Transglutaminases and disease: lessons from genetically engineered mouse models and inherited disorders

Transglutaminase-6 is an autoantigen in progressive multiple sclerosis and is upregulated in reactive astrocytes

BACKGROUND

Transglutaminase-6 (TGM6), a member of the transglutaminase enzyme family, is found predominantly in central nervous system (CNS) neurons under physiological conditions. It has been proposed as an autoimmune target in cerebral palsy, gluten-sensitive cerebellar ataxia, and schizophrenia.

OBJECTIVE

To investigate TGM6 involvement in multiple sclerosis (MS).

METHODS

Antibody levels against TGM6 (TGM6-IgG) were measured in the cerebrospinal fluid (CSF) of 62 primary progressive multiple sclerosis (PPMS), 85 secondary progressive multiple sclerosis (SPMS), and 50 relapsing-remitting multiple sclerosis (RRMS) patients and 51 controls. TGM6 protein expression was analyzed in MS brain autopsy, murine experimental autoimmune encephalomyelitis (EAE), and cultured astrocytes.

RESULTS

CSF levels of TGM6-IgG were significantly higher in PPMS and SPMS compared to RRMS and controls. Notably, patients with clinically active disease had the highest TGM6-IgG levels. Additionally, brain pathology revealed strong TGM6 expression by reactive astrocytes within MS plaques. In EAE, TGM6 expression in the spinal cord correlated with disease course and localized in reactive astrocytes infiltrating white matter lesions. Finally, knocking down TGM6 expression in cultured reactive astrocytes reduced their glial fibrillary acidic protein (GFAP) expression.

CONCLUSION

TGM6-IgG may be a candidate CSF biomarker to predict and monitor disease activity in progressive MS patients. Furthermore, TGM6 expression by reactive astrocytes within both human and mouse lesions suggests its involvement in the mechanisms of glial scar formation.

Recent Progress in the Development of Transglutaminase 2 (TGase2) Inhibitors

Transglutaminase 2 (TGase2, TG2) activity has been implicated in the pathogenesis of a number of unrelated disorders, including celiac, neurological, and renal diseases, and various forms of cancer. It has been suggested that TGase2 activity, such as cross-linking, deamidation, and GTP-related activity, is associated with each disease. Continuing efforts to develop small molecule TG2 inhibitors are ongoing. To develop a new class of TG2 inhibitors, the factors impeding the development of TG2 inhibitors have been identified. Additionally, the conformational effect of TG2 enzyme in regard to its pathological roles, in vitro screening methods, recently discovered TG2 inhibitors, and preclinical evaluations are discussed with a brief summary of current TG2 inhibitor pipelines under the clinical setting.

Biochemical characterization of the medaka (Oryzias latipes) orthologue for mammalian tissue-type transglutaminase (TG2)

Transglutaminase is an enzyme family responsible for post-translational modification such as protein cross-linking and the attachment of primary amine and/or deamidation of glutamine-residue in proteins. Medaka (Oryzias latipes), a recently established model fish, has similar functional proteins to those characterized in mammals. Previously, we found the apparent orthologues that correspond to human transglutaminases in medaka. In this study, regarding the medaka orthologue of human tissue-type transglutaminase (OlTGT), recombinant protein was expressed in an active form in bacteria cultured at low temperature. Using the recombinant protein, we biochemically characterized the enzymatic activity and also obtained a monoclonal antibody that specifically recognized OlTGT. Immunochemical analysis revealed that OlTGT was not expressed ubiquitously, unlike its mammalian orthologue, but in primarily limited tissues such as the eye, brain, spinal cord, and gas gland.

Monocyte behaviour and tissue transglutaminase expression during experimental autoimmune encephalomyelitis in transgenic CX3CR1gfp/gfp mice

Leukocyte infiltration into the central nervous system (CNS) is a key pathological feature in multiple sclerosis (MS) and the MS animal model experimental autoimmune encephalomyelitis (EAE). Recently, preventing leukocyte influx into the CNS of MS patients is the main target of MS therapies and insight into cell behaviour in the circulation is needed for further elucidation of such therapies. In this study, we aimed at in vivo visualization of monocytes in a time-dependent manner during EAE. Using intravital two-photon microscopy (IVM), we imaged CX3CR1^gfp/gfp mice during EAE, visualizing CX3CR1-GFP⁺ monocytes and their dynamics in the spinal cord vasculature. Our observations showed that intraluminal crawling of CX3CR1-GFP⁺ monocytes increased even before the clinical onset of EAE due to immunization of the animals. Furthermore, intraluminal crawling remained elevated during ongoing clinical disease. Besides, the displacement of these cells was larger during the peak of EAE compared to the control animals. In addition, we showed that the enzyme tissue transglutaminase (TG2), which is present in CNS-infiltrated cells in MS patients, is likewise found in CX3CR1-GFP⁺ monocytes in the spinal cord lesions and at the luminal side of the vasculature during EAE. It might thereby contribute to adhesion and crawling of monocytes, facilitating extravasation into the CNS. Thus, we put forward that interference with monocyte adhesion, by e.g. inhibition of TG2, should be applied at a very early stage of EAE and possibly MS, to effectively combat subsequent pathology.

Transglutaminase 2 is dispensable but required for the survival of mice in dextran sulfate sodium-induced colitis

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme that catalyzes crosslinking, polyamination or deamidation of glutamine residues in proteins. It has been reported that TG2 is involved in the pathogenesis of various inflammatory diseases including celiac disease, pulmonary fibrosis, cystic fibrosis, multiple sclerosis and sepsis. Recently, using a mouse model of bleomycin-induced lung fibrosis, we showed that TG2 is required to trigger inflammation via the induction of T helper type 17 (Th17) cell differentiation in response to tissue damage. However, the role of TG2 in inflammatory bowel disease (IBD), which is thought to be a Th17 cell-associated disease, has remained elusive. In this study, we investigated the role of TG2 in dextran sulfate sodium (DSS)-induced colitis, the most widely used mouse model for IBD. Age- and sex-matched wild-type and TG2^-/- mice were fed 2% DSS for 7 days or 3.5% DSS for 5 days in drinking water. An in situ TG activity assay revealed that DSS treatment activates TG2 in various colon cell types, including columnar absorptive cells and goblet cells. DSS-treated TG2^-/- mice showed lower interleukin (IL)-6, but higher IL-17A and RORγt (retinoic acid receptor-related orphan receptor-γt) expression levels in the colon tissues than that in the wild-type mice. Moreover, TG2^-/- mice showed higher mortality than the wild-type mice because of DSS treatment. Nevertheless, we found no significant differences in changes of body weight, colon length, morphology, immune cell infiltration and in vivo intestinal permeability between DSS-treated wild-type and TG2^-/- mice. These results indicate that TG2-mediated Th17 cell differentiation is not required for the pathogenesis of DSS-induced acute colitis.

