An Osteocalcin-deficient mouse strain without endocrine abnormalities
Authors: Diegel Cassandra R. , Hann Steven , Ayturk Ugur M. , Hu Jennifer C.W. , Lim Kyung-eun , Droscha Casey J. , Madaj Zachary B. , Foxa Gabrielle E. , Izaguirre Isaac ,   VARI Vivarium and Transgenics Core, Paracha Noorulain , Pidhaynyy Bohdan , Dowd Terry L. , Robling Alexander G. , Warman Matthew L. , Williams Bart O. 
Published: August 2019

Osteocalcin (OCN), the most abundant non-collagenous protein in the bone matrix, is reported to be a bone-derived endocrine hormone with wide-ranging effects on many aspects of physiology, including glucose metabolism and male fertility. Many of these observations were made using an OCN-deficient mouse allele (Osc-) in which the 2 OCN-encoding genes in mice, Bglap and Bglap2, were deleted in ES cells by homologous recombination. Here we describe mice with a new Bglap and Bglap2 double knockout (dko) allele (Bglap/2p.Pro25fs17Ter) that was generated by CRISPR/Cas9-mediated gene editing. Mice homozygous for this new allele do not express full length Bglap or Bglap2 mRNA and have no immunodetectable OCN in their plasma. FTIR imaging of cortical and trabecular bone in these homozygous knockout animals finds alterations in the crystal size and maturity of the bone mineral, hydroxyapatite, compared to wild-type littermates; however, μCT and 3-point bending tests do not find differences from wild-type littermates with respect to bone mass and strength. In contrast to the previously reported OCN-deficient mice with the Osc- allele, blood glucose levels and male fertility in the OCN-deficient mice with Bglap/2pPro25fs17Ter allele did not have significant differences from wild-type littermates. We cannot explain the absence of endocrine effects in mice with this new knockout allele. Potential explanations include effects of each mutated allele on the transcription of neighboring genes, and differences in genetic background and environment. So that our findings can be confirmed and extended by other interested investigators, we are donating this new Bglap and Bglap2 double knockout strain to The Jackson Laboratory for academic distribution.

Author Summary

Cells that make and maintain bone express proteins that function locally or systemically. The former proteins, such as type 1 collagen, affect the material properties of the skeleton while the latter proteins, such as fibroblast growth factor 23, enable the skeleton to communicate with other organ systems. Mutations that affect the functions of most bone cell expressed proteins cause diseases that have similar features in humans and other mammals, such as mice; for example, brittle bone diseases for type 1 collagen mutations and hypophosphatemic rickets for fibroblast growth factor 23 mutations.

Our study focuses on another bone cell expressed protein, osteocalcin, which has been suggested to function locally to affect bone strength and systemically as hormone. Studies using osteocalcin knockout mice led other investigators to suggest endocrine roles for osteocalcin in regulating blood glucose levels, male fertility, muscle mass, brain development, behavior and cognition. We therefore decided to generate a new strain of osteocalcin knockout mice that could also be used to investigate these non-skeletal effects.

To our surprise the osteocalcin knockout mice we created do not significantly differ from wild-type mice for the 3 phenotypes we examined: bone strength, blood glucose levels, and male fertility. Our data are consistent with findings from osteocalcin knockout rats, but inconsistent with data from the original osteocalcin knockout mice. Because we do not know why our new strain of osteocalcin knockout mice fails to recapitulate phenotypes previously reported for another knockout mouse stain, we have donated our mice to a public repository so that they can be easily obtained and studied in other academic laboratories.