Cell-specific organization and paralog divergence of erythropoietin signaling in Atlantic salmon

Erythropoietin (EPO) is the main hormonal regulator of erythropoiesis in vertebrates and also mediates non-hematopoietic functions. Despite extensive characterization in mammals, the cellular sources and targets of EPO signaling remain poorly defined in non-mammalian vertebrates. The Atlantic salmon provides a valuable model to investigate paralog-specific organization of EPO signaling due to a recent whole genome duplication (WGD). Here, we characterize the tissue- and cell-specific expression of paralogs encoding EPO and the erythropoietin receptor (EPOR) in Atlantic salmon (Salmo salar). Phylogenetic analysis revealed four EPO encoding paralogs retained from the teleost-specific (Ts3R) and salmonid-specific WGD (Ss4R) events, designated epoa1, epoa2, epob1 and epob2, and two Ss4R paralogs encoding EPOR, designated epora1 and epora2. In situ hybridization analysis targeting mRNA revealed that epoa2 was the predominantly expressed paralog in the heart, with cardiomyocytes as the principal source, while epob1 was the most abundant paralog in the kidney with expression mainly in tubular epithelial cells of the trunk kidney. mRNA from the epora genes was most highly expressed in erythropoietic tissues and cells but also detected in non-erythropoietic cell types. Temperature was shown to affect epoa2 mRNA expression in the heart. Collectively, these results identify the cellular sources of epo mRNA in the heart, head kidney and trunk kidney, and demonstrate paralog-specific partitioning of epo expression following the Ss4R WGD in Atlantic salmon. In addition, mRNA for epora genes was detected across a range of hematopoietic and non-hematopoietic cell types. Together, these findings provide a cell-resolved framework for understanding vertebrate EPO signaling, suggesting roles of EPO and EPOR in regulating both erythropoietic and non-hematopoietic processes.