Evaluation of Potential Roles of Zinc Finger Homeobox 3 (Zfhx3) Expressed in Chondrocytes and Osteoblasts on Skeletal Growth in Mice.

Bone formation is tightly modulated by genetically encoded molecular proteins that interact to regulate cellular differentiation and secretion of bony matrix. Many transcription factors are known to coordinate these events by controlling gene transcription within networks. However, not all factors involved are known. Here, we identified a novel function for Zinc Finger Homeobox 3 (Zfhx3), a gene encoding a transcription factor, as a regulator of bone metabolism. We knocked out Zfhx3 conditionally in mice in either chondrocytes or osteoblasts and characterized their bones by micro-CT in 12-week-old mice. We observed a negative effect in linear bone growth in both knockout mice but reduced bone mass only in mice with Zfhx3 deleted in osteoblasts. Loss of Zfhx3 expression in osteoblasts affected trabecular bone mass in femurs and vertebrae in both sexes but influenced cortical bone volume fraction only in females. Moreover, transcriptional analysis of femoral bones in osteoblast Zfhx3 conditional knockout mice revealed a reduced expression of osteoblast genes, and histological evaluation of trabecular bones suggests that Zfhx3 causes changes in bone formation and not resorption. The loss of Zfhx3 causes reductions in trabecular bone area and osteoid volume, but no changes in the expression of osteoclast differentiation markers or number of TRAP stained osteoclasts. These studies introduce Zfhx3 as a relevant factor toward understanding gene regulatory networks that control bone formation and development of peak bone mass.

Sex-Specific Effects of THRß Signaling on Metabolic Responses to High Fat Diet in Mice.

Thyroid hormone (TH) plays a crucial role in regulating the functions of both bone and adipose tissue. Given that TH exerts its cholesterol-lowering effects in hepatic tissue through the TH receptor-ß (TRß), we hypothesized that TRß agonist therapy using MGL3196 (MGL) would be effective in treating increased adiposity and bone loss in response to a 12-week high-fat diet (HFD) in adult C57BL/6J mice. Transcriptional and serum profiling revealed that HFD-induced leptin promoted weight gain in both males and females, but MGL only suppressed leptin induction and weight gain in males. In vitro studies suggest that estrogen suppresses MGL activity in adipocytes, indicating that estrogen might interfere with MGL-TRß function. Compared to systemic adiposity, HFD reduced bone mass in male but not female mice. Paradoxically, MGL treatment reversed macroscopic bone mineral density loss in appendicular bones, but micro-CT revealed that MGL exacerbated HFD-induced trabecular bone loss, and reduced bone strength. In studies on the mechanisms for HFD effects on bone, we found that HFD induced Rankl expression in male femurs that was blocked by MGL. By ex vivo assays, we found that RANKL indirectly represses osteoblast lineage allocation of osteoprogenitors by induction of inflammatory cytokines TNFα, IL-1ß, and CCL2. Finally, we found that MGL functions in both systemic adiposity and bone by nongenomic TRß signaling, as HFD-mediated phenotypes were not rescued in TRß147F knockout mice with normal genomic but defective nongenomic TRß signaling. Our findings demonstrate that the negative effects of HFD on body fat and bone phenotypes are impacted by MGL in a gender-specific manner.

Targeted Overexpression of Claudin 11 in Osteoblasts Increases Trabecular Bone Mass by Stimulating Osteogenesis at the Expense of Adipogenesis in Mice.

Mice lacking Claudin11 (Cldn11) manifest reduced trabecular bone mass. However, the impact of Cldn11 expression in osteoblasts in vivo remains understudied. Herein, we generated osteoblast-specific transgenic (Tg) mice expressing Cldn11 and characterized their skeletal phenotype. Micro-CT analyses of the distal metaphysis of the femur showed a 50% and a 38% increase in trabecular bone mass in Tg male and female mice, respectively, due to a significant increase in trabecular number and a reduction in trabecular separation. Histomorphometry and serum biomarker studies uncovered that increased trabecular bone mass in Cldn11 Tg mice was the consequence of enhanced bone formation. Accordingly, an abundance of bone formation (Alp, Bsp), but not bone resorption (Ctsk), markers were augmented in the femurs of Cldn11 Tg mice. Since the trabecular bone density is known to inversely correlate with the amount of marrow adipose tissue (MAT), we measured the MAT in osmium-tetroxide-labeled bones by micro-CT scanning. We found 86% less MAT in the proximal tibia of the Tg males. Consistently, the expression levels of the adipogenic markers, adiponectin and leptin, were 50% lower in the femurs of the Tg males. Our data are consistent with the possibility that claudin11 exerts anabolic effects in osteoblastic lineage cells that act via promoting the differentiation of marrow stem cells towards osteoblasts at the expense of adipocytes.

