Primary cilia shortening alters osteocyte mechanotransduction: Spaceflight vs. simulated microgravity

Microgravity conditions in space lead to bone loss in the weight-bearing bones of astronauts, with alterations in osteocyte mechanotransduction considered a key cause of this weightlessness-induced bone loss. The primary cilia of osteocytes, which project from their surface, can sense fluid flow and convert shear stress signals into biochemical responses. Our previous studies demonstrated a reduction in both the number of ciliated cells and the ciliary length of MLO-Y4 osteocytes under clinostat-induced simulated microgravity (SMG). In this study, we investigated the effects of simulated microgravity on the transport velocity of intraflagellar transport proteins within cilia and further examined how osteocyte ciliary shortening impacts the downstream Ca²⁺-Calmodulin-NO signaling pathway and subsequent osteogenic regulatory functions. Our results demonstrated that SMG significantly reduced IFT protein trafficking speed in primary cilia. Ciliary shortening was also associated with suppressed downstream osteogenic regulation in osteocytes. To validate these findings, we conducted a 5-day in-orbit experiment of MLO-Y4 osteocytes during the Shenzhou-16 manned mission aboard the China Space Station. Notably, while osteocytes under actual space microgravity exhibited impaired ciliogenesis, they showed no significant reduction in ciliary length, which was inconsistent with the phenotypes under clinostat-induced SMG. Our study reveals that the clinostat-based SMG may not fully replicate the altered mechanotransduction of osteocytes under space microgravity, while underscoring the value of experimental validation in orbital microgravity for advancing space mechanobiology.