Neuronal localization of the TNFα converting enzyme (TACE) in brain tissue and its correlation to amyloid plaques

The tumor necrosis factor (TNF)‐α converting enzyme (TACE) can cleave the cell‐surface ectodomain of the amyloid‐β precursor protein (APP), thus decreasing the generation of amyloid‐β (Aβ) by cultured non‐neuronal cells. While the amyloidogenic processing of APP in neurons is linked to the pathogenesis of Alzheimer's disease (AD), the expression of TACE in neurons has not yet been examined. Thus, we assessed TACE expression in a series of neuronal and non‐neuronal cell types by Western blots. We found that TACE was present in neurons and was only faintly detectable in lysates of astrocytes, oligodendrocytes, and microglial cells. Immunohistochemical analysis was used to determine the cellular localization of TACE in the human brain, and its expression was detected in distinct neuronal populations, including pyramidal neurons of the cerebral cortex and granular cell layer neurons in the hippocampus. Very low levels of TACE were seen in the cerebellum, with Purkinje cells at the granular‐molecular boundary staining faintly. Because TACE was localized predominantly in areas of the brain that are affected by amyloid plaques in AD, we examined its expression in a series of AD brains. We found that AD and control brains showed similar levels of TACE staining, as well as similar patterns of TACE expression. By double labeling for Aβ plaques and TACE, we found that TACE‐positive neurons often colocalized with amyloid plaques in AD brains. These observations support a neuronal role for TACE and suggest a mechanism for its involvement in AD pathogenesis as an antagonist of Aβ formation. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: 40–46, 2001     

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta‐amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein‐coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.     

Neuronal localization of the TNFalpha converting enzyme (TACE) in brain tissue and its correlation to amyloid plaques

The tumor necrosis factor (TNF)-alpha converting enzyme (TACE) can cleave the cell-surface ectodomain of the amyloid-beta precursor protein (APP), thus decreasing the generation of amyloid-beta (Abeta) by cultured non-neuronal cells. While the amyloidogenic processing of APP in neurons is linked to the pathogenesis of Alzheimer's disease (AD), the expression of TACE in neurons has not yet been examined. Thus, we assessed TACE expression in a series of neuronal and non-neuronal cell types by Western blots. We found that TACE was present in neurons and was only faintly detectable in lysates of astrocytes, oligodendrocytes, and microglial cells. Immunohistochemical analysis was used to determine the cellular localization of TACE in the human brain, and its expression was detected in distinct neuronal populations, including pyramidal neurons of the cerebral cortex and granular cell layer neurons in the hippocampus. Very low levels of TACE were seen in the cerebellum, with Purkinje cells at the granular-molecular boundary staining faintly. Because TACE was localized predominantly in areas of the brain that are affected by amyloid plaques in AD, we examined its expression in a series of AD brains. We found that AD and control brains showed similar levels of TACE staining, as well as similar patterns of TACE expression. By double labeling for Abeta plaques and TACE, we found that TACE-positive neurons often colocalized with amyloid plaques in AD brains. These observations support a neuronal role for TACE and suggest a mechanism for its involvement in AD pathogenesis as an antagonist of Abeta formation.     

Oxidative stress in Alzheimer disease

Alzheimer disease (AD) is a progressive dementia affecting a large proportion of the aging population. The histopathological changes in AD include neuronal cell death, formation of amyloid plaques and neurofibrillary tangles. There is also evidence that brain tissue in patients with AD is exposed to oxidative stress (e.g., protein oxidation, lipid oxidation, DNA oxidation and glycoxidation) during the course of the disease. Advanced glycation endproducts (AGEs) are present in amyloid plaques in AD, and its extracellular accumulation may be caused by an accelerated oxidation of glycated proteins. AGEs participate in neuronal death causing direct (chemical) and indirect (cellular) free radical production and consequently increase oxidative stress. The development of drugs for the treatment of AD that breaks the vicious cycles of oxidative stress and neurodegeneration offer new opportunities. These approaches include AGE-inhibitors, antioxidants and anti-inflammatory substances, which prevent free radical production.Key words: ageing, advanced glycation endproducts, Alzheimer disease, amyloid, oxidative stress     

