Structural components of the nonstriated contractile apparatuses in the Caenorhabditis elegans gonadal myoepithelial sheath and their essential roles for ovulation

The α-arrestin SUP-13/ARRD-15 promotes isoform turnover of actin-interacting protein 1 in Caenorhabditis elegans striated muscle

Precise arrangement of actin, myosin, and other regulatory components in a sarcomeric pattern is critical for producing contractile forces in striated muscles. Actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), is one of essential factors that regulate sarcomeric assembly of actin filaments. In the nematode Caenorhabditis elegans, mutation in unc-78, encoding one of the two AIP1 isoforms, causes severe disorganization of sarcomeric actin filaments and near paralysis, but mutation in sup-13 suppresses the unc-78-mutant phenotypes to restore nearly normal sarcomeric actin organization and worm motility. Here, we identified that sup-13 is a nonsense allele of arrd-15 encoding an α-arrestin. The sup-13/arrd-15 mutation suppressed the phenotypes of unc-78 null mutant but required aipl-1 that encodes a second AIP1 isoform. aipl-1 was normally expressed highly in embryos and downregulated in mature muscle. However, in the sup-13/arrd-15 mutant, the AIPL-1 protein was maintained at high levels in adult muscle to compensate for the absence of the UNC-78 protein. The sup-13/arrd-15 mutation caused accumulation of ubiquitinated AIPL-1 protein, suggesting that a normal function of sup-13/arrd-15 is to enhance degradation of ubiquitinated AIPL-1, thereby promoting transition of AIP1 isoforms from AIPL-1 to UNC-78 in developing muscle. These results suggest that α-arrestin is a novel factor to promote isoform turnover by enhancing protein degradation.

ZYX-1/Zyxin plays a minor role in oocyte transit through the spermatheca in C. elegans

In C. elegans, oocytes are ovulated into the spermatheca, where they are fertilized before being pushed into the uterus. Contraction in the C. elegans spermatheca is driven by circumferential acto-myosin fibers. The C. elegans zyxin homolog, zyx-1, is expressed in the body wall muscle, pharynx and spermatheca. To our surprise, a CRISPR-generated zyx-1 deletion allele results in no overt developmental phenotypes, and the spermathecal actin cytoskeleton appears wild type, however, oocyte transit through the spermatheca is slower than in wild type animals. This suggests ZYX-1/Zyxin may regulate spermathecal contraction magnitude or timing of spermathecal bag contraction and/or spermathecal-uterine valve dilation.

Gonadal transcriptomes associated with sex phenotypes provide potential male and female candidate genes of sex determination or early differentiation in Crassostrea gigas, a sequential hermaphrodite mollusc

Background

In the animal kingdom, mollusca is an important phylum of the Lophotrochozoa. However, few studies have investigated the molecular cascade of sex determination/early gonadal differentiation within this phylum. The oyster Crassostrea gigas is a sequential irregular hermaphrodite mollusc of economic, physiological and phylogenetic importance. Although some studies identified genes of its sex-determining/−differentiating pathway, this particular topic remains to be further deepened, in particular with regard to the expression patterns. Indeed, these patterns need to cover the entire period of sex lability and have to be associated to future sex phenotypes, usually impossible to establish in this sequential hermaphrodite. This is why we performed a gonadal RNA-Seq analysis of diploid male and female oysters that have not changed sex for 4 years, sampled during the entire time-window of sex determination/early sex differentiation (stages 0 and 3 of the gametogenetic cycle). This individual long-term monitoring gave us the opportunity to explain the molecular expression patterns in the light of the most statistically likely future sex of each oyster.

Results

The differential gene expression analysis of gonadal transcriptomes revealed that 9723 genes were differentially expressed between gametogenetic stages, and 141 between sexes (98 and 43 genes highly expressed in females and males, respectively). Eighty-four genes were both stage- and sex-specific, 57 of them being highly expressed at the time of sex determination/early sex differentiation. These 4 novel genes including Trophoblast glycoprotein-like, Protein PML-like, Protein singed-like and PREDICTED: paramyosin, while being supported by RT-qPCR, displayed sexually dimorphic gene expression patterns.

Conclusions

This gonadal transcriptome analysis, the first one associated with sex phenotypes in C. gigas, revealed 57 genes highly expressed in stage 0 or 3 of gametogenesis and which could be linked to the future sex of the individuals. While further study will be needed to suggest a role for these factors, some could certainly be original potential actors involved in sex determination/early sex differentiation, like paramyosin and could be used to predict the future sex of oysters.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07838-1.

