Fluid extraction from the left-right organizer uncovers mechanical properties needed for symmetry breaking

Humans and other vertebrates define body axis left-right asymmetry in the early stages of embryo development. The mechanism behind left-right establishment is not fully understood. Symmetry breaking occurs in a dedicated organ called the left-right organizer (LRO) and involves motile cilia generating fluid-flow therein. However, it has been a matter of debate whether the process of symmetry breaking relies on a chemosensory or a mechanosensory mechanism (Shinohara et al., 2012). Novel tailored manipulations for LRO fluid extraction in living zebrafish embryos allowed us to pinpoint a physiological developmental period for breaking left-right symmetry during development. The shortest critical time-window was narrowed to one hour and characterized by a mild counterclockwise flow. The experimental challenge consisted in emptying the LRO of its fluid, abrogating simultaneously flow force and chemical determinants. Our findings revealed an unprecedented recovery capacity of the embryo to re-fil and re-circulate new LRO fluid. The embryos that later developed laterality problems were found to be those that had lower anterior angular velocity and thus less anterior-posterior heterogeneity. Next, aiming to test the presence of any secreted determinant, we replaced the extracted LRO fluid by a physiological buffer. Despite some transitory flow homogenization, laterality defects were absent unless viscosity was altered, demonstrating that symmetry breaking does not depend on the nature of the fluid content but is rather sensitive to fluid mechanics. Altogether, we conclude that the zebrafish LRO is more sensitive to fluid dynamics for symmetry breaking.

Sequential action of JNK genes establishes the embryonic left-right axis

The establishment of the left-right axis is crucial for the placement, morphogenesis and function of internal organs. Left-right specification is proposed to be dependent on cilia-driven fluid flow in the embryonic node. Planar cell polarity (PCP) signalling is crucial for patterning of nodal cilia, yet downstream effectors driving this process remain elusive. We have examined the role of the JNK gene family, a proposed downstream component of PCP signalling, in the development and function of the zebrafish node. We show jnk1 and jnk2 specify length of nodal cilia, generate flow in the node and restrict southpaw to the left lateral plate mesoderm. Moreover, loss of asymmetric southpaw expression does not result in disturbances to asymmetric organ placement, supporting a model in which nodal flow may be dispensable for organ laterality. Later, jnk3 is required to restrict pitx2c expression to the left side and permit correct endodermal organ placement. This work uncovers multiple roles for the JNK gene family acting at different points during left-right axis establishment. It highlights extensive redundancy and indicates JNK activity is distinct from the PCP signalling pathway.

Reemployment of Kupffer's vesicle cells into axial and paraxial mesoderm via transdifferentiation

Kupffer's vesicle (KV) in the teleost embryo is a fluid-filled vesicle surrounded by a layer of epithelial cells with rotating primary cilia. KV transiently acts as the left-right organizer and degenerates after the establishment of left-right asymmetric gene expression. Previous labelling experiments in zebrafish embryos indicated that descendants of KV-epithelial cells are incorporated into mesodermal tissues after the collapse of KV. However, the overall picture of their differentiation potency had been unclear due to the lack of suitable genetic tools and molecular analyses. In the present study, we established a novel zebrafish transgenic line with a promoter of dand5, in which all KV-epithelial cells and their descendants are specifically labelled until the larval stage. We found that KV-epithelial cells undergo epithelial-mesenchymal transition upon KV collapse and infiltrate into adjacent mesodermal progenitors, the presomitic mesoderm and chordoneural hinge. Once incorporated, the descendants of KV-epithelial cells expressed distinct mesodermal differentiation markers and contributed to the mature populations such as the axial muscles and notochordal sheath through normal developmental process. These results indicate that differentiated KV-epithelial cells possess unique plasticity in that they are reemployed into mesodermal lineages through transdifferentiation after they complete their initial role in KV.

CARMIL3 is important for cell migration and morphogenesis during early development in zebrafish

Cell migration is important during early animal embryogenesis. Cell migration and cell shape are controlled by actin assembly and dynamics, which depend on capping proteins, including the barbed-end heterodimeric actin capping protein (CP). CP activity can be regulated by capping-protein-interacting (CPI) motif proteins, including CARMIL (capping protein Arp2/3 myosin-I linker) family proteins. Previous studies of CARMIL3, one of the three highly conserved CARMIL genes in vertebrates, have largely been limited to cells in culture. Towards understanding CARMIL function during embryogenesis in vivo, we analyzed zebrafish lines carrying mutations of carmil3. Maternal-zygotic mutants showed impaired endodermal migration during gastrulation, along with defects in dorsal forerunner cell (DFC) cluster formation, which affected the morphogenesis of Kupffer's vesicle (KV). Mutant KVs were smaller, contained fewer cells and displayed decreased numbers of cilia, leading to defects in left/right (L/R) patterning with variable penetrance and expressivity. The penetrance and expressivity of the KV phenotype in carmil3 mutants correlated well with the L/R heart positioning defect at the end of embryogenesis. This in vivo animal study of CARMIL3 reveals its new role during morphogenesis of the vertebrate embryo. This role involves migration of endodermal cells and DFCs, along with subsequent morphogenesis of the KV and L/R asymmetry.

