How did the cilium evolve?

Evolutionary trajectory for nuclear functions of ciliary transport complex proteins

SUMMARYCilia and the nucleus were two defining features of the last eukaryotic common ancestor. In early eukaryotic evolution, these structures evolved through the diversification of a common membrane-coating ancestor, the protocoatomer. While in cilia, the descendants of this protein complex evolved into parts of the intraflagellar transport complexes and BBSome, the nucleus gained its selectivity by recruiting protocoatomer-like proteins to the nuclear envelope to form the selective nuclear pore complexes. Recent studies show a growing number of proteins shared between the proteomes of the respective organelles, and it is currently unknown how ciliary transport proteins could acquire nuclear functions and vice versa. The nuclear functions of ciliary proteins are still observable today and remain relevant for the understanding of the disease mechanisms behind ciliopathies. In this work, we review the evolutionary history of cilia and nucleus and their respective defining proteins and integrate current knowledge into theories for early eukaryotic evolution. We postulate a scenario where both compartments co-evolved and that fits current models of eukaryotic evolution, explaining how ciliary proteins and nucleoporins acquired their dual functions.

Association of novel DNAH11 variants with asthenoteratozoospermia lead to male infertility

BACKGROUND

Bi-allelic variants in DNAH11 have been identified as causative factors in Primary Ciliary Dyskinesia, leading to abnormal respiratory cilia. Nonetheless, the specific impact of these variants on human sperm flagellar and their involvement in male infertility remain largely unknown.

METHODS

A collaborative effort involving two Chinese reproductive centers conducted a study with 975 unrelated infertile men. Whole-exome sequencing was employed for variant screening, and Sanger sequencing confirmed the identified variants. Morphological and ultrastructural analyses of sperm were conducted using Scanning Electron Microscopy and Transmission Electron Microscopy. Western Blot Analysis and Immunofluorescence Analysis were utilized to assess protein levels and localization. ICSI was performed to evaluate its efficacy in achieving favorable pregnancy outcomes for individuals with DNAH11 variants.

RESULTS

In this study, we identified seven novel variants in the DNAH11 gene in four asthenoteratozoospermia subjects. These variants led the absence of DNAH11 proteins and ultrastructure defects in sperm flagella, particularly affecting the outer dynein arms (ODAs) and adjacent structures. The levels of ODA protein DNAI2 and axoneme related proteins were down regulated, instead of inner dynein arms (IDA) proteins DNAH1 and DNAH6. Two out of four individuals with DNAH11 variants achieved clinical pregnancies through ICSI. The findings confirm the association between male infertility and bi-allelic deleterious variants in DNAH11, resulting in the aberrant assembly of sperm flagella and contributing to asthenoteratozoospermia. Importantly, ICSI emerges as an effective intervention for overcoming reproductive challenges caused by DNAH11 gene variants.

The methylome of motile cilia

Cilia are highly complex motile, sensory, and secretory organelles that contain perhaps 1000 or more distinct protein components, many of which are subject to various posttranslational modifications such as phosphorylation, N-terminal acetylation, and proteolytic processing. Another common modification is the addition of one or more methyl groups to the side chains of arginine and lysine residues. These tunable additions delocalize the side-chain charge, decrease hydrogen bond capacity, and increase both bulk and hydrophobicity. Methylation is usually mediated by S-adenosylmethionine (SAM)-dependent methyltransferases and reversed by demethylases. Previous studies have identified several ciliary proteins that are subject to methylation including axonemal dynein heavy chains that are modified by a cytosolic methyltransferase. Here, we have performed an extensive proteomic analysis of multiple independently derived cilia samples to assess the potential for SAM metabolism and the extent of methylation in these organelles. We find that cilia contain all the enzymes needed for generation of the SAM methyl donor and recycling of the S-adenosylhomocysteine and tetrahydrofolate byproducts. In addition, we find that at least 155 distinct ciliary proteins are methylated, in some cases at multiple sites. These data provide a comprehensive resource for studying the consequences of methyl marks on ciliary biology.

What can insects teach us about hearing loss?

Over the last three decades, insects have been utilized to provide a deep and fundamental understanding of many human diseases and disorders. Here, we present arguments for insects as models to understand general principles underlying hearing loss. Despite ∼600 million years since the last common ancestor of vertebrates and invertebrates, we share an overwhelming degree of genetic homology particularly with respect to auditory organ development and maintenance. Despite the anatomical differences between human and insect auditory organs, both share physiological principles of operation. We explain why these observations are expected and highlight areas in hearing loss research in which insects can provide insight. We start by briefly introducing the evolutionary journey of auditory organs, the reasons for using insect auditory organs for hearing loss research, and the tools and approaches available in insects. Then, the first half of the review focuses on auditory development and auditory disorders with a genetic cause. The second half analyses the physiological and genetic consequences of ageing and short- and long-term changes as a result of noise exposure. We finish with complex age and noise interactions in auditory systems. In this review, we present some of the evidence and arguments to support the use of insects to study mechanisms and potential treatments for hearing loss in humans. Obviously, insects cannot fully substitute for all aspects of human auditory function and loss of function, although there are many important questions that can be addressed in an animal model for which there are important ethical, practical and experimental advantages.

Animal evolution: No cilia, no problem!

Cilia are fundamental organelles of eukaryotes with diverse functions spanning from cell motility to sensory perception. A new study presenting genomes from parasitic horsehair worms reveals highly reduced genomes that have lost the molecular machinery needed to make cilia.

Novel HYDIN variants associated with male infertility in two Chinese families

INTRODUCTION

Infertility is a major disease affecting human life and health, among which male factors account for about half. Asthenoteratozoospermia accounts for the majority of male infertility. High-throughput sequencing techniques have identified numerous variants in genes responsible for asthenoteratozoospermia; however, its etiology still needs to be studied.

METHOD

In this study, we performed whole-exome sequencing on samples from 375 patients with asthenoteratozoospermia and identified two HYDIN compound heterozygous variants, a primary ciliary dyskinesia (PCD)-associated gene, in two unrelated subjects. H&E staining, SEM were employed to analyze the varies on sperm of patients, further, TEM was employed to determine the ultrastructure defects. And westernblot and immunostaining were chose to evaluate the variation of structural protein. ICSI was applied to assist the mutational patient to achieve offspring.

