The Role of Soluble Adenylyl Cyclase in the Regulation of Flagellar Motility in Ascidian Sperm

Flagellar motility in sperm is activated and regulated by factors related to the eggs at fertilization. In the ascidian Ciona intestinalis, a sulfated steroid called the SAAF (sperm activating and attracting factor) induces both sperm motility activation and chemotaxis. Cyclic AMP (cAMP) is one of the most important intracellular factors in the sperm signaling pathway. Adenylyl cyclase (AC) is the key enzyme that synthesizes cAMP at the onset of the signaling pathway in all cellular functions. We previously reported that both transmembrane AC (tmAC) and soluble AC (sAC) play important roles in sperm motility in Ciona. The tmAC plays a major role in the SAAF-induced activation of sperm motility. On the other hand, sAC is involved in the regulation of flagellar beat frequency and the Ca2+-dependent chemotactic movement of sperm. In this study, we focused on the role of sAC in the regulation of flagellar motility in Ciona sperm chemotaxis. The immunochemical analysis revealed that several isoforms of sAC protein were expressed in Ciona sperm, as reported in mammals and sea urchins. We demonstrated that sAC inhibition caused strong and transient asymmetrization during the chemotactic turn, and then sperm failed to turn toward the SAAF. In addition, real-time Ca2+ imaging in sperm flagella revealed that sAC inhibition induced an excessive and prolonged Ca2+ influx to flagella. These results indicate that sAC plays a key role in sperm chemotaxis by regulating the clearance of [Ca2+]i and by modulating Ca2+-dependent flagellar waveform conversion.

The Roles of Two CNG Channels in the Regulation of Ascidian Sperm Chemotaxis

Spermatozoa sense and respond to their environmental signals to ensure fertilization success. Reception and transduction of signals are reflected rapidly in sperm flagellar waveforms and swimming behavior. In the ascidian Ciona intestinalis (type A; also called C. robusta), an egg-derived sulfated steroid called SAAF (sperm activating and attracting factor), induces both sperm motility activation and chemotaxis. Two types of CNG (cyclic nucleotide-gated) channels, Ci-tetra KCNG (tetrameric, cyclic nucleotide-gated, K⁺-selective) and Ci-HCN (hyperpolarization-activated and cyclic nucleotide-gated), are highly expressed in Ciona testis from the comprehensive gene expression analysis. To elucidate the sperm signaling pathway to regulate flagellar motility, we focus on the role of CNG channels. In this study, the immunochemical analysis revealed that both CNG channels are expressed in Ciona sperm and localized to sperm flagella. Sperm motility analysis and Ca²⁺ imaging during chemotaxis showed that CNG channel inhibition affected the changes in flagellar waveforms and Ca²⁺ efflux needed for the chemotactic turn. These results suggest that CNG channels in Ciona sperm play a vital role in regulating sperm motility and intracellular Ca²⁺ regulation during chemotaxis.

Impact of cilia-related genes on mitochondrial dynamics during Drosophila spermatogenesis

Spermatogenesis is a dynamic process of cellular differentiation that generates the mature spermatozoa required for reproduction. Errors that arise during this process can lead to sterility due to low sperm counts and malformed or immotile sperm. While it is estimated that 1 out of 7 human couples encounter infertility, the underlying cause of male infertility can only be identified in 50% of cases. Here, we describe and examine the genetic requirements for missing minor mitochondria (mmm), sterile affecting ciliogenesis (sac), and testes of unusual size (tous), three previously uncharacterized genes in Drosophila that are predicted to be components of the flagellar axoneme. Using Drosophila, we demonstrate that these genes are essential for male fertility and that loss of mmm, sac, or tous results in complete immotility of the sperm flagellum. Cytological examination uncovered additional roles for sac and tous during cytokinesis and transmission electron microscopy of developing spermatids in mmm, sac, and tous mutant animals revealed defects associated with mitochondria and the accessory microtubules required for the proper elongation of the mitochondria and flagella during ciliogenesis. This study highlights the complex interactions of cilia-related proteins within the cell body and advances our understanding of male infertility by uncovering novel mitochondrial defects during spermatogenesis.

