JBMR
COMMENTARY
Coupling Factors: How Many Candidates Can There Be? T John Martin St. Vincent’s Institute of Medical Research, and University of Melbourne Department of Medicine, Melbourne, Australia
O
ur concepts of the cellular events in bone remodeling began in the 1960s with Harold Frost, who described remodeling as a destructive process, wrought by osteoclasts that resorb bone, followed by a productive process ascribed to osteoblasts that form bone.(1) To replace old or damaged bone, bone remodeling takes place asynchronously throughout the skeleton in bone multicellular units (BMUs). An essential feature of the process is that at each BMU, equal volumes of bone are removed by osteoclasts and replaced by osteoblasts.(2–4) Just as osteoblastic lineage cells control osteoclast formation, so too it seemed likely that either the products of bone resorption or products of the osteoclasts themselves promote osteoblast differentiation from precursors in the BMU and, hence, bone formation. It is the latter communication mechanism that is referred to as “coupling.” This term is confined to events taking place within individual BMUs throughout the skeleton; at each BMU the volume of bone removed by resorption is approximately equaled by that replaced by formation. On the other hand, the overall “balance” of bone resorption and formation— the close matching of the whole body rates of these two(5)— represents the summation of the contributions from BMUs throughout the body, where many BMUs are at different stages of maturation. Thus, “coupling” should not be applied as a descriptive term to that overall bone balance. The focus of interest on this topic has led to experiments giving rise to a number of “candidate” coupling factors, a further one provided in this issue of JBMR by Matsuoka and colleagues,(6) who propose that complement factor 3a, the active cleavage product of complement component 3 (C3), is released from active osteoclasts and promotes osteoblast differentiation and bone formation. Treatment of mouse calvarial osteoblasts in culture with concentrated conditioned medium from osteoclast cultures resulted in dose‐dependent increase in alkaline phosphatase (ALP) activity and increased mineralization when the cells were grown under appropriate conditions. Because the activity appeared to be protein in nature, purification was approached using three successive ion exchange chromatography steps. In monitoring purification, they used stimulation of ALP activity in calvarial osteoblasts, the highest activity fraction after purification containing a number of peptide sequences. Among these, the authors identified complement component 3 and used an ELISA with conditioned medium to identify its bioactive cleavage product, C3a. C3a was not detected in conditioned medium from bone marrow macrophages but
increased with differentiation to osteoclasts, and its Gi‐coupled G‐protein receptor, C3aR, was expressed in stromal cell lines and mouse calvarial osteoblasts. The failure to find C3a in bone marrow‐derived macrophage (BMM) conditioned medium in the present experiments is surprising, given the substantial production of complement by other macrophage populations. A C3aR antagonist inhibited the promotion of ALP in calvarial osteoblasts by conditioned medium, and knockdown of C3 in osteoclasts by lentiviral shRNA expression ablated the activity in conditioned medium, whereas a C3aR agonist drug promoted ALP activity in calvarial osteoblast cultures. In in vivo experiments, bone and bone marrow C3 mRNA were significantly increased 4 days after ovariectomy (OVX) in mice, and in marrow after receptor activator of NF‐kB ligand (RANKL) treatment in vivo. In mice examined 14 days after OVX, the bone formation and mineral apposition rates were significantly greater than those in sham‐operated mice or in those treated from day 6 with the C3aR antagonist, SB290157. The latter treatment also significantly enhanced the post‐OVX bone loss, as determined by micro‐CT. The interpretation of these data was that osteoclast C3a production increased after OVX, acting through the C3aR to promote bone formation. The in vivo data rely on a small albeit statistically significant difference in the two bone formation parameters. The data would have benefited from a more thorough analysis, including the