In vitro activation and behavior of the ameboid sperm of Ascaris suum (Nematoda)

MIG-23 is involved in sperm migration by modulating extracellular ATP levels in Ascaris suum

In nematodes, spermiogenesis is a process of sperm activation in which nonmotile spermatids are transformed into crawling spermatozoa. Sperm motility acquisition during this process is essential for successful fertilization, but the underlying mechanisms remain to be clarified. Herein, we have found that extracellular adenosine-5′-triphosphate (ATP) level regulation by MIG-23, which is a homolog of human ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase), was required for major sperm protein (MSP) filament dynamics and sperm motility in the nematode Ascaris suum. During sperm activation, a large amount of ATP was produced in mitochondria and was stored in refringent granules (RGs). Some of the produced ATP was released to the extracellular space through innexin channels. MIG-23 was localized in the sperm plasma membrane and contributed to the ecto-ATPase activity of spermatozoa. Blocking MIG-23 activity resulted in a decrease in the ATP hydrolysis activity of spermatozoa and an increase in the depolymerization rate of MSP filaments in pseudopodia, which eventually affected sperm migration. Overall, our data suggest that MIG-23, which contributes to the ecto-ATPase activity of spermatozoa, regulates sperm migration by modulating extracellular ATP levels.

Released ATP Mediates Spermatozoa Chemotaxis Promoted by Uterus-Derived Factor (UDF) in Ascaris suum

Fertilization requires sperm migration toward oocytes and subsequent fusion. Sperm chemotaxis, a process in which motile sperm are attracted by factors released from oocytes or associated structures, plays a key role in sperm migration to oocytes. Here, we studied sperm chemotaxis in the nematode Ascaris suum. Our data show that uterus-derived factor (UDF), the protein fraction of uterine extracts, can attract spermatozoa. UDF is heat resistant, but its activity is attenuated by certain proteinases. UDF binds to the surface of spermatozoa but not spermatids, and this process is mediated by membranous organelles that fuse with the plasma membrane. UDF induces spermatozoa to release ATP from intracellular storage sites to the extracellular milieu, and extracellular ATP modulates sperm chemotaxis. Moreover, UDF increases protein serine phosphorylation (pS) levels in sperm, which facilitates sperm chemotaxis. Taken together, we revealed that both extracellular ATP and intracellular pS signaling are involved in Ascaris sperm chemotaxis. Our data provide insights into the mechanism of sperm chemotaxis in Ascaris suum.

Transformation: how do nematode sperm become activated and crawl?

Nematode sperm undergo a drastic physiological change during spermiogenesis (sperm activation). Unlike mammalian flagellated sperm, nematode sperm are amoeboid cells and their motility is driven by the dynamics of a cytoskeleton composed of major sperm protein (MSP) rather than actin found in other crawling cells. This review focuses on sperm from Caenorhabditis elegans and Ascaris suum to address the roles of external and internal factors that trigger sperm activation and power sperm motility. Nematode sperm can be activated in vitro by several factors, including Pronase and ionophores, and in vivo through the TRY-5 and SPE-8 pathways. Moreover, protease and protease inhibitors are crucial regulators of sperm maturation. MSP-based sperm motility involves a coupled process of protrusion and retraction, both of which have been reconstituted in vitro. Sperm motility is mediated by phosphorylation signals, as illustrated by identification of several key components (MPOP, MFPs and MPAK) in Ascaris and the characterization of GSP-3/4 in C. elegans.

Nematode sperm maturation triggered by protease involves sperm-secreted serine protease inhibitor (Serpin)

Spermiogenesis is a series of poorly understood morphological, physiological and biochemical processes that occur during the transition of immotile spermatids into motile, fertilization-competent spermatozoa. Here, we identified a Serpin (serine protease inhibitor) family protein (As_SRP-1) that is secreted from spermatids during nematode Ascaris suum spermiogenesis (also called sperm activation) and we showed that As_SRP-1 has two major functions. First, As_SRP-1 functions in cis to support major sperm protein (MSP)-based cytoskeletal assembly in the spermatid that releases it, thereby facilitating sperm motility acquisition. Second, As_SRP-1 released from an activated sperm inhibits, in trans, the activation of surrounding spermatids by inhibiting vas deferens-derived As_TRY-5, a trypsin-like serine protease necessary for sperm activation. Because vesicular exocytosis is necessary to create fertilization-competent sperm in many animal species, components released during this process might be more important modulators of the physiology and behavior of surrounding sperm than was previously appreciated.

