Left-right asymmetry and kinesin superfamily protein KIF3A: new insights in determination of laterality and mesoderm induction by kif3A-/- mice analysis

Foxj1 regulates asymmetric gene expression during left-right axis patterning in mice

Mice with a targeted mutation of the foxj1 gene demonstrate either D- or L-looping of the embryonic cardiac tube. Foxj1 is expressed in ventral cells of the embryonic node prior to asymmetric, left-right expression of other genes. Despite an absence of 9+2 cilia in foxj1(-/-) mice, 9+0 cilia are present in the node of foxj1(-/-) embryos. In foxj1(-/-) embryos, the patterns of expression of the TGF-beta family member nodal and the homeobox family member pitx2 are randomized. No expression of the TGF-beta family member lefty-2 is observed in any foxj1(-/-) early somite stage embryos. Foxj1 thus acts early in left-right axis patterning and regulates asymmetric gene expression. This regulation does not appear to be the result of a direct interaction between Foxj1 and the genes examined.

Targeted deletion of the novel cytoplasmic dynein mD2LIC disrupts the embryonic organiser, formation of the body axes and specification of ventral cell fates

Dyneins have been implicated in left-right axis determination during embryonic development and in a variety of human genetic syndromes. In this paper, we study the recently discovered mouse dynein 2 light intermediate chain (mD2LIC), which is believed to be involved in retrograde intraflagella transport and which, like left-right dynein, is expressed in the node of the mouse embryo. Cells of the ventral node of mouse embryos lacking mD2LIC have an altered morphology and lack monocilia,and expression of Foxa2 and Shh in this structure is reduced or completely absent. At later stages, consistent with the absence of nodal cilia, mD2LIC is required for the establishment of the left-right axis and for normal expression of Nodal, and the ventral neural tube fails to express Shh, Foxa2 and Ebaf. mD2LIC also functions indirectly in the survival of anterior definitive endoderm and in the maintenance of the anterior neural ridge, probably through maintenance of Foxa2/Hnf3β expression. Together, our results indicate that mD2LIC is required to maintain or establish ventral cell fates and for correct signalling by the organiser and midline, and they identify the first embryonic function of a vertebrate cytoplasmic dynein.

Polycystins and mechanosensation in renal and nodal cilia

The external surfaces of the human body, as well as its internal organs, constantly experience different kinds of mechanical stimulations. For example, tubular epithelial cells of the kidney are continuously exposed to a variety of mechanical forces, such as fluid flow shear stress within the lumen of th nephron. The majority of epithelial cells along the nephron, except intercalated cells, possess a primary cilium, an organelle projecting from the cell's apical surface into the luminal space. Despite its discovery over 100 years ago, the primary cilium's function continued to elude researchers for many decades. However, recent studies indicate that renal cilia have a sensory function. Studies on polycystic kidney disease (PKD) have identified many of the molecular players, which should help solve the mystery of how the renal cilium senses fluid flow. In this review, we will summarize the recent breakthroughs in PKD research and discuss the role(s) of the polycystin signaling complex in mediating mechanosensory function by the primary cilium of renal epithelium as well as of the embryonic node.

Developmental mechanism and evolutionary origin of vertebrate left/right asymmetries

The systematically 'handed', or directionally asymmetrical way in which the major viscera are packed within the vertebrate body is known as situs. Other less obvious vertebrate lateralisations concern cognitive neural function, and include the human phenomena of hand-use preference and language-associated cognitive partitioning. An overview, rather than an exhaustive scholarly review, is given of recent advances in molecular understanding of the mechanism that ensures normal development of 'correct' situs. While the asymmetry itself and its left/right direction are clearly vertebrate-conserved characters, data available from various embryo types are compared in order to assess the likelihood that the developmental mechanism is evolutionarily conserved in its entirety. A conserved post-gastrular 'phylotypic' stage, with left- and right-specific cascades of key, orthologous gene expressions, clearly exists. It now seems probable that earlier steps, in which symmetry-breaking information is reliably transduced to trigger these cascades on the correct sides, are also conserved at depth although it remains unclear exactly how these steps operate. Earlier data indicated that the initiation of symmetry-breaking had been transformed, among the different vertebrate classes, as drastically as has the anatomy of pre-gastrular development itself, but it now seems more likely that this apparent diversity is deceptive. Ideas concerning the functional advantages to the vertebrate lifestyle of a systematically asymmetrical visceral packing arrangement, while untestable, are accepted because they form a plausible adaptationist 'just-so' story. Nevertheless, two contrasting beliefs are possible about the evolutionary origins of situs. Major recent advances in analysis of its developmental mechanism are largely due not to zoologists, comparative anatomists or evolutionary systematists, but to molecular geneticists, and these workers have generally assumed that the asymmetry is an evolutionary novelty imposed on a true bilateral symmetry, at or close to the origin of the vertebrate clade. A major purpose of this review is to advocate an alternative view, on the grounds of comparative anatomy and molecular systematics together with the comparative study of expressions of orthologous genes in different forms. This view is that situs represents a co-optation of a pre-existing, evolutionarily ancient non-bilaterality of the adult form in a vertebrate ancestor. Viewed this way, vertebrate or chordate origins are best understood as the novel imposition of an adaptively bilateral locomotory-skeletal-neural system, around a retained non-symmetrical 'visceral' animal. One component of neuro-anatomical asymmetry, the habenular/parapineal one that originates in the diencephalon, has recently been found (in teleosts) to be initiated from the same 'phylotypic' gene cascade that controls situs development. But the function of this particular diencephalic asymmetry is currently unclear. Other left-right partitionings of brain function, including the much more recently evolved, cerebral cortically located one associated with human language and hand-use, may be controlled entirely separately from situs even though their directionality has a particular relation to it in a majority of individuals. Finally, possible relationships are discussed between the vertebrate directional asymmetries and those that occur sporadically among protostome bilaterian forms. These may have very different evolutionary and molecular bases, such that there may have been constraints, in protostome evolution, upon any exploitation of left and right for complex organismic, and particularly cognitive neural function.

