Development of the Renal Arterioles

BRIEF REVIEW www.jasn.org

Maria Luisa S. Sequeira Lopez and R. Ariel Gomez

Department of Pediatrics, University of Virginia School of Medicine, Charlottesville, Virginia

ABSTRACT The kidney is a highly vascularized organ that normally receives a fifth of the first arterioles are seen around 15 to 16 cardiac output. The unique spatial arrangement of the kidney vasculature with each days of gestation. By 18 to 19 days of ges- nephron is crucial for the regulation of renal blood flow, GFR, urine concentration, tation, there is a basic blueprint of arte- and other specialized kidney functions. Thus, the proper and timely assembly of rial and arteriolar development.15 This is kidney vessels with their respective nephrons is a crucial morphogenetic event followed in the ensuing days by a burst of leading to the formation of a functioning kidney necessary for independent extra- branching and elongation of new arteri- uterine life. Mechanisms that govern the development of the kidney vasculature oles that repeat the basic pattern for are poorly understood. In this review, we discuss the anatomical development, about a week after birth, resulting in a embryological origin, lineage relationships, and key regulators of the kidney arte- remarkable increase in the complexity rioles and postglomerular circulation. Because renal disease is associated with and surface area of the vasculature. These deterioration of the kidney microvasculature and/or the reenactment of embryonic orchestrated series of events require that pathways, understanding the morphogenetic events and processes that maintain progenitor cells differentiate, acquire po- the renal vasculature may open new avenues for the preservation of renal structure sitional information, assemble in the and function and prevent the progression of renal disease. right location within the vessel, and seg- regate those cells that participate in J Am Soc Nephrol 22: 2156–2165, 2011. doi: 10.1681/ASN.2011080818 branching. When this process fails, the consequences are devastating as exemplified by the serious developmental de- The mechanisms that govern the devel- into the renal mesenchyme. As the ureter fects described below, which occur (par- opment of the kidney vasculature are branches, the mesenchymal cells con- ticularly in the newborn period when poorly understood. In this brief review, dense around each ureteric tip. The con- branching is at its peak) in animals and we discuss the anatomical development, densate develops into a vesicle, followed humans with ablation of renin cell pre- embryological origin, lineage relation- by a comma-shaped body that subse- cursors, mutations of the renin-angio- ships, and key regulators of the kidney quently develops into an S-shaped body. tensin system, and lack of microRNAs in arterioles and postglomerular circula- Simultaneously, the glomerular cells dif- the renal vasculature, resulting in early tion. For other important regulatory ferentiate until they acquire their adult arterial and arteriolar abnormalities that molecules and mechanisms already features. In humans, nephrogenesis is are followed by deterioration of kidney demonstrated for nonrenal vessels, as complete by 34 to 35 weeks of gestation. structure and function.15–23 In spite of its well as for the development of the glo- In mice and rats, however, nephrogen- importance, very little is known about merular capillaries, the reader is referred esis continues after birth for about 3 to 7 the fate of the vascular precursors and to some excellent reviews.1–14 days, respectively. the mechanisms that lead them to differ- In vivo, vascularization of the kidney entiate and assemble into the kidney ar- is synchronized with epithelial nephro- Anatomical Development of the terioles. genesis. Nephrogenesis of the definitive Renal Arterial Tree kidney results from the reciprocal induc- Using microdissection techniques com- Published online ahead of print. Publication date tive interaction between the primitive ure- bined with histologic assessment, we available at www.jasn.org. teric bud and the metanephric mesen- studied the anatomical development of Correspondence: Dr. Maria Luisa S. Sequeira Lo- chyme (Figure 1). The ureteric bud the renal arterial tree in mice and rats pez, University of Virginia School of Medicine, 409 Lane Road, MR4 Building, Room 2001, Charlottes- induces the mesenchyme to form tubular throughout embryonic and postnatal ville, VA 22908. Phone: 434-924-5065; Fax: 434- and glomerular epithelia. In turn, the life, including adulthood (unpublished 982-4328; E-mail: [email protected] surrounding mesenchyme induces the observations and Figure 2). Those stud- Copyright © 2011 by the American Society of ureter to continue to grow and branch ies reveal that in the mouse kidney, the Nephrology

