The Alternative Splicing Regulator Nova2 Constrains Vascular Erk Signaling to Limit Specification of the Lymphatic Lineage

The correct assignment of cell fate within fields of multipotent progenitors is essential for accurate tissue diversification. The first lymphatic vessels arise from pre-existing veins after venous endothelial cells become specified as lymphatic progenitors. Prox1 specifies lymphatic fate and labels these progenitors; however, the mechanisms restricting Prox1 expression and limiting the progenitor pool remain unknown. We identified a zebrafish mutant that displayed premature, expanded, and prolonged lymphatic specification. The gene responsible encodes the regulator of alternative splicing, Nova2. In zebrafish and human endothelial cells, Nova2 selectively regulates pre-mRNA splicing for components of signaling pathways and phosphoproteins. Nova2-deficient endothelial cells display increased Mapk/Erk signaling, and Prox1 expression is dynamically controlled by Erk signaling. We identify a mechanism whereby Nova2-regulated splicing constrains Erk signaling, thus limiting lymphatic progenitor cell specification. This identifies the capacity of a factor that tunes mRNA splicing to control assignment of cell fate during vascular differentiation.

Biallelic mutation of FBXL7 suggests a novel form of Hennekam syndrome

Hennekam lymphangiectasia-lymphedema syndrome is an autosomal recessive disorder characterized by congenital lymphedema, intestinal lymphangiectasia, facial dysmorphism, and variable intellectual disability. Known disease genes include CCBE1, FAT4, and ADAMTS3. In a patient with clinically diagnosed Hennekam syndrome but without mutations or copy-number changes in the three known disease genes, we identified a homozygous single-exon deletion affecting FBXL7. Specifically, exon 3, which encodes the F-box domain and several leucine-rich repeats of FBXL7, is eliminated. Our analyses of databases representing >100,000 control individuals failed to identify biallelic loss-of-function variants in FBXL7. Published studies in Drosophila indicate Fbxl7 interacts with Fat, of which human FAT4 is an ortholog, and mutation of either gene yields similar morphological consequences. These data suggest that FBXL7 may be the fourth gene for Hennekam syndrome, acting via a shared pathway with FAT4.

Yap1 promotes sprouting and proliferation of lymphatic progenitors downstream of Vegfc in the zebrafish trunk

Lymphatic vascular development involves specification of lymphatic endothelial progenitors that subsequently undergo sprouting, proliferation and tissue growth to form a complex second vasculature. The Hippo pathway and effectors Yap and Taz control organ growth and regulate morphogenesis and cellular proliferation. Yap and Taz control angiogenesis but a role in lymphangiogenesis remains to be fully elucidated. Here we show that YAP displays dynamic changes in lymphatic progenitors and Yap1 is essential for lymphatic vascular development in zebrafish. Maternal and Zygotic (MZ) yap1 mutants show normal specification of lymphatic progenitors, abnormal cellular sprouting and reduced numbers of lymphatic progenitors emerging from the cardinal vein during lymphangiogenesis. Furthermore, Yap1 is indispensable for Vegfc-induced proliferation in a transgenic model of Vegfc overexpression. Paracrine Vegfc-signalling ultimately increases nuclear YAP in lymphatic progenitors to control lymphatic development. We thus identify a role for Yap in lymphangiogenesis, acting downstream of Vegfc to promote expansion of this vascular lineage.

Myosin Vb is required for correct trafficking of N-cadherin and cardiac chamber ballooning

BACKGROUND

During heart morphogenesis, the cardiac chambers undergo ballooning: a process involving regionalized elongation of cardiomyocytes. Cardiomyocyte shape changes require reorganization of the actin cytoskeleton; however, the genetic regulation of this process is not well understood.

RESULTS

From a forward genetic screen, we identified the zebrafish uq ^23ks mutant which manifests chamber ballooning defects. Whole-genome sequencing-mapping identified a truncating mutation in the gene, myo5b. myo5b encodes an atypical myosin required for endosome recycling and, consistent with this, increased vesicles were observed in myo5b mutant cardiomyocytes. Expression of RFP-Rab11a (a recycling endosome marker) confirmed increased recycling endosomes in cardiomyocytes of myo5b mutants. To investigate potential cargo of MyoVb-associated vesicles, we examined the adherens junction protein, N-cadherin. N-cadherin appeared mispatterned at cell junctions, and an increase in the number of intracellular particles was also apparent. Co-localization with RFP-Rab11a confirmed increased N-cadherin-positive recycling endosomes, demonstrating N-cadherin trafficking is perturbed in myo5b mutants. Finally, phalloidin staining showed disorganized F-actin in myo5b cardiomyocytes, suggesting the cytoskeleton fails to remodel, obstructing chamber ballooning.

