Dynamic nuclear polarization of HP1a is correlated to gene expression during cell differentiation

MARÍA ANDREA DESBATS; LUCIANA PAOLA PRENDES; GRACIELA PIWIEN PILLIPUK

Isle sur la Sorgue - Francia

Congreso; EMBO Conference on Nuclear Structure and Dynamics; 2009

EMBO

Cell differentiation is a complex process in which a specific subset of genes is activated while the rest are silenced. In this regard, HP1a has been shown to control gene silencing during differentiation. HP1g was found associated to actively transcribed genes, however little is known about its role during cell differentiation. In 3T3L1 preadipocytes HP1g localizes both in heterochromatic and euchromatic domains, as demonstrated by indirect immunofluorescence and confocal microscopy. Intriguingly, when preadipocytes are induced to differentiate, HP1g rapidly and transiently concentrates in one pole of the nucleus being excluded from heterochromatin. This subnuclear redistribution is exclusive of HP1g; HP1a and HP1b localization remains unchanged as cells differentiate. Nuclear polarization of HP1g is also observed when N2a neuroblastoma and C2C12 myoblast cells are induced to differentiate; in contrast, it is not observed in cells that do not have the capacity to differentiate e.g. NIH-3T3. Interestingly, when 3T3-L1 cells are induced to differentiate HP1g concentrates always opposite to the MTOC, suggesting that transient HP1g polarization takes place in a determined nuclear pole. This HP1g dynamic redistribution depends on active transcription, since treatment of the 3T3-L1 cells with DRB or a-amanitin abrogates HP1g polarization during initial stages of differentiation. We found that epigenetic marks that correlate with active transcription, e.g. 3MeK4H3 and H4Ac, co-localize with polarized HP1g. Noteworthy, RNA polymerase II co-localizes with polarized HP1g and this nuclear domain shows high level of BrUTP incorporation indicating that this constiturtes an area of active transcription. Knock down of HP1g by siRNAs causes a decrease in BrUTP incorporation, phosphorylation of RNA polymerase II on Ser2 (signal) and a marked decrease in the expression level of adipocyte markers. Importantly, to our knowledge this type of dynamic nuclear polarization has not been described for HP1g or any other nuclear factor, and we propose that it may constitute a novel mechanism for regulating the expression of a determined subset of genes required for triggering (during) the initial steps of cell differentiation.