Na+/Ca2+ exchange activity is increased in Alzheimer's disease brain tissues

Exploring the Role of NCX1 and NCX3 in an In Vitro Model of Metabolism Impairment: Potential Neuroprotective Targets for Alzheimer's Disease

Alzheimer's disease (AD) is a widespread neurodegenerative disorder, affecting a large number of elderly individuals worldwide. Mitochondrial dysfunction, metabolic alterations, and oxidative stress are regarded as cooperating drivers of the progression of AD. In particular, metabolic impairment amplifies the production of reactive oxygen species (ROS), resulting in detrimental alterations to intracellular Ca²⁺ regulatory processes. The Na⁺/Ca²⁺ exchanger (NCX) proteins are key pathophysiological determinants of Ca²⁺ and Na⁺ homeostasis, operating at both the plasma membrane and mitochondria levels. Our study aimed to explore the role of NCX1 and NCX3 in retinoic acid (RA) differentiated SH-SY5Y cells treated with glyceraldehyde (GA), to induce impairment of the default glucose metabolism that typically precedes Aβ deposition or Tau protein phosphorylation in AD. By using an RNA interference-mediated approach to silence either NCX1 or NCX3 expression, we found that, in GA-treated cells, the knocking-down of NCX3 ameliorated cell viability, increased the intracellular ATP production, and reduced the oxidative damage. Remarkably, NCX3 silencing also prevented the enhancement of Aβ and pTau levels and normalized the GA-induced decrease in NCX reverse-mode activity. By contrast, the knocking-down of NCX1 was totally ineffective in preventing GA-induced cytotoxicity except for the increase in ATP synthesis. These findings indicate that NCX3 and NCX1 may differently influence the evolution of AD pathology fostered by glucose metabolic dysfunction, thus providing a potential target for preventing AD.

Control of Ca2+ and metabolic homeostasis by the Na+/Ca2+ exchangers (NCXs) in health and disease

Spatial and temporal control of calcium (Ca²⁺) levels is essential for the background rhythms and responses of living cells to environmental stimuli. Whatever other regulators a given cellular activity may have, localized and wider scale Ca²⁺ events (sparks, transients, and waves) are hierarchical determinants of fundamental processes such as cell contraction, excitability, growth, metabolism and survival. Different cell types express specific channels, pumps and exchangers to efficiently generate and adapt Ca²⁺ patterns to cell requirements. The Na⁺/Ca²⁺ exchangers (NCXs) in particular contribute to Ca²⁺ homeostasis by buffering intracellular Ca²⁺ loads according to the electrochemical gradients of substrate ions - i.e., Ca²⁺ and sodium (Na⁺) - and under a dynamic control of redundant regulatory processes. An interesting feature of NCX emerges from the strict relationship that connects transporter activity with cell metabolism: on the one hand NCX operates under constant control of ATP-dependent regulatory processes, on the other hand the ion fluxes generated through NCX provide mechanistic support for the Na⁺-driven uptake of glutamate and Ca²⁺ influx to fuel mitochondrial respiration. Proof of concept evidence highlights therapeutic potential of preserving a timed and balanced NCX activity in a growing rate of diseases (including excitability, neurodegenerative, and proliferative disorders) because of an improved ability of stressed cells to safely maintain ion gradients and mitochondrial bioenergetics. Here, we will summarize and review recent works that have focused on the pathophysiological roles of NCXs in balancing the two-way relationship between Ca²⁺ signals and metabolism.

The Na+/Ca2+ Exchanger 3 Is Functionally Coupled With the NaV1.6 Voltage-Gated Channel and Promotes an Endoplasmic Reticulum Ca2+ Refilling in a Transgenic Model of Alzheimer's Disease

