Development and origins of zebrafish ocular vasculature

Systemic treatment with cigarette smoke extract affects zebrafish visual behaviour, intraocular vasculature morphology and outer segment phagocytosis

Introduction

Cigarette smoking adversely affects multiple aspects of human health including eye disorders such as age-related macular degeneration, cataracts and dry eye disease. However, there remains a knowledge gap in how constituents of cigarette smoke affect vision and retinal biology. We used zebrafish to assess effects of short-term acute exposure to cigarette smoke extract (CSE) on visual behaviour and retinal biology.

Methods

Zebrafish larvae with a developed visual system at three days post-fertilization (dpf) were exposed to CSE for 4, 24 or 48 hours. Visual behaviour, hyaloid vasculature morphology, retinal histology, oxidative stress gene expression and outer segment phagocytosis were investigated using visual behavioural optokinetic and visual motor response assays (OKR and VMR), microscopy (light, fluorescence and transmission electron microscopy), and real-time PCR.

Results

In zebrafish larvae, 48 hours of CSE treatment resulted in significantly reduced visual behaviour. Larvae treated with 10, 15 or 20 μg/mL CSE showed an average of 13.7, 10.7 or 9.4 saccades per minute, respectively, significantly lower compared with 0.05% DMSO controls (p=0.0093, p=0.0004 and p<0.0001, respectively) that exhibited 19.7 saccades per minute. The diameter of intraocular vessels increased from 4.833 μm in 0.05% DMSO controls to 5.885 μm in the 20 μg/mL CSE-treated larvae (p=0.0333). Biometry analysis highlighted a significant axial length elongation in 20 μg/mL CSE-treated larvae (216.9 μm, p<0.0001) compared to 0.05% dimethyl sulfoxide (DMSO) controls (205.1 μm). Larvae exposed to 20 μg/mL CSE had significantly (p=0.0002) higher numbers of RPE phagosomes compared to vehicle controls (0.1425 and 0.093 phagosomes/μm RPE, respectively).

Conclusions

Zebrafish larvae with a developed visual system display apparent defects in visual behaviour and retinal biology after acute exposure to CSE, establishing a valuable in vivo model to investigate ocular disorders related to cigarette smoke.

Intravitreal injection of FGF and TGF-β inhibitors disrupts cranial cartilage development

Cartilage development is a tightly regulated process that requires the interaction of epithelial and mesenchymal tissues layers to initiate the aggregation of mesenchyme in a condensation. Several signaling molecules have been implicated in cartilage formation including FGFs, WNTs, and members of the TGF-β super family. However, little is known about the earliest signals involved in these initial phases of development. Here we aimed to investigate whether direct intravitreal injection of pharmaceutical inhibitors for FGF and TGF-β signaling would perturb cranial cartilages in zebrafish. Via wholemount bone and cartilage staining, we found effects on multiple cranial cartilage elements. We found no effect on scleral cartilage development, however, the epiphyseal bar, basihyal, and basicapsular cartilages were disrupted. Interestingly, the epiphyseal bar arises from the same progenitor pool as the scleral cartilage, namely, the periocular ectomesenchyme. This study adds to the foundational knowledge about condensation induction of cranial cartilage development and provides insight into the timing and signaling involved in the early development of several craniofacial cartilage elements in zebrafish.

Di-(2-ethylhexyl) phthalate impairs angiogenesis and hematopoiesis via suppressing VEGF signaling in zebrafish

Di-(2-ethylhexyl) phthalate (DEHP) is among the most widely used plasticizers in plastic production, which has been detected in various environments. However, DEHP safety remains poorly known. Using zebrafish models, the effects of DEHP on the angiogenesis and hematopoiesis, and the underlying mechanism, were studied. Transgenic zebrafish embryos with specific fluorescence of vascular endothelial cells, myeloid cells, or hematopoietic stem cells were exposed to 0, 100, 150, 200, or 250 nM of DEHP for 22, 46 or 70 h, followed by fluorescence observation, endogenous alkaline phosphatase activity measurement, erythrocyte staining, and gene expression analysis by quantitative PCR and whole mount in situ hybridization. High DEHP concentrations decreased the sprouting rate, average diameter, and length, and the expansion area of the vessels lowered the EAP activity and suppressed the vascular endothelial growth factor (vegf) and hematopoietic marker genes, including c-myb, hbae1, hbbe1, and lyz expressions. DEHP treatment also decreased the number of hematopoietic stem cells, erythrocytes, and myeloid cells at 24 and 72 hpf. These DEHP-induced angiogenetic and hematopoietic defects might be alleviated by vegf overexpression. Our results reveal a plausible mechanistic link between DEHP exposure-induced embryonic angiogenetic defect and hematopoietic impairment.

