Par-4 is a mediator of neuronal degeneration associated with the pathogenesis of Alzheimer disease

Transgenic zebrafish model of neurodegeneration

In Alzheimer's disease (AD), the microtubule-associated protein, tau, is compromised in its normal association with microtubules and forms into paired helical filaments (PHF) that are the hallmark cytoskeletal pathology of the disease. Several posttranslational modifications of tau including phosphorylation have been implicated in AD pathogenesis. In addition, and importantly, mutations in the genes encoding human tau have recently been implicated in a variety of hereditary dementias, collectively termed frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). This has rekindled interest in the importance of tau in neurodegenerative diseases (cf. Vogel [1998] Science 280:1524-1525; Goedert et al. [1998] Neuron 21:955-958; D'Souza et al. [1999] PNAS 96:5598-5603). Despite significant progress in the field of tau biology and neurodegenerative diseases, several important issues remain unresolved. The early functional consequences of tau alterations in living neurons is incompletely understood, and it is not clear how tau in neurodegenerative diseases becomes redistributed from its normal concentration in neuronal axons to pathological inclusions in neuronal soma known as neurofibrillary tangles (NFT). One of the reasons for these gaps in knowledge is the relative paucity of model systems to study these processes. We have developed a transgenic model system to study the functional consequences and trafficking patterns in zebrafish neurons of human tau either mutated on sites associated with hereditary dementias or altered at select posttranslational modification sites. The overall guiding hypothesis is that the model allows dissection of a hierarchy of events relevant to potential mechanisms of neurodegenerative diseases related to critical early stages in development of disease. We showed that a FTDP-17 mutant form of human tau expressed in zebrafish neurons produced a cytoskeletal disruption that closely resembled the NFT in human disease. This model system will prove useful in the study of other mutant taus in vertebrate neurons in vivo, and the approaches developed here will have broad usefulness in the study of functional consequences and potential genetic analyses of introducing into living vertebrate neurons other molecules involved in the pathogenesis of neurodegenerative diseases.

Disruption of neurogenesis by amyloid beta-peptide, and perturbed neural progenitor cell homeostasis, in models of Alzheimer's disease

Neurogenesis occurs in the adult mammalian brain and may play roles in learning and memory processes and recovery from injury, suggesting that abnormalities in neural progenitor cells (NPC) might contribute to the pathogenesis of disorders of learning and memory in humans. The objectives of this study were to determine whether NPC proliferation, survival and neuronal differentiation are impaired in a transgenic mouse model of Alzheimer's disease (AD), and to determine the effects of the pathogenic form of amyloid beta-peptide (Abeta) on the survival and neuronal differentiation of cultured NPC. The proliferation and survival of NPC in the dentate gyrus of the hippocampus was reduced in mice transgenic for a mutated form of amyloid precursor protein that causes early onset familial AD. Abeta impaired the proliferation and neuronal differentiation of cultured human and rodent NPC, and promoted apoptosis of neuron-restricted NPC by a mechanism involving dysregulation of cellular calcium homeostasis and the activation of calpains and caspases. Adverse effects of Abeta on NPC may contribute to the depletion of neurons and cognitive impairment in AD.

p53 inhibitors preserve dopamine neurons and motor function in experimental parkinsonism

Drugs currently used for patients with Parkinson's disease provide temporary relief of symptoms but do not halt or slow the underlying neurodegenerative disease process. Increasing evidence suggests that neurons die in Parkinson's disease by a process called apoptosis, which may be triggered by mitochondrial impairment and oxidative stress. We report that two novel synthetic inhibitors of the tumor suppressor protein p53, pifithrin-alpha (PFT-alpha) and Z-1-117, are highly effective in protecting midbrain dopaminergic neurons and improving behavioral outcome in a mouse model of Parkinson's disease. Mice given intraperitoneal injections of PFT-alpha or Z-1-117 exhibited improved motor function, reduced damage to nigrostriatal dopaminergic neurons and reduced depletion of dopamine and its metabolites after exposure to the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP caused an increase in the level of the proapoptotic protein Bax, which was prevented by giving mice PFT-alpha and Z-1-117. PFT-alpha and Z-1-117 also suppressed Bax production and apoptosis in cultured dopaminergic cells exposed to MPP(+). Our findings demonstrate a pivotal role for p53 in experimental parkinsonism and identify a novel class of synthetic p53 inhibitors with clinical potential.

Structural and functional characterization of the upstream regulatory region of the human gene encoding prostate apoptosis response factor-4

Prostate apoptosis response factor-4 (Par-4) is critical to cell growth and apoptosis. Induction of Par-4 expression has been shown to be required for apoptosis in a diversity of cellular systems, including neurons. Neuronal populations in individuals with degenerative disorders show elevated levels of Par-4 protein in advance of cellular and functional loss. To understand the regulation of par-4 expression, we isolated and characterized 5.7 kb of the human par-4 promoter. We demonstrated that the isolated promoter was functional. Similar to the endogenous par-4 gene, par-4 expression could be induced upon apoptotic insult with thapsigargin following introduction of the promoter DNA into human A375 cells. Also, increased levels of the atypical protein kinase C, zetaPKC, was shown to negatively regulate expression from the ectopic par-4 promoter. A 550 bp sequence immediately upstream to the 5'-untranslated region of the gene was found to be responsible for par-4 promoter induction to apoptosis by thapsigargin.

Modification of brain aging and neurodegenerative disorders by genes, diet, and behavior

Multiple molecular, cellular, structural, and functional changes occur in the brain during aging. Neural cells may respond to these changes adaptively, or they may succumb to neurodegenerative cascades that result in disorders such as Alzheimer's and Parkinson's diseases. Multiple mechanisms are employed to maintain the integrity of nerve cell circuits and to facilitate responses to environmental demands and promote recovery of function after injury. The mechanisms include production of neurotrophic factors and cytokines, expression of various cell survival-promoting proteins (e.g., protein chaperones, antioxidant enzymes, Bcl-2 and inhibitor of apoptosis proteins), preservation of genomic integrity by telomerase and DNA repair proteins, and mobilization of neural stem cells to replace damaged neurons and glia. The aging process challenges such neuroprotective and neurorestorative mechanisms. Genetic and environmental factors superimposed upon the aging process can determine whether brain aging is successful or unsuccessful. Mutations in genes that cause inherited forms of Alzheimer's disease (amyloid precursor protein and presenilins), Parkinson's disease (alpha-synuclein and Parkin), and trinucleotide repeat disorders (huntingtin, androgen receptor, ataxin, and others) overwhelm endogenous neuroprotective mechanisms; other genes, such as those encoding apolipoprotein E(4), have more subtle effects on brain aging. On the other hand, neuroprotective mechanisms can be bolstered by dietary (caloric restriction and folate and antioxidant supplementation) and behavioral (intellectual and physical activities) modifications. At the cellular and molecular levels, successful brain aging can be facilitated by activating a hormesis response in which neurons increase production of neurotrophic factors and stress proteins. Neural stem cells that reside in the adult brain are also responsive to environmental demands and appear capable of replacing lost or dysfunctional neurons and glial cells, perhaps even in the aging brain. The recent application of modern methods of molecular and cellular biology to the problem of brain aging is revealing a remarkable capacity within brain cells for adaptation to aging and resistance to disease.

