A RSC/nucleosome complex determines chromatin architecture and facilitates activator binding.

How is chromatin architecture established and what role does it play in transcription? We show that the yeast regulatory locus UASg bears, in addition to binding sites for the activator Gal4, sites bound by the RSC complex. RSC positions a nucleosome, evidently partially unwound, in a structure that facilitates Gal4 binding to its sites. The complex comprises a barrier that imposes characteristic features of chromatin architecture. In the absence of RSC, ordinary nucleosomes encroach over the UASg and compete with Gal4 for binding. Taken with our previous work, the results show that both prior to and following induction, specific DNA-binding proteins are the predominant determinants of chromatin architecture at the GAL1/10 genes. RSC/nucleosome complexes are also found scattered around the yeast genome. Higher eukaryotic RSC lacks the specific DNA-binding determinants found on yeast RSC, and evidently Gal4 works in those organisms despite whatever obstacle broadly positioned nucleosomes present.

OCT4 and SOX2 Work as Transcriptional Activators in Reprogramming Human Fibroblasts.

SOX2 and OCT4, in conjunction with KLF4 and cMYC, are sufficient to reprogram human fibroblasts to induced pluripotent stem cells (iPSCs), but it is unclear if they function as transcriptional activators or as repressors. We now show that, like OCT4, SOX2 functions as a transcriptional activator. We substituted SOX2-VP16 (a strong activator) for wild-type (WT) SOX2, and we saw an increase in the efficiency and rate of reprogramming, whereas the SOX2-HP1 fusion (a strong repressor) eliminated reprogramming. We report that, at an early stage of reprogramming, virtually all DNA-bound OCT4, SOX2, and SOX2-VP16 were embedded in putative enhancers, about half of which were created de novo. Those associated with SOX2-VP16 were, on average, stronger than those bearing WT SOX2. Many newly created putative enhancers were transient, and many transcription factor locations on DNA changed as reprogramming progressed. These results are consistent with the idea that, during reprogramming, there is an intermediate state that is distinct from both parental cells and iPSCs.

Polycomb Responds to Low Levels of Transcription.

How is Polycomb (Pc), a eukaryotic negative regulator of transcription, targeted to specific mammalian genes? Our genome-wide analysis of the Pc mark H3K27me3 in murine cells revealed that Pc is preferentially associated with CpG island promoters of genes that are transcribed at a low level and less so with promoters of genes that are either silent or more highly expressed. Studies of the CpG island promoter of the Kit gene demonstrate that Pc is largely absent when the gene is silent in myeloid cells, as well as when the gene is highly expressed in mast cells. Manipulations that increase transcription in the former case, and reduce it in the latter, increase Pc occupancy. The average negative effect of Pc, we infer, is about 2-fold. We suggest possible biological roles for such negative effects and propose a mechanism by which Pc might be recruited to weakly transcribed genes.

Endothelin-1 synthesis and secretion in human retinal pigment epithelial cells (ARPE-19): differential regulation by cholinergics and TNF-alpha.

PURPOSE: Endothelin (ET)-1 can produce nerve damage analogous to that in optic neuropathies such as glaucoma. The precise source of endothelin in the posterior segment of the eye remains unclear. The present study was conducted to determine whether the retinal pigment epithelium (RPE), which helps maintain the outer blood-retinal barrier, is a local source for ET-1 and whether the amount of ET-1 secreted by the RPE is be differentially regulated by cholinergics and the cytokine TNF-alpha. METHODS: Human retinal pigment epithelial cells (ARPE-19) were cultured either to a mature state (mRPE) for 4 weeks, with well-defined tight junctions, or to a young state (yRPE) for 4 days, with incompletely formed tight junctions. ET-1-like immunoreactivity was determined by immunocytochemistry, and secreted ET-1 was quantified by radioimmunoassay in both cell types. Cells were stimulated with the cholinergic agonist carbachol or with the cytokine TNF-alpha for specific periods. The expression of muscarinic receptor subtypes M1 and M3 and the peripheral membrane protein ZO-1 were analyzed by immunoblot and immunocytochemistry, respectively. Expression of preproendothelin-1 (ppET-1) mRNA after application of different stimuli at specific time points was determined by real-time RT-PCR. Carbachol-mediated elevation in intracellular calcium ([Ca2+]i) was also measured in the presence or absence of a selective muscarinic receptor antagonist. RESULTS: Constitutive synthesis and secretion of ET-1 was greater in mRPE than in yRPE cells. TNF-alpha caused a significant increase in ppET-1 mRNA levels and ET-1 secretion in both phenotypes. The disruption and subsequent breakdown of the tight junction barrier was evident in either phenotype after treatments with TNF-alpha. There was a concentration-dependent increase in [Ca2+]i in both y- and mRPE cells in response to CCh. CCh at 1 microM significantly increased ET-1 secretion, a response observed in yRPE but not in mRPE cells. This effect may be mediated primarily by the M3 receptor subtype and is phospholipase C (PLC) dependent. CONCLUSIONS: Regulation of ET-1 release in ARPE-19 cells was differentially regulated by TNF-alpha and CCh and was dependent on the age of the culture. RPE may be a source for ET-1 in the retina, and its increased release may become more important during breakdown of the blood-retinal barrier, as seen after TNF-alpha treatment.

