Modelling the biological mechanisms underpinning healthy and pathological bone repair in zebrafish (Danio rerio)

Abstract

Bone is a dynamic, living tissue, maintained by an elaborate and interconnected network of different cell types. These cells synthesise and remodel a highly specialised, mineralised extracellular matrix (ECM). Tightly coupled communication between bone synthesising osteoblasts, bone resorptive osteoclasts and innate immune cells is required to maintain optimal bone turnover and respond to fracture. Osteoporosis is an increasingly prevalent disease with complex aetiology, characterised by reduced bone deposition in relation to bone resorption. This leads to bone fragility and fracture susceptibility; symptoms which carry an extensive socioeconomic impact and high morbidity in the population. Currently, osteoanabolic treatments to promote osteoblast activity in bone are lacking. During healthy fracture repair, osteoblasts precursors rapidly proliferate and differentiate into highly active, bone synthesising osteoblasts. Therefore, studying optimal fracture repair may elucidate the molecular mechanisms which promote osteoanabolic activity, thereby helping to identify new targets for the treatment of osteoporosis and fracture. Here, I employ zebrafish (Danio rerio) as a model system to explore genetic and immunological factors underpinning optimal bone repair. Zebrafish show remarkable genetic, skeletal and immunological similarities with humans, allowing for clinically relevant study of bone. However, unlike mammalian systems, zebrafish display high genetic tractability, fecundity, and fast generation time. Moreover, the translucent caudal fin of adult zebrafish serves as an ideal tissue for live imaging multicellular responses during repair and regeneration of mature bone. Therefore, I explored three factors, with poorly characterised influences on bone repair: Wnt16, neutrophils and Tnf-α. I demonstrate that Wnt16 elicits a protective effect against fracture and supports bone repair by regulating osteoblast differentiation. Furthermore, I highlight a novel, pro-reparative role for neutrophils in fracture stabilisation and rapid ECM production during early bone repair. Finally, I demonstrate that tnfα is expressed by differentiating osteoblasts in regenerating bone and explore the pleiotropic effects of Tnf-α on osteogenesis.

Date of Award	12 May 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Beck J Richardson (Supervisor) & Chrissy L Hammond (Supervisor)