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Failure of anti-amyloid-β peptide (Aβ) therapies against Alzheimer's disease (AD), a neurodegenerative disorder characterized by high amounts of the peptide in the brain, raised the question of the physiological role of Aβ released at low concentrations in the healthy brain. To address this question, we studied the presynaptic and postsynaptic mechanisms underlying the neuromodulatory action of picomolar amounts of oligomeric Aβ 42 (oAβ 42 ) on synaptic glutamatergic function in male and female mice. We found that 200 pm oAβ 42 induces an increase of frequency of miniature EPSCs and a decrease of paired pulse facilitation, associated with an increase in docked vesicle number, indicating that it augments neurotransmitter release at presynaptic level. oAβ 42 also produced postsynaptic changes as shown by an increased length of postsynaptic density, accompanied by an increased expression of plasticity-related proteins such as cAMP-responsive element binding protein phosphorylated at Ser133, calcium-calmodulin-dependent kinase II phosphorylated at Thr286, and brain-derived neurotrophic factor, suggesting a role for Aβ in synaptic tagging. These changes resulted in the conversion of early into late long-term potentiation through the nitric oxide/cGMP/protein kinase G intracellular cascade consistent with a cGMP-dependent switch from short- to long-term memory observed in vivo after intrahippocampal administration of picomolar amounts of oAβ 42 These effects were present upon extracellular but not intracellular application of the peptide and involved α7 nicotinic acetylcholine receptors. These observations clarified the physiological role of oAβ 42 in synaptic function and memory formation providing solid fundamentals for investigating the pathological effects of high Aβ levels in the AD brains. SIGNIFICANCE STATEMENT High levels of oligomeric amyloid-β 42 (oAβ 42 ) induce synaptic dysfunction leading to memory impairment in Alzheimer's disease (AD). However, at picomolar concentrations, the peptide is needed to ensure long-term potentiation (LTP) and memory. Here, we show that extracellular 200 pm oAβ 42 concentrations increase neurotransmitter release, number of docked vesicles, postsynaptic density length, and expression of plasticity-related proteins leading to the conversion of early LTP into late LTP and of short-term memory into long-term memory. These effects require α7 nicotinic acetylcholine receptors and are mediated through the nitric oxide/cGMP/protein kinase G pathway. The knowledge of Aβ function in the healthy brain might be useful to understand the causes leading to its increase and detrimental effect in AD., (Copyright © 2019 the authors.)

Objectives: A neuroinflammatory process, triggered by amyloid-beta (Abeta)-peptide, is thought to play a central role in the neurodegenerative process leading to Alzheimer's disease (AD). Abeta(25-35) retains the functionality of Abeta(42) and was employed to investigate the effects of inflammation-sensitive proteins (ISPs) alpha1-antichymotrypsin (A1ACT) and alpha1-antitrypsin (A1AT) on fibrillar aggregation and cytotoxicity., Design and Methods: Inhibitory concentrations of the ISPs were determined in an established human red blood cell lysis model of Abeta-cytotoxicity. For studies of Abeta-fibrillar aggregation CSF levels of A1ACT (0.041 microM)/A1AT (0.11 microM) were incubated with Congo Red dye 25 microM+Abeta(25-35) 10 microM noting the formation of visible aggregates and spectrophotometric changes over 24 h., Results: A1ACT at CSF reported levels inhibited fibrillar aggregation and cytotoxicity while A1AT at CSF reported levels failed to cause a similar inhibition., Conclusions: A1ACT neutralizes fibrillar aggregation and cytotoxicity of Abeta-peptide more effectively than A1AT. Both proteins are known to be co-deposited with Abeta within senile plaques of AD brains.

Objectives: The aim of this study was to investigate transthyretin (prealbumin) effects on Abeta25-35-induced cytotoxicity., Design and Methods: In view of the well-recognized literature data demonstrating that Abeta25-35 fibrillar aggregates cause in vitro cytotoxicity to human red blood cells and apoptotic changes to SK-N-BE neuroblastoma cells in cultures (ultrastructural evidence), we tested transthyretin effects on these two experimental models., Results: Incubation of Abeta25-35 with transthyretin (at transthyretin concentrations equal to CSF physiological levels) demonstrated both inhibition of red blood cells lysis and neutralization of SK-N-BE neuroblastoma cells ultrastructural apoptotic changes. Moreover, transthyretin was shown to be able to inhibit the formation of fibrillar macroaggregates of Abeta25-35., Conclusions: The findings imply that experimental systems investigating Abeta-induced cytotoxicity consider the protective interaction of transthyretin with Abeta; an interaction to be considered also in vivo in view of the fact that transthyretin immunoreactivity has been previously demonstrated in amyloid plaques of brains from Alzheimer's disease patients.

