An essential link between the kidney and blood pressure control has long been known.
Here, we review evidence supporting the premise that an impaired capacity of the kidney to
excrete sodium in response to elevated blood pressure is a major contributor to
hypertension, irrespective of the initiating cause. In this regard, recent work suggests
that novel pathways controlling key sodium transporters in kidney epithelia have a
critical impact on hypertension pathogenesis, supporting a model in which impaired renal
sodium excretion is a final common pathway through which vascular, neural, and
inflammatory responses raise blood pressure. We also address recent findings calling into
question long-standing notions regarding the relationship between sodium intake and
changes in body fluid volume. Expanded understanding of the role of the kidney as both a
cause and target of hypertension highlights key aspects of pathophysiology and may lead to
identification of new strategies for prevention and treatment.
Diabetic nephropathy is the major cause of end-stage renal disease worldwide. Despite its prevalence, identification of specific factors that cause or predict diabetic nephropathy has been delayed in part by lack of reliable animal models that mimic the disease in humans. The Animal Models of Diabetic Complications Consortium (AMDCC) was created 8 years ago by the National Institutes of Health to develop and characterize models of diabetic nephropathy and other complications. This interim report details the progress made toward that goal, specifically in the development and testing of murine models. Updates are provided on validation criteria for early and advanced diabetic nephropathy, phenotyping methods, the effect of background strain on nephropathy, current best models of diabetic nephropathy, negative models and views of future directions. AMDCC investigators and other investigators in the field have yet to validate a complete murine model of human diabetic kidney disease. Nonetheless, the critical analysis of existing murine models substantially enhances our understanding of this disease process.
Disruption of the regulatory role of the kidneys leads to diverse renal pathologies; one major hallmark is inflammation and fibrosis. Conventional magnitude MRI has been used to study renal pathologies; however, the quantification or even detection of focal lesions caused by inflammation and fibrosis is challenging. We propose that quantitative susceptibility mapping (QSM) may be particularly sensitive for the identification of inflammation and fibrosis. In this study, we applied QSM in a mouse model deficient for angiotensin receptor type 1 (AT1). This model is known for graded pathologies, including focal interstitial fibrosis, cortical inflammation, glomerulocysts and inner medullary hypoplasia. We acquired high-resolution MRI on kidneys from AT1-deficient mice that were perfusion fixed with contrast agent. Two MR sequences were used (three-dimensional spin echo and gradient echo) to produce three image contrasts: T1, T2* (magnitude) and QSM. T1 and T2* (magnitude) images were acquired to segment major renal structures and to provide landmarks for the focal lesions of inflammation and fibrosis in the three-dimensional space. The volumes of major renal structures were measured to determine the relationship of the volumes to the degree of renal abnormalities and magnetic susceptibility values. Focal lesions were segmented from QSM images and were found to be closely associated with the major vessels. Susceptibilities were relatively more paramagnetic in wild-type mice: 1.46 ± 0.36 in the cortex, 2.14 ± 0.94 in the outer medulla and 2.10 ± 2.80 in the inner medulla (10−2 ppm). Susceptibilities were more diamagnetic in knockout mice: −7.68 ± 4.22 in the cortex, −11.46 ± 2.13 in the outer medulla and −7.57 ± 5.58 in the inner medulla (10−2 ppm). This result was consistent with the increase in diamagnetic content, e.g. proteins and lipids, associated with inflammation and fibrosis. Focal lesions were validated with conventional histology. QSM was very sensitive in detecting pathology caused by small focal inflammation and fibrosis. QSM offers a new MR contrast mechanism to study this common disease marker in the kidney.
