Fast label-free cytoskeletal network imaging in living mammalian cells.

We present a full-field technique that allows label-free cytoskeletal network imaging inside living cells. This noninvasive technique allows monitoring of the cytoskeleton dynamics as well as interactions between the latter and organelles on any timescale. It is based on high-resolution quantitative phase imaging (modified Quadriwave lateral shearing interferometry) and can be directly implemented using any optical microscope without modification. We demonstrate the capability of our setup on fixed and living Chinese hamster ovary cells, showing the cytoskeleton dynamics in lamellipodia during protrusion and mitochondria displacement along the cytoskeletal network. In addition, using the quantitative function of the technique, along with simulation tools, we determined the refractive index of a single tubulin microtubule to be ntubu=2.36±0.6 at λ=527 nm.

Patterned hydrophobic domains in the exopolymer matrix of Shewanella oneidensis MR-1 biofilms.

Water-dispersible amphiphilic surface-engineered quantum dots (QDs) were found to be strongly accumulated within discrete zones of the exopolymer network of Shewanella oneidensis MR-1 biofilms, but not on the cell surfaces. These microdomains showed a patterned distribution in the exopolymer matrix, which led to a restricted diffusion of the amphiphilic nanoparticles.

Local cholesterol increase triggers amyloid precursor protein-Bace1 clustering in lipid rafts and rapid endocytosis.

Amyloid peptide (Aß) is generated by sequential cleavage of the amyloid precursor protein (APP) by ß-secretase (Bace1) and γ-secretase. Aß production increases after plasma membrane cholesterol loading through unknown mechanisms. To determine how APP-Bace1 proximity affects this phenomenon, we developed a fluorescence lifetime imaging microscopy-Förster resonance energy transfer (FLIM-FRET) technique for visualization of these molecules either by epifluorescence or at the plasma membrane only using total internal reflection fluorescence. Further, we used fluorescence correlation spectroscopy to determine the lipid rafts partition of APP-yellow fluorescent protein (YFP) and Bace1-green fluorescent protein (GFP) molecules at the plasma membrane of neurons. We show that less than 10 min after cholesterol exposure, Bace1-GFP/APP-mCherry proximity increases selectively at the membrane and APP relocalizes to raft domains, preceded by rapid endocytosis. After longer cholesterol exposures, APP and Bace1 are found in proximity intracellularly. We demonstrate that cholesterol loading does not increase Aß production by having a direct impact on Bace1 catalytic activity but rather by altering the accessibility of Bace1 to its substrate, APP. This change in accessibility is mediated by clustering in lipid rafts, followed by rapid endocytosis.

Optimizing photodynamic therapy by liposomal formulation of the photosensitizer pyropheophorbide-a methyl ester: in vitro and ex vivo comparative biophysical investigations in a colon carcinoma cell line.

Photodynamic therapy (PDT), induced by a photosensitizer (PS) encapsulated in a nanostructure, has emerged as an appropriate treatment to cure a multitude of oncological and non-oncological diseases. Pyropheophorbide-a methyl ester (PPME) is a second-generation PS tested in PDT, and is a potential candidate for future clinical applications. The present study, carried out in a human colon carcinoma cell line (HCT-116), evaluates the improvement resulting from a liposomal formulation of PPME versus free-PPME. Absorption and fluorescence spectroscopies, fluorescence lifetime measurements, subcellular imaging and co-localization analysis have been performed in order to analyze the properties of PPME for each delivery mode. The benefit of drug encapsulation in DMPC-liposomes is clear from our experiments, with a 5-fold higher intracellular drug delivery than that observed with free-PPME at similar concentrations. The reactive oxygen species (ROSs) produced after PPME-mediated photosensitization have been identified and quantified by using electron spin resonance spectroscopy. Our results demonstrate that PPME-PDT-mediated ROSs are composed of singlet oxygen and a hydroxyl radical. The small amounts of PPME inside mitochondria, as revealed by fluorescence co-localization analysis, could maybe explain the very low apoptotic cell death measured in HCT-116 cells.

Membrane Dynamics and Organization of the Phagocyte NADPH Oxidase in PLB-985 Cells.