Background: Fibrillar aggregates of amyloid beta 25-35 (Abeta(25-35)) form rapidly in vitro able to lyse human red blood cells (RBCs). Human sera, albumin, and apolipoprotein E (ApoE) each limit fibrillation and cytotoxicity. Potentially, these substances protect neurons from Abeta(1-40/42) aggregates. Transferrin (TF) is investigated in this study., Methods: The Mattson red blood cells model was employed to determine whether co-incubation of transferrin and Abeta(25-35) prevented lysis. The formation of fibrillar Abeta(25-35) in the presence of transferrin was investigated using Congo red staining and spectrophotometric studies., Results: We found that incubation of 20 muM Abeta(25-35) with physiologic levels of transferrin prevented red blood cells lysis and the formation of macro-aggregates., Conclusions: These in vitro results suggest that transferrin may limit fibrillar beta amyloid formation in vivo and cytotoxicity.

Inhibition of the lysis of human red blood cells (RBCs) exposed to amyloid peptide Abeta25-35 is an model for screening natural and synthetic substances potentially protective against amyloid damage. In this system, human serum and a component, namely apolipoprotein E (apoE), completely prevent RBC lysis. This report demonstrates that albumin, another serum component, is 8-fold more protective: a concentration of 12.5 microg/ml protects RBCs against 20 microM-Abeta25-35, and prevents the formation of fibrillar Abeta25-35 aggregates stainable by Congo Red. The biological relevance of these findings is suggested by the following: (1) a large fraction ( approximately 90%) of circulating Abeta1-42 is bound to albumin; (2) albumin immunoreactivity is present in brain amyloid plaques; and (3) incubation of Abeta with albumin rapidly decreases detectable levels of free Abeta suggesting epitope masking. The results add new and important functional consequences to the amyloid-albumin relationship and imply that experimental systems investigating Abeta cytotoxicity should consider the protective interaction of albumin.

In order to analyze the metabolic response of AD patient platelets to beta-amyloid, we have carried out fluorimetric measurements of intracellular calcium and an ultrastructural survey of platelets exposed to the beta-amyloid active fragment 25-35 (betaA(25-35)). Since it is not possible to analyze directly the damaged neurons in AD, the study of peripheral blood cells, especially platelets, may be of great value for the investigation of the toxic effects of beta-amyloid on AD neuronal cells.

Mattson et al. (9) demonstrated lysis of human red blood cells (RBC) exposed to amyloid peptide Abeta(25-35), a new experimental model for amyloid-beta toxicity. Lysis resulted from poreformation in the RBC membranes and was completely prevented by concurrent exposure to Congo red We demonstrate that human serum, purified ApoE from human plasma, and recombinant isoforms of ApoE neutralize the Abeta(25-35) cytotoxicity: the E2 and E4 isoforms were marginally more effective than E3. Second, we demonstrate that Abeta(25-35) forms fibrils in the reaction mixtures using electron-microscopy. Together these results suggest that the RBC model might be useful in preliminary identification of natural and synthetic substances able to protect against amyloid-beta cytotoxic effects due to fibrillar Abeta(25-35). Such compounds would be candidate molecules for testing in neuronal systems.

A form of beta-amyloid peptide A beta ending at amino acid 42 (A beta42) is the major component of senile amyloid plaques in Alzheimer's Disease (AD). The A beta-peptide earliest modifications are extremely important since they constitute the key events in the progression towards further changes finally leading to fibril formation and to A beta deposits which constitute the core pathological change in AD. Chemical and conformational early modifications of the beta-amyloid peptide are critical steps in AD pathogenesis and have been widely investigated. We now show that a Fenton-type OH-generating system is capable of generating L-Dopa (3,4-dihydroxyphenylalanine) in the tyrosine residue of A beta-peptide via aromatic ring hydroxylation, as the result of hydroxyl radical attack on proteins. Since L-Dopa is not a constituent of mammalian proteins and peptides, the formation of L-Dopa in A beta in vitro constitutes a possible important modification caused by hydroxyl radical attack. These results lay the groundwork for further studies on modification and damage associated with the degenerative disorder in AD where oxidative stress and inflammation are known to occur.

beta-amyloid (A beta) is a normal soluble peptide found in the cerebrospinal fluid (CSF) and other biological fluids. A beta fibrils are associated with Alzheimer's disease (AD) senile plaques. We have used purified soluble A beta (1-42) and A beta (12-28) peptides in order to determine the oxidative modification induced in these peptides by exposure to peroxidase and hydrogen peroxide. We have demonstrated that under these in vitro conditions, dimeric forms of A beta (1-42) can be detected by high-resolution polyacrylamide SDS-PAGE electrophoresis. Further experiments performed by reverse-phase high performance liquid chromatography (RP-HPLC), and monitored by fluorescence detection, showed that the dimeric A beta (1-42) forms induced by the peroxidase reaction are the outcomes of dityrosine bridge formation. This cross-link results from the enzyme catalyzed oxidation. During this reaction, phenolic coupling of tyrosine residues of two A beta (1-42) peptides occurs. No detectable peroxidative modifications were observed with the A beta (12-28) peptide which lacks a tyrosine residue. Since oxidative stress is thought to be associated with AD, the experimental model described here can help in understanding the early events leading to chemical, structural and conformational modifications before the conversion of sA beta to amyloid fibrils and eventually the formation of senile plaques in AD.