The amyloid precursor protein of Alzheimer's disease and the Abeta peptide

Murine models for Down syndrome

The availability of the recently published DNA sequence of human chromosome 21 (HSA21) is a landmark contribution that will have an immediate impact on the study of the role of specific genes to Down syndrome (DS). Trisomy 21 or DS is the only autosomal aneuploidy that is not lethal in the fetal or early postnatal period. DS phenotypes show variable penetrance, affecting many different organs, including brain (mental retardation, early onset of Alzheimer's disease, AD), muscle (hypotonia), skeleton, and blood. DS phenotypes may stem directly from the cumulative effect of overexpression of specific HSA21 gene products or indirectly through the interaction of these gene products with the whole genome, transcriptome, or proteome. Mouse genetic models have played an important role in the elucidation of the contribution of specific genes to the DS phenotype. To date, the strategies used for modeling DS in mice have been three: (1) to assess single-gene contributions to DS phenotype, using transgenic techniques to create models overexpressing single or combinations of genes, (2) to assess the effects of overexpressing large foreign DNA pieces, introduced on yeast artificial chromosomes (YACs) or bacterial artificial chromosomes (BACs) into transgenic mice, and (3) mouse trisomies that carry all or part of MMU16, which has regions of conserved homology with HSA21. Here we review the existing murine models and the relevance of their contribution to DS research.

Amyloid-beta induces chemotaxis and oxidant stress by acting at formylpeptide receptor 2, a G protein-coupled receptor expressed in phagocytes and brain

Amyloid-beta, the pathologic protein in Alzheimer's disease, induces chemotaxis and production of reactive oxygen species in phagocytic cells, but mechanisms have not been fully defined. Here we provide three lines of evidence that the phagocyte G protein-coupled receptor (N-formylpeptide receptor 2 (FPR2)) mediates these amyloid-beta-dependent functions in phagocytic cells. First, transfection of FPR2, but not related receptors, including the other known N-formylpeptide receptor FPR, reconstituted amyloid-beta-dependent chemotaxis and calcium flux in HEK 293 cells. Second, amyloid-beta induced both calcium flux and chemotaxis in mouse neutrophils (which express endogenous FPR2) with similar potency as in FPR2-transfected HEK 293 cells. This activity could be specifically desensitized in both cell types by preincubation with a specific FPR2 agonist, which desensitizes the receptor, or with pertussis toxin, which uncouples it from G(i)-dependent signaling. Third, specific and reciprocal desensitization of superoxide production was observed when N-formylpeptides and amyloid-beta were used to sequentially stimulate neutrophils from FPR -/- mice, which express FPR2 normally. Potential biological relevance of these results to the neuroinflammation associated with Alzheimer's disease was suggested by two additional findings: first, FPR2 mRNA could be detected by PCR in mouse brain; second, induction of FPR2 expression correlated with induction of calcium flux and chemotaxis by amyloid-beta in the mouse microglial cell line N9. Further, in sequential stimulation experiments with N9 cells, N-formylpeptides and amyloid-beta were able to reciprocally cross-desensitize each other. Amyloid-beta was also a specific agonist at the human counterpart of FPR2, the FPR-like 1 receptor. These results suggest a unified signaling mechanism for linking amyloid-beta to phagocyte chemotaxis and oxidant stress in the brain.

Early-onset amyloid deposition and cognitive deficits in transgenic mice expressing a double mutant form of amyloid precursor protein 695

