Intermediate Filament Keratin Dynamics During Oocyte Maturation Requires Maturation/M-Phase Promoting Factor and Mitogen-Activated Protein Kinase Kinase Activities in the Hamster

Research related to intermediate filaments in mammalian oocytes remains poorly advanced. We investigated keratin reorganization in oocytes during meiotic maturation using immunofluorescence, and examined effects of inhibitors for cdc2 and mitogen-activated protein kinase kinase (MAPKK) on keratin assembly. In germinal vesicle (GV) oocytes (n = 26), large and oval-shaped aggregates of non-fibrillar keratin were found in the cortical ooplasm (designated as a 'cortical' pattern). The delicate network of keratin filaments was concentrated in the GV periphery. The large keratin aggregates began to divide into small fragments at the pro-MI/MI stage (n = 22, designated as a 'fragmented' pattern). Some keratin fragments were occasionally broken down into several granules at the peripheral region. In the MII oocytes (n = 24), the filament network was extended over the ooplasm and numerous keratin granules were scattered across the oocyte (designated as a 'granular' pattern). After 12 h of incubation with roscovitine, 66.7% of the oocytes (20/30) were at the GV stage and showed a cortical pattern of keratin. After incubation with U0126, most oocytes (83.9%, 26/31) were at the MII stage; most of them (76.9%, 20/26) showed a fragmented pattern of keratin. The increasing complexity of keratin filament network from the GV to MII stages suggests a possible role in maintaining cell integrity under physical stress after ovulation. In fact, maturation/M-phase promoting factor is necessary for such keratin reorganization, as is meiotic nuclear progression. In addition, MAPKK is involved in keratin reorganization from a fragmented pattern to a granular pattern.