Tissue transglutaminase overexpression in the brain potentiates calcium-induced hippocampal damage

Regulation of specific abnormal calcium signals in the hippocampal CA1 and primary cortex M1 alleviates the progression of temporal lobe epilepsy

Temporal lobe epilepsy is a multifactorial neurological dysfunction syndrome that is refractory, resistant to antiepileptic drugs, and has a high recurrence rate. The pathogenesis of temporal lobe epilepsy is complex and is not fully understood. Intracellular calcium dynamics have been implicated in temporal lobe epilepsy. However, the effect of fluctuating calcium activity in CA1 pyramidal neurons on temporal lobe epilepsy is unknown, and no longitudinal studies have investigated calcium activity in pyramidal neurons in the hippocampal CA1 and primary motor cortex M1 of freely moving mice. In this study, we used a multi-channel fiber photometry system to continuously record calcium signals in CA1 and M1 during the temporal lobe epilepsy process. We found that calcium signals varied according to the grade of temporal lobe epilepsy episodes. In particular, cortical spreading depression, which has recently been frequently used to represent the continuously and substantially increased calcium signals, was found to correspond to complex and severe behavioral characteristics of temporal lobe epilepsy ranging from grade II to grade V. However, vigorous calcium oscillations and highly synchronized calcium signals in CA1 and M1 were strongly related to convulsive motor seizures. Chemogenetic inhibition of pyramidal neurons in CA1 significantly attenuated the amplitudes of the calcium signals corresponding to grade I episodes. In addition, the latency of cortical spreading depression was prolonged, and the above-mentioned abnormal calcium signals in CA1 and M1 were also significantly reduced. Intriguingly, it was possible to rescue the altered intracellular calcium dynamics. Via simultaneous analysis of calcium signals and epileptic behaviors, we found that the progression of temporal lobe epilepsy was alleviated when specific calcium signals were reduced, and that the end-point behaviors of temporal lobe epilepsy were improved. Our results indicate that the calcium dynamic between CA1 and M1 may reflect specific epileptic behaviors corresponding to different grades. Furthermore, the selective regulation of abnormal calcium signals in CA1 pyramidal neurons appears to effectively alleviate temporal lobe epilepsy, thereby providing a potential molecular mechanism for a new temporal lobe epilepsy diagnosis and treatment strategy.

Microglial transglutaminase 2 deficiency causes impaired synaptic remodelling and cognitive deficits in mice

Microglia are the primary source of transglutaminase 2 (TGM2) in the brain; however, the roles of microglial TGM2 in neural development and disease are still not well known. The aim of this study is to elucidate the role and mechanisms of microglial TGM2 in the brain. A mouse line with a specific knockout of Tgm2 in microglia was generated. Immunohistochemistry, Western blot and qRT-PCR assays were performed to evaluate the expression levels of TGM2, PSD-95 and CD68. Confocal imaging, immunofluorescence staining and behavioural analyses were conducted to identify phenotypes of microglial TGM2 deficiency. Finally, RNA sequencing, qRT-PCR and co-culture of neurons and microglia were used to explore the potential mechanisms. Deletion of microglial Tgm2 causes impaired synaptic pruning, reduced anxiety and increased cognitive deficits in mice. At the molecular level, the phagocytic genes, such as Cq1a, C1qb and Tim4, are significantly down-regulated in TGM2-deficient microglia. This study elucidates a novel role of microglial TGM2 in regulating synaptic remodelling and cognitive function, indicating that microglia Tgm2 is essential for proper neural development.

Transient Receptor Potential (TRP) Channels in Pain, Neuropsychiatric Disorders, and Epilepsy

Pharmacomodulation of membrane channels is an essential topic in the study of physiological conditions and disease status. Transient receptor potential (TRP) channels are one such family of nonselective cation channels that have an important influence. In mammals, TRP channels consist of seven subfamilies with a total of twenty-eight members. Evidence shows that TRP channels mediate cation transduction in neuronal signaling, but the full implication and potential therapeutic applications of this are not entirely clear. In this review, we aim to highlight several TRP channels which have been shown to mediate pain sensation, neuropsychiatric disorders, and epilepsy. Recent findings suggest that TRPM (melastatin), TRPV (vanilloid), and TRPC (canonical) are of particular relevance to these phenomena. The research reviewed in this paper validates these TRP channels as potential targets of future clinical treatment and offers patients hope for more effective care.

The Role and Mechanism of Transglutaminase 2 in Regulating Hippocampal Neurogenesis after Traumatic Brain Injury

Traumatic brain injury usually results in neuronal loss and cognitive deficits. Promoting endogenous neurogenesis has been considered as a viable treatment option to improve functional recovery after TBI. However, neural stem/progenitor cells (NSPCs) in neurogenic regions are often unable to migrate and differentiate into mature neurons at the injury site. Transglutaminase 2 (TGM2) has been identified as a crucial component of neurogenic niche, and significantly dysregulated after TBI. Therefore, we speculate that TGM2 may play an important role in neurogenesis after TBI, and strategies targeting TGM2 to promote endogenous neural regeneration may be applied in TBI therapy. Using a tamoxifen-induced Tgm2 conditional knockout mouse line and a mouse model of stab wound injury, we investigated the role and mechanism of TGM2 in regulating hippocampal neurogenesis after TBI. We found that Tgm2 was highly expressed in adult NSPCs and up-regulated after TBI. Conditional deletion of Tgm2 resulted in the impaired proliferation and differentiation of NSPCs, while Tgm2 overexpression enhanced the abilities of self-renewal, proliferation, differentiation, and migration of NSPCs after TBI. Importantly, injection of lentivirus overexpressing TGM2 significantly promoted hippocampal neurogenesis after TBI. Therefore, TGM2 is a key regulator of hippocampal neurogenesis and a pivotal therapeutic target for intervention following TBI.

