Mengrui Wu,Guiqian Chen,2and Yi-Ping Li
TGF-β and BMP signaling in osteoblast,skeletal development,and bone formation,homeostasis and disease
Mengrui Wu1,Guiqian Chen1,2and Yi-Ping Li1
Transforming growth factor-beta(TGF-β)and bone morphogenic protein(BMP)signaling has fundamental roles in both embryonic skeletal development and postnatal bone homeostasis.TGF-βs and BMPs,acting on a tetrameric receptor complex,transduce signals to both the canonical Smad-dependent signaling pathway (that is,TGF-β/BMP ligands,receptors,and Smads)and the non-canonical-Smad-independent signaling pathway(that is,p38 mitogen-activated protein kinase/p38 MAPK)to regulate mesenchymal stem cell differentiation during skeletal development,bone formation and bone homeostasis.Both the Smad and p38 MAPK signaling pathways converge at transcription factors,for example,Runx2 to promote osteoblast differentiation and chondrocyte differentiation from mesenchymal precursor cells.TGF-β and BMP signaling is controlled by multiple factors,including the ubiquitin–proteasome system,epigenetic factors,and microRNA.Dysregulated TGF-β and BMP signaling result in a number of bone disorders in humans. Knockout or mutation of TGF-β and BMP signaling-related genes in mice leads to bone abnormalities of varying severity,which enable a better understanding of TGF-β/BMP signaling in bone and the signaling networks underlying osteoblast differentiation and bone formation.There is also crosstalk between TGF-β/BMP signaling and several critical cytokines'signaling pathways(for example,Wnt,Hedgehog, Notch,PTHrP,and FGF)to coordinate osteogenesis,skeletal development,and bone homeostasis.This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in osteoblast differentiation,chondrocyte differentiation,skeletal development,cartilage formation,bone formation, bone homeostasis,and related human bone diseases caused by the disruption of TGF-β/BMP signaling.
The transforming growth factor-β(TGF-β)superfamily comprises TGF-βs,Activin,bone morphogenetic proteins(BMPs) and other related proteins.1–4TGF-β superfamily members act through a heteromeric receptor complex,comprised of type I and type II receptors at the cell surface that transduce intracellular signals via Smad complex or mitogen-activated protein kinase(MAPK)cascade.1–4At least 29 and probably up to 42 TGF-β superfamily members,f i ve type II receptors and seven type I receptors are encoded by the human genome.2Signals transduced by TGF-β superfamily members regulate the establishment of tissue differentiation through their effects on cell proliferation,differentiation,and migration.1–4
Fetus mammalian skeletal development begins with the condensation of mesenchymal stem cells(MSCs)from neural crest or mesoderm,and is accomplished in two distinct processes:endochondral ossi f i cation and intramembranous ossi f i cation.5–6Endochondral ossi f i cation takes place in skull base and the posterior part of the skull, the axial skeleton,and the appendicular skeleton.Intramembranous ossi f i cation takes place in membranous neuro-and viscerocranium and some parts of the clavicles.5–6During endochondral ossi f i cation,condensedmesenchyme undergoes chondrogenesis and forms cartilage model(anlagen),which is later replaced by mineralized bone.Differentiated chondrocytes were organized into growth plate comprised of different zones, including resting zone,proliferation zone,hypertrophic zone,and calci f i ed zone.5–6During intramembranous ossi f i cation,condensed mesenchyme directly differentiates into osteoblasts.5–6Several cytokines and growth factors orchestrate skeletogenesis(for example,f i broblast growth factor(FGF),Notch,Wnt,Sonic hedgehog(SHH), Indian hedgehog(IHH),parathyroid hormone-related peptide(PTHrP),TGF-β,and BMP).Among them,TGF-βs and BMPs have diverse functions in skeletogenesis,including mesenchyme condensation,skeleton morphogenesis, growth plate development,and osteoblast differentiation.4A wide variety of inheritable developmental bone diseases are caused by human genetic mutations related to TGF-β and BMP signaling.
Besides their roles in bone development,TGF-βs and BMPs also regulate the maintenance of postnatal bone and cartilage.TGF-βs have essential roles in coupling bone construction by osteoblast and bone destruction by osteoclast7–8through osteoclast-mediated Atp6i-speci f i c extracellular acidi f i cation9–10and Cathepsin K-speci f i c extracellular matrix proteins.11–12Multiple BMPs are also potent osteogenic agents that possess signi f i cant clinical implication to accelerate fracture healing in patients.13–14Furthermore,TGF-βs and BMPs regulate postnatal joint cartilage homeostasis,thus dysregulated TGF-β and BMP signaling are often associated with osteoarthritis in both human disease and mouse models.15–19
In this review,we will focus on current understanding of the mechanisms by which different TGF-βs and BMPs transduce signaling and exert their functions in skeletal development and homeostasis(Figures 1 and 2).We will review mouse models(Table 1)and human bone disorders (Table 2)caused by TGF-β/BMP dysregulation.We will also discuss the crosstalk between TGF-β/BMP signaling and other signaling pathways including Wnt,Hedgehog,Notch, and FGF in bone.Those studies have opened new prospects for generating novel prognostics and therapies against bone diseases.
There are three TGF-βs:TGF-β1,TGF-β2,and TGF-β3 in mammals.2TGF-βs elicit their cellular response via binding to a tetrameric receptor complex comprising two TGF-β type I receptors(TβRI/ALK5)and two type II kinase receptors (TβRII)2(Figure 1).TβRII transphosphorylases TβRI resulting in subsequent phosphorylation of receptor-activated Smads(R-Smads),Smad2 and 3(Figure 1).2R-Smads then interact with the common Smad(Co-Smad),Smad4,and translocate into the nucleus,where they recruit co-factors to regulate gene(for example,CREB-binding protein(CBP) or p300)transcription.2Recent studies have revealed that TGF-βs can also activate another group of R-Smad(Smad1, 5,and 8)via binding to ALK1.17,20As an alternative non-Smad-dependent pathway,TGF-β also activates kinase 1 (TAK1)and TAK1-binding protein 1(TAB1)which in turn initiates the MKKs(MAPK pathway member-encoding genes kinases)-p38 MAPK or-Erk(extracellular signalregulated kinase)signaling cascade(Figure 1).4
TGFβs ligands
All TGF-β isoforms are expressed by perichondrium and periosteum21–22as well as epiphyseal growth plate23. However,only Tgfb2-null mice displayed severe skeletal abnormalities in both endochondral and intramembranous bone,24while Tgfb1-de f i cient mice and Tgfb3-null mice had almost normal skeleton.25–27Thus,TGF-β2,but not TGF-β1 or TGF-β3,is essential for embryonic skeleton development.
