Increased amyloidogenic secretion in cerebellar granule cells undergoing apoptosis

Upregulation of TLR4/MyD88 pathway in alcohol-induced Wernicke’s encephalopathy: Findings in preclinical models and in a postmortem human case

Wernicke’s encephalopathy (WE) is a neurologic disease caused by vitamin B1 or thiamine deficiency (TD), being the alcohol use disorder its main risk factor. WE patients present limiting motor, cognitive, and emotional alterations related to a selective cerebral vulnerability. Neuroinflammation has been proposed to be one of the phenomena that contribute to brain damage. Our previous studies provide evidence for the involvement of the innate immune receptor Toll-like (TLR)4 in the inflammatory response induced in the frontal cortex and cerebellum in TD animal models (animals fed with TD diet [TDD] and receiving pyrithiamine). Nevertheless, the effects of the combination of chronic alcohol consumption and TD on TLR4 and their specific contribution to the pathogenesis of WE are currently unknown. In addition, no studies on TLR4 have been conducted on WE patients since brains from these patients are difficult to achieve. Here, we used rat models of chronic alcohol (CA; 9 months of forced consumption of 20% (w/v) alcohol), TD hit (TDD + daily 0.25 mg/kg i.p. pyrithiamine during 12 days), or combined treatment (CA + TDD) to check the activation of the proinflammatory TLR4/MyD88 pathway and related markers in the frontal cortex and the cerebellum. In addition, we characterized for the first time the TLR4 and its coreceptor MyD88 signature, along with other markers of this proinflammatory signaling such as phospo-NFκB p65 and IκBα, in the postmortem human frontal cortex and cerebellum (gray and white matter) of an alcohol-induced WE patient, comparing it with negative (no disease) and positive (aged brain with Alzheimer’s disease) control subjects for neuroinflammation. We found an increase in the cortical TLR4 and its adaptor molecule MyD88, together with an upregulation of the proinflammatory signaling molecules p-NF-ĸB and IĸBα in the CA + TDD animal model. In the patient diagnosed with alcohol-induced WE, we observed cortical and cerebellar upregulation of the TLR4/MyD88 pathway. Hence, our findings provide evidence, both in the animal model and the human postmortem brain, of the upregulation of the TLR4/MyD88 proinflammatory pathway in alcohol consumption–related WE.

Neuroprotective effects of aromatic turmerone on activity deprivation-induced apoptosis in cerebellar granule neurons

Ar-turmerone, which is a major bioactive component found in the essential oil derived from Curcuma longa, has been reported to inhibit proliferation and induce apoptosis in cancer cell lines. Recently, ar-turmerone has been reported to increase the proliferation of neuronal stem cells, in contrast to its actions in cancer cells. These observations raise the possibility that ar-turmerone serves specific functions in neuronal cell lineages. However, the effects of ar-turmerone on postmitotic neurons remain elusive. In the present study, we investigated the neuroprotective functions of ar-turmerone in primary cerebellar granule neuronal cultures. We found that ar-turmerone increased the survival of neurons following activity deprivation. Consistently, the induction of cleaved caspase-3, a hallmark of apoptosis, was prevented by ar-turmerone, although neither the level of reactive oxygen species nor the mitochondrial membrane potential was affected. This study reports a neuroprotective function for ar-turmerone, providing new insights into the potential therapeutic applications of ar-turmerone for neurological disorders.

The Emerging Role of Altered Cerebellar Synaptic Processing in Alzheimer's Disease

The role of the cerebellum in Alzheimer's disease (AD) has been neglected for a long time. Recent studies carried out using transgenic mouse models have demonstrated that amyloid-β (Aβ) is deposited in the cerebellum and affects synaptic transmission and plasticity, sometimes before plaque formation. A wide variability of motor phenotype has been observed in the different murine models of AD, without a consistent correlation with the extent of cerebellar histopathological changes or with cognitive deficits. The loss of noradrenergic drive may contribute to the impairment of cerebellar synaptic function and motor learning observed in these mice. Furthermore, cerebellar neurons, particularly granule cells, have been used as in vitro model of Aβ-induced neuronal damage. An unexpected conclusion is that the cerebellum, for a long time thought to be somehow protected from AD pathology, is actually considered as a region vulnerable to Aβ toxic damage, even at the early stage of the disease, with consequences on motor performance.

BARHL1 Is Downregulated in Alzheimer's Disease and May Regulate Cognitive Functions through ESR1 and Multiple Pathways

The Transcription factor BarH like homeobox 1 (BARHL1) is overexpressed in medulloblastoma and plays a role in neurogenesis. However, much about the BARHL1 regulatory networks and their functions in neurodegenerative and neoplastic disorders is not yet known. In this study, using a tissue microarray (TMA), we report for the first time that BARHL1 is downregulated in hormone-negative breast cancers and Alzheimer’s disease (AD). Furthermore, using an integrative bioinformatics approach and mining knockout mouse data, we show that: (i) BARHL1 and Estrogen Receptor 1 (ESR1) may constitute a network that regulates Neurotrophin 3 (NTF3)- and Brain Derived Neurotrophic Factor (BDNF)-mediated neurogenesis and neural survival; (ii) this is probably linked to AD pathways affecting aberrant post-translational modifications including SUMOylation and ubiquitination; (iii) the BARHL1-ESR1 network possibly regulates β-amyloid metabolism and memory; and (iv) hsa-mir-18a, having common key targets in the BARHL1-ESR1 network and AD pathway, may modulate neuron death, reduce β-amyloid processing and might also be involved in hearing and cognitive decline associated with AD. We have also hypothesized why estrogen replacement therapy improves AD condition. In addition, we have provided a feasible new mechanism to explain the abnormal function of mossy fibers and cerebellar granule cells related to memory and cognitive decline in AD apart from the Tau and amyloid pathogenesis through our BARHL1-ESR1 axis.

Protective Effects of Wogonin against Alzheimer's Disease by Inhibition of Amyloidogenic Pathway

One of the pathogenic systems of Alzheimer's disease (AD) is the formation of β-amyloid plaques in the brains of patients, and amyloidogenic activity becomes one of the therapeutic targets. Here, we report wogonin, one of the major active constituting components in Scutellaria baicalensis, which has the neuroprotective effects on amyloid-β peptides- (Aβ-) induced toxicity. Oral wogonin treatment improved the performance of triple transgenic AD mice (h-APPswe, h-Tau P301L, and h-PS1 M146V) on the Morris water maze, Y-maze, and novel object recognition. Furthermore, wogonin activated the neurite outgrowth of AD cells by increasing neurite length and complexity of Tet-On Aβ₄₂-GFP SH-SY5Y neuroblastoma cells (AD cells) and attenuated amyloidogenic pathway by decreasing the levels of β-secretase, APP β-C-terminal fragment, Aβ-aggregation, and phosphorylated Tau. Wogonin also increased mitochondrial membrane potential (∆ψm) and protected against apoptosis by reducing the expression of Bax and cleaved PARP. Collectively, these results conclude that wogonin may be a promising multifunctional drug candidate for AD.

