Specificity of TGFbeta signaling is conferred by distinct type I receptors and their associated SMAD proteins in Caenorhabditis elegans

BMP-Dependent Mobilization of Fatty Acid Metabolism Promotes Caenorhabditis elegans Survival on a Bacterial Pathogen

The Bone Morphogenetic Proteins (BMPs) are secreted peptide ligands of the Transforming Growth Factor beta (TGF-β) family, initially identified for their roles in development and differentiation across animal species. They are now increasingly recognized for their roles in physiology and infectious disease. In the nematode Caenorhabditis elegans, the BMP ligand DBL-1 controls fat metabolism and immune response, in addition to its roles in body size regulation and development. DBL-1 regulates classical aspects of innate immunity, including the induction of anti-microbial peptides. We theorized that BMP-dependent regulation of fat metabolism could also promote resilience against microbial pathogens. We found that exposure to a bacterial pathogen alters total fat stores, lipid droplet dynamics, and lipid metabolism gene expression in a BMP-dependent manner. We further showed that fatty acid desaturation plays a major role in survival on a bacterial pathogen, while fatty acid β-oxidation plays a more minor role. We conclude that C. elegans mobilizes fatty acid metabolism in response to pathogen exposure to promote survival. Our investigation provides a framework to study potential metabolic interventions that could support therapeutics that are complementary to antibiotic strategies.

ASI-RIM neuronal axis regulates systemic mitochondrial stress response via TGF-β signaling cascade

Morphogens play a critical role in coordinating stress adaptation and aging across tissues, yet their involvement in neuronal mitochondrial stress responses and systemic effects remains unclear. In this study, we reveal that the transforming growth factor beta (TGF-β) DAF-7 is pivotal in mediating the intestinal mitochondrial unfolded protein response (UPRmt) in Caenorhabditis elegans under neuronal mitochondrial stress. Two ASI sensory neurons produce DAF-7, which targets DAF-1/TGF-β receptors on RIM interneurons to orchestrate a systemic UPRmt response. Remarkably, inducing mitochondrial stress specifically in ASI neurons activates intestinal UPRmt, extends lifespan, enhances pathogen resistance, and reduces both brood size and body fat levels. Furthermore, dopamine positively regulates this UPRmt activation, while GABA acts as a systemic suppressor. This study uncovers the intricate mechanisms of systemic mitochondrial stress regulation, emphasizing the vital role of TGF-β in metabolic adaptations that are crucial for organismal fitness and aging during neuronal mitochondrial stress.

TGF-β ligand cross-subfamily interactions in the response of Caenorhabditis elegans to a bacterial pathogen

The Transforming Growth Factor beta (TGF-β) family consists of numerous secreted peptide growth factors that play significant roles in cell function, tissue patterning, and organismal homeostasis, including wound repair and immunity. Typically studied as homodimers, these ligands have the potential to diversify their functions through ligand interactions that may enhance, repress, or generate novel functions. In the nematode Caenorhabditis elegans, there are only five TGF-β ligands, providing an opportunity to dissect ligand interactions in fewer combinations than in vertebrates. As in vertebrates, these ligands can be divided into bone morphogenetic protein (BMP) and TGF-β/Activin subfamilies that predominantly signal through discrete signaling pathways. The BMP subfamily ligand DBL-1 has been well studied for its role in the innate immune response in C. elegans. Here we show that all five TGF-β ligands play a role in survival on bacterial pathogens. We also demonstrate that multiple TGF-β ligand pairs act nonredundantly as part of this response. We show that the two BMP-like ligands-DBL-1 and TIG-2-function independently of each other in the immune response, while TIG-2/BMP and the TGF-β/Activin-like ligand TIG-3 function together. Structural modeling supports the potential for TIG-2 and TIG-3 to form heterodimers. Additionally, we identify TIG-2 and TIG-3 as members of a rare subset of TGF-β ligands lacking the conserved cysteine responsible for disulfide linking mature dimers. Finally, we show that canonical DBL-1/BMP receptor and Smad signal transducers function in the response to bacterial pathogens, while components of the DAF-7 TGF-β/Activin signaling pathway do not play a major role in survival. These results demonstrate a novel potential for BMP and TGF-β/Activin subfamily ligands to interact and may provide a mechanism for distinguishing the developmental and homeostatic functions of these ligands from an acute response such as the innate immune response to bacterial pathogens.

Intestinal Mg2+ accumulation induced by cnnm mutations decreases the body size by suppressing TORC2 signaling in Caenorhabditis elegans

Mg2+ is a vital ion involved in diverse cellular functions by forming complexes with ATP. Intracellular Mg2+ levels are tightly regulated by the coordinated actions of multiple Mg2+ transporters, such as the Mg2+ efflux transporter, cyclin M (CNNM). Caenorhabditis elegans (C. elegans) worms with mutations in both cnnm-1 and cnnm-3 exhibit excessive Mg2+ accumulation in intestinal cells, leading to various phenotypic abnormalities. In this study, we investigated the mechanism underlying the reduction in body size in cnnm-1; cnnm-3 mutant worms. RNA interference (RNAi) of gtl-1, which encodes a Mg2+-intake channel in intestinal cells, restored the worm body size, confirming that this phenotype is due to excessive Mg2+ accumulation. Moreover, RNAi experiments targeting body size-related genes and analyses of mutant worms revealed that the suppression of the target of rapamycin complex 2 (TORC2) signaling pathway was involved in body size reduction, resulting in downregulated DAF-7 expression in head ASI neurons. As the DAF-7 signaling pathway suppresses dauer formation under stress, cnnm-1; cnnm-3 mutant worms exhibited a greater tendency to form dauer upon induction. Collectively, our results revealed that excessive accumulation of Mg2+ repressed the TORC2 signaling pathway in C. elegans worms and suggest the novel role of the DAF-7 signaling pathway in the regulation of their body size.

The adhesion GPCR and PCP component flamingo (FMI-1) alters body size and regulates the composition of the extracellular matrix

The extracellular matrix (ECM) is a network of macromolecules that presents a vital scaffold for cells and enables multiple ways of cellular communication. Thus, it is essential for many physiological processes such as development, tissue morphogenesis, homeostasis, the shape and partially the size of the body and its organs. To ensure these, the composition of the ECM is tissue-specific and highly dynamic. ECM homeostasis is therefore tightly controlled by several mechanisms. Here, we show that FMI-1, the homolog of the Adhesion GPCR Flamingo/CELSR/ADGRC in the nematode Caenorhabditis elegans, modulates the composition of the ECM by controlling the production both of ECM molecules such as collagens and also of ECM modifying enzymes. Thereby, FMI-1 affects the morphology and functionality of the nematode´s cuticle, which is mainly composed of ECM, and also modulates the body size. Mechanistic analyses highlight the fact that FMI-1 exerts its function from neurons non-cell autonomously (trans) solely via its extracellular N terminus. Our data support a model, by which the activity of the receptor, which has a well-described role in the planar cell polarity (PCP) pathway, involves the PCP molecule VANG-1, but seems to be independent of the DBL-1/BMP pathway.

The DBL-1/TGF-β signaling pathway tailors behavioral and molecular host responses to a variety of bacteria in Caenorhabditis elegans

Generating specific, robust protective responses to different bacteria is vital for animal survival. Here, we address the role of transforming growth factor β (TGF-β) member DBL-1 in regulating signature host defense responses in Caenorhabditis elegans to human opportunistic Gram-negative and Gram-positive pathogens. Canonical DBL-1 signaling is required to suppress avoidance behavior in response to Gram-negative, but not Gram-positive bacteria. We propose that in the absence of DBL-1, animals perceive some bacteria as more harmful. Animals activate DBL-1 pathway activity in response to Gram-negative bacteria and strongly repress it in response to select Gram-positive bacteria, demonstrating bacteria-responsive regulation of DBL-1 signaling. DBL-1 signaling differentially regulates expression of target innate immunity genes depending on the bacterial exposure. These findings highlight a central role for TGF-β in tailoring a suite of bacteria-specific host defenses.

