Polaris and Polycystin-2 in dorsal forerunner cells and Kupffer's vesicle are required for specification of the zebrafish left-right axis

PKD2: An Important Membrane Protein in Organ Development

PKD2 was first identified as the pathogenic protein for autosomal dominant polycystic kidney disease (ADPKD) and is widely recognized as an ion channel. Subsequent studies have shown that PKD2 is widely expressed in various animal tissues and plays a crucial role in tissue and organ development. Additionally, PKD2 is conserved from single-celled organisms to vertebrates. Here, we provide an overview of recent advances in the function of PKD2 in key model animals, focusing on the establishment of left-right organ asymmetry, renal homeostasis, cardiovascular development, and signal transduction in reproduction and mating. We specifically focus on the roles of PKD2 in development and highlight future prospects for PKD2 research.

The role of cilia in the development, survival, and regeneration of hair cells

Mutations impacting cilia genes lead to a class of human diseases known as ciliopathies. This is due to the role of cilia in the development, survival, and regeneration of many cell types. We investigated the extent to which disrupting cilia impacted these processes in lateral line hair cells of zebrafish. We found that mutations in two intraflagellar transport (IFT) genes, ift88 and dync2h1, which lead to the loss of kinocilia, caused increased hair cell apoptosis. IFT gene mutants also have a decreased mitochondrial membrane potential, and blocking the mitochondrial uniporter causes a loss of hair cells in wild-type zebrafish but not mutants, suggesting mitochondria dysfunction may contribute to the apoptosis seen in these mutants. These mutants also showed decreased proliferation during hair cell regeneration but did not show consistent changes in support cell number or proliferation during hair cell development. These results show that the loss of hair cells seen following disruption of cilia through either mutations in anterograde or retrograde IFT genes appears to be due to impacts on hair cell survival but not necessarily development in the zebrafish lateral line.

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.

Cardiac Manifestations in Patients with Autosomal Dominant Polycystic Kidney Disease (ADPKD): A Single-Center Study

Key Points

Cardiovascular disease—a key driver of morbidity in CKD—is common in patients with autosomal dominant polycystic kidney disease (ADPKD). Pathologic echocardiography findings, including valvular defects, aortic root dilation, and hypertrophy, are found in most patients with ADPKD. These findings correlate with parameters indicating disease progression in ADPKD. Echocardiography should be offered to all patients with ADPKD.

Background

ADPKD is the most common monogenetic kidney disease and results in kidney failure in >75% of affected individuals. As a systemic disorder, ADPKD is associated with a variety of extrarenal manifestations, including cardiac manifestations, that affect the majority of patients. We characterized the cardiac involvement in patients with ADPKD from the German AD(H)PKD registry and compared them with kidney donor candidates as controls.

Methods

In this single-center cohort study, we evaluated 141 patients with ADPKD (44.17±11.23 years) from the German AD(H)PKD registry and 60 kidney donor candidates (55.08±10.21 years). All patients underwent clinical examination, abdominal MRI, and transthoracic echocardiography.

Results

Of the patients with ADPKD, 65% showed hypertrophy of the left ventricle (as defined by an end-diastolic interventricular septal wall thickness [IVSd] >10 mm) compared with 55% in control patients. Mitral regurgitation was the most common finding among 54% of patients with ADPKD who exhibited valvular dysfunction, albeit mild in most patients. Interestingly, left ventricular ejection fraction (LV-EF) differed significantly between both groups, with higher values in patients with ADPKD (64%±6% versus 60%±6%), whereas other parameters, including IVSd, left ventricular end-diastolic diameter (LVEDD), tricuspid annular plane systolic excursion (TAPSE), and pressure gradients across the aortic and tricuspid valve were similar between groups. Correlations of echocardiographic parameters with markers of disease progression revealed statistically significant associations for aortic root diameter (P =0.01), the pressure gradient across the aortic valve (AV dPmax; P =0.0003), and IVSd (P =0.0001), indicating rapid kidney disease progression may also be associated with cardiac findings.

Conclusion

Cardiovascular abnormalities are prevalent in patients with ADPKD. Considering the importance of cardiovascular disease for outcomes in CKD, early management and possibly prevention are important goals of any treatment scheme. Consequently, echocardiography should be offered to all patients with ADPKD in routine management.

Cilia function as calcium-mediated mechanosensors that instruct left-right asymmetry

The breaking of bilateral symmetry in most vertebrates is critically dependent upon the motile cilia of the embryonic left-right organizer (LRO), which generate a directional fluid flow; however, it remains unclear how this flow is sensed. Here, we demonstrated that immotile LRO cilia are mechanosensors for shear force using a methodological pipeline that combines optical tweezers, light sheet microscopy, and deep learning to permit in vivo analyses in zebrafish. Mechanical manipulation of immotile LRO cilia activated intraciliary calcium transients that required the cation channel Polycystin-2. Furthermore, mechanical force applied to LRO cilia was sufficient to rescue and reverse cardiac situs in zebrafish that lack motile cilia. Thus, LRO cilia are mechanosensitive cellular levers that convert biomechanical forces into calcium signals to instruct left-right asymmetry.

Understanding laterality disorders and the left-right organizer: Insights from zebrafish

Vital internal organs display a left-right (LR) asymmetric arrangement that is established during embryonic development. Disruption of this LR asymmetry-or laterality-can result in congenital organ malformations. Situs inversus totalis (SIT) is a complete concordant reversal of internal organs that results in a low occurrence of clinical consequences. Situs ambiguous, which gives rise to Heterotaxy syndrome (HTX), is characterized by discordant development and arrangement of organs that is associated with a wide range of birth defects. The leading cause of health problems in HTX patients is a congenital heart malformation. Mutations identified in patients with laterality disorders implicate motile cilia in establishing LR asymmetry. However, the cellular and molecular mechanisms underlying SIT and HTX are not fully understood. In several vertebrates, including mouse, frog and zebrafish, motile cilia located in a "left-right organizer" (LRO) trigger conserved signaling pathways that guide asymmetric organ development. Perturbation of LRO formation and/or function in animal models recapitulates organ malformations observed in SIT and HTX patients. This provides an opportunity to use these models to investigate the embryological origins of laterality disorders. The zebrafish embryo has emerged as an important model for investigating the earliest steps of LRO development. Here, we discuss clinical characteristics of human laterality disorders, and highlight experimental results from zebrafish that provide insights into LRO biology and advance our understanding of human laterality disorders.

