Tissue transglutaminase does not contribute to the formation of mutant huntingtin aggregates

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

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

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.

Inhibition of transglutaminase exacerbates polyglutamine-induced neurotoxicity by increasing the aggregation of mutant ataxin-3 in an SCA3 Drosophila model

Transglutaminases (TGs) comprise a family of Ca(2+)-dependent enzymes that catalyze protein cross-linking, which include nine family members in humans but only a single homolog in Drosophila with three conserved domains. Drosophila Tg plays important roles in cuticle morphogenesis, hemolymph clotting, and innate immunity. Mammalian tissue TG (TG2) is involved in polyglutamine diseases (polyQ diseases), and TG6 has been identified as a causative gene of a novel spinocerebellar ataxia, SCA35. Using a well-established SCA3 fly model, we found that RNA interference-mediated suppression of Tg aggravated polyQ-induced neurodegenerative phenotypes. The administration of cystamine, a known effective Tg inhibitor, enhanced ommatidial degeneration in SCA3 flies. We also demonstrated that the aggregates of pathogenic ataxin-3 increased greatly, when the Tg activity was repressed. These findings indicate that Tg is crucial for polyQ-induced neurotoxicity because Tg ablation resulted in more severe neurodegeneration due to the elevated accumulation of insoluble ataxin-3 complexes in the SCA3 Drosophila model.

Aberrant calcium signaling by transglutaminase-mediated posttranslational modification of inositol 1,4,5-trisphosphate receptors

The inositol 1,4,5-trisphosphate receptor (IP3R) in the endoplasmic reticulum mediates calcium signaling that impinges on intracellular processes. IP3Rs are allosteric proteins comprising four subunits that form an ion channel activated by binding of IP3 at a distance. Defective allostery in IP3R is considered crucial to cellular dysfunction, but the specific mechanism remains unknown. Here we demonstrate that a pleiotropic enzyme transglutaminase type 2 targets the allosteric coupling domain of IP3R type 1 (IP3R1) and negatively regulates IP3R1-mediated calcium signaling and autophagy by locking the subunit configurations. The control point of this regulation is the covalent posttranslational modification of the Gln2746 residue that transglutaminase type 2 tethers to the adjacent subunit. Modification of Gln2746 and IP3R1 function was observed in Huntington disease models, suggesting a pathological role of this modification in the neurodegenerative disease. Our study reveals that cellular signaling is regulated by a new mode of posttranslational modification that chronically and enzymatically blocks allosteric changes in the ligand-gated channels that relate to disease states.

Genetic deletion of transglutaminase 2 does not rescue the phenotypic deficits observed in R6/2 and zQ175 mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder caused by expansion of CAG repeats in the huntingtin gene. Tissue transglutaminase 2 (TG2), a multi-functional enzyme, was found to be increased both in HD patients and in mouse models of the disease. Furthermore, beneficial effects have been reported from the genetic ablation of TG2 in R6/2 and R6/1 mouse lines. To further evaluate the validity of this target for the treatment of HD, we examined the effects of TG2 deletion in two genetic mouse models of HD: R6/2 CAG 240 and zQ175 knock in (KI). Contrary to previous reports, under rigorous experimental conditions we found that TG2 ablation had no effect on either motor or cognitive deficits, or on the weight loss. In addition, under optimal husbandry conditions, TG2 ablation did not extend R6/2 lifespan. Moreover, TG2 deletion did not change the huntingtin aggregate load in cortex or striatum and did not decrease the brain atrophy observed in either mouse line. Finally, no amelioration of the dysregulation of striatal and cortical gene markers was detected. We conclude that TG2 is not a valid therapeutic target for the treatment of HD.

Hyperactivity and cortical disinhibition in mice with restricted expression of mutant huntingtin to parvalbumin-positive cells

Recent evidence suggests that interneurons are involved in the pathophysiology of Huntington Disease (HD). Abnormalities in the function of interneurons expressing the calcium buffer parvalbumin (PV) have been observed in multiple mouse models of HD, although it is not clear how PV-positive interneuron dysfunction contributes to behavioral and synaptic deficits. Here, we use the cre-lox system to drive expression of mutant huntingtin (mthtt) in parvalbumin (PV)-positive neurons and find that mutant mice exhibit diffuse mthtt immunoreactivity in PV-rich areas at 10months of age and mthtt aggregates in PV-positive processes at 24months of age. At midlife, mutant mice are hyperactive and display impaired GABA release in the motor cortex, characterized by reduced miniature inhibitory events and severely blunted responses to gamma frequency stimulation, without a loss of PV-positive interneurons. In contrast, 24month-old mutant mice show normalized behavior and responses to gamma frequency stimulation, possibly due to compensatory changes in pyramidal neurons or the formation of inclusions with age. These data indicate that mthtt expression in PV-positive neurons is sufficient to drive a hyperactive phenotype and suggest that mthtt-mediated dysfunction in PV-positive neuronal populations could be a key factor in the hyperkinetic behavior observed in HD. Further clarification of the roles for specific PV-positive populations in this phenotype is warranted to definitively identify cellular targets for intervention.