TGF-β pathways in aging and immunity: lessons from Caenorhabditis elegans

The Transforming Growth Factor-β (TGF-β) superfamily of signaling molecules plays critical roles in development, differentiation, homeostasis, and disease. Due to the conservation of these ligands and their signaling pathways, genetic studies in invertebrate systems including the nematode Caenorhabditis elegans have been instrumental in identifying signaling mechanisms. C. elegans is also a premier organism for research in longevity and healthy aging. Here we summarize current knowledge on the roles of TGF-β signaling in aging and immunity.

TGF-β Ligand Cross-Subfamily Interactions in the Response of Caenorhabditis elegans to Bacterial Pathogens

The Transforming Growth Factor beta (TGF-β) family consists of numerous secreted peptide growth factors that play significant roles in cell function, tissue patterning, and organismal homeostasis, including wound repair and immunity. Typically studied as homodimers, these ligands have the potential to diversify their functions through ligand interactions that are synergistic, cooperative, additive, and/or antagonistic. In the nematode Caenorhabditis elegans, there are only five TGF-β ligands, providing an opportunity to dissect ligand interactions in fewer combinations than in vertebrates. As in vertebrates, these ligands can be divided into bone morphogenetic protein (BMP) and TGF-β/Activin subfamilies that predominantly signal through discrete signaling pathways. The BMP subfamily ligand DBL-1 has been well studied for its role in the innate immune response in C. elegans. Here we show that all five TGF-β ligands play a role in the immune response. We also demonstrate that multiple TGF-β ligands act cooperatively as part of this response. We show that the two BMP-like ligands - DBL-1 and TIG-2 - function independently of each other in the immune response, while TIG-2/BMP and the TGF-β/Activin-like ligand TIG-3 function cooperatively. Structural modeling supports the potential for TIG-2 and TIG-3 to form heterodimers. Finally, we show that canonical DBL-1/BMP receptor and Smad signal transducers function in the response to bacterial pathogens, while components of the DAF-7 TGF-β/Activin signaling pathway do not play a role in survival. These results demonstrate a novel potential for BMP and TGF-β/Activin subfamily ligands to interact, and may provide a mechanism for distinguishing the developmental and homeostatic functions of these ligands from an acute response such as the innate immune response to bacterial pathogens.

Atypical TGF-β signaling controls neuronal guidance in Caenorhabditis elegans

Coordinated expression of cell adhesion and signaling molecules is crucial for brain development. Here, we report that the Caenorhabditis elegans transforming growth factor β (TGF-β) type I receptor SMA-6 (small-6) acts independently of its cognate TGF-β type II receptor DAF-4 (dauer formation-defective-4) to control neuronal guidance. SMA-6 directs neuronal development from the hypodermis through interactions with three, orphan, TGF-β ligands. Intracellular signaling downstream of SMA-6 limits expression of NLR-1, an essential Neurexin-like cell adhesion receptor, to enable neuronal guidance. Together, our data identify an atypical TGF-β-mediated regulatory mechanism to ensure correct neuronal development.

Reciprocal interactions between transforming growth factor beta signaling and collagens: Insights from Caenorhabditis elegans

Studies in genetically tractable organisms such as the nematode Caenorhabditis elegans have led to pioneering insights into conserved developmental regulatory mechanisms. For example, Smad signal transducers for the transforming growth factor beta (TGF-β) superfamily were first identified in C. elegans and in the fruit fly Drosophila. Recent studies of TGF-β signaling and the extracellular matrix (ECM) in C. elegans have forged unexpected links between signaling and the ECM, yielding novel insights into the reciprocal interactions that occur across tissues and spatial scales, and potentially providing new opportunities for the study of biomechanical regulation of gene expression.

BMP pathway regulation of insulin signaling components promotes lipid storage in Caenorhabditis elegans

A small number of peptide growth factor ligands are used repeatedly in development and homeostasis to drive programs of cell differentiation and function. Cells and tissues must integrate inputs from these diverse signals correctly, while failure to do so leads to pathology, reduced fitness, or death. Previous work using the nematode C. elegans identified an interaction between the bone morphogenetic protein (BMP) and insulin/IGF-1-like signaling (IIS) pathways in the regulation of lipid homeostasis. The molecular components required for this interaction, however, were not fully understood. Here we report that INS-4, one of 40 insulin-like peptides (ILPs), is regulated by BMP signaling to modulate fat accumulation. Furthermore, we find that the IIS transcription factor DAF-16/FoxO, but not SKN-1/Nrf, acts downstream of BMP signaling in lipid homeostasis. Interestingly, BMP activity alters sensitivity of these two transcription factors to IIS-promoted cytoplasmic retention in opposite ways. Finally, we probe the extent of BMP and IIS interactions by testing additional IIS functions including dauer formation, aging, and autophagy induction. Coupled with our previous work and that of other groups, we conclude that BMP and IIS pathways have at least three modes of interaction: independent, epistatic, and antagonistic. The molecular interactions we identify provide new insight into mechanisms of signaling crosstalk and potential therapeutic targets for IIS-related pathologies such as diabetes and metabolic syndrome.

Tetraspanins TSP-12 and TSP-14 function redundantly to regulate the trafficking of the type II BMP receptor in Caenorhabditis elegans

Tetraspanins are a unique family of 4-pass transmembrane proteins that play important roles in a variety of cell biological processes. We have previously shown that 2 paralogous tetraspanins in Caenorhabditis elegans, TSP-12 and TSP-14, function redundantly to promote bone morphogenetic protein (BMP) signaling. The underlying molecular mechanisms, however, are not fully understood. In this study, we examined the expression and subcellular localization patterns of endogenously tagged TSP-12 and TSP-14 proteins. We found that TSP-12 and TSP-14 share overlapping expression patterns in multiple cell types, and that both proteins are localized on the cell surface and in various types of endosomes, including early, late, and recycling endosomes. Animals lacking both TSP-12 and TSP-14 exhibit reduced cell-surface levels of the BMP type II receptor DAF-4/BMPRII, along with impaired endosome morphology and mislocalization of DAF-4/BMPRII to late endosomes and lysosomes. These findings indicate that TSP-12 and TSP-14 are required for the recycling of DAF-4/BMPRII. Together with previous findings that the type I receptor SMA-6 is recycled via the retromer complex, our work demonstrates the involvement of distinct recycling pathways for the type I and type II BMP receptors and highlights the importance of tetraspanin-mediated intracellular trafficking in the regulation of BMP signaling in vivo. As TSP-12 and TSP-14 are conserved in mammals, our findings suggest that the mammalian TSP-12 and TSP-14 homologs may also function in regulating transmembrane protein recycling and BMP signaling.

Calcineurin tax-6 regulates male ray development and counteracts with kin-29 kinase in Caenorhabditis elegans

Phosphorylation is one of the critical protein modifications, which can lead to changing the activity of the proteins and regulating a variety of biological processes. Therefore, it is essential to properly maintain the phosphorylation level on proteins by balancing the activity of kinases and phosphatases. In this study, we report that calcineurin, a serine/threonine phosphatase, counteracts with a salt inducible kinase (SIK) to control male tail development in Caenorhabditis elegans. The counteracting regulation is cell lineage-dependent; the number of defective rays from T lineage in animals lacking calcineurin tax-6 is decreased by knock-down of SIK kin-29. This result is in contrast with the knock-down of bone marrow protein (BMP) receptor kinase sma-6, which slightly aggravates the T lineage defect. Also, sma-6 knock-down results in modest defect in ray 1 of V5 lineage in the absence of tax-6 activity. Finally, knock-down of a tyrosine phosphatase cdc-25.3 does not affect the defective ray phenotype of calcineurin tax-6 loss-of-function(lf) mutants. Altogether, these results suggest that balanced phosphorylation mediated by tax-6 and kin-29 is required for proper development of T lineage rays, and tax-6 and sma-6 may function in a parallel pathway in the developmental process of V5 lineage ray 1. This study emphasizes the elaborated developmental process of male ray formation, in which carefully coordinated expression of various genes is essential.

