William Lai

Relationships between circadian rhythms and modulation of gene expression by glucocorticoids in skeletal muscle

The existence and maintenance of biological rhythms linked to the 24-h light-dark cycle are essential to the health and functioning of an organism. Although much is known concerning central clock mechanisms, much less is known about control in peripheral tissues. In this study, circadian regulation of gene expression was examined in rat skeletal muscle. A rich time series involving 54 animals euthanized at 18 distinct time points within the 24-h cycle was performed, and mRNA expression in gastrocnemius muscles was examined using Affymetrix gene arrays. Data mining identified 109 genes that were expressed rhythmically, which could be grouped into eight distinct temporal clusters within the 24-h cycle. These genes were placed into 11 functional categories, which were examined within the context of temporal expression. Transcription factors involved in the regulation of central rhythms were examined, and eight were found to be rhythmically expressed in muscle. Because endogenous glucocorticoids are a major effector of circadian rhythms, genes identified here were compared with those identified in previous studies as glucocorticoid regulated. Of the 109 genes identified here as circadian rhythm regulated, only 55 were also glucocorticoid regulated. Examination of transcription factors involved in circadian control suggests that corticosterone may be the initiator of their rhythmic expression patterns in skeletal muscle.

Gene Expression Analysis of Hepatic Roles in Cause and Development of Diabetes in Goto-Kakizaki Rats

Progression of diabetes was studied in male Goto-Kakizaki (GK) spontaneously diabetic rats between 4 and 20 weeks of age, and compared with Wistar-Kyoto (WKY) controls. Five animals from each strain were killed at 4, 8, 12, 16, and 20 weeks of age. Body weight, plasma glucose, and plasma insulin were measured. WKY rats showed a significantly larger weight gain than GK animals from 8 weeks of age onward. Plasma glucose was relatively stable in WKY. By contrast, plasma glucose was higher in GK than WKY even at 4 weeks and continued to increase up to 12 weeks and then maintained a hyperglycemic plateau throughout the remainder of the experiment. Plasma insulin was relatively stable in WKY from 8 weeks onward but was sharply elevated in GK between 4 and 8 weeks. After 8 weeks, insulin declined in GK with GK concentrations lower than WKY at 20 weeks, suggesting beta-cell failure. Gene expression in liver was explored using Affymetrix 230-2 gene arrays. Data mining identified 395 probe sets out of more than 31,000 that were differentially regulated. Excluding unidentifiable probe sets and considering duplicate probe sets, there were 311 genes that were expressed differently in the liver of the two strains. A functional analysis of these genes indicated that disruption of lipid metabolism in the liver is a major consequence of the chronic hyperglycemia in the GK strain. In addition, the results suggest that chronic inflammation contributes significantly to the development of diabetes in the GK rats.

Circadian variations in rat liver gene expression: relationships to drug actions.

Chronopharmacology is an important but under-explored aspect of therapeutics. Rhythmic variations in biological processes can influence drug action, including pharmacodynamic responses, due to circadian variations in the availability or functioning of drug targets. We hypothesized that global gene expression analysis can be useful in the identification of circadian-regulated genes involved in drug action. Circadian variations in gene expression in rat liver were explored using Affymetrix gene arrays. A rich time series involving animals analyzed at 18 time points within the 24-h cycle was generated. Of the more than 15,000 probe sets on these arrays, 265 exhibited oscillations with a 24-h frequency. Cluster analysis yielded five distinct circadian clusters, with approximately two thirds of the transcripts reaching maximal expression during the dark/active period of the animal. Of the 265 probe sets, 107 were identified as having potential therapeutic importance. The expression levels of clock genes were also investigated in this study. Five clock genes exhibited circadian variation in the liver, and data suggest that these genes may also be regulated by corticosteroids.