FLN-1/filamin is required to anchor the actomyosin cytoskeleton and for global organization of sub-cellular organelles in a contractile tissue

Actomyosin networks are organized in space, direction, size, and connectivity to produce coordinated contractions across cells. We use the C. elegans spermatheca, a tube composed of contractile myoepithelial cells, to study how actomyosin structures are organized. FLN-1/filamin is required for the formation and stabilization of a regular array of parallel, contractile, actomyosin fibers in this tissue. Loss of fln-1 results in the detachment of actin fibers from the basal surface, which then accumulate along the cell junctions and are stabilized by spectrin. In addition, actin and myosin are captured at the nucleus by the linker of nucleoskeleton and cytoskeleton complex (LINC) complex, where they form large foci. Nuclear positioning and morphology, distribution of the endoplasmic reticulum and the mitochondrial network are also disrupted. These results demonstrate that filamin is required to prevent large actin bundle formation and detachment, to prevent excess nuclear localization of actin and myosin, and to ensure correct positioning of organelles.

Two Caenorhabditis elegans calponin-related proteins have overlapping functions that maintain cytoskeletal integrity and are essential for reproduction

Multicellular organisms have multiple genes encoding calponins and calponin-related proteins, some of which are known to regulate actin cytoskeletal dynamics and contractility. However, the functional similarities and differences among these proteins are largely unknown. In the nematode Caenorhabditis elegans, UNC-87 is a calponin-related protein with seven calponin-like (CLIK) motifs and is required for maintenance of contractile apparatuses in muscle cells. Here, we report that CLIK-1, another calponin-related protein that also contains seven CLIK motifs, functionally overlaps with UNC-87 in maintaining actin cytoskeletal integrity in vivo and has both common and different actin-regulatory activities in vitro We found that CLIK-1 is predominantly expressed in the body wall muscle and somatic gonad in which UNC-87 is also expressed. unc-87 mutation caused cytoskeletal defects in the body wall muscle and somatic gonad, whereas clik-1 depletion alone caused no detectable phenotypes. However, simultaneous clik-1 and unc-87 depletion caused sterility because of ovulation failure by severely affecting the contractile actin networks in the myoepithelial sheath of the somatic gonad. In vitro, UNC-87 bundled actin filaments, whereas CLIK-1 bound to actin filaments without bundling them and antagonized UNC-87-mediated filament bundling. We noticed that UNC-87 and CLIK-1 share common functions that inhibit cofilin binding and allow tropomyosin binding to actin filaments, suggesting that both proteins stabilize actin filaments. In conclusion, partially redundant functions of UNC-87 and CLIK-1 in ovulation are likely mediated by their common actin-regulatory activities, but their distinct actin-bundling activities suggest that they also have different biological functions.

A Region of UNC-89 (Obscurin) Lying between Two Protein Kinase Domains Is a Highly Elastic Spring Required for Proper Sarcomere Organization

In Caenorhabditis elegans, unc-89 encodes a set of giant multi-domain proteins (up 8081 residues) localized to the M-lines of muscle sarcomeres and required for normal sarcomere organization and whole-animal locomotion. Multiple UNC-89 isoforms contain two protein kinase domains. There is conservation in arrangement of domains between UNC-89 and its two mammalian homologs, obscurin and SPEG: kinase, a non-domain region of 647-742 residues, Ig domain, Fn3 domain and a second kinase domain. In all three proteins, this non-domain "interkinase region" has low sequence complexity, has high proline content, and lacks predicted secondary structure. We report that a major portion of this interkinase (571 residues out of 647 residues) when examined by single molecule force spectroscopy in vitro displays the properties of a random coil and acts as an entropic spring. We used CRISPR/Cas9 to create nematodes carrying an in-frame deletion of the same 571-residue portion of the interkinase. These animals display severe disorganization of all portions of the sarcomere in body wall muscle. Super-resolution microscopy reveals extra, short-A-bands lying close to the outer muscle cell membrane and between normally spaced A-bands. Nematodes with this in-frame deletion show defective locomotion and muscle force generation. We designed our CRISPR-generatedin-frame deletion to contain an HA tag at the N terminus of the large UNC-89 isoforms. This HA tag results in normal organization of body wall muscle, but approximately half the normal levels of the giant UNC-89 isoforms, dis-organization of pharyngeal muscle, small body size, and reduced muscle force, likely due to poor nutritional uptake.

