Enhanced caffeine-induced Ca2+ release in the 3xTg-AD mouse model of Alzheimer's disease

Analyzing dendritic spine pathology in Alzheimer's disease: problems and opportunities

Synaptic failure is an immediate cause of cognitive decline and memory dysfunction in Alzheimer’s disease. Dendritic spines are specialized structures on neuronal processes, on which excitatory synaptic contacts take place and the loss of dendritic spines directly correlates with the loss of synaptic function. Dendritic spines are readily accessible for both in vitro and in vivo experiments and have, therefore, been studied in great detail in Alzheimer’s disease mouse models. To date, a large number of different mechanisms have been proposed to cause dendritic spine dysfunction and loss in Alzheimer’s disease. For instance, amyloid beta fibrils, diffusible oligomers or the intracellular accumulation of amyloid beta have been found to alter the function and structure of dendritic spines by distinct mechanisms. Furthermore, tau hyperphosphorylation and microglia activation, which are thought to be consequences of amyloidosis in Alzheimer’s disease, may also contribute to spine loss. Lastly, genetic and therapeutic interventions employed to model the disease and elucidate its pathogenetic mechanisms in experimental animals may cause alterations of dendritic spines on their own. However, to date none of these mechanisms have been translated into successful therapeutic approaches for the human disease. Here, we critically review the most intensely studied mechanisms of spine loss in Alzheimer’s disease as well as the possible pitfalls inherent in the animal models of such a complex neurodegenerative disorder.

Glial calcium signalling in Alzheimer's disease

The most accredited (and fashionable) hypothesis of the pathogenesis of Alzheimer Disease (AD) sees accumulation of β-amyloid protein in the brain (in both soluble and insoluble forms) as a leading mechanism of neurotoxicity. How β-amyloid triggers the neurodegenerative disorder is at present unclear, but growing evidence suggests that a deregulation of Ca(2+) homeostasis and deficient Ca(2+) signalling may represent a fundamental pathogenic factor. Given that symptoms of AD are most likely linked to synaptic dysfunction (at the early stages) followed by neuronal loss (at later and terminal phases of the disease), the effects of β-amyloid have been mainly studied in neurones. Yet, it must be acknowledged that neuroglial cells, including astrocytes, contribute to pathological progression of most (if not all) neurological diseases. Here, we review the literature pertaining to changes in Ca(2+) signalling in astrocytes exposed to exogenous β-amyloid or in astrocytes from transgenic Alzheimer disease animals models, characterized by endogenous β-amyloidosis. Accumulated experimental data indicate deregulation of Ca(2+) homeostasis and signalling in astrocytes in AD, which should be given full pathogenetic consideration. Further studies are warranted to comprehend the role of deficient astroglial Ca(2+) signalling in the disease progression.

Identification of tetrahydrocarbazoles as novel multifactorial drug candidates for treatment of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder and the most frequent cause of dementia. To date, there are only a few approved drugs for AD, which show little or no effect on disease progression. Impaired intracellular calcium homeostasis is believed to occur early in the cascade of events leading to AD. Here, we examined the possibility of normalizing the disrupted calcium homeostasis in the endoplasmic reticulum (ER) store as an innovative approach for AD drug discovery. High-throughput screening of a small-molecule compound library led to the identification of tetrahydrocarbazoles, a novel multifactorial class of compounds that can normalize the impaired ER calcium homeostasis. We found that the tetrahydrocarbazole lead structure, first, dampens the enhanced calcium release from ER in HEK293 cells expressing familial Alzheimer's disease (FAD)-linked presenilin 1 mutations. Second, the lead structure also improves mitochondrial function, measured by increased mitochondrial membrane potential. Third, the same lead structure also attenuates the production of amyloid-beta (Aβ) peptides by decreasing the cleavage of amyloid precursor protein (APP) by β-secretase, without notably affecting α- and γ-secretase cleavage activities. Considering the beneficial effects of tetrahydrocarbazoles addressing three key pathological aspects of AD, these compounds hold promise for the development of potentially effective AD drug candidates.

Neuronal calcium signaling: function and dysfunction

Calcium (Ca(2+)) is an universal second messenger that regulates the most important activities of all eukaryotic cells. It is of critical importance to neurons as it participates in the transmission of the depolarizing signal and contributes to synaptic activity. Neurons have thus developed extensive and intricate Ca(2+) signaling pathways to couple the Ca(2+) signal to their biochemical machinery. Ca(2+) influx into neurons occurs through plasma membrane receptors and voltage-dependent ion channels. The release of Ca(2+) from the intracellular stores, such as the endoplasmic reticulum, by intracellular channels also contributes to the elevation of cytosolic Ca(2+). Inside the cell, Ca(2+) is controlled by the buffering action of cytosolic Ca(2+)-binding proteins and by its uptake and release by mitochondria. The uptake of Ca(2+) in the mitochondrial matrix stimulates the citric acid cycle, thus enhancing ATP production and the removal of Ca(2+) from the cytosol by the ATP-driven pumps in the endoplasmic reticulum and the plasma membrane. A Na(+)/Ca(2+) exchanger in the plasma membrane also participates in the control of neuronal Ca(2+). The impaired ability of neurons to maintain an adequate energy level may impact Ca(2+) signaling: this occurs during aging and in neurodegenerative disease processes. The focus of this review is on neuronal Ca(2+) signaling and its involvement in synaptic signaling processes, neuronal energy metabolism, and neurotransmission. The contribution of altered Ca(2+) signaling in the most important neurological disorders will then be considered.

