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

Identification of testis development-related genes by combining Iso-Seq and RNA-Seq in Zeugodacus tau

Introduction: Zeugodacus tau (Walker) is an invasive pest. An effective method to control this pest is the sterile insect technique (SIT). To better apply this technique, it is necessary to understand testis development progression. Methods: Differentially expressed genes (DEGs) during testis development were analyzed by PacBio Iso-Seq and RNA-seq. Results: RNA-Seq library of Z. tau testes on day 1, 6, and 11 post eclosion were constructed. We identified 755 and 865 differentially expressed genes in the comparisons of T6 (testes on day 6) vs. T1 and T11 vs. T1, respectively. The KEGG pathway analysis showed that the DEGs were significantly enriched in retinol metabolism, vitamin B6 metabolism, and ascorbate and aldarate metabolism pathways. Knockdown of retinol dehydrogenase 12-like (rdh12-like), pyridoxal kinase (pdxk) and regucalcin (rgn), the representative gene in each of the above 3 pathways, reduced the hatching rate of Z. tau offspring. In addition, we identified 107 Drosophila spermatogenesis-related orthologous genes in Z. tau, of which innexin 2 (inx2) exhibited significantly up-regulated expression throughout testis development, and the knockdown of this gene reduced offspring hatching rate. Discussion: Our data indicated that rdh12-like, pdxk, rgn, and inx2 genes were related to testis development, and they were conserved in tephritid species. These results suggested that this gene might have the same function in tephritid. The findings provide an insight into testis development and spermatogenesis in tephritid species.

A gonadal gap junction INX-14/Notch GLP-1 signaling axis suppresses gut defense through an intestinal lysosome pathway

Gap junctions mediate intercellular communications across cellular networks in the nervous and immune systems. Yet their roles in intestinal innate immunity are poorly understood. Here, we show that the gap junction/innexin subunit inx-14 acts in the C. elegans gonad to attenuate intestinal defenses to Pseudomonas aeruginosa PA14 infection through the PMK-1/p38 pathway. RNA-Seq analyses revealed that germline-specific inx-14 RNAi downregulated Notch/GLP-1 signaling, while lysosome and PMK-1/p38 pathways were upregulated. Consistently, disruption of inx-14 or glp-1 in the germline enhanced resistance to PA14 infection and upregulated lysosome and PMK-1/p38 activity. We show that lysosome signaling functions downstream of the INX-14/GLP-1 signaling axis and upstream of PMK-1/p38 pathway to facilitate intestinal defense. Our findings expand the understanding of the links between the reproductive system and intestinal defense, which may be evolutionarily conserved in higher organism.

Spatial single-cell sequencing of meiosis I arrested oocytes indicates acquisition of maternal transcripts from the soma

Maternal RNAs are stored from minutes to decades in oocytes throughout meiosis I arrest in a transcriptionally quiescent state. Recent reports, however, propose a role for nascent transcription in arrested oocytes. Whether arrested oocytes launch nascent transcription in response to environmental or hormonal signals while maintaining the meiosis I arrest remains undetermined. We test this by integrating single-cell RNA sequencing, RNA velocity, and RNA fluorescence in situ hybridization on C. elegans meiosis I arrested oocytes. We identify transcripts that increase as the arrested meiosis I oocyte ages, but rule out extracellular signaling through ERK MAPK and nascent transcription as a mechanism for this increase. We report transcript acquisition from neighboring somatic cells as a mechanism of transcript increase during meiosis I arrest. These analyses provide a deeper view at single-cell resolution of the RNA landscape of a meiosis I arrested oocyte and as it prepares for oocyte maturation and fertilization.

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.

Oogenesis in Caenorhabditis elegans

BACKGROUND

The nematode, Caenorhabditis elegans has proven itself as a valuable model for investigating metazoan biology. C. elegans have a transparent body, an invariant cell lineage, and a high level of genetic conservation which makes it a desirable model organism. Although used to elucidate many aspects of somatic biology, a distinct advantage of C. elegans is its well annotated germline which allows all aspects of oogenesis to be observed in real time within a single animal. C. elegans hermaphrodites have two U-shaped gonad arms which produce their own sperm that is later stored to fertilise their own oocytes. These two germlines take up much of the internal space of each animal and germ cells are therefore the most abundant cell present within each animal. This feature and the genetic phenotypes observed for mutant worm gonads have allowed many novel findings that established our early understanding of germ cell dynamics. The mutant phenotypes also allowed key features of meiosis and germ cell maturation to be unveiled.

SUMMARY

This review will focus on the key aspects that make C. elegans an outstanding model for exploring each feature of oogenesis. This will include the fundamental steps associated with germline function and germ cell maturation and will be of use for those interested in exploring reproductive metazoan biology.

KEY MESSAGES

Since germ cell biology is highly conserved in animals, much can be gained from study of a simple metazoan like C. elegans. Past findings have enhanced understanding on topics that would be more laborious or challenging in more complex animal models.

Analysis of the C. elegans Germline Stem Cell Pool

The Caenorhabditis elegans germline is an excellent model for studying the genetic and molecular regulation of stem cell self-renewal and progression of cells from a stem cell state to a differentiated state. The germline tissue is organized in an assembly line with the germline stem cell (GSC) pool at one end and differentiated gametes at the other. A simple mesenchymal niche caps the GSC pool and maintains GSCs in an undifferentiated state by signaling through the conserved Notch pathway. Notch signaling activates transcription of the key GSC regulators lst-1 and sygl-1 proteins in a gradient through the GSC pool. LST-1 and SYGL-1 proteins work with PUF RNA regulators in a self-renewal hub to maintain the GSC pool. In this chapter, we present methods for characterizing the C. elegans GSC pool and early stages of germ cell differentiation. The methods include examination of germlines in living and fixed worms, cell cycle analysis, and analysis of markers. We also discuss assays to separate mutant phenotypes that affect the stem cell vs. differentiation decision from those that affect germ cell processes more generally.

