We report the generation and analysis of functional data from multiple, diverse experiments performed on a targeted 1% of the human genome as part of the pilot phase of the ENCODE Project. These data have been further integrated and augmented by a number of evolutionary and computational analyses. Together, our results advance the collective knowledge about human genome function in several major areas. First, our studies provide convincing evidence that the genome is pervasively transcribed, such that the majority of its bases can be found in primary transcripts, including non-protein-coding transcripts, and those that extensively overlap one another. Second, systematic examination of transcriptional regulation has yielded new understanding about transcription start sites, including their relationship to specific regulatory sequences and features of chromatin accessibility and histone modification. Third, a more sophisticated view of chromatin structure has emerged, including its inter-relationship with DNA replication and transcriptional regulation. Finally, integration of these new sources of information, in particular with respect to mammalian evolution based on inter- and intra-species sequence comparisons, has yielded new mechanistic and evolutionary insights concerning the functional landscape of the human genome. Together, these studies are defining a path for pursuit of a more comprehensive characterization of human genome function.
Transposable elements (TEs) are mobile genetic elements, highly enriched in heterochromatin, that constitute a large percentage of the DNA content of eukaryotic genomes. Aging in Drosophila melanogaster is characterized by loss of repressive heterochromatin structure and loss of silencing of reporter genes in constitutive heterochromatin regions. Using next-generation sequencing, we found that transcripts of many genes native to heterochromatic regions and TEs increased with age in fly heads and fat bodies. A dietary restriction regimen, known to extend life span, repressed the age-related increased expression of genes located in heterochromatin, as well as TEs. We also observed a corresponding age-associated increase in TE transposition in fly fat body cells that was delayed by dietary restriction. Furthermore, we found that manipulating genes known to affect heterochromatin structure, including overexpression of Sir2, Su(var)3-9, and Dicer-2, as well as decreased expression of Adar, mitigated age-related increases in expression of TEs. Increasing expression of either Su(var)3-9 or Dicer-2 also led to an increase in life span. Mutation of Dicer-2 led to an increase in DNA double-strand breaks. Treatment with the reverse transcriptase inhibitor 3TC resulted in decreased TE transposition as well as increased life span in TE-sensitized Dicer-2 mutants. Together, these data support the retrotransposon theory of aging, which hypothesizes that epigenetically silenced TEs become deleteriously activated as cellular defense and surveillance mechanisms break down with age. Furthermore, interventions that maintain repressive heterochromatin and preserve TE silencing may prove key to preventing damage caused by TE activation and extending healthy life span.aging | heterochromatin | transposable elements | dietary restriction | silencing
The most widely used method for detecting genome-wide protein–DNA interactions is chromatin immunoprecipitation on tiling microarrays, commonly known as ChIP-chip. Here, we conducted the first objective analysis of tiling array platforms, amplification procedures, and signal detection algorithms in a simulated ChIP-chip experiment. Mixtures of human genomic DNA and “spike-ins” comprised of nearly 100 human sequences at various concentrations were hybridized to four tiling array platforms by eight independent groups. Blind to the number of spike-ins, their locations, and the range of concentrations, each group made predictions of the spike-in locations. We found that microarray platform choice is not the primary determinant of overall performance. In fact, variation in performance between labs, protocols, and algorithms within the same array platform was greater than the variation in performance between array platforms. However, each array platform had unique performance characteristics that varied with tiling resolution and the number of replicates, which have implications for cost versus detection power. Long oligonucleotide arrays were slightly more sensitive at detecting very low enrichment. On all platforms, simple sequence repeats and genome redundancy tended to result in false positives. LM-PCR and WGA, the most popular sample amplification techniques, reproduced relative enrichment levels with high fidelity. Performance among signal detection algorithms was heavily dependent on array platform. The spike-in DNA samples and the data presented here provide a stable benchmark against which future ChIP platforms, protocol improvements, and analysis methods can be evaluated.
SummaryChromatin structure affects the accessibility of DNA to transcription, repair, and replication. Changes in chromatin structure occur during development, but less is known about changes during aging. We examined the state of chromatin structure and its effect on gene expression during aging in Drosophila at the whole genome and cellular level using whole-genome tiling microarrays of activation and repressive chromatin marks, whole-genome transcriptional microarrays and single-cell immunohistochemistry. We found dramatic reorganization of chromosomal regions with age. Mapping of H3K9me3 and HP1 signals to fly chromosomes reveals in young flies the expected high enrichment in the pericentric regions, the 4th chromosome, and islands of facultative heterochromatin dispersed throughout the genome. With age, there is a striking reduction in this enrichment resulting in a nearly equivalent level of H3K9me3 and HP1 in the pericentric regions, the 4th chromosome, facultative heterochromatin, and euchromatin. These extensive changes in repressive chromatin marks are associated with alterations in age-related gene expression. Large-scale changes in repressive marks with age are further substantiated by single-cell immunohistochemistry that shows changes in nuclear distribution of H3K9me3 and HP1 marks with age. Such epigenetic changes are expected to directly or indirectly impinge upon important cellular functions such as gene expression, DNA repair, and DNA replication. The combination of genome-wide approaches such as whole-genome chromatin immunoprecipitation and transcriptional studies in conjunction with single-cell immunohistochemistry as shown here provide a first step toward defining how changes in chromatin may contribute to the process of aging in metazoans.
