Inactivation of Pten in osteo-chondroprogenitor cells leads to epiphyseal growth plate abnormalities and skeletal overgrowth

Direct activation of PI3K in osteoblasts and osteocytes strengthens murine bone through sex-specific actions on cortical surfaces

Intracellular phosphoinositide 3-kinase (PI3K) signaling is activated by multiple bone-active receptors. Genetic mutations activating PI3K signaling are associated with clinical syndromes of tissue overgrowth in multiple organs, often including the skeleton. While one formation is increased by removing the PI3K inhibitor (phosphatase and TENsin homolog deleted on chromosome 10 (PTEN)), the effect of direct PI3K activation in the osteoblast lineage has not been reported. We introduced a known gain-of-function mutation in Pik3ca, the gene encoding the p110α catalytic subunit of PI3K, in osteocytes and late osteoblasts using the dentin matrix protein-1 Cre (Dmp1Cre) mouse and assessed the skeletal phenotype. Femur shape was grossly normal, but cortical thickness was significantly greater in both male and female Dmp1Cre.Pik3caH1047R mice, leading to almost doubled bone strength at 12 wk of age. Both sexes had smaller marrow areas from 6 wk of age. Female mice also exhibited greater cross-sectional area, which continued to increase until 24 wk of age, resulting in a further increase in bone strength. Although both male and female mice had increased endocortical mineralizing surface, only female mice had increased periosteal mineralizing surface. The bone formed in the Dmp1Cre.Pik3caH1047R mice showed no increase in intracortical remodeling nor any defect in cortical bone consolidation. In contrast, on both endocortical and periosteal surfaces, there was more lamellar bone formation, including highly organized osteocyte networks extending along the entire surface at a greater thickness than in control mice. In conclusion, direct activation of PI3Kα in cells targeted by Dmp1Cre leads to high cortical bone mass and strength with abundant lamellar cortical bone in female and male mice with no increase in intracortical remodeling. This differs from the effect of PTEN deletion in the same cells, suggesting that activating PI3Kα in osteoblasts and osteocytes may be a more suitable target to promote formation of lamellar bone.

Knockdown of Kmt2d leads to growth impairment by activating the Akt/β-catenin signaling pathway

The KMT2D variant-caused Kabuki syndrome (KS) is characterized by short stature as a prominent clinical characteristic. The initiation and progression of body growth are fundamentally influenced by chondrocyte proliferation. Uncertainty persists regarding the possibility that KMT2D deficiency affects growth by impairing chondrocyte proliferation. In this study, we used the CRISPR/Cas13d technique to knockdown kmt2d in zebrafish embryos and lentivirus to create a stable Kmt2d gene knockdown cell line in chondrocytes (ATDC5 cells). We also used CCK8 and flow cytometric studies, respectively, to determine proliferation and cell cycle state. The relative concentrations of phosphorylated Akt (ser473), phosphorylated β-catenin (ser552), and cyclin D1 proteins in chondrocytes and zebrafish embryos were determined by using western blots. In addition, Akt inhibition was used to rescue the phenotypes caused by kmt2d deficiency in chondrocytes, as well as a zebrafish model that was generated. The results showed that a knockdown of kmt2d significantly decreased body length and resulted in aberrant cartilage development in zebrafish embryos. Furthermore, the knockdown of Kmt2d in ATDC5 cells markedly increased proliferation and accelerated the G1/S transition. In addition, the knockdown of Kmt2d resulted in the activation of the Akt/β-catenin signaling pathway in ATDC5 cells. Finally, Akt inhibition could partly rescue body length and chondrocyte development in the zebrafish model. Our study demonstrated that KMT2D modulates bone growth conceivably via regulation of the Akt/β-catenin pathway.

Suppression of Overactive Insulin-Like Growth Factor 1 Attenuates Trauma-Induced Heterotopic Ossification in Mice

Heterotopic ossification (HO) is the ectopic bone formation in soft tissues. Aside from hereditary HO, traumatic HO is common after orthopedic surgery, combat-related injuries, severe burns, or neurologic injuries. Recently, mammalian target of rapamycin (mTOR) was demonstrated to be involved in the chondrogenic and osteogenic processes of HO formation. However, its upstream signaling mechanism remains unknown. The current study used an Achilles tendon puncture-induced HO model to show that overactive insulin-like growth factor 1 (IGF-1) was involved in the progression of HO in mice. Micro-computed tomography imaging showed that IGF-1 not only accelerated the rate of osteogenesis and increased ectopic bone volume but also induced spontaneous ectopic bone formation in undamaged Achilles tendons. Blocking IGF-1 activity with IGF-1 antibody or IGF-1 receptor inhibitor picropodophyllin significantly inhibited HO formation. Mechanistically, IGF-1/IGF-1 receptor activates phosphatidylinositol 3-kinase (PI3K)/Akt signaling to promote the phosphorylation of mTOR, resulting in the chondrogenic and osteogenic differentiation of tendon-derived stem cells into chondrocytes and osteoblasts in vitro and in vivo. Inhibitors of PI3K (LY294002) and mTOR (rapamycin) both suppressed the IGF-1-stimulated mTOR signal and mitigated the formation of ectopic bones significantly. In conclusion, these results indicate that IGF-1 mediated the progression of traumatic HO through PI3K/Akt/mTOR signaling, and suppressing IGF-1 signaling cascades attenuated HO formation, providing a promising therapeutic strategy targeting HO.

Pten knockout in mouse preosteoblasts leads to changes in bone turnover and strength

Bone development and remodeling are controlled by the phosphoinositide-3-kinase (Pi3k) signaling pathway. We investigated the effects of downregulation of phosphatase and tensin homolog (Pten), a negative regulator of Pi3k signaling, in a mouse model of Pten deficiency in preosteoblasts. We aimed to identify mechanisms that are involved in the regulation of bone turnover and are linked to bone disorders. Femora, tibiae, and bone marrow stromal cells (BMSCs) isolated from mice with a conditional deletion of Pten (Pten cKO) in Osterix/Sp7-expressing osteoprogenitor cells were compared to Cre-negative controls. Bone phenotyping was performed by μCT measurements, bone histomorphometry, quantification of bone turnover markers CTX and procollagen type 1 N propeptide (P1NP), and three-point bending test. Proliferation of BMSCs was measured by counting nuclei and Ki-67-stained cells. In vitro, osteogenic differentiation capacity was determined by ALP staining, as well as by detecting gene expression of osteogenic markers. BMSCs from Pten cKO mice were functionally different from control BMSCs. Osteogenic markers were increased in BMSCs derived from Pten cKO mice, while Pten protein expression was lower and Akt phosphorylation was increased. We detected a higher trabecular bone volume and an altered cortical bone morphology in Pten cKO bones with a progressive decrease in bone and tissue mineral density. Pten cKO bones displayed fewer osteoclasts and more osteoblasts (P = .00095) per trabecular bone surface and a higher trabecular bone formation rate. Biomechanical analysis revealed a significantly higher bone strength (P = .00012 for males) and elasticity of Pten cKO femora. On the cellular level, both proliferation and osteogenic differentiation capacity of Pten cKO BMSCs were significantly increased compared to controls. Our findings suggest that Pten knockout in osteoprogenitor cells increases bone stability and elasticity by increasing trabecular bone mass and leads to increased proliferation and osteogenic differentiation of BMSCs.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

