Bone remodeling is tightly regulated by a cross-talk between bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoblasts and osteoclasts communicate with each other to regulate cellular behavior, survival and differentiation through direct cell-to-cell contact or through secretory proteins. Osteoclasts also influence osteoblast formation and differentiation through secretion of soluble factors, including S1P, SEMA4D, CTHRC1 and C3. Here the current knowledge regarding membrane bound- and soluble factors governing cross-talk between osteoblasts and osteoclasts was reviewed.
1. Soluble Factors Released from Osteoblasts
1.1. Macrophage Colony-Stimulating Factor (M-CSF)
M-CSF (also called as CSF1) is a hematopoietic growth factor that allows for survival, proliferation, differentiation, and mobility of mononuclear phagocyte lineages, including osteoclasts
[1][2]. M-CSF, secreted from osteoblasts and bone marrow stromal cells, binds to its cognate receptor C-FMS on the surface of osteoclasts and monocytes/macrophages
[3]. Osteopetrotic (op/op) mice where a thymidine insertion in the
Csf1 gene resulted in M-CSF deficiency show decreased numbers of macrophages and osteoclasts at a young age. However, these phenotypes disappear during aging. Injection of recombinant M-CSF or production of soluble M-CSF in osteoblasts increases osteoclast numbers and rescues osteopetrotic phenotypes in op/op mice, demonstrating that M-CSF is crucial for osteoclast formation at least in young mice, but does not exclude the existence of M-CSF-independent compensatory mechanisms.
1.2. Receptor Activator of NF-κB (Nuclear Factor-Kappa B) Ligand (RANKL)
RANKL is also called osteoclast differentiation factor (ODF), TNF ligand superfamily member 11 (TNFSF11), TNF-related activation-induced cytokine (TRANCE), and OPG ligand (OPGL)
[4][5]. RANKL is highly expressed in osteoblasts, osteocytes, activated T lymphocytes, and lymph nodes
[6][7][8]. RANKL binds to its cognate receptor, receptor activator of NF-κB (RANK) on the surface of osteoclasts and osteoclast precursors, leading to osteoclast differentiation, fusion, and activation
[9][10]. Mice deficient in
Tnfrsf11a (RANK) or
Tnfsf11 (RANKL) are phenocopies of one another, indicating the essential role of this RANKL/RANK signaling axis in bone remodeling
[6][10]. Deletion of RANKL in mice results in severe osteopetrosis due to absence of osteoclasts, whereas overexpression of soluble RANKL leads to severe osteoporosis
[6][11]. Accordingly, blocking RANKL signaling has been proposed as a promising therapeutic target for osteoporotic bone loss and related skeletal disorders.
1.3. Osteoprotegerin (OPG)
OPG is also known as osteoclastogenesis inhibitory factor (OCIF) and TNF receptor superfamily member 11B (TNFRSF11B)
[5][12][13]. OPG was identified as a secreted glycoprotein synthesized by many types of cells, including osteoblasts, lung- or liver-residing cells, and B lymphocytes in the bone marrow
[12][13][14]. Overexpression of OPG results in profound osteopetrosis due to inhibition of osteoclast formation, whereas
Tnfrsf11b (OPG)—deficient mice exhibit rapid postnatal bone loss and severe bone porosity due to an increased osteoclast development
[12][15]. OPG is considered to function as a decoy receptor binding to RANKL, negatively regulating osteoclast differentiation and activation by blocking the RANKL-RANK interaction
[9][16].
1.4. WNT5A
The WNT pathway is crucial for the maintenance of bone homeostasis by regulating osteoblastogenesis and osteoclastogenesis through both β-catenin-dependent (canonical) and -independent (noncanonical) pathways
[17]. A noncanonical WNT ligand, WNT5A, is highly expressed in osteoblast-lineage cells and binds to its cognate receptor, receptor tyrosine kinase-like orphan receptor 2 (ROR2), on the surface of osteoclasts
[18]. Heterozygous deletion of
Wnt5a or
Ror2 in mice resulted in impaired development of bone marrow-derived monocytes (BMM) to mature osteoclasts. Corresponding defects in osteoclastogenesis were also observed in mice with osteoblast-specific deletion of
Wnt5a or osteoclast-specific deletion of
Ror2. WNT5A enhances RANKL-induced osteoclastogenesis by upregulating RANK expression in osteoclasts via activation of the Jun–N-terminal kinase (JNK) MAPK pathway.
1.5. WNT16
The
WNT16 locus is closely associated with bone mineral density (BMD), cortical bone thickness, and fracture risk in humans
[19][20][21]. WNT16 is highly expressed in osteoblast-residing cortical bone, but little to no expression is detected in osteoclasts
[22]. Global deletion of
Wnt16 results in a specific decrease in cortical bone mass and an increase in cortical porosity, along with spontaneous fractures where there is no alteration in trabecular bone. WNT16 suppresses osteoclastogenesis in both a direct and indirect manner. In addition to direct inhibition of osteoclastogenesis via the noncanonical JNK MAPK pathway, WNT16-induced phosphorylation of JUN upregulates expression of OPG in osteoblasts, providing a direct mechanism to suppress osteoclastogenesis. Osteoblast-specific deletion of
Wnt16 in mice phenocopies mice with its global deletion, suggesting osteoblasts are a primary source of WNT16, with an impact on cortical bone and skeletal integrity.
Table 1 provides a list of osteoblast-derived factors that regulate osteoclasts.
Table 1. Summary of the effect of osteoblast-derived factors on osteoclast behavior.