The EGL-13 SOX Domain Transcription Factor Affects the Uterine Cell Lineages in Caenorhabditis elegans

Mechanisms of lineage specification in Caenorhabditis elegans

The studies of cell fate and lineage specification are fundamental to our understanding of the development of multicellular organisms. Caenorhabditis elegans has been one of the premiere systems for studying cell fate specification mechanisms at single cell resolution, due to its transparent nature, the invariant cell lineage, and fixed number of somatic cells. We discuss the general themes and regulatory mechanisms that have emerged from these studies, with a focus on somatic lineages and cell fates. We next review the key factors and pathways that regulate the specification of discrete cells and lineages during embryogenesis and postembryonic development; we focus on transcription factors and include numerous lineage diagrams that depict the expression of key factors that specify embryonic founder cells and postembryonic blast cells, and the diverse somatic cell fates they generate. We end by discussing some future perspectives in cell and lineage specification.

Differential production rates of cytosolic and transmembrane GFP reporters in C. elegans L3 larval uterine cells

Transgene driven protein expression is an important tool for investigating developmental mechanisms in C. elegans . Here, we have assessed protein production rates and levels in L3 larval uterine cells (UCs). Using ubiquitous promoter driven cytosolic and transmembrane tethered GFP, fluorescence recovery after photobleaching, and quantitative fluorescence analysis, we reveal that cytosolic GFP is produced at an ~two-fold higher rate than transmembrane tethered GFP and accumulates at ~five-fold higher levels in UCs. We also provide evidence that cytosolic GFP in the anchor cell, a specialized UC that mediates uterine-vulval connection, is more rapidly degraded through an autophagy-independent mechanism.

The transcriptional corepressor CTBP-1 acts with the SOX family transcription factor EGL-13 to maintain AIA interneuron cell identity in Caenorhabditis elegans

Cell identity is characterized by a distinct combination of gene expression, cell morphology, and cellular function established as progenitor cells divide and differentiate. Following establishment, cell identities can be unstable and require active and continuous maintenance throughout the remaining life of a cell. Mechanisms underlying the maintenance of cell identities are incompletely understood. Here, we show that the gene ctbp-1, which encodes the transcriptional corepressor C-terminal binding protein-1 (CTBP-1), is essential for the maintenance of the identities of the two AIA interneurons in the nematode Caenorhabditis elegans. ctbp-1 is not required for the establishment of the AIA cell fate but rather functions cell-autonomously and can act in later larval stage and adult worms to maintain proper AIA gene expression, morphology and function. From a screen for suppressors of the ctbp-1 mutant phenotype, we identified the gene egl-13, which encodes a SOX family transcription factor. We found that egl-13 regulates AIA function and aspects of AIA gene expression, but not AIA morphology. We conclude that the CTBP-1 protein maintains AIA cell identity in part by utilizing EGL-13 to repress transcriptional activity in the AIAs. More generally, we propose that transcriptional corepressors like CTBP-1 might be critical factors in the maintenance of cell identities, harnessing the DNA-binding specificity of transcription factors like EGL-13 to selectively regulate gene expression in a cell-specific manner.

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

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

Boundary cells restrict dystroglycan trafficking to control basement membrane sliding during tissue remodeling

Epithelial cells and their underlying basement membranes (BMs) slide along each other to renew epithelia, shape organs, and enlarge BM openings. How BM sliding is controlled, however, is poorly understood. Using genetic and live cell imaging approaches during uterine-vulval attachment in C. elegans, we have discovered that the invasive uterine anchor cell activates Notch signaling in neighboring uterine cells at the boundary of the BM gap through which it invades to promote BM sliding. Through an RNAi screen, we found that Notch activation upregulates expression of ctg-1, which encodes a Sec14-GOLD protein, a member of the Sec14 phosphatidylinositol-transfer protein superfamily that is implicated in vesicle trafficking. Through photobleaching, targeted knockdown, and cell-specific rescue, our results suggest that CTG-1 restricts BM adhesion receptor DGN-1 (dystroglycan) trafficking to the cell-BM interface, which promotes BM sliding. Together, these studies reveal a new morphogenetic signaling pathway that controls BM sliding to remodel tissues.

