In vitro aging of beta-amyloid protein causes peptide aggregation and neurotoxicity

Biochemistry of Alzheimer's disease amyloid plaques

One of the principal identifying features of Alzheimer's disease (AD) is the extracellular deposition of fibrous protein aggregates in the form of amyloid plaques. The major component of these deposits is the amyloid beta (A beta) protein that is a proteolytic fragment of the integral membrane amyloid precursor protein (APP). Understanding the pathways responsible for A beta formation and the mechanism by which it accumulates within the brain could provide key answers to AD pathogenesis. This review will explore the biochemistry of A beta and its precursor, the possible causal relationship between amyloid and AD-associated neuronal death, the role of additional cellular elements in amyloid formation, and the potential application of these components in clinical diagnosis.

Physiological production of the beta-amyloid protein and the mechanism of Alzheimer's disease

The progressive deposition of the beta-amyloid peptide in the brain and its microvasculature is an invariant feature of Alzheimer's disease that appears to precede the onset of dementia by many years. It had been assumed that the proteolytic release of beta-amyloid peptide from the transmembrane region of its large precursor protein was an aberrant event, requiring prior membrane injury. However, it has recently been shown that beta-amyloid peptide is continuously secreted from healthy neural and non-neural cells in culture and circulates in human CSF and blood. The finding that beta-amyloid peptide is a normal, soluble product of cellular metabolism has led to many dynamic studies of its formation and clearance in health and in genetic forms of Alzheimer's disease, and should facilitate the design of amyloid-inhibiting therapeutics.

The synaptic organization of the neocortex in Alzheimer's disease

Alzheimer's disease (AD) is characterized by a progressive deterioration of cognitive functions. Recent studies have shown that, in addition to the classically described lesions (plaques and tangles) found in AD, this neurodegenerative disorder is characterized by neuronal and synaptic loss and by synapto-axonal pathology. Stepwise regression analysis has shown that the major correlate of cognitive deficiency is the synapse loss in the prefrontal cortex, contributing about 70% of the strength of the correlation with global psychometric tests. We review evidence that supports the theory that most of the synaptic loss in the neocortex is derived from loss of cortico-cortical associational input into the modules. This hypothesis also predicts that neuritic plaque formation in the neocortical modules could represent an aberrant sprouting reaction of associational fibers responding to abnormal growth stimuli or to local damage. On these bases, it is also proposed that the cellular substrate of AD pathology is synapto-axonal, while in certain other forms of dementia such as Creutzfeldt-Jacob disease (CJD) and HIV encephalitis (HIVE) it is primarily dendritic.

beta-Amyloid neurotoxicity in human cortical culture is not mediated by excitotoxins

beta-Amyloid is a metabolic product of the amyloid precursor protein, which accumulates abnormally in senile plaques in the brains of patients with Alzheimer's disease. The neurotoxicity of beta-amyloid has been observed in cell culture and in vivo, but the mechanism of this effect is unclear. In this report, we describe the direct neurotoxicity of beta-amyloid in high-density primary cultures of human fetal cortex. In 36-day-old cortical cultures, beta-amyloid neurotoxicity was not inhibited by the broad-spectrum excitatory amino acid receptor antagonist kynurenate or the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid under conditions that inhibited glutamate and NMDA neurotoxicity. In 8-day-old cortical cultures, neurons were resistant to glutamate and NMDA toxicity but were still susceptible to beta-amyloid neurotoxicity, which was unaffected by excitatory amino acid receptor antagonists. Treatment with beta-amyloid caused chronic neurodegenerative changes, including neuronal clumping and dystrophic neurites, whereas glutamate treatment caused rapid neuronal swelling and neurite fragmentation. These results suggest that beta-amyloid is directly neurotoxic to primary human cortical neurons by a mechanism that does not involve excitatory amino acid receptors.

beta-Amyloid precursor protein metabolites and loss of neuronal Ca2+ homeostasis in Alzheimer's disease

Recent findings link altered processing of beta-amyloid precursor protein (beta APP) to disruption of neuronal Ca2+ homeostasis and an excitotoxic mechanism of cell death in Alzheimer's disease. A major pathway of beta APP metabolism results in the release of secreted forms of beta APP, APPss. These secreted forms are released in response to electrical activity and can modulate neuronal responses to glutamate, suggesting roles in developmental and synaptic plasticity. beta APP is upregulated in response to neural injury and APPss can protect neurons against excitotoxic or ischemic insults by stabilizing the intracellular Ca2+ concentration [Ca2+]i. An alternative beta APP processing pathway liberates intact beta-amyloid peptide, which can form aggregates that disrupt Ca2+ homeostasis and render neurons vulnerable to metabolic or excitotoxic insults. Genetic abnormalities (e.g. certain beta APP mutations or Down syndrome) and age-related changes in brain metabolism (e.g. reduced energy availability or increased oxidative stress) may favor accumulation of [Ca2+]i-destabilizing beta-amyloid peptide and diminish the release of [Ca2+]i-stabilizing, neuroprotective APPss.