Variable phenotypes and penetrance between and within different zebrafish ciliary transition zone mutants

Meckel syndrome, nephronophthisis, Joubert syndrome and Bardet-Biedl syndrome are caused by mutations in proteins that localize to the ciliary transition zone (TZ). The phenotypically distinct syndromes suggest that these TZ proteins have differing functions. However, mutations in a single TZ gene can result in multiple syndromes, suggesting that the phenotype is influenced by modifier genes. We performed a comprehensive analysis of ten zebrafish TZ mutants, including mks1, tmem216, tmem67, rpgrip1l, cc2d2a, b9d2, cep290, tctn1, nphp1 and nphp4, as well as mutants in ift88 and ift172. Our data indicate that variations in phenotypes exist between different TZ mutants, supporting different tissue-specific functions of these TZ genes. Further, we observed phenotypic variations within progeny of a single TZ mutant, reminiscent of multiple disease syndromes being associated with mutations in one gene. In some mutants, the dynamics of the phenotype became complex with transitory phenotypes that are corrected over time. We also demonstrated that multiple-guide-derived CRISPR/Cas9 F0 'crispant' embryos recapitulate zygotic null phenotypes, and rapidly identified ciliary phenotypes in 11 cilia-associated gene candidates (ankfn1, ccdc65, cfap57, fhad1, nme7, pacrg, saxo2, c1orf194, ttc26, zmynd12 and cfap52).

Eomes function is conserved between zebrafish and mouse and controls left-right organiser progenitor gene expression via interlocking feedforward loops

The T-box family transcription factor Eomesodermin (Eomes) is present in all vertebrates, with many key roles in the developing mammalian embryo and immune system. Homozygous Eomes mutant mouse embryos exhibit early lethality due to defects in both the embryonic mesendoderm and the extraembryonic trophoblast cell lineage. In contrast, zebrafish lacking the predominant Eomes homologue A (Eomesa) do not suffer complete lethality and can be maintained. This suggests fundamental differences in either the molecular function of Eomes orthologues or the molecular configuration of processes in which they participate. To explore these hypotheses we initially analysed the expression of distinct Eomes isoforms in various mouse cell types. Next we compared the functional capabilities of these murine isoforms to zebrafish Eomesa. These experiments provided no evidence for functional divergence. Next we examined the functions of zebrafish Eomesa and other T-box family members expressed in early development, as well as its paralogue Eomesb. Though Eomes is a member of the Tbr1 subfamily we found evidence for functional redundancy with the Tbx6 subfamily member Tbx16, known to be absent from eutherians. However, Tbx16 does not appear to synergise with Eomesa cofactors Mixl1 and Gata5. Finally, we analysed the ability of Eomesa and other T-box factors to induce zebrafish left-right organiser progenitors (known as dorsal forerunner cells) known to be positively regulated by vgll4l, a gene we had previously shown to be repressed by Eomesa. Here we demonstrate that Eomesa indirectly upregulates vgll4l expression via interlocking feedforward loops, suggesting a role in establishment of left-right asymmetry. Conversely, other T-box factors could not similarly induce left-right organiser progenitors. Overall these findings demonstrate conservation of Eomes molecular function and participation in similar processes, but differential requirements across evolution due to additional co-expressed T-box factors in teleosts, albeit with markedly different molecular capabilities. Our analyses also provide insights into the role of Eomesa in left-right organiser formation in zebrafish.

Polycystin-2 (PC2) is a key determinant of in vitro myogenesis

The development of skeletal muscle (myogenesis) is a well-orchestrated process where myoblasts withdraw from the cell cycle and differentiate into myotubes. Signaling by fluxes in intracellular calcium (Ca²⁺) is known to contribute to myogenesis, and increased mitochondrial biogenesis is required to meet the metabolic demand of mature myotubes. However, gaps remain in the understanding of how intracellular Ca²⁺ signals can govern myogenesis. Polycystin-2 (PC2 or TRPP1) is a nonselective cation channel permeable to Ca²⁺. It can interact with intracellular calcium channels to control Ca²⁺ release and concurrently modulates mitochondrial function and remodeling. Due to these features, we hypothesized that PC2 is a central protein in mediating both the intracellular Ca²⁺ responses and mitochondrial changes seen in myogenesis. To test this hypothesis, we created CRISPR/Cas9 knockout (KO) C2C12 murine myoblast cell lines. PC2 KO cells were unable to differentiate into myotubes, had impaired spontaneous Ca²⁺ oscillations, and did not develop depolarization-evoked Ca²⁺ transients. The autophagic-associated pathway beclin-1 was downregulated in PC2 KO cells, and direct activation of the autophagic pathway resulted in decreased mitochondrial remodeling. Re-expression of full-length PC2, but not a calcium channel dead pathologic mutant, restored the differentiation phenotype and increased the expression of mitochondrial proteins. Our results establish that PC2 is a novel regulator of in vitro myogenesis by integrating PC2-dependent Ca²⁺ signals and metabolic pathways.

Bicc1 and Dicer regulate left-right patterning through post-transcriptional control of the Nodal inhibitor Dand5

Rotating cilia at the vertebrate left-right organizer (LRO) generate an asymmetric leftward flow, which is sensed by cells at the left LRO margin. Ciliary activity of the calcium channel Pkd2 is crucial for flow sensing. How this flow signal is further processed and relayed to the laterality-determining Nodal cascade in the left lateral plate mesoderm (LPM) is largely unknown. We previously showed that flow down-regulates mRNA expression of the Nodal inhibitor Dand5 in left sensory cells. De-repression of the co-expressed Nodal, complexed with the TGFß growth factor Gdf3, drives LPM Nodal cascade induction. Here, we show that post-transcriptional repression of dand5 is a central process in symmetry breaking of Xenopus, zebrafish and mouse. The RNA binding protein Bicc1 was identified as a post-transcriptional regulator of dand5 and gdf3 via their 3'-UTRs. Two distinct Bicc1 functions on dand5 mRNA were observed at pre- and post-flow stages, affecting mRNA stability or flow induced translational inhibition, respectively. To repress dand5, Bicc1 co-operates with Dicer1, placing both proteins in the process of flow sensing. Intriguingly, Bicc1 mediated translational repression of a dand5 3'-UTR mRNA reporter was responsive to pkd2, suggesting that a flow induced Pkd2 signal triggers Bicc1 mediated dand5 inhibition during symmetry breakage.

Oncogenic FGFR Fusions Produce Centrosome and Cilia Defects by Ectopic Signaling

A single primary cilium projects from most vertebrate cells to guide cell fate decisions. A growing list of signaling molecules is found to function through cilia and control ciliogenesis, including the fibroblast growth factor receptors (FGFR). Aberrant FGFR activity produces abnormal cilia with deregulated signaling, which contributes to pathogenesis of the FGFR-mediated genetic disorders. FGFR lesions are also found in cancer, raising a possibility of cilia involvement in the neoplastic transformation and tumor progression. Here, we focus on FGFR gene fusions, and discuss the possible mechanisms by which they function as oncogenic drivers. We show that a substantial portion of the FGFR fusion partners are proteins associated with the centrosome cycle, including organization of the mitotic spindle and ciliogenesis. The functions of centrosome proteins are often lost with the gene fusion, leading to haploinsufficiency that induces cilia loss and deregulated cell division. We speculate that this complements the ectopic FGFR activity and drives the FGFR fusion cancers.