Spatial and molecular cues for cell outgrowth during C. elegans uterine development

The Caenorhabditis elegans uterine seam cell (utse) is an H-shaped syncytium that connects the uterus to the body wall. Comprising nine nuclei that move outward in a bidirectional manner, this synctium undergoes remarkable shape change during development. Using cell ablation experiments, we show that three surrounding cell types affect utse development: the uterine toroids, the anchor cell and the sex myoblasts. The presence of the anchor cell (AC) nucleus within the utse is necessary for proper utse development and AC invasion genes fos-1, cdh-3, him-4, egl-43, zmp-1 and mig-10 promote utse cell outgrowth. Two types of uterine lumen epithelial cells, uterine toroid 1 (ut1) and uterine toroid 2 (ut2), mediate proper utse outgrowth and we show roles in utse development for two genes expressed in the uterine toroids: the RASEF ortholog rsef-1 and Trio/unc-73. The SM expressed gene unc-53/NAV regulates utse cell shape; ablation of sex myoblasts (SMs), which generate uterine and vulval muscles, cause defects in utse morphology. Our results clarify the nature of the interactions that exist between utse and surrounding tissue, identify new roles for genes involved in cell outgrowth, and present the utse as a new model system for understanding cell shape change and, putatively, diseases associated with cell shape change.

A proneural gene controls C. elegans neuroblast asymmetric division and migration

Proneural genes control the generation of neuroblasts from the neuroepithelium, but their functions in neuroblast asymmetric division and migration remain elusive. Here, we identified Caenorhabditiselegans mutants of a proneural transcription factor (TF) lin-32, in which Q neuroblasts are produced. We showed that LIN-32 functions in parallel with a storkhead TF, HAM-1, to regulate Q neuroblast asymmetric division, and that Q neuroblast migration is inhibited in lin-32 alleles. Consistently, lin-32 is expressed throughout Q neuroblast lineage, suggesting that LIN-32 may promote different target gene expression. Our studies thus uncovered previously unknown functions of a proneural gene in neuroblast development.

Morphogenesis of the caenorhabditis elegans vulva

Understanding how cells move, change shape, and alter cellular behaviors to form organs, a process termed morphogenesis, is one of the great challenges of developmental biology. Formation of the Caenorhabditis elegans vulva is a powerful, simple, and experimentally accessible model for elucidating how morphogenetic processes produce an organ. In the first step of vulval development, three epithelial precursor cells divide and differentiate to generate 22 cells of 7 different vulval subtypes. The 22 vulval cells then rearrange from a linear array into a tube, with each of the seven cell types undergoing characteristic morphogenetic behaviors that construct the vulva. Vulval morphogenesis entails many of the same cellular activities that underlie organogenesis and tissue formation across species, including invagination, lumen formation, oriented cell divisions, cell–cell adhesion, cell migration, cell fusion, extracellular matrix remodeling, and cell invasion. Studies of vulval development have led to pioneering discoveries in a number of these processes and are beginning to bridge the gap between the pathways that specify cells and their connections to morphogenetic behaviors. The simplicity of the vulva and the experimental tools available in C. elegans will continue to make vulval morphogenesis a powerful paradigm to further our understanding of the largely mysterious mechanisms that build tissues and organs.

Developmental stage-dependent transcriptional regulatory pathways control neuroblast lineage progression

Neuroblasts generate neurons with different functions by asymmetric cell division, cell cycle exit and differentiation. The underlying transcriptional regulatory pathways remain elusive. Here, we performed genetic screens in C. elegans and identified three evolutionarily conserved transcription factors (TFs) essential for Q neuroblast lineage progression. Through live cell imaging and genetic analysis, we showed that the storkhead TF HAM-1 regulates spindle positioning and myosin polarization during asymmetric cell division and that the PAR-1-like kinase PIG-1 is a transcriptional regulatory target of HAM-1. The TEAD TF EGL-44, in a physical association with the zinc-finger TF EGL-46, instructs cell cycle exit after the terminal division. Finally, the Sox domain TF EGL-13 is necessary and sufficient to establish the correct neuronal fate. Genetic analysis further demonstrated that HAM-1, EGL-44/EGL-46 and EGL-13 form three transcriptional regulatory pathways. We have thus identified TFs that function at distinct developmental stages to ensure appropriate neuroblast lineage progression and suggest that their vertebrate homologs might similarly regulate neural development.

Cell fusion in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, 300 of the 959 somatic nuclei present in the adult hermaphrodite are located in syncytia. These syncytia are formed by the fusion of mononucleate cells throughout embryonic and postembryonic development. These cell fusions occur in a well-characterized stereotypical pattern, allowing investigators to study many cell fusion events at the molecular and cellular levels. Using tools that allow visualization of cell membranes, cell junctions, and cell cytoplasm during fusion, genetic screens have identified many C. elegans cell fusion genes, including those that regulate the fusion cell fate decision and two genes that encode components of the cell fusion machinery.

