Alzheimer's PS-1 mutation perturbs calcium homeostasis and sensitizes PC12 cells to death induced by amyloid beta-peptide

Aberrant subcellular neuronal calcium regulation in aging and Alzheimer's disease

In this mini-review/opinion article we describe evidence that multiple cellular and molecular alterations in Alzheimer's disease (AD) pathogenesis involve perturbed cellular calcium regulation, and that alterations in synaptic calcium handling may be early and pivotal events in the disease process. With advancing age neurons encounter increased oxidative stress and impaired energy metabolism, which compromise the function of proteins that control membrane excitability and subcellular calcium dynamics. Altered proteolytic cleavage of the β-amyloid precursor protein (APP) in response to the aging process in combination with genetic and environmental factors results in the production and accumulation of neurotoxic forms of amyloid β-peptide (Aβ). Aβ undergoes a self-aggregation process and concomitantly generates reactive oxygen species that can trigger membrane-associated oxidative stress which, in turn, impairs the functions of ion-motive ATPases and glutamate and glucose transporters thereby rendering neurons vulnerable to excitotoxicity and apoptosis. Mutations in presenilin-1 that cause early-onset AD increase Aβ production, but also result in an abnormal increase in the size of endoplasmic reticulum calcium stores. Some of the events in the neurodegenerative cascade can be counteracted in animal models by manipulations that stabilize neuronal calcium homeostasis including dietary energy restriction, agonists of glucagon-like peptide 1 receptors and drugs that activate mitochondrial potassium channels. Emerging knowledge of the actions of calcium upstream and downstream of Aβ provides opportunities to develop novel preventative and therapeutic interventions for AD. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Oxidative Stress-Mediated Brain Dehydroepiandrosterone (DHEA) Formation in Alzheimer's Disease Diagnosis

Neurosteroids are steroids made by brain cells independently of peripheral steroidogenic sources. The biosynthesis of most neurosteroids is mediated by proteins and enzymes similar to those identified in the steroidogenic pathway of adrenal and gonadal cells. Dehydroepiandrosterone (DHEA) is a major neurosteroid identified in the brain. Over the years we have reported that, unlike other neurosteroids, DHEA biosynthesis in rat, bovine, and human brain is mediated by an oxidative stress-mediated mechanism, independent of the cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1) enzyme activity found in the periphery. This alternative pathway is induced by pro-oxidant agents, such as Fe(2+) and β-amyloid peptide. Neurosteroids are involved in many aspects of brain function, and as such, are involved in various neuropathologies, including Alzheimer's disease (AD). AD is a progressive, yet irreversible neurodegenerative disease for which there are limited means for ante-mortem diagnosis. Using brain tissue specimens from control and AD patients, we provided evidence that DHEA is formed in the AD brain by the oxidative stress-mediated metabolism of an unidentified precursor, thus depleting levels of the precursor in the blood stream. We tested for the presence of this DHEA precursor in human serum using a Fe(2+)-based reaction and determined the amounts of DHEA formed. Fe(2+) treatment of the serum resulted in a dramatic increase in DHEA levels in control patients, whereas only a moderate or no increase was observed in AD patients. The DHEA variation after oxidation correlated with the patients' cognitive and mental status. In this review, we present the cumulative evidence for oxidative stress as a natural regulator of DHEA formation and the use of this concept to develop a blood-based diagnostic tool for neurodegenerative diseases linked to oxidative stress, such as AD.

Mitochondria: the calcium connection

Calcium handling by mitochondria is a key feature in cell life. It is involved in energy production for cell activity, in buffering and shaping cytosolic calcium rises and also in determining cell fate by triggering or preventing apoptosis. Both mitochondria and the mechanisms involved in the control of calcium homeostasis have been extensively studied, but they still provide researchers with long-standing or even new challenges. Technical improvements in the tools employed for the investigation of calcium dynamics have been-and are still-opening new perspectives in this field, and more prominently for mitochondria. In this review we present a state-of-the-art toolkit for calcium measurements, with major emphasis on the advantages of genetically encoded indicators. These indicators can be efficiently and selectively targeted to specific cellular sub-compartments, allowing previously unavailable high-definition calcium dynamic studies. We also summarize the main features of cellular and, in more detail, mitochondrial calcium handling, especially focusing on the latest breakthroughs in the field, such as the recent direct characterization of the calcium microdomains that occur on the mitochondrial surface upon cellular stimulation. Additionally, we provide a major example of the key role played by calcium in patho-physiology by briefly describing the extensively reported-albeit highly controversial-alterations of calcium homeostasis in Alzheimer's disease, casting lights on the possible alterations in mitochondrial calcium handling in this pathology.

H2O2 and PAF mediate Abeta1-42-induced Ca2+ dyshomeostasis that is blocked by EGb761

Calcium (Ca2+) dyshomeostasis may be of pivotal importance in mediating the neurotoxic action of amyloid beta peptide (Abeta), but the mechanism whereby Abeta disrupts Ca2+ homeostasis remains unclear. Using hippocampal neuronal cultures, the present study investigated possible mechanisms underlying Ca2+ dyshomeostasis induced by the oligomeric form of Abeta1-42 and two possible mediators of its toxicity, hydrogen peroxide (H2O2) and platelet-activating factor (PAF). It was found that, both H2O2 and PAF were able to reproduce each of the events induced by oligomeric Abeta1-42, including (a) Ca2+ influx via N-methyl-D-aspartic acid (NMDA) receptors, (b) enhancement of Ca2+ response to NMDA via activation of protein kinase C (PKC), (c) the increase of extracellular concentrations of glutamate and (d) the increase in cytosolic free Ca2+ ([Ca2+]i). Moreover, each of these events could be blocked by Ginkgo biloba extract EGb761, a free radical scavenger with PAF antagonism, and by quercetin, a constituent with well-established free radical scavenging property. In contrast, ginkgolide B, another constituent of EGb761 with well-established PAF-antagonizing activity protected the neurons against Ca2+ dyshomeostasis induced by Abeta1-42 and PAF, but not by H2O2. These results suggested the possibility that Abeta1-42-induced Ca2+ dyshomeostasis might be mediated by formation of toxic mediators such as H2O2 and PAF. Therefore, increased production of toxic mediators such as H2O2 and PAF in the brain may be critical in the pathological mechanism of neurodegenerative diseases, particularly Alzheimer's disease (AD), and may serve as major therapeutic targets for these diseases.

ER calcium and Alzheimer's disease: in a state of flux

The calcium ion (Ca(2+)) plays fundamental roles in orchestrating dynamic changes in the function and structure of nerve cell circuits in the brain. The endoplasmic reticulum (ER), an organelle that actively removes Ca(2+) from the cytoplasm, can release stored Ca(2+) through ER membrane receptor channels responsive either to the lipid messenger inositol trisphosphate (IP(3)) or to cytosolic Ca(2+). Emerging findings suggest that perturbed ER Ca(2+) homeostasis contributes to the dysfunction and degeneration of neurons that occurs in Alzheimer's disease (AD). Presenilin-1 (PS1) is an integral membrane protein in the ER; mutations in PS1 that cause early-onset inherited AD increase the pool of ER Ca(2+) available for release and also enhance Ca(2+) release through ER IP(3)- and ryanodine-sensitive channels. By enhancing Ca(2+) flux across the ER membrane, PS1 mutations may exaggerate Ca(2+) signaling in synaptic terminals and thereby render them vulnerable to dysfunction and degeneration in the settings of aging and amyloid accumulation in AD.

Mitochondria sense with different kinetics the calcium entering into HeLa cells through calcium channels CALHM1 and mutated P86L-CALHM1

The novel Ca(2+) channel CALHM1 (Calcium Homeostasis Modulator 1) generates cytosolic Ca(2+) transients ([Ca(2+)](c)) that regulate the production of amyloid beta (Abeta). Its mutated channel P86L-CALHM1 has been associated to Alzheimer's disease (AD). Using cytosolic- and mitochondrial-targeted aequorins, we have investigated here whether mitochondria sense with similar or different kinetics the Ca(2+) entering into Hela cells and the Ca(2+) released from the endoplasmic reticulum (ER), in control and in cells transfected with CALHM1 and P86L-CALHM1. We have shown that mitochondria sense Ca(2+) entry in the three cell types; however, the [Ca(2+)](c) and mitochondrial Ca(2+) transients [Ca(2+)](m) had substantially slower kinetics in cells expressing P86L-CALHM1. Mitochondria also sensed the ER Ca(2+) released by histamine, but in CALHM1 and P86L-CALHM1 cells the kinetics was faster than that of control cells. Data are compatible with the idea that mutated CALHM1 may cause mitochondrial Ca(2+) overload, suggesting how these cells may become more vulnerable to apoptotic stimuli.