Tissue Transglutaminase Activates Cancer-Associated Fibroblasts and Contributes to Gemcitabine Resistance in Pancreatic Cancer

Resistance to chemotherapy is a hallmark of pancreatic ductal adenocarcinoma (PDA) and has been partly attributed to the dense desmoplastic stroma, which forms a protective niche for cancer cells. Tissue transglutaminase (TG2), a Ca²⁺-dependent enzyme, is secreted by PDA cells and cross-links proteins in the tumor microenvironment (TME) through acyl-transfer between glutamine and lysine residues, promoting PDA growth. The objective of the current study was to determine whether secreted TG2 by PDA cells alters the response of pancreatic tumors to gemcitabine. Orthotopic pancreatic xenografts and co-culture of PDA and stromal cells were employed to determine the mechanisms by which TG2 alters tumor-stroma interactions and response to gemcitabine. Analysis of the pancreatic The Cancer Genome Atlas (TCGA) database demonstrated that increased TG2 expression levels correlate with worse overall survival (hazard ratio = 1.37). Stable TG2 knockdown in PDA cells led to decreased size of pancreatic xenografts and increased sensitivity to gemcitabine in vivo. However, TG2 downregulation did not increase cytotoxicity of gemcitabine in vitro. Additionally, multivessel density and gemcitabine uptake in pancreatic tumor tissue, as measured by mass spectrometry (MS-HPLC), were not significantly different in tumors expressing TG2 versus tumors in which TG2 was knocked down. Fibroblasts, stimulated by TG2 secreted by PDA cells, secrete laminin A1, which protects cancer cells from gemcitabine-induced cytotoxicity. In all, our results demonstrate that TG2 secreted in the pancreatic TME orchestrates the cross talk between cancer cells and stroma, impacting tumor growth and response to chemotherapy. Our study supports TG2 inhibition to increase the antitumor effects of gemcitabine in PDA.

Factor XIII-A transglutaminase deficient mice show signs of metabolically healthy obesity on high fat diet

F13A1 gene, which encodes for Factor XIII-A blood clotting factor and a transglutaminase enzyme, was recently identified as a potential causative gene for obesity in humans. In our previous in vitro work, we showed that FXIII-A regulates preadipocyte differentiation and modulates insulin signaling via promoting plasma fibronectin assembly into the extracellular matrix. To understand the role of FXIII-A in whole body energy metabolism, here we have characterized the metabolic phenotype of F13a1-/- mice. F13a1-/- and F13a1+/+ type mice were fed chow or obesogenic, high fat diet for 20 weeks. Weight gain, total fat mass and fat pad mass, glucose handling, insulin sensitivity, energy expenditure and, morphological and biochemical analysis of adipose tissue was performed. We show that mice lacking FXIII-A gain weight on obesogenic diet, similarly as wild type mice, but exhibit a number of features of metabolically healthy obesity such as protection from developing diet-induced insulin resistance and hyperinsulinemia. Mice also show normal fasting glucose levels, larger adipocytes, decreased extracellular matrix accumulation and inflammation of adipose tissue, as well as decreased circulating triglycerides. This study reveals that FXIII-A transglutaminase can regulate whole body insulin sensitivity and may have a role in the development of diet-induced metabolic disturbances.

Molecular Cues Guiding Matrix Stiffness in Liver Fibrosis

Tissue and matrix stiffness affect cell properties during morphogenesis, cell growth, differentiation, and migration and are altered in the tissue remodeling following injury and the pathological progression. However, detailed molecular mechanisms underlying alterations of stiffness in vivo are still poorly understood. Recent engineering technologies have developed powerful techniques to characterize the mechanical properties of cell and matrix at nanoscale levels. Extracellular matrix (ECM) influences mechanical tension and activation of pathogenic signaling during the development of chronic fibrotic diseases. In this short review, we will focus on the present knowledge of the mechanisms of how ECM stiffness is regulated during the development of liver fibrosis and the molecules involved in ECM stiffness as a potential therapeutic target for liver fibrosis.

Impact of systemic alitretinoin treatment on skin barrier gene and protein expression in patients with chronic hand eczema

BACKGROUND

Chronic hand eczema (CHE) is a common inflammatory skin disease that affects approximately 10% of the population. Systemic alitretinoin has been shown to be effective in patients with CHE who are refractory to topical corticosteroids.

OBJECTIVES

To analyse the impact of alitretinoin on the skin barrier genes and protein expression in the skin lesions of patients with CHE.

MATERIALS AND METHODS

Fifteen patients with CHE were treated with 30 mg daily of alitretinoin for up to 27 weeks. Disease severity was assessed using a clinical score. Skin biopsies from all the patients were evaluated before and after therapy for the expression of Ki-67, various skin barrier genes and thymic stromal lymphopoietin (TSLP) by real-time quantitative polymerase chain reaction and immunohistochemistry.

RESULTS

After alitretinoin application, an improvement in the clinical severity of CHE was observed in the majority of patients. Analysis of skin biopsies before treatment showed a significant increase in Ki-67-positive cells in the suprabasal layer and a dysregulated expression of various skin barrier genes, such as claudin 1, loricrin, filaggrin and cytokeratin 10, which were normalized after treatment. TSLP was significantly upregulated in patients with CHE and also normalized after alitretinoin treatment and negatively correlated with filaggrin.

CONCLUSIONS

Our data indicate that the expression of barrier genes and proteins was normalized following treatment with alitretinoin in patients with CHE. The change in expression levels of these genes correlated with the clinical efficacy, suggesting that alitretinoin exhibits a disease-modifying activity. TSLP is upregulated in CHE and seems to counteract filaggrin expression in the skin.

Phenosafranin inhibits nuclear localization of transglutaminase 2 without affecting its transamidase activity

Transglutaminase 2 (TG2) localizes to the nucleus and induces apoptosis through a crosslinking inactivation of Sp1 in JHH-7 cells treated with acyclic retinoid. We screened an inhibitor suppressing transamidase activity in the nucleus without affecting transamidase activity itself. Phenosafranin was found to inhibit nuclear localization of EGFP-tagged TG2 and dose-dependently reduce nuclear transamidase activity without affecting the activity in a tube. We concluded that phenosafranin was a novel TG2 inhibitor capable of suppressing its nuclear localization.

