Tissue transglutaminase catalyzes the formation of alpha-synuclein crosslinks in Parkinson's disease

Effect of therapeutic mild hypothermia on the genomics of the hippocampus after moderate traumatic brain injury in rats

BACKGROUND

Traumatic brain injury (TBI), a major cause of morbidity and mortality, is a serious public health concern.

OBJECTIVE

To evaluate the effect of mild hypothermia on gene expression in the hippocampus and to try to elucidate molecular mechanisms of hypothermic neuroprotection after TBI.

METHODS

Rats were subjected to mild hypothermia (group 1: n = 3, 33 degrees C, 3H) or normothermia (group 2: n = 3; 37 degrees C, 3H) after TBI. Six genome arrays were applied to detect the gene expression profiles of ipsilateral hippocampus. Functional clustering and gene ontology analysis were then carried out. Another 20 rats were randomly assigned to 4 groups (n = 5 per group): group 3, sham-normothermia; group 4, sham-hypothermia; group 5, TBI-normothermia; and group 6, TBI-hypothermia. Real-time fluorescent quantitative reverse-transcription polymerase chain reaction was used to detect specific selected genes.

RESULTS

We found that 133 transcripts in the hypothermia group were statistically different from those in the normothermia group, including 57 transcripts that were upregulated and 76 that were downregulated after TBI (P < .01). Most of these genes were involved in various pathophysiological processes, and some were critical to cell survival. Analysis showed that 9 gene ontology categories were significantly affected by hypothermia, including the most affected categories: synapse organization and biogenesis (upregulated) and regulation of inflammatory response (downregulated). The mRNA expression of Ank3, Cmbp, Nrxn3, Tgm2, and Fcgr3 was regulated by hypothermia, TBI, or a combination of TBI and hypothermia compared with the sham-normothermia group. Their mRNA expression was significantly regulated by hypothermia in TBI groups.

CONCLUSION

Posttraumatic mild hypothermia has a significant effect on the gene expression profiles of the hippocampus, especially those genes belonging to the 9 gene ontology categories. Differential expression of those genes may be involved in the most fundamental molecular mechanisms of cerebral protection by mild hypothermia after TBI.

Serine 129 Phosphorylation of α-Synuclein Cross-Links with Tissue Transglutaminase to Form Lewy Body-Like Inclusion Bodies

Intraneuronal depositions of α-synuclein have been implicated in the pathogenesis of Parkinsons's disease (PD). Previous reports have identified the crosslinking between α-synuclein and tTG (tissue transglutaminase) in both PD patients and the cellular model. However, no researches have been conducted to further investigate their interaction in physiological conditions. To address this question, we generated the SH-SY5Y cell line which stably expressed the wild-type or mutant (Ser129Ala) α-synuclein. After the treatment with okadaic acid, α-synuclein started forming aggregates upon the activation of tTG. Coimmunoprecipitation assays revealed a decreased interaction of the mutant α-synuclein S129A with tTG compared with the wild-type α-synuclein. Cells expressing the wild-type α-synuclein showed increased eosinophilic cytoplasmic inclusion bodies that resembled Lewy bodies compared with the mutant. Double immunofluorescence staining confirmed the colocalization of the phosphorylated α-synuclein and the tTG in the cells. The S129A mutant demonstrated a lesser degree of colocalization than the wild type.

Anti-inflammatory effects of the R2 peptide, an inhibitor of transglutaminase 2, in a mouse model of allergic asthma, induced by ovalbumin

BACKGROUND AND PURPOSE

Transglutaminase 2 (TGase 2) expression is increased in inflammatory diseases, and TGase 2 inhibitors block these increases. We examined whether the R2 peptide inhibited the expression of TGase 2 in a mouse model of inflammatory allergic asthma.

EXPERIMENTAL APPROACH

C57BL/6 mice were sensitized and challenged by ovalbumin (OVA) to induce asthma. OVA-specific serum IgE and leukotrienes (LTs) levels were measured by enzyme-linked immunosorbent assay. Recruitment of inflammatory cells into bronchoalveolar lavage (BAL) fluid or lung tissues and goblet cell hyperplasia were assessed histologically. Airway hyperresponsiveness was determined in a barometric plethysmographic chamber. Expression of TGase 2, eosinophil major basic protein (EMBP), the adhesion molecule vascular cell adhesion molecule-1, Muc5ac and phospholipase A(2) (PLA(2) ) protein were determined by Western blot. Expression of mRNAs of Muc5ac, cytokines, matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs) were measured by reverse transcriptase-polymerase chain reaction and nuclear factor-κB (NF-κB) by electrophoretic mobility shift assay.

KEY RESULTS

R2 peptide reduced OVA-specific IgE levels; the number of total inflammatory cells, macrophages, neutrophils, lymphocytes and eosinophils in BAL fluid and the number of goblet cells. Airway hyperresponsiveness, TGase 2 and EMBP levels, mRNA levels of interleukin (IL)-4, IL-5, IL-6, IL-8, IL-13, RANTES, tumour necrosis factor-α, and MMP2/9, Muc5ac, NF-κB activity, PLA(2) activity and expressions, and LT levels in BAL cells and lung tissues were all reduced by R2 peptide. R2 peptide also restored expression of TIMP1/2.

