Bioprinted bonoid-on-a-chip model for studying drug effects on human osteocyte differentiation

Osteocytes form an extensive dendritic network within bone tissue that senses mechanical and biochemical stimuli, critical for bone homeostasis. However, the complex interplay between mechanotransduction, oxygen tension, and osteocyte differentiation remains difficult to study due to the lack of physiologically relevant humanin vitromodels. Previously, we developed a microphysiological system (MPS) that integrates perfusion, mechanical loading, and oxygen control within a culture platform that enables the culture of bone-like three-dimensional (3D) constructs [1]. Here, we demonstrate the application of this MPS as Bonoid-on-a-chip (BoC)-platform to investigate osteoblast-to-early osteocyte transition and mechanosensitive signalling in a human-relevant 3D context, under different physical and chemical conditions. We generated bonoids by bioprinting primary human osteoblasts into a gelatine-hydrogel and cultured them under either static conditions or as dynamic BoC. Different BoC environments consisting in controlled perfusion (0.5 ml min^-1), mechanical loading (0% or 10% compression, 1 Hz frequency), and physiological oxygen tension (21% or 12%) over a seven-day period were compared. These different models were additionally treated with dexamethasone (DEXA), to evaluate early osteocyte-targeted effects of drug exposure under these different conditions. We observed high cell viability across all conditions, but clear differences in metabolic activity, morphological changes, and mechanotransduction markers between static and dynamic cultures. Notably, cells within the BoC-platform exhibited extended dendritic processes and significantly increased secretion of prostaglandin E2 and nitrite in response to mechanical stimulation - hallmarks of osteocytic signalling in mechanotransduction. When testing the new BoC as platform for early osteocyte-targeted investigations, we found DEXA exposure disrupting this mechanoresponsive phenotype, highlighting the model's applicability for pharmacological screening. These findings demonstrate the value of our BoC-platform as a human-relevant model for studying bone mechanobiology and drug responsesin vitro. This approach offers a promising alternative to animal models in bone research and opens new avenues for investigating osteocyte function under physiologically dynamic conditions.