TGF-β has an important role in maintaining postnatal bone mass by coupling bone resorption and bone formation(Figure 1).TGF-βs are synthesized as large precursor molecules,composed of mature TGF-β and latency-associated protein(LAP).28LAP remains noncovalently bound to mature TGF-β as it is secreted,rendering it inactive by masking the ECM(extracellular matrix)of many different tissues.28Cleaving LAP by osteoclastic bone resorption release active TGF-β1 to induce enrichment of osteoprogenitor in the bone resorption lacunae.7Tang et al.crossed Tgfb1-null mice with immunode f i cient Rag2-/-mice to overcome the early death of Tgfb1-null mice.8,25The Tgfb1-/-Rag2-/-mice showed a signi f icant loss of trabecular bone density and reduced osteoblast number on the bone surface.8A gain-offunction mutant of TGF-β1 causes Camurati–Engelmann disease(CED)in humans,19–20which is characterized by diaphyseal thickening and f l uctuating bone volume. This mutation results in a similar bone defect in mice (CED mice).8However,TGF-β1 is unable to induce osteogenesis in mesenchymal pluripotent cells,but increases the pool of osteoprogenitors by inducing chemotaxis and proliferation.8Apoptosis of osteoblasts is also blocked by TGF-β1 deletion through maintenance of survival during transdifferentiation into osteocytes.29In addition,TGF-β treatment blocked osteoblast mineralization in culture,30indicating its bi-functions in osteoblast differentiation.On the other hand,active TGF-βs regulate bone resorption in a dose-dependent manner.The gradient of TGF-β created during osteoclast bone resorption can limit further osteoclast activity.28Low concentrations of active TGF-β can induce macrophage migration to the bone resorption pits,whereas high concentrations of active TGF-β inhibit migration of osteoclast precursors.28TGF-β was also shown to promote osteoclast differentiation at low dose and inhibits osteoclast differentiation at high dose through regulating RANKL/OPG ratio secreted by osteoblasts.7,31Furthermore,TGF-β1 via binding to its receptor on osteoclasts activates Smad2/3,which associates directly with TRAF6–TAB1–TAK1 complex and favors osteoclast differentiation.32
TGF-β is a double-edged sword in the maintenance of articular cartilage metabolic homeostasis and the pathogenesis of arthritis33.Loss of TGF-β signaling in cartilage induces chondrocyte hypertrophy,ultimately resulting in cartilage degeneration,and pharmacological activation of the TGF-β pathway has therefore been proposed to preserve articular cartilage integrity during osteoarthritis (OA).19,34–35However,age-dependent switch from TGF-β-ALK5-Smad2/3 to-ALK1-Smad1/5/8 signaling contributes to osteophyte formation and the pathogenesis of osteoarthritis.17,36At the onset of either OA or rheumatoid arthritis(RA),elevated active TGF-β1 resulted from excessive bone resorption recruits MSCs in the subchondral bone marrow and induces the formation of“osteoid islet”,which leads to the degeneration of the overlying articular cartilage.37–38Transgenic expression of active TGF-β1 in osteoblastic cells induced OA,whereas inhibition of TGF-β activity in subchondral bone attenuated the degeneration of articular cartilage.37Similarly,aberrant activation ofTGF-β in subchondral bone is involved at the onset of RA joint cartilage degeneration,whereas either systemic or local blockade of TGF-β activity in the subchondral bone attenuated articular cartilage degeneration in RA.38Furthermore,knockout of the TβRII in nestin-positive MSCs leads to less development of both OA and RA relative to wild-type mice.37–38
Extracellular regulation of TGF-β signaling
ECM proteins regulate TGF-β signaling through controlling ligand availability(Figure 1).LTBPs covalently bind latent TGF-β and modulate tissue levels of TGF-β.Ablation of LTBP-3 reduces both osteogenesis and bone resorption,and resulted in increased bone mass associated with decreased levels of TGF-β.39–40Binding of TGF-β to small leucine-rich proteoglycans(decorin,biglycan, and lumican)restricts the TGF-β in the ECM so as to inhibit its activity.41Bgn and Dcn double de f i ciency results in osteopenia and a striking change in collagen f i bril shape.41Excessive TGF-β signaling,caused by reduced binding of decorin,is also found to be a common mechanism of osteogenesis imperfecta in mouse models.42
TGF-βs receptors
Similarly to TGF-βs,the expression of TGF-β receptors is also detected in the growth plate and perichondrium.21–22,43Mice lacking either TβRI/ALK5 or TβRII in MSC results in short long bone and defects in joint development fusion.30,44–46
MSC-speci f i c Tgfbr2-deletion and MSC-speci f i c Alk5-deletion both promote chondrocyte hypertrophy and decreased chondrocyte proliferation,which was also observed in DN-TβRII transgenic mice.30,34,44–46Thus,TGF-β signaling favors chondrocyte proliferation but blocks the transition from proliferative chondrocyte into hypertrophic chondrocyte.Mice lacking TβRII/ALK5 in the chondrocyte can reproduce defects in the skull base and vertebrae as those observed in the Tgfb2-null mice,24,47but cannot in f l uence normal long bone development.47This phenotype indicates that TGF-β might regulate growth plate development largely through mediating paracrine cytokines secretion by perichondrium cells.
