Zebrafish models for ectopic mineralization disorders: practical issues from morpholino design to post-injection observations

Zebrafish as a Model of Cardiac Pathology and Toxicity: Spotlight on Uremic Toxins

Chronic kidney disease (CKD) is an increasing health care problem. About 10% of the general population is affected by CKD, representing the sixth cause of death in the world. Cardiovascular events are the main mortality cause in CKD, with a cardiovascular risk 10 times higher in these patients than the rate observed in healthy subjects. The gradual decline of the kidney leads to the accumulation of uremic solutes with a negative effect on every organ, especially on the cardiovascular system. Mammalian models, sharing structural and functional similarities with humans, have been widely used to study cardiovascular disease mechanisms and test new therapies, but many of them are rather expensive and difficult to manipulate. Over the last few decades, zebrafish has become a powerful non-mammalian model to study alterations associated with human disease. The high conservation of gene function, low cost, small size, rapid growth, and easiness of genetic manipulation are just some of the features of this experimental model. More specifically, embryonic cardiac development and physiological responses to exposure to numerous toxin substances are similar to those observed in mammals, making zebrafish an ideal model to study cardiac development, toxicity, and cardiovascular disease.

Evaluation of cytogenotoxic potential and embryotoxicity of KRS-Cauvery River water in zebrafish (Danio rerio)

In the Cauvery River (CR), indiscriminate discharge of waste causes unexplained skeletal deformity in some fish species present in the water. To investigate this phenomenon, we analyzed the biological, physical, and chemical parameters present in the water and then evaluated the toxicity effects on the zebrafish (Danio rerio) model. The zebrafish were treated with KRS-CR water samples collected from three stations (fast-flowing water [X], slow-flowing [Y], and stagnant [Z] water), before and after filtration. Firstly, we detected microscopic organisms (MO) such as Cyclops, Daphnia, Spirogyra, Spirochaeta, and total coliform (Escherichia coli), which are bioindicators of water pollution present in the samples. All physicochemical parameters analyzed, including heavy metals before and after filtration of the water with Millipore filter paper (0.45 µm), were within the acceptable limits set by standard organizations, except for decreased dissolved oxygen (DO), and increased biochemical oxygen demand (BOD), and chemical oxygen demand (COD), which are indicators of hypoxic water conditions, as well as the presence of microplastics (polybutene (< 15 µm), polyisobutene (≤ 20 µm), and polymethylpentene (≤3 mm)) and cyclohexyl in CR water samples. Zebrafish embryos treated with the water samples, both before and after filtration exerts the same cytogenotoxic effects by inducing increased reactive oxygen species (ROS) production, which triggers subcellular organelle dysfunctions, DNA damage, apoptosis, pericardial edema, skeletal deformities, and increased mortality. As a result, we observed that both water samples and zebrafish larvae had significantly less oxygen using SEM and EDS. Our findings show that KRS-CR water can induce cytogenotoxic and embryotoxic defects in zebrafish due to hypoxic water conditions triggered by the microplastics influx. The present study would provide valuable insights for health hazards evaluation and future river water treatment strategies.

Clinical and Molecular Delineation of Cutis Laxa Syndromes: Paradigms for Homeostasis

Cutis laxa (CL) syndromes are a large and heterogeneous group of rare connective tissue disorders that share loose redundant skin as a hallmark clinical feature, which reflects dermal elastic fiber fragmentation. Both acquired and congenital-Mendelian- forms exist. Acquired forms are progressive and often preceded by inflammatory triggers in the skin, but may show systemic elastolysis. Mendelian forms are often pleiotropic in nature and classified upon systemic manifestations and mode of inheritance. Though impaired elastogenesis is a common denominator in all Mendelian forms of CL, the underlying gene defects are diverse and affect structural components of the elastic fiber or impair metabolic pathways interfering with cellular trafficking, proline synthesis, or mitochondrial functioning. In this chapter we provide a detailed overview of the clinical and molecular characteristics of the different cutis laxa types and review the latest insights on elastic fiber assembly and homeostasis from both human and animal studies.