A Novel Mutation in an NPXY Motif of β Integrin Reveals Phenotypes Similar to him-4/hemicentin

Integrin, an αβ heterodimeric cell surface receptor for the extracellular matrix (ECM), carries two tyrosine phosphorylation motifs in the cytoplasmic tail of the β subunit. NPXY (Asn-Pro-x-Tyr) is a conserved tyrosine phosphorylation motif that binds to the phospho-tyrosine binding (PTB) domain. We generated a tyrosine to glutamic acid (E) mutation to modify tyrosine (Y) into a negatively charged amino NPXY in the βpat-3 integrin of Caenorhabditis elegans. The transgenic rescue animal displayed defects in gonad migration and tail morphology. Also, the mutant animals produced a high number of males, suggesting that the Y to E mutation in βpat-3 integrin causes a phenotype similar to that of Him mutant. Further analyses revealed that males of pat-3(Y804E) and him-4/hemicentin share additional phenotypes such as abnormal gonad and unsuccessful mating. A pat-3 transgenic rescue mutant with a non-polar phenylalanine (F) in NPXY, pat-3(Y792/804F), suppressed the high male number, defective mating, inviable zygote, and the abnormal gonad of him-4 mutants, indicating that Y to F mutation in both NPXY motifs suppressed the him-4 phenotypes. This finding supports the idea that the ECM determines the activation state in integrin NPXY motifs; him-4/hemicentin may directly or indirectly interact with integrins and maintain the NPXY non-charged. Our findings provide new insight into a suppressive role of an ECM molecule in integrin NPXY phosphorylation.

Kettin, the large actin-binding protein with multiple immunoglobulin domains, is essential for sarcomeric actin assembly and larval development in Caenorhabditis elegans

Among many essential genes in the nematode Caenorhabditis elegans, let-330 is located on the left arm of chromosome V and was identified as the largest target of a mutagen in this region. However, let-330 gene has not been characterized at the molecular level. Here, we report that two sequenced let-330 alleles are nonsense mutations of ketn-1, a previously characterized gene encoding kettin. Kettin is a large actin-binding protein of 472 kDa with 31 immunoglobulin domains and is expressed in muscle cells in C. elegans. let-330/ketn-1 mutants are homozygous lethal at the first larval stage with mild defects in body elongation. These mutants have severe defects in sarcomeric actin and myosin assembly in striated muscle. However, α-actinin and vinculin, which are components of the dense bodies anchoring actin to the membranes, were not significantly disorganized by let-330/ketn-1 mutation. Kettin localizes to embryonic myofibrils before α-actinin is expressed, and α-actinin deficiency does not affect kettin localization in larval muscle. Depletion of vinculin minimally affects kettin localization but significantly reduces colocalization of actin with kettin in embryonic muscle cells. These results indicate that kettin is an essential protein for sarcomeric assembly of actin filaments in muscle cells.

Pairing of homologous chromosomes in C. elegans meiosis requires DEB-1 - an orthologue of mammalian vinculin

During meiosis, homologous chromosomes undergo a dramatic movement in order to correctly align. This is a critical meiotic event but the molecular properties of this 'chromosomal dance' still remainunclear. We identified DEB-1 - an orthologue of mammalian vinculin - as a new component of the mechanistic modules responsible for attaching the chromosomes to the nuclear envelope as apart of the LINC complex. In early meiotic nuclei of C. elegans, DEB-1 is localized to the nuclear periphery and alongside the synaptonemal complex of paired homologues. Upon DEB-1 depletion, chromosomes attached to SUN-1 foci remain highly motile until late pachytene. Although the initiation of homologue pairing started normally, irregularities in the formation of the synaptonemal complex occur, and these results in meiotic defects such as increased number of univalents at diakinesis and high embryonic lethality. Our data identify DEB-1 as a new player regulating chromosome dynamics and pairing during meiotic prophase I.

Regulation of Actin Dynamics in the C. elegans Somatic Gonad

The reproductive system of the hermaphroditic nematode C. elegans consists of a series of contractile cell types—including the gonadal sheath cells, the spermathecal cells and the spermatheca–uterine valve—that contract in a coordinated manner to regulate oocyte entry and exit of the fertilized embryo into the uterus. Contraction is driven by acto-myosin contraction and relies on the development and maintenance of specialized acto-myosin networks in each cell type. Study of this system has revealed insights into the regulation of acto-myosin network assembly and contractility in vivo.

