Transglutaminase activity is related to CAG repeat length in patients with Huntington's disease

The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

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.

SAR Development of Lysine-Based Irreversible Inhibitors of Transglutaminase 2 for Huntington's Disease

We report a series of irreversible transglutaminase 2 inhibitors starting from a known lysine dipeptide bearing an acrylamide warhead. We established new SARs resulting in compounds demonstrating improved potency and better physical and calculated properties. Transglutaminase selectivity profiling and in vitro ADME properties of selected compounds are also reported.

Irreversible 4-Aminopiperidine Transglutaminase 2 Inhibitors for Huntington's Disease

A new series of potent TG2 inhibitors are reported that employ a 4-aminopiperidine core bearing an acrylamide warhead. We establish the structure-activity relationship of this new series and report on the transglutaminase selectivity and in vitro ADME properties of selected compounds. We demonstrate that the compounds do not conjugate glutathione in an in vitro setting and have superior plasma stability over our previous series.

Discovery and structure-activity relationship of potent and selective covalent inhibitors of transglutaminase 2 for Huntington's disease

Tissue transglutaminase 2 (TG2) is a multifunctional protein primarily known for its calcium-dependent enzymatic protein cross-linking activity via isopeptide bond formation between glutamine and lysine residues. TG2 overexpression and activity have been found to be associated with Huntington's disease (HD); specifically, TG2 is up-regulated in the brains of HD patients and in animal models of the disease. Interestingly, genetic deletion of TG2 in two different HD mouse models, R6/1 and R6/2, results in improved phenotypes including a reduction in neuronal death and prolonged survival. Starting with phenylacrylamide screening hit 7d, we describe the SAR of this series leading to potent and selective TG2 inhibitors. The suitability of the compounds as in vitro tools to elucidate the biology of TG2 was demonstrated through mode of inhibition studies, characterization of druglike properties, and inhibition profiles in a cell lysate assay.

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

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

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.

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.

Therapeutic Strategies in Huntington's Disease

This article provides an overview of the therapeutic strategies, from ordinary classical drugs to the modern molecular strategy at experimental level, for Huntington's disease. The disease is characterized by choreic movements, psychiatric disorders, striatal atrophy with selective small neuronal loss, and autosomal dominant inheritance. The genetic abnormality is CAG expansion in huntingtin gene. Mutant huntingtin with abnormally long glutamine stretch aggregates and forms intranuclear inclusions. In this review, I summarize the results of previous trials from the following aspects; 1. symptomatic/palliative therapies including drugs, stereotaxic surgery and repetitive transcranial magnetic stimulation, 2. anti-degenerative therapies including anti-excitotoxicity, reversal of mitochondrial dysfunction and anti-apoptosis, 3. restorative/reparative therapies including neural trophic factors and tissue or stem cell transplantation, and 4. molecular targets in specific and radical therapies including inhibition of truncation of huntingtin, inhibition of aggregate formation, normalization of transcriptional dysregulation, enhancement of autophagic clearance of mutant huntingtin, and specific inhibition of huntingtin expression by sRNAi. Although the strategies mentioned in the latter two categories are mostly at laboratory level at present, we are pleased that one can discuss such "therapeutic strategies", a matter absolutely impossible before the causal gene of Huntington's disease was identified more than 10 years ago. It is also true, however, that some of the "therapeutic strategies" mentioned here would be found difficult to implement and abandoned in the future.

A Rapid Transglutaminase Assay for High-Throughput Screening Applications

Transglutaminases (TGs) are widely distributed enzymes that catalyze posttranslational modification of proteins by Ca2+-dependent cross-linking reactions. The family members of TGs participate in many significant processes of biological functions such as tissue regeneration, cell differentiation, apoptosis, and certain pathologies. A novel technique for TG activity assay was developed in this study. It was based on the rapid capturing, fluorescence quenching, and fast separation of the unreacted fluorescent molecules from the macromolecular product with magnetic dextran-coated charcoal. As few as 3 ng of guinea pig liver transglutaminase (gpTG) could be detected by the method; activities of 96 TG samples could be measured within an hour. The Km of gpTG determined by this method for monodansylcadaverine (dansyl-CAD) and N, N-dimethylcasein was 14 and 5 μM, respectively. A typical competitive inhibition pattern of cystamine on dansyl-CAD for gpTG activity was also demonstrated. The application of this technique is not limited to the use of dansyl-CAD as the fluorescent substrate of TG; other small fluor-labeled TG substrates may substitute dansyl-CAD. Finally, this method is rapid, highly sensitive, and inexpensive. It is suitable not only for high-throughput screening of enzymes or enzyme inhibitors but also for enzyme kinetic analysis.