CRISPR-Cas9-mediated functional dissection of the foxc1 genomic region in zebrafish identifies critical conserved cis-regulatory elements

FOXC1 encodes a forkhead-domain transcription factor associated with several ocular disorders. Correct FOXC1 dosage is critical to normal development, yet the mechanisms controlling its expression remain unknown. Together with FOXQ1 and FOXF2, FOXC1 is part of a cluster of FOX genes conserved in vertebrates. CRISPR-Cas9-mediated dissection of genomic sequences surrounding two zebrafish orthologs of FOXC1 was performed. This included five zebrafish-human conserved regions, three downstream of foxc1a and two remotely upstream of foxf2a/foxc1a or foxf2b/foxc1b clusters, as well as two intergenic regions between foxc1a/b and foxf2a/b lacking sequence conservation but positionally corresponding to the area encompassing a previously reported glaucoma-associated SNP in humans. Removal of downstream sequences altered foxc1a expression; moreover, zebrafish carrying deletions of two or three downstream elements demonstrated abnormal phenotypes including enlargement of the anterior chamber of the eye reminiscent of human congenital glaucoma. Deletions of distant upstream conserved elements influenced the expression of foxf2a/b or foxq1a/b but not foxc1a/b within each cluster. Removal of either intergenic sequence reduced foxc1a or foxc1b expression during late development, suggesting a role in transcriptional regulation despite the lack of conservation at the nucleotide level. Further studies of the identified regions in human patients may explain additional individuals with developmental ocular disorders.

Epigenetic Regulation of Endothelial Cell Lineages During Zebrafish Development-New Insights From Technical Advances

Epigenetic regulation is integral in orchestrating the spatiotemporal regulation of gene expression which underlies tissue development. The emergence of new tools to assess genome-wide epigenetic modifications has enabled significant advances in the field of vascular biology in zebrafish. Zebrafish represents a powerful model to investigate the activity of cis-regulatory elements in vivo by combining technologies such as ATAC-seq, ChIP-seq and CUT&Tag with the generation of transgenic lines and live imaging to validate the activity of these regulatory elements. Recently, this approach led to the identification and characterization of key enhancers of important vascular genes, such as gata2a, notch1b and dll4. In this review we will discuss how the latest technologies in epigenetics are being used in the zebrafish to determine chromatin states and assess the function of the cis-regulatory sequences that shape the zebrafish vascular network.

Mechanisms underlying microglial colonization of developing neural retina in zebrafish

Microglia are brain-resident macrophages that function as the first line of defense in brain. Embryonic microglial precursors originate in peripheral mesoderm and migrate into the brain during development. However, the mechanism by which they colonize the brain is incompletely understood. The retina is one of the first brain regions to accommodate microglia. In zebrafish, embryonic microglial precursors use intraocular hyaloid blood vessels as a pathway to migrate into the optic cup via the choroid fissure. Once retinal progenitor cells exit the cell cycle, microglial precursors associated with hyaloid blood vessels start to infiltrate the retina preferentially through neurogenic regions, suggesting that colonization of retinal tissue depends upon the neurogenic state. Along with blood vessels and retinal neurogenesis, IL34 also participates in microglial precursor colonization of the retina. Altogether, CSF receptor signaling, blood vessels, and neuronal differentiation function as cues to create an essential path for microglial migration into developing retina.

The immune system is comprised of many different cells which protect our bodies from infection and other illnesses. The brain contains its own population of immune cells called microglia. Unlike neurons, these cells form outside the brain during development. They then travel to the brain and colonize specific regions like the retina, the light-sensing part of the eye in vertebrates.

It is poorly understood how newly formed microglia migrate to the retina and whether their entry depends on the developmental state of nerve cells (also known as neurons) in this region. To help answer these questions, Ranawat and Masai attached fluorescent labels that can be seen under a microscope to microglia in the embryos of zebrafish. Developing zebrafish are transparent, making it easy to trace the fluorescent microglia as they travel to the retina and insert themselves among its neurons.

Ranawat and Masai found that blood vessels around the retina act as a pathway that microglia move along. Once they reach the retina, the microglia remain attached and only enter the retina at sites where brain cells are starting to mature in to adult neurons. Further experiments showed that microglia fail to infiltrate and colonize the retina when blood vessels are damaged or neuron maturation is blocked.

These findings reveal some of the key elements that guide microglia to the retina during development. However, further work is needed to establish the molecular and biochemical processes that allow microglia to attach to blood vessels and detect when cells in the retina are starting to mature.

