Developmental genetics of secretory vesicle acidification during Caenorhabditis elegans spermatogenesis

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.

Spermiogenesis in Caenorhabditis elegans: An Excellent Model to Explore the Molecular Basis for Sperm Activation

C. elegans spermiogenesis converts non-motile spermatids into motile, fertilization-competent spermatozoa. Two major events include the building of a pseudopod required for motility and fusion of membranous organelles (MOs)-intracellular secretory vesicles-with the spermatid plasma membrane required for the proper distribution of sperm molecules in mature spermatozoa. The mouse sperm acrosome reaction-a sperm activation event occurring during capacitation-is similar to MO fusion in terms of cytological features and biological significance. Moreover, C. elegans fer-1 and mouse Fer1l5, both encoding members of the ferlin family, are indispensable for MO fusion and acrosome reaction, respectively. Genetics-based studies have identified many C. elegans genes involved in spermiogenesis pathways; however, it is unclear whether mouse orthologs of these genes are involved in the acrosome reaction. One significant advantage of using C. elegans for studying sperm activation is the availability of in vitro spermiogenesis, which enables combining pharmacology and genetics for the assay. If certain drugs can activate both C. elegans and mouse spermatozoa, these drugs would be useful probes to explore the mechanism underlying sperm activation in these two species. By analyzing C. elegans mutants whose spermatids are insensitive to the drugs, genes functionally relevant to the drugs' effects can be identified.

Membrane-associated cytoplasmic granules carrying the Argonaute protein WAGO-3 enable paternal epigenetic inheritance in Caenorhabditis elegans

Epigenetic inheritance describes the transmission of gene regulatory information across generations without altering DNA sequences, enabling offspring to adapt to environmental conditions. Small RNAs have been implicated in this, through both the oocyte and the sperm. However, as much of the cellular content is extruded during spermatogenesis, it is unclear whether cytoplasmic small RNAs can contribute to epigenetic inheritance through sperm. Here we identify a sperm-specific germ granule, termed the paternal epigenetic inheritance (PEI) granule, that mediates paternal epigenetic inheritance by retaining the cytoplasmic Argonaute protein WAGO-3 during spermatogenesis in Caenorhabditis elegans. We identify the PEI granule proteins PEI-1 and PEI-2, which have distinct functions in this process: granule formation, Argonaute selectivity and subcellular localization. We show that PEI granule segregation is coupled to the transport of sperm-specific secretory vesicles through PEI-2 in an S-palmitoylation-dependent manner. PEI-like proteins are found in humans, suggesting that the identified mechanism may be conserved.

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.

Membrane contact site-dependent cholesterol transport regulates Na+/K+-ATPase polarization and spermiogenesis in Caenorhabditis elegans

Spermiogenesis in nematodes is a process whereby round and quiescent spermatids differentiate into asymmetric and crawling spermatozoa. The molecular mechanism underlying this symmetry breaking remains uncharacterized. In this study, we revealed that sperm-specific Na⁺/K⁺-ATPase (NKA) is evenly distributed on the plasma membrane (PM) of Caenorhabditis elegans spermatids but is translocated to and subsequently enters the invaginated membrane of the spermatozoa cell body during sperm activation. The polarization of NKA depends on the transport of cholesterol from the PM to membranous organelles (MOs) via membrane contact sites (MCSs). The inositol 5-phosphatase CIL-1 and the MO-localized PI4P phosphatase SAC-1 may mediate PI4P metabolism to drive cholesterol countertransport via sterol/lipid transport proteins through MCSs. Furthermore, the NKA function is required for C. elegans sperm motility and reproductive success. Our data imply that the lipid dynamics mediated by MCSs might play crucial roles in the establishment of cell polarity. eGraphical abstract.

