Pathogenesis of inclusion bodies in (CAG)n/Qn-expansion diseases with special reference to the role of tissue transglutaminase and to selective vulnerability

Epigenetic regulators of neuronal ferroptosis identify novel therapeutics for neurological diseases: HDACs, transglutaminases, and HIF prolyl hydroxylases

A major thrust of our laboratory has been to identify how physiological stress is transduced into transcriptional responses that feed back to overcome the inciting stress or its consequences, thereby fostering survival and repair. To this end, we have adopted the use of an in vitro model of ferroptosis, a caspase-independent, but iron-dependent form of cell death (Dixon et al., 2012; Ratan, 2020). In this review, we highlight three distinct epigenetic targets that have evolved from our studies and which have been validated in vivo studies. In the first section, we discuss our studies of broad, pan-selective histone deacetylase (HDAC) inhibitors in ferroptosis and how these studies led to the validation of HDAC inhibitors as candidate therapeutics in a host of disease models. In the second section, we discuss our studies that revealed a role for transglutaminase as an epigenetic modulator of proferroptotic pathways and how these studies set the stage for recent elucidation of monoamines as post-translation modifiers of histone function. In the final section, we discuss our studies of iron-, 2-oxoglutarate-, and oxygen-dependent dioxygenases and the role of one family of these enzymes, the HIF prolyl hydroxylases, in mediating transcriptional events necessary for ferroptosis in vitro and for dysfunction in a host of neurological conditions. Overall, our studies highlight the importance of epigenetic proteins in mediating prodeath and prosurvival responses to ferroptosis. Pharmacological agents that target these epigenetic proteins are showing robust beneficial effects in diverse rodent models of stroke, Parkinson's disease, Huntington's disease, and Alzheimer's disease.

A study of molecular changes relating to energy metabolism and cellular stress in people with Huntington's disease: looking for biomarkers

Huntington's disease (HD) is a neurodegenerative disorder characterized by a progressive motor and cognitive decline and the development of psychiatric symptoms. The origin of molecular and biochemical disturbances in HD is a mutation in the HTT gene, which is autosomally dominantly inherited. The altered huntingtin protein is ubiquitously expressed in the CNS, as well as in peripheral tissues. In this study we measured the metabolism changes in gene transcription in blood of HD gene carriers (premanifest and manifest combined) versus 28 healthy controls. The comparison revealed statistically significant Global Pattern Recognition Fold Change (FC) for 6 mRNA transcripts, reflecting an increase of: MAOB (FC = 3.07; p = 0.0005) which encodes an outer mitochondrial membrane-bound enzyme called monoamine oxidase type B; TGM2 (FC = 1.8; p = 0.02) encoding a transglutaminase 2 that mediates cellular stress; SLC2A4 (FC = 1.64; p = 0.02) solute carrier family 2 (facilitated glucose transporter) member 4; branched chain ketoacid dehydrogenase kinase (BCKDK) (FC = 1.34; p = 0.02); decrease of LDHA (FC = -1.16; p = 0.03) lactate dehydrogenase A; and brain-derived neurotrophic factor (BDNF) (FC = -2,11; p = 0.03). These distinguished changes coincided with HD progress. The analyses of gene transcription levels in sub-cohorts confirmed these changes and also revealed 28 statistically significant FCs of gene transcripts involved in ATP production and BCAA metabolism.

Nuclear Inclusions and Pseudoinclusions: Friends or Foes of the Surgical Pathologist?

Abnormal substances in the nuclei that can be observed by light microscopy are often broadly referred to as nuclear inclusions. Although their recognition in the appropriate clinicopathological settings can aid in the diagnosis of some disease entities and tumor types, they can also be a source of error. There are 2 morphologically distinct types of inclusions with different mechanisms of formation and diagnostic significance, including bona fide nuclear inclusions and nuclear pseudoinclusions. Bona fide nuclear inclusions result from accumulation in the nuclei of viral particles, cytoplasmic materials (such as surfactant, immunoglobulin, and glycogen), biotin, nuclear lamins, or polyglutamine. Some of them are diagnostically helpful, such as surfactant inclusion, which can support the pulmonary origin of an adenocarcinoma, whereas others may be misleading, such as biotin inclusion, which can be mistaken for herpes infection. Nuclear pseudoinclusions, which represent invaginations of cytoplasm into the nucleus, are delimited by the nuclear membrane. Although not totally specific, they are particularly common in papillary thyroid carcinoma, meningioma, and usual ductal hyperplasia of the breast and hence may aid in the diagnosis of these entities. Nuclear pseudo-pseudoinclusions, which are artefactual bubbles in the nuclei that mimic nuclear pseudoinclusions or clear nuclei, can lead to misdiagnosis of follicular adenoma or hyperplastic nodule as papillary thyroid carcinoma.

Physio-pathological roles of transglutaminase-catalyzed reactions

Transglutaminases (TGs) are a large family of related and ubiquitous enzymes that catalyze post-translational modifications of proteins. The main activity of these enzymes is the cross-linking of a glutaminyl residue of a protein/peptide substrate to a lysyl residue of a protein/peptide co-substrate. In addition to lysyl residues, other second nucleophilic co-substrates may include monoamines or polyamines (to form mono- or bi-substituted /crosslinked adducts) or -OH groups (to form ester linkages). In the absence of co-substrates, the nucleophile may be water, resulting in the net deamidation of the glutaminyl residue. The TG enzymes are also capable of catalyzing other reactions important for cell viability. The distribution and the physiological roles of TG enzymes have been widely studied in numerous cell types and tissues and their roles in several diseases have begun to be identified. “Tissue” TG (TG2), a member of the TG family of enzymes, has definitely been shown to be involved in the molecular mechanisms responsible for a very widespread human pathology: i.e. celiac disease (CD). TG activity has also been hypothesized to be directly involved in the pathogenetic mechanisms responsible for several other human diseases, including neurodegenerative diseases, which are often associated with CD. Neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, supranuclear palsy, Huntington’s disease and other recently identified polyglutamine diseases, are characterized, in part, by aberrant cerebral TG activity and by increased cross-linked proteins in affected brains. In this review, we discuss the physio-pathological role of TG-catalyzed reactions, with particular interest in the molecular mechanisms that could involve these enzymes in the physio-pathological processes responsible for human neurodegenerative diseases.

