Regulation of rate of cartilage differentiation by Indian hedgehog and PTH-related protein

Rescue of the parathyroid hormone-related protein knockout mouse demonstrates that parathyroid hormone-related protein is essential for mammary gland development

Parathyroid hormone-related protein (PTHrP) was originally discovered as a tumor product that causes humoral hypercalcemia of malignancy. PTHrP is now known to be widely expressed in normal tissues and growing evidence suggests that it is an important developmental regulatory molecule. We had previously reported that overexpression of PTHrP in the mammary glands of transgenic mice impaired branching morphogenesis during sexual maturity and early pregnancy. We now demonstrate that PTHrP plays a critical role in the epithelial-mesenchymal communications that guide the initial round of branching morphogenesis that occurs during the embryonic development of the mammary gland. We have rescued the PTHrP-knockout mice from neonatal death by transgenic expression of PTHrP targeted to chondrocytes. These rescued mice are devoid of mammary epithelial ducts. We show that disruption of the PTHrP gene leads to a failure of the initial round of branching growth that is responsible for transforming the mammary bud into the rudimentary mammary duct system. In the absence of PTHrP, the mammary epithelial cells degenerate and disappear. The ability of PTHrP to support embryonic mammary development is a function of amino-terminal PTHrP, acting via the PTH/PTHrP receptor, for ablation of the PTH/PTHrP receptor gene recapitulates the phenotype of PTHrP gene ablation. We have localized PTHrP expression to the embryonic mammary epithelial cells and PTH/PTHrP receptor expression to the mammary mesenchyme using in situ hybridization histochemistry. Finally, we have rescued mammary gland development in PTHrP-null animals by transgenic expression of PTHrP in embryonic mammary epithelial cells. We conclude that PTHrP is a critical epithelial signal received by the mammary mesenchyme and involved in supporting the initiation of branching morphogenesis.

FGF-, BMP- and Shh-mediated signalling pathways in the regulation of cranial suture morphogenesis and calvarial bone development

The development of calvarial bones is tightly co-ordinated with the growth of the brain and needs harmonious interactions between different tissues within the calvarial sutures. Premature fusion of cranial sutures, known as craniosynostosis, presumably involves disturbance of these interactions. Mutations in the homeobox gene Msx2 as well as the FGF receptors cause human craniosynostosis syndromes. Our histological analysis of mouse calvarial development demonstrated morphological differences in the sagittal suture between embryonic and postnatal stages. In vitro culture of mouse calvaria showed that embryonic, but not postnatal, dura mater regulated suture patency. We next analysed by in situ hybridisation the expression of several genes, which are known to act in conserved signalling pathways, in the sagittal suture during embryonic (E15-E18) and postnatal stages (P1-P6). Msx1 and Msx2 were expressed in the sutural mesenchyme and the dura mater. FGFR2(BEK), as well as Bmp2 and Bmp4, were intensely expressed in the osteogenic fronts and Bmp4 also in the mesenchyme of the sagittal suture and in the dura mater. Fgf9 was expressed throughout the calvarial mesenchyme, the dura mater, the developing bones and the overlying skin, but Fgf4 was not detected in these tissues. Interestingly, Shh and Ptc started to be expressed in patched pattern along the osteogenic fronts at the end of embryonic development and, at this time, the expression of Bmp4 and sequentially those of Msx2 and Bmp2 were reduced, and they also acquired patched expression patterns. The expression of Msx2 in the dura mater disappeared after birth. <P> FGF and BMP signalling pathways were further examined in vitro, in E15 mouse calvarial explants. Interestingly, beads soaked in FGF4 accelerated sutural closure when placed on the osteogenic fronts, but had no such effect when placed on the mid-sutural mesenchyme. BMP4 beads caused an increase in tissue volume both when placed on the osteogenic fronts and on the mid-sutural area, but did not effect suture closure. BMP4 induced the expression of both Msx1 and Msx2 genes in sutural tissue, while FGF4 induced only Msx1. We suggest that the local application of FGF on the osteogenic fronts accelerating suture closure in vitro, mimics the pathogenesis of human craniosynostosis syndromes in which mutations in the FGF receptor genes apparently cause constitutive activation of the receptors. Taken together, our data suggest that conserved signalling pathways regulate tissue interactions during suture morphogenesis and intramembranous bone formation of the calvaria and that morphogenesis of mouse sagittal suture is controlled by different molecular mechanisms during the embryonic and postnatal stages. Signals from the dura mater may regulate the maintenance of sutural patency prenatally, whereas signals in the osteogenic fronts dominate after birth.

Targeted gene misexpression in chick limb buds using avian replication-competent retroviruses

The methods and applications for using avian replication-competent retroviruses to target gene misexpression in the developing limb bud of the chicken are described. These viruses provide the means to exploit the strengths of the chick as a model system in experimental embryology in conjunction with a genetic approach for ectopically expressing a gene of interest. The applications and strengths of the system are detailed. All the steps required to produce a virus carrying a transgene of interest and the methodologies behind designing and carrying out misexpression strategies are outlined, and some useful techniques for analyzing infected embryos are described.

Regulation of growth region cartilage proliferation and differentiation by perichondrium

Endochondral bone formation in vertebrates requires precise coordination between proliferation and differentiation of the participating chondrocytes. We examined the role of perichondrium in this process using an organ culture system of chicken embryonic tibiotarsi. A monoclonal antibody against chicken collagen type X, specifically expressed by hypertrophic chondrocytes, was utilized to monitor the terminal differentiation of chondrocytes. Proliferation of chondrocytes was examined by a BrdU-labeling procedure. The absence of perichondrium is correlated with an extended zone of cartilage expressing collagen type X, suggesting that the perichondrium regulates chondrocyte hypertrophy in a negative manner. Removal of perichondrium, in addition, resulted in an extended zone of chondrocytes incorporating BrdU, indicating that the perichondrium also negatively regulates the proliferation of chondrocytes. Partial removal of perichondrium from one side of the tibiotarsus led to expansion of both the collagen type X-positive domain and the BrdU-positive zone at the site of removal but not where the perichondrium remained intact. This suggests that both types of regulation by the perichondrium are local effects. Furthermore, addition of bovine parathyroid hormone (PTH) to perichondrium-free cultures reversed the expansion of the collagen type X-positive domain but not that of the proliferative zone. This suggests that the regulation of differentiation is dependent upon the PTH/PTHrP receptor and that the regulation of proliferation is likely independent of it. Taken together, these results are consistent with a model where perichondrium regulates both the exit of chondrocytes from the cell cycle, and their subsequent differentiation.

