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

beta-Amyloid converts an acute phase injury response to chronic injury responses

As the brain ages, amyloid deposits accumulate and, as these deposits condense into a beta-sheet conformation, they contribute to the organization of cellular responses and maintain a chronic level of stimulation and injury. Furthermore, accompanying reactions can lead to the production of additional beta-amyloid, the build up of additional fibrillar beta-amyloid, and prolongation of the response. As it accumulates, beta-amyloid appears to develop properties that drive many signal transduction processes in the classic injury cascade and also activate complement, which results in an amplified beta-amyloid AD cascade. In this way several mechanisms, although apparently independent, proceed in parallel, reinforce each other, and perpetuate pathology and structural damage to the brain. Specifically, we suggest that via the activation of complement, initiation, and perpetuation of other cascades, and its own direct toxic actions, beta-amyloid converts an acute response to injury into a chronic damaging inflammatory reaction thereby contributing to neuronal dysfunction and degeneration.

Alzheimer A beta neurotoxicity: promotion by antichymotrypsin, ApoE4; inhibition by A beta-related peptides

Two inflammation-associated proteins found in the Alzheimer amyloid deposits-alpha 1-antichymotrypsin (ACT) and apolipoprotein E4 (apoE4)-have been shown to be genetic risk factors for the development of Alzheimer's disease and to promote the polymerization of the A beta peptide into amyloid filaments in vitro. In the present study, we show that ACT and apoE4 increase the neurotoxicity of the A beta peptide in parallel with their promotion of filament formation. Preincubation of ACT or apoE4 with small A beta-related peptides, or of apoE4 with apoE2, abrogated their subsequent ability to promote both the formation and the neurotoxicity of A beta filaments. These results indicate that ACT and apoE4 may play a stimulatory role in the formation of neurotoxic amyloid in Alzheimer's disease, and that their amyloid promoting activity can be blocked by inhibitory peptides.

The serpin-enzyme complex receptor recognizes soluble, nontoxic amyloid-beta peptide but not aggregated, cytotoxic amyloid-beta peptide

There is now extensive evidence that amyloid-beta peptide is toxic to neurons and that its cytotoxic effects can be attributed to a domain corresponding to amyloid-beta 25-35, GSNKGAIIGLM. We have shown recently that the serine proteinase inhibitor (serpin)-enzyme complex receptor (SEC-R), a receptor initially identified for binding of alpha1-antitrypsin (alpha1-AT) and other serine protease inhibitors, also recognizes the amyloid-beta 25-35 domain. In fact, by recognizing the amyloid-beta 25-35 domain, SEC-R mediates cell surface binding, internalization, and degradation of soluble amyloid-beta peptide. In this study, we examined the possibility that SEC-R mediates the neurotoxic effect of amyloid-beta peptide. A series of peptides based on the sequences of amyloid-beta peptide and alpha1-AT was prepared soluble in dimethyl sulfoxide or insoluble in water and examined in assays for SEC-R binding, for cytotoxicity in neuronal PC12 cells and murine cortical neurons in primary culture, and for aggregation in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. The results show that amyloid-beta peptide 25-35 and amyloid-beta peptide 1-40 prepared soluble in dimethyl sulfoxide compete for binding to SEC-R, are nontoxic, and migrate as monomers in SDS-PAGE analysis. In contrast, the same peptides aged in water did not compete for binding to SEC-R but were toxic and migrated as aggregates in SDS-PAGE. An all-D-amyloid-beta 25-35 peptide was not recognized at all by SEC-R but retained full toxic/aggregating properties. Using a series of deleted, substituted, and chimeric ambeta/alpha1-AT peptides, toxicity correlated well with aggregation but poorly with SEC-R recognition. In a subclone of PC12 cells which developed resistance to the toxic effect of aggregated amyloid-beta 25-35 there was a 2.5-3-fold increase in the number of SEC-R molecules/cell compared with the parent PC12 cell line. These data show that SEC-R does not mediate the cytotoxic effect of aggregated amyloid-beta peptide. Rather, SEC-R could play a protective role by mediating clearance and catabolism of soluble, monomeric amyloid-beta peptide, if soluble amyloid-beta peptide proves to be an in vivo precursor of the insoluble, toxic peptide.

Amyloid beta-protein reduces acetylcholine synthesis in a cell line derived from cholinergic neurons of the basal forebrain

The characteristic features of a brain with Alzheimer disease (AD) include the presence of neuritic plaques composed of amyloid beta-protein (Abeta) and reductions in the levels of cholinergic markers. Neurotoxic responses to Abeta have been reported in vivo and in vitro, suggesting that the cholinergic deficit in AD brain may be secondary to the degeneration of cholinergic neurons caused by Abeta. However, it remains to be determined if Abeta contributes to the cholinergic deficit in AD brain by nontoxic effects. We examined the effects of synthetic Abeta peptides on the cholinergic properties of a mouse cell line, SN56, derived from basal forebrain cholinergic neurons. Abeta 1-42 and Abeta 1-28 reduced the acetylcholine (AcCho) content of the cells in a concentration-dependent fashion, whereas Abeta 1-16 was inactive. Maximal reductions of 43% and 33% were observed after a 48-h treatment with 100 nM of Abeta 1-42 and 50 pM of Abeta 1-28, respectively. Neither Abeta 1-28 nor Abeta 1-42 at a concentration of 100 nM and a treatment period of 2 weeks was toxic to the cells. Treatment of the cells with Abeta 25-28 (48 h; 100 nM) significantly decreased AcCho levels, suggesting that the sequence GSNK (aa 25-28) is responsible for the AcCho-reducing effect of Abeta. The reductions in AcCho levels caused by Abeta 1-42 and Abeta 1-28 were accompanied by proportional decreases in choline acetyltransferase activity. In contrast, acetylcholinesterase activity was unaltered, indicating that Abeta specifically reduces the synthesis of AcCho in SN56 cells. The reductions in AcCho content caused by Abeta 1-42 could be prevented by a cotreatment with all-trans-retinoic acid (10 nM), a compound previously shown to increase choline acetyltransferase mRNA expression in SN56 cells. These results demonstrate a nontoxic, suppressive effect of Abeta on AcCho synthesis, an action that may contribute to the cholinergic deficit in AD brain.

Peripheral cells as an investigational tool for Alzheimer's disease

A number of abnormalities in metabolic and biochemical processes have been found in cultured skin fibroblasts derived from patients affected by Alzheimer's disease (AD). An example of the successful use of peripheral cells to examine a cell biological abnormality in AD are the studies on transduction systems and on APP metabolism, mostly performed on fibroblasts from AD donors. In fact, some of the described alterations mirror events that have also been demonstrated to occur in the AD brain. Within this context data obtained using peripheral cells may help to identify and to test hypotheses on the primary pathophysiological mechanisms leading to AD. In perspective, the identification of peripheral biological markers could provide a useful aid in AD and could allow identification of stages of of the disease or subgrouping of patients, possibly helping to predict the response to treatment.

