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Primary cilia are ubiquitous, microtubule-based organelles that play diverse roles in sensory transduction in many eukaryotic cells. They interrogate the cellular environment through chemosensing, osmosensing, and mechanosensing using receptors and ion channels in the ciliary membrane. Little is known about the mechanical and structural properties of the cilium and how these properties contribute to ciliary perception. We probed the mechanical responses of primary cilia from kidney epithelial cells [Madin-Darby canine kidney-II (MDCK-II)], which sense fluid flow in renal ducts. We found that, on manipulation with an optical trap, cilia deflect by bending along their length and pivoting around an effective hinge located below the basal body. The calculated bending rigidity indicates weak microtubule doublet coupling. Primary cilia of MDCK cells lack interdoublet dynein motors. Nevertheless, we found that the organelles display active motility. 3D tracking showed correlated fluctuations of the cilium and basal body. These angular movements seemed random but were dependent on ATP and cytoplasmic myosin-II in the cell cortex. We conclude that force generation by the actin cytoskeleton surrounding the basal body results in active ciliary movement. We speculate that actin-driven ciliary movement might tune and calibrate ciliary sensory functions.
A complex interplay of viral, host, and ecological factors shapes the spatio-temporal incidence and evolution of human influenza viruses. Although considerable attention has been paid to influenza A viruses, a lack of equivalent data means that an integrated evolutionary and epidemiological framework has until now not been available for influenza B viruses, despite their significant disease burden. Through the analysis of over 900 full genomes from an epidemiological collection of more than 26,000 strains from Australia and New Zealand, we reveal fundamental differences in the phylodynamics of the two co-circulating lineages of influenza B virus (Victoria and Yamagata), showing that their individual dynamics are determined by a complex relationship between virus transmission, age of infection, and receptor binding preference. In sum, this work identifies new factors that are important determinants of influenza B evolution and epidemiology.
p120-catenin (p120) modulates adherens junction (AJ) dynamics by controlling the stability of classical cadherins. Among all p120 isoforms, p120-3A and p120-1A are the most prevalent. Both stabilize cadherins, but p120-3A is preferred in epithelia, whereas p120-1A takes precedence in neurons, fibroblasts, and macrophages. During epithelial-to-mesenchymal transition, E- to N-cadherin switching coincides with p120-3A to -1A alternative splicing. These isoforms differ by a 101-amino acid "head domain" comprising the p120-1A N-terminus. Although its exact role is unknown, the head domain likely mediates developmental and cancer-associated events linked to p120-1A expression (e.g., motility, invasion, metastasis). Here we identified delta-interacting protein A (DIPA) as the first head domain-specific binding partner and candidate mediator of isoform 1A activity. DIPA colocalizes with AJs in a p120-1A- but not 3A-dependent manner. Moreover, all DIPA family members (Ccdc85a, Ccdc85b/DIPA, and Ccdc85c) interact reciprocally with p120 family members (p120, δ-catenin, p0071, and ARVCF), suggesting significant functional overlap. During zebrafish neural tube development, both knockdown and overexpression of DIPA phenocopy N-cadherin mutations, an effect bearing functional ties to a reported mouse hydrocephalus phenotype associated with Ccdc85c. These studies identify a novel, highly conserved interaction between two protein families that may participate either individually or collectively in N-cadherin-mediated development.
© 2014 Markham et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).
Actomyosin contractility plays a key role in tissue morphogenesis. During mammalian development, PTK7 regulates epithelial morphogenesis and planar cell polarity (PCP) through modulation of actomyosin contractility, but the underlying mechanism is unknown. Here, we show that PTK7 interacts with the tyrosine kinase Src and stimulates Src signaling along cell-cell contacts. We further identify ROCK2 as a target of junctional PTK7-Src signaling. PTK7 knockdown in cultured epithelial cells reduced the level of active Src at cell-cell contacts, resulting in delocalization of ROCK2 from cell-cell contacts and decreased junctional contractility, with a concomitant increase in actomyosin on the basal surface. Moreover, we present in vivo evidence that Src family kinase (SFK) activity is critical for PCP regulation in the auditory sensory epithelium and that PTK7-SFK signaling regulates tyrosine phosphorylation of junctional ROCK2. Together, these results delineate a PTK7-Src signaling module for spatial regulation of ROCK activity, actomyosin contractility, and epithelial PCP.
Copyright © 2014 Elsevier Inc. All rights reserved.
A tripartite association of Rab11a with both Rab11-FIP2 and MYO5B regulates recycling endosome trafficking. We sought to define the intermolecular interactions required between Rab11-FIP2 and MYO5B. Using a random mutagenesis strategy, we identified point mutations at S229P or G233E in Rab11-FIP2 that caused loss of interaction with MYO5B in yeast two-hybrid assays as well as loss of interaction of Rab11-FIP2(129-356) with MYO5B tail when expressed in HeLa cells. Single mutations or the double S229P/G233E mutation failed to alter the association of full-length Rab11-FIP2 with MYO5B tail in HeLa cells. While EGFP-Rab11-FIP2 wild type colocalized with endogenous MYO5B staining in MDCK cells, EGFP-Rab11-FIP2(S229P/G233E) showed a significant decrease in localization with endogenous MYO5B. Analysis of Rab11a-containing vesicle movement in live HeLa cells demonstrated that when the MYO5B/Rab11-FIP2 association is perturbed by mutation or by Rab11-FIP2 knockdown, vesicle movement is increased in both speed and track length, consistent with an impairment of MYO5B tethering at the cytoskeleton. These results support a critical role for the interaction of MYO5B with Rab11-FIP2 in stabilizing the functional complex with Rab11a, which regulates dynamic movements of membrane recycling vesicles.
