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

Posttranslational Modifications of -Synuclein, Their Therapeutic Potential, and Crosstalk in Health and Neurodegenerative Diseases

α-Synuclein (α-Syn) aggregation in Lewy bodies and Lewy neurites has emerged as a key pathogenetic feature in Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Various factors, including posttranslational modifications (PTMs), can influence the propensity of α-Syn to misfold and aggregate. PTMs are biochemical modifications of a protein that occur during or after translation and are typically mediated by enzymes. PTMs modulate several characteristics of proteins including their structure, activity, localization, and stability. α-Syn undergoes various posttranslational modifications, including phosphorylation, ubiquitination, SUMOylation, acetylation, glycation, O-GlcNAcylation, nitration, oxidation, polyamination, arginylation, and truncation. Different PTMs of a protein can physically interact with one another or work together to influence a particular physiological or pathological feature in a process known as PTMs crosstalk. The development of detection techniques for the cooccurrence of PTMs in recent years has uncovered previously unappreciated mechanisms of their crosstalk. This has led to the emergence of evidence supporting an association between α-Syn PTMs crosstalk and synucleinopathies. In this review, we provide a comprehensive evaluation of α-Syn PTMs, their impact on misfolding and pathogenicity, the pharmacological means of targeting them, and their potential as biomarkers of disease. We also highlight the importance of the crosstalk between these PTMs in α-Syn function and aggregation. Insight into these PTMS and the complexities of their crosstalk can improve our understanding of the pathogenesis of synucleinopathies and identify novel targets of therapeutic potential. SIGNIFICANCE STATEMENT: α-Synuclein is a key pathogenic protein in Parkinson's disease and other synucleinopathies, making it a leading therapeutic target for disease modification. Multiple posttranslational modifications occur at various sites in α-Synuclein and alter its biophysical and pathological properties, some interacting with one another to add to the complexity of the pathogenicity of this protein. This review details these modifications, their implications in disease, and potential therapeutic opportunities.

Biological Implications and Functional Significance of Transglutaminase Type 2 in Nervous System Tumors

Transglutaminase type 2 (TG2) is the most ubiquitously expressed member of the transglutaminase family. TG2 catalyzes the transamidation reaction leading to several protein post-translational modifications and it is also implicated in signal transduction thanks to its GTP binding/hydrolyzing activity. In the nervous system, TG2 regulates multiple physiological processes, such as development, neuronal cell death and differentiation, and synaptic plasticity. Given its different enzymatic activities, aberrant expression or activity of TG2 can contribute to tumorigenesis, including in peripheral and central nervous system tumors. Indeed, TG2 dysregulation has been reported in meningiomas, medulloblastomas, neuroblastomas, glioblastomas, and other adult-type diffuse gliomas. The aim of this review is to provide an overview of the biological and functional relevance of TG2 in the pathogenesis of nervous system tumors, highlighting its involvement in survival, tumor inflammation, differentiation, and in the resistance to standard therapies.

Pathogenetic Contributions and Therapeutic Implications of Transglutaminase 2 in Neurodegenerative Diseases

Neurodegenerative diseases encompass a heterogeneous group of disorders that afflict millions of people worldwide. Characteristic protein aggregates are histopathological hallmark features of these disorders, including Amyloid β (Aβ)-containing plaques and tau-containing neurofibrillary tangles in Alzheimer's disease, α-Synuclein (α-Syn)-containing Lewy bodies and Lewy neurites in Parkinson's disease and dementia with Lewy bodies, and mutant huntingtin (mHTT) in nuclear inclusions in Huntington's disease. These various aggregates are found in specific brain regions that are impacted by neurodegeneration and associated with clinical manifestations. Transglutaminase (TG2) (also known as tissue transglutaminase) is the most ubiquitously expressed member of the transglutaminase family with protein crosslinking activity. To date, Aβ, tau, α-Syn, and mHTT have been determined to be substrates of TG2, leading to their aggregation and implicating the involvement of TG2 in several pathophysiological events in neurodegenerative disorders. In this review, we summarize the biochemistry and physiologic functions of TG2 and describe recent advances in the pathogenetic role of TG2 in these diseases. We also review TG2 inhibitors tested in clinical trials and discuss recent TG2-targeting approaches, which offer new perspectives for the design of future highly potent and selective drugs with improved brain delivery as a disease-modifying treatment for neurodegenerative disorders.

A Mouse Model to Test Novel Therapeutics for Parkinson's Disease: an Update on the Thy1-aSyn ("line 61") Mice

Development of neuroprotective therapeutics for Parkinson's disease (PD) is facing a lack of translation from pre-clinical to clinical trials. One strategy for improvement is to increase predictive validity of pre-clinical studies by using extensively characterized animal models with a comprehensive set of validated pharmacodynamic readouts. Mice over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter (Thy1-aSyn line 61) reproduce key features of sporadic PD, such as progressive loss of striatal dopamine, alpha-synuclein pathology, deficits in motor and non-motor functions, and elevation of inflammatory markers. Extensive work with this model by multiple laboratories over the past decade further increased confidence in its robustness and validity, especially for analyzing pathomechanisms of alpha-synuclein pathology and down-stream pathways, and for pre-clinical drug testing. Interestingly, while postnatal transgene expression is widespread in central and peripheral neurons, the extent and progression of down-stream pathology differs between brain regions, thereby replicating the characteristic selective vulnerability of neurodegenerative diseases. In-depth characterization of these readouts in conjunction with behavioral deficits has led to more informative endpoints for pre-clinical trials. Each drug tested in Thy1-aSyn line 61 enhances knowledge on how molecular targets, pathology, and functional behavioral readouts are interconnected, thereby further optimizing the platform towards predictive validity for clinical trials. Here, we present the current state of the art using Thy1-aSyn line 61 for drug target discovery, validation, and pre-clinical testing.

