Transglutaminase 2 as a therapeutic target for neurological conditions

Nrf2 and Ferroptosis: Exploring Translational Avenues for Therapeutic Approaches to Neurological Diseases

Nrf2, a crucial protein involved in defense mechanisms, particularly oxidative stress, plays a significant role in neurological diseases (NDs) by reducing oxidative stress and inflammation. NDs, including Alzheimer's, Parkinson's, Huntington's, amyotrophic lateral sclerosis, stroke, epilepsy, schizophrenia, depression, and autism, exhibit ferroptosis, iron-dependent regulated cell death resulting from lipid and iron-dependent reactive oxygen species (ROS) accumulation. Nrf2 has been shown to play a critical role in regulating ferroptosis in NDs. Age-related decline in Nrf2 expression and its target genes (HO-1, Nqo-1, and Trx) coincides with increased iron-mediated cell death, leading to ND onset. The modulation of iron-dependent cell death and ferroptosis by Nrf2 through various cellular and molecular mechanisms offers a potential therapeutic pathway for understanding the pathological processes underlying these NDs. This review emphasizes the mechanistic role of Nrf2 and ferroptosis in multiple NDs, providing valuable insights for future research and therapeutic approaches.

The Search for a Universal Treatment for Defined and Mixed Pathology Neurodegenerative Diseases

The predominant neurodegenerative diseases, Alzheimer's disease, Parkinson's disease, dementia with Lewy Bodies, Huntington's disease, amyotrophic lateral sclerosis, and frontotemporal dementia, are rarely pure diseases but, instead, show a diversity of mixed pathologies. At some level, all of them share a combination of one or more different toxic biomarker proteins: amyloid beta (Aβ), phosphorylated Tau (pTau), alpha-synuclein (αSyn), mutant huntingtin (mHtt), fused in sarcoma, superoxide dismutase 1, and TAR DNA-binding protein 43. These toxic proteins share some common attributes, making them potentially universal and simultaneous targets for therapeutic intervention. First, they all form toxic aggregates prior to taking on their final forms as contributors to plaques, neurofibrillary tangles, Lewy bodies, and other protein deposits. Second, the primary enzyme that directs their aggregation is transglutaminase 2 (TGM2), a brain-localized enzyme involved in neurodegeneration. Third, TGM2 binds to calmodulin, a regulatory event that can increase the activity of this enzyme threefold. Fourth, the most common mixed pathology toxic biomarkers (Aβ, pTau, αSyn, nHtt) also bind calmodulin, which can affect their ability to aggregate. This review examines the potential therapeutic routes opened up by this knowledge. The end goal reveals multiple opportunities that are immediately available for universal therapeutic treatment of the most devastating neurodegenerative diseases facing humankind.

Pharmacological Inhibition of Astrocytic Transglutaminase 2 Facilitates the Expression of a Neurosupportive Astrocyte Reactive Phenotype in Association with Increased Histone Acetylation

Astrocytes play critical roles in supporting structural and metabolic homeostasis in the central nervous system (CNS). CNS injury leads to the development of a range of reactive phenotypes in astrocytes whose molecular determinants are poorly understood. Finding ways to modulate astrocytic injury responses and leverage a pro-recovery phenotype holds promise in treating CNS injury. Recently, it has been demonstrated that ablation of astrocytic transglutaminase 2 (TG2) shifts reactive astrocytes towards a phenotype that improves neuronal injury outcomes both in vitro and in vivo. Additionally, in an in vivo mouse model, pharmacological inhibition of TG2 with the irreversible inhibitor VA4 phenocopied the neurosupportive effects of TG2 deletion in astrocytes. In this study, we extended our comparisons of VA4 treatment and TG2 deletion to provide insights into the mechanisms by which TG2 attenuates neurosupportive astrocytic function after injury. Using a neuron-astrocyte co-culture model, we found that VA4 treatment improves the ability of astrocytes to support neurite outgrowth on an injury-relevant matrix, as we previously showed for astrocytic TG2 deletion. We hypothesize that TG2 mediates its influence on astrocytic phenotype through transcriptional regulation, and our previous RNA sequencing suggests that TG2 is primarily transcriptionally repressive in astrocytes, although it can facilitate both up- and downregulation of gene expression. Therefore, we asked whether VA4 inhibition could alter TG2's interaction with Zbtb7a, a transcription factor that we previously identified as a functionally relevant TG2 nuclear interactor. We found that VA4 significantly decreased the interaction of TG2 and Zbtb7a. Additionally, we assessed the effect of TG2 deletion and VA4 treatment on transcriptionally permissive histone acetylation and found significantly greater acetylation in both experimental groups. Consistent with these findings, our present proteomic analysis further supports the predominant transcriptionally repressive role of TG2 in astrocytes. Our proteomic data additionally unveiled pronounced changes in lipid and antioxidant metabolism in astrocytes with TG2 deletion or inhibition, which likely contribute to the enhanced neurosupportive function of these astrocytes.

