Hyeseon Cho

Pericyte-specific expression of Rgs5: implications for PDGF and EDG receptor signaling during vascular maturation

RGS proteins finely tune heterotrimeric G‐protein signaling. Implying the need for such fine‐tuning in the developing vascular system, in situ hybridization revealed a striking and extensive expression pattern of Rgs5 in the arterial walls of E12.5–E17.5 mouse embryos. The distribution and location of the Rgs5‐positive cells typified that of pericytes and strikingly overlapped the known expression pattern of platelet‐derived growth factor receptor (PDGFR)‐β. Both E14.5 PDGFR‐β‐ and platelet‐derived growth factor (PDGF)‐B‐deficient mice exhibited markedly reduced levels of Rgs5 in their vascular plexa and small arteries. This likely reflects the loss of pericytes in the mutant mice. RGS5 acts as a potent GTPase activating protein for Giα and Gqα and it attenuated angiotensin II‐, endothelin‐1‐, sphingosine‐1‐phosphate‐, and PDGF‐induced ERK‐2 phosphorylation. Together these results indicate that RGS5 exerts control over PDGFR‐β and GPCR‐mediated signaling pathways active during fetal vascular maturation.

Transcription Profiling of Platelet-Derived Growth Factor-B-Deficient Mouse Embryos Identifies RGS5 as a Novel Marker for Pericytes and Vascular Smooth Muscle Cells

All blood capillaries consist of endothelial tubes surrounded by mural cells referred to as pericytes. The origin, recruitment, and function of the pericytes is poorly understood, but the importance of these cells is underscored by the severe cardiovascular defects in mice genetically devoid of factors regulating pericyte recruitment to embryonic vessels, and by the association between pericyte loss and microangiopathy in diabetes mellitus. A general problem in the study of pericytes is the shortage of markers for these cells. To identify new markers for pericytes, we have taken advantage of the platelet-derived growth factor (PDGF)-B knockout mouse model, in which developing blood vessels in the central nervous system are almost completely devoid of pericytes. Using cDNA microarrays, we analyzed the gene expression in PDGF-B null embryos in comparison with corresponding wild-type embryos and searched for down-regulated genes. The most down-regulated gene present on our microarray was RGS5, a member of the RGS family of GTPase-activating proteins for G proteins. In situ hybridization identified RGS5 expression in brain pericytes, and in pericytes and vascular smooth muscle cells in certain other, but not all, locations. Absence of RGS5 expression in PDGF-B and PDGFR beta-null embryos correlated with pericyte loss in these mice. Residual RGS5 expression in rare pericytes suggested that RGS5 is a pericyte marker expressed independently of PDGF-B/R beta signaling. With RGS5 as a proof-of-principle, our data demonstrate the usefulness of microarray analysis of mouse models for abnormal pericyte development in the identification of new pericyte-specific markers.

Ric-8A and Giα Recruit LGN, NuMA, and Dynein to the Cell Cortex To Help Orient the Mitotic Spindle

ABSTRACT In model organisms, resistance to inhibitors of cholinesterase 8 (Ric-8), a G protein α (Gα) subunit guanine nucleotide exchange factor (GEF), functions to orient mitotic spindles during asymmetric cell divisions; however, whether Ric-8A has any role in mammalian cell division is unknown. We show here that Ric-8A and Gαi function to orient the metaphase mitotic spindle of mammalian adherent cells. During mitosis, Ric-8A localized at the cell cortex, spindle poles, centromeres, central spindle, and midbody. Pertussis toxin proved to be a useful tool in these studies since it blocked the binding of Ric-8A to Gαi, thus preventing its GEF activity for Gαi. Linking Ric-8A signaling to mammalian cell division, treatment of cells with pertussis toxin, reduction of Ric-8A expression, or decreased Gαi expression similarly affected metaphase cells. Each treatment impaired the localization of LGN (GSPM2), NuMA (microtubule binding nuclear mitotic apparatus protein), and dynein at the metaphase cell cortex and disturbed integrin-dependent mitotic spindle orientation. Live cell imaging of HeLa cells expressing green fluorescent protein-tubulin also revealed that reduced Ric-8A expression prolonged mitosis, caused occasional mitotic arrest, and decreased mitotic spindle movements. These data indicate that Ric-8A signaling leads to assembly of a cortical signaling complex that functions to orient the mitotic spindle.

