The optimal use of IRES (internal ribosome entry site) in expression vectors

A multi-plex protein expression system for production of complex enzyme formulations in Trichoderma reesei

Heterologous protein production has been challenging in the hyper-cellulolytic fungus, Trichoderma reesei as the species is known for poor transformation efficiency, low homologous recombination frequency, and marginal screening systems for the identification of successful transformants. We have applied the 2A-peptide multi-gene expression system to co-express four proteins, which include three cellulases: a cellobiohydrolase (CBH1), an endoglucanase (EG1), and a β-D-glucosidase (BGL1), as well as the enhanced green fluorescent protein (eGFP) marker protein. We designed a new chassis vector, pTrEno-4X-2A, for this work. Expression of these cellulase enzymes was confirmed by real-time quantitative reverse transcription PCR and immunoblot analysis. The activity of each cellulase was assessed using chromogenic substrates, which confirmed the functionality of the enzymes. Expression and activity of these enzymes were proportional to the level of eGFP fluorescence, thereby validating the reliability of this screening technique. An 18-fold differencein protein expression was observed between the first and third genes within the 2A-peptide construct. The availability of this new multi-gene expression and screening tool is expected to greatly impact multi-enzyme applications, such as the production of complex commercial enzyme formulations and metabolic pathway enzymes, especially those destined for cell-free applications.

Current Practice in Bicistronic IRES Reporter Use: A Systematic Review

Bicistronic reporter assays have been instrumental for transgene expression, understanding of internal ribosomal entry site (IRES) translation, and identification of novel cap-independent translational elements (CITE). We observed a large methodological variability in the use of bicistronic reporter assays and data presentation or normalization procedures. Therefore, we systematically searched the literature for bicistronic IRES reporter studies and analyzed methodological details, data visualization, and normalization procedures. Two hundred fifty-seven publications were identified using our search strategy (published 1994-2020). Experimental studies on eukaryotic adherent cell systems and the cell-free translation assay were included for further analysis. We evaluated the following methodological details for 176 full text articles: the bicistronic reporter design, the cell line or type, transfection methods, and time point of analyses post-transfection. For the cell-free translation assay, we focused on methods of in vitro transcription, type of translation lysate, and incubation times and assay temperature. Data can be presented in multiple ways: raw data from individual cistrons, a ratio of the two, or fold changes thereof. In addition, many different control experiments have been suggested when studying IRES-mediated translation. In addition, many different normalization and control experiments have been suggested when studying IRES-mediated translation. Therefore, we also categorized and summarized their use. Our unbiased analyses provide a representative overview of bicistronic IRES reporter use. We identified parameters that were reported inconsistently or incompletely, which could hamper data reproduction and interpretation. On the basis of our analyses, we encourage adhering to a number of practices that should improve transparency of bicistronic reporter data presentation and improve methodological descriptions to facilitate data replication.

Reprogramming translation for gene therapy

Translational control plays a fundamental role in the regulation of gene expression in eukaryotes. Modulating translational efficiency allows the cell to fine-tune the expression of genes, spatially control protein localization, and trigger fast responses to environmental stresses. Translational regulation involves mechanisms acting on multiple steps of the protein synthesis pathway: initiation, elongation, and termination. Many cis-acting elements present in the 5' UTR of transcripts can influence translation at the initiation step. Among them, the Kozak sequence impacts translational efficiency by regulating the recognition of the start codon; upstream open reading frames (uORFs) are associated with inhibition of translation of the downstream protein; internal ribosomal entry sites (IRESs) can promote cap-independent translation. CRISPR-Cas technology is a revolutionary gene-editing tool that has also been applied to the regulation of gene expression. In this chapter, we focus on the genome editing approaches developed to modulate the translational efficiency with the aim to find novel therapeutic approaches, in particular acting on the cis-elements, that regulate the initiation of protein synthesis.

ER intrabody-mediated inhibition of interferon α secretion by mouse macrophages and dendritic cells

Interferon α (IFNα) counteracts viral infections by activating various IFNα-stimulated genes (ISGs). These genes encode proteins that block viral transport into the host cell and inhibit viral replication, gene transcription and translation. Due to the existence of 14 different, highly homologous isoforms of mouse IFNα, an IFNα knockout mouse has not yet been established by genetic knockout strategies. An scFv intrabody for holding back IFNα isoforms in the endoplasmic reticulum (ER) and thus counteracting IFNα secretion is reported. The intrabody was constructed from the variable domains of the anti-mouse IFNα rat monoclonal antibody 4EA1 recognizing the 5 isoforms IFNα1, IFNα2, IFNα4, IFNα5, IFNα6.