Transglutaminase 2 inhibition ameliorates cardiac fibrosis in myocardial infarction by inducing M2 macrophage polarization in vitro and in vivo

Objective

Macrophages perform vital functions in cardiac remodeling after myocardial infarction (MI). Transglutaminase 2 (TG2) participates in fibrosis. Nevertheless, the role of TG2 in MI and mechanisms underlying macrophage polarization are unclear. This study aimed to discover the functions and possible mechanisms of TG2 in MI.

Material and Methods

C57BL/6 mice were classified into three groups (six mice per group): Sham, MI, and MI+GK921 groups. GK921 acts as a TG2 inhibitor. Cardiac function, myocardial cell apoptosis, fibrosis, and macrophage phenotype in mouse experiments were detected through echocardiography, terminal deoxynucleotidyl transferase dUTP nick end labeling, Masson staining, immunofluorescence, and flow cytometry, respectively. The in vitro study involved the treatment of mouse cardiac fibroblasts isolated from mice with transforming growth factor β1 (TGF-β1) and evaluation of fibrosis through the detection of the expressions of fibrosis-associated proteins using Western blot. Bone marrow-derived macrophages (BMDMs) obtained from mice were triggered by interleukin (IL)-4, and the type of macrophages was determined through flow cytometry.

Results

In in vivo experiments, GK921 substantially improved cardiac injury and fibrosis, induced M2 macrophage polarization, and suppressed the TGF-β1/small mother against decapentaplegic 3 (Smad3) pathway in MI mice. Moreover, TG2 knockdown considerably decreased the expressions of fibrosis-associated proteins in TGF-β1-triggered mouse cardiac fibroblasts, which indicates the repressive effect of TG2 knockdown on fibrosis. In addition, the inhibition effect of TG2 downregulation on the TGF-β1/Smad3 pathway was proven in TGF-β1-treated mouse cardiac fibroblasts in vitro. Moreover, TG2 inhibition remarkably increased M2 macrophage polarization in IL-4-induced BMDMs.

Conclusion

TG2 inhibition facilitated M2 macrophage polarization to provide protection against MI-caused cardiac fibrosis in mice, and these effects may be attained through modulation of the TGF-β1/Smad3 pathway.

Precise Regulations at the Subcellular Level through Intracellular Polymerization, Assembly, and Transformation

A living cell is an intricate machine that creates subregions to operate cell functions effectively. Subcellular dysfunction has been identified as a potential druggable target for successful drug design and therapy. The treatments based on intracellular polymerization, self-assembly, or transformation offer various advantages, including enhanced blood circulation of monomers, long-term drug delivery pharmacokinetics, low drug resistance, and the ability to target deep tissues and organelles. In this review, we discuss the latest developments of intracellular synthesis applied to precisely control cellular functions. First, we discuss the design and applications of endogenous and exogenous stimuli-triggered intracellular polymerization, self-assembly, and dynamic morphology transformation of biomolecules at the subcellular level. Second, we highlight the benefits of these strategies applied in cancer diagnosis and treatment and modulating cellular states or cell metabolism of living systems. Finally, we conclude the recent progress in this field, discuss future perspectives, analyze the challenges of the intracellular functional reactions for regulation, and find future opportunities.