The remodelling of neuronal ionic homeostasis by altered channels and transporters is a critical feature of the Alzheimer's disease (AD) pathogenesis. Different reports converge on the concept that the Na⁺/Ca²⁺ exchanger (NCX), as one of the main regulators of Na⁺ and Ca²⁺ concentrations and signalling, could exert a neuroprotective role in AD. The activity of NCX has been found to be increased in AD brains, where it seemed to correlate with an increased neuronal survival. Moreover, the enhancement of the NCX3 currents (I_NCX) in primary neurons treated with the neurotoxic amyloid β 1-42 (Aβ_1-42) oligomers prevented the endoplasmic reticulum (ER) stress and neuronal death. The present study has been designed to investigate any possible modulation of the I_NCX, the functional interaction between NCX and the Na_V1.6 channel, and their impact on the Ca²⁺ homeostasis in a transgenic in vitro model of AD, the primary hippocampal neurons from the Tg2576 mouse, which overproduce the Aβ_1-42 peptide. Electrophysiological studies, carried in the presence of siRNA and the isoform-selective NCX inhibitor KB-R7943, showed that the activity of a specific NCX isoform, NCX3, was upregulated in its reverse, Ca²⁺ influx mode of operation in the Tg2576 neurons. The enhanced NCX activity contributed, in turn, to increase the ER Ca²⁺ content, without affecting the cytosolic Ca²⁺ concentrations of the Tg2576 neurons. Interestingly, our experiments have also uncovered a functional coupling between NCX3 and the voltage-gated Na_V1.6 channels. In particular, the increased Na_V1.6 currents appeared to be responsible for the upregulation of the reverse mode of NCX3, since both TTX and the Streptomyces griseolus antibiotic anisomycin, by reducing the Na_V1.6 currents, counteracted the increase of the I_NCX in the Tg2576 neurons. In agreement, our immunofluorescence analyses revealed that the NCX3/Na_V1.6 co-expression was increased in the Tg2576 hippocampal neurons in comparison with the WT neurons. Collectively, these findings indicate that NCX3 might intervene in the Ca²⁺ remodelling occurring in the Tg2576 primary neurons thus emerging as a molecular target with a neuroprotective potential, and provide a new outcome of the Na_V1.6 upregulation related to the modulation of the intracellular Ca²⁺ concentrations in AD neurons.

The Na+/Ca2+exchanger in Alzheimer's disease

As a pivotal player in regulating sodium (Na⁺) and calcium (Ca²⁺) homeostasis and signalling in excitable cells, the Na⁺/Ca²⁺ exchanger (NCX) is involved in many neurodegenerative disorders in which an imbalance of intracellular Ca²⁺ and/or Na⁺ concentrations occurs, including Alzheimer's disease (AD). Although NCX has been mainly implicated in neuroprotective mechanisms counteracting Ca²⁺ dysregulation, several studies highlighted its role in the neuronal responses to intracellular Na⁺ elevation occurring in several pathophysiological conditions. Since the alteration of Na⁺ and Ca²⁺ homeostasis significantly contributes to synaptic dysfunction and neuronal loss in AD, it is of crucial importance to analyze the contribution of NCX isoforms in the homeostatic responses at neuronal and synaptic levels. Some studies found that an increase of NCX activity in brains of AD patients was correlated with neuronal survival, while other research groups found that protein levels of two NCX subtypes, NCX2 and NCX3, were modulated in parietal cortex of late stage AD brains. In particular, NCX2 positive synaptic terminals were increased in AD cohort while the number of NCX3 positive terminals were reduced. In addition, NCX1, NCX2 and NCX3 isoforms were up-regulated in those synaptic terminals accumulating amyloid-beta (Aβ), the neurotoxic peptide responsible for AD neurodegeneration. More recently, the hyperfunction of a specific NCX subtype, NCX3, has been shown to delay endoplasmic reticulum stress and apoptotic neuronal death in hippocampal neurons exposed to Aβ insult. Despite some issues about the functional role of NCX in synaptic failure and neuronal loss require further studies, these findings highlight the putative neuroprotective role of NCX in AD and open new strategies to develop new druggable targets for AD therapy.

Evaluation of the effect of cafeteria diet on the kidney Na,K-ATPase activity, and oxidative stress

In this study, renal tissue, subdivided into the cortex and medulla of Wistar rats subjected to a cafeteria diet (CAF) for 24 days or to normal diet, was used to analyze whether the renal enzyme Na,K-ATPase activity was modified by CAF diet, as well as to analyze the α1 subunit of renal Na,K-ATPase expression levels. The lipid profile of the renal plasma membrane and oxidative stress were verified. In the Na,K-ATPase activity evaluation, no alteration was found, but a significant decrease of 30% in the cortex was detected in the α1 subunit expression of the enzyme. There was a 24% decrease in phospholipids in the cortex of rats submitted to CAF, a 17% increase in cholesterol levels in the cortex, and a 23% decrease in the medulla. Lipid peroxidation was significantly increased in the groups submitted to CAF, both in the cortical region, 29%, and in the medulla, 35%. Also, a reduction of 45% in the glutathione levels was observed in the cortex and medulla with CAF. CAF showed a nearly two-fold increase in glutathione peroxidase (GPX) activity in relation to the control group in the cortex and a 59% increase in the GPx activity in the medulla. In conclusion, although the diet was administered for a short period of time, important results were found, especially those related to the lipid profile and oxidative stress, which may directly affect renal function.

