Shared and Distinct Mechanisms of Compartmentalized and Cytosolic Ciliogenesis

Genetic mutation of Cep76 results in male infertility due to abnormal sperm tail composition

The transition zone is a specialised gate at the base of cilia/flagella, which separates the ciliary compartment from the cytoplasm and strictly regulates protein entry. We identified a potential new regulator of the male germ cell transition zone, CEP76. We demonstrated that CEP76 was involved in the selective entry and incorporation of key proteins required for sperm function and fertility into the ciliary compartment and ultimately the sperm tail. In the mutant, sperm tails were shorter and immotile as a consequence of deficits in essential sperm motility proteins including DNAH2 and AKAP4, which accumulated at the sperm neck in the mutant. Severe annulus, fibrous sheath, and outer dense fibre abnormalities were also detected in sperm lacking CEP76. Finally, we identified that CEP76 dictates annulus positioning and structure. This study suggests CEP76 as a male germ cell transition zone protein and adds further evidence to the hypothesis that the spermatid transition zone and annulus are part of the same functional structure.

Regulation of ciliary homeostasis by intraflagellar transport-independent kinesins

Cilia are highly conserved eukaryotic organelles that protrude from the cell surface and are involved in sensory perception, motility, and signaling. Their proper assembly and function rely on the bidirectional intraflagellar transport (IFT) system, which involves motor proteins, including antegrade kinesins and retrograde dynein. Although the role of IFT-mediated transport in cilia has been extensively studied, recent research has highlighted the contribution of IFT-independent kinesins in ciliary processes. The coordinated activities and interplay between IFT kinesins and IFT-independent kinesins are crucial for maintaining ciliary homeostasis. In this comprehensive review, we aim to delve into the specific contributions and mechanisms of action of the IFT-independent kinesins in cilia. By shedding light on their involvement, we hope to gain a more holistic perspective on ciliogenesis and ciliopathies.

CCDC183 is essential for cytoplasmic invagination around the flagellum during spermiogenesis and male fertility

Sperm flagellum plays a crucial role in male fertility. Here, we generated Ccdc183 knockout mice using the CRISPR/Cas9 system to reveal the protein function of the testis-specific protein CCDC183 in spermiogenesis. We demonstrated that the absence of CCDC183 causes male infertility with morphological and motility defects in spermatozoa. Owing to the lack of CCDC183, centrioles after elongation of axonemal microtubules do not connect the cell surface and nucleus during spermiogenesis, which causes subsequent loss of cytoplasmic invagination around the flagellum. As a result, the flagellar compartment does not form properly and cytosol-exposed axonemal microtubules collapse during spermiogenesis. In addition, ectopic localization of accessory structures, such as the fibrous sheath and outer dense fibers, and abnormal head shape as a result of abnormal sculpting by the manchette are observed in Ccdc183 knockout spermatids. Our results indicate that CCDC183 plays an essential role in cytoplasmic invagination around the flagellum to form functional spermatozoa during spermiogenesis.

Genomic Diversity Illuminates the Environmental Adaptation of Drosophila suzukii

Biological invasions carry substantial practical and scientific importance, and represent natural evolutionary experiments on contemporary timescales. Here, we investigated genomic diversity and environmental adaptation of the crop pest Drosophila suzukii using whole-genome sequencing data and environmental metadata for 29 population samples from its native and invasive range. Through a multifaceted analysis of this population genomic data, we increase our understanding of the D. suzukii genome, its diversity and its evolution, and we identify an appropriate genotype-environment association pipeline for our data set. Using this approach, we detect genetic signals of local adaptation associated with nine distinct environmental factors related to altitude, wind speed, precipitation, temperature, and human land use. We uncover unique functional signatures for each environmental variable, such as a prevalence of cuticular genes associated with annual precipitation. We also infer biological commonalities in the adaptation to diverse selective pressures, particularly in terms of the apparent contribution of nervous system evolution to enriched processes (ranging from neuron development to circadian behavior) and to top genes associated with all nine environmental variables. Our findings therefore depict a finer-scale adaptive landscape underlying the rapid invasion success of this agronomically important species.

p25alpha Domain-Containing Proteins of Apicomplexans and Related Taxa

TPPP (tubulin polymerization promoting protein)-like proteins contain one or more p25alpha (Pfam05517) domains. TPPP-like proteins occur in different types as determined by their length (e.g., long-, short-, truncated-, and fungal-type TPPP) and include the protein apicortin, which possesses another domain, doublecortin (DCX, Pfam 03607). These various TPPP-like proteins are found in various phylogenomic groups. In particular, short-type TPPPs and apicortin are well-represented in the Myzozoa, which include apicomplexans and related taxa, chrompodellids, dinoflagellates, and perkinsids. The long-, truncated-, and fungal-type TPPPs are not found in the myzozoans. Apicortins are found in all apicomplexans except one piroplasmid species, present in several other myzozoans, and seem to be correlated with the conoid and apical complex. Short-type TPPPs are predominantly found in myzozoans that have flagella, suggesting a role in flagellum assembly or structure.

Evidence for intraflagellar transport in butterfly spermatocyte cilia

In the model organism insect Drosophila melanogaster short cilia assemble on spermatocytes that elaborate into 1.8 mm long flagella during spermatid differentiation. A unique feature of these cilia/flagella is their lack of dependence on intraflagellar transport (IFT) for their assembly. Here, we show that in the common butterfly Pieris brassicae, the spermatocyte cilia are exceptionally long: about 40 μm compared to less than 1 μm in Drosophila. By transmission electron microscopy, we show that P. brassicae spermatocytes display several features not found in melanogaster, including compelling evidence of IFT structures and features of motile cilia.

The regeneration-responsive element careg monitors activation of Müller glia after MNU-induced damage of photoreceptors in the zebrafish retina

In contrast to mammals, zebrafish can regenerate their damaged photoreceptors. This capacity depends on the intrinsic plasticity of Müller glia (MG). Here, we identified that the transgenic reporter careg, a marker of regenerating fin and heart, also participates in retina restoration in zebrafish. After methylnitrosourea (MNU) treatment, the retina became deteriorated and contained damaged cell types including rods, UV-sensitive cones and the outer plexiform layer. This phenotype was associated with the induction of careg expression in a subset of MG until the reconstruction of the photoreceptor synaptic layer. Single-cell RNA sequencing (scRNAseq) analysis of regenerating retinas revealed a population of immature rods, defined by high expression of rhodopsin and the ciliogenesis gene meig1, but low expression of phototransduction genes. Furthermore, cones displayed deregulation of metabolic and visual perception genes in response to retina injury. Comparison between careg:EGFP expressing and non-expressing MG demonstrated that these two subpopulations are characterized by distinct molecular signatures, suggesting their heterogenous responsiveness to the regenerative program. Dynamics of ribosomal protein S6 phosphorylation showed that TOR signaling became progressively switched from MG to progenitors. Inhibition of TOR with rapamycin reduced the cell cycle activity, but neither affected careg:EGFP expression in MG, nor prevented restoration of the retina structure. This indicates that MG reprogramming, and progenitor cell proliferation might be regulated by distinct mechanisms. In conclusion, the careg reporter detects activated MG, and provides a common marker of regeneration-competent cells in diverse zebrafish organs, including the retina.

