Randomization of left-right asymmetry due to loss of nodal cilia generating leftward flow of extraembryonic fluid in mice lacking KIF3B motor protein

Regulatory mechanisms governing GLI proteins in hedgehog signaling

The Hedgehog (Hh) signaling pathway is critical for regulating cell growth, survival, fate determination, and the overall patterning of both vertebrate and invertebrate body plans. Aberrations in Hh signaling are associated with congenital abnormalities and tumorigenesis. In vertebrates, Hh signaling depends uniquely on primary cilia, microtubule-based organelles that extend from the cell surface. Over the last 2 decades, studies have demonstrated that key molecules regulating Hh signaling dynamically accumulate in primary cilia via intraflagellar transport systems. Moreover, through the primary cilia, extracellular signals are converted to stabilize GLI2 and GLI3 that are transcription factors that play a central role in regulating Hh signaling at the post-translational modification level. Recent in vivo and anatomical studies have uncovered crucial molecules that facilitate the conversion of extracellular signals into the intracellular stabilization of GLI2/GLI3 via primary cilia, emphasizing their essential roles in tissue development and tumorigenesis. This review explores the regulatory mechanisms of GLI2/GLI3 with a focus on mammalian tissue development.

The intraflagellar transport cycle

Primary and motile cilia are eukaryotic organelles that perform crucial roles in cellular signalling and motility. Intraflagellar transport (IFT) contributes to the formation of the highly specialized ciliary proteome by active and selective transport of soluble and membrane proteins into and out of cilia. IFT is performed by the IFT-A and IFT-B protein complexes, which together link cargoes to the microtubule motors kinesin and dynein. In this Review, we discuss recent structural and mechanistic insights on how the IFT complexes are first recruited to the base of the cilium, how they polymerize into an anterograde IFT train, and how this complex imports cargoes from the cytoplasm. We will describe insights into how kinesin-driven anterograde trains are carried to the ciliary tip, where they are remodelled into dynein-driven retrograde trains for cargo export. We will also present how the interplay between IFT-A and IFT-B complexes, motor proteins and cargo adaptors is regulated for bidirectional ciliary transport.

Lagging Brain Gene Expression Patterns of Drosophila melanogaster Young Adult Males Confound Comparisons Between Sexes

Many species, including fruit flies (Drosophila melanogaster), are sexually dimorphic. Phenotypic variation in morphology, physiology, and behavior can affect development, reproduction, health, and aging. Therefore, designating sex as a variable and sex-blocking should be considered when designing experiments. The brain regulates phenotypes throughout the lifespan by balancing survival and reproduction, and sex-specific development at each life stage is likely. Changes in morphology and physiology are governed by differential gene expression, a quantifiable molecular marker for age- and sex-specific variations. We assessed the fruit fly brain transcriptome at three adult ages for gene expression signatures of sex, age, and sex-by-age: 6698 genes were differentially expressed between sexes, with the most divergence at 3 days. Between ages, 31.1% of 6084 differentially expressed genes (1890 genes) share similar expression patterns from 3 to 7 days in females, and from 7 to 14 days in males. Most of these genes (90.5%, 1712) were upregulated and enriched for chemical stimulus detection and/or cilium regulation. Our data highlight an important delay in male brain gene regulation compared to females. Because significant delays in expression could confound comparisons between sexes, studies of sexual dimorphism at phenotypically comparable life stages rather than chronological age should be more biologically relevant.

Notch3 is an asymmetric gene and a modifier of heart looping defects in Nodal mouse mutants

The TGFβ secreted factor NODAL is a major left determinant required for the asymmetric morphogenesis of visceral organs, including the heart. Yet, when this signaling is absent, shape asymmetry, for example of the embryonic heart loop, is not fully abrogated, indicating that there are other factors regulating left-right patterning. Here, we used a tailored transcriptomic approach to screen for genes asymmetrically expressed in the field of heart progenitors. We thus identify Notch3 as a novel left-enriched gene and validate, by quantitative in situ hybridization, its transient asymmetry in the lateral plate mesoderm and node crown, overlapping with Nodal. In mutant embryos, we analyzed the regulatory hierarchy and demonstrate that Nodal in the lateral plate mesoderm amplifies Notch3 asymmetric expression. The function of Notch3 was uncovered in an allelic series of mutants. In single neonate mutants, we observe that Notch3 is required with partial penetrance for ventricle thickness, septation and aortic valve, in addition to its known role in coronary arteries. In compound mutants, we reveal that Notch3 acts as a genetic modifier of heart looping direction and shape defects in Nodal mutants. Whereas Notch3 was previously mainly associated with the CADASIL syndrome, our observations in the mouse and a human cohort support a novel role in congenital heart defects and laterality defects.

Bilateral cellular flows display asymmetry prior to left-right organizer formation in amniote gastrulation

A bilateral body plan is predominant throughout the animal kingdom. Bilaterality of amniote embryos becomes recognizable as midline morphogenesis begins at gastrulation, bisecting an embryonic field into the left and right sides, and left-right (LR) asymmetry patterning follows. While a series of laterality genes expressed after the LR compartmentalization has been extensively studied, the laterality patterning prior to and at the initiation of midline morphogenesis has remained unclear. Here, through a biophysical quantification in a high spatial and temporal resolution, applied to a chick model system, we show that a large-scale bilateral counterrotating cellular flow, termed "polonaise movements", display LR asymmetries in early gastrulation. This cell movement starts prior to the formation of the primitive streak (PS) (the earliest midline structure) and the subsequent appearance of Hensen's node (the LR organizer). The cellular flow speed and vorticity unravel the location and timing of the LR asymmetries. The bilateral flows displayed a Right dominance after 6 h since the start of cell movements. Mitotic arrest that diminishes PS formation resulted in changes in the bilateral flow pattern, but the Right dominance persisted. Our data indicate that the LR asymmetry in amniote gastrula becomes detectable earlier than suggested by current models, which assume that the asymmetric regulation of the laterality signals at the node leads to the LR patterning. More broadly, our results suggest that physical processes can play an unexpected but significant role in influencing LR laterality during embryonic development.

CIROZ is dispensable in ancestral vertebrates but essential for left-right patterning in humans

Four genes-DAND5, PKD1L1, MMP21, and CIROP-form a genetic module that has specifically evolved in vertebrate species that harbor motile cilia in their left-right organizer (LRO). We find here that CIROZ (previously known as C1orf127) is also specifically expressed in the LRO of mice, frogs, and fish, where it encodes a protein with a signal peptide followed by 3 zona pellucida N domains, consistent with extracellular localization. We report 16 individuals from 10 families with bi-allelic CIROZ inactivation variants, which cause heterotaxy with congenital heart defects. While the knockout of Ciroz in mice also leads to situs anomalies, we unexpectedly find that its targeted inactivation in zebrafish and Xenopus does not lead to observable LR anomalies. Moreover, CIROZ is absent or obsolete in select animals with motile cilia at their LRO, including Carnivora, Atherinomorpha fish, or jawless vertebrates. In summary, this evo-devo study identifies CIROZ as an essential gene for breaking bilateral embryonic symmetry in humans and mice, whereas we witness its contemporary pseudogenization in discrete vertebrate species.

