Transglutaminase induces protofibril-like amyloid beta-protein assemblies that are protease-resistant and inhibit long-term potentiation

Role of Serotonylation and SERT Posttranslational Modifications in Alzheimer's Disease Pathogenesis

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is implicated mainly in Alzheimer's disease (AD) and reported to be responsible for several processes and roles in the human body, such as regulating sleep, food intake, sexual behavior, anxiety, and drug abuse. It is synthesized from the amino acid tryptophan. Serotonin also functions as a signal between neurons to mature, survive, and differentiate. It plays a crucial role in neuronal plasticity, including cell migration and cell contact formation. Various psychiatric disorders, such as depression, schizophrenia, autism, and Alzheimer's disease, have been linked to an increase in serotonin-dependent signaling during the development of the nervous system. Recent studies have found 5-HT and other monoamines embedded in the nuclei of various cells, including immune cells, the peritoneal mast, and the adrenal medulla. Evidence suggests these monoamines to be involved in widespread intracellular regulation by posttranslational modifications (PTMs) of proteins. Serotonylation is the calcium-dependent process in which 5-HT forms a long-lasting covalent bond to small cytoplasmic G-proteins by endogenous transglutaminase 2 (TGM2). Serotonylation plays a role in various biological processes. The purpose of our article is to summarize historical developments and recent advances in serotonin research and serotonylation in depression, aging, AD, and other age-related neurological diseases. We also discussed several of the latest developments with Serotonin, including biological functions, pathophysiological implications and therapeutic strategies to treat patients with depression, dementia, and other age-related conditions.

TG2 promotes amyloid beta aggregates: Impact on ER-mitochondria crosstalk, calcium homeostasis and synaptic function in Alzheimer’s disease

Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by cognitive impairment that increasingly affects the elderly. AD's main features have been related to cellular and molecular events, including the aberrant aggregation of the amyloid beta peptide (Aβ), Ca²⁺ dyshomeostasis, and increased mitochondria-associated membranes (MAMs). Transglutaminase type 2 (TG2) is a ubiquitous enzyme whose primary role is the Ca²⁺-dependent proteins transamidation, including the Aβ peptide. TG2 activity has been closely related to cellular damage and death. We detected increased TG2 levels in neuronal cells treated with Aβ oligomers (AβOs) and hippocampal slices from J20 mice using cellular and molecular approaches. In this work, we characterized the capacity of TG2 to interact and promote Aβ toxic aggregates (AβTG2). AβTG2 induced an acute increase in intracellular Ca²⁺, miniature currents, and hiperexcitability, consistent with an increased mitochondrial Ca²⁺ overload, IP3R-VDAC tethering, and mitochondria-endoplasmic reticulum contacts (MERCs). AβTG2 also decreased neuronal viability and excitatory postsynaptic currents, reinforcing the idea of synaptic failure associated with MAMs dysregulation mediated by TG2. Z-DON treatment, TG2 inhibitor, reduced calcium overload, mitochondrial membrane potential loss, and synaptic failure, indicating an involvement of TG2 in a toxic cycle which increases Aβ aggregation, Ca²⁺ overload, and MAMs upregulation. These data provide novel information regarding the role TG2 plays in synaptic function and contribute additional evidence to support the further development of TG2 inhibitors as a disease-modifying strategy for AD.

Extracellular transglutaminase-2, nude or associated with astrocytic extracellular vesicles, modulates neuronal calcium homeostasis

We have uncovered a novel role for astrocytes-derived extracellular vesicles (EVs) in controlling intraneuronal Ca²⁺ concentration ([Ca²⁺]_i) and identified transglutaminase-2 (TG2) as a surface-cargo of astrocytes-derived EVs. Incubation of hippocampal neurons with primed astrocyte-derived EVs have led to an increase in [Ca²⁺]_i, unlike EVs from TG2-knockout astrocytes. Exposure of neurons or brain slices to extracellular TG2 promoted a [Ca²⁺]_i rise, which was reversible upon TG2 removal and was dependent on Ca²⁺ influx through the plasma membrane. Patch-clamp and calcium imaging recordings revealed TG2-dependent neuronal membrane depolarization and activation of inward currents, due to the Na⁺/Ca²⁺-exchanger (NCX) operating in the reverse mode and indirect activation of L-type VOCCs, as indicated by VOCCs/NCX pharmacological inhibitors. A subunit of Na⁺/K⁺-ATPase was selected by comparative proteomics and identified as being functionally inhibited by extracellular TG2, implicating Na⁺/K⁺-ATPase inhibition in NCX reverse mode-switching leading to Ca²⁺ influx and higher basal [Ca²⁺]_i. These data suggest that reactive astrocytes control intraneuronal [Ca²⁺]_i through release of EVs with TG2 as responsible cargo, which could have a significant impact on synaptic activity in brain inflammation.