Suppression of InsP3 receptor-mediated Ca2+ signaling alleviates mutant presenilin-linked familial Alzheimer's disease pathogenesis

Exaggerated intracellular Ca(2+) signaling is a robust proximal phenotype observed in cells expressing familial Alzheimer's disease (FAD)-causing mutant presenilins (PSs). The mechanisms that underlie this phenotype are controversial and their in vivo relevance for AD pathogenesis is unknown. Here, we used a genetic approach to identify the mechanisms involved and to evaluate their role in the etiology of AD in two FAD mouse models. Genetic reduction of the type 1 inositol trisphosphate receptor (InsP3R1) by 50% normalized exaggerated Ca(2+) signaling observed in cortical and hippocampal neurons in both animal models. In PS1M146V knock-in mice, reduced InsP3R1 expression restored normal ryanodine receptor and cAMP response element-binding protein (CREB)-dependent gene expression and rescued aberrant hippocampal long-term potentiation (LTP). In 3xTg mice, reduced InsP3R1 expression profoundly attenuated amyloid β accumulation and tau hyperphosphorylation and rescued hippocampal LTP and memory deficits. These results indicate that exaggerated Ca(2+) signaling, which is associated with FAD PS, is mediated by InsP3R and contributes to disease pathogenesis in vivo. Targeting the InsP3 signaling pathway could be considered a potential therapeutic strategy for patients harboring mutations in PS linked to AD.

Ryanodine receptors: physiological function and deregulation in Alzheimer disease

Perturbed Endoplasmic Reticulum (ER) calcium (Ca2+) homeostasis emerges as a central player in Alzheimer disease (AD). Accordingly, different studies have reported alterations of the expression and the function of Ryanodine Receptors (RyR) in human AD-affected brains, in cells expressing familial AD-linked mutations on the β amyloid precursor protein (βAPP) and presenilins (the catalytic core in γ-secretase complexes cleaving the βAPP, thereby generating amyloid β (Aβ) peptides), as well as in the brain of various transgenic AD mice models. Data converge to suggest that RyR expression and function alteration are associated to AD pathogenesis through the control of: i) βAPP processing and Aβ peptide production, ii) neuronal death; iii) synaptic function; and iv) memory and learning abilities. In this review, we document the network of evidences suggesting that RyR could play a complex dual "compensatory/protective versus pathogenic" role contributing to the setting of histopathological lesions and synaptic deficits that are associated with the disease stages. We also discuss the possible mechanisms underlying RyR expression and function alterations in AD. Finally, we review recent publications showing that drug-targeting blockade of RyR and genetic manipulation of RyR reduces Aβ production, stabilizes synaptic transmission, and prevents learning and memory deficits in various AD mouse models. Chemically-designed RyR "modulators" could therefore be envisioned as new therapeutic compounds able to delay or block the progression of AD.

Increased hippocampal excitability in the 3xTgAD mouse model for Alzheimer's disease in vivo

Mouse Alzheimer's disease (AD) models develop age- and region-specific pathology throughout the hippocampal formation. One recently established pathological correlate is an increase in hippocampal excitability in vivo. Hippocampal pathology also produces episodic memory decline in human AD and we have shown a similar episodic deficit in 3xTg AD model mice aged 3–6 months. Here, we tested whether hippocampal synaptic dysfunction accompanies this cognitive deficit by probing dorsal CA1 and DG synaptic responses in anaesthetized, 4–6 month-old 3xTgAD mice. As our previous reports highlighted a decline in episodic performance in aged control mice, we included aged cohorts for comparison. CA1 and DG responses to low-frequency perforant path stimulation were comparable between 3xTgAD and controls at both age ranges. As expected, DG recordings in controls showed paired-pulse depression; however, paired-pulse facilitation was observed in DG and CA1 of young and old 3xTgAD mice. During stimulus trains both short-latency (presumably monosynaptic: ‘direct’) and long-latency (presumably polysynaptic: ‘re-entrant’) responses were observed. Facilitation of direct responses was modest in 3xTgAD animals. However, re-entrant responses in DG and CA1 of young 3xTgAD mice developed earlier in the stimulus train and with larger amplitude when compared to controls. Old mice showed less DG paired-pulse depression and no evidence for re-entrance. In summary, DG and CA1 responses to low-frequency stimulation in all groups were comparable, suggesting no loss of synaptic connectivity in 3xTgAD mice. However, higher-frequency activation revealed complex change in synaptic excitability in DG and CA1 of 3xTgAD mice. In particular, short-term plasticity in DG and CA1 was facilitated in 3xTgAD mice, most evidently in younger animals. In addition, re-entrance was facilitated in young 3xTgAD mice. Overall, these data suggest that the episodic-like memory deficit in 3xTgAD mice could be due to the development of an abnormal hyper-excitable state in the hippocampal formation.

Differential up-regulation of Vesl-1/Homer 1 protein isoforms associated with decline in visual performance in a preclinical glaucoma model

Glaucoma is a multifactorial progressive ocular pathology, clinically presenting with damage to the retina and optic nerve, ultimately leading to blindness. Retinal ganglion cell loss in glaucoma ultimately results in vision loss. Vesl/Homer proteins are scaffolding proteins that are critical for maintaining synaptic integrity by clustering, organizing and functionally regulating synaptic proteins. Current anti-glaucoma therapies target IOP as the sole modifiable clinical parameters. Long-term pharmacotherapy and surgical treatment do not prevent gradual visual field loss as the disease progresses, highlighting the need for new complementary, alternative and comprehensive treatment approaches. Vesl/Homer expression was measured in the retinae of DBA/2J mice, a preclinical genetic glaucoma model with spontaneous mutations resulting in a phenotype reminiscent of chronic human pigmentary glaucoma. Vesl/Homer proteins were differentially expressed in the aged, glaucomatous DBA/2J retina, both at the transcriptional and translational level. Immunoreactivity for the long Vesl-1L/Homer 1c isoform, but not of the immediate early gene product Vesl-1S/Homer 1a was increased in the synaptic layers of the retina. This increased protein level of Vesl-1L/Homer 1c was correlated with phenotypes of increased disease severity and a decrease in visual performance. The increased expression of Vesl-1L/Homer 1c in the glaucomatous retina likely results in increased intracellular Ca(2+) release through enhancement of synaptic coupling. The ensuing Ca(2+) toxicity may thus activate neurodegenerative pathways and lead to the progressive loss of synaptic function in glaucoma. Our data suggest that higher levels of Vesl-1L/Homer 1c generate a more severe disease phenotype and may represent a viable target for therapy development.

