Signal Transduction in the Erythropoietin Receptor System Don M. Wojchowski,* , † , ‡ , § ,¶ ,1 Richard C. Gregory,§ ,¶ Chris P. Miller,‡ ,¶ Ajay K. Pandit,§ ,¶ and Tony J. Pircher ¶ *Program in Cell & Developmental Biology, †Program in Immunology, and ‡Program in Genetics, §Department of Biochemistry & Molecular Biology and ¶ Department of Veterinary Science, Pennsylvania State University, University Park, Pennsylvania 16802 Events relayed via the single transmembrane recep- tor for erythropoietin (Epo) are essential for the de- velopment of committed erythroid progenitor cells be- yond the colony-forming unit– erythroid stage, and this clearly involves Epo’s inhibition of programmed cell death (PCD). Less well resolved, however, are is- sues regarding the precise nature of Epo-dependent antiapoptotic mechanisms, the extent to which Epo might also promote mitogenesis and/or terminal ery- throid differentiation, and the essential vs modulatory nature of certain Epo receptor cytoplasmic subdo- mains, signal transducing factors, and downstream pathways. Accordingly, this review focuses on the fol- lowing aspects of Epo signal transduction: (1) Epo re- ceptor/Jak2 activation mechanisms; (2) the critical vs dispensable nature of (P)Y sites and SH2 domain-en- coding effectors in survival, growth, and differentia- tion responses; (3) primary mechanisms by which Epo inhibits PCD; (4) the integration of signals relayed by coexpressed and possibly directly interacting cyto- kine receptors; and (5) predictions regarding effector function which are provided by the association of cer- tain primary and familial polycythemias with mutated human Epo receptor forms. © 1999 Academic Press Key Words: Epo; erythropoietin; Epo receptor; erythropoiesis. EPO RECEPTOR STRUCTURE AND BIOLOGY The receptor for erythropoietin (Epo) is a founding member of the type 1 superfamily of single-transmem- brane cytokine receptors [1, 2]. This family shares con- served extracellular ligand-binding regions composed of fibronectin III (FNIII)-like subdomains, as well as a conserved -chain cytoplasmic box 1 motif which binds selectively to Janus kinases [3]. For the Epo receptor per se, crystallographic analyses have revealed that two liganded Epo receptor monomers are bridged by Epo as a symmetrical T-shaped dimer [4, 5]. The ex- tracellular region of each monomer contains two FNIII- like subdomains (D1 and D2) each of which utilizes seven -strands to form two anti-parallel -pleated sheets. The NH 2 -terminal D1 domain forms an h-type fold [6] with a hybrid FNIII/immunoglobulin-like topol- ogy [4]. Within this distal D1 domain, two pairs of cysteine residues (which also are conserved among type 1 receptors) form disulfide bridges. The membrane proximal D2 domain folds with standard s-type FNIII topology [4]. Also in D2 is a conserved WSXWS motif which has been shown to be important for Epo receptor folding [7], yet is located away from sites of Epo bind- ing and interacts with a short NH 2 -terminal helix. Together, these D1 and D2 domains contribute six loops for Epo interactions. Cytoplasmic features in- clude the above-mentioned box 1 domain which in the Epo receptor is specific for Jak2 [8, 9] and eight phos- photyrosine sites which mediate the recruitment of Src homology-2 (SH2) domain-encoding effectors (see be- low). Among type 1 receptors, those for thrombopoietin (Tpo), granulocyte colony-stimulating factor (G-CSF), prolactin (Prl), and growth hormone (GH) are perhaps most closely related to the Epo receptor in that each has been reported to homodimerize [10], bind Jak2 [11–14], and activate STAT5 as a signal transducer and activator of transcription [15–18]. In addition, each exhibits substantial overlap in subsets of phos- photyrosine-associated effectors, and the cytoplasmic domains of each of these receptors have been shown in chimeric constructs to substitute for that of the Epo receptor in supporting red cell production [19 –22]. With regard to Epo receptor function, gene disrup- tion experiments in chimeric mice have shown that while the Epo receptor is not required for the develop- ment of primitive yolk sac-derived erythrocytes, it is essential for definitive erythropoiesis in fetal liver [23]. In Epo receptor -/- mice at day 9.5, normal numbers of embryonic globin-positive yolk sac-derived erythro- cytes are produced, but at later days these erythroid cells are markedly diminished in size and proliferative potential [24]. In fetal liver, burst-forming units– ery- throid (BFUe) and colony-forming units– erythroid 1 To whom correspondence and reprint requests should be ad- dressed at Pennsylvania State University, 115 W. L. Henning Build- ing, University Park, PA 16802. Fax: 814-863-6140. E-mail: dmw1@psu.edu. 0014-4827/99 $30.00 143 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved. Experimental Cell Research 253, 143–156 (1999) Article ID excr.1999.4673, available online at http://www.idealibrary.com on