Composition, organization and mechanisms of the transition zone, a gate for the cilium

The cilium evolved to provide the ancestral eukaryote with the ability to move and sense its environment. Acquiring these functions required the compartmentalization of a dynein-based motility apparatus and signaling proteins within a discrete subcellular organelle contiguous with the cytosol. Here, we explore the potential molecular mechanisms for how the proximal-most region of the cilium, termed transition zone (TZ), acts as a diffusion barrier for both membrane and soluble proteins and helps to ensure ciliary autonomy and homeostasis. These include a unique complement and spatial organization of proteins that span from the microtubule-based axoneme to the ciliary membrane; a protein picket fence; a specialized lipid microdomain; differential membrane curvature and thickness; and lastly, a size-selective molecular sieve. In addition, the TZ must be permissive for, and functionally integrates with, ciliary trafficking systems (including intraflagellar transport) that cross the barrier and make the ciliary compartment dynamic. The quest to understand the TZ continues and promises to not only illuminate essential aspects of human cell signaling, physiology, and development, but also to unravel how TZ dysfunction contributes to ciliopathies that affect multiple organ systems, including eyes, kidney, and brain.

Two distinct compartments of a ctenophore comb plate provide structural and functional integrity for the motility of giant multicilia

Comb plates are large ciliary structures uniquely seen in comb jellies (ctenophores).^1,2,3 A comb plate is constructed from tens of thousands of cilia that are bundled together by structures called compartmenting lamellae (CLs).^4,5,6 We previously reported the first component of the CL, CTENO64, and found that it was specifically found in ctenophores and was essential for the determination of ciliary orientation.³ However, CTENO64 is localized only in the proximal region of the CL; therefore, the molecular architecture of the CL over the entire length of a comb plate has not been elucidated. Here, we identified a second CL component, CTENO189. This ctenophore-specific protein was present in the distal region of comb plates, with a localization clearly segregated from CTENO64. Knockdown of the CTENO189 gene using morpholino antisense oligonucleotides resulted in complete loss of CLs in the distal region of comb plates but did not affect the formation of comb plates or the orientation of each cilium. However, the hexagonal distribution of cilia was disarranged, and the metachronal coordination of comb plates along a comb row was lost in the CTENO189 morphants. The morphant comb plate showed asymmetric ciliary-type movement in normal seawater, and in a high-viscosity solution, it could not maintain the normal waveforms but showed a symmetric flagellar-type movement. Our findings demonstrated two distinct compartments of a comb plate: the proximal CL as the building foundation that rigidly fixes the ciliary orientation, and the distal CL that reinforces the elastic connection among cilia to overcome the hydrodynamic drag of giant multiciliary plates.

Mechanochemical tuning of a kinesin motor essential for malaria parasite transmission

Plasmodium species cause malaria and kill hundreds of thousands annually. The microtubule-based motor kinesin-8B is required for development of the flagellated Plasmodium male gamete, and its absence completely blocks parasite transmission. To understand the molecular basis of kinesin-8B's essential role, we characterised the in vitro properties of kinesin-8B motor domains from P. berghei and P. falciparum. Both motors drive ATP-dependent microtubule gliding, but also catalyse ATP-dependent microtubule depolymerisation. We determined these motors' microtubule-bound structures using cryo-electron microscopy, which showed very similar modes of microtubule interaction in which Plasmodium-distinct sequences at the microtubule-kinesin interface influence motor function. Intriguingly however, P. berghei kinesin-8B exhibits a non-canonical structural response to ATP analogue binding such that neck linker docking is not induced. Nevertheless, the neck linker region is required for motility and depolymerisation activities of these motors. These data suggest that the mechanochemistry of Plasmodium kinesin-8Bs is functionally tuned to support flagella formation.

An Oomycete-Specific Leucine-Rich Repeat-Containing Protein Is Involved in Zoospore Flagellum Development in Phytophthora sojae

A leucine-rich repeat (LRR) is a widespread structural motif of 20 to 30 amino acids with characteristic repetitive sequences rich in leucine. LRR-containing proteins are critical for ligand recognition and binding, participating in plant development and defense. Like plants, oomycetes also harbor genes encoding LRR-containing proteins, but their functions remain largely unknown. We identified a zoospore-upregulated gene from Phytophthora sojae with LRRs and an extra structural maintenance of chromosomes-like domain. We generated knockout and complemented knockout strains of this LRR protein and found that its deletion resulted in a pronounced reduction in zoospore mobility and chemotaxis, cyst germination, and virulence. Interestingly, micro-examination of zoospores under a scanning electron microscope revealed irregularly shaped zoospores without flagella in these deletion mutants. In addition, the reintroduction of this LRR protein into the knockout mutant reversed all the deficiencies. Our data demonstrate a critical role for the Phytophthora LRR protein in modulating zoospore development, which impairs migration to the host soybean and affects the spread of Phytophthora pathogens.

Axonemal Growth and Alignment During Paraspermatogenesis in the Marine Gastropod Strombus luhuanus

Parasperm are non-fertilizing sperm that are produced simultaneously with fertile eusperm. They occur in several animal species and show considerable morphological diversity. We investigated the dynamics of axonemes during paraspermatogenesis in the marine snail S. luhuanus. Mature parasperm were characterized by two lateral undulating membranes for motility and many globular vesicles. Axonemes were first observed as brush-like structures that extruded from the anterior region. Multiple axonemes longer than the brush then started to extend inside the cytoplasm towards the posterior region. The mass of the axonemes separated into two lateral rows and formed an undulating membrane that drives bidirectional swimming in the mature parasperm. The central pair of axonemes was aligned in the undulating membrane, resulting in cooperative bend propagation. During paraspermatogenesis, centrioles were largely diminished and localized to the anterior region. CEP290, a major component of the transition zone, showed a broad distribution in the anterior area. Axonemes in the posterior region showed a 9 + 0 structure with both outer and inner arm dyneins. These observations provide a structural basis for understanding the physiological functions of parasperm in marine reproductive strategies.

Roles of the actin cytoskeleton in ciliogenesis

Primary cilia play a key role in the ability of cells to respond to extracellular stimuli, such as signaling molecules and environmental cues. These sensory organelles are crucial to the development of many organ systems, and defects in primary ciliogenesis lead to multisystemic genetic disorders, known as ciliopathies. Here, we review recent advances in the understanding of several key aspects of the regulation of ciliogenesis. Primary ciliogenesis is thought to take different pathways depending on cell type, and some recent studies shed new light on the cell-type-specific mechanisms regulating ciliogenesis at the apical surface in polarized epithelial cells, which are particularly relevant for many ciliopathies. Furthermore, recent findings have demonstrated the importance of actin cytoskeleton dynamics in positively and negatively regulating multiple stages of ciliogenesis, including the vesicular trafficking of ciliary components and the positioning and docking of the basal body. Finally, studies on the formation of motile cilia in multiciliated epithelial cells have revealed requirements for actin remodeling in this process too, as well as showing evidence of an additional alternative ciliogenesis pathway.

Structure of the ciliogenesis-associated CPLANE complex

Dysfunctional cilia cause pleiotropic human diseases termed ciliopathies. These hereditary maladies are often caused by defects in cilia assembly, a complex event that is regulated by the ciliogenesis and planar polarity effector (CPLANE) proteins Wdpcp, Inturned, and Fuzzy. CPLANE proteins are essential for building the cilium and are mutated in multiple ciliopathies, yet their structure and molecular functions remain elusive. Here, we show that mammalian CPLANE proteins comprise a bona fide complex and report the near-atomic resolution structures of the human Wdpcp-Inturned-Fuzzy complex and of the mouse Wdpcp-Inturned-Fuzzy complex bound to the small guanosine triphosphatase Rsg1. Notably, the crescent-shaped CPLANE complex binds phospholipids such as phosphatidylinositol 3-phosphate via multiple modules and a CPLANE ciliopathy mutant exhibits aberrant lipid binding. Our study provides critical structural and functional insights into an enigmatic ciliogenesis-associated complex as well as unexpected molecular rationales for ciliopathies.