Cross-species genetic screens identify transglutaminase 5 as a regulator of polyglutamine-expanded ataxin-1

Many neurodegenerative disorders are caused by abnormal accumulation of misfolded proteins. In spinocerebellar ataxia type 1 (SCA1), accumulation of polyglutamine-expanded (polyQ-expanded) ataxin-1 (ATXN1) causes neuronal toxicity. Lowering total ATXN1, especially the polyQ-expanded form, alleviates disease phenotypes in mice, but the molecular mechanism by which the mutant ATXN1 is specifically modulated is not understood. Here, we identified 22 mutant ATXN1 regulators by performing a cross-species screen of 7787 and 2144 genes in human cells and Drosophila eyes, respectively. Among them, transglutaminase 5 (TG5) preferentially regulated mutant ATXN1 over the WT protein. TG enzymes catalyzed cross-linking of ATXN1 in a polyQ-length-dependent manner, thereby preferentially modulating mutant ATXN1 stability and oligomerization. Perturbing Tg in Drosophila SCA1 models modulated mutant ATXN1 toxicity. Moreover, TG5 was enriched in the nuclei of SCA1-affected neurons and colocalized with nuclear ATXN1 inclusions in brain tissue from patients with SCA1. Our work provides a molecular insight into SCA1 pathogenesis and an opportunity for allele-specific targeting for neurodegenerative disorders.

A Disulfide-Stabilized Aβ that Forms Dimers but Does Not Form Fibrils

Aβ dimers are a basic building block of many larger Aβ oligomers and are among the most neurotoxic and pathologically relevant species in Alzheimer's disease. Homogeneous Aβ dimers are difficult to prepare, characterize, and study because Aβ forms heterogeneous mixtures of oligomers that vary in size and can rapidly aggregate into more stable fibrils. This paper introduces Aβ_C18C33 as a disulfide-stabilized analogue of Aβ₄₂ that forms stable homogeneous dimers in lipid environments but does not aggregate to form insoluble fibrils. The Aβ_C18C33 peptide is readily expressed in Escherichia coli and purified by reverse-phase HPLC to give ca. 8 mg of pure peptide per liter of bacterial culture. SDS-PAGE establishes that Aβ_C18C33 forms homogeneous dimers in the membrane-like environment of SDS and that conformational stabilization of the peptide with a disulfide bond prevents the formation of heterogeneous mixtures of oligomers. Mass spectrometric (MS) studies in the presence of dodecyl maltoside (DDM) further confirm the formation of stable noncovalent dimers. Circular dichroism (CD) spectroscopy establishes that Aβ_C18C33 adopts a β-sheet conformation in detergent solutions and supports a model in which the intramolecular disulfide bond induces β-hairpin folding and dimer formation in lipid environments. Thioflavin T (ThT) fluorescence assays and transmission electron microscopy (TEM) studies indicate that Aβ_C18C33 does not undergo fibril formation in aqueous buffer solutions and demonstrate that the intramolecular disulfide bond prevents fibril formation. The recently published NMR structure of an Aβ₄₂ tetramer (PDB: 6RHY) provides a working model for the Aβ_C18C33 dimer, in which two β-hairpins assemble through hydrogen bonding to form a four-stranded antiparallel β-sheet. It is anticipated that Aβ_C18C33 will serve as a stable, nonfibrilizing, and noncovalent Aβ dimer model for amyloid and Alzheimer's disease research.

Absence of tissue transglutaminase reduces amyloid-beta pathology in APP23 mice

Aims

Alzheimer's disease (AD) is characterised by amyloid‐beta (Aβ) aggregates in the brain. Targeting Aβ aggregates is a major approach for AD therapies, although attempts have had little to no success so far. A novel treatment option is to focus on blocking the actual formation of Aβ multimers. The enzyme tissue transglutaminase (TG2) is abundantly expressed in the human brain and plays a key role in post‐translational modifications in Aβ resulting in covalently cross‐linked, stable and neurotoxic Aβ oligomers.

In vivo absence of TG2 in the APP23 mouse model may provide evidence that TG2 plays a key role in development and/or progression of Aβ‐related pathology.

Methods

Here, we compared the effects on Aβ pathology in the presence or absence of TG2 using 12‐month‐old wild type, APP23 and a crossbreed of the TG2−/− mouse model and APP23 mice (APP23/TG2−/−).

Results

Using immunohistochemistry, we found that the number of Aβ deposits was significantly reduced in the absence of TG2 compared with age‐matched APP23 mice. To pinpoint possible TG2‐associated mechanisms involved in this observation, we analysed soluble brain Aβ_1–40, Aβ_1–42 and/or Aβ_40/42 ratio, and mRNA levels of human APP and TG2 family members present in brain of the various mouse models. In addition, using immunohistochemistry, both beta‐pleated sheet formation in Aβ deposits and the presence of reactive astrocytes associated with Aβ deposits were analysed.

Conclusions

We found that absence of TG2 reduces the formation of Aβ pathology in the APP23 mouse model, suggesting that TG2 may be a suitable therapeutic target for reducing Aβ deposition in AD.

