The best of both worlds: reaping the benefits from mammalian and bacterial therapeutic circuits Daniel Bojar 1 and Martin Fussenegger 1,2 Synthetic biology has revolutionized the field of biology in the last two decades. By taking apart natural systems and recombining engineered parts in novel constellations, it has not only unlocked a staggering variety of biological control mechanisms but it has also created a panoply of biomedical achievements, such as innovative diagnostics and therapies. The most common mode of action in the field of synthetic biology is mediated by synthetic gene circuits assembled in a systematic and rational manner. This review covers the most recent therapeutic gene circuits implemented in mammalian and bacterial cells designed for the diagnosis and therapy of an extensive array of diseases. Highlighting new tools for therapeutic gene circuits, we describe a future that holds a plethora of potentialities for the medicine of tomorrow. Addresses 1 ETH Zurich, Department of Biosystems Science and Engineering, Mattenstrasse 26, 4058 Basel, Switzerland 2 Faculty of Life Science, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland Corresponding author: Fussenegger, Martin (martin.fussenegger@bsse.ethz.ch) Current Opinion in Chemical Biology 2016, 34:11–19 This review comes from a themed issue on Synthetic biology Edited by Gert Bange and Thorsten Waldminghaus http://dx.doi.org/10.1016/j.cbpa.2016.05.012 1367-5931/# 2016 Elsevier Ltd. All rights reserved. Introduction Synthetic biology, the science of designing novel, biolog- ical systems with the help of engineering [1], is currently at a crossroads. Being on the cusp of the transition from proof-of-concept experiments to ‘real-world’ applications [2], synthetic biology and its genetic circuits now need more precision and robustness than ever [3,4]. Starting from simple modules, such as toggle switches [5] and oscillators [6], the basic components created by research- ers over the years have been recombined extensively to yield complex and innovative genetic circuits that can perform intricate logic operations [7]. One of the main fields of application relevant for syn- thetic biology is biomedical research [8], including ‘smart’ diagnostics and therapies in the form of complex genetic circuits [9–11]. These designed therapies have not only been developed in mammalian cells but also in bacteria, such as the biological model organism Escherichia coli (E. coli) [12,13], which are much simpler and therefore easier to engineer. Utilizing the full potential of synthetic biology, exciting proof-of-concept gene circuits have been assembled in the past, including RNA interference (RNAi) by cancer-invading bacteria [14] and mammalian cells that can process several inputs in order to identify and kill specific cancer cells [15]. This review focuses on the latest, state-of-the-art synthetic gene circuits and will first discuss the new and promising field of ex vivo diagnostics, followed by an in-depth discussion of circuits designed as therapeutics for human diseases in both mammalian as well as in bacterial cells and concluding with an outlook into the future of the synergy between synthetic biology and medicine. The emerging field of ex vivo diagnostics One of the major issues of the transitions from animal models to clinical trials is the limited probability of success [16]. As Seok et al. showed, mice and humans differ substantially in their response to inflammatory stress, por- tending grave differences in the effect of genetic circuits designed to alleviate these stresses [17]. One way to address this conundrum is to replace animal models with ex vivo diagnostics, which have been shown to exhibit higher sensitivity in release assays of IFN-g [18]. One area that suffers from this limited correlation between the results in model systems and those in the human body is the in vitro diagnosis of allergies based on the measure- ment of free immunoglobulin E (IgE) antibodies [19,20]. Here, Ausla ¨nder et al. recently reported the design and implementation of an ex vivo allergy profiler [21  ]. This device detects the characteristic increase in histamine secreted by basophil granulocytes upon addition of the respective allergen (Figure 1a). By rewiring a reporter gene to a histamine-responsive promoter, the authors demon- strated the reliable and sensitive functionality of their system, which was a significant improvement on the above mentioned IgE quantification and the classic, though impractical, skin-prick test [22]. Upcoming bacteriophage-based diagnostics tools are a valuable addition to the existing toolbox, mainly because of their ability to rapidly and precisely identify pathogen- ic bacteria. Two recent studies by Alcaine et al. elucidated this trend very clearly. In the first study, engineered T7 Available online at www.sciencedirect.com ScienceDirect www.sciencedirect.com Current Opinion in Chemical Biology 2016, 34:11–19