Importantly, this direct link indicates that renal fibrosis and anemia could be simultaneously treated by targeting or regulating the cellular properties of REPs. In this review, we provide a summary of recent lines of evidence regarding the role of REPs in health and disease and discuss future research directions for regulating REP functions to treat both fibrosis and anemia. However, these strategies have limited sensitivity and specificity for detecting Epo, hampering the establishment of a consensus as to which cells produce Epo.
To unequivocally determine the identity of REPs, we have utilized two complimentary strategies, BAC transgenic mice and green fluorescent protein GFP reporter knock-in mice Obara et al. Only a few renal interstitial cells are labeled by both strategies under normal conditions, whereas GFP-positive cells robustly emerge and increase in the kidneys under anemic or hypoxic stress. These GFP-positive cells in the interstitium are fibroblast-like cells expressing neural genes [e.
We named these cells renal erythropoietin-producing cells, or REPs Suzuki et al. Epo -knockout mice die at approximately embryonic day We assumed that the embryonic lethality of Epo -null mice could be rescued by transgenic Epo gene expression in the livers of embryos because hepatocytes are the Epo-producing cells at the lethal time point Suzuki et al.
To this end, we have utilized the knowledge regarding Epo gene regulation; i. An 8-kb Epo transgene Tg-Epo 3. To identify the total REPs, we have conducted fate-tracking assays of cells with a history of Epo production using newly generated Tg-EpoCre mice in which the Cre-recombinase is expressed under the regulation of the kb transgene containing the Epo gene regulatory region Souma et al. To detect EpoCre-labeled cell lineages, we have utilized the R26RtdTomato mouse line in which cells that have expressed Cre at least once are marked by tdTomato red fluorescence.
Interestingly, we have observed REPs around the peritubular capillaries, but not in the regions surrounding the larger vessels Figure 1. Figure 1. REPs are peri-capillary CDpositive fibroblast-like cells. The nuclei are stained by DAPI blue. DAPI blue is used for nuclear staining in the merged image.
Abbreviations: A, artery; V, vein. The balance between oxygen supply and demand precisely controls renal Epo production. A salient physiological study using isolated rat kidneys with hypoxic perfusate shows that renal Epo secretion is regulated by tissue oxygen tension Pagel et al. Distant organs, such as skin have been found to participate in regulation of renal Epo synthesis through sensing a hypoxic atmosphere Boutin et al. Recently, Dimke et al. In this mouse model, the kidney has scarce vascularization, resulting in renal hypoxia, increased Epo secretion, and severe polycythemia Dimke et al.
Figure 2. Understanding of the Epo gene regulation has been advanced by the discovery of Epo-producing hepatoma cell lines Hep3B and HepG2 Goldberg et al. In order to understand renal Epo gene regulation, transgenic mouse lines with different regulatory regions have been generated. However, this enhancer is dispensable for renal Epo gene expression, indicating that other cis -elements direct renal Epo production Figure 3. Figure 3. Overview of Epo gene transcriptional regulation.
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.
One of the important lingering questions about Epo gene regulation is the location and characteristics of the regulatory regions for renal Epo gene expression. However, the GATA box is turned out to be indispensable for repressing ectopic Epo gene expression in epithelial lineage cells, including distal tubular cells, bronchial epithelial cells, and cholangiocytes.
Of the transcription factors interacting with these Epo gene regulatory regions, HIFs play the central role in hypoxia-inducible Epo gene expression Haase, ; Suzuki, Table 1.
Summary of gene targeting studies of hypoxia-related factors on renal Epo gene regulation. The asparaginyl hydroxylation is more resistant to suppression by hypoxia than the prolyl hydroxylation Tian et al. FIHdependent hydroxylation is more prone to be inhibited by oxidative stresses than by hypoxic stresses Masson et al. Consistent with these findings, the systemic knockout of FIH-1 does not result in defects in the Epo gene regulation directly Table 1 Zhang et al.
All chronic nephropathies progress with tubular atrophy and interstitial fibrosis along with the relative loss of Epo production Quaggin and Kapus, While fibrosis is an essential biological process for repairing tissue injuries, uncontrolled and persistent injuries lead to sustained fibrogenesis, followed by destruction of tissue architecture and organ failure Quaggin and Kapus, ; Friedman et al.
Therefore, the identification of therapeutics controlling the pathological fibrogenic response would be beneficial for many devastating diseases, such as chronic kidney disease CKD , cirrhosis, and pulmonary fibrosis Friedman et al.
Since a strong correlation between tubulo-interstitial injury and decreased glomerular filtration rate was first described, many researchers have sought the origin of scar-forming cells, i. Using genetic lineage tracing, various cellular sources have been postulated as the origins of myofibroblasts, including pericytes, resident fibroblasts, tubular cells, endothelial cells, fibrocytes, and bone marrow-derived cells, but exact contributions of the sources to renal fibrosis still remain under debate summarized in Table 2 Quaggin and Kapus, ; Mack and Yanagita, A possible direct link between the loss of Epo production and progression of fibrosis was first proposed in Maxwell et al.
