Cloning and expression profiling of small RNAs expressed in the mouse ovary

Small noncoding RNAs have been suggested to play important roles in the regulation of gene expression across all species from plants to humans. To identify small RNAs expressed by the ovary, we generated mouse ovarian small RNA complementary DNA (srcDNA) libraries and sequenced 800 srcDNA clones. We identified 236 small RNAs including 122 microRNAs (miRNAs), 79 piwi-interacting RNAs (piRNAs), and 35 small nucleolar RNAs (snoRNAs). Among these small RNAs, 15 miRNAs, 74 piRNAs, and 21 snoRNAs are novel. Approximately 70% of the ovarian piRNAs are encoded by multicopy genes located within the repetitive regions, resembling previously identified repeat-associated small interference RNAs (rasiRNAs), whereas the remaining approximately 30% of piRNA genes are located in nonrepetitive regions of the genome with characteristics similar to the majority of piRNAs originally cloned from the testis. Since these two types of piRNAs display different structural features, we categorized them into two classes: repeat-associated piRNAs (rapiRNAs, equivalent of the rasiRNAs) and non-repeat-associated piRNAs (napiRNAs). Expression profiling analyses revealed that ovarian miRNAs were either ubiquitously expressed in multiple tissues or preferentially expressed in a few tissues including the ovary. Ovaries appear to express more rapiRNAs than napiRNAs, and sequence analyses support that both may be generated through the "ping-pong" mechanism. Unique expression and structural features of these ovarian small noncoding RNAs suggest that they may play important roles in the control of folliculogenesis and female fertility.