Validity of mouse models for the study of tissue transglutaminase in neurodegenerative diseases

Transglutaminase 6 Is Colocalized and Interacts with Mutant Huntingtin in Huntington Disease Rodent Animal Models

Mammalian transglutaminases (TGs) catalyze calcium-dependent irreversible posttranslational modifications of proteins and their enzymatic activities contribute to the pathogenesis of several human neurodegenerative diseases. Although different transglutaminases are found in many different tissues, the TG6 isoform is mostly expressed in the CNS. The present study was embarked on/undertaken to investigate expression, distribution and activity of transglutaminases in Huntington disease transgenic rodent models, with a focus on analyzing the involvement of TG6 in the age- and genotype-specific pathological features relating to disease progression in HD transgenic mice and a tgHD transgenic rat model using biochemical, histological and functional assays. Our results demonstrate the physical interaction between TG6 and (mutant) huntingtin by co-immunoprecipitation analysis and the contribution of its enzymatic activity for the total aggregate load in SH-SY5Y cells. In addition, we identify that TG6 expression and activity are especially abundant in the olfactory tubercle and piriform cortex, the regions displaying the highest amount of mHTT aggregates in transgenic rodent models of HD. Furthermore, mHTT aggregates were colocalized within TG6-positive cells. These findings point towards a role of TG6 in disease pathogenesis via mHTT aggregate formation.

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.

Complete transglutaminase 2 ablation results in reduced stroke volumes and astrocytes that exhibit increased survival in response to ischemia

Transglutaminase 2 (TG2) is a very multifunctional protein that is ubiquitously expressed in the body. It is a Ca(2+)-dependent transamidating enzyme, a GTPase, as well as a scaffolding protein. TG2 is the predominant form of transglutaminase expressed in the mammalian nervous system. Previously, it was shown that TG2 can affect both cell death and cell survival mechanisms depending on the cell type and the stressor. In the case of ischemic stress, TG2 was previously shown to play a protective role in the models used. For example in hTG2 transgenic mice, where TG2 is overexpressed only in neurons, middle cerebral artery ligation (MCAL) resulted in smaller infarct volumes compared to wild type mice. In this study TG2 knock out mice were used to determine how endogenous TG2 affected stroke volumes. Intriguingly, infarct volumes in TG2 knock out mice were significantly smaller compared to wild type mice. As expected, primary neurons isolated from TG2 knock out mice showed decreased viability in response to oxygen-glucose deprivation. However, primary astrocytes that were isolated from TG2 knock out mice were resistant to oxygen-glucose deprivation in situ. Both wild type and knock out neurons were protected against oxygen glucose deprivation when they were co-cultured with astrocytes from TG2 knockout mice. Therefore, the decreased stroke volumes observed in TG2 knock out mice after MCAL, can be correlated with the protective effects of TG2 knock out in astrocytes in response to oxygen glucose deprivation in situ. These findings suggest that neuron-astrocyte crosstalk plays a significant role in mediating ischemic cell death and that TG2 differentially impacts cell survival depending on cell context.

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 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.

Anti transglutaminase antibodies cause ataxia in mice

BACKGROUND

Celiac disease (CD) is an autoimmune gastrointestinal disorder characterized by the presence of anti-transglutaminase 2 (TG2) and anti-gliadin antibodies. Amongst the neurological dysfunctions associated with CD, ataxia represents the most common one.

METHODS

We analyzed by immunohistochemistry, the anti-neural reactivity of the serum from 20 CD patients. To determine the role of anti-TG2 antibodies in ataxia, two anti-TG2 single chain variable fragments (scFv), isolated from a phage-display IgA antibody library, were characterized by immunohistochemistry and ELISA, and injected in mice to study their effects on motor coordination. We found that 75% of the CD patient population without evidence of neurological involvement, has circulating anti-neural IgA and/or IgG antibodies. Two anti-TG2 scFvs, cloned from one CD patient, stained blood vessels but only one reacted with neurons. This anti-TG2 antibody showed cross reactivity with the transglutaminase isozymes TG3 and TG6. Intraventricular injection of the anti-TG2 or the anti-TG2/3/6 cross-reactive scFv provoked transient, equally intensive ataxia in mice.

