The Ihh (Indian Hedgehog) pathway plays an essential role in facilitating chondrocyte hypertrophy and bone formation during skeletal development. Nkx3.2 (NK3 homeobox 2) is initially induced in chondrocyte precursor cells, maintained in early-stage chondrocytes and down-regulated in terminal-stage chondrocytes. Consistent with these expression patterns, Nkx3.2 has been shown to enhance chondrocyte differentiation and cell survival, while inhibiting chondrocyte hypertrophy and apoptosis. Thus, in the present study, we investigated whether Nkx3.2, an early-stage chondrogenic factor, can be regulated by Ihh, a key regulator for chondrocyte hypertrophy. We show that Ihh signalling can induce proteasomal degradation of Nkx3.2. In addition, we found that Ihh can suppress levels of Lrp (low-density-lipoprotein-receptor-related protein) (Wnt co-receptor) and Sfrp (secreted frizzled-related protein) (Wnt antagonist) expression, which, in turn, may selectively enhance Lrp-independent non-canonical Wnt pathways in chondrocytes. In agreement with these findings, Ihh-induced Nkx3.2 degradation requires Wnt5a, which is capable of triggering Nkx3.2 degradation. Finally, we found that Nkx3.2 protein levels in chondrocytes are remarkably elevated in mice defective in Ihh signalling by deletion of either Ihh or smoothened. Thus these results suggest that Ihh/Wnt5a signalling may play a role in negative regulation of Nkx3.2 for appropriate progression of chondrocyte hypertrophy during chondrogenesis.
- chondrocyte maturation
- Indian Hedgehog (Ihh)
- NK3 homeobox 2 (Nkx3.2)
- parathyroid hormone-related protein (PTHrP)
Nkx3.2 (NK3 homeobox 2), also called Bapx1, is the vertebrate homologue of Drosophila bagpipe, and its initial expression can be observed in chondrocyte precursor cells during embryonic development and, later, its expression is maintained in chondrocytes of various skeletal elements [1–7]. Nkx3.2-deficient mice are perinatal lethal and exhibit significant skeletal phenotypes, including major hypoplasia in vertebrae [8–11]. Interestingly, during cartilage development, Nkx3.2 expression is maintained at high levels in early-stage chondrocytes, whereas it is down-regulated in terminal-stage chondrocytes [1,2,12,13]. Consistent with these expression patterns, Nkx3.2 has been reported to support cell viability of early-stage chondrocytes [13,14], whereas chondrocyte hypertrophy can be inhibited by Nkx3.2 [2,15,16]. Thus, although these previous studies clearly indicate that Nkx3.2 may function in a stage-specific manner during cartilage development, the molecular mechanism for regulation of Nkx3.2 during cartilage development is not well understood.
Ihh (Indian Hedgehog) is a member of the Hh (Hedgehog) family, which has been shown to regulate various developmental processes and cancer pathogenesis; in particular, Ihh signalling plays an essential role in both chondrocyte hypertrophy and bone formation during skeletal development [17–29]. These previous studies have demonstrated that binding of Hh ligand to the cell-surface receptor Ptc (patched), which is normally associated with Smo (smoothened) to suppress signal transmission from Smo, releases Smo from Ptc; subsequently, the liberated Smo triggers downstream signalling cascades to activate Gli (glioma-associated oncogene homologue) family members, which, in turn, control various Hh-dependent cellular events. During cartilage development, expression of Ihh in growth plates is highly restricted in the border areas between proliferative chondrocytes and hypertrophic chondrocytes (i.e. pre-hypertrophic chondrocytes), and a remote cross-talk between Ihh in pre-hypertrophic chondrocytes and PTHrP (parathyroid hormone-related protein) in the perichondrium has been shown to regulate chondrocyte proliferation, hypertrophic maturation, and bone formation [21–25,30–35]. Besides, Ihh has also been suggested to function to promote chondrocyte hypertrophy in a PTHrP-independent manner [24,36]. Thus, although previous studies have verified the importance of the Ihh signalling pathway in endochondral bone development, the precise role of Ihh has not been fully elucidated.
It has been demonstrated that various Wnts can control chondrocyte differentiation, proliferation and hypertrophic maturation during cartilage development; in particular, Wnt5a has been shown to play a critical role in chondrocyte hypertrophy regulation [37–49]. In addition, a broad range of intracellular factors such as Ca2+ ions, protein kinases, transcription factors and ubiquitin E3 ligases have been indicated in various non-canonical Wnt signalling pathways [43,44,50–61]. In the present study, we found that Ihh signalling can induce proteasomal degradation of Nkx3.2 via a Wnt5a-dependent pathway employing CK2 protein kinase and Siah1 (seven in absentia homologue 1) ubiquitin E3 ligase. Furthermore, we support the physiological relevance of these findings by analysing knockout mice with deletion of Ihh or Smo.
