CDC-25.1 stability is regulated by distinct domains to restrict cell division during embryogenesis in C. elegans

Defect-buffering cellular plasticity increases robustness of metazoan embryogenesis

Developmental processes are intrinsically robust so as to preserve a normal-like state in response to genetic and environmental fluctuations. However, the robustness and potential phenotypic plasticity of individual developing cells under genetic perturbations remain to be systematically evaluated. Using large-scale gene perturbation, live imaging, lineage tracing, and single-cell phenomics, we quantified the phenotypic landscape of C. elegans embryogenesis in >2,000 embryos following individual knockdown of over 750 conserved genes. We observed that cellular genetic systems are not sufficiently robust to single-gene perturbations across all cells; rather, gene knockdowns frequently induced cellular defects. Dynamic phenotypic analyses revealed many cellular defects to be transient, with cells exhibiting phenotypic plasticity that serves to alleviate, correct, and accommodate the defects. Moreover, potential developmentally related cell modules may buffer the phenotypic effects of individual cell position changes. Our findings reveal non-negligible contributions of cellular plasticity and multicellularity as compensatory strategies to increase developmental robustness.

Small RNAs couple embryonic developmental programs to gut microbes

Embryogenesis has long been known for its robustness to environmental factors. Although developmental tuning of embryogenesis to the environment experienced by the parent may be beneficial, little is understood on whether and how developmental patterns proactively change. Here, we show that Caenorhabditis elegans undergoes alternative embryogenesis in response to maternal gut microbes. Harmful microbes result in altered endodermal cell divisions; morphological changes, including left-right asymmetric development; double association between intestinal and primordial germ cells; and partial rescue of fecundity. The miR-35 microRNA family, which is controlled by systemic endogenous RNA interference and targets the β-transducin repeat-containing protein/cell division cycle 25 (CDC25) pathway, transmits intergenerational information to regulate cell divisions and reproduction. Our findings challenge the widespread assumption that C. elegans has an invariant cell lineage that consists of a fixed cell number and provide insights into how organisms optimize embryogenesis to adapt to environmental changes through epigenetic control.

Developmental Control of the Cell Cycle: Insights from Caenorhabditis elegans

During animal development, a single fertilized egg forms a complete organism with tens to trillions of cells that encompass a large variety of cell types. Cell cycle regulation is therefore at the center of development and needs to be carried out in close coordination with cell differentiation, migration, and death, as well as tissue formation, morphogenesis, and homeostasis. The timing and frequency of cell divisions are controlled by complex combinations of external and cell-intrinsic signals that vary throughout development. Insight into how such controls determine in vivo cell division patterns has come from studies in various genetic model systems. The nematode Caenorhabditis elegans has only about 1000 somatic cells and approximately twice as many germ cells in the adult hermaphrodite. Despite the relatively small number of cells, C. elegans has diverse tissues, including intestine, nerves, striated and smooth muscle, and skin. C. elegans is unique as a model organism for studies of the cell cycle because the somatic cell lineage is invariant. Somatic cells divide at set times during development to produce daughter cells that adopt reproducible developmental fates. Studies in C. elegans have allowed the identification of conserved cell cycle regulators and provided insights into how cell cycle regulation varies between tissues. In this review, we focus on the regulation of the cell cycle in the context of C. elegans development, with reference to other systems, with the goal of better understanding how cell cycle regulation is linked to animal development in general.

cdc-25.2, a Caenorhabditis elegans ortholog of cdc25, is required for male tail morphogenesis

Cell division cycle 25 (Cdc25) is an evolutionarily conserved phosphatase that promotes cell cycle progression by activating cyclin-dependent kinases (Cdks) which are inactivated by Wee1/Myt1 kinases. It was previously reported that cdc-25.2 promotes oocyte maturation and intestinal cell divisions in Caenorhabditis elegans hermaphrodites. Here, we report a novel function of cdc-25.2 in male tail development which was significantly deformed by cdc-25.2 RNAi depletion and in cdc-25.2 mutant males. The deformation was also observed after RNAi depletion of other cell cycle regulators, cdk-1, cyb-3, cyd-1, and cyl-1. Furthermore, wee-1.3 counteracted cdc-25.2 in male tail development as observed in oocyte maturation and intestine development. The number of cells in ray precursor cell lineages was significantly reduced in cdc-25.2 depleted males. These results indicate that CDC-25.2 is essential for cell divisions in ray precursor cell lineages for proper male tail development.

