The dysregulation of intracellular calcium in Alzheimer disease

The antioxidant N-acetylcysteine prevents the mitochondrial fragmentation induced by soluble amyloid-β peptide oligomers

BACKGROUND

Soluble amyloid-β peptide oligomers (AβOs), which are centrally involved in the pathogenesis of Alzheimer's disease, trigger Ca(2+) influx through N-methyl-D-aspartate receptors and stimulate reactive oxygen species generation in primary hippocampal neurons. We have previously reported that AβOs promote Ca(2+) release mediated by ryanodine receptors (RyR), which in turn triggers mitochondrial fragmentation. We have also reported that the antioxidant N-acetylcysteine (NAC) prevents AβOs-induced Ca(2+) signal generation.

OBJECTIVES

To determine if RyR-mediated Ca(2+) release activated by the specific agonist 4-chloro-m-cresol (4-CMC) induces fragmentation of the mitochondrial network, and to ascertain if NAC prevents the mitochondrial fragmentation induced by AβOs and/or 4-CMC.

METHODS

Mature primary rat hippocampal neurons were incubated for 24 h with sublethal concentrations of AβOs (500 nM) or for 1-3 h with 4-CMC (0.5-1 mM), ± 10 mM NAC. Mitochondrial morphology was assessed by confocal microscopy of fixed neurons stained with anti-mHsp70. Intracellular Ca(2+) levels were determined by time series microscopy of neurons preloaded with Fluo-4 AM.

RESULTS

Preincubation of neurons for 30 min with NAC prevented the mitochondrial fragmentation induced by AβOs or 4-CMC. In addition, we confirmed that preincubation with NAC abolished the stimulation of RyR-mediated Ca(2+) release induced by AβOs or 4-CMC.

CONCLUSION

The present results strongly suggest that the general antioxidant NAC prevents AβO-induced mitochondrial fragmentation by preventing RyR-mediated Ca(2+)-induced Ca(2+) release.

Ca(2+) signaling: an outlook on the characterization of Ca(2+) channels and their importance in cellular functions

Calcium (Ca(2+)) is essential in regulating a plethora of cellular functions that includes cell proliferation and differentiation, axonal guidance and cell migration, neuro/enzyme secretion and exocytosis, development/maintenance of neural circuits, cell death and many more. Since Ca(2+) regulates so many fundamental processes, it could be anticipated that numerous Ca(2+) channels and transporters will assist in regulating Ca(2+) entry across the plasma membrane. Towards this several Ca(2+) channels such as voltage-gated channels, store-operated Ca(2+) entry (SOCE) channels, NMDA, AMPA and other ligand gated channels have been identified. In recent years research focus has been targeted towards identification of the precise function of these essential channels. Furthermore, characterization of these individual Ca(2+) channels has also gained much attention, since specific Ca(2+) channels have been shown to influence a particular cellular response. Moreover, perturbations in these Ca(2+) channels have also been implicated in a spectrum of pathological conditions. Hence, understanding the precise involvement of these Ca(2+) channels in disease conditions would presumably unveil avenues for plausible therapeutic interventions. We thus review the role of Ca(2+) signaling in select -disease conditions and also provide experimental evidence as how they can be characterized in a given cell.

Road to Alzheimer's disease: the pathomechanism underlying

Alzheimer's disease (AD), the most common cause of dementia, results from the interplay of various deregulated mechanisms triggering a complex pathophysiology. The neurons suffer from and slowly succumb to multiple irreversible damages, resulting in cell death and thus memory deficits that characterize AD. In spite of our vast knowledge, it is still unclear as to when the disease process starts and how long the perturbations continue before the disease manifests. Recent studies provide sufficient evidence to prove amyloid β (Aβ) as the primary cause initiating secondary events, but Aβ is also known to be produced under normal conditions and to possess physiological roles, hence, the questions that remain are: What are the factors that lead to abnormal Aβ production? When does Aβ turn into a pathological molecule? What is the chain of events that follows Aβ? The answers are still under debate, and further insight may help us in creating better diagnostic and therapeutic options in AD. The present article attempts to review the current literature regarding AD pathophysiology and proposes a pathophysiologic cascade in AD.