Impaired mitochondrial dynamics and Nrf2 signaling contribute to compromised responses to oxidative stress in striatal cells expressing full-length mutant huntingtin

Huntington disease (HD) is an inherited neurodegenerative disease resulting from an abnormal expansion of polyglutamine in huntingtin (Htt). Compromised oxidative stress defense systems have emerged as a contributing factor to the pathogenesis of HD. Indeed activation of the Nrf2 pathway, which plays a prominent role in mediating antioxidant responses, has been considered as a therapeutic strategy for the treatment of HD. Given the fact that there is an interrelationship between impairments in mitochondrial dynamics and increased oxidative stress, in this present study we examined the effect of mutant Htt (mHtt) on these two parameters. STHdh(Q111/Q111) cells, striatal cells expressing mHtt, display more fragmented mitochondria compared to STHdh(Q7/Q7) cells, striatal cells expressing wild type Htt, concurrent with alterations in the expression levels of Drp1 and Opa1, key regulators of mitochondrial fission and fusion, respectively. Studies of mitochondrial dynamics using cell fusion and mitochondrial targeted photo-switchable Dendra revealed that mitochondrial fusion is significantly decreased in STHdh(Q111/Q111) cells. Oxidative stress leads to dramatic increases in the number of STHdh(Q111/Q111) cells containing swollen mitochondria, while STHdh(Q7/Q7) cells just show increases in the number of fragmented mitochondria. mHtt expression results in reduced activity of Nrf2, and activation of the Nrf2 pathway by the oxidant tBHQ is significantly impaired in STHdh(Q111/Q111) cells. Nrf2 expression does not differ between the two cell types, but STHdh(Q111/Q111) cells show reduced expression of Keap1 and p62, key modulators of Nrf2 signaling. In addition, STHdh(Q111/Q111) cells exhibit increases in autophagy, whereas the basal level of autophagy activation is low in STHdh(Q7/Q7) cells. These results suggest that mHtt disrupts Nrf2 signaling which contributes to impaired mitochondrial dynamics and may enhance susceptibility to oxidative stress in STHdh(Q111/Q111) cells.

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.

Screening of therapeutic strategies for Huntington's disease in YAC128 transgenic mice

Huntington’s disease (HD) is a hereditary neurodegenerative disorder caused by an unstable expansion of cytosine-adenine-guanine (CAG) repeats in the HD gene. The symptoms include cognitive dysfunction and severe motor impairment with loss of voluntary movement coordination that is later replaced by bradykinesia and rigidity. The neuropathology is characterized by neuronal loss mainly in the striatum and cortex, and the appearance of neuronal intranuclear inclusions of mutant huntingtin. The mechanisms responsible for neurodegeneration are still not fully understood although excitotoxicity and a consequent increase in intracellular calcium concentration as well as the activation of caspases and calapins are known to play a key role. There is currently no satisfactory treatment or cure for this disease. The YAC128 transgenic mice express the full-length human HD gene with 128 CAG repeats and constitute a unique model for the study of HD as they replicate the slow and biphasic progression of behavioral deficits characteristic of the human condition and show striatal neuronal loss. As such, these transgenic mice have been an invaluable model not only for the elucidation of the neurodegenerative pathways in HD, but also for the screening and development of new therapeutic approaches. Here, I will review the unique characteristics of this transgenic HD model and will provide a summary of the therapies that have been tested in these mice, namely: potentiation of the protective roles of wild-type huntingtin and mutant huntingtin aggregation, transglutaminase inhibition, inhibition of glutamate- and dopamine-induced toxicity, apoptosis inhibition, use of essential fatty acids, and the novel approach of intrabody gene therapy. The insights obtained from these and future studies will help identify potential candidates for clinical trials and will ultimately contribute to the discovery of a successful treatment for this devastating neurodegenerative disorder.

