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

Structures and functions of cilia during vertebrate embryo development

Cilia are hair-like structures that project from the surface of cells. In vertebrates, most cells have an immotile primary cilium that mediates cell signaling, and some specialized cells assemble one or multiple cilia that are motile and beat synchronously to move fluids in one direction. Gene mutations that alter cilia structure or function cause a broad spectrum of disorders termed ciliopathies that impact virtually every system in the body. A wide range of birth defects associated with ciliopathies underscores critical functions for cilia during embryonic development. In many cases, the mechanisms underlying cilia functions during development and disease remain poorly understood. This review describes different types of cilia in vertebrate embryos and discusses recent research results from diverse model systems that provide novel insights into how cilia form and function during embryo development. The work discussed here not only expands our understanding of in vivo cilia biology, but also opens new questions about cilia and their roles in establishing healthy embryos.

Situs inversus and ciliary abnormalities: 20 years later, what is the connection?

Heterotaxy (also known as situs ambiguous) and situs inversus totalis describe disorders of laterality in which internal organs do not display their typical pattern of asymmetry. First described around 1600 by Girolamo Fabrizio, numerous case reports about laterality disorders in humans were published without any idea about the underlying cause. Then, in 1976, immotile cilia were described as the cause of a human syndrome that was previously clinically described, both in 1904 by AK Siewert and in 1933 by Manes Kartagener, as an association of situs inversus with chronic sinusitis and bronchiectasis, now commonly known as Kartagener’s syndrome. Despite intense research, the underlying defect of laterality disorders remained unclear. Nearly 20 years later in 1995, Björn Afzelius discussed five hypotheses to explain the connection between ciliary defects and loss of laterality control in a paper published in the International Journal of Developmental Biology asking: ‘Situs inversus and ciliary abnormalities: What is the connection?’. Here, nearly 20 research years later, we revisit some of the key findings that led to the current knowledge about the connection between situs inversus and ciliary abnormalities.

Left-right asymmetry is formed in individual cells by intrinsic cell chirality

Many animals show left-right (LR) asymmetric morphology. The mechanisms of LR asymmetric development are evolutionarily divergent, and they remain elusive in invertebrates. Various organs in Drosophila melanogaster show stereotypic LR asymmetry, including the embryonic gut. The Drosophila embryonic hindgut twists 90° left-handedly, thereby generating directional LR asymmetry. We recently revealed that the hindgut epithelial cell is chiral in shape and other properties; this is termed planar cell chirality (PCC). We previously showed by computer modeling that PCC is sufficient to induce the hindgut rotation. In addition, both the PCC and the direction of hindgut twisting are reversed in Myosin31DF (Myo31DF) mutants. Myo31DF encodes Drosophila MyosinID, an actin-based motor protein, whose molecular functions in LR asymmetric development are largely unknown. Here, to understand how PCC directs the asymmetric cell-shape, we analyzed PCC in genetic mosaics composed of cells homozygous for mutant Myo31DF, some of which also overexpressed wild-type Myo31DF. Wild-type cell-shape chirality only formed in the Myo31DF-overexpressing cells, suggesting that cell-shape chirality was established in each cell and reflects intrinsic PCC. A computer model recapitulating the development of this genetic mosaic suggested that mechanical interactions between cells are required for the cell-shape behavior seen in vivo. Our mosaic analysis also suggested that during hindgut rotation in vivo, wild-type Myo31DF suppresses the elongation of cell boundaries, supporting the idea that cell-shape chirality is an intrinsic property determined in each cell. However, the amount and distribution of F-actin and Myosin II, which are known to help generate the contraction force on cell boundaries, did not show differences between Myo31DF mutant cells and wild-type cells, suggesting that the static amount and distribution of these proteins are not involved in the suppression of cell-boundary elongation. Taken together, our results suggest that cell-shape chirality is intrinsically formed in each cell, and that mechanical force from intercellular interactions contributes to its formation and/or maintenance.