Getting to the Heart of Left-Right Asymmetry: Contributions from the Zebrafish Model

The heart is laterally asymmetric. Not only is it positioned on the left side of the body but the organ itself is asymmetric. This patterning occurs across scales: at the organism level, through left-right axis patterning; at the organ level, where the heart itself exhibits left-right asymmetry; at the cellular level, where gene expression, deposition of matrix and proteins and cell behaviour are asymmetric; and at the molecular level, with chirality of molecules. Defective left-right patterning has dire consequences on multiple organs; however, mortality and morbidity arising from disrupted laterality is usually attributed to complex cardiac defects, bringing into focus the particulars of left-right patterning of the heart. Laterality defects impact how the heart integrates and connects with neighbouring organs, but the anatomy of the heart is also affected because of its asymmetry. Genetic studies have demonstrated that cardiac asymmetry is influenced by left-right axis patterning and yet the heart also possesses intrinsic laterality, reinforcing the patterning of this organ. These inputs into cardiac patterning are established at the very onset of left-right patterning (formation of the left-right organiser) and continue through propagation of left-right signals across animal axes, asymmetric differentiation of the cardiac fields, lateralised tube formation and asymmetric looping morphogenesis. In this review, we will discuss how left-right asymmetry is established and how that influences subsequent asymmetric development of the early embryonic heart. In keeping with the theme of this issue, we will focus on advancements made through studies using the zebrafish model and describe how its use has contributed considerable knowledge to our understanding of the patterning of the heart.

Pkd2 Affects Cilia Length and Impacts LR Flow Dynamics and Dand5

The left-right (LR) field recognizes the importance of the mechanism involving the calcium permeable channel Polycystin-2. However, whether the early LR symmetry breaking mechanism is exclusively via Polycystin-2 has not been tested. For that purpose, we need to be able to isolate the effects of decreasing the levels of Pkd2 protein from any eventual effects on flow dynamics. Here we demonstrate that curly-up (cup) homozygous mutants have abnormal flow dynamics. In addition, we performed one cell stage Pkd2 knockdowns and LR organizer specific Pkd2 knockdowns and observed that both techniques resulted in shorter cilia length and abnormal flow dynamics. We conclude that Pkd2 reduction leads to LR defects that cannot be assigned exclusively to its putative role in mediating mechanosensation because indirectly, by modifying cell shape or decreasing cilia length, Pkd2 deficit affects LR flow dynamics.

Aquatic models of human ciliary diseases

Cilia are microtubule-based structures that either transmit information into the cell or move fluid outside of the cell. There are many human diseases that arise from malfunctioning cilia. Although mammalian models provide vital insights into the underlying pathology of these diseases, aquatic organisms such as Xenopus and zebrafish provide valuable tools to help screen and dissect out the underlying causes of these diseases. In this review we focus on recent studies that identify or describe different types of human ciliopathies and outline how aquatic organisms have aided our understanding of these diseases.

Ppargc1a Controls Ciliated Cell Development by Regulating Prostaglandin Biosynthesis

Cilia are microtubule-based organelles that function in a multitude of physiological contexts to perform chemosensing, mechanosensing, and fluid propulsion. The process of ciliogenesis is highly regulated, and disruptions result in disease states termed ciliopathies. Here, we report that peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (ppargc1a) is essential for ciliogenesis in nodal, mono-, and multiciliated cells (MCCs) and for discernment of renal tubule ciliated cell fate during embryogenesis. ppargc1a performs these functions by affecting prostaglandin signaling, whereby cilia formation and renal MCC fate are restored with prostaglandin E₂ (PGE₂) treatment in ppargc1a-deficient animals. Genetic disruption of ppargc1a specifically reduces expression of the prostanoid biosynthesis gene prostaglandin-endoperoxide synthase 1 (ptgs1), and suboptimal knockdown of both genes shows this synergistic effect. Furthermore, ptgs1 overexpression rescues ciliogenesis and renal MCCs in ppargc1a-deficient embryos. These findings position Ppargc1a as a key genetic regulator of prostaglandin signaling during ciliated cell ontogeny.

Fgf-driven Tbx protein activities directly induce myf5 and myod to initiate zebrafish myogenesis

Skeletal muscle derives from dorsal mesoderm formed during vertebrate gastrulation. Fibroblast growth factor (Fgf) signalling cooperates with Tbx transcription factors to promote dorsal mesoderm formation, but their role in myogenesis has been unclear. Using zebrafish, we show that dorsally derived Fgf signals act through Tbx16 and Tbxta to induce slow and fast trunk muscle precursors at distinct dorsoventral positions. Tbx16 binds to and directly activates the myf5 and myod genes, which are required for commitment to myogenesis. Tbx16 activity depends on Fgf signalling from the organiser. In contrast, Tbxta is not required for myf5 expression, but binds a specific site upstream of myod that is not bound by Tbx16 and drives (dependent on Fgf signals) myod expression in adaxial slow precursors, thereby initiating trunk myogenesis. After gastrulation, when similar muscle cell populations in the post-anal tail are generated from tailbud, declining Fgf signalling is less effective at initiating adaxial myogenesis, which is instead initiated by Hedgehog signalling from the notochord. Our findings suggest a hypothesis for ancestral vertebrate trunk myogenic patterning and how it was co-opted during tail evolution to generate similar muscle by new mechanisms.

This article has an associated ‘The people behind the papers’ interview.

Highlighted Article: Tbx16 and Tbxta activate myf5 and myod directly during the earliest myogenesis in zebrafish, and Fgf signalling acts through Tbx16 to drive myogenesis in trunk but not tail.

Polycystin 2 is increased in disease to protect against stress-induced cell death

Polycystin 2 (PC2 or TRPP1, formerly TRPP2) is a calcium-permeant Transient Receptor Potential (TRP) cation channel expressed primarily on the endoplasmic reticulum (ER) membrane and primary cilia of all cell and tissue types. Despite its ubiquitous expression throughout the body, studies of PC2 have focused primarily on its role in the kidney, as mutations in PC2 lead to the development of autosomal dominant polycystic kidney disease (ADPKD), a debilitating condition for which there is no cure. However, the endogenous role that PC2 plays in the regulation of general cellular homeostasis remains unclear. In this study, we measure how PC2 expression changes in different pathological states, determine that its abundance is increased under conditions of cellular stress in multiple tissues including human disease, and conclude that PC2-deficient cells have increased susceptibility to cell death induced by stress. Our results offer new insight into the normal function of PC2 as a ubiquitous stress-sensitive protein whose expression is up-regulated in response to cell stress to protect against pathological cell death in multiple diseases.

