Sequence of C. elegans lag-2 reveals a cell-signalling domain shared with Delta and Serrate of Drosophila

Sulfonate-Modified Polystyrene Nanoparticle at Precited Environmental Concentrations Induces Transgenerational Toxicity Associated with Increase in Germline Notch Signal of Caenorhabditis elegans

Recently, the transgenerational toxicity of nanoplastics has received increasing attention. Caenorhabditis elegans is a useful model to assess the transgenerational toxicity of different pollutants. In nematodes, the possibility of early-life exposure to sulfonate-modified polystyrene nanoparticle (PS-S NP) causing transgenerational toxicity and its underlying mechanisms were investigated. After exposure at the L1-larval stage, transgenerational inhibition in both locomotion behavior (body bend and head thrash) and reproductive capacity (number of offspring and fertilized egg number in uterus) was induced by 1-100 μg/L PS-S NP. Meanwhile, after exposure to 1-100 μg/L PS-S NP, the expression of germline lag-2 encoding Notch ligand was increased not only at the parental generation (P0-G) but also in the offspring, and the transgenerational toxicity was inhibited by the germline RNA interference (RNAi) of lag-2. During the transgenerational toxicity formation, the parental LAG-2 activated the corresponding Notch receptor GLP-1 in the offspring, and transgenerational toxicity was also suppressed by glp-1 RNAi. GLP-1 functioned in the germline and the neurons to mediate the PS-S NP toxicity. In PS-S NP-exposed nematodes, germline GLP-1 activated the insulin peptides of INS-39, INS-3, and DAF-28, and neuronal GLP-1 inhibited the DAF-7, DBL-1, and GLB-10. Therefore, the exposure risk in inducing transgenerational toxicity through PS-S NP was suggested, and this transgenerational toxicity was mediated by the activation of germline Notch signal in organisms.

Higher-order epistasis shapes natural variation in germ stem cell niche activity

To study how natural allelic variation explains quantitative developmental system variation, we characterized natural differences in germ stem cell niche activity, measured as progenitor zone (PZ) size, between two Caenorhabditis elegans isolates. Linkage mapping yielded candidate loci on chromosomes II and V, and we found that the isolate with a smaller PZ size harbours a 148 bp promoter deletion in the Notch ligand, lag-2/Delta, a central signal promoting germ stem cell fate. As predicted, introducing this deletion into the isolate with a large PZ resulted in a smaller PZ size. Unexpectedly, restoring the deleted ancestral sequence in the isolate with a smaller PZ did not increase-but instead further reduced-PZ size. These seemingly contradictory phenotypic effects are explained by epistatic interactions between the lag-2/Delta promoter, the chromosome II locus, and additional background loci. These results provide first insights into the quantitative genetic architecture regulating an animal stem cell system.

Evolving Roles of Notch Signaling in Cortical Development

Expansion of the neocortex is thought to pave the way toward acquisition of higher cognitive functions in mammals. The highly conserved Notch signaling pathway plays a crucial role in this process by regulating the size of the cortical progenitor pool, in part by controlling the balance between self-renewal and differentiation. In this review, we introduce the components of Notch signaling pathway as well as the different mode of molecular mechanisms, including trans- and cis-regulatory processes. We focused on the recent findings with regard to the expression pattern and levels in regulating neocortical formation in mammals and its interactions with other known signaling pathways, including Slit–Robo signaling and Shh signaling. Finally, we review the functions of Notch signaling pathway in different species as well as other developmental process, mainly somitogenesis, to discuss how modifications to the Notch signaling pathway can drive the evolution of the neocortex.

A somatic proteoglycan controls Notch-directed germ cell fate

Communication between the soma and germline optimizes germ cell fate programs. Notch receptors are key determinants of germ cell fate but how somatic signals direct Notch-dependent germ cell behavior is undefined. Here we demonstrate that SDN-1 (syndecan-1), a somatic transmembrane proteoglycan, controls expression of the GLP-1 (germline proliferation-1) Notch receptor in the Caenorhabditis elegans germline. We find that SDN-1 control of a somatic TRP calcium channel governs calcium-dependent binding of an AP-2 transcription factor (APTF-2) to the glp-1 promoter. Hence, SDN-1 signaling promotes GLP-1 expression and mitotic germ cell fate. Together, these data reveal SDN-1 as a putative communication nexus between the germline and its somatic environment to control germ cell fate decisions.

Analysis of GPI-anchored proteins involved in germline stem cell proliferation in the Caenorhabditis elegans germline stem cell niche

Stem cells divide and undergo self-renewal depending on the signals received from the stem cell niche. This phenomenon is indispensable to maintain tissues and organs in individuals. However, not all the molecular factors and mechanisms of self-renewal are known. In our previous study, we reported that glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) synthesized in the distal tip cells (DTCs; the stem cell niche) are essential for germline stem cell proliferation in Caenorhabditis elegans. Here, we characterized the GPI-APs required for proliferation. We selected and verified the candidate GPI-APs synthesized in DTCs by RNA interference screening and found that F57F4.3 (GFI-1), F57F4.4 and F54E2.1 are necessary for germline proliferation. These proteins are likely involved in the same pathway for proliferation and activated by the transcription factor PQM-1. We further provided evidence suggesting that these GPI-APs act through fatty acid remodelling of the GPI anchor, which is essential for association with lipid rafts. These findings demonstrated that GPI-APs, particularly F57F4.3/4 and F54E2.1, synthesized in the germline stem cell niche are located in lipid rafts and involved in promoting germline stem cell proliferation in C. elegans. The findings may thus shed light on the mechanisms by which GPI-APs regulate stem cell self-renewal.

