Generation of specialized blood vessels via lymphatic transdifferentiation

The lineage and developmental trajectory of a cell are key determinants of cellular identity. In the vascular system, endothelial cells (ECs) of blood and lymphatic vessels differentiate and specialize to cater to the unique physiological demands of each organ^1,2. Although lymphatic vessels were shown to derive from multiple cellular origins, lymphatic ECs (LECs) are not known to generate other cell types^3,4. Here we use recurrent imaging and lineage-tracing of ECs in zebrafish anal fins, from early development to adulthood, to uncover a mechanism of specialized blood vessel formation through the transdifferentiation of LECs. Moreover, we demonstrate that deriving anal-fin vessels from lymphatic versus blood ECs results in functional differences in the adult organism, uncovering a link between cell ontogeny and functionality. We further use single-cell RNA-sequencing analysis to characterize the different cellular populations and transition states involved in the transdifferentiation process. Finally, we show that, similar to normal development, the vasculature is rederived from lymphatics during anal-fin regeneration, demonstrating that LECs in adult fish retain both potency and plasticity for generating blood ECs. Overall, our research highlights an innate mechanism of blood vessel formation through LEC transdifferentiation, and provides in vivo evidence for a link between cell ontogeny and functionality in ECs.

Zebrafish mutants provide insights into Apolipoprotein B functions during embryonic development and pathological conditions

Apolipoprotein B (ApoB) is the primary protein of chylomicrons, VLDLs, and LDLs and is essential for their production. Defects in ApoB synthesis and secretion result in several human diseases, including abetalipoproteinemia and familial hypobetalipoproteinemia (FHBL1). In addition, ApoB-related dyslipidemia is linked to nonalcoholic fatty liver disease (NAFLD), a silent pandemic affecting billions globally. Due to the crucial role of APOB in supplying nutrients to the developing embryo, ApoB deletion in mammals is embryonic lethal. Thus, a clear understanding of the roles of this protein during development is lacking. Here, we established zebrafish mutants for 2 apoB genes: apoBa and apoBb.1. Double-mutant embryos displayed hepatic steatosis, a common hallmark of FHBL1 and NAFLD, as well as abnormal liver laterality, decreased numbers of goblet cells in the gut, and impaired angiogenesis. We further used these mutants to identify the domains within ApoB responsible for its functions. By assessing the ability of different truncated forms of human APOB to rescue the mutant phenotypes, we demonstrate the benefits of this model for prospective therapeutic screens. Overall, these zebrafish models uncover what are likely previously undescribed functions of ApoB in organ development and morphogenesis and shed light on the mechanisms underlying hypolipidemia-related diseases.

VEGFC/FLT4-induced cell-cycle arrest mediates sprouting and differentiation of venous and lymphatic endothelial cells

The formation of new vessels requires a tight synchronization between proliferation, differentiation, and sprouting. However, how these processes are differentially activated, often by neighboring endothelial cells (ECs), remains unclear. Here, we identify cell cycle progression as a regulator of EC sprouting and differentiation. Using transgenic zebrafish illuminating cell cycle stages, we show that venous and lymphatic precursors sprout from the cardinal vein exclusively in G1 and reveal that cell-cycle arrest is induced in these ECs by overexpression of p53 and the cyclin-dependent kinase (CDK) inhibitors p27 and p21. We further demonstrate that, in vivo, forcing G1 cell-cycle arrest results in enhanced vascular sprouting. Mechanistically, we identify the mitogenic VEGFC/VEGFR3/ERK axis as a direct inducer of cell-cycle arrest in ECs and characterize the cascade of events that render "sprouting-competent" ECs. Overall, our results uncover a mechanism whereby mitogen-controlled cell-cycle arrest boosts sprouting, raising important questions about the use of cell cycle inhibitors in pathological angiogenesis and lymphangiogenesis.

Beyond cells: The extracellular circulating 20S proteasomes

Accumulating evidence arising from numerous clinical studies indicate that assembled and functional 20S proteasome complexes circulate freely in plasma. Elevated levels of this core proteolytic complex have been found in the plasma of patients suffering from blood, skin and solid cancers, autoimmune disorders, trauma and sepsis. Moreover, in various diseases, there is a positive correlation between circulating 20S proteasome (c20S) levels and treatment efficacy and survival rates, suggesting the involvement of this under-studied c20S complex in pathophysiology. However, many aspects of this system remain enigmatic, as we still do not know the origin, biological role or mechanisms of extracellular transport and regulation of c20S proteasomes. In this review, we provide an overview of the current understanding of the c20S proteasome system and discuss the remaining gaps in knowledge.