Prominins control ciliary length throughout the animal kingdom: New lessons from human prominin-1 and zebrafish prominin-3

Prominins (proms) are transmembrane glycoproteins conserved throughout the animal kingdom. They are associated with plasma membrane protrusions, such as primary cilia, as well as extracellular vesicles derived thereof. Primary cilia host numerous signaling pathways affected in diseases known as ciliopathies. Human PROM1 (CD133) is detected in both somatic and cancer stem cells and is also expressed in terminally differentiated epithelial and photoreceptor cells. Genetic mutations in the PROM1 gene result in retinal degeneration by impairing the proper formation of the outer segment of photoreceptors, a modified cilium. Here, we investigated the impact of proms on two distinct examples of ciliogenesis. First, we demonstrate that the overexpression of a dominant-negative mutant variant of human PROM1 (i.e. mutation Y819F/Y828F) significantly decreases ciliary length in Madin-Darby canine kidney cells. These results contrast strongly to the previously observed enhancing effect of WT PROM1 on ciliary length. Mechanistically, the mutation impeded the interaction of PROM1 with ADP-ribosylation factor-like protein 13B, a key regulator of ciliary length. Second, we observed that in vivo knockdown of prom3 in zebrafish alters the number and length of monocilia in the Kupffer's vesicle, resulting in molecular and anatomical defects in the left-right asymmetry. These distinct loss-of-function approaches in two biological systems reveal that prom proteins are critical for the integrity and function of cilia. Our data provide new insights into ciliogenesis and might be of particular interest for investigations of the etiologies of ciliopathies.

Chemosensing versus mechanosensing in nodal and Kupffer's vesicle cilia and in other left-right organizer organs

How is sensing carried out by cilia in the mouse node, zebrafish Kupffer's vesicle and similar left-right (LR) organizer organs in other species? Two possibilities have been put forward. In the former, cilia would detect some chemical species in the fluid; in the latter, they would detect fluid flow. In either case, the hypothesis is that an imbalance would be detected between this signalling coming from cilia on the left and right sides of the organizer, which would initiate a cascade of signals leading ultimately to the breaking of LR symmetry in the developing body plan of the organism. We review the evidence for both hypotheses. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Gdf3 is required for robust Nodal signaling during germ layer formation and left-right patterning

Vertebrate embryonic patterning depends on signaling from Nodal, a TGFβ superfamily member. There are three Nodal orthologs in zebrafish; southpaw directs left-right asymmetries, while squint and cyclops function earlier to pattern mesendoderm. TGFβ member Vg1 is implicated in mesoderm formation but the role of the zebrafish ortholog, Growth differentiation factor 3 (Gdf3), has not been fully explored. We show that zygotic expression of gdf3 is dispensable for embryonic development, while maternally deposited gdf3 is required for mesendoderm formation and dorsal-ventral patterning. We further show that Gdf3 can affect left-right patterning at multiple stages, including proper development of regional cell morphology in Kupffer’s vesicle and the establishment of southpaw expression in the lateral plate mesoderm. Collectively, our data indicate that gdf3 is critical for robust Nodal signaling at multiple stages in zebrafish embryonic development.

Eph/ephrin signaling maintains the boundary of dorsal forerunner cell cluster during morphogenesis of the zebrafish embryonic left-right organizer

The Kupffer's vesicle (KV) is the so-called left-right organizer in teleost fishes. KV is formed from dorsal forerunner cells (DFCs) and generates asymmetrical signals for breaking symmetry of embryos. It is unclear how DFCs or KV cells are prevented from intermingling with adjacent cells. In this study, we show that the Eph receptor gene ephb4b is highly expressed in DFCs whereas ephrin ligand genes, including efnb2b, are expressed in cells next to the DFC cluster during zebrafish gastrulation. ephb4b knockdown or mutation and efnb2b knockdown cause dispersal of DFCs, a smaller KV and randomization of laterality organs. DFCs often dynamically form lamellipodium-like, bleb-like and filopodium-like membrane protrusions at the interface, which attempt to invade but are bounced back by adjacent non-DFC cells during gastrulation. Upon inhibition of Eph/ephrin signaling, however, the repulsion between DFCs and non-DFC cells is weakened or lost, allowing DFCs to migrate away. Ephb4b/Efnb2b signaling by activating RhoA activity mediates contact and repulsion between DFCs and neighboring cells during gastrulation, preventing intermingling of different cell populations. Therefore, our data uncover an important role of Eph/ephrin signaling in maintaining DFC cluster boundary and KV boundary for normal left-right asymmetrical development.