Disruption of neurogenesis in the subventricular zone of adult mice, and in human cortical neuronal precursor cells in culture, by amyloid beta-peptide: implications for the pathogenesis of Alzheimer's disease

The adult mammalian brain contains populations of stem cells that can proliferate and then differentiate into neurons or glia. The highest concentration of such neural progenitor cells (NPC) is located in the subventricular zone (SVZ) and these cells can produce new olfactory bulb and cerebral cortical neurons. NPC may provide a cellular reservoir for replacement of cells lost during normal cell turnover and after brain injury. However, neurogenesis does not compensate for neuronal loss in age-related neurodegenerative disorders such as Alzheimer's disease (AD), suggesting the possibility that impaired neurogenesis contributes to the pathogenesis of such disorders. We now report that amyloid beta-peptide (Abeta), a self-aggregating neurotoxic protein thought to cause AD, can impair neurogenesis in the SVZ/cerebral cortex of adult mice and in human cortical NPC in culture. The proliferation and migration of NPC in the SVZ of amyloid precursor protein (APP) mutant mice, and in mice receiving an intraventricular infusion of Abeta, were greatly decreased compared to control mice. Studies of NPC neurosphere cultures derived from human embryonic cerebral cortex showed that Abeta can suppress NPC proliferation and differentiation, and can induce apoptosis. The adverse effects of Abeta on neurogenesis were associated with a disruption of calcium regulation in the NPC. Our data show that Abeta can impair cortical neurogenesis, and suggest that this adverse effect of Abeta contributes to the depletion of neurons and the resulting olfactory and cognitive deficits in AD.

Estrogen and brain vulnerability

Accumulated clinical and basic evidence suggests that gonadal steroids affect the onset and progression of several neurodegenerative diseases and schizophrenia, and the recovery from traumatic neurological injury such as stroke. Thus, our view on gonadal hormones in neural function must be broadened to include not only their function in neuroendocrine regulation and reproductive behaviors, but also to include a direct participation in response to degenerative disease or injury. Recent findings indicate that the brain up-regulates both estrogen synthesis and estrogen receptor expression at sites of injury. Genetic or pharmacological inactivation of aromatase, the enzyme involved in estrogen synthesis, indicates that the induction of this enzyme in the brain after injury has a neuroprotective role. Some of the mechanisms underlying the neuroprotective effects of estrogen may be independent of the classically defined nuclear estrogen receptors (ERs). Other neuroprotective effects of estrogen do depend on the classical nuclear ERs, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that non-classical ERs in the membrane or cytoplasm alter phosphorylation cascades, such as those involved in the signaling of insulin-like growth factor-1 (IGF-1). Indeed, ERs and IGF-1 receptor interact in the activation of PI3K and MAPK signaling cascades and in the promotion of neuroprotection. The decrease in estrogen and IGF-1 levels with aging may thus result in an increased risk for neuronal pathological alterations after different forms of brain injury.

Prostate apoptosis response-4 involved in the protective effect of salvianolic acid B against amyloid beta peptide-induced damage in PC12 cells

To observe the effect of salvianolic acid-B (SalB) against the cytoxicity of amyloid beta peptide (A-beta)(25-35) to PC12 cells, the cells were incubated with A-beta, and the cytoxicity was investigated by MTT, flow cytometry and a cell free apoptotic system. The expression of prostate apoptotic response-4 (Par-4) was detected by Western blot. Aged A-beta 10 micromol/L significantly inhibited the MTT reduction of PC12 cells, SalB1 micromol/L inhibited the toxicity induced by A-beta. In flow cytometric analysis, PC12 cells treated with A-beta exhibited degraded DNA content characteristic of apoptosis cells (1.53% vs 19.9%). PC12 cells pretreated with SalB (10 nmol/L, 100 nmol/L, 1 micromol/L) manifested relatively low proportion of apoptosis (15.7%, 13.5%, 11.8%, respectively). SalB (10 nmol/L - 1 micromol/L) when added at the beginning of the cell free apoptotic reaction had no apparent effect on the nuclei apoptosis. Pretreatment of PC12 cells with SalB largely prevented the increase in Par-4 expression of the cells when they were exposed to A-beta. The results suggest that Par-4 is involved in the protective effect of SalB against A-beta-induced damage in PC12 cells.

Cholinergic innervation and function in the prostate gland

The mammalian prostate is densely innervated by hypogastric and pelvic nerves that play an important role in regulating the growth and function of the gland. While there has been much interest in the role of the noradrenergic innervation and adrenoceptors in prostate function, the role of cholinergic neurones in prostate physiology and pathophysiology is not well understood. This review focuses on the role of acetylcholine and cholinoceptors in prostate function. Nitric oxide, vasoactive intestinal polypeptide, and/or neuropeptide Y are co-localised with cholinesterase and/or acetylcholine transporter in some of the nerve fibres supplying the prostate. Their roles are also briefly discussed in this review. A dense network of cholinesterase-staining fibres supplies both prostate epithelium and stroma, suggesting a role of acetylcholine and/or co-localised neuropeptides in the modulation of prostatic secretions, as well as smooth muscle tone. A predominantly epithelial location for prostate muscarinic receptors indicated a major secretomotor role for acetylcholine. The muscarinic receptor subtype mediating muscarinic agonist-induced smooth muscle contraction or enhancement of contractions evoked by nerve stimulation differs in different species. In the human, there is evidence for M(1) receptors on the epithelium, M(2) receptors on the stroma, and both M(1) and M(3) receptors in some prostate cancer cell lines. Several recent investigations indicate that muscarinic receptors may also mediate or modulate normal, benign, and malignant prostate growth. The role of muscarinic agonists and their receptors and the influences of age, testicular, and other steroids in regulating the effects are reviewed.

Folic acid deficiency and homocysteine impair DNA repair in hippocampal neurons and sensitize them to amyloid toxicity in experimental models of Alzheimer's disease

Recent epidemiological and clinical data suggest that persons with low folic acid levels and elevated homocysteine levels are at increased risk of Alzheimer's disease (AD), but the underlying mechanism is unknown. We tested the hypothesis that impaired one-carbon metabolism resulting from folic acid deficiency and high homocysteine levels promotes accumulation of DNA damage and sensitizes neurons to amyloid beta-peptide (Abeta) toxicity. Incubation of hippocampal cultures in folic acid-deficient medium or in the presence of methotrexate (an inhibitor of folic acid metabolism) or homocysteine induced cell death and rendered neurons vulnerable to death induced by Abeta. Methyl donor deficiency caused uracil misincorporation and DNA damage and greatly potentiated Abeta toxicity as the result of reduced repair of Abeta-induced oxidative modification of DNA bases. When maintained on a folic acid-deficient diet, amyloid precursor protein (APP) mutant transgenic mice, but not wild-type mice, exhibited increased cellular DNA damage and hippocampal neurodegeneration. Levels of Abeta were unchanged in the brains of folate-deficient APP mutant mice. Our data suggest that folic acid deficiency and homocysteine impair DNA repair in neurons, which sensitizes them to oxidative damage induced by Abeta.

