An animal or plant has many different types of cells that have specific roles in the life of the organism. These cells are organized into tissues. In most tissues in adult animals, small groups of cells called stem cells are responsible for replacing the other cells that have been lost due to disease, injury, or as part of normal body maintenance.

The ‘germ line’ stem cells of female fruit flies—which produce female sex cells (or eggs)—are an effective system for studying how stem cells are regulated. These cells live in an area of the ovary called a stem cell niche. Each time a stem cell divides, it produces one stem cell and one other daughter cell. This daughter cell then moves into another niche called the ‘differentiation’ niche and undergoes a series of divisions that produce the egg cells. The differentiation niche is formed by escort cells and is crucial for producing the egg cells, but it is not clear how the escort cells promote this process, or how the niche is maintained.

Wang et al. have now studied the differentiation niche in more detail. The experiments show that a cell communication system called Wnt signaling maintains the differentiation niche by controlling the ability of the escort cells to grow and divide. If Wnt signaling is defective, the differentiation niche is lost, which disrupts the formation of egg cells.

Further experiments show that two proteins called Wnt2 and Wnt4 in the differentiation niche—which activate Wnt signaling—act as signals to regulate the niche, mainly by controlling the expression of four particular genes. These four genes encode enzymes that remove ‘reactive oxygen species’ from cells. Wang et al.'s findings have revealed an important role for Wnt signaling in maintaining the differentiation niche. The next step is to figure out the details of how this works.

Stem cells have two important properties, self-renewal and differentiation, which are critical for continuously generating new functional cells to maintain tissue homeostasis. The self-renewal property is controlled in various stem cell systems by interplays between signals from the niche and intrinsic factors (Li and Xie, 2005; Morrison and Spradling, 2008; Losick et al., 2011). Germ line stem cells (GSCs) in the Drosophila ovary and testis are attractive systems for studying stem cell self-renewal at the molecular and cellular level (Fuller and Spradling, 2007; Xie, 2013). Although stem cell differentiation was widely thought to be a developmentally default state, we have recently proposed that GSC lineage differentiation is also controlled extrinsically by a differentiation niche formed by inner germarial sheath cells (ISCs, also known as escort cells). However, it remains unclear how the maintenance and function of the differentiation niche are regulated at the molecular level. In this study, we show that autocrine Wnt2/4 signaling maintains the differentiation niche by regulating ISC proliferation and survival via redox regulation.

In the Drosophila ovary, two or three GSCs at the tip of the germarium, the most anterior region of the Drosophila ovary, continuously self-renew and generate differentiated GSC daughters, cystoblasts (CBs). The CBs further divide four times synchronously with incomplete cytokinesis to form 2-cell, 4-cell, 8-cell, or 16-cell cysts (de Cuevas et al., 1997). GSCs and their differentiated progeny can be reliably identified by their unique morphology of germ line-specific intracellular organelles known as fusomes: GSCs and CBs contain a spherical fusome known as the spectrosome, whereas differentiated germ cell cysts contain a branched fusome (Lin et al., 1994). GSCs can be reliably distinguished from CBs by their direct contact with cap cells (Figure 1A). Cap cells function as the self-renewing niche to maintain GSCs by activating BMP signaling and maintaining E-cadherin-mediated cell adhesion (Song et al., 2002; Xie and Spradling, 1998, 2000). In addition, various classes of intrinsic factors work with BMP signaling and E-cadherin to control GSC self-renewal (Xie, 2013). Therefore, GSC self-renewal is controlled by coordinated functions of niche-initiated signaling pathways and intrinsic factors.

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Following GSC division, differentiating GSC daughters, CBs, are always positioned away from the self-renewal niche. ISCs sit on the surface of the germarium to send their cellular processes to wrap up underneath CBs, mitotic cysts, and early 16-cell cysts, which move posteriorly (Decotto and Spradling, 2005; Kirilly et al., 2011; Morris and Spradling, 2011). Our recent study suggests ISCs and their associate long cellular processes act as the differentiation niche to promote GSC progeny differentiation in the Drosophila ovary because disrupting long ISC processes leads to an accumulation of CB-like cells, indicative of a germ cell differentiation defect (Kirilly et al., 2011). A series of genetic studies have further supported the existence of the differentiation niche.

