RGS4 controls renal blood flow and inhibits cyclosporine-mediated nephrotoxicity

Hydralazine inhibits cysteamine dioxygenase to treat preeclampsia and senesce glioblastoma

The vasodilator hydralazine (HYZ) has been used clinically for ~ 70 years and remains on the World Health Organization's List of Essential Medicines as a therapy for preeclampsia. Despite its longstanding use and the concomitant progress toward a general understanding of vasodilation, the target and mechanism of HYZ have remained unknown. We show that HYZ selectively targets 2-aminoethanethiol dioxygenase (ADO) by chelating its metal cofactor and alkylating one of its ligands. This covalent inactivation slows entry of proteins into the Cys/N-degron pathway that ADO initiates. HYZ's capacity to stabilize regulators of G-protein signaling (RGS4/5) normally marked for degradation by ADO explains its effect on blood vessel tension and comports with prior associations of insufficient RGS levels with human preeclampsia and analogous symptoms in mice. The established importance of ADO in glioblastoma led us to test HYZ in these cell types. Indeed, a single treatment induced senescence, suggesting a potential new HYZ-based therapy for this deadly brain cancer.

Cardiac RGS Proteins in Human Heart Failure and Atrial Fibrillation: Focus on RGS4

The regulator of G protein signaling (RGS) proteins are crucial for the termination of G protein signals elicited by G protein-coupled receptors (GPCRs). This superfamily of cell membrane receptors, by far the largest and most versatile in mammals, including humans, play pivotal roles in the regulation of cardiac function and homeostasis. Perturbations in both the activation and termination of their G protein-mediated signaling underlie numerous heart pathologies, including heart failure (HF) and atrial fibrillation (AFib). Therefore, RGS proteins play important roles in the pathophysiology of these two devasting cardiac diseases, and several of them could be targeted therapeutically. Although close to 40 human RGS proteins have been identified, each RGS protein seems to interact only with a specific set of G protein subunits and GPCR types/subtypes in any given tissue or cell type. Numerous in vitro and in vivo studies in animal models, and also in diseased human heart tissue obtained from transplantations or tissue banks, have provided substantial evidence of the roles various cardiomyocyte RGS proteins play in cardiac normal homeostasis as well as pathophysiology. One RGS protein in particular, RGS4, has been reported in what are now decades-old studies to be selectively upregulated in human HF. It has also been implicated in protection against AFib via knockout mice studies. This review summarizes the current understanding of the functional roles of cardiac RGS proteins and their implications for the treatment of HF and AFib, with a specific focus on RGS4 for the aforementioned reasons but also because it can be targeted successfully with small organic molecule inhibitors.

The therapeutic potential of targeting cardiac RGS4

G protein-coupled receptors (GPCRs) play pivotal roles in regulation of cardiac function and homeostasis. To function properly, every cell needs these receptors to be stimulated only when a specific extracellular stimulus is present, and to be silenced the moment that stimulus is removed. The regulator of G protein signaling (RGS) proteins are crucial for the latter to occur at the cell membrane, where the GPCR normally resides. Perturbations in both activation and termination of G protein signaling underlie numerous heart pathologies. Although more than 30 mammalian RGS proteins have been identified, each RGS protein seems to interact only with a specific set of G protein subunits and GPCR types/subtypes in any given tissue or cell type, and this applies to the myocardium as well. A large number of studies have provided substantial evidence for the roles various RGS proteins expressed in cardiomyocytes play in cardiac physiology and heart disease pathophysiology. This review summarizes the current understanding of the functional roles of cardiac RGS proteins and their implications for the treatment of specific heart diseases, such as heart failure and atrial fibrillation. We focus on cardiac RGS4 in particular, since this isoform appears to be selectively (among the RGS protein family) upregulated in human heart failure and is also the target of ongoing drug discovery efforts for the treatment of a variety of diseases.