Genetic and molecular advances in Alzheimer's disease

The abnormal deposition of amyloid beta protein (A beta) in the brain is the major neuropathological characteristic of Alzheimer's disease (AD). The disease in some early-onset familial cases develops as a result of mutations in the gene coding for the beta-amyloid precursor protein (beta APP) and in the majority of the rest appears to be caused by an unidentified gene on chromosome 14. Only one of the beta APP gene mutations has been associated with aberrant beta APP processing, resulting in an excess production of A beta in vitro, a result suggesting that there might be excessive A beta cleavage from beta APP in AD in vivo. By contrast with the beta APP mutants, no particular allele of the apolipoprotein E (APOE) gene predicts the disease completely but one allele is associated with the disease suggesting APOE is a risk locus for AD. This discovery has been linked to increased deposition of A beta in those cases carrying the risk allele. However, the genetic evidence is currently not sufficient to indicate whether beta APP mismetabolism, direct or indirect A beta neurotoxicity or dysfunction of beta APP (or its derivatives) are central to the AD process.

beta-Amyloid precursor protein mismetabolism and loss of calcium homeostasis in Alzheimer's disease

The suspected involvement of the beta-amyloid precursor protein (beta APP) in the etiology of Alzheimer's disease (AD) has been strengthened by recent genetic evidence, but pursuit of the mechanisms involved will initially require basic cell biology approaches. Several studies have concentrated on toxic activities of beta-amyloid peptide (beta AP) itself, illuminating its contributions to excitotoxicity and calcium-mediated degeneration in general. We now know that generation of beta AP from beta APP also compromises the production of an important set of trophic factors: the secreted forms of beta APP (APPS), which may act--ironically--by conferring protection from calcium-mediated insults. Therefore, conditions which contribute to the formation of beta AP (possibly including ischemia) not only produce an agent which exacerbates calcium-mediated cell death, but also reduce the levels of one of the few factors able to rescue calcium homeostasis. The implications of these postulates and their relationship to the process of aging are discussed.

Calcium-destabilizing and neurodegenerative effects of aggregated beta-amyloid peptide are attenuated by basic FGF

The mechanisms that contribute to neuronal degeneration in Alzheimer's disease (AD) are not understood. Abnormal accumulations of beta-amyloid peptide (beta AP) are thought to be involved in the neurodegenerative process, and recent studies have demonstrated neurotoxic actions of beta APs. We now report that the mechanism of beta AP-mediated neurotoxicity in hippocampal cell culture involves a destabilization of neuronal calcium homeostasis resulting in elevations in intracellular calcium levels ([Ca2+]i) that occur during exposure periods of 6 hr to several days. Both the elevations of [Ca2+]i and neurotoxicity were directly correlated with aggregation of the peptide as assessed by beta AP immunoreactivity and confocal laser scanning microscopy. Exposure of neurons to beta AP resulted in increased sensitivity to the [Ca2+]i-elevating and neurodegenerative effects of excitatory amino acids. Moreover, [Ca2+]i responses to membrane depolarization and calcium ionophore were greatly enhanced in beta AP-treated neurons. Neurons in low cell density cultures were more vulnerable to beta AP toxicity than were neurons in high cell density cultures. Basic fibroblast growth factor (bFGF), but not nerve growth factor (NGF), significantly reduced both the loss of calcium homeostasis and the neuronal damage otherwise caused by beta AP. In AD, beta AP may endanger neurons by destabilizing calcium homeostasis and bFGF may protect neurons by stabilizing intracellular calcium levels. Aggregation of beta AP seems to be a major determinant of its [Ca2+]i-destabilizing and neurotoxic potency.

Apoptosis is induced by beta-amyloid in cultured central nervous system neurons

The molecular mechanism responsible for the neurodegeneration in Alzheimer disease is not known; however, accumulating evidence suggests that beta-amyloid peptide (A beta P) contributes to this degeneration. We now report that synthetic A beta Ps trigger the degeneration of cultured neurons through activation of an apoptotic pathway. Neurons treated with A beta Ps exhibit morphological and biochemical characteristics of apoptosis, including membrane blebbing, compaction of nuclear chromatin, and internucleosomal DNA fragmentation. Aurintricarboxylic acid, an inhibitor of nucleases, prevents DNA fragmentation and delays cell death. Our in vitro results suggest that apoptosis may play a role in the neuronal loss associated with Alzheimer disease.

Cultured GABA-immunoreactive neurons are resistant to toxicity induced by beta-amyloid

Neurodegeneration in Alzheimer's disease is characterized by a selective loss of particular cell populations. Several recent lines of evidence suggest that beta-amyloid protein directly contributes to the disease's progression and is likely responsible for the observed pattern of neuronal death. We have previously demonstrated that aggregated beta-amyloid peptides are neurotoxic to cultured neurons. We now report that a neuronal population exhibiting GABA-immunoreactivity is resistant to beta-amyloid-induced toxicity in vitro, a finding consistent with observations in the Alzheimer brain. Determination of the intrinsic neuronal characteristics responsible for resistance to beta-amyloid may prove beneficial in both understanding the mechanism(s) of beta-amyloid neurotoxicity and halting the disease's progressive neuronal degeneration.

