Regulation of organogenesis by the Caenorhabditis elegans FoxA protein PHA-4

Precise temporal control of the eye regulatory gene Pax6 via enhancer-binding site affinity

How transcription factors interpret the cis-regulatory logic encoded within enhancers to mediate quantitative changes in spatiotemporally restricted expression patterns during animal development is not well understood. Pax6 is a dosage-sensitive gene essential for eye development. Here, we identify the Prep1 (pKnox1) transcription factor as a critical dose-dependent upstream regulator of Pax6 expression during lens formation. We show that Prep1 activates the Pax6 lens enhancer by binding to two phylogenetically conserved lower-affinity DNA-binding sites. Finally, we describe a mechanism whereby Pax6 levels are determined by transcriptional synergy of Prep1 bound to the two sites, while timing of enhancer activation is determined by binding site affinity.

Preparing for the first breath: genetic and cellular mechanisms in lung development

The mammalian respiratory system--the trachea and the lungs--arises from the anterior foregut through a sequence of morphogenetic events involving reciprocal endodermal-mesodermal interactions. The lung itself consists of two highly branched, tree-like systems--the airways and the vasculature--that develop in a coordinated way from the primary bud stage to the generation of millions of alveolar gas exchange units. We are beginning to understand some of the molecular and cellular mechanisms that underlie critical processes such as branching morphogenesis, vascular development, and the differentiation of multipotent progenitor populations. Nevertheless, many gaps remain in our knowledge, the filling of which is essential for understanding respiratory disorders, congenital defects in human neonates, and how the disruption of morphogenetic programs early in lung development can lead to deficiencies that persist throughout life.

Cis-regulatory mechanisms of gene expression in an olfactory neuron type in Caenorhabditis elegans

The generation of cellular diversity is dependent on the precise spatiotemporal regulation of gene expression by both cis- and trans-acting mechanisms. The developmental principles regulating expression of specific gene subsets in individual cell types are not fully understood. Here we define the cis-regulatory mechanisms driving expression of cell-selective and broadly expressed genes in vivo in the AWB olfactory neuron subtype in C. elegans. We identify an element that is necessary to drive expression of neuron-selective chemoreceptor genes in the AWB neurons, and show that this element functions in a context-dependent manner. We find that the expression of broadly expressed sensory neuronal genes in the AWB neurons is regulated by diverse cis- and trans-regulatory mechanisms that act partly in parallel to the pathways governing expression of AWB-selective genes. We further demonstrate that cis-acting mechanisms driving gene expression in the AWB neurons appear to have diverged in related nematode species. Our results provide insights into the cis-regulatory logic driving cell-specific gene expression, and suggest that variations in this logic contribute to the generation of functional diversity.

Genome-wide identification of binding sites defines distinct functions for Caenorhabditis elegans PHA-4/FOXA in development and environmental response

Transcription factors are key components of regulatory networks that control development, as well as the response to environmental stimuli. We have established an experimental pipeline in Caenorhabditis elegans that permits global identification of the binding sites for transcription factors using chromatin immunoprecipitation and deep sequencing. We describe and validate this strategy, and apply it to the transcription factor PHA-4, which plays critical roles in organ development and other cellular processes. We identified thousands of binding sites for PHA-4 during formation of the embryonic pharynx, and also found a role for this factor during the starvation response. Many binding sites were found to shift dramatically between embryos and starved larvae, from developmentally regulated genes to genes involved in metabolism. These results indicate distinct roles for this regulator in two different biological processes and demonstrate the versatility of transcription factors in mediating diverse biological roles.

Impact of DNA-binding position variants on yeast gene expression

Transcription factors (TFs) regulate gene expression by binding to specific binding sites (TFBSs) in gene promoters. TFBS motifs may contain one or more variable positions. Although the prevailing assumption is that nucleotide variants at such positions are functionally equivalent, there is increasing evidence that such variants play a role in regulation of gene expression. In this article, we propose a method for studying the relationship between the expression of target genes and nucleotide variants in TFBS motifs at a genome-wide scale in Saccharomyces cerevisiae, especially the combinatorial effects of variants at two positions. Our analysis shows that nucleotide variations in more than one-third of variable positions and in 20% of dependent position pairs are highly correlated to gene expression. We define such positions as 'functional'. However, some positions are only functional as dependent pairs, but not individually. In addition, a significant proportion of the functional positions have been well conserved across all yeast-related species studied. We also find that some positions require the presence of co-occurring TFs, while others maintain their functionality in the absence of a co-occurring TF. Our analysis supports the importance of nucleotide variants at variable positions of TFBSs in gene regulation.

The C. elegans tailless/Tlx homolog nhr-67 regulates a stage-specific program of linker cell migration in male gonadogenesis

Cell migration is a common event during organogenesis, yet little is known about how migration is temporally coordinated with organ development. We are investigating stage-specific programs of cell migration using the linker cell (LC), a migratory cell crucial for male gonadogenesis of C. elegans. During the L3 and L4 larval stages of wild-type males, the LC undergoes changes in its position along the migratory route, in transcriptional regulation of the unc-5 netrin receptor and zmp-1 zinc matrix metalloprotease, and in cell morphology. We have identified the tailless homolog nhr-67 as a cell-autonomous, stage-specific regulator of timing in LC migration programs. In nhr-67-deficient animals, each of the L3 and L4 stage changes is either severely delayed or never occurs, yet LC development before the early L3 stage or after the mid-L4 stage occurs with normal timing. We propose that there is a basal migration program utilized throughout LC migration that is modified by stage-specific regulators such as nhr-67.

DNA specificity determinants associate with distinct transcription factor functions

To elucidate how genomic sequences build transcriptional control networks, we need to understand the connection between DNA sequence and transcription factor binding and function. Binding predictions based solely on consensus predictions are limited, because a single factor can use degenerate sequence motifs and because related transcription factors often prefer identical sequences. The ETS family transcription factor, ETS1, exemplifies these challenges. Unexpected, redundant occupancy of ETS1 and other ETS proteins is observed at promoters of housekeeping genes in T cells due to common sequence preferences and the presence of strong consensus motifs. However, ETS1 exhibits a specific function in T cell activation; thus, unique transcriptional targets are predicted. To uncover the sequence motifs that mediate specific functions of ETS1, a genome-wide approach, chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-seq), identified both promoter and enhancer binding events in Jurkat T cells. A comparison with DNase I sensitivity both validated the dataset and also improved accuracy. Redundant occupancy of ETS1 with the ETS protein GABPA occurred primarily in promoters of housekeeping genes, whereas ETS1 specific occupancy occurred in the enhancers of T cell–specific genes. Two routes to ETS1 specificity were identified: an intrinsic preference of ETS1 for a variant of the ETS family consensus sequence and the presence of a composite sequence that can support cooperative binding with a RUNX transcription factor. Genome-wide occupancy of RUNX factors corroborated the importance of this partnership. Furthermore, genome-wide occupancy of co-activator CBP indicated tight co-localization with ETS1 at specific enhancers, but not redundant promoters. The distinct sequences associated with redundant versus specific ETS1 occupancy were predictive of promoter or enhancer location and the ontology of nearby genes. These findings demonstrate that diversity of DNA binding motifs may enable variable transcription factor function at different genomic sites.

Genomes contain sequences that encode both gene products and the instructions for where and when each gene is expressed. This gene expression code is critical for normal development and goes awry in disease processes such as cancer. The gene expression code is interpreted by proteins called transcription factors that bind to particular DNA sequences and carry instructions for gene activation or repression. This recognition code is challenged by the presence of highly-similar transcription factors that prefer almost identical DNA sequences. In addition, studies in living cells indicate that individual transcription factors have significant flexibility in sequence recognition. Here, we identify thousands of positions in the genome of human T cells that are bound by the transcription factor ETS1. These data, along with comparisons to other genomic datasets, allow us to identify DNA sequences that specify ETS1 binding while excluding binding of other related transcription factors. Furthermore, we discover that ETS1 binds more than one sequence and that these sequence variants can predict distinct biological functions of ETS1. Thus, this work contributes to our understanding of the gene expression code by addressing both how a transcription factor can bind unique genomic locations and why a transcription factor binds multiple DNA sequences.

