EDITORIAL www.jasn.org

Tubular Vascular Endothelial Interestingly, VEGF tub kidneys were smaller but normal histologically and functionally, in terms of fluid and electro- -A, lyte handling as well as BP regulation. Circulating VEGF-A , and Medullary levels were not different in VEGF tub and control mice. Thus, reduced VEGF tub kidney weight (mostly tubular mass) implies a Vessels: A Trio Linked by autocrine VEGF-A requirement for tubular growth, consistent with data showing that VEGF-A promotes tubulogenesis and Hypoxia renal tubular cell proliferation.4–6 Similarly, decreased VEGF tub medullary microvasculature development Alda Tufro demonstrates a paracrine VEGF-A requirement for peritubular Section of Nephrology, Department of Pediatrics, Yale School of capillary growth and alignment. Together, these findings suggest Medicine, New Haven, Connecticut that VEGF tub mice developed compensatory mechanisms to J Am Soc Nephrol 26: ccc–ccc, 2014. match the vascular supply to metabolic needs of the smaller doi: 10.1681/ASN.2014101004 tubular mass. The drastic renal and circulating EPO increase observed in VEGF tub mice documents hypoxia in the renal medulla. Hypoxia The complex architecture of the renal vasculature is essential due to either decreased pO2 or lack of O2 transport capacity for the organ function and comprises three capillary beds.1,2 (anemia) leads to stabilization of HIF-1a,atranscriptional fl Glomerular capillaries are anked by high-resistance afferent activator of hypoxia-inducible genes, including EPO,nitric and efferent arterioles. Two postglomerular capillary networks oxidesynthase(NOS),VEGF-A, and other angiogenic factors emerge in series from efferent arterioles. Efferent arterioles from (-1, angiopoietin-2, PDGF , FGF , angiopoietin-L4), fi BB b super cial glomeruli give rise to cortical peritubular capillaries, which are expressed in the kidney.7 Despite loss of tubular whereas those from juxtamedullary glomeruli give rise to vasa VEGF-A, the collective hypoxia response restored homeostasis recta. Close alignment of the postglomerular microvasculature (i.e., normal renal histology, function, and BP) in VEGF tub mice, with cortical and medullary renal tubules is critical for O2 de- albeit at the expense of significant polycythemia. EPO regulation fl livery to all nephron segments and for uid and solute reab- by hypoxia mediated by HIF-1a induces EPO transcription and sorption from them. Development of postglomerular capillary translation.7,8 In addition to endocrine stimulation of erythro- beds is thought to be driven and regulated by an array of an- poiesis, EPO promotes endothelial cell survival and proliferation giogenic factors secreted by renal tubules and endothelial or in vitro and in vivo.9,10 Like VEGF tub mice, Epo transgenic mice mural cell precursors, including vascular endothelial growth develop hematocrit of approximately 80%, have normal BP and – factor-A (VEGF-A), , stromal cell derived factor-1, renal function, and have no evidence of cardiovascular disease or erythropoietin (EPO), and angiotensin II. Although the expres- thrombotic events. This remarkable adaptation was found to be sion pattern and function of these factors is well known, their mediated by increased endothelial NOS (eNOS) expression and fi speci c role in the establishment and homeostasis of the renal nitric oxide (NO) levels.11 It appears likely that these mecha- medullary vessels is not clear.2 nisms are involved in VEGF tub mice phenotype as well. In this issue of JASN, Dimke et al.3 examined the role of renal Endogenous NO generated in response to hypoxia is known tubular VEGF-A in vivo using doxycycline-regulated, tubular- to