HiPER1, a phosphatase of the endoplasmic reticulum with a role in chondrocyte maturation

We have previously identified and partially cloned Band 17, a gene expressed in growth plate chondrocytes transiting from proliferation to hypertrophy. We now rename this gene HiPER1, Histidine Phosphatase of the Endoplasmic Reticulum-1, based on the results reported here. HiPER1 encodes two proteins of 318 (HiPER1(318)) and 449 (HiPER1(449)) amino acids, which are 20–21% identical to a group of yeast acid phosphatases that are in the histidine phosphatase family. HiPER1(449) is significantly more abundant than HiPER1(318), correlating with the abundance of the alternatively spliced messages encoding HiPER449 and HiPER318. Anti-HiPER1 antibodies detect two proteins of 53 and 55 kDa in growth plate chondrocytes that are absent in articular chondrocytes. We confirm that the 53 and 55 kDa proteins are HiPER1(449) by heterologous expression of the HiPER1(449) coding sequence in chick embryo fibroblasts. The 53 and 55 kDa proteins are glycosylated forms of HiPER1(449), as N-glycosidase F digestion reduces these proteins to 48 kDa, the predicted size of HiPER1(449) without the N-terminal signal sequence. Immunocytochemistry demonstrates that HiPER1(449) is found in chondrocytes maturing from proliferation to hypertrophy, but is not detectable in resting zone, deep hypertrophic zone or articular chondrocytes, a distribution that is consistent with the message distribution. HiPER1(449) was predicted to localize to the lumen of endoplasmic reticulum by an N-terminal signal sequence and by the C-terminal sequence Ala-Asp-Glu-Leu, which closely matches the consensus signal for ER retention, Lys-Asp-Glu-Leu. We confirm this prediction by demonstrating colocalization of HiPER1(449) with the ER protein HSP47 using dual-label immunofluorescence. PTHrP, a peptide that prevents hypertrophy in chondrocytes, suppressed HiPER1 and HiPER1(449) expression in vitro, an observation that further supports a role for HiPER1 in chondrocyte maturation. The yeast phosphatase homology, localization to the endoplasmic reticulum and pattern of expression suggest that HiPER1 represents a previously unrecognized intracellular pathway, involved in differentiation of chondrocytes.

Toward understanding SOX9 function in chondrocyte differentiation

The transcription factors that trigger the determinative switch to chondrocyte differentiation in mesenchymal cells are still unknown. In humans, mutations in the gene for SOX9, a transcription factor with a DNA-binding domain similar to that of the mammalian testis-determining factor SRY, cause campomelic dysplasia, a severe dwarfism syndrome which affects all cartilage-derived structures. During mouse embryonic development, the Sox9 gene becomes active in all prechondrocytic mesenchymal condensations, and at later stages its expression is maintained at high levels in fully differentiated chondrocytes. A chondrocyte-specific enhancer in the gene for collagen type II (Col2a1), a characteristic marker of chondrocytes, is a direct target for SOX9, and ectopic expression of SOX9 in transgenic mouse embryos is sufficient to activate the endogenous Col2a1 gene in some tissues. These data suggest that SOX9 could have a major role in chondrogenesis. Studies are in progress to identify other target genes for SOX9 in chondrocytes and also other transcription factors that are believed to cooperate with SOX9 in the activation of chondrocyte-specific genes. Defining SOX9 function and the mechanisms that regulate SOX9 gene expression should contribute to a better understanding of chondrocyte differentiation.

Abnormal cholesterol biosynthesis as in Smith-Lemli-Opitz syndrome disrupts normal skeletal development in the rat

Smith-Lemli-Opitz syndrome (SLOS) in human infants is a common autosomal recessive malformation syndrome (estimated incidence, 1:20,000). It is characterized clinically by congenital anomalies, especially craniofacial and limb defects, and biochemically by a defect in 7-dehydrocholesterol-delta7-reductase activity (7DHC-reductase), the final enzyme in cholesterol biosynthesis. In previous studies, early administration of the 7DHC-reductase inhibitor AY9944 to pregnant rats resulted in a high frequency of holoprosencephaly, relevant to craniofacial anomalies of SLOS. In order to test the effect of AY9944 on limb development, we treated dams on gestation day 7 (GD7), which delays the biochemical defect to about GD13 to GD14. Sera were sampled on GD12, GD14, and GD21 and cholesterol and dehydrocholesterols (7DHC and 8DHC) were measured by gas-chromatography-mass spectrometry (GC-MS), as for the diagnosis of SLOS. GD21 fetuses were examined for gross malformations and skeletal development. In treated dams, the SLOS biochemical marker 7DHC accounted for one fourth and one third of total sterols, respectively, on GD12 and GD14, and cholesterolemia on these two gestation days was reduced by 50% and 43%, respectively, as compared with control values. This maternal metabolic defect was associated with decrease in fetal weight and delayed ossification. In addition, scapular malformations were observed in four fetuses from three litters. The malformations could have been caused by the same mechanism as holoprosencephaly after early treatment with AY9944. These cholesterol-deficiency-based malformations could have a common cause in the abnormal expression of Hedgehog or other developmental gene proteins, and may thus explain various congenital polymalformative syndromes in humans, including SLOS.

Chondroadherin expression changes in skeletal development

Chondroadherin is a cartilage protein with cell binding properties. The expression of chondroadherin was studied in rat tissues and during postnatal femoral head development. For design of oligonucleotide probes and primers a 1664 bp, full length, rat chondroadherin cDNA was isolated from a rat chondrosarcoma library and sequenced. Northern blot analysis showed chondroadherin mRNA to be present in femoral head and rib cartilage, as well as in tendon. More sensitive reverse-transcriptase PCR additionally identified the mRNA in calvaria, long bone and bone marrow. Localization of chondroadherin by immunocytochemistry in the developing femoral head from postnatal day 14 to day 60 showed presence of the protein in cartilaginous regions. With increasing age a very distinct localization of chondroadherin was seen in the territorial matrix around late proliferative cells in the growth plate as well as in the developing articular cartilage in the maturing femoral head. Localization of chondroadherin mRNA by in situ hybridization was in agreement with immunocytochemistry with strong hybridization signals in late proliferative cells in the growth plate. In the articular cartilage the expression was restricted to cells in the lower regions. A three-fold increase of cartilage chondroadherin content in the growing femoral head was demonstrated by Western blot analysis. The high expression of this cell binding protein in a dynamic region of cartilage suggests an important role for chondroadherin in the regulation of chondrocyte growth and proliferation.

Control of somite patterning by Sonic hedgehog and its downstream signal response genes

In the avian embryo, previous work has demonstrated that the notochord provides inductive signals to activate myoD and pax1 regulatory genes, which are expressed in the dorsal and ventral somite cells that give rise to myotomal and sclerotomal lineages. Here, we present bead implantation and antisense inhibition experiments that show that Sonic hedgehog is both a sufficient and essential notochord signal molecule for myoD and pax1 activation in somites. Furthermore, we show that genes of the Sonic hedgehog signal response pathway, specifically patched, the Sonic hedgehog receptor, and gli and gli2/4, zinc-finger transcription factors, are activated in coordination with somite formation, establishing that Sonic hedgehog response genes play a regulatory role in coordinating the response of somites to the constitutive notochord Sonic hedgehog signal. Furthermore, the expression of patched, gli and gli2/4 is differentially patterned in the somite, providing mechanisms for differentially transducing the Sonic hedgehog signal to the myotomal and sclerotomal lineages. Finally, we show that the activation of gli2/4 is controlled by the process of somite formation and signals from the surface ectoderm, whereas upregulation of patched and activation of gli is controlled by the process of somite formation and a Sonic hedgehog signal. The Sonic hedgehog signal response genes, therefore, have important functions in regulating the initiation of the Sonic hedgehog response in newly forming somites and in regulating the patterned expression of myoD and pax1 in the myotomal and sclerotomal lineages following somite formation.

The patched gene in development and cancer

The cloning of vertebrate homologues of the Drosophila segment polarity gene patched has led to confirmation of a role for the multipass transmembrane protein which it encodes as a receptor for secreted signalling proteins of the Hedgehog family. In addition, human patched has been identified as a tumour suppressor gene implicated in basal cell carcinomas and medullablastomas.

