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Christophe Zimmer

Publications and source records attributed to Christophe Zimmer.

10 recordsLinked to original sources
PubMed2006

SAGA interacting factors confine sub-diffusion of transcribed genes to the nuclear envelope.

Changes in the transcriptional state of genes have been correlated with their repositioning within the nuclear space. Tethering reporter genes to the nuclear envelope alone can impose repression and recent reports have shown that, after activation, certain genes can also be found closer to the nuclear periphery. The molecular mechanisms underlying these phenomena have remained elusive. Here, with the use of dynamic three-dimensional tracking of a single locus in live yeast (Saccharomyces cerevisiae) cells, we show that the activation of GAL genes (GAL7, GAL10 and GAL1) leads to a confinement in dynamic motility. We demonstrate that the GAL locus is subject to sub-diffusive movement, which after activation can become constrained to a two-dimensional sliding motion along the nuclear envelope. RNA-fluorescence in situ hybridization analysis after activation reveals a higher transcriptional activity for the peripherally constrained GAL genes than for loci remaining intranuclear. This confinement was mediated by Sus1 and Ada2, members of the SAGA histone acetyltransferase complex, and Sac3, a messenger RNA export factor, physically linking the activated GAL genes to the nuclear-pore-complex component Nup1. Deleting ADA2 or NUP1 abrogates perinuclear GAL confinement without affecting GAL1 transcription. Accordingly, transcriptional activation is necessary but not sufficient for the confinement of GAL genes at the nuclear periphery. The observed real-time dynamic mooring of active GAL genes to the inner side of the nuclear pore complex is in accordance with the 'gene gating' hypothesis.

Diffusion↗
PubMed2006

Richardson-Lucy algorithm with total variation regularization for 3D confocal microscope deconvolution.

Confocal laser scanning microscopy is a powerful and popular technique for 3D imaging of biological specimens. Although confocal microscopy images are much sharper than standard epifluorescence ones, they are still degraded by residual out-of-focus light and by Poisson noise due to photon-limited detection. Several deconvolution methods have been proposed to reduce these degradations, including the Richardson-Lucy iterative algorithm, which computes maximum likelihood estimation adapted to Poisson statistics. As this algorithm tends to amplify noise, regularization constraints based on some prior knowledge on the data have to be applied to stabilize the solution. Here, we propose to combine the Richardson-Lucy algorithm with a regularization constraint based on Total Variation, which suppresses unstable oscillations while preserving object edges. We show on simulated and real images that this constraint improves the deconvolution results as compared with the unregularized Richardson-Lucy algorithm, both visually and quantitatively.

Algorithms↗
PubMed2005

Segmenting and tracking fluorescent cells in dynamic 3-D microscopy with coupled active surfaces.

Cell migrations and deformations play essential roles in biological processes, such as parasite invasion, immune response, embryonic development, and cancer. We describe a fully automatic segmentation and tracking method designed to enable quantitative analyses of cellular shape and motion from dynamic three-dimensional microscopy data. The method uses multiple active surfaces with or without edges, coupled by a penalty for overlaps, and a volume conservation constraint that improves outlining of cell/cell boundaries. Its main advantages are robustness to low signal-to-noise ratios and the ability to handle multiple cells that may touch, divide, enter, or leave the observation volume. We give quantitative validation results based on synthetic images and show two examples of applications to real biological data.

Algorithms↗
PubMed2005

Coupled parametric active contours.

We propose an extension of parametric active contours designed to track nonoccluding objects transiently touching each other, a task where both parametric and single level set-based methods usually fail. Our technique minimizes a cost functional that depends on all contours simultaneously and includes a penalty for contour overlaps. This scheme allows us to take advantage of known constraints on object topology, namely, that objects cannot merge. The coupled contours preserve the identity of previously isolated objects during and after a contact event, thus allowing segmentation and tracking to proceed as desired.

Algorithms↗
PubMed2005

Myosin II and the Gal-GalNAc lectin play a crucial role in tissue invasion by Entamoeba histolytica.

Entamoeba histolytica is the human parasite responsible of amoebiasis, during which highly motile trophozoites invade the intestinal epithelium leading to amoebic colitis, and disseminate via the blood circulation causing liver abscesses. The invasive process, central to the pathogenesis, is known to be driven by parasites motility. To investigate molecules responsible for in vivo motion, we performed a high resolution dynamic imaging analysis using two-photon laser scanning microscopy. Image analysis of the parasites during invasion of Caco-2 cell monolayers, an enterocyte-like model, and hamster liver shows that E. histolytica undergoes non-Brownian motion. However, studies of movements of parasite strains dominant negative for myosin II, a central component of the cytoskeleton, and for Gal-GalNAc lectin, a major adhesion molecule, indicate that myosin II is essential for E. histolytica intercellular motility through intestinal cell monolayers and for its motility in liver. In contrast, the Gal-GalNAc lectin exclusively triggers invasion of the liver. These observations are in agreement with emerging studies that highlight marked differences in the way that cells migrate in vitro in two dimensions versus in vivo in three dimensions. The approach that we have developed should be powerful to identify adhesive complexes required for in vivo cell migration in normal and pathogenic situations and may, thereby, lead to new therapeutic drug, for pathologies based on cell motility and adhesion.

