Expression of Indian hedgehog in osteoblasts and its posttranscriptional regulation by transforming growth factor-beta

Key Roles of Gli1 and Ihh Signaling in Craniofacial Development

The Hedgehog (Hh) signaling pathway orchestrates its influence through a dynamic interplay of Hh proteins, the cell surface receptor Ptch1, Smo, and Gli transcription factors, contributing to a myriad of developmental events. Indian Hedgehog (Ihh) and Gli zinc finger transcription factor 1 (Gli1) play crucial roles in developmental regulation within the Hh signaling pathway. Ihh regulates chondrocyte proliferation, differentiation, and bone formation, impacting the development of cranial bones, cartilage, and the temporomandibular joint (TMJ). Losing Ihh results in cranial bone malformation and decreased ossification and affects the formation of cranial base cartilage unions, TMJ condyles, and joint discs. Gli1 is predominantly expressed during early craniofacial development, and Gli1+ cells are identified as the primary mesenchymal stem cells (MSCs) for craniofacial bones, crucial for cell differentiation and morphogenesis. In addition, a complex mutual regulatory mechanism exists between Gli1 and Ihh, ensuring the normal function of the Hh signaling pathway by directly or indirectly regulating each other's expression levels. And the interaction between Ihh and Gli1 significantly impacts the normal development of craniofacial tissues. This review summarizes the pivotal roles of Gli1 and Ihh in the intricate landscape of mammalian craniofacial development and outlines the molecular regulatory mechanisms and intricate interactions governing the growth of bone and cartilage exhibited by Gli1 and Ihh, which provides new insights into potential therapeutic strategies for related diseases or researches of tissue regeneration.

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Inactivation of Ihh in Sp7-Expressing Cells Inhibits Osteoblast Proliferation, Differentiation, and Bone Formation, Resulting in a Dwarfism Phenotype with Severe Skeletal Dysplasia in Mice

Indian hedgehog (Ihh) is an indispensable paracrine factor for proper tissue patterning, skeletogenesis, and cellular proliferation. Recent genetic studies have revealed critical roles of chondrocyte-derived Ihh in regulating chondrocyte proliferation, hypertrophy and cartilage ossification. However, the functions of Sp7-expressing cell-derived Ihh in osteoblast differentiation and bone formation remain unclear. Sp7 is an essential transcription factor for osteoblast differentiation. In the current study, we generated Sp7-iCre; Ihh^fl/fl mice, in which the Ihh gene was specifically deleted in Sp7-expressing cells to investigate the roles of Ihh. Ihh ablation in Sp7-expressing cells resulted in a dwarfism phenotype with severe skeletal dysplasia and lethality at birth, but with normal joint segmentation. Sp7-iCre; Ihh^fl/fl mice had fewer osteoblasts, almost no cortical and trabecular bones, smaller skulls, and wider cranial sutures. Additionally, the levels of osteogenesis- and angiogenesis-related genes, and of major bone matrix protein genes were significantly reduced. These results demonstrated that Ihh regulates bone formation in Sp7-expressing cells. Ihh deficiency in primary osteoblasts cultured in vitro inhibited their proliferation, differentiation, and mineralization ability, and reduced the expression of osteogenesis-related genes. Moreover, the deletion of Ihh also attenuated the Bmp2/Smad/Runx2 pathway in E18.5 tibial and primary osteoblasts. The activity of primary osteoblasts in mutant mice was rescued after treatment with rhBMP2. In summary, our data revealed that Ihh in Sp7-expressing cells plays an indispensable role in osteoblast differentiation, mineralization, and embryonic osteogenesis, further implicated that its pro-osteogenic role may be mediated through the canonical Bmp2/Smad/Runx2 pathway.

The Pleiotropic Intricacies of Hedgehog Signaling: From Craniofacial Patterning to Carcinogenesis

Hedgehog signaling was discovered more than 40 years ago in experiments demonstrating that it is a fundamental mediator of limb development. Since that time, it has been shown to be important in development, homeostasis, and disease. The hedgehog pathway proceeds through a pathway highly conserved throughout animals beginning with the extracellular diffusion of hedgehog ligands, proceeding through an intracellular signaling cascade, and ending with the activation of specific target genes. A vast amount of research has been done elucidating hedgehog signaling mechanisms and regulation. This research has found a complex system of genetics and signaling that helps determine how organisms develop and function. This review provides an overview of what is known about hedgehog genetics and signaling, followed by an in-depth discussion of the role of hedgehog signaling in craniofacial development and carcinogenesis.

