Not all bones are created equal: using zebrafish and other teleost species in osteogenesis research

Developmental osteogenesis and pathologies of mineralized tissues are areas of intense investigations in the mammalian field, but different from other areas of organ formation and developmental biology, zebrafish have been somewhat slow in joining the area of bone research. In recent years, however, genetic screens have provided a number of exciting mutants, and transgenic lines have been developed that permit visualization of osteoblasts and osteoclasts in vivo. We here review some of the recent literature and provide examples where insights from studies in zebrafish have complemented the information available from mammalian models or clinical studies. Furthermore, we provide a comparative overview about different forms of bone within the teleost lineage, and between teleosts and mammals. The vertebrate skeleton serves numerous functions, most notably by providing a stable, but mobile framework against which muscles act. The endoskeleton also has protective functions for the brain and many internal organs, and serves as a store for minerals. The postcranial endoskeleton consists of the axial skeleton, supporting the main body axis, and the appendicular skeleton, supporting the extremities of the body. Bone and cartilage are the main components of the endoskeleton, being produced by osteoblasts and chondrocytes, respectively. Given the vital roles of the skeleton, it is not surprising that in the human clinic a number of diseases and pathologies affect the skeletal system. They include metabolic bone diseases such as osteoporosis and osteoarthritis as well as birth defects such as cleft palate, congenital vertebral malformations, or skeletal dysplasia (for reviews, see McInnes and O'Dell, 2010; Ralston and Uitterlinden, 2010; Zelzer and Olsen, 2003). Particularly osteoporosis (an increased risk of bone fracture due to a decrease in bone density) and osteoarthritis (a degenerative disease affecting joints) are extremely common, and have an enormous bearing on health care costs in an ever-aging society. In many areas of cell biology and organogenesis, zebrafish (Danio rerio) have provided a plethora of useful models that can be employed to study processes with relevance to human disease. In contrast to mouse and chicken, however, zebrafish has a short history as model for bone disease research, or, for that matter, for studying bone formation from a developmental or cellular point of view. Part of this is certainly due to historic reasons. Initially zebrafish were mainly used to understand early developmental processes, and only gradually the use of the system has been expanded to areas of organogenesis and larval development. Another reason is that there is a diffuse notion about teleost bone being "weird" and different from "normal" (i.e., mammalian) bone. Within the vertebrate phylum, teleost species are by far the most successful (at least in terms of species number), so one could actually argue what "normal" is. However, these discussions are, to a degree, doomed to be pointless in an anthropocentric funding environment. Despite a slow start, in recent years a steadily raising number of publications prove the zebrafish as a valuable complementation to the traditional model organisms. As so often before, the availability of mutants has been a driving force in the utilization of zebrafish, and forward genetic screens have already yielded hundreds of mutants (reviewed by Spoorendonk et al., 2010). Fortuitously, some zebrafish mutants survive far longer than knockouts in their murine orthologues (see below), allowing to obtain additional information even in cases where mouse mutants had been available for a number of years. An additional and exciting opportunity for studying skeletal formation and osteogenesis in fish is the possibility to observe bone-forming cells (osteoblasts) in vivo. Transgenic lines for a number of informative markers have been generated by now in zebrafish and medaka (Oryzias latipes) (Renn and Winkler, 2009; Spoorendonk et al., 2008). This is a key advantage of using zebrafish and medaka as model organisms. Among the bone and osteogenesis mutants that have been studied in zebrafish are some instructive examples, two of which are discussed in more detail below. Furthermore, with this chapter, we provide an overview about the different types of skeletal tissues that are present in zebrafish, medaka, and other teleost species