Pyroglutamate-Aβ: role in the natural history of Alzheimer's disease

Metal Ion Binding to Human Glutaminyl Cyclase: A Structural Perspective

Glutaminyl-peptide cyclotransferases (QCs) convert the N-terminal glutamine or glutamate residues of protein and peptide substrates into pyroglutamate (pE) by releasing ammonia or a water molecule. The N-terminal pE modification protects peptides/proteins against proteolytic degradation by amino- or exopeptidases, increasing their stability. Mammalian QC is abundant in the brain and a large amount of evidence indicates that pE peptides are involved in the onset of neural human pathologies such as Alzheimer's and Huntington's disease and synucleinopathies. Hence, human QC (hQC) has become an intensively studied target for drug development against these diseases. Soon after its characterization, hQC was identified as a Zn-dependent enzyme, but a partial restoration of the enzyme activity in the presence of the Co(II) ion was also reported, suggesting a possible role of this metal ion in catalysis. The present work aims to investigate the structure of demetallated hQC and of the reconstituted enzyme with Zn(II) and Co(II) and their behavior in the presence of known inhibitors. Furthermore, our structural determinations provide a possible explanation for the presence of the mononuclear metal binding site of hQC, despite the presence of the same conserved metal binding motifs present in distantly related dinuclear aminopeptidase enzymes.

Associations of Plasma Glutamatergic Metabolites with Alpha Desynchronization during Cognitive Interference and Working Memory Tasks in Asymptomatic Alzheimer's Disease

Electroencephalogram (EEG) studies have suggested compensatory brain overactivation in cognitively healthy (CH) older adults with pathological beta-amyloid(Aβ42)/tau ratios during working memory and interference processing. However, the association between glutamatergic metabolites and brain activation proxied by EEG signals has not been thoroughly investigated. We aim to determine the involvement of these metabolites in EEG signaling. We focused on CH older adults classified under (1) normal CSF Aβ42/tau ratios (CH-NATs) and (2) pathological Aβ42/tau ratios (CH-PATs). We measured plasma glutamine, glutamate, pyroglutamate, and γ-aminobutyric acid concentrations using tandem mass spectrometry and conducted a correlational analysis with alpha frequency event-related desynchronization (ERD). Under the N-back working memory paradigm, CH-NATs presented negative correlations (r = ~-0.74--0.96, p = 0.0001-0.0414) between pyroglutamate and alpha ERD but positive correlations (r = ~0.82-0.95, p = 0.0003-0.0119) between glutamine and alpha ERD. Under Stroop interference testing, CH-NATs generated negative correlations between glutamine and left temporal alpha ERD (r = -0.96, p = 0.037 and r = -0.97, p = 0.027). Our study demonstrated that glutamine and pyroglutamate levels were associated with EEG activity only in CH-NATs. These results suggest cognitively healthy adults with amyloid/tau pathology experience subtle metabolic dysfunction that may influence EEG signaling during cognitive challenge. A longitudinal follow-up study with a larger sample size is needed to validate these pilot studies.

Imbalanced clearance of Aβ peptide cause presynaptic plaque formation

Alzheimer's disease is characterized by abnormal increase of Aβ peptide which is likely as the result of imbalanced homeostasis of its production and clearance mechanisms. Here, we briefly review that the uncleaned extracellular Aβ peptides are loaded into presynaptic neurons. The Aβ oligomers desperately affect pre- and post-synapse neuron activity and turn into plaques inside the presynaptic neurons over the time passes.

Synthesis and Evaluation of a Novel PET Radioligand for Imaging Glutaminyl Cyclase Activity as a Biomarker for Detecting Alzheimer's Disease

Several new lines of research have demonstrated that a significant number of amyloid-β peptides found in Alzheimer's disease (AD) are truncated and undergo post-translational modification by glutaminyl cyclase (QC) at the N-terminal. Notably, QC's products of Abeta-pE3 and Abeta-pE11 have been active targets for investigational drug development. This work describes the design, synthesis, characterization, and in vivo validation of a novel PET radioligand, [18F]PB0822, for targeted imaging of QC. We report herein a simplified and robust chemistry for the synthesis of the standard compound, [19F]PB0822, and the corresponding [18F]PB0822 radioligand. The PET probe was developed with 99.9% radiochemical purity, a molar activity of 965 Ci.mmol-1, and an IC50 of 56.3 nM, comparable to those of the parent PQ912 inhibitor (62.5 nM). Noninvasive PET imaging showed that the probe is distributed in the brain 5 min after intravenous injection. Further, in vivo PET imaging with [18F]PB0822 revealed that AD 5XFAD mice harbor significantly higher QC activity than WT counterparts. The data also suggested that QC activity is found across different brain regions of the tested animals.

Conformation of Pyroglutamated Amyloid β (3-40) and (11-40) Fibrils - Extended or Hairpin?

Amyloid β (Aβ) is a hallmark protein of Alzheimer's disease. One physiologically important Aβ variant is formed by initial N-terminal truncation at a glutamic acid position (either E3 or E11), which is subsequently cyclized to a pyroglutamate (either pE3 or pE11). Both forms have been found in high concentrations in the core of amyloid plaques and are likely of high importance in the pathology of Alzheimer's disease. However, the molecular structure of the fibrils of these variants is not entirely clear. Solid-state NMR spectroscopy studies have reported a molecular contact between Gly25 and Ile31, which would disagree with the conventional hairpin model of wildtype (WT-)Aβ1-40 fibrils, most often described in the literature. We investigated the conformation of the monomeric unit of pE3-Aβ3-40 and pE11-Aβ11-40 (and for comparison also wildtype (WT)-Aβ1-40) fibrils to find out whether the hairpin or a newly suggested extended structure dominates the structure of the Aβ monomers in these fibrils. To this end, solid-state NMR spectroscopy was applied probing the inter-residual contacts between Phe19/Leu34, Ala21/Leu34, and especially Gly25/Ile31 using suitable isotopic labeling schemes. In the second part, the flexible turn of the Aβ40 peptides was replaced by a (3-(3-aminomethyl)phenylazo)phenylacetic acid (AMPP)-based photoswitch, which can predefine the peptide conformation to either an extended (trans) or hairpin (cis) conformation. This enables simultaneous spectroscopic assessment of the conformation of the AMPP-photoswitch, allowing in situ structural investigations during fibrillation in contrast to structural techniques such as NMR spectroscopy or cryo-EM, which can only be applied to stable conformers. Both methods confirm an extended structure for the peptidic monomers in fibrils of all investigated Aβ variants. Especially the Gly25/Ile31 contact is a decisive indicator for the extended structure along with the characteristic absorption spectra of trans-AMPP-Aβ.

Development and evolution of human glutaminyl cyclase inhibitors (QCIs): an alternative promising approach for disease-modifying treatment of Alzheimer's disease

Human glutaminyl cyclase (hQC) is drawing considerable attention and emerging as a potential druggable target for Alzheimer's disease (AD) due to its close involvement in the pathology of AD via the post-translational pyroglutamate modification of amyloid-β. A recent phase 2a study has shown promising early evidence of efficacy for AD with a competitive benzimidazole-based QC inhibitor, PQ912, which also demonstrated favorable safety profiles. This finding has sparked new hope for the treatment of AD. In this review, we briefly summarize the discovery and evolution of hQC inhibitors, with a particular interest in classic Zinc binding group (ZBG)-containing chemicals reported in recent years. Additionally, we highlight several high-potency inhibitors and discuss new trends and challenges in the development of QC inhibitors as an alternative and promising disease-modifying therapy for AD.

