Single-Gene CaMKIIγ or CaMKIIδ Conditional Deletion in Osteoblast-Lineage Cells Does Not Recapitulate Severe Skeletal Phenotype of CaMKIIγ/δ Double Conditional Deletion

Our prior work has demonstrated a role for calcium/calmodulin-dependent protein kinase II (CaMKII) as a signal integrator in osteocytes, especially in the context of mechanical loading. Subsequently, we reported the profound skeletal impacts of conditionally co-deleting both CaMKII γ (gamma) and CaMKIIδ (delta), the two isoforms of CamKII expressed in bone, in osteocytes and mature osteoblasts using the Osteocalcin Cre driver. Camk2delta/gamma double conditional knockout (DCKO) mice express a severe post-natal phenotype of extreme skeletal fragility and low bone mass in the appendicular and axial skeleton, with significant deficits in nearly all measured bone parameters at eight weeks of age. Given the severity of the phenotype of these double cKO mice, we have now developed independent CaMKII γ and CaMKIIδ Osteocalcin Cre conditional knockout (cKO) models to determine the individual roles of the Camk2delta and Camk2gamma isoforms in bone. Preliminary data suggest that the severe phenotype observed in Camk2delta/gamma dCKO mice is not due to the additive loss of CaMKII γ and CaMKIIδ expression. Instead, Camk2delta and Camk2gamma cKO each result in a mild, but not identical, phenotype. Micro-computed tomography has demonstrated that Camk2delta/gamma DCKO experience a ~60-80% decrease in femoral and spinal trabecular bone volume compared to littermate controls, while no significant loss is observed in Camk2delta or Camk2gamma cKO mice. Despite the severe loss of cortical bone mass in Camk2delta/gamma DCKO mice, femoral cortical parameters are decreased only in Camk2delta cKO females, with no significant change in Camk2delta cKO males or Camk2gamma cKO mice of either sex. Mineralization capacity of new bone is greatly diminished in Camk2delta/gamma DCKO mice and mechanical properties testing via 4-point bending has demonstrated that Camk2delta/gamma DCKO femurs are substantially less resistant to force than littermate controls. 4-point bending of Camk2delta or Camk2gamma cKO femurs has revealed diminished resistance to force in only a few parameters, and does not reach the severity of fragility expressed by Camk2delta/gamma DCKO femurs. Together, these data suggest that CaMKII γ and CaMKIIδ have compensatory or redundant functions, as has been observed in other tissues, and that the loss of either isoform in osteocytes and mature osteoblasts does not recapitulate the severe phenotype expressed due to double conditional deletion. Further work is underway to characterize differences in mineralization, endocrine signaling, and mineral homeostasis. In total, CaMKII appears to be fundamentally important to post-natal bone mass and quality, and further investigation may reveal a novel target for clinical intervention during loss of bone mass. AR071614 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.