Kainic acid-induced neuronal cell death in cerebellar granule cells is not prevented by caspase inhibitors

1. We examined the role of non-NMDA receptors in kainic acid (KA)-induced apoptosis in cultures of rat cerebellar granule cells (CGCs). KA (1 - 500 microM) induced cell death in a concentration-dependent manner, which was prevented by NBQX and GYKI 52466, non-NMDA receptor antagonists. Moreover, AMPA blocked KA-induced excitotoxicity, through desensitization of AMPA receptors. 2. Similarly, KA raised the intracellular calcium concentration of CGCs, which was inhibited by NBQX and GYKI 52466. Again, AMPA (100 microM) abolished the KA (100 microM)-induced increase in intracellular calcium concentration. 3. KA-induced cell death in CGCs had apoptotic features, which were determined morphologically, by DNA fragmentation, and by expression of the prostate apoptosis response-4 protein (Par-4). 5. KA (500 microM) slightly (18%) increased caspase-3 activity, which was strongly enhanced by colchicine (1 microM), an apoptotic stimulus. However, neither Z-VAD.fmk, a pan-caspase inhibitor, nor the more specific caspase-3 inhibitor, Ac-DEVD-CHO, prevented KA-induced cell death or apoptosis. In contrast, both drugs inhibited colchicine-induced apoptosis. 5. The calpain inhibitor ALLN had no effect on KA or colchicine-induced neurotoxicity. 6. Our findings indicate that colchicine-induced apoptosis in CGCs is mediated by caspase-3 activation, unlike KA-induced apoptosis.

Urocortin, but not urocortin II, protects cultured hippocampal neurons from oxidative and excitotoxic cell death via corticotropin-releasing hormone receptor type I

Urocortin and urocortin II are members of the corticotropin-releasing hormone (CRH) family of neuropeptides that function to regulate stress responses. Two high-affinity G-protein-coupled receptors have been identified that bind CRH and/or urocortin I and II, designated CRHR1 and CRHR2, both of which are present in hippocampal regions of mammalian brain. The hippocampus plays an important role in regulating stress responses and is a brain region in which neurons are vulnerable during disease and stress conditions, including cerebral ischemia, Alzheimer's disease, and anxiety disorders. Here we report that urocortin exerts a potent protective action in cultured rat hippocampal neurons with concentrations in the range of 0.5-5.0 pm, increasing the resistance of the cells to oxidative (amyloid beta-peptide, 4-hydroxynonenal, ferrous sulfate) and excitotoxic (glutamate) insults. We observed that urocortin is 10-fold more potent than CRH in protecting hippocampal neurons from insult, whereas urocortin II is ineffective. RT-PCR and sequencing analyses revealed the presence of both CRHR1 and CRHR2 in the hippocampal cultures, with CRHR1 being expressed at much higher levels than CRHR2. Using subtype-selective CRH receptor antagonists, we provide evidence that the neuroprotective effect of exogenously added urocortin is mediated by CRHR1. Furthermore, we provide evidence that the signaling pathway that mediates the neuroprotective effect of urocortin involves cAMP-dependent protein kinase, protein kinase C, and mitogen-activated protein kinase. This is the first demonstration of a biological activity of urocortin in hippocampal neurons, suggesting a role for the peptide in adaptive responses of hippocampal neurons to potentially lethal oxidative and excitotoxic insults.

beta-Secretase cleavage of the amyloid precursor protein mediates neuronal apoptosis caused by familial Alzheimer's disease mutations

The amyloid precursor protein (APP) is cleaved by two enzymes, beta-secretase and gamma-secretase, to generate the pathological amyloid beta (Abeta) peptide. Expression of familial Alzheimer's disease (FAD) mutants of APP in primary neurons causes both intracellular accumulation of the C-terminal beta-secretase cleavage product of APP and increased secretion of Abeta, and eventually results in apoptotic death of the cells. To determine whether either of these two processing products of APP is involved in this apoptotic pathway, we first modeled experimentally the accumulation of the beta-secretase cleavage product in neurons. The C-terminal 100 amino acids (C100) of APP, with and without a signal peptide, was expressed in cells via recombinant herpes simplex virus (HSV) vectors. Both transgene products were targeted to the membrane, and both caused apoptosis in the neurons, implicating the beta-secretase cleavage product of APP in apoptosis caused by FAD APPs. Expression in neurons of a mutant of FAD APP that inhibited beta-secretase cleavage inhibited its ability to cause apoptosis. However, expression in neurons of a mutant of FAD APP that inhibited gamma-secretase cleavage did not inhibit the ability of this mutant to cause apoptosis. These data suggested that the C-terminal beta-secretase cleavage product of APP, but not Abeta, mediates the apoptosis caused by FAD mutants of APP. Consistent with this hypothesis, C31, which is generated from the beta-secretase cleavage product, itself caused neuronal apoptosis. Inhibitors of caspases 3, 6 and 8, but not of caspase 9, inhibited the apoptosis caused by FAD mutants of APP. It may be inferred from these data that beta-secretase cleavage of FAD mutants of APP allows the appropriate caspase access to its site of action to produce C31, which directly causes neuronal apoptosis.

pH-induced folding of an apoptotic coiled coil

Par-4 is a 38-kD protein pivotal to the apoptotic pathways of various cell types, most notably prostate cells and neurons, where it has been linked to prostate cancer and various neurodegenerative disorders including Alzheimer's and Huntington's diseases and HIV encephalitis. The C-terminal region of Par-4 is responsible for homodimerization and the ability of Par-4 to interact with proposed effector molecules. In this study, we show that the C-terminal 47 residues of Par-4 are natively unfolded at physiological pH and temperature. Evidence is rapidly accumulating that natively unfolded proteins play an important role in various cellular functions and signaling pathways, and that folding can often be induced on complexation with effector molecules or alteration of environment. Here we use primarily CD studies to show that changes in the environment, particularly pH and temperature, can induce the Par-4 C terminus to form a self-associated coiled coil.

