RNA anchoring in the vegetal cortex of the Xenopus oocyte.

The body plan of the embryo is established by a polarized source of developmental information in the oocyte. The Xenopus laevis oocyte creates polarity by anchoring mRNAs in the vegetal cortex, including Vg1 and Xwnt-11, which might function in body plan specification, and Xcat-2, which might function in germ cell development. To identify components of the RNA anchoring mechanism, we used the manually isolated vegetal cortex (IVC) to assay loss or change in spatial arrangement of mRNAs caused by disruption of cortical elements. The role of cytoskeleton in mRNA anchoring was tested by treating oocytes with inhibitors that selectively disrupted actin microfilaments and cytokeratin filaments. Treatment of oocytes with cytochalasin B caused clumping of Vg1 and Xwnt-11 as revealed by in situ hybridization of the IVC, but did not cause their release, as confirmed by RT-PCR analysis. These mRNA clumps did not match the distribution of actin microfilament clumps, but were distributed similarly to the remnant cytokeratin filaments. Treatment of oocytes with monoclonal anti-cytokeratin antibody C11 released these mRNAs from the cortex. C11 altered the texture of the cytokeratin network, but did not affect the actin meshwork. These results show that Vg1 and Xwnt-11 are retained by a cytokeratin filament-dependent mechanism, and that organization of the cytokeratin network depend on an intact actin meshwork. Colcemid did not disrupt Vg1 and Xwnt-11 retention in the IVC, so anchoring of these mRNAs are independent of microtubules. Membrane disruption in the IVC by Triton X-100 decreased Vg1 and Xwnt-11. Loss of these mRNAs was due mainly to ribonuclease activity released from membrane components. However, when ribonuclease activity was suppressed under cold temperature, a higher amount of Vg1 and Xwnt-11 was recovered in the supernatant. This result suggested that a fraction of these mRNAs required membranes to be retained in the cortex. By contrast, Xcat-2 mRNA was neither released nor degraded following treatments with cytochalasin B, C11, colcemid and Triton X-100 under cold temperature, so no cortical element could be implicated in its anchoring.