Cell culture and transfection
ATDC5 cells were obtained from RIKEN BRC Cell Bank and maintained in DMEM (Dulbecco's modified Eagle's medium)/Ham's F12 (1:1) supplemented with 5% (v/v) FBS (fetal bovine serum). HEK (human embryonic kidney)-293T cells were purchased from the A.T.C.C. (Manassas, VA, U.S.A.) and grown in DMEM supplemented with 10% (v/v) FBS. Cells were transfected under serum-free conditions using VivaMagic (Vivagen) according to the manufacturer's instructions. An empty vector, pCS2, was used to adjust total DNA amounts where necessary.
Chemical reagents and antibodies
PMP (purmorphamine), CCP (cyclopamine), DRB (5,6-dichloro-1-β-D-ribofuranosylbenzimidazole), TBB (4,5,6,7-tetrabromo-1H-benzotriazole), DAPI (4′,6-diamidino-2-phenylindole), MG132 and ionomycin were purchased from Calbiochem. Rabbit polyclonal and monoclonal (9E10) anti-Myc antibodies were purchased from Upstate Biotechnology, anti-HA (haemagglutinin) monoclonal antibody was purchased from Roche, anti-V5 antibody was obtained from Invitrogen, anti-FLAG monoclonal M2 antibody was obtained from Sigma, anti-Ihh was from Santa Cruz Biotechnology, anti-Gli1, anti-Wnt5a and anti-CK2α were purchased from Cell Signaling Technology, anti-PCNA (proliferating-cell nuclear antigen) was obtained from BD Biosciences, anti-Nkx3.2 and anti-CK2β were obtained from Abcam, and anti-GAPDH (glyceraldehyde-3-phosphate dehydrogenase) was purchased from AbFrontier. Secondary antibodies for Western blotting and immunohistochemistry were purchased from Cell Signaling Technology and Jackson ImmunoResearch Laboratories respectively.
All mice were housed in individually ventilated microisolation cages in the specific pathogen-free (SPF) facility of the YLARC (Yonsei Laboratory Animal Research Center). All animals were maintained on a 12 h light/12 h dark cycle with access to food and water ad libitum. All behavioural procedures were conducted during the light phase of the cycle. All experimental procedures were approved by the IACUC (Institutional Animal Care and Use Committee) of the YLARC and performed in accordance with the YLARC IACUC guidelines for the ethical use of animals.
shRNA (short hairpin RNA) and RT (reverse transcription)–PCR
shRNAs for mouse Gli1, Gli2, Gli3, Wnt5a, CK2 and Siah1 were purchased from Open Biosystems and were transfected using FuGENE™ 6 (Roche). Total RNA was isolated using the RNAspin Mini kit (GE Healthcare). The Superscript III cDNA synthesis kit (Invitrogen) was used for cDNA synthesis. PCRs were performed with the oligonucleotides listed in Table 1 consisting of targeted mouse cDNA sequences.
IP (immunoprecipitation) and immunoblotting
Cells were washed in PBS and lysed in buffer containing 50 mM Tris/HCl (pH 7.8), 150 mM NaCl, 1 mM EDTA, 2 mM imidazole, 1 mM NaF, 1.15 mM sodium molybdate (Na2MoO4), 1 mM sodium vanadate (Na3VO4), 4 mM sodium tartrate (C4H4Na2O6), 1.5 mM MgCl2, 1 mM DTT (dithiothreitol), 10% glycerol, 0.5% Nonidet P40 and protease inhibitors (Complete™, EDTA-free Protease Inhibitor Cocktail Tablets; Roche). Cell extracts were centrifuged at 13000 g for 10 min at 4°C, and then supernatants were subjected to IP and WB (Western blot) analysis. Visualization of the immunoblots was performed using the ECL (enhanced chemiluminescence) detection kit from GE Healthcare. For standard WB analysis, cells were lysed in buffer containing 50 mM Tris/HCl (pH 6.8), 1 mM DTT, 8.75% glycerol and 2.5% SDS. After boiling, protein concentrations were determined using the Bio-Rad Laboratories protein assay kit, and equal amounts of protein were subjected to SDS/PAGE and WB analysis.
