Macrophages enhance Vegfa-driven angiogenesis in an embryonic zebrafish tumour xenograft model

Canthin-6-One Inhibits Developmental and Tumour-Associated Angiogenesis in Zebrafish

Tumour-associated angiogenesis play key roles in tumour growth and cancer metastasis. Consequently, several anti-angiogenic drugs such as sunitinib and axitinib have been approved for use as anti-cancer therapies. However, the majority of these drugs target the vascular endothelial growth factor A (VEGFA)/VEGF receptor 2 (VEGFR2) pathway and have shown mixed outcome, largely due to development of resistances and increased tumour aggressiveness. In this study, we used the zebrafish model to screen for novel anti-angiogenic molecules from a library of compounds derived from natural products. From this, we identified canthin-6-one, an indole alkaloid, which inhibited zebrafish intersegmental vessel (ISV) and sub-intestinal vessel development. Further characterisation revealed that treatment of canthin-6-one reduced ISV endothelial cell number and inhibited proliferation of human umbilical vein endothelial cells (HUVECs), suggesting that canthin-6-one inhibits endothelial cell proliferation. Of note, canthin-6-one did not inhibit VEGFA-induced phosphorylation of VEGFR2 in HUVECs and downstream phosphorylation of extracellular signal-regulated kinase (Erk) in leading ISV endothelial cells in zebrafish, suggesting that canthin-6-one inhibits angiogenesis independent of the VEGFA/VEGFR2 pathway. Importantly, we found that canthin-6-one impairs tumour-associated angiogenesis in a zebrafish B16F10 melanoma cell xenograft model and synergises with VEGFR inhibitor sunitinib malate to inhibit developmental angiogenesis. In summary, we showed that canthin-6-one exhibits anti-angiogenic properties in both developmental and pathological contexts in zebrafish, independent of the VEGFA/VEGFR2 pathway and demonstrate that canthin-6-one may hold value for further development as a novel anti-angiogenic drug.

Modelling 3D Tumour Microenvironment In Vivo: A Tool to Predict Cancer Fate

Recently, many studies demonstrated the fundamental role of the tumour microenvironment (TME) in cancer progression. Here, we describe a method to visualize in 3D the behaviour of tumours in zebrafish embryos. We highlight two major actors of the TME, macrophages and vessels. This valuable tool is transposable to Patient-Derived Xenograft imaging in order to predict the fate of malignant tumours according to the dynamics of their TME.

Polarizing Macrophage Functional Phenotype to Foster Cardiac Regeneration

There is an increasing interest in understanding the connection between the immune and cardiovascular systems, which are highly integrated and communicate through finely regulated cross-talking mechanisms. Recent evidence has demonstrated that the immune system does indeed have a key role in the response to cardiac injury and in cardiac regeneration. Among the immune cells, macrophages appear to have a prominent role in this context, with different subtypes described so far that each have a specific influence on cardiac remodeling and repair. Similarly, there are significant differences in how the innate and adaptive immune systems affect the response to cardiac damage. Understanding all these mechanisms may have relevant clinical implications. Several studies have already demonstrated that stem cell-based therapies support myocardial repair. However, the exact role that cardiac macrophages and their modulation may have in this setting is still unclear. The current need to decipher the dual role of immunity in boosting both heart injury and repair is due, at least for a significant part, to unresolved questions related to the complexity of cardiac macrophage phenotypes. The aim of this review is to provide an overview on the role of the immune system, and of macrophages in particular, in the response to cardiac injury and to outline, through the modulation of the immune response, potential novel therapeutic strategies for cardiac regeneration.

Recent Advancement in Breast Cancer Research: Insights from Model Organisms-Mouse Models to Zebrafish

Animal models have been utilized for decades to investigate the causes of human diseases and provide platforms for testing novel therapies. Indeed, breakthrough advances in genetically engineered mouse (GEM) models and xenograft transplantation technologies have dramatically benefited in elucidating the mechanisms underlying the pathogenesis of multiple diseases, including cancer. The currently available GEM models have been employed to assess specific genetic changes that underlay many features of carcinogenesis, including variations in tumor cell proliferation, apoptosis, invasion, metastasis, angiogenesis, and drug resistance. In addition, mice models render it easier to locate tumor biomarkers for the recognition, prognosis, and surveillance of cancer progression and recurrence. Furthermore, the patient-derived xenograft (PDX) model, which involves the direct surgical transfer of fresh human tumor samples to immunodeficient mice, has contributed significantly to advancing the field of drug discovery and therapeutics. Here, we provide a synopsis of mouse and zebrafish models used in cancer research as well as an interdisciplinary 'Team Medicine' approach that has not only accelerated our understanding of varied aspects of carcinogenesis but has also been instrumental in developing novel therapeutic strategies.

How to Generate a Vascular-Labelled Transgenic Zebrafish Model to Study Tumor Angiogenesis and Extravasation

The use of transgenic animals carrying exogenous DNA integrated in their genome is a routine in modern-day laboratories. Nowadays, the zebrafish system represents the most useful tool for transgenesis studies mainly due to easy accessibility and manipulation of the eggs, which are produced in high numbers and over a relatively short generation time. The zebrafish transgenic technology is very straightforward when coupled with angiogenesis studies allowing easy in vivo observation of the vertebrate embryonic vasculature. Here, we describe the most common technique to generate vascular-labelled transgenic zebrafish embryos and their applications to study tumor angiogenesis and visualize tumor extravasation.