Discovering New Progenitor Cell Populations through Lineage Tracing and In Vivo Imaging

Identification of progenitor cells that generate differentiated cell types during development, regeneration, and disease states is central to understanding the mechanisms governing such transitions. For more than a century, different lineage-tracing strategies have been developed, which helped disentangle the complex relationship between progenitor cells and their progenies. In this review, we discuss how lineage-tracing analyses have evolved alongside technological advances, and how this approach has contributed to the identification of progenitor cells in different contexts of cell differentiation. We also highlight a few examples in which lineage-tracing experiments have been instrumental for resolving long-standing debates and for identifying unexpected cellular origins. This discussion emphasizes how this century-old quest to delineate cellular lineage relationships is still active, and new discoveries are being made with the development of newer methodologies.

Cellular Origins of the Lymphatic Endothelium: Implications for Cancer Lymphangiogenesis

The lymphatic system plays important roles in physiological and pathological conditions. During cancer progression in particular, lymphangiogenesis can exert both positive and negative effects. While the formation of tumor associated lymphatic vessels correlates with metastatic dissemination, increased severity and poor patient prognosis, the presence of functional lymphatics is regarded as beneficial for anti-tumor immunity and cancer immunotherapy delivery. Therefore, a profound understanding of the cellular origins of tumor lymphatics and the molecular mechanisms controlling their formation is required in order to improve current strategies to control malignant spread. Data accumulated over the last decades have led to a controversy regarding the cellular sources of tumor-associated lymphatic vessels and the putative contribution of non-endothelial cells to this process. Although it is widely accepted that lymphatic endothelial cells (LECs) arise mainly from pre-existing lymphatic vessels, additional contribution from bone marrow-derived cells, myeloid precursors and terminally differentiated macrophages, has also been claimed. Here, we review recent findings describing new origins of LECs during embryonic development and discuss their relevance to cancer lymphangiogenesis.

Formation and Growth of Cardiac Lymphatics during Embryonic Development, Heart Regeneration, and Disease

The lymphatic system plays crucial roles in regulating fluid homeostasis, immune surveillance, and lipid transport. As is in most of the body's organs, the heart possesses an extensive lymphatic network. Moreover, a robust lymphangiogenic response has been shown to take place following myocardial infarction, highlighting cardiac lymphatics as potential targets for therapeutic intervention. Yet, the unique molecular properties and functions of the heart's lymphatic system have only recently begun to be addressed. In this review, we discuss the mechanisms underlying the formation and growth of cardiac lymphatics during embryonic development and describe their characteristics across species. We further summarize recent findings highlighting diverse cellular origins for cardiac lymphatic endothelial cells and how they integrate to form a single functional lymphatic network. Finally, we outline novel therapeutic avenues aimed at enhancing lymphatic vessel formation and integrity following cardiac injury, which hold great promise for promoting healing of the infarcted heart.

BACH family members regulate angiogenesis and lymphangiogenesis by modulating VEGFC expression

Angiogenesis and lymphangiogenesis are key processes during embryogenesis as well as under physiological and pathological conditions. Vascular endothelial growth factor C (VEGFC), the ligand for both VEGFR2 and VEGFR3, is a central lymphangiogenic regulator that also drives angiogenesis. Here, we report that members of the highly conserved BACH (BTB and CNC homology) family of transcription factors regulate VEGFC expression, through direct binding to its promoter. Accordingly, down-regulation of bach2a hinders blood vessel formation and impairs lymphatic sprouting in a Vegfc-dependent manner during zebrafish embryonic development. In contrast, BACH1 overexpression enhances intratumoral blood vessel density and peritumoral lymphatic vessel diameter in ovarian and lung mouse tumor models. The effects on the vascular compartment correlate spatially and temporally with BACH1 transcriptional regulation of VEGFC expression. Altogether, our results uncover a novel role for the BACH/VEGFC signaling axis in lymphatic formation during embryogenesis and cancer, providing a novel potential target for therapeutic interventions.

