Erythropoietic functions of erythropoietin (EPO) are mediated by its receptor (EPO-R), which is present on the cell surface of erythroid progenitors and induced by hypoxia. We focused on EPO-R from Spalax galili (sEPO-R), one of the four Israeli species of the subterranean blind mole rat, Spalax ehrenbergi superspecies, as a special natural animal model of high tolerance to hypoxia. Led by the intriguing observation that most of the mouse EPO-R (mEPO-R) is retained in the endoplasmic reticulum (ER), we hypothesized that sEPO-R is expressed at higher levels on the cell surface, thus maximizing the response to elevated EPO, which has been reported in this species. Indeed, we found increased cell-surface levels of sEPO-R as compared with mEPO-R by using flow cytometry analysis of BOSC cells transiently expressing HA-tagged EPO-Rs (full length or truncated). We then postulated that unique extracellular sEPO-R sequence features contribute to its processing and cell-surface expression. To map these domains of the sEPO-R that augment receptor maturation, we generated EPO-R derivatives in which parts of the extracellular region of mEPO-R were replaced with the corresponding fragments of sEPO-R. We found that an extracellular portion of sEPO-R, harboring the N-glycosylation site, conferred enhanced maturation and increased transport to the cell surface of the respective chimeric receptor.Taken together, we demonstrate higher surface expression of sEPO-R, attributed at least in part to increased ER exit, mediated by an extracellular region of this receptor. We speculate that these sEPO-R sequence features play a role in the adaptation of Spalax to extreme hypoxia.signal transduction ͉ intracellular trafficking ͉ hypoxia ͉ glycosylation E rythropoietin (EPO) promotes proliferation and differentiation of erythroid cell precursors via activation of its cellsurface receptor (EPO-R) (1, 2). EPO-R is a member of the cytokine-receptor superfamily, characterized by the presence of four conserved Cys residues and a ''WSXWS'' motif in its extracellular domain. The lack of enzymatic activity in the intracellular domain of these receptors necessitates complex formation of ligand-bound receptors with other signaling partners [i.e., Janus kinases (JAKs)] (3) to initiate signaling cascades. EPO-R is synthesized in the endoplasmic reticulum (ER) as a major 64-kDa form with a single endoglycosidase H (Endo H)-sensitive high-mannose oligosaccharide and as a nonglycosylated 62-kDa form. Forty to 60% of the 64-kDa EPO-R molecules undergo further glycan maturation to generate the 66-kDa Endo H-resistant EPO-R (4).In contrast to other type 1 cytokine receptors (5-7), a most striking property of the EPO-R is its low expression on the cell surface (4, 8-10) despite its high intracellular levels. The majority of newly synthesized EPO-R remains sequestered intracellularly and is rapidly degraded (t 1/2 Ϸ 45 min) (11). These metabolic features are similar in both transfected (4) and fetal liver cells that endogenously express the EPO-R (12), suppor...
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