Global and Conditional Disruption of the Igf-I Gene in Osteoblasts and/or Chondrocytes Unveils Epiphyseal and Metaphyseal Bone-Specific Effects of IGF-I in Bone.

To evaluate the relative importance of IGF-I expression in various cell types for endochondral ossification, we quantified the trabecular bone at the secondary spongiosa and epiphysis of the distal femur in 8-12-week-old male mice with a global knockout of the Igf-I gene, as well as the conditional deletion of Igf-I in osteoblasts, chondrocytes, and osteoblasts/chondrocytes and their corresponding wild-type control littermates. The osteoblast-, chondrocyte-, and osteoblast/chondrocyte-specific Igf-I conditional knockout mice were generated by crossing Igf-I floxed mice with Cre transgenic mice in which Cre expression is under the control of either the Col1α2 or Col2α1 promoter. We found that the global disruption of Igf-I resulted in 80% and 70% reductions in bone size, defined as total volume, at the secondary spongiosa and epiphysis of the distal femur, respectively. The abrogation of Igf-I in Col1α2-producing osteoblasts but not Col2α1-producing chondrocytes decreased bone size by 25% at both the secondary spongiosa and epiphysis. In comparison, the deletion of the Igf-I globally or specifically in osteoblasts or chondrocytes reduced trabecular bone mass by 25%. In contrast, the universal deletion of Igf-I in all cells, but not the conditional disruption of Igf-I in osteoblasts and/or chondrocytes reduced trabecular bone mass in the epiphysis. The reduced trabecular bone mass at the secondary spongiosa in osteoblast- and/or chondrocyte-specific Igf-I conditional knockout mice is caused by the reduced trabecular number and increased trabecular separation. Immunohistochemistry studies found that the expression levels of chondrocyte (COL10, MMP13) and osteoblast (BSP) markers were less in the secondary spongiosa and the epiphyses in the global Igf-I deletion mice. Our data indicate that local and endocrine Igf-I act pleiotropically and in a cell type- and bone compartment-dependent manner in bone.

Development of an Animal Model for Traumatic Brain Injury Augmentation of Heterotopic Ossification in Response to Local Injury.

Heterotopic ossification (HO) is the abnormal growth of bone in soft connective tissues that occurs as a frequent complication in individuals with traumatic brain injury (TBI) and in rare genetic disorders. Therefore, understanding the mechanisms behind ectopic bone formation in response to TBI is likely to have a significant impact on identification of novel therapeutic targets for HO treatment. In this study, we induced repetitive mild TBI (mTBI) using a weight drop model in mice and then stimulated HO formation via a local injury to the Achilles tendon or fibula. The amount of ectopic bone, as evaluated by micro-CT analyses, was increased by four-fold in the injured leg of mTBI mice compared to control mice. However, there was no evidence of HO formation in the uninjured leg of mTBI mice. Since tissue injury leads to the activation of hypoxia signaling, which is known to promote endochondral ossification, we evaluated the effect of IOX2, a chemical inhibitor of PHD2 and a known inducer of hypoxia signaling on HO development in response to fibular injury. IOX2 treatment increased HO volume by five-fold compared to vehicle. Since pericytes located in the endothelium of microvascular capillaries are known to function as multipotent tissue-resident progenitors, we determined if activation of hypoxia signaling promotes pericyte recruitment at the injury site. We found that markers of pericytes, NG2 and PDGFRß, were abundantly expressed at the site of injury in IOX2 treated mice. Treatment of pericytes with IOX2 for 72 h stimulated expression of targets of hypoxia signaling (Vegf and Epo), as well as markers of chondrocyte differentiation (Col2α1 and Col10α1). Furthermore, serum collected from TBI mice was more effective in promoting the proliferation and differentiation of pericytes than control mouse serum. In conclusion, our data show that the hypoxic state at the injury site in soft tissues of TBI mice provides an environment leading to increased accumulation and activation of pericytes to form endochondral bone.

Sequential application of small molecule therapy enhances chondrogenesis and angiogenesis in murine segmental defect bone repair.