Aluminum, iron, and zinc ions promote aggregation of physiological concentrations of beta-amyloid peptide

A major pathological feature of Alzheimer's disease (AD) is the presence of a high density of amyloid plaques in the brain tissue of patients. The plaques are predominantly composed of human beta-amyloid peptide beta A4, a 40-mer whose neurotoxicity is related to its aggregation. Certain metals have been proposed as risk factors for AD, but the mechanism by which the metals may exert their effects is unclear. Radioiodinated human beta A4 has been used to assess the effects of various metals on the aggregation of the peptide in dilute solution (10(-10) M). In physiological buffers, 10(-3) M calcium, cobalt, copper, manganese, magnesium, sodium, or potassium had no effect on the rate of beta A4 aggregation. In sharp contrast, aluminum, iron, and zinc under the same conditions strongly promoted aggregation (rate enhancement of 100-1,000-fold). The aggregation of beta A4 induced by aluminum and iron is distinguishable from that induced by zinc in terms of rate, extent, pH and temperature dependence. These results suggest that high concentrations of certain metals may play a role in the pathogenesis of AD by promoting aggregation of beta A4.

Alpha 1-antichymotrypsin binding to Alzheimer A beta peptides is sequence specific and induces fibril disaggregation in vitro

The serine protease inhibitor alpha 1-antichymotrypsin (ACT) consistently colocalizes with amyloid deposits of Alzheimer's disease (AD) and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. AD amyloid fibrils are composed primarily of A beta, which is a proteolytic fragment of the larger beta-amyloid precursor protein. Using negative-stain and immunochemical electron microscopy, we have investigated the binding of ACT to the fibrils formed by four synthetic A beta analogues corresponding to the wild-type human 1-40 sequence [Hwt(1-40)], a 1-40 peptide [HDu(1-40)] containing the Glu22-->Gln mutation found in hereditary cerebral hemorrhage with amyloidosis of the Dutch type, the N-terminal 1-28 residues [beta(1-28)], and an internal fragment of A beta containing residues 11-28 [beta(11-28)]. Each of these peptide analogues assembled into 70-90-A-diameter fibrils resembling native amyloid and, except for beta(11-28), bound ACT, as indicated by the appearance of 80-100-A globular particles that adhered to preformed fibrils and that could be decorated with anti-ACT antibodies. Under the conditions used, ACT binding destabilized the in vitro fibrils and produced a gradual dissolution of the macromolecular assemblies into constituent filaments and shorter fragments. The internal fragment (11-28) did not exhibit ACT binding or any structural changes. These results suggest that a specific sequence likely contained within the N-terminal 10 residues of A beta is responsible for the formation of the ACT-amyloid complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered calcium signaling and neuronal injury: stroke and Alzheimer's disease as examples

Several cellular signaling systems have been implicated in the neuronal death that occurs both in development ("natural" cell death) or in pathological conditions such as stroke and Alzheimer's disease (AD). Here we consider the possibility that neuronal degeneration in an array of disorders including stroke and AD arises from one or more alterations in calcium-regulating systems that result in a loss of cellular calcium homeostasis. A long-standing hypothesis of neuronal injury, the excitatory amino acid (EAA) hypothesis, is revisited in light of new supportive data concerning the roles of EAAs in stroke and the neurofibrillary degeneration in AD. Two quite new concepts concerning mechanisms of neuronal injury and death are presented, namely: 1) growth factors normally "stabilize" intracellular free calcium levels ([Ca2+]i) and protect neurons against ischemic/excitotoxic injury, and 2) aberrant processing of beta-amyloid precursor protein (APP) can cause neurodegeneration by impairing a neuroprotective function of secreted forms of APP (APPs) which normally regulate [Ca2+]i. Altered APP processing also results in the accumulation of beta-amyloid peptide which contributes to neuronal damage by destabilizing calcium homeostasis; in AD beta-amyloid peptide may render neurons vulnerable to excitotoxic conditions that accrue with increasing age (e.g., altered glucose metabolism, ischemia). Growth factors may normally protect neurons against the potentially damaging effects of calcium influx resulting from energy deprivation and overexcitation. For example, bFGF, NGF and IGFs can protect neurons from several brain regions against excitotoxic/ischemic insults. Growth factors apparently stabilize [Ca2+]i by several means including: a reduction in calcium influx; enhanced calcium extrusion or buffering; and maintenance or improvement of mitochondrial function. For example, bFGF can suppress the expression of a N-methyl-D-aspartate (NMDA) receptor protein that mediates excitotoxic damage in hippocampal neurons. Growth factors may also prevent the loss of neuronal calcium homeostasis and the increased vulnerability to neuronal injury caused by beta-amyloid peptide. Since elevated [Ca2+]i can elicit cytoskeletal alterations similar to those seen in AD neurofibrillary tangles, we propose that neuronal damage in AD results from a loss of calcium homeostasis. The data indicate that a variety of alterations in [Ca2+]i regulation may contribute to the neuronal damage in stroke and AD, and suggest possible means of preventing neuronal damage in these disorders.

