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Förster resonance energy transfer (FRET)-based sensors have been powerful tools in cell biologists’ toolkit for decades. Informed by fundamental understanding of fluorescent proteins, protein-protein interactions, and the structural biology of reporter components, researchers have been able to employ creative design approaches to build sensors that are uniquely capable of probing a wide range of phenomena in living cells including visualization of localized calcium signaling, sub-cellular activity gradients, and tension generation to name but a few. While FRET sensors have significantly impacted many fields, one must also be cognizant of the limitations to conventional, intensity-based FRET measurements stemming from variation in probe concentration, sensitivity to photo-bleaching, and bleed-through between the FRET fluorophores. Fluorescence lifetime imaging microscopy (FLIM) largely overcomes the limitations of intensity based FRET measurements. In general terms, FLIM measures the time, which for the reporters described in this chapter is nanoseconds (ns), between photon absorption and emission by a fluorophore. When FLIM is applied to FRET sensors (FLIM-FRET), measurement of the donor fluorophore lifetime provides valuable information such as FRET efficiency and the percentage of reporters engaged in FRET. This chapter introduces fundamental principles of FLIM-FRET towards informing the practical application of the technique and, using two established FRET reporters as proofs of concept, outlines how to use a commercially available FLIM system.

Lipid rafts (LRs) are membrane realms characterized by high concentrations of cholesterol and sphingolipids. Often, they are portrayed as scaffolds on which many different signaling molecules can assemble their cascades. The idea of rafts as scaffolds is garnering significant attention as the consequences of LR disruption have been shown to be manifest in multiple signaling pathways. In this study, LRs in the brain of the twitcher (TWI) mouse, a bona-fide model for infant variants of human globoid cell leukodystrophy or Krabbe disease, were investigated. This mouse has deficient activity of GALC (β-galactosylceramidase) that leads to a progressive accumulation of some galactosyl-sphingolipids in the brain. We hypothesized that the accumulation of psychosine (galactosyl-sphingosine) in the TWI CNS may result in the disruption of rafts in different cell populations such as neurons and oligodendrocytes, both cellular targets during disease. In this communication, we demonstrate that psychosine specifically accumulates in LRs in the TWI brain and sciatic nerve and in samples from brains of human Krabbe patients. It is also shown that this accumulation is accompanied by an increase in cholesterol in these domains and changes in the distribution of the LR markers flotillin-2 and caveolin-1. Finally, we show evidence that this phenomenon may provide a mechanism by which psychosine can exert its known inhibitory effect on protein kinase C. This study provides a previously undescribed biophysical aspect for the mechanism of pathogenesis in Krabbe disease.

Mitochondria have roles in many cellular processes, from energy metabolism and calcium homeostasis to control of cellular lifespan and programmed cell death. These processes affect and are affected by the redox status of and ATP production by mitochondria. Here, we describe the use of two ratiometric, genetically encoded biosensors that can detect mitochondrial redox state and ATP levels at subcellular resolution in living yeast cells. Mitochondrial redox state is measured using redox-sensitive Green Fluorescent Protein (roGFP) that is targeted to the mitochondrial matrix. Mito-roGFP contains cysteines at positions 147 and 204 of GFP, which undergo reversible and environment-dependent oxidation and reduction, which in turn alter the excitation spectrum of the protein. MitGO-ATeam is a Forster resonance energy transfer (FRET) probe in which the e subunit of the FoF1-ATP synthase is sandwiched between FRET donor and acceptor fluorescent proteins. Binding of ATP to the e subunit results in conformation changes in the protein that bring the FRET donor and acceptor in close proximity and allow for fluorescence resonance energy transfer from the donor to acceptor.