Depletion of gipc-1 and gipc-2 causes infertility in Caenorhabditis elegans by reducing sperm motility

The G-protein signaling pathway plays a key role in multiple cellular processes and is well conserved in eukaryotes. Although GIPC (G-protein α subunit interacting protein (GAIP)-interacting protein, C terminus) has been studied in several model organisms, little is known about its role in Caenorhabditis elegans. In the present study, we investigated the roles of gipc-1 and gipc-2 in C. elegans. We observed that they were exclusively expressed in sperm throughout the development and that gipc-1; gipc-2 double mutants were infertile. Further examination of sperm development in gipc-1; gipc-2 mutants revealed defective sperm activation and abnormal pseudopod extension that resulted in reduced sperm motility. Moreover, major sperm protein (MSP) was abnormally segregated between spermatids and residual bodies in gipc-1; gipc-2 mutants. Our findings indicate that gipc-1 and gipc-2 are required for the proper pseudopod extension of sperm during the terminal differentiation of spermatids. During this process, the segregation of MSP into spermatids is important for ensuring normal sperm motility during fertilization.

Live-cell Imaging and Quantitative Analysis of Meiotic Divisions in Caenorhabditis elegans Males

Live-imaging of meiotic cell division has been performed in extracted spermatocytes of a number of species using phase-contrast microscopy. For the nematode Caenorhabditis elegans, removal of spermatocytes from gonads has damaging effects, as most of the extracted spermatocytes show a high variability in the timing of meiotic divisions or simply arrest during the experiment. Therefore, we developed a live-cell imaging approach for in situ filming of spermatocyte meiosis in whole immobilized C. elegans males, thus allowing an observation of male germ cells within an unperturbed environment. For this, we make use of strains with fluorescently labeled chromosomes and centrosomes. Here we describe how to immobilize male worms for live-imaging. Further, we describe the workflow for the acquisition and processing of data to obtain quantitative information about the dynamics of chromosome segregation in spermatocyte meiosis I and II. In addition, our newly developed approach allows us to re-orient filmed spindles in silico, regardless of the initial 3D orientation in the worm, and analyze spindle dynamics in living worms in a statistically robust manner. Our live-imaging approach is also applicable to C. elegans hermaphrodites and should be expandable to other fluorescently labelled nematodes or other fully transparent small model organisms.

Lysosomal activity regulates Caenorhabditis elegans mitochondrial dynamics through vitamin B12 metabolism

Mitochondrial fission and fusion are highly regulated by energy demand and physiological conditions to control the production, activity, and movement of these organelles. Mitochondria are arrayed in a periodic pattern in Caenorhabditis elegans muscle, but this pattern is disrupted by mutations in the mitochondrial fission component dynamin DRP-1. Here we show that the dramatically disorganized mitochondria caused by a mitochondrial fission-defective dynamin mutation is strongly suppressed to a more periodic pattern by a second mutation in lysosomal biogenesis or acidification. Vitamin B12 is normally imported from the bacterial diet via lysosomal degradation of B12-binding proteins and transport of vitamin B12 to the mitochondrion and cytoplasm. We show that the lysosomal dysfunction induced by gene inactivations of lysosomal biogenesis or acidification factors causes vitamin B12 deficiency. Growth of the C. elegans dynamin mutant on an Escherichia coli strain with low vitamin B12 also strongly suppressed the mitochondrial fission defect. Of the two C. elegans enzymes that require B12, gene inactivation of methionine synthase suppressed the mitochondrial fission defect of a dynamin mutation. We show that lysosomal dysfunction induced mitochondrial biogenesis, which is mediated by vitamin B12 deficiency and methionine restriction. S-adenosylmethionine, the methyl donor of many methylation reactions, including histones, is synthesized from methionine by S-adenosylmethionine synthase; inactivation of the sams-1 S-adenosylmethionine synthase also suppresses the drp-1 fission defect, suggesting that vitamin B12 regulates mitochondrial biogenesis and then affects mitochondrial fission via chromatin pathways.

