Protein complex expression by using multigene baculoviral vectors

Elucidation of the molecular basis of protein-interaction networks, in particular in higher eukaryotes, is hampered by insufficient quantities of endogenous multiprotein complexes. Present recombinant expression methods often require considerable investment in both labor and materials before multiprotein expression, and after expression and biochemical analysis these methods do not provide flexibility for expressing an altered multiprotein complex. To meet these demands, we have recently introduced MultiBac, a modular baculovirus-based system specifically designed for eukaryotic multiprotein expression. Here we describe new transfer vectors and a combination of DNA recombination-based methods, which further facilitate the generation of multigene cassettes for protein coexpression (Fig. 1), thus providing a flexible platform for generation of protein expression vectors and their rapid regeneration for revised expression studies. Genes encoding components of a multiprotein complex are inserted into a suite of compatible transfer vectors by homologous recombination. These progenitor constructs are then rapidly joined in the desired combination by Cre-loxP-mediated in vitro plasmid fusion. Protocols for integration of the resulting multigene expression cassettes into the MultiBac baculoviral genome are provided that rely on Tn7 transposition and/or Cre-loxP reaction carried out in vivo in Escherichia coli cells tailored for this purpose. Detailed guidelines for multigene virus generation and amplification, cell culture maintenance and protein production are provided, together with data illustrating the simplicity and remarkable robustness of the present method for multiprotein expression using a composite MultiBac baculoviral vector.     

Generation of transgenic chicks using an oviduct-specific expression system

We successfully replaced the ovalbumin gene of a magnum region in chickens with a human plasminogen activator. We constructed pL-eGFP, pL-tPAGFP and pL-2.8OVtPAGFP vectors and cultured 293FT chicken embryo fibroblasts, chicken primordial germ cells, Hela C127 cells, and oviduct epithelial cells. All vectors were expressed in the transfected cells, except pL-2.8OVtPAGFP vector, which was only expressed in oviduct epithelial cells. A lentivirus with pL-2.8OVtPAGFP was injected in fertilized eggs; 11 chicks hatched in the G? generation, four of them carried the tPAGFP. Two cockerels from the G? generation were crossed with four wild-type hens. Three chicks in G? carried the tPAGFP. We concluded that by using an oviduct-specific vector for transfection, human recombinant plasminogen activator protein can be expressed in the oviducts of laying hens. This character is inherited and can be reproduced with a need for repeated transfection.     

iPS细胞:一种肿瘤细胞

诱导多能干细胞(iPS细胞)可以用于定向分化、动物发育、药物筛选和疾病治疗等研究和应用领域,可以避免ES细胞产生的免疫排斥和伦理道德问题.因此,iPS细胞的产生具有里程碑的意义,并迅速成为生物科学领域中的研究热点.然而,iPS细胞并非非常完美,没有任何瑕疵.研究过程中发现iPS细胞存在诱导频率过低、致瘤性、临床应用安全等一系列问题.本文主要综述有关iPS细胞前期研究成果和iPS细胞存在的一些问题以及iPS细胞与肿瘤细胞之间的联系.     

Integration-free iPS cells engineered using human artificial chromosome vectors

Human artificial chromosomes (HACs) have unique characteristics as gene-delivery vectors, including episomal transmission and transfer of multiple, large transgenes. Here, we demonstrate the advantages of HAC vectors for reprogramming mouse embryonic fibroblasts (MEFs) into induced pluripotent stem (iPS) cells. Two HAC vectors (iHAC1 and iHAC2) were constructed. Both carried four reprogramming factors, and iHAC2 also encoded a p53-knockdown cassette. iHAC1 partially reprogrammed MEFs, and iHAC2 efficiently reprogrammed MEFs. Global gene expression patterns showed that the iHACs, unlike other vectors, generated relatively uniform iPS cells. Under non-selecting conditions, we established iHAC-free iPS cells by isolating cells that spontaneously lost iHAC2. Analyses of pluripotent markers, teratomas and chimeras confirmed that these iHAC-free iPS cells were pluripotent. Moreover, iHAC-free iPS cells with a re-introduced HAC encoding Herpes Simplex virus thymidine kinase were eliminated by ganciclovir treatment, indicating that the HAC safeguard system functioned in iPS cells. Thus, the HAC vector could generate uniform, integration-free iPS cells with a built-in safeguard system.     

