Dual role for RhoA in suppression and induction of cytokines in the human neutrophil

RhoA balances microglial reactivity and survival during neuroinflammation

Microglia are the largest myeloid cell population in the brain. During injury, disease, or inflammation, microglia adopt different functional states primarily involved in restoring brain homeostasis. However, sustained or exacerbated microglia inflammatory reactivity can lead to brain damage. Dynamic cytoskeleton reorganization correlates with alterations of microglial reactivity driven by external cues, and proteins controlling cytoskeletal reorganization, such as the Rho GTPase RhoA, are well positioned to refine or adjust the functional state of the microglia during injury, disease, or inflammation. Here, we use multi-biosensor-based live-cell imaging approaches and tissue-specific conditional gene ablation in mice to understand the role of RhoA in microglial response to inflammation. We found that a decrease in RhoA activity is an absolute requirement for microglial metabolic reprogramming and reactivity to inflammation. However, without RhoA, inflammation disrupts Ca2+ and pH homeostasis, dampening mitochondrial function, worsening microglial necrosis, and triggering microglial apoptosis. Our results suggest that a minimum level of RhoA activity is obligatory to concatenate microglia inflammatory reactivity and survival during neuroinflammation.

The RhoA-ROCK1/ROCK2 Pathway Exacerbates Inflammatory Signaling in Immortalized and Primary Microglia

Neuroinflammation is a unifying factor among all acute central nervous system (CNS) injuries and chronic neurodegenerative disorders. Here, we used immortalized microglial (IMG) cells and primary microglia (PMg) to understand the roles of the GTPase Ras homolog gene family member A (RhoA) and its downstream targets Rho-associated coiled-coil-containing protein kinases 1 and 2 (ROCK1 and ROCK2) in neuroinflammation. We used a pan-kinase inhibitor (Y27632) and a ROCK1- and ROCK2-specific inhibitor (RKI1447) to mitigate a lipopolysaccharide (LPS) challenge. In both the IMG cells and PMg, each drug significantly inhibited pro-inflammatory protein production detected in media (TNF-α, IL-6, KC/GRO, and IL-12p70). In the IMG cells, this resulted from the inhibition of NF-κB nuclear translocation and the blocking of neuroinflammatory gene transcription (iNOS, TNF-α, and IL-6). Additionally, we demonstrated the ability of both compounds to block the dephosphorylation and activation of cofilin. In the IMG cells, RhoA activation with Nogo-P4 or narciclasine (Narc) exacerbated the inflammatory response to the LPS challenge. We utilized a siRNA approach to differentiate ROCK1 and ROCK2 activity during the LPS challenges and showed that the blockade of both proteins may mediate the anti-inflammatory effects of Y27632 and RKI1447. Using previously published data, we show that genes in the RhoA/ROCK signaling cascade are highly upregulated in the neurodegenerative microglia (MGnD) from APP/PS-1 transgenic Alzheimer's disease (AD) mice. In addition to illuminating the specific roles of RhoA/ROCK signaling in neuroinflammation, we demonstrate the utility of using IMG cells as a model for primary microglia in cellular studies.

Lysophosphatidylcholine induces azurophil granule translocation via Rho/Rho kinase/F-actin polymerization in human neutrophils

Translocation of azurophil granules is pivotal for bactericidal activity of neutrophils, the first-line defense cells against pathogens. Previously, we reported that lysophosphatidylcholine (LPC), an endogenous lipid, enhances bactericidal activity of human neutrophils via increasing translocation of azurophil granules. However, the precise mechanism of LPC-induced azurophil granule translocation was not fully understood. Treatment of neutrophil with LPC significantly increased CD63 (an azurophil granule marker) surface expression. Interestingly, cytochalasin B, an inhibitor of action polymerization, blocked LPC-induced CD63 surface expression. LPC increased F-actin polymerization. LPC-induced CD63 surface expression was inhibited by both a Rho specific inhibitor, Tat-C3 exoenzyme, and a Rho kinase (ROCK) inhibitor, Y27632 which also inhibited LPC-induced F-actin polymerization. LPC induced Rho-GTP activation. NSC23766, a Rac inhibitor, however, did not affect LPC-induced CD63 surface expression. Theses results suggest a novel regulatory mechanism for azurophil granule translocation where LPC induces translocation of azurophil granules via Rho/ROCK/F-actin polymerization pathway.