Snapin, positive regulator of stimulation- induced Ca²⁺ release through RyR, is necessary for HIV-1 replication in T cells

To identify critical host factors necessary for human immunodeficiency virus 1 (HIV-1) replication, large libraries of short-peptide-aptamers were expressed retrovirally. The target of one inhibitor peptide, Pep80, identified in this screen was determined to be Snapin, a protein associated with the soluble N-ethyl maleimide sensitive factor adaptor protein receptor (SNARE) complex that is critical for calcium-dependent exocytosis during neurotransmission. Pep80 inhibited Ca²⁺ release from intracellular stores and blocked downstream signaling by direct interruption of the association between Snapin and an intracellular calcium release channel, the ryanodine receptor (RyR). NFAT signaling was preferentially abolished by Pep80. Expression of Snapin overcame Pep80-mediated inhibition of Ca²⁺/NFAT signaling and HIV-1 replication. Furthermore, Snapin induced HIV-1 replication in primary CD4⁺ T cells. Thus, through its interaction with RyR, Snapin is a critical regulator of Ca²⁺ signaling and T cell activation. Use of the genetically selected intracellular aptamer inhibitors allowed us to define unique mechanisms important to HIV-1 replication and T cell biology.

Can the calcium hypothesis explain synaptic loss in Alzheimer's disease?

Alzheimer's disease (AD) is the threat of modern humankind that is provoked by increased human lifespan. Despite extensive studies on AD pathology for more than 100 years, there are no disease-preventing therapies. Growing evidence suggests the role of calcium (Ca2+) in the pathogenesis of AD. The main purpose of the article is to understand whether modern science is able to explain the synapse loss observed in early AD and discuss the role of Ca2+ hypothesis in it. Based on results obtained in our laboratory and others, we propose that familial AD-associated mutations in presenilins cause a Ca2+ overload of endoplasmic reticulum stores which leads to compensatory downregulation of the neuronal store-operated Ca2+ (nSOC) entry pathway. We propose that synaptic nSOC is necessary for the stability of mature synaptic spines and that dysfunction of this pathway may play an important role in synaptic and memory loss in AD.

Presenilins regulate the cellular activity of ryanodine receptors differentially through isotype-specific N-terminal cysteines

Presenilins (PS), endoplasmic reticulum (ER) transmembrane proteins, form the catalytic core of γ-secretase, an amyloid precursor protein processing enzyme. Mutations in PS lead to Alzheimer's disease (AD) by altering γ-secretase activity to generate pathologic amyloid beta and amyloid plaques in the brain. Here, we identified a novel mechanism where binding of a soluble, cytosolic N-terminal domain fragment (NTF) of PS to intracellular Ca(2+) release channels, ryanodine receptors (RyR), controls Ca(2+) release from the ER. While PS1NTF decreased total RyR-mediated Ca(2+) release, PS2NTF had no effect at physiological Ca(2+) concentrations. This differential function and isotype-specificity is due to four cysteines absent in PS1NTF, present, however, in PS2NTF. Site-directed mutagenesis targeting these cysteines converted PS1NTF to PS2NTF function and vice versa, indicating differential RyR binding. This novel mechanism of intracellular Ca(2+) regulation through the PS-RyR interaction represents a novel target for AD drug development and the treatment of other neurodegenerative disorders that critically depend on RyR and PS signaling.

Crosstalk between Endoplasmic Reticulum Stress and Protein Misfolding in Neurodegenerative Diseases

Under physiological conditions, the endoplasmic reticulum (ER) is a central subcellular compartment for protein quality control in the secretory pathway that prevents protein misfolding and aggregation. Instrumental in protein quality control in the ER is the unfolded protein response (UPR), which is activated upon ER stress to reestablish homeostasis through a sophisticated transcriptionally and translationally regulated signaling network. However, this response can lead to apoptosis if the stress cannot be alleviated. The presence of abnormal protein aggregates containing specific misfolded proteins is recognized as the basis of numerous human conformational disorders, including neurodegenerative diseases. Here, I will highlight the overwhelming evidence that the presence of specific aberrant proteins in Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), prion diseases, and Amyotrophic Lateral Sclerosis (ALS) is intimately associated with perturbations in the ER protein quality control machinery that become incompetent to restore protein homeostasis and shift adaptive programs toward the induction of apoptotic signaling to eliminate irreversibly damaged neurons. Increasing our understanding about the deadly crosstalk between ER dysfunction and protein misfolding in these neurodegenerative diseases may stimulate the development of novel therapeutic strategies able to support neuronal survival and ameliorate disease progression.

Ryanodine receptor blockade reduces amyloid-β load and memory impairments in Tg2576 mouse model of Alzheimer disease

In Alzheimer disease (AD), the perturbation of the endoplasmic reticulum (ER) calcium (Ca²⁺) homeostasis has been linked to presenilins, the catalytic core in γ-secretase complexes cleaving the amyloid precursor protein (APP), thereby generating amyloid-β (Aβ) peptides. Here we investigate whether APP contributes to ER Ca²⁺ homeostasis and whether ER Ca²⁺ could in turn influence Aβ production. We show that overexpression of wild-type human APP (APP(695)), or APP harboring the Swedish double mutation (APP(swe)) triggers increased ryanodine receptor (RyR) expression and enhances RyR-mediated ER Ca²⁺ release in SH-SY5Y neuroblastoma cells and in APP(swe)-expressing (Tg2576) mice. Interestingly, dantrolene-induced lowering of RyR-mediated Ca²⁺ release leads to the reduction of both intracellular and extracellular Aβ load in neuroblastoma cells as well as in primary cultured neurons derived from Tg2576 mice. This Aβ reduction can be accounted for by decreased Thr-668-dependent APP phosphorylation and β- and γ-secretases activities. Importantly, dantrolene diminishes Aβ load, reduces Aβ-related histological lesions, and slows down learning and memory deficits in Tg2576 mice. Overall, our data document a key role of RyR in Aβ production and learning and memory performances, and delineate RyR-mediated control of Ca²⁺ homeostasis as a physiological paradigm that could be targeted for innovative therapeutic approaches.

