G protein-mediated neuronal DNA fragmentation induced by familial Alzheimer's disease-associated mutants of APP

A new paradigm for neurotoxicity by FAD mutants of betaAPP: a signaling abnormality

We have demonstrated that normal betaAPP695 behave as a signaling receptor and indicated that point mutations at V642 create autoactive betaAPP in signal transduction. Cellular expression of those familial Alzheimer's disease-associated mutants causes neuronal cells to undergo apoptotic death; and procedures inhibiting the signal of normal betaAPP block the mutant-induced apoptosis. We have also shown that the mutant-induced death is mediated by intracellular G protein activity but not by secretion of Abeta peptides. Accordingly, the mutant-induced death requires a cytoplasmic domain but not the 41st and 42nd residues of the Abeta region. These studies provide a novel insight that betaAPP may play a normal role as a death receptor and that Alzheimer's disease-relevant abnormality occurred in this function may lead neurons to suicidal degeneration.

Effects of specific protease inhibitors on amyloid beta-protein 42 secretion

Amyloid beta-protein (Abeta) is classified into two subspecies defined by its C-terminal length, designated Abeta40 and Abeta42. Although Abeta42 accounts for only approximately 10% of secreted Abeta, this particular species is the most dominant species in Abeta deposits in Alzheimer's disease (AD) and normal aged brains and appears to be the initially deposited species. Secretion levels of Abeta42 have been shown to increase in patients affected by any form of early-onset familial AD. Thus, the suppression of Abeta42 production or secretion could be a therapeutic strategy for AD. In this study, we examined whether protease inhibition affects the Abeta42 secretion ratio (Abeta42: total Abeta). Using specific inhibitors, we determined that the inhibition of calpain but not proteasome induces an increased Abeta42 secretion in cultured cells. These data suggest that calpain differentially affects the gamma-secretases generating Abeta40 and Abeta42 and indicate the possibility of developing compounds that reduce Abeta42 production and secretion though this pathway.

Alzheimer's disease: a re-examination of the amyloid hypothesis

Alzheimer's disease (AD) is a neurodegenerative disorder of the brain characterized by the presence of neuritic amyloid plaques and neurofibrillary tangles. Although it most frequently occurs in the elderly, this disorder also afflicts younger patients. The majority of AD cases are late in onset, lack an obvious genetic etiology and are characterized as sporadic, whereas a small percentage of cases are early in onset and segregate strongly within families (FAD), suggesting a genetic etiology. During the past decade it has become evident that the clinical and histopathological phenotypes of this disease are caused by heterogeneous genetic, and probably environmental, factors. Indeed, several genes have been identified that together appear to cause most of the familial forms of the disease, whereas the epsilon4 allele of the apolipoprotein E (apoE) gene has been shown to be a significant risk factor for the late onset forms of AD. Despite this evidence of heterogeneity, it has been suggested that all of these factors work through a common pathway by triggering the deposition of amyloid in the brain, which is ultimately responsible for the neuronal degeneration of AD. This is a controversial theory, however, primarily because there is a poor correlation between the concentrations and distribution of amyloid depositions in the brain and several parameters of AD pathology, including degree of dementia, loss of synapses, loss of neurons and abnormalities of the cytoskeleton.

The WW domain of neural protein FE65 interacts with proline-rich motifs in Mena, the mammalian homolog of Drosophila enabled

The neural protein FE65 contains two types of protein-protein interaction modules: one WW binding domain and two phosphotyrosine binding domains. The carboxyl-terminal phosphotyrosine binding domain of FE65 interacts in vivo with the beta-amyloid precursor protein, which is implicated in Alzheimer disease. To understand the function of this adapter protein, we identified binding partners for the FE65 WW domain. Proline-rich sequences sharing a proline-proline-leucine-proline core motif were recovered by screening expression libraries for ligands of the FE65 WW domain. Five proteins of molecular masses 60, 75, 80, 140, and 200 kDa could be purified from mouse brain lysates by affinity to the FE65 WW domain. We identified two of these five proteins as the 80- and 140-kDa isoforms encoded by Mena, the mammalian homolog of the Drosophila Enabled gene. Using the SPOTs technique of peptide synthesis, we identified the sequences in Mena that interact with the FE65 WW domain and found that they contain the signature proline-proline-leucine-proline motif. Finally, we demonstrated that Mena binds to FE65 in vivo by coimmunoprecipitation assay from COS cell extracts. The specificity of the Mena-FE65 WW domain association was confirmed by competition assays. Further characterization of the FE65-Mena complex may identify a physiological role for these proteins in beta-amyloid precursor protein biogenesis and may help in understanding the mechanism of molecular changes that underlie Alzheimer disease.

