Amyloidogenesis in Alzheimer's disease: basic biology and animal models

Changes in gamma-secretase activity and specificity caused by the introduction of consensus aspartyl protease active motif in Presenilin 1

Presenilin (PS1 or PS2) is an essential component of the active γ-secretase complex that liberates the Aβ peptides from amyloid precursor protein (APP). PS1 is regarded as an atypical aspartyl protease harboring two essential aspartic acids in the context of the sequence D257LV and D385FI, respectively, rather than the typical DTG...DTG catalytic motif of classical aspartyl proteases. In the present studies, we introduced the sequence DTG in PS1 at and around the catalytic D257 and D385 residues to generate three PS1 mutants: D257TG, D385TG, and the double-mutant D257TG/D385TG. The effects of these changes on the γ-secretase activity in the presence or absence of γ-secretase inhibitors and modulators were investigated. The results showed that PS1 mutants having D385TG robustly enhanced Aβ₄₂ production compared to the wild type (wt), and were more sensitive than wt to inhibition by a classical aspartyl protease transition state mimic, and fenchylamine, a sulfonamide derivative. Unlike wt PS1 and some of its clinical mutants, all three PS1 artificial mutants decreased cleavage of Notch S3-site, suggesting that these artificial mutations may trigger conformational changes at the substrate docking and catalytic site that cause alteration of substrate specificity and inhibition pattern. Consistent with this notion, we have found that NSAID enzymatic inhibitors of COX, known modulators of the γ-secretase activity, cause PS1 mutants containing D385TG to produce higher levels of both Aβ₃₈ and Aβ₄₂, but to reduce levels of Aβ₃₉, showing a pattern of Aβ formation different from that observed with wild type PS1 and its clinical mutants. This study provides an important structural clue for the rational design of drugs to inhibit processing of APP at the γ-site without interfering with Notch processing.

Regulation of amyloid precursor protein secretion by glutamate receptors in human Ntera 2 neurons

The amyloid precursor protein (APP) can be cleaved by a beta-secretase to generate a beta-amyloid peptide, which has been implicated in the pathogenesis of Alzheimer's disease. However, APP can also be cleaved by an alpha-secretase to form a non-amyloidogenic secreted form of APP (APP-S). APP-S secretion can be physiologically regulated. This study examined the glutamatergic regulation of APP in the human neuronal Ntera 2 (NT2N) cell line. Metabotropic glutamate receptor subtypes 1alpha/beta and 5alpha were identified in the NT2N neurons by reverse transcription-polymerase chain reaction. Stimulation of these phosphatidylinositol-linked receptors with glutamate or specific receptor agonists resulted in a dose- and time-dependent increase in the secretion of the amyloid precursor protein (APP-S), measured by the immunoprecipitation of APP-S from the medium of [35S]methionine-labeled NT2N neurons. The glutamate-induced APP-S secretion was maximal at 30 min and at a concentration of 1 mM glutamate. Glutamate-induced APP-S secretion required activation of phospholipase C, which resulted in inositol 1, 4,5-trisphosphate production, as shown by the rapid glutamate-induced accumulation of inositol 1,4,5-trisphosphate. Glutamate also caused an increase in intracellular Ca2+. The protein kinase C activator phorbol 12-myristate 13-acetate, a phorbol ester, as well as 1-oleoyl-2-acetoyl-3-glycerol, a cell-permeable diacylglycerol analog, also stimulated APP-S secretion. These findings suggest that APP-S secretion from NT2N neurons can be regulated by the activation of phosphatidylinositol-linked metabotropic glutamate receptor signaling pathway.

Cis-acting human ApoE tissue expression element is associated with human pattern of intraneuronal ApoE in transgenic mice