Zebrafish—An Optimal Model in Experimental Oncology

A thorough understanding of cancer pathogenesis is a necessary step in the development of more effective and safer therapy. However, due to the complexity of the process and intricate interactions, studying tumor development is an extremely difficult and challenging task. In bringing this issue closer, different scientific models with various advancement levels are helpful. Cell cultures is a system that is too simple and does not allow for multidirectional research. On the other hand, rodent models, although commonly used, are burdened with several limitations. For this reason, new model organisms that will allow for the studying of carcinogenesis stages and factors reliably involved in them are urgently sought after. Danio rerio, an inconspicuous fish endowed with unique features, is gaining in importance in the world of scientific research. Including it in oncological research brings solutions to many challenges afflicting modern medicine. This article aims to illustrate the usefulness of Danio rerio as a model organism which turns out to be a powerful and unique tool for studying the stages of carcinogenesis and solving the hitherto incomprehensible processes that lead to the development of the disease.

Zebrafish Models to Study the Crosstalk between Inflammation and NADPH Oxidase-Derived Oxidative Stress in Melanoma

Melanoma is the deadliest form of skin cancer, and its incidence continues to increase. In the early stages of melanoma, when the malignant cells have not spread to lymph nodes, they can be removed by simple surgery and there is usually low recurrence. Melanoma has a high mortality rate due to its ability to metastasize; once melanoma has spread, it becomes a major health complication. For these reasons, it is important to study how healthy melanocytes transform into melanoma cells, how they interact with the immune system, which mechanisms they use to escape immunosurveillance, and, finally, how they spread and colonize other tissues, metastasizing. Inflammation and oxidative stress play important roles in the development of several types of cancer, including melanoma, but it is not yet clear under which conditions they are beneficial or detrimental. Models capable of studying the relevance of inflammation and oxidative stress in the early steps of melanocyte transformation are urgently needed, as they are expected to help recognize premetastatic lesions in patients by improving both early detection and the development of new therapies.

Targeting tumor-associated macrophages for cancer treatment

Tumor-associated macrophages (TAMs) are abundant, nearly accounting for 30–50% of stromal cells in the tumor microenvironment. TAMs exhibit an immunosuppressive M2-like phenotype in advanced cancer, which plays a crucial role in tumor growth, invasion and migration, angiogenesis and immunosuppression. Consequently, the TAM-targeting therapies are particularly of significance in anti-cancer strategies. The application of TAMs as anti-cancer targets is expected to break through traditional tumor-associated therapies and achieves favorable clinical effect. However, the heterogeneity of TAMs makes the strategy of targeting TAMs variable and uncertain. Discovering the subset specificity of TAMs might be a future option for targeting TAMs therapy. Herein, the review focuses on highlighting the different modalities to modulate TAM’s functions, including promoting the phagocytosis of TAMs, TAMs depletion, blocking TAMs recruitment, TAMs reprogramming and suppressing immunosuppressive tumor microenvironment. We also discuss about several ways to improve the efficacy of TAM-targeting therapy from the perspective of combination therapy and specificity of TAMs subgroups.

Targeting tumor-associated macrophages for cancer treatment

Tumor-associated macrophages (TAMs) are abundant, nearly accounting for 30-50% of stromal cells in the tumor microenvironment. TAMs exhibit an immunosuppressive M2-like phenotype in advanced cancer, which plays a crucial role in tumor growth, invasion and migration, angiogenesis and immunosuppression. Consequently, the TAM-targeting therapies are particularly of significance in anti-cancer strategies. The application of TAMs as anti-cancer targets is expected to break through traditional tumor-associated therapies and achieves favorable clinical effect. However, the heterogeneity of TAMs makes the strategy of targeting TAMs variable and uncertain. Discovering the subset specificity of TAMs might be a future option for targeting TAMs therapy. Herein, the review focuses on highlighting the different modalities to modulate TAM's functions, including promoting the phagocytosis of TAMs, TAMs depletion, blocking TAMs recruitment, TAMs reprogramming and suppressing immunosuppressive tumor microenvironment. We also discuss about several ways to improve the efficacy of TAM-targeting therapy from the perspective of combination therapy and specificity of TAMs subgroups.

Beyond Genetics: Metastasis as an Adaptive Response in Breast Cancer

Metastatic disease represents the primary cause of breast cancer (BC) mortality, yet it is still one of the most enigmatic processes in the biology of this tumor. Metastatic progression includes distinct phases: invasion, intravasation, hematogenous dissemination, extravasation and seeding at distant sites, micro-metastasis formation and metastatic outgrowth. Whole-genome sequencing analyses of primary BC and metastases revealed that BC metastatization is a non-genetically selected trait, rather the result of transcriptional and metabolic adaptation to the unfavorable microenvironmental conditions which cancer cells are exposed to (e.g., hypoxia, low nutrients, endoplasmic reticulum stress and chemotherapy administration). In this regard, the latest multi-omics analyses unveiled intra-tumor phenotypic heterogeneity, which determines the polyclonal nature of breast tumors and constitutes a challenge for clinicians, correlating with patient poor prognosis. The present work reviews BC classification and epidemiology, focusing on the impact of metastatic disease on patient prognosis and survival, while describing general principles and current in vitro/in vivo models of the BC metastatic cascade. The authors address here both genetic and phenotypic intrinsic heterogeneity of breast tumors, reporting the latest studies that support the role of the latter in metastatic spreading. Finally, the review illustrates the mechanisms underlying adaptive stress responses during BC metastatic progression.