Genetic divergence and range expansion in a western North Pacific coral

Coral poleward range expansions have recently been observed in response to warming oceans. Range expansion can lead to reduced genetic diversity and increased frequency of deleterious mutations that were rare in core populations, potentially limiting the ability for adaptation and persistence in novel environments. Successful expansions that overcome these founder effects and colonize new habitat have been attributed to multiple introductions from different sources, hybridization with native populations, or rapid adaptive evolution. Here, we investigate population genomic patterns of the reef-building coral Acropora hyacinthus along a latitudinal cline that includes a well-established range expansion front in Japan using 2b-RAD sequencing. A total of 184 coral samples were collected across seven sites spanning from ~24°N to near its northern range front at ~33°N. We uncover the presence of three cryptic lineages of A. hyacinthus, which occupy discrete reefs within this region. Only one lineage is present along the expansion front and we find evidence for its historical occupation of marginal habitats. Within this lineage we also find evidence of bottleneck pressures associated with expansion events including higher clonality, increased linkage disequilibrium, and lower genetic diversity in range edge populations compared to core populations. Asymmetric migration between populations was also detected with lower migration from edge sites. Lastly, we describe genomic signatures of local adaptation potentially attributed to lower winter temperatures experienced at the more recently expanded northern populations. Together these data illuminate the genomic consequences of range expansion in a coral and highlight how adaptation to discrete environments along expansion fronts may facilitate further range expansion in this temperate coral lineage.

C2cd6-encoded CatSperτ targets sperm calcium channel to Ca2+ signaling domains in the flagellar membrane

In mammalian sperm cells, regulation of spatiotemporal Ca²⁺ signaling relies on the quadrilinear Ca²⁺ signaling nanodomains in the flagellar membrane. The sperm-specific, multi-subunit CatSper Ca²⁺ channel, which is crucial for sperm hyperactivated motility and male fertility, organizes the nanodomains. Here, we report CatSperτ, the C2cd6-encoded membrane-associating C2 domain protein, can independently migrate to the flagella and serve as a major targeting component of the CatSper channel complex. CatSperτ loss of function in mice demonstrates that it is essential for sperm hyperactivated motility and male fertility. CatSpertτ targets the CatSper channel into the quadrilinear nanodomains in the flagella of developing spermatids, whereas it is dispensable for functional channel assembly. CatSperτ interacts with ciliary trafficking machinery in a C2-dependent manner. These findings provide insights into the CatSper channel trafficking to the Ca²⁺ signaling nanodomains and the shared molecular mechanisms of ciliary and flagellar membrane targeting.

Hwang et al. report that the C2 domain protein CatSperτ targets the sperm CatSper Ca²⁺ channel to linear domains of the sperm flagellum during development. The findings provide fundamental insights into CatSper trafficking and the shared molecular mechanisms among ciliary and flagellar membrane targeting.

The Drosophila orthologue of the primary ciliary dyskinesia-associated gene, DNAAF3, is required for axonemal dynein assembly

Ciliary motility is powered by a suite of highly conserved axoneme-specific dynein motor complexes. In humans, the impairment of these motors through mutation results in the disease primary ciliary dyskinesia (PCD). Studies in Drosophila have helped to validate several PCD genes whose products are required for cytoplasmic pre-assembly of axonemal dynein motors. Here we report the characterisation of the Drosophila orthologue of the less-known assembly factor DNAAF3. This gene, CG17669 (Dnaaf3), is expressed exclusively in developing mechanosensory chordotonal (Ch) neurons and the cells that generate spermatozoa, The only two Drosophila cell types bearing cilia/flagella containing dynein motors. Mutation of Dnaaf3 results in larvae that are deaf and adults that are uncoordinated, indicating defective Ch neuron function. The mutant Ch neuron cilia of the antenna specifically lack dynein arms, while Ca imaging in larvae reveals a complete loss of Ch neuron response to vibration stimulus, confirming that mechanotransduction relies on ciliary dynein motors. Mutant males are infertile with immotile sperm whose flagella lack dynein arms and show axoneme disruption. Analysis of proteomic changes suggest a reduction in heavy chains of all axonemal dynein forms, consistent with an impairment of dynein pre-assembly.

Twitchy, the Drosophila orthologue of the ciliary gating protein FBF1/dyf-19, is required for coordinated locomotion and male fertility

Primary cilia are compartmentalised from the rest of the cell by a ciliary gate comprising transition fibres and a transition zone. The ciliary gate allows the selective import and export of molecules such as transmembrane receptors and transport proteins. These are required for the assembly of the cilium, its function as a sensory and signalling centre and to maintain its distinctive composition. Certain motile cilia can also form within the cytosol as exemplified by human and Drosophila sperm. The role of transition fibre proteins has not been well described in the cytoplasmic cilia. Drosophila have both compartmentalised primary cilia, in sensory neurons, and sperm flagella that form within the cytosol. Here, we describe phenotypes for twitchy the Drosophila orthologue of a transition fibre protein, mammalian FBF1/C. elegans dyf-19. Loss-of-function mutants in twitchy are adult lethal and display a severely uncoordinated phenotype. Twitchy flies are too uncoordinated to mate but RNAi-mediated loss of twitchy specifically within the male germline results in coordinated but infertile adults. Examination of sperm from twitchy RNAi-knockdown flies shows that the flagellar axoneme forms, elongates and is post-translationally modified by polyglycylation but the production of motile sperm is impaired. These results indicate that twitchy is required for the function of both sensory cilia that are compartmentalised from the rest of the cell and sperm flagella that are formed within the cytosol of the cell. Twitchy is therefore likely to function as part of a molecular gate in sensory neurons but may have a distinct function in sperm cells.

The regulatory roles of motile cilia in CSF circulation and hydrocephalus

Background

Cerebrospinal fluid (CSF) is an ultra-filtrated colorless brain fluid that circulates within brain spaces like the ventricular cavities, subarachnoid space, and the spine. Its continuous flow serves many primary functions, including nourishment, brain protection, and waste removal.

Main body

The abnormal accumulation of CSF in brain cavities triggers severe hydrocephalus. Accumulating evidence had indicated that synchronized beats of motile cilia (cilia from multiciliated cells or the ependymal lining in brain ventricles) provide forceful pressure to generate and restrain CSF flow and maintain overall CSF circulation within brain spaces. In humans, the disorders caused by defective primary and/or motile cilia are generally referred to as ciliopathies. The key role of CSF circulation in brain development and its functioning has not been fully elucidated.

Conclusions

In this review, we briefly discuss the underlying role of motile cilia in CSF circulation and hydrocephalus. We have reviewed cilia and ciliated cells in the brain and the existing evidence for the regulatory role of functional cilia in CSF circulation in the brain. We further discuss the findings obtained for defective cilia and their potential involvement in hydrocephalus. Furthermore, this review will reinforce the idea of motile cilia as master regulators of CSF movements, brain development, and neuronal diseases.

The paternal toolbox for embryo development and health

The sperm is essential for reconstitution of embryonic diploidy and highly specialized developmental functions. Immediately after gamete fusion, the sperm-borne PLC-zeta triggers activation, generating intracellular free Ca2+ oscillations. Mutations in the PLC-zeta encoding gene are associated with the absence of this factor in mature sperm and inability to achieve fertilization. Sperm play also a role in the greater game of the choreography of fertilization. In the human, the sperm centrioles are introduced into the oocyte environment with gamete fusion. They interact with the oocyte cytoskeletal apparatus to form a functional pair of centrosomes and ultimately regulate pronuclear juxtaposition in preparation for the first cleavage. As a consequence, the fidelity of chromosome segregation during the first cell divisions depends on the function of sperm centrioles. Sperm DNA integrity is essential for embryo development and health. Damaged DNA does not impact on the sperm fertilization ability following ICSI. However, detrimental effects emerge at pre- and post-implantation stages. Sperm-specific epigenetic factors also play an active role in the regulation of embryonic development, as shown by correlations between reduced embryo morphological quality and incorrect chromatin packaging during spermiogenesis or abnormal methylation of sperm CpG islands. This functional landscape demonstrates that the contribution of the sperm to development goes far beyond its well-established role in fertilization. Clinical studies confirm this view and indicate sperm function as a crucial aspect of research to increase the efficacy of assisted reproduction treatments.