Chemosignalling, mechanotransduction and ciliary behaviour in the embryonic node: Computational evaluation of competing theories

Precise specification of left-right asymmetry is essential for patterning the internal organs of vertebrates. Within the embryonic node, posteriorly polarised cilia rotate, causing a leftward fluid flow (nodal flow) that establishes left-right asymmetry. The mechanism by which an embryo senses nodal flow remains uncertain. Existing hypotheses argue that either nodal flow carries morphogen(s) or lipid-bounded vesicles towards the left, thereby generating an asymmetric signal, and/or that mechano-sensory cilia sense this unidirectional flow, stimulating left-sided intracellular calcium signalling. To date, direct and definitive evidence supporting these hypotheses has been lacking. In this study, we conduct a multi-scale study to simulate the nodal cilia and the fluidic environment, analysing left-right signal transmission. By employing computational simulation techniques and solving the relevant three-dimensional unsteady transport equations, we study the flow pattern produced by the rotation of active cilia. By importing dilute species and particles into the computational domain, we investigate the transport of morphogens and nodal vesicular parcels, respectively. Furthermore, by extending the analysis to include the solid mechanics of passive deformable cilia and the coupling of their structural behaviour with the emerging fluid mechanics, we study the response of passive cilia to the nodal flow. Our results reproduce the unidirectional nodal flow, allowing us to evaluate the plausibility of both chemo- and mechano-sensing hypotheses. The quantitative measurements of the flow rate, the molecular transport and distribution provide guidance regarding the necessary morphogen molecular weights to break signalling symmetry. The passive sensory ciliary deformation gives indications regarding the plausibility of this mechano-signalling mechanism.

Salient features of the ciliated organ of asymmetry

Many internal organs develop distinct left and right sides that are essential for their functions. In several vertebrate embryos, motile cilia generate an asymmetric fluid flow that plays an important role in establishing left-right (LR) signaling cascades. These ‘LR cilia’ are found in the ventral node and posterior notochordal plate in mammals, the gastrocoel roof plate in amphibians and Kupffer’s vesicle in teleost fish. I consider these transient ciliated structures as the ‘organ of asymmetry’ that directs LR patterning of the developing embryo. Variations in size and morphology of the organ of asymmetry in different vertebrate species have raised questions regarding the fundamental features that are required for LR determination. Here, I review current models for how LR asymmetry is established in vertebrates, discuss the cellular architecture of the ciliated organ of asymmetry and then propose key features of this organ that are critical for orienting the LR body axis.

Sept6 is required for ciliogenesis in Kupffer's vesicle, the pronephros, and the neural tube during early embryonic development

Septins are conserved filament-forming GTP-binding proteins that act as cellular scaffolds or diffusion barriers in a number of cellular processes. However, the role of septins in vertebrate development remains relatively obscure. Here, we show that zebrafish septin 6 (sept6) is first expressed in the notochord and then in nearly all of the ciliary organs, including Kupffer's vesicle (KV), the pronephros, eye, olfactory bulb, and neural tube. Knockdown of sept6 in zebrafish embryos results in reduced numbers and length of cilia in KV. Consequently, cilium-related functions, such as the left-right patterning of internal organs and nodal/spaw signaling, are compromised. Knockdown of sept6 also results in aberrant cilium formation in the pronephros and neural tube, leading to cilium-related defects in pronephros development and Sonic hedgehog (Shh) signaling. We further demonstrate that SEPT6 associates with acetylated α-tubulin in vivo and localizes along the axoneme in the cilia of zebrafish pronephric duct cells as well as cultured ZF4 cells. Our study reveals a novel role of sept6 in ciliogenesis during early embryonic development in zebrafish.

Analysis of maternal-zygotic ugdh mutants reveals divergent roles for HSPGs in vertebrate embryogenesis and provides new insight into the initiation of left-right asymmetry

Growth factors and morphogens regulate embryonic patterning, cell fate specification, cell migration, and morphogenesis. The activity and behavior of these signaling molecules are regulated in the extracellular space through interactions with proteoglycans (Bernfield et al., 1999; Perrimon and Bernfield 2000; Lander and Selleck 2000; Selleck 2000). Proteoglycans are high molecular-weight proteins consisting of a core protein with covalently linked glycosaminoglycan (GAG) side chains, which are thought to mediate ligand interaction. Drosophila mutant embryos deficient for UDP-glucose dehydrogenase activity (Ugdh, required for GAG synthesis) exhibit abnormal Fgf, Wnt and TGFß signaling and die during gastrulation, indicating a broad and critical role for proteoglycans during early embryonic development (Lin et al., 1999; Lin and Perrimon 2000) (Hacker et al., 1997). Mouse Ugdh mutants also die at gastrulation, however, only Fgf signaling appears disrupted (Garcia-Garcia and Anderson, 2003). These findings suggested a possible divergence in the requirement for proteoglycans during Drosophila and mouse embryogenesis, and that mammals may have evolved alternative means of regulating Wnt and TGFß activity. To further examine the function of proteoglycans in vertebrate development, we have characterized zebrafish mutants devoid of both maternal and zygotic Ugdh/Jekyll activity (MZjekyll). We demonstrate that MZjekyll mutant embryos display abnormal Fgf, Shh, and Wnt signaling activities, with concomitant defects in central nervous system patterning, cardiac ventricular fate specification and axial morphogenesis. Furthermore, we uncover a novel role for proteoglycans in left-right pattern formation. Our findings resolve longstanding questions into the evolutionary conservation of Ugdh function and provide new mechanistic insights into the initiation of left-right asymmetry.