Coordinate regulation of gene expression in the C. elegans excretory cell by the POU domain protein CEH-6

Excretory renal organs are critical in animals for osmoregulation and the elimination of waste. Renal organs across a range of species exhibit cellular and molecular similarities. For example, class III POU-homeodomain transcription factors are expressed in the renal organs of many invertebrates and vertebrates. However, the functional role for these factors is not well characterized. To better understand the role of class III POU-homeodomain proteins in animal excretory systems, we have characterized a set of genes expressed in the Caenorhabditis elegans excretory cell, and determined their regulation by the POU-III transcription factor CEH-6. Our molecular and biochemical studies show that CEH-6 regulates a subset of genes expressed in the excretory cell. Additionally, we find that the CEH-6-dependent genes share two molecular features: they contain at least one octamer regulatory element and they encode for transport and channel proteins. This work suggests that a role for POU-III factors in renal organs is to coordinate the expression of a set of functionally related genes.

Single-cell transcriptional analysis of taste sensory neuron pair in Caenorhabditis elegans

The nervous system is composed of a wide variety of neurons. A description of the transcriptional profiles of each neuron would yield enormous information about the molecular mechanisms that define morphological or functional characteristics. Here we show that RNA isolation from single neurons is feasible by using an optimized mRNA tagging method. This method extracts transcripts in the target cells by co-immunoprecipitation of the complexes of RNA and epitope-tagged poly(A) binding protein expressed specifically in the cells. With this method and genome-wide microarray, we compared the transcriptional profiles of two functionally different neurons in the main C. elegans gustatory neuron class ASE. Eight of the 13 known subtype-specific genes were successfully detected. Additionally, we identified nine novel genes including a receptor guanylyl cyclase, secreted proteins, a TRPC channel and uncharacterized genes conserved among nematodes, suggesting the two neurons are substantially different than previously thought. The expression of these novel genes was controlled by the previously known regulatory network for subtype differentiation. We also describe unique motif organization within individual gene groups classified by the expression patterns in ASE. Our study paves the way to the complete catalog of the expression profiles of individual C. elegans neurons.

Foxa1 and Foxa2 transcription factors regulate differentiation of midbrain dopaminergic neurons

Midbrain dopaminergic neurons (mDA), comprising the substanti nigra pars compacta (A8), the ventral tegmental area (A9) and the retrorubal field (A10) subgroups, are generated from floor plate progenitors, rostral to the isthmic boundary. Floor plate progenitors are specified to become mDA progenitors between embryonic days 8.0 and 10.5. Subsequently these progenitors undergo neuronal differentiation in two phases, termed early and late differentiation to generate immature and mature neurons respectively. Genes that regulate specification, early and late phases of differentiation ofmDA cells have recently been identified. Among them, the forkhead winged helix transcription factors Foxal and Foxa2 (Foxa1/2), have been shown to have essential and dose dependent roles at multiple phases of development. In this chapter, I will summarize recent studies demonstrating a role for Foxa1/2 in regulating the neuronal specification and differentiation ofmDA progenitors and conclude with projections on future directions of this area of research.

Roles of the Wnt effector POP-1/TCF in the C. elegans endomesoderm specification gene network

In C. elegans the 4-cell stage blastomere EMS is an endomesodermal precursor. Its anterior daughter, MS, makes primarily mesodermal cells, while its posterior daughter E generates the entire intestine. The gene regulatory network underlying specification of MS and E has been the subject of study for more than 15 years. A key component of the specification of the two cells is the involvement of the Wnt/beta-catenin asymmetry pathway, which through its nuclear effector POP-1, specifies MS and E as different from each other. Loss of pop-1 function results in the mis-specification of MS as an E-like cell, because POP-1 directly represses the end-1 and end-3 genes in MS, which would otherwise promote an endoderm fate. A long-standing question has been whether POP-1 plays a role in specifying MS fate beyond repression of endoderm fate. This question has been difficult to ask because the only chromosomal lesions that remove both end-1 and end-3 are large deletions removing hundreds of genes. Here, we report the construction of bona fide end-1 end-3 double mutants. In embryos lacking activity of end-1, end-3 and pop-1 together, we find that MS fate is partially restored, while E expresses early markers of MS fate and adopts characteristics of both MS and C. Our results suggest that POP-1 is not critical for MS specification beyond repression of endoderm specification, and reveal that Wnt-modified POP-1 and END-1/3 further reinforce E specification by repressing MS fate in E. By comparison, a previous work suggested that in the related nematode C. briggsae, Cb-POP-1 is not required to repress endoderm specification in MS, in direct contrast with Ce-POP-1, but is critical for repression of MS fate in E. The findings reported here shed new light on the flexibility of combinatorial control mechanisms in endomesoderm specification in Caenorhabditis.

Characterizing the transcriptional regulation of let-721, a Caenorhabditis elegans homolog of human electron flavoprotein dehydrogenase

LET-721 is the Caenorhabditis elegans ortholog of electron-transferring flavoprotein dehydrogenase (ETFDH). We are studying this protein in C. elegans in order to establish a tractable model system for further exploration of ETFDH structure and function. ETFDH is an inner mitochondrial membrane localized enzyme that plays a key role in the beta-oxidation of fatty acids and catabolism of amino acids and choline. ETFDH accepts electrons from at least twelve mitochondrial matrix flavoprotein dehydrogenases via an intermediate dimer protein and transfers the electrons to ubiquinone. In humans, ETFDH mutations result in the autosomal recessive metabolic disorder, multiple acyl-CoA dehydrogenase deficiency. Mutants of let-721 in C. elegans are either maternal effect lethals or semi-sterile. let-721 is transcribed in the pharynx, body wall muscle, hypoderm, intestine and somatic gonad. In addition, the subcellular localization of LET-721 agrees with predictions that it is localized to mitochondria. We identified and confirmed three cis-regulatory sequences (pha-site, rep-site, and act-site). Phylogenetic footprinting of each site indicates that they are conserved between four Caenorhabditis species. The pha-site mapped roughly 1,300 bp upstream of let-721's translational start site and is necessary for expression in pharyngeal tissues. The rep-site mapped roughly 830 bp upstream of the translational start site and represses expression of LET-721 within pharyngeal tissues. The act-site mapped roughly 800 bp upstream of the translational start site and is required for expression within spermatheca, body wall muscle, pharynx, and intestine. Taken together, we find that LET-721 is a mitochondrially expressed protein that is under complex transcriptional controls.

Genes and functions controlled by floral organ identity genes

Floral organ identity genes specify the identity of floral organs in a manner analogous to the specification of body segments by Hox genes in animals. Different combinations of organ identity genes co-ordinate the expression of genes required for the development of each type of floral organ, from organ initiation until final differentiation. Here, I review what is known about the genes and functions subordinate to the organ identity genes. The sets of target genes change as organ development progresses and ultimately organ identity genes modify the expression of thousands of genes with a multitude of predicted functions, particularly in reproductive organs. However, genes involved in transcriptional control and hormone functions feature prominently among the early and direct targets. Functional analysis showed that control of organ-specific tissues and structures can be delegated to specialised intermediate regulators, but organ identity genes also fine-tune genes with general roles in shoot organ development, consistent with the notion that organ identity genes modify a core leaf-like developmental program. Future challenges include obtaining data with cellular resolution, predictive modelling of the regulatory network, and quantitative analysis of how organ identity genes and their targets control cell behaviour and ultimately organ shape.

