Stable beta-secretase activity and presynaptic cholinergic markers during progressive central nervous system amyloidogenesis in Tg2576 mice

Pharmacogenetic considerations when prescribing cholinesterase inhibitors for the treatment of Alzheimer's disease

INTRODUCTION

Cholinergic dysfunction, demonstrated in the late 1970s and early 1980s, led to the introduction of acetylcholinesterase inhibitors (AChEIs) in 1993 (Tacrine) to enhance cholinergic neurotransmission as the first line of treatment against Alzheimer's disease (AD). The new generation of AChEIs, represented by Donepezil (1996), Galantamine (2001) and Rivastigmine (2002), is the only treatment for AD to date, together with Memantine (2003). AChEIs are not devoid of side-effects and their cost-effectiveness is limited. An option to optimize the correct use of AChEIs is the implementation of pharmacogenetics (PGx) in the clinical practice.

AREAS COVERED

(i) The cholinergic system in AD, (ii) principles of AD PGx, (iii) PGx of Donepezil, Galantamine, Rivastigmine, Huperzine and other treatments, and (iv) practical recommendations.

EXPERT OPINION

The most relevant genes influencing AChEI efficacy and safety are APOE and CYPs. APOE-4 carriers are the worst responders to AChEIs. With the exception of Rivastigmine (UGT2B7, BCHE-K), the other AChEIs are primarily metabolized via CYP2D6, CYP3A4, and UGT enzymes, with involvement of ABC transporters and cholinergic genes (CHAT, ACHE, BCHE, SLC5A7, SLC18A3, CHRNA7) in most ethnic groups. Defective variants may affect the clinical response to AChEIs. PGx geno-phenotyping is highly recommended prior to treatment.

Cardiac sympathetic denervation and synucleinopathy in Alzheimer's disease with brain Lewy body disease

Comorbid Lewy body pathology is very common in Alzheimer’s disease and may confound clinical trial design, yet there is no in vivo test to identify patients with this. Tissue (and/or radioligand imaging) studies have shown cardiac sympathetic denervation in Parkinson’s disease and dementia with Lewy bodies, but this has not been explored in Alzheimer’s subjects with Lewy bodies not meeting dementia with Lewy bodies clinicopathological criteria. To determine if Alzheimer’s disease with Lewy bodies subjects show sympathetic cardiac denervation, we analysed epicardial and myocardial tissue from autopsy-confirmed cases using tyrosine hydroxylase and neurofilament immunostaining. Comparison of tyrosine hydroxylase fibre density in 19 subjects with Alzheimer’s disease/dementia with Lewy bodies, 20 Alzheimer’s disease with Lewy bodies, 12 Alzheimer’s disease subjects without Lewy body disease, 19 Parkinson’s disease, 30 incidental Lewy body disease and 22 cognitively normal without Alzheimer’s disease or Lewy body disease indicated a significant group difference (P < 0.01; Kruskal–Wallis analysis of variance) and subsequent pair-wise Mann–Whitney U tests showed that Parkinson’s disease (P < 0.05) and Alzheimer’s disease/dementia with Lewy bodies (P < 0.01) subjects, but not Alzheimer’s disease with Lewy bodies subjects, had significantly reduced tyrosine hydroxylase fibre density as compared with cognitively normal. Both Parkinson’s disease and Alzheimer’s disease/dementia with Lewy bodies subjects also showed significant epicardial losses of neurofilament protein-immunoreactive nerve fibre densities within the fibre bundles as compared with cognitively normal subjects (P < 0.01) and both groups showed high pathologic alpha-synuclein densities (P < 0.0001). Cardiac alpha-synuclein densities correlated significantly with brain alpha-synuclein (P < 0.001), while cardiac tyrosine hydroxylase and neurofilament immunoreactive nerve fibre densities were negatively correlated with the densities of both brain and cardiac alpha-synuclein, as well as Unified Parkinson’s Disease Rating Scale scores (P < 0.05). The clear separation of Alzheimer’s disease/dementia with Lewy bodies subjects from Alzheimer’s disease and cognitively normal, based on cardiac tyrosine hydroxylase fibre density, is the first report of a statistically significant difference between these groups. Our data do not show significant sympathetic cardiac denervation in Alzheimer’s disease with Lewy bodies, but strongly confirm that cardiac nuclear imaging with a noradrenergic radioligand is worthy of further study as a potential means to separate Alzheimer’s disease from Alzheimer’s disease/dementia with Lewy bodies during life.

NGF-Dependent Changes in Ubiquitin Homeostasis Trigger Early Cholinergic Degeneration in Cellular and Animal AD-Model

Basal forebrain cholinergic neurons (BFCNs) depend on nerve growth factor (NGF) for their survival/differentiation and innervate cortical and hippocampal regions involved in memory/learning processes. Cholinergic hypofunction and/or degeneration early occurs at prodromal stages of Alzheimer's disease (AD) neuropathology in correlation with synaptic damages, cognitive decline and behavioral disability. Alteration(s) in ubiquitin-proteasome system (UPS) is also a pivotal AD hallmark but whether it plays a causative, or only a secondary role, in early synaptic failure associated with disease onset remains unclear. We previously reported that impairment of NGF/TrkA signaling pathway in cholinergic-enriched septo-hippocampal primary neurons triggers "dying-back" degenerative processes which occur prior to cell death in concomitance with loss of specific vesicle trafficking proteins, including synapsin I, SNAP-25 and α-synuclein, and with deficit in presynaptic excitatory neurotransmission. Here, we show that in this in vitro neuronal model: (i) UPS stimulation early occurs following neurotrophin starvation (-1 h up to -6 h); (ii) NGF controls the steady-state levels of these three presynaptic proteins by acting on coordinate mechanism(s) of dynamic ubiquitin-C-terminal hydrolase 1 (UCHL-1)-dependent (mono)ubiquitin turnover and UPS-mediated protein degradation. Importantly, changes in miniature excitatory post-synaptic currents (mEPSCs) frequency detected in -6 h NGF-deprived primary neurons are strongly reverted by acute inhibition of UPS and UCHL-1, indicating that NGF tightly controls in vitro the presynaptic efficacy via ubiquitination-mediated pathway(s). Finally, changes in synaptic ubiquitin and selective reduction of presynaptic markers are also found in vivo in cholinergic nerve terminals from hippocampi of transgenic Tg2576 AD mice, even from presymptomatic stages of neuropathology (1-month-old). By demonstrating a crucial role of UPS in the dysregulation of NGF/TrkA signaling on properties of cholinergic synapses, these findings from two well-established cellular and animal AD models provide novel therapeutic targets to contrast early cognitive and synaptic dysfunction associated to selective degeneration of BFCNs occurring in incipient early/middle-stage of disease.

