Renin-expressing cells are associated with branching of the developing kidney vasculature

The transcription factor Tcf21 is required for specifying Foxd1 cells to the juxtaglomerular cell lineage

Renin is a crucial enzyme involved in the regulation of blood pressure and electrolyte balance. It has been shown that renin expressing cells arise from the Foxd1+ stromal progenitors, however the factors involved in guiding Foxd1+ cells towards the renin-secreting cell fate remain poorly understood. Tcf21, also known as Pod1 or Capsulin, is a bHLH transcription factor that is expressed in the metanephric mesenchyme and plays a crucial role in kidney development. We have previously shown that deletion of Tcf21 in Foxd1+ cells ( Foxd1 Cre/+ ;Tcf21 f/f ) results in paucity of vascular mural cells and in disorganized renal arterial tree with fewer, shorter, and thinner arterioles. Here, we sought to examine the relationship between Tcf21 and renin cells during kidney development and test whether Tcf21 is implicated in the regulation of juxtaglomerular cell differentiation. Immunostaining for renin demonstrated that kidneys of Foxd1 Cre/+ ;Tcf21 f/f have fewer renin-positive spots at E16.5 and E18.5 compared with controls. In-situ hybridization for renin mRNA showed reduced expression in Foxd1 Cre/+ ;Tcf21 f/f kidneys at E14.5, E16.5, and E18.5. Together, these data suggest that stromal expression of Tcf21 is required for the emergence of renin cells. To dissect the role of Tcf21 in juxtaglomerular (JG) cells, we deleted Tcf21 upon renin promoter activation ( Ren1 dCre/+ ;Tcf21 f/f ). Interestingly, the Ren1 dCre/+ ;Tcf21 f/f kidney showed normal arterial tree at E16.5 identical to controls. Furthermore, inactivation of Tcf21 upon renin expression did not alter kidney morphology in two- and four-month-old mice. Finally, expression renin mRNA was similar between Ren1 dCre/+ ;Tcf21 f/f and controls at 2 months. Taken together, these findings suggest that Tcf21 expression in Foxd1+ cells is essential for specifying the fate of these cells into juxtaglomerular cells. However, once renin cell identity is assumed, Tcf21 is dispensable. Uncovering the regulation of Foxd1+ cells and their derivatives, including the JG cell lineage, is crucial for understanding the mechanisms underlying renal vasculature formation.

Netrin 1 directs vascular patterning and maturity in the developing kidney

The intricate vascular system of the kidneys supports body fluid and organ homeostasis. However, little is known about how vascular architecture is established during kidney development. More specifically, how signals from the kidney influence vessel maturity and patterning remains poorly understood. Netrin 1 (Ntn1) is a secreted ligand that is crucial for vessel and neuronal guidance. Here, we demonstrate that Ntn1 is expressed by Foxd1+ stromal progenitors in the developing mouse kidney and conditional deletion (Foxd1GC/+;Ntn1fl/fl) results in hypoplastic kidneys with extended nephrogenesis. Wholemount 3D analyses additionally revealed the loss of a predictable vascular pattern in Foxd1GC/+;Ntn1fl/fl kidneys. As vascular patterning has been linked to vessel maturity, we investigated arterialization. Quantification of the CD31+ endothelium at E15.5 revealed no differences in metrics such as the number of branches or branch points, whereas the arterial vascular smooth muscle metrics were significantly reduced at both E15.5 and P0. In support of our observed phenotypes, whole kidney RNA-seq revealed disruptions to genes and programs associated with stromal cells, vasculature and differentiating nephrons. Together, our findings highlight the significance of Ntn1 to proper vascularization and kidney development.

Renin Cells, From Vascular Development to Blood Pressure Sensing

During embryonic and neonatal life, renin cells contribute to the assembly and branching of the intrarenal arterial tree. During kidney arteriolar development renin cells are widely distributed throughout the renal vasculature. As the arterioles mature, renin cells differentiate into smooth muscle cells, pericytes, and mesangial cells. In adult life, renin cells are confined to the tips of the renal arterioles, thus their name juxtaglomerular cells. Juxtaglomerular cells are sensors that release renin to control blood pressure and fluid-electrolyte homeostasis. Three major mechanisms control renin release: (1) β-adrenergic stimulation, (2) macula densa signaling, and (3) the renin baroreceptor, whereby a decrease in arterial pressure leads to increased renin release whereas an increase in pressure results in decrease renin release. Cells from the renin lineage exhibit plasticity in response to hypotension or hypovolemia, whereas relentless, chronic stimulation induces concentric arterial and arteriolar hypertrophy, leading to focal renal ischemia. The renin cell baroreceptor is a nuclear mechanotransducer within the renin cell that transmits external forces to the chromatin to regulate Ren1 gene expression. In addition to mechanotransduction, the pressure sensor of the renin cell may enlist additional molecules and structures including soluble signals and membrane proteins such as gap junctions and ion channels. How these various components integrate their actions to deliver the exact amounts of renin to meet the organism needs is unknown. This review describes the nature and origins of renin cells, their role in kidney vascular development and arteriolar diseases, and the current understanding of the blood pressure sensing mechanism.

Netrin-1 directs vascular patterning and maturity in the developing kidney

Blood filtering by the kidney requires the establishment of an intricate vascular system that works to support body fluid and organ homeostasis. Despite these critical roles, little is known about how vascular architecture is established during kidney development. More specifically, how signals from the kidney influence vessel maturity and patterning remains poorly understood. Netrin-1 (Ntn1) is a secreted ligand critical for vessel and neuronal guidance. Here, we demonstrate that Ntn1 is expressed by stromal progenitors in the developing kidney, and conditional deletion of Ntn1 from Foxd1+ stromal progenitors (Foxd1GC/+;Ntn1fl/fl) results in hypoplastic kidneys that display extended nephrogenesis. Despite expression of the netrin-1 receptor Unc5c in the adjacent nephron progenitor niche, Unc5c knockout kidneys develop normally. The netrin-1 receptor Unc5b is expressed by embryonic kidney endothelium and therefore we interrogated the vascular networks of Foxd1GC/+;Ntn1fl/fl kidneys. Wholemount, 3D analyses revealed the loss of a predictable vascular pattern in mutant kidneys. As vascular patterning has been linked to vessel maturity, we investigated arterialization in these mutants. Quantification of the CD31+ endothelium at E15.5 revealed no differences in metrics such as the number of branches or branch points, whereas the arterial vascular smooth muscle metrics were significantly reduced at both E15.5 and P0. In support of these results, whole kidney RNA-seq showed upregulation of angiogenic programs and downregulation of muscle-related programs which included smooth muscle-associated genes. Together, our findings highlight the significance of netrin-1 to proper vascularization and kidney development.

