Increased vulnerability of hippocampal neurons from presenilin-1 mutant knock-in mice to amyloid beta-peptide toxicity: central roles of superoxide production and caspase activation

Alzheimer's disease: role of aging in pathogenesis

Alzheimer's disease (AD) is characterized by intraneuronal fibrillary tangles, plaques, and cell loss. Brain lesions in both sporadic AD (SAD) and familial AD (FAD) are the same, and in the same distribution pattern, as those in individuals with Down syndrome (DS) and in smaller numbers in nondemented older individuals. Dementia onset is around 40 years for DS, 40-60 years for FAD, and usually over 60 years for SAD. The different categories of AD may be due to processes that augment to different degrees the innate cellular aging rate, that is, mitochondrial superoxide radical (SO) formation. Thus, they increase the rate of accumulation of AD lesions. This lowers the age of onset into the dementia ranges associated with DS, FAD, and SAD, and concomitantly shortens life spans. Faster aging lowers AD onset age by decreasing the onset age for neurofibrillary tangle formation and neuronal loss, and the age when brain intercellular H2O2 can activate microglial cells. The early AD onset in DS is attributed to a defective mitochondrial complex 1. The proteins associated with FAD and their normal counterparts undergo proteolytic processing in the endoplasmic reticulum (ER). The mutated compounds increase the ratio of betaA42 to betaA40 and likely also down-regulate the ER calcium (Ca2+) buffering activity. Decreases in ER Ca2+ content should increase the mitochondrial Ca2+ pool, thus enhancing SO formation. SAD may be due to increased SO formation caused by mutations in the approximately 1000 genes involved in mitochondrial biogenesis and function. The hypothesis suggests measures to prevent and treat.

Induction of cytochrome c-mediated apoptosis by amyloid beta 25-35 requires functional mitochondria

Accumulating data suggest a central role for mitochondria and oxidative stress in neurodegenerative apoptosis. We previously demonstrated that amyloid-beta peptide 25-35 (Abeta 25-35) toxicity in cultured cells is mediated by its effects on functioning mitochondria. In this study, we further explored the hypothesis that Abeta 25-35 might induce apoptotic cell death by altering mitochondrial physiology. Mitochondria in Ntera2 (NT2 rho+) human teratocarcinoma cells exposed to either staurosporine (STS) or Abeta 25-35 were found to release cytochrome c, with subsequent activation of caspases 9 and 3. However, NT2 cells depleted of mitochondrial DNA (rho0 cells), which maintain a normal mitochondrial membrane potential (Deltapsi(m)) despite the absence of a functional electron transport chain (ETC), demonstrated cytochrome c release and caspase activation only with STS. We further observed increased reactive oxygen species (ROS) production and decreased reduced glutathione (GSH) levels in rho+ and rho0 cells treated with STS, but only in rho+ cells treated with Abeta 25-35. We conclude that under in vitro conditions, Abeta can induce oxidative stress and apoptosis only when a functional mitochondrial ETC is present.

Presenilin-1 protects against neuronal apoptosis caused by its interacting protein PAG

Mutations in the presenilin-1 (PS-1) gene account for a significant fraction of familial Alzheimer's disease. The biological function of PS-1 is not well understood. We report here that the proliferation-associated gene (PAG) product, a protein of the thioredoxin peroxidase family, interacts with PS-1. Microinjection of a plasmid expressing PAG into superior cervical ganglion (SCG) sympathetic neurons in primary cultures led to apoptosis. Microinjection of plasmids expressing wild-type PS-1 or a PS-1 mutant with a deletion of exon 10 (PS1dE10) by themselves had no effect on the survival of primary SCG neurons. However, co-injection of wild-type PS-1 with PAG prevented neuronal death, whereas co-injection with the mutant PS-1 did not affect PAG-induced apoptosis. Furthermore, overexpression of PAG accelerated SCG neuronal death induced by nerve growth factor deprivation. This sensitizing effect was also blocked by wild-type PS-1, but not by PS1dE10. These results establish an assay for studying the function of PS-1 in primary neurons, reveal the neurotoxicity of a thioredoxin peroxidase, demonstrate a neuroprotective activity of the wild-type PS-1, and suggest possible involvement of defective neuroprotection by PS-1 mutants in neurodegeneration.

Generation of genetically altered mouse models for aging studies

A number of mouse models have been identified and are being used for aging and age-associated disease research. However, the use of the genetically manipulated mouse model is still a relatively untapped resource for the study of the biology of aging. Genetically altered mice can be powerful tools for biology of aging research because gene expression can be controlled and correlated with established biomarkers. Standard transgene overexpression and gene targeting techniques were modified and used to generate 30 mouse lines during a 4-year period. These lines include models of Werner's syndrome (premature aging or progeria), Alzheimer's disease, other neurodegenerative condition, atherosclerosis, diabetes, immune dysfunction, musculoskeletal disorders, and oxidative stress. These new mouse models are providing additional insights into aging processes and will be useful for developing intervention strategies and collaborative interactions.

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.

