Over-expression of a RhoA-specific guanine nucleotide exchange factor, p190RhoGEF, in mouse dendritic cells negatively regulates cellular responses to bacterial lipopolysaccharide

Over-Expression of p190RhoGEF Regulates the Formation of Atherosclerotic Plaques in the Aorta of ApoE-/- Mice via Macrophage Polarization

The RhoA-specific guanine nucleotide exchange factor p190RhoGEF has been implicated in the control of cell morphology, focal adhesion formation, and cell motility. Previously, we reported that p190RhoGEF is also active in various immune cells. In this study, we examined whether over-expression of p190RhoGEF could affect atherosclerotic plaque formation in mouse aortae. For that purpose, transgenic (TG) mice over-expressing p190RhoGEF were cross-bred with atherosclerosis-prone apolipoprotein E (ApoE)-/- mice to obtain p190RhoGEF-TG mice with ApoE-/- backgrounds (TG/ApoE-/-). Aortic plaque formation was significantly increased in TG/ApoE mice-/- at 30 to 40 weeks of age compared to that in ApoE-/- mice. Serum concentrations of inflammatory cytokines (IL-6 and TNF-α) were greater in TG/ApoE-/- mice than in ApoE-/- mice at ~40 weeks of age. Furthermore, TG/ApoE-/- mice had a greater proportion of peritoneal macrophages within the M1 subset at 30 to 40 weeks of age, together with higher production of inflammatory cytokines and stronger responses to bacterial lipopolysaccharide than ApoE-/- mice. Collectively, these results highlight a crucial role of enhanced p190RhoGEF expression in atherosclerosis progression, including the activation of pro-inflammatory M1 macrophages.

Expression of a RhoA-Specific Guanine Nucleotide Exchange Factor, p190RhoGEF, in Mouse Macrophages Negatively Affects M1 Polarization and Inflammatory Responses

A RhoA-specific guanine nucleotide exchange factor, p190RhoGEF, was first cloned and identified in neuronal cells. In immune cells, we first reported the role of p190RhoGEF in B cells: expression of p190RhoGEF increased after CD40 stimulation and was required for CD40-mediated B cell activation and differentiation. We also showed that over-expression of p190RhoGEF negatively affected dendritic cell function in response to bacterial lipopolysaccharide (LPS). In this study, we examined the role of p190RhoGEF in macrophages using p190RhoGEF over-expressing transgenic (TG) mice. We found macrophages from TG mice to be more round than those from control mice, with enriched polymerized actin at the edge attached to the glass. TG macrophages also responded less to LPS: production of reactive oxygen species, phagocytosis, chemokine-dependent migration, and pro-inflammatory cytokine secretion were all reduced compared with the responses of macrophages from littermate (LTM) control mice. Furthermore, the classical M1 subset population was observed less in the peritoneal macrophages of TG mice than the LTM control mice during LPS-elicited peritoneal inflammation. When the activity of RhoA was inhibited in TG macrophages, their morphology and LPS responses became similar to those of the LTM macrophages. These results suggest that over-expression of p190RhoGEF in macrophages could reduce M1 polarization and inflammatory responses by regulating the actin cytoskeleton.

Selenium Regulation of the Immune Function of Dendritic Cells in Mice Through the ERK, Akt and RhoA/ROCK Pathways

Selenium levels can regulate the function of T cells, macrophages, B cells, natural killer cells and other immune cells. However, the effect of selenium on the immune function of dendritic cells (DCs) isolated from selenium-supplemented mice is unknown. In this study, C57BL/6J mice were randomly divided into three groups and fed diets containing low (0.08 ppm), medium (0.25 ppm) or high (1 ppm) selenium levels for 8 weeks. Immature (imDCs) and mature (mDCs) dendritic cells were then isolated from the bone marrow. Next, the migration, phagocytic capacity and mixed lymphocyte reaction (MLR) for imDCs and mDCs were detected by transwell and flow cytometry. The levels of C-C chemokine receptor type 7 (CCR7), major histocompatibility complex II (MHCII) and reactive oxygen species (ROS) were assayed by flow cytometry. F-actin and superoxide dismutase (SOD) activity was detected by fluorescence microscopy and SOD assay kit, respectively. In addition, the extracellular signal-regulated kinase (ERK), Akt, Ras homolog gene family member A/Rho-associated protein kinase (RhoA/ROCK) signalling, selenoprotein K (SELENOK) and glutathione peroxidase 1 (GPX1) levels were measured by western blot analysis. The results indicated that selenium deficiency enhanced the migration of imDCs by ROS and SELENOK-mediated ERK, Akt and RhoA/ROCK pathways but impaired the antigen uptake of imDCs. Although a high selenium level inhibited the migration of imDCs, it had no effect on phagocytic capacity. For mDCs, low selenium levels impaired free migration, and high levels inhibited the chemotactic migration involved in F-actin and CCR7, respectively. Low and high selenium levels impaired the MLR by inhibiting MHCII surface localisation, which might be related to ROS- and SELENOK-mediated ERK, Akt and RhoA/ROCK signalling pathways. In summary, selenium may regulate the immune function of mouse DCs through the ROS- and SELENOK-mediated ERK, Akt and RhoA/ROCK signalling.