The increasing incidence of physiologic/pathologic conditions that impair the otherwise routine healing of endochondral bone fractures and the occurrence of severe bone injuries necessitate novel approaches to enhance clinically challenging bone fracture repair. To promote the healing of nonunion fractures, we tested an approach that used two small molecules to sequentially enhance cartilage development and conversion to the bone in the callus of a murine femoral segmental defect nonunion model of bone injury. Systemic injections of smoothened agonist 21k (SAG21k) were used to stimulate chondrogenesis through the activation of the sonic hedgehog (SHH) pathway early in bone repair, while injections of the prolyl hydroxylase domain (PHD)2 inhibitor, IOX2, were used to stimulate hypoxia signaling-mediated endochondral bone formation. The expression of SHH pathway genes and Phd2 target genes was increased in chondrocyte cell lines in response to SAG21k and IOX2 treatment, respectively. The segmental defect responded to sequential systemic administration of these small molecules with increased chondrocyte expression of PTCH1, GLI1, and SOX9 in response to SAG and increased expression of hypoxia-induced factor-1α and vascular endothelial growth factor-A in the defect tissues in response to IOX2. At 6 weeks postsurgery, the combined SAG-IOX2 therapy produced increased bone formation in the defect with the bony union over the injury. Clinical significance: This therapeutic approach was successful in promoting cartilage and bone formation within a critical-size segmental defect and established the utility of a sequential small molecule therapy for the enhancement of fracture callus development in clinically challenging bone injuries.

Contrasting effects of Ksr2, an obesity gene, on trabecular bone volume and bone marrow adiposity.

Pathological obesity and its complications are associated with an increased propensity for bone fractures. Humans with certain genetic polymorphisms at the kinase suppressor of ras2 (KSR2) locus develop severe early-onset obesity and type 2 diabetes. Both conditions are phenocopied in mice with Ksr2 deleted, but whether this affects bone health remains unknown. Here we studied the bones of global Ksr2 null mice and found that Ksr2 negatively regulates femoral, but not vertebral, bone mass in two genetic backgrounds, while the paralogous gene, Ksr1, was dispensable for bone homeostasis. Mechanistically, KSR2 regulates bone formation by influencing adipocyte differentiation at the expense of osteoblasts in the bone marrow. Compared with Ksr2's known role as a regulator of feeding by its function in the hypothalamus, pair-feeding and osteoblast-specific conditional deletion of Ksr2 reveals that Ksr2 can regulate bone formation autonomously. Despite the gains in appendicular bone mass observed in the absence of Ksr2, bone strength, as well as fracture healing response, remains compromised in these mice. This study highlights the interrelationship between adiposity and bone health and provides mechanistic insights into how Ksr2, an adiposity and diabetic gene, regulates bone metabolism.

Our bones are living tissues which constantly reshape and renew themselves. This ability relies on stem cells present in the marrow cavity, which can mature into the various types of cells needed to produce new bone material, marrow fat, or other components. Obesity and associated conditions such as type 2 diabetes are often linked to harmful changes in the skeleton. In particular, these metabolic conditions are associated with weight-bearing bones becoming more prone to facture and healing poorly. Mice genetically modified to model obesity and diabetes could help researchers to study exactly how these conditions ­ and the genetic changes that underlie them ­ impact bone health. Gomez et al. aimed to address this question by focusing on KSR2, a gene involved in energy consumption and feeding behavior. Children who carry certain KSR2 mutations are prone to obesity and type 2 diabetes; mice lacking the gene also develop these conditions due to uncontrolled eating. Closely examining mutant mice in which Ksr2 had been deactivated in every cell revealed that the weight-bearing bones of these animals were also more likely to break, and the fractures then healed more slowly. This was the case even though these bones had higher mass and less marrow fat compared to healthy mice. Non-weight bearing bones (such as the spine) did not exhibit these changes. Further experiments revealed that, when expressed normally in the skeleton, Ksr2 skews the stem cell maturation process towards marrow fat cells instead of bone-creating cells. This suggests a new role for Ksr2, which therefore seems to independently regulate both feeding behavior and bone health. In addition, the work by Gomez et al. demonstrate that Ksr2 mutant mice could be a useful model to better understand how obesity and diabetes affect human bones, and to potentially develop new therapies.

Targeted Deletion of the Claudin12 Gene in Mice Increases Articular Cartilage and Inhibits Chondrocyte Differentiation.

