Renal erythropoietin-producing cells in health and disease

Abstract

Erythropoietin (Epo) is an indispensable erythropoietic hormone primarily produced from renal Epo-producing cells (REPs). Epo production in REPs is tightly regulated in a hypoxia-inducible manner to maintain tissue oxygen homeostasis. Insufficient Epo production by REPs causes renal anemia and anemia associated with chronic disorders. Recent studies have broadened our understanding of REPs from prototypic hypoxia-responsive cells to dynamic fibrogenic cells. In chronic kidney disease, REPs are the major source of scar-forming myofibroblasts and actively produce fibrogenic molecules, including inflammatory cytokines. Notably, myofibroblast-transformed REPs (MF-REPs) recover their original physiological properties after resolution of the disease insults, suggesting that renal anemia and fibrosis could be reversible to some extent. Therefore, understanding the plasticity of REPs will lead to the development of novel targeted therapeutics for both renal fibrosis and anemia. This review summarizes the regulatory mechanisms how hypoxia-inducible Epo gene expression is attained in health and disease conditions.

Keywords: erythropoietin; fibrosis; hypoxia; plasticity; renal Epo-producing cell (REP).

Figure 1

REPs are peri-capillary CD73-positive fibroblast-like cells. A kidney section from ISAM-REC (ISAM:R26RtdTomato:Tg-EpoCre) shows ON-REPs expressing EpoGFP (arrowheads, positive for both green and red) in the total REP population (positive for tdTomato fluorescence, red). The nuclei are stained by DAPI (blue). A kidney section from ISAM-REC shows that REPs or CD73-positive fibroblasts are distributed in peri-capillary interstitial spaces but not in peri-arterial interstitial areas of the kidneys. DAPI (blue) is used for nuclear staining in the merged image. Abbreviations: A, artery; V, vein. Scale bar, 100 μm.

Figure 2

ON–OFF regulation of Epo synthesis in REPs. (A) Oxygen deficiency turns OFF-REPs into ON-REPs by turning on “Epo gene switch.” (B) REPs are primarily in the “OFF” state (OFF-REPs, brown dots) under normal oxygen conditions. Under acute anemic conditions, some clusters of OFF-REPs turn into ON-REPs (green dots). The distribution of ON-REPs extends toward hypoxic cortical areas (orange). Strikingly, most OFF-REPs turn into ON-REPs under chronic anemia. This recruitment of ON-REPs determines the total amount of Epo secreted by the kidneys.

Figure 3

Overview of Epo gene transcriptional regulation. Under oxygen-replete conditions, PHDs hydroxylate HIF2α proteins, leading to pVHL-mediated ubiquitination and degradation of HIF2α. Under oxygen-depleted condition, PHDs are inactivated, and HIF2α escapes degradation. The HIF2α–HIF1β heterodimer activates Epo gene transcription by binding to the HRE. The functional HRE for hepatic Epo gene expression (EpoHE) is in the proximal downstream region of the transcription end site, whereas a kidney enhancer has been suggested to be located in a region far upstream of the transcription start site. GATA factors constitutively suppress ectopic Epo gene expression in epithelial lineage cells by binding to the promoter GATA sequence.

Figure 4

Myofibroblast transformation of REPs. Immunohistochemical detection of myofibroblasts with αSMA antibody (red) in ureteral obstructed kidney of ISAM. EpoGFP-positive REPs (green) transform into myofibroblasts (MF-REPs) upon kidney injury induced by ureteral obstruction for 3 days. Blue: DAPI for nuclear staining. EpoGFP protein expression does not reflect ongoing Epo gene transcription due to its longer half-life. EpoGFP mRNA expression is immediately silenced by urinary obstruction (Souma et al., 2013). Scale bar, 100 μm.

Figure 5

The cellular plasticity of REPs governs both fibrosis and anemia. (A) Schematic diagram of the reversible UUO model. The left ureter is obstructed by a vascular clip for 2 days and then re-opened afterwards. (B) The inverse relationship between EpoGFP and αSMA mRNA expression in whole kidneys of ISAM during reversible UUO treatment. (C) Schematic summary showing the plasticity of REPs. MF-REPs produce extracellular matrix (ECM) and inflammatory cytokines in injured kidneys through SMAD and NFκB signaling cascades. After the resolution of environmental cues, MF-REPs revert to their original and physiological Epo-producing phenotype. Switching between ON-REPs and OFF-REPs is determined in a hypoxia-dependent manner via HIF2α activation.