Efficient expression of secreted proteases via recombinant BacMam virus

Baculovirus vectors engineered to contain mammalian cell-active promoter elements have been described as an efficient method for transduction of a broad spectrum of human cell lines at high frequency. In the first large-scale comparative study of secreted protein production using these viral vectors, we have evaluated production of 16 recombinant enzymes--specifically, we exploited these viral vectors, termed 'BacMam' viruses, to drive expression of a panel of proteases selected from all four major mechanistic classes, including secreted, lysosomal, endosomal, and type I transmembrane proteins. To allow a generic purification strategy, coding sequences were truncated to remove transmembrane and/or subcellular retention signals before introduction, in parallel, into a C-terminally Fc-tagged BacMam transfer vector. BacMam viruses were generated and subsequently evaluated for expression of Fc-tagged protein in virus-transduced HEK-F cells. The common Fc-tag enabled single-step affinity purification of secreted recombinant protein from the culture medium. Yields were excellent, with 14 of 16 genes expressed producing 10-30 mg or more purified protein per litre of culture using standardised transduction conditions. At this level, reagent demands for a typical protease high-throughput screen (HTS) could be met from expression cultures as small as 0.1-0.5 L. Our results indicate this expression system offers a highly efficient and scaleable method for production of enzymatically-active secreted proteases and may therefore represent a novel method of protein production for other secreted enzymes with significant advantages over the diverse approaches in current use.     

Generation of glucose-responsive functional islets with a three-dimensional structure from mouse fetal pancreatic cells and iPS cells in vitro

Islets of Langerhans are a pancreatic endocrine compartment consisting of insulin-producing β cells together with several other hormone-producing cells. While some insulin-producing cells or immature pancreatic cells have been generated in vitro from ES and iPS cells, islets with proper functions and a three-dimensional (3D) structure have never been successfully produced. To test whether islets can be formed in vitro, we first examined the potential of mouse fetal pancreatic cells. We found that E16.5 pancreatic cells, just before forming islets, were able to develop cell aggregates consisting of β cells surrounded by glucagon-producing α cells, a structure similar to murine adult islets. Moreover, the transplantation of these cells improved blood glucose levels in hyperglycemic mice. These results indicate that functional islets are formed in vitro from fetal pancreatic cells at a specific developmental stage. By adopting these culture conditions to the differentiation of mouse iPS cells, we developed a two-step system to generate islets, i.e. immature pancreatic cells were first produced from iPS cells, and then transferred to culture conditions that allowed the formation of islets from fetal pancreatic cells. The islets exhibited distinct 3D structural features similar to adult pancreatic islets and secreted insulin in response to glucose concentrations. Transplantation of the islets improved blood glucose levels in hyperglycemic mice. In conclusion, the two-step culture system allows the generation of functional islets with a 3D structure from iPS cells.     

Selecting informative genes for discriminant analysis using multigene expression profiles

Background

Gene expression data extracted from microarray experiments have been used to study the difference between mRNA abundance of genes under different conditions. In one of such experiments, thousands of genes are measured simultaneously, which provides a high-dimensional feature space for discriminating between different sample classes. However, most of these dimensions are not informative about the between-class difference, and add noises to the discriminant analysis.

Results

In this paper we propose and study feature selection methods that evaluate the "informativeness" of a set of genes. Two measures of information based on multigene expression profiles are considered for a backward information-driven screening approach for selecting important gene features. By considering multigene expression profiles, we are able to utilize interaction information among these genes. Using a breast cancer data, we illustrate our methods and compare them to the performance of existing methods.

Conclusion

We illustrate in this paper that methods considering gene-gene interactions have better classification power in gene expression analysis. In our results, we identify important genes with relative large p-values from single gene tests. This indicates that these are genes with weak marginal information but strong interaction information, which will be overlooked by strategies that only examine individual genes.