Ca2+ dysregulation in neurons from transgenic mice expressing mutant presenilin 2

Mutations in amyloid precursor protein (APP), and presenilin-1 and presenilin-2 (PS1 and PS2) have causally been implicated in Familial Alzheimer's Disease (FAD), but the mechanistic link between the mutations and the early onset of neurodegeneration is still debated. Although no consensus has yet been reached, most data suggest that both FAD-linked PS mutants and endogenous PSs are involved in cellular Ca2+ homeostasis. We here investigated subcellular Ca2+ handling in primary neuronal cultures and acute brain slices from wild type and transgenic mice carrying the FAD-linked PS2-N141I mutation, either alone or in the presence of the APP Swedish mutation. Compared with wild type, both types of transgenic neurons show a similar reduction in endoplasmic reticulum (ER) Ca2+ content and decreased response to metabotropic agonists, albeit increased Ca2+ release induced by caffeine. In both transgenic neurons, we also observed a higher ER-mitochondria juxtaposition that favors increased mitochondrial Ca2+ uptake upon ER Ca2+ release. A model is described that integrates into a unifying hypothesis the contradictory effects on Ca2+ homeostasis of different PS mutations and points to the relevance of these findings in neurodegeneration and aging.

Early presynaptic and postsynaptic calcium signaling abnormalities mask underlying synaptic depression in presymptomatic Alzheimer's disease mice

Alzheimer's disease (AD)-linked presenilin (PS) mutations result in pronounced endoplasmic reticulum calcium disruptions that occur before detectable histopathology and cognitive deficits. More subtly, these early AD-linked calcium alterations also reset neurophysiological homeostasis, such that calcium-dependent presynaptic and postsynaptic signaling appear functionally normal yet are actually operating under aberrant calcium signaling systems. In these 3xTg-AD mouse brains, upregulated ryanodine receptor (RyR) activity is associated with a shift toward synaptic depression, likely through a reduction in presynaptic vesicle stores and increased postsynaptic outward currents through small-conductance calcium-activated potassium SK2 channels. The deviant RyR-calcium involvement in the 3xTg-AD mice also compensates for an intrinsic predisposition for hippocampal long-term depression (LTD) and reduced long-term potentiation (LTP). In this study, we detail the impact of disrupted RyR-mediated calcium stores on synaptic transmission properties, LTD, and calcium-activated membrane channels of hippocampal CA1 pyramidal neurons in presymptomatic 3xTg-AD mice. Using electrophysiological recordings in young 3xTg-AD and nontransgenic (NonTg) hippocampal slices, we show that increased RyR-evoked calcium release in 3xTg-AD mice "normalizes" an altered synaptic transmission system operating under a shifted homeostatic state that is not present in NonTg mice. In the process, we uncover compensatory signaling mechanisms recruited early in the disease process that counterbalance the disrupted RyR-calcium dynamics, namely increases in presynaptic spontaneous vesicle release, altered probability of vesicle release, and upregulated postsynaptic SK channel activity. Because AD is increasingly recognized as a "synaptic disease," calcium-mediated signaling alterations may serve as a proximal trigger for the synaptic degradation driving the cognitive loss in AD.

Dysregulation of neural calcium signaling in Alzheimer disease, bipolar disorder and schizophrenia

Neurons have highly developed Ca(2+) signaling systems responsible for regulating a large number of neural functions such as the control of brain rhythms, information processing and the changes in synaptic plasticity that underpin learning and memory. The tonic excitatory drive, which is activated by the ascending arousal system, is particularly important for processes such as sensory perception, cognition and consciousness. The Ca(2+) signaling pathway is a key component of this arousal system that regulates the neuronal excitability responsible for controlling the neural brain rhythms required for information processing and cognition. Dysregulation of the Ca(2+) signaling pathway responsible for many of these neuronal processes has been implicated in the development of some of the major neural diseases in man such as Alzheimer disease, bipolar disorder and schizophrenia. Various treatments, which are known to act by reducing the activity of Ca(2+) signaling, have proved successful in alleviating the symptoms of some of these neural diseases.

Dairy constituents and neurocognitive health in ageing

Age-related cognitive decline (ARCD) and dementia are of increasing concern to an ageing population. In recent years, there has been considerable research focused on effective dietary interventions that may prevent or ameliorate ARCD and dementia. While a number of studies have considered the impact that dairy products may have on physiological health, particularly with regard to the metabolic syndrome and cardiovascular health, further research is currently needed in order to establish the impact that dairy products have in the promotion of healthy brain function during ageing. The present review considers the available evidence for the positive effects of dairy products on the metabolic syndrome and glucose regulation, with consideration of the implications for neurocognitive health. A literature search of current (September 2010) meta-analyses/reviews and original research regarding dairy products and cognition was conducted through SCOPUS using the following search terms for dairy consituents: dairy, milk, cheese, yoghurt, probiotics, whey protein, alpha lactalbumin, calcium, B-12, bioactive peptides and colostrinin (CLN). These search terms for dairy products were combined with the following search terms related to cognition and health: cognition, cognitive decline, dementia, Alzheimer's disease, metabolic syndrome, diabetes, insulin resistance and glucose regulation. Concerns regarding SFA and other fatty acids found in dairy products are also reviewed in relation to different forms of dairy products. The review also considers recent evidence for positive neurocognitive effects associated with bioactive peptides, CLN and proline-rich polypeptides, α-lactalbumin, vitamin B12, calcium and probiotics. Future directions for the extraction and purification of beneficial constituents are also discussed. It is concluded that low-fat dairy products, when consumed regularly as part of a balanced diet, may have a number of beneficial outcomes for neurocognitive health during ageing.