Alzheimer's Disease and Its Possible Evolutionary Origin: Hypothesis

The enormous, 2-3-million-year evolutionary expansion of hominin neocortices to the current enormity enabled humans to take over the planet. However, there appears to have been a glitch, and it occurred without a compensatory expansion of the entorhinal cortical (EC) gateway to the hippocampal memory-encoding system needed to manage the processing of the increasing volume of neocortical data converging on it. The resulting age-dependent connectopathic glitch was unnoticed by the early short-lived populations. It has now surfaced as Alzheimer's disease (AD) in today's long-lived populations. With advancing age, processing of the converging neocortical data by the neurons of the relatively small lateral entorhinal cortex (LEC) inflicts persistent strain and high energy costs on these cells. This may result in their hyper-release of harmless Aβ_1-42 monomers into the interstitial fluid, where they seed the formation of toxic amyloid-β oligomers (AβOs) that initiate AD. At the core of connectopathic AD are the postsynaptic cellular prion protein (PrPC). Electrostatic binding of the negatively charged AβOs to the positively charged N-terminus of PrPC induces hyperphosphorylation of tau that destroys synapses. The spread of these accumulating AβOs from ground zero is supported by Aβ's own production mediated by target cells' Ca2+-sensing receptors (CaSRs). These data suggest that an early administration of a strongly positively charged, AβOs-interacting peptide or protein, plus an inhibitor of CaSR, might be an effective AD-arresting therapeutic combination.

Pharmacophore-driven identification of human glutaminyl cyclase inhibitors from foods, plants and herbs unveils the bioactive property and potential of Azaleatin in the treatment of Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of disabilities in old age and a rapidly growing condition in the elderly population. AD brings significant burden and has a devastating impact on public health, society and the global economy. Thus, developing new therapeutics to combat AD is imperative. Human glutaminyl cyclase (hQC), which catalyzes the formation of neurotoxic pyroglutamate (pE)-modified β-amyloid (Aβ) peptides, is linked to the amyloidogenic process that leads to the initiation of AD. Hence, hQC is an essential target for developing anti-AD therapeutics. Here, we systematically screened and identified hQC inhibitors from natural products by pharmacophore-driven inhibitor screening coupled with biochemical and biophysical examinations. We employed receptor-ligand pharmacophore generation to build pharmacophore models and Phar-MERGE and Phar-SEN for inhibitor screening through ligand-pharmacophore mapping. About 11 and 24 hits identified from the Natural Product and Traditional Chinese Medicine databases, respectively, showed diverse hQC inhibitory abilities. Importantly, the inhibitors TCM1 (Azaleatin; IC₅₀ = 1.1 μM) and TCM2 (Quercetin; IC₅₀ = 4.3 μM) found in foods and plants exhibited strong inhibitory potency against hQC. Furthermore, the binding affinity and molecular interactions were analyzed by surface plasmon resonance (SPR) and molecular modeling/simulations to explore the possible modes of action of Azaleatin and Quercetin. Our study successfully screened and characterized the foundational biochemical and biophysical properties of Azaleatin and Quercetin toward targeting hQC, unveiling their bioactive potential in the treatment of AD.

Upregulation of Glutaminyl Cyclase Contributes to ERS-Induced Apoptosis in PC12 Cells

Glutaminyl cyclase (QC) is responsible for converting the N-terminal glutaminyl and glutamyl of the proteins into pyroglutamate (pE) through cyclization. It has been confirmed that QC catalyzes the formation of neurotoxic pE-modified Aβ in the brain of AD patients. But the effects of upregulated QC in diverse diseases have not been much clear until recently. Here, RNA sequencing was applied to identify differentially expressed genes (DEGs) in PC12 cells with QC overexpressing or knockdown. A total of 697 DEGs were identified in QC overexpressing cells while only 77 in QC knockdown cells. Multiple bioinformatic approaches revealed that the DEGs in QC overexpressing group were enriched in endoplasmic reticulum stress (ERS) related signaling pathways. The gene expression patterns of 23 DEGs were confirmed by RT-qPCR, in which the genes related to ERS showed the highest consistency. We also revealed the protein levels of GRP78, PERK, CHOP, and PARP-1, and caspase family was significantly upregulated by overexpressing QC. Moreover, overexpressing QC significantly increased apoptosis of PC12 cells in a time dependent manner. However, no significant alteration was observed in QC knockdown cells. Therefore, our study indicated that upregulated QC could induce ERS and apoptosis, which consequently trigger diseases by catalyzing the generation of pE-modified mediators.

Role of Hydrophobicity at the N-Terminal Region of Aβ42 in Secondary Nucleation

The self-assembly of the amyloid β 42 (Aβ42) peptide is linked to Alzheimer's disease, and oligomeric intermediates are linked to neuronal cell death during the pathology of the disease. These oligomers are produced prolifically during secondary nucleation, by which the aggregation of monomers is catalyzed on fibril surfaces. Significant progress has been made in understanding the aggregation mechanism of Aβ42; still, a detailed molecular-level understanding of secondary nucleation is lacking. Here, we explore the role of four hydrophobic residues on the unstructured N-terminal region of Aβ42 in secondary nucleation. We create eight mutants with single substitutions at one of the four positions─Ala2, Phe4, Tyr10, and Val12─to decrease the hydrophobicity at respective positions (A2T, A2S, F4A, F4S, Y10A, Y10S, V12A, and V12S) and one mutant (Y10F) to remove the polar nature of Tyr10. Kinetic analyses of aggregation data reveal that the hydrophobicity at the N-terminal region of Aβ42, especially at positions 10 and 12, affects the rate of fibril mass generated via secondary nucleation. Cryo-electron micrographs reveal that most of the mutants with lower hydrophobicity form fibrils that are markedly longer than WT Aβ42, in line with the reduced secondary nucleation rates for these peptides. The dominance of secondary nucleation, however, is still retained in the aggregation mechanism of these mutants because the rate of primary nucleation is even more reduced. This highlights that secondary nucleation is a general phenomenon that is not dependent on any one particular feature of the peptide and is rather robust to sequence perturbations.

Metabolomics Study of Different Germplasm Resources for Three Polygonatum Species Using UPLC-Q-TOF-MS/MS

Rhizomes of the Polygonatum species are well-known in traditional Chinese medicine. The 2020 edition of Chinese Pharmacopoeia includes three different species that possess different pharmacological effects. Due to the lack of standardized discriminant compounds there has often been inadvertently incorrect prescriptions given for these medicines, resulting in serious consequences. Therefore, it is critical to accurately distinguish these herbal Polygonatum species. For this study, UPLC-Q-TOF-MS/MS based metabolomics was employed for the first time to discriminate between three Polygonatum species. Partial least squares discriminant analysis (PLS-DA) models were utilized to select the potential candidate discriminant compounds, after which MS/MS fragmentation patterns were used to identify them. Meanwhile, metabolic correlations were identified using the R language package corrplot, and the distribution of various metabolites was analyzed by box plot and the Z-score graph. As a result, we found that adenosine, sucrose, and pyroglutamic acid were suitable for the identification of different Polygonatum species. In conclusion, this study articulates how various herbal Polygonatum species might be more accurately and efficiently distinguished.

Prediction of Cerebral Amyloid Pathology Based on Plasma Amyloid and Tau Related Markers

Background and Purpose: Pyroglutamate-modified β-amyloid peptide (Aβ_pE) is crucial for AD pathophysiological process. The potential associations of plasma Aβ_pE and total tau (t-tau) with brain Aβ burden and cognitive performance remain to be clarified.

Methods: Forty-six subjects with unimpaired cognition, mild cognitive impairment, or very mild dementia were enrolled. Plasma levels of Aβ_pE3−40, t-tau, and Aβ42 were quantified by immunomagnetic reduction (IMR) assays. We analyzed individual and combined biomarker correlations with neuropsychological scores and Aβ positivity determined by ¹⁸F-florbetapir positron emission tomography (PET).

Results: Both plasma Aβ_pE3−40 levels and Aβ_pE3−40/t-tau ratios correlated negatively with short-term memory and global cognition scores, while correlating positively with PET standardized uptake value ratios (SUVRs). Among the biomarkers analyzed, the combination of Aβ_pE3−40 in a ratio with t-tau had the best discriminatory ability for Aβ PET positivity. Likewise, logistic regression analysis showed that Aβ_pE3−40/t-tau was a highly robust predictor of Aβ PET positivity after controlling for relevant demographic covariates.