Migrasomes provide regional cues for organ morphogenesis during zebrafish gastrulation

Migrasomes are recently identified vesicular organelles that form on retraction fibres behind migrating cells. Whether migrasomes are present in vivo and, if so, the function of migrasomes in living organisms is unknown. Here, we show that migrasomes are formed during zebrafish gastrulation and signalling molecules, such as chemokines, are enriched in migrasomes. We further demonstrate that Tspan4 and Tspan7 are required for migrasome formation. Organ morphogenesis is impaired in zebrafish MZtspan4a and MZtspan7 mutants. Mechanistically, migrasomes are enriched on a cavity underneath the embryonic shield where they serve as chemoattractants to ensure the correct positioning of dorsal forerunner cells vegetally next to the embryonic shield, thereby affecting organ morphogenesis. Our study shows that migrasomes are signalling organelles that provide specific biochemical information to coordinate organ morphogenesis.

Characterization of potential TRPP2 regulating proteins in early Xenopus embryos

Transient receptor potential cation channel-2 (TRPP2) is a nonspecific Ca²⁺ -dependent cation channel with versatile functions including control of extracellular calcium entry at the plasma membrane, release of intracellular calcium ([Ca²⁺ ]i) from internal stores of endoplasmic reticulum, and calcium-dependent mechanosensation in the primary cilium. In early Xenopus embryos, TRPP2 is expressed in cilia of the gastrocoel roof plate (GRP) involved in the establishment of left-right asymmetry, and in nonciliated kidney field (KF) cells, where it plays a central role in early specification of nephron tubule cells dependent on [Ca²⁺ ]i signaling. Identification of proteins binding to TRPP2 in embryo cells can provide interesting clues about the mechanisms involved in its regulation during these various processes. Using mass spectrometry, we have therefore characterized proteins from late gastrula/early neurula stage embryos coimmunoprecipitating with TRPP2. Binding of three of these proteins, golgin A2, protein kinase-D1, and disheveled-2 has been confirmed by immunoblotting analysis of TRPP2-coprecipitated proteins. Expression analysis of the genes, respectively, encoding these proteins, golga2, prkd1, and dvl2 indicates that they are likely to play a role in these two regions. Golga2 and prkd1 are expressed at later stage in the developing pronephric tubule where golgin A2 and protein kinase-D1 might also interact with TRPP2. Colocalization experiments using exogenously expressed fluorescent versions of TRPP2 and dvl2 in GRP and KF reveal that these two proteins are generally not coexpressed, and only colocalized in discrete region of cells. This was observed in KF cells, but does not appear to occur in the apical ciliated region of GRP cells.

The mitochondrial transporter SLC25A25 links ciliary TRPP2 signaling and cellular metabolism

Cilia are organelles specialized in movement and signal transduction. The ciliary transient receptor potential ion channel polycystin-2 (TRPP2) controls elementary cilia-mediated physiological functions ranging from male fertility and kidney development to left-right patterning. However, the molecular components translating TRPP2 channel-mediated Ca2+ signals into respective physiological functions are unknown. Here, we show that the Ca2+-regulated mitochondrial ATP-Mg/Pi solute carrier 25 A 25 (SLC25A25) acts downstream of TRPP2 in an evolutionarily conserved metabolic signaling pathway. We identify SLC25A25 as an essential component in this cilia-dependent pathway using a genome-wide forward genetic screen in Drosophila melanogaster, followed by a targeted analysis of SLC25A25 function in zebrafish left-right patterning. Our data suggest that TRPP2 ion channels regulate mitochondrial SLC25A25 transporters via Ca2+ establishing an evolutionarily conserved molecular link between ciliary signaling and mitochondrial metabolism.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Zebrafish Models of Rare Hereditary Pediatric Diseases

Recent advances in sequencing technologies have made it significantly easier to find the genetic roots of rare hereditary pediatric diseases. These novel methods are not panaceas, however, and they often give ambiguous results, highlighting multiple possible causative mutations in affected patients. Furthermore, even when the mapping results are unambiguous, the affected gene might be of unknown function. In these cases, understanding how a particular genotype can result in a phenotype also needs carefully designed experimental work. Model organism genetics can offer a straightforward experimental setup for hypothesis testing. Containing orthologs for over 80% of the genes involved in human diseases, zebrafish (Danio rerio) has emerged as one of the top disease models over the past decade. A plethora of genetic tools makes it easy to create mutations in almost any gene of the zebrafish genome and these mutant strains can be used in high-throughput preclinical screens for active molecules. As this small vertebrate species offers several other advantages as well, its popularity in biomedical research is bound to increase, with “aquarium to bedside” drug development pipelines taking a more prevalent role in the near future.

In Vivo Regulation of the Zebrafish Endoderm Progenitor Niche by T-Box Transcription Factors

T-box transcription factors T/Brachyury homolog A (Ta) and Tbx16 are essential for correct mesoderm development in zebrafish. The downstream transcriptional networks guiding their functional activities are poorly understood. Additionally, important contributions elsewhere are likely masked due to redundancy. Here, we exploit functional genomic strategies to identify Ta and Tbx16 targets in early embryogenesis. Surprisingly, we discovered they not only activate mesodermal gene expression but also redundantly regulate key endodermal determinants, leading to substantial loss of endoderm in double mutants. To further explore the gene regulatory networks (GRNs) governing endoderm formation, we identified targets of Ta/Tbx16-regulated homeodomain transcription factor Mixl1, which is absolutely required in zebrafish for endoderm formation. Interestingly, we find many endodermal determinants coordinately regulated through common genomic occupancy by Mixl1, Eomesa, Smad2, Nanog, Mxtx2, and Pou5f3. Collectively, these findings augment the endoderm GRN and reveal a panel of target genes underlying the Ta, Tbx16, and Mixl1 mutant phenotypes.