Calcium hypothesis of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder caused by an increase in amyloid metabolism. The calcium hypothesis of AD explores how activation of the amyloidogenic pathway may function to remodel the neuronal Ca(2+) signaling pathways responsible for cognition. Hydrolysis of the beta-amyloid precursor protein (APP) yields two products that can influence Ca(2+) signaling. Firstly, the amyloids released to the outside form oligomers that enhance the entry of Ca(2+) that is pumped into the endoplasmic reticulum (ER). An increase in the luminal level of Ca(2+) within the ER enhances the sensitivity of the ryanodine receptors (RYRs) to increase the amount of Ca(2+) being released from the internal stores. Secondly, the APP intracellular domain may alter the expression of key signaling components such as the RYR. It is proposed that this remodeling of Ca(2+) signaling will result in the learning and memory deficits that occur early during the onset of AD. In particular, the Ca(2+) signaling remodeling may erase newly acquired memories by enhancing the mechanism of long-term depression that depends on activation of the Ca(2+)-dependent protein phosphatase calcineurin. The alteration in Ca(2+) signaling will also contribute to the neurodegeneration that characterizes the later stages of dementia.

Deviant ryanodine receptor-mediated calcium release resets synaptic homeostasis in presymptomatic 3xTg-AD mice

Presenilin mutations result in exaggerated endoplasmic reticulum (ER) calcium release in cellular and animal models of Alzheimer's disease (AD). In this study, we examined whether dysregulated ER calcium release in young 3xTg-AD neurons alters synaptic transmission and plasticity mechanisms before the onset of histopathology and cognitive deficits. Using electrophysiological recordings and two-photon calcium imaging in young (6-8 weeks old) 3xTg-AD and non-transgenic (NonTg) hippocampal slices, we show a marked increase in ryanodine receptor (RyR)-evoked calcium release within synapse-dense regions of CA1 pyramidal neurons. In addition, we uncovered a deviant contribution of presynaptic and postsynaptic ryanodine receptor-sensitive calcium stores to synaptic transmission and plasticity in 3xTg-AD mice that is not present in NonTg mice. As a possible underlying mechanism, the RyR2 isoform was found to be selectively increased more than fivefold in the hippocampus of 3xTg-AD mice relative to the NonTg controls. These novel findings demonstrate that 3xTg-AD CA1 neurons at presymptomatic ages operate under an aberrant, yet seemingly functional, calcium signaling and synaptic transmission system long before AD histopathology onset. These early signaling alterations may underlie the later synaptic breakdown and cognitive deficits characteristic of later stage AD.

Calcium dysregulation in Alzheimer's disease: from mechanisms to therapeutic opportunities

Calcium is involved in many facets of neuronal physiology, including activity, growth and differentiation, synaptic plasticity, and learning and memory, as well as pathophysiology, including necrosis, apoptosis, and degeneration. Though disturbances in calcium homeostasis in cells from Alzheimer's disease (AD) patients have been observed for many years, much more attention was focused on amyloid-beta (Abeta) and tau as key causative factors for the disease. Nevertheless, increasing lines of evidence have recently reported that calcium dysregulation plays a central role in AD pathogenesis. Systemic calcium changes accompany almost the whole brain pathology process that is observed in AD, including synaptic dysfunction, mitochondrial dysfunction, presenilins mutation, Abeta production and Tau phosphorylation. Given the early and ubiquitous involvement of calcium dysregulation in AD pathogenesis, it logically presents a variety of potential therapeutic targets for AD prevention and treatment, such as calcium channels in the plasma membrane, calcium channels in the endoplasmic reticulum membrane, Abeta-formed calcium channels, calcium-related proteins. The review aims to provide an overview of the current understanding of the molecular mechanisms involved in calcium dysregulation in AD, and an insight on how to exploit calcium regulation as therapeutic opportunities in AD.

Mitochondria, calcium and cell death: a deadly triad in neurodegeneration

Mitochondrial Ca(2+) accumulation is a tightly controlled process, in turn regulating functions as diverse as aerobic metabolism and induction of cell death. The link between Ca(2+) (dys)regulation, mitochondria and cellular derangement is particularly evident in neurodegenerative disorders, in which genetic models and environmental factors allowed to identify common traits in the pathogenic routes. We will here summarize: i) the current view of mechanisms and functions of mitochondrial Ca(2+) homeostasis, ii) the basic principles of organelle Ca(2+) transport, iii) the role of Ca(2+) in neuronal cell death, and iv) the new information on the pathogenesis of Alzheimer's, Huntington's and Parkinson's diseases, highlighting the role of Ca(2+) and mitochondria.

Presenilin-2 dampens intracellular Ca2+ stores by increasing Ca2+ leakage and reducing Ca2+ uptake

We have previously shown that familial Alzheimer’s disease mutants of presenilin-2 (PS2) and, to a lesser extent, of presenilin-1 (PS1) lower the Ca²⁺ concentration of intracellular stores. We here examined the mechanism by which wild-type and mutant PS2 affect store Ca²⁺ handling. By using HeLa, SH-SY5Y and MEFs as model cells, and recombinant aequorins as Ca²⁺ probes, we show evidence that transient expression of either wild-type or mutant PS2 increases the passive Ca²⁺ leakage: both ryanodine- and IP₃-receptors contribute to Ca²⁺ exit out of the ER, whereas the ribosome translocon complex is not involved. In SH-SY5Y cells and MEFs, wild-type and mutant PS2 potently reduce the uptake of Ca²⁺ inside the stores, an effect that can be counteracted by over-expression of SERCA-2B. On this line, in wild-type MEFs, lowering the endogenous level of PS2 by RNA interference, increases the Ca²⁺-loading capability of intracellular stores. Furthermore, we show that in PS double knockout MEFs, reduction of Ca²⁺ stores is mimicked by the expression of PS2-D366A, a loss-of-function mutant, uncleaved because also devoid of presenilinase activity but not by co-expression of the two catalytic active fragments of PS2. In summary, both physiological and increased levels of wild-type and mutant PS2 reduce the Ca²⁺ uptake by intracellular stores. To exert this newly described function, PS2 needs to be in its full-length form, even if it can subsequently be cleaved.

Increases of kinin B1 and B2 receptors binding sites after brain infusion of amyloid-beta 1–40 peptide in rats

Although numerous inflammation pathways have been implicated in Alzheimer's disease, the involvement of the kallikrein-kinin system is still under investigation. We anatomically localized and quantified the density of kinin B(1) and B(2) receptors binding sites in the rat brain after the infusion of amyloid-beta (Abeta) peptide in the right lateral brain ventricle for 5 weeks. The conditioned avoidance test showed a significant reduction of memory consolidation in rats infused with Abeta (68.6+/-20.9%, P<0.05) when compared to control group (90.8+/-4.1%; infused with vehicle). Autoradiographic studies performed in brain samples of both groups using [(125)I]HPP-[des-Arg(10)]-Hoe-140 (150pM, 90min, 25 degrees C) showed a significant increase in density of B(1) receptor binding sites in the ventral hippocampal commissure (1.23+/-0.07fmol/mg), fimbria (1.31+/-0.05fmol/mg), CA1 and CA3 hippocampal areas (1.05+/-0.03 and 1.24+/-0.02fmol/mg, respectively), habenular nuclei (1.30+/-0.04fmol/mg), optical tract (1.30+/-0.05fmol/mg) and internal capsule (1.26+/-0.05fmol/mg) in Abeta group. For B(2) receptors ([(125)I]HPP-Hoe-140, 200pM, 90min, 25 degrees C), a significant increase in density of binding sites was observed in optical tract (2.04+/-0.08fmol/mg), basal nucleus of Meynert (1.84+/-0.18fmol/mg), lateral septal nucleus - dorsal and intermediary portions (1.66+/-0.29fmol/mg), internal capsule (1.74+/-0.19fmol/mg) and habenular nuclei (1.68+/-0.11fmol/mg). In control group, none of these nuclei showed [(125)I]HPP-Hoe-140 labeling. This significant increase in densities of kinin B(1) and B(2) receptors in animals submitted to Abeta infusion was observed mainly in brain regions related to cognitive behavior, suggesting the involvement of the kallikrein-kinin system in Alzheimer's disease in vivo.