Evaluation of canonical Hedgehog signaling pathway inhibition in canine osteosarcoma

Canine osteosarcoma (OSA), the most common canine primary bone malignancy, has a highly aggressive biologic behavior. Despite current standard of care therapies, including amputation and adjuvant chemotherapy, most dogs still succumb to metastatic disease. Further investigations into molecular mechanisms and pathways driving OSA are needed to improve therapeutic options. The Hedgehog (HH) cell-signaling pathway has demonstrated involvement in human OSA. Several studies in canine OSA have found changes in expression of some HH pathway genes and demonstrated a role for HH transcription factors. However, the role of this pathway as well as the translational value of its targeting in canine OSA are still undefined. The objectives of this study were to determine the expression of HH components directly in canine OSA tissues and to evaluate the biologic impact of HH signaling inhibition in canine OSA cells. In situ hybridization was used to detect HH family mRNA expression in archived canine OSA tissues and revealed variable expression levels of these mRNAs in canine OSA tissues. The effect of a commercially available Smoothened inhibitor, vismodegib, was studied in established canine OSA cell lines. Alterations in cellular growth as well as assessment of downstream HH targets were evaluated. Although changes in cell growth were noted following Smoothened inhibition, inconsistent decreases in target gene expression were found. While treatment with vismodegib had a negative impact on canine OSA cell growth and viability, the mechanism remains unclear. Further studies are warranted to evaluate the clinical significance of canonical HH signaling in canine OSA.

Evaluation of the articular cartilage in the knees of rats with induced arthritis treated with curcumin

This study was designed to evaluate the anti-inflammatory effects of a curcumin treatment on the knee of rats with induced osteoarthritis. Fifteen adult rats were used and divided in three groups: the osteoarthritis group (OAG), control group (CG-without induction of osteoarthritis), and curcumin-treated osteoarthritis group (COAG). Osteoarthritis was induced in the right knee of rats in the OAG and COAG by administering an intra-articular injection of 1 mg of zymosan. Fourteen days after induction, 50 mg/kg curcumin was administered by gavage daily for 60 days to the COAG. After the treatment period, rats from all groups were euthanized. Medial femoral condyles were collected for light microscopy and immunohistochemical staining. The expression of SOX-5, IHH, MMP-8, MMP-13, and collagen 2 (Col2) was analyzed. The COAG exhibited an increase in the number of chondrocytes in the surface and middle layers compared with that of the OAG and CG, respectively. The COAG also showed a decrease in the thicknesses of the middle and deep layers compared with those of the OAG, and an increase in Col2 expression was observed in all articular layers (surface, middle, and deep) in the COAG compared with that in the OAG. SOX-5 expression was increased in the surface and deep layers of the COAG compared with those in the OAG and CG. Based on the results of this study, the curcumin treatment appeared to exert a protective effect on cartilage, as it did not result in an increase in cartilage thickness or in MMP-8 and MMP-13 expression but led to increased IHH, Col2, and SOX-5 expression and the number of chondrocytes.

Hedgehog Signaling and Embryonic Craniofacial Disorders

Since its initial discovery in a Drosophila mutagenesis screen, the Hedgehog pathway has been revealed to be instrumental in the proper development of the vertebrate face. Vertebrates possess three hedgehog paralogs: Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). Of the three, Shh has the broadest range of functions both in the face and elsewhere in the embryo, while Ihh and Dhh play more limited roles. The Hedgehog pathway is instrumental from the period of prechordal plate formation early in the embryo, until the fusion of the lip and secondary palate, which complete the major patterning events of the face. Disruption of Hedgehog signaling results in an array of developmental disorders in the face, ranging from minor alterations in the distance between the eyes to more serious conditions such as severe clefting of the lip and palate. Despite its critical role, Hedgehog signaling seems to be disrupted through a number of mechanisms that may either be direct, as in mutation of a downstream target of the Hedgehog ligand, or indirect, such as mutation in a ciliary protein that is otherwise seemingly unrelated to the Hedgehog pathway. A number of teratogens such as alcohol, statins and steroidal alkaloids also disrupt key aspects of Hedgehog signal transduction, leading to developmental defects that are similar, if not identical, to those of Hedgehog pathway mutations. The aim of this review is to highlight the variety of roles that Hedgehog signaling plays in developmental disorders of the vertebrate face.

Cdc42 regulates cranial suture morphogenesis and ossification

Cdc42 (cell division cycle 42) is ubiquitously expressed small GTPases belonging to the Rho family of proteins. Previously, we generated limb bud mesenchyme-specific Cdc42 inactivated mice (Cdc42 conditional knockout mice; Cdc42^fl/fl; Prx1-Cre), which showed short limbs and cranial bone deformities, though the mechanism related to the cranium phenotype was unclear. In the present study, we investigated the role of Cdc42 in cranial bone development. Our results showed that loss of Cdc42 caused a defect of intramembranous ossification in cranial bone tissues which is related to decreased expressions of cranial suture morphogenesis genes, including Indian hedgehog (Ihh) and bone morphogenetic proteins (BMPs). These findings demonstrate that Cdc42 plays a crucial role in cranial osteogenesis, and is controlled by Ihh- and BMP-mediated signaling during cranium development.

