Transglutaminase 2: Biology, Relevance to Neurodegenerative Diseases and Therapeutic Implications

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Abstract

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

Introduction

Transglutaminases (TGs) are enzymes that catalyze the formation of isopeptide bonds – a type of covalent linkage between two protein molecules. To date, nine members of the TG family have been identified, of which transglutaminase 2 (TG2) is highly expressed in the human central nervous system (CNS) (Beninati and Piacentini, 2004, Ruan and Johnson, 2007, Siegel and Khosla, 2007). TG2 is unique among this family of proteins in that it has multiple additional enzymatic activities (Martin et al., 2006, Siegel and Khosla, 2007). More importantly, TG2 has been implicated in multiple disease states, including celiac disease and several degenerative disorders of the brain (Martin, et al., 2006).

Neurodegenerative disorders encompass a number of chronic progressive diseases that are characterized by the loss of select neuronal populations in the CNS, which underlie their respective clinical phenotypes, and the aggregation of disease-specific pathogenic proteins in hallmark lesions that highlight the neuropathology of each disorder. The common theme of abnormal protein aggregates and the accumulating evidence that certain forms of these aggregates can be neurotoxic have provided the rationale to develop various approaches to inhibit or clear protein aggregates as potential neuroprotective therapeutic strategies. Factors that promote the formation of these aggregates have also been the subject of intense investigations with the objective of identifying interventions that can slow the progressive disease process. One of these factors is the cross-linking of pathogenic proteins into toxic higher order oligomers by the enzymatic activity of TG2. Other commonalities among these disorders include loss of calcium homeostasis, energy depletion, and increased generation of reactive oxygen species in affected cells, all of which may contribute to the activation of TG2. In this review, we discuss the biochemistry of TG2 and its pathogenic role in neurodegenerative disorders. We also describe efforts towards developing inhibitors of TG2 that could be used as disease modifying therapies for these as yet incurable conditions.

Section snippets

Enzymatic functions of TG2

TG2 is highly versatile functionally with multiple domains (Fig. 1). In addition to being a transamidating enzyme, it has activities as a GTPase, a protein disulfide isomerase, a protein kinase, and an isopeptidase (Chen and Mehta, 1999, Fesus and Piacentini, 2002, Hasegawa et al., 2003, Mishra and Murphy, 2004, Martin et al., 2006, Siegel and Khosla, 2007, Iismaa et al., 2009). These various functions are important in its physiological roles and may each contribute to disease states.

Structural/functional correlates

The gene coding for TG2 is on chromosome 20(q11-12) in humans (Gentile et al., 1994, Chen and Mehta, 1999, Lesort et al., 2000, Martin et al., 2006, Iismaa et al., 2009). It contains 13 exons and 12 introns (Gentile et al., 1991, Fraij and Gonzales, 1997, Chen and Mehta, 1999, Lesort et al., 2000) and is 32.5 kilobases long (Fraij and Gonzales, 1997, Lesort et al., 2000). TGs are found in a wide range of species from bacteria to humans, with the catalytic triad necessary for cross-linking

The impact of TG2 localization on its functions

Of the known TGs, TG2 is the most ubiquitous (Lesort, et al., 2000). It is found in many neural tissues including the brain, spinal cord, and peripheral nerves (Gilad and Varon, 1985, Johnson et al., 1997, Lesort et al., 2000). Within the brain, it is present in many regions including frontal and temporal cortex, hippocampus, substantia nigra and cerebellum (Appelt et al., 1996, Johnson et al., 1997, Kim et al., 1999, Lesort et al., 1999, Lesort et al., 2000, Andringa et al., 2004, Wilhelmus et

Physiologic roles of TG2

As might be expected from the many functions and ubiquitous nature of TG2, diverse physiological roles have been attributed to this protein. Frequently, the stable cross-links formed by TG2 serve a structural purpose, but this transglutaminase activity can also alter the function of substrate molecules (Chen & Mehta, 1999). Additionally, many of the proposed roles of TG2 are independent of its transamidation activity (Siegel & Khosla, 2007).

Regulation of TG2 expression

As might be expected of a protein such as TG2 that is widely expressed and has multiple functions, it is highly regulated at many levels (Chen & Mehta, 1999). A number of regulators increase TG2 expression, many of which vary based on cell type (Chen and Mehta, 1999, Lesort et al., 2000), although some generalizations can be made. One important aspect of TG2 regulation is the frequent involvement of inflammatory mediators. For example TGF-β can both be activated by TG2 (Nunes, et al., 1997),

Mouse models with genetically manipulated TG2

TG2 is implicated in a number of diseases including many neurodegenerative disorders. As a result, genetically altered mice that either have the TG2 gene deleted or are transgenic for TG2 have been generated in order to further study this protein in vivo. Since TG2 is the predominant TG in the mouse forebrain, these studies are expected to be informative about the function of this protein in the brain (Ruan & Johnson, 2007). First, TG2 knock out mice (TG2−/−) were generated independently by two

TG2 in neurodegenerative diseases

TG2 is implicated in a number of neurodegenerative disorders including Huntington's (HD), Alzheimer's (AD), and Parkinson's diseases (PD) (Jeitner et al., 2009b) based on several lines of evidence. First, it is found throughout most brain regions, particularly in neurons (Appelt et al., 1996, Kim et al., 1999, Lesort et al., 1999), and accounts for the majority of transglutaminase activity in the mouse brain (Bailey et al., 2004). Second, the pathophysiology of all these diseases includes the

TG2 and non-neurodegenerative conditions

Although this review focuses on neurodegenerative diseases, we briefly discuss here other disorders in which TG2 is believed to be a contributing factor for two reasons: first, understanding the mechanisms through which TG2 plays a role in non-neurologic diseases can help in designing TG2 inhibitors, and second, future therapeutics aimed at inhibiting TG2 for CNS diseases may have an impact on disease processes in other parts of the body as well.

The disorder in which the role of TG2 is best

TG2 inhibitors

As a result of the putative role of TG2 in a number of disease states, there is considerable interest in developing inhibitors of this enzyme as a disease modifying therapy for these disorders. Of those currently available, cystamine is the most commonly used experimentally to inhibit TG2 activity and has long been known to have this property (Lorand & Conrad, 1984). Cystamine is the disulfide of the aminothiol cysteamine, which has the structure HS-CH2-CH2-NH2 (Thoene et al., 1976) (Lorand &

Conclusions

In conclusion, TG2 is a multifunctional enzyme that has diverse roles throughout the body. A large body of evidence has accumulated supporting the notion that TG2 plays a role in many neurodegenerative disorders, particularly HD, AD, PD, PSP and DLB although further investigations are needed to elucidate the precise role that this enzyme plays in these diseases. However, available evidence indicates that it is the cross-linking function of TG2 that leads to its pathologic activity and,

Conflict of interest

None.

Acknowledgments

M.M.M. is the William Dow Lovett Professor of Neurology and is supported by NIH grants NS059869, NS053517 and NS073994.

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