The identity of the cell type responsive to sclerostin, a negative regulator of 
bone mass, is unknown. Since sclerostin is expressed in vivo by mineral-embedded 
osteocytes, we tested the hypothesis that sclerostin would regulate the behavior 
of cells actively involved in mineralization in adult bone, the preosteocyte. 
Differentiating cultures of human primary osteoblasts exposed to recombinant 
human sclerostin (rhSCL) for 35 days displayed dose- and time-dependent 
inhibition of in vitro mineralization, with late cultures being most responsive 
in terms of mineralization and gene expression. Treatment of advanced (day 35) 
cultures with rhSCL markedly increased the expression of the preosteocyte marker 
E11 and decreased the expression of mature markers DMP1 and SOST. Concomitantly, 
matrix extracellular phosphoglycoprotein (MEPE) expression was increased by 
rhSCL at both the mRNA and protein levels, whereas PHEX was decreased, implying 
regulation through the MEPE-ASARM axis. We confirmed that mineralization by 
human osteoblasts is exquisitely sensitive to the triphosphorylated ASARM-PO4 
peptide. Immunostaining revealed that rhSCL increased the endogenous levels of 
MEPE-ASARM. Importantly, antibody-mediated neutralization of endogenous 
MEPE-ASARM antagonized the effect of rhSCL on mineralization, as did the PHEX 
synthetic peptide SPR4. Finally, we found elevated Sost mRNA expression in the 
long bones of HYP mice, suggesting that sclerostin may drive the increased 
MEPE-ASARM levels and mineralization defect in this genotype. Our results 
suggest that sclerostin acts through regulation of the PHEX/MEPE axis at the 
preosteocyte stage and serves as a master regulator of physiologic bone 
mineralization, consistent with its localization in vivo and its established 
role in the inhibition of bone formation.