RNA metabolism of fertilized Ascaris lumbricoides eggs during uterine development

Transcription in pronuclei and one- to four-cell embryos drives early development in a nematode

BACKGROUND

A long-standing view of development is that transcription is silenced in the oocyte until early divisions in the embryo. The point at which major transcription is reactivated varies between organisms, but is usually after the two-cell stage. However, this model may not be universal.

RESULTS

We used RNA-seq and exploited the protracted development of the parasitic nematode Ascaris suum to provide a comprehensive time course of mRNA expression, degradation, and translation during early development. Surprisingly, we find that ∼4,000 genes are transcribed prior to pronuclear fusion and in the one- to four-cell embryos. Intriguingly, we do not detect maternal contribution of many orthologs of maternal C. elegans mRNAs, but instead find that these are newly transcribed in the A. suum zygote prior to pronuclear fusion. Ribosome profiling demonstrates that, in general, early embryonic mRNAs are not stored for subsequent translation, but are directly translated after their synthesis. The role of maternally contributed and zygotically transcribed genes differs between the nematodes A. suum and C. elegans despite the fact that the two nematodes appear to exhibit highly similar morphological patterns during early development.

CONCLUSIONS

Our study indicates that major transcription can occur immediately after fertilization and prior to pronuclear fusion in metazoa, suggesting that newly transcribed genes appear to drive A. suum early development. Furthermore, the mechanisms used for controlling the timing of the expression of key conserved genes has been altered between the two nematodes, illustrating significant plasticity in the regulatory networks that play important roles in developmental outcomes in nematodes.

Deep small RNA sequencing from the nematode Ascaris reveals conservation, functional diversification, and novel developmental profiles

Eukaryotic cells express several classes of small RNAs that regulate gene expression and ensure genome maintenance. Endogenous siRNAs (endo-siRNAs) and Piwi-interacting RNAs (piRNAs) mainly control gene and transposon expression in the germline, while microRNAs (miRNAs) generally function in post-transcriptional gene silencing in both somatic and germline cells. To provide an evolutionary and developmental perspective on small RNA pathways in nematodes, we identified and characterized known and novel small RNA classes through gametogenesis and embryo development in the parasitic nematode Ascaris suum and compared them with known small RNAs of Caenorhabditis elegans. piRNAs, Piwi-clade Argonautes, and other proteins associated with the piRNA pathway have been lost in Ascaris. miRNAs are synthesized immediately after fertilization in utero, before pronuclear fusion, and before the first cleavage of the zygote. This is the earliest expression of small RNAs ever described at a developmental stage long thought to be transcriptionally quiescent. A comparison of the two classes of Ascaris endo-siRNAs, 22G-RNAs and 26G-RNAs, to those in C. elegans, suggests great diversification and plasticity in the use of small RNA pathways during spermatogenesis in different nematodes. Our data reveal conserved characteristics of nematode small RNAs as well as features unique to Ascaris that illustrate significant flexibility in the use of small RNAs pathways, some of which are likely an adaptation to Ascaris' life cycle and parasitism. The transcriptome assembly has been submitted to NCBI Transcriptome Shotgun Assembly Sequence Database(http://www.ncbi.nlm.nih.gov/genbank/TSA.html) under accession numbers JI163767–JI182837 and JI210738–JI257410.

Structure and development of the spermatozoon of the parasitic nematode, Nematospiroides dubius

Spermatogenesis and sperm maturation in Nematospiroides dubius were studied using electron microscopy. The testis is telegonic and germ cells in the zones of mitosis, growth and meiosis are connected by a central anucleate mass of cytoplasm, the rachis. The early part of spermatogenesis is dominated by the synthesis and growth of membrane-bound vesicles called membranous organelles, which originate from RER-associated Golgi bodies. Following meiosis the spermatids separate from the rachis and their chroinatin, which is no longer bounded by a nuclear envelope, condenses into an arrow-head shape and is extruded to form a tail-like structure. After insemination spermatozoa undergo a profound change called activation. The cytoplasmic region which was previously long and cylindrical becomes spherical and the membranous organelles which lined its perimeter fuse with the plasma membrane and become confined to the posterior hemisphere of the sperm, close to the nuclear tail. The anterior half of the sperm is devoid of organelles but contains many filaments organized into clumps and chains; this region being responsible for amoeboid locomotion of the sperm.

