Role of Major Sperm Protein (MSP) in the Protrusion and Retraction of Ascaris Sperm

Programmed DNA elimination in the parasitic nematode Ascaris

In most organisms, the whole genome is maintained throughout the life span. However, exceptions occur in some species where the genome is reduced during development through a process known as programmed DNA elimination (PDE). In the human and pig parasite Ascaris, PDE occurs during the 4 to 16 cell stages of embryogenesis, when germline chromosomes are fragmented and specific DNA sequences are reproducibly lost in all somatic cells. PDE was identified in Ascaris over 120 years ago, but little was known about its molecular details until recently. Genome sequencing revealed that approximately 1,000 germline-expressed genes are eliminated in Ascaris, suggesting PDE is a gene silencing mechanism. All germline chromosome ends are removed and remodeled during PDE. In addition, PDE increases the number of chromosomes in the somatic genome by splitting many germline chromosomes. Comparative genomics indicates that these germline chromosomes arose from fusion events. PDE separates these chromosomes at the fusion sites. These observations indicate that PDE plays a role in chromosome karyotype and evolution. Furthermore, comparative analysis of PDE in other parasitic and free-living nematodes illustrates conserved features of PDE, suggesting it has important biological significance. We summarize what is known about PDE in Ascaris and its relatives. We also discuss other potential functions, mechanisms, and the evolution of PDE in these parasites of humans and animals of veterinary importance.

Strongyloides spp. eliminate male-determining sperm post-meiotically

If normal male meiosis occurs, it would be expected that 50 % of sperm lack an X chromosome (nullo X) and hence upon fertilisation, result in male progeny. However, for sexual reproduction within the free-living stages of Strongyloides spp. male offspring are absent. We had shown earlier by quantitative whole genome sequencing that within Strongyloides spp., nullo-X sperm are either absent (S. papillosus) or underrepresented (S. ratti) among mature sperm. To investigate how and when this elimination of male-determining sperm occurs, we characterised spermatogenesis and the dynamic localisation of important molecular players such as tubulin, actin and major sperm protein by DIC microscopy, immunohistochemistry, and fluorescent in situ hybridization (FISH) in S. ratti, S. papillosus and Parastrongyloides trichosuri. We found that meiotic divisions in these parasites proceeded as expected for organisms with XO males, resulting in four equally sized spermatocytes, two with and two without an X chromosome. However, mature sperm were found to almost always contain an X chromosome. We also observed structures that contained protein constituents of sperm, such as actin and major sperm protein (MSP) but no DNA. These structures resemble C. elegans residual bodies in appearance and may assume their function. We hypothesize that spermatocytes without an X-chromosome undergo some form of programmed cell death and transform into these residual body-like structures. As in C. elegans, MSP is found in fibrous body-membranous organelles (FB-MOs). Knocking down MSP by RNAi showed that MSP is essential for fertility in S. ratti, as it is in C. elegans.

Sperm-specific glycogen synthase kinase 3 is required for sperm motility and the post-fertilization signal for female meiosis II in Caenorhabditis elegans

In most sexually reproducing animals, sperm entry provides the signal to initiate the final stages of female meiosis. In Caenorhabditis elegans, this signal is required for completion of female anaphase I and entry into meiosis II (MII). memi-1/2/3 (meiosis-to-mitosis) encode maternal components that facilitate this process; memi-1/2/3(RNAi) results in a skipped-MII phenotype. Previously, we used a gain-of-function mutation, memi-1(sb41), to identify genetic suppressors that represent candidates for the sperm-delivered signal. Herein, we characterize two suppressors of memi-1(sb41): gskl-1 and gskl-2. Both genes encode functionally redundant sperm glycogen synthase kinase, type 3 (GSK3) protein kinases. Loss of both genes causes defects in male spermatogenesis, sperm pseudopod treadmilling and paternal-effect embryonic lethality. The two kinases locate within the pseudopod of activated sperm, suggesting that they directly or indirectly regulate the sperm cytoskeletal polymer major sperm protein (MSP). The GSK3 genes genetically interact with another memi-1(sb41) suppressor, gsp-4, which encodes a sperm-specific PP1 phosphatase, previously proposed to regulate MSP dynamics. Moreover, gskl-2 gsp-4; gskl-1 triple mutants often skip female MII, similar to memi-1/2/3(RNAi). The GSK3 kinases and PP1 phosphatases perform similar sperm-related functions and work together for post-fertilization functions in the oocyte that involve MEMI.