Alternative splicing of the Caenorhabditis elegans lev-11 tropomyosin gene is regulated in a tissue-specific manner

Tropomyosin isoforms contribute to generation of functionally divergent actin filaments. In the nematode Caenorhabditis elegans, multiple isoforms are produced from lev-11, the single tropomyosin gene, by combination of two separate promoters and alternative pre-mRNA splicing. In this study, we report that alternative splicing of lev-11 is regulated in a tissue-specific manner so that a particular tropomyosin isoform is expressed in each tissue. Reverse-transcription polymerase chain reaction analysis of lev-11 mRNAs confirms five previously reported isoforms (LEV-11A, LEV-11C, LEV-11D, LEV-11E and LEV-11O) and identifies a new sixth isoform LEV-11T. Using transgenic alternative-splicing reporter minigenes, we find distinct patterns of preferential exon selections in the pharynx, body wall muscles, intestine and neurons. The body wall muscles preferentially process splicing to produce high-molecular-weight isoforms, LEV-11A, LEV-11D and LEV-11O. The pharynx specifically processes splicing to express a low-molecular-weight isoform LEV-11E, whereas the intestine and neurons process splicing to express another low-molecular-weight isoform LEV-11C. The splicing pattern of LEV-11T was not predominant in any of these tissues, suggesting that this is a minor isoform. Our results suggest that regulation of alternative splicing is an important mechanism to express proper tropomyosin isoforms in particular tissue and/or cell types in C. elegans.

Spectrin regulates cell contractility through production and maintenance of actin bundles in the Caenorhabditis elegans spermatheca

Disruption to the contractility of cells, including smooth muscle cells of the cardiovascular system and myoepithelial cells of the glandular epithelium, contributes to the pathophysiology of contractile tissue diseases, including asthma, hypertension, and primary Sjögren's syndrome. Cell contractility is determined by myosin activity and actomyosin network organization and is mediated by hundreds of protein-protein interactions, many directly involving actin. Here we use a candidate RNA interference screen of more than 100 Caenorhabditis elegans genes with predicted actin-binding and regulatory domains to identify genes that contribute to the contractility of the somatic gonad. We identify the spectrin cytoskeleton composed of SPC-1/α-spectrin, UNC-70/β-spectrin, and SMA-1/β heavy-spectrin as required for contractility and actin organization in the myoepithelial cells of the C. elegans spermatheca. We use imaging of fixed and live animals as well as tissue- and developmental-stage-specific disruption of the spectrin cytoskeleton to show that spectrin regulates the production of prominent central actin bundles and is required for maintenance of central actin bundles throughout successive rounds of stretch and contraction. We conclude that the spectrin cytoskeleton contributes to spermathecal contractility by promoting maintenance of the robust actomyosin bundles that drive contraction.

Myosin activity drives actomyosin bundle formation and organization in contractile cells of the Caenorhabditis elegans spermatheca

The contractile myoepithelial cells of the Caenorhabditis elegans somatic gonad are stretched by oocyte entry and subsequently contract to expel the fertilized embryo into the uterus. Formation of aligned, parallel actomyosin bundles during the first ovulation is triggered by oocyte entry and regulated by myosin contractility.

Stress fibers—contractile actomyosin bundles—are important for cellular force production and adaptation to physical stress and have been well studied within the context of cell migration. However, less is known about actomyosin bundle formation and organization in vivo and in specialized contractile cells, such as smooth muscle and myoepithelial cells. The Caenorhabditis elegans spermatheca is a bag-like organ of 24 myoepithelial cells that houses the sperm and is the site of fertilization. During ovulation, spermathecal cells are stretched by oocyte entry and then coordinately contract to expel the fertilized embryo into the uterus. Here we use four-dimensional confocal microscopy of live animals to observe changes to spermathecal actomyosin network organization during cell stretch and contraction. Oocyte entry is required to trigger cell contraction and concomitant production of parallel actomyosin bundles. Actomyosin bundle size, connectivity, spacing, and orientation are regulated by myosin activity. We conclude that myosin drives actomyosin bundle production and that myosin activity is tightly regulated during ovulation to produce an optimally organized actomyosin network in C. elegans spermathecae.