It's never too early to get it Right: A conserved role for the cytoskeleton in left-right asymmetry

For centuries, scientists and physicians have been captivated by the consistent left-right (LR) asymmetry of the heart, viscera, and brain. A recent study implicated tubulin proteins in establishing laterality in several experimental models, including asymmetric chemosensory receptor expression in C. elegans neurons, polarization of HL-60 human neutrophil-like cells in culture, and asymmetric organ placement in Xenopus. The same mutations that randomized asymmetry in these diverse systems also affect chirality in Arabidopsis, revealing a remarkable conservation of symmetry-breaking mechanisms among kingdoms. In Xenopus, tubulin mutants only affected LR patterning very early, suggesting that this axis is established shortly after fertilization. This addendum summarizes and extends the knowledge of the cytoskeleton's role in the patterning of the LR axis. Results from many species suggest a conserved role for the cytoskeleton as the initiator of asymmetry, and indicate that symmetry is first broken during early embryogenesis by an intracellular process.

A unified model for left-right asymmetry? Comparison and synthesis of molecular models of embryonic laterality

Understanding how and when the left-right (LR) axis is first established is a fundamental question in developmental biology. A popular model is that the LR axis is established relatively late in embryogenesis, due to the movement of motile cilia and the resultant directed fluid flow during late gastrulation/early neurulation. Yet, a large body of evidence suggests that biophysical, molecular, and bioelectrical asymmetries exist much earlier in development, some as early as the first cell cleavage after fertilization. Alternative models of LR asymmetry have been proposed that accommodate these data, postulating that asymmetry is established due to a chiral cytoskeleton and/or the asymmetric segregation of chromatids. There are some similarities, and many differences, in how these various models postulate the origin and timing of symmetry breaking and amplification, and these events' linkage to the well-conserved subsequent asymmetric transcriptional cascades. This review examines experimental data that lend strong support to an early origin of LR asymmetry, yet are also consistent with later roles for cilia in the amplification of LR pathways. In this way, we propose that the various models of asymmetry can be unified: early events are needed to initiate LR asymmetry, and later events could be utilized by some species to maintain LR-biases. We also present an alternative hypothesis, which proposes that individual embryos stochastically choose one of several possible pathways with which to establish their LR axis. These two hypotheses are both tractable in appropriate model species; testing them to resolve open questions in the field of LR patterning will reveal interesting new biology of wide relevance to developmental, cell, and evolutionary biology.

Fluid dynamics of heart development

The morphology, muscle mechanics, fluid dynamics, conduction properties, and molecular biology of the developing embryonic heart have received much attention in recent years due to the importance of both fluid and elastic forces in shaping the heart as well as the striking relationship between the heart's evolution and development. Although few studies have directly addressed the connection between fluid dynamics and heart development, a number of studies suggest that fluids may play a key role in morphogenic signaling. For example, fluid shear stress may trigger biochemical cascades within the endothelial cells of the developing heart that regulate chamber and valve morphogenesis. Myocardial activity generates forces on the intracardiac blood, creating pressure gradients across the cardiac wall. These pressures may also serve as epigenetic signals. In this article, the fluid dynamics of the early stages of heart development is reviewed. The relevant work in cardiac morphology, muscle mechanics, regulatory networks, and electrophysiology is also reviewed in the context of intracardial fluid dynamics.

Regulation of basal body and ciliary functions by Diversin

The centrosome is essential for the formation of the cilia and has been implicated in cell polarization and signaling during early embryonic development. A number of Wnt pathway components were found to localize at the centrosome, but how this localization relates to their signaling functions is unclear. In this study, we assessed a role for Diversin, a putative Wnt pathway mediator, in developmental processes that involve cilia. We find that Diversin is specifically localized to the basal body compartment near the base of the cilium in Xenopus multi-ciliated skin cells. Overexpression of Diversin RNA disrupted basal body polarization in these cells, suggesting that tightly regulated control of Diversin levels is crucial for this process. In cells depleted of endogenous Diversin, basal body structure appeared abnormal and this was accompanied by disrupted polarity, shortened or absent cilia and defective ciliary flow. These results are consistent with the involvement of Diversin in processes that are related to the acquisition of cell polarity and require ciliary functions.