Organizer activity in the mouse embryo

The discovery of the embryonic organizer by Hilde Mangold and Hans Spemann in 1924 was one of the most ground-breaking achievements in the 1900 century for developmental biologists and beyond. Ever since the organizer was first described in newts, developmental biologists have been trying to uncover similar structures in other organisms. While the Spemann-Mangold organizer as an axis-inducing centre is evolutionary conserved in vertebrates, similar organizing centres have yet to be observed in mammals. In this review, we will provide a brief historical overview of the discovery of the mouse gastrula organizer and discuss its potential as an organizer throughout early post-implantation mouse development. We discuss cell migrations through the mouse organizer region and morphogenesis of organizer cells and tissues. Finally, we examine the evidence arguing for and against the existence of a head organizer in mice, and the role of the anterior visceral endoderm and the prechordal plate in organizing head structures.

Disruption of Iqsec1 in mice leads to embryonic lethality with reduced large apical vacuoles in the visceral endoderm

Iqsec1 (IQ motif and Sec7 domain-containing protein 1), also known as BRAG2 (Brefeldin A-resistant Arf-GEF 2), is a guanine nucleotide exchange factor that regulates membrane trafficking, cytoskeletal organization, and signal transduction by activating class II and III ADP-ribosylation factors. To investigate the physiological role of Iqsec1 at the whole animal level, we generated Iqsec1-deficient mice using CRISPR/Cas9-mediated gene editing. Nearly all Iqsec1-/- mice (99%) exhibited embryonic lethality with severe growth retardation. Electron microscopy revealed that Iqsec1-/- embryos at embryonic day 8.5 lacked large apical vacuoles in visceral endoderm cells of the yolk sac, compared with controls. These findings suggest that Iqsec1 plays a critical role in embryogenesis, likely through regulation of membrane trafficking in visceral endoderm cells.

ciBAR1 loss in mice causes laterality defects, pancreatic degeneration, and altered glucose tolerance

Bin/Amphiphysin/Rvs (BAR) domains are highly conserved domains found in all eukaryotes. BAR domain proteins form crescent-shaped dimers that sense and sculpt curved lipid membranes and play key roles in various cellular processes. However, their functions in mammalian development are poorly understood. We previously demonstrated that Chibby1-interacting BAR domain-containing 1 (ciBAR1, formerly known as FAM92A) localizes to the ciliary base and plays a critical role in ciliogenesis. Here, we report ciliopathy phenotypes of ciBAR1-KO mice. We found that ∼28% of ciBAR1-KO mice show embryonic lethality because of randomized left-right asymmetry; the rest survive into adulthood with no gross morphological abnormalities. Histological assessments of ciliated tissues revealed exocrine pancreatic lesions. Although overall endocrine islet morphology appeared to be normal, ciBAR1-KO mice showed impaired glucose tolerance. Examination of ductal and islet cilia revealed that cilia number and length were significantly reduced in ciBAR1-KO pancreata. ciBAR1-KO MEFs also exhibited ciliary defects. Our findings indicate that ciBAR1 plays a critical role in ciliogenesis depending on the tissue and cell type in mice.

Mechanism and regulation of kinesin motors

Kinesins are a diverse superfamily of microtubule-based motors that perform fundamental roles in intracellular transport, cytoskeletal dynamics and cell division. These motors share a characteristic motor domain that powers unidirectional motility and force generation along microtubules, and they possess unique tail domains that recruit accessory proteins and facilitate oligomerization, regulation and cargo recognition. The location, direction and timing of kinesin-driven processes are tightly regulated by various cofactors, adaptors, microtubule tracks and microtubule-associated proteins. This Review focuses on recent structural and functional studies that reveal how members of the kinesin superfamily use the energy of ATP hydrolysis to transport cargoes, depolymerize microtubules and regulate microtubule dynamics. I also survey how accessory proteins and post-translational modifications regulate the autoinhibition, cargo binding and motility of some of the best-studied kinesins. Despite much progress, the mechanism and regulation of kinesins are still emerging, and unresolved questions can now be tackled using newly developed approaches in biophysics and structural biology.

Cryo-electron tomography of eel sperm flagella reveals a molecular "minimum system" for motile cilia

Cilia and flagella play a crucial role in the development and function of eukaryotes. The activity of thousands of dyneins is precisely regulated to generate flagellar motility. The complex proteome (600+ proteins) and architecture of the structural core of flagella, the axoneme, have made it challenging to dissect the functions of the different complexes, like the regulatory machinery. Previous reports suggested that the flagellum of American eel sperm lacks many of the canonical axonemal complexes yet is still motile. Here, we use cryo-electron tomography for molecular characterization of this proposed "minimal" motile flagellum. We observed different diameters for the eel sperm flagellum: narrow at the base and wider toward the flagellar tip. Subtomogram averaging revealed the three-dimensional (3D) structure of the eel sperm flagellum. As expected, major complexes were missing, for example, outer dynein arms, radial spokes, and the central pair complex, but we found molecular remnants of most complexes. We also identified bend direction-specific patterns in the inter-DMT distance in actively beating eel sperm flagella and we propose a model for the regulation of dynein activity during their motility. Together, our results shed light on the structure and function of the eel sperm flagellum and provide insight into the minimum requirements for ciliary beating.

Unveiling the potential of lung transplantation for situs inversus

INTRODUCTION

Situs inversus is a rare congenital condition where the organs in the chest and abdomen are reversed, thus complicating surgeries such as lung transplantation. Kartagener syndrome (KS), associated with situs inversus, includes chronic sinusitis and bronchiectasis, which can progress to end-stage lung disease requiring transplantation. This review discusses the unique surgical considerations, technical challenges, and outcomes of lung transplantation in patients with situs inversus, particularly KS.

AREAS COVERED

The review highlights anatomical and physiological challenges in lung transplantation due to reversed organ positioning, requiring customized surgical approaches and intraoperative modifications. Preoperative imaging, anesthesia adjustments, and tailored surgical techniques are crucial for successful transplantation. Postoperative care focuses on managing complications such as primary graft dysfunction, infections, and anastomotic issues. Literature on survival rates, chronic lung allograft dysfunction, and quality of life is analyzed, indicating outcomes comparable to other lung transplant recipients.

EXPERT OPINION

Despite significant challenges, lung transplantation in patients with situs inversus and KS is feasible with outcomes similar to traditional cases. Advances in imaging, surgical planning, and minimally invasive techniques offer promise for improved outcomes. Ongoing research, collaboration, and ethical considerations are essential to optimizing care and expand treatment possibilities for this high-risk patient population.

