Depolarized FRET (depolFRET) on the cell surface: FRET control by photoselection

Flow cytometry based-FRET: basics, novel developments and future perspectives

Förster resonance energy transfer (FRET) is a widespread technology used to analyze and quantify protein interactions in multiple settings. While FRET is traditionally measured by microscopy, flow cytometry based-FRET is becoming popular within the last decade and more commonly used. Flow cytometry based-FRET offers the possibility to assess FRET in a short time-frame in a high number of cells thereby allowing stringent and statistically robust quantification of FRET in multiple samples. Furthermore, established, simple and easy to implement gating strategies facilitate the adaptation of flow cytometry based-FRET measurements to most common flow cytometers. We here summarize the basics of flow cytometry based-FRET, highlight recent novel developments in this field and emphasize on exciting future perspectives.

Customized depolarization spatial patterns with dynamic retardance functions

In this work we demonstrate customized depolarization spatial patterns by imaging a dynamical time-dependent pixelated retarder. A proof-of-concept of the proposed method is presented, where a liquid-crystal spatial light modulator is used as a spatial retarder that emulates a controlled spatially variant depolarizing sample by addressing a time-dependent phase pattern. We apply an imaging Mueller polarimetric system based on a polarization camera to verify the effective depolarization effect. Experimental validation is provided by temporal integration on the detection system. The effective depolarizance results are fully described within a simple graphical approach which agrees with standard Mueller matrix decomposition methods. The potential of the method is discussed by means of three practical cases, which include non-reported depolarization spatial patterns, including exotic structures as a spirally shaped depolarization pattern.

Flow Cytometric FRET Analysis of Protein Interactions

In the past decades, investigation of protein-protein interactions in situ in living or intact cells has gained expanding importance as structure/function relationships proposed from bulk biochemistry and molecular modeling experiments required confirmation at the cellular level. Förster (fluorescence) resonance energy transfer (FRET)-based methods are excellent tools for determining proximity and supramolecular organization of biomolecules at the cell surface or inside the cell. This could well be the basis for the increasing popularity of FRET. In fact, the number of publications exploiting FRET has exploded since the turn of the millennium. Interestingly, most applications are microscope-based, and only a fraction employs flow cytometry, even though the latter offers great statistical power owed to the potentially huge number of individually measured cells. However, with the increased availability of multi-laser flow cytometers, strategies to obtain absolute FRET efficiencies can now be relatively facilely implemented. In this chapter, we intend to provide generally useable protocols for measuring FRET in flow cytometry. After a concise theoretical introduction, recipes are provided for successful labeling techniques and measurement approaches. The simple, quenching-based population-level measurement, the classic ratiometric, intensity-based technique providing cell-by-cell actual FRET efficiencies, and a more advanced version of the latter, allowing for cell-by-cell autofluorescence correction are described. An Excel macro pre-loaded with spectral data of the most commonly used fluorophores is also provided for easy calculation of average FRET efficiencies. Finally, points of caution are given to help design proper experiments and critically interpret the results.