Predicted molecules and signaling pathways for regulating seizures in the hippocampus in lithium-pilocarpine induced acute epileptic rats: A proteomics study

Seizures in rodent models that are induced by lithium-pilocarpine mimic human seizures in a highly isomorphic manner. The hippocampus is a brain region that generates and spreads seizures. In order to understand the early phases of seizure events occurring in the hippocampus, global protein expression levels in the hippocampus on day 1 and day 3 were analyzed in lithium-pilocarpine induced acute epileptic rat models using a tandem mass tag-based proteomic approach. Our results showed that differentially expressed proteins were likely to be enhanced rather than prohibited in modulating seizure activity on days 1 and 3 in lithium-pilocarpine induced seizure rats. The differentially regulated proteins differed on days 1 and 3 in the seizure rats, indicating that different molecules and pathways are involved in seizure events occurring from day 1 to day 3 following lithium-pilocarpine administration. In regard to subcellular distribution, the results suggest that post-seizure cellular function in the hippocampus is possibly regulated in a differential manner on seizure progression. Gene ontology annotation results showed that, on day 1 following lithium-pilocarpine administration, it is likely necessary to regulate macromolecular complex assembly, and cell death, while on day 3, it may be necessary to modulate protein metabolic process, cytoplasm, and protein binding. Protein metabolic process rather than macromolecular complex assembly and cell death were affected on day 3 following lithium-pilocarpine administration. The extracellular matrix, receptors, and the constitution of plasma membranes were altered most strongly in the development of seizure events. In a KEGG pathway enrichment cluster analysis, the signaling pathways identified were relevant to sustained angiogenesis and evading apoptosis, and complement and coagulation cascades. On day 3, pathways relevant to Huntington’s disease, and tumor necrosis factor signaling were most prevalent. These results suggest that seizure events occurring in day 1 modulate macromolecular complex assembly and cell death, and in day 3 modulate biological protein metabolic process. In summary, our study found limited evidence for ongoing seizure events in the hippocampus of lithium-pilocarpine induced animal models; nevertheless, evaluating the global differential expression of proteins and their impacts on bio-function may offer new perspectives for studying epileptogenesis in the future.

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

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

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.

Golgi-specific DHHC type zinc finger protein is decreased in neurons of intractable epilepsy patients and pentylenetetrazole-kindled rats

Golgi-specific DHHC type zinc finger protein (GODZ) is a member of the DHHC protein family, and its enzymatic activity is regulated by fibroblast growth factor or Src kinase-mediated tyrosine phosphorylation. In cultured neurons, GODZ affects the numbers of calcium ions channels, α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors, N-methy-D-aspartate receptors, and γ-aminobutyric acid A receptors on postsynaptic membrane by palmitoylation, thus modulating synaptic plasticity. As the change in synaptic plasticity plays a role in epilepsy, GODZ may play roles in epilepsy. However, the expression of GODZ has never been investigated in brain tissues in vivo, and its change during epilepsy is still unclear. In this study, the cellular distribution of GODZ in brain tissues of both patients and rats was determined using double-labeled immunofluorescence and the levels of GODZ protein and mRNA among intractable epilepsy patients, pentylenetetrazole (PTZ)-kindled rats, and controls were measured using immunohistochemistry, Western blot, and real-time quantitative polymerase chain reaction. GODZ expression was identified on cytomembranes and in the cytoplasm of neurons in the temporal neocortex of intractable epilepsy patients and in the hippocampus and the adjacent temporal cortex of PTZ-kindled rats, but not in astrocytes. Decreased GODZ protein and mRNA were identified in brain tissues of intractable epilepsy patients and PTZ-kindled rats compared with the controls. In conclusion, GODZ is expressed in neurons, but not astrocytes, and epilepsy may reduce the protein and mRNA levels of GODZ, indicating a possible role of GODZ in the pathogenesis or the pathophysiology of epilepsy.

Transglutaminase 2: Friend or foe? The discordant role in neurons and astrocytes

Members of the transglutaminase family catalyze the formation of isopeptide bonds between a polypeptide-bound glutamine and a low molecular weight amine (e.g., spermidine) or the ɛ-amino group of a polypeptide-bound lysine. Transglutaminase 2 (TG2), a prominent member of this family, is unique because in addition to being a transamidating enzyme, it exhibits numerous other activities. As a result, TG2 plays a role in many physiological processes, and its function is highly cell type specific and relies upon a number of factors, including conformation, cellular compartment location, and local concentrations of Ca²⁺ and guanine nucleotides. TG2 is the most abundant transglutaminase in the central nervous system (CNS) and plays a pivotal role in the CNS injury response. How TG2 affects the cell in response to an insult is strikingly different in astrocytes and neurons. In neurons, TG2 supports survival. Overexpression of TG2 in primary neurons protects against oxygen and glucose deprivation (OGD)-induced cell death and in vivo results in a reduction in infarct volume subsequent to a stroke. Knockdown of TG2 in primary neurons results in a loss of viability. In contrast, deletion of TG2 from astrocytes results in increased survival following OGD and improved ability to protect neurons from injury. Here, a brief overview of TG2 is provided, followed by a discussion of the role of TG2 in transcriptional regulation, cellular dynamics, and cell death. The differing roles TG2 plays in neurons and astrocytes are highlighted and compared to how TG2 functions in other cell types.