To study the role of Claudin (CLDN)12 in bone, we developed mice with a targeted deletion of exon2 in the Cldn12 gene for skeletal phenotype analysis. Micro-CT analysis of the secondary spongiosa of distal femurs of mice with targeted disruption of the Cldn12 gene and control littermates showed no significant genotype-specific differences in either cortical or trabecular bone parameters for either gender in 13-week-old mice. Immunohistochemistry revealed that while CLDN12 was expressed in both differentiating chondrocytes and osteoblasts of the secondary spongiosa of 3-week-old wild-type mice, its expression was restricted to differentiating chondrocytes in the articular cartilage and growth plate in adult mice. Articular cartilage area at the knee were increased by 47% in Cldn12 knockout (KO) mice compared to control littermates. Micro-CT analyses found that while the trabecular number was increased by 9% and the trabecular spacing was reduced by 9% in the femoral epiphysis of Cldn12 KO mice, neither bone volume nor bone volume adjusted for tissue volume was different between the two genotypes. The expression levels of Clusterin, Lubricin and Mmp13 were increased by 56%, 46%, and 129%, respectively, in primary articular chondrocytes derived from KO compared to control mice. Our data indicate that targeted deletion of the Cldn12 gene in mice increases articular cartilage, in part, by promoting articular chondrocyte phenotype.

Mice with Targeted Knockout of Tetraspanin 3 Exhibit Reduced Trabecular Bone Mass Caused by Decreased Osteoblast Functions.

Tetraspanin3 (TSPAN3) was identified as a binding partner of claudin11 (CLDN11) in osteoblasts and other cell types. Mice with targeted disruption of Cldn11 exhibited trabecular bone mass deficit caused by reduced bone formation and osteoblast function. To determine if the disruption of CLDN11 interacting protein gene Tspan3 results in a similar skeletal phenotype as that of Cldn11 knockout (KO) mice, we generated homozygous Tspan3 KO and heterozygous control mice and characterized their skeletal phenotypes at 13 weeks of age. Micro-CT measurements of the secondary spongiosa of the distal femur revealed 17% and 29% reduction in trabecular bone volume adjusted for tissue volume (BV/TV) in the male and female mice, respectively. Similarly, trabecular BV/TV of the proximal tibia was reduced by 19% and 20% in the male and female mice, respectively. The reduced trabecular bone mass was caused primarily by reduced trabecular thickness and number, and increased trabecular spacing. Consistent with the reduced bone formation as confirmed by histomorphometry analyses, serum alkaline phosphatase was reduced by 11% in the KO mice as compared with controls. Our findings indicate that TSPAN3 is an important positive regulator of osteoblast function and trabecular bone mass, and the interaction of TSPAN3 with CLDN11 could contribute in part to the bone forming effects of Cldn11 in mice.

Differences in pathways contributing to thyroid hormone effects on postnatal cartilage calcification versus secondary ossification center development.

The proximal and distal femur epiphyses of mice are both weight-bearing structures derived from chondrocytes but differ in development. Mineralization at the distal epiphysis occurs in an osteoblast-rich secondary ossification center (SOC), while the chondrocytes of the proximal femur head (FH), in particular, are directly mineralized. Thyroid hormone (TH) plays important roles in distal knee SOC formation, but whether TH also affects proximal FH development remains unexplored. Here, we found that TH controls chondrocyte maturation and mineralization at the FH in vivo through studies in thyroid stimulating hormone receptor (Tshr-/-) hypothyroid mice by X-ray, histology, transcriptional profiling, and immunofluorescence staining. Both in vivo and in vitro studies conducted in ATDC5 chondrocyte progenitors concur that TH regulates expression of genes that modulate mineralization (Ibsp, Bglap2, Dmp1, Spp1, and Alpl). Our work also delineates differences in prominent transcription factor regulation of genes involved in the different mechanisms leading to proximal FH cartilage calcification and endochondral ossification at the distal femur. The information on the molecular pathways contributing to postnatal cartilage calcification can provide insights on therapeutic strategies to treat pathological calcification that occurs in soft tissues such as aorta, kidney, and articular cartilage.

Lack of Skeletal Effects in Mice with Targeted Disruptionof Prolyl Hydroxylase Domain 1 (Phd1) Gene Expressed in Chondrocytes.

