3Sep/100
H3K9 METHYLATION AND RNA INTERFERENCE REGULATE NUCLEOLAR ORGANIZATION AND REPEATED DNA STABILITY
3Sep/100
FUNCTIONAL INTERACTION BETWEEN PML AND SATB1 REGULATES CHROMATIN-LOOP ARCHITECTURE AND TRANSCRIPTION OF THE MHC CLASS I LOCUS
3Sep/100
SHARP BOUNDARIES OF DPP SIGNALLING TRIGGER LOCAL CELL DEATH REQUIRED FOR DROSOPHILA LEG MORPHOGENESIS
Morphogens are secreted signalling molecules that govern many developmental processes. In the Drosophila wing disc, the transforming growth factor ? (TGF?) homologue Decapentaplegic (Dpp) forms a smooth gradient and specifies cell fate by conferring a defined value of morphogen activity. Thus, neighbouring cells have similar amounts of Dpp protein, and if a sharp discontinuity in Dpp activity is generated between these cells, Jun kinase (JNK)-dependent apoptosis is triggered to restore graded positional information. To date, it has been assumed that this apoptotic process is only activated when normal signalling is distorted. However, we now show that a similar process occurs during normal development: rupture in Dpp activity occurs during normal segmentation of the distal legs of Drosophila. This sharp boundary of Dpp signalling, independently of the absolute level of Dpp activity, induces a JNK?reaper-dependent apoptosis required for the morphogenesis of a particular structure of the leg, the joint. Our results show that Dpp could induce a developmental programme not only in a concentration dependent manner, but also by the creation of a sharp boundary of Dpp activity. Furthermore, the same process could be used either to restore a normal pattern in response to artificial disturbance or to direct a morphogenetic process.3Sep/100
PGC7/STELLA PROTECTS AGAINST DNA DEMETHYLATION IN EARLY EMBRYOGENESIS
DNA methylation is an important means of epigenetic gene regulation and must be carefully controlled as a prerequisite for normal early embryogenesis. Although global demethylation occurs soon after fertilization, it is not evenly distributed throughout the genome. Genomic imprinting and epigenetic asymmetry between parental genomes, that is, delayed demethylation of the maternal genome after fertilization, are clear examples of the functional importance of DNA methylation. Here, we show that PGC7/Stella, a maternal factor essential for early development, protects the DNA methylation state of several imprinted loci and epigenetic asymmetry. After determining that PGC7/Stella binds to Ran binding protein 5 (RanBP5; a nuclear transport shuttle protein), mutant versions of the two proteins were used to examine exactly when and where PGC7/Stella functions within the cell. It is likely that PGC7/Stella protects the maternal genome from demethylation only after localizing to the nucleus, where it maintains the methylation of several imprinted genes. These results demonstrate that PGC7/Stella is indispensable for the maintenance of methylation involved in epigenetic reprogramming after fertilization.3Sep/100
TRIGGERING NEURAL DIFFERENTIATION OF ES CELLS BY SUBTYPE SWITCHING OF IMPORTIN-?
Nuclear proteins are selectively imported into the nucleus by transport factors such as importin-? and importin-?. Here, we show that the expression of importin-? subtypes is strictly regulated during neural differentiation of mouse embryonic stem (ES) cells, and that the switching of importin-? subtype expression is critical for neural differentiation. Moreover, reproducing the switching of importin-? subtype expression in undifferentiated ES cells induced neural differentiation in the presence of leukaemia inhibitory factor (LIF) and serum, coordinated with the regulated expression of Oct3/4, Brn2 and SOX2, which are involved in ES?neural identity determination. These transcription factors were selectively imported into the nucleus by specific subtypes of importin-?. Thus, importin-? subtype switching has a major impact on cell differentiation through the regulated nuclear import of a specific set of transcription factors. This is the first study to propose that transport factors should be considered as major players in cell-fate determination.3Sep/100
REGULATION OF PROTEIN TYROSINE PHOSPHATASE 1B BY SUMOYLATION
Protein-tyrosine phosphatase 1B (PTP1B) is an ubiquitously expressed enzyme that negatively regulates growth-factor signalling and cell proliferation by binding to and dephosphorylating key receptor tyrosine kinases, such as the insulin receptor. It is unclear how the activity of PTP1B is regulated. Using a yeast two-hybrid assay, a protein inhibitor of activated STAT1 (PIAS1) was isolated as a PTP1B-interacting protein. Here, we show that PIAS1, which functions as a small ubiquitin-like modifier (SUMO) E3 ligase, associates with PTP1B in mammalian fibroblasts and catalyses sumoylation of PTP1B. Sumoylation of PTP1B reduces its catalytic activity and inhibits the negative effect of PTP1B on insulin receptor signalling and on transformation by the oncogene v-crk. Insulin-stimulated sumoylation of endogenous PTP1B results in a transient downregulation of the enzyme; this event does not occur when the endogenous enzyme is replaced with a sumoylation-resistant mutant of PTP1B. These results suggest that sumoylation, which has been implicated primarily in processes in the nucleus and nuclear pore, also modulates a key enzyme?substrate signalling complex that regulates metabolism and cell proliferation.3Sep/100
PI(3)K? HAS AN IMPORTANT CONTEXT-DEPENDENT ROLE IN NEUTROPHIL CHEMOKINESIS
The directional movement of cells in a gradient of external stimulus is termed chemotaxis and is important in many aspects of development and differentiated cell function. Phophoinositide 3-kinases (PI(3)Ks) are thought to have critical roles within the gradient-sensing machinery of a variety of highly motile cells, such as mammalian phagocytes, allowing these cells to respond quickly and efficiently to shallow gradients of soluble stimuli. Our analysis of mammalian neutrophil migration towards ligands such as fMLP shows that, although PtdIns(3,4)P2 and PtdIns(3,4,5)P3 accumulate in a PI(3)K?-dependent fashion at the up-gradient leading-edge, this signal is not required for efficient gradient-sensing and gradient-biased movement. PI(3)K? activity is however, a critical determinant of the proportion of cells that can move, that is, respond chemokinetically, in reaction to fMLP. Furthermore, this dependence of chemokinesis on PI(3)K? activity is context dependent, both with respect to the state of priming of the neutrophils and the type of surface on which they are migrating. We propose this effect of PI(3)K? is through roles in the regulation of some aspects of neutrophil polarization that are relevant to movement, such as integrin-based adhesion and the accumulation of polymerized (F)-actin at the leading-edge.3Sep/100
BASAL-TO-APICAL CADHERIN FLOW AT CELL JUNCTIONS
Stable cell?cell adhesion is essential for maintaining tissue integrity, but cells are also able to relocate, implying the existence of mechanisms for coordinating cell adhesion and movement. Here, we show that, in some transformed lines, cadherin adhesion molecules exhibit a flow-like movement in a basal?apical direction at the cell junction and that this flow is associated with reorganizing actin filaments. Such flow also occurs in normal epithelial sheets, but solely at the junctions formed by moving cells. We propose that cadherin flow may provide a mechanism for facilitating the sliding of the two contacting cell membranes in morphogenetically active cell sheets.3Sep/100