Deltex-1 is indispensible for the IL-6 and TGF-β treatment-triggered differentiation of Th17 cells

CSL(CBF1, Su(H) and LAG-1)-dependent Hes-1 signaling plays an important part in regulating Th17 cell differentiation. However, little is known about influence of CSL-independent Deltex-1 signaling on this subset. The current focus is on roles of the Deltex-1 signaling in the Th17 cell differentiation. IL-17-producing CD4⁺ T cell subpopulation could be induced in vitro by treatment of both IL-6 and TGF-β. This could be reversed by knockdown of the deltex-1 gene, following the attenuation of retinoic acid-related orphan receptor γt (RORγt) and its DNA-binding activity in nuclei. Subsequently, Th17-associated cytokines generated by the treated cells were also diminished by the inhibition of Deltex-1 signaling, but the production of IL-10 was enhanced. Contrary to the alteration of RORγt, both zinc-finger transcription factor-3 (GATA3) and transcription factor Forkhead box P3 (Foxp3) were augmented at their mRNA and protein levels as well as DNA-binding activities with the emerging phenotypes of the corresponding cellular subpopulation and T-bet (encoded by TBX21) was not changed. These results reveal for the first time that Deltex-1 is indispensible for the IL-6 and TGF-β treatment-triggered differentiation of Th17 cells, indicating that CSL-independent Deltex-1 signaling favors naïve CD4⁺ T cells to deviate into Th17 cells via the enhancement of RORγt/IL-17A.

Recent Advances in the Genetic, Anatomical, and Environmental Regulation of the C. elegans Germ Line Progenitor Zone

The C. elegans germ line and its gonadal support cells are well studied from a developmental genetics standpoint and have revealed many foundational principles of stem cell niche biology. Among these are the observations that a niche-like cell supports a self-renewing stem cell population with multipotential, differentiating daughter cells. While genetic features that distinguish stem-like cells from their differentiating progeny have been defined, the mechanisms that structure these populations in the germ line have yet to be explained. The spatial restriction of Notch activation has emerged as an important genetic principle acting in the distal germ line. Synthesizing recent findings, I present a model in which the germ stem cell population of the C. elegans adult hermaphrodite can be recognized as two distinct anatomical and genetic populations. This review describes the recent progress that has been made in characterizing the undifferentiated germ cells and gonad anatomy, and presents open questions in the field and new directions for research to pursue.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Delta-like 1 and Delta-like 4 differently require their extracellular domains for triggering Notch signaling in mice

Delta-like (Dll) 1 and Dll4 differently function as Notch ligands in a context-dependent manner. As these ligands share structural properties, the molecular basis for their functional difference is poorly understood. Here, we investigated the superiority of Dll4 over Dll1 with respect to induction of T cell development using a domain-swapping approach in mice. The DOS motif, shared by Notch ligands-except Dll4-contributes to enhancing the activity of Dll for signal transduction. The module at the N-terminus of Notch ligand (MNNL) of Dll4 is inherently advantageous over Dll1. Molecular dynamic simulation revealed that the loop structure in MNNL domain of Dll1 contains unique proline residues with limited range of motion. The Dll4 mutant with Dll1-derived proline residues showed reduced activity. These results suggest that the loop structure-present within the MNNL domain-with a wide range of motion ensures the superiority of Dll4 and uniquely contributes to the triggering of Notch signaling.

Ligand-Induced Cis-Inhibition of Notch Signaling: The Role of an Extracellular Region of Serrate

Cellular development can be controlled by communication between adjacent cells mediated by the highly conserved Notch signaling system. A cell expressing the Notch receptor on one cell can be activated in trans by ligands on an adjacent cell leading to alteration of transcription and cellular fate. Ligands also have the ability to inhibit Notch signaling, and this can be accomplished when both receptor and ligands are coexpressed in cis on the same cell. The manner in which cis-inhibition is accomplished is not entirely clear but it is known to involve several different protein domains of the ligands and the receptor. Some of the protein domains involved in trans-activation are also used for cis-inhibition, but some are used uniquely for each process. In this work, the involvement of various ligand regions and the receptor are discussed in relation to their contributions to Notch signaling.

Biology of the Caenorhabditis elegans Germline Stem Cell System

Stem cell systems regulate tissue development and maintenance. The germline stem cell system is essential for animal reproduction, controlling both the timing and number of progeny through its influence on gamete production. In this review, we first draw general comparisons to stem cell systems in other organisms, and then present our current understanding of the germline stem cell system in Caenorhabditis elegans In contrast to stereotypic somatic development and cell number stasis of adult somatic cells in C. elegans, the germline stem cell system has a variable division pattern, and the system differs between larval development, early adult peak reproduction and age-related decline. We discuss the cell and developmental biology of the stem cell system and the Notch regulated genetic network that controls the key decision between the stem cell fate and meiotic development, as it occurs under optimal laboratory conditions in adult and larval stages. We then discuss alterations of the stem cell system in response to environmental perturbations and aging. A recurring distinction is between processes that control stem cell fate and those that control cell cycle regulation. C. elegans is a powerful model for understanding germline stem cells and stem cell biology.

Dynamics of Notch-Dependent Transcriptional Bursting in Its Native Context

Transcription is well known to be inherently stochastic and episodic, but the regulation of transcriptional dynamics is not well understood. Here, we analyze how Notch signaling modulates transcriptional bursting during animal development. Our focus is Notch regulation of transcription in germline stem cells of the nematode C. elegans. Using the MS2 system to visualize nascent transcripts and live imaging to record dynamics, we analyze bursting as a function of position within the intact animal. We find that Notch-dependent transcriptional activation is indeed "bursty"; that wild-type Notch modulates burst duration (ON-time) rather than duration of pauses between bursts (OFF-time) or mean burst intensity; and that a mutant Notch receptor, which is compromised for assembly into the Notch transcription factor complex, primarily modifies burst size (duration × intensity). These analyses thus visualize the effect of a canonical signaling pathway on metazoan transcriptional bursting in its native context.

The Signaling Network Controlling C. elegans Vulval Cell Fate Patterning

EGF, emitted by the Anchor Cell, patterns six equipotent C. elegans vulval precursor cells to assume a precise array of three cell fates with high fidelity. A group of core and modulatory signaling cascades forms a signaling network that demonstrates plasticity during the transition from naïve to terminally differentiated cells. In this review, we summarize the history of classical developmental manipulations and molecular genetics experiments that led to our understanding of the signals governing this process, and discuss principles of signal transduction and developmental biology that have emerged from these studies.

