Maternal Nanos‐Dependent RNA Stabilization in the Primordial Germ Cells of Drosophila Embryos

Nanos (Nos) is an evolutionary conserved protein expressed in the germline of various animal species. In Drosophila, maternal Nos protein is essential for germline development. In the germline progenitors, or the primordial germ cells (PGCs), Nos binds to the 3′ UTR of target mRNAs to repress their translation. In contrast to this prevailing role of Nos, here we report that the 3′ UTR of CG32425 mRNA mediates Nos‐dependent RNA stabilization in PGCs. We found that the level of mRNA expressed from a reporter gene fused to the CG32425 3′ UTR was significantly reduced in PGCs lacking maternal Nos (nos PGCs) as compared with normal PGCs. By deleting the CG32425 3′ UTR, we identified the region required for mRNA stabilization, which includes Nos‐binding sites. In normal embryos, CG32425 mRNA was maternally supplied into PGCs and remained in this cell type during embryogenesis. However, as expected from our reporter assay, the levels of CG32425 mRNA and its protein product expressed in nos PGCs were lower than in normal PGCs. Thus, we propose that Nos protein has dual functions in translational repression and stabilization of specific RNAs to ensure proper germline development.     

Nanos3 of the frog Rana rugosa: Molecular cloning and characterization

Nanos is expressed in the primordial germ cells (PGCs) and also the germ cells of a variety of organisms as diverse as Drosophila, medaka fish, Xenopus and mouse. In Nanos3‐deficient mice, PGCs fail to incorporate into the gonad and the size of the testis and ovary is thereby dramatically reduced. To elucidate the role of Nanos in an amphibian species, we cloned Nanos3 cDNA from the testis of the R. rugosa frog. RT‐PCR analysis showed strong expression of Nanos3 mRNA in the testis of adult R. rugosa frogs, but expression was not sexually dimorphic during gonadal differentiation. In Nanos3‐knockdown tadpoles produced by the CRISPR/Cas9 system, the number of germ cells decreased dramatically in the gonads of both male and female tadpoles before sex determination and thereafter. This was confirmed by three dimensional imaging of wild‐type and Nanos3 knockdown gonads using serial sections immunostained for Vasa, a marker specific to germ cells. Taken together, these results suggest that Nanos3 protein function is conserved between R. rugosa and mouse.     

Molecular and expression characterization of a nanos1 homologue in Chinese sturgeon, Acipenser sinensis

The nanos gene family was essential for germ line development in diverse organisms. In the present study, the full-length cDNA of a nanos1 homologue in A. sinensis, Asnanos1, was isolated and characterized. The cDNA sequence of Asnanos1 was 1489 base pairs (bp) in length and encoded a peptide of 228 amino acid residues. Multiple sequence alignment showed that the zinc-finger motifs of Nanos1 were highly conserved in vertebrates. By RT-PCR analysis, Asnanos1 mRNAs were ubiquitously detected in all tissues examined except for the fat, including liver, spleen, heart, ovary, kidney, muscle, intestines, pituitary, hypothalamus, telencephalon, midbrain, cerebellum, and medulla oblongata. Moreover, a specific polyclonal antibody was prepared from the in vitro expressed partial AsNanos1 protein. Western blot analysis revealed that the tissue expression pattern of AsNanos1 was not completely coincided with that of its mRNAs, which was not found in fat, muscle and intestines. Additionally, by immunofluoresence localization, it was observed that AsNanos1 protein was in the cytoplasm of primary oocytes and spermatocytes. The presented results indicated that the expression pattern of Asnanos1 was differential conservation and divergence among diverse species.     

