hemingway is required for sperm flagella assembly and ciliary motility in Drosophila

Cilia play major functions in physiology and development, and ciliary dysfunctions are responsible for several diseases in humans called ciliopathies. Cilia motility is required for cell and fluid propulsion in organisms. In humans, cilia motility deficiencies lead to primary ciliary dyskinesia, with upper-airways recurrent infections, left–right asymmetry perturbations, and fertility defects. In Drosophila, we identified hemingway (hmw) as a novel component required for motile cilia function. hmw encodes a 604–amino acid protein characterized by a highly conserved coiled-coil domain also found in the human orthologue, KIAA1430. We show that HMW is conserved in species with motile cilia and that, in Drosophila, hmw is expressed in ciliated sensory neurons and spermatozoa. We created hmw-knockout flies and found that they are hearing impaired and male sterile. hmw is implicated in the motility of ciliated auditory sensory neurons and, in the testis, is required for elongation and maintenance of sperm flagella. Because HMW is absent from mature flagella, we propose that HMW is not a structural component of the motile axoneme but is required for proper acquisition of motile properties. This identifies HMW as a novel, evolutionarily conserved component necessary for motile cilium function and flagella assembly.     

Scoring a backstage pass: mechanisms of ciliogenesis and ciliary access

Cilia are conserved, microtubule-based cell surface projections that emanate from basal bodies, membrane-docked centrioles. The beating of motile cilia and flagella enables cells to swim and epithelia to displace fluids. In contrast, most primary cilia do not beat but instead detect environmental or intercellular stimuli. Inborn defects in both kinds of cilia cause human ciliopathies, diseases with diverse manifestations such as heterotaxia and kidney cysts. These diseases are caused by defects in ciliogenesis or ciliary function. The signaling functions of cilia require regulation of ciliary composition, which depends on the control of protein traffic into and out of cilia.     

Ciliary motility: the components and cytoplasmic preassembly mechanisms of the axonemal dyneins

Motile cilia and flagella are organelles, which function in cell motility and in the transport of fluids over the surface of cells. Motility defects often result in a rare human disease, primary ciliary dyskinesia (PCD). Cell motility depends on axonemal dynein, a molecular motor that drives the beating of cilia and flagella. The dyneins are composed of multiple subunits, which are thought to be preassembled in the cytoplasm before they are transported into cilia and flagella. Axonemal dyneins have been extensively studied in Chlamydomonas. In addition, analyses of human PCDs over the past decade, together with studies in other model animals, have identified the conserved components required for dynein assembly. Recently also, the first cytoplasmic component of dynein assembly, kintoun (ktu), was elucidated through the analysis of a medaka mutant in combination with human genetics and cell biology and biochemical studies of Chlamydomonas. The components of dynein and the proteins involved in its cytoplasmic assembly process are discussed.     

纤毛及纤毛相关疾病研究进展

纤毛是一种以细胞微管为主形成的突出于细胞表面的结构,分布于哺乳动物体内的大多数细胞。近年来研究发现,很多人类疾病都与纤毛结构、长度的失调相关,所以有关纤毛的研究是目前研究的热点领域。越来越多的证据证明,纤毛除了提供流体推动力参与细胞的运动功能之外,还具有信号传导的功能,在细胞生命活动的各个方面发挥着多种关键作用。它参与调控细胞生理活动、增殖与分化以及动物个体发育。因此,深入地探索纤毛调控机理对基础生物学理论的发展和人类纤毛相关疾病的攻克有重要意义。该文简要介绍了纤毛的结构、组装与解聚的机制、参与信号传导的功能以及纤毛缺陷同人类疾病的关系。     