Cardiovascular GPCR regulation by regulator of G protein signaling proteins

G protein-coupled receptors (GPCRs) play pivotal roles in regulation of cardiovascular homeostasis across all vertebrate species, including humans. In terms of normal cellular function, termination of GPCR signaling via the heterotrimeric G proteins is equally (if not more) important to its stimulation. The Regulator of G protein Signaling (RGS) protein superfamily are indispensable for GPCR signaling cessation at the cell membrane, and thus, for cellular control of GPCR signaling and function. Perturbations in both activation and termination of G protein signaling underlie many examples of cardiovascular dysfunction and heart disease pathogenesis. Despite the plethora of over 30 members comprising the mammalian RGS protein superfamily, each member interacts with a specific set of second messenger pathways and GPCR types/subtypes in a tissue/cell type-specific manner. An increasing number of studies over the past two decades have provided compelling evidence for the involvement of various RGS proteins in physiological regulation of cardiovascular GPCRs and, consequently, also in the pathophysiology of several cardiovascular ailments. This chapter summarizes the current understanding of the functional roles of RGS proteins as they pertain to cardiovascular, i.e., heart, blood vessel, and platelet GPCR function, with a particular focus on their implications for chronic heart failure pathophysiology and therapy.

Dual assessment of kidney perfusion and pH by exploiting a dynamic CEST-MRI approach in an acute kidney ischemia-reperfusion injury murine model

Several factors can lead to acute kidney injury, but damage following ischemia and reperfusion injuries is the main risk factor and usually develops into chronic disease. MRI has often been proposed as a method with which to assess renal function. It does so by measuring the renal perfusion of an injected Gd-based contrast agent. The use of pH-responsive agents as part of the CEST (chemical exchange saturation transfer)-MRI technique has recently shown that pH homeostasis is also an important indicator of kidney functionality. However, there is still a need for methods that can provide more than one type of information following the injection of a single contrast agent for the characterization of renal function. Herein we propose, for the first time, dynamic CEST acquisition following iopamidol injection to quantify renal function by assessing both perfusion and pH homeostasis. The aim of this study is to assess renal functionality in a murine unilateral ischemia-reperfusion injury model at two time points (3 and 7 days) after acute kidney injury. The renal-perfusion estimates measured with iopamidol were compared with those obtained with a gadolinium-based agent, via a dynamic contrast enhanced (DCE)-MRI approach, to validate the proposed method. Compared with the contralateral kidneys, the clamped ones showed a significant decrease in renal perfusion, as measured using the DCE-MRI approach, which is consistent with reduced filtration capability. Dynamic CEST-MRI findings provided similar results, indicating that the clamped kidneys displayed significantly reduced renal filtration that persisted up to 7 days after the damage. In addition, CEST-MRI pH imaging showed that the clamped kidneys displayed significantly increased pH values, reflecting the disturbance to pH homeostasis. Our results demonstrate that a single CEST-MRI contrast agent can provide multiple types of information related to renal function and can discern healthy kidneys from pathological ones by combining perfusion measurements with renal pH mapping.

Regulators of G protein signaling in cardiovascular function during pregnancy

G protein-coupled receptor signaling mechanisms are implicated in many aspects of cardiovascular control, and dysfunction of such signaling mechanisms is commonly associated with disease states. Investigators have identified a large number of regulator of G protein signaling (RGS) proteins that variously contribute to the modulation of intracellular second-messenger signaling kinetics. These many RGS proteins each interact with a specific set of second-messenger cascades and receptor types and exhibit tissue-specific expression patterns. Increasing evidence supports the contribution of RGS proteins, or their loss, in the pathogenesis of cardiovascular dysfunctions. This review summarizes the current understanding of the functional contributions of RGS proteins, particularly within the B/R4 family, in cardiovascular disorders of pregnancy including gestational hypertension, uterine artery dysfunction, and preeclampsia.

