Osteocalcin, also known as bone γ-carboxyglutamate
protein (Bglap), is expressed by osteoblasts and is commonly used as a
clinical marker of bone turnover. A mouse model of
osteocalcin deficiency has implicated
osteocalcin as a mediator of changes to the skeleton, endocrine system, reproductive organs and central nervous system. However, differences between mouse and human
osteocalcin at both the genome and
protein levels have challenged the validity of extrapolating findings from the
osteocalcin-deficient mouse model to human disease. The rat
osteocalcin (Bglap) gene locus shares greater synteny with that of humans. To further examine the role of
osteocalcin in disease, we created a rat model with complete loss of
osteocalcin using the CRISPR/Cas9 system. Rat
osteocalcin was modified by injection of CRISPR/Cas9
mRNA into the pronuclei of fertilized single cell Sprague-Dawley embryos, and animals were bred to homozygosity and compound heterozygosity for the mutant alleles. Dual-energy X-ray absorptiometry (DXA),
glucose tolerance testing (GTT),
insulin tolerance testing (ITT), microcomputed tomography (µCT), and a three-point break biomechanical assay were performed on the excised femurs at 5 months of age. Complete loss of
osteocalcin resulted in bones with significantly increased trabecular thickness, density and volume. Cortical bone volume and density were not increased in null animals. The bones had improved functional quality as evidenced by an increase in failure load during the biomechanical stress assay. Differences in
glucose homeostasis were observed between groups, but there were no differences in
body weight or composition. This rat model of complete loss of
osteocalcin provides a platform for further understanding the role of
osteocalcin in disease, and it is a novel model of increased bone formation with potential utility in
osteoporosis and
osteoarthritis research.