CDC-25.2, a C. elegans ortholog of cdc25, is essential for the progression of intestinal divisions

Intestinal divisions in Caenorhabditis elegans take place in 3 stages: (1) cell divisions during embryogenesis, (2) binucleations at the L1 stage, and (3) endoreduplications at the end of each larval stage. Here, we report that CDC-25.2, a C. elegans ortholog of Cdc25, is required for these specialized division cycles between the 16E cell stage and the onset of endoreduplication. Results of our genetic analyses suggest that CDC-25.2 regulates intestinal cell divisions and binucleations by counteracting WEE-1.3 and by activating the CDK-1/CYB-1 complex. CDC-25.2 activity is then repressed by LIN-23 E3 ubiquitin ligase before the onset of intestinal endoreduplication, and this repression is maintained by LIN-35, the C. elegans ortholog of Retinoblastoma (Rb). These findings indicate that timely regulation of CDC-25.2 activity is essential for the progression of specialized division cycles and development of the C. elegans intestine.

cdc-25.4, a Caenorhabditis elegans Ortholog of cdc25, Is Required for Male Mating Behavior

Cell division cycle 25 (cdc25) is an evolutionarily conserved phosphatase that promotes cell cycle progression. Among the four cdc25 orthologs in Caenorhabditis elegans, we found that cdc-25.4 mutant males failed to produce outcrossed progeny. This was not caused by defects in sperm development, but by defects in male mating behavior. The cdc-25.4 mutant males showed various defects during male mating, including contact response, backing, turning, and vulva location. Aberrant turning behavior was the most prominent defect in the cdc-25.4 mutant males. We also found that cdc-25.4 is expressed in many neuronal cells throughout development. The turning defect in cdc-25.4 mutant males was recovered by cdc-25.4 transgenic expression in neuronal cells, suggesting that cdc-25.4 functions in neurons for male mating. However, the neuronal morphology of cdc-25.4 mutant males appeared to be normal, as examined with several neuronal markers. Also, RNAi depletion of wee-1.3, a C. elegans ortholog of Wee1/Myt1 kinase, failed to suppress the mating defects of cdc-25.4 mutant males. These findings suggest that, for successful male mating, cdc-25.4 does not target cell cycles that are required for neuronal differentiation and development. Rather, cdc-25.4 likely regulates noncanonical substrates in neuronal cells.

LIN-23, an E3 Ubiquitin Ligase Component, Is Required for the Repression of CDC-25.2 Activity during Intestinal Development in Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) utilizes two different cell-cycle modes, binucleations during the L1 larval stage and endoreduplications at four larval moltings, for its postembryonic intestinal development. Previous genetic studies indicated that CDC-25.2 is specifically required for binucleations at the L1 larval stage and is repressed before endoreduplications. Furthermore, LIN-23, the C. elegans β-TrCP ortholog, appears to function as a repressor of CDC-25.2 to prevent excess intestinal divisions. We previously reported that intestinal hyperplasia in lin-23(e1883) mutants was effectively suppressed by the RNAi depletion of cdc-25.2. Nevertheless, LIN-23 targeting CDC-25.2 for ubiquitination as a component of E3 ubiquitin ligase has not yet been tested. In this study, LIN-23 is shown to be the major E3 ubiquitin ligase component, recognizing CDC-25.2 to repress their activities for proper transition of cell-cycle modes during the C. elegans postembryonic intestinal development. In addition, for the first time that LIN-23 physically interacts with both CDC-25.1 and CDC-25.2 and facilitates ubiquitination for timely regulation of their activities during the intestinal development.

