Serum- and bradykinin-induced calcium transients in familial Alzheimer's fibroblasts

CRISPR-activated patient fibroblasts for modeling of familial Alzheimer's disease

Analyzing an appropriate disease model system is important to conduct disease research. Analyzing cells obtained from patient tissues could not only help elucidate the pathological mechanisms and to develop novel therapy but also lead to personalized medicine in the future. However, it is generally difficult to collect and culture neuronal cells from patients suffering from neurodegenerative disorders. Skin fibroblasts are easier to collect than neurons but may not show the expected pathology when disease-relevant genes are not sufficiently expressed. In this article, I describe an in vitro model system that enables the facile analysis of neurological disease mechanisms in patient fibroblast cultures by CRISPR transcriptional activation of endogenous disease-relevant genes. This system introduces an additional platform to analyze neurodegenerative disorders.

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

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

The presenilin 2 M239I mutation associated with familial Alzheimer's disease reduces Ca2+ release from intracellular stores

Mutations in presenilin (PS) genes account for the majority of the cases of the familial form of Alzheimer's disease (FAD). PS mutations have been correlated with both over-production of the amyloid-beta-42 (Abeta42) peptide and alterations of cellular Ca(2+) homeostasis. We here show, for the first time, the effect of the recently described PS2 FAD-associated M239I mutation on two major parameters of intracellular Ca(2+) homeostasis: the Ca(2+) storing capacity of the endoplasmic reticulum (ER) and the activation level of capacitative Ca(2+) entry (CCE), the Ca(2+) influx pathway activated by depletion of intracellular stores. Ca(2+) release from intracellular stores was significantly reduced in fibroblasts from FAD patients, compared to that found in cells from healthy individuals or patients affected by sporadic forms of Alzheimer's Disease (AD). No significant difference was however found in CCE between FAD and control fibroblasts. Similar results were obtained in two cell lines (HEK293 and HeLa) stably or transiently expressing the PS2 M239I mutation.

Mutant presenilins disturb neuronal calcium homeostasis in the brain of transgenic mice, decreasing the threshold for excitotoxicity and facilitating long-term potentiation

Mutant human presenilin-1 (PS1) causes an Alzheimer's-related phenotype in the brain of transgenic mice in combination with mutant human amyloid precursor protein by means of increased production of amyloid peptides (Dewachter, I., Van Dorpe, J., Smeijers, L., Gilis, M., Kuiperi, C., Laenen, I., Caluwaerts, N., Moechars, D., Checler, F., Vanderstichele, H. & Van Leuven, F. (2000) J. Neurosci. 20, 6452-6458) that aggravate plaques and cerebrovascular amyloid (Van Dorpe, J., Smeijers, L., Dewachter, I., Nuyens, D., Spittaels, K., van den Haute, C., Mercken, M., Moechars, D., Laenen, I., Kuipéri, C., Bruynseels, K., Tesseur, I., Loos, R., Vanderstichele, H., Checler, F., Sciot, R. & Van Leuven, F. (2000) J. Am. Pathol. 157, 1283-1298). This gain of function of mutant PS1 is approached here in three paradigms that relate to glutamate neurotransmission. Mutant but not wild-type human PS1 (i) lowered the excitotoxic threshold for kainic acid in vivo, (ii) facilitated hippocampal long-term potentiation in brain slices, and (iii) increased glutamate-induced intracellular calcium levels in isolated neurons. Prominent higher calcium responses were triggered by thapsigargin and bradykinin, indicating that mutant PS modulates the dynamic release and storage of calcium ions in the endoplasmatic reticulum. In reaction to glutamate, overfilled Ca(2+) stores resulted in higher than normal cytosolic Ca(2+) levels, explaining the facilitated long-term potentiation and enhanced excitotoxicity. The lowered excitotoxic threshold for kainic acid was also observed in mice transgenic for mutant human PS2[N141I] and was prevented by dantrolene, an inhibitor of Ca(2+) release from the endoplasmic reticulum.