Conclusion: Plasma Aβ_pE3−40/t-tau ratios correlate with cognitive function and cerebral Aβ burden. The suitability of Aβ_pE3−40/t-tau as a candidate clinical biomarker of AD pathology in the brain should be examined further in larger studies.

Inhalational Anesthetics Do Not Deteriorate Amyloid-β-Derived Pathophysiology in Alzheimer's Disease: Investigations on the Molecular, Neuronal, and Behavioral Level

BACKGROUND

Studies suggest that general anesthetics like isoflurane and sevoflurane may aggravate Alzheimer's disease (AD) neuropathogenesis, e.g., increased amyloid-β (Aβ) protein aggregation resulting in synaptotoxicity and cognitive dysfunction. Other studies showed neuroprotective effects, e.g., with xenon.

OBJECTIVE

In the present study, we want to detail the interactions of inhalational anesthetics with Aβ-derived pathology. We hypothesize xenon-mediated beneficial mechanisms regarding Aβ oligomerization and Aβ-mediated neurotoxicity on processes related to cognition.

METHODS

Oligomerization of Aβ 1-42 in the presence of anesthetics has been analyzed by means of TR-FRET and silver staining. For monitoring changes in neuronal plasticity due to anesthetics and Aβ 1-42, Aβ 1-40, pyroglutamate-modified amyloid-(AβpE3), and nitrated Aβ (3NTyrAβ), we quantified long-term potentiation (LTP) and spine density. We analyzed network activity in the hippocampus via voltage-sensitive dye imaging (VSDI) and cognitive performance and Aβ plaque burden in transgenic AD mice (ArcAβ) after anesthesia.

RESULTS

Whereas isoflurane and sevoflurane did not affect Aβ 1-42 aggregation, xenon alleviated the propensity for aggregation and partially reversed AβpE3 induced synaptotoxic effects on LTP. Xenon and sevoflurane reversed Aβ 1-42-induced spine density attenuation. In the presence of Aβ 1-40 and AβpE3, anesthetic-induced depression of VSDI-monitored signaling recovered after xenon, but not isoflurane and sevoflurane removal. In slices pretreated with Aβ 1-42 or 3NTyrAβ, activity did not recover after washout. Cognitive performance and plaque burden were unaffected after anesthetizing WT and ArcAβ mice.

CONCLUSION

None of the anesthetics aggravated Aβ-derived AD pathology in vivo. However, Aβ and anesthetics affected neuronal activity in vitro, whereby xenon showed beneficial effects on Aβ 1-42 aggregation, LTP, and spine density.

Postmortem Neocortical 3H-PiB Binding and Levels of Unmodified and Pyroglutamate Aβ in Down Syndrome and Sporadic Alzheimer's Disease

Individuals with Down syndrome (DS) have a genetic predisposition for amyloid-β (Aβ) overproduction and earlier onset of Aβ deposits compared to patients with sporadic late-onset Alzheimer’s disease (AD). Positron emission tomography (PET) with Pittsburgh Compound-B (PiB) detects fibrillar Aβ pathology in living people with DS and AD, but its relationship with heterogeneous Aβ forms aggregated within amyloid deposits is not well understood. We performed quantitative in vitro³H-PiB binding assays and enzyme-linked immunosorbent assays of fibrillar (insoluble) unmodified Aβ40 and Aβ42 forms and N-terminus truncated and pyroglutamate-modified AβNpE3-40 and AβNpE3-42 forms in postmortem frontal cortex and precuneus samples from 18 DS cases aged 43–63 years and 17 late-onset AD cases aged 62–99 years. Both diagnostic groups had frequent neocortical neuritic plaques, while the DS group had more severe vascular amyloid pathology (cerebral amyloid angiopathy, CAA). Compared to the AD group, the DS group had higher levels of Aβ40 and AβNpE3-40, while the two groups did not differ by Aβ42 and AβNpE3-42 levels. This resulted in lower ratios of Aβ42/Aβ40 and AβNpE3-42/AβNpE3-40 in the DS group compared to the AD group. Correlations of Aβ42/Aβ40 and AβNpE3-42/AβNpE3-40 ratios with CAA severity were strong in DS cases and weak in AD cases. Pyroglutamate-modified Aβ levels were lower than unmodified Aβ levels in both diagnostic groups, but within group proportions of both pyroglutamate-modified Aβ forms relative to both unmodified Aβ forms were lower in the DS group but not in the AD group. The two diagnostic groups did not differ by ³H-PiB binding levels. These results demonstrate that compared to late-onset AD cases, adult DS individuals with similar severity of neocortical neuritic plaques and greater CAA pathology have a preponderance of both pyroglutamate-modified AβNpE3-40 and unmodified Aβ40 forms. Despite the distinct molecular profile of Aβ forms and greater vascular amyloidosis in DS cases, cortical ³H-PiB binding does not distinguish between diagnostic groups that are at an advanced level of amyloid plaque pathology. This underscores the need for the development of CAA-selective PET radiopharmaceuticals to detect and track the progression of cerebral vascular amyloid deposits in relation to Aβ plaques in individuals with DS.

Repurposing FDA-Approved Compounds for the Discovery of Glutaminyl Cyclase Inhibitors as Drugs Against Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative causes of dementia, the pathology of which is still not much clear. It's challenging to discover the disease modifying agents for the prevention and treatment of AD over the years. Emerging evidence has been accumulated to reveal the crucial role of up-regulated glutaminyl cyclase (QC) in the initiation of AD. In the current study, the QC inhibitory potency of a library consisting of 1621 FDA-approved compounds was assessed. A total of 54 hits, 3.33 % of the pool, exhibited QC inhibitory activities. The Ki of the top 5 compounds with the highest QC inhibitory activities were measured. Among these selected hits, compounds affecting neuronal signaling pathways and other mechanisms were recognized. Moreover, several polyphenol derivatives with QC inhibitory activities were also identified. Frameworks and subsets contained in these hits were analyzed. Taken together, our results may contribute to the discovery and development of novel QC inhibitors as potential anti-AD agents.

Exploring the binding mode of PQ912 against secretory glutaminyl cyclase through systematic exploitation of conformational ensembles

Secretory glutaminyl cyclase (sQC) plays an important role in the formation of the pyroglutamate-amyloid beta (pGlu-Aβ) peptide, one of the most abundant variants of Aβ found in the Alzheimer's disease (AD) brain. This post-translationally modified pGlu-Aβ possesses high toxicity and rapid aggregation propensity when compared to the wild-type Aβ (WT-Aβ). Since pGlu-Aβ acts as seed for WT-Aβ, the inhibition of sQC limits the formation of pGlu-Aβ and reduces the overall load of Aβ plaques in the AD brain. PQ912 is a potent inhibitor of sQC and has been enrolled in phase 2b clinical trial of the AD drug development pipeline; however, the binding mode of PQ912 against sQC is not elucidated yet. Understanding the binding mode of PQ912 is important as it helps in the discovery against AD where sQC as a target. To explore the binding mode of PQ912, we employed ensemble docking towards 9 sQC structures that differ either in active site geometry or in the bound ligands. Further pose clustering and binding energy calculations yielded three possible binding modes for PQ912. Finally, all atom molecular dynamics simulations determined the most energetically favorable binding mode for PQ912, in the active site of sQC, which is similar to that of LSB-09, a recently reported sQC inhibitor containing benzimidazole-6-carboxamide moiety.

PEAβ Triggers Cognitive Decline and Amyloid Burden in a Novel Mouse Model of Alzheimer's Disease

Understanding the physiopathology of Alzheimer’s disease (AD) has improved substantially based on studies of mouse models mimicking at least one aspect of the disease. Many transgenic lines have been established, leading to amyloidosis but lacking neurodegeneration. The aim of the current study was to generate a novel mouse model that develops neuritic plaques containing the aggressive pyroglutamate modified amyloid-β (pEAβ) species in the brain. The TAPS line was developed by intercrossing of the pEAβ-producing TBA2.1 mice with the plaque-developing line APPswe/PS1ΔE9. The phenotype of the new mouse line was characterized using immunostaining, and different cognitive and general behavioral tests. In comparison to the parental lines, TAPS animals developed an earlier onset of pathology and increased plaque load, including striatal pEAβ-positive neuritic plaques, and enhanced neuroinflammation. In addition to abnormalities in general behavior, locomotion, and exploratory behavior, TAPS mice displayed cognitive deficits in a variety of tests that were most pronounced in the fear conditioning paradigm and in spatial learning in comparison to the parental lines. In conclusion, the combination of a pEAβ- and a plaque-developing mouse model led to an accelerated amyloid pathology and cognitive decline in TAPS mice, qualifying this line as a novel amyloidosis model for future studies.

