A highly sensitive novel immunoassay specifically detects low levels of soluble Aβ oligomers in human cerebrospinal fluid

INTRODUCTION

Amyloid β-protein oligomers play a key role in Alzheimer's disease (AD), but well-validated assays that routinely detect them in cerebrospinal fluid (CSF) are just emerging. We sought to confirm and extend a recent study using the Singulex Erenna platform that reported increased mean CSF oligomer levels in AD.

METHODS

We tested four antibody pairs and chose one pair that was particularly sensitive, using 1C22, our new oligomer-selective monoclonal antibody, for capture. We applied this new assay to extracts of human brain and CSF.

RESULTS

A combination of 1C22 for capture and 3D6 for detection yielded an Erenna immunoassay with a lower limit of quantification of approximately 0.15 pg/ml that was highly selective for oligomers over monomers and detected a wide size-range of oligomers. Most CSFs we tested had detectable oligomer levels but with a large overlap between AD and controls and a trend for higher mean levels in mild cognitive impairment (MCI) than controls.

CONCLUSION

Aβ oligomers are detectable in most human CSFs, but AD and controls overlap. MCI CSFs may have a modest elevation in mean value by this assay.

Tau immunization: a cautionary tale?

The amyloid β (Aβ)-protein and microtubule-associated protein, tau, are the major components of the amyloid plaques and neurofibrillary tangles that typify Alzheimer's disease (AD) pathology. As such both Aβ and tau have long been proposed as therapeutic targets. Immunotherapy, particularly targeting Aβ, is currently the most advanced clinical strategy for treating AD. However, several Aβ-directed clinical trials have failed, and there is concern that targeting this protein may not be useful. In contrast, there is a growing optimism that tau immunotherapy may prove more efficacious. Here, for the first time, we studied the effects of chronic administration of an anti-tau monoclonal antibody (5E2) in amyloid precursor protein transgenic mice. For our animal model, we chose the J20 mouse line because prior studies had shown that the cognitive deficits in these mice require expression of tau. Despite the fact that 5E2 was present and active in the brains of immunized mice and that this antibody appeared to engage with extracellular tau, 5E2-treatment did not recover age-dependent spatial reference memory deficits. These results indicate that the memory impairment evident in J20 mice is unlikely to be mediated by a form of extracellular tau recognized by 5E2. In addition to the lack of positive effect of anti-tau immunotherapy, we also documented a significant increase in mortality among J20 mice that received 5E2. Because both the J20 mice used here and tau transgenic mice used in prior tau immunotherapy trials are imperfect models of AD our results recommend extensive preclinical testing of anti-tau antibody-based therapies using multiple mouse models and a variety of different anti-tau antibodies.

Exosomes neutralize synaptic-plasticity-disrupting activity of Aβ assemblies in vivo

Background

Exosomes, small extracellular vesicles of endosomal origin, have been suggested to be involved in both the metabolism and aggregation of Alzheimer’s disease (AD)-associated amyloid β-protein (Aβ). Despite their ubiquitous presence and the inclusion of components which can potentially interact with Aβ, the role of exosomes in regulating synaptic dysfunction induced by Aβ has not been explored.

Results

We here provide in vivo evidence that exosomes derived from N2a cells or human cerebrospinal fluid can abrogate the synaptic-plasticity-disrupting activity of both synthetic and AD brain-derived Aβ. Mechanistically, this effect involves sequestration of synaptotoxic Aβ assemblies by exosomal surface proteins such as PrP^C rather than Aβ proteolysis.

Conclusions

These data suggest that exosomes can counteract the inhibitory action of Aβ, which contributes to perpetual capability for synaptic plasticity.

Thermal induction of an alternatively folded state in human IgG-Fc

We report the formation of a non-native, folded state of human IgG4-Fc induced by a high temperature at neutral pH and at a physiological salt concentration. This structure is similar to the molten globule state in that it displays a high degree of secondary structure content and surface-exposed hydrophobic residues. However, it is highly resistant to chemical denaturation. The thermally induced state of human IgG4-Fc is thus associated with typical properties of the so-called alternatively folded state previously described for murine IgG, IgG-Fab, and individual antibody domains (V(L), V(H), C(H)1, and C(H)3) under acidic conditions in the presence of anions. Like some of these molecules, human IgG4-Fc in its alternative fold exists as a mixture of different oligomeric structures, dominated by an equilibrium between monomeric and heptameric species. Heating further induces the formation of fibrous structures in the micrometer range.

Designed surface with tunable IgG density as an in vitro model for immune complex mediated stimulation of leukocytes

We present the design of an in vitro model for immune-complex-mediated stimulation of leukocytes and its functional characteristics with respect to monocyte adhesion. The model was based on the orientation-controlled immobilization of a humanized IgG1 monoclonal antibody (rituximab) via its interaction with a biotinylated peptide epitope derived from the CD20 marker. The peptide was linked to neutravidin covalently attached to a mixed self-assembled monolayer of carboxyl- and methoxy-terminated oligo(ethylene glycol) alkane thiolates on gold. The surface adhesion propensity of human monocytes (cell line U937) was highly dependent on the lateral IgG density and indicated that there exists a distance between IgG-Fc on the surface where interactions with Fc gamma receptors are optimal. This well-defined platform allows for a careful control of the size and orientation of artificial IgG immune complexes, it is easily made compatible with, for example, cellular imaging, and it will become useful for in vitro studies on the importance of Fc gamma receptor interactions in chronic immune-mediated diseases.

A peptide-based, ratiometric biosensor construct for direct fluorescence detection of a protein analyte

We present the design, synthesis, and functional evaluation of peptide-based fluorescent constructs for wavelength-ratiometric biosensing of a protein analyte. The concept was shown using the high-affinity model interaction between the 18 amino acid peptide pTMVP and a recombinant antibody fragment, Fab57P. pTMVP was functionalized in two different positions with 6-bromomethyl-2-(2-furanyl)-3-hydroxychromone, an environmentally sensitive fluorophore with a two-band emission. The equilibrium dissociation constant of the interaction between pTMVP and Fab57P was largely preserved upon labeling. The biosensor ability of the labeled peptide constructs was evaluated in terms of the relative intensity change of the emission bands from the normal (N*) and tautomer (T*) excited-state species of the fluorophore ( I(N*)/I(T*)) upon binding of Fab57P. When the peptide was labeled in the C terminus, the I(N*)/I(T*) ratio changed by 40% upon analyte binding, while labeling close to the residues most important for binding resulted in a construct that completely lacked ratiometric biosensor ability. Integrated biosensor elements for reagentless detection, where peptides and ratiometric fluorophores are combined to ensure robustness in both recognition and signaling, are expected to become an important contribution to the design of future protein quantification assays in immobilized formats.