Enhanced release of secreted form of Alzheimer's amyloid precursor protein from PC12 cells by nicotine

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Evaluation of toxicity of glycerol monooleate nanoparticles on PC12 cell line

An innovative approach to improve drug delivery is the use of glycerol monooleate nanoparticles. Numerous studies describe their high versatility, low toxicity and ability to carry relatively high loads of conjugated compounds including scarcely soluble ones, providing sustained drug release and increasing drug diffusion and half-life. Despite a growing interest in their potential use for therapeutic applications, there are surprisingly few literature data concerning the toxic effects of these nanoparticles at high concentrations in vitro and in vivo, and their effects on cell metabolism. We produced and characterized from a physical-chemical point of view glycerol monooleate nanoparticles and tested them on the PC12 cell line, a rat model of neuronal differentiation. The toxicity of these nanoparticles was evaluated by molecular methods on cell viability, cell cycle, nanoparticle uptake and induction of apoptosis. The results showed that glycerol monooleate nanoparticles up to 100 μg/mL had no toxic effects on PC12 cells, did not induce significant changes in the cell cycle nor cause apoptosis. The nanoparticles entered PC12 cells 8 h after treatment, successfully delivering the conjugate compound inside cells. Overall, glycerol monooleate nanoparticles did not exhibit significant toxicity on PC12 cell line in concentrations up to 100 µg/mL, supporting their therapeutic use as drug delivery systems.

Cholinergic Receptor Binding in Alzheimer Disease and Healthy Aging: Assessment In Vivo with Positron Emission Tomography Imaging

OBJECTIVE

To compare regional nicotinic cholinergic receptor binding in older adults with Alzheimer disease (AD) and healthy older adults in vivo and to assess relationships between receptor binding and clinical symptoms.

METHODS

Using cross-sectional positron emission tomography (PET) neuroimaging and structured clinical assessment, outpatients with mild to moderate AD (N = 24) and healthy older adults without cognitive complaints (C group; N = 22) were studied. PET imaging of α4β2* nicotinic cholinergic receptor binding using 2-[¹⁸F]fluoro-3-(2(S)azetidinylmethoxy)pyridine (2FA) and clinical measures of global cognition, attention/processing speed, verbal memory, visuospatial memory, and neuropsychiatric symptoms were used.

RESULTS

2FA binding was lower in the AD group compared with the C group in the medial thalamus, medial temporal cortex, anterior cingulate, insula/opercula, inferior caudate, and brainstem (p < 0.05, corrected cluster), but binding was not associated with cognition. The C group had significant inverse correlations between 2FA binding in the thalamus (left: r_s = -0.55, p = 0.008; right: r_s = -0.50, p = 0.02; N = 22) and hippocampus (left: r_s = -0.65, p = 0.001; right: r_s = -0.55, p = 0.009; N = 22) and the Trails A score. The AD group had inverse correlation between 2FA binding in anterior cingulate (left: r_s = -0.50, p = 0.01; right: r_s = -0.50, p = 0.01; N = 24) and Neurobehavioral Rating Scale agitation/disinhibition factor score.

CONCLUSION

Cholinergic receptor binding is reduced in specific brain regions in mild to moderate AD and is related to neuropsychiatric symptoms. Among healthy older adults, lower receptor binding may be associated with slower processing speed. Cholinergic receptor binding in vivo may reveal links to other key brain changes associated with aging and AD and may provide a potential molecular treatment target.

Cholinergic Mechanisms in the Cerebral Cortex: Beyond Synaptic Transmission

Functional overviews of cholinergic mechanisms in the cerebral cortex have traditionally focused on the release of acetylcholine with modulator and transmitter effects. Recently, however, data have emerged that extend the role of acetylcholine and cholinergic innervations to a range of housekeeping and metabolic functions. These include regulation of amyloid precursor protein (APP) processing with production of amyloid β (Aβ) and other APP fragments and control of the phosphorylation of microtubule-associated protein (MAP) tau. Evidence has been also presented for receptor-ligand like interactions of cholinergic receptors with soluble Aβ peptide and MAP tau, with modulator and signaling effects. Moreover, high-affinity binding of Aβ to the neurotrophin receptor p75 (p75NTR) enriched in basalo-cortical cholinergic projections has been implicated in clearance of Aβ and nucleation of amyloid plaques. Here, we critically evaluate these unorthodox cholinergic mechanisms and discuss their role in neuronal physiology and the biology of Alzheimer's disease.

Molecular Mechanisms Regulating LPS-Induced Inflammation in the Brain

Neuro-inflammation, one of the pathogenic causes of neurodegenerative diseases, is regulated through the cholinergic anti-inflammatory pathway via the α7 nicotinic acetylcholine receptor (α7 nAChR). We previously showed that either bacterial lipopolysaccharide (LPS) or immunization with the α7(1-208) nAChR fragment decrease α7 nAChRs density in the mouse brain, exacerbating chronic inflammation, beta-amyloid accumulation and episodic memory decline, which mimic the early stages of Alzheimer's disease (AD). To study the molecular mechanisms underlying the LPS and antibody effects in the brain, we employed an in vivo model of acute LPS-induced inflammation and an in vitro model of cultured glioblastoma U373 cells. Here, we report that LPS challenge decreased the levels of α7 nAChR RNA and protein and of acetylcholinesterase (AChE) RNA and activity in distinct mouse brain regions, sensitized brain mitochondria to the apoptogenic effect of Ca(2+) and modified brain microRNA profiles, including the cholinergic-regulatory CholinomiRs-132/212, in favor of anti-inflammatory and pro-apoptotic ones. Adding α7(1-208)-specific antibodies to the LPS challenge prevented elevation of both the anti-inflammatory and pro-apoptotic miRNAs while supporting the resistance of brain mitochondria to Ca(2+) and maintaining α7 nAChR/AChE decreases. In U373 cells, α7-specific antibodies and LPS both stimulated interleukin-6 production through the p38/Src-dependent pathway. Our findings demonstrate that acute LPS-induced inflammation induces the cholinergic anti-inflammatory pathway in the brain, that α7 nAChR down-regulation limits this pathway, and that α7-specific antibodies aggravate neuroinflammation by inducing the pro-inflammatory interleukin-6 and dampening anti-inflammatory miRNAs; however, these antibodies may protect brain mitochondria and decrease the levels of pro-apoptotic miRNAs, preventing LPS-induced neurodegeneration.