For Figure 7(A), E (embryonic day) 16.5 mouse limbs were fixed in 4% (w/v) PFA (paraformaldehyde), and for Figures 7(E) and 7(F), E18.5 mouse limbs were fixed in 10% (v/v) buffered formalin overnight at room temperature (20°C). The fixed embryos were then processed and embedded in paraffin before sectioning at 4 μm (see Figure 7A) and 6 μm (see Figures 7E and 7F) and mounted on silane-coated slides (Muto-Glass). For E18.5 mouse limbs, decalcification was performed with 14% EDTA/PBS (pH 7.4) for 48 h after formalin fixation. Deparaffinization was performed using xylene, a xylene/ethanol mixture, and ethanol. For antigen retrieval, sections were incubated with 10 mM citrate (pH 6.0) and 0.05% Tween 20 for 20 min at 80°C, followed by a double wash in PBS with 0.05% Tween 20. Sections were then pre-incubated with blocking buffer containing 5% (v/v) goat (or bovine) serum, 0.05% sodium azide, and 0.1% Triton X-100 in PBS for 45 min at 25°C, and incubated overnight at 4°C with primary antibodies in antibody dilution buffer containing 1% (v/v) goat (or bovine) serum, 0.05% sodium azide and 0.02% Tween 20. Before microscopy, mounting was performed with anti-fading solution containing 25 mM Tris/HCl (pH 8.7), 10% (v/v) poly(vinyl alcohol), 5% (v/v) glycerol and 2.5% (v/v) DABCO (1,4-diazadicyclo[2.2.2]octane).
Mouse embryo limb bud culture
E11.5 mouse embryos were isolated and the limb buds were dissected and subjected to micromass cultures. Mesenchymal cells were isolated by digestion with 0.1% dispase at 37°C for 90 min, and isolated cells were dissociated by pipetting and filtering through a 40-μm-pore-size nylon cell strainer. Filtered cells were then centrifuged at 120 g for 10 min, and cell pellets were resuspended in DMEM/Ham's F12 (1:1) supplemented with 10% (v/v) FBS. The resuspension volume was 5 μl for each limb bud, and 10 μl aliquots were used for each micromass culture. The cultures were maintained in DMEM/Ham's F12 (1:1) supplemented with 10% (v/v) FBS in a humidified, 5% CO2, 37°C incubator for various times up to 6 days.
Nkx3.2 protein degradation can be induced by Ihh signalling
To explore the possibility that Ihh, which enhances chondrocyte hypertrophy, may negatively regulate Nkx3.2, an inhibitor of chondrocyte maturation, we first tested whether an Hh agonist, PMP [62–64] can modulate Nkx3.2. Interestingly, PMP treatment dramatically reduced the levels of endogenous Nkx3.2 protein in ATDC5 murine chondrogenic cells ; the functional efficacies of PMP were shown by confirming induction of Gli1, a typical Hh-dependent target gene (Figure 1A). In addition, we observed that transient transfection of a constitutively active form of Smo (SmoA1) can significantly decrease the levels of co-transfected Nkx3.2 proteins dose-dependently (Figure 1B). Then, to clarify whether the Hh signalling modulates the expression of Nkx3.2 mRNA expression or Nkx3.2 protein stability, we examined the effects of PMP or SmoA1 overexpression on Nkx3.2 mRNA levels, and found that the levels of Nkx3.2 mRNA were not significantly altered by either PMP treatment or lentiviral infection of SmoA1 (Figures 1C and 1D). Consistent with these results, SmoA1-induced Nkx3.2 protein degradation was effectively blocked by MG132, a proteasome inhibitor (Figure 1E), suggesting that the Hh signalling regulates Nkx3.2 at the level of protein degradation. Finally, we have verified that Ihh signalling does not cause random destabilization of cellular proteins by showing that SmoA1 overexpression does not alter the levels of co-transfected NF-κB (nuclear factor κB) p65, NF-κB p50 or Cbfβ (core binding factor β) proteins (Figure 1F). These results indicate that Ihh signalling can induce proteasome-dependent degradation of Nkx3.2.
Ihh can enhance non-canonical Wnt signalling in chondrocytes
As we found that Ihh can trigger Nkx3.2 protein degradation, we next probed whether a Wnt pathway may participate in this process as well. We suspected a role of Wnt signalling in Ihh-induced Nkx3.2 degradation because various Wnt pathways, along with Ihh, have also been well documented to critically regulate chondrocyte hypertrophy [37,40,41,45–49]. As an initial step, we sought to determine whether Sfrp (secreted frizzled-related protein) 3 (Frzb), a well-known Wnt antagonist [66–69], can affect Nkx3.2 degradation by Ihh. Interestingly, we found that Nkx3.2 degradation triggered by SmoA1 overexpression can be effectively abolished by Sfrp3 co-transfection (Figure 2A); the anti-Wnt functionality of Sfrp3 was verified by confirming its ability to suppress Wnt3a-induced TOPflash reporter activation (results not shown). These results suggest that Ihh-induced Nkx3.2 degradation requires intact Wnt signalling.