The Transglutaminase-2 Interactome in the APP23 Mouse Model of Alzheimer's Disease

Amyloid-beta (Aβ) deposition in the brain is closely linked with the development of Alzheimer’s disease (AD). Unfortunately, therapies specifically targeting Aβ deposition have failed to reach their primary clinical endpoints, emphasizing the need to broaden the search strategy for alternative targets/mechanisms. Transglutaminase-2 (TG2) catalyzes post-translational modifications, is present in AD lesions and interacts with AD-associated proteins. However, an unbiased overview of TG2 interactors is lacking in both control and AD brain. Here we aimed to identify these interactors using a crossbreed of the AD-mimicking APP23 mouse model with wild type and TG2 knock-out (TG2^−/−) mice. We found that absence of TG2 had no (statistically) significant effect on Aβ pathology, soluble brain levels of Aβ_1–40 and Aβ_1–42, and mRNA levels of TG family members compared to APP23 mice at 18 months of age. Quantitative proteomics and network analysis revealed a large cluster of TG2 interactors involved in synaptic transmission/assembly and cell adhesion in the APP23 brain typical of AD. Comparative proteomics of wild type and TG2^−/− brains revealed a TG2-linked pathological proteome consistent with alterations in both pathways. Our data show that TG2 deletion leads to considerable network alterations consistent with a TG2 role in (dys)regulation of synaptic transmission and cell adhesion in APP23 brains.

Extracellular GAPDH Promotes Alzheimer Disease Progression by Enhancing Amyloid-β Aggregation and Cytotoxicity

Neuronal cell death at late stages of Alzheimer's disease (AD) causes the release of cytosolic proteins. One of the most abundant such proteins, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), forms stable aggregates with extracellular amyloid-β (Aβ). We detect these aggregates in cerebrospinal fluid (CSF) from AD patients at levels directly proportional to the progressive stages of AD. We found that GAPDH forms a covalent bond with Q15 of Aβ that is mediated by transglutaminase (tTG). The Q15A substitution weakens the interaction between Aβ and GAPDH and reduces Aβ-GAPDH cytotoxicity. Lentivirus-driven GAPDH overexpression in two AD animal models increased the level of apoptosis of hippocampal cells, neural degeneration, and cognitive dysfunction. In contrast, in vivo knockdown of GAPDH reversed these pathogenic abnormalities suggesting a pivotal role of GAPDH in Aβ-stimulated neurodegeneration. CSF from animals with enhanced GAPDH expression demonstrates increased cytotoxicity in vitro. Furthermore, RX-624, a specific GAPDH small molecular ligand reduced accumulation of Aβ aggregates and reversed memory deficit in AD transgenic mice. These findings argue that extracellular GAPDH compromises Aβ clearance and accelerates neurodegeneration, and, thus, is a promising pharmacological target for AD.

Amyloids: The History of Toxicity and Functionality

Proteins can perform their specific function due to their molecular structure. Partial or complete unfolding of the polypeptide chain may lead to the misfolding and aggregation of proteins in turn, resulting in the formation of different structures such as amyloid aggregates. Amyloids are rigid protein aggregates with the cross-β structure, resistant to most solvents and proteases. Because of their resistance to proteolysis, amyloid aggregates formed in the organism accumulate in tissues, promoting the development of various diseases called amyloidosis, for instance Alzheimer's diseases (AD). According to the main hypothesis, it is considered that the cause of AD is the formation and accumulation of amyloid plaques of Aβ. That is why Aβ-amyloid is the most studied representative of amyloids. Therefore, in this review, special attention is paid to the history of Aβ-amyloid toxicity. We note the main problems with anti-amyloid therapy and write about new views on amyloids that can play positive roles in the different organisms including humans.

Identification of neurotoxic cross-linked amyloid-β dimers in the Alzheimer's brain

The primary structure of canonical amyloid-β-protein was elucidated more than 30 years ago, yet the forms of amyloid-β that play a role in Alzheimer's disease pathogenesis remain poorly defined. Studies of Alzheimer's disease brain extracts suggest that amyloid-β, which migrates on sodium dodecyl sulphate polyacrylamide gel electrophoresis with a molecular weight of ∼7 kDa (7kDa-Aβ), is particularly toxic; however, the nature of this species has been controversial. Using sophisticated mass spectrometry and sensitive assays of disease-relevant toxicity we show that brain-derived bioactive 7kDa-Aβ contains a heterogeneous mixture of covalently cross-linked dimers in the absence of any other detectable proteins. The identification of amyloid-β dimers may open a new phase of Alzheimer's research and allow a better understanding of Alzheimer's disease, and how to monitor and treat this devastating disorder. Future studies investigating the bioactivity of individual dimers cross-linked at known sites will be critical to this effort.