The critical importance of hypoxia-inducible factor (HIF)s in the regulation of endochondral bone formation is now well established. HIF protein levels are closely regulated by the prolyl hydroxylase domain-containing protein (PHD) mediated ubiquitin-proteasomal degradation pathway. Of the three PHD family members expressed in bone, we previously showed that mice with conditional disruption of the Phd2 gene in chondrocytes led to a massive increase in the trabecular bone mass of the long bones. By contrast, loss of Phd3 expression in chondrocytes had no skeletal effects. To investigate the role of Phd1 expressed in chondrocytes on skeletal development, we conditionally disrupted the Phd1 gene in chondrocytes by crossing Phd1 floxed mice with Collagen 2α1-Cre mice for evaluation of a skeletal phenotype. At 12 weeks of age, neither body weight nor body length was significantly different in the Cre+; Phd1flox/flox conditional knockout (cKO) mice compared to Cre−; Phd1flox/flox wild-type (WT) control mice. Micro-CT measurements revealed significant gender differences in the trabecular bone volume adjusted for tissue volume at the secondary spongiosa of the femur and the tibia for both genotypes, but no genotype differences were found for any of the trabecular bone measurements of either femur or tibia. Similarly, cortical bone parameters were not affected in the Phd1 cKO mice compared to control mice. Histomorphometric analyses revealed no significant differences in bone area, bone formation rate or mineral apposition rate in the secondary spongiosa of femurs between cKO and WT control mice. Loss of Phd1 expression in chondrocytes did not affect the expression of markers of chondrocytes (collage 2, collagen 10) or osteoblasts (alkaline phosphatase, bone sialoprotein) in the bones of cKO mice. Based on these and our published data, we conclude that of the three PHD family members, only Phd2 expressed in chondrocytes regulates endochondral bone formation and development of peak bone mass in mice.

Prolyl Hydroxylase Domain-Containing Protein 3 Gene Expression in Chondrocytes Is Not Essential for Bone Development in Mice.

We previously showed that conditional disruption of the Phd2 gene in chondrocytes led to a massive increase in long bone trabecular bone mass. Loss of Phd2 gene expression or inhibition of PHD2 activity by a specific inhibitor resulted in a several-fold compensatory increase in Phd3 expression in chondrocytes. To determine if expression of PHD3 plays a role in endochondral bone formation, we conditionally disrupted the Phd3 gene in chondrocytes by crossing Phd3 floxed (Phd3flox/flox) mice with Col2α1-Cre mice. Loss of Phd3 expression in the chondrocytes of Cre+; Phd3flox/flox conditional knockout (cKO) mice was confirmed by real time PCR. At 16 weeks of age, neither body weight nor body length was significantly different in the Phd3 cKO mice compared to Cre-; Phd3flox/flox wild-type (WT) mice. Areal BMD measurements of total body as well as femur, tibia, and lumbar skeletal sites were not significantly different between the cKO and WT mice at 16 weeks of age. Micro-CT measurements revealed significant gender differences in the trabecular bone volume adjusted for tissue volume at the secondary spongiosa of the femur and the tibia for both genotypes, but no genotype difference was found for any of the trabecular bone measurements of either the femur or the tibia. Trabecular bone volume of distal femur epiphysis was not different between cKO and WT mice. Histology analyses revealed Phd3 cKO mice exhibited a comparable chondrocyte differentiation and proliferation, as evidenced by no changes in cartilage thickness and area in the cKO mice as compared to WT littermates. Consistent with the in vivo data, lentiviral shRNA-mediated knockdown of Phd3 expression in chondrocytes did not affect the expression of markers of chondrocyte differentiation (Col2, Col10, Acan, Sox9). Our study found that Phd2 but not Phd3 expressed in chondrocytes regulates endochondral bone formation, and the compensatory increase in Phd3 expression in the chondrocytes of Phd2 cKO mice is not the cause for increased trabecular bone mass in Phd2 cKO mice.

Conditional disruption of the osterix gene in chondrocytes during early postnatal growth impairs secondary ossification in the mouse tibial epiphysis.