Protein tyrosine phosphatases in skeletal development and diseases

Skeletal development and homeostasis in mammals are modulated by finely coordinated processes of migration, proliferation, differentiation, and death of skeletogenic cells originating from the mesoderm and neural crest. Numerous molecular mechanisms are involved in these regulatory processes, one of which is protein posttranslational modifications, particularly protein tyrosine phosphorylation (PYP). PYP occurs mainly through the action of protein tyrosine kinases (PTKs), modifying protein enzymatic activity, changing its cellular localization, and aiding in the assembly or disassembly of protein signaling complexes. Under physiological conditions, PYP is balanced by the coordinated action of PTKs and protein tyrosine phosphatases (PTPs). Dysregulation of PYP can cause genetic, metabolic, developmental, and oncogenic skeletal diseases. Although PYP is a reversible biochemical process, in contrast to PTKs, little is known about how this equilibrium is modulated by PTPs in the skeletal system. Whole-genome sequencing has revealed a large and diverse superfamily of PTP genes (over 100 members) in humans, which can be further divided into cysteine (Cys)-, aspartic acid (Asp)-, and histidine (His)-based PTPs. Here, we review current knowledge about the functions and regulatory mechanisms of 28 PTPs involved in skeletal development and diseases; 27 of them belong to class I and II Cys-based PTPs, and the other is an Asp-based PTP. Recent progress in analyzing animal models that harbor various mutations in these PTPs and future research directions are also discussed. Our literature review indicates that PTPs are as crucial as PTKs in supporting skeletal development and homeostasis.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Genetic Determinants of Inherited Endocrine Tumors: Do They Have a Direct Role in Bone Metabolism Regulation and Osteoporosis?

Endocrine tumors are neoplasms originating from specialized hormone-secreting cells. They can develop as sporadic tumors, caused by somatic mutations, or in the context of familial Mendelian inherited diseases. Congenital forms, manifesting as syndromic or non-syndromic diseases, are caused by germinal heterozygote autosomal dominant mutations in oncogenes or tumor suppressor genes. The genetic defect leads to a loss of cell growth control in target endocrine tissues and to tumor development. In addition to the classical cancer manifestations, some affected patients can manifest alterations of bone and mineral metabolism, presenting both as pathognomonic and/or non-specific skeletal clinical features, which can be either secondary complications of endocrine functioning primary tumors and/or a direct consequence of the gene mutation. Here, we specifically review the current knowledge on possible direct roles of the genes that cause inherited endocrine tumors in the regulation of bone modeling and remodeling by exploring functional in vitro and in vivo studies highlighting how some of these genes participate in the regulation of molecular pathways involved in bone and mineral metabolism homeostasis, and by describing the potential direct effects of gene mutations on the development of skeletal and mineral metabolism clinical features in patients.

Selection for increased tibia length in mice alters skull shape through parallel changes in developmental mechanisms

Bones in the vertebrate cranial base and limb skeleton grow by endochondral ossification, under the control of growth plates. Mechanisms of endochondral ossification are conserved across growth plates, which increases covariation in size and shape among bones, and in turn may lead to correlated changes in skeletal traits not under direct selection. We used micro-CT and geometric morphometrics to characterize shape changes in the cranium of the Longshanks mouse, which was selectively bred for longer tibiae. We show that Longshanks skulls became longer, flatter, and narrower in a stepwise process. Moreover, we show that these morphological changes likely resulted from developmental changes in the growth plates of the Longshanks cranial base, mirroring changes observed in its tibia. Thus, indirect and non-adaptive morphological changes can occur due to developmental overlap among distant skeletal elements, with important implications for interpreting the evolutionary history of vertebrate skeletal form.

Critical implication of the PTEN/PI3K/AKT pathway during BMP2-induced heterotopic ossification

Heterotopic ossification (HO) is characterized by extraskeletal ossification in soft tissue. Thus far, there is a lack of effective drug therapy against HO. Loss of PTEN in osteoblasts has been reported to accumulate bone mass in skeletal development and promote fracture healing in association with the activation of the PI3K/AKT pathway. However, the role of the PTEN/PI3K/AKT signaling in HO pathogenesis remains unknown. The present study investigated the implication of this pathway during BMP2-induced osteogenic differentiation and ectopic bone formation. It was shown that overexpression of PTEN inhibited proliferation but stimulated apoptosis in mesenchymal pluripotent C3H10T1/2 cells. PTEN also inhibited BMP2-induced osteoblast differentiation, whereas BMP2 repressed PTEN expression and subsequently activated PI3K/AKT. The PI3K inhibitor, LY294002, blocked BMP2-induced osteoblastogenesis, suggesting that the PI3K/AKT pathway is critically required for BMP2 to initiate osteoblastogenesis. In vivo, implantation of BMP2 in muscle induced ectopic endochondral ossification. Strikingly, this bone-forming capacity was notably suppressed by the PI3K inhibitor LY294002. Hence, the results of the present study demonstrated that the PI3K/AKT signaling activity is indispensable for BMP2 to induce ectopic new bone. Targeting the PI3K/AKT pathway using inhibitor(s) may represent a potential molecular therapy for the treatment against HO.

Overview of Skeletal Development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Ortho-silicic acid enhances osteogenesis of osteoblasts through the upregulation of miR-130b which directly targets PTEN

AIMS

Osteoporosis is considered a common skeletal disease. Ortho-silicic acid has been found to enhance the osteogenic differentiation of osteoblasts. However, the molecular mechanism of osteogenesis induced by ortho-silicic acid is still undefined totally. MicroRNAs (miRs) play a key role in osteogenesis of osteoblasts. This study investigated the role of miR-130b in promoting osteogenesis induced by ortho-silicic acid.

MAIN METHODS AND KEY FINDINGS

In this study, we found ortho-silicic acid enhanced osteogenesis of osteoblasts in vitro and promoted preventing and treating osteoporosis in vivo. Furthermore, the expression of miR-130b increased under application of ortho-silicic acid. In vitro, experiments demonstrated miR-130b overexpression or inhibition significantly promoted or suppressed osteogenic differentiation of osteoblasts under application of ortho-silicic acid, respectively. Consistently, downregulation of miR-130b in ovariectomy (OVX) rats dropped off the beneficial effect of ortho-silicic acid against bone loss. Mechanistically, we identified phosphatase and tensin homologue deleted on human chromosome 10 (PTEN) as the direct target of miR-130b during osteogenesis induced by ortho-silicic acid.

SIGNIFICANCE

In conclusion, our findings reveal that ortho-silicic acid promotes the osteogenesis of osteoblasts mediated by the miR-130b/PTEN signaling axis, which identifies a new target to prevent and treat osteoporosis.

