Selectable retrovirus vectors encoding Friend virus gp55 or erythropoietin induce polycythemia with different phenotypic expression and disease progression

Helix packing and orientation in the transmembrane dimer of gp55-P of the spleen focus forming virus

gp55-P is a dimeric membrane protein with a single transmembrane helix that is coded by the env gene of the polycythemic strain of the spleen focus forming virus. gp55-P activates the erythropoietin (Epo) receptor through specific transmembrane helix interactions, leading to Epo-independent growth of erythroid progenitors and eventually promoting erythroleukemia. We describe the use of magic angle spinning deuterium NMR to establish the structure of the transmembrane dimer of gp55-P in model membranes. Comparison of the deuterium lineshapes of leucines in the center (Leu(396-399)) and at the ends (Leu(385), Leu(407)) of the transmembrane sequence shows that gp55-P has a right-handed crossing angle with Leu(399) packed in the dimer interface. We discuss the implications of the structure of the gp55-P transmembrane dimer for activation of the Epo receptor.

Virocrine transformation: the intersection between viral transforming proteins and cellular signal transduction pathways

This review describes a mechanism of viral transformation involving activation of cellular signaling pathways. We focus on four viral oncoproteins: the E5 protein of bovine papillomavirus, which activates the platelet-derived growth factor beta receptor; gp55 of spleen focus forming virus, which activates the erythropoietin receptor; polyoma virus middle T antigen, which resembles an activated receptor tyrosine kinase; and LMP-1 of Epstein-Barr virus, which mimics an activated tumor necrosis factor receptor. These examples indicate that diverse viruses induce cell transformation by activating cellular signal transduction pathways. Study of this mechanism of viral transformation will provide new insights into viral tumorigenesis and cellular signal transduction.

Deregulation of erythropoiesis by the Friend spleen focus-forming virus

The proliferation and differentiation of erythroid cells is a highly regulated process that is controlled primarily at the level of interaction of erythropoietin (Epo) with its specific cell surface receptor (EpoR). However, this process is deregulated in mice infected with the Friend spleen focus-forming virus (SFFV). Unlike normal erythroid cells, erythroid cells from SFFV-infected mice are able to proliferate and differentiate in the absence of Epo, resulting in erythroid hyperplasia and leukemia. Over the past 20 years, studies have been carried out to identify the viral genes responsible for the pathogenicity of SFFV and to understand how expression of these genes leads to the deregulation of erythropoiesis in infected animals. The studies have revealed that SFFV encodes a unique envelope glycoprotein which interacts specifically with the EpoR at the cell surface, resulting in activation of the receptor and subsequent activation of erythroid signal transduction pathways. This leads to the proliferation and differentiation of erythroid precursor cells in the absence of Epo. Although the precise mechanism by which the viral protein activates the EpoR is not yet known, it has been proposed that it causes dimerization of the receptor, resulting in constitutive activation of Epo signal transduction pathways. While interaction of the SFFV envelope glycoprotein with the EpoR leads to Epo-independent erythroid hyperplasia, this is not sufficient to transform these cells. Transformation requires the viral activation of the cellular gene Sfpi-1, whose product is thought to block erythroid cell differentiation. By understanding how SFFV can deregulate erythropoiesis, we may gain insights into the causes and treatment of related diseases in man.

Role of the transmembrane sequence of spleen focus-forming virus gp55 in erythroleukemogenesis

