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

UNC-82/NUAK kinase is required by myosin A, but not myosin B, to assemble and function in the thick filament arms of C. elegans striated muscle

The mechanisms that ensure proper assembly, activity, and turnover of myosin II filaments are fundamental to a diverse range of cellular processes. In Caenorhabditis elegans striated muscle, thick filaments contain two myosins that are functionally distinct and spatially segregated. Using transgenic double mutants, we demonstrate that the ability of increased myosin A expression to restore muscle structure and movement in myosin B mutants requires UNC-82/NUAK kinase activity. Myosin B function appears unaffected in the kinase-impaired unc-82(e1220) mutant: the recessive antimorphic effects on early assembly of paramyosin and myosin A in this mutant are counteracted by increased myosin B expression and exacerbated by loss of myosin B. Using chimeric myosins and motility assays, we mapped the region of myosin A that requires UNC-82 activity to a 531-amino-acid region of the coiled-coil rod. This region includes the 264-amino-acid Region 1, which is sufficient in chimeric myosins to rescue the essential filament-initiation function of myosin A, as well as two sites that interact with myosin head domains in the Interacting Heads Motif. A specific physical interaction between myosin A and UNC-82::GFP is supported by GFP labeling of ectopic myosin A filaments but not thin filaments. We hypothesize that UNC-82 regulates assembly competence of myosin A during parallel assembly in the filament arms.

Transcriptomic analysis of the spatiotemporal axis of oogenesis and fertilization in C. elegans

Caenorhabditis elegans hermaphrodite presents a unique model to study the formation of oocytes. However, the size of the model animal and difficulties in retrieval of specific stages of the germline have obviated closer systematic studies of this process throughout the years. Here, we present a transcriptomic level analysis into the oogenesis of C. elegans hermaphrodites. We dissected a hermaphrodite gonad into seven sections corresponding to the mitotic distal region, the pachytene region, the diplotene region, the early diakinesis region and the 3 most proximal oocytes, and deeply sequenced the transcriptome of each of them along with that of the fertilized egg using a single-cell RNA-seq (scRNA-seq) protocol. We identified specific gene expression events as well as gene splicing events in finer detail along the gonad and provided novel insights into underlying mechanisms of the oogenesis process. Furthermore, through careful review of relevant research literature coupled with patterns observed in our analysis, we delineate transcripts that may serve functions in the interactions between the germline and cells of the somatic gonad. These results expand our knowledge of the transcriptomic space of the C. elegans germline and lay a foundation on which future studies of the germline can be based upon.

Transcriptomic Analysis of the Spatiotemporal Axis of Oogenesis and Fertilization in C. elegans

The oocyte germline of the C. elegans hermaphrodite presents a unique model to study the formation of oocytes. However, the size of the model animal and difficulties in retrieval of specific stages of the germline have obviated closer systematic studies of this process throughout the years. Here, we present a transcriptomic level analysis into the oogenesis of C. elegans hermaphrodites. We dissected a hermaphrodite gonad into seven sections corresponding to the mitotic distal region, the pachytene, the diplotene, the early diakinesis region and the 3 most proximal oocytes, and deeply sequenced the transcriptome of each of them along with that of the fertilized egg using a single-cell RNA-seq protocol. We identified specific gene expression events as well as gene splicing events in finer detail along the oocyte germline and provided novel insights into underlying mechanisms of the oogenesis process. Furthermore, through careful review of relevant research literature coupled with patterns observed in our analysis, we attempt to delineate transcripts that may serve functions in the interaction between the germline and cells of the somatic gonad. These results expand our knowledge of the transcriptomic space of the C. elegans germline and lay a foundation on which future studies of the germline can be based upon.

Comprehensive analysis of differentially expressed profiles of mRNA, lncRNA, and miRNA of Yili geese ovary at different egg-laying stages

Background

The development of the ovaries is an important factor that affects egg production performance in geese. Ovarian development is regulated by genes that are expressed dynamically and stage-specifically. The transcriptome profile analysis on ovarian tissues of goose at different egg laying stages could provide an important basis for screening and identifying key genes regulating ovarian development.

