title: The role of PAR2 in the myeloid compartment and the regulation of osteoclastogenesis
Creators: name: McGrath, Sarah
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: PAR2, protease activated receptor, protease, protease signalling, bone, osteoclast, osteoclastogenesis, osteoblast, osteoimmunology, arthritis, rheumatoid arthritis, bone erosion, inflammatory bone erosion.

description: Protease activated receptor 2 (PAR2) is one member of a family of G-protein coupled receptors. A defining feature of this family of receptors is their protease mediated activation. An emerging role for PAR2 in both the immune system and also in bone biology suggests a potential function for this receptor in osteoimmunology and its associated pathologies. Inflammatory arthritic conditions such as rheumatoid arthritis (RA), are pathologies that include both inflammation and bone destruction, making them a prime model for osteoimmunological studies. Previous murine studies identified that loss of PAR2 results in protection from inflammatory adjuvant-induced arthritis. These animals experienced an attenuated form of arthritis, with significantly reduced joint inflammation and damage. In addition, PAR2 is known to be upregulated in both macrophages (from tissue biopsies) and peripheral blood monocytes in patients with RA. However, the functional impact of protease signalling via PAR2 in monocytes in RA and how this receptor influences joint destruction via monocytes and their tissue differentials is still unknown. The central aim of this doctoral study was to understand the function of PAR2 in monocytes and how protease signalling via PAR2 would influence the differentiation potential of these cells. Specific focus was placed on osteoclasts (OCs), and how protease signalling may further contribute to bone erosion through the action of PAR2 on these bone resorbing cells. Initial work confirmed the expression of the PAR2 on the plasma membrane of monocytes, with the highest expression consistently found on classical monocytes that have the highest osteoclastogenic potential. In order to study the impact of PAR2 signalling on OC formation and activity, in vitro OC differentiation assays were set up using both WT and par2-/- cells. Both standard homeostatic and inflammatory TNF enhanced OC assays were utilised. These studies revealed that during homeostatic OC formation PAR2 contributed to the regulation of OC formation and prevented excessive fusion of precursors (OCPs) into giant cells. The PAR2 mediated regulation of OC formation was found to be important both via OCPs directly, and via the stromal compartment. An absence of PAR2 in osteoblast (OB) -like cells resulted in a more osteoclastogenic stroma and contributed to enhanced OC formation. However, during inflammatory3 driven OC formation, the role of PAR2 in osteoclastogenesis was reversed, and protease signals via PAR2 enhanced the formation of OCs. The observations made in murine in vitro systems were replicated in human cell cultures when monocytes were exposed to PAR2 inhibitors during the process of OC differentiation. Combined, this work indicates that PAR2 has an impact on the process of osteoclastogenesis. Whether PAR2 signalling regulates or enhances OC formation is dependent upon the environment and the combination of signals received by precursor cells.