Neurocytopathic effects of beta-amyloid-stimulated monocytes: a potential mechanism for central nervous system damage in Alzheimer disease

Growing evidence indicates that cells of the mononuclear phagocyte lineage, which includes peripheral blood monocytes (PBM) and tissue macrophages, participate in a variety of neurodestructive events and may play a pivotal role in neurodegenerative conditions such as Alzheimer disease. The present study sought to determine whether exposure of PBM to beta-amyloid peptide (A beta), the major protein of the amyloid fibrils that accumulate in the brain in Alzheimer disease, could induce cytopathic activity in these cells upon their subsequent incubation with neural tissue. PBM were incubated with A beta for 3 days, centrifuged and washed to remove traces of cell-free A beta, and then applied to organotypic cultures of rat brain for varying periods of time. By using a cell-viability assay to quantitate neurocytopathic effect, an increase in the ratio of dead to live cells was detected in cultures containing A beta-stimulated PBM versus control PBM (stimulated with either bovine serum albumin or reverse A beta peptide) as early as 3 days after coculture. The ratio of dead to live cells increased further by 10 days of coculture. By 30 days of coculture, the dead to live cell ratio remained elevated, and the intensity of neurocytopathic effect was such that large areas of brain mass dissociated from the cultures. These results indicate that stimulation of PBM with A beta significantly heightens their neurocytopathic activity and highlight the possibility that inflammatory reactions in the brain play a role in the neurodegeneration that accompanies Alzheimer disease.

Brain amyloid--a physicochemical perspective

The ability to form stable cross-beta fibrils is an intrinsic physicochemical characteristic of the human beta-amyloid peptide (A beta), which forms the brain amyloid of Alzheimer's disease (AD). The high amyloidogenicity and low solubility of this hydrophobic approximately 40-mer have been barriers to its study in the past, but the availability of synthetic peptide and new physical methods has enabled many novel approaches in recent years. Model systems for A beta aggregation (relevant to initial nidus formation) and A beta deposition (relevant to plaque growth and maturation) in vitro have allowed structure/activity relationships and kinetics to be explored quantitatively, and established that these processes are biochemically distinct. Different forms of the peptide, with different physiochemical characteristics, are found in vascular and parenchymal amyloid. Various spectroscopic methods have been used to explore the three-dimensional conformation of A beta both in solution and in solid phase, and demonstrated that the peptide adopts a different configuration in each state. A significant conformational transition is essential to the transformation of A beta from solution to fibril. These observations suggest new therapeutic targets for the treatment of AD.

Protein kinase inhibitor attenuates apoptotic cell death induced by amyloid beta protein in culture of the rat cerebral cortex

Amyloid beta protein (A beta) is deposited characteristically in the brain of patients with Alzheimer's disease. Effects of protein kinase inhibitors (H-89, H-7, KN-62) on A beta-induced neuronal cell death were examined in primary culture of dissociated cerebral cortical cells. beta(25-35), the active fragment of A beta, induced neuronal cell death with apoptotic features including chromatin condensation and DNA fragmentation. The cell death was attenuated by cycloheximide or by H-89, a specific protein kinase A (PKA) inhibitor, but not by H-7 or KN-62. These data suggest that beta(25-35) induces apoptotic cell death through the PKA-mediated pathway.

Conformations of synthetic beta peptides in solid state and in aqueous solution: relation to toxicity in PC12 cells

The secondary structures of peptides beta 25-35 (the active toxic fragment) and beta 35-25 (reverse sequence and non-toxic fragment), as well as of the amidated beta (25-35)-NH2 peptide were investigated in aqueous solution and in the solid state by means of Fourier-transformed infrared spectroscopy and circular dichroism spectroscopy. The conformations of the beta 25-35 and beta 35-25 in solid state were identical and contained mostly beta-sheet structures. In solid state the amidated beta (25-35)-NH2 peptide also contained mostly beta-sheet structures. Freshly prepared aqueous solutions of the beta 25-32 (0.5 - 3.8 mM) contained a mixture of beta-sheet and random coil structures. Within 30-60 min incubation at 37 degrees C in water or in phosphate-buffered saline solution (PBS), beta 25-35 was almost fully converted to a beta-sheet structure. Decreasing the temperature from 37 degrees C to 20 degrees C decreased the rate of conversion from random coil to beta-sheet structures, 1-2 h being required for complete conversion. In contrast beta 35-25 in water or in PBS buffer had mostly a random coil structure and remained so for 6 days. The amidated beta(25-35)-NH2 peptide in water (2.7 mM) was also mostly random coil. However, when this peptide (2-2.7 mM) was dissolved in PBS (pH 7.4) or in 140 mM NaCl, a gel was formed and its conformation was mostly beta-sheet. Decreasing the concentration of beta (25-35)-NH2 peptide in 140 mM NaCl aqueous solution from 2 mM to 1 mM or below favored the conversion from beta-sheet structures to random coil structures. The beta 25-35 was toxic to PC12 cells while beta 35-25 was not. The amidated peptide beta (25-35)-NH2 was at least 500-fold less toxic than beta 25-35. Structural differences between these beta peptides in aqueous solutions may explain the difference in their respective toxicities.

Amyloid fibril toxicity in Alzheimer's disease and diabetes

Several lines of evidence suggest that amyloid deposition in the brain contributes to neuronal degeneration in Alzheimer's disease (AD). In the AD brain, diffuse plaques composed mostly of amorphous beta amyloid (Am-beta A) are inert, whereas compact plaques composed of beta amyloid fibrils (Fib-beta A) are associated with neurodegenerative changes. The effects of these two types of amyloid were tested on primary rat hippocampal neurons. Fib-beta A induced the formation of dystrophic neurites and caused neuronal cell death, whereas Am-beta A was not toxic. In addition, Fib-beta A caused synapse loss in the remaining viable neurons, whereas Am-beta A did not significantly affect synapse number. We also examined the effects of amylin, the primary constituent of the amyloid fibrils that form in the pancreas in adult-onset diabetes. Amylin was toxic to rat and human insulin-producing islet cells in the concentration range of fibril formation. The relative toxic potencies of amylin peptides of different species correlated with their fibril-forming capacity. Soluble amylin was not toxic. The amyloid fibril-binding dye Congo red inhibited the toxicity of both beta A and amylin. Congo red afforded protection against toxicity by a dual mechanism. When present during the phase of fibril polymerization, Congo red could inhibit fibril formation from some peptides. When added to preformed fibrils, Congo red bound to fibrils rendering them nontoxic. These results suggest that fibril formation is necessary for both beta A and amylin toxicity. Congo red appears to be a general inhibitor of amyloid fibril toxicity and may therefore be a useful prototype for drugs targeted to the amyloid pathology of AD and adult-onset diabetes.

