Novel sandwich ELISAs for rat DMP1: Age-related decrease of circulatory DMP1 levels in male rats

Dentin matrix protein 1 (DMP1), a noncollagenous bone matrix protein produced by osteocytes, regulates matrix mineralization and phosphate homeostasis. The lack of a precise assay for circulating DMP1 levels impairs further investigation of the protein's biological significance. Because full-length precursor DMP1 is cleaved into NH2- and COOH-terminal fragments during the secretory process, we developed two new sandwich ELISAs for the NH2- and COOH-terminal fragments of rat DMP1. One of these ELISAs, ELISA 1-2, is based on two affinity-purified polyclonal antibodies against the DMP1-1 and DMP1-2 peptides of the NH2-terminal fragment, whereas the other, ELISA 4-3, is based on two affinity-purified polyclonal antibodies against the DMP1-3 and DMP1-4 peptides of the COOH-terminal fragment. The polyclonal antibodies were characterized in immunohistochemical and liquid chromatography-electrospray ionization tandem mass spectrometry (LC-MS/MS) studies. Immunohistochemical analyses of rat bone using these polyclonal antibodies revealed DMP1 immunoreactivity in osteocytes and pericanalicular matrix, consistent with the previously reported osteocyte-specific expression of DMP1. LC-MS/MS analyses of rat plasma-derived immunoreactive products affinity-extracted with these antibodies revealed the presence of DMP1 in circulating blood. The ELISAs established with these antibodies met accepted standards for reproducibility, repeatability, precision, and accuracy. Circulating DMP1 and levels of other biochemical markers (osteocalcin, Trap5b, Dkk-1, and SOST) were measured in 2-, 4-, 8-, 12-, 18-, 24-, 72-, and 96-week-old Wistar male rats. Circulating DMP1 levels determined by ELISAs 1-2 and 4-3 significantly decreased with age. During rapid skeletal growth (2-12 weeks), DMP1 levels measured by ELISA 4-3 were over three times higher than those measured by ELISA 1-2; however, DMP1 levels in old animals (72 and 96 weeks) were almost the same when measured by either ELISA. DMP1 levels determined by both ELISAs were most highly positively correlated with the level of Dkk-1, second most highly correlated with the level of osteocalcin, and less highly correlated with the levels of Trap5b and SOST. These novel sandwich ELISAs for rat DMP1 are highly specific and allow precise measurements of circulating DMP1, which may be a new biochemical marker for osteocyte-mediated bone turnover.