Distinct origins and molecular mechanisms contribute to lymphatic formation during cardiac growth and regeneration

In recent years, there has been increasing interest in the role of lymphatics in organ repair and regeneration, due to their importance in immune surveillance and fluid homeostasis. Experimental approaches aimed at boosting lymphangiogenesis following myocardial infarction in mice, were shown to promote healing of the heart. Yet, the mechanisms governing cardiac lymphatic growth remain unclear. Here, we identify two distinct lymphatic populations in the hearts of zebrafish and mouse, one that forms through sprouting lymphangiogenesis, and the other by coalescence of isolated lymphatic cells. By tracing the development of each subset, we reveal diverse cellular origins and differential response to signaling cues. Finally, we show that lymphatic vessels are required for cardiac regeneration in zebrafish as mutants lacking lymphatics display severely impaired regeneration capabilities. Overall, our results provide novel insight into the mechanisms underlying lymphatic formation during development and regeneration, opening new avenues for interventions targeting specific lymphatic populations.

Author Correction: The mid-developmental transition and the evolution of animal body plans

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Somatic NRAS mutation in patient with generalized lymphatic anomaly

Generalized lymphatic anomaly (GLA or lymphangiomatosis) is a rare disease characterized by a diffuse proliferation of lymphatic vessels in skin and internal organs. It often leads to progressive respiratory failure and death, but its etiology is unknown. Here, we isolated lymphangiomatosis endothelial cells from GLA tissue. These cells were characterized by high proliferation and survival rates, but displayed impaired capacities for migration and tube formation. We employed whole exome sequencing to search for disease-causing genes and identified a somatic mutation in NRAS. We used mouse and zebrafish model systems to initially evaluate the role of this mutation in the development of the lymphatic system, and we studied the effect of drugs blocking the downstream effectors, mTOR and ERK, on this disease.

Autotaxin-Lysophosphatidic Acid Axis Acts Downstream of Apoprotein B Lipoproteins in Endothelial Cells

OBJECTIVE

As they travel through the blood stream, plasma lipoproteins interact continuously with endothelial cells (ECs). Although the focus of research has mostly been guided by the importance of lipoproteins as risk factors for atherosclerosis, thrombosis, and other cardiovascular diseases, little is known about the mechanisms linking lipoproteins and angiogenesis under physiological conditions, and particularly, during embryonic development. In this work, we performed global mRNA expression profiling of endothelial cells from hypo-, and hyperlipidemic zebrafish embryos with the goal of uncovering novel mediators of lipoprotein signaling in the endothelium.

APPROACH AND RESULTS

Microarray analysis was conducted on fluorescence-activated cell sorting-isolated fli1:EGFP(+) ECs from normal, hypo-, and hyperlipidemic zebrafish embryos. We found that opposed levels of apoprotein B lipoproteins result in differential expression of the secreted enzyme autotaxin in ECs, which in turn affects EC sprouting and angiogenesis. We further demonstrate that the effects of autotaxin in vivo are mediated by lysophosphatidic acid (LPA)-a well-known autotaxin activity product-and that LPA and LPA receptors participate as well in the response of ECs to lipoprotein levels.

CONCLUSIONS

Our findings provide the first in vivo gene expression profiling of ECs facing different levels of plasma apoprotein B lipoproteins and uncover a novel lipoprotein-autotaxin-LPA axis as regulator of EC behavior. These results highlight new roles for lipoproteins as signaling molecules, which are independent of their canonical function as cholesterol transporters.

Development of the lymphatic system: new questions and paradigms

The lymphatic system is a blind-ended network of vessels that plays important roles in mediating tissue fluid homeostasis, intestinal lipid absorption and the immune response. A profound understanding of the development of lymphatic vessels, as well as of the molecular cues governing their formation and morphogenesis, might prove essential for our ability to treat lymphatic-related diseases. The embryonic origins of lymphatic vessels have been debated for over a century, with a model claiming a venous origin for the lymphatic endothelium being predominant. However, recent studies have provided new insights into the origins of lymphatic vessels. Here, we review the molecular mechanisms controlling lymphatic specification and sprouting, and we discuss exciting findings that shed new light on previously uncharacterized sources of lymphatic endothelial cells.

A new role of hindbrain boundaries as pools of neural stem/progenitor cells regulated by Sox2

Background

Compartment boundaries are an essential developmental mechanism throughout evolution, designated to act as organizing centers and to regulate and localize differently fated cells. The hindbrain serves as a fascinating example for this phenomenon as its early development is devoted to the formation of repetitive rhombomeres and their well-defined boundaries in all vertebrates. Yet, the actual role of hindbrain boundaries remains unresolved, especially in amniotes.