Hedgehog signalling in the tumourigenesis and metastasis of osteosarcoma, and its potential value in the clinical therapy of osteosarcoma

The Hedgehog (Hh) signalling pathway is involved in cell differentiation, growth and tissue polarity. This pathway is also involved in the progression and invasion of various human cancers. Osteosarcoma, a subtype of bone cancer, is commonly seen in children and adolescents. Typically, pulmonary osteosarcoma metastases are especially difficult to control. In the present paper, we summarise recent studies on the regulation of osteosarcoma progression and metastasis by downregulating Hh signalling. We also summarise the crosstalk between the Hh pathway and other cancer-related pathways in the tumourigenesis of various cancers. We further summarise and highlight the therapeutic value of potential inhibitors of Hh signalling in the clinical therapy of human cancers.

Suppressor of Fused restraint of Hedgehog activity level is critical for osteogenic proliferation and differentiation during calvarial bone development

Hedgehog signaling plays crucial roles in the development of calvarial bone, relying on the activation of Gli transcription factors. However, the molecular mechanism of the role of regulated Gli protein level in osteogenic specification of mesenchyme still remains elusive. Here, we show by conditionally inactivating Suppressor of Fused (Sufu), a critical repressor of Hedgehog signaling, in Wnt1-Cre-mediated cranial neural crest (CNC) or Dermo1-Cre-mediated mesodermal lineages that Sufu restraint of Hedgehog activity level is critical for differentiation of preosteogenic mesenchyme. Ablation of Sufu results in failure of calvarial bone formation, including CNC-derived bones and mesoderm-derived bones, depending on the Cre line being used. Although mesenchymal cells populate to frontonasal destinations, where they are then condensed, Sufu deletion significantly inhibits the proliferation of osteoprogenitor cells, and these cells no longer differentiate into osteoblasts. We show that there is suppression of Runx2 and Osterix, the osteogenic regulators, in calvarial mesenchyme in the Sufu mutant. We show that down-regulation of several genes upstream to Runx2 and Osterix is manifested within the calvarial primordia, including Bmp2 and its downstream genes Msx1/2 and Dlx5 By contrast, we find that Gli1, the Hedgehog activity readout gene, is excessively activated in mesenchyme. Deletion of Sufu in CNC leads to a discernible decrease in the repressive Gli3 form and an increase in the full-length Gli2. Finally, we demonstrate that simultaneous deletion of Gli2 and Sufu in CNC completely restores calvarial bone formation, suggesting that a sustained level of Hedgehog activity is critical in specification of the osteogenic mesenchymal cells.

miR-15/miR-16 loss, miR-21 upregulation, or deregulation of their target genes predicts poor prognosis in prostate cancer patients

It is clear that several prostate cancers remain indolent whereas others develop into advanced forms. There is a need to improve patient management by identifying biomarkers for personalized treatment. We demonstrated that miR-15/miR-16 loss, miR-21 upregulation, and deregulation of their target genes represent a promising predictive signature of poor patient prognosis.

Transcription Factors in Craniofacial Development: From Receptor Signaling to Transcriptional and Epigenetic Regulation

Craniofacial morphogenesis is driven by spatial-temporal terrains of gene expression, which give rise to stereotypical pattern formation. Transcription factors are key cellular components that control these gene expressions. They are information hubs that integrate inputs from extracellular factors and environmental cues, direct epigenetic modifications, and define transcriptional status. These activities allow transcription factors to confer specificity and potency to transcription regulation during development.

The Hedgehog signalling pathway in bone formation

The Hedgehog (Hh) signalling pathway plays many important roles in development, homeostasis and tumorigenesis. The critical function of Hh signalling in bone formation has been identified in the past two decades. Here, we review the evolutionarily conserved Hh signalling mechanisms with an emphasis on the functions of the Hh signalling pathway in bone development, homeostasis and diseases. In the early stages of embryonic limb development, Sonic Hedgehog (Shh) acts as a major morphogen in patterning the limb buds. Indian Hedgehog (Ihh) has an essential function in endochondral ossification and induces osteoblast differentiation in the perichondrium. Hh signalling is also involved intramembrane ossification. Interactions between Hh and Wnt signalling regulate cartilage development, endochondral bone formation and synovial joint formation. Hh also plays an important role in bone homeostasis, and reducing Hh signalling protects against age-related bone loss. Disruption of Hh signalling regulation leads to multiple bone diseases, such as progressive osseous heteroplasia. Therefore, understanding the signalling mechanisms and functions of Hh signalling in bone development, homeostasis and diseases will provide important insights into bone disease prevention, diagnoses and therapeutics.