Characterization of the eggshell of Haemonchus contortus--I. Structural components

1. By transmission electron microscopy, the eggshell of Haemonchus contortus was seen to be similar to previously studied nematodes, with an outer vitelline layer bounded by a trilaminate membrane, a broad medial region, containing chitin, and an electron dense basal region, containing lipid and protein. 2. Exposure of Haemonchus contortus eggs to proteases resulted in disruption of the shell with removal of components of the outer, medial and basal regions. Exposure to chitinase depleted fibrillar components of the medial region of the shell, while collagenase had no effect. 3. Chloroform/methanol extraction of fresh eggshells caused a minor condensation of the outer, vitelline layer and some depletion of the basal layer. 4. After normal hatching, shells appeared similar to those treated with protease and chitinase, but also lacked the basal, lipid layer. 5. Extracts of isolated unhatched eggshells and hatched eggshells, and extracts of biotin-labelled whole fresh eggs showed three major protein bands when run on sodium dodecyl sulphate-polyacrylamide gels indicating that these three proteins are most likely structural in nature and do not participate in the release of the larva from the eggshell. 6. Biotin-labelled protein bands were degraded by proteases and chitinase, but not collagenase or lipase.

Transcription in nematodes: early Ascaris embryos are transcriptionally active

In this report we analyze early zygotic gene expression in the parasitic nematode Ascaris lumbricoides var. suum. Using synchronous populations of early embryonic stages, nuclei were isolated, and in vitro run-off transcription assays were performed. We find transcriptional activity as early as the 4- to 8-cell stage. The percentage of RNA polymerase II activity, as measured in these assays, is greater than 80% of the total transcription at the 60-cell stage. Furthermore, we show that a specific transcript (actin) can be identified in all early stages tested.

Localization of the in vivo and in vitro transcription initiation site and comparative analysis of the flanking sequences in the two main size classes of Ascaris lumbricoides rDNA

An accurate in vitro transcription system which utilizes the cloned 8.8 and 8.4 kb size classes of Ascaris rRNA genes (pAlr8 and pAlr13) and two kinds of cellular extracts from Ascaris oogonia has been established. Both rDNA containing plasmids are efficiently transcribed in vitro by RNA polymerase I from a unique site of rDNA which corresponds to the in vivo initiation site. The in vitro transcription product has a triphosphorylated 5'-end and starts on a G localized 414 bp (pAlr8) upstream of the beginning of the mature 18S rRNA. The promoter region has been delimited by testing the in vitro template activity of a series of restriction fragments. The region essential for the accuracy of initiation is contained within nucleotides -72 to +65, but full efficiency of transcription requires the additional presence of the region from nucleotides +66 to +84. The sequences upstream from position -72 do not appear to modulate the efficiency of specific in vitro initiation. Furthermore, the sequences flanking the transcription initiation site from position -1500 to +570 have been determined in the two cloned representatives of the two rDNA main size classes.

Chitin synthesis in zygotes of Ascaris suum

In Ascaris suum chitin is formed in the zygote immediately after oocyte fertilization, and its synthesis is completed in the eggs from the distal half of the uterus. Incorporation of radiocarbon [14C] glucose into chitin of the eggshell was 40-fold higher than incorporation of [14C] glucosamine. The same rank order also holds for the incorporation of label from these isotopes into the glycogen of the ovaries. A large part of the radiolabel was incorporated first into oocyte glycogen and only after fertilization was it incorporated into eggshell chitin. Actinomycin D inhibited chitin synthesis in the eggs from the distal half of the uterus and it significantly reduced incorporation of radiocarbon from glucose into chitin.