The periodontal ligament (PDL) and alveolar bone are two critical tissues for understanding orthodontic tooth movement. The current literature is replete with descriptive studies of multiple cell types and their matrices in the PDL and alveolar bone, but is deficient with how stem/progenitor cells differentiate into PDL and alveolar bone cells. Can one type of orthodontic force with a specific magnitude and frequency preferably activate osteoblasts, whereas another force type activates osteoclasts? This chapter will discuss the biology of not only mature cells and their matrices in the periodontal ligament and alveolar bone, but also stem/progenitor cells that differentiate into fibroblasts, osteoblasts and osteoclasts. Key advances in tooth movement rely on further understanding of osteoblast and fibroblast differentiation from mesenchymal stem/progenitor cells, and osteoclastogenesis from the hematopoietic/monocyte lineage.
Heterochromatin formation drives epigenetic mechanisms associated with silenced gene expression. Repressive heterochromatin is established through the RNA interference pathway, triggered by double-stranded RNAs that can be modified via RNA editing. However, the biological consequences of such modifications remain enigmatic. Here we show that RNA editing regulates heterochromatic gene silencing in Drosophila. We utilize the binding activity of an RNA editing enzyme to visualize the in vivo production of a long double-stranded RNA trigger mediated by Hoppel transposable elements. Using homologous recombination, we delete this trigger, dramatically altering heterochromatic gene silencing and chromatin architecture. Furthermore, we show that the trigger RNA is edited, serves as a key regulator of chromatin state, and that dADAR auto-editing generates a natural suppressor of gene silencing. Lastly, systemic differences in RNA editing activity generates inter-individual variation in silencing state within a population. Our data reveal a global role for RNA editing in regulating gene expression.
Organ development requires complex signaling by cells in different tissues. Epithelium and mesenchyme interactions are crucial for the development of skin, hair follicles, kidney, lungs, prostate, major glands, and teeth. Despite myriad literature on cell–cell interactions and ligand–receptor binding, the roles of extracellular vesicles in epithelium–mesenchyme interactions during organogenesis are poorly understood. Here, we discovered that ~100 nm exosomes were secreted by the epithelium and mesenchyme of a developing tooth organ and diffused through the basement membrane. Exosomes were entocytosed by epithelium or mesenchyme cells with preference by reciprocal cells rather than self-uptake. Exosomes reciprocally evoked cell differentiation and matrix synthesis: epithelium exosomes induce mesenchyme cells to produce dentin sialoprotein and undergo mineralization, whereas mesenchyme exosomes induce epithelium cells to produce basement membrane components, ameloblastin and amelogenenin. Attenuated exosomal secretion by Rab27a/b knockdown or GW4869 disrupted the basement membrane and reduced enamel and dentin production in organ culture and reduced matrix synthesis and the size of the cervical loop, which harbors epithelium stem cells, in Rab27aash/ash mutant mice. We then profiled exosomal constituents including miRNAs and peptides and further crossed all epithelium exosomal miRNAs with literature-known miRNA Wnt regulators. Epithelium exosome-derived miR135a activated Wnt/β-catenin signaling and escalated mesenchymal production of dentin matrix proteins, partially reversible by Antago-miR135a attenuation. Our results suggest that exosomes may mediate epithelium–mesenchyme crosstalk in organ development, suggesting that these vesicles and/or the molecular contents they are transporting may be interventional targets for treatment of diseases or regeneration of tissues.
Synopsis The primary goal of regenerative endodontics is to restore the vitality and functions of the dentin-pulp complex, as opposed to filing of the root canal with bioinert materials. Structural restoration is also important but is likely secondary to vitality and functions. Myriads growth factors regulate multiple cellular functions including migration, proliferation, differentiation and apoptosis of several cell types that are intimately involved in dentin-pulp regeneration: odontoblasts, interstitial fibroblasts, vascular-endothelial cells and sprouting nerve fibers. Recent work showing that growth factor delivery, without cell transplantation, can yield pulp-dentin like tissues in vivo provides one of the tangible pathways for regenerative endodontics. This review synthesizes our knowledge on a multitude of growth factors that are known or anticipated to be efficacious in dental pulp-dentin regeneration.
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