CONCLUSION AND IMPLICATIONS

R2 peptide reduced allergic responses by regulating NF-κB/TGase 2 activity in a mouse model of allergic asthma. This peptide may be useful in the treatment of allergic asthma.

Presence of tissue transglutaminase in granular endoplasmic reticulum is characteristic of melanized neurons in Parkinson's disease brain

Parkinson's disease (PD) is characterized by the accumulation of α-synuclein aggregates and degeneration of melanized neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD brain, tTG-induced cross-links have been identified in α-synuclein monomers, oligomers and α-synuclein aggregates. However, whether tTG and α-synuclein occur together in PD affected neurons remains to be established. Interestingly, using immunohistochemistry, we observed a granular distribution pattern of tTG, characteristic of melanized neurons in PD brain. Apart from tTG, these granules were also positive for typical endoplasmic reticulum (ER)-resident chaperones, that is, protein disulphide isomerase, ERp57 and calreticulin, suggesting a direct link to the ER. Additionally, we observed the presence of phosphorylated pancreatic ER kinase (pPERK), a classical ER stress marker, in tTG granule positive neurons in PD brain, although no subcellular colocalization of tTG and pPERK was found. Our data therefore suggest that tTG localization to granular ER compartments is specific for stressed melanized neurons in PD brain. Moreover, as also α-synuclein aggregates were observed in tTG granule positive neurons, these results provide a clue to the cellular site of interaction between α-synuclein and tTG.

Tissue transglutaminase-mediated glutamine deamidation of beta-amyloid peptide increases peptide solubility, whereas enzymatic cross-linking and peptide fragmentation may serve as molecular triggers for rapid peptide aggregation

Tissue transglutaminase (TGase) has been implicated in a number of cellular processes and disease states, where the enzymatic actions of TGase may serve in both, cell survival and apoptosis. To date, the precise functional properties of TGase in cell survival or cell death mechanisms still remain elusive. TGase-mediated cross-linking has been reported to account for the formation of insoluble lesions in conformational diseases. We report here that TGase induces intramolecular cross-linking of β-amyloid peptide (Aβ), resulting in structural changes of monomeric Aβ. Using high resolution mass spectrometry (MS) of cross-linked Aβ peptides, we observed a shift in mass, which is, presumably associated with the loss of NH3 due to enzymatic transamidation activity and hence intramolecular peptide cross-linking. We have observed that a large population of Aβ monomers contained an 0.984 Da increase in mass at a glutamine residue, indicating that glutamine 15 serves as an indispensable substrate in TGase-mediated deamidation to glutamate 15. We provide strong analytical evidence on TGase-mediated Aβ peptide dimerization, through covalent intermolecular cross-linking and hence the formation of Aβ1-40 dimers. Our in depth analyses indicate that TGase-induced post-translational modifications of Aβ peptide may serve as an important seed for aggregation.

Diagnostic cerebrospinal fluid biomarkers for Parkinson's disease: a pathogenetically based approach

The inaccuracy of the early diagnosis of Parkinson's disease (PD) has been a major incentive for studies aimed at the identification of biomarkers. Brain-derived cerebrospinal fluid (CSF) proteins are potential biomarkers considering the major role that proteins play in PD pathogenesis. In this review, we discuss the current hypotheses about the pathogenesis of PD and identify the most promising candidate biomarkers among the CSF proteins studied so far. The list of potential markers includes proteins involved in various pathogenetic processes, such as oxidative stress and protein aggregation. This list will undoubtedly grow in the near future by application of CSF proteomics and subsequent validation of identified proteins. Probably a single biomarker will not suffice to reach high sensitivity and specificity, because PD is pathogenetically heterogeneous and shares etiological factors with other neurodegenerative diseases. Furthermore, identified candidate biomarkers will have to be thoroughly validated before they can be implemented as diagnostic aids.

Protein coding of neurodegenerative dementias: the neuropathological basis of biomarker diagnostics

Neuropathological diagnosis of neurodegenerative dementias evolved by adapting the results of neuroanatomy, biochemistry, and cellular and molecular biology. Milestone findings of intra- and extracellular argyrophilic structures, visualizing protein deposition, initiated a protein-based classification. Widespread application of immunohistochemical and biochemical investigations revealed that (1) there are modifications of proteins intrinsic to disease (species that are phosphorylated, nitrated, oligomers, proteinase-resistant, with or without amyloid characteristics; cleavage products), (2) disease forms characterized by the accumulation of a single protein only are rather the exception than the rule, and (3) some modifications of proteins elude present neuropathological diagnostic procedures. In this review, we summarize how neuropathology, together with biochemistry, contributes to disease typing, by demonstrating a spectrum of disorders characterized by the deposition of various modifications of various proteins in various locations. Neuropathology may help to elucidate how brain pathologies alter the detectability of proteins in body fluids by upregulation of physiological forms or entrapment of different proteins. Modifications of at least the five most relevant proteins (amyloid-beta, prion protein, tau, alpha-synuclein, and TDP-43), aided by analysis of further "attracted" proteins, are pivotal to be evaluated simultaneously with different methods. This should complement the detection of biomarkers associated with pathogenetic processes, and also neuroimaging and genetic analysis, in order to obtain a highly personalized diagnostic profile. Defining clusters of patients based on the patterns of protein deposition and immunohistochemically or biochemically detectable modifications of proteins ("codes") may have higher prognostic predictive value, may be useful for monitoring therapy, and may open new avenues for research on pathogenesis.

