Germline stem cells: origin and destiny

Abstract

Germline stem cells are key to genome transmission to future generations. Over recent years, there have been numerous insights into the regulatory mechanisms that govern both germ cell specification and the maintenance of the germline in adults. Complex regulatory interactions with both the niche and the environment modulate germline stem cell function. This perspective highlights some examples of this regulation to illustrate the diversity and complexity of the mechanisms involved.

Figure 1. Drosophila Ovarian Niche

(A) Drawing of the ovarian GSC compartment. Each ovary is subdivided into 12–16 ovarioles that generate chains of maturing egg chambers. The niche located at the tip of each ovariole contains three different somatic cell types (terminal filament cells, cap cells, and escort or inner germarial sheath [IGS] cells) and two or three GSCs. Asymmetric division of a GSC perpendicular to the niche (arrow) generates a new GSC and a differentiating daughter that exits the niche. The differentiating daughter, called the cystoblast, subsequently undergoes four rounds of division. Each division is incomplete, generating two, four, eight, and sixteen interconnected germline cysts. One cell within the 16-cell cyst differentiates into the oocyte and undergoes meiosis, while the other cells become polyploid nurse cells that feed into the growing oocyte and eventually die. The spectrosome and fusome, germline organelles without membrane that contain alpha-spectrin and adducin, play a role in the orientation of the spindle in the GSC and the regulation of cyst division and oocyte determination in the female. The spectrosome has a round appearance and is found in GSCs and cystoblasts. The fusome extends between dividing cysts. (B) Summary of signaling pathways. Beige: terminal filament. Red: cells of the niche, cap cells, and escort/IGS cells. Blue: GSCs. Green: differentiating germ cells. Yellow: site of adhesion between GSC and niche. TL, regulation by translation; TS, regulation by transcription; S, regulation of stability. Arrow depicts cell-to-cell signaling. In addition to transcriptional regulation via the DPP/GBB receptors, RNA regulators such as Nanos and Pumilio as well as the miRNA machinery act within germ cells and are required for the maintenance of GSCs, likely by repressing RNA translation and stability of germline differentiation genes (Gilboa and Lehmann, 2004; Park et al., 2007; Wang and Lin, 2004). Recent findings propose that in GSCs, Mei-P26 interacts with Nanos and the miRNA machinery to repress RNAs, such as brat, required for differentiation, while in the cystoblast Bam/Bgcn inhibit nanos RNA, thereby favoring Pumilio/Brat interactions, which repress RNAs required for self-renewal and proliferation, such as the BMP-mediated transcription factor Mad and the proliferation factor dMyc (Harris et al., 2011); (Li et al., 2012). Mei-P26 is also required for differentiation and its translation may depend on Vasa (Liu et al., 2009). JAK/STAT signaling is required for GSC maintenance; the ligand Unpaired (UPD) expressed in terminal filament cells activates the Domeless receptor in cap cells, where it activates transcription of dpp (López-Onieva et al., 2008).

Figure 2. Drosophila Testis Niche

(A) Drawing of the testis GSC compartment. Six to twelve GSCs form a rosette around a single cluster of somatic support cells called the hub. Division is oriented (arrow) by the centrosomes (stars). The mother centrosome remains in the GSC (black star). The hub also maintains somatic cyst cell stem cells (CySCs). Division of the CySCs and GSCs is coordinated such that two cyst cells surround each differentiating germ cell. The differentiating germ cell, called a gonioblast, undergoes four rounds of division, before each spermatocyte of the interconnected 16-cell cyst undergoes meiosis. GSCs predominately divide asymmetrically but can also divide symmetrically to replenish the stem cell pool (de Cuevas and Matunis, 2011). The round spectrosome is found in GSC and gonioblast, while the fusome extends equally between dividing cysts. (B) Summary of signaling pathways. Beige: hub cells. Red: cyst stem cells and cyst cells. Blue: GSCs. Green: differentiating germ cells. Yellow: site of adhesion between GSC and niche. TL, regulation by translation; TS, regulation by transcription; pStat, phosphorylated active Stat. The HOW RNA binding protein binds to bam RNA and represses its translation in stem cells. Ectopic HOW delays BAM protein accumulation and leads to additional spermatogonial divisions (Monk et al., 2010). Once a critical level of BAM is reached spermatogonia cease proliferation and differentiate (Insco et al., 2009).

Figure 3. Distal Tip Cell and Stem Cell Compartment of C. elegans Hermaphrodite Gonad

(A) Drawing of GSC compartment of one gonad arm. Mitotically active, undifferentiated germ cells reside in each of the distal ends of the two gonad arms. The distal tip cell (DTC) envelops the gonad tip and serves as a niche. As cells move away from the DTC they cease division and enter meiosis. (B) Summary signaling pathways. Beige: ASI neuron. Red: distal tip cell. Blue: GSCs. Green: differentiating germ cells. TL, regulation by translation. Arrow depicts cell-to-cell signaling. The double pointed arrow indicates the random direction of GSC division.

Figure 4. Mouse Spermatogonial Niche

(A) Drawing of GSC compartment as a quadrant of cross sections of a seminiferous tubule. The entire spermiogenesis process, starting from self-renewing spermatogonia to release of elongated sperm into the lumen of the seminiferous tubule, occurs within the environment of large, somatic Sertoli cells. Tight junctions separate the basal compartment, which contains early spermiogenic stages, from the adluminal compartment filled with later stages. (B) Summary of signaling pathways. Beige: blood vessel and Leydig cells, which may contribute additional factors regulating stem cell maintenance, including Colony stimulating factor 1 (CSF-1) and FGF. Red: Sertoli cells. Note that tight junctions form the blood-testis barrier between the Basal (BC) and Adluminal (AL) compartment, which separates differentiating spermatocytes from undifferentiating SGPs. Blue: SGPs. Green: differentiating germ cells. All depicted interactions are transcriptional. RA: retinoic acid activates KIT receptor during the transition to differentiation. SGPs express high levels of the GDNF receptor GFRα1, the translational repressor Nanos2, and the transcriptional repressor PLZF. Increased levels of the bHLH transcription factors Ngn3 and Nanos3 characterize SGPs that are more likely to differentiate. Etv5 expressed in Sertoli cells leads to production of CCL9 ligand, which is recognized by CCR1 receptor in germ cells and attracts SGPs to Sertoli cells (double headed arrow) (Simon et al., 2010).