Amyloid beta-protein as a substrate interacts with extracellular matrix to promote neurite outgrowth

Progressive deposition of amyloid beta-protein (A beta) in brain parenchyma and blood vessels is a characteristic feature of Alzheimer disease. Recent evidence suggests that addition of solubilized synthetic A beta to medium may produce toxic or trophic effects on cultured hippocampal neurons. Because soluble A beta may not accumulate in significant quantities in brain, we asked whether immobilized A beta peptide as a substrate alters neurite outgrowth from cultured rat peripheral sensory neurons. This paradigm may closely mimic the conditions in Alzheimer disease brain tissue, in which neurites contact insoluble, extracellular aggregates of beta-amyloid. We detected no detrimental effects of A beta substrate on neurite outgrowth. Rather, A beta in combination with low doses of laminin or fibronectin enhanced neurite out-growth from these neuronal explants. Our results suggest that insoluble A beta in the cerebral neuropil may serve as a neurite-promoting matrix, perhaps explaining the apparent regenerative response of neurites observed around amyloid plaques in Alzheimer disease. Moreover, in concert with the recent discovery of A beta production by cultured neurons, our data suggest that A beta plays a normal physiological role in brain by complexing with the extracellular matrix.

An analysis of the effects of Alzheimer's plaques on living neurons

Although senile plaques represent a consistent neuropathological feature in Alzheimer's brains, it is not known what role plaques play in the etiology of the disease. Both growth-promoting and growth-inhibiting influences have been postulated. One of the major components in plaques, beta-amyloid, has been shown to affect neuron survival and neurite outgrowth in vitro. Because plaques consist of other components in addition to beta-amyloid, we undertook the present study to determine whether neuronal survival and neurite outgrowth are affected by the presence of a senile plaque. This was accomplished by using cryostat sections from the cerebral cortex of Alzheimer's patients as a substratum for cultured rat hippocampal neurons. Evaluation of these living neurons on Alzheimer's tissue demonstrated that senile plaques affect the amount, complexity, and direction of neurite outgrowth. In addition, neurons were more likely to extend processes away from plaques rather than toward a plaque. Although cell survival on plaques and in control regions was similar, cell survival was significantly reduced in the peri-plaque region. These observations suggest that senile plaques could have deleterious effects on neural organization in situ.

Neurotoxicity of a prion protein fragment

The cellular prion protein (PrPC) is a sialoglycoprotein of M(r) 33-35K that is expressed predominantly in neurons. In transmissible and genetic neurodegenerative disorders such as scrapie of sheep, spongiform encephalopathy of cattle and Creutzfeldt-Jakob or Gerstmann-Sträussler-Scheinker diseases of humans, PrPC is converted into an altered form (termed PrPSc) which is distinguishable from its normal homologue by its relative resistance to protease digestion. PrPSc accumulates in the central nervous system of affected individuals, and its protease-resistant core aggregates extracellularly into amyloid fibrils. The process is accompanied by nerve cell loss, whose pathogenesis and molecular basis are not understood. We report here that neuronal death results from chronic exposure of primary rat hippocampal cultures to micromolar concentrations of a peptide corresponding to residues 106-126 of the amino-acid sequence deduced from human PrP complementary DNA. DNA fragmentation of degenerating neurons indicates that cell death occurred by apoptosis. The PrP peptide 106-126 has a high intrinsic ability to polymerize into amyloid-like fibrils in vitro. These findings indicate that cerebral accumulation of PrPSc and its degradation products may play a role in the nerve cell degeneration that occurs in prion-related encephalopathies.

Widespread activation of calcium-activated neutral proteinase (calpain) in the brain in Alzheimer disease: a potential molecular basis for neuronal degeneration

Calcium-activated neutral proteinases (CANPs or calpains) are believed to be key enzymes in intracellular signaling cascades and potential mediators of calcium-induced neuronal degeneration. To investigate their involvement in Alzheimer disease, we identified three isoforms of muCANP (calpain I) in human postmortem brain corresponding to an 80-kDa precursor and two autolytically activated isoforms (78 and 76 kDa). As an index of changes in the in vivo activity of muCANP in Alzheimer disease, the ratio of the 76-kDa activated isoform of muCANP to its 80-kDa precursor was measured by immunoassay in selected brain regions from 22 individuals with Alzheimer disease and 18 normal controls. This muCANP activation ratio was elevated 3-fold in the prefrontal cortex from patients with Alzheimer disease but not from patients with Huntington disease. The activation ratio was also significantly elevated, but to a lesser degree, in brain regions where Alzheimer pathology is milder and has not led to overt neuronal degeneration. These findings indicate that muCANP activation is not simply a consequence of cellular degeneration but may be associated with dysfunction in many neurons before gross structural changes occur. The known influences of CANPs on cytoskeleton and membrane dynamics imply that persistent CANP activation may contribute to neurofibrillary pathology and abnormal amyloid precursor protein processing prior to causing synapse loss or cell death in the most vulnerable neuronal populations. Pharmacological modulation of the CANP system may merit consideration as a potential therapeutic strategy in Alzheimer disease.

