Multimodal Control of Bacterial Gene Expression by Red and Blue Light.

By applying sensory photoreceptors, optogenetics realizes the light-dependent control of cellular events and state. Given reversibility, noninvasiveness, and exquisite spatiotemporal precision, optogenetic approaches enable innovative use cases in cell biology, synthetic biology, and biotechnology. In this chapter, we detail the implementation of the pREDusk, pREDawn, pCrepusculo, and pAurora optogenetic circuits for controlling bacterial gene expression by red and blue light, respectively. The protocols provided here guide the practical use and multiplexing of these circuits, thereby enabling graded protein production in bacteria at analytical and semi-preparative scales.

An RNA Motif That Enables Optozyme Control and Light-Dependent Gene Expression in Bacteria and Mammalian Cells.

The regulation of gene expression by light enables the versatile, spatiotemporal manipulation of biological function in bacterial and mammalian cells. Optoribogenetics extends this principle by molecular RNA devices acting on the RNA level whose functions are controlled by the photoinduced interaction of a light-oxygen-voltage photoreceptor with cognate RNA aptamers. Here light-responsive ribozymes, denoted optozymes, which undergo light-dependent self-cleavage and thereby control gene expression are described. This approach transcends existing aptamer-ribozyme chimera strategies that predominantly rely on aptamers binding to small molecules. The optozyme method thus stands to enable the graded, non-invasive, and spatiotemporally resolved control of gene expression. Optozymes are found efficient in bacteria and mammalian cells and usher in hitherto inaccessible optoribogenetic modalities with broad applicability in synthetic and systems biology.

Light-Dependent Control of Bacterial Expression at the mRNA Level.

Sensory photoreceptors mediate numerous light-dependent adaptations across organisms. In optogenetics, photoreceptors achieve the reversible, non-invasive, and spatiotemporally precise control by light of gene expression and other cellular processes. The light-oxygen-voltage receptor PAL binds to small RNA aptamers with sequence specificity upon blue-light illumination. By embedding the responsive aptamer in the ribosome-binding sequence of genes of interest, their expression can be downregulated by light. We developed the pCrepusculo and pAurora optogenetic systems that are based on PAL and allow to down- and upregulate, respectively, bacterial gene expression using blue light. Both systems are realized as compact, single plasmids that exhibit stringent blue-light responses with low basal activity and up to several 10-fold dynamic range. As PAL exerts light-dependent control at the RNA level, it can be combined with other optogenetic circuits that control transcription initiation. By integrating regulatory mechanisms operating at the DNA and mRNA levels, optogenetic circuits with emergent properties can thus be devised. As a case in point, the pEnumbra setup permits to upregulate gene expression under moderate blue light whereas strong blue light shuts off expression again. Beyond providing novel signal-responsive expression systems for diverse applications in biotechnology and synthetic biology, our work also illustrates how the light-dependent PAL-aptamer interaction can be harnessed for the control and interrogation of RNA-based processes.

Trypanosoma cruzi Malic Enzyme Is the Target for Sulfonamide Hits from the GSK Chagas Box.

Chagas disease, an infectious condition caused by Trypanosoma cruzi, lacks treatment with drugs with desired efficacy and safety profiles. To address this unmet medical need, a set of trypanocidal compounds were identified through a large multicenter phenotypic-screening initiative and assembled in the GSK Chagas Box. In the present work, we report the screening of the Chagas Box against T. cruzi malic enzymes (MEs) and the identification of three potent inhibitors of its cytosolic isoform (TcMEc). One of these compounds, TCMDC-143108 (1), came out as a nanomolar inhibitor of TcMEc, and 14 new derivatives were synthesized and tested for target inhibition and efficacy against the parasite. Moreover, we determined the crystallographic structures of TcMEc in complex with TCMDC-143108 (1) and six derivatives, revealing the allosteric inhibition site and the determinants of specificity. Our findings connect phenotypic hits from the Chagas Box to a relevant metabolic target in the parasite, providing data to foster new structure-activity guided hit optimization initiatives.