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Collecting Glomerulus Proximal duct tube

Condensate Distal tube

Metanephric mesenchyme Proximal Vascular tube progenitors Distal tube Ureteric bud Vesicle Comma-shaped

S-shaped

Figure 1. Nephrogenesis results from the interaction between the ureteric bud and the metanephric mesenchyme. Schematic of nephrogenesis. See text for details (adapted from reference 62).

a Foxd1ϩ cell from which all other arterial and perivascular/adventitial cells originate (Figure 3).

Hemogenic Progenitors. It is now accepted that hematopoietic stem cells (HSCs) and ECs originate during embryogenesis from a common progenitor, the hemangioblast.26,27 These cells consist of a subpopulation of the Figure 2. Increased complexity of the renal arterial tree during development. Microdis- primitive streak mesoderm that migrates section of the entire preglomerular arteriolar tree at different developmental time points to the yolk sac where they establish the is shown at the same magnification. primitive hematopoietic system. Primi- tive erythroid progenitors expand within Vascular Progenitors and Arteriolar very early, well before vessels can be dis- the yolk sac and at embryonic day (E)8.5 Development cerned. It is also clear that those precur- enter the newly developing circulation As mentioned above, for nephrons to sors differentiate into all of the cell types and continue to mature. As soon as the function properly, each glomerulus necessary for the development of the kid- liver starts to form, they home to it, must establish its own circulation. ney arterioles, including endothelial cells where they complete their maturation Blood enters the glomerulus through (ECs), smooth muscle cells (SMCs), and and enucleate.28 On the other hand, de- an afferent arteriole that is continued renin cells.24,25 Furthermore, cross- finitive hematopoiesis is established by glomerular capillaries where filtra- transplantation studies of those embry- within the embryo proper in the para- tion occurs and leaves the glomerulus onic prevascular kidneys under the kid- aortic splachnopleural region by regional through an efferent arteriole (Figure 3). ney capsule of a host mouse allowed us hemangioblasts or hemogenic endothe- The establishment of these nephrovascu- and others to demonstrate that precursor lium, which supports the development of lar units is a remarkable morphogenetic cells have the capacity to differentiate, HSCs. Primitive hematopoiesis gener- event requiring spatial and temporal co- acquire the right positional information, ates mainly nucleated primitive erythro- ordination of the cells destined to form and fully assemble to form the kidney ar- cytes, whereas definitive hematopoiesis these structures. Despite the critical rele- terioles.24 The origin, lineage relation- generates all hematopoietic lineages, in- vance of this process, the actual events ships, and morphogenesis of the kidney cluding nucleated and enucleated de- and molecules that control the forma- vasculature are also not well understood. finitive erythrocytes and HSCs with a tion of the kidney arterioles are unclear. Within the stromal compartment, we long-term repopulating activity. Using At the time of the first division of the ure- identified two putative and distinct early multiple techniques including Tie2.Cre; teric bud, within the metanephric mes- progenitor cells that give rise to all cells of R26R lineage tracing, electron micros- enchyme (ϳE12 mouse, E14 rat), the the kidney arterioles and their perivascu- copy, laser capture microdissection, or- embryonic kidneys do not have arteriolar compartment. Those progenitors are gan culture, and cross-transplantation lar vessels. Whereas renal and extrarenal a precursor of hemogenic endothel- experiments, we showed there is a lineage origins have been suggested for the renal ium—provisionally called renal heman- relationship between ECs and HSCs dur- vasculature, it is clear that progenitors gioblasts—capable of giving rise to ery- ing embryonic development and the are present in the metanephric kidney throid and ECs of the renal arteriole and presence of HSCs budding from the en-