New insights into the mechanism of fertilization in nematodes

Fertilization results from the fusion of male and female gametes in all sexually reproducing organisms. Much of nematode fertility work was focused on Caenorhabditis elegans and Ascaris suum. The C. elegans hermaphrodite produces a limited number of sperm initially and then commits to the exclusive production of oocytes. The postmeiotic differentiation called spermiogenesis converts sessile spermatids into motile spermatozoa. The motility of spermatozoa depends on dynamic assembly and disassembly of a major sperm protein-based cytoskeleton uniquely found in nematodes. Both self-derived and male-derived spermatozoa are stored in spermatheca, the site of fertilization in hermaphrodites. The oocyte is arrested in meiotic prophase I until a sperm-derived signal relieves the inhibition allowing the meiotic maturation to occur. Oocyte undergoes meiotic maturation, enters into spermatheca, gets fertilized, completes meiosis, and exits into uterus as a zygote. This review focuses on our current understanding of the events around fertilization in nematodes.

Preparing to move: assembly of the MSP amoeboid motility apparatus during spermiogenesis in Ascaris

We exploited the rapid, inducible conversion of non-motile Ascaris spermatids into crawling spermatozoa to examine the pattern of assembly of the MSP motility apparatus that powers sperm locomotion. In live sperm, the first detectable motile activity is the extension of spikes and, later, blebs from the cell surface. However, examination of cells by EM revealed that the formation of surface protrusions is preceded by assembly of MSP filament tails on the membranous organelles in the peripheral cytoplasm. These organelle-associated filament meshworks assemble within 30 sec after induction of spermiogenesis and persist until the membranous organelles are sequestered into the cell body when the lamellipod extends. The filopodia-like spikes, which are packed with bundles of filaments, extend and retract rapidly but last only a few seconds before giving way to, or converting into, blebs. Coalescence of these blebs, each supported by a dense mesh of filaments, often initiates lamellipod extension, which culminates in the formation of the robust, dynamic MSP fiber complexes that generate sperm motility. The same membrane phosphoprotein that orchestrates assembly of the fiber complexes at the leading edge of the lamellipod of mature sperm is also found at all sites of filament assembly during spermiogenesis. The orderly progression of steps that leads to construction of a functional motility apparatus illustrates the precise spatio-temporal control of MSP filament assembly in the developing cell and highlights the remarkable similarity in organization and plasticity shared by the MSP cytoskeleton and the actin filament arrays in conventional crawling cells.

How the assembly dynamics of the nematode major sperm protein generate amoeboid cell motility

Nematode sperm are amoeboid cells that use a major sperm protein (MSP) cytoskeleton in place of a conventional actin cytoskeleton to power their amoeboid motility. In these simple, specialized cells cytoskeletal dynamics is tightly coupled to locomotion. Studies have capitalized on this feature to explore the key structural properties of MSP and to reconstitute motility both in vivo and in vitro. This review discusses how the mechanistic properties shared by the MSP machinery and actin-based motility systems lead to a "push-pull" mechanism for amoeboid cell motility in which cytoskeletal assembly and disassembly at opposite ends of the lamellipodium are associated with independent forces for protrusion of the leading edge and retraction of the cell body.

Comparative fine structure of sperm of Heterodera schachtii and Punctodera chalcoensis, with phylogenetic implications for Heteroderinae (Nemata: Heteroderidae)

The fine structure of sperm of Heterodera schachtii and Punctodera chalcoensis demonstrates a high degree of interspecific morphological and developmental diversity, particularly in comparison with previously examined Heteroderinae that develop cysts, including Globodera tabacum, as well as out-groups that lack cysts, including species of Verutus, Meloidodera, and Ekphymatodera. Sperm of P. chalcoensis are much smaller and have fewer filopodia than those of species of Heterodera and Globodera. However, the distribution of filopodia on only part of the body, the smooth surface of the filopodia, and the presence of cortical microtubules are traits shared by the three genera with cysts. Unique features shared by P. chalcoensis and Globodera species include the short persistence of fibrous bodies after spermiogenesis and the lack of sperm polarization in the female genital tract. These traits are absent in Heterodera species. Conversely, chromatin remains unchanged with respect to condensation during sperm development in P. chalcoensis and species of Heterodera but not in Globodera. Patterns of evolution of sperm may be useful for testing hypotheses of the phylogeny of Heteroderinae, but the diversity is so great that character coding will be required for a large number of representative species.