Pharmacological inhibition of astrocytic transglutaminase 2 facilitates the expression of a neurosupportive astrocyte reactive phenotype in association with increased histone acetylation

Astrocytes play critical roles in supporting structural and metabolic homeostasis in the central nervous system (CNS). CNS injury leads to the development of a range of reactive phenotypes in astrocytes whose molecular determinants are poorly understood. Finding ways to modulate astrocytic injury responses and leverage a pro-recovery phenotype holds promise in treating CNS injury. Recently, it has been demonstrated that ablation of astrocytic transglutaminase 2 (TG2) modulates the phenotype of reactive astrocytes in a way that improves neuronal injury outcomes both in vitro and in vivo. In an in vivo mouse model, pharmacological inhibition of TG2 with the irreversible inhibitor VA4 phenocopies the neurosupportive effects of TG2 deletion in astrocytes. In this study, we provide insights into the mechanisms by which TG2 deletion or inhibition result in a more neurosupportive astrocytic phenotype. Using a neuron-astrocyte co-culture model, we show that VA4 treatment improves the ability of astrocytes to support neurite outgrowth on an injury-relevant matrix. To better understand how pharmacologically altering TG2 affects its ability to regulate reactive astrocyte phenotypes, we assessed how VA4 inhibition impacts TG2's interaction with Zbtb7a, a transcription factor we have previously identified as a functionally relevant TG2 nuclear interactor. The results of these studies demonstrate that VA4 significantly decreases the interaction of TG2 and Zbtb7a. TG2's interactions with Zbtb7a, as well as a wide range of other transcription factors and chromatin regulatory proteins, suggest that TG2 may act as an epigenetic regulator to modulate gene expression. To begin to understand if TG2-mediated epigenetic modification may impact astrocytic phenotypes in our models, we interrogated the effect of TG2 deletion and VA4 treatment on histone acetylation and found significantly greater acetylation in both experimental groups. Consistent with these findings, previous RNA-sequencing and our present proteomic analysis also supported a predominant transcriptionally suppressive role of TG2 in astrocytes. Our proteomic data additionally unveiled pronounced changes in lipid and antioxidant metabolism in astrocytes with TG2 deletion or inhibition, which likely contribute to the enhanced neurosupportive function of these astrocytes.

Dose-Ranging Effects of the Intracerebral Administration of Atsttrin in Experimental Model of Parkinson's Disease Induced by 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in Mice

Parkinson's disease is one of the most common neurodegenerative disorders characterized by a multitude of motor and non-motor clinical symptoms resulting from the progressive and long-lasting abnormal loss of nigrostriatal dopaminergic neurons. Currently, the available treatments for patients with Parkinson's disease are limited and exert only symptomatic effects, without adequate signs of delaying or stopping the progression of the disease. Atsttrin constitutes the bioengineered protein which ultrastructure is based on the polypeptide chain frame of the progranulin (PGRN), which exerts anti-inflammatory effects through the inhibition of TNFα. The conducted preclinical studies suggest that the therapeutic implementation of Atsttrin may be potentially effective in the treatment of neurodegenerative diseases that are associated with the occurrence of neuroinflammatory processes. The aim of the proposed study was to investigate the effect of direct bilateral intracerebral administration of Atsttrin using stereotactic methods in the preclinical C57BL/6 mouse model of Parkinson's disease inducted by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. The analysis of the dose dependency effects of the increasing doses of Atsttrin has covered a number of parameters and markers regarding neurodegenerative processes and inflammatory responses including IL-1α, TNFα, IL-6, TH, and TG2 mRNA expressions. Accordingly, the evaluation of the changes in the neurochemical profile included DA, DOPAC, 3-MT, HVA, NA, MHPG, 5-HT, and 5-HIAA concentration levels. The intracerebral administration of Atsttrin into the striatum effectively attenuated the neuroinflammatory reaction in evaluated neuroanatomical structures. Furthermore, the partial restoration of monoamine content and its metabolic turnover were observed. In this case, taking into account the previously described pharmacokinetic profile and extrapolated bioavailability as well as the stability characteristics of Atsttrin, an attempt was made to describe as precisely as possible the quantitative and qualitative effects of increasing doses of the compound within the brain tissue microenvironment in the presented preclinical model of the disease. Collectively, this findings demonstrated that the intracerebral administration of Atsttrin may represent a potential novel therapeutic method for the treatment of Parkinson's disease.