The NK-2 class homeodomain factor CEH-51 and the T-box factor TBX-35 have overlapping function in C. elegans mesoderm development

The C. elegans MS blastomere, born at the 7-cell stage of embryogenesis, generates primarily mesodermal cell types, including pharynx cells, body muscles and coelomocytes. A presumptive null mutation in the T-box factor gene tbx-35, a target of the MED-1 and MED-2 divergent GATA factors, was previously found to result in a profound decrease in the production of MS-derived tissues, although the tbx-35(-) embryonic arrest phenotype was variable. We report here that the NK-2 class homeobox gene ceh-51 is a direct target of TBX-35 and at least one other factor, and that CEH-51 and TBX-35 share functions. Embryos homozygous for a ceh-51 null mutation arrest as larvae with pharynx and muscle defects, although these tissues appear to be specified correctly. Loss of tbx-35 and ceh-51 together results in a synergistic phenotype resembling loss of med-1 and med-2. Overexpression of ceh-51 causes embryonic arrest and generation of ectopic body muscle and coelomocytes. Our data show that TBX-35 and CEH-51 have overlapping function in MS lineage development. As T-box regulators and NK-2 homeodomain factors are both important for heart development in Drosophila and vertebrates, our results suggest that these regulators function in a similar manner in C. elegans to specify a major precursor of mesoderm.

A trophic role for Wnt-Ror kinase signaling during developmental pruning in Caenorhabditis elegans

The molecular mechanism by which neurites are selected for elimination or incorporation into the mature circuit during developmental pruning remains unknown. The trophic theory postulates that local cues provided by target or surrounding cells act to inhibit neurite elimination. However, no widely conserved factor mediating this trophic function has been identified. We found that the developmental survival of specific neurites in Caenorhabditis elegans largely depends on detection of the morphogen Wnt by the Ror kinase CAM-1, which is a transmembrane tyrosine kinase with a Frizzled domain. Mutations in Wnt genes or in cam-1 enhanced neurite elimination, whereas overexpression of cam-1 inhibited neurite elimination in a Wnt-dependent manner. Moreover, mutations in these genes counteracted the effect of a mutation in mbr-1, which encodes a transcription factor that promotes neurite elimination. These results reveal the trophic role of an atypical Wnt pathway and reinforce the classical model of developmental pruning.

Global prediction of tissue-specific gene expression and context-dependent gene networks in Caenorhabditis elegans

Tissue-specific gene expression plays a fundamental role in metazoan biology and is an important aspect of many complex diseases. Nevertheless, an organism-wide map of tissue-specific expression remains elusive due to difficulty in obtaining these data experimentally. Here, we leveraged existing whole-animal Caenorhabditis elegans microarray data representing diverse conditions and developmental stages to generate accurate predictions of tissue-specific gene expression and experimentally validated these predictions. These patterns of tissue-specific expression are more accurate than existing high-throughput experimental studies for nearly all tissues; they also complement existing experiments by addressing tissue-specific expression present at particular developmental stages and in small tissues. We used these predictions to address several experimentally challenging questions, including the identification of tissue-specific transcriptional motifs and the discovery of potential miRNA regulation specific to particular tissues. We also investigate the role of tissue context in gene function through tissue-specific functional interaction networks. To our knowledge, this is the first study producing high-accuracy predictions of tissue-specific expression and interactions for a metazoan organism based on whole-animal data.

In animals, a crucial facet of any gene's function is the tissue or cell type in which that gene is expressed and the proteins that it interacts with in that cell. However, genome-wide identification of expression across the multitude of tissues of varying size and complexity is difficult to achieve experimentally. In this paper, we show that microararray data collected from whole animals can be analyzed to yield high-quality predictions of tissue-specific expression. These predictions are of better or comparable accuracy to tissue-specific expression determined from high-throughput experiments. Our results provide a global view of tissue-specific expression in Caenorhabditis elegans, allowing us to address the question of how expression patterns are regulated and to analyze how the functions of genes that are expressed in several tissues are influenced by the cellular context.

HIF-1 modulates dietary restriction-mediated lifespan extension via IRE-1 in Caenorhabditis elegans

Dietary restriction (DR) extends lifespan in various species and also slows the onset of age-related diseases. Previous studies from flies and yeast have demonstrated that the target of rapamycin (TOR) pathway is essential for longevity phenotypes resulting from DR. TOR is a conserved protein kinase that regulates growth and metabolism in response to nutrients and growth factors. While some of the downstream targets of TOR have been implicated in regulating lifespan, it is still unclear whether additional targets of this pathway also modulate lifespan. It has been shown that the hypoxia inducible factor-1 (HIF-1) is one of the targets of the TOR pathway in mammalian cells. HIF-1 is a transcription factor complex that plays key roles in oxygen homeostasis, tumor formation, glucose metabolism, cell survival, and inflammatory response. Here, we describe a novel role for HIF-1 in modulating lifespan extension by DR in Caenorhabditis elegans. We find that HIF-1 deficiency results in extended lifespan, which overlaps with that by inhibition of the RSKS-1/S6 kinase, a key component of the TOR pathway. Using a modified DR method based on variation of bacterial food concentrations on solid agar plates, we find that HIF-1 modulates longevity in a nutrient-dependent manner. The hif-1 loss-of-function mutant extends lifespan under rich nutrient conditions but fails to show lifespan extension under DR. Conversely, a mutation in egl-9, which increases HIF-1 activity, diminishes the lifespan extension under DR. This deficiency is rescued by tissue-specific expression of egl-9 in specific neurons and muscles. Increased lifespan by hif-1 or DR is dependent on the endoplasmic reticulum (ER) stress regulator inositol-requiring protein-1 (IRE-1) and is associated with lower levels of ER stress. Therefore, our results demonstrate a tissue-specific role for HIF-1 in the lifespan extension by DR involving the IRE-1 ER stress pathway.

Dietary restriction (DR) is one of the most robust environmental manipulations that extend lifespan in various species. DR has also been shown to slow the onset of a number of age-related diseases. Studies in model organisms like C. elegans can be used to uncover biological mechanisms that determine the beneficial effects of DR. Previous studies suggest that the nutrient-sensing target of rapamycin (TOR) pathway is required for DR-mediated lifespan extension. However, the downstream mechanisms by which TOR modulates lifespan remain unclear. In mammalian cells, TOR and the downstream S6 kinase (S6K) activate expression of the hypoxia-inducible factor-1 (HIF-1), which is frequently up-regulated in various tumors. Using C. elegans as a model system, we characterized novel functions of HIF-1 in aging. We find that inhibition of HIF-1 extends lifespan under rich nutrient conditions, whereas enhanced levels of HIF-1 only allow partial lifespan extension by DR. We also demonstrated that increased lifespan by hif-1 or DR depends on the endoplasmic reticulum (ER) stress regulator inositol-requiring protein-1 (IRE-1) and is associated with lower levels of ER stress, which is caused by overloading of misfolded/unfolded proteins to ER. Thus, our results support the idea that HIF-1–mediated changes in protein homeostasis play a key role in the lifespan extension by DR.

The polycomb complex protein mes-2/E(z) promotes the transition from developmental plasticity to differentiation in C. elegans embryos

We have used expression profiling and in vivo imaging to characterize Caenorhabditis elegans embryos as they transit from a developmentally plastic state to the onset of differentiation. Normally, this transition is accompanied by activation of developmental regulators and differentiation genes, downregulation of early-expressed genes, and large-scale reorganization of chromatin. We find that loss of plasticity and differentiation onset depends on the Polycomb complex protein mes-2/E(Z). mes-2 mutants display prolonged developmental plasticity in response to heterologous developmental regulators. Early-expressed genes remain active, differentiation genes fail to reach wild-type levels, and chromatin retains a decompacted morphology in mes-2 mutants. By contrast, loss of the developmental regulators pha-4/FoxA or end-1/GATA does not prolong plasticity. This study establishes a model by which to analyze developmental plasticity within an intact embryo. mes-2 orchestrates large-scale changes in chromatin organization and gene expression to promote the timely loss of developmental plasticity. Our findings indicate that loss of plasticity can be uncoupled from cell fate specification.

