Light it up: highly efficient multigene delivery in mammalian cells

MultiBac: Baculovirus-Mediated Multigene DNA Cargo Delivery in Insect and Mammalian Cells

The baculovirus/insect cell system (BICS) is widely used in academia and industry to produce eukaryotic proteins for many applications, ranging from structure analysis to drug screening and the provision of protein biologics and therapeutics. Multi-protein complexes have emerged as vital catalysts of cellular function. In order to unlock the structure and mechanism of these essential molecular machines and decipher their function, we developed MultiBac, a BICS particularly tailored for heterologous multigene transfer and multi-protein complex production. Baculovirus is unique among common viral vectors in its capacity to accommodate very large quantities of heterologous DNA and to faithfully deliver this cargo to a host cell of choice. We exploited this beneficial feature to outfit insect cells with synthetic DNA circuitry conferring new functionality during heterologous protein expression, and developing customized MultiBac baculovirus variants in the process. By altering its tropism, recombinant baculovirions can be used for the highly efficient delivery of a customized DNA cargo in mammalian cells and tissues. Current advances in synthetic biology greatly facilitate the construction or recombinant baculoviral genomes for gene editing and genome engineering, mediated by a MultiBac baculovirus tailored to this purpose. Here, recent developments and exploits of the MultiBac system are presented and discussed.

MultiBacMam Bimolecular Fluorescence Complementation (BiFC) tool-kit identifies new small-molecule inhibitors of the CDK5-p25 protein-protein interaction (PPI)

Protein-protein interactions (PPIs) are at the core of virtually all biological processes in cells. Consequently, targeting PPIs is emerging at the forefront of drug discovery. Cellular assays which closely recapitulate native conditions in vivo are instrumental to understand how small molecule drugs can modulate such interactions. We have integrated MultiBacMam, a baculovirus-based mammalian gene delivery tool we developed, with bimolecular fluorescence complementation (BiFC), giving rise to a highly efficient system for assay development, identification and characterization of PPI modulators. We used our system to analyze compounds impacting on CDK5-p25 PPI, which is implicated in numerous diseases including Alzheimer’s. We evaluated our tool-kit with the known inhibitor p5T, and we established a mini-screen to identify compounds that modulate this PPI in dose-response experiments. Finally, we discovered several compounds disrupting CDK5-p25 PPI, which had not been identified by other screening or structure-based methods before.

Multigene delivery in mammalian cells: Recent advances and applications

Systems for multigene delivery in mammalian cells, particularly in the context of genome engineering, have gained a lot of attention in biomolecular research and medicine. Initially these methods were based on RNA polymerase II promoters and were used for the production of protein complexes and for applications in cell biology such as reprogramming of somatic cells to stem cells. Emerging technologies such as CRISPR/Cas9-based genome engineering, which enable any alteration at the genomic level of an organism, require additional elements including U6-driven expression cassettes for RNA expression and homology constructs for designed genome modifications. For these applications, systems with high DNA capacity, flexibility and transfer rates are needed. In this article, we briefly give an update on some of recent strategies that facilitate multigene assembly and delivery into mammalian cells. Also, we review applications in various fields of biology that rely on multigene delivery systems.

BMP2 expressing genetically engineered mesenchymal stem cells on composite fibrous scaffolds for enhanced bone regeneration in segmental defects

The treatment of critical sized bone defect remains a significant challenge in orthopedics. The objective of the study is to evaluate the effect of the combination of bone morphogenetic protein 2 (BMP2) expressing genetically engineered mesenchymal stem cells (MSCs) [MSCs engineered using a multimam vector, pAceMam1, an emerging gene delivery vector] and an osteoconductive scaffold [silica coated nanohydroxyapatite-gelatin reinforced with fibers] in enhancing bone regeneration in critical sized segmental defects. The scaffold with transfected MSCs showed significantly higher viability, proliferation and osteogenic differentiation in vitro. Further, this group augmented union and new bone formation in critical sized rat femoral segmental defect at 12 weeks when compared to control groups (scaffold with MSCs and scaffold alone). These data demonstrated that the MSCs engineered for transient expression of BMP2 can improve the repair of segmental defects, which paves an avenue for using pAceMam1 as a vector for bone tissue regeneration.

Highly efficient baculovirus-mediated multigene delivery in primary cells

Multigene delivery and subsequent cellular expression is emerging as a key technology required in diverse research fields including, synthetic and structural biology, cellular reprogramming and functional pharmaceutical screening. Current viral delivery systems such as retro- and adenoviruses suffer from limited DNA cargo capacity, thus impeding unrestricted multigene expression. We developed MultiPrime, a modular, non-cytotoxic, non-integrating, baculovirus-based vector system expediting highly efficient transient multigene expression from a variety of promoters. MultiPrime viruses efficiently transduce a wide range of cell types, including non-dividing primary neurons and induced-pluripotent stem cells (iPS). We show that MultiPrime can be used for reprogramming, and for genome editing and engineering by CRISPR/Cas9. Moreover, we implemented dual-host-specific cassettes enabling multiprotein expression in insect and mammalian cells using a single reagent. Our experiments establish MultiPrime as a powerful and highly efficient tool, to deliver multiple genes for a wide range of applications in primary and established mammalian cells.