Neutrophils are the first cells recruited at the site of infections, where they phagocytose the pathogens. Inside the phagosome, pathogens are killed by proteolytic enzymes that are delivered to the phagosome following granule fusion, and by reactive oxygen species (ROS) produced by the NADPH oxidase. The NADPH oxidase complex comprises membrane proteins (NOX2 and p22phox), cytoplasmic subunits (p67phox, p47phox, and p40phox) and the small GTPase Rac. These subunits assemble at the phagosomal membrane upon phagocytosis. In resting neutrophils the catalytic subunit NOX2 is mainly present at the plasma membrane and in the specific granules. We show here that NOX2 is also present in early and recycling endosomes in human neutrophils and in the neutrophil-like cell line PLB-985 expressing GFP-NOX2. In the latter cells, an increase in NOX2 at the phagosomal membrane was detected by live-imaging after phagosome closure, probably due to fusion of endosomes with the phagosome. Using super-resolution microscopy in PLB-985 WT cells, we observed that NOX2 forms discrete clusters in the plasma membrane. The number of clusters increased during frustrated phagocytosis. In PLB-985NCF1ΔGT cells that lack p47phox and do not assemble a functional NADPH oxidase, the number of clusters remained stable during phagocytosis. Our data suggest a role for p47phox and possibly ROS production in NOX2 recruitment at the phagosome.

Time-gated total internal reflection fluorescence microscopy with a supercontinuum excitation source.

We present the instrumental development of a versatile total internal reflection fluorescence lifetime imaging microscopy setup illuminated by a supercontinuum laser source. It enables performing wide-field fluorescence lifetime imaging with subwavelength axial resolution for a large range of fluorophores. The short overall acquisition time and the axial resolution are well suited for dynamic neurobiological applications.

Ex vivo fluorescence imaging of normal and malignant urothelial cells to enhance early diagnosis.

Urinary cytology is a noninvasive and unconstraining technique for urothelial cancer diagnosis but lacks sensitivity for detecting low-grade lesions. In this study, the fluorescence properties of classical Papanicolaou-stained urothelial cytological slides from patients or from cell lines were monitored to investigate metabolic changes in normal and tumoral cells. Time- and spectrally-resolved fluorescence imaging was performed at the single cell level to assess the spectral and temporal properties as well as the spatial distribution of the fluorescence emitted by urothelial cells. The results reveal quite different fluorescence distributions between tumoral urothelial cells, characterized by a perimembrane fluorescence localization, and the normal cells which exhibit an intracellular fluorescence. This is not caused by differences in the fluorescence emission of the endogenous fluorophores NAD(P)H, flavoproteins or porphyrins but by various localization of the EA 50 Papanicolaou stain as revealed by both the spectral and time-resolved parameters. The present results demonstrate that the use of single-cell endofluorescence emission of Papanicolaou-stained urothelial cytological slides can allow an early ex vivo diagnosis of low-grade bladder cancers.

Three-dimensional nanometre localization of nanoparticles to enhance super-resolution microscopy.

Meeting the nanometre resolution promised by super-resolution microscopy techniques (pointillist: PALM, STORM, scanning: STED) requires stabilizing the sample drifts in real time during the whole acquisition process. Metal nanoparticles are excellent probes to track the lateral drifts as they provide crisp and photostable information. However, achieving nanometre axial super-localization is still a major challenge, as diffraction imposes large depths-of-fields. Here we demonstrate fast full three-dimensional nanometre super-localization of gold nanoparticles through simultaneous intensity and phase imaging with a wavefront-sensing camera based on quadriwave lateral shearing interferometry. We show how to combine the intensity and phase information to provide the key to the third axial dimension. Presently, we demonstrate even in the occurrence of large three-dimensional fluctuations of several microns, unprecedented sub-nanometre localization accuracies down to 0.7 nm in lateral and 2.7 nm in axial directions at 50 frames per second. We demonstrate that nanoscale stabilization greatly enhances the image quality and resolution in direct stochastic optical reconstruction microscopy imaging.

Analysis of autofluorescence in polymorphonuclear neutrophils: a new tool for early infection diagnosis.