Role of Trisomy 21 Mosaicism in Sporadic and Familial Alzheimer's Disease

Trisomy 21 and the consequent extra copy of the amyloid precursor protein (APP) gene and increased beta-amyloid (Aβ) peptide production underlie the universal development of Alzheimer's disease (AD) pathology and high risk of AD dementia in people with Down syndrome (DS). Trisomy 21 and other forms of aneuploidy also arise among neurons and peripheral cells in both sporadic and familial AD and in mouse and cell models thereof, reinforcing the conclusion that AD and DS are two sides of the same coin. The demonstration that 90% of the neurodegeneration in AD can be attributed to the selective loss of aneuploid neurons generated over the course of the disease indicates that aneuploidy is an essential feature of the pathogenic pathway leading to the depletion of neuronal cell populations. Trisomy 21 mosaicism also occurs in neurons and other cells from patients with Niemann-Pick C1 disease and from patients with familial or sporadic frontotemporal lobar degeneration (FTLD), as well as in their corresponding mouse and cell models. Biochemical studies have shown that Aβ induces mitotic spindle defects, chromosome mis-segregation, and aneuploidy in cultured cells by inhibiting specific microtubule motors required for mitosis. These data indicate that neuronal trisomy 21 and other types of aneuploidy characterize and likely contribute to multiple neurodegenerative diseases and are a valid target for therapeutic intervention. For example, reducing extracellular calcium or treating cells with lithium chloride (LiCl) blocks the induction of trisomy 21 by Aβ. The latter finding is relevant in light of recent reports of a lowered risk of dementia in bipolar patients treated with LiCl and in the stabilization of cognition in AD patients treated with LiCl.

Glycolytic enzyme upregulation and numbness of mitochondrial activity characterize the early phase of apoptosis in cerebellar granule cells

Alzheimer's disease (AD) and cancer proceed via one or more common molecular mechanisms: a metabolic shift from oxidative phosphorylation to glycolysis-corresponding to the activation of the Warburg effect-occurs in both diseases. The findings reported in this paper demonstrate that, in the early phase of apoptosis, glucose metabolism is enhanced, i.e. key proteins which internalize and metabolize glucose-glucose transporter, hexokinase and phosphofructokinase-are up-regulated, in concomitance with a parallel decrease in oxygen consumption by mitochondria and increase of L-lactate accumulation. Reversal of the glycolytic phenotype occurs in the presence of dichloroacetate, inhibitor of the pyruvate dehydrogenase kinase enzyme, which speeds up apoptosis of cerebellar granule cells, reawakening mitochondria and then modulating glycolytic enzymes. Loss of the adaptive advantage afforded by aerobic glycolysis, which occurs in the late phase of apoptosis, exacerbates the pathological processes underlying neurodegeneration, leading inevitably the cell to death. In conclusion, the data propose that both aerobic, i.e. Warburg effect, essentially due to the protective numbness of mitochondria, and anaerobic glycolysis, rather due to the mitochondrial impairment, characterize the entire time frame of apoptosis, from the early to the late phase, which mimics the development of AD.

Molecular mechanisms of cognitive dysfunction following traumatic brain injury

Traumatic brain injury (TBI) results in significant disability due to cognitive deficits particularly in attention, learning and memory, and higher-order executive functions. The role of TBI in chronic neurodegeneration and the development of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS) and most recently chronic traumatic encephalopathy (CTE) is of particular importance. However, despite significant effort very few therapeutic options exist to prevent or reverse cognitive impairment following TBI. In this review, we present experimental evidence of the known secondary injury mechanisms which contribute to neuronal cell loss, axonal injury, and synaptic dysfunction and hence cognitive impairment both acutely and chronically following TBI. In particular we focus on the mechanisms linking TBI to the development of two forms of dementia: AD and CTE. We provide evidence of potential molecular mechanisms involved in modulating Aβ and Tau following TBI and provide evidence of the role of these mechanisms in AD pathology. Additionally we propose a mechanism by which Aβ generated as a direct result of TBI is capable of exacerbating secondary injury mechanisms thereby establishing a neurotoxic cascade that leads to chronic neurodegeneration.

Cross-linking of cell surface amyloid precursor protein leads to increased β-amyloid peptide production in hippocampal neurons: implications for Alzheimer's disease

The accumulation of the β-amyloid peptide (Aβ) in Alzheimer's disease (AD) is thought to play a causative role in triggering synaptic dysfunction in neurons, leading to their eventual demise through apoptosis. Aβ is produced and secreted upon sequential cleavage of the amyloid precursor protein (APP) by β-secretases and γ-secretases. However, while Aβ levels have been shown to be increased in the brains of AD patients, little is known about how the cleavage of APP and the subsequent generation of Aβ is influenced, or whether the cleavage process changes over time. It has been proposed that Aβ can bind APP and promote amyloidogenic processing of APP, further enhancing Aβ production. Proof of this idea has remained elusive because a clear mechanism has not been identified, and the promiscuous nature of Aβ binding complicates the task of demonstrating the idea. To work around these problems, we used an antibody-mediated approach to bind and cross-link cell-surface APP in cultured rat primary hippocampal neurons. Here we show that cross-linking of APP is sufficient to raise the levels of Aβ in viable neurons with a concomitant increase in the levels of the β-secretase BACE1. This appears to occur as a result of a sorting defect that stems from the caspase-3-mediated inactivation of a key sorting adaptor protein, namely GGA3, which prevents the lysosomal degradation of BACE1. Together, our data suggest the occurrence of a positive pathogenic feedback loop involving Aβ and APP in affected neurons possibly allowing Aβ to spread to nearby healthy neurons.

Nerve growth factor and Alzheimer's disease: new facts for an old hypothesis

Understanding sporadic Alzheimer's disease (AD) onset and progression requires an explanation of what triggers the common core of abnormal processing of the amyloid precursor protein and tau processing. In the quest for upstream drivers of sporadic, late-onset AD neurodegeneration, nerve growth factor (NGF) has a central role. Initially connected to AD on a purely correlative basis, because of its neurotrophic actions on basal forebrain cholinergic neurons, two independent lines of research, reviewed in this article, place alterations of NGF processing and signaling at the center stage of a new mechanism, leading to the activation of amyloidogenesis and tau processing. Thus, experimental studies on NGF deficit induced neurodegeneration in transgenic mice, as well as the mechanistic studies on the anti-amyloidogenic actions of NGF/TrkA signaling in primary neuronal cultures demonstrated a novel causal link between neurotrophic signaling deficits and Alzheimer's neurodegeneration. Around these results, a new NGF hypothesis can be built, with neurotrophic deficits of various types representing an upstream driver of the core AD triad pathology. According to the new NGF hypothesis for AD, therapies aimed at reestablishing a correct homeostatic balance between ligands (and receptors) of the NGF pathway appear to have a clear and strong rationale, not just as long-term cholinergic neuroprotection, but also as a truly disease-modifying approach.

Crosstalk between the ubiquitin-proteasome system and autophagy in a human cellular model of Alzheimer's disease

Alzheimer's disease is the most common progressive neurodegenerative disorder characterized by the abnormal deposition of amyloid plaques, likely as a consequence of an incorrect processing of the amyloid-β precursor protein (AβPP). Dysfunctions in both the ubiquitin-proteasome system and autophagy have also been observed. Recently, an extensive cross-talk between these two degradation pathways has emerged, but the exact implicated processes are yet to be clarified. In this work, we gained insight into such interplay by analyzing human SH-SY5Y neuroblastoma cells stably transfected either with wild-type AβPP gene or 717 valine-to-glycine AβPP-mutated gene. The over-expression of the AβPP mutant isoform correlates with an increase in oxidative stress and a remodeled pattern of protein degradation, with both marked inhibition of proteasome activities and impairment in the autophagic flux. To compensate for this altered scenario, cells try to promote the autophagy activation in a HDAC6-dependent manner. The treatment with amyloid-β(42) oligomers further compromises proteasome activity and also contributes to the inhibition of cathepsin-mediated proteolysis, finally favoring the neuronal degeneration and suggesting the existence of an Aβ(42) threshold level beyond which proteasome-dependent proteolysis becomes definitely dysfunctional.