provision of quantitative osteoclast data. Because there is much more to osteoblast differentiation than a change in ALP at a particular stage, it would be important also to consider other differentiation parameters, rather than relying solely upon promotion of ALP activity in purification and even in assessing the importance of C3a. For the latter, among the many outstanding questions are: What other osteoblast lineage genes are affected by actions through the C3aR in osteoblasts and at what stage of osteoblast differentiation does C3aR optimally act? Because the macrophage is such a prominent target of C3a action, is C3aR mRNA so evident in calvarial osteoblasts because of the macrophage content of mouse calvarial cultures?(7) Macrophage‐mediated actions of C3a need to be considered in both the in vitro and in vivo experiments of the present work, in view of the known activation through Ca3R in monocytes and macrophages of mediators such as IL‐1b through participation of Toll‐like receptor and ATP‐mediated P2X purinoreceptor (P2 7) signals in monocytes and macrophages.(8,9) Further, the fact that C3a circulates in such substantial amounts (approximately
Address correspondence to: T. John Martin, MD, DSc, St Vincent’s Institute of Medical Research and University of Melbourne, 9 Princes Street, Fitzroy 3065, Victoria, Australia. E‐mail: [email protected] This is a Commentary on Matsuoka et al. (J Bone Miner Res. 2014;29:1522–1530. DOI: 10.1002/jbmr.2187). Journal of Bone and Mineral Research, Vol. 29, No. 7, July 2014, pp 1519–1521 DOI: 10.1002/jbmr.2276 © 2014 American Society for Bone and Mineral Research
1519
50 ng/mL in human serum)(10) might call into question a role for C3a only at selected sites in the skeleton as they undergo remodeling. The same research group suggested in a recent report that collagen triple helix repeat containing 1 (CTHRC1) is a coupling factor by virtue of its production by actively resorbing osteoclasts and the fact that it stimulated bone formation in vitro and in vivo.(11) The conclusion from that work, based on in situ hybridization, was that CTHRC1 was not produced by osteoblasts, but by chondrocytes in the embryo, as well as at the growth plate until 3 months of age. In contrast, Kimura and colleagues(12) found CTHRC1 to be a product of the osteoblast lineage, including mesenchymal precursors, that markedly stimulates bone formation. Reconciliation of these discrepant findings will require careful cellular localization studies to establish whether CTHRC acts as an osteoclast product or a signal within the osteoblast lineage. Either way, it could be a participant in local events that contribute to the overall bone remodeling process, whether strictly as one of a number of coupling factors or not. The proposal some decades ago of TGFb and IGF‐1 as coupling agents(13,14) has gained recent support from mouse genetic experiments,(15,16) but there is much emphasis now on the search for osteoclast‐derived control of coupling.(17) When osteoclast conditioned medium stimulated mesenchymal stem cell migration and osteoblastic differentiation, Pedersen and colleagues(18) undertook a microarray study that led them to identify sphingosine‐1‐phosphate (S1P), Wnt 10b, and BMP6 as osteoclast products that might be coupling factors. Whether important as an osteoclast product or not, Wnt10b has been invoked in another context as a T‐cell–derived mediator of the anabolic effect of parathyroid hormone (PTH).(19) The roles of the sphingosine kinase product, S1P, in bone are complex. It can have inhibitory or stimulatory effects on osteoblasts depending on the stage of cell differentiation and on the source of precursors,(18,20,21) can limit bone resorption by increasing recirculation of osteoclast precursors from bone to blood,(22) and can promote osteoprotegerin production by osteoblasts.(23) Data more encouraging of S1P as a coupling agent showed that catK null osteoclasts, with impaired resorption, were nevertheless able to promote aspects of osteoblast differentiation in co‐ culture, an effect inhibited by S1P receptor antagonist.(24) In addition to osteoblast stimulators from osteoclasts, semaphorin 4D was found as an osteoclast product that inhibits osteoblast differentiation and bone formation.