Effect of inhalational anesthetics on cytotoxicity and intracellular calcium differently in rat pheochromocytoma cells (PC12)

Isoflurane, a commonly used inhaled anesthetic, induces apoptosis in rat pheochromocytoma cells (PC12) in a concentration-and time-dependent manner with unknown mechanism. We hypothesized that isoflurane induced apoptosis by causing abnormal calcium release from the endoplasmic reticulum (ER) via activation of inositol 1,4,5-trisphosphate (IP3) receptors. Alzheimer's presenilin-1 (PS1) mutation increased activity of IP3 receptors and therefore rendered cells vulnerable to isoflurane-induced cytotoxicity. Sevoflurane and desflurane had less ability to disrupt intracellular calcium homeostasis and thus being less potent to cause cytotoxicity. This study examined and compared the cytotoxic effects of various inhaled anesthetics on PC12 cells transfected with the Alzheimer's mutated PS1 (L286V) and the disruption of intracellular calcium homeostasis. PC12 cells transfected with wild type (WT) and mutated PS1 (L286V) were treated with equivalent of 1 MAC of isoflurane, sevoflurane and desflurane for 12 h. MTT reduction and LDH release assays were performed to evaluate cell viability. Changes of calcium concentration in cytosolic space ([Ca2+]c) were determined after exposing different types of cells to various inhalational anesthetics. The effects of IP3 receptor antagonist xestospongin C on isoflurane-induced cytotoxicity and calcium release from the ER in L286V PC12 cells were also determined. The results showed that isoflurane at 1 MAC for 12 h induced cytoxicity in L286V but not WT PC12 cells, which was also associated with greater and faster elevation of peak [Ca2+]c in L286V than in the WT cells. Xestospongin C significantly ameliorated isoflurane cytotoxicity in L286V cells, as well as inhibited the calcium release from the ER in L286V cells. Sevoflurane and desflurane at equivalent exposure to isoflurane did not induce similar cytotoxicity or elevation of peak [Ca2+]c in L286V PC12 cells. These results suggested that isoflurane induced cytoxicity by partially causing abnormal calcium release from the ER via activation of IP3 receptors in L286V PC12 cells. Sevoflurane and desflurane at equivalent exposure to isoflurane did not induce similar elevation of [Ca2+]c or neurotoxicity in PC12 cells transfected with the Alzheimer's PS1 mutation.

Neuronal calcium mishandling and the pathogenesis of Alzheimer's disease

Perturbed neuronal Ca(2+) homeostasis is implicated in age-related cognitive impairment and Alzheimer's disease (AD). With advancing age, neurons encounter increased oxidative stress and impaired energy metabolism, which compromise the function of proteins that control membrane excitability and subcellular Ca(2+) dynamics. Toxic forms of amyloid beta-peptide (Abeta) can induce Ca(2+) influx into neurons by inducing membrane-associated oxidative stress or by forming an oligomeric pore in the membrane, thereby rendering neurons vulnerable to excitotoxicity and apoptosis. AD-causing mutations in the beta-amyloid precursor protein and presenilins can compromise these normal proteins in the plasma membrane and endoplasmic reticulum, respectively. Emerging knowledge of the actions of Ca(2+) upstream and downstream of Abeta provides opportunities to develop novel preventative and therapeutic interventions for AD.

Mechanism of Ca2+ disruption in Alzheimer's disease by presenilin regulation of InsP3 receptor channel gating

Mutations in presenilins (PS) are the major cause of familial Alzheimer's disease (FAD) and have been associated with calcium (Ca2+) signaling abnormalities. Here, we demonstrate that FAD mutant PS1 (M146L)and PS2 (N141I) interact with the inositol 1,4,5-trisphosphate receptor (InsP3R) Ca2+ release channel and exert profound stimulatory effects on its gating activity in response to saturating and suboptimal levels of InsP3. These interactions result in exaggerated cellular Ca2+ signaling in response to agonist stimulation as well as enhanced low-level Ca2+signaling in unstimulated cells. Parallel studies in InsP3R-expressing and -deficient cells revealed that enhanced Ca2+ release from the endoplasmic reticulum as a result of the specific interaction of PS1-M146L with the InsP3R stimulates amyloid beta processing,an important feature of AD pathology. These observations provide molecular insights into the "Ca2+ dysregulation" hypothesis of AD pathogenesis and suggest novel targets for therapeutic intervention.

A novel function for the presenilin family member spe-4: inhibition of spermatid activation in Caenorhabditis elegans

BACKGROUND

Sperm cells must regulate the timing and location of activation to maximize the likelihood of fertilization. Sperm from most species, including the nematode Caenorhabditis elegans, activate upon encountering an external signal. Activation for C. elegans sperm occurs as spermatids undergo spermiogenesis, a profound cellular reorganization that produces a pseudopod. Spermiogenesis is initiated by an activation signal that is transduced through a series of gene products. It is now clear that an inhibitory pathway also operates in spermatids, preventing their premature progression to spermatozoa and resulting in fine-scale control over the timing of activation. Here, we describe the involvement of a newly assigned member of the inhibitory pathway: spe-4, a homolog of the human presenilin gene PS1. The spe-4(hc196) allele investigated here was isolated as a suppressor of sterility of mutations in the spermiogenesis signal transduction gene spe-27.

RESULTS

Through mapping, complementation tests, DNA sequencing, and transformation rescue, we determined that allele hc196 is a mutation in the spe-4 gene. Our data show that spe-4(hc196) is a bypass suppressor that eliminates the need for the spermiogenesis signal transduction. On its own, spe-4(hc196) has a recessive, temperature sensitive spermatogenesis-defective phenotype, with mutants exhibiting (i) defective spermatocytes, (ii) defective spermatids, (iii) premature spermatid activation, and (iv) spermatozoa defective in fertilization, in addition to a small number of functional sperm which appear normal microscopically.

CONCLUSION

A fraction of the sperm from spe-4(hc196) mutant males progress directly to functional spermatozoa without the need for an activation signal, suggesting that spe-4 plays a role in preventing spermatid activation. Another fraction of spermatozoa from spe-4(hc196) mutants are defective in fertilization. Therefore, prematurely activated spermatozoa may have several defects: we show that they may be defective in fertilization, and earlier work showed that they obstruct sperm transfer from males at mating. hc196 is a hypomorphic allele of spe-4, and its newly-discovered role inhibiting spermiogenesis may involve known proteolytic and/or calcium regulatory aspects of presenilin function, or it may involve yet-to-be discovered functions.

A presenilin-1 mutation renders neurons vulnerable to isoflurane toxicity

BACKGROUND

Isoflurane, a commonly used inhaled anesthetic, induces apoptosis in rat pheochromocytoma neurosecretory cells (PC12) in a concentration- and time-dependent manner via an as yet unknown mechanism. We hypothesize that isoflurane induces apoptosis by causing abnormal calcium release from the endoplasmic reticulum (ER) via activation of inositol 1,4,5-trisphosphate (IP3) receptors. A presenilin-1 (PS1) mutation associated with familial Alzheimer's disease was shown to increase the activity of IP3 receptors, and therefore may render cells vulnerable to isoflurane-induced cytotoxicity. Sevoflurane and desflurane have less ability to disrupt intracellular calcium homeostasis; and thus we predict they will cause less cytotoxicity.

