The dynamic structure of individual nucleosomes was examined by stretching nucleosomal arrays with a feedback-enhanced optical trap. Forced disassembly of each nucleosome occurred in three stages. Analysis of the data using a simple worm-like chain model yields 76 bp of DNA released from the histone core at low stretching force. Subsequently, 80 bp are released at higher forces in two stages: full extension of DNA with histones bound, followed by detachment of histones. When arrays were relaxed before the dissociated state was reached, nucleosomes were able to reassemble and to repeat the disassembly process. The kinetic parameters for nucleosome disassembly also have been determined.

The interface between cellular systems involving small noncoding RNAs and epigenetic change remains largely unexplored in metazoans. RNA-induced silencing systems have the potential to target particular regions of the genome for epigenetic change by locating specific sequences and recruiting chromatin modifiers. Noting that several genes encoding RNA silencing components have been implicated in epigenetic regulation in Drosophila, we sought a direct link between the RNA silencing system and heterochromatin components. Here we show that PIWI, an ARGONAUTE/PIWI protein family member that binds to Piwi-interacting RNAs (piRNAs), strongly and specifically interacts with heterochromatin protein 1a (HP1a), a central player in heterochromatic gene silencing. The HP1a dimer binds a PxVxL-type motif in the N-terminal domain of PIWI. This motif is required in fruit flies for normal silencing of transgenes embedded in heterochromatin. We also demonstrate that PIWI, like HP1a, is itself a chromatin-associated protein whose distribution in polytene chromosomes overlaps with HP1a and appears to be RNA dependent. These findings implicate a direct interaction between the PIWI-mediated small RNA mechanism and heterochromatin-forming pathways in determining the epigenetic state of the fly genome.[Keywords: PIWI; ARGONAUTE; HP1; heterochromatin; epigenetic; RNAi] Supplemental material is available at http://www.genesdev.org. . In TGS, small RNAs are incorporated into specialized effector complexes able to regulate chromatin modification, resulting in reduced access for the transcriptional machinery to chromatin. The TGS system that contributes to initiation and maintenance of heterochromatin formation in Schizosaccharomyces pombe is particularly well characterized (Verdel and Moazed 2005;Grewal and Jia 2007). In S. pombe, the RNA silencing system targets histone 3 Lys 9 (H3K9) methylation and recruits HP1 homolog Swi6 to the MAT locus and to repetitive elements in pericentric heterochromatin, setting up a heterochromatin spreading/maintenance loop that is dependent on the interaction of Swi6 with the Clr4 histone methyltransferase (HMT) (Grewal and Jia 2007). In plants, a specialized small RNA pathway utilizes heterochromatic RNAs to direct DNA methylation and presumably other chromatin modifications to effect silencing of target loci (for review, see Vaucheret 2007). In Caenorhabditis elegans, a single episode of RNA interference (RNAi) exposure can induce silencing inherited over many generations; mutations that abolish inheritance are all involved in chromatin structure, suggesting a chromatin-based mechanism (Vastenhouw et al. 2006). While the work in fission yeast and plants provides a possible paradigm for RNAi-dependent TGS in metazoans, a specialized TGS effector complex has not yet been characterized in an animal.

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This study evaluated chondrogenesis of mesenchymal progenitor stem cells (MSCs) cultured initially under pre-confluent monolayer conditions exposed to transforming growth factor-81 (TGF-Pl ), and subsequently in three-dimensional cultures containing insulin-like growth factor I (IGF-I). Bone marrow aspirates and chondrocytes were obtained from horses and cultured in monolayer with 0 or 5 ng of TGF-P1 per ml of medium for 6 days. TGIF-81 treated and untreated cultures were distributed to threedimensional fibrin disks containing 0 or 100 ng of IGF-I per ml of medium to establish four treatment groups. After 13 days, cultures were assessed by toluidine blue staining, collagen types I and I1 in situ hybridization and immunohistochemistry, proteoglycan production by [35S]-s~ilfate incorporation, and disk DNA content by fluorometry. Mesenchymal cells in monolayer cultures treated with TGF-81 actively proliferated for the first 4 days, developed cellular rounding, and formed cell clusters. Treated MSC cultures had a two-fold increase in medium proteoglycan content. Pretreatment of MSCs with TGF-81 followed by exposure of cells to IGF-I in three-dimensional culture significantly increased the formation of markers of chondrocytic flmction including disk proteoglycan content and procollagen type I1 mRNA production. However, proteoglycan and procollagen type I1 production by MSC's remained lower than parallel chondrocyte cultures. MSC pretreatment with TGF-P1 without sequential IGF-I was less effective in initiating expression of markers of chondrogenesis. This study indicates that although MSC differentiation was less than complete when compared to mature chondrocytes, chondrogenesis was observed in IGF-I supplemented cultures, particularly when used in concert with TGF-81 pretreatment. 0 9001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.