Table 1.Mouse models of TGF-β/BMP signaling in bone
TGF-β signaling favors bone formation by promoting osteoprogenitor enrichment.Consistently,deletion of Alk5 in MSC resulted in reduced bone collars and trabecular bones associated with decreased osteoblast number.30The study showed that TGF-β,through TβRI,promotes pre-osteoblast commitment and early differentiation.30However,deletion of Tgfbr2 in osteoblasts resulted in an unexpected increase in bone mass due to enhanced PTHrP signaling.43
Table 2.TGF-β/BMP mutations involved in bone diseases
Smad-dependent pathway
Smad2 and Smad3 respond to TGF-β and regulate TGF-βmediated osteoblast and chondrocyte differentiation (Figure 1).Smad2/3 inhibits Runx2 expression,and activated Smad3 also recruits class II histone deacetylases(HDACs)4 and 5 to repress the function of Runx2.48–51TGF-β can no longer inhibit the differentiation of osteoblasts in the absence of Smad3.52–53Although TGF-β-Smad3 negatively regulates osteoblastogenesis,it also inhibits osteoblast apoptosis and the differentiation into osteocyte.Thus, Smad3-null mice are osteopenic associated with increased osteocyte number and apoptosis.52Furthermore,TGF-βinduced inhibition of chondrocyte maturation is potentiated by Smad2/3 signaling,54and abolished by disrupting Smad2-/3 activation.54–55Smad3-null mice also displayed accelerated chondrocyte hypertrophy,at least partially through reduced Noggin level and unopposed BMP signaling.56
In addition,TGF-β-Smad3 signaling is required for postnatal maintenance of articular cartilage homeostasis. Smad3-null mice and chondrocyte-speci f i c Smad3 CKO mice exhibit articular cartilage degeneration and automatically develop OA.57–59Mutations of Smad3 in human caused aneurysms–osteoarthritis syndrome,15,60–61which is presenting with aneurysms,mild craniofacial features,and skeletal and cutaneous anomalies.Mechanistically,TGF-β signals through Smad3 to confer a rapid and dynamic repression of Runx2-induced MMP-13 expression,and signals through p38 to promote Runx2-induced MMP-13 expression in the absence of Smad3.58
Non-Smad-dependent pathway
Non-Smad-dependent pathway initiated by TGF-β also contributes to bone formation.Studying MSC-speci f i c Alk5 CKO mouse model revealed that TGF-β promotes osteoblast proliferation and early differentiation through both Smad2/3 and MAPK signaling pathway.30MAPK signal activated by TGF-β and BMP positively regulates Runx2 expression and function to promote MSC differentiation.62Furthermore,TGF-β2 stimulates cranial suture closure by promoting osteoprogenitor proliferation via the MKK–ERK signaling.63
Previous studies showed that TGF-β,through Alk5-Smad2-/3 or Alk5-MAPK signaling,promotes osteoblast progenitor enrichment and early differentiation,but negatively regulates osteoblast differentiation and mineralization at latterstages.TGF-β is also a molecule coupling bone formation with bone resorption.The relationship between TGF-β and cartilage is more complex.Either loss of TGF-β or excessive active TGF-β contributes to the progress of osteoarthritis. Further studies will elucidate how TGF-β dose-dependently and stage-dependently regulates cartilage integrity.Those fi ndings would facilitate the design of therapeutic approaches for targeting TGF-β in OA treatment.
BMP signaling is mediated through type I and type II BMP receptors(Figure 2).After binding to BMP ligands,homomeric dimers of the type II receptors form a tetrameric complex with homomeric dimers of the type I receptors, and induce transphosphorylation of the type I receptors. This dynamic interaction leads to signal transduced through either Smads or MAPKs,which further activates the transcription of speci f i c target genes involved in osteoblastic differentiation and bone formation(Figure 2).
BMP ligands
Among the 14 BMPs,BMP-2,4,5,6,7,and 9 exhibit high osteogenic activity.64–65Osteogenic capability of BMP-2 and BMP-7 have been vastly studied and the recombinant proteins are currently being investigated in human clinical trials of craniofacial deformities,fracture healing,and spine fusion.13–14BMP-2 vastly increases osteocalcin expression66and a short-term expression of the BMP-2 is necessary and suf f i cient to irreversibly induce bone formation.67BMP-7 induces the expression of osteoblastic differentiation markers,such as ALP and accelerates calcium mineralization.68–70BMP-3 is a‘non-canonical'BMP molecule that transduces type IIB Activin receptor(AcvrIIB)-Smad2/3 signaling to oppose the osteogenic function of other BMPs.71In vivo,BMP-3 is mainly produced by osteoblasts and osteocytes,71and negatively regulates bone mass in vivo,as shown by knockout and transgenic mouse models.72–73Interestingly,a recent study showed that BMP2 addition to culture media rapidly induced expansion of isolated mouse skeletal stem cell(mSSC).74Delivery of BMP2 induces de novo formation of the mSSC and bone in extraskeletal locations.74In addition, co-delivery of BMP2 and sVEGFR1 induces de novo formation of cartilage in extraskeletal locations.74Expression of the BMP antagonist gremlin 1 de f i nes a population of osteochondroreticular stem cells in the bone marrow, which is self-renewal and is able to generate osteoblasts, chondrocytes,and reticular marrow stromal cells,but not adipocytes.75
BMPs have essential roles at many steps in endochondral bone development.Studies showed that BMP signaling promotes chondrocyte proliferation and differentiation,76–78by maintaining the expression of Sox9,a transcription factor that is essential for chondrogenic commitment and differentiation.77,79BMP-7 null mice died shortly after birth and exhibits skeletal patterning defects restricted to the rib cage,skull,and hindlimbs.80Conditional deletion of BMP-2, BMP-4,or BMP-7 in MSC results in normal skeletongenesis.70,81–82Only BMP-2 CKO mice have frequent fractures that fail to heal,which is not observed in either Bmp-4 or Bmp-7 CKO mice.70,83–84The studies indicate that BMPs have duplicate functions in skeleton development,while BMP-2 has a unique role in postnatal bone formation.MSC-speci f i c Bmp-2/-4 DKO mice have extremely malformed limbs and a severe impairment of osteogenesis,which is not observed in Bmp-2/-7 DKO mice.82,85Chondrogenic condensations in some skeletal elements fail in MSC-speci f i c Bmp-2/-4 DKO mice.82Furthermore,chondrocyte-speci f i c Bmp-2 CKO and Bmp-2/4 DKO mice exhibits severe disorganization of chondrocytes within the growth plate region.86Collectively, these studies revealed that coordinated functions of endogenous BMP-2 and BMP-4 are required for osteoblastogenesis as well as chondrocyte proliferation,differentiation and apoptosis during skeletal development.