Oxidative stress in Alzheimer disease

Alzheimer disease (AD) is a progressive dementia affecting a large proportion of the aging population. The histopathological changes in AD include neuronal cell death, formation of amyloid plaques and neurofibrillary tangles. There is also evidence that brain tissue in patients with AD is exposed to oxidative stress (e.g., protein oxidation, lipid oxidation, DNA oxidation and glycoxidation) during the course of the disease. Advanced glycation endproducts (AGEs) are present in amyloid plaques in AD, and its extracellular accumulation may be caused by an accelerated oxidation of glycated proteins. AGEs participate in neuronal death causing direct (chemical) and indirect (cellular) free radical production and consequently increase oxidative stress. The development of drugs for the treatment of AD that breaks the vicious cycles of oxidative stress and neurodegeneration offer new opportunities. These approaches include AGE-inhibitors, antioxidants and anti-inflammatory substances, which prevent free radical production.     

Ca2＋失调与阿尔茨海默病发生发展关系的研究进展

阿尔茨海默病（Alzheimer’s disease,AD）是全球老年人中最常见的神经退行性疾病。在对家族性AD的动物模型和个别AD患者死后脑组织的研究中发现,除了A1342水平升高外,神经元内Ca2＋信号失调,并且Ca2＋信号蛋白表达水平也发生了改变。淀粉样蛋白斑、神经元纤维缠结和神经元丢失是AD的后期标志,它们的共同点可能是破坏神经元钙离子信号。细胞内钙离子水平提高在功能上与淀粉样蛋白斑、早老素突变、tau缠结和突触功能障碍相关,基于这些研究,产生了AD的Ca2＋假说。主要介绍神经元内Ca2＋失调与AD的关系以及钙拮抗剂作为AD的潜在治疗方法。     

Beta-amyloid, neuronal death and Alzheimer's disease

Alzheimer's disease (AD) is a common neurodegenerative disease that affects cognitive function in the elderly. Large extracellular beta-amyloid (Abeta) plaques and tau-containing intraneuronal neurofibrillary tangles characterize AD from a histopathologic perspective. However, the severity of dementia in AD is more closely related to the degree of the associated neuronal and synaptic loss. It is not known how neurons die and synapses are lost in AD; the current review summarizes what is known about this issue. Most evidence indicates that amyloid precursor protein (APP) processing is central to the AD process. The Abeta in plaques is a metabolite of the APP that forms when an alternative (beta-secretase and then gamma-secretase) enzymatic pathway is utilized for processing. Mutations of the APP gene lead to AD by influencing APP metabolism. One leading theory is that the Abeta in plaques leads to AD because Abeta is directly toxic to the adjacent neurons. Other theories advance the notion that neuronal death is triggered by intracellular events that occur during APP processing or by extraneuronal preplaque Abeta oligomers. Some investigators speculate that in many cases there is a more general disorder of protein processing in neurons that leads to cell death. In the later models, Abeta plaques are a byproduct of the disease process, rather than the direct cause of neuronal death. A direct correlation between Abeta plaque burden and neuronal (or synaptic) loss should occur in AD if Abeta plaques cause AD through a direct toxic effect. However, histopathologic studies indicate that the correlation between Abeta plaque burden and neuronal (or synaptic) loss is poor. We conclude that APP processing and Abeta formation is important to the AD process, but that neuronal alterations that underlie symptoms of AD are not due exclusively to a direct toxic effect of the Abeta deposits that occur in plaques. A more general problem with protein processing, damage due to the neuron from accumulation of intraneuronal Abeta or extracellular, preplaque Abeta may also be important as underlying factors in the dementia of AD.     

Aluminium exposure induces Alzheimer's disease-like histopathological alterations in mouse brain

Aluminium (Al) is a neurotoxic metal and Al exposure may be a factor in the aetiology of various neurodegenerative diseases such as Alzheimer's disease (AD). The major pathohistological findings in the AD brain are the presence of neuritic plaques containing beta-amyloid (Abeta) which may interfere with neuronal communication. Moreover, it has been observed that GRP78, a stress-response protein induced by conditions that adversely affect endoplasmic reticulum (ER) function, is reduced in the brain of AD patients. In this study, we investigated the correlation between the expression of Abeta and GRP78 in the brain cortex of mice chronically treated with aluminium sulphate. Chronic exposure over 12 months to aluminium sulphate in drinking water resulted in deposition of Abeta similar to that seen in congophilic amyloid angiopathy (CAA) in humans and a reduction in neuronal expression of GRP78 similar to what has previously been observed in Alzheimer's disease. So, we hypothesise that chronic Al administration is responsible for oxidative cell damage that interferes with ER functions inducing Abeta accumulation and neurodegenerative damage.     