Mutant huntingtin causes defective actin remodeling during stress: defining a new role for transglutaminase 2 in neurodegenerative disease

Huntington's disease (HD) is caused by an expanded CAG tract in the Interesting transcript 15 (IT15) gene encoding the 350 kDa huntingtin protein. Cellular stresses can trigger the release of huntingtin from the endoplasmic reticulum, allowing huntingtin nuclear entry. Here, we show that endogenous, full-length huntingtin localizes to nuclear cofilin–actin rods during stress and is required for the proper stress response involving actin remodeling. Mutant huntingtin induces a dominant, persistent nuclear rod phenotype similar to that described in Alzheimer's disease for cytoplasmic cofilin–actin rods. Using live cell temporal studies, we show that this stress response is similarly impaired when mutant huntingtin is present, or when normal huntingtin levels are reduced. In clinical lymphocyte samples from HD patients, we have quantitatively detected cross-linked complexes of actin and cofilin with complex formation varying in correlation with disease progression. By live cell fluorescence lifetime imaging measurement–Förster resonant energy transfer studies and western blot assays, we quantitatively observed that stress-activated tissue transglutaminase 2 (TG2) is responsible for the actin–cofilin covalent cross-linking observed in HD. These data support a direct role for huntingtin in nuclear actin re-organization, and describe a new pathogenic mechanism for aberrant TG2 enzymatic hyperactivity in neurodegenerative diseases.

Aggregation of expanded huntingtin in the brains of patients with Huntington disease

Huntingtin containing an expanded polyglutamine causes neuronal death and Huntington disease. Although expanded huntingtin is found in virtually every cell type, its toxicity is limited to neurons of certain areas of the brain, such as cortex and caudate/putamen. In affected areas of the brain, expanded huntingtin is not found in its intact monomeric form. It is found instead in the form of N-terminal fragments, oligomers and polymers, all of which accumulate in the cortex. Whereas the oligomer is mostly soluble, the polymers and the fragments associate with each other and with other proteins to form the insoluble inclusions characteristic of the disease. It is likely that the aggregates containing expanded huntingtin are toxic to neurons, but it remains to be determined whether the oligomer or the inclusion is the toxic species.

Dissecting the mechanisms of tissue transglutaminase-induced cross-linking of alpha-synuclein: implications for the pathogenesis of Parkinson disease

Tissue transglutaminase (tTG) has been implicated in the pathogenesis of Parkinson disease (PD). However, exactly how tTG modulates the structural and functional properties of alpha-synuclein (alpha-syn) and contributes to the pathogenesis of PD remains unknown. Using site-directed mutagenesis combined with detailed biophysical and mass spectrometry analyses, we sought to identify the exact residues involved in tTG-catalyzed cross-linking of wild-type alpha-syn and alpha-syn mutants associated with PD. To better understand the structural consequences of each cross-linking reaction, we determined the effect of tTG-catalyzed cross-linking on the oligomerization, fibrillization, and membrane binding of alpha-syn in vitro. Our findings show that tTG-catalyzed cross-linking of monomeric alpha-syn involves multiple cross-links (specifically 2-3). We subjected tTG-catalyzed cross-linked monomeric alpha-syn composed of either wild-type or Gln --> Asn mutants to sequential proteolysis by multiple enzymes and peptide mapping by mass spectrometry. Using this approach, we identified the glutamine and lysine residues involved in tTG-catalyzed intramolecular cross-linking of alpha-syn. These studies demonstrate for the first time that Gln(79) and Gln(109) serve as the primary tTG reactive sites. Mutating both residues to asparagine abolishes tTG-catalyzed cross-linking of alpha-syn and tTG-induced inhibition of alpha-syn fibrillization in vitro. To further elucidate the sequence and structural basis underlying these effects, we identified the lysine residues that form isopeptide bonds with Gln(79) and Gln(109). This study provides mechanistic insight into the sequence and structural basis of the inhibitory effects of tTG on alpha-syn fibrillogenesis in vivo, and it sheds light on the potential role of tTG cross-linking on modulating the physiological and pathogenic properties of alpha-syn.

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.

Differential morphology and composition of inclusions in the R6/2 mouse and PC12 cell models of Huntington’s disease

The histological hallmark feature of Huntington's disease (HD) and other polyglutamine repeat diseases is the presence of intracellular inclusions. Much work has been devoted to trying to determine the relationship between inclusion formation and neuronal injury. However, little attention has been paid to the variability and characteristics of inclusions themselves. Here, we characterize the morphological and biochemical composition of inclusions in both a transgenic mouse model (R6/2 line) and an inducible cell culture model of HD (iPC12Q74). We identified several morphologically distinct kinds of inclusions in different locations (nuclei, cytoplasm and cellular processes). Ubiquitin colocalized completely with all of these inclusions in both the iPC12Q72 and R6/2 models. In the inclusions in iPC12Q74 cells, the 20S and 11S proteasome subunits colocalized variably, and the 19S subunit did not colocalize at all. In inclusions in R6/2 mouse neurons, the 20S subunit colocalized completely, but neither the 11S nor the 19S subunits colocalized at all. While the role of inclusions in the pathogenesis of HD continues to be debated, we suggest that the content and structure of inclusions vary considerably, not only from cell to cell but even within individual cells. Their role in the pathogenesis of HD is likely to depend on their location as well as their composition.

