Genetic analysis of Caenorhabditis elegans glp-1 mutants suggests receptor interaction or competition

glp-1 encodes a member of the highly conserved LIN-12/Notch family of receptors that mediates the mitosis/meiosis decision in the C. elegans germline. We have characterized three mutations that represent a new genetic and phenotypic class of glp-1 mutants, glp-1(Pro). The glp-1(Pro) mutants display gain-of-function germline pattern defects, most notably a proximal proliferation (Pro) phenotype. Each of three glp-1(Pro) alleles encodes a single amino acid change in the extracellular part of the receptor: two in the LIN-12/Notch repeats (LNRs) and one between the LNRs and the transmembrane domain. Unlike other previously described gain-of-function mutations that affect this region of LIN-12/Notch family receptors, the genetic behavior of glp-1(Pro) alleles is not consistent with simple hypermorphic activity. Instead, the mutant phenotype is suppressed by wild-type doses of glp-1. Moreover, a trans-heterozygous combination of two highly penetrant glp-1(Pro) mutations is mutually suppressing. These results lend support to a model for a higher-order receptor complex and/or competition among receptor proteins for limiting factors that are required for proper regulation of receptor activity. Double-mutant analysis with suppressors and enhancers of lin-12 and glp-1 further suggests that the functional defect in glp-1(Pro) mutants occurs prior to or at the level of ligand interaction.     

The establishment of Caenorhabditis elegans germline pattern is controlled by overlapping proximal and distal somatic gonad signals

We investigated the control of proliferation and differentiation in the larval Caenorhabditis elegans hermaphrodite germ line through analysis of glp-1 and lag-2 mutants, cell ablations, and ultrastructural data. After the first several rounds of germ cell division, GLP-1, a receptor of the LIN-12/Notch family, governs germline proliferation. We analyzed the proximal proliferation (Pro) phenotype in glp-1(ar202) and found that initial meiosis was delayed and spatially mispositioned. This is due, at least in part, to a heightened response of the mutant GLP-1 receptor to multiple sources of the somatic ligand LAG-2, including the proximal somatic gonad. We investigated whether proximal LAG-2 affects germline proliferation in the wild type. Our results indicate that (1) LAG-2 is necessary for GLP-1-mediated germline proliferation and prevention of early meiosis, and (2) several distinct anatomical sources of LAG-2 in the larval somatic gonad functionally overlap to promote proliferation and prevent early meiosis. Ultrastructural studies suggest that mitosis is not restricted to areas of direct DTC-germ line contact and that the germ line shares a common cytoplasm in larval stages. We propose that downregulation of the GLP-1 signaling pathway in the proximal germ line at the time of meiotic onset is under tight temporal and spatial control.     

glp-1 is required in the germ line for regulation of the decision between mitosis and meiosis in C. elegans

In the wild-type C. elegans germ line there are both mitotic and meiotic germ cells. Mutations in glp-1 cause germ cells that would normally divide mitotically to enter meiosis. This mutant phenotype mimics the effect of killing the distal tip cell, a somatic cell that interacts with the germ line to regulate the mitotic/meiotic decision. In addition, wild-type glp-1 product is required maternally for embryogenesis. Temperature-shift experiments indicate that the temporal requirement for glp-1 activity in the germ line is the same as that for distal tip cell regulation. Mosaic analyses suggest that glp-1 is produced in the germ line. We propose that glp-1 acts as part of the receiving mechanism in the interaction between the distal tip cell and germ line.     

Enhancers of Glp-1, a Gene Required for Cell-Signaling in Caenorhabditis Elegans, Define a Set of Genes Required for Germline Development

The distal tip cell (DTC) regulates the proliferation or differentiation choice in the Caenorhabditis elegans germline by an inductive mechanism. Cell signaling requires a putative receptor in the germline, encoded by the glp-1 gene, and a putative signal from the DTC, encoded by the lag-2 gene. Both glp-1 and lag-2 belong to multigene gene families whose members are essential for cell signaling during development of various tissues in insects and vertebrates as well as C. elegans. Relatively little is known about how these pathways regulate cell fate choice. To identify additional genes involved in the glp-1 signaling pathway, we carried out screens for genetic enhancers of glp-1. We recovered mutations in five new genes, named ego (enhancer of glp-1), and two previously identified genes, lag-1 and glp-4, that strongly enhance a weak glp-1 loss-of-function phenotype in the germline. Ego mutations cause multiple phenotypes consistent with the idea that gene activity is required for more than one aspect of germline and, in some cases, somatic development. Based on genetic experiments, glp-1 appears to act upstream of ego-1 and ego-3. We discuss the possible functional relationships among these genes in light of their phenotypes and interactions with glp-1.     