small animal preclinical imaging; magnetic susceptibility; quantitative susceptibility mapping; AT1; renal structures; pathology; fibrosis; inflammation
The angiotensin II type 1 receptor (AT1R) mediates most hypertensive actions of angiotensin II. In order to understand the molecular regulation of the AT1 receptor in normal physiology and pathophysiology, methods for sensitive and specific detection of AT1R protein are required. Here, we examined the specificity of a panel of putative anti-AT1R antibodies that are commonly used by investigators in the field. For these studies, we carried out Western blotting and immunocytochemistry with kidney tissue from WT mice and genetically modified mice lacking the major murine AT1R isoform, AT1A (AT1AKO), or with combined deficiency of both the AT1A and AT1B isoforms (AT1ABKO). For the 3 antibodies tested, Western blots of protein homogenates from WT kidneys yielded distinct bands with the expected size range for AT1R. In addition, these bands appeared identical in samples from mice lacking one or both murine AT1R isoforms. Additionally, the pattern of immune histo-chemical staining in kidneys, liver and adrenal glands of WT mice was very similar to that of AT1ABKO mice completely lacking all AT1 receptors. We verified the absence of AT1R subtypes in each mouse line by: 1) quantitative PCR documenting the absence of mRNA species and, 2) functionally by assessing angiotensin II-dependent vasoconstriction, which was substantially blunted in both AT1AKOs and AT1ABKOs. Finally, these antibodies failed to detect epitope-tagged AT1AR protein over-expressed in HEK cells. We conclude that anti-AT1R antibodies available from commercial sources and commonly used in published studies exhibit non-specific binding in mouse tissue that may lead to erroneous results.
angiotensin II type 1 receptor; Western blot; cross-reactivity; AT1A; AT1B
The relative contributions of B lymphocytes and plasma cells during allograft rejection remain unclear. Therefore, the effects of B cell depletion on acute cardiac rejection, chronic renal rejection, and skin graft rejection were compared using CD20 or CD19 mAbs. Both CD20 and CD19 mAbs effectively depleted mature B cells, while CD19 mAb treatment depleted plasmablasts and some plasma cells. B cell depletion did not affect acute cardiac allograft rejection, although CD19 mAb treatment prevented allograft-specific IgG production. Strikingly, CD19 mAb treatment significantly reduced renal allograft rejection and abrogated allograft-specific IgG development, while CD20 mAb treatment did not. By contrast, B cell depletion exacerbated skin allograft rejection and augmented the proliferation of adoptively transferred alloantigen-specific CD4+ T cells, demonstrating that B cells can also negatively regulate allograft rejection. Thereby, B cells can either positively or negatively regulate allograft rejection depending on the nature of the allograft and the intensity of the rejection response. Moreover, CD19 mAb may represent a new approach for depleting both B cells and plasma cells to concomitantly impair T cell activation, inhibit the generation of new allograft-specific Abs, or reduce preexisting allograft-specific Ab levels in transplant patients.
Human clinical trials using type 1 angiotensin (AT1) receptor antagonists indicate that angiotensin II is a critical mediator of cardiovascular and renal disease. However, recent studies have suggested that individual tissue pools of AT1 receptors may have divergent effects on target organ damage in hypertension.
We examined the role of AT1 receptors on T lymphocytes in the pathogenesis of hypertension and its complications.
Methods and Results
Deficiency of AT1 receptors on T cells potentiated kidney injury during hypertension with exaggerated renal expression of chemokines and enhanced accumulation of T cells in the kidney. Kidneys and purified CD4+ T cells from “T cell knockout” mice lacking AT1 receptors on T lymphocytes had augmented expression of Th1-associated cytokines including IFN-γ and TNF-α. Within T lymphocytes, the transcription factors T-bet and GATA-3 promote differentiation toward the Th1 and Th2 lineages, respectively, and AT1 receptor-deficient CD4+ T cells had enhanced T-bet / GATA-3 expression ratios favoring induction of the Th1 response. Inversely, mice that were unable to mount a Th1 response due to T-bet deficiency were protected from kidney injury in our hypertension model.
The current studies identify an unexpected role for AT1 receptors on T lymphocytes to protect the kidney in the setting of hypertension by favorably modulating CD4+ T helper cell differentiation.
Hypertension; Kidney disease; T lymphocytes; Inflammation
The renin–angiotensin system (RAS) exercises fundamental control over sodium and water handling in the kidney. Accordingly, dysregulation of the RAS leads to blood pressure elevation with ensuing renal and cardiovascular damage. Recent studies have revealed that the RAS hormonal cascade is more complex than initially posited with multiple enzymes, effector molecules, and receptors that coordinately regulate the effects of the RAS on the kidney and vasculature. Moreover, recently identified tissue-specific RAS components have pleomorphic effects independent of the circulating RAS that influence critical homeostatic mechanisms including the immune response and fetal development. Further characterization of the diverse interactions between the RAS and other signaling pathways within specific tissues should lead to novel treatments for renal and cardiovascular disease.