Regulation of the Ascaris major sperm protein (MSP) cytoskeleton by intracellular pH

The development and locomotion of the amoeboid sperm of the nematode, Ascaris suum, depend on precise control of the assembly of their unique major sperm protein (MSP) filament system. We used fluorescence ratio imaging of cells loaded with BCECF to show that intracellular pH (pHi) is involved in controlling MSP polymerization in vivo. Spermatogenesis is marked by a cycle of MSP assembly-disassembly-reassembly that coincides with changes in pHi. In spermatocytes, which contain MSP in paracrystalline fibrous bodies, pHi was 6.8, 0.6 units higher than in spermatids, which disassemble the fibrous bodies and contain no assemblies of MSP filaments. Activation of spermatids to complete development resulted in rapid increase in pHi to 6.4 and reappearance of filaments. Treatment of spermatocytes with weak acids caused the fibrous bodies to disassemble whereas incubation of spermatids in weak bases induced MSP assembly. The MSP filaments in spermatozoa are organized into fiber complexes that flow continuously rearward from the leading edge of the pseudopod. These cells established a pseudopodial pH gradient with pHi 0.15 units higher at the leading edge, where fiber complexes assemble, than at the base of the pseudopod, where disassembly occurs. Acidification of these cells caused the MSP cytoskeleton to disassemble and abolished the pH gradient. Acid removal resulted in reassembly of the cytoskeleton, re-establishment of the pH gradient, and re-initiation of motility. MSP assembly in sperm undergoing normal development and motility and in cells responding to chemical manipulation of pHi occurs preferentially at membranes. Thus, we propose that filament assembly in sperm is controlled by pH-sensitive MSP-membrane interaction.

In vitro induction of crawling in the amoeboid sperm of the nematode parasite, Ascaris suum

In a highly synchronous process, the immotile spermatids of Ascaris suum extend pseudopods and become rapidly crawling sperm when treated with an extract from the glandular vas deferens of the male under strict anaerobic conditions. Within 9-12 min, a pseudopod develops, elongates rapidly, and exhibits a continuous flow of membrane specializations, the villipodia, from tip toward base. When attached to acid-washed glass, the pseudopod pulls the cell body along at speeds exceeding 70 microns/min. The pseudopod length remains constant while retrograde flow of villipodia proceeds at the same rate as the sperm's forward movement. Cohorts of about 15 villipodia form at the leading edge, move rearward together, and disappear at the junction of pseudopod and cell body. These are the terminations of branched, refringent fibers, which extend the length of the pseudopod. The latter are the fiber complexes that form its cytoskeleton (Sepsenwol et al.: Journal of Cell Biology 108:55-66, 1989). Locomoting cells sometimes change direction when another crawls by and follow each other. When cells are exposed to air, forward movement ceases in a predictable pattern: the forward extension of the leading edge ceases, the pseudopod shortens from the base, and the cell body continues to be pulled forward. These data contribute to a model for Ascaris sperm amoeboid motility in which independent processes of continuous extension at the leading edge and continuous shortening at the base of the pseudopod act to propel the cell forward.

Amino acid and lipid composition of refringent granules from the ameboid sperm of Ascaris suum (Nematoda)

Transformation of the spermatozoon of Ascaris suum from a spheroidal to an ameboid cell is associated with the formation of a motile pseudopodium and coalescence of the intracellular refringent granules. The pseudopodia of the ameboid spermatozoa contain filaments organized into dense patches, bundles, web-like or lace-like networks, as observed by electron microscopy. The morphology and chemistry of the refringent granules were investigated in subcellular fractions enriched for these structures. Isolated refringent granules were heterogeneous in size measuring from 0.5 X 0.6 to 2.3 X 3.5 microns. Each granule is surrounded by a 110 A thick layer. During fusion, the surfaces of the refringent granules form small extensions resembling micropodia. The process of fusion occurs at many sites on a given granule and simultaneous fusion of several granules was commonly observed. Amino acid analyses of the refringent granule proteins (RGP's) indicated: they are rich in aspartic acid or asparagine (48%), leucine (10%), serine (19%) and aromatic amino acids (11%). Gas-liquid chromatographic analyses of alditol acetate derivatives of monosaccharides released by mild acid hydrolysis showed the predominant sugars to be glucose (7.3 micrograms/mg protein), galactose (9.2 micrograms/mg) and N-acetylglucosamine (5.5 micrograms/mg). Lipid analyses indicated a complex mixture of glycerides, ascarosides and waxes, together with a major component that resembled free fatty acid in mobility on TLC.