The molecular mechanisms of cardiac development and related diseases

Cardiac development is a complex and intricate process involving numerous molecular signals and pathways. Researchers have explored cardiac development through a long journey, starting with early studies observing morphological changes and progressing to the exploration of molecular mechanisms using various molecular biology methods. Currently, advancements in stem cell technology and sequencing technology, such as the generation of human pluripotent stem cells and cardiac organoids, multi-omics sequencing, and artificial intelligence (AI) technology, have enabled researchers to understand the molecular mechanisms of cardiac development better. Many molecular signals regulate cardiac development, including various growth and transcription factors and signaling pathways, such as WNT signaling, retinoic acid signaling, and Notch signaling pathways. In addition, cilia, the extracellular matrix, epigenetic modifications, and hypoxia conditions also play important roles in cardiac development. These factors play crucial roles at one or even multiple stages of cardiac development. Recent studies have also identified roles for autophagy, metabolic transition, and macrophages in cardiac development. Deficiencies or abnormal expression of these factors can lead to various types of cardiac development abnormalities. Nowadays, congenital heart disease (CHD) management requires lifelong care, primarily involving surgical and pharmacological treatments. Advances in surgical techniques and the development of clinical genetic testing have enabled earlier diagnosis and treatment of CHD. However, these technologies still have significant limitations. The development of new technologies, such as sequencing and AI technologies, will help us better understand the molecular mechanisms of cardiac development and promote earlier prevention and treatment of CHD in the future.

Breaking Left-Right Symmetry by the Interplay of Planar Cell Polarity, Calcium Signaling and Cilia

The formation of the embryonic left-right axis is a fundamental process in animals, which subsequently conditions both the shape and the correct positioning of internal organs. During vertebrate early development, a transient structure, known as the left-right organizer, breaks the bilateral symmetry in a manner that is critically dependent on the activity of motile and immotile cilia or asymmetric cell migration. Extensive studies have partially elucidated the molecular pathways that initiate left-right asymmetric patterning and morphogenesis. Wnt/planar cell polarity signaling plays an important role in the biased orientation and rotational motion of motile cilia. The leftward fluid flow generated in the cavity of the left-right organizer is sensed by immotile cilia through complex mechanisms to trigger left-sided calcium signaling and lateralized gene expression pattern. Disrupted asymmetric positioning or impaired structure and function of cilia leads to randomized left-right axis determination, which is closely linked to laterality defects, particularly congenital heart disease. Despite of the formidable progress made in deciphering the critical contribution of cilia to establishing the left-right asymmetry, a strong challenge remains to understand how cilia generate and sense fluid flow to differentially activate gene expression across the left-right axis. This review analyzes mechanisms underlying the asymmetric morphogenesis and function of the left-right organizer in left-right axis formation. It also aims to identify important questions that are open for future investigations.

Targeted Deletion in the Basal Body Protein Talpid3 Leads to Loss of Primary Cilia in Embryonic Stem Cells and Defective Lineage-Specific Differentiation

Talpid3 is a basal body protein required for the formation of primary cilia, an organelle involved in signal transduction. Here, we asked if Talpid3 has a role in the regulation of differentiation and/or self-renewal of ES cells and whether cells lacking cilia due to a deletion in Talpid3 can be reprogrammed to induced pluripotent stem (iPS) cells. We show that mouse embryonic limb fibroblasts which lack primary cilia with a targeted deletion in the Talpid3 (Ta3) gene can be efficiently reprogrammed to iPS cells. Furthermore, vector-free Ta3-/- iPS cells retain ES cell features and are able to self-renew. However, both Ta3-/- iPS and ES cells are unable to form visceral endoderm and differentiate poorly into neurons. The observed defects are not a consequence of reprogramming since Ta3-/- ES cells also exhibit this phenotype. Thus, Talpid3 and primary cilia are required for some differentiation events but appear to be dispensable for stem cell self-renewal and reprogramming.

A New Method for Lateral Visualization of the Primary Cilia on the Surfaces of Cells Cultured on White Glass Rods

Purpose

To investigate the motility of the primary cilia of corneal endothelial cells (CECs), which exist like a hair on the cell surface, using our new in vitro method.

Methods

A white glass rod was heated with a gas burner to produce a rod approximately 0.5 mm in diameter and 20 mm in length and then coated with collagen. A suspension of cultured human CECs (HCECs) was then added to the rod and cultured for 20 days. Cells on the rod's side were then observed using phase-contrast microscopy, and videos and images of the primary cilia were obtained. After fixing the cells cultured on the rod's surface, immunofluorescence staining was performed and fluorescence and phase contrast images were taken.

Results

Hair-like structures were observed on the surface of live HCECs on the rod's surface. Video images revealed that the structures sometimes swayed owing to slight convection of the medium, yet had no motile function, and immunostaining with acetylated α-tubulin antibody confirmed that the structures were primary cilia.

Conclusions

Our new method using white glass rods provided the ability to observe the movement of primary cilia in cultured living HCECs, and the findings clearly showed that the primary cilia of HCECs are passive rather than motile. This novel procedure can be applied widely to other cultured cells as a method to observe the movement of primary cilia from the lateral aspect of the cell.

Translational Relevance

This method may help to clarify the role of primary cilia in the anterior chamber.

Spatiotemporal dynamics of primary and motile cilia throughout lung development

Cilia are specialized structures found on a variety of mammalian cells, with variable roles in the transduction of mechanical and biological signals (by primary cilia, PC), as well as the generation of fluid flow (by motile cilia). Their critical role in the establishment of a left-right axis in early development is well described, as is the innate immune function of multiciliated upper airway epithelium. By contrast, the dynamics of ciliary status during organogenesis and postnatal development is largely unknown. In this study, we define the progression of ciliary status within the endothelium, epithelium, and mesenchyme of the lung. Remarkably, we find that endothelial cells (ECs) lack PC at all stages of development, except in low numbers in the most proximal portions of the pulmonary arteries. In the lung epithelium, a proximodistal ciliary gradient is established over time, as the uniformly mono-ciliated epithelium transitions into proximal, multiciliated cells, and the distal alveolar epithelium loses its cilia. Mesenchymal cells, interestingly, are uniformly ciliated in early development, but with restriction to PDGFRα+ fibroblasts in the adult alveoli. This dynamic process in multiple cellular populations both challenges prior assertions that PC are found on all cells, and highlights a need to understand their spatiotemporal functions.

Pathway to independence: perspectives on the future

In this Perspective, our 2024 Pathway to Independence Fellows provide their thoughts on the future of their field. Covering topics as diverse as plant development, tissue engineering and adaptation to climate change, and using a wide range of experimental organisms, these talented postdocs showcase some of the major open questions and key challenges across the spectrum of developmental biology research.

Left-Right Asymmetry in Invertebrates: From Molecules to Organisms

Although most animals appear symmetric externally, they exhibit chirality within their body cavity, i.e., in terms of asymmetric organ position, directional organ looping, and lateralized organ function. Left-right (LR) asymmetry is determined genetically by intricate molecular interactions that occur during development. Key genes have been elucidated in several species. There are common mechanisms in vertebrates and invertebrates, but some appear to exhibit unique mechanisms. This review focuses on LR asymmetry formation in invertebrates, particularly Drosophila, ascidians, and mollusks. It aims to understand the role of the genes that are key to creating LR asymmetry and how chirality information is converted/transmitted across the hierarchies from molecules to cells and from cells to tissues.

