Detection of antibodies is essential for the diagnosis of many diseases including infections, allergies, and autoimmune diseases. Current heterogeneous immunoassays require multiple time-consuming binding and washing steps, which limits their application in point-of-care diagnostics and high-throughput screening. Here, we report switchable reporter enzymes that allow simple colorimetric detection of antibodies directly in solution. Our approach is based on the antibody-induced disruption of an intramolecular interaction between TEM1 β-lactamase and its inhibitor protein BLIP. Using the HIV1-p17 antibody as an initial target, the interaction between enzyme and inhibitor was carefully tuned to yield a reporter enzyme whose activity increased 10-fold in the presence of pM antibody concentrations. Reporter enzymes for two other antibodies (HA-tag and Dengue virus type I) were obtained by simply replacing the epitope sequences. This new sensor design represents a modular and generic approach to construct antibody reporter enzymes without the cumbersome optimization required by previous engineering strategies.

Genetically encoded FRET-based sensor proteins have significantly contributed to our current understanding of the intracellular functions of Zn2+. However, the external excitation required for these fluorescent sensors can give rise to photobleaching and phototoxicity during long-term imaging, limits applications that suffer from autofluorescence and light scattering, and is not compatible with light-sensitive cells. For these applications, sensor proteins based on Bioluminescence Resonance Energy Transfer (BRET) would provide an attractive alternative. In this work, we used the bright and stable luciferase NanoLuc to create the first genetically encoded BRET sensors for measuring intracellular Zn2+. Using a new sensor approach, the NanoLuc domain was fused to the Cerulean donor domain of two previously developed FRET sensors, eCALWY and eZinCh-2. In addition to preserving the excellent Zn2+ affinity and specificity of their predecessors, these newly developed sensors enable both BRET- and FRET-based detection. While the dynamic range of the BRET signal for the eCALWY-based BLCALWY-1 sensor was limited by the presence of two competing BRET pathways, BRET/FRET sensors based on the eZinCh-2 scaffold (BLZinCh-1 and -2) yielded robust 25–30% changes in BRET ratio. In addition, introduction of a chromophore-silencing mutation resulted in a BRET-only sensor (BLZinCh-3) with increased BRET response (50%) and an unexpected 10-fold increase in Zn2+ affinity. The combination of robust ratiometric response, physiologically relevant Zn2+ affinities, and stable and bright luminescence signal offered by the BLZinCh sensors allowed monitoring of intracellular Zn2+ in plate-based assays as well as intracellular BRET-based imaging in single living cells in real time.

Magnesium has important structural, catalytic and signaling roles in cells, yet few tools exist to image this metal ion in real time and at subcellular resolution. Here we report the first genetically encoded sensor for Mg2+, MagFRET-1. This sensor is based on the high-affinity Mg2+ binding domain of human centrin 3 (HsCen3), which undergoes a transition from a molten-globular apo form to a compactly-folded Mg2+-bound state. Fusion of Cerulean and Citrine fluorescent domains to the ends of HsCen3, yielded MagFRET-1, which combines a physiologically relevant Mg2+ affinity (K d = 148 µM) with a 50% increase in emission ratio upon Mg2+ binding due to a change in FRET efficiency between Cerulean and Citrine. Mutations in the metal binding sites yielded MagFRET variants whose Mg2+ affinities were attenuated 2- to 100-fold relative to MagFRET-1, thus covering a broad range of Mg2+ concentrations. In situ experiments in HEK293 cells showed that MagFRET-1 can be targeted to the cytosol and the nucleus. Clear responses to changes in extracellular Mg2+ concentration were observed for MagFRET-1-expressing HEK293 cells when they were permeabilized with digitonin, whereas similar changes were not observed for intact cells. Although MagFRET-1 is also sensitive to Ca2+, this affinity is sufficiently attenuated (K d of 10 µM) to make the sensor insensitive to known Ca2+ stimuli in HEK293 cells. While the potential and limitations of the MagFRET sensors for intracellular Mg2+ imaging need to be further established, we expect that these genetically encoded and ratiometric fluorescent Mg2+ sensors could prove very useful in understanding intracellular Mg2+ homeostasis and signaling.

Detection of antibodies is essential for the diagnosis of many disease states, including infectious diseases, autoimmune diseases and allergies. Most current antibody detection assays involve multistep detection schemes in which molecular recognition and signal generation are separate processes. A well-known example is the enzyme-linked immunosorbent assay (ELISA), which combines high sensitivity and specificity with strong signal amplification. However, ELISA and other heterogeneous methods require multiple, time-consuming washing and incubation steps, which limits their applicability in point-of-care diagnostics and high-throughput applications. In recent years, several new antibody detection strategies have been developed in which antibody binding and signal generation are integrated within a single biomolecular reporter. These strategies aim to rival ELISA in terms of sensitivity and specificity, while decreasing the time and effort required to perform an assay. Here, we review recent developments in this field according to their mechanism of action and discuss their advantages and limitations.

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