The two motor domains of KIF3A/B coordinate for processive motility and move at different speeds

KIF3A/B, a kinesin involved in intraflagellar transport and Golgi trafficking, is distinctive because it contains two nonidentical motor domains. Our hypothesis is that the two heads have distinct functional properties, which are tuned to maximize the performance of the wild-type heterodimer. To test this, we investigated the motility of wild-type KIF3A/B heterodimer and chimaeric KIF3A/A and KIF3B/B homodimers made by splicing the head of one subunit to the rod and tail of the other. The first result is that KIF3A/B is processive, consistent with its transport function in cells. Secondly, the KIF3B/B homodimer moves at twice the speed of the wild-type motor but has reduced processivity, suggesting a trade-off between speed and processivity. Third, the KIF3A/A homodimer moves fivefold slower than wild-type, demonstrating distinct functional differences between the two heads. The heterodimer speed cannot be accounted for by a sequential head model in which the two heads alternate along the microtubule with identical speeds as in the homodimers. Instead, the data are consistent with a coordinated head model in which detachment of the slow KIF3A head from the microtubule is accelerated roughly threefold by the KIF3B head.

Comparative genomics identifies a flagellar and basal body proteome that includes the BBS5 human disease gene

Cilia and flagella are microtubule-based structures nucleated by modified centrioles termed basal bodies. These biochemically complex organelles have more than 250 and 150 polypeptides, respectively. To identify the proteins involved in ciliary and basal body biogenesis and function, we undertook a comparative genomics approach that subtracted the nonflagellated proteome of Arabidopsis from the shared proteome of the ciliated/flagellated organisms Chlamydomonas and human. We identified 688 genes that are present exclusively in organisms with flagella and basal bodies and validated these data through a series of in silico, in vitro, and in vivo studies. We then applied this resource to the study of human ciliation disorders and have identified BBS5, a novel gene for Bardet-Biedl syndrome. We show that this novel protein localizes to basal bodies in mouse and C. elegans, is under the regulatory control of daf-19, and is necessary for the generation of both cilia and flagella.

The embryonic origins of left-right asymmetry

The bilaterally symmetric body plan of vertebrates features several consistent asymmetries in the placement, structure, and function of organs such as the heart, intestine, and brain. Deviations from the normal pattern result in situs inversus, isomerisms, or heterotaxia (independent randomization), which have significant clinical implications. The invariance of the left-right (LR) asymmetry of normal morphology, neuronal function, and phenotype of several syndromes raises fascinating and fundamental questions in cell, developmental, evolutionary, and neurobiology. While a pathway of asymmetrically expressed signaling factors has been well-characterized in several model systems, very early steps in the establishment of LR asymmetry remain poorly understood. In particular, the origin of consistently oriented asymmetry is unknown. Recently, a candidate for the origins of asymmetry has been suggested: bulk transport of extracellular morphogens by rotating primary cilia during gastrulation. This model is appealing because it 'bootstraps' morphological asymmetry of the embryo from the intrinsic structural (molecular) chirality of motile cilia. However, conceptual and practical problems remain with this hypothesis. Indeed, the genetic data are also consistent with a different mechanism: cytoplasmic transport roles of motor proteins. This review outlines the progress and remaining questions in the field of left-right asymmetry, and focuses on an alternative model for 'Step 1' of asymmetry. More specifically, based on wide-ranging data on ion fluxes and motor protein function in several species, it is suggested that laterality is driven by pH/voltage gradients across the midline, which are established by chiral movement of motor proteins with respect to the cytoskeleton.

The T box transcription factor no tail in ciliated cells controls zebrafish left-right asymmetry

The heart, brain, and gut develop essential left-right (LR) asymmetries. Specialized groups of ciliated cells have been implicated in LR patterning in mouse, chick, frog, and zebrafish embryos. In zebrafish, these ciliated cells are found in Kupffer's vesicle (KV) and are progeny of dorsal forerunner cells (DFCs). However, there is no direct evidence in any vertebrate that the genes involved in LR development are specifically required in ciliated cells. By using a novel method in zebrafish, we knocked down the function of no tail (ntl, homologous to mouse brachyury) in DFCs without affecting its expression in other cells in the embryo. We find that the Ntl transcription factor functions cell autonomously in DFCs to regulate KV morphogenesis and LR determination. This is the first evidence that loss-of-gene function exclusively in ciliated cells perturbs vertebrate LR patterning. Our results demonstrate that the ciliated KV, a transient embryonic organ of previously unknown function, is involved in the earliest known step in zebrafish LR development, suggesting that a ciliary-based mechanism establishes the LR axis in all vertebrate embryos.

Motor or sensor: A new aspect of primary cilia function

Cilia are hair-like structures that protrude from the surface of the cell and are evolutionary well conserved. The characteristic feature of cilia is their motility and, in ciliated epithelia such as the trachea, their principal function is to transport materials along the cell surface. Each epithelial cell has many cilia on its surface. As well as this multiple form of motile cilia seen in the epithelium, there are primary cilia, also known as a monocilium because each cell has only one cilium. These types of cilia are thought to be non-motile because they lack a central pair of microtubules, are anomalous and have no function. However, recent studies have shown that primary cilia are involved in both developmental and pathological processes, including the establishment of left-right asymmetry and polycystic kidney disease. During development, cells in the node rotate their primary cilia to produce an extracellular current that is essential for the determination of left-right asymmetry of the body. In the kidney, primary cilia act as mechanosensors to detect fluid flow. Without such cilia, the kidney develops multiple cysts that eventually destroy kidney function. Furthermore, studies have identified a variety of proteins that are localized in the cilia and their diverse roles in various ciliary functions. These studies suggest the diversity of primary cilia. To elucidate how ciliary proteins interact and perform their functions in primary cilia will help us understand both their function and their diversity.

