A step beyond BRET: Fluorescence by Unbound Excitation from Luminescence (FUEL).

Fluorescence by Unbound Excitation from Luminescence (FUEL) is a radiative excitation-emission process that produces increased signal and contrast enhancement in vitro and in vivo. FUEL shares many of the same underlying principles as Bioluminescence Resonance Energy Transfer (BRET), yet greatly differs in the acceptable working distances between the luminescent source and the fluorescent entity. While BRET is effectively limited to a maximum of 2 times the Förster radius, commonly less than 14 nm, FUEL can occur at distances up to µm or even cm in the absence of an optical absorber. Here we expand upon the foundation and applicability of FUEL by reviewing the relevant principles behind the phenomenon and demonstrate its compatibility with a wide variety of fluorophores and fluorescent nanoparticles. Further, the utility of antibody-targeted FUEL is explored. The examples shown here provide evidence that FUEL can be utilized for applications where BRET is not possible, filling the spatial void that exists between BRET and traditional whole animal imaging.

In vitro and in vivo demonstrations of Fluorescence by Unbound Excitation from Luminescence (FUEL).

Bioluminescence imaging is a powerful technique that allows for deep-tissue analysis in living, intact organisms. However, in vivo optical imaging is compounded by difficulties due to light scattering and absorption. While light scattering is relatively difficult to overcome and compensate, light absorption by biological tissue is strongly dependent upon wavelength. For example, light absorption by mammalian tissue is highest in the blue-yellow part of the visible energy spectrum. Many natural bioluminescent molecules emit photonic energy in this range, thus in vivo optical detection of these molecules is primarily limited by absorption. This has driven efforts for probe development aimed to enhance photonic emission of red light that is absorbed much less by mammalian tissue using either direct genetic manipulation, and/or resonance energy transfer methods. Here we describe a recently identified alternative approach termed Fluorescence by Unbound Excitation from Luminescence (FUEL), where bioluminescent molecules are able to induce a fluorescent response from fluorescent nanoparticles through an epifluorescence mechanism, thereby significantly increasing both the total number of detectable photons as well as the number of red photons produced.

In vivo excitation of nanoparticles using luminescent bacteria.

The lux operon derived from Photorhabdus luminescens incorporated into bacterial genomes, elicits the production of biological chemiluminescence typically centered on 490 nm. The light-producing bacteria are widely used for in vivo bioluminescence imaging. However, in living samples, a common difficulty is the presence of blue-green absorbers such as hemoglobin. Here we report a characterization of fluorescence by unbound excitation from luminescence, a phenomenon that exploits radiating luminescence to excite nearby fluorophores by epifluorescence. We show that photons from bioluminescent bacteria radiate over mesoscopic distances and induce a red-shifted fluorescent emission from appropriate fluorophores in a manner distinct from bioluminescence resonance energy transfer. Our results characterizing fluorescence by unbound excitation from luminescence, both in vitro and in vivo, demonstrate how the resulting blue-to-red wavelength shift is both necessary and sufficient to yield contrast enhancement revealing mesoscopic proximity of luminescent and fluorescent probes in the context of living biological tissues.

Host cell entry by apicomplexa parasites requires actin polymerization in the host cell.

Apicomplexa are obligate intracellular parasites that actively invade host cells using their membrane-associated, actin-myosin motor. The current view is that host cell invasion by Apicomplexa requires the formation of a parasite-host cell junction, which has been termed the moving junction, but does not require the active participation of host actin. Using Toxoplasma gondii tachyzoites and Plasmodium berghei sporozoites, we show that host actin participates in parasite entry. Parasites induce the formation of a ring-shaped F-actin structure in the host cell at the parasite-cell junction, which remains stable during parasite entry. The Arp2/3 complex, an actin-nucleating factor, is recruited at the ring structure and is important for parasite entry. We propose that Apicomplexa invasion of host cells requires not only the parasite motor but also de novo polymerization of host actin at the entry site for anchoring the junction on which the parasite pulls to penetrate the host cell.

Chemotaxis of Entamoeba histolytica towards the pro-inflammatory cytokine TNF is based on PI3K signalling, cytoskeleton reorganization and the Galactose/N-acetylgalactosamine lectin activity.