Caspase cleavage of exon 9 deleted presenilin-1 is an early event in apoptosis induced by calcium ionophore A 23187 in SH-SY5Y neuroblastoma cells

Presenilins (PSs) are mutated in a majority of familial Alzheimer disease (FAD) cases. Mutated PSs may cause FAD by a number of pro-apoptotic mechanisms, or by regulating gamma-secretase activity, a protease involved in beta-amyloid precursor protein processing to the neurotoxic beta-amyloid peptide. Besides their normal endoproteolytic processing, PSs are substrates for caspases, being cleaved to alternative N-terminal and C-terminal fragments. So far little is known about the role of PSs cleavage in the apoptotic machinery. Here, we used SH-SY5Y neuroblastoma cells stably transfected with wild-type or exon 9 deleted presenilin 1 (PS1) in a time-course study after the exposure to the calcium ionophore A23187. During and after exposure to A 23187, intracellular calcium levels were higher in exon 9 deleted PS1 cells as compared with non-transfected and wild-type PS1 transfected cells. Cell death and the enrichment of apoptotic cells after A23187 exposure were increased by overexpression of exon 9 deleted PS1 as compared with the control cell lines. Wild-type PS1 cells were compared with exon 9 deleted PS1 cells and the temporal relationship between PS1 and other caspase substrates cleavages was analyzed. Exon 9 deleted PS1 cells exhibited a higher caspase-3 activation and a greater cleavage of PS1 and poly(ADP-ribose) polymerase (PARP) compared with wild-type PS1 cells. Exon 9 deleted PS1 cleavage occurred earlier than other caspase substrate cleavages (i.e., PARP and gelsolin), simultaneous with minimum detectable caspase-3 activation. Therefore, alternative cleavage of PS1 may play an important role for the regulation of the proteolytic cascade activated during apoptosis.

Posttranscriptional regulation of gene expression in learning by the neuronal ELAV-like mRNA-stabilizing proteins

The view that memory is encoded by variations in the strength of synapses implies that long-term biochemical changes take place within subcellular microdomains of neurons. These changes are thought ultimately to be an effect of transcriptional regulation of specific genes. Localized changes, however, cannot be fully explained by a purely transcriptional control of gene expression. The neuron-specific ELAV-like HuB, HuC, and HuD RNA-binding proteins act posttranscriptionally by binding to adenine- and uridine-rich elements (AREs) in the 3' untranslated region of a set of target mRNAs, and by increasing mRNA cytoplasmic stability and/or rate of translation. Here we show that neuronal ELAV-like genes undergo a sustained up-regulation in hippocampal pyramidal cells only of mice and rats that have learned a spatial discrimination paradigm. This learning-specific increase of ELAV-like proteins was localized within cytoplasmic compartments of the somata and proximal dendrites and was associated with the cytoskeleton. This increase was also accompanied by enhanced expression of the GAP-43 gene, known to be regulated mainly posttranscriptionally and whose mRNA is demonstrated here to be an in vivo ELAV-like target. Antisense-mediated knockdown of HuC impaired spatial learning performance in mice and induced a concomitant down-regulation of GAP-43 expression. Neuronal ELAV-like proteins could exert learning-induced posttranscriptional control of an array of target genes uniquely suited to subserve substrates of memory storage.

Deficiency of presenilin-1 increases calcium-dependent vulnerability of neurons to oxidative stress in vitro

We examined the function of presenilin-1 (PS1) on neuronal resistance to oxidative stress. CNS neurons cultured from PS1-deficient mice exhibited increased vulnerability to H2O2 treatment compared with those from wild-type mice. Antioxidants protected the cultured neurons against the oxidative stress. An intracellular calcium chelator, BAPTA AM, as well as an L-type voltage-dependent calcium channel blocker, nifedipine, rescued the neurons from H2O2-induced death, while an N-type voltage-dependent calcium channel blocker, omega-conotoxin, or calcium release blockers from ER stores, dantrolene and xestospongin C, failed to rescue them. Wild-type and PS1-deficient neurons showed comparable increases of cytoplasmic free calcium levels after exposure to H2O2. Taken together with the data that PS1-deficient neurons exhibited increased vulnerability to glutamate, these findings imply that PS1 confers resistance to oxidative stress on neurons in calcium-dependent manners.

Induction of NADPH cytochrome P450 reductase by the Alzheimer beta-protein. Amyloid as a "foreign body"

A large body of data suggests that the Alzheimer's amyloid peptide (Abeta) causes degeneration and death of neurons by mechanisms that involve reactive oxygen species. The pathways involved in Abeta-mediated oxidative injury are only partially understood. We theorized that abnormal microaggregates and/or pathological conformations of Abeta peptides may behave as xenobiotics and trigger the induction of NADPH cytochrome P450 reductase (CP450r), an enzyme which, if induced by non-physiological substrates (such as xenobiotics like drugs or other 'foreign molecules'), is known to cause oxidative stress. In order to test this hypothesis, i.e. that Abeta can increase the expression of CP450r, SK-N-SH human neuroblastoma cells were exposed to Abeta25-35 and Abeta1-42 and then examined for induction of this enzyme in immunoblots, using specific antibodies. Following exposure to Abeta peptides, neuroblastoma cells showed a clear-cut induction of CP450r. To determine whether this mechanism is operational in vivo, we investigated the expression of CP450r in a transgenic mouse model of Alzheimer's disease (AD) and in brains from patients afflicted with AD, using an immunocytochemical approach. Tissue sections from brains of transgenic mice exhibited strong immunoreactivity for CP450r, surrounding amyloid deposits. The pattern of expression of CP450r was similar to that exhibited by neuritic and oxidative stress markers. Sections from non-transgenic mice showed no detectable immunoreactivity. Immunostaining of sections from four brains with neuropathologically confirmed AD showed a pattern of abnormality different from transgenic mice that was characterized by abnormal immunoreactivity for CP450r within the cytoplasm of cortical neurons. No labeling was seen in sections from aged-matched control brains. The data showed that CP450r is induced by Alzheimer amyloid peptide and that such a response must be considered as one possible mechanism whereby Abeta causes oxidative stress.