Formation of actin-ADF/cofilin rods transiently retards decline of mitochondrial potential and ATP in stressed neurons

When neurons in culture are transiently stressed by inhibition of ATP synthesis, they rapidly form within their neurites rodlike actin inclusions that disappear when the insult is removed. Oxidative stress, excitotoxic insults, and amyloid beta-peptide oligomers also induce rods. Immunostaining of neurites indicates that these rods also contain the majority of the actin filament dynamizing proteins, actin-depolymerizing factor (ADF) and cofilin (AC). If the rods reappear within 24 h after the stress is removed, the neurite degenerates distal to the rod but with no increase in neuronal death. Here, rods were generated in cultured rat E18 hippocampal cells by overexpression of a green fluorescent protein chimera of AC. Surprisingly, we have found that, for a short period (approximately 60 min) immediately after initial rod formation, the loss of mitochondrial membrane potential (Delta Psi(m)) and ATP in neurites with rods is slower than in neurites without them. The Delta Psi(m) was monitored with the fluorescent dye tetramethylrhodamine methyl ester, and ATP was monitored with the fluorescent ion indicator mag-fura 2. Actin in rods is less dynamic than is filamentous actin in other cytoskeletal structures. Because Delta Psi(m) depends on cellular ATP and because ATP hydrolysis associated with actin filament turnover is responsible for a large fraction of neuronal energy consumption (approximately 50%), the formation of rods transiently protects neurites by slowing filament turnover and its associated ATP hydrolysis.

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.

Curcumin suppresses lipopolysaccharide-induced cyclooxygenase-2 expression by inhibiting activator protein 1 and nuclear factor kappab bindings in BV2 microglial cells

Inflammation is a significant component of chronic neurodegenerative diseases. Cyclooxygenase-2 (COX-2) is expressed in activated microglial cells and appears to be an important source of prostaglandins during inflammatory conditions. To investigate the effect of curcumin on COX-2 gene expression in microglial cells, we treated lipopolysaccharide (LPS)-challenged BV2 microglial cells with various concentrations of curcumin. Curcumin significantly inhibited LPS-mediated induction of COX-2 expression in both mRNA and protein levels in a concentration-dependent manner. COX-2 enzyme activity was also inhibited in accordance with mRNA and protein levels. Furthermore, curcumin markedly inhibited LPS-induced nuclear factor kappaB (NF-kappaB) and activator protein 1 (AP-1) DNA bindings. These data suggest that curcumin suppresses LPS-induced COX-2 gene expression by inhibiting NF-kappaB and AP-1 DNA bindings in BV2 microglial cells.

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.

Protective role of tissue transglutaminase in the cell death induced by TNF-alpha in SH-SY5Y neuroblastoma cells

Tissue transglutaminase (tTGase) regulates various biological processes, including extracellular matrix organization, cellular differentiation, and apoptosis. Here we report the protective role of tTGase in the cell death that is induced by the tumor necrosis factor alpha (TNF-alpha) and ceramide, a product of the TNF-alpha signaling pathway, in human neuroblastoma SH-SY5Y cells. Treatment with retinoic acid (RA) induced the differentiation of the neuroblastoma cells with the formation of extended neurites. Immunostaining and Western blot analysis showed the tTGase expression by RA treatment. TNF-alpha or C(2) ceramide, a cell permeable ceramide analog, induced cell death in normal cells, but cell death was largely inhibited by the RA treatment. The inhibition of tTGase by the tTGase inhibitors, monodansylcadaverine and cystamine, eliminated the protective role of RA-treatment in the cell death that is caused by TNF-alpha or C(2)-ceramide. In addition, the co-treatment of TNF-alpha and cycloheximide decreased the protein level of tTGase and cell viability in the RA-treated cells, supporting the role of tTGase in the protection of cell death. DNA fragmentation was also induced by the co-treatment of TNF-alpha and cycloheximide. These results suggest that tTGase expressed by RA treatment plays an important role in the protection of cell death caused by TNF-alpha and ceramide.

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

Tissue transglutaminase (tTG) is a multifunctional enzyme that catalyzes peptide cross-linking and polyamination reactions, and also is a signal-transducing GTPase. tTG protein content and enzymatic activity are upregulated in the brain in Huntington's disease and in other neurological diseases and conditions. Since mouse models are currently being used to study the role of tTG in Huntington's disease and other neurodegenerative diseases, it is critical that the level of its expression in the mouse forebrain be determined. In contrast to human forebrain where tTG is abundant, tTG can only be detected in mouse forebrain by immunoblotting a GTP-binding-enriched protein fraction. tTG mRNA content and transamidating activity are approximately 70% lower in mouse than in human forebrain. However, tTG contributes to the majority of transglutaminase activity within mouse forebrain. Thus, although tTG is expressed at lower levels in mouse compared with human forebrain, it likely plays important roles in neuronal function.