Critical role of transglutaminase and other stress proteins during neurodegenerative processes

Proteolytic stress, resulting from the intracellular accumulation of misfolded or aggregated proteins, which exceed the capacity of the ubiquitin-proteasome system to degrade them, plays a relevant role in neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, and Huntington's chorea. Most of toxic protein aggregates are characterised by the presence of isopeptide bonds (cross-links) catalysed by transglutaminase activity; further, several disease-specific proteins-tau, amyloid-beta, alpha-synuclein, huntingtin-are in vitro and/or in vivo substrates of transglutaminase 2. These findings suggest an important role for transglutaminase 2-mediated cross-linking reactions in neurodegeneration. Therefore, the use of transglutaminase activity inhibitors could ameliorate neuronal cell death. New therapeutic perspectives also arise from the possibility to prevent or reduce protein aggregation by enhancing the activation of heat shock proteins, which have been shown to be potent suppressors of neurodegeneration in cell cultures/animal models. Interestingly, some heat shock proteins have been shown to be in vitro or in vivo cross-linked by transglutaminase 2. These observations seem to suggest that transglutaminase activity could be involved in the stabilization of intracellular protein aggregates by interfering with proteasomal degradation of misfolded proteins. Further studies are needed to validate leading hypotheses and to open new prospects for developing therapeutic tools.

Transglutaminase-mediated intramolecular cross-linking of membrane-bound alpha-synuclein promotes amyloid formation in Lewy bodies

The alpha-synuclein immunopositive and chaotrope-insoluble material from human brains with Lewy body pathology was analyzed by mass spectrometry. From the proteinase K-cleavable peripheral fraction of Lewy bodies, which was densely cross-linked by gamma-glutamyl-epsilon-lysine bonds between HspB1 and ubiquitin in a pattern similar to neurofibrillary tangles (Nemes, Z., Devreese, B., Steinert, P. M., Van Beeumen, J., and Fésüs, L. (2004) FASEB J. 18, 1135-1137), 53 proteins were identified. In the core of Lewy bodies only alpha-synuclein was found, and it contained a low amount of intramolecular cross-links between Gln-99 and Lys-58. In vitro cross-linking of alpha-synuclein by transglutaminases 1-3 and 5 produced a heterogeneous population of variably cross-linked alpha-synucleins in solution, which inhibited the aggregation of the protein into amyloid. However, in the presence of phosphatidylserine-rich membranes and micromolar calcium concentrations, the cross-linking by transglutaminases 1, 2, and 5 showed specificity toward the utilization of Gln-99 and Lys-58. As shown by thioflavin T fluorescence monitoring, the formation of this cross-link accelerated the aggregation of native alpha-synuclein. Chemical cross-linking of residues 58-99 triggered amyloid formation, whereas such bonding of residues 99 to 10 was inhibitory. Our findings reveal the pivotal role of membrane attachment and transglutaminase-mediated intermolecular cross-linking for the propagative misfolding and aggregation of alpha-synuclein.

Transglutaminase activation in neurodegenerative diseases

The following review examines the role of calcium in promoting the in vitro and in vivo activation of transglutaminases in neurodegenerative disorders. Diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease exhibit increased transglutaminase activity and rises in intracellular calcium concentrations, which may be related. The aberrant activation of transglutaminase by calcium is thought to give rise to a variety of pathological moieties in these diseases, and the inhibition has been shown to have therapeutic benefit in animal and cellular models of neurodegeneration. Given the potential clinical relevance of transglutaminase inhibitors, we have also reviewed the recent development of such compounds.

Novel role of transglutaminase 1 in corpora amylacea formation?

Corpora amylacea (CA) are both age and neurodegeneration-related spherical bodies, consisting of polymerized proteins, often thought to be involved in sequestration of hazardous products of cellular metabolism in brain. Although CA formation is associated with cellular stress, the process underlying their formation remains obscure. Transglutaminases (TGs) are stress associated enzymes that induce molecular cross-links, leading to polymerization of substrate proteins. TG expression and activity are elevated in Alzheimer's disease (AD) and Parkinson's disease (PD), and TG-catalyzed cross-links are present in their lesions. Considering the nature of CA, the aim of this study was to investigate the presence of TGs and TG cross-links in CA of healthy aging brain, AD and PD brain, using immunohistochemistry. We observed TG1 and TG cross-links in CA, together with typical cytoskeletal proteins. Furthermore, the presence of proteins associated with AD or PD pathogenesis was not altered in CA of disease brain compared to controls. We propose that TG1-catalyzed cross-linking and consequent polymerization of cytoskeletal and cytoskeleton-associated proteins may underlie CA formation.