Gap junctions mediate discrete regulatory steps during fly spermatogenesis

Gametogenesis requires coordinated signaling between germ cells and somatic cells. We previously showed that Gap junction (GJ)-mediated soma-germline communication is essential for fly spermatogenesis. Specifically, the GJ protein Innexin4/Zero population growth (Zpg) is necessary for somatic and germline stem cell maintenance and differentiation. It remains unknown how GJ-mediated signals regulate spermatogenesis or whether the function of these signals is restricted to the earliest stages of spermatogenesis. Here we carried out comprehensive structure/function analysis of Zpg using insights obtained from the protein structure of innexins to design mutations aimed at selectively perturbing different regulatory regions as well as the channel pore of Zpg. We identify the roles of various regulatory sites in Zpg in the assembly and maintenance of GJs at the plasma membrane. Moreover, mutations designed to selectively disrupt, based on size and charge, the passage of cargos through the Zpg channel pore, blocked different stages of spermatogenesis. Mutations were identified that progressed through early germline and soma development, but exhibited defects in entry to meiosis or sperm individualisation, resulting in reduced fertility or sterility. Our work shows that specific signals that pass through GJs regulate the transition between different stages of gametogenesis.

Innexin function dictates the spatial relationship between distal somatic cells in the Caenorhabditis elegans gonad without impacting the germline stem cell pool

Gap-junctional signaling mediates myriad cellular interactions in metazoans. Yet, how gap junctions control the positioning of cells in organs is not well understood. Innexins compose gap junctions in invertebrates and affect organ architecture. Here, we investigate the roles of gap-junctions in controlling distal somatic gonad architecture and its relationship to underlying germline stem cells in Caenorhabditis elegans. We show that a reduction of soma-germline gap-junctional activity causes displacement of distal sheath cells (Sh1) towards the distal end of the gonad. We confirm, by live imaging, transmission electron microscopy, and antibody staining, that bare regions-lacking somatic gonadal cell coverage of germ cells-are present between the distal tip cell (DTC) and Sh1, and we show that an innexin fusion protein used in a prior study encodes an antimorphic gap junction subunit that mispositions Sh1. We determine that, contrary to the model put forth in the prior study based on this fusion protein, Sh1 mispositioning does not markedly alter the position of the borders of the stem cell pool nor of the progenitor cell pool. Together, these results demonstrate that gap junctions can control the position of Sh1, but that Sh1 position is neither relevant for GLP-1/Notch signaling nor for the exit of germ cells from the stem cell pool.

The C. elegans gonadal sheath Sh1 cells extend asymmetrically over a differentiating germ cell population in the proliferative zone

The Caenorhabditis elegans adult hermaphrodite germline is surrounded by a thin tube formed by somatic sheath cells that support germ cells as they mature from the stem-like mitotic state through meiosis, gametogenesis, and ovulation. Recently, we discovered that the distal Sh1 sheath cells associate with mitotic germ cells as they exit the niche Gordon et al., 2020. Here, we report that these sheath-associated germ cells differentiate first in animals with temperature-sensitive mutations affecting germ cell state, and stem-like germ cells are maintained distal to the Sh1 boundary. We analyze several markers of the distal sheath, which is best visualized with endogenously tagged membrane proteins, as overexpressed fluorescent proteins fail to localize to distal membrane processes and can cause gonad morphology defects. However, such reagents with highly variable expression can be used to determine the relative positions of the two Sh1 cells, one of which often extends further distal than the other.

SAT2 regulates Sertoli cell-germline interactions via STIM1-mediated ROS/WNT/β-catenin signaling pathway

As the main component of seminiferous tubules, Sertoli cells are in close contact with germ cells and generate niche signals, which exhibit pivotal functions in spermatogenesis and male fertility. However, the regulatory mechanisms of Sertoli cell-germline interactions (SGIs) in the testes of neonatal mice (NM) remain largely unclear. Previously, we identified spermidine/spermine N1-acetyl transferase 2 (SAT2) and stromal interaction molecule 1 (STIM1) to be potential regulators of testicular cord formation via comparative proteomics analysis. Here, we demonstrated a novel role of SAT2 for SGIs during testicular development in NM. Testicular explants lacking SAT2 affected the mislocation, but not the quantity, of Sertoli cells, which led to maintenance defects in spermatogonial stem cells (SSCs). Interestingly, SAT2 was essential for the migration of TM4 cells, a Sertoli cell line. Mechanistically, SAT2 was able to bind STIM1, repress its expression, and regulate homeostasis of a reactive oxygen species/wingless type (WNT)/β-catenin pathway in NM testes. Collectively, our study identified that SAT2 was able to regulate SGIs via a STIM1-mediated WNT signaling pathway.

Transcriptome Analysis Reveals miR-302a-3p Affects Granulosa Cell Proliferation by Targeting DRD1 in Chickens

Egg production is an important economic trait in laying chickens as higher yields bring higher profits. Small yellow follicle (SYFL) development is a key determinant of chicken reproductive performance; however, the majority of SYFLs are not selected during the process of chicken reproduction and thus, atresia occurs. Although there have been numerous omic studies focused on egg production, the molecular mechanisms involved are still not well-understood. In this study, we used high-throughput technology to analyze the differences between the SYFL mRNA transcriptomes of high– (H) and low–egg-yielding (L) Taihang layer hens, with the aim of identifying the potential candidate genes involved in controlling the rate of egg production. We constructed six cDNA libraries, three from H and three from L Taihang hens and then performed high-throughput sequencing. Comparison of the H and L groups showed 415 differentially expressed genes (DEGs). In the high-yield group, 226 were upregulated and 189 were downregulated. Differentially enriched biological functions and processes were identified using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) database analysis. Ten of the candidate DEGs we identified (DRD1, MC5R, PCK1, CTSA, TGFBR3, AGO4, SLIT2, RGS1, SCNN1B, and ZP3) have been identified in previous studies as being involved in the development of small yellow follicles. DRD1 was significantly enriched in the gap junction pathway, which is an important pathway in chicken granulosa cells (GCs) to pass nutrition to an oocyte. Homology analysis showed that DRD1 was highly conserved in numerous species, indicating that it may be a productive target for improving egg production. Evidence from bioinformatics analysis revealed that gga-miR-302a-3p putatively targets the 3′UTR region of DRD1. We then identified the functions of gga-miR-302a-3p in follicular granulosa cell proliferation by targeting DRD1. RT-qPCR analysis showed that DRD1 and miR-302a-3p expression were inversely related in the SYLs of high and low egg-yielding chickens. Luciferase assays showed that miR-302a-3p targets the 3′UTR of DRD1, and overexpression of miR-302a-3p significantly inhibits the expression of DRD1 in chicken GCs (p < 0.01). Functional experiments revealed that by targeting DRD1, miR-302a-3p acts as an inhibitor of GC proliferation. Taken together, we concluded that miR-302a-3p affects chicken GC proliferation by targeting DRD1. Our data expanded the knowledge base of genes whose functions are important in egg production and the molecular mechanisms of high-yield egg production in chicken small yellow follicles.

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.

