Osteoblastic Wnts Differentially Regulate Bone Remodeling And The Maintenance Of Bone Marrow Mesenchymalstem Cells

Bone remodeling involves the coordination of the osteoblastic boneformation and osteoclastic bone resorption. Previous studies havedemonstrated that Wnt signaling plays an important role in osteoblastogenesisand bone remodeling. β-catenin is the central mediator of canonical Wntsignaling, while Lrp5is an important coreceptor of Wnt/β-catenin signalingpathway. Loss of β-catenin in osteoblasts/osteocytes leads to osteopenia withenhanced osteoclastogenesis, while inactivating Lrp5in osteocytes decreasesbone mass. However, other researches have shown that inactivating Lrp5inmature osteoblasts causes no change in bone mass, but loss of Lrp5in the gut leads to decreased bone mass. Therefore, the purported mechanisms arecontroversial since Wnt/Lrp5signaling might regulate bone mass locally in thebone or globally in the gut. Meanwhile, inconsistent impact on bone propertyis observed in different genetic alterations of Lrp5and β-catenin. Lrp5mainlyregulates osteoblastogenesis, while β-catenin primarily modulatesosteoclastogenesis. The aim of our research is to elucidate the function of Wntsignaling in regulating bone remodeling and the role of osteoblastic niche inpostnatal bone homeostasis and bone marrow mesenchymal stem cells（BMSCs） maintaining.According to immunohistochemistry examination, several Wnts, suchas canonical Wnt10b and noncanonical Wnt5a/5b are dynamically expressedin differentiating osteoblasts. Real-time PCR confirms the observations, andshow that other Wnt genes, such as Wnt5a and Wnt5b are highly expressed inimmature osteoblasts, while Wnt4, Wnt7b and Wnt16are abundantlyexpressed in mature osteoblast. To understand the global function ofosteoblastic Wnts in postnatal bone remodeling, we block the secretion of Wntproteins from osteoblasts through conditionally inactivating Wntless （Wls）gene by Collagen1α1（Col1-3.6kb）-Cre and Osteocalcin （Osc）-Cre, whichblocks the secretion of nearly all Wnt proteins in the osteoblast lineage. Mostof the Col1^cre/+;Wl^sc/cmice display severe osteopenia, with increased bone resorption, decreased bone formation and impaired maintenance of bonemarrow stem cell （BMSC）. Osc^cre/+;Wl^sc/cmice exhibited similar but muchmilder defects than that of the Col1^cre/+;Wl^sc/cmutant.Further studies show that loss of bone mass in the mutants results fromthe decreased osteoblast proliferation, increased osteoblast apoptosis andimpaired osteoblast differentiation. In addition, these mutants also manifestenhanced bone resorption, coupled with diminished Osteoprotegerin （OPG）expression and elevated receptor activator of nuclear factor-κB （NF-κB） ligand（RNAKL） and macrophage colony stimulating factor（M-CSF） expression.Moreover, blocking Wnt secretion from osteoblasts leads to impairedmaintaining of BMSCs. To further validate the contribution of osteoblasticWnts in bone remodeling, canonical Wnt signaling is cell-autonomouslyelevated while Wnts secretion is simultaneously blocked in osteoblaststhrough generating Col1^cre/+;Wl^sc/c;Catnb^ex3/+double knockout mice. Based onthe comparison of phenotypes in Col1^cre/+;Catnb^ex3/+single andCol1^cre/+;Wl^sc/c;Catnb^ex3/+double mutants, we postulate that osteoblastic Wntsmay directly and locally exert negative effects on osteoclastogenesis.Taken together, these findings support the notion that Wnt/Lrp5signaling locally regulates bone mass accrual through multiple effects ofosteoblastic Wnts on osteoblastic bone formation and osteoclastic bone resorption. Moreover, osteoblastic Wnts confer a niche role for maintenance ofBMSCs, providing additional cues for the definition of BMSCs niche in thebone marrow.