Determinants of renin cell differentiation: a single cell epi-transcriptomics approach

Rationale

Renin cells are essential for survival. They control the morphogenesis of the kidney arterioles, and the composition and volume of our extracellular fluid, arterial blood pressure, tissue perfusion, and oxygen delivery. It is known that renin cells and associated arteriolar cells descend from FoxD1 + progenitor cells, yet renin cells remain challenging to study due in no small part to their rarity within the kidney. As such, the molecular mechanisms underlying the differentiation and maintenance of these cells remain insufficiently understood.

Objective

We sought to comprehensively evaluate the chromatin states and transcription factors (TFs) that drive the differentiation of FoxD1 + progenitor cells into those that compose the kidney vasculature with a focus on renin cells.

Methods and Results

We isolated single nuclei of FoxD1 + progenitor cells and their descendants from FoxD1 ^cre/+ ; R26R-mTmG mice at embryonic day 12 (E12) (n _cells =1234), embryonic day 18 (E18) (n _cells =3696), postnatal day 5 (P5) (n _cells =1986), and postnatal day 30 (P30) (n _cells =1196). Using integrated scRNA-seq and scATAC-seq we established the developmental trajectory that leads to the mosaic of cells that compose the kidney arterioles, and specifically identified the factors that determine the elusive, myo-endocrine adult renin-secreting juxtaglomerular (JG) cell. We confirm the role of Nfix in JG cell development and renin expression, and identified the myocyte enhancer factor-2 (MEF2) family of TFs as putative drivers of JG cell differentiation.

Conclusions

We provide the first developmental trajectory of renin cell differentiation as they become JG cells in a single-cell atlas of kidney vascular open chromatin and highlighted novel factors important for their stage-specific differentiation. This improved understanding of the regulatory landscape of renin expressing JG cells is necessary to better learn the control and function of this rare cell population as overactivation or aberrant activity of the RAS is a key factor in cardiovascular and kidney pathologies.

Generation of functional chimeric kidney containing exogenous progenitor-derived stroma and nephron via a conditional empty niche

Generation of new kidneys can be useful in various research fields, including organ transplantation. However, generating renal stroma, an important component tissue for structural support, endocrine function, and kidney development, remains difficult. Organ generation using an animal developmental niche can provide an appropriate in vivo environment for renal stroma differentiation. Here, we generate rat renal stroma with endocrine capacity by removing mouse stromal progenitor cells (SPCs) from the host developmental niche and transplanting rat SPCs. Furthermore, we develop a method to replace both nephron progenitor cells (NPCs) and SPCs, called the interspecies dual replacement of the progenitor (i-DROP) system, and successfully generate functional chimeric kidneys containing rat nephrons and stroma. This method can generate renal tissue from progenitors and reduce xenotransplant rejection. Moreover, it is a safe method, as donor cells do not stray into nontarget organs, thus accelerating research on stem cells, chimeras, and xenotransplantation.

Comparative Studies of Renin-Null Zebrafish and Mice Provide New Functional Insights

BACKGROUND

The renin-angiotensin system is highly conserved across vertebrates, including zebrafish, which possess orthologous genes coding for renin-angiotensin system proteins, and specialized mural cells of the kidney arterioles, capable of synthesising and secreting renin.

METHODS

We generated zebrafish with CRISPR-Cas9-targeted knockout of renin (ren^-/-) to investigate renin function in a low blood pressure environment. We used single-cell (10×) RNA sequencing analysis to compare the transcriptome profiles of renin lineage cells from mesonephric kidneys of ren^-/- with ren^+/+ zebrafish and with the metanephric kidneys of Ren1^c-/- and Ren1^c+/+ mice.

RESULTS

The ren^-/- larvae exhibited delays in larval growth, glomerular fusion and appearance of a swim bladder, but were viable and withstood low salinity during early larval stages. Optogenetic ablation of renin-expressing cells, located at the anterior mesenteric artery of 3-day-old larvae, caused a loss of tone, due to diminished contractility. The ren^-/- mesonephric kidney exhibited vacuolated cells in the proximal tubule, which were also observed in Ren1^c-/- mouse kidney. Fluorescent reporters for renin and smooth muscle actin (tg(ren:LifeAct-RFP; acta2:EGFP)), revealed a dramatic recruitment of renin lineage cells along the renal vasculature of adult ren^-/- fish, suggesting a continued requirement for renin, in the absence of detectable angiotensin metabolites, as seen in the Ren1YFP Ren1^c-/- mouse. Both phenotypes were rescued by alleles lacking the potential for glycosylation at exon 2, suggesting that glycosylation is not essential for normal physiological function.

CONCLUSIONS

Phenotypic similarities and transcriptional variations between mouse and zebrafish renin knockouts suggests evolution of renin cell function with terrestrial survival.

Renin Cells, the Kidney, and Hypertension

Renin cells are essential for survival perfected throughout evolution to ensure normal development and defend the organism against a variety of homeostatic threats. During embryonic and early postnatal life, they are progenitors that participate in the morphogenesis of the renal arterial tree. In adult life, they are capable of regenerating injured glomeruli, control blood pressure, fluid-electrolyte balance, tissue perfusion, and in turn, the delivery of oxygen and nutrients to cells. Throughout life, renin cell descendants retain the plasticity or memory to regain the renin phenotype when homeostasis is threatened. To perform all of these functions and maintain well-being, renin cells must regulate their identity and fate. Here, we review the major mechanisms that control the differentiation and fate of renin cells, the chromatin events that control the memory of the renin phenotype, and the major pathways that determine their plasticity. We also examine how chronic stimulation of renin cells alters their fate leading to the development of a severe and concentric hypertrophy of the intrarenal arteries and arterioles. Lastly, we provide examples of additional changes in renin cell fate that contribute to equally severe kidney disorders.

Deciphering the Identity of Renin Cells in Health and Disease

Hypotension and changes in fluid-electrolyte balance pose immediate threats to survival. Juxtaglomerular cells respond to such threats by increasing the synthesis and secretion of renin. In addition, smooth muscle cells (SMCs) along the renal arterioles transform into renin cells until homeostasis has been regained. However, chronic unrelenting stimulation of renin cells leads to severe kidney damage. Here, we discuss the origin, distribution, function, and plasticity of renin cells within the kidney and immune compartments and the consequences of distorting the renin program. Understanding how chronic stimulation of these cells in the context of hypertension may lead to vascular pathology will serve as a foundation for targeted molecular therapies.

Ctcf is required for renin expression and maintenance of the structural integrity of the kidney

Renin cells are crucial for the regulation of blood pressure and fluid electrolyte homeostasis. We have recently shown that renin cells possess unique chromatin features at regulatory regions throughout the genome that may determine the identity and memory of the renin phenotype. The 3-D structure of chromatin may be equally important in the determination of cell identity and fate. CCCTC-binding factor (Ctcf) is a highly conserved chromatin organizer that may regulate the renin phenotype by controlling chromatin structure. We found that Ctcf binds at several conserved DNA sites surrounding and within the renin locus, suggesting that Ctcf may regulate the transcriptional activity of renin cells. In fact, deletion of Ctcf in cells of the renin lineage led to decreased endowment of renin-expressing cells accompanied by decreased circulating renin, hypotension, and severe morphological abnormalities of the kidney, including defects in arteriolar branching, and ultimately renal failure. We conclude that control of chromatin architecture by Ctcf is necessary for the appropriate expression of renin, control of renin cell number and structural integrity of the kidney.

