The piwi family genes are crucial for stem cell self-renewal, RNA silencing, and translational regulation in diverse organisms. However, their function in mammals remains unexplored. Here we report the cloning of a murine piwi gene (miwi) essential for spermatogenesis. miwi encodes a cytoplasmic protein specifically expressed in spermatocytes and spermatids. miwi(null) mice display spermatogenic arrest at the beginning of the round spermatid stage, resembling the phenotype of CREM, a master regulator of spermiogenesis. Furthermore, mRNAs of ACT (activator of CREM in testis) and CREM target genes are downregulated in miwi(null) testes. Whereas MIWI and CREM do not regulate each other's expression, MIWI complexes with mRNAs of ACT and CREM target genes. Hence, MIWI may control spermiogenesis by regulating the stability of these mRNAs.

The piwi family genes, which are defined by conserved PAZ and Piwi domains, play important roles in stem cell selfrenewal, RNA silencing, and translational regulation in various organisms. To reveal the function of the mammalian homolog of piwi, we produced and analyzed mice with targeted mutations in the Mili gene, which is one of three mouse homologs of piwi. Spermatogenesis in the MILI-null mice was blocked completely at the early prophase of the first meiosis, from the zygotene to early pachytene, and the mice were sterile. However, primordial germ cell development and female germ cell production were not disturbed. Furthermore, MILI bound to MVH, which is an essential factor during the early spermatocyte stage. The similarities in the phenotypes of the MILI-and MVH-deficient mice and in the physical binding properties of MILI and MVH indicate a functional association of these proteins in post-transcriptional regulation. These data indicate that MILI is essential for the differentiation of spermatocytes. Key words: Mili, Miwi, piwi, Mvh, Spermatogenesis SummaryMili, a mammalian member of piwi family gene, is essential for spermatogenesis

No abstract

In the Drosophila ovary, membrane skeletal proteins such as the adducin-like Hts protein(s), spectrin, and ankyrin are found in the spectrosome, an organelle in germline stem cells (GSC) and their differentiated daughter cells (cystoblasts). These proteins are also components of the fusome, a cytoplasmic structure that spans the cystoblast's progeny that develop to form a germline cyst consisting of 15 nurse cells and an oocyte. Spectrosomes and fusomes are associated with one pole of spindles during mitosis and are implicated in cyst formation and oocyte differentiation. Here we show that the asymmetric behavior of the spectrosome persists throughout the cell cycle of GSC. Eliminating the spectrosome by the htsl mutation leads to randomized spindle orientation, suggesting that the spectrosome anchors the spindle to ensure the asymmetry of GSC division; eliminating the fusome in developing cysts results in defective spindles and randomized spindle orientation as well as asynchronous and reduced cystocyte divisions. These observations suggest that fusomes are required for the proper formation and asymmetric orientation of mitotic spindles. Moreover, they reinforce the notion that fusomes are required for the four synchronous divisions of the cystoblast leading to cyst formation. In htsl cysts which lack fusomes and fail to incorporate a hts gene product(s) into ring canals following cyst formation, polarized microtubule networks do not form, the dynamics of cytoplasmic dynein is disrupted, and oskar and orb RNAs fail to be transported to the future oocyte. These observations support the proposed role of fusomes and ring canals in organizing a polarized microtubule-based transport system for RNA localization that leads to oocyte differentiation.