Micropuncture determination of nephron function in mice without tissue angiotensin-converting enzyme

Modeling normal and nephrotic axial uptake of albumin and other filtered proteins along the proximal tubule

KEY POINTS

We used new and published data to develop a mathematical model that predicts the profile of albumin uptake in the mouse proximal tubule (PT) in normal and nephrotic states, and partially accounts for competitive inhibition of uptake by normally filtered and pathologic ligands. Three pathways, consisting of high-affinity uptake by cubilin receptors, low-affinity uptake by megalin receptors, and fluid phase uptake, contribute to the overall retrieval of filtered proteins. The axial profile and efficiency of protein uptake depend on the initial filtrate composition and the individual protein affinities for megalin and cubilin. Under normal conditions, the majority of albumin is retrieved in S1 but shifts to S2 under nephrotic conditions. Other proteins exhibit different uptake profiles. Our model explains how tubular proteinuria can occur despite a large excess in potential PT uptake capacity.

ABSTRACT

Recent studies indicate that filtered albumin is retrieved in the proximal tubule (PT) via three pathways: receptor-mediated endocytosis via cubilin (high affinity) and megalin (low affinity), and fluid-phase uptake. Expression of megalin is required to maintain all three pathways, making it challenging to determine their respective contributions. Moreover, uptake of filtered molecules varies between the sub-segments (S1, S2, and S3) that make up the PT. Here we used new and published data to develop a mathematical model that predicts the rates of albumin uptake in mouse PT sub-segments in normal and nephrotic states, and partially accounts for competition by β2-microglobulin (β2m) and Immunoglobulin G (IgG). Our simulations indicate that receptor-mediated, rather than fluid-phase uptake, accounts for the vast majority of ligand recovery. Our model predicts that ∼75% of normally filtered albumin is reabsorbed via cubilin; however, megalin-mediated uptake predominates under nephrotic conditions. Our results also suggest that ∼80% of albumin is normally recovered in S1, whereas nephrotic conditions or knockout of cubilin shifts the bulk of albumin uptake to S2. The model predicts β2m and IgG axial recovery profiles qualitatively similar to those of albumin under normal conditions. In contrast with albumin however, the bulk of IgG and β2m uptake still occurs in S1 under nephrotic conditions. Overall, our model provides a kinetic rationale for why tubular proteinuria can occur even though a large excess in potential PT uptake capacity exists, and suggests testable predictions to expand our understanding of the recovery profile of filtered proteins along the PT. Abstract figure legend. Data from mouse models and from cultured proximal tubule (PT) cells were used to create a mathematical model that predicts the uptake profile of albumin and other filtered ligands along the mouse PT in normal and nephrotic states. The distinct contributions of cubilin receptors (magenta), megalin receptors (green), and fluid phase uptake (blue) to total albumin retrieval (black) in S1, S2, and S3 subsegments of the PT are delineated. Under normal conditions, albumin is primarily recovered in the S1 segment by cubilin, whereas the majority is retrieved in S2 under nephrotic conditions. Other proteins exhibit strikingly different uptake profiles. Our model explains how the distribution and capacity of high-affinity and low-affinity uptake pathways enable uptake of albumin over a broad range of filtered concentrations, and how tubular proteinuria can occur despite a large excess in potential PT uptake capacity. Created with BioRender.com. This article is protected by copyright. All rights reserved.

Advances in use of mouse models to study the renin-angiotensin system

The renin-angiotensin system (RAS) is a highly complex hormonal cascade that spans multiple organs and cell types to regulate solute and fluid balance along with cardiovascular function. Much of our current understanding of the functions of the RAS has emerged from a series of key studies in genetically-modified animals. Here, we review key findings from ground-breaking transgenic models, spanning decades of research into the RAS, with a focus on their use in studying blood pressure. We review the physiological importance of this regulatory system as evident through the examination of mouse models for several major RAS components: angiotensinogen, renin, ACE, ACE2, and the type 1 A angiotensin receptor. Both whole-animal and cell-specific knockout models have permitted critical RAS functions to be defined and demonstrate how redundancy and multiplicity within the RAS allow for compensatory adjustments to maintain homeostasis. Moreover, these models present exciting opportunities for continued discovery surrounding the role of the RAS in disease pathogenesis and treatment for cardiovascular disease and beyond.