Timing of Tissue-specific Cell Division Requires a Differential Onset of Zygotic Transcription during Metazoan Embryogenesis

Metazoan development demands not only precise cell fate differentiation but also accurate timing of cell division to ensure proper development. How cell divisions are temporally coordinated during development is poorly understood. Caenorhabditis elegans embryogenesis provides an excellent opportunity to study this coordination due to its invariant development and widespread division asynchronies. One of the most pronounced asynchronies is a significant delay of cell division in two endoderm progenitor cells, Ea and Ep, hereafter referred to as E2, relative to its cousins that mainly develop into mesoderm organs and tissues. To unravel the genetic control over the endoderm-specific E2 division timing, a total of 822 essential and conserved genes were knocked down using RNAi followed by quantification of cell cycle lengths using in toto imaging of C. elegans embryogenesis and automated lineage. Intriguingly, knockdown of numerous genes encoding the components of general transcription pathway or its regulatory factors leads to a significant reduction in the E2 cell cycle length but an increase in cell cycle length of the remaining cells, indicating a differential requirement of transcription for division timing between the two. Analysis of lineage-specific RNA-seq data demonstrates an earlier onset of transcription in endoderm than in other germ layers, the timing of which coincides with the birth of E2, supporting the notion that the endoderm-specific delay in E2 division timing demands robust zygotic transcription. The reduction in E2 cell cycle length is frequently associated with cell migration defect and gastrulation failure. The results suggest that a tissue-specific transcriptional activation is required to coordinate fate differentiation, division timing, and cell migration to ensure proper development.

High-Throughput Profiling of Caenorhabditis elegans Starvation-Responsive microRNAs

MicroRNAs (miRNAs) are non-coding RNAs of ~22 nucleotides in length that regulate gene expression by interfering with the stability and translation of mRNAs. Their expression is regulated during development, under a wide variety of stress conditions and in several pathological processes. In nature, animals often face feast or famine conditions. We observed that subjecting early L4 larvae from Caenorhabditis elegans to a 12-hr starvation period produced worms that are thinner and shorter than well-fed animals, with a decreased lipid accumulation, diminished progeny, reduced gonad size, and an increased lifespan. Our objective was to identify which of the 302 known miRNAs of C. elegans changed their expression under starvation conditions as compared to well-fed worms by means of deep sequencing in early L4 larvae. Our results indicate that 13 miRNAs (miR-34-3p, the family of miR-35-3p to miR-41-3p, miR-39-5p, miR-41-5p, miR-240-5p, miR-246-3p and miR-4813-5p) were upregulated, while 2 miRNAs (let-7-3p and miR-85-5p) were downregulated in 12-hr starved vs. well-fed early L4 larvae. Some of the predicted targets of the miRNAs that changed their expression in starvation conditions are involved in metabolic or developmental process. In particular, miRNAs of the miR-35 family were upregulated 6–20 fold upon starvation. Additionally, we showed that the expression of gld-1, important in oogenesis, a validated target of miR-35-3p, was downregulated when the expression of miR-35-3p was upregulated. The expression of another reported target, the cell cycle regulator lin-23, was unchanged during starvation. This study represents a starting point for a more comprehensive understanding of the role of miRNAs during starvation in C. elegans.