Recapitulation of signals regulating embryonic bone formation during postnatal growth and in fracture repair

A number of proteins have recently been identified which play roles in regulating bone development. One important example is Indian hedgehog (Ihh) which is secreted by the prehyprtrophic chondrocytes. Ihh acts as an activator of a second secreted factor, parathyroid hormone-related protein (PTHrP), which, in turn, negatively regulates the rate of chondrocyte differentiation. Here we examine the expression of these genes and their molecular targets during different stages of bone development. In addition to regulating PTHrP expression in the perichondrium, we find evidence that Ihh may also act on the chondrocytes themselves at particular stages. As bone growth continues postnatally in mammals and the developmental process is reactivated during fracture repair, understanding the molecular basis regulating bone development is of medical relevance. We find that the same molecules that regulate embryonic endochondral ossification are also expressed during postnatal bone growth and fracture healing, suggesting that these processes are controlled by similar mechanisms.

Role of the subchondral vascular system in endochondral ossification: endothelial cells specifically derepress late differentiation in resting chondrocytes in vitro

Endochondral ossification in growth plates proceeds through several consecutive steps of late cartilage differentiation leading to chondrocyte hypertrophy, vascular invasion, and, eventually, to replacement of the tissue by bone. It is well established that the subchondral vascular system is pivotal in the regulation of this process. Cells of subchondral blood vessels act as a source of vascular invasion and, in addition, release factors influencing growth and differentiation of chondrocytes in the avascular growth plate. To elucidate the paracrine contribution of endothelial cells we studied the hypertrophic development of resting chondrocytes from the caudal third of chick embryo sterna in co-culture with endothelial cells. The design of the experiments prevented cell-to-cell contact but allowed paracrine communication between endothelial cells and chondrocytes. Under these conditions, chondrocytes rapidly became hypertrophied in vitro and expressed the stage-specific markers collagen X and alkaline phosphatase. This development also required signaling by thyroid hormone in synergy. Conditioned media could replace the endothelial cells, indicating that diffusible factors mediated this process. By contrast, smooth muscle cells, fibroblasts, or hypertrophic chondrocytes did not secrete this activity, suggesting that the factors were specific for endothelial cells. We conclude that endochondral ossification is under the control of a mutual communication between chondrocytes and endothelial cells. A finely tuned balance between chondrocyte-derived signals repressing cartilage maturation and endothelial signals promoting late differentiation of chondrocytes is essential for normal endochondral ossification during development, growth, and repair of bone. A dysregulation of this balance in permanent joint cartilage also may be responsible for the initiation of pathological cartilage degeneration in joint diseases.

Expression profile of Gli family members and Shh in normal and mutant mouse limb development

Gli genes represent a small family, encoding zinc-finger proteins of the Krüppel-type. The family consists of Gli(1), Gli2, and Gli3, all of which are expressed in the developing mouse limb bud. To assess the role of the Gli family and Sonic hedgehog (Shh) in mouse limb development, we compared the expression domains of all three Gli genes and of Shh. Although each Gli gene has its own distinct expression pattern in limb buds, at 10.5-11.5 dpc all three genes were found not to be expressed in the posterior region, the presumptive Shh expression domain. This transient mutually exclusive expression suggested a potential interaction between Gli genes and Shh. To address this matter, we analysed the expression of Gli genes and Shh in two polydactyly mouse mutants, Extra toes (Xt) and Hemimelic-extra toes (Hx) which express Shh ectopically in the anterior region of the limb field. Since Xt mice lack Gli3 expression, the ectopic Shh expression is genetically linked to the absence of Gli3. In Hx mice we found a down-regulation of Gli3 in the anterior region of the limb bud. In both mutants Gli2 expression pattern was not altered, whereas Gli1 expression was anteriorly up-regulated adjacent to the ectopic Shh domain. These results strongly suggest a positive regulation of Gli1 by Shh and a negative interaction between Shh and Gli3.

Effect of osteogenic protein-1 on the development and mineralization of primary cultures of avian growth plate chondrocytes: modulation by retinoic acid

Osteogenic protein-1 (OP-1), a member of the TGF-beta family of proteins, induces endochondral bone formation. Here we studied the effect of OP-1 on the development of primary cultures of avian growth plate (GP) chondrocytes in either serum-free or serum-containing medium, in the absence or presence of retinoic acid (RA). OP-1 was added on day 7 of culture and continued for 7 days, or until the cultures were harvested, typically on day 21. Alone, OP-1 caused approximately 2-fold increase in proteoglycan synthesis into both the medium and the cell:matrix layer. Additionally, OP-1 caused a dosage-dependent increase in alkaline phosphatase (ALP) activity, and an increase in protein, when given from days 7-14 and examined on day 14. This stimulation was greater in cells grown in serum-free than in serum-containing media (3-5-fold vs. 2-3-fold increase in ALP; approximately 40% vs. approximately 20% increase in protein). Such stimulation of ALP activity and proteoglycan (PG) synthesis in cultured GP cells indicates that OP-1 elicits differentiation of chondrocytes. OP-1 minimally affected cell division (DNA content); however, a slight increase was seen when examined early in the culture. Alone, OP-1 increased mineral (Ca and Pi) content of the cultures by approximately 2-fold in both types of media. As early as day 14, clusters of mineral encircled many of the OP-1 treated cells. Thus, as in vivo, OP-1 strongly promoted mineral formation by the cultured GP chondrocytes. When present together, OP-1 and RA generally blocked the action of the other. Separately OP-1 and RA each stimulated protein synthesis, ALP activity, and Ca2+ deposition; together they were inhibitory to each. Also, RA blocked the stimulation of PG synthesis induced by OP-1; whereas OP-1 decreased cell division engendered by RA. Thus, this GP chondrocyte culture system is a good model for studying factors that influence differentiation and mineral deposition during bone growth in vivo.

Targeted expression of constitutively active receptors for parathyroid hormone and parathyroid hormone-related peptide delays endochondral bone formation and rescues mice that lack parathyroid hormone-related peptide

Mice in which the genes encoding the parathyroid hormone (PTH)-related peptide (PTHrP) or the PTH/PTHrP receptor have been ablated by homologous recombination show skeletal dysplasia due to accelerated endochondral bone formation, and die at birth or in utero, respectively. Skeletal abnormalities due to decelerated chondrocyte maturation are observed in transgenic mice where PTHrP expression is targeted to the growth plate, and in patients with Jansen metaphyseal chondrodysplasia, a rare genetic disorder caused by constitutively active PTH/PTHrP receptors. These and other findings thus indicate that PTHrP and its receptor are essential for chondrocyte differentiation. To further explore the role of the PTH/PTHrP receptor in this process, we generated transgenic mice in which expression of a constitutively active receptor, HKrk-H223R, was targeted to the growth plate by the rat alpha1 (II) collagen promoter. Two major goals were pursued: (i) to investigate how constitutively active PTH/PTHrP receptors affect the program of chondrocyte maturation; and (ii) to determine whether expression of the mutant receptor would correct the severe growth plate abnormalities of PTHrP-ablated mice (PTHrP-/-). The targeted expression of constitutively active PTH/PTHrP receptors led to delayed mineralization, decelerated conversion of proliferative chondrocytes into hypertrophic cells in skeletal segments that are formed by the endochondral process, and prolonged presence of hypertrophic chondrocytes with delay of vascular invasion. Furthermore, it corrected at birth the growth plate abnormalities of PTHrP-/- mice and allowed their prolonged survival. "Rescued" animals lacked tooth eruption and showed premature epiphyseal closure, indicating that both processes involve PTHrP. These findings suggest that rescued PTHrP-/- mice may gain considerable importance for studying the diverse, possibly tissue-specific role(s) of PTHrP in postnatal development.