BMP7 has a pivotal role in postnatal maintenance of articular cartilage.Intra-articular administration of rhBMP7 signi f i cantly inhibited articular cartilage degeneration and blocked production of in f l ammatory cytokines by the synovial membrane.18,87–88Bmp7f/fPrx1-Cre mice,and Bmpr1af/fGdf5-Cre mice exhibit articular cartilage degeneration and automatically develop OA.89
BMP receptors
There are three type I receptors for BMPs,type IA BMP receptor(BMPR1A/ALK3),type IB BMP receptor(BMPR1B/ ALK6),and type I activin receptor(AcvR1/ALK2).BMPs exert their osteogenic signaling through those receptors. Activating mutants of Acvr1 cause ectopic ossi f i cation in mouse90and in human disease(f i brodysplasia ossi f i cans progressive,FOP).91–92Mechanically,active AcvR1 induces epithelial-to-mesenchymal transition and promotes ectopic osteoblastogenesis via canonical Smad1/5 signaling.90,93On the other hand,blocking BMP signaling through overexpression of DN-BMPRII or deletion of Bmpr1a in osteoblast results in low bone mass.94–95Although deletion of Bmpr1a in preosteoblasts causes an unexpected increase in bone mass due to reduced bone resorption,osteoblast differentiation remains low in this condition.96–98
The three type I BMP receptors(BMPR1A,BMPR1B,and ALK6)have largely redundant roles in skeletal development.BMPR1A and BMPR1B share similar expression pattern in chondrocyte condensations and developing skeletons,99–104and AcvR1 is expressed throughout the growth plate.105Activation and dominant-negativeexperiments proved the BMPRIB is essential for cartilage condensation in chicks.103Activation experiments showed that BMPR1A,BMPR1B,and AcvR1 are all able to promote chondrogenesis.77,101,103,106–108However,deletion of each of them alone only results in mild skeletal defects or defects restricted to certain skeletal elements.77,104–105,109–110In contrast,Bmpr1a/Bmpr1b DKO mice,Acvr1/Bmpr1a DKO mice,and Acvr1/Bmpr1b DKO mice exhibit generalized chondrodysplasia that is much more severe than any of the corresponding mutant strains.104–105,110The loss of both BMPR1A and BMPR1B blocks chondrocyte condensation, proliferation,differentiation,survival,and function due to impaired Sox proteins(Sox-5,6,and 9)expression.104,110In conclusion,those mouse models demonstrated that BMP signaling is essential for almost every step during endochondral bone development(Figure 3).
Smad-depedent pathway
Most BMPs activate Smad1/5/8 as their R-Smad(Figure 2). Smad1/5/8-Smad4 complex transcribed Runx2 expression, as they complex with Runx2 to initiate other osteoblast gene expression.62,111–112Smad1 is an important mediator of BMP's osteogenic function.In osteoblast-speci f i c Smad1-CKO mice,BMP signaling was partially inhibited, osteoblast proliferation and differentiation were impaired and mice developed an osteopenic phenotype.113Smad1 and Smad5 together mediate the role of BMP in endochondral bone development.Chondrocyte-speci f i c deletion of Smad1 results in delayed calvarial bone development and shortening of the growth plate.113–114The addition of Smad5 haploinsuf f i ciency or insuf f i ciency leads to more severe chondrodysplasia,while addition of Smad8 insuf f i ciency did not worsen the defects.114–115Thus, Smad8 contributes much less to skeleton development as compared with Smad1 and Smad5.Only BMP3 also activates Smad2/3,which antagonized osteogenic activity of Smad1/5/8 and opposed the function of other BMPs in osteoblast differentiation.116
Both BMPs and TGF-βs signal through Smad4.Smad4 mutation in humans causes Myhre syndrome,a developmental disorder,characterized by short stature and facial dysmorphism.117–118In vitro Smad4 ablation partially suppressed BMP-4-induced osteoblast differentiation.119In vivo abrogation of Smad4 in chondrocytes results in dwar f i sm with a severely disorganized growth plate and ectopicbone collars in perichondrium.120In the Smad4-de f i cient growth plate,resting zone was expanded,while chondrocyte proliferation was reduced and hypertrophic differentiation was accelerated.120Deletion of Smad4 in mature osteoblasts causes lower bone mass and decreased osteoblast proliferation and differentiation up to 6 months of age,while the trabecular bone volume in the mutant mice increased after 7-month-old due to reduced bone resorption.121Embryonic deletion of Smad4 in preosteoblast causes stunted growth,spontaneous fractures and a combination of features seen in osteogenesis imperfecta,cleidocranial dysplasia,and Wnt-de f i ciency syndromes.122Postnatal deletion of Smad4 in preosteoblasts increases mitosis of cells on trabecular bone surfaces as well as in primary osteoblast cultures,while delays differentiation and matrix mineralization by primary osteoblasts associated with altered β-catenin acvitity.123Knockout of Smad4 in osteoblast also reveals that BMP/TGFβs coordinate with Wnt signaling to maintain normal bone mass.In summary,Smad4 has multiples roles in growth plate development and postnatal bone homeostasis.
Non-Smad-dependent pathway
TAK1–MKK–MAPK pathway is also involved in the f i netuning of BMP effects on skeletal development and osteogenic differentiation(Figure 2).Morphological defects of BMP-receptor-de f i cient mice are found to be associated with a decrease in both p38 MAPK and Smad activity.105Ectopic ossi f i cation of FOP patients is associated with an increase in both p38 MAPK and Smad activity.124Either chondrocyte-or MSC-speci f i c Tak1-deletion results in disorganized growth plates as well as a failure to maintain interzone cells of the elbow joint.125Postnatal chondrocytespeci f i c Tak1 deletion results in severe defects of growth plate and articular cartilage development with reduced expression of SOX protein,proteoglycan and type II collagen.126Osteoblast-speci f i c deletion of Tak1 results in clavicular hypoplasia and delays fontanelle fusion,which is a phenotype similar to that of cleidocranial dysplasia,a human disease that is caused by Runx2 de f i ciency.127Mice with deletion of other MAPK pathway member-encoding genes,MAPK kinase 3(Mkk3),Mkk6,p38a,or p38b,also display profoundly reduced bone mass secondary to defective osteoblast differentiation.127Mechanistically, MAPKs phosphorylate a number of osteoblast master transcription factors including Runx2,111,127–128Dlx-5129and Osterix130–132(Figure 2).The phosphorylation facilitates the recruitment of cofactors and promotes their transcriptional activity.111,127–130MAPKs also positively regulate Runx2 and Osterix expression to promote MSC differentiation.62Furthermore,MAPK signaling enhances the canonical BMP–Smad signaling by promoting the interaction between Runx2 and Smad complex.111
In conclusion,most BMP ligands are strong osteogenic agents,through both Smad and non-smad signaling pathway,which synergize at osteogenic transcriptional factors(for example,Runx2,Osx).BMPs also have critical roles in multiple stages of endochondral bone development,including mesenchyme condensation, chondrocyte proliferation,and chondrocyte differentiation.In addition,BMP7 and BMPR1A also have roles in postnatal homeostasis of articular cartilage.Recent studies have shown that BMP2 enhances proliferation and enrichment of mouse skeletal stem cell.Thus,it is of full potential to utilize BMP locally in bone and cartilage regeneration.
BMP and TGF-β signaling are delicately controlled by multiple machineries:Extracellular matrix proteins control the availability of the ligands;Inhibitory Smads antagonize various steps in the Smad-dependent signaling;ubiquitin–proteasome machinery controls stability of various signal transducers and inhibitors;miRNAs regulate the signaling at post-translational level;co-repressors and epigenetic factors regulate the signaling at transcriptional level.