Epigenetic control of somatostatin and cortistatin expression by β amyloid peptide

β Amyloid, present in senile plaques, has been related largely to neuronal loss in the brain of patients with Alzheimer's disease. However, how neurons respond to β amyloid insults is still poorly understood. Here we show that β amyloid increases somatostatin and cortistatin gene expression mainly through an increase in histone 3 lysine 4 methylation (H3K4me3), a modification associated with transcriptional activation. Somatostatin and cortistatin partially decreased β amyloid toxicity in primary cortical neurons in culture. Thus we suggest that neurons respond to β amyloid insults by releasing somatostatin and cortistatin, which will act as a protective agent against β amyloid toxicity. Our results suggest a relevant function for both neuropeptides against β amyloid toxicity, providing new insights into Alzheimer's disease.

Modulation of ultra-low-molecular-weight heparin on [Ca²⁺]i in nervous cells

Heparin is an effective competitive antagonist of inositol 1,4,5-trisphosphate receptors (IP(3)Rs). It binds to IP(3)Rs and affects calcium homeostasis. Ultra-low-molecular-weight heparin (ULMWH) is heparin's derivative, the present study was designed to test the effects of ULMWH on intracellular calcium concentration ([Ca(2+)]i) in primary cultured neurons. [Ca(2+)]i was measured by Multilabel Counter Victor-1420 using Fura-2/AM as the calcium fluorescent probe. The results indicated that ULMWH decreased the resting [Ca(2+)]i with or without extracellular Ca(2+). They had no effects on high K(+)-induced elevation of intracellular Ca(2+) level indicating that ULMWH had no effect on external Ca(2+) influx mediated by voltage-dependent calcium channels. However, they partially reduced the increase in [Ca(2+)]i induced by glutamate. Furthermore, ULMWH significantly inhibited the inositol 1,4,5-trisphosphate (IP(3))-induced increase in [Ca(2+)]i both in cellular and subcellular level. These results suggest that ULMWH may reduce [Ca(2+)]i in neurons through suppressing Ca(2+) release from IP(3)-sensitive stores.

A translational continuum of model systems for evaluating treatment strategies in Alzheimer's disease: isradipine as a candidate drug

A growing body of evidence supports the 'calcium hypothesis' of Alzheimer's disease (AD), which postulates that a variety of insults might disrupt the homeostatic regulation of neuronal calcium (Ca(2+)) in the brain, resulting in the progressive symptoms that typify the disease. However, despite ongoing efforts to develop new methods for testing therapeutic compounds that might be beneficial in AD, no single bioassay permits both rapid screening and in vivo validation of candidate drugs that target specific components of the Ca(2+) regulatory machinery. To address this issue, we have integrated four distinct model systems that provide complementary information about a trial compound: the human neuroblastoma MC65 line, which provides an in vitro model of amyloid toxicity; a transgenic Drosophila model, which develops age-dependent pathologies associated with AD; the 3×TgAD transgenic mouse, which recapitulates many of the neuropathological features that typify AD; and the embryonic nervous system of Manduca, which provides a novel in vivo assay for the acute effects of amyloid peptides on neuronal motility. To demonstrate the value of this 'translational suite' of bioassays, we focused on a set of clinically approved dihydropyridines (DHPs), a class of well-defined inhibitors of L-type calcium channels that have been suggested to be neuroprotective in AD. Among the DHPs tested in this study, we found that isradipine reduced the neurotoxic consequences of β-amyloid accumulation in all four model systems without inducing deleterious side effects. Our results provide new evidence in support of the Ca(2+) hypothesis of AD, and indicate that isradipine represents a promising drug for translation into clinical trials. In addition, these studies also demonstrate that this continuum of bioassays (representing different levels of complexity) provides an effective means of evaluating other candidate compounds that target specific components of the Ca(2+) regulatory machinery and that therefore might be beneficial in the treatment of AD.

Effects of amyloid-β peptides on voltage-gated L-type Ca(V)1.2 and Ca(V)1.3 Ca(2+) channels