Gametogenesis and fertilization in Dirofilaria immitis (Nematoda: Filarioidea)

Gametogenesis in Dirofilaria immitis has been studied principally by means of the aceto-orcein chromosomal squash technique, but with additional ultrastructural observations. A terminal germinative zone, in which a continuous and rapid division of germ cells occurs, has been identified in the gonoduct of both male and female worms. Approximately 20% of cells within these germinative zones were in arrested mitotic division following the incubation in vitro of excised gonads in 0.01% colchicine for 4 h. All primary spermatocytes within a 1-2 cm length of the testis proximal to the germinative zone were at the prophase of the 1st meiotic division. In the corresponding region of the ovary, the primary oocytes were similarly at the prophase of the 1st meiotic division in 75% of female worms examined but in the remaining 25% all primary oocytes possessed markedly less condensed, probably interphase nuclei. A possible hormonal control of the cyclical development of primary oocytes, but not primary spermatocytes in D. immitis is suggested. In most of the remaining length of the gonoducts beyond this region of cells at meiotic prophase, the chromatin material of both primary spermatocytes and oocytes is decondensed. Recondensation of chromosomes in the spermatocytes is observed just prior to entry into the seminal vesicle, where meiosis I is completed and meiosis II takes place. In the primary oocyte, completion of meiosis only occurs after fertilization within the seminal receptacle by an entire male gamete. Following the 2 meiotic divisions in the oocyte and subsequent extrusion of the 2 polar bodies, the haploid chromosome complement of the female unites with that of the male, re-establishing the diploid number of the zygote (2n = 10). Male chromosomes within the oocyte remain visible throughout late oogenesis and fusion occurs without the formation of pronuclei.

Properties and localization of chitin synthase in Ascaris suum eggs

Investigations were carried out into the properties and localization of chitin synthase (EC 2.4.1.16) in connection with the formation of Ascaris suum egg-shells. The chitin synthase of eggs exhibited properties very similar to those found in other chitin-synthesizing organisms. It was stimulated by the presence of ATP, GlcNAc and Mg2+ with maximal activity at pH 7.0. In the phase of active chitin synthesis, during formation of the primary shell layers, chitin synthase was bound to the structures of the eggs. On completion of chitin synthesis in the eggs, the amount of the enzyme increased in the soluble fraction. Two forms of chitin synthase have been found in the eggs, namely an inactive (zymogenic) and an active form. Transformation of these two enzyme forms represents a key mechanism in the regulation of chitin synthesis in A. suum eggs.

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.

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.

Interaction of anthelmintic benzimidazoles with Ascaris suum embryonic tubulin

The ability of mebendazole and fenbendazole to bind to tubulin in cytosolic fractions from 8-day Ascaris suum embryos was determined by inhibition studies with [3H]colchicine. Colchicine binding in the presence of 1 . 10(-6) M mebendazole was completely inhibited during a 6 h incubation period at 37 degrees C. Inhibition of colchicine binding to A. suum embryonic tubulin by mebendazole and fenbendazole appeared to be noncompetitive. The inhibition constants of mebendazole and fenbendazole for A. suum embryonic tubulin were 1.9 . 10(-8) M and 6.5 . 10(-8) M, respectively. Mebendazole and fenbendazole appeared to be competitive inhibitors of colchicine binding to bovine brain tubulin. The inhibition constants of mebendazole and fenbendazole for bovine brain tubulin were 7.3 . 10(-6) M and 1.7 . 10(-5) M, respectively. These values are 250-400 times greater than the inhibition constants of fenbendazole and mebendazole for A. suum embryonic tubulin. Differential binding affinities between nematode tubulin and mammalian tubulin for benzimidazoles may explain the selective toxicity. The importance of tubulin as a receptor for anthelmintic benzimidazoles in animal parasitic nematodes is discussed.