Distribution of mRNA encoding three alpha 2-adrenergic receptor subtypes in the developing mouse embryo suggests a role for the alpha 2A subtype in apoptosis

alpha 2-Adrenergic receptors (alpha 2-ARs) respond to norepinephrine and epinephrine to mediate diverse physiological effects. Using in situ hybridization, the expression pattern of the mRNA encoding the three alpha 2-AR subtypes (alpha 2A, alpha 2B, and alpha 2C) was examined in the mouse embryo. The mRNA encoding the three subtypes was first detected at stage 9.5 days postcoitus (d.p.c.) for the alpha 2A-AR (coincident with norepinephrine availability), 11.5 d.p.c. for the alpha 2B-AR, and 14.5 d.p.c. for the alpha 2C-AR subtype. The mRNA encoding the alpha 2A-AR subtype shows both the earliest and the most widespread expression pattern, including developing stomach and cecum, many craniofacial regions and areas in the central nervous system. Strikingly, the alpha 2A-AR mRNA is expressed in the interdigital mesenchyme between stage 12.5 and 14.5 d.p.c. in parallel with digit separation, raising the possibility that the alpha 2A-AR might contribute to the apoptotic events underlying this process. To test whether alpha 2A-AR can signal apoptotic events, the alpha 2A-AR subtype was introduced into two mouse mesenchymal cell lines, C3H/10t1/2 and NIH-3T3; expression of the alpha 2A-AR correlated with accelerated apoptosis, as detected both by the TUNEL assay and the loss of cell viability. In contrast to the wide distribution of mRNA encoding the alpha 2A-AR subtype, the alpha 2B-AR mRNA was detected only in the developing liver and was most readily detectable between 11.5 and 14.5 d.p.c., when the liver is the principal site of hematopoiesis. The alpha 2C-AR mRNA is detected in the nasal cavity and cerebellar primordium only at > or = 14.5 d.p.c. These studies represent the first characterization of the temporal and spatial expressions of the alpha 2A-AR, alpha 2B-AR, and alpha 2C-AR subtypes during embryogenesis and provide important insights concerning the loci and possible roles of alpha 2-AR-mediated regulation of physiological processes during the developmental program.

Increased apoptosis arising from increased expression of the Alzheimer's disease-associated presenilin-2 mutation (N141I)

Mutations in the genes for presenilin 1 and 2 (PS-1 and PS-2) have been linked to development of early-onset Alzheimer's disease (AD). As neither the normal function of either presenilin is known nor why mutations cause disease, we examined the properties of wild-type, truncated, and mutant PS-2 upon expression in HeLa cells. Although HeLa cells are strongly predisposed to continued mitosis, expression of PS-2 induced programmed cell death (apoptosis). Direct evidence for apoptosis was obtained by double staining for terminal deoxynucleotide transferase nick end labeling (TUNEL) and PS-2 expression and by following green fluorescent protein-tagged PS-2 over time. Deletion analysis indicates that as little as 166 NH2-terminal residues of PS-2 are sufficient for endoplasmic reticulum (ER) localization and apoptosis. Moreover, the AD- associated PS-2 missense mutation (N141I) more efficiently induced cell death compared to wild-type PS-2 despite lower mutant protein accumulation. Expression of the presenilins in several other cell lines and transgenic mice has been accompanied by rapid protein cleavage without the induction of cell death. In contrast, PS-2 expressed in HeLa cells was not cleaved, and cell death occurred. We hypothesize that full-length but not cleaved PS-2 may be important in the regulation or induction of apoptosis.