CC2D1A causes ciliopathy, intellectual disability, heterotaxy, renal dysplasia, and abnormal CSF flow

Intellectual and developmental disabilities result from abnormal nervous system development. Over a 1,000 genes have been associated with intellectual and developmental disabilities, driving continued efforts toward dissecting variant functionality to enhance our understanding of the disease mechanism. This report identified two novel variants in CC2D1A in a cohort of four patients from two unrelated families. We used multiple model systems for functional analysis, including Xenopus, Drosophila, and patient-derived fibroblasts. Our experiments revealed that cc2d1a is expressed explicitly in a spectrum of ciliated tissues, including the left-right organizer, epidermis, pronephric duct, nephrostomes, and ventricular zone of the brain. In line with this expression pattern, loss of cc2d1a led to cardiac heterotaxy, cystic kidneys, and abnormal CSF circulation via defective ciliogenesis. Interestingly, when we analyzed brain development, mutant tadpoles showed abnormal CSF circulation only in the midbrain region, suggesting abnormal local CSF flow. Furthermore, our analysis of the patient-derived fibroblasts confirmed defective ciliogenesis, further supporting our observations. In summary, we revealed novel insight into the role of CC2D1A by establishing its new critical role in ciliogenesis and CSF circulation.

Function of nodal cilia in left-right determination: Mechanical regulation in initiation of symmetry breaking

Visceral organs in vertebrates are arranged with left-right asymmetry; for example, the heart is located on the left side of the body. Cilia at the node of mouse early embryos play an essential role in determining this left-right asymmetry. Using information from the anteroposterior axis, motile cilia at the central region of the node generate leftward nodal flow. Immotile cilia at the periphery of the node mechanically sense the direction of leftward nodal flow in a manner dependent on the polarized localization of Pkd2, which is localized on the dorsal side of cilia. Therefore, only left-side cilia are activated by leftward nodal flow. This activation results in frequent calcium transients in the cilia via the Pkd2 channel, which leads to the degradation of Dand5 mRNA only at the left-side crown-cells. This process is the mechanism of initial determination of the left-side-specific signal. In this review, we provide an overview of initial left-right symmetry breaking that occurs at the node, focusing mainly on a recent biophysical study that revealed the function of nodal immotile cilia using advanced microscopic techniques, such as optical tweezers and super-resolution microscopy.

Cdon is essential for organ left-right patterning by regulating dorsal forerunner cells clustering and Kupffer's vesicle morphogenesis

Cdon and boc are members of the cell adhesion molecule subfamily III Ig/fibronectin. Although they have been reported to be involved in muscle and neural development at late developmental stage, their early roles in embryonic development remain unknown. Here, we discovered that in zebrafish, cdon, but not boc, is expressed in dorsal forerunner cells (DFCs) and the epithelium of Kupffer's vesicle (KV), suggesting a potential role for cdon in organ left-right (LR) patterning. Further data showed that liver and heart LR patterning were disrupted in cdon morphants and cdon mutants. Mechanistically, we found that loss of cdon function led to defect in DFCs clustering, reduced KV lumen, and defective cilia, resulting in randomized Nodal/spaw signaling and subsequent organ LR patterning defects. Additionally, predominant distribution of a cdon morpholino (MO) in DFCs caused defects in DFC clustering, KV morphogenesis, cilia number/length, Nodal/spaw signaling, and organ LR asymmetry, similar to those observed in cdon morphants and cdon -/- embryos, indicating a cell-autonomous role for cdon in regulating KV formation during LR patterning. In conclusion, our data demonstrate that during gastrulation and early somitogenesis, cdon is essential for proper DFC clustering, KV formation, and normal cilia, thereby playing a critical role in establishing organ LR asymmetry.

An atypical basement membrane forms a midline barrier during left-right asymmetric gut development in the chicken embryo

Correct intestinal morphogenesis depends on the early embryonic process of gut rotation, an evolutionarily conserved program in which a straight gut tube elongates and forms into its first loops. However, the gut tube requires guidance to loop in a reproducible manner. The dorsal mesentery (DM) connects the gut tube to the body and directs the lengthening gut into stereotypical loops via left-right (LR) asymmetric cellular and extracellular behavior. The LR asymmetry of the DM also governs blood and lymphatic vessel formation for the digestive tract, which is essential for prenatal organ development and postnatal vital functions including nutrient absorption. Although the genetic LR asymmetry of the DM has been extensively studied, a divider between the left and right DM has yet to be identified. Setting up LR asymmetry for the entire body requires a Lefty1+ midline barrier to separate the two sides of the embryo, without it, embryos have lethal or congenital LR patterning defects. Individual organs including the brain, heart, and gut also have LR asymmetry, and while the consequences of left and right signals mixing are severe or even lethal, organ-specific mechanisms for separating these signals are poorly understood. Here, we uncover a midline structure composed of a transient double basement membrane, which separates the left and right halves of the embryonic chick DM during the establishment of intestinal and vascular asymmetries. Unlike other basement membranes of the DM, the midline is resistant to disruption by intercalation of Netrin4 (Ntn4). We propose that this atypical midline forms the boundary between left and right sides and functions as a barrier necessary to establish and protect organ asymmetry.

The role of primary cilia in congenital heart defect-associated neurological impairments

Congenital heart disease (CHD) has, despite significant improvements in patient survival, increasingly become associated with neurological deficits during infancy that persist into adulthood. These impairments afflict a wide range of behavioral domains including executive function, motor learning and coordination, social interaction, and language acquisition, reflecting alterations in multiple brain areas. In the past few decades, it has become clear that CHD is highly genetically heterogeneous, with large chromosomal aneuploidies and copy number variants (CNVs) as well as single nucleotide polymorphisms (SNPs) being implicated in CHD pathogenesis. Intriguingly, many of the identified loss-of-function genetic variants occur in genes important for primary cilia integrity and function, hinting at a key role for primary cilia in CHD. Here we review the current evidence for CHD primary cilia associated genetic variants, their independent functions during cardiac and brain development and their influence on behavior. We also highlight the role of environmental exposures in CHD, including stressors such as surgical factors and anesthesia, and how they might interact with ciliary genetic predispositions to determine the final neurodevelopmental outcome. The multifactorial nature of CHD and neurological impairments linked with it will, on one hand, likely necessitate therapeutic targeting of molecular pathways and neurobehavioral deficits shared by disparate forms of CHD. On the other hand, strategies for better CHD patient stratification based on genomic data, gestational and surgical history, and CHD complexity would allow for more precise therapeutic targeting of comorbid neurological deficits.

'Three signals - three body axes' as patterning principle in bilaterians

In vertebrates, the three orthogonal body axes, anteroposterior (AP), dorsoventral (DV) and left-right (LR) are determined at gastrula and neurula stages by the Spemann-Mangold organizer and its equivalents. A common feature of AP and DV axis formation is that an evolutionary conserved interplay between growth factors (Wnt, BMP) and their extracellular antagonists (e.g. Dkk1, Chordin) creates signaling gradients for axial patterning. Recent work showed that LR patterning in Xenopus follows the same principle, with R-spondin 2 (Rspo2) as an extracellular FGF antagonist, which creates a signaling gradient that determines the LR vector. That a triad of anti-FGF, anti-BMP, and anti-Wnt governs LR, DV, and AP axis formation reveals a unifying principle in animal development. We discuss how cross-talk between these three signals confers integrated AP-DV-LR body axis patterning underlying developmental robustness, size scaling, and harmonious regulation. We propose that Urbilateria featured three orthogonal body axes that were governed by a Cartesian coordinate system of orthogonal Wnt/AP, BMP/DV, and FGF/LR signaling gradients.