Calcitonin gene-related peptide (CGRP): A novel target for Alzheimer's disease

Alzheimer's disease (AD) is leading cause of death among older characterized by neurofibrillary tangles, oxidative stress, progressive neuronal deficits, and increased levels of amyloid-β (Aβ) peptides. Cholinergic treatment could be the best suitable physiological therapy for AD. Calcitonin gene-related peptide (CGRP) is a thirty-seven-amino acid regulatory neuropeptide resulting from different merging of the CGRP gene, which also includes adrenomedullin, amylin, calcitonin, intermedin, and calcitonin receptor-stimulating peptide. It is a proof for a CGRP receptor within nucleus accumbens of brain that is different from either the CGRP1 or CGRP2 receptor in which it demonstrates similar high-affinity binding for salmon calcitonin, CGRP, and amylin, a possession which is not shared by any extra CGRP receptors. Binding of CGRP to its receptor increases activated cAMP-dependent pkA and PI3 kinase, resulting in N-terminal fragments that are shown to exert complex inhibitory as well facilitator actions on nAChRs. Fragments such as CGRP1-4, CGRP1-5, and CGRP1-6 rapidly as well as reversibly improve agonist sensitivity of nAChRs without straight stimulating those receptors and produce the Ca2+ -induced intracellular Ca2+ mobilization. Renin-angiotensin-aldosterone system (RAAS)-activated angiotensin-type (AT4) receptor is also beneficial in AD. It has been suggested that exogenous administration of CGRP inhibits infiltration of macrophages and expression of various inflammatory mediators such as NFkB, IL-1b, TNF-α, iNOS, matrix metalloproteinase (MMP)-9, and cell adhesion molecules like intercellular adhesion molecule (ICAM)-1 which attenuates consequence of inflammation in AD. Donepezil, a ChEI, inhibits acetylcholinesterase and produces angiogenesis and neurogenesis, in the dentate gyrus of the hippocampus of WT mice after donepezil administration. However, none of the results discovered in CGRP-knockout mice and WT mice exposed to practical denervation. Therefore, selective agonists of CGRP receptors may become the potential candidates for treatment of AD.

Impaired hippocampal acetylcholine release parallels spatial memory deficits in Tg2576 mice subjected to basal forebrain cholinergic degeneration

The Alzheimer's disease (AD) mouse model Tg2576 overexpresses an AD associated mutant variant of human APP and accumulates amyloid beta (Aβ) in an age-dependent manner. Using the selective cholinergic immunotoxin mu p75-saporin (SAP), we induced a partial basal forebrain cholinergic degeneration (BFCD) in 3 months old male Tg2576 mice to co-express cholinergic degeneration with Aβ overexpression as these characteristics constitutes key hallmarks of AD. At 9 months, SAP lesioned Tg2576 mice were cognitively impaired in two spatial paradigms addressing working memory and mid to long-term memory. Conversely, there was no deterioration of cognitive functioning in sham lesioned Tg2576 mice or wild type littermates (wt) receiving the immunotoxin. At 10 months of age, release of acetylcholine (ACh) was addressed by microdialysis in conscious mice. Scopolamine-induced increases in hippocampal ACh efflux was significantly reduced in SAP lesioned Tg2576 mice compared to sham lesioned Tg2576 mice. Intriguingly, there was no significant difference in ACh efflux between wt treatment groups. Following SAP treatment, choline acetyltransferase activity was reduced in the hippocampus and frontal cortex and the reduction was comparable between groups. Our results suggest that partial BFCD acts collectively with increased levels of Aβ to induce cognitive decline and to compromise cholinergic release. Tg2576 mice with BFCD may constitute a new and suitable AD mouse model to study the interrelations between cholinergic deficits and amsyloid deposition.

Glycogen synthase kinase-3 reduces acetylcholine level in striatum via disturbing cellular distribution of choline acetyltransferase in cholinergic interneurons in rats

Cholinergic interneurons, which provide the main source of acetylcholine (ACh) in the striatum, control the striatal local circuits and deeply involve in the pathogenesis of neurodegenerative diseases. Glycogen synthase kinase-3 (GSK-3) is a crucial kinase with diverse fundamental functions and accepted that deregulation of GSK-3 activity also plays important roles in diverse neurodegenerative diseases. However, up to now, there is no direct proof indicating whether GSK-3 activation is responsible for cholinergic dysfunction. In the present study, with combined intracerebroventricular injection of Wortmannin and GF-109203X, we activated GSK-3 and demonstrated the increased phosphorylation level of microtubule-associated protein tau and neurofilaments (NFs) in the rat striatum. The activated GSK-3 consequently decreased ACh level in the striatum as a result of the reduction of choline acetyltransferase (ChAT) activity. The alteration of ChAT activity was due to impaired ChAT distribution rather than its expression. Furthermore, we proved that cellular ChAT distribution was dependent on low phosphorylation level of NFs. Nevertheless, the cholinergic dysfunction in the striatum failed to induce significant neuronal number reduction. In summary, our data demonstrates the link between GSK-3 activation and cholinergic dysfunction in the striatum and provided beneficial evidence for the pathogenesis study of relevant neurodegenerative diseases.

Proteolytically inactive insulin-degrading enzyme inhibits amyloid formation yielding non-neurotoxic aβ peptide aggregates

Insulin-degrading enzyme (IDE) is a neutral Zn²⁺ peptidase that degrades short peptides based on substrate conformation, size and charge. Some of these substrates, including amyloid β (Aβ) are capable of self-assembling into cytotoxic oligomers. Based on IDE recognition mechanism and our previous report of the formation of a stable complex between IDE and intact Aβ in vitro and in vivo, we analyzed the possibility of a chaperone-like function of IDE. A proteolytically inactive recombinant IDE with Glu111 replaced by Gln (IDEQ) was used. IDEQ blocked the amyloidogenic pathway of Aβ yielding non-fibrillar structures as assessed by electron microscopy. Measurements of the kinetics of Aβ aggregation by light scattering showed that 1) IDEQ effect was promoted by ATP independent of its hydrolysis, 2) end products of Aβ-IDEQ co-incubation were incapable of “seeding” the assembly of monomeric Aβ and 3) IDEQ was ineffective in reversing Aβ aggregation. Moreover, Aβ aggregates formed in the presence of IDEQ were non-neurotoxic. IDEQ had no conformational effects upon insulin (a non-amyloidogenic protein under physiological conditions) and did not disturb insulin receptor activation in cultured cells. Our results suggest that IDE has a chaperone-like activity upon amyloid-forming peptides. It remains to be explored whether other highly conserved metallopeptidases have a dual protease-chaperone function to prevent the formation of toxic peptide oligomers from bacteria to mammals.