Renin; Angiotensin; Kidney
Hypertension affects more than 1.5 billion people worldwide but the precise cause of elevated blood pressure (BP) cannot be determined in most affected individuals. Nonetheless, blockade of the renin-angiotensin system (RAS) lowers BP in the majority of patients with hypertension. Despite its apparent role in hypertension pathogenesis, the key cellular targets of the RAS that control BP have not been clearly identified. Here we demonstrate that RAS actions in the epithelium of the proximal tubule have a critical and non-redundant role in determining the level of BP. Abrogation of AT1 angiotensin receptor signaling in the proximal tubule alone is sufficient to lower BP, despite intact vascular responses. Elimination of this pathway reduces proximal fluid reabsorption and alters expression of key sodium transporters, modifying pressure-natriuresis and providing substantial protection against hypertension. Thus, effectively targeting epithelial functions of the proximal tubule of the kidney should be a useful therapeutic strategy in hypertension.
Vascular injury and remodeling are common pathological sequelae of hypertension. Previous studies have suggested that the renin-angiotensin system (RAS) acting through the type I (AT1) angiotensin (AT1)-receptor promotes vascular pathology in hypertension. To study the role of AT1-receptors in this process, we generated mice with cell-specific deletion of AT1-receptors in VSMCs using Cre/Loxp technology. We crossed the SM22α-Cre transgenic mouse line expressing Cre recombinase in smooth muscle cells with a mouse line bearing a conditional allele of the Agtr1a gene (Agtr1a flox), encoding the major murine AT1-receptor isoform (AT1A). In SM22α-Cre+Agtr1a flox/flox (SMKO) mice, AT1A-receptors were efficiently deleted from VSMCs in larger vessels, but not from resistance vessels such as pre-glomerular arterioles. Thus, vasoconstrictor responses to angiotensin II were preserved in SMKOs. To induce hypertensive vascular remodeling, mice were continuously infused with angiotensin II for 4 weeks. During infusion of angiotensin II, blood pressures increased significantly and to a similar extent in SMKOs and controls. In control mice, there was evidence of vascular oxidative stress indicated by enhanced nitrated tyrosine residues in segments of aorta; this was significantly attenuated in SMKOs. Despite these differences in oxidative stress, the extent of aortic medial expansion induced by angiotensin II infusion was virtually identical in both groups. Thus, vascular AT1A-receptors promote oxidative stress in the aortic wall but are not required for remodeling in angiotensin II-dependent hypertension.
angiotensin II; hypertrophy; hyperplasia; aorta; smooth muscle; hypertension
Purpose of the review
The renin-angiotensin system (RAS) is critical for cardiovascular control, impacting normal physiology and disease pathogenesis. Although several biologically active peptides are generated by this system, its major actions are mediated by angiotensin II acting through its type 1 (AT1) and type 2 (AT2) receptors. Along with their effects to influence blood pressure and hemodynamics, recent studies have provided evidence that angiotensin receptors influence a range of processes independent from hemodynamic effects.
This review is focused on new molecular mechanisms underlying actions of AT1 receptors to influence vasoconstriction, inflammation, immune responses, and longevity. Moreover, we also highlight new advances in understanding functions of the AT2 receptor in end-organ damage, emphasizing the AT2 receptor as a potential therapeutic target in cardiovascular diseases.
Here we review recent advances in understanding the role of angiotensin receptors in normal physiology and disease states, focusing on their properties that may contribute to blood pressure regulation, end-organ damage, autoimmune disease and longevity.
Angiotensin receptors; hypertension; aging; vascular function; immunity
Our aim was to determine the contribution of the three angiotensin (Ang) II receptor subtypes (AT1a, AT1b, AT2) to coronary responsiveness, cardiac histopathology, and tissue Ang II levels using mice deficient for one, two, or all three Ang II receptors.