Selection and slippage creating serine homopolymers

Highly repetitive sequence within proteins is an abundant feature yet is considered by some to be the protein equivalent of "junk DNA." Homopolymer sequences, the most highly repetitive of this group, are typically encoded by trinucleotide repeats at the DNA level. It is thought that many of these sequences are produced by a replicative slippage mechanism. Recent studies suggest that these highly mutable regions within proteins may allow for rapid morphological evolution emerging from the increased variability afforded by such coding structures. However, in a homopolymer, it is difficult to determine if the repeated amino acid is due to slippage at the DNA level or due to selection at the protein level. Here we develop and test a model to detect cases for which the homopolymer tract has clearly been selected for, with no evidence of slippage at the DNA level. The polyserine tract within the phosphatidylserine receptor protein is used as an excellent example of one such case.

Cystamine and cysteamine prevent 3-NP-induced mitochondrial depolarization of Huntington's disease knock-in striatal cells

Abstract Cystamine significantly improved motor deficits and extended survival in mouse models of Huntington's disease (HD); however, the precise mechanism(s) by which cystamine and the related compound cysteamine are beneficial remain to be elucidated. Using clonal striatal cell lines from wild-type (STHdhQ7/HdhQ7) and mutant huntingtin knock-in (STHdhQ111/HdhQ111) mice, we have tested the hypothesis that cystamine and cysteamine could be beneficial by preventing the depolarization of mitochondria in cell cultures. Treatment with 3-nitroproprionic acid (3-NP), a mitochondrial complex II inhibitor, induces mitochondrial depolarization and cell death of mutant HD striatal cells but not of wild-type cells. The 3-NP-mediated decrease in the mitochondrial membrane potential was attenuated by 50 microm cystamine and completely inhibited by 250 microm cystamine. Similar results were obtained using cysteamine (50-500 microm). In addition, both cystamine and cysteamine significantly attenuated the 3-NP-induced cell death. Treatment of mutant HD striatal cells with 3-NP resulted in a robust decrease in the cellular and mitochondrial levels of glutathione (GSH) compared with cells exposed to the vehicle alone. Pre-treatment of the cells with cystamine and cysteamine completely prevented the 3-NP-mediated decrease in cellular and mitochondrial GSH levels. Incubation with L-buthionine (S,R) sulfoximine (BSO) 250 microm in combination with cystamine (250 microm) or cysteamine (250 microm) prior to being treated with 3-NP completely prevented the beneficial effects of cystamine and cysteamine on the 3-NP-mediated mitochondrial depolarization. These results demonstrate that cystamine and cysteamine prevent the 3-NP-induced mitochondrial depolarization of HD striatal cell cultures.

Mitochondrial Aconitase is a Transglutaminase 2 Substrate: Transglutamination is a Probable Mechanism Contributing to High-Molecular-Weight Aggregates of Aconitase and Loss of Aconitase Activity in Huntington Disease Brain

Transglutaminase activity was found to be present in highly purified non-synaptosomal rat brain mitochondria. A 78-kDa protein in these organelles was shown to be a transglutaminase 2 substrate, and incubation of a non-synaptosomal mitochondrial lysate with transglutaminase 2 yielded high-Mr proteins. The 78-kDa protein was identified as mitochondrial aconitase by MALDI-TOF analysis. Aconitase activity was decreased in a dose-dependent manner when non-synaptosomal rat brain mitochondria were incubated with transglutaminase 2. Transglutaminase activity is increased about 2-fold in the mitochondrial fraction of HD caudate. Moreover, Western blotting of the mitochondrial fraction revealed that most of the mitochondrial aconitase in HD caudate is present as high-Mr aggregates. Aconitase activity was previously shown to be decreased in Huntington disease (HD) caudate (a region severely damaged by the disease). The present findings suggest that an increase of transglutaminase activity in HD caudate may contribute to mitochondrial dysfunction by incorporating aconitase into inactive polymers.

Neurological proteins are not enriched for repetitive sequences

Proteins associated with disease and development of the nervous system are thought to contain repetitive, simple sequences. However, genome-wide surveys for simple sequences within proteins have revealed that repetitive peptide sequences are the most frequent shared peptide segments among eukaryotic proteins, including those of Saccharomyces cerevisiae, which has few to no specialized developmental and neurological proteins. It is therefore of interest to determine if these specialized proteins have an excess of simple sequences when compared to other sets of compositionally similar proteins. We have determined the relative abundance of simple sequences within neurological proteins and find no excess of repetitive simple sequence within this class. In fact, polyglutamine repeats that are associated with many neurodegenerative diseases are no more abundant within neurological specialized proteins than within nonneurological collections of proteins. We also examined the codon composition of serine homopolymers to determine what forces may play a role in the evolution of extended homopolymers. Codon type homogeneity tends to be favored, suggesting replicative slippage instead of selection as the main force responsible for producing these homopolymers.

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.

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.

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.

Therapeutic effects of cystamine in a murine model of Huntington's disease

The precise cause of neuronal death in Huntington's disease (HD) is unknown. Proteolytic products of the huntingtin protein can contribute to toxic cellular aggregates that may be formed in part by tissue transglutaminase (Tgase). Tgase activity is increased in HD brain. Treatment in R6/2 transgenic HD mice, using the transglutaminase inhibitor cystamine, significantly extended survival, improved body weight and motor performance, and delayed the neuropathological sequela. Tgase activity and N(Sigma)-(gamma-L-glutamyl)-L-lysine (GGEL) levels were significantly altered in HD mice. Free GGEL, a specific biochemical marker of Tgase activity, was markedly elevated in the neocortex and caudate nucleus in HD patients. Both Tgase and GGEL immunoreactivities colocalized to huntingtin aggregates. Cystamine treatment normalized transglutaminase and GGEL levels in R6/2 mice. These findings are consistent with the hypothesis that transglutaminase activity may play a role in the pathogenesis of HD, and they identify cystamine as a potential therapeutic strategy for treating HD patients.