RESULT

We identified two HYDIN compound heterozygous variants. Patient AY078 had novel compound heterozygous splice variants (c.5969-2A>G, c.6316+1G>A), altering the consensus splice acceptor site of HYDIN. He was diagnosed with male infertility and PCD, presenting with decreased sperm progressive motility and morphological abnormalities, and bronchial dilatation in the inferior lobe. Compared to the fertile control, HYDIN levels, acrosome and centrosome markers (ACTL7A, ACROSIN, PLCζ1, and Centrin1), and flagella components (TOMM20, SEPT4, SPEF2, SPAG6, and RSPHs) were significantly reduced in HYDIN-deficient patients. Using intracytoplasmic sperm injection (ICSI), the patient successfully achieved clinical pregnancy. AY079 had deleterious compound heterozygous missense variants, c.9507C>G (p. Asn3169Lys) and c.14081G>A (p. Arg4694His), presenting with infertility; however, semen samples and PCD examination were unavailable.

DISCUSSION

Our findings provide the first evidence that the loss of HYDIN function causes asthenoteratozoospermia presenting with various defects in the flagella structure and the disassembly of the acrosome and neck. Additionally, ICSI could rescue this failure of insemination caused by immobile and malformed sperm induced by HYDIN deficiency.

IFT20: An Eclectic Regulator of Cellular Processes beyond Intraflagellar Transport

Initially discovered as the smallest component of the intraflagellar transport (IFT) system, the IFT20 protein has been found to be implicated in several unconventional mechanisms beyond its essential role in the assembly and maintenance of the primary cilium. IFT20 is now considered a key player not only in ciliogenesis but also in vesicular trafficking of membrane receptors and signaling proteins. Moreover, its ability to associate with a wide array of interacting partners in a cell-type specific manner has expanded the function of IFT20 to the regulation of intracellular degradative and secretory pathways. In this review, we will present an overview of the multifaceted role of IFT20 in both ciliated and non-ciliated cells.

Mechanisms of Regulation in Intraflagellar Transport

Cilia are eukaryotic organelles essential for movement, signaling or sensing. Primary cilia act as antennae to sense a cell's environment and are involved in a wide range of signaling pathways essential for development. Motile cilia drive cell locomotion or liquid flow around the cell. Proper functioning of both types of cilia requires a highly orchestrated bi-directional transport system, intraflagellar transport (IFT), which is driven by motor proteins, kinesin-2 and IFT dynein. In this review, we explore how IFT is regulated in cilia, focusing from three different perspectives on the issue. First, we reflect on how the motor track, the microtubule-based axoneme, affects IFT. Second, we focus on the motor proteins, considering the role motor action, cooperation and motor-train interaction plays in the regulation of IFT. Third, we discuss the role of kinases in the regulation of the motor proteins. Our goal is to provide mechanistic insights in IFT regulation in cilia and to suggest directions of future research.

DNAH5 gene and its correlation with linc02220 expression and sperm characteristics

BACKGROUND

Numerous pieces of evidence show that many environmental and genetic factors can cause male infertility. Much research in recent years has investigated the function of long non-coding RNAs (lncRNAs) in fertility. The main objective of the current study was to investigate the expression of Dynein Axonemal Heavy Chain 5 (DNAH5) as a gene that plays an essential role in sperm motility in individuals with asthenozoospermia and terato-asthenozoospermia. Alterations in linc02220 expression (located close to the DNAH5 gene), its action potential in DNAH5 regulating, and the correlation between their expression and normal sperm morphology and motility were also examined.

METHOD AND MATERIAL

This study examined the semen of 31 asthenozoospermia individuals (AZ), 33 terato-asthenozoospermia (TAZ) individuals, and 33 normospermia (NZ) individuals with normal sperm as a control group. The expression levels of DNAH5 and linc02220 in the sperm samples were analyzed by real-time PCR.

RESULTS

Gene expression analysis revealed a significant association between DNAH5 expression and sperm motility and morphology (p < 0.0001). The DNAH5 expression levels in the TAZ and AZ groups were also significantly reduced; however, linc02220 was significantly upregulated in both TAZ and AZ groups compared to the NZ group (p < 0.0001). DNAH5 expression in the TAZ and AZ groups was negatively correlated with linc02220 expression, thus, DNAH5 downregulation was associated with linc02220 overexpression (p < 0.05).

CONCLUSIONS

The gene linc02220 could be a potential regulatory target for DNAH5, and both could affect sperm's normal motility and morphology.

Self-sustained three-dimensional beating of a model eukaryotic flagellum

Flagella and cilia are common features of a wide variety of biological cells and play important roles in locomotion and feeding at the microscale. The beating of flagella is controlled by molecular motors that exert forces along the length of the flagellum and are regulated by a feedback mechanism coupled to the flagella deformation. We develop a three-dimensional (3D) flagellum beating model based on sliding-controlled motor feedback, accounting for both bending and twist, as well as differential bending resistances along and orthogonal to the major bending plane of the flagellum. We show that beating is generated and sustained spontaneously for a sufficiently high motor activity through an instability mechanism. Isotropic bending rigidities in the flagellum lead to 3D helical beating patterns. By contrast, anisotropic flagella present a rich variety of wave-like beating dynamics, including both 3D beating patterns as well as planar beating patterns. We show that the ability to generate nearly planar beating despite the 3D beating machinery requires only a modest degree of bending anisotropy, and is a feature observed in many eukaryotic flagella such as mammalian spermatozoa.

Quantifying cilia beat frequency using high-speed video microscopy: Assessing frame rate requirements when imaging different ciliated tissues

Motile cilia are found in numerous locations throughout our body and play a critical role in various physiological processes. The most commonly used method to assess cilia motility is to quantify cilia beat frequency (CBF) via video microscopy. However, a large heterogeneity exists within published literature regarding the framerate used to image cilia motility for calculating CBF. The aim of this study was to determine the optimal frame rate required to image cilia motility for CBF assessment, and if the Nyquist theorem may be used to set this rate. One‐second movies of cilia were collected at >600 fps from mouse airways and ependyma at room‐temperature or 37°C. Movies were then down‐sampled to 30–300 fps. CBF was quantified for identical cilia at different framerates by either manual counting or automated MATLAB script. Airway CBF was significantly impaired in 30 fps movies, while ependymal CBF was significantly impaired in both 60 and 30 fps movies. Pairwise comparison showed that video framerate should be at least 150 fps to accurately measure CBF, with minimal improvement in CBF accuracy in movies >150 fps. The automated script was also found to be less accurate for measuring CBF in lower fps movies than manual counting, however, this difference disappeared in higher framerate movies (>150 fps). In conclusion, our data suggest the Nyquist theorem is unreliable for setting sampling rate for CBF measurement. Instead, sampling rate should be 3–4 times faster than CBF for accurate CBF assessment. Especially if CBF calculation is to be automated.