High-energy compounds promote physiological processing of Alzheimer's amyloid-β precursor protein and boost cell survival in culture

Physiological or α-processing of amyloid-β precursor protein (APP) prevents the formation of Aβ, which is deposited in the aging brain and may contribute to Alzheimer's disease. As such, drugs promoting this pathway could be useful for prevention of the disease. Along this line, we searched through a number of substances and unexpectedly found that a group of high-energy compounds (HECs), namely ATP, phosphocreatine, and acetyl coenzyme A, potently increased APP α-processing in cultured SH-SY5Y cells, whereas their cognate counterparts, i.e., ADP, creatine, or coenzyme A did not show the same effects. Other HECs such as GTP, CTP, phosphoenol pyruvate, and S-adenosylmethionine also promoted APP α-processing with varying potencies and the effects were abolished by energy inhibitors rotenone or NaN(3). The overall efficacy of the HECs in the process ranged from three- to four-fold, which was significantly greater than that exhibited by other physiological stimulators such as glutamate and nicotine. This suggested that the HECs were perhaps the most efficient physiological stimulators for APP α-processing. Moreover, the HECs largely offset the inefficient APP α-processing in aged human fibroblasts or in cells impaired by rotenone or H(2) O(2). Most importantly, some HECs markedly boosted the survival rate of SH-SY5Y cells in the death process induced by energy suppression or oxidative stress. These findings suggest a new, energy-dependent regulatory mechanism for the putative α-secretase and thus will help substantially in its identification. At the same time, the study raises the possibility that the HECs may be useful to energize and strengthen the aging brain cells to slow down the progression of Alzheimer's disease.

Evidence supporting the role of calpain in the α-processing of amyloid-β precursor protein

Amyloid plaques are a hallmark of the aging and senile dementia brains, yet their mechanism of origins has remained elusive. A central issue is the regulatory mechanism and identity of α-secretase, a protease responsible for α-processing of amyloid-β precursor protein (APP). A remarkable feature of this enzyme is its high sensitivity to a wide range of cellular stimulators, many of which are agonists for Ca(2+) signaling. This feature, together with previous work in our laboratory, has suggested that calpain, a Ca(2+)-dependent protease, plays a key role in APP α-processing. In this study we report that overexpression of the μ-calpain gene in HEK293 cells resulted in a 2.7-fold increase of the protein levels. Measurements of intracellular calpain enzymatic activity revealed that the calpain overexpressing cells displayed a prominent elevation of the activity compared to wild-type cells. When the cells were stimulated by nicotine, glutamate or phorbol 12,13-dibutylester, the activity increase was even more remarkable and sensitive to calpeptin, a calpain inhibitor. Meanwhile, APP secretion from the calpain overexpressing cells was robustly increased under both resting and stimulated conditions over wild-type cells. Furthermore, cell surface biotinylation experiments showed that μ-calpain was clearly detected among the cell surface proteins. These data together support our view that calpain should be a reasonable candidate for α-secretase for further study. This model is discussed with an interesting fact that three other deposited proteins (tau, spectrin and crystalline) are also the known substrates of calpain. Finally we discuss some current misconceptions in senile dementia research.

Nicotine modifies in vivo and in vitro rat hippocampal amyloid precursor protein processing in young but not old rats

Previous studies have shown that administration of nicotine modifies the expression and secretion of amyloid precursor protein (APP) in various cell lines. The present study investigated the extent to which chronic subcutaneous nicotine administration influences APP levels and processing in cerebral cortex, striatum and hippocampus of young and old rat brains. The results showed that constant nicotine infusion (0.25 or 4.00mg/kg/day) increased the levels of particulate APP (APPp) but not secreted APP (APPs) in the hippocampus of young rats in vivo. This response to nicotine was not observed in the striatum or cerebral cortex of young rats or in any of the brain regions examined in old animals. Subsequent in vitro analysis demonstrated that nicotine enhanced the release of APPs from hippocampal slice preparations and that this increase was attenuated by mecamylamine, a non-selective nicotinic acetylcholine receptor (nAChR) antagonist. The in vitro effect of nicotine on APPs was age-related, being only detected from hippocampal slices derived from the young but not the older animals. These results suggest that nicotine modulates APP expression and secretion in the hippocampus and that the responses observed to the drug are age-dependent being only detected in younger rats.

The nicotinic acetylcholine receptor: smoking and Alzheimer's disease revisited

Epidemiological studies regarding Alzheimer's disease (AD) in smokers currently suggest inconsistent results. The clinicopathological findings also vary as to how AD pathology is affected by smoking behavior. Even though clinicopathological, functional, and epidemiological studies in humans do not present a consistent picture, much of the in vitro data implies that nicotine has neuroprotective effects when used in neurodegenerative disorder models. Current studies of the effects of nicotine and nicotinic agonists on cognitive function in both the non-demented and those with AD are not convincing. More data is needed to determine whether repetitive activation of nAChR with intermittent or acute exposure to nicotine, acute activation of nAChR, or long-lasting inactivation of nAChR secondary to chronic nicotine exposure will have a therapeutic effect and/or explain the beneficial effects of those types of drugs. Other studies show multifaceted connections between nicotine, nicotinic agonists, smoking, and nAChRs implicated in AD etiology. Although many controversies still exist, ongoing studies are revealing how nicotinic receptor changes and functions may be significant to the neurochemical, pathological, and clinical changes that appear in AD.