Chromatin architectures at fission yeast transcriptional promoters and replication origins

We have used micrococcal nuclease (MNase) digestion followed by deep sequencing in order to obtain a higher resolution map than previously available of nucleosome positions in the fission yeast, Schizosaccharomyces pombe. Our data confirm an unusually short average nucleosome repeat length, ∼152 bp, in fission yeast and that transcriptional start sites (TSSs) are associated with nucleosome-depleted regions (NDRs), ordered nucleosome arrays downstream and less regularly spaced upstream nucleosomes. In addition, we found enrichments for associated function in four of eight groups of genes clustered according to chromatin configurations near TSSs. At replication origins, our data revealed asymmetric localization of pre-replication complex (pre-RC) proteins within large NDRs—a feature that is conserved in fission and budding yeast and is therefore likely to be conserved in other eukaryotic organisms.

Genome-wide search identifies Ccnd2 as a direct transcriptional target of Elf5 in mouse mammary gland

BackgroundThe ETS transcription factor Elf5 (also known as ESE-2) is highly expressed in the mammary gland and plays an important role in its development and differentiation. Indeed studies in mice have illustrated an essential role for Elf5 in directing alveologenesis during pregnancy. Although the molecular mechanisms that underlie the developmental block in Elf5 null mammary glands are beginning to be unraveled, this investigation has been hampered by limited information about the identity of Elf5-target genes. To address this shortcoming, in this study we have performed ChIP-cloning experiments to identify the specific genomic segments that are occupied by Elf5 in pregnant mouse mammary glands.ResultsSequencing and genomic localization of cis-regulatory regions bound by Elf5 in vivo has identified several potential target genes covering broad functional categories. A subset of these target genes demonstrates higher expression levels in Elf5-null mammary glands suggesting a repressive functional role for this transcription factor. Here we focus on one putative target of Elf5, the Ccnd2 gene that appeared in our screen. We identify a novel Elf5-binding segment upstream of the Ccnd2 gene and demonstrate that Elf5 can transcriptionally repress Ccnd2 by directly binding to the proximal promoter region. Finally, using Elf5-null mammary epithelial cells and mammary glands, we show that loss of Elf5 in vivo leads to up regulation of Ccnd2 and an altered expression pattern in luminal cells.ConclusionsIdentification of Elf5-targets is an essential first step in elucidating the transcriptional landscape that is shaped by this important regulator. Our studies offer new toolbox in examining the biological role of Elf5 in mammary gland development and differentiation.

Iron-responsive chromatin remodelling and MAPK signalling enhance adhesion in Candida albicans.

Recent cumulative data show that various transcription factors are recruited to the chromatin in an iron‐responsive manner to affect diverse cellular functions in the pathogenic fungus Candida albicans. Here we identified groups of iron‐responsive genes in C. albicans by chromatin remodelling analysis at gene promoters, using micrococcal nuclease (MNase) digestion followed by deep sequencing. Chromatin in the promoter regions of iron uptake and utilization genes showed repressed and active configuration, respectively, under iron‐replete conditions. GO Term enrichment analysis of genes with differentially remodelled chromatin, in respective promoter locales, suggested that many genes involved in adhesion are also iron‐responsive. C. albicans was observed to be more self‐adherent (twofold increase) and formed higher biofilm mass (77% increase) in the presence of iron. Furthermore, we identified various known and novel adhesion‐related genes with iron‐dependent active chromatin profiles that are indicative of potential upregulation under iron‐replete conditions. Transcription factor Cph1 that is activated upon Cek1 phosphorylation also showed an active chromatin profile under iron‐replete conditions and cells showed iron‐responsive Cek1 MAPK phosphorylation in the presence of iron. Thus, iron affects diverse biological functions by modulating chromatin profiles of large gene sets and by signalling through Cek1 MAPK in C. albicans.

ArchAlign: coordinate-free chromatin alignment reveals novel architectures

To facilitate identification and characterization of genomic functional elements, we have developed a chromatin architecture alignment algorithm (ArchAlign). ArchAlign identifies shared chromatin structural patterns from high-resolution chromatin structural datasets derived from next-generation sequencing or tiled microarray approaches for user defined regions of interest. We validated ArchAlign using well characterized functional elements, and used it to explore the chromatin structural architecture at CTCF binding sites in the human genome. ArchAlign is freely available at http://www.acsu.buffalo.edu/~mjbuck/ArchAlign.html.