CONCLUSION

The serum from CD patients contains anti-TG2, TG3 and TG6 antibodies that may potentially cause ataxia.

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.

Intracellular localization and conformational state of transglutaminase 2: implications for cell death

Transglutaminase 2 (TG2) is a multifunctional enzyme that has guanine nucleotide binding and GTP hydrolyzing activity in addition to its transamidating function. Studies show that TG2 is a player in mediating cell death processes. However, there is far from a consensus about the role of this enzyme in cell death processes as it appears to be dependent upon the cell type, stimuli, subcellular localization and conformational state of the enzyme. The purpose of this study was to dissect the role of TG2 in the cell death processes. To this end, we created and characterized 4 distinct point mutants of TG2, each of which differs from the wild type by its conformation or by lacking an important function. We also prepared these mutants as nuclear targeted proteins. By overexpressing mutant or wild type forms of TG2 in HEK 293 cells, we investigated the modulatory role of the protein in the cell death process in response to three stressors: thapsigargin, hyperosmotic stress and oxygen/glucose deprivation (OGD). All of the TG2 constructs, except the R580A mutant (which cannot bind guanine nucleotides and is therefore more prone to exhibit transamidating activity), either did not significantly affect the cell death processes or were protective. However in the case of the R580A mutant, cell death in response to high thapsigargin concentrations, was significantly increased. Intriguingly, nuclear localization of R580A-TG2 was sufficient to counteract the pro-death role of cytoplasmic R580A-TG2. In addition, nuclear localization of TG2 significantly facilitated its protective role against OGD. Our data support the hypothesis that the transamidation activity of TG2, which is mostly quiescent except in extreme stress conditions, is necessary for its pro-death role. In addition, nuclear localization of TG2 generally plays a key role in its protective function against cell death processes, either counteracting the detrimental effect or strengthening the protective role of the protein.

Transcription factor Sp1 dysregulation in Alzheimer's disease

Altered gene expression occurs in central nervous system disorders, including Alzheimer's disease (AD). Transcription factor Sp1 may be involved insofar as it can regulate the expression of several AD-related proteins, including amyloid precursor protein (APP) and tau. Sp1 could itself be regulated by inflammatory and other factors associated with AD, such as interleukin-1beta. We measured an almost threefold elevation in the number of mRNA molecules of this cytokine in the AD frontal cortex. Sp1 mRNA was found to be up-regulated in these AD brains (along with Sp1-regulated COX-2), and the Sp1 increase was also seen at the protein level by Western immunoblotting. To determine whether this would also occur in transgenic mice developing AD pathology, we examined the expression of Sp1 in the cortex and hippocampus and observed higher levels of Sp1 mRNA and protein. These results indicate that elements of regulatory pathways involving transcription factor Sp1 may be useful targets for therapeutic intervention to prevent or reverse AD.

Transglutaminase 2 protects against ischemic insult, interacts with HIF1beta, and attenuates HIF1 signaling

Transglutaminase 2 (TG2) is a multifunctional enzyme that has been implicated in the pathogenesis of neurodegenerative diseases, ischemia, and stroke. The mechanism by which TG2 modulates disease progression have not been elucidated. In this study we investigate the role of TG2 in the cellular response to ischemia and hypoxia. TG2 is up-regulated in neurons exposed to oxygen and glucose deprivation (OGD), and increased TG2 expression protects neurons against OGD-induced cell death independent of its transamidating activity. We identified hypoxia inducible factor 1beta (HIF1beta) as a TG2 binding partner. HIF1beta and HIF1alpha together form the heterodimeric transcription factor hypoxia inducible factor 1 (HIF1). TG2 and the transaminase-inactive mutant C277S-TG2 inhibited a HIF-dependent transcription reporter assay under hypoxic conditions without affecting nuclear protein levels for HIF1alpha or HIF1beta, their ability to form the HIF1 heterodimeric transcription factor, or HIF1 binding to its DNA response element. Interestingly, TG2 attenuates the up-regulation of the HIF-dependent proapoptotic gene Bnip3 in response to OGD but had no effect on the expression of VEGF, which has been linked to prosurvival processes. This study demonstrates for the first time that TG2 protects against OGD, interacts with HIF1beta, and attenuates the HIF1 hypoxic response pathway. These results indicate that TG2 may play an important role in protecting against the delayed neuronal cell death in ischemia and stroke.