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.

Role of oxidative stress in the apoptotic cell death of cultured cerebellar granule neurons

When cultured cerebellar granule neurons (CGN) are transferred from 25 mM KCl (K25) to 5 mM KCl (K5) caspase-3 and caspase-8, but not caspase-1 or caspase-9,activities are induced and cells die apoptotically. CGN death was triggered by a [Ca(2+)](i) modification when [Ca(2+)](i) was reduced from 300 nM to 50 nM in a K5 medium. The [Ca(2+)](i) changes were followed by an increase in ROS levels. The generation of both cytosolic and mitochondrial reactive oxygen species (ROS) occurred at three different times, 10 min, 30 min and 3--4 hr but only those ROS produced after 3--4 hr are involved in the process of cell death. When CGN cultured in a K5 medium are treated with different antioxidants like scavengers of ROS (mannitol, DMSO) or antioxidant enzymes (superoxide dismutase and catalase) phosphatidylserine translocation, caspase activity, chromatin condensation and cell death is markedly diminished. The protective effect of antioxidants is not mediated through a modification in [Ca(2+)](i). Caspase activation, PS translocation and chromatin condensation were downstream of ROS production. In contrast to H(2)O(2), ROS produced by a xanthine/xanthine oxidase system in CGN cultured in K25 were able to directly induce caspase-3 activation and death that resulted sensitive to z-VAD, a caspase inhibitor. These findings indicate that a reduction in [Ca(2+)](i) triggers CGN death by inducing a generation of ROS after 3--4 hr, which could play a critical role in the initial phases of the apoptotic process including PS translocation, chromatin condensation and the activation of initiator and executor caspases.

Elevated vulnerability to oxidative stress-induced cell death and activation of caspase-3 by the Swedish amyloid precursor protein mutation

The Swedish double mutation (KM670/671NL) of amyloid precursor protein (APPsw) is associated with early-onset familial Alzheimer's disease (FAD) and results in from three- to sixfold increased beta-amyloid production. The goal of the present study was to elucidate the effects of APPsw on mechanisms of apoptotic cell death. Therefore, PC12 cells were stably transfected with human APPsw. Here we report that the vulnerability of APPsw-bearing PC12 cells to undergo apoptotic cell death was significantly enhanced after exposure to hydrogen peroxide compared to human wild-type APP-bearing cells, empty vector-transfected cells, and parent untransfected cells. In addition, we have analyzed the potential influence of several mechanisms that can interfere with the execution of the apoptotic cell death program: the inhibition of cell death by the use of caspase inhibitors and the reduction of oxidative stress by the use of (+/-)-alpha-tocopherol (vitamin E). Interestingly, oxidative stress-induced cell death was significantly attenuated in APPsw PC12 cells by pretreatment with caspase-3 inhibitors but not with caspase-1 inhibitors. In parallel, caspase-3 activity was markedly elevated in APPsw PC12 after stimulation with hydrogen peroxide for 6 hr, whereas caspase-1 activity was unaltered. In addition, oxidative stress-induced cell death could be reduced after pretreatment of APPsw cells with (+/-)-alpha-tocopherol. The protective potency of (+/-)-alpha-tocopherol was even greater than that of caspase-3 inhibitors. Our findings further emphasize the role of mutations in the amyloid precursor protein in apoptotic cell death and may provide the fundamental basis for further efforts to elucidate the underlying processes caused by FAD-related mutations.

Alzheimer's disease-like alterations in peripheral cells from presenilin-1 transgenic mice

Many cases of early-onset inherited Alzheimer's disease (AD) are caused by mutations in the presenilin-1 (PS1) gene. Expression of PS1 mutations in cell culture systems and in primary neurons from transgenic mice increases their vulnerability to cell death. Interestingly, enhanced vulnerability to cell death has also been demonstrated for peripheral lymphocytes from AD patients. We now report that lymphocytes from PS1 mutant transgenic mice show a similar hypersensitivity to cell death as do peripheral cells from AD patients and several cell culture systems expressing PS1 mutations. The cell death-enhancing action of mutant PS1 was associated with increased production of reactive oxygen species and altered calcium regulation, but not with changes of mitochondrial cytochrome c. Our study further emphasizes the pathogenic role of mutant PS1 and may provide the fundamental basis for new efforts to close the gap between studies using neuronal cell lines transfected with mutant PS1, neurons from transgenic animals, and peripheral cells from AD patients.

Elevated levels of fragmented DNA nucleosomes in native and activated lymphocytes indicate an enhanced sensitivity to apoptosis in sporadic Alzheimer's disease. Specific differences to vascular dementia

Apoptotic cell death is thought to be the most likely mechanism of cell death contributing to neurodegeneration in Alzheimer's disease (AD). Here, we provide evidence that in sporadic AD cases the vulnerability of peripheral cells to undergo apoptosis is increased compared to non-demented elderly controls and, very importantly, to patients with subcortical vascular encephalopathy (SVE) as another, but demented control group. Quiescent 'native' and 'activated' lymphocytes from AD patients that were predisposed to commit apoptotic cell death by priming the cells with interleukin-2, are shown to accumulate apoptosing cells to a significantly higher extent in spontaneous and in oxidative stress-induced in vitro apoptosis. Our results demonstrate robust differences in cell death sensitivity between AD and vascular dementia. In none of the conditions investigated, lymphocytes from SVE patients were significantly different from non-demented controls. The comparable findings of a higher extent of apoptotic features in neurons and in peripheral blood cells of AD patients are remarkable and may suggest a rather general modulation of apoptotic mechanisms by the disease, which even can be picked up at the level of peripheral lymphocytes under specific in vitro conditions.