γ-Secretase complexes containing caspase-cleaved presenilin-1 increase intracellular Aβ(42) /Aβ(40) ratio

Markers for caspase activation and apoptosis have been shown in brains of Alzheimer's disease (AD) patients and AD-mouse models. In neurons, caspase activation is associated with elevated amyloid β-peptide (Aβ) production. Caspases cleave numerous substrates including presenilin-1 (PS1). The cleavage takes place in the large cytosolic loop of PS1-C-terminal fragment (PS1CTF), generating a truncated PS1CTF lacking half of the loop domain (caspCTF). The loop has been shown to possess important regulatory functions with regard to Aβ(40) and Aβ(42) production. Previously, we have demonstrated that γ-secretase complexes are active during apoptosis regardless of caspase cleavage in the PS1CTF-loop. Here, a PS1/PS2-knockout mouse blastocyst-derived cell line was used to establish stable or transient cell lines expressing either caspCTF or full-length CTF (wtCTF). We show that caspCTF restores γ-secretase activity and forms active γ-secretase complexes together with Nicastrin, Pen-2, Aph-1 and PS1-N-terminal fragment. Further, caspCTF containing γ-secretase complexes have a sustained capacity to cleave amyloid precursor protein (APP) and Notch, generating APP and Notch intracellular domain, respectively. However, when compared to wtCTF cells, caspCTF cells exhibit increased intracellular production of Aβ(42) accompanied by increased intracellular Aβ(42) /Aβ(40) ratio without changing the Aβ secretion pattern. Similarly, induction of apoptosis in wtCTF cells generate a similar shift in intracellular Aβ pattern with increased Aβ(42) /Aβ(40) ratio. In summary, we show that caspase cleavage of PS1 generates a γ-secretase complex that increases the intracellular Aβ(42) /Aβ(40) ratio. This can have implications for AD pathogenesis and suggests caspase inhibitors as potential therapeutic agents.

Amyloid precursor protein binding protein Fe65 is cleaved by caspases during DNA damage-induced apoptosis

Caspases cleave several cellular proteins to execute cell death by apoptosis. The identification of novel substrates of caspases could provide an important clue for elucidation of new apoptosis signaling pathways. In this study, we tested whether an amyloid precursor protein (APP) binding protein Fe65 is proteolytically degraded in neuronal cell death by apoptosis, using a neuron-like cell line, human neuroblastoma SH-SY5Y cells. When treated with DNA damaging agents, etoposide (ETP) and camptothecin (CPT), SH-SY5Y cells underwent apoptosis in a dose-dependent manner. Interestingly, Fe65 (97 kDa) was cleaved to a 65 kDa product during DNA damage-induced apoptosis. Furthermore, the cleavage of Fe65 was accompanied by activation of caspases-9 and -3. The restriction cleavage of Fe65 was completely suppressed by the treatment with a pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe) fluoromethylketone (z-VAD-fmk). These results reveal the restriction cleavage of Fe65 by caspases during DNA damage-induced apoptosis. Since Fe65 has been shown to suppress APP processing to amyloid β (Aβ) production, our findings may provide a new insight into the molecular mechanism by which DNA damage induces Aβ production and subsequent neuronal cell death in Alzheimer's disease (AD).

Apoptosis and in vitro Alzheimer disease neuronal models

Alzheimer disease (AD) is a human neurodegenerative disease characterized by co-existence of extracellular senile plaques (SP) and neurofibrillary tangles (NFT) associated with an extensive neuronal loss, primarily in the cerebral cortex and hippocampus. Several studies suggest that caspase(s)-mediated neuronal death occurs in cellular and animal AD models as well as in human brains of affected patients, although an etiologic role of apoptosis in such neurodegenerative disorder is still debated. This review summarizes the experimental evidences corroborating the possible involvement of apoptosis in AD pathogenesis and discusses the usefulness of ad hoc devised in vitro approaches to study how caspase(s), amyloidogenic processing and tau metabolism might reciprocally interact leading to neuronal death.

Alzheimer's proteins, oxidative stress, and mitochondrial dysfunction interplay in a neuronal model of Alzheimer's disease

In this paper, we discuss the interplay between beta-amyloid (Aβ) peptide, Tau fragments, oxidative stress, and mitochondria in the neuronal model of cerebellar granule neurons (CGNs) in which the molecular events reminiscent of AD are activated. The identification of the death route and the cause/effect relationships between the events leading to death could be helpful to manage the progression of apoptosis in neurodegeneration and to define antiapoptotic treatments acting on precocious steps of the death process. Mitochondrial dysfunction is among the earliest events linked to AD and might play a causative role in disease onset and progression. Recent studies on CGNs have shown that adenine nucleotide translocator (ANT) impairment, due to interaction with toxic N-ter Tau fragment, contributes in a significant manner to bioenergetic failure and mitochondrial dysfunction. These findings open a window for new therapeutic strategies aimed at preserving and/or improving mitochondrial function.

Nitrous oxide plus isoflurane induces apoptosis and increases beta-amyloid protein levels

BACKGROUND

Some anesthetics have been suggested to induce neurotoxicity, including promotion of Alzheimer's disease neuropathogenesis. Nitrous oxide and isoflurane are common anesthetics. The authors set out to assess the effects of nitrous oxide and/or isoflurane on apoptosis and beta-amyloid (Abeta) levels in H4 human neuroglioma cells and primary neurons from naïve mice.

METHODS

The cells or neurons were exposed to 70% nitrous oxide and/or 1% isoflurane for 6 h. The cells or neurons and conditioned media were harvested at the end of the treatment. Caspase-3 activation, apoptosis, processing of amyloid precursor protein, and Abeta levels were determined.

RESULTS

Treatment with a combination of 70% nitrous oxide and 1% isoflurane for 6 h induced caspase-3 activation and apoptosis in H4 naïve cells and primary neurons from naïve mice. The 70% nitrous oxide plus 1% isoflurane, but neither alone, for 6 h induced caspase-3 activation and apoptosis, and increased levels of beta-site amyloid precursor protein-cleaving enzyme and Abeta in H4-amyloid precursor protein cells. In addition, the nitrous oxide plus isoflurane-induced Abeta generation was reduced by a broad caspase inhibitor, Z-VAD. Finally, the nitrous oxide plus isoflurane-induced caspase-3 activation was attenuated by gamma-secretase inhibitor L-685,458, but potentiated by exogenously added Abeta.

CONCLUSION

These results suggest that the common anesthetics nitrous oxide plus isoflurane may promote neurotoxicity by inducing apoptosis and increasing Abeta levels. The generated Abeta may further potentiate apoptosis to form another round of apoptosis and Abeta generation. More studies, especially the in vivo confirmation of these in vitro findings, are needed.