Ultrastructural evidence for an unusual mode of ciliogenesis in mouse multiciliated epithelia

Multiciliogenesis is a cascading process for generating hundreds of motile cilia in single cells. In vertebrates, this process has been investigated in the ependyma of brain ventricles and the ciliated epithelia of the airway and oviduct. Although the early steps to amplify centrioles have been characterized in molecular detail, subsequent steps to establish multicilia have been relatively overlooked. Here, we focused on unusual cilia-related structures previously observed in wild-type mouse ependyma using transmission electron microscopy and analyzed their ultrastructural features and the frequency of their occurrence. In the ependyma, $\sim$5% of cilia existed as bundles; while the majority of the bundles were paired, bundles of more than three cilia were also found. Furthermore, apical protrusions harboring multiple sets of axonemes were occasionally observed (0-2 per section), suggesting an unusual mode of ciliogenesis. In trachea and oviduct epithelia, ciliary bundles were absent, but protrusions containing multiple axonemes were observed. At the base of such protrusions, certain axonemes were completely enwrapped by membranes, whereas others remained incompletely enwrapped. These data suggested that the late steps of multiciliogenesis might include a unique process underlying the development of cilia, which is distinct from the ciliogenesis of primary cilia.

Essential Role of CFAP53 in Sperm Flagellum Biogenesis

The sperm flagellum is essential for male fertility. Despite vigorous research progress toward understanding the pathogenesis of flagellum-related diseases, much remains unknown about the mechanisms underlying the flagellum biogenesis itself. Here, we show that the cilia and flagella associated protein 53 (Cfap53) gene is predominantly expressed in testes, and it is essential for sperm flagellum biogenesis. The knockout of this gene resulted in complete infertility in male mice but not in the females. CFAP53 localized to the manchette and sperm tail during spermiogenesis, the knockout of this gene impaired flagellum biogenesis. Furthermore, we identified two manchette and sperm tail-associated proteins that interacted with CFAP53 during spermiogenesis. Together, our results suggest that CFAP53 is an essential protein for sperm flagellum biogenesis, and its mutations might be associated with multiple morphological abnormalities of the flagella (MMAF).

A missense mutation in IFT74, encoding for an essential component for intraflagellar transport of Tubulin, causes asthenozoospermia and male infertility without clinical signs of Bardet-Biedl syndrome

Cilia and flagella are formed around an evolutionary conserved microtubule-based axoneme and are required for fluid and mucus clearance, tissue homeostasis, cell differentiation and movement. The formation and maintenance of cilia and flagella require bidirectional transit of proteins along the axonemal microtubules, a process called intraflagellar transport (IFT). In humans, IFT defects contribute to a large group of systemic diseases, called ciliopathies, which often display overlapping phenotypes. By performing exome sequencing of a cohort of 167 non-syndromic infertile men displaying multiple morphological abnormalities of the sperm flagellum (MMAF) we identified two unrelated patients carrying a homozygous missense variant adjacent to a splice donor consensus site of IFT74 (c.256G > A;p.Gly86Ser). IFT74 encodes for a core component of the IFT machinery that is essential for the anterograde transport of tubulin. We demonstrate that this missense variant affects IFT74 mRNA splicing and induces the production of at least two distinct mutant proteins with abnormal subcellular localization along the sperm flagellum. Importantly, while IFT74 deficiency was previously implicated in two cases of Bardet-Biedl syndrome, a pleiotropic ciliopathy with variable expressivity, our data indicate that this missense mutation only results in primary male infertility due to MMAF, with no other clinical features. Taken together, our data indicate that the nature of the mutation adds a level of complexity to the clinical manifestations of ciliary dysfunction, thus contributing to the expanding phenotypical spectrum of ciliopathies.

Expansion microscopy provides new insights into the cytoskeleton of malaria parasites including the conservation of a conoid

Malaria is caused by unicellular Plasmodium parasites. Plasmodium relies on diverse microtubule cytoskeletal structures for its reproduction, multiplication, and dissemination. Due to the small size of this parasite, its cytoskeleton has been primarily observable by electron microscopy (EM). Here, we demonstrate that the nanoscale cytoskeleton organisation is within reach using ultrastructure expansion microscopy (U-ExM). In developing microgametocytes, U-ExM allows monitoring the dynamic assembly of axonemes and concomitant tubulin polyglutamylation in whole cells. In the invasive merozoite and ookinete forms, U-ExM unveils the diversity across Plasmodium stages and species of the subpellicular microtubule arrays that confer cell rigidity. In ookinetes, we additionally identify an apical tubulin ring (ATR) that colocalises with markers of the conoid in related apicomplexan parasites. This tubulin-containing structure was presumed to be lost in Plasmodium despite its crucial role in motility and invasion in other apicomplexans. Here, U-ExM reveals that a divergent and considerably reduced form of the conoid is actually conserved in Plasmodium species.

Defects in the cytoplasmic assembly of axonemal dynein arms cause morphological abnormalities and dysmotility in sperm cells leading to male infertility

Axonemal protein complexes, such as outer (ODA) and inner (IDA) dynein arms, are responsible for the generation and regulation of flagellar and ciliary beating. Studies in various ciliated model organisms have shown that axonemal dynein arms are first assembled in the cell cytoplasm and then delivered into axonemes during ciliogenesis. In humans, mutations in genes encoding for factors involved in this process cause structural and functional defects of motile cilia in various organs such as the airways and result in the hereditary disorder primary ciliary dyskinesia (PCD). Despite extensive knowledge about the cytoplasmic assembly of axonemal dynein arms in respiratory cilia, this process is still poorly understood in sperm flagella. To better define its clinical relevance on sperm structure and function, and thus male fertility, further investigations are required. Here we report the fertility status in different axonemal dynein preassembly mutant males (DNAAF2/ KTU, DNAAF4/ DYX1C1, DNAAF6/ PIH1D3, DNAAF7/ZMYND10, CFAP300/C11orf70 and LRRC6). Besides andrological examinations, we functionally and structurally analyzed sperm flagella of affected individuals by high-speed video- and transmission electron microscopy as well as systematically compared the composition of dynein arms in sperm flagella and respiratory cilia by immunofluorescence microscopy. Furthermore, we analyzed the flagellar length in dynein preassembly mutant sperm. We found that the process of axonemal dynein preassembly is also critical in sperm, by identifying defects of ODAs and IDAs in dysmotile sperm of these individuals. Interestingly, these mutant sperm consistently show a complete loss of ODAs, while some respiratory cilia from the same individual can retain ODAs in the proximal ciliary compartment. This agrees with reports of solely one distinct ODA type in sperm, compared to two different ODA types in proximal and distal respiratory ciliary axonemes. Consistent with observations in model organisms, we also determined a significant reduction of sperm flagellar length in these individuals. These findings are relevant to subsequent studies on the function and composition of sperm flagella in PCD patients and non-syndromic infertile males. Our study contributes to a better understanding of the fertility status in PCD-affected males and should help guide genetic and andrological counselling for affected males and their families.