Neurodegeneration of the retina in mouse models of Alzheimer's disease: what can we learn from the retina?

Alzheimer's disease (AD) is an age-related progressive neurodegenerative disease commonly found among elderly. In addition to cognitive and behavioral deficits, vision abnormalities are prevalent in AD patients. Recent studies investigating retinal changes in AD double-transgenic mice have shown altered processing of amyloid precursor protein and accumulation of β-amyloid peptides in neurons of retinal ganglion cell layer (RGCL) and inner nuclear layer (INL). Apoptotic cells were also detected in the RGCL. Thus, the pathophysiological changes of retinas in AD patients are possibly resembled by AD transgenic models. The retina is a simple model of the brain in the sense that some pathological changes and therapeutic strategies from the retina may be observed or applicable to the brain. Furthermore, it is also possible to advance our understanding of pathological mechanisms in other retinal degenerative diseases. Therefore, studying AD-related retinal degeneration is a promising way for the investigation on (1) AD pathologies and therapies that would eventually benefit the brain and (2) cellular mechanisms in other retinal degenerations such as glaucoma and age-related macular degeneration. This review will highlight the efforts on retinal degenerative research using AD transgenic mouse models.

Pre- and post-synaptic cortical cholinergic deficits are proportional to amyloid plaque presence and density at preclinical stages of Alzheimer's disease

Amyloid imaging has identified cognitively normal older people with plaques as a group possibly at increased risk for developing Alzheimer's disease-related dementia. It is important to begin to thoroughly characterize this group so that preventative therapies might be tested. Existing cholinotropic agents are a logical choice for preventative therapy as experimental evidence suggests that they are anti-amyloidogenic and clinical trials have shown that they delay progression of mild cognitive impairment to dementia. A detailed understanding of the status of the cortical cholinergic system in preclinical AD is still lacking, however. For more than 30 years, depletion of the cortical cholinergic system has been known to be one of the characteristic features of AD. Reports to date have suggested that some cholinergic markers are altered prior to cognitive impairment while others may show changes only at later stages of dementia. These studies have generally been limited by relatively small sample sizes, long postmortem intervals and insufficient definition of control and AD subjects by the defining histopathology. We, therefore, examined pre- and post-synaptic elements of the cortical cholinergic system in frontal and parietal cortex in 87 deceased subjects, including non-demented elderly with and without amyloid plaques as well as demented persons with neuropathologically confirmed AD. Choline acetyltransferase (ChAT) activity was used as a presynaptic marker while displacement of (3)H-pirenzepine binding by oxotremorine-M in the presence and absence of GppNHp was used to assess postsynaptic M1 receptor coupling. The results indicate that cortical ChAT activity as well as M1 receptor coupling are both significantly decreased in non-demented elderly subjects with amyloid plaques and are more pronounced in subjects with AD and dementia. These findings confirm that cortical cholinergic dysfunction in AD begins at the preclinical stage of disease and suggest that cholinotropic agents currently used for AD treatment are a logical choice for preventative therapy.

Transcriptomic effects of depleted uranium on acetylcholine and cholesterol metabolisms in Alzheimer's disease model

Some heavy metals, or aluminium, could participate in the development of Alzheimer disease (AD). Depleted uranium (DU), another heavy metal, modulates the cholinergic system and the cholesterol metabolism in the brain of rats, but without neurological disorders. The aim of this study was to determine what happens in organisms exposed to DU that will/are developing the AD. This study was thus performed on a transgenic mouse model for human amyloid precursor protein (APP), the Tg2576 strain. The possible effects of DU through drinking water (20 mg/L) over an 8-month period were analyzed on acetylcholine and cholesterol metabolisms at gene level in the cerebral cortex. The mRNA levels of choline acetyl transferase (ChAT) vesicular acetylcholine transporter (VAChT) and ATP-binding cassette transporter A1 (ABC A1) decreased in control Tg2576 mice in comparison with wild-type mice (respectively -89%, -86% and -44%, p < 0.05). Chronic exposure of Tg2576 mice to DU increased mRNA levels of ChAT (+189%, p < 0.05), VAChT (+120%, p < 0.05) and ABC A1 (+52%, p < 0.05) compared to control Tg2576 mice. Overall, these modifications of acetylcholine and cholesterol metabolisms did not lead to increased disturbances that are specific of AD, suggesting that chronic DU exposure did not worsen the pathology in this experimental model.

The history of the cholinergic hypothesis

The cholinergic hypothesis of cognitive impairment and Alzheimer's disease has been for decades a "polar star" for studies on dementia and neurodegenerative diseases. Aim of the present article is to briefly summarize its birth and its evolution throughout years and discoveries. Putting the cholinergic hypothesis in an historical perspective, allows to appreciate the enormous amount of experimental and clinical research that it has stimulated over years and the impressive extent of knowledge generated by this research. While some of the assumptions at the basis of its original formulation are disputable in the light of recent developments, the cholinergic hypothesis has, however, constituted an invaluable stimulus to better understand not only the anatomy and the biochemistry of the cholinergic systems of brain connections but also its developmental biology, its complex relationships with trophic factors, its role in cognitive functions. Thus, rather than being consigned to history, the cholinergic hypothesis will likely contribute to further understanding dementia and neurodegenerative diseases and will hopefully be integrated in novel therapies and treatments.