Striatal expression of a calmodulin fragment improved motor function, weight loss, and neuropathology in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by a polyglutamine expansion in the huntingtin protein (htt). Increasing evidence suggests that transglutaminase (TGase) plays a critical role in the pathophysiology of HD possibly by stabilizing monomeric, polymeric and aggregated htt. We previously reported that in HEK293 and SH-SY5Y cells expression of a calmodulin (CaM)-fragment, consisting of amino acids 76-121 of CaM, decreased binding of CaM to mutant htt, TGase-modified htt and cytotoxicity associated with mutant htt and normalized intracellular calcium release. In this study, an adeno-associated virus (AAV) that expresses the CaM-fragment was injected into the striatum of HD transgenic R6/2 mice. The CaM-fragment significantly reduced body weight loss and improved motor function as indicated by improved rotarod performance, longer stride length, lower stride frequency, fewer low mobility bouts and longer travel distance than HD controls. A small but insignificant increase in survival was observed in R6/2 mice with CaM-fragment expression. Immunoprecipitation studies show that expression of the CaM-fragment reduced TGase-modified htt in the striatum of R6/2 mice. The percentage of htt-positive nuclei and the size of intranuclear htt aggregates were reduced by the CaM-fragment without striatal volume changes. The effects of CaM-fragment appear to be selective, as activity of another CaM-dependent enzyme, CaM-dependent kinase II, was not altered. Moreover, inhibition of TGase-modified htt was substrate-specific since overall TGase activity in the striatum was not altered by treatment with the CaM-fragment. Together, these results suggest that disrupting CaM-htt interaction may provide a new therapeutic strategy for HD.

Neuroprotective effects of calmodulin peptide 76-121aa: disruption of calmodulin binding to mutant huntingtin

Huntington's disease (HD) is a neurodegenerative disease caused by mutant huntingtin protein containing an expanded polyglutamine tract, which may cause abnormal protein–protein interactions such as increased association with calmodulin (CaM). We previously demonstrated in HEK293 cells that a peptide containing amino acids 76‐121 of CaM (CaM‐peptide) interrupted the interaction between CaM and mutant huntingtin, reduced mutant huntingtin‐induced cytotoxicity and reduced transglutaminase (TG)‐modified mutant huntingtin. We now report that adeno‐associated virus (AAV)‐mediated expression of CaM‐peptide in differentiated neuroblastoma SH‐SY5Y cells, stably expressing an N‐terminal fragment of huntingtin containing 148 glutamine repeats, significantly decreases the amount of TG‐modified huntingtin and attenuates cytotoxicity. Importantly, the effect of the CaM‐peptide shows selectivity, such that total TG activity is not significantly altered by expression of CaM‐peptide nor is the activity of another CaM‐dependent enzyme, CaM kinase II. In vitro, recombinant exon 1 of huntingtin with 44 glutamines (htt‐exon1‐44Q) binds to CaM‐agarose; the addition of 10 µM of CaM‐peptide significantly decreases the interaction of htt‐exon1‐44Q and CaM but not the binding between CaM and calcineurin, another CaM‐binding protein. These data support the hypothesis that CaM regulates TG‐catalyzed modifications of mutant huntingtin and that specific and selective disruption of the CaM‐huntingtin interaction is potentially a new target for therapeutic intervention in HD.

Protective effects of interrupting the binding of calmodulin to mutant huntingtin

There is evidence suggesting that transglutaminase (TG) 2 plays a role in stabilizing monomeric and aggregated huntingtin, thereby contributing to the pathophysiology of Huntington disease. Calmodulin (CaM) regulates TG2 cross-linking of N-terminal mutant huntingtin in cells and colocalizes with TG and huntingtin in inclusions in Huntington disease cortex. The current study examined the effects of small fragments of CaM in human embryonic kidney 293T cells expressing N-terminal mutant huntingtin and transglutaminase 2. Four CaM fragments were developed: first 76 amino acids, last 72 amino acids, 77 amino acids in the center (CaM-center), and the overlapping region of last 72 amino acids and CaM-center (CaM-overlap). The last 72 amino acids, CaM-center, and CaM-overlap significantly decreased amounts of TG-modified huntingtin by 40% to 60%, and cytotoxicity decreased up to 40% compared with cells not expressing any CaM construct. Carbachol-stimulated release of intracellular calcium is significantly higher in cells expressing N-terminal mutant huntingtin and TG2 compared with vector-transfected cells; expression of either CaM-center or CaM-overlap in these cells returned the levels of carbachol-stimulated intracellular calcium release to control values. Furthermore, CaM-overlap expression significantly decreased huntingtin binding to CaM. These data further suggest that CaM regulates TG2 activity, plays a role in the disease-related modifications to mutant huntingtin, and that disruption of CaM-huntingtin interaction is potentially a new target for therapeutic intervention in Huntington disease.

Neuronal toxicity of expanded polyglutamine depends on intracellular distribution in addition to the expression level

The proteins that accumulate in pathologic lesions of neurodegenerative disorders are thought to be closely associated with neuronal cell damage. However, whether or not the formation of cytoplasmic or nuclear inclusions by expanded polyglutamine (polyQ) is directly toxic to neurons has been controversial to date. We prepared a culture model system in which polyQ tracts were transfected into Neuro2a, cells of neuronal origin, to study novel factors involved in cell toxicity of polyQ tracts to neuronal cells. Pathogenic polyQ tracts of 79 repeats (Q79C) when expressed in cytoplasm of Neuro2a cells changed in their intracellular distribution patterns from homogeneous, via punctate aggregates, to massive aggregates with incubation time. Some polyQ tracts formed nuclear inclusions. Cytoplasmic massive aggregates of Q79C tended to be associated with apoptotic fate of Neuro2a cells. Cells exhibiting cytoplasmic massive inclusions had the highest expression level of polyQ tracts among cells with four patterns of intracellular distribution. The elevation in the expression levels of polyQ tracts was not due to the difference in the initial transfection efficiency. When compared among cells expressing polyQ tracts at similar levels, damages were most remarkable in cells with cytoplasmic massive aggregate in terms of shrunken cellular and nuclear sizes. Cells with the other patterns of polyQ tract distribution such as cytoplasmic homogeneous, cytoplasmic punctate and nuclear inclusions were relatively spared. These data suggest that the severity of cell damages depends on the type of intracellular distribution of polyQ tracts, in addition to the expression level of polyQ tracts.