Breaking symmetry: a clinical overview of left-right patterning

It is increasingly recognized that mutations in genes and pathways critical for left-right (L-R) patterning are involved in common isolated congenital malformations such as congenital heart disease, biliary tract anomalies, renal polycystic disease, and malrotation of the intestine, indicating that disorders of L-R development are far more common than a 1 in 10,000 incidence of heterotaxia might suggest. Understanding L-R patterning disorders requires knowledge of molecular biology, embryology, pediatrics, and internal medicine and is relevant to day-to-day clinical genetics practice. We have reviewed data from mammalian (human and mouse) L-R patterning disorders to provide a clinically oriented perspective that might afford the clinician or researcher additional insights into this diagnostically challenging area.

Identification of proteins that interact with the central coiled-coil region of the human protein kinase NEK1

NEK protein kinases are evolutionarily conserved kinases structurally related to the Aspergillus nidulans mitotic regulator NIMA. At least nine members of the NEK family in vertebrates have been described to date, but for most of them the interacting protein partners are unknown. The pleiotropic deleterious effects and the formation of kidney cysts caused by NEK1 mutation in mice emphasize its involvement in the regulation of diverse cellular processes and in the etiology of polycystic kidney disease (PKD), respectively. Here we report the identification of proteins that interacted with the human NEK1 protein kinase in a yeast two-hybrid screen of a human fetal brain cDNA library, using the catalytic and regulatory domains of NEK1 separately as baits. These proteins are known to take part either in the development of PKD, in the double-strand DNA break repair at the G2/M transition phase of the cell cycle, or in neural cell development. The proteins involved in PKD include the motor protein KIF3A and the proteins tuberin and alpha-catulin. Mapping studies of the human NEK1 regulatory domain (NRD) indicated a strong interaction of most of the proteins retrieved from the library with putative coiled coils located in the central region of NRD. Our results give further support to the previous observation that NEK1 is of functional importance for the etiology of PKD.

Left-right asymmetry: nodal cilia make and catch a wave

Asymmetric fluid flow in the mouse node initiates the development of left-right asymmetry. This flow is generated by motile cilia and is detected by immotile mechanosensory cilia, activating an asymmetric calcium spike.

Foxj1 regulates basal body anchoring to the cytoskeleton of ciliated pulmonary epithelial cells

The forkhead box transcription factor Foxj1 is required for cilia formation and left-right axis determination. To define the role of Foxj1 in ciliogenesis, microarray analysis was performed to identify differentially expressed genes in the pulmonary epithelium of foxj1(+/+) and foxj1(-/-) mice. In the absence of Foxj1, the expression of calpastatin, an inhibitor of the protease calpain, decreased. RNase protection confirmed the decrease in calpastatin expression and decreased calpastatin was detected in the proximal pulmonary epithelium of foxj1(-/-) mice by immunohistochemistry. No change was detected in the expression of calpain 2 in the pulmonary epithelium by western blot or immunohistochemistry. By western blot and immunofluorescence, ezrin, a substrate for calpain, was also found to decrease in the pulmonary epithelium of foxj1(-/-) mice. No change in ezrin gene expression was found by RT-PCR. A decrease in ezrin binding phosphoprotein-50 (EBP-50) was also detected by immunofluorescence in the foxj1(-/-) mouse pulmonary epithelium. Immunoelectron microscopy demonstrated ezrin associated with the basal bodies of cilia in the pulmonary epithelium. Treatment of tracheal explants from foxj1(-/-) mice with a calpain inhibitor resulted in a partial reappearance of cilia observed in these mice. Additionally, following treatment of foxj1(-/-) tracheal explants with calpain inhibitor, basal bodies were observed in an apical location along with relocalization of ezrin and EBP-50. Regulation of calpain activity by calpastatin thus provides a mechanism for regulating the anchoring of basal bodies to the apical cytoskeleton in ciliated cells. In the absence of Foxj1, decreased calpastatin expression with decreased ezrin and EBP-50 results in an inability of basal bodies to anchor to the apical cytoskeleton and subsequent failure of axonemal formation.

Intraflagellar transport

It has been a decade since a novel form of microtubule (MT)-based motility, i.e., intraflagellar transport (IFT), was discovered in Chlamydomonas flagella. Subsequent research has supported the hypothesis that IFT is required for the assembly and maintenance of all cilia and flagella and that its underlying mechanism involves the transport of nonmembrane-bound macromolecular protein complexes (IFT particles) along axonemal MTs beneath the ciliary membrane. IFT requires the action of the anterograde kinesin-II motors and the retrograde IFT-dynein motors to transport IFT particles in opposite directions along the MT polymer lattice from the basal body to the tip of the axoneme and back again. A rich diversity of biological processes has been shown to depend upon IFT, including flagellar length control, cell swimming, mating and feeding, photoreception, animal development, sensory perception, chemosensory behavior, and lifespan control. These processes reflect the varied roles of cilia and flagella in motility and sensory signaling.

Kinesin II Mediates Vg1 mRNA Transport in Xenopus Oocytes

The subcellular localization of specific mRNAs is a widespread mechanism for regulating gene expression. In Xenopus oocytes microtubules are required for localization of Vg1 mRNA to the vegetal cortex during the late RNA localization pathway. The factors that mediate microtubule-based RNA transport during the late pathway have been elusive. Here we show that heterotrimeric kinesin II becomes enriched at the vegetal cortex of stage III/IV Xenopus oocytes concomitant with the localization of endogenous Vg1 mRNA. In addition, expression of a dominant negative mutant peptide fragment or injection of a function-blocking antibody, both of which impair the function of heterotrimeric kinesin II, block localization of Vg1 mRNA. We also show that exogenous Vg1 RNA or Xcat-2, another RNA that can use the late pathway, recruits endogenous kinesin II to the vegetal pole and colocalizes with it at the cortex. These data support a model in which kinesin II mediates the transport of specific RNA complexes destined for the vegetal cortex.

