Lok JB (2007) WormBook "Strongyloides stercoralis: a model for translational research on parasitic nematode biology."

Lok JB

[WormBook, 2007]

Because of their free-living life cycle alternatives, Strongyloides and related nematode parasites may represent the best models for translating C. elegans science to the study of nematode parasitism. S. stercoralis, a significant pathogen of humans, can be maintained in laboratory dogs and gerbils. Biosafety precautions necessary for work with S. stercoralis, though unfamiliar to many C. elegans researchers, are straightforward and easily accomplished. Although specialized methods are necessary for large-scale culture of the free-living stages of S. stercoralis, small-scale cultures for experimental purposes may be undertaken using minor modifications of standard C. elegans methods. Similarly, the morphological similarities between C. elegans and the free-living stages of S. stercoralis allow investigational methods such as laser cell ablation and DNA transformation by gonadal microinjection to be easily adapted from C. elegans to S. stercoralis. Comparative studies employing these methods have yielded new insights into the neuronal control of the infective process in parasites and its similarity to regulation of dauer development in C. elegans. Furthermore, we have developed a practical method for transient transformation of S. stercoralis with vector constructs having various tissue- and cell-specific expression patterns and have assembled these into a modular vector kit for distribution to the community.

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.

Min-Ho Lee et al. (2006) WormBook "RNA-binding proteins."

Tim Schedl, Min-Ho Lee

[WormBook, 2006]

The C. elegans genome encodes many RNA-binding proteins (RBPs) with diverse functions in development, indicative of extensive layers of post-transcriptional control of RNA metabolism. A number of C. elegans RBPs have been identified by forward or reverse genetics. They tend to display tissue-specific mutant phenotypes, which underscore their functional importance. In addition, several RBPs that bind regulatory sequences in the 3'' untranslated regions of mRNAs have been identified molecularly. Most C. elegans RBPs are conserved throughout evolution, suggesting that their study in C. elegans may uncover new conserved biological functions. In this review, we primarily discuss RBPs that are associated with well-characterized mutant phenotypes in the germ line, the early embryo, or in somatic tissues. We also discuss the identification of RNA targets of RBPs, which is an important first step to understand how an RBP controls C. elegans development. It is likely that most RBPs regulate multiple RNA targets. Once multiple RNA targets are identified, specific features that distinguish target from non-target RNAs and the type(s) of RNA metabolism that each RBP controls can be determined. Furthermore, one can determine whether the RBP regulates all targets by the same mechanism or different targets by distinct mechanisms. Such studies will provide insights into how RBPs exert coordinate control of their RNA targets, thereby affecting development in a concerted fashion.

Jean-Louis Bessereau (2006) WormBook "Transposons in C. elegans."

Jean-Louis BESSEREAU

[WormBook, 2006]

Transposons are discrete segments of DNA capable of moving through the genome of their host via an RNA intermediate in the case of class I retrotransposon or via a "cut-and-paste" mechanism for class II DNA transposons. Since transposons take advantage of their host''s cellular machinery to proliferate in the genome and enter new hosts, transposable elements can be viewed as parasitic or "selfish DNA". However, transposons may have been beneficial for their hosts as genome evolution drivers, thus providing an example of molecular mutualism. Interactions between transposon and C. elegans research were undoubtedly mutualistic, leading to the advent of needed genomic tools to drive C. elegans research while providing insights into the transposition field. Tc1, the first C. elegans transposon to be identified, turned out to be the founding member of a widespread family of mobile elements: the Tc1/ mariner superfamily. The investigation into transposition regulation in C. elegans has uncovered an unforeseen link between transposition, genome surveillance and RNA interference. Conversely, transposons were utilized soon after their identification to inactivate and clone genes, providing some of the first molecular identities of C. elegans genes. Recent results suggest that transposons might provide a means to engineer site-directed mutations into the C. elegans genome. This article describes the different transposons present in the C. elegans genome with a specific emphasis on the ones that proved to be mobile under laboratory conditions. Mechanisms and control of transposition are discussed briefly. Some tools based on the use of transposons for C. elegans research are presented at the end of this review.

Barbara J. Meyer (2005) WormBook "X-Chromosome dosage compensation."

Barbara J. Meyer

[WormBook, 2005]

In mammals, flies, and worms, sex is determined by distinctive regulatory mechanisms that cause males (XO or XY) and females (XX) to differ in their dose of X chromosomes. In each species, an essential X chromosome-wide process called dosage compensation ensures that somatic cells of either sex express equal levels of X-linked gene products. The strategies used to achieve dosage compensation are diverse, but in all cases, specialized complexes are targeted specifically to the X chromosome(s) of only one sex to regulate transcript levels. In C. elegans, this sex-specific targeting of the dosage compensation complex (DCC) is controlled by the same developmental signal that establishes sex, the ratio of X chromosomes to sets of autosomes (X:A signal). Molecular components of this chromosome counting process have been defined. Following a common step of regulation, sex determination and dosage compensation are controlled by distinct genetic pathways. C. elegans dosage compensation is implemented by a protein complex that binds both X chromosomes of hermaphrodites to reduce transcript levels by one-half. The dosage compensation complex resembles the conserved 13S condensin complex required for both mitotic and meiotic chromosome resolution and condensation, implying the recruitment of ancient proteins to the new task of regulating gene expression. Within each C. elegans somatic cell, one of the DCC components also participates in the separate mitotic/meiotic condensin complex. Other DCC components play pivotal roles in regulating the number and distribution of crossovers during meiosis. The strategy by which C. elegans X chromosomes attract the condensin-like DCC is known. Small, well-dispersed X-recognition elements act as entry sites to recruit the dosage compensation complex and to nucleate spreading of the complex to X regions that lack recruitment sites. In this manner, a repressed chromatin state is spread in cis over short or long distances, thus establishing the global, epigenetic regulation of X chromosomes that is maintained throughout the lifetime of hermaphrodites.

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?