Neurotransmitters and energy metabolites in amyloid-bearing APP(SWE)xPSEN1dE9 Mouse Brain

Alzheimer's disease is characterized by amyloid peptide formation and deposition, neurofibrillary tangles, synaptic loss and central cholinergic dysfunction, dysfunction of energy metabolism, and dementia; however, the interactions between these hallmarks remain poorly defined. We studied a well characterized mouse model of amyloid deposition, the doubly transgenic APP(SWE)xPSEN1dE9 mouse. At 10 to 14 months of age, these mice had high levels of amyloid peptides (6.6 microg/g wet weight) and widespread amyloid plaques. Extracellular levels of acetylcholine (ACh) were determined by microdialysis in the hippocampus and were comparable with nontransgenic mice from the same colony. In the open field, both mouse strains responded with a 3-fold increase of hippocampal ACh release. Exploratory behavior of the transgenic mice appeared normal. Infusion of scopolamine evoked 5- to 6-fold increases of ACh levels in both mouse strains. High-affinity choline uptake and cholinesterase activities were identical in both mouse lines. Extracellular levels of glucose and glycerol were similar in control and transgenic mice, whereas lactate levels were slightly (p = 0.06) and glutamate levels significantly (p = 0.02) lower in transgenic mice. Exploration caused increases of glucose and lactate, whereas infusion of scopolamine (1 microM) increased glucose but not lactate. Glutamate levels were increased by scopolamine, whereas glycerol remained constant under all the conditions. We conclude that amyloid peptide production and plaque deposition causes minor changes in cholinergic function and energy metabolites in transgenic mice in vivo. Amyloid peptide formation and/or deposition may not be sufficient for long-term cholinergic or metabolic dysfunction.

Decreased acetylcholine release is correlated to memory impairment in the Tg2576 transgenic mouse model of Alzheimer's disease

Acetylcholine (ACh) release is one of the key factors in memory mechanisms. To clarify whether beta-amyloid (Abeta) induces a disturbance of the cholinergic system leading to memory impairment, we examined memory impairment and measured hippocampal ACh release in Tg2576 (Tg) mice that over-express the Swedish mutant amyloid precursor protein (APPsw). Furthermore, we examined Abeta burden with aging. Tg mice aged 9-11 months, but not aged 4-6 months, showed memory impairment in the 8-arm radial maze behavior test. Spontaneous ACh release was not altered in Tg mice compared with age-matched control mice at 4-6 or 9-11 months of age. On the other hand, high-K(+)-evoked ACh release was decreased in Tg mice aged 9-11 months, but not in Tg mice aged 4-6 months. Hippocampal Abeta increased in an age-dependent manner, but evident amyloid plaques were not found in the hippocampus of Tg mice aged 11 months. These results suggest that memory impairment in Tg mice could be attributed to cholinergic synapse dysfunction that could not be caused predominantly by amyloid plaques. Measuring ACh release in this model might be a useful index for the screening of new drugs to treat the early-phase of Alzheimer's disease.

Coexisting cholinergic and parahippocampal degeneration: a key to memory loss in dementia and a challenge for transgenic models?

One century after Alzheimer's initial report, a variety of animal models of Alzheimer's disease (AD) are being used to mimic one or more pathological signs viewed as critical for the evolution of cognitive decline in dementia. Among the most common are, (a) traditional lesion models aimed at reproducing the degeneration of one of two key brain regions affected in AD, namely the cholinergic basal forebrain (CBF) and the transentorhinal region, and (b) transgenic mouse models aimed at reproducing AD histopathological hallmarks, namely amyloid plaques and neurofibrillary tangles. These models have provided valuable insights into the development and consequences of the pathology, but they have not consistently reproduced the severity of memory deficits exhibited in AD. The reasons for this lack of correspondence with the severity of expected deficits may include the limited replication of multiple neuropathology in potentially key brain regions. A recent lesion model in the rat found that severe memory impairment was obtained only when the two traditional lesions were combined together (i.e. conjoint CBF and entorhinal cortex lesions), indicative of a dramatic impact on cognitive function when there is coexisting, rather than isolated, damage in these two brain regions. It is proposed that combining AD transgenic mouse models with additional experimental damage to both the CBF and entorhinal regions might provide a unique opportunity to further understand the evolution of the disease and improve treatments of severe cognitive dysfunction in neurodegenerative dementias.

Calpastatin levels affect calpain activation and calpain proteolytic activity in APP transgenic mouse model of Alzheimer's disease

The intracellular Ca(2+)-dependent protease calpain and the specific calpain endogenous inhibitor calpastatin are widely distributed, with the calpastatin/calpain ratio varying among tissues and species. Increased Ca(2+) and calpain activation have been implicated in Alzheimer's disease (AD), with scant data available on calpastatin/calpain ratio in AD. Information is lacking on calpain activation and calpastatin levels in transgenic mice that exhibit AD-like pathology. We studied calpain and calpastatin in Tg2576 mice and in their wild type littermates (control mice). We found that in control mice calpastatin level varies among brain regions; it is significantly higher in the cerebellum than in the hippocampus, frontal and temporal cortex, whereas calpain levels are similar in all these regions. In the Tg2576 mice, calpain is activated, calpastatin is diminished, and calpain-dependent proteolysis is observed in brain regions affected in AD and in transgenic mice (especially hippocampus). In contrast, no differences are observed between the Tg2576 and the control mice in the cerebellum, which does not exhibit AD-like pathology. The results are consistent with the notion that a high level of calpastatin in the cerebellum renders the calpain in this brain region less liable to be activated; in the other brain parts, in which calpastatin is low, calpain is more easily activated in the presence of increased Ca(2+), and in turn the activated calpain leads to further diminution in calpastatin (a known calpain substrate). The results indicate that calpastatin is an important factor in the regulation of calpain-induced protein degradation in the brains of the affected mice, and imply a role for calpastatin in attenuating AD pathology. Promoting calpastatin expression may be used to ameliorate some manifestations of AD.

The place of choline acetyltransferase activity measurement in the "cholinergic hypothesis" of neurodegenerative diseases

The so-called "cholinergic hypothesis" assumes that degenerative dysfunction of the cholinergic system originating in the basal forebrain and innervating several cortical regions and the hippocampus, is related to memory impairment and neurodegeneration found in several forms of dementia and in brain aging. Biochemical methods measuring the activity of the key enzyme for acetylcholine synthesis, choline acetyltransferase, have been used for many years as a reliable marker of the integrity or the damage of the cholinergic pathways. Stereologic counting of the basal forebrain cholinergic cell bodies, has been additionally used to assess neurodegenerative changes of the forebrain cholinergic system. While initially believed to mark relatively early stages of disease, cholinergic dysfunction is at present considered to occur in advanced dementia of Alzheimer's type, while its involvement in mild and prodromal stages of the disease has been questioned. The issue is relevant to better understand the neuropathological basis of the diseases, but it is also of primary importance for therapy. During the last few years, indeed, cholinergic replacement therapies, mainly based on the use of acetylcholinesterase inhibitors to increase synaptic availability of acetylcholine, have been exploited on the assumption that they could ameliorate the progression of the dementia from its initial stages. In the present paper, we review data from human studies, as well as from animal models of Alzheimer's and Down's diseases, focusing on different ways to evaluate cholinergic dysfunction, also in relation to the time point at which these dysfunctions can be demonstrated, and on some discrepancy arising from the use of different methodological approaches. The reviewed literature, as well as some recent data from our laboratories on a mouse model of Down's syndrome, stress the importance of performing biochemical evaluation of choline acetyltransferase activity to assess cholinergic dysfunction both in humans and in animal models.