Developmental Changes in Ciliary Composition during Gametogenesis in Chlamydomonas

Chlamydomonas reinhardtii transitions from mitotically dividing vegetative cells to sexually competent gametes of two distinct mating types following nutrient deprivation. Gametes of opposite mating type interact via their cilia, initiating an intraciliary signaling cascade and ultimately fuse forming diploid zygotes. The process of gametogenesis is genetically encode, and a previous study revealed numerous significant changes in mRNA abundance during this life-cycle transition. Here we describe a proteomic analysis of cilia derived from vegetative and gametic cells of both mating types in an effort to assess the global changes that occur within the organelle during this process. We identify numerous membrane- and/or matrix-associated proteins in gametic cilia that were not detected in cilia from vegetative cells. This includes the pro-protein from which the GATI-amide gametic chemotactic modulator derives, as well as receptors, a dynamin-related protein, ammonium transporters, two proteins potentially involved in the intraciliary signaling cascade-driven increase in cAMP, and multiple proteins with a variety of interaction domains. These changes in ciliary composition likely directly affect the functional properties of this organelle as the cell transitions between life-cycle stages.

Turning the Curve Into Straight: Phenogenetics of the Spine Morphology and Coordinate Maintenance in the Zebrafish

The vertebral column, or spine, provides mechanical support and determines body axis posture and motion. The most common malformation altering spine morphology and function is adolescent idiopathic scoliosis (AIS), a three-dimensional spinal deformity that affects approximately 4% of the population worldwide. Due to AIS genetic heterogenicity and the lack of suitable animal models for its study, the etiology of this condition remains unclear, thus limiting treatment options. We here review current advances in zebrafish phenogenetics concerning AIS-like models and highlight the recently discovered biological processes leading to spine malformations. First, we focus on gene functions and phenotypes controlling critical aspects of postembryonic aspects that prime in spine architecture development and straightening. Second, we summarize how primary cilia assembly and biomechanical stimulus transduction, cerebrospinal fluid components and flow driven by motile cilia have been implicated in the pathogenesis of AIS-like phenotypes. Third, we highlight the inflammatory responses associated with scoliosis. We finally discuss recent innovations and methodologies for morphometrically characterize and analyze the zebrafish spine. Ongoing phenotyping projects are expected to identify novel and unprecedented postembryonic gene functions controlling spine morphology and mutant models of AIS. Importantly, imaging and gene editing technologies are allowing deep phenotyping studies in the zebrafish, opening new experimental paradigms in the morphometric and three-dimensional assessment of spinal malformations. In the future, fully elucidating the phenogenetic underpinnings of AIS etiology in zebrafish and humans will undoubtedly lead to innovative pharmacological treatments against spinal deformities.

KRAB zinc finger protein ZNF676 controls the transcriptional influence of LTR12-related endogenous retrovirus sequences

BACKGROUND

Transposable element-embedded regulatory sequences (TEeRS) and their KRAB-containing zinc finger protein (KZFP) controllers are increasingly recognized as modulators of gene expression. We aim to characterize the contribution of this system to gene regulation in early human development and germ cells.

RESULTS

Here, after studying genes driven by the long terminal repeat (LTR) of endogenous retroviruses, we identify the ape-restricted ZNF676 as the sequence-specific repressor of a subset of contemporary LTR12 integrants responsible for a large fraction of transpochimeric gene transcripts (TcGTs) generated during human early embryogenesis. We go on to reveal that the binding of this KZFP correlates with the epigenetic marking of these TEeRS in the germline, and is crucial to the control of genes involved in ciliogenesis/flagellogenesis, a biological process that dates back to the last common ancestor of eukaryotes.

CONCLUSION

These results illustrate how KZFPs and their TE targets contribute to the evolutionary turnover of transcription networks and participate in the transgenerational inheritance of epigenetic traits.

Electron cryo-tomography structure of axonemal doublet microtubule from Tetrahymena thermophila

Doublet microtubules (DMTs) provide a scaffold for axoneme assembly in motile cilia. Aside from α/β tubulins, the DMT comprises a large number of non-tubulin proteins in the luminal wall of DMTs, collectively named the microtubule inner proteins (MIPs). We used cryoET to study axoneme DMT isolated from Tetrahymena We present the structures of DMT at nanometer and sub-nanometer resolution. The structures confirm that MIP RIB72A/B binds to the luminal wall of DMT by multiple DM10 domains. We found FAP115, an MIP-containing multiple EF-hand domains, located at the interface of four-tubulin dimers in the lumen of A-tubule. It contacts both lateral and longitudinal tubulin interfaces and playing a critical role in DMT stability. We observed substantial structure heterogeneity in DMT in an FAP115 knockout strain, showing extensive structural defects beyond the FAP115-binding site. The defects propagate along the axoneme. Finally, by comparing DMT structures from Tetrahymena and Chlamydomonas, we have identified a number of conserved MIPs as well as MIPs that are unique to each organism. This conservation and diversity of the DMT structures might be linked to their specific functions. Our work provides structural insights essential for understanding the roles of MIPs during motile cilium assembly and function, as well as their relationships to human ciliopathies.

The flagellar germ-line hypothesis: How flagellate and ciliate gametes significantly shaped the evolution of organismal complexity

This essay presents a hypothesis which contends that the development of organismic complexity in the eukaryotes depended extensively on propagation via flagellated and ciliated gametes. Organisms utilizing flagellate and ciliate gametes to propagate their germ line have contributed most of the organismic complexity found in the higher animals. The genes of the flagellum and the flagellar assembly system (intraflagellar transport) have played a disproportionately important role in the construction of complex tissues and organs. The hypothesis also proposes that competition between large numbers of haploid flagellated male gametes rigorously conserved the functionality of a key set of flagellar genes for more than 700 million years. This in turn has insured that a large set (>600) of highly functional cytoskeletal and signal pathway genes is always present in the lineage of organisms with flagellated or ciliated gametes to act as a dependable resource, or "toolkit," for organ elaboration.

Bridging the Gap Between Mammal and Insect Ears – A Comparative and Evolutionary View of Sound-Reception

Insects must wonder why mammals have ears only in their head and why they evolved only one common principle of ear design—the cochlea. Ears independently evolved at least 19 times in different insect groups and therefore can be found in completely different body parts. The morphologies and functional characteristics of insect ears are as wildly diverse as the ecological niches they exploit. In both, insects and mammals, hearing organs are constrained by the same biophysical principles and their respective molecular processes for mechanotransduction are thought to share a common evolutionary origin. Due to this, comparative knowledge of hearing across animal phyla provides crucial insight into fundamental processes of auditory transduction, especially at the biomechanical and molecular level. This review will start by comparing hearing between insects and mammals in an evolutionary context. It will then discuss current findings about sound reception will help to bridge the gap between both research fields.