Peroxidative stress selectively down-regulates the neuronal stress response activated under conditions of endoplasmic reticulum dysfunction

Oxidative stress has been implicated in mechanisms leading to neuronal cell injury in various pathological states of the brain. Here, we investigated the effect of peroxide exposure on the expression of genes coding for cytoplasmic and endoplasmic reticulum (ER) stress proteins. Primary neuronal cell cultures were exposed to H(2)O(2) for 6 h and mRNA levels of hsp70, grp78, grp94, gadd153 were evaluated by quantitative PCR. In addition, peroxide-induced changes in protein synthesis and cell viability were investigated. Peroxide treatment of cells triggered an almost 12-fold increase in hsp70 mRNA levels, but a significant decrease in grp78, grp94 and gadd153 mRNA levels. To establish whether peroxide exposure blocks the ER-resident stress response, cells were also exposed to thapsigargin (Tg, a specific inhibitor of ER Ca(2+)-ATPase) which has been shown to elicit the ER stress response. Tg exposure induced 7.2-fold, 3.6-fold and 8.8-fold increase in grp78, grp94 and gadd153 mRNA levels, respectively. However, after peroxide pre-exposure, the Tg-induced effect on grp78, grp94 and gadd153 mRNA levels was completely blocked. The results indicate that oxidative damage causes a selective down-regulation of the neuronal stress response activated under conditions of ER dysfunction. This down-regulation was only observed in cultures exposed to peroxide levels which induced severe suppression of protein synthesis and cell injury, implying a causative link between peroxide-induced down-regulation of ER stress response system and development of neuronal cell injury. These observations could have implications for our understanding of the mechanisms underlying neuronal cell injury in pathological states of the brain associated with oxidative damage, including Alzheimer's disease where the neuronal stress response activated under conditions of ER dysfunction has been shown to be down-regulated. Down-regulation of ER stress response may increase the sensitivity of neurones to an otherwise nonlethal form of stress.

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.

Presenilin-1 P264L knock-in mutation: differential effects on abeta production, amyloid deposition, and neuronal vulnerability

The pathogenic mechanism linking presenilin-1 (PS-1) gene mutations to familial Alzheimer's disease (FAD) is uncertain, but has been proposed to include increased neuronal sensitivity to degeneration and enhanced amyloidogenic processing of the beta-amyloid precursor protein (APP). We investigated this issue by using gene targeting with the Cre-lox system to introduce an FAD-linked P264L mutation into the endogenous mouse PS-1 gene, an approach that maintains normal regulatory controls over expression. Primary cortical neurons derived from PS-1 homozygous mutant knock-in mice exhibit basal neurodegeneration similar to their PS-1 wild-type counterparts. Staurosporine and Abeta1-42 induce apoptosis, and neither the dose dependence nor maximal extent of cell death is altered by the PS-1 knock-in mutation. Similarly, glutamate-induced neuronal necrosis is unaffected by the PS-1P264L mutation. The lack of effect of the PS-1P264L mutation is confirmed by measures of basal- and toxin-induced caspase and calpain activation, biochemical indices of apoptotic and necrotic signaling, respectively. To analyze the influence of the PS-1P264L knock-in mutation on APP processing and the development of AD-type neuropathology, we created mouse lines carrying mutations in both PS-1 and APP. In contrast to the lack of effect on neuronal vulnerability, cortical neurons cultured from PS-1P264L homozygous mutant mice secrete Abeta42 at an increased rate, whereas secretion of Abeta40 is reduced. Moreover, the PS-1 knock-in mutation selectively increases Abeta42 levels in the mouse brain and accelerates the onset of amyloid deposition and its attendant reactive gliosis, even as a single mutant allele. We conclude that expression of an FAD-linked mutant PS-1 at normal levels does not generally increase cortical neuronal sensitivity to degeneration. Instead, enhanced amyloidogenic processing of APP likely is critical to the pathogenesis of PS-1-linked FAD.

The regulation of presenilin-1 by nerve growth factor

Presenilin-1 (PS1) protein concentration is linked to neuronal development and to the pathogenesis of Alzheimer's disease, yet little is known about the biological factors and mechanisms that control cellular levels of PS1 protein. As PS1 levels are highest in the developing brain, we tested whether neurotrophin-induced differentiation influences PS1 expression using neuronotypic pheochromocytoma (PC12) cells. Treatment of PC12 cells with nerve growth factor (NGF) caused approximately 60-75% increases in the steady-state levels of endogenous PS1 N- and C-terminal fragments. PS1 protein accumulation was dose-responsive to NGF and required the presence of the TrkA NGF receptor tyrosine kinase. NGF also induced PS1 fragment accumulation in cultured explants of rat dorsal root ganglia. Quantitative northern blot analysis using PC12 cultures indicated that NGF did not increase steady-state PS1 mRNA levels. However, pulse-chase experiments indicated that NGF slowed the degradation rate of endogenous PS1 fragments, increasing the half-life from t(1/2) @22.5 to @25.0 h. This increase in half-life was insufficient to account for the approximately 60-75% increase in PS1 fragment levels measured in NGF-treated cells. Thus, NGF may regulate PS1 protein concentration in NGF-responsive cells by a complex mechanism that increases PS1 fragment production independent of holoprotein synthesis.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Cell death mechanisms in neurodegeneration