CONCLUSIONS

MyoVb is required for cardiomyocyte endosomal recycling and appropriate N-cadherin localization during the onset of chamber ballooning. Cardiomyocytes lacking MyoVb are unable to reorganize their actin cytoskeleton, resulting in failed chamber ballooning. Developmental Dynamics 248:284-295, 2019. © 2019 Wiley Periodicals, Inc.

Zebrafish facial lymphatics develop through sequential addition of venous and non-venous progenitors

Lymphatic vessels are known to be derived from veins; however, recent lineage-tracing experiments propose that specific lymphatic networks may originate from both venous and non-venous sources. Despite this, direct evidence of a non-venous lymphatic progenitor is missing. Here, we show that the zebrafish facial lymphatic network is derived from three distinct progenitor populations that add sequentially to the developing facial lymphatic through a relay-like mechanism. We show that while two facial lymphatic progenitor populations are venous in origin, the third population, termed the ventral aorta lymphangioblast (VA-L), does not sprout from a vessel; instead, it arises from a migratory angioblast cell near the ventral aorta that initially lacks both venous and lymphatic markers, and contributes to the facial lymphatics and the hypobranchial artery. We propose that sequential addition of venous and non-venous progenitors allows the facial lymphatics to form in an area that is relatively devoid of veins. Overall, this study provides conclusive, live imaging-based evidence of a non-venous lymphatic progenitor and demonstrates that the origin and development of lymphatic vessels is context-dependent.

Peri-arterial specification of vascular mural cells from naïve mesenchyme requires Notch signaling

Mural cells (MCs) are essential for blood vessel stability and function; however, the mechanisms that regulate MC development remain incompletely understood, in particular those involved in MC specification. Here, we investigated the first steps of MC formation in zebrafish using transgenic reporters. Using pdgfrb and abcc9 reporters, we show that the onset of expression of abcc9, a pericyte marker in adult mice and zebrafish, occurs almost coincidentally with an increment in pdgfrb expression in peri-arterial mesenchymal cells, suggesting that these transcriptional changes mark the specification of MC lineage cells from naïve pdgfrb ^low mesenchymal cells. The emergence of peri-arterial pdgfrb ^high MCs required Notch signaling. We found that pdgfrb-positive cells express notch2 in addition to notch3, and although depletion of notch2 or notch3 failed to block MC emergence, embryos depleted of both notch2 and notch3 lost mesoderm- as well as neural crest-derived pdgfrb ^high MCs. Using reporters that read out Notch signaling and Notch2 receptor cleavage, we show that Notch activation in the mesenchyme precedes specification into pdgfrb ^high MCs. Taken together, these results show that Notch signaling is necessary for peri-arterial MC specification.

Junction-based lamellipodia drive endothelial cell rearrangements in vivo via a VE-cadherin-F-actin based oscillatory cell-cell interaction

Angiogenesis and vascular remodeling are driven by extensive endothelial cell movements. Here, we present in vivo evidence that endothelial cell movements are associated with oscillating lamellipodia-like structures, which emerge from cell junctions in the direction of cell movements. High-resolution time-lapse imaging of these junction-based lamellipodia (JBL) shows dynamic and distinct deployment of junctional proteins, such as F-actin, VE-cadherin and ZO1, during JBL oscillations. Upon initiation, F-actin and VE-cadherin are broadly distributed within JBL, whereas ZO1 remains at cell junctions. Subsequently, a new junction is formed at the front of the JBL, which then merges with the proximal junction. Rac1 inhibition interferes with JBL oscillations and disrupts cell elongation-similar to a truncation in ve-cadherin preventing VE-cad/F-actin interaction. Taken together, our observations suggest an oscillating ratchet-like mechanism, which is used by endothelial cells to move over each other and thus provides the physical means for cell rearrangements.

