The raison d'etre of the germline is to produce oocytes and sperm that pass genetic material and cytoplasmic constituents to the next generation. To achieve this goal, many developmental processes must be executed and coordinated. ERK, the terminal MAP kinase of a number of signaling pathways, controls many aspects of development. Here we present a comprehensive analysis of MPK-1 ERK in Caenorhabditis elegans germline development. MPK-1 functions in four developmental switches: progression through pachytene, oocyte meiotic maturation/ovulation, male germ cell fate specification, and a nonessential function of promoting the proliferative fate. MPK-1 also regulates multiple aspects of cell biology during oogenesis, including membrane organization and morphogenesis: organization of pachytene cells on the surface of the gonadal tube, oocyte organization and differentiation, oocyte growth control, and oocyte nuclear migration. MPK-1 activation is temporally/spatially dynamic and most processes appear to be controlled through sustained activation. MPK-1 thus may act not only in the control of individual processes but also in the coordination of contemporaneous processes and the integration of sequential processes. Knowledge of the dynamic activation and diverse functions of MPK-1 provides the foundation for identification of upstream signaling cascades responsible for region-specific activation and the downstream substrates that mediate the various processes. I N the generation of oocytes and sperm, perhaps the most complex cells in animals, the germline lineage undergoes a multifaceted developmental process that begins in embryogenesis and continues into adulthood. While the details of the steps can differ between species due to differences in reproductive biology, a core set of events occurs in animals: germ cell fate specification, incorporation into the gonad, sexual fate specification, proliferative expansion, and gamete production. Two interconnected differentiation programs define gamete production: (1) meiosis where chromosomes pair, recombine, and then segregate to give a reassorted haploid content and (2) gametogenesis where biosynthetic and morphogenetic processes generate the large nutrient and developmental information-rich oocyte and the small motile sperm. In aggregate, these processes are essential to pass genetic material from generation to generation and to form the totipotent zygote necessary for the development of a new individual. Disruption of germline development can lead to sterility, germline tumors, and birth defects. Thus an important goal is to define the pathways and gene products that control and execute the various steps of germline development.The MAP kinase extracellular signal-regulated kinase (ERK) functions in many aspects of animal development and homeostasis (Marshall 1995;Rubin et al. 1997;Schlessinger 2000;Sundaram 2006). ERK is the terminal regulator of signaling cascades such as canonical receptor tyrosine kinase signaling, which contains core members, including the RA...

RAS-extracellular signal regulated kinase (ERK) signaling governs multiple aspects of cell fate specification, cellular transitions, and growth by regulating downstream substrates through phosphorylation. Understanding how perturbations to the ERK signaling pathway lead to developmental disorders and cancer hinges critically on identification of the substrates. Yet, only a limited number of substrates have been identified that function in vivo to execute ERK-regulated processes. The Caenorhabditis elegans germ line utilizes the well-conserved RAS-ERK signaling pathway in multiple different contexts. Here, we present an integrated functional genomic approach that identified 30 ERK substrates, each of which functions to regulate one or more of seven distinct biological processes during C. elegans germ-line development. Our results provide evidence for three themes that underlie the robustness and specificity of biological outcomes controlled by ERK signaling in C. elegans that are likely relevant to ERK signaling in other organisms: (i) multiple diverse ERK substrates function to control each individual biological process; (ii) different combinations of substrates function to control distinct biological processes; and (iii) regulatory feedback loops between ERK and its substrates help reinforce or attenuate ERK activation. Substrates identified here have conserved orthologs in humans, suggesting that insights from these studies will contribute to our understanding of human diseases involving deregulated ERK activity.functional genomics ͉ signaling ͉ MPK-1 ͉ RNAi screen T he RTK-RAS-ERK pathway relays extracellular signals via a conserved kinase cascade that results in phosphorylation and activation of ERK, the terminal member of this pathway (1, 2). Active ERK in turn phosphorylates substrates to execute many cellular and developmental processes (2) (Fig. 1A). Comprehensive insight into mechanisms underlying ERK-dependent control of biological processes depends on identification of ERK substrates. Although bona fide ERK substrates have been identified in cultured cells (3-5) and potential substrates documented from proteomic studies (6, 7), the function of most of these substrates in vivo has not been defined. Additionally, currently identified substrates do not account for most ERK-dependent events in mammals, making it likely that more substrates remain to be identified. Forward genetic studies in Caenorhabditis elegans and Drosophila have identified a few ERK substrates that act in defined biological contexts (8-10); however, the mutant phenotypes of these genes account for some but not all ERK-regulated processes. To obtain molecular insight into how ERK signaling controls multiple biological processes in vivo through substrates, we devised an integrated bioinformatic, genetic, and biochemical approach by using C. elegans germ-line development as the model system.In C. elegans RAS and ERK are encoded by let-60 and mpk-1, respectively, and loss-of-function or null mutations in these core components abrogate pat...

p53 is a tumor suppressor gene whose regulation is crucial to maintaining genome stability and for the apoptotic elimination of abnormal, potentially cancer-predisposing cells. C. elegans contains a primordial p53 gene, cep-1, that acts as a transcription factor necessary for DNA damage-induced apoptosis. In a genetic screen for negative regulators of CEP-1, we identified a mutation in GLD-1, a translational repressor implicated in multiple C. elegans germ cell fate decisions and related to mammalian Quaking proteins. CEP-1-dependent transcription of proapoptotic genes is upregulated in the gld-1(op236) mutant and an elevation of p53-mediated germ cell apoptosis in response to DNA damage is observed. Further, we demonstrate that GLD-1 mediates its repressive effect by directly binding to the 3'UTR of cep-1/p53 mRNA and repressing its translation. This study reveals that the regulation of cep-1/p53 translation influences DNA damage-induced apoptosis and demonstrates the physiological importance of this mechanism.

Somatic and germline sex determination pathways have diverged significantly in animals, making comparisons between taxa difficult. To overcome this difficulty, we compared the genes in the germline sex determination pathways of Caenorhabditis elegans and C. briggsae, two Caenorhabditis species with similar reproductive systems and sequenced genomes. We demonstrate that C. briggsae has orthologs of all known C. elegans sex determination genes with one exception: fog-2. Hermaphroditic nematodes are essentially females that produce sperm early in life, which they use for self fertilization. In C. elegans, this brief period of spermatogenesis requires FOG-2 and the RNA-binding protein GLD-1, which together repress translation of the tra-2 mRNA. FOG-2 is part of a large C. elegans FOG-2-related protein family defined by the presence of an F-box and Duf38/FOG-2 homogy domain. A fog-2-related gene family is also present in C. briggsae, however, the branch containing fog-2 appears to have arisen relatively recently in C. elegans, post-speciation. The C-terminus of FOG-2 is rapidly evolving, is required for GLD-1 interaction, and is likely critical for the role of FOG-2 in sex determination. In addition, C. briggsae gld-1 appears to play the opposite role in sex determination (promoting the female fate) while maintaining conserved roles in meiotic progression during oogenesis. Our data indicate that the regulation of the hermaphrodite germline sex determination pathway at the level of FOG-2/GLD-1/tra-2 mRNA is fundamentally different between C. elegans and C. briggsae, providing functional evidence in support of the independent evolution of self-fertile hermaphroditism. We speculate on the convergent evolution of hermaphroditism in Caenorhabditis based on the plasticity of the C. elegans germline sex determination cascade, in which multiple mutant paths yield self fertility.