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

Ablation of polyamine catabolic enzymes provokes Purkinje cell damage, neuroinflammation, and severe ataxia

BACKGROUND

Polyamine catabolism plays a key role in maintaining intracellular polyamine pools, yet its physiological significance is largely unexplored. Here, we report that the disruption of polyamine catabolism leads to severe cerebellar damage and ataxia, demonstrating the fundamental role of polyamine catabolism in the maintenance of cerebellar function and integrity.

METHODS

Mice with simultaneous deletion of the two principal polyamine catabolic enzymes, spermine oxidase and spermidine/spermine N1-acetyltransferase (Smox/Sat1-dKO), were generated by the crossbreeding of Smox-KO (Smox-/-) and Sat1-KO (Sat1-/-) animals. Development and progression of tissue injury was monitored using imaging, behavioral, and molecular analyses.

RESULTS

Smox/Sat1-dKO mice are normal at birth, but develop progressive cerebellar damage and ataxia. The cerebellar injury in Smox/Sat1-dKO mice is associated with Purkinje cell loss and gliosis, leading to neuroinflammation and white matter demyelination during the latter stages of the injury. The onset of tissue damage in Smox/Sat1-dKO mice is not solely dependent on changes in polyamine levels as cerebellar injury was highly selective. RNA-seq analysis and confirmatory studies revealed clear decreases in the expression of Purkinje cell-associated proteins and significant increases in the expression of transglutaminases and markers of neurodegenerative microgliosis and astrocytosis. Further, the α-Synuclein expression, aggregation, and polyamination levels were significantly increased in the cerebellum of Smox/Sat1-dKO mice. Finally, there were clear roles of transglutaminase-2 (TGM2) in the cerebellar pathologies manifest in Smox/Sat1-dKO mice, as pharmacological inhibition of transglutaminases reduced the severity of ataxia and cerebellar injury in Smox/Sat1-dKO mice.

CONCLUSIONS

These results indicate that the disruption of polyamine catabolism, via coordinated alterations in tissue polyamine levels, elevated transglutaminase activity and increased expression, polyamination, and aggregation of α-Synuclein, leads to severe cerebellar damage and ataxia. These studies indicate that polyamine catabolism is necessary to Purkinje cell survival, and for sustaining the functional integrity of the cerebellum.

The role of glia in stress: polyamines and brain disorders

This review focuses on the roles of glia and polyamines (PAs) in brain function and dysfunction, highlighting how PAs are one of the principal differences between glia and neurons. The novel role of PAs, such as putrescine, spermidine, and spermine and their precursors and derivatives, is discussed. However, PAs have not yet been a focus of much glial research. They affect many neuronal and glial receptors, channels, and transporters. They are therefore key elements in the development of many diseases and syndromes, thus forming the rationale for PA-focused and glia-focused therapy for these conditions.

The function of tissue transglutaminase in celiac disease

Celiac disease is a chronic small bowel disorder caused by an abnormal immune response to an array of epitopes of wheat gluten and related proteins of rye and barley in genetically susceptible individuals who express the HLA-DQ2/-DQ8 haplotype. Gluten peptides are efficiently presented by celiac disease-specific HLA-DQ2- and HLA-DQ8-positive antigen presenting cells to CD4(+) T-cells that, once activated, drive a T helper cell type 1 response leading to the development of the typical celiac lesion-villous atrophy, crypt hyperplasia and intraepithelial and lamina propria infiltration of inflammatory cells. Tissue transglutaminase (tTG) is a calcium dependent ubiquitous enzyme which catalyses posttranslational modification of proteins and is released from cells during inflammation. tTG is suggested to exert at least two crucial roles in celiac disease: as a deamidating enzyme, that can enhance the immunostimulatory effect of gluten, and as a target autoantigen in the immune response. Since glutamine-rich gliadin peptides are excellent substrates for tTG, and the resulting deamidated and thus negatively charged peptides have much higher affinity for the HLA-DQ2 and HLA-DQ8 molecules, the action of tTG is believed to be a key step in the pathogenesis of celiac disease. This review is focused on the function of tTG in celiac disease, although it also deals with novel advances in tTG-based therapies.

α-Synuclein aggregation and modulating factors

Aggregated a-synuclein is the major component of inclusions in Parkinson's disease and other synucleinopathy brains indicating that a-syn aggregation is associated with the pathogenesis of neurodegenerative disorders. Although the mechanisms underlying a-syn aggregation and toxicity are not fully elucidated, it is clear that a-syn undergoes post-translational modifications and interacts with numerous proteins and other macromolecules, metals, hormones, neurotransmitters, drugs and poisons that can all modulate its aggregation propensity. The current and most recent findings regarding the factors modulating a-syn aggregation process are discussed in detail.

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.

Appearance of tissue transglutaminase in astrocytes in multiple sclerosis lesions: a role in cell adhesion and migration?