Entamoeba histolytica is the protozoan parasite responsible for human amoebiasis. During invasive amoebiasis, migration is an essential process and it has previously been shown that the pro-inflammatory compound tumour necrosis factor (TNF) is produced and that it has a migratory effect on E. histolytica. This paper focuses on the analysis of parasite signalling and cytoskeleton changes leading to directional motility. TNF-induced signalling was PI3K-dependent and could lead to modifications in the polarization of certain cytoskeleton-related proteins. To analyse the effect of TNF signalling on gene expression, we used microarray analysis to screen for genes encoding proteins that were potentially important during chemotaxis towards TNF. Interestingly, we found that elements of the galactose/N-acetylgalactosamine lectin (Gal/GalNAc lectin) were upregulated during chemotaxis as well as genes encoding proteins involved in cytoskeleton dynamics. The alpha-actinin protein appeared to be an important candidate to link the Gal/GalNAc lectin to the cytoskeleton during chemotaxis signalling. Dominant negative parasites blocked for Gal/GalNAc lectin signalling were no longer able to chemotax towards TNF. These results have given us an insight on how E. histolytica changes its cytoskeleton dynamics during chemotaxis and revealed the capital role of PI3K and Gal/GalNAc lectin signalling in chemotaxis.

Bioinformatics and functional analysis of an Entamoeba histolytica mannosyltransferase necessary for parasite complement resistance and hepatical infection.

The glycosylphosphatidylinositol (GPI) moiety is one of the ways by which many cell surface proteins, such as Gal/GalNAc lectin and proteophosphoglycans (PPGs) attach to the surface of Entamoeba histolytica, the agent of human amoebiasis. It is believed that these GPI-anchored molecules are involved in parasite adhesion to cells, mucus and the extracellular matrix. We identified an E. histolytica homolog of PIG-M, which is a mannosyltransferase required for synthesis of GPI. The sequence and structural analysis led to the conclusion that EhPIG-M1 is composed of one signal peptide and 11 transmembrane domains with two large intra luminal loops, one of which contains the DXD motif, involved in the enzymatic catalysis and conserved in most glycosyltransferases. Expressing a fragment of the EhPIG-M1 encoding gene in antisense orientation generated parasite lines diminished in EhPIG-M1 levels; these lines displayed reduced GPI production, were highly sensitive to complement and were dramatically inhibited for amoebic abscess formation. The data suggest a role for GPI surface anchored molecules in the survival of E. histolytica during pathogenesis.

Imaging malaria sporozoites in the dermis of the mammalian host.

The initial phase of malaria infection is the pre-erythrocytic phase, which begins when parasites are injected by the mosquito into the dermis and ends when parasites are released from hepatocytes into the blood. We present here a protocol for the in vivo imaging of GFP-expressing sporozoites in the dermis of rodents, using the combination of a high-speed spinning-disk confocal microscope and a high-speed charge-coupled device (CCD) camera permitting rapid in vivo acquisitions. The steps of this protocol indicate how to infect mice through the bite of infected Anopheles stephensi mosquitoes, record the sporozoites' fate in the mouse ear and to present the data as maximum-fluorescence-intensity projections, time-lapse representations and movie clips. This protocol permits investigating the various aspects of sporozoite behavior in a quantitative manner, such as motility in the matrix, cell traversal, crossing the endothelial barrier of both blood and lymphatic vessels and intravascular gliding. Applied to genetically modified parasites and/or mice, these imaging techniques should be useful for studying the cellular and molecular bases of Plasmodium sporozoite infection in vivo.

In vivo imaging of malaria parasites in the murine liver.

The form of the malaria parasite inoculated by the mosquito, called the sporozoite, transforms inside the host liver into thousands of a new form of the parasite, called the merozoite, which infects erythrocytes. We present here a protocol to visualize in vivo the behavior of Plasmodium berghei parasites in the hepatic tissue of the murine host. The use of GFP-expressing parasites and a high-speed spinning disk confocal microscope allows for the acquisition of four-dimensional images, which provide a time lapse view of parasite displacement and development in tissue volumes. These data can be analyzed to give information on the early events of sporozoite penetration of the hepatic tissue, that is, sporozoite gliding in the liver sinusoids, crossing the sinusoidal barrier, gliding in the parenchyma and traversal of hepatocytes, and invasion of a final hepatocyte, as well as the terminal events of merosome and merozoite release from infected hepatocytes. Combined with the use of mice expressing fluorescent cell types or cell markers, the system will provide useful information not only on the primary infection process, but also on parasite interactions with the host immune cells in the liver.

Initiation of inflammation and cell death during liver abscess formation by Entamoeba histolytica depends on activity of the galactose/N-acetyl-D-galactosamine lectin.