Impaired male fertility is a major issue and affects several men worldwide. Patients may present with reduced number or complete absence of sperm in the ejaculate, as well as functional and/or morphological sperm defects compromising sperm motility. Despite several diagnostic efforts, the underlying causes of these defects often remain unknown („idiopathic“). The beating of sperm flagella as well as motile cilia, such as those of the respiratory tract, is driven by dynein-based motor protein complexes, namely outer and inner dynein arms. In motile cilia these protein complexes are known to be first assembled in the cytoplasm and then delivered into the cilium. In sperm, this process is still poorly understood. Here we analyze sperm cells of male individuals with mutations in distinct genes encoding factors involved in the preassembly of these motor protein complexes. Consistent with defects in their respiratory ciliated cells, these individuals also demonstrate defects in sperm flagella that cause male infertility due to immotile sperm, with a reduction of flagellar length. Our results strengthen the assumption that the preassembly process of outer and inner dynein arms is clinically relevant also in sperm and provide knowledge that should guide genetic and andrological counselling for a subgroup of men with idiopathic infertility.

The Microtubule Cytoskeleton during the Early Drosophila Spermiogenesis

Sperm elongation and nuclear shaping in Drosophila largely depends on the microtubule cytoskeleton that in early spermatids has centrosomal and non-centrosomal origins. We report here an additional γ-tubulin focus localized on the anterior pole of the nucleus in correspondence of the apical end of the perinuclear microtubules that run within the dense complex. The perinuclear microtubules are nucleated by the pericentriolar material, or centriole adjunct, that surrounds the basal body and are retained to play a major role in nuclear shaping. However, we found that both the perinuclear microtubules and the dense complex are present in spermatids lacking centrioles. Therefore, the basal body or the centriole adjunct seem to be dispensable for the organization and assembly of these structures. These observations shed light on a novel localization of γ-tubulin and open a new scenario on the distribution of the microtubules and the organization of the dense complex during early Drosophila spermiogenesis.

Haploid male germ cells-the Grand Central Station of protein transport

BACKGROUND

The precise movement of proteins and vesicles is an essential ability for all eukaryotic cells. Nowhere is this more evident than during the remarkable transformation that occurs in spermiogenesis-the transformation of haploid round spermatids into sperm. These transformations are critically dependent upon both the microtubule and the actin cytoskeleton, and defects in these processes are thought to underpin a significant percentage of human male infertility.

OBJECTIVE AND RATIONALE

This review is aimed at summarising and synthesising the current state of knowledge around protein/vesicle transport during haploid male germ cell development and identifying knowledge gaps and challenges for future research. To achieve this, we summarise the key discoveries related to protein transport using the mouse as a model system. Where relevant, we anchored these insights to knowledge in the field of human spermiogenesis and the causality of human male infertility.

SEARCH METHODS

Relevant studies published in English were identified using PubMed using a range of search terms related to the core focus of the review-protein/vesicle transport, intra-flagellar transport, intra-manchette transport, Golgi, acrosome, manchette, axoneme, outer dense fibres and fibrous sheath. Searches were not restricted to a particular time frame or species although the emphasis within the review is on mammalian spermiogenesis.

OUTCOMES

Spermiogenesis is the final phase of sperm development. It results in the transformation of a round cell into a highly polarised sperm with the capacity for fertility. It is critically dependent on the cytoskeleton and its ability to transport protein complexes and vesicles over long distances and often between distinct cytoplasmic compartments. The development of the acrosome covering the sperm head, the sperm tail within the ciliary lobe, the manchette and its role in sperm head shaping and protein transport into the tail, and the assembly of mitochondria into the mid-piece of sperm, may all be viewed as a series of overlapping and interconnected train tracks. Defects in this redistribution network lead to male infertility characterised by abnormal sperm morphology (teratozoospermia) and/or abnormal sperm motility (asthenozoospermia) and are likely to be causal of, or contribute to, a significant percentage of human male infertility.

WIDER IMPLICATIONS

A greater understanding of the mechanisms of protein transport in spermiogenesis offers the potential to precisely diagnose cases of male infertility and to forecast implications for children conceived using gametes containing these mutations. The manipulation of these processes will offer opportunities for male-based contraceptive development. Further, as increasingly evidenced in the literature, we believe that the continuous and spatiotemporally restrained nature of spermiogenesis provides an outstanding model system to identify, and de-code, cytoskeletal elements and transport mechanisms of relevance to multiple tissues.

The sperm centrioles

Centrioles are eukaryotic subcellular structures that produce and regulate massive cytoskeleton superstructures. They form centrosomes and cilia, regulate new centriole formation, anchor cilia to the cell, and regulate cilia function. These basic centriolar functions are executed in sperm cells during their amplification from spermatogonial stem cells during their differentiation to spermatozoa, and finally, after fertilization, when the sperm fuses with the egg. However, sperm centrioles exhibit many unique characteristics not commonly observed in other cell types, including structural remodeling, centriole-flagellum transition zone migration, and cell membrane association during meiosis. Here, we discuss five roles of sperm centrioles: orchestrating early spermatogenic cell divisions, forming the spermatozoon flagella, linking the spermatozoon head and tail, controlling sperm tail beating, and organizing the cytoskeleton of the zygote post-fertilization. We present the historic discovery of the centriole as a sperm factor that initiates embryogenesis, and recent genetic studies in humans and other mammals evaluating the current evidence for the five functions of sperm centrioles. We also examine information connecting the various sperm centriole functions to distinct clinical phenotypes. The emerging picture is that centrioles are essential sperm components with remarkable functional diversity and specialization that will require extensive and in-depth future studies.

Intraflagellar transport during assembly of flagella of different length in Trypanosoma brucei isolated from tsetse flies

Multicellular organisms assemble cilia and flagella of precise lengths differing from one cell to another, yet little is known about the mechanisms governing these differences. Similarly, protists assemble flagella of different lengths according to the stage of their life cycle. Trypanosoma brucei assembles flagella of 3 to 30 µm during its development in the tsetse fly. This provides an opportunity to examine how cells naturally modulate organelle length. Flagella are constructed by addition of new blocks at their distal end via intraflagellar transport (IFT). Immunofluorescence assays, 3D electron microscopy and live-cell imaging revealed that IFT was present in all T. brucei life cycle stages. IFT proteins are concentrated at the base, and IFT trains are located along doublets 3-4 and 7-8 and travel bidirectionally in the flagellum. Quantitative analysis demonstrated that the total amount of flagellar IFT proteins correlates with the length of the flagellum. Surprisingly, the shortest flagellum exhibited a supplementary large amount of dynamic IFT material at its distal end. The contribution of IFT and other factors to the regulation of flagellum length is discussed.

mRNA localization mediates maturation of cytoplasmic cilia in Drosophila spermatogenesis

Cytoplasmic cilia, a specialized type of cilia in which the axoneme resides within the cytoplasm rather than within the ciliary compartment, are proposed to allow for the efficient assembly of very long cilia. Despite being found diversely in male gametes (e.g., Plasmodium falciparum microgametocytes and human and Drosophila melanogaster sperm), very little is known about cytoplasmic cilia assembly. Here, we show that a novel RNP granule containing the mRNAs for axonemal dynein motor proteins becomes highly polarized to the distal end of the cilia during cytoplasmic ciliogenesis in Drosophila sperm. This allows for the incorporation of these axonemal dyneins into the axoneme directly from the cytoplasm, possibly by localizing translation. We found that this RNP granule contains the proteins Reptin and Pontin, loss of which perturbs granule formation and prevents incorporation of the axonemal dyneins, leading to sterility. We propose that cytoplasmic cilia assembly requires the precise localization of mRNAs encoding key axonemal constituents, allowing these proteins to incorporate efficiently into the axoneme.

