Prx1-Expressing Progenitor Primary Cilia Mediate Bone Formation in response to Mechanical Loading in Mice

Abstract

Increases in mechanical loading can enhance the addition of new bone, altering geometry and density such that bones better withstand higher forces. Bone-forming osteoblasts have long been thought to originate from progenitors, but the exact source is yet to be identified. Previous studies indicate osteogenic precursors arise from Prx1-expressing progenitors during embryonic development and adult fracture repair. However, it is unknown whether this cell population is also a source for mechanically induced active osteoblasts. We first identified that Prx1 is expressed in skeletally mature mouse periosteum, a thin tissue covering the surface of the bone that is rich in osteoprogenitors. We then traced Prx1 progenitor lineage using a transgenic mouse model carrying both a Prx1-driven tamoxifen-inducible Cre and a ROSA-driven lacZ reporter gene. Cells that expressed Prx1 when compressive axial loading was applied were detected within the cortical bone days after stimulation, indicating osteocytes are of Prx1-expressing cell origin. In addition, we evaluated how these cells sense and respond to physical stimulation in vivo by disrupting their primary cilia, which are antenna-like sensory organelles known to enhance mechanical and chemical signaling kinetics. Although Prx1-driven primary cilium disruption did not affect osteoblast recruitment to the bone surface, the relative mineral apposition and bone formation rates were decreased by 53% and 34%, respectively. Thus, this cell population contributes to load-induced bone formation, and primary cilia are needed for a complete response. Interestingly, Prx1-expressing progenitors are easily extracted from periosteum and are perhaps an attractive alternative to marrow stem cells for bone tissue regeneration strategies.

Loss of Adenylyl Cyclase 6 in Leptin Receptor‐Expressing Stromal Cells Attenuates Loading‐Induced Endosteal Bone Formation

“…This diminished mechanoadaptive response is in agreement with work examining a global knockout of AC6 , where AC6 deletion resulted in an inhibited response to ulna loading, ( 29 ) and further strengthens the potential involvement of the primary cilium, to which AC6 localizes, in bone mechanoadaptation. ( 41,42 ) Furthermore, as with the Lepr‐cre;tdTomato mouse, no change was found in the percentage of Lepr‐cre;tdTomato + cells on the bone surface or embedded within the bone. The lack of bone formation and the failure of loading to induce migration of LepR + cells from the marrow to the bone surface in Lepr‐cre;tdTomato;AC6 fl/fl mice are consistent with our hypothesis that loading‐induced bone formation occurs via Lepr‐cre;tdTomato + cells, and that this process requires AC6.…”

Influence of the TGF-β Superfamily on Osteoclasts/Osteoblasts Balance in Physiological and Pathological Bone Conditions

“…The source of osteoblast progenitors in vivo is still under debate. They can be found in bone marrow (MSCs accounting for 0.001 to 0.01 % nucleated cells) and periosteum [ 20 , 21 ]. Recently, new osteoprogenitors called transcortical perivascular cells (2–3% of Lin − cells from the digested cortical bone fraction) were identified [ 22 ].…”

Periosteum progenitors could stimulate bone regeneration in aged murine bone defect model

“…To investigate the degree of Prx1 + MSC participating in bone regeneration in young and aged mice, 4 weeks and 12 months old Prx1CreER-GFP;Rosa26 tdTomato mice (n = 3 per group) were received intraperitoneal injections of 75 mg/kg bodyweight tamoxifen (Sigma-Aldrich) for 5 days before closed left femoral bone fracture was made. 20 At 2 weeks after surgery, the mice were killed, and the femurs were harvested for immunofluorescence.…”

“…18 These Prx1 positive cells (Prx1 + MSCs) have the potential of osteogenic and chondrogenic differentiation and are essential for limb development and bone regeneration. [18][19][20] Duchamp de Lageneste et al found that Prx1 + MSCs can efficiently contribute to cartilage and bone regeneration, while periostin was essential for maintaining the Prx1 + MSCs pool. 21 Therefore, we wondered that if the quantity of Prx1 + MSCs would decrease with age and affect bone regeneration ability in aged mice.…”

“…A previous study found that Prx1 was a paired‐related homeobox gene expressed in a subset of periosteal cells in the cambium layer surrounding the long bones and cartilage 18 . These Prx1 positive cells (Prx1 + MSCs) have the potential of osteogenic and chondrogenic differentiation and are essential for limb development and bone regeneration 18‐20 . Duchamp de Lageneste et al found that Prx1 + MSCs can efficiently contribute to cartilage and bone regeneration, while periostin was essential for maintaining the Prx1 + MSCs pool 21 .…”