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

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.

Neuregulin-1 prevents amyloid β-induced impairment of long-term potentiation in hippocampal slices via ErbB4

Neuregulin-1 (NRG1) participates in numerous neurodevelopmental processes and plasticity of the brain. Despite this, little is known about its role in Alzheimer's disease (AD). Amyloid β (Aβ) peptide is generally believed to play a critical role in the pathogenesis of AD. The present study examined the effect of synthetic Aβ₁₋₄₂ peptides on long-term potentiation (LTP) in the CA1 region of mice hippocampal slices, a cellular model of learning and memory. We found that application of a test dose of Aβ₁₋₄₂ (200 nM) significantly inhibited the development of LTP without affecting basal synaptic transmission. Pretreatment with NRG1 effectively prevented Aβ₁₋₄₂-induced impairment of LTP, an effect that was dose-dependent. This LTP-restoring action of NRG1 was almost completely abolished by blocking ErbB4, a key NRG1 receptor, suggesting that NRG1 acts through ErbB4 to exert its protective action on LTP. The present study thus provides the first demonstration that NRG1/ErbB4 protects against Aβ-induced hippocampal LTP impairment, suggesting that NRG1 may be a promising candidate for the treatment of early-stage AD.

AMP-activated protein kinase: a potential player in Alzheimer's disease

AMP-activated protein kinase (AMPK) stimulates energy production via glucose and lipid metabolism, whereas it inhibits energy consuming functions, such as protein and cholesterol synthesis. Increased cytoplasmic AMP and Ca(2+) levels are the major activators of neuronal AMPK signaling. Interestingly, Alzheimer's disease (AD) is associated with several abnormalities in neuronal energy metabolism, for example, decline in glucose uptake, mitochondrial dysfunctions and defects in cholesterol metabolism, and in addition, with problems in maintaining Ca(2+) homeostasis. Epidemiological studies have also revealed that many metabolic and cardiovascular diseases are risk factors for cognitive impairment and sporadic AD. Emerging studies indicate that AMPK signaling can regulate tau protein phosphorylation and amyloidogenesis, the major hallmarks of AD. AMPK is also a potent activator of autophagic degradation which seems to be suppressed in AD. All these observations imply that AMPK is involved in the pathogenesis of AD. However, the responses of AMPK activation are dependent on stimulation and the extent of activating stress. Evidently, AMPK signaling can repress and delay the appearance of AD pathology but later on, with increasing neuronal stress, it can trigger detrimental effects that augment AD pathogenesis. We will outline the potential role of AMPK function in respect to various aspects affecting AD pathogenesis.

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.

Age-related changes in gene expression are accelerated in Alzheimer's disease

In the normal brain, age is associated with changes in gene expression. Age is also a prominent risk factor for Alzheimer's disease (AD), where clinical features are similar to age-related decreases in cognitive performance. We hypothesized that some age-related changes in gene expression are accelerated in AD patients. To study this, we selected 10 candidate genes earlier shown by microarray analysis to be differentially expressed in AD (Emilsson et al., [2006] Neurobiol Dis 21:618-625). Using real-time PCR analysis and a control based statistical model, we investigated age-related changes in mRNA levels in a large collection of human brain postmortem tissues from AD patients and controls. Our results demonstrate that the age-related changes in gene expression are manifested earlier in AD. Furthermore, five of the genes (ITPKB, RGS4, RAB3A, STMN1, SYNGR3) have in common an involvement in neuronal calcium dependent signaling, a cellular process previously related to both AD and aging. These observations suggest that coordinated transcriptional changes associated with ageing and calcium homeostasis in the human brain are accelerated in patients with AD. Our results point to the possibility that the activity of these genes can be used in the future as a palette of biomarkers for predicting disease risk in young individuals.

Hippocampal hyperactivation in presymptomatic familial Alzheimer's disease

OBJECTIVE

The examination of individuals who carry fully penetrant genetic alterations that result in familial Alzheimer's disease (FAD) provides a unique model for studying the early presymptomatic disease stages. In AD, deficits in episodic and associative memory have been linked to structural and functional changes within the hippocampal system. This study used functional MRI (fMRI) to examine hippocampal function in a group of healthy, young, cognitively-intact presymptomatic individuals (average age 33.7 years) who carry the E280A presenilin-1 (PS1) genetic mutation for FAD. These PS1 subjects will go on to develop the first symptoms of the disease around the age of 45 years. Our objective was to examine hippocampal function years before the onset of clinical symptoms.

METHODS

Twenty carriers of the Alzheimer's-associated E280A PS1 mutation and 19 PS1-negative control subjects participated. Both groups were matched for age, sex, education level, and neuropsychological test performance. All participants performed a face-name associative encoding task while in a Phillips 1.5T fMRI scanner. Analysis focused on the hippocampal system.