Effect of Sodium Selenate on Hippocampal Proteome of 3×Tg-AD Mice-Exploring the Antioxidant Dogma of Selenium against Alzheimer's Disease

Selenium (Se), an antioxidant trace element, is an important nutrient for maintaining brain functions and is reported to be involved in Alzheimer's disease (AD) pathologies. The present study has been designed to elucidate the protein changes in hippocampus of 3×Tg-AD mice after supplementing sodium selenate as an inorganic source of selenium. By using iTRAQ proteomics technology, 113 differentially expressed proteins (DEPs) are found in AD/WT mice with 37 upregulated and 76 downregulated proteins. Similarly, in selenate-treated 3×Tg-AD (ADSe/AD) mice, 115 DEPs are found with 98 upregulated and 17 downregulated proteins. The third group of mice (ADSe/WT) showed 75 DEPs with 46 upregulated and 29 downregulated proteins. Among these results, 42 proteins (40 downregulated and 2 upregulated) in the diseased group showed reverse expression when treated with selenate. These DEPs are analyzed with different bioinformatics tools and are found associated with various AD pathologies and pathways. Based on their functions, selenate-reversed proteins are classified as structural proteins, metabolic proteins, calcium regulating proteins, synaptic proteins, signaling proteins, stress related proteins, and transport proteins. Six altered AD associated proteins are successfully validated by Western blot analysis. This study shows that sodium selenate has a profound effect on the hippocampus of the triple transgenic AD mice. This might be established as an effective therapeutic agent after further investigation.

Intracellular Calcium Dysregulation: Implications for Alzheimer's Disease

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by progressive neuronal loss. AD is associated with aberrant processing of the amyloid precursor protein, which leads to the deposition of amyloid-β plaques within the brain. Together with plaques deposition, the hyperphosphorylation of the microtubules associated protein tau and the formation of intraneuronal neurofibrillary tangles are a typical neuropathological feature in AD brains. Cellular dysfunctions involving specific subcellular compartments, such as mitochondria and endoplasmic reticulum (ER), are emerging as crucial players in the pathogenesis of AD, as well as increased oxidative stress and dysregulation of calcium homeostasis. Specifically, dysregulation of intracellular calcium homeostasis has been suggested as a common proximal cause of neural dysfunction in AD. Aberrant calcium signaling has been considered a phenomenon mainly related to the dysfunction of intracellular calcium stores, which can occur in both neuronal and nonneuronal cells. This review reports the most recent findings on cellular mechanisms involved in the pathogenesis of AD, with main focus on the control of calcium homeostasis at both cytosolic and mitochondrial level.

Reciprocal disruption of neuronal signaling and Aβ production mediated by extrasynaptic NMDA receptors: a downward spiral

It is becoming increasingly clear that aberrant neuronal activity can be the cause and the result of amyloid beta production. Synaptic activation facilitates non-amyloidogenic processing of amyloid precursor protein (APP) and cell survival, primarily through synaptic NMDA receptors (NMDARs) and perhaps specifically those containing GluN2A-subunits. In contrast, extrasynaptic and GluN2B-containing NMDARs promote beta-secretase cleavage of APP into amyloid-beta (Aβ). The opposing nature of these NMDAR populations is reflected in their control over cell survival and death pathways. Subtle changes in glutamate homeostasis may shift the balance between these pathways and could play a role in Alzheimer's disease (AD). Indeed, Aβ production, regional loss of brain connectivity and neurodegeneration correlate with neuronal activity in AD patients. From another perspective, Aβ oligomers (Aβo) alter neuronal signaling through several mechanisms involving NMDARs and intracellular calcium mishandling. While Aβo affect multiple receptors, GluN2B-NMDARs have emerged as primary mediators of altered synaptic plasticity and neurotoxicity. Memantine and its successor, NitroMemantine, are efficient at blocking or reversing the deleterious actions of Aβo largely due to their selectivity for extrasynaptic NMDARs. Recently, Aβo were shown to trigger astrocytic release of glutamate to the extrasynaptic space where it activates NMDARs to promote further Aβ production and synaptic depression. Combined with the reciprocal regulation between neuronal activity and Aβ production, extrasynaptic glutamate release adds to a maladaptive model and ultimately results in synaptotoxicity and neurodegeneration of AD. Extrasynaptic NMDAR antagonists remain as a promising therapeutic avenue by interfering with this cascade.