Transglutaminases and neurodegeneration

Transglutaminases (TGs) are Ca2+-dependent enzymes that catalyze a variety of modifications of glutaminyl (Q) residues. In the brain, these modifications include the covalent attachment of a number of amine-bearing compounds, including lysyl (K) residues and polyamines, which serve to either regulate enzyme activity or attach the TG substrates to biological matrices. Aberrant TG activity is thought to contribute to Alzheimer disease, Parkinson disease, Huntington disease, and supranuclear palsy. Strategies designed to interfere with TG activity have some benefit in animal models of Huntington and Parkinson diseases. The following review summarizes the involvement of TGs in neurodegenerative diseases and discusses the possible use of selective inhibitors as therapeutic agents in these diseases.

Physiological and pathological role of alpha-synuclein in Parkinson's disease through iron mediated oxidative stress; the role of a putative iron-responsive element

Parkinson’s disease (PD) is the second most common progressive neurodegenerative disorder after Alzheimer’s disease (AD) and represents a large health burden to society. Genetic and oxidative risk factors have been proposed as possible causes, but their relative contribution remains unclear. Dysfunction of alpha-synuclein (α-syn) has been associated with PD due to its increased presence, together with iron, in Lewy bodies. Brain oxidative damage caused by iron may be partly mediated by α-syn oligomerization during PD pathology. Also, α-syn gene dosage can cause familial PD and inhibition of its gene expression by blocking translation via a newly identified Iron Responsive Element-like RNA sequence in its 5’-untranslated region may provide a new PD drug target.

Neurochemical approaches in the laboratory diagnosis of Parkinson and Parkinson dementia syndromes: a review

The diagnosis of Parkinson disease (PD) is rendered on the basis of clinical parameters, whereby laboratory chemical tests or morphological imaging is only called upon to exclude other neurodegenerative diseases. The differentiation between PD and other diseases of the basal ganglia, especially the postsynaptic Parkinson syndromes multisystem atrophy (MSA) and progressive supranuclear palsy (PSP), is of decisive importance, on the one hand, for the response to an appropriate therapy, and on the other hand, for the respective prognosis of the disease. However, particularly at the onset of symptoms, it is difficult to precisely distinguish these diseases from each other, presenting with an akinetic‐rigid syndrome. It is not yet possible to conduct a neurochemical differentiation of Parkinson syndromes. Therefore, a reliable biomarker is still to be found that might predict the development of Parkinson dementia. Since this situation is currently the subject of various different studies, the following synopsis is intended to provide a brief summary of the investigations addressing the field of the early neurochemical differential diagnosis of Parkinson syndromes and the early diagnosis of Parkinson dementia, from direct α‐synuclein detection to proteomic approaches. In addition, an overview of the tested biomarkers will be given with regard to their possible introduction as a screening method.

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.

Carnosine and its possible roles in nutrition and health

The dipeptide carnosine has been observed to exert antiaging activity at cellular and whole animal levels. This review discusses the possible mechanisms by which carnosine may exert antiaging action and considers whether the dipeptide could be beneficial to humans. Carnosine's possible biological activities include scavenger of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chelator of zinc and copper ions, and antiglycating and anticross-linking activities. Carnosine's ability to react with deleterious aldehydes such as malondialdehyde, methylglyoxal, hydroxynonenal, and acetaldehyde may also contribute to its protective functions. Physiologically carnosine may help to suppress some secondary complications of diabetes, and the deleterious consequences of ischemic-reperfusion injury, most likely due to antioxidation and carbonyl-scavenging functions. Other, and much more speculative, possible functions of carnosine considered include transglutaminase inhibition, stimulation of proteolysis mediated via effects on proteasome activity or induction of protease and stress-protein gene expression, upregulation of corticosteroid synthesis, stimulation of protein repair, and effects on ADP-ribose metabolism associated with sirtuin and poly-ADP-ribose polymerase (PARP) activities. Evidence for carnosine's possible protective action against secondary diabetic complications, neurodegeneration, cancer, and other age-related pathologies is briefly discussed.

Identification of chemical inhibitors to human tissue transglutaminase by screening existing drug libraries

Human tissue transglutaminase (TGM2) is a calcium-dependent crosslinking enzyme involved in the posttranslational modification of intra- and extracellular proteins and implicated in several neurodegenerative diseases. To find specific inhibitors to TGM2, two structurally diverse chemical libraries (LOPAC and Prestwick) were screened. We found that ZM39923, a Janus kinase inhibitor, and its metabolite ZM449829 were the most potent inhibitors with IC(50) of 10 and 5 nM, respectively. In addition, two other inhibitors, including tyrphostin 47 and vitamin K(3), were found to have an IC(50) in the micromolar range. These agents used in part a thiol-dependent mechanism to inhibit TGM2, consistent with the activation of TGM2 by reduction of an intramolecular disulfide bond. These inhibitors were tested in a polyglutamine-expressing Drosophila model of neurodegeneration and found to improve survival. The TGM2 inhibitors we discovered may serve as valuable lead compounds for the development of orally active TGM2 inhibitors to treat human diseases.