Effects of ATRAP in Renal Proximal Tubules on Angiotensin-Dependent Hypertension

Background We have previously shown that ATRAP (angiotensin II receptor-associated protein; Agtrap) interacts with AT1R (angiotensin II type 1 receptor) and promotes constitutive internalization of AT 1R so as to inhibit hyperactivation of its downstream signaling. In response to angiotensin II , systemic ATRAP deficiency exacerbates angiotensin II -mediated hypertension via hyperactivation of renal tubular AT 1R. Although ATRAP expression is abundant in renal proximal tubules, little is known about the actual function of renal proximal tubule ATRAP in angiotensin-mediated hypertension. Methods and Results In this study, we examined the in vivo functional role of renal proximal tubule ATRAP in angiotensin-dependent hypertension. We succeeded in generating proximal tubule-specific ATRAP knockout ( PT - KO ) mice for the first time using the Cre/loxP system with Pepck-Cre. Detailed analysis of renal ATRAP expression in PT - KO mice estimated by immunohistochemical and laser-capture microdissection analysis revealed that ATRAP mRNA expression decreased by ≈80% in proximal regions of the nephron in PT - KO mice compared with wild-type ( WT ) mice. We compared blood pressure of PT - KO and WT mice using both tail-cuff and radiotelemetric methods. Blood pressure of PT - KO mice was comparable with that of WT mice at baseline. Moreover, no significant differences were noted in pressor response to angiotensin II (600 ng/kg per min or 1000 ng/kg per minute) infusion between PT - KO and WT mice. In addition, angiotensin II -mediated cardiac hypertrophy was identical between PT - KO and WT mice. Conclusions ATRAP deficiency in proximal tubules did not exacerbate angiotensin-dependent hypertension in vivo. The results indicate that renal proximal tubule ATRAP has a minor role in angiotensin-dependent hypertension in vivo.

Intratubular and intracellular renin-angiotensin system in the kidney: a unifying perspective in blood pressure control

The renin-angiotensin system (RAS) is widely recognized as one of the most important vasoactive hormonal systems in the physiological regulation of blood pressure and the development of hypertension. This recognition is derived from, and supported by, extensive molecular, cellular, genetic, and pharmacological studies on the circulating (tissue-to-tissue), paracrine (cell-to-cell), and intracrine (intracellular, mitochondrial, nuclear) RAS during last several decades. Now, it is widely accepted that circulating and local RAS may act independently or interactively, to regulate sympathetic activity, systemic and renal hemodynamics, body salt and fluid balance, and blood pressure homeostasis. However, there remains continuous debate with respect to the specific sources of intratubular and intracellular RAS in the kidney and other tissues, the relative contributions of the circulating RAS to intratubular and intracellular RAS, and the roles of intratubular compared with intracellular RAS to the normal control of blood pressure or the development of angiotensin II (ANG II)-dependent hypertension. Based on a lecture given at the recent XI International Symposium on Vasoactive Peptides held in Horizonte, Brazil, this article reviews recent studies using mouse models with global, kidney- or proximal tubule-specific overexpression (knockin) or deletion (knockout) of components of the RAS or its receptors. Although much knowledge has been gained from cell- and tissue-specific transgenic or knockout models, a unifying and integrative approach is now required to better understand how the circulating and local intratubular/intracellular RAS act independently, or with other vasoactive systems, to regulate blood pressure, cardiovascular and kidney function.

Genetically increased angiotensin I-converting enzyme alters peripheral and renal vascular reactivity to angiotensin II and bradykinin in mice