Progressive cell loss in specific neuronal populations often associated with typical cytoskeletal protein aggregations is a pathological hallmark of neurodegenerative disorders, but the nature, time course and molecular causes of cell death and their relation to cytoskeletal pathologies are still unresolved. Apoptosis or alternative pathways of cell death have been discussed in Alzheimer's disease and other neurodegenerative disorders. Apoptotic DNA fragmentation in human brain as a sign of neuronal injury is found too frequent as to account for continuous neuron loss in these slowly progressive processes. Morphological studies revealed extremely rare apoptotic neuronal death in Alzheimer's disease but yielded mixed results for Parkinson's disease and other neurodegenerative disorders. Based on recent data in human brain, as well as in animal and cell culture models, a picture is beginning to emerge suggesting that, in addition to apoptosis, other forms of programmed cell death may participate in neurodegeneration. Better understanding of the molecular players will further elucidate the mechanisms of cell death in these disorders and their relations to cytoskeletal abnormalities. Susceptible cell populations in a proapoptotic environment show increased vulnerability towards multiple noxious factors discussed in the pathogenesis of neurodegeneration. In conclusion, although many in vivo and in vitro data are in favor of apoptosis involvement in neurodegenerative processes, there is considerable evidence that very complex events may contribute to neuronal death with possible repair mechanisms, the elucidation of which may prove useful for future prevention and therapy of neurodegenerative disorders.

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.

Reduced antioxidant enzyme activity in brains of mice transgenic for human presenilin-1 with single or multiple mutations

Alzheimer's disease-related mutations in the presenilin-1 gene (PS1) are leading to an elevated production of neurotoxic beta-amyloid 1-42 and may additionally enhance oxidative stress. Here, we provide in vivo evidence indicating that brains of transgenic mice expressing different human Alzheimer-linked PS1 mutations exhibit a reduced activity of two antioxidant enzymes. For this purpose, mice transgenic for human PS1 and for single and multiple PS1 mutations were generated. Mice with multiple PS1 mutations showed a significantly decreased activity of the antioxidant enzymes Cu/Zn superoxide dismutase and glutathione reductase already at an age of 3-4 months. As expected, this effect was less pronounced for the mice with a single PS1 mutation. By contrast, animals bearing normal human PS1 showed significantly elevated enzyme activities relative to non-transgenic littermate controls.

Neurons bearing presenilins: weapons for defense or suicide?

Apoptotic machinery designed for cell's organized self-destruction involve different systems of proteases which cleave vital proteins and disassemble nuclear and cytoplasmic structures, committing the cell to death. The most studied apoptotic proteolytic system is the caspase family, but calpains and the proteasome could play important roles as well. Alzheimer's disease associated presenilins showed to be a substrate for such proteolytic systems, being processed early in several apoptotic models, and recent data suggest that alternative presenilin fragments could regulate cell survival. Mutations in genes encoding presenilins proved to sensitize neurons to apoptosis by different mechanisms e.g. increased caspase-3 activation, oxyradicals production and calcium signaling dysregulation. Here we review the data involving presenilins in apoptosis and discuss a possible role of presenilins in the regulation of apoptotic biochemical machinery.

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.

Alzheimer's disease: A hypothesis on pathogenesis

Alzheimer's disease (AD) is the major cause of dementia. It is a systemic disorder whose major manifestations are in the brain. AD cases can be categorized into two groups on the basis of the age of onset-before or after about age 60. The majority of cases, 90-95 percent, are in the late onset category. Early onset cases are largely, if not all, familial (FAD). These are caused by mutations in the genes for the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). In contrast late onset cases are mainly sporadic. The disorder is characterized by intraneuronal fibrillary tangles, plaques, and cell loss. The brain lesions in both early and late-onset AD are the same, and in the same distribution pattern, as those seen in individuals with Down's syndrome (DS) and in smaller numbers in normal older individuals. Extensive studies of AD have yet to result in a generally accepted hypothesis on the pathogenesis of the disorder. Major emphasis has been placed on the role of amyloid, the neurotoxin formed by the action of free radicals on preamyloid. The observation that AD lesions are frequently present in normal older individuals prompted the hypothesis that AD is the result of faster than normal aging of the neurons associated with it. This hypothesis provides plausible explanations for FAD and AD. FAD is associated with mutations in APP, PS1, and PS2. These substances, along with their normal counterparts, undergo proteolytic processing in the endoplasmic reticulum (ER). The mutated compounds, aside from increasing the ratio of βA42 to βA40, may down-regulate the calcium buffering activity of the ER in a manner akin to one or more of the many compounds known to do so. Decreases in the ER calcium pool would cause compensatory increases in other calcium pools, particularly in mitochondria. Increases in mitochondrial calcium levels are associated with enhanced formation of superoxide radical formation, and hence of the rate of aging. SAD may be caused by nuclear and/or mitochondrial DNA mutations beginning early in life that enhance mitochondrial superoxide radical formation in the neurons associated with the disorder. The above explanations for FAD and AD are suggestive of measures to prevent and for treatment.

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.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.

