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

Male germ cell-associated kinase is required for axoneme formation during ciliogenesis in zebrafish photoreceptors

Vertebrate photoreceptors are highly specialized retinal neurons that have cilium-derived membrane organelles called outer segments, which function as platforms for phototransduction. Male germ cell-associated kinase (MAK) is a cilium-associated serine/threonine kinase, and its genetic mutation causes photoreceptor degeneration in mice and retinitis pigmentosa in humans. However, the role of MAK in photoreceptors is not fully understood. Here, we report that zebrafish mak mutants show rapid photoreceptor degeneration during embryonic development. In mak mutants, both cone and rod photoreceptors completely lacked outer segments and underwent apoptosis. Interestingly, zebrafish mak mutants failed to generate axonemes during photoreceptor ciliogenesis, whereas basal bodies were specified. These data suggest that Mak contributes to axoneme development in zebrafish, in contrast to mouse Mak mutants, which have elongated photoreceptor axonemes. Furthermore, the kinase activity of Mak was found to be critical in ciliary axoneme development and photoreceptor survival. Thus, Mak is required for ciliogenesis and outer segment formation in zebrafish photoreceptors to ensure intracellular protein transport and photoreceptor survival.

Rare variants found in multiplex families with orofacial clefts: Does expanding the phenotype make a difference?

Exome sequencing (ES) is now a relatively straightforward process to identify causal variants in Mendelian disorders. However, the same is not true for ES in families where the inheritance patterns are less clear, and a complex etiology is suspected. Orofacial clefts (OFCs) are highly heritable birth defects with both Mendelian and complex etiologies. The phenotypic spectrum of OFCs may include overt clefts and several subclinical phenotypes, such as discontinuities in the orbicularis oris muscle (OOM) in the upper lip, velopharyngeal insufficiency (VPI), microform clefts or bifid uvulas. We hypothesize that expanding the OFC phenotype to include these phenotypes can clarify inheritance patterns in multiplex families, making them appear more Mendelian. We performed exome sequencing to find rare, likely causal genetic variants in 31 multiplex OFC families, which included families with multiple individuals with OFCs and individuals with subclinical phenotypes. We identified likely causal variants in COL11A2, IRF6, SHROOM3, SMC3, TBX3, and TP63 in six families. Although we did not find clear evidence supporting the subclinical phenotype hypothesis, our findings support a role for rare variants in the etiology of OFCs.

Zebrafish null mutants of Sept6 and Sept15 are viable but more susceptible to Shigella infection

Septins are evolutionarily conserved GTP-binding proteins known for their roles in cell division and host defence against Shigella infection. Although septin group members are viewed to function as hetero-oligomeric complexes, the role of individual septins within these complexes or in isolation is poorly understood. Decades of work using mouse models has shown that some septins (including SEPT7) are essential for animal development, while others (including SEPT6) are dispensable, suggesting that some septins may compensate for the absence of others. The zebrafish genome encodes 19 septin genes, representing the full complement of septin groups described in mice and humans. In this report, we characterise null mutants for zebrafish Sept6 (a member of the SEPT6 group) and Sept15 (a member of the SEPT7 group) and test their role in zebrafish development and host defence against Shigella infection. We show that null mutants for Sept6 and Sept15 are both viable, and that expression of other zebrafish septins are not significantly affected by their mutation. Consistent with previous reports using knockdown of Sept2, Sept7b, and Sept15, we show that Sept6 and Sept15 are required for host defence against Shigella infection. These results highlight Shigella infection of zebrafish as a powerful system to study the role of individual septins in vivo.

Transcriptome-Wide Regulation of Key Developmental Pathways in the Mouse Neural Tube by Prenatal Alcohol Exposure

BACKGROUND

Early gestational alcohol exposure is associated with severe craniofacial and CNS dysmorphologies and behavioral abnormalities during adolescence and adulthood. Alcohol exposure during the formation of the neural tube (gestational day [GD] 8 to 10 in mice; equivalent to4th week of human pregnancy) disrupts development of ventral midline brain structures such as the pituitary, septum, and ventricles. This study identifies transcriptomic changes in the rostroventral neural tube (RVNT), the region of the neural tube that gives rise to the midline structures sensitive to alcohol exposure during neurulation.

