A biotin-triggered genetic switch in mammalian cells and mice.

Adjustable and reversible transgene expression systems enabling precise control of metabolic pathways and tunable production of specific target proteins have been essential for conditional reprogramming of mammalian cells to achieve progress in basic and applied bioengineering disciplines. Most of the currently available transgene control modalities have been designed to be responsive to clinically licensed pharmacologically active drugs which were expected to prevail in future clinical trials yet raised concerns about side effects when administered long term at subclinical doses. We have chosen vitamin H, also known as biotin, to control target gene transcription in mammalian cells in a potentially side effect-free manner. BirA, the Escherichia coli repressor of the biotin biosynthesis operon, was fused to the Herpes simplex transactivation domain to generate a biotin-dependent transactivator(BIT), which, in the presence of biotin, binds and activates chimeric target promoters (P(BIT)) harboring BirA-specific operator sites 5' of a minimal promoter. Biotin-inducible transgene expression was functional in a variety of rodent, monkey and human cell lines, showed excellent adjustability and reversibility in transgenic Chinese hamster ovary cell lines, provided precise product gene control in standard bioreactor cultures and enabled dose-dependent vitamin H control of a human glycoprotein in mice. The combination of a side effect-free inducer, precise and reversible transcription tunability and broad functionality in different cell types as well as in entire animals represents a unique asset for the use of biotin-inducible transgene control in future gene therapy, tissue engineering and biopharmaceutical manufacturing scenarios.

Adeno-associated viral vectors engineered for macrolide-adjustable transgene expression in mammalian cells and mice.

BACKGROUND: Adjustable gene expression is crucial in a number of applications such as de- or transdifferentiation of cell phenotypes, tissue engineering, various production processes as well as gene-therapy initiatives. Viral vectors, based on the Adeno-Associated Virus (AAV) type 2, have emerged as one of the most promising types of vectors for therapeutic applications due to excellent transduction efficiencies of a broad variety of dividing and mitotically inert cell types and due to their unique safety features. RESULTS: We designed recombinant adeno-associated virus (rAAV) vectors for the regulated expression of transgenes in different configurations. We integrated the macrolide-responsive E.REX systems (EON and EOFF) into rAAV backbones and investigated the delivery and expression of intracellular as well as secreted transgenes for binary set-ups and for self- and auto-regulated one-vector configurations. Extensive quantitative analysis of an array of vectors revealed a high level of adjustability as well as tight transgene regulation with low levels of leaky expression, both crucial for therapeutical applications. We tested the performance of the different vectors in selected biotechnologically and therapeutically relevant cell types (CHO-K1, HT-1080, NHDF, MCF-7). Moreover, we investigated key characteristics of the systems, such as reversibility and adjustability to the regulating agent, to determine promising candidates for in vivo studies. To validate the functionality of delivery and regulation we performed in vivo studies by injecting particles, coding for compact self-regulated expression units, into mice and adjusting transgene expression. CONCLUSION: Capitalizing on established safety features and a track record of high transduction efficiencies of mammalian cells, adeno- associated virus type 2 were successfully engineered to provide new powerful tools for macrolide-adjustable transgene expression in mammalian cells as well as in mice.

InXy and SeXy, compact heterologous reporter proteins for mammalian cells.

Mammalian reporter proteins are essential for gene-function analysis, drugscreening initiatives and as model product proteins for biopharmaceutical manufacturing. Bacillus subtilis can maintain its metabolism by secreting Xylanase A (XynA), which converts xylan into shorter xylose oligosaccharides. XynA is a family 11 xylanase monospecific for D-xylose containing substrates. Mammalian cells transgenic for constitutive expression of wild-type xynA showed substantial secretion of this prokaryotic enzyme. Deletion analysis confirmed that a prokaryotic signal sequence encoded within the first 81 nucleotides was compatible with the secretory pathway of mammalian cells. Codon optimization combined with elimination of the prokaryotic signal sequence resulted in an exclusively intracellular mammalian Xylanase A variant (InXy) while replacement by an immunoglobulin-derived secretion signal created an optimal secreted Xylanase A derivative (SeXy). A variety of chromogenic and fluorescence-based assays adapted for use with mammalian cells detected InXy and SeXy with high sensitivity and showed that both reporter proteins resisted repeated freeze/thaw cycles, remained active over wide temperature and pH ranges, were extremely stable in human serum stored at room temperature and could independently be quantified in samples also containing other prominent reporter proteins such as the human placental alkaline phosphatase (SEAP) and the Bacillus stearothermophilus-derived secreted alpha-amylase (SAMY). Glycoprofiling revealed that SeXy produced in mammalian cells was N- glycosylated at four different sites, mutation of which resulted in impaired secretion. SeXy was successfully expressed in a variety of mammalian cell lines and primary cells following transient transfection and transduction with adeno-associated virus particles (AAV) engineered for constitutive SeXy expression. Intramuscular injection of transgenic AAVs into mice showed significant SeXy levels in the bloodstream. InXy and SeXy are highly sensitive, compact and robust reporter proteins, fully compatible with pre-existing marker genes and can be assayed in high-throughput formats using very small sample volumes.