Dlk/ZIP kinase-induced apoptosis in human medulloblastoma cells: requirement of the mitochondrial apoptosis pathway

Dlk/ZIP kinase is a member of the Death Associated Protein (DAP) kinase family of pro-apoptotic serine/threonine kinases that have been implicated in regulation of apoptosis and tumour suppression. Expression of both Dlk/ZIP kinase and its interaction partner Par-4 is maintained in four medulloblastoma cell lines investigated, whereas three of seven neuroblastoma cell lines have lost expression of Par-4. Overexpression of a constitutively pro-apoptotic deletion mutant of Dlk/ZIP kinase induced significant apoptosis in D283 medulloblastoma cells. Cell death was characterized by apoptotic membrane blebbing, and a late stage during which the cells had ceased blebbing and were drastically shrunken or disrupted into apoptotic bodies. Over-expression of the anti-apoptotic Bcl-xL protein had no effect on Dlk/ZIP kinase-induced membrane blebbing, but potently inhibited Dlk/ZIP kinase-induced cytochrome c release and transition of cells to late stage apoptosis. Treatment with caspase inhibitors delayed, but did not prevent entry into late stage apoptosis. These results demonstrate that Dlk/ZIP kinase-triggered apoptosis involves the mitochondrial apoptosis pathway. However, cell death proceeded in the presence of caspase inhibitors, suggesting that Dlk/ZIP kinase is able to activate alternative cell death pathways. Alterations of signal transduction pathways leading to Dlk/ZIP kinase induced apoptosis or loss of expression of upstream activators could play important roles in tumour progression and metastasis of neural tumours.

Aberrant induction of Par-4 is involved in apoptosis of hippocampal neurons in presenilin-1 M146V mutant knock-in mice

Mutations in presenilin-1 (PS-1) have been shown to increase neuronal vulnerability to apoptosis in Alzheimer's disease (AD). Par-4 is a novel cell-death-promoting protein associated with neuronal degeneration in AD. We previously reported that, in transfected PC12 cells, Par-4 seems to be involved in the neurodegenerative mechanisms of PS-1 mutations. However, direct evidence for a necessary role of Par-4 in the pathogenic mechanisms of PS-1 mutations in neurons is lacking. We recently generated and characterized presenilin-1 mutant M146V knock-in (PS-1 M146V KI) mice. We now report that expression of the mutant presenilin-1 in these mice induces early and exaggerated increase in Par-4 expression in hippocampal neurons following glucose deprivation (an insult relevant to the pathogenesis of AD). Importantly, inhibition of Par-4 expression by antisense par-4 oligonucleotide treatment counteracts neuronal apoptosis promoted by M146V mutation of PS-1. Mitochondrial dysfunction and caspase-3 activity induced by glucose deprivation was significantly exacerbated in hippocampal neurons expressing the mutant PS-1. Antisense par-4 treatment largely suppressed the adverse effect of the mutant PS-1 on mitochondrial dysfunction and caspase activation. These results provide evidence in hippocampal neurons that Par-4 is involved in the neurodegenerative cascades associated with PS-1 M146V mutation by acting relatively early in the apoptotic process before mitochondrial dysfunction and caspase-3 activation. Since levels of Par-4 are significantly increased in the hippocampus in human AD brain, the results of this study may provide a significant link between aberrant induction of Par-4 and the neurodegenerative cascades promoted by PS-1 mutations in AD.

Iodoacetate protects hippocampal neurons against excitotoxic and oxidative injury: involvement of heat-shock proteins and Bcl-2

Mild metabolic stress may increase resistance of neurons in the brain to subsequent, more severe insults, as demonstrated by the ability of ischemic pre-conditioning and dietary restriction to protect neurons in experimental models of stroke- and age-related neurodegenerative disorders. In the present study we employed iodoacetic acid (IAA), an inhibitor of glyceraldehyde-3-phosphate dehydrogenase, to test the hypothesis that inhibition of glycolysis can protect neurons. Pre-treatment of cultured hippocampal neurons with IAA can protect them against cell death induced by glutamate, iron and trophic factor withdrawal. Surprisingly, protection occurred with concentrations of IAA (2-200 nM) much lower than those required to inhibit glycolysis. Pre-treatment with IAA results in suppression of oxyradical production and stabilization of mitochondrial function in neurons after exposure to oxidative insults. Levels of the stress heat-shock proteins HSP70 and HSP90, and of the anti-apoptotic protein Bcl-2, were increased in neurons exposed to IAA. Our data demonstrate that IAA can stimulate cytoprotective mechanisms within neurons, and suggest the possible use of IAA and related compounds in the prevention and/or treatment of neurodegenerative conditions.

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.

Presenilin mutations and calcium signaling defects in the nervous and immune systems

Presenilin-1 (PS1) is thought to regulate cell differentiation and survival by modulating the Notch signaling pathway. Mutations in PS1 have been shown to cause early-onset inherited forms of Alzheimer's disease (AD) by a gain-of-function mechanism that alters proteolytic processing of the amyloid precursor protein (APP) resulting in increased production of neurotoxic forms of amyloid beta-peptide. The present article considers a second pathogenic mode of action of PS1 mutations, a defect in cellular calcium signaling characterized by overfilling of endoplasmic reticulum (ER) calcium stores and altered capacitive calcium entry; this abnormality may impair synaptic plasticity and sensitize neurons to apoptosis and excitotoxicity. The calcium signaling defect has also been documented in lymphocytes, suggesting a contribution of immune dysfunction to the pathogenesis of AD. A better understanding of the calcium signaling defect resulting from PS1 mutations may lead to the development of novel preventative and therapeutic strategies for disorders of the nervous and immune systems.

Differential expression levels of Par-4 in melanoma

The pro-apoptotic prostate apoptosis response-4 gene product Par-4 sensitizes prostate cells to the induction of programmed cell death. In this study we examined Par-4 expression in human melanoma cell lines and melanoma metastases. The heterogeneous expression detected prompted us to investigate the biological relevance of Par-4 in a human melanoma xenotransplantation model. Overexpression of Par-4 by transfection decreased tumour development in xenotransplanted A375-C6 melanoma cells in SCID mice and correlated to an increase in tumour cell apoptosis. These data suggest that high expression of the pro-apoptotic protein Par-4 could qualify as a prognostic marker in human melanoma.

Apoptosis in amyotrophic lateral sclerosis: a review of the evidence

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease primarily affecting the upper and lower motor neurones of the central nervous system. Recently, a lot of interest has been generated by the possibility that a mechanism of programmed cell death, termed apoptosis, is responsible for the motor neurone degeneration in this condition. Apoptosis is regulated through a variety of different pathways which interact and eventually lead to controlled cell death. Apart from genetic regulation, factors involved in the control of apoptosis include death receptors, caspases, Bcl-2 family of oncoproteins, inhibitor of apoptosis proteins (IAPs), inhibitors of IAPs, the p53 tumour suppressor protein and apoptosis-related molecules. The first part of this article will give an overview of the current knowledge of apoptosis. In the second part of this review, we will examine in detail the evidence for and against the contribution of apoptosis in motor neurone cell death in ALS, looking at cellular-, animal- and human post-mortem tissue-based models. In a chronic neurodegenerative disease such as ALS, conclusive evidence of apoptosis is likely to be difficult to detect, given the rapidity of the apoptotic cell death process in relation to the relatively slow time course of the disease. Although a complete picture of motor neurone death in ALS has not been fully elucidated, there is good and compelling evidence that a programmed cell death pathway operates in this disorder. The strongest body of evidence supporting this comes from the findings that, in ALS, changes in the levels of members of the Bcl-2 family of oncoproteins results in a predisposition towards apoptosis, there is increased expression or activation of caspases-1 and -3, and the dying motor neurones in human cases exhibit morphological features reminiscent of apoptosis. Further supporting evidence comes from the detection of apoptosis-related molecules and anti-Fas receptor antibodies in human cases of ALS. However, the role of the p53 protein in cell death in ALS is at present unclear. An understanding of the mechanism of programmed cell death in ALS may provide important clues for areas of potential therapeutic intervention for neuroprotection in this devastating condition.