Retinoic acid signaling sequentially controls visceral and heart laterality in zebrafish

During zebrafish development, the left-right (LR) asymmetric signals are first established around the Kupffer vesicle (KV), a ciliated organ generating directional fluid flow. Then, LR asymmetry is conveyed and stabilized in the lateral plate mesoderm. Although numerous molecules and signaling pathways are involved in controlling LR asymmetry, mechanistic difference and concordance between different organs during LR patterning are poorly understood. Here we show that RA signaling regulates laterality decisions at two stages in zebrafish. Before the 2-somite stage (2So), inhibition of RA signaling leads to randomized visceral laterality through bilateral expression of nodal/spaw in the lateral plate mesoderm, which is mediated by increases in cilia length and defective directional fluid flow in KV. Fgf8 is required for the regulation of cilia length by RA signaling. Blockage of RA signaling before 2So also leads to mild defects of heart laterality, which become much more severe through perturbation of cardiac bmp4 asymmetry when RA signaling is blocked after 2So. At this stage, visceral laterality and the left-sided Nodal remain unaffected. These findings suggest that RA signaling controls visceral laterality through the left-sided Nodal signal before 2So, and regulates heart laterality through cardiac bmp4 mainly after 2So, first identifying sequential control and concordance of visceral and heart laterality.

Ciliary behaviour and mechano-transduction in the embryonic node: computational testing of hypotheses

Left-right symmetry breaking in the mammalian embryo is believed to occur in a transient embryonic structure, the node: rotational motion of cilia within this structure creates a leftward flow of liquid that is the first asymmetric event observed. A hypothesis, often referred to as the "two-cilia" hypothesis, proposes that the node contains two kinds of primary cilia: motile cilia, driven by motor proteins, that rotate clockwise generating the leftward flow and passive cilia that act as mechano-sensors, reacting mechanically to the emerging flow. The exact mechanism that underlies the initial breaking of symmetry remains unclear, in spite of several studies that have attempted to elucidate the processes involved. In this paper, we present two computational models to (i) simulate the unidirectional flow induced by the active ciliary motion as well as their propulsion on the passive cilia and to (ii) investigate the protein activity that produces the active ciliary rotation-like movement. The models presented incorporate methodologies from computational fluid dynamics, deformable mesh computational techniques and fluid-structure interaction analysis. By solving the three-dimensional unsteady transport equations, with suitable boundary conditions, we confirm that the whirling motion of active cilia is capable of inducing the unidirectional flow and that the passive cilia are pushed by this flow towards the left with a visible deformation of 41.7% of the ciliary length on the tip, supporting the plausibility of the two-cilia hypothesis. Further, by applying finite element analysis and grid deformation techniques, we investigate the ciliary motion triggered by the activation of protein motors and propose a possible dynein activation pattern that is able to produce the clockwise rotation of embryonic cilia.

Sox17 and chordin are required for formation of Kupffer's vesicle and left-right asymmetry determination in zebrafish

Kupffer's vesicle (KV), a ciliated fluid-filled sphere in the zebrafish embryo with a critical role in laterality determination, is derived from a group of superficial cells in the organizer region of the gastrula named the dorsal forerunner cells (DFC). We have examined the role of the expression of sox17 and chordin (chd) in the DFC in KV formation and laterality determination. Whereas sox17 was known to be expressed in DFC, its function in these cells was not studied before. Further, expression of chd in these cells has not been reported previously. Targeted knockdown of Sox17 and Chd in DFC led to aberrant Left-Right (L-R) asymmetry establishment, as visualized by the expression of southpaw and lefty, and heart and pancreas placement in the embryo. These defects correlated with the formation of small KVs with apparently diminished cilia, consistent with the known requirement for ciliary function in the laterality organ for the establishment of L-R asymmetry.