METHODS

PC12 cells transfected with wild type, vector alone (Vector) or mutated PS1 (L286V) were treated with equivalent of 1 MAC of isoflurane, sevoflurane, and desflurane for 12 h. Mitochondria redox activity (MTT reduction) and lactate dehydrogenase release assays were performed to evaluate cell viability. Changes of calcium concentration in cytosolic space ([Ca2+]c) and production of reactive oxygen species (ROS) were determined after exposing different types of cells to various inhaled anesthetics. We also determined the effects of IP3 receptor antagonist xestospongin C on isoflurane-induced cytotoxicity and calcium release from the ER in L286V PC12 cells, and in rat primary cortical neurons.

RESULTS

Isoflurane at 1 MAC for 12 h induced cytotoxicity in L286V but not wild type or vector PC12 cells, and also caused greater and faster increase of peak [Ca2+]c in the L286V cells. Xestospongin C significantly attenuated isoflurane cytotoxicity in both L286V cells and primary cortical neurons and inhibited the calcium release from the ER in L286V cells. Isoflurane did not induce significant changes of ROS production in any type of PC12 cells. Sevoflurane and desflurane at equivalent exposure to isoflurane did not induce similar cytotoxicity or increase of peak [Ca2+]c in L286V PC12 cells.

CONCLUSION

Our results show that the L286V PS1 mutation augments the isoflurane-induced [Ca2+]c increase via calcium release from intracellular stores which, in turn, renders the cells vulnerable to isoflurane neurotoxicity. ROS production was not involved in isoflurane-induced neurotoxicity. Sevoflurane and desflurane, at equivalent exposure to isoflurane, did not induce a similar increase of [Ca2+]c or neurotoxicity in L286V PC12 cells.

Regulation of the inositol 1,4,5-trisphosphate receptor type I by O-GlcNAc glycosylation

The inositol 1,4,5-trisphosphate (InsP3) receptor type I (InsP3R-I) is the principle channel for intracellular calcium (Ca2+) release in many cell types, including central neurons. It is regulated by endogenous compounds like Ca2+ and ATP, by protein partners, and by posttranslational modification. We report that the InsP3R-I is modified by O-linked glycosylation of serine or threonine residues with beta-N-acetylglucosamine (O-GlcNAc). The level of O-GlcNAcylation can be altered in vitro by the addition of the enzymes which add [OGT (O-GlcNActransferase)] or remove (O-GlcNAcase) this sugar or by loading cells with UDP-GlcNAc. We monitored the effects of this modification on InsP3R function at the single-channel level and on intracellular Ca2+ transients. Single-channel activity was monitored with InsP3R incorporated into bilayers; Ca2+ signaling was monitored using cells loaded with a Ca2+-sensitive fluorophore. We found that channel activity was decreased by the addition of O-GlcNAc and that this decrease was reversed by removal of the sugar. Similarly, cells loaded with UDP-GlcNAc had an attenuated response to uncaging of InsP3. These results show that O-GlcNAcylation is an important regulator of the InsP3R-I and suggest a mechanism for neuronal dysfunction under conditions in which O-GlcNAc is high, such as diabetes or physiological stress.

Telomere neurobiology

The ends of chromosomes consist of a hexanucleotide DNA repeat sequence and specialized DNA-binding and telomere-associated proteins. An enzyme activity called telomerase maintains telomere length by using an RNA template (TR) and a reverse transcriptase (TERT) to add the hexanucleotide sequence to the free chromosome end. The structure of telomeres is maintained and modified by telomere repeat-binding factors (TRF1 and TRF2) and proteins known for their role in DNA damage responses, including poly(ADP-ribose) polymerase-1, Werner, and ATM. Telomerase activity can be quantified using a telomere repeat amplification protocol (TRAP) assay, and levels of TERT and telomere-associated proteins are evaluated by immunoblot and immunocytochemical methods. Levels of TERT and telomere-associated proteins can be overexpressed or knocked down using viral vector-based methods. Using the kinds of approaches described here, evidence has been obtained suggesting that telomeres play important roles in regulating neural stem cell proliferation, neuronal differentiation, senescence of glial cells, and apoptosis and DNA damage responses of neural cells.

Calcium dysregulation in Alzheimer's disease

Alzheimer disease (AD) is the most common form of adult dementia. Its pathological hallmarks are synaptic degeneration, deposition of amyloid plaques and neurofibrillary tangles, leading to neuronal loss. A few hypotheses have been proposed to explain AD pathogenesis. The beta-amyloid (Abeta) and hyperphosphorylated tau hypotheses suggest that these proteins are the main players in AD development. Another hypothesis proposes that the dysregulation of calcium homeostasis may be a key factor in accelerating other pathological changes. Although Abeta and tau have been extensively studied, recently published data provide a growing body of evidence supporting the critical role of calcium signalling in AD. For example, presenilins, which are mutated in familial cases of AD, were demonstrated to form low conductance calcium channels in the ER and elevated cytosolic calcium concentration increases amyloid generation. Moreover, memantine, an antagonist of the NMDA-calcium channel receptor, has been found to have a beneficial effect for AD patients offering novel possibilities for a calcium signalling targeted therapy of AD. This review underscores the growing importance of calcium ions in AD development and focuses on the relevant aspects of calcium homeostasis.

Isoflurane preconditioning inhibited isoflurane-induced neurotoxicity

The commonly used inhaled anesthetic isoflurane has been shown to be both neuroprotective and neurotoxic in various cell cultures and animal models. We hypothesize that, like cerebral ischemia, isoflurane is inherently neurotoxic. Limited exposure of isoflurane provides neuroprotection via induction of endogenous neuroprotective mechanisms (preconditioning), while prolonged exposure of isoflurane induces neurotoxicity directly by its inherent neurotoxic effects. To test this hypothesis, we treated rat primary cortical neurons at different days in vitro (DIV) and rat pheochromocytoma neurosecretory (PC12) cells with or without Alzheimer's mutated presenilin-1 (PS1) with 2.4% isoflurane for 24 h to induce cell damage determined by both MTT (3-(4,5-dimethyithiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) reduction and LDH (lactate dehydrogenase) release assays. For isoflurane preconditioning, we treated the above cells with isoflurane at 0.6%, 1.2% and 2.4% for 60 min, 4 h prior to a prolonged exposure of 2.4% isoflurane for 24 h. One hour of preconditioning with isoflurane dose-dependently inhibited neurotoxicity induced by 2.4% isoflurane for 24 h in both primary cortical neurons and PC12 cells. This neuroprotection was most dramatically observed in matured cortical neurons (DIV 16) and PC12 cells with over expression of Alzheimer's mutated PS1 (L286V). Preconditioning L286V PC12 cells with equivalent two minimal alveolar concentrations (MAC) of halothane (1.5%), but not sevoflurane (4%), also abolished the neurotoxicity induced by 2.4% isoflurane for 24 h. Overall, these results suggest that isoflurane may be both neuroprotective and neurotoxic, depending on the exposure concentrations and duration.

Presenilin-mediated signal transduction

Presenilin proteins, mutated forms of which cause early onset familial Alzheimer's disease, are capable of modulating various cell signal transduction pathways, the most extensively studied of which has been intracellular calcium signalling. Disease causing presenilin mutations can potentiate inositol(1,4,5)trisphosphate (InsP3) mediated endoplasmic reticulum release due to calcium overload in this organelle, as well as attenuate capacitative calcium entry. Our own studies have shown a novel function for presenilins that involves regulation of acetylcholine muscarinic receptor-stimulated phospholipase C upstream of InsP3 regulated calcium release. This article reviews the mechanisms by which presenilins modulate intracellular calcium signalling and the role that deregulated calcium homeostasis could play in the pathogenesis of Alzheimer's disease.