Chromosome diminution in Ascaris suum. Two-fold increase of nucleosomal histone to DNA ratios during development

The swine intestinal nematode, Ascaris suum, eliminates chromatin material from its primordial somatic cells during early embryogenesis. A technique for isolation of nuclei from pre- and post-diminution stage embryos has been developed and these isolated nuclei were used in investigations of nuclear events during diminution. The amount of DNA per nucleus determined by diphenylamine assays and isotope dilutions was 0.66 pg and 0.29 pg in pre- and post-diminution nuclei, respectively. Thus, A. suum loses 56% of its nuclear DNA during diminution. The loss of nuclear DNA enabled in vivo examination of histone to DNA ratios as a function of changes in DNA quantities. Ascaris histones were identified by acid extractability and tryptic fingerprint comparison with rat liver histones. Measurement of histone quantities was accomplished using linearity of Coomassie blue binding to histones separated in dodecyl sulfate gels. Ascaris nucleosomal histones levels were relatively constant in pre- and post-diminution nuclei. However, nucleosomal histone to DNA ratios approximately doubled during diminution.

Quantitative analysis of RNA, glycogen and nucleotides from different developmental stages of Ascaris lumbricoides (var. suum)

Cold acid phenol extractions (Kirby, K.S. (1968) in Methods in Enzymology, Vol. XII B, pp. 87--99, Academic Press, New York) of homogenates from oocytes, zygotes and larval stages of Ascaris lumbricoides yield predominantly glycogen, some glycogen . phosphate complexes and guanine nucleotides, but only small amounts of RNA. Oocytes contain 10.0 pg of extractable 19 S and 26 S rRNA and 1.1 pg 4 S and 5 S RNA per cell. During the maturation of the oocyte to the zygote stage, the amount of 19 S and 26 S rRNA increases by 57%, and that of 4 S and 5 S RNA by 130%. Larval stages contain 62% more 19 S and 26 S rRNA, and 47% more 4 S and 5 S RNA than zygotes. The amount of the extractable glycogen decreases by about 80% and nucleotides by about 70% from oogenesis to the larval stage. The Kirby extract from homogenates of spermatids contains only traces of glycogen, glycogen . phosphate complexes and nucleotides, and 115 times less RNA per spermatid than oocytes. The nucleotide pool sizes of oocytes, zygotes and larval stages were determined; the pools consist of 71--88% guanine nucleotides. Spermatids contain only adenine nucleotides and an unidentified, nucleotide-like compound.

Nuclear protein changes in the maternally and paternally derived chromatin at fertilization

The proteins which become associated with the paternally derived chromatin during fertilization may be instrumental in its activation and in the dramatic structural metamorphosis of the sperm nucleus during pronuclear development. Proteins associated with sperm and zygote nuclei and male and female pronuclei of fertilized sea urchin eggs were analysed by polyacrylamide gel electrophoresis in order to examine nuclear protein changes in the paternally and maternally derived chromatin following insemination. Results demonstrate major changes in both the solubility characteristics and polypeptide profiles of sperm nuclei upon insemination. Evidence is presented which indicates that at fertilization the paternally derived chromatin acquires proteins of molecular weights greater than 80,000 and a nuclear protein composition similar to that of the female pronucleus. The nuclear proteins associated with zygote nuclei were compared to those of combined male and female pronuclei and showed many similarities and some differences. Several polypeptides were present in zygote nuclei which were not observed in pronuclear extracts.

Synthesis of RNA by male pronuclei of fertilized sea urchin eggs

Experiments have been conducted in an effort to determine when the paternally derived genome of sea urchin embryos becomes active in RNA synthesis. Participation of the paternal genome in RNA transcription as early as fertilization has been indicated by the following: (1) polyspermic zygotes consistently demonstrate greater levels of incorporation of labelled precursor (3H-uridine) into RNA when compared to monospermic eggs. (2) Monospermic and polyspermic eggs treated with actinomycin D show the same levels of incorporation of label into RNA. (3) Analysis of autoradiographs demonstrates the incorporation of 3H-uridine into male and female pronuclei.