In our previous studies, we have found that the prepubertal increase in thyroid hormone levels induces osterix (Osx) signaling in hypertrophic chondrocytes to transdifferentiate them into osteoblasts. To test if Osx expressed in chondrocytes directly contributes to transdifferentiation and secondary ossification, we generated Osx flox/flox ; Col2-Cre-ERT2 mice and knocked out Osx with a single injection of tamoxifen at postnatal day (P) 3 prior to evaluation of the epiphyseal bone phenotype by µCT, histology, and immunohistochemistry (IHC) at P21. Vehicle (oil)-treated Osx flox/flox ; Col2-Cre-ERT2 and tamoxifen-treated, Cre-negative Osx flox/flox mice were used as controls. µCT analysis of tibial epiphyses revealed that trabecular bone mass was reduced by 23% in the Osx conditional knockout (cKO) compared with control mice. Trabecular number and thickness were reduced by 28% and 8%, respectively, while trabecular separation was increased by 24% in the cKO mice. Trichrome staining of longitudinal sections of tibial epiphyses showed that bone area and bone area adjusted for total area were decreased by 22% and 18%, respectively. IHC studies revealed the presence of abundant Osx-expressing prehypertrophic chondrocytes in the epiphyses of control mice at P10, but not in the cKO mice. Furthermore, expression levels of MMP13, COL10, ALP, and BSP were considerably reduced in the epiphyses of cKO mice. We also found that Osx overexpression in ATDC5 chondrocytes increased expression of Col10, Mmp13, Alp, and Bsp. Our data indicate that Osx expressed in chondrocytes plays a significant role in secondary ossification by regulating expression of genes involved in chondrocyte hypertrophy and osteoblast transdifferentiation.

Novel Role for Claudin-11 in the Regulation of Osteoblasts via Modulation of ADAM10-Mediated Notch Signaling.

The claudin (Cldn) family comprises 27 members of 20 to 34 kDa transmembrane tight junction proteins. In addition to Cldns' established canonical role as barriers controlling paracellular flow of molecules, a distinct noncanonical role for them as mediators of cell signaling is now emerging. In our studies evaluating Cldn family expression levels during osteoblast differentiation, Cldn-11 showed the largest increase (60-fold). Immunohistochemistry studies revealed high Cldn-11 expression in trabecular (Tb) bone lining cells. Micro-CT analysis of femurs and vertebrae of Cldn-11 knock-out (KO) mice at 12 weeks of age exhibited a 40% (p < 0.01) reduction in Tb bone volume adjusted for tissue volume compared with control mice, a change caused by significant reductions in Tb number and thickness and increase in Tb separation. Histomorphometry and serum biomarker studies revealed that reduced bone formation, not increased resorption, is the cause for reduced Tb bone volume in the Cldn-11 KO mice. Cldn-11 KO osteoblasts expressed reduced ALP and BSP, whereas Cldn-11 overexpression in MC3T3-E1 cells increased expression of ALP and BSP. Mechanistically, Cldn-11 interacted with tetraspanin (Tspan)3 in osteoblasts, and Tspan3 knockdown reduced osteoblast differentiation. Because members of the Tspan family regulate cell functions via Notch signaling, we evaluated whether Cldn-11/Tspan3 regulates Notch signaling in osteoblasts. Accordingly, Notch targets Hey1 and Hey2 were significantly upregulated in Cldn-11 overexpressing cultures but downregulated in both Cldn-11 KO and Tspan3 knockdown osteoblasts. Because ADAM10 has been shown to interact with Tspan family members to regulate Notch signaling, we evaluated whether Cldn-11 regulates ADAM10 expression. Cldn-11 overexpressing cells express more mature ADAM10, and an ADAM10 inhibitor blocked the Cldn-11 effect on osteoblast differentiation. Based on these data, we propose Cldn-11 as a novel component of an osteoblast cell surface protein complex, comprising Tspan3 and ADAM10, which regulates Notch signaling and cell differentiation. © 2019 American Society for Bone and Mineral Research.

Conditional Deletion of the Phd2 Gene in Articular Chondrocytes Accelerates Differentiation and Reduces Articular Cartilage Thickness.

Based on our findings that PHD2 is a negative regulator of chondrocyte differentiation and that hypoxia signaling is implicated in the pathogenesis of osteoarthritis, we investigated the consequence of disruption of the Phd2 gene in chondrocytes on the articular cartilage phenotype in mice. Immunohistochemistry detected high expression of PHD2 in the superficial zone (SZ), while PHD3 and HIF-1α (target of PHD2) are mainly expressed in the middle-deep zone (MDZ). Conditional deletion of the Phd2 gene (cKO) in chondrocytes accelerated the transition of progenitors to hypertrophic (differentiating) chondrocytes as revealed by reduced SZ thickness, and increased MDZ thickness, as well as increased chondrocyte hypertrophy. Immunohistochemistry further revealed decreased levels of progenitor markers but increased levels of hypertrophy markers in the articular cartilage of the cKO mice. Treatment of primary articular chondrocytes, in vitro, with IOX2, a specific inhibitor of PHD2, promoted articular chondrocyte differentiation. Knockdown of Hif-1α expression in primary articular chondrocytes using lentiviral vectors containing Hif-1α shRNA resulted in reduced expression levels of Vegf, Glut1, Pgk1, and Col10 compared to control shRNA. We conclude that Phd2 is a key regulator of articular cartilage development that acts by inhibiting the differentiation of articular cartilage progenitors via modulating HIF-1α signaling.

Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Regulates Chondrocyte Differentiation and Secondary Ossification in Mice.

Endochondral ossification plays an important role in the formation of the primary ossification centers (POCs) and secondary ossification centers (SOCs) of mammalian long bones. However, the molecular mechanisms that regulate POC and SOC formation are different. We recently demonstrated that Prolyl Hydroxylase Domain-containing Protein 2 (Phd2) is a key mediator of vitamin C effects on bone. We investigated the role of Phd2 on endochondral ossification of the epiphyses by conditionally deleting the Phd2 gene in osteoblasts and chondrocytes. We found that the deletion of Phd2 in osteoblasts did not cause changes in bone parameters in the proximal tibial epiphyses in 5 week old mice. In contrast, deletion of Phd2 in chondrocytes resulted in increased bone mass and bone formation rate (normalized to tissue volume) in long bone epiphyses, indicating that Phd2 expressed in chondrocytes, but not osteoblasts, negatively regulates secondary ossification of epiphyses. Phd2 deletion in chondrocytes elevated mRNA expression of hypoxia-inducible factor (HIF) signaling molecules including Hif-1α, Hif-2α, Vegfa, Vegfb, and Epo, as well as markers for chondrocyte hypertrophy and mineralization such as Col10, osterix, alkaline phosphatase, and bone sialoprotein. These data suggest that Phd2 expressed in chondrocytes inhibits endochondral ossification at the epiphysis by suppressing HIF signaling pathways.

Effects of Thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3) and their Metabolites on Osteoblast Differentiation.

Studies involving human genetic mutations and mutant mouse models have provided irrevocable evidence for a key role for thyroid hormones (THs) in the regulation of skeletal growth. While T3 binds to TH receptors with higher affinity than T4, T4 occupied TH receptors have also been reported in the nucleus under euthyroid conditions raising the possibility that T4 bound nuclear receptors may be biologically relevant in thyroid syndromes with elevated free T4 and reduced T3 levels. We, therefore, evaluated the direct effects of T4, T3, and their metabolites (rT3 and T2) in stimulating osteoblast differentiation using MC3T3-E1 preosteoblasts which do not produce detectable levels of deiodinases. Under serum-free conditions, a 24-h treatment of MC3T3-E1 cells with THs and their metabolites caused a dose-dependent increase in the expression of osteoblast differentiation markers, osterix, and osteocalcin. Circulating concentrations of T3 (~1 ng/ml) and T4 (~30 ng/ml) showed similar potency in stimulating osteoblast differentiation marker expression, while rT3 and T2 were less potent than T3 and T4. Moreover, T3 and T4 treatments elevated the IGF-1 mRNA level suggesting the involvement of IGF-1 signaling in the TH regulation of osteoblast differentiation. We conclude that an elevated T4 level in the absence of T3 may exert stimulatory effects on osteoblast differentiation. The establishment of cell-specific effects of T4 on osteoblasts may provide a strategy to generate T4 mimics that exert skeletal specific effects without the confounding T3 effects on other tissues.

Thyroid hormone receptor-ß1 signaling is critically involved in regulating secondary ossification via promoting transcription of the Ihh gene in the epiphysis.