Tissue transglutaminase activation modulates inflammation in cystic fibrosis via PPARgamma down-regulation

Cystic fibrosis (CF), the most common life-threatening inherited disease in Caucasians, is due to mutations in the CF transmembrane conductance regulator (CFTR) gene and is characterized by airways chronic inflammation and pulmonary infections. The inflammatory response is not secondary to the pulmonary infections. Indeed, several studies have shown an increased proinflammatory activity in the CF tissues, regardless of bacterial infections, because inflammation is similarly observed in CFTR-defective cell lines kept in sterile conditions. Despite recent studies that have indicated that CF airway epithelial cells can spontaneously initiate the inflammatory cascade, we still do not have a clear insight of the molecular mechanisms involved in this increased inflammatory response. In this study, to understand these mechanisms, we investigated ex vivo cultures of nasal polyp mucosal explants of CF patients and controls, CFTR-defective IB3-1 bronchial epithelial cells, C38 isogenic CFTR corrected, and 16HBE normal bronchial epithelial cell lines. We have shown that a defective CFTR induces a remarkable up-regulation of tissue transglutaminase (TG2) in both tissues and cell lines. The increased TG2 activity leads to functional sequestration of the anti-inflammatory peroxisome proliferator-activated receptor gamma and increase of the classic parameters of inflammation, such as TNF-alpha, tyrosine phosphorylation, and MAPKs. Specific inhibition of TG2 was able to reinstate normal levels of peroxisome proliferator-activated receptor-gamma and dampen down inflammation both in CF tissues and CFTR-defective cells. Our results highlight an unpredicted central role of TG2 in the mechanistic pathway of CF inflammation, also opening a possible new wave of therapies for sufferers of chronic inflammatory diseases.

Tissue transglutaminase modulates alpha-synuclein oligomerization

We have studied the interaction of the enzyme tissue transglutaminase (tTG), catalyzing cross-link formation between protein-bound glutamine residues and primary amines, with Parkinson's disease-associated alpha-synuclein protein variants at physiologically relevant concentrations. We have, for the first time, determined binding affinities of tTG for wild-type and mutant alpha-synucleins using surface plasmon resonance approaches, revealing high-affinity nanomolar equilibrium dissociation constants. Nanomolar tTG concentrations were sufficient for complete inhibition of fibrillization by effective alpha-synuclein cross-linking, resulting predominantly in intramolecularly cross-linked monomers accompanied by an oligomeric fraction. Since oligomeric species have a pathophysiological relevance we further investigated the properties of the tTG/alpha-synuclein oligomers. Atomic force microscopy revealed morphologically similar structures for oligomers from all alpha-synuclein variants; the extent of oligomer formation was found to correlate with tTG concentration. Unlike normal alpha-synuclein oligomers the resultant structures were extremely stable and resistant to GdnHCl and SDS. In contrast to normal beta-sheet-containing oligomers, the tTG/alpha-synuclein oligomers appear to be unstructured and are unable to disrupt phospholipid vesicles. These data suggest that tTG binds equally effective to wild-type and disease mutant alpha-synuclein variants. We propose that tTG cross-linking imposes structural constraints on alpha-synuclein, preventing the assembly of structured oligomers required for disruption of membranes and for progression into fibrils. In general, cross-linking of amyloid forming proteins by tTG may prevent the progression into pathogenic species.

Transglutaminase-catalyzed transamidation: a novel mechanism for Rac1 activation by 5-hydroxytryptamine2A receptor stimulation

Transglutaminase (TGase)-induced activation of small G proteins via 5-hydroxytryptamine (HT)(2A) receptor signaling leads to platelet aggregation (Cell 115:851-862, 2003). We hypothesize that stimulation of 5-HT(2A) receptors in neurons activates TGase, resulting in transamidation of serotonin to a small G protein, Rac1, thereby constitutively activating Rac1. Using immunoprecipitation and immunoblotting, we show that, in rat cortical cell line A1A1v, serotonin increases TGase-catalyzed transamidation of Rac1. This transamidation occurs in both undifferentiated and differentiated cells. Treatment with a 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine, but not the 5-HT(1A) receptor agonist 5-hydroxy-2-dipropylamino tetralin, increases transamidation of Rac1 by TGase. In A1A1v cells, 5-HT(2A) receptors mediate the transamidation reaction because expression of 5-HT(2C) receptors was not detectable and the selective 5-HT(2A) receptor antagonist blocked transamidation. Time course studies demonstrate that transamidation of Rac1 is significantly elevated after 5 and 15 min of serotonin treatment, but returns it to control levels after 30 min. The activity of Rac1 is also transiently increased following serotonin stimulation. Inhibition of TGase by cystamine or small interfering RNA reduces TGase modification of Rac1, and cystamine also prevents Rac1 activation. Serotonin itself is bound to Rac1 by TGase following 5-HT(2A) receptor stimulation as demonstrated by coimmunoprecipitation experiments and a dose-dependent decrease of serotonin-associated Rac1 by cystamine. These data support the hypothesis that Rac1 activity is transiently increased due to TGase-catalyzed transamidation of serotonin to Rac1 via stimulation of 5-HT(2A) receptors. Activation of Rac1 via TGase is a novel effector and second messenger of the 5-HT(2A) receptor-signaling cascade in neurons.