Polycystic liver diseases

Polycystic liver diseases (PCLDs) are genetic disorders with heterogeneous etiologies and a range of phenotypic presentations. PCLD exhibits both autosomal or recessive dominant pattern of inheritance and is characterized by the progressive development of multiple cysts, isolated or associated with polycystic kidney disease, that appear more extensive in women. Cholangiocytes have primary cilia, functionally important organelles (act as mechanosensors) that are involved in both normal developmental and pathological processes. The absence of polycystin-1, 2, and fibrocystin/polyductin, normally localized to primary cilia, represent a potential mechanism leading to cyst formation, associated with increased cell proliferation and apoptosis, enhanced fluid secretion, abnormal cell-matrix interactions, and alterations in cell polarity. Proliferative and secretive activities of cystic epithelium can be regulated by estrogens either directly or by synergizing growth factors including nerve growth factor, IGF1, FSH and VEGF. The abnormalities of primary cilia and the sensitivity to proliferative effects of estrogens and different growth factors in PCLD cystic epithelium provide the morpho-functional basis for future treatment targets, based on the possible modulation of the formation and progression of hepatic cysts.

Situs inversus in Dpcd/Poll-/-, Nme7-/- , and Pkd1l1-/- mice

Situs inversus (SI) is a congenital condition characterized by left-right transposition of thoracic and visceral organs and associated vasculature. The usual asymmetrical positioning of organs is established early in development in a transient structure called the embryonic node. The 2-cilia hypothesis proposes that 2 kinds of primary cilia in the embryonic node determine left-right asymmetry: motile cilia that generate a leftward fluid flow, and immotile mechanosensory cilia that respond to the flow. Here, we describe 3 mouse SI models that provide support for the 2-cilia hypothesis. In addition to having SI, Dpcd/Poll(-/-) mice (for: deleted in a mouse model of primary ciliary dyskinesia) and Nme7(-/-) mice (for: nonmetastatic cells 7) had lesions consistent with deficient ciliary motility: Hydrocephalus, sinusitis, and male infertility developed in Dpcd/Poll(-/-) mice, whereas hydrocephalus and excessive nasal exudates were seen in Nme7(-/-) mice. In contrast, the absence of respiratory tract lesions, hydrocephalus, and male infertility in Pkd1l1(-/-) mice (for: polycystic kidney disease 1 like 1) suggested that dysfunction of motile cilia was not involved in the development of SI in this line. Moreover, the gene Pkd1l1 has considerable sequence similarity with Pkd1 (for: polycystic kidney disease 1), which encodes a protein (polycystin-1) that is essential for the mechanosensory function of immotile primary cilia in the kidney. The markedly reduced viability of Pkd1l1(-/-) mice is somewhat surprising given the absence of any detected abnormalities (other than SI) in surviving Pkd1l1(-/-) mice subjected to a comprehensive battery of phenotype-screening exams. However, the heart and great vessels of Pkd1l1(-/-) mice were not examined, and it is possible that the decreased viability of Pkd1l1(-/-) mice is due to undiagnosed cardiovascular defects associated with heterotaxy.

Mouse dynein axonemal intermediate chain 2: cloning and expression

Ovarian follicular development is a complex process. Investigation of the mechanisms regulating the initiation of follicular growth, and the growth and differentiation of preantral follicles is of great interest. In an effort to clone follicular development-related genes, we selected a partial cDNA fragment by differential display reverse-transcription PCR using total RNA extracted from 5-day-old and 10-day-old mouse ovaries, and its open reading frame was obtained by rapid amplification of cDNA ends. Sequencing showed that the fragment is the mouse dynein axonemal intermediate chain 2 gene (Dnaic2), which has an 87% homology with human DNAI2, a candidate gene for primary ciliary dyskinesia. Northern and western analyses indicate that Dnaic2 produces an approximate 3 kb mRNA that is translated into an approximate 70 kDa protein. The mRNA is predominantly expressed in mouse ovary, testis, and lung. In mouse ovaries, Dnaic2 mRNA was detected at high levels in vivo on day 10, with a subsequent decrease on days 15 and 20, in adult and old ovaries. However, Dnaic2 expression was weak on day 5. Dnaic2 protein was localized on the surface of the oocyte. No obvious fluorescence signal was detected in primordial and primary follicles, while strong signals were detected on the oocyte surface of secondary and antral follicles, in particular for secondary follicles in day 10. These data suggest that Dnaic2 plays a role in ovarian follicular development.