TheC. elegansCBFβ homologue BRO-1 interacts with the Runx factor, RNT-1, to promote stem cell proliferation and self-renewal

In this report, we investigate the C. elegans CBFβ homologue,BRO-1. bro-1 mutants have a similar male-specific sensory ray loss phenotype to rnt-1 (the C. elegans homologue of the mammalian CBFβ-interacting Runx factors), caused by failed cell divisions in the seam lineages. Our studies indicate that BRO-1 and RNT-1 form a cell proliferation-promoting complex, and that BRO-1 increases both the affinity and specificity of RNT-1-DNA interactions. Overexpression of bro-1,like rnt-1, leads to an expansion of seam cell number and co-overexpression of bro-1 and rnt-1 results in massive seam cell hyperplasia. Finally, we find that BRO-1 appears to act independently of RNT-1 in certain situations. These studies provide new insights into the function and regulation of this important cancer-associated DNA-binding complex in stem cells and support the view that Runx/CBFβ factors have oncogenic potential.

Co-regulation by Notch and Fos is required for cell fate specification of intermediate precursors during C. elegans uterine development

The Notch pathway is the key signal for many cell fate decisions in the nematode Caenorhabditis elegans including the uterine pi cell fate, crucial for a proper uterine-vulval connection and egg laying. Expression of the egl-13 SOX domain transcription factor is specifically upregulated upon induction of the pi lineage and not in response to other LIN-12/Notch-mediated decisions. We determined that dual regulation by LIN-12 and FOS-1 is required for egl-13 expression at specification and for complete rescue of egl-13 mutants. We found that fos-1 mutants exhibit uterine defects and fail to express pi markers. We show that FOS-1 is expressed at pi cell specification and can bind in vitro to egl-13 upstream regulatory sequence (URS) as a heterodimer with C. elegans Jun.

Modulation of Caenorhabditis elegans transcription factor activity by HIM-8 and the related Zinc-Finger ZIM proteins

The previously reported negative regulatory activity of HIM-8 on the Sox protein EGL-13 is shared by the HIM-8-related ZIM proteins. Furthermore, mutation of HIM-8 can modulate the effects of substitution mutations in the DNA-binding domains of at least four other transcription factors, suggesting broad regulatory activity by HIM-8.

Regulation of anchor cell invasion and uterine cell fates by the egl-43 Evi-1 proto-oncogene in Caenorhabditis elegans

Cell invasion is a tightly controlled process occurring during development and tumor progression. The nematode Caenorhabditis elegans serves as a genetic model to study cell invasion during normal development. In the third larval stage, the anchor cell in the somatic gonad first induces and then invades the adjacent epidermal vulval precursor cells. The homolog of the Evi-1 oncogene, egl-43, is necessary for basement membrane destruction and anchor cell invasion. egl-43 is part of a regulatory network mediating cell invasion downstream of the fos-1 proto-oncogene. In addition, EGL-43 is required to specify the cell fates of ventral uterus cells downstream of or in parallel with LIN-12 NOTCH. Comparison with mammalian Evi-1 suggests a conserved pathway controlling cell invasion and cell fate specification.

AFF-1, a FOS-1-regulated fusogen, mediates fusion of the anchor cell in C. elegans

Cell fusion is fundamental for reproduction and organ formation. Fusion between most C. elegans epithelial cells is mediated by the EFF-1 fusogen. However, fusion between the anchor cell and the utse syncytium that establishes a continuous uterine-vulval tube proceeds normally in eff-1 mutants. By isolating mutants where the anchor-cell fails to fuse, we identified aff-1. AFF-1 ectopic expression results in fusion of cells that normally do not fuse in C. elegans. The fusogen activity of AFF-1 was further confirmed by its ability to fuse heterologous cells. AFF-1 and EFF-1 differ in their fusogenic activity and expression patterns but share eight conserved predicted disulfide bonds in their ectodomains, including a putative TGF-beta-type-I-Receptor domain. We found that FOS-1, the Fos transcription factor ortholog that controls anchor-cell invasion during nematode development, is a specific activator of aff-1-mediated anchor-cell fusion. Thus, FOS-1 links cell invasion and fusion in a developmental cascade.