A dynein-associated photoreceptor protein prevents ciliary acclimation to blue light

Light-responsive regulation of ciliary motility is known to be conducted through modulation of dyneins, but the mechanism is not fully understood. Here, we report a novel subunit of the two-headed f/I1 inner arm dynein, named DYBLUP, in animal spermatozoa and a unicellular green alga. This subunit contains a BLUF (sensors of blue light using FAD) domain that appears to directly modulate dynein activity in response to light. DYBLUP (dynein-associated BLUF protein) mediates the connection between the f/I1 motor domain and the tether complex that links the motor to the doublet microtubule. Chlamydomonas lacking the DYBLUP ortholog shows both positive and negative phototaxis but becomes acclimated and attracted to high-intensity blue light. These results suggest a mechanism to avoid toxic strong light via direct photoregulation of dyneins.

Genetic Variation in Pan Species Is Shaped by Demographic History and Harbors Lineage-Specific Functions

Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are the closest living relatives of humans, but the two species show distinct behavioral and physiological differences, particularly regarding female reproduction. Despite their recent rapid decline, the demographic histories of the two species have been different during the past 1–2 Myr, likely having an impact on their genomic diversity. Here, we analyze the inferred functional consequences of genetic variation across 69 individuals, making use of the most complete data set of genomes in the Pan clade to date. We test to which extent the demographic history influences the efficacy of purifying selection in these species. We find that small historical effective population sizes (N_e) correlate not only with low levels of genetic diversity but also with a larger number of deleterious alleles in homozygosity and an increased proportion of deleterious changes at low frequencies. To investigate the putative genetic basis for phenotypic differences between chimpanzees and bonobos, we exploit the catalog of putatively deleterious protein-coding changes in each lineage. We show that bonobo-specific nonsynonymous changes are enriched in genes related to age at menarche in humans, suggesting that the prominent physiological differences in the female reproductive system between chimpanzees and bonobos might be explained, in part, by putatively adaptive changes on the bonobo lineage.

Crystal structure of a Ca2+-dependent regulator of flagellar motility reveals the open-closed structural transition

Sperm chemotaxis toward a chemoattractant is very important for the success of fertilization. Calaxin, a member of the neuronal calcium sensor protein family, directly acts on outer-arm dynein and regulates specific flagellar movement during sperm chemotaxis of ascidian, Ciona intestinalis. Here, we present the crystal structures of calaxin both in the open and closed states upon Ca²⁺ and Mg²⁺ binding. The crystal structures revealed that three of the four EF-hands of a calaxin molecule bound Ca²⁺ ions and that EF2 and EF3 played a critical role in the conformational transition between the open and closed states. The rotation of α7 and α8 helices induces a significant conformational change of a part of the α10 helix into the loop. The structural differences between the Ca²⁺- and Mg²⁺-bound forms indicates that EF3 in the closed state has a lower affinity for Mg²⁺, suggesting that calaxin tends to adopt the open state in Mg²⁺-bound form. SAXS data supports that Ca²⁺-binding causes the structural transition toward the closed state. The changes in the structural transition of the C-terminal domain may be required to bind outer-arm dynein. These results provide a novel mechanism for recognizing a target protein using a calcium sensor protein.

Genetic signatures of natural selection in a model invasive ascidian

Invasive species represent promising models to study species’ responses to rapidly changing environments. Although local adaptation frequently occurs during contemporary range expansion, the associated genetic signatures at both population and genomic levels remain largely unknown. Here, we use genome-wide gene-associated microsatellites to investigate genetic signatures of natural selection in a model invasive ascidian, Ciona robusta. Population genetic analyses of 150 individuals sampled in Korea, New Zealand, South Africa and Spain showed significant genetic differentiation among populations. Based on outlier tests, we found high incidence of signatures of directional selection at 19 loci. Hitchhiking mapping analyses identified 12 directional selective sweep regions, and all selective sweep windows on chromosomes were narrow (~8.9 kb). Further analyses indentified 132 candidate genes under selection. When we compared our genetic data and six crucial environmental variables, 16 putatively selected loci showed significant correlation with these environmental variables. This suggests that the local environmental conditions have left significant signatures of selection at both population and genomic levels. Finally, we identified “plastic” genomic regions and genes that are promising regions to investigate evolutionary responses to rapid environmental change in C. robusta.

CRISPR/Cas9-Mediated Rapid Generation of Multiple Mouse Lines Identified Ccdc63 as Essential for Spermiogenesis

Spermatozoa are flagellated cells whose role in fertilization is dependent on their ability to move towards an oocyte. The structure of the sperm flagella is highly conserved across species, and much of what is known about this structure is derived from studies utilizing animal models. One group of proteins essential for the movement of the flagella are the dyneins. Using the advanced technology of CRISPR/Cas9 we have targeted three dynein group members; Dnaic1, Wdr63 and Ccdc63 in mice. All three of these genes are expressed strongly in the testis. We generated mice with amino acid substitutions in Dnaic1 to analyze two specific phosphorylation events at S124 and S127, and generated simple knockouts of Wdr63 and Ccdc63. We found that the targeted phosphorylation sites in Dnaic1 were not essential for male fertility. Similarly, Wdr63 was not essential for male fertility; however, Ccdc63 removal resulted in sterile male mice due to shortened flagella. This study demonstrates the versatility of the CRISPR/Cas9 system to generate animal models of a highly complex system by introducing point mutations and simple knockouts in a fast and efficient manner.