Deletion of M1 muscarinic acetylcholine receptors increases amyloid pathology in vitro and in vivo

Alzheimer's disease (AD) is a progressive neurological disorder that causes dementia and poses a major public health crisis as the population ages. Aberrant processing of the amyloid precursor protein (APP) is strongly implicated as a proximal event in AD pathophysiology, but the neurochemical signals that regulate APP processing in the brain are not completely understood. Activation of muscarinic acetylcholine receptors (mAChRs) has been shown to affect APP processing and AD pathology, but less is known about the roles of specific mAChR subtypes. In this study, we used M(1) mAChR knock-out mice (M(1)KO) to isolate the effects of the M(1) mAChR on APP processing in primary neurons and on the development of amyloid pathology in a transgenic mouse model of AD. We demonstrate that the loss of M(1) mAChRs increases amyloidogenic APP processing in neurons, as evidenced by decreased agonist-regulated shedding of the neuroprotective APP ectodomain APPsalpha and increased production of toxic Abeta peptides. Expression of M(1) mAChRs on the M(1)KO background rescued this phenotype, indicating that M(1) mAChRs are sufficient to modulate nonamyloidogenic APP processing. In APP(Swe/Ind) transgenic mice, the loss of M(1) mAChRs resulted in increased levels of brain Abeta and greater accumulation of amyloid plaque pathology. Analysis of APP metabolites in APP(Swe/Ind) brain tissue indicates that the loss of M(1) mAChRs increases amyloidogenic APP processing. These results indicate that the M(1) mAChR is an important regulator of amyloidogenesis in the brain and provide strong support for targeting the M(1) mAChR as a therapeutic candidate in AD.

Allelic variation of calsyntenin 2 (CLSTN2) modulates the impact of developmental tobacco smoke exposure on mnemonic processing in adolescents

BACKGROUND

Exposure to nicotine in tobacco smoke during development has been linked to subsequent deficits in attention and memory. The present study tested for evidence that genetic variation may contribute to individual differences in vulnerability to the effects of developmental exposure to tobacco smoke on memory and medial temporal lobe function in adolescents.

METHODS

Verbal and visuospatial memory were assessed and functional magnetic resonance imaging (fMRI) data were acquired in 101 adolescents systematically characterized for prenatal and adolescent exposure to tobacco smoke, while they performed an encoding and recognition memory task. The impact of allelic variation at loci within CLSTN2 (encoding synaptic protein calsyntenin 2) and KIBRA, shown previously to modulate early and delayed recall of words, on the dependent measures was examined.

RESULTS

KIBRA genotype did not exert significant main or interacting effects with prenatal or adolescent exposure to tobacco smoke on verbal or visuospatial memory. Previous observations of a beneficial effect of the CLSTN2 C allele on verbal recall were replicated. Adolescent exposure to tobacco smoke reversed this beneficial effect and was associated with increased activation of parahippocampal gyrus during early and delayed recognition in CLTSN2 C allele carriers. While the CLSTN2 C allele conferred enhanced functional connectivity between brain regions subserving accurate verbal recognition, adolescent exposure to tobacco smoke reversed this effect.

CONCLUSIONS

These findings extend previous work demonstrating that calsyntenins play an essential role in learning and indicate that this role is modulated both by CLSTN2 genotype and, during adolescent development, by exposure to tobacco smoke.

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection

Alzheimer's disease (AD), the major contributor to dementia in the elderly, involves accumulation in the brain of extracellular plaques containing the beta-amyloid protein (Abeta) and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. AD is also characterized by a loss of neurons, particularly those expressing nicotinic acetylcholine receptors (nAChRs), thereby leading to a reduction in nAChR numbers. The Abeta(1-42) protein, which is toxic to neurons, is critical to the onset and progression of AD. The discovery of new drug therapies for AD is likely to be accelerated by an improved understanding of the mechanisms whereby Abeta causes neuronal death. We examine the evidence for a role in Abeta(1-42) toxicity of nAChRs; paradoxically, nAChRs can also protect neurons when activated by nicotinic ligands. Abeta peptides and nicotine differentially activate several intracellular signaling pathways, including the phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene homolog pathway, the extracellular signal-regulated kinase/mitogen-activated protein kinase, and JAK-2/STAT-3 pathways. These pathways control cell death or survival and the secretion of Abeta peptides. We propose that understanding the differential activation of these pathways by nicotine and/or Abeta(1-42) may offer the prospect of new routes to therapy for AD.

Cholinergic treatments with emphasis on m1 muscarinic agonists as potential disease-modifying agents for Alzheimer's disease

The only prescribed drugs for treatment of Alzheimer's disease (AD) are acetylcholinesterase inhibitors (e.g., donepezil, rivastigmine, galantamine, and tacrine) and memantine, an NMDA antagonist. These drugs ameliorate mainly the symptoms of AD, such as cognitive impairments, rather than halting or preventing the causal neuropathology. There is currently no cure for AD and there is no way to stop its progression, yet there are numerous therapeutic approaches directed against various pathological hallmarks of AD that are extensively being pursued. In this context, the three major hallmark characteristics of AD (i.e., the CNS cholinergic hypofunction, formation of beta-amyloid plaques, and tangles containing hyperphosphorylated tau proteins) are apparently linked. Such linkages may have therapeutic implications, and this review is an attempt to analyze these versus the advantages and drawbacks of some cholinergic compounds, such as acetylcholinesterase inhibitors, M1 muscarinic agonists, M2 antagonists, and nicotinic agonists. Among the reviewed treatments, M1 selective agonists emerge, in particular, as potential disease modifiers.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Regulation of the NMDA receptor-mediated synaptic response by acetylcholinesterase inhibitors and its impairment in an animal model of Alzheimer's disease

The cholinergic system is crucial for cognitive processes and the deficient acetylcholine (ACh) function has been implicated in Alzheimer's disease (AD). Inhibitors of acetylcholinesterase (AChE), which act to enhance cholinergic function by prolonging the action of endogenously released ACh, have been used as the major therapy of AD. To understand the functional roles of cholinergic enhancement in prefrontal cortex (PFC), a key brain region for cognition, we examined the impact of AChE inhibitors in PFC neurons on synaptic responses mediated by the NMDA receptor (NMDAR), an important player in learning and memory. We found that AChE inhibitors produced a strong and persistent reduction of the amplitude of NMDA receptor-mediated excitatory postsynaptic current (NMDAR-EPSC). This effect was mainly mediated by nicotinic ACh receptors, and through a Ca(2+)-dependent mechanism. Inhibition of extracellular signal-regulated kinases (ERK) abolished the regulation of NMDAR function by AChE inhibitors, suggesting the involvement of ERK. In the transgenic mouse model of AD overexpressing mutant beta-amyloid precursor protein (APP), the effect of AChE inhibitors on NMDAR-EPSC was significantly impaired, which was associated with their diminished effect on ERK activation. Taken together, these results suggest that one of the key targets of endogenous ACh involved in cognition is the NMDAR-mediated transmission. Loss of the regulation of synaptic NMDAR responses by endogenous ACh may contribute to the cognitive deficiency in AD.

