Simon Ausländer

From gene switches to mammalian designer cells: present and future prospects

Nature has evolved a treasury of biological molecules that are logically connected to networks, enabling cells to maintain their functional integrity. Similar to electronic circuits, cells operate as information-processing systems that dynamically integrate and respond to distinct input signals. Synthetic biology aims to standardize and expand the natural toolbox of biological building blocks to engineer novel synthetic networks in living systems. Mammalian cells harboring integrated designer circuits could work as living biocomputers that execute predictable metabolic and therapeutic functions. This review presents design principles of mammalian gene circuits, highlights recent developments, and discusses future challenges and prospects.

A Synthetic Multifunctional Mammalian pH Sensor and CO2 Transgene-Control Device

All metabolic activities operate within a narrow pH range that is controlled by the CO2-bicarbonate buffering system. We hypothesized that pH could serve as surrogate signal to monitor and respond to the physiological state. By functionally rewiring the human proton-activated cell-surface receptor TDAG8 to chimeric promoters, we created a synthetic signaling cascade that precisely monitors extracellular pH within the physiological range. The synthetic pH sensor could be adjusted by organic acids as well as gaseous CO2 that shifts the CO2-bicarbonate balance toward hydrogen ions. This enabled the design of gas-programmable logic gates, provided remote control of cellular behavior inside microfluidic devices, and allowed for CO2-triggered production of biopharmaceuticals in standard bioreactors. When implanting cells containing the synthetic pH sensor linked to production of insulin into type 1 diabetic mice developing diabetic ketoacidosis, the prosthetic network automatically scored acidic pH and coordinated an insulin expression response that corrected ketoacidosis.

Smart medication through combination of synthetic biology and cell microencapsulation

Recent advances in the field of synthetic biology have led to the design of a new generation of complex, man-made biological networks that operate inside living cells in a desired manner. Key elements of these systems are often controllable genetic switches that are capable of processing therapeutic signals by sensing and responding to the environment. For biomedical applications, however, it is necessary to seal these engineered cells in order to protect them from the host immune system and enable straightforward removal after completion of the therapy. A promising and successful approach is the microencapsulation of defined cells into a semi-permeable and biocompatible microcapsule. Shielding from the external environment still allows exchange to occur on a molecular basis. Thus, the powerful combination of synthetic biology and microencapsulation has been opening the door to novel and innovative cell-based biomedical applications, such as smart implantable drug delivery systems. This review highlights recent developments in the overlap of these two areas, thereby presenting promising developments and perspectives for future treatment strategies.

Rational design of a small molecule-responsive intramer controlling transgene expression in mammalian cells

Aptamers binding proteins or small molecules have been shown to be versatile and powerful building blocks for the construction of artificial genetic switches. In this study, we present a novel aptamer-based construct regulating the Tet Off system in a tetracycline-independent manner thus achieving control of transgene expression. For this purpose, a TetR protein-inhibiting aptamer was engineered for use in mammalian cells, enabling the RNA-responsive control of the tetracycline-dependent transactivator (tTA). By rationally attaching the theophylline aptamer as a sensor, the inhibitory TetR aptamer and thus tTA activity became dependent on the ligand of the sensor aptamer. Addition of the small molecule theophylline resulted in enhanced binding to the corresponding protein in vitro and in inhibition of reporter gene expression in mammalian cell lines. By using aptamers as adaptors in order to control protein activity by a predetermined small molecule, we present a simple and straightforward approach for future applications in the field of Chemical Biology. Moreover, aptamer-based control of the widely used Tet system introduces a new layer of regulation thereby facilitating the construction of more complex gene networks.

Engineering a ribozyme cleavage-induced split fluorescent aptamer complementation assay

Abstract Hammerhead ribozymes are self-cleaving RNA molecules capable of regulating gene expression in living cells. Their cleavage performance is strongly influenced by intra-molecular loop–loop interactions, a feature not readily accessible through modern prediction algorithms. Ribozyme engineering and efficient implementation of ribozyme-based genetic switches requires detailed knowledge of individual self-cleavage performances. By rational design, we devised fluorescent aptamer-ribozyme RNA architectures that allow for the real-time measurement of ribozyme self-cleavage activity in vitro. The engineered nucleic acid molecules implement a split Spinach aptamer sequence that is made accessible for strand displacement upon ribozyme self-cleavage, thereby complementing the fluorescent Spinach aptamer. This fully RNA-based ribozyme performance assay correlates ribozyme cleavage activity with Spinach fluorescence to provide a rapid and straightforward technology for the validation of loop–loop interactions in hammerhead ribozymes.