Human Marfan and Marfan-like Syndrome associated mutations lead to altered trafficking of the Type II TGFβ receptor in Caenorhabditis elegans

The transforming growth factor-β (TGFβ) family plays an important role in many developmental processes and when mutated often contributes to various diseases. Marfan syndrome is a genetic disease with an occurrence of approximately 1 in 5,000. The disease is caused by mutations in fibrillin, which lead to an increase in TGFβ ligand activity, resulting in abnormalities of connective tissues which can be life-threatening. Mutations in other components of TGFβ signaling (receptors, Smads, Schnurri) lead to similar diseases with attenuated phenotypes relative to Marfan syndrome. In particular, mutations in TGFβ receptors, most of which are clustered at the C-terminal end, result in Marfan-like (MFS-like) syndromes. Even though it was assumed that many of these receptor mutations would reduce or eliminate signaling, in many cases signaling is active. From our previous studies on receptor trafficking in C. elegans, we noticed that many of these receptor mutations that lead to Marfan-like syndromes overlap with mutations that cause mis-trafficking of the receptor, suggesting a link between Marfan-like syndromes and TGFβ receptor trafficking. To test this hypothesis, we introduced three of these key MFS and MFS-like mutations into the C. elegans TGFβ receptor and asked if receptor trafficking is altered. We find that in every case studied, mutated receptors mislocalize to the apical surface rather than basolateral surface of the polarized intestinal cells. Further, we find that these mutations result in longer animals, a phenotype due to over-stimulation of the nematode TGFβ pathway and, importantly, indicating that function of the receptor is not abrogated in these mutants. Our nematode models of Marfan syndrome suggest that MFS and MFS-like mutations in the type II receptor lead to mis-trafficking of the receptor and possibly provides an explanation for the disease, a phenomenon which might also occur in some cancers that possess the same mutations within the type II receptor (e.g. colon cancer).

The Caenorhabditis elegans SMOC-1 Protein Acts Cell Nonautonomously To Promote Bone Morphogenetic Protein Signaling

Bone morphogenetic protein (BMP) signaling regulates many different developmental and homeostatic processes in metazoans. The BMP pathway is conserved in Caenorhabditis elegans, and is known to regulate body size and mesoderm development. We have identified the C. elegans smoc-1 (Secreted MOdular Calcium-binding protein-1) gene as a new player in the BMP pathway. smoc-1(0) mutants have a small body size, while overexpression of smoc-1 leads to a long body size and increased expression of the RAD-SMAD (reporter acting downstream of SMAD) BMP reporter, suggesting that SMOC-1 acts as a positive modulator of BMP signaling. Using double-mutant analysis, we showed that SMOC-1 antagonizes the function of the glypican LON-2 and acts through the BMP ligand DBL-1 to regulate BMP signaling. Moreover, SMOC-1 appears to specifically regulate BMP signaling without significant involvement in a TGFβ-like pathway that regulates dauer development. We found that smoc-1 is expressed in multiple tissues, including cells of the pharynx, intestine, and posterior hypodermis, and that the expression of smoc-1 in the intestine is positively regulated by BMP signaling. We further established that SMOC-1 functions cell nonautonomously to regulate body size. Human SMOC1 and SMOC2 can each partially rescue the smoc-1(0) mutant phenotype, suggesting that SMOC-1's function in modulating BMP signaling is evolutionarily conserved. Together, our findings highlight a conserved role of SMOC proteins in modulating BMP signaling in metazoans.

Mutagenesis and Imaging Studies of BMP Signaling Mechanisms in C. elegans

C. elegans has played a central role in the elucidation of the TGFβ pathway over the last two decades. This is due to the high conservation of the pathway components and the power of genetic and cell biological approaches applied toward understanding how the pathway signals. In Subheading 3, we detail approaches to study the BMP branch of the TGFβ pathway in C. elegans.

From "the Worm" to "the Worms" and Back Again: The Evolutionary Developmental Biology of Nematodes

Since the earliest days of research on nematodes, scientists have noted the developmental and morphological variation that exists within and between species. As various cellular and developmental processes were revealed through intense focus on Caenorhabditis elegans, these comparative studies have expanded. Within the genus Caenorhabditis, they include characterization of intraspecific polymorphisms and comparisons of distinct species, all generally amenable to the same laboratory culture methods and supported by robust genomic and experimental tools. The C. elegans paradigm has also motivated studies with more distantly related nematodes and animals. Combined with improved phylogenies, this work has led to important insights about the evolution of nematode development. First, while many aspects of C. elegans development are representative of Caenorhabditis, and of terrestrial nematodes more generally, others vary in ways both obvious and cryptic. Second, the system has revealed several clear examples of developmental flexibility in achieving a particular trait. This includes developmental system drift, in which the developmental control of homologous traits has diverged in different lineages, and cases of convergent evolution. Overall, the wealth of information and experimental techniques developed in C. elegans is being leveraged to make nematodes a powerful system for evolutionary cellular and developmental biology.

The TGF-β Family in Caenorhabditis elegans

Transforming growth factor β (TGF-β) and related ligands have potent effects on an enormous diversity of biological functions in all animals examined. Because of the strong conservation of TGF-β family ligand functions and signaling mechanisms, studies from multiple animal systems have yielded complementary and synergistic insights. In the nematode Caenorhabditis elegans, early studies were instrumental in the elucidation of TGF-β family signaling mechanisms. Current studies in C. elegans continue to identify new functions for the TGF-β family in this organism as well as new conserved mechanisms of regulation.

Cell-to-cell spread of microsporidia causes Caenorhabditis elegans organs to form syncytia

The growth of pathogens is dictated by their interactions with the host environment1. Obligate intracellular pathogens undergo several cellular decisions as they progress through their life cycles inside host cells2. We have studied this process for microsporidian species in the genus Nematocida as they grew and developed inside their co-evolved animal host, Caenorhabditis elegans3-5. We found that microsporidia can restructure multicellular host tissues into a single contiguous multinucleate cell. In particular, we found that all three Nematocida species we studied were able to spread across the cells of C. elegans tissues before forming spores, with two species causing syncytial formation in the intestine and one species causing syncytial formation in the muscle. We also found that the decision to switch from replication to differentiation in Nematocida parisii was altered by the density of infection, suggesting that environmental cues influence the dynamics of the pathogen life cycle. These findings show how microsporidia can maximize the use of host space for growth and that environmental cues in the host can regulate a developmental switch in the pathogen.

dbl-1/TGF-β and daf-12/NHR Signaling Mediate Cell-Nonautonomous Effects of daf-16/FOXO on Starvation-Induced Developmental Arrest

Nutrient availability has profound influence on development. In the nematode C. elegans, nutrient availability governs post-embryonic development. L1-stage larvae remain in a state of developmental arrest after hatching until they feed. This “L1 arrest” (or "L1 diapause") is associated with increased stress resistance, supporting starvation survival. Loss of the transcription factor daf-16/FOXO, an effector of insulin/IGF signaling, results in arrest-defective and starvation-sensitive phenotypes. We show that daf-16/FOXO regulates L1 arrest cell-nonautonomously, suggesting that insulin/IGF signaling regulates at least one additional signaling pathway. We used mRNA-seq to identify candidate signaling molecules affected by daf-16/FOXO during L1 arrest. dbl-1/TGF-β, a ligand for the Sma/Mab pathway, daf-12/NHR and daf-36/oxygenase, an upstream component of the daf-12 steroid hormone signaling pathway, were up-regulated during L1 arrest in a daf-16/FOXO mutant. Using genetic epistasis analysis, we show that dbl-1/TGF-β and daf-12/NHR steroid hormone signaling pathways are required for the daf-16/FOXO arrest-defective phenotype, suggesting that daf-16/FOXO represses dbl-1/TGF-β, daf-12/NHR and daf-36/oxygenase. The dbl-1/TGF-β and daf-12/NHR pathways have not previously been shown to affect L1 development, but we found that disruption of these pathways delayed L1 development in fed larvae, consistent with these pathways promoting development in starved daf-16/FOXO mutants. Though the dbl-1/TGF-β and daf-12/NHR pathways are epistatic to daf-16/FOXO for the arrest-defective phenotype, disruption of these pathways does not suppress starvation sensitivity of daf-16/FOXO mutants. This observation uncouples starvation survival from developmental arrest, indicating that DAF-16/FOXO targets distinct effectors for each phenotype and revealing that inappropriate development during starvation does not cause the early demise of daf-16/FOXO mutants. Overall, this study shows that daf-16/FOXO promotes developmental arrest cell-nonautonomously by repressing pathways that promote larval development.