What Zebrafish and Nanotechnology Can Offer for Cancer Treatments in the Age of Personalized Medicine

Cancer causes millions of deaths each year and thus urgently requires the development of new therapeutic strategies. Nanotechnology-based anticancer therapies are a promising approach, with several formulations already approved and in clinical use. The evaluation of these therapies requires efficient in vivo models to study their behavior and interaction with cancer cells, and to optimize their properties to ensure maximum efficacy and safety. In this way, zebrafish is an important candidate due to its high homology with the human genoma, its large offspring, and the ease in developing specific cancer models. The role of zebrafish as a model for anticancer therapy studies has been highly evidenced, allowing researchers not only to perform drug screenings but also to evaluate novel therapies such as immunotherapies and nanotherapies. Beyond that, zebrafish can be used as an “avatar” model for performing patient-derived xenografts for personalized medicine. These characteristics place zebrafish in an attractive position as a role model for evaluating novel therapies for cancer treatment, such as nanomedicine.

Mitochondrial-targeting antioxidant MitoQ modulates angiogenesis and promotes functional recovery after spinal cord injury

BACKGROUND

In traumatic spinal cord injury (SCI), secondary injuries, including vascular injury, cellular death, mitochondrial dysfunction, and vascular injury, have been considered as important causes of impaired functional recovery after SCI. Postinjury angiogenesis has been considered to be a potential strategy for SCI treatment. New-born vessels may play a key role in nerve regeneration, which indicates the importance of angiogenesis in nerve regeneration. Recent studies have revealed the crosstalk between reactive oxygen species (ROS) and angiogenesis. As the main source of cellular ROS, mitochondria have been proven to be essential to the angiogenesis process.

METHODS

SCI was established in a T10 clip-compression animal model. Then, the animals received an intraperitoneal injection of MitoQ (1 mg/ml) on Days 0, 1, and 2 after surgery. The Basso Mouse Scale (BMS) score and footprint analysis (CatWalk analysis) were performed to evaluate functional recovery after SCI. Immunofluorescence assay (LEL-FITC/CD31/Iba-1/Neurofilament) was performed to evaluate angiogenesis, microglia activation and neural regeneration. RT-qPCR (VEGFR-1, VEGFR-2 and VEGFA) was performed to evaluate angiogenesis-related factor in injured spinal cord. ATP production assay and western-blotting assay (Mfn-1 and Drp-1) were performed to evaluate mitochondrial function in the injured spinal cord. BV2 cells were used as in vitro cell models. After receiving TBHP or TBHP-MitoQ treatment, ELISA and immunofluorescence assays were used to evaluate the level of VEGFA secretion from BV2 cells. A coculture system of HUVECs and BV2 cells was established. Tube formation assays and immunofluorescence assays (CD31) were performed on HUVECs in a coculture system to evaluate angiogenesis promotion. ATP production assays were performed to evaluate mitochondrial function in BV2 cells. MitoSOX Red and DCFH-DA staining were performed to evaluate mitochondrial and cellular ROS.

RESULTS

In vitro MitoQ promoted the secretion of VEGFA from BV2 cells, which was verified through ELISA and immunofluorescence assays. The angiogenic promotion of MitoQ-treated BV2 cells was evaluated by tube formation and immunofluorescence assays (CD31) in a coculture system of BV2 cells and HUVECs. MitoQ inhibited cellular and mitochondrial-derived ROS in TBHP-treated BV2 cells. ATP production was increased in MitoQ-treated BV2 cells. To verify MitoQ's effect in vivo, a T10 clip-compression animal model was established successfully. MitoQ significantly promoted functional recovery, as shown by the BMS assay and gait analyser. The promotion of neural regeneration was identified through immunofluorescence assay of neurofilament. Immunofluorescence assay (LEL-FITC/CD31/Iba-1) and RT-qPCR (VEGFR-1, VEGFR-2 and VEGFA) indicated that MitoQ could promote angiogenesis and inhibit macrophage/microglia activation in lesion-site after SCI. Enhanced ATP production and increased Mfn-1 with decreased Drp-1 protein expression showed MitoQ could promote mitochondrial function in SCI.

CONCLUSION

The mitochondrial-specific antioxidant MitoQ promotes functional recovery and tissue preservation through the enhancement of angiogenesis with the modification of mitochondrial function after SCI.

M-CSFR/CSF1R signaling regulates myeloid fates in zebrafish via distinct action of its receptors and ligands

csf1ra and csf1rb are indispensable for macrophage differentiation and, together with csf1a, regulate embryonic macrophage fates.

Il34 regulates zebrafish granulocyte development through csf1rb.

Macrophage colony-stimulating factor receptor (M-CSFR/CSF1R) signaling is crucial for the differentiation, proliferation, and survival of myeloid cells. The CSF1R pathway is a promising therapeutic target in many human diseases, including neurological disorders and cancer. Zebrafish are commonly used for human disease modeling and preclinical therapeutic screening. Therefore, it is necessary to understand the proper function of cytokine signaling in zebrafish to reliably model human-related diseases. Here, we investigate the roles of zebrafish Csf1rs and their ligands (Csf1a, Csf1b, and Il34) in embryonic and adult myelopoiesis. The proliferative effect of exogenous Csf1a on embryonic macrophages is connected to both receptors, Csf1ra and Csf1rb, however there is no evident effect of Csf1b in zebrafish embryonic myelopoiesis. Furthermore, we uncover an unknown role of Csf1rb in zebrafish granulopoiesis. Deregulation of Csf1rb signaling leads to failure in myeloid differentiation, resulting in neutropenia throughout the whole lifespan. Surprisingly, Il34 signaling through Csf1rb seems to be of high importance as both csf1rb^Δ4bp-deficient and il34^Δ5bp-deficient zebrafish larvae lack granulocytes. Our single-cell RNA sequencing analysis of adult whole kidney marrow (WKM) hematopoietic cells suggests that csf1rb is expressed mainly by blood and myeloid progenitors, and the expression of csf1ra and csf1rb is nonoverlapping. We point out differentially expressed genes important in hematopoietic cell differentiation and immune response in selected WKM populations. Our findings could improve the understanding of myeloid cell function and lead to the further study of CSF1R pathway deregulation in disease, mostly in cancerogenesis.

