Expression of G protein αq Subunit is Decreased in Lymphocytes from Patients with Rheumatoid Arthritis and is Correlated with Disease Activity

Gαq modulates the energy metabolism of osteoclasts

Introduction

The bacterial protein toxin Pasteurella multocida toxin (PMT) mediates RANKL-independent osteoclast differentiation. Although these osteoclasts are smaller, their resorptive activity is high which helps in efficient destruction of nasal turbinate bones of pigs.

Methods

The proteome of bone marrow-derived macrophages differentiated into osteoclasts with either RANKL or PMT was analysed. The results were verified by characterizing the metabolic activity using Seahorse analysis, a protein translation assay, immunoblots, real-time PCR as well as flow cytometry-based monitoring of mitochondrial activity and ROS production. A Gαq overexpression system using ER-Hoxb8 cells was used to identify Gαq-mediated metabolic effects on osteoclast differentiation and function.

Results

PMT induces the upregulation of metabolic pathways, which included strong glycolytic activity, increased expression of GLUT1 and upregulation of the mTOR pathway. As OxPhos components were expressed more efficiently, cells also displayed increased mitochondrial respiration. The heterotrimeric G protein Gαq plays a central role in this hypermetabolic cell activation as it triggers mitochondrial relocalisation of pSerSTAT3 and an increase in OPA1 expression. This seems to be caused by a direct interaction between STAT3 and OPA1 resulting in enhanced mitochondrial respiration. Overexpression of Gαq mimicked the hypermetabolic phenotype observed for PMT-induced osteoclasts and resulted in higher glycolytic and mitochondrial activity as well as increased bone resorptive activity. In addition, rheumatoid arthritis (RA) patients showed an increase in GNAQ expression, especially in the synovial fluid.

Discussion

Our study suggests that Gαq plays a key role in PMT-induced osteoclastogenesis. Enhanced expression of GNAQ at the site of inflammation in RA patients indicates its pathophysiological relevance in the context of inflammatory bone disorders.

The p53 status in rheumatoid arthritis with focus on fibroblast-like synoviocytes

P53 is a transcription factor that regulates many signaling pathways like apoptosis, cell cycle, DNA repair, and cellular stress responses. P53 is involved in inflammatory responses through the regulation of inflammatory signaling pathways, induction of cytokines, and matrix metalloproteinase expression. Also, p53 regulates immune responses through modulating Toll-like receptors expression and innate and adaptive immune cell differentiation and maturation. P53 is a modulator of the apoptosis and proliferation processes through regulating multiple anti and pro-apoptotic genes. Rheumatoid arthritis (RA) is categorized as an invasive inflammatory autoimmune disease with irreversible deformity of joints and bone resorption. Different immune and non-immune cells contribute to RA pathogenesis. Fibroblast-like synoviocytes (FLSs) have been recently introduced as a key player in the pathogenesis of RA. These cells in RA synovium produce inflammatory cytokines and matrix metalloproteinases which results in synovitis and joint destruction. Besides, hyper proliferation and apoptosis resistance of FLSs lead to synovial hyperplasia and bone and cartilage destruction. Given the critical role of p53 in inflammation, apoptosis, and cell proliferation, lack of p53 function (due to mutation or low expression) exerts a prominent role for this gene in the pathogenesis of RA. This review focuses on the role of p53 in different mechanisms and cells (specially FLSs) that involved in RA pathogenesis.

STAT4: an immunoregulator contributing to diverse human diseases

Signal transducer and activator of transcription 4 (STAT4) is a member of the STAT family and localizes to the cytoplasm. STAT4 is phosphorylated after a variety of cytokines bind to the membrane, and then dimerized STAT4 translocates to the nucleus to regulate gene expression. We reviewed the essential role played by STAT4 in a wide variety of cells and the pathogenesis of diverse human diseases, especially many kinds of autoimmune and inflammatory diseases, via activation by different cytokines through the Janus kinase (JAK)-STAT signaling pathway.

