Mechanical loading under estrogen deficiency enhances mineralization and osteoblast maturation in an advanced 3D vascularized bone organoid driven by hypertrophy and apoptosis

Bone is a highly mechanosensitive tissue that responds to mechanical cues and adapts to maintain a homeostatic balance between bone formation and resorption, thereby preserving structural integrity and strength. Estrogen is the key modulator of skeletal homeostasis that regulates the mechanosensitivity and survival of osteocytes, promotes osteoblast activity and reduces osteoclastogenesis. The decline in circulating estrogen after menopause leads to increased bone resorption, widely associated with postmenopausal osteoporosis, but is also associated with vascular dysregulation, mineral heterogeneity and impaired osteocyte mechanobiology. Recently, we developed a vascularized and mineralized human in vitro bone model and applied this model to mimic postmenopausal estrogen withdrawal. This model provided a mechanistic understanding of changes in vascularization and bone mineralization in estrogen deficiency, which were associated with hypertrophy and apoptosis. However, this model did not account for mechanical loading, so it was unclear how mechanobiological factors would affect the pathophysiological responses of the model under estrogen withdrawal. So, here we advance our in vitro vascularized bone model by incorporating cyclic physiologically relevant mechanical stimulation using a bioreactor. The results revealed that mechanical loading with estrogen (healthy condition) enhanced mineral production, alongside physiological hypertrophy, apoptosis and vascularization. In contrast, mechanical loading during estrogen withdrawal (disease condition) increased collagen I and accelerated the maturation of osteoblasts (downregulated Runx2/Opn, upregulated DMP1 gene expression), which was associated with pathological hypertrophy and apoptosis. This study provides a translationally relevant platform to study bone mechanobiology in healthy and disease states and for preclinical testing of therapeutic strategies targeting osteogenic dysregulation due to estrogen deficiency.