Kinesin motors and primary cilia

Cilia and flagella play important roles in human health by contributing to cellular motility as well as sensing and responding to environmental cues. Defects in ciliary assembly and/or function can lead to a range of human diseases, collectively known as the ciliopathies, including polycystic kidney, liver and pancreatic diseases, sterility, obesity, situs inversus, hydrocephalus and retinal degeneration. A basic understanding of how cilia form and function is essential for deciphering ciliopathies and generating therapeutic treatments. The cilium is a unique compartment that contains a distinct complement of protein and lipid. However, the molecular mechanisms by which soluble and membrane protein components are targeted to and trafficked into the cilium are not well understood. Cilia are generated and maintained by IFT (intraflagellar transport) in which IFT cargoes are transported along axonemal microtubules by kinesin and dynein motors. A variety of genetic, biochemical and cell biological approaches has established the heterotrimeric kinesin-2 motor as the 'core' IFT motor, whereas other members of the kinesin-2, kinesin-3 and kinesin-4 families function as 'accessory' motors for the transport of specific cargoes in diverse cell types. Motors of the kinesin-9 and kinesin-13 families play a non-IFT role in regulating ciliary beating or axonemal length, respectively. Entry of kinesin motors and their cargoes into the ciliary compartment requires components of the nuclear import machinery, specifically importin-β2 (transportin-1) and Ran-GTP (Ran bound to GTP), suggesting that similar mechanisms may regulate entry into the nuclear and ciliary compartments.     

Mechanisms of mammalian ciliary motility: Insights from primary ciliary dyskinesia genetics

Motile cilia and flagella are organelles that, historically, have been poorly understood and inadequately investigated. However, cilia play critical roles in fluid clearance in the respiratory system and the brain, and flagella are required for sperm motility. Genetic studies involving human patients and mouse models of primary ciliary dyskinesia over the last decade have uncovered a number of important ciliary proteins and have begun to elucidate the mechanisms underlying ciliary motility. When combined with genetic, biochemical, and cell biological studies in Chlamydomonas reinhardtii, these mammalian genetic analyses begin to reveal the mechanisms by which ciliary motility is regulated.     

Ciliogenesis: building the cell's antenna

The cilium is a complex organelle, the assembly of which requires the coordination of motor-driven intraflagellar transport (IFT), membrane trafficking and selective import of cilium-specific proteins through a barrier at the ciliary transition zone. Recent findings provide insights into how cilia assemble and disassemble in synchrony with the cell cycle and how the balance of ciliary assembly and disassembly determines the steady-state ciliary length, with the inherent length-dependence of IFT rendering the ciliary assembly rate a decreasing function of length. As cilia are important in sensing and processing developmental signals and directing the flow of fluids such as mucus, defects in ciliogenesis and length control are likely to underlie a range of cilium-related human diseases.     

BLD10/CEP135 Is a Microtubule-Associated Protein that Controls the Formation of the Flagellum Central Microtubule Pair

Cilia and flagella are involved in a variety of processes and human diseases, including ciliopathies and sterility. Their motility is often controlled by?a central microtubule (MT) pair localized within the ciliary MT-based skeleton, the axoneme. We characterized the formation of the motility apparatus in detail in Drosophila spermatogenesis. We show that assembly of the central MT pair starts prior to the meiotic divisions, with nucleation of a singlet MT within the basal body of a small cilium, and that the second MT of the pair only assembles much later, upon flagella formation. BLD10/CEP135, a conserved player in centriole and flagella biogenesis, can bind and stabilize MTs and is required for the early steps of central MT pair formation. This work describes a genetically tractable system to study motile cilia formation and provides an explanation for BLD10/CEP135's role in assembling highly stable MT-based structures, such as motile axonemes and centrioles.     

A chemical screen identifies class a g-protein coupled receptors as regulators of cilia

Normal cilia length and motility are critical for proper cellular function. Prior studies of the regulation of ciliary structure and length have primarily focused on the intraflagellar transport machinery and motor proteins required for ciliary assembly and disassembly. However, several mutants with abnormal length flagella highlight the importance of signaling proteins as well. In this study, an unbiased chemical screen was performed to uncover signaling pathways that are critical for ciliogenesis and length regulation using flagella of the green alga Chlamydomonas reinhardtii as a model. The annotated Sigma LOPAC1280 chemical library was screened for effects on flagellar length, motility, and severing as well as cell viability. Assay data were clustered to identify pathways regulating flagella. The most frequent target found to be involved in flagellar length regulation was the family of dopamine binding G-protein coupled receptors (GPCRs). In mammalian cells, cilium length could indeed be altered with expression of the dopamine D1 receptor. Our screen thus reveals signaling pathways that are potentially critical for ciliary formation, resorption, and length maintenance, which represent candidate targets for therapeutic intervention of disorders involving ciliary malformation and malfunction.     