New insights on the reparative cells in bone regeneration and repair

Bone possesses a remarkable repair capacity to regenerate completely without scar tissue formation. This unique characteristic, expressed during bone development, maintenance and injury (fracture) healing, is performed by the reparative cells including skeletal stem cells (SSCs) and their descendants. However, the identity and functional roles of SSCs remain controversial due to technological difficulties and the heterogeneity and plasticity of SSCs. Moreover, for many years, there has been a biased view that bone marrow is the main cell source for bone repair. Together, these limitations have greatly hampered our understanding of these important cell populations and their potential applications in the treatment of fractures and skeletal diseases. Here, we reanalyse and summarize current understanding of the reparative cells in bone regeneration and repair and outline recent progress in this area, with a particular emphasis on the temporal and spatial process of fracture healing, the sources of reparative cells, an updated definition of SSCs, and markers of skeletal stem/progenitor cells contributing to the repair of craniofacial and long bones, as well as the debate between SSCs and pericytes. Finally, we also discuss the existing problems, emerging novel technologies and future research directions in this field.

A new homozygous HERC1 gain-of-function variant in MDFPMR syndrome leads to mTORC1 hyperactivation and reduced autophagy during cell catabolism

The giant 532 kDa HERC1 protein is a ubiquitin ligase that interacts with tuberous sclerosis complex subunit 2 (TSC2), a negative upstream regulator of the mammalian target of rapamycin complex 1 (mTORC1). TSC2 regulates anabolic cell growth through its influence on protein synthesis, cell growth, proliferation, autophagy, and differentiation. TSC subunit 1 (TSC1) stabilizes TSC2 by inhibiting the interaction between TSC2 and HERC1, forming a TSC1-TSC2 complex that negatively regulates mTORC1. HERC1-TSC2 interaction destabilizes and degrades TSC2. Recessive mutations in HERC1 have been reported in patients with intellectual disability. Some patients exhibit epilepsy, macrocephaly, somatic overgrowth, and dysmorphic facial features as well. Here we describe two sisters from a consanguineous marriage with a novel homozygous missense variant in the C-terminal HECT domain of HERC1 [chr15:g63,907,989C>G GRCh37.p11 | c.14,072G>C NM_003922 | p.(Arg4,691Pro)]. Symptoms compris global developmental delay, macrocephaly, somatic overgrowth, intellectual disability, seizures, schizoaffective disorder, and pyramidal tract signs. We functionally assessed the HERC1 mutation by investigation of patient and control fibroblasts under normal and nutrient starving conditions. During catabolic state, mTORC1 activity remained high in patient fibroblasts, which stands in stark contrast to its downregulation in controls. This was corroborated by an abnormally high phosphorylation of S6K1-kinase, a direct downstream target of mTORC1, in patients. Moreover, autophagy, usually enhanced in catabolic states, was down-regulated in patient fibroblasts. These data confirm that the missense variant found in both patients results in a gain-of-function for the mutant HERC1 protein.

ADAM10 is indispensable for longitudinal bone growth in mice

Skeletal development is a highly sophisticated process in which the expression of a variety of growth factors, signaling molecules, and extracellular matrix proteins is spatially and temporally orchestrated. In the present study, we show that ADAM10, a transmembrane protease that is critically involved in the functional regulation of various membrane-bound molecules, plays an essential role in the longitudinal growth of long bones and in skeletal development. We found that mutant mice lacking ADAM10 in osteochondroprogenitors exhibited marked growth retardation and had shorter long bones than the control mice. Histomorphometric analysis revealed that the mutant mice had a shorter hypertrophic zone and that their hypertrophic chondrocytes were smaller in size than those of the control mice. Unexpectedly, we found that the mRNA expression of the chemokine CXCL12 and its receptor CXCR4 were significantly reduced in cartilage tissues lacking ADAM10. Further, exogenous supplementation of recombinant CXCL12 rescued the defect in the ADAM10-deficient growth plate in an ex vivo culture model. Taken together, our data show a previously unknown role for ADAM10 in skeletal development that involves its regulation of the CXCL12 and CXCR4 signaling pathway.

The Roles of FoxO Transcription Factors in Regulation of Bone Cells Function

Forkhead box class O family member proteins (FoxOs) are evolutionarily conserved transcription factors for their highly conserved DNA-binding domain. In mammalian species, all the four FoxO members, FoxO1, FoxO3, FoxO4, and FoxO6, are expressed in different organs. In bone, the first three members are extensively expressed and more studied. Bone development, remodeling, and homeostasis are all regulated by multiple cell lineages, including osteoprogenitor cells, chondrocytes, osteoblasts, osteocytes, osteoclast progenitors, osteoclasts, and the intercellular signaling among these bone cells. The disordered FoxOs function in these bone cells contribute to osteoarthritis, osteoporosis, or other bone diseases. Here, we review the current literature of FoxOs for their roles in bone cells, focusing on helping researchers to develop new therapeutic approaches and prevent or treat the related bone diseases.

PTEN promotes intervertebral disc degeneration by regulating nucleus pulposus cell behaviors

Intervertebral disc degeneration (IDD) is induced by multiple factors including increased apoptosis, decreased survival, and reduced extracellular matrix (ECM) synthesis in the nucleus pulposus (NP) cells. The tumor suppressor phosphatase and tensin homolog deleted from chromosome 10 (PTEN) is the only known lipid phosphatase counteracting the PI3K/AKT pathway. Loss of PTEN leads to activated PI3K/AKT signaling, which plays a key role in a variety of cancers. However, the role of PTEN/PI3K/AKT signaling nexus in IDD remains unknown. Here, we report that PTEN is overexpressed in degenerative NP, which correlates with inactivated AKT. Using the PTEN knockdown approach by lentivirus-mediated short interfering RNA gene transfer technique, we report that PTEN decreases survival but induces apoptosis and senescence of NP cells. PTEN also inhibits expression and production of ECM components including collagen II, aggrecan, and proteoglycan. Furthermore, PTEN modulates the expression of ECM regulatory molecules SOX-9 and matrix metalloproteinase-3 (MMP-3). Using small-molecule AKT inhibitor GDC-0068, we confirm that PTEN regulates NP cell behaviors through its direct targeting of PI3K/AKT. These findings demonstrate for the first time that PTEN/PI3K/AKT signaling axis plays an important role in the pathogenesis of IDD. Targeting PTEN using gene therapy may represent a promising therapeutic approach against disc degenerative diseases.

Sustained Akt signaling in articular chondrocytes causes osteoarthritis via oxidative stress-induced senescence in mice

Osteoarthritis (OA) is an age-related disorder that is strongly associated with chondrocyte senescence. The causal link between disruptive PTEN/Akt signaling and chondrocyte senescence and the underlying mechanism are unclear. In this study, we found activated Akt signaling in human OA cartilage as well as in a mouse OA model with surgical destabilization of the medial meniscus. Genetic mouse models mimicking sustained Akt signaling in articular chondrocytes via PTEN deficiency driven by either Col2a1-Cre or Col2a1-Cre^ERT2 developed OA, whereas restriction of Akt signaling reversed the OA phenotypes in PTEN-deficient mice. Mechanistically, prolonged activation of Akt signaling caused an accumulation of reactive oxygen species and triggered chondrocyte senescence as well as a senescence-associated secretory phenotype, whereas chronic administration of the antioxidant N-acetylcysteine suppressed chondrocyte senescence and mitigated OA progression in PTEN-deficient mice. Therefore, inhibition of Akt signaling by PTEN is required for the maintenance of articular cartilage. Disrupted Akt signaling in articular chondrocytes triggers oxidative stress-induced chondrocyte senescence and causes OA.