Nuclear lipid droplets and nuclear damage in Caenorhabditis elegans

Fat stored in the form of lipid droplets has long been considered a defining characteristic of cytoplasm. However, recent studies have shown that nuclear lipid droplets occur in multiple cells and tissues, including in human patients with fatty liver disease. The function(s) of stored fat in the nucleus has not been determined, and it is possible that nuclear fat is beneficial in some situations. Conversely, nuclear lipid droplets might instead be deleterious by disrupting nuclear organization or triggering aggregation of hydrophobic proteins. We show here that nuclear lipid droplets occur normally in C. elegans intestinal cells and germ cells, but appear to be associated with damage only in the intestine. Lipid droplets in intestinal nuclei can be associated with novel bundles of microfilaments (nuclear actin) and membrane tubules that might have roles in damage repair. To increase the normal, low frequency of nuclear lipid droplets in wild-type animals, we used a forward genetic screen to isolate mutants with abnormally large or abundant nuclear lipid droplets. Genetic analysis and cloning of three such mutants showed that the genes encode the lipid regulator SEIP-1/seipin, the inner nuclear membrane protein NEMP-1/Nemp1/TMEM194A, and a component of COPI vesicles called COPA-1/α-COP. We present several lines of evidence that the nuclear lipid droplet phenotype of copa-1 mutants results from a defect in retrieving mislocalized membrane proteins that normally reside in the endoplasmic reticulum. The seip-1 mutant causes most germ cells to have nuclear lipid droplets, the largest of which occupy more than a third of the nuclear volume. Nevertheless, the nuclear lipid droplets do not trigger apoptosis, and the germ cells differentiate into gametes that produce viable, healthy progeny. Thus, our results suggest that nuclear lipid droplets are detrimental to intestinal nuclei, but have no obvious deleterious effect on germ nuclei.

Several human disorders such as obesity are associated with abnormal fat storage. Cells normally store fat in cytoplasmic organelles called lipid droplets. However, recent studies have shown that fat can also form inside of the cell nucleus, and the effects of nuclear fat are not known. Here we use the cell biology and genetics of the model organism C. elegans to study the causes and consequences of nuclear fat. We show that intestinal cells can contain nuclear fat, particularly during high-low-high changes in cytoplasmic fat that involve de novo fat synthesis. Nuclear fat is associated with multiple changes in intestinal nuclei that appear to represent damage and repair. Germ nuclei that normally differentiate into oocytes can also contain nuclear fat. In germ cells, however, even high levels of nuclear fat appear to cause little or no damage. Our results suggest that intestinal nuclei and germ cell nuclei might have different responses to nuclear fat in part because they differ in chromosomal organization at the nuclear envelope.

Non-autonomous regulation of germline stem cell proliferation by somatic MPK-1/MAPK activity in C. elegans

Extracellular-signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) is a major positive regulator of cell proliferation, which is often upregulated in cancer. However, few studies have addressed ERK/MAPK regulation of proliferation within a complete organism. The Caenorhabditis elegans ERK/MAPK ortholog MPK-1 is best known for its control of somatic organogenesis and germline differentiation, but it also stimulates germline stem cell proliferation. Here, we show that the germline-specific MPK-1B isoform promotes germline differentiation but has no apparent role in germline stem cell proliferation. By contrast, the soma-specific MPK-1A isoform promotes germline stem cell proliferation non-autonomously. Indeed, MPK-1A functions in the intestine or somatic gonad to promote germline proliferation independent of its other known roles. We propose that a non-autonomous role of ERK/MAPK in stem cell proliferation may be conserved across species and various tissue types, with major clinical implications for cancer and other diseases.

The prevailing paradigm is that ERK/MAPK functions autonomously to promote cell proliferation upon mitogen stimulation. Robinson-Thiewes et al. now demonstrate that C. elegans ERK/MAPK acts within somatic tissues to non-autonomously promote the proliferation of germline stem cells. Germline ERK/MAPK is thus dispensable for germline stem cell proliferation.

What Is Lost in the Weismann Barrier?

The Weismann barrier has long been regarded as a basic tenet of biology. However, upon close examination of its historical origins and August Weismann's own writings, questions arise as to whether such a status is warranted. As scientific research has advanced, the persistence of the concept of the barrier has left us with the same dichotomies Weismann contended with over 100 years ago: germ or soma, gene or environment, hard or soft inheritance. These dichotomies distract from the more important questions we need to address going forward. In this review, we will examine the theories that have shaped Weismann's thinking, how the concept of the Weismann barrier emerged, and the limitations that it carries. We will contrast the principles underlying the barrier with recent and less recent findings in developmental biology and transgenerational epigenetic inheritance that have profoundly eroded the oppositional view of germline vs. soma. Discarding the barrier allows us to examine the interactive processes and their response to environmental context that generate germ cells in the first place, determine the entirety of what is inherited through them, and set the trajectory for the health status of the progeny they bear.

A Limited and Diverse Set of Suppressor Mutations Restore Function to INX-8 Mutant Hemichannels in the Caenorhabditis elegans Somatic Gonad

In Caenorhabditis elegans, gap junctions couple cells of the somatic gonad with the germline to support germ cell proliferation and gametogenesis. A strong loss-of-function mutation (T239I) affects the second extracellular loop (EL2) of the somatic INX-8 hemichannel subunit. These mutant hemichannels form non-functional gap junctions with germline-expressed innexins. We conducted a genetic screen for suppressor mutations that restore germ cell proliferation in the T239I mutant background and isolated seven intragenic mutations, located in diverse domains of INX-8 but not the EL domains. These second-site mutations compensate for the original channel defect to varying degrees, from nearly complete wild-type rescue, to partial rescue of germline proliferation. One suppressor mutation (E350K) supports the innexin cryo-EM structural model that the channel pore opening is surrounded by a cytoplasmic dome. Two suppressor mutations (S9L and I36N) may form leaky channels that support germline proliferation but cause the demise of somatic sheath cells. Phenotypic analyses of three of the suppressors reveal an equivalency in the rescue of germline proliferation and comparable delays in gametogenesis but a graded rescue of fertility. The mutations described here may be useful for elucidating the biochemical pathways that produce the active biomolecules transiting through soma-germline gap junctions.