Nicotinic component of galantamine in the regulation of amyloid precursor protein processing

Current therapies for Alzheimer's disease treatment rely mainly on acetylcholinesterase inhibitors, improving central cholinergic neurotransmission. Among these molecules, galantamine (GAL) has an interesting pharmacological profile as it is both a reversible acetylcholinesterase inhibitor and an allosteric potentiator of nicotinic cholinergic receptors. We investigated the effect of GAL on the metabolism of the amyloid precursor protein (APP) in differentiated SH-SY5Y neuroblastoma cells. The rationale was based on the suggestion that cholinergic activity may also be involved in the regulation of APP metabolism. We studied the acute effect on APP metabolism measuring the secretion of sAPPalpha in the conditioned medium of cells. Following 2h treatment, GAL 10microM promoted a strong increase in the release of sAPPalpha, the maximal effect approaching on average three-fold baseline value. The compound appeared to increase the release of sAPPalpha, with a mechanism dependent upon an indirect cholinergic stimulation. The effect of GAL was prevented by pre-treatment with alpha-bungarotoxin (40nM) but not low (nanomolar) atropine concentrations, suggesting the specific involvement of nicotinic cholinergic receptors.

Nicotine modulates expression of amyloid precursor protein and amyloid precursor-like protein 2 in mouse brain and in SH-SY5Y neuroblastoma cells

Epidemiological studies indicate that tobacco smoking can be protective against neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). The objective of the present study was to examine the changes in gene expression induced by chronic oral nicotine administration (100 mug/ml in 2% saccharin for 14 days), with special emphasis on amyloid precursor protein (APP) and its homologue, amyloid precursor-like protein 2 (APLP2), in different brain regions of C57BL/6 mice using a pathway-focused microarray. Our results revealed that nicotine stimulated mRNA expression of APP in the amygdala (64%; P = 0.003) and hippocampus (32%; P = 0.034) and of APLP2 in the amygdala (39%; P = 0.002). These results were verified by quantitative real-time RT-PCR except that expression of APLP2 was also significantly upregulated by nicotine in the hippocampus. In addition, in vitro nicotine treatment of SH-SY5Y neuroblastoma cells resulted in a significant increase in expression of APP protein, soluble APP, and APLP2, whereas co-treatment with mecamylamine (an antagonist of nicotinic acetylcholine receptors) attenuated the stimulating effect of nicotine on APP and APLP2 expression. These findings suggest that nicotine treatment facilitates the increase in the expression of mRNA and protein of the APP and APLP2 genes in rat brain and SH-SY5Y neuroblastoma cells.

Attenuation of Abeta deposition in the entorhinal cortex of normal elderly individuals associated with tobacco smoking

Investigating correlates of tobacco smoking provides the only currently available opportunity of examining effects of long-term exposure of nicotinic receptors on a specific nicotinic agonist in human. Alzheimer-type pathology (Abeta and abnormally phosphorylated tau assessed on the basis of AT8 immunoreactivity) together with vascular markers has been compared in age-matched groups of normal elderly smokers and non-smokers in the entorhinal cortex, an area of noted age-related pathology. The density of total Abeta and diffuse Abeta immunoreactivity, together with formic acid-extractable Abeta42 but not Abeta40, was reduced in smokers (n = 10-18) compared with non-smokers (n = 10-20) (P < 0.05). There was also a reduced percentage of cortical and leptomeningeal vessels with associated Abeta immunoreactivity in smokers (n = 13) compared with non-smokers (n = 14) (P < 0.005 and 0.05, respectively). There was a significant inverse correlation between formic acid-extractable Abeta42 and pack years (n = 34, r = -0.389, P = 0.025), with a similar trend for total Abeta immunoreactivity which did not reach statistical significance (n = 30, r = -0.323, P = 0.082). In contrast, there were no significant group differences for vascular markers (collagen IV, alpha-actin or glucose transporter 1), AT8 immunoreactivity or phosphate-buffered saline-soluble Abeta peptides, and no significant associations with gender for any of the measured parameters. These findings are consistent with previously reported reductions in histologically assessed amyloid plaques in aged human brain associated with tobacco use and dramatic lessening of Abeta deposits in APPsw mice after nicotine treatment. Development of nicotinic drugs to protect against beta-amyloidosis as one of the principal pathological hallmarks of brain ageing and Alzheimer's disease is indicated.

Treatment of dementia with neurotransmission modulation

The prevalence of dementia is growing in developed countries where elderly patients are increasing in numbers. Neurotransmission modulation is one approach to the treatment of dementia. Cholinergic precursors, anticholinesterases, nicotine receptor agonists and muscarinic M(2) receptor antagonists are agents that enhance cholinergic neurotransmission and that depend on having some intact cholinergic innervation to be effective in the treatment of dementia. The cholinergic precursor choline alfoscerate may be emerging as a potential useful drug in the treatment of dementia, with few adverse effects. Of the anticholinesterases, donepezil, in addition to having a similar efficacy to tacrine in mild-to-moderate Alzheimer's disease (AD), appears to have major advantages; its use is associated with lower drop-out rates in clinical trials, a lower incidence of cholinergic-like side effects and no liver toxicity. Rivastigmine is efficacious in the treatment in dementia with Lewy bodies, a condition in which the other anticholinesterases have not been tested extensively to date. Galantamine is an anticholinesterase and also acts as an allosteric potentiating modulator at nicotinic receptors to increase the release of acetylcholine. Pooled data from clinical trials of patients with mild-to-moderate AD suggest that the benefits and safety profile of galantamine are similar to those of the anticholinesterases. Selective nicotine receptor agonists are being developed that enhance cognitive performance without influencing autonomic and skeletal muscle function, but these have not yet entered clinical trial for dementia. Unlike the cholinergic enhancers, the M(1) receptor agonists do not depend upon intact cholinergic nerves but on intact M(1) receptors for their action, which are mainly preserved in AD and dementia with Lewy bodies. The M(1) receptor-selective agonists developed to date have shown limited efficacy in clinical trials and have a high incidence of side effects. A major recent advancement in the treatment of dementia is memantine, a non-competitive antagonist at NMDA receptors. Memantine is beneficial in the treatment of severe and moderate-to-severe AD and may also be of some benefit in the treatment of mild-to-moderate vascular dementia. Drugs that modulate 5-HT, somatostatin and noradrenergic neurotransmission are also being considered for the treatment of dementia.