Summary: During formation of the Kupffer's vesicle (KV) – the left-right organizer in zebrafish – Eph/ephrin signaling prevents KV cells from intermingling with adjacent cells.

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.

nlz1 is required for cilia formation in zebrafish embryogenesis

The formation of cilia is a fundamental developmental process affecting diverse functions such as cellular signaling, tissue morphogenesis and body patterning. However, the mechanisms of ciliogenesis during vertebrate development are not fully understood. In this report we describe a novel role of the Nlz1 protein in ciliogenesis. We demonstrate morpholino-mediated knockdown of nlz1 in zebrafish causes abnormal specification of the cells of Kupffer's vesicle (KV); a severe reduction of the number of cilia in KV, the pronephros, and the neural floorplate; and a spectrum of later phenotypes reminiscent of human ciliopathies. In vitro and in vivo data indicate that Nlz1 acts downstream of Foxj1a and Wnt8a/presumed canonical Wnt signaling. Furthermore, Nlz1 contributes to motile cilia formation by positively regulating Wnt11/presumed non-canonical Wnt signaling. Together, our data suggest a novel role of nlz1 in ciliogenesis and the morphogenesis of multiple tissues.

Cell collectivity regulation within migrating cell cluster during Kupffer's vesicle formation in zebrafish

Although cell adhesion is thought to fasten cells tightly, cells that adhere to each other can migrate directionally. This group behavior, called “collective cell migration,” is observed during normal development, wound healing, and cancer invasion. Loss-of-function of cell adhesion molecules in several model systems of collective cell migration results in delay or inhibition of migration of cell groups but does not lead to dissociation of the cell groups, suggesting that mechanisms of cells staying assembled as a single cell cluster, termed as “cell collectivity,” remain largely unknown. During the formation of Kupffer's vesicle (KV, an organ of laterality in zebrafish), KV progenitors form a cluster and migrate together toward the vegetal pole. Importantly, in this model system of collective cell migration, knockdown of cell adhesion molecules or signal components leads to failure of cell collectivity. In this review, we summarize recent findings in cell collectivity regulation during collective migration of KV progenitor cells and describe our current understanding of how cell collectivity is regulated during collective cell migration.

Organized chaos in Kupffer's vesicle: how a heterogeneous structure achieves consistent left-right patterning

Successful establishment of left-right asymmetry is crucial to healthy vertebrate development. In many species this process is initiated in a ciliated, enclosed cavity, for example Kupffer's vesicle (KV) in zebrafish. The microarchitecture of KV is more complex than that present in the left-right organizer of many other species. While swirling flow in KV is recognized as essential for left-right patterning, its generation, nature and conversion to asymmetric gene expression are only beginning to be fully understood. We recently [Sampaio, P et al. Dev Cell 29:716-728] combined imaging, genetics and fluid dynamics simulation to characterize normal and perturbed ciliary activity, and their correlation to asymmetric charon expression and embryonic organ fate. Randomness in cilia number and length have major implications for robust flow generation; even a modest change in mean cilia length has a major effect on flow speed to due to nonlinear scaling arising from fluid mechanics. Wildtype, and mutant embryos with normal liver laterality, exhibit stronger flow on the left prior to asymmetric inhibition of charon. Our discovery of immotile cilia, taken with data on morphant embryos with very few cilia, further support the role of mechanosensing in initiating and/or enhancing flow conversion into gene expression.