Deletion of mineralocorticoid receptors in smooth muscle cells blunts renal vascular resistance following acute cyclosporine administration

Calcineurin inhibitors such as cyclosporine A (CsA) are still commonly used after renal transplantation, despite CsA--induced nephrotoxicity (CIN), which is partly related to vasoactive mechanisms. The mineralocorticoid receptor (MR) is now recognized as a key player in the control of vascular tone, and both endothelial cell- and vascular smooth muscle cell (SMC)-MR modulate the vasoactive responses to vasodilators and vasoconstrictors. Here we tested whether vascular MR is involved in renal hemodynamic changes induced by CsA. The relative contribution of vascular MR in acute CsA treatment was evaluated using mouse models with targeted deletion of MR in endothelial cell or SMC. Results indicate that MR expressed in SMC, but not in endothelium, contributes to the increase of plasma urea and creatinine, the appearance of isometric tubular vacuolization, and overexpression of a kidney injury biomarker (neutrophil gelatinase--associated lipocalin) after CsA treatment. Inactivation of MR in SMC blunted CsA--induced phosphorylation of contractile proteins. Finally, the in vivo increase of renal vascular resistance induced by CsA was blunted when MR was deleted from SMC cells, and this was associated with decreased L-type Ca2D channel activity. Thus, our study provides new insights into the role of vascular MR in renal hemodynamics during acute CIN, and provides rationale for clinical studies of MR antagonism to manage the side effects of calcineurin inhibitors.

Human vascular progenitor cells derived from renal arteries are endothelial-like and assist in the repair of injured renal capillary networks

Vascular progenitor cells show promise for the treatment of microvasculature endothelial injury. We investigated the function of renal artery progenitor cells derived from radical nephrectomy patients, in animal models of acute ischemic and hyperperfusion injuries. Present in human adventitia, CD34positive/CD105negative cells were clonal and expressed transcription factors Sox2/Oct4 as well as surface markers CXCR4 (CD184)/KDR(CD309) consistent with endothelial progenitor cells. Termed renal artery-derived vascular progenitor cells (RAPC), injected cells were associated with decreased serum creatinine after ischemia/reperfusion, reduced albuminuria after hyperperfusion, and improved blood flow in both models. A small population of RAPC integrated with the renal microvasculature following either experimental injury. At a cellular level, RAPC promoted local endothelial migration in co-culture. Profiling of RAPC microRNA identified high levels of miRNA 218; also found at high levels in exosomes isolated from RAPC conditioned media after cell contact for 24 hours. After hydrogen peroxide-induced endothelial injury, RAPC exosomes harbored Robo-1 transcript; a gene known to be regulated by mir218. Such exosomes enhanced endothelial cell migration in culture in the absence of RAPC. Thus, our work shows the feasibility of pre-emptive pro-angiogenic progenitor cell procurement from a targeted patient population and potential therapeutic use in the form of autologous cell transplantation.

Cyclosporin A upregulates ETB receptor in vascular smooth muscle via activation of mitogen-activating protein kinases and NF-κB pathways

Hypertension is one of the most frequent complications of solid organ transplantation, and cyclosporin A (CsA) plays a predominant role in the pathophysiology of post-transplant hypertension. However, the exact molecular mechanisms of CsA-induced hypertension remain obscure. We previously showed that CsA increased the mRNA expression and contractile function of endothelin B (ETB) receptor in vascular smooth muscle cells. The present study was designed to investigate the underlying mechanisms of CsA-induced upregulation of ETB receptor in vasculature. Rat mesenteric arteries were incubated with CsA in an organ culture system, and results showed that CsA enhanced ETB receptor mRNA in the time- and dose-dependent manner, and increased protein expression levels of ETB receptor after treatment with CsA 10(-5)M for 6h. Furthermore, CsA induced phosphorylation of extracellular regulated protein kinases 1 and 2 (ERK1/2), p38, and translocation of nuclear factor-kappaB (NF-κB) p65 in vasculature. Blocking ERK1/2, p38, or NF-κB activation with their specific inhibitors markedly attenuated CsA-induced upregulation of ETB receptor mRNA expression and protein levels, and ETB receptor-mediated contraction. In summary, this study showed that mitogen-activating protein kinases (ERK1/2 and p38) and the downstream transcriptional factor NF-κB pathways were involved in CsA-induced upregulation of ETB receptor in arterial smooth muscle cells.

