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

Long-term and low-dose exposure to nanopolystyrene induces a protective strategy to maintain functional state of intestine barrier in nematode Caenorhabditis elegans

Functional state of intestinal barrier plays an important role for environmental animals in being against various toxicants. We investigated GATA transcriptional factor ELT-2-mediated intestinal response to nanopolystyrere in Caenorhabditis elegans. Prolonged exposure to nanopolystyrene (≥1 μg/L) induced an increase in expression of ELT-2, and intestinal RNA interference (RNAi) knockdown of elt-2 caused enhancement in intestinal permeability. Meanwhile, mutation of elt-2 resulted in susceptibility to nanopolystyrene toxicity, and ELT-2 functioned in intestine to regulate the nanopolystyrene toxicity. ERM-1, CLEC-63, and CLEC-85 were identified as targets of ELT-2 in regulating the nanopolystyrene toxicity. ERM-1 was required for maintaining functional state in intestinal barrier, and functioned synergistically with CLEC-63 or CLEC-85 to regulate nanopolystyrene toxicity. Therefore, activation of intestinal ELT-2 by nanopolystyrere could mediate a protective strategy to maintain the functional state of intestinal barrier. During this process, intestinal ELT-2 activated two different molecular signals (ERM-1 signal and CLEC-63/85 signal) for nematodes against the nanopolystyrene toxicity.

Alternative splicing of the Caenorhabditis elegans lev-11 tropomyosin gene is regulated in a tissue-specific manner

Tropomyosin isoforms contribute to generation of functionally divergent actin filaments. In the nematode Caenorhabditis elegans, multiple isoforms are produced from lev-11, the single tropomyosin gene, by combination of two separate promoters and alternative pre-mRNA splicing. In this study, we report that alternative splicing of lev-11 is regulated in a tissue-specific manner so that a particular tropomyosin isoform is expressed in each tissue. Reverse-transcription polymerase chain reaction analysis of lev-11 mRNAs confirms five previously reported isoforms (LEV-11A, LEV-11C, LEV-11D, LEV-11E and LEV-11O) and identifies a new sixth isoform LEV-11T. Using transgenic alternative-splicing reporter minigenes, we find distinct patterns of preferential exon selections in the pharynx, body wall muscles, intestine and neurons. The body wall muscles preferentially process splicing to produce high-molecular-weight isoforms, LEV-11A, LEV-11D and LEV-11O. The pharynx specifically processes splicing to express a low-molecular-weight isoform LEV-11E, whereas the intestine and neurons process splicing to express another low-molecular-weight isoform LEV-11C. The splicing pattern of LEV-11T was not predominant in any of these tissues, suggesting that this is a minor isoform. Our results suggest that regulation of alternative splicing is an important mechanism to express proper tropomyosin isoforms in particular tissue and/or cell types in C. elegans.

The multipotency-to-commitment transition in Caenorhabditis elegans-implications for reprogramming from cells to organs

In animal embryos, cells transition from a multipotential state, with the capacity to adopt multiple fates, into an irreversible, committed state of differentiation. This multipotency-to-commitment transition (MCT) is evident from experiments in which cell fate is reprogrammed by transcription factors for cell type-specific differentiation, as has been observed extensively in Caenorhabditis elegans. Although factors that direct differentiation into each of the three germ layer types cannot generally reprogram cells after the MCT in this animal, transcription factors for endoderm development are able to do so in multiple differentiated cell types. In one case, these factors can redirect the development of an entire organ in the process of "transorganogenesis". Natural transdifferentiation also occurs in a small number of differentiated cells during normal C. elegans development. We review these reprogramming and transdifferentiation events, highlighting the cellular and developmental contexts in which they occur, and discuss common themes underlying direct cell lineage reprogramming. Although certain aspects may be unique to the model system, growing evidence suggests that some mechanisms are evolutionarily conserved and may shed light on cellular plasticity and disease in humans.