Interaction between tissue transglutaminase and amyloid-beta: Protein-protein binding versus enzymatic crosslinking

Self-interaction, chaperone binding and posttranslational modification of amyloid-beta (Aβ) is essential in the initiation and propagation of Aβ aggregation. Aggregation results in insoluble Aβ deposits characteristic of Alzheimer's disease (AD) brain lesions, i.e. senile plaques and cerebral amyloid angiopathy. Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes posttranslational modifications including the formation of covalent ε-(γ-glutamyl)lysine isopeptide bonds (molecular crosslinks), and colocalizes with Aβ deposits in AD. Two independent groups recently found that apart from the induction of Aβ oligomerization, the blood-derived transglutaminase member FXIIIa forms stable protein-protein complexes with Aβ independent of the transamidation reaction. Here, we investigated whether also tTG forms rigid protein complexes with Aβ in the absence of catalytic activation. We found that both Aβ_1-40 and Aβ_1-42 are substrates for tTG-catalyzed crosslinking. In addition, in the absence of calcium or the presence of a peptidergic inhibitor of tTG, stable tTG-Aβ_1-40 complexes were found. Interestingly, the stable complexes between tTG and Aβ_1-40, were only found at 'physiological' concentrations of Aβ_1-40. Together, our data suggest that depending on the Aβ species at hand, and on the concentration of Aβ, rigid protein-complexes are formed between tTG and Aβ_1-40 without the involvement of the crosslinking reaction.

Opening up about Tissue Transglutaminase: When Conformation Matters More than Enzymatic Activity

Tissue transglutaminase (tTG), also referred to as type 2 transglutaminase or Gα_h, can bind and hydrolyze GTP, as well as function as a protein crosslinking enzyme. tTG is widely expressed and can be detected both inside cells and in the extracellular space. In contrast to many enzymes, the active and inactive conformations of tTG are markedly different. The catalytically inactive form of tTG adopts a compact “closed-state” conformation, while the catalytically active form of the protein adopts an elongated “open-state” conformation. tTG has long been appreciated as an important player in numerous diseases, including celiac disease, neuronal degenerative diseases, and cancer, and its roles in these diseases often depend as much upon its conformation as its catalytic activity. While its ability to promote these diseases has been traditionally thought to be dependent on its protein crosslinking activity, more recent findings suggest that the conformational state tTG adopts is also important for mediating its effects. In particular, we and others have shown that the closed-state of tTG is important for promoting cell growth and survival, while maintaining tTG in the open-state is cytotoxic. In this review, we examine the two unique conformations of tTG and how they contribute to distinct biological processes. We will also describe how this information can be used to generate novel therapies to treat diseases, with a special focus on cancer.

Coagulation factor XIIIa cross-links amyloid β into dimers and oligomers and to blood proteins

In cerebral amyloid angiopathy (CAA) and Alzheimer's disease (AD), the amyloid β (Aβ) peptide deposits along the vascular lumen, leading to degeneration and dysfunction of surrounding tissues. Activated coagulation factor XIIIa (FXIIIa) covalently cross-links proteins in blood and vasculature, such as in blood clots and on the extracellular matrix. Although FXIIIa co-localizes with Aβ in CAA, the ability of FXIIIa to cross-link Aβ has not been demonstrated. Using Western blotting, kinetic assays, and microfluidic analyses, we show that FXIIIa covalently cross-links Aβ40 into dimers and oligomers (k _cat/K_m = 1.5 × 10⁵ m^-1s^-1), as well as to fibrin, platelet proteins, and blood clots under flow in vitro Aβ40 also increased the stiffness of platelet-rich plasma clots in the presence of FXIIIa. These results suggest that FXIIIa-mediated cross-linking may contribute to the formation of Aβ deposits in CAA and Alzheimer's disease.

New insight into transglutaminase 2 and link to neurodegenerative diseases

Formation of toxic protein aggregates is a common feature and mainly contributes to the pathogenesis of neurodegenerative diseases (NDDs), which include amyotrophic lateral sclerosis (ALS), Alzheimer’s, Parkinson’s, Huntington’s, and prion diseases. The transglutaminase 2 (TG2) gene encodes a multifunctional enzyme, displaying four types of activity, such as transamidation, GTPase, protein disulfide isomerase, and protein kinase activities. Many studies demonstrated that the calcium-dependent transamidation activity of TG2 affects the formation of insoluble and toxic amyloid aggregates that mainly consisted of NDD-related proteins. So far, many important and NDD-related substrates of TG2 have been identified, including amlyoid-β, tau, α-synuclein, mutant huntingtin, and ALS-linked trans-activation response (TAR) DNA-binding protein 43. Recently, the formation of toxic inclusions mediated by several TG2 substrates were efficiently inhibited by TG2 inhibitors. Therefore, the development of highly specific TG2 inhibitors would be an important tool in alleviating the progression of TG2-related brain disorders. In this review, the authors discuss recent advances in TG2 biochemistry, several mechanisms of molecular regulation and pleotropic signaling functions, and the presumed role of TG2 in the progression of many NDDs.

