Plasma membrane polarization during mating in yeast cells

The yeast mating cell provides a simple paradigm for analyzing mechanisms underlying the generation of surface polarity. Endocytic recycling and slow diffusion on the plasma membrane were shown to facilitate polarized surface distribution of Snc1p (Valdez-Taubas, J., and H.R. Pelham. 2003. Curr. Biol. 13:1636–1640). Here, we found that polarization of Fus1p, a raft-associated type I transmembrane protein involved in cell fusion, does not depend on endocytosis. Instead, Fus1p localization to the tip of the mating projection was determined by its cytosolic domain, which binds to peripheral proteins involved in mating tip polarization. Furthermore, we provide evidence that the lipid bilayer at the mating projection is more condensed than the plasma membrane enclosing the cell body, and that sphingolipids are required for this lipid organization

Developmental regulation of apical endocytosis controls epithelial patterning in vertebrate tubular organs

© 2015 Macmillan Publishers Limited. Epithelial organs develop through tightly coordinated events of cell proliferation and differentiation in which endocytosis plays a major role. Despite recent advances, how endocytosis regulates the development of vertebrate organs is still unknown. Here we describe a mechanism that facilitates the apical availability of endosomal SNARE receptors for epithelial morphogenesis through the developmental upregulation of plasmolipin (pllp) in a highly endocytic segment of the zebrafish posterior midgut. The protein PLLP (Pllp in fish) recruits the clathrin adaptor EpsinR to sort the SNARE machinery of the endolysosomal pathway into the subapical compartment, which is a switch for polarized endocytosis. Furthermore, PLLP expression induces apical Crumbs internalization and the activation of the Notch signalling pathway, both crucial steps in the acquisition of cell polarity and differentiation of epithelial cells. We thus postulate that differential apical endosomal SNARE sorting is a mechanism that regulates epithelial patterning.MINECO (BFU2011-22622) and CONSOLIDER (CSD2009-00016); Fundación Obra Social `La Caixa' PhD fellowship. G.A. was supported by the Amarouto Program for senior researchers from the Comunidad Autónoma de Madrid.Peer Reviewe

Smoothelin-like 2 Inhibits Coronin-1B to Stabilize the Apical Actin Cortex during Epithelial Morphogenesis

The actin cortex is involved in many biological processes and needs to be significantly remodeled during cell differentiation. Developing epithelial cells construct a dense apical actin cortex to carry out their barrier and exchange functions. The apical cortex assembles in response to three-dimensional (3D) extracellular cues, but the regulation of this process during epithelial morphogenesis remains unknown. Here, we describe Smoothelin-like 2 (SMTNL2) function, a member of the smooth-muscle related Smoothelin protein family, in apical cortex maturation. SMTNL2 is induced during the development of multiple epithelial tissues and localizes to the apical and junctional actin cortex in intestinal and kidney epithelial cells. SMTNL2 deficiency leads to membrane herniations in the apical domain of epithelial cells, indicative of cortex abnormalities. We find that SMTNL2 binds to actin filaments and is required to slow down the turnover of apical actin. We also characterize the SMTNL2 proximal interactome and find that SMTNL2 executes its functions partly through inhibition of Coronin-1B. While Coronin-1B-mediated actin dynamics are required for early morphogenesis, its sustained activity is detrimental for the mature apical shape. SMTNL2 binds to Coronin-1B through its N-terminal coiled-coil region and negates its function to stabilize the apical cortex. In sum, our results unveil a mechanism for regulating actin dynamics during epithelial morphogenesis, providing critical insights on the developmental control of the cellular corte

Microbial Colonization Induces Dynamic Temporal and Spatial Patterns of NF-κB Activation in the Zebrafish Digestive Tract

The nuclear factor κ-light-chain enhancer of activated B cells (NF-κB) transcription factor pathway is activated in response to diverse microbial stimuli to regulate expression of genes involved in immune responses and tissue homeostasis. However, the temporal and spatial activation of NF-κB in response to microbial signals have not been determined in whole living organisms, and the molecular and cellular details of these responses are not well understood. We used in vivo imaging and molecular approaches to analyze NF-κB activation in response to the commensal microbiota in transparent gnotobiotic zebrafish

