Molecular basis of sperm flagellar axonemes: structural and evolutionary aspects

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

DNAJB13 is a radial spoke protein of mouse '9+2' axoneme

It has been shown that DNAJB13, a type II heat shock protein 40, is highly expressed in the testis and is an axonemal component of mouse mature spermatozoa. By multi-tissue reverse transcription polymerase chain reaction, we found that Dnajb13 gene was expressed not only in the testis but also in several other ciliated cell-containing tissues like brain, lung and oviduct. Immunohistochemistry on mouse trachea and oviduct sections shown that DNAJB13 was present in the motile cilia of those tissues. To define further its localization in the axoneme, immunoelectron microscopy of mouse sperm flagella was performed and shown that DNAJB13 was localized to radial spokes of the axoneme. Taken together, our data indicate that DNAJB13 is a radial spoke protein of the mouse '9+2' axoneme.

Spag16, an axonemal central apparatus gene, encodes a male germ cell nuclear speckle protein that regulates SPAG16 mRNA expression

Spag16 is the murine orthologue of Chlamydomonas reinhardtii PF20, a protein known to be essential to the structure and function of the "9+2" axoneme. In Chlamydomonas, the PF20 gene encodes a single protein present in the central pair of the axoneme. Loss of PF20 prevents central pair assembly/integrity and results in flagellar paralysis. Here we demonstrate that the murine Spag16 gene encodes two proteins: 71 kDa SPAG16L, which is found in all murine cells with motile cilia or flagella, and 35 kDa SPAG16S, representing the C terminus of SPAG16L, which is expressed only in male germ cells, and is predominantly found in specific regions within the nucleus that also contain SC35, a known marker of nuclear speckles enriched in pre-mRNA splicing factors. SPAG16S expression precedes expression of SPAG16L. Mice homozygous for a knockout of SPAG16L alone are infertile, but show no abnormalities in spermatogenesis. Mice chimeric for a mutation deleting the transcripts for both SPAG16L and SPAG16S have a profound defect in spermatogenesis. We show here that transduction of SPAG16S into cultured dispersed mouse male germ cells and BEAS-2B human bronchial epithelial cells increases SPAG16L expression, but has no effect on the expression of several other axoneme components. We also demonstrate that the Spag16L promoter shows increased activity in the presence of SPAG16S. The distinct nuclear localization of SPAG16S and its ability to modulate Spag16L mRNA expression suggest that SPAG16S plays an important role in the gene expression machinery of male germ cells. This is a unique example of a highly conserved axonemal protein gene that encodes two protein products with different functions.

Roles of osmolality, calcium - Potassium antagonist and calcium in activation and flagellar beating pattern of sturgeon sperm

The present study shows the roles of osmolality, calcium (Ca(2+))-potassium (K(+)) antagonist and Ca(2+) in sperm activation and flagellar beating of a sturgeon species, sterlet (Acipenser ruthenus). Sperm motility was activated at hypoosmolality relative to seminal plasma and suppressed at 175 mOsmol kg(-1). Sperm activation was totally suppressed by 0.35mM K(+), but Ca(2+) could fully reverse K(+) inhibitory effect at Ca(2+): K(+) ratio of 0.25. Neither EGTA (a chelator of Ca(2+) ions) nor nifedipine (a Ca(2+) channel blocker) prevented sperm activation. But, sperm motility and velocity were significantly decreased by EGTA, nifedipine and an inhibitor for Ca(2+)/calmodulin activated phosphodiesterase (w-7) that suggest role of Ca(2+) signaling after triggering sperm activation through hypoosmolality. Symmetric flagellar beating was also turned to asymmetric after activation in w-7, which is an evidence for modulation of Ca(2+)-binding proteins activity. Sturgeon sperm, similar to salmonids, is immotile in seminal plasma due to high K(+) concentrations, but the mechanism of sperm activation seems to be closer to other fish species where osmolality prohibits sperm activation in seminal plasma. In these species, hypoosmolality is the primary signal for sperm Ca(2+)-dependent signaling of axonemal beating.

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.

Atypical protein kinase C controls sea urchin ciliogenesis

The distribution and function of aPKC are examined during sea urchin ciliogenesis. The kinase concentrates in a ring at the transition zone between the basal body and the elongating axoneme. Inhibition of aPKC results in mislocalization of the kinase and defective ciliogenesis. Thus aPKC controls the growth of motile cilia in invertebrate embryos.