Transglutaminase 2 overexpression induces depressive-like behavior and impaired TrkB signaling in mice

Serotonin (5-hydroxytryptamine, 5-HT) and brain-derived neurotrophic factor (BDNF) are two signaling molecules that have important regulatory roles in the development and plasticity of neural circuits that are known to be altered in depression. However, the mechanism by which 5-HT regulates BDNF signaling is unknown. In the present study, we found that 5-HT treatment increases BDNF receptor, TrkB (tropomyosin related kinase B), levels in mouse primary cortical neurons via a Rac1 (RAS-related C3 botulinum toxin substrate 1)-dependent mechanism. Significant increases in the levels of type-2 transglutaminase (TG2, which is implicated in transamidation of 5-HT to Rac1) are observed in the mouse prefrontal cortex (PFC) following chronic exposure to stress. We also found that TG2 levels are increased in the post-mortem PFC of depressed suicide subjects relative to matched controls. Moreover, in mice, neuronal overexpression of TG2 resulted in the atrophy of neurons and reduced levels of TrkB in the PFC as well as a depressive-like phenotype. Overexpression of TG2 in mouse cortical neurons reduced TrkB levels as a result of impaired endocytosis of TrkB. TG2 inhibition by either a viral particle or pharmacological approach attenuated behavioral deficits caused by chronic unpredictable stress. Moreover, the overexpression of TrkB in the mouse PFC ameliorated the depressive-like phenotype of TG2-overexpressed mice. Taken together, these post-mortem and preclinical findings identify TG2 as a critical mediator of the altered TrkB expression and depressive-like behaviors associated with chronic exposure to stress and suggest that TG2 may represent a novel therapeutic target in depression.

Effect of Some Growth Factors on Tissue Transglutaminase Overexpression Induced by β-Amyloid in Olfactory Ensheathing Cells

Herein, we assessed in a particular glial cell type, called olfactory ensheathing cells (OECs), the effect of some growth factors (GFs) on tissue transglutaminase (TG2) overexpression induced by amyloid-beta (Aβ) with native full-length peptide 1-42 or by fragments, 25-35 or 35-25, as control. Previously, we demonstrated that TG2 overexpression induced by some stressors was down-regulated by GFs exposure in OECs. To monitor cell viability, an MTT test was used, while TG2 expression was examined using immunocytochemical and Western blot analysis. We also considered the involvement of the TG2-mediated apoptotic pathway. Vimentin expression was evaluated as well. Reactive oxygen species and reduced glutathione levels were utilized to test the oxidative intracellular status. Lactate dehydrogenase released into the medium, as a marker of necrotic cell death, was evaluated. We found that in OECs exposed to Aβ(1-42) or Aβ(25-35) for 24 h, TG2 expression increased, and we observed that the protein appeared prevalently localized in the cytosol. The pre-treatment with GFs, basic fibroblast growth factor (bFGF) or glial-derived neurotrophic factor (GDNF), down-regulated the TG2 level, which was prevalently limited to the nuclear compartment. Vimentin expression and caspase cleavage showed a significant enhancement in Aβ(1-42) and Aβ(25-35) exposed cells. The pre-treatment with bFGF or GDNF was able to restore the levels of the proteins to control values, and the intracellular oxidative status modified by the exposure to Aβ(1-42) or Aβ(25-35). Our data suggest that both bFGF or GDNF could be an innovative mechanism to contrast TG2 expression, which plays a key role in Alzheimer's disease.

INCREASED TISSUE TRANSGLUTAMINASE LEVELS ARE ASSOCIATED WITH INCREASED EPILEPTIFORM ACTIVITY IN ELECTROENCEPHALOGRAPHY AMONG PATIENTS WITH CELIAC DISEASE

BACKGROUND

Celiac disease is an autoimmune systemic disorder in genetically predisposed individuals precipitated by gluten ingestion.

OBJECTIVE

In this study, we aimed to determine asymptomatic spike-and-wave findings on electroencephalography in children with celiac disease.

METHODS

A total of 175 children with the diagnosis of celiac disease (study group) and 99 age- and sex-matched healthy children as controls (control group) were included in the study. In order to determine the effects of gluten free diet on laboratory and electroencephalography findings, the celiac group is further subdivided into two as newly-diagnosed and formerly-diagnosed patients. Medical histories of all children and laboratory findings were all recorded and neurologic statuses were evaluated. All patients underwent a sleep and awake electroencephalography.

RESULTS

Among 175 celiac disease patients included in the study, 43 were newly diagnosed while 132 were formerly-diagnosed patients. In electroencephalography evaluation of patients the epileptiform activity was determined in 4 (9.3%) of newly diagnosed and in 2 (1.5%) of formerly diagnosed patients; on the other hand the epileptiform activity was present in only 1 (1.0%) of control cases. There was a statistically significant difference between groups in regards to the presence of epileptiform activity in electroencephalography. Pearson correlation analysis revealed that epileptiform activity in both sleep and awake electroencephalography were positively correlated with tissue transglutaminase levels (P=0.014 and P=0.019, respectively).