Amnesia induced in mice by centrally administered beta-amyloid peptides involves cholinergic dysfunction

Substantial evidences suggest that the increased cerebral deposition, and neurotoxic action of the beta-amyloid peptide, the major constituent of senile plaques, may represent the underlying cause of the cognitive deficits observed in Alzheimer's disease. Herein, we attempted to verify this hypothesis by inducing a potential Alzheimer's-type amnesia after direct intracerebroventricular administration of aggregated beta 25-35-amyloid peptide in mice. In this aim, mnesic capacities were evaluated after 6-13 days, using spontaneous alternation in the Y-maze, step-down type passive avoidance and place learning in a water-maze. Pretraining administration of aggregated beta 25-35 peptide induced dose-dependent decreases in both alternation behaviour and passive avoidance, at doses of 3 and 9 nmol/mouse. A reduced but still significant impairment was observed when the peptide was not aggregated, or 'aged', by preincubation for 4 days at 37 degrees C. The beta 1-28 peptide, at 3 nmol/mouse, also induced a marked decrease in step-down latency. Posttraining, but not preretention, administration of beta 25-35 peptide also significantly impaired learning. The beneficial effects of cholinergic agents on beta 25-35-induced amnesia was examined using the cholinesterase inhibitor tacrine (THA, 1.3 and 4.3 mumol/kg i.p.) and the nicotinic receptor agonist (-)-nicotine (NIC, 0.06 and 0.2 mumol/kg i.p.). Both drugs induced a dose-dependent abrogation of the beta 25-35-induced decreases in alternation behaviour and passive avoidance. Furthermore, THA, at 1.3 mumol/kg, and NIC, at 0.2 mumol/kg, also reversed the beta 25-35-induced impairment of place learning and retention in the water-maze. Histological examination of Cresyl violet-stained brain sections indicated a moderate but significant cell loss within the frontoparietal cortex and the hippocampal formation of mice treated with aged beta 25-35 peptide (9 nmol). Examination of Congo red-stained sections in the same animals demonstrated the presence of numerous amyloid deposits throughout these brain areas. These results confirm that the deposition of beta-amyloid peptide in the brain is in some way related to impairment of learning and cholinergic degeneration and suggest that the [25-35] fragment of the beta-amyloid protein, sufficient to induce neuronal death in cultures, also induces an Alzheimer's-type amnesia in mice.

Monoclonal antibodies inhibit in vitro fibrillar aggregation of the Alzheimer beta-amyloid peptide

The beta-amyloid peptide, the hallmark of Alzheimer disease, forms fibrillar toxic aggregates in brain tissue that can be dissolved only by strong denaturing agents. To study beta-amyloid formation and its inhibition, we prepared immune complexes with two monoclonal antibodies (mAbs), AMY-33 and 6F/3D, raised against beta-amyloid fragments spanning amino acid residues 1-28 and 8-17 of the beta-amyloid peptide chain, respectively. In vitro aggregation of beta-amyloid peptide was induced by incubation for 3 h at 37 degrees C and monitored by ELISA, negative staining electron microscopy, and fluorimetric studies. We found that the mAs prevent the aggregation of beta-amyloid peptide and that the inhibitory effect appears to be related to the localization of the antibody-binding sites and the nature of the aggregating agents. Preparation of mAbs against "aggregating epitopes," defined as sequences related to the sites where protein aggregation is initiated, may lead to the understanding and prevention of protein aggregation. The results of this study may provide a foundation for using mAbs in vivo to prevent the beta-amyloid peptide aggregation that is associated with Alzheimer disease.

Deposits of A beta fibrils are not toxic to cortical and hippocampal neurons in vitro

Amyloid beta peptide (A beta), which is deposited as insoluble fibrils in senile plaques, is thought to play a role in the neuropathology of Alzheimer's disease. We have developed a model in which rat embryonic cerebral cortical or hippocampal neurons are seeded onto culture dishes containing deposits of substrate-bound, fibrillar A beta. The neurons attached rapidly to A beta 1-40 and A beta 1-42 substrates and extended long, branching neurites. Quantitative assessment demonstrated that survival of neurons on the A beta matrices was equivalent to or better than on control substrates of poly L-lysine or poly L-ornithine. In contrast, preparations of A beta fibrils added directly to the culture medium caused neuronal death as previously reported in the literature. These results reveal that the response of neurons to deposited A beta 1-40 and A beta 1-42 is substantially different from that observed with suspensions of the amyloid peptides, with the former serving as growth-promoting substrates for cortical and hippocampal neurons. This may thus imply that fibrillar A beta of senile plaques is not sufficient by itself to cause the plaque-associated neuronal degeneration characteristic of AD.

Protein folding, nucleation phenomena and delayed neurodegeneration in Alzheimer's disease

This hypothesis attempts to explain how Alzheimer's disease can be both sporadic and autosomal dominant with catastrophic neurodegeneration occurring after decades of normal function. The production of A beta peptide, the subunit of amyloid plaques, from the ubiquitous amyloid precursor protein is discussed. Conformational changes are argued to be crucial to the formation of these amyloid plaques and to their neurotoxicity. Parallels are drawn with prion disease where similarly a normal cellular protein becomes pathogenic once a conformational change is induced. Post-mitotic neurons in the brain are susceptible to this destructive process which is initiated by nucleation phenomena and is then self propagating. An understanding of the conformational changes involved in plaque formation may open new therapeutic avenues in Alzheimer's disease.

beta-Amyloid 25-35 and/or quinolinic acid injections into the basal forebrain of young male Fischer-344 rats: behavioral, neurochemical and histological effects

beta-Amyloid peptides have been shown to potentiate the neurotoxic effect of excitatory amino acids in vitro. In order to determine if this occurs in vivo, four experiments were performed. We injected beta-amyloid 25-35 (beta A 25-35) and/or quinolinic acid (QA) bilaterally into the ventral pallidum/substantia innominata (VP/SI) of rats. Control rats received vehicle infusions. A high dose of QA (75.0 nmol/3 microliters) increased open field activity and impaired spatial learning in the Morris water maze, but did not affect the acquisition of a one-way conditioned avoidance response. These changes were associated with histological evidence of neurotoxicity and a reduction in amygdaloid but not frontal cortical or hippocampal choline acetyltransferase (ChAT) activity. A lower dose of QA (37.5 nmol/3 microliters) produced no behavioral effects. It reduced amygdaloid ChAT activity to a lesser extent than the higher dose (15% vs. 29-37%), and caused less histological damage. beta A 25-35 (1.0 or 8.0 nmol/3 microliters) failed to produce behavioral, histological or neurochemical signs of toxicity. Neither dose of beta A 25-35 potentiated the effects of QA (37.5 nmol) on behavior or amygdaloid ChAT activity, and did not appear to increase the histological damage caused by QA. These results suggest that in vivo beta A 25-35 is not neurotoxic and does not potentiate the neurotoxicity of QA in the VP/SI. Further, the histological effects of a high dose of beta A 25-35 (8.0 nmol/3 microliters; a cavitation containing a Congo red positive proteinaceous material) are quite distinct from those produced by a high dose of QA (75.0 nmol/3 microliters; widespread neuronal loss and gliosis).