Activation of in situ tissue transglutaminase by intracellular reactive oxygen species

We have investigated the novel function of intracellular reactive oxygen species (ROS) in the activation of in situ tissue transglutaminase (tTGase) by lysophosphatidic acid (LPA) and transforming growth factor-beta (TGF-beta) in Swiss 3T3 fibroblasts. LPA induced a transient increase of intracellular ROS with a maximal increase at 10 min, which was blocked by ROS scavengers, N-acetyl-L-cysteine and catalase. LPA activated tTGase with a maximal increase at 1h, which was inhibited by cystamine and ROS scavengers. Incubation with exogenous H(2)O(2) activated tTGase. TGF-beta also activated tTGase with a maximal activation at 2h and the tTGase activation was inhibited by the ROS scavengers. Scrape-loading of C3 transferase inhibited the ROS production and in situ tTGase activation by LPA and TGF-beta, and the inhibitory effect of C3 transferase was reversed by exogenous H(2)O(2). Microinjection of GTPgammaS inhibited transamidating activity of tTGase stimulated by LPA, TGF-beta, and maitotoxin. These results suggested that intracellular ROS was essential for the activation of in situ tTGase in response to LPA and TGF-beta.

Are Huntington's and polyglutamine-based ataxias proteasome storage diseases?

To date, 10 neurological diseases, including Huntington's and several ataxias, are caused by a lengthening of glutamine (Q) tracts in various proteins. Even though the Q expansions arise in unrelated proteins, the diseases share three striking features: (1) 35 contiguous glutamines constitutes the pathological threshold for 9 of the 10 diseases; (2) the Q-expanded proteins are expressed in many tissues, yet pathology is largely restricted to neurons; and (3) the Q-expanded proteins or fragments thereof form nuclear inclusions that also contain ubiquitin, proteasomes and chaperones. Our studies of the proteasome activator REGgamma suggest a possible explanation for these shared properties. REGgamma is highly expressed in brain, located in the nucleus and actually suppresses the proteasome active sites principally responsible for cleaving glutamine-MCA bonds. These observations coupled with reports that peptides longer than 35 residues, the polyQ pathology threshold, are unable to diffuse out of the proteasome suggest the following hypothesis. Proteins containing long glutamine tracts are efficiently pumped into REGgamma-capped 26S proteasomes, but REGgamma suppression of cleavage after glutamine produces polyQ fragments too long to diffuse out of the 20S proteolytic core thereby inactivating the 26S proteasome. In effect, we hypothesize that the polyQ pathologies may be proteasomal storage diseases analogous to disorders of lysosome catabolism.

Oligomeric and polymeric aggregates formed by proteins containing expanded polyglutamine

Neurological diseases resulting from proteins containing expanded polyglutamine (polyQ) are characteristically associated with insoluble neuronal inclusions, usually intranuclear, and neuronal death. We describe here oligomeric and polymeric aggregates formed in cells by expanded polyQ. These aggregates are not dissociated by concentrated formic acid, an extremely effective solvent for otherwise insoluble proteins. Perinuclear inclusions formed in cultured cells by expanded polyQ can be completely dissolved in concentrated formic acid, but a soluble protein oligomer containing the expanded polyQ and released by the formic acid is not dissociated to monomer. In Huntington's disease, a formic acid-resistant oligomer is present in cerebral cortex, but not in cerebellum. Cortical nuclei contain a polymeric aggregate of expanded polyQ that is insoluble in formic acid, does not enter polyacrylamide gels, but is retained on filters. This finding shows that the process of polymerization is more advanced in the cerebral cortex than in cultured cells. The resistance of oligomer and polymer to formic acid suggests the participation of covalent bonds in their stabilization.

Cystamine inhibits caspase activity. Implications for the treatment of polyglutamine disorders

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an abnormally expended polyglutamine domain. There is no effective treatment for HD; however, inhibition of caspase activity or prevention of mitochondria dysfunction delays disease progression in HD mouse models. Similarly administration of cystamine, which can inhibit transglutaminase, prolonged survival of HD mice, suggesting that inhibition of transglutaminase might provide a new treatment strategy. However, it has been suggested that cystamine may inhibit other thiol-dependent enzymes in addition to transglutaminase. In this study we show that cystamine inhibits recombinant active caspase-3 in a concentration-dependent manner. At low concentrations cystamine is an uncompetitive inhibitor of caspase-3 activity, becoming a non-competitive inhibitor at higher concentrations. The IC(50) for cystamine-mediated inhibition of caspase-3 activity in vitro was 23.6 microm. In situ cystamine inhibited in a concentration-dependent manner the activation of caspase-3 by different pro-apoptotic agents. Additionally, cystamine inhibited caspase-3 activity to the same extent in cell lines stably overexpressing wild type tissue transglutaminase (tTG), a mutant inactive tTG, or an antisense for tTG, demonstrating that cystamine inhibits caspase activity independently of any effects it may have on the transamidating activity of tTG. Finally, treatment with cystamine resulted in a robust increase in the levels of glutathione. These findings demonstrate that cystamine may prolong neuronal survival and delay the onset of HD by inhibiting caspases and increasing the level of antioxidants such as glutathione.