Goosecoid misexpression alters the morphology and Hox gene expression of the developing chick limb bud

The homeobox-containing gene goosecoid (gsc) has been implicated in a variety of embryonic processes from gastrulation to rib patterning. We have analyzed the role it plays during chick limb development. Expression is initially observed at stage 20 in a proximal-anterior-ventral domain of the early limb bud which expands during subsequent stages. Later in limb development a second domain of expression appears distally which resolves to regions which surround the condensing cartilage. In order to understand the function of gsc in limb development, we have examined the effect of misexpressing gsc throughout the limb. Two striking phenotypes are observed. The first, evident at stage 24, is an alteration in the angle of femur outgrowth from the main body axis. The second, which can be detected at day 10 of development, is an overall decrease in the size of the limb with bones that are small, misshapen and bent. These phenotypes correlate with a decrease in levels of Hox gene expression in gsc-infected limb buds. From these results we suggest that gsc may normally function to regulate growth and patterning of the limb, perhaps through regulation of Hox gene expression.

Parathyroid hormone-related protein (PTHrP) as a regulating factor of endochondral bone formation

Parathyroid hormone-related protein (PTHrP) was first identified as a pathogenetic factor for the hypercalcemia of malignancy. Recently PTHrP is focused as a physiological paracrine factor regulating cell proliferation and differentiation in many tissues during fetal and postnatal growth. Evidence for the skeletal origin of PTHrP comes from several sources and targeted disruption of the PTHrP gene in mice has resulted in a phenotype with accelerated endochondral bone formation, suggesting PTHrP as a factor regulating chondrocyte differentiation. Indian hedgehog, one of the conserved family of hedgehog regulating segmentation of Drosophila, is found to be an upstream factor of PTHrP in a regulating pathway of chondrocyte differentiation. Moreover, Bcl-2, a protein that controls programmed cell death in several cell types, is suggested to lie downstream of PTHrP in this pathway. A point mutation of PTH/PTHrP receptor is identified in a patient with Jansen-type metaphyseal chondrodysplasia and constitutive, ligand independent activation is indicated in this mutant receptor.

Expression of indian hedgehog, bone morphogenetic protein 6 and gli during skeletal morphogenesis

A complex signaling pathway involving members of the Hedgehog, Bone morphogenetic protein (Bmp) and Gli families regulates early patterning events in fetal skeletogenesis (Hui and Joyner, 1993. A mouse model of Greig cephalopolysyndactyly syndrome: the extra-toes mutation contains an intragenic deletion of the Gli3 gene. Nat. Genet. 3, 241-246; Bitgood and McMahon, 1995. Hedgehog and Bmp genes are coexpressed at many diverse sites of cell-cell interaction in the mouse embryo. Dev. Biol. 172, 126-138; Lanske et al., 1996. PTH/PTHrP receptor in early development and Indian hedgehog-regulated bone growth. Science 273, 663-666; Vortkamp et al., 1996. Regulation of rate of cartilage differentiation by Indian hedgehog and PTH-related protein. Science 273, 613-622). Hedgehog genes encode secreted proteins that mediate patterning and growth through the induction of secondary signals (reviewed in Hammerschmidt et al., 1997. The world according to hedgehog. Trends Genet. 13, 14-21). Two potential targets of Ihh are bmp6 and gli (Johnson et al., 1995. Patched overexpression alters wing disc size and pattern: transcriptional and post-transcriptional effects on hedgehog targets. Development 121, 4161-4170; Dominguez et al., 1996. Sending and receiving the hedgehog signal: control by the Drosophila Gli protein Cubitus interruptus. Science 272, 1621-1625; Marigo et al., 1996. Sonic hedgehog differentially regulates expression of GLI and GLI3 during limb development. Dev. Biol. 180, 273-283). We investigated the molecular similarities and differences between fetal and postnatal skeletal development by analyzing the coincident and complimentary expression domains of indian hedgehog (ihh), bmp6 and gli in adjacent sections throughout the process of skeletogenesis. In almost all of the skeletal tissues examined, the expression domains of ihh and bmp6 were adjacent to one another and this region was surrounded by gli-expressing cells. These observations are in keeping with the proposed function of gli as a negative regulator of Ihh signaling and the induction of Bmps by Hedgehog proteins (Roberts et al., 1995. Sonic hedgehog is an endodermal signal inducing Bmp-4 and Hox genes during induction and regionalization of the chick hindgut. Development 121, 3163-3174; Kawakami et al., 1996. BMP signaling during bone pattern determination in the developing limb. Development 122, 3557-3566). By puberty, ihh, bmp6 and gli transcripts were no longer detected in the growth plate, despite the fact that physeal chondrocytes continued to hypertrophy and differentiate. Although bmp6 was expressed, ihh transcripts were not found in primordia of intramembranous bones, nor in cells lining the future articular surfaces. Collectively our findings suggest that ihh participates in, but is not required for chondrocyte hypertrophy.

Parathyroid hormone enhances early and suppresses late stages of osteogenic and chondrogenic development in a BMP-dependent mesenchymal differentiation system (C3H10T1/2)

The role of parathyroid hormone (PTH) upon osteo-/chondrogenic development was investigated in a bone morphogenetic protein (BMP)-dependent differentiation system involving the recombinant expression of BMPs in mesenchymal progenitor cells (C3H10T1/2). The constitutive expression of the PTH/PTH related protein receptor in this system led to a marked stimulation of chondrogenic and osteogenic development, while the permanent application of the ligand PTH(1-34) resulted in opposite responses by stimulating the early and suppressing the late stages of osteo-/chondrogenic development. These contrasting effects of PTH(1-34) on osteogenic and chondrocytic development seem, therefore, to depend on the cellular state of differentiation. The osteogenic and chondrocytic differentiation potential was substantiated histologically and by genetic analyses of marker genes like c-fos, alkaline phosphatase, osteocalcin, collagen alpha1(I), and collagen alpha1(II). The capacity to regulate osteogenic and chondrogenic development is located in the amino-terminal (1-34) region of the PTH molecule and seems to be mediated by the cyclic adenosine monophosphate signaling cascade. The application of other PTH domains like PTH(28-48) and PTH(53-84) did not exhibit significant responses. PTH acts as an essential factor in mesenchymal development controlling rates of differentiation into the osteogenic or chondrogenic lineage. The analysis of PTH effects in this system demonstrates the value of recombinant mesenchymal progenitor cells in the in vitro analysis of osteo-/chondrogenic development.

Parathyroid hormone-related protein (PTHrP) inhibits proliferation of Chinese hamster ovary cells stably transfected with a PTH/PTHrP receptor cDNA

The effect of parathyroid hormone-related protein (PTHrP) on proliferation was examined in Chinese hamster ovary (CHO) cells transfected with a PTH/PTHrP receptor encoding cDNA. Treatment with chicken PTHrP(1-36) (chPTHrP) lowered the cell number to 49 +/- 2% of untreated controls after 6 days with a half-maximal effect at 1 nM. The effect was mimicked by human (h) PTH(1-34), Br-cAMP and forskolin, but not by the receptor antagonist hPTH(3-34). Reduction of cell number was accompanied by increased PTH/PTHrP receptor expression and persistently activated adenylyl cyclase, together with altered cell morphology from epithelial to spindle-like forms, clustered growth and increased phosphate uptake. chPTHrP increased [Ca2+]i, but failed to activate membrane bound protein kinase C (PKC). Pretreatment with chPTHrP did not affect phorbol ester stimulated PKC activity, and chPTHrP or serum evoked increases in [Ca2+]i. In conclusion, PTHrP induced inhibition of proliferation and altered cell morphology is mediated by increased adenylyl cyclase rather than by PKC-dependent mechanisms in CHO cells.