NGF and BDNF signaling control amyloidogenic route and Abeta production in hippocampal neurons

Here, we report that interruption of NGF or BDNF signaling in hippocampal neurons rapidly activates the amyloidogenic pathway and causes neuronal apoptotic death. These events are associated with an early intracellular accumulation of PS1 N-terminal catalytic subunits and of APP C-terminal fragments and a progressive accumulation of intra- and extracellular Abeta aggregates partly released into the culture medium. The released pool of Abeta induces an increase of APP and PS1 holoprotein levels, creating a feed-forward toxic loop that might also cause the death of healthy neurons. These events are mimicked by exogenously added Abeta and are prevented by exposure to beta- and gamma-secretase inhibitors and by antibodies directed against Abeta peptides. The same cultured neurons deprived of serum die, but APP and PS1 overexpression does not occur, Abeta production is undetectable, and cell death is not inhibited by anti-Abeta antibodies, suggesting that hippocampal amyloidogenesis is not a simple consequence of an apoptotic trigger but is due to interruption of neurotrophic signaling.

The inhalation anesthetic desflurane induces caspase activation and increases amyloid beta-protein levels under hypoxic conditions

Perioperative factors including hypoxia, hypocapnia, and certain anesthetics have been suggested to contribute to Alzheimer disease (AD) neuropathogenesis. Desflurane is one of the most commonly used inhalation anesthetics. However, the effects of desflurane on AD neuropathogenesis have not been previously determined. Here, we set out to assess the effects of desflurane and hypoxia on caspase activation, amyloid precursor protein (APP) processing, and amyloid beta-protein (Abeta) generation in H4 human neuroglioma cells (H4 naïve cells) as well as those overexpressing APP (H4-APP cells). Neither 12% desflurane nor hypoxia (18% O(2)) alone affected caspase-3 activation, APP processing, and Abeta generation. However, treatment with a combination of 12% desflurane and hypoxia (18% O(2)) (desflurane/hypoxia) for 6 h induced caspase-3 activation, altered APP processing, and increased Abeta generation in H4-APP cells. Desflurane/hypoxia also increased levels of beta-site APP-cleaving enzyme in H4-APP cells. In addition, desflurane/hypoxia-induced Abeta generation could be reduced by the broad caspase inhibitor benzyloxycarbonyl-VAD. Finally, the Abeta aggregation inhibitor clioquinol and gamma-secretase inhibitor L-685,458 attenuated caspase-3 activation induced by desflurane/hypoxia. In summary, desflurane can induce Abeta production and caspase activation, but only in the presence of hypoxia. Pending in vivo confirmation, these data may have profound implications for anesthesia care in elderly patients, and especially those with AD.

DNA damage-inducing agents elicit gamma-secretase activation mediated by oxidative stress

According to the amyloid cascade hypothesis, Alzheimer's disease is the consequence of neuronal cell death induced by beta-amyloid (Abeta), which accumulates by abnormal clearance or production. On the other hand, recent studies have shown cell death-induced alteration in amyloid precursor protein (APP) processing, suggesting potential mutual interactions between APP processing and cell death. We have shown previously that the cell death caused by DNA damage-inducing agents (DDIAs) facilitated gamma-secretase activity and Abeta generation in a Bax/Bcl-2-dependent, but caspase-independent manner. Here, we attempted to elucidate the downstream mechanism that modulates gamma-secretase activity in DDIA-treated cells. N-acetyl cysteine, a potent antioxidant, attenuated DDIA-induced enhancement of gamma-secretase activity but failed to rescue cell death. Overexpression of heat shock protein 70, which blocks cytochrome c release from mitochondria, also reduced gamma-secretase activity. Moreover, glutathione depletion significantly facilitated gamma-secretase activity and Abeta generation by enhancing the formation of higher molecular weight gamma-secretase complex before signs of cell death developed. Finally, Abeta treatment, a known inducer of oxidative stress, also increased gamma-secretase activity. Taken together, these results indicate that DDIA-induced gamma-secretase activation is dependent on augmented oxidative stress, and that Abeta and gamma-secretase may activate each other. On the basis of these results, we propose a feed-back loop between oxidative stress and Abeta generation mediated by gamma-secretase activation.

Mechanism of glial differentiation of neural progenitor cells by amyloid precursor protein

BACKGROUND

We found that human neural progenitor cells (HNPCs) exposed to high concentrations of amyloid precursor protein (APP) or transplanted into APP transgenic mice (APP23) primarily differentiated into astrocytes, suggesting that pathological alterations of APP processing in Alzheimer's disease (AD) may prevent neuronal differentiation of HNPCs.

OBJECTIVES

To investigate the mechanism of APP-induced glial differentiation of HNPCs.

METHODS

We treat HNPCs with APP and analyze the expression and phosphorylation of signaling molecules using PCR and Western blots. To confirm the involvement of the factors, RNA interference of the signaling molecule is conducted.

RESULTS

APP treatment caused inductions of CNTF, gp130 and JAK1 gene expressions, and STAT3 phosphorylation, while silencing of these genes by RNA interference suppressed the glial differentiation of the cells, indicating involvement of the IL-6/gp130 pathway. APP also increased the generation of notch intracellular domain and gene expression of Hes1, indicating that glial differentiation of HNPCs may be mediated by the notch signaling.

CONCLUSION

These results indicate that APP may regulate HNPC differentiation through activation of both the IL-6/gp130 and notch signaling pathway. Although the importance of adult neurogenesis is not clear, glial differentiation of HNPCs may cause problems in maintaining normal brain function and may contribute to the AD pathology.

Down’s syndrome and Alzheimer’s disease: two sides of the same coin

All Down’s syndrome individuals develop Alzheimer’s disease (AD) neuropathology by the age of 40 years. To unite the two diseases under one hypothesis, we have suggested that classical AD, both of the genetic and late-onset sporadic forms, might be promoted by small numbers of trisomy 21 cells developing during the life of the affected individual. Recent evidence from several laboratories will be presented, which strongly supports the trisomy 21 hypothesis that defects in mitosis, and particularly in chromosome segregation, may be a part of the AD process. Specifically, genetic mutations that cause familial AD disrupt the cell cycle and lead to chromosome aneuploidy, including trisomy 21, in transgenic mice and transfected cells; cells from both familial and sporadic AD patients exhibit chromosome aneuploidy, including trisomy 21. The possibility that many cases of AD are mosaic for trisomy 21 suggests novel approaches to diagnosis and therapy.

Dicer expression and localization in post-mitotic neurons

RNA-mediated gene silencing is recently emerged as a fundamental mechanism of regulation of gene expression in many organisms and tissues, with special emphasis with respect to the nervous system. With the aim to study the components of RNA silencing machinery, we have investigated the expression profile and localization of dicer protein RNase III endonuclease in cultures of post-mitotic neurons. Dicer catalyzes the processing of double-stranded RNAs (dsRNAs) into approximately 21-25 nucleotide-long small interfering (si)RNAs and micro (mi)RNAs, and it represents an essential step in the biogenesis of these small non-coding RNA molecules. We show that in rat primary neurons dicer is localized in the somatodendritic compartment, at the Golgi-reticulum area network level. This peculiar distribution was altered by brefeldin A treatment. Moreover the Golgi-reticulum dicer signal was observed also in primary astroglial cells. In addiction dicer was observed to be regulated during the embryogenesis and development in several tissues. In fact its expression is developmentally regulated in cultured cerebellar granule neurons. This is the first study in which dicer is shown preferentially distributed in the Golgi-reticulum area in post-mitotic terminally differentiated neuronal and glial cells and that its profile is modulated during maturation and development of in vitro cultured cerebellar granule neurons.

