Studies of Dentin Matrix Protein 1 (DMP1) Regulation and Function in Vivo

Dentin matrix protein 1 (DMP1) is a highly phosphorylated extracellular matrix protein, predominantly expressed in osteocytes in bone. Full-length DMP1 is enzymatically processed into a 37 kDa N- and a 57 kDa C-terminal fragment in vivo. The goal of this study was to identify the cis-regulatory regions that control osteoctye-specific expression of the Dmpl gene, to determine the in vivo function of the 57 kDa C-terminal fragment and to determine the role of DMP1 in control of phosphate homeostasis.

In vitro transient transfection assays showed that a 2.4-kb cis-regulatory region of the Dmpl gene contains basal promoter activity and that a 9.6-kb cis-regulatory region has the highest promoter activity. Transgenic mice bearing a lacZ reporter driven by these two different cis-regulatory regions separately were then generated and characterized. The region between -9.6 and -2.4-kb of the Dmpl gene has been identified as an osteocyte-specific module, which confers the osteocyte-specific expression of the lacZ reporter gene in vivo.

Dmpl-null mice display a rachitic bone phenotype, elevated circulating FGF23 levels and hypophosphatemia. Transient transfection of the full-length DMP1 into various cell lines showed that DMP1 is processed similarly in vitro as in vivo into 37 kDa and 57 kDa fragments. Targeted re-expression of the full-length DMP1 or the 57 kDa fragment driven by a 3.6-kb Collal promoter in Dmpl-null mice rescues the skeletal abnormalities, suggesting that the 57 kDa fragment is the essential functional domain of DMP1.

FGF23, a potent phosphaturic hormone, was sharply increased in Dmpl-null osteocytes and serum and re-expression of DMP1 restored FGF23 to normal. Furthermore, a high phosphate diet rescued the rachitic phenotype in the growth plate and dramatically improved mineralization abnormalities in Dmpl-null mice. However, co-expression of PHEX, a membrane protease whose mutations lead to a similar phenotype as that of Dmpl-null mice, and DMP1 showed that PHEX had no effect on DMP1 cleavage in vitro, suggesting that both genes function independently in control of phosphate homeostasis.

In conclusion, the 57 kDa C-terminal fragment is the essential functional domain of DMP1 which plays a critical role in control of phosphate homeostasis through regulation of FGF23.

Year	Entry
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Year

Entry

2004

Shimada T, Hasegawa H, Yamazaki Y, Muto T, Hino R, Takeuchi Y, Fujita T, Nakahara K, Fukumoto S, Yamashita T. FGF-23 is a potent regulator of vitamin D metabolism and phosphate homeostasis. J Bone Miner Res. March 2004;19(3):429-435.

2005

Li X, Zhang Y, Kang H, Liu W, Liu P, Zhang J, Harris SE, Wu D. Sclerostin binds to LRP5/6 and antagonizes canonical Wnt signaling. J Biol Chem. May 20, 2005;280(20):19883-19887.

2005

Poole KES, Van Bezooijen RL, Loveridge N, Hamersma H, Papapoulos SE, Löwik CW, Reeve J. Sclerostin is a delayed secreted product of osteocytes that inhibits bone formation. FASEB J. August 2005;19(10):1842-1844.

1977

Parfitt AM. The cellular basis of bone turnover and bone loss: a rebuttal of the osteocytic resorption—bone flow theory. Clin Orthop Relat Res. September 1977;127:236-247.

2004

van Bezooijen RL, Roelen BAJ, Visser A, van der Wee-Pals L, de Wilt E, Karperien M, Hamersma H, Papapoulos SE, ten Dijke P, Löwik CWGM. Sclerostin is an osteocyte-expressed negative regulator of bone formation, but not a classical BMP antagonist. J Exp Med. March 15, 2004;199(6):805-814.