Overload of intracellular Ca(2+) has been implicated in the pathogenesis of neuronal disorders, such as Alzheimer's disease. Various mechanisms produce abnormalities in intracellular Ca(2+) homeostasis systems. L-type Ca(2+) channels have been known to be closely involved in the mechanisms underlying the neurodegenerative properties of amyloid-β (Aβ) peptides. However, most studies of L-type Ca(2+) channels in Aβ-related mechanisms have been limited to Ca(V)1.2, and surprisingly little is known about the involvement of Ca(V)1.3 in Aβ-induced neuronal toxicity. In the present study, we examined the expression patterns of Ca(V)1.3 after Aβ(25-35) exposure for 24 h and compared them with the expression patterns of Ca(V)1.2. The expression levels of Ca(V)1.3 were not significantly changed by Aβ(25-35) at both the mRNA levels and the total protein level in cultured hippocampal neurons. However, surface protein levels of Ca(V)1.3 were significantly increased by Aβ(25-35), but not by Aβ(35-25). We next found that acute treatment with Aβ(25-35) increased Ca(V)1.3 channel activities in HEK293 cells using whole-cell patch-clamp recordings. Furthermore, using GTP pulldown and co-immunoprecipitation assays in HEK293 cell lysates, we found that amyloid precursor protein interacts with β(3) subunits of Ca(2+) channels instead of Ca(V)1.2 or Ca(V)1.3 α(1) subunits. These results show that Aβ(25-35) chronically or acutely upregulates Ca(V)1.3 in the rat hippocampal and human kidney cells (HEK293). This suggests that Ca(V)1.3 has a potential role along with Ca(V)1.2 in the pathogenesis of Alzheimer's disease.

Spinocerebellar ataxia type 13 mutant potassium channel alters neuronal excitability and causes locomotor deficits in zebrafish

Whether changes in neuronal excitability can cause neurodegenerative disease in the absence of other factors such as protein aggregation is unknown. Mutations in the Kv3.3 voltage-gated K(+) channel cause spinocerebellar ataxia type 13 (SCA13), a human autosomal-dominant disease characterized by locomotor impairment and the death of cerebellar neurons. Kv3.3 channels facilitate repetitive, high-frequency firing of action potentials, suggesting that pathogenesis in SCA13 is triggered by changes in electrical activity in neurons. To investigate whether SCA13 mutations alter excitability in vivo, we expressed the human dominant-negative R420H mutant subunit in zebrafish. The disease-causing mutation specifically suppressed the excitability of Kv3.3-expressing, fast-spiking motor neurons during evoked firing and fictive swimming and, in parallel, decreased the precision and amplitude of the startle response. The dominant-negative effect of the mutant subunit on K(+) current amplitude was directly responsible for the reduced excitability and locomotor phenotype. Our data provide strong evidence that changes in excitability initiate pathogenesis in SCA13 and establish zebrafish as an excellent model system for investigating how changes in neuronal activity impair locomotor control and cause cell death.

Presenilins as endoplasmic reticulum calcium leak channels and Alzheimer's disease pathogenesis

Alzheimer disease (AD) is the most common neurodegenerative disorder worldwide and is at present, incurable. The accumulation of toxic amyloid-beta (Aβ) peptide aggregates in AD brain is thought to trigger the extensive synaptic loss and neurodegeneration linked to cognitive decline, an idea that underlies the 'amyloid hypothesis' of AD etiology in both the familal (FAD) and sporadic forms of the disease. Genetic mutations causing FAD also result in the dysregulation of neuronal calcium (Ca(2+)) handling and may contribute to AD pathogenesis, an idea termed the 'calcium hypothesis' of AD. Mutations in presenilin proteins account for majority of FAD cases. Presenilins function as catalytic subunit of γ-secretase involved in generation of Aβ peptide Recently, we discovered that presenilns function as low-conductance, passive ER Ca(2+) leak channels, independent of γ-secretase activity. We further discovered that many FAD mutations in presenilins result in loss of ER Ca(2+) leak function activity and Ca(2+) overload in the ER. These results provided potential explanation for abnormal Ca(2+) signaling observed in FAD cells with mutations in presenilns. Our latest work on studies of ER Ca(2+) leak channel function of presenilins and implications of these findings for understanding AD pathogenesis are discussed in this article.

Calcium homoeostasis modulator 1 (CALHM1) reduces the calcium content of the endoplasmic reticulum (ER) and triggers ER stress