Modulation of the cell membrane lipid milieu by peroxisomal β-oxidation induces Rho1 signaling to trigger inflammatory responses

Phagocytosis, signal transduction, and inflammatory responses require changes in lipid metabolism. Peroxisomes have key roles in fatty acid homeostasis and in regulating immune function. We find that Drosophila macrophages lacking peroxisomes have perturbed lipid profiles, which reduce host survival after infection. Using lipidomic, transcriptomic, and genetic screens, we determine that peroxisomes contribute to the cell membrane glycerophospholipid composition necessary to induce Rho1-dependent signals, which drive cytoskeletal remodeling during macrophage activation. Loss of peroxisome function increases membrane phosphatidic acid (PA) and recruits RhoGAPp190 during infection, inhibiting Rho1-mediated responses. Peroxisome-glycerophospholipid-Rho1 signaling also controls cytoskeleton remodeling in mouse immune cells. While high levels of PA in cells without peroxisomes inhibit inflammatory phenotypes, large numbers of peroxisomes and low amounts of cell membrane PA are features of immune cells from patients with inflammatory Kawasaki disease and juvenile idiopathic arthritis. Our findings reveal potential metabolic markers and therapeutic targets for immune diseases and metabolic disorders.

Enpp2 Expression by Dendritic Cells Is a Key Regulator in Migration

Enpp2 is an enzyme that catalyzes the conversion of lysophosphatidylcholine (LPC) to lysophosphatidic acid (LPA), which exhibits a wide variety of biological functions. Here, we examined the biological effects of Enpp2 on dendritic cells (DCs), which are specialized antigen-presenting cells (APCs) characterized by their ability to migrate into secondary lymphoid organs and activate naïve T-cells. DCs were generated from bone marrow progenitors obtained from C57BL/6 mice. Enpp2 levels in DCs were regulated using small interfering (si)RNA or recombinant Enpp2. Expression of Enpp2 in LPS-stimulated mature (m)DCs was high, however, knocking down Enpp2 inhibited mDC function. In addition, the migratory capacity of mDCs increased after treatment with rmEnpp2; this phenomenon was mediated via the RhoA-mediated signaling pathway. Enpp2-treated mDCs showed a markedly increased capacity to migrate to lymph nodes in vivo. These findings strongly suggest that Enpp2 is necessary for mDC migration capacity, thereby increasing our understanding of DC biology. We postulate that regulating Enpp2 improves DC migration to lymph nodes, thus improving the effectiveness of cancer vaccines based on DC.

Immunomodulatory Yersinia outer proteins (Yops)-useful tools for bacteria and humans alike

Human-pathogenic Yersinia produce plasmid-encoded Yersinia outer proteins (Yops), which are necessary to down-regulate anti-bacterial responses that constrict bacterial survival in the host. These Yops are effectively translocated directly from the bacterial into the target cell cytosol by the type III secretion system (T3SS). Cell-penetrating peptides (CPPs) in contrast are characterized by their ability to autonomously cross cell membranes and to transport cargo - independent of additional translocation systems. The recent discovery of bacterial cell-penetrating effector proteins (CPEs) - with the prototype being the T3SS effector protein YopM - established a new class of autonomously translocating immunomodulatory proteins. CPEs represent a vast source of potential self-delivering, anti-inflammatory therapeutics. In this review, we give an update on the characteristic features of the plasmid-encoded Yops and, based on recent findings, propose the further development of these proteins for potential therapeutic applications as natural or artificial cell-penetrating forms of Yops might be of value as bacteria-derived biologics.

S1P2/G12/13 Signaling Negatively Regulates Macrophage Activation and Indirectly Shapes the Atheroprotective B1-Cell Population

OBJECTIVES

Monocyte/macrophage recruitment and activation at vascular predilection sites plays a central role in the pathogenesis of atherosclerosis. Heterotrimeric G proteins of the G12/13 family have been implicated in the control of migration and inflammatory gene expression, but their function in myeloid cells, especially during atherogenesis, is unknown.