Extracellular transglutaminase-2, nude or associated with astrocytic extracellular vesicles, modulates neuronal calcium homeostasis

We have uncovered a novel role for astrocytes-derived extracellular vesicles (EVs) in controlling intraneuronal Ca²⁺ concentration ([Ca²⁺]_i) and identified transglutaminase-2 (TG2) as a surface-cargo of astrocytes-derived EVs. Incubation of hippocampal neurons with primed astrocyte-derived EVs have led to an increase in [Ca²⁺]_i, unlike EVs from TG2-knockout astrocytes. Exposure of neurons or brain slices to extracellular TG2 promoted a [Ca²⁺]_i rise, which was reversible upon TG2 removal and was dependent on Ca²⁺ influx through the plasma membrane. Patch-clamp and calcium imaging recordings revealed TG2-dependent neuronal membrane depolarization and activation of inward currents, due to the Na⁺/Ca²⁺-exchanger (NCX) operating in the reverse mode and indirect activation of L-type VOCCs, as indicated by VOCCs/NCX pharmacological inhibitors. A subunit of Na⁺/K⁺-ATPase was selected by comparative proteomics and identified as being functionally inhibited by extracellular TG2, implicating Na⁺/K⁺-ATPase inhibition in NCX reverse mode-switching leading to Ca²⁺ influx and higher basal [Ca²⁺]_i. These data suggest that reactive astrocytes control intraneuronal [Ca²⁺]_i through release of EVs with TG2 as responsible cargo, which could have a significant impact on synaptic activity in brain inflammation.

Absence of tissue transglutaminase reduces amyloid-beta pathology in APP23 mice

Aims

Alzheimer's disease (AD) is characterised by amyloid‐beta (Aβ) aggregates in the brain. Targeting Aβ aggregates is a major approach for AD therapies, although attempts have had little to no success so far. A novel treatment option is to focus on blocking the actual formation of Aβ multimers. The enzyme tissue transglutaminase (TG2) is abundantly expressed in the human brain and plays a key role in post‐translational modifications in Aβ resulting in covalently cross‐linked, stable and neurotoxic Aβ oligomers.

In vivo absence of TG2 in the APP23 mouse model may provide evidence that TG2 plays a key role in development and/or progression of Aβ‐related pathology.

Methods

Here, we compared the effects on Aβ pathology in the presence or absence of TG2 using 12‐month‐old wild type, APP23 and a crossbreed of the TG2−/− mouse model and APP23 mice (APP23/TG2−/−).

Results

Using immunohistochemistry, we found that the number of Aβ deposits was significantly reduced in the absence of TG2 compared with age‐matched APP23 mice. To pinpoint possible TG2‐associated mechanisms involved in this observation, we analysed soluble brain Aβ_1–40, Aβ_1–42 and/or Aβ_40/42 ratio, and mRNA levels of human APP and TG2 family members present in brain of the various mouse models. In addition, using immunohistochemistry, both beta‐pleated sheet formation in Aβ deposits and the presence of reactive astrocytes associated with Aβ deposits were analysed.

Conclusions

We found that absence of TG2 reduces the formation of Aβ pathology in the APP23 mouse model, suggesting that TG2 may be a suitable therapeutic target for reducing Aβ deposition in AD.

Therapeutics in the Pipeline Targeting α-Synuclein for Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder and the fastest growing neurologic disease in the world, yet no disease-modifying therapy is available for this disabling condition. Multiple lines of evidence implicate the protein α-synuclein (α-Syn) in the pathogenesis of PD, and as such, there is intense interest in targeting α-Syn for potential disease modification. α-Syn is also a key pathogenic protein in other synucleionpathies, most commonly dementia with Lewy bodies. Thus, therapeutics targeting this protein will have utility in these disorders as well. Here we discuss the various approaches that are being investigated to prevent and mitigate α-Syn toxicity in PD, including clearing its pathologic aggregates from the brain using immunization strategies, inhibiting its misfolding and aggregation, reducing its expression level, enhancing cellular clearance mechanisms, preventing its cell-to-cell transmission within the brain and perhaps from the periphery, and targeting other proteins associated with or implicated in PD that contribute to α-Syn toxicity. We also discuss the therapeutics in the pipeline that harness these strategies. Finally, we discuss the challenges and opportunities for the field in the discovery and development of therapeutics for disease modification in PD. SIGNIFICANCE STATEMENT: PD is the second most common neurodegenerative disorder, for which disease-modifying therapies remain a major unmet need. A large body of evidence points to α-synuclein as a key pathogenic protein in this disease as well as in dementia with Lewy bodies, making it of leading therapeutic interest. This review discusses the various approaches being investigated and progress made to date toward discovering and developing therapeutics that would slow and stop progression of these disabling diseases.

Cooperation of cell adhesion and autophagy in the brain: Functional roles in development and neurodegenerative disease

Cellular adhesive connections directed by the extracellular matrix (ECM) and maintenance of cellular homeostasis by autophagy are seemingly disparate functions that are molecularly intertwined, each regulating the other. This is an emerging field in the brain where the interplay between adhesion and autophagy functions at the intersection of neuroprotection and neurodegeneration. The ECM and adhesion proteins regulate autophagic responses to direct protein clearance and guide regenerative programs that go awry in brain disorders. Concomitantly, autophagic flux acts to regulate adhesion dynamics to mediate neurite outgrowth and synaptic plasticity with functional disruption contributed by neurodegenerative disease. This review highlights the cooperative exchange between cellular adhesion and autophagy in the brain during health and disease. As the mechanistic alliance between adhesion and autophagy has been leveraged therapeutically for metastatic disease, understanding overlapping molecular functions that direct the interplay between adhesion and autophagy might uncover therapeutic strategies to correct or compensate for neurodegeneration.

Neurite outgrowth inhibitory levels of organophosphates induce tissue transglutaminase activity in differentiating N2a cells: evidence for covalent adduct formation

Organophosphate compounds (OPs) induce both acute and delayed neurotoxic effects, the latter of which is believed to involve their interaction with proteins other than acetylcholinesterase. However, few OP-binding proteins have been identified that may have a direct role in OP-induced delayed neurotoxicity. Given their ability to disrupt Ca²⁺ homeostasis, a key aim of the current work was to investigate the effects of sub-lethal neurite outgrowth inhibitory levels of OPs on the Ca²⁺-dependent enzyme tissue transglutaminase (TG2). At 1-10 µM, the OPs phenyl saligenin phosphate (PSP) and chlorpyrifos oxon (CPO) had no effect cell viability but induced concentration-dependent decreases in neurite outgrowth in differentiating N2a neuroblastoma cells. The activity of TG2 increased in cell lysates of differentiating cells exposed for 24 h to PSP and chlorpyrifos oxon CPO (10 µM), as determined by biotin-cadaverine incorporation assays. Exposure to both OPs (3 and/or 10 µM) also enhanced in situ incorporation of the membrane permeable substrate biotin-X-cadaverine, as indicated by Western blot analysis of treated cell lysates probed with ExtrAvidin peroxidase and fluorescence microscopy of cell monolayers incubated with FITC-streptavidin. Both OPs (10 µM) stimulated the activity of human and mouse recombinant TG2 and covalent labelling of TG2 with dansylamine-labelled PSP was demonstrated by fluorescence imaging following SDS-PAGE. A number of TG2 substrates were tentatively identified by mass spectrometry, including cytoskeletal proteins, chaperones and proteins involved protein synthesis and gene regulation. We propose that the elevated TG2 activity observed is due to the formation of a novel covalent adduct between TG2 and OPs.