Endoplasmic reticulum enrollment in Alzheimer's disease

Alzheimer's disease (AD) poses a huge challenge for society and health care worldwide as molecular pathogenesis of the disease is poorly understood and curative treatment does not exist. The mechanisms leading to accelerated neuronal cell death in AD are still largely unknown, but accumulation of misfolded disease-specific proteins has been identified as potentially involved. In the present review, we describe the essential role of endoplasmic reticulum (ER) in AD. Despite the function that mitochondria may play as the central major player in the apoptotic process, accumulating evidence highlights ER as a critical organelle in AD. Stress that impairs ER physiology leads to accumulation of unfolded or misfolded proteins, such as amyloid β (Aβ) peptide, the major component of amyloid plaques. In an attempt to ameliorate the accumulation of unfolded proteins, ER stress triggers a protective cellular mechanism, which includes the unfolded protein response (UPR). However, when activation of the UPR is severe or prolonged enough, the final cellular outcome is pathologic apoptotic cell death. Distinct pathways can be activated in this process, involving stress sensors such as the JNK pathway or ER chaperones such as Bip/GRP94, stress modulators such as Bcl-2 family proteins, or even stress effectors such as caspase-12. Here, we detail the involvement of the ER and associated stress pathways in AD and discuss potential therapeutic strategies targeting ER stress.

Consequences of inhibiting amyloid precursor protein processing enzymes on synaptic function and plasticity

Alzheimer's disease (AD) is a neurodegenerative disease, one of whose major pathological hallmarks is the accumulation of amyloid plaques comprised of aggregated β-amyloid (Aβ) peptides. It is now recognized that soluble Aβ oligomers may lead to synaptic dysfunctions early in AD pathology preceding plaque deposition. Aβ is produced by a sequential cleavage of amyloid precursor protein (APP) by the activity of β- and γ-secretases, which have been identified as major candidate therapeutic targets of AD. This paper focuses on how Aβ alters synaptic function and the functional consequences of inhibiting the activity of the two secretases responsible for Aβ generation. Abnormalities in synaptic function resulting from the absence or inhibition of the Aβ-producing enzymes suggest that Aβ itself may have normal physiological functions which are disrupted by abnormal accumulation of Aβ during AD pathology. This interpretation suggests that AD therapeutics targeting the β- and γ-secretases should be developed to restore normal levels of Aβ or combined with measures to circumvent the associated synaptic dysfunction(s) in order to have minimal impact on normal synaptic function.

A reversible early oxidized redox state that precedes macromolecular ROS damage in aging nontransgenic and 3xTg-AD mouse neurons

The brain depends on redox electrons from nicotinamide adenine dinucleotide (reduced form; NADH) to produce ATP and oxyradicals (reactive oxygen species [ROS]). Because ROS damage and mitochondrial dysregulation are prominent in aging and Alzheimer's disease (AD) and their relationship to the redox state is unclear, we wanted to know whether an oxidative redox shift precedes these markers and leads to macromolecular damage in a mouse model of AD. We used the 3xTg-AD mouse model, which displays cognitive deficits beginning at 4 months. Hippocampal/cortical neurons were isolated across the age span and cultured in common nutrients to control for possible hormonal and vascular differences. We found an increase of NAD(P)H levels and redox state in nontransgenic (non-Tg) neurons until middle age, followed by a decline in old age. The 3xTg-AD neurons maintained much lower resting NAD(P)H and redox states after 4 months, but the NADH regenerating capacity continuously declined with age beginning at 2 months. These redox characteristics were partially reversible with nicotinamide, a biosynthetic precursor of NAD+. Nicotinamide also protected against glutamate excitotoxicity. Compared with non-Tg neurons, 3xTg-AD neurons had more mitochondria/neuron and lower glutathione (GSH) levels that preceded age-related increases in ROS levels. These GSH deficits were again reversible with nicotinamide in 3xTg-AD neurons. Surprisingly, low macromolecular ROS damage was only elevated after 4 months in the 3xTg-AD neurons if antioxidants were removed. The present data suggest that a more oxidized redox state and a lower antioxidant GSH defense can be dissociated from neuronal ROS damage, changes that precede the onset of cognitive deficits in the 3xTg-AD model.

Mitochondrial- and endoplasmic reticulum-associated oxidative stress in Alzheimer's disease: from pathogenesis to biomarkers

Alzheimer's disease (AD) is the most common cause of dementia in the elderly, affecting several million of people worldwide. Pathological changes in the AD brain include the presence of amyloid plaques, neurofibrillary tangles, loss of neurons and synapses, and oxidative damage. These changes strongly associate with mitochondrial dysfunction and stress of the endoplasmic reticulum (ER). Mitochondrial dysfunction is intimately linked to the production of reactive oxygen species (ROS) and mitochondrial-driven apoptosis, which appear to be aggravated in the brain of AD patients. Concomitantly, mitochondria are closely associated with ER, and the deleterious crosstalk between both organelles has been shown to be involved in neuronal degeneration in AD. Stimuli that enhance expression of normal and/or folding-defective proteins activate an adaptive unfolded protein response (UPR) that, if unresolved, can cause apoptotic cell death. ER stress also induces the generation of ROS that, together with mitochondrial ROS and decreased activity of several antioxidant defenses, promotes chronic oxidative stress. In this paper we discuss the critical role of mitochondrial and ER dysfunction in oxidative injury in AD cellular and animal models, as well as in biological fluids from AD patients. Progress in developing peripheral and cerebrospinal fluid biomarkers related to oxidative stress will also be summarized.

Toxic role of K+ channel oxidation in mammalian brain

Potassium (K(+)) channels are essential to neuronal signaling and survival. Here we show that these proteins are targets of reactive oxygen species in mammalian brain and that their oxidation contributes to neuropathy. Thus, the KCNB1 (Kv2.1) channel, which is abundantly expressed in cortex and hippocampus, formed oligomers upon exposure to oxidizing agents. These oligomers were ∼10-fold more abundant in the brain of old than young mice. Oxidant-induced oligomerization of wild-type KCNB1 enhanced apoptosis in neuronal cells subject to oxidative insults. Consequently, a KCNB1 variant resistant to oxidation, obtained by mutating a conserved cysteine to alanine, (C73A), was neuroprotective. The fact that oxidation of KCNB1 is toxic, argues that this mechanism may contribute to neuropathy in conditions characterized by high levels of oxidative stress, such as Alzheimer's disease (AD). Accordingly, oxidation of KCNB1 channels was exacerbated in the brain of a triple transgenic mouse model of AD (3xTg-AD). The C73A variant protected neuronal cells from apoptosis induced by incubation with β-amyloid peptide (Aβ(1-42)). In an invertebrate model (Caenorhabditis elegans) that mimics aspects of AD, a C73A-KCNB1 homolog (C113S-KVS-1) protected specific neurons from apoptotic death induced by ectopic expression of human Aβ(1-42). Together, these data underscore a novel mechanism of toxicity in neurodegenerative disease.

