Increased cell surface expression of C-terminal truncated erythropoietin receptors in polycythemia

Cytokine receptor splice variants in hematologic diseases

Cytokine and cytokine receptors are important regulators of hematopoiesis. Hematopoietic stem cells (HSCs) and progenitors differentiate into the myeloid or lymphoid lineage in response to specific cytokines. Cell-type specific receptors are expressed on committed progenitors that bind to other late-acting cytokines that are involved in terminal differentiation of hematopoietic cells. In normal hematopoiesis, these receptors undergo alternative splicing and are developmentally regulated. Splicing changes can significantly affect the structure and function of the receptors resulting in alterations of either the extracellular ligand binding domain or the cytoplasmic signaling domain responsible for cellular growth and differentiation. Most alternatively spliced isoforms generally lose the ability to promote differentiation. Evidently, overexpression of naturally occurring cytokine receptor alternate isoforms are observed in multiple myeloid diseases such as myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), and polycythemia vera (PV). The purpose of this review is to introduce the various isoforms of key cytokine receptors that play a crucial role in myeloid development and their potential role in myeloid diseases.

Oxygen-dependent Regulation of Erythropoietin Receptor Turnover and Signaling

von Hippel-Lindau (VHL) disease is a rare familial cancer predisposition syndrome caused by a loss or mutation in a single gene,VHL, but it exhibits a wide phenotypic variability that can be categorized into distinct subtypes. The phenotypic variability has been largely argued to be attributable to the extent of deregulation of the α subunit of hypoxia-inducible factor α, a well established target of VHL E3 ubiquitin ligase, ECV (Elongins/Cul2/VHL). Here, we show that erythropoietin receptor (EPOR) is hydroxylated on proline 419 and 426 via prolyl hydroxylase 3. EPOR hydroxylation is required for binding to the β domain of VHL and polyubiquitylation via ECV, leading to increased EPOR turnover. In addition, several type-specific VHL disease-causing mutants, including those that have retained proper binding and regulation of hypoxia-inducible factor α, showed a severe defect in binding prolyl hydroxylated EPOR peptides. These results identify EPOR as the secondbona fidehydroxylation-dependent substrate of VHL that potentially influences oxygen homeostasis and contributes to the complex genotype-phenotype correlation in VHL disease.

Dynamic ligand modulation of EPO receptor pools, and dysregulation by polycythemia-associated EPOR alleles

Erythropoietin (EPO) and its cell surface receptor (EPOR) are essential for erythropoiesis; can modulate non-erythroid target tissues; and have been reported to affect the progression of certain cancers. Basic studies of EPOR expression and trafficking, however, have been hindered by low-level EPOR occurrence, and the limited specificity of anti-EPOR antibodies. Consequently, these aspects of EPOR biology are not well defined, nor are actions of polycythemia- associated mutated EPOR alleles. Using novel rabbit monoclonal antibodies to intracellular, PY- activated and extracellular EPOR domains, the following properties of the endogenous hEPOR in erythroid progenitors first are unambiguously defined. 1) High- Mr EPOR forms become obviously expressed only when EPO is limited. 2) EPOR-68K plus -70K species sequentially accumulate, and EPOR-70K comprises an apparent cell surface EPOR population. 3) Brefeldin A, N-glycanase and associated analyses point to EPOR-68K as a core-glycosylated intracellular EPOR pool (of modest size). 4) In contrast to recent reports, EPOR inward trafficking is shown (in UT7epo cells, and primary proerythroblasts) to be sharply ligand-dependent. Beyond this, when C-terminal truncated hEPOR-T mutant alleles as harbored by polycythemia patients are co-expressed with the wild-type EPOR in EPO-dependent erythroid progenitors, several specific events become altered. First, EPOR-T alleles are persistently activated upon EPO- challenge, yet are also subject to apparent turn-over (to low-Mr EPOR products). Furthermore, during exponential cell growth EPOR-T species become both over-represented, and hyper-activated. Interestingly, EPOR-T expression also results in an EPO dose-dependent loss of endogenous wild-type EPOR's (and, therefore, a squelching of EPOR C-terminal- mediated negative feedback effects). New knowledge concerning regulated EPOR expression and trafficking therefore is provided, together with new insight into mechanisms via which mutated EPOR-T polycythemia alleles dysregulate the erythron. Notably, specific new tools also are characterized for studies of EPOR expression, activation, action and metabolism.