Transglutaminase 2 Depletion Attenuates α-Synuclein Mediated Toxicity in Mice

α-Synuclein (α-Syn) is a key pathogenic protein in α-synucleinopathies including Parkinson disease (PD) and Dementia with Lewy Bodies. The aggregation of α-Syn is believed to be deleterious and a critical step leading to neuronal dysfunction and death. One of the factors that may contribute to the initial steps of this aggregation is crosslinking through transglutaminase 2 (TG2). We previously demonstrated that overexpression of TG2 exacerbates α-Syn toxicity in mice and yeast by increasing the higher-order species of α-Syn. Herein, we investigated whether deletion of the TG2 encoding gene could mitigate the toxicity of α-Syn in vivo. Compared with α-Syn transgenic (Syn^Tg) mice, TG2 null /α-Syn transgenic mice (TG2^KO/Syn^Tg) exhibited a reduced amount of phosphorylated α-Syn aggregates and fewer proteinase K-resistant α-Syn aggregates in sections of brain tissue. Neuritic processes that are depleted in Syn^Tg mice compared to wild-type mice were preserved in double TG2^KO/Syn^Tg mice. Additionally, the neuroinflammatory reaction to α-Syn was attenuated in TG2^KO/Syn^Tg animals. These neuropathological markers of diminished α-Syn toxicity in the absence of TG2 were associated with better motor performance on the rotarod and balance beam. These results suggest that deleting TG2 reduces the toxicity of α-Syn in vivo and improves the behavioral performance of Syn^Tg mice. Accordingly, these findings collectively support pharmacological inhibition of TG2 as a potential disease modifying therapeutic strategy for α-synucleinopathies.

Characterisation of a novel cold-adapted calcium-activated transglutaminase: implications for medicine and food processing

Transglutaminases are a family of enzymes that catalyse the cross‐linking of proteins by forming covalent bonds between lysine and glutamine residues in various polypeptides. Cross‐linking reactions are involved in blood clots, skin formation, embryogenesis and apoptosis. Clinically, these enzymes appear to be implicated in neurodegenerative diseases, tumours and coeliac diseases. Transglutaminases have great potential for use in the food industry because of their ability to cross‐link proteins that are not normally linked. Here, a gene coding for transglutaminase from Atlantic cod was cloned into a bacterial expression vector and used to transform protein expression in a strain of Escherichia coli. The successful expression of recombinant transglutaminase protein from Atlantic cod (AcTG‐1) as a soluble protein upon induction at low temperature was confirmed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, immunoblotting and mass spectrometry analysis. Biochemical characterisation demonstrated that the transglutaminase was active between 0 and 65 °C, but was completely inactivated after 20‐min incubation at 70 °C. Interestingly, the enzyme displayed cold‐adapted features, such as temperature instability combined with high catalytic efficiency at low temperatures (8–16 °C). In addition, the enzyme had optimal activity at 50 °C, a new feature for a cold‐adapted enzyme. AcTG‐1 was active in the pH range from 6 to 9, with an optimum at pH 8, and required 5 mm calcium for maximum activity. Potential calcium‐binding sites in the enzyme were predictable, making the enzyme an appropriate model for studying structure–function relationships in the calcium‐dependent transglutaminase family. In vitro gel analysis revealed that transglutaminase cross‐linked casein, collagen and gelatin. The binding of fish fillets in the presence of recombinant AcTG‐1 provided further macroscopic proof for the potential application of AcTG‐1 as a biological cross‐linker in the food industry. Once binding occurred, fish fillets withstood further processing such as frying, boiling, freeze‐thawing and chilling. The low‐temperature activity and new enzymatic properties of AcTG‐1 appear to offer advantages over commercially available enzymatic glues in the food industry.

Disulfiram facilitates ataxin-3 nuclear translocation and potentiates the cytotoxicity in a cell model of SCA3

Spinocerebellar ataxia type 3 (SCA3) is caused by the expansion of a glutamine-encoding CAG repeat in the ATXN3 gene encoding the protein ataxin-3. The nuclear presence of polyglutamine-expanded ataxin-3 is of critical importance for the pathogenesis of SCA3. Disulfiram, an FDA-approved drug for alcoholism, has also garnered attention in cancer treatment. However, it has shown toxicity in the nervous system. Bearing this in mind, we treated cells expressing ataxin-3 with disulfiram to measure several pathogenic cascades of SCA3, including aggregate formation, soluble ataxin-3 expression and nuclear localization of ataxin-3 and the cytotoxicity, which assess the direct effect of disulfiram on SCA3 cell models. To our knowledge, this is direct evidence that disulfiram elevated the nuclear localization of polyglutamine-expanded ataxin-3 and enhanced the cytotoxicity in a cell model of SCA3. Furthermore, disulfiram did not affect the aggregate formation of polyglutamine-expanded ataxin-3 at least at a single dose. Our findings repurpose disulfiram as a modulator of ataxin-3 nuclear transport that aggravates the pathology of SCA3, which is a new target for disulfiram. This study also represents an important example of determining novel side effects in pre-existing drugs. This study suggests that caution may be warranted when this compound is used to treat alcohol abuse or cancer in patients carrying a SCA3-causing mutation.

Transglutaminase 2 Induces Deficits in Social Behavior in Mice

Impairments in social behavior are highly implicated in many neuropsychiatric disorders. Recent studies indicate a role for endoplasmic reticulum (ER) stress in altering social behavior, but the underlying mechanism is not known. In the present study, we examined the role of transglutaminase 2 (TG2), a calcium-dependent enzyme known to be induced following ER stress, in social behavior in mice. ER stress induced by tunicamycin administration increased TG2 protein levels in the mouse prefrontal cortex (PFC). PFC-specific inhibition of TG2 attenuated ER stress-induced deficits in social behavior. Conversely, overexpression of TG2 in the PFC resulted in social behavior impairments in mice. In addition, systemic administration of cysteamine, a TG2 inhibitor, attenuated social behavior deficits. Our preliminary findings using postmortem human brain samples found increases in TG2 mRNA and protein levels in the middle frontal gyrus of subjects with autism spectrum disorder. These findings in mice and human postmortem brain samples identify changes in TG2 activity in the possible dysregulation of social behavior.