Renin-Expressing Cells Require β1-Integrin for Survival and for Development and Maintenance of the Renal Vasculature

Juxtaglomerular cells are crucial for blood pressure and fluid-electrolyte homeostasis. The factors that maintain the life of renin cells are unknown. In vivo, renin cells receive constant cell-to-cell, mechanical, and neurohumoral stimulation that maintain their identity and function. Whether the presence of this niche is crucial for the vitality of the juxtaglomerular cells is unknown. Integrins are the largest family of cell adhesion molecules that mediate cell-to-cell and cell-to-matrix interactions. Of those, β1-integrin is the most abundant in juxtaglomerular cells. However, its role in renin cell identity and function has not been ascertained. To test the hypothesis that cell-matrix interactions are fundamental not only to maintain the identity and function of juxtaglomerular cells but also to keep them alive, we deleted β1-integrin in vivo in cells of the renin lineage. In mutant mice, renin cells died by apoptosis, resulting in decreased circulating renin, hypotension, severe renal-vascular abnormalities, and renal failure. Results indicate that cell-to-cell and cell-to-matrix interactions via β1-integrin is essential for juxtaglomerular cells survival, suggesting that the juxtaglomerular niche is crucial not only for the tight regulation of renin release but also for juxtaglomerular cell survival-a sine qua non condition to maintain homeostasis.

Stromal prorenin receptor is critical for normal kidney development

Formation of the metanephric kidney requires coordinated interaction among the stroma, ureteric bud, and cap mesenchyme. The transcription factor Foxd1, a specific marker of renal stromal cells, is critical for normal kidney development. The prorenin receptor (PRR), a receptor for renin and prorenin, is also an accessory subunit of the vacuolar proton pump V-ATPase. Global loss of PRR is embryonically lethal in mice, indicating an essential role of the PRR in embryonic development. Here, we report that conditional deletion of the PRR in Foxd1⁺ stromal progenitors in mice (cKO) results in neonatal mortality. The kidneys of surviving mice show reduced expression of stromal markers Foxd1 and Meis1 and a marked decrease in arterial and arteriolar development with the subsequent decreased number of glomeruli, expansion of Six2⁺ nephron progenitors, and delay in nephron differentiation. Intrarenal arteries and arterioles in cKO mice were fewer and thinner and showed a marked decrease in the expression of renin, suggesting a central role for the PRR in the development of renin-expressing cells, which in turn are essential for the proper formation of the renal arterial tree. We conclude that stromal PRR is crucial for the appropriate differentiation of the renal arterial tree, which in turn may restrict excessive expansion of nephron progenitors to promote a coordinated and proper morphogenesis of the nephrovascular structures of the mammalian kidney.

Investigating the RAS can be a fishy business: interdisciplinary opportunities using Zebrafish

The renin-angiotensin system (RAS) is highly conserved, and components of the RAS are present in all vertebrates to some degree. Although the RAS has been studied since the discovery of renin, its biological role continues to broaden with the identification and characterization of new peptides. The evolutionarily distant zebrafish is a remarkable model for studying the kidney due to its genetic tractability and accessibility for in vivo imaging. The zebrafish pronephros is an especially useful kidney model due to its structural simplicity yet complex functionality, including capacity for glomerular and tubular filtration. Both the pronephros and mesonephros contain renin-expressing perivascular cells, which respond to RAS inhibition, making the zebrafish an excellent model for studying the RAS. This review summarizes the physiological and genetic tools currently available for studying the zebrafish kidney with regards to functionality of the RAS, using novel imaging techniques such as SPIM microscopy coupled with targeted single cell ablation and synthesis of vasoactive RAS peptides.

Changes in cell fate determine the regenerative and functional capacity of the developing kidney before and after release of obstruction

Congenital obstructive nephropathy is a major cause of chronic kidney disease (CKD) in children. The contribution of changes in the identity of renal cells to the pathology of obstructive nephropathy is poorly understood. Using a partial unilateral ureteral obstruction (pUUO) model in genetically modified neonatal mice, we traced the fate of cells derived from the renal stroma, cap mesenchyme, ureteric bud (UB) epithelium, and podocytes using Foxd1Cre, Six2Cre, HoxB7Cre, and Podocyte.Cre mice respectively, crossed with double fluorescent reporter (membrane-targetted tandem dimer Tomato (mT)/membrane-targetted GFP (mG)) mice. Persistent obstruction leads to a significant loss of tubular epithelium, rarefaction of the renal vasculature, and decreased renal blood flow (RBF). In addition, Forkhead Box D1 (Foxd1)-derived pericytes significantly expanded in the interstitial space, acquiring a myofibroblast phenotype. Degeneration of Sine Oculis Homeobox Homolog 2 (Six2) and HoxB7-derived cells resulted in significant loss of glomeruli, nephron tubules, and collecting ducts. Surgical release of obstruction resulted in striking regeneration of tubules, arterioles, interstitium accompanied by an increase in blood flow to the level of sham animals. Contralateral kidneys with remarkable compensatory response to kidney injury showed an increase in density of arteriolar branches. Deciphering the mechanisms involved in kidney repair and regeneration post relief of obstruction has potential therapeutic implications for infants and children and the growing number of adults suffering from CKD.

Development of the renal vasculature

The kidney vasculature has a unique and complex architecture that is central for the kidney to exert its multiple and essential physiological functions with the ultimate goal of maintaining homeostasis. An appropriate development and coordinated assembly of the different vascular cell types and their association with the corresponding nephrons is crucial for the generation of a functioning kidney. In this review we provide an overview of the renal vascular anatomy, histology, and current knowledge of the embryological origin and molecular pathways involved in its development. Understanding the cellular and molecular mechanisms involved in renal vascular development is the first step to advance the field of regenerative medicine.