Etorphine inhibits cell growth and induces apoptosis in SK-N-SH cells: involvement of pertussis toxin-sensitive G proteins

Opiates have been used extensively in the treatment of pain but with the severe side effect of addiction, which is believed to be related to opiates' direct (primary) or indirect (secondary) neurotoxicity. In this study, the effects of opioids on cell growth and apoptosis have been examined in human neuroblastoma cell line SK-N-SH. Etorphine, a wide-spectrum and potent agonist of opioid receptors, was found to significantly inhibit cell growth and to induce apoptosis. The inhibitory and apoptotic activities of etorphine followed a dose- and time-dependent manner. The more specific agonists of opioid receptors such as morphine, [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAGO), [D-Pen2, D-Pen5]-enkephalin (DPDPE), dynorphin A and nociceptin/orphanin FQ did not show similar toxic activities under the same conditions. In addition, the effects of etorphine could not be blocked by the opioid receptor antagonist naloxone, suggesting that the effects of etorphine might not be mediated by a classical opioid receptor. However, pretreatment of SK-N-SH cells with pertussis toxin (PTX) blocked the inhibition of cell growth and apoptosis induced by etorphine, indicating the involvement of PTX-sensitive G proteins in the processes. It was also shown that etorphine-induced apoptosis was prevented by actinomycin D (AD) and interleukin-1beta converting enzyme inhibitor I. Interestingly, etorphine was similarly potent to inhibit growth of pheochromocytoma (PC12) cells but less effective in SH-SY5Y neuroblastoma cells and C6 glioma cells. We propose that inhibition of cell growth and induction of apoptosis may be one mechanism of opioid neurotoxicity.

A purine nucleoside phosphorylase (PNP) inhibitor induces apoptosis via caspase-3-like protease activity in MOLT-4 T cells

Children with congenital homozygous deficiency of purine nucleoside phosphorylase (PNP) have abnormalities in purine metabolism that result in T-cell selective immune deficiency. The mechanism of action for cell death has been attributed to intracellular accumulation of dGTP, a potent inhibitor of ribonucleotide reductase and subsequently DNA synthesis, in thymocytes and T-cells but not B-cells. However, the mode of cell death has not been determined to be either necrosis or apoptosis. To examine the involvement of apoptosis in T-cells following PNP inhibition, MOLT-4 cells, a human T cell leukemia cell line, were co-treated with the PNP inhibitor, CI-1000 (2-amino 3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]-pyrimidin-4-one HCl), and 2'-deoxyguanosine (dGuo) which resulted in a concentration-dependent loss of cell viability (trypan blue) and inhibition of tritiated thymidine ([3H]-TdR) uptake. Staining of cells with the DNA dye Hoechst 33,258 showed nuclear morphology characteristic of apoptosis. Western blots (24 h lysates) were probed with antibodies against several proteins implicated in apoptosis. Anti-PARP revealed the presence of an 85 kD PARP breakdown product while, anti-alpha-spectrin revealed the accumulation a 120 kD breakdown product, both suggestive of CPP32 cleavage (caspase-3; an ICE-like cysteine protease). Western blots also detected the loss of the intact 32 kD caspase-3 isoform, a biochemical event associated with caspase-3 activation. Corresponding fluorometric activity assays detected a marked increase in caspase-3-like activity using the substrate Ac-DEVD-MCA. Lastly, a pan caspase inhibitor (Z-D-DCB) and 2'-deoxycytidine (dCyd), which is known to prevent dGTP accumulation following PNP inhibition, were able to prevent cell death and all indicators of caspase-3-like activity in MOLT-4 cells co-treated with dGuo and CI-1000. In summary, we provided several lines of evidence for the role of apoptosis and the contribution of caspase-3-like proteases in T-cell death following PNP inhibition.