Expression of Gαq Is Decreased in Lymphocytes from Primary Sjögren's Syndrome Patients and Related to Increased IL-17A Expression

Primary Sjögren's syndrome (pSS) is a rheumatic disease characterized by the destruction of salivary and lacrimal glands, and its pathogenesis mechanism remains unclear. Gαq is the α-subunit of the heterotrimeric Gq protein. Researches demonstrated that Gαq was involved in the pathogenesis regulation of several rheumatic diseases. This study explored the role of Gαq in pSS. Gαq mRNA levels in peripheral blood mononuclear cells (PBMCs) from 39 patients and 26 healthy controls (HCs) were investigated using real-time PCR. IL-17A serum concentrations in 22 pSS patients and 23 HCs were tested by ELISA, and the clinical significance of Gαq was analyzed. The association of Gαq with interleukin-17A (IL-17A) expression was also analyzed in patients with pSS. We showed that Gαq expression in PBMCs from patients with pSS was significantly lower than that in PBMCs from HCs. Gαq expression level was closely associated with pSS disease activity. Furthermore, a negative association was also found in IL-17A and Gαq expression level. These data suggest that Gαq is involved in pSS pathogenesis regulation, possibly due to its regulation of Th17. These results provide new insights into the pSS pathogenesis mechanism involving abnormal Th17 regulation.

Pasteurella multocida Toxin Triggers RANKL-Independent Osteoclastogenesis

Bone remodeling is a continuous process to retain the structural integrity and function of the skeleton. A tight coupling is maintained between osteoclast-mediated resorption of old or damaged bones and osteoblast-mediated formation of new bones for bone homeostasis. While osteoblasts differentiate from mesenchymal stem cells, osteoclasts are hematopoietic in origin and derived from myeloid precursor cells. Osteoclast differentiation is driven by two cytokines, cytokine receptor activator of NF-κB ligand (RANKL), and macrophage colony-stimulating factor. Imbalances in the activity of osteoblasts and osteoclasts result in the development of bone disorders. Bacterially caused porcine atrophic rhinitis is characterized by a loss of nasal ventral conche bones and a distortion of the snout. While Bordetella bronchiseptica strains cause mild and reversible symptoms, infection of pigs with toxigenic Pasteurella multocida strains causes a severe and irreversible decay. The responsible virulence factor Pasteurella multocida toxin (PMT) contains a deamidase activity in its catalytical domain that constitutively activates specific heterotrimeric G proteins to induce downstream signaling cascades. While osteoblasts are inhibited by the toxin, osteoclasts are activated, thus skewing bone remodeling toward excessive bone degradation. Still, the mechanism by which PMT interferes with bone homeostasis, and the reason for this unusual target tissue is not yet well understood. Here, we show that PMT has the potential to differentiate bone marrow-derived macrophages into functional osteoclasts. This toxin-mediated differentiation process is independent of RANKL, a cytokine believed to be indispensable for triggering osteoclastogenesis, as addition of osteoprotegerin to PMT-treated macrophages does not show any effect on PMT-induced osteoclast formation. Although RANKL is not a prerequisite, toxin-primed macrophages show enhanced responsiveness to low concentrations of RANKL, suggesting that the PMT-generated microenvironment offers conditions where low concentrations of RANKL lead to an increase in the number of osteoclasts resulting in increased resorption. PMT-mediated release of the osteoclastogenic cytokines such as IL-6 and TNF-α, but not IL-1, supports the differentiation process. Although the production of cytokines and the subsequent activation of signaling cascades are necessary for PMT-mediated differentiation into osteoclasts, they are not sufficient and PMT-induced activation of G protein signaling is essential for efficient osteoclastogenesis.