The chirality of gut rotation derives from left-right asymmetric changes in the architecture of the dorsal mesentery

We have investigated the structural basis by which the counterclockwise direction of the amniote gut is established. The chirality of midgut looping is determined by left-right asymmetries in the cellular architecture of the dorsal mesentery, the structure that connects the primitive gut tube to the body wall. The mesenchymal cells of the dorsal mesentery are more condensed on the left side than on the right and, additionally, the overlying epithelium on the left side exhibits a columnar morphology, in contrast to a cuboidal morphology on the right. These properties are instructed by a set of transcription factors: Pitx2 and Isl1 specifically expressed on the left side, and Tbx18 expressed on the right, regulated downstream of the secreted protein Nodal which is present exclusively on the left side. The resultant differences in cellular organization cause the mesentery to assume a trapezoidal shape, tilting the primitive gut tube leftward.

Ttrap is an essential modulator of Smad3-dependent Nodal signaling during zebrafish gastrulation and left-right axis determination

During vertebrate development, signaling by the TGFbeta ligand Nodal is critical for mesoderm formation, correct positioning of the anterior-posterior axis, normal anterior and midline patterning, and left-right asymmetric development of the heart and viscera. Stimulation of Alk4/EGF-CFC receptor complexes by Nodal activates Smad2/3, leading to left-sided expression of target genes that promote asymmetric placement of certain internal organs. We identified Ttrap as a novel Alk4- and Smad3-interacting protein that controls gastrulation movements and left-right axis determination in zebrafish. Morpholino-mediated Ttrap knockdown increases Smad3 activity, leading to ectopic expression of snail1a and apparent repression of e-cadherin, thereby perturbing cell movements during convergent extension, epiboly and node formation. Thus, although the role of Smad proteins in mediating Nodal signaling is well-documented, the functional characterization of Ttrap provides insight into a novel Smad partner that plays an essential role in the fine-tuning of this signal transduction cascade.

Endothelial primary cilia in areas of disturbed flow are at the base of atherosclerosis

Atherosclerosis develops in the arterial system at sites of low as well as low and oscillating shear stress. Previously, we demonstrated a shear-related distribution of ciliated endothelial cells in the embryonic cardiovascular system and postulated that the primary cilium is a component of the shear stress sensor, functioning as a signal amplifier. This shear-related distribution is reminiscent of the atherosclerotic predilection sites. Thus, we determined whether a link exists between location and frequency of endothelial primary cilia and atherogenesis. We analyzed endothelial ciliation of the adult aortic arch and common carotid arteries of wild type C57BL/6 and apolipoprotein-E-deficient mice. Primary cilia are located at the atherosclerotic predilection sites, where flow is disturbed, in wild type mice and they occur on and around atherosclerotic lesions in apolipoprotein-E-deficient mice, which have significantly more primary cilia in the aortic arch than wild type mice. In addition, common carotid arteries were challenged for shear stress by application of a restrictive cast, resulting in the presence of primary cilia only at sites of induced low and disturbed shear. In conclusion, these data relate the presence of endothelial primary cilia to regions of atherogenesis, where they increase in number under hyperlipidemia-induced lesion formation. Experimentally induced flow disturbance leads to induction of primary cilia, and subsequently to atherogenesis, which suggests a role for primary cilia in endothelial activation and dysfunction.

The role of shear stress on ET-1, KLF2, and NOS-3 expression in the developing cardiovascular system of chicken embryos in a venous ligation model

In this review, the role of wall shear stress in the chicken embryonic heart is analyzed to determine its effect on cardiac development through regulating gene expression. Therefore, background information is provided for fluid dynamics, normal chicken and human heart development, cardiac malformations, cardiac and vitelline blood flow, and a chicken model to induce cardiovascular anomalies. A set of endothelial shear stress-responsive genes coding for endothelin-1 (ET-1), lung Krüppel-like factor (LKLF/KLF2), and endothelial nitric oxide synthase (eNOS/NOS-3) are active in development and are specifically addressed.

Cholangiocyte cilia detect changes in luminal fluid flow and transmit them into intracellular Ca2+ and cAMP signaling

BACKGROUND & AIMS

Cholangiocytes have primary cilia extending from the apical plasma membrane into the ductal lumen. While the physiologic significance of cholangiocyte cilia is unknown, studies in renal epithelia suggest that primary cilia possess sensory functions. Here, we tested the hypothesis that cholangiocyte cilia are sensory organelles that detect and transmit luminal bile flow stimuli into intracellular Ca2+ ([Ca2+]i) and adenosine 3',5'-cyclic monophosphate (cAMP) signaling.