To elucidate further the relationship between Ihh and Wnt, we next investigated whether Ihh signalling can modulate expression levels of various Wnt ligands (Wnt1, Wnt3a, Wnt4, Wnt5a and Wnt7a), Wnt co-receptors [Lrp (low-density-lipoprotein-receptor-related protein) 5 and Lrp6] and Wnt antagonists (Sfrp1, Sfrp2 and Sfrp3) in chondrocytes. Then, we found that PMP treatment can cause marginal up-regulation of Wnt5a and slight down-regulation of Wnt4 in ATDC5 cells (Figure 2B). Interestingly, in addition, we noticed that PMP treatment can remarkably suppress levels of Lrp6, Sfrp2 and Sfrp3 expression (Figure 2B). On the other hand, expression levels of Wnt1, Wnt3a, Wnt7a, Lrp5 and Sfrp1 were quite low and not significantly altered by PMP treatment in ATDC5 cells (Figure 2B). Besides, lentivirus-mediated overexpression of SmoA1 was also capable of similarly modulating expression patterns of these genes (results not shown). Taken together, these results indicate that Ihh can cause significant down-regulation of Lrp6 (Wnt co-receptor) and Sfrp2/Sfrp3 (Wnt antagonist) in chondrocytes.
To assess the physiological relevance of our results (i.e. down-regulation of Lrp and Sfrp by Ihh), we analysed expression of Sfrp1/Sfrp2/Sfrp3 and Lrp5/Lrp6 in Ihh expression-negative and -positive tissues isolated from E16.5 mouse growth plate; the tissues above and below the broken line in the left-hand panel in Figure 2(C) were dissected separately as Ihh expression-negative (i.e. proliferating zone) and positive (i.e. maturing zone) chondrocytes respectively from E16.5 mouse hindlimbs. Consistent with the results obtained from ATDC5 in vitro cultures, expression levels of Sfrp2, Sfrp3, Lrp5 and Lrp6 were remarkably reduced in Ihh expression-enriched chondrocytes (Figure 2C, right-hand panels). Therefore these results show that elevated levels of Ihh expression and decreased levels of Lrp (Wnt co-receptor) and Sfrp (anti-Wnt) expression have a tight correlation in vivo as well as in vitro.
As a matter of fact, these findings suggest the following intriguing scenario. First, down-regulation of Sfrp (anti-Wnt) expression by Ihh would, in principle, increase the efficacies of both canonical and non-canonical Wnt signalling because reduced levels of anti-Wnts can indirectly enhance the interactions between Wnt ligands and Wnt receptors [70,71]. Secondly, suppression of Lrp (Wnt co-receptor) expression by Ihh would selectively attenuate canonical Wnt pathways since Lrp co-receptors are required for Wnt receptors to optimally interact with canonical Wnt ligands and transmit canonical Wnt signals [70,72,73]. On the other hand, non-canonical Wnt ligands can bind to the receptors in the absence of Lrp co-receptors and can transmit non-canonical (i.e. β-catenin-independent) downstream signals via a Lrp5/Lrp6-independent manner [70,72,73]. Therefore simultaneous suppression of Wnt co-receptors and Wnt antagonists (i.e. Lrps and Sfrps) would be beneficial only to non-canonical Wnt signalling pathways because reduced levels of Lrp co-receptors would result in substantial attenuation of canonical Wnt signalling pathways in spite of decreased levels of Sfrp anti-Wnts.
Wnt5a is required for Ihh-induced Nkx3.2 degradation
So far, our results indicate that Wnt signalling is required for Ihh-induced Nkx3.2 degradation (Figure 2A) and that Ihh can enhance and attenuate non-canonical and canonical Wnt signalling respectively (Figures 2B and 2C). Therefore it is a realistic possibility that Ihh may employ a non-canonical Wnt pathway to trigger Nkx3.2 protein degradation. In this context, we hypothesized that Wnt5a, a non-canonical Wnt, which has been shown to be a key regulator for chondrocyte hypertrophy [37,40,41,45–49], may modulate Nkx3.2 protein stability. In agreement with this hypothesis, we found that the protein levels of Nkx3.2 can be significantly decreased by Wnt5a co-transfection, whereas Cbfβ protein levels are unaffected (Figure 3A). Thus these results prompted us to examine whether Wnt5a is required for Ihh-induced Nkx3.2 degradation. Interestingly, Wnt5a knockdown was effective in suppressing both overexpressed Nkx3.2 degradation by SmoA1 co-transfection (Figure 3B) and endogenous Nkx3.2 protein destabilization by PMP treatment (Figure 3C); the efficacies of Wnt5a shRNAs were verified as shown in Figure 3(D).
