Stem cell-mediated osteogenesis: therapeutic potential for bone tissue engineering

Figures & data

Figure 1 Osteoclast differentiation is induced by macrophage colony-stimulating factor (M-CSF), receptor activator of nuclear factor-κB [NF-κB] ligand (RANKL) and its costimulatory factor, immunoglobulin (Ig)-like receptor. (A) Precursor-cell stage. The binding of M-CSF to its receptor, cFMS, activates the proliferation and survival of osteoclast precursor cells of the monocyte–macrophage lineage that express receptor activator of nuclear factor-κB (RANK). The costimulatory receptors might be stimulated from early stages, although ligands of costimulatory receptors have yet to be identified. (B) Proximal RANK signals. RANKL binding to RANK results in the recruitment of tumor-necrosis-factor-receptor-associated factor 6 (TRAF6). At the same time, RANK activation results in the phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) in DAP12 and Fc-receptor common γ-subunit (FcRγ), both of which are adaptor proteins associating with distinct Ig-like receptors. Ig-like receptor signals are called costimulatory signals for RANK. (C) Initial induction of nuclear factor of activated T cells, cytoplasmic 1 (NFATc1). NFATc1, a key transcription factor for osteoclastogenesis, is initially induced by TRAF6-activated NF-κB and NFATc2 that is present in the cell before RANKL stimulation. Phosphorylation of the ITAM in DAP12 (or FcRγ) results in the recruitment of spleen tyrosine kinases (SYKs) that activate calcium signaling through phospholipase Cγ (PLCγ). (D) Auto amplification of NFATc1. Calcium signal–mediated persistent activation of NFATc1, as well as cooperation with activator protein 1 (AP1), is a prerequisite for the robust induction of NFATc1. AP1 activation is mediated by the induction and activation of cFOS by calcium/calmodulin-dependent protein kinase type IV (CaMKIV)-stimulated cyclic adenosine monophosphate responsive-element-binding protein (CREB) and cFMS. The NFATc1 promoter is epigenetically activated through histone acetylation and NFATc1 binds to an NFAT-binding site of its own promoter. (E) In the nucleus, NFATc1 works together with other transcription factors, such as AP1, PU.1, microphthalmia-associated transcription factor (MITF) and CREB, to induce various osteoclast-specific genes, including tartrate-resistant acid phosphatase, cathepsin K, and calcitonin receptor.

Note: Reprinted by permission from Macmillan Publishers Ltd: Nature Review Immunology, H Takayanagi, Osteoimmunology: shared mechanisms and crosstalk between the immune and bone systems, 7(4):292–304, copyright 2007.²⁹

Figure 2 The immune and skeletal systems share cytokines, receptors, signaling molecules, and transcription factors, all of which cooperatively regulate osteoclasts and osteoblasts as well as their interactions. Osteoblasts regulate osteoclastogenesis through receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-receptor activator of nuclear factor-κB (RANK) (and its decoy receptor osteoprotegerin [OPG]) interactions, macrophage colony-stimulating factor (M-CSF)–cFMS interactions and immunoglobulin (Ig)-like receptors associated with immunoreceptor tyrosine-based activation motif-harboring adaptor molecules (such as DAP12 and Fc-receptor common γ-subunit [FcRγ], the ligands of which are not well characterized). Although not depicted, semaphorin 6D, and its receptor plexin A1, and ephrin receptor B4 and ephrin B2 were newly identified as mediators of osteoblast–osteoclast interactions. There are extensive signaling pathways in osteoclasts. RANK and Ig-like receptors stimulate downstream signaling cascades (such as tumor necrosis factor [TNF] receptor-associated factor 6 [TRAF6], NF-κB, mitogen-activated protein kinases [MAPKs], activator protein 1 [AP1], calcineurin, and nuclear factor of activated T cells cytoplasmic 1 [NFATc1]), which are influenced by a number of immunoregulatory molecules including CD40 ligand (CD40L), interleukin-1 (IL-1), interferon-β (IFNβ), IFNγ, TNF, and lipopolysaccharide (LPS). Dendritic-cell-specific transmembrane protein (DC-STAMP) and ATP6V0D2 are necessary for the fusion of osteoclast precursor cells. Phosphoinositide 3-kinase (PI3K)-AKT and growth-factor-receptor-bound protein 2–extracellular-signal- regulated kinase (GRB2–ERK) pathways are important for the proliferation and survival of the osteoclast lineage, whereas VAV3, cSRC, and Casitas B-lineage lymphoma (cCBL) are included in the molecules required for cytoskeletal reorganization and bone-resorbing osteoclasts. Osteoclast activity is dependent on acidifying proton pump ATP6I and chloride channel 7 (ClC7), as well as matrix-degrading enzymes such as cathepsin K and matrix metalloproteinase 9 (MMP9). The following molecules are known to be involved in both the bone system and the immune system: NF-κB, RANKL, RANK, OPG, cFMS, M-CSF, Ig-like receptors, FcRγ, DAP12, TRAF6, MAPKs, AP1, calcineurin, NFATc1, CD40L, IL-1, IFNγ, IFNβ, TNF, LPS, DC-STAMP, PI3K, AKT, ERK, VAV3, cSRC, and cCBL.