Inflammatory mediators resulting from transglutaminase 2 expressed in mast cells contribute to the development of Parkinson's disease in a mouse model

This study aimed to investigate the role of transglutaminase 2 (TG2) expressed in mast cells in substantia nigra (SN) in Parkinson's disease (PD) model or human PD patients. C57BL/6 mice received 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) by ip injection to induce PD. Bone marrow-derived mast cells (BMMCs) were adoptively transferred to TG2 knockout (KO or TG2^-/-) mice by iv injection 1 day before MPTP injection or stimulated by 1 methyl-4-phenylpyridinium (MMP⁺). KO-MPTP mice showed reduced expression of tyrosine hydroxylase (TH) and dopamine (DA) transporter (DAT) and loss of TH⁺ DA neurons, and expression of markers (c-kit, tryptase, FcεRI), mediators' release (histamine, leukotrienes, cytokines), and TG2 related to mast cells, and co-localization of DA neuronal cells and mast cells in SN tissues or release of mediators and TG2 activity in SN tissues and sera versus those in WT (wild type)-MPTP or BM + KO-MPTP mice. KO-MPTP mice reversed the alterations of behavior. KO-BMMCs-transferred KO-MPTP (BM + KO-MPTP) mice had restoration of all the responses versus the KO-MPTP mice. MPP⁺-stimulated BMMCs had increased mediators' release, which were inhibited by TG2 inhibitor (R2 peptide). All the mediators and TG2 activity were also increased in the sera of human PD patients. The data suggest that TG2 expressed in mast cells recruited into SN tissues might contribute to neuroinflammation, which is known as one of the important features in pathogenesis of PD, via up-regulating the release of various mediators.

Calcium-dependent activation of transglutaminase 2 by nanosecond pulsed electric fields

Exposure of cultured human cells to nanosecond pulsed electric fields (nsPEFs) elicits various cellular events, including Ca²⁺ influx and cell death. Recently, nsPEFs have been regarded as a novel physical treatment useful for biology and medicine, but the underlying mechanism of action remains to be fully elucidated. In this study, we investigated the effect of nsPEFs on transglutaminases (TGs), enzymes that catalyze covalent protein modifications such as protein-protein crosslinking. Cellular TG activity was monitored by conjugation of cellular proteins with biotin-cadaverine, a cell-permeable pseudosubstrate for TGs. We applied nsPEFs to HeLa S3 cells and found that overall catalytic activity of cellular TGs was greatly increased in a Ca²⁺-dependent manner. The Ca²⁺ ionophore ionomycin significantly augmented nsPEF-induced TG activation, further supporting the importance of Ca²⁺. Among human TG family members, TG2 is known to be the most ubiquitously expressed, and its catalytic activity requires elevated intracellular Ca²⁺. Given the requirement of Ca²⁺ for TG activation by nsPEFs, we performed depletion of TG2 by RNA interference (RNAi). We observed that TG2 RNAi suppressed the nsPEF-induced TG activation and partially alleviated the cytotoxic effects of nsPEFs. These findings demonstrate that TG2 activation is a Ca²⁺-dependent event in nsPEF-exposed cells and exerts negative effects on cell physiology.

Transglutaminase 2 modulation of NF-κB signaling in astrocytes is independent of its ability to mediate astrocytic viability in ischemic injury

Transglutaminase 2 (TG2) is a multifunctional protein that can contribute to cell death and cell survival processes in a variety of disease contexts. Within the brain, TG2 has been shown to promote cell death in ischemic injury when expressed in astrocytes (Colak and Johnson, 2012). However, the specific functions and characteristics of astrocytic TG2 that mediate this effect are largely unknown. Therefore, the goal of this study was to investigate the role of astrocytic TG2 in mediating cellular viability processes in the context of ischemic injury, with a specific focus on its contributions to intracellular signaling cascades. We show that, in response to oxygen/glucose deprivation (OGD), acute lentiviral-mediated knockdown of TG2, as well as inhibition with an irreversible TG2 inhibitor, enhances cell survival. We also show that TG2 depletion increases nuclear factor-κB (NF-κB) signaling, whereas inhibition reduces NF-κB activity. Despite its clear contribution to NF-κB signaling, however, TG2 modulation of NF-κB signaling is not likely to be a major contributor to its ability to mediate astrocytic viability in this context. Overall, the results of this study provide insight into the role of TG2 in astrocytes and suggest possible avenues for future study of the relationship between astrocytic TG2 and ischemic injury.

Microglia Endocytose Amyloid β Through the Binding of Transglutaminase 2 and Milk Fat Globule EGF Factor 8 Protein

Activation of glial cells has been observed in neurodegenerative diseases including Alzheimer's disease (AD). Aggregation of amyloid β (Aβ) is profusely observed as characteristic pathology in AD brain. In our previous study using microglial cell line BV-2, tissue-type transglutaminase (TG2) was found to be involved in phagocytosis (Kawabe et al., in Neuroimmunomodulation 22(4):243-249, 2015; Kawabe et al., Neurochem Res 2017). In the present study, we examined whether TG2 and milk fat globule EGF factor 8 protein (MFG-E8), an adaptor protein promotes macrophage to engulf apoptotic cells, were involved in Aβ endocytosis. When the neuronal/glial mixed culture was stimulated freshly prepared Aβ_1-42 for 3 days, the incorporation of Aβ was observed by immunofluorescence staining technique in Iba-1-positive microglia. Cystamine, a broad competitive inhibitor of TGs, suppressed it. When aggregated Aβ was added to the mixed culture, the immunoreactivity of MFG-E8 surrounding Aβ was observed, and then followed by microglial endocytosis. Using western blotting technique, MFG-E8 was detected in cell lysate of astrocyte culture, and was also detected in the medium. When microglia culture was incubated with astrocyte conditioned medium, MFG-E8 levels in microglia tended to increase. It is likely that microglia might utilize MFG-E8 released from astrocytes as well as that expressed in themselves in order to endocytose Aβ aggregation. Furthermore, we confirmed that MFG-E8 could bind with TG2 in microglia culture by immunoprecipitate technique. These results suggest that microglia might uptake Aβ as a complex of aggregated Aβ/MFG-E8/TG2.