Gαq Regulates the Development of Rheumatoid Arthritis by Modulating Th1 Differentiation

The Gαq-containing G protein, an important member of G_q/11 class, is ubiquitously expressed in mammalian cells. Gαq has been found to play an important role in immune regulation and development of autoimmune disease such as rheumatoid arthritis (RA). However, how Gαq participates in the pathogenesis of RA is still not fully understood. In the present study, we aimed to find out whether Gαq controls RA via regulation of Th1 differentiation. We observed that the expression of Gαq was negatively correlated with the expression of signature Th1 cytokine (IFN-γ) in RA patients, which suggests a negative role of Gαq in differentiation of Th1 cells. By using Gαq knockout (Gnaq-/-) mice, we demonstrated that loss of Gαq led to enhanced Th1 cell differentiation. Gαq negative regulated the differentiation of Th1 cell by modulating the expression of T-bet and the activity of STAT4. Furthermore, we detected the increased ratio of Th1 cells in Gnaq-/- bone marrow (BM) chimeras spontaneously developing inflammatory arthritis. In conclusion, results presented in the study demonstrate that loss of Gαq promotes the differentiation of Th1 cells and contributes to the pathogenesis of RA.

Decreased Gαq expression in T cells correlates with enhanced cytokine production and disease activity in systemic lupus erythematosus

Aberrant T cell immune responses appear central to the development of systemic lupus erythematosus (SLE). We previously reported that Gαq, the alpha subunit of Gq, regulates T and B cell immune responses, promoting autoimmunity. To address whether Gαq contributes to the pathogenesis of SLE, Gαq mRNA expression was studied using real time-PCR in PBMCs and T cells from SLE patients as well as age- and sex-matched healthy controls. Our results showed that Gαq mRNA expression was decreased in PBMCs and T cells from SLE patients compared to healthy individuals. Correlation analyses showed that Gαq expression in T cells from SLE patients was associated with disease severity (as per SLE Disease Activity Index), the presence of lupus nephritis, and expression of Th1, Th2 and Th17 cytokines. In keeping with clinical results, T-helper cell subsets (Th1, Th2 and Th17) were over-represented in Gαq knockout mice. In addition, Gαq expression in SLE T cells was negatively correlated with the expression of Bcl-2, an anti-apoptotic gene, and positively correlated with the expression of Bax, a pro-apoptotic gene. These data suggest that reduced Gαq levels in T cells may promote enhanced and prolonged T cell activation, contributing to the clinical manifestations of SLE.

Gq-Coupled Receptors in Autoimmunity

Heterotrimeric G proteins can be divided into Gi, Gs, Gq/11, and G12/13 subfamilies according to their α subunits. The main function of G proteins is transducing signals from G protein coupled receptors (GPCRs), a family of seven transmembrane receptors. In recent years, studies have demonstrated that GPCRs interact with Gq, a member of the Gq/11 subfamily of G proteins. This interaction facilitates the vital role of this family of proteins in immune regulation and autoimmunity, particularly for Gαq, which is considered the functional α subunit of Gq protein. Therefore, understanding the mechanisms through which Gq-coupled receptors control autoreactive lymphocytes is critical and may provide insights into the treatment of autoimmune disorders. In this review, we summarize recent advances in studies of the role of Gq-coupled receptors in autoimmunity, with a focus on their pathologic role and downstream signaling.