ArchTEx: accurate extraction and visualization of next-generation sequence data

MOTIVATION The extension of mapped sequence tags is a common step in the analysis of single-end next-generation sequencing (NGS) data from protein localization and chromatin studies. The optimal extension can vary depending on experimental and technical conditions. Improper extension of sequence tags can obscure or mislead the interpretation of NGS results. We present an algorithm, ArchTEx (Architectural Tag Extender), which identifies the optimal extension of sequence tags based on the maximum correlation between forward and reverse tags and extracts and visualizes sites of interest using the predicted extension. AVAILABILITY AND IMPLEMENTATION ArchTEx requires Java 1.6 or newer. Source code and the compiled program are freely available at http://sourceforge.net/projects/archtex/ CONTACT mjbuck@buffalo.edu SUPPLEMENTARY INFORMATION Supplementary data are available at Bioinformatics online.

Chromatin Mediation of a Transcriptional Memory Effect in Yeast

Previous studies have described a transcriptional “memory effect,” whereby transcript levels of many Abf1-regulated genes in the budding yeast Saccharomyces cerevisiae are undiminished even after Abf1 has dissociated from its regulatory sites. Here we provide additional support for this effect and investigate its molecular basis. We show that the effect is observed in a distinct abf1 ts mutant from that used in earlier studies, demonstrating that it is robust, and use chromatin immunoprecipitation to show that Abf1 association is decreased similarly from memory effect and transcriptionally responsive genes at the restrictive temperature. We also demonstrate that the association of TATA-binding protein and Pol II decreases after the loss of Abf1 binding for transcriptionally responsive genes but not for memory effect genes. Examination of genome-wide nucleosome occupancy data reveals that although transcriptionally responsive genes exhibit increased nucleosome occupancy in abf1 ts yeast, the promoter regions of memory effect targets show no change in abf1 ts mutants, maintaining an open chromatin conformation even after Abf1 eviction. This contrasting behavior reflects different inherent propensity for nucleosome formation between the two classes, driven by the presence of A/T-rich sequences upstream of the Abf1 site in memory effect gene promoters. These sequence-based differences show conservation in closely related fungi and also correlate with different gene expression noise, suggesting a physiological basis for greater access to “memory effect” promoter regions. Thus, our results establish a conserved mechanism underlying a transcriptional memory effect whereby sequences surrounding Abf1 binding sequences affect local nucleosome occupancy following loss of Abf1 binding. Furthermore, these findings demonstrate that sequence-based differences in the propensity for nucleosome occupancy can influence the transcriptional response of genes to an altered regulatory signal.

An Integrative Approach to Understanding the Combinatorial Histone Code at Functional Elements

SUMMARY The rapid advancement of genomic technology has revealed the enormous complexity and combinatorial nature of chromatin modifications. To facilitate interpretation of the combinatorial nature of chromatin, we have developed a novel method to integrate all chromatin datasets into distinct nucleosome types (nucleosome alphabet). We have applied this approach to Saccharomyces cerevisiae, generating a nucleosome alphabet, which forms chromatin motifs when mapped back to the genome. By applying novel chromatin alignment and global word search approaches, we have defined distinctive chromatin motifs for introns, origins of replication, tRNAs, antisense transcripts, double-strand-break hotspots and DNase hypersensitive sites, and can distinguish genes by expression level. We have also uncovered strong associations between transcription factor binding and specific types of nucleosomes. Our results demonstrate the uses and functionality of defining a chromatin alphabet and provide a unique and novel framework for exploring chromatin architecture. CONTACT mjbuck@buffalo.edu SUPPLEMENTARY INFORMATION  Supplementary data are available at Bioinformatics online.