Myosin1G promotes Nodal signaling to control zebrafish left-right asymmetry

Myosin1D (Myo1D) has recently emerged as a conserved regulator of animal Left-Right (LR) asymmetry that governs the morphogenesis of the vertebrate central LR Organizer (LRO). In addition to Myo1D, the zebrafish genome encodes the closely related Myo1G. Here we show that while Myo1G also controls LR asymmetry, it does so through an entirely different mechanism. Myo1G promotes the Nodal-mediated transfer of laterality information from the LRO to target tissues. At the cellular level, Myo1G is associated with endosomes positive for the TGFβ signaling adapter SARA. myo1g mutants have fewer SARA-positive Activin receptor endosomes and a reduced responsiveness to Nodal ligands that results in a delay of left-sided Nodal propagation and tissue-specific laterality defects in organs that are most distant from the LRO. Additionally, Myo1G promotes signaling by different Nodal ligands in specific biological contexts. Our findings therefore identify Myo1G as a context-dependent regulator of the Nodal signaling pathway.

SENP3-regulated Nodal signaling plays a potential role in cardiac left-right asymmetry development

Congenital heart disease (CHD) is a type of major defect that occurs during embryonic development. Although significant advances have been made in the treatment of CHD, its etiology and molecular mechanism remain unclear. To identify the critical role of SUMOylation in cardiac development, we generated SENP3 knockout mice and showed that SENP3 knockout mice die on embryonic day 8.5 with an open neural tube and reversed left-right cardiac asymmetry. Moreover, SENP3 knockout promoted apoptosis and senescence of H9C2 cells. Further studies showed that Nodal, a critical gene that forms left-right asymmetry, is regulated by SENP3 and that SENP3 regulates cell apoptosis and senescence in a Nodal-dependent manner. Furthermore, Nodal was hyper-SUMOylated after SENP3 knockout, and SUMOylation of Nodal inhibited its ubiquitination and ubiquitin-proteasome degradation pathway. Nodal overexpression enhanced cell apoptosis and senescence; however, the mutation at the SUMOylation site of Nodal reversed its effect on the apoptosis and senescence of H9C2 cells. More importantly, the SENP3-Nodal axis regulates cell senescence by inducing cell autophagy. These results suggest that the SENP3-Nodal signaling axis regulates cardiac senescence-autophagy homeostasis, which in turn affects cardiac development and results in the occurrence of CHD.

Localisation and function of key axonemal microtubule inner proteins and dynein docking complex members reveal extensive diversity among vertebrate motile cilia

Vertebrate motile cilia are classified as (9+2) or (9+0), based on the presence or absence of the central pair apparatus, respectively. Cryogenic electron microscopy analyses of (9+2) cilia have uncovered an elaborate axonemal protein composition. The extent to which these features are conserved in (9+0) cilia remains unclear. CFAP53, a key axonemal filamentous microtubule inner protein (fMIP) and a centriolar satellites component, is essential for motility of (9+0), but not (9+2) cilia. Here, we show that in (9+2) cilia, CFAP53 functions redundantly with a paralogous fMIP, MNS1. MNS1 localises to ciliary axonemes, and combined loss of both proteins in zebrafish and mice caused severe outer dynein arm loss from (9+2) cilia, significantly affecting their motility. Using immunoprecipitation, we demonstrate that, whereas MNS1 can associate with itself and CFAP53, CFAP53 is unable to self-associate. We also show that additional axonemal dynein-interacting proteins, two outer dynein arm docking (ODAD) complex members, show differential localisation between types of motile cilia. Together, our findings clarify how paralogous fMIPs, CFAP53 and MNS1, function in regulating (9+2) versus (9+0) cilia motility, and further emphasise extensive structural diversity among these organelles.

The evolution of developmental biology through conceptual and technological revolutions

Developmental biology-the study of the processes by which cells, tissues, and organisms develop and change over time-has entered a new golden age. After the molecular genetics revolution in the 80s and 90s and the diversification of the field in the early 21st century, we have entered a phase when powerful technologies provide new approaches and open unexplored avenues. Progress in the field has been accelerated by advances in genomics, imaging, engineering, and computational biology and by emerging model systems ranging from tardigrades to organoids. We summarize how revolutionary technologies have led to remarkable progress in understanding animal development. We describe how classic questions in gene regulation, pattern formation, morphogenesis, organogenesis, and stem cell biology are being revisited. We discuss the connections of development with evolution, self-organization, metabolism, time, and ecology. We speculate how developmental biology might evolve in an era of synthetic biology, artificial intelligence, and human engineering.

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Neurons dispose of hyperactive kinesin into glial cells for clearance

Microtubule-based kinesin motor proteins are crucial for intracellular transport, but their hyperactivation can be detrimental for cellular functions. This study investigated the impact of a constitutively active ciliary kinesin mutant, OSM-3CA, on sensory cilia in C. elegans. Surprisingly, we found that OSM-3CA was absent from cilia but underwent disposal through membrane abscission at the tips of aberrant neurites. Neighboring glial cells engulf and eliminate the released OSM-3CA, a process that depends on the engulfment receptor CED-1. Through genetic suppressor screens, we identified intragenic mutations in the OSM-3CA motor domain and mutations inhibiting the ciliary kinase DYF-5, both of which restored normal cilia in OSM-3CA-expressing animals. We showed that conformational changes in OSM-3CA prevent its entry into cilia, and OSM-3CA disposal requires its hyperactivity. Finally, we provide evidence that neurons also dispose of hyperactive kinesin-1 resulting from a clinic variant associated with amyotrophic lateral sclerosis, suggesting a widespread mechanism for regulating hyperactive kinesins.

Optical microscopic imaging, manipulation, and analysis methods for morphogenesis research

Morphogenesis is a developmental process of organisms being shaped through complex and cooperative cellular movements. To understand the interplay between genetic programs and the resulting multicellular morphogenesis, it is essential to characterize the morphologies and dynamics at the single-cell level and to understand how physical forces serve as both signaling components and driving forces of tissue deformations. In recent years, advances in microscopy techniques have led to improvements in imaging speed, resolution and depth. Concurrently, the development of various software packages has supported large-scale, analyses of challenging images at the single-cell resolution. While these tools have enhanced our ability to examine dynamics of cells and mechanical processes during morphogenesis, their effective integration requires specialized expertise. With this background, this review provides a practical overview of those techniques. First, we introduce microscopic techniques for multicellular imaging and image analysis software tools with a focus on cell segmentation and tracking. Second, we provide an overview of cutting-edge techniques for mechanical manipulation of cells and tissues. Finally, we introduce recent findings on morphogenetic mechanisms and mechanosensations that have been achieved by effectively combining microscopy, image analysis tools and mechanical manipulation techniques.

