👤 Regine Losson

Recent advances reveal emerging unique functions of poly(ADP-ribose) polymerase-1 (Parp-1) and Parp-2 in heterochromatin integrity and cell differentiation. However, the chromatin-mediated molecular and cellular events involved remain elusive. Here we describe specific physical and functional interactions of Parp-1 and Parp-2 with the transcriptional intermediary factor (TIF1beta) and the heterochromatin proteins (HP1) that affect endodermal differentiation. We show that Parp-2 binds to TIF1beta with high affinity both directly and through HP1alpha. Both partners colocalize at pericentric heterochromatin in primitive endoderm-like cells. Parp-2 also binds to HP1beta but not to HP1gamma. In contrast Parp-1 binds weakly to TIF1beta and HP1beta only. Both Parps selectively poly(ADP-ribosyl)ate HP1alpha. Using shRNA approaches, we provide evidence for distinct participation of both Parps in endodermal differentiation. Whereas Parp-2 and its activity are required for the relocation of TIF1beta to heterochromatic foci during primitive endodermal differentiation, Parp-1 and its activity modulate TIF1beta-HP1alpha association with consequences on parietal endodermal differentiation. Both Parps control TIF1beta transcriptional activity. In addition, this work identifies both Parps as new modulators of the HP1-mediated subcode histone.-Quénet, D., Gasser, V., Fouillen, L., Cammas, F., Sanglier-Cianferani, S., Losson, R., Dantzer, F. The histone subcode: poly(ADP-ribose) polymerase-1 (Parp-1) and Parp-2 control cell differentiation by regulating the transcriptional intermediary factor TIF1beta and the heterochromatin protein HP1alpha. Show less

Mechanisms of transcriptional repression are important during cell differentiation. Mammalian heterochromatin protein 1 isoforms HP1alpha, HP1beta, and HP1gamma play important roles in the regulation of chromatin structure and function. We explored the possibility of different roles for the three HP1 isoforms in an integrated system, skeletal muscle terminal differentiation. In this system, terminal differentiation is initiated by the transcription factor MyoD, whose target genes remain mainly silent until myoblasts are induced to differentiate. Here we show that HP1alpha and HP1beta isoforms, but not HP1gamma, interact with MyoD in myoblasts. This interaction is direct, as shown using recombinant proteins in vitro. A gene reporter assay revealed that HP1alpha and HP1beta, but not HP1gamma, inhibit MyoD transcriptional activity, suggesting a model in which MyoD could serve as a bridge between nucleosomes and chromatin-binding proteins such as HDACs and HP1. Chromatin immunoprecipitation assays show a preferential recruitment of HP1 proteins on MyoD target genes in proliferating myoblasts. Finally, modulation of HP1 protein level impairs MyoD target gene expression and muscle terminal differentiation. Together, our data show a nonconventional interaction between HP1 and a tissue-specific transcription factor, MyoD. In addition, they strongly suggest that HP1 isoforms play important roles during muscle terminal differentiation in an isoform-dependent manner. Show less

Mammalian heterochromatin protein 1 (HP1) alpha, HP1beta and HP1gamma are closely related non-histone chromosomal proteins that function in gene silencing, presumably by organizing higher order chromatin structures. Here, we show by co-immunoprecipitation that HP1alpha, but neither HP1beta nor HP1gamma, forms a complex with the BRG1 chromatin-remodeling factor in HeLa cells. In vitro, BRG1 interacts directly and preferentially with HP1alpha. The region conferring this preferential binding has been mapped to residues 106-180 of the HP1alpha C-terminal chromoshadow domain. Using site-directed mutagenesis, we have identified three amino acid residues I113, A114 and C133 in HP1alpha (K, P and S in HP1beta and HP1gamma) that are essential for the selective interaction of HP1alpha with BRG1. Interestingly, these residues were also shown to be critical for the silencing activity of HP1alpha. Taken together, these results demonstrate that mammalian HP1 proteins are biochemically distinct and suggest an entirely novel function for BRG1 in modulating HP1alpha-containing heterochromatic structures. Show less

We report the cloning and characterization of a novel member of the Transcriptional Intermediary Factor 1 (TIF1) gene family, human TIF1gamma. Similar to TIF1alpha and TIF1beta, the structure of TIF1beta is characterized by multiple domains: RING finger, B boxes, Coiled coil, PHD/TTC, and bromodomain. Although structurally related to TIF1alpha and TIF1beta, TIF1gamma presents several functional differences. In contrast to TIF1alpha, but like TIF1beta, TIF1 does not interact with nuclear receptors in yeast two-hybrid or GST pull-down assays and does not interfere with retinoic acid response in transfected mammalian cells. Whereas TIF1alpha and TIF1beta were previously found to interact with the KRAB silencing domain of KOX1 and with the HP1alpha, MODI (HP1beta) and MOD2 (HP1gamma) heterochromatinic proteins, suggesting that they may participate in a complex involved in heterochromatin-induced gene repression, TIF1gamma does not interact with either the KRAB domain of KOX1 or the HP1 proteins. Nevertheless, TIF1gamma, like TIF1alpha and TIF1beta, exhibits a strong silencing activity when tethered to a promoter. Since deletion of a novel motif unique to the three TIF1 proteins, called TIF1 signature sequence (TSS), abrogates transcriptional repression by TIF1gamma, this motif likely participates in TIF1 dependent repression. Show less