We have created early-onset transgenic (Tg) models by exploiting the synergistic effects of familial Alzheimer's disease mutations on amyloid beta-peptide (Abeta) biogenesis. TgCRND8 mice encode a double mutant form of amyloid precursor protein 695 (KM670/671NL+V717F) under the control of the PrP gene promoter. Thioflavine S-positive Abeta amyloid deposits are present at 3 months, with dense-cored plaques and neuritic pathology evident from 5 months of age. TgCRND8 mice exhibit 3,200-4,600 pmol of Abeta42 per g brain at age 6 months, with an excess of Abeta42 over Abeta40. High level production of the pathogenic Abeta42 form of Abeta peptide was associated with an early impairment in TgCRND8 mice in acquisition and learning reversal in the reference memory version of the Morris water maze, present by 3 months of age. Notably, learning impairment in young mice was offset by immunization against Abeta42 (Janus, C., Pearson, J., McLaurin, J., Mathews, P. M., Jiang, Y., Schmidt, S. D., Chishti, M. A., Horne, P., Heslin, D., French, J., Mount, H. T. J., Nixon, R. A., Mercken, M., Bergeron, C., Fraser, P. E., St. George-Hyslop, P., and Westaway, D. (2000) Nature 408, 979-982). Amyloid deposition in TgCRND8 mice was enhanced by the expression of presenilin 1 transgenes including familial Alzheimer's disease mutations; for mice also expressing a M146L+L286V presenilin 1 transgene, amyloid deposits were apparent by 1 month of age. The Tg mice described here suggest a potential to investigate aspects of Alzheimer's disease pathogenesis, prophylaxis, and therapy within short time frames.

How well does the CSF inform upon pathology in the brain in Creutzfeldt-Jakob and Alzheimer's diseases?

Analysis of lumbar cerebrospinal fluid (CSF) plays a major role in the investigation of central nervous system disease, but how well do the changes in the CSF reflect pathology within the brain and spinal cord parenchyma? Both Creutzfeldt-Jakob (CJD) and Alzheimer's disease (AD) are characterized by the deposition of insoluble beta-pleated sheet peptides [prion protein (PrP) and beta-amyloid (Abeta), respectively] in the extracellular spaces of grey matter in the brain, but there is discordance in both diseases between the peptide levels in the brain and in the CSF. Experimental studies using tracers have shown that interstitial fluid (ISF) drains through very narrow intercellular spaces within grey matter into bulk flow perivascular channels that surround penetrating arteries. ISF then flows to the surface of the brain and joins CSF to drain to cervical lymph nodes. Such drainage of ISF and CSF to regional lymph nodes in the rat plays a significant role in B-cell and T-cell immune reactions within the brain. In man, the pia mater separates the periarterial ISF drainage pathways from the CSF in the subarachnoid space. The almost complete lack of insoluble protease-resistant PrP entering the CSF from the brain in patients with CJD, reported by Wong et al. in this issue of the Journal of Pathology, illustrates the limitations of ISF drainage pathways for the elimination of insoluble peptides from brain tissue. Insoluble Abeta accumulates in the extracellular spaces as plaques in AD and in periarterial ISF drainage pathways as cerebral amyloid angiopathy. Soluble Abeta appears to become entrapped by the insoluble Abeta in the ISF drainage pathways; thus, as the level of soluble Abeta in the brain rises in AD, the level in the CSF falls. Thus, the changes in the CSF do not accurately reflect the accumulation of the abnormal peptides in the brain parenchyma in either CJD or AD. In both diseases, facilitation of ISF drainage and elimination of PrP and Abeta peptides from the extracellular spaces of the brain may lead to practical therapeutic strategies for these devastating disorders.

Astroglial expression of human alpha(1)-antichymotrypsin enhances alzheimer-like pathology in amyloid protein precursor transgenic mice

Proteases and their inhibitors play key roles in physiological and pathological processes. Cerebral amyloid plaques are a pathological hallmark of Alzheimer's disease (AD). They contain amyloid-ss (Ass) peptides in tight association with the serine protease inhibitor alpha(1)-antichymotrypsin.(1,2) However, it is unknown whether the increased expression of alpha(1)-antichymotrypsin found in AD brains counteracts or contributes to the disease. We used regulatory sequences of the glial fibrillary acidic protein gene(3) to express human alpha(1)-antichymotrypsin (hACT) in astrocytes of transgenic mice. These mice were crossed with transgenic mice that produce human amyloid protein precursors (hAPP) and Ass in neurons.(4,5) No amyloid plaques were found in transgenic mice expressing hACT alone, whereas hAPP transgenic mice and hAPP/hACT doubly transgenic mice developed typical AD-like amyloid plaques in the hippocampus and neocortex around 6 to 8 months of age. Co-expression of hAPP and hACT significantly increased the plaque burden at 7 to 8, 14, and 20 months. Both hAPP and hAPP/hACT mice showed significant decreases in synaptophysin-immunoreactive presynaptic terminals in the dentate gyrus, compared with nontransgenic littermates. Our results demonstrate that hACT acts as an amyloidogenic co-factor in vivo and suggest that the role of hACT in AD is pathogenic.