SKN-1/Nrf, stress responses, and aging in Caenorhabditis elegans

The mammalian Nrf/CNC proteins (Nrf1, Nrf2, Nrf3, p45 NF-E2) perform a wide range of cellular protective and maintenance functions. The most thoroughly described of these proteins, Nrf2, is best known as a regulator of antioxidant and xenobiotic defense, but more recently has been implicated in additional functions that include proteostasis and metabolic regulation. In the nematode Caenorhabditis elegans, which offers many advantages for genetic analyses, the Nrf/CNC proteins are represented by their ortholog SKN-1. Although SKN-1 has diverged in aspects of how it binds DNA, it exhibits remarkable functional conservation with Nrf/CNC proteins in other species and regulates many of the same target gene families. C. elegans may therefore have considerable predictive value as a discovery model for understanding how mammalian Nrf/CNC proteins function and are regulated in vivo. Work in C. elegans indicates that SKN-1 regulation is surprisingly complex and is influenced by numerous growth, nutrient, and metabolic signals. SKN-1 is also involved in a wide range of homeostatic functions that extend well beyond the canonical Nrf2 function in responses to acute stress. Importantly, SKN-1 plays a central role in diverse genetic and pharmacologic interventions that promote C. elegans longevity, suggesting that mechanisms regulated by SKN-1 may be of conserved importance in aging. These C. elegans studies predict that mammalian Nrf/CNC protein functions and regulation may be similarly complex and that the proteins and processes that they regulate are likely to have a major influence on mammalian life- and healthspan.

Developmental robustness in the Caenorhabditis elegans embryo

Developmental robustness is the ability of an embryo to develop normally despite many sources of variation, from differences in the environment to stochastic cell-to-cell differences in gene expression. The nematode Caenorhabditis elegans exhibits an additional level of robustness: Unlike most other animals, the embryonic pattern of cell divisions is nearly identical from animal to animal. The endoderm (gut) lineage is an ideal model for studying such robustness as the juvenile gut has a simple anatomy, consisting of 20 cells that are derived from a single cell, E, and the gene regulatory network that controls E specification shares features with developmental regulatory networks in many other systems, including genetic redundancy, parallel pathways, and feed-forward loops. Early studies were initially concerned with identifying the genes in the network, whereas recent work has focused on understanding how the endoderm produces a robust developmental output in the face of many sources of variation. Genetic control exists at three levels of endoderm development: Progenitor specification, cell divisions within the developing gut, and maintenance of gut differentiation. Recent findings show that specification genes regulate all three of these aspects of gut development, and that mutant embryos can experience a "partial" specification state in which some, but not all, E descendants adopt a gut fate. Ongoing studies using newer quantitative and genome-wide methods promise further insights into how developmental gene-regulatory networks buffer variation.

G1/S Inhibitors and the SWI/SNF Complex Control Cell-Cycle Exit during Muscle Differentiation

The transition from proliferating precursor cells to post-mitotic differentiated cells is crucial for development, tissue homeostasis, and tumor suppression. To study cell-cycle exit during differentiation in vivo, we developed a conditional knockout and lineage-tracing system for Caenorhabditis elegans. Combined lineage-specific gene inactivation and genetic screening revealed extensive redundancies between previously identified cell-cycle inhibitors and the SWI/SNF chromatin-remodeling complex. Muscle precursor cells missing either SWI/SNF or G1/S inhibitor function could still arrest cell division, while simultaneous inactivation of these regulators caused continued proliferation and a C. elegans tumor phenotype. Further genetic analyses support that SWI/SNF acts in concert with hlh-1 MyoD, antagonizes Polycomb-mediated transcriptional repression, and suppresses cye-1 Cyclin E transcription to arrest cell division of muscle precursors. Thus, SWI/SNF and G1/S inhibitors provide alternative mechanisms to arrest cell-cycle progression during terminal differentiation, which offers insight into the frequent mutation of SWI/SNF genes in human cancers.

A model of the regulatory network involved in the control of the cell cycle and cell differentiation in the Caenorhabditis elegans vulva

BACKGROUND

There are recent experimental reports on the cross-regulation between molecules involved in the control of the cell cycle and the differentiation of the vulval precursor cells (VPCs) of Caenorhabditis elegans. Such discoveries provide novel clues on how the molecular mechanisms involved in the cell cycle and cell differentiation processes are coordinated during vulval development. Dynamic computational models are helpful to understand the integrated regulatory mechanisms affecting these cellular processes.

RESULTS

Here we propose a simplified model of the regulatory network that includes sufficient molecules involved in the control of both the cell cycle and cell differentiation in the C. elegans vulva to recover their dynamic behavior. We first infer both the topology and the update rules of the cell cycle module from an expected time series. Next, we use a symbolic algorithmic approach to find which interactions must be included in the regulatory network. Finally, we use a continuous-time version of the update rules for the cell cycle module to validate the cyclic behavior of the network, as well as to rule out the presence of potential artifacts due to the synchronous updating of the discrete model. We analyze the dynamical behavior of the model for the wild type and several mutants, finding that most of the results are consistent with published experimental results.