Functional diversity for REST (NRSF) is defined by in vivo binding affinity hierarchies at the DNA sequence level

The molecular events that contribute to, and result from, the in vivo binding of transcription factors to their cognate DNA sequence motifs in mammalian genomes are poorly understood. We demonstrate that variations within the DNA sequence motifs that bind the transcriptional repressor REST (NRSF) encode in vivo DNA binding affinity hierarchies that contribute to regulatory function during lineage-specific and developmental programs in fundamental ways. First, canonical sequence motifs for REST facilitate strong REST binding and control functional classes of REST targets that are common to all cell types, whilst atypical motifs participate in weak interactions and control those targets, which are cell- or tissue-specific. Second, variations in REST binding relate directly to variations in expression and chromatin configurations of REST's target genes. Third, REST clearance from its binding sites is also associated with variations in the RE1 motif. Finally, and most surprisingly, weak REST binding sites reside in DNA sequences that show the highest levels of constraint through evolution, thus facilitating their roles in maintaining tissue-specific functions. These relationships have never been reported in mammalian systems for any transcription factor.

Structure and evolution of the C. elegans embryonic endomesoderm network

The specification of the Caenorhabditis elegans endomesoderm has been the subject of study for more than 15 years. Specification of the 4-cell stage endomesoderm precursor, EMS, occurs as a result of the activation of a transcription factor cascade that starts with SKN-1, coupled with input from the Wnt/beta-catenin asymmetry pathway through the nuclear effector POP-1. As development proceeds, transiently-expressed cell fate factors are succeeded by stable, tissue/organ-specific regulators. The pathway is complex and uses motifs found in all transcriptional networks. Here, the regulators that function in the C. elegans endomesoderm network are described. An examination of the motifs in the network suggests how they may have evolved from simpler gene interactions. Flexibility in the network is evident from the multitude of parallel functions that have been identified and from apparent changes in parts of the corresponding network in Caenorhabditis briggsae. Overall, the complexities of C. elegans endomesoderm specification build a picture of a network that is robust, complex, and still evolving.

Different dietary restriction regimens extend lifespan by both independent and overlapping genetic pathways in C. elegans

Dietary restriction (DR) has the remarkable ability to extend lifespan and healthspan. A variety of DR regimens have been described in species ranging from yeast to mammals. However, whether different DR regimens extend lifespan via universal, distinct, or overlapping pathways is still an open question. Here we examine the genetic pathways that mediate longevity by different DR regimens in Caenorhabditis elegans. We have previously shown that the low-energy sensing AMP-activated protein kinase AMPK/aak-2 and the Forkhead transcription factor FoxO/daf-16 are necessary for longevity induced by a DR regimen that we developed (sDR). Here we find that AMPK and FoxO are necessary for longevity induced by another DR regimen, but are dispensable for the lifespan extension induced by two different DR methods. Intriguingly, AMPK is also necessary for the lifespan extension elicited by resveratrol, a natural polyphenol that mimics some aspects of DR. Conversely, we test if genes previously reported to mediate longevity by a variety of DR methods are necessary for sDR-induced longevity. Although clk-1, a gene involved in ubiquinone biosynthesis, is also required for sDR-induced lifespan extension, we find that four other genes (sir-2.1, FoxA/pha-4, skn-1, and hsf-1) are all dispensable for longevity induced by sDR. Consistent with the observation that different DR methods extend lifespan by mostly independent genetic mechanisms, we find that the effects on lifespan of two different DR regimens are additive. Understanding the genetic network by which different DR regimens extend lifespan has important implications for harnessing the full benefits of DR on lifespan and healthspan.

The evolution of Fox genes and their role in development and disease

The forkhead box (Fox) family of transcription factors, which originated in unicellular eukaryotes, has expanded over time through multiple duplication events, and sometimes through gene loss, to over 40 members in mammals. Fox genes have evolved to acquire a specialized function in many key biological processes. Mutations in Fox genes have a profound effect on human disease, causing phenotypes as varied as cancer, glaucoma and language disorders. We summarize the salient features of the evolution of the Fox gene family and highlight the diverse contribution of various Fox subfamilies to developmental processes, from organogenesis to speech acquisition.

ELT-2 is the predominant transcription factor controlling differentiation and function of the C. elegans intestine, from embryo to adult

Starting with SAGE-libraries prepared from C. elegans FAC-sorted embryonic intestine cells (8E-16E cell stage), from total embryos and from purified oocytes, and taking advantage of the NextDB in situ hybridization data base, we define sets of genes highly expressed from the zygotic genome, and expressed either exclusively or preferentially in the embryonic intestine or in the intestine of newly hatched larvae; we had previously defined a similarly expressed set of genes from the adult intestine. We show that an extended TGATAA-like sequence is essentially the only candidate for a cis-acting regulatory motif common to intestine genes expressed at all stages. This sequence is a strong ELT-2 binding site and matches the sequence of GATA-like sites found to be important for the expression of every intestinal gene so far analyzed experimentally. We show that the majority of these three sets of highly expressed intestinal-specific/intestinal-enriched genes respond strongly to ectopic expression of ELT-2 within the embryo. By flow-sorting elt-2(null) larvae from elt-2(+) larvae and then preparing Solexa/Illumina-SAGE libraries, we show that the majority of these genes also respond strongly to loss-of-function of ELT-2. To test the consequences of loss of other transcription factors identified in the embryonic intestine, we develop a strain of worms that is RNAi-sensitive only in the intestine; however, we are unable (with one possible exception) to identify any other transcription factor whose intestinal loss-of-function causes a phenotype of comparable severity to the phenotype caused by loss of ELT-2. Overall, our results support a model in which ELT-2 is the predominant transcription factor in the post-specification C. elegans intestine and participates directly in the transcriptional regulation of the majority (>80%) of intestinal genes. We present evidence that ELT-2 plays a central role in most aspects of C. elegans intestinal physiology: establishing the structure of the enterocyte, regulating enzymes and transporters involved in digestion and nutrition, responding to environmental toxins and pathogenic infections, and regulating the downstream intestinal components of the daf-2/daf-16 pathway influencing aging and longevity.

Similar regulatory logic in Ciona intestinalis for two Wnt pathway modulators, ROR and SFRP-1/5

Anteroposterior patterning of the ectoderm in the invertebrate chordate Ciona intestinalis first relies on key zygotic activators, such as FoxA, and later on the coordinated responses to inducing signals from the underlying mesendoderm. Here, we focus on a mechanism of coordination of these responses by looking at the cis-regulatory logics of Ci-Rora and Ci-Rorb, which are coding for putative non-canonical transmembrane Wnt receptors, and are present in tandem along the C. intestinalis chromosome 08q. We showed that during cleavage stages, both Ci-Rora and Ci-Rorb genes are initially expressed in all blastomeres of the anterior ectoderm (a-line), as sFRP1/5 (Lamy, C., Rothbächer, U., Caillol, D., Lemaire, P., 2006. Ci-FoxA-a is the earliest zygotic determinant of the ascidian anterior ectoderm and directly activates Ci-sFRP1/5. Development 133, 2835-2844.). We then carried out a functional analysis of cis-regulatory regions and showed that both genes have elements enriched in Ci-FoxA-a binding sites. We dissected one of these early enhancers, and showed that it is directly activated by Ci-FoxA-a, as one sFRP1/5 cis-regulatory element. We also showed that although FoxA binding sites are abundant in genomes, dense clusters of these sites are found upstream from very few genes, and have a high predictive value of a-line expression. These data indicate an important role for FoxA in anterior specification, via the transcriptional regulation of target genes belonging to various signalling pathways.