INF2- and FHOD-related formins promote ovulation in the somatic gonad of C. elegans

Formins are regulators of actin filament dynamics. We demonstrate here that two formins, FHOD-1 and EXC-6, are important in the nematode Caenorhabditis elegans for ovulation, during which actomyosin contractions push a maturing oocyte from the gonad arm into a distensible bag-like organ, the spermatheca. EXC-6, a homolog of the disease-associated mammalian formin INF2, is highly expressed in the spermatheca, where it localizes to cell-cell junctions and to circumferential actin filament bundles. Loss of EXC-6 does not noticeably affect the organization the actin filament bundles, and causes only a very modest increase in the population of junction-associated actin filaments. Despite absence of a strong cytoskeletal phenotype, approximately half of ovulations in exc-6 mutants exhibit extreme defects, including failure of the oocyte to enter the spermatheca, or breakage of the oocyte as the distal spermatheca entrance constricts during ovulation. Loss of FHOD-1 alone has little effect, and we cannot detect FHOD-1 in the spermatheca. However, combined loss of these formins in double fhod-1;exc-6 mutants results in profound ovulation defects, with significant slowing of the entry of oocytes into the spermatheca, and failure of nearly 80% of ovulations. We suggest that EXC-6 plays a role directly in the spermatheca, perhaps by modulating the ability of the spermatheca wall to rapidly accommodate an incoming oocyte, while FHOD-1 may play an indirect role relating to its known importance in the growth and function of the egg-laying muscles. © 2016 The Authors. Cytoskeleton Published by Wiley Periodicals, Inc.

Two distinct myosin II populations coordinate ovulatory contraction of the myoepithelial sheath in the Caenorhabditis elegans somatic gonad

In the nematode somatic gonad, nonmuscle myosin and muscle myosin form distinct filaments and coordinate ovulatory contraction of the myoepithelial sheath. Nonmuscle myosin regulatory light chain is phosphoregulated, and its phosphorylation and dephosphorylation are critical for successful ovulation.

The myoepithelial sheath in the somatic gonad of the nematode Caenorhabditis elegans has nonstriated contractile actomyosin networks that produce highly coordinated contractility for ovulation of mature oocytes. Two myosin heavy chains are expressed in the myoepithelial sheath, which are also expressed in the body-wall striated muscle. The troponin/tropomyosin system is also present and essential for ovulation. Therefore, although the myoepithelial sheath has smooth muscle–like contractile apparatuses, it has a striated muscle–like regulatory mechanism through troponin/tropomyosin. Here we report that the myoepithelial sheath has a distinct myosin population containing nonmuscle myosin II isoforms, which is regulated by phosphorylation and essential for ovulation. MLC-4, a nonmuscle myosin regulatory light chain, localizes to small punctate structures and does not colocalize with large, needle-like myosin filaments containing MYO-3, a striated-muscle myosin isoform. RNA interference of MLC-4, as well as of its upstream regulators, LET-502 (Rho-associated coiled-coil forming kinase) and MEL-11 (a myosin-binding subunit of myosin phosphatase), impairs ovulation. Expression of a phosphomimetic MLC-4 mutant mimicking a constitutively active state also impairs ovulation. A striated-muscle myosin (UNC-54) appears to provide partially compensatory contractility. Thus the results indicate that the two spatially distinct myosin II populations coordinately regulate ovulatory contraction of the myoepithelial sheath.

Robust Distal Tip Cell Pathfinding in the Face of Temperature Stress Is Ensured by Two Conserved microRNAS in Caenorhabditis elegans

Biological robustness, the ability of an organism to maintain a steady-state output as genetic or environmental inputs change, is critical for proper development. MicroRNAs have been implicated in biological robustness mechanisms through their post-transcriptional regulation of genes and gene networks. Previous research has illustrated examples of microRNAs promoting robustness as part of feedback loops and genetic switches and by buffering noisy gene expression resulting from environmental and/or internal changes. Here we show that the evolutionarily conserved microRNAs mir-34 and mir-83 (homolog of mammalian mir-29) contribute to the robust migration pattern of the distal tip cells in Caenorhabditis elegans by specifically protecting against stress from temperature changes. Furthermore, our results indicate that mir-34 and mir-83 may modulate the integrin signaling involved in distal tip cell migration by potentially targeting the GTPase cdc-42 and the beta-integrin pat-3. Our findings suggest a role for mir-34 and mir-83 in integrin-controlled cell migrations that may be conserved through higher organisms. They also provide yet another example of microRNA-based developmental robustness in response to a specific environmental stress, rapid temperature fluctuations.