Ionic and signal transduction alterations in Alzheimer's disease: relevance of studies on peripheral cells

Several lines of evidence indicate that Alzheimer's disease (AD) has systemic expression. Systemic changes are manifested as alterations in a number of molecular and cellular processes. Although, these alterations appear to have little or no consequence in peripheral systems, their parallel expression in the central nervous system (CNS) could account for the principal clinical manifestations of the disease. Recent research seems to indicate that alterations in ion channels, calcium homeostasis, and protein kinase C (PKC) can be linked and thereby constitute a model of pathophysiological relevance. Considering the difficulties of studying dynamic pathophysiological processes in the disease-ridden postmortem AD brain, peripheral tissues such as fibroblasts provide a suitable model to study molecular and cellular aspects of the disease.

Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Deltapsim) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca2+ level ([Ca2+]i). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca2+ release in response to its modulator, caffeine; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin-dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-kappaB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca2+]i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+]i changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Calcium responses in fibroblasts from asymptomatic members of Alzheimer's disease families

We have previously identified alterations of K+ channel function, IP3-mediated calcium release, and Cp20 (a memory-associated GTP binding protein) in fibroblasts from Alzheimer's disease (AD) patients vs controls. Some of these alterations can be integrated into an index that distinguishes AD patients from controls with both high specificity and high sensitivity. We report here that alterations in IP3-mediated calcium responses are present in a large proportion of AD family members (i.e., individuals at high risk) before clinical symptoms of Alzheimer's disease are present. This was not the case if such members later "escaped" AD symptoms. This preclinical calcium signal correlate of later AD does not reflect, however, the presence of the PS1 familial AD gene.

Fibroblast models of neurological disorders: fluorescence measurement studies

Biochemical studies of human fibroblasts from patients with neurological disorders have revealed a wealth of information on how such disorders occur. In this review, Gerald Connolly describes how recently developed fluorescence video imaging techniques have been used to study the physiology of skin fibroblasts isolated from patients with certain neurological disorders, including those produced by Alzheimer's disease, Lesch-Nyhan syndrome, mitochondrial disorders, amyotrophic lateral sclerosis and lysosomal disorders. The results of these studies indicate disruptions in cell homeostasis, particularly specific changes in Ca2+ homeostasis and autofluorescence, which mirror changes thought to occur in the CNS of neurologically impaired patients. More extensive studies of these 'systemic changes' using new fluorescent indicators, combined with advances in imaging techniques, are predicted to increase the potential usefulness of human skin fibroblasts as experimental models and to help diagnose and treat neurological disorders.

Effects of the peroxisome proliferator ciprofibrate and prostaglandin F2 alpha combination treatment on second messengers in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepatic tumors in rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolizes lipids, including eicosanoids. Peroxisome proliferators transiently induce increased cell proliferation in vivo. However, peroxisome proliferators are weakly mitogenic and are not co-mitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Earlier study found that the peroxisome proliferator ciprofibrate is comitogenic with eicosanoids. In order to study possible mechanisms of the comitogenicity of peroxisome proliferator ciprofibrate and eicosanoids, we hypothesized that the co-mitogenicity may result from synergistic or additive increases of second messengers in mitogenic signal pathways. We therefore examined the effect of the peroxisome proliferator ciprofibrate, prostaglandin F2 alpha (PGF2 alpha) and the combination of ciprofibrate and PGF2 alpha with or without growth factors on the protein kinase C (PKC) activity, and inositol-1, 4, 5-triphosphate (IP3) and intracellular calcium ([Ca2+]i) concentrations in cultured rat hepatocytes. The combination of ciprofibrate and PGF2 alpha significantly increased particulate PKC activity. The combination of ciprofibrate and PGF2 alpha also significantly increased EGF, transforming growth factor-alpha (TGF-alpha) and hepatic growth factor (HGF)-induced particulate PKC activity. The combination of ciprofibrate and PGF2 alpha greatly increased [Ca2+]i. However, the increases of PKC activity and [Ca2+]i by ciprofibrate and PGF2 alpha alone were much smaller. Neither ciprofibrate or PGF2 alpha alone nor the combination of ciprofibrate and PGF2 alpha significantly increased the formation of IP3. The combination of ciprofibrate and PGF2 alpha, however, blocked the inhibitory effect of TGF-beta on particulate PKC activity and formation of IP3 induced by EGF. These results show that co-mitogenicity of the peroxisome proliferator ciprofibrate and eicosanoids may result from the increase in particulate PKC activity and intracellular calcium concentration but not from the formation of IP3.