As these results indicate that Wnt5a is required for Ihh to destabilize Nkx3.2 proteins, we next evaluated whether Ihh is necessary for Wnt5a to trigger Nkx3.2 degradation. In contrast with the results in Figures 3(B) and 3(C), Ihh signalling inhibition by using CCP, a well-established Hh antagonist, did not abrogate Nkx3.2 protein degradation caused by Wnt5a overexpression (Figure 3E). Thus these results indicate that endogenous Wnt5a is required for exogenously forced Ihh signals (i.e. SmoA1 overexpression or PMP treatment) to cause Nkx3.2 destabilization, whereas endogenous Ihh is not necessary for overexpressed Wnt5a to trigger Nkx3.2 degradation.
Taken together, our results in Figures 2 and 3 support the following hypothesis: Ihh enhances non-canonical Wnt (e.g. Wnt5a) signalling efficacies via parallel inhibition of Lrp co-receptors and Sfrp antagonists (Figures 2B and 2C), and, as a result, augmented Wnt5a signalling can trigger Nkx3.2 protein degradation (Figure 3A). Furthermore, for this hypothesis to be correct, it is indeed expected that Wnt5a is required for Ihh signalling to cause Nkx3.2 degradation (Figures 3B and 3C), whereas Nkx3.2 degradation can occur in the absence of Ihh signalling if Wnt5a is provided in excess (Figure 3E).
CK2 is necessary for Ihh/Wnt5a to induce proteasomal degradation of Nkx3.2
Because our results indicate that Wnt5a can render Nkx3.2 protein degradation, we next sought to identify intracellular components employed in this process. To do this, we first examined various intracellular components including PKC (protein kinase C), TAK1 [TGF (transforming growth factor)-β-activated kinase 1], CK1 (casein kinase 1), protein kinase CK2 and Ca2+ ions, all of which have been indicated in various Wnt5a-mediated signalling pathways [50–52,54,56–58]. Then, we found that PKCα, Ca2+, TAK1 and CK1α/δ/ϵ may not be closely associated with Nkx3.2 protein stability control (Figures 4A–4D). However, interestingly, ectopic expression of CK2α/β dramatically reduced the levels of co-expressed Nkx3.2 proteins, but not Runx2 (runt-related transcription factor 2) (Figure 4E). In addition, we observed the same effects on Nkx3.2 protein stability by co-expressing CK2α'/β (results not shown). Conversely, KD (kinase-dead) forms of CK2α and/or CK2α' could not induce Nkx3.2 protein degradation (results not shown), suggesting that the catalytic activity of CK2 is necessary for CK2 to cause Nkx3.2 protein destabilization.
We then tried to determine whether CK2 is required for Ihh/Wnt5a-induced Nkx3.2 degradation. Interestingly, we found that KD forms of CK2α and/or CK2α' can efficiently abolish Nkx3.2 protein degradation triggered by SmoA1 (Figure 5A) or by Wnt5a (Figure 5B). Consistent with these findings, pharmacological inhibition of CK2 using DRB or TBB resulted in successful blockage of Nkx3.2 protein degradation caused by SmoA1 (Figure 5C) or by Wnt5a (Figure 5D). In addition, we observed that endogenous Nkx3.2 protein destabilization by PMP treatment can be effectively attenuated by pharmacological inhibition of CK2 using DRB or TBB treatment (Figure 5E) or by RNA knockdown of CK2α/β (Figure 5F). Finally, we found that CK2α/β overexpression can remarkably elevate the levels of Nkx3.2 ubiquitination (Figure 5G). Together, these results indicate that CK2 is engaged in Ihh-induced Wnt5a-dependent proteasomal degradation of Nkx3.2.
Siah1 plays a role in Nkx3.2 ubiquitination triggered by Ihh signalling
Because our results suggest that proteasomal degradation of Nkx3.2 can be induced by Ihh signalling, we next sought to identify the E3 ligase engaged in this process. To this end, we first examined involvement of Siah1, the mammalian homologue of Drosophila Sina [74,75], because Ihh-induced Nkx3.2 degradation requires Wnt5a signals, and Siah1 has been shown to play a role in various Wnt5a-induced proteasomal degradation processes in chondrocytes [43,61,76]. Interestingly, we found that overexpression of Siah1, but not βTrCP1 (β-transducin repeat-containing protein 1), substantially reduced the protein levels of co-expressed Nkx3.2 (Figure 6A). As we observed that Siah1 is capable of inducing Nkx3.2 protein degradation, we next investigated whether Siah1 is required for Ihh-induced Nkx3.2 degradation. We found that a dominant-negative form of Siah1 (i.e. a RING-domain-deletion mutant) can abolish SmoA1-induced Nkx3.2 degradation (Figure 6B), and Siah1 knockdown disabled SmoA1 to destabilize Nkx3.2 proteins (Figure 6C); the efficacies of Siah1 shRNAs were estimated as shown in Figure 6(D). Together, these results indicate that Siah1 may function in the Ihh-induced Nkx3.2-degradation pathway.