Nppa and Nppb act redundantly during zebrafish cardiac development to confine AVC marker expression and reduce cardiac jelly volume

Atrial natriuretic peptide (nppa/anf) and brain natriuretic peptide (nppb/bnp) form a gene cluster with expression in the chambers of the developing heart. Despite restricted expression, a function in cardiac development has not been demonstrated by mutant analysis. This is attributed to functional redundancy; however, their genomic location in cis has impeded formal analysis. Using genome editing, we have generated mutants for nppa and nppb, and found that single mutants were indistinguishable from wild type, whereas nppa/nppb double mutants displayed heart morphogenesis defects and pericardial oedema. Analysis of atrioventricular canal (AVC) markers show expansion of bmp4, tbx2b, has2 and versican expression into the atrium of double mutants. This expanded expression correlates with increased extracellular matrix in the atrium. Using a biosensor for hyaluronic acid to measure the cardiac jelly (cardiac extracellular matrix), we confirmed cardiac jelly expansion in nppa/nppb double mutants. Finally, bmp4 knockdown rescued the expansion of has2 expression and cardiac jelly in double mutants. This definitively shows that nppa and nppb function redundantly during cardiac development to restrict gene expression to the AVC, preventing excessive cardiac jelly synthesis in the atrial chamber.

A blood capillary plexus-derived population of progenitor cells contributes to genesis of the dermal lymphatic vasculature during embryonic development

Despite the essential role of the lymphatic vasculature in tissue homeostasis and disease, knowledge of the organ-specific origins of lymphatic endothelial progenitor cells remains limited. The assumption that most murine embryonic lymphatic endothelial cells (LECs) are venous derived has recently been challenged. Here, we show that the embryonic dermal blood capillary plexus constitutes an additional, local source of LECs that contributes to the formation of the dermal lymphatic vascular network. We describe a novel mechanism whereby rare PROX1-positive endothelial cells exit the capillary plexus in a Ccbe1-dependent manner to establish discrete LEC clusters. As development proceeds, these clusters expand and further contribute to the growing lymphatic system. Lineage tracing and analyses of Gata2-deficient mice confirmed that these clusters are endothelial in origin. Furthermore, ectopic expression of Vegfc in the vasculature increased the number of PROX1-positive progenitors within the capillary bed. Our work reveals a novel source of lymphatic endothelial progenitors employed during construction of the dermal lymphatic vasculature and demonstrates that the blood vasculature is likely to remain an ongoing source of LECs during organogenesis, raising the question of whether a similar mechanism operates during pathological lymphangiogenesis.

Brain drains: new insights into brain clearance pathways from lymphatic biology

The lymphatic vasculature act as the drainage system for most of our tissues and organs, clearing interstitial fluid and waste and returning them to the blood circulation. This is not the case for the central nervous system (CNS), which is devoid of parenchymal lymphatic vessels. Nevertheless, the brain is responsible for 25% of the body's metabolism and only compromises 2% of the body's mass. This high metabolic load requires an efficient system to remove waste products and maintain homeostasis. Well-described mechanisms of waste clearance include phagocytic immune cell functions as well as perivascular fluid flow; however, the need for active drainage of waste from the brain is becoming increasingly appreciated. Recent developments in lymphatic vascular biology challenge the proposition that the brain lacks lymphatic drainage or an equivalent. In this review, we describe the roles of the glymphatic system (a key drainage mechanism in the absence of lymphatics), the recently characterized meningeal lymphatic vessels, and explore an enigmatic cell population found in zebrafish called mural lymphatic endothelial cells. These systems may play important individual and collective roles in draining and clearing wastes from the brain.

Visualization and Tools for Analysis of Zebrafish Lymphatic Development

The accessibility and optical transparency of the zebrafish embryo offers a unique platform for live-imaging of developmental lymphangiogenesis. Transgenic lines labelling lymphatic progenitors and vessels enable researchers to visualize cellular processes and ask how they contribute to lymphatic development in genetic models. Furthermore, validated immunofluorescence staining for key signaling and cell fate markers (phosphorylated Erk and Prox1) allow single cell resolution studies of lymphatic differentiation. Here, we describe in detail how zebrafish embryos and larvae can be mounted for high resolution, staged imaging of lymphatic networks, how lymphangiogenesis can be reliably quantified and how immunofluorescence can reveal lymphatic signaling and differentiation. These methods offer researchers the opportunity to experimentally dissect developmental lymphangiogenesis with outstanding resolution.