Generation of beta-amyloid in the secretory pathway in neuronal and nonneuronal cells

The cellular mechanism underlying the generation of beta-amyloid in Alzheimer disease and its relationship to the normal metabolism of the amyloid precursor protein are unknown. In this report, we show that 3- and 4-kDa peptides derived from amyloid precursor protein are normally secreted. Epitope mapping and radiolabel sequence analysis suggest that the 4-kDa peptide is closely related to full-length beta-amyloid and the 3-kDa species is a heterogeneous set of peptides truncated at the beta-amyloid N terminus. The beta-amyloid peptides are secreted in parallel with amyloid precursor protein. Inhibitors of Golgi processing inhibit secretion of beta-amyloid peptides, whereas lysosomal inhibitors have no effect. The secretion of beta-amyloid-related peptides occurs in a wide variety of cell types, but which peptides are produced and their absolute levels are dependent on cell type. Human astrocytes generated higher levels of beta-amyloid than any other cell type examined. These results suggest that beta-amyloid is generated in the secretory pathway and provide evidence that glial cells are a major source of beta-amyloid production in the brain.

Evidence for excitoprotective and intraneuronal calcium-regulating roles for secreted forms of the beta-amyloid precursor protein

The beta-amyloid precursor protein (beta APP) is a membrane-spanning glycoprotein that is the source of the beta-amyloid peptide (beta AP) which accumulates as senile plaques in the brains of patients with Alzheimer's disease. beta APP is normally processed such that a cleavage occurs within the beta AP, liberating secreted forms of beta APP (APPss) from the cell. The neuronal functions of these forms are unknown. We now report that APPss have a potent neuroprotective action in cultured rat hippocampal and septal neurons and in human cortical neurons. APPs695 and APPs751 protected neurons against hypoglycemic damage, and the neuroprotection was abolished by antibodies to a specific region common to both APPs695 and APPs751. APPss caused a rapid and prolonged reduction in [Ca2+]i and prevented the rise in [Ca2+]i that normally mediated hypoglycemic damage. APPss also protected neurons against glutamate neurotoxicity, effectively raising the excitotoxic threshold. APPss may normally play excitoprotective and neuromodulatory roles. Alternative processing of APPss in Alzheimer's disease may contribute to neuronal degeneration by compromising the normal function of APPss and by promoting the deposition of beta AP.

Alzheimer disease amyloid beta protein forms calcium channels in bilayer membranes: blockade by tromethamine and aluminum

Amyloid beta protein (A beta P) is the 40- to 42-residue polypeptide implicated in the pathogenesis of Alzheimer disease. We have incorporated this peptide into phosphatidylserine liposomes and then fused the liposomes with a planar bilayer. When incorporated into bilayers the A beta P forms channels, which generate linear current-voltage relationships in symmetrical solutions. A permeability ratio, PK/PCl, of 11 for the open A beta P channel was estimated from the reversal potential of the channel current in asymmetrical KCl solutions. The permeability sequence for different cations, estimated from the reversal potential of the A beta P-channel current for each system of asymmetrical solutions, is Pcs > PLi > PCa > or = PK > PNa. A beta P-channel current (either CS+ or Ca2+ as charge carriers) is blocked reversibly by tromethamine (millimolar range) and irreversibly by Al3+ (micromolar range). The inhibition of the A beta P-channel current by these two substances depends on transmembrane potential, suggesting that the mechanism of blockade involves direct interaction between tromethamine (or Al3+) and sites within the A beta P channel. Hitherto, A beta P has been presumed to be neurotoxic. On the basis of the present data we suggest that the channel activity of the polypeptide may be responsible for some or all of its neurotoxic effects. We further propose that a useful strategy for drug discovery for treatment of Alzheimer disease may include screening compounds for their ability to block or otherwise modify A beta P channels.

Amyloid beta protein does not interact with tachykinin receptors coupled to inositol phospholipid hydrolysis in human astrocytoma cells

We have tested the interaction between amyloid beta protein (A beta P) and tachykinin receptors in cultured UC-11MG astrocytoma cells, which express high affinity substance P receptors and respond to substance P with an unusually large stimulation of polyphosphoinositide hydrolysis. Both the full-length A beta P (A beta P1-40) and the fragment 25-35 (A beta P25-35) did not affect the stimulation of [3H]inositolmonophosphate (InsP) formation by substance P. A beta P25-35 was also inactive when applied to the cultures 18 or 72 h prior to the assay. In addition, A beta P25-35 did not displace specifically bound [3H]SarMet substance P from its recognition sites in intact UC-11MG cells. These results suggest that, at least in this specific cell type, amyloid peptides do not interact with substance P receptors.