Constitutively active Galphaq and Galpha13 trigger apoptosis through different pathways

We investigated the effect of expression of constitutively active Galpha mutants on cell survival. Transfection of constitutively active Galphaq and Galpha13 in two different cell lines caused condensation of genomic DNA and nuclear fragmentation. Endonuclease cleavage of genomic DNA was followed by labeling the DNA fragments and subsequent flow cytometric analysis. The observed cellular phenotype was identical to the phenotype displayed by cells undergoing apoptosis. To distinguish between the apoptosis-inducing ability of the two Galpha-subunits, the signaling pathways involved in this cellular function were investigated. Whereas Galphaq induced apoptosis via a protein kinaseC-dependent pathway, Galpha13 caused programmed cell death through a pathway involving the activation of the small G-protein Rho. Both of the pathways leading to apoptosis were blocked by overexpression of bcl-2. In contrast to other apoptosis-inducing systems, expression of constitutively active Galphaq and Galpha13 triggered apoptosis in high serum as well as in defined medium.

Aberrant expression of bcl-2 gene family in Down's syndrome brains

Down's syndrome (DS) patient brains are known to develop prematurely the same degenerative changes as those seen in Alzheimer's disease (AD). On the assumption that the apoptotic mechanism is involved in the neuronal loss in DS, we have investigated the expression of the bcl-2 gene family in DS brains and found marked alterations. The most prominent changes were in the temporal lobes where neuronal loss was greatest. Our findings suggest that a apoptotic process is involved in the neuronal loss in DS.

Apoptosis, neurotrophic factors and neurodegeneration

Apoptosis is an active process of cell death characterized by distinct morphological features, and is often the end result of a genetic programme of events, i.e. programmed cell death (PCD). There is growing evidence supporting a role for apoptosis in some neurodegenerative diseases. This conclusion is based on DNA fragmentation studies and findings of increased levels of pro-apoptotic genes in human brain and in in vivo and in vitro model systems. Additionally, there is some evidence for a loss of neurotrophin support in neurodegenerative diseases. In Alzheimer's disease, in particular, there is strong evidence from human brain studies, transgenic models and in vitro models to suggest that the mode of nerve cell death is apoptotic. In this review we describe the evidence implicating apoptosis in neurodegenerative diseases with a particular emphasis on Alzheimer's disease.

Recombinant expression of caveolin-1 in oncogenically transformed cells abrogates anchorage-independent growth

Caveolae are plasma membrane-attached vesicular organelles. Caveolin-1, a 21-24-kDa integral membrane protein, is a principal component of caveolae membranes in vivo. Both caveolae and caveolin are most abundantly expressed in terminally differentiated cells: adipocytes, endothelial cells, and muscle cells. Conversely, caveolin-1 mRNA and protein expression are lost or reduced during cell transformation by activated oncogenes such as v-abl and H-ras (G12V); caveolae are absent from these cell lines. However, its remains unknown whether down-regulation of caveolin-1 protein and caveolae organelles contributes to their transformed phenotype. Here, we have expressed caveolin-1 in oncogenically transformed cells under the control of an inducible-expression system. Regulated induction of caveolin-1 expression was monitored by Western blot analysis and immunofluorescence microscopy. Our results indicate that caveolin-1 protein is expressed well using this system and correctly localizes to the plasma membrane. Induction of caveolin-1 expression in v-Abl-transformed and H-Ras (G12V)-transformed NIH 3T3 cells abrogated the anchorage-independent growth of these cells in soft agar and resulted in the de novo formation of caveolae as seen by transmission electron microscopy. Consistent with its antagonism of Ras-mediated cell transformation, caveolin-1 expression dramatically inhibited both Ras/MAPK-mediated and basal transcriptional activation of a mitogen-sensitive promoter. Using an established system to detect apoptotic cell death, it appears that the effects of caveolin-1 may, in part, be attributed to its ability to initiate apoptosis in rapidly dividing cells. In addition, we find that caveolin-1 expression levels are reversibly down-regulated by two distinct oncogenic stimuli. Taken together, our results indicate that down-regulation of caveolin-1 expression and caveolae organelles may be critical to maintaining the transformed phenotype in certain cell populations.

