Huitema, L.F., Apschner, A., Logister, I., Spoorendonk, K.M., Bussmann, J., Hammond, C.L., and Schulte-Merker, S. (2012) Entpd5 is essential for skeletal mineralization and regulates phosphate homeostasis in zebrafish. Proceedings of the National Academy of Sciences of the United States of America. 109(52):21372-21377.
Bone mineralization is an essential step during the embryonic development of vertebrates, and bone serves vital functions
in human physiology. To systematically identify unique gene functions essential for osteogenesis, we performed a forward genetic
screen in zebrafish and isolated a mutant, no bone (nob), that does not form any mineralized bone. Positional cloning of nob identified the causative gene to encode ectonucleoside triphosphate/diphosphohydrolase 5 (entpd5); analysis of its expression pattern demonstrates that entpd5 is specifically expressed in osteoblasts. An additional mutant, dragonfish (dgf), exhibits ectopic mineralization in the craniofacial and axial skeleton and encodes a loss-of-function allele of ectonucleotide pyrophosphatase phosphodiesterase 1 (enpp1). Intriguingly, generation of double-mutant nob/dgf embryos restored skeletal mineralization in nob mutants, indicating that mechanistically, Entpd5 and Enpp1 act as reciprocal regulators of phosphate/pyrophosphate homeostasis
in vivo. Consistent with this, entpd5 mutant embryos can be rescued by high levels of inorganic phosphate, and phosphate-regulating factors, such as fgf23 and npt2a, are significantly affected in entpd5 mutant embryos. Our study demonstrates that Entpd5 represents a previously unappreciated essential player in phosphate homeostasis
and skeletal mineralization.