Intracellular amyloid and the neuronal origin of Alzheimer neuritic plaques

Genetic analysis of familial forms of Alzheimer's disease (AD) causally links the proteolytic processing of the amyloid precursor protein (APP) and AD. However, the specific type of amyloid and mechanisms of amyloid pathogenesis remain unclear. We conducted a detailed analysis of intracellular amyloid with an aggregation specific conformation dependent monoclonal antibody, M78, raised against fibrillar Aß42. M78 immunoreactivity colocalizes with Aß and the carboxyl terminus of APP (APP-CTF) immunoreactivities in perinuclear compartments at intermediate times in 10month 3XTg-AD mice, indicating that this represents misfolded and aggregated protein rather than normally folded APP. At 12months, M78 immunoreactivity also accumulates in the nucleus. Neuritic plaques at 12months display the same spatial organization of centrally colocalized M78, diffuse chromatin and neuronal nuclear NeuN staining surrounded by peripheral M78 and APP-CTF immunoreactivity as observed in neurons, indicating that neuritic plaques arise from degenerating neurons with intracellular amyloid immunoreactivity. The same staining pattern was observed in neuritic plaques in human AD brains, showing elevated intracellular M78 immunoreactivity at intermediate stages of amyloid pathology (Braak A and B) compared to no amyloid pathology and late stage amyloid pathology (Braak 0 and C, respectively). These results indicate that intraneuronal protein aggregation and amyloid accumulation is an early event in AD and that neuritic plaques are initiated by the degeneration and death of neurons by a mechanism that may be related to the formation of extracellular traps by neutrophils.

Fibril specific, conformation dependent antibodies recognize a generic epitope common to amyloid fibrils and fibrillar oligomers that is absent in prefibrillar oligomers

BACKGROUND

Amyloid-related degenerative diseases are associated with the accumulation of misfolded proteins as amyloid fibrils in tissue. In Alzheimer disease (AD), amyloid accumulates in several distinct types of insoluble plaque deposits, intracellular Abeta and as soluble oligomers and the relationships between these deposits and their pathological significance remains unclear. Conformation dependent antibodies have been reported that specifically recognize distinct assembly states of amyloids, including prefibrillar oligomers and fibrils.

RESULTS

We immunized rabbits with a morphologically homogeneous population of Abeta42 fibrils. The resulting immune serum (OC) specifically recognizes fibrils, but not random coil monomer or prefibrillar oligomers, indicating fibrils display a distinct conformation dependent epitope that is absent in prefibrillar oligomers. The fibril epitope is also displayed by fibrils of other types of amyloids, indicating that the epitope is a generic feature of the polypeptide backbone. The fibril specific antibody also recognizes 100,000 x G soluble fibrillar oligomers ranging in size from dimer to greater than 250 kDa on western blots. The fibrillar oligomers recognized by OC are immunologically distinct from prefibrillar oligomers recognized by A11, even though their sizes overlap broadly, indicating that size is not a reliable indicator of oligomer conformation. The immune response to prefibrillar oligomers and fibrils is not sequence specific and antisera of the same specificity are produced in response to immunization with islet amyloid polypeptide prefibrillar oligomer mimics and fibrils. The fibril specific antibodies stain all types of amyloid deposits in human AD brain. Diffuse amyloid deposits stain intensely with anti-fibril antibody although they are thioflavin S negative, suggesting that they are indeed fibrillar in conformation. OC also stains islet amyloid deposits in transgenic mouse models of type II diabetes, demonstrating its generic specificity for amyloid fibrils.

CONCLUSION

Since the fibril specific antibodies are conformation dependent, sequence-independent, and recognize epitopes that are distinct from those present in prefibrillar oligomers, they may have broad utility for detecting and characterizing the accumulation of amyloid fibrils and fibrillar type oligomers in degenerative diseases.

Rapid Characterization of Amyloid- Side-Chain Oxidation by Tandem Mass Spectrometry and the Scoring Algorithm for Spectral Analysis

PURPOSE

Amyloid-beta (Abeta) is a self-aggregating protein found in senile plaques in Alzheimer's disease (AD) brain and is thought to play a major role in the disease process. Oxidative stress may be a predominant cause of the formation of these Abeta aggregates. This study aims at identifying possible sites of copper-catalyzed oxidation of Abeta1-40 using liquid chromatography tandem mass spectrometry (LC/MS/MS) and scoring algorithm for spectral analysis (SALSA). Traditionally, identification of post-translational modifications by tandem mass spectrometric analysis requires users to inspect manually thousands of MS/MS spectra, which can be a tedious and time-consuming process. With the use of SALSA, users can automatically search for post-translational modifications based on the spacing of the m/z values associated with the ion series of an amino acid sequence.