A soluble form of the intrabody had a K_D of 39 nM to IFNα4. It could be demonstrated that the anti-IFNα intrabody inhibits clearly recombinant IFNα4 secretion by HEK293T cells. In addition, the secretion of IFNα4 was effectively inhibited in stably transfected intrabody expressing RAW 264.7 macrophages and dendritic D1 cells. Colocalization of the intrabody with IFNα4 and the ER marker calnexin in HEK293T cells indicated complex formation of intrabody and IFNα4 inside the ER. Intracellular binding of intrabody and antigen was confirmed by co-immunoprecipitation. Complexes of endogenous IFNα and intrabody could be visualized in the ER of Poly (I:C) stimulated RAW 264.7 macrophages and D1 dendritic cells. Infection of macrophages and dendritic cells with the vesicular stomatitis virus VSV-AV2 is attenuated by IFNα and IFNβ. The intrabody increased virus proliferation in RAW 264.7 macrophages and D1 dendritic cells under IFNβ-neutralizing conditions. To analyze if all IFNα isoforms are recognized by the intrabody was not in the focus of this study. Provided that binding of the intrabody to all isoforms was confirmed, the establishment of transgenic intrabody mice would be promising for studying the function of IFNα during viral infection and autoimmune diseases.

Modifying inter-cistronic sequence significantly enhances IRES dependent second gene expression in bicistronic vector: Construction of optimised cassette for gene therapy of familial hypercholesterolemia

Internal ribosome entry site (IRES) sequences have become a valuable tool in the construction of gene transfer and therapeutic vectors for multi-cistronic gene expression from a single mRNA transcript. The optimal conditions for effective use of this sequence to construct a functional expression vector are not precisely defined but it is generally assumed that the internal ribosome entry site dependent expression of the second gene in such as cassette is less efficient than the cap-dependent expression of the first gene. Mainly tailoring inter-cistronic sequence significantly enhances IRES dependent second gene expression in bicistronic vector further in construction of optimised cassette for gene therapy of familial hypercholesterolemia. We tailored the size of the inter-cistronic spacer sequence at the 5′ region of the internal ribosome entry site sequence using sequential deletions and demonstrated that the expression of the 3′ gene can be significantly increased to similar levels as the cap-dependent expression of the 5’ gene. Maximum expression efficiency of the downstream gene was obtained when the spacer is composed of 18–141 base pairs. In this case a single mRNA transcriptional unit containing both the first and the second Cistron was detected. Whilst constructs with spacer sequences of 216 bp or longer generate a single transcriptional unit containing only the first Cistron. This suggests that long spacers may affect transcription termination. When the spacer is 188 bp, both transcripts were produced simultaneously in most transfected cells, while a fraction of them expressed only the first but not the second gene. Expression analyses of vectors containing optimised cassettes clearly confirm that efficiency of gene transfer and biological activity of the expressed transgenic proteins in the transduced cells can be achieved. Furthermore, Computational analysis was carried out by molecular dynamics (MD) simulation to determine the most emerges as viable containing specific binding site and bridging of 5′ and 3′ ends involving direct RNA-RNA contacts and RNA-protein interactions. These results provide a mechanistic basis for translation stimulation and RNA resembling for the synergistic stimulation of cap-dependent translation.

Using the 2A Protein Coexpression System: Multicistronic 2A Vectors Expressing Gene(s) of Interest and Reporter Proteins

To date, a huge range of different proteins-many with cotranslational and posttranslational subcellular localization signals-have been coexpressed together with various reporter proteins in vitro and in vivo using 2A peptides. The pros and cons of 2A co-expression technology are considered below, followed by a simple example of a "how to" protocol to concatenate multiple genes of interest, together with a reporter gene, into a single gene linked via 2As for easy identification or selection of transduced cells.

MGMT enrichment and second gene co-expression in hematopoietic progenitor cells using separate or dual-gene lentiviral vectors

The DNA repair gene O(6)-methylguanine-DNA methyltransferase (MGMT) allows efficient in vivo enrichment of transduced hematopoietic stem cells (HSC). Thus, linking this selection strategy to therapeutic gene expression offers the potential to reconstitute diseased hematopoietic tissue with gene-corrected cells. However, different dual-gene expression vector strategies are limited by poor expression of one or both transgenes. To evaluate different co-expression strategies in the context of MGMT-mediated HSC enrichment, we compared selection and expression efficacies in cells cotransduced with separate single-gene MGMT and GFP lentivectors to those obtained with dual-gene vectors employing either encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) or foot and mouth disease virus (FMDV) 2A elements for co-expression strategies. Each strategy was evaluated in vitro and in vivo using equivalent multiplicities of infection (MOI) to transduce 5-fluorouracil (5-FU) or Lin(-)Sca-1(+)c-kit(+) (LSK)-enriched murine bone marrow cells (BMCs). The highest dual-gene expression (MGMT(+)GFP(+)) percentages were obtained with the FMDV-2A dual-gene vector, but half of the resulting gene products existed as fusion proteins. Following selection, dual-gene expression percentages in single-gene vector cotransduced and dual-gene vector transduced populations were similar. Equivalent MGMT expression levels were obtained with each strategy, but GFP expression levels derived from the IRES dual-gene vector were significantly lower. In mice, vector-insertion averages were similar among cells enriched after dual-gene vectors and those cotransduced with single-gene vectors. These data demonstrate the limitations and advantages of each strategy in the context of MGMT-mediated selection, and may provide insights into vector design with respect to a particular therapeutic gene or hematologic defect.