Transglutaminase is linked to neurodegenerative diseases

Transglutaminase catalyzes a covalent bond between peptide-bound glutamine residues and either lysine-bound peptide residues or mono- or polyamines. Multiple lines of evidence suggest that transglutaminase is involved in neurodegenerative diseases including Alzheimer disease, progressive supranuclear palsy, Huntington disease (HD), and Parkinson disease. In all of the neurodegenerative diseases examined to date, transglutaminase enzyme activity is upregulated in selectively vulnerable brain regions, transglutaminase proteins are associated with inclusion bodies characteristic of the diseases, and prominent proteins in the inclusion bodies are modified by transglutaminase enzymes. These prominent proteins in the inclusion bodies, including tau, alpha-synuclein, and huntingtin protein, are modified by transglutaminase in vitro and alpha-synuclein and huntingtin protein are modified in cells in culture. Similar changes in transglutaminase and transglutaminase-modified proteins are replicated in transgenic mouse models of the neurodegenerative diseases, including Huntington disease and progressive supranuclear palsy. Lastly, inhibition of transglutaminase either via drug treatments or molecular approaches is beneficial for the treatment of HD transgenic mice but has yet to be explored for the other neurodegenerative diseases. Further research is needed to determine the specific role(s) that transglutaminase plays in the pathophysiology of neurodegenerative diseases with possible implications for transglutaminase as a therapeutic target.

Phage display selection of efficient glutamine-donor substrate peptides for transglutaminase 2

Understanding substrate specificity and identification of natural targets of transglutaminase 2 (TG2), the ubiquitous multifunctional cross-linking enzyme, which forms isopeptide bonds between protein-linked glutamine and lysine residues, is crucial in the elucidation of its physiological role. As a novel means of specificity analysis, we adapted the phage display technique to select glutamine-donor substrates from a random heptapeptide library via binding to recombinant TG2 and elution with a synthetic amine-donor substrate. Twenty-six Gln-containing sequences from the second and third biopanning rounds were susceptible for TG2-mediated incorporation of 5-(biotinamido)penthylamine, and the peptides GQQQTPY, GLQQASV, and WQTPMNS were modified most efficiently. A consensus around glutamines was established as pQX(P,T,S)l, which is consistent with identified substrates listed in the TRANSDAB database. Database searches showed that several proteins contain peptides similar to the phage-selected sequences, and the N-terminal glutamine-rich domain of SWI1/SNF1-related chromatin remodeling proteins was chosen for detailed analysis. MALDI/TOF and tandem mass spectrometry-based studies of a representative part of the domain, SGYGQQGQTPYYNQQSPHPQQQQPPYS (SnQ1), revealed that Q(6), Q(8), and Q(22) are modified by TG2. Kinetic parameters of SnQ1 transamidation (K(M)(app) = 250 microM, k(cat) = 18.3 sec(-1), and k(cat)/K(M)(app) = 73,200) classify it as an efficient TG2 substrate. Circular dichroism spectra indicated that SnQ1 has a random coil conformation, supporting its accessibility in the full-length parental protein. Added together, here we report a novel use of the phage display technology with great potential in transglutaminase research.

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.

Mutant Huntingtin Protein: A Substrate for Transglutaminase 1, 2, and 3

The most prominent neuropathologic hallmarks of Huntington disease (HD) are cortical and striatal perinuclear cytoplasmic aggregates and intranuclear inclusions of mutant huntingtin. Our laboratory previously demonstrated that huntingtin protein colocalizes with transglutaminase 2 and its product, the epsilon-(gamma-glutamyl)lysine bond in intranuclear inclusions in HD frontal cortex. We also found that transglutaminase 2 cross-links N-terminal fragments of mutant huntingtin (htt-N63-148Q-myc) in cells in culture. We now report a significant increase in transglutaminase 2 mRNA in HD cortex (225% of controls) and striatum (399% of controls). Expression of the short transglutaminase 2 mRNA splice variant was not detectable in HD, although previous studies demonstrated upregulation in Alzheimer disease and progressive supranuclear palsy. Cells co-transfected with GFP-tagged transglutaminase 1, 2, or 3 and htt-N63-148Q-myc exhibit increased cross-linked huntingtin in the insoluble fraction of cell lysates. Treatment of cells with cystamine, a chemical inhibitor of transglutaminase, decreased aggregated and cross-linked huntingtin and increased viability of cells that were transfected with transglutaminase 2 and htt-N63-148Q-myc. These data suggest that transglutaminase 1, 2, and 3 could be involved in cross-linking of huntingtin into intranuclear inclusions in HD and that inhibiting transglutaminase should be explored as a potential treatment strategy for HD.

Calmodulin regulates transglutaminase 2 cross-linking of huntingtin

Striatal and cortical intranuclear inclusions and cytoplasmic aggregates of mutant huntingtin are prominent neuropathological hallmarks of Huntington's disease (HD). We demonstrated previously that transglutaminase 2 cross-links mutant huntingtin in cells in culture and demonstrated the presence of transglutaminase-catalyzed cross-links in the HD cortex that colocalize with transglutaminase 2 and huntingtin. Because calmodulin regulates transglutaminase activity in erythrocytes, platelets, and the gizzard, we hypothesized that calmodulin increases cross-linking of huntingtin in the HD brain. We found that calmodulin colocalizes at the confocal level with transglutaminase 2 and with huntingtin in HD intranuclear inclusions. Calmodulin coimmunoprecipitates with transglutaminase 2 and huntingtin in cells transfected with myc-tagged N-terminal huntingtin fragments containing 148 polyglutamine repeats (htt-N63-148Q-myc) and transglutaminase 2 but not in cells transfected with myc-tagged N-terminal huntingtin fragments containing 18 polyglutamine repeats. Our previous studies demonstrated that transfection with both htt-N63-148Q-myc and transglutaminase 2 resulted in cross-linking of mutant huntingtin protein fragments and the formation of insoluble high-molecular-weight aggregates of huntingtin protein fragments. Transfection with transglutaminase 2 and htt-N63-148Q-myc followed by treatment of cells with N-(6-aminohexyl)-1-naphthalenesulfonamide, a calmodulin inhibitor, resulted in a decrease in cross-linked huntingtin. Inhibiting the interaction of calmodulin with transglutaminase and huntingtin protein could decrease cross-linking and diminish huntingtin aggregate formation in the HD brain.

Effect of tissue transglutaminase on the solubility of proteins containing expanded polyglutamine repeats

The expansion of a polyglutamine (polyQ) domain in neuronal proteins is the molecular genetic cause of at least eight neurodegenerative diseases. Proteins with a polyQ domain that is greater than 40 Q (Q40) residues form insoluble intranuclear and cytoplasmic inclusions. Expanded polyQ proteins self-associate by non-covalent interactions and become insoluble. They can also be covalently cross-linked by tissue transglutaminase (TTG), a calcium-dependent enzyme present in cells throughout the nervous system. However, it remains unclear whether TTG cross-linking directly contributes to the insolubility of the expanded polyQ proteins. Using an in vitro solubility assay, we found TTG cross-linked Q62 monomers into high molecular weight soluble complexes in a calcium-dependent reaction. Inhibition of TTG cross-linking by primary amine substrates including putrescine and biotinylated pentylamine antagonized TTG's ability to form soluble complexes. In contrast, primary amines (histamine and lysine) that were less effective inhibitors of TTG cross-linking did not inhibit Q62 from becoming insoluble. In summary, TTG can increase the solubility of expanded polyQ proteins by catalyzing intermolecular cross-links. This demonstrates directly that TTG will reduce the ability of expanded polyQ proteins from becoming insoluble. Furthermore, the effectiveness of a primary amine substrate at inhibiting formation of insoluble inclusions may be related to their ability to inhibit intermolecular cross-linking by TTG.