Regulation of structure and function of sarcomeric actin filaments in striated muscle of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been used as a valuable system to study structure and function of striated muscle. The body wall muscle of C. elegans is obliquely striated muscle with highly organized sarcomeric assembly of actin, myosin, and other accessory proteins. Genetic and molecular biological studies in C. elegans have identified a number of genes encoding structural and regulatory components for the muscle contractile apparatuses, and many of them have counterparts in mammalian cardiac and skeletal muscles or striated muscles in other invertebrates. Applicability of genetics, cell biology, and biochemistry has made C. elegans an excellent system to study mechanisms of muscle contractility and assembly and maintenance of myofibrils. This review focuses on the regulatory mechanisms of structure and function of actin filaments in the C. elegans body wall muscle. Sarcomeric actin filaments in C. elegans muscle are associated with the troponin-tropomyosin system that regulates the actin-myosin interaction. Proteins that bind to the side and ends of actin filaments support ordered assembly of thin filaments. Furthermore, regulators of actin dynamics play important roles in initial assembly, growth, and maintenance of sarcomeres. The knowledge acquired in C. elegans can serve as bases to understand the basic mechanisms of muscle structure and function.

UNC-87 isoforms, Caenorhabditis elegans calponin-related proteins, interact with both actin and myosin and regulate actomyosin contractility

Two UNC-87 isoforms with seven calponin-like repeats are expressed widely in muscle and nonmuscle cells in Caenorhabditis elegans. They bind to actin and myosin and inhibit actomyosin motility in vitro. unc-87 mutation enhances contraction of nonstriated muscle in vivo, suggesting that UNC-87 isoforms are negative regulators of actomyosin contractility.

Calponin-related proteins are widely distributed among eukaryotes and involved in signaling and cytoskeletal regulation. Calponin-like (CLIK) repeat is an actin-binding motif found in the C-termini of vertebrate calponins. Although CLIK repeats stabilize actin filaments, other functions of these actin-binding motifs are unknown. The Caenorhabditis elegans unc-87 gene encodes actin-binding proteins with seven CLIK repeats. UNC-87 stabilizes actin filaments and is essential for maintenance of sarcomeric actin filaments in striated muscle. Here we show that two UNC-87 isoforms, UNC-87A and UNC-87B, are expressed in muscle and nonmuscle cells in a tissue-specific manner by two independent promoters and exhibit quantitatively different effects on both actin and myosin. Both UNC-87A and UNC-87B have seven CLIK repeats, but UNC-87A has an extra N-terminal extension of ∼190 amino acids. Both UNC-87 isoforms bind to actin filaments and myosin to induce ATP-resistant actomyosin bundles and inhibit actomyosin motility. UNC-87A with an N-terminal extension binds to actin and myosin more strongly than UNC-87B. UNC-87B is associated with actin filaments in nonstriated muscle in the somatic gonad, and an unc-87 mutation causes its excessive contraction, which is dependent on myosin. These results strongly suggest that proteins with CLIK repeats function as a negative regulator of actomyosin contractility.

Two actin-interacting protein 1 isoforms function redundantly in the somatic gonad and are essential for reproduction in Caenorhabditis elegans

The somatic gonad of the nematode Caenorhabditis elegans exhibits highly regulated contractility during ovulation, which is essential for successful reproduction. Nonstriated actin filament networks in the myoepithelial sheath at the proximal ovary provide contractile forces to push a mature oocyte for ovulation, but the mechanism of assembly and regulation of the contractile actin networks is poorly understood. Here, we show that actin-interacting protein 1 (AIP1) is essential for the assembly of the contractile actin networks in the myoepithelial sheath. AIP1 promotes disassembly of actin filaments in the presence of actin depolymerizing factor (ADF)/cofilin. C. elegans has two AIP1 genes, unc-78 and aipl-1. Mutation or RNA interference of a single AIP1 isoform causes only minor impacts on reproduction. However, simultaneous depletion of the two AIP1 isoforms causes sterility. AIP1-depleted animals show very weak contractility of the myoepithelial sheath and fail to ovulate a mature oocyte, which results in accumulation of endomitotic oocytes in the ovary. Depletion of AIP1 prevents assembly of actin networks and causes abnormal aggregation of actin as well as ADF/cofilin in the myoepithelial sheath. These results indicate that two AIP1 isoforms have redundant roles in assembly of the contractile apparatuses necessary for C. elegans reproduction.