Sperm dysfunction and ciliopathy

Sperm motility is driven by motile cytoskeletal elements in the tail, called axonemes. The structure of axonemes consists of 9 + 2 microtubules, molecular motors (dyneins), and their regulatory structures. Axonemes are well conserved in motile cilia and flagella through eukaryotic evolution. Deficiency in the axonemal structure causes defects in sperm motility, and often leads to male infertility. It has been known since the 1970s that, in some cases, male infertility is linked with other symptoms or diseases such as Kartagener syndrome. Given that these links are mostly caused by deficiencies in the common components of cilia and flagella, they are called "immotile cilia syndrome" or "primary ciliary dyskinesia," or more recently, "ciliopathy," which includes deficiencies in primary and sensory cilia. Here, we review the structure of the sperm flagellum and epithelial cilia in the human body, and discuss how male fertility is linked to ciliopathy.

Branchial cilia and sperm flagella recruit distinct axonemal components

Eukaryotic cilia and flagella have highly conserved 9 + 2 structures. They are functionally diverged to play cell-type-specific roles even in a multicellular organism. Although their structural components are therefore believed to be common, few studies have investigated the molecular diversity of the protein components of the cilia and flagella in a single organism. Here we carried out a proteomic analysis and compared protein components between branchial cilia and sperm flagella in a marine invertebrate chordate, Ciona intestinalis. Distinct feature of protein recruitment in branchial cilia and sperm flagella has been clarified; (1) Isoforms of α- and β-tubulins as well as those of actins are distinctly used in branchial cilia or sperm flagella. (2) Structural components, such as dynein docking complex, tektins and an outer dense fiber protein, are used differently by the cilia and flagella. (3) Sperm flagella are specialized for the cAMP- and Ca2+-dependent regulation of outer arm dynein and for energy metabolism by glycolytic enzymes. Our present study clearly demonstrates that flagellar or ciliary proteins are properly recruited according to their function and stability, despite their apparent structural resemblance and conservation.

Distinct roles of soluble and transmembrane adenylyl cyclases in the regulation of flagellar motility in Ciona sperm

Adenylyl cyclase (AC) is a key enzyme that synthesizes cyclic AMP (cAMP) at the onset of the signaling pathway to activate sperm motility. Here, we showed that both transmembrane AC (tmAC) and soluble AC (sAC) are distinctly involved in the regulation of sperm motility in the ascidian Ciona intestinalis. A tmAC inhibitor blocked both cAMP synthesis and the activation of sperm motility induced by the egg factor sperm activating and attracting factor (SAAF), as well as those induced by theophylline, an inhibitor of phoshodiesterase. It also significantly inhibited cAMP-dependent phosphorylation of a set of proteins at motility activation. On the other hand, a sAC inhibitor does not affect on SAAF-induced transient increase of cAMP, motility activation or protein phosphorylation, but it reduced swimming velocity to half in theophylline-induced sperm. A sAC inhibitor KH-7 induced circular swimming trajectory with smaller diameter and significantly suppressed chemotaxis of sperm to SAAF. These results suggest that tmAC is involved in the basic mechanism for motility activation through cAMP-dependent protein phosphorylation, whereas sAC plays distinct roles in increase of flagellar beat frequency and in the Ca2+-dependent chemotactic movement of sperm.

Waveform generation is controlled by phosphorylation and swimming direction is controlled by Ca2+ in sperm from the mosquito Culex quinquefasciatus