Superoxide anion regulates the mitochondrial free Ca2+ through uncoupling proteins

Mitochondrial dysfunction, which is closely related to intracellular calcium overload and excessive free radicals, is an important cause of Alzheimer's disease (AD). However, molecular mechanisms of the mitochondrial Ca(2+) disregulation induced by oxidative stress in AD are still obscure. In an effort to gain a further understanding of this problem, we investigated the effects of superoxide anion, a primary free radical, on the expression of uncoupling proteins (UCPs) and the mitochondrial free Ca(2+) levels in the neuroblastoma SH-SY5Y cell line (neo) and stably expressed wild-type human APP(APP) and APP-Swedish mutation (APPsw) SH-SY5Y cells. It was found that UCP2 and UCP4 protein levels were upregulated in neo but downregulated in APP and APPsw cells by the superoxide anion. Our results show that the superoxide anion can regulate protein levels of UCP2 and UCP4 in SH-SY5Y cells, and the mitochondrial free Ca(2+) shifted their levels, tightly coupled with the protein levels of UCPs. When UCP2 and UCP4 were knocked down by siRNA, the result was reversed. These data suggest that the superoxide anion can regulate the mitochondrial free Ca(2+) by regulating the expression of UCPs. These observations also indicate that UCPs can be potential targets in pathotherapy prevention of AD.

Role of calcium in the pathogenesis of Alzheimer's disease and transgenic models

Alzheimer's disease (AD) is a progressive neurodegenerative disorder of the elderly that is characterized by memory loss. Neuropathologically, the AD brain is marked by an increased AP burden, hyperphosphorylated tau aggregates, synaptic loss, and inflammatory responses. Disturbances in calcium homeostasis are also one of the earliest molecular changes that occur in AD patients, alongside alterations in calcium-dependent enzymes in the post-mortem brain. The sum of these studies suggests that calcium dyshomeostasis is an integral part of the pathology, either influencing AP production, mediating its effects or both. Increasing evidence from in vitro studies demonstrates that the AP peptide could modulate a number of ion channels increasing calcium influx, including voltage-gated calcium and potassium channels, the NMDA receptor, the nicotinic receptor, as well as forming its own calcium-conducting pores. In vivo evidence has shown that A3 impairs both LTP and cognition, whereas all of these ion channels cluster at the synapse and underlie synaptic transmission and hence cognition. Here we consider the evidence that AP causes cognitive deficits through altering calcium homeostasis at the synapse, thus impairing synaptic transmission and LTP. Furthermore, this disruption appearr to occur without overt or extensive neuronal loss, as it is observed in transgenic mouse models of AD, but may contribute to the synaptic loss, which is an early event that correlates best with cognitive decline.

Enhanced learning and memory in mice lacking Na+/Ca2+ exchanger 2

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) plays a role in regulation of intracellular Ca(2+) concentration via the forward mode (Ca(2+) efflux) or the reverse mode (Ca(2+) influx). To define the physiological function of the exchanger in vivo, we generated mice deficient for NCX2, the major isoform in the brain. Mutant hippocampal neurons exhibited a significantly delayed clearance of elevated Ca(2+) following depolarization. The frequency threshold for LTP and LTD in the hippocampal CA1 region was shifted to a lowered frequency in the mutant mice, thereby favoring LTP. Behaviorally, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. These results demonstrate that NCX2 can be a temporal regulator of Ca(2+) homeostasis and as such is essential for the control of synaptic plasticity and cognition.

Evidence for a zinc/proton antiporter in rat brain

The data presented in this paper are consistent with the existence of a plasma membrane zinc/proton antiport activity in rat brain. Experiments were performed using purified plasma membrane vesicles isolated from whole rat brain. Incubating vesicles in the presence of various concentrations of 65Zn2+ resulted in a rapid accumulation of 65Zn2+. Hill plot analysis demonstrated a lack of cooperativity in zinc activation of 65Zn2+ uptake. Zinc uptake was inhibited in the presence of 1 mM Ni2+, Cd2+, or CO2+. Calcium (1 mM) was less effective at inhibiting 65Zn2+ uptake and Mg2+ and Mn2+ had no effect. The initial rate of vesicular 65Zn2+ uptake was inhibited by increasing extravesicular H+ concentration. Vesicles preloaded with 65Zn2+ could be induced to release 65Zn2+ by increasing extravesicular H+ or addition of 1 mM nonradioactive Zn2+. Hill plot analysis showed a lack of cooperativity in H+ activation of 65Zn2+ release. Based on the Hill analyses, the stoichiometry of transport may include Zn2+/Zn2+ exchange and Zn2+/H+ antiport, the latter being potentially electrogenic. Zinc/proton antiport may be an important mode of zinc uptake into neurons and contribute to the reuptake of zinc to replenish presynaptic vesicle stores after stimulation.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Decreased brain protein levels of cytochrome oxidase subunits in Alzheimer's disease and in hereditary spinocerebellar ataxia disorders: a nonspecific change?