Effect of General Anesthetic Agents on Microglia

The effects of general anesthetic agents (GAAs) on microglia and their potential neurotoxicity have attracted the attention of neuroscientists. Microglia play important roles in the inflammatory process and in neuromodulation of the central nervous system. Microglia-mediated neuroinflammation is a key mechanism of neurocognitive dysfunction during the perioperative period. Microglial activation by GAAs induces anti-inflammatory and pro-inflammatory effects in microglia, suggesting that GAAs play a dual role in the mechanism of postoperative cognitive dysfunction. Understanding of the mechanisms by which GAAs regulate microglia may help to reduce the incidence of postoperative adverse effects. Here, we review the actions of GAAs on microglia and the consequent changes in microglial function. We summarize clinical and animal studies associating microglia with general anesthesia and describe how GAAs interact with neurons via microglia to further explore the mechanisms of action of GAAs in the nervous system.

Biological Implications and Functional Significance of Transglutaminase Type 2 in Nervous System Tumors

Transglutaminase type 2 (TG2) is the most ubiquitously expressed member of the transglutaminase family. TG2 catalyzes the transamidation reaction leading to several protein post-translational modifications and it is also implicated in signal transduction thanks to its GTP binding/hydrolyzing activity. In the nervous system, TG2 regulates multiple physiological processes, such as development, neuronal cell death and differentiation, and synaptic plasticity. Given its different enzymatic activities, aberrant expression or activity of TG2 can contribute to tumorigenesis, including in peripheral and central nervous system tumors. Indeed, TG2 dysregulation has been reported in meningiomas, medulloblastomas, neuroblastomas, glioblastomas, and other adult-type diffuse gliomas. The aim of this review is to provide an overview of the biological and functional relevance of TG2 in the pathogenesis of nervous system tumors, highlighting its involvement in survival, tumor inflammation, differentiation, and in the resistance to standard therapies.

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.

Development and Utilization of Novel Transglutaminase 2 Inhibitors for Potential Treatment of Autoimmune Disease and Gastrointestinal Disorder

Transglutaminases, classified explicitly as "protein-glutamine: amine γ-glutamyl transferases", catalyze the formation of isopeptide bonds between the γ-amino group of lysine and the γ-glutamyl group of glutamine, leading to the release of ammonia. These enzymes play pivotal roles in diverse medical conditions such as cardiovascular, autoimmune, neurodegenerative, and dermatological diseases, as well as in wound healing and inflammatory diseases. This Patent Highlight presents novel inhibitors of transglutaminases, specifically transglutaminase 2 (TG2), and their synthetic processes. In addition, these inhibitors are effective treatments for diseases linked to TG2, such as celiac disease and fibrotic disorders. Therefore, these inhibitors can be active agents in pharmaceutical treatments.

Cell-Impermeable Inhibitors Confirm That Intracellular Human Transglutaminase 2 Is Responsible for the Transglutaminase-Associated Cancer Phenotype