Regulators in the ECM
ECM proteins regulate TGF-β signaling through controlling ligand availability(Figure 1).LTBPs covalently bind latent TGF-β and modulate tissue levels of TGF-β.Ablation of LTBP-3 reduces both osteogenesis and bone resorption, and resulted in increased bone mass associated with decreased levels of TGF-β.39–40Binding of TGF-β to small leucine-rich proteoglycans(decorin,biglycan,and lumican)restricts the TGF-β in the ECM so as to inhibit its activity.41Bgn and Dcn double de f i ciency results in osteopenia and a striking change in collagen f i bril shape.41ExcessiveTGF-β signaling caused by reduced binding of decorin is also found to be a common mechanism of osteogenesis imperfecta in mouse models.42
BMP signaling is regulated by a group of cognate binding proteins(for example,Noggin,Chordin,Gremlin, and Follistatin)that competitively bound BMPs to prevent their binding to receptors133–135(Figure 2).Noggin opposes BMP's osteogenic function in vivo136and in vitro.137Nognull mice died at birth with severe malformed skeletons due to unopposed BMP signaling,138–139and addition of de f i ciency of other BMP antagonists(Grem1 or Follistatin) aggravated the skeleton malformation of Nog-/-mice.140–141Ectopic expression of Noggin in osteoblast impairs osteoblastogenesis and results in osteopenia in mice.142–143Ectopic expression of Noggin is also able toblock cartilage development,change skeletal morphology,and prevent cranial suture fusion.144–145Noggin expression is induced by BMPs,and acts in a negative regulatory loop to inhibit BMP activity.133Other signals including Sox9 and FGFs also regulate BMP signaling via manipulating Noggin expression.145–148Chordin and Chordin-like proteins(CHL)were identi f i ed as a factor dorsalizing the Xenopus embryo.149–152Chordin and CHL treatment prevented BMP-induced osteoblast differentiation and mineralization,152–153as well as BMP-induced chondrocyte maturation.149,152,154Furthermore,Chordin-/-Nog+/-mice exhibit defects restricted to the head associated with increased BMP activity.155
I-Smad
Inhibitory Smads(I-Smad,Smad6,and 7)inhibits BMPs and TGF-βs signal in multiple ways:preventing R-Smad nuclei translocation,competitively binding with type I receptor to prevent R-Smad phosphorylation,and promoting R-smads or receptor degradation by recruiting E3 ubiquitin ligases Smurf1/2.156–157Smad6 preferentially inhibits BMP signaling (Figure 2),while Smad7 interferes with both BMP and TGFβ signaling157(Figures 1 and 2).Smad6 and 7 overexpression blocks BMPs-induced osteoblast and chondrocyte differentiation in vitro,107,158–161and vice versa.160Chondrocytespeci f i c Smad6 transgenic mice show postnatal dwar f i sm, osteopenia,and delayed chondrocyte hypertrophy due to inhibition of Smad1/5/8 signaling.162Besides its role in BMP signaling,Smad6 and Smurf1 also induce Runx2 degradation in an ubiquitin-proteasome-dependent manner,which directly inhibits osteoblast differentiation.163Thus, Smad6 expression is also mediated by BMP–Smad1–Runx2 signaling at transcriptional level and regulates BMP and Runx2 activity in a negative feedback loop.160,164Conditional-overexpression of Smad7 in MSC or stagespeci f i c chondrocytes results in defective mesenchymal condensation,chondrocyte proliferation and chondrocyte maturation,respectively.165
Ubiquitin–proteasomal degradation pathway
Stability of protein transducers within TGFβ and BMP signaling is under control of ubiquitin enzymes and deubiquitin enzymes(Figures 1 and 2).Smurf2 is an E3 ubiquitin ligase that targets TGFβreceptor and Smad2/3 for ubiquitin–proteasomal degradation.166–167Ectopic overexpression of Smurf2 in condensing chondrogenic mesenchyme of chicken wing bud accelerates chondrocyte maturation and ossi f i cation.166Smurf1 is also an E3 ubiquitin ligase with a wide range of a targets,including BMP type I receptors,Smad1/5,Runx2,and MKK2.162–163,167–168Smad6/Smurf1 double transgenic pups exhibit delayed endochondral bone formation that is more severe than Smad6 transgenic pups.162Smurf1-de f i cient mice were born normal but exhibited a temporal increase of bone mass as they aged,due to accumulation of phosphorylated MKK2 and activation of the downstream JNK signaling cascade.167Arkadia,an E3 ubiquitin ligase that induces ubiquitylation and proteasome-dependent degradation of TGF-β and BMP suppressors,such as Smad6, Smad7,and c-Ski/SnoN,promotes osteoblast mineralization and differentiation induced by BMPs.169SUMO(small ubiquitin-related modi f i er)and Ubc9(ubiquitin conjugating enzyme 9)target Smad4 for degradation and inhibit osteoblastic differentiation induced by BMP2.170–171Furthermore,deubiquitylating enzyme USP15 mediates K48-linked deubiquitylation of ALK3,to promote ALK3-smad1 signal and BMP-induced osteoblast differentiation.172
Transcriptional repressors
A group of co-repressors controls transcriptional activity of the R-Smad/co-Smad complex(Figure 2).Ski/SnoN interacts with R-Smad proteins to repress their activity. c-Ski/SnoN,evoked by TGF-βsuppresses the BMP signaling and hypertrophic chondrocyte maturation.SnoN is highly expressed in articular cartilage and may involve in TGFβ-mediated cartilage homeostasis.Tob as a corepressor is another negative regulator of BMP/Smad signaling in osteoblasts.173BMP2-signal is elevated in the absence of Tob and repressed by overproduction of Tob.174–175Mice carrying a targeted deletion of the Tob gene had a greater bone mass resulting from increased numbers of osteoblasts and were protected from OVX-induced osteoporosis.174–175
MicroRNAs
