Marcela Herrera

Background: Prostaglandin E ₂ ( PGE ₂ ) is a major prostanoid with multiple actions that potentially affect blood pressure ( BP ). PGE ₂ acts through 4 distinct E‐prostanoid ( EP ) receptor isoforms: EP 1 to EP 4. The EP 4 receptor ( EP 4R) promotes PGE ₂ ‐dependent vasodilation, but its role in the pathogenesis of hypertension is not clear. Methods and Results: To address this issue, we studied mice after temporal‐ and cell‐specific deletion of EP 4R. First, using a mouse line with loss of EP 4 expression induced universally after birth, we confirm that EP 4R mediates a major portion of the acute vasodilatory effects of infused PGE ₂ . In addition, EP 4 contributes to control of resting BP , which was increased by 5±1 mm Hg in animals with generalized deficiency of this receptor. We also show that EP 4 is critical for limiting elevations in BP caused by high salt feeding and long‐term infusion of angiotensin II . To more precisely identify the mechanism for these actions, we generated mice in which EP 4R loss is induced after birth and is limited to smooth muscle. In these mice, acute PGE ₂ ‐dependent vasodilation was attenuated, indicating that this response is mediated by EP 4R in vascular smooth muscle cells. However, absence of EP 4R only in this vascular compartment had a paradoxical effect of lowering resting BP , whereas the protective effect of EP 4R on limiting angiotensin II–dependent hypertension was unaffected. Conclusions: Taken together, our findings support a complex role for EP 4R in regulation of BP and in hypertension, which appears to involve actions of the EP 4R in tissues beyond vascular smooth muscle cells.

The renal collecting duct (CD), as the terminal segment of the nephron, is responsible for the final adjustments to the amount of sodium excreted in urine. While angiotensin II modulates reabsorptive functions of the CD, the contribution of these actions to physiological homeostasis is not clear. To examine this question, we generated mice with cell-specific deletion of AT _1A receptors from the CD. Elimination of AT _1A receptors from both principal and intercalated cells (CDKO mice) had no effect on blood pressures at baseline or during successive feeding of low- or high-salt diets. In contrast, the severity of hypertension caused by chronic infusion of angiotensin II was paradoxically exaggerated in CDKO mice compared with controls. In wild-type mice, angiotensin II induced robust expression of cyclooxygenase-2 (COX-2) in renal medulla, primarily localized to intercalated cells. Upregulation of COX-2 was diminished in CDKO mice, resulting in reduced generation of vasodilator prostanoids. This impaired expression of COX-2 has physiological consequences, since administration of a specific COX-2 inhibitor to CDKO and control mice during angiotensin II infusion equalized their blood pressures. Stimulation of COX-2 was also triggered by exposure of isolated preparations of medullary CDs to angiotensin II. Deletion of AT _1A receptors from principal cells alone did not affect angiotensin II–dependent COX2 stimulation, implicating intercalated cells as the main source of COX2 in this setting. These findings suggest a novel paracrine role for the intercalated cell to attenuate the severity of hypertension. Strategies for preserving or augmenting this pathway may have value for improving the management of hypertension.

Diabetic Nephropathy (DN) remains an unmet medical challenge as its prevalence is projected to continue to increase and specific medicines for treatment remain undeveloped. Activation of the immune system, in particular T-cells, is emerging as a possible mechanism underlying DN disease progression in humans and animal models. We hypothesized that inhibition of T-cell activation will ameliorate DN. Interaction of B7-1 (CD80) on the surface of antigen presenting cells with its binding partners, CTLA4 (CD152) and CD28 on T-cells, is essential for T-cell activation. In this study we used the soluble CTLA4-Fc fusion protein Abatacept to block cell surface B7-1, preventing the cellular interaction and inhibiting T-cell activation. When Abatacept was dosed in an animal model of diabetes-induced albuminuria, it reduced albuminuria in both prevention and intervention modes. The number of T-cells infiltrating the kidneys of DN animals correlated with the degree of albuminuria and treatment with Abatacept reduced the number of renal T-cells. As B7-1 induction has been recently proposed to underlie podocyte damage in DN, Abatacept could be efficacious in DN by protecting podocytes. However, this does not appear to be the case as B7-1 was not expressed in: 1) kidneys of DN animals; 2) stimulated human podocytes in culture; or 3) glomeruli of DN patients. We conclude that Abatacept ameliorates DN by blocking systemic T-cell activation and not by interacting with podocytes.