Renin cells in homeostasis, regeneration and immune defence mechanisms

An accumulating body of evidence suggests that renin-expressing cells have developed throughout evolution as a mechanism to preserve blood pressure and fluid volume homeostasis as well as to counteract a number of homeostatic and immunological threats. In the developing embryo, renin precursor cells emerge in multiple tissues, where they differentiate into a variety of cell types. The function of those precursors and their progeny is beginning to be unravelled. In the developing kidney, renin-expressing cells control the morphogenesis and branching of the renal arterial tree. The cells do not seem to fully differentiate but instead retain a degree of developmental plasticity or molecular memory, which enables them to regenerate injured glomeruli or to alter their phenotype to control blood pressure and fluid-electrolyte homeostasis. In haematopoietic tissues, renin-expressing cells might regulate bone marrow differentiation and participate in a circulating leukocyte renin-angiotensin system, which acts as a defence mechanism against infections or tissue injury. Furthermore, renin-expressing cells have an intricate lineage and functional relationship with erythropoietin-producing cells and are therefore central to two endocrine systems - the renin-angiotensin and erythropoietin systems - that sustain life by controlling fluid volume and composition, perfusion pressure and oxygen delivery to tissues. However, loss of the homeostatic control of these systems following dysregulation of renin-expressing cells can be detrimental, with serious pathological events.

Chronic Stimulation of Renin Cells Leads to Vascular Pathology

Experimental or spontaneous genomic mutations of the renin-angiotensin system or its pharmacological inhibition in early life leads to renal abnormalities, including poorly developed renal medulla, papillary atrophy, hydronephrosis, inability to concentrate the urine, polyuria, polydipsia, renal failure, and anemia. At the core of such complex phenotype is the presence of unique vascular abnormalities: the renal arterioles do not branch or elongate properly and they have disorganized, concentric hypertrophy. This lesion has been puzzling because it is often found in hypertensive individuals whereas mutant or pharmacologically inhibited animals are hypotensive. Remarkably, when renin cells are ablated with diphtheria toxin, the vascular hypertrophy does not occur, suggesting that renin cells per se may contribute to the vascular disease. To test this hypothesis, on a Ren1^c-/- background, we generated mutant mice with reporter expression (Ren1^c-/-;Ren1^c-Cre;R26R.mTmG and Ren1^c-/-;Ren1^c-Cre;R26R.LacZ) to trace the fate of renin^null cells. To assess whether renin^null cells maintain their renin promoter active, we used Ren1^c-/-;Ren1^c-YFP mice that transcribe YFP (yellow fluorescent protein) directed by the renin promoter. We also followed the expression of Akr1b7 and miR-330-5p, markers of cells programmed for the renin phenotype. Contrary to what we expected, renin^null cells did not die or disappear. Instead, they survived, increased in number along the renal arterial tree, and maintained an active molecular memory of the myoepitheliod renin phenotype. Furthermore, null cells of the renin lineage occupied the walls of the arteries and arterioles in a chaotic, directionless pattern directly contributing to the concentric arterial hypertrophy.

Juxtaglomerular Cell Phenotypic Plasticity

Renin is the first and rate-limiting step of the renin-angiotensin system. The exclusive source of renin in the circulation are the juxtaglomerular cells of the kidney, which line the afferent arterioles at the entrance of the glomeruli. Normally, renin production by these cells suffices to maintain homeostasis. However, under chronic stimulation of renin release, for instance during a low-salt diet or antihypertensive therapy, cells that previously expressed renin during congenital life re-convert to a renin-producing cell phenotype, a phenomenon which is known as “recruitment”. How exactly such differentiation occurs remains to be clarified. This review critically discusses the phenotypic plasticity of renin cells, connecting them not only to the classical concept of blood pressure regulation, but also to more complex contexts such as development and growth processes, cell repair mechanisms and tissue regeneration.

Transcriptome Analysis of Human Reninomas as an Approach to Understanding Juxtaglomerular Cell Biology

Renin, a key component in the regulation of blood pressure in mammals, is produced by the rare and highly specialized juxtaglomerular cells of the kidney. Chronic stimulation of renin release results in a recruitment of new juxtaglomerular cells by the apparent conversion of adjacent smooth muscle cells along the afferent arterioles. Because juxtaglomerular cells rapidly dedifferentiate when removed from the kidney, their developmental origin and the mechanism that explains their phenotypic plasticity remain unclear. To overcome this limitation, we have performed RNA expression analysis on 4 human renin-producing tumors. The most highly expressed genes that were common between the reninomas were subsequently used for in situ hybridization in kidneys of 5-day-old mice, adult mice, and adult mice treated with captopril. From the top 100 genes, 10 encoding for ligands were selected for further analysis. Medium of human embryonic kidney 293 cells transfected with the mouse cDNA encoding these ligands was applied to (pro)renin-synthesizing As4.1 cells. Among the ligands, only platelet-derived growth factor B reduced the medium and cellular (pro)renin levels, as well as As4.1 renin gene expression. In addition, platelet-derived growth factor B-exposed As4.1 cells displayed a more elongated and aligned shape with no alteration in viability. This was accompanied by a downregulated expression of α-smooth muscle actin and an upregulated expression of interleukin-6, suggesting a phenotypic shift from myoendocrine to inflammatory. Our results add 36 new genes to the list that characterize renin-producing cells and reveal a novel role for platelet-derived growth factor B as a regulator of renin-synthesizing cells.

Zebrafish mesonephric renin cells are functionally conserved and comprise two distinct morphological populations

Zebrafish provide an excellent model in which to assess the role of the renin-angiotensin system in renal development, injury, and repair. In contrast to mammals, zebrafish kidney organogenesis terminates with the mesonephros. Despite this, the basic functional structure of the nephron is conserved across vertebrates. The relevance of teleosts for studies relating to the regulation of the renin-angiotensin system was established by assessing the phenotype and functional regulation of renin-expressing cells in zebrafish. Transgenic fluorescent reporters for renin (ren), smooth muscle actin (acta2), and platelet-derived growth factor receptor-beta (pdgfrb) were studied to determine the phenotype and secretory ultrastructure of perivascular renin-expressing cells. Whole kidney ren transcription responded to altered salinity, pharmacological renin-angiotensin system inhibition, and renal injury. Mesonephric ren-expressing cells occupied niches at the preglomerular arteries and afferent arterioles, forming intermittent epithelioid-like multicellular clusters exhibiting a granular secretory ultrastructure. In contrast, renin cells of the efferent arterioles were thin bodied and lacked secretory granules. Renin cells expressed the perivascular cell markers acta2 and pdgfrb Transcriptional responses of ren to physiological challenge support the presence of a functional renin-angiotensin system and are consistent with the production of active renin. The reparative capability of the zebrafish kidney was harnessed to demonstrate that ren transcription is a marker for renal injury and repair. Our studies demonstrate substantive conservation of renin regulation across vertebrates, and ultrastructural studies of renin cells reveal at least two distinct morphologies of mesonephric perivascular ren-expressing cells.

Novel Functions of Renin Precursors in Homeostasis and Disease

Renin progenitors appear early and are found in multiple tissues throughout the embryo. Besides their well known role in blood pressure and fluid homeostasis, renin progenitors participate in tissue morphogenesis, repair, and regeneration, and may integrate immune and endocrine responses. In the bone marrow, renin cells offer clues to understand normal and neoplastic hematopoiesis.