Neurotrophic interactions in the nervous system

Traditional views have suggested that information flow in the nervous system is dependent on millisecond-to-millisecond communication occurring in a point-to-point manner. However, recent evidence suggests that growth and trophic functions are central to development and maintenance of function in the brain. In turn, trophism occurs over days to weeks to months and appears to underlie processes as diverse as learning, memory and development, on the one hand, and the pathogenesis of disease, on the other. This work group focussed on molecular and cellular mechanisms underlying trophic function in the brain.

Fibril formation by primate, rodent, and Dutch-hemorrhagic analogues of Alzheimer amyloid beta-protein

Deposition of extraneuronal fibrils that assemble from the 39-43 residue beta/A4 amyloid protein is one of the earliest histopathological features of Alzheimer's disease. We have used negative-stain electron microscopy, Fourier-transform infrared (FT-IR) spectroscopy, and fiber X-ray diffraction to examine the structure and properties of synthetic peptides corresponding to residues 1-40 of the beta/A4 protein of primate [Pm(1-40); human and monkey], rodent [Ro(1-40); with Arg5-->Gly, Tyr10-->Phe, and His13-->Arg], and hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D) [Du(1-40); with Glu22-->Gln]. As controls, we examined a reverse primate sequence [Pm*(40-1)] and an extensively substituted primate peptide [C(1-40); with Glu3-->Arg, Arg5-->Glu, Asp7-->Val, His13-->Lys, Lys16-->His, Val18-->Asp, Phe19-->Ser, Phe20-->Tyr, Ser26-->Pro, Ala30-->Val, Ile31-->Ala, Met35-->norLeu, Gly38-->Ile, Val39-->Ala, and Val40-->Gly]. The assembly of these peptides was studied to understand the relationship between species-dependent amyloid formation and beta/A4 sequence and the effect of a naturally occurring point mutation of fibrillogenesis. The three N-terminal amino acid differences between Pm(1-40) and Ro(1-40) had virtually no effect on the morphology or organization of the fibrils formed by these peptides, indicating that the lack of amyloid deposits in rodent brain is not due directly to specific changes in its beta/A4 sequence. beta-Sheet and fibril formation, judged by FT-IR, was maximal within the pH range 5-8 for Pm(1-40), pH 5-10.5 for Du(1-40), and pH 2.5-8 for Ro(1-40).(ABSTRACT TRUNCATED AT 250 WORDS)

Protease inhibitors generate cytotoxic fragments from Alzheimer amyloid protein precursor in cDNA-transfected glioma cells

A human glioma cell line (Bu-17) was stably transfected with full-length cDNA encoding beta/A4 amyloid protein precursor (APP). When the transfectants were treated with protease inhibitors (leupeptin, E-64, and antipain) and the lysosomotropic agent chloroquine, aberrantly processed fragments of APP having molecular sizes of 8-30 kDa were detected with an antibody against the carboxyl-terminal sequence of APP. Immunocytochemistry revealed that these fragments were localized in the lysosome-like organelles. Treatment of the APP cDNA transfectants with chloroquine for 3 days caused cellular degeneration, and leupeptin and E-64 enhanced chloroquine-induced cytotoxicity. These results suggest that inhibition of lysosomal hydrolases impairs intracellular APP metabolism to generate aberrantly processed fragments that induce cytotoxicity.

The acute neurotoxicity and effects upon cholinergic axons of intracerebrally injected beta-amyloid in the rat brain

The acute neurotoxicity and effects upon cholinergic axons of an intracerebrally injected synthetic peptide corresponding to the first 1-40 amino acids of beta-amyloid protein (beta AP1-40) was studied in rats. A synthetic peptide with the reverse sequence (beta AP40-1) or the vehicle alone were injected in the contralateral hemisphere as control. The size of the resulting lesions was quantified in serial sections using an image analyzer. Counts of cholinergic and noradrenergic fibers were also obtained around the lesion area. The results revealed that beta AP1-40 was significantly more toxic than both reverse peptide and the vehicle. The latter two, however, also caused considerable neurotoxicity. beta AP1-40 was toxic to both cholinergic and noradrenergic fibers to the same extent, and this toxicity was limited to the immediate vicinity of the lesion. This study confirms and extends the results of previous studies reporting neurotoxic effects of intracerebrally injected beta-amyloid in the rat. Our results also show that beta AP1-40 itself is not the source of the altered acetylcholinesterase enzyme activity that has been described in the plaques and tangles of Alzheimer's disease.

Lack of Alzheimer pathology after beta-amyloid protein injections in rat brain

In order to establish a direct relationship between beta-amyloid protein (beta AP) and in vivo neurotoxicity, we made intraparenchymal injections and Alzet pump infusions of beta AP into the hippocampus and cortex of adult rats. We tested a number of synthetic beta AP peptides (beta AP 1-40, 1-38, and 25-35) and peptide controls (scrambled and reversed 1-40, and scrambled and reversed 25-35) over a wide range of concentrations and in a variety of vehicles. The rats were sacrificed from 2-35 days following the implant, and the brains examined by standard immunohistochemical and histological methods used to evaluate the pathologies associated with Alzheimer's disease. We report the lack of Alzheimer related pathology and no significant morphological differences between the beta AP peptide and the peptide and vehicle control injections. These observations indicate that the simple intraparenchymal injection of beta AP in the rat brain is not an appropriate model of Alzheimer-related neurotoxicity.

