Pre-osteoclast, as an another mechano-sensing cell, has a critical role in mechano-shaping of skeleton

It is unclear that osteoclast lineages can sense mechanical stimuli, mainly by lack of information about the presence or absence of mechano-sensing organelles in these cells. Primary cilium is a microtubule-based organelle, which can translate mechanical loading signals into biochemical and transcriptional responses. We first identified the presence of primary cilia in tartrate-resistant acid phosphatase (TRAP)-positive mononuclear cells (preosteoclasts) prior to form TRAP-positive multinuclear cells (osteoclasts), suggesting that preosteoclasts can sense mechanical stimuli. To understand a role of primary cilia in preosteoclasts, we generated osteoclast-specific Ift88 or Kif3a knockout Cathepsin K-Cre transgenic mice. Deletion of Ift88 or Kif3a reduced the number of cilia in preosteoclasts. Micro-CT analysis display that mice with conditional deletion of Ift88 or Kif3a in osteoclast lineages led to a decrease in femoral cortical bone tissue area and bone marrow area under exercise conditions, implying that disruption of primary cilia in preosteoclasts/osteoclasts narrowed the femoral cortical bone shape. In contrast, the trabecular and cortical bone mass was not altered in these mice. Mechanistic studies showed that shear stress suppressed osteoclastogenesis by c-fos degradation. Also, shear stress increased periostin levels in conditioned media from preosteoclasts, resulting in increased differentiation and bone nodule formation of osteoblasts through activation of LRP6/?-catenin. All shear stress-induced changes were near completely blocked by knock-downs of Ift88 and Kif3a in preosteoclasts. These findings first suggest that preosteoclasts may sense mechanical stimuli with a primary cilium, and that mechanical stress-mediated primary cilia activation of preosteoclasts may play an important role in controlling bone modeling and shaping by modulating both bone resorption and formation. Overall design: We investigated the role of Ift88 in osteoclast-lineage cells upon shear stress stimulation. We generated TRAP positive mononucleated cells (pre-osteoclasts) by knockdown with Ift88 shRNA and then treating the cells with MCSF and RANKL for 2 days. Then, we applied orbital shear stress stimulation to TRAP positive mononucleated cells and analyzed the gene expression regulated compared to control through RNA-seq. In addition, gene expression regulated after orbital shear stress stimulation in Ift88 knockdown cells were also obtained through RNA-seq analysis.