CALHM1 (calcium homoeostasis modulator 1), a membrane protein with similarity to NMDA (N-methyl-D-aspartate) receptor channels that localizes in the plasma membrane and the ER (endoplasmic reticulum) of neurons, has been shown to generate a plasma-membrane Ca2+ conductance and has been proposed to influence Alzheimer's disease risk. In the present study we have investigated the effects of CALHM1 on intracellular Ca2+ handling in HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] cells by using targeted aequorins for selective monitorization of Ca2+ transport by organelles. We find that CALHM1 increases Ca2+ leak from the ER and, more importantly, reduces ER Ca2+ uptake by decreasing both the transport capacity and the Ca2+ affinity of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase). As a result, the Ca2+ content of the ER is drastically decreased. This reduction in the Ca2+ content of the ER triggered the UPR (unfolded protein response) with induction of several ER stress markers, such as CHOP [C/EBP (CCAAT/enhancer-binding protein)-homologous protein], ERdj4, GRP78 (glucose-regulated protein of 78 kDa) and XBP1 (X-box-binding protein 1). Thus CALHM1 might provide a relevant link between Ca2+ homoeostasis disruption, ER stress and cell damage in the pathogenesis of neurodegenerative diseases

Presenilins function in ER calcium leak and Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is the most common neurodegenerative disorder worldwide and is at present, incurable. The accumulation of toxic amyloid-beta (Aβ) peptide aggregates in AD brain is thought to trigger the extensive synaptic loss and neurodegeneration linked to cognitive decline, an idea that underlies the 'amyloid hypothesis' of AD etiology in both the familal (FAD) and sporadic forms of the disease. Genetic mutations causing FAD also result in the dysregulation of neuronal calcium (Ca(2+)) handling and may contribute to AD pathogenesis, an idea termed the 'calcium hypothesis' of AD. Mutations in presenilin proteins account for the majority of FAD cases. Presenilins function as catalytic subunits of γ-secretase involved in the generation of Aβ peptide. Recently, we discovered that presenilns function as low-conductance, passive ER Ca(2+) leak channels, independent of γ-secretase activity. We further discovered that many FAD mutations in presenilins results in the loss of ER Ca(2+) leak function activity and Ca(2+) overload in the ER. These results provided potential explanation for abnormal Ca(2+) signaling observed in FAD cells with mutations in presenilns. The implications of these findings for understanding AD pathogenesis are discussed in this article.

Endoplasmic-reticulum calcium depletion and disease

The endoplasmic reticulum (ER) as an intracellular Ca(2+) store not only sets up cytosolic Ca(2+) signals, but, among other functions, also assembles and folds newly synthesized proteins. Alterations in ER homeostasis, including severe Ca(2+) depletion, are an upstream event in the pathophysiology of many diseases. On the one hand, insufficient release of activator Ca(2+) may no longer sustain essential cell functions. On the other hand, loss of luminal Ca(2+) causes ER stress and activates an unfolded protein response, which, depending on the duration and severity of the stress, can reestablish normal ER function or lead to cell death. We will review these various diseases by mainly focusing on the mechanisms that cause ER Ca(2+) depletion.

CALHM1 P86L polymorphism modulates CSF Aβ levels in cognitively healthy individuals at risk for Alzheimer's disease

The calcium homeostasis modulator 1 (CALHM1) gene codes for a novel cerebral calcium channel controlling intracellular calcium homeostasis and amyloid-β (Aβ) peptide metabolism, a key event in the etiology of Alzheimer's disease (AD). The P86L polymorphism in CALHM1 (rs2986017) initially was proposed to impair CALHM1 functionally and to lead to an increase in Aβ accumulation in vitro in cell lines. Recently, it was reported that CALHM1 P86L also may influence Aβ metabolism in vivo by increasing Aβ levels in human cerebrospinal fluid (CSF). Although the role of CALHM1 in AD risk remains uncertain, concordant data have now emerged showing that CALHM1 P86L is associated with an earlier age at onset of AD. Here, we have analyzed the association of CALHM1 P86L with CSF Aβ in samples from 203 AD cases and 46 young cognitively healthy individuals with a positive family history of AD. We failed to detect an association between the CALHM1 polymorphism and CSF Aβ levels in AD patients. Our data, however, revealed a significant association of CALHM1 P86L with elevated CSF Aβ42 and Aβ40 in the normal cohort at risk for AD. This work shows that CALHM1 modulates CSF Aβ levels in presymptomatic individuals, strengthening the notion that CALHM1 is involved in AD pathogenesis. These data further demonstrate the utility of endophenotype-based approaches focusing on CSF biomarkers for the identification or validation of risk factors for AD.