Enhanced ryanodine-mediated calcium release in mutant PS1-expressing Alzheimer's mouse models

Intracellular Ca(2+) signaling involves Ca(2+) liberation through both inositol triphosphate and ryanodine receptors (IP(3)R and RyR). However, little is known of the functional interactions between these Ca(2+) sources in either neuronal physiology, or during Ca(2+) disruptions associated with Alzheimer's disease (AD). By the use of whole-cell recordings and 2-photon Ca(2+) imaging in cortical slices we distinguished between IP(3)R- and RyR-mediated Ca(2+) components in nontransgenic (non-Tg) and AD mouse models and demonstrate powerful signaling interactions between these channels. Ca(2+)-induced Ca(2+) release (CICR) through RyR contributed modestly to Ca(2+) signals evoked by photoreleased IP(3) in cortical neurons from non-Tg mice. In contrast, the exaggerated signals in 3xTg-AD and PS1(KI) mice resulted primarily from enhanced CICR through RyR, rather than through IP(3)R, and were associated with increased RyR expression levels. Moreover, membrane hyperpolarizations evoked by IP(3) in neurons from AD mouse models were even greater than expected simply from the exaggerated Ca(2+) signals, pointing to an increased coupling efficiency between cytosolic [Ca(2+)] and K(+) channel regulation. Our results highlight the critical roles of RyR-mediated Ca(2+) signaling in both neuronal physiology and pathophysiology, and point to presenilin-linked disruptions in RyR signaling as an important genetic factor in AD.

Presenilin dependence of phospholipase C and protein kinase C signaling

Presenilins (PSs) are involved in processing several proteins such as the amyloid precursor protein (APP), as well as in pathways for cell death and survival. We previously showed that some familial Alzheimer's disease PS mutations cause increased basal and acetylcholine muscarinic receptor-stimulated phospholipase C (PLC) activity which was gamma-secretase dependent. To further evaluate the dependence of PLC on PSs we measured PLC activity and the activation of variant protein kinase C (PKC) isoforms in mouse embryonic fibroblasts (MEFs) lacking either PS1, PS2, or both. PLC activity and PKCalpha and PKCgamma activations were significantly lower in PS1 and PS2 double knockout MEFs after PLC stimulation. Protein levels of PKCalpha and PKCgamma were lower in PS1 and PS2 double knockout MEFs. In contrast, PKCdelta levels were significantly elevated in PS1 and PS2 double knockout as well as in PS1 knockout MEFs. Also, PKCdelta levels were lowered after transfection of PS1 into PS1 knockout or PS double knockout MEFs. Using APP knockout MEFs we showed that the expression of PKCalpha, but not the other PKC isoforms is partially dependent on APP and can be regulated by APP intracellular domain (AICD). These results show that PLC and PKC activations are modulated by PS and also that PSs differentially regulate the expression of PKC isoforms by both APP/AICD-dependent and independent mechanisms.

Transient receptor potential channels in Alzheimer's disease

Cognitive impairment and emotional disturbances in Alzheimer's disease (AD) result from the degeneration of synapses and neuronal death in the limbic system and associated regions of the cerebral cortex. An alteration in the proteolytic processing of the amyloid precursor protein (APP) results in increased production and accumulation of amyloid beta-peptide (Abeta) in the brain. Abeta can render neurons vulnerable to excitotoxicity and apoptosis by disruption of cellular Ca(2+) homeostasis and neurotoxic factors including reactive oxygen species (ROS), nitric oxide (NO), and cytokines. Many lines of evidence have suggested that transient receptor potential (TRP) channels consisting of six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) are involved in Ca(2+) homeostasis disruption. Thus, emerging evidence of the pathophysiological role of TRP channels has yielded promising candidates for molecular entities mediating Ca(2+) homeostasis disruption in AD. In this review, we focus on the TRP channels in AD and highlight some TRP "suspects" for which a role in AD can be anticipated. An understanding of the involvement of TRP channels in AD may lead to the development of new target therapies.

Susceptibility of hippocampal neurons to Abeta peptide toxicity is associated with perturbation of Ca2+ homeostasis

Neuritic dystrophy, loss of synapses and neuronal death in the cerebral cortex and hippocampus are hallmarks of Alzheimer's disease. The aim of the present study was to investigate the differential susceptibility of cortical and hippocampal neurons to amyloid-beta (Abeta)-induced toxicity. For that, we have used primary neuronal cultures prepared from rat brain cortex and hippocampus which were treated with the synthetic peptides Abeta25-35 or Abeta1-40. Abeta-induced apoptotic cell death was analyzed by determining caspase-3-like activity. Neuritic dystrophy was evaluated by cobalt staining and MAP2 immunoreactivity. Perturbation of Ca(2+) homeostasis caused by exposure to Abeta was evaluated by determining basal cytosolic calcium levels in the whole neuronal population and by single cell calcium imaging under basal and KCl-depolarization conditions. Finally, levels of GluR2 subunit of glutamate AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate) receptors were quantified by western blotting. Our results demonstrated that hippocampal neurons in culture are more susceptible than cortical neurons to Abeta-induced apoptosis and also that this mechanism involves the perturbation of Ca(2+) homeostasis. Accordingly, the exposure of hippocampal neurons to Abeta peptides decreases the protein levels of the GluR2 subunit of glutamate AMPA receptors that may be associated with a significant rise of cytosolic Ca(2+) concentration, leading to dendritic dystrophy and activation of apoptotic neuronal death.

Reduction in CHT1-mediated choline uptake in primary neurons from presenilin-1 M146V mutant knock-in mice

The memory loss in Alzheimer's disease (AD) has been linked to cholinergic hypoactivity. Mutations in presenilin-1 (PS-1) may regulate cholinergic signaling, although their precise roles in cholinergic neurotransmission in AD are unsettled. Neuronal uptake of choline via the high affinity choline transporter (CHT1) is essential for cholinergic neurotransmission. CHT1 is a Na+-dependent, hemicholinium-3 (HC-3)-sensitive choline transporter. Although cholinergic neurons in the nucleus basalis of Meynert are a major source of cholinergic projections for the cerebral cortex, it is unclear whether cortical neurons exhibit intrinsic CHT1 activity that is altered in AD. We now report that primary cortical neurons express intrinsic and biologically active CHT1, and that, in these neurons, CHT1-mediated choline uptake activity is significantly reduced in PS-1 M146V mutant knock-in mice. Further kinetic studies using HC-3 binding and cell surface biotinylation assays showed that the PS-1 mutation inhibits CHT1 mediated choline uptake by reducing the ligand binding affinity of CHT1 without significantly altering levels of CHT1 expression in the plasma membrane. Since human neocortex has recently been shown to possess intrinsic cholinergic innervation, our results indicate that alterations in CHT1-mediated high affinity choline uptake in cortical neurons may contribute to Alzheimer's dementia.

Presenilin-1 inhibits delta-catenin-induced cellular branching and promotes delta-catenin processing and turnover

Although delta-catenin/neural plakophilin-related armadillo protein (NPRAP) was reported to interact with presenilin-1 (PS-1), the effects of PS-1 on delta-catenin have not been established. In this study, we report that overexpression of PS-1 inhibits the delta-catenin-induced dendrite-like morphological changes in NIH 3T3 cells and promotes delta-catenin processing and turnover. The effects of PS-1 on endogenous delta-catenin processing were confirmed in hippocampal neurons overexpressing PS-1, as well as in the transgenic mice expressing the disease-causing mutant PS-1 (M146V). In addition, disease-causing mutant PS-1 (M146V and L286V) enhanced delta-catenin processing, whereas PS-1/gamma-secretase inhibitors could block the formation of processed forms of delta-catenin. Together, our findings suggest that PS-1 can affect delta-catenin-induced morphogenesis possibly through the regulation of its processing and stability.

Hypocapnia induces caspase-3 activation and increases Abeta production

BACKGROUND

At least half of all cases of early onset (<60) familial Alzheimer's disease (FAD) are caused by any of over 150 mutations in three genes: the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). Mutant forms of PS1 have been shown to sensitize cells to apoptotic cell death.

OBJECTIVE

We investigated the effects of hypocapnia, a risk factor for both cognitive and neurodevelopment deficits, on caspase-3 activation, apoptosis, and amyloid beta-protein (Abeta) production, and assessed the influence of the PS1Delta9 FAD mutation on these effects.

METHOD

For this purpose, we exposed stably transfected H4 human neuroglioma cells to conditions consistent with hypocapnia (PCO2<40 mm Hg) and hypocapnia plus hypoxia (PO2<21%).