Alzheimer Aβ Amyloid Forms an Inhibitory Neuronal Substrate

Abstract: Alzheimer's disease (AD) is identified by the accumulation of amyloid plaques, neurofibrillary degeneration, and the accompanying neuronal loss. AD amyloid assembles into compact fibrous deposits from the amyloid β(Aβ) protein, which is a proteo-lytic fragment of the membrane-associated amyloid precursor protein. To examine the effects of amyloid on neuron growth, a hybrid mouse motoneuron cell line (NSC34) exhibiting spontaneous process formation was exposed to artificial "plaques" created from aggregated synthetic Aβ peptides. These correspond to full-length Aβ residues 1–40 (Aβ1–40), an internal β-sheet region comprising residues 11–28 (Aβ11–28), and a proposed toxic fragment comprising residues 25–35 (Aβ25–35). Fibers were immobilized onto culture dishes, and addition of cells to these in vitro plaques revealed that Aβ was not a permissive substrate for cell adhesion. Neurites in close contact with these deposits displayed abnormal swelling and a tendency to avoid contact with the Aβ fibers. In contrast, Aβ did not affect the adhesion or growth of rat astrocytes, implicating a specific Aβ-neuron relationship. The inhibitory effects were also unique to Aβ as no response was observed to deposits of pancreatic islet amyloid poly-peptide fibers. Considering the importance of cell adhesion in neurite elongation and axonal guidance, the antiadhesive properties of Aβ amyloid plaques found in vivo may contribute to the neuronal loss responsible for the clinical manifestations of AD.     

MAP-2 immunolabeling can distinguish diffuse from dense-core amyloid plaques in brains with Alzheimer's disease

Alzheimer's disease (AD) neuropathology is characterized by the presence of diffuse and dense-core (neuritic) amyloid plaques in specific areas of the brain. The origin of these plaques and the relationship between them is poorly understood. Current methods to identify clearly these types of plaques in the AD brains are largely dependent upon morphological characteristics. Dense-core amyloid plaques in the entorhinal cortex and hippocampus of AD brains might arise from the lysis of neurons overburdened by excessive intracellular deposition of amyloid beta_1-42 (Aβ42) peptide. The local release of active lysosomal enzymes, which persist within these plaques, might degrade most of the released intracellular proteins, leaving behind only those that are resistant to proteolytic digestion, such as ubiquitin, tau, neurofilament proteins and amyloid. To test the possibility that proteins that are sensitive to proteolysis may be degraded selectively in plaques, we used immunohistochemistry to examine the distribution of microtubule-associated protein-2 (MAP-2), a protein localized primarily in neuronal dendrites and known to be sensitive to proteolysis. Uniform MAP-2 immunolabeling was detected throughout the somatodendritic compartment of neurons in age-matched control cortical brain tissues as well as throughout areas of Aβ42-positive diffuse plaques in AD brains. In contrast, analysis of serial sections revealed that MAP-2 was absent from Aβ42-positive dense-core plaques in AD brains. Our results indicate that this differential MAP-2 immunolabeling pattern among plaques may be employed as a reliable and sensitive method to distinguish dense-core plaques from diffuse plaques within AD brain tissue. Furthermore, this biochemical distinction indicates that dense-core and diffuse plaques are formed by different mechanisms.     

Morphologically distinct types of amyloid plaques point the way to a better understanding of Alzheimer’s disease pathogenesis