In silico analysis, seasonal variation and gonadotropic regulation of jag1 and its receptor notch1 in testis of spotted snakehead Channa punctatus

The present study in seasonally breeding spotted snakehead Channa punctatus, for the first time in nonmammalian vertebrates, demonstrated correlation between reproductive phase-dependent testicular expression of ligand Jag1/receptor Notch1 and spermatogenic events. Testicular transcriptome sequencing data from our earlier study in C. punctatus was used in the present study to select the best transcript for jag1 (cpjag1) and notch1 (cpnotch1). The transcripts cpjag1 and cpnotch1 encoded full-length putative proteins of 1215 (cpJag1) and 2475 (cpNotch1) amino acids, respectively. A marked homology in the extracellular domains of Jag1 and Notch1 was observed following their alignment with respective proteins from different vertebrates, suggesting conservation in ligand-receptor interaction in C. punctatus. Both cpJag1 and cpNotch1 showed phylogenetic closeness with their teleostean counterparts, especially with that of Perciformes. Temporal expression of cpjag1 and cpnotch1 in testis depending on reproductive phases showed an appreciably high expression during spermatogenically inactive resting and postspawning phases when seminiferous lobules consisted of spermatogonial stem cells and undifferentiated spermatogonia. Their expression sharply declined during spermatogenically active preparatory and spawning phases. It appears that involvement of cpjag1/cpnotch1 is restricted to inactive phases when spermatogonial stem cells renew themselves and replenish undifferentiated spermatogonia. This assumption is ascertained by an experimental study in which high level of testicular cpjag1/cpnotch1 expression in control fish of resting phase markedly decreased after administration of human chorionic gonadotropin that is known to induce proliferation and differentiation of spermatogonia and spawning of spermatozoa.

MIB-1 Is Required for Spermatogenesis and Facilitates LIN-12 and GLP-1 Activity in Caenorhabditis elegans

Covalent attachment of ubiquitin to substrate proteins changes their function or marks them for proteolysis, and the specificity of ubiquitin attachment is mediated by the numerous E3 ligases encoded by animals. Mind Bomb is an essential E3 ligase during Notch pathway signaling in insects and vertebrates. While Caenorhabditis elegans encodes a Mind Bomb homolog (mib-1), it has never been recovered in the extensive Notch suppressor/enhancer screens that have identified numerous pathway components. Here, we show that C. elegans mib-1 null mutants have a spermatogenesis-defective phenotype that results in a heterogeneous mixture of arrested spermatocytes, defective spermatids, and motility-impaired spermatozoa. mib-1 mutants also have chromosome segregation defects during meiosis, molecular null mutants are intrinsically temperature-sensitive, and many mib-1 spermatids contain large amounts of tubulin. These phenotypic features are similar to the endogenous RNA intereference (RNAi) mutants, but mib-1 mutants do not affect RNAi. MIB-1 protein is expressed throughout the germ line with peak expression in spermatocytes followed by segregation into the residual body during spermatid formation. C. elegans mib-1 expression, while upregulated during spermatogenesis, also occurs somatically, including in vulva precursor cells. Here, we show that mib-1 mutants suppress both lin-12 and glp-1 (C. elegans Notch) gain-of-function mutants, restoring anchor cell formation and a functional vulva to the former and partly restoring oocyte production to the latter. However, suppressed hermaphrodites are only observed when grown at 25°, and they are self-sterile. This probably explains why mib-1 was not previously recovered as a Notch pathway component in suppressor/enhancer selection experiments.

The anti-aging and anti-oxidation effects of tea water extract in Caenorhabditis elegans

Tea includes puer tea, black tea, green tea and many others. By using model organism Caenorhabditis elegans, the anti-aging and anti-oxidation effects of tea water extract were systemically examined in this study. We found that water extract of puer tea, black tea and green tea all increased the lifespan of worms, postponed Aβ-induced progressive paralysis in Alzheimer's disease transgenic worms, and improved the tolerance of worms to the oxidative stress induced by heavy metal Cr⁶⁺. Moreover, the anti-oxidation effects of tea water extract at low concentration were different among 4 kinds of brands of green tea. The underlying mechanisms were further explored using genetically manipulated-mutant worms. The anti-oxidative stress effects of green tea water extract depend on the dietary restriction and germline signaling pathways, but not the FOXO and mitochondrial respiratory chain signals. Therefore, tea water extract provides benefits of anti-aging, anti-AD and anti-oxidation.

Regulation of the Balance Between Proliferation and Differentiation in Germ Line Stem Cells

In many animals, reproductive fitness is dependent upon the production of large numbers of gametes over an extended period of time. This level of gamete production is possible due to the continued presence of germ line stem cells. These cells can produce two types of daughter cells, self-renewing daughter cells that will maintain the stem cell population and differentiating daughter cells that will become gametes. A balance must be maintained between the proliferating self-renewing cells and those that differentiate for long-term gamete production to be maintained. Too little proliferation can result in depletion of the stem cell population, while too little differentiation can lead to a lack of gamete formation and possible tumor formation. In this chapter, we discuss our current understanding of how the balance between proliferation and differentiation is achieved in three well-studied germ line model systems: the Drosophila female, the mouse male, and the C. elegans hermaphrodite. While these three systems have significant differences in how this balance is regulated, including differences in stem cell population size, signaling pathways utilized, and the use of symmetric and/or asymmetric cell divisions, there are also similarities found between them. These similarities include the reliance on a predominant signaling pathway to promote proliferation, negative feedback loops to rapidly shutoff proliferation-promoting cues, close association of the germ line stem cells with a somatic niche, cytoplasmic connections between cells, projections emanating from the niche cell, and multiple mechanisms to limit the spatial influence of the niche. A comparison between different systems may help to identify elements that are essential for a proper balance between proliferation and differentiation to be achieved and elements that may be achieved through various mechanisms.