Angiotensin I-converting enzyme (ACE) levels in humans are under strong genetic influence. Genetic variation in ACE has been linked to risk for and progression of cardiovascular and renal diseases. Causality has been documented in genetically modified mice, but the mechanisms underlying causality are not completely elucidated. To further document the vascular and renal consequences of a moderate genetic increase in ACE synthesis, we studied genetically modified mice carrying three copies of the ACE gene (three-copy mice) and littermate wild-type animals (two-copy mice). We investigated peripheral and renal vascular reactivity to angiotensin II and bradykinin in vivo by measuring blood pressure and renal blood flow after intravenous administration and also reactivity of isolated glomerular arterioles by following intracellular Ca2+ mobilization. Carrying three copies of the ACE gene potentiated the systemic and renal vascular responses to angiotensin II over the whole range of peptide concentration tested. Consistently, the response of isolated glomerular afferent arterioles to angiotensin II was enhanced in three-copy mice. In these mice, signaling pathways triggered by endothelial activation by bradykinin or carbachol in glomerular arterioles were also altered. Although the nitric oxide (NO) synthase (NOS)/NO pathway was not functional in arterioles of two-copy mice, in muscular efferent arterioles of three-copy mice NOS3 gene expression was induced and NO mediated the effect of bradykinin or carbachol. These data document new and unexpected vascular consequences of a genetic increase in ACE synthesis. Enhanced vasoconstrictor effect of angiotensin II may contribute to the risk for cardiovascular and renal diseases linked to genetically high ACE levels.

NEW & NOTEWORTHY A moderate genetic increase in angiotensin I-converting enzyme (ACE) in mice similar to the effect of the ACE gene D allele in humans unexpectedly potentiates the systemic and renal vasoconstrictor responses to angiotensin II. It also alters the endothelial signaling pathways triggered by bradykinin or carbachol in glomerular efferent arterioles.

Concurrent activation of multiple vasoactive signaling pathways in vasoconstriction caused by tubuloglomerular feedback: a quantitative assessment

Tubuloglomerular feedback (TGF) describes the negative relationship between (a) NaCl concentration at the macula densa and (b) glomerular filtration rate or glomerular capillary pressure. TGF-induced vasoconstriction of the afferent arteriole results from the enhanced effect of several vasoconstrictors with an effect size sequence of adenosine = 20-HETE > angiotensin II > thromboxane = superoxide > renal nerves > ATP. TGF-mediated vasoconstriction is limited by the simultaneous release of several vasodilators with an effect size sequence of nitric oxide > carbon monoxide = kinins > adenosine. The sum of the constrictor effects exceeds that of the dilator effects by the magnitude of the TGF response. The validity of the additive model used in this analysis can be tested by determining the effect of combined inhibition of some or all agents contributing to TGF. Multiple independent contributors to TGF are consistent with the variability of TGF and of the factors contributing to TGF resetting.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

ACE overexpression in myelomonocytic cells: effect on a mouse model of Alzheimer's disease

While it is well known that angiotensin converting enzyme (ACE) plays an important role in blood pressure control, ACE also has effects on renal function, hematopoiesis, reproduction, and aspects of the immune response. ACE 10/10 mice overexpress ACE in myelomonocytic cells. Macrophages from these mice have an increased polarization towards a pro-inflammatory phenotype that results in a very effective immune response to challenge by tumors or bacterial infection. In a mouse model of Alzheimer's disease (AD), the ACE 10/10 phenotype provides significant protection against AD pathology, including reduced inflammation, reduced burden of the neurotoxic amyloid-β protein and preserved cognitive function. Taken together, these studies show that increased myelomonocytic ACE expression in mice alters the immune response to better defend against many different types of pathologic insult, including the cognitive decline observed in an animal model of AD.

Renal angiotensin-converting enzyme is essential for the hypertension induced by nitric oxide synthesis inhibition

The kidney is an important source of angiotensin-converting enzyme (ACE) in many species, including humans. However, the specific effects of local ACE on renal function and, by extension, BP control are not completely understood. We previously showed that mice lacking renal ACE, are resistant to the hypertension induced by angiotensin II infusion. Here, we examined the responses of these mice to the low-systemic angiotensin II hypertensive model of nitric oxide synthesis inhibition with L-NAME. In contrast to wild-type mice, mice without renal ACE did not develop hypertension, had lower renal angiotensin II levels, and enhanced natriuresis in response to L-NAME. During L-NAME treatment, the absence of renal ACE was associated with blunted GFR responses; greater reductions in abundance of proximal tubule Na(+)/H(+) exchanger 3, Na(+)/Pi co-transporter 2, phosphorylated Na(+)/K(+)/Cl(-) cotransporter, and phosphorylated Na(+)/Cl(-) cotransporter; and greater reductions in abundance and processing of the γ isoform of the epithelial Na(+) channel. In summary, the presence of ACE in renal tissue facilitates angiotensin II accumulation, GFR reductions, and changes in the expression levels and post-translational modification of sodium transporters that are obligatory for sodium retention and hypertension in response to nitric oxide synthesis inhibition.