Changes in ciliary neurotrophic factor content in the rat brain after continuous intracerebroventricular infusion of beta-amyloid(1-40) protein

We have previously shown that the continuous intracerebroventricular infusion of beta-amyloid(1-40) protein results in memory impairments in rats, associated with a reduction of choline acetyltransferase activity in the frontal cortex and hippocampus. In the present study, we examined whether the infusion of beta-amyloid(1-40) protein affected the content of ciliary neurotrophic factor (CNTF) in the rat brain. The beta-amyloid(1-40) infusion increased CNTF content in the frontal cortex, hippocampus, and the cerebellum, but decreased its content in the brain stem. These results suggest that accumulation of beta-amyloid(1-40) in the brain may affect CNTF production in vivo.

Amyloid beta peptide potentiates cytokine secretion by interleukin-1 beta-activated human astrocytoma cells

Neurodegenerative processes in Alzheimer disease (AD) are thought to be driven in part by the deposition of amyloid beta (A beta), a 39- to 43-amino acid peptide product resulting from an alternative cleavage of amyloid precursor protein. Recent descriptions of in vitro neurotoxic effects of A beta support this hypothesis and suggest toxicity might be mediated by A beta-induced neuronal calcium disregulation. In addition, it has been reported that "aging" A beta results in increased toxic potency due to peptide aggregation and formation of a beta-sheet secondary structure. In addition, A beta might also promote neuropathology indirectly by activating immune/inflammatory pathways in affected areas of the brain (e.g., cortex and hippocampus). Here we report that A beta can modulate cytokine secretion [interleukins 6 and 8 (IL-6 and IL-8)] from human astrocytoma cells (U-373 MG). Freshly prepared and aged A beta modestly stimulated IL-6 and IL-8 secretion from U-373 MG cells. However, in the presence of interleukin-1 beta (IL-1 beta), aged, but not fresh, A beta markedly potentiated (3- to 8-fold) cytokine release. In contrast, aged A beta did not potentiate substance P (NK-1)- or histamine (H1)-stimulated cytokine production. Further studies showed that IL-1 beta-induced cytokine release was potentiated by A beta-(25-35), while A beta-(1-16) was inactive. Calcium disregulation may be responsible for the effects of A beta on cytokine production, since the calcium ionophore A23187 similarly potentiated IL-1 beta-induced cytokine secretion and EGTA treatment blocked either A beta or A23187 activity. Thus, chronic neurodegeneration in AD-affected brain regions may be mediated in part by the ability of A beta to exacerbate inflammatory pathways in a conformation-dependent manner.

Heparan sulfate and chondroitin sulfate glycosaminoglycan attenuate beta-amyloid(25-35) induced neurodegeneration in cultured hippocampal neurons

beta-Amyloid peptide has been reported to be toxic to neurons in vitro and in vivo. The fragment of the beta 1-42 peptide believed to be responsible for this toxicity consists of amino acids 25 to 35. beta-amyloid protein, heparan sulfate (HS) glycosaminoglycan (GAG), and proteoglycan (PG) are all localized throughout the senile plaques found in Alzheimer's disease. Chondroitin sulfate (CS) and dermatan sulfate have also been found at the periphery of senile plaques. We have found that both HS and CS prevented neurite fragmentation and toxicity normally induced by beta 25-35. HS and CS by themselves did not have a significant influence on cell viability, indicating that their protective actions were not due to a general trophic effect. In contrast, cultures treated with HS and beta 1-42 did not show significantly reduced toxicity compared to cultures treated with beta 1-42 alone despite specific binding interactions. These data indicate that one function of GAGs in the brain may be to protect neurons from select toxic insults and injury, and additionally suggest that HS interacts differently with different beta-amyloid fragments. These data further suggest that different beta-amyloid fragments may induce distinct mechanisms of toxicity in vitro.

Amino-terminal deletions enhance aggregation of beta-amyloid peptides in vitro

beta-Amyloid protein, which assembles into pathological aggregates deposited in Alzheimer's disease brain tissue, exhibits N-terminal heterogeneity both in vitro and in vivo. To investigate the effects of this N-terminal heterogeneity on the assembly characteristics and biophysical properties of beta-amyloid, we synthesized a series of peptides with progressively shortened N termini (initial residues at positions beta 1, beta 4, beta 8, beta 12, and beta 17) and C termini extending to residue beta 40 or beta 42. We report that peptides with N-terminal deletions exhibit enhanced peptide aggregation relative to full-length species, as quantitatively assessed by sedimentation analyses. Overall, sedimentation levels were greater for peptides terminating at residue beta 42 than for those terminating at residue beta 40. To determine if established biophysical features of the full-length protein were maintained in the truncated peptides, structural and bioactive properties of these peptides were examined and compared. Full-length and truncated peptides exhibiting aggregation showed circular dichroism spectra consistent with predominant beta-sheet conformation, fibrillar morphology under transmission electron microscopy, and significant toxicity in cultures of rat hippocampal neurons. These data demonstrate that N-terminal deletions enhance aggregation of beta-amyloid into neurotoxic, beta-sheet fibrils and suggest that such peptides may initiate and/or nucleate the pathological deposition of beta-amyloid.

GM1 ganglioside-bound amyloid beta-protein (A beta): a possible form of preamyloid in Alzheimer's disease

The earliest event so far known that occurs in the brain affected with Alzheimer's disease (AD) is the deposition and fibril formation of amyloid beta-protein (A beta). A beta is cleaved from a glycosylated membrane protein, called beta-amyloid protein precursor, and normally secreted into the extracellular space. Here we report on the presence of membrane-bound A beta that tightly binds GM1 ganglioside. This suggests that this novel A beta species, rather than secreted A beta, may act as a 'seed' for amyloid and further that intracellular abnormalities in the membrane recycling already exist at the stage of amyloidogenesis.

The toxicity in vitro of beta-amyloid protein

Images

NACP, the precursor protein of the non-amyloid beta/A4 protein (A beta) component of Alzheimer disease amyloid, binds A beta and stimulates A beta aggregation

NACP, a 140-amino acid presynaptic protein, is the precursor of NAC [the non-amyloid beta/A4 protein (A beta) component of Alzheimer disease (AD) amyloid], a peptide isolated from and immunologically localized to brain amyloid of patients afflicted with AD. NACP produced in Escherichia coli bound to A beta peptides, the major component of AD amyloid. NACP bound to A beta 1-38 and A beta 25-35 immobilized on nitrocellulose but did not bind to A beta 1-28 on the filter under the same conditions. NACP binding to A beta 1-38 was abolished by addition of A beta 25-35 but not by A beta 1-28, suggesting that the hydrophobic region of the A beta peptide is critical to this binding. NACP-112, a shorter splice variant of NACP containing the NAC sequence, bound to A beta, but NACP delta, a deletion mutant of NACP lacking the NAC domain, did not bind A beta 1-38. Furthermore, binding between NACP-112 and A beta 1-38 was decreased by addition of peptide Y, a peptide that covers the last 15 residues of NAC. In an aqueous solution, A beta 1-38 aggregation was observed when NACP was also present in an incubation mixture at a ratio of 1:125 (NACP/A beta), whereas A beta 1-38 alone or NACP alone did not aggregate under the same conditions, suggesting that the formation of a complex between A beta and NACP may promote aggregation of A beta. Thus, NACP can bind A beta peptides through the specific sequence and can promote A beta aggregation, raising the possibility that NACP may play a role in the development of AD amyloid.