RESULTS

Despite identical behavioral performance, presymptomatic PS1 mutation carriers exhibited increased activation of the right anterior hippocampus during encoding of novel face-name associations compared to matched controls.

INTERPRETATION

Our results demonstrate that functional changes within the hippocampal memory system occur years before cognitive decline in FAD. These presymptomatic changes in hippocampal physiology in FAD suggest that hippocampal fMRI patterns during associative encoding may also provide a preclinical biomarker in sporadic AD.

Natural products as a source of Alzheimer's drug leads

This review focuses on recent developments in the use of natural products as therapeutics for Alzheimer's disease. The compounds span a diverse array of structural classes and are organized according to their mechanism of action, with the focus primarily on the major hypotheses. Overall, the review discusses more than 180 compounds and summarizes 400 references.

Roles of β-adrenergic receptors in Alzheimer's disease: implications for novel therapeutics

Alzheimer's disease (AD), the most common cause of age-related dementia, is a progressive neurodegenerative disorder with an enormous unmet medical need. In recent years, several unexpected longitudinal and cross-sectional epidemiological studies reveal that beta-blockers treatment reduces the prevalence of AD in patients suffering from hypertension. Now, a newly population-based study of individuals with incident AD demonstrates that beta-blockers are also associated with delay of functional decline. Furthermore, accumulated convincing evidences from cell culture experiments and animal studies have also suggested that β-adrenergic receptors (β-ARs) may involve in the AD pathogenesis through effects on amyloid-β (Aβ) production or inflammation. This review explores clinical and experimental studies that might help to explain the roles of β-ARs in the AD pathogenesis and the potential underlying mechanisms and whether treatment with β-ARs antagonists provides a new therapeutic option for AD.

Hsp90 regulates tau pathology through co-chaperone complexes in Alzheimer's disease

Alzheimer's disease is a tauopathy which involves the deposition of microtubule-associated tau proteins into neurofibrillary tangles. Post-translational modifications, in particular site-specific phosphorylations, affect the conformation of tau protein which is an intrinsically disordered protein. These structural changes significantly increase the affinity of tau protein for certain molecular chaperones. Hsp90 is a major cellular chaperone which assembles large complexes with a variety of co-chaperones. The main function of Hsp90 complexes is to maintain protein quality control and assist in protein degradation via proteasomal and autophagic-lysosomal pathways. Tau protein is a client protein for these Hsp90 complexes. If the tau protein is in an abnormal or modified form, then it can trigger the recruitment of CHIP protein, a co-chaperone with E3 activity, to the complex which induces the ubiquitination of tau protein and activates its downstream degradation processes. Large immunophilins, FKBP51 and FKBP52 are also co-chaperones of Hsp90-tau complexes. These proteins contain peptidylprolyl cis/trans isomerase activity which catalyzes phosphorylation-dependent rotation in pSer/Thr-Pro peptide bond. The proline switch in the tau conformation triggers dephosphorylation of Ser/Thr residues phosphorylated, e.g. by two well-known tau kinases Cdk5 and GSK-3β. Binding of PP5 protein phosphatase to Hsp90 complex, can also dephosphorylate tau protein. Subsequently, dephosphorylated tau protein can be shuttled back to the microtubules. It seems that high-affinity binding of abnormal tau to Hsp90 complexes may have some counteracting effects on the aggregation process, since Hsp90 inhibitors can ameliorate the aggregation process in several neurodegenerative diseases. We will review the role of Hsp90 chaperone network in the regulation of tau biology and pathology in Alzheimer's disease.

Modulation of mitochondrial calcium as a pharmacological target for Alzheimer's disease

Perturbed neuronal calcium homeostasis is a prominent feature in Alzheimer's disease (AD). Mitochondria accumulate calcium ions (Ca(2+)) for cellular bioenergetic metabolism and suppression of mitochondrial motility within the cell. Excessive Ca(2+) uptake into mitochondria often leads to mitochondrial membrane permeabilization and induction of apoptosis. Ca(2+) is an interesting second messenger which can initiate both cellular life and death pathways in mitochondria. This review critically discusses the potential of manipulating mitochondrial Ca(2+) concentrations as a novel therapeutic opportunity for treating AD. This review also highlights the neuroprotective role of a number of currently available agents that modulate different mitochondrial Ca(2+) transport pathways. It is reasoned that these mitochondrial Ca(2+) modulators are most effective in combination with agents that increase the Ca(2+) buffering capacity of mitochondria. Modulation of mitochondrial Ca(2+) handling is a potential pharmacological target for future development of AD treatments.