Results

Here, we report that hindbrain boundaries in the chick embryo consist of a subset of cells expressing the key neural stem cell (NSC) gene Sox2. These cells co-express other neural progenitor markers such as Transitin (the avian Nestin), GFAP, Pax6 and chondroitin sulfate proteoglycan. The majority of the Sox2⁺ cells that reside within the boundary core are slow-dividing, whereas nearer to and within rhombomeres Sox2⁺ cells are largely proliferating. In vivo analyses and cell tracing experiments revealed the contribution of boundary Sox2⁺ cells to neurons in a ventricular-to-mantle manner within the boundaries, as well as their lateral contribution to proliferating Sox2⁺ cells in rhombomeres. The generation of boundary-derived neurospheres from hindbrain cultures confirmed the typical NSC behavior of boundary cells as a multipotent and self-renewing Sox2⁺ cell population. Inhibition of Sox2 in boundaries led to enhanced and aberrant neural differentiation together with inhibition in cell-proliferation, whereas Sox2 mis-expression attenuated neurogenesis, confirming its significant function in hindbrain neuronal organization.

Conclusions

Data obtained in this study deciphers a novel role of hindbrain boundaries as repetitive pools of neural stem/progenitor cells, which provide proliferating progenitors and differentiating neurons in a Sox2-dependent regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0277-y) contains supplementary material, which is available to authorized users.

The mid-developmental transition and the evolution of animal body plans

Animals are grouped into ~35 'phyla' based upon the notion of distinct body plans. Morphological and molecular analyses have revealed that a stage in the middle of development--known as the phylotypic period--is conserved among species within some phyla. Although these analyses provide evidence for their existence, phyla have also been criticized as lacking an objective definition, and consequently based on arbitrary groupings of animals. Here we compare the developmental transcriptomes of ten species, each annotated to a different phylum, with a wide range of life histories and embryonic forms. We find that in all ten species, development comprises the coupling of early and late phases of conserved gene expression. These phases are linked by a divergent 'mid-developmental transition' that uses species-specific suites of signalling pathways and transcription factors. This mid-developmental transition overlaps with the phylotypic period that has been defined previously for three of the ten phyla, suggesting that transcriptional circuits and signalling mechanisms active during this transition are crucial for defining the phyletic body plan and that the mid-developmental transition may be used to define phylotypic periods in other phyla. Placing these observations alongside the reported conservation of mid-development within phyla, we propose that a phylum may be defined as a collection of species whose gene expression at the mid-developmental transition is both highly conserved among them, yet divergent relative to other species.

Venous-derived angioblasts generate organ-specific vessels during zebrafish embryonic development

Formation and remodeling of vascular beds are complex processes orchestrated by multiple signaling pathways. Although it is well accepted that vessels of a particular organ display specific features that enable them to fulfill distinct functions, the embryonic origins of tissue-specific vessels and the molecular mechanisms regulating their formation are poorly understood. The subintestinal plexus of the zebrafish embryo comprises vessels that vascularize the gut, liver and pancreas and, as such, represents an ideal model in which to investigate the early steps of organ-specific vessel formation. Here, we show that both arterial and venous components of the subintestinal plexus originate from a pool of specialized angioblasts residing in the floor of the posterior cardinal vein (PCV). Using live imaging of zebrafish embryos, in combination with photoconvertable transgenic reporters, we demonstrate that these angioblasts undergo two phases of migration and differentiation. Initially, a subintestinal vein forms and expands ventrally through a Bone Morphogenetic Protein-dependent step of collective migration. Concomitantly, a Vascular Endothelial Growth Factor-dependent shift in the directionality of migration, coupled to the upregulation of arterial markers, is observed, which culminates with the generation of the supraintestinal artery. Together, our results establish the zebrafish subintestinal plexus as an advantageous model for the study of organ-specific vessel development and provide new insights into the molecular mechanisms controlling its formation. More broadly, our findings suggest that PCV-specialized angioblasts contribute not only to the formation of the early trunk vasculature, but also to the establishment of late-forming, tissue-specific vascular beds.

Highlighted article: A specialized pool of angioblasts is the origin of the zebrafish subintestinal plexus, a structure that gives rise to the organ-specific vessels of the gut, liver and pancreas.