Caenorhabditis elegans cisRED: a catalogue of conserved genomic elements

The availability of completely sequenced genomes from eight species of nematodes has provided an opportunity to identify novel cis-regulatory elements in the promoter regions of Caenorhabditis elegans transcripts using comparative genomics. We determined orthologues for C. elegans transcripts in C. briggsae, C. remanei, C. brenneri, C. japonica, Pristionchus pacificus, Brugia malayi and Trichinella spiralis using the WABA alignment algorithm. We pooled the upstream region of each transcript in C. elegans with the upstream regions of its orthologues and identified conserved DNA sequence elements by de novo motif discovery. In total, we discovered 158 017 novel conserved motifs upstream of 3847 C. elegans transcripts for which three or more orthologues were available, and identified 82% of 44 experimentally proven regulatory elements from ORegAnno. We annotated 26% of the motifs as similar to known binding sequences of transcription factors from ORegAnno, TRANSFAC and JASPAR. This is the first catalogue of annotated conserved upstream elements for nematodes and can be used to find putative regulatory elements, improve gene models, discover novel RNA genes, and understand the evolution of transcription factors and their binding sites in phylum Nematoda. The annotated motifs provide novel binding site candidates for both characterized transcription factors and orthologues of characterized mammalian transcription factors.

The molecular basis of organ formation: insights from the C. elegans foregut

The digestive tracts of many animals are epithelial tubes with specialized compartments to break down food, remove wastes, combat infection, and signal nutrient availability. C. elegans possesses a linear, epithelial gut tube with foregut, midgut, and hindgut sections. The simple anatomy belies the developmental complexity that is involved in forming the gut from a pool of heterogeneous precursor cells. Here, I focus on the processes that specify cell fates and control morphogenesis within the embryonic foregut (pharynx) and the developmental roles of the pharynx after birth. Maternally donated factors in the pregastrula embryo converge on pha-4, a FoxA transcription factor that specifies organ identity for pharyngeal precursors. Positive feedback loops between PHA-4 and other transcription factors ensure commitment to pharyngeal fate. Binding-site affinity of PHA-4 for its target promoters contributes to the progression of the pharyngeal precursors towards differentiation. During morphogenesis, the pharyngeal precursors form an epithelial tube in a process that is independent of cadherins, catenins, and integrins but requires the kinesin zen-4/MKLP1. After birth, the pharynx and/or pha-4 are involved in repelling pathogens and controlling aging.

Comparative developmental expression profiling of two C. elegans isolates

Gene expression is known to change during development and to vary among genetically diverse strains. Previous studies of temporal patterns of gene expression during C. elegans development were incomplete, and little is known about how these patterns change as a function of genetic background. We used microarrays that comprehensively cover known and predicted worm genes to compare the landscape of genetic variation over developmental time between two isolates of C. elegans. We show that most genes vary in expression during development from egg to young adult, many genes vary in expression between the two isolates, and a subset of these genes exhibit isolate-specific changes during some developmental stages. This subset is strongly enriched for genes with roles in innate immunity. We identify several novel motifs that appear to play a role in regulating gene expression during development, and we propose functional annotations for many previously unannotated genes. These results improve our understanding of gene expression and function during worm development and lay the foundation for linkage studies of the genetic basis of developmental variation in gene expression in this important model organism.

Pioneer factors, genetic competence, and inductive signaling: programming liver and pancreas progenitors from the endoderm

The endoderm is a multipotent progenitor cell population in the embryo that gives rise to the liver, pancreas, and other cell types and provides paradigms for understanding cell-type specification. Studies of isolated embryo tissue cells and genetic approaches in vivo have defined fibroblast growth factor/mitogen-activated protein kinase (FGF/MAPK) and bone morphogenetic protein (BMP) signaling pathways that induce liver and pancreatic fates in the endoderm. In undifferentiated endoderm cells, the FoxA and GATA transcription factors are among the first to engage silent genes, helping to endow competence for cell-type specification. FoxA proteins can bind their target sites in highly compacted chromatin and open up the local region for other factors to bind; hence, they have been termed "pioneer factors." We recently found that FoxA proteins remain bound to chromatin in mitosis, as an epigenetic mark. In embryonic stem cells, which lack FoxA, FoxA target sites can be occupied by FoxD3, which in turn helps to maintain a local demethylation of chromatin. By these means, a cascade of Fox factors helps to endow progenitor cells with the competence to activate genes in response to tissue-inductive signals. Understanding such epigenetic mechanisms for transcriptional competence coupled with knowledge of the relevant signals for cell-type specification should greatly facilitate efforts to predictably differentiate stem cells to liver and pancreatic fates.

The HLH-6 transcription factor regulates C. elegans pharyngeal gland development and function

The Caenorhabditis elegans pharynx (or foregut) functions as a pump that draws in food (bacteria) from the environment. While the “organ identity factor” PHA-4 is critical for formation of the C. elegans pharynx as a whole, little is known about the specification of distinct cell types within the pharynx. Here, we use a combination of bioinformatics, molecular biology, and genetics to identify a helix-loop-helix transcription factor (HLH-6) as a critical regulator of pharyngeal gland development. HLH-6 is required for expression of a number of gland-specific genes, acting through a discrete cis-regulatory element named PGM1 (Pharyngeal Gland Motif 1). hlh-6 mutants exhibit a frequent loss of a subset of glands, while the remaining glands have impaired activity, indicating a role for hlh-6 in both gland development and function. Interestingly, hlh-6 mutants are also feeding defective, ascribing a biological function for the glands. Pharyngeal pumping in hlh-6 mutants is normal, but hlh-6 mutants lack expression of a class of mucin-related proteins that are normally secreted by pharyngeal glands and line the pharyngeal cuticle. An interesting possibility is that one function of pharyngeal glands is to secrete a pharyngeal lining that ensures efficient transport of food along the pharyngeal lumen.

To make an organ, cells must be instructed to be part of a common structure yet must also be assigned specific roles or identities within that structure. For example, the stomach contains a variety of different kinds of cells, including muscles, nerves, and glands. This same complexity is seen even in relatively simple organs, like the pharynx (foregut) of the nematode C. elegans. The pharynx is a neuromuscular organ that pumps in food (bacteria) from the environment. This organ is relatively simple (containing only 80 cells) yet contains five distinct kinds of cells. How these different cells are specified is unclear but likely involves combinations of developmental regulators known as transcription factors. Here, we examine one cell type, the pharyngeal glands, and identify a key regulator of their development, the transcription factor HLH-6. Interestingly, HLH-6 is closely related to a mammalian transcription factor, Sgn1, which is involved in development of mammalian salivary glands, suggesting that C. elegans pharyngeal glands are evolutionarily related to mammalian salivary glands. A further connection is that the pharyngeal glands of C. elegans appear to be required for efficient feeding, possibly by secreting mucin-like proteins that ensure the smooth passage of food along the digestive tract.

The Target of Rapamycin pathway antagonizes pha-4/FoxA to control development and aging

BACKGROUND

FoxA factors are critical regulators of embryonic development and postembryonic life, but little is know about the upstream pathways that modulate their activity. C. elegans pha-4 encodes a FoxA transcription factor that is required to establish the foregut in embryos and to control growth and longevity after birth. We previously identified the AAA+ ATPase homolog ruvb-1 as a potent suppressor of pha-4 mutations.

RESULTS

Here we show that ruvb-1 is a component of the Target of Rapamycin (TOR) pathway in C. elegans (CeTOR). Both ruvb-1 and let-363/TOR control nucleolar size and promote localization of box C/D snoRNPs to nucleoli, suggesting a role in rRNA maturation. Inactivation of let-363/TOR or ruvb-1 suppresses the lethality associated with reduced pha-4 activity. The CeTOR pathway controls protein homeostasis and also contributes to adult longevity. We find that pha-4 is required to extend adult lifespan in response to reduced CeTOR signaling. Mutations in the predicted CeTOR target rsks-1/S6 kinase or in ife-2/eIF4E also reduce protein biosynthesis and extend lifespan, but only rsks-1 mutations require pha-4 for adult longevity. In addition, rsks-1, but not ife-2, can suppress the larval lethality associated with pha-4 loss-of-function mutations.