Mitochondrial permeability transition in Ca(2+)-dependent apoptosis and necrosis

A variety of stimuli utilize an increase of cytosolic free Ca(2+) concentration as a second messenger to transmit signals, through Ca(2+) release from the endoplasmic reticulum or opening of plasma membrane Ca(2+) channels. Mitochondria contribute to the tight spatiotemporal control of this process by accumulating Ca(2+), thus shaping the return of cytosolic Ca(2+) to resting levels. The rise of mitochondrial matrix free Ca(2+) concentration stimulates oxidative metabolism; yet, in the presence of a variety of sensitizing factors of pathophysiological relevance, the matrix Ca(2+) increase can also lead to opening of the permeability transition pore (PTP), a high conductance inner membrane channel. While transient openings may serve the purpose of providing a fast Ca(2+) release mechanism, persistent PTP opening is followed by deregulated release of matrix Ca(2+), termination of oxidative phosphorylation, matrix swelling with inner membrane unfolding and eventually outer membrane rupture with release of apoptogenic proteins and cell death. Thus, a rise in mitochondrial Ca(2+) can convey both apoptotic and necrotic death signals by inducing opening of the PTP. Understanding the signalling networks that govern changes in mitochondrial free Ca(2+) concentration, their interplay with Ca(2+) signalling in other subcellular compartments, and regulation of PTP has important implications in the fine comprehension of the main biological routines of the cell and in disease pathogenesis.

Mutagenesis mapping of the presenilin 1 calcium leak conductance pore

Missense mutations in presenilin 1 (PS1) and presenilin 2 (PS2) proteins are a major cause of familial Alzheimer disease. Presenilins are proteins with nine transmembrane (TM) domains that function as catalytic subunits of the γ-secretase complex responsible for the cleavage of the amyloid precursor protein and other type I transmembrane proteins. The water-filled cavity within presenilin is necessary to mediate the intramembrane proteolysis reaction. Consistent with this idea, cysteine-scanning mutagenesis and NMR studies revealed a number of water-accessible residues within TM7 and TM9 of mouse PS1. In addition to γ-secretase function, presenilins also demonstrate a low conductance endoplasmic reticulum Ca(2+) leak function, and many familial Alzheimer disease presenilin mutations impair this function. To map the potential Ca(2+) conductance pore in PS1, we systematically evaluated endoplasmic reticulum Ca(2+) leak activity supported by a series of cysteine point mutants in TM6, TM7, and TM9 of mouse PS1. The results indicate that TM7 and TM9, but not TM6, could play an important role in forming the conductance pore of PS1. These results are consistent with previous cysteine-scanning mutagenesis and NMR analyses of PS1 and provide further support for our hypothesis that the hydrophilic catalytic cavity of presenilins may also constitute a Ca(2+) conductance pore.

Membrane Incorporation, Channel Formation, and Disruption of Calcium Homeostasis by Alzheimer's β-Amyloid Protein

Oligomerization, conformational changes, and the consequent neurodegeneration of Alzheimer's β-amyloid protein (AβP) play crucial roles in the pathogenesis of Alzheimer's disease (AD). Mounting evidence suggests that oligomeric AβPs cause the disruption of calcium homeostasis, eventually leading to neuronal death. We have demonstrated that oligomeric AβPs directly incorporate into neuronal membranes, form cation-sensitive ion channels (“amyloid channels”), and cause the disruption of calcium homeostasis via the amyloid channels. Other disease-related amyloidogenic proteins, such as prion protein in prion diseases or α-synuclein in dementia with Lewy bodies, exhibit similarities in the incorporation into membranes and the formation of calcium-permeable channels. Here, based on our experimental results and those of numerous other studies, we review the current understanding of the direct binding of AβP into membrane surfaces and the formation of calcium-permeable channels. The implication of composition of membrane lipids and the possible development of new drugs by influencing membrane properties and attenuating amyloid channels for the treatment and prevention of AD is also discussed.

Redox control of brain calcium in health and disease

Calcium ion is a highly versatile cellular messenger. Calcium signals-defined as transient increments in intracellular-free calcium concentration-elicit a multiplicity of responses that depend on cell type and signal properties such as their intensity, duration, cellular localization, and frequency. The vast literature available on the role of calcium signals in brain cells, chiefly centered on neuronal cells, indicates that calcium signals regulate essential neuronal functions, including synaptic transmission, gene expression, synaptic plasticity processes underlying learning and memory, and survival or death. The eight articles comprising this forum issue address different and novel aspects of calcium signaling in normal neuronal function, including how calcium signals interact with the generation of reactive species of oxygen/nitrogen with various functional consequences, and focus also on how abnormal calcium homeostasis and signaling, plus oxidative stress, affect overall brain physiology during aging and in neurodegenerative conditions such as Alzheimer's or Parkinson's disease.

