Expression from cell type-specific enhancer-modified retroviral vectors after transduction: influence of marker gene stability

Pantropic retroviruses as a transduction tool for sea urchin embryos

Sea urchins are an important model for experiments at the intersection of development and systems biology, and technical innovations that enhance the utility of this model are of great value. This study explores pantropic retroviruses as a transduction tool for sea urchin embryos, and demonstrates that pantropic retroviruses infect sea urchin embryos with high efficiency and genomically integrate at a copy number of one per cell. We successfully used a self-inactivation strategy to both insert a sea urchin-specific enhancer and disrupt the endogenous viral enhancer. The resulting self-inactivating viruses drive global and persistent gene expression, consistent with genomic integration during the first cell cycle. Together, these data provide substantial proof of principle for transduction technology in sea urchin embryos.

Examination of mesenchymal stem cell-mediated RNAi transfer to Huntington's disease affected neuronal cells for reduction of huntingtin

Huntington's disease (HD) is a fatal, autosomal dominant neurodegenerative disorder caused by an expanded trinucleotide (CAG) repeat in exon 1 of the huntingtin gene (Htt). This expansion creates a toxic polyglutamine tract in the huntingtin protein (HTT). Currently, there is no treatment for either the progression or prevention of the disease. RNA interference (RNAi) technology has shown promise in transgenic mouse models of HD by reducing expression of mutant HTT and slowing disease progression. The advancement of RNAi therapies to human clinical trials is hampered by problems delivering RNAi to affected neurons in a robust and sustainable manner. Mesenchymal stem cells (MSC) have demonstrated a strong safety profile in both completed and numerous ongoing clinical trials. MSC exhibit a number of innate therapeutic effects, such as immune system modulation, homing to injury, and cytokine release into damaged microenvironments. The ability of MSC to transfer larger molecules and even organelles suggested their potential usefulness as delivery vehicles for therapeutic RNA inhibition. In a series of model systems we have found evidence that MSC can transfer RNAi targeting both reporter genes and mutant huntingtin in neural cell lines. MSC expressing shRNA antisense to GFP were found to decrease expression of GFP in SH-SY5Y cells after co-culture when assayed by flow cytometry. Additionally MSC expressing shRNA antisense to HTT were able to decrease levels of mutant HTT expressed in both U87 and SH-SY5Y target cells when assayed by Western blot and densitometry. These results are encouraging for expanding the therapeutic abilities of both RNAi and MSC for future treatments of Huntington's disease.

Interaction of hypoxia-inducible factor-1α and Notch signaling regulates medulloblastoma precursor proliferation and fate

Medulloblastoma (MDB) is the most common brain malignancy of childhood. It is currently thought that MDB arises from aberrantly functioning stem cells in the cerebellum that fail to maintain proper control of self-renewal. Additionally, it has been reported that MDB cells display higher endogenous Notch signaling activation, known to promote the survival and proliferation of neoplastic neural stem cells and to inhibit their differentiation. Although interaction between hypoxia-inducible factor-1α (HIF-1α) and Notch signaling is required to maintain normal neural precursors in an undifferentiated state, an interaction has not been identified in MDB. Here, we investigate whether hypoxia, through HIF-1α stabilization, modulates Notch1 signaling in primary MDB-derived cells. Our results indicate that MDB-derived precursor cells require hypoxic conditions for in vitro expansion, whereas acute exposure to 20% oxygen induces tumor cell differentiation and death through inhibition of Notch signaling. Importantly, stimulating Notch1 activation with its ligand Dll4 under hypoxic conditions leads to expansion of MDB-derived CD133(+) and nestin(+) precursors, suggesting a regulatory effect on stem cells. In contrast, MDB cells undergo neuronal differentiation when treated with γ-secretase inhibitor, which prevents Notch activation. These results suggest that hypoxia, by maintaining Notch1 in its active form, preserves MDB stem cell viability and expansion.

Molecular mechanisms of HIF-1alpha modulation induced by oxygen tension and BMP2 in glioblastoma derived cells

Background

Glioblastoma multiforme (GBM) is one of most common and still poorly treated primary brain tumors. In search for new therapeutic approaches, Bone Morphogenetic Proteins (BMPs) induce astroglial commitment in GBM-derived cells in vitro. However, we recently suggested that hypoxia, which is characteristic of the brain niche where GBM reside, strongly counter-acts BMP effects. It seems apparent that a more complete understanding of the biology of GBM cells is needed, in particular considering the role played by hypoxia as a signaling pathways regulator. HIF-1α is controlled at the transcriptional and translational level by mTOR and, alike BMP, also mTOR pathway modulates glial differentiation in central nervous system (CNS) stem cells.