Identification of renin progenitors in the mouse bone marrow that give rise to B-cell leukaemia

The cell of origin and triggering events for leukaemia are mostly unknown. Here we show that the bone marrow contains a progenitor that expresses renin throughout development and possesses a B-lymphocyte pedigree. This cell requires RBP-J to differentiate. Deletion of RBP-J in these renin-expressing progenitors enriches the precursor B-cell gene programme and constrains lymphocyte differentiation, facilitated by H3K4me3 activating marks in genes that control the pre-B stage. Mutant cells undergo neoplastic transformation, and mice develop a highly penetrant B-cell leukaemia with multi-organ infiltration and early death. These renin-expressing cells appear uniquely vulnerable as other conditional models of RBP-J deletion do not result in leukaemia. The discovery of these unique renin progenitors in the bone marrow and the model of leukaemia described herein may enhance our understanding of normal and neoplastic haematopoiesis.

Vascular versus tubular renin: role in kidney development

Renin, the key regulated enzyme of the renin-angiotensin system regulates blood pressure, fluid-electrolyte homeostasis, and renal morphogenesis. Whole body deletion of the renin gene results in severe morphological and functional derangements, including thickening of renal arterioles, hydronephrosis, and inability to concentrate the urine. Because renin is found in vascular and tubular cells, it has been impossible to discern the relative contribution of tubular versus vascular renin to such a complex phenotype. Therefore, we deleted renin independently in the vascular and tubular compartments by crossing Ren1(c fl/fl) mice to Foxd1-cre and Hoxb7-cre mice, respectively. Deletion of renin in the vasculature resulted in neonatal mortality that could be rescued with daily injections of saline. The kidneys of surviving mice showed the absence of renin, hypertrophic arteries, hydronephrosis, and negligible levels of plasma renin. In contrast, lack of renin in the collecting ducts did not affect kidney morphology, intra-renal renin, or circulating renin in basal conditions or in response to a homeostatic stress, such as sodium depletion. We conclude that renin generated in the renal vasculature is fundamental for the development and integrity of the kidney, whereas renin in the collecting ducts is dispensable for normal kidney development and cannot compensate for the lack of renin in the vascular compartment. Further, the main source of circulating renin is the kidney vasculature.

Renin expression in developing zebrafish is associated with angiogenesis and requires the Notch pathway and endothelium

Although renin is a critical regulatory enzyme of the cardiovascular system, its roles in organogenesis and the establishment of cardiovascular homeostasis remain unclear. Mammalian renin-expressing cells are widespread in embryonic kidneys but are highly restricted, specialized endocrine cells in adults. With a functional pronephros, embryonic zebrafish are ideal for delineating the developmental functions of renin-expressing cells and the mechanisms governing renin transcription. Larval zebrafish renin expression originates in the mural cells of the juxtaglomerular anterior mesenteric artery and subsequently at extrarenal sites. The role of renin was determined by assessing responses to renin-angiotensin system blockade, salinity variation, and renal perfusion ablation. Renin expression did not respond to renal flow ablation but was modulated by inhibition of angiotensin-converting enzyme and altered salinity. Our data in larval fish are consistent with conservation of renin's physiological functions. Using transgenic renin reporter fish, with mindbomb and cloche mutants, we show that Notch signaling and the endothelium are essential for developmental renin expression. After inhibition of angiogenesis, renin-expressing cells precede angiogenic sprouts. Arising from separate lineages, but relying on mutual interplay with endothelial cells, renin-expressing cells are among the earliest mural cells observed in larval fish, performing both endocrine and paracrine functions.

Unwrapping the origins and roles of the renal endothelium

The renal vasculature, like all vessels, is lined by a thin layer of simple squamous epithelial cells called an endothelium. These endothelial-lined vessels can be subdivided into four major compartments: arteries, veins, capillaries and lymphatics. The renal vasculature is a highly integrated network that forms through the active processes of angiogenesis and vasculogenesis. Determination of the precise contribution of these two processes and of the molecular signaling that governs the differentiation, specification and maturation of these critical cell populations is the focus of an actively evolving field of research. Although much of the focus has concentrated on the origin of the glomerular capillaries, in this review we extend the investigation to the origins of the endothelial cells throughout the entire kidney and the signaling events that cause their distinct functional and molecular profiles. A thorough understanding of endothelial cell biology may play a critical role in a better understanding of renal vascular diseases.

The earliest metanephric arteriolar progenitors and their role in kidney vascular development

The development of the kidney arterioles is poorly understood. Mature arterioles contain several functionally and morphologically distinct cell types, including smooth muscle, endothelial, and juxtaglomerular cells, and they are surrounded by interconnected pericytes, fibroblasts, and other interstitial cells. We have shown that the embryonic kidney possesses all of the necessary precursors for the development of the renal arterial tree, and those precursors assemble in situ to form the kidney arterioles. However, the identity of those precursors was unclear. Within the embryonic kidney, several putative progenitors marked by the expression of either the winged-forkhead transcription factor 1 (Foxd1+ progenitor), the aspartyl-protease renin (Ren+ progenitor), and/or hemangioblasts (Scl+ progenitor) are likely to differentiate and endow most of the cells of the renal arterial tree. However, the lineage relationships and the role of these distinct progenitors in renal vascular morphogenesis have not been delineated. We, therefore, designed a series of experiments to ascertain the hierarchical lineage relationships between Foxd1+ and Ren+ progenitors and the role of these two precursors in the morphogenesis and patterning of the renal arterial tree. Results show that 1) Foxd1+ cells are the precursors for all the mural cells (renin cells, smooth muscle cells, perivascular fibroblasts, and pericytes) of the renal arterial tree and glomerular mesangium, and 2) Foxd1 per se directs the origin, number, orientation, and cellular composition of the renal vessels.

Development of renal renin-expressing cells does not involve PDGF-B-PDGFR-β signaling

Apart from their endocrine functions renin-expressing cells play an important functional role as mural cells of the developing preglomerular arteriolar vessel tree in the kidney. The recruitment of renin-expressing cells from the mesenchyme to the vessel wall is not well understood. Assuming that it may follow more general lines of pericyte recruitment to endothelial tubes we have now investigated the relevance of the platelet-derived growth factor (PDGF)-B-PDGFR-β signaling pathway in this context. We studied renin expression in kidneys lacking PDGFR-β in these cells and in kidneys with reduced endothelial PDGF-B expression. We found that expression of renin in the kidneys under normal and stimulated conditions was not different from wild-type kidneys. As expected, PDGFR-β immunoreactivity was found in mesangial, adventitial and tubulo-interstitial cells but not in renin-expressing cells. These findings suggest that the PDGF-B-PDGFR-β signaling pathway is not essential for the recruitment of renin-expressing cells to preglomerular vessel walls in the kidney.