Recent researches have raised the importance of micro-RNA(miRNA)in skeleton homeostasis and in TGF-β and BMP signaling pathway(Figures 1 and 2).BMP treatment regulates multiple miRNA expression during osteoblastogenesis,and a number of those miRNAs feedback to regulate BMP signaling:176–179miR-133 targets Runx2 and Smad5 to inhibit BMP-induced osteogenesis;176miR-30 family members negatively regulate BMP-2-induced osteoblast differentiation by targeting Smad1 and Runx2;177–178miR-322 targets Tob and enhances BMP response.179In addition,some non-BMP-regulated miRNAs also have regulatory roles in BMP signaling:miR-141 and-200a remarkably modulate the BMP-2-induced pre-osteoblast differentiation through the translational repression of Dlx5;180miR-542-3p targets BMP7 and represses BMP7-induced osteoblast differentiation and survival;181miR-20a promotes osteogenic differentiation through upregulation of BMP/Runx2 signaling by targeting PPARγ,Bambi,and Crim1;182miR-140 targets a mild inhibitor of BMP Dnpep,and loss of miR-140 in mice causes growth defects of endochondral bones and craniofacial deformities.183Furthermore,several miRNAs are reported to regulate chondrocyte differentiation by targeting TGF-β and BMP signaling.Pro f i ling identi f i ed expression of several miRNAs that changed signi f i cantly in human OA chondrocyte compared with normal cells targets Smad-signaling, including miR-20b,miR-146a,and miR-345.184–186miR-146a, a miRNA increased in OA,targets Smad4 and induces chondrocyte apoptosis and cellular responsiveness to TGF-β.187–188miR-199a(*)targets Smad1 and adversely regulates early chondrocyte differentiation.189
Epigenetic regulation
The Sox9-related transcriptional apparatus activates its target gene expression through p300-mediated histone acetylation on chromatin,and TGF-β has an important role in recruiting p300 to the promoter.190TGF-β also induces the expression of KDM4B,which removes the silencing H3K9me3 marks on the Sox9 promoter and increased Smad3 occupancy on the Sox9 promoter,so as to enhance Sox9 expression.191HDAC inhibitor suberoylanilide hydroxamic acid(SAHA;vorinostat)increases the Runx2 promoter acetylation to intrigue Runx2 expression and osteoblastogenesis,in a BMP2-dependent manner.192The Bmp2 promoter regions of the genes are epigenetically locked with increased CpG methylation and decreased H3K9 acetylation.193And CpG-demethylating agent or the HDAC inhibitor trichostatin-A renders Bmp2 expression.193At the protein level,HDACs deacetylate Smad7 to protect it against ubiquitination and degradation.4BMP-2 signaling stimulates p300-mediated Runx2 acetylation,which increases its stability and transactivation activity.4HDAC4 and HDAC5 deacetylate Runx2 and renders its degradation through ubiquitin–proteasomal pathway.4
In summary,traditional regulators(for example,ECM, I-Smad,ubiquitin-related proteins,transcriptional co-factors)of BMP and TGF-β signaling have been extensively studied and the regulatory network is f i nely established. Prosperous progress will be made to discover novel regulation of BMP and TGF-β signaling by miRNA and epigenetic factors,and will provide more clues to design new treatment for bone diseases through targeting BMP and TGF-β signaling.
Coordinated signaling networks controlled by multiple cytokines regulate skeletogenesis and osteoblast differentiation.To regulate bone homeostasis,TGF-β and BMP signaling dynamically crosstalks with other pathways including Wnt,Hedgehog,FGF,Notch and PTHrP(Figures 3 and 4;Table 3).
Crosstalk between TGF-β and BMP signaling with Wnt signaling
TGF-β cooperates with Wnt to stimulate both osteoblast and chondrocyte differentiation in a positive regulatory loop.TGF-β upregulates the expression of Wnts(Wnt-2,-4, -5a,-7a,and-10a)as well as Wnt co-receptor LRP5,and suppresses the expression of β-catenin inhibitor Axin1/2,to promote Wnt/β-catenin signaling.194–195On the other hand, Wnt signaling induces Runx2-mediated TGFβRI expression in aβ-catenin-independent way to promote TGF-β signaling.196Bosch et al.20showed that canonical Wnt signaling skews TGF-βsignaling towards signaling via ALK1-Smad1/5/8 to promote chondrocyte hypertrophy.
BMPs have dual functions in regulating Wnt signaling. BMP induces Dkk1 and Sost expression through MAPK and Smad signaling,respectively,via BMPRIA to inhibit Wnt signaling in osteoblast,and negatively regulated bone mass.96,98Smad4 competitively binds β-catenin and prevents its binding with Tcf/Lef transcription complex.123Thus,ablation of Smad4 either in the crest neuron stem cell or in osteoblast results in upregulation of canonical Wnt signaling so as to promote bone formation.60,123BMP2-induced formation of Smad1-Dvl1 complex also restricts β-catenin and inhibits its activity.197On the other hand,BMPs stimulate osteoblast and chondrocyte differentiation synergistically with Wnt signaling.BMP-2 promotes canonical Wnt signaling by inducing Wnt3a,Wnt1,Lrp5 expression and inhibiting the expression of β-TrCP,the F-box E3 ligase was found responsible forβ-catenin degradation in osteoblasts.198–199Ablation of Smad4 in the preosteoblasts depletes LRP5 expression and impairs both BMP and Wnt signaling.123BMP-2-induced LRP-5 expression also contributes to chondrocyte hypertrophy and osteoarthritis progress.200Osteoblastogenesis is maximized in the presence of both BMP and Wnt signaling.198,201–202Smad and TCF/LEF/β-catenin cooperatively complex with each other on the promoter to achieve the highest expression of osteoblast genes(Dlx5,Msx2,and Runx2).202Moreover, activation of Wnt signaling by Wnt3a or overexpression of βcatenin/TCF4 promotes BMP signaling by stimulating BMP2 transcription.203
Table 3.Crosstalk between TGF-β/BMP signaling and other signaling molecules in bone
Crosstalk between TGF-β and BMP signaling with FGF signaling
FGF and BMP signal synergistically promote osteoblast differentiation.BMP signaling is essential for FGF-induced osteoblast differentiation,204and the osteogenic effect of BMP2 is also repressed in the absence of FGF2.205FGF2 is responsible for BMP-induced nuclei translocation and accumulation of Runx2 and P-Smad1/5/8.206FGF2,FGF9, and FGF18 induce while DN-FGFR reduces BMP2 expression.204,207–208Disruption of the FGF2 in mice impairs the expression of BMP2 and reduces bone mass.205However,FGF2 has a critical role in osteoblast proliferation but inhibits mineralization while BMP2 is instrumental in stimulating mineralization.209–210Thus,besides cooperative functions,FGF2 and BMPs have different roles at different stages of osteoblast differentiation.