Composition and function of ciliary inner-dynein-arm subunits studied in Chlamydomonas reinhardtii

Motile cilia (also interchangeably called "flagella") are conserved organelles extending from the surface of many animal cells and play essential functions in eukaryotes, including cell motility and environmental sensing. Large motor complexes, the ciliary dyneins, are present on ciliary outer-doublet microtubules and drive movement of cilia. Ciliary dyneins are classified into two general types: the outer dynein arms (ODAs) and the inner dynein arms (IDAs). While ODAs are important for generation of force and regulation of ciliary beat frequency, IDAs are essential for control of the size and shape of the bend, features collectively referred to as waveform. Also, recent studies have revealed unexpected links between IDA components and human diseases. In spite of their importance, studies on IDAs have been difficult since they are very complex and composed for several types of IDA motors, each unique in composition and location in the axoneme. Thanks in part to genetic, biochemical, and structural analysis of Chlamydomonas reinhardtii, we are beginning to understand the organization and function of the ciliary IDAs. In this review, we summarize the composition of Chlamydomonas IDAs particularly focusing on each subunit, and discuss the assembly, conservation, and functional role(s) of these IDA subunits. Furthermore, we raise several additional questions/challenges regarding IDAs, and discuss future perspectives of IDA studies.

Origins of eukaryotic excitability

All living cells interact dynamically with a constantly changing world. Eukaryotes, in particular, evolved radically new ways to sense and react to their environment. These advances enabled new and more complex forms of cellular behaviour in eukaryotes, including directional movement, active feeding, mating, and responses to predation. But what are the key events and innovations during eukaryogenesis that made all of this possible? Here we describe the ancestral repertoire of eukaryotic excitability and discuss five major cellular innovations that enabled its evolutionary origin. The innovations include a vastly expanded repertoire of ion channels, the emergence of cilia and pseudopodia, endomembranes as intracellular capacitors, a flexible plasma membrane and the relocation of chemiosmotic ATP synthesis to mitochondria, which liberated the plasma membrane for more complex electrical signalling involved in sensing and reacting. We conjecture that together with an increase in cell size, these new forms of excitability greatly amplified the degrees of freedom associated with cellular responses, allowing eukaryotes to vastly outperform prokaryotes in terms of both speed and accuracy. This comprehensive new perspective on the evolution of excitability enriches our view of eukaryogenesis and emphasizes behaviour and sensing as major contributors to the success of eukaryotes. This article is part of the theme issue 'Basal cognition: conceptual tools and the view from the single cell'.

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.

Evolution of the centrosome, from the periphery to the center

Centrosomes are central organelles that organize microtubules (MTs) in animals, fungi and several other eukaryotic lineages. Despite an important diversity of structure, the centrosomes of different lineages share the same functions and part of their molecular components. To uncover how divergent centrosomes are related to each other, we need to trace the evolutionary history of MT organization. Careful assessment of cytoskeletal architecture in extant eukaryotic species can help us infer the ancestral state and identify the subsequent changes that took place during evolution. This led to the finding that the last common ancestor of all eukaryotes was very likely a biflagellate cell with a surprisingly complex cytoskeletal organization. Centrosomes are likely derived from the basal bodies of such flagellate, but when and how many times this happened remains unclear. Here, we discuss different hypotheses for how centrosomes evolved in a eukaryotic lineage called Amorphea, to which animals, fungi and amoebozoans belong.

SPEF2- and HYDIN-Mutant Cilia Lack the Central Pair-associated Protein SPEF2, Aiding Primary Ciliary Dyskinesia Diagnostics

Primary ciliary dyskinesia (PCD) is a genetically heterogeneous chronic destructive airway disease. PCD is traditionally diagnosed by nasal nitric oxide measurement, analysis of ciliary beating, transmission electron microscopy (TEM), and/or genetic testing. In most genetic PCD variants, laterality defects can occur. However, it is difficult to establish a diagnosis in individuals with PCD and central pair (CP) defects, and alternative strategies are required because of very subtle ciliary beating abnormalities, a normal ciliary ultrastructure, and normal situs composition. Mutations in HYDIN are known to cause CP defects, but the genetic analysis of HYDIN variants is confounded by the pseudogene HYDIN2, which is almost identical in terms of intron/exon structure. We have previously shown that several types of PCD can be diagnosed via immunofluorescence (IF) microscopy analyses. Here, using IF microscopy, we demonstrated that in individuals with PCD and CP defects, the CP-associated protein SPEF2 is absent in HYDIN-mutant cells, revealing its dependence on functional HYDIN. Next, we performed IF analyses of SPEF2 in respiratory cells from 189 individuals with suspected PCD and situs solitus. Forty-one of the 189 individuals had undetectable SPEF2 and were subjected to a genetic analysis, which revealed one novel loss-of-function mutation in SPEF2 and three reported and 13 novel HYDIN mutations in 15 individuals. The remaining 25 individuals are good candidates for new, as-yet uncharacterized PCD variants that affect the CP apparatus. SPEF2 mutations have been associated with male infertility but have not previously been identified to cause PCD. We identified a mutation of SPEF2 that is causative for PCD with a CP defect. We conclude that SPEF2 IF analyses can facilitate the detection of CP defects and evaluation of the pathogenicity of HYDIN variants, thus aiding the molecular diagnosis of CP defects.

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.

Propulsive nanomachines: the convergent evolution of archaella, flagella and cilia

Echoing the repeated convergent evolution of flight and vision in large eukaryotes, propulsive swimming motility has evolved independently in microbes in each of the three domains of life. Filamentous appendages – archaella in Archaea, flagella in Bacteria and cilia in Eukaryotes – wave, whip or rotate to propel microbes, overcoming diffusion and enabling colonization of new environments. The implementations of the three propulsive nanomachines are distinct, however: archaella and flagella rotate, while cilia beat or wave; flagella and cilia assemble at their tips, while archaella assemble at their base; archaella and cilia use ATP for motility, while flagella use ion-motive force. These underlying differences reflect the tinkering required to evolve a molecular machine, in which pre-existing machines in the appropriate contexts were iteratively co-opted for new functions and whose origins are reflected in their resultant mechanisms. Contemporary homologies suggest that archaella evolved from a non-rotary pilus, flagella from a non-rotary appendage or secretion system, and cilia from a passive sensory structure. Here, we review the structure, assembly, mechanism and homologies of the three distinct solutions as a foundation to better understand how propulsive nanomachines evolved three times independently and to highlight principles of molecular evolution.

