Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Cancer stem cells (CSC) are characterized by high self-renewal capacity, tumor-initiating potential, and therapy resistance. Aldehyde dehydrogenase (ALDH)+ cell population serves as an indicator of prostate CSCs with increased therapy resistance, enhanced DNA double-strand break repair, and activated epithelial-mesenchymal transition (EMT) and migration. Numerous ALDH genes contribute to ALDH enzymatic activity; however, only some of them showed clinical relevance. We found that ALDH1A1 and ALDH1A3 genes functionally regulate CSC properties and radiation sensitivity of PCa. We revealed a negative correlation between ALDH1A1 and ALDH1A3 expression in publicly available prostate cancer (PCa) datasets and demonstrated that ALDH1A1 and ALDH1A3 have opposing predictive value for biochemical recurrence-free survival. Our data suggest an association of ALDH1A1 with the metastatic burden, elucidating the role of ALDH genes in the metastatic spread and homing to the bone, which can be, at least partially, attributed to regulating the transforming growth factor beta 1 (TGFB1) and matrix metalloproteinases (MMPs). ALDH genes play a diverse role in PCa development under AR and β-catenin-dependent regulation, with ALDH1A1 becoming dominant in later stages of tumor development when PCa cells gain androgen independence. Taken together, our results indicate that ALDH1A1 and ALDH1A3 modulate PCa radiosensitivity, regulate CSCs phenotype, and spread of PCa cells to the bone, therefore having clinical implication for identifying patients at high risk for progression to metastatic disease.

In zebrafish, transgenic labeling approaches, robust regenerative responses and excellent in vivo imaging conditions enable precise characterization of immune cell behavior in response to injury. Here, we monitored osteoblast-immune cell interactions in bone, a tissue which is particularly difficult to in vivo image in tetrapod species. Ablation of individual osteoblasts leads to recruitment of neutrophils and macrophages in varying numbers, depending on the extent of the initial insult, and initiates generation of cathepsin K+ osteoclasts from macrophages. Osteoblast ablation triggers the production of pro-inflammatory cytokines and reactive oxygen species, which are needed for successful macrophage recruitment. Excess glucocorticoid signaling as it occurs during the stress response inhibits macrophage recruitment, maximum speed and changes the macrophage phenotype. Although osteoblast loss is compensated for within a day by contribution of committed osteoblasts, macrophages continue to populate the region. Their presence is required for osteoblasts to fill the lesion site. Our model enables visualization of bone repair after microlesions at single-cell resolution and demonstrates a pro-osteogenic function of tissue-resident macrophages in non-mammalian vertebrates.