Peripheral markers in testing pathophysiological hypotheses and diagnosing Alzheimer's disease

Alterations in amyloid precursor protein (APP) metabolism, calcium regulation, oxidative metabolism, and transduction systems have been implicated in Alzheimer's disease (AD). Limitations to the use of postmortem brain for examining molecular mechanisms underscore the need to develop a human tissue model representative of the pathophysiological processes that characterize AD. The use of peripheral tissues, particularly of cultured skin fibroblasts derived from AD patients, could complement studies of autopsy samples and provide a useful tool with which to investigate such dynamic processes as signal transduction systems, ionic homeostasis, oxidative metabolism, and APP processing. Peripheral cells as well as body fluids (i.e., plasma and CSF) could also provide peripheral biological markers for the diagnosis of AD. The criteria required for a definite diagnosis of AD presently include clinical criteria in association with histopathologic evidence obtained from biopsy or autopsy. Thus, the use of peripheral markers as a diagnostic tool, either to predict or at least to confirm a diagnosis, may be of great importance.

Compromised mitochondrial function leads to increased cytosolic calcium and to activation of MAP kinases

We have investigated in rat pheochromacytoma PC12 cells the activation of the mitogen-activated protein kinases ERK1 and ERK2 by the mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP). This treatment slowly decreases ATP levels to 30% of control, whereas the internal calcium level rises very rapidly to 250% of control, derived from internal stores. Tyrosine phosphorylation of ERK1 and ERK2 increases gradually, starting after 5 min of treatment, to reach a maximum at 30 min; the kinase activity reaches 250% when measured after 1 hr of treatment. The drop in ATP levels is slower still. Comparison of the time courses of the rapid rise in cytosolic calcium with the slower increase in ERK1 and ERK2 activation suggests one or more intermediate stages in this pathway. Chelation of cytosolic calcium with dimethyl bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid abolished the FCCP-stimulated rise in internal calcium, as well as the tyrosine phosphorylation and the activation of the ERKs. Surprisingly, caffeine, which releases calcium from different internal stores, did not increase the tyrosine phosphorylation and did not activate the ERKs. The FCCP effect on calcium storage may be related to mitochondrial dysfunction in Alzheimer disease, which might result in ineffective buffering of cytosolic calcium that leads to mitogen-activated protein kinase activation and subsequent protein phosphorylations.

Calcium homeostasis and reactive oxygen species production in cells transformed by mitochondria from individuals with sporadic Alzheimer's disease

Alzheimer's disease (AD) is associated with defects in mitochondrial function. Mitochondrial-based disturbances in calcium homeostasis, reactive oxygen species (ROS) generation, and amyloid metabolism have been implicated in the pathophysiology of sporadic AD. The cellular consequences of mitochondrial dysfunction, however, are not known. To examine these consequences, mitochondrially transformed cells (cybrids) were created from AD patients or disease-free controls. Mitochondria from platelets were fused to rho0 cells created by depleting the human neuroblastoma line SH-SY5Y of its mitochondrial DNA (mtDNA). AD cybrids demonstrated a 52% decrease in electron transport chain (ETC) complex IV activity but no difference in complex I activity compared with control cybrids or SH-SY5Y cells. This mitochondrial dysfunction suggests a transferable mtDNA defect associated with AD. ROS generation was elevated in the AD cybrids. AD cybrids also displayed an increased basal cytosolic calcium concentration and enhanced sensitivity to inositol-1,4, 5-triphosphate (InsP3)-mediated release. Furthermore, they recovered more slowly from an elevation in cytosolic calcium induced by the InsP3 agonist carbachol. Mitochondrial calcium buffering plays a major role after this type of perturbation. beta-amyloid (25-35) peptide delayed the initiation of calcium recovery to a carbachol challenge and slowed the recovery rate. Nerve growth factor reduced the carbachol-induced maximum and moderated the recovery kinetics. Succinate increased ETC activity and partially restored the AD cybrid recovery rate. These subtle alterations in calcium homeostasis and ROS generation might lead to increased susceptibility to cell death under circumstances not ordinarily toxic.