Calcium signalling and Alzheimer's disease

New insights into how Ca(2+) regulates learning and memory have begun to provide clues as to how the amyloid-dependent remodelling of neuronal Ca(2+) signalling pathways can disrupt the mechanisms of learning and memory in Alzheimer's disease (AD). The calcium hypothesis of AD proposes that activation of the amyloidogenic pathway remodels the neuronal Ca(2+) signalling pathways responsible for cognition by enhancing the entry of Ca(2+) and/or the release of internal Ca(2+) by ryanodine receptors or InsP(3) receptors. The specific proposal is that Ca(2+) signalling remodelling results in a persistent elevation in the level of Ca(2+) that constantly erases newly acquired memories by enhancing the mechanism of long-term depression (LTD). Neurons can still form memories through the process of LTP, but this stored information is rapidly removed by the persistent activation of LTD. Further dysregulation in Ca(2+) signalling will then go on to induce the neurodegeneration that characterizes the later stages of dementia.

PSAPP mice exhibit regionally selective reductions in gliosis and plaque deposition in response to S100B ablation

BACKGROUND

Numerous studies have reported that increased expression of S100B, an intracellular Ca2+ receptor protein and secreted neuropeptide, exacerbates Alzheimer's disease (AD) pathology. However, the ability of S100B inhibitors to prevent/reverse AD histopathology remains controversial. This study examines the effect of S100B ablation on in vivo plaque load, gliosis and dystrophic neurons.

METHODS

Because S100B-specific inhibitors are not available, genetic ablation was used to inhibit S100B function in the PSAPP AD mouse model. The PSAPP/S100B-/- line was generated by crossing PSAPP double transgenic males with S100B-/- females and maintained as PSAPP/S100B+/- crosses. Congo red staining was used to quantify plaque load, plaque number and plaque size in 6 month old PSAPP and PSAPP/S100B-/- littermates. The microglial marker Iba1 and astrocytic marker glial fibrillary acidic protein (GFAP) were used to quantify gliosis. Dystrophic neurons were detected with the phospho-tau antibody AT8. S100B immunohistochemistry was used to assess the spatial distribution of S100B in the PSAPP line.

RESULTS

PSAPP/S100B-/- mice exhibited a regionally selective decrease in cortical but not hippocampal plaque load when compared to PSAPP littermates. This regionally selective reduction in plaque load was accompanied by decreases in plaque number, GFAP-positive astrocytes, Iba1-positive microglia and phospho-tau positive dystrophic neurons. These effects were not attributable to regional variability in the distribution of S100B. Hippocampal and cortical S100B immunoreactivity in PSAPP mice was associated with plaques and co-localized with astrocytes and microglia.

CONCLUSIONS

Collectively, these data support S100B inhibition as a novel strategy for reducing cortical plaque load, gliosis and neuronal dysfunction in AD and suggest that both extracellular as well as intracellular S100B contribute to AD histopathology.

Prolonged Abeta treatment leads to impairment in the ability of primary cortical neurons to maintain K+ and Ca2+ homeostasis

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterised by the formation of insoluble amyloidogenic plaques and neurofibrillary tangles. Beta amyloid (Abeta) peptide is one of the main constituents in Abeta plaques, and is thought to be a primary causative agent in AD. Neurons are likely to be exposed to chronic, sublethal doses of Abeta over an extended time during the pathogenesis of AD, however most studies published to date using in vitro models have focussed on acute studies. To experimentally model the progressive pathogenesis of AD, we exposed primary cortical neurons daily to 1 muM of Abeta1-40 over 7 days and compared their survival with age-similar untreated cells. We also investigated whether chronic Abeta exposure affects neuronal susceptibility to the subsequent acute excitotoxicity induced by 10 muM glutamate and assessed how Ca2+ and K+ homeostasis were affected by either treatment.

RESULTS

We show that continuous exposure to 1 muM Abeta1-40 for seven days decreased survival of cultured cortical neurons by 20%. This decrease in survival correlated with increased K+ efflux from the cells. One day treatment with 1 muM Abeta followed by glutamate led to a substantially higher K+ efflux than in the age-similar untreated control. This difference further increased with the duration of the treatment. K+ efflux also remained higher in Abeta treated cells 20 min after glutamate application leading to 2.8-fold higher total K+ effluxed from the cells compared to controls. Ca2+ uptake was significantly higher only after prolonged Abeta treatment with 2.5-fold increase in total Ca2+ uptake over 20 min post glutamate application after six days of Abeta treatment or longer (P < 0.05).