Intracellular generation and accumulation of amyloid beta-peptide terminating at amino acid 42

Amyloid beta-peptide (Abeta) is known to accumulate in senile plaques of Alzheimer's disease (AD) patients and is now widely believed to play a major role in the disease. Two populations of peptides occur terminating either at amino acid 40 or at amino acid 42 (Abeta1-40 and Abeta1-42). Alternative N-terminal cleavages produce additional heterogeneity (Abetax-40 and Abetax-42). Peptides terminating at amino acid 42 are believed to be the major player in sporadic AD as well as familial AD (FAD). Whereas the cellular mechanism for the generation of Abeta terminating at amino acid 40 is well understood, very little is known about the cleavage of Abeta after amino acid 42. By using two independent methods we demonstrate intracellular Abeta1-42 as well as Abetax-42 but less Abetax-40 and Abeta1-40 in kidney 293 cells stably transfected with wild type beta-amyloid precursor protein (betaAPP) or the FAD-associated Val/Gly mutation. Moreover, retention of betaAPP within the endoplasmic reticulum (ER) by treatment with brefeldin A does not block the cleavage at amino acid 42 but results in an increased production of all species of Abeta terminating at amino acid 42. This indicates that the cleavage after amino acid 42 can occur within the ER. Treatment of cells with monensin, which blocks transport of (betaAPP) within the Golgi causes a marked accumulation of intracellular Abetax-42 and Abetax-40. Therefore these experiments indicate that the gamma-secretase cleavage of Abeta after amino acid 42 can occur within the ER and later within the secretory pathway within the Golgi. Moreover inhibition of reinternalization by cytoplasmic deletions of betaAPP as well as inhibition of intracellular acidification by NH4Cl does not block intracellular Abeta1-42 or Abetax-42 production.

The toxicity of the Alzheimer's beta-amyloid peptide correlates with a distinct fiber morphology

In an attempt to elucidate the relationship among aggregation properties, fiber morphology, and cellular toxicity several beta-amyloid peptides (A beta) were prepared according to a standardized procedure. Peptides either carried mutations inside the membrane anchor segment around amino acid position 35 or their carboxy terminus was shortened from 42 to 41, 40, or 39 amino acids. The time-dependent self-assembly of monomeric A beta into fibers was simultaneously monitored by electron microscopy, circular dichroism spectroscopy, analytical ultracentrifugation, and A beta-mediated cellular toxicity using the reduction of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) to measure cell viability. The transition of A beta monomers into fibers was analyzed by more than 600 electron micrographs. Distinct morphological changes from seed-like structures to immature and mature fibers were observed. Seeds were of spherical appearance. Immature fibers were typically elongated structures with a rough surface and with varying thickness depending on the A beta sequence. Mature fibers were characterized by a periodic variation of their thickness along the fiber axis. The proportion of these different structures and the total amount of aggregated A beta was amino acid sequence-dependent. Wild-type A beta 1-42 and its oxidized derivative carrying a methionine sulfoxide residue at position 35 showed the highest rate of fiber formation and exerted toxic activity in the MTT assay at very low nanomolar concentrations. The fibers formed by these two peptides were predominantly of the mature type. In contrast, carboxyl-terminus truncated peptides A beta 1-41, A beta 1-40, and A beta 1-39 or most A beta 1-42 derivatives mutated around amino acid position 35 showed a reduced aggregation rate, the immature fibers predominated, and the toxicity was orders of magnitude lower. Thus, a correlation can be drawn among the chemical structure, aggregation properties, fiber morphology, and cellular toxicity.

Participation of presenilin 2 in apoptosis: enhanced basal activity conferred by an Alzheimer mutation

Overexpression of the familial Alzheimer's disease gene Presenilin 2 (PS2) in nerve growth factor-differentiated PC12 cells increased apoptosis induced by trophic factor withdrawal or beta-amyloid. Transfection of antisense PS2 conferred protection against apoptosis induced by trophic withdrawal in nerve growth factor-differentiated or amyloid precursor protein-expressing PC12 cells. The apoptotic cell death induced by PS2 protein was sensitive to pertussis toxin, suggesting that heterotrimeric GTP-binding proteins are involved. A PS2 mutation associated with familial Alzheimer's disease was found to generate a molecule with enhanced basal apoptotic activity. This gain of function might accelerate the process of neurodegeneration that occurs in Alzheimer's disease, leading to the earlier age of onset characteristic of familial Alzheimer's disease.