Residues in the membrane-spanning and extracellular loop regions of the parathyroid hormone (PTH)-2 receptor determine signaling selectivity for PTH and PTH-related peptide

The parathyroid hormone (PTH)-2 receptor displays strong ligand selectivity in that it responds fully to PTH but not at all to PTH-related peptide (PTHrP). In contrast, the PTH-1 receptor (PTH/PTHrP receptor) responds fully to both ligands. Previously it was shown that two divergent residues in PTH and PTHrP account for PTH-2 receptor selectivity; position 23 (Trp in PTH and Phe in PTHrP) determines binding selectivity and position 5 (Ile in PTH and His in PTHrP) determines signaling selectivity. To identify sites in the PTH-2 receptor involved in discriminating between His5 and Ile5, we constructed PTH-2 receptor/PTH-1 receptor chimeras, expressed them in COS-7 cells, and tested for cAMP responsiveness to [Trp23] PTHrP-(1-36), and to the nondiscriminating peptide [Ile5, Trp23]PTHrP-(1-36) (the Phe23 --> Trp modification enabled high affinity binding of each ligand to the PTH-2 receptor). The chimeras revealed that the membrane-spanning/loop region of the receptor determined His5/Ile5 signaling selectivity. Subsequent analysis of smaller cassette substitutions and then individual point mutations led to the identification of two single residues that function as major determinants of residue 5 signaling selectivity. These residues, Ile244 at the extracellular end of transmembrane helix 3, and Tyr318 at the COOH-terminal portion of extracellular loop 2, are replaced by Leu and Ile in the PTH-1 receptor, respectively. The results thus indicate a functional interaction between two residues in the core region of the PTH-2 receptor and residue 5 of the ligand.

Osteopetrosis in mice lacking NF-kappaB1 and NF-kappaB2

The nfkb1 and nfkb2 genes encode closely related products regulating immune and inflammatory responses. Their role during development and differentiation remains unclear. The generation of nfkb1 null mice (p50-/-) resulted in altered immune responses, but had no effect on development. Similarly, nfkb2 knockout mice (p52-/-) did not show developmental defects (J.C. et al., manuscript submitted). We have investigated the potential for in vivo compensatory functions of these genes by generating double-knockout mice. The surprising result was that the animals developed osteopetrosis because of a defect in osteoclast differentiation, suggesting redundant functions of NF-kappaB1 and NF-kappaB2 proteins in the development of this cell lineage. The osteopetrotic phenotype was rescued by bone marrow transplantation, indicating that the hematopoietic component was impaired. These results define a new mouse osteopetrotic mutant and implicate NF-kappaB proteins in bone development, raising new directions in the treatment of bone disorders.

Chondrocyte survival and differentiation in situ are integrin mediated

Chondrocytes in specific areas of the chick sternum have different developmental fates. Cephalic chondrocytes become hypertrophic and secrete type X collagen into the extracellular matrix prior to bone deposition. Middle and caudal chondrocytes remain cartilaginous throughout development and continue to secrete collagen types II, IX, and XI. The interaction of integrin receptors with extracellular matrix molecules has been shown to affect cytoskeleton organization, proliferation, differentiation, and gene expression in other cell types. We hypothesized that chondrocyte survival and differentiation including the deposition into interstitial matrix of type X collagen may be integrin receptor mediated. To test this hypothesis, a serum-free organ culture sternal model that recapitulates normal development and maintains the three-dimensional relationships of the tissue was developed. We examined chondrocyte differentiation by five parameters: type X collagen deposition into interstitial matrix, sternal growth, actin distribution, cell shape, and cell diameter changes. Additional sterna were analyzed for apoptosis using a fragmented DNA assay. Sterna were organ cultured with blocking antibodies specific for integrin subunits (alpha2, alpha3, or beta1). In the presence of anti-beta1 integrin (25 microg/ml, clone W1B10), type X collagen deposition into interstitial matrix and sternal growth were significantly inhibited. In addition, all chondrocytes were significantly smaller, the actin was disrupted, and there was a significant increase in apoptosis throughout the specimens. Addition of anti-alpha2 (10 microg/ml, clone P1E6) or anti-alpha3 (10 microg/ml, clone P1B5) integrin partially inhibited type X collagen deposition into interstitial matrix; however, sternal growth and cell size were significantly decreased. These data are the first obtained from intact tissue and demonstrate that the interaction of chondrocytes with extracellular matrix is required for chondrocyte survival and differentiation.

OP-1 (BMP-7) affects mRNA expression of type I, II, X collagen, and matrix Gla protein in ossifying long bones in vitro

In long bone development, a regulating role of OP-1 is suggested by the local correlated expression of both OP-1 ligand and OP-1 binding receptors in developing mouse hind limbs. OP-1 is expressed in the interdigital mesenchyme, whereas OP-1 binding receptors are found in the bordering perichondrium, and both OP-1 ligand and receptors are present in the zone of (pre)hypertrophic chondrocytes. We investigated the role of OP-1 in long bone development experimentally by treating organ cultures of embryonic mouse metatarsals with rhOP-1. The mRNA expression patterns of type I, II, X collagen, and matrix Gla protein (MGP) were studied using in situ hybridization and cell proliferation using [3H]thymidine and BrdU labeling. In the epiphyseal perichondrium, treatment with 40 ng/ml OP-1 enhanced cell proliferation after day 2, while 6-day treatment caused a shift in expression from type I collagen to type II collagen mRNA. This supports previous histochemical findings that OP-1 induced the transition of perichondrium into cartilage. In the center of the rudiment, OP-1 inhibited the expression of type X collagen mRNA, indicating inhibition of chondrocyte hypertrophy. An arrest of differentiation at the (pre)hypertrophic chondrocyte stage was also indicated by the large area of cells expressing MGP mRNA in the OP-1-treated rudiments. We conclude that OP-1 affected the expression of marker genes of chondrocyte differentiation by acting on two steps in endochondral ossification. First, cell proliferation was enhanced, particularly so in the perichondrium where cells started to express the chondrocyte phenotype. Second, the terminal differentiation of mature chondrocytes into hypertrophic chondrocytes was inhibited. These results, combined with the known pattern of OP-1 ligand and BMP receptor expression in the embryo, suggest that OP-1 plays a local role in the cascade of events during endochondral ossification.