Prospects for the pharmacotherapy of amyotrophic lateral sclerosis : old strategies and new paradigms for the third millennium

Biomedical researchers interested in amyotrophic lateral sclerosis (ALS) must invoke newly developing technologies if we are to discover pharmaceutical treatments that will help a significant population of patients with the disease. The focus of ALS research over the last 10 years has been on reactive oxygen species (ROS) and glutamate excitotoxicity, resulting in several clinical trials and the launch of the only drug currently available for the treatment of ALS, riluzole. Unfortunately, the therapeutic benefits have been minimal, at best, and the prognosis for patients with ALS has not improved beyond very modest retardation of the disease course. By emphasising ROS and glutamate excitotoxicity, current ALS research has only partially been able to attenuate the rate of motor decline and neuronal loss associated with this illness. Clues to additional therapeutic potentialities will come from an increased understanding of the mode of cell death (apoptotic or other) and the pathways leading to neuronal demise. If death is apoptotic, inhibiting caspases may be useful. The regulatory modifications for cell death at the molecular level remain to be determined and exploited to prevent neuronal loss, although novel pathways have been recently elucidated that impact on protein aggregation and processing. Oxidative stress, seen in both familial and sporadic forms of ALS, may be only one post-translational mechanism likely to affect specific proteins essential for the health and stability of motor neurons. Protein cross-linking by transglutaminase paralleling that may lead to defects in proteasome function may also be a significant mechanism. The latest capabilities to screen protein changes in specific cells represent the kinds of advances needed to combat ALS in the third millennium.

Glutamate-induced increases in transglutaminase activity in primary cultures of astroglial cells

Glutamate exposure of astroglial cells caused ligand-gated channel receptor activation, associated with excitotoxic cell response. We investigated the effects of 24 h glutamate exposure on transglutaminase in astrocytes primary cultures at 7, 14, and 21 days in vitro (DIV). Increases in enzyme activity were observed as a function of cell differentiation stage in glutamate-treated cultures. These effects were significantly reduced when GYKI 52466, an AMPA/KA receptors inhibitor, was added to the culture medium prior to incubation with glutamate. Microscopy observation on transglutaminase-mediated, fluorescent dansylcadaverine incorporation in living cells was consistent with these results. Western blotting analysis with monoclonal antibody showed that glutamate also up-regulated tissue transglutaminase expression, which reached the highest values in 14 DIV cultures. Confocal laser scanning microscopy analysis of immunostained astroglial cells showed a mainly cytoplasmic localisation of the enzyme both in control and treated cultures; nevertheless, counterstaining with the nuclear dye acridine orange demonstrated the presence of tissue transglutaminase also into the nucleus of glutamate-exposed and 21 DIV cells. The increases in enzyme expression and localisation in the nucleus of glutamate-treated astroglial cells may be part of biochemical alterations induced by excitotoxic stimulus.

Glucose/mitochondria in neurological conditions

Impairments of glucose and mitochondrial function are important causes of brain dysfunction and therefore of brain disease. Abnormalities have been found in association with disease of the nervous system in most of the components of glucose/mitochondrial metabolism. In many, molecular genetic abnormalities have been defined. Brain glucose oxidation is abnormal in common diseases of the nervous system, including Alzheimer disease and other dementias, Parkinson disease, delirium, probably schizophrenia and other psychoses, and of course cerebrovascular disease. Defects in a single component and even a single mutation can be associated with different clinical phenotypes. The same clinical phenotype can result from different genotypes. The complex relationship between biological abnormality in brain glucose utilization and clinical disorder is similar to that in other disorders that have been intensively studied at the genetic level. Genes for components of the pathways of brain glucose oxidation are good candidate genes for disease of the brain. Preliminary data support the proposal that treatments to normalize abnormalities in brain glucose oxidation may benefit many patients with common brain diseases.

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.

Transglutaminase cross-links in intranuclear inclusions in Huntington disease

Cortical and striatal perinuclear cytoplasmic aggregates and intranuclear inclusions of mutant huntingtin are neuropathological hallmarks of Huntington disease (HD). Although the mechanisms involved in the formation of these aggregates are unclear, a recent hypothesis implicates cross-linking of mutant huntingtin protein into aggregates by transglutaminase. This study explores the hypothesis that transglutaminase catalyzes cross-linking of huntingtin into intranuclear inclusions. Using immunofluorescence and confocal microscopy we demonstrate 99% colocalization of transglutaminase-catalyzed epsilon-(gamma-glutamyl) lysine covalent cross-links with nuclear aggregates of huntingtin protein in the frontal cortex of postmortem HD brain tissue. Furthermore, the transglutaminase 2 isoform colocalizes with both huntingtin protein and epsilon-(gamma-glutamyl) lysine covalent cross-links in HD intranuclear inclusions. Transient transfection of N-terminally truncated huntingtin with an expanded glutamine domain (htt-N63-148Q-myc) with and without and transglutaminase 2 into HEK 293T cells resulted in an increase in cross-linked huntingtin in the insoluble formic acid-treated pellet in comparison to transfection of N-terminally truncated huntingtin with normal length glutamine domain (htt-N63-18Q-myc). Transfection with both htt-N63-148Q-myc and transglutaminase 2 resulted in high molecular weight huntingtin in the insoluble fraction. These data support the hypothesis that transglutaminase catalyzed cross-linking of mutant huntingtin is involved in the formation and/or stabilization of huntingtin protein aggregates in HD. Based on these and other studies, modulation of transglutaminase activity could be explored as a treatment for HD.