Controversy exists as to the clinical importance, cause, and disease specificity of the cytochrome oxidase (CO) activity reduction observed in some patients with Alzheimer's disease (AD). Although it is assumed that the enzyme is present in normal amount in AD, no direct measurements of specific CO protein subunits have been conducted. We measured protein levels of CO subunits encoded by mitochondrial (COX I, COX II) and nuclear (COX IV, COX VIc) DNA in autopsied brain of patients with AD whom we previously reported had decreased cerebral cortical CO activity. To assess disease specificity, groups of patients with spinocerebellar ataxia type I and Friedreich's ataxia were also included. As compared with the controls, mean protein concentrations of all four CO subunits were significantly decreased (-19 to -47%) in temporal and parietal cortices in the AD group but were not significantly reduced (-12 to -17%) in occipital cortex. The magnitude of the reduction in protein levels of the CO subunits encoded by mitochondrial DNA (-42 to -47%) generally exceeded that encoded by nuclear DNA (-19 to -43%). In the spinocerebellar ataxia disorders, COX I and COX II levels were significantly decreased in cerebellar cortex (-22 to -32%) but were normal or close to normal in cerebral cortex, an area relatively unaffected by neurodegeneration. We conclude that protein levels of mitochondrial- and nuclear-encoded CO subunits are moderately reduced in degenerating but not in relatively spared brain areas in AD and that the decrease is not specific to this disorder. The simplest explanation for our findings is that CO is decreased in human brain disorders as a secondary event in brain areas having reduced neuronal activity or neuronal/synaptic elements consequent to the primary neurodegenerative process.

Alzheimer's-specific effects of soluble beta-amyloid on protein kinase C-alpha and -gamma degradation in human fibroblasts

Alzheimer's disease (AD) is a multifactorial disease in which beta-amyloid peptide (betaAP) plays a critical role. We report here that the soluble fraction 1-40 of betaAP differentially degrades protein kinase C-alpha and -gamma (PKCalpha and PKCgamma) isoenzymes in normal (age-matched controls, AC) and AD fibroblasts most likely through proteolytic cascades. Treatment with nanomolar concentrations of betaAP(1-40) induced a 75% decrease in PKCalpha, but not PKCgamma, immunoreactivity in AC fibroblasts. In the AD fibroblasts, a 70% reduction of the PKCgamma, but not PKCalpha, immunoreactivity was observed after betaAP treatment. Preincubation of AC or AD fibroblasts with 50 microM lactacystine, a selective proteasome inhibitor, prevented beta-AP(1-40)-mediated degradation of PKCalpha in the AC cells, and PKCgamma in the AD fibroblasts. The effects of betaAP(1-40) on PKCalpha in AC fibroblasts were prevented by inhibition of protein synthesis and reversed by PKC activation. A 3-hr treatment with 100 nM phorbol 12-myristate 13-acetate restored the PKCalpha signal in treated AC cells but it did not reverse the effects of betaAP(1-40) on PKCgamma in the AD fibroblasts. Pretreatment with the protein synthesis inhibitor, cycloheximide (CHX, 100 microM), inhibited the effects of betaAP(1-40) on PKCalpha and blocked the rescue effect of phorbol 12-myristate 13-acetate in AC fibroblasts but did not modify PKCgamma immunoreactivity in AD cells. These results suggest that betaAP(1-40) differentially affects PKC regulation in AC and AD cells via proteolytic degradation and that PKC activation exerts a protective role via de novo protein synthesis in normal but not AD cells.

Brain energy metabolizing enzymes in Alzheimer's disease: alpha-ketoglutarate dehydrogenase complex and cytochrome oxidase

PET observations of reduced cerebral glucose metabolism in AD could be explained by a defect in key energy metabolizing enzymes. In particular, levels of two enzymes, cytochrome oxidase (CO) and alpha-ketoglutarate dehydrogenase complex (alpha KGDHC) are generally assumed to be reliably reduced in postmortem brain of patients with AD. How strong is the evidence that brain CO and alpha KGDHC are reduced in AD? In our study CO activity and alpha KGDHC activity and protein subunit levels were measured in cerebral cortex of 19-29 AD patients and 29 control subjects. We found that mean CO activity in cerebral cortex was reduced by 16-26% in the AD group but with almost complete overlap between control and patient ranges. Since our publication in 1992, mean brain CO activity in AD was modestly reduced in 9 independent studies (p < 0.05 in 5). Activity of alpha KGDHC varied widely in control/AD subjects and is not useful as an enzyme marker. Cerebral cortical protein levels of E1-3 subunits, which showed much less variance, were reduced by 23-41% but with large overlap between control/patient groups. We concluded that decreased (i.e., below normal) brain CO and alpha KGDHC is a feature of some, but not all patients with AD. The possible causes and significance of the enzyme changes are discussed.