Transcriptome analysis provides a blueprint of coral egg and sperm functions

BACKGROUND

Reproductive biology and the evolutionary constraints acting on dispersal stages are poorly understood in many stony coral species. A key piece of missing information is egg and sperm gene expression. This is critical for broadcast spawning corals, such as our model, the Hawaiian species Montipora capitata, because eggs and sperm are exposed to environmental stressors during dispersal. Furthermore, parental effects such as transcriptome investment may provide a means for cross- or trans-generational plasticity and be apparent in egg and sperm transcriptome data.

METHODS

Here, we analyzed M. capitata egg and sperm transcriptomic data to address three questions: (1) Which pathways and functions are actively transcribed in these gametes? (2) How does sperm and egg gene expression differ from adult tissues? (3) Does gene expression differ between these gametes?

RESULTS

We show that egg and sperm display surprisingly similar levels of gene expression and overlapping functional enrichment patterns. These results may reflect similar environmental constraints faced by these motile gametes. We find significant differences in differential expression of egg vs. adult and sperm vs. adult RNA-seq data, in contrast to very few examples of differential expression when comparing egg vs. sperm transcriptomes. Lastly, using gene ontology and KEGG orthology data we show that both egg and sperm have markedly repressed transcription and translation machinery compared to the adult, suggesting a dependence on parental transcripts. We speculate that cell motility and calcium ion binding genes may be involved in gamete to gamete recognition in the water column and thus, fertilization.

Nek8445, a protein kinase required for microtubule regulation and cytokinesis in Giardia lamblia

Giardia has 198 Nek kinases whereas humans have only 11. Giardia has a complex microtubule cytoskeleton that includes eight flagella and several unique microtubule arrays that are utilized for parasite attachment and facilitation of rapid mitosis and cytokinesis. The need to regulate these structures may explain the parallel expansion of the number of Nek family kinases. Here we use live and fixed cell imaging to uncover the role of Nek8445 in regulating Giardia cell division. We demonstrate that Nek8445 localization is cell cycle regulated and this kinase has a role in regulating overall microtubule organization. Nek8445 depletion results in short flagella, aberrant ventral disk organization, loss of the funis, defective axoneme exit, and altered cell shape. The axoneme exit defect is specific to the caudal axonemes, which exit from the posterior of the cell, and this defect correlates with rounding of the cell posterior and loss of the funis. Our findings implicate a role for the funis in establishing Giardia’s cell shape and guiding axoneme docking. On a broader scale our results support the emerging view that Nek family kinases have a general role in regulating microtubule organization.

Centrioles and Ciliary Structures during Male Gametogenesis in Hexapoda: Discovery of New Models

Centrioles are-widely conserved barrel-shaped organelles present in most organisms. They are indirectly involved in the organization of the cytoplasmic microtubules both in interphase and during the cell division by recruiting the molecules needed for microtubule nucleation. Moreover, the centrioles are required to assemble cilia and flagella by the direct elongation of their microtubule wall. Due to the importance of the cytoplasmic microtubules in several aspects of the cell life, any defect in centriole structure can lead to cell abnormalities that in humans may result in significant diseases. Many aspects of the centriole dynamics and function have been clarified in the last years, but little attention has been paid to the exceptions in centriole structure that occasionally appeared within the animal kingdom. Here, we focused our attention on non-canonical aspects of centriole architecture within the Hexapoda. The Hexapoda is one of the major animal groups and represents a good laboratory in which to examine the evolution and the organization of the centrioles. Although these findings represent obvious exceptions to the established rules of centriole organization, they may contribute to advance our understanding of the formation and the function of these organelles.

Microtubule organelles in Giardia

Giardia lamblia is a widespread parasitic protist with a complex MT cytoskeleton that is critical for motility, attachment, mitosis and cell division, and transitions between its two life cycle stages-the infectious cyst and flagellated trophozoite. Giardia trophozoites have both highly dynamic and highly stable MT organelles, including the ventral disc, eight flagella, the median body and the funis. The ventral disc, an elaborate MT organelle, is essential for the parasite's attachment to the intestinal villi to avoid peristalsis. Giardia's four flagellar pairs enable swimming motility and may also promote attachment. They are maintained at different equilibrium lengths and are distinguished by their long cytoplasmic regions and novel extra-axonemal structures. The functions of the median body and funis, MT organelles unique to Giardia, remain less understood. In addition to conserved MT-associated proteins, the genome is enriched in ankyrins, NEKs, and novel hypothetical proteins that also associate with the MT cytoskeleton. High-resolution ultrastructural imaging and a current inventory of more than 300 proteins associated with Giardia's MT cytoskeleton lay the groundwork for future mechanistic analyses of parasite attachment to the host, motility, cell division, and encystation/excystation. Giardia's unique MT organelles exemplify the capacity of MT polymers to generate intricate structures that are diverse in both form and function. Thus, beyond its relevance to pathogenesis, the study of Giardia's MT cytoskeleton informs basic cytoskeletal biology and cellular evolution. With the availability of new molecular genetic tools to disrupt gene function, we anticipate a new era of cytoskeletal discovery in Giardia.

Length-dependent disassembly maintains four different flagellar lengths in Giardia

With eight flagella of four different lengths, the parasitic protist Giardia is an ideal model to evaluate flagellar assembly and length regulation. To determine how four different flagellar lengths are maintained, we used live-cell quantitative imaging and mathematical modeling of conserved components of intraflagellar transport (IFT)-mediated assembly and kinesin-13-mediated disassembly in different flagellar pairs. Each axoneme has a long cytoplasmic region extending from the basal body, and transitions to a canonical membrane-bound flagellum at the 'flagellar pore'. We determined that each flagellar pore is the site of IFT accumulation and injection, defining a diffusion barrier functionally analogous to the transition zone. IFT-mediated assembly is length-independent, as train size, speed, and injection frequencies are similar for all flagella. We demonstrate that kinesin-13 localization to the flagellar tips is inversely correlated to flagellar length. Therefore, we propose a model where a length-dependent disassembly mechanism controls multiple flagellar lengths within the same cell.

The Role of Sperm Centrioles in Human Reproduction - The Known and the Unknown

Each human spermatozoon contains two remodeled centrioles that it contributes to the zygote. There, the centrioles reconstitute a centrosome that assembles the sperm aster and participate in pronuclei migration and cleavage. Thus, centriole abnormalities may be a cause of male factor infertility and failure to carry pregnancy to term. However, the precise mechanisms by which sperm centrioles contribute to embryonic development in humans are still unclear, making the search for a link between centriole abnormalities and impaired male fecundity particularly difficult. Most previous investigations into the role of mammalian centrioles during fertilization have been completed in murine models; however, because mouse sperm and zygotes appear to lack centrioles, these studies provide information that is limited in its applicability to humans. Here, we review studies that examine the role of the sperm centrioles in the early embryo, with particular emphasis on humans. Available literature includes case studies and case-control studies, with a few retrospective studies and no prospective studies reported. This literature has provided some insight into the morphological characteristics of sperm centrioles in the zygote and has allowed identification of some centriole abnormalities in rare cases. Many of these studies suggest centriole involvement in early embryogenesis based on phenotypes of the embryo with only indirect evidence for centriole abnormality. Overall, these studies suggest that centriole abnormalities are present in some cases of sperm with asthenoteratozoospermia and unexplained infertility. Yet, most previously published studies have been restricted by the laborious techniques (like electron microscopy) and the limited availability of centriolar markers, resulting in small-scale studies and the lack of solid causational evidence. With recent progress in sperm centriole biology, such as the identification of the unique composition of sperm centrioles and the discovery of the atypical centriole, it is now possible to begin to fill the gaps in sperm centriole epidemiology and to identify the etiology of sperm centriole dysfunction in humans.

