High throughput FRET screening of the plasma membrane based on TIRFM

Understanding FRET as a research tool for cellular studies

Communication of molecular species through dynamic association and/or dissociation at various cellular sites governs biological functions. Understanding these physiological processes require delineation of molecular events occurring at the level of individual complexes in a living cell. Among the few non-invasive approaches with nanometer resolution are methods based on Förster Resonance Energy Transfer (FRET). FRET is effective at a distance of 1-10 nm which is equivalent to the size of macromolecules, thus providing an unprecedented level of detail on molecular interactions. The emergence of fluorescent proteins and SNAP- and CLIP- tag proteins provided FRET with the capability to monitor changes in a molecular complex in real-time making it possible to establish the functional significance of the studied molecules in a native environment. Now, FRET is widely used in biological sciences, including the field of proteomics, signal transduction, diagnostics and drug development to address questions almost unimaginable with biochemical methods and conventional microscopies. However, the underlying physics of FRET often scares biologists. Therefore, in this review, our goal is to introduce FRET to non-physicists in a lucid manner. We will also discuss our contributions to various FRET methodologies based on microscopy and flow cytometry, while describing its application for determining the molecular heterogeneity of the plasma membrane in various cell types.

Lysosomal rerouting of Hsp70 trafficking as a potential immune activating tool for targeting melanoma

Tumor specific cell surface localization and release of the stress inducible heat shock protein 70 (Hsp70) stimulate the immune system against cancer cells. A key immune stimulatory function of tumor-derived Hsp70 has been exemplified with the murine melanoma cell model, B16 overexpressing exogenous Hsp70. Despite the therapeutic potential mechanism of Hsp70 transport to the surface and release remained poorly understood. We investigated principles of Hsp70 trafficking in B16 melanoma cells with low and high level of Hsp70. In cells with low level of Hsp70 apparent trafficking of Hsp70 was mediated by endosomes. Excess Hsp70 triggered a series of changes such as a switch of Hsp70 trafficking from endosomes to lysosomes and a concomitant accumulation of Hsp70 in lysosomes. Moreover, lysosomal rerouting resulted in an elevated concentration of surface Hsp70 and enabled active release of Hsp70. In fact, hyperthermia, a clinically applicable approach triggered immediate active lysosomal release of soluble Hsp70 from cells with excess Hsp70. Furthermore, excess Hsp70 enabled targeting of internalized surface Hsp70 to lysosomes, allowing in turn heat-induced secretion of surface Hsp70. Altogether, we show that excess Hsp70 expressed in B16 melanoma cells diverts Hsp70 trafficking from endosomes to lysosomes, thereby supporting its surface localization and lysosomal release. Controlled excess-induced lysosomal rerouting and secretion of Hsp70 is proposed as a promising tool to stimulate anti-tumor immunity targeting melanoma.

Uniform total internal reflection fluorescence illumination enables live cell fluorescence resonance energy transfer microscopy

Fluorescence resonance energy transfer (FRET) microscopy is a powerful technique to quantify dynamic protein-protein interactions in live cells. Total internal reflection fluorescence (TIRF) microscopy can selectively excite molecules within about 150 nm of the glass-cell interface. Recently, these two approaches were combined to enable high-resolution FRET imaging on the adherent surface of living cells. Here, we show that interference fringing of the coherent laser excitation used in TIRF creates lateral heterogeneities that impair quantitative TIRF-FRET measurements. We overcome this limitation by using a two-dimensional scan head to rotate laser beams for donor and acceptor excitation around the back focal plane of a high numerical aperture objective. By setting different radii for the circles traced out by each laser in the back focal plane, the penetration depth was corrected for different wavelengths. These modifications quell spatial variations in illumination and permit calibration for quantitative TIRF-FRET microscopy. The capability of TIRF-FRET was demonstrated by imaging assembled cyan and yellow fluorescent protein-tagged HIV-Gag molecules in single virions on the surfaces of living cells. These interactions are shown to be distinct from crowding of HIV-Gag in lipid rafts.

Optical systems for single cell analyses

BACKGROUND

Data extracted from a population of cells represent the average response from all cells within the population. Even when the cells are genetically identical, cell-to-cell variations and genetic noise can make the cells respond in completely different ways. To understand the mechanisms behind the behaviour of a population, the cells must also be analysed on an individual basis.

