Fate‐mapping of erythropoietin‐producing cells in mouse models of hypoxaemia and renal tissue remodelling reveals repeated recruitment and persistent functionality

Abstract

Aim Fibroblast‐like renal erythropoietin (Epo) producing (REP) cells of the corticomedullary border region “sense” a decrease in blood oxygen content following anaemia or hypoxaemia. Burst‐like transcription of Epo during tissue hypoxia is transient and is lost during fibrotic tissue remodelling, as observed in chronic kidney disease. The reason for this loss of Epo expression is under debate. Therefore, we tested the hypothesis that REP cell migration, loss and/or differentiation may cause Epo inhibition. Methods Using a reporter mouse that allows permanent labelling of active REP cells at any given time point, we analysed the spatiotemporal fate of REP cells following their initial hypoxic recruitment in models of hypoxaemia and renal tissue remodelling. Results In long‐term tracing experiments, tagged REP reporter cells neither died, proliferated, migrated nor transdifferentiated into myofibroblasts. Approximately 60% of tagged cells re‐expressed Epo upon a second hypoxic stimulus. In an unilateral model of tissue remodelling, tagged cells proliferated and ceased to produce Epo before a detectable increase in myofibroblast markers. Treatment with a hypoxia‐inducible factor (HIF) stabilizing agent (FG‐4592/roxadustat) re‐induced Epo expression in the previously active REP cells of the damaged kidney to a similar extent as in the contralateral healthy kidney. Conclusions Rather than cell death or differentiation, these results suggest cell‐intrinsic transient inhibition of Epo transcription: following long‐term dormancy, REP cells can repeatedly be recruited by tissue hypoxia, and during myofibrotic tissue remodelling, dormant REP cells are efficiently rescued by a pharmaceutic HIF stabilizer, demonstrating persistent REP cell functionality even during phases of Epo suppression.

The effect of HIF on metabolism and immunity

“…In addition to the systemic, adaptive response to hypoxia elicited by chemosensors, hypoxia induces a ubiquitous and highly evolutionarily conserved pathway that activates an adaptive transcriptional response at the cellular level. This pathway was first identified in studies that investigated how specific cells of the kidney sense hypoxia 25 – 28 , revealing that in response to hypoxia, these cells could increase their production of erythropoietin (EPO) through activation of HIF signalling 29 – 33 . This process amplified erythrocyte production to increase the oxygen-carrying capacity of the blood and to counteract a systemic decrease in pO 2 .…”

Erythropoietin‐producing cells in the kidney: Novel insights in their long‐term fate during hypoxaemia and renal tissue remodelling

“…An elegant study published in the current issue of Acta Physiologica analyses the spatio‐temporal fate of previously recruited renal erythropoietin (Epo)‐producing (REP) cells during systemic hypoxaemia and renal tissue remodelling 1 . Reduced blood haemoglobin content because of impaired erythropoietin (Epo) production poses a serious threat in chronic kidney disease (CKD).…”

“…An elegant study published in the current issue of Acta Physiologica analyses the spatio-temporal fate of previously recruited renal erythropoietin (Epo)-producing (REP) cells during systemic hypoxaemia and renal tissue remodelling. 1 Reduced blood haemoglobin content because of impaired erythropoietin (Epo) production poses a serious threat in chronic kidney disease (CKD). Renal anaemia is commonly treated by subcutaneous injections with recombinant human Epo and, since recently, by oral administration of so-called hypoxia-inducible factor (HIF) stabilizers.…”

Neurogenic and pericytic plasticity of conditionally immortalized cells derived from renal erythropoietin‐producing cells

“…Intriguingly, renal Epo production and blood Epo levels diminish rapidly despite ongoing hypoxia, long before a corrective response in blood oxygen transport capacity occurs (Dahl et al, 2022 ; Eckardt et al, 1990 ). Transcriptional Epo activity in REP cells seems to follow a poorly understood “on‐off” pattern (Souma et al, 2015 ; Suzuki & Yamamoto, 2016 ).…”

“…As REP cells in vivo normally do not proliferate (Dahl et al, 2022 ; Souma et al, 2013 ), affecting oxygen consumption and energy metabolism, it is possible that cellular quiescence is required for maintaining normal oxygen sensing and signaling in cultured REPD cells. Similar observations have been made before in other cell lines, for example, derived from podocytes or kidney proximal tubule cells: conditional immortalization using a heat‐labile SV40 large T antigen followed by cultivation under nonpermissive conditions (i.e., an increase in the temperature from 33°C to 37–39°C) resulted in a differentiation pattern more closely resembling the respective cell type in vivo (Loghman‐Adham et al, 1997 ; Saleem et al, 2002 ).…”