APPROACH AND RESULTS

Mice with myeloid-specific deficiency for G12/13 show reduced atherosclerosis with a clear shift to anti-inflammatory gene expression in aortal macrophages. These changes are because of neither altered monocyte/macrophage migration nor reduced activation of inflammatory gene expression; on the contrary, G12/13-deficient macrophages show an increased nuclear factor-κB-dependent gene expression in the resting state. Chronically increased inflammatory gene expression in resident peritoneal macrophages results in myeloid-specific G12/13-deficient mice in an altered peritoneal micromilieu with secondary expansion of peritoneal B1 cells. Titers of B1-derived atheroprotective antibodies are increased, and adoptive transfer of peritoneal cells from mutant mice conveys atheroprotection to wild-type mice. With respect to the mechanism of G12/13-mediated transcriptional control, we identify an autocrine feedback loop that suppresses nuclear factor-κB-dependent gene expression through a signaling cascade involving sphingosine 1-phosphate receptor subtype 2, G12/13, and RhoA.

CONCLUSIONS

Together, these data show that selective inhibition of G12/13 signaling in macrophages can augment atheroprotective B-cell populations and ameliorate atherosclerosis.

Viral activation of stress-regulated Rho-GTPase signaling pathway disrupts sites of mRNA degradation to influence cellular gene expression

Viruses are useful tools that often reveal previously unrecognized levels of control within a cell. By studying the oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV), we discovered a new signaling axis in endothelial cells (ECs) that links actin cytoskeleton dynamics to post-transcriptional control of gene expression. Translational repression and rapid decay of mRNAs containing AU-rich elements (AREs) occurs in cytoplasmic RNA granules known as processing bodies (PBs). Rho-GTPase activity influences PB dynamics but mechanistic details remain obscure. We have previously shown that the KSHV Kaposin B protein blocks the degradation of ARE-mRNAs that encode potent cytokines and angiogenic factors, at least in part by preventing PB formation. Moreover, Kaposin B is sufficient to cause marked alterations in endothelial cell physiology including the formation of long parallel actin stress fibers and accelerated migration and angiogenic phenotypes. All of these phenotypes depend on Kaposin B-mediated activation of a non-canonical signaling pathway comprising the stress-inducible kinase MK2, hsp27, p115RhoGEF and RhoA. Accelerated endothelial cell migration and angiogenesis depends on the subsequent activation of the RhoA-dependent kinase ROCK, but PB disruption is ROCK-independent. In this Commentary, we discuss implications of the activation of this signaling axis, and propose mechanistic links between RhoA activation and PB dynamics.

Contribution of human neutrophils in the development of protective immune response during in vitro Leishmania major infection

Stimulation of neutrophils may potentiate immunity to Leishmania major. CpG-containing oligodeoxynucleotide (ODN) has immune stimulatory effects and has been suggested as adjuvants and therapeutics to potentiate efficacy of vaccines and treatments against leishmaniasis. Here, we examined the stimulatory effect of synthetic ODN containing CpG motifs class A and B on cytokine production by neutrophils. Neutrophils from healthy donors responded to CpG-ODN type A, but not to class B, with secretion of IL-8 and following GM-CSF pretreatment with TNF-α production. To test whether neutrophil responses were altered in cutaneous leishmaniasis (CL) and to better understand the role of neutrophils in susceptibility and resistance to disease, we evaluated cytokine responses in GM-CSF preconditioned neutrophils from asymptomatic (Leishmanin skin test positive, LST+) and nonhealing CL individuals to CpG-ODN class A and assessed the expression levels of toll-like receptors (TLR2), 4 and 9. LST+ and healthy donor, but not nonhealing CL neutrophils, responded with TNF-α secretion. Neutrophils from nonhealing CL displayed increased mRNA expression levels of TLR2, 4 and 9 compared to neutrophils from LST+ or healthy donors. Therefore, failure to cure CL is associated with reduced ability of neutrophils to secrete TNF-α and correlates with high TLR 2, 4 and 9 expressions.