Results

In this study, 4 ovary tissues at each breeding period of pre-laying (PP), laying (LP), and ceased-laying period (CP), respectively, with significant morphology difference, were used for RNA extraction and mRNAs, lncRNAs, and miRNAs comparison in Yili geese. CeRNA regulatory network was constructed for key genes screening. A total of 337, 1136, and 525 differentially expressed DE mRNAs, 466, 925, and 742 DE lncRNAs and 258, 1131 and 909 DE miRNAs were identified between PP and LP, between CP and LP, and between CP and PP groups, respectively. Functional enrichment analysis showed that the differentially expressed mRNAs and non-coding RNA target genes were mainly involved in the cell process, cytokine-cytokine receptor interaction, phagosome, calcium signaling pathway, steroid biosynthesis and ECM-receptor interaction. Differential genes and non-coding RNAs, PDGFRB, ERBB4, LHCGR, MSTRG.129094.34, MSTRG.3524.1 and gga-miR-145–5p, related to reproduction and ovarian development were highly enriched. Furthermore, lncRNA-miRNA-mRNA regulatory networks related to ovary development were constructed.

Conclusions

Our study found dramatic transcriptomic differences in ovaries of Yili geese at different egg-laying stages, and a differential lncRNA-miRNA-mRNA regulatory network related to cell proliferation, differentiation and apoptosis and involved in stromal follicle development were established and preliminarily validated, which could be regarded as a key regulatory pathway of ovarian development in Yili geese.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08774-4.

Mutual dependence between tropomodulin and tropomyosin in the regulation of sarcomeric actin assembly in Caenorhabditis elegans striated muscle

Tropomodulin and tropomyosin are important components of sarcomeric thin filaments in striated muscles. Tropomyosin decorates the side of actin filaments and enhances tropomodulin capping at the pointed ends of the filaments. Their functional relationship has been extensively characterized in vitro, but in vivo and cellular studies in mammals are often complicated by the presence of functionally redundant isoforms. Here, we used the nematode Caenorhabditis elegans, which has a relatively simple composition of tropomodulin and tropomyosin genes, and demonstrated that tropomodulin (unc-94) and tropomyosin (lev-11) are mutually dependent on each other in their sarcomere localization and regulation of sarcomeric actin assembly. Mutation of tropomodulin caused sarcomere disorganization with formation of actin aggregates. However, the actin aggregation was suppressed when tropomyosin was depleted in the tropomodulin mutant. Tropomyosin was mislocalized to the actin aggregates in the tropomodulin mutants, while sarcomere localization of tropomodulin was lost when tropomyosin was depleted. These results indicate that tropomodulin and tropomyosin are interdependent in the regulation of organized sarcomeric assembly of actin filaments in vivo.

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.

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.

Tropomyosin isoforms differentially affect muscle contractility in the head and body regions of the nematode Caenorhabditis elegans

Tropomyosin, one of the major actin filament-binding proteins, regulates actin-myosin interaction and actin-filament stability. Multicellular organisms express a number of tropomyosin isoforms, but understanding of isoform-specific tropomyosin functions is incomplete. The nematode Caenorhabditis elegans has a single tropomyosin gene, lev-11, which has been reported to express four isoforms by using two separate promoters and alternative splicing. Here, we report a fifth tropomyosin isoform, LEV-11O, which is produced by alternative splicing that includes a newly identified seventh exon, exon 7a. By visualizing specific splicing events in vivo, we find that exon 7a is predominantly selected in a subset of the body wall muscles in the head, while exon 7b, which is the alternative to exon 7a, is utilized in the rest of the body. Point mutations in exon 7a and exon 7b cause resistance to levamisole--induced muscle contraction specifically in the head and the main body, respectively. Overexpression of LEV-11O, but not LEV-11A, in the main body results in weak levamisole resistance. These results demonstrate that specific tropomyosin isoforms are expressed in the head and body regions of the muscles and contribute differentially to the regulation of muscle contractility.