Cytosolic lysine residues enhance anterograde transport and activation of the erythropoietin receptor

Lysine residues are key residues in many cellular processes, in part due to their ability to accept a wide variety of post-translational modifications. In the present study, we identify the EPO-R [EPO (erythropoietin) receptor] cytosolic lysine residues as enhancers of receptor function. EPO-R drives survival, proliferation and differentiation of erythroid progenitor cells via binding of its ligand EPO. We mutated the five EPO-R cytosolic lysine residues to arginine residues (5KR EPO-R), eliminating putative lysine-dependent modifications. Overexpressed 5KR EPO-R displayed impaired ubiquitination and improved stability compared with wt (wild-type) EPO-R. Unexpectedly, fusion proteins consisting of VSVGtsO45 (vesicular stomatitis virus glycoprotein temperature-sensitive folding mutant) with wt or 5KR EPO-R cytosolic domains demonstrated delayed glycan maturation kinetics upon substitution of the lysine residues. Moreover, VSVG-wt EPO-R, but not VSVG-5KR EPO-R, displayed endoplasmic reticulum-associated ubiquitination. Despite similar cell-surface EPO-binding levels of both receptors and the lack of EPO-induced ubiquitination by 5KR EPO-R, the lysine-less mutant produced weaker receptor activation and signalling than the wt receptor. We thus propose that EPO-R cytosolic lysine residues enhance receptor function, most probably through ubiquitination and/or other post-translational modifications.

Correction of murine β-thalassemia after minimal lentiviral gene transfer and homeostatic in vivo erythroid expansion

A challenge for gene therapy of genetic diseases is to maintain corrected cell populations in subjects undergoing transplantation in cases in which the corrected cells do not have intrinsic selective advantage over nontransduced cells. For inherited hematopoietic disorders, limitations include inefficient transduction of stem cell pools, the requirement for toxic myelosuppression, and a lack of optimal methods for cell selection after transduction. Here, we have designed a lentiviral vector that encodes human β-globin and a truncated erythropoietin receptor, both under erythroid-specific transcriptional control. This truncated receptor confers enhanced sensitivity to erythropoietin and a benign course in human carriers. Transplantation of marrow transduced with the vector into syngenic thalassemic mice, which have elevated plasma erythropoietin levels, resulted in long-term correction of the disease even at low ratios of transduced/untransduced cells. Amplification of the red over the white blood cell lineages was self-controlled and averaged ∼ 100-fold instead of ∼ 5-fold for β-globin expression alone. There was no detectable amplification of white blood cells or alteration of hematopoietic homeostasis. Notwithstanding legitimate safety concerns in the context of randomly integrating vectors, this approach may prove especially valuable in combination with targeted integration or in situ homologous recombination/repair and may lower the required level of pretransplantation myelosuppression.

Insertion of an NPVY sequence into the cytosolic domain of the erythropoietin receptor selectively affects erythropoietin-mediated signalling and function

EPO (erythropoietin), the major hormone regulating erythropoiesis, functions via activation of its cell-surface receptor (EPO-R) present on erythroid progenitor cells. One of the most striking properties of EPO-R is its low expression on the cell surface, as opposed to its high intracellular levels. The low cell-surface expression of EPO-R may thus limit the efficacy of EPO that is routinely used to treat primary and secondary anaemia. In a recent study [Nahari, Barzilay, Hirschberg and Neumann (2008) Biochem. J. 410, 409–416] we have shown that insertion of an NPVY sequence into the intracellular domain of EPO-R increases its cell-surface expression. In the present study we demonstrate that this NPVY EPO-R insert has a selective effect on EPO-mediated downstream signalling in Ba/F3 cells expressing this receptor (NPVY-EPO-R). This is monitored by increased phosphorylation of the NPVY-EPO-R (on Tyr479), Akt, JAK2 (Janus kinase 2) and ERK1/2 (extracellular-signal-regulated kinase 1/2), but not STAT5 (signal transducer and activator of transcription 5), as compared with cells expressing wild-type EPO-R. This enhanced signalling is reflected in augmented proliferation at low EPO levels (0.05 units/ml) and protection against etoposide-induced apoptosis. Increased cell-surface levels of NPVY-EPO-R are most probably not sufficient to mediate these effects as the A234E-EPO-R mutant that is expressed at high cell-surface levels does not confer an augmented response to EPO. Taken together, we demonstrate that insertion of an NPVY sequence into the cytosolic domain of the EPO-R confers not only improved maturation, but also selectively affects EPO-mediated signalling resulting in an improved responsiveness to EPO reflected in cell proliferation and protection against apoptosis.