(25) In addition to such secreted positive and negative regulators of coupling, membrane activities have been invoked, including ephrinB2, membrane‐bound in the mature osteoclast, and suggested to act on its receptor, EphB4, in the osteoblast lineage to promote differentiation, a process that would require cell contact.(26) Such contact would also be required to support the hypothesized role of RANK driving reverse signaling through RANKL in the osteoblast lineage.(27) These and other “candidates” are considered in more detail in recent reviews that highlight the complexity of coupling in the BMU, including the likely contribution of products of cells other than osteoclasts.(28,29) How many candidates can there be? It seems likely that there are many factors derived from matrix and cells in the BMU—not only osteoclasts, but T cells, macrophages, and osteoblasts themselves—that contribute to some extent to the process of coupling. The likelihood of finding a dominant, single factor analogous to RANKL in promotion of osteoclasts seems remote. The precision in time and space required to program osteoblast
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lineage cells in the BMU likely requires a number of local factors acting at different stages of osteoblast differentiation. No studies so far address this question: At what stages of osteoblast differentiation do “candidate” coupling factors exert their actions, and what are those actions? Most of the candidate coupling factors have been arrived at through in vitro studies of osteoblast cells at particular stages of differentiation or occasionally ex vivo studies on cells from genetically manipulated mice. As can be said of most—or even all—of the other candidate coupling factors so far, there are aspects of interest in the present work,(6) but more would be required to be convincing of a critical role for C3a in remodeling within the BMU and how it might operate. The work provides another link with the immune system and even with inflammation, something that might not necessarily be so surprising. In describing bone remodeling, Frost likened it to the healing of a soft tissue wound, an event associated with the release of inflammatory mediators,(1) and certain similarities have been pointed out between bone remodeling and inflammation.(30) The biological effects of activated complement result from the release of cytokine signals from macrophages, T cells, and dendritic cells. C3a leads to inflammation by amplifying the immune response(31) in asthma, sepsis, liver regeneration, and autoimmune encephalomyelitis.(31,32) Mice null for the C3aR gene had a marked reduction in macrophage content of adipose tissue, reduced adiposity, and enhanced insulin sensitivity.(33) Any possible contribution of activated complement to enhanced osteoblast differentiation in the cellular events in remodeling in the BMU should take into account the participation of other cells.
Disclosures The author states that he has no conflicts of interest.
Acknowledgments The author acknowledges support from the National Health and Medical Research Council, Australia, and the Victorian Government OIS Program to St. Vincent’s Institute of Medical Research.
References 1. Hattner R, Epker BN, Frost HM. Suggested sequential mode of control of changes in cell behaviour in adult bone remodelling. Nature. 1965;206(983):489–90. 2. Frost HM. Dynamics of bone remodeling. In: Frost HM, editor. Bone biodynamics. Boston: Little‐Brown; 1964. p.315–33. 3. Parfitt AM. The cellular basis of bone remodeling: the quantum concept reexamined in light of recent advances in the cell biology of bone. Calcif Tissue Int. 1984;36(Suppl 1):S37–45. 4. Eriksen EF. Normal and pathological remodeling of human trabecular bone: three dimensional reconstruction of the remodeling sequence in normals and in metabolic bone disease. Endocr Rev. 1986;7(4): 379–408. 5. Harris WH, Heaney RP. Skeletal renewal and metabolic bone disease. N Engl J Med. 1969;280(4):193–202. 6. Matsuoka K, Park KA, Ito M, Ikeda K, Takeshita S. Osteoclast‐derived complement component 3a stimulates osteoblast differentiation. J Bone Miner Res. 2014;29(7):1520–28. 7. Chang MK, Raggatt LJ, Alexander KA, et al. Osteal tissue macrophages are intercalated throughout human and mouse bone lining tissues and regulate osteoblast function in vitro and in vivo. J Immunol. 2008;181(2):1232–44.