Nicotine reduces A beta in the brain and cerebral vessels of APPsw mice

Ten days treatment with nicotine reduced insoluble amyloid A beta 1-40 and Alpha beta 1-42 peptides by 80% in the cortex of 9-month-old APPsw mice, which is more than that observed in 14.5-month-old mice following nicotine treatment for 5.5 months. A reduction in A beta associated with cerebral vessels was observed in addition to that deposited as parenchymal plaques after 5.5 months treatment. The diminution in A beta peptides observed was not accompanied by changes in brain alpha, beta or gamma secretase-like activities, NGF or BDNF protein expression measured in brain homogenates. A significant increase in sAPP was observed after nicotine treatment of SH-SY5Yneuroblastoma cells that could be blocked by the nicotinic antagonist mecamylamine. Attenuation of elevated [(125)I]-alpha bungarotoxin binding (alpha 7) in APPsw mice was observed after 5.5 months nicotine treatment. Both these observations suggest that the reduction in insoluble A beta by nicotine might be in part mediated via the alpha 7 nicotinic receptor. Further studies are required to identify potential mechanisms of the nicotine's amyloid-reducing effect.

Reduced levels of Abeta 40 and Abeta 42 in brains of smoking controls and Alzheimer's patients

The effects of nicotine on levels of Abeta 40 and Abeta 42 and nicotinic receptor binding sites were studied in brains from nonsmoking and smoking patients with Alzheimer's disease (AD) and aged-matched controls. The levels of soluble and insoluble Abeta 40 and Abeta 42 in frontal cortex and Abeta 40 in temporal cortex and hippocampus were significantly decreased in smoking AD patients compared to nonsmokers with AD. In smoking controls the levels of soluble and insoluble Abeta 40 and Abeta 42 in the frontal and temporal cortex were significantly lower than in nonsmoking controls. The binding of [(3)H]cytisine in temporal cortex was significantly increased in smokers with AD compared to nonsmokers with AD. In smoking controls [(3)H]cytisine and [(3)H]epibatidine binding were significantly increased from 1.5- to 2-fold compared to nonsmoking controls whereas binding sites for [(125)I]alpha-bungarotoxin was less up-regulated. These results indicate that selective nicotinic receptor agonists may be a novel protective therapy in AD by reducing Abeta levels as well as the loss of nicotinic receptors in AD brain.

Stimulation of β-amyloid precursor protein α-processing by phorbol ester involves calcium and calpain activation

Normal processing of Alzheimer's beta-amyloid precursor protein (APP) is markedly stimulated by phorbol esters, but the underlying mechanisms have yet to be fully understood. In this study, we observed that: (a) Phorbol 12,13-dibutyrate (PDBu)-stimulated APP secretion in cultured SH-SY5Y neuroblastoma and fibroblast cells was blocked by EGTA and calpain inhibitors in a concentration-dependent manner, but not by other protease inhibitors. (b) Secretion of fibronectin, another secretory protein tested for comparison, was enhanced by PDBu, but insensitive to calpain inhibitors. (c) PDBu stimulated intracellular calpain activity as measured by the hydrolysis of a fluorogenic calpain substrate. (d) PDBu also induced rapid proteolysis of two endogenous substrates of calpains, i.e., tau and microtubule-associated protein-2 (MAP-2) and the proteolysis was blocked by EGTA and calpain inhibitors. Taken together, these results suggest that stimulation of APP alpha-processing by PDBu is through a mechanism that involves the activation of Ca(2+) and, most notably, calpain. The implications of the findings are discussed in relation to the regulatory mechanism of APP alpha-processing.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Nicotinic receptors in aging and dementia

Activation of neuronal nicotinic acetylcholine receptors (nAChRs) has been shown to maintain cognitive function following aging or the development of dementia. Nicotine and nicotinic agonists have been shown to improve cognitive function in aged or impaired subjects. Smoking has also been shown in some epidemiological studies to be protective against the development of neurodegenerative diseases. This is supported by animal studies that have shown nicotine to be neuroprotective both in vivo and in vitro. Treatment with nicotinic agonists may therefore be useful in both slowing the progression of neurodegenerative illnesses, and improving function in patients with the disease. While increased nicotinic function has been shown to be beneficial, loss of cholinergic markers is often seen in patients with dementia, suggesting that decreased cholinergic function could contribute to both the cognitive deficits, and perhaps the neuronal degeneration, associated with dementia. In this article we will review the literature on each of these areas. We will also present hypotheses that might address the mechanisms underlying the ability of nAChR function to protect against neurodegeneration or improve cognition, two potentially distinct actions of nicotine.