© 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
Establishment and maintenance of apico-basolateral trafficking pathways are critical to epithelial homeostasis. Loss of polarity and trafficking fidelity are thought to occur as a consequence of transformation; however, here we report that selective mistrafficking of the epidermal growth factor receptor (EGFR) ligand epiregulin (EREG) from the basolateral to the apical cell surface drives transformation. Normally, EREG is preferentially delivered to the basolateral surface of polarized Madin-Darby canine kidney cells. EREG basolateral trafficking is regulated by a conserved tyrosine-based basolateral sorting motif in its cytoplasmic domain (YXXΦ: Y(156)ERV). Both Y156 and V159 are required for basolateral sorting of EREG, because Y156A and V159G substitutions redirect EREG to the apical cell surface. We also show that basolateral sorting of EREG is adaptor protein 1B-independent. Apical mistrafficking of EREG has a distinctive phenotype. In contrast to transient EGFR tyrosine phosphorylation after basolateral EREG stimulation, apical EREG leads to prolonged EGFR tyrosine phosphorylation, which may be related, at least in part, to a lack of negative regulatory Y1045 phosphorylation and subsequent ubiquitylation. Notably, Madin-Darby canine kidney cells stably expressing apically mistrafficked EREG form significantly larger, hyperproliferative, poorly differentiated, and locally invasive tumors in nude mice compared with WT EREG-expressing cells.
Several polarity proteins, including Scribble (Scrb) have been implicated in control of vesicle traffic, and in particular the endocytosis of E-cadherin, but through unknown mechanisms. We now show that depletion of Scrb enhances endocytosis of E-cadherin by weakening the E-cadherin-p120catenin interaction. Unexpectedly, however, the internalized E-cadherin is not degraded but accumulates in the Golgi apparatus. Silencing p120-catenin causes degradation of E-cadherin in lysosomes, but degradation is blocked by the co-depletion of Scrb, which diverts the internalized E-cadherin to the Golgi. Loss of Scrb also enhances E-cadherin binding to retromer components, and retromer is required for Golgi accumulation of Scrb, and E-cadherin stability. These data identify a novel and unanticipated function for Scrb in blocking retromer-mediated diversion of E-cadherin to the Golgi. They provide evidence that polarity proteins can modify the intracellular itinerary for endocytosed membrane proteins.
Evidence from multiple studies suggests that Clostridium perfringens ε-toxin is a pore-forming toxin, assembling into oligomeric complexes in the plasma membrane of sensitive cells. In a previous study, we used gene-trap mutagenesis to identify mammalian factors contributing to toxin activity, including caveolin-2 (CAV2). In this study, we demonstrate the importance of caveolin-2 and its interaction partner, caveolin-1 (CAV1), in ε-toxin-induced cytotoxicity. Using CAV2-specific shRNA in a toxin-sensitive human kidney cell line, ACHN, we confirmed that cells deficient in CAV2 exhibit increased resistance to ε-toxin. Similarly, using CAV1-specific shRNA, we demonstrate that cells deficient in CAV1 also exhibit increased resistance to the toxin. Immunoprecipitation of CAV1 and CAV2 from ε-toxin-treated ACHN cells demonstrated interaction of both CAV1 and -2 with the toxin. Furthermore, blue-native PAGE indicated that the toxin and caveolins were components of a 670 kDa protein complex. Although ε-toxin binding was only slightly perturbed in caveolin-deficient cells, oligomerization of the toxin was dramatically reduced in both CAV1- and CAV2-deficient cells. These results indicate that CAV1 and -2 potentiate ε-toxin induced cytotoxicity by promoting toxin oligomerization - an event which is requisite for pore formation and, by extension, cell death.
The double-stranded RNA virus mammalian reovirus displays broad cell, tissue, and host tropism. A critical checkpoint in the reovirus replication cycle resides within viral cytoplasmic inclusions, which are biosynthetic centers of genome multiplication and new-particle assembly. Replication of strain type 3 Dearing (T3) is arrested in Madin-Darby canine kidney (MDCK) cells at a step subsequent to inclusion development and prior to formation of genomic double-stranded RNA. This phenotype is primarily regulated by viral replication protein μ2. To understand how reovirus inclusions differ in productively and abortively infected MDCK cells, we used confocal immunofluorescence and thin-section transmission electron microscopy (TEM) to probe inclusion organization and particle morphogenesis. Although no abnormalities in inclusion morphology or viral protein localization were observed in T3-infected MDCK cells using confocal microscopy, TEM revealed markedly diminished production of mature progeny virions. T3 inclusions were less frequent and smaller than those formed by T3-T1M1, a productively replicating reovirus strain, and contained decreased numbers of complete particles. T3 replication was enhanced when cells were cultivated at 31°C, and inclusion ultrastructure at low-temperature infection more closely resembled that of a productive infection. These results indicate that particle assembly in T3-infected MDCK cells is defective, possibly due to a temperature-sensitive structural or functional property of μ2. Thus, reovirus cell tropism can be governed by interactions between viral replication proteins and the unique cell environment that modulate efficiency of particle assembly.