Control of oocyte growth and meiotic maturation in Caenorhabditis elegans

In sexually reproducing animals, oocytes arrest at diplotene or diakinesis and resume meiosis (meiotic maturation) in response to hormones. Chromosome segregation errors in female meiosis I are the leading cause of human birth defects, and age-related changes in the hormonal environment of the ovary are a suggested cause. Caenorhabditis elegans is emerging as a genetic paradigm for studying hormonal control of meiotic maturation. The meiotic maturation processes in C. elegans and mammals share a number of biological and molecular similarities. Major sperm protein (MSP) and luteinizing hormone (LH), though unrelated in sequence, both trigger meiotic resumption using somatic Gα(s)-adenylate cyclase pathways and soma-germline gap-junctional communication. At a molecular level, the oocyte responses apparently involve the control of conserved protein kinase pathways and post-transcriptional gene regulation in the oocyte. At a cellular level, the responses include cortical cytoskeletal rearrangement, nuclear envelope breakdown, assembly of the acentriolar meiotic spindle, chromosome segregation, and likely changes important for fertilization and the oocyte-to-embryo transition. This chapter focuses on signaling mechanisms required for oocyte growth and meiotic maturation in C. elegans and discusses how these mechanisms coordinate the completion of meiosis and the oocyte-to-embryo transition.

Biochemical and cell biological analysis of actin in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has long been a useful model organism for muscle research. Its body wall muscle is obliquely striated muscle and exhibits structural similarities with vertebrate striated muscle. Actin is the core component of the muscle thin filaments, which are highly ordered in sarcomeric structures in striated muscle. Genetic studies have identified genes that regulate proper organization and function of actin filaments in C. elegans muscle, and sequence of the worm genome has revealed a number of conserved candidate genes that may regulate actin. To precisely understand the functions of actin-binding proteins, such genetic and genomic studies need to be complemented by biochemical characterization of these actin-binding proteins in vitro. This article describes methods for purification and biochemical characterization of actin from C. elegans. Although rabbit muscle actin is commonly used to characterize actin-binding proteins from many eukaryotic organisms, we detect several quantitative differences between C. elegans actin and rabbit muscle actin, highlighting that use of actin from an appropriate source is important in some cases. Additionally, we describe probes for cell biological analysis of actin in C. elegans.

FLN-1/filamin is required for maintenance of actin and exit of fertilized oocytes from the spermatheca in C. elegans

Filamin, known primarily for its actin cross-linking function, is a stretch-sensitive structural and signaling scaffold that binds transmembrane receptors and a wide variety of intracellular signaling proteins. The Caenorhabditis elegans filamin ortholog, FLN-1, has a well conserved overall structure, including an N-terminal actin-binding domain, and a series of 20 immunoglobulin (Ig)-like repeats. FLN-1 partially colocalizes with actin filaments in spermathecal and uterine cells. Analysis of phenotypes resulting from a deletion allele and RNAi depletion indicates FLN-1 is required to maintain the actin cytoskeleton in the spermatheca and uterus, and to allow the exit of embryos from the spermatheca. FLN-1 deficient animals accumulate embryos in the spermatheca, lay damaged and unfertilized eggs, and consequently exhibit dramatically reduced brood sizes. The phospholipase PLC-1 is also required for the exit of embryos from the spermatheca, and analysis of doubly mutant animals suggests that PLC-1 and FLN-1 act in the same pathway to promote proper transit of embryos from the spermatheca to the uterus. Given the modular protein structure, subcellular localization, genetic interaction with PLC-1, and known mechanosensory functions of filamin, we postulate that FLN-1 may be required to convert mechanical information about the presence of the oocyte into a biochemical signal, thereby allowing timely exit of the embryo from the spermatheca.