Amyloid beta-induced neuronal death is bax-dependent but caspase-independent

Fibrillar amyloid beta (Abeta) peptides are major constituents of senile plaques in Alzheimer disease (AD) brain and cause neuronal apoptosis in vitro. Bax and caspase-3 have been implicated in the pathogenesis of AD and are components of a well-defined molecular pathway of neuronal apoptosis. To determine whether Abeta-induced neuronal apoptosis involves bax and/or caspase-3 activation, we examined the effect of Abeta on wild-type, bax-deficient, and caspase-3-deficient telencephalic neurons in vitro. In wild-type cultures, Abeta produced time- and concentration-dependent caspase-3 activation, apoptotic nuclear changes, and neuronal death. These neurotoxic effects of Abeta were not observed in bax-deficient cultures. Caspase-3 deficiency, or pharmacological inhibition of caspase activity, prevented caspase-3 activation and blocked the appearance of apoptotic nuclear features but not Abeta-induced neuronal death. Neither calpain inhibition nor microtubule stabilization with Taxol protected telencephalic neurons from Abeta-induced caspase activation or apoptosis. These results have potential implications regarding the underlying pathophysiology of AD and towards AD treatment strategies.

Inherent abnormalities in energy metabolism in Alzheimer disease. Interaction with cerebrovascular compromise

Alzheimer disease (AD) is a form of the dementia syndrome. AD appears to have a variety of fundamental etiologies that lead to the neuropathological manifestations which define the disease. Patients who are at high risk to develop AD typically show impairments of cerebral metabolic rate in vivo even before they show any evidence of the clinical disease on neuropsychological, electrophysiological, and neuroimaging examinations. Therefore, impairment in energy metabolism in AD can not be attributed to loss of brain substance or to electrophysiological abnormalities. Among the characteristic abnormalities in the AD brain are deficiencies in several enzyme complexes which participate in the mitochondrial oxidation of substrates to yield energy. There include the pyruvate dehydrogenase complex (PDHC), the alpha-ketoglutarate dehydrogenase complex (KGDHC), and Complex IV of the electron transport chain (COX). The deficiency of KGDHC may be due to a mixture of causes including damage by free radicals and perhaps to genetic variation in the DLST gene encoding the core protein of this complex. Inherent impairment of glucose oxidation by the AD brain may reasonably be expected to interact synergistically with an impaired supply of oxygen and glucose to the AD brain, in causing brain damage. These considerations lead to the hypothesis that cerebrovascular compromise and inherent abnormalities in the brain's ability to oxidize substrates can interact to favor the development of AD, in individuals who are genetically predisposed to develop neuritic plaques.

Presenilins and Alzheimer's disease: biological functions and pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia in the elderly population. Dementia is associated with massive accumulation of fibrillary aggregates in various cortical and subcortical regions of the brain. These aggregates appear intracellularly as neurofibrillary tangles, extracellularly as amyloid plaques and perivascular amyloid in cerebral blood vessels. The causative factors in AD etiology implicate both, genetic and environmental factors. The large majority of early-onset familial Alzheimer's disease (FAD) cases are linked to mutations in the genes coding for presenilin 1 (PS1) and presenilin 2 (PS2). The corresponding proteins are 467 (PS1) and 448 (PS2) amino-acids long, respectively. Both are membrane proteins with multiple transmembrane regions. Presenilins show a high degree of conservation between species and a presenilin homologue with definite conservation of the hydrophobic structure has been identified even in the plant Arabidopsis thaliana. More than 50 missense mutations in PS1 and two missense mutations in PS2 were identified which are causative for FAD. PS mutations lead to the same functional consequence as mutations on amyloid precursor protein (APP), altering the processing of APP towards the release of the more amyloidogenic form 1-42 of Abeta (Abeta42). In this regard, the physical interaction between APP and presenilins in the endoplasmic reticulum has been demonstrated and might play a key role in Abeta42 production. It was hypothesized that PS1 might directly cleave APP. However, extracellular amyloidogenesis and Abeta production might not be the sole factor involved in AD pathology and several lines of evidence support a role of apoptosis in the massive neuronal loss observed. Presenilins were shown to modify the apoptotic response in several cellular systems including primary neuronal cultures. Some evidence is accumulating which points towards the beta-catenin signaling pathways to be causally involved in presenilin mediated cell death. Increased degradation of beta-catenin has been shown in brain of AD patients with PS1 mutations and reduced beta-catenin signaling increased neuronal vulnerability to apoptosis in cell culture models. The study of presenilin physiological functions and the pathological mechanisms underlying their role in pathogenesis clearly advanced our understanding of cellular mechanisms underlying the neuronal cell death and will contribute to the identification of novel drug targets for the treatment of AD.

Caspase-2 mediates neuronal cell death induced by beta-amyloid

beta-amyloid (Abeta) has been proposed to play a role in the pathogenesis of Alzheimer's disease (AD). Deposits of insoluble Abeta are found in the brains of patients with AD and are one of the pathological hallmarks of the disease. It has been proposed that Abeta induces death by oxidative stress, possibly through the generation of peroxynitrite from superoxide and nitric oxide. In our current study, treatment with nitric oxide generators protected against Abeta-induced death, whereas inhibition of nitric oxide synthase afforded no protection, suggesting that formation of peroxynitrite is not critical for Abeta-mediated death. Previous studies have shown that aggregated Abeta can induce caspase-dependent apoptosis in cultured neurons. In all of the neuronal populations studied here (hippocampal neurons, sympathetic neurons, and PC12 cells), cell death was blocked by the broad spectrum caspase inhibitor N-benzyloxycarbonyl-val-ala-asp-fluoromethyl ketone and more specifically by the downregulation of caspase-2 with antisense oligonucleotides. In contrast, downregulation of caspase-1 or caspase-3 did not block Abeta(1-42)-induced death. Neurons from caspase-2 null mice were totally resistant to Abeta(1-42) toxicity, confirming the importance of this caspase in Abeta-induced death. The results indicate that caspase-2 is necessary for Abeta(1-42)-induced apoptosis in vitro.