Disruption of foxc1 genes in zebrafish results in dosage-dependent phenotypes overlapping Axenfeld-Rieger syndrome

The Forkhead Box C1 (FOXC1) gene encodes a forkhead/winged helix transcription factor involved in embryonic development. Mutations in this gene cause dysgenesis of the anterior segment of the eye, most commonly Axenfeld-Rieger syndrome (ARS), often with other systemic features. The developmental mechanisms and pathways regulated by FOXC1 remain largely unknown. There are two conserved orthologs of FOXC1 in zebrafish, foxc1a and foxc1b. To further examine the role of FOXC1 in vertebrates, we generated foxc1a and foxc1b single knockout zebrafish lines and bred them to obtain various allelic combinations. Three genotypes demonstrated visible phenotypes: foxc1a-/- single homozygous and foxc1-/- double knockout homozygous embryos presented with similar characteristics comprised of severe global vascular defects and early lethality, as well as microphthalmia, periocular edema and absence of the anterior chamber of the eye; additionally, fish with heterozygous loss of foxc1a combined with homozygosity for foxc1b (foxc1a+/-;foxc1b-/-) demonstrated craniofacial defects, heart anomalies and scoliosis. All other single and combined genotypes appeared normal. Analysis of foxc1 expression detected a significant increase in foxc1a levels in homozygous and heterozygous mutant eyes, suggesting a mechanism for foxc1a upregulation when its function is compromised; interestingly, the expression of another ARS-associated gene, pitx2, was responsive to the estimated level of wild-type Foxc1a, indicating a possible role for this protein in the regulation of pitx2 expression. Altogether, our results support a conserved role for foxc1 in the formation of many organs, consistent with the features observed in human patients, and highlight the importance of correct FOXC1/foxc1 dosage for vertebrate development.

Selective Requirements for Vascular Endothelial Cells and Circulating Factors in the Regulation of Retinal Neurogenesis

Development of the vertebrate eye requires signaling interactions between neural and non-neural tissues. Interactions between components of the vascular system and the developing neural retina have been difficult to decipher, however, due to the challenges of untangling these interactions from the roles of the vasculature in gas exchange. Here we use the embryonic zebrafish, which is not yet reliant upon hemoglobin-mediated oxygen transport, together with genetic strategies for (1) temporally-selective depletion of vascular endothelial cells, (2) elimination of blood flow through the circulation, and (3) elimination of cells of the erythroid lineage, including erythrocytes. The retinal phenotypes in these genetic systems were not identical, with endothelial cell-depleted retinas displaying laminar disorganization, cell death, reduced proliferation, and reduced cell differentiation. In contrast, the lack of blood flow resulted in a milder retinal phenotype showing reduced proliferation and reduced cell differentiation, indicating that an endothelial cell-derived factor(s) is/are required for laminar organization and cell survival. The lack of erythrocytes did not result in an obvious retinal phenotype, confirming that defects in retinal development that result from vascular manipulations are not due to poor gas exchange. These findings underscore the importance of the cardiovascular system supporting and controlling retinal development in ways other than supplying oxygen. In addition, these findings identify a key developmental window for these interactions and point to distinct functions for vascular endothelial cells vs. circulating factors.

Zebrafish Vascular Development: General and Tissue-Specific Regulation

Circulation is required for the delivery of oxygen and nutrition to tissues and organs, as well as waste collection. Therefore, the heart and vessels develop first during embryogenesis. The circulatory system consists of the heart, blood vessels, and blood cells, which originate from the mesoderm. The gene expression pattern required for blood vessel development is predetermined by the hierarchical and sequential regulation of genes for the differentiation of mesodermal cells. Herein, we review how blood vessels form distinctly in different tissues or organs of zebrafish and how vessel formation is universally or tissue-specifically regulated by signal transduction pathways and blood flow. In addition, the unsolved issues of mutual contacts and interplay of circulatory organs during embryogenesis are discussed.

Elucidating the early signaling cues involved in zebrafish chondrogenesis and cartilage morphology

