Triple Fluorescence Anisotropy Reporter Imaging in Living Cells

Fluorescence Anisotropy as a Self-Referencing Readout for Ion-Selective Sensing and Imaging Using Homo-FRET between Chromoionophores

Fluorescence anisotropy has been widely used in developing biosensors and immunoassays, by virtue of the self-reference and environment-sensitive properties. However, fluorescence anisotropic chemical sensors on inorganic ions are limited by the total anisotropy change. To this end, we demonstrate here fluorescence anisotropic ion-selective optodes based on the homo-FRET (Förster resonance energy transfer) of the crowded chromoionophores. The conventional fluorescence on-off mode is transformed into the anisotropic mode. Variation of the target ion concentration changes the inter-chromoionophore distance in the organic sensing phase, leading to different extents of homo-FRET and steady-state anisotropy. A theoretical model is developed by coupling homo-FRET and anisotropy. Anisotropic detections of pH, K⁺, and Na⁺ are demonstrated as examples based on the different ionophores for H⁺, K⁺, and Na⁺, respectively. Further, fluorescence imaging of the nano-optodes, plasticized poly(vinyl chloride) sensing films, and live cells are demonstrated using a homemade fluorescence anisotropic imaging platform. The results form the basis of an ion-selective analytical method operating in the fluorescence anisotropic mode, which could potentially be applied to other fluorescence on-off probes based on homo-FRET.

Comparative Ca2+ channel contributions to intracellular Ca2+ levels in the circadian clock

Circadian rhythms in mammals are coordinated by the central clock in the brain, located in the suprachiasmatic nucleus (SCN). Multiple molecular and cellular signals display a circadian variation within SCN neurons, including intracellular Ca2+, but the mechanisms are not definitively established. SCN cytosolic Ca2+ levels exhibit a peak during the day, when both action potential firing and Ca2+ channel activity are increased, and are decreased at night, correlating with a reduction in firing rate. In this study, we employ a single-color fluorescence anisotropy reporter (FLARE), Venus FLARE-Cameleon, and polarization inverted selective-plane illumination microscopy to measure rhythmic changes in cytosolic Ca2+ in SCN neurons. Using this technique, the Ca2+ channel subtypes contributing to intracellular Ca2+ at the peak and trough of the circadian cycle were assessed using a pharmacological approach with Ca2+ channel inhibitors. Peak (218 ± 16 nM) and trough (172 ± 13 nM) Ca2+ levels were quantified, indicating a 1.3-fold circadian variance in Ca2+ concentration. Inhibition of ryanodine-receptor-mediated Ca2+ release produced a larger relative decrease in cytosolic Ca2+ at both time points compared to voltage-gated Ca2+channels. These results support the hypothesis that circadian Ca2+ rhythms in SCN neurons are predominantly driven by intracellular Ca2+ channels, although not exclusively so. The study provides a foundation for future experiments to probe Ca2+ signaling in a dynamic biological context using FLAREs.

PIE-FLIM Measurements of Two Different FRET-Based Biosensor Activities in the Same Living Cells

We report the use of pulsed interleaved excitation (PIE)-fluorescence lifetime imaging microscopy (FLIM) to measure the activities of two different biosensor probes simultaneously in single living cells. Many genetically encoded biosensors rely on the measurement of Förster resonance energy transfer (FRET) to detect changes in biosensor conformation that accompany the targeted cell signaling event. One of the most robust ways of quantifying FRET is to measure changes in the fluorescence lifetime of the donor fluorophore using FLIM. The study of complex signaling networks in living cells demands the ability to track more than one of these cellular events at the same time. Here, we demonstrate how PIE-FLIM can separate and quantify the signals from different FRET-based biosensors to simultaneously measure changes in the activity of two cell signaling pathways in the same living cells in tissues. The imaging system described here uses selectable laser wavelengths and synchronized detection gating that can be tailored and optimized for each FRET pair. Proof-of-principle studies showing simultaneous measurement of cytosolic calcium and protein kinase A activity are shown, but the PIE-FLIM approach is broadly applicable to other signaling pathways.

Single-color, ratiometric biosensors for detecting signaling activities in live cells

Genetically encoded fluorescent biosensors have revolutionized the study of signal transduction by enabling the real-time tracking of signaling activities in live cells. Investigating the interaction between signaling networks has become increasingly important to understanding complex cellular phenomena, necessitating an update of the biosensor toolkit to allow monitoring and perturbing multiple activities simultaneously in the same cell. We therefore developed a new class of fluorescent biosensors based on homo-FRET, deemed FLuorescence Anisotropy REporters (FLAREs), which combine the multiplexing ability of single-color sensors with a quantitative, ratiometric readout. Using an array of color variants, we were able to demonstrate multiplexed imaging of three activity reporters simultaneously in the same cell. We further demonstrate the compatibility of FLAREs for use with optogenetic tools as well as intravital two-photon imaging.