Effects of wild-type and mutant human amyloid precursor protein on cortical afferent network

Alzheimer's disease is characterized by severe neuronal disintegration supposed to be partly associated with amyloid pathology. Recently, we described morphological alterations of pyramidal cell structure in transgenic mice expressing wild-type or mutant human amyloid precursor protein (hAPP) (strains B6-Py8.9 and Tg2576), which are unrelated to direct plaque-associated changes. In this study, we focused on the pattern of cortical afferent connections in these transgenic mice. The quantity of cholinergic afferents is increased in both transgenic lines. Glutamatergic intra- and interhemispheric afferents are augmented in B6-Py8.9 mice but decreased in Tg2576 mice. Furthermore, perisomatic inhibition of pyramidal neurons was found to be reduced in Tg2576 mice. Findings suggest different effects of wild-type and mutant hAPP on neuronal connectivity.

Phenserine

Phenserine, a derivative of physostigmine, was first described as an inhibitor of acetylcholinesterase (AChE) and was shown to improve cognition in various experimental paradigms in rodents and dogs. It was clinically tested for Alzheimer's disease, with moderate success in initial Phase II studies. Phenserine deserves attention for an additional quality of action: in addition to inhibiting AChE, it modulates the amount of beta-amyloid precursor protein (APP) in neuronal cell culture by reducing APP translation. This effect probably involves interaction of phenserine with a regulatory element in the 5'-untranslated region of the APP gene that controls APP expression. Phenserine apparently reduces translational efficiency of APP mRNA into protein, a process that may involve an interaction with iron and/or an iron-responsive element. As a consequence, phenserine reduces beta-amyloid peptide (Abeta) formation in vitro and in vivo. Phenserine is also unique because of differing actions of its enantiomers: (-)-phenserine is the active enantiomer for inhibition of AChE, whereas (+)-phenserine ('posiphen') has weak activity as an AChE inhibitor and can be dosed much higher. Both enantiomers are equipotent in downregulating APP expression. (+)-Posiphen may be a promising drug, either alone or in combination with (-)-phenserine, to attenuate the progression of Alzheimer's disease.

Impairment of muscarinic transmission in transgenic APPswe/PS1dE9 mice

We assessed the integrity of cholinergic neurotransmission in parietal cortex of young adult (7 months) and aged (17 months) transgenic APPswe/PS1dE9 female mice compared to littermate controls. Choline acetyltransferase and acetylcholinesterase activity declined age-dependently in both genotypes, whereas both age- and genotype-dependent decline was found in butyrylcholinesterase activity, vesicular acetylcholine transporter density, muscarinic receptors and carbachol stimulated binding of GTP gamma S in membranes as a functional indicator of muscarinic receptor coupling to G-proteins. Notably, vesicular acetylcholine transporter levels and muscarinic receptor-G-protein coupling were impaired in transgenic mice already at the age of 7 months compared to wild type littermates. Thus, brain amyloid accumulation in this mouse model is accompanied by a serious deterioration of muscarinic transmission already before the mice manifest significant cognitive deficits.

Transcriptional and translational regulation of BACE1 expression--implications for Alzheimer's disease

The proteolytical processing of the amyloid precursor protein (APP) gives rise to beta-amyloid peptides, which accumulate in brains of Alzheimer's disease (AD) patients. Different soluble or insoluble higher molecular weight forms of beta-amyloid peptides have been postulated to trigger a complex pathological cascade that may cause synaptic dysfunction, inflammatory processes, neuronal loss, cognitive impairment, and finally the onset of the disease. The generation of beta-amyloid peptides requires the proteolytical cleavage of APP by an aspartyl protease named beta-site APP-cleaving enzyme 1 (BACE1). The expression and enzymatic activity of BACE1 are increased in brains of AD patients. Here we discuss the importance of a number of recently identified transcription factors as well as post-transcriptional modifications and activation of intracellular signaling molecules for the regulation of BACE1 expression in brain. Importantly, some of these factors are known to be involved in the inflammatory and chronic stress responses of the brain, which are compromised during aging. Moreover, recent evidence indicates that beneficial effects of non-steriodal anti-inflammatory drugs on the progression of AD are mediated--at least in part--by effects on the peroxisome proliferator-activated receptor-gamma response element present in the BACE1 promoter. The identification of the cell type-specific expression and activation of NF-kappaB, Sp1 and YY1 transcription factors may provide a basis to specifically interfere with BACE1 expression and, thereby, to lower the concentrations of beta-amyloid peptides, which may prevent neuronal cell loss and cognitive decline in AD patients.

Altered synaptic function in Alzheimer's disease

Alzheimer's disease is the leading cause of dementia in the elderly, presenting itself clinically by progressive loss of memory and learning. Since synaptic density correlates more closely with cognitive impairment than any other pathological lesion observable in the disease pathology, an increased understanding of the mechanisms behind synaptic disconnection is of vital importance. Our lab investigated the neurotransmitter-specific status of distinct cortical presynaptic bouton populations in various transgenic mouse models of the Alzheimer's-like amyloid pathology in order to assess their involvement throughout the progression of the pathology. These studies have revealed that the amyloid pathology appears to progress in a neurotransmitter-specific manner where the cholinergic terminals appear most vulnerable, followed by the glutamatergic terminals and finally by the somewhat more resilient GABAergic terminals. This review will discuss additional studies which also provide evidence of a neurotransmitter-specific pathology as well as comment on the potential explanations for the observed vulnerabilities, touching upon metabolic demand, trophic support and receptor mediated activation.

