Number Of Germline Stem Cells Research Articles

A wicked gene shrinks numbers of germline stem cells

Ribosome biogenesis regulates asymmetric growth and self-renewal of these cells in Drosophila

Male Germline Stem Cell Division and Spermatocyte Growth Require Insulin Signaling in Drosophila

Spermatogenesis in Drosophila commences with cell division of germline stem cells (GSCs) to produce male germline cells at the tip of the testis. However, molecular mechanisms inducing division of male GSCs have not been reported. Insulin-like peptides are known to play an essential role in stimulation of proliferation and growth of somatic cells, and it has recently been reported that such peptides promote cell division in female Drosophila GSCs. However, their effects on male germline cells have not been characterized. We found that inhibition of insulin production and insulin signaling mutations resulted in decreased numbers of germline cells in Drosophila testes. GSC numbers were maintained in young mutant males, with a gradual decrease in abundance of GSCs with age. Furthermore, in mutants, fewer germline cysts originated from GSCs and a lower frequency of GSC division was seen. Insulin signaling was found to promote cell cycle progression of the male GSCs at the G(2)/M phase. The cell volume of spermatocytes increases up to 25 times before initiation of meiosis in Drosophila. We examined whether insulin signaling extrinsically induces the greatest cell growth in Drosophila diploid cells and found that spermatocyte growth was affected in mutants. The results indicate that in addition to its function in somatic cells, insulin signaling plays an essential role in cell proliferation and growth during male Drosophila gametogenesis and that sperm production is regulated by hormonal control via insulin-like peptides.

Nuclear Lamin for Gene Silencing

In the Drosophila ovary, germline stem cells (GSCs) divide to produce one cell that differentiates (a cystoblast) and one stem cell. The stem cell population is maintained by the release of Decapentaplegic (Dpp, a member of the bone morphogenetic protein family) from the stromal somatic cap cells (niche cells), which silences the gene bag-of-marbles ( bam ), which is required for cystoblast differentiation. Jiang et al . identified otefin ( ote ), which encodes a nuclear lamin protein of the LEM family (containing an LEM domain), in a screen for female sterile flies. The germaria (the site of the stem cell population and cystoblast formation) of the mutant flies contained very few germ cells. When GSCs were marked with green fluorescent protein (GFP), the authors found that the number of GSCs declined to nearly absent in Ote-deficient GSCs, whereas wild-type GSCs persisted, which is consistent with excessive differentiation and lack of self-renewal of the GSC population in the ote mutants. Ote was required in the GSCs because reintroduction of ote into the somatic cells only failed to rescue the mutant ovary phenotype of the Ote-deficient animals. Overexpression of Ote led to an increased number of GSC-like cells, which suggests that excess Ote may suppress or delay differentiation. Bam was detected by expression of a fusion protein, and in Ote-deficient GSCs Bam-GFP was abundant, whereas in wild-type GSCs Bam-GFP was undetectable. Bam expression is inhibited by Dpp signaling. The authors showed that a genetic interaction occurred between signaling components of the Dpp pathway and Ote, and that when an inhibitor of Dpp signaling was overexpressed, forced overexpression of Ote led to a loss of GSCs. Thus, Dpp signaling appeared to be required for Ote to stimulate stem cell self-renewal. Ote required nuclear membrane localization and the LEM domain to function in the GSCs because introduction of Ote lacking the transmembrane domain or the LEM domain or targeted to the plasma membrane with a different transmembrane domain failed to rescue the mutant ovary phenotype. Endogenous Ote and Smad4 (called Medea in flies) coimmunoprecipitated when a tagged form of Medea was transfected into S2 cells; this interaction required the LEM domain of Ote. Ote and Medea were also found bound to the Bam silencer region in chromatin immunoprecipitation experiments with S2 cells, and the binding of Ote was lost if endogenous Medea was knocked down with siRNA. Thus, signal-dependent gene silencing in the GSCs appears to involve an interaction between the chromatin, the nuclear envelope through Ote, and the transcription factor Medea. X. Jiang, L. Xia, D. Chen, Y. Yang, H. Huang, L. Yang, Q. Zhao, L. Shen, J. Wang, D. Chen, Otefin, a nuclear membrane protein, determines the fate of germline stem cells in Drosophila via interaction with Smad complexes. Dev. Cell 14 , 494-506 (2008). [PubMed]

Aging and the Germ Line: Where Mortality and Immortality Meet

Germ cells are highly specialized cells that form gametes, and they are the only cells within an organism that contribute genes to offspring. Germline stem cells (GSCs) sustain gamete production, both oogenesis (egg production) and spermatogenesis (sperm production), in many organisms. Since the genetic information contained within germ cells is passed from generation to generation, the germ line is often referred to as immortal. Therefore, it is possible that germ cells possess unique strategies to protect and transmit the genetic information contained within them indefinitely. However, aging often leads to a dramatic decrease in gamete production and fecundity. In addition, single gene mutations affecting longevity often have a converse effect on reproduction. Recent studies examining age-related changes in GSC number and activity, as well as changes to the stem cell microenvironment, provide insights into the mechanisms underlying the observed reduction in gametogenesis over the lifetime of an organism.

Notch signaling controls germline stem cell niche formation in theDrosophilaovary

Stem cells, which can self-renew and generate differentiated cells, have been shown to be controlled by surrounding microenvironments or niches in several adult tissues. However, it remains largely unknown what constitutes a functional niche and how niche formation is controlled. In the Drosophila ovary, germline stem cells (GSCs), which are adjacent to cap cells and two other cell types, have been shown to be maintained in the niche. In this study, we show that Notch signaling controls formation and maintenance of the GSC niche and that cap cells help determine the niche size in the Drosophila ovary. Expanded Notch activation causes the formation of more cap cells and bigger niches, which support more GSCs, whereas compromising Notch signaling during niche formation decreases the cap cell number and niche size and consequently the GSC number. Furthermore, the niches located away from their normal location can still sufficiently sustain GSC self-renewal by maintaining high local BMP signaling and repressing bam as in normal GSCs. Finally, loss of Notch function in adults results in rapid loss of the GSC niche, including cap cells and thus GSCs. Our results indicate that Notch signaling is important for formation and maintenance of the GSC niche, and that cap cells help determine niche size and function.

