A G-CSF receptor-gyrase B fusion gene: A new type of molecular switch for expansion of genetically modified hematopoietic cells

A chemical switch for controlling viral infectivity

Chemically triggered molecular switches for controlling the fate and function of biological systems are fundamental to the emergence of synthetic biology and the development of biomedical applications. We here present the first chemically triggered switch for controlling the infectivity of adeno-associated viral (AAV) vectors.

Forced dimerization increases the activity of ΔEGFR/EGFRvIII and enhances its oncogenicity

Delta epidermal growth factor receptor (ΔEGFR), an in-frame deletion mutant of the extracellular ligand-binding domain, which occurs in about 30% of glioblastoma, is a potent oncogene that promotes tumor growth and progression. The signaling of ΔEGFR is ligand-independent and low intensity, allowing it to evade the normal mechanisms of internalization and degradation by the endocytic machinery and hence is persistent. The basis of the oncogenic potential of ΔEGFR remains incompletely understood, including whether dimerization plays an important role in its signal and whether its oncogenic potential is dependent on its relatively low intensity, when compared with the acutely activated wild-type receptor. To examine these two important questions, we have generated a chimeric ΔEGFR that allows forced dimerization via domains derived from variants of the FKBP12 protein that are brought together by FK506 derivatives. Forced dimerization of chimeric ΔEGFR significantly increased the intensity of its signal, as measured by receptor phosphorylation levels, suggesting that the naturally occurring ΔEGFR does not form strong or stable dimers as part of its low level signal. Interestingly, the increased activity of dimerized, chimeric ΔEGFR did not promote receptor internalization, implying that reduced rate of endocytic downregulation of ΔEGFR is an inherent characteristic. Significantly, forced dimerization enhanced the oncogenic signal of the receptor, implying that the ΔEGFR is a potent oncogene despite, not because of its low intensity.

Growth promotion of genetically modified hematopoietic progenitors using an antibody/c-Mpl chimera

Thrombopoietin is a potent cytokine that exerts proliferation of hematopoietic stem cells (HSCs) through its cognate receptor, c-Mpl. Therefore, mimicry of c-Mpl signaling by a receptor recognizing an artificial ligand would be attractive to attain specific expansion of genetically modified HSCs. Here we propose a system enabling selective expansion of genetically modified cells using an antibody/receptor chimera that can be activated by a specific antigen. We constructed an antibody/c-Mpl chimera, in which single-chain Fv (ScFv) of an anti-fluorescein antibody was tethered to the extracellular D2 domain of the erythropoietin receptor and transmembrane/cytoplasmic domains of c-Mpl. When the chimera was expressed in interleukin (IL)-3-dependent pro-B cell line Ba/F3, genetically modified cells were selectively expanded in the presence of fluorescein-conjugated BSA (BSA-FL) as a specific antigen. Furthermore, highly purified mouse HSCs transduced with the retrovirus carrying antibody/c-Mpl chimera gene proliferated in vitro in response to BSA-FL, and the cells retained in vivo long-term repopulating abilities. These results demonstrate that the antibody/c-Mpl chimera is capable of signal transduction that mimics wild-type c-Mpl signaling.

Design and Applications of Bifunctional Small Molecules: Why Two Heads Are Better Than One

Induction of protein--protein interactions is a daunting challenge, but recent studies show promise for small molecules that specifically bring two or more protein molecules together for enhanced or novel biological effect. The first such bifunctional molecules were the rapamycin- and FK506-based "chemical inducers of dimerization", but the field has since expanded with new molecules and new applications in chemical genetics and cell biology. Examples include coumermycin-mediated gyrase B dimerization, proteolysis targeting chimeric molecules (PROTACs), drug hybrids, and strategies for exploiting multivalency in toxin binding and antibody recruitment. This Review discusses these and other advances in the design and use of bifunctional small molecules and potential strategies for future systems.

Novel gene switches

Controlling gene activity in space and time represents a cornerstone technology in gene and cell therapeutic applications, bioengineering, drug discovery as well as fundamental and applied research. This chapter provides a comprehensive overview of the different approaches for regulating gene activity and product protein formation at different biosynthetic levels, from genomic rearrangements over transcription and translation control to strategies for engineering inducible secretion and protein activity with a focus on the development during the past 2 years. Recent advances in designing second-generation gene switches, based on novel inducer administration routes (gas phase) as well as on the combination of heterologous switches with endogenous signals, will be complemented by an overview of the emerging field of mammalian synthetic biology, which enables the design of complex synthetic and semisynthetic gene networks. This article will conclude with an overview of how the different gene switches have been applied in gene therapy studies, bioengineering and drug discovery.

