Barbara Conradt (2005) WormBook "Programmed cell death."

BARBARA CONRADT

[WormBook, 2005]

Programmed cell death is an integral component of C. elegans development. Genetic studies in C. elegans have led to the identification of more than two dozen genes that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular and biochemical studies have revealed the underlying conserved mechanisms that control these three phases of programmed cell death. In particular, an interplay of transcriptional regulatory cascades and networks involving CES-1 , CES-2 , HLH-1 / HLH-2 , TRA-1 , and other transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 in cells destined to die. A protein interaction cascade involving EGL-1 , CED-9 , CED-4 and CED-3 results in the activation of the key cell death protease CED-3 . The activation of CED-3 initiates the cell disassembly process and nuclear DNA fragmentation, which is mediated by the release of apoptogenic mitochondrial factors ( CPS-6 and WAH-1 ) and which involves multiple endo- and exo-nucleases such as NUC-1 and seven CRN nucleases. The recognition and removal of the dying cell is mediated by two partially redundant signaling pathways involving CED-1 , CED-6 and CED-7 in one pathway and CED-2 , CED-5 , CED-10 , CED-12 and PSR-1 in the other pathway. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in multicellular organisms.

David Zarkower (2006) WormBook "Somatic sex determination."

David Zarkower

[WormBook, 2006]

C. elegans occurs in two natural sexes, the XX hermaphrodite and the XO male, which differ extensively in anatomy, physiology, and behavior. All somatic differences between the sexes result from the differential activity of a global sex determination regulatory pathway. This pathway also controls X chromosome dosage compensation, which is coordinated with sex determination by the action of the three SDC proteins. The SDC proteins control somatic and germline sex by transcriptional repression of the her-1 gene. HER-1 is a secreted protein that controls a regulatory module consisting of a transmembrane receptor, TRA-2 , three intracellular FEM proteins, and the zinc finger transcription factor TRA-1 . The molecular workings of this regulatory module are still being elucidated. Similarity of TRA-2 to patched receptors and of TRA-1 to GLI proteins suggests that parts of the global pathway originally derived from a Hedgehog signaling pathway. TRA-1 controls all aspects of somatic sexual differentiation, presumably by regulating a variety of tissue- and cell-specific downstream targets, including the cell death regulator EGL-1 and the male sexual regulator MAB-3 . Sex determination evolves rapidly, and conservation of sexual regulators between phyla has been elusive. An apparent exception involves DM domain proteins, including MAB-3 , which control sexual differentiation in nematodes, arthropods, and vertebrates. Important issues needing more study include the detailed molecular mechanisms of the global pathway, the identities of additional sexual regulators acting in the global pathway and downstream of TRA-1 , and the evolutionary history of the sex determination pathway. Recently developed genetic and genomic technologies and comparative studies in divergent species have begun to address these issues.

Alan M. Zahler (2005) WormBook "Alternative splicing in C. elegans."

Alan M. Zahler

[WormBook, 2005]

Alternative splicing is a common mechanism for the generation of multiple isoforms of proteins. It can function to expand the proteome of an organism and can serve as a way to turn off gene expression post-transcriptionally. This review focuses on splicing and its regulation in C. elegans. The fully-sequenced C. elegans genome combined with its elegant genetics offers unique advantages for exploring alternative splicing regulation in metazoans. The topics covered in this review include constitutive splicing factors, identification of alternatively spliced genes, examples of alternative splicing in C. elegans, and alternative splicing regulation. Key genes whose regulated alternative splicing are reviewed include let-2 , unc-32 , unc-52 , egl-15 and xol-1 . Factors involved in alternative splicing that are discussed include mec-8 , smu-1 , smu-2 , fox-1 , exc-7 and unc-75 .

John Yochem et al. (2005) WormBook "Genetic mosaics."

John Yochem, Robert K. Herman

[WormBook, 2005]

Genetic mosaics can be used to gain insight into the cell specificity of gene function. How Caenorhabditis elegans mosaics are typically generated is reviewed, and several examples with relevance to developmental studies are mentioned. One example is mpk-1 , which encodes a member of the Ras-MAP-kinase pathway. mpk-1 mosaics have been a means of studying the distinct cells that require the gene for distinct fates during development. The gene bre-5 is used as an example of the usefulness of mosaic analysis for non-developmental studies. Potential problems with mosaic analysis are discussed, and the power of combining mosaic analysis with cell- or tissue-specific promoters is mentioned.

Benjamin Podbilewicz (2006) WormBook "Cell fusion."

