Significance of Premature Vertebral Mineralization in Zebrafish Models in Mechanistic and Pharmaceutical Research on Hereditary Multisystem Diseases

Abstract

Zebrafish are increasingly becoming an important model organism for studying the pathophysiological mechanisms of human diseases and investigating how these mechanisms can be effectively targeted using compounds that may open avenues to novel treatments for patients. The zebrafish skeleton has been particularly instrumental in modeling bone diseases as-contrary to other model organisms-the lower load on the skeleton of an aquatic animal enables mutants to survive to early adulthood. In this respect, the axial skeletons of zebrafish have been a good read-out for congenital spinal deformities such as scoliosis and degenerative disorders such as osteoporosis and osteoarthritis, in which aberrant mineralization in humans is reflected in the respective zebrafish models. Interestingly, there have been several reports of hereditary multisystemic diseases that do not affect the vertebral column in human patients, while the corresponding zebrafish models systematically show anomalies in mineralization and morphology of the spine as their leading or, in some cases, only phenotype. In this review, we describe such examples, highlighting the underlying mechanisms, the already-used or potential power of these models to help us understand and amend the mineralization process, and the outstanding questions on how and why this specific axial type of aberrant mineralization occurs in these disease models.

Keywords: axial skeleton; multisystemic disorders; premature mineralization; zebrafish model.

Figure 3

Signaling pathways involved in ectopic mineralization in zebrafish in physiological circumstances (blue) and disease (pink) for each of the discussed models, depicted at the site of highest expression for each gene. In the pathophysiological illustrations, arrows indicate increased or decreased expression of the involved mediators. AMP: adenosine monophosphate; ATP: adenosine triphosphate; BMP: bone morphogenetic protein; CPP: calciprotein particle; ECM: extracellular matrix; LAP: latency-associated peptide; LTBP: latent transforming growth factor β-binding proteins; PTH: parathyroid hormone; P: phosphorylated; Pi: inorganic phosphate; PPi: inorganic pyrophosphate; SARA: smad anchor for receptor activation; vit D: vitamin D. Adapted from [37,90,128,129,138,152,153,155].

Figure 1

(A) Overview of the composition of the axial skeleton of adult zebrafish (modified from Dietrich et al. (2021)) [20]. Inserts illustrate a sagittal (B) and transversal (C) view of the morphology of the vertebral body and the intervertebral disc (IVD) (adapted from Cotti et al. (2022)) [21].

Figure 2

Graphical overview of the discussed zebrafish models, including the targeted gene, the genome-editing tool(s) used, and a schematic representation of the larval and adult ectopic mineralization phenotype (indicated in black). All PXE/GACI models showed advanced vertebral mineralization in the larvae [55,57,58,59,60], except for the model developed by Li et al [53]. In the adult models, excessive (inter)vertebral calcification was observed. Similarly, excessive vertebral mineralization was seen in the hmx1−/− larvae [37]. This recurring phenotype was not present in the klotho−/− or golgb1−/− larvae, but the adult fish showed extensive vertebral mineralization [61,62]. This was also observed in the adult ltbp1-knockout fish, including calcification of the ribs [63]. Finally, the color scheme indicates the similarities and differences between the different adult models in terms of more detailed phenotypic characteristics such as craniofacial hypermineralization, rib abnormalities, the presence of a curved spine, vertebral fusions and calcified nodules on the vertebrae and/or ribs, skin and bulbus arteriosus calcification, heart fibrosis, and eye abnormalities. Filled circles indicate that the corresponding phenotypic feature is present.