Proteinase K is an activator for the male-dependent spermiogenesis pathway in Caenorhabditis elegans: Its application to pharmacological dissection of spermiogenesis

Caenorhabditis elegans spermiogenesis involves spermatid activation into spermatozoa. Activation occurs through either SPE-8 class-dependent or class-independent pathways. Pronase (Pron) activates the SPE-8 class-dependent pathway, whereas no in vitro tools are available to stimulate the SPE-8 class-independent pathway. Thus, whether there is a functional relationship between these two pathways is currently unclear. In this study, we found that proteinase K (ProK) can activate the SPE-8 class-independent pathway. In vitro spermiogenesis assays using Pron and ProK suggested that SPE-8 class proteins act in the hermaphrodite- and male-dependent spermiogenesis pathways and that some spermatid proteins presumably working downstream of spermiogenesis pathways, including MAP kinases, are preferentially involved in the SPE-8 class-dependent pathway. We screened a library of chemicals, and a compound that we named DDI-1 inhibited both Pron- and ProK-induced spermiogenesis. To our surprise, several DDI-1 analogues that are structurally similar to DDI-1 blocked Pron, but not ProK, induced spermiogenesis. Although the mechanism by which DDI-1 blocks spermiogenesis is yet unknown, we have begun to address this issue by selecting two DDI-1-resistant mutants. Collectively, our data support a model in which C. elegans male and hermaphrodite spermiogenesis each has its own distinct, parallel pathway.

The zinc transporter ZIPT-7.1 regulates sperm activation in nematodes

Sperm activation is a fascinating example of cell differentiation, in which immotile spermatids undergo a rapid and dramatic transition to become mature, motile sperm. Because the sperm nucleus is transcriptionally silent, this transition does not involve transcriptional changes. Although Caenorhabditis elegans is a leading model for studies of sperm activation, the mechanisms by which signaling pathways induce this transformation remain poorly characterized. Here we show that a conserved transmembrane zinc transporter, ZIPT-7.1, regulates the induction of sperm activation in Caenorhabditis nematodes. The zipt-7.1 mutant hermaphrodites cannot self-fertilize, and males reproduce poorly, because mutant spermatids are defective in responding to activating signals. The zipt-7.1 gene is expressed in the germ line and functions in germ cells to promote sperm activation. When expressed in mammalian cells, ZIPT-7.1 mediates zinc transport with high specificity and is predominantly located on internal membranes. Finally, genetic epistasis places zipt-7.1 at the end of the spe-8 sperm activation pathway, and ZIPT-7.1 binds SPE-4, a presenilin that regulates sperm activation. Based on these results, we propose a new model for sperm activation. In spermatids, inactive ZIPT-7.1 is localized to the membranous organelles, which contain higher levels of zinc than the cytoplasm. When sperm activation is triggered, ZIPT-7.1 activity increases, releasing zinc from internal stores. The resulting increase in cytoplasmic zinc promotes the phenotypic changes characteristic of activation. Thus, zinc signaling is a key step in the signal transduction process that mediates sperm activation, and we have identified a zinc transporter that is central to this activation process.

Sperm are specialized cells with transcriptionally silent DNA that has been packaged for delivery into the egg. In their final step of development, immature sperm undergo a rapid transition from nonmotile cells to mature, motile sperm capable of fertilization. The signals that trigger this change are not clearly understood. By identifying mutants in the roundworm Caenorhabditis elegans that are defective in sperm activation, we discovered a conserved transmembrane protein, ZIPT-7.1, that transports zinc and promotes sperm activation in both sexes. ZIPT-7.1 is expressed in the germ line and functions there to control sperm activation. When expressed ectopically in mammalian cells, the protein specifically transports zinc across membranes and localizes primarily to membranes within the cell. Previous genetic studies had identified two pathways that mediate sperm activation in C. elegans, and our results suggest that zipt-7.1 acts at the end of one of these two, the spe-8 pathway. We propose that when this pathway triggers sperm activation, it acts through ZIPT-7.1, which mediates the release of zinc from internal stores in the immature sperm. This released zinc functions as a second messenger to promote the differentiation of mature, motile sperm.