Transorganogenesis and transdifferentiation in C. elegans are dependent on differentiated cell identity

The differentiated cell identities and structure of fully formed organs are generally stable after their development. In contrast, we report here that development of the C. elegans proximal somatic gonad (hermaphrodite uterus and spermathecae, and male vas deferens) can be redirected into intestine-like organs by brief expression of the ELT-7 GATA transcription factor. This process converts one developing organ into another and can hence be considered "transorganogenesis." We show that, following pulsed ELT-7 expression, cells of the uterus activate and maintain intestine-specific gene expression and are transformed at the ultrastructural level to form an epithelial tube resembling the normal intestine formed during embryogenesis. Ubiquitous ELT-7 expression activates intestinal markers in many different cell types but only cells in the somatic gonad and pharynx appear to become fully reprogrammed. We found that ectopic expression of other endoderm-promoting transcription factors, but not muscle- or ectoderm- promoting transcription factors, redirects the fate of these organs, suggesting that pharyngeal and somatic gonad cells are specifically competent to adopt intestine identity. Although the intestine, pharynx, and somatic gonad are derived from distant cell lineages, they all express the PHA-4/FoxA transcription factor. While we found that post-embryonic PHA-4 is not necessary for pharynx or uterus reprogramming and PHA-4 is not sufficient in combination with ELT-7 to induce reprogramming in other cells types, knock down of PHA-4 during embryogenesis, which abolishes normal pharynx differentiation, prevents pharyngeal precursors from being reprogrammed into intestine. These results suggest that differentiated cell identity determines susceptibility to transdifferentiation and highlight the importance of cellular context in controlling competency for reprogramming.

Regulation of UNC-130/FOXD-mediated mesodermal patterning in C. elegans

Spatial polarity cues in animals are used repeatedly during development for many processes, including cell fate determination, cell migration, and axon guidance. In Caenorhabditis elegans, the body wall muscle extends the length of the animal in four distinct quadrants and generates an UNC-129/TGF-β-related signal that is much higher in the dorsal two muscle quadrants compared to their ventral counterparts. This pattern of unc-129 expression requires the activity of the proposed transcriptional repressor UNC-130/FOXD whose body wall muscle activity is restricted to the ventral two body wall muscle quadrants. To understand how these dorsal-ventral differences in UNC-130 activity are established and maintained, we have analyzed the regulation of unc-130 expression and the distribution of UNC-130 protein. We have identified widespread, cis-acting elements in the unc-130 promoter that function to positively regulate ventral body wall muscle expression and negatively regulate dorsal body wall muscle expression. We have defined the temporal distribution of UNC-130 protein in body wall muscle cells during embryogenesis, demonstrated that this pattern is required to establish the dorsal-ventral polarity of UNC-129/TGF-β, and shown that UNC-130 is not required post-embryonically to maintain the asymmetry of body wall muscle unc-129 expression. Finally, we have tested the impact of the depletion of a variety of transcription factors, repressors, and signaling molecules to identify additional regulators of body wall muscle UNC-130 polarity. Our results confirm and extend earlier studies to clarify the mechanisms by which UNC-130 is controlled and affects the pattern of unc-129 expression in body wall muscle. These results further our understanding of the transcriptional logic behind the generation of polarity cues involving this poorly understood subclass of Forkhead factors.

Expression profiles and unc-27 mutation rescue of the striated muscle type troponin I isoform-3 in Caenorhabditis elegans

Transcription control of multiple genes in tissue- and stage-specific patterns is still of major interest. We show here that troponin I (TNI) is expressed under the control of upstream non-coding sequences and had functions as an isoform of intermediate type between pharynx and body-wall of the gene. In Caenorhabditis elegans, three striated muscle TNIs are expressed in body-wall muscles and a cardiac isoform is expressed in the pharynx. We have analyzed the gene expression mechanisms of tni-3 gene and motility function of its protein product. Promoter deletion analysis of the tni-3 gene identified muscle enhancers including the head enhancer. The CBF1/Su(H)/LAG-1-binding motif was included in the head enhancer. Yeast one-hybrid screening isolated the lag-1 clone in five candidates. Functional differences between the three striated muscle TNIs were investigated by the expression of promoter-fusion genes into tni-2/unc-27(e155) null mutant animals. The results suggest that the cis-elements in the promoters of the three genes are important for their tissue-specific expression and that from the function of TNI-3, the tni-3 gene would be an intermediate in the evolution of these genes by gene duplication. Mechanisms of tni-3 expression and its molecular function may contribute to our understanding of gene evolution and developmental programs.