CONCLUSION

We have determined an increased epileptiform activity frequency among newly-diagnosed celiac disease patients compared with formerly-diagnosed celiac disease patients and control cases. Moreover the tissue transglutaminase levels were also correlated with the presence of epileptiform activity in electroencephalography. Among newly diagnosed celiac disease patients, clinicians should be aware of this association and be alert about any neurological symptoms.

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.

Transglutaminase 2 and neuroinflammation

Neuroinflammatory processes seem to play a pivotal role in various chronic neurodegenerative diseases, characterized also by the pathogenetic accumulation of specific protein aggregates. Several of these proteins have been shown to be substrates of transglutaminases, calcium-dependent enzymes that catalyze protein crosslinking reactions. However, it has recently been demonstrated that transglutaminase 2 (TG2) may also be involved in molecular mechanisms underlying inflammation. In the central nervous system, astrocytes and microglia are the cell types mainly involved in the inflammatory process. This review is focused on the increases of TG2 protein expression and enzyme activity that occur in astroglial, microglial and monocyte cell models in response to inflammatory stimuli. The transcription factor NF-κB is considered the main regulator of inflammation, being activated by a variety of stimuli including calcium influx, oxidative stress and inflammatory cytokines. Under these conditions, the over-expression of TG2 results in the sustained activation of NF-κB. Several findings emphasize the possible role of the TG2/NF-κB activation pathway in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, multiple sclerosis and amyotrophic lateral sclerosis. Although further studies are needed to characterize the TG2/NF-κB cross-talk in monocytes/macrophages/microglia within the central nervous system, some results show that TG2 and NF-κB are co-localized in cell compartments. Together, evidence suggests that TG2 plays a role in neuroinflammation and contributes to the production of compounds that are potentially deleterious to neuronal cells.

Transglutaminase 2 exacerbates α-synuclein toxicity in mice and yeast

α-Synuclein is a key pathogenic protein that aggregates in hallmark lesions in Parkinson's disease and other α-synucleinopathies. Prior in vitro studies demonstrated that it is a substrate for cross-linking by transglutaminase 2 (TG2) into higher-order species. Here we investigated whether this increased aggregation occurs in vivo and whether TG2 exacerbates α-synuclein toxicity in Mus musculus and Saccharomyces cerevisiae. Compared with α-synuclein transgenic (Syn(Tg)) mice, animals double transgenic for human α-synuclein and TG2 (TG2(Tg)/Syn(Tg)) manifested greater high-molecular-weight insoluble species of α-synuclein in brain lysates and developed α-synuclein aggregates in the synaptic vesicle fraction. In addition, larger proteinase K-resistant aggregates developed, along with increased thioflavin-S-positive amyloid fibrils. This correlated with an exaggerated neuroinflammatory response, as seen with more astrocytes and microglia. Further neuronal damage was suggested by greater morphological disruption of nerve fibers and a trend toward decreased c-Fos immunoreactive neurons. Finally, the performance of TG2(Tg)/Syn(Tg) animals on motor behavioral tasks was worse relative to Syn(Tg) mice. Greater toxicity of α-synuclein was also demonstrated in yeast cells coexpressing TG2. Our findings demonstrate that TG2 promotes the aggregation of α-synuclein in vivo and that this is associated with aggravated toxicity of α-synuclein and its downstream neuropathologic consequences.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

Calcium signaling and epilepsy

Calcium signaling is involved in a multitude of physiological and pathophysiological mechanisms. Over the last decade, it has been increasingly recognized as an important factor in epileptogenesis, and it is becoming obvious that the excess synchronization of neurons that is characteristic for seizures can be linked to various calcium signaling pathways. These include immediate effects on membrane excitability by calcium influx through ion channels as well as delayed mechanisms that act through G-protein coupled pathways. Calcium signaling is able to cause hyperexcitability either by direct modulation of neuronal activity or indirectly through calcium-dependent gliotransmission. Furthermore, feedback mechanisms between mitochondrial calcium signaling and reactive oxygen species are able to cause neuronal cell death and seizures. Unravelling the complexity of calcium signaling in epileptogenesis is a daunting task, but it includes the promise to uncover formerly unknown targets for the development of new antiepileptic drugs.

Transglutaminase is a therapeutic target for oxidative stress, excitotoxicity and stroke: a new epigenetic kid on the CNS block

Transglutaminases (TGs) are multifunctional, calcium-dependent enzymes that have been recently implicated in stroke pathophysiology. Classically, these enzymes are thought to participate in cell injury and death in chronic neurodegenerative conditions via their ability to catalyze covalent, nondegradable crosslinks between proteins or to incorporate polyamines into protein substrates. Accumulating lines of inquiry indicate that specific TG isoforms can shuttle into the nucleus when they sense pathologic changes in calcium or oxidative stress, bind to chromatin and thereby transduce these changes into transcriptional repression of genes involved in metabolic or oxidant adaptation. Here, we review the evidence that supports principally a role for one isoform of this family, TG2, in cell injury and death associated with hemorrhagic or ischemic stroke. We also outline an evolving model in which TG2 is a critical mediator between pathologic signaling and epigenetic modifications that lead to gene repression. Accordingly, the salutary effects of TG inhibitors in stroke may derive from their ability to restore homeostasis by removing inappropriate deactivation of adaptive genetic programs by oxidative stress or extrasynaptic glutamate receptor signaling.

