The influence of Interleukin-33 and Galanin on bone biology

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Public Health and Human Biosciences, Faculty of Health Sciences, La Trobe University, Bundoora.

Bone is required for mechanical support and movement of the body, as well as playing an important role in maintaining calcium homeostasis. Bone remodelling relies on the action of several specialised cell types; osteoblasts, which form bone, osteoclasts, which break down and resorb bone and osteocytes, which form a mechanosensory network within the bone and are thought to regulate bone mass. The Interleukin-1 family member Interleukin-33 (IL-33), and its receptor ST2L, were shown to be expressed by osteoblasts. IL-33 treatment of osteoblasts increased in vitro mineralisation, while inhibiting in vitro osteoclast formation. This thesis aimed to further investigate the regulation and effect of IL-33 in bone. IL-33 mRNA levels and protein levels were found to be induced by Parathyroid Hormone (PTH) and Oncostatin M (OSM) treatment. The increase in mineralisation previously observed in vitro with IL-33 treatment was confirmed. IL-33 was found to suppress mRNA levels of the osteocyte produced factor sclerostin in vitro, ex vivo and in vivo. This suppression of sclerostin may explain the osteogenic effect of IL-33 found in vitro. Our group previously found that IL-33 failed to generate osteoclasts from mouse precursors, however some controversial work by Mun and colleagues indicated that IL-33 could stimulate osteoclastic cell formation from human monocyte precursors in a RANKL independent manner. The work described in this thesis indicated that IL-33 did indeed possess this ability; however the osteoclastic cell formation response was inconsistent. Despite finding that IL-33 could affect osteoblasts and osteoclasts, and that IL-33 mRNA levels are increased in bone fracture, no difference in bone mass or biomechanical characteristics of healed bone fractures was found in IL-33 receptor knockout (ST2KO) mice. Finally, our lab previously found that the neuropeptide galanin and its receptors are present in bone cells and galanin treatment improved bone injury healing. However, this thesis could not identify a direct effect of galanin on bone cells.