Extracellular Matrix Remodelling in Adolescent Bone Development

R. Luke Harris, PhD

Assistant Professor, School of Health Sciences University of Northern British Columbia

The goal of my research program is to investigate physiological mechanisms underlying musculoskeletal adaptations to changes in mobility. In skeletal tissues, ECM proteoglycans are essential in the development and maintenance of bone and cartilage matrix, and in cartilage inflammatory responses. The contents and physiological roles of versican and aggrecan have never been explored in adult bone tissue, despite their importance in the cell biology of several other remodeling tissues. Bisphosphonate drugs (BPs) are a core treatment modality for osteoporosis and are used in the treatment of bone metastases because they inhibit the activity of the osteoclasts that normally break down bone. BPs’ effects on bone ECM turnover have never been studied. Preliminary data from my lab indicate that versican and aggrecan contents are altered in the final stages of adolescent bone development and are further altered by BP treatment. Whole epiphyseal bone homogenates were collected and prepared from 12 or 24 wk- old mice that had been treated with the BP zoledronate or saline vehicle for, respectively, 3 or 15 wk (n=3/group, 12 mice total). Versican and aggrecan isoform contents were measured in the bone samples using Western blotting, and significance was accepted at P < 0.05. With zoledronate treatment compared to vehicle, there were large difference in mouse bones contents of 340 and 60 kDa versican isoforms and in 100 and 60 kDa aggrecan isoforms (drug effect). In 24 week-old mice (15 weeks of treatment) compared to 12 week-old mice (3 weeks of treatment), large decreases were observed in the bone contents of 340, 80, 60 and 11 kDa versican isoforms, and of 100, 70 and 40 kDa aggrecan isoforms (age effect). There was a statistically significant zoledronate-age interaction for the 330 kDa aggrecan core protein. During maturation from 12 to 24 wk old, mice undergo normal hindlimb epiphyseal growth plate ossification, so our pilot data from this model indicate that versican content in bone ECM decreases not only due to BP treatment but also during normal bone development during late adolescence. Therefore, advancing our understanding of proteoglycan expression and turnover in bone remodeling will contribute to improved therapies for bone injury and disease, as well as to improved management of the side effects of bone treatments such as bisphosphonates.