Basal forebrain cholinergic dysfunction in Alzheimer's disease--interrelationship with beta-amyloid, inflammation and neurotrophin signaling

Alzheimer's disease, the most common neurodegenerative disorder of senile dementia, is characterized by two major morpho-pathological hallmarks. Deposition of extracellular neuritic, beta-amyloid peptide-containing plaques (senile plaques) in cerebral cortical regions of Alzheimer patients is accompanied by the presence of intracellular neurofibrillary tangles in cerebral pyramidal neurons. Basal forebrain cholinergic dysfunction is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed in patients with Alzheimer's disease. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology in Alzheimer's disease by affecting expression and processing of the beta-amyloid precursor protein (APP). Vice versa, low level of soluble beta-amyloid has been observed to inhibit cholinergic synaptic function. Deposition of beta-amyloid plaques in Alzheimer's disease is also accompanied by a significant plaque-associated glial up-regulation of interleukin-1, which has been attributed to affect expression and metabolism of APP and to interfere with cholinergic transmission. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, as well as local inflammatory upregulation, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Brain cholinergic vulnerability: Relevance to behavior and disease

The major populations of cholinergic neurons in the brain include two "projection" systems, located in the pontine reticular formation and in the basal forebrain. These two complexes comprise, in part, the anatomical substrates for the "ascending reticular activating system" (ARAS). The pontine cholinergic system relays its rostral influences mainly through thalamic intralaminar nuclei, but it also connects to the basal forebrain and provides a minor innervation of cortex. The basal forebrain cholinergic complex (BFCC) projects directly to cortex and hippocampus, and has a minor connection with the thalamus. Recent data reveal that a parallel system of basal forebrain GABAergic projection neurons innervates cortex/hippocampus in a way that seems to complement the BFCC. Generally, the picture developed from more than 50 years of research is consistent with a "global" influence of these two ascending cholinergic projections on cortical and hippocampal regions. Seemingly, the BFCC acts in tandem or in parallel with the pontine cholinergic projection to activate the electro-encephalogram, increase cerebral blood flow, regulate sleep-wake cycling, and modulate cognitive function. There are quite a number and variety of human brain conditions, notably including Alzheimer's disease, in which degeneration of basal forebrain cholinergic neurons has been documented. Whether the corticopetal GABA system is affected by disease has not been established. Studies of degeneration of the pontine projection are limited, but the available data suggest that it is relatively preserved in Alzheimer's disease. Hypotheses of BFCC degeneration include growth factor deprivation, intracellular calcium dysfunction, amyloid excess, inflammation, and mitochondrial abnormalities/oxidative stress. But, despite considerable research conducted over several decades, the exact mechanisms underlying brain cholinergic vulnerability in human disease remain unclear.

Interactions between beta-amyloid and central cholinergic neurons: implications for Alzheimer's disease

Alzheimer's disease is an age-related neurodegenerative disorder that is characterized by a progressive loss of memory and deterioration of higher cognitive functions. The brain of an individual with Alzheimer's disease exhibits extracellular plaques of aggregated beta-amyloid protein (Abeta), intracellular neurofibrillary tangles that contain hyperphosphorylated tau protein and a profound loss of basal forebrain cholinergic neurons that innervate the hippocampus and the neocortex. Abeta accumulation may trigger or contribute to the process of neurodegeneration. However, the mechanisms whereby Abeta induces basal forebrain cholinergic cell loss and cognitive impairment remain obscure. Physiologically relevant concentrations of Abeta-related peptides have acute, negative effects on multiple aspects of acetylcholine (ACh) synthesis and release, without inducing toxicity. These data suggest a neuromodulatory influence of the peptides on central cholinergic functions. Long-term exposure to micromolar Abeta induces cholinergic cell toxicity, possibly via hyperphosphorylation of tau protein. Conversely, activation of selected cholinergic receptors has been shown to alter the processing of the amyloid precursor protein as well as phosphorylation of tau protein. A direct interaction between Abeta and nicotinic ACh receptors has also been demonstrated. This review addresses the role of Abeta-related peptides in regulating the function and survival of central cholinergic neurons and the relevance of these effects to cholinergic deficits in Alzheimer's disease. Understanding the functional interrelations between Abeta peptides, cholinergic neurons and tau phosphorylation will unravel the biologic events that precede neurodegeneration and may lead to the development of more effective pharmacotherapies for Alzheimer's disease.

Transgenic mice overexpressing amyloid beta protein are an incomplete model of Alzheimer disease

We compared lesions in elderly transgenic (tg) mice carrying the Swedish double mutation KM670/671NL with lesions in Alzheimer disease (AD) by histochemical and immunohistochemical techniques. Highly similar staining for beta-amyloid protein (Abeta) was observed in AD and the mouse models. The abundant amyloid deposits in tg mice were in a consolidated state as revealed by strong Congo red birefringence. In both tg mice and AD, amyloid deposits were ApoE-positive and were surrounded by activated astrocytes. However, Bielschowsky silver staining and immunostaining with tau antibodies revealed no neurofibrillary tangles (NFTs) in the mice as opposed to abundant NFTs in AD. The microglial pattern was also distinctly different. Tg mice had only weakly activated microglia, which expressed low levels of the complement receptor CD11b. They were gathered around the periphery of the deposits. In contrast, AD lesions had strongly activated microglia, which expressed high levels of CD11b. They were associated with the plaque core. Immunostaining for complement proteins was weak in tg mice but very strong in AD deposits. We conclude that the weak inflammatory response and absence of NFTs indicate that tg mice are only a limited model of AD. Therapeutic strategies for the treatment of AD based on tg mouse models that overexpress Abeta may be limited in their application.

Alterations in cholinergic and non-cholinergic neurotransmitter receptor densities in transgenic Tg2576 mouse brain with beta-amyloid plaque pathology

Cholinergic deficits in Alzheimer's disease are accompanied by a number of alterations in other transmitter systems including glutamate, noradrenaline and serotonin, suggesting the involvement also of other neurotransmitter systems in the pathogenesis of the disease. To address the question whether beta-amyloid may contribute to these deficits, brain tissue from transgenic Tg2576 mice with Alzheimer plaque pathology at ages of 5 (still no significant plaque load) and 17 months (moderate to high cortical beta-amyloid plaque load) were examined for a number of cholinergic and non-cholinergic markers. Transgenic mice with no significant plaque load demonstrated reduced hemicholinium-3 (HCh-3) binding to choline uptake sites in anterior brain regions as compared to non-transgenic littermates, while in aged transgenic mice with high number of plaque deposits decreased HCh-3 binding levels were accompanied by increased vesicular acetylcholine transporter binding in selected cortical brain regions. In aged transgenic mice GABA(A), NMDA, AMPA, kainate, and beta-adrenergic as well 5-HT(1A)- and 5-HT(2A)-receptor binding levels were hardly affected, whereas alpha(1)- and alpha(2)-adrenoceptor binding was increased in selected cerebral cortical regions as compared to non-transgenic littermates. The development of changes in both cholinergic and non-cholinergic markers in transgenic Tg2576 mouse brain already before the onset of progressive plaque deposition provides in vivo evidence of a modulatory role of soluble beta-amyloid on cortical neurotransmission and may be referred to the deficits in learning and memory observed in these mice also before significant plaque load.