Transglutaminase 2 facilitates or ameliorates HIF signaling and ischemic cell death depending on its conformation and localization

Transglutaminase 2 (TG2) is a widely expressed and multifunctional protein that modulates cell death/survival processes. We have previously shown that TG2 binds to hypoxia inducible factor 1β (HIF1β) and decreases the upregulation of HIF responsive genes; however, the relationship between these observations was not investigated. In this study, we investigated whether endogenous TG2 is sufficient to suppress HIF activity and whether the interaction between TG2 and HIF1β is required for this suppression. shRNA-mediated silencing of TG2 significantly enhanced HIF activation in response to hypoxia. In addition, nuclear localization of TG2 is required for its suppressive effect on HIF activity, with TG2 being recruited to HIF responsive promoters in hypoxic conditions. These observations suggest that TG2 directly regulates hypoxic transcriptional machinery; however, its interaction with HIF1β was not required for this regulation. We also examined whether TG2's effect on cell death/survival processes in ischemia is due to its effects on HIF signaling. Our results indicate that TG2 mediated HIF suppression can be separated from TG2's effect on cell survival in hypoxic/hypoglycemic conditions. Lastly, here we show that nuclear TG2 in the closed conformation and non-nuclear TG2 in the open conformation have opposing effects on hypoxic/hypoglycemic cell death, which could explain previous controversial results. Overall, our results further clarify the role of TG2 in mediating the cellular response to ischemia and suggest that manipulating the conformation of TG2 might be of pharmacological interest as a therapeutic strategy for the treatment of ischemia-related pathologies.

Tissue transglutaminase overexpression does not modify the disease phenotype of the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a devastating autosomal-dominant neurodegenerative disorder initiated by an abnormally expanded polyglutamine in the huntingtin protein. Determining the contribution of specific factors to the pathogenesis of HD should provide rational targets for therapeutic intervention. One suggested contributor is the type 2 transglutaminase (TG2), a multifunctional calcium dependent enzyme. A role for TG2 in HD has been suggested because a polypeptide-bound glutamine is a rate-limiting factor for a TG2-catalyzed reaction, and TG2 can cross-link mutant huntingtin in vitro. Further, TG2 is up regulated in brain areas affected in HD. The objective of this study was to further examine the contribution of TG2 as a potential modifier of HD pathogenesis and its validity as a therapeutic target in HD. In particular our goal was to determine whether an increase in TG2 level, as documented in human HD brains, modulates the well-characterized phenotype of the R6/2 HD mouse model. To accomplish this objective a genetic cross was performed between R6/2 mice and an established transgenic mouse line that constitutively expresses human TG2 (hTG2) under control of the prion promoter. Constitutive expression of hTG2 did not affect the onset and progression of the behavioral and neuropathological HD phenotype of R6/2 mice. We found no alterations in body weight changes, rotarod performances, grip strength, overall activity, and no significant effect on the neuropathological features of R6/2 mice. Overall the results of this study suggest that an increase in hTG2 expression does not significantly modify the pathology of HD.

Transglutaminase 2: biology, relevance to neurodegenerative diseases and therapeutic implications

Neurodegenerative disorders are characterized by progressive neuronal loss and the aggregation of disease-specific pathogenic proteins in hallmark neuropathologic lesions. Many of these proteins, including amyloid Αβ, tau, α-synuclein and huntingtin, are cross-linked by the enzymatic activity of transglutaminase 2 (TG2). Additionally, the expression and activity of TG2 is increased in affected brain regions in these disorders. These observations along with experimental evidence in cellular and mouse models suggest that TG2 can contribute to the abnormal aggregation of disease causing proteins and consequently to neuronal damage. This accumulating evidence has provided the impetus to develop inhibitors of TG2 as possible neuroprotective agents. However, TG2 has other enzymatic activities in addition to its cross-linking function and can modulate multiple cellular processes including apoptosis, autophagy, energy production, synaptic function, signal transduction and transcription regulation. These diverse properties must be taken into consideration in designing TG2 inhibitors. In this review, we discuss the biochemistry of TG2, its various physiologic functions and our current understanding about its role in degenerative diseases of the brain. We also describe the different approaches to designing TG2 inhibitors that could be developed as potential disease-modifying therapies.

Transglutaminase 2: a molecular Swiss army knife

Transglutaminase 2 (TG2) is the most widely distributed member of the transglutaminase family with almost all cell types in the body expressing TG2 to varying extents. In addition to being widely expressed, TG2 is an extremely versatile protein exhibiting transamidating, protein disulphide isomerase and guanine and adenine nucleotide binding and hydrolyzing activities. TG2 can also act as a protein scaffold or linker. This unique protein also undergoes extreme conformational changes and exhibits localization diversity. Being mainly a cytosolic protein; it is also found in the nucleus, associated with the cell membrane (inner and outer side) and with the mitochondria, and also in the extracellular matrix. These different activities, conformations and localization need to be carefully considered while assessing the role of TG2 in physiological and pathological processes. For example, it is becoming evident that the role of TG2 in cell death processes is dependent upon the cell type, stimuli, subcellular localization and conformational state of the protein. In this review we discuss in depth the conformational and functional diversity of TG2 in the context of its role in numerous cellular processes. In particular, we have highlighted how differential localization, conformation and activities of TG2 may distinctly mediate cell death processes.