Diagnosing bacterial infection (BI) remains a challenge for the attending physician. An ex vivo infection model based on human fixed polymorphonuclear neutrophils (PMNs) gives an autofluorescence signal that differs significantly between stimulated and unstimulated cells. We took advantage of this property for use in an in vivo pneumonia mouse model and in patients hospitalized with bacterial pneumonia. A 2-fold decrease was observed in autofluorescence intensity for cytospined PMNs from broncho-alveolar lavage (BAL) in the pneumonia mouse model and a 2.7-fold decrease was observed in patients with pneumonia when compared with control mice or patients without pneumonia, respectively. This optical method provided an autofluorescence mean intensity cut-off, allowing for easy diagnosis of BI. Originally set up on a confocal microscope, the assay was also effective using a standard epifluorescence microscope. Assessing the autofluorescence of PMNs provides a fast, simple, cheap and reliable method optimizing the efficiency and the time needed for early diagnosis of severe infections. Rationalized therapeutic decisions supported by the results from this method can improve the outcome of patients suspected of having an infection.

A fluorescence-based assay for monitoring clinical drug resistance.

BACKGROUND AND AIMS: Multidrug resistance (MDR) limits effectiveness in treating malignancy by modifying internalisation and/or externalisation of drugs through cancer cell membranes. In this study we describe an assay to monitor patients' responses to chemotherapy. METHODS: The assay is based on the fluorescent properties of doxorubicin alone as well as in combination with methotrexate, vinblastine, doxorubicin and cisplatin (MVAC). The slide-based cell imaging technique was first optimised using a panel of breast and urothelial cancer cell lines and then extended to fine needle breast aspiration biopsy and urine cytology. RESULTS: The drug fluorescence behaviour observed on smears of clinical specimens is identical to that obtained using fixed cultured cells. The fluorescence of sensitive cells to chemotherapy is mainly localised in the nucleus, whereas resistant cells show a weak fluorescence signal localised in the cytoplasm. The difference in terms of fluorescence intensity is also highlighted through fluorescence spectra. CONCLUSIONS: The results suggest that the assay provides clinically valuable information in predicting responses to doxorubicin and/or MVAC therapy. Originally set up on a confocal microscope, the assay was also effective using a standard epifluorescence microscope; as such it is technically simple, reliable and inexpensive.

Homodimerization of amyloid precursor protein at the plasma membrane: a homoFRET study by time-resolved fluorescence anisotropy imaging.

Classical FRET (Förster Resonance Energy Transfer) using two fluorescent labels (one for the donor and another one for the acceptor) is not efficient for studying the homodimerization of a protein as only half of the homodimers formed can be identified by this technique. We thus resorted to homoFRET detected by time-resolved Fluorescence Anisotropy IMaging (tr-FAIM). To specifically image the plasma membrane of living cells, an original combination of tr-FAIM and Total Internal Reflection Fluorescence Lifetime Imaging Microscope (TIRFLIM) was implemented. The correcting factor accounting for the depolarization due to the high numerical aperture (NA) objective, mandatory for TIRF microscopy, was quantified on fluorescein solutions and on HEK293 cells expressing enhanced Green Fluorescence Protein (eGFP). Homodimerization of Amyloid Precursor Protein (APP), a key mechanism in the etiology of Alzheimer's disease, was measured on this original set-up. We showed, both in epifluorescence and under TIRF excitation, different energy transfer rates associated with the homodimerization of wild type APP-eGFP or of a mutated APP-eGFP, which forms constitutive dimers. This original set-up thus offers promising prospects for future studies of protein homodimerization in living cells in control and pathological conditions.

Functional defect of regulatory CD4(+)CD25+ T cells in the thymus of patients with autoimmune myasthenia gravis.