Cyclin D2 and the CDK substrate p220NPAT are required for self‐renewal of human embryonic stem cells

Self‐renewal of pluripotent human embryonic stem (hES) cells utilizes an abbreviated cell cycle that bypasses E2F/pRB‐dependent growth control. We investigated whether self‐renewal is alternatively regulated by cyclin/CDK phosphorylation of the p220^NPAT/HiNF‐P complex to activate histone gene expression at the G1/S phase transition. We show that cyclin D2 is prominently expressed in pluripotent hES cells, but cyclin D1 eclipses cyclin D2 during differentiation. Depletion of cyclin D2 or p220^NPAT causes a cell cycle defect in G1 reflected by diminished phosphorylation of p220^NPAT, decreased cell cycle dependent histone H4 expression and reduced S phase progression. Thus, cyclin D2 and p220^NPAT are principal cell cycle regulators that determine competency for self‐renewal in pluripotent hES cells. While pRB/E2F checkpoint control is relinquished in human ES cells, fidelity of physiological regulation is secured by cyclin D2 dependent activation of the p220^NPAT/HiNF‐P mechanism that may explain perpetual proliferation of hES cells without transformation or tumorigenesis. J. Cell. Physiol. 222: 456–464, 2010. © 2009 Wiley‐Liss, Inc.     

Stomaching Notch

The self‐renewal and differentiation of tissue stem cells must be tightly controlled. Unrestrained self‐renewal leads to over‐proliferation of stem cells, which may cause tumor formation, while uncontrolled differentiation leads to depletion of the stem cell pool. In this issue of The EMBO Journal, Demitrack et al (2015) show that the Notch pathway is a key regulator of Lgr5 antral stem cell self‐renewal and differentiation. Notch signaling controls the proliferation and differentiation of stem cells as well as gastric tissue growth, while uncontrolled Notch activity in stem cells leads to polyp formation.     

Smurf2 regulates hematopoietic stem cell self‐renewal and aging

The age‐dependent decline in the self‐renewal capacity of stem cells plays a critical role in aging, but the precise mechanisms underlying this decline are not well understood. By limiting proliferative capacity, senescence is thought to play an important role in age‐dependent decline of stem cell self‐renewal, although direct evidence supporting this hypothesis is largely lacking. We have previously identified the E3 ubiquitin ligase Smurf2 as a critical regulator of senescence. In this study, we found that mice deficient in Smurf2 had an expanded hematopoietic stem cell (HSC) compartment in bone marrow under normal homeostatic conditions, and this expansion was associated with enhanced proliferation and reduced quiescence of HSCs. Surprisingly, increased cycling and reduced quiescence of HSCs in Smurf2‐deficient mice did not lead to premature exhaustion of stem cells. Instead, HSCs in aged Smurf2‐deficient mice had a significantly better repopulating capacity than aged wild‐type HSCs, suggesting that decline in HSC function with age is Smurf2 dependent. Furthermore, Smurf2‐deficient HSCs exhibited elevated long‐term self‐renewal capacity and diminished exhaustion in serial transplantation. As we found that the expression of Smurf2 was increased with age and in response to regenerative stress during serial transplantation, our findings suggest that Smurf2 plays an important role in regulating HSC self‐renewal and aging.     

SIRT1 regulates macrophage self‐renewal

Mature differentiated macrophages can self‐maintain by local proliferation in tissues and can be extensively expanded in culture under specific conditions, but the mechanisms of this phenomenon remain only partially defined. Here, we show that SIRT1, an evolutionary conserved regulator of life span, positively affects macrophage self‐renewal ability in vitro and in vivo. Overexpression of SIRT1 during bone marrow‐derived macrophage differentiation increased their proliferative capacity. Conversely, decrease of SIRT1 expression by shRNA inactivation, CRISPR/Cas9 mediated deletion and pharmacological inhibition restricted macrophage self‐renewal in culture. Furthermore, pharmacological SIRT1 inhibition in vivo reduced steady state and cytokine‐induced proliferation of alveolar and peritoneal macrophages. Mechanistically, SIRT1 inhibition negatively regulated G1/S transition, cell cycle progression and a network of self‐renewal genes. This included inhibition of E2F1 and Myc and concomitant activation of FoxO1, SIRT1 targets mediating cell cycle progression and stress response, respectively. Our findings indicate that SIRT1 is a key regulator of macrophage self‐renewal that integrates cell cycle and longevity pathways. This suggests that macrophage self‐renewal might be a relevant parameter of ageing.     