Polymorphism and structural maturation of bunyamwera virus in Golgi and post-Golgi compartments

The Golgi apparatus is the assembly site for a number of complex enveloped viruses. Using high-preservation methods for electron microscopy, we have detected two previously unknown maturation steps in the morphogenesis of Bunyamwera virus in BHK-21 cells. The first maturation takes place inside the Golgi stack, where annular immature particles transform into dense, compact structures. Megalomicin, a drug that disrupts the trans side of the Golgi complex, reversibly blocks transformation, showing that a functional trans-Golgi is needed for maturation. The second structural change seems to take place during the egress of viral particles from cells, when a coat of round-shaped spikes becomes evident. A fourth viral assembly was detected in infected cells: rigid tubular structures assemble in the Golgi region early in infection and frequently connect with mitochondria. In Vero cells, the virus induces an early and spectacular fragmentation of intracellular membranes while productive infection progresses. Assembly occurs in fragmented Golgi stacks and generates tubular structures, as well as the three spherical viral forms. These results, together with our previous studies with nonrelated viruses, show that the Golgi complex contains key factors for the structural transformation of a number of enveloped viruses that assemble intracellularly.

Transient osmotic stress facilitates mutant huntingtin aggregation

Human neuroblastoma SH-SY5Y cell lines stably expressing mutant truncated huntingtin with 82 (mutant) glutamine repeats (N63-82Q) were briefly exposed to hyperosmotic conditions which decrease cell volume and therefore transiently increased the concentration of N63-82Q, as well as activating specific stress-induced pathways. Transient hyperosmotic treatment significantly increased the number of cells with aggregates. When the N63-82Q cells were subsequently returned to iso-osmotic medium after the treatment, the number of cells with aggregates remained constant up to 12 h. However, between 12 and 24 h another significant increase in aggregate frequency was observed, with approximately 55% of the cells containing aggregates after 24 h. This may be due in part to the formation of microaggregates during hyperosmotic conditions that act as seeds for the aggregate formation. Further, treatment of cells with geldanamycin, which activates a heat shock response, significantly attenuated the hyperosmotic-induced increase in aggregate formation.

Cerebrospinal fluid tissue transglutaminase in vascular dementia

The enzyme tissue transglutaminase (tTG), an indicator of acute cell death, is found in brains of Alzheimer's and Huntington's disease patients. tTG, as a specific marker for apoptosis, may therefore be a powerful biochemical marker of the acute degenerating process in vivo and may be useful in discrimination between vascular dementia (VaD) and Alzheimer's disease (AD). It may serve as completion of CSF analysis in diagnosis of dementing disorders and be a simple way of assessing the efficacy of possible new anti-apoptotic drugs.

Cerebrospinal fluid tissue transglutaminase as a biochemical marker for Alzheimer's disease

Tissue transglutaminase (tTG) is an indicator of acute cell death in vitro. An increase in tTG protein level is found in postmortem Alzheimer's disease (AD) brains as well as in Huntington's disease. No study revealed tTG in vivo so far. We investigated the concentrations of tTG in the cerebrospinal fluid (CSF) obtained from 84 patients using ELISA assays. We compared 33 patients with probable AD to 18 patients with probable vascular dementia (VaD) and 33 control patients without neuropsychological deficit. Diagnosis was supported by CSF parameter and neuroimaging. We found a highly significant difference (P = 0.001) between the concentration of tTG in the AD groups (7.58 pg/ml) and controls (2.99 pg/ml). There was no statistical difference between controls and VaD (2.93 pg/ml). Interestingly, tTG did not show an association with tau protein, Abeta42, apoE4, neuropsychological items, or age. Males showed lower tTG values than females; however, this difference did not reach statistical significance. To our knowledge, this is the first demonstration that tTG is increased in AD in vivo. Our results suggest that tTG may be a powerful biochemical marker of the acute degenerating process in vivo. It may serve as completion of CSF analysis in the diagnosis of dementing disorders and may be a simple way of assessing the efficacy of possible new antiapoptotic drugs.