Shifting the focus of zebrafish toward a model of the tumor microenvironment

Cancer cells exist in a complex ecosystem with numerous other cell types in the tumor microenvironment (TME). The composition of this tumor/TME ecosystem will vary at each anatomic site and affects phenotypes such as initiation, metastasis, and drug resistance. A mechanistic understanding of the large number of cell-cell interactions between tumor and TME requires models that allow us to both characterize as well as genetically perturb this complexity. Zebrafish are a model system optimized for this problem, because of the large number of existing cell-type-specific drivers that can label nearly any cell in the TME. These include stromal cells, immune cells, and tissue resident normal cells. These cell-type-specific promoters/enhancers can be used to drive fluorophores to facilitate imaging and also CRISPR cassettes to facilitate perturbations. A major advantage of the zebrafish is the ease by which large numbers of TME cell types can be studied at once, within the same animal. While these features make the zebrafish well suited to investigate the TME, the model has important limitations, which we also discuss. In this review, we describe the existing toolset for studying the TME using zebrafish models of cancer and highlight unique biological insights that can be gained by leveraging this powerful resource.

High-Resolution, 3D Imaging of the Zebrafish Gill-Associated Lymphoid Tissue (GIALT) Reveals a Novel Lymphoid Structure, the Amphibranchial Lymphoid Tissue

The zebrafish is extensively used as an animal model for human and fish diseases. However, our understanding of the structural organization of its immune system remains incomplete, especially the mucosa-associated lymphoid tissues (MALTs). Teleost MALTs are commonly perceived as diffuse and scattered populations of immune cells throughout the mucosa. Yet, structured MALTs have been recently discovered in Atlantic salmon (Salmo salar L.), including the interbranchial lymphoid tissue (ILT) in the gills. The existence of the ILT was only recently identified in zebrafish and other fish species, highlighting the need for in-depth characterizations of the gill-associated lymphoid tissue (GIALT) in teleosts. Here, using 3-D high-resolution microscopy, we analyze the GIALT of adult zebrafish with an immuno-histology approach that reveals the organization of lymphoid tissues via the labeling of T/NK cells with an antibody directed to a highly conserved epitope on the kinase ZAP70. We show that the GIALT in zebrafish is distributed over at least five distinct sub-regions, an organization found in all pairs of gill arches. The GIALT is diffuse in the pharyngeal part of the gill arch, the interbranchial septum and the filaments/lamellae, and structured in two sub-regions: the ILT, and a newly discovered lymphoid structure located along each side of the gill arch, which we named the Amphibranchial Lymphoid Tissue (ALT). Based on RAG2 expression, neither the ILT nor the ALT constitute additional thymi. The ALT shares several features with the ILT such as presence of abundant lymphoid cells and myeloid cells embedded in a network of reticulated epithelial cells. Further, the ILT and the ALT are also a site for T/NK cell proliferation. Both ILT and ALT show structural changes after infection with Spring Viraemia of Carp Virus (SVCV). Together, these data suggest that ALT and ILT play an active role in immune responses. Comparative studies show that whereas the ILT seems absent in most neoteleosts ("Percomorphs"), the ALT is widely present in cyprinids, salmonids and neoteleosts, suggesting that it constitutes a conserved tissue involved in the protection of teleosts via the gills.

Bufalin suppresses tumour microenvironment-mediated angiogenesis by inhibiting the STAT3 signalling pathway

BACKGROUND

Antiangiogenic therapy has increasingly become an important strategy for the treatment of colorectal cancer. Recent studies have shown that the tumour microenvironment (TME) promotes tumour angiogenesis. Bufalin is an active antitumour compound whose efficacy has been indicated by previous studies. However, there are very few studies on the antiangiogenic effects of bufalin.

METHODS

Herein, human umbilical vein endothelial cell (HUVEC) tube formation, migration and adhesion tests were used to assess angiogenesis in vitro. Western blotting and quantitative PCR were used to detect relevant protein levels and mRNA expression levels. A subcutaneous xenograft tumour model and a hepatic metastasis model were established in mice to investigate the influence of bufalin on angiogenesis mediated by the TME in vivo.

RESULTS

We found that angiogenesis mediated by cells in the TME was significantly inhibited in the presence of bufalin. The results demonstrated that the proangiogenic genes in HUVECs, such as VEGF, PDGFA, E-selectin and P-selectin, were downregulated by bufalin and that this downregulation was mediated by inhibition of the STAT3 pathway. Overexpression of STAT3 reversed the inhibitory effects of bufalin on angiogenesis. Furthermore, there was little reduction in angiogenesis when bufalin directly acted on the cells in the tumour microenvironment.

CONCLUSION

Our findings demonstrate that bufalin suppresses tumour microenvironment-mediated angiogenesis by inhibiting the STAT3 signalling pathway in vascular endothelial cells, revealing that bufalin may be used as a new antiangiogenic adjuvant therapy medicine to treat colorectal cancer.