A Genome-Wide RNAi Screen for Enhancers of a Germline Tumor Phenotype Caused by Elevated GLP-1/Notch Signaling in Caenorhabditis elegans

Stem cells are tightly controlled in vivo Both the balance between self-renewal and differentiation and the rate of proliferation are often regulated by multiple factors. The Caenorhabditis elegans hermaphrodite germ line provides a simple and accessible system for studying stem cells in vivo In this system, GLP-1/Notch activity prevents the differentiation of distal germ cells in response to ligand production from the nearby distal tip cell, thereby supporting a stem cell pool. However, a delay in germline development relative to somatic gonad development can cause a pool of undifferentiated germ cells to persist in response to alternate Notch ligands expressed in the proximal somatic gonad. This pool of undifferentiated germ cells forms a proximal tumor that, in adulthood, blocks the oviduct. This type of "latent niche"-driven proximal tumor is highly penetrant in worms bearing the temperature-sensitive weak gain-of-function mutation glp-1(ar202) at the restrictive temperature. At the permissive temperature, few worms develop tumors. Nevertheless, several interventions elevate the penetrance of proximal tumor formation at the permissive temperature, including reduced insulin signaling or the ablation of distal-most sheath cells. To systematically identify genetic perturbations that enhance proximal tumor formation, we sought genes that, upon RNAi depletion, elevate the percentage of worms bearing proximal germline tumors in glp-1(ar202) at the permissive temperature. We identified 43 genes representing a variety of functional classes, the most enriched of which is "translation". Some of these genes also influence the distal germ line, and some are conserved genes for which genetic interactions with Notch were not previously known in this system.

Molecular basis of reproductive senescence: insights from model organisms

PURPOSE

Reproductive decline due to parental age has become a major barrier to fertility as couples have delayed having offspring into their thirties and forties. Advanced parental age is also associated with increased incidence of neurological and cardiovascular disease in offspring. Thus, elucidating the etiology of reproductive decline is of clinical importance.

METHODS

Deciphering the underlying processes that drive reproductive decline is particularly challenging in women in whom a discrete oocyte pool is established during embryogenesis and may remain dormant for tens of years. Instead, our understanding of the processes that drive reproductive senescence has emerged from studies in model organisms, both vertebrate and invertebrate, that are the focus of this literature review.

CONCLUSIONS

Studies of reproductive aging in model organisms not only have revealed the detrimental cellular changes that occur with age but also are helping identify major regulator proteins controlling them. Here, we discuss what we have learned from model organisms with respect to the molecular mechanisms that maintain both genome integrity and oocyte quality.

Gap junctions deliver malonyl-CoA from soma to germline to support embryogenesis in Caenorhabditis elegans

Gap junctions are ubiquitous in metazoans and play critical roles in important biological processes, including electrical conduction and development. Yet, only a few defined molecules passing through gap junction channels have been linked to specific functions. We isolated gap junction channel mutants that reduce coupling between the soma and germ cells in the Caenorhabditis elegans gonad. We provide evidence that malonyl-CoA, the rate-limiting substrate for fatty acid synthesis (FAS), is produced in the soma and delivered through gap junctions to the germline; there it is used in fatty acid synthesis to critically support embryonic development. Separation of malonyl-CoA production from its site of utilization facilitates somatic control of germline development. Additionally, we demonstrate that loss of malonyl-CoA production in the intestine negatively impacts germline development independently of FAS. Our results suggest that metabolic outsourcing of malonyl-CoA may be a strategy by which the soma communicates nutritional status to the germline.

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.

Stem cell niche exit in C. elegans via orientation and segregation of daughter cells by a cryptic cell outside the niche

Stem cells reside in and rely upon their niche to maintain stemness but must balance self-renewal with the production of daughters that leave the niche to differentiate. We discovered a mechanism of stem cell niche exit in the canonical C. elegans distal tip cell (DTC) germ stem cell niche mediated by previously unobserved, thin, membranous protrusions of the adjacent somatic gonad cell pair (Sh1). A disproportionate number of germ cell divisions were observed at the DTC-Sh1 interface. Stem-like and differentiating cell fates segregated across this boundary. Spindles polarized, pairs of daughter cells oriented between the DTC and Sh1, and Sh1 grew over the Sh1-facing daughter. Impeding Sh1 growth by RNAi to cofilin and Arp2/3 perturbed the DTC-Sh1 interface, reduced germ cell proliferation, and shifted a differentiation marker. Because Sh1 membrane protrusions eluded detection for decades, it is possible that similar structures actively regulate niche exit in other systems.

Integration of Migratory Cells into a New Site In Vivo Requires Channel-Independent Functions of Innexins on Microtubules

During embryonic development and cancer metastasis, migratory cells must establish stable connections with new partners at their destinations. Here, we establish the Drosophila border cells as a model for this multistep process. During oogenesis, border cells delaminate from the follicular epithelium and migrate. When they reach their target, the oocyte, they undergo a stereotypical series of steps to adhere to it, then connect with another migrating epithelium. We identify gap-junction-forming innexin proteins as critical. Surprisingly, the channel function is dispensable. Instead, Innexins 2 and 3 function within the border cells, and Innexin 4 functions within the germline, to regulate microtubules. The microtubule-dependent border cell-oocyte interaction is essential to brace the cells against external morphogenetic forces. Thus, we establish an experimental model and use genetic, thermogenetic, and live-imaging approaches to uncover the contributions of Innexins and microtubules to a cell-biological process important in development and cancer.

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.

Gaps and barriers: Gap junctions as a channel of communication between the soma and the germline

Gap junctions, expressed in most tissues of the body, allow for the cytoplasmic coupling of adjacent cells and promote tissue cooperation. Gap junctions connect also the soma and the germline in many animals, and transmit somatic signals that are crucial for germline maturation and integrity. In this review, we examine the involvement of gap junctions in the relay of information between the soma and the germline, and ask whether such communication could have consequences for the progeny. While the influence of parental experiences on descendants is of great interest, the possibility that gap junctions participate in the transmission of information across generations is largely unexplored.

α-Integrins dictate distinct modes of type IV collagen recruitment to basement membranes

Basement membranes (BMs) are cell-associated extracellular matrices that support tissue integrity, signaling, and barrier properties. Type IV collagen is critical for BM function, yet how it is directed into BMs in vivo is unclear. Through live-cell imaging of endogenous localization, conditional knockdown, and misexpression experiments, we uncovered distinct mechanisms of integrin-mediated collagen recruitment to Caenorhabditis elegans postembryonic gonadal and pharyngeal BMs. The putative laminin-binding αINA-1/βPAT-3 integrin was selectively activated in the gonad and recruited laminin, which directed moderate collagen incorporation. In contrast, the putative Arg-Gly-Asp (RGD)-binding αPAT-2/βPAT-3 integrin was activated in the pharynx and recruited high levels of collagen in an apparently laminin-independent manner. Through an RNAi screen, we further identified the small GTPase RAP-3 (Rap1) as a pharyngeal-specific PAT-2/PAT-3 activator that modulates collagen levels. Together, these studies demonstrate that tissues can use distinct mechanisms to direct collagen incorporation into BMs to precisely control collagen levels and construct diverse BMs.