EGO-1, a putative RNA-directed RNA polymerase, promotes germline proliferation in parallel with GLP-1/notch signaling and regulates the spatial organization of nuclear pore complexes and germline P granules in Caenorhabditis elegans

Caenorhabditis elegans EGO-1, a putative cellular RNA-directed RNA polymerase, promotes several aspects of germline development, including proliferation, meiosis, and gametogenesis, and ensures a robust response to RNA interference. In C. elegans, GLP-1/Notch signaling from the somatic gonad maintains a population of proliferating germ cells, while entry of germ cells into meiosis is triggered by the GLD-1 and GLD-2 pathways. GLP-1 signaling prevents germ cells from entering meiosis by inhibiting GLD-1 and GLD-2 activity. We originally identified the ego-1 gene on the basis of a genetic interaction with glp-1. Here, we investigate the role of ego-1 in germline proliferation. Our data indicate that EGO-1 does not positively regulate GLP-1 protein levels or GLP-1 signaling activity. Moreover, GLP-1 signaling does not positively regulate EGO-1 activity. EGO-1 does not inhibit expression of GLD-1 protein in the distal germline. Instead, EGO-1 acts in parallel with GLP-1 signaling to influence the proliferation vs. meiosis fate choice. Moreover, EGO-1 and GLD-1 act in parallel to ensure germline health. Finally, the size and distribution of nuclear pore complexes and perinuclear P granules are altered in the absence of EGO-1, effects that disrupt germ cell biology per se and probably limit germline growth.     

Caenorhabditis elegans germline patterning requires coordinated development of the somatic gonadal sheath and the germ line

Interactions between the somatic gonad and the germ line influence the amplification, maintenance, and differentiation of germ cells. In Caenorhabditis elegans, the distal tip cell/germline interaction promotes a mitotic fate and/or inhibits meiosis through GLP-1/Notch signaling. However, GLP-1-mediated signaling alone is not sufficient for a wild-type level of germline proliferation. Here, we provide evidence that specific cells of the somatic gonadal sheath lineage influence amplification, differentiation, and the potential for tumorigenesis of the germ line. First, an interaction between the distal-most pair of sheath cells and the proliferation zone of the germ line is required for larval germline amplification. Second, we show that insufficient larval germline amplification retards gonad elongation and thus delays meiotic entry. Third, a more severe delay in meiotic entry, as is exhibited in certain mutant backgrounds, inappropriately juxtaposes undifferentiated germ cells with cells of the proximal sheath lineage, leading to the formation of a proximal germline tumor derived from undifferentiated germ cells. Tumors derived from dedifferentiated germ cells, however, respond to the proximal interaction differently depending on the mutant background. Our study underscores the importance of strict developmental coordination between neighboring tissues. We discuss these results in the context of mechanisms that may underlie tumorigenesis.     

Two homologous regulatory genes, lin-12 and glp-1, have overlapping functions.

Two homologous genes, lin-12 and glp-1, encode transmembrane proteins required for regulatory cell interactions during C. elegans development. Based on their single mutant phenotypes, each gene has been thought to govern a distinct set of cell fates. We show here that lin-12 and glp-1 are functionally redundant during embryogenesis: Unlike either single mutant, the lin-12 glp-1 double mutant dies soon after hatching. Numerous cellular defects can be observed in these Lag (for lin-12 and glp-1) double mutants. Furthermore, we have identified two genes, lag-1 and lag-2, that appear to be required for both lin-12 and glp-1-mediated cell interactions. Strong loss-of-function lag mutants are phenotypically indistinguishable from the lin-12 glp-1 double; weak lag mutants have phenotypes typical of lin-12 and glp-1 single mutants. We speculate that the lin-12 and glp-1 proteins are biochemically interchangeable and that their divergent roles in development may rely largely on differences in gene expression.     