Rgs5 Targeting Leads to Chronic Low Blood Pressure and a Lean Body Habitus

ABSTRACT RGS5 is a potent GTPase-activating protein for Giα and Gqα that is expressed strongly in pericytes and is present in vascular smooth muscle cells. To study the role of RGS5 in blood vessel physiology, we generated Rgs5-deficient mice. The Rgs5−/− mice developed normally, without obvious defects in cardiovascular development or function. Surprisingly, Rgs5−/− mice had persistently low blood pressure, lower in female mice than in male mice, without concomitant cardiac dysfunction, and a lean body habitus. The examination of the major blood vessels revealed that the aortas of Rgs5−/− mice were dilated compared to those of control mice, without altered wall thickness. Isolated aortic smooth muscle cells from the Rgs5−/− mice exhibited exaggerated levels of phosphorylation of vasodilator-stimulated phosphoprotein and extracellular signal-regulated kinase in response to stimulation with either sodium nitroprusside or sphingosine 1-phosphate. The results of this study, along with those of previous studies demonstrating that RGS5 stability is under the control of nitric oxide via the N-end rule pathway, suggest that RGS5 may balance vascular tone by attenuating vasodilatory signaling in vivo in opposition to RGS2, another RGS (regulator of G protein signaling) family member known to inhibit G protein-coupled receptor-mediated vasoconstrictor signaling. Blocking the function or the expression of RGS5 may provide an alternative approach to treat hypertension.

The aorta and heart differentially express RGS (regulators of G-protein signalling) proteins that selectively regulate sphingosine 1-phosphate, angiotensin II and endothelin-1 signalling

Normal cardiovascular development and physiology depend in part upon signalling through G-protein-coupled receptors (GPCRs), such as the angiotensin II type 1 (AT(1)) receptor, sphingosine 1-phosphate (S1P) receptors and endothelin-1 (ET-1) receptor. Since regulator of G-protein signalling (RGS) proteins function as GTPase-activating proteins for the G alpha subunit of heterotrimeric G-proteins, these proteins undoubtedly have functional roles in the cardiovascular system. In the present paper, we show that human aorta and heart differentially express RGS1, RGS2, RGS3S (short-form), RGS3L (long-form), PDZ-RGS3 (PDZ domain-containing) and RGS4. The aorta prominently expresses mRNAs for all these RGS proteins except PDZ-RGS3. Various stimuli that are critical for both cardiovascular development and function regulate dynamically the mRNA levels of several of these RGS proteins in primary human aortic smooth muscle cells. Both RGS1 and RGS3 inhibit signalling through the S1P(1) (formerly known as EDG-1), S1P(2) (formerly known as EDG-5) and S1P(3) (formerly known as EDG-3) receptors, whereas RGS2 and RGS4 selectively attenuate S1P(2)-and S1P(3)-receptor signalling respectively. All of the tested RGS proteins inhibit AT(1)-receptor signalling, whereas only RGS3 and, to a lesser extent, RGS4 inhibit ET(A)-receptor signalling. The conspicuous expression of RGS proteins in the cardiovascular system and their selective effects on relevant GPCR-signalling pathways provide additional evidence that they have functional roles in cardiovascular development and physiology.

Localization of Giα proteins in the centrosomes and at the midbody: implication for their role in cell division

At the plasma membrane, heterotrimeric G proteins act as molecular switches to relay signals from G protein–coupled receptors; however, Gα subunits also have receptor-independent functions at intracellular sites. Regulator of G protein signaling (RGS) 14, which enhances the intrinsic GTPase activity of Giα proteins, localizes in centrosomes, which suggests the coexpression of Giα. We show expression of Giα1, Giα2, and Giα3 in the centrosomes and at the midbody. Fluorescence resonance energy transfer analysis confirms a direct interaction between RGS14 and Giα1 in centrosomes. Expression of GTPase-deficient Giα1 results in defective cytokinesis, whereas that of wild-type or GTPase-deficient Giα3 causes prolonged mitosis. Cells treated with pertussis toxin, with reduced expression of Giα1, Giα2, and Giα3 or with decreased expression of RGS14 also exhibit cytokinesis defects. These results suggest that Giα proteins and their regulators at these sites may play essential roles during mammalian cell division.

HBx targeting to mitochondria and ROS generation are necessary but insufficient for HBV-induced cyclooxygenase-2 expression.