Phagocytosis of dying cells: influence of smoking and static magnetic fields

It is becoming evident that failure in the removal of dying cells causes and/or promotes the onset of chronic diseases. Impairment of phagocytosis of apoptotic cells can be due not only to genetic or molecular malfunctioning but also to external/environmental factors. Two of these environmental factors have been recently reported to down regulate the clearance of apoptotic cells: cigarette smoke and static magnetic fields. Cigarette smoke contains highly reactive carbonyls that modify proteins which directly/indirectly affects cellular function. Human macrophages interacting with carbonyl or cigarette smoke modified extracellular matrix (ECM) proteins dramatically down regulated their ability to phagocytose apoptotic neutrophils. It was postulated that changes in the ECM environment as a result of cigarette smoke affect the ability of macrophages to remove apoptotic cells. This decreased phagocytic activity was as a result of sequestration of receptors involved in the uptake of apoptotic cells towards that of recognition of carbonyl adducts on the modified ECM proteins leading to increased macrophage adhesion. Downregulation of the phagocytosis of apoptotic cells was also described when performed in presence of static magnetic fields (SMFs) of moderate intensity. SMFs have been reported to perturb distribution of membrane proteins and glycoproteins, receptors, cytoskeleton and trans-membrane fluxes of different ions, especially calcium [Ca(2+)]i, that in turn, interfere with many different physiological activities, including phagocytosis. The effects of cigarette smoke and SMF on the phagocytosis of dying cells will be here discussed.

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.

Effects of long-term memantine on memory and neuropathology in Ts65Dn mice, a model for Down syndrome

Memantine is a partial NMDA receptor antagonist that has been shown to improve learning and memory in several animal models, and is approved for the treatment of Alzheimer's disease (AD). Chronic treatments using memantine in animal models of Alzheimer's disease show disease-modifying effects and suggest a potential neuroprotective function. The present study assessed the effects of both short- and long-term memantine treatment in a mouse model of Down syndrome (DS), the Ts65Dn mouse. The Ts65Dn mouse contains a partial trisomy of murine chromosome 16, and exhibits hippocampal-dependent memory deficits, as well as progressive degeneration of basal forebrain cholinergic neurons (BCFNs). Ts65Dn mice were treated with memantine for a period of 6 months, beginning at 4 months of age. At the end of treatment the mice underwent memory testing using novel object recognition and water radial arm maze tasks, and then histologically analyzed for markers of neurodegeneration. Memantine treatment improved spatial and recognition memory performance in the Ts65Dn mice, though not to the level of normosomic littermate controls. Despite these memory improvements, histological analysis found no morphological signs of neuroprotection of basal forebrain cholinergic or locus coeruleus neurons in memantine-treated Ts65Dn mice. However, memantine treatment of Ts65Dn mice gave rise to elevated brain-derived neurotrophic factor expression in the hippocampus and frontal cortex, suggesting a mechanism of behavioral modification. Thus, our findings provide further evidence for memory facilitation of memantine, but suggest pharmacological rather than neuroprotective effects of memantine both after acute and chronic treatment in this mouse model.

Basic mechanisms of neurodegeneration: a critical update

Neurodegenerative diseases are characterized by progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra and intracellular accumulation of misfolded proteins, the hallmarks of many neurodegenerative proteinopathies. Major basic processes include abnormal protein dynamics due to deficiency of the ubiquitin-proteosome-autophagy system, oxidative stress and free radical formation, mitochondrial dysfunction, impaired bioenergetics, dysfunction of neurotrophins, 'neuroinflammatory' processes and (secondary) disruptions of neuronal Golgi apparatus and axonal transport. These interrelated mechanisms lead to programmed cell death is a long run over many years. Neurodegenerative disorders are classified according to known genetic mechanisms or to major components of protein deposits, but recent studies showed both overlap and intraindividual diversities between different phenotypes. Synergistic mechanisms between pathological proteins suggest common pathogenic mechanisms. Animal models and other studies have provided insight into the basic neurodegeneration and cell death programs, offering new ways for future prevention/treatment strategies.

Mitochondrial medicine for neurodegenerative diseases

Mitochondrial dysfunction has been reported in a wide array of neurological disorders ranging from neuromuscular to neurodegenerative diseases. Recent studies on neurodegenerative diseases have revealed that mitochondrial pathology is generally found in inherited or sporadic neurodegenerative diseases and is believed to be involved in the pathophysiological process of these diseases. Commonly seen types of mitochondrial dysfunction in neurodegenerative diseases include excessive free radical generation, lowered ATP production, mitochondrial permeability transition, mitochondrial DNA lesions, perturbed mitochondrial dynamics and apoptosis. Mitochondrial medicine as an emerging therapeutic strategy targeted to mitochondrial dysfunction in neurodegenerative diseases has been proven to be of value, though this area of research is still at in its early stage. In this article, we report on recent progress in the development of several mitochondrial therapies including antioxidants, blockade of mitochondrial permeability transition, and mitochondrial gene therapy as evidence that mitochondrial medicine has promise in the treatment of neurodegenerative diseases.