Gαq controls rheumatoid arthritis via regulation of Th17 differentiation

Gαq, the α-subunit of Gq protein, is ubiquitously expressed in mammalian cells. It initially attracted attention for its physiological significance in cardiovascular system. In recent years, studies have also indicated the important roles of Gαq in regulating immunity, supplying us a new insight into the mechanism of immune regulation. T helper type 17 (Th17) cells are potent inducers of tissue inflammation. Many studies have shown that Th17 cells are major effector cells in the pathogenesis of many experimental autoimmune diseases and human inflammatory conditions such as rheumatoid arthritis (RA). One of our previous studies has shown that Gαq negatively controls the disease activity of RA. However, how Gαq controls the pathogenesis of autoimmune disease is not clear. Whether this effect is via the regulation of Th17 differentiation is still not known. We aimed to find out the role of Gαq in control of Th17 differentiation. We investigated the relationship between Gαq and Th17 in RA patients. We then investigated the mechanism of how Gαq regulated Th17 differentiation by using Gnaq(-/-) mice. We observed that the expression of Gαq was negatively associated with interleukin-17A expression in RA patients, indicating that Gαq negatively controlled the differentiation of Th17 cells. By using Gnaq(-/-) mice, we demonstrated that Gαq inhibited the differentiation of Th17 cell via regulating the activity of extracellular signal-regulated kinase-1/2 to control the expression of STAT3 (signal transducer and activator of transcription 3) and RORα (RAR-related orphan receptor-α). These data suggest the possibility of targeting Gαq to develop a novel therapeutic regimen for autoimmune disease.

The deficiency of Gαq leads to enhanced T-cell survival

We have previously reported that Gαq, the α subunit of the Gq protein, had important roles in dendritic cell migration, B-cell survival and autoimmunity. In this study, we showed that the deficiency of Gαq led to enhanced T-cell survival. Cultured Gnaq(-/-) T cells exhibited survival advantages both in medium alone and in the presence of anti-CD3 stimulation. Gnaq(-/-) T cells still exhibited a survival advantage when they were cultured in the presence of interleukin (IL)-2 or IL-7. Gnaq(-/-) T cells were more resistant to activation-induced cell death (AICD) in vitro. The survival advantage of Gnaq(-/-) T cells was further confirmed by transferring T cells into syngeneic hosts in vivo. Gαq deficiency might promote T-cell survival by upregulated Bcl-xL expression and downregulated Fas and FasL expressions. Furthermore, upon T-cell receptor (TCR) ligation, Akt activity was increased in Gnaq(-/-) T cells in comparison with wild-type (WT) T cells. The survival advantage of Gnaq(-/-) T cells was significantly attenuated after adding Akt inhibitor. Taken together, our data demonstrated a negative role of Gαq in regulating T-cell survival.

Implications of non-canonical G-protein signaling for the immune system

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which consist of three subunits α, β, and γ, function as molecular switches to control downstream effector molecules activated by G protein-coupled receptors (GPCRs). The GTP/GDP binding status of Gα transmits information about the ligand binding state of the GPCR to intended signal transduction pathways. In immune cells heterotrimeric G proteins impact signal transduction pathways that directly, or indirectly, regulate cell migration, activation, survival, proliferation, and differentiation. The cells of the innate and adaptive immune system abundantly express chemoattractant receptors and lesser amounts of many other types of GPCRs. But heterotrimeric G-proteins not only function in classical GPCR signaling, but also in non-canonical signaling. In these pathways the guanine exchange factor (GEF) exerted by a GPCR in the canonical pathway is replaced or supplemented by another protein such as Ric-8A. In addition, other proteins such as AGS3-6 can compete with Gβγ for binding to GDP bound Gα. This competition can promote Gβγ signaling by freeing Gβγ from rapidly rebinding GDP bound Gα. The proteins that participate in these non-canonical signaling pathways will be briefly described and their role, or potential one, in cells of the immune system will be highlighted.

The regulating function of heterotrimeric G proteins in the immune system

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which consist of an α-, a β- and a γ-subunit, have crucial roles as molecular switches in the regulation of the downstream effector molecules of multiple G protein-coupled receptor signalling pathways, such as phospholipase C and adenylyl cyclase. According to the structural and functional similarities of their α-subunits, G proteins can be divided into four subfamilies: Gαs, Gαi/o, Gαq/11 and Gα12/13. Most of the α- and the βγ-subunits are abundantly expressed on the surface of immune cells. Recent studies have demonstrated that G proteins are a group of important immunomodulatory factors that regulate the migration, activation, survival, proliferation, differentiation and cytokine secretion of immune cells. In this review, we summarise the recent findings on the functions of G proteins in immune regulation and autoimmunity.