ITPR1 Mutation Contributes to Hemifacial Microsomia Spectrum

Hemifacial microsomia (HM) is a craniofacial congenital defect involving the first and second branchial arch, mainly characterized by ocular, ear, maxilla-zygoma complex, mandible, and facial nerve malformation. HM follows autosomal dominant inheritance. Whole-exome sequencing of a family revealed a missense mutation in a highly conserved domain of ITPR1. ITPR1 is a calcium ion channel. By studying ITPR1's expression pattern, we found that ITPR1 participated in craniofacial development, especially the organs that corresponded to the phenotype of HM. In zebrafish, itpr1b, which is homologous to human ITPR1, is closely related to craniofacial bone formation. The knocking down of itpr1b in zebrafish could lead to a remarkable decrease in craniofacial skeleton formation. qRT-PCR suggested that knockdown of itpr1b could increase the expression of plcb4 while decreasing the mRNA level of Dlx5/6. Our findings highlighted ITPR1's role in craniofacial formation for the first time and suggested that ITPR1 mutation contributes to human HM.

Zebrafish Models of Human Skeletal Disorders: Embryo and Adult Swimming Together

Danio rerio (zebrafish) is an elective model organism for the study of vertebrate development because of its high degree of homology with human genes and organs, including bone. Zebrafish embryos, because of the optical clarity, small size, and fast development, can be easily used in large-scale mutagenesis experiments to isolate mutants with developmental skeletal defects and in high-throughput screenings to find new chemical compounds for the ability to revert the pathological phenotype. On the other hand, the adult zebrafish represents another powerful resource for pathogenic and therapeutic studies about adult human bone diseases. In fish, some characteristics such as bone turnover, reparation, and remodeling of the adult bone tissue cannot be found at the embryonic stage. Several pathological models have been established in adult zebrafish such as bone injury models, osteoporosis, and genetic diseases such as osteogenesis imperfecta. Given the growing interest for metabolic diseases and their complications, adult zebrafish models of type 2 diabetes and obesity have been recently generated and analyzed for bone complications using scales as model system. Interestingly, an osteoporosis-like phenotype has been found to be associated with metabolic alterations suggesting that bone complications share the same mechanisms in humans and fish. Embryo and adult represent powerful resources in rapid development to study bone physiology and pathology from different points of view.

Persistence of the ABCC6 genes and the emergence of the bony skeleton in vertebrates

The ATP-binding cassette transporter 6 (ABCC6) gene encodes a cellular transmembrane protein transporter (MRP6) that is involved in the regulation of tissue calcification in mammals. Mutations in ABCC6 are associated with human ectopic calcification disorders. To gain insight into its evolution and involvement in tissue calcification we conducted a comparative analysis of the ABCC6 gene and the related gene ABCC1 from invertebrates to vertebrates where a bony endoskeleton first evolved. Taking into consideration the role of ABCC6 in ectopic calcification of human skin we analysed the involvement of both genes in the regeneration of scales, mineralized structures that develop in fish skin. The ABCC6 gene was only found in bony vertebrate genomes and was absent from Elasmobranchs, Agnatha and from invertebrates. In teleost fish the abcc6 gene duplicated but the two genes persisted only in some teleost genomes. Six disease causing amino acid mutations in human MRP6 are a normal feature of abcc6 in fish, suggesting they do not have a deleterious effect on the protein. After scale removal the abcc6 (5 and 10 days) and abcc1 (10 days) gene expression was up-regulated relative to the intact control skin and this coincided with a time of intense scale mineralization.

The zebrafish operculum: A powerful system to assess osteogenic bioactivities of molecules with pharmacological and toxicological relevance

Bone disorders affect millions of people worldwide and available therapeutics have a limited efficacy, often presenting undesirable side effects. As such, there is a need for novel molecules with bone anabolic properties. The aim of this work was to establish a rapid, reliable and reproducible method to screen for molecules with osteogenic activities, using the zebrafish operculum to assess bone formation. Exposure parameters were optimized through morphological analysis of the developing operculum of larvae exposed to calcitriol, a molecule with known pro-osteogenic properties. An exposure of 3days initiated at 3days post-fertilization was sufficient to stimulate operculum formation, while not affecting survival or development of the larvae. Dose-dependent pro- and anti-osteogenic effects of calcitriol and cobalt chloride, respectively, demonstrated the sensitivity of the method and the suitability of the operculum system. A double transgenic reporter line expressing fluorescent markers for early and mature osteoblasts was used to gain insights into the effects of calcitriol and cobalt at the cellular level, with osteoblast maturation shown to be stimulated and inhibited, respectively, in the operculum of exposed fish. The zebrafish operculum represents a consistent, robust and rapid screening system for the discovery of novel molecules with osteogenic, anti-osteoporotic or osteotoxic activity.