Maxwell et al. Interestingly, tamoxifen, a selective estrogen receptor modulator, is found to improve the Epo-producing ability of myofibroblasts Asada et al. To better understand the contribution of REPs to renal fibrosis and the link between fibrosis and anemia, ISAM have been utilized as the most efficient reporter mouse model for the Epo-producing ability. These results indicate that the renal fibrogenic milieu strongly represses Epo gene transcription in REPs during their myofibroblast transformation process Souma et al.
Consistent with the finding that renal myofibroblasts contribute to the inflammatory milieu, damage-associated molecular patterns DAMPs induce IL-6 and MCP1 productions in myofibroblasts Campanholle et al. Additionally, the loss of local Epo production might have deteriorating effects on fibrogenesis and inflammation in the kidneys, because cytoprotective function of Epo beyond erythropoiesis has been predicted Noguchi et al.
Figure 4. Myofibroblast transformation of REPs. Blue: DAPI for nuclear staining. EpoGFP protein expression does not reflect ongoing Epo gene transcription due to its longer half-life. Interestingly, the product of the hemoglobin Hb concentration times the Epo concentration in the peripheral blood of patients with diabetic nephropathy correlates well with the stages of diabetic nephropathy and predicts future chronic renal failure in overt diabetic nephropathy Inomata et al. Although confirmation of this argument waits for larger studies, we surmise that Epo would be a good biomarker to estimate the severity of interstitial injury and to predict the prognosis of damaged kidneys based on the short half-life 4—8 h of Epo Jelkmann, One aspect that makes the determination of the origins of myofibroblasts difficult is the complexity regarding the identity of the interstitial cells, i.
It has been shown that FoxD1-tagged pericytes are the major source of renal myofibroblasts Humphreys et al. As mentioned above, we found that REPs are the major source of renal myofibroblasts through EpoCre-based functional lineage tracing Souma et al. Recently, Kramann et al. Some clinical and experimental reports have shown that renal structural damage, including fibrosis, is reversible Zeisberg et al. Furthermore, dexamethasone facilitates this reversion, possibly through enhancing the resolution of inflammation in injured kidneys Souma et al.
These results indicate that REPs possess plasticity in response to environmental cues. Consistent with this observation, hepatic myofibroblasts can revert to their normal cellular character hepatic stellate cells during the regression of fibrosis Kisseleva et al.
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.
C Schematic summary showing the plasticity of REPs. After the resolution of environmental cues, MF-REPs revert to their original and physiological Epo-producing phenotype. A genome-wide transcriptome analysis of sham-treated kidneys, obstructed kidneys, and recovering kidneys indicates that the atherosclerotic and acute phase response signals are the top two up-regulated pathways and that valine, leucine, and isoleucine degradation and fatty acid metabolisms are the top two down-regulated pathways Souma et al.
Recent transcriptome analyses using kidney samples from human CKD patients reveals that the gene expressions of fatty acid metabolism are decreased and inflammatory signaling is increased in kidney diseases Kang et al. Based on the fact that metabolic intermediates play an important role in gene regulation, it is of great interest to determine whether deranged fatty acid metabolism would affect Epo production and whether correcting the metabolism, e. Uremic toxins are a group of compounds that are normally excreted by healthy kidneys, but accumulate upon kidney injuries.
These results further emphasize the importance of correcting the unbalanced microenvironment in injured kidneys. It is hypothesized that the kidney functions as a critmeter by sensing the relative volumes of each component of the blood through the common signal of tissue oxygen tension. The kidney's unique ability to sense ECF volume through tissue oxygen signal allows it to coordinate these two volumes to produce the normal hematocrit.
Hence, it may be the kidneys ability to report a measure of ECF volume as a tissue oxygen signal and thus to regulate the hematocrit that establishes it as the logical site of erythropoietin production. The critmeter is proposed to be a functional unit located at the tip of the cortical labyrinth at the juxta-medullary region of the kidney where erythropoietin is made physiologically. Within these organs, the cells synthesizing Epo were identified by using in situ hybridization in hypoxic animals with an increased Epo mRNA expression.
Epo-producing cells in the kidney were peritubular cells, most likely endothelial cells of the cortex and outer medulla. Glomerular and tubular cells were not labeled. The importance of these other influences is poorly understood.
Erythropoietin is necessary for life in vertebrates. Mice that are homozygous for deletions of the erythropoietin gene die early in gestation from severe anemia. A common cause of erythropoietin deficiency is chronic kidney disease. When the kidneys are damaged, their ability to produce erythropoietin is compromised and anemia ensues. The anemia associated with chronic kidney disease can be alleviated by treatment with erythropoietin. Such treatment is also valuable in a number of other types of disease associated with deficits in red blood cells production.
0コメント