A review of hedgehog signaling in cranial bone development

During craniofacial development, the Hedgehog (HH) signaling pathway is essential for mesodermal tissue patterning and differentiation. The HH family consists of three protein ligands: Sonic Hedgehog (SHH), Indian Hedgehog (IHH), and Desert Hedgehog (DHH), of which two are expressed in the craniofacial complex (IHH and SHH). Dysregulations in HH signaling are well documented to result in a wide range of craniofacial abnormalities, including holoprosencephaly (HPE), hypotelorism, and cleft lip/palate. Furthermore, mutations in HH effectors, co-receptors, and ciliary proteins result in skeletal and craniofacial deformities. Cranial suture morphogenesis is a delicate developmental process that requires control of cell commitment, proliferation and differentiation. This review focuses on both what is known and what remains unknown regarding HH signaling in cranial suture morphogenesis and intramembranous ossification. As demonstrated from murine studies, expression of both SHH and IHH is critical to the formation and fusion of the cranial sutures and calvarial ossification. SHH expression has been observed in the cranial suture mesenchyme and its precise function is not fully defined, although some postulate SHH to delay cranial suture fusion. IHH expression is mainly found on the osteogenic fronts of the calvarial bones, and functions to induce cell proliferation and differentiation. Unfortunately, neonatal lethality of IHH deficient mice precludes a detailed examination of their postnatal calvarial phenotype. In summary, a number of basic questions are yet to be answered regarding domains of expression, developmental role, and functional overlap of HH morphogens in the calvaria. Nevertheless, SHH and IHH ligands are integral to cranial suture development and regulation of calvarial ossification. When HH signaling goes awry, the resultant suite of morphologic abnormalities highlights the important roles of HH signaling in cranial development.

Chondrocytic Atf4 regulates osteoblast differentiation and function via Ihh

Atf4 is a leucine zipper-containing transcription factor that activates osteocalcin (Ocn) in osteoblasts and indian hedgehog (Ihh) in chondrocytes. The relative contribution of Atf4 in chondrocytes and osteoblasts to the regulation of skeletal development and bone formation is poorly understood. Investigations of the Atf4(-/-);Col2a1-Atf4 mouse model, in which Atf4 is selectively overexpressed in chondrocytes in an Atf4-null background, demonstrate that chondrocyte-derived Atf4 regulates osteogenesis during development and bone remodeling postnatally. Atf4 overexpression in chondrocytes of the Atf4(-/-);Col2a1-Atf4 double mutants corrects the reduction in stature and limb in Atf4(-/-) embryos and rectifies the decrease in Ihh expression, Hh signaling, proliferation and accelerated hypertrophy that characterize the Atf4(-/-) developing growth plate cartilages. Unexpectedly, this genetic manipulation also restores the expression of osteoblastic marker genes, namely Ocn and bone sialoprotein, in Atf4(-/-) developing bones. In Atf4(-/-);Col2a1-Atf4 adult mice, all the defective bone parameters found in Atf4(-/-) mice, including bone volume, trabecular number and thickness, and bone formation rate, are rescued. In addition, the conditioned media of ex vivo cultures from wild-type or Atf4(-/-);Col2a1-Atf4, but not Atf4(-/-) cartilage, corrects the differentiation defects of Atf4(-/-) bone marrow stromal cells and Ihh-blocking antibody eliminates this effect. Together, these data indicate that Atf4 in chondrocytes is required for normal Ihh expression and for its paracrine effect on osteoblast differentiation. Therefore, the cell-autonomous role of Atf4 in chondrocytes dominates the role of Atf4 in osteoblasts during development for the control of early osteogenesis and skeletal growth.