Molecular motor proteins of the kinesin superfamily proteins (KIFs): structure, cargo and disease

Intracellular organelle transport is essential for morphogenesis and functioning of the cell. Kinesins and kinesin-related proteins make up a large superfamily of molecular motors that transport cargoes such as vesicles, organelles (e.g. mitochondria, peroxisomes, lysosomes), protein complexes (e.g. elements of the cytoskeleton, virus particles), and mRNAs in a microtubule- and ATP-dependent manner in neuronal and non-neuronal cells. Until now, more than 45 kinesin superfamily proteins (KIFs) have been identified in the mouse and human genomes. Elucidating the transport pathways mediated by kinesins, the identities of the cargoes moved, and the nature of the proteins that link kinesin motors to cargoes are areas of intense investigation. This review focuses on the structure, the binding partners of kinesins and kinesin-based human diseases.

Cell Biology of Normal and Abnormal Ciliogenesis in the Ciliated Epithelium

Ciliogenesis is divided into four stages: (1) generation of centrioles, (2) migration of duplicated centrioles, (3) formation of the basal body-associated structures, and (4) elongation of cilia. The ultrastructural profile of ciliogenesis is fundamentally the same among various kinds of animal species. In acentriolar centriologenesis, centrioles are generated around deuterosomes by the use of fibrous granules. Components of the centriolar precursor structures, and genes that regulate the differentiation of ciliated cells, have been revealed. Ciliary abnormalities are classified into two categories: specific congenital defects of ciliary structure and acquired nonspecific anomalies of the ciliary apparatus. When ciliogenesis is disturbed, various nonspecific ciliary abnormalities develop in the cell. Inhibition of centriole migration results in the development of intracytoplasmic axonemes, cilia within periciliary sheaths, and intracellular ciliated vacuoles. Swollen cilia and the bulging type of compound cilia are formed during ciliary budding and elongation. Primary cilia can also develop from one of a pair of centrioles. They lack dynein arms and are immobile, but work as a mechanosensor and play a role during morphogenesis of the kidney. Abnormal function or structure of primary cilia results in the development of polycystic kidney disease. The axonemes of primary cilia or monocilia in the embryonic node cells are associated with dynein arms and move vortically. They have a role in determining the left-right (L-R) asymmetry of the fetus. This review also discusses the ciliogenesis of a primary cilium in the cell.

Hedgehog signalling in the mouse requires intraflagellar transport proteins

Intraflagellar transport (IFT) proteins were first identified as essential factors for the growth and maintenance of flagella in the single-celled alga Chlamydomonas reinhardtii. In a screen for embryonic patterning mutations induced by ethylnitrosourea, here we identify two mouse mutants, wimple (wim) and flexo (fxo), that lack ventral neural cell types and show other phenotypes characteristic of defects in Sonic hedgehog signalling. Both mutations disrupt IFT proteins: the wim mutation is an allele of the previously uncharacterized mouse homologue of IFT172; and fxo is a new hypomorphic allele of polaris, the mouse homologue of IFT88. Genetic analysis shows that Wim, Polaris and the IFT motor protein Kif3a are required for Hedgehog signalling at a step downstream of Patched1 (the Hedgehog receptor) and upstream of direct targets of Hedgehog signalling. Our data show that IFT machinery has an essential and vertebrate-specific role in Hedgehog signal transduction.

Genetics of human laterality disorders: insights from vertebrate model systems

Many internal organs in the vertebrate body are asymmetrically oriented along the left-right (L-R) body axis. Organ asymmetry and some components of the molecular signaling pathways that direct L-R development are highly conserved among vertebrate species. Although individuals with full reversal of organ L-R asymmetry (situs inversus totalis) are healthy, significant morbidity and mortality is associated with perturbations in laterality that result in discordant orientation of organ systems and complex congenital heart defects. In humans and other vertebrates, genetic alterations of L-R signaling pathways can result in a wide spectrum of laterality defects. In this review we categorize laterality defects in humans, mice, and zebrafish into specific classes based on altered patterns of asymmetric gene expression, organ situs defects, and midline phenotypes. We suggest that this classification system provides a conceptual framework to help consolidate the disparate laterality phenotypes reported in humans and vertebrate model organisms, thereby refining our understanding of the genetics of L-R development. This approach helps suggest candidate genes and genetic pathways that might be perturbed in human laterality disorders and improves diagnostic criteria.

Motor protein control of ion flux is an early step in embryonic left-right asymmetry

The invariant left-right asymmetry of animal body plans raises fascinating questions in cell, developmental, evolutionary, and neuro-biology. While intermediate mechanisms (e.g., asymmetric gene expression) have been well-characterized, very early steps remain elusive. Recent studies suggested a candidate for the origins of asymmetry: rotary movement of extracellular morphogens by cilia during gastrulation. This model is intellectually satisfying, because it bootstraps asymmetry from the intrinsic biochemical chirality of cilia. However, conceptual and practical problems remain with this hypothesis, and the genetic data is consistent with a different mechanism. Based on wide-ranging data on ion fluxes and motor protein action in a number of species, a model is proposed whereby laterality is generated much earlier, by asymmetric transport of ions, which results in pH/voltage gradients across the midline. These asymmetries are in turn generated by a new candidate for "step 1": asymmetric localization of electrogenic proteins by cytoplasmic motors.