Plasticity of the Electrical Connectome of C. elegans

The specific patterns and functional properties of electrical synapses of a nervous system are defined by the neuron-specific complement of electrical synapse constituents. We systematically examined the molecular composition of the electrical connectome of the nematode C. elegans through a genome- and nervous-system-wide analysis of the expression patterns of the invertebrate electrical synapse constituents, the innexins. We observe highly complex combinatorial expression patterns throughout the nervous system and found that these patterns change in a strikingly neuron-type-specific manner throughout the nervous system when animals enter an insulin-controlled diapause arrest stage under harsh environmental conditions, the dauer stage. By analyzing several individual synapses, we demonstrate that dauer-specific electrical synapse remodeling is responsible for specific aspects of the altered locomotory and chemosensory behavior of dauers. We describe an intersectional gene regulatory mechanism involving terminal selector and FoxO transcription factors mediating dynamic innexin expression plasticity in a neuron-type- and environment-specific manner.

Innexins: Expression, Regulation, and Functions

The innexin (Inx) proteins form gap junction channels and non-junctional channels (named hemichannels) in invertebrates. These channels participate in cellular communication playing a relevant role in several physiological processes. Pioneer studies conducted mainly in worms and flies have shown that innexins participate in embryo development and behavior. However, recent studies have elucidated new functions of innexins in Arthropoda, Nematoda, Annelida, and Cnidaria, such as immune response, and apoptosis. This review describes emerging data of possible new roles of innexins and summarizes the data available to date.

Spatiotemporal Gene Expression Analysis of the Caenorhabditis elegans Germline Uncovers a Syncytial Expression Switch

Developmental programs are executed by tightly controlled gene regulatory pathways. Here, we combined the unique sample retrieval capacity afforded by laser capture microscopy with analysis of mRNA abundance by CEL-Seq (cell expression by linear amplification and sequencing) to generate a spatiotemporal gene expression map of the Caenorhabditis elegans syncytial germline from adult hermaphrodites and males. We found that over 6000 genes exhibit spatiotemporally dynamic expression patterns throughout the hermaphrodite germline, with two dominant groups of genes exhibiting reciprocal shifts in expression at late pachytene during meiotic prophase I. We found a strong correlation between restricted spatiotemporal expression and known developmental and cellular processes, indicating that these gene expression changes may be an important driver of germ cell progression. Analysis of the male gonad revealed a shift in gene expression at early pachytene and upregulation of subsets of genes following the meiotic divisions, specifically in early and late spermatids, mostly transcribed from the X chromosome. We observed that while the X chromosome is silenced throughout the first half of the gonad, some genes escape this control and are highly expressed throughout the germline. Although we found a strong correlation between the expression of genes corresponding to CSR-1-interacting 22G-RNAs during germ cell progression, we also found that a large fraction of genes may bypass the need for CSR-1-mediated germline licensing. Taken together, these findings suggest the existence of mechanisms that enable a shift in gene expression during prophase I to promote germ cell progression.

A C. elegans Model for the Study of RAGE-Related Neurodegeneration

The receptor for advanced glycation products (RAGE) is a cell surface, multi-ligand receptor belonging to the immunoglobulin superfamily; this receptor is implicated in a variety of maladies, via inflammatory pathways and induction of oxidative stress. Currently, RAGE is being studied using a limited number of mammalian in vivo, and some complementary in vitro, models. Here, we present a Caenorhabditis elegans model for the study of RAGE-related pathology: a transgenic strain, expressing RAGE in all neurons, was generated and subsequently tested behaviorally, developmentally, and morphologically. In addition to RAGE expression being associated with a significantly shorter lifespan, the following behavioral observations were made when RAGE-expressing worms were compared to the wild type: RAGE-expressing worms showed an impaired dopaminergic system, evaluated by measuring the fluorescent signal of GFP tagging; these worms exhibited decreased locomotion-both general and following ethanol exposure-as measured by counting body bends in adult worms; RAGE expression was also associated with impaired recovery of quiescence and pharyngeal pumping secondary to heat shock, as a significantly smaller fraction of RAGE-expressing worms engaged in these behaviors in the 2 h immediately following the heat shock. Finally, significant developmental differences were also found between the two strains: RAGE expression leads to a significantly smaller fraction of hatched eggs 24 h after laying and also to a significantly slower developmental speed overall. As evidence for the role of RAGE in a variety of neuropathologies accumulates, the use of this novel and expedient model should facilitate the elucidation of relevant underlying mechanisms and also the development of efficient therapeutic strategies.

Overlapping expression patterns and functions of three paralogous P5B ATPases in Caenorhabditis elegans

P5B ATPases are present in the genomes of diverse unicellular and multicellular eukaryotes, indicating that they have an ancient origin, and that they are important for cellular fitness. Inactivation of ATP13A2, one of the four human P5B ATPases, leads to early-onset Parkinson's disease (Kufor-Rakeb Syndrome). The presence of an invariant PPALP motif within the putative substrate interaction pocket of transmembrane segment M4 suggests that all P5B ATPases might have similar transport specificity; however, the identity of the transport substrate(s) remains unknown. Nematodes of the genus Caenorhabditis possess three paralogous P5B ATPase genes, catp-5, catp-6 and catp-7, which probably originated from a single ancestral gene around the time of origin of the Caenorhabditid clade. By using CRISPR/Cas9, we have systematically investigated the expression patterns, subcellular localization and biological functions of each of the P5B ATPases of C. elegans. We find that each gene has a unique expression pattern, and that some tissues express more than one P5B. In some tissues where their expression patterns overlap, different P5Bs are targeted to different subcellular compartments (e.g., early endosomes vs. plasma membrane), whereas in other tissues they localize to the same compartment (plasma membrane). We observed lysosomal co-localization between CATP-6::GFP and LMP-1::RFP in transgenic animals; however, this was an artifact of the tagged LMP-1 protein, since anti-LMP-1 antibody staining of native protein revealed that LMP-1 and CATP-6::GFP occupy different compartments. The nematode P5Bs are at least partially redundant, since we observed synthetic sterility in catp-5(0); catp-6(0) and catp-6(0) catp-7(0) double mutants. The double mutants exhibit defects in distal tip cell migration that resemble those of ina-1 (alpha integrin ortholog) and vab-3 (Pax6 ortholog) mutants, suggesting that the nematode P5Bs are required for ina-1and/or vab-3 function. This is potentially a conserved regulatory interaction, since mammalian ATP13A2, alpha integrin and Pax6 are all required for proper dopaminergic neuron function.