An Early Function of Polycystin-2 for Left-Right Organizer Induction in Xenopus

Nodal signaling controls asymmetric organ placement during vertebrate embryogenesis. Nodal is induced by a leftward fluid flow at the ciliated left-right organizer (LRO). The mechanism of flow sensing, however, has remained elusive. pkd2 encodes the calcium channel Polycystin-2, which is required for kidney development and laterality, and may act in flow perception. Here, we have studied the role of Polycystin-2 in Xenopus and show that pkd2 is indispensable for left-right (LR) asymmetry. Knockdown of pkd2 prevented left-asymmetric nodal cascade induction in the lateral plate mesoderm. Defects were due to failure of LRO specification, morphogenesis, and, consequently, absence of leftward flow. Polycystin-2 synergizes with the unconventional nodal-type signaling molecule Xnr3 to induce the LRO precursor tissue before gastrulation, upstream of symmetry breakage. Our data uncover an unknown function of pkd2 in LR axis formation, which we propose represents an ancient role of Polycystin-2 during LRO induction in lower vertebrates.

•

Loss of Polycystin-2 in Xenopus results in LR asymmetry defects upstream of leftward flow

•

LR defects are caused by lack of LR organizer induction

•

Polycystin-2 is required upstream of foxj1 for specification of superficial mesoderm

•

Polycystin-2 and Xnr3 synergistically induce foxj1 in the superficial mesoderm

Vertebrate Left-Right Asymmetry: What Can Nodal Cascade Gene Expression Patterns Tell Us?

Laterality of inner organs is a wide-spread characteristic of vertebrates and beyond. It is ultimately controlled by the left-asymmetric activation of the Nodal signaling cascade in the lateral plate mesoderm of the neurula stage embryo, which results from a cilia-driven leftward flow of extracellular fluids at the left-right organizer. This scenario is widely accepted for laterality determination in wildtype specimens. Deviations from this norm come in different flavors. At the level of organ morphogenesis, laterality may be inverted (situs inversus) or non-concordant with respect to the main body axis (situs ambiguus or heterotaxia). At the level of Nodal cascade gene activation, expression may be inverted, bilaterally induced, or absent. In a given genetic situation, patterns may be randomized or predominantly lacking laterality (absence or bilateral activation). We propose that the distributions of patterns observed may be indicative of the underlying molecular defects, with randomizations being primarily caused by defects in the flow-generating ciliary set-up, and symmetrical patterns being the result of impaired flow sensing, on the left, the right, or both sides. This prediction, the reasoning of which is detailed in this review, pinpoints functions of genes whose role in laterality determination have remained obscure.

Antagonistic interactions in the zebrafish midline prior to the emergence of asymmetric gene expression are important for left-right patterning

Left-right (L-R) asymmetry of the internal organs of vertebrates is presaged by domains of asymmetric gene expression in the lateral plate mesoderm (LPM) during somitogenesis. Ciliated L-R coordinators (LRCs) are critical for biasing the initiation of asymmetrically expressed genes, such as nodal and pitx2, to the left LPM. Other midline structures, including the notochord and floorplate, are then required to maintain these asymmetries. Here we report an unexpected role for the zebrafish EGF-CFC gene one-eyed pinhead (oep) in the midline to promote pitx2 expression in the LPM. Late zygotic oep (LZoep) mutants have strongly reduced or absent pitx2 expression in the LPM, but this expression can be rescued to strong levels by restoring oep in midline structures only. Furthermore, removing midline structures from LZoep embryos can rescue pitx2 expression in the LPM, suggesting the midline is a source of an LPM pitx2 repressor that is itself inhibited by oep Reducing lefty1 activity in LZoep embryos mimics removal of the midline, implicating lefty1 in the midline-derived repression. Together, this suggests a model where Oep in the midline functions to overcome a midline-derived repressor, involving lefty1, to allow for the expression of left side-specific genes in the LPM.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Claudin5a is required for proper inflation of Kupffer's vesicle lumen and organ laterality

Left-right asymmetric organ development is critical to establish a proper body plan of vertebrates. In zebrafish, the Kupffer’s vesicle (KV) is a fluid-filled sac which controls asymmetric organ development, and a properly inflated KV lumen by means of fluid influx is a prerequisite for the asymmetric signal transmission. However, little is known about the components that support the paracellular tightness between the KV luminal epithelial cells to sustain hydrostatic pressure during KV lumen expansion. Here, we identified that the claudin5a (cldn5a) is highly expressed at the apical surface of KV epithelial cells and tightly seals the KV lumen. Downregulation of cldn5a in zebrafish showed a failure in organ laterality that resulted from malformed KV. In addition, accelerated fluid influx into KV by combined treatment of forskolin and 3-isobutyl-1-methylxanthine failed to expand the partially-formed KV lumen in cldn5a morphants. However, malformed KV lumen and defective heart laterality in cldn5a morphants were significantly rescued by exogenous cldn5a mRNA, suggesting that the tightness between the luminal epithelial cells is important for KV lumen formation. Taken together, these findings suggest that cldn5a is required for KV lumen inflation and left-right asymmetric organ development.

Klf8 regulates left-right asymmetric patterning through modulation of Kupffer's vesicle morphogenesis and spaw expression

BACKGROUND

Although vertebrates are bilaterally symmetric organisms, their internal organs are distributed asymmetrically along a left-right axis. Disruption of left-right axis asymmetric patterning often occurs in human genetic disorders. In zebrafish embryos, Kupffer's vesicle, like the mouse node, breaks symmetry by inducing asymmetric expression of the Nodal-related gene, spaw, in the left lateral plate mesoderm (LPM). Spaw then stimulates transcription of itself and downstream genes, including lft1, lft2, and pitx2, specifically in the left side of the diencephalon, heart and LPM. This developmental step is essential to establish subsequent asymmetric organ positioning. In this study, we evaluated the role of krüppel-like factor 8 (klf8) in regulating left-right asymmetric patterning in zebrafish embryos.

METHODS

Zebrafish klf8 expression was disrupted by both morpholino antisense oligomer-mediated knockdown and a CRISPR-Cas9 system. Whole-mount in situ hybridization was conducted to evaluate gene expression patterns of Nodal signalling components and the positions of heart and visceral organs. Dorsal forerunner cell number was evaluated in Tg(sox17:gfp) embryos and the length and number of cilia in Kupffer's vesicle were analyzed by immunocytochemistry using an acetylated tubulin antibody.

RESULTS

Heart jogging, looping and visceral organ positioning were all defective in zebrafish klf8 morphants. At the 18-22 s stages, klf8 morphants showed reduced expression of genes encoding Nodal signalling components (spaw, lft1, lft2, and pitx2) in the left LPM, diencephalon, and heart. Co-injection of klf8 mRNA with klf8 morpholino partially rescued spaw expression. Furthermore, klf8 but not klf8△zf overexpressing embryos showed dysregulated bilateral expression of Nodal signalling components at late somite stages. At the 10s stage, klf8 morphants exhibited reductions in length and number of cilia in Kupffer's vesicle, while at 75% epiboly, fewer dorsal forerunner cells were observed. Interestingly, klf8 mutant embryos, generated by a CRISPR-Cas9 system, showed bilateral spaw expression in the LPM at late somite stages. This observation may be partly attributed to compensatory upregulation of klf12b, because klf12b knockdown reduced the percentage of klf8 mutants exhibiting bilateral spaw expression.