Injection of okadaic acid into the meynert nucleus basalis of rat brain induces decreased acetylcholine level and spatial memory deficit

Abnormal hyperphosphorylation of tau and cholinergic deficit occur in the early stage of Alzheimer's disease (AD) and relate to the dementia symptom. Hyperphosphorylation of tau, neurofilament (NF) and other proteins in AD brain appears to be caused by a down-regulation of protein phosphatase 2A (PP2A), but the mechanism leading to cholinergic deficit is still unknown. In this study, we selectively inhibited PP2A by injection of okadaic acid (OA) into the Meynert nucleus basalis of rats. We found that injection of OA induced hyperphosphorylation of tau and NF and decreased acetylcholine (ACh) level in the nucleus basalis of Meynert. These alterations were accompanied by spatial memory deficit in OA-injected rats. We also demonstrated that the OA-induced ACh reduction may be due to a failure of intraneuronal transport of choline acetyltransferase (ChAT) from cell body to the neuronal terminals rather than an alteration of activity of ChAT or acetylcholinesterase. This study suggests that a down-regulation of PP2A may underlie both abnormal hyperphosphorylation of cytoskeletal proteins leading to neurofibrillary degeneration and cholinergic deficiency in AD.

Central cholinergic functions in human amyloid precursor protein knock-in/presenilin-1 transgenic mice

Alzheimer's disease is characterized by amyloid peptide formation and deposition, neurofibrillary tangles, central cholinergic dysfunction, and dementia; however, the relationship between these parameters is not well understood. We studied the effect of amyloid peptide formation and deposition on central cholinergic function in knock-in mice carrying the human amyloid precursor protein (APP) gene with the Swedish/London double mutation (APP-SL mice) which were crossbred with transgenic mice overexpressing normal (PS1wt) or mutated (M146L; PS1mut) human presenilin-1. APP-SLxPS1mut mice had increased levels of Abeta peptides at 10 months of age and amyloid plaques at 14 months of age while APP-SLxPS1wt mice did not have increased peptide levels and did not develop amyloid plaques. We used microdialysis in 15-27 months old mice to compare hippocampal acetylcholine (ACh) levels in the two mouse lines and found that extracellular ACh levels were slightly but significantly reduced in the APP-SLxPS1mut mice (-26%; P=0.044). Exploratory activity in the open field increased hippocampal ACh release by two-fold in both mouse lines; total and relative increases were not significantly different for the two strains under study. Similarly, infusion of scopolamine (1 microM) increased hippocampal ACh release to a similar extent (3-5-fold) in both groups. High-affinity choline uptake, a measure of the ACh turnover rate, was identical in both mouse lines. Neurons expressing choline acetyltransferase were increased in the septum of APP-SLxPS1mut mice (+26%; P=0.046). We conclude that amyloid peptide production causes a small decrease of extracellular ACh levels. The deposition of amyloid plaques, however, does not impair stimulated ACh release and proceeds without major changes of central cholinergic function.

Prevention of age-related spatial memory deficits in a transgenic mouse model of Alzheimer's disease by chronic Ginkgo biloba treatment

Alzheimer's disease (AD) is characterized by cognitive decline and deposition of beta-amyloid (Abeta) plaques in cortex and hippocampus. A transgenic mouse AD model (Tg2576) that overexpresses a mutant form of human Abeta precursor protein exhibits age-related cognitive deficits, Abeta plaque deposition, and oxidative damage in the brain. We tested the ability of Ginkgo biloba, a flavonoid-rich antioxidant, to antagonize the age-related behavioral impairment and neuropathology exhibited by Tg2576 mice. At 8 months of age, 16 female Tg2576 and 15 female wild-type (wt) littermate mice were given ad lib access to tap water or Ginkgo biloba (70 mg/kg/day in water). After 6 months of treatment, all mice received Morris water maze training (4 trials/day for 10 days) to assess hippocampal dependent spatial learning. All mice received a 60-s probe test of spatial memory retention 24 h after the 40th trial. Untreated Tg2576 mice exhibited a spatial learning impairment, relative to wt mice, while Ginkgo biloba-treated Tg2576 mice exhibited spatial memory retention comparable to wt during the probe test. Spatial learning was not different between Ginkgo biloba-treated and untreated wt mice. There were no group differences in learning to swim to a visible platform. Soluble Abeta and hippocampal Abeta plaque burden did not differ between the Tg2576 groups. Brain levels of protein carbonyls were paradoxically elevated in Ginkgo biloba-treated mice. These data indicate that chronic Ginkgo biloba treatment can block an age-dependent decline in spatial cognition without altering Abeta levels and without suppressing protein oxidation in a transgenic mouse model of AD.

Neurotoxic mechanisms caused by the Alzheimer's disease-linked Swedish amyloid precursor protein mutation: oxidative stress, caspases, and the JNK pathway

Autosomal dominant forms of familial Alzheimer's disease (FAD) are caused by mutations of the amyloid precursor protein (APP) gene and by mutations of the genes encoding for presenilin 1 or presenilin 2. Simultaneously, evidence is provided that increased oxidative stress might play a crucial role in the rapid progression of the Swedish FAD. Here we investigated the effect of the Swedish double mutation (K670M/N671L) in the beta-amyloid precursor protein on oxidative stress-induced cell death mechanisms in PC12 cells. Western blot analysis and cleavage studies of caspase substrates revealed an elevated activity of the executor caspase 3 after treatment with hydrogen peroxide in cells containing the Swedish APP mutation. This elevated activity is the result of the enhanced activation of both intrinsic and extrinsic apoptosis pathways, including activation of caspase 2 and caspase 8. Furthermore, we observed an enhanced activation of JNK pathway and an attenuation of apoptosis by SP600125, a JNK inhibitor, through protection of mitochondrial dysfunction and reduction of caspase 9 activity. Our findings provide evidence that the massive neurodegeneration in early age of FAD patients could be a result of an increased vulnerability of neurons through activation of different apoptotic pathways as a consequence of elevated levels of oxidative stress.