Methods and results
Hearts of knockout mice and their wild-type controls were collected for histochemistry or perfused according to Langendorff, and kidneys were removed to measure tissue Ang II. Ang II dose-dependently decreased coronary flow (CF) and left ventricular systolic pressure (LVSP), and these effects were absent in all genotypes deficient for AT1a, independently of AT1b and AT2. The deletion of Ang II receptors had an effect neither on the morphology of medium-sized vessels in the heart nor on the development of fibrosis. However, the lack of both AT1 subtypes was associated with atrophic changes in the myocardium, a reduced CF and a reduced LVSP. AT1a deletion alone, independently of the presence or absence of AT1b and/or AT2, reduced renal Ang II by 50% despite a five-fold rise of plasma Ang II. AT1b deletion, on top of AT1a deletion (but not alone), further decreased tissue Ang II, while increasing plasma Ang II. In mice deficient for all three Ang II receptors, renal Ang II was located only extracellularly.
The lack of both AT1 subtypes led to a baseline reduction of CF and LVSP, and the effects of Ang II on CF and LVSP were found to be exclusively mediated via AT1a. The lack of AT1a or AT1b does not influence the development or maintenance of normal cardiac morphology, whereas deficiency for both receptors led to atrophic changes in the heart. Renal Ang II levels largely depend on AT1 binding of extracellularly generated Ang II, and in the absence of all three Ang II receptors, renal Ang II is only located extracellularly.
Angiotensin II; G protein-coupled receptors; Genetically modified animals
Prostaglandin (PG) E2 has multiple actions that may affect blood pressure. It is synthesized from arachidonic acid by the sequential actions of phospholipases, cyclooxygenases, and PGE synthases. While microsomal PGE synthase 1 (mPGES1) is the only genetically-verified PGE synthase, results of previous studies examining the consequences of mPGES1-deficiency on blood pressure (BP) are conflicting. To determine whether genetic background modifies the impact of mPGES1 on BP, we generated mPGES1−/− mice on two distinct inbred backgrounds, DBA/1lacJ and 129/SvEv. On the DBA/1 background, baseline BP was similar between wild-type (WT) and mPGES1−/− mice. By contrast, on the 129 background, baseline BPs were significantly higher in mPGES1−/− animals than WT controls. During angiotensin II infusion, the DBA/1 mPGES1−/− and WT mice developed mild hypertension of similar magnitude, while 129-mPGES1−/− mice developed more severe hypertension than WT controls. DBA/1 animals developed only minimal albuminuria in response to angiotensin II infusion. By contrast, WT 129 mice had significantly higher levels of albumin excretion than WT DBA/1 and the extent of albuminuria was further augmented in 129 mPGES1−/− animals. In WT mice of both strains, the increase in urinary excretion of PGE2 with angiotensin II was attenuated in mPGES1−/− animals. Urinary excretion of thromboxane was unaffected by angiotensin II in the DBA/1 lines but increased more than 4-fold in 129 mPGES1−/− mice. These data indicate that genetic background significantly modifies the BP response to mPGES1 deficiency. Exaggerated production of thromboxane may contribute to the robust hypertension and albuminuria in 129 mPGES1-deficient mice.
prostanoids; PGE synthase; blood pressure; strain; hypertension
Drugs and antibodies that interrupt vascular endothelial growth factor (VEGF) signaling pathways improve outcomes in patients with a variety of cancers by inhibiting tumor angiogenesis. A major adverse effect of these treatments is hypertension, suggesting a critical role for VEGF in blood pressure (BP) regulation. However, the physiological mechanisms underlying the control of BP by VEGF are unclear. To address this question, we administered a specific antibody against the major VEGF receptor, VEGFR2, to normal mice and assessed the consequences on BP. Compared to vehicle-treated controls, administration of the anti-VEGFR2 antibody caused a rapid and sustained increase in BP of ≈10 mm Hg. This increase in BP was associated with a significant reduction in renin mRNA expression in the kidney (p=0.019) and in urinary excretion of aldosterone (p<0.05). Treatment with the anti-VEGFR2 antibody also caused marked reduction in expression of endothelial and neuronal nitric oxide synthases (eNOS and nNOS) in the kidney. To examine the role of nitric oxide (NO) in the hypertension caused by blocking VEGFR2, mice were treated with Nω-nitro-L-arginine methyl ester (L-NAME) (20 mg/kg/day), an inhibitor of NO production. L-NAME administration abolished the difference in blood pressure between the vehicle- and anti-VEGFR2-treated groups. Our data suggest that VEGF, acting via VEGFR2, plays a critical role in blood pressure control by promoting NOS expression and NO activity. Interfering with this pathway is likely to be one mechanism underlying hypertension caused by anti-angiogenic agents targeting VEGF.