Zebrafish Models for the Safety and Therapeutic Testing of Nanoparticles with a Focus on Macrophages

New nanoparticles and biomaterials are increasingly being used in biomedical research for drug delivery, diagnostic applications, or vaccines, and they are also present in numerous commercial products, in the environment and workplaces. Thus, the evaluation of the safety and possible therapeutic application of these nanomaterials has become of foremost importance for the proper progress of nanotechnology. Due to economical and ethical issues, in vitro and in vivo methods are encouraged for the testing of new compounds and/or nanoparticles, however in vivo models are still needed. In this scenario, zebrafish (Danio rerio) has demonstrated potential for toxicological and pharmacological screenings. Zebrafish presents an innate immune system, from early developmental stages, with conserved macrophage phenotypes and functions with respect to humans. This fact, combined with the transparency of zebrafish, the availability of models with fluorescently labelled macrophages, as well as a broad variety of disease models offers great possibilities for the testing of new nanoparticles. Thus, with a particular focus on macrophage-nanoparticle interaction in vivo, here, we review the studies using zebrafish for toxicological and biodistribution testing of nanoparticles, and also the possibilities for their preclinical evaluation in various diseases, including cancer and autoimmune, neuroinflammatory, and infectious diseases.

The Use of Zebrafish Xenotransplant Assays to Analyze the Role of lncRNAs in Breast Cancer

Breast cancer represents a great challenge since it is the first cause of death by cancer in women worldwide. LncRNAs are a newly described class of non-coding RNAs that participate in cancer progression. Their use as cancer markers and possible therapeutic targets has recently gained strength. Animal xenotransplants allows for in vivo monitoring of disease development, molecular elucidation of pathogenesis and the design of new therapeutic strategies. Nevertheless, the cost and complexities of mice husbandry makes medium to high throughput assays difficult. Zebrafishes (Danio rerio) represent a novel model for these assays, given the ease with which xenotransplantation trials can be performed and the economic and experimental advantages it offers. In this review we propose the use of xenotransplants in zebrafish to study the role of breast cancer lncRNAs using low to medium high throughput assays.

The Zebrafish Cardiac Endothelial Cell-Roles in Development and Regeneration

In zebrafish, the spatiotemporal development of the vascular system is well described due to its stereotypical nature. However, the cellular and molecular mechanisms orchestrating post-embryonic vascular development, the maintenance of vascular homeostasis, or how coronary vessels integrate into the growing heart are less well studied. In the context of cardiac regeneration, the central cellular mechanism by which the heart regenerates a fully functional myocardium relies on the proliferation of pre-existing cardiomyocytes; the epicardium and the endocardium are also known to play key roles in the regenerative process. Remarkably, revascularisation of the injured tissue occurs within a few hours after cardiac damage, thus generating a vascular network acting as a scaffold for the regenerating myocardium. The activation of the endocardium leads to the secretion of cytokines, further supporting the proliferation of the cardiomyocytes. Although epicardium, endocardium, and myocardium interact with each other to orchestrate heart development and regeneration, in this review, we focus on recent advances in the understanding of the development of the endocardium and the coronary vasculature in zebrafish as well as their pivotal roles in the heart regeneration process.

The Zebrafish Xenograft Models for Investigating Cancer and Cancer Therapeutics

Simple Summary

The identification and development of new anti-cancer drugs requires extensive testing in animal models to establish safety and efficacy of drug candidates. The transplantation of human tumor tissue into mouse (tumor xenografts) is commonly used to study cancer progression and to test potential drugs for their anti-cancer activity. Mouse models do not afford the ability to test a large number of drug candidates quickly as it takes several weeks to conduct these experiments. In contrast, tumor xenograft studies in zebrafish provide an efficient platform for rapid testing of safety and efficacy in less than two weeks.

Abstract

In order to develop new cancer therapeutics, rapid, reliable, and relevant biological models are required to screen and validate drug candidates for both efficacy and safety. In recent years, the zebrafish (Danio rerio) has emerged as an excellent model organism suited for these goals. Larval fish or immunocompromised adult fish are used to engraft human cancer cells and serve as a platform for screening potential drug candidates. With zebrafish sharing ~80% of disease-related orthologous genes with humans, they provide a low cost, high-throughput alternative to mouse xenografts that is relevant to human biology. In this review, we provide background on the methods and utility of zebrafish xenograft models in cancer research.

Benefits of Zebrafish Xenograft Models in Cancer Research

As a promising in vivo tool for cancer research, zebrafish have been widely applied in various tumor studies. The zebrafish xenograft model is a low-cost, high-throughput tool for cancer research that can be established quickly and requires only a small sample size, which makes it favorite among researchers. Zebrafish patient-derived xenograft (zPDX) models provide promising evidence for short-term clinical treatment. In this review, we discuss the characteristics and advantages of zebrafish, such as their transparent and translucent features, the use of vascular fluorescence imaging, the establishment of metastatic and intracranial orthotopic models, individual pharmacokinetics measurements, and tumor microenvironment. Furthermore, we introduce how these characteristics and advantages are applied other in tumor studies. Finally, we discuss the future direction of the use of zebrafish in tumor studies and provide new ideas for the application of it.

Unveiling Tumor Microenvironment Interactions Using Zebrafish Models

The tumor microenvironment (TME) is a rich and active arena that is strategically evolved overtime by tumors to promote their survival and dissemination. Over the years, attention has been focused to characterize and identify the tumor-supporting roles and subsequent targeting potentials of TME components. Nevertheless, recapitulating the human TME has proved inherently challenging, leaving much to be explored. In this regard, in vivo model systems like zebrafish, with its optical clarity, ease of genetic manipulation, and high engraftment, have proven to be indispensable for TME modeling and investigation. In this review, we discuss the recent ways by which zebrafish models have lent their utility to provide new insights into the various cellular and molecular mechanisms driving TME dynamics and tumor support. Specifically, we report on innate immune cell interactions, cytokine signaling, metastatic plasticity, and other processes within the metastatic cascade. In addition, we reflect on the arrival of adult zebrafish models and the potential of patient-derived xenografts.