play an important role in the homeostasis of the oxygen supply fi tub speci c VEGF-A deletion (VEGF ) during development and to the renal medulla and to prevent hypoxic injury.12 Moreover, in adult mice, combined with reporter mice. These elegant stud- intrinsic NO and reactive oxygen species (ROS) regulate de- ies revealed that tubular VEGF-A is necessary during develop- scending vasa recta contraction, whereby inhibition of NOS v ment to establish medullary microvessels and to reach normal with N -nitro-L-arginine methyl ester (L-NAME) constricts kidney size. Loss of tubular VEGF-A after weaning impairs descending vasa recta, an effect that is abrogated by NO donors the maintenance of medullary microvasculature. In addition, or cGMP and blunted by NAPDH inhibitor and superoxide loss of tubular VEGF-A leads to EPO induction resulting in dismutase mimetics.13 These findings suggest that NO/SNO polycythemia. donors might mitigate the peritubular capillary dropout in VEGF tub mice. Published online ahead of print. Publication date available at www.jasn.org. Other factors may also be involved in VEGF tub phenotype. Correspondence: Dr. Alda Tufro, Section of Nephrology, Department of Pe- Although angiopoietin-1 contributes to the angiogenic re- diatrics, Yale School of Medicine, 333 Cedar Street, LMP3086, New Haven, CT sponse to hypoxia, angiopoietin-2 leads to vessel regression 06520-8064. Email: [email protected] in the absence of VEGF-A.14 The developmental VEGF tub Copyright © 2014 by the American Society of Nephrology phenotype in the renal medulla is remarkably similar to that

J Am Soc Nephrol 26: ccc–ccc,2014 ISSN : 1046-6673/2605-ccc 1 EDITORIAL www.jasn.org induced by an angiotensin II type 1 antagonist in neonatal rats, 2. Herzlinger D, Hurtado R: Patterning the renal vascular bed [published which downregulates VEGF-A, angiopoietin-1, angiopoietin-2, online ahead of print August 13, 2014]. Semin Cell Dev Biol doi:10.1016/j. and the tie-2 receptor, suggesting that angiotensin II/AT1 semcdb.2014.08.002 A 3. Dimke H, Sparks MA, Thomson BR, Frische S, Coffman TM, Quaggin signaling promotes expansion of postglomerular capillary SE: Tubulovascular cross-talk by vascular endothelial growth factor A through several angiogenic factors.15 Proliferation of maintains peritubular microvasculature in kidney. JAmSocNephrol26: SMA1 cells along with renal EPO producing cells in the inter- XXX–XXX, 2014 stitium of VEGF tub occurred only after an approximately 20% 4. Tufro A, Norwood VF, Carey RM, Gomez RA: Vascular endothelial drop in hematocrit, suggesting that this fibrosis-like response is growth factor induces nephrogenesis and vasculogenesis. JAmSoc Nephrol 10: 2125–2134, 1999 EPO independent (EPO mRNA change is similar to C57 con- 5. Villegas G, Lange-Sperandio B, Tufro A: Autocrine and paracrine trols), and may involve increased PDGF receptor (PDGFR) sig- functions of vascular endothelial growth factor (VEGF) in renal tubular naling, known to be induced by acute hypoxia and to cause epithelial cells. Kidney Int 67: 449–457, 2005 myofibroblast proliferation (SMA1/PDGFRb1 cells).16 Fur- 6. Kanellis J, Fraser S, Katerelos M, Power DA: Vascular endothelial growth factor is a survival factor for renal tubular epithelial cells. Am J ther studies are needed to evaluate whether this process is re- – fi Physiol Renal Physiol 278: F905 F915, 2000 versible or leads to brosis and CKD, as reported in unilateral 7. Semenza GL: Hypoxia-inducible factors in physiology and medicine. 