Epac-based FRET sensors have been widely used for the detection of cAMP concentrations in living cells. Originally developed by us as well as others, we have since then reported several important optimizations that make these sensors favourite among many cell biologists. We here report cloning and characterization of our fourth generation of cAMP sensors, which feature outstanding photostability, dynamic range and signal-to-noise ratio. The design is based on mTurquoise2, currently the brightest and most bleaching-resistant donor, and a new acceptor cassette that consists of a tandem of two cp173Venus fluorophores. We also report variants with a single point mutation, Q270E, in the Epac moiety, which decreases the dissociation constant of cAMP from 9.5 to 4 μM, and thus increases the affinity ~ 2.5-fold. Finally, we also prepared and characterized dedicated variants with non-emitting (dark) acceptors for single-wavelength FLIM acquisition that display an exceptional near-doubling of fluorescence lifetime upon saturation of cAMP levels. We believe this generation of cAMP outperforms all other sensors and therefore recommend these sensors for all future studies.

FRET-based sensors for cyclic Adenosine Mono Phosphate (cAMP) have revolutionized the way in which this important intracellular messenger is studied. The currently prevailing sensors consist of the cAMP-binding protein Epac1, sandwiched between suitable donor- and acceptor fluorescent proteins (FPs). Through a conformational change in Epac1, alterations in cellular cAMP levels lead to a change in FRET that is most commonly detected by either Fluorescence Lifetime Imaging (FLIM) or by Sensitized Emission (SE), e.g., by simple ratio-imaging. We recently reported a range of different Epac-based cAMP sensors with high dynamic range and signal-to-noise ratio. We showed that constructs with cyan FP as donor are optimal for readout by SE, whereas other constructs with green FP donors appeared much more suited for FLIM detection. In this study, we present a new cAMP sensor, termed TEpacVV, which employs mTurquoise as donor. Spectrally very similar to CFP, mTurquoise has about doubled quantum efficiency and unlike CFP, its fluorescence decay is strictly single-exponential. We show that TEpacVV appears optimal for detection both by FLIM and SE, that it has outstanding FRET span and signal-to-noise ratio, and improved photostability. Hence, TEpacVV should become the cAMP sensor of choice for new experiments, both for FLIM and ratiometric detection.

Clathrin lattices at the plasma membrane coat both invaginated and flat regions forming clathrin-coated pits and clathrin plaques, respectively. The function and regulation of clathrin-coated pits in endocytosis are well understood but clathrin plaques remain enigmatic nanodomains. Here we use super-resolution microscopy, molecular genetics and cell biology to show that clathrin plaques contain the machinery for clathrin-mediated endocytosis and cell adhesion, and associate with both clathrin-coated pits and filamentous actin. We also find that actin polymerization promoted by N-WASP through the Arp2/3 complex is crucial for the regulation of plaques but not pits. Clathrin plaques oppose cell migration and undergo actin- and N-WASP-dependent disassembly upon activation of LPA receptor 1, but not EGF receptor. Most importantly, plaque disassembly correlates with the endocytosis of LPA receptor 1 and down-modulation of AKT activity. Thus, clathrin plaques serve as dynamic actin-controlled hubs for clathrin-mediated endocytosis and signalling that exhibit receptor specificity.

Single-chain alkylphospholipids, unlike conventional chemotherapeutic drugs, act on cell membranes to induce apoptosis in tumor cells. We tested four different alkylphospholipids, i.e., edelfosine, perifosine, erucylphosphocholine, and compound D-21805, as inducers of apoptosis in the mouse lymphoma cell line S49. We compared their mechanism of cellular entry and their potency to induce apoptosis through inhibition of de novo biosynthesis of phosphatidylcholine at the endoplasmic reticulum. Alkylphospholipid potency closely correlated with the degree of phosphatidylcholine synthesis inhibition in the order edelfosine > D-21805 > erucylphosphocholine > perifosine. In all cases, exogenous lysophosphatidylcholine, an alternative source for cellular phosphatidylcholine production, could partly rescue cells from alkylphospholipid-induced apoptosis, suggesting that phosphatidylcholine biosynthesis is a direct target for apoptosis induction. Cellular uptake of each alkylphospholipid was dependent on lipid rafts because pretreatment of cells with the raft-disrupting agents, methyl-B-cyclodextrin, filipin, or bacterial sphingomyelinase, reduced alkylphospholipid uptake and/or apoptosis induction and alleviated the inhibition of phosphatidylcholine synthesis. Uptake of all alkylphospholipids was inhibited by small interfering RNA (siRNA) -mediated blockage of sphingomyelin synthase (SMS1), which was previously shown to block raft-dependent endocytosis. Similar to edelfosine, perifosine accumulated in (isolated) lipid rafts independent on raft sphingomyelin content per se. However, perifosine was more susceptible than edelfosine to back-extraction by fatty acid-free serum albumin, suggesting a more peripheral location in the cell due to less effective internalization. Overall, our results suggest that lipid rafts are critical membrane portals for cellular entry of alkylphospholipids depending on SMS1 activity and, therefore, are potential targets for alkylphospholipid anticancer therapy. [Mol Cancer Ther 2007;6(8):2337 -45]

The ALP (alkyl-lysophospholipid) edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine; Et-18-OCH3) induces apoptosis in S49 mouse lymphoma cells. To this end, ALP is internalized by lipid raft-dependent endocytosis and inhibits phosphatidylcholine synthesis. A variant cell-line, S49AR, which is resistant to ALP, was shown previously to be unable to internalize ALP via this lipid raft pathway. The reason for this uptake failure is not understood. In the present study, we show that S49AR cells are unable to synthesize SM (sphingomyelin) due to down-regulated SMS1 (SM synthase 1) expression. In parental S49 cells, resistance to ALP could be mimicked by small interfering RNA-induced SMS1 suppression, resulting in SM deficiency and blockage of raft-dependent internalization of ALP and induction of apoptosis. Similar results were obtained by treatment of the cells with myriocin/ISP-1, an inhibitor of general sphingolipid synthesis, or with U18666A, a cholesterol homoeostasis perturbing agent. U18666A is known to inhibit Niemann-Pick C1 protein-dependent vesicular transport of cholesterol from endosomal compartments to the trans-Golgi network and the plasma membrane. U18666A reduced cholesterol partitioning in detergent-resistant lipid rafts and inhibited SM synthesis in S49 cells, causing ALP resistance similar to that observed in S49AR cells. The results are explained by the strong physical interaction between (newly synthesized) SM and available cholesterol at the Golgi, where they facilitate lipid raft formation. We propose that ALP internalization by lipid-raft-dependent endocytosis represents the retrograde route of a constitutive SMS1- and lipid-raft-dependent membrane vesicular recycling process.