METHODS

Female C57BL/6J mice were administered 2 doses of alcohol (2.9 g/kg) or vehicle 4 hours apart on GD 9.0. The RVNTs of embryos were collected 6 or 24 hours after the first dose and processed for RNA-seq.

RESULTS

Six hours following GD 9.0 alcohol exposure (GD 9.25), over 2,300 genes in the RVNT were determined to be differentially regulated by alcohol. Enrichment analysis determined that PAE affected pathways related to cell proliferation, p53 signaling, ribosome biogenesis, and immune activation. In addition, over 100 genes involved in primary cilia formation and function and regulation of morphogenic pathways were altered 6 hours after alcohol exposure. The changes to gene expression were largely transient, as only 91 genes identified as differentially regulated by prenatal alcohol at GD 10 (24 hours postexposure). Functionally, the differentially regulated genes at GD 10 were related to organogenesis and cell migration.

CONCLUSIONS

These data give a comprehensive view of the changing landscape of the embryonic transcriptome networks in regions of the neural tube that give rise to brain structures impacted by a neurulation-stage alcohol exposure. Identification of gene networks dysregulated by alcohol will help elucidate the pathogenic mechanisms of alcohol's actions.

TBX20 Regulates Angiogenesis Through the Prokineticin 2-Prokineticin Receptor 1 Pathway

BACKGROUND

Angiogenesis is integral for embryogenesis, and targeting angiogenesis improves the outcome of many pathological conditions in patients. TBX20 is a crucial transcription factor for embryonic development, and its deficiency is associated with congenital heart disease. However, the role of TBX20 in angiogenesis has not been described.

METHODS

Loss- and gain-of-function approaches were used to explore the role of TBX20 in angiogenesis both in vitro and in vivo. Angiogenesis gene array was used to identify key downstream targets of TBX20.

RESULTS

Unbiased gene array survey showed that TBX20 knockdown profoundly reduced angiogenesis-associated PROK2 (prokineticin 2) gene expression. Indeed, loss of TBX20 hindered endothelial cell migration and in vitro angiogenesis. In a murine angiogenesis model using subcutaneously implanted Matrigel plugs, we observed that TBX20 deficiency markedly reduced PROK2 expression and restricted intraplug angiogenesis. Furthermore, recombinant PROK2 administration enhanced angiogenesis and blood flow recovery in murine hind-limb ischemia. In zebrafish, transient knockdown of tbx20 by morpholino antisense oligos or genetic disruption of tbx20 by CRISPR/Cas9 impaired angiogenesis. Furthermore, loss of prok2 or its cognate receptor prokr1a also limited angiogenesis. In contrast, overexpression of prok2 or prokr1a rescued the impaired angiogenesis in tbx20-deficient animals.

CONCLUSIONS

Our study identifies TBX20 as a novel transcription factor regulating angiogenesis through the PROK2-PROKR1 (prokineticin receptor 1) pathway in both development and disease and reveals a novel mode of angiogenic regulation whereby the TBX20-PROK2-PROKR1 signaling cascade may act as a "biological capacitor" to relay and sustain the proangiogenic effect of vascular endothelial growth factor. This pathway may be a therapeutic target in the treatment of diseases with dysregulated angiogenesis.