'Tissue' transglutaminase ablation reduces neuronal death and prolongs survival in a mouse model of Huntington's disease

By crossing Huntington's disease (HD) R6/1 transgenic mice with 'tissue' transglutaminase (TG2) knock-out mice, we have demonstrated that this multifunctional enzyme plays an important role in the neuronal death characterising this disorder in vivo. In fact, a large reduction in cell death is observed in R6/1, TG2(-/-) compared with R6/1 transgenic mice. In addition, we have shown that the formation of neuronal intranuclear inclusions (NII) is potentiated in absence of the 'tissue' transglutaminase. These phenomena are paralleled by a significant improvement both in motor performances and survival of R6/1, TG2(-/-) versus R6/1 mice. Taken together these findings suggest an important role for tissue transglutaminase in the regulation of neuronal cell death occurring in Huntington's disease.

Protein aggregation in Huntington's disease

The presence of an expanded polyglutamine produces a toxic gain of function in huntingtin. Protein aggregation resulting from this gain of function is likely to be the cause of neuronal death. Two main mechanisms of aggregation have been proposed: hydrogen bonding by polar-zipper formation and covalent bonding by transglutaminase-catalyzed cross-linking. In cell culture models of Huntington's disease, aggregates are mostly stabilized by hydrogen bonds, but covalent bonds are also likely to occur. Nothing is known about the nature of the bonds that stabilize the aggregates in the brain of patients with Huntington's disease. It seems that the nature of the bond stabilizing the aggregates is one of the most important questions, as the answer would condition the therapeutic approach to Huntington's disease.

Mutant huntingtin aggregates do not sensitize cells to apoptotic stressors

It has been postulated that neuronal inclusions composed of mutant huntingtin may play a causative role in the pathogenesis of Huntington's disease. To study the putative role of aggregates in modulating apoptotic vulnerability, SH-SY5Y cell lines stably expressing truncated huntingtin with 18 (wild-type) (N63-18Q) or 82 (mutant) (N63-82Q) glutamine repeats were established. Aggregates were observed in approximately 13% of the N63-82Q cells; no aggregates were observed in the N63-18Q cells. In response to apoptotic stimuli such as staurosporine or hyperosmotic stress, caspase-3 activity was significantly greater in the N63-82Q cells compared to the N63-18Q cells. However, double immunostaining for huntingtin and active caspase-3 revealed that the presence of aggregates did not correlate with the presence of active caspase-3, indicating that aggregates do not contribute to the increase in apoptosis in the N63-82Q cells.

Arfaptin 2 regulates the aggregation of mutant huntingtin protein

Huntington's disease (HD) is an inherited neurodegenerative disorder. Here we demonstrate that expression of arfaptin 2/POR1 (partner of Rac1) in cultured cells induces the formation of pericentriolar and nuclear aggregates, which morphologically resemble mutant huntingtin aggregates characteristic of HD. Endogenous arfaptin 2 localizes to aggregates induced by expression of an abnormal amino-terminal fragment of huntingtin that contains polyglutamine (polyQ) expansions. A dominant inhibitory mutant of arfaptin 2 inhibits aggregation of mutant huntingtin, but not in the presence of proteasome inhibitors. Using cell-free biochemical assays, we show that arfaptin 2 inhibits proteasome activity. Finally, we show that expression of arfaptin 2 is increased at sites of neurodegeneration and the protein localizes to huntingtin aggregates in HD transgenic mouse brains. Our data suggest that arfaptin 2 is involved in regulating huntingtin protein aggregation, possibly by impairing proteasome function.

Does tissue transglutaminase play a role in Huntington's disease?

Tissue transglutaminase (tTG) likely plays a role in numerous processes in the nervous system. tTG posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines (Glns). This reaction results in the incorporation of polyamines into substrate proteins or the formation of protein crosslinks, modifications that likely have significant effects on neural function. Huntington's disease is a genetic disorder caused by an expansion of the polyglutamine domain in the huntingtin protein. Because a polypeptide bound Gln is the determining factor for a tTG substrate, and mutant huntingtin aggregates have been found in Huntington's disease brain, it has been hypothesized that tTG may contribute to the pathogenesis of Huntington's disease. In vitro, polyglutamine constructs and huntingtin are substrates of tTG. Further, the levels of tTG and TG activity are elevated in Huntington's disease brain and immunohistochemical studies have demonstrated that there is an increase in tTG reactivity in affected neurons in Huntington's disease. These findings suggest that tTG may play a role in Huntington's disease. However in situ, neither wild type nor mutant huntingtin is modified by tTG. Further, immunocytochemical analysis revealed that tTG is totally excluded from the huntingtin aggregates, and modulation of the expression level of tTG had no effect on the frequency of the aggregates in the cells. Therefore, tTG is not required for the formation of huntingtin aggregates, and likely does not play a role in this process in Huntington's disease brain. However, tTG interacts with truncated huntingtin, and selectively polyaminates proteins that are associated with mutant truncated huntingtin. Given the fact that the levels of polyamines in cells is in the millimolar range and the crosslinking and polyaminating reactions catalyzed by tTG are competing reactions, intracellularly polyamination is likely to be the predominant reaction. Polyamination of proteins is likely to effect their function, and therefore it can be hypothesized that tTG may play a role in the pathogenesis of Huntington's disease by modifying specific proteins and altering their function and/or localization. Further research is required to define the specific role of tTG in Huntington's disease.