Journal of Bone and Mineral Research
8. Asgari E, Le Friec G, Yamamoto H, et al. C3a modulates IL‐1beta secretion in human monocytes by regulating ATP efflux and subsequent NLRP3 inflammasome activation. Blood. 2013;122(20): 3473–81. 9. Dinarello CA. The C3a receptor, caspase‐1, and release of IL‐1beta. Blood. 2013;122(20):3394–5. 10. Wlazlo N, van Greevenbroek MM, Ferreira I, et al. Activated complement factor 3 is associated with liver fat and liver enzymes: the CODAM study. Eur J Clin Invest. 2013;43(7):679–88. 11. Takeshita S, Fumoto T, Matsuoka K, et al. Osteoclast‐secreted CTHRC1 in the coupling of bone resorption to formation. J Clin Invest. 2013;123(9):3914–24. 12. Kimura H, Kwan KM, Zhang Z, et al. Cthrc1 is a positive regulator of osteoblastic bone formation. PLoS One. 2008;3(9):e3174. 13. Howard GA, Bottemiller BL, Turner RT, Rader JI, Baylink DJ. Parathyroid hormone stimulates bone formation and resorption in organ culture: evidence for a coupling mechanism. Proc Natl Acad Sci USA. 1981;78(5):3204–8. 14. Baylink DJ, Finkelman RD, Mohan S. Growth factors to stimulate bone formation. J Bone Miner Res. 1993;8(Suppl 2):S565–72. 15. Tang Y, Wu X, Lei W, et al. TGF‐beta1‐induced migration of bone mesenchymal stem cells couples bone resorption with formation. Nat Med. 2009;15(7):757–65. 16. Xian L, Wu X, Pang L, et al. Matrix IGF‐1 maintains bone mass by activation of mTOR in mesenchymal stem cells. Nat Med. 2012; 18(7):1095–101. 17. Martin TJ, Sims NA. Osteoclast‐derived activity in the coupling of bone formation to resorption. Trends Mol Med. 2005;11(2):76–81. 18. Pederson L, Ruan M, Westendorf JJ, Khosla S, Oursler MJ. Regulation of bone formation by osteoclasts involves Wnt/BMP signaling and the chemokine sphingosine‐1‐phosphate. Proc Natl Acad Sci USA. 2008;105(52):20764–9. 19. Bedi B, Li JY, Tawfeek H, et al. Silencing of parathyroid hormone (PTH) receptor 1 in T cells blunts the bone anabolic activity of PTH. Proc Natl Acad Sci USA. 2012;109(12):E725–33. 20. Ryu J, Kim HJ, Chang EJ, Huang H, Banno Y, Kim HH. Sphingosine 1‐phosphate as a regulator of osteoclast differentiation and osteoclast‐osteoblast coupling. Embo J. 2006;25(24):5840–51. 21. Quint P, Ruan M, Pederson L, et al. Sphingosine 1‐phosphate (S1P) receptors 1 and 2 coordinately induce mesenchymal cell migration
Journal of Bone and Mineral Research
through S1P activation of complementary kinase pathways. J Biol Chem. 2013;288(8):5398–406. 22. Ishii M, Kikuta J, Shimazu Y, Meier‐Schellersheim M, Germain RN. Chemorepulsion by blood S1P regulates osteoclast precursor mobilization and bone remodeling in vivo. J Exp Med. 2010;207(13): 2793–8. 23. Matsuzaki E, Hiratsuka S, Hamachi T, et al. Sphingosine‐1‐phosphate promotes the nuclear translocation of beta‐catenin and thereby induces osteoprotegerin gene expression in osteoblast‐like cell lines. Bone. 2013;55(2):315v24. 24. Lotinun S, Kiviranta R, Matsubara T, et al. Osteoclast‐specific cathepsin K deletion stimulates S1P‐dependent bone formation. J Clin Invest. 2013;123(2):666–81. 25. Negishi‐Koga T, Shinohara M, Komatsu N, et al. Suppression of bone formation by osteoclastic expression of semaphorin 4D. Nat Med. 2011;17(11):1473–80. 26. Zhao C, Irie N, Takada Y, et al. Bidirectional ephrinB2‐EphB4 signaling controls bone homeostasis. Cell Metab. 2006;4(2):111–21. 27. Furuya Y, Inagaki A, Khan M, et al. Stimulation of bone formation in cortical bone of mice treated with a receptor activator of nuclear factor‐kappaB ligand (RANKL)‐binding peptide that possesses osteoclastogenesis inhibitory activity. J Biol Chem. 2013;288(8): 5562–71. 28. Henriksen K, Karsdal MA, Martin TJ. Osteoclast‐derived coupling factors in bone remodeling. Calcif Tissue Int. 2014;94(1):88–97. 29. Sims NA, Martin TJ. Coupling the activities of bone formation and resorption: a multitude of signals within the basic multicellular unit. Bonekey Rep. 2014;3:481. 30. Martin TJ, Rodan GA. Coupling of bone formation and resorption during bone remodeling. In: Marcus R, Feldman D, Kelsey J, editors. Osteoporosis. 2nd ed. Vol. I. London: Academic Press; 2001. 31. Carroll MC. The complement system in regulation of adaptive immunity. Nat Immunol. 2004;5(10):981–6. 32. Markiewski MM, Lambris JD. The role of complement in inflammatory diseases from behind the scenes into the spotlight. Am J Pathol. 2007;171(3):715–27. 33. Mamane Y, Chung Chan C, et al. The C3a anaphylatoxin receptor is a key mediator of insulin resistance and functions by modulating adipose tissue macrophage infiltration and activation. Diabetes. 2009;58(9):2006–17.