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beled mice with traceable reporters allowed further experiments in mice. Hemangioblast Foxd1+ cell When embryonic kidneys are trans- Flk-1+ + planted into the anterior chamber of the Flk-1 Tie2+ + Tie2 Kit+ + eye or under the kidney capsule of adult Kit VE-cadherin+ + VE-cadherin Tie2+ hosts, the embryonic kidneys develop a + SMA proper vasculature with all of the vascu- Endothelial Hemogenic FibroblastVascular Renin cell Mesangial cell endothelium smooth precursor cell lar cell types (ECs, SMCs, and renin cells) muscle cell originating from within the donor embryonic kidney (Figure 4).24,36 However, when prevascular embryonic kidneys are transplanted under the kidney capsule of newborn mice, cells derived from the host, still undergoing nephrogenesis, contribute to the ECs of developing vessels and glomeruli.36 Those studies suggest that when the embryonic kidney is transplanted into sites actively undergoing vascular development (such as the Efferent avian chorioallantoic membrane or the arteriole nephrogenic cortex of the newborn mouse), vascular precursors from the Figure 3. Two main progenitors give rise to the renal arteriole: the hemangioblast and host respond to cues from the trans- ϩ the Foxd1 progenitor cell. Solid arrows indicate current knowledge from others and our planted, developing metanephric mesen- lab. Dashed arrows indicate possible lineage relationships. Phenotypic markers are chyme, migrate, and differentiate into indicated in gray. The renin cell precursor gives rise to a subset of vascular SMCs. ECs and form part of the transplanted kidney vasculature. Thus, the presence, dothelium.29 Recently, those studies SMCs, not originating from renin cells, stage of development, and location of were confirmed by others using an in- mural/interstitial pericytes, and adventi- vascular progenitors determines where ducible VE-cadherin.Cre mouse line.30 tial fibroblasts (unpublished, Figure 3). the vasculature originates. Within the Furthermore, time-lapse imaging with Whereas the lineage relationship among embryo proper, the embryonic kidney is live markers and genetic analysis of dif- all of these cell types still needs to be ex- surrounded by a loose developing mes- ferentiating ESCs also confirms that even amined in detail, those studies suggest enchyme that also contains vascular pre- ϩ non-aortic-derived ECs are hemo- that Foxd1 cells are upstream in the dif- cursors. It may be possible that some ex- genic.31,32 Our studies also suggest the in- ferentiation pathway of mural cells of the trarenal vascular precursors originate trinsic capacity of the metanephros for arteriole. from outside the embryonic kidney and hemo-vasculogenesis, the concomitant migrate to form renal vessels. Therefore, development of a blood vessel with its Embryonic Origin of the Renal renal vessels may have a dual, chimeric own blood.29 Our conceptualization of Vasculature: Renal, Extrarenal, or origin as discussed below. the contribution of hemogenic precur- Both? sors to the development of the kidney ar- Whether the renal vessels originate from Angiogenesis versus terioles is shown in Figure 3. outside or within the kidney has been a Vasculogenesis long-standing controversial issue. Inter- Both angiogenesis and vasculogenesis Progenitors of Mural Cells, Pericytes, and species cross transplantation experi- have been described as major processes Interstitial Cells. ments of prevascular embryonic kidneys for vessel formation, and both are likely Foxd1 is a winged-helix transcription were originally performed into quail to occur during normal development as factor expressed in developing ventral chorioallantoic membrane because of well as in pathologic conditions. Vascu- diencephalon and in stromal cells of the the presence of a characteristic quail nu- logenesis consists in the local differentia- developing kidney.33 Recently, we traced clear marker that could easily track the tion and assembly of endothelial precur- the lineage of Foxd1 cells within the kid- contribution of the host tissue.25,35 Those sors into endothelial tubes followed by ney using a FoxD1.GFP.Cre knock-in experiments show the presence of host- recruitment of local mesenchymal cells ϩ mouse34 and found that Foxd1 cells dif- derived ECs and mesangial cells within that differentiate into SMCs, which coat ferentiate into renin cells, which in turn the embryonic kidney and suggest an ex- the growing endothelial tube and pro- differentiate into a subset of vascular trarenal source for ECs and mesangial vide stability to the newly formed arteri- SMCs and mesangial cells, and the rest of cells. The development of genetically-la- ole. Angiogenesis is the formation of new