Gli3Xt-J/Xt-J mice exhibit lambdoid suture craniosynostosis which results from altered osteoprogenitor proliferation and differentiation

Gli3 is a zinc-finger transcription factor whose activity is dependent on the level of hedgehog (Hh) ligand. Hh signaling has key roles during endochondral ossification; however, its role in intramembranous ossification is still unclear. In this study, we show that Gli3 performs a dual role in regulating both osteoprogenitor proliferation and osteoblast differentiation during intramembranous ossification. We discovered that Gli3Xt-J/Xt-J mice, which represent a Gli3-null allele, exhibit craniosynostosis of the lambdoid sutures and that this is accompanied by increased osteoprogenitor proliferation and differentiation. These cellular changes are preceded by ectopic expression of the Hh receptor Patched1 and reduced expression of the transcription factor Twist1 in the sutural mesenchyme. Twist1 is known to delay osteogenesis by binding to and inhibiting the transcription factor Runx2. We found that Runx2 expression in the lambdoid suture was altered in a pattern complimentary to that of Twist1. We therefore propose that loss of Gli3 results in a Twist1-, Runx2-dependent expansion of the sutural osteoprogenitor population as well as enhanced osteoblastic differentiation which results in a bony bridge forming between the parietal and interparietal bones. We show that FGF2 will induce Twist1, normalize osteoprogenitor proliferation and differentiation and rescue the lambdoid suture synostosis in Gli3Xt-J/Xt-J mice. Taken together, we define a novel role for Gli3 in osteoblast development; we describe the first mouse model of lambdoid suture craniosynostosis and show how craniosynostosis can be rescued in this model.

Chondrocyte-specific modulation of Cyp27b1 expression supports a role for local synthesis of 1,25-dihydroxyvitamin D3 in growth plate development

The Cyp27b1 enzyme (25-hydroxyvitamin D-1alpha-hydroxylase) that converts 25-hydroxyvitamin D into the active metabolite, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], is expressed in kidney but also in other cell types such as chondrocytes. This suggests that local production of 1,25(OH)(2)D(3) could play an important role in the differentiation of these cells. To test this hypothesis, we engineered mutant mice that do not express the Cyp27b1 gene in chondrocytes. Inactivation of both alleles of the Cyp27b1 gene led to decreased RANKL expression and reduced osteoclastogenesis, increased width of the hypertrophic zone of the growth plate at embryonic d 15.5, increased bone volume in neonatal long bones, and increased expression of the chondrocytic differentiation markers Indian Hedgehog and PTH/PTHrP receptor. The expression of the angiogenic marker VEGF was decreased, accompanied by decreased platelet/endothelial cell adhesion molecule-1 staining in the neonatal growth plate, suggesting a delay in vascularization. In parallel, we engineered strains of mice overexpressing a Cyp27b1 transgene in chondrocytes by coupling the Cyp27b1 cDNA to the collagen alpha(1)(II) promoter. The transgenic mice showed a mirror image phenotype when compared with the tissue-specific inactivation, i.e. a reduction in the width of the hypertrophic zone of the embryonic growth plate, decreased bone volume in neonatal long bones, and inverse expression patterns of chondrocytic differentiation markers. These results support an intracrine role of 1,25(OH)(2)D(3) in endochondral ossification and chondrocyte development in vivo.

Ihh/Gli2 signaling promotes osteoblast differentiation by regulating Runx2 expression and function

Genetic and cell biological studies have indicated that Indian hedgehog (Ihh) plays an important role in bone development and osteoblast differentiation. However, the molecular mechanism by which Ihh regulates osteoblast differentiation is complex and remains to be fully elucidated. In this study, we investigated the role of Ihh signaling in osteoblast differentiation using mesenchymal cells and primary osteoblasts. We observed that Ihh stimulated alkaline phosphatase (ALP) activity, osteocalcin expression, and calcification. Overexpression of Gli2- but not Gli3-induced ALP, osteocalcin expression, and calcification of these cells. In contrast, dominant-negative Gli2 markedly inhibited Ihh-dependent osteoblast differentiation. Ihh treatment or Gli2 overexpression also up-regulated the expression of Runx2, an essential transcription factor for osteoblastogenesis, and enhanced the transcriptional activity and osteogenic action of Runx2. Coimmunoprecipitation analysis demonstrated a physical interaction between Gli2 and Runx2. Moreover, Ihh or Gli2 overexpression failed to increase ALP activity in Runx2-deficient mesenchymal cells. Collectively, these results suggest that Ihh regulates osteoblast differentiation of mesenchymal cells through up-regulation of the expression and function of Runx2 by Gli2.

Indian Hedgehog produced by postnatal chondrocytes is essential for maintaining a growth plate and trabecular bone

Indian hedgehog (Ihh) is essential for chondrocyte and osteoblast proliferation/differentiation during prenatal endochondral bone formation. The early lethality of various Ihh-ablated mutant mice, however, prevented further analysis of its role in postnatal bone growth and development. In this study, we describe the generation and characterization of a mouse model in which the Ihh gene was successfully ablated from postnatal chondrocytes in a temporal/spatial-specific manner; postnatal deletion of Ihh resulted in loss of columnar structure, premature vascular invasion, and formation of ectopic hypertrophic chondrocytes in the growth plate. Furthermore, destruction of the articular surface in long bones and premature fusion of growth plates of various endochondral bones was evident, resulting in dwarfism in mutant mice. More importantly, these mutant mice exhibited continuous loss of trabecular bone over time, which was accompanied by reduced Wnt signaling in the osteoblastic cells. These results demonstrate, for the first time, that postnatal chondrocyte-derived Ihh is essential for maintaining the growth plate and articular surface and is required for sustaining trabecular bone and skeletal growth.

