Cell-cell communication mechanism in the corpus luteum

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In the present study, the expression of annexin V in the corpus luteum was investigated on the basis of the hypothesis that annexin V would be involved in cell-cell communication in the corpus luteum. The expression of annexin 5 was examined by immunocytochemistry, in situ hybridization, and Northern blot analysis with special reference to the functional stage of corpus luteum. Annexin 5, a member of annexin family protein, has been shown to be distributed p referentially to the corpus luteum in the ovary. As granulosa cells do not express annexin 5, an unknown regulatory mechanism of annexin 5 synthesis in the ovarian tissue is presumed. Although immunocytochemical analysis of annexin 5 demonstrated the distribution of annexin 5 to most luteal cells, in situ hybridization showed that only large luteal cells express annexin 5 mRNA.The expression of annexin 5 mRNA was decreased significantly at mid-pseudopregnancy and this change was shown to be due to a decrease in luteal expression. When the expression of annexin 5 was compared in the ovarian tissue on the 12th day of both pregnancy and pseudopregnancy, the pregnant ovaries showed a lower expression of annexin 5 mRNA.While the membrane association of annexin 5 was distinctly observed in the pregnant corpus luteum, it was very slight in the regressing luteal cells of pseudopregnancy. Administration of bromoergocriptin at mid-pseudopregnancy increased annexin 5 mRNA of the ovary. On the other hand, administering prolactin to cycling rats twice daily from estrus significantly decreased the level at diestrus. The present results demonstrate the preferential expression of annexin 5 in the large luteal cells of the corpus luteum and that this is negatively regulated during the functional luteal phase by prolactin. Collectively, these data suggest a significant role of annexin 5 in the regulation of luteal function by means of cell-cell communication in the corpus luteum.