Animals↗
PubMed2004

Imaging movement of malaria parasites during transmission by Anopheles mosquitoes.

Malaria is contracted when Plasmodium sporozoites are inoculated into the vertebrate host during the blood meal of a mosquito. In infected mosquitoes, sporozoites are present in large numbers in the secretory cavities of the salivary glands at the most distal site of the salivary system. However, how sporozoites move through the salivary system of the mosquito, both in resting and feeding mosquitoes, is unknown. Here, we observed fluorescent Plasmodium berghei sporozoites within live Anopheles stephensi mosquitoes and their salivary glands and ducts. We show that sporozoites move in the mosquito by gliding, a type of motility associated with their capacity to invade host cells. Unlike in vitro, sporozoite gliding inside salivary cavities and ducts is modulated in speed and motion pattern. Imaging of sporozoite discharge through the proboscis of salivating mosquitoes indicates that sporozoites need to locomote from cavities into ducts to be ejected and that their progression inside ducts favours their early ejection. These observations suggest that sporozoite gliding allows not only for cell invasion but also for parasite locomotion in host tissues, and that it may control parasite transmission.

Animals↗
PubMed2003

EhPAK, a member of the p21-activated kinase family, is involved in the control of Entamoeba histolytica migration and phagocytosis.

Entamoeba histolytica migration is essential for the development of amoebiasis, a human disease characterised by invasion and destruction of tissues. Amoebic motility requires both polarisation of the cell and formation of a predominant pseudopod. As p21-activated kinases PAKs are known to regulate eukaryotic cell motility and morphology, we investigated the role of PAK in E. histolytica. We showed that the C-terminal domain of EhPAK comprised a constitutive kinase activity in vitro and that overproduction of this fragment, in E. histolytica, caused a significant reduction in amoeboid migration, as measured by dynamic image analysis, indicating an involvement of EhPAK in this process. A dramatic loss of polarity, as indicated by the increased number of membrane extensions all around E. histolytica, was also observed, suggesting that the N-terminal domain of EhPAK was necessary for maintenance of cell polarity. To support this view, we showed that despite the absence of the consensus motif to bind to Rac and Cdc42, the N-terminal domain of EhPAK bound to Rac1, suggesting that the N-terminal region was a regulatory domain. In addition, we also found an increased rate of human red blood cell phagocytosis, suggesting for the first time an active role for a PAK protein in this process. Taking together, the results suggest strongly that EhPAK is a key regulatory element in polarity, motility and phagocytosis of E. histolytica.

Actin Cytoskeleton↗
PubMed2002

Membrane-cytoskeleton interactions during the formation of the immunological synapse and subsequent T-cell activation.

Upon antigen recognition, T cells undergo substantial membrane and cytoskeletal rearrangements that lead to the formation of the immunological synapse and are necessary for subsequent T-cell activation. However, little is known about how membrane and cytoskeletal molecules interact during these processes. Here we discuss the involvement of the membrane-microfilament linker ezrin. We propose that ezrin is a component of the cytoskeleton-mediated architecture of the immunological synapse that plays a role in T-cell receptor clustering, protein kinase C theta translocation and intracellular signaling.

4-Butyrolactone↗
PubMed2002

Segmentation and tracking of migrating cells in videomicroscopy with parametric active contours: a tool for cell-based drug testing.

This paper presents a segmentation and tracking method for quantitative analysis of cell dynamics from in vitro videomicroscopy data. The method is based on parametric active contours and includes several adaptations that address important difficulties of cellular imaging, particularly the presence of low-contrast boundary deformations known as pseudopods, and the occurence of multiple contacts between cells. First, we use an edge map based on the average intensity dispersion that takes advantage of relative background homogeneity to facilitate the detection of both pseudopods and interfaces between adjacent cells. Second, we introduce a repulsive interaction between contours that allows correct segmentation of objects in contact and overcomes the shortcomings of previously reported techniques to enforce contour separation. Our tracking technique was validated on a realistic data set by comparison with a manually defined ground-truth and was successfully applied to study the motility of amoebae in a biological research project.

Algorithms↗