Centrosomal protein FOR20 is essential for cilia-dependent development in zebrafish embryos

Centrosomal proteins play critical roles in ciliogenesis. Mutations in many centrosomal proteins have been documented to contribute to developmental defects and cilium-related diseases. Centrosomal protein fibroblast growth factor receptor 1 oncogene partner-related protein of 20 kDa (FOR20) is crucial for ciliogenesis in mammalian cells and the unicellular eukaryote Paramecium; however, the biologic significance of FOR20 in vertebrate development remains unclear. We cloned the zebrafish homolog of the for20 gene and found that for20 mRNA is enriched in ciliated tissues during early zebrafish development. Knockdown of for20 by morpholino oligonucleotides in zebrafish results in multiple ciliary phenotypes, including curved body, hydrocephaly, pericardial edema, kidney cysts, and left-right asymmetry defects. for20 morphants show reduced number and length of cilia in Kupffer's vesicle and pronephric ducts. High-speed video microscopy reveals that cilia in most for20 morphants are consistently paralyzed or beat arrhythmically. To confirm the ciliary phenotypes of for20 morphants, we used the CRISPR/Cas9 system to disrupt for20 gene in zebrafish. for20 mutants exhibit multiple ciliary phenotypes resembling the defects in for20 morphants. All of these phenotypes in for20 morphants and mutants are significantly reversed by exogenous expression of for20 mRNA. Taken together, these data suggest that FOR20 is required for cilium-mediated processes during zebrafish embryogenesis.-Xie, S., Jin, J., Xu, Z., Huang, Y., Zhang, W., Zhao, L., Lo, L. J., Peng, J., Liu, W., Wang, F., Shu, Q., Zhou, T. Centrosomal protein FOR20 is essential for cilia-dependent development in zebrafish embryos.

Septins in kidney: A territory little explored

Septins are a conserved family of GTP‐binding proteins that assemble into cytoskeletal filaments to function in a highly sophisticated and physiologically regulated manner. Originally septins were discovered in the budding yeast as membrane‐associated filaments that affect cell polarity and cytokinesis. In the last decades, much progress has been made in understanding the biochemical properties and cell biological functions of septins. In line with this, mammalian septins have been shown to be involved in various cellular processes, including regulation of cell polarity, cytoskeletal organization, vesicle trafficking, ciliogenesis, and cell–pathogen interactions. A growing number of studies have shown that septins play important roles in tissue and organ development and physiology; yet, little is known about their role in the kidney. In the following review, we discuss the structure and functions of septins in general and summarize the evidence for their presence and roles in the kidney.

A Soft Microenvironment Protects from Failure of Midbody Abscission and Multinucleation Downstream of the EMT-Promoting Transcription Factor Snail

Multinucleation is found in more than one third of tumors and is linked to increased tolerance for mutation, resistance to chemotherapy, and invasive potential. The integrity of the genome depends on proper execution of the cell cycle, which can be altered through mechanotransduction pathways as the tumor microenvironment stiffens during tumorigenesis. Here, we show that signaling downstream of matrix metalloproteinase-3 (MMP3) or TGFβ, known inducers of epithelial-mesenchymal transition (EMT), also promotes multinucleation in stiff microenvironments through Snail-dependent expression of the filament-forming protein septin-6, resulting in midbody persistence, abscission failure, and multinucleation. Consistently, we observed elevated expression of Snail and septin-6 as well as multinucleation in a human patient sample of metaplastic carcinoma of the breast, a rare classification characterized by deposition of collagen fibers and active EMT. In contrast, a soft microenvironment protected mammary epithelial cells from becoming multinucleated by preventing Snail-induced upregulation of septin-6. Our data suggest that tissue stiffening during tumorigenesis synergizes with oncogenic signaling to promote genomic abnormalities that drive cancer progression.Significance: These findings reveal tissue stiffening during tumorigenesis synergizes with oncogenic signaling to promote genomic abnormalities that drive cancer progression. Cancer Res; 78(9); 2277-89. ©2018 AACR.