The parasite Entamoeba histolytica colonizes the human intestine causing amoebic colitis and disseminates through the vascular route to form liver abscesses. The Gal/GalNAc lectin is an adhesion protein complex which sustains tissue invasion by E. histolytica. Disruption of the Gal/GalNAc lectin function in engineered parasites (HGL-2 trophozoites) changed the pathophysiology of hamster liver abscess formation. HGL-2 trophozoites produced numerous small inflammatory foci located in the vicinity of blood vessels. The low penetration of HGL-2 trophozoites into hepatic tissue was shown to be associated with weak attraction of neutrophils and macrophages to the infiltrated areas and absence of pro-inflammatory tumour necrosis factor, in contrast to wild type or control vector infections. The low host inflammatory response in HGL-2 infections correlated with a delay in apoptosis of hepatic cells, whereas apoptosis of endothelial cells was not detected. Triggering of apoptosis in both host cell types most likely has a central role in modulating inflammation, a major landmark in hepatic amoebiasis. These data highlight the key role of the Gal/GalNAc lectin in initiation of E. histolytica hepatic infection.

Human tumor necrosis factor is a chemoattractant for the parasite Entamoeba histolytica.

In an analysis of the molecular factors triggering amebiasis, we investigated the chemotaxis of Entamoeba histolytica toward tumor necrosis factor (TNF) in vitro, using quantitative imaging techniques. Our findings enabled us to propose a hitherto unknown role for TNF as a chemokinetic and chemoattractant agent for this parasite.

Regulation of the cerebellar inositol 1,4,5-trisphosphate receptor by univalent cations.

In the present study we investigated the effects of K and other univalent cations on [3H]InsP3 [[3H]Ins(1,4,5)P3] binding to sheep cerebellar microsomes. In equilibrium binding experiments performed over 4 s at pH 7.1 and 20 degrees C, the addition of K to the binding medium decreased the affinity and increased the total number of binding sites for InsP3 in a dose-dependent manner. At low InsP3 concentration (0.5 nM) these effects resulted in a biphasic dose-response curve, with maximal binding at about 75 mM K. In contrast, the dose-response curve calculated for InsP3 at the physiological concentration of 5 mM, was linear up to 200 mM K. Univalent inorganic cations stimulated [3H]InsP3 binding to various extents, with the following descending order of efficiency at 75 mM: Cs approximately Rb approximately K>Na>Li. The effect of K on InsP3R affinity was rapidly reversed upon cation removal. We were therefore also able to demonstrate that K increased Bmax (maximal specific binding) by pre-treating microsomes with K before measuring [3H]InsP3 binding in the absence of that cation. The increase in Bmax was reversible, but this reversal occurred less rapidly than the change in affinity. These results are consistent with a process by which K reversibly converted very low-affinity sites into sites with higher affinity, making them detectable in competitive binding experiments. They suggest that interconversion between these two affinity states constitutes the basis of a K-controlled regulatory mechanism for cerebellar InsP3R.

Supplementation with gliadin-combined plant superoxide dismutase extract promotes antioxidant defences and protects against oxidative stress.

The potential benefits to health of antioxidant enzymes supplied either through dietary intake or supplementation is still a matter of controversy. The development of dietary delivery systems using wheat gliadin biopolymers as a natural carrier represents a new alternative. Combination of antioxidant enzymes with this natural carrier not only delayed their degradation (i.e. the superoxide dismutase, SOD) during the gastrointestinal digestive process, but also promoted, in vivo, the cellular defences by strengthening the antioxidant status. The effects of supplementation for 28 days with a standardized melon SOD extract either combined (Glisodin) or not with gliadin, were evaluated on various oxidative-stress biomarkers. As already described there was no change either in superoxide dismutase, catalase or glutathione peroxidase activities in blood circulation or in the liver following non-protected SOD supplementation. However, animals supplemented with Glisodin showed a significant elevation in circulated antioxidant enzymes activities, correlated with an increased resistance of red blood cells to oxidative stress-induced hemolysis. In the presence of Sin-1, a chemical donor of peroxynitrites, mitochondria from hepatocytes regularly underwent membrane depolarization as the primary biological event of the apoptosis cascade. Hepatocytes isolated from animals supplemented with Glisodin presented a delayed depolarization response and an enhanced resistance to oxidative stress-induced apoptosis. It is concluded that supplementation with gliadin-combined standardized melon SOD extract (Glisodin) promoted the cellular antioxidant status and protected against oxidative stress-induced cell death.