Multiple Sclerosis (MS) is a neuroinflammatory disease mainly affecting young adults. A major pathological hallmark of MS is the presence of demyelinated lesions in the central nervous system. In the active phase of the disease, astrocytes become activated, migrate and contribute to local tissue remodeling that ultimately can result in an astroglial scar. This process is facilitated by extracellular matrix proteins, including fibronectin. Tissue Transglutaminase (TG2) is a multifunctional enzyme with a ubiquitous tissue distribution and it has been shown that inflammatory cytokines can induce TG2 activity. In addition, TG2 is known to mediate cell adhesion and migration. We therefore hypothesized that TG2 is present in MS lesions and plays a role in cell adhesion and/or migration. Our studies showed that TG2 immunoreactivity appeared in astrocytes in active and chronic active MS lesions. These TG2 positive astrocytes partly co-localized with fibronectin. Additional in vitro studies showed that TG2 mediated astrocytoma adhesion to and migration on the extracellular matrix protein fibronectin. We therefore speculate that TG2 mediates the enhanced interaction of astrocytes with fibronectin in the extracellular matrix of MS lesions, thereby contributing to astrocyte adhesion and migration, and thus in tissue remodeling and possibly glial scarring.

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

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

Involvement of mitochondrial complex II defects in neuronal death produced by N-terminus fragment of mutated huntingtin

Alterations of mitochondrial function may play a central role in neuronal death in Huntington's disease (HD). However, the molecular mechanisms underlying such functional deficits of mitochondria are not elucidated yet. We herein showed that the expression of two important constituents of mitochondrial complex II, the 30-kDa iron-sulfur (Ip) subunit and the 70-kDa FAD (Fp) subunit, was preferentially decreased in the striatum of HD patients compared with controls. We also examined several mitochondrial proteins in striatal neurons that were infected with lentiviral vectors coding for the N-terminus part of huntingtin (Htt) with either a pathological (Htt171-82Q) or physiological (Htt171-19Q) polyglutamine tract. Compared with Htt171-19Q, expression of Htt171-82Q preferentially decreased the levels of Ip and Fp subunits and affected the dehydrogenase activity of the complex. The Htt171-82Q-induced preferential loss of complex II was not associated with a decrease in mRNA levels, suggesting the involvement of a posttranscriptional mechanism. Importantly, the overexpression of either Ip or Fp subunit restored complex II levels and blocked mitochondrial dysfunction and striatal cell death induced by Htt171-82Q in striatal neurons. The present results strongly suggest that complex II defects in HD may be instrumental in striatal cell death.

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

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

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

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

Identification of combinatorial drug regimens for treatment of Huntington's disease using Drosophila

We explore the hypothesis that pathology of Huntington's disease involves multiple cellular mechanisms whose contributions to disease are incrementally additive or synergistic. We provide evidence that the photoreceptor neuron degeneration seen in flies expressing mutant human huntingtin correlates with widespread degenerative events in the Drosophila CNS. We use a Drosophila Huntington's disease model to establish dose regimens and protocols to assess the effectiveness of drug combinations used at low threshold concentrations. These proof of principle studies identify at least two potential combinatorial treatment options and illustrate a rapid and cost-effective paradigm for testing and optimizing combinatorial drug therapies while reducing side effects for patients with neurodegenerative disease. The potential for using prescreening in Drosophila to inform combinatorial therapies that are most likely to be effective for testing in mammals is discussed.

Neostriatal glutamatergic system is involved in the pathogenesis of picrotoxin-induced choreomyoclonic hyperkinesis

Administration of dizocilpine (MK-801, noncompetitive antagonist of NMDA glutamate receptors) into the neostriatum decreased the reproducibility and duration of hyperkinesis in rats induced by repeated microinjections of GABA(A) receptor antagonist picrotoxin. By contrast, glutamate potentiated the hyperkinetic and convulsive effect of picrotoxin and promoted the inhibition of conditioned avoidance response. Our results indicate that the striatal glutamatergic system is involved in the development of locomotor and cognitive disorders associated with deficiency of the neostriatal GABAergic system and playing a role in the pathogenesis of Huntington's chorea.

Autosomal dominant cerebellar ataxias: clinical features, genetics, and pathogenesis

Autosomal dominant cerebellar ataxias are hereditary neurodegenerative disorders that are known as spinocerebellar ataxias (SCA) in genetic nomenclature. In the pregenomic era, ataxias were some of the most poorly understood neurological disorders; the unravelling of their molecular basis enabled precise diagnosis in vivo and explained many clinical phenomena such as anticipation and variable phenotypes even within one family. However, the discovery of many ataxia genes and loci in the past decade threatens to cause more confusion than optimism among clinicians. Therefore, the provision of guidance for genetic testing according to clinical findings and frequencies of SCA subtypes in different ethnic groups is a major challenge. The identification of ataxia genes raises hope that essential pathogenetic mechanisms causing SCA will become more and more apparent. Elucidation of the pathogenesis of SCA hopefully will enable the development of rational therapies for this group of disorders, which currently can only be treated symptomatically.