Inhibition of phosphatidylinositol 3-kinase α (PI3Kα) prevents heterotopic ossification

Heterotopic ossification (HO) is the pathological formation of ectopic endochondral bone within soft tissues. HO occurs following mechanical trauma, burns, or congenitally in patients suffering from fibrodysplasia ossificans progressiva (FOP). FOP patients carry a conserved mutation in ACVR1 that becomes neomorphic for activin A responses. Here, we demonstrate the efficacy of BYL719, a PI3Kα inhibitor, in preventing HO in mice. We found that PI3Kα inhibitors reduce SMAD, AKT, and mTOR/S6K activities. Inhibition of PI3Kα also impairs skeletogenic responsiveness to BMPs and the acquired response to activin A of the Acvr1^R206H allele. Further, the efficacy of PI3Kα inhibitors was evaluated in transgenic mice expressing Acvr1^Q207D . Mice treated daily or intermittently with BYL719 did not show ectopic bone or cartilage formation. Furthermore, the intermittent treatment with BYL719 was not associated with any substantial side effects. Therefore, this work provides evidence supporting PI3Kα inhibition as a therapeutic strategy for HO.

Lipid phosphatase SHIP-1 regulates chondrocyte hypertrophy and skeletal development

SH2‐containing inositol‐5′‐phosphatase‐1 (SHIP‐1) controls the phosphatidylinositol‐3′‐kinase (PI3K) initiated signaling pathway by limiting cell membrane recruitment and activation of Akt. Despite the fact that many of the growth factors important to cartilage development and functions are able to activate the PI3K signal transduction pathway, little is known about the role of PI3K signaling in chondrocyte biology and its contribution to mammalian skeletogenesis. Here, we report that the lipid phosphatase SHIP‐1 regulates chondrocyte hypertrophy and skeletal development through its expression in osteochondroprogenitor cells. Global SHIP‐1 knockout led to accelerated chondrocyte hypertrophy and premature formation of the secondary ossification center in the bones of postnatal mice. Drastically higher vascularization and greater number of c‐kit + progenitors associated with sinusoids in the bone marrow also indicated more advanced chondrocyte hypertrophic differentiation in SHIP‐1 knockout mice than in wild‐type mice. In corroboration with the in vivo phenotype, SHIP‐1 deficient PDGFRα + Sca‐1 + osteochondroprogenitor cells exhibited rapid differentiation into hypertrophic chondrocytes under chondrogenic culture conditions in vitro. Furthermore, SHIP‐1 deficiency inhibited hypoxia‐induced cellular activation of Akt and extracellular‐signal‐regulated kinase (Erk) and suppressed hypoxia‐induced cell proliferation. These results suggest that SHIP‐1 is required for hypoxia‐induced growth signaling under physiological hypoxia in the bone marrow. In conclusion, the lipid phosphatase SHIP‐1 regulates skeletal development by modulating chondrogenesis and the hypoxia response of the osteochondroprogenitors during endochondral bone formation.

HIF-1α Regulates Glucocorticoid-Induced Osteoporosis Through PDK1/AKT/mTOR Signaling Pathway

Long-term and high dose glucocorticoid treatment can cause decreased viability and function of osteoblasts, which leads to osteoporosis and osteonecrosis. In this study, we investigated the role and mechanism of action of HIF-1α in glucocorticoid-induced osteogenic inhibition in MC3T3-E1 cells. Our results showed that HIF-1α protein expression was reduced when MC3T3-E1 cells were exposed to dexamethasone (Dex) at varying concentrations ranging from 10^-9 to 10^-6 M. PDK1 expression was also decreased in MC3T3-E1 cells after dexamethasone treatment. MC3T3-E1 cells when treated with the glucocorticoid receptor antagonist RU486 along with dexamethasone showed enhanced HIF-1α expression. In addition, upregulated expression of HIF-1α was capable of promoting the osteogenic ability of MC3T3-E1 cells and PDK1 expression. However, the HIF-1α antagonist 2-methoxyestradiol (2-ME) had a reverse effect in MC3T3-E1 cells exposed to dexamethasone. Furthermore, the PDK1 antagonist dichloroacetate could repress the osteogenic ability of MC3T3-E1 cells, although HIF-1α was upregulated when transduced with adenovirus-HIF-1α construct. The PDK1 agonist PS48 was able to promote the osteogenic ability of MC3T3-E1 cells treated with dexamethasone. Importantly, the protein levels of p-AKT and p-mTOR were increased in MC3T3-E1 cells treated with dexamethasone after PS48 treatment. in vivo, the PDK1 agonist PS48 could maintain the bone mass of mice treated with dexamethasone. This study provides a new understanding of the mechanism of glucocorticoid-induced osteoporosis.

Role of Forkhead Box O Transcription Factors in Oxidative Stress-Induced Chondrocyte Dysfunction: Possible Therapeutic Target for Osteoarthritis?

Chondrocyte dysfunction occurs during the development of osteoarthritis (OA), typically resulting from a deleterious increase in oxidative stress. Accordingly, strategies for arresting oxidative stress-induced chondrocyte dysfunction may lead to new potential therapeutic targets for OA treatment. Forkhead box O (FoxO) transcription factors have recently been shown to play a protective role in chondrocyte dysfunction through the regulation of inflammation, autophagy, aging, and oxidative stress. They also regulate growth, maturation, and matrix synthesis in chondrocytes. In this review, we discuss the recent progress made in the field of oxidative stress-induced chondrocyte dysfunction. We also discuss the protective role of FoxO transcription factors as potential molecular targets for the treatment of OA. Understanding the function of FoxO transcription factors in the OA pathology may provide new insights that will facilitate the development of next-generation therapies to prevent OA development and to slow OA progression.

Role of Forkhead Box O Transcription Factors in Oxidative Stress-Induced Chondrocyte Dysfunction: Possible Therapeutic Target for Osteoarthritis?

Chondrocyte dysfunction occurs during the development of osteoarthritis (OA), typically resulting from a deleterious increase in oxidative stress. Accordingly, strategies for arresting oxidative stress-induced chondrocyte dysfunction may lead to new potential therapeutic targets for OA treatment. Forkhead box O (FoxO) transcription factors have recently been shown to play a protective role in chondrocyte dysfunction through the regulation of inflammation, autophagy, aging, and oxidative stress. They also regulate growth, maturation, and matrix synthesis in chondrocytes. In this review, we discuss the recent progress made in the field of oxidative stress-induced chondrocyte dysfunction. We also discuss the protective role of FoxO transcription factors as potential molecular targets for the treatment of OA. Understanding the function of FoxO transcription factors in the OA pathology may provide new insights that will facilitate the development of next-generation therapies to prevent OA development and to slow OA progression.

