Erythropoietin (EPO), a cytokine hormone, is responsible for the regulation of red blood cell (erythrocyte) production. EPO is a glycoprotein produced by cells of the peritubular capillary endothelium of the kidney and, to a lesser extent, by liver hepatocytes. EPO production is stimulated by reduced oxygen content in the renal arterial circulation, mediated by a transcription factor that is oxygen-sensitive.
Secreted EPO (165 amino acids) binds to EPO receptors on the surface of bone marrow erythroid precursors, resulting in their rapid replication and maturation to functional red blood cells. This stimulation results in a rapid rise in erythrocyte counts and a consequent rise in blood oxygen. Altered levels of EPO or mutations in EPO receptors are linked to changes in the hematocrit (% of red blood cells in blood). For example, kidney failure leads directly to severe anemia due to low EPO levels and hence reduced hematopoiesis.
In a bizarre case of benign erythrocytosis (overproduction of red cells), the"disease" was linked to a heterozygous mutation in the EPO receptor gene of a three-time Olympic gold medalist in cross country skiing. His hematocrit was ~60%, well above the standard 45%, due to loss of a negative control region in the EPO receptor.
Shown at left is the structure of the cytokine Erythropoietin (EPO), bound by the extracellular domains of two identical EPO receptors, designated here as EPObp2 and EPObp1 (PDB ID 1cn4, Syed, et al., 1998). Several structural features of this cytokine and its receptors are illustrated by clicking on the buttons , in order.
II. Erythropoeitin Structure
EPO contains a four-helical bundle with a topology shared with other cytokines. The four helices of this bundle are termed A, B, C, and D. The A and D helices are linked by a disulphide bridge. The B and C helices are linked by a short loop. In addition to the A-D, helices, EPO contains two short helices, B' and C'.
The structure of EPO is further stabilized by numerous hydrophobic interactions. For example, aromatic and hydrophobic amino acids of the D-helix pack against hydrophobic residues of helices A, B, and C, helping to form the hydrophobic core of EPO.
III. EPO Receptor Structure and EPO Binding
The structure of both of the extracellular domains contains two, seven-stranded beta sandwich subdomains, like other cytokine receptors. Each extracellular domain has an N-terminal alpha helix positioned in the "elbow" between the beta sandwich subdomains. This helix has sidechains that interact with residues of two conserved regions, and this is thought to be instrumental in stabilizing the folded EPO receptor.
EPO binding imposes a distinct orientation of the two identical EPO receptors, providing for optimal signal transduction. Each of two opposite faces of EPO interacts with one binding site on each EPO receptor. Interestingly, these binding sites are non identical, even though the receptors are identical. Site 1 involves complementary interactions between six loops (L1-L6) of EPObp1 and EPO helices A, B', D, plus part of the loop between the A and B helices. Site 2, a lower affinity binding site, involves interaction between five loops (L1-L3, L5, L6) of EPObp2 and residues on helices A and C of EPO.
de la Chapelle A, Sistonen P, Lehväslaiho H, Ikkala E, Juvonen E. 1993. Familial erythrocytosis genetically linked to the erythropoietin receptor gene. Lancet 341:82-84.
Syed, R. S., Reid, S. W., Li, C., Cheetham, J. C., Aoki, K. H., Liu, B., Zhan, H., Osslund, T. D., Chirino, A. J., Zhang, J., Finer-Moore, J., Elliott, S., Sitney, K., Katz, B. A., Matthews, D. J., Wendoloski, J. J., Egrie, J., Stroud, R. M.. 1998. Efficiency of Signalling Through Cytokine Receptors Depends Critically on Receptor Orientation. Nature 395: 511