Perturbed collagen deposition and bone homeostasis in mice with an osteoblast specific deletion of the zinc receptor

We previously demonstrated that global zinc receptor (ZnR/GPR39) loss in mice disrupts bone homeostasis due to changes in matrix composition. The cellular and molecular processes underlying ZnR/GPR39 regulation of bone matrix composition remain unknown. Therefore, we hypothesized that ZnR/GPR39 activity in osteoblasts could control bone integrity. To test this hypothesis, we generated mice with the ZnR/GPR39 floxed allele and crossed them with mice expressing Cre recombinase under the control of the osteoblast-specific SP7 promoter. Then we investigated the bone structural and histological parameters in female mice with osteoblast-specific deletion of ZnR/GPR39. MicroCT analysis of vertebral trabecular bone of osteoblast specific ZnR/GPR39 knockout female mice showed a significant alteration in bone mass of these mice compared to wildtype groups. These mice had a significant increase in osteoclasts as well as a high rate of mineral formation. To determine the molecular mechanisms regulated by ZnR/GPR39 in osteoblasts we compared collagen levels in protein fractions of wildtype and ZnR/GPR39 deficient osteoblast cultures at different differentiation time points. Western blot analysis showed a consistent decrease in mature collagen levels in the extracellular matrix (ECM) of osteoblasts from ZnR/GPR39 deficient mice compared to wild type counterpart. Additionally, when ZnR/GPR39 was overexpressed in an osteoblast cell line, ECM collagen levels increased, indicating that GPR39 primarily affects collagen through regulation of collagen deposition. Furthermore, zinc pulse increased the expression of COL1A2 in normal osteoblasts but not GPR39 deficient osteoblasts. Overall, our findings imply that ZnR/GPR39 govern bone composition via regulating collagen processing and deposition by osteoblasts.