The initiation of spermiogenesis in the nematode Caenorhabditis elegans

Spermiogenesis in nematodes involves the activation of sessile spherical spermatids to motile bipolar amoeboid spermatozoa. In Caenorhabditis elegans males spermiogenesis is normally induced by copulation. Spermatids transferred to hermaphrodites as well as some of those left behind in the male become spermatozoa a few minutes after mating. Spermiogenesis can also be induced in vitro by the ionophore monensin (G.A. Nelson and S. Ward, 1980, Cell 19, 457-464) and by weak bases such as triethanolamine. Both triethanolamine and monensin cause a rapid increase in intracellular pH from 7.1 to 7.5 or 8.0. This pH increase precedes the subsequent morphological events of spermiogenesis. Triethanolamine or monensin must be present throughout spermiogenesis for all cells to form pseudopods, but once pseudopods are formed the inducers are unnecessary for subsequent motility. The pH induced spermiogenesis is inhibited by drugs that block mitochondria or glycolysis. Protease treatment can also induce spermiogenesis without increasing intracellular pH, apparently bypassing the pH-dependent steps in activation and the requirement for glycolysis. These results show that the initiation of spermiogenesis in C. elegans, like some steps in egg activation and the initiation of sea urchin sperm motility, can be induced by an increase in intracellular pH, but this pH change can be bypassed by proteolysis.

Subcellular fractions and the refringent granules of the spermatozoa of Ascaris suum (Nematoda)

Six subcellular fractions were isolated by differential centrifugation of the homogenate of spermatozoa of Ascaris suum. The cellular constituents of pelleted fractions, as identified by electron microscopy, were membranes and membranous organelles (fraction A1), microsomal (A2), cytoplasmic (A3), large refringent granules (B1), small refringent granules (B2) and a detergent-soluble fraction (B3). Polypeptide analysis of SDS-PAGE showed that the 18,400-dalton band, one of the major spermatozoan proteins, is detectable in all of the fractions. However, the cytoplasmic (A1) and refringent-granule (B1) fractions contained the highest level. The isolated refringent granules consisted of 2-6% lipid while the nonlipid fraction formed an insoluble matrix with a fibrillar network morphology. This fibrillar matrix contained three polypeptides of small molecular weight (7,000-14,000) in addition to the 18,400-dalton polypeptide. These small polypeptides (7,000--14,000 MW) are detectable only in fractions of the refringent granules and are therefore called the refringent-granule proteins (RGP). These RGP are sensitive to tryptic hydrolysis and have solubility properties similar to the protein, ascaridine.

Sperm morphogenesis in wild-type and fertilization-defective mutants of Caenorhabditis elegans

Taking advantage of conditions that allow spermatogenesis in vitro, the timing and sequence of morphological changes leading from the primary spermatocyte to the spermatozoon is described by light and electron microscopy. Together with previous studies, this allows a detailed description of the nuclear, cytoplasmic, and membrane changes occurring during spermatozoan morphogenesis. By comparison with wild type, abnormalities in spermatogenesis leading to aberrant infertile spermatozoa are found in six fertilization-defective (fer) mutants. In fer-1 mutant males, spermatids appear normal, but during spermiogenesis membranous organelles (MO) fail to fuse with the sperm plasma membrane and a short, though motile. pseudopod is formed. In fer-2, fer-3, and fer-4 mutants, spermatids accumulate 48-nm tubules around their nuclei where the centriole and an RNA containing perinuclear halo would normally be. In all three mutants, spermatids still activate to spermatozoa with normal fusion of their MOs, but the pseudopods formed are aberrant in most fer-2 and fer-4 spermatozoa and in some fer-3 spermatozoa. In fer-5 mutant males, spermatozoa do not form. Instead, defective spermatids with crystalline inclusions and abnormal internal laminar membranes accumulate. In fer-6 mutant males, only a few spermatozoa form and these have defective pseudopods. These spermatozoa retain their fibrous bodies, a structure which normally disassembles in the spermatid. The time of appearance of developmental abnormalities in all of these mutants correlates with the temperature-sensitive periods for development of infertility. The observation that each of these mutants has a different and discreet set of morphological defects, a structure which normally disassembles in the spermatid. The time of appearance of developmental abnormalities in all of these mutants correlates with the temperature-sensitive periods for development of infertility. The observation that each of these mutants has a different and discreet set of morphological defects, a structure which normally disassembles in the spermatid. The time of appearance of developmental abnormalities in all of these mutants correlates with the temperature-sensitive periods for development of infertility. The observation that each of these mutants has a different and discreet set of morphological defects shows that the strict sequence of morphogenetic events that occurs during wild-type spermatogenesis cannot arise because each event is dependent on previous events. Instead, spermatozoa, like bacteriophages, must be formed by multiple independent pathways of morphogenesis.