RESULTS

Hypocapnia (20 mm Hg CO2 for 6 h) induced caspase-3 activation and apoptosis; the PS1Delta9 FAD mutation significantly potentiated these effects. Moreover, the combination of hypocapnia (20 mm Hg CO2) and hypoxia (5%O2) induced caspase-3 activation and apoptosis in a synergistic manner. Hypocapnia (5 and 20 mm Hg CO2 for 6 h) also led to an increased Abeta production.

CONCLUSION

The findings suggest that hypocapnia (e.g. during general anesthesia) could exacerbate AD neuropathogenesis.

An endoplasmic-reticulum-specific apoptotic pathway is involved in prion and amyloid-beta peptides neurotoxicity

Prion (PrP) and amyloid-beta (Abeta) peptides are involved in the neuronal loss that occurs in Prion disorders (PrD) and Alzheimer's disease (AD), respectively, partially due to Ca(2+) dysregulation. Besides, the endoplasmic reticulum (ER) stress has an active role in the neurotoxic mechanisms that lead to these pathologies. Here, we analyzed whether the ER-mediated apoptotic pathway is involved in the toxic effect of synthetic PrP and Abeta peptides. In PrP106-126- and Abeta1-40-treated cortical neurons, the release of Ca(2+) through ER ryanodine (RyR) and inositol 1,4,5-trisphosphate (IP(3)R) receptors induces ER stress and leads to increased cytosolic Ca(2+) and reactive oxygen species (ROS) levels and subsequently to apoptotic death involving mitochondrial cytochrome c release and caspases activation. These results demonstrate that the early PrP- and Abeta-induced perturbation of ER Ca(2+) homeostasis is a death message that leads to neuronal loss, suggesting that the regulation of ER Ca(2+) levels may be a potential therapeutical target for PrD and AD.

Presenilin mutations linked to familial Alzheimer's disease reduce endoplasmic reticulum and Golgi apparatus calcium levels

Presenilin-1 and -2 (PS1 and PS2) mutations, the major cause of familial Alzheimer's disease (FAD), have been causally implicated in the pathogenesis of neuronal cell death through a perturbation of cellular Ca(2+) homeostasis. We have recently shown that, at variance with previous suggestions obtained in cells expressing other FAD-linked PS mutations, PS2-M239I and PS2-T122R cause a reduction and not an increase in cytosolic Ca(2+) rises induced by Ca(2+) release from stores. In this contribution we have used different cell models: human fibroblasts from controls and FAD patients, cell lines (SH-SY5Y, HeLa, HEK293, MEFs) and rat primary neurons expressing a number of PS mutations, e.g. P117L, M146L, L286V, and A246E in PS1 and M239I, T122R, and N141I in PS2. The effects of FAD-linked PS mutations on cytosolic Ca(2+) changes have been monitored either by using fura-2 or recombinant cytosolic aequorin as the probe. Independently of the cell model or the employed probe, the cytosolic Ca(2+) increases, caused by agonist stimulation or full store depletion by drug treatment, were reduced or unchanged in cells expressing the PS mutations. Using aequorins, targeted to the endoplasmic reticulum or the Golgi apparatus, we here show that FAD-linked PS mutants lower the Ca(2+) content of intracellular stores. The phenomenon was most prominent in cells expressing PS2 mutants, and was observed also in cells expressing the non-pathogenic, "loss-of-function" PS2-D366A mutation. Taken as a whole, our findings, while confirming the capability of presenilins to modify Ca(2+) homeostasis, suggest a re-evaluation of the "Ca(2+) overload" hypothesis in AD and a new working hypothesis is presented.

Enhanced ryanodine receptor recruitment contributes to Ca2+ disruptions in young, adult, and aged Alzheimer's disease mice

Neuronal Ca2+ signaling through inositol triphosphate receptors (IP3R) and ryanodine receptors (RyRs) must be tightly regulated to maintain cell viability, both acutely and over a lifetime. Exaggerated intracellular Ca2+ levels have been associated with expression of Alzheimer's disease (AD) mutations in young mice, but little is known of Ca2+ dysregulations during normal and pathological aging processes. Here, we used electrophysiological recordings with two-photon imaging to study Ca2+ signaling in nontransgenic (NonTg) and several AD mouse models (PS1KI, 3xTg-AD, and APPSweTauP301L) at young (6 week), adult (6 months), and old (18 months) ages. At all ages, the PS1KI and 3xTg-AD mice displayed exaggerated endoplasmic reticulum (ER) Ca2+ signals relative to NonTg mice. The PS1 mutation was the predominant "calciopathic" factor, because responses in 3xTg-AD mice were similar to PS1KI mice, and APPSweTauP301L mice were not different from controls. In addition, we uncovered powerful signaling interactions and differences between IP3R- and RyR-mediated Ca2+ components in NonTg and AD mice. In NonTg mice, RyR contributed modestly to IP3-evoked Ca2+, whereas the exaggerated signals in 3xTg-AD and PS1KI mice resulted primarily from enhanced RyR-Ca2+ release and were associated with increased RyR expression across all ages. Moreover, IP3-evoked membrane hyperpolarizations in AD mice were even greater than expected from exaggerated Ca2+ signals, suggesting increased coupling efficiency between cytosolic [Ca2+] and K+ channel regulation. We conclude that lifelong ER Ca2+ disruptions in AD are related to a modulation of RyR signaling associated with PS1 mutations and represent a discrete "calciumopathy," not merely an acceleration of normal aging.

Loss of nicastrin elicits an apoptotic phenotype in mouse embryos

Nicastrin is a member of the high molecular weight presenilin complex that plays a central role in gamma-secretase cleavage of numerous type-1 membrane-associated proteins required for cell signaling, proliferation and lineage development. We have generated a nicastrin-null mouse line by disruption of exon 3. Similar to previously described nicastrin-null mice, these animals demonstrate severe growth retardation, mortality beginning at embryonic age 10.5 days, and marked developmental abnormalities indicative of a severe Notch phenotype. Preceding their mortality, 10.5-day-old nicastrin-null embryos were found to also exhibit specific apoptosis within regions showing profound deformities, particularly in the developing heart and brain. This result suggests that complete disruption of presenilin complexes elicits programmed cell death, in addition to a Notch phenotype, which may contribute to the developmental abnormalities and embryonic mortality of nicastrin-null mice and possibly neurodegeneration in Alzheimer's disease.

Apoptotic and anti-apoptotic synaptic signaling mechanisms

Although several prominent morphological features of apoptosis are evident in the cell body (e.g., cell shrinkage, membrane blebbing, and nuclear DNA condensation and fragmentation) the biochemical and molecular cascades that constitute the cell death machinery can be engaged in synaptic terminals and neurites. Initiating events such as oxyradical production and calcium influx, and effector processes such as Par-4 production, mitochondrial alterations and caspase activation, can be induced in synapses and neurites. Several prominent signal transduction pathways in synaptic terminals play important roles in either promoting or preventing neuronal death in physiological and pathological conditions. For example, activation of glutamate receptors in postsynaptic spines can induce neuronal apoptosis, whereas local activation of neurotrophic factor receptors in presynaptic terminals can prevent neuronal death. Factors capable of inducing nuclear chromatin condensation and fragmentation can be produced locally in synaptic terminals and neurites, and may propogate to the cell body. Recent findings suggest that, beyond their roles in inducing or preventing cell death, apoptotic and anti-apoptotic cascades play roles in synaptic plasticity (structural remodelling and long-term functional changes). For example, caspase activation results in proteolysis of glutamate receptor (AMPA) subunits, which results in altered neuronal responsivity to glutamate. Activation of neurotrophic factor receptors in synaptic terminals can result in local changes in energy metabolism and calcium homeostasis, and can induce long-term changes in synaptic transmission. The emerging data therefore suggest that synapses can be considered as autonomous compartments in which both pro- and anti-apoptotic signaling pathways are activated resulting in structural and functional changes in neuronal circuits. A better understanding of such synaptic signaling mechanisms may reveal novel approaches for preventing and treating an array of neurodegenerative conditions that are initiated by perturbed synaptic homeostasis.