MIB-1 Is Required for Spermatogenesis and Facilitates LIN-12 and GLP-1 Activity in Caenorhabditis elegans

Covalent attachment of ubiquitin to substrate proteins changes their function or marks them for proteolysis, and the specificity of ubiquitin attachment is mediated by the numerous E3 ligases encoded by animals. Mind Bomb is an essential E3 ligase during Notch pathway signaling in insects and vertebrates. While Caenorhabditis elegans encodes a Mind Bomb homolog (mib-1), it has never been recovered in the extensive Notch suppressor/enhancer screens that have identified numerous pathway components. Here, we show that C. elegans mib-1 null mutants have a spermatogenesis-defective phenotype that results in a heterogeneous mixture of arrested spermatocytes, defective spermatids, and motility-impaired spermatozoa. mib-1 mutants also have chromosome segregation defects during meiosis, molecular null mutants are intrinsically temperature-sensitive, and many mib-1 spermatids contain large amounts of tubulin. These phenotypic features are similar to the endogenous RNA intereference (RNAi) mutants, but mib-1 mutants do not affect RNAi. MIB-1 protein is expressed throughout the germ line with peak expression in spermatocytes followed by segregation into the residual body during spermatid formation. C. elegans mib-1 expression, while upregulated during spermatogenesis, also occurs somatically, including in vulva precursor cells. Here, we show that mib-1 mutants suppress both lin-12 and glp-1 (C. elegans Notch) gain-of-function mutants, restoring anchor cell formation and a functional vulva to the former and partly restoring oocyte production to the latter. However, suppressed hermaphrodites are only observed when grown at 25°, and they are self-sterile. This probably explains why mib-1 was not previously recovered as a Notch pathway component in suppressor/enhancer selection experiments.

PL10 DEAD-Box Protein is Expressed during Germ Cell Differentiation in the Reptile Podarcis sicula (Family Lacertidae)

Among genes involved in the regulation of germ cell differentiation, those of DDX4/Vasa and the Ded1/DDX3 subfamilies encode for DEAD-box ATP-dependent RNA helicases, proteins involved in many mechanisms related to RNA processing. For the first time in reptiles, using specific antibodies at confocal microscopy, we analysed the localization pattern of a Ded1/DDX3 subfamily member in testis and ovary of Podarcis sicula (Ps-PL10) during the reproductive cycle. In testis, Ps-PL10 is expressed in the cytoplasm of spermatocytes and it is not detected in spermatogonia. Differently from Ps-VASA, in round spermatids, Ps-PL10 is not segregated in the chromatoid body but it accumulates in the cytoplasm of residual bodies, and mature spermatozoa are unstained. These observations suggest that in males, Ps-PL10 (1) is involved in spermatogenesis and (2) is then eliminated with residual bodies. In the ovary, Ps-PL10 is present with granules in the cytoplasm of early meiotic cells of the germinal bed (GB), while it is not present in oogonia and somatic cells of the GB stroma. In follicular cells of ovarian follicles, Ps-PL10 expression starts after their fusion with the oocyte. Numerous Ps-PL10 spots are visible in pyriform (nurse-like) cells concomitantly with the protein accumulation in the cytoplasm of differentiating oocyte. In pyriform cells, Ps-PL10 spots are present in the cytoplasm and nuclei, as observed for Ps-VASA, and in the nucleoli, suggesting for Ps-PL10 a role in rRNA processing and in the transport of molecules from the nucleus to cytoplasm and from nurse cells to the oocyte.

