Fluorescence lifetime imaging in biosciences: technologies and applications

Label-free spatial analysis of free and enzyme-bound NAD(P)H in the presence of high concentrations of melanin

The analysis of autofluorescence, often regarded as undesired noise during the imaging of biological samples, allows label free, unbiased detection of NAD(P)H and melanin in native samples. Because both the emission and absorption spectra of these fluorophores overlap and they can hence not be differentiated using emission filters or with different excitation wavelengths, fluorescence lifetime imaging microscopy (FLIM) is used to differentiate between them. In the present paper the application of two-photon excitation microscopy is presented to investigate the autofluorescence of fungal spores. The model organism which was examined is Aspergillus ochraceus. Furthermore a strategy is developed which allows to quantitatively analyze the fluorescence lifetimes of melanin, free NAD(P)H and protein-bound NAD(P)H using forward convolution of a multiexponential decay function with the instrument response function (IRF) and subsequent fitting to the experimental fluorescence data. As a consequence proteins, which are able to bind NAD(P)H, are located with sub-cellular resolution. Furthermore a spatial differentiation of the fluorophores NAD(P)H and melanin inside the spores, is revealed.

Can new optical techniques for in vivo imaging and flow cytometry of the microcirculation benefit sickle cell disease research?

Intravital microscopy is a valuable tool for research into sickle cell disease with studies being carried out on transgenic mice and human volunteers. The method has helped to develop an explanation for sickle crises based on cell adhesion to the vascular endothelium followed by logjamming of rigid sickle cells and has stimulated much research into new treatments. In recent years there have been numerous new optical techniques developed for imaging the microcirculation and understanding the circulation of cells within the body, many of which have been further developed into in vivo flow cytometry techniques. This brief review highlights some of the progress made to date in the understanding of sickle cell disease using intravital microscopy. New techniques for imaging the microcirculation and their potential uses in understanding sickle cell disease are discussed.

Selective detection of NADPH oxidase in polymorphonuclear cells by means of NAD(P)H-based fluorescence lifetime imaging

NADPH oxidase (NOX2) is a multisubunit membrane-bound enzyme complex that, upon assembly in activated cells, catalyses the reduction of free oxygen to its superoxide anion, which further leads to reactive oxygen species (ROS) that are toxic to invading pathogens, for example, the fungus Aspergillus fumigatus. Polymorphonuclear cells (PMNs) employ both nonoxidative and oxidative mechanisms to clear this fungus from the lung. The oxidative mechanisms mainly depend on the proper assembly and function of NOX2. We identified for the first time the NAD(P)H-dependent enzymes involved in such oxidative mechanisms by means of biexponential NAD(P)H-fluorescence lifetime imaging (FLIM). A specific fluorescence lifetime of 3670±140 picoseconds as compared to 1870 picoseconds for NAD(P)H bound to mitochondrial enzymes could be associated with NADPH bound to oxidative enzymes in activated PMNs. Due to its predominance in PMNs and due to the use of selective activators and inhibitors, we strongly believe that this specific lifetime mainly originates from NOX2. Our experiments also revealed the high site specificity of the NOX2 assembly and, thus, of the ROS production as well as the dynamic nature of these phenomena. On the example of NADPH oxidase, we demonstrate the potential of NAD(P)H-based FLIM in selectively investigating enzymes during their cellular function.