Structural characterization of transglutaminase-catalyzed cross-linking between glyceraldehyde 3-phosphate dehydrogenase and polyglutamine repeats

The accumulation of abnormal polyglutamine-containing protein aggregates within the cytosol and nuclei of affected neurons is a hallmark of the progressive neurodegenerative disorders caused by an elongated (CAG)(n) repeat in the genome. The polyglutamine domains are excellent substrates for the enzyme transglutaminase type 2 (tissue), resulting in the formation of cross-links with polypeptides containing lysyl groups. Enzymatic activity toward the Q(n) domains increases greatly upon lengthening of such Q(n) stretches (n > 40). Among the possible amine donors, the glycolytic enzyme glyceraldehyde-3-phosphate-dehydrogenase was shown to tightly bind several proteins involved in polyglutamine expansion diseases. Recently, the authors have shown that K191, K268, and K331, out of the 26 lysines present in glyceraldehyde-3-phosphate-dehydrogenase, are the reactive amine-donor sites forming cross-links with substance P, which bears the simplest Q(n) domain (n = 2). The present study reports that synthetic peptides of both pathological and nonpathological length (n = 43 and 17, respectively) form cross-links with the same K residues located in the C-terminal region of glyceraldehyde-3-phosphate-dehydrogenase. In addition, it is shown that extra K residues present in the C termini of glyceraldehyde-3-phosphate-dehydrogenase are susceptible to cross-linking in the presence of transglutaminase. The present results indicate a possible modulating effect of Q(n) stretches on tissue transglutaminase substrate specificity and mechanism of recognition.

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.

Transglutaminases: nature's biological glues

Transglutaminases (Tgases) are a widely distributed group of enzymes that catalyse the post-translational modification of proteins by the formation of isopeptide bonds. This occurs either through protein cross-linking via epsilon-(gamma-glutamyl)lysine bonds or through incorporation of primary amines at selected peptide-bound glutamine residues. The cross-linked products, often of high molecular mass, are highly resistant to mechanical challenge and proteolytic degradation, and their accumulation is found in a number of tissues and processes where such properties are important, including skin, hair, blood clotting and wound healing. However, deregulation of enzyme activity generally associated with major disruptions in cellular homoeostatic mechanisms has resulted in these enzymes contributing to a number of human diseases, including chronic neurodegeneration, neoplastic diseases, autoimmune diseases, diseases involving progressive tissue fibrosis and diseases related to the epidermis of the skin. In the present review we detail the structural and regulatory features important in mammalian Tgases, with particular focus on the ubiquitous type 2 tissue enzyme. Physiological roles and substrates are discussed with a view to increasing and understanding the pathogenesis of the diseases associated with transglutaminases. Moreover the ability of these enzymes to modify proteins and act as biological glues has not gone unnoticed by the commercial sector. As a consequence, we have included some of the present and future biotechnological applications of this increasingly important group of enzymes.

Selective colocalization of transglutaminase-like activity in ubiquitinated intranuclear inclusions of hereditary dentatorubral-pallidoluysian atrophy

To investigate the role of transglutaminase (TG) in the pathophysiology of dentatorubral-pallidoluysian atrophy (DRPLA), the distributions of ubiquitin-positive neuronal intranuclear inclusions (Ub-NII) and TG activity were studied in three patients with DRPLA and four disease controls. In the cerebellar granule cells of DRPLA, 2.5-4.9% of neurons had Ub-NII, and 7.5-9.8% of them were TG positive. In the frontal cortex; however, the ratio of neurons with Ub-NII was relatively low compared with those in the cerebellar cortex, and no Ub-NII was TG positive. There was no distinct difference in the ratio of neurons with Ub-NII and their TG positivity between the cases with homozygous or heterozygous DRPLA patients. The selective and good colocalization of Ub-NII and TG in the cerebellar granule cells may reveal a role of TG in the neurodegenerative process in DRPLA.

Injury-induced "switch" from GTP-regulated to novel GTP-independent isoform of tissue transglutaminase in the rat spinal cord

We recently found that alternative transcripts of tissue transglutaminase (tTG or TG2) were present in hippocampal brain regions of Alzheimer's disease (AD), but not in control, non-demented, age-matched brains. Since antecedent non-severe trauma has been implicated in AD and other neurodegenerative diseases, such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), we were interested in whether alternative transcripts might be detected in a model of neurotrauma, controlled-contusion spinal cord injury (SCI) in the rat. Implicated in diverse roles from growth and differentiation to apoptotic cell death, only bifunctional tTG, of the nine member TG family, has dual catalytic activities: guanine trinucleotide (GTP) hydrolyzing activity (GTPase), as well as protein cross-linking. These functions imply two physiological functions: programmed cell life and death. These may have profound roles in the nervous system since studies in cultured astrocytes found tTG short (S) mRNA transcripts induced by treatment with injury-related cytokines. In the developing rat spinal cord, tTG activity is concentrated in ventral horn alpha motoneurons, but neither studies of spinal cord tTG gene expression, nor evaluation of the GTP-regulated isoforms in tissues, have been reported. We now report increased tTG protein and gene expression occurring rapidly after SCI. In parallel, novel appearance of a second, short form transcript, in addition to the normal long (L) isoform, occurs by 8 h of injury. Up-regulation of tTG message and activity following neural injury. with appearance of a truncated GTP-unregulated S form, may represent new approaches to drug targets in neurotrauma.

Co-overexpression of folding modulators improves the solubility of the recombinant guinea pig liver transglutaminase expressed in Escherichia coli

Transglutaminases (EC 2.3.2.13) catalyze the formation of epsilon-(gamma-glutamyl)lysine cross-links and the substitution of primary amines for the gamma-carboxamide groups of protein bound glutamine residues, and are involved in many biological phenomena. Transglutaminase reactions are also applicable in applied enzymology. Here, we established an expression system of recombinant mammalian tissue-type transglutaminase with high productivity. Overexpression of guinea pig liver transglutaminase in Escherichia coli, using a plasmid pET21-d, mostly resulted in the accumulation of insoluble and inactive enzyme protein. By the expression culture at lower temperatures (25 and 18 degrees C), however, a fraction of the soluble and active enzyme protein slightly increased. Co-overexpression of a molecular chaperone system (DnaK-DnaJ-GrpE) and/or a folding catalyst (trigger factor) improved the solubility of the recombinant enzyme produced in E. coli cells. The specific activity, the affinity to the amine substrate, and the sensitivity to the calcium activation and GTP inhibition of the purified soluble recombinant enzyme were lower than those of the natural liver enzyme. These results indicated that co-overexpression of folding modulators tested improved the solubility of the overproduced recombinant mammalian tissue-type transglutaminase, but the catalytic properties of the soluble recombinant enzyme were not exactly the same as those of the natural enzyme.