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vessels from preexisting ones and in- volves the sprouting, migration, and proliferation of already differentiated ECs followed by recruitment of perivascular cells along the way. Whereas both processes are markedly different, some studies support the notion that the kidney arterioles actually form by a combination of vasculogenesis and angiogenesis.37 We generated chimeric mice by injecting either ROSA26 ES cells (all cells are blue upon 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside [X-gal] reaction) into wild-type blastocysts or vice versa and examined their kidneys at different stages of development. By assess- ing the contribution of labeled versus un- labeled cells to a particular vessels or nephron segment, the technique permits us to infer its clonality or lack thereof. Our unpublished experiments suggest Figure 4. The embryonic kidney possesses precursors for the renal arterioles. Embry- that the renal vasculature develops by a onic kidney (embryonic day 12) section from a ROSA 26 mouse transplanted under the combination of both processes, with a kidney capsule of an adult wild-type mouse for 7 days subjected to the 5-bromo-4- prominence of vasculogenesis in the ␤ chloro-3-indolyl- -D-galactopyranoside (X-gal) reaction and immunostained for early embryonic kidney and a strong an- ␣-smooth muscle actin, which labels the SMCs of the arterioles and developing giogenic component during the elonga- interstitial pericytes. Glomeruli were properly vascularized, and all of the vascular cell types originated from within the donor embryonic kidney. tion of the vasculature at later stages. In- terestingly, whereas the endothelial layer of the vessels appears to be clonal, the smooth muscle layer of the arterial tree is usually chimeric, indicating that differentiation of the smooth muscle wall occurs by vasculogenesis (Figure 5). Although vasculogenesis and angiogenesis were originally thought to occur in the developing embryo, there now is evidence that these processes also occur during physiologic (endometrial angiogenesis) and pathologic (tumor vascularization) stresses of adult life. It is possible therefore that vasculogenesis and angiogenesis are activated during remodeling of the vasculature after pathologic conditions including ischemic renal injury.38

Regulation of Vascular Development The major mechanisms that control vascular development in other systems have been the subject of excellent reviews1–14 and in the case of arteriolar development Figure 5. The renal arterial tree develops by vasculogenesis. This kidney section from involve an active cross-talk between peri- a ROSA26/129S SvEv chimeric mouse subjected to the 5-bromo-4-chloro-3-indolyl-␤- cytes and ECs. Pericytes coat endothelial D-galactopyranoside (X-gal) reaction shows a renal artery formed by contribution of tubes, providing mechanical stability for both blastocyst and injected ESC-derived cells, suggesting a vasculogenic origin. the mature vessel, but they also partici-