RGS4 inhibits angiotensin II signaling and macrophage localization during renal reperfusion injury independent of vasospasm

Vascular inflammation is a major contributor to the severity of acute kidney injury. In the context of vasospasm-independent reperfusion injury we studied the potential anti-inflammatory role of the Gα-related RGS protein, RGS4. Transgenic RGS4 mice were resistant to 25 minute injury, although post-ischemic renal arteriolar diameter was equal to the wild type early after injury. A 10 minute unilateral injury was performed to study reperfusion without vasospasm. Eighteen hours after injury blood flow was decreased in the inner cortex of wild type mice with preservation of tubular architecture. Angiotensin II levels in the kidneys of wild type and transgenic mice were elevated in a sub-vasoconstrictive range 12 and 18 hours after injury. Angiotensin II stimulated pre-glomerular vascular smooth muscle cells (VSMC) to secrete the macrophage chemoattractant, RANTES; a process decreased by angiotensin II R2 (AT2) inhibition. However, RANTES increased when RGS4 expression was suppressed implicating Gα protein activation in an AT2-RGS4-dependent pathway. RGS4 function, specific to VSMC, was tested in a conditional VSMC-specific RGS4 knockout showing high macrophage density by T2 MRI compared to transgenic and non-transgenic mice after the 10 minute injury. Arteriolar diameter of this knockout was unchanged at successive time points after injury. Thus, RGS4 expression, specific to renal VSMC, inhibits angiotensin II-mediated cytokine signaling and macrophage recruitment during reperfusion, distinct from vasomotor regulation.

Selective clinical and immune response of the oligoclonal autoreactive T cells in Omenn patients after cyclosporin A treatment

The immunological hallmark of Omenn syndrome (OS) is the expansion and activation of an oligoclonal population of autoreactive T cells. These cells should be controlled rapidly by immunosuppressive agents, such as cyclosporin A (CsA), to avoid tissue infiltration and to improve the general outcome of the patients. Here we studied the clinical and the immune response to CsA in two Omenn patients and also examined the gene expression profile associated with good clinical response to such therapy. T cell receptor diversity was studied in cells obtained from OS patients during CsA therapy. Characterization of gene expression in these cells was carried out by using the TaqMan low-density array. One patient showed complete resolution of his symptoms after CsA therapy. The other patient showed selective response of his oligoclonal T cell population and combination therapy was required to control his symptoms. Transcriptional profile associated with good clinical response to CsA therapy revealed significant changes in 26·6% of the tested genes when compared with the transcriptional profile of the cells before treatment. Different clinical response to CsA in two OS patients is correlated with their immunological response. Varying clonal expansions in OS patients can cause autoimmune features and can respond differently to immunosuppressive therapy; therefore, additional treatment is sometimes indicated. CsA for OS patients causes regulation of genes that are involved closely with self-tolerance and autoimmunity.

Delayed graft function in the kidney transplant

Acute kidney injury occurs with kidney transplantation and too frequently progresses to the clinical diagnosis of delayed graft function (DGF). Poor kidney function in the first week of graft life is detrimental to the longevity of the allograft. Challenges to understand the root cause of DGF include several pathologic contributors derived from the donor (ischemic injury, inflammatory signaling) and recipient (reperfusion injury, the innate immune response and the adaptive immune response). Progressive demand for renal allografts has generated new organ categories that continue to carry high risk for DGF for deceased donor organ transplantation. New therapies seek to subdue the inflammatory response in organs with high likelihood to benefit from intervention. Future success in suppressing the development of DGF will require a concerted effort to anticipate and treat tissue injury throughout the arc of the transplantation process.

RGS4, a GTPase activator, improves renal function in ischemia-reperfusion injury

Acute kidney dysfunction after ischemia-reperfusion injury (IRI) may be a consequence of persistent intrarenal vasoconstriction. Regulators of G-protein signaling (RGSs) are GTPase activators of heterotrimeric G proteins that can regulate vascular tone. RGS4 is expressed in vascular smooth muscle cells in the kidney; however, its protein levels are low in many tissues due to N-end rule-mediated polyubiquitination and proteasomal degradation. Here, we define the role of RGS4 using a mouse model of IRI comparing wild-type (WT) with RGS4-knockout mice. These knockout mice were highly sensitized to the development of renal dysfunction following injury exhibiting reduced renal blood flow as measured by laser-Doppler flowmetry. The kidneys from knockout mice had increased renal vasoconstriction in response to endothelin-1 infusion ex vivo. The intrinsic renal activity of RGS4 was measured following syngeneic kidney transplantation, a model of cold renal IRI. The kidneys transplanted between knockout and WT mice had significantly reduced reperfusion blood flow and increased renal cell death. WT mice administered MG-132 (a proteasomal inhibitor of the N-end rule pathway) resulted in increased renal RGS4 protein and in an inhibition of renal dysfunction after IRI in WT but not in knockout mice. Thus, RGS4 antagonizes the development of renal dysfunction in response to IRI.