Direct and indirect gene regulation by a life-extending FOXO protein in C. elegans: roles for GATA factors and lipid gene regulators

In long-lived C. elegans insulin/IGF-1 pathway mutants, the life-extending FOXO transcription factor DAF-16 is present throughout the animal, but we find that its activity in a single tissue can delay the aging of other tissues and extend the animal's life span. To better understand the topography of DAF-16 action among the tissues, we analyzed a collection of DAF-16-regulated genes. DAF-16 regulated most of these genes in a cell-autonomous fashion, often using tissue-specific GATA factors to direct their expression to specific tissues. DAF-16 could also act cell nonautonomously to influence gene expression. DAF-16 affected gene expression in other cells, at least in part, via the lipid-gene regulator MDT-15. DAF-16, and probably MDT-15, could act cell nonautonomously in the endoderm to ameliorate the paralysis caused by expressing Alzheimer's Aβ protein in muscles. These findings suggest that MDT-15-dependent intercellular signals, possibly lipid signals, can help to coordinate tissue physiology, enhance proteostasis, and extend life in response to DAF-16/FOXO activity.

HIF-1 regulates iron homeostasis in Caenorhabditis elegans by activation and inhibition of genes involved in iron uptake and storage

Caenorhabditis elegans ftn-1 and ftn-2, which encode the iron-storage protein ferritin, are transcriptionally inhibited during iron deficiency in intestine. Intestinal specific transcription is dependent on binding of ELT-2 to GATA binding sites in an iron-dependent enhancer (IDE) located in ftn-1 and ftn-2 promoters, but the mechanism for iron regulation is unknown. Here, we identify HIF-1 (hypoxia-inducible factor -1) as a negative regulator of ferritin transcription. HIF-1 binds to hypoxia-response elements (HREs) in the IDE in vitro and in vivo. Depletion of hif-1 by RNA interference blocks transcriptional inhibition of ftn-1 and ftn-2 reporters, and ftn-1 and ftn-2 mRNAs are not regulated in a hif-1 null strain during iron deficiency. An IDE is also present in smf-3 encoding a protein homologous to mammalian divalent metal transporter-1. Unlike the ftn-1 IDE, the smf-3 IDE is required for HIF-1–dependent transcriptional activation of smf-3 during iron deficiency. We show that hif-1 null worms grown under iron limiting conditions are developmentally delayed and that depletion of FTN-1 and FTN-2 rescues this phenotype. These data show that HIF-1 regulates intestinal iron homeostasis during iron deficiency by activating and inhibiting genes involved in iron uptake and storage.

Due to its presence in proteins involved in hemoglobin synthesis, DNA synthesis, and mitochondrial respiration, eukaryotic cells require iron for survival. Excess iron can lead to oxidative damage, while iron deficiency reduces cell growth and causes cell death. Dysregulation of iron homeostasis in humans caused by iron deficiency or excess leads to anemia, diabetes, and neurodegenerative disorders. All organisms have thus developed mechanisms to sense, acquire, and store iron. We use Caenorhabditis elegans as a model organism to study mechanisms of iron regulation. Our previous studies show that the iron-storage protein ferritin (FTN-1, FTN-2) is transcriptionally inhibited in intestine during iron deficiency, but the mechanisms regulating iron regulation are not known. Here, we find that hypoxia-inducible factor 1 (HIF-1) transcriptionally inhibits ftn-1 and ftn-2 during iron deficiency. We also show that HIF-1 activates the iron uptake gene smf-3. Transcriptional activation and inhibition by HIF-1 is dependent on an iron enhancer in the promoters of these genes. HIF-1 is a known transcriptional activator, but its role in transcriptional inhibition is not well understood. Our data show that HIF-1 regulates iron homeostasis by activating and inhibiting iron uptake and storage genes, and they provide insight into HIF-1 transcriptional inhibition.

