The current primary cilium-related diseases are relatively rare, genetic disorders (e.g. Bardet-Biedl syndrome, Meckel-Gruber syndrome, and polycystic kidney disease), however, it is possible that primary cilia play a role in complex, multi-factorial diseases as well. For example, osteoporosis, a common and costly disease that fundamentally results from an imbalance of bone formation and bone resorption, can result from low levels of sex hormones, vitamin and mineral deficiencies, and lack of physical loading 
. In this study, we found that loading of Colα1(I) 2.3-Cre;Kif3afl/fl
mutant ulnae resulted in reduced loading-induced bone formation when compared to controls. We have previously shown that cultured osteoblasts and osteocytes respond to mechanical stimulation with primary cilia-dependent osteogenic gene expression 
. Taken together these data support the idea that primary cilia may be more important in formation of bone in adults and in complex multi-factorial bone diseases, such as osteoporosis, than was previously appreciated.
In addition to their established role in development, our study provides further evidence that primary cilia play a role in maintaining adult tissue, specifically adult bone, due to mechanical loading. Recently, mice bearing global mutations in Pkd1 have been shown to form less cortical and trabecular bone with lower bone density and reduced osteogenic gene expression during skeletal development 
. Further evidence suggests that Pkd1 might mediate sensing of mechanical stresses that occur with skeletal development 
. In our study, there was no skeletal defect resulting from disrupting IFT in osteocytes and osteoblasts, suggesting that bone primary cilia do not participate in skeletogenesis. However, our loading results demonstrate that these primary cilia play an important role in mechanosensing in adult bone. There is precedence for the mechanisms of skeletal development and skeletal remodeling being distinct in the osteopontin-dependent response to mechanical loading 
and integrin-dependent responses to mechanical loading and unloading 
. Thus, although the mechanisms of ciliary mechanotransduction in adult bone are not fully understood, our findings suggest that the primary cilium plays a significant role in the connection between extracellular sensing and bone formation in adult bone.
While numerous in vivo
studies have shown that primary cilia act as chemosensors in several important chemical signaling pathways, there has been less in vivo
evidence that primary cilia are involved in mechanosensing. Studies of primary cilia and mechanosensing have shown that there is a primary cilium-dependent Ca2+
response when cultured kidney cells are mechanically stimulated 
. More recently, ex vivo
studies of primary cilia in the embryonic node and liver ducts implicated primary cilia as sensors of fluid flow 
. In addition, Xiao et al. examined bone morphology of mice heterozygous for mutation of pkd1
, a gene that encodes polycystin 1 
; polycystin 1 is a transmembrane protein that is found at the cilium, and the polycystin 1/2 complex is thought to have mechanosensing capabilities 
. They found that these mice are osteopenic; however, mechanical loads were not applied directly 
. Our results provide the first direct in vivo
evidence that primary cilia in bone sense physical extracellular signals. They extend prior studies by demonstrating that primary cilia in bone sense physical extracellular signals and are important in cellular mechanosensing in bone in adults via a mechanism that might be distinct from the role of Pkd1 in skeletal development.
Because disrupting intraflagellar transport via the deletion of Kif3a may or may not result in missing or stunted primary cilia, demonstration of ciliary dysfunction is challenging. In vivo imaging of primary cilia would be inconclusive, and the associated technical difficulties have greatly limited the assessment of primary cilia in bone. Culturing primary osteocytes is another strategy, but the isolation of these cells and their return to a proliferative state would again make presence or absence of primary cilia, per se, inconclusive. Thus, verification of the Kif3a-deleted genotype, as shown in this study, may be the more effective approach.
This work raises a number of intriguing points for future research. There are a number of studies showing that disruption of IFT has led to a clear breakdown in signaling pathways, such as the hedgehog signaling 
and Wnt signaling 
. In this study, we showed that response to loading was decreased in Colα1(I) 2.3-Cre; Kif3afl/fl
mice, and indeed, a number of studies have explored primary cilium-dependent mechanotransduction pathways in musculoskeletal systems 
. However, we did not link disruption of any specific pathway to the deletion of Kif3a or primary cilia disruption in this study. In addition, while Kif3a is widely accepted to be essential to IFT in the primary cilium 
and Kif3a deletion to be a method of disrupting cilia formation and function 
, there are studies that show that Kif3a is associated with non-ciliary microtubules in some cell types 
. Finally, there are a number of potential mechanosensors in bone, including integrins 
and mechanically-activated membrane channels 
. The existence of other mechanosensors is suggested by our studies since deletion of the Kif3a resulted in a decrease in loading-induced response when compared to wild-type mice, rather than a failure to respond to loading.
In summary, these findings represent an in vivo demonstration of a link between the disruption of primary cilia function and decreased bone mechano-responsiveness, suggesting that the primary cilium acts as cellular mechanosensors. Our results also suggest that primary cilia may play a broader role in regulating bone formation in adults and in complex multi-factorial bone diseases than was previously appreciated.