Roles of the cytoskeleton and motor proteins in endocytic sorting

After internalization, endocytic material is actively transported through the cytoplasm, predominantly by microtubule motor proteins. Microtubule-based endocytic transport facilitates sorting of endocytic contents, vesicle fusion and fission, delivery to lysosomes, cytosolic dispersal, as well as nuclear uptake and cytosolic egress of pathogens. Endosomes, like most organelles, move bidirectionally through the cytosol and regulate their cellular location by controlling the activity of motor proteins, and potentially by controlling microtubule and actin polymerization. Control of motor protein activity is manifest by increased microtubule "run lengths", and the binding of motor proteins to organelles can be regulated by motor protein receptors. A mechanistic understanding of how organelles control motor protein activity to allow for endocytic sorting presents an exciting avenue for future research.

Zebrafish pronephros: A model for understanding cystic kidney disease

The embryonic kidney of the zebrafish is the pronephros. The ease of genetic analysis and experimentation in zebrafish, coupled with the simplicity of the pronephros, make the zebrafish an ideal model system for studying kidney development and function. Several mutations have been isolated in zebrafish genetic screens that result in cyst formation in the pronephros. Cloning and characterization of these mutations will provide insight into kidney development but may also provide understanding of the molecular basis of cystic kidney diseases. In this review, we focus on the zebrafish as a model for understanding cystic kidney disease and the links between cystic kidney disease and left-right patterning.

Biochemical and molecular characterization of diseases linked to motor proteins

Recent studies have revealed that kinesin, dynein and myosin each form large superfamilies and participate in many different intracellular transport systems. Importantly, these motor proteins play significant roles in the pathogenesis of a variety of diseases. Studies using knockout mice for kinesin KIF1B have led to the identification of the cause of a human hereditary neuropathy, Charcot-Marie-Tooth disease type 2A. The function of members of the dynein superfamily whose existence has previously only been confirmed through genome databases, has been revealed by studies of immotile cilia syndrome. Unconventional myosins have been shown to function in the inner-ear cells by examination of hereditary human hearing impairment and studies using mouse models. In addition, some diseases are caused by mutations, not in the motor itself, but in the proteins associated with the motor proteins. Here, we discuss the relationship of these motor proteins and how they contribute to disease in molecular terms.

Fusicoccin signaling reveals 14-3-3 protein function as a novel step in left-right patterning during amphibian embryogenesis

To gain insight into the molecular mechanisms underlying the control of morphogenetic signals by H+ flux during embryogenesis, we tested Fusicoccin-A (FC), a compound produced by the fungus Fusicoccum amygdali Del. In plant cells, FC complexes with 14-3-3 proteins to activate H+ pumping across the plasma membrane. It has long been thought that FC acts on higher plants only; here, we show that exposing frog embryos to FC during early development specifically results in randomization of the asymmetry of the left-right (LR) axis (heterotaxia). Biochemical and molecular-genetic evidence is presented that 14-3-3-family proteins are an obligate component of Xenopus FC receptors and that perturbation of 14-3-3 protein function results in heterotaxia. The subcellular localization of 14-3-3 mRNAs and proteins reveals novel cytoplasmic destinations, and a left-right asymmetry at the first cell division. Using gain-of-function and loss-of-function experiments, we show that 14-3-3E protein is likely to be an endogenous and extremely early aspect of LR patterning. These data highlight a striking conservation of signaling pathways across kingdoms, suggest common mechanisms of polarity establishment between C. elegans and vertebrate embryos, and uncover a novel entry point into the pathway of left-right asymmetry determination.

IFT20 links kinesin II with a mammalian intraflagellar transport complex that is conserved in motile flagella and sensory cilia

Intraflagellar transport (IFT) is an evolutionarily conserved mechanism thought to be required for the assembly and maintenance of all eukaryotic cilia and flagella. Although IFT proteins are present in cells with sensory cilia, the organization of IFT protein complexes in those cells has not been analyzed. To determine whether the IFT complex is conserved in the sensory cilia of photo-receptors, we investigated protein interactions among four mammalian IFT proteins: IFT88/Polaris, IFT57/Hippi, IFT52/NGD5, and IFT20. We demonstrate that IFT proteins extracted from bovine photoreceptor outer segments, a modified sensory cilium, co-fractionate at approximately 17 S, similar to IFT proteins extracted from mouse testis. Using antibodies to IFT88 and IFT57, we demonstrate that all four IFT proteins co-immunoprecipitate from lysates of mouse testis, kidney, and retina. We also extended our analysis to interactions outside of the IFT complex and demonstrate an ATP-regulated co-immunoprecipitation of heterotrimeric kinesin II with the IFT complex. The internal architecture of the IFT complex was investigated using the yeast two-hybrid system. IFT20 exhibited a strong interaction with IFT57/Hippi and the kinesin II subunit, KIF3B. Our data indicate that all four mammalian IFT proteins are part of a highly conserved complex in multiple ciliated cell types. Furthermore, IFT20 appears to bridge kinesin II with the IFT complex.

Left-right asymmetry: nodal points

The striking left-right asymmetry of visceral organs is known to depend on left- and right-side-specific cascades of gene expression during early embryogenesis. Now, developmental biologists are characterizing the earliest steps in asymmetry determination that dictate the sidedness of asymmetric gene expression. The proteins and structures involved control fascinating physiological processes, such as extracellular fluid flow and membrane voltage potential and yet little is known about how their activities are coordinated to control laterality. By analogy with intercellular signalling in certain epithelial and endothelial cells, however, it is reasonable to speculate that at least three of these players, monocilia, gap junction communication and the Ca2+ channel polycystin-2, participate in a signalling pathway that propagates left-right cues through multicellular fields.