Thymus-derived CD4(+)CD25+ regulatory T (Treg) cells are essential for the maintenance of immunologic self-tolerance. Despite their critical role in the active suppression of experimental autoimmune disorders, little is known about their involvement in human autoimmune diseases. Myasthenia gravis (MG) is a CD4+ T cell-dependent autoimmune disease and the thymus is assumed to be the initiation site. To identify possible defects in the Treg cells in MG, we analyzed CD4(+)CD25+ cells in thymi from patients with MG compared to those from healthy subjects. We found a normal CD4(+)CD25+ number but a severe functional defect in their regulatory activity together with a decreased expression of the transcription factor, Foxp3, which is essential for T-cell regulatory function. The phenotypic analysis of CD4(+)CD25+ thymocytes revealed an increased number of activated effector cells with strong Fas expression in patients with MG. However, whatever their level of Fas, CD4(+)CD25+ thymocytes from patients with MG remained unable to suppress the proliferation of responding cells, indicating that the impaired Treg cell function is not due to contamination by activated effector T cells. These data are the first to demonstrate a severe functional impairment of thymic Treg cells in MG, which could contribute to the onset of this autoimmune disease.

IL-22, in contrast to IL-10, does not induce Ig production, due to absence of a functional IL-22 receptor on activated human B cells.

IL-22 is an IL-10 homologue that binds to and signals via the class II cytokine receptor (R) heterodimer IL-22RA1/CFR2-4 (IL-10R2), the latter chain being part of the IL-10R complex. Here, we report that, despite its structural similarity with IL-10, as well as its use of the common IL-10R2 chain, IL-22, in contrast to IL-10, is unable to induce Ig production by activated human B cells. Whereas culture of anti-CD40 mAb-stimulated splenic or tonsillar B cells in the presence of rIL-10 resulted in the production of IgG, IgG1, IgG3 and IgA, rIL-22, at concentrations ranging from 4 to 100 ng/ml, did not induce the production of any of these isotypes. Moreover, unlike rIL-10 which enhanced rIL-4-induced IgG4 and IgE production, rIL-22 was ineffective. Although activated B cells expressed transcripts for a soluble IL-22-binding protein (IL-22RA2), no mRNA for a transmembrane IL-22R (IL-22RA1) could be detected. The latter result was confirmed by the demonstration that rIL-22 failed to induce activation of STAT-3 and -5 in resting or activated B cells. Together, these data show that IL-22, in contrast to its homologue IL-10, is not involved in the immunological activity of B cells, which is due to the absence of a functional IL-22R at the surface of these cells.

Cutting edge: IL-21 is a switch factor for the production of IgG1 and IgG3 by human B cells.

IL-21 is a cytokine that regulates the activation of T and NK cells and promotes the proliferation of B cells activated via CD40. In this study, we show that rIL-21 strongly induces the production of all IgG isotypes by purified CD19(+) human spleen or peripheral blood B cells stimulated with anti-CD40 mAb. Moreover, it was found to specifically induce the production of IgG(1) and IgG(3) by CD40-activated CD19(+)CD27(-) naive human B cells. Although stimulation of CD19(+) B cells via CD40 alone induced gamma 1 and gamma 3 germline transcripts, as well as the expression of activation-induced cytidine deaminase, only stimulation with both anti-CD40 mAb and rIL-21 resulted in the production of S gamma/S mu switch circular DNA. These results show that IL-21, in addition to promoting growth and differentiation of committed B cells, is a specific switch factor for the production of IgG(1) and IgG(3).

Activated, but not resting human Th2 cells, in contrast to Th1 and T regulatory cells, produce soluble ST2 and express low levels of ST2L at the cell surface.

The T1/ST2 gene encodes, as a result of differential splicing, a cell surface protein (transmembrane form of T1/ST2, ST2L) and a soluble, secreted, protein (ST2). Here, we show that transcripts for both ST2L and ST2 are present in activated human Th2 clones, but not in Th1 and T regulatory clones. This activation-dependent expression of ST2L/ST2 transcripts was also found in short-term in vitro differentiated, activated CD4(+) Th2 cells. No expression of ST2L or ST2 mRNA was detected in any of the resting T cell subsets. Low cell surface expression of ST2L was detected on activated Th2 clones, and on freshly isolated non-IFN-gamma-producing CD4(+) peripheral blood T cells, activated with anti-CD3 and anti-CD28 mAb. Finally, ST2 could be detected in the culture supernatants of activated, but not resting, Th2 clones. Taken together, these results show that the T1/ST2 gene products are inducible proteins and that human Th2 cells, in addition to expressing ST2L at their cell surface, secrete ST2 following activation.