Dual and opposing roles for the kinesin-2 motor, KIF17, in Hedgehog-dependent cerebellar development

While the kinesin-2 motors KIF3A and KIF3B have essential roles in ciliogenesis and Hedgehog (HH) signal transduction, potential role(s) for another kinesin-2 motor, KIF17, in HH signaling have yet to be explored. Here, we investigated the contribution of KIF17 to HH-dependent cerebellar development, where Kif17 is expressed in both HH-producing Purkinje cells and HH-responding cerebellar granule neuron progenitors (CGNPs). Germline Kif17 deletion in mice results in cerebellar hypoplasia due to reduced CGNP proliferation, a consequence of decreased HH pathway activity mediated through decreased Sonic HH (SHH) protein. Notably, Purkinje cell-specific Kif17 deletion partially phenocopies Kif17 germline mutants. Unexpectedly, CGNP-specific Kif17 deletion results in the opposite phenotype-increased CGNP proliferation and HH target gene expression due to altered GLI transcription factor processing. Together, these data identify KIF17 as a key regulator of HH-dependent cerebellar development, with dual and opposing roles in HH-producing Purkinje cells and HH-responding CGNPs.

Motor proteins, spermatogenesis and testis function

The role of motor proteins in supporting intracellular transports of vesicles and organelles in mammalian cells has been known for decades. On the other hand, the function of motor proteins that support spermatogenesis is also well established since the deletion of motor protein genes leads to subfertility and/or infertility. Furthermore, mutations and genetic variations of motor protein genes affect fertility in men, but also a wide range of developmental defects in humans including multiple organs besides the testis. In this review, we seek to provide a summary of microtubule and actin-dependent motor proteins based on earlier and recent findings in the field. Since these two cytoskeletons are polarized structures, different motor proteins are being used to transport cargoes to different ends of these cytoskeletons. However, their involvement in germ cell transport across the blood-testis barrier (BTB) and the epithelium of the seminiferous tubules remains relatively unknown. It is based on recent findings in the field, we have provided a hypothetical model by which motor proteins are being used to support germ cell transport across the BTB and the seminiferous epithelium during the epithelial cycle of spermatogenesis. In our discussion, we have highlighted the areas of research that deserve attention to bridge the gap of research in relating the function of motor proteins to spermatogenesis.

Characterization of the disease-causing mechanism of KIF3B mutations from ciliopathy patients

The heterodimeric kinesin-2 motor (KIF3A/KIF3B with accessory protein KAP3) drives intraflagellar transport, essential for ciliogenesis and ciliary function. Three point mutations in the KIF3B subunit have recently been linked to disease in humans (E250Q and L523P) and Bengal cats (A334T) (Cogné et al., Am. J. Hum. Genet., 2020, 106, 893-904). Patients display retinal atrophy and, in some cases, other ciliopathy phenotypes. However, the molecular mechanism leading to disease is currently unknown. Here, we used Kif3a -/- ;Kif3b -/- (knockout) 3T3 cells, which cannot make cilia, to characterize these mutations. While reexpression of KIF3B(E250Q) and KIF3B(L523P) did not rescue ciliogenesis, reexpression of wildtype or KIF3B(A334T) restored ciliogenesis to wildtype levels. Fluorescent tagging revealed that the E250Q mutant decorated microtubules and thus is a rigor mutation. The L523P mutation, in the alpha-helical stalk domain, surprisingly did not affect formation of the KIF3A/KIF3B/KAP3 complex but instead impaired motility along microtubules. Lastly, expression of the A334T motor was reduced in comparison to all other motors, and this motor displayed an impaired ability to disperse the Golgi complex when artificially linked to this high-load cargo. In summary, this work uses cell-based assays to elucidate the molecular effects of disease-causing mutations in the KIF3B subunit on the kinesin-2 holoenzyme.

A novel in-frame deletion in KIF5C gene causes infantile onset epilepsy and psychomotor retardation

Motor proteins, encoded by Kinesin superfamily (KIF) genes, are critical for brain development and plasticity. Increasing studies reported KIF's roles in neurodevelopmental disorders. Here, a 6 years and 3 months-old Chinese boy with markedly symptomatic epilepsy, intellectual disability, brain atrophy, and psychomotor retardation was investigated. His parents and younger sister were phenotypically normal and had no disease-related family history. Whole exome sequencing identified a novel heterozygous in-frame deletion (c.265_267delTCA) in exon 3 of the KIF5C in the proband, resulting in the removal of evolutionarily highly conserved p.Ser90, located in its ATP-binding domain. Sanger sequencing excluded the proband's parents and family members from harboring this variant. The activity of ATP hydrolysis in vitro was significantly reduced as predicted. Immunofluorescence studies showed wild-type KIF5C was widely distributed throughout the cytoplasm, while mutant KIF5C was colocalized with microtubules. The live-cell imaging of the cargo-trafficking assay revealed that mutant KIF5C lost the peroxisome-transporting ability. Drosophila models also confirmed p.Ser90del's essential role in nervous system development. This study emphasized the importance of the KIF5C gene in intracellular cargo-transport as well as germline variants that lead to neurodevelopmental disorders and might enable clinicians for timely and accurate diagnosis and disease management in the future.

Three-Dimensional Chiral Morphogenesis of Active Fluids

Chirality is an essential nature of biological systems. However, it remains obscure how the handedness at the microscale is translated into chiral morphogenesis at the tissue level. Here, we investigate three-dimensional (3D) tissue morphogenesis using an active fluid theory invoking chirality. We show that the coordination of achiral and chiral stresses, arising from microscopic interactions and energy input of individual cells, can engender the self-organization of 3D papillary and helical structures. The achiral active stress drives the nucleation of asterlike topological defects, which initiate 3D out-of-plane budding, followed by rodlike elongation. The chiral active stress excites vortexlike topological defects, which favor the tip spheroidization and twisting of the elongated rod. These results unravel the chiral morphogenesis observed in our experiments of 3D organoids generated by human embryonic stem cells.

Perspectives on polarity - exploring biological asymmetry across scales

In this Perspective, Journal of Cell Science invited researchers working on cell and tissue polarity to share their thoughts on unique, emerging or open questions relating to their field. The goal of this article is to feature 'voices' from scientists around the world and at various career stages, to bring attention to innovative and thought-provoking topics of interest to the cell biology community. These voices discuss intriguing questions that consider polarity across scales, evolution, development and disease. What can yeast and protists tell us about the evolution of cell and tissue polarity in animals? How are cell fate and development influenced by emerging dynamics in cell polarity? What can we learn from atypical and extreme polarity systems? How can we arrive at a more unified biophysical understanding of polarity? Taken together, these pieces demonstrate the broad relevance of the fascinating phenomenon of cell polarization to diverse fundamental biological questions.