Glutaminyl Cyclase, Diseases, and Development of Glutaminyl Cyclase Inhibitors

Pyroglutamate (pE) modification, catalyzed mainly by glutaminyl cyclase (QC), is prevalent throughout nature and is particularly important in mammals including humans for the maturation of hormones, peptides, and proteins. In humans, the upregulation of QC is involved in multiple diseases and conditions including Alzheimer's disease, Huntington's disease, melanomas, thyroid carcinomas, accelerated atherosclerosis, septic arthritics, etc. This upregulation catalyzes the generation of modified mediators such as pE-amyloid beta (Aß) and pE-chemokine ligand 2 (CCL2) peptides. Not surprisingly, QC has emerged as a reasonable target for the development of therapeutics to combat these diseases and conditions. In this manuscript the deleterious effects of upregulated QC resulting in disease manifestation are reviewed, along with progress on the development of QC inhibitors.

Amyloid Beta Hypothesis in Alzheimer's Disease: Major Culprits and Recent Therapeutic Strategies

Alzheimer's disease (AD) is one of the most common forms of dementia and has been a global concern for several years. Due to the multi-factorial nature of the disease, AD has become irreversible, fatal and imposes a tremendous socio-economic burden. Even though experimental medicines suggested moderate benefits, AD still lacks an effective treatment strategy for the management of symptoms or cure. Among the various hypotheses that describe development and progression of AD, the amyloid hypothesis has been a long-term adherent to the AD due to the involvement of various forms of Amyloid beta (Aβ) peptides in the impairment of neuronal and cognitive functions. Hence, majority of the drug discovery approaches in the past have focused on the prevention of the accumulation of Aβ peptides. Currently, there are several agents in the phase III clinical trials that target Aβ or the various macromolecules triggering Aβ deposition. In this review, we present the state of the art knowledge on the functional aspects of the key players involved in the amyloid hypothesis. Furthermore, we also discuss anti-amyloid agents present in the Phase III clinical trials.

Piperidine-4-carboxamide as a new scaffold for designing secretory glutaminyl cyclase inhibitors

Alzheimer's disease (AD), a common chronic neurodegenerative disease, has become a major public health concern. Despite years of research, therapeutics for AD are limited. Overexpression of secretory glutaminyl cyclase (sQC) in AD brain leads to the formation of a highly neurotoxic pyroglutamate variant of amyloid beta, pGlu-Aβ, which acts as a potential seed for the aggregation of full length Aβ. Preventing the formation of pGlu-Aβ through inhibition of sQC has become an attractive disease-modifying therapy in AD. In this current study, through a pharmacophore assisted high throughput virtual screening, we report a novel sQC inhibitor (Cpd-41) with a piperidine-4-carboxamide moiety (IC₅₀ = 34 μM). Systematic molecular docking, MD simulations and X-ray crystallographic analysis provided atomistic details of the binding of Cpd-41 in the active site of sQC. The unique mode of binding and moderate toxicity of Cpd-41 make this molecule an attractive candidate for designing high affinity sQC inhibitors.

Structural characteristics of oligomers formed by pyroglutamate-modified amyloid β peptides studied by solid-state NMR

Neuronal plaques of amyloid β (Aβ) peptides of varying length carrying different posttranslational modifications represent a molecular hallmark of Alzheimer's disease. It is believed that transient oligomeric Aβ assemblies associating in early fibrillation events represent particularly cytotoxic peptide aggregates. Also, N-terminally truncated (in position 3 or 11) and pyroglutamate modified peptides exhibited an increased toxicity compared to the wildtype. In the current study, the molecular structure of oligomeric species of pGlu3-Aβ(3-40) and pGlu11-Aβ(11-40) was investigated using solid-state NMR spectroscopy. On the secondary structure level, for both modified peptides a large similarity between oligomers and mature fibrils of the modified peptides was found mainly based on 13C NMR chemical shift data. Some smaller structural differences were detected in the vicinity of the respective modification site. Also, the crucial early folding molecular contact between residues Phe19 and Leu34 could be observed for the oligomers of both modified peptide species. Therefore, it has to be concluded that the major secondary structure elements of Aβ are already present in oligomers of pGlu3-Aβ(3-40) and pGlu11-Aβ(11-40). These posttranslationally modified peptides arrange in a similar fashion as observed for wild type Aβ(1-40).

Amyloid aggregates accumulate in melanoma metastasis modulating YAP activity

Melanoma progression is generally associated with increased transcriptional activity mediated by the Yes-associated protein (YAP). Mechanical signals from the extracellular matrix are sensed by YAP, which then activates the expression of proliferative genes, promoting melanoma progression and drug resistance. Which extracellular signals induce mechanotransduction, and how this is mediated, is not completely understood. Here, using secretome analyses, we reveal the extracellular accumulation of amyloidogenic proteins, i.e. premelanosome protein (PMEL), in metastatic melanoma, together with proteins that assist amyloid maturation into fibrils. We also confirm the accumulation of amyloid-like aggregates, similar to those detected in Alzheimer disease, in metastatic cell lines, as well as in human melanoma biopsies. Mechanistically, beta-secretase 2 (BACE2) regulates the maturation of these aggregates, which in turn induce YAP activity. We also demonstrate that recombinant PMEL fibrils are sufficient to induce mechanotransduction, triggering YAP signaling. Finally, we demonstrate that BACE inhibition affects cell proliferation and increases drug sensitivity, highlighting the importance of amyloids for melanoma survival, and the use of beta-secretase inhibitors as potential therapeutic approach for metastatic melanoma.

High-yield Production of Amyloid-β Peptide Enabled by a Customized Spider Silk Domain

During storage in the silk gland, the N-terminal domain (NT) of spider silk proteins (spidroins) keeps the aggregation-prone repetitive region in solution at extreme concentrations. We observe that NTs from different spidroins have co-evolved with their respective repeat region, and now use an NT that is distantly related to previously used NTs, for efficient recombinant production of the amyloid-β peptide (Aβ) implicated in Alzheimer’s disease. A designed variant of NT from Nephila clavipes flagelliform spidroin, which in nature allows production and storage of β-hairpin repeat segments, gives exceptionally high yields of different human Aβ variants as a solubility tag. This tool enables efficient production of target peptides also in minimal medium and gives up to 10 times more isotope-labeled monomeric Aβ peptides per liter bacterial culture than previously reported.

Dipeptidyl-Peptidase Activity of Meprin β Links N-truncation of Aβ with Glutaminyl Cyclase-Catalyzed pGlu-Aβ Formation

The formation of amyloid-β (Aβ) peptides is causally involved in the development of Alzheimer's disease (AD). A significant proportion of deposited Aβ is N-terminally truncated and modified at the N-terminus by a pGlu-residue (pGlu-Aβ). These forms show enhanced neurotoxicity compared to full-length Aβ. Although the truncation may occur by aminopeptidases after formation of Aβ, recently discovered processing pathways of amyloid-β protein precursor (AβPP) by proteases such as meprin β may also be involved. Here, we assessed a role of meprin β in forming Aβ3-40/42, which is the precursor of pGlu-Aβ3-40/42 generated by glutaminyl cyclase (QC). Similar to QC, meprin β mRNA is significantly upregulated in postmortem brain from AD patients. A histochemical analysis supports the presence of meprin β in neurons and astrocytes in the vicinity of pGlu-Aβ containing deposits. Cleavage of AβPP-derived peptides by meprin β in vitro results in peptides Aβ1-x, Aβ2-x, and Aβ3-x. The formation of N-truncated Aβ by meprin β was also corroborated in cell culture. A subset of the generated peptides was converted into pGlu-Aβ3-40 by an addition of glutaminyl cyclase, supporting the preceding formation of Aβ3-40. Further analysis of the meprin β cleavage revealed a yet unknown dipeptidyl-peptidase-like activity specific for the N-terminus of Aβ1-x. Thus, our data suggest that meprin β contributes to the formation of N-truncated Aβ by endopeptidase and exopeptidase activity to generate the substrate for QC-catalyzed pGlu-Aβ formation.