Across the teleost skeleton, cartilages are diverse in their composition suggesting subtle differences in their developmental mechanisms. This study aims to elucidate the regulatory role of bone morphogenetic protein (BMPs) during the morphogenesis of two cartilage elements in zebrafish: the scleral cartilage in the eye and the caudal fin endoskeleton. Zebrafish larvae were exposed to a BMP inhibitor (LDN193189) at a series of timepoints preceding the initial appearance of the scleral cartilage and caudal fin endoskeleton. Morphological assessments of the cartilages in later stages, revealed that BMP-inhibited fish harbored striking disruptions in caudal fin endoskeletal morphology, regardless of the age at which the inhibitor treatment was performed. In contrast, scleral cartilage morphology was unaffected in all age groups. Morphometric and principal component analysis, performed on the caudal fin endoskeleton, revealed differential clustering of principal components one and two in BMP-inhibited and control fish. Additionally, the expression of sox9a and sox9b were reduced in BMP-inhibited fish when compared to controls, indicating that LDN193189 acts via a Sox9-dependent pathway. Further examination of notochord flexion also revealed a disruptive effect of BMP inhibition on this process. This study provides a detailed characterization of the effects of BMP inhibition via LDN193189 on zebrafish cartilage morphogenesis and development. It highlights the specific, localized role of the BMP-signaling pathways during the development of different cartilage elements and sheds some light on the morphological characteristics of fossil teleosts that together suggest an uncoupling of the developmental processes between the upper and lower lobes of the caudal fin.

Endothelial Cell Dynamics in Vascular Development: Insights From Live-Imaging in Zebrafish

The formation of the vertebrate vasculature involves the acquisition of endothelial cell identities, sprouting, migration, remodeling and maturation of functional vessel networks. To understand the cellular and molecular processes that drive vascular development, live-imaging of dynamic cellular events in the zebrafish embryo have proven highly informative. This review focusses on recent advances, new tools and new insights from imaging studies in vascular cell biology using zebrafish as a model system.

Hyaloid vasculature and mmp2 activity play a role during optic fissure fusion in zebrafish

Vertebrate retinal development requires timely and precise fusion of the optic fissure (OF). Failure of this event leads to congenital vision impairment in the form of coloboma. Recent studies have suggested hyaloid vasculature to be involved in OF fusion. In order to examine this link, we analyzed OF fusion and hyaloid vasculogenesis in the zebrafish pax2a noi mutant line. We first determined that pax2a-/- embryos fail to accumulate F-actin in the OF prior to basement membrane (BM) degradation. Furthermore, using 3D and live imaging we observed reduced OF hyaloid vascularization in pax2a-/- embryos. When examining the connection between pax2a loss of function and hyaloid vasculature, we observed significant reduction of talin1 expression, a regulator of hyaloid vasculature. In addition, cranial VEGF expression was found to be reduced in pax2a-/- embryos. Pharmacological inhibition of VEGF signaling phenocopied the pax2a-/- vasculature, F-actin and BM degradation phenotypes. Lastly, we determined that OF associated hyaloid vasculature is a source of mmp2, mmp14a and mmp14b expression and showed that mmp2 is functionally necessary for degradation of OF BM. Taken together we propose a pax2a driven mechanism that ensures proper and timely hyaloid vasculature invasion of the OF in order to facilitate availability of the BM remodeler mmp2.

Bisphenol S Induces Ectopic Angiogenesis in Embryos via VEGFR2 Signaling, Leading to Lipid Deposition in Blood Vessels of Larval Zebrafish

Bisphenol S (BPS), used as a bisphenol A substitute, has been detected in various environments. However, the safety of BPS is still unclear. Here, zebrafish embryos were exposed to BPS (0, 1, 10, and 100 μg/L) for 24, 48, 72, 96 h, and 15 days. BPS induced ectopic sprouting of budding blood vessels in embryos, but the blood flow velocity within these lesions was unchanged at 48 h. At 72 h postfertilization (hpf), by observing the subintestinal venous plexus responsible for yolk absorption, we found that VEGFR2 transduced an angiogenic signal and that the subsequent reduction in blood flow velocity inhibited yolk absorption. At 96 hpf, yolk consumption was still delayed because of the disturbed transportation route, resulting in transient extensive lipid retention in the blood vessels. After feeding, obvious atherogenic lipids were discovered in the blood vessels, especially in bends, bifurcations, and stenoses. This dynamic visualization of the pathogenesis demonstrates a plausible mechanistic link between BPS exposure-induced embryonic vessel overgrowth and an increased atherosclerosis risk.

In Vivo Analysis of Optic Fissure Fusion in Zebrafish: Pioneer Cells, Basal Lamina, Hyaloid Vessels, and How Fissure Fusion is Affected by BMP

Colobomata, persistent optic fissures, frequently cause congenital blindness. Here, we focused on optic fissure fusion using in vivo time-lapse imaging in zebrafish. We identified the fusion initiating cells, which we termed “pioneer cells.” Based on morphology, localization, and downregulation of the neuroretinal (NR) precursor marker rx2, these cells could be considered as retinal pigment epithelial (RPE) progenitors. Notably, pioneer cells regain rx2 expression and integrate into the NR after fusion, indicating that they do not belong to the pool of RPE progenitors, supported by the lack of RPE marker expression in pioneer cells. They establish the first cellular contact between the margins in the proximal fissure region and separate the hyaloid artery and vein. After initiation, the fusion site is progressing distally, increasing the distance between the hyaloid artery and vein. A timed BMP (Bone Morphogenetic Protein) induction, resulting in coloboma, did not alter the morphology of the fissure margins, but it did affect the expression of NR and RPE markers within the margins. In addition, it resulted in a persisting basal lamina and persisting remnants of periocular mesenchyme and hyaloid vasculature within the fissure, supporting the necessity of BMP antagonism within the fissure margins. The hampered fissure fusion had severe effects on the vasculature of the eye.