IRES-incorporated lactococcal bicistronic vector for target gene expression in a eukaryotic system

Plasmid DNAs isolated from lactic acid bacteria (LAB) such as Lactococcus lactis (L. lactis) has been gaining more interests for its positive prospects in genetic engineering-related applications. In this study, the lactococcal plasmid, pNZ8048 was modified so as to be able to express multiple genes in the eukaryotic system. Therefore, a cassette containing an internal ribosome entry site (IRES) was cloned between VP2 gene of a very virulent infectious bursal disease (vvIBDV) UPM 04190 of Malaysian local isolates and the reporter gene, green fluorescent protein (GFP) into pNZ:CA, a newly constructed derivative of pNZ8048 harboring the cytomegalovirus promoter (Pcmv) and polyadenylation signal. The new bicistronic vector, denoted as pNZ:vig was subjected to in vitro transcription/translation system followed by SDS-PAGE and Western blot analysis to rapidly verify its functionality. Immunoblotting profiles showed the presence of 49 and 29kDa bands that corresponds to the sizes of the VP2 and GFP proteins respectively. This preliminary result shows that the newly constructed lactococcal bicistronic vector can co-express multiple genes in a eukaryotic system via the IRES element thus suggesting its feasibility to be used for transfection of in vitro cell cultures and vaccine delivery.

An ion transport-independent role for the cation-chloride cotransporter KCC2 in dendritic spinogenesis in vivo

The neuron-specific K-Cl cotransporter, KCC2, is highly expressed in the vicinity of excitatory synapses in pyramidal neurons, and recent in vitro data suggest that this protein plays a role in the development of dendritic spines. The in vivo relevance of these observations is, however, unknown. Using in utero electroporation combined with post hoc iontophoretic injection of Lucifer Yellow, we show that premature expression of KCC2 induces a highly significant and permanent increase in dendritic spine density of layer 2/3 pyramidal neurons in the somatosensory cortex. Whole-cell recordings revealed that this increased spine density is correlated with an enhanced spontaneous excitatory activity in KCC2-transfected neurons. Precocious expression of the N-terminal deleted form of KCC2, which lacks the chloride transporter function, also increased spine density. In contrast, no effect on spine density was observed following in utero electroporation of a point mutant of KCC2 (KCC2-C568A) where both the cotransporter function and the interaction with the cytoskeleton are disrupted. Transfection of the C-terminal domain of KCC2, a region involved in the interaction with the dendritic cytoskeleton, also increased spine density. Collectively, these results demonstrate a role for KCC2 in excitatory synaptogenesis in vivo through a mechanism that is independent of its ion transport function.

Construction of Foot-and-mouth disease virus 2A-based bicistronic expression vector and coexpression of two genes in goat mammary epithelial cells

Using animal mammary glands as bioreactors for producing commercially important proteins is a cutting-edge direction in the field of biotechnology development and application. Dairy goats are an important dairy livestock, with roughage-resistance, fast propagation, long lactation periods and high milk production per bodyweight; these characteristics make dairy goats ideal for use as mammary gland bioreactors. Foot-and-mouth disease virus 2A (FMDV 2A) is an efficient viral cleavage element that mediates proteolytic cleavage independent of the presence of other FMDV sequences. It is often incorporated into recombinant vectors to generate cleavage in the presence of heterologous sequences. To achieve specific co-expression of two heterologous genes in goat mammary gland epithelial (GMGE) cells, a mammary gland-specific bicistronic expression vector, pFIEβ, containing the β-casein 5′ flanking sequence and FMDV 2A, was successfully constructed and the specific expression of human interleukin 2 (hIL-2) and enhanced green fluorescent protein (EGFP) was conducted in primary GMGE cells. Another bicistronic expression vector, pFIEC, driven by the cytomegalovirus promoter, was constructed as a positive control. In cells transfected with pFIEβ and pFIEC, RT-PCR verified the existence of recombinant fusion mRNA of hIL-2 upstream of EGFP within the FMDV 2A cassette fragment and western blot analysis showed the existence of the fusion between hIL-2 and EGFP. It is concluded that FMDV 2A generated specific co-expression of multiple genes for the first time in primary GMGE cells driven by the β-casein promoter.

Problems encountered in bicistronic IRES-GFP expression vectors employed in functional analyses of GC-induced genes