The role of transforming growth factor beta signaling in messenger RNA stability

Transforming growth factor beta (TGF-beta) is a biologically multipotent regulatory protein implicated in functions that include the regulation of cellular growth, differentiation, extracellular matrix formation, and wound healing. It also plays a role in the pathologies of Alzheimer's disease, cancer and autoimmune disorders. TGF-beta modulates gene expression by affecting transcriptional activation and mRNA turnover rate. Steady-state mRNA levels depend on both the transcriptional activity and mRNA half-life. The stability of mRNA can be modified by the binding of trans-acting factors to cis-elements on the message. These can protect the mRNA from cleavage by RNAses, or they may promote mRNA cleavage. Changes in mRNA stability can lead to changes in the proteome and subsequently in cellular metabolism. The SMAD family of proteins has been implicated in the transduction of the TGF-beta signal, where they regulate transcriptional activity. This review attempts to provide new insights into the role played by TGF-beta in the regulation of mRNA turnover.

Conditional deletion of Indian hedgehog from collagen type 2alpha1-expressing cells results in abnormal endochondral bone formation

Indian hedgehog (Ihh) is actively involved in endochondral bone formation. Although expression of Ihh is mostly restricted to pre-hypertrophic chondrocytes, the role of chondrocyte-derived Ihh in endochondral bone formation is not completely understood. To address such unresolved issues, we used the Cre/loxP approach to generate mice (Col2alpha1Cre; Ihhd/Ihhd) in which the Ihh gene was selectively ablated from collagen type II expressing cells. Mutant mice were born with the expected ratio of Mendelian inheritance, but died shortly after birth and were smaller in size, exhibiting malformed and retarded growth of limbs with severe skeletal deformities. Alizarin red S staining showed abnormal mineralization of axial and appendicular bones. Histological analysis of mutant long bones revealed abnormal endochondral bone formation with loss of a normal growth plate. In addition, in vivo bromo-deoxyuridine (BrdU) labelling showed a marked decrease in chondrocyte proliferation. A delay in chondrocyte hypertrophy in Col2alpha1Cre; Ihhd/Ihhd mice was detected by the expression of collagen type X and osteopontin, using in situ hybridization. Furthermore, there was no expression of bone markers such as collagen type I, bone Gla protein, Runx2/Cbfa1 or PTH-R in the perichondrium of mutant mice, indicating the absence of osteoblasts from endochondral bones. Thus, selective loss of chondrocyte-derived Ihh recapitulated the defects in Ihh(-/-) animals, providing direct in vivo evidence that Ihh not only regulates chondrocyte proliferation and differentiation but also exerts effects on osteoblast differentiation. Understanding the exact functions of the molecules involved in endochondral bone formation will form the basis for further study to determine the molecular mechanisms of skeletal diseases involving various cellular components of bone.

Indian hedgehog signaling pathway: expression and regulation in pancreatic cancer

Pancreatic cancer is an aggressive malignancy that exhibits a number of genetic and epigenetic alterations. Indian hedgehog (Ihh) and its 2 signaling receptors, patched (Ptc) and smoothened (Smo), are involved in pancreatic development and regulation of beta-cell function as well as in certain human tumors. In the current study, we analyzed the expression, distribution and function of Ihh and its receptors in pancreatic cancer. Quantitative RT-PCR and immunohistochemistry were utilized to analyze the expression, localization and transcriptional regulation of Ihh, Ptc and Smo. The effects of inhibition and stimulation of the hedgehog signaling pathway on pancreatic cancer cell growth were examined by the MTT cell growth assay. By quantitative RT-PCR, Ihh, Ptc and Smo mRNA levels were increased 35-, 1.2- and 1.6-fold, respectively, in pancreatic cancer tissues in comparison to normal pancreatic tissues. By immunohistochemistry, Ihh, Ptc and Smo were expressed in the islet cells of normal and cancerous tissues and in pancreatic cancer cells. The growth of pancreatic cancer cells was dose-dependently inhibited by the hedgehog antagonist cyclopamine through G0/G1 arrest. In contrast, Ihh agonists exhibited no significant effect on pancreatic cancer cell growth. TGF-beta1 repressed Ihh transcription in a TGF-beta1-responsive pancreatic cancer cell line, but had no effect on the other tested cell lines. In conclusion, Ihh and its receptors Ptc and Smo are expressed in pancreatic cancer, and blockage of hedgehog signaling results in inhibition of pancreatic cancer cell growth, suggesting that aberrant activation of the Ihh signaling pathway contributes to tumor development in this malignancy.