Gamete interactions require transmembranous immunoglobulin-like proteins with conserved roles during evolution

C. elegans spe-9 class genes are male germline-enriched in their expression and indispensable during sperm-oocyte fusion. Identification of mammalian orthologs that exhibit similar functions to these C. elegans genes has been a challenge. The mouse Izumo1 gene encodes a sperm-specific, immunoglobulin (Ig)-like transmembrane (TM) protein that is required for gamete fusion. We recently identified the C. elegans spe-45 gene, which shows male germline-enriched expression and encodes an Ig-like TM protein. spe-45 mutant worms produced otherwise normal spermatozoa that cannot fuse with oocytes, causing essentially the same phenotype as that seen in the Izumo1-knockout mice. By counting the number of self-sperm in the spermatheca of spe-45 hermaphrodites, it was found that this gene might be involved in sperm guidance from the uterus into the spermatheca, as well as gamete fusion. Moreover, we discovered that SPE-45 and IZUMO1 share certain functions for gamete fusion, which are presumably related to binding with cis- and/or trans-partners. Intriguingly, various organisms have Ig-like TM proteins that act during gamete interactions, indicating the wide-spread utility of Ig-like domains during fertilization.

SLC6 family transporter SNF-10 is required for protease-mediated activation of sperm motility in C. elegans

Motility of sperm is crucial for their directed migration to the egg. The acquisition and modulation of motility are regulated to ensure that sperm move when and where needed, thereby promoting reproductive success. One specific example of this phenomenon occurs during differentiation of the ameboid sperm of Caenorhabditis elegans as they activate from a round spermatid to a mature, crawling spermatozoon. Sperm activation is regulated by redundant pathways to occur at a specific time and place for each sex. Here, we report the identification of the solute carrier 6 (SLC6) transporter protein SNF-10 as a key regulator of C. elegans sperm activation in response to male protease activation signals. We find that SNF-10 is present in sperm and is required for activation by the male but not by the hermaphrodite. Loss of both snf-10 and a hermaphrodite activation factor render sperm completely insensitive to activation. Using in vitro assays, we find that snf-10 mutant sperm show a specific deficit in response to protease treatment but not to other activators. Prior to activation, SNF-10 is present in the plasma membrane, where it represents a strong candidate to receive signals that lead to subcellular morphogenesis. After activation, it shows polarized localization to the cell body region that is dependent on membrane fusions mediated by the dysferlin FER-1. Our discovery of snf-10 offers insight into the mechanisms differentially employed by the two sexes to accomplish the common goal of producing functional sperm, as well as how the physiology of nematode sperm may be regulated to control motility as it is in mammals.

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.

Calcium signaling surrounding fertilization in the nematode Caenorhabditis elegans

Calcium plays a prominent role during fertilization in many animals. This review focuses on roles of Ca(2+) during the events around fertilization in the model organism, Caenorhabditis elegans. Specifically, the role of Ca(2+) in sperm, oocytes and the surrounding somatic tissues during fertilization will be discussed, with the focus on sperm activation, meiotic maturation of oocytes, ovulation, sperm-egg interaction and fertilization.

V-ATPase V1 sector is required for corpse clearance and neurotransmission in Caenorhabditis elegans

The vacuolar-type ATPase (V-ATPase) is a proton pump composed of two sectors, the cytoplasmic V(1) sector that catalyzes ATP hydrolysis and the transmembrane V(o) sector responsible for proton translocation. The transmembrane V(o) complex directs the complex to different membranes, but also has been proposed to have roles independent of the V(1) sector. However, the roles of the V(1) sector have not been well characterized. In the nematode Caenorhabditis elegans there are two V(1) B-subunit genes; one of them, vha-12, is on the X chromosome, whereas spe-5 is on an autosome. vha-12 is broadly expressed in adults, and homozygotes for a weak allele in vha-12 are viable but are uncoordinated due to decreased neurotransmission. Analysis of a null mutation demonstrates that vha-12 is not required for oogenesis or spermatogenesis in the adult germ line, but it is required maternally for early embryonic development. Zygotic expression begins during embryonic morphogenesis, and homozygous null mutants arrest at the twofold stage. These mutant embryos exhibit a defect in the clearance of apoptotic cell corpses in vha-12 null mutants. These observations indicate that the V(1) sector, in addition to the V(o) sector, is required in exocytic and endocytic pathways.