More than two sides of a coin? How to detect the multiple activities of type 2 transglutaminase

Programmed cell death (PCD) by apoptosis has been widely characterized as a process in which the expression and protein activation of a gene must be regulated in a very precise way in order to achieve the elimination of the dying cell without disturbing the neighborhoods. One of the first genes observed to be induced during the onset of PCD is the one coding for type 2 transglutaminase (TG2). Since the late 1990s, the unveiling of different new properties and enzymatic activities suggested the involvement of TG2 in a variety of cellular processes other than PCD and rendered the study of this protein more and more complicated.

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.

The protective effects of cystamine in the R6/2 Huntington's disease mouse involve mechanisms other than the inhibition of tissue transglutaminase

Tissue transglutaminase (tTG) is a multifunctional enzyme that contributes to disease progression in mouse models of Huntington's disease (HD), an inherited neurodegenerative disease that shows an age-related onset. Moreover, administration of the transglutaminase inhibitor cystamine delays the onset of pathology in the R6/2 HD mouse model. However, the contribution of tTG inhibition towards the therapeutic effects of cystamine has not been determined, as this compound likely has multiple mechanisms of action in the R6/2 mouse. In this study, we found that administration of cystamine in drinking water delayed the age of onset for motor dysfunction and extended lifespan to a similar extent in R6/2 mice that had a normal genetic complement of tTG compared with R6/2 mice that did not express tTG. Since the magnitude of cystamine's therapeutic effects was not affected by the genetic deletion of tTG, these results suggest that the mechanism of action for cystamine in this HD mouse model involves targets other than tTG inhibition.

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

Tissue transglutaminase (tTG) post-translationally modifies proteins in a calcium-dependent manner by incorporation of polyamines, deamination or crosslinking. Moreover, tTG can also bind and hydrolyze GTP. tTG is the major transglutaminase in the mammalian nervous system, localizing predominantly in neurons. Although tTG has been clearly demonstrated to be elevated in neurodegenerative diseases and in response to acute CNS injury, its role in these pathogenic processes remains unclear. Transgenic mice that overexpress human tTG (htTG) primarily in CNS neurons were generated to explore the role of tTG in the nervous system and its contribution to neuropathological processes. tTG transgenic mice were phenotypically normal and were born with the expected Mendelian frequency. However, when challenged systemically with kainic acid, tTG transgenic mice, in comparison to wild-type (WT) mice, developed more extensive hippocampal neuronal damage. This was evidenced by a decreased number of healthy neurons, and increased terminal deoxynucleotidyl dUTP nick end labeling (TUNEL) labeling as an indicator of neuronal cell death in the kainic acid-treated transgenic mice. Moreover, the duration and severity of seizures developed by htTG transgenics in response to kainic acid administration were significantly more pronounced than those observed in WT mice. These data indicate for the first time that tTG may play an active role in excitatory amino acid-induced neuronal cell death, which has been postulated to be an important component of acute CNS injury and chronic CNS neurodegenerative conditions.