Apoptosis and the nervous system

Apoptosis is now recognized as a normal feature in the development of the nervous system and may also play a role in neurodegenerative diseases and aging. This phenomenon has been investigated intensively during the last 6-7 years, and the progress made in this field is reviewed here. Besides a few in vivo studies, a variety of neuronal preparations from various parts of the brain, the majority of which were primary cultures, and some cell lines have been investigated. Several apoptosis-inducing agents have been identified, and these include lack of neurotrophic support, neurotransmitters, neurotoxicants, modulators of protein phosphorylation and calcium homeostasis, DNA-damaging agents, oxidative stress, nitric oxide, and ceramides. The precise signaling cascade is not well established, and there are lacunae in many suggested pathways. However, it appears certain that the Bcl family of proteins is involved in the apoptotic pathway, and these proteins in turn affect the processing of interleukin-1beta converting enzyme (ICE)/caspases. The available evidence suggests that there may be several apoptotic pathways that may depend on the cell type and the inducing agent, and most of the pathways may converge at the ICE/caspases step.

The lipid peroxidation product 4-hydroxy-2,3-nonenal increases AP-1-binding activity through caspase activation in neurons

The transcription factor activator protein-1 (AP-1) is activated in response to physiological activity in neuronal circuits and in response to neuronal injury associated with various acute and chronic neurodegenerative conditions. The membrane lipid peroxidation product 4-hydroxy-2,3-nonenal (HNE) is increasingly implicated in the disruption of neuronal calcium homeostasis that occurs in various paradigms of neuronal excitotoxicity and apoptosis. The possible mechanistic links between lipid peroxidation and alterations in gene transcription during neuronal apoptosis have not previously been examined. We now report that exposure of cultured rat cortical neurons to an apoptotic concentration of HNE results in a large increase in AP-1 DNA-binding activity. The protein synthesis inhibitor cycloheximide blocked the induction of AP-1, consistent with a requirement for induction of expression of AP-1 family members. The broad-spectrum caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone and the caspase-3 inhibitor N-acetyl-Asp-Glu-Val-Asp-aldehyde blocked HNE-induced increases in AP-1 DNA-binding activity, demonstrating a requirement for caspase activation in the activation of AP-1. HNE induced phosphorylation of c-Jun N-terminal kinase (JNK), which was prevented by caspase inhibitors, indicating that HNE was acting at or upstream of JNK phosphorylation. The intracellular calcium chelator BAPTA-acetoxymethyl ester completely prevented stimulation of AP-1 DNA-binding by HNE, indicating a requirement for calcium. Moreover, agents that suppress mitochondrial calcium uptake (ruthenium red) and membrane permeability transition (cyclosporin A) attenuated AP-1 activation by HNE, suggesting a contribution of mitochondrial alterations to AP-1 activation. Collectively, our data suggest a scenario in which HNE disrupts neuronal calcium homeostasis and perturbs mitochondrial function, resulting in caspase activation. Activated caspases, in turn, induce activation of JNK, resulting in stimulation of AP-1 DNA-binding protein production. This transcriptional pathway induced by HNE may modulate the cell death process.

Arbiter of differentiation and death: Notch signaling meets apoptosis

Notch-ligand interactions are a highly conserved mechanism that regulates cell fate decisions. Over the past few years, numerous observations have shown that this mechanism operates to regulate cell differentiation in an enormous variety of developmental and cell maturation processes. Recent studies indicate that in addition to cell differentiation, Notch signaling has direct effects on proliferation and programmed cell death. The picture emerging from these findings suggests that, depending on cellular and developmental context, Notch signaling may function as a general "arbiter" of cell fate, regulating differentiation potential, rate of proliferation, and apoptotic cell death. In this review, we briefly summarize the current knowledge of the structure and function of Notch receptors and discuss the recent evidence that Notch signaling regulates apoptotic cell death. The possible mechanisms of this effect and its potential implications for developmental biology, immunobiology, neuropathology, and tumor biology are discussed.

Activation of caspase-3 in single neurons and autophagic granules of granulovacuolar degeneration in Alzheimer's disease. Evidence for apoptotic cell death

Neuronal loss is prominent in Alzheimer's disease (AD), and its mechanisms remain unresolved. Apoptotic cell death has been implicated on the basis of studies demonstrating DNA fragmentation and an up-regulation of proapoptotic proteins in the AD brain. However, DNA fragmentation in neurons is too frequent to account for the continuous neuronal loss in a degenerative disease extending over many years. Furthermore, the typical apoptotic morphology has not been convincingly documented in AD neurons with fragmented DNA. We report the detection of the activated form of caspase-3, the central effector enzyme of the apoptotic cascade, in AD and Down's syndrome (DS) brain using an affinity-purified antiserum. In AD and DS, single neurons with apoptotic morphology showed cytoplasmic immunoreactivity for activated caspase-3, whereas no neurons were labeled in age-matched controls. Apoptotic neurons were identified at an approximate frequency of 1 in 1100 to 5000 neurons in the cases examined. Furthermore, caspase-3 immunoreactivity was detected in granules of granulovacuolar degeneration. Our results provide direct evidence for apoptotic neuronal death in AD with a frequency compatible with the progression of neuronal degeneration in this chronic disease and identify autophagic vacuoles of granulovacuolar degeneration as possible means for the protective segregation of early apoptotic alterations in the neuronal cytoplasm.