Engineering Gene Circuits for Mammalian Cell–Based Applications

Synthetic gene switches are basic building blocks for the construction of complex gene circuits that transform mammalian cells into useful cell-based machines for next-generation biotechnological and biomedical applications. Ligand-responsive gene switches are cellular sensors that are able to process specific signals to generate gene product responses. Their involvement in complex gene circuits results in sophisticated circuit topologies that are reminiscent of electronics and that are capable of providing engineered cells with the ability to memorize events, oscillate protein production, and perform complex information-processing tasks. Microencapsulated mammalian cells that are engineered with closed-loop gene networks can be implanted into mice to sense disease-related input signals and to process this information to produce a custom, fine-tuned therapeutic response that rebalances animal metabolism. Progress in gene circuit design, in combination with recent breakthroughs in genome engineering, may result in tailored engineered mammalian cells with great potential for future cell-based therapies.

Synthetic RNA-based switches for mammalian gene expression control

Synthetic ribonucleic acid (RNA)-based gene switches control RNA functions in a ligand-responsive manner. Key building blocks are aptamers that specifically bind to small molecules or protein ligands. Engineering approaches often combine rational design and high-throughput screening to identify optimal connection sites or sequences. In this report, we discuss basic principles and emerging design strategies for the engineering of RNA-based gene switches in mammalian cells. Their small size compared with those of transcriptional gene switches, together with advancements in design strategies and performance, may bring RNA-based switches to the forefront of biomedical and biotechnological applications.

Designer exosomes produced by implanted cells intracerebrally deliver therapeutic cargo for Parkinson’s disease treatment

Exosomes are cell-derived nanovesicles (50–150 nm), which mediate intercellular communication, and are candidate therapeutic agents. However, inefficiency of exosomal message transfer, such as mRNA, and lack of methods to create designer exosomes have hampered their development into therapeutic interventions. Here, we report a set of EXOsomal transfer into cells (EXOtic) devices that enable efficient, customizable production of designer exosomes in engineered mammalian cells. These genetically encoded devices in exosome producer cells enhance exosome production, specific mRNA packaging, and delivery of the mRNA into the cytosol of target cells, enabling efficient cell-to-cell communication without the need to concentrate exosomes. Further, engineered producer cells implanted in living mice could consistently deliver cargo mRNA to the brain. Therapeutic catalase mRNA delivery by designer exosomes attenuated neurotoxicity and neuroinflammation in in vitro and in vivo models of Parkinson’s disease, indicating the potential usefulness of the EXOtic devices for RNA delivery-based therapeutic applications.Exosomes function as intercellular information transmitters and are candidates for delivery of therapeutic agents. Here the authors present EXOtic, a synthetic biology device for in-situ production of designer exosomes and demonstrate in vivo application in models of Parkinsons disease.

Engineered UV-A light-responsive gene expression system for measuring sun cream efficacy in mammalian cell culture.

Light-dependent gene regulation systems are advantageous as they allow for precise spatio-temporal control of target gene expression. In this paper, we present a novel UV-A and blue-light-inducible gene control system that is based on the light-dependent heterodimerization of the CRY2 and C1BN domains. Upon their interaction, a transcription factor is released from the cell membrane and initiates target gene expression. Capitalizing on that, sun cream UV-A protection properties were measured intracellularly.

Dynamic genome engineering in living cells

Engineered gene expression systems provide cells with a molecular memory of their past [Also see Research Article by Farzadfard and Lu] Living cells continuously measure, process, and store cellular and environmental information in response to specific signals. Bioengineers are now starting to use these systems to build customized genetic regulatory circuits that can control targeted biological processes. They are also using them to develop new regulatory modes and novel biochemical pathways (1). In combination with new genome-editing tools (2), this technology holds great promise for the development of biomedical and biotechnological applications of specially engineered “designer” cells. On page 825, Farzadfard and Lu (3) move us closer to this goal by constructing a cellular memory device that is based on a conditional gene-editing platform. In this way they have gained access to the enormous storage capacity of genomic DNA to record analog information.