Apolipoprotein E polymorphic variants (ApoE-epsilon2, epsilon3, and epsilon4) are associated with the age of onset distribution and risk of Alzheimer disease. The question of whether ApoE is expressed at a comparatively low level in human neurons compared to astrocytes, or whether ApoE enters neuronal cytoplasm via altered endosomal metabolism is important to understanding potential pathogenic roles for ApoE as a susceptibility gene in Alzheimer disease. ApoE deficient ("knock-out") mice received large human genomic DNA fragment transgenes for each of the three common apoE alleles. All transgenic mice demonstrated glial/astrocytic (normal rodent pattern), as well as cortical intraneuronal ApoE immunoreactivity with all three human isoforms and at multiple ApoE human allele doses (Xu et al. (32)). To test whether ApoE intraneuronal immunoreactivity was due to ApoE gene sequences between mouse and human, we examined another set of mice constructed using targeted replacement so that the human ApoE gene was placed under mouse gene promoters. Current analyses show that targeted replacement recombinant mice show normal rodent glial expression pattern, but no ApoE neuronal immunoreactivity through six months of age compared to large human genomic DNA fragment transgenic mice, which show neuronal content of ApoE throughout adult life. We conclude that cis-acting DNA sequences, rather than the specific sequence of the ApoE gene, may be responsible for low levels of transcription activity in cortical neurons.

Altered expression of amyloid protein precursor mRNA in the rat hippocampus following trimethyltin intoxication: an in situ hybridization study

Previous studies have reported increased levels of amyloid protein precursor (APP) and APP mRNA in the hippocampus and basal forebrain of patients with Alzheimer's disease. Similar changes have been found in the brains of aged rodents and transgenic mice. It now appears that alterations in the expression of individual isoforms of APP mRNA may have a role to play in amyloid-pathogenesis. Here we examined the effect of acute administration of the limbic system neurotoxin trimethyltin (TMT) (8 mg/kg i.p.) on APP-751 and APP-695 mRNA expression in the rat hippocampus (CA1, CA2, CA3 and CA4) using in situ hybridization techniques. We found that following TMT treatment the expression of APP-751 mRNA was increased in CA1 pyramidal cells while that of APP-695 mRNA remained unchanged. TMT also increased the numbers of APP-751 and APP-695 mRNA positively hybridized cells in the CA1 pyramidal layer. These findings suggest that an alteration in APP mRNA expression is involved in the response of the rodent brain to TMT intoxication.

Beta-amyloid(1-40) effects on behavior and memory

Beta amyloid 1-40 is a primary protein in plaques found in the brains of patients with Alzheimer's disease. There is evidence that unaggregated soluble beta-amyloid may be neurotoxic and may have behavioral effects on some types of memory. In the current study, the 1-40 fragment of beta-amyloid protein (beta A4), or vehicle, was bilaterally injected into the rostral hippocampus of rats performing under stable food-maintained schedules of reinforcement or under a delayed conditional discrimination procedure. Under the first procedure, rats were trained to stability under a multiple fixed interval 15 s, fixed ratio 30 reinforcement schedule. This reinforcement schedule has proven sensitive to low-dose drug effects. Acute bilateral hippocampal beta A4 (1.0, 2.0 and 3.0 microliters of 10(-3) M) administration did not significantly alter responding, compared to vehicle, under either reinforcement condition. Following the acute single-injection regimen, rats were administered chronic daily beta A4 (1 microliter of 10(-3) M), bilaterally, for 15 days. No significant changes in lever-pressing performance were observed during the chronic injection regimen, but performance declined significantly 30 days after termination of the chronic daily regimen. Histological examination revealed three of six rats showed positive reactions under Thioflavin S staining in and around the area of cannulae termination. The second assessment employed a delayed conditional discrimination procedure to evaluate the effects of intrahippocampal injections of beta A4 on short-term working memory. This conditional discrimination procedure assesses appropriate responding, dependent on a previously presented stimulus, after delays of various lengths have been imposed between the stimulus and the response opportunity.(ABSTRACT TRUNCATED AT 250 WORDS)

beta-Amyloid precursor protein (APP)-like immunoreactivity in the human sympathetic ganglia

The localization of the beta/A4 amyloid precursor protein (APP) was studied in the human lumbar paravertebral sympathetic ganglia of subjects of different ages, free of neurologic disease, using combined immunohistochemistry and image analysis techniques (optic microdensitometry). To ascertain which cells displayed APP-like immunoreactivity (APP-LI), S-100 and neurofilament proteins were studied in parallel to label the supporting glial cells and the neuron perikarya, respectively. Specific APP-LI was observed labelling both neuron cell bodies and supporting glial cells independently of age. In all cases, the intensity of immunostaining was stronger in glial cells than in neurons. Moreover, the intensity of APP-LI was independent of both age and neuron size. Present results provide evidence for the presence of APP-LI in the human sympathetic ganglia, and for the absence of changes in the expression of this protein, or proteins, with aging. The functional and clinical relevance of these findings remains to be clarified.