CONCLUSIONS

Our model shows that the regulation of Notch signaling by the cell cycle preserves the potential of the VPCs and the three vulval fates to differentiate and de-differentiate, allowing them to remain completely responsive to the concentration of LIN-3 and lateral signal in the extracellular microenvironment.

E3 ubiquitin ligases promote progression of differentiation during C. elegans embryogenesis

Regulated choice between cell fate maintenance and differentiation provides decision points in development to progress toward more restricted cell fates or to maintain the current one. Caenorhabditis elegans embryogenesis follows an invariant cell lineage where cell fate is generally more restricted upon each cell division. EMS is a progenitor cell in the four-cell embryo that gives rise to the endomesoderm. We recently found that when ubiquitin-mediated protein degradation is compromised, the anterior daughter of EMS, namely MS, reiterates the EMS fate. This observation demonstrates an essential function of ubiquitin-mediated protein degradation in driving the progression of EMS-to-MS differentiation. Here we report a genome-wide screen of the ubiquitin pathway and extensive lineage analyses. The results suggest a broad role of E3 ligases in driving differentiation progression. First, we identified three substrate-binding proteins for two Cullin-RING ubiquitin ligase (CRL) E3 complexes that promote the progression from the EMS fate to MS, namely LIN-23/β-TrCP and FBXB-3 for the CRL1/SCF complex and ZYG-11/ZYG-11B for the CRL2 complex. Genetic analyses suggest these E3 ligases function through a multifunctional protein OMA-1 and the endomesoderm lineage specifier SKN-1 to drive differentiation. Second, we found that depletion of components of the CRL1/SCF complex induces fate reiteration in all major founder cell lineages. These data suggest that regulated choice between self-renewal and differentiation is widespread during C. elegans embryogenesis as in organisms with regulative development, and ubiquitin-mediated protein degradation drives the choice towards differentiation. Finally, bioinformatic analysis of time series gene expression data showed that expression of E3 genes is transiently enriched during time windows of developmental stage transitions. Transcription factors show similar enrichment, but not other classes of regulatory genes. Based on these findings we propose that ubiquitin-mediated protein degradation, like many transcription factors, function broadly as regulators driving developmental progression during embryogenesis in C. elegans.

Uncoupling different characteristics of the C. elegans E lineage from differentiation of intestinal markers

In the 4-cell C. elegans embryo, a signal from P₂ to its anterior sister, EMS, specifies the posterior daughter of EMS, E, as the sole founder cell for intestine. The P₂-to-EMS signal restricts high level zygotic expression of the redundant GATA transcription factors, END-1 and END-3, to only the E lineage. Expression of END-1 or END-3 in early blastomeres is sufficient to drive intestinal differentiation. We show here that a number of E lineage characteristics, which are also regulated through P₂-EMS signaling, can be uncoupled from intestine development, and each with a different sensitivity to specific perturbations of the P₂-EMS signal. For example, we show that the extended cell cycle in Ea and Ep depends on the P₂-induced high level expression of the cell cycle regulator, WEE-1.1, in E. A mutation in wee-1.1 results in shortened Ea and Ep cell cycles, but has no effect upon intestinal differentiation or embryogenesis. Furthermore, it has been shown previously that the total number of E lineage cell divisions is regulated by a mechanism dependent upon E being specified as the intestinal founder cell. We now show, however, that cell division counting can be uncoupled from intestine differentiation in the E lineage. Many mutations in P₂-EMS signal genes exhibit nonfully-penetrant defects in intestinal differentiation. When embryos with those mutations generate intestinal cells, they often make too many intestinal cells. In addition, at the level of individual embryos, expression of end-1 and end-3, and another very early E-specific zygotic gene, sdz-23, exhibit stochastic and discordant defects in P₂-EMS signaling mutants. We show here that sdz-23 is expressed close to wildtype levels in embryos deleted of both end-1 and end-3. sdz-23 does not appear to function in intestine development, raising the intriguing possibility that the P₂-EMS interaction has downstream molecular consequences within the E lineage independent of end-1/3 and intestinal development.

