Amino-terminally elongated Aß peptides are generated by the secreted metalloprotease ADAMTS4 and deposit in a subset of Alzheimer's disease brains.

AIMS: The aggregation and deposition of amyloid-ß (Aß) peptides in the brain is thought to be the initial driver in the pathogenesis of Alzheimer's disease (AD). Aside from full-length Aß peptides starting with an aspartate residue in position 1, both N-terminally truncated and elongated Aß peptides are produced by various proteases from the amyloid precursor protein (APP) and have been detected in brain tissues and body fluids. Recently, we demonstrated that the particularly abundant N-terminally truncated Aß4-x peptides are generated by ADAMTS4, a secreted metalloprotease that is exclusively expressed in the oligodendrocyte cell population. In this study, we investigated whether ADAMTS4 might also be involved in the generation of N-terminally elongated Aß peptides. METHODS: We used cell-free and cell-based assays in combination with matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF) and electrochemiluminescence sandwich immunoassays to identify and quantify N-terminally elongated Aß peptide variants. Antibodies against these Aß variants were characterised by peptide microarrays and employed for the immunohistochemical analyses of human brain samples. RESULTS: In this study, we discovered additional ADAMTS4 cleavage sites in APP. These were located N-terminal to Asp-(1) in the Aß peptide sequence between residues Glu-(-7) and Ile-(-6) as well as Glu-(-4) and Val-(-3), resulting in the release of N-terminally elongated Aß-6-x and Aß-3-x peptides, of which the latter serve as a component in a promising Aß-based plasma biomarker. Aß-6/-3-40 peptides were detected in supernatants of various cell lines and in the cerebrospinal fluid (CSF), and ADAMTS4 enzyme activity promoted the release of Aß-6/-3-x peptides. Furthermore, by immunohistochemistry, a subset of AD cases displayed evidence of extracellular and vascular localization of N-terminally elongated Aß-6/-3-x peptides. DISCUSSION: The current findings implicate ADAMTS4 in both the pathological process of Aß peptide aggregation and in the early detection of amyloid pathology in AD.

Matrix Development for the Detection of Phosphorylated Amyloid-ß Peptides by MALDI-TOF-MS.

Amyloid-ß (Aß) peptides, including post-translationally modified variants thereof, are believed to play a key role in the onset and progression of Alzheimer's disease. Suggested modified Aß species with potential disease relevance include Aß peptides phosphorylated at serine in position eight (pSer8-Aß) or 26 (pSer26-Aß). However, the published studies on those Aß peptides essentially relied on antibody-based approaches. Thus, complementary analyses by mass spectrometry, as shown for other modified Aß variants, will be necessary not only to unambiguously verify the existence of phosphorylated Aß species in brain samples but also to reveal their exact identity as to phosphorylation sites and potential terminal truncations. With the aim of providing a novel tool for addressing this still-unresolved issue, we developed a customized matrix formulation, referred to as TOPAC, that allows for improved detection of synthetic phosphorylated Aß species by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. When TOPAC was compared with standard matrices, we observed higher signal intensities but minimal methionine oxidation and phosphate loss for intact pSer8-Aß(1-40) and pSer26-Aß(1-40). Similarly, TOPAC also improved the mass spectrometric detection and sequencing of the proteolytic cleavage products pSer8-Aß(1-16) and pSer26-Aß(17-28). We expect that TOPAC will facilitate future efforts to detect and characterize endogenous phosphorylated Aß species in biological samples and that it may also find its use in phospho-proteomic approaches apart from applications in the Aß field.

Detection and quantification of Aß-3-40 (APP669-711) in cerebrospinal fluid.