On the pathway of mineral deposition in larval zebrafish caudal fin bone

A poorly understood aspect of bone biomineralization concerns the mechanisms whereby ions are sequestered from the environment, concentrated, and deposited in the extracellular matrix. In this study, we follow mineral deposition in the caudal fin of the zebrafish larva in vivo. Using fluorescence and cryo-SEM-microscopy, in combination with Raman and XRF spectroscopy, we detect the presence of intracellular mineral particles located between bones, and in close association with blood vessels. Calcium-rich particles are also located away from the mineralized bone, and these are also in close association with blood vessels. These observations challenge the view that mineral formation is restricted to osteoblast cells juxtaposed to bone, or to the extracellular matrix. Our results, derived from observations performed in living animals, contribute a new perspective to the comprehensive mechanism of bone formation in vertebrates, from the blood to the bone. More broadly, these findings may shed light on bone mineralization processes in other vertebrates, including humans.

Development and origins of zebrafish ocular vasculature

Background

The developing eye receives blood supply from two vascular systems, the intraocular hyaloid system and the superficial choroidal vessels. In zebrafish, a highly stereotypic and simple set of vessels develops on the surface of the eye prior to development of choroidal vessels. The origins and formation of this so-called superficial system have not been described.

Results

We have analyzed the development of superficial vessels by time-lapse imaging and identified their origins by photoconversion experiments in kdrl:Kaede transgenic embryos. We show that the entire superficial system is derived from a venous origin, and surprisingly, we find that the hyaloid system has, in addition to its previously described arterial origin, a venous origin for specific vessels. Despite arising solely from a vein, one of the vessels in the superficial system, the nasal radial vessel (NRV), appears to acquire an arterial identity while growing over the nasal aspect of the eye and this happens in a blood flow-independent manner.

Conclusions

Our results provide a thorough analysis of the early development and origins of zebrafish ocular vessels and establish the superficial vasculature as a model for studying vascular patterning in the context of the developing eye.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0066-9) contains supplementary material, which is available to authorized users.

Zebrafish as a model for apolipoprotein biology: comprehensive expression analysis and a role for ApoA-IV in regulating food intake

Improved understanding of lipoproteins, particles that transport lipids throughout the circulation, is vital to developing new treatments for the dyslipidemias associated with metabolic syndrome. Apolipoproteins are a key component of lipoproteins. Apolipoproteins are proteins that structure lipoproteins and regulate lipid metabolism through control of cellular lipid exchange. Constraints of cell culture and mouse models mean that there is a need for a complementary model that can replicate the complex in vivo milieu that regulates apolipoprotein and lipoprotein biology. Here, we further establish the utility of the genetically tractable and optically clear larval zebrafish as a model of apolipoprotein biology. Gene ancestry analyses were implemented to determine the closest human orthologs of the zebrafish apolipoprotein A-I (apoA-I), apoB, apoE and apoA-IV genes and therefore ensure that they have been correctly named. Their expression patterns throughout development were also analyzed, by whole-mount mRNA in situ hybridization (ISH). The ISH results emphasized the importance of apolipoproteins in transporting yolk and dietary lipids: mRNA expression of all apolipoproteins was observed in the yolk syncytial layer, and intestinal and liver expression was observed from 4-6 days post-fertilization (dpf). Furthermore, real-time PCR confirmed that transcription of three of the four zebrafish apoA-IV genes was increased 4 hours after the onset of a 1-hour high-fat feed. Therefore, we tested the hypothesis that zebrafish ApoA-IV performs a conserved role to that in rat in the regulation of food intake by transiently overexpressing ApoA-IVb.1 in transgenic larvae and quantifying ingestion of co-fed fluorescently labeled fatty acid during a high-fat meal as an indicator of food intake. Indeed, ApoA-IVb.1 overexpression decreased food intake by approximately one-third. This study comprehensively describes the expression and function of eleven zebrafish apolipoproteins and serves as a springboard for future investigations to elucidate their roles in development and disease in the larval zebrafish model.

Simultaneous Raman microspectroscopy and fluorescence imaging of bone mineralization in living zebrafish larvae

Confocal Raman microspectroscopy and fluorescence imaging are two well-established methods providing functional insight into the extracellular matrix and into living cells and tissues, respectively, down to single molecule detection. In living tissues, however, cells and extracellular matrix coexist and interact. To acquire information on this cell-matrix interaction, we developed a technique for colocalized, correlative multispectral tissue analysis by implementing high-sensitivity, wide-field fluorescence imaging on a confocal Raman microscope. As a proof of principle, we study early stages of bone formation in the zebrafish (Danio rerio) larvae because the zebrafish has emerged as a model organism to study vertebrate development. The newly formed bones were stained using a calcium fluorescent marker and the maturation process was imaged and chemically characterized in vivo. Results obtained from early stages of mineral deposition in the zebrafish fin bone unequivocally show the presence of hydrogen phosphate containing mineral phases in addition to the carbonated apatite mineral. The approach developed here opens significant opportunities in molecular imaging of metabolic activities, intracellular sensing, and trafficking as well as in vivo exploration of cell-tissue interfaces under (patho-)physiological conditions.