Tissue transglutaminase: a novel pharmacological target in preventing toxic protein aggregation in neurodegenerative diseases

Alzheimer's disease, Parkinson's disease and Huntington's disease are neurodegenerative diseases, characterized by the accumulation and deposition of neurotoxic protein aggregates. The capacity of specific proteins to self-interact and form neurotoxic aggregates seems to be a common underlying mechanism leading to pathology in these neurodegenerative diseases. This process might be initiated and/or accelerated by proteins that interact with these aggregating proteins. The transglutaminase (TG) family of proteins are calcium-dependent enzymes that catalyze the formation of covalent epsilon-(gamma-glutamyl)lysine isopeptide bonds, which can result in both intra- and intermolecular cross-links. Intramolecular cross-links might modify self-interacting proteins, and make them more prone to aggregate. In addition, intermolecular cross-links could link self-aggregating proteins and thereby initiate and/or stimulate the aggregation process. So far, increased levels and activity of tissue transglutaminase (tTG), the best characterized member of the TG family, have been observed in many neurodegenerative diseases, and the self-interacting proteins, characteristic of Alzheimer's disease, Parkinson's disease and Huntington's disease, are known substrates of tTG. Here, we focus on the role of tTG in the initiation of the aggregation process of self-interacting proteins in these diseases, and promote the notion that tTG might be an attractive novel target for treatment of neurodegenerative diseases.

Identification and characterization of a new alpha-synuclein isoform and its role in Lewy body diseases

Alternative splicing is an important mechanism to generate a large number of mRNAs, thus increasing proteome diversity and tissue specificity. Three transcript variants of alpha-synuclein, a neuronal protein mainly involved in synapses, have been described so far. Whereas alpha-synuclein 140 is the whole and main transcript, alpha-synuclein 112 and 126 are short proteins that result from in-frame deletions of exons 3 and 5, respectively. Because the aforesaid alpha-synuclein isoforms show differential expression changes in Lewy body diseases (LBDs), in the present work, we searched for a fourth alpha-synuclein isoform and studied its expression levels in LBD brains. By using isoform-specific primers, isoform co-amplification and direct sequencing, we identified alpha-synuclein 98, which lacks exons 3 and 5. mRNA expression analyses in non-neuronal tissue revealed that alpha-synuclein 98 is a brain-specific splice variant with varying expression levels in different areas of fetal and adult brain. Additionally, we studied alpha-synuclein 98 expression levels by real-time semi-quantitative RT-PCR in the frontal cortices of LBD patients and compared them with those of Alzheimer disease (AD) patients and control subjects. Overexpression of alpha-synuclein 98 in LBD and AD brains would indicate its specific involvement in the pathogenesis of these neurodegenerative disorders.

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.

Transglutaminase 2 regulates mallory body inclusion formation and injury-associated liver enlargement

BACKGROUND & AIMS

Mallory body (MB) inclusions are a characteristic feature of several liver disorders and share similarities with cytoplasmic inclusions observed in neural diseases and myopathies. MBs consist primarily of keratins 8 and 18 (K8/K18), require a K8-greater-than-K18 ratio for their formation, and contain glutamine-lysine cross-links generated by transglutaminase (TG). We hypothesized that protein transamidation is essential for MB formation.

METHODS

Because TG2 is the most abundant hepatocyte TG, we tested our hypothesis using TG2(-/-) and their wild-type counterpart mice fed 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), an established MB inducer. Keratin cross-linking was further examined using recombinant proteins or transgenic mice that overexpress K8 or K18.

RESULTS

TG2(-/-) livers have markedly reduced TG2 activity as compared with TG2(+/+) livers. The DDC-fed TG2(-/-) mice have dramatic decreases in MB formation and liver hypertrophy response as contrasted with DDC-fed TG2(+/+) mice. Despite similar hepatocellular damage, TG2(-/-) mice had more gallstones, jaundice, and ductal proliferation than wild-type mice. Inhibition of MB formation in TG2(-/-) mice was associated with marked attenuation of ubiquitination and K8-containing protein cross-linking. MB formation and resolution paralleled the generation then disappearance of cross-linked K8, respectively. K8 is a preferential TG2 substrate when compared to K18, as examined in vitro or in DDC-fed transgenic mice that overexpress K8 or K18.

CONCLUSIONS

We demonstrate an essential role for TG2 in determining injury-mediated liver enlargement and the necessity of K8 and TG2 for generating cross-linked keratins and MBs. The role of TG in inclusion formation might extend to nonkeratin intermediate filament protein-related diseases.

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.