Molecular evolution of troponin I and a role of its N-terminal extension in nematode locomotion

The troponin complex, composed of troponin T (TnT), troponin I (TnI), and troponin C (TnC), is the major calcium-dependent regulator of muscle contraction, which is present widely in both vertebrates and invertebrates. Little is known about evolutionary aspects of troponin in the animal kingdom. Using a combination of data mining and functional analysis of TnI, we report evidence that an N-terminal extension of TnI is present in most of bilaterian animals as a functionally important domain. Troponin components have been reported in species in most of representative bilaterian phyla. Comparison of TnI sequences shows that the core domains are conserved in all examined TnIs, and that N- and C-terminal extensions are variable among isoforms and species. In particular, N-terminal extensions are present in all protostome TnIs and chordate cardiac TnIs but lost in a subset of chordate TnIs including vertebrate skeletal-muscle isoforms. Transgenic rescue experiments in Caenorhabditis elegans striated muscle show that the N-terminal extension of TnI (UNC-27) is required for coordinated worm locomotion but not in sarcomere assembly and single muscle-contractility kinetics. These results suggest that N-terminal extensions of TnIs are retained from a TnI ancestor as a functional domain.

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.

Lasting effects of early exposure to temperature on the gonadal transcriptome at the time of sex differentiation in the European sea bass, a fish with mixed genetic and environmental sex determination

BACKGROUND

Sex in fish is plastic and in several species can be influenced by environmental factors. In sensitive species, elevated temperatures have a masculinizing effect. Previous studies on the effects of temperature on gene expression have been restricted to a few cognate genes, mostly related to testis or ovarian development, and analyzed in gonads once they had completed the process of sex differentiation. However, studies on the effect of temperature at the whole gonadal transcriptomic level are scarce in fish and, in addition, temperature effects at the time of sex differentiation at the transcriptomic level are also unknown. Here, we used the European sea bass, a gonochoristic teleost with a polygenic sex determination system influenced by temperature, and exposed larvae to elevated temperature during the period of early gonad formation. Transcriptomic analysis of the gonads was carried out about three months after the end of temperature exposure, shortly after the beginning of the process of sex differentiation.

RESULTS

Elevated temperature doubled the number of males with respect to untreated controls. Transcriptomic analysis of early differentiating female gonads showed how heat caused: 1) an up-regulation of genes related to cholesterol transport (star), the stress response (nr3c1) and testis differentiation (amh, dmrt, etc.), 2) a decrease in the expression of genes related to ovarian differentiation such as cyp19a1a, and 3) an increase in the expression of several genes related to epigenetic regulatory mechanisms (hdac11, dicer1, ehmt2, jarid2a, pcgf2, suz12, mettl22).

CONCLUSIONS

Taken together, the results of this study contribute to the understanding of how the early environment sets permanent changes that result in long-lasting consequences, in this case in the sexual phenotype. Results also show the usefulness of comparing the effects of heat on the behavior of cognate genes related to sex differentiation as well as that of genes involved in establishing and maintaining cell identity through epigenetic mechanisms.

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.

HSF-1-mediated cytoskeletal integrity determines thermotolerance and life span

The conserved heat shock transcription factor-1 (HSF-1) is essential to cellular stress resistance and life-span determination. The canonical function of HSF-1 is to regulate a network of genes encoding molecular chaperones that protect proteins from damage caused by extrinsic environmental stress or intrinsic age-related deterioration. In Caenorhabditis elegans, we engineered a modified HSF-1 strain that increased stress resistance and longevity without enhanced chaperone induction. This health assurance acted through the regulation of the calcium-binding protein PAT-10. Loss of pat-10 caused a collapse of the actin cytoskeleton, stress resistance, and life span. Furthermore, overexpression of pat-10 increased actin filament stability, thermotolerance, and longevity, indicating that in addition to chaperone regulation, HSF-1 has a prominent role in cytoskeletal integrity, ensuring cellular function during stress and aging.

Sea lily muscle lacks a troponin-regulatory system, while it contains paramyosin

Troponin, a Ca(2+)-dependent regulator of striated muscle contraction, has been characterized in vertebrates, protochordates (amphioxus and ascidian), and many invertebrate animals that are categorized in protostomes, but it has not been detected in echinoderms, such as sea urchin and sea cucumber, members of subphylum Eleutherozoa. In this study, we examined the muscle of a species of isocrinid sea lilies, a member of subphylum Pelmatozoa, that constitute the most basal group of extant echinoderms to clarify whether troponin is lacking from the early evolution of echinoderms. Native thin filaments were released from the muscle homogenates in a relaxing buffer containing ATP and EGTA, a Ca(2+)-chelator, and were collected by ultra-centrifugation. Actin and tropomyosin, but not a troponin-like protein, were detected in the filament preparation. The filaments increased Mg(2+)-ATPase activity of rabbit skeletal muscle myosin irrespective of the presence or absence of Ca(2+). The results indicate that Ca(2+)-sensitive factor, troponin, is lacking in the thin filaments of sea lily muscle as in those of the other echinoderms, sea urchin and sea cucumber. On the other hand, a paramyosin-like protein that is absent from chordates was detected in sea lily muscle as in the muscles of the other echinoderms and invertebrate animals of protostomes.