Chronic inflammation can be a major risk factor for cancer development and may contribute to the high worldwide incidence of hepatocellular carcinoma (HCC). Cyclooxygenase-2 (COX-2) is known to be an important mediator of inflammatory responses; however, its link to hepatitis B virus (HBV)-mediated inflammatory responses has not been established. Here, we demonstrate that the expression of COX-2 mRNA and protein was significantly elevated in cells transfected by HBV replicon but not in cells transfected by HBV genome lacking the HBx gene. Notably, COX-2 induction was correlated with HBx’s ability to increase reactive oxygen species (ROS) levels. Consistently with this, antioxidant treatment and ectopic expression of manganese superoxide dismutase or catalase completely abolished COX-2 induction. Interestingly, a mitochondria localization-defective mutant of HBx (HBxΔ68–117) neither increased intracellular ROS levels nor induced COX-2 expression. HBx68–117, which encodes only amino acids 68–117 and is sufficient for mitochondria localization, increased ROS levels but did not induce COX-2 expression. Similarly, HBx targeting to the outer membrane of mitochondria (Mito-HBx) increased ROS but also failed to increase COX-2 expression, suggesting that other cytoplasmic signaling pathways are involved in HBx-mediated COX-2 induction. Indeed, inhibition of cytoplasmic calcium signaling by cyclosporine A, blocking mitochondrial permeability transition pore, and herbimycin, and inhibition of calcium-dependent tyrosine kinase suppressed HBV-mediated COX-2 induction. Thus, the data indicate that both mitochondrial ROS and cytoplasmic calcium signaling are necessary for the COX-2 induction. Our studies revealed a pathophysiological link between HBV infection and hepatic inflammation, and this chain of events might contribute to early steps in HBV-associated liver carcinogenesis.

The mitochondrial ubiquitin ligase MARCH5 resolves MAVS aggregates during antiviral signalling.

Mitochondria serve as platforms for innate immunity. The mitochondrial antiviral signalling (MAVS) protein forms aggregates that elicit robust type-I interferon induction on viral infection, but persistent MAVS signalling leads to host immunopathology; it remains unknown how these signalling aggregates are resolved. Here we identify the mitochondria-resident E3 ligase, MARCH5, as a negative regulator of MAVS aggregates. March5(+/-) mice and MARCH5-deficient immune cells exhibit low viral replication and elevated type-I interferon responses to RNA viruses. MARCH5 binds MAVS only during viral stimulation when MAVS forms aggregates, and these interactions require the RING domain of MARCH5 and the CARD domain of MAVS. MARCH5, but not its RING mutant (MARCH5(H43W)), reduces the level of MAVS aggregates. MARCH5 transfers ubiquitin to Lys7 and Lys500 of MAVS and promotes its proteasome-mediated degradation. Our results indicate that MARCH5 modulates MAVS-mediated antiviral signalling, preventing excessive immune reactions.

The role of type I interferons in intestinal infection, homeostasis, and inflammation.

Type I interferons are a widely expressed family of effector cytokines that promote innate antiviral and antibacterial immunity. Paradoxically, they can also suppress immune responses by driving production of anti‐inflammatory cytokines, and dysregulation of these cytokines can contribute to host‐mediated immunopathology and disease progression. Recent studies describe their anti‐inflammatory role in intestinal inflammation and the locus containing IFNAR, a heterodimeric receptor for the type I interferons has been identified as a susceptibility region for human inflammatory bowel disease. This review focuses on the role of type I IFNs in the intestine in health and disease and their emerging role as immune modulators. Clear understanding of type I IFN‐mediated immune responses may provide avenues for fine‐tuning existing IFN treatment for infection and intestinal inflammation.

Chapter 9 Regulation of Immune Function by G Protein‐Coupled Receptors, Trimeric G Proteins, and RGS Proteins

Receptors for chemokines and a variety of ligands such as histamine, nucleosides, and bioactive lipids signal through heterotrimeric G proteins and play critical roles in immune function. Heterotrimeric G protein signaling pathways are subjected to many layers of regulation including regulators of G protein signaling (RGS) proteins that mainly function to attenuate these signaling pathways. This review focuses on the overall importance of G protein-coupled receptor-heterotrimeric G protein-RGS protein signaling in immune function with emphasis on lymphocyte trafficking and motility. Considerable portion is devoted to discussing mechanisms by which chemoattractant receptors activate downstream signaling pathways that function during leukocyte migration. Studies using intravital imaging techniques to monitor lymphocyte trafficking and motility as well as ones probing intracellular spatiotemporal dynamics of trimeric signaling components are also discussed as they increasingly provide mechanistic insights into trimeric G protein signaling networks.