ORGANIZZAZIONE DEL CITOSCHELETRO, ESPRESSIONE EDISTRIBUZIONE INTRACITOPLASMATICA DI STAUFEN(RNA-BINDING PROTEIN) E CALRETICULINA IN OVOCITIUMANI DURANTE IL PROCESSO MATURATIVO

Before the mammalian oocyte engages in the fertilization process, it must acquire an array of molecular and cellular assets defining its developmental potential. These properties specify competencies to complete meiosis and initiate mitosis. Meiotic maturation requires the acquisition both of nuclear and cytoplasmic competence and this complex mechanism involves most of the organelles, the cytoskeleton and molecules that are relocated from the nucleus to the cytoplasm. Messenger RNAs of maternal origin are accumulated in the oocyte throughout its growth in the ovary. These transcripts are then shuttled to specific sites of the ooplasm, where local translation is promoted. The oocyte maturation marks the beginning of this phase of activation of maternal transcripts and proteins and involves the "unmasking" of maternal mRNA concomitantly to post-translational changes of stored proteins. This process loosens the stabilization mechanisms that had been imposed during oogenesis. At the end of this route maternal information will be available to the developing embryo. In this scenario, Staufen RNA Binding Protein may play an important role in the light of its function of vector of mRNAs from the nucleus to the sites of translation. Therefore, we hypothesized that Staufen is expressed during oocyte maturation and that the localization of Staufen protein changes during the meiotic transition from GV to MII. We also postulated that the pattern of Staufen distribution reflects its physical interaction with the endoplasmic reticulum, an organelle directly involved in the process of mRNA translation whose relocalization is central to the process of oocyte maturation. To test the above hypotheses, we assessed the expression of Staufen and Calreticulin (CALR), the latter adopted as a marker of the endoplasmic reticulum, in human oocytes at different stages of maturation: GV, MI and MII. The oocytes were obtained from supernumerary material of assisted reproductive cycles and were subjected to polymerase chain reaction in order to investigate the expression of Staufen, CALR and related RNAs. The corresponding protein products were identified by immunofluorescence and confocal laser scanning microscopy. Our findings indicate that Staufen and CALR are constantly expressed and selectively localized during oocyte maturation. The different pattern of Staufen distribution at the GV, MI and MII stages implicates that localization of protein translation is one of the levels at which gene expression is regulated in the maturing oocyte. On the other hand, localization of CALR at the MII stage reveals novel aspects of physical interaction with the MII spindle and suggests a previously unrecognised role of the endoplasmic reticulum in support of cytoskeletal function in the human oocyte.