Nicotine and nicotinic receptors; relevance to Parkinson's disease

The development of nicotinic agonists for therapy in neurodegenerative disorders such as Parkinson's disease is an area currently receiving considerable attention. The rationale for such work stems from findings that reveal a loss of nicotinic receptors in Parkinson's disease brains. These results, coupled with reports that nicotine treatment relieves some of the symptoms of this disorder, provides support for the contention that nicotine and/or nicotinic agonists may be beneficial for acute symptomatic treatment. Moreover, the observation that there is a decreased incidence of Parkinson's disease with tobacco use, possibly due to the nicotine in tobacco products, may imply that such drugs are useful for long-term neuroprotection. However, there are multiple nicotinic receptor populations in the brain with different functional properties. Identification of the subtypes involved in nigrostriatal dopaminergic activity is therefore critical for the rational use of selective therapeutic agents for symptomatic treatment and/or neuroprotection. Accumulating evidence, both in rodents and nonhuman primates now indicate that alpha6* nicotinic receptors are present on nigrostriatal dopaminergic neurons, and furthermore, that receptors containing this subunit may be most vulnerable to nigrostriatal damage, at least in nonhuman primates. These data suggest that nicotinic receptor ligands directed to alpha6* nicotinic receptors might be particularly relevant for Parkinson's disease therapeutics.

Nicotine reduces the secretion of Alzheimer's beta-amyloid precursor protein containing beta-amyloid peptide in the rat without altering synaptic proteins

Alzheimer's disease (AD) is characterized by cerebrovascular deposition of the amyloid beta-peptide (A beta), which is derived from a larger beta-amyloid precursor protein (beta APP). Altered metabolism of beta APP, resulting in increased A beta production, appears central in the neuropathology of AD. The processing of the holoprotein beta APP by different "secretase" enzymes results in three major carboxyl-truncated species. One species, which results from the cleavage of beta APP by gamma-secretase, is secreted into the cerebrospinal fluid (CSF) and is called sAPP gamma as it contains an intact A beta domain. Moreover, AD is characterized by cholinergic dysfunction and the loss of synaptic proteins. Reports of an inverse relation between nicotine intake, due to cigarette smoking, and the incidence of AD prompted us to investigate the effects of nicotine on beta APP processing and synaptic proteins in rats and in cell culture. Nicotine, 1 and 8 mg/kg/day, doses commensurate with cigarette smoking, and a higher but well tolerated dose, respectively, was administered over 14 days to rats. Levels of sAPP in the CSF sample were evaluated by Western blot analysis. The higher dose significantly increased levels of total sAPP; however, both doses significantly reduced sAPP gamma, which contains the amyloidogenic portion of A beta. These actions were blocked by nicotinic receptor antagonism. Nicotinic antagonists alone had no effect on either total sAPP or sAPP gamma levels in CSF. Nicotine did not significantly change the intracellular levels of total beta APP in rat brain extracts, which is consistent with neuronal cell culture data. Similarly, levels of vesicular protein, such as synaptophysin, and presynaptic terminal protein SNAP-25 were unaffected by nicotine treatment both in vivo and in cell culture experiments. Taken together, these results suggest that nicotine modifies beta APP processing away from the formation of potentially amyloidogenic products, without altering the levels of synaptic proteins, and that this can potentially offer therapeutic potential for Alzheimer's disease.

Chronic nicotine treatment reduces beta-amyloidosis in the brain of a mouse model of Alzheimer's disease (APPsw)

Alzheimer's disease neuropathology is characterised by beta-amyloid plaques and neurofibrillary tangles. Inhibition of beta-amyloid accumulation may be essential for effective therapy in Alzheimer's disease. In this study we have treated transgenic mice carrying the Swedish mutation of human amyloid precursor protein [Tg(Hu.APP695.K670N-M671L)2576], which develop brain beta-amyloid deposits, with nicotine in drinking fluid (200 microg/mL) from 9-14.5 months of age (5.5 months). A significant reduction in amyloid beta peptide 1-42 positive plaques by more than 80% (p < 0.03) was observed in the brains of nicotine treated compared to sucrose treated transgenic mice. In addition, there was a selective reduction in extractable amyloid beta peptides in nicotine treated mice; cortical insoluble 1-40 and 1-42 peptide levels were lower by 48 and 60%, respectively (p < 0.005), whilst there was no significant change in soluble 1-40 or 1-42 levels. The expression of glial fibrillary acidic protein was not affected by nicotine treatment. These results indicate that nicotine may effectively reduce amyloid beta peptide aggregation in brain and that nicotinic drug treatment may be a novel protective therapy in Alzheimer's disease.

The effects of long-term treatment with metrifonate, a cholinesterase inhibitor, on cholinergic activity, amyloid pathology, and cognitive function in APP and PS1 doubly transgenic mice

Recent studies in cell cultures have shown that modulating the cholinergic activity can influence the processing and metabolism of amyloid precursor protein (APP). To investigate whether acetylcholinesterase inhibitors (ChEIs) could decrease production of amyloid beta-peptide (A(beta)) and slow down the accumulation of A(beta) also in vivo, we chronically administered metrifonate (100 mg/kg, po), a second-generation ChEI, to 7-month-old doubly transgenic APP+PS1 mice and their nontransgenic littermate controls for 7 months. Behavioral studies, including open field test, T maze, and water maze, were conducted after 6 months treatment with metrifonate, and the mice were sacrificed at the age of 14 months for biochemical and histological analyses. The long-term treatment with metrifonate failed to inhibit the marked overproduction and deposition of A(beta) in the APP+PS1 mice; in contrast, it increased both A(beta)40 and A(beta)42 levels in the hippocampus. However, the A(beta)42 to 40 ratio was significantly reduced by the treatment. In addition, the number of amyloid plaques in the hippocampus did not differ between the treatment and the control groups. Tolerance to cholinesterase inhibition might be induced in the mouse brain because the inhibition rate of AChE was attenuated from about 80 to 50% during the experiment in both APP+PS1 and nontransgenic mice. The metrifonate treatment did not affect cognitive testing parameters but reduced swimming speed and locomotor activity in both genotypes. Our results do not support the idea that ChEIs would slow down the progression of amyloid pathology in Alzheimer's disease.