Renal angiotensin-converting enzyme and blood pressure control

PURPOSE OF REVIEW

This review presents novel findings regarding the renal angiotensin-converting enzyme (ACE) and its role in blood pressure (BP) control.

RECENT FINDINGS

The textbook flow diagram of the renin-angiotensin system (RAS) shows the pulmonary endothelium as the main source of the ACE that converts angiotensin I to angiotensin II. However, ACE is made in large quantities by the kidneys, which raises the important question of what precisely is the function of renal ACE? Recent studies in gene-targeted mice indicates that renal ACE plays a dominant role in regulating the response of the kidney to experimental hypertension. In particular, renal ACE and locally generated angiotensin II affect the activity of several key sodium transporters and the induction of sodium and water retention resulting in the elevation of BP.

SUMMARY

New experimental data link the renal ACE/angiotensin II pathway and the local regulation of sodium transport as key elements in the development of hypertension.

New frontiers in the intrarenal Renin-Angiotensin system: a critical review of classical and new paradigms

The renin-angiotensin system (RAS) is well-recognized as one of the oldest and most important regulators of arterial blood pressure, cardiovascular, and renal function. New frontiers have recently emerged in the RAS research well beyond its classic paradigm as a potent vasoconstrictor, an aldosterone release stimulator, or a sodium-retaining hormone. First, two new members of the RAS have been uncovered, which include the renin/(Pro)renin receptor (PRR) and angiotensin-converting enzyme 2 (ACE2). Recent studies suggest that prorenin may act on the PRR independent of the classical ACE/ANG II/AT1 receptor axis, whereas ACE2 may degrade ANG II to generate ANG (1-7), which activates the Mas receptor. Second, there is increasing evidence that ANG II may function as an intracellular peptide to activate intracellular and/or nuclear receptors. Third, currently there is a debate on the relative contribution of systemic versus intrarenal RAS to the physiological regulation of blood pressure and the development of hypertension. The objectives of this article are to review and discuss the new insights and perspectives derived from recent studies using novel transgenic mice that either overexpress or are deficient of one key enzyme, ANG peptide, or receptor of the RAS. This information may help us better understand how ANG II acts, both independently or through interactions with other members of the system, to regulate the kidney function and blood pressure in health and disease.

Rediscovering ACE: novel insights into the many roles of the angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is best known for the catalytic conversion of angiotensin I to angiotensin II. However, the use of gene-targeting techniques has led to mouse models highlighting many other biochemical properties and actions of this enzyme. This review discusses recent studies examining the functional significance of ACE tissue-specific expression and the presence in ACE of two independent catalytic sites with distinct substrates and biological effects. It is these features which explain why ACE makes important contributions to many different physiological processes including renal development, blood pressure control, inflammation, and immunity.

Fluid reabsorption in proximal convoluted tubules of mice with gene deletions of claudin-2 and/or aquaporin1

Deletions of claudin-2 (Cldn2) and aquaporin1 (AQP1) reduce proximal fluid reabsorption (PFR) by about 30% and 50%, respectively. Experiments were done to replicate these observations and to determine in AQP1/claudin-2 double knockout mice (DKO) if the effects of deletions of these established water pores are additive. PFR was determined in inactin/ketamine-anesthetized mice by free-flow micropuncture using single-nephron I(125)-iothalamate (io) clearance. Animal means of PFR [% of glomerular filtration rate (GFR)] derived from TF/Piothalamate ratios in 12 mice in each of four groups [wild type (WT), Cldn2(-/-), AQP1(-/-), and DKO) were 45.8 ± 0.85 (51 tubules), 35.4 ± 1 (54 tubules; P < 0.01 vs. WT), 36.8 ± 1 (63 tubules; P < 0.05 vs. WT), and 33.9 ± 1.4 (69 tubules; P < 0.01 vs. WT). Kidney and single-nephron GFRs (SNGFR) were significantly reduced in all mutant strains. The direct relationship between PFR and SNGFR was maintained in mutant mice, but the slope of this relationship was reduced in the absence of Cldn2 and/or AQP1. Transtubular osmotic pressure differences were not different between WT and Cldn2(-/-) mice, but markedly increased in DKO. In conclusion, the deletion of Cldn2, AQP1, or of both Cldn2 and AQP1 reduces PFR by 22.7%, 19.6%, and 26%, respectively. Our data are consistent with an up to 25% paracellular contribution to PFR. The reduced osmotic water permeability caused by absence of AQP1 augments luminal hypotonicity. Aided by a fall in filtered load, the capacity of non-AQP1-dependent transcellular reabsorption is sufficient to maintain PFR without AQP1 and claudin-2 at 75% of control.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Proximal tubule-dominant transfer of AT(1a) receptors induces blood pressure responses to intracellular angiotensin II in AT(1a) receptor-deficient mice