COUPLING FACTORS
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COMMENTARY
Coupling Factors: How Many Candidates Can There Be? T John Martin St. Vincent’s Institute of Medical Research, and University of Melbourne Department of Medicine, Melbourne, Australia
O
ur concepts of the cellular events in bone remodeling began in the 1960s with Harold Frost, who described remodeling as a destructive process, wrought by osteoclasts that resorb bone, followed by a productive process ascribed to osteoblasts that form bone.(1) To replace old or damaged bone, bone remodeling takes place asynchronously throughout the skeleton in bone multicellular units (BMUs). An essential feature of the process is that at each BMU, equal volumes of bone are removed by osteoclasts and replaced by osteoblasts.(2–4) Just as osteoblastic lineage cells control osteoclast formation, so too it seemed likely that either the products of bone resorption or products of the osteoclasts themselves promote osteoblast differentiation from precursors in the BMU and, hence, bone formation. It is the latter communication mechanism that is referred to as “coupling.” This term is confined to events taking place within individual BMUs throughout the skeleton; at each BMU the volume of bone removed by resorption is approximately equaled by that replaced by formation. On the other hand, the overall “balance” of bone resorption and formation— the close matching of the whole body rates of these two(5)— represents the summation of the contributions from BMUs throughout the body, where many BMUs are at different stages of maturation. Thus, “coupling” should not be applied as a descriptive term to that overall bone balance. The focus of interest on this topic has led to experiments giving rise to a number of “candidate” coupling factors, a further one provided in this issue of JBMR by Matsuoka and colleagues,(6) who propose that complement factor 3a, the active cleavage product of complement component 3 (C3), is released from active osteoclasts and promotes osteoblast differentiation and bone formation. Treatment of mouse calvarial osteoblasts in culture with concentrated conditioned medium from osteoclast cultures resulted in dose‐dependent increase in alkaline phosphatase (ALP) activity and increased mineralization when the cells were grown under appropriate conditions. Because the activity appeared to be protein in nature, purification was approached using three successive ion exchange chromatography steps. In monitoring purification, they used stimulation of ALP activity in calvarial osteoblasts, the highest activity fraction after purification containing a number of peptide sequences. Among these, the authors identified complement component 3 and used an ELISA with conditioned medium to identify its bioactive cleavage product, C3a. C3a was not detected in conditioned medium from bone marrow macrophages but
increased with differentiation to osteoclasts, and its Gi‐coupled G‐protein receptor, C3aR, was expressed in stromal cell lines and mouse calvarial osteoblasts. The failure to find C3a in bone marrow‐derived macrophage (BMM) conditioned medium in the present experiments is surprising, given the substantial production of complement by other macrophage populations. A C3aR antagonist inhibited the promotion of ALP in calvarial osteoblasts by conditioned medium, and knockdown of C3 in osteoclasts by lentiviral shRNA expression ablated the activity in conditioned medium, whereas a C3aR agonist drug promoted ALP activity in calvarial osteoblast cultures. In in vivo experiments, bone and bone marrow C3 mRNA were significantly increased 4 