Animals must cope with feast and famine in the wild. Environmental fluctuations require a balancing act between development in favorable conditions and survival during starvation. Disruption of the pathways that govern this balance can lead to cancer, where cells proliferate when they should not, and metabolic diseases, where nutrient sensing is impaired. In the roundworm Caenorhabditis elegans, larval development is controlled by nutrient availability. Larvae are able to survive starvation by stopping development and starting again after feeding. Stopping and starting development in this multicellular animal requires signaling to coordinate development across tissues and organs. How such coordination is accomplished is poorly understood. Insulin/insulin-like growth factor (IGF) signaling governs larval development in response to nutrient availability. Here we show that insulin/IGF signaling activity in one tissue can affect the development of other tissues, suggesting regulation of additional signaling pathways. We identified two pathways that promote development in fed larvae and are repressed by lack of insulin/IGF signaling in starved larvae. Repression of these pathways is crucial to stopping development throughout the animal during starvation. These three pathways are widely conserved and associated with disease, suggesting the nutrient-dependent regulatory network they comprise is important to human health.

Promotion of bone morphogenetic protein signaling by tetraspanins and glycosphingolipids

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor β (TGFβ) superfamily of secreted molecules. BMPs play essential roles in multiple developmental and homeostatic processes in metazoans. Malfunction of the BMP pathway can cause a variety of diseases in humans, including cancer, skeletal disorders and cardiovascular diseases. Identification of factors that ensure proper spatiotemporal control of BMP signaling is critical for understanding how this pathway is regulated. We have used a unique and sensitive genetic screen to identify the plasma membrane-localized tetraspanin TSP-21 as a key new factor in the C. elegans BMP-like “Sma/Mab” signaling pathway that controls body size and postembryonic M lineage development. We showed that TSP-21 acts in the signal-receiving cells and genetically functions at the ligand-receptor level. We further showed that TSP-21 can associate with itself and with two additional tetraspanins, TSP-12 and TSP-14, which also promote Sma/Mab signaling. TSP-12 and TSP-14 can also associate with SMA-6, the type I receptor of the Sma/Mab pathway. Finally, we found that glycosphingolipids, major components of the tetraspanin-enriched microdomains, are required for Sma/Mab signaling. Our findings suggest that the tetraspanin-enriched membrane microdomains are important for proper BMP signaling. As tetraspanins have emerged as diagnostic and prognostic markers for tumor progression, and TSP-21, TSP-12 and TSP-14 are all conserved in humans, we speculate that abnormal BMP signaling due to altered expression or function of certain tetraspanins may be a contributing factor to cancer development.

The bone morphogenetic protein (BMP) signaling pathway is required for multiple developmental processes during metazoan development. Various diseases, including cancer, can result from mis-regulation of the BMP pathway. Thus, it is critical to identify factors that ensure proper regulation of BMP signaling. Using the nematode C. elegans, we have devised a highly specific and sensitive genetic screen to identify new modulators in the BMP pathway. Through this screen, we identified three conserved tetraspanin molecules as novel factors that function to promote BMP signaling in a living organism. We further showed that these three tetraspanins likely form a complex and function together with glycosphingolipids to promote BMP signaling. Recent studies have implicated several tetraspanins in cancer initiation, progression and metastasis in mammals. Our findings suggest that the involvement of tetraspanins in cancer may partially be due to their function in modulating the activity of BMP signaling.

Regulation of extracellular matrix organization by BMP signaling in Caenorhabditis elegans

In mammals, Bone Morphogenetic Protein (BMP) pathway signaling is important for the growth and homeostasis of extracellular matrix, including basement membrane remodeling, scarring, and bone growth. A conserved BMP member in Caenorhabditis elegans, DBL-1, regulates body length in a dose-sensitive manner. Loss of DBL-1 pathway signaling also results in increased anesthetic sensitivity. However, the physiological basis of these pleiotropic phenotypes is largely unknown. We created a DBL-1 over-expressing strain and show that sensitivity to anesthetics is inversely related to the dose of DBL-1. Using pharmacological, genetic analyses, and a novel dye permeability assay for live, microwave-treated animals, we confirm that DBL-1 is required for the barrier function of the cuticle, a specialized extracellular matrix. We show that DBL-1 signaling is required to prevent animals from forming tail-entangled aggregates in liquid. Stripping lipids off the surface of wild-type animals recapitulates this phenotype. Finally, we find that DBL-1 signaling affects ultrastructure of the nematode cuticle in a dose-dependent manner, as surface lipid content and cuticular organization are disrupted in animals with genetically altered DBL-1 levels. We propose that the lipid layer coating the nematode cuticle normally prevents tail entanglement, and that reduction of this layer by loss of DBL-1 signaling promotes aggregation. This work provides a physiological mechanism that unites the DBL-1 signaling pathway roles of not only body size regulation and drug responsiveness, but also the novel Hoechst 33342 staining and aggregation phenotypes, through barrier function, content, and organization of the cuticle.

CEH-28 activates dbl-1 expression and TGF-β signaling in the C. elegans M4 neuron

M4 is a multifunctional neuron in the Caenorhabditis elegans pharynx that can both stimulate peristaltic contractions of the muscles in the pharyngeal isthmus and function systemically to regulate an enhanced sensory response under hypoxic conditions. Here we identify a third function for M4 that depends on activation of the TGF-β family gene dbl-1 by the homeodomain transcription factor CEH-28. dbl-1 is expressed in M4 and a subset of other neurons, and we show CEH-28 specifically activates dbl-1 expression in M4. Characterization of the dbl-1 promoter indicates that CEH-28 targets an M4-specific enhancer within the dbl-1 promoter region, while expression in other neurons is mediated by separate regulatory sequences. Unlike ceh-28 mutants, dbl-1 mutants do not exhibit M4 synaptic and signaling defects. Instead, both ceh-28 and dbl-1 mutants exhibit morphological defects in the g1 gland cells located adjacent to M4 in the pharynx, and these defects can be partially rescued by M4-specific expression of dbl-1 in these mutants. Identical gland cell defects are observed in sma-6 and daf-4 mutants defective in the receptor for DBL-1, but they are not observed in sma-2 and sma-3 mutants lacking the R-Smads functioning downstream of this receptor. Together these results identify a novel neuroendocrine function for M4 and provide evidence for an R-Smad-independent mechanism for DBL-1 signaling in C. elegans.

The olfactory neuron AWC promotes avoidance of normally palatable food following chronic dietary restriction

Changes in metabolic state alter foraging behavior and food preference in animals. Here, I show that normally attractive food becomes repulsive to Caenorhabditis elegans if animals are chronically undernourished as a result of alimentary tract defects. This behavioral plasticity is achieved in two ways: increased food leaving and induction of aversive behavior towards food. A particularly strong food avoider is defective in the chitin synthase that makes the pharyngeal lining. Food avoidance induced by underfeeding is mediated by cGMP signaling in the olfactory neurons AWC and AWB, and the gustatory neurons ASJ and ASK. Food avoidance is enhanced by increased population density and is reduced if the animals are unable to correctly interpret their nutritional state as a result of defects in the AMP kinase or TOR/S6kinase pathways. The TGF-β/DBL-1 pathway suppresses food avoidance and the cellular basis for this is distinct from its role in aversive olfactory learning of harmful food. This study suggests that nutritional state feedback via nutrient sensors, population size and olfactory neurons guides food preference in C. elegans.