The epidermal growth factor (EGF) signaling pathway is active in ISCs to promote GSC lineage differentiation partly by repressing dally expression (Schultz et al., 2002; Liu et al., 2010). In addition, Rho signaling is also required in ISCs to promote GSC differentiation partly by repressing dally and dpp expression. dally encodes a proteoglycan protein, which is capable of promoting Dpp/BMP diffusion to the differentiation niche (Guo and Wang, 2009; Hayashi et al., 2009). Ecdysteroid signaling also operates in ISCs to promote germ cell differentiation because inactivating ecdysteroid receptors EcR and Usp in ISCs disrupts cyst formation (Morris and Spradling, 2012). One potential mechanism is that ecdysteroid signaling controls the formation of ISC cellular processes, thereby promoting the interaction between ISCs and germ cells (Konig and Shcherbata, 2015). Gap junction protein Inx2 functions in ISCs to promote germ cell differentiation, but its transmitted substances between ISCs and germ cells remain identified (Mukai et al., 2011). The importance of gap junctions between ISCs and germ cells could also explain why ISC cellular processes are important for germ cell differentiation. Therefore, physical interactions and signaling-mediated communications between ISC cellular processes and GSC progeny likely contribute to GSC progeny differentiation collectively.

In addition, chromatin regulators are also important in ISCs to promote GSC differentiation. Eggless, a Drosophila H3K9 trimethyltransferase, maintains ISCs and represses dally and dpp expression in ISCs, thereby promoting germ cell differentiation (Wang et al., 2011). Similarly, Piwi functions in ISCs likely as a chromatin regulator to control germ cell differentiation partly by repressing dpp expression (Jin et al., 2013; Ma et al., 2014). dBre1 (a E3 ubiquitin ligase) and dSet1 (a H3K4 trimethylase) together control H3K4 trimethylation in ISCs and promote germ cell differentiation partly by limiting BMP signaling from the differentiation niche (Xuan et al., 2013). The potential chromatin factor without children (Woc) maintains the ISC-germ cell physical interaction via regulation of Stat-Zfh1 (Maimon et al., 2014). The histone demethylase Lsd1 is required in ISCs to promote germ cell differentiation by maintaining ISC survival, maintaining ISC morphology, and preventing BMP signaling from the differentiation niche (Eliazer et al., 2011, 2014). Therefore, ISCs function as the differentiation niche by preventing BMP signaling and direct communication.

A recent study showed that tyrosine kinase Btk29A maintains Wnt signaling in ISCs by phosphorylating Drosophila β-catenin homolog Armadillo (Arm) (Hamada-Kawaguchi et al., 2014). It also argues that Wnt4 activates Wnt signaling to maintain Piwi expression and repress E-cadherin expression in ISCs, thereby promoting germ cell differentiation. In contrast, this study has demonstrated that both ISC-expressing Wnt2 and Wnt4, but not Wnt4 alone, act through known Wnt pathway components to maintain active Wnt signaling, promoting germ cell differentiation. More importantly, we show that Wnt signaling is required to maintain ISCs by promoting ISC survival and proliferation. Surprisingly, Wnt signaling is dispensable for Piwi expression. Instead, we demonstrate that Wnt signaling controls the expression of four Gst genes to maintain the reduced redox, thereby promoting ISC maintenance and germ cell differentiation. Finally, knocking down one of the Gst gene, GstD2, in ISCs leads to the germ cell differentiation defect. Therefore, our study has revealed a novel mechanism which autocrine Wnt signaling utilizes to maintain ISCs and promote germ cell differentiation.