Lack of evidence for presenilins as endoplasmic reticulum Ca2+ leak channels

Familial Alzheimer disease (FAD) is linked to mutations in the presenilin (PS) homologs. FAD mutant PS expression has several cellular consequences, including exaggerated intracellular Ca(2+) ([Ca(2+)](i)) signaling due to enhanced agonist sensitivity and increased magnitude of [Ca(2+)](i) signals. The mechanisms underlying these phenomena remain controversial. It has been proposed that PSs are constitutively active, passive endoplasmic reticulum (ER) Ca(2+) leak channels and that FAD PS mutations disrupt this function resulting in ER store overfilling that increases the driving force for release upon ER Ca(2+) release channel opening. To investigate this hypothesis, we employed multiple Ca(2+) imaging protocols and indicators to directly measure ER Ca(2+) dynamics in several cell systems. However, we did not observe consistent evidence that PSs act as ER Ca(2+) leak channels. Nevertheless, we confirmed observations made using indirect measurements employed in previous reports that proposed this hypothesis. Specifically, cells lacking PS or expressing a FAD-linked PS mutation displayed increased area under the ionomycin-induced [Ca(2+)](i) versus time curve (AI) compared with cells expressing WT PS. However, an ER-targeted Ca(2+) indicator revealed that this did not reflect overloaded ER stores. Monensin pretreatment selectively attenuated the AI in cells lacking PS or expressing a FAD PS allele. These findings contradict the hypothesis that PSs form ER Ca(2+) leak channels and highlight the need to use ER-targeted Ca(2+) indicators when studying ER Ca(2+) dynamics.

Calcium channel blocking as a therapeutic strategy for Alzheimer's disease: the case for isradipine

Alzheimer's disease is the most devastating neurodegenerative disorder in the elderly, yet treatment options are severely limited. The drug development effort to modify Alzheimer's disease pathology by intervention at beta amyloid production sites has been largely ineffective or inconclusive. The greatest challenge has been to identify and define downstream mechanisms reliably predictive of clinical symptoms. Beta amyloid accumulation leads to dysregulation of intracellular calcium by plasma membrane L-type calcium channels located on neuronal somatodendrites and axons in the hippocampus and cortex. Paradoxically, L-type calcium channel subtype Ca(v)1.2 also promotes synaptic plasticity and spatial memory. Increased intracellular calcium modulates amyloid precursor protein processing and affects multiple downstream pathways including increased hyperphosphorylated tau and suppression of autophagy. Isradipine is a Federal Drug Administration-approved dihydropyridine calcium channel blocker that binds selectively to Ca(v)1.2 in the hippocampus. Our studies have shown that isradipine in vitro attenuates beta amyloid oligomer toxicity by suppressing calcium influx into cytoplasm and by suppressing Ca(v)1.2 expression. We have previously shown that administration of isradipine to triple transgenic animal model for Alzheimer's disease was well-tolerated. Our results further suggest that isradipine became bioavailable, lowered tau burden, and improved autophagy function in the brain. A better understanding of brain pharmacokinetics of calcium channel blockers will be critical for designing new experiments with appropriate drug doses in any future clinical trials for Alzheimer's disease. This review highlights the importance of Ca(v)1.2 channel overexpression, the accumulation of hyperphosphorylated tau and suppression of autophagy in Alzheimer's disease and modulation of this pathway by isradipine.

13-Desmethyl spirolide-C is neuroprotective and reduces intracellular Aβ and hyperphosphorylated tau in vitro

Spirolides are marine compounds of the cyclic imine group. Although the mechanism of action is not fully elucidated yet, cholinergic (muscarinic and nicotinic) receptors have been proposed as the main targets of these toxins. In this study we examined the effect of 13-desmethyl spirolide-C (SPX) on amyloid-beta (Aβ) accumulation and tau hyperphosphorylation in a neuronal model from triple transgenic mice (3xTg) for Alzheimer disease (AD). In vitro treatment of 3xTg cortical neurons with SPX reduced intracellular Aβ accumulation and the levels of phosphorylated tau. SPX treatment did not affect the steady-state levels of neither the M1 and M2 muscarinic nor the α7 nicotinic acetylcholine receptors (AChRs), while it decreased the amplitude of acetylcholine-evoked responses and increased ACh (acetylcholine) levels in 3xTg neurons. Additionally, SPX treatment decreased the levels of two protein kinases involved in tau phosphorylation, glycogen synthase kinase 3β (GSK-3β) and extracellular-regulated kinase (ERK). Also SPX abolished the glutamate-induced neurotoxicity in both control and 3xTg neurons. The results presented here constitute the first report indicating that exposure of 3xTg neurons to nontoxic concentrations of SPX produces a simultaneous reduction in the main pathological characteristics of AD. In spite of the few reports analyzing the mode of action of the toxin we suggest that SPX could ameliorate AD pathology increasing the intracellular ACh levels and simultaneously diminishing the levels of kinases involved in tau phosphorylation.

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.

Early calcium dysregulation in Alzheimer's disease: setting the stage for synaptic dysfunction

Alzheimer's disease (AD) is an irreversible and progressive neurodegenerative disorder with no known cure or clear understanding of the mechanisms involved in the disease process. Amyloid plaques, neurofibrillary tangles and neuronal loss, though characteristic of AD, are late stage markers whose impact on the most devastating aspect of AD, namely memory loss and cognitive deficits, are still unclear. Recent studies demonstrate that structural and functional breakdown of synapses may be the underlying factor in AD-linked cognitive decline. One common element that presents with several features of AD is disrupted neuronal calcium signaling. Increased intracellular calcium levels are functionally linked to presenilin mutations, ApoE4 expression, amyloid plaques, tau tangles and synaptic dysfunction. In this review, we discuss the role of AD-linked calcium signaling alterations in neurons and how this may be linked to synaptic dysfunctions at both early and late stages of the disease.