Alzheimer's presenilin mutation sensitizes neural cells to apoptosis induced by trophic factor withdrawal and amyloid beta-peptide: involvement of calcium and oxyradicals

Most autosomal dominant inherited forms of early onset Alzheimer's disease (AD) are caused by mutations in the presenilin-1 (PS-1) gene on chromosome 14. PS-1 is an integral membrane protein with six to nine membrane-spanning domains and is expressed in neurons throughout the brain wherein it is localized mainly in endoplasmic reticulum (ER). The mechanism or mechanisms whereby PS-1 mutations promote neuron degeneration in AD are unknown. Recent findings suggest links among deposition of amyloid beta-peptide (Abeta), oxidative stress, disruption of ion homeostasis, and an apoptotic form of neuron death in AD. We now report that expression of the human PS-1 L286V mutation in PC12 cells increases their susceptibility to apoptosis induced by trophic factor withdrawal and Abeta. Increases in oxidative stress and intracellular calcium levels induced by the apoptotic stimuli were exacerbated greatly in cells expressing the PS-1 mutation, as compared with control cell lines and lines overexpressing wild-type PS-1. The antiapoptotic gene product Bcl-2 prevented apoptosis after NGF withdrawal from differentiated PC12 cells expressing mutant PS-1. Elevations of [Ca2+]i in response to thapsigargin, an inhibitor of the ER Ca2+-ATPase, were increased in cells expressing mutant PS-1, and this adverse effect was abolished in cells expressing Bcl-2. Antioxidants and blockers of calcium influx and release from ER protected cells against the adverse consequences of the PS-1 mutation. By perturbing cellular calcium regulation and promoting oxidative stress, PS-1 mutations may sensitize neurons to apoptotic death in AD.

Lymphocytes and neutrophils as peripheral models to study the effect of beta-amyloid on cellular calcium signalling in Alzheimer's disease

According to the calcium hypothesis of brain aging, disturbances of free intracellular calcium homeostasis ([Ca2+]i) play a key role in pathology of Alzheimer's disease (AD). Recent data from neuronal tissue culture support the contribution of the beta-amyloid peptide (beta A) to neurodegeneration in AD, probably by disruption of the intracellular Ca2+ regulation. On the basis of this premise, we used peripheral blood cells to examine the role of beta A on Ca2+ signalling, not only to obtain an experimental approach to investigate these effects of beta A in man, but also to search for AD-specific alterations of the effects of beta A on Ca2+ signalling. This approach is based on observations indicating that the phytohemagglutinin (PHA)-induced Ca2+ response in circulating human lymphocytes of healthy volunteers is affected by beta A and its fragment 25-35 in a fashion similar to its effects on central neurons, whereas we found no effect of beta A on receptor-activated Ca2+ response in neutrophils. Therefore, we used human blood lymphocytes as peripheral model systems to search directly for AD-related abnormalities of Ca2+ regulation, for alterations of beta A effects on Ca2+ signalling and on membrane fluidity, and for possible changes of potassium channels. In accordance with our data in neutrophils, we were unable to identify any relevant change of the PHA-induced Ca2+ elevations in lymphocytes, which is not supporting the assumption of general alterations of cellular Ca2+ regulation in AD. On the other hand, the amplifying effect of beta A on Ca2+ signalling was significantly reduced in lymphocytes from AD patients. Moreover, Ca2+ responses to beta A25-35 were not different between early- and late-onset AD patients. Our findings indicate that the sensitivity of the lymphocyte for the effects of beta A is reduced in a high percentage of patients with probable or possible AD. As possible explanation we observed a similar reduction of the sensitivity of the lymphocyte membrane for the fluidity-decreasing properties of beta A. Finally, the inhibition of the PHA-induced Ca2+ response by tetraethylammonium (TEA) was lower in the AD group compared to aged controls. This could suggest the presence of a K+ channel dysfunction on AD lymphocytes, as it has been shown on skin fibroblasts of AD patients.