CONCLUSIONS

Our results demonstrate that zebrafish Klf8 regulates left-right asymmetric patterning by modulating both Kupffer's vesicle morphogenesis and spaw expression in the left LPM.

Left-Right Patterning: Breaking Symmetry to Asymmetric Morphogenesis

Vertebrates exhibit striking left-right (L-R) asymmetries in the structure and position of the internal organs. Symmetry is broken by motile cilia-generated asymmetric fluid flow, resulting in a signaling cascade - the Nodal-Pitx2 pathway - being robustly established within mesodermal tissue on the left side only. This pathway impinges upon various organ primordia to instruct their side-specific development. Recently, progress has been made in understanding both the breaking of embryonic L-R symmetry and how the Nodal-Pitx2 pathway controls lateralized cell differentiation, migration, and other aspects of cell behavior, as well as tissue-level mechanisms, that drive asymmetries in organ formation. Proper execution of asymmetric organogenesis is critical to health, making furthering our understanding of L-R development an important concern.

TRPP2 ion channels: Critical regulators of organ morphogenesis in health and disease

Ion channels control the membrane potential and mediate transport of ions across membranes. Archetypical physiological functions of ion channels include processes such as regulation of neuronal excitability, muscle contraction, or transepithelial ion transport. In that regard, transient receptor potential ion channel polycystin 2 (TRPP2) is remarkable, because it controls complex morphogenetic processes such as the establishment of properly shaped epithelial tubules and left-right-asymmetry of organs. The fascinating question of how an ion channel regulates morphogenesis has since captivated the attention of scientists in different disciplines. Four loosely connected key insights on different levels of biological complexity ranging from protein to whole organism have framed our understanding of TRPP2 physiology: 1) TRPP2 is a non-selective cation channel; 2) TRPP2 is part of a receptor-ion channel complex; 3) TRPP2 localizes to primary cilia; and 4) TRPP2 is required for organ morphogenesis. In this review, we will discuss the current knowledge in these key areas and highlight some of the challenges ahead.

Identification and Expression Analysis of the Complete Family of Zebrafish pkd Genes

Polycystic kidney disease (PKD) proteins are trans-membrane proteins that have crucial roles in many aspects of vertebrate development and physiology, including the development of many organs as well as left–right patterning and taste. They can be divided into structurally-distinct PKD1-like and PKD2-like proteins and usually one PKD1-like protein forms a heteromeric polycystin complex with a PKD2-like protein. For example, PKD1 forms a complex with PKD2 and mutations in either of these proteins cause Autosomal Dominant Polycystic Kidney Disease (ADPKD), which is the most frequent potentially-lethal single-gene disorder in humans. Here, we identify the complete family of pkd genes in zebrafish and other teleosts. We describe the genomic locations and sequences of all seven genes: pkd1, pkd1b, pkd1l1, pkd1l2a, pkd1l2b, pkd2, and pkd2l1. pkd1l2a/pkd1l2b are likely to be ohnologs of pkd1l2, preserved from the whole genome duplication that occurred at the base of the teleosts. However, in contrast to mammals and cartilaginous and holostei fish, teleosts lack pkd2l2, and pkdrej genes, suggesting that these have been lost in the teleost lineage. In addition, teleost, and holostei fish have only a partial pkd1l3 sequence, suggesting that this gene may be in the process of being lost in the ray-finned fish lineage. We also provide the first comprehensive description of the expression of zebrafish pkd genes during development. In most structures we detect expression of one pkd1-like gene and one pkd2-like gene, consistent with these genes encoding a heteromeric protein complex. For example, we found that pkd2 and pkd1l1 are expressed in Kupffer's vesicle and pkd1 and pkd2 are expressed in the developing pronephros. In the spinal cord, we show that pkd1l2a and pkd2l1 are co-expressed in KA cells. We also identify potential co-expression of pkd1b and pkd2 in the floor-plate. Interestingly, and in contrast to mouse, we observe expression of all seven pkd genes in regions that may correspond to taste receptors. Taken together, these results provide a crucial catalog of pkd genes in an important model system for elucidating cell and developmental processes and modeling human diseases and the most comprehensive analysis of embryonic pkd gene expression in any vertebrate.

Bi-allelic Mutations in PKD1L1 Are Associated with Laterality Defects in Humans

Disruption of the establishment of left-right (L-R) asymmetry leads to situs anomalies ranging from situs inversus totalis (SIT) to situs ambiguus (heterotaxy). The genetic causes of laterality defects in humans are highly heterogeneous. Via whole-exome sequencing (WES), we identified homozygous mutations in PKD1L1 from three affected individuals in two unrelated families. PKD1L1 encodes a polycystin-1-like protein and its loss of function is known to cause laterality defects in mouse and medaka fish models. Family 1 had one fetus and one deceased child with heterotaxy and complex congenital heart malformations. WES identified a homozygous splicing mutation, c.6473+2_6473+3delTG, which disrupts the invariant splice donor site in intron 42, in both affected individuals. In the second family, a homozygous c.5072G>C (p.Cys1691Ser) missense mutation was detected in an individual with SIT and congenital heart disease. The p.Cys1691Ser substitution affects a highly conserved cysteine residue and is predicted by molecular modeling to disrupt a disulfide bridge essential for the proper folding of the G protein-coupled receptor proteolytic site (GPS) motif. Damaging effects associated with substitutions of this conserved cysteine residue in the GPS motif have also been reported in other genes, namely GPR56, BAI3, and PKD1 in human and lat-1 in C. elegans, further supporting the likely pathogenicity of p.Cys1691Ser in PKD1L1. The identification of bi-allelic PKD1L1 mutations recapitulates previous findings regarding phenotypic consequences of loss of function of the orthologous genes in mice and medaka fish and further expands our understanding of genetic contributions to laterality defects in humans.

