A SWI/SNF-like factor from chicken liver that disrupts nucleosomes and transfers histone octamers in cis and trans

ATP-dependent chromatin remodeling factors have been implicated in nuclear processes involving DNA. Here we report partial purification and characterization of an ATP-dependent chromatin remodeling activity from chicken liver. Nuclear extract from chicken liver was fractionated chromatographically to enrich proteins immunoreacting to antibodies against components of human SWI/SNF, namely BRG1, BAF170, BAF155, and BAF57. Immunoreactivity to these antibodies elutes with a mass of about 2MDa on Sepharose CL-6B gel filtration, suggesting that they constitute a SWI/SNF-like complex (SLC). The SLC displays three chromatin-remodeling activities, viz. nucleosome disruption, octamer transfer, and nucleosome sliding (octamer transfer in cis). We further show that components of SLC, as revealed by immunoreactivity to the above antibodies, display a dynamic nucleocytoplasmic distribution and colocalize with RNA polymerase II in the liver nuclei. This report contributes to the understanding of phylogenetic generality of chromatin remodeling factors in eukaryotes.     

Mechanisms of palatal epithelial seam disintegration by transforming growth factor (TGF) beta3

TGFbeta3 signaling initiates and completes sequential phases of cellular differentiation that is required for complete disintegration of the palatal medial edge seam, that progresses between 14 and 17 embryonic days in the murine system, which is necessary in establishing confluence of the palatal stroma. Understanding the cellular mechanism of palatal MES disintegration in response to TGFbeta3 signaling will result in new approaches to defining the causes of cleft palate and other facial clefts that may result from failure of seam disintegration. We have isolated MES primary cells to study the details of MES disintegration mechanism by TGFbeta3 during palate development using several biochemical and genetic approaches. Our results demonstrate a novel mechanism of MES disintegration where MES, independently yet sequentially, undergoes cell cycle arrest, cell migration and apoptosis to generate immaculate palatal confluency during palatogenesis in response to robust TGFbeta3 signaling. The results contribute to a missing fundamental element to our base knowledge of the diverse roles of TGFbeta3 in functional and morphological changes that MES undergo during palatal seam disintegration. We believe that our findings will lead to more effective treatment of facial clefting.     

Activation of the human alpha1(I) collagen promoter by leptin is not mediated by transforming growth factor beta responsive elements

Leptin increases human alpha1 (I) collagen mRNA and type I collagen production and enhances hepatic fibrosis in animal models of hepatic fibrosis. These effects of leptin on fibrogenesis may be mediated by TGFbeta1, since leptin increases the TGFbeta type II receptor and augments the effect of TGFbeta1 on collagen production by stellate cells. In this study, leptin increased the activity of the human alpha1 (I) collagen promoter in transfected stellate cells. Leptin did not further enhance the activation of the promoter induced by TGFbeta1. Leptin had no effects on the transfected TGFbeta-responsive p3TP-LUX plasmid, which contains 3 CAGA elements that are essential and sufficient for the induction by TGFbeta. Leptin did not increase significantly the binding of proteins to two TGFbeta1 responsive elements in the human alpha1 (I) collagen promoter. In conclusion, this study shows that leptin activates the alpha1 (I) collagen gene and that this effect is not mediated by TGFbeta responsive elements.     

XNP-1/ATR-X acts with RB, HP1 and the NuRD complex during larval development in C. elegans

Mutations in the XNP/ATR-X gene cause several X-linked mental retardation syndromes in humans. The XNP/ATR-X gene encodes a DNA-helicase belonging to the SNF2 family. It has been proposed that XNP/ATR-X might be involved in chromatin remodelling. The lack of a mouse model for the ATR-X syndrome has, however, hampered functional studies of XNP/ATR-X. C. elegans possesses one homolog of the XNP/ATR-X gene, named xnp-1. By analysing a deletion mutant, we show that xnp-1 is required for the development of the embryo and the somatic gonad. Moreover, we show that abrogation of xnp-1 function in combination with inactivation of genes of the NuRD complex, as well as lin-35/Rb and hpl-2/HP1 leads to a stereotyped block of larval development with a cessation of growth but not of cell division. We also demonstrate a specific function for xnp-1 together with lin-35 or hpl-2 in the control of transgene expression, a process known to be dependent on chromatin remodelling. This study thus demonstrates that in vivo XNP-1 acts in association with RB, HP1 and the NuRD complex during development.     