The unity and diversity of the ciliary central apparatus

Nearly all motile cilia and flagella (terms here used interchangeably) have a '9+2' axoneme containing nine outer doublet microtubules and two central microtubules. The central pair of microtubules plus associated projections, termed the central apparatus (CA), is involved in the control of flagellar motility and is essential for the normal movement of '9+2' cilia. Research using the green alga Chlamydomonas reinhardtii, an important model system for studying cilia, has provided most of our knowledge of the protein composition of the CA, and recent work using this organism has expanded the number of known and candidate CA proteins nearly threefold. Here we take advantage of this enhanced proteome to examine the genomes of a wide range of eukaryotic organisms, representing all of the major phylogenetic groups, to identify predicted orthologues of the C. reinhardtii CA proteins and explore how widely the proteins are conserved and whether there are patterns to this conservation. We also discuss in detail two contrasting groups of CA proteins-the ASH-domain proteins, which are broadly conserved, and the PAS proteins, which are restricted primarily to the volvocalean algae. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

On the unity and diversity of cilia

Cilia are specialized cellular organelles that are united in structure and implicated in diverse key life processes across eukaryotes. In both unicellular and multicellular organisms, variations on the same ancestral form mediate sensing, locomotion and the production of physiological flows. As we usher in a new, more interdisciplinary era, the way we study cilia is changing. This special theme issue brings together biologists, biophysicists and mathematicians to highlight the remarkable range of systems in which motile cilia fulfil vital functions, and to inspire and define novel strategies for future research. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Emerging Roles of the Intraflagellar Transport System in the Orchestration of Cellular Degradation Pathways

Ciliated cells exploit a specific transport system, the intraflagellar transport (IFT) system, to ensure the traffic of molecules from the cell body to the cilium. However, it is now clear that IFT activity is not restricted to cilia-related functions. This is strikingly exemplified by the observation that IFT proteins play important roles in cells lacking a primary cilium, such as lymphocytes. Indeed, in T cells the IFT system regulates the polarized transport of endosome-associated T cell antigen receptors and signaling mediators during assembly of the immune synapse, a specialized interface that forms on encounter with a cognate antigen presenting cell and on which T cell activation and effector function crucially depend. Cellular degradation pathways have recently emerged as new extraciliary functions of the IFT system. IFT proteins have been demonstrated to regulate autophagy in ciliated cells through their ability to recruit the autophagy machinery to the base of the cilium. We have now implicated the IFT component IFT20 in another central degradation process that also controls the latest steps in autophagy, namely lysosome function, by regulating the cation-independent mannose-6-phosphate receptor (CI-MPR)-dependent lysosomal targeting of acid hydrolases. This involves the ability of IFT20 to act as an adaptor coupling the CI-MPR to dynein for retrograde transport to the trans-Golgi network. In this short review we will discuss the emerging roles of IFT proteins in cellular degradation pathways.

Sperm motility in fishes: (III) diversity of regulatory signals from membrane to the axoneme

Fish spermatozoa acquire potential for motility in the sperm duct where they are immotile. Osmolality of the seminal plasma is a key factor to maintain spermatozoa in the quiescent state in either freshwater or marine fishes. However, potassium (K⁺) ions prevent spermatozoa motility in salmonid and sturgeon fishes, while CO₂ inhibits spermatozoa motility in flatfishes. Once, spermatozoa are released at spawning, their motility is initiated in hypo-osmotic and hyper-osmotic environments in freshwater and marine fishes, respectively. Some substances produced by the testes (a progestin), or released from oocytes (peptides) induce spermatozoa hypermotility in some marine fishes including the Atlantic croaker and Pacific herrings, respectively. Duration of spermatozoa motility is short, lasting for a few seconds to few minutes in most fishes due to rapid depletion of energy required for the beating of the motility apparatus called axoneme. In the osmotic-activated spermatozoa, K⁺ and water effluxes occur in freshwater and marine fishes, respectively, which trigger spermatozoa motility signaling. In general, initiation of axonemal beating is associated with an increase in intracellular calcium (Ca²⁺) ions in spermatozoa of both freshwater and marine fishes and a post- or pre-increase in intracellular pH, while cyclic adenosine monophosphate (cAMP) remains unchanged. However, axonemal beating is cAMP-dependent in demembranated spermatozoa of salmonid and sturgeon fishes. Calcium from extracellular environment or intracellular stores supply required Ca²⁺ concentration for axonemal beating. Several axonemal proteins have been so far identified in fishes that are activated by Ca²⁺ and cAMP, directly or mediated by protein kinase C and protein kinase A, respectively. The present study reviews differences and similarities in complex regulatory signals controlling spermatozoa motility initiation in fishes, and notes physiological mechanisms that await elucidation.

The intraflagellar transport protein IFT20 controls lysosome biogenesis by regulating the post-Golgi transport of acid hydrolases

The assembly and function of the primary cilium depends on multimolecular intraflagellar transport (IFT) complexes that shuttle their cargo along the axonemal microtubules through their interaction with molecular motors. The IFT system has been moreover recently implicated in a reciprocal interplay between autophagy and ciliogenesis. We have previously reported that IFT20 and other components of the IFT complexes participate in the assembly of the immune synapse in the non-ciliated T cell, suggesting that other cellular processes regulated by the IFT system in ciliated cells, including autophagy, may be shared by cells lacking a cilium. Starting from the observation of a defect in autophagic clearance and an accumulation of lipid droplets in IFT20-deficient T cells, we show that IFT20 is required for lysosome biogenesis and function by controlling the lysosomal targeting of acid hydrolases. This function involves its ability to regulate the retrograde traffic of the cation-independent mannose-6-phosphate receptor (CI-MPR) to the trans-Golgi network, which is achieved by coupling recycling CI-MPRs to the microtubule motor dynein. Consistent with the lysosomal defect, an upregulation of the TFEB-dependent expression of the lysosomal gene network can be observed in IFT20-deficient cells, which is associated with defective tonic T-cell antigen receptor signaling and mTOR activity. We additionally show that the lysosome-related function of IFT20 extends to non-ciliated cells other than T cells, as well as to ciliated cells. Our findings provide the first evidence that a component of the IFT system that controls ciliogenesis is implicated in the biogenesis of lysosomes.