Transglutaminase 6: a protein associated with central nervous system development and motor function

Transglutaminases (TG) form a family of enzymes that catalyse various post-translational modifications of glutamine residues in proteins and peptides including intra- and intermolecular isopeptide bond formation, esterification and deamidation. We have characterized a novel member of the mammalian TG family, TG6, which is expressed in a human carcinoma cell line with neuronal characteristics and in mouse brain. Besides full-length protein, alternative splicing results in a short variant lacking the second β-barrel domain in man and a variant with truncated β-sandwich domain in mouse. Biochemical data show that TG6 is allosterically regulated by Ca²⁺ and guanine nucleotides. Molecular modelling indicates that TG6 could have Ca²⁺ and GDP-binding sites related to those of TG3 and TG2, respectively. Localization of mRNA and protein in the mouse identified abundant expression of TG6 in the central nervous system. Analysis of its temporal and spatial pattern of induction in mouse development indicates an association with neurogenesis. Neuronal expression of TG6 was confirmed by double-labelling of mouse forebrain cells with cell type-specific markers. Induction of differentiation in mouse Neuro 2a cells with NGF or dibutyryl cAMP is associated with an upregulation of TG6 expression. Familial ataxia has recently been linked to mutations in the TGM6 gene. Autoantibodies to TG6 were identified in immune-mediated ataxia in patients with gluten sensitivity. These findings suggest a critical role for TG6 in cortical and cerebellar neurons.

Transglutaminase 2 interaction with small heat shock proteins mediate cell survival upon excitotoxic stress

Transglutaminase 2 has been postulated to be involved in the pathogenesis of central nervous system neurodegenerative disorders. However, its role in neuronal cell death remains to be elucidated. Excitotoxicity is a common event underlying neurodegeneration. We aimed to evaluate the protein targets for transglutaminase 2 in cell response to NMDA-induced excitotoxic stress, using SH-SY5Y neuroblastoma cells which express high tranglutaminase 2 levels upon retinoic acid-driven differentiation toward neurons. NMDA-evoked calcium increase led to transglutaminase 2 activation that mediated cell survival, as at first suggested by the exacerbation of NMDA toxicity in the presence of R283, a synthetic competitive inhibitor of transglutaminase active site. Assays of R283-mediated transglutaminase inhibition showed the involvement of enzyme activity in NMDA-induced reduction in protein basal levels of pro-apoptotic caspase-3 and the stress protein Hsp20. However, this occurred in a way different from protein cross-linking, given that macromolecular assemblies were not observed in our experimental conditions for both proteins. Co-immunoprecipitation experiments provided evidence for the interaction, in basal conditions, between transglutaminase 2 and Hsp20, as well as between Hsp20 and Hsp27, a major anti-apoptotic protein promoting caspase-3 inactivation and degradation. NMDA treatment disrupted both these interactions that were restored upon transglutaminase 2 inhibition with R283. These results suggest that transglutaminase 2 might be protective against NMDA-evoked excitotoxic insult in neuronal-like SH-SY5Y cells in a way, independent from transamidation that likely involves its interaction with the complex Hsp20/Hsp27 playing a pro-survival role.

Red blood cell lysis and brain tissue-type transglutaminase upregulation in a hippocampal model of intracerebral hemorrhage

Red blood cell (RBC) lysis and iron release contribute to intracerebral hemorrhage (ICH)-induced brain injury. Tissue-type transglutaminase (tTG), which has a role in neurodegeneration, is upregulated after ICH. The current study investigated the effect of RBC lysis and iron release on brain tTG levels and neuronal death in a rat model of ICH. This study had three parts: (1) Male Sprague-Dawley rats received an intrahippocampal injection of 10 μL of either packed RBCs or lysed RBCs; (2) rats had a 10 μL injection of either saline, hemoglobin or FeCl2; (3) rats received a 10 μL injection of hemoglobin and were treated with an iron chelator, deferoxamine or vehicle. All rats were killed 24 h later, and the brains were sectioned for tTG and Fluoro-Jade C staining. Lysed but not packed RBCs caused marked tTG upregulation (p<0.05) and neuronal death (p<0.05) in the ipsilateral hippocampus CA-1 region. Both hemoglobin and iron mimicked the effects of lysed RBCs, resulting in tTG expression and neuronal death (p<0.05). Hemoglobin-induced tTG upreglution and neuronal death were reduced by deferoxamine (p<0.05). These results indicate that RBC lysis and iron toxicity contribute to neurodegeneration after ICH.