The membrane glycoprotein encoded by the env gene of either the polycythemia- or anemia-inducing spleen focus-forming virus (SFFVp or SFFVa, respectively) is responsible for the induction of erythroleukemia in mice. It has been shown that the SFFVp glycoprotein, gp55, interacts with the erythropoietin receptor (EPO-R) and promotes EPO-independent proliferation of an EPO-R-expressing hematopoietic cell line, Ba/F3 (Li et al., Nature 343:762, 1990). We show here that when residues within the transmembrane (TM) sequence of an SFFVp gp55 are altered based on the sequences of the anemia-inducing gp55s by a methionine-to-isoleucine (M-I) substitution, a di-leucine deletion (dLL), or both, the resulting mutants display an attenuated phenotype that resembles an SFFVa: they induce milder erythroproliferative disease without polycythemia in vivo and are unable to promote EPO-independent cell proliferation in vitro. The dLL mutation directly interferes with EPO-R binding by decreasing the affinity of gp55 for the receptor. On the other hand, the M-I mutation hampers the full mitogenic activation of EPO-R while having no effect on receptor binding and asserts a dominant negative effect over the wild-type SFFVp gp55. Two other sequence changes within the TM sequence did not affect the biological activities of the SFFVp gp55. These results indicate that the TM sequence of the SFFV env glycoprotein plays a prominent role in SFFV-induced erythroleukemogenesis through its influence on the mitogenic activation of EPO-R.

Erythropoietin and Friend virus gp55 activate different JAK/STAT pathways through the erythropoietin receptor in erythroid cells

Abnormal erythropoietin (EPO)-independent cell growth is induced after infection of erythroid progenitor cells with a polycythemic strain of Friend virus (FVp). Binding of its Env-related glycoprotein (gp55) to the EPO receptor (EPOR) mimics the activation of the EPOR with EPO. We investigated the gp55-EPOR signaling in erythroblastoid cells from mice infected with FVp and in cells of FVp-induced or gp55-transgenic-mouse-derived erythroleukemia cell lines, comparing it with the EPO-EPOR signaling in EPO-responsive erythroblastoid cells. While the Janus protein tyrosine kinase JAK2 and the transcription factor STAT5 became tyrosine phosphorylated with the EPO stimulation in EPO-responsive erythroblastoid cells from anemic mice, JAK1 and STAT5 were constitutively tyrosine phosphorylated in all of these FVp gp55-induced erythroblastoid or erythroleukemic cells. Moreover, this constitutively tyrosine-phosphorylated STAT5 was unable to bind to its specific DNA sequences and did not translocate to the nucleus. Nuclear translocation and DNA binding of this STAT5 species required EPO stimulation. These findings clearly indicate that the FVp gp55-EPOR signaling is distinct from the EPO-EPOR signaling and suggest that STAT5 may not play an essential role in the transmission of the cell growth signals in FVp gp55-induced erythroleukemia cells.

Redundant and Selective Roles for Erythropoietin Receptor Tyrosines in Erythropoiesis In Vivo

Cytokine receptors have been shown in cell culture systems to use phosphotyrosine residues as docking sites for certain signal transduction intermediates. Studies using various cellular backgrounds have yielded conflicting information about the importance of such residues. The present studies were undertaken to determine whether or not tyrosine residues within the erythropoietin receptor (EPOR) are essential for biologic activity during hematopoiesis in vivo. A variant of the EPOR was constructed that contains both a substitution (R129C) causing constitutive receptor activation as well as replacement of all eight cytoplasmic tyrosines by phenylalanines (cEPORYF). A comparison between animals exposed to recombinant retroviruses expressing cEPOR and cEPORYF showed that efficient red blood cell (RBC) development in vivo is dependent on the presence of tyrosine residues in the cytoplasmic domain of the EPOR. In addition, an inefficient EPOR tyrosine independent pathway supporting RBC development was detected. Tyrosine add-back mutants showed that multiple individual tyrosines have the capacity to restore full erythropoietic potential to the EPOR as determined in whole animals. The analysis of primary erythroid progenitors transduced with the various cEPOR tyrosine mutants and tyrosine add-backs showed that only tyrosine 343 (Y1) and tyrosine 479 (Y8) were capable of supporting immature burst-forming unit–erythroid progenitor development. Thus, this receptor is characterized by striking functional redundancy of tyrosines in a biologically relevant context. However, selective tyrosine residues may be uniquely important for early signals supporting erythroid development.