Down-regulation of tricarboxylic acid (TCA) cycle genes blocks progression through the first mitotic division in Caenorhabditis elegans embryos

The cell cycle is a highly regulated process that enables the accurate transmission of chromosomes to daughter cells. Here we uncover a previously unknown link between the tricarboxylic acid (TCA) cycle and cell cycle progression in the Caenorhabditis elegans early embryo. We found that down-regulation of TCA cycle components, including citrate synthase, malate dehydrogenase, and aconitase, resulted in a one-cell stage arrest before entry into mitosis: pronuclear meeting occurred normally, but nuclear envelope breakdown, centrosome separation, and chromosome condensation did not take place. Mitotic entry is controlled by the cyclin B-cyclin-dependent kinase 1 (Cdk1) complex, and the inhibitory phosphorylation of Cdk1 must be removed in order for the complex to be active. We found that following down-regulation of the TCA cycle, cyclin B levels were normal but CDK-1 remained inhibitory-phosphorylated in one-cell stage-arrested embryos, indicative of a G2-like arrest. Moreover, this was not due to an indirect effect caused by checkpoint activation by DNA damage or replication defects. These observations suggest that CDK-1 activation in the C. elegans one-cell embryo is sensitive to the metabolic state of the cell, and that down-regulation of the TCA cycle prevents the removal of CDK-1 inhibitory phosphorylation. The TCA cycle was previously shown to be necessary for the development of the early embryo in mammals, but the molecular processes affected were not known. Our study demonstrates a link between the TCA cycle and a specific cell cycle transition in the one-cell stage embryo.

Gene transcription is coordinated with, but not dependent on, cell divisions during C. elegans embryonic fate specification

Cell differentiation and proliferation are coordinated during animal development, but the link between them remains uncharacterized. To examine this relationship, we combined single-molecule RNA imaging with time-lapse microscopy to generate high-resolution measurements of transcriptional dynamics in Caenorhabditis elegans embryogenesis. We found that globally slowing the overall development rate of the embryo by altering temperature or by mutation resulted in cell proliferation and transcription slowing, but maintaining, their relative timings, suggesting that cell division may directly control transcription. However, using mutants with specific defects in cell cycle pathways that lead to abnormal lineages, we found that the order between cell divisions and expression onset can switch, showing that expression of developmental regulators is not strictly dependent on cell division. Delaying cell divisions resulted in only slight changes in absolute expression time, suggesting that expression and proliferation are independently entrained to a separate clock-like process. These changes in relative timing can change the number of cells expressing a gene at a given time, suggesting that timing may help determine which cells adopt particular transcriptional patterns. Our results place limits on the types of mechanisms that are used during normal development to ensure that division timing and fate specification occur at appropriate times.

A novel mutation in β integrin reveals an integrin-mediated interaction between the extracellular matrix and cki-1/p27KIP1

The cell-extracellular matrix (ECM) interaction plays an essential role in maintaining tissue shapes and regulates cell behaviors such as cell adhesion, differentiation and proliferation. The mechanism by which the ECM influences the cell cycle in vivo is poorly understood. Here we demonstrate that the β integrin PAT-3 regulates the localization and expression of CKI-1, a C. elegans homologue of the cyclin dependent kinase inhibitor p27(KIP1). In nematodes expressing wild type PAT-3, CKI-1::GFP localizes primarily to nucleoli in hypodermal cells, whereas in animals expressing mutant pat-3 with a defective splice junction, CKI-1::GFP appears clumped and disorganized in nucleoplasm. RNAi analysis links cell adhesion genes to the regulation of CKI-1. RNAi of unc-52/perlecan, ina-1/α integrin, pat-4/ILK, and unc-97/PINCH resulted in abnormal CKI-1::GFP localization. Additional RNAi experiments revealed that the SCF E3 ubiquitin-ligase complex genes, skpt-1/SKP2, cul-1/CUL1 and lin-23/F-box, are required for the proper localization and expression of CKI-1, suggesting that integrin signaling and SCF E3 ligase work together to regulate the cellular distribution of CKI-1. These data also suggest that integrin plays a major role in maintaining proper CKI-1/p27(KIP1) levels in the cell. Perturbed integrin signaling may lead to the inhibition of SCF ligase activity, mislocalization and elevation of CKI-1/p27(KIP1). These results suggest that adhesion signaling is crucial for cell cycle regulation in vivo.