Novel insights into the function of β-cell M3 muscarinic acetylcholine receptors: therapeutic implications

Impaired function of pancreatic β-cells is one of the hallmarks of type 2 diabetes. β-cell function is regulated by the activity of many hormones and neurotransmitters, which bind to specific cell surface receptors. The M(3) muscarinic acetylcholine receptor (M3R) belongs to the superfamily of G protein-coupled receptors and, following ligand dependent activation, selectively activates G proteins of the G(q/11) family. Recent studies with M3R mutant mice strongly suggest that β-cell M3Rs play a central role in promoting insulin release and maintaining correct glucose homeostasis. In this review, we highlight recent studies indicating that β-cell M3Rs and components of downstream signaling pathways might represent promising new targets for the treatment of type 2 diabetes.

Regulator of G protein signaling proteins as drug targets: current state and future possibilities

Regulators of G protein signaling (RGS) proteins have emerged in the past two decades as novel drug targets in many areas of research. Their importance in regulating signaling via G protein-coupled receptors has become evident as numerous studies have been published on the structure and function of RGS proteins. A number of genetic models have also been developed, demonstrating the potential clinical importance of RGS proteins in various disease states, including central nervous system disorders, cardiovascular disease, diabetes, and several types of cancer. Apart from their classical mechanism of action as GTPase-activating proteins (GAPs), RGS proteins can also serve other noncanonical functions. This opens up a new approach to targeting RGS proteins in drug discovery as the view on the function of these proteins is constantly evolving. This chapter summarizes the latest development in RGS protein drug discovery with special emphasis on noncanonical functions and regulatory mechanisms of RGS protein expression. As more reports are being published on this group of proteins, it is becoming clear that modulation of GAP activity might not be the only way to therapeutically target RGS proteins.

RGS4 is a negative regulator of insulin release from pancreatic beta-cells in vitro and in vivo

Therapeutic strategies that augment insulin release from pancreatic beta-cells are considered beneficial in the treatment of type 2 diabetes. We previously demonstrated that activation of beta-cell M(3) muscarinic receptors (M3Rs) greatly promotes glucose-stimulated insulin secretion (GSIS), suggesting that strategies aimed at enhancing signaling through beta-cell M3Rs may become therapeutically useful. M3R activation leads to the stimulation of G proteins of the G(q) family, which are under the inhibitory control of proteins known as regulators of G protein signaling (RGS proteins). At present, it remains unknown whether RGS proteins play a role in regulating insulin release. To address this issue, we initially demonstrated that MIN6 insulinoma cells express functional M3Rs and that RGS4 was by far the most abundant RGS protein expressed by these cells. Strikingly, siRNA-mediated knockdown of RGS4 expression in MIN6 cells greatly enhanced M3R-mediated augmentation of GSIS and calcium release. We obtained similar findings using pancreatic islets prepared from RGS4-deficient mice. Interestingly, RGS4 deficiency had little effect on insulin release caused by activation of other beta-cell GPCRs. Finally, treatment of mutant mice selectively lacking RGS4 in pancreatic beta-cells with a muscarinic agonist (bethanechol) led to significantly increased plasma insulin and reduced blood glucose levels, as compared to control littermates. Studies with beta-cell-specific M3R knockout mice showed that these responses were mediated by beta-cell M3Rs. These findings indicate that RGS4 is a potent negative regulator of M3R function in pancreatic beta-cells, suggesting that RGS4 may represent a potential target to promote insulin release for therapeutic purposes.