Is monocyte- and macrophage-derived tissue transglutaminase involved in inflammatory processes?

Monocytes and macrophages are key players in inflammatory processes following an infection or tissue damage. Monocytes adhere and extravasate into the inflamed tissue, differentiate into macrophages, and produce inflammatory mediators to combat the pathogens. In addition, they take up dead cells and debris and, therefore, take part in the resolution of inflammation. The multifunctional enzyme tissue Transglutaminase (TG2, tTG) is known to participate in most of those monocyte- and macrophage-mediated processes. Moreover, TG2 expression and activity can be regulated by inflammatory mediators. In the present review, we selectively elaborate on the expression, regulation, and contribution of TG2 derived from monocytes and macrophages to inflammatory processes mediated by those cells. In addition, we discuss the role of TG2 in certain pathological conditions, in which inflammation and monocytes and/or macrophages are prominently present, including atherosclerosis, sepsis, and multiple sclerosis. Based on the studies and considerations reported in this review, we conclude that monocyte- and macrophage-derived TG2 is clearly involved in various processes contributing to inflammation. However, TG2’s potential as a therapeutic target to counteract the possible detrimental effects or stimulate the potential beneficial effects on monocyte and macrophage responses during inflammation should be carefully considered. Alternatively, as TG2-related parameters can be used as a marker of disease, e.g., in celiac disease, or of disease-stage, e.g., in cancer, we put forward that this could be subject of research for monocyte- or macrophage-derived TG2 in inflammatory diseases.

Expression of Transglutaminase in Foreskin of Children with Balanitis Xerotica Obliterans

Balanitis xerotica obliterans (BXO) is a chronic inflammatory skin disorder of unclear etiology. The etiology and the exact molecular mechanisms underlying the disease are still unknown. The human transglutaminase (TG) family consists of several proteins with catalytic activity essential for biological processes. In the present research we investigated the transcript levels of three TGs in patients operated on for congenital phimosis without or with histologically confirmed BXO; Thirty children with acquired phimosis were enrolled. The removed foreskins were sent both for histological diagnosis and for quantitative real-time PCR to evaluate the transcript levels of keratinocyte (TG1), tissue (TG2), and epidermal (TG3) transglutaminase; We observed a decrease in TG1 and TG3 transcripts by about 70% (p < 0.001) in foreskins from patients with BXO (n = 15) in comparison with patients without BXO (n = 15) and an increase in TG2 mRNA levels by 2.9 folds (p < 0.001); Reduced expression of both TG1 and TG3 was associated with the altered structure of the foreskin in BXO and can be a consequence of damage to keratinocytes. Increased expression of TG2 can be the result of chronic inflammation. TG2 overexpression can play a pivotal role in triggering and maintaining the inflammatory response in BXO patients.

Celiac anti-type 2 transglutaminase antibodies induce differential effects in fibroblasts from celiac disease patients and from healthy subjects

Type 2 transglutaminase (TG2) has an important pathogenic role in celiac disease (CD), an inflammatory intestinal disease that is caused by the ingestion of gluten-containing cereals. Indeed, TG2 deamidates specific gliadin peptides, thus enhancing their immunogenicity. Moreover, the transamidating activity seems to provoke an autoimmune response, where TG2 is the main autoantigen. Many studies have highlighted a possible pathogenetic role of anti-TG2 antibodies, because they modulate TG2 enzymatic activity and they can interact with cell-surface TG2, triggering a wide range of intracellular responses. Autoantibodies also alter the uptake of the alpha-gliadin peptide 31-43 (p31-43), responsible of the innate immune response in CD, thus partially protecting cells from p31-43 damaging effects in an intestinal cell line. Here, we investigated whether anti-TG2 antibodies protect cells from p31-43-induced damage in a CD model consisting of primary dermal fibroblasts. We found that the antibodies specifically reduced the uptake of p31-43 by fibroblasts derived from healthy subjects but not in those derived from CD patients. Analyses of TG2 expression and enzymatic activity did not reveal any significant difference between fibroblasts from healthy and celiac subjects, suggesting that other features related to TG2 may be responsible of such different behaviors, e.g., trafficking or subcellular distribution. Our findings are in line with the concept that a "celiac cellular phenotype" exists and that TG2 may contribute to this phenotype. Moreover, they suggest that the autoimmune response to TG2, which alone may damage the celiac mucosa, also fails in its protective role in celiac cells.

FRET-based detection of isozyme-specific activities of transglutaminases

Transglutaminases (TGs) comprise a protein family in which the members catalyze the formation of isopeptide bonds between glutamine and lysine residues in various proteins. Expression studies on its three major members, FXIII, TG1, and TG2, have been performed in a relatively large number of mammalian tissues in comparison with those on the other isozymes. We previously identified a highly reactive substrate peptide, including glutamine, for each isozyme from a phage display library and developed a method for detecting isozyme-specific activities by incorporating a labeled substrate peptide into lysine residues of proteins. Here, we describe genetically encoded Förster resonance energy transfer (FRET)-based probes composed of each fluorescence protein (Cerulean and EVenus) fused with substrate peptides. The probe pairs, designated as Trac-MTG (His-CerΔ11-LQ/EV-K-His) containing linker and substrate peptide sequence for microbial TG (MTG), increased the EVenus:Cerulean fluorescence intensity ratio by more than 1.5-fold. Furthermore, we demonstrated that Trac-TG1 (His-CerΔ11-K5) and Trac-TG2 (His-CerΔ11-T26) containing substrate peptide sequence for mammalian TGs successfully detected the isozyme-specific activity of TG1 and TG2, respectively. In this study, we developed a rapid and convenient experimental system for measuring the isozyme-specific activity of TGs. The application of these probes for analyses in cells and tissues will be helpful for elucidating the physiological and pathological functions of TGs.

P2X7 receptor-mediated TG2 externalization: a link to inflammatory arthritis?

Transglutaminases have important roles in stabilizing extracellular protein assemblies in tissue repair processes but some reaction products can stimulate immune activation, leading to chronic inflammatory conditions or autoimmunity. Exacerbated disease in models of inflammatory arthritis has been ascribed to sustained extracellular enzyme activity alongside formation of select protein modifications. Here, we review the evidence, with a focus on the link between P2X7R signaling and TG2 export, a pathway that we have recently discovered which ties extracellular protein modifications into the danger signal-mediated innate immune response. These recent insights offer new opportunities for therapeutic intervention.