Identification of Specific Inhibitors of Trypanosoma cruzi Malic Enzyme Isoforms by Target-Based HTS.

Trypanosoma cruzi is the causative agent of Chagas disease. The lack of an efficient and safe treatment supports the research into novel metabolic targets, with the malic enzyme (ME) representing one such potential candidate. T. cruzi expresses a cytosolic (TcMEc) and a mitochondrial (TcMEm) ME isoform, with these activities functioning to generate NADPH, a key source of reducing equivalents that drives a range of anabolic and protective processes. To identify specific inhibitors that target TcMEs, two independent high-throughput screening strategies using a diversity library containing 30,000 compounds were employed. IC50 values of 262 molecules were determined for both TcMEs, as well as for three human ME isoforms, with the inhibitors clustered into six groups according to their chemical similarity. The most potent hits belonged to a sulfonamide group that specifically target TcMEc. Moreover, several selected inhibitors of both TcMEs showed a trypanocidal effect against the replicative forms of T. cruzi. The chemical diversity observed among those compounds that inhibit TcMEs activity emphasizes the druggability of these enzymes, with a sulfonamide-based subset of compounds readily able to block TcMEc function at a low nanomolar range.

Discovery of new uncompetitive inhibitors of glucose-6-phosphate dehydrogenase.

The enzyme glucose-6-phosphate dehydrogenase (G6PDH) catalyzes the first step of the oxidative branch of the pentose phosphate pathway, which provides cells with NADPH, an essential cofactor for many biosynthetic pathways and antioxidizing enzymes. In Trypanosoma cruzi, the G6PDH has being pursued as a relevant target for the development of new drugs against Chagas disease. At present, the best characterized inhibitors of T. cruzi G6PDH are steroidal halogenated compounds derivatives from the mammalian hormone precursor dehydroepiandrosterone, which indeed are also good inhibitors of the human homologue enzyme. The lack of target selectivity might result in hemolytic side effects due to partial inhibition of human G6PDH in red blood cells. Moreover, the treatment of Chagas patients with steroidal drugs might also cause undesired androgenic side effects. Aiming to identify of new chemical classes of T. cruzi G6PDH inhibitors, we performed a target-based high-throughput screen campaign against a commercial library of diverse compounds. Novel TcG6PDH inhibitors were identified among thienopyrimidine and quinazolinone derivatives. Preliminary structure activity relationships for the identified hits are presented, including structural features that contribute for selectivity toward the parasite enzyme. Our results indicate that quinazolinones are promising hits that should be considered for further optimization.

Recognition by the thyroid hormone receptor of canonical DNA response elements.

To shed more light on the molecular requirements for recognition of thyroid response elements (TREs) by thyroid receptors (TRs), we compared the specific aspects of DNA TRE recognition by different TR constructs. Using fluorescence anisotropy, we performed a detailed and hierarchical study of TR-TRE binding. This was done by comparing the binding affinities of three different TR constructs for four different TRE DNA elements, including palindromic sequences and direct repeats (F2, PAL, DR-1, and DR-4) as well as their interactions with nonspecific DNA sequences. The effect of MgCl(2) on suppressing of nonselective DNA binding to TR was also investigated. Furthermore, we determined the dissociation constants of the hTRbeta DBD (DNA binding domain) and hTRbeta DBD-LBD (DNA binding and ligand binding domains) for specific TREs. We found that a minimum DNA recognition peptide derived from DBD (H1TR) is sufficient for recognition and interaction with TREs, whereas scrambled DNA sequences were unrecognized. Additionally, we determined that the TR DBD binds to F2, PAL, and DR-4 with high affinity and similar K(d) values. The TR DBD-LBD recognizes all the tested TREs but binds preferentially to F2, with even higher affinity. Finally, our results demonstrate the important role played by LBDs in modulating TR-DNA binding.