Modeling ZNF408-Associated FEVR in Zebrafish Results in Abnormal Retinal Vasculature

Purpose

Familial exudative vitreoretinopathy (FEVR) is an inherited retinal disease in which the retinal vasculature is affected. Patients with FEVR typically lack or have abnormal vasculature in the peripheral retina, the outcome of which can range from mild visual impairment to complete blindness. A missense mutation (p.His455Tyr) in ZNF408 was identified in an autosomal dominant FEVR family. Little, however, is known about the molecular role of ZNF408 and how its defect leads to the clinical features of FEVR.

Methods

Using CRISPR/Cas9 technology, two homozygous mutant zebrafish models with truncated znf408 were generated, as well as one heterozygous and one homozygous missense znf408 model in which the human p.His455Tyr mutation is mimicked.

Results

Intriguingly, all three znf408-mutant zebrafish strains demonstrated progressive retinal vascular pathology, initially characterized by a deficient hyaloid vessel development at 5 days postfertilization (dpf) leading to vascular insufficiency in the retina. The generation of stable mutant lines allowed long-term follow up studies, which showed ectopic retinal vascular hyper-sprouting at 90 dpf and extensive vascular leakage at 180 dpf.

Conclusions

Together, our data demonstrate an important role for znf408 in the development and maintenance of the vascular system within the retina.

Synchronized tissue-scale vasculogenesis and ubiquitous lateral sprouting underlie the unique architecture of the choriocapillaris

The choriocapillaris is an exceptionally high density, two-dimensional, sheet-like capillary network, characterized by the highest exchange rate of nutrients for waste products per area in the organism. These unique morphological and physiological features are critical for supporting the extreme metabolic requirements of the outer retina needed for vision. The developmental mechanisms and processes responsible for generating this unique vascular network remain, however, poorly understood. Here we take advantage of the zebrafish as a model organism for gaining novel insights into the cellular dynamics and molecular signaling mechanisms involved in the development of the choriocapillaris. We show for the first time that zebrafish have a choriocapillaris highly similar to that in mammals, and that it is initially formed by a novel process of synchronized vasculogenesis occurring simultaneously across the entire outer retina. This initial vascular network expands by un-inhibited sprouting angiogenesis whereby all endothelial cells adopt tip-cell characteristics, a process which is sustained throughout embryonic and early post-natal development, even after the choriocapillaris becomes perfused. Ubiquitous sprouting was maintained by continuous VEGF-VEGFR2 signaling in endothelial cells delaying maturation until immediately before stages where vision becomes important for survival, leading to the unparalleled high density and lobular structure of this vasculature. Sprouting was throughout development limited to two dimensions by Bruch's membrane and the sclera at the anterior and posterior surfaces respectively. These novel cellular and molecular mechanisms underlying choriocapillaris development were recapitulated in mice. In conclusion, our findings reveal novel mechanisms underlying the development of the choriocapillaris during zebrafish and mouse development. These results may explain the uniquely high density and sheet-like organization of this vasculature.

Frizzled 4 regulates ventral blood vessel remodeling in the zebrafish retina

BACKGROUND

Familial exudative vitreoretinopathy (FEVR) is a rare congenital disorder characterized by a lack of blood vessel growth to the periphery of the retina with secondary fibrovascular proliferation at the vascular-avascular junction. These structurally abnormal vessels cause leakage and hemorrhage, while the fibroproliferative scarring results in retinal dragging, detachment and blindness. Mutations in the FZD4 gene represent one of the most common causes of FEVR.

METHODS

A loss of function mutation resulting from a 10-nucleotide insertion into exon 1 of the zebrafish fzd4 gene was generated using transcription activator-like effector nucleases (TALENs). Structural and functional integrity of the retinal vasculature was examined by fluorescent microscopy and optokinetic responses.

RESULTS

Zebrafish retinal vasculature is asymmetrically distributed along the dorsoventral axis, with active vascular remodeling on the ventral surface of the retina throughout development. fzd4 mutants exhibit disorganized ventral retinal vasculature with discernable tubular fusion by week 8 of development. Furthermore, fzd4 mutants have impaired optokinetic responses requiring increased illumination.