E3-ubiquitin ligase NEDD4 enhances bone formation by removing TGFβ1-induced pSMAD1 in immature osteoblast

Neural precursor cell expressed developmentally downregulated protein 4 (NEDD4) is an E3 ubiquitin ligase that regulates animal growth and development. To investigate the role of NEDD4 in skeletogenesis in vivo, we established immature osteoblast-specific 2.3-kb Collagen Type I Alpha 1 chain (Col1α1) promoter-driven Nedd4 transgenic (Nedd4-TG, Col1α1-Nedd4^Tg/+) mice and conditional knockout (Nedd4-cKO, Col1α1-Cre;Nedd4^fl/fl) mice. The Nedd4-TG mice displayed enhanced bone mass accrual and upregulated gene expression of osteogenic markers in bone. In addition, bone formation was decreased in the Nedd4-cKO mice compared to that in their littermates. The proliferation of primary osteoblasts isolated from calvaria and the number and surface area of tibial osteoblasts were higher in the Nedd4-TG mice than those in their littermates. Throughout the osteoblast differentiation, the expression of Nedd4 and Tgfb1 were high at early stage of osteoblast maturation, but decreased at the later stage when Bmp2 expression level is high. TGFβ1 signaling was consolidated by degradation of pSMAD1, which was transiently induced by TGFβ1, in NEDD4-overexpressing osteoblasts. Furthermore, pERK1/2 signaling was enhanced in osteoblast from TG mice than those in their littermates. These results suggest that NEDD4 enhances osteoblast proliferation by removing pSMAD1 activated by TGFβ1, and potentiating pSMAD2 and pERK1/2 pathways at early stage of bone formation.

Lin28a overexpression reveals the role of Erk signaling in articular cartilage development

Adult articular cartilage shows limited tissue turnover, and therefore development of the proper structure of articular cartilage is crucial for life-long joint function. However, the mechanism by which the articular cartilage structure is developmentally regulated is poorly understood. In this study, we show evidence that activation of extracellular signal-regulated kinases (Erk1/2) in articular chondrocyte progenitors during developmental stages control articular cartilage thickness. We found that overexpression of Lin28a, an RNA-binding protein that regulates organismal growth and metabolism, in articular chondrocyte progenitor cells upregulated Erk signaling and increased articular cartilage thickness. Overexpression of a constitutively active Kras mimicked Lin28a overexpression, and inhibition of Erk signaling during embryonic stages normalized the cartilage phenotype of both Kras- and Lin28a-overexpressing mice. These results suggest that articular cartilage thickness is mainly determined during the process of embryonic synovial joint development, which is positively regulated by Erk signaling.

The Role of FoxOs in Bone Health and Disease

Recent studies with murine models of cell-specific loss- or gain-of-function of FoxOs have provided novel insights into the function and signaling of these transcription factors on the skeleton. They have revealed that FoxO actions in chondrocytes are critical for normal skeletal development, and FoxO actions in cells of the osteoclast or osteoblast lineage greatly influence bone resorption and formation and, consequently, bone mass. FoxOs also act in osteoblast progenitors to inhibit Wnt signaling and bone formation. Additionally, FoxOs decrease bone resorption via direct antioxidant effects on osteoclasts and upregulation of the antiosteoclastogenic cytokine OPG in cells of the osteoblast lineage. Deacetylation of FoxOs by the NAD-dependent histone deacetylase Sirt1 in both osteoblasts and osteoclasts stimulates bone formation and inhibits bone resorption, making Sirt1 activators promising therapeutic agents for diseases of low bone mass. In this chapter, we review these advances and discuss their implications for the pathogenesis and treatment of estrogen deficiency-, Type 1 diabetes-, and age-related osteoporosis.

Pten regulates neural crest proliferation and differentiation during mouse craniofacial development

BACKGROUND

The phosphatase and tensin homolog deleted on chromosome TEN (Pten) is implicated in a broad range of developmental events and diseases. However, its role in neural crest and craniofacial development has not been well illustrated.

RESULTS

Using genetically engineered mouse models, we showed that inactivating Pten specifically in neural crest cells causes malformation of craniofacial structures. Pten conditional knockout mice exhibit perinatal lethality with overgrowth of craniofacial structures. At the cellular level, Pten deficiency increases cell proliferation rate and enhances osteoblast differentiation. Our data further revealed that inactivating Pten elevates PI3K/Akt signaling activity in neural crest derivatives, and confirmed that attenuation of PI3K/Akt activity led to decreased neural crest cell proliferation and differentiation both in vitro and in vivo.

CONCLUSIONS

Our study revealed that Pten is essential for craniofacial morphogenesis in mice. Inactivating Pten in neural crest cells increases proliferation rate and promotes their differentiation toward osteoblasts. Our data further indicate that Pten acts via modulating PI3K/Akt activity during these processes. Developmental Dynamics 247:304-314, 2018. © 2017 Wiley Periodicals, Inc.

miR-322 stabilizes MEK1 expression to inhibit RAF/MEK/ERK pathway activation in cartilage

Cartilage originates from mesenchymal cell condensations that differentiate into chondrocytes of transient growth plate cartilage or permanent cartilage of the articular joint surface and trachea. MicroRNAs fine-tune the activation of entire signaling networks and thereby modulate complex cellular responses, but so far only limited data are available on miRNAs that regulate cartilage development. Here, we characterize a miRNA that promotes the biosynthesis of a key component in the RAF/MEK/ERK pathway in cartilage. Specifically, by transcriptome profiling we identified miR-322 to be upregulated during chondrocyte differentiation. Among the various miR-322 target genes in the RAF/MEK/ERK pathway, only Mek1 was identified as a regulated target in chondrocytes. Surprisingly, an increased concentration of miR-322 stabilizes Mek1 mRNA to raise protein levels and dampen ERK1/2 phosphorylation, while cartilage-specific inactivation of miR322 in mice linked the loss of miR-322 to decreased MEK1 levels and to increased RAF/MEK/ERK pathway activation. Such mice died perinatally due to tracheal growth restriction and respiratory failure. Hence, a single miRNA can stimulate the production of an inhibitory component of a central signaling pathway to impair cartilage development.

Bone Size and Quality Regulation: Concerted Actions of mTOR in Mesenchymal Stromal Cells and Osteoclasts

The bone size and quality, acquired during adolescent growth under the influence of anabolic hormones, growth factors, and nutrients, determine the height and bone stability and forecast osteoporosis risks in late life. Yet bone size and quality control mechanisms remain enigmatic. To study the roles of mammalian target of rapamycin (mTOR) signaling, sensor of growth factors and nutrients, in bone size and quality regulation, we ablated Tsc1, a suppressor of mTOR, in mesenchymal stromal cells (MSCs), monocytes, or their progenies osteoblasts and osteoclasts. mTOR activation in MSCs, but much less in osteoblasts, increased bone width and mass due to MSC hyperproliferation, but decreased bone length and mineral contents due to defective MSC differentiation. mTOR activation promotes bone mineral accretion by inhibiting osteoclast differentiation and activity directly or via coupling with MSCs. Tuberous sclerosis complex patient studies confirmed these findings. Thus, mTOR regulates bone size via MSCs and bone quality by suppressing catabolic activities of osteoclasts.

Oriented clonal cell dynamics enables accurate growth and shaping of vertebrate cartilage

Cartilaginous structures are at the core of embryo growth and shaping before the bone forms. Here we report a novel principle of vertebrate cartilage growth that is based on introducing transversally-oriented clones into pre-existing cartilage. This mechanism of growth uncouples the lateral expansion of curved cartilaginous sheets from the control of cartilage thickness, a process which might be the evolutionary mechanism underlying adaptations of facial shape. In rod-shaped cartilage structures (Meckel, ribs and skeletal elements in developing limbs), the transverse integration of clonal columns determines the well-defined diameter and resulting rod-like morphology. We were able to alter cartilage shape by experimentally manipulating clonal geometries. Using in silico modeling, we discovered that anisotropic proliferation might explain cartilage bending and groove formation at the macro-scale.

