Live imaging of primary ocular vasculature formation in zebrafish

An Orthotopic Model of Uveal Melanoma in Zebrafish Embryo: A Novel Platform for Drug Evaluation

Uveal melanoma is a highly metastatic tumor, representing the most common primary intraocular malignancy in adults. Tumor cell xenografts in zebrafish embryos may provide the opportunity to study in vivo different aspects of the neoplastic disease and its response to therapy. Here, we established an orthotopic model of uveal melanoma in zebrafish by injecting highly metastatic murine B16-BL6 and B16-LS9 melanoma cells, human A375M melanoma cells, and human 92.1 uveal melanoma cells into the eye of zebrafish embryos in the proximity of the developing choroidal vasculature. Immunohistochemical and immunofluorescence analyses showed that melanoma cells proliferate during the first four days after injection and move towards the eye surface. Moreover, bioluminescence analysis of luciferase-expressing human 92.1 uveal melanoma cells allowed the quantitative assessment of the antitumor activity exerted by the canonical chemotherapeutic drugs paclitaxel, panobinostat, and everolimus after their injection into the grafted eye. Altogether, our data demonstrate that the zebrafish embryo eye is a permissive environment for the growth of invasive cutaneous and uveal melanoma cells. In addition, we have established a new luciferase-based in vivo orthotopic model that allows the quantification of human uveal melanoma cells engrafted in the zebrafish embryo eye, and which may represent a suitable tool for the screening of novel drug candidates for uveal melanoma therapy.

Semaphorin 3fa Controls Ocular Vascularization From the Embryo Through to the Adult

Purpose

Pathological blood vessel growth in the eye is implicated in several diseases that result in vision loss, including age-related macular degeneration and diabetic retinopathy. The limits of current disease therapies have created the need to identify and characterize new antiangiogenic drugs. Here, we identify the secreted chemorepellent semaphorin-3fa (Sema3fa) as an endogenous anti-angiogenic in the eye.

Methods

We generated a CRISPR/Cas9 sema3fa zebrafish mutant line, sema3faca304/304. We assessed the retinal and choroidal vasculature in both larval and adult wild-type and sema3fa mutant zebrafish.

Results

We find sema3fa mRNA is expressed by the ciliary marginal zone, neural retina, and retinal pigment epithelium of zebrafish larvae as choroidal vascularization emerges and the hyaloid/retinal vasculature is remodeled. The hyaloid vessels of sema3fa mutants develop appropriately but fail to remodel during the larval period, with adult mutants exhibiting a denser network of capillaries in the retinal periphery than seen in wild-type. The choroid vasculature is also defective in that it develops precociously, and aberrant, leaky sprouts are present in the normally avascular outer retina of both sema3faca304/304 larvae and adult fish.

Conclusions

Sema3fa is a key endogenous signal for maintaining an avascular retina and preventing pathologic vascularization. Furthermore, we provide a new experimentally accessible model for studying choroid neovascularization (CNV) resulting from primary changes in the retinal environment that lead to downstream vessel infiltration.

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 as an Animal Model for Ocular Toxicity Testing: A Review of Ocular Anatomy and Functional Assays

Xenobiotics make their way into organisms from diverse sources including diet, medication, and pollution. Our understanding of ocular toxicities from xenobiotics in humans, livestock, and wildlife is growing thanks to laboratory animal models. Anatomy and physiology are conserved among vertebrate eyes, and studies with common mammalian preclinical species (rodent, dog) can predict human ocular toxicity. However, since the eye is susceptible to toxicities that may not involve a histological correlate, and these species rely heavily on smell and hearing to navigate their world, discovering visual deficits can be challenging with traditional animal models. Alternative models capable of identifying functional impacts on vision and requiring minimal amounts of chemical are valuable assets to toxicology. Human and zebrafish eyes are anatomically and functionally similar, and it has been reported that several common human ocular toxicants cause comparable toxicity in zebrafish. Vision develops rapidly in zebrafish; the tiny larvae rely on visual cues as early as 4 days, and behavioral responses to those cues can be monitored in high-throughput fashion. This article describes the comparative anatomy of the zebrafish eye, the notable differences from the mammalian eye, and presents practical applications of this underutilized model for assessment of ocular toxicity.

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.

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.

Recent insights into vascular development from studies in zebrafish

PURPOSE OF REVIEW

Zebrafish has provided a powerful platform to study vascular biology over the past 25 years, owing to their distinct advantages for imaging and genetic manipulation. In this review, we summarize recent progress in vascular biology with particular emphasis on vascular development in zebrafish.

RECENT FINDINGS

The advent of transcription activator-like effector nuclease and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 genome-editing technologies has dramatically facilitated reverse genetic approaches in zebrafish, as in other models. Here, we highlight recent studies on vascular development in zebrafish which mainly employed forward or reverse genetics combined with high-resolution imaging. These studies have advanced our understanding of diverse areas in vascular biology, including transcriptional regulation of endothelial cell differentiation, endothelial cell signaling during angiogenesis and lymphangiogenesis, vascular bed-specific developmental mechanisms, and perivascular cell recruitment.

SUMMARY

The unique attributes of the zebrafish model have allowed critical cellular and molecular insights into fundamental mechanisms of vascular development. Knowledge acquired through recent zebrafish work further advances our understanding of basic mechanisms underlying vascular morphogenesis, maintenance, and homeostasis. Ultimately, insights provided by the zebrafish model will help to understand the genetic, cellular, and molecular underpinnings of human vascular malformations and diseases.