Comprehensive Characterization of the Pyroglutamate Amyloid-β Induced Motor Neurodegenerative Phenotype of TBA2.1 Mice

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is being intensively investigated using a broad variety of animal models. Many of these models express mutant versions of human amyloid-β protein precursor (AβPP) that are associated with amyloid-β protein (Aβ)-induced early onset familial AD. Most of these models, however, do not develop bold neurodegenerative pathology and the respective phenotypes. Nevertheless, this may well be essential for their suitability to identify therapeutically active compounds that have the potential for a curative or at least disease-modifying therapy in humans. In this study, the new transgenic mouse model TBA2.1 was explored in detail to increase knowledge about the neurodegenerative process induced by the presence of pyroglutamate modified human Aβ3-42 (pEAβ3-42). Analysis of the sensorimotor phenotype, motor coordination, Aβ pathology, neurodegeneration, and gliosis revealed formation and progression of severe pathology and phenotypes including massive neuronal loss in homozygous TBA2.1 mice within a few months. In contrast, the start of a slight phenotype was observed only after 21 months in heterozygous mice. These data highlight the role of pEAβ3-42 in the disease development and progression of AD. Based on the findings of this study, homozygous TBA2.1 mice can be utilized to gain deeper understanding in the underlying mechanisms of pEAβ3-42 and might be suitable as an animal model for treatment studies targeting toxic Aβ species, complementary to the well described transgenic AβPP mouse models.

In vitroandin silicodetermination of glutaminyl cyclase inhibitors

Alzheimer's disease (AD) is the most common form of neurodegenerative disease currently. It is widely accepted that AD is characterized by the self-assembly of amyloid beta (Aβ) peptides. The human glutaminyl cyclase (hQC) enzyme is characterized by association with Aβ peptide generation. The development of hQC inhibitors could prevent the self-aggregation of Aβ peptides, resulting in impeding AD. Utilizing structural knowledge of the hQC substrates and known hQC inhibitors, new heterocyclic and peptidomimetic derivatives were synthesized and were able to inhibit the hQC enzyme. The inhibiting abilities of these compounds were evaluated using a fluorometric assay. The binding mechanism at the atomic level was estimated using molecular docking, free energy perturbation, and quantum chemical calculation methods. The predicted log(BBB) and human intestinal absorption values indicated that these compounds are able to permeate the blood–brain barrier and be well-absorbed through the gastrointestinal tract. Overall, 5,6-dimethoxy-N-(3-(5-methyl-1H-imidazol-1-yl)propyl)-1H-benzo[d]imidazol-2-amine (1_2) was indicated as a potential drug for AD treatment.

Rational design of new hQC inhibitors.

Current state of Alzheimer's fluid biomarkers

Alzheimer’s disease (AD) is a progressive neurodegenerative disease with a complex and heterogeneous pathophysiology. The number of people living with AD is predicted to increase; however, there are no disease-modifying therapies currently available and none have been successful in late-stage clinical trials. Fluid biomarkers measured in cerebrospinal fluid (CSF) or blood hold promise for enabling more effective drug development and establishing a more personalized medicine approach for AD diagnosis and treatment. Biomarkers used in drug development programmes should be qualified for a specific context of use (COU). These COUs include, but are not limited to, subject/patient selection, assessment of disease state and/or prognosis, assessment of mechanism of action, dose optimization, drug response monitoring, efficacy maximization, and toxicity/adverse reactions identification and minimization. The core AD CSF biomarkers Aβ42, t-tau, and p-tau are recognized by research guidelines for their diagnostic utility and are being considered for qualification for subject selection in clinical trials. However, there is a need to better understand their potential for other COUs, as well as identify additional fluid biomarkers reflecting other aspects of AD pathophysiology. Several novel fluid biomarkers have been proposed, but their role in AD pathology and their use as AD biomarkers have yet to be validated. In this review, we summarize some of the pathological mechanisms implicated in the sporadic AD and highlight the data for several established and novel fluid biomarkers (including BACE1, TREM2, YKL-40, IP-10, neurogranin, SNAP-25, synaptotagmin, α-synuclein, TDP-43, ferritin, VILIP-1, and NF-L) associated with each mechanism. We discuss the potential COUs for each biomarker.

Deceleration of the neurodegenerative phenotype in pyroglutamate-Aβ accumulating transgenic mice by oral treatment with the Aβ oligomer eliminating compound RD2

Alzheimer's disease, a multifactorial incurable disorder, is mainly characterised by progressive neurodegeneration, extracellular accumulation of amyloid-β protein (Aβ), and intracellular aggregation of hyperphosphorylated tau protein. During the last years, Aβ oligomers have been claimed to be the disease causing agent. Consequently, development of compounds that are able to disrupt already existing Aβ oligomers is highly desirable. We developed d-enantiomeric peptides, consisting solely of d-enantiomeric amino acid residues, for the direct and specific elimination of toxic Aβ oligomers. The drug candidate RD2 did show high oligomer elimination efficacy in vitro and the in vivo efficacy of RD2 was demonstrated in treatment studies by enhanced cognition in transgenic mouse models of amyloidosis. Here, we report on the in vitro and in vivo efficacy of the compound towards pyroglutamate-Aβ, a particular aggressive Aβ species. Using the transgenic TBA2.1 mouse model, which develops pyroglutamate-Aβ_(3-42) induced neurodegeneration, we are able to show that oral RD2 treatment resulted in a significant deceleration of the progression of the phenotype. The in vivo efficacy against this highly toxic Aβ species further validates RD2 as a drug candidate for the therapeutic use in humans.

Identifying hQC Inhibitors of Alzheimer's Disease by Effective Customized Pharmacophore-Based Virtual Screening, Molecular Dynamic Simulation, and Binding Free Energy Analysis

Human glutaminyl cyclase (hQC) appeared as a promising new target with its inhibitors attracted much attention for the treatment of Alzheimer's disease (AD) in recent years. But so far, only a few compounds have been reported as hQC inhibitors. To find novel and potent hQC inhibitors, a high-specificity ZBG (zinc-binding groups)-based pharmacophore model comprising customized ZBG feature was first generated using HipHop algorithm in Discovery Studio software for screening out hQC inhibitors from the SPECS database. After purification by docking studies and drug-like ADMET properties filters, four potential hit compounds were retrieved. Subsequently, these hit compounds were subjected to 30-ns molecular dynamic (MD) simulations to explore their binding modes at the active side of hQC. MD simulations demonstrated that these hit compounds formed a chelating interaction with the zinc ion, which was consistent with the finding that the electrostatic interaction was the major driving force for binding to hQC confirmed with MMPBSA energy decomposition. Higher binding affinities of these compounds were also verified by the binding free energy calculations comparing with the references. Thus, these identified compounds might be potential hQC candidates and could be used for further investigation.

The Impact of APP on Alzheimer-like Pathogenesis and Gene Expression in Down Syndrome iPSC-Derived Neurons

Early-onset Alzheimer disease (AD)-like pathology in Down syndrome is commonly attributed to an increased dosage of the amyloid precursor protein (APP) gene. To test this in an isogenic human model, we deleted the supernumerary copy of the APP gene in trisomic Down syndrome induced pluripotent stem cells or upregulated APP expression in euploid human pluripotent stem cells using CRISPRa. Cortical neuronal differentiation shows that an increased APP gene dosage is responsible for increased β-amyloid production, altered Aβ42/40 ratio, and deposition of the pyroglutamate (E3)-containing amyloid aggregates, but not for several tau-related AD phenotypes or increased apoptosis. Transcriptome comparisons demonstrate that APP has a widespread and temporally modulated impact on neuronal gene expression. Collectively, these data reveal an important role for APP in the amyloidogenic aspects of AD but challenge the idea that increased APP levels are solely responsible for increasing specific phosphorylated forms of tau or enhanced neuronal cell death in Down syndrome-associated AD pathogenesis.