Transcriptional activation by Oct4 is sufficient for the maintenance and induction of pluripotency.

Oct4 is an essential regulator of pluripotency in vivo and in vitro in embryonic stem cells, as well as a key mediator of the reprogramming of somatic cells into induced pluripotent stem cells. It is not known whether activation and/or repression of specific genes by Oct4 is relevant to these functions. Here, we show that fusion proteins containing the coding sequence of Oct4 or Xlpou91 (the Xenopus homolog of Oct4) fused to activating regions, but not those fused to repressing regions, behave as Oct4, suppressing differentiation and promoting maintenance of undifferentiated phenotypes in vivo and in vitro. An Oct4 activation domain fusion supported embryonic stem cell self-renewal in vitro at lower concentrations than that required for Oct4 while alleviating the ordinary requirement for the cytokine LIF. At still lower levels of the fusion, LIF dependence was restored. We conclude that the necessary and sufficient function of Oct4 in promoting pluripotency is to activate specific target genes.

Thrombin-induced endothelin-1 synthesis and secretion in retinal pigment epithelial cells is rho kinase dependent.

PURPOSE: The retinal pigment epithelium (RPE) is a major source for endothelin-1 (ET-1), a potent vasoactive peptide, at the outer blood­retinal barrier. Factors that regulate ET-1 synthesis at this site may help identify its normal function and its role in pathologic states accompanying retinal injury. Thrombin is one such factor that might act on the RPE after injury and breakdown of the blood­retinal barrier. The present study was conducted to identify signaling intermediates in thrombin-induced ET-1 synthesis and secretion in primary human RPE (hRPE) and transformed RPE cells (ARPE-19) and a possible pharmacological strategy to block excess release of ET-1. METHODS: Cultured hRPE cells were treated with different concentrations of thrombin and thrombin receptor agonists, and a time course to measure levels of preproET-1 (ppET-1) mRNA and secreted mature ET-1 was performed. Levels of secondary messengers [Ca²+]i and RhoA were measured and pharmacologically inhibited to determine how receptor-mediated thrombin activity lead to changes in ET-1 levels. RESULTS: Thrombin primarily acts via the protease-activated receptor-1 (PAR-1) subtype in RPE to induce ET-1 synthesis. Thrombin and other receptor agonists increased both [Ca²+]<]i and active RhoA. PAR-1-dependent rho/Rho kinase activation led to increase in ppET-1 mRNA and mature ET-1 secretion. CONCLUSIONS: Transient intracellular calcium mobilization and protein kinase C activation by thrombin play a minor role, if any, in ET-1 synthesis in RPE. Instead, rho/Rho kinase activation after PAR-1 stimulation strongly increased ppET-1 mRNA and ET-1 secretion in hRPE cells.

Effect of elevated intraocular pressure on endothelin-1 in a rat model of glaucoma.

The role of endothelin-1 (ET-1) a potent vasoactive peptide, in glaucoma pathogenesis is receiving increasing attention, particularly in astroglial activation in optic nerve damage. Our laboratory has also shown that ET-1 treatment causes proliferation of cultured human optic nerve head astrocytes to possibly initiate astrogliosis. ET-1 is distributed in retina, optic nerve, and ciliary epithelium, however the effects of elevated intraocular pressure (IOP) (as seen in glaucoma) on ET-1 and ET(B) receptors are not clearly understood. In the present study, the levels of immunoreactive ET-1 (ir-ET-1) in aqueous humour (AH) and optic nerve head (ONH) were determined in the Morrison elevated IOP model of glaucoma. Additionally in the ONH of these rats, immunohistochemical analyses of ET(B) receptors and glial fibrillary acidic protein (GFAP; a marker for astroglial cells and for astrogliosis) were performed. There was 2- to 2.5-fold increase in AH ir-ET-1 levels for rats subjected to elevated IOP, compared to their respective controls. In the Morrison rat model of glaucoma, elevated IOP increased optic nerve ir-ET-1 with concomitant increases in ir-ET(B) and ir-GFAP labelling (possibly indicative of astrogliosis and hypertrophy). As seen in brain astrocytes subjected to neurotrauma, the present findings are suggestive of ET-1's role in astroglial activation, particularly in response to elevated IOP in glaucoma.