Recapitulation of the parathyroid hormone-related peptide-Indian hedgehog pathway in the regenerating deer antler

Parathyroid hormone (PTH)-related peptide (PTHrP) and the PTH/PTHrP receptor (PPR) play an essential role in controlling growth plate development. The aim of the present study was to use the deer antler as a model to determine whether PTHrP and PPR may also have a function in regulating cartilage and bone regeneration in an adult mammal. Antlers are the only mammalian appendages that are able to undergo repeated cycles of regeneration, and their growth from a blastema involves a modified endochondral process. Immunohistochemistry was used to establish sites of localization of PTHrP and PPR in antlers at different stages of development. The pattern of Indian Hedgehog (IHH) and transforming growth factor-beta1 (TGF beta1) distribution was also investigated, because PTHrP expression in the developing limb is regulated by IHH and during embryonic growth plate formation TGF beta1 acts upstream of PTHrP to regulate the rate of chondrocyte differentiation. In the antler blastema (<10 days of development), PTHrP, PPR, and TGF beta1 were localized in epidermis, dermis, regenerating epithelium, and in mesenchymal cells but IHH expression was not detected. In the rapidly growing antler (weeks 4-8 of development), PTHrP, PPR, and TGF beta1 were localized in skin, perichondrium, undifferentiated mesenchyme, recently differentiated chondrocytes, and in perivascular cells in cartilage but not in fully differentiated hyperytrophic chondrocytes. IHH was restricted to recently differentiated chondrocytes and to perivascular cells in cartilage. In mineralized cartilage and bone, PTHrP, PPR, IHH, and TGF beta1 were immunolocalized in perivascular cells and differentiated osteoblasts. PTHrP and PPR were also present in the periosteum. TGF beta1 in vitro stimulated PTHrP synthesis by cells from blastema, perichondrium, and cartilage. The findings of this study suggest that molecules which regulate embryonic skeletal development and postnatal epiphyseal growth may also control blastema formation, chondrogenesis, and bone formation in the regenerating deer antler. This finding is further evidence that developmental signaling pathways are recapitulated during adult mammalian bone regeneration.

Posttranscriptional regulation of TSC-22 (TGF-beta-stimulated clone-22) gene by TGF-beta 1

TSC-22 gene was composed of three exons and its length was approximately 5.5 kb including 2.9 kb promoter region. The transcription starting site was located at 7 and 29 bp downstream from TATA box. Promoter analysis revealed that 2146 bp of TSC-22 promoter was activated by several differentiation inducing drugs. Although originally TSC-22 was isolated as a TGF-beta-inducible gene, TSC-22 promoter was not activated by the enhanced TGF-beta signaling. We found 3 copies of the Shaw-Kamens sequence (AUUUA) in the human TSC-22 mRNA 3'-UTR and identified three proteins (40, 20, and 15 kDa) which bound to this. Only the 40 kDa protein-RNA complex was decreased by treatment with TGF-beta 1. Moreover, the TSC-22 mRNA 3'-UTR destabilized the heterologous luciferase mRNA, but the destabilization was recovered with TGF-beta 1. These observations suggest that up-regulation of TSC-22 mRNA by TGF-beta 1 is achieved by mRNA stabilization, but not by transcriptional activation.

Stomach regional specification requiresHoxa5-driven mesenchymal-epithelial signaling

The genetic control of gut regionalization relies on a hierarchy of molecular events in which the Hox gene family of transcription factors is suspected to be key participant. We have examined the role of Hox genes in gut patterning using the Hoxa5–/– mice as a model. Hoxa5 is expressed in a dynamic fashion in the mesenchymal component of the developing gut. Its loss of function results in gastric enzymatic anomalies in Hoxa5–/– surviving mutants that are due to perturbed cell specification during stomach development. Histological, biochemical and molecular characterization of the mutant stomach phenotype may be compatible with a homeotic transformation of the gastric mucosa. As the loss of mesenchymal Hoxa5 function leads to gastric epithelial defects, Hoxa5 should exert its action by controlling molecules involved in mesenchymal-epithelial signaling. Indeed, in the absence of Hoxa5 function, the expression of genes encoding for signaling molecules such as sonic hedgehog, Indian hedgehog, transforming growth factor β family members and fibroblast growth factor 10, is altered. These findings provide insight into the molecular controls of patterning events of the stomach, supporting the notion that Hoxa5 acts in regionalization and specification of the stomach by setting up the proper domains of expression of signaling molecules.