Solvent effects on beta protein toxicity in vivo

Human beta (1-40) and rat beta (1-42) were dissolved in three different solvents and stereotaxically injected into rat hippocampus with the contralateral side injected with control reverse sequence peptide or vehicle alone. Results at 1 week showed gross toxicity of the 35% acetonitrile solvent which was markedly enhanced by 3 nmol of beta protein but not by reverse sequence peptide. Beta peptide in water also appeared more toxic than reverse sequence, but the results were less clear cut. In contrast, 3 nmol of beta peptide in a cyclodextrin/PBS solution produced no marked short-term toxic effects. Peripheral injection of substance P failed to prevent toxicity. We conclude that solvent effects play a major role in acute beta protein neurotoxicity.

beta-Amyloid neurotoxicity: a discussion of in vitro findings

Significant advances in Alzheimer's disease (AD) research require definitive, reproducible findings from all employed paradigms. Recently, the existing in vitro data addressing the possible contribution of beta-amyloid protein to AD neuropathology have been the subject of controversy. We summarize and interpret existing data and discuss relevant methodological issues. We suggest that in vitro data support the conclusion that beta-amyloid peptides decrease the viability of cultured neurons and that this effect can be enhanced by subsequent insults.

Cerebrovascular injuries induced by activation of platelets and leukocytes in vivo and their correction by neurotropin

Ischemia-like brain damage was induced in cats by selective injection of 4 beta-phorbol-12 beta-myristate-13 alpha-acetate (PMA) into the left carotid artery. PMA-injection provoked significant decreases in platelet and neutrophil counts due to their intravascular aggregation. Platelet and neutrophil aggregates caused brain edema with accumulation of sodium fluorescein in the cerebrospinal fluid and ipsilateral derangement of the cerebral energy state in the parietal cortex. Neurotropin administration decreased the changes in platelet and neutrophil counts and prevented the developments of both brain edema and cerebral energy failure.

Effect of beta amyloid peptides on neurons in hippocampal slice cultures

Several investigators have described the neurotrophic and neurotoxic effects of beta amyloid peptide fragments on dissociated hippocampal neurons in culture. In these prior studies, the peptides were added to dissociated cultures between day 0 and day 4 in vitro, before hippocampal neurons are fully mature. We have analyzed the neurotrophic and neurotoxic effects of beta amyloid fragments beta 1-28, beta 25-35 and beta 1-40 on hippocampal slice cultures, whose physiology and morphology resembles the intact hippocampus. Addition of beta 1-28 or beta 25-35 to the growth medium did not produce significant changes in dendritic length or number of branches. Nerve growth factor, previously reported to enhance the neurotoxic effects of beta 1-40 on dissociated hippocampal neurons in culture, did not significantly enhance the neurotrophic effects of beta 1-28. To achieve high local concentrations of peptides and to avoid potential access problems in the cultures, we injected beta 1-28, beta 25-35, and beta 1-40 directly into the cultures. Amyloid-mediated neurotoxicity was not observed for beta 1-28 or beta 25-35, but beta 1-40 appeared to produce neurodegeneration around the site of injection.

In vivo neurotoxicity of beta-amyloid [beta(1-40)] and the beta(25-35) fragment

We examined the histological changes produced by injections of beta-amyloid [beta(1-40)], and control peptides in rat and monkey cerebral cortex. beta(25-35) injections were also studied in rat cortex. Standard immunoperoxidase procedures were used to detect the distribution of tau, MAP2, beta(1-40) and ALZ 50 immunoreactivity. All injections produced localized necrosis at the injection site surrounded by a zone of neuronal loss and gliosis. In rat cortex, lesions produced by solubilized beta(1-40) and beta(25-35) in water were generally larger than those produced by control peptides. Tau and ALZ 50 antibodies labeled neurites and diffusely positive perikarya around beta(1-40) injections, whereas MAP2 staining was reduced, paralleling the distribution of neuronal loss and gliosis. In aged primate cortex, beta(1-40) lesion size was dose dependent. Hyalinized, ALZ 50 positive neurons, and abnormal neurites were prominent around the injection site. Although beta-amyloid is acutely neurotoxic in both rat and monkey cerebral cortex, neuronal degeneration in the primate more closely resembles that found in AD.