Germline Stem Cell Differentiation Entails Regional Control of Cell Fate Regulator GLD-1 in Caenorhabditis elegans

Germline stem cell differentiation in Caenorhabditis elegans is controlled by glp-1 Notch signaling. Cell fate regulator GLD-1 is sufficient to induce meiotic entry and expressed at a high level during meiotic prophase, inhibiting mitotic gene activity. glp-1 signaling and other regulators control GLD-1 levels post-transcriptionally (low in stem cells, high in meiotic prophase), but many aspects of GLD-1 regulation are uncharacterized, including the link between glp-1-mediated transcriptional control and post-transcriptional GLD-1 regulation. We established a sensitive assay to quantify GLD-1 levels across an ∼35-cell diameter field, where distal germline stem cells differentiate proximally into meiotic prophase cells in the adult C. elegans hermaphrodite, and applied the approach to mutants in known or proposed GLD-1 regulators. In wild-type GLD-1 levels elevated ∼20-fold in a sigmoidal pattern. We found that two direct transcriptional targets of glp-1 signaling, lst-1 and sygl-1, were individually required for repression of GLD-1. We determined that lst-1 and sygl-1 act in the same genetic pathway as known GLD-1 translational repressor fbf-1, while lst-1 also acts in parallel to fbf-1, linking glp-1-mediated transcriptional control and post-transcriptional GLD-1 repression. Additionally, we estimated the position in wild-type gonads where germ cells irreversibly commit to meiotic development based on GLD-1 levels in worms where glp-1 activity was manipulated to cause an irreversible fate switch. Analysis of known repressors and activators, as well as modeling the sigmoidal accumulation pattern, indicated that regulation of GLD-1 levels is largely regional, which we integrated with the current view of germline stem cell differentiation.

Transcription factor hlh-2/E/Daughterless drives expression of α integrin ina-1 during DTC migration in C. elegans

Integrins are involved in a vast number of cell behaviors due to their roles in adhesion and signaling. The regulation of integrin expression is of particular interest as a mechanism to drive developmental events and for the role of altered integrin expression profiles in cancer. Dynamic regulation of the expression of integrin receptors is required for the migration of the distal tip cell (DTC) during gonadogenesis in Caenorhabditis elegans. α integrin ina-1 is required for DTC motility, yet is up-regulated by an unknown mechanism. Analysis of the promoter for α integrin ina-1 identified two E-box sequences that are required for ina-1 expression in the DTC. Knockdown of transcription factor hlh-2, an established E-box binding partner and ortholog of E/Daughterless, prevented expression of a transcriptional fusion of the ina-1 promoter to RFP and blocked DTC migration. Similarly, knockdown of hlh-2 also prevented expression of a translational fusion of the genomic ina-1 gene to GFP while blocking DTC migration. Knockdown of HLH-2 binding partner MIG-24 also reduced ina-1 expression and DTC migration. Overall, these results show that the transcription factor hlh-2 is required for up-regulation of ina-1 at the onset of DTC migration.

Two classes of gap junction channels mediate soma-germline interactions essential for germline proliferation and gametogenesis in Caenorhabditis elegans

In all animals examined, somatic cells of the gonad control multiple biological processes essential for germline development. Gap junction channels, composed of connexins in vertebrates and innexins in invertebrates, permit direct intercellular communication between cells and frequently form between somatic gonadal cells and germ cells. Gap junctions comprise hexameric hemichannels in apposing cells that dock to form channels for the exchange of small molecules. Here we report essential roles for two classes of gap junction channels, composed of five innexin proteins, in supporting the proliferation of germline stem cells and gametogenesis in the nematode Caenorhabditis elegans. Transmission electron microscopy of freeze-fracture replicas and fluorescence microscopy show that gap junctions between somatic cells and germ cells are more extensive than previously appreciated and are found throughout the gonad. One class of gap junctions, composed of INX-8 and INX-9 in the soma and INX-14 and INX-21 in the germ line, is required for the proliferation and differentiation of germline stem cells. Genetic epistasis experiments establish a role for these gap junction channels in germline proliferation independent of the glp-1/Notch pathway. A second class of gap junctions, composed of somatic INX-8 and INX-9 and germline INX-14 and INX-22, is required for the negative regulation of oocyte meiotic maturation. Rescue of gap junction channel formation in the stem cell niche rescues germline proliferation and uncovers a later channel requirement for embryonic viability. This analysis reveals gap junctions as a central organizing feature of many soma–germline interactions in C. elegans.

Protein kinase CK2 both promotes robust proliferation and inhibits the proliferative fate in the C. elegans germ line

Stem cells are capable of both self-renewal (proliferation) and differentiation. Determining the regulatory mechanisms controlling the balance between stem cell proliferation and differentiation is not only an important biological question, but also holds the key for using stem cells as therapeutic agents. The Caenorhabditis elegans germ line has emerged as a valuable model to study the molecular mechanisms controlling stem cell behavior. In this study, we describe a large-scale RNAi screen that identified kin-10, which encodes the β subunit of protein kinase CK2, as a novel factor regulating stem cell proliferation in the C. elegans germ line. While a loss of kin-10 in an otherwise wild-type background results in a decrease in the number of proliferative cells, loss of kin-10 in sensitized genetic backgrounds results in a germline tumor. Therefore, kin-10 is not only necessary for robust proliferation, it also inhibits the proliferative fate. We found that kin-10's regulatory role in inhibiting the proliferative fate is carried out through the CK2 holoenzyme, rather than through a holoenzyme-independent function, and that it functions downstream of GLP-1/Notch signaling. We propose that a loss of kin-10 leads to a defect in CK2 phosphorylation of its downstream targets, resulting in abnormal activity of target protein(s) that are involved in the proliferative fate vs. differentiation decision. This eventually causes a shift towards the proliferative fate in the stem cell fate decision.