Benjamin Podbilewicz

[WormBook, 2006]

Selective cell fusion is a natural part of development. It is found in sexually reproducing organisms that require fertilization to propagate and in muscles, placenta, bones, lens of the eye and stem cells. Cell fusion is particularly important in the development of C. elegans: in addition to 300 sperm and oocytes that fuse during fertilization, 300 of the 1090 somatic cells born, fuse throughout development. Studies of cell fusion in C. elegans have shown that although different types of cells fuse, cell membrane merger is initiated through a common mechanism involving the action of one gene, eff-1 . In worms with mutations that inactivate eff-1 , almost none of the 300 somatic cells that normally fuse do so, but appear to differentiate, attach and behave in the same way as fusing cells. Such worms develop and survive but have numerous morphological, behavioral and fertility defects associated to cell fusion failure in the epidermis, pharynx, male tail, vulva and uterus. Cell fusion in embryonic dorsal epithelial cells has been analyzed in great detail by confocal microscopy using membrane fluorescent probes, apical junction markers and cytoplasmic aqueous fluorescent probes allowing the direct observation of membrane disappearance, pore expansion and cytoplasmic content mixing. The complete elimination of the membranes between two fusing cells takes about 30 min and involves vesiculation of the fusing membranes. Genetic and cell biological evidence indicates that eff-1 activity is both necessary and sufficient to fuse epithelial and myoepithelial cells in vivo. Based on electron microscopic analyses of intermediates of cell fusion in eff-1 mutants, it appears that eff-1 is required for both initiation and expansion of fusion pores, similar to the fusogen of Influenza virus. While only one gene encoding a novel candidate component of the cell membrane fusion machinery has been found, the nematode''s cell fusion program is under the control of many cell-specific transcriptional regulators. A large number of these conserved regulators prevent cell fusion by repressing eff-1 activity. For example, if either ceh-16 /engrailed or the GATA factor EGL-18 /ELT-5 is inactivated, the lateral epidermal cells that normally do not fuse in the embryo will fuse causing embryonic lethality. And if either the Hox protein lin-39 /Deformed or its cofactor ceh-20 /Extradenticle is inactivated, the ventral epidermal vulval precursor cells that normally do not fuse in the larvae will fuse and the hermaphrodite will have no vulva. In addition, there is evidence for coordinated and complex regulation of lin-39 in the ventral epidermis by Ras, Wnt, Rb/E2F, NuRD and lin-15 pathways. It appears that in many cells that normally do not fuse, specific transcription complexes repress eff-1 expression preventing cell fusion. ref-2 (REgulator of Fusion-2) encodes a Zn-finger protein that is required to generate ventral Pn.p cells and to keep them unfused both in males and hermaphrodites. ref-2 is necessary, but not sufficient, to maintain Pn.p cells unfused. This review shows that far from cell fusion being an unusual phenomenon, there is the clear prospect that animal cells in all tissues are intrinsically programmed to fuse, and are only prevented from fusing by transcriptional and post-transcriptional control mechanisms. There are three major questions that remain open for future research: (1) How does eff-1 fuse cells? (2) How do Ras, Wnt, Rb, NuRD, E2F, heterochronic and other pathways control cell fusion? (3) What are the implications of cell fusion beyond worms?

Judith Kimble et al. (2005) WormBook "Germline proliferation and its control."

Sarah L. Crittenden, Judith Kimble

[WormBook, 2005]

The C. elegans germ line proliferates from one primordial germ cell (PGC) set aside in the early embryo to over a thousand cells in the adult. Most germline proliferation is controlled by the somatic distal tip cell, which provides a stem cell niche at the distal end of the adult gonad. The distal tip cell signals to the germ line via the Notch signaling pathway, which in turn controls a network of RNA regulators. The FBF-1 and FBF-2 RNA-binding proteins promote continued mitoses in germ cells located close to the distal tip cell, while the GLD-1 , GLD-2 , GLD-3 , and NOS-3 RNA regulators promote entry into meiosis as germ cells leave the stem cell niche. In addition to these key regulators, many other genes affect germline proliferation.

Sander van den Heuvel (2005) WormBook "Cell-cycle regulation."