Intracerebral beta-amyloid(25-35) produces tissue damage: is it neurotoxic?

beta-Amyloid (1-40) and (25-35) have been reported to be toxic to primary cultured neurons. beta-Amyloid (1-40) was also reported to induce neurodegeneration following intracerebral injection. We attempted to replicate and extend these findings by injecting both the full length amyloid peptide and the 25-35 fragment. beta 1-40 (3 nmol in 1 microliter) or beta 25-35 (20 nmol in 2 microliters) in a vehicle of 10% DMSO (3 and 10 mM concentration, respectively) induced tissue loss and neurodegeneration. We also attempted to prevent the amyloid-induced damage by coinjecting 200 nmol of Substance P. There was no obvious reduction in the size of the lesions. Other studies, however, have reported antagonism of amyloid toxicity with tachykinin agonists. Since beta-amyloid does not appear to bind to tachykinin receptors, there is some question as to the site of the putative interaction of these peptides and, therefore, the mechanism by which beta-amyloid induces tissue damage. Our own results and published cell culture toxicity studies suggest that aggregation of the peptide and physical displacement of tissue may be responsible for both the neuronal and tissue loss, although this hypothesis is not consistent with other published findings.

Methodological variables in the assessment of beta amyloid neurotoxicity

Cell culture systems for evaluating the biological effects of the beta-amyloid protein are potentially important tools in the study of the pathogenesis of Alzheimer's disease. In this report, methodological considerations in the assessment of beta-amyloid neurotoxicity are discussed. Chronic incubation of beta 1-40 in primary human cortical cultures results in progressive neuronal degeneration. The neurodegenerative process occurs in association with localized deposition of beta-amyloid on the neuronal soma ultimately resulting in the formation of compact beta-amyloid deposits. A beta 1-40 preparation from another laboratory was tested that did not form neuronal beta-amyloid deposits and was not neurotoxic. Thus, the conformational state of the beta 1-40 peptide leading to the formation of neuronal amyloid deposits is an important determinant of neurotoxicity. Variables in peptide preparation that influence this property may account for variation in neurotoxic potency.

Reversible in vitro growth of Alzheimer disease beta-amyloid plaques by deposition of labeled amyloid peptide

The salient pathological feature of Alzheimer disease (AD) is the presence of a high density of amyloid plaques in the brain tissue of victims. The plaques are predominantly composed of human beta-amyloid peptide (beta A4), a 40-mer whose neurotoxicity is related to its aggregation. Radioiodinated human beta A4 is rapidly deposited in vitro from a dilute (less than 10 pM) solution onto neuritic and diffuse plaques and cerebrovascular amyloid in AD brain tissue, whereas no deposition is detectable in tissue without performed plaques. This growth of plaques by deposition of radiolabeled beta A4 to plaques is reversible, with a dissociation half-time of approximately 1 h. The fraction of grey matter occupied by plaques that bind radiolabeled beta A4 in vitro is dramatically larger in AD cortex (23 +/- 11%) than in age-matched normal controls (less than 2%). In contrast to the human peptide, rat/mouse beta A4 (differing at three positions from human beta A4) does not affect the deposition of radiolabeled human beta A4. beta A4 has no detectable interaction with tachykinin receptors in rat or human brain. The use of radioiodinated beta A4 provides an in vitro system for the quantitative evaluation of agents or conditions that may inhibit or enhance the growth or dissolution of AD plaques. This reagent also provides an extremely sensitive method for visualizing various types of amyloid deposits and a means for characterizing and locating sites of amyloid peptide binding to cells and tissues.

Aggregation of the amyloid precursor protein within degenerating neurons and dystrophic neurites in Alzheimer's disease

Using a monoclonal antibody raised against purified, native, human protease nexin-2/amyloid precursor protein, which recognizes an amino terminal epitope on the amyloid precursor protein and detects all major isoforms of amyloid precursor protein, we examined the localization of the amyloid precursor protein within Alzheimer's and aged control brains. Very light cytoplasmic neuronal amyloid precursor protein staining but no neuritic staining was visible in control brains. In the Alzheimer's brain, we detected numerous amyloid precursor protein-immunopositive neurons with moderate to strong staining in select regions. Many neurons also contained varying levels of discrete granular, intracellular accumulations of amyloid precursor protein, and a few pyramidal neurons in particular appeared completely filled with amyloid precursor protein granules. "Ghost"-like deposits of amyloid precursor protein granules arranged in pyramidal, plaque-like shapes were identified. We detected long, amyloid precursor protein-immunopositive neurites surrounding and entering plaques. Many contained swollen varicosities along their length or ended in bulbous tips. Amyloid precursor protein immunoreactivity in the Alzheimer's brain was primarily present as granular deposits (plaques). The amyloid precursor protein granules do not appear to co-localize within either astrocytes or microglia, as evidenced by double-labeling immunohistochemistry with anti-glial fibrillary acidic protein and anti-leukocyte common antigen antibodies or Rinucus cummunicus agglutin lectin. Amyloid precursor protein could occasionally be detected in blood vessels in Alzheimer's brains. The predominantly neuronal and neuritic localization of amyloid precursor protein immunoreactivity indicates a neuronal source for much of the amyloid precursor protein observed in Alzheimer's disease pathology, and suggests a time-course of plaque development beginning with neuronal amyloid precursor protein accumulation, then deposition into the extracellular space, subsequent processing by astrocytes or microglia, and resulting in beta-amyloid peptide accumulation in plaques.