Relationship between dose, distance and time in Sonic Hedgehog-mediated regulation of anteroposterior polarity in the chick limb

Anteroposterior polarity in the vertebrate limb is thought to be regulated in response to signals derived from a specialized region of distal posterior mesenchyme, the zone of polarizing activity. Sonic Hedgehog (Shh) is expressed in the zone of polarizing activity and appears to mediate the action of the zone of polarizing activity. Here we have manipulated Shh signal in the limb to assess whether it acts as a long-range signal to directly pattern all the digits. Firstly, we demonstrate that alterations in digit development are dependent upon the dose of Shh applied. DiI-labeling experiments indicate that cells giving rise to the extra digits lie within a 300 microm radius of a Shh bead and that the most posterior digits come from cells that lie very close to the bead. A response to Shh involves a 12–16 hour period in which no irreversible changes in digit pattern occur. Increasing the time of exposure to Shh leads to specification of additional digits, firstly digit 2, then 3, then 4. Cell marking experiments demonstrate that cells giving rise to posterior digits are first specified as anterior digits and later adopt a more posterior character. To monitor the direct range of Shh signalling, we developed sensitive assays for localizing Shh by attaching alkaline phosphatase to Shh and introducing cells expressing these forms into the limb bud. These experiments demonstrate that long-range diffusion across the anteroposterior axis of the limb is possible. However, despite a dramatic difference in their diffusibility in the limb mesenchyme, the two forms of alkaline phosphatase-tagged Shh proteins share similar polarizing activity. Moreover, Shh-N (aminoterminal peptide of Shh)-coated beads and Shh-expressing cells also exhibit similar patterning activity despite a significant difference in the diffusibility of Shh from these two sources. Finally, we demonstrate that when Shh-N is attached to an integral membrane protein, cells transfected with this anchored signal also induce mirror-image pattern duplications in a dose-dependent fashion similar to the zone of polarizing activity itself. These data suggest that it is unlikely that Shh itself signals digit formation at a distance. Beads soaked in Shh-N do not induce Shh in anterior limb mesenchyme ruling out direct propagation of a Shh signal. However, Shh induces dose-dependent expression of Bmp genes in anterior mesenchyme at the start of the promotion phase. Taken together, these results argue that the dose-dependent effects of Shh in the regulation of anteroposterior pattern in the limb may be mediated by some other signal(s). BMPs are plausible candidates.

Expression of a truncated, kinase-defective TGF-beta type II receptor in mouse skeletal tissue promotes terminal chondrocyte differentiation and osteoarthritis

Members of the TGF-beta superfamily are important regulators of skeletal development. TGF-betas signal through heteromeric type I and type II receptor serine/threonine kinases. When over-expressed, a cytoplasmically truncated type II receptor can compete with the endogenous receptors for complex formation, thereby acting as a dominant-negative mutant (DNIIR). To determine the role of TGF-betas in the development and maintenance of the skeleton, we have generated transgenic mice (MT-DNIIR-4 and -27) that express the DNIIR in skeletal tissue. DNIIR mRNA expression was localized to the periosteum/perichondrium, syno-vium, and articular cartilage. Lower levels of DNIIR mRNA were detected in growth plate cartilage. Transgenic mice frequently showed bifurcation of the xiphoid process and sternum. They also developed progressive skeletal degeneration, resulting by 4 to 8 mo of age in kyphoscoliosis and stiff and torqued joints. The histology of affected joints strongly resembled human osteo-arthritis. The articular surface was replaced by bone or hypertrophic cartilage as judged by the expression of type X collagen, a marker of hypertrophic cartilage normally absent from articular cartilage. The synovium was hyperplastic, and cartilaginous metaplasia was observed in the joint space. We then tested the hypothesis that TGF-beta is required for normal differentiation of cartilage in vivo. By 4 and 8 wk of age, the level of type X collagen was increased in growth plate cartilage of transgenic mice relative to wild-type controls. Less proteoglycan staining was detected in the growth plate and articular cartilage matrix of transgenic mice. Mice that express DNIIR in skeletal tissue also demonstrated increased Indian hedgehog (IHH) expression. IHH is a secreted protein that is expressed in chondrocytes that are committed to becoming hypertrophic. It is thought to be involved in a feedback loop that signals through the periosteum/ perichondrium to inhibit cartilage differentiation. The data suggest that TGF-beta may be critical for multifaceted maintenance of synovial joints. Loss of responsiveness to TGF-beta promotes chondrocyte terminal differentiation and results in development of degenerative joint disease resembling osteoarthritis in humans.

The matrix metalloproteinase-14 (MMP-14) gene is structurally distinct from other MMP genes and is co-expressed with the TIMP-2 gene during mouse embryogenesis

The matrix metalloproteinases (MMPs) are a family of zinc-containing matrix degrading endopeptidases. A subfamily of membrane type (MT) -MMPs has been described recently. We have determined the structure of the gene (Mmp14) encoding the first MT-MMP to be described, MT1-MMP (MMP-14), and mapped it to mouse chromosome 14. The mouse MMP-14 protein is encoded by ten exons. The novel C-terminal peptide domains of MMP-14 are encoded by a single large exon that also encodes the 3'-untranslated region. The structure of the exons encoding the catalytic domain and pro-domain of MMP-14 is distinct from previously described MMP genes, whereas the exons encoding the hemopexin-like domains are similar to those of most other MMP genes. Mmp14 and the gene for tissue inhibitor of metalloproteinases-2 (Timp2) show a temporally and spatially co-regulated expression during mouse development. They are co-expressed during vascular and urogenital development and during the development of osteocartilaginous and musculotendinous structures. The stringent co-expression of these two genes suggests common regulatory pathways that may have important functional implications for the activation of pro-gelatinase A in health and disease.

Further observations on programmed cell death in the epiphyseal growth plate: comparison of normal and dyschondroplastic epiphyses

The objective of the investigation was to provide information on apoptosis in the normal epiphysis and to assess apoptosis in the plate of the dyschondroplastic chick. Apoptosis was evaluated using two terminal deoxynucleotide transferase end-labeling procedures, DNA fragmentation and nuclear morphology. We found that there was a minimal level of apoptosis in the dyschondroplastic cartilage. In the tibial dyschondroplastic (TD) lesion itself, only about 3% of cells are positive in the articular and proliferative regions; 11% of prehypertrophic chondrocytes are stained by the end-labeling procedure, and most of the cells are localized around vascular channels at the calcifying front. This finding suggests that dyschondroplasia is linked to impairment of apoptosis, and as a result the tissue contains immature cells that have outlived their normal life span. In contrast, in the normal plate, we noted that when the proliferative period was complete, the cells became terminal transferase positive; in addition, chondrocytes in the normal plate exhibited DNA fragmentation. Semiquantitative analysis of stained chondrocytes in the growth plate indicate that in the proliferative zone 15.5% of cells are terminal deoxynucleotidyl transferase (TUNEL) positive; in contrast, 44% of postmitotic chondrocytes are stained by the TUNEL procedure. The presence of a sharp border between the pre- and postmitotic zones suggests that the stimulus for apoptosis is maturation dependent and reflects local metabolic control. We also examined apoptosis in metaphyseal osteoblasts. We found that adjacent to the epiphysis, many osteoblasts were undergoing apoptosis. In more mature sites in the metaphysis, there was less cell death, indicating that osteoblast apoptosis was delayed and cells were completing their normal life cycle. Although terminal transferase end-labeled cells were not seen in articular cartilage, we noted that fibroblasts, in the perichondrial ligament surrounding the articular as well as the epiphyseal regions of the plate, were undergoing apoptosis. Apoptosis at this site may be related to lateral expansion of the cartilages, reflect a high cell turnover rate at the junction between the tissues, and result from paracrine signals received from the underlying cartilage.