Subcellular patterns of SPE-6 localization reveal unexpected complexities in Caenorhabditis elegans sperm activation and sperm function

To acquire and maintain directed cell motility, Caenorhabditis elegans sperm must undergo extensive, regulated cellular remodeling, in the absence of new transcription or translation. To regulate sperm function, nematode sperm employ large numbers of protein kinases and phosphatases, including SPE-6, a member of C. elegans' highly expanded casein kinase 1 superfamily. SPE-6 functions during multiple steps of spermatogenesis, including functioning as a "brake" to prevent premature sperm activation in the absence of normal extracellular signals. Here, we describe the subcellular localization patterns of SPE-6 during wild-type C. elegans sperm development and in various sperm activation mutants. While other members of the sperm activation pathway associate with the plasma membrane or localize to the sperm's membranous organelles, SPE-6 surrounds the chromatin mass of unactivated sperm. During sperm activation by either of two semiautonomous signaling pathways, SPE-6 redistributes to the front, central region of the sperm's pseudopod. When disrupted by reduction-of-function alleles, SPE-6 protein is either diminished in a temperature-sensitive manner (hc187) or is mislocalized in a stage-specific manner (hc163). During the multistep process of sperm activation, SPE-6 is released from its perinuclear location after the spike stage in a process that does not require the fusion of membranous organelles with the plasma membrane. After activation, spermatozoa exhibit variable proportions of perinuclear and pseudopod-localized SPE-6, depending on their location within the female reproductive tract. These findings provide new insights regarding SPE-6's role in sperm activation and suggest that extracellular signals during sperm migration may further modulate SPE-6 localization and function.

Transcriptional profiles in Strongyloides stercoralis males reveal deviations from the Caenorhabditis sex determination model

The human and canine parasitic nematode Strongyloides stercoralis utilizes an XX/XO sex determination system, with parasitic females reproducing by mitotic parthenogenesis and free-living males and females reproducing sexually. However, the genes controlling S. stercoralis sex determination and male development are unknown. We observed precocious development of rhabditiform males in permissive hosts treated with corticosteroids, suggesting that steroid hormones can regulate male development. To examine differences in transcript abundance between free-living adult males and other developmental stages, we utilized RNA-Seq. We found two clusters of S. stercoralis-specific genes encoding predicted transmembrane proteins that are only expressed in free-living males. We additionally identified homologs of several genes important for sex determination in Caenorhabditis species, including mab-3, tra-1, fem-2, and sex-1, which may have similar functions. However, we identified three paralogs of gld-1; Ss-qki-1 transcripts were highly abundant in adult males, while Ss-qki-2 and Ss-qki-3 transcripts were highly abundant in adult females. We also identified paralogs of pumilio domain-containing proteins with sex-specific transcripts. Intriguingly, her-1 appears to have been lost in several parasite lineages, and we were unable to identify homologs of tra-2 outside of Caenorhabditis species. Together, our data suggest that different mechanisms control male development in S. stercoralis and Caenorhabditis species.

Cytoskeletal diversification across 1 billion years: What red algae can teach us about the cytoskeleton, and vice versa

The cytoskeleton has a central role in eukaryotic biology, enabling cells to organize internally, polarize, and translocate. Studying cytoskeletal machinery across the tree of life can identify common elements, illuminate fundamental mechanisms, and provide insight into processes specific to less-characterized organisms. Red algae represent an ancient lineage that is diverse, ecologically significant, and biomedically relevant. Recent genomic analysis shows that red algae have a surprising paucity of cytoskeletal elements, particularly molecular motors. Here, we review the genomic and cell biological evidence and propose testable models of how red algal cells might perform processes including cell motility, cytokinesis, intracellular transport, and secretion, given their reduced cytoskeletons. In addition to enhancing understanding of red algae and lineages that evolved from red algal endosymbioses (e.g., apicomplexan parasites), these ideas may also provide insight into cytoskeletal processes in animal cells.

Genomics of the Parasitic Nematode Ascaris and Its Relatives

Nematodes of the genus Ascaris are important parasites of humans and swine, and the phylogenetically related genera (Parascaris, Toxocara, and Baylisascaris) infect mammals of veterinary interest. Over the last decade, considerable genomic resources have been established for Ascaris, including complete germline and somatic genomes, comprehensive mRNA and small RNA transcriptomes, as well as genome-wide histone and chromatin data. These datasets provide a major resource for studies on the basic biology of these parasites and the host-parasite relationship. Ascaris and its relatives undergo programmed DNA elimination, a highly regulated process where chromosomes are fragmented and portions of the genome are lost in embryonic cells destined to adopt a somatic fate, whereas the genome remains intact in germ cells. Unlike many model organisms, Ascaris transcription drives early development beginning prior to pronuclear fusion. Studies on Ascaris demonstrated a complex small RNA network even in the absence of a piRNA pathway. Comparative genomics of these ascarids has provided perspectives on nematode sex chromosome evolution, programmed DNA elimination, and host-parasite coevolution. The genomic resources enable comparison of proteins across diverse species, revealing many new potential drug targets that could be used to control these parasitic nematodes.

The intrinsically disordered protein SPE-18 promotes localized assembly of MSP in Caenorhabditis elegans spermatocytes

Many specialized cells use unconventional strategies of cytoskeletal control. Nematode spermatocytes discard their actin and tubulin following meiosis, and instead employ the regulated assembly/disassembly of the Major Sperm Protein (MSP) to drive sperm motility. However, prior to the meiotic divisions, MSP is sequestered through its assembly into paracrystalline structures called fibrous bodies (FBs). The accessory proteins that direct this sequestration process have remained mysterious. This study reveals SPE-18 as an intrinsically disordered protein that is essential for MSP assembly within FBs. In spe-18 mutant spermatocytes, MSP forms disorganized cortical fibers, and the cells arrest in meiosis without forming haploid sperm. In wild-type spermatocytes, SPE-18 localizes to pre-FB complexes and functions with the kinase SPE-6 to localize MSP assembly. Changing patterns of SPE-18 localization uncover previously unappreciated complexities in FB maturation. Later, within newly individualized spermatids, SPE-18 is rapidly lost, yet SPE-18 loss alone is insufficient for MSP disassembly. Our findings reveal an alternative strategy for sequestering cytoskeletal elements, not as monomers but in localized, bundled polymers. Additionally, these studies provide an important example of disordered proteins promoting ordered cellular structures.

A transcriptomic snapshot of early molecular communication between Pasteuria penetrans and Meloidogyne incognita

BACKGROUND

Southern root-knot nematode Meloidogyne incognita (Kofoid and White, 1919), Chitwood, 1949 is a key pest of agricultural crops. Pasteuria penetrans is a hyperparasitic bacterium capable of suppressing the nematode reproduction, and represents a typical coevolved pathogen-hyperparasite system. Attachment of Pasteuria endospores to the cuticle of second-stage nematode juveniles is the first and pivotal step in the bacterial infection. RNA-Seq was used to understand the early transcriptional response of the root-knot nematode at 8 h post Pasteuria endospore attachment.

RESULTS

A total of 52,485 transcripts were assembled from the high quality (HQ) reads, out of which 582 transcripts were found differentially expressed in the Pasteuria endospore encumbered J2 s, of which 229 were up-regulated and 353 were down-regulated. Pasteuria infection caused a suppression of the protein synthesis machinery of the nematode. Several of the differentially expressed transcripts were putatively involved in nematode innate immunity, signaling, stress responses, endospore attachment process and post-attachment behavioral modification of the juveniles. The expression profiles of fifteen selected transcripts were validated to be true by the qRT PCR. RNAi based silencing of transcripts coding for fructose bisphosphate aldolase and glucosyl transferase caused a reduction in endospore attachment as compared to the controls, whereas, silencing of aspartic protease and ubiquitin coding transcripts resulted in higher incidence of endospore attachment on the nematode cuticle.

CONCLUSIONS

Here we provide evidence of an early transcriptional response by the nematode upon infection by Pasteuria prior to root invasion. We found that adhesion of Pasteuria endospores to the cuticle induced a down-regulated protein response in the nematode. In addition, we show that fructose bisphosphate aldolase, glucosyl transferase, aspartic protease and ubiquitin coding transcripts are involved in modulating the endospore attachment on the nematode cuticle. Our results add new and significant information to the existing knowledge on early molecular interaction between M. incognita and P. penetrans.

The secreted MSP domain of C. elegans VAPB homolog VPR-1 patterns the adult striated muscle mitochondrial reticulum via SMN-1

The major sperm protein domain (MSPd) has an extracellular signaling function implicated in amyotrophic lateral sclerosis. Secreted MSPds derived from the C. elegans VAPB homolog VPR-1 promote mitochondrial localization to actin-rich I-bands in body wall muscle. Here we show that the nervous system and germ line are key MSPd secretion tissues. MSPd signals are transduced through the CLR-1 Lar-like tyrosine phosphatase receptor. We show that CLR-1 is expressed throughout the muscle plasma membrane, where it is accessible to MSPd within the pseudocoelomic fluid. MSPd signaling is sufficient to remodel the muscle mitochondrial reticulum during adulthood. An RNAi suppressor screen identified survival of motor neuron 1 (SMN-1) as a downstream effector. SMN-1 acts in muscle, where it colocalizes at myofilaments with ARX-2, a component of the Arp2/3 actin-nucleation complex. Genetic studies suggest that SMN-1 promotes Arp2/3 activity important for localizing mitochondria to I-bands. Our results support the model that VAPB homologs are circulating hormones that pattern the striated muscle mitochondrial reticulum. This function is crucial in adults and requires SMN-1 in muscle, likely independent of its role in pre-mRNA splicing.

The C. elegans VAPB homolog VPR-1 is a permissive signal for gonad development

VAMP/synaptobrevin-associated proteins (VAPs) contain an N-terminal major sperm protein domain (MSPd) that is associated with amyotrophic lateral sclerosis. VAPs have an intracellular housekeeping function, as well as an extracellular signaling function mediated by the secreted MSPd. Here we show that the C. elegans VAP homolog VPR-1 is essential for gonad development. vpr-1 null mutants are maternal effect sterile due to arrested gonadogenesis following embryo hatching. Somatic gonadal precursor cells and germ cells fail to proliferate fully and complete their respective differentiation programs. Maternal or zygotic vpr-1 expression is sufficient to induce gonadogenesis and fertility. Genetic mosaic and cell type-specific expression studies indicate that vpr-1 activity is important in the nervous system, germ line and intestine. VPR-1 acts in parallel to Notch signaling, a key regulator of germline stem cell proliferation and differentiation. Neuronal vpr-1 expression is sufficient for gonadogenesis induction during a limited time period shortly after hatching. These results support the model that the secreted VPR-1 MSPd acts at least in part on gonadal sheath cell precursors in L1 to early L2 stage hermaphrodites to permit gonadogenesis.

Highlighted Article:vpr-1 null mutants are sterile upon hatching, a defect rescued by the expression of MSPd from almost any tissue except for the somatic gonad itself. See also the companion paper by Schultz et al.

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.

Self-Pressurised Rapid Freezing (SPRF): an easy-to-use and low-cost alternative cryo-fixation method for nematodes

Self-Pressurised Rapid Freezing (SPRF), an easy-to-use and low-cost alternative cryo-fixation method, was evaluated based on a comparative analysis of the ultrastructure of spermatozoa of the nematodes Acrobeles complexus and Caenorhabditis elegans. Sealed copper tubes, packed with active nematodes in water, were plunged into nitrogen slush, a semi-solid form of nitrogen. The water inside the capillary copper tube expands upon cooling due to the formation of hexagonal ice, thereby generating high pressure intrinsically for cryo-fixation of the sample. For sperm cells cryo-fixed by SPRF, the preservation of the ultrastructure was comparable to that achieved with high pressure freezing. This was evidenced by the clear details in mitochondria, membranous organelles and cytoskeleton in the pseudopod. It was demonstrated that SPRF fixation did not destroy antigenicity, based on the results of the immunolocalisation of the major sperm protein in both species. In conclusion, SPRF is a low-cost alternative cryo-fixation method for nematodes.

Actoclampin (+)-end-tracking motors: How the pursuit of profilin's role(s) in actin-based motility twice led to the discovery of how cells crawl

The path to the discovery of the actoclampins began with efforts to define profilin's role in actin-based pathogen and endosome rocketing. That research identified a set of FPPPP-containing cargo proteins and FPPPP-binding proteins that are consistently stationed within the polymerization zone during episodes of active motility. The very same biophysical clues that forced us to abandon Brownian Ratchet models guided us to the Actoclampin Hypothesis, which asserts that every propulsive filament possesses a (+)-end-tracking motor that generates the forces cells need to crawl. Each actoclampin motor is a multi-arm oligomeric complex, employing one arm to recruit/deliver Profilin•Actin•ATP to a growth-site located at the (+)-end of the lagging subfilament, while a second arm maintains an affinity-modulated binding interaction with the extreme (+)-end of the other subfilament. The alternating actions of these arms define a true molecular motor, the processivity of which explains why propelling filaments maintain full possession of their cargo. The Actoclampin Hypothesis also suggests how the energetics of tracker interactions with the (+)-end determines whether a given actoclampin is a passive (low force-producing) or active (high force-producing) motor, the latter requiring the Gibbs free energy of ATP hydrolysis. Another aim of this review is to acknowledge an earlier notional model that emerged from efforts to comprehend profilin's pivotal role(s) in actin-based cell motility.