Studying the Tumor Microenvironment in Zebrafish

The tumor microenvironment significantly contributes to tumor initiation, progression, neo-angiogenesis, and metastasis, and a better understanding of the role of the different cellular players would facilitate the development of novel therapeutic strategies for cancer treatment. Towards this goal, intravital imaging is a powerful method to unravel interaction partners of tumor cells. Among vertebrate model organisms, zebrafish is uniquely suited for in vivo imaging studies. In recent years zebrafish has also become a valuable model in cancer research. In this chapter, we will summarize, how zebrafish has been used to characterize cells of the tumor microenvironment. We will cover both genetically engineered cancer models and xenograft models in zebrafish. The majority of work has been done on the role of innate immune cells and their role during tumor initiation and metastasis, but we will also cover studies focusing on adipocytes, fibroblasts, and endothelial cells. Taken together, we will highlight the versatile use of the zebrafish model for in vivo tumor microenvironment studies.

ZenoFishDb v1.1: A Database for Xenotransplantation Studies in Zebrafish

Rapidly accumulating literature has proven feasibility of the zebrafish xenograft models in cancer research. Nevertheless, online databases for searching the current zebrafish xenograft literature are in great demand. Herein, we have developed a manually curated database, called ZenoFishDb v1.1 (https://konulab.shinyapps.io/zenofishdb), based on R Shiny platform aiming to provide searchable information on ever increasing collection of zebrafish studies for cancer cell line transplantation and patient-derived xenografts (PDXs). ZenoFishDb v1.1 user interface contains four modules: DataTable, Visualization, PDX Details, and PDX Charts. The DataTable and Visualization pages represent xenograft study details, including injected cell lines, PDX injections, molecular modifications of cell lines, zebrafish strains, as well as technical aspects of the xenotransplantation procedures in table, bar, and/or pie chart formats. The PDX Details module provides comprehensive information on the patient details in table format and can be searched and visualized. Overall, ZenoFishDb v1.1 enables researchers to effectively search, list, and visualize different technical and biological attributes of zebrafish xenotransplantation studies particularly focusing on the new trends that make use of reporters, RNA interference, overexpression, or mutant gene constructs of transplanted cancer cells, stem cells, and PDXs, as well as distinguished host modifications.

Modeling Cancer Using Zebrafish Xenografts: Drawbacks for Mimicking the Human Microenvironment

The first steps towards establishing xenografts in zebrafish embryos were performed by Lee et al., 2005 and Haldi et al., 2006, paving the way for studying human cancers using this animal species. Since then, the xenograft technique has been improved in different ways, ranging from optimizing the best temperature for xenografted embryo incubation, testing different sites for injection of human tumor cells, and even developing tools to study how the host interacts with the injected cells. Nonetheless, a standard protocol for performing xenografts has not been adopted across laboratories, and further research on the temperature, microenvironment of the tumor or the cell-host interactions inside of the embryo during xenografting is still needed. As a consequence, current non-uniform conditions could be affecting experimental results in terms of cell proliferation, invasion, or metastasis; or even overestimating the effects of some chemotherapeutic drugs on xenografted cells. In this review, we highlight and raise awareness regarding the different aspects of xenografting that need to be improved in order to mimic, in a more efficient way, the human tumor microenvironment, resulting in more robust and accurate in vivo results.

Zebrafish In Vivo Models of Cancer and Metastasis

Metastasis, the dispersal of cancer cells from a primary tumor to secondary sites within the body, is the leading cause of cancer-related death. Animal models have been an indispensable tool to investigate the complex interactions between the cancer cells and the tumor microenvironment during the metastatic cascade. The zebrafish (Danio rerio) has emerged as a powerful vertebrate model for studying metastatic events in vivo. The zebrafish has many attributes including ex-utero development, which facilitates embryonic manipulation, as well as optically transparent tissues, which enables in vivo imaging of fluorescently labeled cells in real time. Here, we summarize the techniques which have been used to study cancer biology and metastasis in the zebrafish model organism, including genetic manipulation and transgenesis, cell transplantation, live imaging, and high-throughput compound screening. Finally, we discuss studies using the zebrafish, which have complemented and benefited metastasis research.

Modeling oncolytic virus dynamics in the tumor microenvironment using zebrafish

We have adapted a zebrafish (Danio rerio) tumor xenograft model for use in the study of oncolytic virotherapy. Following implantation of mammalian cancer cells into the perivitelline space of developing zebrafish embryos, both local and intravenous oncolytic virus treatments produce a tumor-specific infection with measurable antitumor effects. Tumor cells are injected at 48 h post fertilization, with oncolytic virus treatment then being administered 24 h later to allow for an initial period of tumor development and angiogenesis. Confocal fluorescent imaging is used to quantify dynamics within the tumor environment. The natural translucency of zebrafish at the embryo stage, coupled with the availability of strains with fluorescent immune and endothelial cell reporter lines, gives the model broad potential to allow for real time, in vivo investigation of important events within tumors throughout the course of virotherapy. Zebrafish xenografts offer a system with biologic fidelity to processes in human cancer development that influence oncolytic virus efficacy, and to our knowledge this is the first demonstration of the model's use in the context of virotherapy. Compared with other models, our protocol offers a powerful, inexpensive approach to evaluating novel oncolytic viruses and oncolytic virus-based combination therapies, with potential application to investigating the impacts of virotherapy on immune response, tumor vasculature, and metastatic disease.

