Retinoic acid accelerates downregulation of the Xist repressor, Oct4, and increases the likelihood of Xist activation when Tsix is deficient

<p>Abstract</p> <p>Background</p> <p>Imbalances in X-linked gene dosage between the sexes are resolved by transcriptionally silencing one of two X-chromosomes in female cells of the early mammalian embryo. X-inactivation is triggered by expression of the non-coding <it>Xist </it>gene. In turn, <it>Xist </it>is dually regulated by the antisense Tsix RNA and by the Oct4 pluripotency factor. Although there is general agreement that <it>Tsix </it>is an inhibitor of <it>Xist</it>, some laboratories have observed ectopic <it>Xist </it>induction in differentiating male ES cells when <it>Tsix </it>is mutated, whereas we have not observed significant changes in <it>Xist</it>. These observational differences have led to fundamentally diverse models of X-chromosome counting. Here, we investigate if different methods of cell differentiation and use of all <it>-trans </it>retinoic acid (RA) could be causative factors and how they might impact <it>Xist </it>expression.</p> <p>Results</p> <p>We compared suspension and cell-adhesion cultures in the presence or absence of RA and find that RA significantly impacts <it>Xist </it>expression in <it>Tsix</it>-mutant male cells. Whereas the standard embryoid body method infrequently leads to ectopic Xist expression, adding RA generates a significant number of Xist-positive male cells. However, while normal Xist clouds in wild-type female cells are robust and well-circumscribed, those found in the RA-treated mutant males are loosely dispersed. Furthermore, ectopic Xist expression does not generally lead to complete gene silencing. We attribute the effect of RA on <it>Xist </it>to RA's repressive influence on Oct4, a pluripotency factor recently shown to regulate <it>Tsix </it>and <it>Xist</it>. RA-treated ES cells exhibit accelerated decreases in Oct4 RNA levels and also display accelerated loss of binding to <it>Xist </it>intron 1. When <it>Tsix </it>is deficient, the faster kinetics of Oct4 loss tip the equilibrium towards <it>Xist </it>expression. However, the aberrant Xist clusters are unlikely to explain elevated cell death, as X-linked silencing does not necessarily correlate with the qualitatively aberrant Xist clusters.</p> <p>Conclusions</p> <p>We conclude that RA treatment leads to premature downregulation of Oct4 and partial derepression of <it>Xist </it>irrespective of X-chromosome counting. RA-induced Xist clusters in male cells do not result in global or stable silencing, and excess cell death is not observed. These data and RA's known pleiotropic effects on ES transcription networks suggest that RA differentation bypasses normal X-inactivation controls and should be used judiciously. We propose that the likelihood of <it>Xist </it>expression is determined by a balance of multiple <it>Xist </it>activators and repressors, and that levels of Oct4 and Tsix are crucial toward achieving this balance.</p

The Large N 't Hooft Limit of Kazama-Suzuki Model

We consider N=2 Kazama-Suzuki model on CP^N=SU(N+1)/SU(N)xU(1). It is known that the N=2 current algebra for the supersymmetric WZW model, at level k, is a nonlinear algebra. The N=2 W_3 algebra corresponding to N=2 was recovered from the generalized GKO coset construction previously. For N=4, we construct one of the higher spin currents, in N=2 W_5 algebra, with spins (2, 5/2, 5/2, 3). The self-coupling constant in the operator product expansion of this current and itself depends on N as well as k explicitly. We also observe a new higher spin primary current of spins (3, 7/2, 7/2, 4). From the behaviors of N=2, 4 cases, we expect the operator product expansion of the lowest higher spin current and itself in N=2 W_{N+1} algebra. By taking the large (N, k) limit on the various operator product expansions in components, we reproduce, at the linear order, the corresponding operator product expansions in N=2 classical W_{\infty}^{cl}[\lambda] algebra which is the asymptotic symmetry of the higher spin AdS_3 supergravity found recently.Comment: 44 pages; the two typos in the first paragraph of page 23 corrected and to appear in JHE

Meta-Stable Brane Configurations with Seven NS5-Branes

We present the intersecting brane configurations consisting of NS-branes, D4-branes(and anti D4-branes) and O6-plane, of type IIA string theory corresponding to the meta-stable nonsupersymmetric vacua in four dimensional N=1 supersymmetric SU(N_c) x SU(N_c') x SU(N_c'') gauge theory with a symmetric tensor field, a conjugate symmetric tensor field and bifundamental fields. We also describe the intersecting brane configurations of type IIA string theory corresponding to the nonsupersymmetric meta-stable vacua in the above gauge theory with an antisymmetric tensor field, a conjugate symmetric tensor field, eight fundamental flavors and bifundamentals. These brane configurations consist of NS-branes, D4-branes(and anti D4-branes), D6-branes and O6-planes.Comment: 34pp, 9 figures; Improved the draft and added some footnotes; Figure 1, footnote 7 and captions of Figures 7,8,9 added or improved and to appear in CQ