Comprehensive single-cell transcriptional profiling of a multicellular organism

To resolve cellular heterogeneity, we developed a combinatorial indexing strategy to profile the transcriptomes of single cells or nuclei, termed sci-RNA-seq (single-cell combinatorial indexing RNA sequencing). We applied sci-RNA-seq to profile nearly 50,000 cells from the nematode Caenorhabditis elegans at the L2 larval stage, which provided >50-fold "shotgun" cellular coverage of its somatic cell composition. From these data, we defined consensus expression profiles for 27 cell types and recovered rare neuronal cell types corresponding to as few as one or two cells in the L2 worm. We integrated these profiles with whole-animal chromatin immunoprecipitation sequencing data to deconvolve the cell type-specific effects of transcription factors. The data generated by sci-RNA-seq constitute a powerful resource for nematode biology and foreshadow similar atlases for other organisms.

Control of oocyte meiotic maturation in C. elegans

In virtually all sexually reproducing animals, oocytes arrest in meiotic prophase and resume meiosis in a conserved biological process called meiotic maturation. Meiotic arrest enables oocytes, which are amongst the largest cells in an organism, to grow and accumulate the necessary cellular constituents required to support embryonic development. Oocyte arrest can be maintained for a prolonged period, up to 50 years in humans, and defects in the meiotic maturation process interfere with the faithful segregation of meiotic chromosomes, representing the leading cause of human birth defects and female infertility. Hormonal signaling and interactions with somatic cells of the gonad control the timing of oocyte meiotic maturation. Signaling activates the CDK1/cyclin B kinase, which plays a central role in regulating the nuclear and cytoplasmic events of meiotic maturation. Nuclear maturation encompasses nuclear envelope breakdown, meiotic spindle assembly, and chromosome segregation whereas cytoplasmic maturation involves major changes in oocyte protein translation and cytoplasmic organelles and is less well understood. Classically, meiotic maturation has been studied in organisms with large oocytes to facilitate biochemical analysis. Recently, the nematode Caenorhabditis elegans is emerging as a genetic paradigm for studying the regulation of oocyte meiotic maturation. Studies in this system have revealed conceptual, anatomical, and molecular links to oocytes in all animals including humans. This review focuses on the signaling mechanisms required to control oocyte growth and meiotic maturation in C. elegans and discusses how the downstream regulation of protein translation coordinates the completion of meiosis and the oocyte-to-embryo transition.

Linking the environment, DAF-7/TGFβ signaling and LAG-2/DSL ligand expression in the germline stem cell niche

The developmental accumulation of proliferative germ cells in the C. elegans hermaphrodite is sensitive to the organismal environment. Previously, we found that the TGFβ signaling pathway links the environment and proliferative germ cell accumulation. Neuronal DAF-7/TGFβ causes a DAF-1/TGFβR signaling cascade in the gonadal distal tip cell (DTC), the germline stem cell niche, where it negatively regulates a DAF-3 SMAD and DAF-5 Sno-Ski. LAG-2, a founding DSL ligand family member, is produced in the DTC and activates the GLP-1/Notch receptor on adjacent germ cells to maintain germline stem cell fate. Here, we show that DAF-7/TGFβ signaling promotes expression of lag-2 in the DTC in a daf-3-dependent manner. Using ChIP and one-hybrid assays, we find evidence for direct interaction between DAF-3 and the lag-2 promoter. We further identify a 25 bp DAF-3 binding element required for the DTC lag-2 reporter response to the environment and to DAF-7/TGFβ signaling. Our results implicate DAF-3 repressor complex activity as a key molecular mechanism whereby the environment influences DSL ligand expression in the niche to modulate developmental expansion of the germline stem cell pool.

Planarian regeneration in space: Persistent anatomical, behavioral, and bacteriological changes induced by space travel

Regeneration is regulated not only by chemical signals but also by physical processes, such as bioelectric gradients. How these may change in the absence of the normal gravitational and geomagnetic fields is largely unknown. Planarian flatworms were moved to the International Space Station for 5 weeks, immediately after removing their heads and tails. A control group in spring water remained on Earth. No manipulation of the planaria occurred while they were in orbit, and space‐exposed worms were returned to our laboratory for analysis. One animal out of 15 regenerated into a double‐headed phenotype—normally an extremely rare event. Remarkably, amputating this double‐headed worm again, in plain water, resulted again in the double‐headed phenotype. Moreover, even when tested 20 months after return to Earth, the space‐exposed worms displayed significant quantitative differences in behavior and microbiome composition. These observations may have implications for human and animal space travelers, but could also elucidate how microgravity and hypomagnetic environments could be used to trigger desired morphological, neurological, physiological, and bacteriomic changes for various regenerative and bioengineering applications.

LIN-41 and OMA Ribonucleoprotein Complexes Mediate a Translational Repression-to-Activation Switch Controlling Oocyte Meiotic Maturation and the Oocyte-to-Embryo Transition in Caenorhabditis elegans

An extended meiotic prophase is a hallmark of oogenesis. Hormonal signaling activates the CDK1/cyclin B kinase to promote oocyte meiotic maturation, which involves nuclear and cytoplasmic events. Nuclear maturation encompasses nuclear envelope breakdown, meiotic spindle assembly, and chromosome segregation. Cytoplasmic maturation involves major changes in oocyte protein translation and cytoplasmic organelles and is poorly understood. In the nematode Caenorhabditis elegans, sperm release the major sperm protein (MSP) hormone to promote oocyte growth and meiotic maturation. Large translational regulatory ribonucleoprotein (RNP) complexes containing the RNA-binding proteins OMA-1, OMA-2, and LIN-41 regulate meiotic maturation downstream of MSP signaling. To understand the control of translation during meiotic maturation, we purified LIN-41-containing RNPs and characterized their protein and RNA components. Protein constituents of LIN-41 RNPs include essential RNA-binding proteins, the GLD-2 cytoplasmic poly(A) polymerase, the CCR4-NOT deadenylase complex, and translation initiation factors. RNA sequencing defined messenger RNAs (mRNAs) associated with both LIN-41 and OMA-1, as well as sets of mRNAs associated with either LIN-41 or OMA-1. Genetic and genomic evidence suggests that GLD-2, which is a component of LIN-41 RNPs, stimulates the efficient translation of many LIN-41-associated transcripts. We analyzed the translational regulation of two transcripts specifically associated with LIN-41 which encode the RNA regulators SPN-4 and MEG-1. We found that LIN-41 represses translation of spn-4 and meg-1, whereas OMA-1 and OMA-2 promote their expression. Upon their synthesis, SPN-4 and MEG-1 assemble into LIN-41 RNPs prior to their functions in the embryo. This study defines a translational repression-to-activation switch as a key element of cytoplasmic maturation.

Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Gap junctions (GJs) are aqueous channels that allow cells to communicate via physiological signals directly. The role of gap junctional connectivity in determining single-cell functions has long been recognized. However, GJs have another important role: the regulation of large-scale anatomical pattern. GJs are not only versatile computational elements that allow cells to control which small molecule signals they receive and emit, but also establish connectivity patterns within large groups of cells. By dynamically regulating the topology of bioelectric networks in vivo, GJs underlie the ability of many tissues to implement complex morphogenesis. Here, a review of recent data on patterning roles of GJs in growth of the zebrafish fin, the establishment of left-right patterning, the developmental dysregulation known as cancer, and the control of large-scale head-tail polarity, and head shape in planarian regeneration has been reported. A perspective in which GJs are not only molecular features functioning in single cells, but also enable global neural-like dynamics in non-neural somatic tissues has been proposed. This view suggests a rich program of future work which capitalizes on the rapid advances in the biophysics of GJs to exploit GJ-mediated global dynamics for applications in birth defects, regenerative medicine, and morphogenetic bioengineering. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 643-673, 2017.

Gap junctions in C. elegans: Their roles in behavior and development

The nematode Caenorhabditis elegans utilizes gap junctions in different fashions in virtually all of its cells. This model animal has a surprisingly large number of innexin genes within its genome, and many nematode cell types can express multiple innexins at once, leading to the formation of diverse junction types and enough redundancy to limit the effect of single gene knockdowns on animal development or behavioral phenotypes. Here, we review the general properties of these junctions, their expression patterns, and their known roles in tissue development and in the animal's connectome. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 587-596, 2017.

Functional analysis of microRNA pathway genes in the somatic gonad and germ cells during ovulation in C. elegans

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play critical roles in animal development and physiology, though functions for most miRNAs remain unknown. Worms with reduced miRNA biogenesis due to loss of Drosha or Pasha/DGCR8 activity are sterile and fail to ovulate, indicating that miRNAs are required for the process of oocyte maturation and ovulation. Starting with this penetrant sterile phenotype and using new strains created to perform tissue specific RNAi, we characterized the roles of the C. elegans Pasha, pash-1, and two miRNA-specific Argonautes, alg-1 and alg-2, in somatic gonad cells and in germ cells in the regulation of ovulation. Conditional loss of pash-1 activity resulted in a reduced rate of ovulation and in basal and ovulatory sheath contractions. Similarly, knockdown of miRNA-specific Argonautes in the cells of the somatic gonad by tissue-specific RNAi results in a reduction of the ovulation rate and in basal and ovulatory sheath contractions. Reduced miRNA pathway gene activity resulted in a range of defects, including oocytes that were pinched upon entry of the oocyte into the distal end of the spermatheca in about 42% of the ovulation events observed following alg-1 RNAi. This phenotype was not observed on worms exposed to control RNAi. In contrast, knockdown of alg-1 and alg-2 in germ cells results in few defects in oocyte maturation and ovulation. These data identify specific steps in the process of ovulation that require miRNA pathway gene activity in the somatic gonad cells.

DAF-18/PTEN signals through AAK-1/AMPK to inhibit MPK-1/MAPK in feedback control of germline stem cell proliferation

Under replete growth conditions, abundant nutrient uptake leads to the systemic activation of insulin/IGF-1 signalling (IIS) and the promotion of stem cell growth/proliferation. Activated IIS can stimulate the ERK/MAPK pathway, the activation of which also supports optimal stem cell proliferation in various systems. Stem cell proliferation rates can further be locally refined to meet the resident tissue's need for differentiated progeny. We have recently shown that the accumulation of mature oocytes in the C. elegans germ line, through DAF-18/PTEN, inhibits adult germline stem cell (GSC) proliferation, despite high systemic IIS activation. We show here that this feedback occurs through a novel cryptic signalling pathway that requires PAR-4/LKB1, AAK-1/AMPK and PAR-5/14-3-3 to inhibit the activity of MPK-1/MAPK, antagonize IIS, and inhibit both GSC proliferation and the production of additional oocytes. Interestingly, our results imply that DAF-18/PTEN, through PAR-4/LKB1, can activate AAK-1/AMPK in the absence of apparent energy stress. As all components are conserved, similar signalling cascades may regulate stem cell activities in other organisms and be widely implicated in cancer.

Sperm Navigation Mechanisms in the Female Reproductive Tract

Fertilization, the union of an oocyte and a sperm, is a fundamental process that restores the diploid genome and initiates embryonic development. For the sperm, fertilization is the end of a long journey, one that starts in the male testis before transitioning to the female reproductive tract's convoluted tubule architecture. Historically, motile sperm were thought to complete this journey using luck and numbers. A different picture of sperm has emerged recently as cells that integrate complex sensory information for navigation. Chemical, physical, and thermal cues have been proposed to help guide sperm to the waiting oocyte. Molecular mechanisms are being delineated in animal models and humans, revealing common features, as well as important differences. Exposure to pheromones and nutritional signals can modulate guidance mechanisms, indirectly impacting sperm motility performance and fertility. These studies highlight the importance of sensory information and signal transduction in fertilization.

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.

Exogenous Molecule and Organelle Delivery in Oogenesis

Recent discoveries on the delivery of small- and large-size molecules and organelles to the oocytes/eggs from external sources, such as surrounding somatic cells, body fluids, and sperm, change our understanding of female germ cells' (oocytes and eggs) self-containment and individuality. In this chapter, we will summarize present-day knowledge on sources and presumptive functions of different types of exogenous molecules and organelles delivered to the animal oocytes and eggs.