The functional anatomy of the human spermatozoon: relating ultrastructure and function

The Internet, magazine articles, and even biomedical journal articles, are full of cartoons of spermatozoa that bear minimal resemblance to real spermatozoa, especially human spermatozoa, and this had led to many misconceptions about what spermatozoa look like and how they are constituted. This review summarizes the historical and current state of knowledge of mammalian sperm ultrastructure, with particular emphasis on and relevance to human spermatozoa, combining information obtained from a variety of electron microscopic (EM) techniques. Available information on the composition and configuration of the various ultrastructural components of the spermatozoon has been related to their mechanistic purpose and roles in the primary aspects of sperm function and fertilization: motility, hyperactivation, capacitation, the acrosome reaction and sperm-oocyte fusion.

The Microtubule-Depolymerizing Kinesin-13 Klp10A Is Enriched in the Transition Zone of the Ciliary Structures of Drosophila melanogaster

The precursor of the flagellar axoneme is already present in the primary spermatocytes of Drosophila melanogaster. During spermatogenesis each primary spermatocyte shows a centriole pair that moves to the cell membrane and organizes an axoneme-based structure, the cilium-like region (CLR). The CLRs persist through the meiotic divisions and are inherited by young spermatids. During spermatid differentiation the ciliary caps elongate giving rise to the sperm axoneme. Mutations in Klp10A, a kinesin-13 of Drosophila, results in defects of centriole/CLR organization in spermatocytes and of ciliary cap assembly in elongating spermatids. Reduced Klp10A expression also results in strong structural defects of sensory type I neurons. We show, here, that this protein displays a peculiar localization during male gametogenesis. The Klp10A signal is first detected at the distal ends of the centrioles when they dock to the plasma membrane of young primary spermatocytes. At the onset of the first meiotic prometaphase, when the CLRs reach their full size, Klp10A is enriched in a distinct narrow area at the distal end of the centrioles and persists in elongating spermatids at the base of the ciliary cap. We conclude that Klp10A could be a core component of the ciliary transition zone in Drosophila.

Ciliary and cytoskeletal functions of an ancient monooxygenase essential for bioactive amidated peptide synthesis

Many secreted peptides used for cell-cell communication require conversion of a C-terminal glycine to an amide for bioactivity. This reaction is catalyzed only by the integral membrane protein peptidylglycine α-amidating monooxygenase (PAM). PAM has been highly conserved and is found throughout the metazoa; PAM-like sequences are also present in choanoflagellates, filastereans, unicellular and colonial chlorophyte green algae, dinoflagellates and haptophytes. Recent studies have revealed that in addition to playing a key role in peptidergic signaling, PAM also regulates ciliogenesis in vertebrates, planaria and chlorophyte algae, and is required for the stability of actin-based microvilli. Here we briefly introduce the basic principles involved in ciliogenesis, the sequential reactions catalyzed by PAM and the trafficking of PAM through the secretory and endocytic pathways. We then discuss the multi-faceted roles this enzyme plays in the formation and maintenance of cytoskeleton-based cellular protrusions and propose models for how PAM protein and amidating activity might contribute to ciliogenesis. Finally, we consider why some ciliated organisms lack PAM, and discuss the potential ramifications of ciliary localized PAM for the endocrine features commonly observed in patients with ciliopathies.

The Centriolar Adjunct⁻Appearance and Disassembly in Spermiogenesis and the Potential Impact on Fertility

During spermiogenesis, the proximal centriole forms a special microtubular structure: the centriolar adjunct. This structure appears at the spermatid stage, which is characterized by a condensed chromatin nucleus. We showed that the centriolar adjunct disappears completely in mature porcine spermatozoa. In humans, the centriolar adjunct remnants are present in a fraction of mature spermatids. For the first time, the structure of the centriolar adjunct in the cell, and its consequent impact on fertility, were examined. Ultrastructural analysis using transmission electron microscopy was performed on near 2000 spermatozoa per person, in two patients with idiopathic male sterility (IMS) and five healthy fertile donors. We measured the average length of the “proximal centriole + centriolar adjunct” complex in sections, where it had parallel orientation in the section plane, and found that it was significantly longer in the spermatozoa of IMS patients than in the spermatozoa of healthy donors. This difference was independent of chromatin condensation deficiency, which was also observed in the spermatozoa of IMS patients. We suggest that zygote arrest may be related to an incompletely disassembled centriolar adjunct in a mature spermatozoon. Therefore, centriolar adjunct length can be potentially used as a complementary criterion for the immaturity of spermatozoa in the diagnostics of IMS patients.

Cell type-specific structural plasticity of the ciliary transition zone in C. elegans

BACKGROUND INFORMATION

The current consensus on cilia development posits that the ciliary transition zone (TZ) is formed via extension of nine centrosomal microtubules. In this model, TZ structure remains unchanged in microtubule number throughout the cilium life cycle. This model does not however explain structural variations of TZ structure seen in nature and could also lend itself to the misinterpretation that deviations from nine-doublet microtubule ultrastructure represent an abnormal phenotype. Thus, a better understanding of events that occur at the TZ in vivo during metazoan development is required.

RESULTS

To address this issue, we characterized ultrastructure of two types of sensory cilia in developing Caenorhabditis elegans. We discovered that, in cephalic male (CEM) and inner labial quadrant (IL2Q) sensory neurons, ciliary TZs are structurally plastic and remodel from one structure to another during animal development. The number of microtubule doublets forming the TZ can be increased or decreased over time, depending on cilia type. Both cases result in structural TZ intermediates different from TZ in cilia of adult animals. In CEM cilia, axonemal extension and maturation occurs concurrently with TZ structural maturation.

CONCLUSIONS AND SIGNIFICANCE

Our work extends the current model to include the structural plasticity of metazoan transition zone, which can be structurally delayed, maintained or remodelled in cell type-specific manner.

It takes two (centrioles) to tango

Cells that divide during embryo development require precisely two centrioles during interphase and four centrioles during mitosis. This precise number is maintained by allowing each centriole to nucleate only one centriole per cell cycle (i.e. centriole duplication). Yet, how the first cell of the embryo, the zygote, obtains two centrioles has remained a mystery in most mammals and insects. The mystery arose because the female gamete (oocyte) is thought to have no functional centrioles and the male gamete (spermatozoon) is thought to have only one functional centriole, resulting in a zygote with a single centriole. However, recent studies in fruit flies, beetles and mammals, including humans, suggest an alternative explanation: spermatozoa have a typical centriole and an atypical centriole. The sperm typical centriole has a normal structure but distinct protein composition, whereas the sperm atypical centriole is distinct in both. During fertilization, the atypical centriole is released into the zygote, nucleates a new centriole and participates in spindle pole formation. Thus, the spermatozoa's atypical centriole acts as a second centriole in the zygote. Here, we review centriole biology in general and especially in reproduction, we describe the discovery of the spermatozoon atypical centriole, and we provide an updated model for centriole inherence during sexual reproduction. While we focus on humans and other non-rodent mammals, we also provide a broader evolutionary perspective.