R-Spondin 2 governs Xenopus left-right body axis formation by establishing an FGF signaling gradient

Establishment of the left-right (LR, sinistral, dextral) body axis in many vertebrate embryos relies on cilia-driven leftward fluid flow within an LR organizer (LRO). A cardinal question is how leftward flow triggers symmetry breakage. The chemosensation model posits that ciliary flow enriches a signaling molecule on the left side of the LRO that promotes sinistral cell fate. However, the nature of this sinistralizing signal has remained elusive. In the Xenopus LRO, we identified the stem cell growth factor R-Spondin 2 (Rspo2) as a symmetrically expressed, sinistralizing signal. As predicted for a flow-mediated signal, Rspo2 operates downstream of leftward flow but upstream of the asymmetrically expressed gene dand5. Unexpectedly, in LR patterning, Rspo2 acts as an FGF receptor antagonist: Rspo2 via its TSP1 domain binds Fgfr4 and promotes its membrane clearance by Znrf3-mediated endocytosis. Concordantly, we find that at flow-stage, FGF signaling is dextralizing and forms a gradient across the LRO, high on the dextral- and low on the sinistral side. Rspo2 gain- and loss-of function equalize this FGF signaling gradient and sinistralize and dextralize development, respectively. We propose that leftward flow of Rspo2 produces an FGF signaling gradient that governs LR-symmetry breakage.

Molecular Pathways and Animal Models of Defects in Situs

Left-right patterning is among the least well understood of the three axes defining the body plan, and yet it is no less important, with left-right patterning defects causing structural birth defects with high morbidity and mortality, such as complex congenital heart disease, biliary atresia, or intestinal malrotation. The cell signaling pathways governing left-right asymmetry are highly conserved and involve multiple components of the TGF-β superfamily of cell signaling molecules. Central to left-right patterning is the differential activation of Nodal on the left, and BMP signaling on the right. In addition, a plethora of other cell signaling pathways including Shh, FGF, and Notch also contribute to the regulation of left-right patterning. In vertebrate embryos such as the mouse, frog, or zebrafish, the specification of left-right identity requires the left-right organizer (LRO) containing cells with motile and primary cilia that mediate the left-sided propagation of Nodal signaling, followed by left-sided activation of Lefty and then Pitx2, a transcription factor that specifies visceral organ asymmetry. While this overall scheme is well conserved, there are striking species differences, including the finding that motile cilia do not play a role in left-right patterning in some vertebrates. Surprisingly, the direction of heart looping, one of the first signs of organ left-right asymmetry, was recently shown to be specified by intrinsic cell chirality, not Nodal signaling, possibly a reflection of the early origin of Nodal signaling in radially symmetric organisms. How this intrinsic chirality interacts with downstream molecular pathways regulating visceral organ asymmetry will need to be further investigated to elucidate how disturbance in left-right patterning may contribute to complex CHD.

Formation and function of multiciliated cells

In vertebrates, multiciliated cells (MCCs) are terminally differentiated cells that line the airway tracts, brain ventricles, and reproductive ducts. Each MCC contains dozens to hundreds of motile cilia that beat in a synchronized manner to drive fluid flow across epithelia, the dysfunction of which is associated with a group of human diseases referred to as motile ciliopathies, such as primary cilia dyskinesia. Given the dynamic and complex process of multiciliogenesis, the biological events essential for forming multiple motile cilia are comparatively unelucidated. Thanks to advancements in genetic tools, omics technologies, and structural biology, significant progress has been achieved in the past decade in understanding the molecular mechanism underlying the regulation of multiple motile cilia formation. In this review, we discuss recent studies with ex vivo culture MCC and animal models, summarize current knowledge of multiciliogenesis, and particularly highlight recent advances and their implications.

Cilia and Nodal Flow in Asymmetry: An Engineering Perspective

Over the past several years, cilia in the primitive node have become recognized more and more for their contribution to development, and more specifically, for their role in axis determination. Although many of the mechanisms behind their influence remain undocumented, it is known that their presence and motion in the primitive node of developing embryos is the determinant of the left-right axis. Studies on cilial mechanics and nodal fluid dynamics have provided clues as to how this asymmetry mechanism works, and more importantly, have shown that direct manipulation of the flow field in the node can directly influence physiology. Although relatively uncommon, cilial disorders have been shown to have a variety of impacts on individuals from chronic respiratory infections to infertility, as well as situs inversus which is linked to congenital heart disease. After first providing background information pertinent to understanding nodal flow and information on why this discussion is important, this paper aims to give a review of the history of nodal cilia investigations, an overview of cilia mechanics and nodal flow dynamics, as well as a review of research studies current and past that sought to understand the mechanisms behind nodal cilia's involvement in symmetry-breaking pathways through a biomedical engineering perspective. This discussion has the additional intention to compile interdisciplinary knowledge on asymmetry and development such that it may encourage more collaborative efforts between the sciences on this topic, as well as provide insight on potential paths forward in the field.

Establishment of Cardiac Laterality

Formation of the vertebrate heart with its complex arterial and venous connections is critically dependent on patterning of the left-right axis during early embryonic development. Abnormalities in left-right patterning can lead to a variety of complex life-threatening congenital heart defects. A highly conserved pathway responsible for left-right axis specification has been uncovered. This pathway involves initial asymmetric activation of a nodal signaling cascade at the embryonic node, followed by its propagation to the left lateral plate mesoderm and activation of left-sided expression of the Pitx2 transcription factor specifying visceral organ asymmetry. Intriguingly, recent work suggests that cardiac laterality is encoded by intrinsic cell and tissue chirality independent of Nodal signaling. Thus, Nodal signaling may be superimposed on this intrinsic chirality, providing additional instructive cues to pattern cardiac situs. The impact of intrinsic chirality and the perturbation of left-right patterning on myofiber organization and cardiac function warrants further investigation. We summarize recent insights gained from studies in animal models and also some human clinical studies in a brief overview of the complex processes regulating cardiac asymmetry and their impact on cardiac function and the pathogenesis of congenital heart defects.

Gastrulation-stage alcohol exposure induces similar rates of craniofacial malformations in male and female C57BL/6J mice

BACKGROUND

Prenatal alcohol exposure during gastrulation (embryonic day [E] 7 in mice, ~3rd week of human pregnancy) impairs eye, facial, and cortical development, recapitulating birth defects characteristic of Fetal Alcohol Syndrome (FAS). However, it is not known whether the prevalence or severity of craniofacial features associated with FAS is affected by biological sex.

METHODS

The current study administered either alcohol (2.9 g/kg, two i.p. doses, 4 hr apart) or vehicle to pregnant C57BL/6J females on E7, prior to gonadal sex differentiation, and assessed fetal morphology at E17.

RESULTS

Whereas sex did not affect fetal size in controls, alcohol-exposed females were smaller than both control females and alcohol-treated males. Alcohol exposure increased the incidence of eye defects to a similar degree in males and females. Together, these data suggest that females might be more sensitive to the general developmental effects of alcohol, but not effects specific to the craniofacies. Whole transcriptomic analysis of untreated E7 embryos found 214 differentially expressed genes in females vs. males, including those in pathways related to cilia and mitochondria, histone demethylase activity, and pluripotency.