Alzheimer's disease and post-operative cognitive dysfunction

Alzheimer's disease (AD), an insidious and progressive neurodegenerative disorder accounting for the vast majority of dementia, is characterized by global cognitive decline and the robust accumulation of amyloid deposits and neurofibrillary tangles in the brain. This review article is based on the currently published literature regarding molecular studies of AD and the potential involvement of AD neuropathogenesis in post-operative cognitive dysfunction (POCD). Genetic evidence, confirmed by neuropathological and biochemical studies, indicates that excessive beta-amyloid protein (Abeta) generated from amyloidogenic processing of the beta-amyloid precursor protein (APP) plays a fundamental role in the AD neuropathogenesis. Abeta is produced from APP by beta-secretase, and then gamma-secretase complex, consisting of presenilins, nicastrin (NCSTN), APH-1 and PEN-2. Additionally, Abeta clearance and APP adaptor proteins can contribute to AD neuropathogenesis via affecting Abeta levels. Finally, cellular apoptosis may also be involved in AD neuropathogenesis. Surgery and anesthesia can cause cognitive disorders, especially in elderly patients. Even the molecular mechanisms underlying these disorders are largely unknown; several perioperative factors such as hypoxia, hypocapnia and anesthetics may be associated with AD and render POCD via trigging AD neuropathogenesis. More studies to assess the potential relationship between anesthesia/surgery and AD dementia are, therefore, urgently needed.

Effect of gamma-secretase inhibitors on muscarinic receptor-mediated calcium signaling in human salivary epithelial cells

Altered intracellular Ca(2+) signaling has been observed in cells derived from Alzheimer's disease patients, and a possible link between gamma-secretase activity and the content of intracellular Ca(2+) stores has been suggested. To test this hypothesis we studied the effects of several gamma-secretase inhibitors on muscarinic receptor-mediated intracellular calcium release in the human salivary gland cell line HSG. Although several inhibitors in the peptide aldehyde class partially blocked carbachol-induced Ca(2+) transients, these effects did not appear to be due to gamma-secretase inhibition, and overall we found no evidence that inhibition of gamma-secretase activity had any significant effect on agonist-induced intracellular calcium release in HSG cells. In complementary experiments with presenilin-null cells we found that the reconstitution of gamma-secretase activity by transfection with wild-type presenilin 1 likewise had no significant effect on thapsigargin-induced Ca(2+) release. In a test of the specific hypothesis that the level of APP intracellular domain (AICD), the intracellular fragment of the beta-amyloid precursor protein (APP) resulting from gamma-secretase cleavage, can modulate the Ca(2+) content of the endoplasmic reticulum, we were unable to demonstrate any effect of APP small interfering RNA on the magnitude of carbachol-induced intracellular calcium release in HSG cells. Together our data cast considerable doubt on the hypothesis that there is a direct link between gamma-secretase activity and the content of intracellular Ca(2+) stores.

Expression of a familial Alzheimer's disease-linked presenilin-1 variant enhances perforant pathway lesion-induced neuronal loss in the entorhinal cortex

Alzheimer's disease (AD) is characterized by neuronal loss in the hippocampus and entorhinal cortex that is manifested by progressive memory impairment and cognitive decline. Autosomal-dominant, familial forms of AD (FAD) are caused by mutations in genes encoding amyloid precursor protein, presenilin-1 (PS1), and presenilin 2. Although it is established that expression of mutant PS1 variants leads to increased production of highly fibrillogenic amyloidbeta42 (Abeta42) peptides that deposit in the brains of patients with AD, the mechanism(s) by which Abeta deposition and expression of mutant genes induce lamina- and region-specific vulnerability of neuronal populations is not known. We have examined the hypothesis that expression of transgene-encoded FAD-linked mutant PS1 variants in entorhinal cortex neurons exacerbates the vulnerability of these cells to lesion-induced neuronal loss. To test this notion, we transected the perforant pathway (PP) of transgenic mice harboring either wild-type human PS1 (PS1HWT) or the FAD-linked mutant PS1DeltaE9 variant and examined neuronal survival in layer II of the entorhinal cortex (ECL2). Remarkably, PP transections lead to marked reductions in the numbers of ECL2 neurons in the ECL2 of mice expressing mutant PS1, compared with ECL2 neurons in PP-lesioned PS1HWT mice. Finally, and in contrast to studies in nontransgenic mice and in mice expressing PS1HWT, ECL2 neurons that express mutant PS1 and the calcium binding protein calbindin-D28k in ECL2 are also susceptible to lesion-induced neuronal loss. We conclude that expression of FAD-linked mutant PS1 variants enhances the vulnerability of neurons in the entorhinal cortex to PP lesion-induced cytotoxicity.

Perturbations in calcium-mediated signal transduction in microglia from Alzheimer's disease patients

Calcium-sensitive fluorescence microscopy has been used to study Ca2+-dependent signal transduction pathways in microglia obtained from Alzheimer's disease (AD) patients and non-demented (ND) individuals. Data were obtained from nine AD cases and seven ND individuals and included basal levels of intracellular Ca2+ [Ca2+]i, peak amplitudes (Delta[Ca2+]i) and time courses of adenosine triphosphate (ATP) responses and amplitudes of an initial transient response and a subsequent second component of Ca2+ influx through store-operated channels (SOC) induced by platelet-activating factor (PAF). Overall, AD microglia were characterized by significantly higher (20%) basal Ca2+ [Ca2+]i relative to ND cells. The Delta[Ca2+]i of ATP and initial phase of PAF responses, which reflect rapid depletion of Ca2+ from endoplasmic reticulum stores, were reduced by respective values of 63% and 59% in AD cells relative to amplitudes recorded from ND microglia. Additionally, AD microglia showed diminished amplitudes (reduction of 61%) of SOC-mediated Ca2+ entry induced by PAF and prolonged time courses (increase of 60%) of ATP responses with respect to ND microglia. We have generally replicated these results with exposure of human fetal microglia to beta amyloid (5 microM Abeta1-42 applied for 24 hr). Overall, these data indicate significant abnormalities are present in Ca2+-mediated signal transduction in microglia isolated from AD patients.

Presenilin 1 deficiency alters the activity of voltage-gated Ca2+ channels in cultured cortical neurons

Presenilins 1 and 2 (PS1 and PS2, respectively) play a critical role in mediating gamma-secretase cleavage of the amyloid precursor protein (APP). Numerous mutations in the presenilins are known to cause early-onset familial Alzheimer's disease (FAD). In addition, it is well established that PS1 deficiency leads to altered intracellular Ca(2+) homeostasis involving endoplasmic reticulum Ca(2+) stores. However, there has been little evidence suggesting Ca(2+) signals from extracellular sources are influenced by PS1. Here we report that the Ca(2+) currents carried by voltage-dependent Ca(2+) channels are increased in PS1-deficient cortical neurons. This increase is mediated by a significant increase in the contributions of L- and P-type Ca(2+) channels to the total voltage-mediated Ca(2+) conductance in PS1 (-/-) neurons. In addition, chelating intracellular Ca(2+) with 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) produced an increase in Ca(2+) current amplitude that was comparable to the increase caused by PS1 deficiency. In contrast to this, BAPTA had no effect on voltage-dependent Ca(2+) conductances in PS1-deficient neurons. These data suggest that PS1 deficiency may influence voltage-gated Ca(2+) channel function by means that involve intracellular Ca(2+) signaling. These findings reveal that PS1 functions at multiple levels to regulate and stabilize intracellular Ca(2+) levels that ultimately control neuronal firing behavior and influence synaptic transmission.

Calcium dysregulation in Alzheimer's disease: Recent advances gained from genetically modified animals

Alzheimer's disease is a progressive and irreversible neurodegenerative disorder that leads to cognitive, memory and behavioural impairments. Two decades of research have implicated disturbances of intracellular calcium homeostasis as playing a proximal pathological role in the neurodegeneration associated with Alzheimer's disease. A large preponderance of evidence has been gained from the use of a diverse range of cell lines. Whilst useful in understanding the principal mechanism of neurotoxicity associated with Alzheimer's disease, technical differences, such as cell type or even the form of amyloid-beta used often underlie conflicting results. In this review, we discuss recent contributions that transgenic technology has brought to this field. For example, the triple transgenic mouse model of Alzheimer's disease has implicated intraneuronal accumulation of the amyloid-beta peptide as an initiating factor in synaptic dysfunction and behavioural deficits. Importantly, this synaptic dysfunction occurs prior to cell loss or extracellular amyloid plaque accumulation. The cause of synaptic dysfunction is unknown but it is likely that amyloid-beta and its ability to disrupt intracellular calcium homeostasis plays a key role in this process.