Procollagen I-expressing renin cell precursors

Renin-expressing cells in the kidney normally appear as mural cells of developing preglomerular vessels and finally impose as granulated juxtaglomerular cells in adult kidneys. The differentiation of renin-expressing cells from the metanephric mesenchyme in general and the potential role of special precursor stages in particular is not well understood. Therefore, it was the aim of this study to search for renin cell precursors in the kidney. As an experimental model, we used kidneys of aldosterone synthase-deficient mice, which display a prominent compensatory overproduction of renin cells that are arranged in multilayered perivascular cell clusters. We found that the perivascular cell clusters contained two apparently distinct cell types, one staining positive for renin and another one staining positive for type I procollagen (PC1). It appeared as if PC1 and renin expression were inversely related at the cellular level. The proportion of renin-positive to PC1-positive cells in the clusters was inversely linked to the rate of salt intake, as was overall renin expression. Our findings suggest that the cells in the perivascular cell clusters can reversibly switch between PC1 and renin expression and that PC1-expressing cells might be precursors of renin cells. A few of those PC1-positive cells were found also in adult wild-type kidneys in the juxtaglomerular lacis cell area, in which renin expression can be induced on demand.

[The cortical collecting duct plays a pivotal role in the kidney's local renin-angiotensin system]

The renin-angiotensin system is one of the most important hormone systems in the body, and the regulations as well as the role in the juxtaglomerular apparatus are well known. The present review focuses on renin secretion in a recently described localization, the cortical collecting duct. The authors display it in parallel of the copying strategy of an adult and a developing kidney. Furthermore, based on different animal studies it highlights the local role of renin released from the collecting duct. In chronic angiotensin II-infused, 2-kidney, 1-clip hypertensive model as well as in diabetic rats the major source of (pro)renin is indeed the collecting duct. In this localization this hormone can reach both the systemic circulation and the interstitial renin-angiotensin system components including the newly described (pro)renin receptor, by which (pro)renin is able to locally activate pro-fibrotic intracellular signal pathways. Consequently, one can postulate that in the future renin may serve either as a new therapeutic target in nephropathy associated with both hypertension and diabetes or as an early diagnostic marker in chronic diseases leading to nephropathy.

Postnatal development of the renal medulla; role of the renin-angiotensin system

Adverse events during foetal development can predispose the individual for cardiovascular disease later in life, a correlation known as foetal programming of adult hypertension. The 'programming' events have been associated with the kidneys due to the significant role in extracellular volume control and long-term blood pressure regulation. Previously, nephron endowment and functional consequences of a low nephron number have been extensively investigated without achieving a full explanation of the underlying pathophysiological mechanisms. In this review, we will focus on mechanisms of postnatal development in the renal medulla with regard to the programming effects. The renin-angiotensin system is critically involved in mammalian kidney development and impaired signalling gives rise to developmental renal lesions that have been associated with hypertension later in life. A consistent finding in both experimental animal models and in human case reports is atrophy of the renal medulla with developmental lesions to both medullary nephron segments and vascular development with concomitant functional disturbances reaching into adulthood. A review of current knowledge of the role of the renin-angiotensin system for renal medullary development will be given.

The effect of metanephros transplantation on blood pressure in anephric rats with induced acute hypotension

BACKGROUND

The kidney is an important organ for maintaining blood pressure. We have previously reported that transplanted metanephroi can reproduce some kidney functions. The aim of the present study was to determine the metabolic function of transplanted metanephroi with particular reference to maintaining blood pressure.

METHODS

Male Wistar rats were transplanted with metanephroi (transplanted group, n = 28), following unilateral nephrectomy. For comparison, we performed unilateral nephrectomy without transplantation in 32 rats (non-transplanted group, n = 18; haeminephrectomy control group, n = 14). The remaining kidney was removed 2 weeks after the initial operation, while control rats had a sham operation. Hypotension was induced by intravenous infusion of diltiazem hydrochloride or rapid withdrawal of blood. Mean arterial blood pressure (MAP) was invasively monitored and plasma renin activity (PRA) was analysed at multiple time points. Renin expression by metanephroi was evaluated by real-time polymerase chain reaction and immunohistochemistry.

RESULTS

Metanephroi in the transplanted group expressed renin messenger RNA. Metanephros transplantation significantly raised PRA and maintained MAP compared with the non-transplanted group. No significant differences between the transplanted and control groups were found with respect to PRA or MAP. PRA was positively correlated with metanephroi weight as well as MAP in the transplanted group.

CONCLUSION

The present study shows that transplantation of metanephroi produces PRA and contributes to raising MAP in a rat model of acute hypotension.

Development of the renal arterioles

The kidney is a highly vascularized organ that normally receives a fifth of the cardiac output. The unique spatial arrangement of the kidney vasculature with each nephron is crucial for the regulation of renal blood flow, GFR, urine concentration, and other specialized kidney functions. Thus, the proper and timely assembly of kidney vessels with their respective nephrons is a crucial morphogenetic event leading to the formation of a functioning kidney necessary for independent extrauterine life. Mechanisms that govern the development of the kidney vasculature are poorly understood. In this review, we discuss the anatomical development, embryological origin, lineage relationships, and key regulators of the kidney arterioles and postglomerular circulation. Because renal disease is associated with deterioration of the kidney microvasculature and/or the reenactment of embryonic pathways, understanding the morphogenetic events and processes that maintain the renal vasculature may open new avenues for the preservation of renal structure and function and prevent the progression of renal disease.

Genes that confer the identity of the renin cell

Renin-expressing cells modulate BP, fluid-electrolyte homeostasis, and kidney development, but remarkably little is known regarding the genetic regulatory network that governs the identity of these cells. Here we compared the gene expression profiles of renin cells with most cells in the kidney at various stages of development as well as after a physiologic challenge known to induce the transformation of arteriolar smooth muscle cells into renin-expressing cells. At all stages, renin cells expressed a distinct set of genes characteristic of the renin phenotype, which was vastly different from other cell types in the kidney. For example, cells programmed to exhibit the renin phenotype expressed Akr1b7, and maturing cells expressed angiogenic factors necessary for the development of the kidney vasculature and RGS (regulator of G-protein signaling) genes, suggesting a potential relationship between renin cells and pericytes. Contrary to the plasticity of arteriolar smooth muscle cells upstream from the glomerulus, which can transiently acquire the embryonic phenotype in the adult under physiologic stress, the adult juxtaglomerular cell always possessed characteristics of both smooth muscle and renin cells. Taken together, these results identify the gene expression profile of renin-expressing cells at various stages of maturity, and suggest that juxtaglomerular cells maintain properties of both smooth muscle and renin-expressing cells, likely to allow the rapid control of body fluids and BP through both contractile and endocrine functions.

The renin phenotype: roles and regulation in the kidney

PURPOSE OF REVIEW

Renin cells are fundamental for the control of blood pressure, fluid electrolyte homeostasis and kidney development. This review discusses recent discoveries regarding the mechanisms that control the identity and fate of renin cells and their role in the maintenance of kidney architecture and function.