Antagonistic BMP and FGF pathways control the differentiation and proliferation rate of chondrocytes in the growth plate.76,110Although BMP signal promotes chondrogenesis,FGF signal is a negative regulator of chondrogenesis and multiple mutations with constitutive activity of FGFR3 result in achondroplasia in humans.211BMP signaling is required to inhibit activation of FGF signaling (for example,STAT and ERK1/2 activation)at least in part by inhibiting the expression of FGFR1.110FGF signals are downregulated by ectopic BMP expression or Chordin/ Noggin de f i ciency,and upregulated by CHL1 ectopic expression.153,212Chondrocyte-speci f i c activation of Fgfr3 in mice induced premature synchondrosis closure and enhanced osteoblast differentiation around synchondrosis associated with promoted BMP signaling,due to increased BMP ligand and decreased BMP antagonist expression.211Chondrocyte-speci f i c deletion of BMPRIA rescued the bone overgrowth phenotype observed in Fgfr3-de f i cient mice by reducing chondrocyte differentiation.168
Crosstalk between TGF-β and BMP signaling with Hedgehog signaling
BMPs and Hedgehog signaling cooperatively regulate osteoblastogenesis in long bones.213–214IHH is required for BMP-induced osteogenesis in vitro.213Hedgehog-Gli activators direct osteo-chondrogenic function of BMPs toward osteogenesis in the perichondrium.215SHH-Gli2 stimulates BMP-2 expression at transcription level to enhance osteoblast differentiation.216–217BMP2 also induces expression of IHH and downregulates expression of patched 1(PTC1),a negative regulator of Hedgehog signaling,during osteoblast differentiation in a positive feedback loop.213
During growth plate development,IHH-PTHrP regulatory loop keeps chondrocyte in the proliferation pool and favors elongation of long bones.Previous studies have shown that BMP signaling upregulates IHH–PTHrP signaling in vivo,and maintains a normal chondrocyte proliferation rate in parallel with IHH.218–219Expression of IHH,PTC1,and PTHrP receptor(PPR)were decreased in Smad4-de f i cient growth plates,but were enhanced in CA-ALK2-, CA-BMPRIA-,or CA-BMPRIB-overexpressed chick limb bud.108,120In cultured mouse limb explants,BMP2 application induces and Noggin antagonizes IHH expression.218BMP6 also promotes IHH expression to favor chondrocyte hypertrophy.220Furthermore,chondrocyte-speci f i c Smad1-/5 CKO mice develop chondrodysplasia and exhibit abnormal growth at the end of bones,probably owing to an imbalance in the crosstalk between the BMP,FGF, and IHH/PTHrP pathways.213On the other hand,IHH also promotes chondrocyte hypertrophy independent of PTHrP via integrating with BMP and Wnt signaling.221Chondrocyte-speci f i c deletion of PTC1 upregulated BMP expression and BMP–Smad signaling.222IHH de f i ciency downregulates BMP expression and reduces cranial bone size and all markers.214
In axial skeleton development,sequential SHH and BMP signals are required for speci f i cation of a chondrogenic fate in presomitic tissue.223In this process,SHH promotes BMP expression,and initiates an Nkx3.2/Sox9 autoregulatory loop that is maintained by BMP signals to induce chondrogenesis.223On the other hand,BMP activity negatively regulates SHH transcription and a BMP-SHH negative-feedback loop serves to con f i ne SHH expression during limb development.153,212SHH signal was impaired in the defected craniobone in the absence of Chordin and Noggin,the two BMP cognate binding partners.155
Crosstalk between TGF-β and BMP signaling with Notch signaling
Notch activation favors BMP-induced osteoblast differentiation.224–225Notch inhibition represses expression of BMP target genes.226BMP-2 and TGF-β regulates expression of signaling proteins in Notch pathway(for example,Lfng, Hey1,and Hes1).227Conditional deletion of Jag1,the Notch ligand in CNC,revealed altered collagen deposition,delayed ossi f i cation,and reduced expression of early and late determinants of osteoblast development during maxillary ossi f i cation,associated with dysregulation of BMP receptor expression in the Maxillary mesenchymal stem cell.
Crosstalk between TGF-β and BMP signaling with PTH signaling
Osteoblast-speci f i c Tgfbr2 CKO mice have increased bone mass which is caused by hyperactivition of PTH type I receptor(PTH1R)and could be rescued by disruption of PTH signaling by injection of PTH(7-34)or ablation of PTH1R.43,228Mechanistic studies show that Tgfbr2 directly phosphorylates the PTH1R,which modulates PTH-induced endocytosis of both receptors and attenuates both TGF-β and PTH signaling in vivo.43,228PTH enhances MSC differentiation into the osteoblast lineage through endocytosis of the PTH1R/Lrp6 complex,which enhances the BMPsreceptor binding af f i nicty.229Positive regulation of PTHrP expression by TGFβ/BMP signaling can be observed in Smad4 mutant mice,Smad1/5 mutant mice and chick limbs overexpressing constitutively active BMP receptors.It has also been proposed that TGF-βpromotes PTHrP expression.22Both TGF-β and PTH inhibit terminal differentiation of chondrocyte in the growth plate,while BMPs promotes this process.230
Taken together,these studies demonstrate that TGF-βs and BMPs cooperate with other cytokines(Wnt, Hedgehog,FGF,Notch,and PTHrP)to regulate osteoblast and chondrocyte differentiation.Further studies will not only elucidate the speci f i c interactions between those signaling pathways in bone,but also explore the potential of combined treatment for bone diseases by targeting multiple signaling pathways to achieve optimal outcome.