Inflammatory Responses during Tumour Initiation: From Zebrafish Transgenic Models of Cancer to Evidence from Mouse and Man

The zebrafish is now an important model organism for cancer biology studies and provides unique and complementary opportunities in comparison to the mammalian equivalent. The translucency of zebrafish has allowed in vivo live imaging studies of tumour initiation and progression at the cellular level, providing novel insights into our understanding of cancer. Here we summarise the available transgenic zebrafish tumour models and discuss what we have gleaned from them with respect to cancer inflammation. In particular, we focus on the host inflammatory response towards transformed cells during the pre-neoplastic stage of tumour development. We discuss features of tumour-associated macrophages and neutrophils in mammalian models and present evidence that supports the idea that these inflammatory cells promote early stage tumour development and progression. Direct live imaging of tumour initiation in zebrafish models has shown that the intrinsic inflammation induced by pre-neoplastic cells is tumour promoting. Signals mediating leukocyte recruitment to pre-neoplastic cells in zebrafish correspond to the signals that mediate leukocyte recruitment in mammalian tumours. The activation state of macrophages and neutrophils recruited to pre-neoplastic cells in zebrafish appears to be heterogenous, as seen in mammalian models, which provides an opportunity to study the plasticity of innate immune cells during tumour initiation. Although several potential mechanisms are described that might mediate the trophic function of innate immune cells during tumour initiation in zebrafish, there are several unknowns that are yet to be resolved. Rapid advancement of genetic tools and imaging technologies for zebrafish will facilitate research into the mechanisms that modulate leukocyte function during tumour initiation and identify targets for cancer prevention.

Zebrafish Xenografts for Drug Discovery and Personalized Medicine

Cancer is the second leading cause of death in the world. Given that cancer is a highly individualized disease, predicting the best chemotherapeutic treatment for individual patients can be difficult. Ex vivo models such as mouse patient-derived xenografts (PDX) and organoids are being developed to predict patient-specific chemosensitivity profiles before treatment in the clinic. Although promising, these models have significant disadvantages including long growth times that introduce genetic and epigenetic changes to the tumor. The zebrafish xenograft assay is ideal for personalized medicine. Imaging of the small, transparent fry is unparalleled among vertebrate organisms. In addition, the speed (5-7 days) and small patient tissue requirements (100-200 cells per animal) are unique features of the zebrafish xenograft model that enable patient-specific chemosensitivity analyses.

Determining macrophage versus neutrophil contributions to innate immunity using larval zebrafish

The specific roles of the two major innate immune cell types – neutrophils and macrophages – in response to infection and sterile inflammation are areas of great interest. The larval zebrafish model of innate immunity, and the imaging capabilities it provides, is a source of new research and discoveries in this field. Multiple methods have been developed in larval zebrafish to specifically deplete functional macrophages or neutrophils. Each of these has pros and cons, as well as caveats, that often make it difficult to directly compare results from different studies. The purpose of this Review is to (1) explore the pros, cons and caveats of each of these immune cell-depleted models; (2) highlight and place into a broader context recent key findings on the specific functions of innate immune cells using these models; and (3) explore future directions in which immune cell depletion methods are being expanded.

Summary: Macrophages and neutrophils are distinct innate immune cells with diverse roles in diverse inflammatory contexts. Recent research in larval zebrafish using cell-specific depletion methods has revealed new insights into these cells' functions.

Spatial Distribution of Macrophages During Callus Formation and Maturation Reveals Close Crosstalk Between Macrophages and Newly Forming Vessels

Macrophages are essential players in the process of fracture healing, acting by remodeling of the extracellular matrix and enabling vascularization. Whilst activated macrophages of M1-like phenotype are present in the initial pro-inflammatory phase of hours to days of fracture healing, an anti-inflammatory M2-like macrophage phenotype is supposed to be crucial for the induction of downstream cascades of healing, especially the initiation of vascularization. In a mouse-osteotomy model, we provide a comprehensive characterization of vessel (CD31⁺, Emcn⁺) and macrophage phenotypes (F4/80, CD206, CD80, Mac-2) during the process of fracture healing. To this end, we phenotype the phases of vascular regeneration-the expansion phase (d1-d7 after injury) and the remodeling phase of the endothelial network, until tissue integrity is restored (d14-d21 after injury). Vessels which appear during the bone formation process resemble type H endothelium (CD31^hiEmcn^hi), and are closely connected to osteoprogenitors (Runx2⁺, Osx⁺) and F4/80⁺ macrophages. M1-like macrophages are present in the initial phase of vascularization until day 3 post osteotomy, but they are rare during later regeneration phases. M2-like macrophages localize mainly extramedullary, and CD206⁺ macrophages are found to express Mac-2⁺ during the expansion phase. VEGFA expression is initiated by CD80⁺ cells, including F4/80⁺ macrophages, until day 3, while subsequently osteoblasts and chondrocytes are main contributors to VEGFA production at the fracture site. Using Longitudinal Intravital Microendoscopy of the Bone (LIMB) we observe changes in the motility and organization of CX3CR1⁺ cells, which infiltrate the injury site after an osteotomy. A transient accumulation, resulting in spatial polarization of both, endothelial cells and macrophages, in regions distal to the fracture site, is evident. Immunofluorescence histology followed by histocytometric analysis reveals that F4/80⁺CX3CR1⁺ myeloid cells precede vascularization.