Intrathecal synthesis of autoantibodies against tissue transglutaminase

Anti-tissue transglutaminase (tTG) antibodies (AtTGA) are typically found in serum of patients with untreated coeliac disease (CD). tTG catalyses crosslinking of peptides an activity supposed to be important in neurological disorders. tTG occurs in cerebrospinal fluid (CSF) and its assay in CSF was suggested to be diagnostically useful. However, nothing is known about AtTGA in CSF. Therefore, in 129 unselected CSF-serum pairs IgA- and IgG-AtTGA were assayed by ELISA using human recombinant tTG. For comparison, IgA- and IgG-anti-gliadin antibodies (AGA), typically coexisting with AtTGA were measured. Albumin, total IgA and IgG and further parameters were determined according to routine programme recommended by the European CSF consensus group. AtTGA were detected in 27 (IgA) and in 63 (IgG) CSF samples. Antibody indices (AI) could be calculated for AtTGA from 21 (IgA) and from 61 (IgG) sample pairs. AI for AtTGA was >2 in 11 (IgA) and in 22 (IgG) sample pairs, hinting to intrathecal antibody synthesis. AI for AGA was >2 only for 1 (IgA) and 2 (IgG) sample pairs. Patients with normal routine findings had significantly higher AI for IgA-AtTGA than patients with abnormal findings. This is the first demonstration of AtTGA in CSF and their intrathecal synthesis. The pathogenetic relevance of this new autoantibody species remains to be clarified.

Immunoblot analysis reveals that isopeptide antibodies do not specifically recognize the epsilon-(gamma-glutamyl)lysine bonds formed by transglutaminase activity

Transglutaminases (TGs) posttranslationally modify proteins by transamidation of specific polypeptide bond glutamines. This reaction results in deamination, polyamine incorporation or the formation of isopeptide bonds. Transglutaminase activity in the brain is increased in several neurodegenerative diseases. Because insoluble inclusions occur in these neurodegenerative diseases, it has been hypothesized that transglutaminase contributes to the formation of the inclusions by catalyzing the formation of isopeptide bonds resulting in crosslinked, insoluble protein aggregates. To demonstrate a role for transglutaminase in the formation of these inclusions, the primary approach has been to show increased immunoreactivity with antibodies that recognize the isopeptide bonds. However, the specificity of these antibodies for isopeptide crosslinks within or between proteins has not been clearly established. In this report we demonstrate that the two most commonly used isopeptide antibodies do not specifically recognize the isopeptide bonds formed by transglutaminase when they are within or between proteins.

Can flies help humans treat neurodegenerative diseases?

Neurodegenerative diseases are becoming increasingly common as life expectancy increases. Recent years have seen tremendous progress in the identification of genes that cause these diseases. While mutations have been found and cellular processes defined that are altered in the disease state, the identification of treatments and cures has proven more elusive. The process of finding drugs and therapies to treat human diseases can be slow, expensive and frustrating. Can model organisms such as Drosophila speed the process of finding cures and treatments for human neurodegenerative diseases? We pose three questions, (1) can one mimic the essential features of human diseases in an organism like Drosophila, (2) can one cure a model organisms of human disease and (3) will these efforts accelerate the identification of useful therapies for testing in mice and ultimately humans? Here we focus on the use of Drosophila to identify potential treatments for neurodegenerative diseases such as Huntington's and we discuss how well these therapies translate into mammalian systems.

Pathogenesis of polyglutamine disorders: aggregation revisited

Expansion of CAG trinucleotide repeats coding for polyglutamine in unrelated proteins causes at least nine late-onset progressive neurodegenerative disorders, including Huntington's disease and a number of spinocerebellar ataxias. Expanded polyglutamine provokes a dominant gain-of-function neurotoxicity, regardless of the specific protein context within which it resides. Nevertheless, the protein context does modulate polyglutamine toxicity, as evidenced by the distinct clinical and pathological features of the various disorders. Importantly, polyglutamine toxicity might derive from its ability to aggregate. Indeed, aggregation probably underlies some defining attributes of the polyglutamine disorders, such as their late onset, progressive nature, and the dependence of onset age on polyglutamine length. However, the central role of aggregation in polyglutamine pathogenesis has been challenged by several studies, which instead argued that the soluble form of the disease proteins is responsible for neuronal damage. Thus, the question whether polyglutamine aggregates are deleterious, harmless or protective remains the most passionately disputed issue in the study of these diseases. In this review, we attempt to reconcile some of these controversies.

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.