Our laboratory has developed a series of Gateway(®) compatible lentiviral expression systems for constitutive and conditional gene knock-down and over-expression. For tetracycline-regulated transgenic expression, we constructed a lentiviral "DEST" plasmid (pHR-TetCMV-Dest-IRES-GFP5) containing a tetracycline-responsive minimal CMV promoter, followed by an attP site-flanked DEST cassette (for efficient cloning of cDNAs by "Gateway(®)" recombination cloning) and green fluorescent protein (GFP) driven by an internal ribosomal entry site (IRES).This lentiviral bicistronic plasmid allows immediate FACS identification and characterization of successfully transfected cell lines. Although this system worked well with several cDNAs, we experienced serious problems with SLA, Bam and BMF. Particularly, we cloned the cDNA for human SLA (Src-like adapter), a candidate gene in GC-induced apoptosis, into this plasmid. The resulting construct (pHR-TetCMV-SLA-IRES-GFP5) was transfected into HEK 293-T packaging cells to produce viral particles for transduction of CEM-C7H2-2C8 cells. Although the construct produced many green fluorescent colonies at the HEK 293-T and the CEM-C7H2-2C8 level, we could not detect any SLA protein with α-SLA antibody from corresponding cell lysates. In contrast, the antibody readily detected SLA in whole cell lysate of HEK 293-T cells transfected with a GST-flagged SLA construct lacking IRES-GFP. To directly address the potential role of the IRES-GFP sequence, we cloned the SLA coding region into pHR-TetCMV-Dest, a vector that differs from pHR-TetCMV-Dest-IRES-GFP5 just by the absence of the IRES-GFP cassette. The resulting pHR-TetCMV-SLA construct was used for transfection of HEK 293-T cells. Corresponding lysates were assayed with α-SLA antibody and found positive. These data, in concert with previous findings, suggest that the IRES-GFP cassette may interfere with translation of certain smaller size cDNAs (like SLA) or generate fusion proteins and entail defective virus production in an unpredictable manner.

Construction of a bicistronic vector for the co-expression of two genes in Caenorhabditis elegans using a newly identified IRES

The nematode Caenorhabditis elegans is an important model animal for biological research. Currently, transgenic C. elegans strains are mainly generated by injecting DNA encoding a gene of interest, in combination with a reporter gene, into the gonad. With this approach, the interpretation of negative results, such as the failure to observe reporter expression, is frequently required. Single, selectable vectors are urgently required. Internal ribosome entry site (IRES) elements are known to bind the eukaryotic ribosomal translation initiation complex and independently promote translation initiation. Bioinformatic analysis predicted an IRES motif upstream of the start codon of the C. elegans Hsp-3 gene. While this sequence has a Y-shaped double-hairpin secondary structure characteristic of IRES elements, it was unclear if it could function as an IRES. In the present study, this predicted Hsp-3 IRES was incorporated into a bicistronic vector driven by the myo-3 promoter, which allowed co-expression of RFP and GFP genes in the muscle tissue of C. elegans and thereby demonstrated that this IRES element is functional. This vector provides a novel, powerful tool for C. elegans research.

Pathologic and phenotypic alterations in a mouse expressing a connexin47 missense mutation that causes Pelizaeus-Merzbacher-like disease in humans

Gap junction channels are intercellular conduits that allow diffusional exchange of ions, second messengers, and metabolites. Human oligodendrocytes express the gap junction protein connexin47 (Cx47), which is encoded by the GJC2 gene. The autosomal recessive mutation hCx47M283T causes Pelizaeus-Merzbacher-like disease 1 (PMLD1), a progressive leukodystrophy characterized by hypomyelination, retarded motor development, nystagmus, and spasticity. We introduced the human missense mutation into the orthologous position of the mouse Gjc2 gene and inserted the mCx47M282T coding sequence into the mouse genome via homologous recombination in embryonic stem cells. Three-week-old homozygous Cx47M282T mice displayed impaired rotarod performance but unchanged open-field behavior. 10-15-day-old homozygous Cx47M282T and Cx47 null mice revealed a more than 80% reduction in the number of cells participating in glial networks after biocytin injections into oligodendrocytes in sections of corpus callosum. Homozygous expression of mCx47M282T resulted in reduced MBP expression and astrogliosis in the cerebellum of ten-day-old mice which could also be detected in Cx47 null mice of the same age. Three-month-old homozygous Cx47M282T mice exhibited neither altered open-field behavior nor impaired rotarod performance anymore. Adult mCx47M282T expressing mice did not show substantial myelin alterations, but homozygous Cx47M282T mice, additionally deprived of connexin32, which is also expressed in oligodendrocytes, died within six weeks after birth and displayed severe myelin defects accompanied by astrogliosis and activated microglia. These results strongly suggest that PMLD1 is caused by the loss of Cx47 channel function that results in impaired panglial coupling in white matter tissue.

Combined VEGF and LMP-1 delivery enhances osteoprogenitor cell differentiation and ectopic bone formation

A novel strategy to enhance bone repair is to combine angiogenic factors and osteogenic factors. We combined vascular endothelial growth factor (VEGF) and LIM mineralization protein-1 (LMP-1) by using an internal ribosome entry site to link the genes within a single plasmid. We then evaluated the effects on osteoblastic differentiation in vitro and ectopic bone formation in vivo with a subcutaneously placed PLAGA scaffold loaded with a cloned mouse osteoprogenitor cell line, D1, transfected with plasmids containing VEGF and LMP-1 genes. The cells expressing both genes elevated mRNA expression of RunX2 and β-catenin and alkaline phosphatase activity compared to cells from other groups. In vivo, X-ray and micro-CT analysis of the retrieved implants revealed more ectopic bone formation at 2 and 3 weeks but not at 4 weeks compared to other groups. The results indicate that the combination of the therapeutic growth factors potentiates cell differentiation and may promote osteogenesis.