DSH-2 regulates asymmetric cell division in the early C. elegans somatic gonad

Like other organs, the C. elegans gonad develops from a simple primordium that must undergo axial patterning to generate correct adult morphology. Proximal/distal (PD) polarity in the C. elegans gonad is established early during gonadogenesis by the somatic gonad precursor cells, Z1 and Z4. Z1 and Z4 each divide asymmetrically to generate one daughter with a proximal fate and one with a distal fate. PD polarity of the Z1/Z4 lineages requires the activity of a Wnt pathway that activates the TCF/LEF homolog pop-1. How the gonadal pathway controlling pop-1 is regulated by upstream factors has been unclear, as neither Wnt nor Dishevelled (Dsh) proteins have been shown to be required. Here we show that the C. elegansdsh homolog dsh-2 controls gonadal polarity. As in pop-1 mutants, dsh-2 hermaphrodites have Z1 and Z4 lineage defects indicative of defective PD polarity and are missing gonadal arms. Males have an elongated but disorganized gonad, also with lineage defects. DSH-2 protein is expressed in the Z1/Z4 gonadal precursor cells. Asymmetric distribution of nuclear GFP::POP-1 in Z1 and Z4 daughter cells is reversed in dsh-2 mutants, with higher levels in distal than proximal daughters. dsh-2 and the frizzled receptor homolog lin-17 have a strong genetic interaction, suggesting that they act in a common pathway. We suggest that DSH-2 functions as an upstream regulator of POP-1 in the somatic gonad to control asymmetric cell division, thereby establishing proximal-distal polarity of the developing organ.     

The Caenorhabditis Elegans Sel-1 Gene, a Negative Regulator of Lin-12 and Glp-1, Encodes a Predicted Extracellular Protein

The Caenorhabditis elegans lin-12 and glp-1 genes encode members of the LIN-12/NOTCH family of receptors. The sel-1 gene was identified as an extragenic suppressor of a lin-12 hypomorphic mutant. We show in this report that the sel-1 null phenotype is wild type, except for an apparent elevation in lin-12 and glp-1 activity in sensitized genetic backgrounds, and that this genetic interaction seems to be lin-12 and glp-1 specific. We also find that sel-1 encodes a predicted extracellular protein, with a domain sharing sequence similarity to predicted proteins from humans and yeast. SEL-1 may interact with the LIN-12 and GLP-1 receptors and/or their respective ligands to down-regulate signaling.     

glp-1 and lin-12, genes implicated in distinct cell-cell interactions in C. elegans, encode similar transmembrane proteins

Genomic DNA closely related in sequence to lin-12, a gene that specifies certain cell fates during C. elegans development, was isolated from a C. elegans library by low stringency hybridization. DNA sequencing of genomic and cDNA clones predicts the new sequence to encode an integral membrane protein that shares three repeated amino acid sequence motifs with the lin-12 product and the Drosophila Notch product: an epidermal growth factor-like motif, the "lin-12/Notch Repeat," and a motif present in two yeast gene products that have cell cycle dependent functions. Austin and Kimble (see accompanying paper) present evidence that this sequence corresponds to glp-1, a gene implicated in cell-cell interactions distinct from those involving lin-12. Possible implications of the predicted structure of the glp-1 product with respect to these cell-cell interactions are discussed.     

Ultrastructural features of the adult hermaphrodite gonad of Caenorhabditis elegans: relations between the germ line and soma.

Genetic and embryological experiments have established the Caenorhabditis elegans adult hermaphrodite gonad as a paradigm for studying the control of germline development and the role of soma-germline interactions. We describe ultrastructural features relating to essential germline events and the soma-germline interactions upon which they depend, as revealed by electron and fluorescence microscopy. Gap junctions were observed between oocytes and proximal gonadal sheath cells that contract to ovulate the oocyte. These gap junctions must be evanescent since individual oocytes lose contact with sheath cells when they are ovulated. In addition, proximal sheath cells are coupled to each other by gap junctions. Within proximal sheath cells, actin/myosin bundles are anchored to the plasma membrane at plaque-like structures we have termed hemi-adherens junctions, which in turn are closely associated with the gonadal basal lamina. Gap junctions and hemi-adherens junctions are likely to function in the coordinated series of contractions required to ovulate the mature oocyte. Proximal sheath cells are fenestrated with multiple small pores forming conduits from the gonadal basal lamina to the surface of the oocyte, passing through the sheath cell. In most instances where pores occur, extracellular yolk particles penetrate the gonadal basal lamina to directly touch the underlying oocytes. Membrane-bounded yolk granules were generally not found in the sheath cytoplasm by either electron microscopy or fluorescence microscopy. Electron microscopic immunocytochemistry was used to confirm and characterize the appearance of yolk protein in cytoplasmic organelles within the oocyte and in free particles in the pseudocoelom. The primary route of yolk transport apparently proceeds from the intestine into the pseudocoelom, then through sheath pores to the surface of the oocyte, where endocytosis occurs. Scanning electron microscopy was used to directly visualize the distal tip cell which extends tentacle-like processes that directly contact distal germ cells. These distal tip cell processes are likely to play a critical role in promoting germline mitosis. Scanning electron microscopy also revealed thin filopodia extending from the distal sheath cells. Distal sheath filopodia were also visualized using a green fluorescent protein reporter gene fusion and confocal microscopy. Distal sheath filopodia may function to stretch the sheath over the distal arm.     