Spectrophotometric assays for L-lysine alpha-oxidase and gamma-glutamylamine cyclotransferase

A new assay for l-lysine alpha-oxidase is described. In this assay, the oxidized product generated from l-lysine is reacted with semicarbazide to form alpha-keto-epsilon-aminocaproate semicarbazone. Formation of the alpha-keto acid semicarbazone is continuously monitored spectrophotometrically at 248 nm (epsilon 10,160 +/- 240 M(-1) cm(-1)). The method was adapted to provide a new assay for gamma-glutamylamine cyclotransferase. This enzyme catalyzes the conversion of many l-gamma-glutamylamines to 5-oxo-l-proline and free amine. A biologically important substrate is N(epsilon)-(gamma-l-glutamyl)-l-lysine, which is converted to 5-oxo-l-proline and l-lysine by the action of gamma-glutamylamine cyclotransferase. The l-lysine generated from N(epsilon)-(gamma-l-glutamyl)-l-lysine in an endpoint assay is converted to alpha-keto epsilon-aminocaproate semicarbazone in the presence of semicarbazide, excess l-lysine alpha-oxidase, and catalase. The methods were applied to the determination of gamma-glutamylamine cyclotransferase activity of partially purified preparations of the bovine kidney enzyme and to detect gamma-glutamylamine cyclotransferase activity in rat kidney and liver homogenates.

Expression in Escherichia coli and purification of hexahistidine-tagged human tissue transglutaminase

Recent evidence suggests that aberrant transglutaminase activity is associated with a wide variety of diseases. Tissue transglutaminase is the most widely distributed of the six well-characterized transglutaminases in humans. We describe a method for expressing hexahistidine-tagged human tissue transglutaminase in Escherichia coli BL21(DE3) using the pET-30 Ek/LIC expression vector. Purification of the expressed enzyme from suspensions of E. coli cells treated with CelLytic B Bacterial Cell Lysis/Extraction Reagent was accomplished by immobilized metal (Ni2+) affinity column chromatography. The procedure typically yields highly purified and highly active recombinant human tissue transglutaminase in about 1 day (about 0.6 mg/from a 1-liter culture).

Identification of tissue transglutaminase-reactive lysine residues in glyceraldehyde-3-phosphate dehydrogenase

Polyglutamine domains are excellent substrates for tissue transglutaminase resulting in the formation of cross-links with polypeptides containing lysyl residues. This finding suggests that tissue transglutaminase may play a role in the pathology of neurodegenerative diseases associated with polyglutamine expansion. The glycolytic enzyme GAPDH previously was shown to tightly bind several proteins involved in such diseases. The present study confirms that GAPDH is an in vitro lysyl donor substrate of tissue transglutaminase. A dansylated glutamine-containing peptide was used as probe for labeling the amino-donor sites. SDS gel electrophoresis of a time-course reaction mixture revealed the presence of both fluorescent GAPDH monomers and high molecular weight polymers. Western blot analysis performed using antitransglutaminase antibodies reveals that tissue transglutaminase takes part in the formation of heteropolymers. The reactive amino-donor sites were identified using mass spectrometry. Here, we report that of the 26 lysines present in GAPDH, K191, K268, and K331 were the only amino-donor residues modified by tissue transglutaminase.

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.

Protein crosslinking, tissue transglutaminase, alternative splicing and neurodegeneration

Increasing interest and awareness of protein aggregation as being implicated in neurodegenerative processes has developed in recent years. One novel mechanism for this may be transglutaminase (TGase)-mediated protein crosslinking, that is involved in a variety of natural processes ranging from the stabilization of fibrin clots to production of the epidermal cell envelope and the fluid barrier of the skin. TGases are also implicated in both function and dysfunction of the central (CNS) and peripheral (PNS) nervous systems. The most ubiquitously expressed member of the TGase family, known as tissue TGase (tTG) or TG2, which, in addition to catalyzing the production of epsilon-lysine to gamma-glutaminyl isodipeptide bonds, serves a dual function as the G-protein Galpha(h) and is both expressed and active in PNS and CNS. It differs from other members of the TGase gene family in this regard and may implicate it in 'switches' from life or trophic signaling to those associated with apoptosis. In this regard, recent data indicate that one or more TGases are involved in neurodegenerative disorders such as the Qn/CAG repeat disorders, as well as Alzheimer's and Parkinson's diseases. As do many genes, particularly those highly expressed in the CNS, tTG undergoes alternative processing. Elevated expression and alternative splicing, resulting in a short (S) isoform of tTG with more active crosslinking activity, are associated with increased neuronal loss in affected regions in the demented brain. Our recent and novel data indicate that tTG mRNA, protein, and TGase activity are elevated in certain neurodegenerative diseases, and are accompanied by transcription of this S splice variant that results in unregulated crosslinking, unique to neurodegenerative disorders.

Tissue transglutaminase and coeliac disease pathogenesis: potential molecular mechanisms for other human diseases

Coeliac disease (CD) is one of the most common food intolerances described in the western population. The main food agent that provokes the strong and diffuse clinical symptoms has been known for several years to be gliadin, a protein present in a very large number of human foods derived from vegetables. Only recently, some biochemical and immunological aspects of this very common disease have been clarified, and tissue transglutaminase, a multifunctional and ubiquitous enzyme, has been identified as one of the major factors. This enzyme, through its catalytic activities, produces the main biochemical and immunological effects observed in patients affected by this disease. The aim of this review is to summarize the most recent findings concerning the relationships between the biochemical properties of tissue transglutaminase and the basic molecular mechanisms responsible for CD. In addition, we present some frequent clinical associations of CD with other human diseases, with particular reference to neuropsychiatric disorders. Possible molecular links between CD, neuropsychiatric disorders and biochemical activities of transglutaminase enzymes are discussed.

N(epsilon)-(gamma-L-glutamyl)-L-lysine (GGEL) is increased in cerebrospinal fluid of patients with Huntington's disease

Pathological-length polyglutamine (Q(n)) expansions, such as those that occur in the huntingtin protein (htt) in Huntington's disease (HD), are excellent substrates for tissue transglutaminase in vitro, and transglutaminase activity is increased in post-mortem HD brain. However, direct evidence for the participation of tissue transglutaminase (or other transglutaminases) in HD patients in vivo is scarce. We now report that levels of N(epsilon)-(gamma-L-glutamyl)-L-lysine (GGEL)--a 'marker' isodipeptide produced by the transglutaminase reaction--are elevated in the CSF of HD patients (708 +/- 41 pmol/mL, SEM, n = 36) vs. control CSF (228 +/- 36, n = 27); p < 0.0001. These data support the hypothesis that transglutaminase activity is increased in HD brain in vivo.

