Mechanical loading is essential for skeletal growth, maintenance, and repair, yet the mechanisms that regulate bone mechanoadaptation remain incompletely understood. Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), downstream of the Hippo pathway, are emerging regulators of bone cell biology, but their in vivo role in skeletal mechanoadaptation has not been fully defined.

To address this, we combined pharmacological inhibition of YAP/TAZ with models of prenatal and postnatal C57BL/6 mice. We began with a short in vivo study to test the effectiveness of verteporfin (VP) in suppressing YAP/TAZ activity and to optimize dynamic histomorphometry and high-resolution osteocyte imaging. VP is a small-molecule inhibitor that interferes with YAP/TAZ–TEAD transcriptional activity. Next, we examined prenatal load-induced bone formation using an ex vivo bioreactor system, in which E15.5 embryonic hindlimbs were subjected to cyclic compression to induce ossification of the primary centers. Samples were also cultured in media containing VP to test whether YAP/TAZ inhibition suppresses load-induced prenatal bone formation.
Finally, we investigated adult mechanoadaptation using a cyclic compressive in vivo tibial loading model in C57BL/6 mice. Animals were injected with DMSO (vehicle), VP, or MGH-CP1 (CP1) and loaded over two weeks. CP1 is a TEAD autopalmitoylation inhibitor that suppresses YAP/TAZ–TEAD transcriptional activity.

Our results demonstrated that YAP/TAZ inhibition abrogated load-induced bone formation. Prenatally, rudiment mineral lengthening was suppressed. In adults, the anabolic effects of loading—increased periosteal and endosteal bone formation parameters, cortical thickness, and trabecular architecture were supprerssed when YAP/TAZ were inhibited. Mechanical loading increased the fraction of positively labeled osteocyte lacunae and enhanced osteocyte cytoskeletal branching, but these effects were abolished under VP or CP1 treatment. Finally, we confirmed the mechanosensitivity of bone marrow cells: loading upregulated Cyr61 and Ctgf, well-established YAP/TAZ target genes, but this response was suppressed by VP or CP1.

These findings establish YAP/TAZ as critical regulators of load-induced bone formation across developmental stages. By coordinating mechanosensitive responses among osteoblasts, osteoclasts, osteocytes, and marrow stromal cells, YAP/TAZ act as central regulators of skeletal mechanotransduction. This work not only provides mechanistic insight into the cellular basis of bone mechanoadaptation but also highlights YAP/TAZ as potential therapeutic targets in conditions of skeletal fragility, including osteoporosis and osteogenesis imperfecta.