OBJECTIVE

This review highlights the use of optical manipulation, microfluidics and advanced fluorescence imaging techniques for the acquisition of single cell data.

CONCLUSION

By implementation of these three techniques, it is possible to achieve a deeper insight into the principles underlying cellular functioning and a more thorough understanding of the phenomena often observed in cell populations, thus facilitating research in drug discovery.

TripleFRET measurements in flow cytometry

A frequently used method for viewing protein interactions and conformation, Förster (fluorescence) resonance energy transfer (FRET), has traditionally been restricted to two fluorophores. Lately, several methods have been introduced to expand FRET methods to three species. We present a method that allows the determination of FRET efficiency in three-dye systems on a flow cytometer. TripleFRET accurately reproduces energy transfer efficiency values measured in two-dye systems, and it can indicate the presence of trimeric complexes, which is not possible with conventional FRET methods. We also discuss the interpretation of energy transfer values obtained with tripleFRET in relation to spatial distribution of labeled molecules, specifically addressing the limitations of using total energy transfer to determine molecular distance.

A PDMS-based biochip with integrated sub-micrometre position control for TIRF microscopy of the apical cell membrane

A poly(dimethylsiloxane) (PDMS)-based biochip with an integrated pressure controlled positioning system with sub-micrometre precision was realized. The biochip was easy and cheap to manufacture and enabled positioning in a wet environment. It allowed the application of total internal reflection fluorescence (TIRF) microscopy at the dorsal cell membrane, which is not adhering to a support. Specifically, the chip enabled TIRF microscopy at the apical membrane of polarized epithelial cells. Thereby, the device allowed us for the first time to monitor individual fusion events of GPI-GFP bearing vesicles at the apical membrane in live Madin-Darby canine kidney II (MDCK II) cells. Moreover, a mapping of fusion sites became feasible and revealed that the whole apical membrane is fusion competent. In total, the biochip offers an all-in-one solution for apical TIRF microscopy and contributes a novel tool to study trafficking processes close to the apical plasma membrane in polarized epithelial cells.

Strength in numbers: effects of acceptor abundance on FRET efficiency

Fluorescence resonance energy transfer (FRET) is a strongly distance-dependent process between a donor and an acceptor molecule, which can be used for sensitive distance measurements and characterization of molecular interactions at the nanometer level. The original mathematical description of this process, however, is only valid for the interaction of one donor with one acceptor. This criterion is not always met, especially in biological systems, where multiple structures can interact simultaneously, often making distance estimations based on transfer efficiency values error-prone. Herein we investigate how the interaction of multiple acceptors and donors influences the transfer efficiency value in an intramolecular cellular FRET system by manipulating the fluorophore/protein ratio of the fluorophore-conjugated antibodies. We show that the labeling ratio of the acceptor has the largest influence on measured transfer efficiency and decreasing or increasing the acceptor labeling ratio can be utilized to manipulate the FRET response of the acceptor-donor pair and therefore is a tool for optimizing sensitivity of FRET measurements.

Evaluation of intensity-based ratiometric FRET in image cytometry--approaches and a software solution

The intensity-based ratiometric FRET (fluorescence resonance energy transfer) method is a powerful technique for following molecular interactions in living cells. Since it is not based on irreversibly destroying the donor or the acceptor fluorophores, the time course of changes in FRET efficiency values can be monitored by this method. ImageJ, a sophisticated software tool for many types of image processing allows users to extend it with programs for various purposes. Implementing intensity-based ratiometric FRET with ImageJ vastly enhances the applicability of the FRET method. We developed an efficient ImageJ plugin, RiFRET, which calculates FRET efficiency on a pixel-by-pixel basis from ratiometric FRET images. It allows the user to correct for channel cross-talk (bleed-through) and to calculate FRET from image stacks, i.e., from 3D data sets. Semiautomatic processing for larger datasets is also included in the program. Furthermore, several options for calibrating FRET efficiency calculations were tested and their applicability to various expression systems is discussed. Although the ratiometric FRET method is widely applied, our plugin is the first freely available software for evaluating such FRET data. The program is user friendly and provides reliable, standardized results.