Fungus-derived hydroxyl radicals kill hepatic cells by enhancing nuclear transglutaminase

We previously reported the importance of induced nuclear transglutaminase (TG) 2 activity, which results in hepatic cell death, in ethanol-induced liver injury. Here, we show that co-incubation of either human hepatic cells or mouse primary hepatocytes derived from wild-type but not TG2-/- mice with pathogenic fungi Candida albicans and C. glabrata, but not baker's yeast Saccharomyces cerevisiae, induced cell death in host cells by enhancing cellular, particularly nuclear, TG activity. Further pharmacological and genetic approaches demonstrated that this phenomenon was mediated partly by the production of reactive oxygen species (ROS) such as hydroxyl radicals, as detected by a fluorescent probe and electron spin resonance. A ROS scavenger, N-acetyl cysteine, blocked enhanced TG activity primarily in the nuclei and inhibited cell death. In contrast, deletion of C. glabrata nox-1, which encodes a ROS-generating enzyme, resulted in a strain that failed to induce the same phenomena. A similar induction of hepatic ROS and TG activities was observed in C. albicans-infected mice. An antioxidant corn peptide fraction inhibited these phenomena in hepatic cells. These results address the impact of ROS-generating pathogens in inducing nuclear TG2-related liver injuries, which provides novel therapeutic targets for preventing and curing alcoholic liver disease.

The Aggregation Paths and Products of Aβ42 Dimers Are Distinct from Those of the Aβ42 Monomer

Extracts of Alzheimer's disease (AD) brain that contain what appear to be sodium dodecyl sulfate-stable amyloid β-protein (Aβ) dimers potently block LTP and impair memory consolidation. Brain-derived dimers can be physically separated the Aβ monomer, consist primarily of Aβ42, and resist denaturation by chaotropic agents. In nature, covalently cross-linked Aβ dimers could be generated in two ways: by the formation of a dityrosine (DiY) or an isopeptide ε-(γ-glutamyl)-lysine (Q-K) bond. We enzymatically cross-linked recombinant Aβ42 monomer to produce DiY and Q-K dimers and then used a range of biophysical methods to study their aggregation. Both Q-K and DiY dimers aggregate to form soluble assemblies distinct from the fibrillar aggregates formed by the Aβ monomer. The results suggest that the cross-links disfavor fibril formation from Aβ dimers, thereby enhancing the concentration of soluble aggregates akin to those in aqueous extracts of AD brain. Thus, it seems that Aβ dimers may play an important role in determining the formation of soluble rather than insoluble aggregates.

Alzheimer disease: modeling an Aβ-centered biological network

In genetically complex diseases, the search for missing heritability is focusing on rare variants with large effect. Thanks to next generation sequencing technologies, genome-wide characterization of these variants is now feasible in every individual. However, a lesson from current studies is that collapsing rare variants at the gene level is often insufficient to obtain a statistically significant signal in case-control studies, and that network-based analyses are an attractive complement to classical approaches. In Alzheimer disease (AD), according to the prevalent amyloid cascade hypothesis, the pathology is driven by the amyloid beta (Aβ) peptide. In past years, based on experimental studies, several hundreds of proteins have been shown to interfere with Aβ production, clearance, aggregation or toxicity. Thanks to a manual curation of the literature, we identified 335 genes/proteins involved in this biological network and classified them according to their cellular function. The complete list of genes, or its subcomponents, will be of interest in ongoing AD genetic studies.

Catalytically active tissue transglutaminase colocalises with Aβ pathology in Alzheimer's disease mouse models

Alzheimer's disease (AD) is characterised by amyloid-beta (Aβ) protein deposition in the brain. Posttranslational modifications in Aβ play an important role in Aβ deposition. Tissue transglutaminase (tTG) is an enzyme involved in posttranslational cross-linking of proteins. tTG levels and activity are increased in AD brains, and tTG is associated with Aβ deposits and lesion-associated astrocytes in AD cases. Furthermore, Aβ is a substrate of tTG-catalysed cross-linking. To study the role of tTG in Aβ pathology, we compared tTG distribution and activity in both the APPSWE/PS1ΔE9 and APP23 mice models with human AD. Using immunohistochemistry, we found association of both tTG and in situ active tTG with Aβ plaques and vascular Aβ, in early and late stages of Aβ deposition. In addition, tTG staining colocalised with Aβ-associated reactive astrocytes. Thus, alike human AD cases, tTG was associated with Aβ depositions in these AD models. Although, distribution pattern and spatial overlay of both tTG and its activity with Aβ pathology was substantially different from human AD cases, our findings provide evidence for an early role of tTG in Aβ pathology. Yet, species differences should be taken into account when using these models to study the role of tTG in Aβ pathology.