Identification of DNAJA1 as a novel interacting partner and a substrate of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed multifunctional member of the transglutaminase enzyme family. It has been implicated to have roles in many physiological and pathological processes such as differentiation, apoptosis, signal transduction, adhesion and migration, wound healing and inflammation. Previous studies revealed that TG2 has various intra- and extra-cellular interacting partners, which contribute to these processes. In the present study, we identified a molecular co-chaperone, DNAJA1, as a novel interacting partner of human TG2 using a GST pull-down assay and subsequent mass spectrometry analysis, and further confirmed this interaction via ELISA and surface plasmon resonance measurements. Interaction studies were also performed with domain variants of TG2 and results suggest that the catalytic core domain of TG2 is essential for the TG2-DNAJA1 interaction. Cross-linking activity was not essential for the interaction since DNAJA1 was also found to interact with the catalytically inactive form of TG2. Furthermore, we have showed that DNAJA1 interacts with the open form of TG2 and regulates its transamidation activity under both in vitro and in situ conditions. We also found that DNAJA1 is a glutamine donor substrate of TG2. Since DNAJA1 and TG2 are reported to regulate common pathological conditions such as neurodegenerative disorders and cancer, the findings in the present paper open up possibilities to explore molecular mechanisms behind TG2-regulated functions.

Essential Role of Transglutaminase 2 in Vascular Endothelial Growth Factor-Induced Vascular Leakage in the Retina of Diabetic Mice

Diabetic retinopathy is predominantly caused by vascular endothelial growth factor (VEGF)-induced vascular leakage; however, the underlying mechanism is unclear. Here we designed an in vivo transglutaminase (TGase) activity assay in mouse retina and demonstrated that hyperglycemia induced vascular leakage by activating TGase2 in diabetic retina. VEGF elevated TGase2 activity through sequential elevation of intracellular Ca(2+) and reactive oxygen species (ROS) concentrations in endothelial cells. The TGase inhibitors cystamine and monodansylcadaverin or TGase2 small interfering RNA (siRNA) prevented VEGF-induced stress fiber formation and vascular endothelial (VE)-cadherin disruption, which play a critical role in modulating endothelial permeability. Intravitreal injection of two TGase inhibitors or TGase2 siRNA successfully inhibited hyperglycemia-induced TGase activation and microvascular leakage in the retinas of diabetic mice. C-peptide or ROS scavengers also inhibited TGase activation in diabetic mouse retinas. The role of TGase2 in VEGF-induced vascular leakage was further supported using diabetic TGase2(-/-) mice. Thus, our findings suggest that ROS-mediated activation of TGase2 plays a key role in VEGF-induced vascular leakage by stimulating stress fiber formation and VE-cadherin disruption.

Real-time kinetic method to monitor isopeptidase activity of transglutaminase 2 on protein substrate

Transglutaminase 2 (TG2) is a ubiquitously expressed multifunctional protein with Ca(2+)-dependent transamidase activity forming protease-resistant N(ε)-(γ-glutamyl) lysine crosslinks between proteins. It can also function as an isopeptidase cleaving the previously formed crosslinks. The biological significance of this activity has not been revealed yet, mainly because of the lack of a protein-based method for its characterization. Here we report the development of a novel kinetic method for measuring isopeptidase activity of human TG2 by monitoring decrease in the fluorescence polarization of a protein substrate previously formed by crosslinking fluorescently labeled glutamine donor FLpepT26 to S100A4 at a specific lysine residue. The developed method could be applied to test mutant enzymes and compounds that influence isopeptidase activity of TG2.

Transglutaminase 2 has opposing roles in the regulation of cellular functions as well as cell growth and death

Transglutaminase 2 (TG2) is primarily known as the most ubiquitously expressed member of the transglutaminase family with Ca²⁺-dependent protein crosslinking activity; however, this enzyme exhibits multiple additional functions through GTPase, cell adhesion, protein disulfide isomerase, kinase, and scaffold activities and is associated with cell growth, differentiation, and apoptosis. TG2 is found in the extracellular matrix, plasma membrane, cytosol, mitochondria, recycling endosomes, and nucleus, and its subcellular localization is an important determinant of its function. Depending upon the cell type and stimuli, TG2 changes its subcellular localization and biological activities, playing both anti- and pro-apoptotic roles. Increasing evidence indicates that the GTP-bound form of the enzyme (in its closed form) protects cells from apoptosis but that the transamidation activity of TG2 (in its open form) participates in both facilitating and inhibiting apoptosis. A difficulty in the study and understanding of this enigmatic protein is that opposing effects have been reported regarding its roles in the same physiological and/or pathological systems. These include neuroprotective or neurodegenerative effects, hepatic cell growth-promoting or hepatic cell death-inducing effects, exacerbating or having no effect on liver fibrosis, and anti- and pro-apoptotic effects on cancer cells. The reasons for these discrepancies have been ascribed to TG2's multifunctional activities, genetic variants, conformational changes induced by the immediate environment, and differences in the genetic background of the mice used in each of the experiments. In this article, we first report that TG2 has opposing roles like the protagonist in the novel Dr. Jekyll and Mr. Hyde, followed by a summary of the controversies reported, and finally discuss the possible reasons for these discrepancies.

The Genetic Basis of Baculum Size and Shape Variation in Mice

The rapid divergence of male genitalia is a preeminent evolutionary pattern. This rapid divergence is especially striking in the baculum, a bone that occurs in the penis of many mammalian species. Closely related species often display diverse baculum morphology where no other morphological differences can be discerned. While this fundamental pattern of evolution has been appreciated at the level of gross morphology, nearly nothing is known about the genetic basis of size and shape divergence. Quantifying the genetic basis of baculum size and shape variation has been difficult because these structures generally lack obvious landmarks, so comparing them in three dimensions is not straightforward. Here, we develop a novel morphometric approach to quantify size and shape variation from three-dimensional micro-CT scans taken from 369 bacula, representing 75 distinct strains of the BXD family of mice. We identify two quantitative trait loci (QTL) that explain ∼50% of the variance in baculum size, and a third QTL that explains more than 20% of the variance in shape. Together, our study demonstrates that baculum morphology may diverge relatively easily, with mutations at a few loci of large effect that independently modulate size and shape. Based on a combination of bioinformatic investigations and new data on RNA expression, we prioritized these QTL to 16 candidate genes, which have hypothesized roles in bone morphogenesis and may enable future genetic manipulation of baculum morphology.