Transglutaminase 2 protects against ischemic stroke

Transglutaminase 2 (TG2) is a multifunctional protein that modulates cell survival and death pathways. It is upregulated in numerous ischemic models, and protects primary neurons from oxygen and glucose deprivation. TG2 binds to the hypoxia inducible factor (HIF) 1beta and decreases the upregulation of hypoxic-induced proapoptotic genes. To investigate the role of TG2 in ischemic stroke in vivo, we used the murine, permanent middle cerebral artery (MCA) ligation model. TG2 mRNA levels are increased after MCA ligations, and transgenic mice that express human TG2 in neurons had significantly smaller infarct volumes than wild type littermates. Further, TG2 translocates into the nucleus within 2h post ligation. Nuclear-localized TG2 is also apparent in human stroke cases. TG2 suppressed the upregulation of the HIF-induced, proapoptotic gene, Noxa. The findings of this study indicate that TG2 plays a role in attenuating ischemic-induced cell death possibly by modulating hypoxic-induced transcriptional processes.

A metallothionein mimetic peptide protects neurons against kainic acid-induced excitotoxicity

Metallothioneins I and II (MTI/II) are metal-binding proteins overexpressed in response to brain injury. Recently, we have designed a peptide, termed EmtinB, which is modeled after the beta-domain of MT-II and mimics the biological effects of MTI/II in vitro. Here, we demonstrate the neuroprotective effect of EmtinB in the in vitro and in vivo models of kainic acid (KA)-induced neurotoxicity. We show that EmtinB passes the blood-brain barrier and is detectable in plasma for up to 24 hr. Treatment with EmtinB significantly attenuates seizures in C57BL/6J mice exposed to moderate (20 mg/kg) and high (30 mg/kg) KA doses and tends to decrease mortality induced by the high KA dose. Histopathological evaluation of hippocampal (CA3 and CA1) and cortical areas of mice treated with 20 mg/kg KA shows that EmtinB treatment reduces KA-induced neurodegeneration in the CA1 region. These findings establish EmtinB as a promising target for therapeutic development.

TG2 protects neuroblastoma cells against DNA-damage-induced stress, suppresses p53 activation

Tissue transglutaminase (TG2) is a multifunctional member of the transglutaminase (TGase) family (E.C.2.3.2.13), which catalyzes in a calcium-dependent reaction the formation of covalent bonds between the gamma-carboxamide groups of peptide-bound glutamine residues and various primary amines. Here, we investigated the role of TG2 in a response of the neuroblastoma SH-SY5Y cells to topoisomerase II inhibitor etoposide, known to trigger DNA-damage cell response. We found an early and transient (approximately 2 h) increase of the TG2 protein in SH-SY5Y cells treated with etoposide, along with the increase of phosphorylated and total levels of the p53 protein. Next, we showed that SH-SY5Y cells, which overexpress wild-type TG2 were significantly protected against etoposide-induced cell death. The TG2 protective effect was associated only with the transamidation active form of TG2, because overexpression the wild-type TG2, but not its transamidation inactive C277S form, resulted in a pronounced suppression of caspase-3 activity as well as p53 phosphorylation during the etoposide-induced stress. In addition, exacerbation of cell death with a significant increase in caspase-3 and p53 activation was observed in SH/anti-TG2 cells, in which expression of the endogenous TG2 protein has been greatly reduced by the antisense cDNA construct. Though the cell signaling and molecular mechanisms of the TG2-driven suppression of the cell death machinery remain to be investigated, our findings strongly suggest that TG2 plays an active role in the response of neuroblastoma cells to DNA-damage-induced stress by exerting a strong protective effect, likely by the suppression of p53 activation and p53-driven cell signaling events.

Transglutaminase participates in the blockade of neurotransmitter release by tetanus toxin: evidence for a novel biological function

Inhibition of neuroexocytosis by tetanus neurotoxin (TeNT) involves VAMP-2/synaptobrevin-2 cleavage. However, deletion of the TeNT activity does not completely abolish its inhibitory action. TeNT is a potent activator of the cross-linking enzyme transglutaminase 2 (TGase 2) in vitro. The role of the latter mechanism in TeNT poisoning was investigated in isolated nerve terminals and intact neurons. TeNT-induced inhibition of glutamate release from rat cortical synaptosomes was associated with a simultaneous activation of neuronal transglutaminase (TGase) activity. The TeNT-induced blockade of neuroexocytosis was strongly attenuated by pretreatment of either live Aplysia neurons or isolated nerve terminals with specific TGase inhibitors or neutralizing antibodies. The same treatments completely abolished the residual blockade of neuroexocytosis of a non-proteolytic mutant of TeNT light chain. Electrophysiological studies indicated that TGase activation occurs at an early step of TeNT poisoning and contributes to the inhibition of transmitter release. Bioinformatics and biochemical analyses identified synapsin I and SNAP-25 as potential presynaptic TGase substrates in isolated nerve terminals, which are potentially involved in the inhibitory action of TeNT. The results suggest that neuronal TGase activity plays an important role in the regulation of neuroexocytosis and is one of the intracellular targets of TeNT in neurons.

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

The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

Myenteric neuronal loss in rats with experimental colitis: role of tissue transglutaminase-induced apoptosis

BACKGROUND AND AIMS

Transglutaminases are tissue enzymes involved in different neuronal processes including maintenance and signalling. However, their up-regulation elicited by a variety of noxae contributes to neurodegeneration. This study tested the hypothesis that experimental inflammation evoked transglutaminase up-regulation in myenteric neurons and that this event had an impact on neuronal survival.