Eyeing endothelins: a cellular perspective.

Endothelin is an endogenous vasoactive peptide that is considered among the most potent vasoconstrictor substances known. In addition to its vascular effects, endothelins and their receptors have been shown to be present in the eye and to have a number of ocular actions that may be important for ocular homeostasis, but, in excess can be a potential contributor to ocular neuropathy in glaucoma. The current review focuses on the cellular and molecular aspects of endothelins and its receptors in the eye with an emphasis on its relationship to ocular function and its potential role in the etiology of glaucoma pathophysiology.

Endothelin-1 distribution and basolateral secretion in the retinal pigment epithelium.

Endothelins are a family of conserved vasoactive peptides that are widely expressed in different biological systems including the eye. In the cell culture model of retinal pigment epithelium, ARPE-19, the synthesis and secretion of endothelin-1 (ET-1) is regulated by cholinergics and TNF-alpha. In the present study we investigated the expression of ET-1 in RPE in situ, in rat and human eyes. Additionally, we have employed the human retinal pigment epithelial (ARPE-19) cells to delineate the apical and basolateral ET-1 expression and secretion by confocal microscopy and radioimmunoassay, respectively. Our results suggest a possible conservation of ET-1 expression predominantly in the mammalian RPE underlining its importance at this site. Additionally, our results suggest that constitutive ET-1 secretion is predominantly towards the basolateral side in cultured RPE possibly allowing ET-1 to activate its receptors located in the choroidal blood vessels and regulate retinal and choroidal blood flow.

Dexamethasone regulates endothelin-1 and endothelin receptors in human non-pigmented ciliary epithelial (HNPE) cells.

Endothelin-1 (ET-1) lowers intraocular pressure (IOP) in animal models by regulating aqueous humour dynamics through both inflow and outflow mechanisms. Moreover, ET's concentration is elevated in glaucoma patients and in animal models of glaucoma. Glucocorticoid therapy often can lead to increase IOP in susceptible individuals including patients with primary open angle glaucoma (POAG). In this study, we examined the effects of dexamethasone (Dex), a frequently used anti-inflammatory glucocorticoid, on the synthesis and release of endothelin-1 and on the expression of endothelin receptors in human non-pigmented ciliary epithelial (HNPE) cells, an established source for ET-1 in the anterior chamber. As measured by ET-1 immunoreactivity, ET-1 was concentration-dependently increased following 24hr Dex treatment, with a maximum concentration (100 nM) causing a threefold increase of ET-1 release. Western blot analysis of HNPE cells showed the expression of endothelin receptor A (ET(A)) and endothelin receptor B (ET(B)) with approximate molecular weights of 40 kDa. Dex treatment decreased ET(A) receptor expression at all Dex doses, but up-regulated ET(B) receptors with 10nM Dex having the greatest effect. Quantitative PCR demonstrated that Dex also increased the mRNA of pre-pro-ET-1 (ppET-1) and ET(B) but decreased the mRNA of ET(A). RU486, a glucocorticoid receptor antagonist, was able to block Dex's actions on ET release and ET(B) receptor expression, but did not block its action on ET(A) receptor expression. Endothelin receptors were minimally expressed in HNPE cells as determined in binding experiments (B(max): ET(A) 17, ET(B) 25 fmolmg(-1) membrane protein). However Dex treatment stimulated a dramatic increase in ET(B) receptor density while decreasing ET(A) receptors (B(max): ET(A) 11, ET(B) 116 fmolmg(-1) membrane protein). The regulation of endothelin and its receptors could be a novel mechanism associated with glucocorticoid's effects on intraocular pressure. The increase in ET-1 and disproportionate regulation in ET receptor expression by Dex could promote dysregulation in ET's mechanism on both inflow and outflow, thus affecting aqueous humour dynamics in the anterior chamber of the eye.