Prolyl-4-hydroxylases 2 and 3 control erythropoietin production in renin-expressing cells of mouse kidneys.

Activation of the hypoxia-signalling pathway induced by deletion of the ubiquitin-ligase von Hippel-Lindau protein causes an endocrine shift of renin-producing cells to erythropoietin (EPO)-expressing cells. However, the underlying mechanisms have not yet been investigated. Since oxygen-regulated stability of hypoxia-inducible transcription factors relevant for EPO expression is dependent on the activity of prolyl-4-hydroxylases (PHD) 2 and 3, this study aimed to determine the relevance of different PHD isoforms for the EPO expression in renin-producing cells in vivo. For this purpose, mice with inducible renin cell-specific deletions of different PHD isoforms were analysed. Our study shows that there are two subgroups of renal renin-expressing cells, juxtaglomerular renin+ cells and platelet-derived growth factor receptor-ß+ interstitial renin+ cells. These interstitial renin+ cells belong to the cell pool of native EPO-producing cells and are able to express EPO and renin in parallel. In contrast, co-deletion of PHD2 and PHD3, but not PHD2 deletion alone, induces EPO expression in juxtaglomerular and hyperplastic renin+ cells and downregulates renin expression. A strong basal PHD3 expression in juxtaglomerular renin+ cells seems to prevent the hypoxia-inducible transcription factor-2-dependent phenotype shift into EPO cells. In summary, PHDs seem important for the stabilization of the juxtaglomerular renin cell phenotype. Moreover, these findings reveal tubulointerstitial cells as a novel site of renal renin expression and suggest a high endocrine plasticity of these cells. Our data concerning the distinct expression patterns and functions of PHD2 and PHD3 provide new insights into the regulation of renin-producing cells and highlight the need for selective PHD inhibitors. KEY POINTS: Renal renin-expressing cells can be clearly distinguished into two subgroups, the typical juxtaglomerular renin-producing cells and interstitial renin+ cells. Interstitial renin+ cells belong to the cell pool of native erythropoietin (EPO)-producing cells, show a fast EPO response to acute hypoxia-inducible factor-2 (HIF-2) stabilization and are able to express EPO and renin in parallel. Only co-deletion of the prolyl-4-hydroxylases (PHD) 2 and 3, but not PHD2 deletion alone, induces EPO expression in juxtaglomerular renin+ cells. Chronic HIF-2 stabilization in juxtaglomerular renin-expressing cells leads to their phenotypic shift into EPO-producing cells. A strong basal PHD3 expression in juxtaglomerular renin+ cells seems to prevent a HIF-2-dependent phenotype shift into EPO cells suggesting PHD3 fulfils a stabilizer function for the juxtaglomerular renin cell phenotype.

Flexible and multifaceted: the plasticity of renin-expressing cells.

The protease renin, the key enzyme of the renin-angiotensin-aldosterone system, is mainly produced and secreted by juxtaglomerular cells in the kidney, which are located in the walls of the afferent arterioles at their entrance into the glomeruli. When the body's demand for renin rises, the renin production capacity of the kidneys commonly increases by induction of renin expression in vascular smooth muscle cells and in extraglomerular mesangial cells. These cells undergo a reversible metaplastic cellular transformation in order to produce renin. Juxtaglomerular cells of the renin lineage have also been described to migrate into the glomerulus and differentiate into podocytes, epithelial cells or mesangial cells to restore damaged cells in states of glomerular disease. More recently, it could be shown that renin cells can also undergo an endocrine and metaplastic switch to erythropoietin-producing cells. This review aims to describe the high degree of plasticity of renin-producing cells of the kidneys and to analyze the underlying mechanisms.

Inhibition of transforming growth factor ß1 signaling in resident interstitial cells attenuates profibrotic gene expression and preserves erythropoietin production during experimental kidney fibrosis in mice.

Kidney fibrosis is characterized by the development of myofibroblasts originating from resident kidney and immigrating cells. Myofibroblast formation and extracellular matrix production during kidney damage are triggered by various cytokines. Among these, transforming growth factor ß1 (TGFß1) is considered a central trigger for kidney fibrosis. We found a highly upregulated expression of TGFß1 and TGFß receptor 2 (TGFß-R2) mRNAs in kidney interstitial cells in experimental fibrosis. Here, we investigated the contribution of TGFß1 signaling in resident kidney interstitial cells to organ fibrosis using the models of adenine induced nephropathy and unilateral ureteral occlusion in mice. For this purpose TGFß1 signaling was interrupted by inducible deletion of the TGFß-R2 gene in interstitial cells expressing the fibroblast marker platelet derived growth factor receptor-ß. Expression of profibrotic genes was attenuated up to 50% in kidneys lacking TGFß-R2 in cells positive for platelet derived growth factor receptor-ß. Additionally, deletion of TGFß-R2 prevented the decline of erythropoietin production in ureter ligated kidneys. Notably, fibrosis associated expression of α-smooth muscle actin as a myofibroblast marker and deposits of extracellular collagens were not altered in mice with targeted deletion of TGFß-R2. Thus, our findings suggest an enhancing effect of TGFß1 signaling in resident interstitial cells that contributes to profibrotic gene expression and the downregulation of erythropoietin production, but not to the development of myofibroblasts during kidney fibrosis.