VEGF and BMP-6 enhance bone formation mediated by cloned mouse osteoprogenitor cells

New strategies such as combined utilization of growth factors may provide a better treatment for difficult fractures. We have demonstrated enhanced angiogenesis and osteogenesis through the actions of vascular endothelial growth factor (VEGF) and bone morphogenetic protein-6 (BMP-6) on the osteogenic differentiation of a cloned mouse osteoprogenitor cell in vitro and ectopic bone formation in vivo. Human VEGF and BMP-6 genes expressed together produced a significant increase in alkaline phosphatase activity, expression of the RunX2 and osteocalcin genes and mineralization. Microcomputed tomographic analysis of subcutaneous implants consisting of cells transfected with VEGF and BMP-6 cDNA and delivered on a 3D poly (lactic-co-glycolic acid) scaffold confirmed the additive effects between VEGF and BMP-6. Ectopic bone formation in the VEGF plus BMP-6 group was greatest compared to that in either VEGF or BMP-6 alone. This is the first study that demonstrates osteogenesis in vitro and in vivo through the additive effects of VEGF and BMP-6.

Improvement of reporter activity by IRES-mediated polycistronic reporter system

Many in vitro and in vivo applications for transgenesis require co-expression of heterologous genes. The use of internal ribosome entry sites (IRESs) in dicistronic expression vectors enables the expression of two genes controlled by one promoter in target cells or whole organisms. Here we describe the expansion of IRES exploitation to generate multicistronic vectors capable of expressing multiple reporter genes, especially to improve the fluorescence yield of autofluorescent reporter gene products such as green fluorescent protein (GFP). We found that the increase in fluorescence output of GFP is proportional to the number of IRES-GFP repeats in the multicistronic vector. At least four genes can be expressed from a multicistonic vector by using tandem IRES elements, with no significant alteration of the expression level of the cap-dependent translated gene. Moreover, gene expression under the control of multiple IRES element has no effect on the posttranscriptional regulation through 3′-untranslated regions (3′UTR). Thus, endogenous gene expression and regulation, especially those controlled by weak promoters, can be analyzed with our IRES-dependent polycistronic reporter gene expression system.

Some oculodentodigital dysplasia-associated Cx43 mutations cause increased hemichannel activity in addition to deficient gap junction channels

Oculodentodigital dysplasia (ODDD) is a dominantly inherited human disorder associated with different symptoms like craniofacial anomalies, syndactyly and heart dysfunction. ODDD is caused by mutations in the GJA1 gene encoding the gap junction protein connexin43 (Cx43). Here, we have characterized four Cx43 mutations (I31M, G138R, G143S and H194P) after stable expression in HeLa cells. In patients, the I31M and G138R mutations showed all phenotypic characteristics of ODDD, whereas G143S did not result in facial abnormalities and H194P mutated patients exhibited no syndactylies. In transfected HeLa cells, these mutations led to lack of the P2 phosphorylation state of the Cx43 protein, complete inhibition of gap junctional coupling measured by neurobiotin transfer and increased hemichannel activity. In addition, altered trafficking and delayed degradation were found in these mutants by immunofluorescence and pulse-chase analyses. In G138R and G143S mutants, the increased hemichannel activity correlated with an increased half-time of the Cx43 protein. However, the I31M mutated protein showed no extended half-time. Thus, the increased hemichannel activity may be directly caused by an altered conformation of the mutated channel forming protein. We hypothesize that increased hemichannel activity may aggravate the phenotypic abnormalities in ODDD patients who are deficient in Cx43 gap junction channels.

Targeted-simultaneous expression of Gas1 and p53 using a bicistronic adenoviral vector in gliomas

The targeted expression of transgenes is one of the principal goals of gene therapy, and it is particularly relevant for the treatment of brain tumors. In this study, we examined the effect of the overexpression of human gas1 (growth arrest specific 1) and human p53 cDNAs, both under the transcriptional control of a promoter of the human glial fibrillary acidic protein (gfa2), employing adenoviral expression vectors, in glioma cells. We showed that the targeted overexpression of gas1 and p53 (AdSGas1 and AdSp53, respectively) in rat glioma cells (C6) reduced the number of viable cells and induced apoptosis. Moreover, the adenovirally targeted expression of these genes also reduced tumor growth in vivo. Unexpectedly, there was no additive effect when both gas1 and p53 were simultaneously expressed in the same cells using a bicistronic adenoviral vector. We suggest that Gas1 does not act in combination with p53 in the C6 and U373 glioma cell lines, inducing apoptosis and cell cycle arrest. Our results indicate that the targeted expression of tumor suppressor genes (gas1 and p53) regulated by the gfa2 promoter, together with adenoviral vectors may provide an interesting approach for adjuvant selective glioma gene therapy.