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.

Calcium responses in human fibroblasts: a diagnostic molecular profile for Alzheimer's disease

We have previously identified alterations of K+ channel function, IP3-mediated calcium release, and Cp20 (a memory-associated GTP binding protein) in fibroblasts from AD patients vs. controls. In the present study we introduce a scoring system based on these response alterations that integrates two or more alterations (and their degree) in AD vs. control fibroblasts. This scoring system generates an index that distinguishes AD patients from controls with both high specificity and sensitivity. We also show that low doses of bradykinin elicit intracellular calcium release almost exclusively in AD cell lines in an all or none fashion that provide a clear measurement of enhanced IP3-mediated function in AD vs. controls.

Soluble beta-amyloid induces Alzheimer's disease features in human fibroblasts and in neuronal tissues

It has been shown that K+ channels, Cp20 (a 20kD GTP-binding protein), and intracellular calcium release, play a key role in associative memory storage. These same elements have been shown to be altered in fibroblasts from Alzheimer's Disease (AD) patients. In addition, it has been shown that PKC, also implicated in memory storage and closely related to the above mentioned components, is also altered in AD fibroblasts. Moreover, beta-amyloid was capable of inducing an AD-like phenotype for K+ channels and Cp20 in otherwise normal fibroblasts, providing additional evidence for the potential involvement of these components in AD and suggesting a possible pathological consequence of soluble beta-amyloid elevation in AD. Preliminary evidence shows that comparable changes in potassium channel function are also present in human olfactory neuroblasts from AD patients. These results indicate that the observed changes not only occur in peripheral tissues such as fibroblasts, but also in neural tissue, the primary site of AD pathology.

Calcium stores in cultured fibroblasts and their changes with Alzheimer's disease

The experiments in this paper identify multiple calcium compartments in cultured human fibroblasts and reveal abnormalities in one of these pools in cells from Alzheimer patients. In the presence of external calcium, bradykinin (BK) increased cytosolic free calcium ([Ca2+]i) about 3-fold and then [Ca2+]i rapidly declined. Omission of calcium from the media did not affect the BK-induced peak, which indicates that the peak reflects internal stores. Other compounds that also released calcium from internal stores included A23187 (a calcium ionophore), thapsigargin (Tg; an inhibitor of endoplasmic reticulum ATPase), and FCCP (an uncoupler of oxidative phosphorylation). The [Ca2+]i response to sequential addition of compounds in calcium-free media identified discrete internal calcium stores. BK depleted internal calcium pools such that subsequent stimulation with BK, FCCP or bombesin did not increase [Ca2+]i. However, A23187 or thapsigargin still elicited responses. A23187 depleted essentially all internal calcium pools. Either Tg or FCCP reduced the calcium stores that could be released by BK or A23187. Thus, cellular calcium compartments that respond to BK and A23187 partially overlap. The common pool includes Tg-and FCCP-sensitive compartments. Calcium stores were examined in cells from Alzheimer disease patients, because previous studies suggest that their calcium homeostasis is altered. A23187 addition to BK-treated cells produced a 95% greater response in cell lines from Alzheimer patients (n = 7) than in those from controls (n = 5). Thus, various calcium stores can be pharmacologically distinguished in fibroblasts and at least one of these compartments is abnormal in Alzheimer's disease.

