Putting crystals in place: the regulation of biomineralization in zebrafish

In humans the skeleton has a number of crucial functions: It provides protection and mechanical support to the body, is an important metabolic organ and represents the place of adult hematopoiesis. There are a number of human diseases related to the muscoskeletal system; prominent examples are osteoporosis and osteoarthritis. Osteoporosis is characterized by a gradual reduction of bone mass due to an imbalance between bone formation and resorption, which leads to an increased risk of fracture. In osteoarthritis, loss of joint cartilage tissue with subsequent inflammation and formation of bone spurs leads to chronic pain and loss of joint flexibility. These diseases represent a major burden to the health systems in an ever-aging society. Another aspect related to skeletal biology is the regulation of bio-mineralization. Ectopic (displaced) mineralizations can occur in most soft tissues and are a burden for patients with systemic mineral imbalance such as in chronic kidney disease, but are also seen as a consequence of rare, monogenetic diseases, injury or aging. Tissues of the cardiovascular system are particularly inclined to ectopic mineralization. Such ectopic mineralization in the cardio-vasculature correlates with severe clinical symptoms such as myocardial infarction. In recent years it has become clear that, mechanistically, bio-mineralization is a process that has to be actively inhibited as a default state. This inhibition must be released in a rigidly controlled manner in order for mineralization to occur in skeletal elements. A central aspect of this concept is the tightly controlled balance between phosphate, a constituent of the biomineral hydroxyapatite, and pyrophosphate, a physiochemical inhibitor of mineralization. The research described in my thesis investigated zebrafish mutants showing defective bone-mineralization. The mutant “no bone” (nob), completely lacks mineralization, and the mutant “dragonfish” (dgf), shows excessive and pathologic mineralization in multiple tissues. The respective mutations could be attributed to the genes entpd5 (nob) and enpp1 (dgf), respectively. The well-established role of enpp1 as an enzyme generating pyrophosphate, allowed us to characterize entpd5, as a previously unknown and critical factor regulating bio-mineralization in zebrafish via the phosphate/pyrophosphate axis. A further key finding was that in response to ectopic mineralization in dgf mutants a rapid cellular response occurs by cells with osteoclastic (mineral resorbing) properties. This finding is of relevance for patients with a disease named generalized-arterial-calcification-of-infancy (GACI), which is caused by mutations in ENPP1. One treatment option for them is bisphosphonates which can inhibit further mineralizations, but are also known for their inhibitory effect on osteoclasts, which is exploited in the treatment of osteoporosis. Hence novel drugs inhibiting mineralization are desirable. We show in a proof-of-concept that dgf mutants can be a useful and easy to evaluate model for testing candidate compounds. Additionally we describe a zebrafish mutant for osterix, a key transcription factor in osteoblasts; and discovered that while the formation of the head skeleton in zebrafish depends on osterix, the first mineralized pattern in the axial skeleton occurs independently of osterix, pointing towards the existence of a distinct population of osteogenic cells that differs from the typical osteoblast.