Female germline stem cell niches of earwigs are structurally simple and different from those of Drosophila melanogaster

Stem cells function in niches, which consist of somatic cells that control the stem cells' self‐renewal, proliferation, and differentiation. Drosophila ovary germline niche consists of the terminal filament (TF) cells, cap cells, and escort stem cells; signaling from the TF cells and the cap cells is essential for maintenance of germline stem cells (GSCs). Here, we show that in the earwig Opisthocosmia silvestris, the female GSC niche is morphologically simple and consist of the TF cells and several structurally uniform escort cells. The most posterior cell of the TF (the basal cell of the TF) differs from remaining TF cells and is separated from the anterior region of the germarium by the processes of the escort cells, and consequently, does not contact the GSCs directly. We also show that between somatic cells of earwig niche argosome‐like vesicles and cytoneme‐like extensions are present. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Nanos3 maintains the germ cell lineage in the mouse by suppressing both Bax-dependent and -independent apoptotic pathways

Cell death in the germ line is controlled by both positive and negative mechanisms that maintain the appropriate number of germ cells and that prevent the possible formation of germ cell tumors. In the mouse embryo, Steel/c-Kit signaling is required to prevent migrating primordial germ cells (PGCs) from undergoing Bax-dependent apoptosis. In our current study, we show that migrating PGCs also undergo apoptosis in Nanos3-null embryos. We assessed whether the Bax-dependent apoptotic pathway is responsible for this cell death by knocking out the Bax gene together with the Nanos3 gene. Differing from Steel-null embryos, however, the Bax elimination did not completely rescue PGC apoptosis in Nanos3-null embryos, and only a portion of the PGCs survived in the double knockout embryo. We further established a mouse line, Nanos3-Cre-pA, to undertake lineage analysis and our results indicate that most of the Nanos3-null PGCs die rather than differentiate into somatic cells, irrespective of the presence or absence of Bax. In addition, a small number of surviving PGCs in Nanos3/Bax-null mice are maintained and differentiate as male and female germ cells in the adult gonads. Our findings thus suggest that heterogeneity exists in the PGC populations and that Nanos3 maintains the germ cell lineage by suppressing both Bax-dependent and Bax-independent apoptotic pathways.     

Barbronia weberi (Clitellata,Hirudinida, Salifidae) has ovary cords of the Erpobdella type

The organization of the ovaries in representative of the Salifidae (Hirudinida, Erpobdelliformes) was studied at the ultrastructural level for the first time. Like in other leeches, the ovaries of Barbronia weberi are composed of an outer envelope (i.e., an ovisac made up of two coelomic epithelia, muscle cells, and connective tissue) and several internal units, which are broadly similar to the ovary cords found in representatives of the Erpobdellidae. There are usually 6–8 ovary cords that are twisted or cambered with a narrow apical part and a broader, irregularly shaped distal end in each ovisac of B. weberi. Each ovary cord is built from somatic and germ‐line cells and the latter tend to form multicellular cysts that are equipped with a central cytoplasmic core (cytophore). There are two morphologically different subpopulations of germ‐line cells: oocytes and more numerous nurse cells. Growing oocytes form protuberances on the ovary cord surface and eventually detach from the cord and float freely in the ovisac lumen, whereas the other components of germ‐line cysts (i.e., nurse cells and cytophore) degenerate. It should be pointed out that there is a prominent gradient of germ‐cell development along the long axis of the cord. The somatic cells form the ovary cord envelope (the so‐called spongiosa cells) and also penetrate the spaces between germ‐line cells. Both kinds of the somatic cells, that is, those forming the cord envelope and the somatic cells that are associated with oocytes (follicular cells) have a well‐developed system of intercellular channels. Additionally, one prominent somatic cell, the apical cell, was found at the apical tip of each ovary cord. Because the aforementioned features of ovary cords found in B. weberi are very similar (with a few minor exceptions) to the ovary cords that have been described in Erpobdella octoculata and E. johanssoni, we propose the term “ovary cords of the Erpobdella type” for them. Our results support a close phylogenetic relationship between Salifidae and Erpobdellidae. J. Morphol. 275:479–488, 2014. © 2013 Wiley Periodicals, Inc.