High-resolution NMR characterization of low abundance oligomers of amyloid-β without purification

Alzheimer's disease is characterized by the misfolding and self-assembly of the amyloidogenic protein amyloid-β (Aβ). The aggregation of Aβ leads to diverse oligomeric states, each of which may be potential targets for intervention. Obtaining insight into Aβ oligomers at the atomic level has been a major challenge to most techniques. Here, we use magic angle spinning recoupling (1)H-(1)H NMR experiments to overcome many of these limitations. Using (1)H-(1)H dipolar couplings as a NMR spectral filter to remove both high and low molecular weight species, we provide atomic-level characterization of a non-fibrillar aggregation product of the Aβ1-40 peptide using non-frozen samples without isotopic labeling. Importantly, this spectral filter allows the detection of the specific oligomer signal without a separate purification procedure. In comparison to other solid-state NMR techniques, the experiment is extraordinarily selective and sensitive. A resolved 2D spectra could be acquired of a small population of oligomers (6 micrograms, 7% of the total) amongst a much larger population of monomers and fibers (93% of the total). By coupling real-time (1)H-(1)H NMR experiments with other biophysical measurements, we show that a stable, primarily disordered Aβ1-40 oligomer 5-15 nm in diameter can form and coexist in parallel with the well-known cross-β-sheet fibrils.

The blood clotting Factor XIIIa forms unique complexes with amyloid-beta (Aβ) and colocalizes with deposited Aβ in cerebral amyloid angiopathy

AIMS

Cerebral amyloid angiopathy (CAA) is a key pathological hallmark of Alzheimer's disease (AD) characterized by accumulation of amyloid-beta (Aβ) protein in blood vessel walls. CAA impairs vessel functioning, affects blood brain barrier integrity and accelerates cognitive decline of AD patients. Unfortunately, mechanisms underlying Aβ deposition in the vessel wall remain largely unknown. Factor XIIIa (FXIIIa) is a blood-derived transglutaminase crucial in blood coagulation by cross-linking fibrin molecules. Evidence is mounting that blood-derived factors are present in CAA and may play a role in protein deposition in the vessel wall. We therefore investigated whether FXIIIa is present in CAA and if FXIIIa cross-link activity affects Aβ aggregation.

METHODS

Using immunohistochemistry, we investigated the distribution of FXIIIa, its activator thrombin and in situ FXIIIa activity in CAA in post-mortem AD tissue. We used surface plasmon resonance and Western blot analysis to study binding of FXIIIa to Aβ and the formation of FXIIIa-Aβ complexes, respectively. In addition, we studied cytotoxicity of FXIIIa-Aβ complexes to cerebrovascular cells.

RESULTS

FXIIIa, thrombin and in situ FXIIIa activity colocalize with the Aβ deposition in CAA. Furthermore, FXIIIa binds to Aβ with a higher binding affinity for Aβ1-42 compared with Aβ1-40 . Moreover, highly stable FXIIIa-Aβ complexes are formed independently of FXIIIa cross-linking activity that protected cerebrovascular cells from Aβ-induced toxicity in vitro.

CONCLUSIONS

Our data showed that FXIIIa colocalizes with Aβ in CAA and that FXIIIa forms unique protein complexes with Aβ that might play an important role in Aβ deposition and persistence in the vessel wall.

Glutathionylspermidine in the modification of protein SH groups: the enzymology and its application to study protein glutathionylation

Cysteine is very susceptible to reactive oxygen species. In response; posttranslational thiol modifications such as reversible disulfide bond formation have arisen as protective mechanisms against undesired in vivo cysteine oxidation. In Gram-negative bacteria a major defense mechanism against cysteine overoxidation is the formation of mixed protein disulfides with low molecular weight thiols such as glutathione and glutathionylspermidine. In this review we discuss some of the mechanistic aspects of glutathionylspermidine in prokaryotes and extend its potential use to eukaryotes in proteomics and biochemical applications through an example with tissue transglutaminase and its S-glutathionylation.

Secreted amyloid β-proteins in a cell culture model include N-terminally extended peptides that impair synaptic plasticity

Evidence for a central role of amyloid β-protein (Aβ) in the genesis of Alzheimer’s disease (AD) has led to advanced human trials of Aβ-lowering agents. The “amyloid hypothesis” of AD postulates deleterious effects of small, soluble forms of Aβ on synaptic form and function. Because selectively targeting synaptotoxic forms of soluble Aβ could be therapeutically advantageous, it is important to understand the full range of soluble Aβ derivatives. We previously described a Chinese hamster ovary (CHO) cell line (7PA2 cells) that stably expresses mutant human amyloid precursor protein (APP). Here, we extend this work by purifying an sodium dodecyl sulfate (SDS)-stable, ∼8 kDa Aβ species from the 7PA2 medium. Mass spectrometry confirmed its identity as a noncovalently bonded Aβ40 homodimer that impaired hippocampal long-term potentiation (LTP) in vivo. We further report the detection of Aβ-containing fragments of APP in the 7PA2 medium that extend N-terminal from Asp1 of Aβ. These N-terminally extended Aβ-containing monomeric fragments are distinct from soluble Aβ oligomers formed from Aβ1-40/42 monomers and are bioactive synaptotoxins secreted by 7PA2 cells. Importantly, decreasing β-secretase processing of APP elevated these alternative synaptotoxic APP fragments. We conclude that certain synaptotoxic Aβ-containing species can arise from APP processing events N-terminal to the classical β-secretase cleavage site.