We next explored the possibility that CK2 may regulate the interaction between Nkx3.2 and Siah1 E3 ligase. Intriguingly, co-IP assays revealed that Nkx3.2 could form a complex with Siah1 in the presence of the proteasome inhibitor MG132, and this interaction could be remarkably attenuated by pharmacological inhibitors of CK2 such as DRB and TBB (Figure 6E). In addition, we have confirmed that CK2-induced Nkx3.2 protein degradation can be effectively protected by dominant-negative Siah1 (Figure 6F). Therefore these results suggest that CK2 can enhance the interaction between Nkx3.2 and Siah1 to cause Nkx3.2 ubiquitination, which, in turn, promotes proteasomal degradation of Nkx3.2.
Ihh signalling regulates Nkx3.2 protein levels during chondrogenesis
To validate the physiological relevance of Ihh-induced Nkx3.2 protein degradation phenomena, we first performed F-IHC (fluorescence immunohistochemistry) of mouse E16.5 growth plates for Ihh and Nkx3.2 proteins. Consistent with our in vitro findings, Nkx3.2 protein levels were notably reduced in Ihh expression-positive tissues (Figure 7A, compare panels c and d, or g and h); the staining patterns of DAPI and PCNA indicate that the areas above and below the broken line represent proliferating and maturing zones of the growth plates respectively (Figure 7A, compare panels a and b, or e and f). To provide additional evidence for this link, we next monitored the relationship between levels of Nkx3.2 protein and Ihh signalling during in vitro chondrocyte maturation; here, we considered the induction of Gli1 as a marker for Ihh signalling (Figures 7B and 7C, top panels). In agreement with the in vivo expression patterns observed from mouse embryonic tissues (Figure 7A), levels of Nkx3.2 protein were decreased rapidly upon the appearance of Ihh signalling (i.e. Gli1 expression) during ATDC5 maturation (Figure 7B, middle panel) or mouse E11.5 limb bud culture (Figure 7C, middle panel). In addition, this typical reduction of Nkx3.2 protein levels during chondrocyte maturation can be significantly delayed by the presence of CCP, an Hh antagonist (Figure 7D, middle panel, compare lanes 4 and 5 with lanes 7 and 8); as expected, CCP treatment blocked naturally occurring Gli induction (i.e. Ihh signal activation; Figure 7D, top panel). Thus these results suggest that Nkx3.2 down-regulation during chondrocyte maturation can be closely associated with Ihh signalling.
To substantiate further this connection between Nkx3.2 protein levels and Ihh signalling in vivo, we next analysed mouse E18.5 cartilage growth plate from Ihh-knockout mice  or Col2-Cre;Smon/c mice . Briefly, Smon/c mice contain one copy of the null allele and one copy of the conditional allele, which can be deleted by Cre recombinase. As reported previously [23,24], both of these mutant mice displayed severe skeletal phenotypes, including significant inhibition of hypertrophic maturation of chondrocyte and osteoblast differentiation in endochondral bones. Most relevant to the present study, we have carefully analysed the expression patterns of Nkx3.2 proteins in these mutant mice. Consistent with our results so far, the characteristic decrease in Nkx3.2 protein levels in hypertrophic chondrocytes of normal growth plates (Figure 7E, panel b for Ihh+/− mice; Figure 7F, panel b for Smo+/+ mice) was not observed in either the Ihh−/− growth plate (Figure 7E, panel c) or the Col2-Cre;Smon/c growth plate (Figure 7F, panel c). Instead, in these mutant mice, Nkx3.2 protein levels were substantially elevated throughout the growth plate, including both proliferative and hypertrophic chondrocytes.