Zebrafish as a model for monocarboxyl transporter 8-deficiency

Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor retardation characterized by neurological impairment and abnormal thyroid hormone (TH) levels. Mutations in the TH transporter, monocarboxylate transporter 8 (MCT8), are associated with AHDS. MCT8 knock-out mice exhibit impaired TH levels; however, they lack neurological defects. Here, the zebrafish mct8 gene and promoter were isolated, and mct8 promoter-driven transgenic lines were used to show that, similar to humans, mct8 is primarily expressed in the nervous and vascular systems. Morpholino-based knockdown and rescue experiments revealed that MCT8 is strictly required for neural development in the brain and spinal cord. This study shows that MCT8 is a crucial regulator during embryonic development and establishes the first vertebrate model for MCT8 deficiency that exhibits a neurological phenotype.

S1P1 inhibits sprouting angiogenesis during vascular development

Coordination between the vascular system and forming organs is essential for proper embryonic development. The vasculature expands by sprouting angiogenesis, during which tip cells form filopodia that incorporate into capillary loops. Although several molecules, such as vascular endothelial growth factor A (Vegfa), are known to induce sprouting, the mechanism that terminates this process to ensure neovessel stability is still unknown. Sphingosine-1-phosphate receptor 1 (S1P(1)) has been shown to mediate interaction between endothelial and mural cells during vascular maturation. In vitro studies have identified S1P(1) as a pro-angiogenic factor. Here, we show that S1P(1) acts as an endothelial cell (EC)-autonomous negative regulator of sprouting angiogenesis during vascular development. Severe aberrations in vessel size and excessive sprouting found in limbs of S1P(1)-null mouse embryos before vessel maturation imply a previously unknown, mural cell-independent role for S1P(1) as an anti-angiogenic factor. A similar phenotype observed when S1P(1) expression was blocked specifically in ECs indicates that the effect of S1P(1) on sprouting is EC-autonomous. Comparable vascular abnormalities in S1p(1) knockdown zebrafish embryos suggest cross-species evolutionary conservation of this mechanism. Finally, genetic interaction between S1P(1) and Vegfa suggests that these factors interplay to regulate vascular development, as Vegfa promotes sprouting whereas S1P(1) inhibits it to prevent excessive sprouting and fusion of neovessels. More broadly, because S1P, the ligand of S1P(1), is blood-borne, our findings suggest a new mode of regulation of angiogenesis, whereby blood flow closes a negative feedback loop that inhibits sprouting angiogenesis once the vascular bed is established and functional.

ApoB-containing lipoproteins regulate angiogenesis by modulating expression of VEGF receptor 1

Despite the clear major contribution of hyperlipidemia to the prevalence of cardiovascular disease in the developed world, the direct effects of lipoproteins on endothelial cells have remained obscure and are under debate. Here we report a previously uncharacterized mechanism of vessel growth modulation by lipoprotein availability. Using a genetic screen for vascular defects in zebrafish, we initially identified a mutation, stalactite (stl), in the gene encoding microsomal triglyceride transfer protein (mtp), which is involved in the biosynthesis of apolipoprotein B (ApoB)-containing lipoproteins. By manipulating lipoprotein concentrations in zebrafish, we found that ApoB negatively regulates angiogenesis and that it is the ApoB protein particle, rather than lipid moieties within ApoB-containing lipoproteins, that is primarily responsible for this effect. Mechanistically, we identified downregulation of vascular endothelial growth factor receptor 1 (VEGFR1), which acts as a decoy receptor for VEGF, as a key mediator of the endothelial response to lipoproteins, and we observed VEGFR1 downregulation in hyperlipidemic mice. These findings may open new avenues for the treatment of lipoprotein-related vascular disorders.

