Tissue transglutaminase-catalysed cross-linking induces Apolipoprotein E multimers inhibiting Apolipoprotein E's protective effects towards amyloid-beta-induced toxicity

Defining the conformational states that enable transglutaminase 2 to promote cancer cell survival versus cell death

Transglutaminase 2 (TG2) is a GTP-binding/protein-crosslinking enzyme that has been investigated as a therapeutic target for Celiac disease, neurological disorders, and aggressive cancers. TG2 has been suggested to adopt two conformational states that regulate its functions: a GTP-bound, closed conformation, and a calcium-bound, crosslinking-active open conformation. TG2 mutants that constitutively adopt an open conformation are cytotoxic to cancer cells. Thus, small molecules that maintain the open conformation of TG2 could offer a new therapeutic strategy. Here, we investigate TG2, using static and time-resolved small-angle X-ray scattering (SAXS) and single-particle cryoelectron microscopy (cryo-EM), to determine the conformational states responsible for conferring its biological effects. We also describe a newly developed TG2 inhibitor, LM11, that potently kills glioblastoma cells and use SAXS to investigate how LM11 affects the conformational states of TG2. Using SAXS and cryo-EM, we show that guanine nucleotide-bound TG2 adopts a monomeric closed conformation while calcium-bound TG2 assumes an open conformational state that can form higher order oligomers. SAXS analysis also suggests how a TG2 mutant that constitutively adopts the open state binds nucleotides through an alternative mechanism to wildtype TG2. Furthermore, we use time-resolved SAXS to show that LM11 increases the ability of calcium to drive TG2 to an open conformation, which is not reversible by guanine nucleotides and is cytotoxic to cancer cells. Taken together, our findings demonstrate that the conformational dynamics of TG2 are more complex than previously suggested and highlight how conformational stabilization of TG2 by LM11 maintains TG2 in a cytotoxic conformational state.

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.

Increased transcription of transglutaminase 1 mediates neuronal death in in vitro models of neuronal stress and Aβ1-42-mediated toxicity

Alzheimer's disease (AD) is the most common cause of dementia. At the pre-symptomatic phase of the disease, the processing of the amyloid precursor protein (APP) produces toxic peptides, called amyloid-β 1-42 (Aβ 1-42). The downstream effects of Aβ 1-42 production are not completely uncovered. Here, we report the involvement of transglutaminase 1 (TG1) in in vitro AD models of neuronal toxicity. TG1 was increased at late stages of the disease in the hippocampus of a mouse model of AD and in primary cortical neurons undergoing stress. Silencing of TGM1 gene was sufficient to prevent Aβ-mediated neuronal death. Conversely, its overexpression enhanced cell death. TGM1 upregulation was mediated at the transcriptional level by an activator protein 1 (AP1) binding site that when mutated halted TGM1 promoter activation. These results indicate that TG1 acts downstream of Aβ-toxicity, and that its stress-dependent increase makes it suitable for pharmacological intervention.

Neuro-Immune Hemostasis: Homeostasis and Diseases in the Central Nervous System

Coagulation and the immune system interact in several physiological and pathological conditions, including tissue repair, host defense, and homeostatic maintenance. This network plays a key role in diseases of the central nervous system (CNS) by involving several cells (CNS resident cells, platelets, endothelium, and leukocytes) and molecular pathways (protease activity, complement factors, platelet granule content). Endothelial damage prompts platelet activation and the coagulation cascade as the first physiological step to support the rescue of damaged tissues, a flawed rescuing system ultimately producing neuroinflammation. Leukocytes, platelets, and endothelial cells are sensitive to the damage and indeed can release or respond to chemokines and cytokines (platelet factor 4, CXCL4, TNF, interleukins), and growth factors (including platelet-derived growth factor, vascular endothelial growth factor, and brain-derived neurotrophic factor) with platelet activation, change in capillary permeability, migration or differentiation of leukocytes. Thrombin, plasmin, activated complement factors and matrix metalloproteinase-1 (MMP-1), furthermore, activate intracellular transduction through complement or protease-activated receptors. Impairment of the neuro-immune hemostasis network induces acute or chronic CNS pathologies related to the neurovascular unit, either directly or by the systemic activation of its main steps. Neurons, glial cells (astrocytes and microglia) and the extracellular matrix play a crucial function in a “tetrapartite” synaptic model. Taking into account the neurovascular unit, in this review we thoroughly analyzed the influence of neuro-immune hemostasis on these five elements acting as a functional unit (“pentapartite” synapse) in the adaptive and maladaptive plasticity and discuss the relevance of these events in inflammatory, cerebrovascular, Alzheimer, neoplastic and psychiatric diseases. Finally, based on the solid reviewed data, we hypothesize a model of neuro-immune hemostatic network based on protein–protein interactions. In addition, we propose that, to better understand and favor the maintenance of adaptive plasticity, it would be useful to construct predictive molecular models, able to enlighten the regulating logic of the complex molecular network, which belongs to different cellular domains. A modeling approach would help to define how nodes of the network interact with basic cellular functions, such as mitochondrial metabolism, autophagy or apoptosis. It is expected that dynamic systems biology models might help to elucidate the fine structure of molecular events generated by blood coagulation and neuro-immune responses in several CNS diseases, thereby opening the way to more effective treatments.