Identification of transglutaminase 2 kinase substrates using a novel on-chip activity assay

Transglutaminase 2 (TG2) is an enzyme that plays a critical role in a wide variety of cellular processes through its multifunctional activities. TG2 kinase has emerged as an important regulator of apoptosis, as well as of chromatin structure and function. However, systematic investigation of TG2 kinase substrates is limited due to a lack of a suitable TG2 kinase activity assays. Thus, we developed a novel on-chip TG2 kinase activity assay for quantitative determination of TG2 kinase activity and for screening TG2 kinase substrate proteins in a high-throughput manner. Quantitative TG2 kinase activity was determined by selective detection of substrate protein phosphorylation on the surface of well-type amine arrays. The limit of detection (LOD) of this assay was 4.34μg/ml. We successfully applied this new activity assay to the kinetic analysis of 27 TG2-related proteins for TG2 kinase activity in a high-throughput manner and determined Michaelis-Menten constants (Km) of these proteins. We used the Km values and cellular locations of the TG2-related proteins to construct a substrate affinity map for TG2 kinase. Therefore, this on-chip TG2 kinase activity assay has a strong potential for the systematic investigation of substrate proteins and will be helpful for studying new physiological functions.

A Pharmacogenetic Discovery: Cystamine Protects Against Haloperidol-Induced Toxicity and Ischemic Brain Injury

Haloperidol is an effective antipsychotic agent, but it causes Parkinsonian-like extrapyramidal symptoms in the majority of treated subjects. To address this treatment-limiting toxicity, we analyzed a murine genetic model of haloperidol-induced toxicity (HIT). Analysis of a panel of consomic strains indicated that a genetic factor on chromosome 10 had a significant effect on susceptibility to HIT. We analyzed a whole-genome SNP database to identify allelic variants that were uniquely present on chromosome 10 in the strain that was previously shown to exhibit the highest level of susceptibility to HIT. This analysis implicated allelic variation within pantetheinase genes (Vnn1 and Vnn3), which we propose impaired the biosynthesis of cysteamine, could affect susceptibility to HIT. We demonstrate that administration of cystamine, which is rapidly metabolized to cysteamine, could completely prevent HIT in the murine model. Many of the haloperidol-induced gene expression changes in the striatum of the susceptible strain were reversed by cystamine coadministration. Since cystamine administration has previously been shown to have other neuroprotective actions, we investigated whether cystamine administration could have a broader neuroprotective effect. Cystamine administration caused a 23% reduction in infarct volume after experimentally induced cerebral ischemia. Characterization of this novel pharmacogenetic factor for HIT has identified a new approach for preventing the treatment-limiting toxicity of an antipsychotic agent, which could also be used to reduce the extent of brain damage after stroke.

The Extracellular Matrix in Bronchopulmonary Dysplasia: Target and Source

Bronchopulmonary dysplasia (BPD) is a common complication of preterm birth that contributes significantly to morbidity and mortality in neonatal intensive care units. BPD results from life-saving interventions, such as mechanical ventilation and oxygen supplementation used to manage preterm infants with acute respiratory failure, which may be complicated by pulmonary infection. The pathogenic pathways driving BPD are not well-delineated but include disturbances to the coordinated action of gene expression, cell-cell communication, physical forces, and cell interactions with the extracellular matrix (ECM), which together guide normal lung development. Efforts to further delineate these pathways have been assisted by the use of animal models of BPD, which rely on infection, injurious mechanical ventilation, or oxygen supplementation, where histopathological features of BPD can be mimicked. Notable among these are perturbations to ECM structures, namely, the organization of the elastin and collagen networks in the developing lung. Dysregulated collagen deposition and disturbed elastin fiber organization are pathological hallmarks of clinical and experimental BPD. Strides have been made in understanding the disturbances to ECM production in the developing lung, but much still remains to be discovered about how ECM maturation and turnover are dysregulated in aberrantly developing lungs. This review aims to inform the reader about the state-of-the-art concerning the ECM in BPD, to highlight the gaps in our knowledge and current controversies, and to suggest directions for future work in this exciting and complex area of lung development (patho)biology.

Elevated Transglutaminase Activity Triggers Angiotensin Receptor Activating Autoantibody Production and Pathophysiology of Preeclampsia

BACKGROUND

Preeclampsia (PE) is a life-threatening hypertensive disorder of pregnancy associated with autoantibodies, termed AT1-AA, that activate the AT1 angiotensin receptor. Although the pathogenic nature of these autoantibodies has been extensively studied, little is known about the molecular cause of their generation.

METHODS AND RESULTS

Here we show that tissue transglutaminase (TG2), an enzyme that conducts posttranslational modification of target proteins, directly modified the 7-amino acid (7-aa) epitope peptide that localizes to the second extracellular loop of the AT1 receptor. These findings led us to further discover that plasma transglutaminase activity was induced and contributed to the production of AT1-AA and disease development in an experimental model of PE induced by injection of LIGHT, a tumor necrosis factor superfamily member. Key features of PE were regenerated by adoptive transfer of purified IgG from LIGHT-injected pregnant mice and blocked by the 7-amino acid epitope peptide. Translating our mouse research to humans, we found that plasma transglutaminase activity was significantly elevated in PE patients and was positively correlated with AT1-AA levels and PE features.

CONCLUSIONS

Overall, we provide compelling mouse and human evidence that elevated transglutaminase underlies AT1-AA production in PE and highlight novel pathogenic biomarkers and innovative therapeutic possibilities for the disease.