Di-tyrosine cross-link decreases the collisional cross-section of aβ peptide dimers and trimers in the gas phase: an ion mobility study

Oligomeric forms of Aβ peptide are most likely the main synaptotoxic and neurotoxic agent in Alzheimer’s disease. Toxicity of various Aβ oligomeric forms has been confirmed in vivo and also in vitro. However, in vitro preparations were found to be orders of magnitude less toxic than oligomers obtained from in vivo sources. This difference can be explained by the presence of a covalent cross-link, which would stabilize the oligomer. In the present work, we have characterized the structural properties of Aβ dimers and trimers stabilized by di- and tri-tyrosine cross-links. Using ion mobility mass spectrometry we have compared the collisional cross-section of non-cross-linked and cross-linked species. We have found that the presence of cross-links does not generate new unique forms but rather shifts the equilibrium towards more compact oligomer types that can also be detected for non-cross-linked peptide. In consequence, more extended forms, probable precursors of off-pathway oligomeric species, become relatively destabilized in cross-linked oligomers and the pathway of oligomer evolution becomes redirected towards fibrillar structures.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Discovery of two transglutaminases derived from same zymogen for the Streptomyces hygroscopicus and analysis of their formation processes

BACKGROUND

Transglutaminase (TGase) is secreted as a zymogen (Pro-TGase) and is then processed by removal of its N-terminal region through exogenous proteolytic activity. In this study it was discovered that the Pro-TGase from Streptomyces hygroscopicus was also activated by its TGase (processed through exogenous proteolytic activity), resulting in a different active form.

RESULTS

The two TGases exhibited different ionic strengths, hydrophobicities, Km values and stabilities. Circular dichroism spectral analysis showed that the two enzymes had non-identical secondary structures, while liquid chromatography/mass spectrometry (LC-MS) analysis indicated that they differed in molecular mass by 111 Da. The formation of the TGase activated from Pro-TGase by TGase was delayed compared with that of TGase processed through exogenous proteolytic activity. Furthermore, it was found that the TGase activated from Pro-TGase by TGase did not activate Pro-TGase.

CONCLUSION

Two TGases derived from the same zymogen from S. hygroscopicus were discovered. These two active forms of TGase may be due to different activation processes: one of them is catalysed by its own active TGase, while the other is activated by an exogenous protease.

Globular and protofibrillar aβ aggregates impair neurotransmission by different mechanisms

In Alzheimer's disease, substantial evidence indicates the causative role of soluble amyloid β (Aβ) aggregates. Although a variety of Aβ assemblies have been described, the debate about their individual relevance is still ongoing. One critical issue hampering this debate is the use of different methods for the characterization of endogenous and synthetic peptide and their intrinsic limitations for distinguishing Aβ aggregates. Here, we used different protocols for the establishment of prefibrillar Aβ assemblies with varying morphologies and sizes and compared them in a head-to-head fashion. Aggregation was characterized via the monomeric peptide over time until spheroidal, protofibrillar, or fibrillar Aβ aggregates were predominant. It could be shown that a change in the ionic environment induced a structural rearrangement, which consequently confounds the delineation of a measured neurotoxicity toward a distinct Aβ assembly. Here, neuronal binding and hippocampal neurotransmission were found to be suitable to account for the synaptotoxicity to different Aβ assemblies, based on the stability of the applied Aβ aggregates in these settings. In contrast to monomeric or fibrillar Aβ, different prefibrillar Aβ aggregates targeted neurons and impaired hippocampal neurotransmission with nanomolar potency, albeit by different modalities. Spheroidal Aβ aggregates inhibited NMDAR-dependent long-term potentiation, as opposed to protofibrillar Aβ aggregates, which inhibited AMPAR-dominated basal neurotransmission. In addition, a provoked structural conversion of spheroidal to protofibrillar Aβ assemblies resulted in a time-dependent suppression of basal neurotransmission, indicative of a mechanistic switch in synaptic impairment. Thus, we emphasize the importance of addressing the metastability of prefacto characterized Aβ aggregates in assigning a biological effect.