Methodology/Principal Findings

Here, we investigate the role of mTOR signaling in the regulation of HIF-1α stability in primary GBM-derived cells maintained under hypoxia (2% oxygen). We found that GBM cells, when acutely exposed to high oxygen tension, undergo Akt/mTOR pathway activation and that BMP2 acts in an analogous way. Importantly, repression of Akt/mTOR signaling is maintained by HIF-1α through REDD1 upregulation. On the other hand, BMP2 counter-acts HIF-1α stability by modulating intracellular succinate and by controlling proline hydroxylase 2 (PHD2) protein through inhibition of FKBP38, a PHD2 protein regulator.

Conclusions/Significance

In this study we elucidate the molecular mechanisms by which two pro-differentiating stimuli, BMP2 and acute high oxygen exposure, control HIF-1α stability. We previously reported that both these stimuli, by inducing astroglial differentiation, affect GBM cells growth. We also found differences in high oxygen and BMP2 sensitivity between GBM cells and normal cells that should be further investigated to better define tumor cell biology.

Hypoxia and HIF1alpha repress the differentiative effects of BMPs in high-grade glioma

Hypoxia commonly occurs in solid tumors of the central nervous system (CNS) and often interferes with therapies designed to stop their growth. We found that pediatric high-grade glioma (HGG)-derived precursors showed greater expansion under lower oxygen tension, typical of solid tumors, than normal CNS precursors. Hypoxia inhibited p53 activation and subsequent astroglial differentiation of HGG precursors. Surprisingly, although HGG precursors generated endogenous bone morphogenetic protein (BMP) signaling that promoted mitotic arrest under high oxygen tension, this signaling was actively repressed by hypoxia. An acute increase in oxygen tension led to Smad activation within 30 minutes, even in the absence of exogenous BMP treatment. Treatment with BMPs further promoted astroglial differentiation or death of HGG precursors under high oxygen tension, but this effect was inhibited under hypoxic conditions. Silencing of hypoxia-inducible factor 1alpha (HIF1alpha) led to Smad activation even under hypoxic conditions, indicating that HIF1alpha is required for BMP repression. Conversely, BMP activation at high oxygen tension led to reciprocal degradation of HIF1alpha; this BMP-induced degradation was inhibited in low oxygen. These results show a novel, mutually antagonistic interaction of hypoxia-response and neural differentiation signals in HGG proliferation, and suggest differences between normal and HGG precursors that may be exploited for pediatric brain cancer therapy.

Generation of stably transfected Mammalian cell lines as fluorescent screening assay for NF-kappaB activation-dependent gene expression

Cellular stress protection responses lead to increased transcription of several genes via modulation of transcription factors. Activation of the Nuclear Factor kappaB (NF-kappaB) pathway as a possible antiapoptotic route represents one important cellular stress response. To identify conditions that are capable of modifying this pathway, a screening assay for detection of NF-kappaB-dependent gene activation using the reporter protein Enhanced Green Fluorescent Protein (EGFP) and its destabilized variant (d2EGFP) was developed. Human Embryonic Kidney (HEK/293) cells were stably transfected with a vector carrying EGFP or d2EGFP under control of a synthetic promoter containing 4 copies of the NF-kappaB response element. Treatment with tumor necrosis factor alpha (TNF-alpha) gave rise to substantial EGFP/d2EGFP expression in up to 90% of the cells and was therefore used to screen different stably transfected clones for induction of NF-kappaB-dependent gene expression. The time course of NF-kappaB activation leading to d2EGFP expression was measured in an oligonucleotide-based NF-kappaB-ELISA. NF-kappaB binding in-creased after 15-min incubation with TNF-alpha. In parallel, d2EGFP increased after 3 h and reached its maximum at 24 h. These results show (1) the time lag between NF-kappaB activation and d2EGFP transcription, translation, and protein folding and (2) the increased reporter gene expression after treatment with TNF-alpha to be caused by the activation of NF-kappaB. The detection of d2EGFP expression required FACS analysis or fluorescence microscopy, while EGFP could also be measured in the microplate reader, rendering the assay useful for high-throughput screening.