The C. elegans F-box proteins LIN-23 and SEL-10 antagonize centrosome duplication by regulating ZYG-1 levels

The correct segregation of DNA during cell division requires formation of a bipolar spindle, organized at each pole by a centrosome. The regulation of centrosome duplication such that each mitotic cell has exactly two centrosomes is therefore of central importance to cell division. Deregulation of centrosome duplication causes the appearance of supernumerary centrosomes, which are a hallmark of many cancer cells and can contribute to tumorigenesis. Overexpression of the kinase Plk4, which is required for centrosome duplication, causes the formation of extra centrosomes, and aberrant Plk4 expression levels are associated with cancer. Data from Drosophila and human cells show that Plk4 levels are regulated by the SCF ubiquitin ligase and proteasomal degradation. Recognition of Plk4 by the SCF complex is mediated by the F-box protein Slimb/βTrCP. We show that levels of the C. elegans Plk4 homolog ZYG-1 are elevated by impairing proteasome or SCF function, indicating that ZYG-1 is regulated by a conserved mechanism. In C. elegans, similar to Drosophila and humans, we find that the Slimb/βTrCP homolog LIN-23 regulates ZYG-1 levels. In addition, we show that a second F-box protein, SEL-10, also contributes to ZYG-1 regulation. Co-depletion of LIN-23 and SEL-10 suggests these proteins function cooperatively. Because SEL-10 is the homolog of human FBW7, which is frequently mutated in cancer, our findings have implications for understanding tumorigenesis.

mir-35 is involved in intestine cell G1/S transition and germ cell proliferation in C. elegans

MicroRNA (miRNA) regulates gene expression in many cellular events, yet functions of only a few miRNAs are known in C. elegans. We analyzed the function of mir-35-41 unique to the worm, and show here that mir-35 regulates the G1/S transition of intestinal cells and germ cell proliferation. Loss of mir-35 leads to a decrease of nuclei numbers in intestine and distal mitotic gonad, while re-introduction of mir-35 rescues the mutant phenotypes. Genetic analysis indicates that mir-35 may act through Rb/E2F and SCF pathways. Further bioinformatic and functional analyses demonstrate that mir-35 targets evolutionally conserved lin-23 and gld-1. Together, our study reveals a novel function of mir-35 family in cell division regulation.

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.

Fate specification and tissue-specific cell cycle control of the Caenorhabditis elegans intestine

The Caenorhabditis elegans β-TrCP orthologue LIN-23 of maternal origin regulates a progressive decline of CDC-25.1 abundance over several embryonic cell-cycles and specifies cell number of one tissue, the embryonic intestine.

Coordination between cell fate specification and cell cycle control in multicellular organisms is essential to regulate cell numbers in tissues and organs during development, and its failure may lead to oncogenesis. In mammalian cells, as part of a general cell cycle checkpoint mechanism, the F-box protein β-transducin repeat-containing protein (β-TrCP) and the Skp1/Cul1/F-box complex control the periodic cell cycle fluctuations in abundance of the CDC25A and B phosphatases. Here, we find that the Caenorhabditis elegans β-TrCP orthologue LIN-23 regulates a progressive decline of CDC-25.1 abundance over several embryonic cell cycles and specifies cell number of one tissue, the embryonic intestine. The negative regulation of CDC-25.1 abundance by LIN-23 may be developmentally controlled because CDC-25.1 accumulates over time within the developing germline, where LIN-23 is also present. Concurrent with the destabilization of CDC-25.1, LIN-23 displays a spatially dynamic behavior in the embryo, periodically entering a nuclear compartment where CDC-25.1 is abundant.