Characterization of muscarinic acetylcholine receptors in cultured adult skin fibroblasts: effects of the Swedish Alzheimer's disease APP 670/671 mutation on binding levels

We have characterised the muscarinic receptor subtypes found in human skin fibroblasts and compared binding levels in cell lines from members of the Alzheimer's disease family with the Swedish amyloid precursor protein (APP) 670/671 mutation. Binding studies with [3H] quinuclidinyl benzilate ([3H]QNB) and the M2/M4 selective antagonist [3H] (+/-)-5,11-dihydro-11-([(2-[(di-propylamino)methyl]-1- piperidinyl]ethyl)amino]carbonyl)-6H-pyrido(2,3-b)(1,4)benzodiazepine-6- one ([3H]AF-DX 384) revealed the presence of a single population of muscarinic receptors on lysed fibroblast membranes. [3H]QNB binding was displaced by a number of selective muscarinic ligands with a rank order of potency: atropine > himbacine > methoctramine > (+/-)-p-fluoro-hexahydro-sila-difenidol hydrochloride > pirenzepine > muscarinic-toxin-3. APP 670/671 mutation carrying cell lines showed 25-35% lower levels of muscarinic receptors labelled with [3H]QNB, [3H]N-methyl scopolamine and [3H]AF-DX 384, compared to controls. This difference was not statistically significant due to large individual variation. It is concluded that muscarinic receptors on adult skin fibroblasts are predominantly of the M2 subtype. Since these cells do not possess M1 and M3 receptor subtypes, they are unlikely to provide a good model for studying muscarinic receptor regulation of APP processing.

Molecular mechanisms of memory and the pathophysiology of Alzheimer's disease

Research on molecular and biophysical mechanisms of associative learning and memory storage identified a number of key elements that are phylogenetically conserved. In both vertebrates and invertebrates, K+ channels, PKC, Cp20, and intracellular Ca2+ regulation play a fundamental role in memory mechanisms. Because memory loss is the hallmark and perhaps the earliest sign of Alzheimer's disease, we hypothesized that these normal memory mechanisms might be altered in AD. With the use of a variety of experimental methodologies, our results revealed that one of the critical elements in memory storage, K+ channels, are dysfunctional in AD fibroblasts. Moreover, beta-amyloid induced the same K+ dysfunction in normal cells. Intracellular Ca2+ release, also associated with molecular memory mechanisms, was found altered in fibroblasts from patients with AD. The results therefore strongly suggest that biophysical and molecular mechanisms of associative learning could be altered in AD and that they may contribute to the memory loss observed early in the disease.

Internal Ca2+ mobilization is altered in fibroblasts from patients with Alzheimer disease

The recent demonstration of K+ channel dysfunction in fibroblasts from Alzheimer disease (AD) patients and past observations of Ca(2+)-mediated K+ channel modulation during memory storage suggested that AD, which is characterized by memory loss and other cognitive deficits, might also involve dysfunction of intracellular Ca2+ mobilization. Bombesin-induced Ca2+ release, which is inositol trisphosphate-mediated, is shown here to be greatly enhanced in AD fibroblasts compared with fibroblasts from control groups. Bradykinin, another activator of phospholipase C, elicits similar enhancement of Ca2+ signaling in AD fibroblasts. By contrast, thapsigargin, an agent that releases Ca2+ by direct action on the endoplasmic reticulum, produced no differences in Ca2+ increase between AD and control fibroblasts. Depolarization-induced Ca2+ influx data previously demonstrated the absence of between-group differences of Ca2+ pumping and/or buffering. There was no correlation between the number of passages in tissue culture and the observed Ca2+ responses. Furthermore, cells of all groups were seeded and analyzed at the same densities. Radioligand binding experiments indicated that the number and affinity of bombesin receptors cannot explain the observed differences. These and previous observations suggest that the differences in bombesin and bradykinin responses in fibroblasts and perhaps other cell types are likely to be due to alteration of inositol trisphosphate-mediated release of intracellular Ca2+.