Sex-specific differences in direct osteoclastic versus indirect osteoblastic effects underlay the low bone mass of Pannexin1 deletion in TRAP-expressing cells in mice

Pannexin1 (Panx1) is a hemichannel-forming protein that participates in the communication of cells with the extracellular space. To characterize the role of osteoclastic Panx1 on bone, Panx1(fl/fl);TRAP-Cre (Panx1(ΔOc)) mice were generated, and compared to Panx1(fl/fl) littermates at 6 weeks of age. Total and femoral BMD was ~20% lower in females and males whereas spinal BMD was lower only in female Panx1(ΔOc) mice. μCT analyses showed that cortical bone of the femoral mid-diaphysis was not altered in Panx1(ΔOc) mice. In contrast, cancellous bone in the distal femur and lumbar vertebra was significantly decreased in both female and male Panx1(ΔOc) mice compared to Panx1(fl/fl) controls and was associated with higher osteoclast activity in female Panx1(ΔOc) mice, with no changes in the males. On the other hand, vertebral bone formation was decreased for both sexes, resulting from lower mineral apposition rate in the females and lower mineralizing surface in the males. Consistent with an osteoclastic effect in female Panx1(ΔOc) mice, osteoclast differentiation with RANKL/M-CSF and osteoclast bone resorbing activity in vitro were higher in female, but not male, Panx1(ΔOc) mice, compared to Panx1(fl/fl) littermates. Surprisingly, although Panx1 expression was normal in bone marrow stromal-derived osteoblasts from male and female Panx1(ΔOc) mice, mineral deposition by male (but not female) Panx1(ΔOc) osteoblasts was lower than controls, and it was reduced in male Panx1(fl/fl) osteoblasts when conditioned media prepared from male Panx1(ΔOc) osteoclast cultures was added to the cell culture media. Thus, deletion of Panx1 in TRAP-expressing cells in female mice leads to low bone mass primarily through a cell autonomous effect in osteoclast activity. In contrast, our evidence suggests that changes in the osteoclast secretome drive reduced osteoblast function in male Panx1(ΔOc) mice, resulting in low bone mass.