Cortical and cytoplasmic flows driven by actin microfilaments polarize the cortical ER-mRNA domain along the a-v axis in ascidian oocytes.
Cellular mechanisms generating the polarized redistribution of maternal Type I postplasmic/PEM mRNAs in ascidian oocytes remain unknown. We have previously shown that PEM-1 mRNA is associated with a network of rough cortical Endoplasmic Reticulum (cER) polarized along the animal-vegetal (a-v) axis forming a cER-mRNA domain in mature oocytes. We now investigate the a-v polarization of this cER-mRNA domain during meiotic maturation using H. roretzi and C. intestinalis. We show that the cER and Hr-PEM-1 aggregate as interconnected cortical patches at the cell periphery before maturation, which uniformly spread out during maturation and form a reticulated organization enriched in the vegetal hemisphere at the end of maturation. Time-lapse video recordings coupled with micromanipulations reveal that stereotyped surface, cortical and cytoplasmic flows accompany the vegetal shift of the cER-mRNA domain and mitochondria-rich myoplasm. Treatments with cytochalasin B and nocodazole indicate that both polarization of the cER-mRNA domain and mitochondria-rich myoplasm and cortical and cytoplasmic flows depend on actin cytoskeleton, but not microtubules. Using cortical fragments prepared from maturing oocytes coupled with high resolution immuno/in situ localization, we have further analyzed the effects of these inhibitors on the reorganizations the cER network and Hr-PEM-1 mRNA. We show that before maturation starts, Hr-PEM-1 mRNAs are already associated with the cER, and actin cytoskeleton inhibitors disturb their association. Finally, we hypothesize that Germinal Vesicle Break Down (GVBD) triggers an actomyosin-dependent cortical flow which directs the a-v polarization of ascidian oocytes.
Prodon
François
F
Department of Biological Sciences, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan. francois.prodon@obs-vlfr.fr
Sardet
Christian
C
Nishida
Hiroki
H
D008841:Actin Cytoskeleton / Q000502:physiology*; D000818:Animals; D019521:Body Patterning / Q000502:physiology*; D016764:Cell Polarity / Q000502:physiology*; D003593:Cytoplasm / Q000502:physiology*; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000378:metabolism / Q000502:physiology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005260:Female; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008954:Models, Biological; D009865:Oocytes / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000378:metabolism; D014561:Urochordata / Q000196:embryology*
D012333:RNA, Messenger