Maternal MEMI Promotes Female Meiosis II in Response to Fertilization in Caenorhabditis elegans

In most animals, female meiosis completes only after fertilization. Sperm entry has been implicated in providing a signal for the initiation of the final meiotic processes; however, a maternal component required for this process has not been previously identified. We report the characterization of a novel family of three highly similar paralogs (memi-1, memi-2, memi-3) that encode oocyte-specific proteins. A hyper-morphic mutation memi-1(sb41) results in failure to exit female meiosis II properly; however, loss of all three paralogs results in a "skipped meiosis II" phenotype. Mutations that prevent fertilization, such as fer-1(hc1), also cause a skipped meiosis II phenotype, suggesting that the MEMI proteins represent a maternal component of a postfertilization signal that specifies the meiosis II program. MEMI proteins are degraded before mitosis and sensitive to ZYG-11, a substrate-specific adapter for cullin-based ubiquitin ligase activity, and the memi-1(sb41) mutation results in inappropriate persistence of the MEMI-1 protein into mitosis. Using an RNAi screen for suppressors of memi-1(sb41), we identified a sperm-specific PP1 phosphatase, GSP-3/4, as a putative sperm component of the MEMI pathway. We also found that MEMI and GSP-3/4 proteins can physically interact via co-immunoprecipitation. These results suggest that sperm-specific PP1 and maternal MEMI proteins act in the same pathway after fertilization to facilitate proper meiosis II and the transition into embryonic mitosis.

UBR-5, a Conserved HECT-Type E3 Ubiquitin Ligase, Negatively Regulates Notch-Type Signaling in Caenorhabditis elegans

Notch-type signaling mediates cell-cell interactions important for animal development. In humans, reduced or inappropriate Notch signaling activity is associated with various developmental defects and disease states, including cancers. Caenorhabditis elegans expresses two Notch-type receptors, GLP-1 and LIN-12. GLP-1 mediates several cell-signaling events in the embryo and promotes germline proliferation in the developing and adult gonad. LIN-12 acts redundantly with GLP-1 in certain inductive events in the embryo and mediates several cell-cell interactions during larval development. Recovery of genetic suppressors and enhancers of glp-1 or lin-12 loss- or gain-of-function mutations has identified numerous regulators of GLP-1 and LIN-12 signaling activity. Here, we report the molecular identification of sog-1, a gene identified in screens for recessive suppressors of conditional glp-1 loss-of-function mutations. The sog-1 gene encodes UBR-5, the sole C. elegans member of the UBR5/Hyd family of HECT-type E3 ubiquitin ligases. Molecular and genetic analyses indicate that the loss of ubr-5 function suppresses defects caused by reduced signaling via GLP-1 or LIN-12. In contrast, ubr-5 mutations do not suppress embryonic or larval lethality associated with mutations in a downstream transcription factor, LAG-1. In the gonad, ubr-5 acts in the receiving cells (germ cells) to limit GLP-1 signaling activity. SEL-10 is the F-box component of SCF(SEL-10) E3 ubiquitin-ligase complex that promotes turnover of Notch intracellular domain. UBR-5 acts redundantly with SEL-10 to limit Notch signaling in certain tissues. We hypothesize that UBR-5 activity limits Notch-type signaling by promoting turnover of receptor or limiting its interaction with pathway components.

Cell cycle features of C. elegans germline stem/progenitor cells vary temporally and spatially

Many organisms accumulate a pool of germline stem cells during development that is maintained in later life. The dynamics of establishment, expansion and homeostatic maintenance of this pool are subject to both developmental and physiological influences including the availability of a suitable niche microenvironment, nutritional status, and age. Here, we investigated the dynamics of germline proliferation during stages of expansion and homeostasis, using the C. elegans germ line as a model. The vast majority of germ cells in the proliferative zone are in interphase stages of mitosis (G1, S, G2) rather than in the active mitotic (M) phase. We examined mitotic index and DNA content, comparing different life stages, mutants, and physiological conditions. We found that germ cells in larval stages cycle faster than in adult stages, but that this difference could not be attributed to sexual fate of the germ cells. We also found that larval germ cells exhibit a lower average DNA content compared to adult germ cells. We extended our analysis to consider the effects of distance from the niche and further found that the spatial pattern of DNA content differs between larval and adult stages in the wild type and among mutants in pathways that interfere with cell cycle progression, cell fate, or both. Finally, we characterized expansion of the proliferative pool of germ cells during adulthood, using a regeneration paradigm (ARD recovery) in which animals are starved and re-fed. We compared adult stage regeneration and larval stage expansion, and found that the adult germ line is capable of rapid accumulation but does not sustain a larval-level mitotic index nor does it recapitulate the larval pattern of DNA content. The regenerated germ line does not reach the number of proliferative zone nuclei seen in the continuously fed adult. Taken together, our results suggest that cell cycle dynamics are under multiple influences including distance from the niche, age and/or maturation of the germ line, nutrition and, possibly, latitude for physical expansion.

Cell-cycle quiescence maintains Caenorhabditis elegans germline stem cells independent of GLP-1/Notch

Many types of adult stem cells exist in a state of cell-cycle quiescence, yet it has remained unclear whether quiescence plays a role in maintaining the stem cell fate. Here we establish the adult germline of Caenorhabditis elegans as a model for facultative stem cell quiescence. We find that mitotically dividing germ cells--including germline stem cells--become quiescent in the absence of food. This quiescence is characterized by a slowing of S phase, a block to M-phase entry, and the ability to re-enter M phase rapidly in response to re-feeding. Further, we demonstrate that cell-cycle quiescence alters the genetic requirements for stem cell maintenance: The signaling pathway required for stem cell maintenance under fed conditions--GLP-1/Notch signaling--becomes dispensable under conditions of quiescence. Thus, cell-cycle quiescence can itself maintain stem cells, independent of the signaling pathway otherwise essential for such maintenance.

Mechano-logical model of C. elegans germ line suggests feedback on the cell cycle

The Caenorhabditis elegans germ line is an outstanding model system in which to study the control of cell division and differentiation. Although many of the molecules that regulate germ cell proliferation and fate decisions have been identified, how these signals interact with cellular dynamics and physical forces within the gonad remains poorly understood. We therefore developed a dynamic, 3D in silico model of the C. elegans germ line, incorporating both the mechanical interactions between cells and the decision-making processes within cells. Our model successfully reproduces key features of the germ line during development and adulthood, including a reasonable ovulation rate, correct sperm count, and appropriate organization of the germ line into stably maintained zones. The model highlights a previously overlooked way in which germ cell pressure may influence gonadogenesis, and also predicts that adult germ cells might be subject to mechanical feedback on the cell cycle akin to contact inhibition. We provide experimental data consistent with the latter hypothesis. Finally, we present cell trajectories and ancestry recorded over the course of a simulation. The novel approaches and software described here link mechanics and cellular decision-making, and are applicable to modeling other developmental and stem cell systems.