Unraveling mitochondrial piRNAs in mouse embryonic gonadal cells

Although mitochondria are widely studied organelles, the recent interest in the role of mitochondrial small non-coding RNAs (sncRNA) is providing new functional perspectives in germ cell development and differentiation. PIWI-interacting RNAs (piRNAs) are single-stranded sncRNAs of 20-35 nt generated from the processing of pre-piRNAs longer molecules to be functionally bound to PIWI proteins. Initially ascribed to germ cells, as protectors against transposons, we now know that they are also expressed in somatic cells. In mammals, germ cells differentiate at early stages of embryonic development in the primitive gonads as primordial germ cells (PGCs), initiating divergent pathways in both sexes, accompanied by somatic cells (SCs) of the ovary or testis. Previously, we identified mitochondrial piRNAs (mito-piRNAs) in mouse germ and somatic cells. However, a comparative analysis of mito-piRNAs between PGCs and SCs, between sexes and during early gonadal development has not been performed. We leverage NGS data obtained from PGCs and SCs purified from early differentiating embryonic ovaries and testis from E11.5 to E13.5. Using bioinformatic tools, here we unravel: the origin (from nuclear or mitochondrial genome), the levels of expression, the potential role of mito-piRNAs, as well as their association with genomic regions encoding other sncRNAs (such as tRNAs and rRNAs) and the mitochondrial regulatory region (D-loop). Finally, our results indicate nucleo-mitochondrial communication at both anterograde and retrograde signaling, mediated by mito-piRNAs. Mitochondrial piRNA of 12 different mouse samples, corresponding to PGCs and gonadal somatic cells from E11.5, E12.5 and E13.5 male and female gonads, generated by RNA-seq.尽管线粒体是广受研究的细胞器，近年来针对线粒体小分子非编码RNA（small non-coding RNAs，sncRNA）功能角色的研究兴趣，为生殖细胞的发育与分化提供了崭新的功能视角。PIWI互作RNA（PIWI-interacting RNAs，piRNAs）是一类长度为20~35 nt的单链sncRNA，由更长的前piRNA分子加工生成，并可与PIWI蛋白形成功能性结合复合物。最初这类RNA被认为仅特异性存在于生殖细胞中，作为转座子的抑制保护因子，而今我们已证实其在体细胞中也有表达。在哺乳动物体内，生殖细胞会在胚胎发育早期于原始性腺中分化为原始生殖细胞（primordial germ cells，PGCs），随后启动两性分化通路，并与卵巢或睾丸的体细胞（somatic cells，SCs）协同发挥作用。本团队此前已在小鼠的生殖细胞与体细胞中鉴定出线粒体piRNA（mitochondrial piRNAs，mito-piRNAs）。然而，目前尚未有针对PGCs与体细胞之间、不同性别之间以及早期性腺发育进程中mito-piRNAs的比较分析研究。本研究利用从E11.5至E13.5时期的胚胎卵巢和睾丸中纯化分离得到的PGCs与体细胞所获取的下一代测序（next-generation sequencing，NGS）数据，通过生物信息学工具，揭示了mito-piRNAs的起源（源自细胞核基因组或线粒体基因组）、表达水平、潜在功能，以及其与编码其他非编码RNA（如转运RNA（tRNAs）和核糖体RNA（rRNAs））的基因组区域和线粒体调控区域（D环，D-loop）的关联。最终研究结果表明，mito-piRNAs介导了顺行与逆行信号传导的核线粒体通讯。本研究涵盖12份小鼠样本的线粒体piRNA数据，样本分别来自E11.5、E12.5及E13.5时期雌雄性腺中的PGCs与性腺体细胞，所有数据均通过RNA测序（RNA-seq）获取。