Redundant and Selective Roles for Erythropoietin Receptor Tyrosines in Erythropoiesis In Vivo

Cytokine receptors have been shown in cell culture systems to use phosphotyrosine residues as docking sites for certain signal transduction intermediates. Studies using various cellular backgrounds have yielded conflicting information about the importance of such residues. The present studies were undertaken to determine whether or not tyrosine residues within the erythropoietin receptor (EPOR) are essential for biologic activity during hematopoiesis in vivo. A variant of the EPOR was constructed that contains both a substitution (R129C) causing constitutive receptor activation as well as replacement of all eight cytoplasmic tyrosines by phenylalanines (cEPORYF). A comparison between animals exposed to recombinant retroviruses expressing cEPOR and cEPORYF showed that efficient red blood cell (RBC) development in vivo is dependent on the presence of tyrosine residues in the cytoplasmic domain of the EPOR. In addition, an inefficient EPOR tyrosine independent pathway supporting RBC development was detected. Tyrosine add-back mutants showed that multiple individual tyrosines have the capacity to restore full erythropoietic potential to the EPOR as determined in whole animals. The analysis of primary erythroid progenitors transduced with the various cEPOR tyrosine mutants and tyrosine add-backs showed that only tyrosine 343 (Y1) and tyrosine 479 (Y8) were capable of supporting immature burst-forming unit–erythroid progenitor development. Thus, this receptor is characterized by striking functional redundancy of tyrosines in a biologically relevant context. However, selective tyrosine residues may be uniquely important for early signals supporting erythroid development.

Dominant selection of hematopoietic progenitor cells with retroviral MDR1 co-expression vectors

When transferring the human multidrug resistance 1 (MDR1) cDNA, FMEV retroviral vectors mediate high-dose multidrug resistance and, thus, background-free selection in primary human hematopoietic progenitor cells. Here, we analyzed strategies for co-expression of a second gene from an FMEV:MDR1 vector. When linking the cDNAs with the internal ribosomal entry site (IRES) of poliovirus or retroviral splice signals, almost all multidrug-resistant hematopoietic colonies simultaneously coexpressed the 3' positioned second gene, neomycin-phosphotransferase (neoR). The IRES strategy allowed functional co-transfer of a 4.2-kb lacZ-neoR fusion gene, resulting in a total proviral genome size of 11 kb, corresponding to the packaging limit of retroviral vectors. Preselection based on multidrug resistance elevated the expression of the second gene in IRES constructs, but not in splice vectors. Moreover, three intriguing observations were made. First, up to 30% of cells preselected for functional transfer of the 3' positioned cDNA (neoR) showed infunctional MDR1; this occurred irrespective of the linking principle and was associated with instability of the MDR1 transcription unit. Second, the levels of multidrug resistance achieved with the co-expression vectors were moderately lower (15-30% reduced) than those mediated by the monocistronic counterpart. Third, transduction with FMEV:MDR1 co-expression vectors still resulted in high-dose cancer drug resistance and background-free selection of hematopoietic progenitor cells (including primary human CD34+ colony-forming units). Thus, for the first time, we describe MDR1 co-expression vectors that maintain their desired function in early and primary human hematopoietic cells. However, careful interpretation of the data reveals that further vector improvements are required to obtain clinically useful MDR1 co-expression vectors.

Cell surface organization of the erythropoietin receptor complex differs depending on its mode of activation

During erythroid development erythropoietin (EPO) binds specifically to a receptor primarily present on committed erythroid progenitors, stimulating mitogenic, survival, and differentiative growth response pathways. Other modes of erythropoietin receptor (EPO-R) activation, such as interaction with the env gene Friend virus envelope glycoprotein (F-gp55) of spleen focus-forming virus or specific mutations in the extracellular domain of the EPO-R, give rise to pathological consequences, in vivo and EPO-independent proliferation and differentiation of cultured cells. Activating extracellular receptor mutations result in covalently linked receptor homodimers. These observations and others have led to the proposal that EPO activates the EPO-R by inducing dimer formation on the cell surface. It has been assumed that F-gp55 also induces dimer formation of the EPO-R; however, clear evidence of this is lacking. In addition, EPO and F-gp55 stimulation of the EPO-R elicit different biological responses. To probe whether the cell surface EPO-R is structurally different with these activators, we contrasted the cell surface EPO-R complex formed following receptor activation by EPO, F-gp55, and mutations in the extracellular domain of the receptor. Our results indicate that cell surface forms of activated EPO-R differ, as judged by their differential association with F-gp55 and pattern of associated cell surface proteins. Interestingly, we find that the env gene of an anemic strain of Friend virus, Rauscher virus envelope glycoprotein, does not interact with the EPO-R at the cell surface. Thus, the mode of Rauscher virus envelope glycoprotein-induced erythroblastosis may be distinct from F-gp55-induced erythroblastosis and possibly not involve the EPO-R.