Rapid chondrocyte maturation by serum-free culture with BMP-2 and ascorbic acid

In serum-containing medium, ascorbic acid induces maturation of prehypertrophic chick embryo sternal chondrocytes. Recently, cultured chondrocytes have also been reported to undergo maturation in the presence of bone morphogenetic proteins or in serum-free medium supplemented with thyroxine. In the present study, we have examined the combined effect of ascorbic acid, BMP-2, and serum-free conditions on the induction of alkaline phosphatase and type X collagen in chick sternal chondrocytes. Addition of either ascorbate or rhBMP-2 to nonconfluent cephalic sternal chondrocytes produced elevated alkaline phosphatase levels within 24-72 h, and simultaneous exposure to both ascorbate and BMP yielded enzyme levels at least threefold those of either inducer alone. The effects of ascorbate and BMP were markedly potentiated by culture in serum-free medium, and alkaline phosphatase levels of preconfluent serum-free cultures treated for 48 h with BMP+ascorbate were equivalent to those reached in serum-containing medium only after confluence. While ascorbate addition was required for maximal alkaline phosphatase activity, it did not induce a rapid increase in type X collagen mRNA. In contrast, BMP added to serum-free medium induced a three- to fourfold increase in type X collagen mRNA within 24 h even in the presence of cyclohexamide, indicating that new protein synthesis was not required. Addition of thyroid hormone to serum-free medium was required for maximal ascorbate effects but not for BMP stimulation. Neither ascorbate nor BMP induced alkaline phosphatase activity in caudal sternal chondrocytes, which do not undergo hypertrophy during embryonic development. These results indicate that ascorbate+BMP in serum-free culture induces rapid chondrocyte maturation of prehypertrophic chondrocytes. The mechanisms for ascorbate and BMP action appear to be distinct, while BMP and thyroid hormone may share a similar mechanism for induction.

Rapid and long-term effects of PTH(1-34) on growth plate chondrocytes are mediated through two different pathways in a cell-maturation-dependent manner

The aims of this study were to clarify the role of cell maturation stage on chondrocyte response to parathyroid hormone (PTH) by examining the effect of PTH(1-34) on alkaline-phosphatase-specific activity (ALPase) of chondrocyte cultures at two distinct stages of maturation, and to determine the signaling pathways used by the cells to mediate this effect. Confluent, fourth passage rat costochondral resting zone (RC) and growth zone (GC) chondrocytes were used. ALPase was measured in the cell layer, as well as in matrix vesicles (MV) and plasma membranes (PM), after the addition of 10(-7) 10(-11) mol/L bovine PTH(1-34), the active peptide, or bovine PTH(3-34), the inactive peptide, to the cultures. PTH(1-34) increased ALPase in the GC cultures at two separate times: between 5 and 180 min, with maximal stimulation at 10 min, and 36 to 48 h. In contrast, PTH(3-34) had no effect. At 10 min and 48 h, PTH(1-34) produced a dose-dependent increase in ALPase of both MV and PM isolated from GC cultures. Addition of forskolin and IBMX to increase cAMP increased ALPase in GC cultures to a level similar to that seen after addition of PTH(1-34). In contrast, the addition of PTH(1-34) to RC cells only increased ALPase between 5 and 60 min, with peak activity at 10 min. As with GC, PTH increased ALPase in both MV and PM. Moreover, the addition of PTH(3-34) or forskolin and IBMX had no effect on ALPase in RC. PTH(1-34) had no effect on GC protein kinase C (PKC) activity; however, the addition of PTH(1-34) to RC caused a dose-dependent increase in PKC activity. H8, an inhibitor of PKA, had no effect on PTH-stimulated ALPase in RC cells, but inhibited the PTH-dependent response in GC cells. In contrast, chelerythrine, an inhibitor of PKC activity, inhibited PTH-stimulated ALPase in RC cells, but had no effect on PTH-stimulated ALPase in GC cells. This study shows that the effect of PTH(1-34) on RC and GC cells is maturation dependent in terms of time course and mechanism. Whereas both cell types exhibit a rapid response to PTH, only GC cells show a long-term response. In GC, the effects of PTH are associated with changes in cAMP and may also involve at least one other pathway, whereas, in RC, the PTH effects appear to be associated with changes in PKC.

Notochord induction of zebrafish slow muscle mediated by Sonic hedgehog

The patterning of vertebrate somitic muscle is regulated by signals from neighboring tissues. We examined the generation of slow and fast muscle in zebrafish embryos and show that Sonic hedgehog (Shh) secreted from the notochord can induce slow muscle from medial cells of the somite. Slow muscle derives from medial adaxial myoblasts that differentiate early, whereas fast muscle arises later from a separate myoblast pool. Mutant fish lacking shh expression fail to form slow muscle but do form fast muscle. Ectopic expression of shh, either in wild-type or mutant embryos, leads to ectopic slow muscle at the expense of fast. We suggest that Shh acts to induce myoblasts committed to slow muscle differentiation from uncommitted presomitic mesoderm.

Distinct roles of type I bone morphogenetic protein receptors in the formation and differentiation of cartilage

The bone morphogenetic proteins (BMPs), TGF beta superfamily members, play diverse roles in embryogenesis, but how the BMPs exert their action is unclear and how different BMP receptors (BMPRs) contribute to this process is not known. Here we demonstrate that the two type I BMPRs, BMPR-IA and BMPR-IB, regulate distinct processes during chick limb development. BmpR-IB expression in the embryonic limb prefigures the future cartilage primordium, and its activity is necessary for the initial steps of chondrogenesis. During later chondrogenesis, BmpR-IA is specifically expressed in prehypertrophic chondrocytes. BMPR-IA regulates chondrocyte differentiation, serving as a downstream mediator of Indian Hedgehog (IHH) function in both a local signaling loop and a longer-range relay system to PTHrP. BMPR-IB also regulates apoptosis: Expression of activated BMPR-IB results in increased cell death, and we showed previously that dominant-negative BMPR-IB inhibits apoptosis. Our studies indicate that in TGF beta signaling systems, different type I receptor isoforms are dedicated to specific functions during embryogenesis.

Induction of osteogenic differentiation by hedgehog proteins

In this study we asked whether members of the hedgehog gene family are involved in osteogenesis. C3H10T1/2 cells and MC3T3-E1 cells expressed the putative hedgehog receptor patched (Ptc) gene. Medium conditioned by chicken embryo fibroblast cultures expressing either Indian hedgehog or Sonic hedgehog stimulated alkaline phosphatase (APase) activity in cultures of the mouse mesenchymal cell line C3H10T1/2 and the osteoblastic cell line MC3T3-E1. These stimulatory effects were synergistically enhanced by bone morphogenetic protein-2 (BMP-2). Treatment with the amino-terminal portion of recombinant Sonic hedgehog proteins (rShh-N) up-regulated the expression of the Ptc gene within 12 h and increased production of APase in C3H10T1/2. rShh-N and BMP-2 also synergistically stimulated APase activity. rShh-N treatment did not affect the expression levels of Bmp-2, -4, -5, -6 and -7 genes. These findings indicate that hedgehog proteins directly act on osteogenic precursor cells and osteoblasts and stimulate osteogenic differentiation of these cells in co-operation with BMPs.

Evidence for the involvement of the Gli gene family in embryonic mouse lung development

Murine Gli, Gli2, and Gli3 are zinc finger genes related to Drosophila cubitus interuptus, a component of the hedgehog signal transduction pathway. In the embryonic lung, all three Gli genes are strongly expressed at the pseudoglandular stage, in distinct but overlapping domains of the mesoderm. Expression of Gli and Gli3, but not of Gli2, is subsequently downregulated at the canalicular stage, coincident with a decline in the expression of sonic hedgehog (Shh) and the hedgehog receptor gene, patched (Ptc). Overexpression of Shh in the lung results in increased levels of Ptc mRNA. Gli, but not Gli2, is also upregulated, suggesting a differential involvement of the Gli genes in the regulation of Ptc by SHH during lung development. Gli3 is not upregulated by Shh overexpression. However, its importance for lung development is shown by the finding that Gli3XtJ embryos, homozygous for a mutation involving a deletion of the Gli3 gene, have a stereotypic pattern of abnormalities in lung morphogenesis. The pulmonary defects in these embryos, consisting of localized shape changes and size reductions, correlate with normal Gli3 expression. Thus, our data indicate that one of the Gli genes, Gli3, is essential for normal lung development, and that another, Gli, can be placed downstream of Shh signaling in the lung.