Neurochemical biomarkers can support the diagnosis of Alzheimer's disease and may facilitate clinical trials. In blood plasma, the ratio of the amyloid-ß (Aß) peptides Aß-3-40/Aß1-42 can predict cerebral amyloid-ß pathology with high accuracy (Nakamura et al., 2018). Whether or not Aß-3-40 (aka. amyloid precursor protein (APP) 669-711) is also present in cerebrospinal fluid (CSF) is not clear. Here, we investigated whether Aß-3-40 can be detected in CSF and to what extent the CSF Aß-3-40/Aß42 ratio is able to differentiate between individuals with or without amyloid-ß positron emission tomography (PET) evidence of brain amyloid. The occurrence of Aß-3-40 in human CSF was assessed by immunoprecipitation followed by mass spectrometry. For quantifying the CSF concentrations of Aß-3-40 in 23 amyloid PET-negative and 17 amyloid PET-positive subjects, we applied a sandwich-type immunoassay. Our findings provide clear evidence of the presence of Aß-3-40 and Aß-3-38 in human CSF. While there was no statistically significant difference in the CSF concentration of Aß-3-40 between the two diagnostic groups, the CSF Aß-3-40/Aß42 ratio was increased in the amyloid PET-positive individuals. We conclude that Aß-3-40 appears to be a regular constituent of CSF and may potentially serve to accentuate the selective decrease in CSF Aß42 in Alzheimer's disease.

The CNS Myelin Proteome: Deep Profile and Persistence After Post-mortem Delay.

Myelin membranes are dominated by lipids while the complexity of their protein composition has long been considered to be low. However, numerous additional myelin proteins have been identified since. Here we revisit the proteome of myelin biochemically purified from the brains of healthy c56Bl/6N-mice utilizing complementary proteomic approaches for deep qualitative and quantitative coverage. By gel-free, label-free mass spectrometry, the most abundant myelin proteins PLP, MBP, CNP, and MOG constitute 38, 30, 5, and 1% of the total myelin protein, respectively. The relative abundance of myelin proteins displays a dynamic range of over four orders of magnitude, implying that PLP and MBP have overshadowed less abundant myelin constituents in initial gel-based approaches. By comparisons with published datasets we evaluate to which degree the CNS myelin proteome correlates with the mRNA and protein abundance profiles of myelin and oligodendrocytes. Notably, the myelin proteome displays only minor changes if assessed after a post-mortem delay of 6 h. These data provide the most comprehensive proteome resource of CNS myelin so far and a basis for addressing proteomic heterogeneity of myelin in mouse models and human patients with white matter disorders.

Delineating the Molecular Basis of the Calmodulin‒bMunc13-2 Interaction by Cross-Linking/Mass Spectrometry-Evidence for a Novel CaM Binding Motif in bMunc13-2.

Exploring the interactions between the Ca2+ binding protein calmodulin (CaM) and its target proteins remains a challenging task. Members of the Munc13 protein family play an essential role in short-term synaptic plasticity, modulated via the interaction with CaM at the presynaptic compartment. In this study, we focus on the bMunc13-2 isoform expressed in the brain, as strong changes in synaptic transmission were observed upon its mutagenesis or deletion. The CaM‒bMunc13-2 interaction was previously characterized at the molecular level using short bMunc13-2-derived peptides only, revealing a classical 1‒5‒10 CaM binding motif. Using larger protein constructs, we have now identified for the first time a novel and unique CaM binding site in bMunc13-2 that contains an N-terminal extension of a classical 1‒5‒10 CaM binding motif. We characterize this motif using a range of biochemical and biophysical methods and highlight its importance for the CaM‒bMunc13-2 interaction.

The Calmodulin Binding Region of the Synaptic Vesicle Protein Mover Is Required for Homomeric Interaction and Presynaptic Targeting.

Neurotransmitter release is mediated by an evolutionarily conserved machinery. The synaptic vesicle (SV) associated protein Mover/TPRGL/SVAP30 does not occur in all species and all synapses. Little is known about its molecular properties and how it may interact with the conserved components of the presynaptic machinery. Here, we show by deletion analysis that regions required for homomeric interaction of Mover are distributed across the entire molecule, including N-terminal, central and C-terminal regions. The same regions are also required for the accumulation of Mover in presynaptic terminals of cultured neurons. Mutating two phosphorylation sites in N-terminal regions did not affect these properties. In contrast, a point mutation in the predicted Calmodulin (CaM) binding sequence of Mover abolished both homomeric interaction and presynaptic targeting. We show that this sequence indeed binds Calmodulin, and that recombinant Mover increases Calmodulin signaling upon heterologous expression. Our data suggest that presynaptic accumulation of Mover requires homomeric interaction mediated by regions distributed across large areas of the protein, and corroborate the hypothesis that Mover functionally interacts with Calmodulin signaling.