Ligand-induced EpoR internalization is mediated by JAK2 and p85 and is impaired by mutations responsible for primary familial and congenital polycythemia

Epo-induced endocytosis of EpoR plays important roles in the down-regulation of EpoR signaling and is the primary means that regulates circulating Epo concentrations. Here we show that cell-surface EpoR is internalized via clathrin-mediated endocytosis. Both JAK2 kinase activity and EpoR cytoplasmic tyrosines are important for ligand-dependent EpoR internalization. Phosphorylated Y429, Y431, and Y479 in the EpoR cytoplasmic domain bind p85 subunit of PI3 kinase on Epo stimulation and individually are sufficient to mediate Epo-dependent EpoR internalization. Knockdown of p85alpha and p85beta or expression of their dominant-negative forms, but not inhibition of PI3 kinase activity, dramatically impaired EpoR internalization, indicating that p85alpha and p85beta may recruit proteins in the endocytic machinery on Epo stimulation. Furthermore, mutated EpoRs from primary familial and congenital polycythemia (PFCP) patients lacking the 3 important tyrosines do not bind p85 or internalize on stimulation. Addition of residues encompassing Y429 and Y431 to these truncated receptors restored p85beta binding and Epo sensitivity. Our results identify a novel PI3 kinase activity-independent function of p85 in EpoR internalization and support a model that defects of internalization in truncated EpoRs from PFCP patients contribute to Epo hypersensitivity and prolonged signaling.

A transplanted NPVY sequence in the cytosolic domain of the erythropoietin receptor enhances maturation

Activation of the EPO-R [EPO (erythropoietin) receptor] by its ligand EPO promotes erythropoiesis. Low cell surface EPO-R levels are traditionally attributed to inefficient folding mediated by the receptor extracellular domain. In the present study, we addressed the role of the EPO-R intracellular domain in exit from the ER (endoplasmic reticulum) and surface expression. A fusion protein between the thermo-reversible folding mutant of VSVG (vesicular-stomatitis-virus glycoprotein) (VSVGtsO45) and the EPO-R cytosolic domain [VSVG-WT (wild-type)] displayed delayed intracellular trafficking as compared with the parental VSVGtsO45, suggesting that the EPO-R cytosolic domain can hamper ER exit. Although NPXY-based motifs were originally associated with clathrin binding and endocytosis, they may also function in other contexts of the secretory pathway. A fusion protein between VSVGtsO45 and the cytosolic portion of EPO-R containing an NPVY insert (VSVG-NPVY) displayed enhanced glycan maturation and surface expression as compared with VSVG-WT. Notably, the NPVY insert also conferred improved maturation and augmented cell surface EPO-R. Our findings highlight three major concepts: (i) the EPO-R cytosolic domain is involved in ER exit of the receptor. (ii) Sequence motifs that participate in endocytosis can also modulate transport along the secretory pathway. (iii) VSVG-fusion proteins may be employed to screen for intracellular sequences that regulate transport.

The Src homology 2 domain tyrosine phosphatases SHP-1 and SHP-2: diversified control of cell growth, inflammation, and injury

Interest in the diverse biology of protein tyrosine phosphatases that are encoded by more than 100 genes in the human genome continues to grow at an accelerated pace. In particular, two cytoplasmic protein tyrosine phosphatases composed of two Src homology 2 (SH2) NH2-terminal domains and a C-terminal protein-tyrosine phosphatase domain referred to as SHP-1 and SHP-2 are known to govern a host of cellular functions. SHP-1 and SHP-2 modulate progenitor cell development, cellular growth, tissue inflammation, and cellular chemotaxis, but more recently the role of SHP-1 and SHP-2 to directly control cell survival involving oxidative stress pathways has come to light. SHP-1 and SHP-2 are fundamental for the function of several growth factor and metabolic pathways yielding far reaching implications for disease pathways and disorders such as diabetes, neurodegeneration, and cancer. Although SHP-1 and SHP-2 can employ similar or parallel cellular pathways, these proteins also clearly exert opposing effects upon downstream cellular cascades that affect early and late apoptotic programs. SHP-1 and SHP-2 modulate cellular signals that involve phosphatidylinositol 3-kinase, Akt, Janus kinase 2, signal transducer and activator of transcription proteins, mitogen-activating protein kinases, extracellular signal-related kinases, c-Jun-amino terminal kinases, and nuclear factor-kappaB. Our progressive understanding of the impact of SHP-1 and SHP-2 upon multiple cellular environments and organ systems should continue to facilitate the targeted development of treatments for a variety of disease entities.