An essential role of STIM1, Orai1, and S100A8-A9 proteins for Ca2+ signaling and FcγR-mediated phagosomal oxidative activity

Phagocytosis is a process of innate immunity that allows for the enclosure of pathogens within the phagosome and their subsequent destruction through the production of reactive oxygen species (ROS). Although these processes have been associated with increases of intracellular Ca(2+) concentrations, the mechanisms by which Ca(2+) could regulate the different phases of phagocytosis remain unknown. The aim of this study was to investigate the Ca(2+) signaling pathways involved in the regulation of FcγRs-induced phagocytosis. Our work focuses on IgG-opsonized zymosan internalization and phagosomal ROS production in DMSO-differentiated HL-60 cells and neutrophils. We found that chelation of intracellular Ca(2+) by BAPTA or emptying of the intracellular Ca(2+) store by thapsigargin reduced the efficiency of zymosan internalization. Using an small interfering RNA strategy, our data establish that the observed Ca(2+) release occurs through two isoforms of inositol 1,4,5-triphosphate receptors, ITPR1 and ITPR3. In addition, we provide evidence that phagosomal ROS production is dependent on extracellular Ca(2+) entry. We demonstrate that the observed Ca(2+) influx is supported by ORAI calcium release-activated calcium modulator 1 (Orai1) and stromal interaction molecule 1 (STIM1). This result suggests that extracellular Ca(2+) entry, which is required for ROS production, is mediated by a store-operated Ca(2+) mechanism. Finally, our data identify the complex formed by S100A8 and S100A9 (S100 calcium-binding protein A8 and A9 complex), two Ca(2+)-binding proteins, as the site of interplay between extracellular Ca(2+) entry and intraphagosomal ROS production. Thus, we demonstrate that FcγR-mediated phagocytosis requires intracellular Ca(2+) store depletion for the internalization phase. Then phagosomal ROS production requires extracellular Ca(2+) entry mediated by Orai1/STIM1 and relayed by S100A8-A9 as Ca(2+) sensor.

Rac1 activation induces tumour necrosis factor-α expression and cardiac dysfunction in endotoxemia

Induction of tumour necrosis factor-α (TNF-α) expression leads to myocardial depression during sepsis. However, the underlying molecular mechanisms are not fully understood. The aim of this study was to investigate the role of Rac1 in TNF-α expression and cardiac dysfunction during endotoxemia and to determine the involvement of phosphoinositide-3 kinase (PI3K) in lipopolysaccharide (LPS)-induced Rac1 activation. Our results showed that LPS-induced Rac1 activation and TNF-α expression in cultured neonatal mouse cardiomyocytes. The response was inhibited in Rac1 deficient cardiomyocytes or by a dominant-negative Rac1 (Rac1N17). To determine whether PI3K regulates Rac1 activation, cardiomyocytes were treated with LY294002, a PI3K selective inhibitor. Treatment with LY294002 decreased Rac1 activity as well as TNF-α expression stimulated by LPS. Furthermore, inhibition of PI3K and Rac1 activity decreased LPS-induced superoxide generation which was associated with a significant reduction in ERK1/2 phosphorylation. To investigate the role of Rac1 in myocardial depression during endotoxemia in vivo, wild-type and cardiomyocyte-specific Rac1 deficient mice were treated with LPS (2 mg/kg, i.p.). Deficiency in Rac1 significantly decreased myocardial TNF-α expression and improved cardiac function during endotoxemia. We conclude that PI3K-mediated Rac1 activation is required for induction of TNF-α expression in cardiomyocytes and cardiac dysfunction during endotoxemia. The effect of Rac1 on TNF-α expression seems to be mediated by increased NADPH oxidase activity and ERK1/2 phosphorylation.