Differential degradation of amyloid beta genetic variants associated with hereditary dementia or stroke by insulin-degrading enzyme

Inherited amino acid substitutions at position 21, 22, or 23 of amyloid beta (Abeta) lead to presenile dementia or stroke. Insulin-degrading enzyme (IDE) can hydrolyze Abeta wild type, yet whether IDE is capable of degrading Abeta bearing pathogenic substitutions is not known. We studied the degradation of all of the published Abeta genetic variants by recombinant rat IDE (rIDE). Monomeric Abeta wild type, Flemish (A21G), Italian (E22K), and Iowa (D23N) variants were readily degraded by rIDE with a similar efficiency. However, proteolysis of Abeta Dutch (E22Q) and Arctic (E22G) was significantly lower as compared with Abeta wild type and the rest of the mutant peptides. In the case of Abeta Dutch, inefficient proteolysis was related to a high content of beta structure as assessed by circular dichroism. All of the Abeta variants were cleaved at Glu3-Phe4 and Phe4-Arg5 in addition to the previously described major sites within positions 13-15 and 18-21. SDS-stable Abeta dimers were highly resistant to proteolysis by rIDE regardless of the variant, suggesting that IDE recognizes a conformation that is available for interaction only in monomeric Abeta. These results raise the possibility that upregulation of IDE may promote the clearance of soluble Abeta in hereditary forms of Abeta diseases.

Protective effect of the heparin-derived oligosaccharide C3, on AF64A-induced cholinergic lesion in rats

Alzheimer's disease (AD) literature indicates that glycosaminoglycans (GAGs) may prevent proteoglycan-induced amyloid-beta (Abeta) aggregation, decrease Abeta-induced tau-2 immunoreactivity, and increase the axonal growth and arborization of hippocampal neurons. However, there is no information about the impact of GAGs on cholinergic lesions. Here, AF64A was administered stereotaxically into the lateral ventricles of rats, at doses that are selective for cholinotoxicity (1 and 2 nmol). The heparin-derived oligosaccharide (HDO), C3 (25mg/kg), was administered orally, once daily for 7 days before, and 7 days after AF64A administration. Choline acetyltransferase (ChAT) immunohistochemistry revealed that C3 administration reduced AF64A-induced cholinergic damage in the septum and cingulum bundle. Quantitative neuronal cell counts showed that C3 attenuated, by 60%, the decrease in cell number in the medial septum. Enzyme analysis showed that C3 also significantly restored ChAT (30%) and acetylcholinesterase (AChE) enzyme activity (45%), which had been diminished by AF64A. Our data suggest that, in addition to its effects of anti-Abeta aggregation, anti-Abeta-induced tau-2 immunoreactivity, and neurotrophic effects, C3 also effectively reduces AF64A-induced cholinergic damage; hence it may have potential therapeutic value in AD patients.

Lithium inhibits Abeta-induced stress in endoplasmic reticulum of rabbit hippocampus but does not prevent oxidative damage and tau phosphorylation

The goal of this study was to assess the in vivo effect of Abeta on apoptosis pathways involving the endoplasmic reticulum and mitochondria, and its relationship to the induction of tau phosphorylation and DNA oxidative damage. In rabbits treated intracisternally with aggregated Abeta(1-42), clear evidence of endoplasmic reticulum stress was observed by the activation of caspase-12 and cleavage of caspase-3 in the endoplasmic reticulum. Mitochondrial injury was evident from the release of cytochrome c into the cytosol and the induction of oxidized mitochondrial DNA. Tau phosphorylation and nuclear translocation of NF-kappaB and GSK-3beta were also observed. Treatment with lithium, an inhibitor of GSK-3beta, inhibited caspase activation but did not prevent mitochondrial DNA damage or tau hyperphosphorylation, suggesting that the translocation of GSK-3beta may represent an upstream event that leads to caspase activation but is unrelated to tau hyperphosphorylation or mitochondrial DNA oxidative damage. We propose that treatment by lithium alone is not sufficient to protect against the multiple adverse effects of Abeta, and the use of agents that prevent oxidative DNA damage and tau hyperphosphorylation, together with lithium, may provide better protection from the neurotoxic effect of Abeta.

Overexpression of hAPPswe impairs rewarded alternation and contextual fear conditioning in a transgenic mouse model of Alzheimer's disease

One of the hallmarks of the pathology in Alzheimer's disease is the deposition of amyloid plaques throughout the brain, especially within the hippocampus and amygdala. Transgenic mice that overexpress the Swedish mutation of human amyloid precursor protein (hAPPswe; Tg2576) show age-dependent memory deficits in hippocampus-dependent learning tasks. However, the performance of aged Tg2576 mice in amygdala-dependent learning tasks has not been thoroughly assessed. We trained young (2-4 mo) and old (16-18 mo) Tg2576 and wild-type mice in a T-maze alternation task (hippocampus-dependent) and a Pavlovian fear-conditioning task (amygdala- and hippocampus-dependent). As previously reported, old Tg2576 mice showed impaired acquisition of rewarded alternation; none of these mice reached the criterion of at least five out of six correct responses over three consecutive days. In contrast, old Tg2576 mice showed normal levels of conditional freezing to an auditory conditional stimulus (CS) and acquired a contextual discrimination normally. However, when the salience of the fear-conditioning context was decreased, old (12-14 mo) Tg2576 mice were impaired at acquiring fear to the conditioning context, but not to the tone CS. Histological examination of a subset of the mice verified the existence of amyloid plaques in the cortex, hippocampus, and amygdala of old, but not young, Tg2576 mice. Hence, learning and memory deficits in old Tg2576 mice are limited to hippocampus-dependent tasks, despite widespread amyloid deposition in cortex, hippocampus, and amygdala.

Inherited dementias

Dementia, defined as progressive cognitive decline, is a feature of a wide variety of genetic disorders. For example, a search of "dementia" in the Online Mendelian Inheritance in Man (www.ncbi.nlm.nih.gov/Omim) reveals 162 entries. Therefore this article cannot be encyclopedic and will be necessarily restricted to more prevalent or illustrative etiologies of familial dementia in adults. These disorders also have in common an initial and primarily dementing clinical presentation. Thus, this article is limited to: familial Alzheimer's disease (AD) and related amyloid angiopathies, frontotemporal dementias (FTD) and related tauopathies, familial prion diseases, British and Danish familial dementias, and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL).