The genetics and cell biology of fertilization

SPE-51, a sperm-secreted protein with an immunoglobulin-like domain, is required for fertilization in C. elegans

Fertilization is a fundamental process in sexual reproduction during which gametes fuse to combine their genetic material and start the next generation in their life cycle. Fertilization involves species-specific recognition, adhesion, and fusion between the gametes.1,2 In mammals and other model species, some proteins are known to be required for gamete interactions and have been validated with loss-of-function fertility phenotypes.3,4 Yet, the molecular basis of sperm-egg interaction is not well understood. In a forward genetic screen for fertility mutants in Caenorhabditis elegans, we identified spe-51. Mutant worms make sperm that are unable to fertilize the oocyte but otherwise normal by all available measurements. The spe-51 gene encodes a secreted protein that includes an immunoglobulin (Ig)-like domain and a hydrophobic sequence of amino acids. The SPE-51 protein acts cell autonomously and localizes to the surface of the spermatozoa. We further show that the gene product of the mammalian sperm function gene Sof1 is likewise secreted. This is the first example of a secreted protein required for the interactions between the sperm and egg with genetic validation for a specific function in fertilization in C. elegans (also see spe-365). This is also the first experimental evidence that mammalian SOF1 is secreted. Our analyses of these genes begin to build a paradigm for sperm-secreted or reproductive-tract-secreted proteins that coat the sperm surface and influence their survival, motility, and/or the ability to fertilize the egg.

RNA-binding protein complex AMG-1/SLRP-1 mediates germline development and spermatogenesis by maintaining mitochondrial homeostasis in Caenorhabditis elegans

The mechanisms of RNA-binding proteins (RBPs)-mediated post-transcriptional regulation of pre-existing mRNAs, which is essential for spermatogenesis, remain poorly understood. In this study, we identify that a germline-specific mitochondrial RBP AMG-1(abnormal mitochondria in germline 1), a homolog of mammalian leucine-rich PPR motif-containing protein (LRPPRC), is required for spermatogenesis in Caenorhabditis elegans. The amg-1 mutation hinders germline development without affecting somatic development and leads to the aberrant mitochondrial morphology and structure associated with mitochondrial dysfunctions specifically in the germline. We demonstrate that AMG-1 is most frequently bound to mtDNA-encoded 12S and 16S ribosomal RNA, the essential components of mitochondrial ribosomes, and that 12S rRNA expression mediated by AMG-1 is crucial for germline mitochondrial protein homeostasis. Furthermore, steroid receptor RNA activator (SRA) stem loop interacting RNA binding protein (SLRP-1), a homolog of mammalian SRA stem loop interacting RNA binding protein (SLIRP) in C. elegans, interacts with AMG-1 genetically to regulate germline development and reproductive success in C. elegans. Overall, these findings reveal the novel function of mtRBP, specifically in regulating germline development.

CRISPR/Cas9 Mediated Fluorescent Tagging of Caenorhabditis elegans SPE-38 Reveals a Complete Localization Pattern in Live Spermatozoa

The Caenorhabditis elegans spe-38 gene encodes a four-pass transmembrane molecule that is required in sperm for fertilization. In previous work, the localization of the SPE-38 protein was examined using polyclonal antibodies on spermatids and mature amoeboid spermatozoa. SPE-38 is localized to unfused membranous organelles (MOs) in nonmotile spermatids. Different fixation conditions revealed that SPE-38 either localized to fused MOs and the cell body plasma membrane or the pseudopod plasma membrane of mature sperm. To address this localization paradox in mature sperm, CRISPR/Cas9 genome editing was used to tag endogenous SPE-38 with fluorescent wrmScarlet-I. Homozygous male and hermaphrodite worms encoding SPE-38::wrmScarlet-I were fertile indicating the fluorescent tag does not interfere with SPE-38 function during sperm activation or fertilization. We found that SPE-38::wrmScarlet-I localized to MOs in spermatids consistent with previous antibody localization. In mature and motile spermatozoa we found SPE-38::wrmScarlet-I in fused MOs, the cell body plasma membrane, and the pseudopod plasma membrane. We conclude that the localization pattern observed with SPE-38::wrmScarlet-I represents the complete distribution of SPE-38 in mature spermatozoa and this localization pattern is consistent with a hypothesized role of SPE-38 directly in sperm-egg binding and/or fusion.

The molecular underpinnings of fertility: Genetic approaches in Caenorhabditis elegans

The study of mutations that impact fertility has a catch-22. Fertility mutants are often lost since they cannot simply be propagated and maintained. This has hindered progress in understanding the genetics of fertility. In mice, several molecules are found to be required for the interactions between the sperm and egg, with JUNO and IZUMO1 being the only known receptor pair on the egg and sperm surface, respectively. In Caenorhabditis elegans, a total of 12 proteins on the sperm or oocyte have been identified to mediate gamete interactions. Majority of these genes were identified through mutants isolated from genetic screens. In this review, we summarize the several key screening strategies that led to the identification of fertility mutants in C. elegans and provide a perspective about future research using genetic approaches. Recently, advancements in new technologies such as high-throughput sequencing and Crispr-based genome editing tools have accelerated the molecular, cell biological, and mechanistic analysis of fertility genes. We review how these valuable tools advance our understanding of the molecular underpinnings of fertilization. We draw parallels of the molecular mechanisms of fertilization between worms and mammals and argue that our work in C. elegans complements fertility research in humans and other species.

SLC17A6/7/8 Vesicular Glutamate Transporter Homologs in Nematodes

Members of the superfamily of solute carrier (SLC) transmembrane proteins transport diverse substrates across distinct cellular membranes. Three SLC protein families transport distinct neurotransmitters into synaptic vesicles to enable synaptic transmission in the nervous system. Among them is the SLC17A6/7/8 family of vesicular glutamate transporters, which endows specific neuronal cell types with the ability to use glutamate as a neurotransmitter. The genome of the nematode Caenorhabditis elegans encodes three SLC17A6/7/8 family members, one of which, eat-4/VGLUT, has been shown to be involved in glutamatergic neurotransmission. Here, we describe our analysis of the two remaining, previously uncharacterized SLC17A6/7/8 family members, vglu-2 and vglu-3 These two genes directly neighbor one another and are the result of a recent gene duplication event in C. elegans, but not in other Caenorhabditis species. Compared to EAT-4, the VGLU-2 and VGLU-3 protein sequences display a more distant similarity to canonical, vertebrate VGLUT proteins. We tagged both genomic loci with gfp and detected no expression of vglu-3 at any stage of development in any cell type of both C. elegans sexes. In contrast, vglu-2::gfp is dynamically expressed in a restricted set of distinct cell types. Within the nervous system, vglu-2::gfp is exclusively expressed in a single interneuron class, AIA, where it localizes to vesicular structures in the soma, but not along the axon, suggesting that VGLU-2 may not be involved in synaptic transport of glutamate. Nevertheless, vglu-2 mutants are partly defective in the function of the AIA neuron in olfactory behavior. Outside the nervous system, VGLU-2 is expressed in collagen secreting skin cells where VGLU-2 most prominently localizes to early endosomes, and to a lesser degree to apical clathrin-coated pits, the trans-Golgi network, and late endosomes. On early endosomes, VGLU-2 colocalizes most strongly with the recycling promoting factor SNX-1, a retromer component. Loss of vglu-2 affects the permeability of the collagen-containing cuticle of the worm, and based on the function of a vertebrate VGLUT1 protein in osteoclasts, we speculate that vglu-2 may have a role in collagen trafficking in the skin. We conclude that C. elegans SLC17A6/7/8 family members have diverse functions within and outside the nervous system.

Natural Variation and Genetic Determinants of Caenorhabditis elegans Sperm Size

The diversity in sperm shape and size represents a powerful paradigm to understand how selection drives the evolutionary diversification of cell morphology. Experimental work on the sperm biology of the male-hermaphrodite nematode Caenorhabditis elegans has elucidated diverse factors important for sperm fertilization success, including the competitive superiority of larger sperm. Yet despite extensive research, the molecular mechanisms regulating C. elegans sperm size and the genetic basis underlying natural variation in sperm size remain unknown. To address these questions, we quantified male sperm size variation of a worldwide panel of 97 genetically distinct C. elegans strains, allowing us to uncover significant genetic variation in male sperm size. Aiming to characterize the molecular genetic basis of C. elegans male sperm size variation using a genome-wide association study, we did not detect any significant quantitative trait loci. We therefore focused on the genetic analysis of pronounced sperm size differences observed between recently diverged laboratory strains (N2 vs. LSJ1/2). Using mutants and quantitative complementation tests, we demonstrate that variation in the gene nurf-1 underlies the evolution of small sperm in the LSJ lineage. Given the previous discovery that this same nurf-1 variation was central for hermaphrodite laboratory adaptation, the evolution of reduced male sperm size in LSJ strains likely reflects a pleiotropic consequence. Together, our results provide a comprehensive quantification of natural variation in C. elegans sperm size and first insights into the genetic determinants of Caenorhabditis sperm size, pointing at an involvement of the NURF chromatin remodeling complex.

Secreted d-aspartate oxidase functions in C. elegans reproduction and development

d-Aspartate oxidase (DDO) is a degradative enzyme that acts stereospecifically on free acidic D-amino acids such as d-aspartate and d-glutamate. d-Aspartate plays an important role in regulating neurotransmission, developmental processes, hormone secretion, and reproductive functions in mammals. In contrast, the physiological role of d-glutamate in mammals remains unclear. In Caenorhabditis elegans, the enzyme responsible for in vivo metabolism of d-glutamate is DDO-3, one of the three DDO isoforms, which is also required for normal self-fertility, hatching, and lifespan. In general, eukaryotic DDOs localize to subcellular peroxisomes in a peroxisomal targeting signal type 1 (PTS1)-dependent manner. However, DDO-3 does not contain a PTS1, but instead has a putative N-terminal signal peptide (SP). In this study, we found that DDO-3 is a secreted DDO, the first such enzyme to be described in eukaryotes. In hermaphrodites, DDO-3 was secreted from the proximal gonadal sheath cells in a SP-dependent manner and transferred to the oocyte surface. In males, DDO-3 was secreted from the seminal vesicle into the seminal fluid in a SP-dependent manner during mating with hermaphrodites. In both sexes, DDO-3 was secreted from the cells where it was produced into the body fluid and taken up by scavenger coelomocytes. Full-length DDO-3 transgene rescued all phenotypes elicited by the deletion of ddo-3, whereas a DDO-3 transgene lacking the putative SP did not. Together, these results indicate that secretion of DDO-3 is essential for its physiological functions.

Convergent evolution of sperm gigantism and the developmental origins of sperm size variability in Caenorhabditis nematodes

Sperm cells provide essential, if usually diminutive, ingredients to successful sexual reproduction. Despite this conserved function, sperm competition and coevolution with female traits can drive spectacular morphological change in these cells. Here, we characterize four repeated instances of convergent evolution of sperm gigantism in Caenorhabditis nematodes using phylogenetic comparative methods on 26 species. Species at the extreme end of the 50-fold range of sperm-cell volumes across the genus have sperm capable of comprising up to 5% of egg-cell volume, representing severe attenuation of the magnitude of anisogamy. Furthermore, we uncover significant differences in mean and variance of sperm size among genotypes, between sexes, and within and between individuals of identical genotypes. We demonstrate that the developmental basis of sperm size variation, both within and between species, becomes established during an early stage of sperm development at the formation of primary spermatocytes, while subsequent meiotic divisions contribute little further sperm size variability. These findings provide first insights into the developmental determinants of inter- and intraspecific sperm size differences in Caenorhabditis. We hypothesize that life history and ecological differences among species favored the evolution of alternative sperm competition strategies toward either many smaller sperm or fewer larger sperm.

Biochemistry, function, and deficiency of vitamin B12 in Caenorhabditis elegans

Caenorhabditis elegans is a nematode that has been widely used as an animal for investigation of diverse biological phenomena. Vitamin B12 is essential for the growth of this worm, which contains two cobalamin-dependent enzymes, methylmalonyl-CoA mutase and methionine synthase. A full complement of gene homologs encoding the enzymes associated with the mammalian intercellular metabolic processes of vitamin B12 is identified in the genome of C elegans However, this worm has no orthologs of the vitamin B12-binders that participate in human intestinal absorption and blood circulation. When the worm is treated with a vitamin B12-deficient diet for five generations (15 days), it readily develops vitamin B12 deficiency, which induces worm phenotypes (infertility, delayed growth, and shorter lifespan) that resemble the symptoms of mammalian vitamin B12 deficiency. Such phenotypes associated with vitamin B12 deficiency were readily induced in the worm.

Forward Genetics Identifies a Requirement for the Izumo-like Immunoglobulin Superfamily spe-45 Gene in Caenorhabditis elegans Fertilization

Fertilization is a conserved process in all sexually reproducing organisms whereby sperm bind and fuse with oocytes. Despite the importance of sperm-oocyte interactions in fertilization, the molecular underpinnings of this process are still not well understood. The only cognate ligand-receptor pair identified in the context of fertilization is sperm-surface Izumo and egg-surface Juno in the mouse [1]. Here we describe a genetic screening strategy to isolate fertilization mutants in Caenorhabditis elegans in order to generate a more complete inventory of molecules required for gamete interactions. From this screening strategy, we identified, cloned, and characterized spe-45, a gene that encodes an Izumo-like immunoglobulin superfamily protein. Mammalian Izumo is required for male fertility and has the same basic mutant phenotype as spe-45. Worms lacking spe-45 function produce morphologically normal and motile sperm that cannot fuse with oocytes despite direct contact in the reproductive tract. The power of this screen to identify proteins with ancient sperm functions suggests that characterization of additional mutants from our screen may reveal other deeply conserved components in fertility pathways and complement studies in other organisms.

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.

Dramatic fertility decline in aging C. elegans males is associated with mating execution deficits rather than diminished sperm quality

Although much is known about female reproductive aging, fairly little is known about the causes of male reproductive senescence. We developed a method that facilitates culture maintenance of Caenorhabditis elegans adult males, which enabled us to measure male fertility as populations age, without profound loss of males from the growth plate. We find that the ability of males to sire progeny declines rapidly in the first half of adult lifespan and we examined potential factors that contribute towards reproductive success, including physical vigor, sperm quality, mating apparatus morphology, and mating ability. Of these, we find little evidence of general physical decline in males or changes in sperm number, morphology, or capacity for activation, at time points when reproductive senescence is markedly evident. Rather, it is the loss of efficient mating ability that correlates most strongly with reproductive senescence. Low insulin signaling can extend male ability to sire progeny later in life, although insulin impact on individual facets of mating behavior is complex. Overall, we suggest that combined modest deficits, predominantly affecting the complex mating behavior rather than sperm quality, sum up to block effective C. elegans male reproduction in middle adult life.

Vitamin B12 deficiency in Caenorhabditis elegans results in loss of fertility, extended life cycle, and reduced lifespan

Vitamin B₁₂ (B₁₂) deficiency has been linked to developmental disorders, metabolic abnormalities, and neuropathy; however, the mechanisms involved remain poorly understood. Caenorhabditis elegans grown under B₁₂-deficient conditions for five generations develop severe B₁₂ deficiency associated with various phenotypes that include decreased egg-laying capacity (infertility), prolonged life cycle (growth retardation), and reduced lifespan. These phenotypes resemble the consequences of B₁₂ deficiency in mammals, and can be induced in C. elegans in only 15 days. Thus, C. elegans is a suitable animal model for studying the biological processes induced by vitamin deficiency.