Fluorescence-activated droplet sequencing (FAD-seq) directly provides sequences of screening hits in antibody discovery.

Droplet microfluidics has become a very powerful tool in high-throughput screening, including antibody discovery. Screens are usually carried out by physically sorting droplets hosting cells of the desired phenotype, breaking them, recovering the encapsulated cells, and sequencing the paired antibody light and heavy chain genes at the single-cell level. This series of multiple consecutive manipulation steps of rare screening hits is complex and challenging, resulting in a significant loss of clones with the desired phenotype or large fractions of cells with incomplete antibody information. Here, we present fluorescence-activated droplet sequencing, in which droplets showing the desired phenotype are selectively picoinjected with reagents for RT-PCR. Subsequently, light and heavy chain genes are natively paired, fused into a single-chain fragment variant format, and amplified before off-chip transfer and downstream nanopore sequencing. This workflow is sufficiently sensitive for obtaining different paired full-length antibody sequences from as little as five droplets, fulfilling the desired phenotype. Replacing physical sorting by specific sequencing overcomes a general bottleneck in droplet microfluidic screening and should be compatible with many more applications.

Design and construction of a microfluidics workstation for high-throughput multi-wavelength fluorescence and transmittance activated droplet analysis and sorting.

Droplet microfluidics has revolutionized quantitative high-throughput bioassays and screening, especially in the field of single-cell analysis where applications include cell characterization, antibody discovery and directed evolution. However, droplet microfluidic platforms capable of phenotypic, fluorescence-based readouts and sorting are still mostly found in specialized labs, because their setup is complex. Complementary to conventional FACS, microfluidic droplet sorters allow the screening of cell libraries for secreted factors, or even for the effects of secreted or surface-displayed factors on a second cell type. Furthermore, they also enable PCR-activated droplet sorting for the isolation of genetic material harboring specific markers. In this protocol, we provide a detailed step-by-step guide for the construction of a high-throughput droplet analyzer and sorter, which can be accomplished in ~45 working hours by nonspecialists. The resulting instrument is equipped with three lasers to excite the fluorophores in droplets and photosensors that acquire fluorescence signals in the blue (425-465 nm), green (505-545 nm) and red (580-630 nm) spectrum. This instrument also allows transmittance-activated droplet sorting by analyzing the brightfield light intensity transmitting through the droplets. The setup is validated by sorting droplets containing fluorescent beads at 200 Hz with 99.4% accuracy. We show results from an experiment where droplets hosting single cells were sorted on the basis of increased matrix metalloprotease activity as an application of our workstation in single-cell molecular biology, e.g., to analyze molecular determinants of cancer metastasis.

Technological and computational advances driving high-throughput oncology.

Engineering and computational advances have opened many new avenues in cancer research, particularly when being exploited in interdisciplinary approaches. For example, the combination of microfluidics, novel sequencing technologies, and computational analyses has been crucial to enable single-cell assays, giving a detailed picture of tumor heterogeneity for the very first time. In a similar way, these 'tech' disciplines have been elementary for generating large data sets in multidimensional cancer 'omics' approaches, cell-cell interaction screens, 3D tumor models, and tissue level analyses. In this review we summarize the most important technology and computational developments that have been or will be instrumental for transitioning classical cancer research to a large data-driven, high-throughput, high-content discipline across all biological scales.

Fluorogenic RNA-Based Biosensor to Sense the Glycolytic Flux in Mammalian Cells.

The visualization of metabolic flux in real time requires sensor molecules that transduce variations of metabolite concentrations into an appropriate output signal. In this regard, fluorogenic RNA-based biosensors are promising molecular tools as they fluoresce only upon binding to another molecule. However, to date no such sensor is available that enables the direct observation of key metabolites in mammalian cells. Toward this direction, we selected and characterized an RNA light-up sensor designed to respond to fructose 1,6-bisphosphate and applied it to probe glycolytic flux variation in mammal cells.

A dimerization-based fluorogenic dye-aptamer module for RNA imaging in live cells.

Live-cell imaging of RNA has remained a challenge because of the lack of naturally fluorescent RNAs. Recently developed RNA aptamers that can light-up small fluorogenic dyes could overcome this limitation, but they still suffer from poor brightness and photostability. Here, we propose the concept of a cell-permeable fluorogenic dimer of self-quenched sulforhodamine B dyes (Gemini-561) and the corresponding dimerized aptamer (o-Coral) that can drastically enhance performance of the current RNA imaging method. The improved brightness and photostability, together with high affinity of this complex, allowed direct fluorescence imaging in live mammalian cells of RNA polymerase III transcription products as well as messenger RNAs labeled with a single copy of the aptamer; that is, without tag multimerization. The developed fluorogenic module enables fast and sensitive detection of RNA inside live cells, while the proposed design concept opens the route to new generation of ultrabright RNA probes.