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.

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.

Of mice and men: solving the molecular mysteries of Huntington's disease

Recent advances in the manipulation of mouse embryos provide opportunities for the disciplines of neuroscience and molecular genetics to join forces and tackle some previously intractable questions in this area of research. Even Huntington's disease has started to yield clues to its complex pathophysiology as a result of the recent application of transgenic technologies. This short review, while necessarily providing some background clinical information on Huntington's disease, will focus on how modifications of the mouse genome have contributed, and are continuing to contribute, to our understanding of the complex disease process. Such new insights may well turn the hope of developing the first effective treatment for this devastating disease into reality.

The length of polyglutamine tract, its level of expression, the rate of degradation, and the transglutaminase activity influence the formation of intracellular aggregates

A common feature of CAG-expansion neurodegenerative diseases is the presence of intranuclear aggregates in neuronal cells. We have used a synthetic fusion protein containing at the NH2 terminus the influenza hemoagglutinin epitope (HA), a polyglutamine stretch (polyQ) of various size (17, 36, 43 CAG) and a COOH tail encoding the green fluorescent protein (GFP). The fusion proteins were expressed in COS-7 and neuroblastoma SK-N-BE cells. We found that the formation of aggregates largely depends on the length of polyglutamine tracts and on the levels of expression of the fusion protein. Moreover, transglutaminase overexpression caused an increase of insoluble aggregates only in cells expressing the mutant expanded protein. Conversely, treatment of cells with cystamine, a transglutaminase inhibitor, reduced the percentage of aggregates. We found also that the inhibition of the proteasome ubiquitin-dependent degradation increased the formation of intranuclear aggregates. These data suggest that length of polyglutamine tract, its expression, unbalance between cellular transglutaminase activity, and the ubiquitin-degradation pathway are key factors in the formation of intranuclear aggregates.

Transglutaminase aggregates huntingtin into nonamyloidogenic polymers, and its enzymatic activity increases in Huntington's disease brain nuclei

The protein huntingtin (htt), aggregated in neuronal nuclear inclusions, is pathognomonic of Huntington's disease (HD). Constructs, translated in vitro from the N terminus of htt, containing either polyQ23 from a normal individual, or polyQ41 or polyQ67 from an HD patient, were all soluble. Transglutaminase (TGase) crosslinked these proteins, and the aggregations did not have the staining properties of amyloid. More TGase-catalyzed aggregates formed when the polyglutamine domain of htt exceeded the pathologic threshold of polyQ36. Furthermore, shorter htt constructs, containing 135 aa or fewer, formed more aggregates than did larger htt constructs. TGase activity in the HD brain was increased compared with the control, with notable increases in cell nuclei. The increased TGase activity was brain specific. In lymphoblastoid cells from HD patients, TGase activity was decreased. TGase-mediated crosslinking of htt may be involved in the formation of the nonamyloidogenic nuclear inclusions found in the HD brain. The staining properties of nuclear inclusions in the HD brain revealed that they were not amyloid.

Recent advances on the pathogenesis of Huntington's disease

We review recent advances regarding the pathogenesis of Huntington's disease (HD). This genetic neurodegenerative disorder is caused by an expanded CAG repeat in a gene coding for a protein, with unknown function, called huntingtin. There is selective death of striatal and cortical neurons. Both in patients and a transgenic mouse model of the disease, neuronal intranuclear inclusions, immunoreactive for huntingtin and ubiquitin, develop. Huntingtin interacts with the proteins GAPDH, HAP-1, HIP1, HIP2, and calmodulin, and a mutant huntingtin is specifically cleaved by the proapoptotic enzyme caspase 3. The pathogenetic mechanism is not known, but it is presumed that there is a toxic gain of function of the mutant huntingtin. Circumstantial evidence suggests that excitotoxicity, oxidative stress, impaired energy metabolism, and apoptosis play a role.

Huntington disease: clinical, genetic, and social aspects

Huntington disease (HD) is a fascinating neurodegenerative disorder whose features straddle the boundaries of psychiatry, neurology, and genetics. The clinical symptoms of HD consist of a triad of motor, cognitive, and psychiatric/behavioral disturbances. In 1993, the HD Collaborative Research Group identified the gene and the mutation responsible for HD. HD was one of the first neurodegenerative disorders discovered to be caused by a novel mutational mechanism known as trinucleotide repeat expansion. Since then, HD has been the model for autosomal dominant neurogenetic disorders. The clinical, pathological, and genetic aspects of the disease are reviewed and some of the questions that remain to be answered by researchers of the 21st century are outlined.