The role of intracellular ANG II in proximal tubules of the kidney remains poorly understood. We tested the hypothesis that proximal tubule-dominant transfer of AT(1a) receptors in the cortex mediates intracellular ANG II-induced blood pressure responses in AT(1a) receptor-deficient (Agtr1a-/-) mice. A GFP-tagged AT(1a) receptor, AT(1a)R/GFP, and an enhanced cyan fluorescent intracellular ANG II fusion protein, ECFP/ANG II, were expressed in proximal tubules of Agtr1a-/- mouse kidneys via the adenoviral transfer using a sodium and glucose cotransporter 2 promoter. Transfer of AT(1a)R/GFP alone or with ECFP/ANG II induced proximal tubule-dominant expression of AT(1a)R/GFP and/or ECFP/ANG II with a peak response at 2 wk. No significant AT(1a)R/GFP and/or ECFP/ANG II expression was observed in the glomeruli, medulla, or extrarenal tissues. Transfer of AT(1a)R/GFP alone, but not ECFP/ANG II, increased systolic blood pressure by 12 ± 2 mmHg by day 14 (n = 9, P < 0.01). However, cotransfer of AT(1a)R/GFP with ECFP/ANG II increased blood pressure by 18 ± 2 mmHg (n = 12, P < 0.01). Twenty-four hour urinary sodium excretion was decreased by day 7 with proximal tubule-dominant transfer of AT(1a)R/GFP alone (P < 0.01) or with AT(1a)R/GFP and ECFP/ANG II cotransfer (P < 0.01). These responses were associated with twofold increases in phosphorylated ERK1/2, lysate, and membrane NHE-3 proteins in freshly isolated proximal tubules (P < 0.01). By contrast, transfer of control CMV-GFP (a recombinant human adenovirus type 5 expresses enhanced green fluorescent protein under the control of a cytomegalovirus (CMV) promoter), ECFP/ANG II, or a scrambled control ECFP/ANG IIc alone in proximal tubules had no effect on all indices. These results suggest that AT(1a) receptors and intracellular ANG II in proximal tubules of the kidney play an important physiological role in blood pressure regulation.

Enhanced angiotensin receptor-associated protein in renal tubule suppresses angiotensin-dependent hypertension

We have previously shown that angiotensin II type 1 receptor-associated protein (ATRAP/Agtrap) interacts with the angiotensin II type 1 receptor and promotes constitutive internalization of the receptor so as to inhibit the pathological activation of its downstream signaling but preserve baseline physiological signaling activity. The present study was designed to investigate the role of renal ATRAP in angiotensin II-dependent hypertension. We generated transgenic mice dominantly expressing ATRAP in the renal tubules, including renal distal tubules. The renal ATRAP transgenic mice exhibited no significant change in blood pressure at baseline on normal salt diet. However, in the renal ATRAP transgenic mice compared with wild-type mice, the following took place: (1) the development of high blood pressure in response to angiotensin II infusion was significantly suppressed based on radiotelemetry, (2) the extent of daily positive sodium balance was significantly reduced during angiotensin II infusion in metabolic cage analysis, and (3) the renal Na+ -Cl- cotransporter activation and α-subunit of the epithelial sodium channel induction by angiotensin II infusion were inhibited. Furthermore, adenoviral overexpression of ATRAP suppressed the angiotensin II-mediated increase in the expression of α-subunit of the epithelial sodium channel in mouse distal convoluted tubule cells. These results indicate that renal tubule-dominant ATRAP activation provokes no evident effects on blood pressure at baseline but exerts an inhibitory effect on the pathological elevation of blood pressure in response to angiotensin II stimulation, thereby suggesting that ATRAP is a potential target of interest in blood pressure modulation under pathological conditions.