A mutation of cdc-25.1 causes defects in germ cells but not in somatic tissues in C. elegans

By screening C. elegans mutants for severe defects in germline proliferation, we isolated a new loss-of-function allele of cdc-25.1, bn115. bn115 and another previously identified loss-of-function allele nr2036 do not exhibit noticeable cell division defects in the somatic tissues but have reduced numbers of germ cells and are sterile, indicating that cdc-25.1 functions predominantly in the germ line during postembryonic development, and that cdc-25.1 activity is probably not required in somatic lineages during larval development. We analyzed cell division of germ cells and somatic tissues in bn115 homozygotes with germline-specific anti-PGL-1 immunofluorescence and GFP transgenes that express in intestinal cells, in distal tip cells, and in gonadal sheath cells, respectively. We also analyzed the expression pattern of cdc-25.1 with conventional and quantitative RT-PCR. In the presence of three other family members of cdc-25 in C. elegans defects are observed only in the germ line but not in the somatic tissues in cdc-25.1 single mutants, and cdc-25.1 is expressed predominantly, if not exclusively, in the germ line during postembryonic stages. Our findings indicate that the function of cdc-25.1 is unique in the germ line but likely redundant with other members in the soma.

Inhibition of overexpressed CDC-25.1 phosphatase activity by flavone in Caenorhabditis elegans

We previously reported that flavone induces embryonic lethality in Caenorhabditis elegans, which appeared to be the result of cell cycle arrest during early embryogenesis. To test this possibility, here we examined whether flavone inhibits the activity of a key cell cycle regulator, CDC-25.1 in C. elegans. A gain-of-function cdc-25.1 mutant, rr31, which exhibits extra cell divisions in intestinal cells, was used to test the inhibitory effects of flavone on CDC-25 activity. Flavone inhibited the extra cell divisions of intestinal cells in rr31, and modifications of flavone reduced the inhibitory effects. The inhibitory effects of flavone on CDC-25.1 were partly, if not completely, due to transcriptional repression.

Suppressors of the cdc-25.1(gf)-associated intestinal hyperplasia reveal important maternal roles for prp-8 and a subset of splicing factors in C. elegans

The maternal contribution of gene products enables embryos to initiate their developmental program in the absence of zygotic gene expression. In Caenorhabditis elegans, maternal CDC-25.1 levels are tightly regulated to promote early cell divisions, while stabilization of this phosphatase by gain-of-function mutations gives rise to intestinal-specific hyperplasia. To identify regulators of CDC-25.1 levels and/or function, we performed a modifier screen of the cdc-25.1(gf)-dependent hyperplasia. One of the isolated suppressor mutants possesses a donor splice site mutation in prp-8, a key splicing factor of the U5-specific snRNP. prp-8(rr40) produces aberrant prp-8 splice variants that generate C-terminal truncations at the expense of wild-type prp-8. Levels of maternal transcripts are reduced, including cdc-25.1, while zygotic transcripts appear unperturbed, suggesting a germ-line-specific role for this splicing factor in regulating the splicing, and consequently, the steady-state levels of maternal transcripts. Using a novel feeding RNAi strategy we found that only a subset of splicing factors suppress cdc-25.1(gf), suggesting that they too may play specific roles in germ-line spliceosome function. In humans, mutations in the corresponding hPrp8 C-terminal domain result in retinitis pigmentosa, a retinal-specific disorder. Intriguingly, despite affecting the general splicing apparatus, both human and C. elegans show tissue-specific defects resulting from mutations in this key splicing component. Our findings suggest that in addition to its important regulatory function in the C. elegans germ line, prp-8(rr40) may provide further insight into the etiology of this splicing-associated human disorder.