Use of zebrafish to study Shigella infection

Shigella is a leading cause of dysentery worldwide, responsible for up to 165 million cases of shigellosis each year. Shigella is also recognised as an exceptional model pathogen to study key issues in cell biology and innate immunity. Several infection models have been useful to explore Shigella biology; however, we still lack information regarding the events taking place during the Shigella infection process in vivo Here, we discuss a selection of mechanistic insights recently gained from studying Shigella infection of zebrafish (Danio rerio), with a focus on cytoskeleton rearrangements and cellular immunity. We also discuss how infection of zebrafish can be used to investigate new concepts underlying infection control, including emergency granulopoiesis and the use of predatory bacteria to combat antimicrobial resistance. Collectively, these insights illustrate how Shigella infection of zebrafish can provide fundamental advances in our understanding of bacterial pathogenesis and vertebrate host defence. This information should also provide vital clues for the discovery of new therapeutic strategies against infectious disease in humans.

The requirement of SEPT2 and SEPT7 for migration and invasion in human breast cancer via MEK/ERK activation

Septins are a novel class of GTP-binding cytoskeletal proteins evolutionarily conserved from yeast to mammals and have now been found to play a contributing role in a broad range of tumor types. However, their functional importance in breast cancer remains largely unclear. Here, we demonstrated that pharmaceutical inhibition of global septin dynamics would greatly suppress proliferation, migration and invasiveness in breast cancer cell lines. We then examined the expression and subcellular distribution of the selected septins SEPT2 and SEPT7 in breast cancer cells, revealing a rather variable localization of the two proteins with cell cycle progression. To determine the role of both septins in mediating malignant behavior of cancer cells, we used RNA silencing to specifically deplete endogenous SEPT2 or SEPT7 in highly invasive breast cancer cell line MDA-MB-231. Our findings showed that SEPT2/7 depletion had virtually identical inhibitory effects on cellular proliferation, apoptosis, migration and invasion. Moreover, the opposite performance in migration and invasion was observed after enforced expression of SEPT2/7 in the same cell line. We further demonstrated MEK/ERK activation, but not other MAPKs and AKT, was positively correlated with the protein levels of SEPT2 and SEPT7. Additionally, in SEPT2/7-overexpressing cells, the MEK specific inhibitor U0126 was able to correct the high active status of MEK/ERK while normalizing the increased invasive behaviors of these cells. Taken together, these results strongly suggest that SEPT2 and SEPT7 are involved in breast carcinogenesis and may serve as valuable therapeutic targets for breast cancer.

Depletion of Tissue-Specific Ion Transporters Causes Differential Expression of PRL Targets in Response to Increased Levels of Endogenous PRL

Prolactin (PRL) has been considered a key regulator of ion uptake in zebrafish. The genes slc12a10.2 and slc12a3, which are Na⁺ and chloride Cl^- co-transporters, have been reported to be regulated by PRL in freshwater fish. The integrative network of PRL signaling dissected from the knockout of tissue-specific downstream PRL ion transporters remains poor. In the present study, zebrafish models with increased endogenous levels of PRL were generated through the knockout of slc12a10.2 or slc12a3, and the developmental consequences were analyzed. The increased levels of pituitary PRL were observed in both slc12a10.2- and slc12a3-deficient fish. Unlike the slc12a3-deficient fish, which could survive to adulthood, the slc12a10.2-deficient fish began to die at 9 days post-fertilization (dpf) and did not survive beyond 17 dpf. This survival defect is a result of defective Cl^- uptake in this mutant, indicating that Slc12a10.2 plays an essential role in Cl^- uptake. Intriguingly, compared to the levels in control fish, no significant differences in the levels of Na⁺ in the body were observed in slc12a10.2- or slc12a3-deficient zebrafish. The upregulations of the PRL downstream transporters, slc9a3.2, slc12a10.2, and atp1a1a.5 were observed in slc12a3-deficient fish in both the gills/skin and the pronephric duct. However, this type of response was not observed in the pronephric duct of slc12a10.2-deficient fish, except under Na⁺-deprived conditions. Our results show that PRL is susceptible to deficiencies in downstream ion transporters. Moreover, both the gills/skin and pronephric duct show differential expression of downstream PRL targets in response to increased levels of pituitary PRL caused by the depletion of tissue-specific ion transporters.

