Establishment of Polycomb silencing requires a transient interaction between PC and ESC

Two distinct types of Polycomb complexes have been identified in flies and in vertebrates, one containing ESC and one containing PC. Using LexA fusions, we show that PC and ESC can establish silencing of a reporter gene but that each requires the presence of the other. In early embryonic extracts, we find PC transiently associated with ESC in a complex that includes EZ, PHO, PH, GAGA, and RPD3 but not PSC. In older embryos, PC is found in a complex including PH, PSC, GAGA, and RPD3, whereas ESC is in a separate complex including EZ, PHO, and RPD3.

The sea urchin sns insulator blocks CMV enhancer following integration in human cells

Insulators are a new class of genetic elements that attenuate enhancer function directionally. Previously, we characterized in sea urchin a 265-bp-long insulator, termed sns. To test insulator activity following stable integration in human cells, we placed sns between the CMV enhancer and a tk promoter upstream of a GFP transgene of plasmid or retroviral vectors. In contrast to controls, cells transfected or transduced with insulated constructs displayed a barely detectable fluorescence. Southern blot and PCR ruled out vector rearrangement following integration into host DNA; RNase protection confirmed the enhancer blocking activity. Finally, we demonstrate that two cis-acting sequences, previously characterized in sea urchin, are also specific binding sites for human proteins. We conclude that sns interferes with enhancer promoter interaction also in a human chromatin context. The relatively small size, evolutionary conservation and apparent lack of enhancer specificity might result useful in gene transfer experiments in human cells.

Homeobox genes and sea urchin development

We describe the expression of three Paracentrotus lividus homeobox-containing genes of the dispersed class during sea urchin embryogenesis and discuss their possible roles in the mechanisms of cell specification and embryo morphogenesis. PlHbox12 represents the first regulator identified in sea urchin that belongs to the zygotic class of transcription factors. Its early and transient expression and the localization of transcripts suggests that PlHbox12 is involved in cell fate specification of the oral or aboral ectodermal territories at the early cleavage stages. PlHbox9 is expressed just after the completion of gastrulation in a narrow stripe of cells at the ectoderm-endoderm boundary. It probably organizes a novel spatial boundary which definitely separates the archenteron and the aboral ectoderm. Finally, the spatial and temporal expression of the PlOtp gene strongly indicate that this regulator is conditionally activated in few cells of the oral ectoderm and is involved in patterning of this territory at late stages. Furthermore, our data indicate that PlOtp acts upstream of signaling systems that lead to the activation of the primary mesenchyme cell gene expression program and skeletal morphogenesis.

Functional characterization of the enhancer blocking element of the sea urchin early histone gene cluster reveals insulator properties and three essential cis-acting sequences

Insulator elements can be functionally identified by their ability to shield promoters from regulators in a position-dependent manner or their ability to protect adjacent transgenes from position effects. We have previously reported the identification of a 265 bp sns DNA fragment at the 3' end of the sea urchin H2A early histone gene that blocked expression of a reporter gene in transgenic embryos when placed between the enhancer and the promoter. Here we show that sns interferes with enhancer-promoter interaction in a directional manner. When sns is placed between the H2A modulator and the inducible tet operator, the modulator is barred from interaction with the basal promoter. However, the tet activator (tTA) can still activate the promoter, even in the presence of sns, demonstrating that sns does not interfere with activity of a downstream enhancer. In addition, the H2A modulator can still drive expression of a divergently oriented transcription unit, suggesting that sns does not inhibit binding of transcription factor(s) to the enhancer. To identify cis-acting sequence elements within sns which are responsible for insulator activity, we have performed in vitro DNase I footprinting and EMSA analysis, and in vivo functional assays by microinjection into sea urchin embryos. We have identified three binding sites for protein complexes: a palindrome, a direct repeat, and a C+T sequence that corresponds to seven GAGA motifs on the transcribed strand. Insulator function requires all three cis-acting elements. Based on these results, we conclude that sns displays properties similar to the best characterized insulators and suggest that directional blocking of enhancer-activated transcription by sns depends on the assembly of distinct DNA-protein complexes.

Apoptosis in recent myocardial infarction

PURPOSE

Apoptosis is considered a common pathological feature in acute myocardial infarction (MI) and heart failure; however its role in the later phases post MI has not been characterized. The goal of our study was to investigate by pathological examination human hearts at 20 to 30 days post MI and identify signs of ongoing cell apoptosis.