Autophagy and regulation of cilia function and assembly

Motile and primary cilia (PC) are microtubule-based structures located at the cell surface of many cell types. Cilia govern cellular functions ranging from motility to integration of mechanical and chemical signaling from the environment. Recent studies highlight the interplay between cilia and autophagy, a conserved cellular process responsible for intracellular degradation. Signaling from the PC recruits the autophagic machinery to trigger autophagosome formation. Conversely, autophagy regulates ciliogenesis by controlling the levels of ciliary proteins. The cross talk between autophagy and ciliated structures is a novel aspect of cell biology with major implications in development, physiology and human pathologies related to defects in cilium function.     

Drosophila Bld10 Is a Centriolar Protein That Regulates Centriole, Basal Body, and Motile Cilium Assembly

Cilia and flagella play multiple essential roles in animal development and cell physiology. Defective cilium assembly or motility represents the etiological basis for a growing number of human diseases. Therefore, how cilia and flagella assemble and the processes that drive motility are essential for understanding these diseases. Here we show that Drosophila Bld10, the ortholog of Chlamydomonas reinhardtii Bld10p and human Cep135, is a ubiquitous centriolar protein that also localizes to the spermatid basal body. Mutants that lack Bld10 assemble centrioles and form functional centrosomes, but centrioles and spermatid basal bodies are short in length. bld10 mutant flies are viable but male sterile, producing immotile sperm whose axonemes are deficient in the central pair of microtubules. These results show that Drosophila Bld10 is required for centriole and axoneme assembly to confer cilium motility.     

Mechanisms of Weight Control by Primary Cilia

A primary cilium, a hair-like protrusion of the plasma membrane, is a pivotal organelle for sensing external environmental signals and transducing intracellular signaling. An interesting linkage between cilia and obesity has been revealed by studies of the human genetic ciliopathies Bardet-Biedl syndrome and Alström syndrome, in which obesity is a principal manifestation. Mouse models of cell type-specific cilia dysgenesis have subsequently demonstrated that ciliary defects restricted to specific hypothalamic neurons are sufficient to induce obesity and hyperphagia. A potential mechanism underlying hypothalamic neuron cilia-related obesity is impaired ciliary localization of G protein-coupled receptors involved in the regulation of appetite and energy metabolism. A well-studied example of this is melanocortin 4 receptor (MC4R), mutations in which are the most common cause of human monogenic obesity. In the paraventricular hypothalamus neurons, a blockade of ciliary trafficking of MC4R as well as its downstream ciliary signaling leads to hyperphagia and weight gain. Another potential mechanism is reduced leptin signaling in hypothalamic neurons with defective cilia. Leptin receptors traffic to the periciliary area upon leptin stimulation. Moreover, defects in cilia formation hamper leptin signaling and actions in both developing and differentiated hypothalamic neurons. The list of obesity-linked ciliary proteins is expending and this supports a tight association between cilia and obesity. This article provides a brief review on the mechanism of how ciliary defects in hypothalamic neurons facilitate obesity.     

