No abstract

Summary Embryonic development relies on the capacity of progenitor cells to appropriately respond to inductive cues; a cellular property known as developmental competence. Here we report that epigenetic priming of enhancers signifies developmental competence during endodermal lineage diversification. Chromatin mapping during pancreatic and hepatic differentiation of human embryonic stem cells revealed the en masse acquisition of a poised chromatin state at enhancers specific to endoderm-derived cell lineages in gut tube intermediates. Experimentally, the acquisition of this poised enhancer state predicts the ability of endodermal intermediates to respond to inductive signals. Furthermore, these enhancers are first recognized by the pioneer transcription factors FOXA1 and FOXA2 when competence is acquired, while subsequent recruitment of lineage-inductive transcription factors, such as PDX1, leads to enhancer and target gene activation. Together, our results identify the acquisition of a poised chromatin state at enhancers as a mechanism by which progenitor cells acquire developmental competence.

The interaction between the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the Pluronic triblock copolymer F127 was investigated. F127 is a nonionic surfactant with structural formula EO97PO69EO97, where EO represents the ethylene oxide block and PO represents the propylene oxide block. A combination of experiments involving a TTAB selective electrode (electromotive force), isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), and light scattering have shown that TTAB binds to both monomeric and micellar F127. TTAB forms a polymer/micellar TTAB complex with monomeric F127. In addition, TTAB binds to F127 micelles leading to the transformation of the aggregated F127 into mixed micelles followed by a breakdown of these aggregates into smaller mixed F127/TTAB aggregates as more TTAB is added. This process continues until all the aggregated F127 is dissociated. DSC measurements have also shown that small amounts of TTAB (typically 10-4 mol dm-3) can decrease the critical micelle temperature (cmt) of F127. This represents a third mode of binding in which TTAB induces F127 to form micelles at temperatures several degrees below its “pure” cmt. Possible mechanisms for these processes involving different modes of interaction of TTAB with F127 are introduced and discussed.

Growing axons navigate by responding to chemical guidance cues. Here we report that growth cones of rat cerebellar axons in culture turned away from a gradient of SDF-1, a chemokine that attracts migrating leukocytes and cerebellar granule cells via a G protein-coupled receptor (GPCR). Similarly, Xenopus spinal growth cones turned away from a gradient of baclofen, an agonist of the GABA(B) receptor. This response was mediated by G(i) and subsequent activation of phospholipase C (PLC), which triggered two pathways: protein kinase C (PKC) led to repulsion, and inositol 1,4,5-triphosphate (IP(3)) receptor activation led to attractive turning. Under normal culture conditions, PKC-dependent repulsion dominated, but the repulsion could be converted to attraction by inhibiting PKC or by elevating cytosolic cGMP. Thus, GPCRs can mediate both repulsive and attractive axon guidance in vitro, and chemokines may serve as guidance cues for axon pathfinding.