The overexpression of presenilin2 and Alzheimer's-disease-linked presenilin2 variants influences TRPC6-enhanced Ca2+ entry into HEK293 cells

Mutations in the presenilin (PS) genes are linked to the development of early-onset Alzheimer's disease by a gain-of-function mechanism that alters proteolytic processing of the amyloid precursor protein (APP). Recent work indicates that Alzheimer's-disease-linked mutations in presenilin1 and presenilin2 attenuate calcium entry and augment calcium release from the endoplasmic reticulum (ER) in different cell types. However, the regulatory mechanisms underlying the altered profile of Ca(2+) signaling are unknown. The present study investigated the influence of two familial Alzheimer's-disease-linked presenilin2 variants (N141I and M239V) and a loss-of-function presenilin2 mutant (D263A) on the activity of the transient receptor potential canonical (TRPC)6 Ca(2+) entry channel. We show that transient coexpression of Alzheimer's-disease-linked presenilin2 mutants and TRPC6 in human embryonic kidney (HEK) 293T cells abolished agonist-induced TRPC6 activation without affecting agonist-induced endogenous Ca(2+) entry. The inhibitory effect of presenilin2 and the Alzheimer's-disease-linked presenilin2 variants was not due to an increase in amyloid beta-peptides in the medium. Despite the strong negative effect of the presenilin2 and Alzheimer's-disease-linked presenilin2 variants on agonist-induced TRPC6 activation, conformational coupling between inositol 1,4,5-trisphosphate receptor type 3 (IP(3)R3) and TRPC6 was unaffected. In cells coexpressing presenilin2 or the FAD-linked presenilin2 variants, Ca(2+) entry through TRPC6 could still be induced by direct activation of TRPC6 with 1-oleoyl-2-acetyl-sn-glycerol (OAG). Furthermore, transient coexpression of a loss-of-function PS2 mutant and TRPC6 in HEK293T cells enhanced angiotensin II (AngII)- and OAG-induced Ca(2+) entry. These results clearly indicate that presenilin2 influences TRPC6-mediated Ca(2+) entry into HEK293 cells.

Mitochondrial enzymes and endoplasmic reticulum calcium stores as targets of oxidative stress in neurodegenerative diseases

Considerable evidence indicates that oxidative stress accompanies age-related neurodegenerative diseases. Specific mechanisms by which oxidative stress leads to neurodegeneration are unknown. Two targets of oxidative stress that are known to change in neurodegenerative diseases are the mitochondrial enzyme alpha-ketoglutarate dehydrogenase complex (KGDHC) and endoplasmic reticulum calcium stores. KGDHC activities are diminished in all common neurodegenerative diseases and the changes are particularly well documented in Alzheimer's disease (AD). A second change that occurs in cells from AD patients is an exaggerated endoplasmic reticulum calcium store [i.e., bombesin-releasable calcium stores (BRCS)]. H(2)O(2), a general oxidant, changes both variables in the same direction as occurs in disease. Other oxidants selectively alter these variables. Various antioxidants were used to help define the critical oxidant species that modifies these responses. All of the antioxidants diminish the oxidant-induced carboxy-dichlorofluorescein (cDCF) detectable reactive oxygen species (ROS), but have diverse actions on these cellular processes. For example, alpha-keto-beta-methyl-n-valeric acid (KMV) diminishes the H(2)O(2) effects on BRCS, while trolox and DMSO exaggerate the response. Acute trolox treatment does not alter H(2)O(2)-induced changes in KGDHC, whereas chronic treatment with trolox increases KGDHC almost threefold. The results suggest that KGDHC and BRCS provide targets by which oxidative stress may induce neurodegeneration and a useful tool for selecting antioxidants for reversing age-related neurodegeneration.

Mean age of onset in familial Alzheimer's disease is determined by amyloid beta 42

More than 130 known mutations in the presenilin-1 (PS1) gene result in familial Alzheimer's disease (FAD) with a mutation specific age of disease onset. These mutations increase amyloid beta 42 (A beta42) levels, and this increase has been validated in recent years as one pathogenic factor in FAD. However, further malfunctions of mutant presenilin-1 are discussed as well. In order to assess the weight of A beta42 regarding the pathogenesis of FAD, we expressed mutant forms of PS1 (30-65 years onset age) in COS-7 cells and analyzed amyloid beta levels by a novel ELISA. We found a strong correlation (r = 0.98; p<0.001) between the A beta40/42-ratio and mean age of disease onset indicating a substantial extent of A beta42 contribution to FAD pathology. Our data strongly suggest that A beta42 is the decisive factor for age of onset in FAD.

Expression of calsenilin in neurons and astrocytes in the Alzheimer??s disease brain

Calsenilin, a multifunctional Ca2+-binding protein, has been identified as an Alzheimer's disease-associated presenilin interactor. Here, we investigated the histochemical localization of calsenilin and its expression levels in the brains of sporadic Alzheimer's disease. Both messenger RNA and protein expression of calsenilin were observed in neurons of the cerebral cortex and hippocampus of control brains, and more intense staining was in Alzheimer's disease brains. Although calsenilin is primarily expressed in neurons, its immunoreactivity was also detected in reactive astrocytes of the Alzheimer's disease brains. In Alzheimer's disease brains, the caspase-derived fragment of calsenilin was only detected in cytosolic fraction. Our findings suggest that calsenilin overexpression in both neurons and reactive astrocytes may play an important role in apoptosis and in Alzheimer's disease pathology.

Calcium Dysregulation and Membrane Disruption as a Ubiquitous Neurotoxic Mechanism of Soluble Amyloid Oligomers*♦

Increasing evidence suggests that amyloid peptides associated with a variety of degenerative diseases induce neurotoxicity in their intermediate oligomeric state, rather than as monomers or fibrils. To test this hypothesis and investigate the possible involvement of Ca2+ signaling disruptions in amyloid-induced cytotoxicity, we made homogeneous preparations of disease-related amyloids (Abeta, prion, islet amyloid polypeptide, polyglutamine, and lysozyme) in various aggregation states and tested their actions on fluo-3-loaded SH-SY5Y cells. Application of oligomeric forms of all amyloids tested (0.6-6 microg ml-1) rapidly (approximately 5 s) elevated intracellular Ca2+, whereas equivalent amounts of monomers and fibrils did not. Ca2+ signals evoked by Abeta42 oligomers persisted after depletion of intracellular Ca2+ stores, and small signals remained in Ca2+-free medium, indicating contributions from both extracellular and intracellular Ca2+ sources. The increased membrane permeability to Ca2+ cannot be attributed to activation of endogenous Ca2+ channels, because responses were unaffected by the potent Ca2+-channel blocker cobalt (20 microm). Instead, observations that Abeta42 and other oligomers caused rapid cellular leakage of anionic fluorescent dyes point to a generalized increase in membrane permeability. The resulting unregulated flux of ions and molecules may provide a common mechanism for oligomer-mediated toxicity in many amyloidogenic diseases, with dysregulation of Ca2+ ions playing a crucial role because of their strong trans-membrane concentration gradient and involvement in cell dysfunction and death.

Biomarkers for apoptosis in Alzheimer's disease

Alzheimer's disease (AD) affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. Today there is no cure for this devastating and always lethal disease and therefore it is of great interest for patients, relatives and societies to find new drugs that can hinder the disease process. During the progression of AD a substantial amount of neurons degenerate in the brain. The mechanisms of cell death involved in AD have not been fully elucidated. However, there are several reports showing that neurons die partly by apoptosis in the AD brain. Drugs blocking apoptosis could therefore be potentially useful for early prevention of neuronal cell death. Biomarkers for apoptosis should be important tools in the evaluation of drug effects and in the diagnostics of AD. Here we review the current knowledge in the field and discuss potential biomarkers for apoptosis in AD.