Structure and functional reselection of the Mango-III fluorogenic RNA aptamer.

Several turn-on RNA aptamers that activate small-molecule fluorophores have been selected in vitro. Among these, the ~30 nucleotide Mango-III is notable because it binds the thiazole orange derivative TO1-Biotin with high affinity and fluoresces brightly (quantum yield 0.55). Uniquely among related aptamers, Mango-III exhibits biphasic thermal melting, characteristic of molecules with tertiary structure. We report crystal structures of TO1-Biotin complexes of Mango-III, a structure-guided mutant Mango-III(A10U), and a functionally reselected mutant iMango-III. The structures reveal a globular architecture arising from an unprecedented pseudoknot-like connectivity between a G-quadruplex and an embedded non-canonical duplex. The fluorophore is restrained into a planar conformation by the G-quadruplex, a lone, long-range trans Watson-Crick pair (whose A10U mutation increases quantum yield to 0.66), and a pyrimidine perpendicular to the nucleobase planes of those motifs. The improved iMango-III and Mango-III(A10U) fluoresce ~50% brighter than enhanced green fluorescent protein, making them suitable tags for live cell RNA visualization.

Optimization of fluorogenic RNA-based biosensors using droplet-based microfluidic ultrahigh-throughput screening.

Biosensors are biological molecules able to detect and report the presence of a target molecule by the emission of a signal. Nucleic acids are particularly appealing for the design of such molecule since their great structural plasticity makes them able to specifically interact with a wide range of ligands and their structure can rearrange upon recognition to trigger a reporting event. A biosensor is typically made of three main domains: a sensing domain that is connected to a reporting domain via a communication module in charge of transmitting the sensing event through the molecule. The communication module is therefore an instrumental element of the sensor. This module is usually empirically developed through a trial-and-error strategy with the testing of only a few combinations judged relevant by the experimenter. In this work, we introduce a novel method combining the use of droplet-based microfluidics and next generation sequencing. This method allows to functionally characterize up to a million of different sequences in a single set of experiments and, by doing so, to exhaustively test every possible sequence permutations of the communication module. Here, we demonstrate the efficiency of the approach by isolating a set of optimized RNA biosensors able to sense theophylline and to convert this recognition into fluorescence emission.

Fluorogenic RNA Mango aptamers for imaging small non-coding RNAs in mammalian cells.

Despite having many key roles in cellular biology, directly imaging biologically important RNAs has been hindered by a lack of fluorescent tools equivalent to the fluorescent proteins available to study cellular proteins. Ideal RNA labelling systems must preserve biological function, have photophysical properties similar to existing fluorescent proteins, and be compatible with established live and fixed cell protein labelling strategies. Here, we report a microfluidics-based selection of three new high-affinity RNA Mango fluorogenic aptamers. Two of these are as bright or brighter than enhanced GFP when bound to TO1-Biotin. Furthermore, we show that the new Mangos can accurately image the subcellular localization of three small non-coding RNAs (5S, U6, and a box C/D scaRNA) in fixed and live mammalian cells. These new aptamers have many potential applications to study RNA function and dynamics both in vitro and in mammalian cells.

Crystal structure and fluorescence properties of the iSpinach aptamer in complex with DFHBI.

Fluorogenic RNA aptamers are short nucleic acids able to specifically interact with small molecules and strongly enhance their fluorescence upon complex formation. Among the different systems recently introduced, Spinach, an aptamer forming a fluorescent complex with the 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI), is one of the most promising. Using random mutagenesis and ultrahigh-throughput screening, we recently developed iSpinach, an improved version of the aptamer, endowed with an increased folding efficiency and thermal stability. iSpinach is a shorter version of Spinach, comprising five mutations for which the exact role has not yet been deciphered. In this work, we cocrystallized a reengineered version of iSpinach in complex with the DFHBI and solved the X-ray structure of the complex at 2 Å resolution. Only a few mutations were required to optimize iSpinach production and crystallization, underlying the good folding capacity of the molecule. The measured fluorescence half-lives in the crystal were 60% higher than in solution. Comparisons with structures previously reported for Spinach sheds some light on the possible function of the different beneficial mutations carried by iSpinach.