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

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

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.

Altered ryanodine receptor expression in mild cognitive impairment and Alzheimer's disease

Intracellular Ca(2+) dysregulation is an underlying component of Alzheimer's disease (AD) pathophysiology, and recent evidence implicates the ryanodine receptor (RyR) in the disease pathway. Three genes code for different RyR isoforms and each gene transcript gives rise to several alternatively spliced messenger RNAs (mRNAs). These variants confer distinct functionality to the RyR channel, such as altering Ca(2+) release properties or subcellular localization. Changes in RyR isoform expression and alternative splicing have not been examined for potential roles in AD pathogenesis. Here, we compare mRNA levels of the RyR2 and RyR3 isoforms as well as specific alternatively spliced variants across vulnerable brain regions from postmortem samples of individuals with no cognitive impairment (NCI), mild cognitive impairment (MCI), and AD. We find an increase in RyR2 transcripts in MCI brains compared with no cognitive impairment. In addition, there is a reduction in a RyR2 splice variant, associated with an antiapoptotic function, in MCI and AD brains. These alterations in RyR expression at early disease stages may reflect the onset of pathologic mechanisms leading to later neurodegeneration.

Enhanced ROS generation mediated by Alzheimer's disease presenilin regulation of InsP3R Ca2+ signaling

Familial Alzheimer's disease (FAD) is caused by mutations in amyloid precursor protein and presenilins (PS1, PS2). Many FAD-linked PS mutations affect intracellular calcium (Ca(2+)) homeostasis by proximal mechanisms independent of amyloid production by dramatically enhancing gating of the inositol trisphosphate receptor (InsP(3)R) intracellular Ca(2+) release channel by a gain-of-function effect that mirrors genetics of FAD and is independent of secretase activity. Electrophysiological recordings of InsP(3)R in FAD patient B cells, cortical neurons of asymptomatic PS1-AD mice, and other cells revealed they have higher occupancy in a high open probability burst mode, resulting in enhanced Ca(2+) signaling. Exaggerated Ca(2+) signaling through this mechanism results in enhanced generation of reactive oxygen species, believed to be an important component in AD pathogenesis. Exaggerated Ca(2+) signaling through InsP(3)R-PS interaction is a disease specific and robust proximal mechanism in AD that may contribute to the pathology of AD by enhanced generation of reactive oxygen species.

The ER stress factor XBP1s prevents amyloid-beta neurotoxicity

Alzheimer's disease (AD) is an incurable neurodegenerative disorder clinically characterized by progressive cognitive impairment. A prominent pathologic hallmark in the AD brain is the abnormal accumulation of the amyloid-β 1-42 peptide (Aβ), but the exact pathways mediating Aβ neurotoxicity remain enigmatic. Endoplasmic reticulum (ER) stress is induced during AD, and has been indirectly implicated as a mediator of Aβ neurotoxicity. We report here that Aβ activates the ER stress response factor X-box binding protein 1 (XBP1) in transgenic flies and in mammalian cultured neurons, yielding its active form, the transcription factor XBP1s. XBP1s shows neuroprotective activity in two different AD models, flies expressing Aβ and mammalian cultured neurons treated with Aβ oligomers. Trying to identify the mechanisms mediating XBP1s neuroprotection, we found that in PC12 cells treated with Aβ oligomers, XBP1s prevents the accumulation of free calcium (Ca(2+)) in the cytosol. This protective activity can be mediated by the downregulation of a specific isoform of the ryanodine Ca(2+) channel, RyR3. In support of this observation, a mutation in the only ryanodine receptor (RyR) in flies also suppresses Aβ neurotoxicity, indicating the conserved mechanisms between the two AD models. These results underscore the functional relevance of XBP1s in Aβ toxicity, and uncover the potential of XBP1 and RyR as targets for AD therapeutics.

Alzheimer's disease-associated ubiquilin-1 regulates presenilin-1 accumulation and aggresome formation

The Alzheimer's disease (AD)-associated ubiquilin-1 regulates proteasomal degradation of proteins, including presenilin (PS). PS-dependent γ-secretase generates β-amyloid (Aβ) peptides, which excessively accumulate in AD brain. Here, we have characterized the effects of naturally occurring ubiquilin-1 transcript variants (TVs) on the levels and subcellular localization of PS1 and other γ-secretase complex components and subsequent γ-secretase function in human embryonic kidney 293, human neuroblastoma SH-SY5Y and mouse primary cortical cells. Full-length ubiquilin-1 TV1 and TV3 that lacks the proteasome-interaction domain increased full-length PS1 levels as well as induced accumulation of high-molecular-weight PS1 and aggresome formation. Accumulated PS1 colocalized with TV1 or TV3 in the aggresomes. Electron microscopy indicated that aggresomes containing TV1 or TV3 were targeted to autophagosomes. TV1- and TV3-expressing cells did not accumulate other unrelated proteasome substrates, suggesting that the increase in PS1 levels was not because of a general impairment of the ubiquitin-proteasome system. Furthermore, PS1 accumulation and aggresome formation coincided with alterations in Aβ levels, particularly in cells overexpressing TV3. These effects were not related to altered γ-secretase activity or PS1 binding to TV3. Collectively, our results indicate that specific ubiquilin-1 TVs can cause PS1 accumulation and aggresome formation, which may impact AD pathogenesis or susceptibility.

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.

Presenilin mouse and zebrafish models for dementia: focus on neurogenesis

Autosomal dominant mutations in the presenilin gene PSEN cause familial Alzheimer's disease (AD), a neurological disorder pathologically characterized by intraneuronal accumulation and extracellular deposition of amyloid-β in plaques and intraneuronal, hyperphosphorylated tau aggregation in neurofibrillary tangles. Presenilins (PS/PSENs) are part of the proteolytic γ-secretase complex, which cleaves substrate proteins within the membrane. Cleavage of the amyloid precursor protein (APP) by γ-secretase releases amyloid-β peptides. Besides its role in the processing of APP and other transmembrane proteins, presenilin plays an important role in neural progenitor cell maintenance and neurogenesis. In this review, we discuss the role of presenilin in relation to neurogenesis and neurodegeneration and review the currently available presenilin animal models. In addition to established mouse models, zebrafish are emerging as an attractive vertebrate model organism to study the role of presenilin during the development of the nervous system and in neurodegenerative disorders involving presenilin. Zebrafish is a suitable model organism for large-scale drug screening, making this a valuable model to identify novel therapeutic targets for AD.

