Title: Investigating the role of a small leucine rich protein asporin in cartilage ageing and age-related disease
Funder: Dunhill Medical Trust

Abstract: Asporin, a member of the small leucine rich protein (SLRP) family, is an extracellular matrix (ECM) protein associated with collagen mineralisation and found in perichondrium, periosteum and periodontal ligament, but not in healthy cartilage (Henry et al. 2001, Kou et al. 2007). Asporin’s expression is increased in osteoarthritic lesions, osteophytes and subchondral bone in OA patients (Juchtmans et al. 2015, Nakajima et al. 2007) and polymorphisms in the aspartate-rich region of asporin have been shown to correlate with high risk of osteoarthritis (OA) and intervertebral disc degeneration in several populations (Kizawa et al. 2005, Min et al. 2014, Mustafa et a. 2005). Asporin can suppress TGFß1-mediated expression of cartilage ECM genes, prevent mineralisation of periodontal ligaments, and regulate deposition of hydroxyapatite in the bone collagen matrix (Yu et al. 2018, Kalamajski et al. 2009, Yamada et al. 2007). It has been postulated that asporin and asporin-mediated modulation of TGFß1 signalling may present a therapeutic target for prevention or modulation of OA; however, the role of asporin in skeletal development was never investigated in detail. In a recent pilot study, we have found that asporin knock-out mice have normal epiphyseal and articular cartilages and their long bone growth is not affected. However, asporin knock-out bones are denser, tougher and hypermineralised, indicating a novel role for asporin in bone development. Furthermore, we have verified our data using siRNA knock-down of asporin in mesenchymal stem cells (MSCs) differentiation and confirmed that lack of asporin affects the osteogenesis and mineralisation of MSC cells, but not chondrogenesis. The data is currently being prepared for publication. Based on the expression pattern of asporin we postulate that asporin expression in the osteoarthritic cartilage may be a result of dedifferentiation and plays a detrimental role in OA progression by modulating TGFß1 signalling and inhibiting cartilage ECM deposition. Our pilot data suggest that deletion of asporin from developing healthy cartilage has no effect on cartilage morphology and ECM composition. We therefore hypothesise that deletion or inhibition of asporin in adult cartilage may present a therapeutic avenue for osteoarthritis. We propose to test this hypothesis by studying the age-related and trauma-induced (destabilisation of medial meniscus, DMM) OA progression in animals in which asporin has been deleted in developing and adult cartilage, using histology, immunohistochemistry and transcriptomic techniques in order to decipher the role of asporin in OA progression.

Lay abstract: Osteoarthritis (OA) is an age-related debilitating condition that presents a severe health and economic burden on the modern ageing society. There currently is no cure for OA other than pain management and terminal stage total join replacement surgery. OA results from a combination of factors, including age, lifestyle, injury and genetic susceptibility. This complexity together with scarcity of suitable human samples significantly hampers the study of OA progression and discovery of novel therapeutic targets. Interestingly, certain genetic changes may contribute to OA susceptibility. One of such changes has been found in the gene encoding asporin, a protein that is not present in healthy cartilage tissue but can be found in OA cartilage and is thought to dampen the production of new cartilage in OA-affected joints. The changes found in the asporin gene increase its function and thus increase OA susceptibility. We therefore propose that deleting asporin from cartilage may in fact be protective against OA. We have recently analysed a mouse model in which asporin was deleted during development and observed no changes in cartilage morphology or function, indicating that asporin is not required for skeletal development, but may play a role in cartilage ageing and disease. In this project, we propose to induce OA through ageing or injury in mice with asporin deleted in cartilage, and study the effect of lack of asporin on OA progression. We believe that our study will allow us to uncover novel therapeutic approaches for OA and increase understanding of disease progression and of ageing in healthy cartilage.

Research activity codes: 2.1 BIOLOGICAL AND ENDOGENOUS FACTORS;1.1 NORMAL BIOLOGICAL DEVELOPMENT AND FUNCTIONING
Health categories: MUSCULOSKELETAL