Amphotericin B Increases Transglutaminase 2 Expression Associated with Upregulation of Endocytotic Activity in Mouse Microglial Cell Line BV-2

Amphotericin B (AmB), a polyene antibiotic, is reported to cause the microglial activation to induce nitric oxide (NO) production and proinflammatory cytokines expression, and change neurotrophic factors expression in cultured microglia (Motoyoshi et al. in Neurochem Int 52:1290-1296, 2008). On the other hand, tissue-type transglutaminase (TG2) is involved in connection to phagocytes with apoptotic cells. Engulfment of neurons by activated microglia is thought to cause neurodegenerative diseases but detail is unclear, and involvement of TG2 in phagocytosis has been reported in our previous study using lipopolysaccharide-stimulated BV-2 cells (Kawabe et al. in Neuroimmunomodulation 22(4):243-249, 2015). In the present study, we examined the changes of TG2 expression, phagocytosis and pinocytosis in BV-2 cells stimulated by AmB. AmB stimulation increased TG2 expression and TG activity. Phagocytosis of dead cells and pinocytosis of fluorescent microbeads were also up-regulated by AmB stimulation in BV-2 cells. Blockade of TG activity by cystamine, an inhibitor of TGs, suppressed AmB-enhanced TG2 expression, TG activity, NO production, phagocytosis and pinocytosis. Excessive NO production from microglia and/or facilitation of phagocytosis might be involved in neuronal death. To control TG activity might make possible to protect neurons and care for CNS diseases.

Assessing the Catalytic Activity of Transglutaminases in the Context of Autophagic Responses

The human transglutaminases (TGases) are a widely distributed and peculiar group of enzymes that catalyze the posttranslational modification of proteins by the formation of isopeptide bonds. Tissue or type 2 transglutaminase (TG2) represents the most ubiquitous isoform belonging to TGases family. The vast array of biochemical functions catalyzed by TG2 distinguishes it from the other members of the TGase family. In the presence of high calcium levels TG2 catalyzes a vast array of protein posttranslational modifications, including protein-protein cross-linking, incorporation of primary amines into proteins, as well as glutamine deamination. In the last few years, it has become evident that TG2 is involved in the final maturation of autolysosomes. The TG2 regulation of autophagy occurs by its transamidating activity and its inhibition results in the intracellular increase of ubiquitinated protein aggregates. In this chapter, we describe the methods used in our laboratories to assess the catalytic activity of TG2 in the autophagic process.

Transglutaminase type 2-dependent selective recruitment of proteins into exosomes under stressful cellular conditions

Numerous studies are revealing a role of exosomes in intercellular communication, and growing evidence indicates an important function for these vesicles in the progression and pathogenesis of cancer and neurodegenerative diseases. However, the biogenesis process of exosomes is still unclear. Tissue transglutaminase (TG2) is a multifunctional enzyme with different subcellular localizations. Particularly, under stressful conditions, the enzyme has been also detected in the extracellular matrix, but the mechanism(s) by which TG2 is released outside the cells requires further investigation. Therefore, the goal of the present study was to determine whether exosomes might be a vehicle for TG2 to reach the extracellular space, and whether TG2 could be involved in exosomes biogenesis. To address this issue, we isolated and characterized exosomes derived from cells either expressing or not TG2, under stressful conditions (i.e. proteasome impairment or expressing a mutated form of huntingtin (mHtt) containing 84 polyglutamine repeats). Our results show that TG2 is present in the exosomes only upon proteasome blockade, a condition in which TG2 interacts with TSG101 and ALIX, two key proteins involved in exosome biogenesis. Interestingly, we found that TG2 favours the assembly of a protein complex including mHtt, ALIX, TSG101 and BAG3, a co-chaperone involved in the clearance of mHtt. The formation of this complex is paralleled by the selective recruitment of mHtt and BAG3 in the exosomes derived from TG2 proficient cells only. Overall, our data indicate that TG2 is an important player in the biogenesis of exosomes controlling the selectivity of their cargo under stressful cellular conditions. In addition, these vesicles represent the way by which cells can release TG2 into the extracellular space under proteostasis impairment.

A Pharmacogenetic Discovery: Cystamine Protects Against Haloperidol-Induced Toxicity and Ischemic Brain Injury

Haloperidol is an effective antipsychotic agent, but it causes Parkinsonian-like extrapyramidal symptoms in the majority of treated subjects. To address this treatment-limiting toxicity, we analyzed a murine genetic model of haloperidol-induced toxicity (HIT). Analysis of a panel of consomic strains indicated that a genetic factor on chromosome 10 had a significant effect on susceptibility to HIT. We analyzed a whole-genome SNP database to identify allelic variants that were uniquely present on chromosome 10 in the strain that was previously shown to exhibit the highest level of susceptibility to HIT. This analysis implicated allelic variation within pantetheinase genes (Vnn1 and Vnn3), which we propose impaired the biosynthesis of cysteamine, could affect susceptibility to HIT. We demonstrate that administration of cystamine, which is rapidly metabolized to cysteamine, could completely prevent HIT in the murine model. Many of the haloperidol-induced gene expression changes in the striatum of the susceptible strain were reversed by cystamine coadministration. Since cystamine administration has previously been shown to have other neuroprotective actions, we investigated whether cystamine administration could have a broader neuroprotective effect. Cystamine administration caused a 23% reduction in infarct volume after experimentally induced cerebral ischemia. Characterization of this novel pharmacogenetic factor for HIT has identified a new approach for preventing the treatment-limiting toxicity of an antipsychotic agent, which could also be used to reduce the extent of brain damage after stroke.

Thiol-Disulfide Exchange Reactions in the Mammalian Extracellular Environment

Disulfide bonds represent versatile posttranslational modifications whose roles encompass the structure, catalysis, and regulation of protein function. Due to the oxidizing nature of the extracellular environment, disulfide bonds found in secreted proteins were once believed to be inert. This notion has been challenged by the discovery of redox-sensitive disulfides that, once cleaved, can lead to changes in protein activity. These functional disulfides are twisted into unique configurations, leading to high strain and potential energy. In some cases, cleavage of these disulfides can lead to a gain of function in protein activity. Thus, these motifs can be referred to as switches. We describe the couples that control redox in the extracellular environment, examine several examples of proteins with switchable disulfides, and discuss the potential applications of disulfides in molecular biology.

Expression of Functional Recombinant Human Tissue Transglutaminase (TG2) Using the Bac-to-Bac Baculovirus Expression System

PURPOSE

Tissue transglutaminase (TG2) is a unique multifunctional enzyme. The enzyme possesses enzymatic activities such as transamidation/crosslinking and non-enzymatic functions such as cell migration and signal transduction. TG2 has been shown to be involved in molecular mechanisms of cancers and several neurodegenerative diseases such as Alzheimer's disease. The present study aimed at cloning and expression of full length human TG2 in Bac-to-Bac baculovirus expression system and evaluation of its activity.