Pharmacologic transgene control systems for gene therapy

Pharmacologic transgene-expression dosing is considered essential for future gene therapy scenarios. Genetic interventions require precise transcription or translation fine-tuning of therapeutic transgenes to enable their titration into the therapeutic window, to adapt them to daily changing dosing regimes of the patient, to integrate them seamlessly into the patient's transcriptome orchestra, and to terminate their expression after successful therapy. In recent years, decisive progress has been achieved in designing high-precision trigger-inducible mammalian transgene control modalities responsive to clinically licensed and inert heterologous molecules or to endogenous physiologic signals. Availability of a portfolio of compatible transcription control systems has enabled assembly of higher-order control circuitries providing simultaneous or independent control of several transgenes and the design of (semi-)synthetic gene networks, which emulate digital expression switches, regulatory transcription cascades, epigenetic expression imprinting, and cellular transcription memories. This review provides an overview of cutting-edge developments in transgene control systems, of the design of synthetic gene networks, and of the delivery of such systems for the prototype treatment of prominent human disease phenotypes.

Selection of genetically modified cell population using hapten-specific antibody/receptor chimera

Efficient selection of the genetically modified cell population is a critical step to obtain the cells with desired properties. In this study, we propose an antigen-mediated genetically modified cell amplification (AMEGA) system employing an antibody/receptor chimera that triggers a growth signal in response to a non-toxic hapten dimer. An anti-fluorescein single-chain Fv fused to the extracellular D2 domain of erythropoietin receptor and transmembrane/intracellular domains of gp130 was expressed together with a model transgene, enhanced green fluorescent protein (EGFP) downstream of IRES sequence, by retroviral infection to IL-3-dependent Ba/F3 cells. Addition of fluorescein dimers connected by various oligo-DNA linkers induced selective growth of transfectants, thus leading to efficient expansion of EGFP-positive cell population. Also, digestion of the oligonucleotides by specific restriction endonuclease completely suppressed cell growth. Because these hapten dimers are not harmful for normal cells, the approach will be especially useful for reversible in vitro or in vivo expansion of genetically modified cell population employed for cell therapy and tissue engineering.

AMEGA: antigen-mediated genetically modified cell amplification

Selection of genetically modified cells is a critical step to engineer the cells with desired properties. While antibiotic selection has been commonly used, administration of cytotoxic drugs often leads to deleterious effects not only to inert cells but also to transfected or transduced ones. To overcome this problem, a positive screening method for genetically modified cells is proposed using a pair of chimeric receptors that trigger a growth signal in response to a specific antigen. Either V(H) or V(L) region of anti-hen egg lysozyme (HEL) antibody HyHEL-10 was fused to extracellular D2 domain of erythropoietin receptor (EpoR) and transmembrane/cytoplasmic domains of either EpoR or gp130. A model transgene, enhanced green fluorescent protein (EGFP) and the chimeric receptor genes that reconstituted functional Fv were retrovirally co-infected to interleukin (IL)-3-dependent Ba/F3 cells, followed by direct HEL selection in the absence of IL-3. Consequently, a single round of selection led to a single population of EGFP-positive cells. The detailed protocol of the method termed antigen-mediated genetically modified cell amplification (AMEGA) is described.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

Chimeric VEGFRs are activated by a small-molecule dimerizer and mediate downstream signalling cascades in endothelial cells

Despite much interest in vascular endothelial growth factor (VEGF) and its receptors (VEGFRs -1 and -2), VEGF-induced signalling cascades remain incompletely defined. Attempts to assign individual responses to a particular receptor have used either transfected cell lines, receptor-specific growth factors or antisense oligonucleotides. Such studies have attributed the majority of VEGF-induced responses to activation of VEGFR-2. As a consequence of poor growth factor-induced VEGFR-1 autophosphorylation however, observations from these studies may instead reflect the relative activation of the two receptors. We have generated novel chimeric VEGF receptors in which the dimerization domain of the B subunit of DNA gyrase is fused to the cytoplasmic domain of VEGFRs -1 and -2. When expressed in porcine aortic endothelial cells, both chimeric VEGFR-1 and -2 autophosphorylate in response to addition of the small-molecule dimerizing agent, coumermycin. Once activated, both receptors induce downstream signalling cascades, exemplified here by the activation of MAPK, PLCgamma and PKB/Akt. Furthermore, we demonstrate that the Y1175 residue of VEGFR-2 is essential for the activation of PLCgamma mediated by this chimeric receptor. In contrast to previous reports which show a limited ability of VEGFR-1 to mediate signalling cascades, we show that once sufficiently activated, VEGFR-1 signals in a similar manner to VEGFR-2 in endothelial cells.