Loss of MEC-17 leads to microtubule instability and axonal degeneration

Axonal degeneration arises as a consequence of neuronal injury and is a common hallmark of a number of neurodegenerative diseases. However, the genetic causes and the cellular mechanisms that trigger this process are still largely unknown. Based on forward genetic screening in C. elegans, we have identified the α-tubulin acetyltransferase gene mec-17 as causing spontaneous, adult-onset, and progressive axonal degeneration. Loss of MEC-17 leads to microtubule instability, a reduction in mitochondrial number, and disrupted axonal transport, with altered distribution of both mitochondria and synaptic components. Furthermore, mec-17-mediated axonal degeneration occurs independently from its acetyltransferase domain; is enhanced by mutation of coel-1, a tubulin-associated molecule; and correlates with the animal's body length. This study therefore identifies a critical role for the conserved microtubule-associated protein MEC-17 in preserving axon integrity and preventing axonal degeneration.

The neogenin/DCC homolog UNC-40 promotes BMP signaling via the RGM protein DRAG-1 in C. elegans

The deleted in colorectal cancer (DCC) homolog neogenin functions in both netrin- and repulsive guidance molecule (RGM)-mediated axon guidance and in bone morphogenetic protein (BMP) signaling. How neogenin functions in mediating BMP signaling is not well understood. We show that the sole C. elegans DCC/neogenin homolog UNC-40 positively modulates a BMP-like pathway by functioning in the signal-receiving cells at the ligand/receptor level. This function of UNC-40 is independent of its role in netrin-mediated axon guidance, but requires its association with the RGM protein DRAG-1. We have identified the key residues in the extracellular domain of UNC-40 that are crucial for UNC-40-DRAG-1 interaction and UNC-40 function. Surprisingly, the extracellular domain of UNC-40 is sufficient to promote BMP signaling, in clear contrast to the requirement of its intracellular domain in mediating axon guidance. Mouse neogenin lacking the intracellular domain is also capable of mediating BMP signaling. These findings reveal an unexpected mode of action for neogenin regulation of BMP signaling.

RNAseq analysis of the parasitic nematode Strongyloides stercoralis reveals divergent regulation of canonical dauer pathways

The infectious form of many parasitic nematodes, which afflict over one billion people globally, is a developmentally arrested third-stage larva (L3i). The parasitic nematode Strongyloides stercoralis differs from other nematode species that infect humans, in that its life cycle includes both parasitic and free-living forms, which can be leveraged to investigate the mechanisms of L3i arrest and activation. The free-living nematode Caenorhabditis elegans has a similar developmentally arrested larval form, the dauer, whose formation is controlled by four pathways: cyclic GMP (cGMP) signaling, insulin/IGF-1-like signaling (IIS), transforming growth factor β (TGFβ) signaling, and biosynthesis of dafachronic acid (DA) ligands that regulate a nuclear hormone receptor. We hypothesized that homologous pathways are present in S. stercoralis, have similar developmental regulation, and are involved in L3i arrest and activation. To test this, we undertook a deep-sequencing study of the polyadenylated transcriptome, generating over 2.3 billion paired-end reads from seven developmental stages. We constructed developmental expression profiles for S. stercoralis homologs of C. elegans dauer genes identified by BLAST searches of the S. stercoralis genome as well as de novo assembled transcripts. Intriguingly, genes encoding cGMP pathway components were coordinately up-regulated in L3i. In comparison to C. elegans, S. stercoralis has a paucity of genes encoding IIS ligands, several of which have abundance profiles suggesting involvement in L3i development. We also identified seven S. stercoralis genes encoding homologs of the single C. elegans dauer regulatory TGFβ ligand, three of which are only expressed in L3i. Putative DA biosynthetic genes did not appear to be coordinately regulated in L3i development. Our data suggest that while dauer pathway genes are present in S. stercoralis and may play a role in L3i development, there are significant differences between the two species. Understanding the mechanisms governing L3i development may lead to novel treatment and control strategies.

Parasitic nematodes infect over one billion people worldwide and cause many diseases, including strongyloidiasis, filariasis, and hookworm disease. For many of these parasites, including Strongyloides stercoralis, the infectious form is a developmentally arrested and long-lived thirdstage larva (L3i). Upon encountering a host, L3i quickly resume development and mature into parasitic adults. In the free-living nematode Caenorhabditis elegans, a similar developmentally arrested third-stage larva, known as the dauer, is regulated by four key cellular mechanisms. We hypothesized that similar cellular mechanisms control L3i arrest and activation. Therefore, we used deep-sequencing technology to characterize the S. stercoralis transcriptome (RNAseq), which allowed us to identify S. stercoralis homologs of components of these four mechanisms and examine their temporal regulation. We found similar temporal regulation between S. stercoralis and C. elegans for components of two mechanisms, but dissimilar temporal regulation for two others, suggesting conserved as well as novel modes of developmental regulation for L3i. Understanding L3i development may lead to novel control strategies as well as new treatments for strongyloidiasis and other diseases caused by parasitic nematodes.

DBL-1, a TGF-β, is essential for Caenorhabditis elegans aversive olfactory learning

The TGF-β superfamily is conserved throughout metazoan, and its members play essential roles in development and disease. TGF-β has also been implicated in adult neural plasticity. However, the underlying mechanisms are not well understood. Here we report that DBL-1, a Caenorhabditis elegans TGF-β homolog known to control body morphology and immunity, is essential for aversive olfactory learning of potentially harmful bacteria food. We show that DBL-1 generated by the AVA command interneurons, which are critical for sensorimotor responses, regulates aversive olfactory learning, and that the activity of the type I TGF-β receptor SMA-6 in the hypodermis is needed during adulthood to generate olfactory plasticity. These spatial and temporal mechanisms are critical for the DBL-1 signaling to achieve its diverse functions in development and adult neural plasticity. Interestingly, aversive training decreases AVA calcium response, leading to an increase in the DBL-1 signal secreted from AVA, revealing an experience-dependent change that can underlie the role of TGF-β signaling in mediating plasticity.

C. elegans ADAMTS ADT-2 regulates body size by modulating TGFβ signaling and cuticle collagen organization

Organismal growth and body size are influenced by both genetic and environmental factors. We have utilized the strong molecular genetic techniques available in the nematode Caenorhabditis elegans to identify genetic determinants of body size. In C. elegans, DBL-1, a member of the conserved family of secreted growth factors known as the Transforming Growth Factor β superfamily, is known to play a major role in growth control. The mechanisms by which other determinants of body size function, however, is less well understood. To identify additional genes involved in body size regulation, a genetic screen for small mutants was previously performed. One of the genes identified in that screen was sma-21. We now demonstrate that sma-21 encodes ADT-2, a member of the ADAMTS (a disintegrin and metalloprotease with thrombospondin motifs) family of secreted metalloproteases. ADAMTS proteins are believed to remodel the extracellular matrix and may modulate the activity of extracellular signals. Genetic interactions suggest that ADT-2 acts in parallel with or in multiple size regulatory pathways. We demonstrate that ADT-2 is required for normal levels of expression of a DBL-1-responsive transcriptional reporter. We further demonstrate that adt-2 regulatory sequences drive expression in glial-like and vulval cells, and that ADT-2 activity is required for normal cuticle collagen fibril organization. We therefore propose that ADT-2 regulates body size both by modulating TGFβ signaling activity and by maintaining normal cuticle structure.

mab-31 and the TGF-beta pathway act in the ray lineage to pattern C. elegans male sensory rays

Background

C. elegans TGF-β-like Sma/Mab signaling pathway regulates both body size and sensory ray patterning. Most of the components in this pathway were initially identified by genetic screens based on the small body phenotype, and many of these mutants display sensory ray patterning defect. At the cellular level, little is known about how and where these components work although ray structural cell has been implicated as one of the targets. Based on the specific ray patterning abnormality, we aim to identify by RNAi approach additional components that function specifically in the ray lineage to elucidate the regulatory role of TGF-β signaling in ray differentiation.