Altered oxidation and signal transduction systems in fibroblasts from Alzheimer patients

Abnormalities in calcium regulation, amyloid-beta-protein (A beta) production and oxidative metabolism have been implicated in Alzheimer's disease (AD). The use of cultured fibroblasts complement post-mortem and genetic approaches in clarifying the interaction of these processes and the underlying mechanism for the changes in AD. Definition of gene defects in particular Alzheimer families (FAD) permits elucidation of the role of those genetic abnormalities in altered signal transduction in cell lines from those families. Abnormalities in calcium regulation, ion channels, cyclic AMP, the phosphatidylinositide cascade and oxidative metabolism are well documented in fibroblasts from patients with primary genetic defects in the presenilins. Recent studies in AD fibroblasts that demonstrate abnormal secretion of A beta, a protein known to form the characteristic extracellular amyloid deposits in AD brain, further supports the use of these cells in AD research. Comparison of changes in calcium signaling, mitochondrial oxidation and A beta production in these cells suggests that changes in signal transduction including calcium may be a more consistent observation than altered A beta production in fibroblasts from some FAD families. An understanding of these abnormalities in fibroblasts may provide further insights into the pathophysiology of AD, new diagnostic measures and perhaps innovative therapeutic approaches.

Frontal lobe phosphorus metabolism and neuropsychological function in aging and in Alzheimer's disease

31P Magnetic resonance spectroscopy of the frontal lobe was performed in 17 patients with Alzheimer's disease (AD), 8 elderly controls (EC), and 17 young controls (YC). The phosphocreatine/inorganic phosphate (PCr/Pi) ratio in AD (2.32 +/- 0.26 SD) was significantly lower than in EC (2.65 +/- 0.41). In AD patients, a correlation was observed between the PCr/Pi ratio and the dementia rating scale (r = -0.50, p = 0.04). A significant positive correlation between PCr/Pi ratio and age was observed in both AD (r = 0.67, p = 0.003) and YC (r = 0.63, p = 0.006) groups, however, suggesting caution in interpretation of this ratio in AD. We did not find differences between AD, EC, or YC in any other spectroscopic measure. A significant sex difference in the phosphomonoester/phosphodiester ratio (PME/PDE) ratio was observed in AD brain. Females had a lower PME/PDE ratio than males.

Studies of the mechanism underlying increased Na+/Ca2+ exchange activity in Alzheimer's disease brain

The Na+/Ca2+ exchanger was characterized in plasma membrane vesicles derived from frozen human postmortem tissues. The frontal cortex, temporal cortex and cerebellum of control and Alzheimer's disease (AD) tissues were compared. Na+/Ca2+ exchange activity was defined as the change in vesicular Ca2+ content seen after Na+ loaded vesicles were diluted into choline buffer. The time course of changes in Ca2+ content after dilution was similar in all three regions of control brain. In AD brain, both frontal and temporal cortex vesicles showed elevated Ca2+ content, most evident as an increased peak Ca2+ content at 2 min. The AD cerebellar cortex time course was similar to control and did not show an elevated peak at 2 min. No differences were seen in the passive permeability to Ca2+ when comparing plasma membrane vesicles prepared from control and AD brain. Vesicles from the frontal and temporal cortex of AD brain showed increases in the Vmax of the initial velocity of Ca2+ uptake when compared to control brain, whereas, the cerebellum did not. There were no significant effects of AD on the Km for Ca2+ activation of the initial velocity. Ca2+ influx measured during the rise in vesicular Ca2+ content was elevated in vesicles from AD temporal cortex when compared to control. Two known inhibitors (exchange inhibitory peptide and dichlorobenzamil) of the cardiac Na+/Ca2+ exchanger inhibited the human brain exchanger equally well in control and AD vesicles. Increased Na+/Ca2+ exchange activity was not due to astrocytic gliosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential alterations of ion channel binding sites in temporal and occipital regions of the cerebral cortex in Alzheimer's disease