A complex regulatory network coordinating cell cycles during C. elegans development is revealed by a genome-wide RNAi screen

The development and homeostasis of multicellular animals requires precise coordination of cell division and differentiation. We performed a genome-wide RNA interference screen in Caenorhabditis elegans to reveal the components of a regulatory network that promotes developmentally programmed cell-cycle quiescence. The 107 identified genes are predicted to constitute regulatory networks that are conserved among higher animals because almost half of the genes are represented by clear human orthologs. Using a series of mutant backgrounds to assess their genetic activities, the RNA interference clones displaying similar properties were clustered to establish potential regulatory relationships within the network. This approach uncovered four distinct genetic pathways controlling cell-cycle entry during intestinal organogenesis. The enhanced phenotypes observed for animals carrying compound mutations attest to the collaboration between distinct mechanisms to ensure strict developmental regulation of cell cycles. Moreover, we characterized ubc-25, a gene encoding an E2 ubiquitin-conjugating enzyme whose human ortholog, UBE2Q2, is deregulated in several cancers. Our genetic analyses suggested that ubc-25 acts in a linear pathway with cul-1/Cul1, in parallel to pathways employing cki-1/p27 and lin-35/pRb to promote cell-cycle quiescence. Further investigation of the potential regulatory mechanism demonstrated that ubc-25 activity negatively regulates CYE-1/cyclin E protein abundance in vivo. Together, our results show that the ubc-25-mediated pathway acts within a complex network that integrates the actions of multiple molecular mechanisms to control cell cycles during development.

Preadipocytes proliferate and differentiate under the guidance of Delta-like 1 homolog (DLK1)

Obesity occurs when an excessive dietary fat intake leads to expansion of adipose tissue, which mainly consists of adipocytes that arise from proliferating and differentiating adipose stem cells, the preadipocytes. Obesity is a consequence of both adipocyte hypertrophy and hyperplasia. Knowledge about preadipocyte differentiation is relatively well established, whereas the mechanism responsible for preadipocyte proliferation is incompletely understood and only in the early stage of comprehension. In this regard, we have recently identified that Delta-like 1 homolog (Dlk1) (also known as Preadipocyte factor 1 [Pref-1]) inhibits preadipocyte proliferation by regulating their entry into G1/S-phase. This novel disclosure, adding to the previous published data on Dlk1 repression of preadipocyte differentiation, has given us the chance to firmly place Dlk1 as a master regulator of preadipocyte homeostasis and adipose tissue expansion. Dlk1 manipulation may, therefore, open new perspectives in obesity treatments.

Cell-cycle regulation of NOTCH signaling during C. elegans vulval development

C. elegans vulval development is one of the best-characterized systems to study cell fate specification during organogenesis. The detailed knowledge of the signaling pathways determining vulval precursor cell (VPC) fates permitted us to create a computational model based on the antagonistic interactions between the epidermal growth factor receptor (EGFR)/RAS/MAPK and the NOTCH pathways that specify the primary and secondary fates, respectively. A key notion of our model is called bounded asynchrony, which predicts that a limited degree of asynchrony in the progression of the VPCs is necessary to break their equivalence. While searching for a molecular mechanism underlying bounded asynchrony, we discovered that the termination of NOTCH signaling is tightly linked to cell-cycle progression. When single VPCs were arrested in the G1 phase, intracellular NOTCH failed to be degraded, resulting in a mixed primary/secondary cell fate. Moreover, the G1 cyclins CYD-1 and CYE-1 stabilize NOTCH, while the G2 cyclin CYB-3 promotes NOTCH degradation. Our findings reveal a synchronization mechanism that coordinates NOTCH signaling with cell-cycle progression and thus permits the formation of a stable cell fate pattern.

Stem cell proliferation versus meiotic fate decision in Caenorhabditis elegans

The C. elegans germ line has emerged as an important model for -understanding how a stem cell population is maintained throughout the life of the animal while still producing the gametes necessary for propagation of the species. The stem cell population in the adult hermaphrodite is relatively large, with stem cells giving rise to daughters that appear intrinsically equivalent; however, some of the daughters retain the proliferative fate while others enter meiotic prophase. While machinery exists for cells to progress through the mitotic cell cycle and machinery exists for cells to progress through meiotic prophase, central to understanding germ line development is identifying the genes and regulatory processes that determine whether the mitotic cell cycle or meiotic prophase machinery will be utilized; in other words, the genes that regulate the switch of germ cells from the proliferative stem cell fate to the meiotic development fate. Whether a germ cell self-renews or enters meiotic prophase is largely determined by its proximity to the distal tip cell (DTC), which is the somatic niche cell that caps the distal end of the gonad. Germ cells close to the DTC have high levels of GLP-1 Notch signaling, which promotes the proliferative fate, while cells further from the DTC have high activity levels of the GLD-1 and GLD-2 redundant RNA regulatory pathways, as well as a third uncharacterized pathway, each of which direct cells to enter meiotic prophase. Other factors and pathways modulate this core genetic pathway, or work in parallel to it, presumably to ensure that a tight balance is maintained between proliferation and meiotic entry.

Germline stem cells and their regulation in the nematode Caenorhabditis elegans

C. elegans germline stem cells exist within a stem cell pool that is maintained by a single-celled mesenchymal niche and Notch signaling. Downstream of Notch signaling, a regulatory network governs stem cells and differentiation. Central to that network is the FBF RNA-binding protein, a member of the widely conserved PUF family that functions by either of two broadly conserved mechanisms to repress its target mRNAs. Without FBF, germline stem cells do not proliferate and they do not maintain their naïve, undifferentiated state. Therefore, FBF is a pivotal regulator of germline self-renewal. Validated FBF targets include several key differentiation regulators as well as a major cell cycle regulator. A genomic analysis identifies many other developmental and cell cycle regulators as likely FBF targets and suggests that FBF is a broad-spectrum regulator of the genome with >1,000 targets. A comparison of the FBF target list with similar lists for human PUF proteins, PUM1 and PUM2, reveals ∼200 shared targets. The FBF hub works within a network controlling self-renewal vs. differentiation. This network consists of classical developmental cell fate regulators and classical cell cycle regulators. Recent results have begun to integrate developmental and cell cycle regulation within the network. The molecular dynamics of the network remain a challenge for the future, but models are proposed. We suggest that molecular controls of C. elegans germline stem cells provide an important model for controls of stem cells more broadly.

The NOTCH receptor and its ligands

An intricate interplay of signalling pathways dictates the acquisition of specific cell fates during development. The NOTCH receptor is the central element in a cell-interaction mechanism that controls the fate of a very broad spectrum of precursor cells. Conservation across species implies that signalling through this receptor is a tool frequently used by metazoans to modulate the fate of precursor cells. This article describes recent advances in the genetic and molecular dissection of this developmentally fundamental pathway that have provided new insights into the mechanism by which extracellular signals act through the NOTCH receptor to determine or alter cellular fate.