Motoneurons are essential for vascular pathfinding

The neural and vascular systems share common guidance cues that have direct and independent signaling effects on nerves and endothelial cells. Here, we show that zebrafish Netrin 1a directs Dcc-mediated axon guidance of motoneurons and that this neural guidance function is essential for lymphangiogenesis. Specifically, Netrin 1a secreted by the muscle pioneers at the horizontal myoseptum (HMS) is required for the sprouting of dcc-expressing rostral primary motoneuron (RoP) axons and neighboring axons along the HMS, adjacent to the future trajectory of the parachordal chain (PAC). These axons are required for the formation of the PAC and, subsequently, the thoracic duct. The failure to form the PAC in netrin 1a or dcc morphants is phenocopied by laser ablation of motoneurons and is rescued both by cellular transplants and overexpression of dcc mRNA. These results provide a definitive example of the requirement of axons in endothelial guidance leading to the parallel patterning of nerves and vessels in vivo.

Endothelial cells promote migration and proliferation of enteric neural crest cells via beta1 integrin signaling

Enteric neural crest-derived cells (ENCCs) migrate along the intestine to form a highly organized network of ganglia that comprises the enteric nervous system (ENS). The signals driving the migration and patterning of these cells are largely unknown. Examining the spatiotemporal development of the intestinal neurovasculature in avian embryos, we find endothelial cells (ECs) present in the gut prior to the arrival of migrating ENCCs. These ECs are patterned in concentric rings that are predictive of the positioning of later arriving crest-derived cells, leading us to hypothesize that blood vessels may serve as a substrate to guide ENCC migration. Immunohistochemistry at multiple stages during ENS development reveals that ENCCs are positioned adjacent to vessels as they colonize the gut. A similar close anatomic relationship between vessels and enteric neurons was observed in zebrafish larvae. When EC development is inhibited in cultured avian intestine, ENCC migration is arrested and distal aganglionosis results, suggesting that ENCCs require the presence of vessels to colonize the gut. Neural tube and avian midgut were explanted onto a variety of substrates, including components of the extracellular matrix and various cell types, such as fibroblasts, smooth muscle cells, and endothelial cells. We find that crest-derived cells from both the neural tube and the midgut migrate avidly onto cultured endothelial cells. This EC-induced migration is inhibited by the presence of CSAT antibody, which blocks binding to beta1 integrins expressed on the surface of crest-derived cells. These results demonstrate that ECs provide a substrate for the migration of ENCCs via an interaction between beta1 integrins on the ENCC surface and extracellular matrix proteins expressed by the intestinal vasculature. These interactions may play an important role in guiding migration and patterning in the developing ENS.

Imaging the developing lymphatic system using the zebrafish

The lymphatic system is essential for immune responses, fluid homeostasis, and fat absorption, and is involved in many pathological processes, including tumour metastasis and lymphoedema. Despite its importance, progress in understanding the origins and early development of this system has been hampered by difficulties in observing lymphatic cells in vivo and performing genetic and experimental manipulation of the lymphatic system. These difficulties stem in part from the lack of a model organism combining these features. The zebrafish is a genetically accessible vertebrate with an optically clear embryo permitting high-resolution in vivo imaging, but the existence of a lymphatic vascular system has not been previously reported in this model organism. Using a series of morphological, molecular and functional studies we have visualized and characterized lymphatic vessels in the developing zebrafish. Our studies show that the zebrafish possesses a lymphatic system that shares many of the characteristic features of lymphatic vessels found in other vertebrates. Using multiphoton time-lapse imaging we have carried out in vivo cell tracking experiments to trace the origins of lymphatic endothelial cells. Our data provide conclusive new evidence supporting a venous origin for primitive lymphatic endothelial cells.

Live imaging of lymphatic development in the zebrafish

The lymphatic system has become the subject of great interest in recent years because of its important role in normal and pathological processes. Progress in understanding the origins and early development of this system, however, has been hampered by difficulties in observing lymphatic cells in vivo and in performing defined genetic and experimental manipulation of the lymphatic system in currently available model organisms. Here, we show that the optically clear developing zebrafish provides a useful model for imaging and studying lymphatic development, with a lymphatic system that shares many of the morphological, molecular and functional characteristics of the lymphatic vessels found in other vertebrates. Using two-photon time-lapse imaging of transgenic zebrafish, we trace the migration and lineage of individual cells incorporating into the lymphatic endothelium. Our results show lymphatic endothelial cells of the thoracic duct arise from primitive veins through a novel and unexpected pathway.