Multiple quantum solid-state NMR indicates a parallel, not antiparallel, organization of beta-sheets in Alzheimer's beta-amyloid fibrils

Senile plaques associated with Alzheimer's disease contain deposits of fibrils formed by 39- to 43-residue beta-amyloid peptides with possible neurotoxic effects. X-ray diffraction measurements on oriented fibril bundles have indicated an extended beta-sheet structure for Alzheimer's beta-amyloid fibrils and other amyloid fibrils, but the supramolecular organization of the beta-sheets and other structural details are not well established because of the intrinsically noncrystalline, insoluble nature of amyloid fibrils. Here we report solid-state NMR measurements, using a multiple quantum (MQ) (13)C NMR technique, that probe the beta-sheet organization in fibrils formed by the full-length, 40-residue beta-amyloid peptide (Abeta(1-40)). Although an antiparallel beta-sheet organization often is assumed and is invoked in recent structural models for full-length beta-amyloid fibrils, the MQNMR data indicate an in-register, parallel organization. This work provides site-specific, atomic-level structural constraints on full-length beta-amyloid fibrils and applies MQNMR to a significant problem in structural biology.

Proteomics: new perspectives, new biomedical opportunities

Proteomics-based approaches, which examine the expressed proteins of a tissue or cell type, complement the genome initiatives and are increasingly being used to address biomedical questions. Proteins are the main functional output, and the genetic code cannot always indicate which proteins are expressed, in what quantity, and in what form. For example, post-translational modifications of proteins, such as phosphorylation or glycosylation, are very important in determining protein function. Similarly, the effects of environmental factors or multigenic processes such as ageing or disease cannot be assessed simply by examination of the genome alone. This review describes the underlying technology and illustrates several areas of biomedical research, ranging from pathogenesis of neurological disorders to drug and vaccine design, in which potential clinical applications are being explored.

Solid-state NMR as a probe of amyloid fibril structure

Amyloid fibrils are intrinsically noncrystalline, insoluble, high-molecular-weight aggregates of peptides and proteins, with considerable biomedical and biophysical significance. Solid-state NMR techniques are uniquely capable of providing high-resolution, site-specific structural constraints for amyloid fibrils, at the level of specific interatomic distances and torsion angles. So far, a relatively small number of solid-state NMR studies of amyloid fibrils have been reported. These have addressed issues about the supramolecular organization of beta-sheets in the fibrils and the peptide conformation in the fibrils, and have concentrated on the beta-amyloid peptide of Alzheimer's disease. Many additional applications of solid-state NMR to amyloid fibrils from a variety of sources are anticipated in the near future, as these systems are ideally suited for the technique and are of widespread current interest.

The use of seldi proteinchip arrays to monitor production of Alzheimer's betaamyloid in transfected cells

beta-Amyloid (Abeta), a 39-43 residue peptide, is the principal component of senile plaques found in the brains of patients with Alzheimer's disease (AD). There are two main lines of evidence that its deposition is the cause of neurodegeneration. First, mutations found in three genes in familial Alzheimer's cases give rise to increased production of the longest, most toxic, form, Abeta 1-42. Second. aggregated Abeta is toxic to neuronal cells in culture. Inhibitors of the proteases involved in its release from the amyloid precursor protein are, therefore, of major therapeutic interest. The best candidates for the releasing proteases are both aspartyl proteases, which are integrated into the membranes of the endoplasmic reticulum and Golgi network. A sensitive assay using Ciphergen's Seldi system has been developed to measure all the variants of Abeta in culture supernatants, which will be of great value in screening inhibitors of these proteases. With this assay, it has been shown that increasing intracellular cholesterol increases the activities of both beta-secretase, and gamma-secretase 42. Moreover, changing the intracellular targeting of amyloid precursor glycoprotein (APP) yields increased alpha-secretase cleavage, and increases in the amounts of oxidized/nitrated forms of Abeta.