CONCLUSIONS

The data suggest that pha-4 and the CeTOR pathway antagonize one another to regulate postembryonic development and adult longevity. We suggest a model in which nutrients promote TOR and S6 kinase signaling, which represses pha-4/FoxA, leading to a shorter lifespan. A similar regulatory hierarchy may function in other animals to modulate metabolism, longevity, or disease.

The genomic distribution and function of histone variant HTZ-1 during C. elegans embryogenesis

In all eukaryotes, histone variants are incorporated into a subset of nucleosomes to create functionally specialized regions of chromatin. One such variant, H2A.Z, replaces histone H2A and is required for development and viability in all animals tested to date. However, the function of H2A.Z in development remains unclear. Here, we use ChIP-chip, genetic mutation, RNAi, and immunofluorescence microscopy to interrogate the function of H2A.Z (HTZ-1) during embryogenesis in Caenorhabditis elegans, a key model of metazoan development. We find that HTZ-1 is expressed in every cell of the developing embryo and is essential for normal development. The sites of HTZ-1 incorporation during embryogenesis reveal a genome wrought by developmental processes. HTZ-1 is incorporated upstream of 23% of C. elegans genes. While these genes tend to be required for development and occupied by RNA polymerase II, HTZ-1 incorporation does not specify a stereotypic transcription program. The data also provide evidence for unexpectedly widespread independent regulation of genes within operons during development; in 37% of operons, HTZ-1 is incorporated upstream of internally encoded genes. Fewer sites of HTZ-1 incorporation occur on the X chromosome relative to autosomes, which our data suggest is due to a paucity of developmentally important genes on X, rather than a direct function for HTZ-1 in dosage compensation. Our experiments indicate that HTZ-1 functions in establishing or maintaining an essential chromatin state at promoters regulated dynamically during C. elegans embryogenesis.

Structural basis for DNA recognition by FoxO1 and its regulation by posttranslational modification

FoxO transcription factors regulate the transcription of genes that control metabolism, cellular proliferation, stress tolerance, and possibly life span. A number of posttranslational modifications within the forkhead DNA-binding domain regulate FoxO-mediated transcription. We describe the crystal structures of FoxO1 bound to three different DNA elements and measure the change in FoxO1-DNA affinity with acetylation and phosphorylation. The structures reveal additional contacts and increased DNA distortion for the highest affinity DNA site. The flexible wing 2 region of the forkhead domain was not observed in the structures but is necessary for DNA binding, and we show that p300 acetylation in wing 2 reduces DNA affinity. We also show that MST1 phosphorylation of FoxO1 prevents high-affinity DNA binding. The observation that FoxO-DNA affinity varies between response elements and with posttranslational modifications suggests that modulation of FoxO-DNA affinity is an important component of FoxO regulation in health and misregulation in disease.

Aging and survival: the genetics of life span extension by dietary restriction

Reducing food intake to induce undernutrition but not malnutrition extends the life spans of multiple species, ranging from single-celled organisms to mammals. This increase in longevity by dietary restriction (DR) is coupled to profound beneficial effects on age-related pathology. Historically, much of the work on DR has been undertaken using rodent models, and 70 years of research has revealed much about the physiological changes DR induces. However, little is known about the genetic pathways that regulate the DR response and whether or not they are conserved between species. Elucidating these pathways may facilitate the design of targeted pharmaceutical treatments for a range of age-related pathologies. Here, we discuss how recent work in nonmammalian model organisms has revealed new insight into the genetics of DR and how the discovery of DR-specific transcription factors will advance our understanding of this phenomenon.

An elt-3/elt-5/elt-6 GATA transcription circuit guides aging in C. elegans

To define the C. elegans aging process at the molecular level, we used DNA microarray experiments to identify a set of 1294 age-regulated genes and found that the GATA transcription factors ELT-3, ELT-5, and ELT-6 are responsible for age regulation of a large fraction of these genes. Expression of elt-5 and elt-6 increases during normal aging, and both of these GATA factors repress expression of elt-3, which shows a corresponding decrease in expression in old worms. elt-3 regulates a large number of downstream genes that change expression in old age, including ugt-9, col-144, and sod-3. elt-5(RNAi) and elt-6(RNAi) worms have extended longevity, indicating that elt-3, elt-5, and elt-6 play an important functional role in the aging process. These results identify a transcriptional circuit that guides the rapid aging process in C. elegans and indicate that this circuit is driven by drift of developmental pathways rather than accumulation of damage.

Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network

Neuronal subtype specification in the vertebrate neural tube is one of the best-studied examples of embryonic pattern formation. Distinct neuronal subtypes are generated in a precise spatial order from progenitor cells according to their location along the anterior-posterior and dorsal-ventral axes. Underpinning this organization is a complex network of multiple extrinsic and intrinsic factors. This review focuses on the molecular mechanisms and general strategies at play in ventral regions of the forming spinal cord, where sonic hedgehog-based morphogen signaling is a key determinant. We discuss recent advances in our understanding of these events and highlight unresolved questions.

The CSL transcription factor LAG-1 directly represses hlh-6 expression in C. elegans

The Caenorhabditis elegans gene hlh-6 is expressed specifically in pharyngeal glands, one of five distinct pharyngeal cell types. Expression of hlh-6 is controlled by a discrete set of cis-regulatory elements, including a negative element called HRL1. Here we demonstrate that HRL1 is a functional binding site for LAG-1, the CSL transcriptional effector of Notch in C. elegans, and that regulation of hlh-6 by LAG-1 is direct. Regulation of hlh-6 by LAG-1 is strictly negative: removal of HRL1 or LAG-1 regulation results in ectopic expression of hlh-6, but does not affect expression in pharyngeal glands. Furthermore, direct regulation of hlh-6 expression does not appear to involve Notch signaling, contrary to the canonical mechanism by which CSL factors regulate target genes. We also identify an additional cis-regulatory element in the hlh-6 promoter that, together with previously identified elements, is sufficient to overcome repression by LAG-1 and activate hlh-6 expression in pharyngeal glands.

Bimolecular fluorescence complementation (BiFC) analysis of protein interactions in Caenorhabditis elegans

Protein interactions are essential components of signal transduction in cells. With the progress in genome-wide yeast two hybrid screens and proteomics analyses, many protein interaction networks have been generated. These analyses have identified hundreds and thousands of interactions in cells and organisms, creating a challenge for further validation under physiological conditions. The bimolecular fluorescence complementation (BiFC) assay is such an assay that meets this need. The BiFC assay is based on the principle of protein fragment complementation, in which two non-fluorescent fragments derived from a fluorescent protein are fused to a pair of interacting partners. When the two partners interact, the two non-fluorescent fragments are brought into proximity and an intact fluorescent protein is reconstituted. Hence, the reconstituted fluorescent signals reflect the interaction of two proteins under study. Over the past six years, the BiFC assay has been used for visualization of protein interactions in living cells and organisms, including our application of the BiFC assay to the transparent nematode Caenorhabditis elegans. We have demonstrated that BiFC analysis in C. elegans provides a direct means to identify and validate protein interactions in living worms and allows visualization of temporal and spatial interactions. Here, we provide a guideline for the implementation of BiFC analysis in living worms and discuss the factors that are critical for BiFC analysis.

Tuning in to the signals: noncoding sequence conservation in vertebrate genomes

Aligning and comparing genomic sequences enables the identification of conserved sequence signatures and can enrich for coding and noncoding functional regions. In vertebrates, the comparison of human and rodent genomes and the comparison of evolutionarily distant genomes, such as human and pufferfish, have identified specific sets of 'ultraconserved' sequence elements associated with the control of early development. However, is this just the tip of a 'conservation iceberg' or do these sequences represent a specific class of regulatory element? Studies on the zebrafish phox2b gene region and the ENCODE project suggest that many regulatory elements are not highly conserved, posing intriguing questions about the relationship between noncoding sequence conservation and function and the evolution of regulatory sequences.

Wormnet: a crystal ball for Caenorhabditis elegans

An integrated gene network for Caenorhabditis elegans using data from multiple genome-wide screens encompasses most protein-coding genes and can accurately predict their phenotypes.