Cystamine treatment is neuroprotective in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult onset neurodegenerative disorder characterized by selective atrophy and cell loss within the striatum. There is currently no treatment that can prevent the striatal neuropathology. Transglutaminase (TG) activity is increased in HD patients, is associated with cell death, and has been suggested to contribute to striatal neuronal loss in HD. This work assesses the therapeutic potential of cystamine, an inhibitor of TG activity with additional potentially beneficial effects. Specifically, we examine the effect of cystamine on striatal neuronal loss in the YAC128 mouse model of HD. We demonstrate here for the first time that YAC128 mice show a forebrain-specific increase in TG activity compared with wild-type (WT) littermates which is decreased by oral delivery of cystamine. Treatment of symptomatic YAC128 mice with cystamine starting at 7 months prevented striatal neuronal loss. Cystamine treatment also ameliorated the striatal volume loss and striatal neuronal atrophy observed in these animals, but was unable to prevent motor dysfunction or the down-regulation of dopamine and cyclic adenosine monophsophate-regulated phosphoprotein (DARPP-32) expression in the striatum. While the exact mechanism responsible for the beneficial effects of cystamine in YAC128 mice is uncertain, our findings suggest that cystamine is neuroprotective and may be beneficial in the treatment of HD.

Tissue transglutaminase during mouse central nervous system development: lack of alternative RNA processing and implications for its role(s) in murine models of neurotrauma and neurodegeneration

Tissue transglutaminase (tTG) is a member of a multigene family principally involved in catalyzing the formation of protein cross-links. Unlike other members of the transglutaminase family, tTG is multifunctional since it also serves as a guanosine triphosphate (GTP) binding protein (Galpha(h)) and participates in cell adhesion. Different isoforms of tTG can be produced by proteolysis or alternative splicing. We find that tTG mRNA is expressed at low levels in the mouse CNS relative to other tissues, and at lower levels in the CNS of mouse in comparison to that of human or rat. tTG mRNA levels are higher in the heart compared to the CNS, for example, and much higher in the liver. Within the CNS, tTG message is lowest in the adult cerebellum and thalamus and highest in the frontal cortex and striatum. In the hippocampus, tTG expression is highest during embryonic development and falls off dramatically after 1 week of life. We did not find alternative splicing of the mouse tTG. At the protein level, the predominant isoform is approximately 62 kDa. In summary, tTG, an important factor in neuronal survival, is expressed at low levels in the mouse CNS and, unlike rat and human tTG, does not appear to be regulated by alternative splicing. These findings have implications for analyses of rodent tTG expression in human neurodegenerative and neurotrauma models where alternative processing may be an attractive pathogenetic mechanism. They further impact on drug discovery paradigms, where modulation of activity may have therapeutic value.

Transglutaminase activity is present in highly purified nonsynaptosomal mouse brain and liver mitochondria

Several active transglutaminase (TGase) isoforms are known to be present in human and rodent tissues, at least three of which, namely, TGase 1, TGase 2 (tissue transglutaminase), and TGase 3, are present in the brain. TGase activity is known to be present in the cytosolic, nuclear, and extracellular compartments of the brain. Here, we show that highly purified mouse brain nonsynaptosomal mitochondria and mouse liver mitochondria and mitoplast fractions derived from these preparations possess TGase activity. Western blotting and experiments with TGase 2 knock-out (KO) mice ruled out the possibility that most of the mitochondrial/mitoplast TGase activity is due to TGase 2, the TGase isoform responsible for the majority of the activity ([14C]putrescine-binding assay) in whole brain and liver homogenates. The identity of the mitochondrial/mitoplast TGase(s) is not yet known. Possibly, the activity may be due to one of the other TGase isoforms or perhaps to a protein that does not belong to the classical TGase family. This activity may play a role in regulation of mitochondrial function both in normal physiology and in disease. Its nature and regulation deserve further study.