RhoA Signaling in Immune Cell Response and Cardiac Disease

Chronic inflammation, the activation of immune cells and their cross-talk with cardiomyocytes in the pathogenesis and progression of heart diseases has long been overlooked. However, with the latest research developments, it is increasingly accepted that a vicious cycle exists where cardiomyocytes release cardiocrine signaling molecules that spiral down to immune cell activation and chronic state of low-level inflammation. For example, cardiocrine molecules released from injured or stressed cardiomyocytes can stimulate macrophages, dendritic cells, neutrophils and even T-cells, which then subsequently increase cardiac inflammation by co-stimulation and positive feedback loops. One of the key proteins involved in stress-mediated cardiomyocyte signal transduction is a small GTPase RhoA. Importantly, the regulation of RhoA activation is critical for effective immune cell response and is being considered as one of the potential therapeutic targets in many immune-cell-mediated inflammatory diseases. In this review we provide an update on the role of RhoA at the juncture of immune cell activation, inflammation and cardiac disease.

RhoA as a Key Regulator of Innate and Adaptive Immunity

RhoA is a ubiquitously expressed cytoplasmic protein that belongs to the family of small GTPases. RhoA acts as a molecular switch that is activated in response to binding of chemokines, cytokines, and growth factors, and via mDia and the ROCK signaling cascade regulates the activation of cytoskeletal proteins, and other factors. This review aims to summarize our current knowledge on the role of RhoA as a general key regulator of immune cell differentiation and function. The contribution of RhoA for the primary functions of innate immune cell types, namely neutrophils, macrophages, and conventional dendritic cells (DC) to (i) get activated by pathogen-derived and endogenous danger signals, (ii) migrate to sites of infection and inflammation, and (iii) internalize pathogens has been fairly established. In activated DC, which constitute the most potent antigen-presenting cells of the immune system, RhoA is also important for the presentation of pathogen-derived antigen and the formation of an immunological synapse between DC and antigen-specific T cells as a prerequisite to induce adaptive T cell responses. In T cells and B cells as the effector cells of the adaptive immune system Rho signaling is pivotal for activation and migration. More recently, mutations of Rho and Rho-modulating factors have been identified to predispose for autoimmune diseases and as causative for hematopoietic malignancies.

EBI2-mediated bridging channel positioning supports splenic dendritic cell homeostasis and particulate antigen capture

Splenic dendritic cells (DCs) present blood-borne antigens to lymphocytes to promote T cell and antibody responses. The cues involved in positioning DCs in areas of antigen exposure in the spleen are undefined. Here we show that CD4⁺ DCs highly express EBI2 and migrate to its oxysterol ligand, 7α,25-OHC. In mice lacking EBI2 or the enzymes needed for generating normal distributions of 7α,25-OHC, CD4⁺ DCs are reduced in frequency and the remaining cells fail to situate in marginal zone bridging channels. The CD4⁺ DC deficiency can be rescued by LTβR agonism. EBI2-mediated positioning in bridging channels promotes DC encounter with blood-borne particulate antigen. Upon exposure to antigen, CD4⁺ DCs move rapidly to the T-B zone interface and promote induction of helper T cell and antibody responses. These findings establish an essential role for EBI2 in CD4⁺ DC positioning and homeostasis and in facilitating capture and presentation of blood-borne particulate antigens.

DOI:http://dx.doi.org/10.7554/eLife.00757.001

One of the main roles of the spleen is to make the antibodies that protect the body against viruses, bacteria and other microorganisms. Antibodies are made by B cells, which are a type of white blood cell, after they have been exposed to antigens. For most antibody responses, it is also necessary for the B cells to get help from other white blood cells called T cells that have been exposed to antigens. Specialized cells called dendritic cells have a central role in bringing the antigens—which are usually fragments of the infectious agents that have invaded the body—to the T cells.

One subset of dendritic cells, called CD4⁺ dendritic cells, are found in large numbers in a part of the spleen called the bridging channel, but the process by which these cells become localized in this channel has not been fully understood. Now, Yi and Cyster show that a receptor called EBI2, which is found on the surface of CD4⁺ dendritic cells, binds to a type of organic molecule called an oxysterol that is produced in the bridging channel.

In mice that had been genetically engineered to lack EBI2 or the enzymes needed to make this particular oxysterol—which is known as 7α,25-dihydroxycholesterol, or 7α,25-OHC for short—the CD4⁺ dendritic cells were no longer clustered in the bridging channel and their number was markedly decreased. This showed that the interaction between EBI2 and the oxysterol was essential for ensuring that the CD4⁺ dendritic cells were in the right place. The correct positioning of the CD4⁺ dendritic cells was, in turn, necessary for maintaining cell numbers. Moreover, these mice had a weakened immune response because of the very low number of antigens that were being presented to the T cells.

A number of autoimmune diseases, such as lupus, are caused by the body developing an immune response to its own cells and tissues. One implication of the work of Yi and Cyster is that if small molecule inhibitors of EBI2 could be designed, they might be able to suppress the onset of such autoimmune responses.

DOI:http://dx.doi.org/10.7554/eLife.00757.002