Three ion channel binding sites were examined by means of quantitative ligand binding autoradiography in temporal and occipital cortex from 9 patients with neuropathologically confirmed Alzheimer's disease (AD) and 7 matched control subjects. The following ligands were used: 125I-apamin to label a population of Ca(2+)-sensitive K+ channels; [3H]PN200-110 to label L-type voltage-sensitive Ca2+ channels and [3H]glibenclamide to label ATP-sensitive K+ channels. Ion channel binding sites were compared to: choline acetyltransferase (ChAT) activity and plaque densities measured in the same tissue. In the temporal cortex in AD 125I-apamin binding was increased compared to controls (e.g. superficial layers: control = 0.71 +/- 0.07; AD = 1.02 +/- 0.07, mean +/- S.E.M. pmol/g tissue). In contrast, in adjacent sections [3H]glibenclamide binding was reduced in AD compared to controls (e.g. superficial layers: control = 25.3 +/- 1.7; AD = 17.9 +/- 1.4 pmol/g tissue). [3H]PN200-110 binding in temporal cortex was not altered in AD compared to controls. In the occipital cortex 125I-apamin binding was increased in AD while both [3H]glibenclamide and [3H]PN-200-110 binding sites in this cortical area were not different from controls. Plaque density (per mm2) was higher in temporal (e.g. layers I-III, 43 +/- 6) than in occipital cortex (layers I-III, 27 +/- 4) in the AD patients while ChAT activity was reduced by 40% in temporal cortex and by 50% in occipital cortex compared to controls. The results suggest that the three ion channel binding sites are located on structural elements in the brain which are differentially affected by the pathophysiology of AD.

Calcium-destabilizing and neurodegenerative effects of aggregated beta-amyloid peptide are attenuated by basic FGF

The mechanisms that contribute to neuronal degeneration in Alzheimer's disease (AD) are not understood. Abnormal accumulations of beta-amyloid peptide (beta AP) are thought to be involved in the neurodegenerative process, and recent studies have demonstrated neurotoxic actions of beta APs. We now report that the mechanism of beta AP-mediated neurotoxicity in hippocampal cell culture involves a destabilization of neuronal calcium homeostasis resulting in elevations in intracellular calcium levels ([Ca2+]i) that occur during exposure periods of 6 hr to several days. Both the elevations of [Ca2+]i and neurotoxicity were directly correlated with aggregation of the peptide as assessed by beta AP immunoreactivity and confocal laser scanning microscopy. Exposure of neurons to beta AP resulted in increased sensitivity to the [Ca2+]i-elevating and neurodegenerative effects of excitatory amino acids. Moreover, [Ca2+]i responses to membrane depolarization and calcium ionophore were greatly enhanced in beta AP-treated neurons. Neurons in low cell density cultures were more vulnerable to beta AP toxicity than were neurons in high cell density cultures. Basic fibroblast growth factor (bFGF), but not nerve growth factor (NGF), significantly reduced both the loss of calcium homeostasis and the neuronal damage otherwise caused by beta AP. In AD, beta AP may endanger neurons by destabilizing calcium homeostasis and bFGF may protect neurons by stabilizing intracellular calcium levels. Aggregation of beta AP seems to be a major determinant of its [Ca2+]i-destabilizing and neurotoxic potency.

Analysis of Na+/Ca2+ exchange activity in human brain: the effect of normal aging

Na+/Ca2+ exchange activity and passive permeability to Ca2+ were analyzed in plasma membrane vesicles (PMV) purified from whole rat brain and three regions of human brain: frontal cortex, temporal cortex, and cerebellum. Accumulation of Ca2+ due to Na+/Ca2+ exchange activity showed a characteristic pattern of an initial rapid rise in Ca2+ content followed by a stable plateau in both rat and human brain. Total Ca2+ accumulation in rat brain PMV was on average three-fold higher than in human brain. Passive permeability to Ca2+ was measured as the rate of Ca2+ release from PMV first loaded with 45Ca by Na+/Ca2+ exchange and then exposed to 1 mM EGTA. The Ca2+ permeabilities of human and rat brain PMV were similar. Ca2+ release from rat brain PMV was faster overall and was resolved into fast and slow components while in human brain a single slow component was found. Post mortem delay up to 4 h had no effect on Na+/Ca2+ exchange Km for Ca2+, Vmax, and peak uptake and Ca2+ release rate in rat brain PMV. Human frontal cortex was shown to have a greater Na+/Ca2+ exchange activity than that found in the cerebellum. The frontal cortex, temporal cortex and cerebellum had similar Ca2+ permeabilities. Age-related effects on Na+/Ca2+ exchange activity and Ca2+ permeability were determined in 15 tissues from human frontal cortex (age at death 21 to 93 years). No significant age related effects were seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in calcium's role as a messenger during aging in neuronal and nonneuronal cells