DOI:http://dx.doi.org/10.7554/eLife.25902.001

Role and mechanism of PTEN in adiponectin-induced osteogenesis in human bone marrow mesenchymal stem cells

Human bone marrow-derived stromal cells (hBMSC) are multi-potent stem cells that can differentiate into osteogenic and adipogenic lineages. Adiponectin (APN) is an adipocyte-derived hormone that modulates a series of metabolic processes. Recent studies revealed a relationship between APN and bone regeneration, though the underlying mechanism was not fully examined. Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a tumor suppressor and a therapeutic target for the metabolic syndrome. Its deletion mutants increase osteoblast activity and bone mineral density. Both APN and PTEN are involved in osteogenic differentiation. However, whether PTEN is involved in APN-induced bone metabolism remains unclear. This project was designed to study whether PTEN was involved in APN-mediated osteogenesis of hBMSCs. We found that APN downregulated PTEN expression and that both it and an inhibitor of PTEN (SF1670) increased the expression of osteogenic markers such as osteocalcin, alkaline phosphatase, and runt-related transcription factor-2 in APN-treated hBMSCs. Our results suggested that APN enhanced osteogenic differentiation of hBMSCs in vitro partially by inhibiting PTEN expression. APN could be a therapeutic agent in tissue regeneration engineering and bone regeneration by inhibiting PTEN expression and then promoting the osteogenic differentiation of hBMSCs.

Forkhead box O transcription factors in chondrocytes regulate endochondral bone formation

The differentiation of embryonic mesenchymal cells into chondrocytes and the subsequent formation of a cartilaginous scaffold that enables the formation of long bones are hallmarks of endochondral ossification. During this process, chondrocytes undergo a remarkable sequence of events involving proliferation, differentiation, hypertrophy and eventually apoptosis. Forkhead Box O (FoxO) transcription factors (TFs) are well-known regulators of such cellular processes. Although FoxO3a was previously shown to be regulated by 1,25-dihydroxyvitamin D₃ in osteoblasts, a possible role for this family of TFs in chondrocytes during endochondral ossification remains largely unstudied. By crossing Collagen2-Cre mice with FoxO1^lox/lox;FoxO3a^lox/lox;FoxO4^lox/lox mice, we generated mice in which the three main FoxO isoforms were deleted in growth plate chondrocytes (chondrocyte triple knock-out; CTKO). Intriguingly, CTKO neonates showed a distinct elongation of the hypertrophic zone of the growth plate. CTKO mice had increased overall body and tail length at eight weeks of age and suffered from severe skeletal deformities at older ages. CTKO chondrocytes displayed decreased expression of genes involved in redox homeostasis. These observations illustrate the importance of FoxO signaling in chondrocytes during endochondral ossification.

Class I PI-3-Kinase Signaling Is Critical for Bone Formation Through Regulation of SMAD1 Activity in Osteoblasts

Bone formation and homeostasis is carried out by osteoblasts, whose differentiation and activity are regulated by osteogenic signaling networks. A central mediator of these inputs is the lipid kinase phosphatidylinositol 3-kinase (PI3K). However, at present, there are no data on the specific role of distinct class IA PI3K isoforms in bone biology. Here, we performed osteoblast-specific deletion in mice to show that both p110α and p110β isoforms are required for survival and differentiation and function of osteoblasts and thereby control bone formation and postnatal homeostasis. Impaired osteogenesis arises from increased GSK3 activity and a depletion of SMAD1 protein levels in PI3K-deficient osteoblasts. Accordingly, pharmacological inhibition of GSK3 activity or ectopic expression of SMAD1 or SMAD5 normalizes bone morphogenetic protein (BMP) transduction and osteoblast differentiation. Together, these results identify the PI3K-GSK3-SMAD1 axis as a central node integrating multiple signaling networks that govern bone formation and homeostasis. © 2016 American Society for Bone and Mineral Research.

Activation of mTORC1 in B Lymphocytes Promotes Osteoclast Formation via Regulation of β-Catenin and RANKL/OPG

The cytokine receptor activator of nuclear factor-κB ligand (RANKL) induces osteoclast formation from monocyte/macrophage lineage cells. However, the mechanisms by which RANKL expression is controlled in cells that support osteoclast differentiation are still unclear. We show that deletion of TSC1 (tuberous sclerosis complex 1) in murine B cells causes constitutive activation of mechanistic target of rapamycin complex 1 (mTORC1) and stimulates RANKL but represses osteoprotegerin (OPG) expression and subsequently promotes osteoclast formation and causes osteoporosis in mice. Furthermore, the regulation of RANKL/OPG and stimulation of osteoclastogenesis by mTORC1 was confirmed in a variety of RANKL-expressing cells and in vivo. Mechanistically, mTORC1 controls RANKL/OPG expression through negative feedback inactivation of Akt, destabilization of β-catenin mRNA, and downregulation of β-catenin. Our findings demonstrate that mTORC1 activation-stimulated RANKL expression in B cells is sufficient to induce bone loss and osteoporosis. The study also established a link between mTORC1 and the RANKL/OPG axis via negative regulation of β-catenin. © 2016 American Society for Bone and Mineral Research.

mTORC1 regulates PTHrP to coordinate chondrocyte growth, proliferation and differentiation

Precise coordination of cell growth, proliferation and differentiation is essential for the development of multicellular organisms. Here, we report that although the mechanistic target of rapamycin complex 1 (mTORC1) activity is required for chondrocyte growth and proliferation, its inactivation is essential for chondrocyte differentiation. Hyperactivation of mTORC1 via TSC1 gene deletion in chondrocytes causes uncoupling of the normal proliferation and differentiation programme within the growth plate, resulting in uncontrolled cell proliferation, and blockage of differentiation and chondrodysplasia in mice. Rapamycin promotes chondrocyte differentiation and restores these defects in mutant mice. Mechanistically, mTORC1 downstream kinase S6K1 interacts with and phosphorylates Gli2, and releases Gli2 from SuFu binding, resulting in nuclear translocation of Gli2 and transcription of parathyroid hormone-related peptide (PTHrP), a key regulator of bone development. Our findings demonstrate that dynamically controlled mTORC1 activity is crucial to coordinate chondrocyte proliferation and differentiation partially through regulating Gli2/PTHrP during endochondral bone development.

Regulation of Long Bone Growth in Vertebrates; It Is Time to Catch Up

The regulation of organ size is essential to human health and has fascinated biologists for centuries. Key to the growth process is the ability of most organs to integrate organ-extrinsic cues (eg, nutritional status, inflammatory processes) with organ-intrinsic information (eg, genetic programs, local signals) into a growth response that adapts to changing environmental conditions and ensures that the size of an organ is coordinated with the rest of the body. Paired organs such as the vertebrate limbs and the long bones within them are excellent models for studying this type of regulation because it is possible to manipulate one member of the pair and leave the other as an internal control. During development, growth plates at the end of each long bone produce a transient cartilage model that is progressively replaced by bone. Here, we review how proliferation and differentiation of cells within each growth plate are tightly controlled mainly by growth plate-intrinsic mechanisms that are additionally modulated by extrinsic signals. We also discuss the involvement of several signaling hubs in the integration and modulation of growth-related signals and how they could confer remarkable plasticity to the growth plate. Indeed, long bones have a significant ability for "catch-up growth" to attain normal size after a transient growth delay. We propose that the characterization of catch-up growth, in light of recent advances in physiology and cell biology, will provide long sought clues into the molecular mechanisms that underlie organ growth regulation. Importantly, catch-up growth early in life is commonly associated with metabolic disorders in adulthood, and this association is not completely understood. Further elucidation of the molecules and cellular interactions that influence organ size coordination should allow development of novel therapies for human growth disorders that are noninvasive and have minimal side effects.