METHODS

Rats with or without trinitro-benzene-sulphonic acid-induced colitis were used. One week after colitis induction, longitudinal muscle-myenteric plexus preparations were obtained from left colon to assess tissue-transglutaminase activity, protein and mRNA expression. Double labelling immunofluorescence using antibodies to neuron-specific enolase and transglutaminase was performed to identify myenteric neurons expressing transglutaminase. Additional sets of experiments evaluated the involvement of transglutaminase in the apoptotic process of cultured myenteric neurons.

RESULTS

Compared to controls, rats with colitis showed several tranglutaminase/neuron-specific enolase positive myenteric neurons. Western blot analysis and RT-PCR confirmed that in rats with colitis, the increased neuronal transglutaminase-immunoreactivity was associated with an increased enzyme expression. Similarly, transglutaminase activity was significantly higher than in controls (1100+/-280 m U/g vs. 725+/-119 m U/g, p<0.05). In cultured myenteric neurons incubation with the specific transglutaminase inducer, retinoic acid, significantly increased neuronal apoptosis, whereas the presence of cystamine significantly reduced the number of apoptotic neurons.

CONCLUSIONS

Experimental colitis evoked transglutaminase up-regulation and increased activity in myenteric neurons. This mechanism enhances neuronal susceptibility to apoptosis and could contribute to neuropathic changes during gut inflammation.

Tissue-type transglutaminase and the effects of cystamine on intracerebral hemorrhage-induced brain edema and neurological deficits

Neurodegeneration occurs after intracerebral hemorrhage (ICH) and tissue-type transglutaminase (tTG) has a role in neurodegenerative disorders. The present study investigated tTG expression after ICH and the effects of a tTG inhibitor, cystamine, on ICH-induced brain edema and neurological deficits. This study has two parts. In the first, male Sprague-Dawley rats received an intracaudate injection of 100 microL autologous whole blood or a needle insertion (sham). Rats were killed 3 days later and the brains used for immunohistochemistry, Western blots and real-time quantitative polymerase chain reaction. In the second set, ICH rats were treated intraperitoneally with either a tTG inhibitor, cystamine, or vehicle. Rats underwent behavioral testing and were killed at day-3 for measurement of brain swelling. tTG positive cells were found in the ipsilateral basal ganglia after ICH and most of those cells were neuron-like. Western blot analysis showed a 3-fold increase in tTG in the ipsilateral basal ganglia (p<0.01 vs. sham) after ICH. tTG mRNA levels were also significantly higher (8.5-fold increase vs. sham). Cystamine treatment attenuated ICH-induced brain swelling (day 3: 14.4+/-3.2 vs. 21.4+/-4.0% in vehicle-treated rats, p<0.01), neuronal death and improved functional outcome (forelimb placing score: 47+/-23 vs. 17+/-16% in vehicle-treated rats, p<0.05). ICH induces perihematomal tTG upregulation and cystamine, a tTG inhibitor, reduces ICH-induced brain swelling and neurological deficits.

Type 2 transglutaminase differentially modulates striatal cell death in the presence of wild type or mutant huntingtin

Huntington's disease (HD), which is caused by an expanded polyglutamine tract in huntingtin (htt), is characterized by extensive loss of striatal neurons. The dysregulation of type 2 transglutaminase (TG2) has been proposed to contribute to the pathogenesis in HD as TG2 is up-regulated in HD brain and knocking out TG2 in mouse models of HD ameliorates the disease process. To understand the role of TG2 in the pathogenesis of HD, immortalized striatal cells established from mice in which mutant htt with a polyglutamine stretch of 111 Gln had been knocked-in and wild type (WT) littermates, were stably transfected with human TG2 in a tetracycline inducible vector. Overexpression of TG2 in the WT striatal cells resulted in significantly greater cell death under basal conditions as well as in response to thapsigargin treatment, which causes increased intracellular calcium concentrations. Furthermore, in WT striatal cells TG2 overexpression potentiated mitochondrial membrane depolarization, intracellular reactive oxygen species production, and apoptotic cell death in response to thapsigargin. In contrast, in mutant striatal cells, TG2 overexpression did not increase cell death, nor did it potentiate thapsigargin-induced mitochondrial membrane depolarization or intracellular reactive oxygen species production. Instead, TG2 overexpression in mutant striatal cells attenuated the thapsigargin-activated apoptosis. When in situ transglutaminase activity was quantitatively analyzed in these cell lines, we found that in response to thapsigargin treatment TG2 was activated in WT, but not mutant striatal cells. These data suggest that mutant htt alters the activation of TG2 in response to certain stimuli and therefore differentially modulates how TG2 contributes to cell death processes.

Tissue transglutaminase and the stress response

The expression of the protein crosslinking enzyme tissue transglutaminase (TG2, tTG), the ubiquitous member of transglutaminase family, can be regulated by multiple factors. Although it has been suggested that TG2 can be involved in apoptotic cell death, high levels of enzyme have also been associated with cell survival in response to different stimuli. Furthermore, evidence indicates that increases in TG2 production cause enzyme translocation to cell membrane. Cell stress can also lead to TG2 accumulation on the cell surface and in the extracellular matrix resulting in changes in cell-matrix interactions.Here, we discuss the underlying mechanisms of TG2 up-regulation induced by various stimuli including glutamate exposure, calcium influx, oxidative stress, UV, and inflammatory cytokines. These findings agree with a postulated role for transglutaminases in molecular mechanisms involved in several diseases suggesting that cross-linking reactions could be a relevant part of the biochemical changes observed in pathological conditions.