Multiple modifications allow high-titer production of retroviral vectors carrying heterologous regulatory elements

Tumor-specific expression of therapeutic genes is a prerequisite in many approaches to retrovirus-mediated cancer gene therapy. However, tissue specificity is often associated with a reduction in viral titer. To overcome this problem, we constructed a series of murine leukemia virus (MLV)-based retroviral promoter conversion (ProCon) vectors carrying either the simian virus 40 poly(A) signal trimer (3pA) inserted in the 3' long terminal repeat (LTR) of these vectors or the human cytomegalovirus enhancer region (CMVe) inserted 5' and 3' of the retroviral LTRs. Furthermore, an extended AT stretch/attachment site (AT/att) of wild-type MLV was introduced into the vector. In the vector-producing cells, insertion of the CMVe and/or the 3pA resulted in a three- to fourfold-enhanced marker gene expression compared to the parental vector, whereas insertion of the AT/att gave a slight decrease in expression. The combination of all three modifications had no additional effects. In contrast, however, neomycin selection of infected cells revealed only a slight increase in virus titer with vectors carrying the 3pA modification; the titer was increased by 1 with vectors containing the extended AT/att, although the viral DNA copy numbers in infected cells were similar with both types of vectors. Thus, insufficient integration rather than insufficient reverse transcription and/or production of virus RNA is the major cause for the low titer obtained with the ProCon vectors. The combination of all three modifications resulted in a 2- to 3-log increase in the virus titer. These modifications result in expression targeted ProCon vectors with titers similar to those of nonmodified MLV-based vectors.

Retroviral vectors for high-level transgene expression in T lymphocytes

Efficient expression of genes transferred by retroviral vectors is a prerequisite for gene therapy, especially when the biological effect depends on the amount of transgene product. High-level gene expression is desirable for several gene therapy approaches involving T lymphocytes. We evaluated standard retroviral vectors with cis-regulatory control elements of the Moloney murine leukemia virus (Mo-MLV) with or without the human T cell-specific CD2 enhancer. For comparison, vectors containing the long terminal repeat (LTR) of myeloproliferative sarcoma virus (MPSV) and an improved 5' untranslated region were used (MP71 vectors), with or without the woodchuck hepatitis virus posttranscriptional regulatory element (PRE). All vectors expressed the enhanced green fluorescent protein (GFP) to measure transgene expression. In mouse T cells MP71 vectors with and without the PRE yielded an up to 10-fold higher expression level compared with the Mo-MLV-based vectors currently used for gene transfer into T lymphocytes. A high multiplicity of infection (MOI) of standard Mo-MLV vectors could not reach expression levels obtained with a low MOI of MP71 vector. Ex vivo-transduced mouse T lymphocytes maintained the vector-dependent differences in level of transgene expression in Rag-1-deficient mice when adoptively transferred. In four human T cell lines and human primary T lymphocytes MP71 vectors yielded an up to 75-fold higher GFP expression level in comparison with the standard Mo-MLV vector. In contrast to mouse T cells, the integration of the PRE into MP71 vectors induced in human T cells a further significant increase in transgene expression level. Southern blot analysis of CEM T cells revealed that the superior performance of MP71 vectors was not due to a higher rate of viral integration. In summary, MP71 vectors are useful tools for stable, high-level gene expression in T lymphocytes, for example, in the expression of T cell receptor genes.

In vivo targeting of tumor endothelial cells by systemic delivery of lentiviral vectors

Tumor angiogenesis is a rate-limiting factor for tumor growth, and the endothelial cells of tumor vessels display specific features that can be exploited for the selective delivery of cancer therapeutics. To specifically target exogenous genes to angiogenic tumor vessels, we generated a panel of vesicular stomatitis virus-pseudotyped lentiviral vectors (LVs) engineered for endothelial cell (EC)-specific expression. We cloned a wide repertoire of transcription regulatory sequences from genes preferentially expressed in ECs (Tie1, Tie2, Flk-1, VE-Cad, and ICAM-2) into self-inactivating LVs to drive expression of the marker gene encoding green fluorescent protein (GFP) or of the conditionally toxic gene encoding nitroreductase, and compared them with the ubiquitously expressing phosphoglycerate kinase (PGK) and cytomegalovirus (CMV) promoters. We evaluated the efficiency and specificity of vector expression in vitro in a panel of human primary cultures, including ECs, fibroblasts, neurons, lymphocytes, and hematopoietic progenitors, and in tumor cell lines. We found that vectors containing promoter and enhancer sequences from the Tie2 gene achieved remarkable specificity of expression in ECs in vitro and in vivo. On intravenous delivery into tumor-bearing mice, the Tie2 vector targeted expression to the ECs of tumor vessels. In contrast, LVs carrying the PGK or CMV promoter gave widespread GFP marking in ECs and non-ECs of tumors and other organs. The previously reported upregulation of the Tie2 gene in ECs activated for angiogenesis may explain the remarkable selectivity of expression of the Tie2 vector in ECs of tumor vessels. The new vector provides the means for selective delivery of gene therapy to tumor sites in vivo.