CONCLUSION

We have generated a visually impaired zebrafish FEVR model exhibiting abnormal retinal vasculature. These fish provide a tractable system for studying vascular biology in retinovascular disorders, and demonstrate the feasibility of using zebrafish for evaluating future FEVR genes identified in humans.

Intussusceptive Vascular Remodeling Precedes Pathological Neovascularization

Supplemental Digital Content is available in the text.

Objective—

Pathological neovascularization is crucial for progression and morbidity of serious diseases such as cancer, diabetic retinopathy, and age-related macular degeneration. While mechanisms of ongoing pathological neovascularization have been extensively studied, the initiating pathological vascular remodeling (PVR) events, which precede neovascularization remains poorly understood. Here, we identify novel molecular and cellular mechanisms of preneovascular PVR, by using the adult choriocapillaris as a model.

Approach and Results—

Using hypoxia or forced overexpression of VEGF (vascular endothelial growth factor) in the subretinal space to induce PVR in zebrafish and rats respectively, and by analyzing choriocapillaris membranes adjacent to choroidal neovascular lesions from age-related macular degeneration patients, we show that the choriocapillaris undergo robust induction of vascular intussusception and permeability at preneovascular stages of PVR. This PVR response included endothelial cell proliferation, formation of endothelial luminal processes, extensive vesiculation and thickening of the endothelium, degradation of collagen fibers, and splitting of existing extravascular columns. RNA-sequencing established a role for endothelial tight junction disruption, cytoskeletal remodeling, vesicle- and cilium biogenesis in this process. Mechanistically, using genetic gain- and loss-of-function zebrafish models and analysis of primary human choriocapillaris endothelial cells, we determined that HIF (hypoxia-induced factor)-1α-VEGF-A-VEGFR2 signaling was important for hypoxia-induced PVR.

Conclusions—

Our findings reveal that PVR involving intussusception and splitting of extravascular columns, endothelial proliferation, vesiculation, fenestration, and thickening is induced before neovascularization, suggesting that identifying and targeting these processes may prevent development of advanced neovascular disease in the future.

Veins and Arteries Build Hierarchical Branching Patterns Differently: Bottom-Up versus Top-Down

A tree-like hierarchical branching structure is present in many biological systems, such as the kidney, lung, mammary gland, and blood vessels. Most of these organs form through branching morphogenesis, where outward growth results in smaller and smaller branches. However, the blood vasculature is unique in that it exists as two trees (arterial and venous) connected at their tips. Obtaining this organization might therefore require unique developmental mechanisms. As reviewed here, recent data indicate that arterial trees often form in reverse order. Accordingly, initial arterial endothelial cell differentiation occurs outside of arterial vessels. These pre-artery cells then build trees by following a migratory path from smaller into larger arteries, a process guided by the forces imparted by blood flow. Thus, in comparison to other branched organs, arteries can obtain their structure through inward growth and coalescence. Here, new information on the underlying mechanisms is discussed, and how defects can lead to pathologies, such as hypoplastic arteries and arteriovenous malformations.

Fine-tuning of fgf8a expression through alternative polyadenylation has a selective impact on Fgf-associated developmental processes

The formation of distinct 3'UTRs through alternative polyadenylation is a mechanism of gene expression regulation that has been implicated in many physiological and pathological processes. However, its functions in the context of vertebrate embryonic development have been largely unaddressed, in particular with a gene-specific focus. Here we show that the most abundant 3'UTR for the zebrafish fgf8a gene in the developing embryo mediates a strong translational repression, when compared to a more sparsely used alternative 3'UTR, which supports a higher translational efficiency. By inducing a shift in the selection efficiency of the associated polyadenylation sites, we show a temporally and spatially specific impact of fgf8a 3'UTR usage on embryogenesis, in particular at late stages during sensory system development. In addition, we identified a previously undescribed role for Fgf signalling in the initial stages of superficial retinal vascularization. These results reveal a critical functional importance of gene-specific alternative 3'UTRs in vertebrate embryonic development.

Temporal and Spatial Post-Transcriptional Regulation of Zebrafish tie1 mRNA by Long Noncoding RNA During Brain Vascular Assembly

Supplemental Digital Content is available in the text.

Objective—

Tie1 (tyrosine kinase containing immunoglobulin and epidermal growth factor homology 1), an endothelial and hematopoietic cell–specific receptor tyrosine kinase, is an important regulator of angiogenesis and critical for maintaining vascular integrity. The post-transcriptional regulation of tie1 mRNA expression is not understood, but it might partly explain Tie1’s differential expression pattern in endothelium. Following up on our previous work that identified natural antisense transcripts from the tie1 locus—tie1 antisense (tie1AS), which regulates tie1 mRNA levels in zebrafish—we attempted to identify the mechanism of this regulation.