METHODS

pFastBac HTA donor vector containing coding sequence of human TG2 was constructed. The construct was transformed to DH10Bac for generating recombinant bacmid. The verified bacmid was transfected to insect cell line (Sf9). Expression of recombinant TG2 was examined by RT-PCR, SDS-PAGE and western blot analysis. Functional analysis was evaluated by fluorometric assay and gel electrophoresis.

RESULTS

Recombinant bacmid was verified by amplification of a band near to 4500 bp. Expression analysis showed that the enzyme was expressed as a protein with a molecular weight near 80 kDa. Western blot confirmed the presence of TG2 and the activity assays including flurometric assay indicated that the recombinant TG2 was functional. The electrophoresis assay conformed that the expressed TG2 was the indeed capable of crosslinking in the presence of physiological concentration calcium ions.

CONCLUSION

Human TG2 was expressed efficiently in the active biological form in the Bac-to-Bac baculovirus expression system. The expressed enzyme could be used for medical diagnostic, or studies which aim at finding novel inhibitors of the enzymes . To best of our knowledge, this is probably the first report of expression of full length human tissue transglutaminase (TG2) using the Bac-to-Bac expression system.

Expression of Functional Recombinant Human Tissue Transglutaminase (TG2) Using the Bac-to-Bac Baculovirus Expression System

PURPOSE

Tissue transglutaminase (TG2) is a unique multifunctional enzyme. The enzyme possesses enzymatic activities such as transamidation/crosslinking and non-enzymatic functions such as cell migration and signal transduction. TG2 has been shown to be involved in molecular mechanisms of cancers and several neurodegenerative diseases such as Alzheimer's disease. The present study aimed at cloning and expression of full length human TG2 in Bac-to-Bac baculovirus expression system and evaluation of its activity.

METHODS

pFastBac HTA donor vector containing coding sequence of human TG2 was constructed. The construct was transformed to DH10Bac for generating recombinant bacmid. The verified bacmid was transfected to insect cell line (Sf9). Expression of recombinant TG2 was examined by RT-PCR, SDS-PAGE and western blot analysis. Functional analysis was evaluated by fluorometric assay and gel electrophoresis.

RESULTS

Recombinant bacmid was verified by amplification of a band near to 4500 bp. Expression analysis showed that the enzyme was expressed as a protein with a molecular weight near 80 kDa. Western blot confirmed the presence of TG2 and the activity assays including flurometric assay indicated that the recombinant TG2 was functional. The electrophoresis assay conformed that the expressed TG2 was the indeed capable of crosslinking in the presence of physiological concentration calcium ions.

CONCLUSION

Human TG2 was expressed efficiently in the active biological form in the Bac-to-Bac baculovirus expression system. The expressed enzyme could be used for medical diagnostic, or studies which aim at finding novel inhibitors of the enzymes . To best of our knowledge, this is probably the first report of expression of full length human tissue transglutaminase (TG2) using the Bac-to-Bac expression system.

Development of carbon-11 labeled acryl amides for selective PET imaging of active tissue transglutaminase

INTRODUCTION

Tissue transglutaminase (TG2) is a ubiquitously expressed enzyme capable of forming metabolically and mechanically stable crosslinks between the γ-carboxamide of a glutamine acyl-acceptor substrate and the ε-amino functionality of a lysine acyl-donor substrate resulting in protein oligomers. High TG2 crosslinking activity has been implicated in the pathogenesis of various diseases including celiac disease, cancer and fibrotic and neurodegenerative diseases. Development of a PET tracer specific for active TG2 provides a novel tool to further investigate TG2 biology in vivo in disease states. Recently, potent irreversible active site TG2 inhibitors carrying an acrylamide warhead were synthesized and pharmacologically characterized.

METHODS

Three of these inhibitors, compound 1, 2 and 3, were successfully radiolabeled with carbon-11 on the acrylamide carbonyl position using a palladium mediated [(11)C]CO aminocarbonylation reaction. Ex vivo biodistribution and plasma stability were evaluated in healthy Wistar rats. Autoradiography was performed on MDA-MB-231 tumor sections.

RESULTS

[(11)C]1, -2 and -3 were obtained in decay corrected radiochemical yields of 38-55%. Biodistribution showed low uptake in peripheral tissues, with the exception of liver and kidney. Low brain uptake of <0.05% ID/g was observed. Blood plasma analysis demonstrated that [(11)C]1 and [(11)C]2 were rapidly metabolized, whereas [(11)C]3 was metabolized at a more moderate rate (63.2 ± 6.8 and 28.7 ± 10.8% intact tracer after 15 and 45 min, respectively). Autoradiography with [(11)C]3 on MDA-MB-231 tumor sections showed selective and specific binding of the radiotracer to the active state of TG2.

CONCLUSIONS

Taken together, these results identify [(11)C]3 as the most promising of the three compounds tested for development as PET radiotracer for the in vivo investigation of TG2 activity.

Transglutaminase 2--a novel inhibitor of adipogenesis

Differentiation of preadipocytes to lipid storing adipocytes involves extracellular signaling pathways, matrix remodeling and cytoskeletal changes. A number of factors have been implicated in maintaining the preadipocyte state and preventing their differentiation to adipocytes. We have previously reported that a multifunctional and protein crosslinking enzyme, transglutaminase 2 (TG2) is present in white adipose tissue. In this study, we have investigated TG2 function during adipocyte differentiation. We show that TG2 deficient mouse embryonic fibroblasts (Tgm2-/- MEFs) display increased and accelerated lipid accumulation due to increased expression of major adipogenic transcription factors, PPARγ and C/EBPα. Examination of Pref-1/Dlk1, an early negative regulator of adipogenesis, showed that the Pref-1/Dlk1 protein was completely absent in Tgm2-/- MEFs during early differentiation. Similarly, Tgm2-/- MEFs displayed defective canonical Wnt/β-catenin signaling with reduced β-catenin nuclear translocation. TG2 deficiency also resulted in reduced ROCK kinase activity, actin stress fiber formation and increased Akt phosphorylation in MEFs, but did not alter fibronectin matrix levels or solubility. TG2 protein levels were unaltered during adipogenic differentiation, and was found predominantly in the extracellular compartment of MEFs and mouse WAT. Addition of exogenous TG2 to Tgm2+/+ and Tgm2-/- MEFs significantly inhibited lipid accumulation, reduced expression of PPARγ and C/EBPα, promoted the nuclear accumulation of β-catenin, and recovered Pref-1/Dlk1 protein levels. Our study identifies TG2 as a novel negative regulator of adipogenesis.