Molecular mechanism by which acyclic retinoid induces nuclear localization of transglutaminase 2 in human hepatocellular carcinoma cells

Nuclear accumulation of transglutaminase 2 (TG2) is an important step in TG2-dependent cell death. However, the underlying molecular mechanisms for nuclear translocation of TG2 are still poorly understood. In this study, we demonstrated that acyclic retinoid (ACR) induced nuclear accumulation of TG2 in JHH-7 cells, a hepatocellular carcinoma (HCC) leading to their apoptosis. We further demonstrated molecular mechanism in nuclear-cytoplasmic trafficking of TG2 and an effect of ACR on it. We identified a novel 14-amino acid nuclear localization signal (NLS) (466)AEKEETGMAMRIRV(479) in the 'C' domain and a leucine-rich nuclear export signal (NES) (657)LHMGLHKL(664) in the 'D' domain that allowed TG2 to shuttle between the nuclear and cytosolic milieu. Increased nuclear import of GAPDH myc-HIS fused with the identified NLS was observed, confirming its nuclear import ability. Leptomycin B, an inhibitor of exportin-1 as well as point mutation of all leucine residues to glutamine residues in the NES of TG2 demolished its nuclear export. TG2 formed a trimeric complex with importin-α and importin-β independently from transamidase activity which strongly suggested the involvement of a NLS-based translocation of TG2 to the nucleus. ACR accelerated the formation of the trimeric complex and that may be at least in part responsible for enhanced nuclear localization of TG2 in HCC cells treated with ACR.

Assays for Posttranslational Modifications of Intermediate Filament Proteins

Intermediate filament (IF) proteins are known to be regulated by a number of posttranslational modifications (PTMs). Phosphorylation is the best-studied IF PTM, whereas ubiquitination, sumoylation, acetylation, glycosylation, ADP-ribosylation, farnesylation, and transamidation are less understood in functional terms but are known to regulate specific IFs under various contexts. The number and diversity of IF PTMs is certain to grow along with rapid advances in proteomic technologies. Therefore, the need for a greater understanding of the implications of PTMs to the structure, organization, and function of the IF cytoskeleton has become more apparent with the increased availability of data from global profiling studies of normal and diseased specimens. This chapter will provide information on established methods for the isolation and monitoring of IF PTMs along with the key reagents that are necessary to carry out these experiments.

P2X7 receptor activation regulates rapid unconventional export of transglutaminase-2

Transglutaminases (denoted TG or TGM) are externalized from cells via an unknown unconventional secretory pathway. Here, we show for the first time that purinergic signaling regulates active secretion of TG2 (also known as TGM2), an enzyme with a pivotal role in stabilizing extracellular matrices and modulating cell–matrix interactions in tissue repair. Extracellular ATP promotes TG2 secretion by macrophages, and this can be blocked by a selective antagonist against the purinergic receptor P2X7 (P2X7R, also known as P2RX7). Introduction of functional P2X7R into HEK293 cells is sufficient to confer rapid, regulated TG2 export. By employing pharmacological agents, TG2 release could be separated from P2X7R-mediated microvesicle shedding. Neither Ca²⁺ signaling alone nor membrane depolarization triggered TG2 secretion, which occurred only upon receptor membrane pore formation and without pannexin channel involvement. A gain-of-function mutation in P2X7R associated with autoimmune disease caused enhanced TG2 externalization from cells, and this correlated with increased pore activity. These results provide a mechanistic explanation for a link between active TG2 secretion and inflammatory responses, and aberrant enhanced TG2 activity in certain autoimmune conditions.

Summary: Purinergic signaling regulates unconventional secretion of transglutaminase-2 (TG2) and explains the link between aberrant protein modifications and inflammatory responses in TG2-dependent autoimmunity.

Distribution of transglutaminase family members in mouse whole body sections

Transglutaminases (TGs) comprise a protein family in which the members catalyze the formation of isopeptide bonds between glutamine and lysine residues in various proteins. Eight enzymes have been identified and designated as factor XIII (FXIII) and TG1-7. Expression studies of four major members, i.e., FXIII, TG1, TG2, and TG3, have been performed in a relatively large number of mammalian tissues in comparison with those on the other isozymes. The structural and biochemical characteristics of these individual isozymes and expression analyses of TG family in some tissue extracts have been reported, but there have been no simultaneous comparative analyses of both their mRNA and protein expression patterns in tissues distributions. Thus, we developed novel experimental systems for in situ hybridization using cryofilm attached to whole body sections of neonatal mice, thereby obtaining data regarding the tissue distributions of the major TG isozymes. In this study, we performed the first detailed comparative analysis of the mRNA and protein distribution studies of TG family members in a wide range of mouse tissues. These data will be helpful for elucidating the unknown physiological and pathological functions of TGs.

Decreased Mechanical Strength and Collagen Content in SPARC-Null Periodontal Ligament Is Reversed by Inhibition of Transglutaminase Activity

The periodontal ligament (PDL) is a critical tissue that provides a physical link between the mineralized outer layer of the tooth and the alveolar bone. The PDL is composed primarily of nonmineralized fibrillar collagens. Expression of secreted protein acidic and rich in cysteine (SPARC/osteonectin), a collagen-binding matricellular protein, has been shown to be essential for collagen homeostasis in PDL. In the absence of SPARC, PDL collagen fibers are smaller and less dense than fibers that constitute WT PDL. The aim of this study was to identify cellular mechanisms by which SPARC affected collagen fiber assembly and morphology in PDL. Cross-linking of fibrillar collagens is one parameter that is known to affect insoluble collagen incorporation and fiber morphology. Herein, the reduction in collagen fiber size and quantity in the absence of SPARC expression was shown to result in a PDL with reduced molar extraction force in comparison to that of WT mice (C57Bl/6J). Furthermore, an increase in transglutaminase activity was found in SPARC-null PDL by biochemical analyses that was supported by immunohistochemical results. Specifically, collagen I was identified as a substrate for transglutaminase in PDL and transglutaminase activity on collagen I was found to be greater in SPARC-null tissues in comparison to WT. Strikingly, inhibition of transglutaminase activity in SPARC-null PDL resulted in increases in both collagen fiber thickness and in collagen content, whereas transglutaminase inhibitors injected into WT mice resulted in increases in collagen fiber thickness only. Furthermore, PDL treated with transglutaminase inhibitors exhibited increases in molar extraction force in WT and in SPARC-null mice. Thus, SPARC is proposed to act as a critical regulator of transglutaminase activity on collagen I with implications for mechanical strength of tissues.