Two populations of node monocilia initiate left-right asymmetry in the mouse

The vertebrate body plan has conserved handed left-right (LR) asymmetry that is manifested in the heart, lungs, and gut. Leftward flow of extracellular fluid at the node (nodal flow) is critical for normal LR axis determination in the mouse. Nodal flow is generated by motile node cell monocilia and requires the axonemal dynein, left-right dynein (lrd). In the absence of lrd, LR determination becomes random. The cation channel polycystin-2 is also required to establish LR asymmetry. We show that lrd localizes to a centrally located subset of node monocilia, while polycystin-2 is found in all node monocilia. Asymmetric calcium signaling appears at the left margin of the node coincident with nodal flow. These observations suggest that LR asymmetry is established by an entirely ciliary mechanism: motile, lrd-containing monocilia generate nodal flow, and nonmotile polycystin-2 containing cilia sense nodal flow initiating an asymmetric calcium signal at the left border of the node.

Notch activity induces Nodal expression and mediates the establishment of left-right asymmetry in vertebrate embryos

Left-sided expression of Nodal in the lateral plate mesoderm is a conserved feature necessary for the establishment of normal left-right asymmetry during vertebrate embryogenesis. By using gain- and loss-of-function experiments in zebrafish and mouse, we show that the activity of the Notch pathway is necessary and sufficient for Nodal expression around the node, and for proper left-right determination. We identify Notch-responsive elements in the Nodal promoter, and unveil a direct relationship between Notch activity and Nodal expression around the node. Our findings provide evidence for a mechanism involving Notch activity that translates an initial symmetry-breaking event into asymmetric gene expression.

Loss of the Tg737 protein results in skeletal patterning defects

Tg737 mutant mice exhibit pathologic conditions in numerous tissues along with skeletal patterning defects. Herein, we characterize the skeletal pathologic conditions and confirm a role for Tg737 in skeletal patterning through transgenic rescue. Analyses were conducted in both the hypomorphic Tg737(orpk) allele that results in duplication of digit one and in the null Tg737(delta2-3betaGal) allele that is an embryonic lethal mutation exhibiting eight digits per limb. In early limb buds, Tg737 expression is detected throughout the mesenchyme becoming concentrated in precartilage condensations at later stages. In situ analyses indicate that the Tg737(orpk) mutant limb defects are not associated with changes in expression of Shh, Ihh, HoxD11-13, Patched, BMPs, or Glis. Likewise, in Tg737(delta2-3betaGal) mutant embryos, there was no change in Shh expression. However, in both alleles, Fgf4 was ectopically expressed on the anterior apical ectodermal ridge. Collectively, the data argue for a dosage effect of Tg737 on the limb phenotypes and that the polydactyly is independent of Shh misexpression.

The left-right determinant Inversin is a component of node monocilia and other 9+0 cilia

Inversin (Inv), a protein that contains ankyrin repeats, plays a key role in left-right determination during mammalian embryonic development, but its precise function remains unknown. Transgenic mice expressing an Inv and green fluorescent protein (GFP) fusion construct (Inv::GFP) were established to facilitate characterization of the subcellular localization of Inv. The Inv::GFP transgene rescued the laterality defects and polycystic kidney disease of Inv/Inv mice, indicating that the fusion protein is functional. In transgenic embryos, Inv::GFP protein was detected in the node monocilia. The fusion protein was also present in other 9+0 monocilia, including those of kidney epithelial cells and the pituitary gland, but it was not localized to 9+2 cilia. The N-terminal region of Inv (InvDeltaC) including the ankyrin repeats also localized to the node cilia and rescued the left-right defects of Inv/Inv mutants. Although no obvious abnormalities were detected in the node monocilia of Inv/Inv embryos, the laterality defects of such embryos were corrected by an artificial leftward flow of fluid in the node, suggesting that nodal flow is impaired by the Inv mutation. These results suggest that the Inv protein contributes to left-right determination as a component of monocilia in the node and is essential for the generation of normal nodal flow.

Kidney-specific inactivation of the KIF3A subunit of kinesin-II inhibits renal ciliogenesis and produces polycystic kidney disease

Polycystic kidney disease (PKD) is the most common genetic cause of renal failure in humans. Several proteins that are encoded by genes associated with PKD have recently been identified in primary cilia in renal tubular epithelia. These findings have suggested that abnormalities in cilia formation and function may play a role in the pathogenesis of PKD. To directly determine whether cilia are essential to maintain tubular integrity, we conditionally inactivated KIF3A, a subunit of kinesin-II that is essential for cilia formation, in renal epithelia. Constitutive inactivation of KIF3A produces abnormalities of left-right axis determination and embryonic lethality. Here we show that tissue-specific inactivation of KIF3A in renal tubular epithelial cells results in viable offspring with normal-appearing kidneys at birth. Cysts begin to develop in the kidney at postnatal day 5 and cause renal failure by postnatal day 21. The cyst epithelial cells lack primary cilia and exhibit increased proliferation and apoptosis, apical mislocalization of the epidermal growth factor receptor, increased expression of beta-catenin and c-Myc, and inhibition of p21(CIP1). These results demonstrate that the absence of renal cilia produces both the clinical and cell biological findings associated with PKD. Most generally, the phenotype of Kif3a mutant mice suggests a role for primary cilia in the maintenance of lumen-forming epithelial differentiation.

Lateralization defects and ciliary dyskinesia: lessons from algae

Flagella and cilia are two very similar organelles that "beat" to move cells and to propel fluid over tissues. They are highly conserved, being found in organisms ranging from prokaryotes to plant and animal eukaryotes. In humans, cilia are present in almost every organ, and several human conditions involve dysfunctional cilia; for example, lateralization defects, where the positions of organs are reversed, and primary ciliary dyskinesia, a rare condition where patients suffer from recurrent respiratory infections. In this article, we will discuss how information gained from studies on algae has aided research into these human diseases. These studies found a variety of functions that was previously unsuspected, renewing interest in cilia.

