Cellular functions of tissue transglutaminase

Abstract

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Figure 1.1

TG2 acting as transglutaminase catalyzes several types of posttranslational modifications of proteins. (1) Protein cross-linking. TG2-mediated transamidation reactions proceed via formation of a N^ε(γ-glutamyl)lysine isopeptide bond between the acceptor Gln residue of the protein 1 and deprotonated Lys donor residue of the protein 2. TG2 displays specificities toward both their Gln and Lys substrates. (2) Protein aminylation. TG2-mediated transamidation reactions occur via incorporation of an amine (H₂NR) into the Gln residue of the acceptor protein. Diamines and polyamines may act as a tether in a bis-glutaminyl adduct between two protein molecules. (3) Deamidation of proteins. TG2-mediated hydrolysis reactions in the absence of amine cosubstrates convert the Gln residues of the reactive protein into the Glu residues. Electron movements are shown by curved arrows. The de novo formed covalent bonds are shown by curved lines.

Figure 1.2

Biological consequences of transglutaminase activity of TG2 on protein substrates. (1) Self-cross-linking of TG2 to protein substrates. TG2 incorporates itself into covalent complexes with protein substrates. (2) TG2 catalyzes the formation of intramolecular isopeptide cross-links between the selected Gln and Lys residues of protein substrates. In (1, 2), TG2 alters the conformation, stability, and functions of protein substrates. (3a) TG2-catalyzed de novo polymerization of protein substrates involves the formation of covalent isopeptide bonds between the protein monomers. (3b) Reinforcement of preexisting noncovalent protein polymers by TG2-mediated covalent cross-linking of protein monomers (enzymatic spotwelding). In (3a, 3b) TG2 modifies the properties of covalently cross-linked protein polymers compared with those of protein monomers (3a) or noncovalent polymers (3b). (4) TG2-mediated monoaminylation of protein substrates. (5) TG2-mediated protein deamidation. In (4, 5) TG2-induced protein modifications alter the activities of protein substrates. Altered biological activities of TG2-modified protein monomers are reflected by darker shades (1, 2, 4, 5); altered biological activities of TG2-modified protein polymers are shown as grid patterns (3a, 3b). The de novo formed covalent bonds are shown by curved lines.

Figure 1.3

Regulation of biological activities of protein substrates by TG2-mediated modifications and their pathophysiological implications. (1) TG2-mediated covalent cross-linking of IκBα leads to proteasomal degradation of the IκBα polymers and depletion of the active monomeric IκBα, causing a constitutive activation of NFκB. This TG2-related mechanism has important consequences for chronic inflammation and cancer. (2) Monoamine hormones (serotonine, norepinephrine, dopamine, etc.) delivered into the cell via monoamine transporters are covalently linked by TG2 to cytoplasmic target proteins, such as small regulatory GTPases Rho1, Rac1, Rab3A, Rab4a, Rab27A, or cytoskeletal components such as α-actin. These TG2-induced posttranslational modifications alter the biological activities of target proteins. The diverse biological effects of these TG2-driven modifications have important implications for diabetes and arterial hypertension. (3) TG2-mediated deamidation is described for several protein substrates such as gliadin peptides, B-crystallins, and Hsp20. These TG2-catalyzed protein modifications appear important for pathogenesis of celiac disease and cataract formation.

Figure 1.4

GTPase activity of TG2/Ghα: the signaling cascade and regulation. GDP-bound heterodimer TG2/Ghα-calreticulin/Ghβ is inactive. (1) Agonist stimulation of transmembrane GPCRs induces exchange of GDP to GTP and dissociation of GTP-bound TG2/Ghα from calreticulin/Ghβ. (2) GTP-bound TG2/Ghα activates PLCδ1. (3, 4) Signal termination occurs with GTP hydrolysis (3) and reassociation of GDP-bound TG2/Ghα with free calreticulin/Ghβ (4). (5) PLCδ1 promotes coupling efficiency of this signaling system through its GEF function and stabilization of GTP-bound TG2/Ghα. (6) PLCδ1 catalyzes hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) to diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP₃), causing an increase in intracellular [Ca²⁺]. (7) The switch of GTPase activity of TG2/Ghα to transglutaminase activity of TG2 in cells is triggered by elevation of intracellular [Ca²⁺] and decrease of guanine nucleotides.