     

Construction of a baculovirus-silkworm multigene expression system and its application on producing virus-like particles

A new baculovirus-silkworm multigene expression system named Bombyx mori MultiBac is developed and described here, by which multiple expression cassettes can be introduced into the Bombyx mori nuclear polyhedrosis virus (BmNPV) genome efficiently. The system consists of three donor vectors (pCTdual, pRADM and pUCDMIG) and an invasive diaminopimelate (DAP) auxotrophic recipient E. coli containing BmNPV-Bacmid (BmBacmid) with a homologous recombination region, an attTn7 site and a loxp site. Two genes carried by pCTdual are firstly inserted into BmBacmid by homologous recombination, while the other eight genes in pRADM and pUCDMIG are introduced into BmBacmid through Tn7 transposition and cre-loxp recombination. Then the invasive and DAP auxotrophic E. coli carrying recombinant BmBacmid is directly injected into silkworm for expressing heterologous genes in larvae or pupae. Three structural genes of rotavirus and three fluorescent genes have been simultaneously expressed in silkworm larvae using our new system, resulting in the formation of virus-like particles (VLPs) of rotavirus and the color change of larvae. The VLPs were purified from hemolymph by ultracentrifugation using CsCl gradients, with a yield of 12.7 μg per larva. For the great capacity of foreign genes and the low cost of feeding silkworm, this high efficient BmMultiBac expression system provides a suitable platform to produce VLPs or protein complexes.     

Efficient Generation of Virus-Free iPS Cells Using Liposomal Magnetofection

The generation of induced pluripotent stem (iPS) cells is a powerful tool in regenerative medicine, and advances in nanotechnology clearly have great potential to enhance stem cell research. Here, we introduce a liposomal magnetofection (LMF) method for iPS cell generation. Efficient conditions for generating virus-free iPS cells from mouse embryonic fibroblast (MEF) cells were determined through the use of different concentrations of CombiMag nanoparticle-DNA(pCX-OKS-2A and pCX-cMyc)-lipoplexes and either one or two cycles of the LMF procedure. The cells were prepared in a short reprogramming time period (≤8 days, 0.032–0.040%). Among the seven LMF-iPS cell lines examined, two were confirmed to be integration-free, and an integration-free LMF-iPS cell line was produced under the least toxic conditions (single LMF cycle with a half-dose of plasmid). This cell line also displayed in vitro/in vivo pluripotency, including teratoma formation and chimeric mouse production. In addition, the safety of CombiMag-DNA lipoplexes for the transfection of MEF cells was confirmed through lactate dehydrogenase activity assay and transmission electron microscopy. These results demonstrated that the LMF method is simple, effective, and safe. LMF may represent a superior technique for the generation of virus-free or integration-free iPS cell lines that could lead to enhanced stem cell therapy in the future.     

iPS cells: a game changer for future medicine

The induced pluripotent stem cell (iPSC) technology is instrumental in advancing the fields of disease modeling and cell transplantation. We herein discuss the various issues regarding disease modeling and cell transplantation presented in previous reports, and also describe new iPSC‐based medicine including iPSC clinical trials. In such trials, iPSCs from patients can be used to predict drug responders/non‐responders by analyzing the efficacy of the drug on iPSC‐derived cells. They could also be used to stratify patients after actual clinical trials, including those with sporadic diseases, based on the drug responsiveness of each patient in the clinical trials. iPSC‐derived cells can be used for the identification of response markers, leading to increased success rates in such trials. Since iPSCs can be used in micromedicine for drug discovery, and in macromedicine for actual clinical trials, their use would tightly connect both micro‐ and macromedicine. The use of iPSCs in disease modeling, cell transplantation, and clinical trials could therefore lead to significant changes in the future of medicine.     