CONCLUSIONS

Our data suggest that long term exposure to Abeta is detrimental because it reduces the ability of cortical neurons to maintain K+ and Ca2+ homeostasis in response to glutamate challenge, a response that might underlie the early symptoms of AD. The observed inability to maintain K+ homeostasis might furthermore be useful in future studies as an early indicator of pathological changes in response to Abeta.

Role of presenilins in neuronal calcium homeostasis

Alzheimer's disease (AD) is a progressive and irreversible neurodegenerative disorder. Familial AD (FAD) mutations in presenilins have been linked to calcium (Ca(2+)) signaling abnormalities. To explain these results, we previously proposed that presenilins function as endoplasmic reticulum (ER) passive Ca(2+) leak channels. To directly investigate the role of presenilins in neuronal ER Ca(2+) homeostasis, we here performed a series of Ca(2+) imaging experiments with primary neuronal cultures from conditional presenilin double-knock-out mice (PS1(dTAG/dTAG), PS2(-/-)) and from triple-transgenic AD mice (KI-PS1(M146V), Thy1-APP(KM670/671NL), Thy1-tau(P301L)). Obtained results provided additional support to the hypothesis that presenilins function as ER Ca(2+) leak channels in neurons. Interestingly, we discovered that presenilins play a major role in ER Ca(2+) leak function in hippocampal but not in striatal neurons. We further discovered that, in hippocampal neurons, loss of presenilin-mediated ER Ca(2+) leak function was compensated by an increase in expression and function of ryanodine receptors (RyanRs). Long-term feeding of the RyanR inhibitor dantrolene to amyloid precursor protein-presenilin-1 mice (Thy1-APP(KM670/671NL), Thy1-PS1(L166P)) resulted in an increased amyloid load, loss of synaptic markers, and neuronal atrophy in hippocampal and cortical regions. These results indicate that disruption of ER Ca(2+) leak function of presenilins may play an important role in AD pathogenesis.

The S100B/RAGE Axis in Alzheimer's Disease

Increasing evidence suggests that the small EF-hand calcium-binding protein S100B plays an important role in Alzheimer's disease. Among other evidences are the increased levels of both S100B and its receptor, the Receptor for Advanced Glycation Endproducts (RAGEs) in the AD diseased brain. The regulation of RAGE signaling by S100B is complex and probably involves other ligands including the amyloid beta peptide (Aβ), the Advanced Glycation Endproducts (AGEs), or transtheyretin. In this paper we discuss the current literature regarding the role of S100B/RAGE activation in Alzheimer's disease.

Neuronal calcium signaling, mitochondrial dysfunction, and Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder among the aged worldwide. AD is characterized by extensive synaptic and neuronal loss that leads to impaired memory and cognitive decline. The cause of AD is not completely understood and no effective therapy has been developed. The accumulation of toxic amyloid-beta42 (Abeta42) peptide oligomers and aggregates in AD brain has been proposed to be primarily responsible for the pathology of the disease, an idea dubbed the 'amyloid hypothesis' of AD etiology. In addition to the increase in Abeta42 levels, disturbances in neuronal calcium (Ca2+) signaling and alterations in expression levels of Ca2+ signaling proteins have been observed in animal models of familial AD and in studies of postmortem brain samples from sporadic AD patients. Based on these data, the 'Ca2+ hypothesis of AD' has been proposed. In particular, familial AD has been linked with enhanced Ca2+ release from the endoplasmic reticulum and elevated cytosolic Ca2+ levels. The augmented cytosolic Ca2+ levels can trigger signaling cascades that affect synaptic stability and function and can be detrimental to neuronal health, such as activation of calcineurin and calpains. Here we review the latest results supporting the 'Ca2+ hypothesis' of AD pathogenesis. We further argue that over time, supranormal cytosolic Ca2+ signaling can impair mitochondrial function in AD neurons. We conclude that inhibitors and stabilizers of neuronal Ca2+ signaling and mitochondrial function may have therapeutic potential for AD treatment. We also discuss latest and planned AD therapeutic trials of agents targeting Ca2+ channels and mitochondria.