Gene array profile identifies collagen type XV as a novel human osteoblast-secreted matrix protein

Bone mass remains plastic throughout life and is subjected to changes due to the relative rate of bone formation and bone resorption (Harada and Rodan, [2003]; Teitelbaum and Ross, [2003]; Teitelbaum, [2006]). The rate of bone formation is dependent on the commitment and replication of osteoprogenitor cells, their differentiation into functional osteoblasts and the life span of mature osteoblasts (Deng et al., [2008]). Bone marrow is the tissue filling the space between the vascular sinus and bone surfaces in the pores of cancellous bone (Burkhardt et al., [1984]; Hauge et al., [2001]). The bone cavities are filled with soft bone marrow mainly composed of undifferentiated bone marrow stromal cells (MSCs) forming a delicate connective tissue stroma that separates haematopoietic and adipose marrow compartments, blood stem cells and vessels (Yin and Li, [2006]). Bone marrow-derived stromal cells are multipotent cells that can give rise to the majority of MSC lineages, including osteoblasts, chondrocytes, fibroblasts, adipocytes, endothelial cells and myocytes (Pittenger et al., [1999]; Caplan, [2005], [2007]). Pre-osteoblasts and osteoblasts (Ducy et al., [2000]), also defined as bone-forming cells, are located along the backbone of the trabeculae. The anatomic location of these cells in the bone tissue is well defined using conventional histological staining (Parfitt, [2001]; Teitelbaum and Ross, [2003]; Eriksen et al., [2007]) and multiple markers have been found to be immunoreactive against in vitro isolated MSCs or osteoblasts (Tuli et al., [2003]; Sakaguchi et al., [2004]; Sacchetti et al., [2007]; Delorme et al., [2008]). In vitro isolated MSCs or osteoblasts have been used extensively in studies concerning bone cell biology (Wang et al., [2006]; Bielby et al., [2007]; Lisignoli et al., [2007]) and other various purposes, such as transplantation, inflammation or tissue engineering (Nauta and Fibbe, [2007]; Pountos et al., [2007]; Tian et al., [2008]). Only a recent study (Sacchetti et al., [2007]), identifying clonogenic skeletal progenitors with the ability of self-renewal, compared bone MSCs with other osteogenic cell strains and demonstrated, both in vitro and in vivo, that among the different markers evaluated (CD49a, CD63, CD90, CD105, CD140b, CD146, STRO-1 and alkaline phosphatase) only CD146 was differently expressed in bone MSCs. However, also MSCs from other sources expressed CD146 at different levels (De Bari et al., [2008]; Delorme et al., [2008]; Widemann et al., [2008]). The identification and characterisation of novel osteoblast-related molecules could help to distinguish MSCs (immature cells) from osteoblasts (mature cells). Therefore, we isolated both MSCs and osteoblasts from bone marrow and trabecular bone of the same subject using methods widely described in literature (Robey and Termine, [1985]; Tuli et al., [2003]; Sakaguchi et al., [2004]; De Bari et al., [2008]; Delorme et al., [2008]). We investigated their phenotype and proliferation rate and performed a gene expression profiling analysis. Notably, gene expression analysis identified collagen XV as the most up-regulated gene in osteoblasts compared to MSCs, leading us to further investigate its expression and localisation in vivo. The expression of collagen type XV was confirmed at the protein level on isolated osteoblasts and it was found to be significantly increased during osteogenic differentiation of MSCs in vitro. Free ionised extracellular calcium significantly down-modulated its expression on osteoblasts. Moreover, by light and electron microscopy on bone tissue biopsies we found high expression of collagen type XV mainly in working osteoblasts forming new bone tissue and in lining osteoblasts. Our data demonstrate that collagen type XV, so far described in the basement membrane of other cell types, is also expressed by human osteoblasts both in vitro and in vivo.