RECENT FINDINGS

It is now established that cyclic AMP is a crucial factor for the regulation of the renin phenotype. Furthermore, additional factors such as microRNAs and gap junctions have recently emerged as key regulators for the maintenance and proper functioning of renin cells.

SUMMARY

Experiments described in this review will hopefully raise new questions regarding the mechanisms that control the identity, plasticity and function of renin cells.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

The microRNA-processing enzyme dicer maintains juxtaglomerular cells

Juxtaglomerular cells are highly specialized myoepithelioid granulated cells located in the glomerular afferent arterioles. These cells synthesize and release renin, which distinguishes them from other cells. How these cells maintain their identity, restricted localization, and fate is unknown and is fundamental to the control of BP and homeostasis of fluid and electrolytes. Because microRNAs may control cell fate via temporal and spatial gene regulation, we generated mice with a conditional deletion of Dicer, the RNase III endonuclease that produces mature microRNAs in cells of the renin lineage. Deletion of Dicer severely reduced the number of juxtaglomerular cells, decreased expression of the renin genes (Ren1 and Ren2), lowered plasma renin concentration, and decreased BP. As a consequence of the disappearance of renin-producing cells, the kidneys developed striking vascular abnormalities and prominent striped fibrosis. We conclude that microRNAs maintain the renin-producing juxtaglomerular cells and the morphologic integrity and function of the kidney.

Methods for imaging Renin-synthesizing, -storing, and -secreting cells

Renin-producing cells have been the object of intense research efforts for the past fifty years within the field of hypertension. Two decades ago, research focused on the concept and characterization of the intrarenal renin-angiotensin system. Early morphological studies led to the concept of the juxtaglomerular apparatus, a minute organ that links tubulovascular structures and function at the single nephron level. The kidney, thus, appears as a highly "topological organ" in which anatomy and function are intimately linked. This point is reflected by a concurrent and constant development of functional and structural approaches. After summarizing our current knowledge about renin cells and their distribution along the renal vascular tree, particularly along glomerular afferent arterioles, we reviewed a variety of imaging techniques that permit a fine characterization of renin synthesis, storage, and release at the single-arteriolar, -cell, or -granule level. Powerful tools such as multiphoton microscopy and transgenesis bear the promises of future developments of the field.

Origination angle of renal artery has no role in renal artery pattern

INTRODUCTION

Renal artery (RA) anatomy plays a critical role in selecting donors. During the present study, we sought to evaluate the physiological role of RA origination angle for the presence of an accessory RA or its early branching.

METHODS

During the present cross-sectional study (August 2005-October 2007), 143 candidates for kidney donation underwent RA angiography by 64 multidetector computed tomographic angiography. We assessed the RA diameter, distance to first branching, presence of accessory RA, and early branching, as well as the origination angle of RA from aorta in coronal plane (alpha angle).

RESULTS

The male-to-female ratio was 96:47 with an overall mean age of 27.42 +/- 4.55 years. The alpha angle, sine, cosine of the alpha angle and the deviation factor were not significantly different between kidneys with versus without an accessory artery or between the RA with versus without an early branching. Only the RA diameter (P = .047) and the distance of RA to the branching (P < .001) in kidneys with an accessory RA were significantly lower and higher than those without an accessory RA, respectively. Also the distance of the RA to the branching was significantly less in kidneys with an early branching (P < .001). The RA diameter directly correlated with the RA origination angle (r = .191, P = .001), while there was no correlation between the distance to RA branching and the RA origination angle (r = -.060, P = .311).

CONCLUSION

The origination angle of the RA from aorta has no role in the early branching or accessory RA development. There was a direct correlation between the RA diameter and the RA origination angle.

Development of vascular renin expression in the kidney critically depends on the cyclic AMP pathway

During metanephric kidney development, renin expression in the renal vasculature begins in larger vessels, shifting to smaller vessels and finally remaining restricted to the terminal portions of afferent arterioles at the entrance into the glomerular capillary network. The mechanisms determining the successive expression of renin along the vascular axis of the kidney are not well understood. Since the cAMP signaling cascade plays a central role in the regulation of both renin secretion and synthesis in the adult kidney, it seemed feasible that this pathway might also be critical for renin expression during kidney development. In the present study we determined the spatiotemporal development of renin expression and the development of the preglomerular arterial tree in mouse kidneys with renin cell-specific deletion of G(s)alpha, a core element for receptor activation of adenylyl cyclases. We found that in the absence of the G(s)alpha protein, renin expression was largely absent in the kidneys at any developmental stage, accompanied by alterations in the development of the preglomerular arterial tree. These data indicate that the maintenance of renin expression following a specific spatiotemporal pattern along the preglomerular vasculature critically depends on the availability of G(s)alpha. We infer from our data that the cAMP signaling pathway is not only critical for the regulation of renin synthesis and secretion in the mature kidney but that it also is critical for establishing the juxtaglomerular expression site of renin during development.

Do mechanical fluid laws dictate the branching pattern of the renal artery?

INTRODUCTION

Anatomy of the renal artery is an important issue in the renal transplantation era. Multi-detector computed tomography angiography (MDCTA) is an accurate modality for the preoperative assessment of live renal donors, and it provides excellent details of donor arterial anatomy. We studied the relationship between the angle of emergence of the renal artery from the aorta and its branching pattern.

METHODS

In this study, the MDCTA images obtained from the 138 kidneys of 77 potential renal transplant donors were studied. The courses of the right and left renal arteries from the aorta to the kidney hilus were delineated. The branching angle of the renal artery from the aorta (beta, angle) and the length of the renal artery from the aorta until its first division were measured (Delta, distance). The renal artery deviation from the perpendicular plane of the aorta (D, factor of deviation) was calculated by the following formula: D = (1 - sin [beta]). The cosine of this angle (cos [beta]) was also calculated. Statistical analyses were performed with Pearson correlation tests. The P value was set at .05.

RESULTS

The mean age of patients was 28.7 +/- 4.3 with a male to female ratio of 63:14. The mean Delta distance and small de, Cyrillic diameter were 34.37 +/- 10.68 mm (range, 10-58) and 6.13 +/- 1.37 mm (range, 2.8-9.9), respectively. The mean beta angle, factor of deviation, and cos (beta) were 62.19 degrees +/- 16.44, 0.15 +/- 0.14, and 0.45 +/- 0.25, respectively. Significant negative correlations were found between the beta angle, and Delta distance (r = -0.308; P < .001), and small de, Cyrillic diameter (r = -0.303; P = .003). Factor of deviation and cos (beta) were directly associated Delta distance and small de, Cyrillic diameter.

CONCLUSION

These findings indicated that with the main renal artery axis deviating from the perpendicular plane of the aorta or with a smaller branching angle, this artery had a greater diameter and underwent late branching. This study suggested that the renal artery diameter and branching pattern might be determined by the mechanical fluid laws.