Live imaging molecular changes in junctional tension upon VE-cadherin in zebrafish

Forces play diverse roles in vascular development, homeostasis and disease. VE-cadherin at endothelial cell-cell junctions links the contractile acto-myosin cytoskeletons of adjacent cells, serving as a tension-transducer. To explore tensile changes across VE-cadherin in live zebrafish, we tailored an optical biosensor approach, originally established in vitro. We validate localization and function of a VE-cadherin tension sensor (TS) in vivo. Changes in tension across VE-cadherin observed using ratio-metric or lifetime FRET measurements reflect acto-myosin contractility within endothelial cells. Furthermore, we apply the TS to reveal biologically relevant changes in VE-cadherin tension that occur as the dorsal aorta matures and upon genetic and chemical perturbations during embryonic development.

Mechanical forces play a crucial role during morphogenesis, but how these are sensed and transduced in vivo is not fully understood. Here the authors apply a FRET tension sensor to live zebrafish and study changes in VE-cadherin tension at endothelial cell-cell junctions during arterial maturation.

How to Plumb a Pisces: Understanding Vascular Development and Disease Using Zebrafish Embryos

Our vasculature plays diverse and critical roles in homeostasis and disease. In recent decades, the use of zebrafish has driven our understanding of vascular development into new areas, identifying new genes and mechanisms controlling vessel formation and allowing unprecedented observation of the cellular and molecular events that shape the developing vasculature. Here, we highlight key mechanisms controlling formation of the zebrafish vasculature and investigate how knowledge from this highly tractable model system has informed our understanding of vascular disease in humans.

Tmem2 Regulates Embryonic Vegf Signaling by Controlling Hyaluronic Acid Turnover

Angiogenesis is responsible for tissue vascularization during development, as well as in pathological contexts, including cancer and ischemia. Vascular endothelial growth factors (VEGFs) regulate angiogenesis by acting through VEGF receptors to induce endothelial cell signaling. VEGF is processed in the extracellular matrix (ECM), but the complexity of ECM control of VEGF signaling and angiogenesis remains far from understood. In a forward genetic screen, we identified angiogenesis defects in tmem2 zebrafish mutants that lack both arterial and venous Vegf/Vegfr/Erk signaling. Strikingly, tmem2 mutants display increased hyaluronic acid (HA) surrounding developing vessels. Angiogenesis in tmem2 mutants was rescued, or restored after failed sprouting, by degrading this increased HA. Furthermore, oligomerized HA or overexpression of Vegfc rescued angiogenesis in tmem2 mutants. Based on these data, and the known structure of Tmem2, we find that Tmem2 regulates HA turnover to promote normal Vegf signaling during developmental angiogenesis.

Pharmacological targeting of the transcription factor SOX18 delays breast cancer in mice

Pharmacological targeting of transcription factors holds great promise for the development of new therapeutics, but strategies based on blockade of DNA binding, nuclear shuttling, or individual protein partner recruitment have yielded limited success to date. Transcription factors typically engage in complex interaction networks, likely masking the effects of specifically inhibiting single protein-protein interactions. Here, we used a combination of genomic, proteomic and biophysical methods to discover a suite of protein-protein interactions involving the SOX18 transcription factor, a known regulator of vascular development and disease. We describe a small-molecule that is able to disrupt a discrete subset of SOX18-dependent interactions. This compound selectively suppressed SOX18 transcriptional outputs in vitro and interfered with vascular development in zebrafish larvae. In a mouse pre-clinical model of breast cancer, treatment with this inhibitor significantly improved survival by reducing tumour vascular density and metastatic spread. Our studies validate an interactome-based molecular strategy to interfere with transcription factor activity, for the development of novel disease therapeutics.