Insights into photoreceptor ciliogenesis revealed by animal models

Photoreceptors are polarized neurons, with very specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment, the site of photon capture that initiates vision, an inner segment that houses the biosynthetic machinery and a synaptic terminal for signal transmission to downstream neurons. Outer segments and inner segments are connected by a connecting cilium (CC), the equivalent of a transition zone (TZ) of primary cilia. The connecting cilium is part of the basal body/axoneme backbone that stabilizes the outer segment. This report will update the reader on late developments in photoreceptor ciliogenesis and transition zone formation, specifically in mouse photoreceptors, focusing on early events in photoreceptor ciliogenesis. The connecting cilium, an elongated and narrow structure through which all outer segment proteins and membrane components must traffic, functions as a gate that controls access to the outer segment. Here we will review genes and their protein products essential for basal body maturation and for CC/TZ genesis, sorted by phenotype. Emphasis is given to naturally occurring mouse mutants and gene knockouts that interfere with CC/TZ formation and ciliogenesis.

Coordination of eukaryotic cilia and flagella

Propulsion by slender cellular appendages called cilia and flagella is an ancient means of locomotion. Unicellular organisms evolved myriad strategies to propel themselves in fluid environments, often involving significant differences in flagella number, localisation and modes of actuation. Remarkably, these appendages are highly conserved, occurring in many complex organisms such as humans, where they may be found generating physiological flows when attached to surfaces (e.g. airway epithelial cilia), or else conferring motility to male gametes (e.g. undulations of sperm flagella). Where multiple cilia arise, their movements are often observed to be highly coordinated. Here I review the two main mechanisms for motile cilia coordination, namely, intracellular and hydrodynamic, and discuss their relative importance in different ciliary systems.

Role for intraflagellar transport in building a functional transition zone

Genetic disorders caused by cilia dysfunction, termed ciliopathies, frequently involve the intraflagellar transport (IFT) system. Mutations in IFT subunits-including IFT-dynein motor DYNC2H1-impair ciliary structures and Hedgehog signalling, typically leading to "skeletal" ciliopathies such as Jeune asphyxiating thoracic dystrophy. Intriguingly, IFT gene mutations also cause eye, kidney and brain ciliopathies often linked to defects in the transition zone (TZ), a ciliary gate implicated in Hedgehog signalling. Here, we identify a C. elegans temperature-sensitive (ts) IFT-dynein mutant (che-3; human DYNC2H1) and use it to show a role for retrograde IFT in anterograde transport and ciliary maintenance. Unexpectedly, correct TZ assembly and gating function for periciliary proteins also require IFT-dynein. Using the reversibility of the novel ts-IFT-dynein, we show that restoring IFT in adults (post-developmentally) reverses defects in ciliary structure, TZ protein localisation and ciliary gating. Notably, this ability to reverse TZ defects declines as animals age. Together, our findings reveal a previously unknown role for IFT in TZ assembly in metazoans, providing new insights into the pathomechanism and potential phenotypic overlap between IFT- and TZ-associated ciliopathies.

Chirality of the cytoskeleton in the origins of cellular asymmetry

Self-assembly of two important components of the cytoskeleton of eukaryotic cells, actin microfilaments and microtubules (MTs) results in polar filaments of one chirality. As is true for bacterial flagella, in actin microfilaments, screw direction is important for assembly processes and motility. For MTs, polar orientation within the cell is paramount. The alignment of these elements in the cell cytoplasm gives rise to emergent properties, including the potential for cell differentiation and specialization. Complex MTs with a characteristic chirality are found in basal bodies and centrioles; this chirality is preserved in cilia. In motile cilia, it is reflected in the direction of the effective stroke. The positioning of the basal body or cilia on the cell surface depends on polarity proteins. In evolution, survival depends on global polarity information relayed to the cell in part by orientation of the MT and actin filament cytoskeletons and the chirality of the basal body to determine left and right coordinates within a defined anterior-posterior cell and tissue axis.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Intraflagellar transport: mechanisms of motor action, cooperation, and cargo delivery

Intraflagellar transport (IFT) is a form of motor-dependent cargo transport that is essential for the assembly, maintenance, and length control of cilia, which play critical roles in motility, sensory reception, and signal transduction in virtually all eukaryotic cells. During IFT, anterograde kinesin-2 and retrograde IFT dynein motors drive the bidirectional transport of IFT trains that deliver cargo, for example, axoneme precursors such as tubulins as well as molecules of the signal transduction machinery, to their site of assembly within the cilium. Following its discovery in Chlamydomonas, IFT has emerged as a powerful model system for studying general principles of motor-dependent cargo transport and we now appreciate the diversity that exists in the mechanism of IFT within cilia of different cell types. The absence of heterotrimeric kinesin-2 function, for example, causes a complete loss of both IFT and cilia in Chlamydomonas, but following its loss in Caenorhabditis elegans, where its primary function is loading the IFT machinery into cilia, homodimeric kinesin-2-driven IFT persists and assembles a full-length cilium. Generally, heterotrimeric kinesin-2 and IFT dynein motors are thought to play widespread roles as core IFT motors, whereas homodimeric kinesin-2 motors are accessory motors that mediate different functions in a broad range of cilia, in some cases contributing to axoneme assembly or the delivery of signaling molecules but in many other cases their ciliary functions, if any, remain unknown. In this review, we focus on mechanisms of motor action, motor cooperation, and motor-dependent cargo delivery during IFT.

DGKδ triggers endoplasmic reticulum release of IFT88-containing vesicles destined for the assembly of primary cilia

The morphogenic factor Sonic hedgehog (Shh) signals through the primary cilium, which relies on intraflagellar transport to maintain its structural integrity and function. However, the process by which protein and lipid cargos are delivered to the primary cilium from their sites of synthesis still remains poorly characterized. Here, we report that diacylglycerol kinase δ (DGKδ), a residential lipid kinase in the endoplasmic reticulum, triggers the release of IFT88-containing vesicles from the ER exit sites (ERES), thereby setting forth their movement to the primary cilium. Encoded by the gene whose mutations originally implicated the primary cilium as the venue of Shh signaling, IFT88 is known to be part of the complex B that drives the anterograde transport within cilia. We show that IFT88 interacts with DGKδ, and is associated with COPII-coated vesicles at the ERES. Using a combination of RNAi silencing and gene knockout strategies, we further show that DGKδ is required for supporting Shh signaling both in vitro and in vivo, demonstrating the physiological significance of this regulation.