The influence of psychotropic drugs on cerebral cell death: female neurovulnerability to antipsychotics

Tissue transglutaminase (tTG) is a marker for apoptosis, and its protein level is known to be increased in post-mortem Alzheimer's and Huntington's disease brains. tTG is increased in the cerebrospinal fluid of patients with Alzheimer's disease. However, the influence of psychotropic medication on acute cell death has not been studied so far in vivo, although some experiments performed in vitro suggest that antipsychotic drugs are neurotoxic. The protein level of tTG was examined in the cerebrospinal fluid obtained from 29 patients under neuroleptic medication in the last 24 h before lumbar puncture (eight patients diagnosed with Alzheimer's disease and 21 patients with other neurological diseases), and compared with those from 55 patients without antipsychotic medication (25 Alzheimer's patients and 30 others). In addition, the influence of several other psychotropic drugs on apoptosis was analysed. A significant influence (P<0.01) of antipsychotic drugs for both the Alzheimer's and the non-Alzheimer's group was found with respect to tTG protein levels in cerebrospinal fluid. By contrast to the male subgroups, the female groups showed a strong influence of neuroleptics on cerebral cell death. Surprisingly, atypical antipsychotics did not differ from typical neuroleptics in neurotoxicity. By contrast, no influence of antidepressants, cholinesterase-inhibitors, nootropics, tranquilizers and tramadol on cerebral cell death was found. The results suggest that typical and atypical antipsychotic drugs may induce cerebral cell death, especially in female patients. Subjects with Alzheimer's disease might be even more vulnerable to any antipsychotic. Therefore, subsequent research should aim to identify atypical neuroleptics without neurotoxicity. A limit on the use of first- and second-generation antipsychotics in elderly patients is proposed. Finally, the possible connection between the observed increased cerebral cell death and tardive dyskinesia, the most threatening side-effect in antipsychotic therapy, is discussed.

Tissue transglutaminase contributes to disease progression in the R6/2 Huntington's disease mouse model via aggregate-independent mechanisms

Huntington's disease (HD) is caused by an expansion of CAG repeats within the huntingtin gene and is characterized by intraneuronal mutant huntingtin protein aggregates. In order to determine the role of tissue transglutaminase (tTG) in HD aggregate formation and disease progression, we cross-bred the R6/2 HD mouse model with a tTG knockout mouse line. R6/2 mice that were tTG heterozygous knockouts (R6/2 : tTG+/-) and tTG homozygous knockouts (R6/2 : tTG-/-) showed a very similar increase in aggregate number within the striatum compared with R6/2 mice that were wild-type with respect to tTG (R6/2 : tTG+/+). Interestingly, a significant delay in the onset of motor dysfunction and death occurred in R6/2 : tTG-/- mice compared with both R6/2 : tTG+/+ and R6/2 : tTG+/- mice. As aggregate number was similarly increased in the striatum of both R6/2 : tTG+/- and R6/2 : tTG-/- mice, whereas only R6/2 : tTG-/- mice showed delayed disease progression, these data suggest that the contribution of tTG towards motor dysfunction and death in the R6/2 mouse is independent of its ability to negatively regulate aggregate formation. Moreover, the combined results from this study suggest that the formation of striatal huntingtin aggregates does not directly influence motor dysfunction or death in this HD mouse model.

Developmental regulation of tissue transglutaminase in the mouse forebrain

Tissue transglutaminase (tTG) is a multifunctional enzyme that catalyzes both transamidation and GTPase reactions. In cell culture models tTG-mediated transamidation positively regulates many processes that occur in vivo during the mammalian brain growth spurt (BGS), including neuronal differentiation, neurite outgrowth, synaptogenesis and cell death mechanisms. However, little is known about the levels of tTG expression and transglutaminase (TG) activity during mammalian brain development. In this study, C57BL/6 mouse forebrains were collected at embryonic day (E) 12, E14, E17, postnatal day (P) 0, P7 and P56 and analyzed for tTG expression and TG activity. RT-PCR analysis demonstrated that tTG mRNA content increases during mouse forebrain development, whereas immunoblot analysis demonstrated that tTG protein content decreases during this time. TG activity was low in prenatal mouse forebrain but increased fivefold to peak at P0, which corresponds with the beginning of the mouse BGS. Further analysis demonstrated that the lack of temporal correlation between tTG protein content and TG activity is the result of an endogenous inhibitor of tTG that is present in prenatal but not postnatal mouse forebrain. These results demonstrate for the first time that tTG enzymatic activity in the mammalian forebrain is developmentally regulated by post-translational mechanisms.