Septins restrict inflammation and protect zebrafish larvae from Shigella infection

Shigella flexneri, a Gram-negative enteroinvasive pathogen, causes inflammatory destruction of the human intestinal epithelium. Infection by S. flexneri has been well-studied in vitro and is a paradigm for bacterial interactions with the host immune system. Recent work has revealed that components of the cytoskeleton have important functions in innate immunity and inflammation control. Septins, highly conserved cytoskeletal proteins, have emerged as key players in innate immunity to bacterial infection, yet septin function in vivo is poorly understood. Here, we use S. flexneri infection of zebrafish (Danio rerio) larvae to study in vivo the role of septins in inflammation and infection control. We found that depletion of Sept15 or Sept7b, zebrafish orthologs of human SEPT7, significantly increased host susceptibility to bacterial infection. Live-cell imaging of Sept15-depleted larvae revealed increasing bacterial burdens and a failure of neutrophils to control infection. Strikingly, Sept15-depleted larvae present significantly increased activity of Caspase-1 and more cell death upon S. flexneri infection. Dampening of the inflammatory response with anakinra, an antagonist of interleukin-1 receptor (IL-1R), counteracts Sept15 deficiency in vivo by protecting zebrafish from hyper-inflammation and S. flexneri infection. These findings highlight a new role for septins in host defence against bacterial infection, and suggest that septin dysfunction may be an underlying factor in cases of hyper-inflammation.

Folate-dependent methylation of septins governs ciliogenesis during neural tube closure

Periconception maternal folic acid (vitamin B₉) supplementation can reduce the prevalence of neural tube defects (NTDs), although just how folates benefit the developing embryo and promote closing of the neural tube and other morphologic processes during development remains unknown. Folate contributes to a 1-carbon metabolism, which is essential for purine biosynthesis and methionine recycling and affects methylation of DNA, histones, and nonhistone proteins. Herein, we used animal models and cultured mammalian cells to demonstrate that disruption of the methylation pathway mediated by folate compromises normal neural tube closure (NTC) and ciliogenesis. We demonstrate that the embryos with NTD failed to adequately methylate septin2, a key regulator of cilium structure and function. We report that methylation of septin2 affected its GTP binding activity and formation of the septin2-6-7 complex. We propose that folic acid promotes normal NTC in some embryos by regulating the methylation of septin2, which is critical for normal cilium formation during early embryonic development.-Toriyama, M., Toriyama, M., Wallingford, J. B., Finnell, R. H. Folate-dependent methylation of septins governs ciliogenesis during neural tube closure.

Uncovering the Roles of Septins in Cilia

Septins are a family of GTP-binding proteins that associate with cellular membranes and the cytoskeleton. Their ability to polymerize into filamentous structures permits them to serve as diffusion barriers for membrane proteins and as multi-molecular scaffolds that recruit components of signaling pathways. At the cellular level, septins contribute to the regulation of numerous processes, including cytokinesis, cell polarity, cell migration, and many others. In this review, we discuss emerging evidence for roles of mammalian septins in the biogenesis and function of flagella and cilia, and how this may impact human diseases such as ciliopathies.

Expression of sept3, sept5a and sept5b in the Developing and Adult Nervous System of the Zebrafish (Danio rerio)

Septins are a highly conserved family of small GTPases that form cytoskeletal filaments. Their cellular functions, especially in the nervous system, still remain largely enigmatic, but there are accumulating lines of evidence that septins play important roles in neuronal physiology and pathology. In order to further dissect septin function in the nervous system a detailed temporal resolved analysis in the genetically well tractable model vertebrate zebrafish (Danio rerio) is crucially necessary. To close this knowledge gap we here provide a reference dataset describing the expression of selected septins (sept3, sept5a and sept5b) in the zebrafish central nervous system. Strikingly, proliferation zones are devoid of expression of all three septins investigated, suggesting that they have a role in post-mitotic neural cells. Our finding that three septins are mainly expressed in non-proliferative regions was further confirmed by double-stainings with a proliferative marker. Our RNA in situ hybridization (ISH) study, detecting sept3, sept5a and sept5b mRNAs, shows that all three septins are expressed in largely overlapping regions of the developing brain. However, the expression of sept5a is much more confined compared to sept3 and sept5b. In contrast, the expression of all the three analyzed septins is largely similar in the adult brain.