Challenge with mammary tumor cells expressing MHC class II and CD80 prevents the development of spontaneously arising tumors in MMTV-neu transgenic mice

The HER-2/Neu oncogene has been implicated in human and mouse breast cancer. Indeed, transgenic MMTV-neu mice expressing this oncogene from the mammary tumor virus long terminal repeat develop spontaneous mammary tumors and die within 1 year of life. We have expressed the class II transactivator (CIITA) and/or the costimulatory molecule CD80 (B7.1) in a mammary carcinoma cell line (MCNeuA) derived from these mice. Class II transactivator directs the expression of MHC class II and the machinery for antigen processing and presentation by this pathway. When injected into MMTV-neu mice, tumor cells expressing CD80 or CD80 and CIITA, were rejected completely. In addition, following the rejection of dual expressing cells, 75% of the mice were protected against the development of subsequent spontaneous tumors. Cells expressing only CD80 or CIITA were not as effective as antitumor vaccines in preventing the development of spontaneous tumors. Thus, converting cancer cells into antigen presenting cells could represent an effective immunotherapy for breast cancer.

Reversion of the biochemical defects in murine embryonic Sandhoff neurons using a bicistronic lentiviral vector encoding hexosaminidase alpha and beta

Sandhoff disease, a neurodegenerative disorder characterized by the intracellular accumulation of GM2 ganglioside, is caused by mutations in the hexosaminidase beta-chain gene resulting in a hexosaminidase A (alphabeta) and B (betabeta) deficiency. A bicistronic lentiviral vector encoding both the hexosaminidase alpha and beta chains (SIV.ASB) has previously been shown to correct the beta-hexosaminidase deficiency and to reduce GM2 levels both in transduced and cross-corrected human Sandhoff fibroblasts. Recent advances in determining the neuropathophysiological mechanisms in Sandhoff disease have shown a mechanistic link between GM2 accumulation, neuronal cell death, reduction of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA) activity, and axonal outgrowth. To examine the ability of the SIV.ASB vector to reverse these pathophysiological events, hippocampal neurons from embryonic Sandhoff mice were transduced with the lentivector. Normal axonal growth rates were restored, as was the rate of Ca(2+) uptake via the SERCA and the sensitivity of the neurons to thapsigargin-induced cell death, concomitant with a decrease in GM2 and GA2 levels. Thus, we have demonstrated that the bicistronic vector can reverse the biochemical defects and down-stream consequences in Sandhoff neurons, reinforcing its potential for Sandhoff disease in vivo gene therapy.

Repressible transgenic model of NRAS oncogene–driven mast cell disease in the mouse

To create a model in which to study the effects of RAS dysregulation in hematopoietic disease, we developed separate founder lines of transgenic mice, with the tetracycline transactivator (tTA) driven by the Vav hematopoietic promoter in one line and NRASV12 driven by the tetracycline responsive element (TRE2) in the other. When these lines are crossed, doubly transgenic animals uniformly develop a disease similar to human aggressive systemic mastocytosis (ASM) or mast cell leukemia (MCL) when they are between 2 and 4 months of age. Disease is characterized by tissue infiltrates of large, well-differentiated mast cells in the spleen, liver, skin, lung, and thymus. Analysis of bone sections shows small to large foci of similarly well-differentiated mast cells. Results also show that transgene expression and diseases are repressible through the administration of doxycycline in the drinking water of affected animals, indicating that NRASV12 expression is required to initiate and maintain disease in doubly transgenic mice. Our inducible system of transgenes, developed as a model of mutant NRASV12 oncogene–driven myeloid disease, will be useful for studying the role of RAS dysregulation in hematopoietic disease in general and in discrete human diseases, specifically ASM and MCL.

Factors limiting autogene-based cytoplasmic expression systems

The relatively low levels of transfection that can be achieved by current gene delivery systems have limited the therapeutic utility of gene transfer. This is especially true for non-viral gene delivery systems, where the levels of gene expression achieved are usually well below the levels achieved by viral gene transfer systems. Previous work from our laboratory describes an enhanced dual promoter autogene-based cytoplasmic expression system that gives rise to levels of gene expression 20-fold higher than that of a CMV nuclear expression plasmid control. Here various strategies are described to increase the levels of autogene-based gene expression by changing variables such as the type of nuclear promoter, phage RNAP gene, and IRES element. Although insights into the function of various IRES elements were gained, none of these changes demonstrated a significant increase in gene expression. However, determination of the mRNA levels achieved using quantitative RNase protection assays and immunofluorescence experiments revealed that transgene mRNA levels were saturated at up to 10 times higher than all other mRNA in the transfected cell combined. It follows that mRNA production, as well as translation, are important factors limiting autogene-based cytoplasmic expression.

Use and comparison of different internal ribosomal entry sites (IRES) in tricistronic retroviral vectors

BACKGROUND

Polycistronic retroviral vectors that contain several therapeutic genes linked via internal ribosome entry sites (IRES), provide new and effective tools for the co-expression of exogenous cDNAs in clinical gene therapy protocols. For example, tricistronic retroviral vectors could be used to genetically modify antigen presenting cells, enabling them to express different co-stimulatory molecules known to enhance tumor cell immunogenicity.