Result

We report here the characterization of a new member of the Sma/Mab pathway, mab-31, recovered from a genome-wide RNAi screen. mab-31 mutants showed ray cell cluster patterning defect and mis-specification of the ray identity. mab-31 encodes a nuclear protein expressed in descendants of ray precursor cells impacting on the ray cell's clustering properties, orientation of cell division plane, and fusion of structural cells. Genetic experiments also establish its relationship with other Sma/Mab pathway components and transcription factors acting upstream and downstream of the signaling event.

Conclusion

mab-31 function is indispensable in Sma/Mab signal recipient cells during sensory rays specification. Both mab-31 and sma-6 are required in ray lineage at the late larval stages. They act upstream of C. elegans Pax-6 homolog and repress its function. These findings suggested mab-31 is a key factor that can integrate TFG-β signals in male sensory ray lineage to define organ identity.

The RGM protein DRAG-1 positively regulates a BMP-like signaling pathway in Caenorhabditis elegans

The bone morphogenetic protein (BMP) signaling pathway regulates multiple developmental and homeostatic processes. Mutations in the pathway can cause a variety of somatic and hereditary disorders in humans. Multiple levels of regulation, including extracellular regulation, ensure proper spatiotemporal control of BMP signaling in the right cellular context. We have identified a modulator of the BMP-like Sma/Mab pathway in C. elegans called DRAG-1. DRAG-1 is the sole member of the repulsive guidance molecule (RGM) family of proteins in C. elegans, and is crucial in regulating body size and mesoderm development. Using a combination of molecular genetic and biochemical analyses, we demonstrate that DRAG-1 is a membrane-associated protein that functions at the ligand-receptor level to modulate the Sma/Mab pathway in a cell-type-specific manner. We further show that DRAG-1 positively modulates this BMP-like pathway by using a novel Sma/Mab-responsive reporter. Our work provides a direct link between RGM proteins and BMP signaling in vivo and a simple and genetically tractable system for mechanistic studies of RGM protein regulation of BMP pathways.

Regulation of genes affecting body size and innate immunity by the DBL-1/BMP-like pathway in Caenorhabditis elegans

BACKGROUND

Bone morphogenetic proteins (BMPs) are members of the conserved transforming growth factor beta (TGFbeta superfamily, and play many developmental and homeostatic roles. In C. elegans, a BMP-like pathway, the DBL-1 pathway, controls body size and is involved in innate immunity. How these functions are carried out, though, and what most of the downstream targets of this pathway are, remain unknown.

RESULTS

We performed a microarray analysis and compared expression profiles of animals lacking the SMA-6 DBL-1 receptor, which decreases pathway signaling, with animals that overexpress DBL-1 ligand, which increases pathway signaling. Consistent with a role for DBL-1 in control of body size, we find positive regulation by DBL-1 of genes involved in physical structure, protein synthesis and degradation, and metabolism. However, cell cycle genes were mostly absent from our results. We also identified genes in a hedgehog-related pathway, which may comprise a secondary signaling pathway downstream of DBL-1 that controls body size. In addition, DBL-1 signaling up-regulates pro-innate immunity genes. We identified a reporter for DBL-1 signaling, which is normally repressed but is up-regulated when DBL-1 signaling is reduced.

CONCLUSIONS

Our results indicate that body size in C. elegans is controlled in part by regulation of metabolic processes as well as protein synthesis and degradation. This supports the growing body of evidence that suggests cell size is linked to metabolism. Furthermore, this study discovered a possible role for hedgehog-related pathways in transmitting the BMP-like signal from the hypodermis, where the core DBL-1 pathway components are required, to other tissues in the animal. We also identified the up-regulation of genes involved in innate immunity, clarifying the role of DBL-1 in innate immunity. One of the highly regulated genes is expressed at very low levels in wild-type animals, but is strongly up-regulated in Sma/Mab mutants, making it a useful reporter for DBL-1/BMP-like signaling in C. elegans.

Caenorhabditis elegans SMA-10/LRIG is a conserved transmembrane protein that enhances bone morphogenetic protein signaling

Bone morphogenetic protein (BMP) pathways control an array of developmental and homeostatic events, and must themselves be exquisitely controlled. Here, we identify Caenorhabditis elegans SMA-10 as a positive extracellular regulator of BMP-like receptor signaling. SMA-10 acts genetically in a BMP-like (Sma/Mab) pathway between the ligand DBL-1 and its receptors SMA-6 and DAF-4. We cloned sma-10 and show that it has fifteen leucine-rich repeats and three immunoglobulin-like domains, hallmarks of an LRIG subfamily of transmembrane proteins. SMA-10 is required in the hypodermis, where the core Sma/Mab signaling components function. We demonstrate functional conservation of LRIGs by rescuing sma-10(lf) animals with the Drosophila ortholog lambik, showing that SMA-10 physically binds the DBL-1 receptors SMA-6 and DAF-4 and enhances signaling in vitro. This interaction is evolutionarily conserved, evidenced by LRIG1 binding to vertebrate receptors. We propose a new role for LRIG family members: the positive regulation of BMP signaling by binding both Type I and Type II receptors.

Regulation of protein turnover by longevity pathways

Cellular homeostasis, which is needed for the cells to survive, requires a well-controlled balance in protein turnover. Both protein synthesis and degradation are influenced by distinct genetic pathways that control aging in divergent eukaryotic species. These conserved mechanisms involve the insulin/IGF-1 (Insulin-like Growth Factor-i), TGF-I (Transforming Growth Factor-beta), JNK (c-Jun terminal kinase), RTK/Ras/MAPK (Receptor Tyrosine Kinase/ Ras/Mitogen-Activated Protein Kinase) and TOR (kinase Target Of Rapamycin) signaling cascades and the mitochondrial respiratory system-each of them promotes protein synthesis; as well as the intracellular protein degradation machineries, including the ubiquitin-proteasome system and lysosome-mediated autophagy. In addition to providing building blocks for generation of new proteins and fuelling the cell with energy under starvation, the protein degradation processes eliminate damaged, nonfunctional proteins, the accumulation of which serves as the primary contributory factor to aging. Interestingly, a complex, intimate regulatory relationship exists between mechanisms promoting protein synthesis and those mediating protein degradation: under certain circumstances the former downregulate the latter. Thus, conditions that favor protein synthesis can enhance the rate at which damaged proteins accumulate. This may explain why genetic interventions and environmental factors (e.g., dietary restriction) that reduce protein synthesis, at least to tolerable levels, extend lifespan and increase resistance to cellular stress in various experimental model organisms of aging. In this chapter, the molecular mechanisms by which protein synthesis-promoting longevity pathways and protein degradation pathways interact with each other are discussed.

TGF-beta Sma/Mab signaling mutations uncouple reproductive aging from somatic aging

Female reproductive cessation is one of the earliest age-related declines humans experience, occurring in mid-adulthood. Similarly, Caenorhabditis elegans' reproductive span is short relative to its total life span, with reproduction ceasing about a third into its 15–20 day adulthood. All of the known mutations and treatments that extend C. elegans' reproductive period also regulate longevity, suggesting that reproductive span is normally linked to life span. C. elegans has two canonical TGF-ß signaling pathways. We recently found that the TGF-ß Dauer pathway regulates longevity through the Insulin/IGF-1 Signaling (IIS) pathway; here we show that this pathway has a moderate effect on reproductive span. By contrast, TGF-ß Sma/Mab signaling mutants exhibit a substantially extended reproductive period, more than doubling reproductive span in some cases. Sma/Mab mutations extend reproductive span disproportionately to life span and act independently of known regulators of somatic aging, such as Insulin/IGF-1 Signaling and Dietary Restriction. This is the first discovery of a pathway that regulates reproductive span independently of longevity and the first identification of the TGF-ß Sma/Mab pathway as a regulator of reproductive aging. Our results suggest that longevity and reproductive span regulation can be uncoupled, although they appear to normally be linked through regulatory pathways.