MATERIALS AND METHODS

Two hearts were collected at autopsy from patients who died 20 to 30 days from the onset of MI (Cases 1 and 2). Gross anatomy and light microscopy examination of the hearts was performed to define the infarcted area and the infarct-related artery. The in situ end-labeling of DNA fragmentation (TUNEL) was performed to identify apoptotic cells and the apoptotic rate (AR) was calculated.

RESULTS

There were no signs of acute necrosis in any of the specimens examined. A high number of myocardiocyte were positive at TUNEL examination in specimens obtained at sites of infarction, mean AR = 44%, but not in specimens derived from the same patients at regions remote from the MI, AR = 0.

CONCLUSIONS

High grade apoptosis is present at sites of infarction and not in regions remote from the infarcted area in the later phases post MI. These data support persistent myocardiocyte loss and identify a possible explanation of progressive left ventricular dysfunction in the subacute phases of MI.

Spatially restricted expression of PlOtp, a Paracentrotus lividus orthopedia-related homeobox gene, is correlated with oral ectodermal patterning and skeletal morphogenesis in late-cleavage sea urchin embryos

Several homeobox genes are expressed in the sea urchin embryo but their roles in development have yet to be elucidated. Of particular interest are homologues of homeobox genes that in mouse and Drosophila are involved in patterning the developing central nervous system (CNS). Here, we report the cloning of an orthopedia (Otp)-related gene from Paracentrotus lividus, PlOtp. Otp is a single copy zygotic gene that presents a unique and highly restricted expression pattern. Transcripts were first detected at the mid-gastrula stage in two pairs of oral ectoderm cells located in a ventrolateral position, overlying primary mesenchyme cell (PMC) clusters. Increases in both transcript abundance and the number of Otp-expressing cells were observed at prism and pluteus stages. Otp transcripts are symmetrically distributed in a few ectodermal cells of the oral field. Labelled cells were observed close to sites of active skeletal rod growth (tips of the budding oral and anal arms), and at the juxtaposition of stomodeum and foregut. Chemicals known to perturb PMC patterning along animal-vegetal and oral-aboral axes altered the pattern of Otp expression. Vegetalization by LiCl caused a shift in Otp-expressing cells toward the animal pole, adjacent to shifted PMC aggregates. Nickel treatment induced expression of the Otp gene in an increased number of ectodermal cells, which adopted a radialized pattern. Finally, ectopic expression of Otp mRNA affected patterning along the oral-aboral axis and caused skeletal abnormalities that resembled those exhibited by nickel-treated embryos. From these results, we conclude that the Otp homeodomain gene is involved in short-range cell signalling within the oral ectoderm for patterning the endoskeleton of the larva through epithelial-mesenchymal interactions.

Regulation of the sea urchin early H2A histone gene expression depends on the modulator element and on sequences located near the 3' end

Transcription of the sea urchin early histone genes occurs transiently during early cleavage, reaching the maximum at the morula stage and declining to an undetectable level at the gastrula stage. To identify the regulatory elements responsible for the timing and the levels of transcription of the H2A gene, we used promoter binding studies in nuclear extracts and microinjection of a CAT transgene driven by the early H2A promoter. We found that morula and gastrula nuclear proteins produced indistinguishable DNase I footprint patterns on the H2A promoter. Two sites of interactions, centred on the modulator/enhancer and on the CCAAT box respectively, were detected. Deletion of the modulator or coinjection of an excess of modulator sequences severely affected the expression of two transgenes driven by the enhancer-less and modulator-containing H2A promoter. Finally, a DNA fragment containing 3' coding and post-H2A spacer sequences, where upon silencing three micrococcal nuclease hypersensitive sites were previously mapped, specifically repressed at the gastrula stage the expression of the transgene driven by the H2A promoter. These results indicate that the modulator is essential for the expression of early H2A gene and that sequences for downregulation are localized near the 3' end of the H2A gene.

Sea urchin HSF activity in vitro and in transgenic embryos

Evidence is provided for the presence at the physiological temperature of 20 degrees C of a heat shock transcriptor factor, HSF, in the nuclei of P.lividus embryos. This HSF is able to specifically bind in vitro the heat shock element, HSE, of the promoter of the hsp70 gene i.v., as suggested by DNA-protein binding reactions and DNAse I protection assays. Upon heat-shock, at the temperature of 31 degrees C, its ability to bind the HSE units becomes much higher. The HSF activated by heat-shock drives in vivo the transcription of the beta-galactosidase reporter gene in transgenic sea urchin gastrulae. An ATF-like transcription factor, widely described in other organisms but not at all in sea urchins, is also present in the nuclear extracts and is able to bind the consensus individuated in the hsp70 i.v. gene promoter.