Although the differentiation niche is critical for promoting GSC progeny differentiation, little is known about its regulation. Here, we have identified Wnt2 and Wnt4 as autocrine signals to maintain the GSC differentiation niche partly through redox regulation (Figure 7I). First, Wnt signaling is required in ISCs for their maintenance by promoting cell proliferation and survival. Defective Wnt signaling causes a severe ISC loss, thereby preventing germ cell differentiation. Second, Wnt signaling is required to maintain the reduced redox state by sustaining the expression of Gst genes. This represents a novel connection between Wnt signaling and redox control. In addition, this study has also revealed that the reduced redox state is critical for ISC survival and thus for promoting germ cell differentiation. Third, Wnt signaling in ECs promotes GSC progeny differentiation partly by repressing BMP signaling in differentiated GSC progeny. Fourth, Wnt2 and Wnt4 represent redundant autocrine signals for maintaining Wnt signaling in the germ cell differentiation niche. Wnt signaling has been shown to control stem cell self-renewal directly (Holland et al., 2013), whereas ROS has shown to prime hematopoietic progenitor differentiation in Drosophila (Owusu-Ansah and Banerjee, 2009). This study has demonstrated the importance of Wnt signaling in maintaining the GSC differentiation niche by reducing ROS and thus promoting GSC lineage differentiation. Therefore, this study not only has identified critical signals for maintaining the GSC differentiation niche but also has revealed a novel function of Wnt signaling in the regulation of cellular redox.

This study has demonstrated that autocrine Wnt signaling controls ISC maintenance, thereby promoting germ cell differentiation. First, canonical Wnt signaling is required in ISCs to promote germ cell differentiation. c587-mediated knockdown of Wnt signal transducers Fz/fz2, dsh, and arm as well as c587-directed overexpression of Wnt signaling inhibitors sgg and axn leads to similar germ cell differentiation defects. Moreover, c587-directed overexpression of a constitutively active form of arm* can fully rescue the germ cell differentiation defect caused by dsh knockdown. Second, Wnt signaling maintains ISCs by promoting proliferation and survival. c587-mediated knockdown of dsh and arm leads to a severe ISC loss, whereas c587-mediated knockdown of sgg and axn or c587-directed arm* overexpression expands the ISC population. In addition, hyperactive Wnt signaling increases ISC proliferation and decreases apoptosis, whereas Wnt signaling downregulation increases ISC apoptosis and decreases proliferation. Thus, canonical Wnt signaling maintains the differentiation niche by promoting ISC proliferation and survival. However, we could not completely rule out the possibility that defective Wnt signaling leads to the loss of adult ISCs due to early developmental defects. Third, ISC-expressing Wnt2 and Wnt4 function redundantly in the differentiation niche to promote germ cell differentiation. Our RNA-seq results show that wnt2 and wnt4 mRNAs are present in the purified ISCs at high levels, while other wnt genes are expressed at much lower levels. c587-mediated wnt2 and wnt4 double knockdown results in more severe germ cell differentiation defects than wnt2 or wnt4 single knockdown.

Piwi has recently been shown to be required in the differentiation niche for promoting germ cell differentiation partly by repressing dpp expression (Jin et al., 2013; Ma et al., 2014). Although a recent study proposes that Wnt signaling controls germ cell differentiation by regulating piwi expression (Hamada-Kawaguchi et al., 2014), this study shows that Piwi protein and mRNAs are not downregulated in Wnt signaling-defective ISCs, suggesting that Wnt signaling does not sustain piwi expression in the differentiation niche to promote GSC progeny differentiation. Instead, this study has further revealed that Wnt signaling maintains the differentiation niche by controlling the cellular redox. First, our RNA-seq results show that GstD2, GstD4, GstD10, and GstE3 mRNA expression levels are significantly downregulated in the purified Wnt signaling-defective ISCs in comparison with the control ISCs. Second, defective Wnt signaling in ISCs elevates ROS levels in themselves and underneath germ cells, indicating that Wnt signaling is required in the differentiation niche to maintain low ROS levels. It remains unclear if increased ROS levels in early germ cells contribute to their differentiation defects. Third, c587-directed GST2, SOD1, and CAT overexpression can dramatically suppress the ROS elevation, the germ cell differentiation retardation, and the ISC loss caused by defective Wnt signaling, indicating that ROS elevation is responsible for the germ cell differentiation defect and the ISC loss. Finally, c587-mediated knockdown of GstD2 and Cat results in the ISC loss and the germ cell differentiation defect, indicating that ROS elevation in ISCs is sufficient to cause ISC loss and retard germ cell differentiation. This study has, for the first time, demonstrated that autocrine Wnt signaling controls cellular redox state in the differentiation niche and thus promotes germ cell differentiation.