Female reproductive cessation is the earliest aging phenotype humans experience, and its importance as a clinical issue is growing as more women opt to have children later in life. While much work has been done to understand the general aging process, little is currently known about the regulation of reproductive aging. Like longevity, the ability to produce progeny with advanced age is likely to be genetically regulated. Thus, understanding the processes that regulate reproductive aging may allow us to address the problems of maternal age-related infertility and birth defects. C. elegans and humans both have long post-reproductive life spans, leaving open the possibility that their reproductive spans might be extendable. C. elegans has been used previously to discover conserved regulators of aging, and here we use worms to identify a new regulator of reproductive aging, a highly conserved TGF-ß signaling pathway. We find that TGF-ß signaling regulates reproductive aging independently of somatic aging. This is the first identification of a pathway that breaks the coupling that normally links the two processes. Our work will provide new insights into the improvement of human fertility and prevention of age-related birth defects, and it has implications for the evolutionary relationship between reproduction and longevity regulation.

Information-based methods for predicting gene function from systematic gene knock-downs

BACKGROUND

The rapid annotation of genes on a genome-wide scale is now possible for several organisms using high-throughput RNA interference assays to knock down the expression of a specific gene. To date, dozens of RNA interference phenotypes have been recorded for the nematode Caenorhabditis elegans. Although previous studies have demonstrated the merit of using knock-down phenotypes to predict gene function, it is unclear how the data can be used most effectively. An open question is how to optimally make use of phenotypic observations, possibly in combination with other functional genomics datasets, to identify genes that share a common role.

RESULTS

We compared several methods for detecting gene-gene functional similarity from phenotypic knock-down profiles. We found that information-based measures, which explicitly incorporate a phenotype's genomic frequency when calculating gene-gene similarity, outperform non-information-based methods. We report the presence of newly predicted modules identified from an integrated functional network containing phenotypic congruency links derived from an information-based measure. One such module is a set of genes predicted to play a role in regulating body morphology based on their multiply-supported interactions with members of the TGF-beta signaling pathway.

CONCLUSION

Information-based metrics significantly improve the comparison of phenotypic knock-down profiles, based upon their ability to enhance gene function prediction and identify novel functional modules.

Neural and molecular dissection of a C. elegans sensory circuit that regulates fat and feeding

A major challenge in understanding energy balance is deciphering the neural and molecular circuits that govern behavioral, physiological, and metabolic responses of animals to fluctuating environmental conditions. The neurally expressed TGF-beta ligand DAF-7 functions as a gauge of environmental conditions to modulate energy balance in C. elegans. We show that daf-7 signaling regulates fat metabolism and feeding behavior through a compact neural circuit that allows for integration of multiple inputs and the flexibility for differential regulation of outputs. In daf-7 mutants, perception of depleting food resources causes fat accumulation despite reduced feeding rate. This fat accumulation is mediated, in part, through neural metabotropic glutamate signaling and upregulation of peripheral endogenous biosynthetic pathways that direct energetic resources into fat reservoirs. Thus, neural perception of adverse environmental conditions can promote fat accumulation without a concomitant increase in feeding rate.

RNT-1 regulation in C. elegans

RUNXs are important transcription factors, which are involved in animal development and human carcinogenesis. RNT-1, the only homologue of RUNXs, in Caenorhabditis elegans (C. elegans) has been identified and viable mutant animals of rnt-1 gene have been isolated and characterized recently. Genetic analyses using rnt-1 mutants have shown that RNT-1 is regulated by TGFbeta- and Wnt-signaling pathways in the body size regulation and male tail development. Here, we review our current understanding of RNT-1 functions in these signaling pathways. Furthermore, future prospects of RNT-1 and BRO-1 studies in C. elegans are discussed in this review.

Regulation of rnt-1 expression mediated by the opposing effects of BRO-1 and DBL-1 in the nematode Caenorhabditis elegans

During development of Caenorhabditis elegans, expression of the RUNX homolog, rnt-1, is tightly regulated both spatially and temporally. In this study, we investigated the mechanism underlying the temporal regulation of rnt-1. We found that rnt-1 contained evolutionarily conserved consensus RUNX binding sequences within one of its introns, and that RNT-1 bound to these intronic sequences both in vitro and in vivo in the presence of BRO-1, suggesting that RNT-1 together with BRO-1 represses its own transcription. Fine deletion and substitution experiments revealed a binding site within the intron that was critical for rnt-1 regulation. Importantly, we found that the TGFbeta homolog, DBL-1, was required for counteracting the repressive activity of BRO-1 at postembryonic stages. Accordingly, ectopic expression of DBL-1 induced transcription of rnt-1 in the lateral hypodermis and other tissues even at the postembryonic stages. Taken together, our data suggest that rnt-1 expression is regulated by the balance between DBL-1-mediated activation and BRO-1-mediated repression at the postembryonic stages.

Autophagy genes unc-51 and bec-1 are required for normal cell size in Caenorhabditis elegans

Here we show that in the nematode Caenorhabditis elegans mutational inactivation of two autophagy genes unc-51/atg1 and bec-1/atg6/beclin1 results in small body size without affecting cell number. Furthermore, loss-of-function mutations in unc-51 and bec-1 suppress the giant phenotype of mutant animals with aberrant insulin-like growth factor-1 (insulin/IGF-1) or transforming growth factor-beta (TGF-beta) signaling. This function for unc-51 and bec-1 in cell size control and their interaction with these two growth modulatory pathways may represent a link between the hormonal and nutritional regulation of cell growth.

crm-1 facilitates BMP signaling to control body size in Caenorhabditis elegans

We have identified in Caenorhabditis elegans a homologue of the vertebrate Crim1, crm-1, which encodes a putative transmembrane protein with multiple cysteine-rich (CR) domains known to have bone morphogenetic proteins (BMPs) binding activity. Using the body morphology of C. elegans as an indicator, we showed that attenuation of crm-1 activity leads to a small body phenotype reminiscent of that of BMP pathway mutants. We showed that the crm-1 loss-of-function phenotype can be rescued by constitutive supply of sma-4 activity. crm-1 can enhance BMP signaling and this activity is dependent on the presence of the DBL-1 ligand and its receptors. crm-1 is expressed in neurons at the ventral nerve cord, where the DBL-1 ligand is produced. However, ectopic expression experiments reveal that crm-1 gene products act outside the DBL-1 producing cells and function non-autonomously to facilitate dbl/sma pathway signaling to control body size.

Cell size and invasion in TGF-beta-induced epithelial to mesenchymal transition is regulated by activation of the mTOR pathway

Epithelial to mesenchymal transition (EMT) occurs during development and cancer progression to metastasis and results in enhanced cell motility and invasion. Transforming growth factor-β (TGF-β) induces EMT through Smads, leading to transcriptional regulation, and through non-Smad pathways. We observe that TGF-β induces increased cell size and protein content during EMT. This translational regulation results from activation by TGF-β of mammalian target of rapamycin (mTOR) through phosphatidylinositol 3-kinase and Akt, leading to the phosphorylation of S6 kinase 1 and eukaryotic initiation factor 4E–binding protein 1, which are direct regulators of translation initiation. Rapamycin, a specific inhibitor of mTOR complex 1, inhibits the TGF-β–induced translation pathway and increase in cell size without affecting the EMT phenotype. Additionally, rapamycin decreases the migratory and invasive behavior of cells that accompany TGF-β–induced EMT. The TGF-β–induced translation pathway through mTOR complements the transcription pathway through Smads. Activation of mTOR by TGF-β, which leads to increased cell size and invasion, adds to the role of TGF-β–induced EMT in cancer progression and may represent a therapeutic opportunity for rapamycin analogues in cancer.

Mutants carrying two sma mutations are super small in the nematode C. elegans

Body size determination is critical for multicellular organisms; however, the mechanisms remain largely unknown. Mutations that alter body size were studied to solve the mechanisms, for example, in mouse, fruit fly and the nematode Caenorhabditis elegans. In C. elegans, a large mutant and several small body size (sma) mutants are known. Of the latter, sma-2, sma-3, sma-4, sma-6, dbl-1 and daf-4 have a mutation in the components of the DBL-1/TGFbeta signal pathway, and sma-5 in a MAP kinase homologue. We have constructed double mutants carrying two of such small body size mutations, sma-5 and sma-4 or sma-2. They are much smaller than either of the parental single mutants, indicating that the sma-5 gene functions independently of the DBL-1/TGFbeta pathway. We show that their body volumes are as small as 1/10 of that of the wild-type, and that the sizes of major organs are much reduced, by the methods previously developed by us. But the numbers of cells are not changed, suggesting that the cells are very small. These results highlight surprising flexibility of body size and cell size in a multicellular organism, which will give a novel insight into the mechanisms of body size control.