Figure 1.5

Regulation of TG2 expression. A number of stressors, hormones, growth factors, cytokines, chemokines, and oncogenes impact TG2 mRNA expression levels through transcriptional regulation via several regulatory elements in the promoter region of the gene, or posttranslationally modulate TG2 protein levels by modulating the rate of its proteasomal degradation. Solid lines represent the established transcriptional or posttranslational regulatory cascades while dotted lines reflect currently undetermined pathways. Dashed line depicts the EGF-mediated effect of shifting cytoplasmic TG2 to the inner side of plasma membrane at the leading edge.

Figure 1.6

Enzymatic and nonenzymatic activities of TG2 in diverse cellular compartments. The adapter/scaffolding nonenzymatic function of TG2 (Ad/Sc) and its transglutaminase (TG), GTPase/ATPase (G/A), protein disulfide isomerase (PDI), and protein kinase (PK) enzymatic activities are shown for the protein localized in the cytoplasm, underneath the plasma membrane (PM), in the nucleus, in mitochondria (MT), in early/late/recycling endosomes (E/L/RE) and lysosomes (LY), and on the cell surface, in the ECM, and in extracellular microvesicles (MV).

Figure 1.7

The TG2-containing adhesive/signaling complexes on the cell surface. Solid black lines indicate TG2-mediated activation of cytoplasmic targets by transmembrane signaling receptors. Dotted black line marks binding of activated PKCα to the integrin cytoplasmic tails that causes their redistribution on the cell surface. Dashed gray lines outline the activation of syndecan-2 by intracellular PKCα and syndecan-2-mediated activation of ROCK that induces stress fiber and focal adhesion formation. Dashed black line marks the nuclear translocation of β-catenin that leads to its complex formation with Tcf/Lef and activation of gene transcription. Curved black line indicates the principal pathway of surface TG2 internalization. Dashed double black line depicts the unknown pathway of GPR56-induced Gα_q activation that inhibits tumor cell growth and metastasis.

Figure 1.8

Dynamic regulation of cell-surface TG2 levels and functions. (1) TG2 externalization. The unconventional pathway of cytoplasmic TG2 secretion involves phospholipid-dependent delivery into recycling endosomes. Solid lines mark the major endosomal recycling pathway that operates via the perinuclear recycling endosomal compartment. Dashed line indicates the PI(3)P-dependent recruitment of cytoplasmic TG2 (hexagons) to the membranes of the perinuclear recycling compartment. (2) Endocytosis of TG2. The constitutive LRP1-dependent internalization and lysosomal degradation of cell-surface TG2. Solid lines mark the major endosomal recycling and lysosomal degradative pathways.

Figure 1.9

TG2 as a novel transcriptional coregulator in the nucleus. (1) TG2-dependent enzymatic cross-linking and polymerization of the SP1 transcription factor in the nucleus causes its inactivation and inhibits SP1-mediated transcription of the prosurvival c-Met gene in hepatocytes. This transamidation-dependent mechanism mediated by nuclear TG2 is involved in liver steatohepatitis. (2) TG2 binds noncovalently to c-Jun in the nucleus and prevents c-Jun/c-Fos dimerization, thereby decreasing AP1-dependent transcription of the MMP9 gene in cardiomyoblasts. This nonenzymatic mechanism mediated by nuclear TG2 is thought to be involved in ECM remodeling. (3) TG2 interacts noncovalently with HIF1β in the nucleus and prevents its dimerization with HIF1α, thus inhibiting HIF1 binding to the HRE in the promoter region of Bnip3 gene and decreasing its transcription in neuronal cells. This nonenzymatic nuclear TG2-driven mechanism is implicated in the prosurvival effect of TG2 in stroke.