Age-related fibrillar deposits in brains of C57BL/6 mice. A review of localization, staining characteristics, and strain specificity

The present article reviews findings regarding the age-related occurrence of clusters of unusual granules in the brains of C57BL/6 (B6) mice and discusses the potential relevance of this phenomenon as a model of specific aspects of brain aging in humans. The granules occur predominantly in the hippocampus of B6 mice and represent aggregations of fibrillar material that are mostly associated with astrocytes. The deposits become evident at about 4 to 6 mo of age, and increase markedly in both number and size thereafter. Similar structures have been observed in adult senescence accelerated mice (SAM) and have been noted, although very rarely, in older mice from other strains. The deposits appear to manifest dominant genetic heritability. Heparan sulfate proteoglycan and laminin or related molecules have been identified as components of the granular material. Although the deposits do not represent senile plaques with beta-amyloid deposition, they might mimic the deposition of extracellular matrix molecules that is thought to be an early event in amyloidogenesis in the aged brain and in Alzheimer's disease.

Effects of an exogenous beta-amyloid peptide on retention for spatial learning

Three experiments assessed the effects of beta-amyloid 1-40 (beta A4) on spatial learning in Sprague-Dawley rats. In Experiment 1, rats were trained on a signaled footshock avoidance in a Y-maze. Rats received a single injection of beta A4 or vehicle in both sides of the hippocampus immediately after the fifth trial. The beta A4 group took significantly longer than the vehicle group to learn to avoid the shock when trained to criterion 1 week later, suggesting a detrimental effect of beta A4 on memory consolidation. Experiment 2 used a food reinforcer rather than shock relief under procedures similar to Experiment 1. Again, the beta A4 group took longer to learn the maze to criterion. This shocks that the effect in Experiment 1 was not specific to shock-maintained learning. In Experiment 3, rats were trained to retrieve a food pellet from each arm of an eight-arm radial maze. After training to criterion, beta A4 or vehicle was administered intrahippocampally 30 min before the daily session for 26 sessions. There were no acute or chronic effects of beta A4 injection on radial maze performance, and no aggregation of beta A4 or significant necrosis was observed upon postmortem histological analysis. These experiments suggest that single injections of beta A4 impair memory consolidation, but repeated injections of beta A4 over an extended period do not affect well-learned behavior.

Cellular and molecular biology of Alzheimer's disease and animal models

Alzheimer's disease (AD), the most common dementing disorder of late life, is a major cause of disability and death in the elderly. Neurobiological, genetic, and molecular studies have defined the vulnerable neural systems, abnormalities in cytoskeletal proteins in neurons, the biology of the beta-amyloid precursor protein (APP) and beta-amyloid (A beta, beta A4), and several APP mutations linked to the disease. More recently, investigators have begun to develop animal models essential for delineating pathogenetic mechanisms and for developing and testing new therapies for treating AD in humans. This review focuses primarily on recent progress in investigation of animal models of AD (including aged nonhuman primates and transgenic mice), which have begun to clarify some of the questions raised by investigation of the disease in humans.

Introduction and expression of the 400 kilobase amyloid precursor protein gene in transgenic mice [corrected]

Overexpression of the gene encoding the beta-amyloid precursor protein (APP) may have a key role in the pathogenesis of both Alzheimer's disease (AD) and Down Syndrome (DS). We have therefore introduced a 650 kilobase (kb) yeast artificial chromosome (YAC) that contains the entire, unrearranged 400 kb human APP gene into mouse embryonic stem (ES) cells by lipid-mediated transfection. ES lines were generated that contain a stably integrated, unrearranged human APP gene. Moreover, we demonstrate germ line transmission of the APP YAC in transgenic mice and expression of human APP mRNA and protein at levels comparable to endogenous APP. This transgenic strategy may prove invaluable for the development of mouse models for AD and DS.

Alzheimer disease and the prion disorders amyloid beta-protein and prion protein amyloidoses

Alzheimer disease and the prion disorders/spongiform encephalopathies share many common features. These chronic, progressive, sometimes familial diseases of the central nervous system are characterized by the presence of different types of amyloid deposits in the brain. This review provides a perspective on these two types of neurodegenerative disorders.