Structural and Functional Characterization of an Ancient Bacterial Transglutaminase Sheds Light on the Minimal Requirements for Protein Cross-Linking

Transglutaminases are best known for their ability to catalyze protein cross-linking reactions that impart chemical and physical resilience to cellular structures. Here, we report the crystal structure and characterization of Tgl, a transglutaminase from the bacterium Bacillus subtilis. Tgl is produced during sporulation and cross-links the surface of the highly resilient spore. Tgl-like proteins are found only in spore-forming bacteria of the Bacillus and Clostridia classes, indicating an ancient origin. Tgl is a single-domain protein, produced in active form, and the smallest transglutaminase characterized to date. We show that Tgl is structurally similar to bacterial cell wall endopeptidases and has an NlpC/P60 catalytic core, thought to represent the ancestral unit of the cysteine protease fold. We show that Tgl functions through a unique partially redundant catalytic dyad formed by Cys116 and Glu187 or Glu115. Strikingly, the catalytic Cys is insulated within a hydrophobic tunnel that traverses the molecule from side to side. The lack of similarity of Tgl to other transglutaminases together with its small size suggests that an NlpC/P60 catalytic core and insulation of the active site during catalysis may be essential requirements for protein cross-linking.

Transglutaminase 2--a novel inhibitor of adipogenesis

Differentiation of preadipocytes to lipid storing adipocytes involves extracellular signaling pathways, matrix remodeling and cytoskeletal changes. A number of factors have been implicated in maintaining the preadipocyte state and preventing their differentiation to adipocytes. We have previously reported that a multifunctional and protein crosslinking enzyme, transglutaminase 2 (TG2) is present in white adipose tissue. In this study, we have investigated TG2 function during adipocyte differentiation. We show that TG2 deficient mouse embryonic fibroblasts (Tgm2-/- MEFs) display increased and accelerated lipid accumulation due to increased expression of major adipogenic transcription factors, PPARγ and C/EBPα. Examination of Pref-1/Dlk1, an early negative regulator of adipogenesis, showed that the Pref-1/Dlk1 protein was completely absent in Tgm2-/- MEFs during early differentiation. Similarly, Tgm2-/- MEFs displayed defective canonical Wnt/β-catenin signaling with reduced β-catenin nuclear translocation. TG2 deficiency also resulted in reduced ROCK kinase activity, actin stress fiber formation and increased Akt phosphorylation in MEFs, but did not alter fibronectin matrix levels or solubility. TG2 protein levels were unaltered during adipogenic differentiation, and was found predominantly in the extracellular compartment of MEFs and mouse WAT. Addition of exogenous TG2 to Tgm2+/+ and Tgm2-/- MEFs significantly inhibited lipid accumulation, reduced expression of PPARγ and C/EBPα, promoted the nuclear accumulation of β-catenin, and recovered Pref-1/Dlk1 protein levels. Our study identifies TG2 as a novel negative regulator of adipogenesis.

Isopeptidase activity of human transglutaminase 2: disconnection from transamidation and characterization by kinetic parameters

Transglutaminase 2 (TG2) is a multifunctional protein with diverse catalytic activities and biological roles. Its best studied function is the Ca(2+)-dependent transamidase activity leading to formation of γ-glutamyl-ε-lysine isopeptide crosslinks between proteins and γ-glutamyl-amine derivatives. TG2 has a poorly studied isopeptidase activity cleaving these bonds. We have developed and characterised TG2 mutants which are significantly deficient in transamidase activity while have normal or increased isopeptidase activity (W332F) and vice versa (W278F). The W332F mutation led to significant changes of both the K m and the V max kinetic parameters of the isopeptidase reaction of TG2 while its calcium and GTP sensitivity was similar to the wild-type enzyme. The W278F mutation resulted in six times elevated amine incorporating transamidase activity demonstrating the regulatory significance of W278 and W332 in TG2 and that mutations can change opposed activities located at the same active site. The further application of our results in cellular systems may help to understand TG2-driven physiological and pathological processes better and lead to novel therapeutic approaches where an increased amount of crosslinked proteins correlates with the manifestation of degenerative disorders.

Cellular Factor XIIIA Transglutaminase Localizes in Caveolae and Regulates Caveolin-1 Phosphorylation, Homo-oligomerization and c-Src Signaling in Osteoblasts

Transglutaminases (TGs) are a family of widely distributed enzymes that catalyze protein crosslinking by forming a covalent isopeptide bond between the substrate proteins. We have shown that MC3T3-E1 osteoblasts express Factor XIII-A (FXIII-A), and that the extracellular crosslinking activity of FXIII-A is involved in regulating matrix secretion and deposition. In this study, we have investigated the localization and potential role of intracellular FXIII-A. Conventional immunofluorescence microscopy and TIRF microscopy analyses showed that FXIII-A co-localizes with caveolin-1 in specialized membrane structures, caveolae, in differentiating osteoblasts. The caveolae-disrupting agent methyl-β-cyclodextrin abolished FXIII-A staining and co-localization with caveolin-1 from the osteoblast plasma membrane. The presence of FXIII-A in caveolae was confirmed by preparing caveolae-enriched cellular fractions using sucrose density gradient ultracentrifugation followed by western blotting. Despite this association of FXIII-A with caveolae, there was no detectable transglutaminase activity in caveolae, as measured by monodansylcadaverine incorporation. TG inhibitor NC9--which can alter TG enzyme conformation--localized to caveolae and displaced FXIII-A from these structures when added to the osteoblast cultures. The decreased FXIII-A levels in caveolae after NC9 treatment increased c-Src activation, which resulted in caveolin-1 phosphorylation, homo-oligomerization and Akt phosphorylation, suggesting cellular FXIII-A has a role in regulating c-Src signaling in osteoblasts.

Regulation of endothelial cell inflammation and lung polymorphonuclear lymphocyte infiltration by transglutaminase 2

We addressed the role of transglutaminase 2 (TG2), a calcium-dependent enzyme that catalyzes cross-linking of proteins, in the mechanism of endothelial cell (EC) inflammation and lung polymorphonuclear lymphocyte (PMN) infiltration. Exposure of EC to thrombin, a procoagulant and proinflammatory mediator, resulted in activation of the transcription factor nuclear factor κB (NF-κB) and its target genes, vascular cell adhesion molecule 1, monocyte chemotactic protein 1, and interleukin 6. RNAi knockdown of TG2 inhibited these responses. Analysis of NF-κB activation pathway showed that TG2 knockdown was associated with inhibition of thrombin-induced DNA binding as well as serine phosphorylation of RelA/p65, a crucial event that controls transcriptional capacity of the DNA-bound RelA/p65. These results implicate an important role for TG2 in mediating EC inflammation by promoting DNA-binding and transcriptional activity of RelA/p65. Because thrombin is released in high amounts during sepsis, and its concentration is elevated in plasma and lavage fluids of patients with acute respiratory distress syndrome, we determined the in vivo relevance of TG2 in a mouse model of sepsis-induced lung PMN recruitment. A marked reduction in NF-κB activation, adhesion molecule expression, and lung PMN sequestration was observed in TG2 knockout mice compared with wild-type mice exposed to endotoxemia. Together, these results identify TG2 as an important mediator of EC inflammation and lung PMN sequestration associated with intravascular coagulation and sepsis.