N-Terminal Truncated Aß4-42 Is a Substrate for Neprilysin Degradation in vitro and in vivo.

In sporadic Alzheimer's disease (AD), an imbalance between production and clearance of amyloid-ß (Aß) peptides seems to account for enhanced Aß accumulation. The metalloprotease neprilysin (NEP) is an important Aß degrading enzyme as shown by a variety of in vitro and in vivo studies. While the degradation of full-length Aß peptides such as Aß1-40 and Aß1-42 is well established, it is less clear whether NEP is also capable of degrading N-terminally truncated Aß species such as the common variant Aß4-42. In the present report, we confirmed the degradation of Aß4-x species by neprilysin using in vitro digestion and subsequent analysis using gel-based assays and mass spectrometry. By crossing Tg4-42 mice expressing only Aß4-42 peptides with homozygous NEP-knock-out mice (NEP-/-), we were able to demonstrate that NEP deficiency increased hippocampal intraneuronal Aß levels and aggravated neuron loss in the Tg4-42 transgenic mouse model of AD.

Partial Immunoblotting of 2D-Gels: A Novel Method to Identify Post-Translationally Modified Proteins Exemplified for the Myelin Acetylome.

Post-translational modifications (PTMs) play a key role in regulating protein function, yet their identification is technically demanding. Here, we present a straightforward workflow to systematically identify post-translationally modified proteins based on two-dimensional gel electrophoresis. Upon colloidal Coomassie staining the proteins are partially transferred, and the investigated PTMs are immunodetected. This strategy allows tracking back the immunopositive antigens to the corresponding spots on the original gel, from which they are excised and mass spectrometrically identified. Candidate proteins are validated on the same membrane by immunodetection using a second fluorescence channel. We exemplify the power of partial immunoblotting with the identification of lysine-acetylated proteins in myelin, the oligodendroglial membrane that insulates neuronal axons. The excellent consistency of the detected fluorescence signals at all levels allows the differential comparison of PTMs across multiple conditions. Beyond PTM screening, our multi-level workflow can be readily adapted to clinical applications such as identifying auto-immune antigens or host-pathogen interactions.

A presequence-binding groove in Tom70 supports import of Mdl1 into mitochondria.

The translocase of the outer mitochondrial membrane (TOM complex) is the general entry gate into mitochondria for almost all imported proteins. A variety of specific receptors allow the TOM complex to recognize targeting signals of various precursor proteins that are transported along different import pathways. Aside from the well-characterized presequence receptors Tom20 and Tom22 a third TOM receptor, Tom70, binds proteins of the carrier family containing multiple transmembrane segments. Here we demonstrate that Tom70 directly binds to presequence peptides using a dedicated groove. A single point mutation in the cavity of this pocket (M551R) reduces the presequence binding affinity of Tom70 ten-fold and selectively impairs import of the presequence-containing precursor Mdl1 but not the ADP/ATP carrier (AAC). Hence Tom70 contributes to the presequence import pathway by recognition of the targeting signal of the Mdl1 precursor.

TMPRSS2 and ADAM17 cleave ACE2 differentially and only proteolysis by TMPRSS2 augments entry driven by the severe acute respiratory syndrome coronavirus spike protein.