Cse1l Is a Negative Regulator of CFTR-Dependent Fluid Secretion

Transport of chloride through the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) channel is a key step in regulating fluid secretion in vertebrates[1, 2]. Loss of CFTR function leads to cystic fibrosis (CF)[1, 3, 4], a disease that affects the lungs, pancreas, liver, intestine and vas deferens. Conversely, un-controlled activation of the channel leads to increased fluid secretion and plays a major role in several diseases and conditions including cholera[5, 6] and other secretory diarrheas [7] as well as Polycystic Kidney Disease (PKD)[8–10]. Understanding how CFTR activity is regulated in vivo has been limited by the lack of a genetic model. Here, we used a forward genetic approach in zebrafish to uncover CFTR regulators. We report the identification, isolation and characterization of a mutation in the zebrafish cse1l gene that leads to the sudden and dramatic expansion of the gut tube. We show that this phenotype results from a rapid accumulation of fluid due to the un-controlled activation of the CFTR channel. Analyses in zebrafish embryos and mammalian cells indicate that Cse1l is a negative regulator of CFTR-dependent fluid secretion. This work demonstrates the importance of fluid homeostasis in development and establishes the zebrafish as a much needed model system to study CFTR regulation in vivo

Parallelized computational 3D video microscopy of freely moving organisms at multiple gigapixels per second

To study the behavior of freely moving model organisms such as zebrafish (Danio rerio) and fruit flies (Drosophila) across multiple spatial scales, it would be ideal to use a light microscope that can resolve 3D information over a wide field of view (FOV) at high speed and high spatial resolution. However, it is challenging to design an optical instrument to achieve all of these properties simultaneously. Existing techniques for large-FOV microscopic imaging and for 3D image measurement typically require many sequential image snapshots, thus compromising speed and throughput. Here, we present 3D-RAPID, a computational microscope based on a synchronized array of 54 cameras that can capture high-speed 3D topographic videos over a 135-cm^2 area, achieving up to 230 frames per second at throughputs exceeding 5 gigapixels (GPs) per second. 3D-RAPID features a 3D reconstruction algorithm that, for each synchronized temporal snapshot, simultaneously fuses all 54 images seamlessly into a globally-consistent composite that includes a coregistered 3D height map. The self-supervised 3D reconstruction algorithm itself trains a spatiotemporally-compressed convolutional neural network (CNN) that maps raw photometric images to 3D topography, using stereo overlap redundancy and ray-propagation physics as the only supervision mechanism. As a result, our end-to-end 3D reconstruction algorithm is robust to generalization errors and scales to arbitrarily long videos from arbitrarily sized camera arrays. The scalable hardware and software design of 3D-RAPID addresses a longstanding problem in the field of behavioral imaging, enabling parallelized 3D observation of large collections of freely moving organisms at high spatiotemporal throughputs, which we demonstrate in ants (Pogonomyrmex barbatus), fruit flies, and zebrafish larvae

Función de las plataformas lipídicas en la distribución proteica y en la polaridad celular en Saccharomyces cerevisiae