Nephron filtration rate and proximal tubular fluid reabsorption in the Akita mouse model of type I diabetes mellitus

An increase of glomerular filtration rate (hyperfiltration) is an early functional change associated with type I or type II diabetes mellitus in patients and animal models. The causes underlying glomerular hyperfiltration are not entirely clear. There is evidence from studies in the streptozotocin model of diabetes in rats that an increase of proximal tubular reabsorption results in the withdrawal of a vasoconstrictor input exerted by the tubuloglomerular feedback (TGF) mechanism. In the present study, we have used micropuncture to assess single nephron function in wild type (WT) mice and in two strains of type I diabetic Ins2+/- mice in either a C57Bl/6 (Akita) or an A1AR-/- background (Akita/A1AR-/-) in which TGF is non-functional. Kidney glomerular filtration rate (GFR) of anesthetized mice was increased by 25% in Akita mice and by 52% in Akita/A1AR-/-, but did not differ between genotypes when corrected for kidney weight. Single nephron GFR (SNGFR) measured by end-proximal fluid collections averaged 11.8 ± 1 nl/min (n=17), 13.05 ± 1.1 nl/min (n=23; p=0.27), and 15.4 ± 0.84 nl/min (n=26; p=0.009 compared to WT; p=0.09 compared to Akita) in WT, Akita, and Akita/A1AR-/- mice respectively. Proximal tubular fluid reabsorption was not different between WT and diabetic mice and correlated with SNGFR in all genotypes. We conclude that glomerular hyperfiltration is a primary event in the Akita model of type I diabetes, perhaps driven by an increased filtering surface area, and that it is ameliorated by TGF to the extent that this regulatory system is functional.

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

AT1A angiotensin receptors in the renal proximal tubule regulate blood pressure

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 nonredundant role in determining the level of BP. Abrogation of AT(1) 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.

The Sweet Pee model for Sglt2 mutation

Inhibiting renal glucose transport is a potential pharmacologic approach to treat diabetes. The renal tubular sodium-glucose transporter 2 (SGLT2) reabsorbs approximately 90% of the filtered glucose load. An animal model with sglt2 dysfunction could provide information regarding the potential long-term safety and efficacy of SGLT2 inhibitors, which are currently under clinical investigation. Here, we describe Sweet Pee, a mouse model that carries a nonsense mutation in the Slc5a2 gene, which results in the loss of sglt2 protein function. The phenotype of Sweet Pee mutants was remarkably similar to patients with mutations in the Scl5a2 gene. The Sweet Pee mutants had improved glucose tolerance, higher urinary excretion of calcium and magnesium, and growth retardation. Renal physiologic studies demonstrated a prominent distal osmotic diuresis without enhanced natriuresis. Sweet Pee mutants did not exhibit increased KIM-1 or NGAL, markers of acute tubular injury. After induction of diabetes, Sweet Pee mice had better overall glycemic control than wild-type control mice, but had a higher risk for infection and an increased mortality rate (70% in homozygous mutants versus 10% in controls at 20 weeks). In summary, the Sweet Pee model allows study of the long-term benefits and risks associated with inhibition of SGLT2 for the management of diabetes. Our model suggests that inhibiting SGLT2 may improve glucose control but may confer increased risks for infection, malnutrition, volume contraction, and mortality.

Maintained tubuloglomerular feedback responses during acute inhibition of P2 purinergic receptors in mice