Dietary restriction protects hippocampal neurons against the death-promoting action of a presenilin-1 mutation

Alzheimer's disease (AD) is an age-related disorder that involves degeneration of synapses and neurons in brain regions involved in learning and memory processes. Some cases of AD are caused by mutations in presenilin-1 (PS1), an integral membrane protein located in the endoplasmic reticulum. Previous studies have shown that PS1 mutations increase neuronal vulnerability to excitotoxicity and apoptosis. Although dietary restriction (DR) can increase lifespan and reduce the incidence of several age-related diseases in rodents, the possibility that DR can modify the pathogenic actions of mutations that cause AD has not been examined. The vulnerability of hippocampal neurons to excitotoxic injury was increased in PS1 mutant knockin mice. PS1 mutant knockin mice and wild-type mice maintained on a DR regimen for 3 months exhibited reduced excitotoxic damage to hippocampal CA1 and CA3 neurons compared to mice fed ad libitum; the DR regimen completely counteracted the endangering effect of the PS1 mutation. The magnitude of increase in levels of the lipid peroxidation product 4-hydroxynonenal following the excitotoxic insult was lower in DR mice compared to mice fed ad libitum, suggesting that suppression of oxidative stress may be one mechanism underlying the neuroprotective effect of DR. These findings indicate that the neurodegeneration-promoting effect of an AD-linked mutation is subject to modification by diet.

Evidence for caspase-mediated cleavage of AMPA receptor subunits in neuronal apoptosis and Alzheimer's disease

In Alzheimer's disease (AD) synapses degenerate and neurons die in brain regions involved in learning and memory processes. Although the cellular and molecular mechanisms underlying the neurodegenerative process in AD are unclear, increasing evidence suggests roles for amyloid beta-peptide (Abeta) and biochemical cascades associated with a form of programmed cell death called apoptosis. Cysteine proteases of the caspase family are activated in neurons undergoing apoptosis and apparently play a major role in the cell death process by cleaving yet-to-be-identified substrates. We now report that caspase activity is increased in brain tissue and neurons from AD patients, and in cultured hippocampal neurons undergoing apoptosis after exposure to amyloid beta-peptide (Abeta). Western blot analyses using antibodies against different subunits of 2-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) types of ionotropic glutamate receptors indicate that AMPA receptor subunits (GluR1, GluR2/3, and GluR4), but not NMDA receptor subunits (NR1 and NR2A), are proteolytically cleaved after exposure of hippocampal neurons to apoptotic insults, including Abeta, and that the caspase inhibitor zVAD-fmk suppresses such cleavage. Western blot analysis of brain tissue from AD patients and age-matched controls revealed evidence for increased proteolysis of AMPA receptor subunits in AD. Our data suggest roles for caspase-mediated cleavage of AMPA receptor subunits in modifying neuronal responsivity to glutamate and in the neurodegenerative process in AD.

Neurotrophic factors [activity-dependent neurotrophic factor (ADNF) and basic fibroblast growth factor (bFGF)] interrupt excitotoxic neurodegenerative cascades promoted by a PS1 mutation

Although an excitotoxic mechanism of neuronal injury has been proposed to play a role in chronic neurodegenerative disorders such as Alzheimer's disease, and neurotrophic factors have been put forward as potential therapeutic agents, direct evidence is lacking. Taking advantage of the fact that mutations in the presenilin-1 (PS1) gene are causally linked to many cases of early-onset inherited Alzheimer's disease, we generated PS1 mutant knock-in mice and directly tested the excitotoxic and neurotrophic hypotheses of Alzheimer's disease. Primary hippocampal neurons from PS1 mutant knock-in mice exhibited increased production of amyloid beta-peptide 42/43 and increased vulnerability to excitotoxicity, which occurred in a gene dosage-dependent manner. Neurons expressing mutant PS1 exhibited enhanced calcium responses to glutamate and increased oxyradical production and mitochondrial dysfunction. Pretreatment with either basic fibroblast growth factor or activity-dependent neurotrophic factor protected neurons expressing mutant PS1 against excitotoxicity. Both basic fibroblast growth factor and activity-dependent neurotrophic factor stabilized intracellular calcium levels and abrogated the increased oxyradical production and mitochondrial dysfunction otherwise caused by the PS1 mutation. Our data indicate that neurotrophic factors can interrupt excitotoxic neurodegenerative cascades promoted by PS1 mutations.

Altered calcium homeostasis and mitochondrial dysfunction in cortical synaptic compartments of presenilin-1 mutant mice

Alzheimer's disease is characterized by amyloid beta-peptide deposition, synapse loss, and neuronal death, which are correlated with cognitive impairments. Mutations in the presenilin-1 gene on chromosome 14 are causally linked to many cases of early-onset inherited Alzheimer's disease. We report that synaptosomes prepared from transgenic mice harboring presenilin-1 mutations exhibit enhanced elevations of cytoplasmic calcium levels following exposure to depolarizing agents, amyloid beta-peptide, and a mitochondrial toxin compared with synaptosomes from nontransgenic mice and mice overexpressing wild-type presenilin-1. Mitochondrial dysfunction and caspase activation following exposures to amyloid beta-peptide and metabolic insults were exacerbated in synaptosomes from presenilin-1 mutant mice. Agents that buffer cytoplasmic calcium or that prevent calcium release from the endoplasmic reticulum protected synaptosomes against the adverse effect of presenilin-1 mutations on mitochondrial function. Abnormal synaptic calcium homeostasis and mitochondrial dysfunction may contribute to the pathogenic mechanism of presenilin-1 mutations.