Cell autonomy of lin-12 function in a cell fate decision in C. elegans

The lin-12 gene of C. elegans encodes a predicted transmembrane protein that controls a decision by two cells, Z1.ppp and Z4.aaa, between the anchor cell (AC) and ventral uterine precursor cell (VU) fates. We performed laser ablation experiments to demonstrate that specification of the VU fate of Z1.ppp or Z4.aaa depends on an "AC-to-VU" signal from the presumptive AC. We generated genetic mosaics in which defined cells lacked lin-12 activity. By correlating the fates of Z1.ppp and Z4.aaa with the lin-12 genotype of nearly every cell in these mosaics, we conclude that lin-12 function is VU cell autonomous. We present a model in which lin-12 functions in the receiving mechanism for the "AC-to-VU" signal leading to the specification of the AC and VU fates of Z1.ppp and Z4.aaa.     

Suppressors of a Lin-12 Hypomorph Define Genes That Interact with Both Lin-12 and Glp-1 in Caenorhabditis Elegans

The lin-12 gene of Caenorhabditis elegans is thought to encode a receptor which mediates cell-cell interactions required to specify certain cell fates. Reversion of the egg-laying defective phenotype caused by a hypomorphic lin-12 allele identified rare extragenic suppressor mutations in five genes, sel-1, sel-9, sel-10, sel-11 and sel(ar40) (sel = suppressor and/or enhancer of lin-12). Mutations in each of these sel genes suppress defects associated with reduced lin-12 activity, and enhance at least one defect associated with elevated lin-12 activity. None of the sel mutations cause any obvious phenotype in a wild-type background. Gene dosage experiments suggest that sel-1 and sel(ar40) mutations are reduction-of-function mutations, while sel-9 and sel-11 mutations are gain-of-function mutations. sel-1, sel-9, sel-11 and sel(ar40) mutations do not suppress amorphic lin-12 alleles, while sel-10 mutations are able to bypass partially the requirement for lin-12 activity in at least one cell fate decision. sel-1, sel-9, sel-10, sel-11 and sel(ar40) mutations are also able to suppress the maternal-effect lethality caused by a partial loss-of-function allele of glp-1, a gene that is both structurally and functionally related to lin-12. These sel genes may therefore function in both lin-12 and glp-1 mediated cell fate decisions.     

On the control of germ cell development in Caenorhabditis elegans

After hatching, the germ line progenitor cells in C. elegans begin to divide mitotically; later, some of the germ line cells enter meiosis and differentiate into gametes. In the adult, mitotic germ cells, or stem cells, are found at one end (the distal end) and meiotic cells occupy the rest of the elongate gonad. Removal of two somatic gonadal cells, the distal tip cells, by laser microsurgery has a dramatic effect on germ cell development. In either sex, this operation leads to the arrest of mitosis and the initiation of meiosis in germ cells. The function of the distal tip cell in the intact animal appears to be the inhibition of meiosis (or stimulation of mitosis) in nearby germ cells. During development, this permits growth and, in the adult, it maintains the germ line stem cell population. A change in the position of the distal tip cell in the gonad at an early point in development is correlated with a change in the axial polarity of the germ line tissue. This suggests that the localization of the distal tip cell's inhibitory activity at the distal end of the gonad establishes the axial polarity of the germ line tissue in the intact animal.     

Identification and Characterization of Genes That Interact with Lin-12 in Caenorhabditis Elegans

We identified and characterized 14 extragenic mutations that suppressed the dominant egg-laying defect of certain lin-12 gain-of-function mutations. These suppressors defined seven genes: sup-17, lag-2, sel-4, sel-5, sel-6, sel-7 and sel-8. Mutations in six of the genes are recessive suppressors, whereas the two mutations that define the seventh gene, lag-2, are semi-dominant suppressors. These suppressor mutations were able to suppress other lin-12 gain-of-function mutations. The suppressor mutations arose at a very low frequency per gene, 10-50 times below the typical loss-of-function mutation frequency. The suppressor mutations in sup-17 and lag-2 were shown to be rare non-null alleles, and we present evidence that null mutations in these two genes cause lethality. Temperature-shift studies for two suppressor genes, sup-17 and lag-2, suggest that both genes act at approximately the same time as lin-12 in specifying a cell fate. Suppressor alleles of six of these genes enhanced a temperature-sensitive loss-of-function allele of glp-1, a gene related to lin-12 in structure and function. Our analysis of these suppressors suggests that the majority of these genes are part of a shared lin-12/glp-1 signal transduction pathway, or act to regulate the expression or stability of lin-12 and glp-1.