From Planar Cell Polarity to Ciliogenesis and Back: The Curious Tale of the PPE and CPLANE proteins

Why some genes are more popular than others remains an open question, but one example of this phenomenon involves the genes controlling planar cell polarity (PCP), the polarization of cells within a plane of a tissue. Indeed, the so-called 'core' PCP genes such as dishevelled, frizzled, and prickle have been extensively studied both in animal models and by human genetics. By contrast, other genes that influence PCP signaling have received far less attention. Among the latter are inturned, fuzzy, and fritz, but recent work should bring these once obscure regulators into the limelight. We provide here a brief history of planar polarity effector (PPE) and CPLANE (ciliogenesis and planar polarity effector) proteins, discuss recent advances in understanding their molecular mechanisms of action, and describe their roles in human disease.

Investigation of septin biology in vivo using zebrafish

The zebrafish (Danio rerio) is an important animal model to study cell biology in vivo. Benefits of the zebrafish include a fully annotated reference genome, an easily manipulable genome (for example, by morpholino oligonucleotide or CRISPR-Cas9), and transparent embryos for noninvasive, real-time microscopy using fluorescent transgenic lines. Zebrafish have orthologues of most human septins, and studies using larvae were used to investigate the role of septins in vertebrate development. The zebrafish larva is also an established model to study the cell biology of infection and has recently been used to visualize septin assembly during bacterial infection in vivo. Here, we describe protocols for the study of septins in zebrafish, with emphasis on techniques used to investigate the role of septins in host defense against bacterial infection.

Analysis of cilia structure and function in zebrafish

Cilia are microtubule-based protrusions on the surface of most eukaryotic cells. They are found in most, if not all, vertebrate organs. Prominent cilia form in sensory structures, the eye, the ear, and the nose, where they are crucial for the detection of environmental stimuli, such as light and odors. Cilia are also involved in developmental processes, including left-right asymmetry formation, limb morphogenesis, and the patterning of neurons in the neural tube. Some cilia, such as those found in nephric ducts, are thought to have mechanosensory roles. Zebrafish proved very useful in genetic analysis and imaging of cilia-related processes, and in the modeling of mechanisms behind human cilia abnormalities, known as ciliopathies. A number of zebrafish defects resemble those seen in human ciliopathies. Forward and reverse genetic strategies generated a wide range of cilia mutants in zebrafish, which can be studied using sophisticated genetic and imaging approaches. In this chapter, we provide a set of protocols to examine cilia morphology, motility, and cilia-related defects in a variety of organs, focusing on the embryo and early postembryonic development.

Asymmetric Distribution of Primary Cilia Allocates Satellite Cells for Self-Renewal

Regeneration of vertebrate skeletal muscles requires satellite cells, a population of stem cells that are quiescent in normal conditions and divide, differentiate, and self-renew upon activation triggered by exercise, injury, and degenerative diseases. Satellite cell self-renewal is essential for long-term tissue homeostasis, and previous work has identified a number of external cues that control this process. However, little is known of the possible intrinsic control mechanisms of satellite cell self-renewal. Here, we show that quiescent satellite cells harbor a primary cilium, which is rapidly disassembled upon entry into the cell cycle. Contrasting with a commonly accepted belief, cilia reassembly does not occur uniformly in cells exiting the cell cycle. We found that primary cilia reassemble preferentially in cells committed to self-renew, and disruption of cilia reassembly causes a specific deficit in self-renewing satellite cells. These observations indicate that primary cilia provide an intrinsic cue essential for satellite cell self-renewal.