Ciliary Extracellular Vesicles: Txt Msg Organelles

Cilia are sensory organelles that protrude from cell surfaces to monitor the surrounding environment. In addition to its role as sensory receiver, the cilium also releases extracellular vesicles (EVs). The release of sub-micron sized EVs is a conserved form of intercellular communication used by all three kingdoms of life. These extracellular organelles play important roles in both short and long range signaling between donor and target cells and may coordinate systemic responses within an organism in normal and diseased states. EV shedding from ciliated cells and EV–cilia interactions are evolutionarily conserved phenomena, yet remarkably little is known about the relationship between the cilia and EVs and the fundamental biology of EVs. Studies in the model organisms Chlamydomonas and Caenorhabditis elegans have begun to shed light on ciliary EVs. Chlamydomonas EVs are shed from tips of flagella and are bioactive. Caenorhabditis elegans EVs are shed and released by ciliated sensory neurons in an intraflagellar transport-dependent manner. Caenorhabditis elegans EVs play a role in modulating animal-to-animal communication, and this EV bioactivity is dependent on EV cargo content. Some ciliary pathologies, or ciliopathies, are associated with abnormal EV shedding or with abnormal cilia–EV interactions. Until the 21st century, both cilia and EVs were ignored as vestigial or cellular junk. As research interest in these two organelles continues to gain momentum, we envision a new field of cell biology emerging. Here, we propose that the cilium is a dedicated organelle for EV biogenesis and EV reception. We will also discuss possible mechanisms by which EVs exert bioactivity and explain how what is learned in model organisms regarding EV biogenesis and function may provide insight to human ciliopathies.     

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.     

Cilia and ciliopathies: From Chlamydomonas and beyond

The biological function of motile cilia/flagella has long been recognized. The non-motile primary cilium, once regarded as a vestigial organelle, however, has been found recently to play unexpected roles in mammalian physiology and development. Defects in cilia have profound impact on human health. Diseases related to cilia, collectively called ciliopathies include male infertility, primary cilia dyskinesia, renal cyst formation, blindness, polydactyly, obesity, hypertension, and even mental retardation. Our current understanding of cilia and ciliopathies has been fueled by basic research employing various model organisms including Chlamydomonas, a unicellular green alga. This review article provides a general introduction to the cell biology of cilia and an overview of various cilia-related diseases.     

Intraflagellar transport

Eukaryotic cilia and flagella, including primary cilia and sensory cilia, are highly conserved organelles that project from the surfaces of many cells. The assembly and maintenance of these nearly ubiquitous structures are dependent on a transport system--known as 'intraflagellar transport' (IFT)--which moves non-membrane-bound particles from the cell body out to the tip of the cilium or flagellum, and then returns them to the cell body. Recent results indicate that defects in IFT might be a primary cause of some human diseases.     

IFT52 plays an essential role in sensory cilia formation and neuronal sensory function in Drosophila

Cilia are microtubule-based, hair-like organelles involved in sensory function or motility, playing critical roles in many physiological processes such as reproduction, organ development, and sensory perception. In insects, cilia are restricted to certain sensory neurons and sperms, being important for chemical and mechanical sensing, and fertility. Although great progress has been made regarding the mechanism of cilia assembly, the formation of insect cilia remains poorly understand, even in the insect model organism Drosophila. Intraflagellar transport (IFT) is a cilia-specific complex that traffics protein cargos bidirectionally along the ciliary axoneme and is essential for most cilia. Here we investigated the role of IFT52, a core component of IFT-B, in cilia/flagellar formation in Drosophila. We show that Drosophila IFT52 is distributed along the sensory neuronal cilia, and is essential for sensory cilia formation. Deletion of Ift52 results in severe defects in cilia-related sensory behaviors. It should be noted that IFT52 is not detected in spermatocyte cilia or sperm flagella of Drosophila. Accordingly, ift52 mutants can produce sperms with normal motility, supporting a dispensable role of IFT in Drosophila sperm flagella formation. Altogether, IFT52 is a conserved protein essential for sensory cilia formation and sensory neuronal function in insects.     

Swimming with protists: perception, motility and flagellum assembly

In unicellular and multicellular eukaryotes, fast cell motility and rapid movement of material over cell surfaces are often mediated by ciliary or flagellar beating. The conserved defining structure in most motile cilia and flagella is the '9+2' microtubule axoneme. Our general understanding of flagellum assembly and the regulation of flagellar motility has been led by results from seminal studies of flagellate protozoa and algae. Here we review recent work relating to various aspects of protist physiology and cell biology. In particular, we discuss energy metabolism in eukaryotic flagella, modifications to the canonical assembly pathway and flagellum function in parasite virulence.