METHODS

Abeta1-40 was subjected to copper-catalyzed oxidative stress. LC/MS/MS and SALSA analyses were used to determine the sites of post-translational modification within the tryptic fragments.

RESULTS

Oxidation was found to occur preferentially at the histidine residues Hisl3 and Hisl4 and at the methionine residue (Met35) of Abeta1-40.

CONCLUSIONS

The combination of LC/MS/MS and SALSA searches could dramatically improve the efficiency and accuracy of determining the specific sites of oxidation of in vitro, copper-oxidized Abeta1-40 as well as other oxidized proteins.

Complement component C1q modulates the phagocytosis of Abeta by microglia

Recent studies showing that microglia internalize the amyloid beta-peptide (Abeta) suggest that these cells have the potential for clearing Abeta deposits in Alzheimer's disease, and mechanisms that regulate the removal of Abeta may therefore be of clinical interest. Previous studies from this laboratory showing that C1q enhances phagocytosis of cellular targets by rat microglia prompted the current investigations characterizing the effects of C1q on microglial phagocytosis of Abeta. Microglia were shown to phagocytose Abeta1-42, in agreement with observations of other investigators. Uptake of Abeta1-42 was observed for concentrations of 5-50 microM, and phagocytosis of peptides containing (14)C or fluorescein (FM) labels was not affected by the interaction of microglia with C1q-coated surfaces. However, inclusion of C1q (125 nM-1.4 microM) in solutions of 50 microM Abeta1-42 inhibited the uptake of (14)C-Abeta1-42 and FM-Abeta1-42, suggesting that C1q blocks the interaction of Abeta with microglia. Uptake of Abeta was partially blocked by the scavenger receptor ligands polyinosinic acid and maleylated BSA. Inhibition of Abeta uptake by C1q may contribute to the accumulation of fibrillar, C1q-containing plaques that occurs in parallel with disease progression. These data suggest that mechanisms which interfere with the binding of C1q to Abeta may be of therapeutic value both through inhibition of the inflammatory events resulting from complement activation and via altered access of Abeta sites necessary for ingestion by microglia.

Loss of endosomal/lysosomal membrane impermeability is an early event in amyloid Abeta1-42 pathogenesis

Previous studies have implicated the failure to degrade aggregated Abeta1-42 in late endosomes or secondary lysosomes as a mechanism for the accumulation of beta-amyloid in Alzheimer's disease. We examined the consequences of intracellular accumulation of Abeta1-42 on the integrity of the endosomal/lysosomal compartment by monitoring Lucifer Yellow fluorescence and the release of lysosomal hydrolases into the soluble, cytosolic fraction. In control cells, the Lucifer Yellow fluorescence is observed as punctate staining in a perinuclear distribution with no apparent cytoplasmic fluorescence, consistent with its localization in late endosomes or secondary lysosomes. After incubation with Abeta1-42 for 6 hr, a loss of lysosomal membrane impermeability is observed as evidenced by redistribution of the fluorescence to a diffuse, cytoplasmic pattern. The loss of lysosomal membrane impermeability is correlated with Abeta1-42 accumulation, since incubation of the cells with the nonaccumulating isoform of amyloid, Abeta1-40, does not induce leakage. The same results were obtained using the release of soluble lysosomal hydrolases, cathepsin D and beta-hexosaminidase, into the cytosol as an assay for the leakage of lysosomal contents. Together, our results suggest that the loss of lysosomal membrane impermeability may be an early event in Abeta pathogenesis, and provide an explanation for the miscompartmentalization of extracellular and cytoplasmic components observed in Alzheimer's disease (AD). The release of hydrolases may further cause the breakdown of the cytoskeleton and the blebbing of the plasma membrane, and the leakage of heparan sulfate glycosaminoglycans from the lysosome may ultimately promote the assembly of tau into neurofibrillary tangles (NFT).