Photoacoustic imaging as a tool to probe the tumour microenvironment

The tumour microenvironment (TME) is a complex cellular ecosystem subjected to chemical and physical signals that play a role in shaping tumour heterogeneity, invasion and metastasis. Studying the roles of the TME in cancer progression would strongly benefit from non-invasive visualisation of the tumour as a whole organ in vivo, both preclinically in mouse models of the disease, as well as in patient tumours. Although imaging techniques exist that can probe different facets of the TME, they face several limitations, including limited spatial resolution, extended scan times and poor specificity from confounding signals. Photoacoustic imaging (PAI) is an emerging modality, currently in clinical trials, that has the potential to overcome these limitations. Here, we review the biological properties of the TME and potential of existing imaging methods that have been developed to analyse these properties non-invasively. We then introduce PAI and explore the preclinical and clinical evidence that support its use in probing multiple features of the TME simultaneously, including blood vessel architecture, blood oxygenation, acidity, extracellular matrix deposition, lipid concentration and immune cell infiltration. Finally, we highlight the future prospects and outstanding challenges in the application of PAI as a tool in cancer research and as part of a clinical oncologist's arsenal.

Summary: This Review details the potential of photoacoustic imaging to visualise features of the tumour microenvironment such as blood vessels, hypoxia, fibrosis and immune infiltrate to provide unprecedented insight into tumour biology.

Spotlight on zebrafish: the next wave of translational research

Five years after the launch of the Disease Models & Mechanisms (DMM) Special Issue on zebrafish as a disease model, the field has progressed significantly. Zebrafish have been used to precisely model human genetic variants, to unpick the mechanisms of metabolic and other diseases, to study infection, inflammation and cancer, and to develop and test new therapeutic approaches. In this Editorial, we highlight recent research published in DMM that uses zebrafish to develop new experimental tools and to provide new insight into disease mechanism and therapy. The broad spectrum of subjects and approaches covered in these articles underscores the versatility of zebrafish in translational research. Further, it highlights the zebrafish community's ethos of creativity and collaboration in translating basic biological research into clinically relevant advances affecting how we understand and treat human disease.

Summary: Zebrafish are a highly versatile and relevant organism for human disease modelling. This Editorial highlights the recent zebrafish research published in DMM.

From gene to treatment: supporting rare disease translational research through model systems

Individual rare diseases may affect only a few people, making them difficult to recognize, diagnose or treat by studying humans alone. Instead, model organisms help to validate genetic associations, understand functional pathways and develop therapeutic interventions for rare diseases. In this Editorial, we point to the key parameters in face, construct, predictive and target validity for accurate disease modelling, with special emphasis on rare disease models. Raising the experimental standards for disease models will enhance successful clinical translation and benefit rare disease research.

Summary: This Editorial discusses the importance of model systems with accurate face, construct, target and predictive validity for rare disease research.

Embryonic zebrafish xenograft assay of human cancer metastasis

Cancer metastasis is the most important prognostic factor determining patient survival, but currently there are very few drugs or therapies that specifically inhibit the invasion and metastasis of cancer cells. Currently, human cancer metastasis is largely studied using transgenic and immunocompromised mouse xenograft models, which are useful for analysing end-point tumour growth but are unable to accurately and reliably monitor in vivo invasion, intravasation, extravasation or secondary tumour formation of human cancer cells. Furthermore, limits in our ability to accurately monitor early stages of tumour growth and detect micro-metastases likely results in pain and suffering to the mice used for cancer xenograft experiments. Zebrafish ( Danio rerio) embryos, however, offer many advantages as a model system for studying the complex, multi-step processes involved during cancer metastasis. This article describes a detailed method for the analysis of human cancer cell invasion and metastasis in zebrafish embryos before they reach protected status at 5 days post fertilisation. Results demonstrate that human cancer cells actively invade within a zebrafish microenvironment, and form metastatic tumours at secondary tissue sites, suggesting that the mechanisms involved during the different stages of metastasis are conserved between humans and zebrafish, supporting the use of zebrafish embryos as a viable model of human cancer metastasis. We suggest that the embryonic zebrafish xenograft model of human cancer is a tractable laboratory model that can be used to understand cancer biology, and as a direct replacement of mice for the analysis of drugs that target cancer invasion and metastasis.

Embryonic zebrafish xenograft assay of human cancer metastasis

Cancer metastasis is the most important prognostic factor determining patient survival, but currently there are very few drugs or therapies that specifically inhibit the invasion and metastasis of cancer cells. Currently, human cancer metastasis is largely studied using transgenic and immunocompromised mouse xenograft models, which are useful for analysing end-point tumour growth but are unable to accurately and reliably monitor in vivo invasion, intravasation, extravasation or secondary tumour formation of human cancer cells. Furthermore, limits in our ability to accurately monitor early stages of tumour growth and detect micro-metastases likely results in pain and suffering to the mice used for cancer xenograft experiments. Zebrafish ( Danio rerio) embryos, however, offer many advantages as a model system for studying the complex, multi-step processes involved during cancer metastasis. This article describes a detailed method for the analysis of human cancer cell invasion and metastasis in zebrafish embryos before they reach protected status at 5 days post fertilisation. Results demonstrate that human cancer cells actively invade within a zebrafish microenvironment, and form metastatic tumours at secondary tissue sites, suggesting that the mechanisms involved during the different stages of metastasis are conserved between humans and zebrafish, supporting the use of zebrafish embryos as a viable model of human cancer metastasis. We suggest that the embryonic zebrafish xenograft model of human cancer is a tractable laboratory model that can be used to understand cancer biology, and as a direct replacement of mice for the analysis of drugs that target cancer invasion and metastasis.