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.

Cystamine and cysteamine as inhibitors of transglutaminase activity in vivo

Cystamine is commonly used as a transglutaminase inhibitor. This disulphide undergoes reduction in vivo to the aminothiol compound, cysteamine. Thus, the mechanism by which cystamine inhibits transglutaminase activity in vivo could be due to either cystamine or cysteamine, which depends on the local redox environment. Cystamine inactivates transglutaminases by promoting the oxidation of two vicinal cysteine residues on the enzyme to an allosteric disulphide, whereas cysteamine acts as a competitive inhibitor for transamidation reactions catalyzed by this enzyme. The latter mechanism is likely to result in the formation of a unique biomarker, N-(γ-glutamyl)cysteamine that could serve to indicate how cyst(e)amine acts to inhibit transglutaminases inside cells and the body.

The Netherlands Brain Bank for Psychiatry

The Netherlands Brain Bank (NBB) performs rapid autopsies of donors who gave written informed consent during life for the use of their brain tissue and medical files for research. The NBB initiated the Netherlands Brain Bank for Psychiatry (NBB-Psy), a prospective donor program for psychiatric diseases. NBB-Psy wants to expand the tissue collections in order to provide a strong incentive to increase research in psychiatry. The ultimate goal of NBB-Psy is to reduce the burden of psychiatric disorders for patients, their families, and for society as a whole. NBB-Psy consists of an antemortem and postmortem donor program. This chapter focuses on the design of NBB-Psy and the antemortem donor program, where patients and relatives are actively informed on the possibility to become a brain donor. Since the initiation of NBB-Psy, the number of registered donors with a psychiatric diagnosis has increased from 149 in 2010 to 1018 in May 2016.

Neuro-Coagulopathy: Blood Coagulation Factors in Central Nervous System Diseases

Blood coagulation factors and other proteins, with modulatory effects or modulated by the coagulation cascade have been reported to affect the pathophysiology of the central nervous system (CNS). The protease-activated receptors (PARs) pathway can be considered the central hub of this regulatory network, mainly through thrombin or activated protein C (aPC). These proteins, in fact, showed peculiar properties, being able to interfere with synaptic homeostasis other than coagulation itself. These specific functions modulate neuronal networks, acting both on resident (neurons, astrocytes, and microglia) as well as circulating immune system cells and the extracellular matrix. The pleiotropy of these effects is produced through different receptors, expressed in various cell types, in a dose- and time-dependent pattern. We reviewed how these pathways may be involved in neurodegenerative diseases (amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases), multiple sclerosis, ischemic stroke and post-ischemic epilepsy, CNS cancer, addiction, and mental health. These data open up a new path for the potential therapeutic use of the agonist/antagonist of these proteins in the management of several central nervous system diseases.

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.

Development of carbon-11 labeled acryl amides for selective PET imaging of active tissue transglutaminase

INTRODUCTION

Tissue transglutaminase (TG2) is a ubiquitously expressed enzyme capable of forming metabolically and mechanically stable crosslinks between the γ-carboxamide of a glutamine acyl-acceptor substrate and the ε-amino functionality of a lysine acyl-donor substrate resulting in protein oligomers. High TG2 crosslinking activity has been implicated in the pathogenesis of various diseases including celiac disease, cancer and fibrotic and neurodegenerative diseases. Development of a PET tracer specific for active TG2 provides a novel tool to further investigate TG2 biology in vivo in disease states. Recently, potent irreversible active site TG2 inhibitors carrying an acrylamide warhead were synthesized and pharmacologically characterized.

METHODS

Three of these inhibitors, compound 1, 2 and 3, were successfully radiolabeled with carbon-11 on the acrylamide carbonyl position using a palladium mediated [(11)C]CO aminocarbonylation reaction. Ex vivo biodistribution and plasma stability were evaluated in healthy Wistar rats. Autoradiography was performed on MDA-MB-231 tumor sections.

RESULTS

[(11)C]1, -2 and -3 were obtained in decay corrected radiochemical yields of 38-55%. Biodistribution showed low uptake in peripheral tissues, with the exception of liver and kidney. Low brain uptake of <0.05% ID/g was observed. Blood plasma analysis demonstrated that [(11)C]1 and [(11)C]2 were rapidly metabolized, whereas [(11)C]3 was metabolized at a more moderate rate (63.2 ± 6.8 and 28.7 ± 10.8% intact tracer after 15 and 45 min, respectively). Autoradiography with [(11)C]3 on MDA-MB-231 tumor sections showed selective and specific binding of the radiotracer to the active state of TG2.

CONCLUSIONS

Taken together, these results identify [(11)C]3 as the most promising of the three compounds tested for development as PET radiotracer for the in vivo investigation of TG2 activity.