Transglutaminase 2 (TG2) is a multifunctional enzyme primarily responsible for crosslinking proteins. Ubiquitously expressed in humans, TG2 can act either as a transamidase by crosslinking two substrates through formation of an Nε(ɣ-glutaminyl)lysine bond or as an intracellular G-protein. These discrete roles are tightly regulated by both allosteric and environmental stimuli and are associated with dramatic changes in the conformation of the enzyme. The pleiotropic nature of TG2 and multi-faceted activities have resulted in TG2 being implicated in numerous disease pathologies including celiac disease, fibrosis, and cancer. Targeted TG2 therapies have not been selective for subcellular localization, such that currently no tools exist to selectively target extracellular over intracellular TG2. Herein, we have designed novel TG2-selective inhibitors that are not only highly potent and irreversible, but also cell impermeable, targeting only extracellular TG2. We have also further derivatized the scaffold to develop probes that are intrinsically fluorescent or bear an alkyne handle, which target both intra- and extracellular TG2, in order to facilitate cellular labelling and pull-down assays. The fluorescent probes were internalized and imaged in cellulo, and provide the first implicit experimental evidence that by comparison with their cell-impermeable analogues, it is specifically intracellular TG2, and presumably its G-protein activity, that contributes to transglutaminase-associated cancer progression.

Allosteric Activation of Transglutaminase 2 via Inducing an "Open" Conformation for Osteoblast Differentiation

Osteoblasts play an important role in the regulation of bone homeostasis throughout life. Thus, the damage of osteoblasts can lead to serious skeletal diseases, highlighting the urgent need for novel pharmacological targets. This study introduces chemical genetics strategy by using small molecule forskolin (FSK) as a probe to explore the druggable targets for osteoporosis. Here, this work reveals that transglutaminase 2 (TGM2) served as a major cellular target of FSK to obviously induce osteoblast differentiation. Then, this work identifies a previously undisclosed allosteric site in the catalytic core of TGM2. In particular, FSK formed multiple hydrogen bonds in a saddle-like domain to induce an "open" conformation of the β-sandwich domain in TGM2, thereby promoting the substrate protein crosslinks by incorporating polyamine. Furthermore, this work finds that TGM2 interacted with several mitochondrial homeostasis-associated proteins to improve mitochondrial dynamics and ATP production for osteoblast differentiation. Finally, this work observes that FSK effectively ameliorated osteoporosis in the ovariectomy mice model. Taken together, these findings show a previously undescribed pharmacological allosteric site on TGM2 for osteoporosis treatment, and also provide an available chemical tool for interrogating TGM2 biology and developing bone anabolic agent.

Novel irreversible peptidic inhibitors of transglutaminase 2

Transglutaminase 2 (TG2), also referred to as tissue transglutaminase, plays crucial roles in both protein crosslinking and cell signalling. It is capable of both catalysing transamidation and acting as a G-protein, these activities being conformation-dependent, mutually exclusive, and tightly regulated. The dysregulation of both activities has been implicated in numerous pathologies. TG2 is expressed ubiquitously in humans and is localized both intracellularly and extracellularly. Targeted TG2 therapies have been developed but have faced numerous hurdles including decreased efficacy in vivo. Our latest efforts in inhibitor optimization involve the modification of a previous lead compound's scaffold by insertion of various amino acid residues into the peptidomimetic backbone, and derivatization of the N-terminus with substituted phenylacetic acids, resulting in 28 novel irreversible inhibitors. These inhibitors were evaluated for their ability to inhibit TG2 in vitro and their pharmacokinetic properties, and the most promising candidate 35 (k inact/K I = 760 × 103 M-1 min-1) was tested in a cancer stem cell model. Although these inhibitors display exceptional potency versus TG2, with k inact/K I ratios nearly ten-fold higher than their parent compound, their pharmacokinetic properties and cellular activity limit their therapeutic potential. However, they do serve as a scaffold for the development of potent research tools.