The conserved role and divergent regulation of foxa, a pan-eumetazoan developmental regulatory gene

Foxa is a forkhead transcription factor that is expressed in the endoderm lineage across metazoans. Orthologs of foxa are expressed in cells that intercalate, polarize, and form tight junctions in the digestive tracts of the mouse, the sea urchin, and the nematode and in the chordate notochord. The loss of foxa expression eliminates these morphogenetic processes. The remarkable similarity in foxa phenotypes in these diverse organisms raises the following questions: why is the developmental role of Foxa so highly conserved? Is foxa transcriptional regulation as conserved as its developmental role? Comparison of the regulation of foxa orthologs in sea urchin and in Caenorhabditis elegans shows that foxa transcriptional regulation has diverged significantly between these two organisms, particularly in the cells that contribute to the C. elegans pharynx formation. We suggest that the similarity of foxa phenotype is due to its role in an ancestral gene regulatory network that controlled intercalation followed by mesenchymal-to-epithelial transition. foxa transcriptional regulation had evolved to support the developmental program in each species so foxa would play its role controlling morphogenesis at the necessary embryonic address.

In vitro and in vivo characterization of Caenorhabditis elegans PHA-4/FoxA response elements

Caenorhabditis elegans PHA-4 is a member of the FoxA group of transcription factors. PHA-4 is critical for development of the C. elegans pharynx and directly regulates most or all pharyngeal genes. The consensus binding site of PHA-4 has not been identified, with previous analysis of PHA-4 targets relying on the mammalian FoxA consensus. Here, we use in vitro and in vivo analyses to demonstrate three features of PHA-4 response elements. First, the PHA-4 consensus matches that of other FoxA proteins, but only a subset of possible sites is active in an in vivo assay. Second, sequence flanking the core PHA-4 site can influence the strength of reporter expression in vivo, as seen for other Fox proteins. Third, in the context of some pharyngeal promoters, PHA-4 response elements are flanked by distinct cis-regulatory elements that modulate response to PHA-4, generating gene expression in specific pharyngeal cell types.

Cloning and expression profiles of two isoforms of a CHH-like gene specifically expressed in male Chinese shrimp, Fenneropenaeus chinensis

Two full-length cDNA sequences (Fc-CHH1, Fc-CHH2) encoding a crustacean hyperglycemic hormone (CHH) precursor homolog and their DNA sequences were cloned from Chinese shrimp Fenneropenaeus chinensis. The deduced amino acid sequences of them are predicted to contain a signal peptide and a mature peptide. The mature peptides of Fc-CHH1 and Fc-CHH2 shared 78% identity, but they showed low identities (less than 40%) to CHH peptides from other species. Both Fc-CHH1 and Fc-CHH2 proteins contain six highly conserved cysteine residues which are characteristic of the CHH family peptides. The transcripts of Fc-CHH1 and Fc-CHH2 were shown to be specifically present in the spermatophore sac of mature male Chinese shrimp through reverse transcription-polymerase chain reaction (RT-PCR) detection. The transcripts of Fc-CHH1 and Fc-CHH2 begin to appear at the immature stage (115 days after the first post-larvae stage) when the spermatophore sac was first observed to be appeared. In situ hybridization analyses showed that Fc-CHH1 and Fc-CHH2 transcripts located at the epithelial cells in the internal wall of the spermatophore sac. In the cloned DNA sequences of Fc-CHH1 and Fc-CHH2, the predicted transcription factor binding sites in the 5' flanking sequences are different from those previously reported for CHH family genes of crustacean. To our knowledge, these are novel CHH-like genes expressed specifically in male shrimp. Their function needs to be further investigated.

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.