Depletion of GGA3 stabilizes BACE and enhances beta-secretase activity

Beta-site APP-cleaving enzyme (BACE) is required for production of the Alzheimer's disease (AD)-associated Abeta protein. BACE levels are elevated in AD brain, and increasing evidence reveals BACE as a stress-related protease that is upregulated following cerebral ischemia. However, the molecular mechanism responsible is unknown. We show that increases in BACE and beta-secretase activity are due to posttranslational stabilization following caspase activation. We also found that during cerebral ischemia, levels of GGA3, an adaptor protein involved in BACE trafficking, are reduced, while BACE levels are increased. RNAi silencing of GGA3 also elevated levels of BACE and Abeta. Finally, in AD brain samples, GGA3 protein levels were significantly decreased and inversely correlated with increased levels of BACE. In summary, we have elucidated a GGA3-dependent mechanism regulating BACE levels and beta-secretase activity. This mechanism may explain increased cerebral levels of BACE and Abeta following cerebral ischemia and existing in AD.

The inhalation anesthetic isoflurane induces a vicious cycle of apoptosis and amyloid beta-protein accumulation

The anesthetic isoflurane has been reported to induce apoptosis and increase Abeta generation and aggregation. However, the molecular mechanism underlying these effects remains unknown. We therefore set out to assess whether the effects of isoflurane on apoptosis are linked to amyloid beta-protein (Abeta) generation and aggregation. For this purpose, we assessed the effects of isoflurane on beta-site amyloid beta precursor protein (APP)-cleaving enzyme (BACE) and gamma-secretase, the proteases responsible for Abeta generation. We also tested the effects of inhibitors of Abeta aggregation (iAbeta5, a beta-sheet breaker peptide; clioquinol, a copper-zinc chelator) on the ability of isoflurane to induce apoptosis. All of these studies were performed on naive human H4 neuroglioma cells as well as those overexpressing APP (H4-APP cells). Isoflurane increased the levels of BACE and gamma-secretase and secreted Abeta in the H4-APP cells. Isoflurane-induced Abeta generation could be blocked by the broad-based caspase inhibitor Z-VAD. The Abeta aggregation inhibitors, iAbeta5 and clioquinol, selectively attenuated caspase-3 activation induced by isoflurane. However, isoflurane was able to induce caspase-3 activation in the absence of any detectable alterations of Abeta generation in naive H4 cells. Finally, Abeta potentiated the isoflurane-induced caspase-3 activation in naive H4 cells. Collectively, these findings suggest that isoflurane can induce apoptosis, which, in turn, increases BACE and gamma-secretase levels and Abeta secretion. Isoflurane also promotes Abeta aggregation. Accumulation of aggregated Abeta in the media can then promote apoptosis. The result is a vicious cycle of isoflurane-induced apoptosis, Abeta generation and aggregation, and additional rounds of apoptosis, leading to cell death.

Role of water-soluble amyloid-beta in the pathogenesis of Alzheimer's disease

Water-soluble amyloid-beta (wsAbeta) is present in cerebral cortex of subjects at risk of Alzheimer's disease (AD) as well as in normal elderly subjects as a mixture of three major amyloid-beta (Abeta) species: 1-42, py3-42 and py11-42. The three wsAbeta species are nondetectable in brains of young people, free of immunohistochemically detectable amyloid plaques. In the brains of Down's syndrome and APP-mutant transgenic mice, wsAbeta appears long time before amyloid deposition, indicating that it represent the first form of Abeta aggregation and accumulation. In normal brain, wsAbeta is bound to apolipoprotein E that favours its degradation by proteases. The composition of wsAbeta, in terms of the ratio between the full-length 1-42 and the py3-42 peptides, correlates with the severity of clinical and pathological phenotype in familial early onset AD. Water-soluble Abeta is the native counterpart of the Abeta small aggregates (soluble oligomers) that show in vitro an early and high neuronal toxicity.

The gonadotropin connection in Alzheimer's disease

Although not traditionally thought of as regulators of neuronal function, the hypothalamic-pituitary-gonadal (HPG) hormones luteinizing hormone (LH), gonadotropin-releasing hormone (GnRH), and activins possess neuronal receptors. These receptors are found throughout the limbic system on a number of different cell types, and, like reproductive tissues, the expression of these receptors is regulated by hormonal feedback loops. These hormones and their receptors regulate structure and a diverse range of functions in the brain. Therefore, it is not surprising that the dysregulation of the HPG axis with menopause and andropause (leading to elevated LH, GnRH, and activin signaling but decreased sex steroid signaling) might promote alterations in both the structure and function of neuronal cells. To date, most evidence has accumulated for a role of LH in promoting neurodegenerative changes. LH is known to cross the blood-brain barrier, receptors for LH are most concentrated in the hippocampus, that region of the brain most vulnerable to Alzheimer's disease (AD) and LH is significantly elevated in both the serum and the pyramidal neurons of AD subjects. LH promotes the amyloidogenic processing of the amyloid-beta precursor protein in vitro, and the antigonadotropin leuprolide acetate decreases amyloid generation in mice. Moreover, leuprolide acetate improves the cognitive performance and decreases amyloid-beta deposition in aged transgenic mice carrying the Swedish AbetaPP mutation. Therefore, the elevation of LH with the dysregulation of the HPG axis at menopause and andropause is a physiologically relevant signal that could promote neurodegeneration. Epidemiological support for a role of LH/GnRH in AD is evidenced by a reduction in neurodegenerative disease among prostate cancer patients a group known to GnRH agonists. Clinical trials are underway for the treatment of AD using GnRH analogs and should provide further insights into the gonadotropin connection in AD.

The endocytotic pathway is required for increased A beta 42 secretion during apoptosis

Secretion and progressive cerebral accumulation of beta-amyloid peptides (A beta), which derive by endoproteolytic ('amyloidogenic') processing of beta-amyloid precursor protein (APP), are felt to represent collectively an early and necessary event in the pathogenesis of Alzheimer's disease. APP amyloidogenic processing can occur via secretory or endocytotic pathways, but the relative contribution of these pathways to A beta secretion remains to be established. The effect of apoptosis on amyloidogenic processing and A beta secretion similarly is incompletely understood. We tested the hypothesis that APP processing by the endocytotic pathway represents a stress-related neural cell response, by comparing A beta secretion after induction of apoptosis in PC12 cells transfected either for endocytosis-competent or -deficient APP. Newly prepared adenoviral vectors encompassing targeted mutagenesis of the cytoplasmic tail YENP tetrapeptide sequence, which serves as the principal APP internalization signal, were used to express endocytosis-deficient holoprotein. We report that the endocytotic pathway is required for the generation and secretion of A beta 42, and that secretion of this neurotoxic peptide increases significantly during apoptosis. We demonstrate additionally that more A beta 40 apparently is generated in secretory compartments during apoptosis when APP processing by the endocytotic pathway is impaired.

Gene expression profiles of apoptotic neurons

The multigenic program underlying neuronal apoptosis is mostly unknown. To study the program, we used genome-scale screening by oligonucleotide microarrays during serum and potassium deprivation-induced apoptosis of cerebellar granule neurons. From the 8740 genes interrogated by the arrays, 423 genes were found to be regulated at both the transcriptional and the posttranscriptional level and segregated into distinct clusters. Semantic clustering based on gene ontologies showed coordinated expression of genes with common biological functions and metabolic pathways. Among the genes implicated in apoptotic cerebellar granule neurons, 70 were in common with those differentially expressed in cortical neurons exposed to amyloid beta-protein, indicating the existence of common mechanisms responsible for neuronal cell death. Our results offer a genomic view of the changes that accompany neuronal apoptosis and yield new insights into the underlying molecular basis.