TEG-1 CD2BP2 regulates stem cell proliferation and sex determination in the C. elegans germ line and physically interacts with the UAF-1 U2AF65 splicing factor

BACKGROUND

For a stem cell population to exist over an extended period, a balance must be maintained between self-renewing (proliferating) and differentiating daughter cells. Within the Caenorhabditis elegans germ line, this balance is controlled by a genetic regulatory pathway, which includes the canonical Notch signaling pathway.

RESULTS

Genetic screens identified the gene teg-1 as being involved in regulating the proliferation versus differentiation decision in the C. elegans germ line. Cloning of TEG-1 revealed that it is a homolog of mammalian CD2BP2, which has been implicated in a number of cellular processes, including in U4/U6.U5 tri-snRNP formation in the pre-mRNA splicing reaction. The position of teg-1 in the genetic pathway regulating the proliferation versus differentiation decision, its single mutant phenotype, and its enrichment in nuclei, all suggest TEG-1 also functions as a splicing factor. TEG-1, as well as its human homolog, CD2BP2, directly bind to UAF-1 U2AF65, a component of the U2 auxiliary factor.

CONCLUSIONS

TEG-1 functions as a splicing factor and acts to regulate the proliferation versus meiosis decision. The interaction of TEG-1 CD2BP2 with UAF-1 U2AF65, combined with its previously described function in U4/U6.U5 tri-snRNP, suggests that TEG-1 CD2BP2 functions in two distinct locations in the splicing cascade.

The putative Notch ligand HyJagged is a transmembrane protein present in all cell types of adult Hydra and upregulated at the boundary between bud and parent

Background

The Notch signalling pathway is conserved in pre-bilaterian animals. In the Cnidarian Hydra it is involved in interstitial stem cell differentiation and in boundary formation during budding. Experimental evidence suggests that in Hydra Notch is activated by presenilin through proteolytic cleavage at the S3 site as in all animals. However, the endogenous ligand for HvNotch has not been described yet.

Results

We have cloned a cDNA from Hydra, which encodes a bona-fide Notch ligand with a conserved domain structure similar to that of Jagged-like Notch ligands from other animals. Hyjagged mRNA is undetectable in adult Hydra by in situ hybridisation but is strongly upregulated and easily visible at the border between bud and parent shortly before bud detachment. In contrast, HyJagged protein is found in all cell types of an adult hydra, where it localises to membranes and endosomes. Co-localisation experiments showed that it is present in the same cells as HvNotch, however not always in the same membrane structures.

Conclusions

The putative Notch ligand HyJagged is conserved in Cnidarians. Together with HvNotch it may be involved in the formation of the parent-bud boundary in Hydra. Moreover, protein distribution of both, HvNotch receptor and HyJagged indicate a more widespread function for these two transmembrane proteins in the adult hydra, which may be regulated by additional factors, possibly involving endocytic pathways.

Notch signaling and Notch signaling modifiers

Originally discovered nearly a century ago, the Notch signaling pathway is critical for virtually all developmental programs and modulates an astounding variety of pathogenic processes. The DSL (Delta, Serrate, LAG-2 family) proteins have long been considered canonical activators of the core Notch pathway. More recently, a wide and expanding network of non-canonical extracellular factors has also been shown to modulate Notch signaling, conferring newly appreciated complexity to this evolutionarily conserved signal transduction system. Here, I review current concepts in Notch signaling, with a focus on work from the last decade elucidating novel extracellular proteins that up- or down-regulate signal potency.

γ-Secretase-regulated mechanisms similar to notch signaling may play a role in signaling events, including APP signaling, which leads to Alzheimer's disease

Although γ-secretase was first identified as a protease that cleaves amyloid precursor protein (APP) within the transmembrane domain, thus producing Aβ peptides that are thought to be pathogenic in Alzheimer's disease (AD), its physiological functions have not been fully elucidated. In the canonical Notch signaling pathway, intramembrane cleavage by γ-secretase serves to release an intracellular domain of Notch that shows activity in the nucleus through binding to transcription factors. Many type 1 transmembrane proteins, including Notch, Delta, and APP, have recently been shown to be substrates for γ-secretase, and their intracellular domains are released from the cell membrane following cleavage by γ-secretase. The common enzyme γ-secretase modulates proteolysis and the turnover of possible signaling molecules, which has led to the attractive hypothesis that mechanisms similar to Notch signaling contribute widely to proteolysis-regulated signaling pathways. APP is also likely to have a signaling mechanism, although the physiological functions of APP have not been elucidated. Indeed, we have shown that the intracellular domain of APP alters gene expression and induces neuron-specific apoptosis. These results suggest that APP signaling responds to the onset of AD. Here, we review the possibility of γ-secretase-regulated signaling, including APP signaling, which leads to AD.

Claudin family proteins in Caenorhabditis elegans

In the last decade, the claudin family of integral membrane proteins has been identified as the major protein component of the tight junctions in all vertebrates. The claudin superfamily proteins also function to regulate channel activity, intercellular signaling, and cell morphology. Subsequently, claudin homologues have been identified in invertebrates, including Drosophila and Caenorhabditis elegans. Recent studies demonstrate that the C. elegans claudins, clc-1 to clc-5, and similar proteins in the greater PMP22/EMP/claudin/calcium channel γ subunit family, including nsy-1-nsy-4 and vab-9, while highly divergent at a sequence level from each other and from the vertebrate claudins, in some cases play roles similar to those traditionally assigned to their vertebrate homologues. These include regulating cell adhesion and passage of small molecules through the paracellular space. The claudin superfamily proteins also function to regulate channel activity, intercellular signaling, and cell morphology. Study of claudin superfamily proteins in C. elegans should continue to provide clues as to how core claudin protein function can be modified to serve various specific roles at regions of cell-cell contact in metazoans.