Revisiting in vivo staining with alizarin red S--a valuable approach to analyse zebrafish skeletal mineralization during development and regeneration

BACKGROUND

The correct evaluation of mineralization is fundamental for the study of skeletal development, maintenance, and regeneration. Current methods to visualize mineralized tissue in zebrafish rely on: 1) fixed specimens; 2) radiographic and μCT techniques, that are ultimately limited in resolution; or 3) vital stains with fluorochromes that are indistinguishable from the signal of green fluorescent protein (GFP)-labelled cells. Alizarin compounds, either in the form of alizarin red S (ARS) or alizarin complexone (ALC), have long been used to stain the mineralized skeleton in fixed specimens from all vertebrate groups. Recent works have used ARS vital staining in zebrafish and medaka, yet not based on consistent protocols. There is a fundamental concern on whether ARS vital staining, achieved by adding ARS to the water, can affect bone formation in juvenile and adult zebrafish, as ARS has been shown to inhibit skeletal growth and mineralization in mammals.

RESULTS

Here we present a protocol for vital staining of mineralized structures in zebrafish with a low ARS concentration that does not affect bone mineralization, even after repetitive ARS staining events, as confirmed by careful imaging under fluorescent light. Early and late stages of bone development are equally unaffected by this vital staining protocol. From all tested concentrations, 0.01% ARS yielded correct detection of bone calcium deposits without inducing additional stress to fish.

CONCLUSIONS

The proposed ARS vital staining protocol can be combined with GFP fluorescence associated with skeletal tissues and thus represents a powerful tool for in vivo monitoring of mineralized structures. We provide examples from wild type and transgenic GFP-expressing zebrafish, for endoskeletal development and dermal fin ray regeneration.

Zebrafish: A possible tool to evaluate bioactive ions

Zebrafish is a well-established model organism with a skeletal structure that highly resembles mammalian bone. Yet its use in the research field of biomaterials has been limited. One area that could benefit from this model system is the evaluation of ionic dissolution products from different materials. As a proof of concept we have evaluated the effect of silicate ions on the zebrafish larvae and compared it to a well-known osteoblastic cell line, MC3T3-E1 subclone 14. We have shown that sodium metasilicate (125 μM and 625 μM) induces more mineralisation in a dose-dependent manner in zebrafish larvae, 9 days post fertilisation as compared to the non-treated group. Moreover the same trends were seen when adding sodium metasilicate to MC3T3-E1 cultures, with more mineralisation and higher ALP levels with higher doses of silicate (25, 125 and 625 μM). These results indicate the feasibility of zebrafish larvae for ionic dissolution studies. The zebrafish model is superior to isolated cell cultures in the aspect that it includes the whole bone remodelling system, with osteoblasts, osteoclasts and osteocytes. Zebrafish could thus provide a powerful in vivo tool and be a bridge between cell culture systems and mammalian models.

Morpholino oligonucleotide knockdown of the extracellular calcium-sensing receptor impairs early skeletal development in zebrafish

The complex vertebrate skeleton depends on regulated cell activities to lay down protein matrix and mineral components of bone. As a distinctive vertebrate characteristic, bone is a storage site for physiologically-important calcium ion. The extracellular calcium-sensing receptor (CaSR) is linked to homeostatic regulation of calcium through its expression in endocrine glands that secrete calcium homeostatic hormones, in Ca(2+)- and ion-transporting epithelia, and in skeleton. Since CaSR is restricted in its presence to the chordate-vertebrate evolutionary lineage, we propose there to be important functional ties between CaSRs and vertebrate skeleton in the context of that group's characteristic form of calcium-mineralized skeleton. Since little is known about CaSR in the skeletal biology of non-mammalian vertebrates, reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization and immunohistochemistry were applied to adult and embryonic zebrafish to reveal CaSR transcript and protein expression in several tissues, including, among these, chondrocytes and developing bone and notochord as components in skeletal development. Morpholino oligonucleotide (MO) knockdown technique was used to probe CaSR role(s) in the zebrafish model system. By RT-PCR assessment, injection of a splice-inhibiting CaSR MO reduced normally-spliced Casr gene transcript expression measured at 2days postfertilization (dpf). Corresponding to the knockdown of normally-spliced mRNA by the CaSR MO, we observed a morphant phenotype characterized by stunted growth and disorganization of the notochord and axial skeleton by 1dpf. We conclude that, like its critically important role in normal bone development in mammals, CaSR is essential in skeletogenesis in fishes.