Activity-dependent increase in β-amyloid precursor protein mRNA expression in neurons

Although beta-amyloid precursor protein (APP) has been suggested to play a role in neuronal survival and plasticity, the mRNA expression of APP has not been studied in terms of neuronal activity. In cultures of mouse cerebellar granule cells, we found that the levels of APP mRNA increased when a high concentration of potassium was present in the medium. A deprivation of membrane depolarization caused by lowering the K+ concentration decreased both mRNA expression and protein synthesis of APP. Increasing the concentration, however, restored mRNA expression, which was driven by the influx of Ca2+ through L-type voltage-dependent calcium channels and mediated by de novo protein synthesis. Thus, APP mRNA expression is controlled in an activity-dependent manner in neurons.

In vitro cultured neurons for molecular studies correlating apoptosis with events related to Alzheimer disease

This short review analyses the possible molecular events linking a general program of death such as apoptosis to highly specific intracellular pathways involving the function and degradation of two proteins--tau and amyloid precursor protein--which in their aggregated state constitute the hallmark of Alzheimer disease. By surveying the recent studies carried out in 'in vitro' neuronal cultures--with special emphasis to cerebellar granule neurons--the apparent correlation between onset of apoptosis, tau cleavage with formation of potential toxic fragments, and activation of an amyloidogenic route are discussed. Within this framework, proteasomes seem to play a crucial role upstream of the proteolytic cascade involving calpain(s) and caspase(s) by contributing to tau and amyloid precursor protein-altered breakdown and consequent tendency to aggregation of their degradation fragments. Thus, apoptotic death due to altered supply of anti apoptotic agents, neurotrophic factors, deafferentiation or other causes, may constitute a major trigger of the onset of Alzheimer disease.

Neuronal fibrillogenesis: amyloid fibrils from primary neuronal cultures impair long-term memory in the crab Chasmagnathus

Amyloid beta protein (Abeta) fibrillogenesis is considered one of the crucial steps of Alzheimer's disease (AD) pathogenesis. The effect of endogenous neuronal amyloid fibrils on memory processes is unknown. To investigate this issue, we first characterised the Abeta fibrillar aggregates secreted by cerebellar granule cells and then we evaluated the effect of neuronal fibrils on an invertebrate model of memory. An increase of fibril formation, assessed by Thioflavin T (ThT) fluorescence, was observed in the conditioned medium of apoptotic neurons during 48 h of the apoptotic process. Moreover, the depolarisation-stimulated secretion of cerebellar granule cells contains monomers of endogenous Abeta, which undergo cell-free fibrillogenesis over several days of incubation. The pattern of single endogenous fibrils, examined by electron microscopy, was similar to that of synthetic Abeta while a tighter and more complex interfibrillar organization was observed in endogenous fibrils. The biological effect of neuronal fibrils was studied in a long-term memory (LTM) paradigm, namely the context-signal learning of the crab Chasmagnathus. Pre-training injection of neuronal fibril extract (protein concentration, 1 microg/ml) induced amnesia in a dose-dependent manner. On the contrary, no effect on retention was observed with the administration of two orders higher doses (100 microg/ml) of synthetic Abeta1-40. These results indicate that only naturally secreted fibrils, but not synthetic Abeta, clearly interfere with memory process.

Rb binding protein Che-1 interacts with Tau in cerebellar granule neurons. Modulation during neuronal apoptosis

Che-1 is a recently identified human Rb binding protein that inhibits the Rb growth-suppressing function and regulates cell proliferation. Che-1 contacts the Rb and competes with HDAC1 for Rb-binding site, removing HDAC1 from the Rb/E2F cell cycle-regulated promoters. We have investigated the expression of Che-1 in neuronal cells and we showed that Che-1 directly interacts with Tau. Tau is a microtubule-associated protein involved in the assembly and stabilization of neuronal microtubules network that plays a crucial role modulating neuronal morphogenesis, axonal shape, and transport. In rat cerebellar granule neurons (CGNs) Che-1 partially colocalizes with Tau in the cytoplasm. Che-1 binds the amino-terminal region of Tau protein, which is not involved in microtubule interactions. Tau and Che-1 endogenous proteins coimmunoprecipitate from CGNs cellular lysates. In addition, Che-1/Tau interaction was demonstrated both in overexpressing COS-7 cells and CGNs by FRET analysis. Finally, we observed that Tau/Che-1 interaction is modulated during neuronal apoptosis.

Caspase activation increases beta-amyloid generation independently of caspase cleavage of the beta-amyloid precursor protein (APP)

The amyloid precursor protein (APP) undergoes "alternative" proteolysis mediated by caspases. Three major caspase recognition sites have been identified in the APP, i.e. one at the C terminus (Asp720) and two at the N terminus (Asp197 and Asp219). Caspase cleavage at Asp720 has been suggested as leading to increased production of Abeta. Thus, we set out to determine which putative caspase sites in APP, if any, are cleaved in Chinese hamster ovary cell lines concurrently with the increased Abeta production that occurs during apoptosis. We found that cleavage at Asp720 occurred concurrently with caspase 3 activation and the increased production of total secreted Abeta and Abeta1-42 in association with staurosporine- and etoposide-induced apoptosis. To investigate the contribution of caspase cleavage of APP to Abeta generation, we expressed an APP mutant truncated at Asp720 that mimics APP caspase cleavage at the C-terminal site. This did not increase Abeta generation but, in contrast, dramatically decreased Abeta production in Chinese hamster ovary cells. Furthermore, the ablation of caspase-dependent cleavage at Asp720, Asp197, and Asp219 (by site-directed mutagenesis) did not prevent enhanced Abeta production following etoposide-induced apoptosis. These findings indicate that the enhanced Abeta generation associated with apoptosis does not require cleavage of APP at its C-terminal (Asp720) and/or N-terminal caspase sites.

Potential use of stem cells in neuroreplacement therapies for neurodegenerative diseases

The use of stem cells for neuroreplacement therapy is no longer science fiction--it is science fact. We have succeeded in the development of neural and mesenchymal stem cell transplantation to produce neural cells in the brain. We have also seen improvement in cognitive function following stem cell transplantation in a memory-impaired aged animal model. These results promise a bright future for stem cell therapies in neurodegenerative diseases. Before we begin to think about clinical applications beyond the present preclinical studies, we have to consider the pathophysiological environment of individual diseases and weigh the factors that affect stem cell biology. Here, I not only review potential therapeutic applications of stem cell strategies in neurodegenerative diseases, but also discuss stem cell biology regarding factors that are altered under disease conditions.