Given the important roles of TGF-β and BMP in chondrogenesis and osteogenesis,mutations in TGF-β and BMP signaling cause a wide range of skeletal disorders in human (Table 2).Not only do these studies reveal the roles of TGF-β and BMP signaling in bone formation and homeostasis, but also they decipher the molecular mechanisms and pathogenesis underlying related bone diseases and,most importantly,provide signi f i cant insights into the development of novel therapies.Fibrodysplasia ossi f i cans progressiva(FOP;OMIM 135100) FOP is a rare autosomal dominant disease characterized by progressive ectopic ossi f i cation of connective tissues and skeletal muscles.FOP is caused by activating mutations in BMP type I receptor Acvr1.Among them,R206H is the f i rst characterized and most prevalent mutation,while other site mutations were also reported and contribute to the clinical variability and diverse severity of the disease (for example,R258S,L196P,and G328E).91–92,231–233
Brachydactyly type A2(BDA2;OMIM 112600)
BDA2 is characterized by hypoplasia of the second middle phalanx of the index fi nger and sometimes the little fi nger.234Inactivating mutations of Bmpr1b gene(for example,I200K,R486K,and R486Q)235–236or GDF5 gene (for example,L441P and R380Q)237–238were reported in BDA2 patients.Dathe et al.239reported another genetic cause of BDA2:duplication of a regulatory element downstream of BMP2 repressed BMP2 expression.GDF5 mutations were also found to be the cause of a number of cartilage disorders,including symphalangsism,238chondrodysplasia,240and osteoarthritis.16
Myhre syndrome(MIM 139210)
Inactivated SMAD4 mutation causes Myhre syndrome, which is a developmental disorder characterized by short stature,short hands and feet,facial dysmorphism,muscular hypertrophy,deafness,and cognitive delay.117–118Myhre syndrome patients carry an I500 mutation at Smad4 that results in a stabilized but unfunctional mutant.118,241
Noggin-mutation-related diseases
As the major BMP antagonist,mutation of Noggin in human causes multiple bone disorders.Missense mutations in BMP antagonist Noggin cause tyrosine kinase-like orphan receptor 2(ROR2)-negative brachydactyly type B(BDB2; OMIM611377),which is characterized by hypoplasia/aplasia of distal phalanges in combination with distal symphalangism and fusion of carpal/tarsal bones242.Three missense mutations of Noggin were reported to cause Tarsal–carpal coalition syndrome(TCC;OMIM 186570),an autosomal dominant disorder characterized by short f i rst metacarpals causing brachydactyly as well as fusion of the carpals, tarsals,phalanges,and humeroradial.243Truncating mutation in the NOG gene in two families caused autosomal dominant stapes ankylosis with broad thumbs and toes, hyperopia,and skeletal anomalies(OMIM 184460).244Heterozygous noggin mutations were reported to cause segregating proximal symphalangism(SYM1;OMIM 185800),which is characterized by ankylosis of the proximal interphalangeal joints,carpal and tarsal bone fusion or segregating multiple synostoses syndrome(SYNS1;OMIM 186500),which is characterized by multiple joint fusions.138
Other human bone diseases caused by mutations in TGF-β/BMP signaling
Several mutations in the TGF-β1 gene were reported to cause Camurati–Engelmann disease(CED;OMIM131300), a rare autosomal dominant disease characterized by cortical thickening of the diaphyses of long bones, hyperostosis,and bone pain.245–246Nonsense,missense, and frameshift mutations of Smad3 were reported to cause aneurysms–osteoarthritis syndrome(AOS;OMIM 613795), which is presented with aneurysms,dissections,and tortuosity throughout the arterial tree in association with mild craniofacial features as well as skeletal and cutaneous anomalies.15,60–61
Both BMP and TGF-β possess clinical implications to treat bone diseases including bone healing,rheumatoid arthritis ect.TβRI kinase inhibitor downregulates rheumatoid synoviocytes and prevents the arthritis.247A small molecule inhibitor of BMP type I receptor activity has been demonstrated to be useful in treating FOP and heterotopic ossi f i cation syndromes.248Currently,two BMP products have been approved by the Food and Drug Administration for clinical applications to treat fractures of long bones and improve intervertebral disk regeneration through a puri f i ed collagen matrix,respectively,infused with BMP-2(Medtronic)or OP-1 BMP-7(Stryker Biotech)and implanted at the site of the fracture.4With the use of BMPs increasingly accepted in spinal fusion surgeries,other therapeutic approaches targeting BMP signaling are emerging beyond applications to skeletal disorders.4In addition,GDF-5 has emerged as a therapeutic target for rheumatic diseases.249Recent applications graft BMP peptides corresponding to residues 73–92,89–117,and 68–87 of BMP-2, BMP-7,and BMP-9 as adhesion peptides(GRGDSPC)onto polyethylene terephthatalate surfaces to enhance osteogenic differentiation and mineralization of pre-osteoblastic cells.250These engineered biomaterials for enhanced bone regeneration are in the initial trial stage of development.
To sum up,mutations in the BMP and TGF-β signalingrelated genes result in multiple inheritable bone diseases in humans,including ossi f i cation disorders,joint diseases,and skeleton developmental defects.In the future,more molecules or peptides targeting BMP and TGF-β signaling will be developed to treat genetic bone disorders,fracture healing,and osteoarthritis diseases.
BMP and TGF-β signaling pathways have important roles in skeletal development and postnatal skeleton homeostasis,by crosstalking with multiple signaling pathways, such as Wnt,Hedgehog,Notch,and FGF.Our understanding of BMP and TGF-β signaling is advanced with the generation of related mouse models,heritable humandisease genetic studies and other new technologies including high-throughput screening.Speci f i city and versatility of BMP and TGF-βsignaling are controlled by various ligand-receptor combinations,and transduced by co-Smad/R-Smad complex or MAPK cascade.The signaling is also dedicatedly controlled by factors including extracellular cognate binding proteins,I-Smad,epigenetic factors,and microRNA.Disruption of BMP and TGF-β signaling results in various bone disorders,such as osteoarthritis,FOP and Myhre syndrome.Manipulating BMP and TGF-β signaling pathways possess clinical implications for the treatment of multiple bone diseases including fracture healing,osteoarthritis,osteoporosis,and FOP.So far,BMP-2-and BMP-7-containing osteogenic implants have been used in over one million patients worldwide for the treatment of long bone nonunions,spinal fusions,and acute fractures.More importantly,with the aging population expected to double over the next decade,the number of patients suffering from osteoarthritis and osteoporosis is likely to increase dramatically and so is the cost of Medicare.Thus,it is compelling to elucidate the pathophysiology and the molecular mechanisms underlying skeleton health and bone diseases.Further,TGF and BMP signaling study will provide signi f i cant insights into the mechanism underlying how TGF and BMP signaling regulates osteoblast and chondrocyte proliferation,differentiation,maturation and activity in bone and cartilage formation,under both physiological and pathological condition(for example,osteoarthritis,osteoporosis,and bone cancer metastases.This study will also facilitate the design of novel therapeutic approaches for bone diseases.
We thank Dr Joel Jules for his extensive reading and discussion of our manuscript.We apologize to the many researchers whose work could not be cited due to space limitations.Work in our laboratory is supported by grants by NIH grant AR-044741(Y-PL)and R01DE023813(Y-PL).
The authors declare no con f l ict of interest.
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©The Author(s)2016
Research(2016)4,16009;
10.1038/boneres.2016.9;Published online:26 April 2016
1Department of Pathology,University of Alabama at Birmingham,Birmingham,USA and2Department of neurology,Bruke Medical Research Institute,Weil Cornell Medicine of Cornell University,White Plains,USA
Correspondence:Y-P Li(ypli@uab.edu)
Received:29 December 2015;Revised:4 March 2016;Accepted:7 March 2016