Bone morphogenetic protein regulation of forkhead/winged helix transcription factor Foxc2 (Mfh1) in a murine mesodermal cell line C1 and in skeletal precursor cells

Mfh1/Foxc2 is a member of forkhead/winged helix transcription factor family in which its members serve as key regulators in embryogenesis and cell differentiation in various species. Mutant mice null for Mfh1 show defects in axial and cranial skeletogenesis, suggesting requirement of Mfh1 for skeletal tissue development. However, the roles of Mfh1 and its regulation during early skeletogenesis have not been understood fully yet. In this study, we investigated developmental regulation of Mfh1 expression during embryonic skeletogenesis in vivo and in vitro chondrogenic cell differentiation using a mesodermal progenitor-like cell line C1. We first examined expression patterns of Mfh1 in relation to the cartilage phenotype-related molecules including bone morphogenetic proteins (BMPs) during mouse embryogenesis by in situ hybridization. In 10.5 days postcoitum (dpc) mouse limb, Mfh1 messenger RNA (mRNA) was expressed in the mesenchymal cells in the tissues that later give rise to skeleton. In 11.5 dpc embryos, Mfh1 transcripts were expressed in the cell condensation of skeletal blastemas. BMP2 transcripts were expressed in the cell condensation proximal to the Mfh1-expressing cells in the limbs and those of BMP-7 were expressed in the mesenchymal tissue surrounding the Mfh1-positive cell condensation. In 12.5 dpc and 13.5 dpc embryos, the expression of Mfh1 was localized to the perichondrium, which surrounds cells that express noggin and SOX9 mRNA. BMP-2 expression was overlapped with that of Mfh1 in the peripheral layer of 12.5 dpc and 13.5 dpc limb skeletal blastemas. Mfh1 expression persisted in the perichondrium of 15.5 dpc embryos though its level was reduced. We then examined the expression of Mfh1 in the mouse mesodermal cell line C1 that differentiates into chondrocytes in vitro. Mfh1 mRNA was expressed constitutively at low levels in C1 cells before the induction of its differentiation. On the differentiation of C1 cells into chondrocytes by the treatment with dexamethasone (Dex), Mfh1 expression was increased and peaked on day 4 of Dex treatment. Treatment with BMP-4/7 and BMP-7 protein also enhanced Mfh1 expression in C1 cells. To further examine the causative relationship between BMP and Mfh1 in mesenchymal tissue, we performed a mouse limb bud organ culture to implant BMP proteins with carriers into the mesenchymal tissue of the limb bud. Implantation of BMP-7 protein in the limb bud of 11.5 dpc embryos induced Mfh1 expression, suggesting that BMP regulates Mfh1 expression in limb mesenchyme. These results indicate that Mfh1 expression is associated with the early stage of chondrogenic differentiation both in vivo and in vitro and that BMPs regulate Mfh1 expression in skeletal precursor cells.

Actions of hedgehog proteins on skeletal cells

Recent advances in developmental and molecular biology during embryogenesis and organogenesis have provided new insights into the mechanism of bone formation. Members of the hedgehog gene family were initially characterized as patterning factors in embryonic development, but recently they have been shown to regulate skeletal formation in vertebrates. The amino terminal fragment of Sonic hedgehog (Shh-N), which is an active domain of Shh, has the ability to induce ectopic cartilage and bone formation in vivo. Shh-N stimulates chondrogenic differentiation in cultures of chondrogenic cell line cells in vitro and inhibits chondrogenesis in primary limb bud cells. These findings suggest that the regulation of chondrogenesis by hedgehog proteins depends on the cell populations being studied. Indian hedgehog (Ihh) is prominently expressed in developing cartilage. Ectopic expression of Ihh decreases type X collagen expression and induces the up-regulation of parathyroid hormone-related peptide (PTHrp) gene expression in perichondrium cells. A negative feedback loop consisting of Ihh and PTHrp, induced by Ihh, appears to regulate the rate of chondrocyte maturation. The direct actions of Shh and Ihh on stimulation of osteoblast differentiation are evidenced by the findings that these factors stimulate alkaline phosphatase activity in cultures of pluripotent mesenchymal cell line cells and osteoblastic cells and that these cells express putative receptors of hedgehog proteins. In conclusion, hedgehog proteins seem to be significantly involved in skeletal formation through multiple actions on chondrogenic mesenchymal cells, chondrocytes, and osteogenic cells.