Cross linking of polyglutamine domains catalyzed by tissue transglutaminase is greatly favored with pathological-length repeats: does transglutaminase activity play a role in (CAG)(n)/Q(n)-expansion diseases?

Protein aggregates are a hallmark of Huntington's disease (HD) and other inherited neurodegenerative diseases caused by an elongated (CAG)(n) repeat in the genome and to a corresponding increase in the size of the Q(n) domain in the expressed protein. When the protein associated with HD (huntingtin) contains <35 Q repeats disease does not occur. However, an n>/=40 leads to disease. Some investigators have proposed that aggregates in the nuclei of affected cells are toxic, but other workers have suggested that the aggregates may be neutral or even protective. Whether or not they are toxic, an understanding of the processes whereby the aggregates develop may shed light on the neuropathological processes involved in the (CAG)(n)/Q(n)-expansion disorders. Q(n) domains have a tendency to non-covalently self align as 'polar zippers' rendering them less soluble, but evidence that such polar zippers occur in the aggregates in intact HD brain has so far been limited. The human brain contains at least three Ca(2+)-dependent enzymes (transglutaminases, TGases) that catalyze protein cross-linking reactions, namely TGase 1, TGase 2 (tissue transglutaminase, tTGase) and TGase 3. Q(n) aggregates have been found by several groups to be excellent substrates of tTGase. Moreover, the activity toward the Q(n) domains increases greatly as n is increased to 40 or beyond. tTGase mRNA and total TGase activity are elevated in HD brain. Moreover, some evidence suggests that Ca(2+) homeostasis is disrupted in HD brain. We propose that the combination of increased huntingtin (or huntingtin fragment containing the Q(n) domain) in the nucleus, increased the ability of the Q(n) domains to act as substrate, increased Ca(2+) levels and increased inherent TGase activity all contribute to increased cross-linking of proteins in HD brain. At first the proteasome machinery can recognize and degrade the cross-linked proteins, but over time the proteasome machinery may be overwhelmed and protein aggregates will accumulate.

Transglutaminase-dependent formation of protein aggregates as possible biochemical mechanism for polyglutamine diseases

Transglutaminases (Enzyme Commission 2.3.2.13) are a large family of enzymes that show the common capacity to catalyze cross-linking of protein substrates. Some members of this family of enzymes are also capable of catalyzing other reactions important for the cell life. The distribution and the role of these enzymes have been widely studied in numerous cell types and tissues, but only recently their expression and functions started to be investigated in the central nervous system. One of the main biochemical properties of the transglutaminase enzymes is to form large protein aggregates that are insoluble in all known protein detergents, such as urea, guanidinium, and sodium dodecyl sulfate. Recently, the transglutaminase activity has been hypothesized to be involved in the pathogenetic mechanisms responsible for the formation of cellular inclusions present in Huntington disease and in all the other polyglutamine (polyQ) diseases hitherto identified, such as spinobulbar muscular atrophy or Kennedy disease, spinocerebellar ataxias (SCA-1, SCA-2, SCA-3 or Machado-Joseph disease, SCA-6 and SCA-7) and dentatorubropallidoluysian atrophy. In this review we describe the biochemical properties of the transglutaminase enzymes and some recent findings about the physiopathological roles played by these enzymes in the central nervous system.

A microtiter plate assay for polyglutamine aggregate extension

Polyglutamine (polyGln) aggregates are neuropathological markers of expanded CAG repeat disorders, and may also play a critical role in the development of these diseases. We have established a highly sensitive, fast, reproducible, and specific assay capable of monitoring aggregate-dependent deposition of polyglutamine peptides. This assay allows detailed studies on various aspects of aggregation kinetics, and also makes possible the detection and quantitation of low levels of "extension-competent" aggregates. In the simplest form of this assay, polyGln aggregates are made from chemically synthesized peptides and immobilized onto microplate wells. These wells are incubated for different times with low concentrations of a soluble biotinylated polyGln peptide. Europium-streptavidin complexation of the immobilized biotin, followed by time-resolved fluorescence detection of the deposited europium, allows us to calculate the rate (fmol/h) of incorporation of polyGln peptides into polyGln aggregates. This assay will make possible basic studies on the assembly mechanism of polyGln aggregates and on critical features of the reaction, such as polyGln length dependence. The assay also will be a valuable tool for screening and characterizing anti-aggregation inhibitors. It will also be useful for detection and quantitation of aggregation-competent polyGln aggregates in biological materials, which may prove to be of critical importance in understanding the disease mechanism.