The atypical protein kinase C (aPKC) is part of the conserved aPKC/PAR6/PAR3 protein complex, which regulates many cell polarity events, including the formation of a primary cilium at the apical surface of epithelial cells. Cilia are highly organized, conserved, microtubule-based structures involved in motility, sensory processes, signaling, and cell polarity. We examined the distribution and function of aPKC in the sea urchin embryo, which forms a swimming blastula covered with motile cilia. We found that in the early embryo aPKC is uniformly cortical and becomes excluded from the vegetal pole during unequal cleavages at the 8- to 64-cell stages. During the blastula and gastrula stages the kinase localizes at the base of cilia, forming a ring at the transition zone between the basal body and the elongating axoneme. A dose-dependent and reversible inhibition of aPKC results in mislocalization of the kinase, defective ciliogenesis, and lack of swimming. Thus, as in the primary cilium of differentiated mammalian cells, aPKC controls the growth of motile cilia in invertebrate embryos. We suggest that aPKC might function to phosphorylate kinesin and so activate the transport of intraflagellar vesicles.

ankAT-1 is a novel gene mediating the apical tuft formation in the sea urchin embryo

In sea urchin embryos, the apical tuft forms within the neurogenic animal plate. When FoxQ2, one of the earliest factors expressed specifically in the animal plate by early blastula stage, is knocked down, the structure of the apical tuft is altered. To determine the basis of this phenotype, we identified FoxQ2-dependent genes using microarray analysis. The most strongly down-regulated gene in FoxQ2 morphants encodes a protein with ankyrin repeats region in its N-terminal domain. We named this gene ankAT-1, Ankyrin-containing gene specific for Apical Tuft. Initially its expression in the animal pole region of very early blastula stage embryos is FoxQ2-independent but becomes FoxQ2-dependent beginning at mesenchyme blastula stage and continuing in the animal plate of 3-day larvae. Furthermore, like FoxQ2, this gene is expressed throughout the expanded apical tuft region that forms in embryos lacking nuclear β-catenin. When AnkAT-1 is knocked-down by injecting a morpholino, the cilia at the animal plate in the resulting embryos are much shorter and their motility is less than that of motile cilia in other ectoderm cells, and remains similar to that of long apical tuft cilia. We conclude that AnkAT-1 is involved in regulating the length of apical tuft cilia.

Evolution and spermatogenesis

Sexual reproduction depends on the production of haploid gametes, and their fusion to form diploid zygotes. Here, we discuss sperm production and function in a molecular and functional evolutionary context, drawing predominantly from studies in model organisms (mice, Drosophila, Caenorhabditis elegans). We consider the mechanisms involved in establishing and maintaining a germline stem cell population in testes, as well as the factors that regulate their contribution to the pool of differentiating cells. These processes involve considerable interaction between the germline and the soma, and we focus on regulatory signalling events in a variety of organisms. The male germline has a unique transcriptional profile, including expression of many testis-specific genes. The evolutionary pressures associated with gene duplication and acquisition of testis function are discussed in the context of genome organization and transcriptional regulation. Post-meiotic differentiation of spermatids involves very dramatic changes in cell shape and acquisition of highly specialized features. We discuss the variety of sperm motility mechanisms and how various reproductive strategies are associated with the diversity of sperm forms found in animals.

Morphological sperm defects analyzed by light microscopy and transmission electron microscopy and their correlation with sperm motility

OBJECTIVES

To compare sperm defects as assessed by light microscopy (LM) and transmission electron microscopy (TEM), and to correlate them with sperm motility.

METHODS

A cohort of 40 male partners of infertile couples was selected. Group 1 (n = 31) included subjects with motility >5 and <50%, group 2 (n = 9) included those with motility <5% and the control group consisted of 10 normospermic subjects. Semen analysis of morphological parameters was carried out by LM and TEM.

RESULTS

A linear correlation between LM and TEM regarding head defects and excess residual cytoplasm (r = 0.87 and 0.90) was found, whereas there was a poor correlation between tail and midpiece anomalies (r = 0.46 and 0.21). No significant variations were detected by LM and TEM regarding sperm head defects and excess residual cytoplasm, whereas TEM showed a significantly greater percentage of tail and midpiece alterations compared with LM in groups 1 and 2, as well as controls (P < 0.05). The microtubular pattern '<9 + 2' represented the most frequent axonemal morphological alteration.

CONCLUSIONS

TEM might represent an additional diagnostic tool in the presence of severe sperm hypomotility or absence of motility.