The RNA-binding protein Vg1 RBP is required for cell migration during early neural development

After mid-blastula transition, populations of cells within the Xenopus embryo become motile. Using antisense morpholino oligonucleotides, we find that Vg1 RBP, an RNA-binding protein implicated in RNA localization in oocytes, is required for the migration of cells forming the roof plate of the neural tube and, subsequently, for neural crest migration. These cells are properly determined but remain at their site of origin. Consistent with a possible role in cell movement, Vg1 RBP asymmetrically localizes to extended processes in migrating neural crest cells. Given that Vg1 RBP is a member of the conserved VICKZ family of proteins, expressed in embryonic and neoplastic cells, these data shed light on the likely role of these RNA-binding proteins in regulating cell movements during both development and metastasis.

The involvement of a conserved family of RNA binding proteins in embryonic development and carcinogenesis

Vg1 RBP is a member of a family of highly conserved proteins that appear to be involved in RNA localization, stability, and/or translational control in a wide variety of cell types and organisms. Over the last few years, the human homologs of these proteins have been found to be overexpressed in an increasing number of different kinds of cancers. Although the role of these proteins in neoplasia is not understood, results from several labs, including our own, are beginning to suggest that many of these proteins may be important in cell motility, a necessary requirement for metastasis. This paper will review these data and suggest a model for the role of Vg1 RBP and its homologs in embryonic development and carcinogenesis.

Defining cis-acting elements and trans-acting factors in RNA localization

Research over the last 10 to 15 years has revealed that intracellular RNA localization is a widespread phenomenon found in a large range of different cell types in an equally impressive number of different organisms (Bashirullah et al., 1998; St. Johnston, 1995). Efforts have focused both on the molecular mechanisms involved in localizing RNAs to particular intracellular targets and on the functional importance (to the cell) of placing certain RNAs at particular cellular sites. In many cases, an understanding of the role of RNA localization seems to be predicated on a careful analysis of how a particular RNA achieves its characteristic distribution. A generalized model of RNA localization usually invokes cellular factors recognizing RNA target sequences. This review will focus on several systems in which cis-acting elements and trans-acting factors recognizing these elements are involved in RNA localization: how they have been defined, how they relate to each other, and how they interact and function to help achieve defined intracellular localization. Conservation of both RNA elements and protein factors across species suggests that RNA localization is probably a fundamental cellular process.

Vg1 RBP intracellular distribution and evolutionarily conserved expression at multiple stages during development

We have analyzed the expression and intracellular distribution, during oogenesis and embryogenesis, of Vg1 RBP, a protein implicated in the intracellular localization of Vg1 mRNA to the vegetal cortex of Xenopus oocytes. Vg1 RBP (protein) colocalizes with Vg1 RNA at all stages of oogenesis. Vg1 RBP RNA, however, localizes to the animal pole during late oogenesis, and remains in the animal blastomeres and ectodermal precursors until its zygotic transcription is activated, around stage 12. Vg1 RBP mRNA then becomes expressed throughout the neural epithelium. Vg1 RBP mRNA expression is also detected in what appears to be neural crest cells undergoing delamination and lateral migration. By tailbud stages, Vg1 RBP expression is present in the branchial arches, otic vesicle, pronephros, and along the neural tube. To examine the expression pattern in different species, we cloned the zebrafish homolog of Vg1 RBP by using a highly homologous EST clone to screen an embryonic cDNA library. In situ hybridization reveals that Vg1 RBP RNA localizes early in oogenesis to the animal pole. Although Vg1 RBP RNA is detected in all blastomeres of the early embryo, the expression pattern in the one day old zebrafish embryo is almost identical to that of the equivalent stage Xenopus embryo. These results indicate that the zygotic expression pattern is similar in frogs and fish, and that there is a conserved zygotic expression of Vg1 RBP distinct from its expression in the oocyte.

RNA-binding protein conserved in both microtubule- and microfilament-based RNA localization

Vg1 mRNA translocation to the vegetal cortex of Xenopus oocytes requires intact microtubules, and a 3' UTR cis-acting element (termed VLE), which also mediates sequence-specific binding of several proteins. One protein, the 69-kD Vg1 RBP, associates Vg1 RNA to microtubules in vitro. Here we show that Vg1 RBP-binding sites correlate with vegetal localization. Purification and cloning of Vg1 RBP revealed five RNA-binding motifs: four KH and one RRM domains. Surprisingly, Vg1 RBP is highly homologous to the zipcode binding protein implicated in the microfilament-mediated localization of beta actin mRNA in fibroblasts. These data support Vg1 RBP's direct role in vegetal localization and suggest the existence of a general, evolutionarily conserved mechanism for mRNA targeting.