The vertebrate primary cilium is a sensory organelle

The primary cilium is a generally non-motile cilium that occurs singly on most cells in the vertebrate body. The function of this organelle, which has been the subject of much speculation but little experimentation, has been unknown. Recent findings reveal that the primary cilium is an antenna displaying specific receptors and relaying signals from these receptors to the cell body. For example, kidney primary cilia display polycystin-2, which forms part of a Ca2+ channel that initiates a signal that controls cell differentiation and proliferation. Kidney primary cilia also are mechanosensors that, when bent, initiate a Ca2+ signal that spreads throughout the cell and to neighboring cells. Primary cilia on other cell types specifically display different receptors, including those for somatostatin and serotonin.

Intraflagellar transport and cilia-dependent diseases

Intraflagellar transport involves the movement of large protein particles along ciliary microtubules and is required for the assembly and maintenance of eukaryotic cilia and flagella. Intraflagellar-transport defects in the mouse cause a range of diseases including polycystic kidney disease, retinal degeneration and the laterality abnormality situs inversus, highlighting the important role that motile, sensory and primary cilia play in vertebrates.

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.

Asymmetries in H+/K+-ATPase and cell membrane potentials comprise a very early step in left-right patterning

A pharmacological screen identified the H+ and K+ ATPase transporter as obligatory for normal orientation of the left-right body axis in Xenopus. Maternal H+/K+-ATPase mRNA is symmetrically expressed in the 1-cell Xenopus embryo but becomes localized during the first two cell divisions, demonstrating that asymmetry is generated within two hours postfertilization. Although H+/K+-ATPase subunit mRNAs are symmetrically localized in chick embryos, an endogenous H+/K+-ATPase-dependent difference in membrane voltage potential exists between the left and right sides of the primitive streak. In both species, pharmacologic or genetic perturbation of endogenous H+/K+-ATPase randomized the sided pattern of asymmetrically expressed genes and induced organ heterotaxia. Thus, LR asymmetry determination depends on a very early differential ion flux created by H+/K+-ATPase activity.

BMP signaling positively regulates Nodal expression during left right specification in the chick embryo

Exogenous application of BMP to the lateral plate mesoderm (LPM) of chick embryos at the early somite stage had a positive effect on Nodal expression. BMP applications into the right LPM were followed by a rapid activation of Nodal, while applications into the left LPM resulted in expansion of the normal domain of Nodal expression. Conversely, blocking of BMP signaling by Noggin in the left LPM interfered with the activation of Nodal expression. These results support a positive role for endogenous BMP on Nodal expression in the LPM. We also report that BMP positively regulates the expression of Caronte, Snail and Cfc in both the left and right LPM. BMP-treated embryos had molecular impairment of the midline with downregulation of Lefty1, Brachyury and Shh but we also show that the midline defect was not sufficient to induce ectopic Nodal expression. We discuss our findings in the context of the known molecular control of the specification of left-right asymmetry.

Determination of left-right patterning of the mouse embryo by artificial nodal flow

Substantial insight has recently been achieved into the mechanisms responsible for the generation of left-right (L-R) asymmetry in the vertebrate body plan. However, the mechanism that underlies the initial breaking of symmetry has remained unclear. In the mouse, a leftward fluid flow on the ventral side of the node caused by the vortical motion of cilia (referred to as nodal flow) is implicated in symmetry breaking, but direct evidence for the role of this flow has been lacking. Here we describe the development of a system in which mouse embryos are cultured under an artificial fluid flow and with which we have examined how flow affects L-R patterning. An artificial rightward flow that was sufficiently rapid to reverse the intrinsic leftward nodal flow resulted in reversal of situs in wild-type embryos. The artificial flow was also able to direct the situs of mutant mouse embryos with immotile cilia. These results provide the first direct evidence for the role of mechanical fluid flow in L-R patterning.

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

A healthy understanding of intraflagellar transport

A microtubule-dependent motility process called intraflagellar transport (IFT) occurs beneath the plasma membrane of cilia and flagella. IFT was first observed in Chlamydomonas, and orthologs of some of the polypeptides involved in IFT have recently been identified in other organisms, including C. elegans and the mouse. In addition to a role in the assembly and maintenance of cilia and flagella, evidence is reviewed here that indicates defects in the process of IFT may be related to problems with human health. Moreover, recent data suggest the possibility of two new roles for IFT in cell function. The first is in transcriptional control of the genes encoding ciliary and flagellar proteins. IFT could provide a mechanism whereby the cell senses the presence or absence of its cilia or flagella and responds by turning on gene transcription resulting in replacement of the missing organelle. The second role is in signal transduction, whereby cilia act as sensors of the external cellular environment and transduce information about the surroundings into intracellular signals that are sent via IFT to the cell body, thus inducing an appropriate cellular response to the environment.

A complex syndrome of left-right axis, central nervous system and axial skeleton defects in Zic3 mutant mice

X-linked heterotaxy (HTX1) is a rare developmental disorder characterized by disturbances in embryonic laterality and other midline developmental field defects. HTX1 results from mutations in ZIC3, a member of the GLI transcription factor superfamily. A targeted deletion of the murine Zic3 locus has been created to investigate its function and interactions with other molecular components of the left-right axis pathway. Embryonic lethality is seen in approximately 50% of null mice with an additional 30% lethality in the perinatal period. Null embryos have defects in turning, cardiac development and neural tube closure. Malformations in live born null mice include complex congenital heart defects, pulmonary reversal or isomerism, CNS defects and vertebral/rib anomalies. Investigation of nodal expression in Zic3-deficient mice indicates that, although nodal is initially expressed symmetrically in the node, there is failure to maintain expression and to shift to asymmetric expression. Subsequent nodal and Pitx2 expression in the lateral plate mesoderm in these mice is randomized, indicating that Zic3 acts upstream of these genes in the determination of left-right asymmetry. The phenotype of these mice correctly models the defects found in human HTX1 and indicates an important role for Zic3 in both left-right and axial patterning.