Utilising polymorphisms to achieve allele-specific genome editing in zebrafish

The advent of genome editing has significantly altered genetic research, including research using the zebrafish model. To better understand the selectivity of the commonly used CRISPR/Cas9 system, we investigated single base pair mismatches in target sites and examined how they affect genome editing in the zebrafish model. Using two different zebrafish strains that have been deep sequenced, CRISPR/Cas9 target sites containing polymorphisms between the two strains were identified. These strains were crossed (creating heterozygotes at polymorphic sites) and CRISPR/Cas9 complexes that perfectly complement one strain injected. Sequencing of targeted sites showed biased, allele-specific editing for the perfectly complementary sequence in the majority of cases (14/19). To test utility, we examined whether phenotypes generated by F0 injection could be internally controlled with such polymorphisms. Targeting of genes bmp7a and chordin showed reduction in the frequency of phenotypes in injected ‘heterozygotes’ compared with injecting the strain with perfect complementarity. Next, injecting CRISPR/Cas9 complexes targeting two separate sites created deletions, but deletions were biased to selected chromosomes when one CRISPR/Cas9 target contained a polymorphism. Finally, integration of loxP sequences occurred preferentially in alleles with perfect complementarity. These experiments demonstrate that single nucleotide polymorphisms (SNPs) present throughout the genome can be utilised to increase the efficiency of in cis genome editing using CRISPR/Cas9 in the zebrafish model.

Summary: Heterozygous single nucleotide polymorphisms in CRISPR/Cas9 target sites bias genome editing in favour of alleles with perfect complementarity to gRNAs, a feature which can be exploited for chromosome-specific editing.

Vegfd modulates both angiogenesis and lymphangiogenesis during zebrafish embryonic development

Vascular endothelial growth factors (VEGFs) control angiogenesis and lymphangiogenesis during development and in pathological conditions. In the zebrafish trunk, Vegfa controls the formation of intersegmental arteries by primary angiogenesis and Vegfc is essential for secondary angiogenesis, giving rise to veins and lymphatics. Vegfd has been largely thought of as dispensable for vascular development in vertebrates. Here, we generated a zebrafish vegfd mutant by genome editing. vegfd mutants display significant defects in facial lymphangiogenesis independent of vegfc function. Strikingly, we find that vegfc and vegfd cooperatively control lymphangiogenesis throughout the embryo, including during the formation of the trunk lymphatic vasculature. Interestingly, we find that vegfd and vegfc also redundantly drive artery hyperbranching phenotypes observed upon depletion of Flt1 or Dll4. Epistasis and biochemical binding assays suggest that, during primary angiogenesis, Vegfd influences these phenotypes through Kdr (Vegfr2) rather than Flt4 (Vegfr3). These data demonstrate that, rather than being dispensable during development, Vegfd plays context-specific indispensable and also compensatory roles during both blood vessel angiogenesis and lymphangiogenesis.

Vegfa signaling promotes zebrafish intestinal vasculature development through endothelial cell migration from the posterior cardinal vein

The mechanisms underlying organ vascularization are not well understood. The zebrafish intestinal vasculature forms early, is easily imaged using transgenic lines and in-situ hybridization, and develops in a stereotypical pattern thus making it an excellent model for investigating mechanisms of organ specific vascularization. Here, we demonstrate that the sub-intestinal vein (SIV) and supra-intestinal artery (SIA) form by a novel mechanism from angioblasts that migrate out of the posterior cardinal vein and coalesce to form the intestinal vasculature in an anterior to posterior wave with the SIA forming after the SIV. We show that vascular endothelial growth factor aa (vegfaa) is expressed in the endoderm at the site where intestinal vessels form and therefore likely provides a guidance signal. Vegfa/Vegfr2 signaling is required for early intestinal vasculature development with mutation in vegfaa or loss of Vegfr2 homologs causing nearly complete inhibition of the formation of the intestinal vasculature. Vegfc and Vegfr3 function, however, are dispensable for intestinal vascularization. Interestingly, ubiquitous overexpression of Vegfc resulted in an overgrowth of the SIV, suggesting that Vegfc is sufficient to induce SIV development. These results argue that Vegfa signaling directs endothelial cells to migrate out of existing vasculature and coalesce to form the intestinal vessels. It is likely that a similar mechanism is utilized during vascularization of other organs.

Active contractility at E-cadherin junctions and its implications for cell extrusion in cancer

Cellular contractility regulates tissue cohesion and morphogenesis. In epithelia, E-cadherin adhesion couples the contractile cortices of neighboring cells together to produce tension at junctions that can be transmitted across the epithelium in a planar fashion. We have recently demonstrated that contractility is also patterned in the apical-lateral axis within epithelial junctions. Our findings highlight the role that cytoskeletal regulation plays in controlling the levels of intra-junctional tension. Of note, dysregulation of this apicolateral pattern of tension can drive oncogenic cell extrusion. In this article, we provide a detailed description of the actomyosin cytoskeleton organization during oncogenic extrusion and discuss the implications of cell extrusion in cancer.