days after ovariectomy (OVX) in mice, and in marrow after receptor activator of NF‐kB ligand (RANKL) treatment in vivo. In mice examined 14 days after OVX, the bone formation and mineral apposition rates were significantly greater than those in sham‐operated mice or in those treated from day 6 with the C3aR antagonist, SB290157. The latter treatment also significantly enhanced the post‐OVX bone loss, as determined by micro‐CT. The interpretation of these data was that osteoclast C3a production increased after OVX, acting through the C3aR to promote bone formation. The in vivo data rely on a small albeit statistically significant difference in the two bone formation parameters. The data would have benefited from a more thorough analysis, including the provision of quantitative osteoclast data. Because there is much more to osteoblast differentiation than a change in ALP at a particular stage, it would be important also to consider other differentiation parameters, rather than relying solely upon promotion of ALP activity in purification and even in assessing the importance of C3a. For the latter, among the many outstanding questions are: What other osteoblast lineage genes are affected by actions through the C3aR in osteoblasts and at what stage of osteoblast differentiation does C3aR optimally act? Because the macrophage is such a prominent target of C3a action, is C3aR mRNA so evident in calvarial osteoblasts because of the macrophage content of mouse calvarial cultures?(7) Macrophage‐mediated actions of C3a need to be considered in both the in vitro and in vivo experiments of the present work, in view of the known activation through Ca3R in monocytes and macrophages of mediators such as IL‐1b through participation of Toll‐like receptor and ATP‐mediated P2X purinoreceptor (P2 7) signals in monocytes and macrophages.(8,9) Further, the fact that C3a circulates in such substantial amounts (approximately
Address correspondence to: T. John Martin, MD, DSc, St Vincent’s Institute of Medical Research and University of Melbourne, 9 Princes Street, Fitzroy 3065, Victoria, Australia. E‐mail: [email protected] This is a Commentary on Matsuoka et al. (J Bone Miner Res. 2014;29:1522–1530. DOI: 10.1002/jbmr.2187). Journal of Bone and Mineral Research, Vol. 29, No. 7, July 2014, pp 1519–1521 DOI: 10.1002/jbmr.2276 © 2014 American Society for Bone and Mineral Research
1519
50 ng/mL in human serum)(10) might call into question a role for C3a only at selected sites in the skeleton as they undergo remodeling. The same research group suggested in a recent report that collagen triple helix repeat containing 1 (CTHRC1) is a coupling factor by virtue of its production by actively resorbing osteoclasts and the fact that it stimulated bone formation in vitro and in vivo.(11) The conclusion from that work, based on in situ hybridization, was that CTHRC1 was not produced by osteoblasts, but by chondrocytes in the embryo, as well as at the growth plate until 3 months of age. In contrast, Kimura and colleagues(12) found CTHRC1 to be a product of the osteoblast lineage, including mesenchymal precursors, that markedly stimulates bone formation. Reconciliation of these discrepant findings will require careful cellular localization studies to establish whether CTHRC acts as an osteoclast product or a signal within the osteoblast lineage. Either way, it could be a participant in local events that contribute to the overall bone remodeling process, whether strictly as one of a number of coupling factors or not. The proposal some decades ago of TGFb and IGF‐1 as coupling agents(13,14) has gained recent support from mouse genetic experiments,(15,16) but there is much emphasis now on the search for osteoclast‐derived control of coupling.(17) When osteoclast conditioned medium stimulated mesenchymal stem cell migration and osteoblastic differentiation, Pedersen and colleagues(18) undertook a microarray study that led them to identify sphingosine‐1‐phosphate (S1P), Wnt 10b, and BMP6 as osteoclast products that might be coupling factors. Whether important as an osteoclast product or not, Wnt10b has been invoked in another context as a T‐cell–derived mediator of the anabolic effect of parathyroid hormone (PTH).