Inappropriate activation of the type 1A angiotensin (AT_1A) receptor contributes to the pathogenesis of hypertension and its associated complications. To define the role for actions of vascular AT_1A receptors in BP regulation and hypertension pathogenesis, we generated mice with cell-specific deletion of AT_1A receptors in smooth muscle cells (SMKO mice) using Loxp technology and Cre transgenes with robust expression in both conductance and resistance arteries. We found that elimination of AT_1A receptors from vascular smooth muscle cells (VSMCs) caused a modest (approximately 7 mmHg) yet significant reduction in baseline BP and exaggerated sodium sensitivity in mice. Additionally, the severity of angiotensin II (Ang II)–dependent hypertension was dramatically attenuated in SMKO mice, and this protection against hypertension was associated with enhanced urinary excretion of sodium. Despite the lower BP, acute vasoconstrictor responses to Ang II in the systemic vasculature were largely preserved (approximately 80% of control levels) in SMKO mice because of exaggerated activity of the sympathetic nervous system rather than residual actions of AT_1B receptors. In contrast, Ang II–dependent responses in the renal circulation were almost completely eliminated in SMKO mice (approximately 5%–10% of control levels). These findings suggest that direct actions of AT_1A receptors in VSMCs are essential for regulation of renal blood flow by Ang II and highlight the capacity of Ang II–dependent vascular responses in the kidney to effect natriuresis and BP control.

Despite the importance of juxtaglomerular cell recruitment in the pathophysiology of cardiovascular diseases, the mechanisms that underlie renin production under conditions of chronic stimulation remain elusive. We have previously shown that CD44+ mesenchymal-like cells (CD44+ cells) exist in the adult kidney. Under chronic sodium deprivation, these cells are recruited to the juxtaglomerular area and differentiate to new renin-expressing cells. Given the proximity of macula densa to the juxtaglomerular area and the importance of macula densa released prostanoids in renin synthesis and release, we hypothesized that chronic sodium deprivation induces macula densa release of prostanoids, stimulating renal CD44+ cell activation and differentiation. CD44+ cells were isolated from adult kidneys and cocultured with the macula densa cell line, MMDD1, in normal or low-sodium medium. Low sodium stimulated prostaglandin E2 production by MMDD1 and induced migration of CD44+ cells. These effects were inhibited by addition of a cyclooxygenase 2 inhibitor (NS398) or an E-prostanoid receptor 4 antagonist (AH23848) to MMDD1 or CD44+ cells, respectively. Addition of prostaglandin E2 to CD44+ cells increased cell migration and induced renin expression. In vivo activation of renal CD44+ cells during juxtaglomerular recruitment was attenuated in wild-type mice subjected to salt restriction in the presence of cyclooxygenase 2 inhibitor rofecoxib. Similar results were observed in E-prostanoid receptor 4 knockout mice subjected to salt restriction. These results show that the prostaglandin E2/E-prostanoid receptor 4 pathway plays a key role in the activation of renal CD44+ mesenchymal stromal cell–like cells during conditions of juxtaglomerular recruitment; highlighting the importance of this pathway as a key regulatory mechanism of juxtaglomerular recruitment.

Renal damage resulting from acute and chronic kidney injury poses an important problem to public health. Currently, patients with end-stage renal disease rely solely on kidney transplantation or dialysis for survival. Emerging therapies aiming to prevent and reverse kidney damage are thus in urgent need. Although the kidney was initially thought to lack the capacity for self-repair, several studies have indicated that this might not be the case; progenitor and stem cells appear to play important roles in kidney repair under various pathological conditions. In this review, we summarize recent findings on the role of progenitor/stem cells on kidney repair as well as discuss their potential as a therapeutic approach for kidney diseases.

The angiotensin II type 1 receptor (AT ₁ R) mediates most hypertensive actions of angiotensin II. To understand the molecular regulation of the AT ₁ R in normal physiology and pathophysiology, methods for sensitive and specific detection of AT ₁ R protein are required. Here, we examined the specificity of a panel of putative AT ₁ R antibodies that are commonly used by investigators in the field. For these studies, we carried out Western blotting and immunohistochemistry with kidney tissue from wild-type mice and genetically modified mice lacking the major murine AT ₁ R isoform, AT _1A (AT _1A KO), or with combined deficiency of both the AT _1A and AT _1B isoforms (AT _1AB KO). For the 3 antibodies tested, Western blots of protein homogenates from wild-type kidneys yielded distinct bands with the expected size range for AT ₁ R. In addition, these bands appeared identical in samples from mice lacking 1 or both murine AT ₁ R isoforms. Additionally, the pattern of immunohistochemical staining in kidneys, liver, and adrenal glands of wild-type mice was very similar to that of AT _1AB KO mice completely lacking all AT ₁ R. We verified the absence of AT ₁ R subtypes in each mouse line by the following: (1) quantitative polymerase chain reaction documenting the absence of mRNA species, and (2) functionally by assessing angiotensin II–dependent vasoconstriction, which was substantially blunted in both AT _1A KOs and AT _1AB KOs. Finally, these antibodies failed to detect epitope-tagged AT _1A R protein overexpressed in human embryonic kidney cells. We conclude that anti-AT ₁ R antibodies available from commercial sources and commonly used in published studies exhibit nonspecific binding in mouse tissue that may lead to erroneous results.