hypertension; angiogenesis; cancer; vascular endothelial growth factor; nitric oxide
Studies in humans and animal models indicate a key contribution of angiotensin II to the pathogenesis of glomerular diseases. To examine the role of type 1 angiotensin (AT1) receptors in glomerular inflammation associated with autoimmune disease, we generated MRL-Faslpr/lpr (lpr) mice lacking the major murine type 1 angiotensin receptor (AT1A); lpr mice develop a generalized autoimmune disease with glomerulonephritis that resembles SLE. Surprisingly, AT1A deficiency was not protective against disease but instead substantially accelerated mortality, proteinuria, and kidney pathology. Increased disease severity was not a direct effect of immune cells, since transplantation of AT1A-deficient bone marrow did not affect survival. Moreover, autoimmune injury in extrarenal tissues, including skin, heart, and joints, was unaffected by AT1A deficiency. In murine systems, there is a second type 1 angiotensin receptor isoform, AT1B, and its expression is especially prominent in the renal glomerulus within podocytes. Further, expression of renin was enhanced in kidneys of AT1A-deficient lpr mice, and they showed evidence of exaggerated AT1B receptor activation, including substantially increased podocyte injury and expression of inflammatory mediators. Administration of losartan, which blocks all type 1 angiotensin receptors, reduced markers of kidney disease, including proteinuria, glomerular pathology, and cytokine mRNA expression. Since AT1A-deficient lpr mice had low blood pressure, these findings suggest that activation of type 1 angiotensin receptors in the glomerulus is sufficient to accelerate renal injury and inflammation in the absence of hypertension.
The renin-angiotensin system plays a role in the etiology of hypertension and the pathophysiology of cardiac and renal diseases in humans. Ang II is the central product of this system and is involved in regulating immune responses, inflammation, cell growth, and proliferation by acting through Ang II type 1 receptors (AT1 and AT2). Here, we show that targeted disruption of the Agtr1a gene that encodes AT1A results in marked prolongation of life span in mice. Agtr1a–/– mice developed less cardiac and vascular injury, and multiple organs from these mice displayed less oxidative damage than wild-type mice. The longevity phenotype was associated with an increased number of mitochondria and upregulation of the prosurvival genes nicotinamide phosphoribosyltransferase (Nampt) and sirtuin 3 (Sirt3) in the kidney. In cultured tubular epithelial cells, Ang II downregulated Sirt3 mRNA, and this effect was inhibited by an AT1 antagonist. These results demonstrate that disruption of AT1 promotes longevity in mice, possibly through the attenuation of oxidative stress and overexpression of prosurvival genes, and suggests that the Ang II/AT1 pathway may be targeted to influence life span in mammals.
Chagas' disease is caused by infection with the parasite Trypanosoma cruzi. We report that infected, but not uninfected, human endothelial cells (ECs) released thromboxane A2 (TXA2). Physical chromatography and liquid chromatography-tandem mass spectrometry revealed that TXA2 is the predominant eicosanoid present in all life stages of T. cruzi. Parasite-derived TXA2 accounts for up to 90% of the circulating levels of TXA2 in infected wild-type mice, and perturbs host physiology. Mice in which the gene for the TXA2 receptor (TP) has been deleted, exhibited higher mortality and more severe cardiac pathology and parasitism (fourfold) than WT mice after infection. Conversely, deletion of the TXA2 synthase gene had no effect on survival or disease severity. TP expression on somatic cells, but not cells involved in either acquired or innate immunity, was the primary determinant of disease progression. The higher intracellular parasitism observed in TP-null ECs was ablated upon restoration of TP expression. We conclude that the host response to parasite-derived TXA2 in T. cruzi infection is possibly an important determinant of mortality and parasitism. A deeper understanding of the role of TXA2 may result in novel therapeutic targets for a disease with limited treatment options.