Note: Reprinted by permission from Macmillan Publishers Ltd: Nature Review Immunology, H Takayanagi, Osteoimmunology: shared mechanisms and crosstalk between the immune and bone systems, 7(4):292–304, copyright 2007.²⁹
Abbreviations: CaMKIV, calcium/calmodulin-dependent protein kinase type IV; CREB, cyclic adenosine monophosphate responsive-element-binding protein; PLC, phospholipase C; MITF, microphthalmia-associated transcription factor; IKK, inhibitor of NF-κB (IκB) kinase; SYK, spleen tyrosine kinase.

Figure 3 Bone remodeling after fracture. Mesenchymal stem cells are recruited within lesions and induced to form new bone following both endochondral and intramembranous pathways. During bone formation, Indian hedgehog (Ihh) acts at a very early stage to induce the expression of runt-related transcription factor 2 (Runx2), which needs to be phosphorylated to be active. Bone morphogenetic proteins (BMPs) are also necessary to lead osteoblastic commitment and to drive osteoblastic maturation, notably through Runx2 and distal-less homeobox 5 (Dlx5) induction. Mitogen-activated protein kinase (MAPK) can phosphorylate Dlx5 and Runx2. The osteochondroblastic progenitors can express Ihh, which induces secretion of parathyroid hormone-related protein (PTHrP) and also acts on preosteoblastic cells positive for collagen 1a1 (col1a1), alkaline phosphatase (ALP), and PTH receptor 1 (PTH-R1) to increase their maturation. Msh homeobox 2 (Msx2) is a factor preferentially found within proliferative progenitors, whereas Dlx5 leads to maturation. In addition, Dlx5 and Msx2 compete for DNA binding. Therefore, the Dlx5:Msx2 content drives the maturation of cells. In addition, Msx2 induces apoptosis in later stages of maturation. Wnt proteins can induce the proliferation of osteochondroblastic progenitors and preosteoblasts. When osteoblasts mature, they can express Wnt inhibitors such as Dickkopf-related protein 1 (Dkk1) molecules. Osteoclasts are capable of degrading mineralized bone and are hematopoietic-derived cells. They are generated through receptor activator of nuclear kappa-B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) cytokines secreted by activated T- and B-lymphocytes and by preosteoblasts. Osteoclast activities can be regulated by interleukin-10 (IL-10) and by osteoprotegerin (OPG), a decoy receptor of RANKL. OPG is preferentially expressed by mature osteoblasts (positive for osteocalcin [OSC], bone sialoprotein [BSP], PTH-R1, and osteonectin [SPARC]). Finally, degradation of the bone matrix releases several cytokines and growth factors, such as BMPs and insulin-like growth factor (IGF), which in turn can activate immature cells.

Note: Reprinted and modified by permission from Trends in Molecular Medicine, F Deschaseaux, L Sensebe, D Heymann, Mechanisms of bone repair and regeneration, Vol. 15, Issue 9, Pages 417–429, Copyright 1999, with permission from Elsevier.³
Abbreviations: TRAP, tartrate-resistant acid phosphatase; Osx, osterix; Col10a1/2a1/1a1, collagen 10a1/2a1/1a1; GSK3β, glycogen synthase kinase 3β; TGFβ, transforming growth factor β; PPARg, peroxisome proliferator-activated receptor gamma; C/EBPa, CCAAT-enhancer-binding protein alpha.

TakayanagiHOsteoimmunology: shared mechanisms and crosstalk between the immune and bone systemsNat Rev Immunol20077429230417380158

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DeschaseauxFSensebeLHeymannDMechanisms of bone repair and regenerationTrends Mol Med200915941742919740701

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