Novel 3-arylethynyl-substituted thieno[3,4-b]pyrazine derivatives as human transglutaminase 2 inhibitors

In the process of optimization, we developed a novel core skeleton of thieno[3,4-b]pyrazine via GK-13. The derivatives synthesized were shown to inhibit TGase 2 activity in cancer cells. Some of the hit compounds such as the arylethynyl group-coupled thieno[3,4-b]pyrazine derivatives were shown to exhibit promising activity for use as potential therapeutic small-molecules in renal cancer by inhibiting TGase 2 activity.

An unprecedented dual antagonist and agonist of human Transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed, Ca(2+)-activated extracellular enzyme in mammals that is maintained in a catalytically dormant state by multiple mechanisms. Although its precise physiological role in the extracellular matrix remains unclear, aberrantly up-regulated TG2 activity is a hallmark of several maladies, including celiac disease. Previously, we reported the discovery of a class of acylideneoxoindoles as potent, reversible inhibitors of human TG2. Detailed analysis of one of those inhibitors (CK-IV-55) led to an unprecedented and striking observation. Whereas this compound was a non-competitive inhibitor (3.3±0.9 μM) of human TG2 at saturating Ca(2+) concentrations, it activated TG2 in the presence of sub-saturating but physiologically relevant Ca(2+) concentrations (0.5-0.7 mM). This finding was validated in a cellular model of TG2 activation and inhibition. Mutant TG2 analysis suggested that CK-IV-55 and its analogs bound to a low-affinity Ca(2+) binding site on the catalytic core of TG2. A mechanistic model for the dual agonistic/antagonistic action of CK-IV-55 on TG2 is presented, and the pathophysiological implications of basal activation of intestinal TG2 by small molecules are discussed.

Support for patients with celiac disease: A literature review

BACKGROUND

Celiac disease (CD) is a lifelong disorder. Patients are at increased risk of complications and comorbidity.

OBJECTIVES

We conducted a review of the literature on patient support and information in CD and aim to issue recommendations about patient information with regards to CD.

METHODS

DATA SOURCE

We searched PubMed for English-language articles published between 1900 and June 2014, containing terms related to costs, economics of CD, or education and CD.

STUDY SELECTION

Papers deemed relevant by any of the participating authors were included in the study.

DATA SYNTHESIS

No quantitative synthesis of data was performed. Instead we formulated a consensus view of the information that should be offered to all patients with CD.

RESULTS

There are few randomized clinical trials examining the effect of patient support in CD. Patients and their families receive information from many sources. It is important that health care personnel guide the patient through the plethora of facts and comments on the Internet. An understanding of CD is likely to improve dietary adherence. Patients should be educated about current knowledge about risk factors for CD, as well as the increased risk of complications. Patients should also be advised to avoid other health hazards, such as smoking. Many patients are eager to learn about future non-dietary treatments of CD. This review also comments on novel therapies but it is important to stress that no such treatment is available at present.

CONCLUSION

Based on mostly observational data, we suggest that patient support and information should be an integral part of the management of CD, and is likely to affect the outcome of CD.

Lipopolysaccharide-Stimulated Transglutaminase 2 Expression Enhances Endocytosis Activity in the Mouse Microglial Cell Line BV-2

OBJECTIVES

In peripheral macrophages, tissue-type transglutaminase (TG2) is reported to be involved in phagocytosis of apoptotic cells. However, the contribution of TG2 to microglial phagocytosis has not been investigated. In this study, using a microglial cell line, BV-2, we examined the changes in TG2 expression, phagocytosis and pinocytosis in cells stimulated by lipopolysaccharide (LPS).

METHODS

Cells of the mouse microglial cell line BV-2 were stimulated by LPS with or without cystamine, an inhibitor of TG enzyme activity, for 24 h. TG2 expression was measured by real-time RT-PCR and Western blotting. TG activity was evaluated using biotinylated pentylamine as a substrate. Pinocytosis was determined by uptake of 1-µm fluorescent microbeads. Phagocytosis was assessed by uptake of dead cells, human neuroblastoma SH-SY5Y cells, which were pretreated with H2O2 for 24 h.

RESULTS

Phagocytosis of dead cells and pinocytosis of fluorescent microbeads were up-regulated by LPS stimulation together with TG2 expression. Blockade of TG enzyme activity by cystamine suppressed TG2 expression, phagocytosis and pinocytosis.

CONCLUSIONS

These results suggested that LPS-induced TG2 was involved in the mechanism of pinocytosis and phagocytosis in microglia.

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

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

Discovery of potent and specific dihydroisoxazole inhibitors of human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed enzyme that catalyzes the posttranslational modification of glutamine residues on protein or peptide substrates. A growing body of literature has implicated aberrantly regulated activity of TG2 in the pathogenesis of various human inflammatory, fibrotic, and other diseases. Taken together with the fact that TG2 knockout mice are developmentally and reproductively normal, there is growing interest in the potential use of TG2 inhibitors in the treatment of these conditions. Targeted-covalent inhibitors based on the weakly electrophilic 3-bromo-4,5-dihydroisoxazole (DHI) scaffold have been widely used to study TG2 biology and are well tolerated in vivo, but these compounds have only modest potency, and their selectivity toward other transglutaminase homologues is largely unknown. In the present work, we first profiled the selectivity of existing inhibitors against the most pertinent TG isoforms (TG1, TG3, and FXIIIa). Significant cross-reactivity of these small molecules with TG1 was observed. Structure–activity and −selectivity analyses led to the identification of modifications that improved potency and isoform selectivity. Preliminary pharmacokinetic analysis of the most promising analogues was also undertaken. Our new data provides a clear basis for the rational selection of dihydroisoxazole inhibitors as tools for in vivo biological investigation.

Type 2 Transglutaminase, mitochondria and Huntington's disease: menage a trois

Mitochondria produce the bulk of cellular energy and work as decisional "hubs" for cellular responses by integrating different input signals. The determinant in the physiopathology of mammals, they attract major attention, nowadays, for their contribution to brain degeneration. How they can withstand or succumb to insults leading to neuronal death is an object of great attention increasing the need for a better understanding of the interplay between inner and outer mitochondrial pathways residing in the cytosol. Of the latter, those dictating protein metabolism and therefore influencing the quality function and control of the organelle are of our most immediate interest and here we describe the Transglutaminase type 2 (TG2) contribution to mitochondrial function, dysfunction and neurodegeneration. Besides reviewing the latest evidences we share also the novel ones on the IF1 pathway depicting a molecular conduit governing mitochondrial turnover and homeostasis relevant to envisaging preventive and therapeutic strategies to respectively predict and counteract deficiencies associated with deregulated mitochondrial function in neuropathology.