Tubuloglomerular feedback (TGF), the change of afferent arteriolar resistance initiated by changes of luminal NaCl concentration, is thought to be related to NaCl-dependent release of ATP by macula densa cells. In the present study, we have explored the possibility that the released ATP may directly interact with vasoconstrictor P2 purinergic receptors in the vicinity of the glomerular vascular pole. In two different strains of wild-type mice (SWR/J and FVB), TGF responses were determined in vivo by measuring the stop flow pressure (P(SF)) change caused by a saturating increase in loop of Henle flow rate before and during the administration of the P2 receptor inhibitors PPADS (12 mg/kg + 35 mg·kg(-1)·h(-1) iv) or suramin (50 mg/kg + 150 mg·kg(-1)·h(-1)). Both agents significantly reduced the blood pressure response to the P2X agonist α,β-methylene ATP. In SWR/J and FVB mice, elevating flow to 30 nl/min reduced P(SF) by 16.4 ± 2.2 and 17.1 ± 1.8%. During infusion of PPADS, P(SF) fell by 18.8 ± 2 (P = 0.4) and 16.5 ± 1.5% (P = 0.82) in the two strains of mice. During suramin infusion, P(SF) decreased by 14.7 ± 2.4 (P = 0.62) and 15 ± 1.3% (P = 0.4) in SWR/J and FVB mice, respectively. Including PPADS (10(-4) M) in the loop perfusate did not significantly alter the P(SF) response (18.9 ± 1.8%; P = 0.54). Arterial blood pressure was not systematically affected by the P2 inhibitors. As measured by free-flow micropuncture, PPADS significantly reduced proximal tubular fluid reabsorption in both fractional and absolute terms. These results indicate that the direct activation of P2 purinergic receptors by ATP is not a major cause of TGF-induced vasoconstriction in vivo.

Renal failure in mice with Gsalpha deletion in juxtaglomerular cells

BACKGROUND

Mice with deletion of Gsalpha in renin-producing cells (RC/FF mice) have been shown to have greatly reduced renin production and lack of responsiveness of renin secretion to acute stimuli. In addition, young RC/FF mice are hypotensive and have a vasopressin-resistant concentrating defect. In the present study we have determined the long-term effect on renal function, blood pressure, and renal pathology in this low renin and diuretic mouse model.

METHODS AND RESULTS

Urine osmolarity of RC/FF mice was decreased in all age groups. GFR measured at 7, 14 and 20 weeks of age declined progressively. Single nephron GFR similarly declined while fractional proximal fluid absorption was maintained. Expression levels of extracellular matrix proteins (collagen I, IV and fibronectin) and alpha-smooth muscle actin were increased in kidneys of RC/FF mice at 20 weeks, and this was accompanied by focal segmental glomerulosclerosis and periglomerular interstitial fibrosis. RC/FF mice showed a progressive reduction of body weight, an increase in urine albumin excretion, and an increase of blood pressure with aging.

CONCLUSION

A chronic reduction of renin production in mice may be a risk factor in its own right, and does not protect renal function against the profibrotic influence of a chronically elevated urine flow.

Enhanced tubuloglomerular feedback in mice with vascular overexpression of A1 adenosine receptors

Adenosine 1 receptors (A1AR) in the kidney are expressed in the vasculature and the tubular system. Pharmacological inhibition or global genetic deletion of A1AR causes marked reductions or abolishment of tubuloglomerular feedback (TGF) responses. To assess the function of vascular A1AR in TGF, we generated transgenic mouse lines in which A1AR expression in smooth muscle was augmented by placing A1AR under the control of a 5.38-kb fragment of the rat smooth muscle alpha-actin promoter and first intron (12). Two founder lines with highest expression in the kidney [353 +/- 42 and 575 +/- 43% compared with the wild type (WT)] were used in the experiments. Enhanced expression of A1AR at the expected site in these lines was confirmed by augmented constrictor responses of isolated afferent arterioles to administration of the A1AR agonist N6-cyclohexyladenosine. Maximum TGF responses (0-30 nl/min flow step) were increased from 8.4 +/- 0.9 mmHg in WT (n = 21) to 14.2 +/- 0.7 mmHg in A1AR-transgene (tg) 4 (n = 22; P < 0.0001), and to 12.6 +/- 1.2 mmHg in A1AR-tg7 (n = 12; P < 0.02). Stepwise changes in perfusion flow caused greater numerical TGF responses in A1AR-tg than WT in all flow ranges with differences reaching levels of significance in the intermediate flow ranges of 7.5-10 and 10-15 nl/min. Proximal-distal single-nephron glomerular filtration rate (SNGFR) differences (free-flow micropuncture) were also increased in A1AR-tg, averaging 6.25 +/- 1.5 nl/min compared with 2.6 +/- 0.51 nl/min in WT (P = 0.034). Basal plasma renin concentrations as well as the suppression of renin secretion after volume expansion were similar in A1AR-tg and WT mice, suggesting lack of transgene expression in juxtaglomerular cells. These data indicate that A1AR expression in vascular smooth muscle cells is a critical component for TGF signaling and that changes in renal vascular A1AR expression may determine the magnitude of TGF responses.