Sander Van Den Heuvel

[WormBook, 2005]

Cell-division control affects many aspects of development. Caenorhabditis elegans cell-cycle genes have been identified over the past decade, including at least two distinct Cyclin-Dependent Kinases (CDKs), their cyclin partners, positive and negative regulators, and downstream targets. The balance between CDK activation and inactivation determines whether cells proceed through G 1 into S phase, and from G 2 to M, through regulatory mechanisms that are conserved in more complex eukaryotes. The challenge is to expand our understanding of the basic cell cycle into a comprehensive regulatory network that incorporates environmental factors and coordinates cell division with growth, differentiation and tissue formation during development. Results from several studies indicate a critical role for CKI-1 , a CDK inhibitor of the Cip/Kip family, in the temporal control of cell division, potentially acting downstream of heterochronic genes and dauer regulatory pathways.

Susan Strome (2005) WormBook "Specification of the germ line."

Susan Strome

[WormBook, 2005]

In C. elegans, the germ line is set apart from the soma early in embryogenesis. Several important themes have emerged in specifying and guiding the development of the nascent germ line. At early stages, the germline blastomeres are maintained in a transcriptionally silent state by the transcriptional repressor PIE-1 . When this silencing is lifted, it is postulated that correct patterns of germline gene expression are controlled, at least in part, by MES-mediated regulation of chromatin state. Accompanying transcriptional regulation by PIE-1 and the MES proteins, RNA metabolism in germ cells is likely to be regulated by perinuclear RNA-rich cytoplasmic granules, termed P granules. This chapter discusses the molecular nature and possible roles of these various germline regulators, and describes a recently discovered mechanism to protect somatic cells from following a germline fate.

Paul W. Sternberg (2005) WormBook "Vulval development."

Paul W. Sternberg

[WormBook, 2005]

C. elegans vulval development is an intensively studied example of animal organogenesis. A network of intercellular signaling, signal transduction, and transcriptional regulation underlies the precise formation of this organ, which is the connection between the hermaphrodite uterus and the outside of the nematode. A single cell of the somatic gonad, the anchor cell, organizes the development of the vulva from epidermal precursors as well as the physical connection of the epidermis with the uterus. WNT signaling acting via the HOX gene lin-39 renders six epidermal precursor cells competent to respond to other developmental signals. The anchor cell induces nearby epidermal precursor cells to generate vulval cells via an epidermal growth factor (EGF) signaling pathway. The precise pattern of vulval precursor cell fates involves the graded action of the EGF signaling and LIN-12 (Notch) mediated lateral signaling. EGF promotes the 1 fate while LIN-12 promotes the 2 fate. Both EGF and LIN-12 prevent precursor cells from adopting the 3 fate, which generates non-specialized epidermis. EGF-receptor and Notch signaling are antagonistic: EGF-receptor signaling leads to down-regulation of the Notch-like receptor LIN-12 , while LIN-12 signaling induces negative regulators of EGF-receptor signaling such as MAP kinase phosphatase LIP-1 and the tyrosine kinase ARK-1. The 1 precursor cell generates vulE and vulF mature vulval cells; the pattern of vulE and vulF cells requires an additional signal from the anchor cell as well as WNT signaling. The two 2 precursor cells generate vulA, vulB1, vulB2, vulC and vulD cells but in mirror symmetric polar patterns: ABCD and DCBA. The reversed polarity of the posterior 2 precursor cell lineage requires WNT signaling mediated by both Frizzled class and Ryk class WNT-receptors LIN-17 and LIN-18 , respectively. A network of transcription factors controls the seven mature adult cell types; these include the LIM domain protein LIN-11 , the Pax2/5/8 protein EGL-38 , the zinc finger protein LIN-29 , and the Nkx6.1/6.2 protein COG-1. The anchor cell also patterns nearby uterine cells, via the DSL ligand LAG-2 and LIN-12 , to generate the four uv1 cells that form the tight connection with the vulva. This connection is initiated by the anchor cell, which invades between the vulF cells in a process analogous to invasive behavior of metastatic tumor cells. During this invasion process, the basement membranes between the gonad and body wall are degraded. The extensive information about vulval development in C. elegans has helped it become a paradigmatic case for identifying and studying a variety of regulatory pathways.

James R. Priess (2005) WormBook "Notch signaling in the C. elegans embryo."

James R. Priess

[WormBook, 2005]

Cell-cell interactions mediated by the Notch signaling pathway occur throughout C. elegans embryogenesis. These interactions have major roles in specifying cell fates and in tissue morphogenesis. The network of Notch interactions is linked in part through the Notch-regulated expression of components of the pathway, allowing one interaction to pattern subsequent ones. The Notch signal transduction pathway is highly conserved in animal embryogenesis. The REF-1 family of bHLH transcription factors are major targets of Notch signaling in the C. elegans embryo, and are distantly related to HES proteins that are targets of Notch signaling in Drosophila and vertebrates.