An extracellular region of the erythropoietin receptor of the subterranean blind mole rat Spalax enhances receptor maturation

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.

Molecular markers for the diagnosis of Philadelphia chromosome negative myeloproliferative disorders

Polycythemia vera, essential thrombocythemia, idiopathic myelofibrosis and chronic myelogenous leukemia have been collectively termed the myeloproliferative disorders due to similarities in their clinical presentation. With the exception of chronic myelogenous leukemia, which is characterized by the presence of the Philadelphia chromosome, the myeloproliferative disorders display no consistent cytogenetic abnormalities. Hence, the diagnosis of Polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis to date relies on clinical criteria. However, several molecular aberrations have been described, which can be used as molecular markers for the diagnosis of these clinical entities. This review outlines the diagnostic assays developed and highlights the advantages and disadvantages of the following markers: (1). Endogenous Erythroid Colonies, (2). Clonality, (3). Reduced c-Mpl protein expression and (4). PRV-1 mRNA over expression.

Basic sciences of the myeloproliferative diseases: pathogenic mechanisms of ET and PV

The molecular pathogenesis of ET and PV is unknown, although the relatively indolent clinical course observed in most patients suggests that the defect may be subtle and difficult to establish. Clonality analysis using X-chromosome inactivation patterns in females on purified CD34+ cells have confirmed that a defect is present in the hematopoietic stem cell. However, at least in ET, a significant proportion of patients have polyclonal hemopoiesis, and this presumably reflects the heterogeneous nature of the disorder(s). Attention has focussed on the potential disruption of the physiological regulators EPO and TPO and their respective receptors. In familial disorders, pathological mutations have been identified in some, but by no means all, cases: EPO receptor mutations in PFCP, TPO mutations in FT and, conversely, TPO receptor (c-mpl) mutations in CAMT. Equivalent ligand or receptor mutations have not been detected in ET or PV patients. However, there is evidence to suggest that c-mpl expression may be dysregulated, with low or absent c-mpl mRNA or protein reported in ET and/or PV patients. At present it is not clear whether this is the cause or consequence of the paradoxically normal/increased TPO levels found with both primary and secondary thrombocytosis. In vitro culture analysis has demonstrated both cytokine independence and hyper-sensitivity as a generalised feature of progenitor cells from many patients, but differences exist depending on the assays used and there is little understanding of the mechanism(s) underlying these responses. Two genes have recently been identified with increased mRNA expression in PV granulocytes: PRV-1, a novel cell surface receptor closely related to the uPAR/Ly6/CD59/snake toxin family of proteins, and NFI-B, a member of the nuclear factor I family which may be associated with TGF-beta resistance. Investigation of their regulation and biological effects may assist in determining the pathobiology of these elusive disorders.

A transmembrane trap method for efficient cloning of genes encoding proteins possessing transmembrane domain

To facilitate searching for genes encoding cell membrane proteins, we developed a method for isolating cDNAs that contain sequences for hydrophobic transmembrane runs. This cloning strategy, termed the "transmembrane (TM) trap method," utilizes a vector that directs the cell surface expression of mouse CD4 fusion protein when an insert encoding hydrophobic transmembrane sequences is cloned in-frame with correct orientation. We applied this novel method to isolation of cytokine receptor cDNAs. Our strategy enabled efficient isolation of relatively rare species encoding receptors such as IL-2Rgamma, IL-3Rbeta, IL-4Ralpha, IL-5Ralpha, and IL-6Ralpha. This method also could be used to isolate cDNAs for intracellular molecules with a transmembrane region, e.g., bcl-2. These results indicate that the TM trap method provides an efficient cloning strategy for identification of various families of genes encoding proteins with one or more transmembrane regions.