Notch: Implications of endogenous inhibitors for therapy

Soluble components of Notch signalling can be applied to manipulate a central pathway essential for the development of metazoans and often deregulated in illnesses such as stroke, cancer or cardiovascular diseases. Commonly, the Notch cascade is inhibited by small compound inhibitors, which either block the proteolysis of Notch receptors by gamma-secretases or interfere with the transcriptional activity of the Notch intracellular domain. Specific antibodies can also be used to inhibit ligand-induced activation of Notch receptors. Alternatively, naturally occurring endogenous inhibitors of Notch signalling might offer a specific way to block receptor activation. Examples are the soluble variants of the canonical Notch ligand Jagged1 and the non-canonical Notch ligand Dlk1, both deprived of their transmembrane regions upon ectodomain shedding, or the bona fide secreted molecule EGFL7. We present frequently used methods to decrease Notch signalling, and we discuss how soluble Notch inhibitors may be used to treat diseases.

Cancer models in Caenorhabditis elegans

Although now dogma, the idea that nonvertebrate organisms such as yeast, worms, and flies could inform, and in some cases even revolutionize, our understanding of oncogenesis in humans was not immediately obvious. Aided by the conservative nature of evolution and the persistence of a cohort of devoted researchers, the role of model organisms as a key tool in solving the cancer problem has, however, become widely accepted. In this review, we focus on the nematode Caenorhabditis elegans and its diverse and sometimes surprising contributions to our understanding of the tumorigenic process. Specifically, we discuss findings in the worm that address a well-defined set of processes known to be deregulated in cancer cells including cell cycle progression, growth factor signaling, terminal differentiation, apoptosis, the maintenance of genome stability, and developmental mechanisms relevant to invasion and metastasis.

PRP-17 and the pre-mRNA splicing pathway are preferentially required for the proliferation versus meiotic development decision and germline sex determination in Caenorhabditis elegans

In C. elegans, the decision between germline stem cell proliferation and entry into meiosis is controlled by GLP-1 Notch signaling, which promotes proliferation through repression of the redundant GLD-1 and GLD-2 pathways that direct meiotic entry. We identify prp-17 as another gene functioning downstream of GLP-1 signaling that promotes meiotic entry, largely by acting on the GLD-1 pathway, and that also functions in female germline sex determination. PRP-17 is orthologous to the yeast and human pre-mRNA splicing factor PRP17/CDC40 and can rescue the temperature-sensitive lethality of yeast PRP17. This link to splicing led to an RNAi screen of predicted C. elegans splicing factors in sensitized genetic backgrounds. We found that many genes throughout the splicing cascade function in the proliferation/meiotic entry decision and germline sex determination indicating that splicing per se, rather than a novel function of a subset of splicing factors, is necessary for these processes.

Role of preadipocyte factor 1 in adipocyte differentiation

Preadipocyte factor 1 (Pref-1) is an EGF-repeat-containing transmembrane protein that inhibits adipogenesis. The extracellular domain of Pref-1 is cleaved by TNF-α converting enzyme to generate the biologically active soluble form of Pref-1. The role of Pref-1 in adipogenesis has been firmly established by in vitro and in vivo studies. Pref-1 activates ERK/MAPK and upregulates Sox9 expression to inhibit adipocyte differentiation. Sox9 directly binds to the promoter regions of CCAAT/enhancer-binding protein-β and CCAAT/enhancer-binding protein-δ in order to suppress their promoter activities in preventing adipocyte differentiation. Here, we describe the function of Pref-1 in adipocyte differentiation and the recent findings on the mechanisms by which Pref-1 inhibits adipocyte differentiation.

Canonical and non-canonical Notch ligands

Notch signaling induced by canonical Notch ligands is critical for normal embryonic development and tissue homeostasis through the regulation of a variety of cell fate decisions and cellular processes. Activation of Notch signaling is normally tightly controlled by direct interactions with ligand-expressing cells, and dysregulated Notch signaling is associated with developmental abnormalities and cancer. While canonical Notch ligands are responsible for the majority of Notch signaling, a diverse group of structurally unrelated noncanonical ligands has also been identified that activate Notch and likely contribute to the pleiotropic effects of Notch signaling. Soluble forms of both canonical and noncanonical ligands have been isolated, some of which block Notch signaling and could serve as natural inhibitors of this pathway. Ligand activity can also be indirectly regulated by other signaling pathways at the level of ligand expression, serving to spatiotemporally compartmentalize Notch signaling activity and integrate Notch signaling into a molecular network that orchestrates developmental events. Here, we review the molecular mechanisms underlying the dual role of Notch ligands as activators and inhibitors of Notch signaling. Additionally, evidence that Notch ligands function independent of Notch is presented. We also discuss how ligand posttranslational modification, endocytosis, proteolysis, and spatiotemporal expression regulate their signaling activity.

DLL4 blockade inhibits tumor growth and reduces tumor-initiating cell frequency

Previous studies have shown that blocking DLL4 signaling reduced tumor growth by disrupting productive angiogenesis. We developed selective anti-human and anti-mouse DLL4 antibodies to dissect the mechanisms involved by analyzing the contributions of selectively targeting DLL4 in the tumor or in the host vasculature and stroma in xenograft models derived from primary human tumors. We found that each antibody inhibited tumor growth and that the combination of the two antibodies was more effective than either alone. Treatment with anti-human DLL4 inhibited the expression of Notch target genes and reduced proliferation of tumor cells. Furthermore, we found that specifically inhibiting human DLL4 in the tumor, either alone or in combination with the chemotherapeutic agent irinotecan, reduced cancer stem cell frequency, as shown by flow cytometric and in vivo tumorigenicity studies.