Ultrastructure of spermiogenesis and spermatozoa in Marchalina hellenica (Gennadius) (Hemiptera: Sternorrhyncha, Marchalinidae)

The spermiogenesis, the sperm structure and the sperm motility of Marchalina hellenica (Gennadius) were examined. In the early spermiogenesis a centriolar apparatus was identified, but this structure is not involved in the production of the sperm flagellum. As in other Coccoidea, the flagellar axoneme originates by the activity of the thickened tip of the numerous microtubules surrounding the nuclear anterior region close to the periphery of the cell. This region pushes against a narrow cytoplasmic layer, giving rise to a papilla. In this region a novel structure, consisting of a regular network of thin filaments, arranged orthogonally to the bundle of microtubules, is visible. The sperm flagellum consists of a series of about 260 microtubules, regularly arranged in rings around the axial nucleus. This latter extends in the middle part of the sperm length. As usual in scale insects, sperm form a bundle, which in M. hellenica is composed of 64 sperm cells, surrounded by somatic cyst cells. The sperm bundle has an helicoidal array, with a cap of dense material at its apex, lending the anterior and the posterior region of the sperm bundle with a different structural organization. This difference is responsible of the different speed gradient observed in the helical wave propagating along the sperm bundle.

Evidence for a self-organized compliant mechanism for the spontaneous steady beating of cilia

Cilia or eukaryotic flagella are slender 200-nm-diameter organelles that move the immersing fluid relative to a cell and sense the environment. Their core structure is nine doublet microtubules (DMTs) arranged around a central-pair. When motile, thousands of tiny motors slide the DMTs relative to each other to facilitate traveling waves of bending along the cilium's length. These motors provide the energy to change the shape of the cilium and overcome the viscous forces of moving in the surrounding fluid. In planar beating, motors walk toward where the cilium is attached to the cell body. Traveling waves are initiated by motors bending the elastic cilium back and forth, a self-organized mechanical oscillator. We found remarkably that the energy in a wave is nearly constant over a wide range of (ATP) and medium viscosities and inter-doublet springs operate only in the central and not in the basal region. Since the energy in a wave does not depend on its rate of formation, the control mechanism is likely purely mechanical. Further the torque per length generated by the motors acting on the doublets is proportional to and nearly in phase with the microtubule sliding velocity with magnitude dependent on the medium. We determined the frequency-dependent elastic moduli and strain energies of beating cilia. Incorporation of these in an energy-based model explains the beating frequency, wavelength, limiting of the wave amplitude and the overall energy of the traveling wave. Our model describes the intricacies of the basal-wave initiation as well as the traveling wave.

Evolution of Cilia

Anton van Leeuwenhoek's startling microscopic observations in the 1600s first stimulated fascination with the way that cells use cilia to generate currents and to swim in a fluid environment. Research in recent decades has yielded deep knowledge about the mechanical and biochemical nature of these organelles but only opened a greater fascination about how such beautifully intricate and multifunctional structures arose during evolution. Answers to this evolutionary puzzle are not only sought to satisfy basic curiosity, but also, as stated so eloquently by Dobzhansky (Am Zool 4: 443 [1964]), because "nothing in biology makes sense except in the light of evolution." Here I attempt to summarize current knowledge of what ciliary organelles of the last eukaryotic common ancestor (LECA) were like, explore the ways in which cilia have evolved since that time, and speculate on the selective processes that might have generated these organelles during early eukaryotic evolution.

Cytoskeletal architecture and its evolutionary significance in amoeboid eukaryotes and their mode of locomotion

The cytoskeleton is the hallmark of eukaryotic evolution. The molecular and architectural aspects of the cytoskeleton have been playing a prominent role in our understanding of the origin and evolution of eukaryotes. In this study, we seek to investigate the cytoskeleton architecture and its evolutionary significance in understudied amoeboid lineages belonging to Amoebozoa. These amoebae primarily use cytoplasmic extensions supported by the cytoskeleton to perform important cellular processes such as movement and feeding. Amoeboid structure has important taxonomic significance, but, owing to techniques used, its potential significance in understanding diversity of the group has been seriously compromised, leading to an under-appreciation of its value. Here, we used immunocytochemistry and confocal microscopy to study the architecture of microtubules (MTs) and F-actin in diverse groups of amoebae. Our results demonstrate that all Amoebozoa examined are characterized by a complex cytoskeletal array, unlike what has been previously thought to exist. Our results not only conclusively demonstrate that all amoebozoans possess complex cytoplasmic MTs, but also provide, for the first time, a potential synapomorphy for the molecularly defined Amoebozoa clade. Based on this evidence, the last common ancestor of amoebozoans is hypothesized to have had a complex interwoven MT architecture limited within the granular cell body. We also generate several cytoskeleton characters related to MT and F-actin, which are found to be robust for defining groups in deep and shallow nodes of Amoebozoa.

From damage response to action potentials: early evolution of neural and contractile modules in stem eukaryotes

Eukaryotic cells convert external stimuli into membrane depolarization, which in turn triggers effector responses such as secretion and contraction. Here, we put forward an evolutionary hypothesis for the origin of the depolarization-contraction-secretion (DCS) coupling, the functional core of animal neuromuscular circuits. We propose that DCS coupling evolved in unicellular stem eukaryotes as part of an 'emergency response' to calcium influx upon membrane rupture. We detail how this initial response was subsequently modified into an ancient mechanosensory-effector arc, present in the last eukaryotic common ancestor, which enabled contractile amoeboid movement that is widespread in extant eukaryotes. Elaborating on calcium-triggered membrane depolarization, we reason that the first action potentials evolved alongside the membrane of sensory-motile cilia, with the first voltage-sensitive sodium/calcium channels (Nav/Cav) enabling a fast and coordinated response of the entire cilium to mechanosensory stimuli. From the cilium, action potentials then spread across the entire cell, enabling global cellular responses such as concerted contraction in several independent eukaryote lineages. In animals, this process led to the invention of mechanosensory contractile cells. These gave rise to mechanosensory receptor cells, neurons and muscle cells by division of labour and can be regarded as the founder cell type of the nervous system.