METHODS

Scanning electron microscopy, transmission electron microscopy, and immunofluorescent confocal microscopy of rat isolated intrahepatic bile duct units (IBDUs) were used to detect and characterize cholangiocyte cilia. The fluid flow-induced changes in Ca2+ and cAMP levels in cholangiocytes of microperfused IBDUs were detected by epifluorescence microscopy and a fluorescence assay, respectively.

RESULTS

In microperfused IBDUs, luminal fluid flow induced an increase in [Ca2+]i and caused suppression of the forskolin-stimulated cAMP increase. The fluid flow-induced changes in [Ca2+]i and cAMP levels were significantly reduced or abolished when cilia were removed by chloral hydrate or when ciliary-associated proteins polycystin-1 (a mechanoreceptor), polycystin-2 (a Ca2+ channel), and the Ca2+-inhibitable adenylyl cyclase isoform 6 were individually down-regulated by small interfering RNAs.

CONCLUSIONS

Cholangiocyte cilia are sensory organelles containing polycystin-1, polycystin-2, and adenylyl cyclase isoform 6 through which luminal fluid flow affects both [Ca2+]i and cAMP signaling in the cell. The data suggest a new model for regulation of ductal bile secretion involving cholangiocyte cilia.

The primary cilium as the cell's antenna: signaling at a sensory organelle

Almost every vertebrate cell has a specialized cell surface projection called a primary cilium. Although these structures were first described more than a century ago, the full scope of their functions remains poorly understood. Here, we review emerging evidence that in addition to their well-established roles in sight, smell, and mechanosensation, primary cilia are key participants in intercellular signaling. This new appreciation of primary cilia as cellular antennae that sense a wide variety of signals could help explain why ciliary defects underlie such a wide range of human disorders, including retinal degeneration, polycystic kidney disease, Bardet-Biedl syndrome, and neural tube defects.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

Lack of a laterality phenotype in Pkd1 knock-out embryos correlates with absence of polycystin-1 in nodal cilia

The invariant asymmetric placement of thoracic and abdominal organs in the vertebrates is controlled by the left-asymmetric activity of the Nodal signaling cascade during embryogenesis. In the mouse embryo asymmetric induction of nodal is thought to be dependent on functional monocilia on the ventral node cells and on the Pkd2 gene, which encodes the calcium channel polycystin-2 (PC2). In humans mutations in PKD2 and PKD1 give rise to polycystic kidney disease. The PC1 and PC2 proteins are thought to function as part of a multifactorial complex. Localization of both proteins to the primary renal cilium suggested a function on cilia of the ventral node. Here we investigated Pkd1 knock-out embryos for laterality defects and found wild-type organ morphogenesis and normal expression of nodal and Pitx2. While PC2 localized to nodal cilia, no ciliary localization of PC1 was detected in mouse embryos. This finding was confirmed in an archetypical mammalian blastodisc, the rabbit embryo. Thus, absence of PC1 localization to cilia corresponded with a lack of laterality defects in Pkd1 knock-out embryos. Our results demonstrate a PC1-independent function of PC2 in left-right axis formation, and indirectly support a ciliary role of PC2 in this process.

Alpha-actinin associates with polycystin-2 and regulates its channel activity

Polycystin-2 (PC2) is the product of the PKD2 gene, which is mutated in 10-15% patients of autosomal dominant polycystic kidney disease (ADPKD). PC2 is an integral transmembrane protein and acts as a calcium-permeable cation channel. The functional modulation of this channel by other protein partners remains largely unknown. In the present study, using a yeast two-hybrid approach, we discovered that both intracellular N- and C-termini of PC2 associate with alpha-actinins, actin-binding and actin-bundling proteins important in cytoskeleton organization, cell adhesion, proliferation and migration. The PC2-alpha-actinin association was confirmed by in vitro glutathione S-transferase pull-down and dot blot overlay assays. In addition, the in vivo interaction between endogenous PC2 and alpha-actinins was demonstrated by co-immunoprecipitation in human embryonic kidney 293 and Madin-Darby canine kidney (MDCK) cells, rat kidney and heart tissues and human syncytiotrophoblast (hST) apical membrane vesicles. Immunofluorescence experiments showed that PC2 and alpha-actinin were partially co-localized in epithelial MDCK and inner medullary collecting duct cells, NIH 3T3 fibroblasts and hST vesicles. We studied the functional modulation of PC2 by alpha-actinin in a lipid bilayer electrophysiology system using in vitro translated PC2 and found that alpha-actinin substantially stimulated the channel activity of reconstituted PC2. A similar stimulatory effect of alpha-actinin on PC2 was also observed when hST vesicles were reconstituted in lipid bilayer. Thus, physical and functional interactions between PC2 and alpha-actinin may play an important role in abnormal cell adhesion, proliferation and migration observed in ADPKD.