RESULTS

We have constructed and compared different retroviral vectors containing two co-stimulatory molecules (CD70, CD80) and selectable marker genes linked to different IRES sequences (IRES from EMCV, c-myc, FGF-2 and HTLV-1). The tricistronic recombinant amphotropic viruses containing the IRES from EMCV, FGF-2 or HTLV-1 were equally efficient in inducing the expression of an exogenous gene in the transduced murine or human cells, without displaying any cell type specificity. The simultaneous presence of several IRESes on the same mRNA, however, can induce the differential expression of the various cistrons. Here we show that the IRESes of HTLV-1 and EMCV interfere with the translation induced by other IRESes in mouse melanoma cells. The IRES from FGF-2 did however induce the expression of exogenous cDNA in human melanoma cells without any positive or negative regulation from the other IRESs present within the vectors. Tumor cells that were genetically modified with the tricistronic retroviral vectors, were able to induce an in vivo anti-tumor immune response in murine models.

CONCLUSION

Translation of the exogenous gene is directed by the IRES and its high level of expression not only depends on the type of cell that is transduced but also on the presence of other genetic elements within the vector.

Modeling the dosage effect of oncogenes in leukemogenesis

PURPOSE OF REVIEW

This review discusses the dosage effects of some oncogenes in leukemogenesis and compares various methods that model human hematologic malignancies in mice by introducing genetic lesions in a cell type-specific, time-controlled, and dosage-relevant manner.

RECENT FINDINGS

Recent evidence indicates that optimal dosage of cancer-related gene products plays an important role in the induction of mouse tumors that recapitulate their human counterparts.

SUMMARY

The mouse is a very valuable model system for experimentally dissecting the in vivo pathogenesis of cancer, for identifying pharmacological targets of cancer and for evaluating cancer therapies. In modeling human cancer, it has been shown that both the timing of introducing/activating oncogenic mutation(s) and the cell types into which the genetic lesion(s) is targeted are critical for cancer development. Recent studies also showed that efficient induction of relevant human leukemia in mice by certain oncogenes, such as PML/RARalpha and TEL/ABL, only occurred when they were expressed at a low level or close to pathophysiologically relevant level. These studies stress the importance of studying oncoprotein function at pathophysiologically relevant expression levels. Conditional gene expression systems are powerful tools for developing mouse models for human cancer by introducing genetic lesions in a cell type-specific, time-controlled and dosage-relevant manner. The bone marrow retroviral transduction and transplantation system can also mimic the cell and temporally specific origin of hematological malignancies by targeting oncogenes into sorted hematopoietic cells. This versatile approach is particularly powerful in structure-function analysis of oncogenes in vivo. However, overexpression of a transgene driven by retroviral vectors may alter the biological outcomes of the transgene in vivo. My colleagues and I have shown that generating vectors with modulated transgene expression can overcome this limitation of the retroviral transduction system in modeling human cancer in mice. Conditional gene expression and the modified retroviral transduction systems will be complimentary in studying human cancers in mice.

Induction of tubulogenesis in telomerase-immortalized human microvascular endothelial cells by glioblastoma cells

To facilitate the study of human endothelial cells we have used a replication defective retrovirus encoding the catalytic subunit of telomerase (hTERT) to derive populations of telomerase-immortalized human microvascular endothelial (TIME) cells. Whereas parental HMVECs became senescent on average within 35-45 population doublings (PDs), TIME cells have continued to proliferate for at least 200 PDs. TIME cells express readily detectable telomerase activity but display only a modest increase in telomere length. Karyotypic analysis reveals the cells to have a normal complement of human chromosomes with no evidence of gross genetic abnormalities. Furthermore, TIME cells retain many of the characteristics of the primary endothelial cells from which they were derived. For example, they express a panel of characteristic endothelial cell surface marker proteins such as CD31/PECAM-1 and alpha(v)beta3-integrin. In addition, TIME cells express receptors for low-density lipoprotein (LDL) receptor as they are competent for receptor-mediated endocytosis of fluorescent acetylated LDL. Importantly, when plated on matrigel, TIME cells undergo tubule formation. Moreover, when cocultured in the presence of human glioma cells, but not primary human astrocytes, TIME cells are induced to form stable tubules. Detachment of TIME cells from extracellular matrix leads to a form of programmed cell death known as anoikis. Conditional activation of the protein kinase Akt (Akt:ER*) significantly inhibited the onset of TIME cell anoikis under these conditions. We believe that the ability of hTERT to immortalize primary human endothelial cells, and the fact that such cells retain the endothelial characteristics of the cells from which they were derived, will greatly facilitate the analysis of human endothelial cell biology in vitro.

Efficient ablation by immunotoxin-mediated cell targeting of the cell types that express human interleukin-2 receptor depending on the internal ribosome entry site

BACKGROUND

Immunotoxin-mediated cell targeting (IMCT) is a technique for conditional genetic ablation of specific cell types. IMCT provides a useful approach for generating animal models for human neurodegenerative disorders. The strategy of IMCT depends on the cytotoxic activity of antiTac-based recombinant immunotoxins that selectively target cells expressing the human interleukin-2 receptor alpha-subunit (IL-2Ralpha). Transgenic mice were generated that express the IL-2Ralpha under the control of an appropriate tissue-specific gene promoter, and they were treated with the recombinant immunotoxins resulting in the ablation of the target cell types. To restrict the expression of IL-2Ralpha transgene in the cell types of interest, it is useful to knock-in the IL-2Ralpha expression cassette into the specific marker gene locus with gene targeting. Moreover, the knock-in of the IL-2Ralpha cassette located downstream of an internal ribosome entry site (IRES) into the 3'-untranslated region of the marker gene enables IL-2Ralpha expression in the restricted cell types while preserving the intact marker gene expression. However, there is a possibility that IRES-dependent expression of the receptor may be less efficient than cap-dependent expression.

