Yamashita T et al . Osteoblasts and <strong>osteoclast</strong> <strong>differentiation</strong>Cell cycle c-FmsRANKProgression ArrestHematopoieticprogenitorsM-CSFIL-34Hematopoietic organs(bone, spleen)QOPsOsteoblastsCirculat<strong>in</strong>gBlood vesselHom<strong>in</strong>gDifferentiationFigure 2 In vivo dynamics <strong>of</strong> <strong>osteoclast</strong> precursors. Cellsexpress<strong>in</strong>g both receptor activator <strong>of</strong> nuclear factor-κB (RANK)and c-Fms are cell cycle-arrested quiescent <strong>osteoclast</strong> precursors(QOPs) <strong>in</strong> vivo. QOPs are detected <strong>in</strong> hematopoieticorgans such as the spleen and bone. macrophage colonystimulat<strong>in</strong>gfactor (M-CSF) and/or <strong>in</strong>terleuk<strong>in</strong> 34 (IL-34) appearto be <strong><strong>in</strong>volved</strong> <strong>in</strong> the <strong>differentiation</strong> <strong>of</strong> hematopoietic progenitorcells <strong>in</strong>to QOPs. Some QOPs circulate to f<strong>in</strong>d bone. Osteoblastsplay a role <strong>in</strong> the hom<strong>in</strong>g <strong>of</strong> QOPs to bone. QOPs <strong>in</strong>bone differentiate <strong>in</strong>to <strong>osteoclast</strong>s without cell cycle progression<strong>in</strong> response to M-CSF/IL-34 and RANK ligand.QOPsOsteoclastscycle-arrested quiescent <strong>osteoclast</strong> progenitors (QOPs)”(Figure 2). QOPs were also isolated from bone marrowand peripheral blood [22] . Bone marrow-derived QOPsfailed to express macrophage-associated markers such asF4/80 and CD11b. Bone marrow-derived QOPs showedno phagocytic activity and did not proliferate <strong>in</strong> responseto M-CSF. They differentiated <strong>in</strong>to <strong>osteoclast</strong>s, but not<strong>in</strong>to dendritic cells. Therefore, it has been concluded thatQOPs are committed <strong>osteoclast</strong> precursors [22] .ROLE OF IL-34 IN OSTEOCLASTOGENESISRecently, L<strong>in</strong> et al [23] discovered IL-34 as a new ligandfor c-Fms. The am<strong>in</strong>o acid sequence <strong>of</strong> IL-34 was quitedifferent from that <strong>of</strong> M-CSF, but IL-34 promoted macrophagecolony formation similar to M-CSF. Chiharaet al [24] reported that M-CSF and IL-34 used differentsignal<strong>in</strong>g to <strong>in</strong>duce the expression <strong>of</strong> several chemok<strong>in</strong>esand suggested that they differentially regulated macrophagefunction. However, IL-34 as well as M-CSF, <strong>in</strong>comb<strong>in</strong>ation with RANKL, <strong>in</strong>duced <strong>osteoclast</strong> formation<strong>in</strong> mouse and human cell culture systems [25] . IL-34was specifically expressed <strong>in</strong> splenic tissues, predom<strong>in</strong>antly<strong>in</strong> the red pulp region. Recently, Nakamichi et al [26]showed that RANK and c-Fms double-positive QOPsdid not exist <strong>in</strong> bone, but existed <strong>in</strong> the spleen <strong>of</strong> op/opmice (Figure 2). IL-34 was highly expressed <strong>in</strong> vascularendothelial cells <strong>in</strong> the spleen. Vascular endothelial cells<strong>in</strong> bone also expressed IL-34, but its expression levelwas much lower than that <strong>in</strong> the spleen, suggest<strong>in</strong>g arole <strong>of</strong> IL-34 <strong>in</strong> the splenic ma<strong>in</strong>tenance <strong>of</strong> QOPs. Indeed,removal <strong>of</strong> the spleen (splenectomy) completelyblocked M-CSF-<strong>in</strong>duced <strong>osteoclast</strong> formation <strong>in</strong> op/opmice. Osteoclasts appeared <strong>in</strong> aged op/op mice with upregulation<strong>of</strong> IL-34 expression <strong>in</strong> the spleen and bone.Splenectomy also blocked the age-associated appearance<strong>of</strong> <strong>osteoclast</strong>s [26] . These results suggest that IL-34plays a pivotal role <strong>in</strong> ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g the splenic reservoir<strong>of</strong> QOPs, which are transferred to bone <strong>in</strong> response toM-CSF adm<strong>in</strong>istration <strong>in</strong> op/op mice (Figure 2). Recently,sph<strong>in</strong>gos<strong>in</strong>e-1-phosphate, a lipid mediator regulat<strong>in</strong>gimmune cell traffick<strong>in</strong>g, was shown to regulate the migration<strong>of</strong> <strong>osteoclast</strong> precursors [22,27] . Osteoblasts appearto help hom<strong>in</strong>g <strong>of</strong> QOPs to bone. Thus, <strong>osteoblasts</strong>determ<strong>in</strong>e the distribution <strong>of</strong> QOPs, which decide thecorrect sites <strong>of</strong> <strong>osteoclast</strong> formation.WNT5A-RECEPTOR TYROSINE KINASE-LIKE ORPHAN RECEPTOR 2 SIGNALINGAND OSTEOCLASTOGENESISImmunohistochemical analysis revealed that RANKexpression <strong>in</strong> <strong>osteoclast</strong> precursors was much strongerthan that <strong>in</strong> bone marrow and the spleen [21,28] . Recently,Maeda et al [29] reported that Wnt5a produced by <strong>osteoblasts</strong>promoted RANK expression <strong>in</strong> <strong>osteoclast</strong> precursors(Figure 3).Wnt b<strong>in</strong>ds to two dist<strong>in</strong>ct receptor complexes: acomplex <strong>of</strong> Frizzled and low density lipoprote<strong>in</strong> receptor-relatedprote<strong>in</strong> 5/6 (LRP5/6) and another complex<strong>of</strong> Frizzled and receptor tyros<strong>in</strong>e k<strong>in</strong>ase-like orphanreceptors (Rors) [30] . The b<strong>in</strong>d<strong>in</strong>g <strong>of</strong> Wnts to these Wntreceptors activates two classes <strong>of</strong> signal<strong>in</strong>g pathways: aβ-caten<strong>in</strong>-dependent (canonical) pathway and β-caten<strong>in</strong><strong>in</strong>dependent(non-canonical) pathway. The importance<strong>of</strong> the canonical pathway <strong>in</strong> bone metabolism has beenemphasized by the identification <strong>of</strong> a l<strong>in</strong>k between bonemass and mutations <strong>in</strong> the LRP5 gene. Loss-<strong>of</strong>-functionmutations <strong>in</strong> LRP5 reduced the number <strong>of</strong> <strong>osteoblasts</strong>and caused osteoporosis [31] . Glass et al [32] developed miceexpress<strong>in</strong>g a stabilized form <strong>of</strong> β-caten<strong>in</strong> <strong>in</strong> <strong>osteoblasts</strong>(β-caten<strong>in</strong> mutant mice) and reported that β-caten<strong>in</strong>mutant mice developed severe osteopetrosis due to theup-regulation <strong>of</strong> OPG expression. Thus, Wnt/β-caten<strong>in</strong>signal<strong>in</strong>g is crucial <strong>in</strong> osteoblastogenesis and <strong>osteoclast</strong>ogenesis.However, the role <strong>of</strong> the non-canonical Wntpathway <strong>in</strong> bone resorption rema<strong>in</strong>s largely unknown.Maeda et al [29] showed that Wnt5a-receptor tyros<strong>in</strong>ek<strong>in</strong>ase-like orphan receptor 2 (Ror2) signal<strong>in</strong>g between<strong>osteoblasts</strong> and <strong>osteoclast</strong> precursors enhanced <strong>osteoclast</strong>ogenesis.Ror2-deficient mice exhibited impaired<strong>osteoclast</strong>ogenesis. A deficiency <strong>in</strong> Wnt5a, a ligand <strong>of</strong>Ror2, caused a similar defect <strong>in</strong> mice. Osteoblasts expressedWnt5a, while <strong>osteoclast</strong> precursors expressedRor2, a co-receptor <strong>of</strong> Wnt5a. Wnt5a enhanced RANKexpression <strong>in</strong> <strong>osteoclast</strong> precursors through co-receptorRor2 signal<strong>in</strong>g. RANK promoter-driven luciferase activi-WJO|www.wjgnet.com177 November 18, 2012|Volume 3|Issue 11|