17 ureteral obstruction. Cell 148: 399–408, 2012 The report by Dimke et al.3 represents an exciting contri- 8. Tsui AK, Marsden PA, Mazer CD, Sled JG, Lee KM, Henkelman RM, Cahill bution to understanding the role of tubular VEGF in kidney LS,ZhouYQ,ChanN,LiuE,HareGM:DifferentialHIFandNOSre- fi fi development and disease pathogenesis. Furthermore, this study sponses to acute anemia: De ning organ-speci c hemoglobin thresholds for tissue hypoxia. Am J Physiol Regul Integr Comp Physiol demonstrates that tubular VEGF-A is necessary for full devel- 307: –R25, 2014 opment of the renal medulla, and loss of tubular VEGF-A 9. Wang XQ, Vaziri ND: Erythropoietin depresses nitric oxide synthase ex- elicits a homeostatic response to regional hypoxia involving pression by human endothelial cells. Hypertension 33: 894–899, 1999 EPO, resulting in mild microvascular dropout in the mature 10. Wang L, Di L, Noguchi CT: Erythropoietin, a novel versatile player kidney with no functional abnormalities. Additional work will regulating energy metabolism beyond the erythroid system. Int J Biol Sci 10: 921–939, 2014 be necessary to ascertain the mechanistic links among VEGF-A fi 11. Ruschitzka FT, Wenger RH, Stallmach T, Quaschning T, de Wit C, signaling, hypoxia/anemia, and renal brosis in humans. This Wagner K, Labugger R, Kelm M, Noll G, Rülicke T, Shaw S, Lindberg RL, study raises appealing questions to be addressed in futures stud- Rodenwaldt B, Lutz H, Bauer C, Lüscher TF, Gassmann M: Nitric oxide ies: Are there tubular VEGF-independent mechanisms that prevents cardiovascular disease and determines survival in polyglobulic guide peritubular capillary development in the cortex? Do mice overexpressing erythropoietin. Proc Natl Acad Sci U S A 97: 11609– tub 11613, 2000 medullary microvessels from VEGF mice lack endothelial 12. Brezis M, Heyman SN, Dinour D, Epstein FH, Rosen S: Role of nitric cell fenestrations, as observed in mice treated with VEGF re- oxide in renal medullary oxygenation. Studies in isolated and intact rat ceptor 2 kinase inhibitors?18 Elucidation of the role of NO, kidneys. JClinInvest88: 390–395, 1991 eNOS, and ROS in the adaptive hypoxia response observed in 13. Cao C, Edwards A, Sendeski M, Lee-Kwon W, Cui L, Cai CY, Patzak A, this model may uncover novel therapeutic targets and set the Pallone TL: Intrinsic nitric oxide and superoxide production regulates stage for translational research. descending vasa recta contraction. Am J Physiol Renal Physiol 299: F1056–F1064, 2010 14. Hanahan D: Signaling vascular morphogenesis and maintenance. Sci- ence 277: 48–50, 1997 15. Madsen K, Marcussen N, Pedersen M, Kjaersgaard G, Facemire C, ACKNOWLEDGMENTS Coffman TM, Jensen BL: Angiotensin II promotes development of the renal microcirculation through AT1 receptors. JAmSocNephrol21: This work was supported by grants from the National Institutes of 448–459, 2010 Health (R01-DK059333 and R01-DK098824). 16. Humphreys BD, Lin SL, Kobayashi A, Hudson TE, Nowlin BT, Bonventre JV, Valerius MT, McMahon AP, Duffield JS: Fate tracing reveals the and not epithelial origin of myofibroblasts in kidney fibrosis. Am J Pathol 176: 85–97, 2010 DISCLOSURES 17. Souma T, Yamazaki S, Moriguchi T, Suzuki N, Hirano I, Pan X, Minegishi N, None. Abe M, Kiyomoto H, Ito S, Yamamoto M: Plasticity of renal erythropoietin- producing cells governs fibrosis. J Am Soc Nephrol 24: 1599–1616, 2013 18. Kamba T, Tam BY, Hashizume H, Haskell A, Sennino B, Mancuso MR, REFERENCES Norberg SM, O’Brien SM, Davis RB, Gowen LC, Anderson KD, Thurston G, Joho S, Springer ML, Kuo CJ, McDonald DM: VEGF-dependent 1. Kriz W: Structural organization of renal medullary circulation. Nephron plasticity of fenestrated capillaries in the normal adult microvascula- 31: 290–295, 1982 ture. 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