J Am Soc Nephrol 22: 2156–2165, 2011 Development of the Renal Arterioles 2159 BRIEF REVIEW www.jasn.org pate in the regulation of blood flow. Dur- cells, which promotes pericyte prolifera- genitors. As the vessels mature, renin ing arteriolar development, pericytes tion by acting on PDGF-B receptor. In cells differentiate into SMCs and mesan- stimulate EC proliferation and migra- fact, mice deficient for PDGF-B or its re- gial cells. Finally when elongation is tion. In turn, ECs activate the pericyte ceptor die in utero with widespread vas- complete, a few renin cells remain near precursor cell population.11 The para- cular defects consisting of vessel dilation the glomerulus, the juxtaglomerular cells crine interactions between these two cell and microaneurisms: sprouting capillar- usually found in the adult animal. These types involve vascular endothelial ies in these null mice cannot attract peri- experiments suggest that renin cells ei- growth factor (VEGF), angiopoietins, cytes.43,44 Furthermore, within the kid- ther directly or indirectly regulate the PDGF-B, TGF␤, Notch, Ephrins, and ney, the lack of PDGF-B or its receptor branching and elongation of the renal ar- sphingosine 1 phosphate (S1P) signaling fail to develop mesangial cells (which are terial tree through local generation of an- pathways, among others. VEGF (A–D a subtype of pericytes) and show abnor- giotensin. Support for this hypothesis is and placental growth factor) and its ty- mal glomerular capillaries.45 derived from separate experiments de- rosine kinase receptors are key regulators TGF␤ signaling is also involved in scribed below. of vascular development (vasculogen- vascular development. Both ECs and Treatment of rats during the first 12 esis, angiogenesis, and lymphangiogen- vascular SMCs express TGF␤ receptors. days of postnatal life, when arteriolar esis). VEGF-A induces the migration and Activation of this pathway results in the branching is at its peak, with the AT1 proliferation of pericytes and stimulates differentiation of mesenchymal progen- blocker losartan results in marked im- EC synthesis of nitric oxide, which also itors to pericytes. Endothelial or vascular pairment of kidney vascular develop- stimulates pericytes recruitment. Inter- SMC deletion of TGF␤ receptors results ment characterized by fewer, shorter, estingly, pericytes and renin cells (closely in early vascular defects and embryonic and thicker radial and afferent arteri- related to pericytes) produce VEGF, lethality.46 oles. This is accompanied by reduced which in turn may stabilize ECs in the The S1P pathway is also a key regula- glomerular size and tubular atrophy renal growing arteriole. tor of EC cell migration and growth. S1P with attendant reduction in overall The angiopoietin system also plays a is a bioactive sphingolipid metabolite kidney growth. Interestingly, inhibi- critical role in vascular development. It crucial in many biologic processes, in- tion of angiotensin generation in rana is composed by two glycoproteins (an- cluding angiogenesis. S1P generated by catesbiana tadpoles undergoing pro- giopoietin-1 and -2) with opposite ef- two sphingosine kinases, SphK1 and metamorphosis results in even more fects that bind to the endothelial cell- SphK2, activates a family of five G-pro- marked renal abnormalities with per- specific tyrosine-protein kinase receptor tein–coupled receptors, S1P1–S1P5. The sistence of pronephric tubes and areas (TEK).39–41 Angiopoietin-1 mediates re- variety of responses mediated by S1P re- of undifferentiated mesenchyme.49 ciprocal interactions between the develop- ceptors depends on the type of receptor These experiments, as well as those of ing endothelium and perivascular cells. and its coupled downstream effectors ex- other investigators, indicate that not Deletion of angiopoietin-1 or TEK results pressed in a given cell. Deletion of S1P1 only is angiotensin II necessary for in embryonic lethality caused by cardio- in mice results in embryonic lethality at nephrovascular development but also vascular abnormalities. It has been sug- E12.5 to 14.5 caused by abnormal forma- its vascular growth actions are con- gested that the role of angiopoietin-1 in the tion of blood vessels, possibly because of served across the phylogenetic scale.49 maintenance of vascular integrity was by failure of migration and/or differentia- In agreement with those findings, deletion recruitment of pericytes surrounding the tion of vascular SMCs and pericytes, of any of the genes of the renin-angiotensin capillary tubes. However, a recent thor- demonstrating the critical role of S1P1 in system (angiotensin-converting enzyme, ough study using inducible conditional de- vascular maturation.47 However, the role renin, angiotensinogen, and AT1AϩB) letion of angiopoietin-1 at different devel- of this pathway in the development of the results in similar abnormalities (Figure opmental stages and in different cell renal vasculature has not yet been inves- 6). The growth effects of angiotensin II compartments demonstrates, when the tigated. are not limited to the renal arterioles deletion occurs between E10.5 and E12.5, and extend to the postglomerular cir- that vascular abnormalities include an in- Renin-Angiotensin in Kidney Vascular culation. Recent work from Madsen et crease in the number and diameter of de- Development. al.50 indicate that treatment with can- veloping vessels, but recruitment of peri- We have previously shown the associa- desartan for 2 weeks reduces the cytes is not affected. Whereas deletion of tion of renin-expressing cells with length, volume, and surface area of angiopoietin-1 after E13.5 did not show an branching and elongation of the renal ar- capillaries in both the cortex and the evident phenotype, in response to injury or terial tree.48 Development of a new arte- medulla and inhibits the proper orga- microvascular stress there is an increase in rial branch is preceded by the appearance nization of vasa recta bundles. These angiogenesis and fibrosis resulting in an ac- of renin-expressing cells at the point of findings are accompanied by inhibi- celerated progression of the disease, result- branching, followed by out pouching tion of VEGF, angiopoietin-1 and -2, ing in more profound organ damage.42 and elongation of a new arteriole covered and the angiopoietin receptor Tie-2. As PDGF-B is secreted by endothelial almost exclusively with renin cell pro- a result, medullary abnormalities en-