Altered processing of amyloid precursor protein in the human neuroblastoma SH-SY5Y by chronic hypoxia

Alzheimer's disease (AD) is more prevalent following an ischemic or hypoxic episode, such as stroke. Indeed, brain levels of amyloid precursor protein (APP) and the cytotoxic amyloid beta peptide (Abeta) fragment are enhanced in these patients and in animal models following experimental ischaemia. We have investigated the effect of chronic hypoxia (CH; 2.5% O2, 24 h) on processing of APP in the human neuroblastoma, SH-SY5Y. We demonstrate that constitutive and muscarinic-receptor-enhanced secretion of the alpha-secretase cleaved fragment of APP, sAPPalpha, was reduced by approximately 60% in CH cells. The caspase inhibitor BOC-D(Ome)FMK did not reverse this effect of CH, and CH cells were as viable as controls, based on MTT assays. Thus, loss of sAPPalpha is not related to cell death or caspase processing of APP. Pre-incubation with antioxidants did not reverse the effect of CH, and the effect could not be mimicked by H2O2, discounting the involvement of reactive oxygen species in hypoxic loss of sAPPalpha. CH did not affect muscarinic activation of extracellular-signal regulated kinase. However, expression of ADAM 10 (widely believed to be alpha-secretase) was decreased approximately 50% following CH. Thus, CH selectively decreases processing of APP by the alpha-secretase pathway, most likely by decreasing levels of ADAM 10.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

The amyloidogenic pathway of amyloid precursor protein (APP) is independent of its cleavage by caspases

Amyloid beta-protein (A beta) is the main constituent of senile plaques in Alzheimer's disease and is derived by proteolysis from the amyloid precursor protein (APP). Generation and secretion of both A beta 40 and A beta 42 isoforms depend largely on internalization of APP and occurs mainly in the endocytic pathway. Evidence has also been presented (Gervais, F. G., Xu, D., Robertson, G. S., Vaillancourt, J. P., Zhu, Y., Huang, J., LeBlanc, A., Smith, D., Rigby, M., Shearman, M. S., Clarke, E. E., Zheng, H., Van der Ploeg, L. H. T., Ruffolo, S. C., Thornberry, N. A., Xanthoudakis, S., Zamboni, R. J., Roy, S., and Nicholson, D. W. (1999) Cell, 97, 395--406) that caspase cleavage of APP at its cytosolic tail affects its processing such that it is redirected to a more amyloidogenic pathway, resulting in enhanced A beta generation. However, caspase cleavage of APP also results in loss of its endocytosis signal (YENP), an event that would predict a decline in internalization and a concomitant decrease, not an increase, in A beta generation. In the present work, we examined whether caspase cleavage of APP is relevant to amyloidogenesis. We found that 1) caspase cleavage of APP results in reduced internalization and, accordingly, a decline in A beta secretion; 2) masking of the caspase site in APP did not affect A beta levels and, 3) caspase activation in cells by serum withdrawal did not increase A beta secretion. Thus, caspase cleavage of APP is unlikely to play a direct role in amyloidogenesis.

Transfer of the apoptotic message in sister cultures of cerebellar neurons

In vitro cultured cerebellar granule cells (cgc) undergo apoptotic death when the depolarising concentration of KCl 25 mM is adjusted to 5 mM. We investigated whether the apoptotic message can be transferred from a group of neurons to neighbouring but separated sister cultures by resorting to two different culture systems, one based on the use of concentric dishes, the other employing the transwells. The extent of transferred death was 38.6% in the two dishes system and 31.3% in the transwell system. The transfer of such death-inducing signal(s), accompanied by chromatin condensation, was inhibited by actinomicin D. Preliminary experiments aimed at identifying such apoptosis-inducing signal(s) suggest the involvement of beta-amyloid fragment(s).

Up-regulation of Bcl-xL in response to subtoxic beta-amyloid: role in neuronal resistance against apoptotic and oxidative injury

Neuron death in Alzheimer's disease is believed to be triggered by an increased production of amyloidogenic beta-amyloid peptides, involving both increased oxidative stress and activation of a conserved death program. Bcl-xL, an anti-apoptotic protein of the Bcl-2 family, is expressed at high levels in the adult nervous system. Exposure of neuronal cultures to subtoxic concentrations of beta-amyloid peptide 1-40 (1-10microM) or the fragment 25-35 (1-10microM) up-regulated both bcl-xL mRNA and Bcl-xL protein levels, determined by reverse transcriptase-polymerase chain reaction and western blot analysis. Bcl-xL protein was also up-regulated during oxidative stress induced by exposure to hydrogen peroxide (3-100microM) or ferric ions (1-10microM). In contrast, apoptotic stimuli (exposure to staurosporine or serum withdrawal) actually decreased neuronal Bcl-xL expression. To investigate the role of Bcl-xL in cell death relevant to Alzheimer's disease, we stably overexpressed Bcl-xL in human SH-SY5Y neuroblastoma cells. Cells overexpressing Bcl-xL were significantly protected from beta-amyloid neurotoxicity and staurosporine-induced apoptosis compared to vector-transfected controls. In contrast, Bcl-xL overexpression only conferred a mild protection against oxidative injury induced by hydrogen peroxide. We conclude that up-regulation of Bcl-xL expression in response to subtoxic concentrations of beta-amyloid is a stress response that increases the resistance of neurons to beta-amyloid neurotoxicity primarily by inhibiting apoptotic processes.

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.

Calcium ionophore A23187 specifically decreases the secretion of beta-secretase cleaved amyloid precursor protein during apoptosis in primary rat cortical cultures

Alzheimer's disease (AD) is characterized by the degeneration and loss of neurons, intracellular neurofibrillary tangles and the accumulation of extracellular senile plaques consisting mainly of beta-amyloid (A beta). A beta is generated from the amyloid precursor protein (APP) by sequential beta- and gamma-secretase cleavage. Alternatively, APP may be cleaved within the A beta region by alpha-secretase, preventing A beta formation. Here we investigated APP processing and secretion in primary neurons, using either colchicine or the calcium ionophore A23187 to induce apoptosis. Cell viability was determined by MTT measurements and apoptosis was further confirmed by annexin V and propidium iodide staining. We found that exposure to A23187 significantly decreased the secretion of soluble beta-secretase cleaved APP (beta-sAPP) in a caspase-dependent manner, although the secretion of total soluble APP beta sAPP) did not change. In addition, caspase inhibition restored cell viability to control levels. Exposure to colchicine did not change the amount of either secreted beta-sAPP or total sAPP and caspase inhibition was only partially able to restore cell viability. We conclude that calcium homeostasis is an important apoptotic effector specifically affecting the beta-secretase cleavage of APP.

Oxidative stress induces increase in intracellular amyloid beta-protein production and selective activation of betaI and betaII PKCs in NT2 cells

Amyloid beta-protein (Abeta) aggregation produces an oxidative stress in neuronal cells that, in turn, may induce an amyloidogenic shift of neuronal metabolism. To investigate this hypothesis, we analyzed intra- and extracellular Abeta content in NT2 differentiated cells incubated with 4-hydroxy-2,3-nonenal (HNE), a major product of lipid peroxidation. In parallel, we evaluated protein kinase C (PKC) isoenzymes activity, a signaling system suspected to modulate amyloid precursor protein (APP) processing. Low HNE concentrations (0.1-1 microM) induced a 2-6 fold increase of intracellular Abeta production that was concomitant with selective activation of betaI and betaII PKC isoforms, without affecting either cell viability or APP full-length expression. Selective activation of the same PKC isoforms was observed following NT2 differentiation. Our findings suggest that PKC beta isoenzymes are part of cellular mechanisms that regulate production of the intracellular Abeta pool. Moreover, they indicate that lipid peroxidation fosters intracellular Abeta accumulation, creating a vicious neurodegenerative loop.