Development of renin expression in the mouse kidney

During metanephric kidney development, renin is expressed in the walls of larger intrarenal arteries, but is restricted to the terminal part of the preglomerular arterioles in the adult kidney. Our study describes the three-dimensional development of renin expression in mouse kidneys during fetal and postnatal life. Renin immunoreactivity first appeared at day 14 of development in the cells expressing alpha-smooth muscle actin (alphaSMA) in the arcuate arteries. Before adulthood, the branching of the arcuate arterial tree increased exponentially and renin expression shifted from proximal to distal parts of the tree. Renin expression at branching points or in the cones of growing vessels was not seen. Instead, renin expression appeared after vessel walls and branches were already established, disappeared a few days later, and remained only in the juxtaglomerular regions of afferent arterioles. In these arterioles, coexpression of renin and alphaSMA disappeared gradually, with the terminal cells expressing only renin. At all stages of kidney development, renin expression among comparable vessel segments was heterogeneous. Renin expression remained stable after it reached the terminal parts of afferent arterioles.

Expression profiles of congenital renal dysplasia reveal new insights into renal development and disease

Congenital renal dysplasia (RD) is a major cause of renal failure in the pediatric population. Although molecular and genetic aspects of RD have been studied in animal models, limited studies have been done in human RD primarily due to lack of available material. To identify novel genes that are associated with RD and normal kidney development, we performed microarray analysis on total RNA extracted from age-matched fetal kidneys of normal and RD patients. In midgestational RD kidneys, we found 180 upregulated and 104 downregulated transcripts compared with normal kidneys. Among the increased transcripts in the dysplastic kidneys were matrix-degrading enzymes (MMP7, MMP19, TIMP1), inflammation- and immunity-related genes, and growth factors. Expression of genes known to be essential for normal kidney development, such as WT1, BMP7, renin, angiotensin receptor 2 (AGTR2), SAL-like 1 (SALL1) and glypican 3 (GPC3), were decreased in dysplastic kidneys. Expression of selected gene products (BMP7, renin, and MMP7) was further confirmed in parallel sections and in several normal and human dysplastic kidneys, supporting the role of these genes as putative RD biomarkers. These results are among the first to reveal disrupted expression profiles during gestation in human RD patients.

Renin-angiotensin system affects endothelial morphology and permeability of renal afferent arteriole

The afferent arteriole (AA) is an important regulatory site of renal function and blood pressure. We have demonstrated endothelial fenestration and high permeability in the vicinity of renin granulated epithelioid cells in the juxtaglomerular portion of the afferent arteriole in different mammals. The permeability of fenestrated endothelium of afferent arteriole may be important in connection to various physiologic and pathophysiologic processes. We have assumed that the permeable fenestration may serve as a communication channel between the intravascular circulation and a pathway for renin secretion. Utilising the multiphoton image technique we were able to visualise the endothelial fenestration and renin granules of the in vitro microperfused AA and in vivo AA. We demonstrated that ferritin-positive, i.e., permeable portion of the afferent arteriole, under control conditions is on average 45 microm, which is about one-third to half of the total length of the afferent arteriole. The length of this portion is not constant and can change by physiologic and pharmacologic manipulation of renin formation. The permeability of the afferent arteriole is not changing only parallel with the pharmacologically stimulated renin secretion as already demonstrated in adult rats, but also with the change of renin appearance in afferent arteriole within the very first few days of life after birth. Independently from the age there is a significant correlation between the renin-positive and permeable portion of the AA. Further studies are necessary to clarify the physiological significance of afferent arteriolar permeability and its changes in the postnatal development of the kidney, as well as in correlation with activity of renin- angiotensin system.

Activation of the Rat Renin Promoter by HOXD10·PBX1b·PREP1, Ets-1, and the Intracellular Domain of Notch

Renin gene expression is subject to complex developmental and tissue-specific regulation. A comparison of the promoter sequences of the human, rat, and mouse renin genes has revealed a highly conserved sequence homologous to the DNA recognition sequence for CBF1 (CSL/RBP-Jkappa/Su(H)/LAG1/RBPSUH). Electrophoretic mobility shift assays document that As4.1 cell nuclear protein complex binding to the putative rat renin CBF1-binding site (-175 to -168 bp) contains CBF1. Transient transfection analyses in COS-7 cells further document that a CBF1-VP16 fusion protein and the intracellular domain of Notch1 robustly activate a promoter containing multiple copies of the rat renin CBF1-binding site. An Ets-binding site (-143 to -138 bp) has also been identified in the rat renin promoter by sequence comparisons and electrophoretic mobility shift assays. Transcription factor Ets-1 is capable of activating the rat renin promoter through the Ets-binding site. Mutation of the CBF-binding site significantly increases transcriptional activity of the rat renin promoter in Calu-6 and COS-7 cells but not in As4.1 cells, whereas mutation of the Ets-binding site reduces promoter activity of the rat renin gene in all three cell lines. Finally, we show that the intracellular domain of Notch1, Ets-1, and HOXD10.PBX1b.PREP1 activate the rat renin promoter cooperatively in COS-7 cells. These results strongly suggest that the renin gene is a downstream target of the Notch signaling pathway.

Dysmorphogenesis of kidney cortical peritubular capillaries in angiopoietin-2-deficient mice

Angiopoietin-2 (Ang-2) modulates Tie-2 receptor activation. In mouse kidney maturation, Ang-2 is expressed in arteries, with lower levels in tubules, whereas Tie-2 is expressed by endothelia. We hypothesized that Ang-2 deficiency disrupts kidney vessel patterning. The normal renal cortical peritubular space contains fenestrated capillaries, which have few pericytes; they receive water and solutes which proximal tubules reclaim from the glomerular filtrate. In wild-type neonates, alpha smooth muscle actin (alpha SMA), platelet-derived growth factor receptor beta (PDGFR beta), and desmin-expressing cells were not prominent in this compartment. In Ang-2 null mutants, alpha SMA, desmin, and PDGFR beta prominently immunolocalized in cortical peritubular locations. Some alpha SMA-positive cells were closely associated with CD31- and Tie-2-positive peritubular capillary endothelia, and some of the alpha SMA-positive cells expressed PDGFR beta, desmin, and neural/glial cell 2 (NG2), consistent with a pericyte-like identity. Immunoblotting suggested an increase of total and tyrosine-phosphorylated Tie-2 proteins in null mutant versus wild-type kidneys, and electron microscopy confirmed disorganized capillaries and adjacent cells in cortical peritubular spaces in mutant neonate kidneys. Hence, Ang-2 deficiency causes dysmorphogenesis of cortical peritubular capillaries, with adjacent cells expressing pericyte-like markers; we speculate the latter effect is caused by disturbed paracrine signaling between endothelial and surrounding mesenchymal precursor cells.