Neurodegeneration mediated by glutamate and beta-amyloid peptide: a comparison and possible interaction

In Alzheimer's disease, abnormal extracellular accumulations of beta-amyloid (a major component of the senile plaques) and of the excitatory amino acid glutamate are both thought to be associated with degeneration of nerve cells. In the present study, using cultured cortical or hippocampal neurons as an in vitro model, we compared the effects of various factors influencing neurodegeneration mediated by glutamate or by beta-amyloid peptide (A beta). We also asked the question: does long-term treatment with sublethal doses of A beta-(25-35) potentiate glutamate-mediated excitotoxicity? Neuronal cell death was quantified using the lactate dehydrogenase (LDH) method. Since extracellular LDH remains stable for days, the magnitude of relative afflux of LDH correlates in a linear fashion with the number of damaged neurons in cultures. When applied singly, both glutamate (for 15 min) and A beta-(25-35) or its parent peptide A beta-(1-40) (continuously) produced a dose-dependent neuronal degeneration. In the case of glutamate, the half-maximal effects were observed at about 0.08 mM glutamate for both cerebral cortical and hippocampal neurons (cultured for 13 days in vitro, DIV). The effect of A beta-(25-35) was also time-dependent, while neurons grown in a chemically defined medium showed relatively greater susceptibility to A beta-(25-35) than those cultured in a serum-containing medium. These differential effects were not related to the presence of different numbers of glial cells in the cultures. Treatment with different doses of the antimitotic inhibitor, cytosine arabinoside, for 24 h (6-7 DIV) produced at 13 DIV cortical neuronal cultures with varying numbers of astrocytes, as determined by the astrocyte-specific enzyme glutamine synthetase. The presence of astrocytes decreased the toxicity of glutamate for neurons. The modulation was due to uptake of glutamate by astrocytes, thereby reducing its effective concentration, as the effect was seen at 0.1 mM and not at 10 mM glutamate. Incorporation of an NMDA receptor mediated Ca2+ ion channel blocker, MK-801, together with glutamate completely inhibited degeneration of cortical neurons, and pretreatment of cultures with basic fibroblast growth factor for 2 days did so partially. However, these compounds had no effect on neurotoxicity mediated by A beta-(25-35). Lastly, the effect of glutamate interacted with that of A beta-(25-35). Pretreatment of cortical neurons for 2 days with 10 microM A beta-(25-35) by itself had no appreciable effect, but it potentiated significantly the degeneration of these neurons mediated by glutamate.(ABSTRACT TRUNCATED AT 400 WORDS)

Amyloid beta-protein aggregation nullifies its pathologic properties in cultured cerebrovascular smooth muscle cells

Alzheimer's disease and related disorders are characterized by deposition of aggregated amyloid beta-protein (A beta) and accompanying pathologic changes in the neuropil and in the walls of cerebral blood vessels. A beta induces neurotoxicity in vitro, and this effect is markedly enhanced when the peptide is preaggregated. Recently, we reported that freshly solubilized A beta 1-42 can induce cellular degeneration and a striking increase in the levels of cellular amyloid beta-protein precursor and soluble A beta peptide in cultured cerebrovascular smooth muscle cells (Davis-Salinas, J., Saporito-Irwin, S. M., Cotman, C. W., and Van Nostrand, W. E. (1995) J. Neurochem. 65, 931-934). In the present study, we show that preaggregation of A beta 1-42 abolishes the ability of the peptide to induce these cellular pathologic responses in these cells in vitro. These findings suggest that distinct mechanisms for A beta-induced cytotoxicity exist for cultured neurons and cerebrovascular smooth muscle cells, supporting that different processes may be involved in the parenchymal and cerebrovascular pathology of Alzheimer's disease and related disorders.

Beta amyloid is neurotoxic in hippocampal slice cultures

We examined the neurotoxicity of the 40 amino acid fragment of beta amyloid peptide (A beta 1-40) in cultured hippocampal slices. When injected into area CA3, A beta 1-40 produced widespread neuronal damage. Injection of the reverse sequence peptide, A beta 40-1, or vehicle alone produced little damage. The distribution A beta 1-40 was highly correlated with the area of neuronal damage. Thioflavine S and electron microscopic analysis confirmed that injected A beta 1-40 formed 7-9 nm AD type amyloid fibrils in the cultures. A beta 1-40 also altered the number of GFAP immunoreactive astrocytes and ED-1 immunoreactive microglia/macrophages within and around the A beta 1-40 deposit. The observed neurotoxicity of A beta 1-40 in hippocampal slice cultures provides evidence that this peptide may be responsible for the neurodegeneration observed in AD.

beta-Amyloid peptide fragment 25-35 potentiates the calcium-dependent release of excitatory amino acids from depolarized hippocampal slices

beta-Amyloid protein (beta AP) has been frequently associated with the neuropathology of Alzheimer's disease (AD), although the mechanisms by which it can induce neurodegeneration are still unknown. Some studies in hippocampal cultured neurons suggest that beta AP, particularly its fragment 25-35, may induce neural growth or render neurons more vulnerable to excitotoxic insults by a mechanism involving intracellular Ca2+ dyshomeostasis. We have studied the effect of fragment 25-35 on the release of endogenous amino acids from hippocampal slices of young adult (3-3.5-month-old) and aged (23-25-month-old) rats, under basal, K(+)-depolarization, and post-depolarization conditions, in the presence and absence of Ca2+. In both young and aged tissue, the basal release of amino acids was not affected by the peptide. By contrast, 1-hr preincubation of slices from young animals with 10 microM 25-35 fragment resulted in a 140% increase of glutamate and aspartate release stimulated by K+ depolarization, compared with the control-stimulated release. These effects were strictly dependent on external Ca2+. Neither the K(+)-stimulated release of gamma-amino butyric acid (GABA) nor the release of glycine, glutamine, taurine, or alanine, which was not stimulated by high K+, were affected. Substance P and a scrambled sequence of the 25-35 fragment were without any effect per se, but substance P blocked the stimulatory effect of fragment 25-35 on glutamate and aspartate release. In slices from aged rats the basal release of glutamate was significantly higher (260%) than that in young tissue, and the K(+)-induced release of both aspartate and glutamate was also higher.(ABSTRACT TRUNCATED AT 250 WORDS)

Age- and peroxidative stress-related modifications of the cerebral enzymatic activities linked to mitochondria and the glutathione system

The aging brain undergoes a process of enhanced peroxidative stress, as shown by reports of altered membrane lipids, oxidized proteins, and damaged DNA. The aims of this review are to examine: (1) the possible contribution of mitochondrial processes to the formation and release of reactive oxygen species (ROS) in the aging brain; and (2) the age-related changes of antioxidant defenses, both enzymatic and nonenzymatic. It will focus on studies investigating the role of the electron transfer chain as the site of ROS formation in brain aging and the alterations of the glutathione system, also in relation to the effects of exogenous pro-oxidant agents. The possible role of peroxidative stress in age-related neurodegenerative diseases will also be discussed.

