A key feature of many adult stem cell lineages is that stem cell daughters destined for differentiation undergo several transit amplifying (TA) divisions before initiating terminal differentiation, allowing few and infrequently dividing stem cells to produce many differentiated progeny. Although the number of progenitor divisions profoundly affects tissue (re)generation, and failure to control these divisions may contribute to cancer, the mechanisms that limit TA proliferation are not well understood. Here, we use a model stem cell lineage, the Drosophila male germ line, to investigate the mechanism that counts the number of TA divisions. bam ͉ spermatogenesis ͉ Drosophila ͉ transit amplifying cell division A dult stem cells act throughout life to replenish differentiated cells lost to normal turnover or injury. In many adult stem cell lineages, stem cell daughters undergo transit amplifying (TA) mitotic division before terminal differentiation. The number of TA divisions strongly influences the capacity of adult stem cells to regenerate and repair tissues (1). In addition, strict limits on TA cell proliferation may help prevent accumulation of oncogenic replication errors. Defects in the mechanisms that count and limit the number of TA divisions may therefore predispose to cancer. Indeed, recent evidence points toward cancer initiating events occurring in TA cells in leukemia (2, 3).Despite the importance for normal tissue homeostasis and cancer, the mechanisms that specify the number of TA divisions are not understood. Here, we use the Drosophila male germ line model adult stem cell lineage to investigate the mechanisms that normally set developmentally programmed limits on proliferation of TA cells. Drosophila melanogaster male germ line stem cells (GSCs) lie in a niche at the tip of the testis, attached to somatic hub cells, and are maintained by signals from the hub and f lanking somatic stem cells (4 -7). When a GSC divides, one daughter remains in the niche and self-renews, while the other is displaced away and initiates differentiation. The resulting differentiating gonialblast, which is enveloped by a pair of somatic cells, founds a clone of 16 spermatogonia through four synchronous TA divisions with incomplete cytokinesis. Soon after the fourth TA division, the resulting 16 germ cells undergo premeiotic DNA synthesis in synchrony and switch to the spermatocyte program of cell growth, meiosis, and terminal differentiation. As spermatocytes, the cells increase in volume 25-fold, take on a distinctive morphology, and turn on a unique gene expression program for spermatid differentiation (8) (Fig. 1A).The anatomy of developing germ cell cysts makes the Drosophila germ line especially well suited for investigating how the number of TA divisions is controlled. Because TA sister cells descended from a common gonialblast are contained within a common somatic cell envelope and divide in synchrony, the number of rounds of TA division executed prior to differentiation can be assessed by counting the number of dif...

Summary In adult stem cell lineages, progenitor cells commonly undergo mitotic transit amplifying (TA) divisions before terminal differentiation, allowing production of many differentiated progeny per stem cell division. Mechanisms that limit TA divisions and trigger the switch to differentiation may protect against cancer by preventing accumulation of oncogenic mutations in the proliferating population. Here we show that the switch from TA proliferation to differentiation in the Drosophila male germline stem cell lineage is mediated by translational control. The TRIM-NHL tumor suppressor homolog Mei-P26 facilitates accumulation of the differentiation regulator Bam in TA cells. In turn, Bam and its partner Bgcn bind the mei-P26 3′UTR and repress translation of mei-P26 in late TA cells. Thus, germ cells progress through distinct, sequential regulatory states, from Mei-P26 on/Bam off to Bam on/Mei-P26 off. TRIM-NHL homologs across species facilitate the switch from proliferation to differentiation, suggesting a novel and conserved developmentally-programmed tumor suppressor mechanism.

Ubiquitin-protein ligases (E3s) are often in the precarious position of ubiquitinating themselves, mediating their own destruction. The intrinsically disordered E3 San1 prevents its own autoubiquitination and degradation by minimizing Lys residues and hydrophobic stretches in its disordered regions.

Stem cell behavior is regulated by extrinsic signals from specialized microenvironments, or niches, and intrinsic factors required for execution of context-appropriate responses to niche signals. Here we show that function of the transcriptional regulator longitudinals lacking (lola) is required cell autonomously for germline stem cell and somatic cyst stem cell maintenance in the Drosophila testis. In addition, lola is also required for proper execution of key developmental transitions during male germ cell differentiation, including the switch from transit amplifying progenitor to spermatocyte growth and differentiation, as well as meiotic cell cycle progression and spermiogenesis. Different lola isoforms, each having unique C-termini and zinc finger domains, may control different aspects of proliferation and differentiation in the male germline and somatic cyst stem cell lineages.

During meiosis, germ cell and stage-specific components impose additional layers of regulation on the core cell cycle machinery to set up an extended G2 period termed meiotic prophase. In Drosophila males, meiotic prophase lasts 3.5 days, during which spermatocytes turn up expression of over 3000 genes and grow 25-fold in volume. Previous work showed that the core cell cycle regulator Cyclin B (CycB) is subject to translational repression in immature Drosophila spermatocytes, mediated by the RNA-binding protein Rbp4 and its partner Fest. Here we show that another spermatocyte-specific protein, Lut, is required for translational repression of cycB in an 8-hour window just before spermatocytes are fully mature. In males mutant for rbp4 or lut, spermatocytes enter and exit the meiotic divisions 6-8 hours earlier than in wild-type. In addition, we show that spermatocyte-specific isoforms of Syncrip (Syp) are required for expression of CycB protein and normal entry into the meiotic divisions. Both Lut and Syp interact with Fest in an RNA-independent manner. Thus a complex of spermatocyte-specific regulators choreograph the timing of expression of CycB protein during male meiotic prophase.