Abstract

The details of the sequence of pathological events leading to neuron death in Alzheimer’s disease (AD) are not known. Even the formation of amyloid plaques, one of the major histopathological hallmarks of AD, is not clearly understood; both the origin of the amyloid and the means of its deposition remain unclear. It is still widely considered, however, that amyloid plaques undergo gradual growth in the interstitial space of the brain via continual extracellular deposition of amyloid beta peptides at “seeding sites,” and that these growing plaques encroach progressively on neurons and their axons and dendritic processes, eventually leading to neuronal death. Actually, histopathological evidence to support this mechanism is sparse and of uncertain validity. The fact that the amyloid deposits in AD brains that are collectively referred to as plaques are of multiple types and that each seems to have a different origin often is overlooked. We have shown experimentally that many of the so-called “diffuse amyloid plaques,” which lack associated inflammatory cells, are either the result of leaks of amyloid from blood vessels at focal sites of blood-brain barrier breaches or are artifacts resulting from grazing sections through the margins of dense core plaques. In addition, we have provided experimental evidence that neuronal death via necrosis leaves a residue that takes the form of a spheroid “cloud” of amyloid, released by cell lysis, surrounding a dense core that often contains neuronal nuclear material. Support for a neuronal origin for these “dense core amyloid plaques” includes their ability to attract inflammatory cells (microglia and immigrant macrophages) and that they contain nuclear and cytoplasmic components that are somewhat resistant to proteolysis by lysosomes released during neuronal cell lysis. We discuss here the clinical and therapeutic importance of recognizing that amyloid deposition occurs both within neurons (intracellular) and in the interstitial (extracellular) space of the brain. For dense core plaques, we propose that the latter location largely follows from the former. This scenario suggests that blocking intraneuronal amyloid deposition should be a primary therapeutic target. This strategy also would be effective for blocking the gradual compromise of neuronal function resulting from this intraneuronal deposition, and the eventual death and lysis of these amyloid-burdened neurons that leads to amyloid release and the appearance of dense core amyloid plaques in the interstitium of AD brains.     

Calcium signaling in Alzheimer's disease & therapies

Alzheimer's disease (AD) is the most common type of dementia and is characterized by the accumulation of amyloid (Aβ) plaques and neurofibrillary tangles in the brain. Much attention has been given to develop AD treatments based on the amyloid cascade hypothesis; however, none of these drugs had good efficacy at improving cognitive functions in AD patients suggesting that Aβ might not be the disease origin. Thus, there are urgent needs for the development of new therapies that target on the proximal cause of AD. Cellular calcium (Ca²⁺) signals regulate important facets of neuronal physiology. An increasing body of evidence suggests that age-related dysregulation of neuronal Ca²⁺ homeostasis may play a proximal role in the pathogenesis of AD as disrupted Ca²⁺ could induce synaptic deficits and promote the accumulation of Aβ plaques and neurofibrillary tangles. Given that Ca²⁺ disruption is ubiquitously involved in all AD pathologies, it is likely that using chemical agents or small molecules specific to Ca²⁺ channels or handling proteins on the plasma membrane and membranes of intracellular organelles to correct neuronal Ca²⁺ dysregulation could open up a new approach to AD prevention and treatment. This review summarizes current knowledge on the molecular mechanisms linking Ca²⁺ dysregulation with AD pathologies and discusses the possibility of correcting neuronal Ca²⁺ disruption as a therapeutic approach for AD.     

Dysfunction of amyloid precursor protein signaling in neurons leads to DNA synthesis and apoptosis

The classic neuropathological diagnostic markers for AD are amyloid plaques and neurofibrillary tangles, but their role in the etiology and progression of the disease remains incompletely defined. Research over the last decade has revealed that cell cycle abnormalities also represent a major neuropathological feature of AD. These abnormalities appear very early in the disease process, prior to the appearance of plaques and tangles; and it has been suggested that neuronal cell cycle regulatory failure may be a significant component of the pathogenesis of AD. The amyloid precursor protein (APP) is most commonly known as the source of the beta-amyloid (Abeta) peptides that accumulate in the brains of patients with AD. However, a large body of work supports the idea that APP is also a signaling receptor. Most recently, it has been shown that familial AD (FAD) mutations in APP or simple overexpression of wild type APP cause dysfunction of APP signaling, resulting in initiation of DNA synthesis in neurons and consequent apoptosis. In this article, we review the evidence that APP has the potential to activate aberrant neuronal cell cycle re-entry in AD, and we describe a signal transduction pathway that may mediate this abnormal activation of the cell cycle.     

