Transglutaminase 2 Depletion Attenuates α-Synuclein Mediated Toxicity in Mice

Pros and cons of narrow- versus wide-compartment rotarod apparatus: An experimental study in mice

The rotarod test, a sensorimotor assessment that allows for quantitative evaluation of motor coordination in rodents, has extensive application in many research fields. The test results exhibit extreme between-study variability, sometimes making it challenging to conclude the validity of certain disease models and related therapeutic effects. Although the variation in test paradigms may account for this disparity, some features of rotarod apparatus including rod diameter make differences. However, it is unknown whether the width of animal compartment has a role in rotarod performance. Here we comprehensively evaluated the active rotarod performance and adverse incidents in multiple strains of mice on an 11-cm- or a 5-cm-wide compartment apparatus. We found that mouse behaviors on these apparatuses were surprisingly different. It took a markedly longer time to train mice on the narrow- than wide-compartment rotarod. Further, non-transgenic B6129S and tau knockout mice aged 11 months and beyond showed different levels of improvement based on the compartment width. These mice had no overt improvements on accelerating rotarod over 4-5 training sessions on the narrow compartment, contrary to marked progress on the wide counterpart. The incidents of mice passively somersaulting round and fragmented running occurred significantly more on the wide than narrow compartment during accelerating rotarod sessions. Mice fell off rod more frequently on narrow than wide compartments upon attempt to turn around and when moving backward on rod. The pros and cons of narrow versus wide compartments are informative as to how to choose a rotarod apparatus that best fits the animal models used.

Pathogenetic Contributions and Therapeutic Implications of Transglutaminase 2 in Neurodegenerative Diseases

Neurodegenerative diseases encompass a heterogeneous group of disorders that afflict millions of people worldwide. Characteristic protein aggregates are histopathological hallmark features of these disorders, including Amyloid β (Aβ)-containing plaques and tau-containing neurofibrillary tangles in Alzheimer's disease, α-Synuclein (α-Syn)-containing Lewy bodies and Lewy neurites in Parkinson's disease and dementia with Lewy bodies, and mutant huntingtin (mHTT) in nuclear inclusions in Huntington's disease. These various aggregates are found in specific brain regions that are impacted by neurodegeneration and associated with clinical manifestations. Transglutaminase (TG2) (also known as tissue transglutaminase) is the most ubiquitously expressed member of the transglutaminase family with protein crosslinking activity. To date, Aβ, tau, α-Syn, and mHTT have been determined to be substrates of TG2, leading to their aggregation and implicating the involvement of TG2 in several pathophysiological events in neurodegenerative disorders. In this review, we summarize the biochemistry and physiologic functions of TG2 and describe recent advances in the pathogenetic role of TG2 in these diseases. We also review TG2 inhibitors tested in clinical trials and discuss recent TG2-targeting approaches, which offer new perspectives for the design of future highly potent and selective drugs with improved brain delivery as a disease-modifying treatment for neurodegenerative disorders.

Automated procedure to detect subtle motor alterations in the balance beam test in a mouse model of early Parkinson's disease

Parkinson's disease (PD) is the most common motor neurodegenerative disorder, characterised by aggregated α-synuclein (α-syn) constituting Lewy bodies. We aimed to investigate temporal changes in motor impairments in a PD mouse model induced by overexpression of α-syn with the conventional manual analysis of the balance beam test and a novel approach using machine learning algorithms to automate behavioural analysis. We combined automated animal tracking using markerless pose estimation in DeepLabCut, with automated behavioural classification in Simple Behavior Analysis. Our automated procedure was able to detect subtle motor deficits in mouse performances in the balance beam test that the manual analysis approach could not assess. The automated model revealed time-course significant differences for the "walking" behaviour in the mean interval between each behavioural bout, the median event bout duration and the classifier probability of occurrence in male PD mice, even though no statistically significant loss of tyrosine hydroxylase in the nigrostriatal system was found in either sex. These findings are valuable for early detection of motor impairment in early PD animal models. We provide a user-friendly, step-by-step guide for automated assessment of mouse performances in the balance beam test, which aims to be replicable without any significant computational and programming knowledge.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

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.

Genetic modifiers of synucleinopathies-lessons from experimental models

α-Synuclein is a pleiotropic protein underlying a group of progressive neurodegenerative diseases, including Parkinson's disease and dementia with Lewy bodies. Together, these are known as synucleinopathies. Like all neurological diseases, understanding of disease mechanisms is hampered by the lack of access to biopsy tissues, precluding a real-time view of disease progression in the human body. This has driven researchers to devise various experimental models ranging from yeast to flies to human brain organoids, aiming to recapitulate aspects of synucleinopathies. Studies of these models have uncovered numerous genetic modifiers of α-synuclein, most of which are evolutionarily conserved. This review discusses what we have learned about disease mechanisms from these modifiers, and ways in which the study of modifiers have supported ongoing efforts to engineer disease-modifying interventions for synucleinopathies.