MSP and GLP-1/Notch signaling coordinately regulate actomyosin-dependent cytoplasmic streaming and oocyte growth in C. elegans

Fertility depends on germline stem cell proliferation, meiosis and gametogenesis, yet how these key transitions are coordinated is unclear. In C. elegans, we show that GLP-1/Notch signaling functions in the germline to modulate oocyte growth when sperm are available for fertilization and the major sperm protein (MSP) hormone is present. Reduction-of-function mutations in glp-1 cause oocytes to grow abnormally large when MSP is present and Galpha(s)-adenylate cyclase signaling in the gonadal sheath cells is active. By contrast, gain-of-function glp-1 mutations lead to the production of small oocytes. Surprisingly, proper oocyte growth depends on distal tip cell signaling involving the redundant function of GLP-1 ligands LAG-2 and APX-1. GLP-1 signaling also affects two cellular oocyte growth processes, actomyosin-dependent cytoplasmic streaming and oocyte cellularization. glp-1 reduction-of-function mutants exhibit elevated rates of cytoplasmic streaming and delayed cellularization. GLP-1 signaling in oocyte growth depends in part on the downstream function of the FBF-1/2 PUF RNA-binding proteins. Furthermore, abnormal oocyte growth in glp-1 mutants, but not the inappropriate differentiation of germline stem cells, requires the function of the cell death pathway. The data support a model in which GLP-1 function in MSP-dependent oocyte growth is separable from its role in the proliferation versus meiotic entry decision. Thus, two major germline signaling centers, distal GLP-1 activation and proximal MSP signaling, coordinate several spatially and temporally distinct processes by which germline stem cells differentiate into functional oocytes.

Identification of a molecular marker for type A spermatogonia by microarray analysis using gonadal cells from pvasa-GFP transgenic rainbow trout (Oncorhynchus mykiss)

The spermatogonia of fish can be classified as being either undifferentiated type A spermatogonia or differentiated type B spermatogonia. Although type A spermatogonia, which contain spermatogonial stem cells, have been demonstrated to be a suitable material for germ cell transplantation, no molecular markers for distinguishing between type A and type B spermatogonia in fish have been developed to date. We therefore sought to develop a molecular marker for type A spermatogonia in rainbow trout. Using GFP-dependent flow cytometry (FCM), enriched fractions of type A and type B spermatogonia, testicular somatic cells, and primordial germ cells were prepared from rainbow trout possessing the green fluorescent protein (GFP) gene driven by trout vasa regulatory regions (pvasa-GFP rainbow trout). The gene-expression profiles of each cell fraction were then compared with a microarray containing cDNAs representing 16,006 genes from several salmonid species. Genes exhibiting high expression for type A spermatogonia relative to above-mentioned other types of gonadal cells were identified and subjected to RT-PCR and quatitative PCR analysis. Since only the rainbow trout notch1 homologue showed significantly high expression in the type A spermatogonia-enriched fraction, we propose that notch1 may be a useful molecular marker for type A spermatogonia. The combination of GFP-dependent FCM and microarray analysis of pvasa-GFP rainbow trout can therefore be applied to the identification of potentially useful molecular markers of germ cells in fish.

Minireview: Pref-1: role in adipogenesis and mesenchymal cell fate

Preadipocyte factor-1 [Pref-1; also called Dlk1 (Delta-like protein 1)] is made as an epidermal growth factor-repeat containing transmembrane protein that produces a biologically active soluble form by TNF-alpha-converting enzyme (TACE)-mediated cleavage. Soluble Pref-1 activates the MAPK kinase/ERK pathway. In adipose tissue, Pref-1 is specifically expressed in preadipocytes but not in adipocytes and thus is used as a preadipocyte marker. Inhibition of adipogenesis by Pref-1 has been well established in vitro as well as in vivo by ablation and overexpression of Pref-1. SRY (sex determining region Y)-box 9 (Sox9), a transcription factor expressed in preadipocytes to suppress CCAAT enhancer binding protein beta and (C/EBP) delta expression, is required to be down-regulated before adipocyte differentiation can proceed. By activating MAPK kinase/ERK, Pref-1 prevents down-regulation of Sox9, resulting in inhibition of adipogenesis. Furthermore, by inducing Sox9, Pref-1 promotes chondrogenic induction of mesenchymal cells but prevents chondrocyte maturation as well as osteoblast differentiation. Thus, Pref-1 directs multipotent mesenchymal cells toward the chondrogenic lineage but inhibits differentiation into adipocytes as well as osteoblasts and chondrocytes. Pref-1, encoded by an imprinted gene, has also been detected in progenitor cells in various tissues during regeneration and therefore may have a more general role in maintaining cells in an undifferentiated state.

Epidermal growth factor-like domain 7 (EGFL7) modulates Notch signalling and affects neural stem cell renewal

Epidermal growth factor-like domain 7 (EGFL7) is a secreted factor implicated in cellular responses such as cell migration and blood vessel formation; however the molecular mechanisms underlying the effects of EGFL7 are largely unknown. Here we have identified transmembrane receptors of the Notch family as EGFL7-binding molecules. Secreted EGFL7 binds to a region in Notch involved in ligand-mediated receptor activation, thus acting as an antagonist of Notch signalling. Expression of EGFL7 in neural stem cells (NSCs) in vitro decreased Notch-specific signalling and consequently, reduced proliferation and self-renewal of NSCs. Such altered Notch signalling caused a shift in the differentiation pattern of cultured NSCs towards an excess of neurons and oligodendrocytes. We identified neurons as a source of EGFL7 in the brain, suggesting that brain-derived EGFL7 acts as an endogenous antagonist of Notch signalling that regulates proliferation and differentiation of subventricular zone-derived adult NSCs.

C. elegans HLH-2/E/Daughterless controls key regulatory cells during gonadogenesis

The Caenorhabditis elegans distal tip cell (DTC) provides a niche for germline stem cells in both hermaphrodites and males. The hermaphrodite distal tip cell (hDTC) also provides "leader" function to control gonadal elongation and shape, while in males, leader function is allocated to the linker cell (LC). Therefore, the male distal tip cell (mDTC) serves as a niche but not as a leader. The C. elegans homolog of E/Daughterless, HLH-2, was previously implicated in hDTC specification. Here we report that HLH-2 is also critical for hDTC maintenance, hDTC niche function and hDTC expression of a lag-2/DSL ligand reporter. We also find that HLH-2 functions in males to direct linker cell specification and to promote both mDTC maintenance and the mDTC niche function. We conclude that HLH-2 functions in both sexes to promote leader cell specification and DTC niche function.