Tissue-transglutaminase in rat and human brain: light and electron immunocytochemical analysis and in situ hybridization study

Tissue-type transglutaminases constitute a family of enzymes having a dual role. They catalyze the post-translational modification of proteins and play a role in signal transduction pathways, several isoforms have been cloned in the brain. Many in vitro experiments and post-mortem studies have claimed that the enzyme plays a central role in the development of neurodegenerative disorders, especially in CAG-triplet diseases. In the present investigation, we conducted an immunocytochemical study using two different antibodies raised against tissue-type transglutaminase. To confirm the enzyme expression, non-radioactive in situ hybridization was performed on adjacent sections. The study was completed by analyzing the ultrastructural localization of the enzyme by electron microscopy. Tissue-type transglutaminase was widely expressed in both the human and rat brain. Many positive cells exhibiting neuronal features were found in the brain and cerebellum. There was a preferential expression in elements of pyramidal and extrapyramidal pathways with less expression in the somatosensory system. The mRNA detection confirmed the distribution of the enzyme. The ultrastructural approach revealed the presence of the enzyme in all neuronal compartments. Light and electron microscopy studies showed the ubiquitous nature of the enzyme and its putative role in functional as well as putative pathological processes.

Tissue transglutaminase selectively modifies proteins associated with truncated mutant huntingtin in intact cells

The cause of Huntington's disease (HD) is a pathological expansion of the polyglutamine domain within the N-terminal region of huntingtin. Neuronal intranuclear inclusions and cytoplasmic aggregates composed of the mutant huntingtin within certain neuronal populations are a characteristic hallmark of HD. However, how the expanded polyglutamine repeats of mutant huntingtin cause HD is not known. Because in vitro expanded polyglutamine repeats are excellent glutaminyl-donor substrates of tissue transglutaminase (tTG), it has been hypothesized that tTG may contribute to the formation of these aggregates in HD. However, an association between huntingtin and tTG or modification of huntingtin by tTG has not been demonstrated in cells. To examine the interactions between tTG and huntingtin human neuroblastoma SH-SY5Y cells were stably transfected with full-length huntingtin containing 23 (FL-Q23) (wild type) or 82 (FL-Q82) (mutant) glutamine repeats or a truncated N-terminal huntingtin construct containing 23 (Q23) (wild type) or 62 (Q62) (mutant) glutamine repeats. Aggregates were rarely observed in the cells expressing full-length mutant huntingtin, and no specific colocalization of full-length huntingtin and tTG was observed. In contrast, in cells expressing truncated mutant huntingtin (Q62) there were numerous complexes of truncated mutant huntingtin and many of these complexes co-localized with tTG. However, the complexes were not insoluble structures. Further, truncated huntingtin coimmunoprecipitated with tTG, and this association increased when tTG was activated. Activation of tTG did not result in the modification of either truncated or full-length huntingtin, however proteins that were associated with truncated mutant huntingtin were selectively modified by tTG. This study is the first to demonstrate that tTG specifically interacts with a truncated form of huntingtin, and that activated tTG selectively modifies mutant huntingtin-associated proteins. These data suggest that proteolysis of full-length mutant huntingtin likely precedes its interaction with tTG and this process may facilitate the modification of huntingtin-associated proteins and thus contribute to the etiology of HD.

A Sensitive Fluorometric Assay for Tissue Transglutaminase

We have devised a highly sensitive fluorometric well plate assay for tissue transglutaminase that is suitable for multiple kinetic analyses/high-throughput screening of chemical inventories for inhibitors of this enzyme. The procedure measures the rate of fluorescence enhancement (lambda(exc) 260 nm, lambda(em) 538 nm) when 1-N-(carbobenzoxy-l-glutaminylglycyl)-5-N-(5'N'N'-dimethylaminonaphthalenesulfonyl)diamidopentane (glutaminyl substrate) is cross-linked to dansyl cadaverine (amine substrate). The assay procedure can be used to measure the activity of as little as 60 microU of purified guinea pig liver tissue transglutaminase (4.2 ng or 54 fmol of enzyme).

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

The cause of Huntington's disease (HD) is a pathological expansion of the polyglutamine domain within the NH(2)-terminal region of huntingtin. Neuronal intranuclear inclusions and cytoplasmic aggregates composed of the mutant huntingtin within certain neuronal populations are a characteristic hallmark of HD. Because in vitro expanded polyglutamine repeats are glutaminyl-donor substrates of tissue transglutaminase (tTG), it has been hypothesized that tTG may contribute to the formation of these aggregates in HD. Therefore, it is of fundamental importance to establish whether tTG plays a significant role in the formation of mutant huntingtin aggregates in the cell. Human neuroblastoma SH-SY5Y cells were stably transfected with truncated NH(2)-terminal huntingtin constructs containing 18 (wild type) or 82 (mutant) glutamines. In the cells expressing the mutant truncated huntingtin construct, numerous SDS-resistant aggregates were present in the cytoplasm and nucleus. Even though numerous aggregates were present in the mutant huntingtin-expressing cells, tTG did not coprecipitate with mutant truncated huntingtin. Further, tTG was totally excluded from the aggregates, and significantly increasing tTG expression had no effect on the number of aggregates or their intracellular localization (cytoplasm or nucleus). When a YFP-tagged mutant truncated huntingtin construct was transiently transfected into cells that express no detectable tTG due to stable transfection with a tTG antisense construct, there was extensive aggregate formation. These findings clearly demonstrate that tTG is not required for aggregate formation, and does not facilitate the process of aggregate formation. Therefore, in HD, as well as in other polyglutamine diseases, tTG is unlikely to play a role in the formation of aggregates.

Intron-exon swapping of transglutaminase mRNA and neuronal Tau aggregation in Alzheimer's disease

In order to understand the mechanism for insoluble neurotoxic protein polymerization in Alzheimer's disease (AD) brain neurons, we examined protein and gene expression for transglutaminase (TGase 2; tissue transglutaminase (tTG)) in hippocampus and isocortex. We found co-localization of tTG protein and activity with tau-positive neurofibrillary tangles, whereas mRNA and sequence analysis indicated an absolute increase in tTG synthesized. Although apoptosis in AD hippocampus is now an established mode of neuronal cell death, no definite underlying mechanism(s) is known. Since TGase-mediated protein aggregation is implicated in polyglutamine ((CAG)(n)/Q(n) expansion) disorder apoptosis, and expanded Q(n) repeats are excellent TGase substrates, a role for TGase in AD is possible. However, despite such suggestions almost 20 years ago, the molecular mechanism remained elusive. We now present one possible molecular mechanism for tTG-mediated, neurotoxic protein polymerization leading to neuronal apoptosis in AD that involves not its substrates (like Q(n) repeats) but rather the unique presence of alternative transcripts of tTG mRNA. In addition to a full-length (L) isoform in aged non-demented brains, we found a short isoform (S) lacking a binding domain in all AD brains. Our current results identify intron-exon "switching" between L and S isoforms, implicating G-protein-coupled signaling pathways associated with tTG that may help to determine the dual roles of this enzyme in neuronal life and death processes.