Histochemical study of acute and chronic intraperitoneal nicotine effects on several glycolytic and Krebs cycle dehydrogenase activities in the frontoparietal cortex and subcortical nuclei of the rat brain

The effects of nicotine on the activity of different dehydrogenases in frontoparietal regions and subcortical nuclei of the rat brain have been studied using histochemical methods. Nicotine sulphate was intraperitoneally administered in acute (4 mg/kg/day x 3 days) or chronic (ALZET osmotic pump providing 2 mg/kg/day x 15 days) doses. The enzymes analyzed were glyceraldehyde-3-phosphate, lactate, malate and succinate dehydrogenases (gly3PDH, LDH, MDH, and SDH, respectively). The results demonstrate that chronic as well as acute administration of nicotine produced strong increases in all these enzymatic activities in the superior layers (I, II and III) of the frontoparietal cortex (cingulate, motor and somatosensory regions); but high increases were not seen in the deeper layers of the cortex or in the subcortical nuclei (substantia nigra, caudate-putamen, nucleus accumbens or nucleus basalis magnocellularis). These hyperactivities were produced in brain regions with normally low enzymatic activity (cortex), but not in those with great intensity (subcortical nuclei). The results are in rough agreement with previous reports on nicotine-induced increases in glucose utilization, gly3PDH genic expression and neuronal hyperactivity in the brain cortex; but significant discrepancies between the cortical enzymatic maps and those obtained both in these studies and others on nicotine(N)-receptor localization have been appreciated. The results support the hypothesis that nicotinic cholinergic drugs can have metabolic, long-lasting stimulant effects on cortical neurons at specific points (probably layer III pyramidal cells and structures with alpha7-N-receptors) of the cortical circuits that could be of great interest in improving altered cognitive functions that are present in Alzheimer disease, as well as in other less severe mental disturbances. Mitochondrial hyperfunction should also be evaluated as a possible side-effect (as an oxidative stress inductor) of these kinds of drugs.

Effects of nicotine on APP secretion and Abeta- or CT(105)-induced toxicity

Several lines of evidence indicated that overexpression or aberrant processing of amyloid precursor protein (APP) is causally related to Alzheimer's disease (AD). Amyloid precursor protein is principally cleaved within the amyloid beta protein domain to release a large soluble ectodomain (APPs), known to have a wide range of trophic functions. The central hypothesis guiding this review is that nicotine may play an important role in APP secretion and protection against toxicity induced by APP metabolic fragments (beta-amyloid [Abeta], carboxyl terminal [CT]). Findings from our experiments have shown that nicotine enhances the release of APPs, which has neurotrophic and neuroprotective activities in concentration-dependent (>50 micromol/L) and time-dependent (>2 hours) manners. In addition, pretreatment of nicotine (>10 micromol/L for 24 hours) partially prevented Abeta or CT(105)-induced cytotoxicity in primary cultured neuron cells, and the effects of nicotine-induced protection were inhibited by the pretreatment with a nicotine alpha-bungarotoxin. Nicotine (>10 micromol/L for 24 hours) partially inhibited CT(105)-induced cytotoxicity when PC12 cells was transfected with CT(105). From these results, we proposed that nicotine or nicotinic receptor agonist treatment might improve the cognitive functions not only by supplementation of cholinergic neurotransmission, but also by protecting Abeta- or CT(105)-induced neurotoxicity probably through the increased release of APPs and the activation of nicotinic receptors.

Nicotine and its interaction with beta-amyloid protein: a short review

Two features of Alzheimer's disease (AD) are beta-amyloid protein (betaAP) deposition and a severe cholinergic deficit. beta-Amyloid protein is a 39- to 43-amino acid transmembrane fragment of a larger precursor molecule, amyloid precursor protein. It is a major constituent of senile plaque, a neuropathologic hallmark of AD, and has been shown to be neurotoxic in vivo and in vitro. The cholinergic neurotransmission system is seen as the primary target of AD. However, other systems are also found to show functional deficit. An association between cholinergic deficit and betaAP is suggested by a negative correlation between cigarette smoking and AD. Evidence hitherto suggests that betaAP causes neuronal death possibly via apoptosis by disrupting calcium homeostasis, which may involve direct activation or enhancement of ligand-gated or voltage-dependent calcium channels. Selective second messengers such as protein kinases are triggered that signal neuronal death. Nicotine or acetylcholinesterase inhibitors can partially prevent the neurotoxicity of betaAP in vivo and in vitro. However, the exact mechanism by which nicotine provides its protective effects is not fully understood, but clearly there are protective roles for nicotine. Here, some aspects of betaAP neurotoxicity and nicotinic intervention as a protective agent are discussed.

On neurodegenerative diseases, models, and treatment strategies: lessons learned and lessons forgotten a generation following the cholinergic hypothesis

Life's Journey If life is indeed a journey, then poetry must be the map that reveals all its topographic possibilitiesellipsis while science is the compass that keeps us from getting lost. -R. T. Bartus, Simple Words for Complex Lives, (c) 1998 In the nearly 20 years since the cholinergic hypothesis was initially formulated, significant progress has been achieved. Initial palliative treatments for Alzheimer's disease (AD) have proven beneficial and have gained FDA approval, the use of animal models for studying AD and other neurodegenerative diseases has achieved wider acceptance, and important insight into the potential causes and pathogenic variables associated with various neurodegenerative diseases continues to increase. This paper reviews the current status of the cholinergic hypothesis in the context of continuing efforts to improve upon existing treatments for AD and explores the role that animal models might continue to play. Using the benefit of hindsight, particular emphasis is placed on an analysis of the approaches, strategies, and assumptions regarding animal models that proved useful in developing the initial treatments and those that did not. Additionally, contemporary issues of AD are discussed within the context of the cholinergic hypothesis, with particular attention given to how they may impact the further refinement of animal models, and the development of even more effective treatments. Finally, arguments are presented that, despite the deserved enthusiasm and optimism for identifying means of halting the pathogenesis of AD, a clear need for more effective palliative treatments will continue, long after successful pathogenic treatments are available. This review, therefore, focuses on issues and experiences intended to: (a) facilitate further development and use of animal models for AD and other neurodegenerative diseases, and (b) accelerate the identification of newer, even more effective treatments.