The Drosophila Brahma (SWI/SNF) chromatin remodeling complex exhibits cell-type specific activation and repression functions

The Brahma (Brm) complex of Drosophila melanogaster is a SWI/SNF-related chromatin remodeling complex required to correctly maintain proper states of gene expression through ATP-dependent effects on chromatin structure. The SWI/SNF complexes are comprised of 8-11 stable components, even though the SWI2/SNF2 (BRM, BRG1, hBRM) ATPase subunit alone is partially sufficient to carry out chromatin remodeling in vitro. The remaining subunits are required for stable complex assembly and/or proper promoter targeting in vivo. Our data reveals that SNR1 (SNF5-Related-1), a highly conserved subunit of the Brm complex, is required to restrict complex activity during the development of wing vein and intervein cells, illustrating a functional requirement for SNR1 in modifying whole complex activation functions. Specifically, we found that snr1 and brm exhibited opposite mutant phenotypes in the wing and differential misregulation of genes required for vein and intervein cell development, including rhomboid, decapentaplegic, thick veins, and blistered, suggesting possible regulatory targets for the Brm complex in vivo. Our genetic results suggest a novel mechanism for SWI/SNF-mediated gene repression that relies on the function of a 'core' subunit to block or shield BRM (SWI2/SNF2) activity in specific cells. The SNR1-mediated repression is dependent on cooperation with histone deacetylases (HDAC) and physical associations with NET, a localized vein repressor.     

Insights into role of bromodomain, testis-specific (Brdt) in acetylated histone H4-dependent chromatin remodeling in mammalian spermiogenesis

Mammalian spermiogenesis is of considerable biological interest especially due to the unique chromatin remodeling events that take place during spermatid maturation. Here, we have studied the expression of chromatin remodeling factors in different spermatogenic stages and narrowed it down to bromodomain, testis-specific (Brdt) as a key molecule participating in chromatin remodeling during rat spermiogenesis. Our immunocytochemistry experiments reveal that Brdt colocalizes with acetylated H4 in elongating spermatids. Remodeling assays showed an acetylation-dependent but ATP-independent chromatin reorganization property of Brdt in haploid round spermatids. Furthermore, Brdt interacts with Smarce1, a member of the SWI/SNF family. We have studied the genomic organization of smarce1 and identified that it has two splice variants expressed during spermatogenesis. The N terminus of Brdt is involved in the recognition of Smarce1 as well as in the reorganization of hyperacetylated round spermatid chromatin. Interestingly, the interaction between Smarce1 and Brdt increases dramatically upon histone hyperacetylation both in vitro and in vivo. Thus, our results indicate this interaction to be a vital step in the chromatin remodeling process during mammalian spermiogenesis.     

Altered primordial germ cell migration in the absence of transforming growth factor beta signaling via ALK5

Transforming growth factor beta (TGFbeta) inhibits proliferation and promotes the migration of primordial germ cells (PGCs) towards explants of gonadal ridges in vitro. However, its effects in vivo are still unclear. Here, we analyzed the behavior of PGCs in embryos lacking TGFbeta signaling via the type I receptor ALK5. TGFbeta in vivo was neither a chemoattractant for PGCs, nor did it affect their proliferation during migration towards the gonadal ridges up to embryonic day (E)10. Unexpectedly, the absence of TGFbeta signaling in fact resulted in significant facilitation of PGC migration out of the hindgut, due to the reduced deposition of collagen type I surrounding the gut of Alk5-deficient mutant embryos. Migratory PGCs adhere strongly to collagen; therefore, reduced collagen type I along the gut may result in reduced adhesion, facilitating migration into the dorsal mesenterium and gonadal ridges. Our results provide new evidence for the role of TGFbeta signaling in migration of PGCs in vivo distinct from that described previously.     

CHB2, a member of the SWI3 gene family,is a global regulator in Arabidopsis

The SWI/SNF complex is an ATP-dependent chromatin remodeling complex that plays an important role in the regulation of eukaryotic gene expression. Very little is known about the function of SWI/SNF complex in plants compared with animals and yeast. SWI3 is one of the core components of the SWI/SNF chromatin remodeling complexes in yeast. We have identified a putative SWI3-like cDNA clone, CHB2 (AtSWI3B), from Arabidopsis thaliana by screening the expressed sequence tag database. CHB2 encodes a putative protein of 469 amino acids and shares 23% amino acid sequence identity and 64% similarity with the yeast SWI3. The Arabidopsis genome contains four SWI3-like genes, namely CHB1 (AtSWI3A), CHB2 (AtSWI3B), CHB3 (AtSWI3C) and CHB4 (AtSWI3D). The expression of CHB2, CHB3 and CHB4 mRNA was detected in all tissues analyzed by RT-PCR. The expression of CHB1 mRNA, however, could not be detected in the siliques, suggesting that there is differential expression among CHB genes in different Arabidopsis tissues. To investigate the role of CHB2 in plants, Arabidopsis plants were transformed with a gene construct comprising a CHB2 cDNA in the antisense orientation driven by the CaMV 35S promoter. Repression of CHB2 expression resulted in pleiotropic developmental abnormalities including abnormal seedling and leaf phenotypes, dwarfism, delayed flowering and no apical dominance, suggesting a global role for CHB2 in the regulation of gene expression. Our results indicate that CHB2 plays an essential role in plant growth and development.