Loss of ift122, a Retrograde Intraflagellar Transport (IFT) Complex Component, Leads to Slow, Progressive Photoreceptor Degeneration Due to Inefficient Opsin Transport

In the retina, aberrant opsin transport from cell bodies to outer segments leads to retinal degenerative diseases such as retinitis pigmentosa. Opsin transport is facilitated by the intraflagellar transport (IFT) system that mediates the bidirectional movement of proteins within cilia. In contrast to functions of the anterograde transport executed by IFT complex B (IFT-B), the precise functions of the retrograde transport mediated by IFT complex A (IFT-A) have not been well studied in photoreceptor cilia. Here, we analyzed developing zebrafish larvae carrying a null mutation in ift122 encoding a component of IFT-A. ift122 mutant larvae show unexpectedly mild phenotypes, compared with those of mutants defective in IFT-B. ift122 mutants exhibit a slow onset of progressive photoreceptor degeneration mainly after 7 days post-fertilization. ift122 mutant larvae also develop cystic kidney but not curly body, both of which are typically observed in various ciliary mutants. ift122 mutants display a loss of cilia in the inner ear hair cells and nasal pit epithelia. Loss of ift122 causes disorganization of outer segment discs. Ectopic accumulation of an IFT-B component, ift88, is observed in the ift122 mutant photoreceptor cilia. In addition, pulse-chase experiments using GFP-opsin fusion proteins revealed that ift122 is required for the efficient transport of opsin and the distal elongation of outer segments. These results show that IFT-A is essential for the efficient transport of outer segment proteins, including opsin, and for the survival of retinal photoreceptor cells, rendering the ift122 mutant a unique model for human retinal degenerative diseases.

The endoderm indirectly influences morphogenetic movements of the zebrafish head kidney through the posterior cardinal vein and VegfC

Integration of blood vessels and organ primordia determines organ shape and function. The head kidney in the zebrafish interacts with the dorsal aorta (DA) and the posterior cardinal vein (PCV) to achieve glomerular filtration and definitive hematopoiesis, respectively. How the head kidney co-develops with both the axial artery and vein remains unclear. We found that in endodermless sox32-deficient embryos, the head kidney associated with the PCV but not the DA. Disrupted convergent migration of the PCV and the head kidney in sox32-deficient embryos was rescued in a highly coordinated fashion through the restoration of endodermal cells. Moreover, grafted endodermal cells abutted the host PCV endothelium in the transplantation assay. Interestingly, the severely-disrupted head kidney convergence in the sox32-deficient embryo was suppressed by both the cloche mutation and the knockdown of endothelial genes, indicating that an interaction between the endoderm and the PCV restricts the migration of the head kidney. Furthermore, knockdown of either vegfC or its receptor vegfr3 suppressed the head kidney convergence defect in endodermless embryos and perturbed the head kidney-PCV association in wild-type embryos. Our findings thus underscore a role for PCV and VegfC in patterning the head kidney prior to organ assembly and function.

Antennas of organ morphogenesis: the roles of cilia in vertebrate kidney development

Cilia arose early during eukaryotic evolution, and their structural components are highly conserved from the simplest protists to complex metazoan species. In recent years, the role of cilia in the ontogeny of vertebrate organs has received increasing attention due to a staggering correlation between human disease and dysfunctional cilia. In particular, the presence of cilia in both the developing and mature kidney has become a deep area of research due to ciliopathies common to the kidney, such as polycystic kidney disease (PKD). Interestingly, mutations in genes encoding proteins that localize to the cilia cause similar cystic phenotypes in kidneys of various vertebrates, suggesting an essential role for cilia in kidney organogenesis and homeostasis as well. Importantly, the genes so far identified in kidney disease have conserved functions across species, whose kidneys include both primary and motile cilia. Here, we aim to provide a comprehensive description of cilia and their role in kidney development, as well as highlight the usefulness of the zebrafish embryonic kidney as a model to further understand the function of cilia in kidney health.

Genetic Analysis Reveals a Hierarchy of Interactions between Polycystin-Encoding Genes and Genes Controlling Cilia Function during Left-Right Determination

During mammalian development, left-right (L-R) asymmetry is established by a cilia-driven leftward fluid flow within a midline embryonic cavity called the node. This 'nodal flow' is detected by peripherally-located crown cells that each assemble a primary cilium which contain the putative Ca2+ channel PKD2. The interaction of flow and crown cell cilia promotes left side-specific expression of Nodal in the lateral plate mesoderm (LPM). Whilst the PKD2-interacting protein PKD1L1 has also been implicated in L-R patterning, the underlying mechanism by which flow is detected and the genetic relationship between Polycystin function and asymmetric gene expression remains unknown. Here, we characterize a Pkd1l1 mutant line in which Nodal is activated bilaterally, suggesting that PKD1L1 is not required for LPM Nodal pathway activation per se, but rather to restrict Nodal to the left side downstream of nodal flow. Epistasis analysis shows that Pkd1l1 acts as an upstream genetic repressor of Pkd2. This study therefore provides a genetic pathway for the early stages of L-R determination. Moreover, using a system in which cultured cells are supplied artificial flow, we demonstrate that PKD1L1 is sufficient to mediate a Ca2+ signaling response after flow stimulation. Finally, we show that an extracellular PKD domain within PKD1L1 is crucial for PKD1L1 function; as such, destabilizing the domain causes L-R defects in the mouse. Our demonstration that PKD1L1 protein can mediate a response to flow coheres with a mechanosensation model of flow sensation in which the force of fluid flow drives asymmetric gene expression in the embryo.

Little fish, big catch: zebrafish as a model for kidney disease

The zebrafish, Danio rerio, is a relevant vertebrate model for biomedical research and translational studies because of its broad genetic conservation with humans. In recent years, scientists have formulated a growing list of zebrafish kidney disease paradigms, the study of which has contributed a multitude of insights into the basic biology of human conditions and even identified potential therapeutic agents. Conversely, there are also distinctive aspects of zebrafish biology lacking in higher vertebrates, such as the capacity to heal without lasting scar formation after tissue damage and the ability to generate nephrons throughout their lifespan, which makes the zebrafish uniquely suited to study regeneration in the context of the kidney. Here, we review several informative zebrafish models of kidney disease and discuss their future applications in nephrology.

The zebrafish Kupffer's vesicle as a model system for the molecular mechanisms by which the lack of Polycystin-2 leads to stimulation of CFTR

In autosomal dominant polycystic kidney disease (ADPKD), cyst inflation and continuous enlargement are associated with marked transepithelial ion and fluid secretion into the cyst lumen via cystic fibrosis transmembrane conductance regulator (CFTR). Indeed, the inhibition or degradation of CFTR prevents the fluid accumulation within cysts. The in vivo mechanisms by which the lack of Polycystin-2 leads to CFTR stimulation are an outstanding challenge in ADPKD research and may bring important biomarkers for the disease. However, hampering their study, the available ADPKD in vitro cellular models lack the three-dimensional architecture of renal cysts and the ADPKD mouse models offer limited access for live-imaging experiments in embryonic kidneys. Here, we tested the zebrafish Kupffer's vesicle (KV) as an alternative model-organ. KV is a fluid-filled vesicular organ, lined by epithelial cells that express both CFTR and Polycystin-2 endogenously, being each of them easily knocked-down. Our data on the intracellular distribution of Polycystin-2 support its involvement in the KV fluid-flow induced Ca²⁺-signalling. Mirroring kidney cysts, the KV lumen inflation is dependent on CFTR activity and, as we clearly show, the knockdown of Polycystin-2 results in larger KV lumens through overstimulation of CFTR. In conclusion, we propose the zebrafish KV as a model organ to study the renal cyst inflation. Favouring its use, KV volume can be easily determined by in vivo imaging offering a live readout for screening compounds and genes that may prevent cyst enlargement through CFTR inhibition.

