Combining TIR and FRET in Molecular Test Systems

Lasers in Live Cell Microscopy

Due to their unique properties—coherent radiation, diffraction limited focusing, low spectral bandwidth and in many cases short light pulses—lasers play an increasing role in live cell microscopy. Lasers are indispensable tools in 3D microscopy, e.g., confocal, light sheet or total internal reflection microscopy, as well as in super-resolution microscopy using wide-field or confocal methods. Further techniques, e.g., spectral imaging or fluorescence lifetime imaging (FLIM) often depend on the well-defined spectral or temporal properties of lasers. Furthermore, laser microbeams are used increasingly for optical tweezers or micromanipulation of cells. Three exemplary laser applications in live cell biology are outlined. They include fluorescence diagnosis, in particular in combination with Förster Resonance Energy Transfer (FRET), photodynamic therapy as well as laser-assisted optoporation, and demonstrate the potential of lasers in cell biology and—more generally—in biomedicine.

Probing Small Distances in Live Cell Imaging

For probing small distances in living cells, methods of super-resolution microscopy and molecular sensing are reported. A main requirement is low light exposure to maintain cell viability and to avoid photobleaching of relevant fluorophores. From this point of view, Structured Illumination Microscopy (SIM), Axial Tomography, Total Internal Reflection Fluorescence Microscopy (TIRFM) and often a combination of these methods are used. To show the high potential of these techniques, measurements on cell-substrate topology as well as on intracellular translocation of the glucose transporter GLUT4 are described. In addition, molecular parameters can be deduced from spectral data, fluorescence lifetimes or non-radiative energy transfer (FRET) between a donor and an acceptor molecule. As an example, FRET between the epidermal growth factor receptor (EGFR) and the growth factor receptor-bound protein 2 (Grb2) is described. Since this interaction, as well as further processes of cellular signaling (e.g., translocation of GLUT4) are sensitive to stimulation by pharmaceutical agents, methods (e.g., TIRFM) are transferred from a fluorescence microscope to a multi-well reader system for simultaneous detection of large cell populations.

Fluorescence Microscopy-Based Quantitation of GLUT4 Translocation: High Throughput or High Content?

Due to the global rise of type 2 diabetes mellitus (T2DM) in combination with insulin resistance, novel compounds to efficiently treat this pandemic disease are needed. Screening for compounds that induce the translocation of glucose transporter 4 (GLUT4) from the intracellular compartments to the plasma membrane in insulin-sensitive tissues is an innovative strategy. Here, we compared the applicability of three fluorescence microscopy-based assays optimized for the quantitation of GLUT4 translocation in simple cell systems. An objective-type scanning total internal reflection fluorescence (TIRF) microscopy approach was shown to have high sensitivity but only moderate throughput. Therefore, we implemented a prism-type TIR reader for the simultaneous analysis of large cell populations grown in adapted microtiter plates. This approach was found to be high throughput and have sufficient sensitivity for the characterization of insulin mimetic compounds in live cells. Finally, we applied confocal microscopy to giant plasma membrane vesicles (GPMVs) formed from GLUT4-expressing cells. While this assay has only limited throughput, it offers the advantage of being less sensitive to insulin mimetic compounds with high autofluorescence. In summary, the combined implementation of different fluorescence microscopy-based approaches enables the quantitation of GLUT4 translocation with high throughput and high content.

Förster resonance energy transfer-what can we learn and how can we use it?

The present manuscript gives a short overview on Förster Resonance Energy Transfer (FRET) of molecular interactions in the nanometre range. First, its principle is described and a short historical overview is given. Subsequently some principal methods and applications of FRET sensing and imaging are described (with some emphasis on fluorescence lifetime imaging, FLIM), and finally two innovative FRET techniques are presented in more detail. Applications are focused on measurements of living cells.