Changes of beta-amyloid precursor protein after compression trauma to the spinal cord: an experimental study in the rat using immunohistochemistry

We evaluated by immunohistochemistry the changes of beta-amyloid precursor protein (beta APP) and beta-amyloid peptide (beta A) in the spinal cord of rats with compression injury at Th8-9 of mild, moderate, and severe degrees. The spinal cord of normal rats and animals with laminectomy revealed immunoreactivity to beta APP in nerve cell bodies, the initial part of a few axons of the gray matter, and in scattered glial cells. At 4 h after compression, beta APP-immunoreactivity occurred in a few swollen axons of the longitudinal tracts; such beta APP-immunoreactive axons remained throughout the experimental period of 9 days. The number of immunoreactive axons and the intensity of their immunoreactivity were increased in rats with moderate and severe compression. The caudal Th10 segment exhibited more pronounced accumulation of beta APP immunoreactivity than the cranial Th segment. There was no evidence of beta A accumulation after compression injury. In conclusion, there is a rapidly occurring, long-lasting accumulation of immunoreactive beta-amyloid precursor protein after compression injury of rat spinal cord. This accumulation is related to the degree of impact to the cord.

Differential effects of amyloid peptides beta-(1-40) and beta-(25-35) injections into the rat nucleus basalis

The nucleus basalis of male Charles River Wistar rats was injected with 10 micrograms of the beta-amyloid peptides beta-(1-40) and beta-(25-35) and changes in the morphology of the lesioned area, the release of acetylcholine from the cortex, and in behavior were investigated. Injections of saline and a scrambled (25-35) peptide were used as controls. One week after lesioning, a Congo Red-positive deposit of aggregated material was found at the beta-peptides injection site, which lasted for about 21 days in the case of the beta-(25-35) peptide and at least two months for beta-(1-40). No deposit was detected after scrambled peptide injection. At one week post injection, an extensive glial reaction surrounded the injection site of all peptides and saline as well. Such a reaction was still present but rather attenuated after two months. A decrease in the number of cholinergic neurons was detected in the nucleus basalis after one week with all treatments except saline. After two months, a reduction in the number of choline acetyltransferase-immunopositive neurons was still detectable in the rats injected with beta-(1-40) but not in the beta-(25-35)-or scrambled-injected. The reduction in choline acetyltransferase immunoreactivity was closely paralleled by a decrease in basal acetylcholine release from the parietal cortex ipsilateral to the lesion. Disruption of object recognition was observed in the first weeks after beta-(25-35) peptide injection, whereas the beta-(1-40) peptide impaired the performance only two months after lesion. Rats with lesions induced by beta-peptides may be a useful animal model of amyloid deposition for investigation of the pathogenetic mechanisms leading to Alzheimer's disease.

Amyloid beta protein (25-35) stimulation of phospholipases A, C and D activities of LA-N-2 cells

[3H]Myristic acid prelabeled LA-N-2 cells were exposed to varying concentrations of amyloid beta protein (25-35), from 20 to 250 micrograms/ml, and the activation of phospholipases A and D estimated. A progressive increase in phosphatidylethanol formation, a measure of phospholipase D activity, and of free fatty acid release, a measure of phospholipase A activity, was observed over a time-course of 60 min. [3H]Inositol prelabeled LA-N-2 cells were exposed to varying concentrations of A beta P, from 20 to 125 micrograms/ml, and phospholipase C activation was measured. There was an increased release of inositol phosphates in the presence of amyloid beta protein as a function of incubation time. The effects of adrenergic, metabotropic amino acid and bombesin antagonists on the A beta P mediated stimulation of phospholipase C activity was investigated. Propranolol, a beta adrenergic antagonist, 7-chloro-kynurenic acid, a metabotropic amino acid antagonist, and [Tyr4-D-Phe12]bombesin, a bombesin antagonist, blunted the A beta P stimulation of phospholipase C activity in [3H]inositol prelabeled LA-N-2 cells. This suggests that amyloid beta protein activation of phospholipase C may be receptor mediated. The phospholipase C inhibitor U 71322 prevented the activation of phospholipase C by A beta P. However, this activation was not effected by tocopherol, propylgallate, or vitamin C.

Aggregation of secreted amyloid beta-protein into sodium dodecyl sulfate-stable oligomers in cell culture

Filamentous aggregates of the 40-42-residue amyloid beta-protein (A beta) accumulate progressively in the limbic and cerebral cortex in Alzheimer's disease, where they are intimately associated with neuronal and glial cytopathology. Attempts to model this cytotoxicity in vitro using synthetic peptides have shown that monomeric A beta is relatively inert, whereas aggregated A beta reproducibly exerts a variety of neurotoxic effects. The processes that mediate the conversion of monomeric A beta into a toxic aggregated state are thus of great interest. Previous studies of this conversion have employed high concentrations (10(-5)-10(-3) M) of synthetic A beta peptides under nonbiological conditions. We report here the detection of small amounts (< 10(-9) M) of SDS-stable A beta oligomers in the culture media of Chinese hamster ovary cells expressing endogenous or transfected amyloid beta-protein precursor genes. The identity of these oligomers (primarily dimers and trimers) was established by immunoprecipitation with a panel of A beta antibodies, by electrophoretic comigration with synthetic A beta oligomers, and by amino acid sequencing. The oligomeric A beta species comprised approximately 10-20% of the total immunoprecipitable A beta in these cultures. A truncated A beta species beginning at Arg 5 was enriched in the oligomers, suggesting that amino-terminal heterogeneity can influence A beta oligomerization in this system. Addition of Congo red (10 microM) during metabolic labeling of the cells led to increased monomeric and decreased oligomeric A beta. The ability to detect and quantitate oligomers of secreted A beta peptides in cell culture should facilitate dynamic studies of the critical process of initial A beta aggregation under physiological conditions.

beta-amyloid fibrils induce tau phosphorylation and loss of microtubule binding

A central issue in the pathogenesis of Alzheimer's disease (AD) is the relationship between amyloid deposition and neurofibrillary tangle formation. To determine whether amyloid fibril formation affects the phosphorylation state of tau, primary cultures of fetal rat hippocampal and human cortical neurons were treated with beta-amyloid (beta A) in a soluble, amorphous-aggregated, or fibrillar form. Fibrillar beta A, but not soluble or amorphous-aggregated beta A, markedly induces the phosphorylation of tau at Ser-202 and Ser-396/Ser-404, resulting in a shift in the tau M(r) in human cortical neurons. Hyperphosphorylated tau accumulates in the somatodendritic compartment of fibrillar beta A-treated neurons in a soluble form that is not associated with microtubules and is incapable of binding to microtubules in vitro. Dephosphorylation of beta A-induced tau restores its capacity to bind to microtubules. Thus, amyloid fibril formation alters the phosphorylation state of tau, resulting in the loss of microtubule binding capacity and somatodendritic accumulation, properties also exhibited by tau in the AD brain. Amyloid fibril formation may therefore be a cause of abnormal tau phosphorylation in AD.

