Gap junction mediated regulation of osteocytes to osteoblastic alkaline phosphatase activity is independent of microgravity

Decreased bone formation is one of the main causes of bone loss under microgravity. As a mechanical perceiver, osteocyte regulates bone formation by sending mechanical signals to osteoblast. However, the regulation of osteoblastic bone formation by osteocytes is less known so far under the microgravity conditions. The aim of this study was to investigate the regulation of bone formation/loss by detection of the osteocytic regulatory effects on the bone-specific alkaline phosphatase (ALP) activity of osteoblasts under altered gravitational environment. The altered gravitational environment was provided by a large gradient high magnetic field that could produce high magneto-gravitational environment (HMGE) and provided three apparent gravity levels (mu g, 1g, and 2g). After the MLO-Y4 and 2T3 cells were cocultured in direct physical contact for 24 h and established gap junction intercellular communication under HMGE, a highly significant rapid increase in ALP activity was observed in mu g, 1g, 2g, and control group (p < 0.001). Conversely, application of 50-mu M beta-glycyrrhetinic acid gap junction inhibitor or remote coculture significantly decreased the ALP activity. There were no obvious differences of osteoblastic 2T3 ALP activity among mu g, 1g, 2g, and control groups. Based on these findings, we interpreted that gap junction was probably the main route of osteoblastic ALP activity regulation by osteocyte. Gravity had no significant impact on the osteoblastic ALP activity regulated by osteocyte in a short period of time.