CONCLUSION

Gastrulation-stage alcohol induces craniofacial malformations in male and female mouse fetuses at similar rates and severity, though growth deficits are more prevalent females. These findings support the investigation of biological sex as a contributing factor in prenatal alcohol studies.

Functions of cilia in cardiac development and disease

Errors in embryonic cardiac development are a leading cause of congenital heart defects (CHDs), including morphological abnormalities of the heart that are often detected after birth. In the past few decades, an emerging role for cilia in the pathogenesis of CHD has been identified, but this topic still largely remains an unexplored area. Mouse forward genetic screens and whole exome sequencing analysis of CHD patients have identified enrichment for de novo mutations in ciliary genes or non-ciliary genes, which regulate cilia-related pathways, linking cilia function to aberrant cardiac development. Key events in cardiac morphogenesis, including left-right asymmetric development of the heart, are dependent upon cilia function. Cilia dysfunction during left-right axis formation contributes to CHD as evidenced by the substantial proportion of heterotaxy patients displaying complex CHD. Cilia-transduced signaling also regulates later events during heart development such as cardiac valve formation, outflow tract septation, ventricle development, and atrioventricular septa formation. In this review, we summarize the role of motile and non-motile (primary cilia) in cardiac asymmetry establishment and later events during heart development.

Calcium signaling mediates proliferation of the precursor cells that give rise to the ciliated left-right organizer in the zebrafish embryo

Several of our internal organs, including heart, lungs, stomach, and spleen, develop asymmetrically along the left-right (LR) body axis. Errors in establishing LR asymmetry, or laterality, of internal organs during early embryonic development can result in birth defects. In several vertebrates-including humans, mice, frogs, and fish-cilia play a central role in establishing organ laterality. Motile cilia in a transient embryonic structure called the "left-right organizer" (LRO) generate a directional fluid flow that has been proposed to be detected by mechanosensory cilia to trigger asymmetric signaling pathways that orient the LR axis. However, the mechanisms that control the form and function of the ciliated LRO remain poorly understood. In the zebrafish embryo, precursor cells called dorsal forerunner cells (DFCs) develop into a transient ciliated structure called Kupffer's vesicle (KV) that functions as the LRO. DFCs can be visualized and tracked in the embryo, thereby providing an opportunity to investigate mechanisms that control LRO development. Previous work revealed that proliferation of DFCs via mitosis is a critical step for developing a functional KV. Here, we conducted a targeted pharmacological screen to identify mechanisms that control DFC proliferation. Small molecule inhibitors of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) were found to reduce DFC mitosis. The SERCA pump is involved in regulating intracellular calcium ion (Ca2+) concentration. To visualize Ca2+ in living embryos, we generated transgenic zebrafish using the fluorescent Ca2+ biosensor GCaMP6f. Live imaging identified dynamic cytoplasmic Ca2+ transients ("flux") that occur unambiguously in DFCs. In addition, we report Ca2+ flux events that occur in the nucleus of DFCs. Nuclear Ca2+ flux occurred in DFCs that were about to undergo mitosis. We find that SERCA inhibitor treatments during DFC proliferation stages alters Ca2+ dynamics, reduces the number of ciliated cells in KV, and alters embryo laterality. Mechanistically, SERCA inhibitor treatments eliminated both cytoplasmic and nuclear Ca2+ flux events, and reduced progression of DFCs through the S/G2 phases of the cell cycle. These results identify SERCA-mediated Ca2+ signaling as a mitotic regulator of the precursor cells that give rise to the ciliated LRO.

The role of cilia during organogenesis in zebrafish

Cilia are hair-like organelles that protrude from the surface of eukaryotic cells and are present on the surface of nearly all human cells. Cilia play a crucial role in signal transduction, organ development and tissue homeostasis. Abnormalities in the structure and function of cilia can lead to a group of human diseases known as ciliopathies. Currently, zebrafish serves as an ideal model for studying ciliary function and ciliopathies due to its relatively conserved structure and function of cilia compared to humans. In this review, we will summarize the different types of cilia that present in embryonic and adult zebrafish, and provide an overview of the advantages of using zebrafish as a vertebrate model for cilia research. We will specifically focus on the roles of cilia during zebrafish organogenesis based on recent studies. Additionally, we will highlight future prospects for ciliary research in zebrafish.

Autonomous and non-cell autonomous role of cilia in structural birth defects in mice

Ciliopathies are associated with wide spectrum of structural birth defects (SBDs), indicating important roles for cilia in development. Here, we provide novel insights into the temporospatial requirement for cilia in SBDs arising from deficiency in Ift140, an intraflagellar transport (IFT) protein regulating ciliogenesis. Ift140-deficient mice exhibit cilia defects accompanied by wide spectrum of SBDs including macrostomia (craniofacial defects), exencephaly, body wall defects, tracheoesophageal fistula (TEF), randomized heart looping, congenital heart defects (CHDs), lung hypoplasia, renal anomalies, and polydactyly. Tamoxifen inducible CAGGCre-ER deletion of a floxed Ift140 allele between E5.5 to 9.5 revealed early requirement for Ift140 in left-right heart looping regulation, mid to late requirement for cardiac outflow septation and alignment, and late requirement for craniofacial development and body wall closure. Surprisingly, CHD were not observed with 4 Cre drivers targeting different lineages essential for heart development, but craniofacial defects and omphalocele were observed with Wnt1-Cre targeting neural crest and Tbx18-Cre targeting epicardial lineage and rostral sclerotome through which trunk neural crest cells migrate. These findings revealed cell autonomous role of cilia in cranial/trunk neural crest-mediated craniofacial and body wall closure defects, while non-cell autonomous multi-lineage interactions underlie CHD pathogenesis, revealing unexpected developmental complexity for CHD associated with ciliopathies.

The two-step cargo recognition mechanism of heterotrimeric kinesin

Kinesin-driven intracellular transport is essential for various cell biological events and thus plays a crucial role in many pathological processes. However, little is known about the molecular basis of the specific and dynamic cargo-binding mechanism of kinesins. Here, an integrated structural analysis of the KIF3/KAP3 and KIF3/KAP3-APC complexes unveils the mechanism by which KIF3/KAP3 can dynamically grasp APC in a two-step manner, which suggests kinesin-cargo recognition dynamics composed of cargo loading, locking, and release. Our finding is the first demonstration of the two-step cargo recognition and stabilization mechanism of kinesins, which provides novel insights into the intracellular trafficking machinery.

Emerging principles of primary cilia dynamics in controlling tissue organization and function

Primary cilia project from the surface of most vertebrate cells and are key in sensing extracellular signals and locally transducing this information into a cellular response. Recent findings show that primary cilia are not merely static organelles with a distinct lipid and protein composition. Instead, the function of primary cilia relies on the dynamic composition of molecules within the cilium, the context-dependent sensing and processing of extracellular stimuli, and cycles of assembly and disassembly in a cell- and tissue-specific manner. Thereby, primary cilia dynamically integrate different cellular inputs and control cell fate and function during tissue development. Here, we review the recently emerging concept of primary cilia dynamics in tissue development, organization, remodeling, and function.