Thus, theDspptransgene partially corrected the lacunocanalicular defects of theDmp1-deficient mice. == Fig. assessed the effects of transgenicDsppon osteogenesis by analyzing the formation and mineralization of the long bones inDmp1/mice that expresses a transgene encoding full-length DSPP driven by a 3. 6-kb ratCol1a1promoter (referred as Dmp1/; Dspp-Tg mice). We characterized the long bones of theDmp1/; Dspp-Tg mice at different ages and compared them with those fromDmp1/andDmp1+/(normal control) mice. Our analyses showed that the long bones ofDmp1/; Dspp-Tg mice had a significant increase in cortical bone thickness, bone volume and mineral density along with a remarkable restoration of Oxotremorine M iodide trabecular thickness compared to those of theDmp1/mice. The long bones ofDmp1/; Dspp-Tg mice underwent a dramatic reduction in the amount of osteoid, significant improvement of Oxotremorine M iodide the collagen fibrillar network, and better organization Oxotremorine M iodide of the lacunocanalicular system, compared to theDmp1/mice. The elevated levels of biglycan, bone sialoprotein and osteopontin inDmp1/mice were also noticeably corrected by the transgenic expression ofDspp. These findings suggest that DSPP and DMP1 may function synergistically within the complex milieus of bone matrices. Keywords: Bone, Dentin Matrix Protein 1, Dentin Sialophosphoprotein, Mineralization, Transgenic Mice == INTRODUCTION == The organic components in the extracellular matrix (ECM) of bone are composed of collagen type I and a number of non-collagenous proteins (NCPs). One family of NCPs is the Small Integrin-Binding Ligand N-linked Glycoprotein (SIBLING) family, which consists of osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP) and matrix extracellular phosphoglycoprotein (MEPE) [1]. These SIBLING family members play important biological roles in the formation and mineralization of bone and dentin [1-4], as evidenced by the observations that mutations in or ablation of their genes are associated with developmental abnormalities in the two tissues [5-7]. DMP1, first cloned from a rat odontoblast cDNA library, has been identified in dentin, bone and cementum as well as in some non-mineralized tissues [8-10]. In the appendicular skeleton, DMP1 is expressed by osteocytes, osteoblasts and hypertrophic chondrocytes [11-13]. Dmp1-deficient mice displayed severe defects in the cartilage and bone, which resembled the manifestations of autosomal recessive hypophosphatemic rickets (ARHR), a human hereditary disease caused Oxotremorine M iodide by mutations in theDMP1gene. This condition was characterized by the elevation of serum fibroblast growth factor 23 (FGF23) and a reduction of serum phosphorus, along with malformed and hypomineralized bone and dentin [14, 15]. Although DSPP was originally thought to HILDA be exclusively expressed by odontoblasts, the dentin-forming cells, later on its expression was detected in bone, cementum and certain non-mineralizing tissues including the salivary glands and kidneys [16-19]. Our previous studies showed that the expression level of DSPP in the rat long bone is approximately 1/400thof that in the rat dentin [19]. Mouse and human genetic studies have associatedDSPPmutations or its ablation with dentinogenesis imperfecta, characterized by thinner dentin, enlarged pulp chamber and widened predentin [5, 6, 20, 21]. WhileDsppknockout mice have severe defects in the formation and mineralization of dentin, the changes in the long bones ofDspp-deficient mice are mild [22]. Certain pieces of evidence suggest a possible functional relationship between DMP1 and DSPP [9, 16, 23]. The dentin defects inDmp1/mice are similar to those inDspp/mice [20, 24] and the dentin of theDmp1/mice has a reduced level of DSPP expression [24]. In vitrostudies revealed that DMP1 localizes in the nucleus during the differentiation of odontoblasts and is bound to and activates theDspppromoter in odontoblast cell lines [25]. While DMP1 has been shown to play a crucial role in osteogenesis [14, 26, Oxotremorine M iodide 27] and chondrogenesis [7], Dsppablation in mice or its mutations in human dentinogenesis imperfecta subjects do not cause severe defects in the long bone, althoughDspp-knockout mice have obvious alveolar bone abnormalities [22, 28]. Previously, we showed that the transgenic expression ofDspprescued the dentin and alveolar bone defects ofDmp1/mice [29]. To further.