Troponin I controls ovulatory contraction of non-striated actomyosin networks in the C. elegans somatic gonad

The myoepithelial sheath of the Caenorhabditis elegans somatic gonad has non-striated actomyosin networks that provide contractile forces during ovulation, a process in which a mature oocyte is expelled from the ovary. Troponin T and troponin C are known regulators of contraction of the myoepithelial sheath. These are two of the three components of the troponin complex that is generally considered as a striated-muscle-specific regulator of actomyosin contraction. Here, we report identification of troponin I as the third component of the troponin complex that regulates ovulatory contraction of the myoepithelial sheath. C. elegans has four genes encoding troponin-I isoforms. We found that tni-1 and unc-27 (also known as tni-2) encode two major troponin-I isoforms in the myoepithelial sheath. Combination of RNA interference and mutation of tni-1 and unc-27 resulted in loss of the troponin-I protein in the gonad and caused sterility due to defective contraction of the myoepithelial sheath. Troponin-I-depleted gonads were hypercontracted, which is consistent with the function of troponin I as an inhibitor of actomyosin contraction. Troponin I was associated with non-striated actin networks in a tropomyosin-dependent manner. Our results demonstrate that troponin I regulates contraction of non-striated actomyosin networks and is an essential cytoskeletal component of the C. elegans reproductive system.

Stage-dependent transcriptional changes and characterization of paramyosin of the bovine lungworm Dictyocaulus viviparus

The bovine lungworm Dictyocaulus viviparus is of major economic importance in cattle farming in the temperate zones. The invertebrate protein paramyosin is one of the main components of muscle thick filaments but can also exhibit immunomodulatory functions. It represents a promising vaccine candidate in parasitic helminths. In this study, D. viviparus paramyosin (DvPmy) was characterized on the transcriptional as well as genomic level. The identified genomic sequence comprises 19 introns compared to only 10 introns in the Caenorhabditis elegans orthologue. Quantitative real time PCR transcriptional analysis revealed paramyosin transcription throughout the whole parasite's life cycle with the highest transcription rate in the agile moving first-stage larvae and the lowest in motionless hypobiosis induced third stage larvae. Recombinantly expressed DvPmy was found to bind collagen and IgG. Thereby the present study is the first showing that nematode paramyosin has the capability for immunomodulation and thus may be involved in host immune defence.

Essential role of ADF/cofilin for assembly of contractile actin networks in the C. elegans somatic gonad

The somatic gonad of the nematode Caenorhabditis elegans contains a myoepithelial sheath, which surrounds oocytes and provides contractile forces during ovulation. Contractile apparatuses of the myoepithelial-sheath cells are non-striated and similar to those of smooth muscle. We report the identification of a specific isoform of actin depolymerizing factor (ADF)/cofilin as an essential factor for assembly of contractile actin networks in the gonadal myoepithelial sheath. Two ADF/cofilin isoforms, UNC-60A and UNC-60B, are expressed from the unc-60 gene by alternative splicing. RNA interference of UNC-60A caused disorganization of the actin networks in the myoepithelial sheath. UNC-60B, which is known to function in the body-wall muscle, was not necessary or sufficient for actin organization in the myoepithelial sheath. However, mutant forms of UNC-60B with reduced actin-filament-severing activity rescued the UNC-60A-depletion phenotype. UNC-60A has a much weaker filament-severing activity than UNC-60B, suggesting that an ADF/cofilin with weak severing activity is optimal for assembly of actin networks in the myoepithelial sheath. By contrast, strong actin-filament-severing activity of UNC-60B was required for assembly of striated myofibrils in the body-wall muscle. Our results suggest that an optimal level of actin-filament-severing activity of ADF/cofilin is required for assembly of actin networks in the somatic gonad.

Role of phosphatidylinositol-4-phosphate 5' kinase (ppk-1) in ovulation of Caenorhabditis elegans

During Caenorhabditis elegans ovulation, the somatic gonad integrates signals from germ cells and propels a mature oocyte into the spermatheca for fertilization. Previous work suggests that phosphoinositide signaling plays important roles in C. elegans fertility. To fully understand inositol-1,4,5-trisphosphate (IP(3)) signaling in ovulation, we have examined the function of phosphatidylinositol-4-phosphate 5' kinase (PIP5K) in C. elegans. Our results show that the C. elegans PIP5K homolog, ppk-1, is essential for ovulation in C. elegans; ppk-1 is mainly expressed in somatic gonad, and depletion of ppk-1 expression causes defective ovulation, reduced gonad sheath contractility, and sterility. Increased IP(3) signaling compensates for ppk-1 (RNAi)-induced sterility, suggesting that ppk-1 is linked to IP(3) signaling. These results demonstrate that ppk-1 plays an essential role in IP(3) signaling and cytoskeleton organization in somatic gonad.