Thyroid hormone (TH) action is mediated through two nuclear TH receptors, THRα and THRß. Although the role of THRα is well established in bone, less is known about the relevance of THRß-mediated signaling in bone development. On ther basis of our recent finding that TH signaling is essential for initiation and formation of secondary ossification center, we evaluated the role of THRs in mediating TH effects on epiphysial bone formation. Two-day treatment of TH-deficient Tshr(-/-) mice with TH increased THRß1 mRNA level 3.4-fold at day 7 but had no effect on THRα1 mRNA level at the proximal tibia epiphysis. Treatment of serum-free cultures of tibias from 3-day-old mice with T3 increased THRß1 expression 2.1- and 13-fold, respectively, at 24 and 72 h. Ten-day treatment of Tshr(-/-) newborns (days 5-14) with THRß1 agonist GC1 at 0.2 or 2.0 µg/day increased BV/TV at day 21 by 225 and 263%, respectively, compared with vehicle treatment. Two-day treatment with GC1 (0.2 µg/day) increased expression levels of Indian hedgehog (Ihh) 100-fold, osterix 15-fold, and osteocalcin 59-fold compared with vehicle at day 7 in the proximal tibia epiphysis. Gel mobility shift assay demonstrated that a putative TH response element in the distal promoter of mouse Ihh gene interacted with THRß1. GC1 treatment (1 nM) increased Ihh distal promoter activity 20-fold after 48 h in chondroctyes. Our data suggest a novel role for THRß1 in secondary ossification at the epiphysis that involves transcriptional upregulation of Ihh gene.

Conditional Deletion of Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Gene Reveals Its Essential Role in Chondrocyte Function and Endochondral Bone Formation.

The hypoxic growth plate cartilage requires hypoxia-inducible factor (HIF)-mediated pathways to maintain chondrocyte survival and differentiation. HIF proteins are tightly regulated by prolyl hydroxylase domain-containing protein 2 (Phd2)-mediated proteosomal degradation. We conditionally disrupted the Phd2 gene in chondrocytes by crossing Phd2 floxed mice with type 2 collagen-α1-Cre transgenic mice and found massive increases (>50%) in the trabecular bone mass of long bones and lumbar vertebra of the Phd2 conditional knockout (cKO) mice caused by significant increases in trabecular number and thickness and reductions in trabecular separation. Cortical thickness and tissue mineral density at the femoral middiaphysis of the cKO mice were also significantly increased. Dynamic histomorphometric analyses revealed increased longitudinal length and osteoid surface per bone surface in the primary spongiosa of the cKO mice, suggesting elevated conversion rate from hypertrophic chondrocytes to mineralized bone matrix as well as increased bone formation in the primary spongiosa. In the secondary spongiosa, bone formation measured by mineralizing surface per bone surface and mineral apposition rate were not changed, but resorption was slightly reduced. Increases in the mRNA levels of SRY (sex determining region Y)-box 9, osterix (Osx), type 2 collagen, aggrecan, alkaline phosphatase, bone sialoprotein, vascular endothelial growth factor, erythropoietin, and glycolytic enzymes in the growth plate of cKO mice were detected by quantitative RT-PCR. Immunohistochemistry revealed an increased HIF-1α protein level in the hypertrophic chondrocytes of cKO mice. Infection of chondrocytes isolated from Phd2 floxed mice with adenoviral Cre resulted in similar gene expression patterns as observed in the cKO growth plate chondrocytes. Our findings indicate that Phd2 suppresses endochondral bone formation, in part, via HIF-dependent mechanisms in mice.

RE1-Silencing Transcription Factor (Rest) is a Novel Regulator of Osteoblast Differentiation.

RE1-silencing transcription factor (Rest) has been identified as a master negative regulator of neuronal differentiation. Nothing is known about Rest function in bone cells. In this study, we examined the Rest expression levels and role during osteoblast differentiation. We found that Rest is abundantly expressed in bone marrow stromal cells, calvarial osteoblasts, and MC3T3-E1 osteoblasts. Treatment of primary osteoblasts with ascorbic acid (AA) down regulated Rest mRNA expression at an early stage, but not in later stages of differentiation. Consistent with treatment of primary cultures, AA treatment of MC3T3-E1 cells significantly reduced Rest protein expression at day 3 and at day 8 after initiation of osteoblast differentiation. Treatment of bone marrow stromal cells with BMP-2 and dexamethasone, but not IGF-I for 3 days greatly decreased Rest mRNA expression. To test the function of Rest during osteoblast differentiation, Rest expression was knocked down in MC3T3-E1 cell subclones segregated on the basis of ALP activity (differentiation status) using lentivirus expressing shRNA against Rest. An 80% knockdown of Rest expression decreased Osterix (Osx) expression by 52-57% and as a result, increased both basal and AA induced ALP expression and activity in the subclone that expresses low basal level of ALP (undifferentiated). By contrast, a 98% knockdown of Rest expression in cells that express high basal levels of ALP (differentiated cells) caused a significant reduction in Osx expression, basal and AA induced ALP expression and activity. These data suggest that Rest regulates early osteoblast differentiation via modulating Rest expression that is independent of Osx expression.