Stabilization of actin filaments prevents germinal vesicle breakdown and affects microtubule organization in Xenopus oocytes

Abstract

In Xenopus oocytes, extremely giant nuclei, termed germinal vesicles, contain a large amount of actin filaments most likely for mechanical integrity. Here, we show that microinjection of phalloidin, an F-actin-stabilizing drug, prevents the germinal vesicle breakdown (GVBD) in oocytes treated with progesterone. These nuclei remained for more 12 h after control oocytes underwent GVBD. Immunostaining showed significant elevation of actin in the remaining nuclei and many actin filament bundles in the cytoplasm. Furthermore, microtubules formed unusual structures in both nuclei and cytoplasm of phalloidin-injected oocytes stimulated by progesterone. Cytoplasmic microtubule arrays and intranuclear microtubules initially formed in phalloidin-injected oocytes as control oocytes exhibited white maturation spots; these structures gradually disappeared and finally converged upon intranuclear short bundles when control oocytes completed maturation. In contrast, treatment of oocytes with jasplakinolide, a cell membrane-permeable actin filament-stabilizing drug, did not affect GVBD. This drug preferentially induced accumulation of actin filaments at the cortex without any increase in cytoplasmic actin staining. Based on these results, intranuclear and cytoplasmic actin filament dynamics appear to be required for the completion of GVBD and critically involved in the regulation of microtubule assembly during oocyte maturation in Xenopus laevis.

Activation of ADF/cofilin by phosphorylation-regulated Slingshot phosphatase is required for the meiotic spindle assembly inXenopus laevisoocytes

“…To clarify this point, we treated oocytes with jasplakinolide to stabilize actin filaments and examined its effect on spindle formation (Figure 6E). As previously shown (Okada et al ., 2012), jasplakinolide treatment induced the formation of thick actin layer at the oocyte cortex (Figure 6F). In these oocytes, fine, radial microtubule structures were also scattered in the cytoplasm, which resembled structures formed in anti-XSSH-antibody–injected oocytes.…”

Regulation of GVBD in mouse oocytes by miR-125a-3p and Fyn kinase through modulation of actin filaments

“…Oocytes that underwent GVBD exhibited abnormal spindle and decrease in the intensity of cytoplasmic actin filaments labeling 46 . In Xenopus oocytes, actin filaments were found in the nucleus 47, 48 and stabilization of actin filaments by microinjection of phalloidin prevented GVBD and altered the organization of microtubules 49 . In starfish oocytes, in which the process of GVBD is not dependent on microtubules 50 , a mesh of actin filaments polymerizes transiently on the inner nuclear membrane and drives GVBD 51 .…”

Structure of the formin-interaction domain of the actin nucleation-promoting factor Bud6

“…Dimeric Bud6 is expected to recruit a total of four actin monomers in association with the Bni1 dimer, whereas Spire could array as many as eight actin subunits with its multiple WH2 domains in association with Cappuccino dimer. APC dimerizes, like Bud6, and then binds at least four actin monomers with high affinity (49,50). There is no known homolog of Bud6 in higher eukaryotes, but the mechanistic (12,13).…”