Cytophotometric study of nuclear proteins during embryogenesis in two nematode species, Ascaris lumbricoides and Panagrellus silusiae

Patterns of nuclear protein changes during embryogenesis in two nematode systems: Ascaris lumbricoides and Panagrellus silusiae were examined microspectrophotometrically. In both species, development is accompanied by an extensive loss of basic nuclear protein as noted by the diminished intranuclear binding of both the Sakaguchi reagent and fluorodinitrobenzene chromophore. The alkaline fast green (pH 8.1) procedure stained both nuclear and cytoplasmic regions of interphase cells to a similar extent throughout embryogenesis. In cells at metaphase the chromosomes bound the fast green dye intensely while the cytoplasm continued to give a positive reaction. Relative Feulgen stainability of nuclei during embryonic development varied between species. In Ascaris, the one-celled stage nucleus was Feulgen-negative and the LI juvenile somatic nuclei yielded a mean Feulgen-DNA value which was considerably less than a postdiminution diploid DNA equivilent. In Panagrellus, the nuclei of one-celled stage embryos contain DNA in excess of a 4C quantity which subsequently is reduced during embryonic divisions. As in Ascaris the nuclei of Panagrellus LI juveniles give hypodiploid Feulgen-DNA values. The significance of these changes remains to be determined.

Karyotype analysis of Ascaris lumbricoides var. suum. Male and female pachytene nuclei by 3-D reconstruction from electron microscopy of serial sections

Twelve synaptonemal complexes are present in both oocyte and spermatocyte pachytene nuclei of Ascaris lumbricoides var. suum, as determined by 3-D reconstruction of the nuclear contents from electron microscropy of serial sections and therefore, n=12 in the strain of Ascaris described here. In the female the heterochromatic end of each synaptonemal complex is attached to the nuclear envelope and the other end is free in the nucleoplasm. In the male neither end of the synaptonemal complex is attached, but there is a heterochromatic knob at one end of each complex. Five additional large heterochromatic masses are present in the spermatocyte nucleus and these may be the sex chromosomes described by earlier workers.

Properties and synthesis of ribosomal RNA in the free-living nematode Panagrellus silusiae

The normal S values of ribosomes and ribosomal subunits extracted from the free-living nematode. Panagrellus silusiae are similar to those of other eukaryotic organisms. Upon heating (70 degrees C, 15 min) a 5.5-S RNA is released from the larger ribosomal RNA (27 S; 1.40-10(6) daltons). Unlike other protostomes, however, the 27-S rRNA does not dissociate in an orderly manner to yield 18-S components. The electrophoretic mobility of the small ribosomal RNA species (18 S; 0.72-10(6) daltons) is unaltered by heat treatment. The base composition of both Panagrellus rRNA species has a 51% G+C content. Labelling worms with either [Me-3H] methionine or [14C] uridine at various stages of postembryonic development and subsequent analysis of the extracted RNA revealed eight radioactive fractions, two of which were the mature ribosomal RNAs. From both the time sequence of appearance of the eight RNA species and the relative degree of methylation of the various RNA fractions two maturation pathways for ribosomal RNA processing were deduced. A 37-S (3.6-10(6) daltons) RNA is the precursor to both 27-S and 18-S RNA. During processing about two-fifths of the original transcriptional unit is lost. In this organism the "transcribed spacer" regions may be methylated although alternate interpretations have not been precluded.

Oogenesis and fertilization in Dipetalonema viteae (Nematoda: Filarioidea)

Oogenesis and fertilization in the filarial worm Dipetalonema viteae have been described. The female gametes are developed from a syncytium situated at the posterior tip of the ovary. The syncytium grows along the length of the ovary as an anucleate rachis carrying the germ cells with it. In the germinal region of the ovary the rachis is a central axial structure, but in the growth region it becomes very branched. The female gametes retain their attachment with the rachis until they have become mature primary oocytes; they acquire a surface coat whilst in the growth zone of the ovary. At fertilization the successful spermatozoon produces numerous ribosomes. Residual spermatozoa are apparently destroyed by the oocyte. The fertilized oocyte divides meiotically to yield an ovum and two polar bodies. In species destined to be unsheathed in the blood the surface coat and the original oolemma separate from the surface of the ovum. A new oolemma is produced, presumably under the stimulus of fertilization.