DMP1 is essential for osteocyte formation and function.

Abstract

DMP1 is an acidic phosphorylated extracellular matrix protein that was originally identified from a rat incisor cDNA library and thought to have a primary function in regulation of dentinogenesis 1 . DMP1 shares similar sequence, biochemical, and genomic DNA organizational properties with a family of proteins, called SIBLINGs for Small; Integrin-Binding LIgand, N-linked Glycoprotein. The members of this family include bone sialoprotein (BSP), osteopontin (OPN), dentin sialophosphoprotein (DSPP) and matrix extracellular phosphoglycoprotein (MEPE). All of these family members contain an RGD sequence for integrin binding and can bind to hydroxyapatite 2,3 . BSP, OPN and DMP1 can also bridge complement Factor H to cell surface receptors, an alternative complement pathway in order to prevent cell lysis 2,3 . OPN and DMP1 4 have been demonstrated to bind to CD44, a membrane bound protein thought to interact with the ERM (ezrin, radixin, moesin) family of adapter proteins that link to actin in the cytoskeleton. Our recent observations on the expression pattern of DMP1 and of the phenotype in mice lacking the DMP1 gene underscore the critical importance of DMP1 in osteocyte function. These findings include: 1) although DMP1 is expressed in all tissues that undergo mineralization, its expression in osteocytes is much higher than in any other cell type as determined by in situ hybridization, lac Z knock-in expression, and immunostaining; 2) by immunostaining, DMP1 appears to be highly abundant in the dendritic processes of osteocytes and by immuno-EM appears to be localized on the canalicular walls along the lamina limitans; 3) a dramatic increase in DMP1 expression is observed in osteocytes in response to mechanical loading both in vitro and in vivo; 4) DMP1 null mice show major abnormalities in osteocyte morphology, with a 2-fold increase in lacunar size, abnormal buckling of the membrane surface of the dendrites, loss of integrity of the lamina limitans and obliteration of the canalicular space; 5) DMP1 null mice show a severe impairment in mineralization, with patchy and poorly organized mineral and an apparent delay in the transition from osteoblasts to osteocytes; 6) mechanical loading of the ulna from DMP1 null mice at 60 Hz produces strains 1.7 times higher than the strains in wild type and heterozygous littermates, indicating a significant change in the material (elastic moduli) and/or structural (stiffness) properties of the bones; and 7) a further striking observation in these mice is the progressive change in the skeletal properties with age with bony protrusions forming over time, appearing primarily at sites of muscle insertion, suggesting an abnormal response to mechanical loading in postnatal animals. We propose that osteocytes in the DMP1 null mice exist in a hyperstimulated state, due to the mechanically compromised skeleton in these mice and that the formation of abnormal