is the primary determining factor for a tTG substrate, it has been hypothesized that due to the increase in Gln content, mutant htt may modified by tTG and this event may contribute to the pathogenesis of HD, possibly by facilitating the formation of htt aggregates. tTG is increased in HD, suggesting involvement in the pathogenic process. However, tTG is not required for aggregate formation. Further, tTG is excluded from htt aggregates and increasing or decreasing tTG has no effect on the frequency or localization of the aggregates. Considering these and other data, tTG is unlikely to play a major role in the formation of htt inclusions in HD brain. tTG may play a role in modulating neuronal cell death in response to specific stressors. If a stress increases the transamidating activity of tTG (e.g. increases in Ca++ levels), then tTG facilitates the cell death process. In contrast, if a stress does not result in an increase in the transamidating activity of tTG, then tTG protects against cell death. The protective effects of tTG are independent of its transamidating and hence likely dependent on its GTP/ATP binding and hydrolytic activity. Therefore the increase in tTG levels in HD brain could either be helpful or harmful depending on the cellular mechanisms that contribute to neuronal death.Acknowledgements: Supported by NIH grant AG12396.

Caspases, Apoptosis, and Alzheimer Disease: Causation, Correlation, and Confusion

Extensive neuron loss occurs in Alzheimer disease (AD) brain and some authors have speculated that dysregulation of apoptotic death pathways is etiologically responsible for the disease. Apoptosis is regulated in mammalian cells by a family of cysteine proteases called caspases. At least 7 different caspases (caspases 1, 2, 3, 6, 8, 9, and 12) have been implicated in regulating neuronal cell death in response to amyloid beta (A beta) exposure in vitro, in animal models of neurodegenerative diseases, and in AD brain itself. Despite this seemingly impressive array of data implicating caspases and apoptosis as etiologic factors in AD, the direct involvement of caspase-dependent neuronal apoptosis in AD pathogenesis remains uncertain. Alternative explanations for some findings, contradictory experimental observations, and lack of morphologically convincing apoptotic neurons in the vast majority of AD brains has led to the revised hypothesis that apoptosis-associated molecular events cause neuronal dysfunction in the absence of, or prior to, neuronal death. Unfortunately, this new view renders the term "apoptosis-associated" functionally meaningless since it bears no relationship with apoptotic death and fails to focus scientific investigation on the molecular insults that trigger the "apoptosis-associated" response in AD neurons. On balance, an etiologic role for caspases in AD is far from proven. It remains possible, however, that caspase-dependent neuronal death contributes to AD neuron loss and thus, caspase inhibition offers some hope for extending AD neuron survival so that other agents, targeting upstream events, may delay or reverse primary AD pathology.

Novel Role for JNK as a Stress-activated Bcl2 Kinase

Interleukin (IL)-3-induced Bcl2 phosphorylation at Ser(70) may be required for its full and potent antiapoptotic activity. However, in the absence of IL-3, increased expression of Bcl2 can also prolong cell survival. To determine how Bcl2 may be functionally phosphorylated following IL-3 withdrawal, a stress-activated Bcl2 kinase (SAK) was sought. Results indicate that anisomycin, a potent activator of the stress kinase JNK/SAPK, can induce Bcl2 phosphorylation at Ser(70) and that JNK1 can be latently activated following IL-3 withdrawal to mediate Bcl2 phosphorylation. JNK1 directly phosphorylates Bcl2 in vitro, co-localizes with Bcl2, and collaborates with Bcl-2 to mediate prolonged cell survival in the absence of IL-3 or following various stress applications. Dominant-negative (DN)-JNK1 can block both anisomycin and latent IL-3 withdrawal-induced Bcl2 phosphorylation (>90%) and potently enhances cell death. Furthermore, low dose okadaic acid (OA), a potent protein phosphatase 1 and 2A inhibitor, can activate the mitogen-activated protein kinases JNK1 and ERK1/2, but not p38 kinase, to induce Bcl2 phosphorylation and prolong cell survival in factor-deprived cells. Since PD98059, a specific MEK inhibitor, can only partially inhibit OA-induced Bcl2 phosphorylation but completely blocks OA-induced Bcl2 phosphorylation in cells expressing DN-JNK1, this supports the conclusion that OA may stimulate Bcl2 phosphorylation via a mechanism involving both JNK1 and ERK1/2. Collectively, these findings indicate a novel role for JNK1 as a SAK and may explain, at least in part, how functional phosphorylation of Bc12 can occur in the absence of growth factor.

Bax Involvement in p53-Mediated Neuronal Cell Death

The tumor suppressor gene p53 has been implicated in the loss of neuronal viability, but the signaling events associated with p53-mediated cell death in cortical and hippocampal neurons are not understood. Previous work has shown that adenovirus-mediated delivery of the p53 gene causes cortical and hippocampal neuronal cell death with some features typical of apoptosis. In the present study we determined whether p53-initiated changes in neuronal viability were dependent on members of the Bcl-2 family of cell death regulators. Primary cultures of cortical neurons were derived from animals containing Bax (+/+ and +/-) or those deficient in Bax (-/-). Cell damage was assessed by direct cell counting and by measurements of MTT activity. Neurons containing at least one copy of the Bax gene were damaged severely by exposure to excitotoxins or by the induction of DNA damage. In contrast, Bax-deficient neurons (-/-) exhibited significant protection from both types of injury. Bax protein expression was elevated significantly by glutamate exposure, but not by camptothecin-induced DNA damage in wild-type neurons. The glutamate-induced increase in Bax protein was dependent on the presence of the p53 gene. However, increased p53 expression, using adenovirus-mediated transduction, was not sufficient by itself to elevate Bax protein levels. These results demonstrate that Bax is required for neuronal cell death in response to some forms of cytotoxic injury and further support the key role for p53 activation in response to excitotoxic and genotoxic injury.

Reduction of CuZn-Superoxide Dismutase Activity Exacerbates Neuronal Cell Injury and Edema Formation after Transient Focal Cerebral Ischemia

Apoptotic neuronal cell death has recently been associated with the development of infarction after cerebral ischemia. In a variety of studies, CuZn-superoxide dismutase (CuZn-SOD) has been shown to protect the brain from ischemic injury. A possible role for CuZn-SOD-related modulation of neuronal viability is suggested by the finding that CuZn-SOD inhibits apoptotic neuronal cell death in response to some forms of cellular damage. We evaluated this possibility in the model of transient focal cerebral ischemia in mice bearing a disruption of the CuZn-SOD gene (Sod1). Homozygous mutant (Sod1 -/-) mice had no detectable CuZn-SOD activity, and heterozygous mutants (Sod1 +/-) showed a 50% decrease compared with wild-type mice. Sod1 -/- mice showed a high level of blood-brain barrier disruption soon after 1 hr of middle cerebral artery occlusion and 100% mortality at 24 hr after ischemia. Sod1 +/- mice showed 30% mortality at 24 hr after ischemia, and neurological deficits were exacerbated compared with wild-type controls. The Sod1 +/- animals also had increased infarct volume and brain swelling, accompanied by increased apoptotic neuronal cell death as indicated by the in situ nick-end labeling technique to detect DNA fragmentation and morphological criteria. These results suggest that oxygen-free radicals, especially superoxide anions, are an important factor for the development of infarction by brain edema formation and apoptotic neuronal cell death after focal cerebral ischemia and reperfusion.