If the advent of single cell technologies has taught us anything over the past few years, it’s that biologists have up until now gotten only a birds-eye view of most systems and processes. Case in point: in a new study that used combined single cell gene expression and epigenetic analyses, researchers in Israel were able to identify a single, unique cell type behind the production of erythropoietin (Epo) in the kidney. They believe their discovery could open up the treatment possibilities for anemia. Keep reading to learn more about their work.
Single cell tools unveil unique cell type behind Epo production in the kidney
All cells require oxygen, and the process by which the body delivers this essential life-giving element is through red blood cells, also known as erythrocytes. Erythropoiesis is the process of making erythrocytes, with the goal of maintaining a balanced oxygen level in the body. While erythropoiesis can happen in certain bones, renal (kidney) Epo is the master regulator of the red blood cell creation process—and the kidney is the main source of this endocrine hormone in adults (1). Epo moves from the kidney into the blood, finds progenitors in the bone marrow to bind to, and helps them proliferate and differentiate—which ultimately helps keep red blood cell count at sufficient levels. Despite its important role, not much is known about Epo gene expression regulation outside the petri dish. In fact, no cell type has been definitively identified as the source of renal Epo in humans or animal models (2).
In their recent Nature Medicine study (3), a team led by Bjørt Kragesteen, PhD, at the Weizmann Institute of Science in Israel, performed single cell RNA sequencing (scRNA-seq) and single cell assay for transposase-accessible chromatin sequencing (scATAC-seq) of an Epo reporter mouse to identify cells that produce Epo under hypoxic conditions (3). Employing a previously developed Epo-CreERT mouse model (that permanently labels actively producing cells) (4), they used 10x Genomics Chromium Single Cell Multiome ATAC + Gene Expression on single nuclei derived from mice kidney cells to identify what they called “Norn” cells (named after “mythological Norse entities believed to govern human fates,” they write). This single population of unique, conserved kidney fibroblast-like cells governs Epo production, they found.
Digging deep: A closer look at Norn cells with scRNA-seq
Their data unveiled much detail about this newly discovered cell type. While Norn cells do share expression of key fibroblast and pericyte markers, including Pdgfra, Pdgfrb, Rgs5, and Nt5e, Norn cells differ from these other cell types in that they have hundreds of differentially expressed genes compared to each of the other stromal cell populations. Norn cells highly express Hsd11b1 (an enzyme regulating cortisol activity), Cxcl14 and Cxcl12 (ligands of the Cxc chemokine system involved in the growth of new blood vessels), Cygb (a cytoglobin that plays a role in intracellular oxygen transport), and Cfh (a complement pathway regulator)—all of which are not expressed by other kidney cell types.
Surveying the regulatory landscape of Norn cells
In addition, their data showed that Norn cells have a unique epigenetic identity. They found thousands of unique candidate enhancer regions that may contribute to the Norn cellular identity and function. These regions are enriched in motifs of transcription factors TCF21, C/EBPδ, and GATA6, which suggests these factors belong to a new piece of the Epo gene regulation puzzle, thereby also playing a part in determining Norn cell identity.
Toward new therapies for anemia
Their detailed cell atlas of Epo regulation is of pressing need when it comes to treating the common condition of anemia. Recombinant Epo is often used to treat patients with end-stage renal disease (5); however, prolonged Epo exposure through injections can lead to severe side effects, including an increased risk of blood clots (6). Their atlas, they hope, has only just begun to open avenues toward alternative therapeutic approaches for treating anemia and other conditions that arise due to deficient Epo levels.
- Wenger RH & Kurtz A. Erythropoietin. Compr Physiol 1: 1759–94 (2011).
- Broeker KAE, et al. Different subpopulations of kidney interstitial cells produce erythropoietin and factors supporting tissue oxygenation in response to hypoxia in vivo. Kidney Int 98: 918–931 (2020).
- Kragesteen BK, et al. The transcriptional and regulatory identity of erythropoietin producing cells. Nat Med 29: 1191–1200 (2023).
- Imeri F, et al. Generation of renal Epo-producing cell lines by conditional gene tagging reveals rapid HIF-2 driven Epo kinetics, cell autonomous feedback regulation, and a telocyte phenotype. Kidney Int 95: 375–387 (2019).
- Haase VH. Hypoxia-inducible factor-prolyl hydroxylase inhibitors in the treatment of anemia of chronic kidney disease. Kidney Int Suppl 11: 8–25 (2011).
- Gordeuk VR, et al. Thrombotic risk in congenital erythrocytosis due to up-regulated hypoxia sensing is not associated with elevated hematocrit. Haematologica 105: e87–e90 (2020).