So far, various studies have revealed a number of important signaling pathways and factors in ISCs to promote germ cell differentiation by maintaining ISC cellular process-mediated germ cell–soma interaction and preventing BMP signaling (Xie, 2013). This study shows that Wnt signaling is required for preventing BMP signaling in differentiated germ cells and for maintaining long ISC cellular processes. Rho, Eggless, Woc, Lsd1, Piwi, EGFR signaling, and ecdysone signaling have been shown to be required in ISCs to maintain long ISC cellular processes encasing germ cells (Schultz et al., 2002; Kirilly et al., 2011; Wang et al., 2011; Eliazer et al., 2014; Ma et al., 2014; Maimon et al., 2014; Konig and Shcherbata, 2015). Since properly differentiated germ cells are also required to maintain long ISC cellular processes (Kirilly et al., 2011), it is difficult to distinguish the cause and effect of the germ cell differentiation defect and the ISC cellular process loss. In contrast, three known strategies operate in the differentiation niche to prevent BMP signaling, thereby producing a permissive environment for germ cell differentiation. First, Lsd1, Rho, and Piwi are required in ISCs to repress dpp mRNA expression, thereby directly preventing BMP signaling in the differentiation niche (Eliazer et al., 2011; Kirilly et al., 2011; Jin et al., 2013; Ma et al., 2014). dpp encodes a BMP ligand, which activates BMP signaling important for GSC self-renewal (Xie and Spradling, 1998). Second, Rho, Eggless, and EGFR signaling function in ISCs to repress the expression of dally, preventing BMP diffusion from the self-renewal niche to the differentiation niche (Liu et al., 2010; Kirilly et al., 2011; Wang et al., 2011). Dally, which is a proteoglycan protein facilitating BMP differentiation and signaling, is expressed in cap cells, the GSC self-renewal niche, to restrict BMP signaling to GSCs (Akiyama et al., 2008; Guo and Wang, 2009). Third, a recent study has proposed that cap cell-initiated Wnt signaling maintains the expression of tkv encoding a type I BMP receptor in ISCs, thereby preventing BMP signaling in differentiated germ cells (Luo et al., 2015). Our findings from this study have also supported the idea that autocrine Wnt signaling in ISCs promotes GSC progeny differentiation partly by repressing BMP signaling. However, our findings have also argued that inactivating Wnt signaling does not regulate the mRNA expression levels of tkv and other BMP pathway components in ISCs. Therefore, future research will be needed to investigate the molecular mechanisms for Wnt signaling in the differentiation niche to prevent BMP signaling and maintain long ISC cellular processes.

The Drosophila stocks used in this study include: c587 (Kirilly et al., 2011), PZ1444 (Kirilly et al., 2011), UAS-SOD1 (Pan et al., 2007), UAS-axn (BL7225), UAS-sgg (BL6818), armRNAi (BL31304; BL31305), dshRNAi (BL31306; BL31307), axnRNAi (BL31705), sggRNAi (BL35364), Wnt2RNAi (BL36722; TH00483; TH00484), and Wnt4RNAi (BL29442; TH00485). TH lines for Wnt2 and Wnt4 were constructed for this study according to the published procedure (Ni et al., 2011); the coding region of Gst2 and Cat were cloned into a UASp vector to generate UAS-Gst2 and UAS-Cat using standard molecular biology techniques. Drosophila strains were maintained and crossed at room temperature on standard cornmeal/molasses/agar media unless specified. To maximize the RNAi-mediated knockdown effect, newly eclosed flies were cultured at 29°C for a week before the analysis of ovarian phenotypes.

BrdU incorporation, TUNEL labeling, and DHE staining were performed according to the published procedures (Xie and Spradling, 1998; Owusu-Ansah and Banerjee, 2009; Wang et al., 2011). Immunohistochemistry was performed according to our previously published procedures (Xie and Spradling, 1998). The following antibodies were used in this study: rabbit polyclonal anti-β-galactosidase antibody (1:100, Cappel), mouse monoclonal anti-Hts antibody (1:50, DSHB), Guinea pig polyclonal anti-Piwi antibodies (1:100; provided by H. Lin), rabbit monoclonal anti-pS423/425 Smad3 antibody (1:100, Epitomics), and rat monoclonal anti-Vasa antibody (1:50, DSHB). All images were taken with a Leica TCS SP5 confocal microscope.