Regional expression of Par-4 mRNA and protein after fluid percussion brain injury in the rat

Regional levels of prostate apoptosis response-4 (Par-4) protein and mRNA were measured after lateral fluid percussion (FP) brain injury in rats. Immunochemical studies indicated that Par-4 immunoreactivity (ir) is present in cortical neurons and hippocampal CA1-CA3 pyramidal neurons in uninjured rats. Increases of Par-4-ir were observed in the CA3 neurons of the ipsilateral hippocampus (IH), but not in injured left cortex (IC) at 48 h after FP brain injury. Levels of the Par-4 mRNA measured by RT-PCR assay and protein measured by Western blot procedure were significantly increased in the injured IC and IH, but not in the contralateral right cortex and hippocampus after brain injury. Levels of both Par-4 protein and mRNA were significantly increased in the IC and IH as early as 2 h and stayed elevated at 24 and 48 h after injury. These data show that the induction of proapoptotic Par-4 mRNA and protein occurs only in the IC and IH that have been observed to undergo apoptosis and neuronal cell loss after lateral FP brain injury. Because increased expression of Par-4 has been observed to contribute to apoptosis and cell death in cultured neurons, the present temporal pattern of Par-4 expression is consistent with a role for Par-4 in apoptosis and neuronal cell death after traumatic brain injury.

Par-4 is a synaptic protein that regulates neurite outgrowth by altering calcium homeostasis and transcription factor AP-1 activation

Although Par-4 (prostate apoptosis response-4) is involved in initiation of neurodegenerative cascades associated with certain neurodegenerative disorders, normal physiological roles of Par-4 in neurons have remained elusive. It was recently reported that Par-4 protein levels could be regulated at translational level in synaptic terminals following apoptotic insults, suggesting that Par-4 might play a role in synaptic function. We report that Par-4 is a synaptic protein preferably localized in postsynaptic density (PSD). The expression of Par-4 in synaptosome preparations and PSDs are developmentally and regionally regulated. Synaptic Par-4 is enriched in the cerebral cortex and the hippocampus, but not in the cerebellum. In vitro as well as in vivo experiments demonstrate that the levels of synaptic Par-4 increase as the neurons mature. Overexpression of Par-4 in transfected PC12 cells inhibits nerve growth factor (NGF)-induced cellular differentiation and neurite outgrowth by a mechanism involving aberrant elevation of intracellular calcium levels and suppression of activation of the transcription factor AP-1. The actions of Par-4 were consistently blocked by co-expression of the dominant negative regulator of Par-4 activity (the leucine zipper domain of Par-4). Since the leucine zipper domain of Par-4 (Leu.zip) may mediate protein--protein interactions, the results indicate that the actions of Par-4 require its interaction with other protein(s) or dimerization with itself. These results suggest that Par-4 may play an important role in postsynaptic signal transduction and regulation of cellular pathways associated with cellular differentiation and neurite outgrowth. Identification of Par-4 as a novel synaptic protein may have significant implications in understanding the mechanisms of synaptic functions in physiological and pathological settings.

Prostate apoptosis response-4 enhances secretion of amyloid beta peptide 1-42 in human neuroblastoma IMR-32 cells by a caspase-dependent pathway

Prostate apoptosis response-4 (Par-4) is a leucine zipper protein that promotes neuronal cell death in Alzheimer's disease (AD). Neuronal degeneration in AD may result from extracellular accumulation of amyloid beta peptide (Abeta) 1-42. To examine the effect of Par-4 on Abeta secretion and to reconcile amyloid/apoptosis hypotheses of AD, we generated IMR-32 cell lines that overexpress Par-4 and/or its leucine zipper domain. Overexpression of Par-4 did not significantly affect levels of the endogenously expressed beta amyloid precursor protein but drastically increased the Abeta(1-42)/Abeta(total) ratio in the conditioned media about 6-8 h after trophic factor withdrawal. Time course analysis of caspase activation reveals that Par-4 overexpression exacerbated caspase activation, which is detectable within 2 h after trophic factor withdrawal. Furthermore, inhibition of caspase activity by the broad spectrum caspase inhibitor BD-fmk significantly attenuated the Par-4-induced increase in Abeta 1-42 production. In addition, the effects of Par-4 on secretion of Abeta 1-42 were consistently blocked by co-expression of the leucine zipper domain, indicating that the effect of Par-4 on Abeta secretion may require its interaction with other protein(s). These results suggest that Par-4 increases secretion of Abeta 1-42 largely through a caspase-dependent pathway after apoptotic cascades are initiated.

The DAP kinase family of pro-apoptotic proteins: novel players in the apoptotic game

The DAP (Death Associated Protein) kinase family is a novel subfamily of pro-apoptotic serine/threonine kinases. All five DAP kinase family members identified to date are ubiquitously expressed in various tissues and are capable of inducing apoptosis. The sequence homology of the five kinases is largely restricted to the N-terminal kinase domain. In contrast, the adjacent C-terminal regions are very diverse and link individual family members to specific signal transduction pathways. There is increasing evidence that DAP kinase family members are involved in both extrinsic and intrinsic pathways of apoptosis and may play a role in tumor progression. This review will focus on structural composition and subcellular localization of DAP kinase family members and on signal transduction pathways leading to their activation. Potential mechanisms of DAP kinase family-mediated apoptosis will be discussed. BioEssays 23:352-358, 2001.

Evidence for the involvement of Par-4 in ischemic neuron cell death

After a stroke many neurons in the ischemic brain tissue die by a process called apoptosis, a form of cell death that may be preventable. The specific molecular cascades that mediate ischemic neuronal death are not well understood. The authors recently identified prostate apoptosis response-4 (Par-4) as a protein that participates in the death of cultured hippocampal neurons induced by trophic factor withdrawal and exposure to glutamate. Here, the authors show that Par-4 levels increase in vulnerable populations of hippocampal and striatal neurons in rats after transient forebrain ischemia; Par-4 levels increased within 6 hours of reperfusion and remained elevated in neurons undergoing apoptosis 3 days later. After transient focal ischemia in mice, Par-4 levels were increased 6 to 12 hours after reperfusion in the infarcted cortex and the striatum, and activation of caspase-8 occurred with a similar time course. Par-4 immunoreactivity was localized predominantly in cortical neurons at the border of the infarct area. A Par-4 antisense oligonucleotide protected cultured hippocampal neurons against apoptosis induced by chemical hypoxia and significantly reduced focal ischemic damage in mice. The current data suggest that early up-regulation of Par-4 plays a pivotal role in ischemic neuronal death in animal models of stroke and cardiac arrest.