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.

Transcriptome profiling of female alates and egg-laying queens of the Formosan subterranean termite

Termites are known to have an extraordinary reproductive plasticity and capacity, but the underlying genetic patterns of termite reproductive biology are relatively understudied. The goal of this study was to identify genes for which expression levels differ between dealated precopulatory females (virgins) and egg-laying queens of the Formosan subterranean termite, Coptotermes formosanus Shiraki. We constructed a normalized polyphenic expressed sequence tag (EST) library that represents genomic material from most of the castes and life stages of the Formosan subterranean termite. Microarrays were designed using probes from this EST library and public genomic resources. Virgin females and queens were competitively hybridized to these microarrays and differentially expressed candidate genes were identified. Differential expression of eight genes was subsequently confirmed via reverse transcriptase quantitative PCR (RT-QPCR). When compared to virgins, queens had higher expression of genes coding for proteins related to immunity (gram negative binding protein), nutrition (e.g., termite-derived endo-beta-1,4-glucanase), protein storage, regulation of caste differentiation and reproduction (hexamerin, juvenile hormone binding protein). Queens also had higher transcript levels for genes involved in metabolism of xenobiotics, fat, and juvenile hormone (glutathione-S-transferase-like proteins, and cytochrome P450), among others. In particular, hexamerin, juvenile hormone binding protein, and a cytochrome P450 from the 4C subfamily are likely to be involved in initiating the inactive period during the reproductive cycle of the queen. Vice versa, virgins had higher expression than queens of genes related to respiration, probably due to recent flight activity, and several genes of unknown function.

Comparative studies on troponin, a Ca²⁺-dependent regulator of muscle contraction, in striated and smooth muscles of protochordates

Troponin is well known as a Ca(2+)-dependent regulator of striated muscle contraction and it has been generally accepted that troponin functions as an inhibitor of muscle contraction or actin-myosin interaction at low Ca(2+) concentrations, and Ca(2+) at higher concentrations removes the inhibitory action of troponin. Recently, however, troponin became detectable in non-striated muscles of several invertebrates and in addition, unique troponin that functions as a Ca(2+)-dependent activator of muscle contraction has been detected in protochordate animals, although troponin in vertebrate striated muscle is known as an inhibitor of the contraction in the absence of a Ca(2+). Further studies on troponin in invertebrate muscle, especially in non-striated muscle, would provide new insight into the evolution of regulatory systems for muscle contraction and diverse function of troponin and related proteins. The methodology used for preparation and characterization of functional properties of protochordate striated and smooth muscles will be helpful for further studies of troponin in other invertebrate animals.

Detection of a troponin I-like protein in non-striated muscle of the tardigrades (water bears)

Tardigrades, also known as water bears, have somatic muscle fibers that are responsible for movement of their body and legs. These muscle fibers contain thin and thick filaments in a non-striated pattern. However, the regulatory mechanism of muscle contraction in tardigrades is unknown. In the absence of extensive molecular and genomic information, we detected a protein of 31 kDa in whole lysates of tardigrades that cross-reacted with the antibody raised against nematode troponin I (TnI). TnI is a component of the troponin complex that regulates actin-myosin interaction in a Ca(2+)-dependent and actin-linked manner. This TnI-like protein was co-extracted with actin in a buffer containing ATP and EGTA, which is known to induce relaxation of a troponin-regulated contractile system. The TnI-like protein was specifically expressed in the somatic muscle fibers in adult animals and partially co-localized with actin filaments in a non-striated manner. Interestingly, the pharyngeal muscle did not express this protein. These observations suggest that the non-striated somatic muscle of tardigrades has an actin-linked and troponin-regulated system for muscle contraction.