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. 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.