Behavioral phenotype of individuals with Down syndrome

Evidence is reviewed for a developmentally-emerging behavioral phenotype in individuals with Down syndrome that includes significant delay in nonverbal cognitive development accompanied by additional, specific deficits in speech, language production, and auditory short-term memory in infancy and childhood, but fewer adaptive behavior problems than individuals with other cognitive disabilities. Evidence of dementia emerges for up to half the individuals studied after age 50. Research issues affecting control group selection in establishing phenotypic characteristics are discussed, as well as the possible genetic mechanisms underlying variation in general cognitive delay, specific language impairment, and adult dementia. MRDD Research Reviews 2000;6:84-95. Wiley-Liss, Inc.

Par-4 induces cholinergic hypoactivity by suppressing ChAT protein synthesis and inhibiting NGF-inducibility of ChAT activity

Profound reductions in choline acetyl-transferase (ChAT) activity are reliable markers for cholinergic hypoactivity associated with cognitive function deficit in Alzheimer's disease (AD). Par-4 (prostate apoptosis response-4) is a novel mediator of neuronal apoptosis associated with the pathogenesis of AD. Par-4 contains a leucine zipper domain (Leu.zip) that presumably mediates protein-protein interactions critical for its functions in apoptosis. Par-4 activity can be effectively blocked by overexpression of Leu. zip because it exerts a dominant negative action possibly by competitively blocking the interaction of Par-4 with other proteins. Whether Par-4 participates in regulation of cholinergic signaling has not been determined. We report that overexpression of Par-4 results in apoptotic and non-apoptotic reductions in ChAT activity in transfected PC12 cells following exposure to a toxic concentration (50 microM) of aggregated amyloid beta peptide 1-42 (Abeta 1-42) and a non-toxic concentration (1 microM) of soluble Abeta 1-42, respectively. Non-apoptotic reduction in ChAT activity induced by Par-4 can be completely blocked by co-overexpression of Leu.zip, indicating that enhanced Par-4 activity is a necessary event for cholinergic hypoactivity in PC12 cells. Further studies found that Par-4 induces non-apoptotic reduction in ChAT activity by: (1) reducing ChAT protein levels following exposure to non-toxic concentration of Abeta, and (2) blocking the cellular capability to increase ChAT activity following exposure to nerve growth factor (NGF). The role of Par-4 in inducing cholinergic hypoactivity may have significant implications in the understanding and the treatment of memory impairment in AD.

High-level neuronal expression of abeta 1-42 in wild-type human amyloid protein precursor transgenic mice: synaptotoxicity without plaque formation

Amyloid plaques are a neuropathological hallmark of Alzheimer's disease (AD), but their relationship to neurodegeneration and dementia remains controversial. In contrast, there is a good correlation in AD between cognitive decline and loss of synaptophysin-immunoreactive (SYN-IR) presynaptic terminals in specific brain regions. We used expression-matched transgenic mouse lines to compare the effects of different human amyloid protein precursors (hAPP) and their products on plaque formation and SYN-IR presynaptic terminals. Four distinct minigenes were generated encoding wild-type hAPP or hAPP carrying mutations that alter the production of amyloidogenic Abeta peptides. The platelet-derived growth factor beta chain promoter was used to express these constructs in neurons. hAPP mutations associated with familial AD (FAD) increased cerebral Abeta(1-42) levels, whereas an experimental mutation of the beta-secretase cleavage site (671(M-->I)) eliminated production of human Abeta. High levels of Abeta(1-42) resulted in age-dependent formation of amyloid plaques in FAD-mutant hAPP mice but not in expression-matched wild-type hAPP mice. Yet, significant decreases in the density of SYN-IR presynaptic terminals were found in both groups of mice. Across mice from different transgenic lines, the density of SYN-IR presynaptic terminals correlated inversely with Abeta levels but not with hAPP levels or plaque load. We conclude that Abeta is synaptotoxic even in the absence of plaques and that high levels of Abeta(1-42) are insufficient to induce plaque formation in mice expressing wild-type hAPP. Our results support the emerging view that plaque-independent Abeta toxicity plays an important role in the development of synaptic deficits in AD and related conditions.