Parva germina, a gene involved in germ cell maintenance during male and female Drosophila gametogenesis

We report the initial characterization of a gene, parva germina (pag), required for germ cell maintenance in both males and females. pag gonads contain a small number of germline stem cells at the onset of gametogenesis. In contrast, adult mutant gonads are either empty or have a very small number of germ cells that never develop in 16-cell cysts. Ovarioles and testes, therefore, are rudimentary, and the very few germ cells they contain are unable to differentiate into eggs or sperm. Germline stem cells are progressively depleted over time. The average number of germ cells, therefore, decreases in pag mutant ovarioles with the age of the mother, whereas the proportion of agametic germaria goes up. These observations suggest that the pag gene product is involved in germ cell maintenance in both male and female gametogenesis.

Germline stem cell number in the Drosophila ovary is regulated by redundant mechanisms that control Dpp signaling.

The available experimental data support the hypothesis that the cap cells (CpCs) at the anterior tip of the germarium form an environmental niche for germline stem cells (GSCs) of the Drosophila ovary. Each GSC undergoes an asymmetric self-renewal division that gives rise to both a GSC, which remains associated with the CpCs, and a more posterior located cystoblast (CB). The CB upregulates expression of the novel gene, bag of marbles (bam), which is necessary for germline differentiation. Decapentaplegic (Dpp), a BMP2/4 homologue, has been postulated to act as a highly localized niche signal that maintains a GSC fate solely by repressing bam transcription. Here, we further examine the role of Dpp in GSC maintenance. In contrast to the above model, we find that an enhancer trap inserted near the Dpp target gene, Daughters against Dpp (Dad), is expressed in additional somatic cells within the germarium, suggesting that Dpp protein may be distributed throughout the anterior germarium. However, Dad-lacZ expression within the germline is present only in GSCs and to a lower level in CBs, suggesting there are mechanisms that actively restrict Dpp signaling in germ cells. We demonstrate that one function of Bam is to block Dpp signaling downstream of Dpp receptor activation, thus establishing the existence of a negative feedback loop between the action of the two genes. Moreover, in females doubly mutant for bam and the ubiquitin protein ligase Smurf, the number of germ cells responsive to Dpp is greatly increased relative to the number observed in either single mutant. These data indicate that there are multiple, genetically redundant mechanisms that act within the germline to downregulate Dpp signaling in the Cb and its descendants, and raise the possibility that a Cb and its descendants must become refractory to Dpp signaling in order for germline differentiation to occur.

Yb Modulates the Divisions of Both Germline and Somatic Stem Cells through piwi- and hh-Mediated Mechanisms in the Drosophila Ovary

The coordinated division of distinctive types of stem cells within an organ is crucial for organogenesis and homeostasis. Here we show genetic interactions among fs(1)Yb ( Yb), piwi, and hedgehog ( hh) that regulate the division of both germline stem cells (GSCs) and somatic stem cells (SSCs), the two constituent stem cell populations of the Drosophila ovary. Yb is required for both GSC and SSC divisions; loss of Yb function eliminates GSCs and reduces SSC division, while Yb overexpression increases GSC number and causes SSC overproliferation. We also show that Yb acts via the piwi- and hh-mediated signaling pathways that emanate from the same signaling cells to control GSC and SSC division, respectively. hh signaling also has a minor effect in GSC division.

Somatic support cells restrict germline stem cell self-renewal and promote differentiation.

Stem cells maintain populations of highly differentiated, short-lived cell-types, including blood, skin and sperm, throughout adult life. Understanding the mechanisms that regulate stem cell behaviour is crucial for realizing their potential in regenerative medicine. A fundamental characteristic of stem cells is their capacity for asymmetric division: daughter cells either retain stem cell identity or initiate differentiation. However, stem cells are also capable of symmetric division where both daughters remain stem cells, indicating that mechanisms must exist to balance self-renewal capacity with differentiation. Here we present evidence that support cells surrounding the stem cells restrict self-renewal and control stem cell number by ensuring asymmetric division. Loss of function of the Drosophila Epidermal growth factor receptor in somatic cells disrupted the balance of self-renewal versus differentiation in the male germline, increasing the number of germline stem cells. We propose that activation of this receptor specifies normal behaviour of somatic support cells; in turn, the somatic cells play a guardian role, providing information that prevents self-renewal of stem cell identity by the germ cell they enclose.

Piwi encodes a nucleoplasmic factor whose activity modulates the number and division rate of germline stem cells.

piwi represents the first class of genes known to be required for stem cell self-renewal in diverse organisms. In the Drosophila ovary, piwi is required in somatic signaling cells to maintain germline stem cells. Here we show that piwi encodes a novel nucleoplasmic protein present in both somatic and germline cells, with the highly conserved C-terminal region essential for its function. Removing PIWI protein from single germline stem cells significantly decreases the rate of their division. This suggests that PIWI has a second role as a cell-autonomous promoter of germline stem cell division. Consistent with its dual function, over-expression of piwi in somatic cells causes an increase both in the number of germline stem cells and the rate of their division. Thus, PIWI is a key regulator of stem cell division - its somatic expression modulates the number of germline stem cells and the rate of their division, while its germline expression also contributes to promoting stem cell division in a cell-autonomous manner.