The GFP-positive dshKD or axn-overexpressing ISCs were sorted by FACS, and mRNA isolation was carried out according to the published procedure (Ma et al., 2014). Following manufacturer's directions and using the Illumina TruSeq Stranded mRNA LT Kit (Illumina, MA, United States; Cat. No. RS-122-2101/2), short fragment libraries were constructed. The resulting libraries were purified using Agencourt AMPure XP system (Backman Coulter, CA, United States; Cat. No. A63880) and were then quantified using a LabChip® GX (PerkinElmer, MA, United States) and a Qubit Fluorometer (Life Technologies, CA, United States). All libraries were pooled, re-quantified, and run as 50 bp single-end lanes on an Illumina HiSeq 2500 instrument, using HiSeq Control Software 2.0.5 and Real-Time Analysis (RTA) version 1.17.20.0. Secondary Analysis version CASAVA-1.8.2 was run to demultiplex reads and generate FASTQ files.

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Author details

1. Su Wang

   1. Stowers Institute for Medical Research, Kansas City, United States
   2. Department of Anatomy and Cell Biology, University of Kansas School of Medicine, Kansas City, United States

   Contribution

   SW, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

2. Yuan Gao

   Center for Life Sciences, College of Life Sciences, School of Medical Sciences, Tsinghua University, Beijing, China

   Contribution

   YG, Acquisition of data, Analysis and interpretation of data

   Competing interests

   The authors declare that no competing interests exist.

3. Xiaoqing Song

   Stowers Institute for Medical Research, Kansas City, United States

   Contribution

   XS, Acquisition of data

   Competing interests

   The authors declare that no competing interests exist.

4. Xing Ma

   1. Stowers Institute for Medical Research, Kansas City, United States
   2. Department of Anatomy and Cell Biology, University of Kansas School of Medicine, Kansas City, United States

   Contribution

   XM, Acquisition of data

   Competing interests

   The authors declare that no competing interests exist.

5. Xiujuan Zhu

   Stowers Institute for Medical Research, Kansas City, United States

   Contribution

   XZ, Acquisition of data

   Competing interests

   The authors declare that no competing interests exist.

6. Ying Mao

   Center for Life Sciences, College of Life Sciences, School of Medical Sciences, Tsinghua University, Beijing, China

   Contribution

   YM, Acquisition of data

   Competing interests

   The authors declare that no competing interests exist.

7. Zhihao Yang

   Center for Life Sciences, College of Life Sciences, School of Medical Sciences, Tsinghua University, Beijing, China

   Contribution

   ZY, Contributed unpublished essential data or reagent

   Competing interests

   The authors declare that no competing interests exist.

8. Jianquan Ni

   Center for Life Sciences, College of Life Sciences, School of Medical Sciences, Tsinghua University, Beijing, China

   Contribution

   JN, Contributed unpublished essential data or reagent

   Competing interests

   The authors declare that no competing interests exist.

9. Hua Li

   Stowers Institute for Medical Research, Kansas City, United States

   Contribution

   HL, Analysis and interpretation of data

   Competing interests

   The authors declare that no competing interests exist.

10. Kathryn E Malanowski

    Stowers Institute for Medical Research, Kansas City, United States

    Contribution

    KEM, Acquisition of data

    Competing interests

    The authors declare that no competing interests exist.

11. Perera Anoja

    Stowers Institute for Medical Research, Kansas City, United States

    Contribution

    PA, Acquisition of data

    Competing interests

    The authors declare that no competing interests exist.

12. Jungeun Park

    Stowers Institute for Medical Research, Kansas City, United States

    Contribution

    JP, Acquisition of data

    Competing interests

    The authors declare that no competing interests exist.

13. Jeff Haug

    Stowers Institute for Medical Research, Kansas City, United States

    Contribution

    JH, Acquisition of data

    Competing interests

    The authors declare that no competing interests exist.

14. Ting Xie

    1. Stowers Institute for Medical Research, Kansas City, United States
    2. Department of Anatomy and Cell Biology, University of Kansas School of Medicine, Kansas City, United States

    Contribution

    TX, Conception and design, Analysis and interpretation of data, Drafting or revising the article

    For correspondence

    tgx@stowers.org

    Competing interests

    The authors declare that no competing interests exist.

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