Finally, to support the biological importance of Nkx3.2–Ihh cross-talk, we examined the effects of Nkx3.2 overexpression and/or PMP administration on chondrocyte maturation. First, stable overexpression of Nkx3.2 substantially diminished type X collagen induction during ATDC5 maturation (Figure 7G, top panel, compare lanes 3 and 8), suggesting that forced expression of Nkx3.2 can inhibit chondrocyte maturation. Secondly, as would be expected, the elevation of Ihh signalling by PMP treatment significantly potentiated the induction of both type X collagen, a typical marker for chondrocyte hypertrophy, and Gli1, a typical marker for Ihh signalling (Figure 7G, top and second panel from top, compare lanes 3 and 5). Thirdly, blockage of type X collagen induction by Nkx3.2 overexpression was partially rescued by PMP treatment (Figure 7G, top panel, compare lanes 8 and 10), suggesting that inhibition of chondrocyte maturation caused by the elevated levels of Nkx3.2 can be attenuated by Ihh-induced down-regulation of Nkx3.2. Fourthly, unlike type X collagen, Gli1 induction by PMP was not significantly altered by stable overexpression of Nkx3.2 (Figure 7G, second panel from top, compare lanes 4 and 5 with lanes 9 and 10), suggesting that Nkx3.2 may not inhibit Ihh signalling (e.g. Gli12 induction by Ihh). Taken together, Nkx3.2 degradation triggered by Ihh may be a necessary event for optimal progression of hypertrophic maturation in chondrocytes.
Chondrocyte maturation control via Ihh-induced Nkx3.2 degradation
Although many previous studies have indicated that Ihh plays an essential role in endochondral bone development, complete understanding of the precise actions of Ihh pathways will require further study. In the present study, we have revealed that Ihh signalling can negatively regulate Nkx3.2 by triggering proteasomal degradation of Nkx3.2. In fact, these results provide an additional molecular explanation of how Ihh functions to promote chondrocyte hypertrophy during cartilage development. As stated above, defects in Ihh signalling have been shown to result in disorganized and incomplete chondrocyte hypertrophy [20,22–25], and our results may suggest that inappropriate accumulation of Nkx3.2 could be one of the consequences caused by loss of Ihh signalling. Thus it is a realistic possibility that Nkx3.2–Ihh cross-talk may have contributed to cause the phenotypes observed from Ihh signalling-defective mice. Consistent with this hypothesis, the present and previous [2,15] studies have indicated that forced expression of Nkx3.2 can delay chondrocyte hypertrophy in vitro. Thus these findings together logically support the biological significance of Nkx3.2–Ihh cross-talk in normal progression of chondrocyte maturation during cartilage development.
On the other hand, previous studies have demonstrated that Nkx3.2 (Bapx1) deletion in mice results in severe defects in axial skeletons, whereas limb development is relatively normal [8–11]. Nonetheless, our results suggest the possibility that a functional cross-talk between Ihh and Nkx3.2 may play a role in appendicular cartilage development as well. Thus it is conceivable that a redundant factor may have compensated for the absence of Nkx3.2 in appendicular cartilage development in Nkx3.2-knockout mice reported previously. Our future studies may focus on understanding the effects of chondrocyte-specific gain-of-function or loss-of-function mutations of Nkx3.2 on Ihh signalling and cartilage development in vivo.
A cross-talk between Ihh and Wnt pathways during cartilage hypertrophy
Functional interactions between Ihh and Wnt pathways have been indicated previously in diverse events, including cartilage growth plate control and joint segmentation [17,20,27,36,42,44,46,55,77–80]. In the present study, we found that Ihh can cause parallel inhibition of Lrp (Wnt co-receptors) and Sfrp (Wnt antagonists) in chondrocytes, which, in turn, may result in selective enhancement of Wnt5a signalling in the pre-hypertrophic zone of cartilage growth plate. Consistent with these observations, we demonstrate that Ihh-induced Nkx3.2 degradation requires intact Wnt5a expression (Figure 4). On the other hand, it has been indicated that the levels of Wnt5a expression in Ihh-knockout or Col2-Cre;Smon/c mice were not notably altered [23,24]. Nevertheless, we found that Nkx3.2 protein levels in the growth plates are evidently up-regulated in these Ihh signalling-deficient mice (Figure 8). Thus these results indicate that Wnt5a expression alone is not sufficient to down-regulate Nkx3.2 in Ihh-knockout or Col2-Cre;Smon/c mice because Ihh-mediated enhancement of Wnt5a signalling would be absent from these Ihh signalling-defective mice. Taken together, it is plausible that a collaborative action involving Ihh–Wnt5a cross-talk may play a role in optimal progression of chondrocyte hypertrophy during cartilage development (Figure 8). Additionally, cellular components such as CK2 and Siah1, which we have revealed to be necessary factors for Ihh-induced Wnt5a-dependent Nkx3.2 protein degradation, may also target other proteins during cartilage development to mediate the signals involving Ihh–Wnt5a cross-talk (Figure 8).