(19) The roles of the sphingosine kinase product, S1P, in bone are complex. It can have inhibitory or stimulatory effects on osteoblasts depending on the stage of cell differentiation and on the source of precursors,(18,20,21) can limit bone resorption by increasing recirculation of osteoclast precursors from bone to blood,(22) and can promote osteoprotegerin production by osteoblasts.(23) Data more encouraging of S1P as a coupling agent showed that catK null osteoclasts, with impaired resorption, were nevertheless able to promote aspects of osteoblast differentiation in co‐ culture, an effect inhibited by S1P receptor antagonist.(24) In addition to osteoblast stimulators from osteoclasts, semaphorin 4D was found as an osteoclast product that inhibits osteoblast differentiation and bone formation.(25) In addition to such secreted positive and negative regulators of coupling, membrane activities have been invoked, including ephrinB2, membrane‐bound in the mature osteoclast, and suggested to act on its receptor, EphB4, in the osteoblast lineage to promote differentiation, a process that would require cell contact.(26) Such contact would also be required to support the hypothesized role of RANK driving reverse signaling through RANKL in the osteoblast lineage.(27) These and other “candidates” are considered in more detail in recent reviews that highlight the complexity of coupling in the BMU, including the likely contribution of products of cells other than osteoclasts.(28,29) How many candidates can there be? It seems likely that there are many factors derived from matrix and cells in the BMU—not only osteoclasts, but T cells, macrophages, and osteoblasts themselves—that contribute to some extent to the process of coupling. The likelihood of finding a dominant, single factor analogous to RANKL in promotion of osteoclasts seems remote. The precision in time and space required to program osteoblast
1520
MARTIN
lineage cells in the BMU likely requires a number of local factors acting at different stages of osteoblast differentiation. No studies so far address this question: At what stages of osteoblast differentiation do “candidate” coupling factors exert their actions, and what are those actions? Most of the candidate coupling factors have been arrived at through in vitro studies of osteoblast cells at particular stages of differentiation or occasionally ex vivo studies on cells from genetically manipulated mice. As can be said of most—or even all—of the other candidate coupling factors so far, there are aspects of interest in the present work,(6) but more would be required to be convincing of a critical role for C3a in remodeling within the BMU and how it might operate. The work provides another link with the immune system and even with inflammation, something that might not necessarily be so surprising. In describing bone remodeling, Frost likened it to the healing of a soft tissue wound, an event associated with the release of inflammatory mediators,(1) and certain similarities have been pointed out between bone remodeling and inflammation.(30) The biological effects of activated complement result from the release of cytokine signals from macrophages, T cells, and dendritic cells. C3a leads to inflammation by amplifying the immune response(31) in asthma, sepsis, liver regeneration, and autoimmune encephalomyelitis.(31,32) Mice null for the C3aR gene had a marked reduction in macrophage content of adipose tissue, reduced adiposity, and enhanced insulin sensitivity.(33) Any possible contribution of activated complement to enhanced osteoblast differentiation in the cellular events in remodeling in the BMU should take into account the participation of other cells.
Disclosures The author states that he has no conflicts of interest.
Acknowledgments The author acknowledges support from the National Health and Medical Research Council, Australia, and the Victorian Government OIS Program to St. Vincent’s Institute of Medical Research.