Losartan chemistry and its effects via AT1 mechanisms in the kidney

Besides the importance of the renin-angiotensin system (RAS) in the circulation and other organs, the local RAS in the kidney has attracted a great attention in research in last decades. The renal RAS plays an important role in the body fluid homeostasis and long-term cardiovascular regulation. All major components and key enzymes for the establishment of a local RAS as well as two important angiotensin II (Ang II) receptor subtypes, AT1 and AT2 receptors, have been confirmed in the kidney. In additional to renal contribution to the systemic RAS, the intrarenal RAS plays a critical role in the regulation of renal function as well as in the development of kidney disease. Notably, kidney AT1 receptors locating at different cells and compartments inside the kidney are important for normal renal physiological functions and abnormal pathophysiological processes. This mini-review focuses on: 1) the local renal RAS and its receptors, particularly the AT1 receptor and its mechanisms in physiological and pathophysiological processes; and 2) the chemistry of the selective AT1 receptor blocker, losartan, and the potential mechanisms for its actions in the renal RAS-mediated disease.

Tubuloglomerular feedback: mechanistic insights from gene-manipulated mice

Tubuloglomerular feedback (TGF) describes a causal and direct relationship between tubular NaCl concentration at the end of the ascending limb of the loop of Henle and afferent arteriolar tone. The use of genetically altered mice has led to an expansion of our understanding of the mechanisms underlying the functional coupling of epithelial, mesangial, and vascular cells in TGF. Studies in mice with deletions of the A or B isoform of NKCC2 (Na,K,2Cl cotransporter) and of ROMK indicate that NaCl uptake is required for response initiation. A role for transcellular salt transport is suggested by the inhibitory effect of ouabain in mutant mice with an ouabain-sensitive alpha1 Na,K-ATPase. No effect on TGF was observed in NHE2- and H/K-ATPase-deficient mice. TGF responses are abolished in A1 adenosine receptor-deficient mice, and studies in mice with null mutations in NTPDase1 or ecto-5'-nucleotidase indicate that adenosine involved in TGF is mainly derived from dephosphorylation of released ATP. Angiotensin II is a required cofactor for the elicitation of TGF responses, as AT1 receptor or angiotensin-converting enzyme deficiencies reduce TGF responses, mostly by reducing adenosine effectiveness. Overall, the evidence from these studies in genetically altered mice indicates that transcellular NaCl transport induces the generation of adenosine that, in conjunction with angiotensin II, elicits afferent arteriolar constriction.

Nephron function in transgenic mice with selective vascular or tubular expression of Angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) null mice display aberrant renal pathology. Inadequate formation of angiotensin II (Ang II) results in hypotension, loss of fluid homeostasis, lack of urine concentration, and failure to regulate GFR through the tubuloglomerular feedback (TGF) mechanism. For examining the tissue-specific role of ACE in renal structure and regulation of renal filtrate formation, single-nephron GFR, proximal tubular fluid reabsorption, and TGF responsiveness were determined in mice that expressed ACE in only one tissue. Maximum TGF responses in mice that expressed somatic ACE (sACE) in proximal tubule cells (Gs strain) or germinal ACE in the serum (Pg strain) were reduced significantly compared with wild-type (WT) mice. In contrast, TGF responses in mice that expressed sACE in vascular endothelial cells (Ts strain) were not different from control. Single-nephron GFR was reduced in Ts compared with WT mice, but fractional reabsorption and therefore glomerulotubular balance were not distinguishable. BP responses to exogenous Ang I were diminished in Ts, Gs, and Pg mice, whereas those to Ang II were the same in the different strains. Plasma and renal tissue Ang I of all transgenic mouse strains was significantly higher than WT, whereas Ang II levels were generally lower; aldosterone levels were significantly lower than WT in Ts mice but not in the two other transgenic strains. Our results demonstrate that vascular expression of sACE can largely but not completely restore TGF regulation of GFR. Proximal fluid reabsorption in the chronic absence of proximal tubule ACE is normal.