RhoH/TTF negatively regulates leukotriene production in neutrophils

Leukotriene B(4) (LTB(4)) is an important proinflammatory lipid mediator generated by neutrophils upon activation. GM-CSF stimulation is known to enhance agonist-mediated LTB(4) production of neutrophils within minutes, a process called "priming". In this study, we demonstrate that GM-CSF also limits the production of LTB(4) by neutrophils via a transcriptional mechanism at later time points. We identified hemopoietic-specific Ras homologous (RhoH)/translocation three four (TTF), which was induced following GM-CSF stimulation in neutrophils, as a key regulator in this process. Neutrophils derived from RhoH/TTF-deficient (Rhoh(-/-)) mice demonstrated increased LTB(4) production upon activation compared with normal mouse neutrophils. Moreover, neutrophils from cystic fibrosis patients expressed enhanced levels of RhoH/TTF and generated less LTB(4) upon activation compared with normal human neutrophils. Taken together, these data suggest that RhoH/TTF represents an inducible feedback inhibitor in neutrophils that is involved in the limitation of innate immune responses.

Cell Defence and Survival

Central to immune defence mechanisms is the role of transcription factor nuclear factor kappa B (NF-kB). This is a complex biochemical topic with ever more controls revealed. NF-kB determines the production of proinflammatory cytokines and chemokines. Pharmacologists step in with possible means of control. Other systems involved in defence include the cyclooxygenase 2 (Cox-2) enzyme and perioxisome proliferator-activated receptors. Insulin receptor activation needs to be seen in context. The mTOR system directs uptake of nutrients by cells. mTOR is suppressed by rapamycin, whose usage is now quite considerable in the control of transplant rejection.

A novel method for long term bone marrow culture and genetic modification of murine neutrophils via retroviral transduction

Neutrophils are a critical component of the innate immune response to invading microbial pathogens. However, an excessive and/or prolonged neutrophil response can result in tissue injury that is thought to underlie the pathogenesis of various inflammatory diseases. The development of novel therapeutic strategies for inflammatory diseases depends on an improved understanding of regulation of neutrophil function. However, investigations into neutrophil function have been constrained in part by the difficulty of genetically modifying neutrophils using current techniques. To overcome this, we have developed a novel method for the genetic modification of murine bone marrow derived progenitor cells using retroviral transduction followed by long term bone marrow culture to generate mature neutrophils. These neutrophils are functionally mature as determined by morphology, surface marker (Gr1, CD11b, CD62L and CXCR2) expression, and functional attributes including the ability to generate superoxide, exocytose granule contents, chemotax, and phagocytose and kill bacteria. Further, the in vitro matured neutrophils are capable of migrating to an inflammatory site in vivo. We utilized this system to express the Bcl-2 transgene in mature neutrophils using the retroviral vectors pMIG and pMIT. Bcl-2 overexpression conferred a substantial delay in spontaneous apoptosis of neutrophils as assessed by annexin V and 7-amino-actinomycin D (7AAD) staining. Moreover, Bcl-2 overexpression did not alter granulopoiesis, as assessed by morphology and surface marker expression. This system enables the genetic manipulation of progenitor cells that can be differentiated in vitro to mature neutrophils that are functional in vitro and in vivo.

Effects of liver X receptor agonist treatment on pulmonary inflammation and host defense

Liver X receptor (LXR) alpha and beta are members of the nuclear receptor superfamily of ligand-activated transcription factors. Best known for triggering "reverse cholesterol transport" gene programs upon their activation by endogenous oxysterols, LXRs have recently also been implicated in regulation of innate immunity. In this study, we define a role for LXRs in regulation of pulmonary inflammation and host defense and identify the lung and neutrophil as novel in vivo targets for pharmacologic LXR activation. LXR is expressed in murine alveolar macrophages, alveolar epithelial type II cells, and neutrophils. Treatment of mice with TO-901317, a synthetic LXR agonist, reduces influx of neutrophils to the lung triggered by inhaled LPS, intratracheal KC chemokine, and intratracheal Klebsiella pneumoniae and impairs pulmonary host defense against this bacterium. Pharmacologic LXR activation selectively modulates airspace cytokine expression induced by both LPS and K. pneumoniae. Moreover, we report for the first time that LXR activation impairs neutrophil motility and identify inhibition of chemokine-induced RhoA activation as a putative underlying mechanism. Taken together, these data define a novel role for LXR in lung pathophysiology and neutrophil biology and identify pharmacologic activation of LXR as a potential tool for modulation of innate immunity in the lung.