Alzheimer's disease: cholesterol, membrane rafts, isoprenoids and statins

Alzheimer''s disease (AD) is a heterogeneous neurodegenerative disorder and the most prevalent form of dementia worldwide. AD is characterized pathologically by amyloid-β plaques, neurofibrillary tangles and neuronal loss, and clinically by a progressive loss of cognitive abilities. At present, the fundamental molecular mechanisms underlying the disease are unclear and no treatment for AD is known. Epidemiological evidence continues to mount linking vascular diseases, such as hypertension and diabetes, and hypercholesterolaemia with an increased risk for developing AD. A growing amount of evidence suggests a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques in AD development. Cholesterol and statins clearly modulate β-amyloid precursor protein (βAPP) processing in cell culture and animal models. Statins not only reduce endogenous cholesterol synthesis but also exert other various pleiotrophic effects, such as the reduction in protein isoprenylation. Through these effects statins modulate a variety of cellular functions involving both cholesterol (and membrane rafts) and isoprenylation. Although clearly other factors, such as vascular inflammation, oxidative stress and genetic factors, are intimately linked with the progression of AD, this review focuses on the present research findings describing the effect of cholesterol, membrane rafts and isoprenylation in regulating βAPP processing and in particular γ-secretase complex assembly and function and AD progression, along with consideration for the potential role statins may play in modulating these events.     

Acetylcholinesterase induces neuronal cell loss,astrocyte hypertrophy and behavioral deficits in mammalian hippocampus

Previous studies have demonstrated that acetylcholinesterase (AChE) promotes the assembly of amyloid-beta-peptides into neurotoxic amyloid fibrils and is toxic for chick retina neuronal cultures and neuroblastoma cells. Moreover, AChE is present in senile plaques in Alzheimer's disease (AD) brains. Here we have studied the effect of AChE on astrocytes and hippocampal neurons in vivo. Morphological as well as behavioral disturbances were analyzed after intrahippocampal injection of AChE. Rats were trained in the Morris water maze and assayed for behavioral parameters. Neuronal cell loss was found in the upper leaf of the dentate gyrus in rats injected with AChE in comparison with control animals. Glial fibrillary acidic protein immunoreactivity showed astrocytic hypertrophy and the magnitude of the response was associated with neuronal cell loss. Behavioral results show that injection of AChE produces cognitive impairment demonstrated by an altered water maze performance including (i) a higher escape latency score, (ii) a decreased spatial acuity and (iii) a shorter time of swimming in the platform quadrant. These findings indicate that a local increment in neuronal AChE concentration at the mammalian hippocampus, such as those present in amyloid deposits, may play a role in triggering neuropathological and behavioral changes such as those observed in AD brains.     

An aluminum-based rat model for Alzheimer's disease exhibits oxidative damage, inhibition of PP2A activity, hyperphosphorylated tau, and granulovacuolar degeneration

In Alzheimer's disease (AD), oxidative damage leads to the formation of amyloid plaques while low PP2A activity results in hyperphosphorylated tau that polymerizes to form neurofibrillary tangles. We probed these early events, using brain tissue from a rat model for AD that develops memory deterioration and AD-like behaviors in old age after chronically ingesting 1.6 mg aluminum/kg bodyweight/day, equivalent to the high end of the human dietary aluminum range. A control group consumed 0.4 mg aluminum/kg/day. We stained brain sections from the cognitively-damaged rats for evidence of amyloid plaques, neurofibrillary tangles, aluminum, oxidative damage, and hyperphosphorylated tau. PP2A activity levels measured 238.71+/-17.56 pmol P(i)/microg protein and 580.67+/-111.70 pmol P(i)/microg protein (p<0.05) in neocortical/limbic homogenates prepared from cognitively-damaged and control rat brains, respectively. Thus, PP2A activity in cognitively-damaged brains was 41% of control value. Staining results showed: (1) aluminum-loading occurs in some aged rat neurons as in some aged human neurons; (2) aluminum-loading in rat neurons is accompanied by oxidative damage, hyperphosphorylated tau, neuropil threads, and granulovacuolar degeneration; and (3) amyloid plaques and neurofibrillary tangles were absent from all rat brain sections examined. Known species difference can reasonably explain why plaques and tangles are unable to form in brains of genetically-normal rats despite developing the same pathological changes that lead to their formation in human brain. As neuronal aluminum can account for early stages of plaque and tangle formation in an animal model for AD, neuronal aluminum could also initiate plaque and tangle formation in humans with AD.     