Involvement of PTHrP signalling in Ihh-induced Nkx3.2 degradation
The Ihh and PTHrP pathways have been demonstrated to closely communicate to control both chondrocyte proliferation and hypertrophic maturation via a remote cross-talk in the growth plate [18,20–22,24,29,77,81–83]. However, as would be expected, to demonstrate that Ihh could induce Nkx3.2 protein degradation, additional modulation of PTHrP signalling was not required in our routine experimental settings. Nevertheless, alterations of Ihh signalling might have modified PTHrP signalling by affecting expression of PTHrP and/or PPR [PTH (parathyroid hormone)/PTHrP receptor] or the activity of various cellular components. To clarify this issue, we have investigated whether exogenous Ihh signalling (i.e. PMP treatment or SmoA1 overexpression) can modulate expression of PTHrP and/or PPR under the experimental conditions employed in the present study. Then we found that neither PTHrP nor PPR expression is appreciably altered by providing exogenous Ihh signalling in ATDC5 cells as revealed by RT–PCR and microarray analyses (results not shown). Consistent with these findings, we have observed that pharmacological inhibition of PKA (protein kinase A), a well-established PTHrP downstream component, did not block Ihh-induced Nkx3.2 protein degradation (results not shown). Thus these results together suggest that PTHrP signalling is unlikely to be necessary for Ihh to trigger Nkx3.2 protein degradation.
From a different perspective, it has been demonstrated that appropriate expression of PTHrP in vivo is dependent on intact Ihh signalling [21,22,24,25,29]. In addition, previous work has indicated that Nkx3.2 mRNA expression in the proliferating columnar chondrocytes is specifically and remarkably diminished in PTHrP signalling-deficient mice . Thus, on the basis of these findings, it could be predicted that Nkx3.2 mRNA levels may be very low in Ihh−/− or Col2-Cre;Smon/c mice. Nonetheless, we have found that Nkx3.2 protein levels in these mutant mice are substantially elevated (Figure 8). Therefore the findings of the previous and present studies together prompted us to speculate the following. First, PTHrP signalling may play a role in potentiating and maintaining Nkx3.2 expression rather than initial induction; thus much reduced, but appreciable, levels of Nkx3.2 mRNA may still exist in Ihh signalling-deficient chondrocytes. Secondly, whereas Ihh deletion would result in a reduction of Nkx3.2 mRNA expression via PTHrP inhibition, Ihh-induced Nkx3.2 protein degradation would also be blocked in Ihh signalling-defective chondrocytes; therefore, under these circumstances, low levels of Nkx3.2 mRNA expression might be sufficient to give rise to detectable accumulation of Nkx3.2 proteins. Thirdly, it is also feasible that Nkx3.2 expression may require PTHrP signalling only when Ihh signalling is normally operated.
Seung-Won Choi contributed to performing the experiments in Figures 1–6 and to writing the paper. Da-Un Jeong, Boyoung Lee and Jeong-Ah Kim conducted the experiments for Figure 7. Kyu Sang Joeng and Fanxin Long provided paraffin sections for Figures 7(E) and 7(F). Dae-Won Kim supervised the experiments and wrote the paper.
This study was supported by the Korean Ministry of Education, Science and Technology [grant numbers 2011-0020409, FPR08B1-260, 2011-0001029 and 2011-0027837].
Abbreviations: Cbfβ, core binding factor β; CCP, cyclopamine; CK1, casein kinase 1; DAPI, 4′,6-diamidino-2-phenylindole; DMEM, Dulbecco's modified Eagle's medium; DRB, 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole; DTT, dithiothreitol; E, embryonic day; FBS, fetal bovine serum; F-IHC, fluorescence immunohistochemistry; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; Gli, glioma-associated oncogene homologue; HA, haemagglutinin; HEK, human embryonic kidney; Hh, Hedgehog; Ihh, Indian Hedgehog; IP, immunoprecipitation; KD, kinase-dead; Lrp, low-density-lipoprotein-receptor-related protein; NF-κB, nuclear factor κB; Nkx3.2, NK3 homeobox 2; PCNA, proliferating-cell nuclear antigen; PKC, protein kinase C; PMP, purmorphamine; Ptc, patched; PTHrP, parathyroid hormone-related protein; PPR, PTH (parathyroid hormone)/PTHrP receptor; RT, reverse transcription; Runx2, runt-related transcription factor 2; Sfrp, secreted frizzled-related protein; shRNA, short hairpin RNA; Siah1, seven in absentia homologue 1; Smo, smoothened; TAK1, TGF (transforming growth factor)-β-activated kinase 1; TBB, 4,5,6,7-tetrabromo-1H-benzotriazole; βTrCP1, β-transducin repeat-containing protein 1; WB, Western blot
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