Enhancer blocking activity located near the 3' end of the sea urchin early H2A histone gene

The sea urchin early histone repeating unit contains one copy of each of the five histone genes whose coordinate expression during development is regulated by gene-specific elements. To learn how within the histone repeating unit a gene-specific activator can be prevented to communicate with the heterologous promoters, we searched for domain boundaries by using the enhancer blocking assay. We focused on the region near the 3' end of the H2A gene where stage-specific nuclease cleavage sites appear upon silencing of the early histone genes. We demonstrated that a DNA fragment of 265 bp in length, defined as sns (for silencing nucleoprotein structure), blocked the enhancer activity of the H2A modulator in microinjected sea urchin embryos only when placed between the enhancer elements and the promoter. We also found that sns silenced the modulator elements even when placed at 2.7 kb from the promoter. By contrast, the enhancer activity of the modulator sequences, located downstream to the coding region, was not affected when sns was positioned in close proximity to the promoter. Finally, the H2A sns fragment placed between the simian virus 40 regulative region and the tk promoter repressed chloramphenicol acetyltransferase expression in transfected human cell lines. We conclude that 3' end of the H2A gene contains sequence elements that behave as functional barriers of enhancer function in the enhancer blocking assay. Furthermore, our results also indicate that the enhancer blocking function of sns lacks enhancer and species specificity and that it can act in transient assays.

Homeobox-containing gene transiently expressed in a spatially restricted pattern in the early sea urchin embryo

In the sea urchin embryo, the lineage founder cells whose polyclonal progenies will give rise to five different territories are segregated at the sixth division. To investigate the mechanisms by which the fates of embryonic cells are first established, we looked for temporal and spatial expression of homeobox genes in the very early cleavage embryos. We report evidence that PlHbox12, a paired homeobox-containing gene, is expressed in the embryo from the 4-cell stage. The abundance of the transcripts reaches its maximum when the embryo has been divided into the five polyclonal territories--namely at the 64-cell stage--and it abruptly declines at later stages of development. Blastomere dissociation experiments indicate that maximal expression of PlHbox12 is dependent on intercellular interactions, thus suggesting that signal transduction mechanisms are responsible for its transcriptional activation in the early cleavage embryo. Spatial expression of PlHbox12 was determined by whole-mount in situ hybridization. PlHbox12 transcripts in embryos at the fourth, fifth, and sixth divisions seem to be restricted to the conditionally specified ectodermal lineages. These results suggest a possible role of the PlHbox12 gene in the early events of cell specification of the presumptive ectodermal territories.

Expression of homeobox-containing genes in the sea urchin (Parancentrotus lividus) embryo

Two homeobox-containing genes that belong to different homeodomain classes have been isolated from a sea urchin genomic library. One, PlHbox11, is the sea urchin homologue of the human and mouse Hox B3 gene, the other, PlHbox12, shows about 55% identity with paired class genes. Expression profile analysis of the two sea urchin Hbox genes suggests that they play different roles during embryogenesis. In fact, PlHbox11 transcripts are rare and are detected only in the pluteus larva and in the Aristotle's lantern and intestine of the adult. The PlHbox12 gene is, on the contrary, transiently expressed in the very early embryo already at the four cell stage; it accumulates at the 64 cell stage and disappears at later stages of development. In situ hybridization experiments to 16 and 32 cell stage whole mount embryos showed localization of the PlHbox12 mRNA to part of the mesomere-macromere region of the early cleavage embryo. These observations suggest a possible role of this gene in early events of cell specification.

A study of freeze-dried (lyophilized) dentin used as an apical barrier in adult monkey teeth

Thirty-two fully formed vital teeth of four adult cynomolgus monkeys were endodontically prepared to simulate conditions of an open apex. The apical 2 to 5 mm. of 24 teeth were filled with freeze-dried dentin. The eight remaining teeth received no implant material and served as controls. All the teeth were obturated with gutta-percha. The monkeys were sacrificed at 6, 13, 23, and 27 week periods, and the specimens were prepared for histologic examination. The early specimens showed acute inflammation apically; later specimens showed osseous healing. Some experimental teeth had partial cementum bridging against the implant material. Freeze-dried dentin was found to be a biocompatible material which can be used effectively as a substitute barrier against which gutta-percha can be condensed in mechanically prepared open-apexed monkey teeth.