Can small molecule inhibitors of glutaminyl cyclase be used as a therapeutic for Alzheimer's disease?

Alzheimer's disease (AD) is a multifactorial and socioeconomically burdensome disease. In view of the failures of anti-AD candidates, we should try to rethink what we did before and what we should do next, in part at least. Research shows that the more neurotoxic factor, pyroglutamate-Aβs, and the more important inflammatory mediators, pyroglutamate-CCL2, both contribute to the initiation of AD specifically and the generation of N-terminal intramolecular cyclization catalyzed by glutaminyl cyclase quality control, the over-expression of which correlates positively with the severity of AD. Subsequently, lowering pyroglutamate-Aβs and pyroglutamate-CCL2 levels by quality control inhibition using small molecule inhibitors could be expected as an amazing strategy for the prevention and treatment of AD.

APP/Aβ structural diversity and Alzheimer's disease pathogenesis

The amyloid cascade hypothesis of Alzheimer's disease (AD) proposes amyloid- β (Aβ) is a chief pathological element of dementia. AD therapies have targeted monomeric and oligomeric Aβ 1-40 and 1-42 peptides. However, alternative APP proteolytic processing produces a complex roster of Aβ species. In addition, Aβ peptides are subject to extensive posttranslational modification (PTM). We propose that amplified production of some APP/Aβ species, perhaps exacerbated by differential gene expression and reduced peptide degradation, creates a diverse spectrum of modified species which disrupt brain homeostasis and accelerate AD neurodegeneration. We surveyed the literature to catalog Aβ PTM including species with isoAsp at positions 7 and 23 which may phenocopy the Tottori and Iowa Aβ mutations that result in early onset AD. We speculate that accumulation of these alterations induce changes in secondary and tertiary structure of Aβ that favor increased toxicity, and seeding and propagation in sporadic AD. Additionally, amyloid-β peptides with a pyroglutamate modification at position 3 and oxidation of Met35 make up a substantial portion of sporadic AD amyloid deposits. The intrinsic physical properties of these species, including resistance to degradation, an enhanced aggregation rate, increased neurotoxicity, and association with behavioral deficits, suggest their emergence is linked to dementia. The generation of specific 3D-molecular conformations of Aβ impart unique biophysical properties and a capacity to seed the prion-like global transmission of amyloid through the brain. The accumulation of rogue Aβ ultimately contributes to the destruction of vascular walls, neurons and glial cells culminating in dementia. A systematic examination of Aβ PTM and the analysis of the toxicity that they induced may help create essential biomarkers to more precisely stage AD pathology, design countermeasures and gauge the impacts of interventions.

Factors affecting the physical stability (aggregation) of peptide therapeutics

The number of biological therapeutic agents in the clinic and development pipeline has increased dramatically over the last decade and the number will undoubtedly continue to increase in the coming years. Despite this fact, there are considerable challenges in the development, production and formulation of such biologics particularly with respect to their physical stabilities. There are many cases where self-association to form either amorphous aggregates or highly structured fibrillar species limits their use. Here, we review the numerous factors that influence the physical stability of peptides including both intrinsic and external factors, wherever possible illustrating these with examples that are of therapeutic interest. The effects of sequence, concentration, pH, net charge, excipients, chemical degradation and modification, surfaces and interfaces, and impurities are all discussed. In addition, the effects of physical parameters such as pressure, temperature, agitation and lyophilization are described. We provide an overview of the structures of aggregates formed, as well as our current knowledge of the mechanisms for their formation.

Synthesis and Evaluation of Diphenyl Conjugated Imidazole Derivatives as Potential Glutaminyl Cyclase Inhibitors for Treatment of Alzheimer's Disease

High expression of glutaminyl cyclase (QC) contributes to the initiation of Alzheimer's disease (AD) by catalyzing the generation of neurotoxic pyroglutamate (pE)-modified β-amyloid (Aβ) peptides. Preventing the generation of pE-Aβs by QC inhibition has been suggested as a novel approach to a disease-modifying therapy for AD. In this work, a series of diphenyl conjugated imidazole derivatives (DPCIs) was rationally designed and synthesized. Analogues with this scaffold exhibited potent inhibitory activity against human QC (hQC) and good in vitro blood-brain barrier (BBB) permeability. Further assessments corroborated that the selected hQC inhibitor 28 inhibits the activity of hQC, dramatically reduces the generation of pE-Aβs in cultured cells and in vivo, and improves the behavior of AD mice.

Pyroglutamate-modified Aβ(3-42) affects aggregation kinetics of Aβ(1-42) by accelerating primary and secondary pathways

The aggregation into amyloid fibrils of amyloid-β (Aβ) peptides is a hallmark of Alzheimer's disease. A variety of Aβ peptides have been discovered in vivo, with pyroglutamate-modified Aβ (pEAβ) forming a significant proportion. pEAβ is mainly localized in the core of plaques, suggesting a possible role in inducing and facilitating Aβ oligomerization and accumulation. Despite this potential importance, the aggregation mechanism of pEAβ and its influence on the aggregation kinetics of other Aβ variants have not yet been elucidated. Here we show that pEAβ(3-42) forms fibrils much faster than Aβ(1-42) and the critical concentration above which aggregation was observed was drastically decreased by one order of magnitude compared to Aβ(1-42). We elucidated the co-aggregation mechanism of Aβ(1-42) with pEAβ(3-42). At concentrations at which both species do not aggregate as homofibrils, mixtures of pEAβ(3-42) and Aβ(1-42) aggregate, suggesting the formation of mixed nuclei. We show that the presence of pEAβ(3-42) monomers increases the rate of primary nucleation of Aβ(1-42) and that fibrils of pEAβ(3-42) serve as highly efficient templates for elongation and catalytic surfaces for secondary nucleation of Aβ(1-42). On the other hand, the addition of Aβ(1-42) monomers drastically decelerates the primary and secondary nucleation of pEAβ(3-42) while not altering the pEAβ(3-42) elongation rate. In addition, even moderate concentrations of fibrillar Aβ(1-42) prevent pEAβ(3-42) aggregation, likely due to non-reactive binding of pEAβ(3-42) monomers to the surfaces of Aβ(1-42) fibrils. Thus, pEAβ(3-42) accelerates aggregation of Aβ(1-42) by affecting all individual reaction steps of the aggregation process while Aβ(1-42) dramatically slows down the primary and secondary nucleation of pEAβ(3-42).

Mammalian Metallothionein-3: New Functional and Structural Insights

Metallothionein-3 (MT-3), a member of the mammalian metallothionein (MT) family, is mainly expressed in the central nervous system (CNS). MT-3 possesses a unique neuronal growth inhibitory activity, and the levels of this intra- and extracellularly occurring metalloprotein are markedly diminished in the brain of patients affected by a number of metal-linked neurodegenerative disorders, including Alzheimer’s disease (AD). In these pathologies, the redox cycling of copper, accompanied by the production of reactive oxygen species (ROS), plays a key role in the neuronal toxicity. Although MT-3 shares the metal-thiolate clusters with the well-characterized MT-1 and MT-2, it shows distinct biological, structural and chemical properties. Owing to its anti-oxidant properties and modulator function not only for Zn, but also for Cu in the extra- and intracellular space, MT-3, but not MT-1/MT-2, protects neuronal cells from the toxicity of various Cu(II)-bound amyloids. In recent years, the roles of zinc dynamics and MT-3 function in neurodegeneration are slowly emerging. This short review focuses on the recent developments regarding the chemistry and biology of MT-3.