The Pax2/5/8 gene egl-38 coordinates organogenesis of the C. elegansegg-laying system

Organogenesis requires coordinated development between different tissues and cells. The Pax family of transcription factors coordinates multiple developmental events in organs including the kidney, thyroid and the eye. Studying Pax factors in different organisms should identify unifying characteristics of organ development with implications to both development and disease. Here we investigate the function of the Pax2/5/8 transcription factor EGL-38 in coordinating development of the C. elegans egg-laying system. A functional egg-laying system requires cell fate specification events in the epithelial cells of the vulva as well as the mesodermal cells in the uterus of the somatic gonad. Using gene expression studies, genetic mutant analysis and genetic mosaics, we show that egl-38 has functions in both tissues of the organ to promote its development. We incorporate these results together with previous results to propose that EGL-38 plays multiple roles in the development of the egg-laying system, acting to both promote cell fate and to coordinate the development between different cell types. As the Pax2 gene performs similar roles in the development of the mammalian kidney, we show that coordinating organogenesis is a conserved function for Pax2/5/8 transcription factors.

Molecular characterization of the Caenorhabditis elegans REF-1 family member, hlh-29/hlh-28

Members of the Caenorhabditis elegans REF-1 family of bHLH proteins are atypical in that each protein contains two bHLH domains. In this study we describe a functional and molecular characterization of the REF-1 family members, hlh-29/hlh-28. 5'-RACE results confirm the presence of two bHLH domain coding regions in a single transcript and quantitative PCR (qPCR) shows that hlh-29/hlh-28 mRNA is detected in wild-type animals throughout development. A promoter fusion of hlh-29 to the green fluorescent protein shows post-embryonic reporter activity in cells of the vulva, the somatic gonad, the intestine and in neuronal cells of the head and tail. Loss of hlh-29/hlh-28 function via RNA interference (RNAi) results in multiple phenotypes including late embryonic lethality, yolk protein accumulation, everted vulva, bordering behavior, and alter chemosensory responses.

N-ethylmaleimide sensitive factor is required for fusion of the C. elegans uterine anchor cell

The fusion of the Caenorhabditis elegans uterine anchor cell (AC) with the uterine-seam cell (utse) is an excellent model system for studying cell-cell fusion, which is essential to animal development. We obtained an egg-laying defective (Egl) mutant in which the AC fails to fuse with the utse. This defect is highly specific: other aspects of utse development and other cell fusions appear to occur normally. We find that defect is due to a missense mutation in the nsf-1 gene, which encodes N-ethylmaleimide-sensitive factor (NSF), an intracellular membrane fusion factor. There are two NSF-1 isoforms, which are expressed in distinct tissues through two separate promoters. NSF-1L is expressed in the uterus, including the AC. We find that nsf-1 is required cell-autonomously in the AC for its fusion with the utse. Our results establish AC fusion as a paradigm for studying cell fusion at single cell resolution and demonstrate that the NSF ATPase is a key player in this process.

C. elegans HIM-8 functions outside of meiosis to antagonize EGL-13 Sox protein function

egl-13 encodes a Sox domain protein that is required for proper uterine seam cell development in Caenorhabditis elegans. We demonstrate that mutations of the C2H2 zinc fingers encoded by the him-8 (high incidence of males) gene partially suppress the egg-laying and connection-of-gonad morphology defects caused by incompletely penetrant alleles of egl-13. him-8 alleles have previously characterized recessive effects on recombination and segregation of the X chromosome during meiosis due to failure of X chromosome homolog pairing and subsequent synapsis. However, we show that him-8 alleles are semi-dominant suppressors of egl-13, and the semi-dominant effect is due to haplo-insufficiency of the him-8 locus. Thus, we conclude that the wild-type him-8 gene product acts antagonistically to EGL-13. Null alleles of egl-13 cannot be suppressed, suggesting that this antagonistic interaction most likely occurs either upstream of or in parallel with EGL-13. Moreover, we conclude that suppression of egl-13 is due to a meiosis-independent function of him-8 because suppression is observed in mutants that have severely reduced meiotic germ cell populations and suppression does not depend on the function of him-8 in the maternal germ line. We also show that the chromosomal context of egl-13 seems important in the him-8 suppression mechanism. Interactions between these genes can give insight into function of Sox family members, which are important in many aspects of metazoan development, and into functions of him-8 outside of meiosis.

Chromatin and RNAi factors protect the C. elegans germline against repetitive sequences

Protection of genomes against invasion by repetitive sequences, such as transposons, viruses, and repetitive transgenes, involves strong and selective silencing of these sequences. During silencing of repetitive transgenes, a trans effect ("cosuppression") occurs that results in silencing of cognate endogenous genes. Here we report RNA interference (RNAi) screens performed to catalog genes required for cosuppression in the Caenorhabditis elegans germline. We find factors with a putative role in chromatin remodeling and factors involved in RNAi. Together with molecular data also presented in this study, these results suggest that in C. elegans repetitive sequences trigger transcriptional gene silencing using RNAi and chromatin factors.