Proteasome involvement and accumulation of ubiquitinated proteins in cerebellar granule neurons undergoing apoptosis

We investigated the potential role of the ubiquitin proteolytic system in the death of cerebellar granule neurons induced by reduction of extracellular potassium. Inhibitors of proteasomal function block apoptosis if administered at onset of this process, but they do not exert such effect when added 2-3 hr later. The same inhibitors also prevent caspase-3 activity and calpain-caspase-3-mediated processing of tau protein, suggesting that proteasomes are involved upstream of the caspase activation. Although the proteasomes seem to play an early primary role in programmed cell death, we found that with progression of apoptosis, during the execution phase, a perturbation in normal ubiquitin-proteasome function occurs, and high levels of ubiquitinated proteins accumulate in the cytoplasm of dying cells. Such accumulation correlates with a progressive decline of proteasome chymotrypsin and trypsin-like activities and, to a lower extent, of postacidic-like activity. Both intracytoplasmic accumulation of ubiquitinated proteins and decline of proteasome function are reversed by the pan-caspase inhibitor Z-VAD-fmk. The decline in proteasome function is accompanied by, and likely attributable to, a marked and progressive decline of deubiquitinating activities. The finding that the proteasomes are early involved in apoptosis and that ubiquitinated proteins accumulate during this process prospect granule neurons as a model system aimed at correlating these events with neurodegenerative diseases.

Granule neuron DNA damage following deafferentation in adult rats cerebellar cortex: a lesion model

Neuronal programmed cell death is regulated by a neurotrophic supply from targets and afferent inputs. The relative contribution of each component varies according to neuronal type and age. We have previously reported that primary cultures of cerebellar granule cells undergo apoptosis when deprived of depolarising KCl concentrations, suggesting a significant role of afferent inputs in the control of cerebellar granule cells survival. This issue was investigated by setting up various in vivo lesional paradigms in order to obtain partial or total deafferentation of the cerebellar granule layer in adult rats. At different times after surgery, cerebellar sections were subjected to TUNEL staining in order to detect possible DNA damage. One week after unilateral pedunculotomy, few scattered groups of apoptotic granule neurons were observed in the homolateral hemisphere. On the contrary, total deafferentation obtained by a new experimental paradigm based on an "L-cut" lesion induced massive and widespread apoptotic death in the granule layer of the deafferentated area. The time window of DNA fragmentation in granule layer was one to seven days after the "L-cut". Selective Purkinje cell deafferentation obtained by 3-acetylpyridine injection did not result in TUNEL staining in the cerebellar cortex. The current finding that mossy fiber axotomy induces granule cell apoptotic death points out for the first time the crucial role of afferent inputs in mature granule cell survival. Moreover, the in vivo lesional model described here may prove to be an useful tool for investigating cellular and molecular mechanisms of neuronal death triggered by deafferentation.

Shedding of membrane type 1 matrix metalloproteinase in a human breast carcinoma cell line

Membrane type 1 matrix metalloproteinase (MT1-MMP) with a transmembrane domain is a new member of the MMP gene family and is expressed on the cell surfaces of many carcinoma cells to activate the zymogen of MMP-2 (gelatinase A). We have previously reported that MT1-MMP is released into culture media in a complex form with tissue inhibitor of metalloproteinases 2 (TIMP-2) from a human breast carcinoma cell line, MDA-MB-231, treated with concanavalin A (Con A). In the present study, we further studied the release mechanism of MT1-MMP. Immunoblot analysis indicated that the amounts of MT1-MMP in culture media increase with the time of exposure and the concentration of Con A, and those in cell lysates conversely decrease in a similar way. Time- and dose-dependent release of MT1-MMP into the media was confirmed by a sandwich enzyme immunoassay specific to MT1-MMP. The molecular weight of the immunoreactive MTI-MMP in the media was Mr 56,000, which was 4,000-Mr smaller than that in the cell lysates. Northern blot analysis demonstrated that the mRNA expression level of MT1-MMP is about 3-fold enhanced after a 24 h-exposure to Con A and this is maintained up to 72-h exposure. The release of MT1-MMP from the Con A-treated cells was inhibited by metalloproteinase inhibitors such as EDTA and o-phenanthroline, but not by MMP inhibitors including TIMP-1, TIMP-2 and BB94 or other proteinase inhibitors of serine, cysteine and aspartic proteinases. During the Con A treatment of the cells, cell viability decreased time- and dose-dependently and dead cells reacted positively in the TdT-mediated dUTP Nick-End Labeling (TUNEL) method. Con A-treated MDA cells showed apoptotic morphology when stained with Hoechst dye and hematoxylin and eosin. DNA ladder formation was detected by electrophoresis of the DNA from Con A-treated MDA cells. These results suggest that MT1-MMP release from Con A-treated cells is due to shedding mediated by metalloproteinase(s) other than MMPs, and is associated with apoptosis.

Recent advances on neuronal caspases in development and neurodegeneration

In view of a large and growing literature, this overview emphasizes recent advances in neuronal caspases and their role in cell death. To provide historical perspective, morphology and methods are surveyed with emphasis on early studies on interleukin converting enzyme (ICE) as a prototype for identifying zymogen subunits. The unexpected homology of ICE (caspase-1) to Caenorhabditis elegans death gene CED-3 provided early clues linking caspases to programmed cell death, and led later to discovery of bcl-2 proteins (CED-9 homologs) and 'apoptosis associated factors' (Apafs). Availability of substrates, inhibitors, and cDNAs led to identification of up to 16 caspases as a new superfamily of unique cysteine proteinases targeting Asp groups. Those acting as putative death effectors dismantle neurons by catabolism of proteins essential for survival. Caspases degrade amyloid precursor protein (APP), presenilins (PS1, PS2), tau, and huntingtin, raising questions on their role in neurodegeneration. Brain contains 'inhibitors of apoptosis proteins' (IAPs) survivin and NAIP associated also with some neuronal disorders. Apoptotic stress in neurons initiates a chain of events leading to activation of distal caspases by pathways that remain to be fully mapped. Neuronal caspases play multiple roles for initiation and execution of cell death, for morphogenesis, and in non-mitotic neurons for homeostasis. Recent studies focus on cytochrome c as pivotal in mediating conversion of procaspase-9 as a major initiator for apoptosis. Identifying signaling pathways and related events paves the way to design useful therapeutic remedies to prevent neuronal loss in disease or aging.

Alternative, non-secretase processing of Alzheimer's beta-amyloid precursor protein during apoptosis by caspase-6 and -8

Alzheimer's disease (AD) is a progressive neurodegenerative disorder. Although the pathogenesis of AD is unknown, it is widely accepted that AD is caused by extracellular accumulation of a neurotoxic peptide, known as Abeta. Mutations in the beta-amyloid precursor protein (APP), from which Abeta arises by proteolysis, are associated with some forms of familial AD (FAD) and result in increased Abeta production. Two other FAD genes, presenilin-1 and -2, have also been shown to regulate Abeta production; however, studies examining the biological role of these FAD genes suggest an alternative theory for the pathogenesis of AD. In fact, all three genes have been shown to regulate programmed cell death, hinting at the possibility that dysregulation of apoptosis plays a primary role in causing neuronal loss in AD. In an attempt to reconcile these two hypotheses, we investigated APP processing during apoptosis and found that APP is processed by the cell death proteases caspase-6 and -8. APP is cleaved by caspases in the intracellular portion of the protein, in a site distinct from those processed by secretases. Moreover, it represents a general effect of apoptosis, because it occurs during cell death induced by several stimuli both in T cells and in neuronal cells.