Approach and Results—

Through in vitro and in vivo ribonucleoprotein binding studies, we demonstrated that tie1AS long noncoding RNA interacts with an RNA binding protein—embryonic lethal and abnormal vision Drosophila-like 1 (Elavl1)—that regulates tie1 mRNA levels. When we disrupted the interaction between tie1AS and Elavl1 by using constitutively active antisense morpholino oligonucleotides or photoactivatable morpholino oligonucleotides, tie1 mRNA levels increased between 26 and 31 hours post-fertilization, particularly in the head. This increase correlated with dilation of primordial midbrain channels, smaller eyes, and reduced ventricular space. We also observed these phenotypes when we used CRISPR (clustered regularly interspaced short palindromic repeats)–mediated CRISPRi (CRISPR-mediated interference) to knock down tie1AS. Treatment of the morpholino oligonucleotide–injected embryos with a small molecule that decreased tie1 mRNA levels rescued all 3 abnormal phenotypes.

Conclusions—

We identified a novel mode of temporal and spatial post-transcriptional regulation of tie1 mRNA. It involves long noncoding RNA, tie1AS, and Elavl1 (an interactor of tie1AS).

Morphogenetic defects underlie Superior Coloboma, a newly identified closure disorder of the dorsal eye

The eye primordium arises as a lateral outgrowth of the forebrain, with a transient fissure on the inferior side of the optic cup providing an entry point for developing blood vessels. Incomplete closure of the inferior ocular fissure results in coloboma, a disease characterized by gaps in the inferior eye and recognized as a significant cause of pediatric blindness. Here, we identify eight patients with defects in tissues of the superior eye, a congenital disorder that we term superior coloboma. The embryonic origin of superior coloboma could not be explained by conventional models of eye development, leading us to reanalyze morphogenesis of the dorsal eye. Our studies revealed the presence of the superior ocular sulcus (SOS), a transient division of the dorsal eye conserved across fish, chick, and mouse. Exome sequencing of superior coloboma patients identified rare variants in a Bone Morphogenetic Protein (Bmp) receptor (BMPR1A) and T-box transcription factor (TBX2). Consistent with this, we find sulcus closure defects in zebrafish lacking Bmp signaling or Tbx2b. In addition, loss of dorsal ocular Bmp is rescued by concomitant suppression of the ventral-specific Hedgehog pathway, arguing that sulcus closure is dependent on dorsal-ventral eye patterning cues. The superior ocular sulcus acts as a conduit for blood vessels, with altered sulcus closure resulting in inappropriate connections between the hyaloid and superficial vascular systems. Together, our findings explain the existence of superior coloboma, a congenital ocular anomaly resulting from aberrant morphogenesis of a developmental structure.

Ocular coloboma is a disease characterized by gaps in the lower portion of the eye and can affect the iris, lens, or retina, and cause loss of vision. Coloboma arises from incomplete closure of a transient fissure on the underside of the developing eye. Therefore, our identification of patients with similar tissue defects, but restricted to the superior half of eye, was surprising. Here, we describe an ocular developmental structure, the superior ocular sulcus, as a potential origin for the congenital disorder superior coloboma. Formation and closure of the sulcus are directed by dorsal-ventral eye patterning, and altered patterning interferes with the role of the sulcus as a pathway for blood vessel growth onto the eye.

Physiological and ecological implications of ocean deoxygenation for vision in marine organisms

Climate change has induced ocean deoxygenation and exacerbated eutrophication-driven hypoxia in recent decades, affecting the physiology, behaviour and ecology of marine organisms. The high oxygen demand of visual tissues and the known inhibitory effects of hypoxia on human vision raise the questions if and how ocean deoxygenation alters vision in marine organisms. This is particularly important given the rapid loss of oxygen and strong vertical gradients in oxygen concentration in many areas of the ocean. This review evaluates the potential effects of low oxygen (hypoxia) on visual function in marine animals and their implications for marine biota under current and future ocean deoxygenation based on evidence from terrestrial and a few marine organisms. Evolutionary history shows radiation of eye designs during a period of increasing ocean oxygenation. Physiological effects of hypoxia on photoreceptor function and light sensitivity, in combination with morphological changes that may occur throughout ontogeny, have the potential to alter visual behaviour and, subsequently, the ecology of marine organisms, particularly for fish, cephalopods and arthropods with 'fast' vision. Visual responses to hypoxia, including greater light requirements, offer an alternative hypothesis for observed habitat compression and shoaling vertical distributions in visual marine species subject to ocean deoxygenation, which merits further investigation.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Endothelial Notch signalling limits angiogenesis via control of artery formation