Biomarker evidence for uncoupling of amyloid build-up and toxicity in Alzheimer's disease

Certain preparations of Alzheimer-associated amyloid beta (Aβ) exhibit rapid (within minutes) synaptotoxicity when applied to hippocampal slices or neuronal cell cultures, or when injected into the central nervous system of rodents. In addition, it is well known that some elderly people have brain amyloidosis without showing signs of cognitive impairment or neurodegeneration beyond the age norm. Biomarkers, reviewed extensively in a recent Perspectives article in Alzheimer's & Dementia, suggest that amyloid-positive individuals are at higher risk of Alzheimer's disease than similarly aged individuals without evidence of brain amyloidosis, provided they live long enough. But how can the brain resist amyloid pathology for many years? Here, we expand on recent biomarker studies suggesting that Aβ build-up and toxicity may occur in two phases. We hypothesize that the first phase may involve an autocatalytic build-up of a nontoxic Aβ reservoir, tentatively named the Aβ(Cat) pool, and that gain of toxicity may require brain incubation of Aβ in the water-deprived plaque milieu over years to produce modified forms of the protein that are truly neurotoxic (Aβ(Tox)). We argue for the need to describe the molecular natures of Aβ(Cat) and Aβ(Tox) in greater detail as a means to gain success in anti-Aβ disease-modifying drug development.

Tissue transglutaminase colocalizes with extracellular matrix proteins in cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is a key histopathological hallmark of Alzheimer's disease (AD) and hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D). CAA is characterized by amyloid-beta (Aβ) depositions and remodeling of the extracellular matrix (ECM) in brain vessels and plays an important role in the development and progression of both AD and HCHWA-D. Tissue transglutaminase (tTG) modulates the ECM by molecular cross-linking of ECM proteins. Here, we investigated the distribution pattern, cellular source, and activity of tTG in CAA in control, AD, and HCHWA-D cases. We observed increased tTG immunoreactivity and colocalization with Aβ in the vessel wall in early stage CAA, whereas in later CAA stages, tTG and its cross-links were present in halos enclosing the Aβ deposition. In CAA, tTG and its cross-links at the abluminal side of the vessel were demonstrated to be either of astrocytic origin in parenchymal vessels, of fibroblastic origin in leptomeningeal vessels, and of endothelial origin at the luminal side of the deposited Aβ. Furthermore, the ECM proteins fibronectin and laminin colocalized with the tTG-positive halos surrounding the deposited Aβ in CAA. However, we observed that in situ tTG activity was present throughout the vessel wall in late stage CAA. Together, our data suggest that tTG and its activity might play a differential role in the development and progression of CAA, possibly evolving from direct modulation of Aβ aggregation to cross-linking of ECM proteins resulting in ECM restructuring.

Alzheimer's disease Aβ assemblies mediating rapid disruption of synaptic plasticity and memory

Alzheimer’s disease (AD) is characterized by episodic memory impairment that often precedes clinical diagnosis by many years. Probing the mechanisms of such impairment may provide much needed means of diagnosis and therapeutic intervention at an early, pre-dementia, stage. Prior to the onset of significant neurodegeneration, the structural and functional integrity of synapses in mnemonic circuitry is severely compromised in the presence of amyloidosis. This review examines recent evidence evaluating the role of amyloid-ß protein (Aβ) in causing rapid disruption of synaptic plasticity and memory impairment. We evaluate the relative importance of different sizes and conformations of Aβ, including monomer, oligomer, protofibril and fibril. We pay particular attention to recent controversies over the relevance to the pathophysiology of AD of different water soluble Aβ aggregates and the importance of cellular prion protein in mediating their effects. Current data are consistent with the view that both low-n oligomers and larger soluble assemblies present in AD brain, some of them via a direct interaction with cellular prion protein, cause synaptic memory failure. At the two extremes of aggregation, monomers and fibrils appear to act in vivo both as sources and sinks of certain metastable conformations of soluble aggregates that powerfully disrupt synaptic plasticity. The same principle appears to apply to other synaptotoxic amyloidogenic proteins including tau, α-synuclein and prion protein.

γ-Glutamylamines and neurodegenerative diseases

Transglutaminases catalyze the formation of γ-glutamylamines utilizing glutamyl residues and amine-bearing compounds such as lysyl residues and polyamines. These γ-glutamylamines can be released from proteins by proteases in an intact form. The free γ-glutamylamines can be catabolized to 5-oxo-L-proline and the free amine by γ-glutamylamine cyclotransferase. Free γ-glutamylamines, however, accumulate in the CSF and affected areas of Huntington Disease brain. This observation suggests transglutaminase-derived γ-glutamylamines may play a more significant role in neurodegeneration than previously thought. The following monograph reviews the metabolism of γ-glutamylamines and examines the possibility that these species contribute to neurodegeneration.

Tissue transglutaminase: a novel therapeutic target in cerebral amyloid angiopathy

Accumulation of amyloid-β (Aβ) in brain vessel walls, known as cerebral amyloid angiopathy (CAA), plays a key role in Alzheimer's disease pathogenesis. CAA might result from impaired transport of Aβ out of the brain. Although the mechanisms underlying reduced Aβ transport are largely unknown, thickening of basement membrane extracellular matrix (ECM) is likely involved. Tissue transglutaminase (tTG) is an enzyme capable of modulating the ECM by covalently cross-linking ECM proteins. Recently, our group found that tTG and its cross-linking activity are associated with CAA pathology, suggesting a role for tTG in ECM modulation in CAA. Therefore, inhibition of tTG activity might be a promising novel therapeutic target to counteract CAA.