Most animal sperm are quiescent in the male reproductive tract and become activated after mixing with accessory secretions from the male and/or female reproductive tract. Sperm from the mosquito Culex quinquefasciatus initiate flagellar motility after mixing with male accessory gland components, and the sperm flagellum displays three distinct motility patterns over time: a low amplitude, a long wavelength form (Wave A), a double waveform consisting of two superimposed waveforms over the length of the flagellum (Wave B), and finally, a single helical waveform that propels the sperm at high velocity (Wave C). This flagellar behavior is replicated by treating quiescent sperm with trypsin. When exposed to either broad spectrum or tyrosine kinase inhibitors, sperm activated by accessory gland secretions exhibited motility through Wave B but were unable to progress to Wave C. The MEK1/2 inhibitor UO126 and the ERK1/2 inhibitor FR180204 each blocked the transition from Wave B to Wave C, indicating a role for MAPK activity in the control of waveform and, accordingly, progressive movement. Furthermore, a MAPK substrate antibody stained the flagellum of activated sperm. In the absence of extracellular Ca(2+), a small fraction of sperm swam backwards, whereas most could not be activated by either accessory glands or trypsin and were immotile. However, the phosphatase inhibitor okadaic acid in the absence of extracellular Ca(2+) induced all sperm to swim backwards with a flagellar waveform similar to Wave A. These results indicate that flagellar waveform generation and direction of motility are controlled by protein phosphorylation and Ca(2+) levels, respectively.

Lipid rafts function in Ca2+ signaling responsible for activation of sperm motility and chemotaxis in the ascidian Ciona intestinalis

Lipid rafts are specialized membrane microdomains that function as signaling platforms across plasma membranes of many animal and plant cells. Although there are several studies implicating the role of lipid rafts in capacitation of mammalian sperm, the function of these structures in sperm motility activation and chemotaxis remains unknown. In the ascidian Ciona intestinalis, egg-derived sperm activating- and attracting-factor (SAAF) induces both activation of sperm motility and sperm chemotaxis to the egg. Here we found that a lipid raft disrupter, methyl-β-cyclodextrin (MCD), inhibited both SAAF-induced sperm motility activation and chemotaxis. MCD inhibited both SAAF-promoted synthesis of intracellular cyclic AMP and sperm motility induced by ionophore-mediated Ca(2+) entry, but not that induced by valinomycin-mediated hyperpolarization. Ca(2+)-imaging revealed that lipid raft disruption inhibited Ca(2+) influx upon activation of sperm motility. The Ca(2+)-activated adenylyl cyclase was clearly inhibited by MCD in isolated lipid rafts. The results suggest that sperm lipid rafts function in signaling upstream of cAMP synthesis, most likely in SAAF-induced Ca(2+) influx, and are required for Ca(2+)-dependent pathways underlying activation and chemotaxis in Ciona sperm.

Building blocks of the nexin-dynein regulatory complex in Chlamydomonas flagella

The directional flow generated by motile cilia and flagella is critical for many processes, including human development and organ function. Normal beating requires the control and coordination of thousands of dynein motors, and the nexin-dynein regulatory complex (N-DRC) has been identified as an important regulatory node for orchestrating dynein activity. The nexin link appears to be critical for the transformation of dynein-driven, linear microtubule sliding to flagellar bending, yet the molecular composition and mechanism of the N-DRC remain largely unknown. Here, we used proteomics with special attention to protein phosphorylation to analyze the composition of the N-DRC and to determine which subunits may be important for signal transduction. Two-dimensional electrophoresis and MALDI-TOF mass spectrometry of WT and mutant flagellar axonemes from Chlamydomonas identified 12 N-DRC-associated proteins, including all seven previously observed N-DRC components. Sequence and PCR analyses identified the mutation responsible for the phenotype of the sup-pf-4 strain, and biochemical comparison with a radial spoke mutant revealed two components that may link the N-DRC and the radial spokes. Phosphoproteomics revealed eight proteins with phosphorylated isoforms for which the isoform patterns changed with the genotype as well as two components that may play pivotal roles in N-DRC function through their phosphorylation status. These data were assembled into a model of the N-DRC that explains aspects of its regulatory function.

Sperm flagella: comparative and phylogenetic perspectives of protein components

Sperm motility is necessary for the transport of male DNA to eggs in species with both external and internal fertilization. Flagella comprise several proteins for generating and regulating motility. Central cytoskeletal structures called axonemes have been well conserved through evolution. In mammalian sperm flagella, two accessory structures (outer dense fiber and the fibrous sheath) surround the axoneme. The axonemal bend movement is based on the active sliding of axonemal doublet microtubules by the molecular motor dynein, which is divided into outer and inner arm dyneins according to positioning on the doublet microtubule. Outer and inner arm dyneins play different roles in the production and regulation of flagellar motility. Several regulatory mechanisms are known for both dyneins, which are important in motility activation and chemotaxis at fertilization. Although dynein itself has certain properties that contribute to the formation and propagation of flagellar bending, other axonemal structures-specifically, the radial spoke/central pair apparatus-have essential roles in the regulation of flagellar bending. Recent genetic and proteomic studies have explored several new components of axonemes and shed light on the generation and regulation of sperm motility during fertilization.