Network excitability dysfunction in Alzheimer's disease: insights from in vitro and in vivo models

Recent reports have drawn attention to dysfunctions of intrinsic neuronal excitability and network activity in Alzheimer disease (AD). Here we review the possible causes of these basic dysfunctions and implications for AD, based on in vitro and in vivo findings. We then review the current therapeutic approaches particularly linked to the issue of neuronal excitability in AD.

CONCLUSION

AD is a complex, neurodegenerative disorder. Hippocampal synaptic dysfunction is an early feature of the degenerative process that is clearly linked to memory impairment, the first and major symptom of AD. A growing body of evidence points toward a dysfunction of neuronal networks. Intrinsic neuronal excitability, mainly through profound dysregulation of calcium homeostasis, appears to be largely affected. Consequently, neuronal communication is disturbed. Such cellular defects might underlie cognitive manifestations like fluctuations in cognitive impairment and might also explain several observations obtained with EEG, MEG, MRI, or PET studies, leading to the concept of a disconnection syndrome in AD.

NMDA-mediated Ca(2+) influx drives aberrant ryanodine receptor activation in dendrites of young Alzheimer's disease mice

Deficits in synaptic function, particularly through NMDA receptors (NMDARs), are linked to late-stage cognitive impairments in Alzheimer's disease (AD). At earlier disease stages, however, there is evidence for altered endoplasmic reticulum (ER) calcium signaling in human cases and in neurons from AD mouse models. Despite the fundamental importance of calcium to synaptic function, neither the extent of ER calcium dysregulation in dendrites nor its interaction with synaptic function in AD pathophysiology is known. Identifying the mechanisms underlying early synaptic calcium dysregulation in AD pathogenesis is likely a key component to understanding, and thereby preventing, the synapse loss and downstream cognitive impairments. Using two-photon calcium imaging, flash photolysis of caged glutamate, and patch-clamp electrophysiology in cortical brain slices, we examined interactions between synaptically and ER-evoked calcium release at glutamatergic synapses in young AD transgenic mice. We found increased ryanodine receptor-evoked calcium signals within dendritic spine heads, dendritic processes, and the soma of pyramidal neurons from 3xTg-AD and TAS/TPM AD mice relative to NonTg controls. In addition, synaptically evoked postsynaptic calcium responses were larger in the AD strains, as were calcium signals generated from NMDAR activation. However, calcium responses triggered by back-propagating action potentials were not different. Concurrent activation of ryanodine receptors (RyRs) with either synaptic or NMDAR stimulation generated a supra-additive calcium response in the AD strains, suggesting an aberrant calcium-induced calcium release (CICR) effect within spines and dendrites. We propose that presenilin-linked disruptions in RyR signaling and subsequent CICR via NMDAR-mediated calcium influx alters synaptic function and serves as an early pathogenic factor in AD.

A mitocentric view of Alzheimer's disease suggests multi-faceted treatments

Alzheimer's disease (AD) is defined by senile plaques made of amyloid-beta peptide (Abeta), neurofibrillary tangles made of hyperphosphorylated tau proteins, and memory deficits. Thus, the events initiating the cascade leading to these end points may be more effective therapeutic targets than treating each facet individually. In the small percentage of cases of AD that are genetic (or animal models that reflect this form of AD), the factor initiating AD is clear (e.g., genetic mutations lead to high Abeta1-42 or hyperphosphorylated tau proteins). In the vast majority of AD cases, the cause is unknown. Substantial evidence now suggests that abnormalities in glucose metabolism/mitochondrial function/oxidative stress (GMO) are an invariant feature of AD and occur at an early stage of the disease process in both genetic and non-genetic forms of AD. Indeed, decreases in brain glucose utilization are diagnostic for AD. Changes in calcium homeostasis also precede clinical manifestations of AD. Abnormal GMO can lead to plaques, tangles, and the calcium abnormalities that accompany AD. Abnormalities in GMO diminish the ability of the brain to adapt. Therapies targeting mitochondria may ameliorate abnormalities in plaques, tangles, calcium homeostasis, and cognition that comprise AD.

Using a genetically targeted sensor to investigate the role of presenilin-1 in ER Ca2+ levels and dynamics

The ER plays a fundamental role in storing cellular Ca(2+), generating Ca(2+) signals, and modulating Ca(2+) in both the cytosol and mitochondria. Genetically encoded Ca(2+) sensors can be explicitly targeted to the ER to directly define Ca(2+) levels and monitor fluxes of Ca(2+) within this organelle. In this study we use an ER-targeted Ca(2+) sensor to define both the level and dynamics of ER Ca(2+) in cells expressing mutant presenilin proteins. Growing evidence suggests the enigmatic presenilin-1 plays a role in regulating ER Ca(2+). Presenilin-1 was initially identified in a screen for genetic causes of inherited familial Alzheimer's disease (fAD). The connection between presenilin-1, calcium regulation, and Alzheimer's disease may provide the key to understanding the long-observed, but poorly understood, link between Alzheimer's disease and Ca(2+) dysregulation. In this study we examined seven fAD-causing mutations in presenilin-1 to define how they influence ER Ca(2+) levels and dynamics. We observed that some, but not all, mutations in PS1 decrease the level of Ca(2+) within the ER and this difference depends on the enzymatic activity of PS1. Two mutations tested altered the kinetics of Ca(2+) release from the ER upon ATP stimulation, resulting in faster spiking. Combined, these results indicate that mutations in PS1 can alter the balance of Ca(2+) in cells and have the potential to influence the nature of Ca(2+) signals.

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.

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.