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In skeletal muscles, primary cilia are associated with quiescent satellite cells

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Primary cilia disassemble when satellite cells enter the cell cycle

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Primary cilia reassemble preferentially in self-renewing satellite cells

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Disruption of primary cilia reassembly impairs satellite cell self-renewal

Immunofluorescent staining of septins in primary cilia

Primary cilia are cellular antennae that receive and transduce extracellular cues. These microtubule-rich structures are comprised of at least three distinct ciliary compartments: basal bodies, transition zone, and axoneme. Septins have been implicated in cilia function at the transition zone, but accumulating evidence suggests that they localize predominantly within the axoneme. Here, we describe three fixation conditions that preserve the substructure of primary cilia and demonstrate known ciliary proteins that localize to these distinct ciliary substructures. Finally, we show immunostaining and live microscopy methods to detect septins within the axoneme.

Visualization of in vivo septin ultrastructures by platinum replica electron microscopy

Septins are cytoskeletal proteins involved in diverse biological processes including cytokinesis, cell morphogenesis, motility, and ciliogenesis. Septins form various filamentous structures in vitro and in vivo, but the higher-order architecture of septin structures in vivo remains poorly defined. The best understood system in this respect is the budding yeast Saccharomyces cerevisiae, where septins form a ring structure that undergoes multiple stages of remodeling during the cell cycle. In this chapter, we describe a method for visualizing supramolecular septin structures in yeast at high spatial resolution using platinum replica electron microscopy. This approach can be applied to further understand the regulation of assembly and remodeling of septin higher-order structures, as well as the relationship between septin architecture and function.

Kupffer's vesicle size threshold for robust left-right patterning of the zebrafish embryo

BACKGROUND

Motile cilia in the "organ of asymmetry" create directional fluid flows that are vital for left-right (LR) asymmetric patterning of vertebrate embryos. Organ function often depends on tightly regulated organ size control, but the role of organ of asymmetry size in LR patterning has remained unknown. Observations of the organ of asymmetry in the zebrafish, called Kupffer's vesicle (KV), have suggested significant variations in KV size in wild-type embryos, raising questions about the impact of KV organ size on LR patterning.

RESULTS

To understand the relationship between organ of asymmetry size and its function, we characterized variations in KV at several developmental stages and in several different zebrafish strains. We found that the number of KV cilia and the size of the KV lumen were highly variable, whereas the length of KV cilia showed less variation. These variabilities were similar among different genetic backgrounds. By specifically modulating KV size and analyzing individual embryos, we identified a size threshold that is necessary for KV function.

CONCLUSIONS

Together these results indicate the KV organ of asymmetry size is not tightly controlled during development, but rather must only exceed a threshold to direct robust LR patterning of the zebrafish embryo.

Orphan G-protein coupled receptor 22 (Gpr22) regulates cilia length and structure in the zebrafish Kupffer's vesicle

GPR22 is an orphan G protein-coupled receptor (GPCR). Since the ligand of the receptor is currently unknown, its biological function has not been investigated in depth. Many GPCRs and their intracellular effectors are targeted to cilia. Cilia are highly conserved eukaryotic microtubule-based organelles that protrude from the membrane of most mammalian cells. They are involved in a large variety of physiological processes and diseases. However, the details of the downstream pathways and mechanisms that maintain cilia length and structure are poorly understood. We show that morpholino knock down or overexpression of gpr22 led to defective left-right (LR) axis formation in the zebrafish embryo. Specifically, defective LR patterning included randomization of the left-specific lateral plate mesodermal genes (LPM) (lefty1, lefty2, southpaw and pitx2a), resulting in randomized cardiac looping. Furthermore, gpr22 inactivation in the Kupffer’s vesicle (KV) alone was still able to generate the phenotype, indicating that Gpr22 mainly regulates LR asymmetry through the KV. Analysis of the KV cilia by immunofluorescence and transmission electron microscopy (TEM), revealed that gpr22 knock down or overexpression resulted in changes of cilia length and structure. Further, we found that Gpr22 does not act upstream of the two cilia master regulators, Foxj1a and Rfx2. To conclude, our study characterized a novel player in the field of ciliogenesis.