Quantitative description of fluid flows produced by left-right cilia in zebrafish

Motile cilia generate directional flows that move mucus through airways, cerebrospinal fluid through brain ventricles, and oocytes through fallopian tubes. In addition, specialized monocilia beat in a rotational pattern to create asymmetric flows that are involved in establishing the left-right (LR) body axis during embryogenesis. These monocilia, which we refer to as "left-right cilia," produce a leftward flow of extraembryonic fluid in a transient "organ of asymmetry" that directs asymmetric signaling and development of LR asymmetries in the cardiovascular system and gastrointestinal tract. The asymmetric flows are thought to establish a chemical gradient and/or activate mechanosensitive cilia to initiate calcium ion signals and a conserved Nodal (TGFβ) pathway on the left side of the embryo, but the mechanisms underlying this process remain unclear. The zebrafish organ of asymmetry, called Kupffer's vesicle, provides a useful model system for investigating LR cilia and cilia-powered fluid flows. Here, we describe methods to visualize flows in Kupffer's vesicle using fluorescent microspheres and introduce a new and freely available MATLAB particle tracking code to quantitatively describe these flows. Analysis of normal and aberrant flows indicates this approach is useful for characterizing flow properties that impact LR asymmetry and may be more broadly applicable for quantifying other cilia flows.

Are there conserved roles for the extracellular matrix, cilia, and junctional complexes in left-right patterning?

Many different types of molecules have essential roles in patterning the left-right axis and directing asymmetric morphogenesis. In particular, the relationship between signaling molecules and transcription factors has been explored extensively. Another group of proteins implicated in left-right patterning are components of the extracellular matrix, apical junctions, and cilia. These structural molecules have the potential to participate in the conversion of morphogenetic cues from the extracellular environment into morphogenetic patterning via their interactions with the actin cytoskeleton. Although it has been relatively easy to temporally position these proteins within the hierarchy of the left-right patterning pathway, it has been more difficult to define how they mechanistically fit into these pathways. Consequently, our understanding of how these factors impart patterning information to influence the establishment of the left-right axis remains limited. In this review, we will discuss those structural molecules that have been implicated in early phases of left-right axis development.

Small heat shock proteins are necessary for heart migration and laterality determination in zebrafish

Small heat shock proteins (sHsps) regulate cellular functions not only under stress, but also during normal development, when they are expressed in organ-specific patterns. Here we demonstrate that two small heat shock proteins expressed in embryonic zebrafish heart, hspb7 and hspb12, have roles in the development of left-right asymmetry. In zebrafish, laterality is determined by the motility of cilia in Kupffer's vesicle (KV), where hspb7 is expressed; knockdown of hspb7 causes laterality defects by disrupting the motility of these cilia. In embryos with reduced hspb7, the axonemes of KV cilia have a 9+0 structure, while control embyros have a predominately 9+2 structure. Reduction of either hspb7 or hspb12 alters the expression pattern of genes that propagate the signals that establish left-right asymmetry: the nodal-related gene southpaw (spaw) in the lateral plate mesoderm, and its downstream targets pitx2, lefty1 and lefty2. Partial depletion of hspb7 causes concordant heart, brain and visceral laterality defects, indicating that loss of KV cilia motility leads to coordinated but randomized laterality. Reducing hspb12 leads to similar alterations in the expression of downstream laterality genes, but at a lower penetrance. Simultaneous reduction of hspb7 and hspb12 randomizes heart, brain and visceral laterality, suggesting that these two genes have partially redundant functions in the establishment of left-right asymmetry. In addition, both hspb7 and hspb12 are expressed in the precardiac mesoderm and in the yolk syncytial layer, which supports the migration and fusion of mesodermal cardiac precursors. In embryos in which the reduction of hspb7 or hspb12 was limited to the yolk, migration defects predominated, suggesting that the yolk expression of these genes rather than heart expression is responsible for the migration defects.

Small heat shock proteins Hspb7 and Hspb12 regulate early steps of cardiac morphogenesis

Cardiac morphogenesis is a complex multi-stage process, and the molecular basis for controlling distinct steps remains poorly understood. Because gata4 encodes a key transcriptional regulator of morphogenesis, we profiled transcript changes in cardiomyocytes when Gata4 protein is depleted from developing zebrafish embryos. We discovered that gata4 regulates expression of two small heat shock genes, hspb7 and hspb12, both of which are expressed in the embryonic heart. We show that depletion of Hspb7 or Hspb12 disrupts normal cardiac morphogenesis, at least in part due to defects in ventricular size and shape. We confirmed that gata4 interacts genetically with the hspb7/12 pathway, but surprisingly, we found that hspb7 also has an earlier, gata4-independent function. Depletion perturbs Kupffer's vesicle (KV) morphology leading to a failure in establishing the left-right axis of asymmetry. Targeted depletion of Hspb7 in the yolk syncytial layer is sufficient to disrupt KV morphology and also causes an even earlier block to heart tube formation and a bifid phenotype. Recently, several genome-wide association studies found that HSPB7 SNPs are highly associated with idiopathic cardiomyopathies and heart failure. Therefore, GATA4 and HSPB7 may act alone or together to regulate morphogenesis with relevance to congenital and acquired human heart disease.