laboratoire de physique statistique
laboratoire de physique statistique




P A R M I :

Transcriptional Memory in the Drosophila Embryo - Ferraro, Teresa and Esposito, Emilia and Mancini, Laure and Ng, Sam and Lucas, Tanguy and Coppey, Mathieu and Dostatni, Nathalie and Walczak, Aleksandra M. and Levine, Michael and Lagha, Mounia
CURRENT BIOLOGY 26212-218 (2016)

Abstract : Transmission of active transcriptional states from mother to daughter cells has the potential to foster precision in the gene expression programs underlying development. Such transcriptional memory has been specifically proposed to promote rapid reactivation of complex gene expression profiles after successive mitoses in Drosophila development [1]. By monitoring transcription in living Drosophila embryos, we provide the first evidence for transcriptional memory in animal development. We specifically monitored the activities of stochastically expressed transgenes in order to distinguish active and inactive mother cells and the behaviors of their daughter nuclei after mitosis. Quantitative analyses reveal that there is a 4-fold higher probability for rapid reactivation after mitosis when the mother experienced transcription. Moreover, memory nuclei activate transcription twice as fast as neighboring inactive mothers, thus leading to augmented levels of gene expression. We propose that transcriptional memory is a mechanism of precision, which helps coordinate gene activity during embryogenesis.
Live Imaging of Bicoid-Dependent Transcription in Drosophila Embryos - Lucas, Tanguy and Ferraro, Teresa and Roelens, Baptiste and Chanes, Jose De Las Heras and Walczak, Aleksandra M. and Coppey, Mathieu and Dostatni, Nathalie
CURRENT BIOLOGY 232135-2139 (2013)

Abstract : The early Drosophila embryo is an ideal model to understand the transcriptional regulation of well-defined patterns of gene expression in a developing organism [1]. In this system, snapshots of transcription measurements obtained by RNA FISH on fixed samples [2,3] cannot provide the temporal resolution needed to distinguish spatial heterogeneity [3] from inherent noise [4, 5]. Here, we used the MS2-MCP system [6, 7] to visualize in living embryos nascent transcripts expressed from the canonical hunchback (hb) promoter [8, 9] under the control of Bicoid (Bcd) [10]. The hb-MS2 reporter is expressed as synchronously as endogenous hb in the anterior half of the embryo, but unlike hb it is also active in the posterior, though more heterogeneously and more transiently than in the anterior. The length and intensity of active transcription periods in the anterior are strongly reduced in absence of Bcd, whereas posterior ones are mostly Bcd independent. This posterior noisy signal decreases progressively through nuclear divisions, so that the MS2 reporter expression mimics the known anterior hb pattern at cellularization. We propose that the establishment of the hb pattern relies on Bcd-dependent lengthening of transcriptional activity periods in the anterior and may require two distinct repression mechanisms in the posterior.
A General Theoretical Framework to Infer Endosomal Network Dynamics from Quantitative Image Analysis - Foret, Lionel and Dawson, Jonathan E. and Villasenor, Roberto and Collinet, Claudio and Deutsch, Andreas and Brusch, Lutz and Zerial, Marino and Kalaidzidis, Yannis and Juelicher, Frank
CURRENT BIOLOGY 221381-1390 (2012)

Abstract : Background: Endocytosis allows the import and distribution of cargo into a series of endosomes with distinct morphological and biochemical characteristics. Our current understanding of endocytic cargo trafficking is based on the kinetics of net cargo transport between endosomal compartments without considering individual endosomes. However, endosomes form a dynamic network of membranes undergoing fusion and fission, thereby continuously exchanging and redistributing cargo. The macroscopic kinetic properties, i.e., the properties of the endosomal network as a whole, result from the collective behaviors of many individual endosomes, a problem so far largely unaddressed. Results: Here, we developed a general theoretical framework to describe the dynamics of cargo distributions in the endosomal network. We combined the theory with quantitative experiments to study how the macroscopic kinetic properties of the endosomal network emerge from microscopic processes at the level of individual endosomes. We compared our theory predictions to experimental data in which dynamic distributions of endocytosed low-density lipoprotein (LDL) were quantified. Conclusions: Our theory can quantitatively describe the observed cargo distributions as a function of time. Remarkably, the theory allows determining microscopic kinetic parameters such as the fusion rate between endosomes from still images of cargo distributions at different times of internalization. We show that this method is robust and sensitive because cargo distributions result from an average over many stochastic events in many cells. Our results provide theoretical and experimental support to the ``funnel model'' of endosome progression and suggest that the conversion of early to late endosomes is the major mode of LDL trafficking.
Mechanical Forces of Fission Yeast Growth - Minc, Nicolas and Boudaoud, Arezki and Chang, Fred
CURRENT BIOLOGY 191096-1101 (2009)

Abstract : Mechanical properties contribute to the control of cell size, morphogenesis, development, and lifestyle of fungal cells [1-4]. Tip growth can be understood by a viscoplastic model, in which growth is derived by high internal turgor pressure and cell-wall elasticity [2, 5]. To understand how these properties regulate growth in the rod-shaped fission yeast Schizosaccaromyces pombe, we devised femtoliter cylindrical polydimethylsiloxane (PDMS) microchambers with varying elasticity as force sensors for single cells. By buckling cells in these chambers, we determine the elastic surface modulus of the cell wall to be 20.2 +/- 6.1 N.m(-1). By analyzing the growth of the cells as they push against the walls of the chamber, we derive force-velocity relationships and values for internal effective turgor pressure of 0.85 +/- 0.15 MPa and a growth-stalling force of 11 +/- 3 mu N. The behavior of cells buckling under the force of their own growth provides an independent test of this model and parameters. Force generation is dependent on turgor pressure and a glycerol synthesis gene, gpd1(+) (glycerol-3-phosphate dehydrogenase), and is independent of actin cables. This study develops a quantitative framework for tip cell growth and characterizes mechanisms of force generation that contribute to fungal invasion into host tissues.