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of the cAMP pathway have a more profound phenotypic consequence. In fact, deletion of CBP and p300, two well known coactivators of CREB pos- sessing histone acetyl transferase activity, results in severe vascular and nephron alterations accompanying a nearly complete depletion of renin cells. Overall, the aforementioned studies indicate that the cAMP pathway is fundamental in the differentiation of mural cells (renin and SMCs) of the renal arterioles.

Role of MicroRNAs in Vascular Develop- ment. Endogenous miRNAs are small, noncod- ing RNAs of about 18 to 22 nucleotides in Figure 6. Gene deletion of any component of the renin angiotensin system results in length. miRNAs exhibit tissue and cell renal arteriolar abnormalities. Representative Masson’s Trichrome staining showing a specificity, and some are regulated devel- marked increase in the size of the vascular wall and diminished lumen. opmentally. They control a myriad of cellular processes and are involved in cell differentiation and morphogenetic events. sue, encompassing hypoplasia of the velopment early in fetal life. Condi- The generation of mature miRNAs re- ␣ papilla, thickening of vessels, and accu- tional deletion of G s in cells from the quires the sequential processing of a mulation of ␣SMCs in the outer med- renin lineage such as arteriolar SMCs looped primary transcript (pri-miR, 100- ullary interstitium. Functionally, this is and mesangial cells results in almost 1000 nucleotides) to a single-stranded miR reflected by a marked reduction in re- complete absence of renin-expressing by two RNase III endonuclease complexes: nal blood flow. As the authors suggest, cells in the fetal kidney accompanied Drosha-DGCR8 in the nucleus and Dicer it seems that postnatal development of by blunted development of the preglo- in the cytoplasm. miRNAs mediate post- the kidney depends on appropriate merular arterial tree.53 Thus, the cAMP transcriptional events including mRNA regulation of angiogenesis. pathway is an early regulator of the de- degradation and translational repression. velopment and maturation of mural They may also have chromatin effects that Role of the cAMP Pathway. cells of the kidney. Whether the effect is ultimately influence gene expression. We have previously shown that cAMP direct in the SMC or as a result of di- Deletion of Dicer in renin cells results in signaling is necessary for renin cell minished expression of renin and/or a severe reduction in the number of renin- identity and is also linked to develop- the presence of renin cells remains un- expressing cells accompanied by decreased ment of kidney arterioles. Ligand bind- clear. Interestingly, studies performed circulating renin, hypotension, and strik- ing of G-protein–coupled receptors re- on ␤1/␤2-adrenergic receptor-defi- ing nephrovascular abnormalities charac- sults in activation of adenylyl cyclases, cient mice show a marked reduction of terized by a unique type of striped cortico- which in turn generate cAMP from renin expression in the vasculature in medullary fibrosis.23 Within the fibrotic ATP. cAMP activates protein kinase A, the embryo without changes in the stripes, renal arterioles are unusually which in turn phosphorylates cAMP number of juxtaglomerular cells in the formed, replaced by interstitial cells in dif- response element–binding (CREB) adult, suggesting a distinct responsive- ferent states of development, phenotypic protein, a transcription factor that reg- ness to ␤-adrenergic stimulation be- conversion, and /or degeneration.23 The ulates the promoter activity of many tween these two cell populations. The mechanisms for the peculiar striped fibro- genes, including the renin gene. Re- vasculature in those mice appears nor- sis remain to be studied, but it is likely to cently, in vitro and in vivo studies dem- mal.54 Given that deletion of a more result from the combination of abnormal onstrate the crucial role of the cAMP downstream element of the cAMP vessel architecture, which together with ar- ␣ pathway on the acquisition and main- pathway, as in the case of G s, does im- terial hypotension leads to localized tissue tenance of the renin cell identity.51,52 pair vascular development, it seems hypoperfusion and ischemia along the Activation of the cAMP pathway that other mechanisms may compen- path where the renal vessels traverse from was proven to be of importance not sate for the absence of the ␤ receptors, the juxtamedullary area through the renal only for the well known stimulation of whereas deletion of more proximal el- cortex (Figure 7).23 renin release but also for vascular de- ements (with respect to the renin gene) Contrary to the phenotype encoun-