Glutaminyl Cyclase Inhibitor PQ912 Improves Cognition in Mouse Models of Alzheimer's Disease-Studies on Relation to Effective Target Occupancy

Numerous studies suggest that the majority of amyloid-β (Aβ) peptides deposited in Alzheimer's disease (AD) are truncated and post-translationally modified at the N terminus. Among these modified species, pyroglutamyl-Aβ (pE-Aβ, including N3pE-Aβ40/42 and N11pE-Aβ40/42) has been identified as particularly neurotoxic. The N-terminal modification renders the peptide hydrophobic, accelerates formation of oligomers, and reduces degradation by peptidases, leading ultimately to the accumulation of the peptide and progression of AD. It has been shown that the formation of pyroglutamyl residues is catalyzed by glutaminyl cyclase (QC). Here, we present data about the pharmacological in vitro and in vivo efficacy of the QC inhibitor (S)-1-(1H-benzo[d]imidazol-5-yl)-5-(4-propoxyphenyl)imidazolidin-2-one (PQ912), the first-in-class compound that is in clinical development. PQ912 inhibits human, rat, and mouse QC activity, with K_i values ranging between 20 and 65 nM. Chronic oral treatment of hAPP_SLxhQC double-transgenic mice with approximately 200 mg/kg/day via chow shows a significant reduction of pE-Aβ levels and concomitant improvement of spatial learning in a Morris water maze test paradigm. This dose results in a brain and cerebrospinal fluid concentration of PQ912 which relates to a QC target occupancy of about 60%. Thus, we conclude that >50% inhibition of QC activity in the brain leads to robust treatment effects. Secondary pharmacology experiments in mice indicate a fairly large potency difference for Aβ cyclization compared with cyclization of physiologic substrates, suggesting a robust therapeutic window in humans. This information constitutes an important translational guidance for predicting the therapeutic dose range in clinical studies with PQ912.

Pyroglutamate-Modified Amyloid-β(3-42) Shows α-Helical Intermediates before Amyloid Formation

Pyroglutamate-modified amyloid-β (pEAβ) has been described as a relevant Aβ species in Alzheimer's-disease-affected brains, with pEAβ (3-42) as a dominant isoform. Aβ (1-40) and Aβ (1-42) have been well characterized under various solution conditions, including aqueous solutions containing trifluoroethanol (TFE). To characterize structural properties of pEAβ (3-42) possibly underlying its drastically increased aggregation propensity compared to Aβ (1-42), we started our studies in various TFE-water mixtures and found striking differences between the two Aβ species. Soluble pEAβ (3-42) has an increased tendency to form β-sheet-rich structures compared to Aβ (1-42), as indicated by circular dichroism spectroscopy data. Kinetic assays monitored by thioflavin-T show drastically accelerated aggregation leading to large fibrils visualized by electron microscopy of pEAβ (3-42) in contrast to Aβ (1-42). NMR spectroscopy was performed for backbone and side-chain chemical-shift assignments of monomeric pEAβ (3-42) in 40% TFE solution. Although the difference between pEAβ (3-42) and Aβ (1-42) is purely N-terminal, it has a significant impact on the chemical environment of >20% of the total amino acid residues, as revealed by their NMR chemical-shift differences. Freshly dissolved pEAβ (3-42) contains two α-helical regions connected by a flexible linker, whereas the N-terminus remains unstructured. We found that these α-helices act as a transient intermediate to β-sheet and fibril formation of pEAβ (3-42).

Unmodified and pyroglutamylated amyloid β peptides form hypertoxic hetero-oligomers of unique secondary structure

Amyloid β (Aβ) peptide plays a major role in Alzheimer's disease (AD) and occurs in multiple forms, including pyroglutamylated Aβ (AβpE). Identification and characterization of the most cytotoxic Aβ species is necessary for advancement in AD diagnostics and therapeutics. While in brain tissue multiple Aβ species act in combination, structure/toxicity studies and immunotherapy trials have been focused on individual forms of Aβ. As a result, the molecular composition and the structural features of "toxic Aβ oligomers" have remained unresolved. Here, we have used a novel approach, hydration from gas phase coupled with isotope-edited Fourier transform infrared (FTIR) spectroscopy, to identify the prefibrillar assemblies formed by Aβ and AβpE and to resolve the structures of both peptides in combination. The peptides form unusual β-sheet oligomers stabilized by intramolecular H-bonding as opposed to intermolecular H-bonding in the fibrils. Time-dependent morphological changes in peptide assemblies have been visualized by atomic force microscopy. Aβ/AβpE hetero-oligomers exert unsurpassed cytotoxic effect on PC12 cells as compared to oligomers of individual peptides or fibrils. These findings lead to a novel concept that Aβ/AβpE hetero-oligomers, not just Aβ or AβpE oligomers, constitute the main neurotoxic conformation. The hetero-oligomers thus present a new biomarker that may be targeted for development of more efficient diagnostic and immunotherapeutic strategies to combat AD.

Identification of Glutaminyl Cyclase Genes Involved in Pyroglutamate Modification of Fungal Lignocellulolytic Enzymes

The breakdown of plant biomass to simple sugars is essential for the production of second-generation biofuels and high-value bioproducts. Currently, enzymes produced from filamentous fungi are used for deconstructing plant cell wall polysaccharides into fermentable sugars for biorefinery applications. A post-translational N-terminal pyroglutamate modification observed in some of these enzymes occurs when N-terminal glutamine or glutamate is cyclized to form a five-membered ring. This modification has been shown to confer resistance to thermal denaturation for CBH-1 and EG-1 cellulases. In mammalian cells, the formation of pyroglutamate is catalyzed by glutaminyl cyclases. Using the model filamentous fungus Neurospora crassa, we identified two genes (qc-1 and qc-2) that encode proteins homologous to mammalian glutaminyl cyclases. We show that qc-1 and qc-2 are essential for catalyzing the formation of an N-terminal pyroglutamate on CBH-1 and GH5-1. CBH-1 and GH5-1 produced in a Δqc-1 Δqc-2 mutant, and thus lacking the N-terminal pyroglutamate modification, showed greater sensitivity to thermal denaturation, and for GH5-1, susceptibility to proteolytic cleavage. QC-1 and QC-2 are endoplasmic reticulum (ER)-localized proteins. The pyroglutamate modification is predicted to occur in a number of additional fungal proteins that have diverse functions. The identification of glutaminyl cyclases in fungi may have implications for production of lignocellulolytic enzymes, heterologous expression, and biotechnological applications revolving around protein stability.

Pyroglutamate modification is the post-translational conversion of N-terminal glutamine or glutamate into a cyclized amino acid derivative. This modification is well studied in animal systems but poorly explored in fungal systems. In Neurospora crassa, we show that this modification takes place in the ER and is catalyzed by two well-conserved enzymes, ubiquitously conserved throughout the fungal kingdom. We demonstrate that the modification is important for the structural stability and aminopeptidase resistance of CBH-1 and GH5-1, two important cellulase enzymes utilized in industrial plant cell wall deconstruction. Many additional fungal proteins predicted in the genome of N. crassa and other filamentous fungi are predicted to carry an N-terminal pyroglutamate modification. Pyroglutamate addition may also be a useful way to stabilize secreted proteins and peptides, which can be easily produced in fungal production systems.