KSR-Based Medium Improves the Generation of High-Quality Mouse iPS Cells

Induced pluripotent stem (iPS) cells from somatic cells have great potential for regenerative medicine. The efficiency in generation of iPS cells has been significantly improved in recent years. However, the generation of high-quality iPS cells remains of high interest. Consistently, we demonstrate that knockout serum replacement (KSR)-based medium accelerates iPS cell induction and improves the quality of iPS cells, as confirmed by generation of chimeras and all iPS cell-derived offspring with germline transmission competency. Both alkaline phosphatase (AP) activity assay and expression of Nanog have been used to evaluate the efficiency of iPS cell induction and formation of ES/iPS cell colonies; however, appropriate expression of Nanog frequently indicates the quality of ES/iPS cells. Interestingly, whereas foetal bovine serum (FBS)-based media increase iPS cell colony formation, as revealed by AP activity, KSR-based media increase the frequency of iPS cell colony formation with Nanog expression. Furthermore, inhibition of MAPK/ERK by a specific inhibitor, PD0325901, in KSR- but not in FBS-based media significantly increases Nanog-GFP⁺ iPS cells. In contrast, addition of bFGF in KSR-based media decreases proportion of Nanog-GFP⁺ iPS cells. Remarkably, PD can rescue Nanog-GFP⁺ deficiency caused by bFGF. These data suggest that MAPK/ERK pathway influences high quality mouse iPS cells and that KSR- and PD-based media could enrich homogeneous authentic pluripotent stem cells.     

BacMam recombinant baculovirus in transporter expression: a study of BCRP and OATP1B1

Human BCRP and OATP1B1 have recently been identified as important transporters in the absorption, distribution, and elimination of clinically significant drugs. In this report, we illustrate the use of modified baculoviruses, termed BacMam viruses for the expression of functional BCRP and OATP1B1 in mammalian cells. We show a variety of host cells efficiently transduced to express BCRP including HEK 293, LLC-PK, and U-2 OS, where protein levels on the cell-surface were modulated by titrating different amounts of viral inoculum. In addition, using the BODIPY-prazosin efflux assay and the BacMam reagent we illustrate inhibition of BCRP activity with GF120918 or Fumitremorgin C. Furthermore, we present data demonstrating simultaneous expression of BCRP and OATP1B1 in BacMam transduced mammalian cells by simply adding viral inoculum of each transporter. Thus these results indicate that BacMam mediated gene delivery provides a novel and efficient research tool for the investigation of single or multiple transporters in vitro.     

Generation of induced pluripotent stem cells using elastin like polypeptides as a non-viral gene delivery system

Induced pluripotent stem cells (iPSCs) have been generated from various somatic cells using different approaches; however, a major restriction of reprogramming methods is the use of viral vectors, which have the risk of causing genome-integration of viral DNA. Here, without a viral vector, we generated iPSCs from mouse fibroblasts using an elastin-like polypeptide (ELP)-based transfection method. Our findings support the possible use of ELPs for delivery of the reprogramming genes in to somatic cells for generation of iPSCs. Results of gel retardation assay demonstrated efficient complexation of ELPs with a plasmid containing the four Yamanaka stem cell factors, Oct-4, Klf4, c-myc, and Sox2. After transfection, the iPSCs showed embryonic stem cell-like characteristics, including expression of endogenous pluripotency genes, differentiation into three germ layer lineages, and formation of teratomas in vivo. Our results demonstrate that ELP-based gene delivery may provide a safe method for use in generation of virus-free and exogenous DNA-free iPSCs, which will be crucial for future applications in stem cell-based therapies.     

Generation of a cytotoxic T-lymphocyte response using a Salmonella antigen-delivery system

We have constructed a general-use vector for the cloning and stable expression of foreign genes in the chromosome of attenuated Salmonella typhimurium. Using this chromosomal expression vector (CEV), we expressed the circumsporozoite (CS) gene of the mouse malaria Plasmodium yoelii in an aroA S. typhimurium strain. Mice immunized with CS-expressing Salmonella recombinants mount a CS-specific cytotoxic T-lymphocyte (CTL) response. This is the first demonstration that attenuated Salmonella can elicit a specific CTL response to a foreign protein in mice. The ability to easily and stably express foreign genes from the Salmonella chromosome and the generation of specific CTL greatly expands the potential of Salmonella as an antigen-delivery system.