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NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 6 | MARCH 2005 | 195

It is well known that the extensive passaging ofmost normal cells in culture induces a state of irreversible cell-cycle arrest that is known ascellular senescence. Previous studies have shownthat the entry of cells into senescence isaccompanied by marked changes in nuclearmorphology — the chromatin of senescent cellscondenses to form several transcriptionallyinactive senescence-associated heterochromatinfoci (SAHF) that are enriched in heterochromatinprotein-1 (HP1). It has been suggested that thegenes that promote cell proliferation are stablysilenced by sequestration to SAHF, but the precisecontribution of SAHF to the permanent cell-cyclearrest that typifies senescence is unclear. Now, bycharacterizing the composition and assembly ofSAHF, Peter Adams and colleagues have shed lighton the relationship between chromatin structureand senescence and they report their findings inDevelopmental Cell.

Adams and co-workers induced senescence inprimary human fibroblasts, and analysed thechanges that took place in the nucleus of thesecells using immunofluorescence microscopy. Theyfound that, in addition to HP1, SAHF containedthe variant histone macroH2A. MacroH2A isenriched on the mammalian inactive Xchromosome and is strongly associated withtranscriptional repression. So, SAHF mightrepresent sites in the nucleus where proliferation-promoting genes are packaged into repressiveheterochromatin — thereby ensuring that thesegenes are refractory to mitogenic stimuli.

Interestingly, the promyelocytic leukaemia(PML) body — a nuclear organelle that has beenimplicated in the promotion of senescence —seems to have a role in the formation of SAHF.Adams and his team observed that, shortly afterthe induction of senescence, HP1 proteinsaccumulated in PML bodies. Subsequently,HP1 relocated from PML bodies to SAHF.Furthermore, the authors observed that thehistone-chaperone protein HIRA localized toPML bodies coincident with HP1. HIRA forms acomplex with another histone-chaperone proteinASF1a, and the yeast orthologues of HIRA andASF1a are implicated in the assembly ofheterochromatin. The authors therefore proposedthat the HIRA–ASF1a complex might promote the formation of SAHF. Consistent with thisprediction, the overexpression of HIRA andASF1a in primary human fibroblasts induced theformation of macroH2A- and HP1-containingSAHF. And, when human fibroblasts were inducedto senesce, the knockdown of ASF1a by short

hairpin RNA inhibited the formation of SAHFand delayed cell-cycle exit. This supports the viewthat the HIRA–ASF1a-mediated formation ofSAHF is necessary for efficient cell-cycle exitduring senescence.

So how does the HIRA–ASF1a complexpromote the deposition of macroH2A in SAHF?As HIRA seemingly does not interact directly with macroH2A, the authors reasoned that thiscomplex is unlikely to directly incorporatemacroH2A into chromatin. Instead, they suggestthat human ASF1a and HIRA function at a pointupstream of macroH2A deposition, whichindirectly facilitates the subsequent incorporationof macroH2A by other histone chaperones. Theyalso suggest that the localization of HP1 andHIRA to PML bodies, prior to SAHF formation,modifies these proteins in some way to facilitateSAHF assembly. Importantly, the participation ofPML bodies in the reorganization of chromatinthat accompanies senescence might explain thetumour-suppressor activity of these enigmaticnuclear bodies.

Shannon Amoils

References and linksORIGINAL RESEARCH PAPER Zhang, R. et al. Formation ofmacroH2A-containing senescence-associated heterochromatin fociand senescence driven by ASF1a and HIRA. Dev. Cell 8, 19–30 (2005)FURTHER READING Schulz, L. & Tyler, J. Heterochromatin focuseson senescence. Mol. Cell 17, 168–170 (2005)

An organized exit

N U C L E A R O R G A N I Z AT I O Nthereby stabilizing it and allowingRNA-dependent RNA polymerase(RdRP) to amplify the response bygenerating abundant secondarysiRNAs. This possibility is consistentwith data indicating that RDE-3 isnot required for RNAi that is initi-ated by large amounts of transgene-expressed dsRNA. In addition, the fission yeast RDE-3 homologue Cid12interacts with RdRP, and the Mellogroup has unpublished data showingthat the detectable accumulation ofsiRNA during RNAi requires RdRPactivity. Clearly, other explanationsare possible and functional studies areneeded to resolve this question.

Arianne Heinrichs

References and linksORIGINAL RESEARCH PAPER Chen, C.-C. G. et al. A member of the polymeraseβ nucleotidyltransferase superfamily is required forRNA interference in C. elegans. Curr. Biol. 13 Jan 2005 (doi:10.1016/S0960982205000370)

Two senescent primary human fibroblasts showing SAHF (blue), HIRA(red) and PML bodies (green). The HIRA and PML bodies colocalize(yellow). Image kindly provided by Peter Adams, Fox Chase CancerCenter, Philadelphia, USA.