Rifampicin protects PC12 cells against MPP+-induced apoptosis and inhibits the expression of an alpha-Synuclein multimer

The potential cytoprotective effects of the anti-leprosy antibiotic rifampicin were investigated in rat pheochromocytoma (PC12) cells prior to intoxication with 1-Methyl-4-phenyl pyridinium (MPP(+)). MPP(+) induced both apoptotic and necrotic cell death, and increased the expression of a 57 kDa species of alpha-Synuclein. This species of alpha-Synuclein is larger than the monomer, and is therefore an oligomer or an aggregated form of the protein. Rifampicin significantly increased survival of these catecholaminergic cells in a concentration-dependent manner. The expression of the higher molecular mass alpha-Synuclein was increased by MPP(+) exposure, and its expression was inversely related to cell survival in the rifampicin-treated cells. Importantly, rifampicin suppressed apoptosis almost completely, without shifting the death cascade to necrosis, which is a problem that has been reported with caspase inhibitors of apoptosis (Hartmann, A., Troadec, J.D., Hunot, S., Kikly, K., Faucheux, B.A., Mouatt-Prigent, A., Ruberg, M. Agid, Y., Hirsch, E.C., 2001. Caspase-8 is an effector in apoptotic death of dopaminergic neurons in Parkinson's disease, but pathway inhibition results in neuronal necrosis. J. Neurosci. 21, 2247-2255). These results suggest that rifampicin improves survival of catecholamine- and alpha-Synuclein-containing cells, which degenerate in Parkinson's disease (PD), and thus may be therapeutic in this disease.

Alpha-synuclein structure, posttranslational modification and alternative splicing as aggregation enhancers

Alpha-synuclein aggregation is thought to be a key event in the pathogenesis of synucleinopathies. Although different alpha-synuclein alterations and modifications have been proposed to be responsible for early aggregation steps, the mechanisms underlying these events remain unclarified. Alpha-synuclein is a small protein localized to synaptic terminals and its intrinsic structure has been claimed to be an important factor for self-oligomerization and self-aggregation. Alpha-synuclein expression studies in cell cultures have demonstrated that posttranslational modifications, such as phosphorylation, oxidation, and sumoylation, are primarily involved in alpha-synuclein aggregation. Furthermore, in the last few years accumulating evidence has pointed to alternative splicing as a crucial mechanism in the development of neurodegenerative disorders. At least three different alpha-synuclein isoforms have been described as products of alternative splicing. Two of these isoforms (alpha-synuclein 112 and alpha-synuclein 126) are shorter proteins with probably altered functions and aggregation propensity. The present review attempts to summarize the data so far available on alpha-synuclein structure, posttranslational modifications, and alternative splicing as possible enhancers of aggregation.

Changed distribution pattern of the constitutive rather than the inducible HSP70 chaperone in neuromelanin-containing neurones of the Parkinsonian midbrain

Aberrant protein aggregation has been recognized as an important factor in the degeneration of melanized dopaminergic neurones in Parkinson's disease (PD). The constitutive (HSP73) and (heat)-inducible (HSP72) proteins of the heat shock 70 family form a major defence system against pathological protein aggregation. However, the distribution patterns of these chaperones in nigral neuromelanin-laden neurones are largely unknown. The present study determined the distribution of HSP72 and HSP73 in control and Parkinsonian substantia nigra, using immunohistochemistry. In the neuromelanin-laden neurones of controls, HSP72 was nondetectable, whereas HSP73 was weakly expressed in both the cytosol and the nucleus. Surprisingly, in PD subjects, marked nuclear HSP73, but not HSP72 immunoreactivity was observed, while cytosolic immunoreactivity of the two chaperones resembled the labelling pattern observed in controls. Furthermore, HSP73 immunoreactivity was observed in a subset of the Lewy bodies (LBs) detected in the substantia nigra of PD subjects, whereas only few of these LBs were labelled with HSP72. Interestingly, HSP72 and to a lesser extent HSP73 immunoreactivity was much stronger in nonmelanized neurones as compared with melanized neurones in this area. Thus, we conclude that the distribution pattern of HSP73 rather than HSP72 is changed in the nigral neuromelanin-laden neurones of PD subjects as compared with control subjects. The impaired ability of aged, dopaminergic neurones to express high levels of chaperones, may contribute to the preferential vulnerability of the latter cells in PD.

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

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

Lewy bodies

Lewy bodies (LB) in the substantia nigra are a cardinal pathological feature of Parkinson's disease, but they occur in a number of neurodegenerative diseases and can be widespread in the nervous system. The characteristics, locations, and composition of LB are reviewed, with particular attention to alpha-synuclein (alpha-SYN), which appears to be the major component of LB. The propensity for alpha-SYN, a presynaptic protein widely expressed in the brain, to aggregate is because of an amyloidogenic central region. The factors that favor the aggregation of alpha-SYN and mechanisms of toxicity are examined, and a mechanism through which aggregates of alpha-SYN could induce mitochondrial dysfunction and/or release of proapoptotic molecules is proposed.