Alterations in calcium transport appear to be functionally significant. Treatment with drugs that promote calcium uptake partially reverse some of the age-related deficits in calcium-dependent processes. Thus, the relevance of decreased calcium coupled receptor binding is supported by the ability of 3,4-diaminopyridine to promote acetylcholine release by forebrain slices from aged mice. This drug also reduces the age-related depression in synaptosomal calcium uptake in aged rats and mice. 3,4-Diaminopyridine also reverses the age-related deficit in calcium transport, the age-related deficits in the tight rope test, and 8 arm maze performance. 3,4-Diaminopyridine is also effective in nonexcitable tissues, such as cultured skin fibroblasts; it increases the decreased cytosolic-free calcium. Depressed cell spreading of fibroblasts can be reversed by treatment of cells with the calcium ionophore A23187 which promotes calcium influx. 4-Aminopyridine, a similarly related compound, partially reverses short-term memory deficits in patients with Alzheimer's disease. Tetrahydroaminoacridine, an aminopyridine analog with anticholinesterase properties, produces clinical improvement in behavioral deficits due to Alzheimer's disease. Only recently has the aging brain become a subject of intense study. Evidently, the neurobiology of aging needs to develop its own theories to account for the unique aspects of brain aging as well as integrate them with the peripheral changes. An exciting but unexplored area of research in the aging brain concerns the coupling between calcium and the final end product, the induction of genes. Still unknown are the molecular events that set these processes in motion. In addition, whether conditions such as dietary restriction that increase longevity in certain rodents also retard age-related changes in calcium remains to be determined.

A correlative study of calcium channel antagonist binding and local neuropathological features in the hippocampus in Alzheimer's disease

[3H]PN200-110 binding sites were studied by means of quantitative autoradiography in hippocampal sections of patients with Alzheimer's disease and age-matched control subjects. Choline acetyltransferase activity, plaque, tangle and cell densities were also determined in the same tissue samples used for autoradiographic studies. Quantitative autoradiographic analysis of [3H]PN200-110 binding in control hippocampus revealed a heterogeneous pattern similar to that described in rodents, being particularly high in the dentate gyrus. In Alzheimer's disease, [3H]PN200-110 binding was markedly reduced in the subiculum (control = 9.85 +/- 1.41 pmol/g; Alzheimer = 3.41 +/- 0.54 pmol/g, mean +/- S.E.M., P less than 0.001). In the subiculum there was a disproportionate reduction of [3H]PN200-110 binding in comparison to cell loss in Alzheimer's disease. The activity of choline acetyltransferase in the hippocampus was markedly reduced in Alzheimer's disease (controls 6.9 +/- 1.0; Alzheimer 2.7 +/- 0.9 nmol/h/mg protein, mean +/- S.E.M., P less than 0.01). There was a strong correlation between choline acetyltransferase activity and [3H]PN200-110 binding in the subiculum. [3H]PN200-110 binding did not correlate with plaque density in the subiculum. The discrete reduction and preservation of [3H]PN200-110 binding in the present study is consistent with the pattern of selective cellular vulnerability in the hippocampal region in Alzheimer's disease.

Effects of microtubule stabilization and destabilization on tau immunoreactivity in cultured hippocampal neurons

Tau immunoreactivity is altered in neurofibrillary tangles (NFT) and degenerating neurites in Alzheimer's disease (AD). In addition, cytoskeletal proteins including tau are excessively phosphorylated in AD. Previous data indicated that calcium influx can cause antigenic changes in tau in cultured rat hippocampal and human cortical neurons similar to those seen in NFT. The present study used cultured hippocampal neurons to test the hypothesis that disruption of microtubules is a key event leading to altered antigenic properties of tau that result from calcium influx. As previously reported, we found that glutamate (100-500 microM) and calcium ionophore A23187 (0.5-1 microM) elevated intraneuronal calcium levels and caused a reduction in microtubules, a marked increase in staining with Alz-50 and 5E2, and a decrease in tau-1 immunoreactivity. The microtubule-disrupting agent colchicine (1 microM) caused increased immunoreactivity of neurons towards tau antibodies Alz-50 and 5E2, and these effects of colchicine occurred in the absence of an increase in intracellular calcium levels. The microtubule-stabilizing drug taxol (100 nM) reduced neuronal immunoreactivity towards Alz-50 and 5E2 in untreated cultures and in cultures exposed to glutamate or A23187. Western blot analysis indicated that A23187 caused a reduction in tau levels which was partially prevented by taxol, suggesting that tau associated with microtubules is less susceptible to calcium-mediated degradation. Acid phosphatase treatment increased neuronal immunoreactivity towards tau-1 and reduced immunoreactivity towards Alz-50. The calcium-induced alterations in tau immunoreactivity were, and the colchicine-induced alterations were not, affected by acid phosphatase treatment. Taken together, the data indicate that microtubule depolymerization can cause antigenic changes in tau similar to those seen in NFT independently of an increase in intraneuronal calcium levels. Stabilization of microtubules prevented the antigenic changes in tau suggesting that microtubules affect the availability and/or properties of epitopes on tau that are recognized by antibodies that stain NFT.