Integrin signaling via the PI3-kinase-Akt pathway increases neuronal resistance to glutamate-induced apoptosis

Integrins are integral membrane proteins that mediate adhesive interactions of cells with the extracellular matrix and with other cells. Integrin engagement results in activation of intracellular signaling cascades that effect several different cellular responses including motility, proliferation and survival. Although integrins are known to provide cell survival signaling in various types of non-neuronal cells, the possibility that integrins modulate neuron survival has not been explored. We now report data demonstrating a neuroprotective function of integrins in embryonic hippocampal neurons. Neurons grown on laminin, an integrin ligand, exhibit increased resistance to glutamate-induced apoptosis compared with neurons grown on polylysine. Neurons expressed integrin beta1 and treatment of cultures with an antibody against integrin beta1 abolished the protective effect of laminin. Neurons maintained on laminin exhibited a sustained activation of the Akt signaling pathway demonstrated in immunoblot analyses using an antibody that selectively recognizes phosphorylated Akt. The neuroprotective effect of integrin engagement by laminin was mimicked by an IKLLI-containing integrin-binding peptide and was abolished by treatment of neurons with the PI3 kinase inhibitor wortmanin. Levels of the anti-apoptotic protein Bcl-2 were increased in neurons grown on laminin and decreased by wortmanin, suggesting a mechanism for the neuroprotective effect of integrin-mediated signaling. The ability of integrin-mediated signaling to prevent glutamate-induced apoptosis suggests a mechanism whereby neuron-substrate interactions can promote neuron survival under conditions of glutamate receptor overactivation.

Markers of apoptosis and models of programmed cell death in Alzheimer's disease

Alzheimer's disease (AD) is neuropathologically marked by the presence of senile plaques composed of beta-amyloid peptide and by neurofibrillary tangles formed by abnormally phosphorylated tau protein. Many authors have also reported a neuronal loss in affected regions of the brain in AD patients. This neuronal degeneration could be linked to the triggering of intracellular pathways leading to apoptosis. Previous works were focused on the links between neuronal apoptosis and tau and amyloid precursor protein (APP) metabolisms. We have analyzed tau gene expression in primary neuronal cultures submitted to an apoptotic stress produced by excitotoxicity or serum deprivation. Glutamate induces an enhancement of tau gene expression in resistant neurons whereas a reduced expression is noted in apoptotic cells. This decrease is similar to what is observed after trophic support withdrawal in neuronal cultures. Neurons expressing phosphorylated tau are more resistant to experimental apoptosis than neurons positively labeled for dephosphorylated tau protein (AT8/Tau 1 epitope). In vitro apoptotic neurons are able to produce membrane blebbings (strongly immunopositive for APP and amyloidogenic fragments) that are secondary released in the extracellular space. Finally neurons overexpressing human mutated presenilin 1 (M146 L) are more prone to degenerate than neurons overexpressing human wild-type presenilin 1 after apoptosis induction.

Existing data suggest that Alzheimer's disease is preventable

The ultimate goal of Alzheimer's disease (AD) research is to prevent the onset of the neurodegenerative process and thereby allow successful aging without cognitive decline. Herein I argue that a simple and effective preventative approach for AD may be in hand. AD is a disorder associated with the aging process and is, accordingly, characterized by cellular and molecular changes that occur in age-related diseases in other organ systems. Such changes include increased levels of oxidative stress, perturbed energy metabolism, and accumulation of insoluble (oxidatively modified) proteins (prominent among which are amyloid beta-peptide and tau). The risk of several other prominent age-related disorders, including cardiovascular disease, cancer, and diabetes, is known to be influenced by the level of food intake--high food intake increases risk, and low food intake reduces risk. An overwhelming body of data from studies of rodents and monkeys has documented the profound beneficial effects of dietary restriction (DR) in extending life span and reducing the incidence of age-related diseases. Reduced levels of cellular oxidative stress and enhancement of energy homeostasis contribute to the beneficial effects of DR. Recent findings suggest that DR may enhance resistance of neurons in the brain to metabolic, excitotoxic, and oxidative insults relevant to the pathogenesis of AD and other neurodegenerative disorders. While further studies will be required to establish the extent to which DR will reduce the incidence of AD, it would seem prudent (based on existing data) to recommend DR as widely applicable preventative approach for age-related disorders including neurodegenerative disorders.

Mechanisms of cell death in neurodegenerative disorders

OBJECTIVE

Progressive cell loss in specific neuronal populations is the prominent pathological hallmark of neurodegenerative diseases, but its molecular basis remains unresolved. Apoptotic cell death has been implicated as a general mechanism in Alzheimer disease (AD) and other neurodegenerative disorders. However, DNA fragmention in neurons is too frequent to account for the continuous loss in these slowly progressive diseases.

MATERIAL AND METHODS

In 9 cases of morphologically confirmed AD (CERAD criteria, Braak stages 5 or 6), 5 cases of Parkinson disease (PD) and 3 cases each of Dementia with Lewy bodies (DLB), Progressive Supranuclear Palsy (PSP), and Multiple System Atrophy (MSA), and 7 age-matched controls, the TUNEL method was used to detect DNA fragmentation, and immunohistochemistry for an array of apoptosis-related proteins (ARP), protooncogenes, and activated caspase-3 were performed.

RESULTS

In AD, a considerable number of hippocampal neurons showed DNA fragmentation with a 3 to 5.7 fold increase related to neurofibrillary tangles and amyloid deposits, but only exceptional neurons displayed apoptotic morphology (1 in 1100-5000) and cytoplasmic immunoreactivity for ARPs and activated caspase-3 (1 in 2600 to 5650 hippocampal neurons), whereas no neurons were labeled in age-matched controls. Caspase-3 immunoreactivity was seen in granules of granulovacuolar degeneration, only rarely colocalized with tau-immunoreactivity. In PD, DLB, and MSA, TUNEL positivity and expression of ARPs or activated caspase-3 was only seen in microglia, rare astrocytes and in oligodendroglia with cytoplasmic inclusions in MSA, but not in nigral or other neurons with or without Lewy bodies. In PSP, only single neurons but oligodendrocytes, some with tau deposits, in brainstem tegmentum and pontine nuclei were TUNEL-positive and expressed both ARPs and activated caspase-3.

CONCLUSIONS

These data provide evidence for extremely rare apoptotic neuronal death in AD compatible with the progression of neuronal degeneration in this chronic disease. In other neurodegenerative disorders, apoptosis mainly involves microglia and oligodendroglia, while alternative mechanisms of neuronal death may occur. Susceptible cell populations in a proapoptotic environment show increased vulnerability towards metabolic and other pathogenic factors, with autophagy as a possible protective mechanism in early stages of programmed cell death. The intracellular cascade leading to cell death still awaits elucidation.