Current viral gene delivery systems are limited in the amount of foreign DNA they can deliver to cells. Here the authors develop MultiPrime, a baculovirus-based vector system capable of multigene delivery into a wide variety of cells, and use Multiprime for genome engineering by CRISPR/Cas9.

Effective isotope labeling of proteins in a mammalian expression system

Isotope labeling of biologically interesting proteins is a prerequisite for structural and dynamics studies by NMR spectroscopy. Many of these proteins require mammalian cofactors, chaperons, or posttranslational modifications such as myristoylation, glypiation, disulfide bond formation, or N- or O-linked glycosylation; and mammalian cells have the necessary machinery to produce them in their functional forms. Here, we describe recent advances in mammalian expression, including an efficient adenoviral vector-based system, for the production of isotopically labeled proteins. This system enables expression of mammalian proteins and their complexes, including proteins that require posttranslational modifications. We describe a roadmap to produce isotopically labeled (15)N and (13)C posttranslationally modified proteins, such as the outer domain of HIV-1 gp120, which has four disulfide bonds and 15 potential sites of N-linked glycosylation. These methods should allow NMR spectroscopic analysis of the structure and function of posttranslationally modified and secreted, cytoplasmic, or membrane-bound proteins.

An internal ribosome entry site (IRES) mutant library for tuning expression level of multiple genes in mammalian cells

A set of mutated Encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) elements with varying strengths is generated by mutating the translation initiation codons of 10^th, 11^th, and 12^th AUG to non-AUG triplets. They are able to control the relative expression of multiple genes over a wide range in mammalian cells in both transient and stable transfections. The relative strength of each IRES mutant remains similar in different mammalian cell lines and is not gene specific. The expressed proteins have correct molecular weights. Optimization of light chain over heavy chain expression by these IRES mutants enhances monoclonal antibody expression level and quality in stable transfections. Uses of this set of IRES mutants can be extended to other applications such as synthetic biology, investigating interactions between proteins and its complexes, cell engineering, multi-subunit protein production, gene therapy, and reprogramming of somatic cells into stem cells.

Baculovirus: an insect-derived vector for diverse gene transfer applications

Insect-derived baculoviruses have emerged as versatile and safe workhorses of biotechnology. Baculovirus expression vectors (BEVs) have been applied widely for crop and forest protection, as well as safe tools for recombinant protein production in insect cells. However, BEVs ability to efficiently transduce noninsect cells is still relatively poorly recognized despite the fact that efficient baculovirus-mediated in vitro and ex vivo gene delivery into dormant and dividing vertebrate cells of diverse origin has been described convincingly by many authors. Preliminary proof of therapeutic potential has also been established in preclinical studies. This review summarizes the advantages and current status of baculovirus-mediated gene delivery. Stem cell transduction, preclinical animal studies, tissue engineering, vaccination, cancer gene therapy, viral vector production, and drug discovery are covered.

Dynamic clonal analysis of murine hematopoietic stem and progenitor cells marked by 5 fluorescent proteins using confocal and multiphoton microscopy

We demonstrate a methodology for tracing the clonal history of hematopoietic stem and progenitor cells (HSPCs) behavior in live tissues in 4 dimensions (4D). This integrates genetic combinatorial marking using lentiviral vectors encoding various fluorescent proteins (FPs) with advanced imaging methods. Five FPs: Cerulean, EGFP, Venus, tdTomato, and mCherry were concurrently used to create a diverse palette of color-marked cells. A key advantage of imaging using a confocal/2-photon hybrid microscopy approach is the simultaneous assessment of uniquely 5FP-marked cells in conjunction with structural components of the tissues at high resolution. Volumetric analyses revealed that spectrally coded HSPC-derived cells can be detected noninvasively in various intact tissues, including the bone marrow, for extensive periods of time after transplantation. Live studies combining video-rate multiphoton and confocal imaging in 4D demonstrate the possibility of dynamic cellular and clonal tracking in a quantitative manner. This methodology has applications in the understanding of clonal architecture in normal and perturbed hematopoiesis.

Mammalian expression of isotopically labeled proteins for NMR spectroscopy

NMR spectroscopic characterization of biologically interesting proteins generally requires the incorporation of (15)N/(13)C and/or (2)H stable isotopes. While prokaryotic incorporation systems are regularly used, mammalian ones are not: of the nearly 9,000 NMR macromolecular structures currently deposited in the Protein Data Bank, only a handful (<0.5%) were solved with proteins expressed in mammalian systems. This low number of structures is largely a reflection of the difficulty in producing uniformly labeled, mammalian-expressed proteins. This is unfortunate, as many interesting proteins require mammalian cofactors, chaperons, or post-translational modifications such as N-linked glycosylation, and mammalian cells have the necessary machinery to produce them correctly. Here we describe recent advances in mammalian expression, including an efficient adenoviral vector-based system, for the production of isotopically enriched proteins. This system allows for the expression of mammalian proteins and their complexes, including proteins that require post-translational modifications. We describe how this system can produce isotopically labeled (15)N and (13)C post-translationally modified proteins, such as the outer domain of HIV-1 gp120, which has 15 sites of N-linked glycosylation. Selective amino-acid labeling is also described. These developments should reduce barriers to the determination of NMR structures with isotopically labeled proteins from mammalian expression systems.