Angiogenic sprouting needs to be tightly controlled. It has been suggested that the Notch ligand dll4 expressed in leading tip cells restricts angiogenesis by activating Notch signalling in trailing stalk cells. Here, we show using live imaging in zebrafish that activation of Notch signalling is rather required in tip cells. Notch activation initially triggers expression of the chemokine receptor cxcr4a. This allows for proper tip cell migration and connection to the pre-existing arterial circulation, ultimately establishing functional arterial-venous blood flow patterns. Subsequently, Notch signalling reduces cxcr4a expression, thereby preventing excessive blood vessel growth. Finally, we find that Notch signalling is dispensable for limiting blood vessel growth during venous plexus formation that does not generate arteries. Together, these findings link the role of Notch signalling in limiting angiogenesis to its role during artery formation and provide a framework for our understanding of the mechanisms underlying blood vessel network expansion and maturation.

Bipotent progenitors as embryonic origin of retinal stem cells

In lower vertebrates, retinal stem cells (RSCs) capable of producing all retinal cell types are a resource for retinal tissue growth throughout life. However, the embryonic origin of RSCs remains largely elusive. Using a Zebrabow-based clonal analysis, we characterized the RSC niche in the ciliary marginal zone of zebrafish retina and illustrate that blood vessels associated with RSCs are required for the maintenance of actively proliferating RSCs. Full lineage analysis of RSC progenitors reveals lineage patterns of RSC production. Moreover, in vivo lineage analysis demonstrates that these RSC progenitors are the direct descendants of a set of bipotent progenitors in the medial epithelial layer of developing optic vesicles, suggesting the involvement of the mixed-lineage states in the RSC lineage specification.

Live imaging of primary ocular vasculature formation in zebrafish

Ocular vasculature consists of the central retinal and ciliary vascular systems, which are essential to maintaining visual function. Many researchers have attempted to determine their origins and development; however, the detailed, stepwise process of ocular vasculature formation has not been established. In zebrafish, two angioblast clusters, the rostral and midbrain organizing centers, form almost all of the cranial vasculature, including the ocular vasculature, and these are from where the cerebral arterial and venous angioblast clusters, respectively, differentiate. In this study, we first determined the anatomical architecture of the primary ocular vasculature and then followed its path from the two cerebral angioblast clusters using a time-lapse analysis of living Tg(flk1:EGFP)k7 zebrafish embryos, in which the endothelial cells specifically expressed enhanced green fluorescent protein. We succeeded in capturing images of the primary ocular vasculature formation and were able to determine the origin of each ocular vessel. In zebrafish, the hyaloid and ciliary arterial systems first organized independently, and then anastomosed via the inner optic circle on the surface of the lens by the lateral transfer of the optic vein. Finally, the choroidal vascular plexus formed around the eyeball to complete the primary ocular vasculature formation. To our knowledge, this study is the first to report successful capture of circular integration of the optic artery and vein, lateral transfer of the optic vein to integrate the hyaloidal and superficial ocular vasculatures, and formation of the choroidal vascular plexus. Furthermore, this new morphological information enables us to assess the entire process of the primary ocular vasculature formation, which will be useful for its precise understanding.

Abnormal retinal development in Cloche mutant zebrafish

BACKGROUND

Functions for the early embryonic vasculature in regulating development of central nervous system tissues, such as the retina, have been suggested by in vitro studies and by in vivo manipulations that caused additional ocular vessels to develop. Here, we use an avascular zebrafish embryo, cloche-/- (clo-/-), to begin to identify necessary developmental functions of the ocular vasculature in regulating development and patterning of the neural retina, in vivo. These studies are possible in zebrafish embryos, which do not yet rely upon the vasculature for tissue oxygenation.

RESULTS

clo-/- embryos lacked early ocular vasculature and were microphthalmic, with reduced retinal cell proliferation and cell survival. Retinas of clo mutants were disorganized, with irregular synaptic layers, mispatterned expression domains of retinal transcription factors, morphologically abnormal Müller glia, reduced differentiation of specific retinal cell types, and sporadically distributed cone photoreceptors. Blockade of p53-mediated cell death did not completely rescue this phenotype and revealed ectopic cones in the inner nuclear layer. clo-/- embryos did not upregulate a molecular marker for hypoxia.

CONCLUSIONS

The disorganized retinal phenotype of clo-/- embryos is consistent with a neural and glial developmental patterning role for the early ocular vasculature that is independent of its eventual function in gas exchange.