The war on hTG2: warhead optimization in small molecule human tissue transglutaminase inhibitors

Human tissue transglutaminase (hTG2) is a multifunctional enzyme with protein cross-linking and G-protein activity, both of which have been implicated in the progression of diseases such as fibrosis and cancer stem cell propagation when dysregulated, prompting the development of small molecule targeted covalent inhibitors (TCIs) possessing a crucial electrophilic 'warhead'. In recent years there have been significant advances in the library of warheads available for the design of TCIs; however, the exploration of warhead functionality in hTG2 inhibitors has remained relatively stagnant. Herein, we describe a structure-activity relationship study entailing rational design and synthesis for systematic variation of the warhead on a previously reported small molecule inhibitor scaffold, and rigorous kinetic evaluation of inhibitory efficiency, selectivity, and pharmacokinetic stability. This study reveals a strong influence on the kinetic parameters k inact and K I with even subtle variation in warhead structure, suggesting that the warhead plays a significant role in not only reactivity, but also binding affinity, which consequently extends to isozyme selectivity. Warhead structure also influences in vivo stability, which we model by measuring intrinsic reactivity with glutathione, as well as stability in hepatocytes and in whole blood, giving insight into degradation pathways and relative therapeutic potential of different functional groups. This work provides fundamental structural and reactivity information highlighting the importance of strategic warhead design for the development of potent hTG2 inhibitors.

The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

Comparative Profiling of TG2 and Its Effectors in Human Relapsing Remitting and Progressive Multiple Sclerosis

Multiple Sclerosis (MS) is a chronic CNS autoimmune disease characterized by immune-mediated demyelination, axon loss, and disability. Dysregulation of transglutaminase-2 (TG2) has been implicated in disease initiation and progression. Herein, TG2 expression in post-mortem human brain tissue from Relapsing Remitting MS (RRMS) or Progressive MS (PMS) individuals were examined and correlated with the presence of TG2 binding partners and effectors implicated in the processes of inflammation, scar formation, and the antagonism of repair. Tissues from Relapsing-Remitting Multiple Sclerosis (RRMS; n = 6), Progressive Multiple Sclerosis (PMS; n = 5), and non-MS control (n = 6) patients underwent immunohistochemistry for TG2, PLA2, COX-2, FN, CSPG, and HSPG. TG2 was strongly upregulated in active RRMS and PMS lesions, within blood vessels and the perivascular tissue of sclerotic plaques. TG2 colocalization was observed with GFAP+ astrocytes and ECM, including FN, HSPG, and CSPG, which also increased in either RRMS or PMS lesions. Although TG2 was not colocalized with inflammatory mediators COX-2 and PLA2, or the macrophage-microglia marker Iba1, its increased expression correlated with their elevation in active RRMS and PMS lesions. In summary, the correlation of strong TG2 induction in either RRMS or PMS with some of its binding partners but not others implicates potentially different roles for TG2 in disparate MS forms that may warrant further investigation.

Structure-activity relationships of hydrophobic alkyl acrylamides as tissue transglutaminase inhibitors

Tissue transglutaminase (TG2) is a multifunctional protein that plays biological roles based on its ability to catalyse protein cross-linking and to function as a non-canonical G-protein known as Ghα. The non-regulated activity of TG2 has been implicated in fibrosis, celiac disease and the survival of cancer stem cells, underpinning the therapeutic potential of cell permeable small molecule inhibitors of TG2. In the current study, we designed a small library of inhibitors to explore the importance of a terminal hydrophobic moiety, as well as the length of the tether to the irreversible acrylamide warhead. Subsequent kinetic evaluation using an in vitro activity assay provided values for the k _inact and K _I parameters for each of these irreversible inhibitors. The resulting structure-activity relationship (SAR) clearly indicated the affinity conferred by dansyl and adamantyl moieties, as well as the efficiency provided by the shortest warhead tether. We also provide the first direct evidence of the capability of these inhibitors to suppress the GTP binding ability of TG2, at least partially. However, it is intriguing to note that the SAR trends observed herein are opposite to those predicted by molecular modelling - namely that longer tether groups should improve binding affinity by allowing for deeper insertion of the hydrophobic moiety into a hydrophobic pocket on the enzyme. This discrepancy leads us to question whether the existing crystallographic structures of TG2 are appropriate for docking non-peptidic inhibitors. In the absence of a more relevant crystallographic structure, the data from rigorous kinetic studies, such as those provided herein, are critically important for the development of future small molecule TG2 inhibitors.