A Mouse Model to Test Novel Therapeutics for Parkinson's Disease: an Update on the Thy1-aSyn ("line 61") Mice

Development of neuroprotective therapeutics for Parkinson's disease (PD) is facing a lack of translation from pre-clinical to clinical trials. One strategy for improvement is to increase predictive validity of pre-clinical studies by using extensively characterized animal models with a comprehensive set of validated pharmacodynamic readouts. Mice over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter (Thy1-aSyn line 61) reproduce key features of sporadic PD, such as progressive loss of striatal dopamine, alpha-synuclein pathology, deficits in motor and non-motor functions, and elevation of inflammatory markers. Extensive work with this model by multiple laboratories over the past decade further increased confidence in its robustness and validity, especially for analyzing pathomechanisms of alpha-synuclein pathology and down-stream pathways, and for pre-clinical drug testing. Interestingly, while postnatal transgene expression is widespread in central and peripheral neurons, the extent and progression of down-stream pathology differs between brain regions, thereby replicating the characteristic selective vulnerability of neurodegenerative diseases. In-depth characterization of these readouts in conjunction with behavioral deficits has led to more informative endpoints for pre-clinical trials. Each drug tested in Thy1-aSyn line 61 enhances knowledge on how molecular targets, pathology, and functional behavioral readouts are interconnected, thereby further optimizing the platform towards predictive validity for clinical trials. Here, we present the current state of the art using Thy1-aSyn line 61 for drug target discovery, validation, and pre-clinical testing.

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.

Therapeutics in the Pipeline Targeting α-Synuclein for Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder and the fastest growing neurologic disease in the world, yet no disease-modifying therapy is available for this disabling condition. Multiple lines of evidence implicate the protein α-synuclein (α-Syn) in the pathogenesis of PD, and as such, there is intense interest in targeting α-Syn for potential disease modification. α-Syn is also a key pathogenic protein in other synucleionpathies, most commonly dementia with Lewy bodies. Thus, therapeutics targeting this protein will have utility in these disorders as well. Here we discuss the various approaches that are being investigated to prevent and mitigate α-Syn toxicity in PD, including clearing its pathologic aggregates from the brain using immunization strategies, inhibiting its misfolding and aggregation, reducing its expression level, enhancing cellular clearance mechanisms, preventing its cell-to-cell transmission within the brain and perhaps from the periphery, and targeting other proteins associated with or implicated in PD that contribute to α-Syn toxicity. We also discuss the therapeutics in the pipeline that harness these strategies. Finally, we discuss the challenges and opportunities for the field in the discovery and development of therapeutics for disease modification in PD. SIGNIFICANCE STATEMENT: PD is the second most common neurodegenerative disorder, for which disease-modifying therapies remain a major unmet need. A large body of evidence points to α-synuclein as a key pathogenic protein in this disease as well as in dementia with Lewy bodies, making it of leading therapeutic interest. This review discusses the various approaches being investigated and progress made to date toward discovering and developing therapeutics that would slow and stop progression of these disabling diseases.

Transglutaminase 2 as a therapeutic target for neurological conditions

INTRODUCTION

Transglutaminase 2 (TG2) has been implicated in numerous neurological conditions, including neurodegenerative diseases, multiple sclerosis, and CNS injury. Early studies on the role of TG2 in neurodegenerative conditions focused on its ability to 'crosslink' proteins into insoluble aggregates. However, more recent studies have suggested that this is unlikely to be the primary mechanism by which TG2 contributes to the pathogenic processes. Although the specific mechanisms by which TG2 is involved in neurological conditions have not been clearly defined, TG2 regulates numerous cellular processes through which it could contribute to a specific disease. Given the fact that TG2 is a stress-induced gene and elevated in disease or injury conditions, TG2 inhibitors may be useful neurotherapeutics.

AREAS COVERED

Overview of TG2 and different TG2 inhibitors. A brief review of TG2 in neurodegenerative diseases, multiple sclerosis and CNS injury and inhibitors that have been tested in different models. Database search: https://pubmed.ncbi.nlm.nih.gov prior to 1 July 2021.

EXPERT OPINION

Currently, it appears unlikely that inhibiting TG2 in the context of neurodegenerative diseases would be therapeutically advantageous. However, for multiple sclerosis and CNS injuries, TG2 inhibitors may have the potential to be therapeutically useful and thus there is rationale for their further development.

Role of Transglutaminase 2 in Cell Death, Survival, and Fibrosis

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme catalyzing the crosslinking between Gln and Lys residues and involved in various pathophysiological events. Besides this crosslinking activity, TG2 functions as a deamidase, GTPase, isopeptidase, adapter/scaffold, protein disulfide isomerase, and kinase. It also plays a role in the regulation of hypusination and serotonylation. Through these activities, TG2 is involved in cell growth, differentiation, cell death, inflammation, tissue repair, and fibrosis. Depending on the cell type and stimulus, TG2 changes its subcellular localization and biological activity, leading to cell death or survival. In normal unstressed cells, intracellular TG2 exhibits a GTP-bound closed conformation, exerting prosurvival functions. However, upon cell stimulation with Ca²⁺ or other factors, TG2 adopts a Ca²⁺-bound open conformation, demonstrating a transamidase activity involved in cell death or survival. These functional discrepancies of TG2 open form might be caused by its multifunctional nature, the existence of splicing variants, the cell type and stimulus, and the genetic backgrounds and variations of the mouse models used. TG2 is also involved in the phagocytosis of dead cells by macrophages and in fibrosis during tissue repair. Here, we summarize and discuss the multifunctional and controversial roles of TG2, focusing on cell death/survival and fibrosis.