Entpd5 is essential for skeletal mineralization and regulates phosphate homeostasis in zebrafish

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-offunction
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.