MEK5, a new target of the atypical protein kinase C isoforms in mitogenic signaling

The MEK5-extracellular signal-regulated kinase (ERK5) tandem is a novel mitogen-activated protein kinase cassette critically involved in mitogenic activation by the epidermal growth factor (EGF). The atypical protein kinase C isoforms (aPKCs) have been shown to be required for cell growth and proliferation and have been reported to interact with the adapter protein p62 through a short stretch of acidic amino acids termed the aPKC interaction domain. This region is also present in MEK5, suggesting that it may be an aPKC-binding partner. Here we demonstrate that the aPKCs interact in an EGF-inducible manner with MEK5 and that this interaction is required and sufficient for the activation of MEK5 in response to EGF. Consistent with the role of the aPKCs in the MEK5-ERK5 pathway, we show that zetaPKC and lambda/iotaPKC activate the Jun promoter through the MEF2C element, a well-established target of ERK5. From all these results, we conclude that MEK5 is a critical target of the aPKCs during mitogenic signaling.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

The cell cycle Cdc25A tyrosine phosphatase is activated in degenerating postmitotic neurons in Alzheimer's disease

The Cdc25 phosphatases play key roles in cell-cycle progression by activating cyclin-dependent kinases. The latter are absent from neurons that are terminally differentiated in adult brain. However, accumulation of mitotic phosphoepitopes, and re-expression and activation of the M phase regulator, Cdc2/cyclin B, have been described in neurons undergoing degeneration in Alzheimer's disease (AD). To explain this atypical mitotic activation in neurons we investigated the Cdc2-activating Cdc25A phosphatase in human brain. The structural hallmarks of AD neurodegeneration, neurofibrillary tangles and neuritic plaques, were prominently immunolabeled with Cdc25A antibodies. In addition numerous neurons without visible structural alterations were also intensely stained, whereas control brain was very weakly positive. After immunoprecipitation from control and AD tissue, we found that the tyrosine dephosphorylating activity of Cdc25A against exogenous Cdc2 substrate was elevated in AD. Accordingly, Cdc25A from AD tissue displayed increased immunoreactivity with the mitotic phosphoepitope-specific antibody, MPM-2, and co-localized with MPM-2 immunoreactivity in AD neurons. These data suggest that Cdc25A participates in mitotic activation during neurodegeneration. The involvement of Cdc25A in cellular transformation, modulation of the DNA damage checkpoint, and linkage of mitogenic signaling to cell cycle machinery, also implicates one of these cell-cycle pathways in AD pathogenesis.

The atypical protein kinase Cs. Functional specificity mediated by specific protein adapters

Since its discovery more than 10 years ago, the atypical PKC (aPKC) subfamily has attracted great interest. A number of reports have shown that the kinases of this subfamily play critical roles in signaling pathways that control cell growth, differentiation and survival. Recently, several investigators have identified a number of aPKC-interacting proteins whose characterization is helping to unravel the mechanisms of action and functions of these kinases. These interactors include p62, Par-6, MEK5 and Par-4. The details of how these adapters serve to link the aPKCs to different receptor signaling pathways and substrates in response to specific stimuli are crucial not only for developing an understanding of the roles and functions of the aPKCs themselves, but also for more generally establishing a view of how specificity in signal transduction is achieved.

In vivo 2-deoxyglucose administration preserves glucose and glutamate transport and mitochondrial function in cortical synaptic terminals after exposure to amyloid beta-peptide and iron: evidence for a stress response

Mild metabolic stress can increase resistance of neurons in the brain to subsequent more severe insults, as exemplified by the beneficial effects of heat shock and ischemic preconditioning. Studies of Alzheimer's disease and other age-related neurodegenerative disorders indicate that dysfunction and degeneration of synapses occur early in the cell death process, and that oxidative stress and mitochondrial dysfunction are central events in this pathological process. It was recently shown that administration of 2-deoxy-d-glucose (2DG), a nonmetabolizable glucose analog that induces metabolic stress, to rats and mice can increase resistance of neurons in the brain to excitotoxic, ischemic, and oxidative injury. We now report that administration of 2DG to adult rats (daily i.p. injections of 100 mg/kg body weight) increases resistance of synaptic terminals to dysfunction and degeneration induced by amyloid beta-peptide and ferrous iron, an oxidative insult. The magnitude of impairment of glucose and glutamate transport induced by amyloid beta-peptide and iron was significantly reduced in cortical synaptosomes from 2DG-treated rats compared to saline-treated control rats. Mitochondrial dysfunction, as indicated by increased levels of reactive oxygen species and decreased transmembrane potential, was significantly attenuated after exposure to amyloid beta-peptide and iron in synaptosomes from 2DG-treated rats. Levels of the stress proteins HSP-70 and GRP-78 were increased in synaptosomes from 2DG-treated rats, suggesting a mechanism whereby 2DG protects synaptic terminals. We conclude that 2DG bolsters cytoprotective mechanisms within synaptic terminals, suggesting novel preventative and therapeutic approaches for neurodegenerative disorders.

Apoptosis in neurodegenerative disorders

Neuronal death underlies the symptoms of many human neurological disorders, including Alzheimer's, Parkinson's and Huntington's diseases, stroke, and amyotrophic lateral sclerosis. The identification of specific genetic and environmental factors responsible for these diseases has bolstered evidence for a shared pathway of neuronal death--apoptosis--involving oxidative stress, perturbed calcium homeostasis, mitochondrial dysfunction and activation of cysteine proteases called caspases. These death cascades are counteracted by survival signals, which suppress oxyradicals and stabilize calcium homeostasis and mitochondrial function. With the identification of mechanisms that either promote or prevent neuronal apoptosis come new approaches for preventing and treating neurodegenerative disorders.

Unique structural and functional properties of the ATP-binding domain of atypical protein kinase C-iota

Atypical protein kinase C-iota (aPKCiota) plays an important role in mitogenic signaling, actin cytoskeleton organization, and cell survival. Apart from the differences in the regulatory domain, the catalytic domain of aPKCiota differs considerably from other known kinases, because it contains a modification within the glycine-rich loop motif (GXGXXG) that is found in the nucleotide-binding fold of virtually all nucleotide-binding proteins including PKCs, Ras, adenylate kinase, and the mitochondrial F1-ATPase. We have used site-directed mutagenesis and kinetic analysis to investigate whether these sequence differences affect the nucleotide binding properties and catalytic activity of aPKCiota. When lysine 274, a residue essential for ATP binding and activity conserved in most protein kinases, was replaced by arginine (K274R mutant), aPKCiota retained its normal kinase activity. This is in sharp contrast to results published for any other PKC or even distantly related kinases like phosphoinositide 3-kinase gamma, where the same mutation completely abrogated the kinase activity. Furthermore, the sensitivity of aPKCiota for inhibition by GF109203X, a substance acting on the ATP-binding site, was not altered in the K274R mutant. In contrast, replacement of Lys-274 by tryptophan (K274W) completely abolished the kinase activity of PKCiota. In accordance with results obtained with other kinase-defective PKC mutants, in cultured cells aPKCiota-K274W acted in a dominant negative fashion on signal transduction pathways involving endogenous aPKCiota, whereas the effect of the catalytically active K274R mutant was identical to the wild type enzyme. In summary, aPKCiota differs from classical and novel PKCs also in the catalytic domain. This information could be of significant value for the development of specific inhibitors of aPKCiota as a key factor in central signaling pathways.