Early association of reactive astrocytes with senile plaques in Alzheimer's disease

The fibrillar beta-amyloid protein (A beta) plaques of Alzheimer's disease (AD) are associated with reactive astrocytes and dystrophic neurites and have been suggested to contribute to neurodegenerative events in the disease. We recently reported parallel in vitro and in situ findings, suggesting that the adoption of a reactive phenotype and the colocalization of astrocytes with plaques in AD may be mediated in large part by aggregated A beta. Thus, A beta-mediated effects on astrocytes may directly affect disease progression by modifying the degenerative plaque environment. Alternatively, plaque-associated reactive astrocytosis may primarily represent a glial response to the neural injury associated with plaques and not significantly contribute to AD pathology. To investigate the validity of these two positions, we examined the differential colocalization of reactive astrocytes and dystrophic neurites with plaques. Hippocampal sections from AD brains--ranging in neuropathology from mild to severe--were triple-labeled with antibodies recognizing A beta protein, reactive astrocytes, and dystrophic neurites. We observed not only plaques containing both or neither cell type, but also plaques containing (1) reactive astrocytes but not dystrophic neurites and (2) dystrophic neurites but not reactive astrocytes. The relative proportion of plaques colocalized with reactive astrocytes in the absence of dystrophic neurites is relatively high in mild AD but significantly decreases over the course of the disease, suggesting that plaque-associated astrocytosis may be an early and perhaps contributory event in AD pathology rather than merely a response to neuronal injury. These data underscore the potentially significant contributions of reactive astrocytosis in modifying the plaque environment in particular and disease progression in general.

Anticonvulsants attenuate amyloid beta-peptide neurotoxicity, Ca2+ deregulation, and cytoskeletal pathology

Increasing evidence supports the involvement of amyloid beta-peptide (A beta) and an excitotoxic mechanism of neuronal injury in the pathogenesis of Alzheimer's disease. However, approaches aimed at preventing A beta toxicity and neurofibrillary degeneration are undeveloped. We now report that anticonvulsants (carbamazepine, phenytoin, and valproic acid) can protect cultured rat hippocampal neurons against A beta- and glutamate-induced injury. Each of the anticonvulsants attenuated the elevation of intracellular free calcium levels [(Ca2+)i] elicited by A beta or glutamate suggesting that their neuroprotective mechanism of action involved stabilization of [Ca2+]i. These compounds were effective at clinically relevant concentrations (carbamazepine, 100 nM-10 microM; phenytoin, 100 nM-1 microM; valproic acid, 100 nM-100 microM). The anticonvulsants suppressed glutamate-induced alterations in tau and buiquitin immunoreactivities. Compounds that stabilize [Ca2+]i may afford protection against the kinds of insults believed to underlie neuronal injury in Alzheimer's disease.

Particulate forms of APP in the extracellular milieu of cultured cells

The principle externalized forms of amyloid precursor protein (APP) are soluble and well-characterized, but some evidence has suggested the additional presence of externalized APP in a nonsoluble form. To further assess this possibility, the current study has applied high resolution microscopy protocols in addition to immunoprecipitation to characterize externalized APP in three commonly used cell culture models (SH-SY5Y human neuroblastoma cells, fetal rat brain cells, and HEK 293 human embryonic kidney cells). Confocal immunofluorescence microscopy, using an antiserum against the c-terminal domain of APP, showed typical cell-associated APP, but hot spots of APP also were evident in cell-free areas, apparently associated with the culture substrata. These hot-spots were examined for evidence of cellular deterioration by whole mount transmission electron microscopy. Neither cell debris nor disrupted cells were present. Instead, the hot spots of substratum-bound APP comprised discrete microparticles, approximately 50-100 nm across. These microparticles also could be found near cells and in some cases were attached to cell surface fibrils. Substratum-bound APP also could be found clustered within the extracellular matrix made by primary cell cultures. Occurrence of APP in extracellular microparticles was verified by centrifugation-immunoprecipitation analysis of media conditioned by APP-transfected cells. Radiolabeling data showed that particulate APP was from metabolically active cells. Metabolic labeling of particle-associated APP, as well as the absence of cellular debris near the APP-containing particles, suggests that the occurrence of nonsoluble APP in the extracellular milieu derives from a physiologically active process.

The Ca2+ influx induced by beta-amyloid peptide 25-35 in cultured hippocampal neurons results from network excitation

Although a neurotoxic role has been postulated for the beta-amyloid protein (beta AP), which accumulates in brain tissues in Alzheimer's disease, a precise mechanism underlying this toxicity has not been identified. The peptide fragment consisting of amino acid residues 25 through 35 (beta AP25-35), in particular, has been reported to be toxic in cultured neurons. We report that beta AP25-35, applied to rat hippocampal neurons in culture, caused reversible and repeatable increases in the intracellular Ca2+ concentration ([Ca2+]i), as measured by fura 2 fluorimetry. Furthermore, beta AP25-35 induced bursts of excitatory potentials and action potential firing in individual neurons studied with whole cell current clamp recordings. The beta AP25-35-induced [Ca2+]i elevations and electrical activity were enhanced by removal of extracellular Mg2+, and they could be blocked by tetrodotoxin, by non-N-methyl-D-aspartate (NMDA) and NMDA glutamate receptor antagonists, and by the L-type Ca2+ channel antagonist nimodipine. Similar responses of bursts of action potentials and [Ca2+]i increases were evoked by beta AP1-40. Responses to beta AP25-35 were not prevented by pretreatment with pertussis toxin. Excitatory responses and [Ca2+]i elevations were not observed in cerebellar neuron cultures in which inhibitory synapses predominate. Although the effects of beta AP25-35 depended on the activation of glutamatergic synapses, there was no enhancement of kainate- or NMDA-induced currents by beta AP25-35 in voltage-clamp studies. We conclude that beta AP25-35 enhances excitatory activity in glutamatergic synaptic networks, causing excitatory potentials and Ca2+ influx. This property may explain the toxicity of beta AP25-35.

Calretinin-immunoreactive neurons are resistant to beta-amyloid toxicity in vitro

The molecular pathways by which beta-amyloid protein (A beta) induces neurotoxicity in vitro are unknown. We report that cultured hippocampal neurons exhibiting immunoreactivity for the calcium binding protein calretinin are relatively resistant to degeneration resulting from exposure to either beta 25-35 or beta 1-42. These findings suggest that intrinsic characteristics of calretinin cells, possibly including enhanced calcium buffering capacity, underlie the resistance of these cells to A beta toxicity in vitro and perhaps similar insults in Alzheimer's disease.