Behavioral and synaptic defects in C. elegans lacking the NK-2 homeobox gene ceh-28

C. elegans pharyngeal behavior consists of two distinct types of muscle contractions, termed pumping and peristalsis. Pumping ingests and concentrates bacteria in the anterior pharyngeal lumen, and it is occasionally followed by a transient peristaltic contraction that carries ingested bacteria through the posterior pharyngeal isthmus. These behaviors are controlled by a small pharyngeal nervous system consisting of 20 neurons that is almost completely independent of the extra-pharyngeal nervous system. The cholinergic motor neuron M4 controls peristalsis via synapses with the posterior isthmus muscles. Here we show that the NK-2 family homeobox gene ceh-28 is expressed in M4, where it regulates synapse assembly and peristalsis. ceh-28 mutants exhibit frequent and prolonged peristalses, and treatment with agonists or antagonists of muscarinic acetylcholine receptors can phenocopy or suppress ceh-28 mutant defects, respectively. Synapses in ceh-28 mutant M4 cells are irregularly spaced and sized, and they are abnormally located along the full length of the isthmus. We suggest that CEH-28 inhibits synaptogenesis, and that ceh-28 mutant behavioral defects result from excessive or ectopic stimulation of muscarinic acetylcholine receptors in the isthmus muscles.

Developmental expression of foxA and gata genes during gut formation in the polychaete annelid, Capitella sp. I

Most bilaterian animals have evolved a through gut that is regionally specialized along the anterior-posterior axis. In the polychaete annelid, Capitella sp. I, the alimentary canal is subdivided into a buccal cavity, pharynx, esophagus, midgut, and hindgut. Members of the Fox and GATA families of transcription factors have conserved functions in patterning ectodermal and endodermal gut components. We have isolated and characterized expression of one FoxA gene (CapI-foxA) and four GATA genes (CapI-gataB1, CapI-gataB2, CapI-gataB3, and CapI-gataA1) from Capitella sp. I. Both gene families are expressed in the developing gut of this polychaete. CapI-foxA, an ortholog of the FoxA subgroup, is expressed in vegetal hemisphere micromeres of cleavage-stage embryos, in multiple blastomeres within and surrounding the blastopore during gastrulation, and throughout morphogenesis of the pharynx, esophagus, and hindgut. The CapI-gataB genes group within the vertebrate GATA4/5/6 subfamily, appear to be products of lineage-specific gene duplication, and are expressed in specific domains of endomesoderm. CapI-gataB1 is expressed in endoderm precursors and throughout developing midgut endoderm, and is particularly prominent at anterior and posterior midgut boundaries. CapI-gataB2 is co-expressed with CapI-gataB1 in midgut endoderm, and is also expressed in visceral mesoderm. CapI-gataB3 is limited to and coexpressed with CapI-gataB2 in visceral mesoderm. CapI-gataA1 groups within the vertebrate GATA1/2/3 subfamily and is expressed primarily in ectodermal tissues of the brain, ventral nerve cord, lateral trunk, and both pharyngeal and esophageal regions of the foregut. Collectively, the CapI-foxA and CapI-gata genes show patterns of expression that span almost the entire length of the developing alimentary canal, consistent with a role in gut development.

Gene activation using FLP recombinase in C. elegans

The FLP enzyme catalyzes recombination between specific target sequences in DNA. Here we use FLP to temporally and spatially control gene expression in the nematode C. elegans. Transcription is blocked by the presence of an "off cassette" between the promoter and the coding region of the desired product. The "off cassette" is composed of a transcriptional terminator flanked by FLP recognition targets (FRT). This sequence can be excised by FLP recombinase to bring together the promoter and the coding region. We have introduced two fluorescent reporters into the system: a red reporter for promoter activity prior to FLP expression and a green reporter for expression of the gene of interest after FLP expression. The constructs are designed using the multisite Gateway system, so that promoters and coding regions can be quickly mixed and matched. We demonstrate that heat-shock-driven FLP recombinase adds temporal control on top of tissue specific expression provided by the transgene promoter. In addition, the temporal switch is permanent, rather than acute, as is usually the case for heat-shock driven transgenes. Finally, FLP expression can be driven by a tissue specific promoter to provide expression in a subset of cells that can only be addressed as the intersection of two available promoters. As a test of the system, we have driven the light chain of tetanus toxin, a protease that cleaves the synaptic vesicle protein synaptobrevin. We show that we can use this to inactivate synaptic transmission in all neurons or a subset of neurons in a FLP-dependent manner.

Function of the PHA-4/FOXA transcription factor during C. elegans post-embryonic development

BACKGROUND

pha-4 encodes a forkhead box (FOX) A transcription factor serving as the C. elegans pharynx organ identity factor during embryogenesis. Using Serial Analysis of Gene Expression (SAGE), comparison of gene expression profiles between growing stages animals and long-lived, developmentally diapaused dauer larvae revealed that pha-4 transcription is increased in the dauer stage.

RESULTS

Knocking down pha-4 expression by RNAi during post-embryonic development showed that PHA-4 is essential for dauer recovery, gonad and vulva development. daf-16, which encodes a FOXO transcription factor regulated by insulin/IGF-1 signaling, shows overlapping expression patterns and a loss-of-function post-embryonic phenotype similar to that of pha-4 during dauer recovery. pha-4 RNAi and daf-16 mutations have additive effects on dauer recovery, suggesting these two regulators may function in parallel pathways. Gene expression studies using RT-PCR and GFP reporters showed that pha-4 transcription is elevated under starvation, and a conserved forkhead transcription factor binding site in the second intron of pha-4 is important for the neuronal expression. The vulval transcription of lag-2, which encodes a ligand for the LIN-12/Notch lateral signaling pathway, is inhibited by pha-4 RNAi, indicating that LAG-2 functions downstream of PHA-4 in vulva development.

CONCLUSION

Analysis of PHA-4 during post-embryonic development revealed previously unsuspected functions for this important transcriptional regulator in dauer recovery, and may help explain the network of transcriptional control integrating organogenesis with the decision between growth and developmental arrest at the dauer entry and exit stages.

A Caenorhabditis motif compendium for studying transcriptional gene regulation

BACKGROUND

Controlling gene expression is fundamental to biological complexity. The nematode Caenorhabditis elegans is an important model for studying principles of gene regulation in multi-cellular organisms. A comprehensive parts list of putative regulatory motifs was yet missing for this model system. In this study, we compile a set of putative regulatory motifs by combining evidence from conservation and expression data.

DESCRIPTION

We present an unbiased comparative approach to a regulatory motif compendium for Caenorhabditis species. This involves the assembly of a new nematode genome, whole genome alignments and assessment of conserved k-mers counts. Candidate motifs are selected from a set of 9,500 randomly picked genes by three different motif discovery strategies. Motif candidates have to pass a conservation enrichment filter. Motif degeneracy and length are optimized. Retained motif descriptions are evaluated by expression data using a non-parametric test, which assesses expression changes due to the presence/absence of individual motifs. Finally, we also provide condition-specific motif ensembles by conditional tree analysis.

CONCLUSION

The nematode genomes align surprisingly well despite high neutral substitution rates. Our pipeline delivers motif sets by three alternative strategies. Each set contains less than 400 motifs, which are significantly conserved and correlated with 214 out of 270 tested gene expression conditions. This motif compendium is an entry point to comprehensive studies on nematode gene regulation. The website: http://corg.eb.tuebingen.mpg.de/CMC has extensive query capabilities, supplements this article and supports the experimental list.

mef2 activity levels differentially affect gene expression during Drosophila muscle development

Cell differentiation is controlled by key transcription factors, and a major question is how they orchestrate cell-type-specific genetic programs. Muscle differentiation is a well studied paradigm in which the conserved Mef2 transcription factor plays a pivotal role. Recent genomic studies have identified a large number of mef2-regulated target genes with distinct temporal expression profiles during Drosophila myogenesis. However, the question remains as to how a single transcription factor can control such diverse patterns of gene expression. In this study we used a strategy combining genomics and developmental genetics to address this issue in vivo during Drosophila muscle development. We found that groups of mef2-regulated genes respond differently to changes in mef2 activity levels: some require higher levels for their expression than others. Furthermore, this differential requirement correlates with when the gene is first expressed during the muscle differentiation program. Genes that require higher levels are activated later. These results implicate mef2 in the temporal regulation of muscle gene expression, and, consistent with this, we show that changes in mef2 activity levels can alter the start of gene expression in a predictable manner. Together these results indicate that Mef2 is not an all-or-none regulator; rather, its action is more subtle, and levels of its activity are important in the differential expression of muscle genes. This suggests a route by which mef2 can orchestrate the muscle differentiation program and contribute to the stringent regulation of gene expression during myogenesis.