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura: 20-05-2002El establecimiento y mantenimiento de los distintos compartimentos celulares requiere la existencia de mecanismos que aseguren el conecto transporte y distribuci6n de proteínas. En dlulas epiteliales MDCK los glicoesfingolipidos son tamsportados a la membrana apical. Esto dio lugar a la propuesta de que complejos de glicoesfingolipidos forman plataformas para el transporte polarizado de proteínas. Estas plataformas lipidicas se forman por el estrecho empaquetamiento de los ácidos grasos de los glicoesfingolípidos, mayormente saturados, con colesterol a las que distintas proteínas se asocian específicamente. Poco es lo que se conoce acerca de los factores y mecanismos que regulan este proceso. En este trabajo he utilizado la levadura Saccharomyces cerevisiae como modelo experimental para estudiar la biogénesis y función de las plataformas lipídicas . Los resultados aquí presentados demuestran la existencia de plataformas lipídicas en levaduras. En este organismo las plataformas lipidicas estan compuestas de esfingolípidos y ergosterol. El ensamblaje de las plataformas lipídicas en levaduras empieza en el retículo endoplasmático, a diferencia de los mamíferos donde ocurre en el complejo de Golgi. En esta tesis demuestro el papel fundamental de las plataformas lipidicas en el transporte a la superficie cellular de Pmalp, la proteína más abundante de la membrana plasmática. La asociación de Pmalp con las plataformas lipidicas es mediada por la unini6n de la proteína Astlp que determina la formaci6n de un coplejo que luego es transportado a la superficie. Las plataformas lipídicas están implicadas en la generación y el mantenimiento de la polaridad celular. Cuando levaduras haploides perciben la presencia de feromonas secretadas por otra célula se produce un crecimiento polarizado de ambas en la direccion de la otra cklula. Durante esta respuesta se produce una reorganizaci6n de la membrana, dependiente de plataformas lipidicas , que resulta en la retención de determinadas proteínas en la punta de las proyecciones. Esta tesis presenta evidencia que demuestra el papel de la interacción con plataformas lipidicas en el transporte de proteínas y en establecimiento de polaridad celular

Aberrant Processing of the WSC Family and Mid2p Cell Surface Sensors Results in Cell Death of Saccharomyces cerevisiae O-Mannosylation Mutants

Protein O mannosylation is a crucial protein modification in uni- and multicellular eukaryotes. In humans, a lack of O-mannosyl glycans causes congenital muscular dystrophies that are associated with brain abnormalities. In yeast, protein O mannosylation is vital; however, it is not known why impaired O mannosylation results in cell death. To address this question, we analyzed the conditionally lethal Saccharomyces cerevisiae protein O-mannosyltransferase pmt2 pmt4Δ mutant. We found that pmt2 pmt4Δ cells lyse as small-budded cells in the absence of osmotic stabilization and that treatment with mating pheromone causes pheromone-induced cell death. These phenotypes are partially suppressed by overexpression of upstream elements of the protein kinase C (PKC1) cell integrity pathway, suggesting that the PKC1 pathway is defective in pmt2 pmt4Δ mutants. Congruently, induction of Mpk1p/Slt2p tyrosine phosphorylation does not occur in pmt2 pmt4Δ mutants during exposure to mating pheromone or elevated temperature. Detailed analyses of the plasma membrane sensors of the PKC1 pathway revealed that Wsc1p, Wsc2p, and Mid2p are aberrantly processed in pmt mutants. Our data suggest that in yeast, O mannosylation increases the activity of Wsc1p, Wsc2p, and Mid2p by enhancing their stability. Reduced O mannosylation leads to incorrect proteolytic processing of these proteins, which in turn results in impaired activation of the PKC1 pathway and finally causes cell death in the absence of osmotic stabilization

O-Glycosylation as a Sorting Determinant for Cell Surface Delivery in Yeast

Little is known about the mechanisms that determine localization of proteins to the plasma membrane in Saccharomyces cerevisiae. The length of the transmembrane domains and association of proteins with lipid rafts have been proposed to play a role in sorting to the cell surface. Here, we report that Fus1p, an O-glycosylated integral membrane protein involved in cell fusion during yeast mating, requires O-glycosylation for cell surface delivery. In cells lacking PMT4, encoding a mannosyltransferase involved in the initial step of O-glycosylation, Fus1p was not glycosylated and accumulated in late Golgi structures. A chimeric protein lacking O-glycosylation motif was missorted to the vacuole and accumulated in late Golgi in wild-type cells. Exocytosis of this protein could be restored by addition of a 33-amino acid portion of an O-glycosylated sequence from Fus1p. Our data suggest that O-glycosylation functions as a sorting determinant for cell surface delivery of Fus1p