Components of the renin-angiotensin system (RAS) are expressed in a number of areas in the brain involved in cardiovascular control. However, it has been difficult to link RAS actions in circumscribed brain regions to specific physiological functions. In a study appearing in this issue of the JCI, Sakai and associates use a combination of sophisticated transgenic techniques and stereotaxic microinjection of recombinant viral vectors to demonstrate a pivotal role in the regulation of thirst and salt appetite of angiotensin II generated in the subfornical organ in the brain (see the related article beginning on page 1088).
The carboxypeptidase ACE2 is a homologue of angiotensin-converting enzyme (ACE). To clarify the physiological roles of ACE2, we generated mice with targeted disruption of the Ace2 gene. ACE2-deficient mice were viable, fertile, and lacked any gross structural abnormalities. We found normal cardiac dimensions and function in ACE2-deficient animals with mixed or inbred genetic backgrounds. On the C57BL/6 background, ACE2 deficiency was associated with a modest increase in blood pressure, whereas the absence of ACE2 had no effect on baseline blood pressures in 129/SvEv mice. After acute Ang II infusion, plasma concentrations of Ang II increased almost 3-fold higher in ACE2-deficient mice than in controls. In a model of Ang II–dependent hypertension, blood pressures were substantially higher in the ACE2-deficient mice than in WT. Severe hypertension in ACE2-deficient mice was associated with exaggerated accumulation of Ang II in the kidney, as determined by MALDI-TOF mass spectrometry. Although the absence of functional ACE2 causes enhanced susceptibility to Ang II–induced hypertension, we found no evidence for a role of ACE2 in the regulation of cardiac structure or function. Our data suggest that ACE2 is a functional component of the renin-angiotensin system, metabolizing Ang II and thereby contributing to regulation of blood pressure.
The aspartyl protease renin was first isolated from the kidney by Tigerstedt more than a century ago. In the kidney, renin secretion is tightly linked to sodium intake and renal perfusion pressure, reflecting the important role of the renin-angiotensin system (RAS) in controlling body fluid volume and blood pressure. The study by Mackins et al. in this issue of the JCI describes a novel source of renin: the mast cell (see the related article beginning on page 1063). This discovery suggests a distinct pathway for activation of the RAS that may have a particular impact on the pathogenesis of chronic tissue injury as well as more acute pathology such as arrhythmias in the heart.
Lipopolysaccharide (LPS) causes apoptotic deletion of CD4+ CD8+ thymocytes, a phenomenon that has been linked to immune dysfunction and poor survival during sepsis. Given the abundance of thromboxane-prostanoid (TP) receptors in CD4+ CD8+ thymocytes and in vitro evidence that thromboxane A2 (TXA2) causes apoptosis of these cells, we tested whether enhanced generation of TXA2 plays a role in LPS-induced thymocyte apoptosis. Mice injected with 50 μg of LPS intraperitoneally displayed a marked increase in generation of TXA2 and prostaglandin E2 in the thymus as well as apoptotic deletion of CD4+ CD8+ thymocytes. Administration of indomethacin or rofecoxib inhibited prostanoid synthesis but did not affect thymocyte death. In contrast, thymocyte apoptosis in response to LPS was significantly attenuated in TP-deficient mice. These studies indicate that TXA2 mediates a portion of apoptotic thymocyte death caused by LPS. The absence of an effect of global inhibition of prostanoid synthesis suggests a complex role for prostanoids in this model.
Angiotensin II, acting through type 1 angiotensin (AT1) receptors, has potent effects that alter renal excretory mechanisms. Control of sodium excretion by the kidney has been suggested to be the critical mechanism for blood pressure regulation by the renin-angiotensin system (RAS). However, since AT1 receptors are ubiquitously expressed, precisely dissecting their physiological actions in individual tissue compartments including the kidney with conventional pharmacological or gene targeting experiments has been difficult. Here, we used a cross-transplantation strategy and AT1A receptor–deficient mice to demonstrate distinct and virtually equivalent contributions of AT1 receptor actions in the kidney and in extrarenal tissues to determining the level of blood pressure. We demonstrate that regulation of blood pressure by extrarenal AT1A receptors cannot be explained by altered aldosterone generation, which suggests that AT1 receptor actions in systemic tissues such as the vascular and/or the central nervous systems make nonredundant contributions to blood pressure regulation. We also show that interruption of the AT1 receptor–mediated short-loop feedback in the kidney is not sufficient to explain the marked stimulation of renin production induced by global AT1 receptor deficiency or by receptor blockade. Instead, the renin response seems to be primarily determined by renal baroreceptor mechanisms triggered by reduced blood pressure. Thus, the regulation of blood pressure by the RAS is mediated by AT1 receptors both within and outside the kidney.
