Cell cycle dependent phosphorylation and subnuclear organization of the histone gene regulator p220NPAT in human embryonic stem cells

✍ Scribed by Prachi N. Ghule; Klaus A. Becker; J. Wade Harper; Jane B. Lian; Janet L. Stein; Andre J. van Wijnen; Gary S. Stein

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 537 KB
• Volume: 213
• Category: Article
• ISSN: 0021-9541
• DOI: 10.1002/jcp.21119

✦ Synopsis

Abstract

Human embryonic stem (ES) cells have an expedited cell cycle (∼15 h) due to an abbreviated G1 phase (∼2.5 h) relative to somatic cells. One principal regulatory event during cell cycle progression is the G1/S phase induction of histone biosynthesis to package newly replicated DNA. In somatic cells, histone H4 gene expression is controlled by CDK2 phosphorylation of p220^NPAT^ and localization of HiNF‐P/p220^NPAT^ complexes with histone genes at Cajal body related subnuclear foci. Here we show that this ‘S point’ pathway is operative in situ in human ES cells (H9 cells; NIH‐designated WA09). Immunofluorescence microscopy shows an increase in p220^NPAT^ foci in G1 reflecting the assembly of histone gene regulatory complexes in situ. In contrast to somatic cells where duplication of p220^NPAT^ foci is evident in S phase, the increase in the number of p220^NPAT^ foci in ES cells appears to precede the onset of DNA synthesis as measured by BrdU incorporation. Phosphorylation of p220^NPAT^ at CDK dependent epitopes is most pronounced in S phase when cells exhibit elevated levels of cyclins E and A. Our data indicate that subnuclear organization of the HiNF‐P/p220^NPAT^ pathway is rapidly established as ES cells emerge from mitosis and that p220^NPAT^ is subsequently phosphorylated in situ. Our findings establish that the HiNF‐P/p220^NPAT^ gene regulatory pathway operates in a cell cycle dependent microenvironment that supports expression of DNA replication‐linked histone genes and chromatin assembly to accommodate human stem cell self‐renewal. J. Cell. Physiol. 213: 9–17, 2007. © 2007 Wiley‐Liss, Inc.