Proteases and lipoprotein receptors in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of senile dementia, and is a complex disorder. The pathological hallmarks of AD were discovered by Dr. Alois Alzheimer in 1907, and include deposits of amyloid or senile plaques and neurofibrillar tangles. Plaques are composed of a peptide, termed the Abeta peptide, that is derived by proteolytic processing of the amyloid precursor protein (APP), while neurofibrillar tangles result from a hyperphosphorylation of the tau protein. Mechanisms associated with the formation of plaques and neurofibrillar tangles and their respective contributions to the disease process have been intensely investigated. Proteolytic processing of APP that results in the generation of the Abeta peptide is now well understood and is influenced by several proteins. Recent evidence suggests that the Abeta levels are carefully regulated, and several proteases play an important role in removing the Abeta peptide. Finally, it is becoming apparent that several members of the LDL receptor family play important roles in the brain, and may modulate the course of AD.     

Renin inhibitor aliskiren exerts neuroprotection against amyloid beta-peptide toxicity in rat cortical neurons

Accumulation of amyloid β-peptide (Aβ) in senile plaques, a pathological hallmark of Alzheimer's disease (AD), has been implicated in neuronal degeneration. Renin-angiotensin system (RAS) blockers, including the renin inhibitor aliskiren, are a group of clinically relevant anti-hypertensive agents. The present study was initiated to investigate whether aliskiren may modulate Aβ neurotoxicity as an additional function aside from its established property of lowering blood pressure. We found aliskiren conferred neuronal resistance to Aβ toxicity in primary rat cortical cultures. Moreover, both Aβ25-35 and Aβ1-42 induced renin expression in cortical neurons; in parallel, a heightened expression of renin was detected in the cerebral cortices of 9-month-old AD transgenic mice. Notably, aliskiren blocked Aβ-mediated neuronal induction of renin. We therefore concluded that aliskiren may carry neuroprotective action against Aβ toxicity. Furthermore, the aliskiren effects may involve downregulation of renin expression induced by Aβ.     

Tau PET Imaging for Staging of Alzheimer's Disease in Down Syndrome

Alzheimer's disease (AD) pathology and early‐onset dementia develop almost universally in Down syndrome (DS). AD is defined neuropathologically by the presence of extracellular plaques of aggregated amyloid β protein and intracellular neurofibrillary tangles (NFTs) of aggregated hyperphosphorylated tau protein. The development of radiolabeled positron emission tomography (PET) ligands for amyloid plaques and tau tangles enables the longitudinal assessment of the spatial pattern of their accumulation in relation to symptomatology. Recent work indicates that amyloid pathology develops 15–20 years before neurodegeneration and symptom onset in the sporadic and autosomal dominant forms of AD, while tau pathology correlates more closely with symptomatic stages evidenced by cognitive decline and dementia. Recent work on AD biomarkers in DS illustrates similarities between DS and sporadic AD. It may soon be possible to apply recently developed staging classifications to DS to obtain a more nuanced understanding of the development AD in DS and to provide more accurate diagnosis and prognosis in the clinic.     

Production of intracellular amyloid-containing fragments in hippocampal neurons expressing human amyloid precursor protein and protection against amyloidogenesis by subtle amino acid substitutions in the rodent sequence.

A distinguishing feature of Alzheimer's disease (AD) is the deposition of amyloid plaques in brain parenchyma. These plaques arise by the abnormal accumulation of beta A4, a proteolytic fragment of amyloid precursor protein (APP). Despite the fact that neurons are dramatically affected in the course of the disease, little is known about the neuronal processing of APP. To address this question we have expressed in fully mature, synaptically active rat hippocampal neurons, the neuronal form of human APP (APP695), two mutant forms of human APP associated with AD, and the mouse form of APP (a species known not to develop amyloid plaques). Protein expression was achieved via the Semliki Forest Virus system. Expression of wild type human APP695 resulted in the secretion of beta A4-amyloid peptide and the intracellular accumulation of potential amyloidogenic and non-amyloidogenic fragments. The relative amount of amyloid-containing fragments increased dramatically during expression of the clinical mutants, while it decreased strongly when the mouse form of APP was expressed. 'Humanizing' the rodent APP sequence by introducing three mutations in the beta A4-region also led to increased production of amyloid peptide to levels similar to those obtained with human APP. The single Gly601 to Arg substitution alone was sufficient to triple the ratio of beta A4-peptide to non-amyloidogenic p3-peptide. Due to the capacity of these cells to secrete and accumulate intracellular amyloid fragments, we hypothesize that in the pathogenesis of AD there is a positive feed-back loop where neurons are both producers and victims of amyloid, leading to neuronal degeneration and dementia.(ABSTRACT TRUNCATED AT 250 WORDS)