Transcription factors GATA/ELT-2 and forkhead/HNF-3/PHA-4 regulate the tropomyosin gene expression in the pharynx and intestine of Caenorhabditis elegans

Gene regulation during development is an important biological activity that leads to synthesis of biomolecules at specific locations and specific times. The single tropomyosin gene of Caenorhabditis elegans, tmy-1/lev-11, produces four isoforms of protein: two from the external promoter and two from the internal promoter. We investigated the internal promoter of tropomyosin to identify sequences that regulate expression of tmy-1 in the pharynx and intestine. By promoter deletion of tmy-1 reporters as well as by database analyses, a 100-bp fragment that contained binding sequences for a GATA factor, for a chicken CdxA homolog, and for a forkhead factor was identified. Both the forkhead and CdxA binding sequences contributed to pharyngeal and intestinal expression. In addition, the GATA site also influenced intestinal expression of tmy-1 reporter. We showed that ELT-2 and PHA-4 proteins interact directly with the GATA and forkhead binding sequences, respectively, in gel mobility shift assays. RNA interference knockdown of elt-2 diminished tmy-1::gfp expression in the intestine. In contrast to RNA interference knockdown of pha-4, expression of tmy-1::gfp in pha-4;smg-1 mutants was slightly weaker than that of the wild type. Ectopic expression of PHA-4 and ELT-2 by heat shock was sufficient to elicit widespread expression of tmy-1::lacZ reporter in embryos. We found no indication of a synergistic relation between ELT-2 and PHA-4. Based on our data, PHA-4 and CdxA function as general transcription factors for pharyngeal and intestinal regulation of tmy-1. We present models by which ELT-2, PHA-4, and CdxA orchestrate expression from the internal promoter of tmy-1.

Conversion of a replication origin to a silencer through a pathway shared by a Forkhead transcription factor and an S phase cyclin

Silencing of the mating-type locus HMR in Saccharomyces cerevisiae requires DNA elements called silencers. To establish HMR silencing, the origin recognition complex binds the HMR-E silencer and recruits the silent information regulator (Sir)1 protein. Sir1 in turn helps establish silencing by stabilizing binding of the other Sir proteins, Sir2-4. However, silencing is semistable even in sir1Delta cells, indicating that SIR1-independent establishment mechanisms exist. Furthermore, the requirement for SIR1 in silencing a sensitized version of HMR can be bypassed by high-copy expression of FKH1 (FKH1(hc)), a conserved forkhead transcription factor, or by deletion of the S phase cyclin CLB5 (clb5Delta). FKH1(hc) caused only a modest increase in Fkh1 levels but effectively reestablished Sir2-4 chromatin at HMR as determined by Sir3-directed chromatin immunoprecipitation. In addition, FKH1(hc) prolonged the cell cycle in a manner distinct from deletion of its close paralogue FKH2, and it created a cell cycle phenotype more reminiscent to that caused by a clb5Delta. Unexpectedly, and in contrast to SIR1, both FKH1(hc) and clb5Delta established silencing at HMR using the replication origins, ARS1 or ARSH4, as complete substitutes for HMR-E (HMRDeltaE::ARS). HMRDeltaE::ARS1 was a robust origin in CLB5 cells. However, initiation by HMRDeltaE::ARS1 was reduced by clb5Delta or FKH1(hc), whereas ARS1 at its native locus was unaffected. The CLB5-sensitivity of HMRDeltaE::ARS1 did not result from formation of Sir2-4 chromatin because sir2Delta did not rescue origin firing in clb5Delta cells. These and other data supported a model in which FKH1 and CLB5 modulated Sir2-4 chromatin and late-origin firing through opposing regulation of a common pathway.

Temporal ChIP-on-chip reveals Biniou as a universal regulator of the visceral muscle transcriptional network

Smooth muscle plays a prominent role in many fundamental processes and diseases, yet our understanding of the transcriptional network regulating its development is very limited. The FoxF transcription factors are essential for visceral smooth muscle development in diverse species, although their direct regulatory role remains elusive. We present a transcriptional map of Biniou (a FoxF transcription factor) and Bagpipe (an Nkx factor) activity, as a first step to deciphering the developmental program regulating Drosophila visceral muscle development. A time course of chromatin immunoprecipitatation followed by microarray analysis (ChIP-on-chip) experiments and expression profiling of mutant embryos reveal a dynamic map of in vivo bound enhancers and direct target genes. While Biniou is broadly expressed, it regulates enhancers driving temporally and spatially restricted expression. In vivo reporter assays indicate that the timing of Biniou binding is a key trigger for the time span of enhancer activity. Although bagpipe and biniou mutants phenocopy each other, their regulatory potential is quite different. This network architecture was not apparent from genetic studies, and highlights Biniou as a universal regulator in all visceral muscle, regardless of its developmental origin or subsequent function. The regulatory connection of a number of Biniou target genes is conserved in mice, suggesting an ancient wiring of this developmental program.

Notch-GATA synergy promotes endoderm-specific expression of ref-1 in C. elegans

The Notch signaling pathway is involved in a wide variety of cell-fate decisions during development. The diverse behavior of Notch-activated cells is thought to depend on tissue- or cell-type-specific transcription factors, yet the identities of such factors and the mechanism of cooperation with the Notch pathway are largely unknown. We identify here an enhancer in the promoter of ref-1, a C. elegans Notch target, which promotes Notch-dependent expression in mesodermal and endodermal cells. The enhancer contains predicted binding sites for the Notch transcriptional effector LAG-1/CSL that are essential for expression, a non-CSL site required for mesodermal expression, and four predicted binding sites for GATA transcription factors that are required for endodermal expression. We show that endodermal expression involves the GATA transcription factor ELT-2, and that ELT-2 can bind LAG-1/CSL in vitro. In many types of Notch-activated embryonic cells, ectopic ELT-2 is sufficient to drive expression of reporters containing the enhancer.

Genetic suppressors of Caenorhabditis elegans pha-4/FoxA identify the predicted AAA helicase ruvb-1/RuvB

FoxA transcription factors are critical regulators of gut development and function. FoxA proteins specify gut fate during early embryogenesis, drive gut differentiation and morphogenesis at later stages, and affect gut function to mediate nutritional responses. The level of FoxA is critical for these roles, yet we know relatively little about regulators for this family of proteins. To address this issue, we conducted a genetic screen for mutants that suppress a partial loss of pha-4, the sole FoxA factor of Caenorhabditis elegans. We identified 55 mutants using either chemical or insertional mutagenesis. Forty-two of these were informational suppressors that affected nonsense-mediated decay, while the remaining 13 were pha-4 suppressors. These 13 alleles defined at least six different loci. On the basis of mutational frequencies for C. elegans and the genetic dominance of four of the suppressors, we predict that many of the suppressors are either unusual loss-of-function mutations in negative regulators or rare gain-of-function mutations in positive regulators. We characterized one dominant suppressor molecularly and discovered the mutation alters a likely cis-regulatory region within pha-4 itself. A second suppressor defined a new locus, the predicted AAA+ helicase ruvb-1. These results indicate that our screen successfully found cis- or trans-acting regulators of pha-4.