Polaris, a protein disrupted in orpk mutant mice, is required for assembly of renal cilium

Cilia are organelles that play diverse roles, from fluid movement to sensory reception. Polaris, a protein associated with cystic kidney disease in Tg737(o)(rpk) mice, functions in a ciliogenic pathway. Here, we explore the role of polaris in primary cilia on Madin-Darby canine kidney cells. The results indicate that polaris localization and solubility change dramatically during cilia formation. These changes correlate with the formation of basal bodies and large protein rafts at the apical surface of the epithelia. A cortical collecting duct cell line has been derived from mice with a mutation in the Tg737 gene. These cells do not develop normal cilia, which can be corrected by reexpression of the wild-type Tg737 gene. These data suggest that the primary cilia are important for normal renal function and/or development and that the ciliary defect may be a contributing factor to the cystic disease in Tg737(o)(rpk) mice. Further characterization of these cells will be important in elucidating the physiological role of renal cilia and in determining their relationship to cystic disease.

Mutations in DNAH5 cause primary ciliary dyskinesia and randomization of left-right asymmetry

Primary ciliary dyskinesia (PCD, MIM 242650) is characterized by recurrent infections of the respiratory tract due to reduced mucociliary clearance and by sperm immobility. Half of the affected offspring have situs inversus (reversed organs), which results from randomization of left-right (LR) asymmetry. We previously localized to chromosome 5p a PCD locus containing DNAH5, which encodes a protein highly similar to the Chlamydomonas gamma-dynein heavy chain. Here we characterize the full-length 14-kb transcript of DNAH5. Sequence analysis in individuals with PCD with randomization of LR asymmetry identified mutations resulting in non-functional DNAH5 proteins.

Molecular motor KIF1C is not essential for mouse survival and motor-dependent retrograde Golgi apparatus-to-endoplasmic reticulum transport

KIF1C is a new member of the kinesin superfamily of proteins (KIFs), which act as microtubule-based molecular motors involved in intracellular transport. We cloned full-length mouse kif1C cDNA, which turned out to have a high homology to a mitochondrial motor KIF1Balpha and to be expressed ubiquitously. To investigate the in vivo significance of KIF1C, we generated kif1C(-/-) mice by knocking in the beta-galactosidase gene into the motor domain of kif1C gene. On staining of LacZ, we detected its expression in the heart, liver, hippocampus, and cerebellum. Unexpectedly, kif1C(-/-) mice were viable and showed no obvious abnormalities. Because immunocytochemistry showed partial colocalization of KIF1C with the Golgi marker protein, we compared the organelle distribution in primary lung fibroblasts from kif1C(+/+) and kif1C(-/-) mice. We found that there was no significant difference in the distribution of the Golgi apparatus or in the transport from the Golgi apparatus to the endoplasmic reticulum (ER) facilitated by brefeldin A between the two cells. This retrograde membrane transport was further confirmed to be normal by time-lapse analysis. Consequently, KIF1C is dispensable for the motor-dependent retrograde transport from the Golgi apparatus to the ER.

Establishment of vertebrate left-right asymmetry

The generation of morphological, such as left-right, asymmetry during development is an integral part of the establishment of a body plan. Until recently, the molecular basis of left-right asymmetry was a mystery, but studies indicate that Nodal and the Lefty proteins, transforming growth factor-beta-related molecules, have a central role in generating asymmetric signals. Although the initial mechanism of symmetry breaking remains unknown, developmental biologists are beginning to analyse the pathway that leads to left-right asymmetry establishment and maintenance.

Left-right asymmetry determination in vertebrates

A distinctive and essential feature of the vertebrate body is a pronounced left-right asymmetry of internal organs and the central nervous system. Remarkably, the direction of left-right asymmetry is consistent among all normal individuals in a species and, for many organs, is also conserved across species, despite the normal health of individuals with mirror-image anatomy. The mechanisms that determine stereotypic left-right asymmetry have fascinated biologists for over a century. Only recently, however, has our understanding of the left-right patterning been pushed forward by links to specific genes and proteins. Here we examine the molecular biology of the three principal steps in left-right determination: breaking bilateral symmetry, propagation and reinforcement of pattern, and the translation of pattern into asymmetric organ morphogenesis.

Metazoan motor models: kinesin superfamily in C. elegans

In eukaryotic cells members of the kinesin family mediate intracellular transport by carrying cellular cargo on microtubule tracks. The nematode Caenorhabditis elegans genome encodes 21 members of the kinesin family, which show significant homology to their mammalian orthologs. Based on motor domain sequence homology and placement of the motor domain in the protein, the C. elegans kinesins have been placed in eight distinct groups; members of which participate in embryonic development, protein transport, synaptic membrane vesicles movement and in the axonal growth. Among 21 kinesins, at least 11 play a central role in spindle movement and chromosomal segregation. Understanding the function of C. elegans kinesins and related proteins may help navigate through the intricacies of intracellular traffic in a simple animal.

Intact lysosome transport and phagosome function despite kinectin deficiency

The mechanism of cargo coupling to kinesin motor proteins is a fundamental issue in organelle transport along microtubules. Kinectin has been postulated to function as a membrane anchor protein that attaches various organelles to the prototype motor protein kinesin. To verify the biological relevance of kinectin in vivo, the murine kinectin gene was disrupted by homologous recombination. Unexpectedly, kinectin-deficient mice were viable and fertile, and no gross abnormalities were observed up to 1 year of age. The assembly of the endoplasmic reticulum was essentially unaffected in kinectin-deficient cells. Mitochondria appeared to be correctly distributed throughout the cytoplasm along the microtubules. Furthermore, the stationary distribution and the bidirectional movement of lysosomes did not depend on kinectin. Kinectin-deficient phagocytes internalized and cleared bacteria, indicating that phagosome trafficking and maturation are functional without kinectin. Thus, these data unequivocally indicate that kinectin is not essential for trafficking of lysosomes, phagosomes, and mitochondria in vivo.