Cross-talk between the bone morphogenetic protein pathway and other major signaling pathways results in tightly regulated cell-specific outcomes

Members of the bone morphogenetic protein (BMP) family of ligands have been identified in a variety of vertebrate and invertebrate species and have been shown to play an essential role in a range of biological processes, including mesodermal patterning and organ development, as well as the formation of bone and cartilage. Although the interaction of BMP ligands with specific type I and II serine-threonine kinase receptor complexes is crucial to the initiation of signaling, it is the interaction of intracellular signaling molecules, called Smads, with receptor complexes and other transcription factors that defines the repertoire of biological responses associated with the BMPs. But although all BMP ligands appear to interact with specific type I and II receptors, growing biochemical and developmental evidence supports the notion that alternative non-Smad pathways also participates in BMP signaling. Here, we review the interaction or 'cross-talk' between the BMP and other major signaling pathways (transforming growth factor beta/activin, Notch, p38 mitogen-activated protein kinase and Toll) that results in tightly regulated cell-specific outcomes.

Regulation of Caenorhabditis elegans body size and male tail development by the novel gene lon-8

BACKGROUND

In C. elegans and other nematode species, body size is determined by the composition of the extracellular cuticle as well as by the nuclear DNA content of the underlying hypodermis. Mutants that are defective in these processes can exhibit either a short or a long body size phenotype. Several mutations that give a long body size (Lon) phenotype have been characterized and found to be regulated by the DBL-1/TGF-beta pathway, that controls post-embryonic growth and male tail development.

RESULTS

Here we characterize a novel gene affecting body size. lon-8 encodes a secreted product of the hypodermis that is highly conserved in Rhabditid nematodes. lon-8 regulates larval elongation as well as male tail development. In both processes, lon-8 appears to function independently of the Sma/Mab pathway. Rather, lon-8 genetically interacts with dpy-11 and dpy-18, which encode cuticle collagen modifying enzymes.

CONCLUSION

The novel gene lon-8 encodes a secreted product of the hypodermis that controls body size and male ray morphology in C. elegans. lon-8 genetically interacts with enzymes that affect the composition of the cuticle.

Glypican LON-2 is a conserved negative regulator of BMP-like signaling in Caenorhabditis elegans

Bone morphogenetic protein (BMP) pathways are required for a wide variety of developmental and homeostatic decisions, and mutations in signaling components are associated with several diseases. An important aspect of BMP control is the extracellular regulation of these pathways. We show that LON-2 negatively regulates a BMP-like signaling pathway that controls body length in C. elegans. lon-2 acts genetically upstream of the BMP-like gene dbl-1, and loss of lon-2 function results in animals that are longer than normal. LON-2 is a conserved member of the glypican family of heparan sulfate proteoglycans, a family with several members known to regulate growth-factor signaling in many organisms. LON-2 is functionally conserved because the Drosophila glypican gene dally rescues the lon-2(lf) body-size defect. We show that the LON-2 protein binds BMP2 in vitro, and a mutant variation of LON-2 found in lon-2(e2140) animals diminishes this interaction. We propose that LON-2 binding to DBL-1 negatively regulates this pathway in C. elegans by attenuating ligand-receptor interactions. This is the first report of a glypican directly interacting with a growth-factor pathway in C. elegans and provides a mechanistic model for glypican regulation of growth-factor pathways.

Structural and functional evidence for a singular repertoire of BMP receptor signal transducing proteins in the lophotrochozoan Crassostrea gigas suggests a shared ancestral BMP/activin pathway

The transforming growth factor beta (TGF-beta) superfamily includes bone morphogenetic proteins, activins and TGF-betasensu stricto (s.s). These ligands, which transduce their signal through a heteromeric complex of type I and type II receptors, have been shown to play a key role in numerous biological processes including early embryonic development in both deuterostomes and ecdyzozoans. Lophochotrozoans, the third major group of bilaterian animals, have remained in the background of the molecular survey of metazoan development. We report the cloning and functional study of the central part of the BMP pathway machinery in the bivalve mollusc Crassostrea gigas (Cg-BMPR1 type I receptor and Cg-TGFbetasfR2 type II receptor), showing an unusual functional mode of signal transduction for this superfamily. The use of the zebrafish embryo as a reporter organism revealed that Cg-BMPR1, Cg-TGFbetasfR2, Cg-ALR I, an activin Type I receptor or their dominant negative acting truncated forms, when overexpressed during gastrulation, resulted in a range of phenotypes displaying severe disturbance of anterioposterior patterning, due to strong modulations of ventrolateral mesoderm patterning. The results suggest that Cg-BMPR1, and to a certain degree Cg-TGFbetasfR2 proteins, function in C. gigas in a similar way to their zebrafish orthologues. Finally, based on phylogenetic analyses, we propose an evolutionary model within the complete TGF-beta superfamily. Thus, evidence provided by this study argues for a possible conserved endomesoderm/ectomesoderm inductive mechanism in spiralians through an ancestral BMP/activin pathway in which the singular, promiscuous and probably unique Cg-TGFbetasfR2 would be the shared type II receptor interface for both BMP and activin ligands.

Molecular signals regulating proliferation of stem and progenitor cells in mouse olfactory epithelium

To understand how signaling molecules regulate the generation of neurons from proliferating stem cells and neuronal progenitors in the developing and regenerating nervous system, we have studied neurogenesis in a model neurogenic epithelium, the olfactory epithelium (OE) of the mouse. Our studies have employed a candidate approach to test signaling molecules of potential importance in regulating neurogenesis and have utilized methods that include tissue culture, in situ hybridization and mouse genetics. Using these approaches, we have identified three distinct stages of stem and transit amplifying progenitor cells in the differentiation pathway of olfactory receptor neurons (ORNs) and have identified mechanisms by which the development of each of these progenitor cell types is regulated by signals produced both within the OE itself and by its underlying stroma. Our results indicate that regulation of olfactory neurogenesis is critically dependent on multiple signaling molecules from two different polypeptide growth factor superfamilies, the fibroblast growth factors and the transforming growth factor beta (TGF-beta) group. In addition, they indicate that these signaling molecules interact in at least two important ways: first, opposing signals converge on cells at specific developmental stages in the ORN pathway to regulate proliferation and differentiation; and second, these signaling molecules--particularly the TGF-betas and their antagonists--play key roles in feedback loops that regulate the size of progenitor cell pools and thereby neuron number, during development and regeneration.

cGMP and a germ-line signal control body size in C. elegans through cGMP-dependent protein kinase EGL-4

Mechanisms involved in the control of body size are largely unknown. In the nematode C. elegans, several small body size mutants were isolated, and the responsible genes were reported to encode putative components of a TGFbeta signalling pathway. Recently, mutants in the egl-4 gene encoding cGMP-dependent protein kinases were found to have a larger body size, and it was suggested that EGL-4 down-regulates the TGFbeta/DBL-1 pathway. We show that a permeable cGMP analogue 8-Br-cGMP significantly reduces body size of the wild-type but not that of an egl-4 mutant, indicating that cGMP controls body size through EGL-4. Laser ablation of germ-line cells revealed that a germ-line signal and EGL-4 function in the same pathway. Targeted expression of EGL-4 indicates that EGL-4 can function in hypodermis, neurones and intestine both cell-autonomously and cell-nonautonomously to control organ and body size. We propose a signal cascade for the control of body size that involves a germ-line signal, cGMP, G-kinase EGL-4 and DBL-1/TGFbeta pathway. It is interesting that two important pathways involving cGMP and TGFbeta, respectively, are related. Also, the results suggest a novel mechanism for the control of organ and body size in which hypodermis plays a key role