Unveiling the establishment of left-right asymmetry in the chick embryo

Vertebrates display striking left-right asymmetries in the placement of internal organs, which are concealed by a seemingly bilaterally symmetric body plan. The establishment of asymmetries about the left-right axis occurs early during embryo development and requires the concerted and sequential action of several epigenetic, genetic and cellular mechanisms. Experiments in the chick embryo model have contributed crucially to our current understanding of such mechanisms and are reviewed here. Particular emphasis is given to the elucidation of a genetic network that conveys left-right information from Hensen's node to the organ primordia, characterized to a significant degree of detail in the chick embryo. We also point out a number of early and late events in the determination of left-right asymmetries that are currently poorly understood and for whose study the chick embryo model presents several advantages. We anticipate that the availability of the chick genome sequence will be combined with multidisciplinary approaches from experimental embryology, biophysics, live-cell imaging, and mathematical modeling to boost up our knowledge of left-right organ asymmetry in the near future.

Kupffer's vesicle is a ciliated organ of asymmetry in the zebrafish embryo that initiates left-right development of the brain, heart and gut

Monocilia have been proposed to establish the left-right (LR) body axis in vertebrate embryos by creating a directional fluid flow that triggers asymmetric gene expression. In zebrafish, dorsal forerunner cells (DFCs) express a conserved ciliary dynein gene (left-right dynein-related1, lrdr1) and form a ciliated epithelium inside a fluid-filled organ called Kupffer's vesicle (KV). Here, videomicroscopy demonstrates that cilia inside KV are motile and create a directional fluid flow just prior to the onset of asymmetric gene expression in lateral cells. Laser ablation of DFCs and surgical disruption of KV provide direct evidence that ciliated KV cells are required during early somitogenesis for subsequent LR patterning in the brain, heart and gut. Antisense morpholinos against lrdr1 disrupt KV fluid flow and perturb LR development. Furthermore, lrdr1 morpholinos targeted to DFC/KV cells demonstrate that Lrdr1 functions in these ciliated cells to control LR patterning. This provides the first direct evidence, in any vertebrate, that impairing cilia function in derivatives of the dorsal organizer, and not in other cells that express ciliogenic genes, alters LR development. Finally, genetic analysis reveals novel roles for the T-box transcription factor no tail and the Nodal signaling pathway as upstream regulators of lrdr1 expression and KV morphogenesis. We propose that KV is a transient embryonic 'organ of asymmetry' that directs LR development by establishing a directional fluid flow. These results suggest that cilia are an essential component of a conserved mechanism that controls the transition from bilateral symmetry to LR asymmetry in vertebrates.

Monocilia on chicken embryonic endocardium in low shear stress areas

During cardiovascular development, fluid shear stress patterns change dramatically due to extensive remodeling. This biomechanical force has been shown to drive gene expression in endothelial cells and, consequently, is considered to play a role in cardiovascular development. The mechanism by which endothelial cells sense shear stress is still unidentified. In this study, we postulate that primary cilia function as fluid shear stress sensors of endothelial cells. Such a function already has been attributed to primary cilia on epithelial cells of the adult kidney and of Hensen's node in the embryo where they transduce mechanical signals into an intracellular Ca2+ signaling response. Recently, primary cilia were observed on human umbilical vein endothelial cells. These primary cilia disassembled when subjected to high shear stress levels. Whereas endocardial-endothelial cells have been reported to be more shear responsive than endothelial cells, cilia are not detected, thus far, on endocardial cells. In the present study, we use field emission scanning electron microscopy to show shear stress-related regional differences in cell protrusions within the cardiovasculature of the developing chicken. Furthermore, we identify one of these cell protrusions as a monocilium with monoclonal antibodies against acetylated and detyrosinated alpha-tubulin. The distribution pattern of the monocilia was compared to the chicken embryonic expression pattern of the high shear stress marker Krüppel-like factor-2. We demonstrate the presence of monocilia on endocardial-endothelial cells in areas of low shear stress and postulate that they are immotile primary cilia, which function as fluid shear stress sensors.

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.