Cytokine receptor signal transduction and the control of hematopoietic cell development

The cytokine receptor superfamily is characterized by structural motifs in the exoplasmic domain and by the absence of catalytic activity in the cytosolic segment. Activated by ligand-triggered multimerization, these receptors in turn activate a number of cytosolic signal transduction proteins, including protein tyrosine kinases and phosphatases, and affect an array of cellular functions that include proliferation and differentiation. Molecular study of these receptors is revealing the roles they play in the control of normal hematopoiesis and in the development of disease.

Improved retroviral vectors for hematopoietic stem cell protection and in vivo selection

Therapeutic gene transfer into hematopoietic cells is critically dependent on the evolution of methods that allow ex vivo expansion, high-frequency transduction, and selection of gene-modified long-term repopulating cells. Progress in this area needs elaboration of defined culture and transduction conditions for long-term repopulating cells and improvement of gene transfer systems. We have optimized retroviral vector constructions based on murine leukemia viruses (MuLV) to overcome the transcriptional repression encountered with the use of conventional Moloney MuLV (MoMuLV) vectors in early hematopoietic progenitor cells (HPC). Novel retroviral vectors, termed FMEV (for Friend-MCF/MESV hybrid vectors), were cloned that mediate greatly improved gene expression in the myeloerythroid compartment. Transfer of the selectable marker multidrug resistance 1 (mdr1), FMEV, in contrast to conventional MoMuLV-related vectors currently in use for clinical protocols, mediated background-free selectability of transduced human HPC in the presence of myeloablative doses of the cytostatic agent paclitaxel in vitro. Furthermore, FMEV also greatly improved chemo-protection of hematopoietic progenitor cells in a murine model system in vivo. Finally, when a second gene was transferred along with mdr1 in an FMEV-backbone, close to 100% coexpression was observed in multidrug-resistant colonies. These observations have significant consequences for a number of ongoing and planned gene therapy trials, for example, stem cell protection to reduce the myelotoxic side effects of anticancer chemotherapy, correction of inherited disorders involving hematopoietic cells, and antagonism of HIV infection.

Novel retroviral vectors for efficient expression of the multidrug resistance (mdr-1) gene in early hematopoietic cells

We present data that retroviral gene expression in early hematopoietic cells is subjected to transcriptional controls similar to those previously described for embryonic stem cells. Transient transfection experiments revealed that both the viral enhancer region in the U3 region of the long terminal repeat as well as a repressor element coincident with the primer binding site of Moloney leukemia viruses are limiting for expression in hematopoietic cells in a differentiation-dependent manner. Within the group of Moloney leukemia virus-related viruses, only the myeloproliferative sarcoma virus showed high enhancer activity in myeloid (including erythroid) cells. In contrast, enhancer regions related to the Friend mink cell focus-forming viruses mediate much higher gene expression levels in both multipotent and lineage-committed myeloid cells. In addition, transcriptional repression related to sequences in the primer binding site of Moloney leukemia virus-derived vectors is also found in early hematopoietic cells and can be overcome by using the corresponding sequences of the murine embryonic stem cell virus. On the basis of these results, two types of novel retroviral hybrid vectors were developed; they combine the U3 regions of either the Friend mink cell focus-forming virus family or the myeloproliferative sarcoma virus with the primer binding site of the murine embryonic stem cell virus. When used to express the human multiple drug resistance gene, these vectors substantially improve protection to cytostatic drugs in transduced hematopoietic cell lines FDC-Pmix, TF-1, and K-562 in comparison with Moloney leukemia virus-derived vectors presently used for the stem cell protection approach in somatic gene therapy.