Direct evidence of oxidative injury produced by the Alzheimer's beta-amyloid peptide (1-40) in cultured hippocampal neurons

The beta-Amyloid peptide (A beta) is hypothesized to mediate the neurodegeneration seen in Alzheimer's disease. Recently, we proposed a new hypothesis to explain the toxicity of A beta based on the free-radical generating capacity of A beta. We have recently demonstrated using electron paramagnetic resonance (EPR) spectroscopy that A beta (1-40) generates free radicals in solution. It was therefore suggested that A beta radicals can attack cell membranes, initiate lipoperoxidation, damage membrane proteins, and compromise ion homeostasis resulting in neurodegeneration. To evaluate this hypothesis, the ability of A beta to induce neuronal oxidation, changes in calcium levels, enzyme inactivation, and neuronal death were compared with the ability of A beta to produce free-radicals. Using hippocampal neurons in culture, several methods for detection of oxidation were utilized such as the conversion of 2,7-dichlorofluorescin to 2,7-dichlorofluorescein, and a new fluorescence microscopic method for the detection of carbonyls. The ability of A beta to produce free-radicals was determined using EPR with the spin-trapping compound N-tert-butyl-alpha-phenylnitrone. Consistent with previous studies, we found that preincubation of A beta increased the toxicity of the peptide. There is a strong correlation between the intensity of radical generation by A beta and neurotoxicity. The highest neuronal oxidation and toxicity was seen at a time when A beta was capable of generating the most intense radical signal. Furthermore, little oxidation and toxicity was seen when cultures were treated with freshly dissolved A beta, which did not generate a detectable radical signal. These data are consistent with the hypothesis that free-radical-based oxidative damage induced by A beta contributes to the neurodegeneration of Alzheimer's disease.

A potential role for apoptosis in neurodegeneration and Alzheimer's disease

Previous studies have shown that beta-amyloid (A beta) peptides are neurotoxic. Recent data suggest that neurons undergoing A beta-induced cell death exhibit characteristics that correspond to the classical features of apoptosis, suggesting that these cells may initiate a program of cell death. This chapter explores the criteria and precautions that must be applied to evaluate mechanisms of cell death in vitro and in vivo, discusses the evidence supporting an apoptotic mechanism of cell death in response to A beta in cultured neurons, and describes potential correlations for these findings in the Alzheimer's disease brain. In addition, cellular signaling pathways that may be associated with apoptosis in response to A beta are examined, and support for apoptosis as a mechanism of cell death for other neurodegeneration-inducing stimuli (e.g., oxidative injury) is described. The connection of multiple stimuli that induce neuronal cell death to an apoptotic mechanism suggests that apoptosis could play a central role in neurodegeneration in the brain.

beta-Amyloid peptide produced in vitro is degraded by proteinases released by cultured cells

The primary histopathological feature of Alzheimer's disease is the accumulation of beta-amyloid in the brains of afflicted individuals. This peptide has been shown to be produced and liberated both in vitro and in vivo by normal physiological processes. The mechanism by which beta-amyloid is formed, as well as that by which it may be cleared, are events likely to impact on the development and progression of this disease. Thus, the fate of beta-amyloid peptides secreted by cultured mammalian cells was investigated. It was found that levels of the soluble peptide are reduced over time due to the activity of multiple types of proteinases including those from the metallo, aspartyl, and thiol classes. Inhibitors to each class of proteinase can only partially block beta-amyloid degradation, but, if used in combination, they can fully prevent its catabolism. The Kunitz serine proteinase inhibitor domain, present on two beta-amyloid precursor protein isoforms, was found to be an effective inhibitor of beta-amyloid peptide degradation. These data indicate that modulations in expression of secreted proteinases and/or beta-amyloid precursor isoforms may influence levels of beta-amyloid.

Differential regulation by beta-amyloid peptides of intracellular free Ca2+ concentration in cultured rat microglia

We have previously shown that exposure to beta-amyloid peptides alters microglial activity and viability. It is thought that beta-amyloid peptides induce toxicity in neuronal cultures by destabilizing Ca2+ homeostasis. To investigate the effects of beta-amyloid peptides on intracellular free Ca2+ concentration ([Ca2+]i) in cultured microglia, we used Fura-2 imaging. Exposure to 25 microM beta-amyloid-(25-35) induced increases in 2+]i within 1 h. In contrast, exposure to 25 microM beta-amyloid-(1-42), the full-length homolog to the beta-amyloid protein deposited in plaques, does not, over the same time period. However, the average [Ca2+]i of microglia is increased by a 6 h exposure to beta-amyloid-(1-42). Thus, beta-amyloid-(25-35) can alter [Ca2+]i in microglia on a different time scale than beta-amyloid-(1-42), indicating a specificity in the response of these cells as compared to neurons.

Neurotoxicity of A beta amyloid protein in vitro is not altered by calcium channel blockade

In cortical cultures, A beta protein destabilizes calcium homeostasis, but direct neurotoxicity of A beta is not observed. In hippocampal cultures, we and others find treatment with A beta protein decreases neuronal survival, but the mechanism of neurotoxicity is unknown. We have used low-density, serum-free cultures of hippocampal neurons to determine whether the neurotoxicity of A beta protein in vitro can be altered by voltage- or ligand-gated calcium channel antagonists or cyclic nucleotides. In these cultures, neither omega-conotoxin, nifedipine, verapamil, APV, nor MK-801 altered the survival of neurons exposed to synthetic A beta 1-40. The N-channel antagonist diltiazem decreased A beta 1-40 toxicity repeatedly, but slightly, perhaps by indirectly contributing to increased neuronal viability. Treatment of cultures with dibutyryl cAMP, 8-bromo cAMP, dibutyryl cGMP, and 8-bromo cGMP also failed to alter A beta toxicity. Thus, the toxicity of beta protein in low-density hippocampal cultures was not directly altered either by calcium channel blockers or by the addition of cyclic nucleotides.

Evidence for an endogenous neurocidin in the Manduca sexta ventral nerve cord

Half of the neurons in the abdominal nervous system of the moth Manduca sexta die after adult eclosion. Two physiological signals regulate post-eclosion neuronal death in adult moths. The first is endocrine: a decline in blood ecdysteroids is necessary for the death of neurons in the segmental ganglia. The second signal, which is highly specific for a pair of motoneurons found at the posterior midline in each of the three unfused abdominal ganglia, originates in the nervous system. It is transmitted from the fused pterothoracic ganglion to abdominal ganglion A3 via the intersegmental connectives. To characterize the signal of neural origin, we have developed an in vitro bioassay for neuron-killing factors ("neurocidins"). Aqueous extracts of pterothoracic ganglia were prepared and applied to cultured ventral nerve cords. These extracts exhibited concentration-dependent effectiveness in killing motoneurons. The active component of the extract was heat-stable and protease-sensitive. Size fractionation studies suggested that the active component has a molecular mass between 10 and 30 kD. This is the first report of an endogenous neuron-killing protein from an insect nervous system.