Cerebral amyloid angiopathy: accumulation of A beta in interstitial fluid drainage pathways in Alzheimer's disease

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of beta-amyloid (A beta) peptides in the walls of arteries both in the cortex and meninges. Here, we test the hypothesis that CAA results from the progressive accumulation of A beta in the perivascular interstitial fluid drainage pathways of the brain. Experimental studies have shown that interstitial fluid (ISF) from the rat brain flows along periarterial spaces to join the cerebrospinal fluid (CSF) to drain to cervical lymph nodes. Such lymphatic drainage plays a key role in B-cell and T-cell mediated immunity of the brain. Anatomical studies have defined periarterial ISF drainage pathways in the human brain that are homologous with the lymphatic pathways in the rat brain but are largely separate from the CSF. Periarterial channels in the brain in man are in continuity with those of leptomeningeal arteries and can be traced from the brain to the extracranial portions of the internal carotid arteries related to deep cervical lymph nodes. The pattern of deposition of A beta in senile plaques and in CAA suggests that A beta accumulates in pericapillary and periarterial ISF drainage pathways. A beta could accumulate in CAA due to either (i) increased production of A beta, (ii) reduced solubility of A beta peptides, or (iii) impedance of drainage of A beta along periarterial ISF drainage pathways within the brain and leptomeninges due to aging factors in cerebral arteries. Elucidation of factors that reduce elimination of A beta via perivascular drainage pathways may lead to their rectification and to new strategies for treatment of Alzheimer's disease.

Multiple antioxidants in the prevention and treatment of neurodegenerative disease: analysis of biologic rationale

Parkinson's disease and Alzheimer's disease are major progressive neurologic disorders, the risk of which increases with advancing age (65 years and over). In familial cases, however, early onset of disease (35-65 years) is observed. In spite of extensive basic and chemical research on Parkinson's disease and Alzheimer's disease, no preventive or long-term effective treatment strategies are available. The analysis of existing literature suggests that oxidative stress is a major intermediary risk factor for the action of diverse groups of neurotoxins that are involved in these neurodegenerative diseases. In this review, it is proposed that the epigenetic components (mitochondria, other organelles, membranes, protein modification) rather than nuclear genes of neurons are the primary targets for the action of neurotoxins, including free radicals. In addition, a scientific rationale for using multiple antioxidants in clinical trials for the prevention of Parkinson's disease and Alzheimer's disease among high-risk populations, and as an adjunct to standard therapy in the treatment of these diseases is presented.

REVIEW: tau protein pathology in Alzheimer's disease and related disorders

Abundant neurofibrillary lesions made of hyperphosphorylated microtubule-associated protein tau constitute one of the defining neuropathological features of Alzheimer's disease. However, tau containing filamentous inclusions in neurones and/or glial cells also define a number of other neurodegenerative disorders clinically characterized by dementia and/or motor syndromes. All these disorders, therefore, are grouped under the generic term of tauopathies. In the first part of this review we outline the morphological and biochemical features of some major tauopathies, e. g. Alzheimer's disease, argyrophilic grain disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration. The impact of the recent finding of tau gene mutations in familial frontotemporal dementia and parkinsonism linked to chromosome 17 on other tauopathies is discussed in the second part. The review closes with a look towards a new understanding of neurodegenerative disorders characterized by filamentous nerve cell inclusions. The recent identification of the major protein component of their respective inclusions led to a surprising convergence of seemingly unrelated disorders. The new findings now allow us to classify neurodegenerative disorders with filamentous nerve cell inclusions into four main categories: (i) the tauopathies; (ii) the alpha-synucleinopathies; (iii) the polyglutamine disorders; and (iv) the iquitin disorders'. Within the proposed classification scheme, tauopathies constitute the most frequent type of disorder.