Rb1 and Pten Co-Deletion in Osteoblast Precursor Cells Causes Rapid Lipoma Formation in Mice

The Rb and Pten tumor suppressor genes are important regulators of bone development and both are frequently mutated in the bone cancer osteosarcoma (OS). To determine if Rb1 and Pten synergize as tumor suppressor genes for osteosarcoma, we co-deleted them in osteoprogenitor cells. Surprisingly, we observed rapid development of adipogenic but not osteosarcoma tumors in the ΔRb1/Pten mice. ΔPten solo deleted mice also developed lipoma tumors but at a much reduced frequency and later onset than those co-deleted for Rb1. Pten deletion also led to a marked increase in adipocytes in the bone marrow. To better understand the function of Pten in bone development in vivo, we conditionally deleted Pten in OSX⁺ osteoprogenitor cells using OSX-Cre mice. μCT analysis revealed a significant thickening of the calvaria and an increase in trabeculae volume and number in the femur, consistent with increased bone formation in these mice. To determine if Pten and Rb1 deletion actively promotes adipogenic differentiation, we isolated calvarial cells from Pten^fl/fl and Pten^fl/fl; Rb1^fl/fl mice, infected them with CRE or GFP expressing adenovirus, treated with differentiation media. We observed slightly increased adipogenic, and osteogenic differentiation in the ΔPten cells. Both phenotypes were greatly increased upon Rb1/Pten co-deletion. This was accompanied by an increase in expression of genes required for adipogenesis. These data indicate that Pten deletion in osteoblast precursors is sufficient to promote frequent adipogenic, but only rare osteogenic tumors. Rb1 hetero- or homo-zygous co-deletion greatly increases the incidence and the rapidity of onset of adipogenic tumors, again, with only rare osteosarcoma tumors.

Wntless spatially regulates bone development through β-catenin-dependent and independent mechanisms

BACKGROUND

Canonical and noncanonical Wnt signaling pathways both play pivotal roles in bone development. Wntless/GPR177 is a chaperone protein that is required for secretion of all Wnt ligands. We previously showed that deletion of Wntless within mature osteoblasts severely impaired postnatal bone homeostasis.

RESULTS

In this study, we systemically evaluated how deletion of Wntless in different stages of osteochondral differentiation affected embryonic bone development, by crossing Wntless (Wls)-flox/flox mice with strains expressing cre recombinase behind the following promoters: Osteocalcin, Collagen 2a1, or Dermo1. Ex vivo µCT and whole-mount skeletal staining were performed to examine skeletal mineralization. Histology and immunohistochemistry were used to evaluate cellular differentiation and alterations in Wnt signaling. In this work, we found that Wntless regulated chondrogenesis and osteogenesis through both canonical and noncanonical Wnt signaling.

CONCLUSIONS

These findings provide more insight into the requirements of different Wnt-secretion cell types critical for skeletal development.

Phosphatase and Tensin Homologue: Novel Regulation by Developmental Signaling

Phosphatase and tensin homologue (PTEN) is a critical cell endogenous inhibitor of phosphoinositide signaling in mammalian cells. PTEN dephosphorylates phosphoinositide trisphosphate (PIP3), and by so doing PTEN has the function of negative regulation of Akt, thereby inhibiting this key intracellular signal transduction pathway. In numerous cell types, PTEN loss-of-function mutations result in unopposed Akt signaling, producing numerous effects on cells. Numerous reports exist regarding mutations in PTEN leading to unregulated Akt and human disease, most notably cancer. However, less is commonly known about nonmutational regulation of PTEN. This review focuses on an emerging literature on the regulation of PTEN at the transcriptional, posttranscriptional, translational, and posttranslational levels. Specifically, a focus is placed on the role developmental signaling pathways play in PTEN regulation; this includes insulin-like growth factor, NOTCH, transforming growth factor, bone morphogenetic protein, wnt, and hedgehog signaling. The regulation of PTEN by developmental mediators affects critical biological processes including neuronal and organ development, stem cell maintenance, cell cycle regulation, inflammation, response to hypoxia, repair and recovery, and cell death and survival. Perturbations of PTEN regulation consequently lead to human diseases such as cancer, chronic inflammatory syndromes, developmental abnormalities, diabetes, and neurodegeneration.

Mesenchymal progenitor cells for the osteogenic lineage

Mesenchymal progenitors of the osteogenic lineage provide the flexibility for bone to grow, maintain its function and homeostasis. Traditionally, colony-forming-unit fibroblasts (CFU-Fs) have been regarded as surrogates for mesenchymal progenitors; however, this definition cannot address the function of these progenitors in their native setting. Transgenic murine models including lineage-tracing technologies based on the cre-lox system have proven to be useful in delineating mesenchymal progenitors in their native environment. Although heterogeneity of cell populations of interest marked by a promoter-based approach complicates overall interpretation, an emerging complexity of mesenchymal progenitors has been revealed. Current literatures suggest two distinct types of bone progenitor cells; growth-associated mesenchymal progenitors contribute to explosive growth of bone in early life, whereas bone marrow mesenchymal progenitors contribute to the much slower remodeling process and response to injury that occurs mainly in adulthood. More detailed relationships of these progenitors need to be studied through further experimentation.

Fibroblast growth factor signaling in skeletal development and disease

Fibroblast growth factor (FGF) signaling pathways are essential regulators of vertebrate skeletal development. FGF signaling regulates development of the limb bud and formation of the mesenchymal condensation and has key roles in regulating chondrogenesis, osteogenesis, and bone and mineral homeostasis. This review updates our review on FGFs in skeletal development published in Genes & Development in 2002, examines progress made on understanding the functions of the FGF signaling pathway during critical stages of skeletogenesis, and explores the mechanisms by which mutations in FGF signaling molecules cause skeletal malformations in humans. Links between FGF signaling pathways and other interacting pathways that are critical for skeletal development and could be exploited to treat genetic diseases and repair bone are also explored.

RECQL4 Regulates p53 Function In Vivo During Skeletogenesis

RECQ DNA helicases play critical roles in maintaining genomic stability, but their role in development has been less well studied. Rothmund-Thomson syndrome, RAPADILINO, and Baller-Gerold syndrome are rare genetic disorders caused by mutations in the RECQL4 gene. These patients have significant skeletal developmental abnormalities including radial ray, limb and craniofacial defects. To investigate the role of Recql4 in the developing skeletal system, we generated Recql4 conditional knockout mice targeting the skeletal lineage. Inactivation of Recql4 using the Prx1-Cre transgene led to limb abnormalities and craniosynostosis mimicking the major bone findings in human RECQL4 patients. These Prx1-Cre(+) ;Recql4(fl/fl) mice as well as Col2a1-Cre(+) ;Recql4(fl/fl) mice exhibited growth plate defects and an increased p53 response in affected tissues. Inactivation of Trp53 in these Recql4 mutants resulted in genetic rescue of the skeletal phenotypes, indicating an in vivo interaction between Recql4 and Trp53, and p53 activation as an underlying mechanism for the developmental bone abnormalities in RECQL4 disorders. Our findings show that RECQL4 is critical for skeletal development by modulating p53 activity in vivo.