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A Deletion of Dicer in renin cells

↓Renin cells

Abnormal vessel Hypotension architecture

Hypoperfusion 50 µm B

Localized ischemia

Striped fibrosis

Figure 7. Schematic of possible mechanisms for the pathogenesis of striped renal fibrosis in mice with deletion of Dicer in the Figure 8. Renin cells per se are responsible for the thickening of the arterial wall. (A) Ϫ Ϫ renin cell lineage (from reference 23). Kidney sections from mice with deletion of angiotensinogen (Atg / ) and ablation of renin cells (DTA/DTA) immunostained for ␣-smooth muscle actin showing an enlarge- Ϫ Ϫ ment and increase in thickness of the arteriolar wall in Atg / , whereas in the ones with tered in mice with targeted deletions of the ablation of renin cells, the vessel wall is not thickened (from reference 16). (B) Schematic renin angiotensin system genes,17–22 in showing the localization of renin cells (with dark dots) surrounding the thickened smooth Ϫ Ϫ which SMCs proliferate around the blood muscle layer (red circles) of the arteriolar wall in the Atg / , whereas the DTA/DTA lacks vessel (Figure 6), DicerϪ/Ϫ animals do not renin cells and the smooth muscle layer remains thin. have arteriolar thickening, displaying in- stead adventitial fibroplasia.23 The reasons for the apparent decrease in SMCs is un- likely to be due to decreased renin because mice with homozygous deletion of the renin gene also have arteriolar thickening.17,18 The answer to this question is suggested from experiments in which the renin cells were ablated using diphtheria toxin targeted to the renin locus. Mice expressing diphtheria toxin A in renin cells have thin vessels and do not show hyperplasia of SMCs surrounding the arterioles (Figure 8).16 These experiments suggest that renin cells produce factors that Figure 9. The renin cell expresses angiogenic and trophic factors (circled in red). stimulate the abnormal concentric growth of the kidney interlobular and afferent arterioles. In fact, recent gene pro- the branching and elongation of the re- enous activation of renal vasoconstric- filing indicates that renin cells are capa- nal arterial tree and in pathologic condi- tors as described above. ble of producing more than 14 different tions to aberrant, circumferential growth well known angiogenic and trophic factors, of the renal arterioles as demonstrated in Vascular Development and Renal including VEGF, angiopoietin, IGF2, and situations where there is an absence of Disease autotaxin, among others (Figure 9).55 angiotensin II actions in the presence of In adults, the incidence of chronic re- These factors may contribute in health to renin cell hyperplasia, ischemia, or exog- nal disease is about 16%.56 The number