The type II transmembrane serine proteases TMPRSS2 and HAT can cleave and activate the spike protein (S) of the severe acute respiratory syndrome coronavirus (SARS-CoV) for membrane fusion. In addition, these proteases cleave the viral receptor, the carboxypeptidase angiotensin-converting enzyme 2 (ACE2), and it was proposed that ACE2 cleavage augments viral infectivity. However, no mechanistic insights into this process were obtained and the relevance of ACE2 cleavage for SARS-CoV S protein (SARS-S) activation has not been determined. Here, we show that arginine and lysine residues within ACE2 amino acids 697 to 716 are essential for cleavage by TMPRSS2 and HAT and that ACE2 processing is required for augmentation of SARS-S-driven entry by these proteases. In contrast, ACE2 cleavage was dispensable for activation of the viral S protein. Expression of TMPRSS2 increased cellular uptake of soluble SARS-S, suggesting that protease-dependent augmentation of viral entry might be due to increased uptake of virions into target cells. Finally, TMPRSS2 was found to compete with the metalloprotease ADAM17 for ACE2 processing, but only cleavage by TMPRSS2 resulted in augmented SARS-S-driven entry. Collectively, our results in conjunction with those of previous studies indicate that TMPRSS2 and potentially related proteases promote SARS-CoV entry by two separate mechanisms: ACE2 cleavage, which might promote viral uptake, and SARS-S cleavage, which activates the S protein for membrane fusion. These observations have interesting implications for the development of novel therapeutics. In addition, they should spur efforts to determine whether receptor cleavage promotes entry of other coronaviruses, which use peptidases as entry receptors.

Analysis of amino-terminal variants of amyloid-ß peptides by capillary isoelectric focusing immunoassay.

Here we present a novel assay for the separation and detection of amino-terminal amyloid-ß (Aß) peptide variants by capillary isoelectric focusing (CIEF) immunoassay. Specific amino-terminally truncated Aß peptides appear to be generated by ß-secretase (BACE1)-independent mechanisms and have previously been observed in cerebrospinal fluid (CSF) after BACE1 inhibitor treatment in an animal model. CIEF immunoassay sensitivity is sufficient to detect total Aß in CSF without preconcentration. To analyze low-abundance amino-terminally truncated Aß peptides from cell culture supernatants, we developed a CIEF-compatible immunoprecipitation protocol, allowing for selective elution of Aß peptides with very low background. CIEF immunoassay and immunoprecipitation mass spectrometry analysis identified peptides starting at residue Arg(5) as the main amino-terminal Aß variants produced in the presence of tripartite BACE1 inhibitor in our cell culture model. The CIEF immunoassay allows for robust relative quantification of Aß peptide patterns in biological samples. To assess the future possibility of absolute quantification, we have prepared the Aß peptides Aß(x-10), Aß(x-16), and Aß(5-38(D23S)) by using solid phase peptide synthesis as internal standards for the CIEF immunoassay.

Structural insights into the calmodulin-Munc13 interaction obtained by cross-linking and mass spectrometry.

Munc13 proteins are essential regulators of synaptic vesicle priming and play a key role in adaptive synaptic plasticity phenomena. We recently identified and characterized the Ca(2+)-dependent interaction of Munc13 and calmodulin (CaM) as the molecular mechanism linking changes in residual Ca(2+) concentrations to presynaptic vesicle priming and short-term plasticity. Here, we used peptidic photoprobes covering the established CaM-binding motif of Munc13 for photoaffinity labeling (PAL) of CaM, followed by structural characterization of the covalent photoadducts. Our innovative analytical workflow based on isotopically labeled CaM and mass spectrometry revealed that, in the bound state, the hydrophobic anchor residue of the CaM-binding motif in Munc13s contacts two distinct methionine residues in the C-terminal domain of CaM. To address the orientation of the peptide during binding, we obtained additional distance constraints from the mass spectrometric analysis of chemically cross-linked CaM-Munc13 peptide adducts. The constraints from both complementary cross-linking approaches were integrated into low-resolution three-dimensional structure models of the CaM-Munc13 peptide complexes. Our experimental data are best compatible with the structure of the complex formed by CaM and a CaM-binding peptide derived from neuronal NO synthase and show that Munc13-1 and ubMunc13-2 bind to CaM in an antiparallel orientation through a 1-5-8 motif. The structural information about the CaM-Munc13 peptide complexes will facilitate the design of Munc13 variants with altered CaM affinity and thereby advance the detailed functional analysis of the role of Munc13 proteins in synaptic transmission and plasticity.