Modulation of beta-amyloid precursor protein processing and tau phosphorylation by acetylcholine receptors

Neurofibrillary lesions and senile plaques that are composed mainly of hyperphosphorylated tau protein and the amyloid-beta peptide derived from the amyloid precursor protein, respectively, are classical hallmarks of Alzheimer's disease. A number of studies strongly suggests that amyloid-beta formation and amyloid depositions are linked to the pathogenesis of Alzheimer's disease. Recent findings suggest that very low concentrations of the amyloid-beta can inhibit various cholinergic neurotransmitter functions independently of apparent neurotoxicity. Many factors have been shown to influence the processing of amyloid precursor protein, including activation of muscarinic and nicotinic receptors. This review focus on some recent studies concerning the regulation of amyloid precursor protein processing and modulation of tau phosphorylation by acetylcholine receptor stimulation and how cholinergic deficits and amyloid-beta might be related to one another.

Nicotine enhances the biosynthesis and secretion of transthyretin from the choroid plexus in rats: implications for beta-amyloid formation

Epidemiological studies indicated that cigarette smoking protects against the development of several neurodegenerative disorders, including Alzheimer's disease (AD). However, the molecular mechanism(s) underlying this is poorly understood. To gain insight into these protective effects, we used differential display PCR (DD-PCR) to amplify RNA from various brain regions of rats self-administering (SA) nicotine compared with yoked-saline controls. We found that the transthyretin (TTR) gene, whose product has been shown to bind to amyloid beta (Abeta) protein and prevent Abeta aggregation, was more abundantly expressed ( approximately 1.5- to 2.0-fold) in the brainstem and hippocampus (areas containing choroid plexus) of nicotine SA rats. Subsequently, quantitative reverse transcription-PCR analysis confirmed these DD-PCR findings and demonstrated that nicotine increased TTR mRNA levels in these regions in a time- and dose-dependent manner. Significantly higher TTR protein concentrations were also detected in the ventricular CSF of nicotine-treated rats. In contrast, no differences either in plasma TTR or in CSF and plasma retinol-binding protein were detected. Immunohistochemical analysis showed that immunoreactive TTR was 41.5% lower in the choroid plexus of nicotine-treated rats compared with the saline controls. On the basis of these data, we speculate that the protective effects of nicotine on the development of AD may be attributable, in part, to the increased biosynthesis and secretion of TTR from the choroid plexus. These findings also point toward new approaches that may take advantage of the potentially novel therapeutic effects of nicotinic agonists in patients with AD.

Mitogen-activated protein kinase is involved in N-methyl-D-aspartate receptor regulation of amyloid precursor protein cleavage

Glutamate is the principal excitatory neurotransmitter in the mammalian brain. Several lines of evidence suggest that glutamatergic hypoactivity exists in the Alzheimer's disease brain, where it may contribute to both brain amyloid burden and cognitive dysfunction. Although metabotropic glutamate receptors have been shown to alter cleavage of the amyloid precursor protein, little attention has been paid to the role of N-methyl-D-aspartate receptors in this process. We now report that activation of N-methyl-D-aspartate receptors in transiently transfected human embryonic kidney 293 cells increases production of the soluble amyloid precursor protein derivative. Moreover, using both pharmacological and gene transfer techniques, we show that this effect is largely due to activation of the mitogen-activated protein kinase cascade, specifically the pathway leading to activation of extracellular signal-regulated protein kinase but not other mitogen-activated protein kinases. These observations further our understanding of the pathways that regulate amyloid precursor protein cleavage, and buttress the notion that regulation of amyloid precursor protein cleavage is critically dependent upon the mitogen-activated protein kinase cascade.

Neuroprotection in Alzheimer's disease - new strategies for treatment

Alzheimer's disease is the most common dementia disorder characterized by multiple pathological changes in the brain leading to a progressive memory loss and other cognitive symptoms producing occupational and social disabilities. Although a great deal of progress has been made in recent years in further understanding the genetic aberrations and patho-physiological processes of Alzheimer's disease there is still no cure of the disease. The transmitter replacement therapy is so far the most explored therapy. Three cholinesterase inhibitors have so far been approved and presently in clinical use in many countries. Although the cholinesterase inhibitors generally appear to produce symptomatic effects with palliative effect on existing cognitive disturbances recent data suggest that they also may have effect on progression of the disease including possible neuroprotective effects. Possible interactions between Abeta and cholinergic neurotransmission may exist. Treatment of cells with Abeta causes decreased cholinergic activity. Pretreatment of PC12 cells with cholinesterase inhibitors such as tacrine and donepezil in clinical relevant concentrations can attenuate Abeta (25-35) toxicity through mechanisms which may be mediated via nicotinic receptors. Estrogen has been shown to protect against Abeta toxicity in different cell lines and also to reduce the formation of Abeta. Its mechanism for the neuroprotective effect is however not fully clarified. A potentiation of the clinical effect of cholinesterase inhibitors in Alzheimer patients has been given together with estrogen. Experimental data suggest that the neuroprotective effect of estrogen as studied in PC12 cells was mediated at least partly via the alpha(7) nicotinic receptor. Treatment with Abeta in nanomolar concentrations for 7 days in PC12 cells significantly decreased the number of nicotinic receptor binding sites and mRNA levels. The effects by Abeta on nicotinic receptors are prevented by nicotine pretreatment. The finding suggests a possible link between Abeta and nicotinic receptor deficits in Alzheimer patients in the early course of the disease.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Regulation of amyloid precursor protein cleavage

Multiple lines of evidence suggest that increased production and/or deposition of the beta-amyloid peptide, derived from the amyloid precursor protein, contributes to Alzheimer's disease. A growing list of neurotransmitters, growth factors, cytokines, and hormones have been shown to regulate amyloid precursor protein processing. Although traditionally thought to be mediated by activation of protein kinase C, recent data have implicated other signaling mechanisms in the regulation of this process. Moreover, novel mechanisms of regulation involving cholesterol-, apolipoprotein E-, and stress-activated pathways have been identified. As the phenotypic changes associated with Alzheimer's disease encompass many of these signaling systems, it is relevant to determine how altered cell signaling may be contributing to increasing brain amyloid burden. We review the myriad ways in which first messengers regulate amyloid precursor protein catabolism as well as the signal transduction cascades that give rise to these effects.