Ion channels and calcium signaling in motile cilia

The beating of motile cilia generates fluid flow over epithelia in brain ventricles, airways, and Fallopian tubes. Here, we patch clamp single motile cilia of mammalian ependymal cells and examine their potential function as a calcium signaling compartment. Resting motile cilia calcium concentration ([Ca²⁺] ~170 nM) is only slightly elevated over cytoplasmic [Ca²⁺] (~100 nM) at steady state. Ca²⁺ changes that arise in the cytoplasm rapidly equilibrate in motile cilia. We measured Ca_V1 voltage-gated calcium channels in ependymal cells, but these channels are not specifically enriched in motile cilia. Membrane depolarization increases ciliary [Ca²⁺], but only marginally alters cilia beating and cilia-driven fluid velocity within short (~1 min) time frames. We conclude that beating of ependymal motile cilia is not tightly regulated by voltage-gated calcium channels, unlike that of well-studied motile cilia and flagella in protists, such as Paramecia and Chlamydomonas.

DOI:http://dx.doi.org/10.7554/eLife.11066.001

Certain specialized cells in the brain, airways and Fallopian tubes have large numbers of hair-like structures called motile cilia on their surface. By beating in a synchronized manner, these cilia help to move fluids across the surface of the cells: for example, cilia on lung cells beat to clear mucus away, while those in the brain help the cerebrospinal fluid to circulate.

Motile cilia in mammals are structurally similar to the flagella that propel sperm cells and certain single-celled organisms around their environments. These flagella have specialized pore-forming proteins called ion channels in their membrane through which calcium ions can move. This flow of calcium ions controls the beating of the flagella. However, it is unclear whether a similar movement of calcium ions across the cilia membrane regulates motile cilia beating in mammals.

Doerner et al. have now used a method called patch clamping to study the movement of calcium ions across the membrane of the motile cilia found on a particular type of mouse brain cell. This revealed that unlike flagella, these motile cilia have very few voltage-gated calcium channels; instead, the vast majority of these ion channels reside in the main body of the cell. Furthermore, the level of calcium ions in the motile cilia follows changes in calcium ion levels that originate in the cell body.

Overall, Doerner et al. demonstrate that the activity of voltage-gated calcium channels does not control the beating rhythm of the motile cilia in the mouse brain or how quickly the fluid above the cell surface moves. Future work should investigate whether this is also the case for the cells that line the trachea and Fallopian tubes.

DOI:http://dx.doi.org/10.7554/eLife.11066.002

A Cilia Independent Role of Ift88/Polaris during Cell Migration

Ift88 is a central component of the intraflagellar transport (Ift) complex B, essential for the building of cilia and flagella from single cell organisms to mammals. Loss of Ift88 results in the absence of cilia and causes left-right asymmetry defects, disordered Hedgehog signaling, and polycystic kidney disease, all of which are explained by aberrant ciliary function. In addition, a number of extraciliary functions of Ift88 have been described that affect the cell-cycle, mitosis, and targeting of the T-cell receptor to the immunological synapse. Similarly, another essential ciliary molecule, the kinesin-2 subunit Kif3a, which transports Ift-B in the cilium, affects microtubule (MT) dynamics at the leading edge of migrating cells independently of cilia. We now show that loss of Ift88 impairs cell migration irrespective of cilia. Ift88 is required for the polarization of migrating MDCK cells, and Ift88 depleted cells have fewer MTs at the leading edge. Neither MT dynamics nor MT nucleation are dependent on Ift88. Our findings dissociate the function of Ift88 from Kif3a outside the cilium and suggest a novel extraciliary function for Ift88. Future studies need to address what unifying mechanism underlies the different extraciliary functions of Ift88.

A microtubule organizing centre (MTOC) is responsible for the production of the sperm flagellum in Matsucoccus feytaudi (Hemiptera: Coccoidea)

A microtubule organizing centre (MTOC) has been described in the spermatid of the hemipteran Matsucoccus feytaudi (Coccoidea). This structure, revealed as a fluorescent ring by treatment with γ-tubulin antibody, gives rise to a bundle of microtubules which surrounds the elongated cylindrical nucleus. This microtubule bundle has been considered an atypical sperm flagellum provided with sperm motility. A comparison of the M. feytaudi MTOC with the material associated with the centriole of Drosophila melanogaster spermatids confirms the great similarity between the two structures, both involved in the nucleation of microtubules. Like the D. melanogaster material associated with the centriole, the M. feytaudi MTOC is a transient structure which disappears or degenerates at the end of spermiogenesis and is no longer visible in the mature sperm.

Calcium sensors of ciliary outer arm dynein: functions and phylogenetic considerations for eukaryotic evolution

The motility of eukaryotic cilia and flagella is modulated in response to several extracellular stimuli. Ca(2+) is the most critical intracellular factor for these changes in motility, directly acting on the axonemes and altering flagellar asymmetry. Calaxin is an opisthokont-specific neuronal calcium sensor protein first described in the sperm of the ascidian Ciona intestinalis. It binds to a heavy chain of two-headed outer arm dynein in a Ca(2+)-dependent manner and regulates 'asymmetric' wave propagation at high concentrations of Ca(2+). A Ca(2+)-binding subunit of outer arm dynein in Chlamydomonas reinhardtii, the light chain 4 (LC4), which is a Ca(2+)-sensor phylogenetically different from calaxin, shows Ca(2+)-dependent binding to a heavy chain of three-headed outer arm dynein. However, LC4 appears to participate in 'symmetric' wave propagation at high concentrations of Ca(2+). LC4-type dynein light chain is present in bikonts, except for some subclasses of the Excavata. Thus, flagellar asymmetry-symmetry conversion in response to Ca(2+) concentration represents a 'mirror image' relationship between Ciona and Chlamydomonas. Phylogenetic analyses indicate the duplication, divergence, and loss of heavy chain and Ca(2+)-sensors of outer arm dynein among excavate species. These features imply a divergence point with respect to Ca(2+)-dependent regulation of outer arm dynein in cilia and flagella during the evolution of eukaryotic supergroups.

Microtubule-depolymerizing kinesins in the regulation of assembly, disassembly, and length of cilia and flagella

Defects in ciliary assembly, maintenance, and signaling are associated with various human diseases and developmental disorders, termed ciliopathies. Eukaryotic flagella and cilia (interchangeable terms) are microtubule-based organelles. Thus, microtubule dynamics and microtubule-dependent transport are predicted to affect the structural integrity and functionality of cilia profoundly. Kinesin-2 is well known for its role in intraflagellar transport to transport ciliary precursors and signaling molecules. Recently, microtubule-depolymerizing kinesins found in kinesin-8, -13, and -14A families have emerged as regulators of cilia. We first discuss ciliary kinesins identified in the flagellar or ciliary proteome, and then focus on the function and regulation of microtubule-depolymerizing kinesins. Lastly, we review the recent advances of microtubule-depolymerizing kinesins in controlling ciliary assembly, disassembly, and length.