Scratching the niche that controls Caenorhabditis elegans germline stem cells

Abstract

The Caenorhabditis elegans gonad provides a well-defined model for a stem cell niche and its control of self-renewal and differentiation. The distal tip cell (DTC) forms a mesenchymal niche that controls germline stem cells (GSCs), both to generate the germline tissue during development and to maintain it during adulthood. The DTC uses GLP-1/Notch signaling to regulate GSCs; germ cells respond to Notch signaling with a network of RNA regulators to control the decision between self-renewal and entry into the meiotic cell cycle.

Fig. 1

The C. elegans germline and its DTC niche. (A-B) Nomarski micrographs show gonad arms in situ. (A) Adult hermaphrodite possesses two gonadal arms; the posterior arm is in focus and enclosed by a pink oval (micrograph from M. Gallegos). (B) Adult male has a single gonadal arm (blue oval). (C-D) Cartoons showing DTC niche (red) and germline in the two sexes. Shown are mitotically dividing germ cells (yellow) at the distal end, which include GSCs, germ cells in early meiotic prophase (green) and developing gametes. Insets: DTC ablation (X) abolishes all germline mitotic divisions and causes all germ cells to differentiate. (C) Adult hermaphrodite germline possesses one DTC distally and developing oocytes (pink) proximally. (D) Adult male germline has two DTCs distally and developing sperm (blue) proximally. (E-F) Notch mutant germlines (conventions same as in C and D). (E) When Notch signaling is removed in adults, germline self-renewal is not maintained: germ cells leave the mitotic cell cycle and enter meiosis. (F) When Notch signaling is unregulated, germline differentiation fails: germ cells remain in the mitotic cell cycle and form a tumor. (G-I) DTC position and morphology. (G) Nuclear-localized GFP (green) reveals DTC nucleus at distal end. (H) Cytoplasmic GFP (green) reveals DTC processes that extend proximally along the germline surface (DAPI stained nuclei, blue). (I) Membrane-localized GFP (green) shows intercalation of DTC processes between germ cells (Hoechst stained nuclei, blue) in a single transverse confocal slice through the gonad arm.

Fig. 2

Control of DTC specification and function. (A) DTC lineage. Hermaphrodite (left) and male (right) SGPs (grey) generate DTCs (red) by asymmetric divisions. Other daughters (blue) generate other components of the somatic gonad. (B) SGP division is symmetrical with DTC loss and extra other somatic gonadal cells when the Wnt β-catenin asymmetry pathway or ceh-22 is depleted. (C) SGP division is symmetrical with extra DTCs and loss of other gonadal cells when SYS-1/β-catenin or ceh-22 is overexpressed. (D) Genetic pathway for control of DTC fate specification and DTC niche function (see text for explanation).

Fig. 3

Control of germ cell self-renewal and proliferation. (A) Graph showing total germ cell number with time. Wild-type and mutants are depicted with distinct symbols: solid symbols show that germline self-renewal is continuing; empty symbols show that germline self-renewal has failed and that all germ cells have differentiated. In wild-type and fog-1 mutants, larval proliferation is equivalent; in wild-type, germline self-renewal maintains germ cell number during adulthood; in fog-1 mutants, germ cell counts have not been done in later adults, but the size of the germline appears to be maintained. In other key mutants, larval proliferation produces 10 germ cells (fog-1; fbf-1 fbf-2), 120 germ cells (fbf-1 fbf-2), or 500 germ cells (fog-1/+; fbf-1 fbf-2), but those divisions stop by the stage indicated as an open symbol. Therefore, germline proliferation is limited and self-renewal fails, either in L2 larvae (fog-1; fbf-1 fbf-2), L4 larvae (fbf-1 fbf-2), or adults (fog-1/+; fbf-1 fbf-2). (B) Network of regulators for germ cell self-renewal and differentiation. See text for explanation. Gene X indicates that Notch signaling likely controls additional genes that are not yet identified.