Impaired mitochondrial function results in increased tissue transglutaminase activity in situ

Tissue transglutaminase (tTG) is a transamidating enzyme that is elevated in Huntington's disease (HD) brain and may be involved in the etiology of the disease. Further, there is evidence of impaired mitochondrial function in HD. Therefore, in this study, we examined the effects of mitochondrial dysfunction on the transamidating activity of tTG. Neuroblastoma SH-SY5Y cells stably overexpressing human tTG or mutated inactive tTG were treated with 3-nitropropionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase. 3-NP treatment of tTG-expressing cells resulted in a significant increase of TG activity in situ. In vitro measurements demonstrated that 3-NP had no direct effect on tTG activity. However, 3-NP treatment resulted in a significant decrease of the levels of GTP and ATP, two potent inhibitors of the transamidating activity of tTG. No significant changes in the intracellular levels of calcium were observed in 3-NP-treated cells. Treatment with 3-NP in combination with antioxidants significantly reduced the 3-NP-induced increase in in situ TG activity, demonstrating that oxidative stress is a contributing factor to the increase of TG activity. This study demonstrates for the first time that impairment of mitochondrial function significantly increases TG activity in situ, a finding that may have important relevance to the etiology of HD.

Regulation of the dual function tissue transglutaminase/Galpha(h) during murine neuromuscular development: gene and enzyme isoform expression

Coagulation Factor XIII (F. VIII), a member of the transglutaminase (TGase) superfamily, is activated by thrombin, cross-links fibrin and stabilizes clots. Another member of this family, tissue TGase (tTG), having similar enzymatic activity, is implicated in neural development and synapse stabilization. Our previous studies indicated that synapse formation and maintenance at the neuromuscular junction (NMJ) involved components of the coagulation cascade in development. Others then showed that either F. XIII or tTG were localized at NMJs in a developmentally-regulated fashion. In the current studies, we addressed the temporal course of skeletal muscle tTG gene expression and found maximal expression at birth and continuing into the immediate postnatal period. Subcellular fractionation revealed a relatively constant particulate isoform of TGase activity which predominated in early embryonic muscle development. In contrast, cytosolic TGase specific activity became the major isoform in the postnatal period. The timing of muscle TGase activity correlated well with expression of tTG mRNA and we now present novel data of Tgm 2 gene expression for tTG in skeletal muscle. Confirming and extending the previous studies, TGase becomes localized at NMJs in the early, further ramifying in the late, neonatal period. These data suggest that the early pulse of particulate activity could coincide with the period of myoblast cell death in embryonic muscle. On the other hand, the peak cytosolic TGase activity occurs in the neonatal period, correlating temporally with muscle prothrombin expression during activity-dependent synapse elimination and possibly the source of the enzyme localized to the NMJ extracellular matrix resulting in synaptic stabilization.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Polyglutamine-mediated aggregation and cell death

The expansion of CAG repeats is the genetic defect underlying eight neurodegenerative diseases. A common feature of these disorders is the presence of intracellular aggregates in neuronal cells. It is still unclear the significance of these cellular inclusions in the neurodegenerative process, since cell death without aggregate formation has been reported. We have constructed a synthetic fusion protein containing 17 or 43 CAG repeats and the green fluorescent protein that recapitulates the features of CAG-expanded alleles. Expression of 43, but not 17 CAG repeats results in formation of nuclear aggregates in human neuroblastoma cells. Moreover, the normal allele (17 CAG) is sequestered in the inclusion bodies. The presence of nuclear inclusions tightly correlates with apoptosis in cells expressing the protein encoding 43 CAG repeats. Cells harboring nuclear aggregates stop proliferation and undergo apoptosis. Moreover, the inhibition of protein degradation pathway increases intracellular aggregates and cell death. These data indicate that intranuclear aggregates induce apoptosis and suggest that the degradation of unfolded proteins improves cell survival.

Bioenergetics in Huntington's disease

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder with relentless course and prototypical clinical symptoms. In 1993 HD was associated with an expanded CAG triplet repeat stretch on chromosome 4 in the coding region of its target protein, huntington. The length of the resulting polyglutamine++ extensions correlates with lower age of onset and a higher density of ubiquitin-positive neuronal intranuclear inclusions. Recently it has been proposed that mutant huntington induces progressive neuronal cell death by an apoptotic mechanism. There is strong evidence that disturbances in cellular energy homeostasis and oxidative damage contribute to neurodegeneration. This review will summarize and discuss the current concepts that point towards an involvement of free radical-induced oxidative stress, glutamate excitotoxicity and mitochondrial respiratory chain defects in pathogenesis of HD.

The alpha-ketoglutarate dehydrogenase complex

The alpha-ketoglutarate dehydrogenase complex (KGDHC) is an important mitochondrial constituent, and deficiency of KGDHC is associated with a number of neurological disorders. KGDHC is composed of three proteins, each encoded on a different and well-characterized gene. The sequences of the human proteins are known. The organization of the proteins into a large, ordered multienzyme complex (a "metabolon") has been well studied in prokaryotic and eukaryotic species. KGDHC catalyzes a critical step in the Krebs tricarboxylic acid cycle, which is also a step in the metabolism of the potentially excitotoxic neurotransmitter glutamate. A number of metabolites modify the activity of KGDHC, including inactivation by 4-hydroxynonenal and other reactive oxygen species (ROS). In human brain, the activity of KGDHC is lower than that of any other enzyme of energy metabolism, including phosphofructokinase, aconitase, and the electron transport complexes. Deficiencies of KGDHC are likely to impair brain energy metabolism and therefore brain function, and lead to manifestations of brain disease. In general, the clinical manifestations of KGDHC deficiency relate to the severity of the deficiency. Several such disorders have been recognized: infantile lactic acidosis, psychomotor retardation in childhood, intermittent neuropsychiatric disease with ataxia and other motor manifestations, Friedreich's and other spinocerebellar ataxias, Parkinson's disease, and Alzheimer's disease (AD). A KGDHC gene has been associated with the first two and last two of these disorders. KGDHC is not uniformly distributed in human brain, and the neurons that appear selectively vulnerable in human temporal cortex in AD are enriched in KGDHC. We hypothesize that variations in KGDHC that are not deleterious during reproductive life become deleterious with aging, perhaps by predisposing this mitochondrial metabolon to oxidative damage.