Homozygous missense mutation L673P in adenylate kinase 7 (AK7) leads to primary male infertility and multiple morphological anomalies of the flagella but not to primary ciliary dyskinesia

Motile cilia and sperm flagella share an extremely conserved microtubule-based cytoskeleton, called the axoneme, which sustains beating and motility of both organelles. Ultra-structural and/or functional defects of this axoneme are well-known to cause primary ciliary dyskinesia (PCD), a disorder characterized by recurrent respiratory tract infections, chronic otitis media, situs inversus, male infertility and in most severe cases, hydrocephalus. Only recently, mutations in genes encoding axonemal proteins with preferential expression in the testis were identified in isolated male infertility; in those cases, individuals displayed severe asthenozoospermia due to Multiple Morphological Abnormalities of the sperm Flagella (MMAF) but not PCD features. In this study, we performed genetic investigation of two siblings presenting MMAF without any respiratory PCD features, and we report the identification of the c.2018T > G (p.Leu673Pro) transversion in AK7, encoding an adenylate kinase, expressed in ciliated tissues and testis. By performing transcript and protein analyses of biological samples from individual carrying the transversion, we demonstrate that this mutation leads to the loss of AK7 protein in sperm cells but not in respiratory ciliated cells, although both cell types carry the mutated transcript and no tissue-specific isoforms were detected. This work therefore, supports the notion that proteins shared by both cilia and sperm flagella may have specific properties and/or function in each organelle, in line with the differences in their mode of assembly and organization. Overall, this work identifies a novel genetic cause of asthenozoospermia due to MMAF and suggests that in humans, more deleterious mutations of AK7 might induce PCD.

The "transition zone" of the cilium-like regions in the Drosophila spermatocytes and the role of the C-tubule in axoneme assembly

The fruit-fly Drosophila melanogaster harbours different types of ciliary structures: ciliary projections associated with neurons of type I and cilium-like regions (CLRs) found during male gametogenesis. The latter deserve particular attention since they are morphologically similar to vertebrate primary cilia and transform into the sperm axonemes during spermiogenesis. Although, all the centrioles are able to organize the CLRs, we found that the mother centriole docks first to the plasma membrane suggesting a new intrinsic functional asymmetry between the parent centrioles. We also show that the CLRs lack the Y-links that connect the axoneme doublets with the plasma membrane in conventional primary cilia. Moreover, the C-tubules, that are lacking in the axoneme of the primary cilia, persisted along the CLRs albeit modified into longitudinal blades. Remarkably, mutant flies in which the CLRs are devoid of the C-tubules or their number is reduced lack sperm axonemes or have incomplete axonemes. Therefore, the C-tubules are dispensable for the assembly of the CLRs but are essential for sperm axoneme elongation and maintenance in Drosophila.

To beat, or not to beat, that is question! The spectrum of ciliopathies

Cilia are widely distributed throughout the human body, and have numerous roles in physiology, development, and disease. Ciliary ultrastructure is complex, consisting of nine parallel microtubules doublets, with or without motor dynein arms and a central pair of microtubules. Classification of cilia has evolved over time, and currently, four main classes are described: motile and non-motile cilia with a "9 + 2" structure, and motile and non-motile cilia with a "9 + 0" structure, which depend on the presence or absence of dynein arms and a central pair. Ciliopathies are inherited multisystem disorders of cilia, and may present with a varied spectrum of genotypes and phenotypes. Motor and sensory ciliopathies were historically considered as distinct dysfunctions of motile and non-motile cilia, but recent data indicate that the classical features of motor and sensory cilia may overlap.

Rapid Evolution of Sperm Produces Diverse Centriole Structures that Reveal the Most Rudimentary Structure Needed for Function

Centrioles are ancient subcellular protein-based organelles that maintain a conserved number and structure across many groups of eukaryotes. Centriole number (two per cells) is tightly regulated; each pre-existing centriole nucleates only one centriole as the cell prepares for division. The structure of centrioles is barrel-shaped, with a nine-fold symmetry of microtubules. This organization of microtubules is essential for the ancestral function of centriole–cilium nucleation. In animal cells, centrioles have gained an additional role: recruiting pericentriolar material (PCM) to form a centrosome. Therefore, it is striking that in animal spermatozoa, the centrioles have a remarkable diversity of structures, where some are so anomalous that they are referred to as atypical centrioles and are barely recognizable. The atypical centriole maintains the ability to form a centrosome and nucleate a new centriole, and therefore reveals the most rudimentary structure that is needed for centriole function. However, the atypical centriole appears to be incapable of forming a cilium. Here, we propose that the diversity in sperm centriole structure is due to rapid evolution in the shape of the spermatozoa head and neck. The enhanced diversity may be driven by a combination of direct selection for novel centriole functions and pleiotropy, which eliminates centriole properties that are dispensable in the spermatozoa function.

Reconstitution reveals motor activation for intraflagellar transport

The human body represents a striking example of ciliary diversification. Extending from the surface of most cells, cilia accomplish an astonishingly diverse set of tasks. Predictably, mutations in ciliary genes cause a wide range of human diseases such as male infertility or blindness. In C. elegans sensory cilia, this functional diversity appears to be traceable to the differential regulation of the kinesin-2-powered intraflagellar transport (IFT) machinery. Here, we reconstituted the first functional, multi-component IFT complex that is deployed in the sensory cilia of C. elegans. Our bottom-up approach revealed the molecular basis of specific motor recruitment to the IFT trains. We identified the key component that incorporates homodimeric kinesin-2 into its physiologically relevant context which in turn allosterically activates the motor for efficient transport. These results lay the groundwork for a molecular delineation of IFT regulation that eluded understanding since its ground-breaking discovery more than two decades ago.

Insights into Ciliary Genes and Evolution from Multi-Level Phylogenetic Profiling

Cilia (flagella) are important eukaryotic organelles, present in the Last Eukaryotic Common Ancestor, and are involved in cell motility and integration of extracellular signals. Ciliary dysfunction causes a class of genetic diseases, known as ciliopathies, however current knowledge of the underlying mechanisms is still limited and a better characterization of genes is needed. As cilia have been lost independently several times during evolution and they are subject to important functional variation between species, ciliary genes can be investigated through comparative genomics. We performed phylogenetic profiling by predicting orthologs of human protein-coding genes in 100 eukaryotic species. The analysis integrated three independent methods to predict a consensus set of 274 ciliary genes, including 87 new promising candidates. A fine-grained analysis of the phylogenetic profiles allowed a partitioning of ciliary genes into modules with distinct evolutionary histories and ciliary functions (assembly, movement, centriole, etc.) and thus propagation of potential annotations to previously undocumented genes. The cilia/basal body localization was experimentally confirmed for five of these previously unannotated proteins (LRRC23, LRRC34, TEX9, WDR27, and BIVM), validating the relevance of our approach. Furthermore, our multi-level analysis sheds light on the core gene sets retained in gamete-only flagellates or Ecdysozoa for instance. By combining gene-centric and species-oriented analyses, this work reveals new ciliary and ciliopathy gene candidates and provides clues about the evolution of ciliary processes in the eukaryotic domain. Additionally, the positive and negative reference gene sets and the phylogenetic profile of human genes constructed during this study can be exploited in future work.

Transition Zone Migration: A Mechanism for Cytoplasmic Ciliogenesis and Postaxonemal Centriole Elongation

The cilium is an elongated and continuous structure that spans two major subcellular domains. The cytoplasmic domain contains a short centriole, which serves to nucleate the main projection of the cilium. This projection, known as the axoneme, remains separated from the cytoplasm by a specialized gatekeeping complex within a ciliary subdomain called the transition zone. In this way, the axoneme is compartmentalized. Intriguingly, however, this general principle of cilium biology is altered in the sperm cells of many animals, which instead contain a cytoplasmic axoneme domain. Here, we discuss the hypothesis that the formation of specialized sperm giant centrioles and cytoplasmic cilia is mediated by the migration of the transition zone from its typical location as part of a structure known as the annulus and examine the intrinsic properties of the transition zone that may facilitate its migratory behavior.