Activation function 2 (AF2) domain of estrogen receptor-α regulates mechanotransduction during bone fracture healing in estrogen-competent mice

External mechanostimulation applied by whole-body low-magnitude high-frequency vibration (LMHFV) was demonstrated to cause no or negative effects on fracture healing in estrogen-competent rodents, while in ovariectomized (OVX), estrogen-deficient rodents bone formation after fracture was improved. Using mice with an osteoblast-specific deletion of the estrogen receptor alpha (ER alpha), we demonstrated that ER alpha signaling in osteoblasts is required for both the anabolic and catabolic effects of LMHFV during bone fracture healing in OVX and non-OVX mice, respectively. Because the vibration effects mediated by ER alpha were strictly dependent on the estrogen status, we hypothesized different roles of ligand-dependent and -independent ER alpha signaling. To investigate this assumption in the present study, we used mice with a deletion of the C-terminal activation function (AF) domain-2 of the ER alpha receptor, which mediated ligand-dependent ER alpha signaling (ER alpha AF-20). OVX and non-OVX ER alpha AF-20 animals were subjected to femur osteotomy followed by vibration treatment. We revealed that estrogen-competent mice lacking the AF-2 domain were protected from LMHFV-induced impaired bone regeneration, while the anabolic effects of vibration in OVX mice were not affected by the AF-2 knockout. RNA sequencing further showed that genes involved in Hippo/Yap1-Taz and Wnt signaling were significantly downregulated upon LMHFV in the presence of estrogen in vitro. In conclusion, we demonstrated that the AF-2 domain is crucial for the negative effects of vibration during bone fracture healing in estrogen-competent mice suggesting that the osteoanabolic effects of vibration are rather mediated by ligand-independent ER alpha signaling.