TGFβ signaling in cartilage development and maintenance

Members of the transforming growth factor beta (TGFβ) superfamily of secreted factors play essential roles in nearly every aspect of cartilage formation and maintenance. However, the mechanisms by which TGFβs transduce their effects in cartilage in vivo remain poorly understood. Mutations in several TGFβ family members, their receptors, extracellular modulators, and intracellular transducers have been described, and these usually impact the development of the cartilaginous skeleton. Furthermore, genome-wide association studies have linked components of the (TGFβ) superfamily to susceptibility to osteoarthritis. This review focuses on recent discoveries from genetic studies in the mouse regarding the regulation of TGFβ signaling in developing growth plate and articular cartilage, as well as the different modes of crosstalk between canonical and noncanonical TGFβ signaling. These new insights into TGFβ signaling in cartilage may open new prospects for therapies that maintain healthy articular cartilage.

Advances in Skeletal Dysplasia Genetics

Skeletal dysplasias result from disruptions in normal skeletal growth and development and are a major contributor to severe short stature. They occur in approximately 1/5,000 births, and some are lethal. Since the most recent publication of the Nosology and Classification of Genetic Skeletal Disorders, genetic causes of 56 skeletal disorders have been uncovered. This remarkable rate of discovery is largely due to the expanded use of high-throughput genomic technologies. In this review, we discuss these recent discoveries and our understanding of the molecular mechanisms behind these skeletal dysplasia phenotypes. We also cover potential therapies, unusual genetic mechanisms, and novel skeletal syndromes both with and without known genetic causes. The acceleration of skeletal dysplasia genetics is truly spectacular, and these advances hold great promise for diagnostics, risk prediction, and therapeutic design.

Mind the gap: genetic manipulation of basicranial growth within synchondroses modulates calvarial and facial shape in mice through epigenetic interactions

Phenotypic integration patterns in the mammalian skull have long been a focus of intense interest as a result of their suspected influence on the trajectory of hominid evolution. Here we test the hypothesis that perturbation of cartilage growth, which directly affects only the chondrocranium during development, will produce coordinated shape changes in the adult calvarium and face regardless of mechanism. Using two murine models of cartilage undergrowth that target two very different mechanisms, we show that strong reduction in cartilage growth produces a short, wide, and more flexed cranial base. This in turn produces a short, wide face in both models. Cranial base and face are already correlated early in ontogeny, and the relationship between these modules gains structure through postnatal growth and development. These results provide further evidence that there exist physical interactions between developing parts of the phenotype that produce variation at a distance from the actual locus upon which a particular selective pressure is acting. Phenotypic changes observed over the course of evolution may not all require adaptationist explanations; rather, it is likely that a substantial portion of observed phenotypic variation over the history of a clade is not directly adaptive but rather a secondary consequence of some local response to selection.

Association of bone loss with the upregulation of survival-related genes and concomitant downregulation of Mammalian target of rapamycin and osteoblast differentiation-related genes in the peripheral blood of late postmenopausal osteoporotic women

We aimed to identify bone related markers in the peripheral blood of osteoporotic (OP) patients that pointed toward molecular mechanisms underlying late postmenopausal bone loss. Whole blood from 22 late postmenopausal OP patients and 26 healthy subjects was examined. Bone mineral density (BMD) was measured by DXA. Protein levels of p70-S6K, p21, MMP-9, TGFβ1, and caspase-3 were quantified by ELISA. Gene expression was measured using real-time RT-PCR. OP registered by low BMD indices in late postmenopausal patients was associated with a significant upregulation of autophagy protein ULK1, cyclin-dependent kinase inhibitor p21, and metalloproteinase MMP-9 gene expression in the blood compared to the healthy controls and in a significant downregulation of mTOR (mammalian target of rapamycin), RUNX2, and ALPL gene expression, while expression of cathepsin K, caspase-3, transforming growth factor (TGF) β1, interleukin- (IL-) 1β, and tumor necrosis factor α (TNFα) was not significantly affected. We also observed a positive correlation between TGFβ1 and RUNX2 expression and BMD at femoral sites in these patients. Therefore, bone loss in late postmenopausal OP patients is associated with a significant upregulation of survival-related genes (ULK1 and p21) and MMP-9, as well as the downregulation of mTOR and osteoblast differentiation-related genes (RUNX2 and ALPL) in the peripheral blood compared to the healthy controls.

Ubiquitous expression of the Pik3caH1047R mutation promotes hypoglycemia, hypoinsulinemia, and organomegaly

Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of PI3K, are among the most common mutations found in human cancer and have also recently been implicated in a range of overgrowth syndromes in humans. We have used a novel inducible "exon-switch" approach to knock in the constitutively active Pik3ca(H1047R) mutation into the endogenous Pik3ca gene of the mouse. Ubiquitous expression of the Pik3ca(H1047R) mutation throughout the body resulted in a dramatic increase in body weight within 3 weeks of induction (mutant 150 ± 5%; wild-type 117 ± 3%, mean ± sem), which was associated with increased organ size rather than adiposity. Severe metabolic effects, including a reduction in blood glucose levels to 59 ± 4% of baseline (11 days postinduction) and undetectable insulin levels, were also observed. Pik3ca(H1047R) mutant mice died earlier (median survival 46.5 d post-mutation induction) than wild-type control mice (100% survival > 250 days). Although deletion of Akt2 increased median survival by 44%, neither organ overgrowth, nor hypoglycemia were rescued, indicating that both the growth and metabolic functions of constitutive PI3K activity can be Akt2 independent. This mouse model demonstrates the critical role of PI3K in the regulation of both organ size and glucose metabolism at the whole animal level.

A subset of chondrogenic cells provides early mesenchymal progenitors in growing bones

The hallmark of endochondral bone development is the presence of cartilaginous templates, in which osteoblasts and stromal cells are generated to form mineralized matrix and support bone marrow hematopoiesis. However, the ultimate source of these mesenchymal cells and the relationship between bone progenitors in fetal life and those in later life are unknown. Fate-mapping studies revealed that cells expressing cre-recombinases driven by the collagen II (Col2) promoter/enhancer and their descendants contributed to, in addition to chondrocytes, early perichondrial precursors prior to Runx2 expression and, subsequently, to a majority of osteoblasts, Cxcl12 (chemokine (C-X-C motif) ligand 12)-abundant stromal cells and bone marrow stromal/mesenchymal progenitor cells in postnatal life. Lineage-tracing experiments using a tamoxifen-inducible creER system further revealed that early postnatal cells marked by Col2-creER, as well as Sox9-creER and aggrecan (Acan)-creER, progressively contributed to multiple mesenchymal lineages and continued to provide descendants for over a year. These cells are distinct from adult mesenchymal progenitors and thus provide opportunities for regulating the explosive growth that occurs uniquely in growing mammals.