Transglutaminase 4 as a prostate autoantigen in male subfertility

Autoimmune polyendocrine syndrome type 1 (APS1), a monogenic disorder caused by AIRE gene mutations, features multiple autoimmune disease components. Infertility is common in both males and females with APS1. Although female infertility can be explained by autoimmune ovarian failure, the mechanisms underlying male infertility have remained poorly understood. We performed a proteome-wide autoantibody screen in APS1 patient sera to assess the autoimmune response against the male reproductive organs. By screening human protein arrays with male and female patient sera and by selecting for gender-imbalanced autoantibody signals, we identified transglutaminase 4 (TGM4) as a male-specific autoantigen. Notably, TGM4 is a prostatic secretory molecule with critical role in male reproduction. TGM4 autoantibodies were detected in most of the adult male APS1 patients but were absent in all the young males. Consecutive serum samples further revealed that TGM4 autoantibodies first presented during pubertal age and subsequent to prostate maturation. We assessed the animal model for APS1, the Aire-deficient mouse, and found spontaneous development of TGM4 autoantibodies specifically in males. Aire-deficient mice failed to present TGM4 in the thymus, consistent with a defect in central tolerance for TGM4. In the mouse, we further link TGM4 immunity with a destructive prostatitis and compromised secretion of TGM4. Collectively, our findings in APS1 patients and Aire-deficient mice reveal prostate autoimmunity as a major manifestation of APS1 with potential role in male subfertility.

Novel insights into the function and dynamics of extracellular matrix in liver fibrosis

Emerging evidence suggests that altered components and posttranslational modifications of proteins in the extracellular matrix (ECM) may both initiate and drive disease progression. The ECM is a complex grid consisting of multiple proteins, most of which play a vital role in containing the essential information needed for maintenance of a sophisticated structure anchoring the cells and sustaining normal function of tissues. Therefore, the matrix itself may be considered as a paracrine/endocrine entity, with more complex functions than previously appreciated. The aims of this review are to 1) explore key structural and functional components of the ECM as exemplified by monogenetic disorders leading to severe pathologies, 2) discuss selected pathological posttranslational modifications of ECM proteins resulting in altered functional (signaling) properties from the original structural proteins, and 3) discuss how these findings support the novel concept that an increasing number of components of the ECM harbor signaling functions that can modulate fibrotic liver disease. The ECM entails functions in addition to anchoring cells and modulating their migratory behavior. Key ECM components and their posttranslational modifications often harbor multiple domains with different signaling potential, in particular when modified during inflammation or wound healing. This signaling by the ECM should be considered a paracrine/endocrine function, as it affects cell phenotype, function, fate, and finally tissue homeostasis. These properties should be exploited to establish novel biochemical markers and antifibrotic treatment strategies for liver fibrosis as well as other fibrotic diseases.

Celiac disease autoimmunity and hip fracture risk: findings from a prospective cohort study

The impact of celiac disease autoimmunity on bone health is unclear. We investigated the associations of seropositivity for tissue transglutaminase antibodies (tTGA) and endomysial antibodies (EMA) with incident hip fractures using data from a prospective cohort study, Mini-Finland Health Survey. Baseline serum samples, taken in 1978-80, were tested for tTGA and EMA. Incident hip fractures up to the year 2011 were ascertained from a national hospitalization register. Associations between seropositivity and hip fractures were modeled using Cox proportional hazards regression adjusted for age, sex, body mass index, vitamin D, gamma-glutamyl transferase, smoking, and self-rated health. Our analyses were based on 6919 men and women who had no record of celiac disease or hip fracture before the study baseline. A total of 382 individuals had a hip fracture during a median follow-up of 30 years. Compared with the tTGA-negative individuals (n = 6350), tTGA-positive participants (n = 569; with hip fracture, n = 51) had a higher risk of hip fractures (hazard ratio [HR] = 1.59, 95% confidence interval [CI] 1.17, 2.14). The findings were similar for another tTGA test (n 200; with hip fracture, n = 26; HR = 2.23, 95% CI 1.49, 3.34). We found no evidence for an association between EMA positivity and hip fracture risk (HR = 0.92, 95% CI 0.34, 2.47; n = 74; with hip fracture, n = 4). In our prospective population-based study of Finnish adults, seropositivity for tTGA was associated with an increased hip fracture risk.

Post-translational modifications of tubulin: pathways to functional diversity of microtubules

Tubulin and microtubules are subject to a remarkable number of post-translational modifications. Understanding the roles these modifications play in determining the functions and properties of microtubules has presented a major challenge that is only now being met. Many of these modifications are found concurrently, leading to considerable diversity in cellular microtubules, which varies with development, differentiation, cell compartment, and cell cycle. We now know that post-translational modifications of tubulin affect, not only the dynamics of the microtubules, but also their organization and interaction with other cellular components. Many early suggestions of how post-translational modifications affect microtubules have been replaced with new ideas and even new modifications as our understanding of cellular microtubule diversity comes into focus.

Discovery of potent and specific dihydroisoxazole inhibitors of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme that catalyzes the posttranslational modification of glutamine residues on protein or peptide substrates. A growing body of literature has implicated aberrantly regulated activity of TG2 in the pathogenesis of various human inflammatory, fibrotic, and other diseases. Taken together with the fact that TG2 knockout mice are developmentally and reproductively normal, there is growing interest in the potential use of TG2 inhibitors in the treatment of these conditions. Targeted-covalent inhibitors based on the weakly electrophilic 3-bromo-4,5-dihydroisoxazole (DHI) scaffold have been widely used to study TG2 biology and are well tolerated in vivo, but these compounds have only modest potency, and their selectivity toward other transglutaminase homologues is largely unknown. In the present work, we first profiled the selectivity of existing inhibitors against the most pertinent TG isoforms (TG1, TG3, and FXIIIa). Significant cross-reactivity of these small molecules with TG1 was observed. Structure–activity and −selectivity analyses led to the identification of modifications that improved potency and isoform selectivity. Preliminary pharmacokinetic analysis of the most promising analogues was also undertaken. Our new data provides a clear basis for the rational selection of dihydroisoxazole inhibitors as tools for in vivo biological investigation.