Lower bone matrix mineralization in cases of sclerostin deficiency

Abstract

Type I procollagen molecules synthesized by osteoblasts contain globular protein extensions on their Cand N-terminals. These propeptides are cleaved in the pericellular space by specific proteases to permit proper assembly of the collagen molecules into fibrils. Mutations in the region that encode the C-terminal cleavage sites lead to a recently described type of osteogenesis imperfecta with increased bone mineral density (BMD). To understand the effect of this type of mutation on bone matrix mineralization we used quantitative backscattered electron imaging (qBEI) to examine transiliac bone biopsy samples from two adult siblings (41 years old male, 39 years old female). Both had a mutation in one COL1A1 allele (c.3652GNA) which resulted in a substitution of alanine by threonine in the C-propeptide cleavage site (p.Ala1218Thr). The siblings had incurred dozens of long bone fractures, and had elevated DXA BMD, significant hearing loss, normal stature, white sclerae and normal dentition. The bone mineralization density distribution (BMDD) in both individuals, as measured by qBEI, had a marked shift towards higher mineralization in both cancellous and cortical bone. The mean (CaMean) and mode (CaPeak) calcium content compared to the normative reference data base were increased by 19% and 21%, respectively. Most remarkably, about 80% of the bone area was mineralized beyond 25.3 wt.% Ca, the physiological plateau level in normal bone matrix. These data quantitate the extreme matrix hyper-mineralization and suggest that it is the basis of bone brittleness and fractures. doi:10.1016/j.bone.2012.08.045 O45 Lower bone matrix mineralization in cases of sclerostin deficiency A. Roschger, E. Paschalis, P. Roschger, I. Kramer, M. Kneissel, K. Klaushofer, S. Papapoulos Ludwig Boltzmann Institute of Osteology at Hanusch Hospital of WGKK and AUVA Trauma Centre Meidling, 1st Medical Department, Hanusch Hospital, Vienna, Austria Musculoskeletal Disease Area, Novartis Institutes for BioMedical Research, CH-4002 Basel, Switzerland Leiden University Medical Center, Department of Endocrinology & Metabolic Diseases, Leiden 2333 ZA, Netherlands Abstract: Sclerostin is known to inhibit WNT signaling and therefore to act as a negative regulator for bone formation. Consequently, sclerostin deficiency leads to increased bone formation and mass. Until now there is no information about the mineralization status of such an extensively formed bone matrix. However, mineralization is an important factor for the mechanical competence of bone material. Bone biopsy samples from patients with sclerosteosis (SC) (a rare bone disease caused by lacking of sclerostin due to a mutation in the SOST gene) and bone Sclerostin is known to inhibit WNT signaling and therefore to act as a negative regulator for bone formation. Consequently, sclerostin deficiency leads to increased bone formation and mass. Until now there is no information about the mineralization status of such an extensively formed bone matrix. However, mineralization is an important factor for the mechanical competence of bone material. Bone biopsy samples from patients with sclerosteosis (SC) (a rare bone disease caused by lacking of sclerostin due to a mutation in the SOST gene) and bone necropsies samples from Sost-KO mice were investigated by quantitative backscattered electron imaging (qBEI). The bone mineralization density distribution (BMDD) as measured by qBEI was determined from mandible, mastoid, or ear canal bone of SC-patients (n=4, age 8 to 14 years and n=2, age 24 and 43 years) and from femoral midshaft of Sost-KO mice (n=10, age 16 weeks) and corresponding wildtype mice (n=11, age 16 weeks). The BMDD outcome, weighted mean mineral content (CaMean), revealed: i) the young SC-patients exhibited a reduced CaMean compared to a historical young reference database (−4.6%, pb0.01). ii) the adult SC-patients displayed a reduced CaMean (−15%) compared to control samples (n=4, age matched, similar skeletal sites). iii) in the Sost-KO mice the cortical CaMean was significantly reduced compared to wildtype (−1.9%, pb0.001). The data indicate, that in sclerostin deficiency the increased bone formation is accompanied by a reduction in bone matrix mineralization. Recent studies on Sost-KO mice suggest this to be a consequence of alterations in mineralization kinetics. doi:10.1016/j.bone.2012.08.046 O46 The fine tuning regulation of osteoblast–osteoclast cross-talk A. Teti University of L'Aquila, Department of Biotechnological and Applied Clinical Sciences, Italy Abstract: Bone remodeling relies on the balanced activities of osteoblasts and osteoclasts. Several molecules regulate osteoblast–osteoclast cross-talk, including soluble factors and cell–cell interacting mechanisms. We identified two novel osteoblast products that regulate osteoclast activity, the insulin-like growth factor Bone remodeling relies on the balanced activities of osteoblasts and osteoclasts. Several molecules regulate osteoblast–osteoclast cross-talk, including soluble factors and cell–cell interacting mechanisms. We identified two novel osteoblast products that regulate osteoclast activity, the insulin-like growth factor binding protein 5 (IGFBP5) and the matrix protein PRELP. IGFBP5 stimulates osteoclastogenesis and bone resorption and is released by both immature and mature osteoblasts under the control, in the former, of the non-receptor tyrosine kinase c-Src and, in the latter, of the transcription factor Runx2. In immature osteoblasts c-Src phosphorylates STAT3, a transcription factor inducing IL-6 and IGFBP5, that contribute to the maintenance of a poor osteoblast differentiation. In mature osteoblasts, c-Src expression is down-regulated, de-repressing the expression of Runx2, which turned out to induce IGFBP5 as well. IGFBP5 further stimulates c-Src phosphorylation in immature osteoblasts and has no effect on mature osteoblasts, while it stimulates osteoclasts in both conditions. In contrast, the osteoblast-derived glycosaminoglycan binding protein PRELP inhibits osteoclastogenesis with a mechanism requiring chondroitin sulphateand annexin 2-dependent internalization of its N-terminal domain, translocation to the nucleus and inhibition of the pro-osteoclast transcription factor, NF-kappaB. Both IGFBP5 and PRELP play a role in vivo, the former being stimulated under the control of c-Src in conditions in which IL-6 is overexpressed, such as in inflammatory diseases, the latter preventing bone loss in bone diseases, including osteoporosis and bone metastases. The discovery of these two novel osteoblast/ osteoclast regulators may have important translational implications, especially for the therapy of bone diseases. doi:10.1016/j.bone.2012.08.047 O47 On the importance of selenium for bone physiology N. Pietschmann, E. Rijntjes, A. Hoeg, L. Schomburg Institute for Experimental Endocrinology, Charite Universitats-medizin