The impact of ciliary length on the mechanical response of osteocytes to fluid shear stress

Background: Osteocytes are crucial for detecting mechanical stimuli and translating them into biochemical responses within the bone. The primary cilium, a cellular 'antenna,' plays a vital role in this process. However, there is a lack of direct correlation between cilium length changes and osteocyte mechanosensitivity changes. This study aims to reveal the relationship between ciliary length and nitric oxide (NO) release in osteocytes to show how primary cilia may be involved in reducing osteocyte mechanosensitivity caused by microgravity. Materials and methods: We used the MLO-Y4 cell line and primary osteoblasts to adjust the ciliary length using chloral hydrate (CH) for shortening and lithium ions (Li⁺) for elongation. We then examined the impact of varied ciliary lengths on osteocyte response to fluid shear stress, focusing on the PC1/PC2–Ca²⁺-NO signaling pathway. Co-culture systems assessed downstream effects on osteoblast function, including collagen secretion and mineralization. Results: We observed a significant correlation between ciliary length and osteocyte mechanosensitivity, with longer primary cilia enhancing Ca²⁺ influx and NO release in response to fluid shear stress. However, contrary to expectations, calmodulin (CaM) expression did not increase with ciliary length, suggesting alternative pathways, such as PKC or Akt/PKB, may modulate p-eNOS activity. Co-cultured osteoblasts showed altered osteogenic functions regulated by osteocyte-derived signals influenced by primary cilia length. Conclusion: Our findings clarify the role of primary cilia length in modulating osteocyte mechanosensitivity and their influence on osteoblast function, highlighting a complex regulatory network that may not solely rely on CaM for NO release. These insights contribute to a deeper understanding of bone mechanotransduction and could have implications for developing therapeutic targets for osteocyte-related disorders.