X-box-binding protein 1 (XBP1) is an important regulator of a subset of genes active during endoplasmic reticulum (ER) stress. In the present study, we analyzed XBP1 level and location to explore the effect of ER stress on oocyte maturation and developmental competency of porcine embryos in an in vitro culture system. First, we examined the localization of XBP1 at different meiotic stages of porcine oocytes and at early stages of parthenogenetic embryo development. Fluorescence staining showed that expression of functional XBP1 was weak in mature oocytes and at the 1-, 2-, and 8-cell stages of embryos but abundant at the germinal vesicle (GV), 4-cell, morula, and blastocyst stages. In addition, RT-PCR revealed that both spliced XBP1 (XBP1-s) and unspliced XBP1 (XBP1-u) were expressed at the GV, 4-cell, morula, and blastocyst stages. Tunicamycin, an ER stress inducer, induced active XBP1 protein in nuclei of 4-cell embryos. Next, porcine embryos cultured in the presence of tauroursodeoxycholate, an ER stress inhibitor, were studied. Total cell numbers and the extent of the inner cell mass increased (P < 0.05), whereas the rate of nuclear apoptosis decreased (P < 0.05). Moreover, expression of the antiapoptotic gene BCL2 increased, whereas expression of the proapoptotic genes BCL2L1 (Bcl-xl) and TP53 decreased. The results indicated that inhibition of ER stress enhanced porcine oocyte maturation and embryonic development by preventing ER stress-mediated apoptosis in vitro.
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Vol. 86 • No. 4