Different subpopulations of kidney interstitial cells produce erythropoietin and factors supporting tissue oxygenation in response to hypoxia in vivo.

Genetic induction of hypoxia signaling by deletion of the von Hippel-Lindau (Vhl) protein in mesenchymal PDGFR-ß+ cells leads to abundant HIF-2 dependent erythropoietin (EPO) expression in the cortex and outer medulla of the kidney. This rather unique feature of kidney PDGFR-ß+ cells promote questions about their special characteristics and general functional response to hypoxia. To address these issues, we characterized kidney PDGFR-ß+ EPO expressing cells based on additional cell markers and their gene expression profile in response to hypoxia signaling induced by targeted deletion of Vhl or exposure to low oxygen and carbon monoxide respectively, and after unilateral ureteral obstruction. CD73+, Gli1+, tenascin C+ and interstitial SMMHC+ cells were identified as zonally distributed subpopulations of PDGFR-ß+ cells. EPO expression could be induced by Vhl deletion in all PDGFR-ß+ subpopulations. Under hypoxemic conditions, recruited EPO+ cells were mostly part of the CD73+ subpopulation. Besides EPO production, expression of adrenomedullin and regulator of G-protein signaling 4 was upregulated in PDGFR-ß+ subpopulations in response to the different hypoxic stimuli. Thus, different kidney interstitial PDGFR-ß+ subpopulations exist, capable of producing EPO in response to different stimuli. Activation of hypoxia signaling in these cells also induces factors likely contributing to improved kidney interstitial tissue oxygenation.

Endocrine functions of the renal interstitium.

This review aims to summarize the knowledge about the sensor and endocrine response functions of resident interstitial cells of the kidney. By the production of renin, erythropoietin and arachidonate metabolites (medullipin) subsets of renal interstitial fibroblasts and pericytes in different kidney zones play a central role in salt, blood pressure and oxygen homeostasis of the body. Common to these endocrine functions is that their regulation mainly occurs by (de)recruitment of active cells.

Renal interstitial cells to the rescue.

The ability of the adult zebrafish to replace damaged nephrons in the kidney depends on renal progenitor cells and renal interstitial cells working closely together.

Renal interstitial cells promote nephron regeneration by secreting prostaglandin E2.

In organ regeneration, progenitor and stem cells reside in their native microenvironment, which provides dynamic physical and chemical cues essential to their survival, proliferation, and differentiation. However, the types of cells that form the native microenvironment for renal progenitor cells (RPCs) have not been clarified. Here, single-cell sequencing of zebrafish kidney reveals fabp10a as a principal marker of renal interstitial cells (RICs), which can be specifically labeled by GFP under the control of fabp10a promoter in the fabp10a:GFP transgenic zebrafish. During nephron regeneration, the formation of nephrons is supported by RICs that form a network to wrap the RPC aggregates. RICs that are in close contact with RPC aggregates express cyclooxygenase 2 (Cox2) and secrete prostaglandin E2 (PGE2). Inhibiting PGE2 production prevents nephrogenesis by reducing the proliferation of RPCs. PGE2 cooperates with Wnt4a to promote nephron maturation by regulating ß-catenin stability of RPC aggregates. Overall, these findings indicate that RICs provide a necessary microenvironment for rapid nephrogenesis during nephron regeneration.

The Role of Renal Interstitial Cells in Proximal Tubular Regeneration.

The healthy kidney is considered to be a relatively stable organ with little baseline cell turnover. Nevertheless, cells are constantly replaced to conserve the structural and functional integrity of the organ. The mechanisms of the baseline regenerative processes may also be relevant in situations of insults to the kidney, when the need for cellular replacement considerable exceeds the baseline cell turnover. This review will focus on the mechanisms of the regeneration of the tubular system, in particular the proximal tubule. Specifically, we will cover new aspects of (i), the regenerative capacity of the proximal tubule in health and disease, (ii) the relevant cell populations of proximal tubular regeneration, and (iii) the supportive role of renal interstitial cells in regenerative processes of the tubular system.