METHODS AND RESULTS

The efficiency of IRES-dependent IL-2Ralpha expression and immunotoxin responsiveness of the cells expressing the receptor were examined. The IL-2Ralpha gene fused to green fluorescence protein (GFP) (IL-2R/ GFP) was used as the target receptor. Embryonic stem cell clones were isolated that carry two types of bicistronic vectors in which the IL-2R/GFP fusion gene or the chloramphenicol acetyltransferase gene was connected upstream or downstream of IRES. The expression level of IL-2R/GFP protein in the cell clones was evaluated by GFP fluorescence detection and Western blot analysis. The IRES-dependent expression produced the same level of receptor protein as cap-dependent expression. The immunotoxin responsiveness of the cloned cells was evaluated by measuring the colony-forming efficiency in medium containing various amounts of a recombinant immunotoxin. The colony-forming efficiency of the cells expressing IL-2R/ GFP through IRES-dependent expression was reduced together with increasing immunotoxin concentration in a similar dose-dependent manner to the cells expressing the receptor through cap-dependent expression.

CONCLUSIONS

The present results indicate that it is possible to effectively use the IRES-dependent expression system for IMCT. The system permits expression of the target receptor in selective cell types by introducing the IRES-driven expression cassette into the 3'-untranslated region of the marker gene locus.

Composition and arrangement of genes define the strength of IRES-driven translation in bicistronic mRNAs

In addition to the cap-dependent mechanism, eukaryotic initiation of translation can occur by a cap-independent mechanism which directs ribosomes to defined start codons enabled by internal ribosome entry site (IRES) elements. IRES elements from poliovirus and encephalomyocarditis virus are often used to construct bi- or oligocistronic expression vectors to co-express various genes from one mRNA. We found that while cap-dependent translation initiation from bicistronic mRNAs remains comparable to monocistronic expression, internal initiation mediated by these viral IRESs is often very inefficient. Expression of bicistronic expression vectors containing the hepatitis B virus core antigen (HBcAg) together with various cytokines in the second cistron of bicistronic mRNAs gave rise to very low levels of the tested cytokines. On the other hand, the HBcAg was well expressed when positioned in the second cistron. This suggests that the arrangement of cistrons in a bicistronic setting is crucial for IRES-dependent translation of the second cistron. A systematic examination of expression of reporter cistrons from bicistronic mRNAs with respect to position was carried out. Using the dual luciferase assay system we show that the composition of reading frames on a bicistronic mRNA and the order in which they are arranged define the strength of IRES-dependent translation. Although the cellular environment and the nature of the IRES element influence translation strength the dominant determinant is the nature and the arrangement of cistrons on the mRNA.

Hybrid vector designs to control the delivery, fate and expression of transgenes

One of the greatest challenges to gene therapy is the targetting of gene delivery selectively to the sites of disease and regulation of transgene expression without adverse effects. Ultimately, the successful realization of these goals is dependent upon improvements in vector design. Over the years, viral vector design has progressed from various types of replication-defective viral mutants to replication-conditioned viruses and, more recently, to 'gutted' and hybrid vectors, which have, respectively, eliminated expression of non-relevant or toxic viral genes and incorporated desired elements of different viruses so as to increase the efficacy of gene delivery in vivo. This review will focus on the different viral and cellular elements which have been incorporated into virus vectors to: improve transduction efficiencies; alter the entry specificity of virions; control the fate of transgenes in the host cells; and regulate transgene expression.

Internal ribosome entry sites (IRESs): reality and use

IRESs are known to recruit ribosomes directly, without a previous scanning of untranslated region of mRNA by the ribosomes. IRESs have been found in a number of viral and cellular mRNAs. Experimentally, IRESs are commonly used to direct the expression of the second cistrons of bicistronic mRNAs. The mechanism of action of IRESs is not fully understood and a certain number of laboratories were not successful in using them in a reliable manner. Three observations done in our laboratory suggested that IRESs might not work as functionally as it was generally believed. Stem loops added before IRESs inhibited mRNA translation. When added into bicistronic mRNAs, IRESs initiated translation of the second cistrons efficiently only when the intercistronic region contained about 80 nucleotides, and they did not work any more effectively with intercistronic regions containing at least 300-400 nucleotides. Conversely, IRESs inserted at any position into the coding region of a cistron interrupted its translation and initiated translation of the following cistron. The first two data are hardly compatible with the idea that IRESs are able to recruit ribosomes without using the classical scanning mechanism. IRESs are highly structured and cannot be scanned by the 40S ribosomal subunit. We suggest that IRESs are short-circuited and are essentially potent stimulators favoring translation in particular physiological situations.