Calcium-Sensing Receptors of Human Neural Cells Play Crucial Roles in Alzheimer's Disease

In aged subjects, late-onset Alzheimer's disease (LOAD) starts in the lateral entorhinal allocortex where a failure of clearance mechanisms triggers an accumulation of neurotoxic amyloid-β42 oligomers (Aβ42-os). In neurons and astrocytes, Aβ42-os enhance the transcription of Aβ precursor protein (APP) and β-secretase/BACE1 genes. Thus, by acting together with γ-secretase, the surpluses of APP and BACE1 amplify the endogenous production of Aβ42-os which pile up, damage mitochondria, and are oversecreted. At the plasmalemma, exogenous Aβ42-os bind neurons' and astrocytes' calcium-sensing receptors (CaSRs) activating a set of intracellular signaling pathways which upkeep Aβ42-os intracellular accumulation and oversecretion by hindering Aβ42-os proteolysis. In addition, Aβ42-os accumulating in the extracellular milieu spread and reach mounting numbers of adjacent and remoter teams of neurons and astrocytes which in turn are recruited, again via Aβ42-os•CaSR-governed mechanisms, to produce and release additional Aβ42-os amounts. This relentless self-sustaining mechanism drives AD progression toward upper cortical areas. Later on accumulating Aβ42-os elicit the advent of hyperphosphorylated (p)-Tau oligomers which acting together with Aβ42-os and other glial neurotoxins cooperatively destroy wider and wider cognition-related cortical areas. In parallel, Aβ42-os•CaSR signals also elicit an excess production and secretion of nitric oxide and vascular endothelial growth factor-A from astrocytes, of Aβ42-os and myelin basic protein from oligodendrocytes, and of proinflammatory cytokines, nitric oxide and (likely) Aβ42-os from microglia. Activated astrocytes and microglia survive the toxic onslaught, whereas neurons and oligodendrocytes increasingly die. However, we have shown that highly selective allosteric CaSR antagonists (calcilytics), like NPS 2143 and NPS 89626, efficiently suppress all the neurotoxic effects Aβ42-os•CaSR signaling drives in cultured cortical untransformed human neurons and astrocytes. In fact, calcilytics increase Aβ42 proteolysis and discontinue the oversecretion of Aβ42-os, nitric oxide, and vascular endothelial growth factor-A from both astrocytes and neurons. Seemingly, calcilytics would also benefit the other types of glial cells and cerebrovascular cells otherwise damaged by the effects of Aβ42-os•CaSR signaling. Thus, given at amnestic minor cognitive impairment (aMCI) or initial symptomatic stages, calcilytics could prevent or terminate the propagation of LOAD neuropathology and preserve human neurons' viability and hence patients' cognitive abilities.

Generation of aggregation prone N-terminally truncated amyloid β peptides by meprin β depends on the sequence specificity at the cleavage site

BACKGROUND

The metalloprotease meprin β cleaves the Alzheimer's Disease (AD) relevant amyloid precursor protein (APP) as a β-secretase reminiscent of BACE-1, however, predominantly generating N-terminally truncated Aβ2-x variants.

RESULTS

Herein, we observed increased endogenous sAPPα levels in the brains of meprin β knock-out (ko) mice compared to wild-type controls. We further analyzed the cellular interaction of APP and meprin β and found that cleavage of APP by meprin β occurs prior to endocytosis. The N-terminally truncated Aβ2-40 variant shows increased aggregation propensity compared to Aβ1-40 and acts even as a seed for Aβ1-40 aggregation. Additionally, we observed that different APP mutants affect the catalytic properties of meprin β and that, interestingly, meprin β is unable to generate N-terminally truncated Aβ peptides from Swedish mutant APP (APPswe).

CONCLUSION

Concluding, we propose that meprin β may be involved in the generation of N-terminally truncated Aβ2-x peptides of APP, but acts independently from BACE-1.

Structural Analysis and Aggregation Propensity of Pyroglutamate Aβ(3-40) in Aqueous Trifluoroethanol

A hallmark of Alzheimer's disease (AD) is the accumulation of extracellular amyloid-β (Aβ) plaques in the brains of patients. N-terminally truncated pyroglutamate-modified Aβ (pEAβ) has been described as a major compound of Aβ species in senile plaques. pEAβ is more resistant to degradation, shows higher toxicity and has increased aggregation propensity and β-sheet stabilization compared to non-modified Aβ. Here we characterized recombinant pEAβ(3-40) in aqueous trifluoroethanol (TFE) solution regarding its aggregation propensity and structural changes in comparison to its non-pyroglutamate-modified variant Aβ(1-40). Secondary structure analysis by circular dichroism spectroscopy suggests that pEAβ(3-40) shows an increased tendency to form β-sheet-rich structures in 20% TFE containing solutions where Aβ(1-40) forms α-helices. Aggregation kinetics of pEAβ(3-40) in the presence of 20% TFE monitored by thioflavin-T (ThT) assay showed a typical sigmoidal aggregation in contrast to Aβ(1-40), which lacks ThT positive structures under the same conditions. Transmission electron microscopy confirms that pEAβ(3-40) aggregated to large fibrils and high molecular weight aggregates in spite of the presence of the helix stabilizing co-solvent TFE. High resolution NMR spectroscopy of recombinantly produced and uniformly isotope labeled [U-15N]-pEAβ(3-40) in TFE containing solutions indicates that the pyroglutamate formation affects significantly the N-terminal region, which in turn leads to decreased monomer stability and increased aggregation propensity.

Detection and Quantification of β-Amyloid, Pyroglutamyl Aβ, and Tau in Aged Canines

Canine cognitive dysfunction syndrome is an age-associated disorder that resembles many aspects of human Alzheimer disease. The characterization of canine cognitive dysfunction syndrome has been restricted to selected laboratory dogs and mongrels, thereby limiting our knowledge of potential breed-related and age-related differences. We examined the brains of 24 dogs from various breeds. The frontal cortex, hippocampus, and entorhinal cortex were investigated. Deposits of β-amyloid (Aβ) and tau were analyzed phenotypically and quantified stereologically. In all dogs aged 10 years or older, plaques containing pyroglutamyl Aβ and Aβ8-17 were detected. Within the ventral hippocampus, significantly more pyroglutamyl Aβ plaques were deposited in small and medium dogs than in large dogs. Hyperphosphorylated tau with formation of neurofibrillary tangles was observed in 3 animals aged 13 to 15 years. This study provides the first investigation of pyroglutamyl Aβ in comparison with total Aβ (as shown by Aβ8-17 immunoreactivity) in dogs of different breeds, sizes, and ages. Our results indicate that canine cognitive dysfunction syndrome is relatively common among aged canines, thereby emphasizing the relevance of such populations to translational Alzheimer disease research.

Purification and Characterization of Recombinant N-Terminally Pyroglutamate-Modified Amyloid-β Variants and Structural Analysis by Solution NMR Spectroscopy

Alzheimer’s disease (AD) is the leading cause of dementia in the elderly and is characterized by memory loss and cognitive decline. Pathological hallmark of AD brains are intracellular neurofibrillary tangles and extracellular amyloid plaques. The major component of these plaques is the highly heterogeneous amyloid-β (Aβ) peptide, varying in length and modification. In recent years pyroglutamate-modified amyloid-β (pEAβ) peptides have increasingly moved into the focus since they have been described to be the predominant species of all N-terminally truncated Aβ. Compared to unmodified Aβ, pEAβ is known to show increased hydrophobicity, higher toxicity, faster aggregation and β-sheet stabilization and is more resistant to degradation. Nuclear magnetic resonance (NMR) spectroscopy is a particularly powerful method to investigate the conformations of pEAβ isoforms in solution and to study peptide/ligand interactions for drug development. However, biophysical characterization of pEAβ and comparison to its non-modified variant has so far been seriously hampered by the lack of highly pure recombinant and isotope-enriched protein. Here we present, to our knowledge, for the first time a reproducible protocol for the production of pEAβ from a recombinant precursor expressed in E. coli in natural isotope abundance as well as in uniformly [U-¹⁵N]- or [U-¹³C, ¹⁵N]-labeled form, with yields of up to 15 mg/l E. coli culture broth. The chemical state of the purified protein was evaluated by RP-HPLC and formation of pyroglutamate was verified by mass spectroscopy. The recombinant pyroglutamate-modified Aβ peptides showed characteristic sigmoidal aggregation kinetics as monitored by thioflavin-T assays. The quality and quantity of produced pEAβ40 and pEAβ42 allowed us to perform heteronuclear multidimensional NMR spectroscopy in solution and to sequence-specifically assign the backbone resonances under near-physiological conditions. Our results suggest that the presented method will be useful in obtaining cost-effective high-quality recombinant pEAβ40 and pEAβ42 for further physiological and biochemical studies.