Purification of dyneins from sperm flagella

Metazoan spermatozoa, especially those from marine invertebrates and fish, are excellent sources for isolating axonemal dyneins because of their cellular homogeneity and the large amounts that can be collected. Sperm flagella can be easily isolated by homogenization and subsequent centrifugation. Axonemes are obtained by demembranation of flagella with the nonionic detergent Triton X-100. The outer arm dyneins have been most widely studied because they are specifically extracted by a high-salt solution and can be isolated as a relatively pure fraction of ~20S two-headed dynein by sucrose density gradient centrifugation. Only a few reports have described the isolation of inner arm dyneins from sperm and the protocol has room for improvement. Sperm show clear changes in motility at fertilization, which are exerted through the regulation of axonemal dyneins by protein phosphorylation and Ca(2+) binding. Therefore dyneins from sperm flagella are an excellent biochemically tractable source for studying the regulation of axonemal dyneins. Here we describe protocols used for purification of flagellar dyneins from sperm of tunicates, sea urchins, and fish. The techniques described here could be applied to other species with appropriate modifications.

The regulation of dynein-driven microtubule sliding in Chlamydomonas flagella by axonemal kinases and phosphatases

The purpose of this chapter is to review the methodology and advances that have revealed conserved signaling proteins that are localized in the 9+2 ciliary axoneme for regulating motility. Diverse experimental systems have revealed that ciliary and eukaryotic flagellar motility is regulated by second messengers including calcium, pH, and cyclic nucleotides. In addition, recent advances in in vitro functional studies, taking advantage of isolated axonemes, pharmacological approaches, and biochemical analysis of axonemes have demonstrated that otherwise ubiquitous, conserved protein kinases and phosphatases are transported to and anchored in the axoneme. Here, we focus on the functional/pharmacological, genetic, and biochemical approaches in the model genetic system Chlamydomonas that have revealed highly conserved kinases, anchoring proteins (e.g., A-kinase anchoring proteins), and phosphatases that are physically located in the axoneme where they play a direct role in control of motility.

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.

Relocation of myosin and actin, kinesin and tubulin in the acrosome reaction of bovine spermatozoa

The mammalian acrosome reaction is a specialised exocytotic event. Although molecular motors are known to be involved in exocytosis in many cell types, their potential involvement in the acrosome reaction has remained unknown. Here, it has been shown that actin is localised within the equatorial segment and in the marginal acrosomal ridge of the heads of unreacted bull spermatozoa. Myosins IIA and IIB are found within the anterior acrosomal margins of virtually all sperm cells and, less prominently, within the equatorial segment. Tubulin was detected in the equatorial segment and around the periphery of the acrosome while kinesin was prominent in the equatorial segment. After induction of the acrosome reaction by means of the calcium ionophore A23187, the number of cells exhibiting actin fluorescence intensity in the anterior acrosomal margin decreased four-fold and those displaying equatorial segment fluorescence decreased 3.5-fold; myosin IIA immunofluorescence decreased in intensity with most spermatozoa losing equatorial staining, whereas there was little change in the distribution or intensity of myosin IIB immunofluorescence, except for approximately 20% decrease in the number of cells exhibiting acrosomal staining. Tubulin became largely undetectable within the head and kinesin staining spread rostrally over the main acrosome region. A possible sequence of events that ties in these observations of molecular motor involvement with the known participation of SNARE proteins is provided.

Global gene expression profiles for life stages of the deadly amphibian pathogen Batrachochytrium dendrobatidis

Amphibians around the world are being threatened by an emerging pathogen, the chytrid fungus Batrachochytrium dendrobatidis (Bd). Despite intensive ecological study in the decade since Bd was discovered, little is known about the mechanism by which Bd kills frogs. Here, we compare patterns of global gene expression in controlled laboratory conditions for the two phases of the life cycle of Bd: the free-living zoospore and the substrate-embedded sporangia. We find zoospores to be transcriptionally less complex than sporangia. Several transcripts more abundant in zoospores provide clues about how this motile life stage interacts with its environment. Genes with higher levels of expression in sporangia provide new hypotheses about the molecular pathways involved in metabolic activity, flagellar function, and pathogenicity in Bd. We highlight expression patterns for a group of fungalysin metallopeptidase genes, a gene family thought to be involved in pathogenicity in another group of fungal pathogens that similarly cause cutaneous infection of vertebrates. Finally we discuss the challenges inherent in developing a molecular toolkit for chytrids, a basal fungal lineage separated by vast phylogenetic distance from other well characterized fungi.