Summary: Here, we tested the zebrafish Kupffer's vesicle (KV) as a model organ to study, through in vivo imaging of KV volume, the stimulation of cystic fibrosis transmembrane conductance regulator (CFTR) in autosomal dominant polycystic kidney disease ADPKD.

X-Linked Retinitis Pigmentosa 2 Is a Novel Maternal-Effect Gene Required for Left-Right Asymmetry in Zebrafish

Retinitis pigmentosa 2 (RP2) gene is responsible for up to 20% of X-linked retinitis pigmentosa, a severe heterogeneous genetic disorder resulting in progressive retinal degeneration in humans. In vertebrates, several bodies of evidence have clearly established the role of Rp2 protein in cilia genesis and/or function. Unexpectedly, some observations in zebrafish have suggested the oocyte-predominant expression of the rp2 gene, a typical feature of maternal-effect genes. In the present study, we investigate the maternal inheritance of rp2 gene products in zebrafish eggs in order to address whether rp2 could be a novel maternal-effect gene required for normal development. Although both rp2 mRNA and corresponding protein are expressed during oogenesis, rp2 mRNA is maternally inherited, in contrast to Rp2 protein. A knockdown of the protein transcribed from both rp2 maternal and zygotic mRNA results in delayed epiboly and severe developmental defects, including eye malformations, that were not observed when only the protein from zygotic origin was knocked down. Moreover, the knockdown of maternal and zygotic Rp2 revealed a high incidence of left-right asymmetry establishment defects compared to only zygotic knockdown. Here we show that rp2 is a novel maternal-effect gene exclusively expressed in oocytes within the zebrafish ovary and demonstrate that maternal rp2 mRNA is essential for successful embryonic development and thus contributes to egg developmental competence. Our observations also reveal that Rp2 protein translated from maternal mRNA is important to allow normal heart loop formation, thus providing evidence of a direct maternal contribution to left-right asymmetry establishment.

Intraciliary calcium oscillations initiate vertebrate left-right asymmetry

BACKGROUND

Bilateral symmetry during vertebrate development is broken at the left-right organizer (LRO) by ciliary motility and the resultant directional flow of extracellular fluid. However, how ciliary motility is perceived and transduced into asymmetrical intracellular signaling at the LRO remains controversial. Previous work has indicated that sensory cilia and polycystin-2 (Pkd2), a cation channel, are required for sensing ciliary motility, yet their function and the molecular mechanism linking both to left-right signaling cascades are unknown.

RESULTS

Here we report novel intraciliary calcium oscillations (ICOs) at the LRO that connect ciliary sensation of ciliary motility to downstream left-right signaling. Utilizing cilia-targeted genetically encoded calcium indicators in live zebrafish embryos, we show that ICOs depend on Pkd2 and are left-biased at the LRO in response to ciliary motility. Asymmetric ICOs occur with onset of LRO ciliary motility, thus representing the earliest known LR asymmetric molecular signal. Suppression of ICOs using a cilia-targeted calcium sink reveals that they are essential for LR development.

CONCLUSIONS

These findings demonstrate that intraciliary calcium initiates LR development and identify cilia as a functional ion signaling compartment connecting ciliary motility and flow to molecular LR signaling.

TRPP2-dependent Ca2+ signaling in dorso-lateral mesoderm is required for kidney field establishment in Xenopus

In Xenopus laevis embryos, kidney field specification is dependent on retinoic acid (RA) and coincides with a dramatic increase of Ca(2+) transients, but the role of Ca(2+) signaling in the kidney field is unknown. Here, we identify TRPP2, a member of the transient receptor potential (TRP) superfamily of channel proteins encoded by the pkd2 gene, as a central component of Ca(2+) signaling in the kidney field. TRPP2 is strongly expressed at the plasma membrane where it might regulate extracellular Ca(2+) entry. Knockdown of pkd2 in the kidney field results in the downregulation of pax8, but not of other kidney field genes (lhx1, osr1 and osr2). We further show that inhibition of Ca(2+) signaling with an inducible Ca(2+) chelator also causes downregulation of pax8, and that pkd2 knockdown results in a severe inhibition of Ca(2+) transients in kidney field explants. Finally, we show that disruption of RA results both in an inhibition of intracellular Ca(2+) signaling and of TRPP2 incorporation into the plasma membrane of kidney field cells. We propose that TRPP2-dependent Ca(2+) signaling is a key component of pax8 regulation in the kidney field downstream of RA-mediated non-transcriptional control of TRPP2.

Phosphodiesterase 1A modulates cystogenesis in zebrafish

Substantial evidence indicates the importance of elevated cAMP in polycystic kidney disease (PKD). Accumulation of cAMP in cystic tissues may be, in part, caused by enhanced adenylyl cyclase activity, but inhibition of cAMP degradation by phosphodiesterases (PDE) likely has an important role, because cAMP is inactivated much faster than it is synthesized. PDE1 is the only PDE family activated by Ca(2+), which is reduced in PKD cells. To assess the contribution of the PDE1A subfamily to renal cyst formation, we examined the expression and function of PDE1A in zebrafish. We identified two splice isoforms with alternative starts corresponding to human PDE1A1 and PDE1A4. Expression of the two isoforms varied in embryos and adult tissues, and both isoforms hydrolyzed cAMP with Ca(2+)/calmodulin dependence. Depletion of PDE1A in zebrafish embryos using splice- and translation-blocking morpholinos (MOs) caused pronephric cysts, hydrocephalus, and body curvature. Human PDE1A RNA and the PKA inhibitors, H89 and Rp-cAMPS, partially rescued phenotypes of pde1a morphants. Additionally, MO depletion of PDE1A aggravated phenotypes in pkd2 morphants, causing more severe body curvature, and human PDE1A RNA partially rescued pkd2 morphant phenotypes, pronephric cysts, hydrocephalus, and body curvature. Together, these data indicate the integral role of PDE1A and cAMP signaling in renal development and cystogenesis, imply that PDE1A activity is altered downstream of polycystin-2, and suggest that PDE1A is a viable drug target for PKD.