Protein kinase C iota protects neural cells against apoptosis induced by amyloid beta-peptide

Protein kinase C (PKC) isoforms are increasingly recognized as playing important roles in the regulation of neuronal plasticity and survival. Recent findings from studies of non-neuronal cells suggest that atypical isoforms of PKC can modulate apoptosis in various paradigms. Because increasing data support a role for neuronal apoptosis in the pathogenesis of Alzheimer's disease (AD), we tested the hypothesis that PKCiota (PKCiota) can modify vulnerability of neural cells to apoptosis induced by amyloid beta-peptide (ABP), a cytotoxic peptide linked to neuronal degeneration in AD. Overexpression of PKCiota increased the resistance of PC12 cells to apoptosis induced by ABP. Associated with the increased resistance to apoptosis were improved mitochondrial function and reduced activity of caspases. In addition, ABP-induced increases in levels of oxidative stress and intracellular calcium levels were attenuated in cells overexpressing PKCiota. These findings suggest that PKCiota prevents apoptosis induced by ABP by interrupting the cell death process at a very early step, thereby allowing the cells to maintain ion homeostasis and mitochondrial function.

Apoptosis in neural development and disease

Cell death via apoptosis is a prominent feature in mammalian neural development. Recent studies into the basic mechanism of apoptosis have revealed biochemical pathways that control and execute apoptosis in mammalian cells. Protein factors in these pathways play important roles during development in regulating the balance between neuronal life and death. Additionally, mounting evidence indicates such pathways may also be activated during several neurodegenerative diseases, resulting in improper loss of neurons.

Participation of par-4 in the degeneration of striatal neurons induced by metabolic compromise with 3-nitropropionic acid

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by chorea, psychiatric disturbances, and dementia. It is caused by a polyglutamine repeat expansion in the huntingtin protein. The striatum is a major site of neuronal loss in HD, but the mechanisms underlying the neurodegenerative process have not been established. Systemic administration of the succinate dehydrogenase inhibitor 3-nitropropionic acid (3NP) to rodents results in motor dysfunction and degeneration of striatal neurons with features similar to those of HD. Here we report that levels of prostate apoptosis response-4 (Par-4; a protein recently linked to neuronal apoptosis) increase in striatum, and to a lesser extent in cortex and hippocampus, after systemic administration of 3NP to adult rats. The increase in Par-4 levels occurred within 6 h of 3NP administration and was followed by an increase in caspase activation which preceded neuronal loss. Exposure of cultured primary striatal neurons to 3NP induced a rapid increase of Par-4 levels and caspase activation. Treatment of striatal neurons with a Par-4 antisense oligonucleotide blocked Par-4 induction by 3NP, suppressed caspase activation, and attenuated neuronal apoptosis. The caspase-3 inhibitor DEVD suppressed 3NP-induced apoptosis of striatal neurons, but did not prevent induction of Par-4, indicating that Par-4 acts upstream of caspase-3 activation in the cell death pathway. Our results suggest that Par-4 plays an important role in the degeneration of striatal neurons in an experimental model of HD.

Par-4 induces cholinergic hypoactivity by suppressing ChAT protein synthesis and inhibiting NGF-inducibility of ChAT activity

Profound reductions in choline acetyl-transferase (ChAT) activity are reliable markers for cholinergic hypoactivity associated with cognitive function deficit in Alzheimer's disease (AD). Par-4 (prostate apoptosis response-4) is a novel mediator of neuronal apoptosis associated with the pathogenesis of AD. Par-4 contains a leucine zipper domain (Leu.zip) that presumably mediates protein-protein interactions critical for its functions in apoptosis. Par-4 activity can be effectively blocked by overexpression of Leu. zip because it exerts a dominant negative action possibly by competitively blocking the interaction of Par-4 with other proteins. Whether Par-4 participates in regulation of cholinergic signaling has not been determined. We report that overexpression of Par-4 results in apoptotic and non-apoptotic reductions in ChAT activity in transfected PC12 cells following exposure to a toxic concentration (50 microM) of aggregated amyloid beta peptide 1-42 (Abeta 1-42) and a non-toxic concentration (1 microM) of soluble Abeta 1-42, respectively. Non-apoptotic reduction in ChAT activity induced by Par-4 can be completely blocked by co-overexpression of Leu.zip, indicating that enhanced Par-4 activity is a necessary event for cholinergic hypoactivity in PC12 cells. Further studies found that Par-4 induces non-apoptotic reduction in ChAT activity by: (1) reducing ChAT protein levels following exposure to non-toxic concentration of Abeta, and (2) blocking the cellular capability to increase ChAT activity following exposure to nerve growth factor (NGF). The role of Par-4 in inducing cholinergic hypoactivity may have significant implications in the understanding and the treatment of memory impairment in AD.

Pro-apoptotic action of PAR-4 involves inhibition of NF-kappaB activity and suppression of BCL-2 expression

Par-4(1) (prostate apoptosis response 4) is known to function at an early stage in apoptosis in several different cell types, including neurons. On the other hand, activation of the transcription factor NF-kappaB can prevent apoptosis in various cancer cells and neurons. We now report that overexpression of full-length Par-4 in cultured PC12 cells results in a suppression of basal NF-kappaB DNA-binding activity and NF-kappaB activation following trophic factor withdrawal (TFW). The decreased NF-kappaB activity is correlated with enhanced apoptosis. Conversely, NF-kappaB activity is increased and vulnerability to apoptosis reduced in cells overexpressing a dominant-negative form of Par-4. Par-4 overexpression or functional blockade had no effect on AP-1 DNA-binding activity. Expression of the antiapoptotic protein Bcl-2 was dramatically reduced in PC12 cells overexpressing Par-4. Our data suggest that suppression of NF-kappaB activation plays a major role in the proapoptotic function of Par-4.

The catalytic subunit of telomerase protects neurons against amyloid beta-peptide-induced apoptosis

The catalytic subunit of telomerase (TERT) is a specialized reverse transcriptase that has been associated with cell immortalization and cancer. It was reported recently that TERT is expressed in neurons throughout the brain in embryonic and early postnatal development, but is absent from neurons in the adult brain. We now report that suppression of TERT levels and function in embryonic mouse hippocampal neurons in culture using antisense technology and the telomerase inhibitor 3' -azido-2' 3' -dideoxythymidine significantly increases their vulnerability to cell death induced by amyloid beta-peptide, a neurotoxic protein believed to promote neuronal degeneration in Alzheimer's disease. Neurons in which TERT levels were reduced exhibited increased levels of oxidative stress and mitochondrial dysfunction following exposure to amyloid beta-peptide. Overexpression of TERT in pheochromocytoma cells resulted in decreased vulnerability to amyloid beta-peptide-induced apoptosis. Our findings demonstrate a neuroprotective function of TERT in an experimental model relevant to Alzheimer's disease, and suggest the possibility that restoration of TERT expression in neurons in the adult brain may protect against age-related neurodegeneration.