References 1. Hattner R, Epker BN, Frost HM. Suggested sequential mode of control of changes in cell behaviour in adult bone remodelling. Nature. 1965;206(983):489–90. 2. Frost HM. Dynamics of bone remodeling. In: Frost HM, editor. Bone biodynamics. Boston: Little‐Brown; 1964. p.315–33. 3. Parfitt AM. The cellular basis of bone remodeling: the quantum concept reexamined in light of recent advances in the cell biology of bone. Calcif Tissue Int. 1984;36(Suppl 1):S37–45. 4. Eriksen EF. Normal and pathological remodeling of human trabecular bone: three dimensional reconstruction of the remodeling sequence in normals and in metabolic bone disease. Endocr Rev. 1986;7(4): 379–408. 5. Harris WH, Heaney RP. Skeletal renewal and metabolic bone disease. N Engl J Med. 1969;280(4):193–202. 6. Matsuoka K, Park KA, Ito M, Ikeda K, Takeshita S. Osteoclast‐derived complement component 3a stimulates osteoblast differentiation. J Bone Miner Res. 2014;29(7):1520–28. 7. Chang MK, Raggatt LJ, Alexander KA, et al. Osteal tissue macrophages are intercalated throughout human and mouse bone lining tissues and regulate osteoblast function in vitro and in vivo. J Immunol. 2008;181(2):1232–44.
Journal of Bone and Mineral Research
8. Asgari E, Le Friec G, Yamamoto H, et al. C3a modulates IL‐1beta secretion in human monocytes by regulating ATP efflux and subsequent NLRP3 inflammasome activation. Blood. 2013;122(20): 3473–81. 9. Dinarello CA. The C3a receptor, caspase‐1, and release of IL‐1beta. Blood. 2013;122(20):3394–5. 10. Wlazlo N, van Greevenbroek MM, Ferreira I, et al. Activated complement factor 3 is associated with liver fat and liver enzymes: the CODAM study. Eur J Clin Invest. 2013;43(7):679–88. 11. Takeshita S, Fumoto T, Matsuoka K, et al. Osteoclast‐secreted CTHRC1 in the coupling of bone resorption to formation. J Clin Invest. 2013;123(9):3914–24. 12. Kimura H, Kwan KM, Zhang Z, et al. Cthrc1 is a positive regulator of osteoblastic bone formation. PLoS One. 2008;3(9):e3174. 13. Howard GA, Bottemiller BL, Turner RT, Rader JI, Baylink DJ. Parathyroid hormone stimulates bone formation and resorption in organ culture: evidence for a coupling mechanism. Proc Natl Acad Sci USA. 1981;78(5):3204–8. 14. Baylink DJ, Finkelman RD, Mohan S. Growth factors to stimulate bone formation. J Bone Miner Res. 1993;8(Suppl 2):S565–72. 15. Tang Y, Wu X, Lei W, et al. TGF‐beta1‐induced migration of bone mesenchymal stem cells couples bone resorption with formation. Nat Med. 2009;15(7):757–65. 16. Xian L, Wu X, Pang L, et al. Matrix IGF‐1 maintains bone mass by activation of mTOR in mesenchymal stem cells. Nat Med. 2012; 18(7):1095–101. 17. Martin TJ, Sims NA. Osteoclast‐derived activity in the coupling of bone formation to resorption. Trends Mol Med. 2005;11(2):76–81. 18. Pederson L, Ruan M, Westendorf JJ, Khosla S, Oursler MJ. Regulation of bone formation by osteoclasts involves Wnt/BMP signaling and the chemokine sphingosine‐1‐phosphate. Proc Natl Acad Sci USA. 2008;105(52):20764–9. 19. Bedi B, Li JY, Tawfeek H, et al. Silencing of parathyroid hormone (PTH) receptor 1 in T cells blunts the bone anabolic activity of PTH. Proc Natl Acad Sci USA. 2012;109(12):E725–33. 20. Ryu J, Kim HJ, Chang EJ, Huang H, Banno Y, Kim HH. Sphingosine 1‐phosphate as a regulator of osteoclast differentiation and osteoclast‐osteoblast coupling. Embo J. 2006;25(24):5840–51. 21. Quint P, Ruan M, Pederson L, et al. Sphingosine 1‐phosphate (S1P) receptors 1 and 2 coordinately induce mesenchymal cell migration
Journal of Bone and Mineral Research
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