Biglycan deficiency increases osteoclast differentiation and activity due to defective osteoblasts Yanming Bi a,2 , Karina L. Nielsen b,2 , Tina M. Kilts a , Anne Yoon a , Morten A. Karsdal b , Helen F. Wimer c , Edward M. Greenfield d , Anne-Marie Heegaard c,1 , Marian F. Young a, ⁎ a Craniofacial and Skeletal Diseases Branch, Building 30 Room 225, National Institute of Dental and Craniofacial Research, National Institutes of Health, MSC 4320, 9000 Rockville Pike, Bethesda, MD 20892, USA b Nordic Bioscience A/S and Department of Radiation Biology, The Finsen Center, National University Hospital, Copenhagen, Denmark c Department of Orthopaedics, Case Western Reserve University, Cleveland, OH 44106, USA d National Museum of Natural History, Smithsonian Institution, NW Washington, DC 20560, USA Received 3 June 2005; revised 28 October 2005; accepted 2 November 2005 Available online 20 December 2005 Abstract Bone mass is maintained by a fine balance between bone formation by osteoblasts and bone resorption by osteoclasts. Although osteoblasts and osteoclasts have different developmental origins, it is generally believed that the differentiation, function, and survival of osteoclasts are regulated by osteogenic cells. We have previously shown that the extracellular matrix protein, biglycan (Bgn), plays an important role in the differentiation of osteoblast precursors. In this paper, we showed that Bgn is involved in regulating osteoclast differentiation through its effect on osteoblasts and their precursors using both in vivo and in vitro experiments. The in vivo osteolysis experiment showed that LPS (lipopolisaccharide)-induced osteolysis occurred more rapidly and extensively in bgn deficient mice compared to wild type (WT) mice. To further understand the mechanism of action, we determined the effects of Bgn on 1α, 25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 )-induced osteoclast differentiation and bone resorption in an co- culture of calvariae-derived pre-osteoblasts and osteoclast precursors derived from spleen or bone marrow. Time course and dose response experiments showed that tartrate-resistant acid phosphatase-positive multinuclear cells appeared earlier and more extensively in the co-cultures containing calvarial cells from bgn deficient mice than WT mice, regardless of the genotype of osteoclast precursors. The osteoblast abnormality that stimulated osteoclast formation appeared to be independent of the differential production of soluble RANKL and OPG and, instead, due to a decrease in osteoblast maturation accompanied by increase in osteoblastic proliferation. In addition to the imbalance between differentiation and proliferation, there was a differential decrease in secretory leukocyte protease inhibitor (slpi) in bgn deficient osteoblasts treated with 1,25-(OH) 2 D 3 . These findings point to a novel molecular factor made by osteoblasts that could potentially be involved in LPS-induced osteolysis. Published by Elsevier Inc. Keywords: Biglycan; Proteoglycan; Osteoclast; Osteolysis; ECM Introduction Bgn is an extracellular matrix (ECM) small proteoglycan, a part of the family of genes called small leucine rich proteogly- can (SLRPs), and is abundant in skeletal tissues [5,38,40]. Understanding of the role of ECM in controlling skeletal func- tion has made substantial progress by studying human diseases or mouse models defective in specific ECM components. The Bgn gene is localized to the X (and not Y) chromosome [18] and is differentially transcribed in patients with X-chromosome anomalies including Turners (XO) and Kleinfelters Syndrome (XXY) [20,23], diseases characterized by short and long skele- tal stature, respectively. To directly test whether there is a connection between the expression of Bgn and skeletal function, deficient mice were generated [41]. Those mice develop an osteoporosis-like phenotype with significant decreases in tra- becular bone volume, mineral apposition rate, and bone forma- tion rate by 6 months of age [41]. Further experiments showed that there is an age dependent decrease in the number and differentiation of osteogenic precursors in the bone marrow of Bone 38 (2006) 778 – 786 www.elsevier.com/locate/bone ⁎ Corresponding author. Fax: +1 301 402 0824. E-mail address: myoung@dir.nidcr.nih.gov (M.F. Young). 1 Present address: Department of Pharmacology, The Danish University of Pharmaceutical Sciences Universitetsparken 2 DK-2100 Copenhagen, Denmark. 2 Co-first authors. 8756-3282/$ - see front matter. Published by Elsevier Inc. doi:10.1016/j.bone.2005.11.005