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of patients with end-stage renal disease nance of the renal vasculature is a logical lar development and angiogenesis. Arterio- continues to increase by 8 to 14% per step in developing strategies to preserve scler Thromb Vasc Biol 29: 630–638, 2009 2. Gridley T: Notch signaling in vascular devel- year and is expected to double during renal structure and function. opment and physiology. Development 134: the next 15 years. Many kidney diseases 2709–2718, 2007 have either primary or secondary vas- Future Opportunities 3. Kono M, Allende ML, Proia RL: Sphingosine- cular lesions. Renal disease is associated It is now more accepted that both ECs 1-phosphate regulation of mammalian de- with alterations in vascular remodeling, and and HSCs derive from a common pre- velopment. Biochim Biophys Acta 1781: 435–441, 2008 progression of renal disease is accompanied cursor cell, the hemangioblast. 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Int J Dev Biol 55: spontaneous models of hypertension-as- contribute to the endowment of renal 261–268, 2011 sociated renal damage.61 It is unknown vascular cells and how those cells differ- 12. Argraves KM, Wilkerson BA, Argraves WS: what mechanisms establish phenotypic entiate and assemble to form and main- Sphingosine-1-phosphate signaling in vasculogenesis and angiogenesis. World J Biol variability in the vascular beds and how tain the renal arterioles, a frontier basi- Chem 1: 291–297, 2010 these might function for the mainte- cally unexplored has the potential to 13. Quaggin SE, Kreidberg JA: Development of nance of a healthy vascular system. When benefit children and adults with congen- the renal glomerulus: Good neighbors and formation of the kidney vasculature is al- ital and acquired kidney diseases, vascu- good fences. Development 135: 609–620, tered, the consequences are devastating, lar diseases, and hypertension. 2008 14. Vaughan MR, Quaggin SE: How do mesan- as exemplified by the serious develop- gial and endothelial cells form the glomeru- mental defects described above, occur- lar tuft? J Am Soc Nephrol 19: 24–33, 2008 ring in animals and humans with abla- ACKNOWLEDGMENTS 15. Gomez RA, Sequeira-Lopez ML: Vascular tion of renin cell precursors, mutations development of the kidney. In: Assembly of of the renin-angiotensin system, and a This work was supported by National Insti- the vasculature and its regulation, edited by Tomanek RJ, Boston, Birkhauser, 2002, pp lack of miRNAs in the renal vasculature, tutes of Health Grants KO8 DK75481 (to 193–210 resulting in early arterial and arteriolar M.L.S.S.L.) and R37 HL066242 and RO1 16. Pentz ES, Moyano MA, Thornhill BA, Se- abnormalities that are followed by the HL096735 (to R.A.G). queira Lopez ML, Gomez RA: Ablation of deterioration of kidney structure and renin-expressing juxtaglomerular cells re- function.15–23 sults in a distinct kidney phenotype. Am J Physiol Regul Integr Comp Physiol 286: Generation of new vessels in the kid- DISCLOSURES R474–R483, 2004 ney is not confined to embryonic devel- None. 17. Yanai K, Saito T, Kakinuma Y, Kon Y, Hirota opment but seems to be a mechanism K, Taniguchi-Yanai K, Nishijo N, Shigematsu also activated in response to injury. Y, Horiguchi H, Kasuya Y, Sugiyama F, Given the fact that a variety of renal dis- Yagami K, Murakami K, Fukamizu A: Renin- REFERENCES dependent cardiovascular functions and eases have abnormalities in the renal vas- renin-independent blood-brain barrier func- culature, understanding the mechanisms 1. Gaengel K, Genove G, Armulik A, Betsholtz tions revealed by renin-deficient mice. J Biol underlying the formation and mainte- C: Endothelial-mural cell signaling in vascu- Chem 275: 5–8, 2000

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