Members of the family of prostanoids, made up of prostaglandins and thromboxanes, are generated via COX-mediated metabolism of arachidonic acid. These lipid mediators exhibit wide-ranging biological actions that include regulating both vasomotor tone and renal sodium excretion. As COX inhibition is often associated with sodium retention leading to edema and hypertension, prostanoids appear to have a role in preventing the development of high blood pressure. On the other hand, prostaglandin E2 (PGE2) and PGI2 have also been implicated as determinants of renin secretion. A new study suggests that PGI2 plays a critical role in stimulating renin release and promoting hypertension following renal artery stenosis.
A family of orphan transporters has been discovered that are structurally related to the Na+-Cl−-dependent neurotransmitter transporters, including the dopamine transporter. One member of this family, the mouse XT2 gene, is predominantly expressed in the kidney and has 95% homology to rat ROSIT (renal osmotic stress-induced Na+-Cl− organic solute cotransporter). To study the physiological functions of this transporter, we generated XT2-knockout mice by gene targeting. XT2−/− mice develop and survive normally with no apparent abnormalities. To attempt to identify potential substrates for XT2, we screened urine from XT2-knockout mice by high-pressure liquid chromatography and mass spectrometry and found significantly elevated concentrations of glycine. To study glycine handling, XT2+/+ and XT2−/− mice were injected with radiolabeled glycine, and urine samples were collected to monitor glycine excretion. After 2 h, XT2−/− mice were found to excrete almost twice as much glycine as the XT2+/+ controls (P = 0.03). To determine whether the absence of the XT2 transporter affected sodium and fluid homeostasis, we measured systolic blood pressure by computerized tail-cuff manometry. Systolic blood pressure was significantly higher in XT2−/− mice (127 ± 3 mmHg) than in wild-type controls (114 ± 2 mmHg; P < 0.001). This difference in systolic blood pressure was maintained on high and low salt feeding. To examine whether the alteration in blood pressure and the defect in glycine handling were related, we measured systolic blood pressure in the XT2−/− mice during dietary glycine supplementation. Glycine loading caused systolic blood pressure to fall in the XT2−/− mice from 127 ± 3 to 115 ± 3 mmHg (P < 0.001), a level virtually identical to that of the wild-type controls. These data suggest that the XT2 orphan transporter is involved in glycine reabsorption and that the absence of this transporter is sufficient to cause hypertension.
Production of prostaglandin E2 (PGE2) is enhanced during inflammation, and this lipid mediator can dramatically modulate immune responses. There are four receptors for PGE2 (EP1–EP4) with unique patterns of expression and different coupling to intracellular signaling pathways. To identify the EP receptors that regulate cellular immune responses, we used mouse lines in which the genes encoding each of the four EP receptors were disrupted by gene targeting. Using the mixed lymphocyte response (MLR) as a model cellular immune response, we confirmed that PGE2 has potent antiproliferative effects on wild-type responder cells. The absence of either the EP1 or EP3 receptors did not alter the inhibitory response to PGE2 in the MLR. In contrast, when responder cells lacked the EP2 receptor, PGE2 had little effect on proliferation. Modest resistance to PGE2 was also observed in EP4–/– responder cells. Reconstitution experiments suggest that EP2 receptors primarily inhibit the MLR through direct actions on T cells. Furthermore, PGE2 modulates macrophage function by activating the EP4 receptor and thereby inhibiting cytokine release. Thus, PGE2 regulates cellular immune responses through distinct EP receptors on different immune cell populations: EP2 receptors directly inhibit T cell proliferation while EP2 and EP4 receptors regulate antigen presenting cells functions.