COMP-1 promotes competitive advantage of nematode sperm

Competition among sperm to fertilize oocytes is a ubiquitous feature of sexual reproduction as well as a profoundly important aspect of sexual selection. However, little is known about the cellular mechanisms sperm use to gain competitive advantage or how these mechanisms are regulated genetically. In this study, we utilize a forward genetic screen in Caenorhabditis elegans to identify a gene, comp-1, whose function is specifically required in competitive contexts. We show that comp-1 functions in sperm to modulate their migration through and localization within the reproductive tract, thereby promoting their access to oocytes. Contrary to previously described models, comp-1 mutant sperm show no defects in size or velocity, thereby defining a novel pathway for preferential usage. Our results indicate not only that sperm functional traits can influence the outcome of sperm competition, but also that these traits can be modulated in a context-dependent manner depending on the presence of competing sperm.

The regulation of spermatogenesis and sperm function in nematodes

In the nematode C. elegans, both males and self-fertile hermaphrodites produce sperm. As a result, researchers have been able to use a broad range of genetic and genomic techniques to dissect all aspects of sperm development and function. Their results show that the early stages of spermatogenesis are controlled by transcriptional and translational processes, but later stages are dominated by protein kinases and phosphatases. Once spermatids are produced, they participate in many interactions with other cells - signals from the somatic gonad determine when sperm activate and begin to crawl, signals from the female reproductive tissues guide the sperm, and signals from sperm stimulate oocytes to mature and be ovulated. The sperm also show strong competitive interactions with other sperm and oocytes. Some of the molecules that mediate these processes have conserved functions in animal sperm, others are conserved proteins that have been adapted for new roles in nematode sperm, and some are novel proteins that provide insights into evolutionary change. The advent of new techniques should keep this system on the cutting edge of research in cellular and reproductive biology.

Role of posttranslational modifications in C. elegans and ascaris spermatogenesis and sperm function

Generally, spermatogenesis and sperm function involve widespread posttranslational modification of regulatory proteins in many different species. Nematode spermatogenesis has been studied in detail, mostly by genetic/molecular genetic techniques in the free-living Caenorhabditis elegans and by biochemistry/cell biology in the pig parasite Ascaris suum. Like other nematodes, both of these species produce sperm that use a form of amoeboid motility termed crawling, and many aspects of spermatogenesis are likely to be similar in both species. Consequently, work in these two nematode species has been largely complementary. Work in C. elegans has identified a number of spermatogenesis-defective genes and, so far, 12 encode enzymes that are implicated as catalysts of posttranslational protein modification. Crawling motility involves extension of a single pseudopod and this process is powered by a unique cytoskeleton composed of Major Sperm Protein (MSP) and accessory proteins, instead of the more widely observed actin. In Ascaris, pseudopod extension and crawling motility can be reconstituted in vitro, and biochemical studies have begun to reveal how posttranslational protein modifications, including phosphorylation, dephosphorylation and proteolysis, participate in these processes.

Comparative profiling of microRNAs in male and female adults of Ascaris suum

Ascaris nematodes, which cause ascariasis in humans and pigs, are among the most important nematodes from both health and economic perspectives. microRNA (miRNA) is now recognized as key regulator of gene expression at posttranscription level. The public availability of the genome and transcripts of Ascaris suum provides powerful resources for the research of miRNA profiles of the parasite. Therefore, we investigated and compared the miRNA profiles of male and female adult A. suum using Solexa deep sequencing combined with bioinformatic analysis and stem-loop reverse transcription polymerase chain reaction. Deep sequencing of small RNAs yielded 11.71 and 11.72 million raw reads from male and female adults of A. suum, respectively. Analysis showed that the noncoding RNA of the two genders, including tRNA, rRNA, snRNA, and snoRNA, were similar. By mapping to the A. suum genome, we obtained 494 and 505 miRNA candidates from the female and male parasite, respectively, and 87 and 82 of miRNA candidates were consistent with A. suum miRNAs deposited in the miRBase database. Among the miRNA candidates, 154 were shared by the two genders, and 340 and 351 were female and male specific with their target numbers ranged from one to thousands, respectively. Functional prediction revealed a set of elongation factors, heat shock proteins, and growth factors from the targets of gender-specific miRNAs, which were essential for the development of the parasite. Moreover, major sperm protein and nematode sperm cell motility protein were found in targets of the male-specific miRNAs. Ovarian message protein was found in targets of the female-specific miRNAs. Enrichment analysis revealed significant differences among Gene Ontology terms of miRNA targets of the two genders, such as electron carrier and biological adhesion process. The regulating functions of gender-specific miRNAs was therefore not only related to the fundamental functions of cells but also were essential to the germ development of the parasite. The present study provides a framework for further research of Ascaris miRNAs, and consequently leads to the development of potential nucleotide vaccines against Ascaris of human and animal health significance.

Identification and characterization of new major sperm protein genes from Oesophagostomum dentatum and Oesophagostomum quadrispinulatum from pigs in China

The present study identified and characterized new major sperm protein (MSP) genes from the two nodule worms Oesophagostomum dentatum and Oesophagostomum quadrispinulatum collected from pigs in China. Total genomic DNA was extracted individually from 10 male nematode samples representing O. dentatum, and 4 male nematode samples representing O. quadrispinulatum. A pair of primers (OMSP1F/MSP1R) was designed based on the MSP gene sequences of Ascaris suum and O. dentatum available in GenBank, and used to amplify the MSP genes from the two porcine nodule worms. The PCR products were purified and subsequently cloned into pGEM-T Easy vector. Recombinants were identified by PCR and sequenced. Sequence analysis revealed that there were two different types of MSP sequences in O. dentatum and O. quadrispinulatum, one contained intron, and the other did not. The lengths of the MSP sequences containing introns were 433 bp or 439 bp in O. dentatum, and 436 bp, 439 bp or 446 bp in O. quadrispinulatum, containing 1 or 2 introns. Five and three new members of the MSP multigene family were identified in O. dentatum and O. quadrispinulatum in this study, respectively. The MSP sequences without introns were 381 bp in length, and can be deduced into 126 amino acids. The sequences of MSP genes containing introns seem to be more conserved than those without introns. The identities of deduced amino acid sequences of the MSP genes containing introns were 96.0-100% within and between the two nodule worms, and were 81.1-93.7% compared with other published MSP sequences of the representative nematodes. The present study identified new MSP genes with introns from O. dentatum and O. quadrispinulatum for the first time. The identification and characterization of newly described MSP genes from O. dentatum and O. quadrispinulatum have implications for further studies of molecular biology and reproduction control of Oesophagostomum spp.

SACY-1 DEAD-Box helicase links the somatic control of oocyte meiotic maturation to the sperm-to-oocyte switch and gamete maintenance in Caenorhabditis elegans

In sexually reproducing animals, oocytes arrest at diplotene or diakinesis and resume meiosis (meiotic maturation) in response to hormones. In Caenorhabditis elegans, major sperm protein triggers meiotic resumption through a mechanism involving somatic Gα_s–adenylate cyclase signaling and soma-to-germline gap-junctional communication. Using genetic mosaic analysis, we show that the major effector of Gα_s–adenylate cyclase signaling, protein kinase A (PKA), is required in gonadal sheath cells for oocyte meiotic maturation and dispensable in the germ line. This result rules out a model in which cyclic nucleotides must transit through sheath-oocyte gap junctions to activate PKA in the germ line, as proposed in vertebrate systems. We conducted a genetic screen to identify regulators of oocyte meiotic maturation functioning downstream of Gα_s–adenylate cyclase–PKA signaling. We molecularly identified 10 regulatory loci, which include essential and nonessential factors. sacy-1, which encodes a highly conserved DEAD-box helicase, is an essential germline factor that negatively regulates meiotic maturation. SACY-1 is a multifunctional protein that establishes a mechanistic link connecting the somatic control of meiotic maturation to germline sex determination and gamete maintenance. Modulatory factors include multiple subunits of a CoREST-like complex and the TWK-1 two-pore potassium channel. These factors are not absolutely required for meiotic maturation or its negative regulation in the absence of sperm, but function cumulatively to enable somatic control of meiotic maturation. This work provides insights into the genetic control of meiotic maturation signaling in C. elegans, and the conserved factors identified here might inform analysis in other systems through either homology or analogy.