Tissue transglutaminase: an emerging target for therapy and imaging

Tissue transglutaminase (transglutaminase 2) is a multifunctional enzyme with many interesting properties resulting in versatile roles in both physiology and pathophysiology. Herein, the particular involvement of the enzyme in human diseases will be outlined with special emphasis on its role in cancer and in tissue interactions with biomaterials. Despite recent progress in unraveling the different cellular functions of transglutaminase 2, several questions remain. Transglutaminase 2 features in both confirmed and some still ambiguous roles within pathological conditions, raising interest in developing inhibitors and imaging probes which target this enzyme. One important prerequisite for identifying and characterizing such molecular tools are reliable assay methods to measure the enzymatic activity. This digest Letter will provide clarification about the various assay methods described to date, accompanied by a discussion of recent progress in the development of inhibitors and imaging probes targeting transglutaminase 2.

Transglutaminase 2 exacerbates α-synuclein toxicity in mice and yeast

α-Synuclein is a key pathogenic protein that aggregates in hallmark lesions in Parkinson's disease and other α-synucleinopathies. Prior in vitro studies demonstrated that it is a substrate for cross-linking by transglutaminase 2 (TG2) into higher-order species. Here we investigated whether this increased aggregation occurs in vivo and whether TG2 exacerbates α-synuclein toxicity in Mus musculus and Saccharomyces cerevisiae. Compared with α-synuclein transgenic (Syn(Tg)) mice, animals double transgenic for human α-synuclein and TG2 (TG2(Tg)/Syn(Tg)) manifested greater high-molecular-weight insoluble species of α-synuclein in brain lysates and developed α-synuclein aggregates in the synaptic vesicle fraction. In addition, larger proteinase K-resistant aggregates developed, along with increased thioflavin-S-positive amyloid fibrils. This correlated with an exaggerated neuroinflammatory response, as seen with more astrocytes and microglia. Further neuronal damage was suggested by greater morphological disruption of nerve fibers and a trend toward decreased c-Fos immunoreactive neurons. Finally, the performance of TG2(Tg)/Syn(Tg) animals on motor behavioral tasks was worse relative to Syn(Tg) mice. Greater toxicity of α-synuclein was also demonstrated in yeast cells coexpressing TG2. Our findings demonstrate that TG2 promotes the aggregation of α-synuclein in vivo and that this is associated with aggravated toxicity of α-synuclein and its downstream neuropathologic consequences.

The emerging potential of autophagy-based therapies in the treatment of cystic fibrosis lung infections

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR), a channel that normally transports anions across epithelial cell membranes. The most common manifestation of CF is buildup of mucus in the airways and bacterial colonization of the lower respiratory tract, accompanied by chronic inflammation. Antibiotics are used to control CF-associated opportunistic infections, but lengthy antibiotic treatment risks the emergence of multiple-drug resistant (MDR) strains. New antimicrobial strategies are needed to prevent and treat infections in these high-risk individuals. Autophagy contributes to the control of a variety of microbial infections. For this reason, the recent discovery of functional impairment of autophagy in CF provides a new basis for understanding susceptibility to severe infections. Here, we review the role of autophagy in host defense against CF-associated bacterial and fungal pathogens, and survey pharmacologic approaches to restore normal autophagy function in these individuals. Autophagy restoration therapy may improve pathogen clearance and mitigate lung inflammation in CF airways.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Pharmacological potential of biogenic amine-polyamine interactions beyond neurotransmission

Histamine, serotonin and dopamine are biogenic amines involved in intercellular communication with multiple effects on human pathophysiology. They are products of two highly homologous enzymes, histidine decarboxylase and l-aromatic amino acid decarboxylase, and transmit their signals through different receptors and signal transduction mechanisms. Polyamines derived from ornithine (putrescine, spermidine and spermine) are mainly involved in intracellular effects related to cell proliferation and death mechanisms. This review summarizes structural and functional evidence for interactions between components of all these amine metabolic and signalling networks (decarboxylases, transporters, oxidases, receptors etc.) at cellular and tissue levels, distinct from nervous and neuroendocrine systems, where the crosstalk among these amine-related components can also have important pathophysiological consequences. The discussion highlights aspects that could help to predict and discuss the effects of intervention strategies.

Tissue transglutaminase is not a biochemical marker for Alzheimer's disease

Typical hallmarks of Alzheimer's disease (AD) are pathologic deposits in cortical and subcortical regions consisting of self-aggregated proteins such as amyloid-beta (Aβ) or tau. Tissue transglutaminase (tTG) catalyses calcium-dependent cross-linking between proteins (transamidation) resulting in protease-resistant isopeptide bonds. Because of this ability, tTG was suspected to participate in AD pathogenesis. Aβ and tau can be cross-linked by tTG in vitro. In AD neocortex, messenger RNA expression of tTG is increased. However, data on transamidation in AD specimens--activity of not only tTG but also other transglutaminases--are contradictory. The aim of our study was to investigate if tTG is involved in AD development and may be useful as biomarker for AD. We studied human brain samples for tTG concentration, tTG localization, and transamidation activity and cerebrospinal fluid (CSF) for tTG content by novel sensitive and highly specific methods. Neither tTG concentration nor transamidation was increased in AD brain homogenates. Immunohistologically, we found no colocalization of tTG in neocortex sections with tau or Aβ deposits but with blood vessels. Only in rare cases, tTG was detectable in CSF samples. This could be attributed to liberation from erythrocytes. Our data contradict the view that tTG is a potential biochemical marker for AD.

Post-translational control of protein function by disulfide bond cleavage

Protein action in nature is largely controlled by the level of expression and by post-translational modifications. Post-translational modifications result in a proteome that is at least two orders of magnitude more diverse than the genome. There are three basic types of post-translational modifications: covalent modification of an amino acid side chain, hydrolytic cleavage or isomerization of a peptide bond, and reductive cleavage of a disulfide bond. This review addresses the modification of disulfide bonds. Protein disulfide bonds perform either a structural or a functional role, and there are two types of functional disulfide: the catalytic and allosteric bonds. The allosteric disulfide bonds control the function of the mature protein in which they reside by triggering a change when they are cleaved. The change can be in ligand binding, substrate hydrolysis, proteolysis, or oligomer formation. The allosteric disulfides are cleaved by oxidoreductases or by thiol/disulfide exchange, and the configurations of the disulfides and the secondary structures that they link share some recurring features. How these bonds are being identified using bioinformatics and experimental screens and what the future holds for this field of research are also discussed.