CIPRO 2.5: Ciona intestinalis protein database, a unique integrated repository of large-scale omics data, bioinformatic analyses and curated annotation, with user rating and reviewing functionality

The Ciona intestinalis protein database (CIPRO) is an integrated protein database for the tunicate species C. intestinalis. The database is unique in two respects: first, because of its phylogenetic position, Ciona is suitable model for understanding vertebrate evolution; and second, the database includes original large-scale transcriptomic and proteomic data. Ciona intestinalis has also been a favorite of developmental biologists. Therefore, large amounts of data exist on its development and morphology, along with a recent genome sequence and gene expression data. The CIPRO database is aimed at collecting those published data as well as providing unique information from unpublished experimental data, such as 3D expression profiling, 2D-PAGE and mass spectrometry-based large-scale analyses at various developmental stages, curated annotation data and various bioinformatic data, to facilitate research in diverse areas, including developmental, comparative and evolutionary biology. For medical and evolutionary research, homologs in humans and major model organisms are intentionally included. The current database is based on a recently developed KH model containing 36 034 unique sequences, but for higher usability it covers 89 683 all known and predicted proteins from all gene models for this species. Of these sequences, more than 10 000 proteins have been manually annotated. Furthermore, to establish a community-supported protein database, these annotations are open to evaluation by users through the CIPRO website. CIPRO 2.5 is freely accessible at http://cipro.ibio.jp/2.5.

Armadillo-repeat protein functions: questions for little creatures

Armadillo (ARM)-repeat proteins form a large family with diverse and fundamental functions in many eukaryotes. ARM-repeat proteins have largely been characterised in multicellular organisms and much is known about how a subset of these proteins function. The structure of ARM-repeats allows proteins containing them to be functionally very versatile. Are the ARM-repeat proteins in 'little creatures' as multifunctional as their better-studied relatives? The time is now right to start analysing ARM-repeat proteins in these new systems to better understand their cell biology. Here, we review recent advances in understanding the many cellular roles of both well-known and novel ARM-repeat proteins.

Structural and functional implications of phosphorylation and acetylation in the regulation of the AAA+ protein p97

p97, also known as VCP (valosin-containing protein), is a hexameric AAA+ ATPase that participates in a variety of cellular processes. It is believed that p97 mediates these processes through the binding of various adaptor proteins. Many factors govern adaptor binding and the regulatory mechanisms are not yet well understood. Sites of phosphorylation and acetylation on p97 have been identified and such post-translational modifications may be involved in regulating p97 function. Phosphorylation and, to a lesser extent, acetylation of p97 have been shown to modify its properties - for example, by modulating adaptor binding and directing subcellular localization. These modifications have been implicated in a number of p97-mediated processes, including misfolded protein degradation, membrane fusion, and transcription factor activation. This review describes the known phosphorylation and acetylation sites on p97 and discusses their possible structural and functional implications.

Regulation of sperm flagellar motility activation and chemotaxis caused by egg-derived substance(s) in sea cucumber

The sea cucumber Holothuria atra is a broadcast spawner. Among broadcast spawners, fertilization occurs by means of an egg-derived substance(s) that induces sperm flagellar motility activation and chemotaxis. Holothuria atra sperm were quiescent in seawater, but exhibited flagellar motility activation near eggs with chorion (intact eggs). In addition, they moved in a helical motion toward intact eggs as well as a capillary filled with the water layer of the egg extracts, suggesting that an egg-derived compound(s) causes motility activation and chemotaxis. Furthermore, demembranated sperm flagella were reactivated in high pH (> 7.8) solution without cAMP, and a phosphorylation assay using (gamma-32P)ATP showed that axonemal protein phosphorylation and dephosphorylation also occurred in a pH-dependent manner. These results suggest that the activation of sperm motility in holothurians is controlled by pH-sensitive changes in axonemal protein phosphorylation. Ca2+ concentration affected the swimming trajectory of demembranated sperm, indicating that Ca2+-binding proteins present at the flagella may be associated with regulation of flagellar waveform. Moreover, the phosphorylation states of several axonemal proteins were Ca2+-sensitive, indicating that Ca2+ impacts both kinase and phosphatase activities. In addition, in vivo sperm protein phosphorylation occurred after treatment with a water-soluble egg extract. Our results suggest that one or more egg-derived compounds activate motility and subsequent chemotactic behavior via Ca2+-sensitive flagellar protein phosphorylation.