p21-activated kinase 1 activity is required for histone H3 Ser phosphorylation and chromatin condensation in mouse oocyte meiosis.

p21-activated kinase 1 (Pak1) is essential for a variety of cellular events, including gene transcription, cytoskeletal organisation, cell proliferation and apoptosis. Pak1 is activated upon autophosphorylation on many amino residues; in particular, phosphorylation on Thr maintains maximal Pak1 activation. In the present study we investigated the protein expression, subcellular localisation and function of Pak1 phosphorylated on Thr (pPak1Thr) in mouse oocytes. pPak1Thr was detected upon meiotic resumption and localised on the condensing chromatin. Thr phosphorylation was markedly suppressed by the Pak1 ATP-competitive inhibitor PF-3758309, but not by the allosteric inhibitors IPA-3 (2.5 μM and 10μM) (1, 1'-dithiobis-2-naphthalenol) and TAT-PAK18 (10 μM), which prevent the binding of Pak1 to its upstream activators GTPase Cdc42/Rac and Pak-interacting exchange factor (PIX), respectively, implying that Pak1 activation may be independent of GTPase and PIX in oocyte meiosis. Inhibition of Pak1 activation concomitantly restrained histone H3 phosphorylation on Ser and consequently inhibited chromatin condensation; however, this phenotype was reversed by concomitant administration of the Pak1 activator FTY720. The changes in the pattern of expression of phosphorylated extracellular signal-regulated kinase 1/2 in response to PF-3758309 or FTY720 were the same as seen for pPak1Thr. These results show that activated Pak1 regulates chromatin condensation by promoting H3 Ser phosphorylation in oocytes after the resumption of meiotic progression.