Tau protein is cross-linked by transglutaminase in P301L tau transgenic mice

The microtubule-associated protein tau is highly soluble under physiological conditions. However, in tauopathies, tau protein aggregates into insoluble filaments and neurofibrillary tangles (NFTs). The mechanisms underlying the formation of tau filaments and NFTs in tauopathies remain unclear. Several lines of evidence suggest that transglutaminase may cross-link tau into stable, insoluble aggregates, leading to the formation of NFTs in Alzheimer's disease and progressive supranuclear palsy. To further determine the contribution of transglutaminase in the formation of NFTs, we compared the levels of cross-linked tau protein from P301L tau transgenic mice that develop NFTs to four-repeat wild-type (4RWT) tau transgenic and nontransgenic mice that do not develop NFT pathology. Immunoprecipitation and immunoblotting experiments show that transglutaminase cross-links phosphorylated tau in the hindbrain of P301L tau transgenic mice but not in mice overexpressing 4RWT tau and nontransgenic mice. Cross-linked, phosphorylated tau from P301L tau transgenic mice runs as high-molecular mass aggregates on Western blots, similar to cross-linked tau from paired helical filaments of Alzheimer's disease. We also used double-label immunofluorescence to demonstrate colocalization of PHF-1-immunoreactive tau and the transglutaminase-catalyzed cross-link in the hindbrain, spinal cord, and cortex of P301L tau transgenic mice. In the spinal cord, 87% of PHF-1-labeled cells colocalize with the transglutaminase-catalyzed cross-link. Additionally, transglutaminase enzymatic activity is significantly elevated in the spinal cord of P301L tau transgenic mice. These studies further implicate transglutaminase in the formation and/or stabilization of NFT and paired helical filaments and provide a model system to investigate the therapeutic potential of transglutaminase inhibitors in tauopathies.

The role of α-synuclein in neurodegenerative diseases

Alpha-synuclein is a 140 amino acid neuronal protein that has been associated with several neurodegenerative diseases. A point mutation in the gene coding for the alpha-synuclein protein was the first discovery linking this protein to a rare familial form of Parkinson's disease (PD). Subsequently, other mutations in the alpha-synuclein gene have been identified in familial PD. The aggregated proteinaceous inclusions called Lewy bodies found in PD and cortical Lewy body dementia (LBD) were discovered to be predominantly alpha-synuclein. Aberrant aggregation of alpha-synuclein has been detected in an increasing number of neurodegenerative diseases, collectively known as synucleopathies. Alpha-synuclein exists physiologically in both soluble and membrane-bound states, in unstructured and alpha-helical conformations, respectively. The physiological function of alpha-synuclein appears to require its translocation between these subcellular compartments and interconversion between the 2 conformations. Abnormal processing of alpha-synuclein is predicted to lead to pathological changes in its binding properties and function. In this review, genetic and environmental risk factors for alpha-synuclein pathology are described. Various mechanisms for in vitro and in vivo alpha-synuclein aggregation and neurotoxicity are summarized, and their relevance to neuropathology is explored.

Transglutaminase 2 Induces Nuclear Factor-κB Activation via a Novel Pathway in BV-2 Microglia

Transglutaminase 2 (TGase 2) expression is increased in inflammatory diseases. We demonstrated previously that inhibitors of TGase 2 reduce nitric oxide (NO) generation in a lipopolysaccharide (LPS)-treated microglial cell line. However, the precise mechanism by which TGase 2 promotes inflammation remains unclear. We found that TGase 2 activates the transcriptional activator nuclear factor (NF)-kappaB and thereby enhances LPS-induced expression of inducible nitric-oxide synthase. TGase 2 activates NF-kappaB via a novel pathway. Rather than stimulating phosphorylation and degradation of the inhibitory subunit alpha of NF-kappaB (I-kappaBalpha), TGase2 induces its polymerization. This polymerization results in dissociation of NF-kappaB and its translocation to the nucleus, where it is capable of up-regulating a host of inflammatory genes, including inducible nitric-oxide synthase and tumor necrosis factor alpha (TNF-alpha). Indeed, TGase inhibitors prevent depletion of monomeric I-kappaBalpha in the cytosol of cells overexpressing TGase 2. In an LPS-induced rat brain injury model, TGase inhibitors significantly reduced TNF-alpha synthesis. The findings are consistent with a model in which LPS-induced NF-kappaB activation is the result of phosphorylation of I-kappaBalpha by I-kappaB kinase as well as I-kappaBalpha polymerization by TGase 2. Safe and stable TGase2 inhibitors may be effective agents in diseases associated with inflammation.

Tissue transglutaminase is not involved in the aggregate formation of stably expressed α-synuclein in sh-sy5y human neuroblastoma cells

Intraneuronal deposition containing alpha-synuclein is implicated in the pathogenesis of synuclein-opathies including Parkinsons disease (PD). Although it has been demonstrated that cytoplasmic inclusions of wild type alpha-synuclein are observed in the brain of PD patients and that alpha-synuclein mutations such as A30P and A53T accelerate aggregate formation, the exact mechanism by which alpha-synuclein forms insoluble aggregates is still controversial. In the present study, to understand the possible involvement of tissue transglutaminase (tTG) in aggregate formation of alpha-synuclein, SH-SY5Y cell lines stably expressing wild type or mutant (A30P or A53T) alpha-synuclein were created and aggregate formation of alpha-synuclein was observed upon activation of tTG. The data demonstrated that alpha-synuclein negligibly interacted with tTG and that activation of tTG did not result in the aggregate formation of alpha-synuclein in SH-SY5Y cells overexpressing either wild type or mutant alpha-synuclein. In addition, alpha-synuclein was not modified by activated tTG in situ. These data suggest that tTG is unlikely to be a contributing factor to the formation of aggregates of alpha-synuclein in a stable cell model.