Cellular hypertrophy and calcification of embryonal carcinoma-derived chondrogenic cell line ATDC5 in vitro

During the process of endochondral bone formation, proliferating chondrocytes give rise to hypertrophic cells, which then deposit a mineralized matrix to form calcified cartilage prior to replacement by bone. Previously, we reported that a clonal cell line, ATDC5, undergoes efficient chondrogenic differentiation through a cellular condensation stage. Here we report that the differentiated ATDC5 cells became hypertrophic at the center of cartilage nodules, when the cells ceased to grow. Formation of hypertrophic chondrocytes took place in association with type X collagen gene expression and a dramatic elevation of alkaline phosphate (ALPase) activity. After 5 weeks of culture, mineralization of the culture could be discerned as Alizarin red-positive spots, which spread throughout the nodules even in the absence of beta-glycerophosphate. Electron microscopy and electron probe microanalysis revealed that calcification was first initiated at matrix vesicles in the territorial matrix and that it advanced progressively along the collagen fibers in a manner similar to that which occurs in vivo. The infrared spectrum of the mineralized nodules indicated two absorption doublets around 1030 cm-1 and 600 cm-1, which are characteristic of apatitic mineral. Calcifying cultures of ATDC5 cells retained responsiveness to parathyroid hormone (PTH): PTH markedly inhibited elevation of ALPase activity and calcification in the culture in a dose-dependent manner. Thus, we demonstrated that ATDC5 cells keep track of the multistep differentiation process encompassing the stages from mesenchymal condensation to calcification in vitro. ATDC5 cells provide an excellent model to study the molecular mechanism underlying regulation of cartilage differentiation during endochondral bone formation.

Parallel expression of Sox9 and Col2a1 in cells undergoing chondrogenesis

To assess the role of the transcription factor Sox9 in cartilage formation we have compared the expression pattern of Sox9 and Col2a1 at various stages of mouse embryonic development. Expression of Col2a1 colocalized with expression of Sox9 in all chondroprogenitor cells. In the sclerotomal compartment of somites the onset of Sox9 expression preceded that of Col2a1. A perfect correlation was also seen between high levels of Sox9 expression and high levels of Col2a1 expression in chondrocytic cells. However, no Sox9 expression was detected in hypertrophic chondrocytes; only low levels of Col2a1 RNA were found in the upper hypertrophic zone. Coexpression of Sox9 and Col2a1 was also seen in the notochord. At E11.5 Sox9 expression in the brain and spinal neural tube was more widespread than that of Col2a1 although at E14.5 Sox9 and Col2a1 transcripts were colocalized in discrete areas of the brain. Distinct differences between Sox9 and Col2a1 expression were observed in the otic vesicle at E11.5. At E8.5, expression of Sox9 but not of Col2a1 was seen in the dorsal tips of the neural folds and after neural tube closure also in presumptive crest cells emigrating from the dorsal pole of the neural tube. No Col2a1 expression was detected in gonadal ridges in which high levels of Sox9 expression were detected. Together with our previous results showing that the chondrocyte-specific enhancer element of the Col2a1 gene is a direct target for Sox9, these results suggest that Sox9 plays a major role in expression of Col2a1. The correlation between high expression levels of Sox9 and high expression levels of Col2a1 in chondrocytes suggests the hypothesis that high levels of Sox9 are needed for full expression of the chondrocyte phenotype; lower levels of Sox9 such as in neuronal tissues which are also associated with lower expression levels of Col2a1 would be compatible with other cell specifications.

Recent advances in molecular analysis of skeletal dysplasia

Recent advances in molecular and cellular biology have contributed significantly to the elucidation of the pathogenesis of many kinds of skeletal dysplasia. The number of skeletal dysplastic diseases that are identified to have associated abnormalities in genes has increased. Some diseases such as achondroplasia, thanatophoric dysplasia and hypochondroplasia are shown to be allelic. In addition to those diseases associated with mutations of the fibroblast growth factor receptor 3 gene, the abnormalities in collagen, Gs alpha, vitamin D receptor and tissue nonspecific alkaline phosphatase genes are briefly reviewed in this article.

Hypoparathyroidism and pseudohypoparathyroidism

Hypoparathyroidism is a clinical disorder characterized by hypocalcemia and hyperphosphatemia in the absence of renal failure and hypomagnesemia. The causes of hypoparathyroidism can be classified as two groups: (i) insufficient parathyroid hormone (PTH) secretion in relation to the serum calcium level (hypoparathyroidism); and (ii) impaired PTH action (pseudohypoparathyroidism). The main emphasis in this report is to distinguish subgroups based on the etiology and pathophysiology of the various aspects of the disease.

A role for Indian hedgehog in extraembryonic endoderm differentiation in F9 cells and the early mouse embryo

Hedgehog genes in Drosophila and vertebrates control patterning of a number of different structures during embryogenesis. They code for secreted signaling proteins that are cleaved into an active aminopeptide and a carboxypeptide. The aminopeptide can mediate local and long range events and can act as a morphogen, inducing differentiation of distinct cell types in a concentration-dependent manner. We demonstrate here that the expression of Indian hedgehog mRNA and protein is upregulated dramatically as F9 cells differentiate in response to retinoic acid, into either parietal endoderm or embryoid bodies, containing an outer visceral endoderm layer. The ES cell line D3 forms embryoid bodies in suspension culture without addition of retinoic acid and also upregulates Indian hedgehog expression. RT-PCR analysis of blastocyst outgrowth cultures demonstrates that whereas little or no Indian hedgehog message is present in blastocysts, significant levels appear upon subsequent days of culture, coincident with the emergence of parietal endoderm cells. In situ hybridization analysis for Indian hedgehog mRNA expression demonstrates the presence of elevated levels of message in the outer visceral endoderm cells relative to the core cells in mature embryoid bodies and in the visceral endoderm of Day 6.5 embryos. Whole-mount in situ hybridization analysis of Day 7.5 and 8.5 embryos indicates that Indian hedgehog expression is highest in the visceral yolk sac at this stage. F9 cell lines expressing a full length Indian hedgehog cDNA express a number of characteristics of differentiated cells, in the absence of retinoic acid. Taken together, these data suggest that Indian hedgehog is involved in mediating differentiation of extraembryonic endoderm during early mouse embryogenesis.

Control of cell pattern in the neural tube by the zinc finger transcription factor and oncogene Gli-1

Sonic hedgehog (Shh) is a putative morphogen secreted by the floor plate and notochord, which specifies the fate of multiple cell types in the ventral aspect of the vertebrate nervous system. Since in Drosophila the actions of Hh have been shown to be transduced by Cubitus interruptus (Ci), a zinc finger transcription factor, we examined whether a vertebrate homolog of this protein can mediate the functions of Shh in the vertebrate nervous system. Here, we demonstrate that expression of Gli-1, one of three vertebrate homologs of Ci, can be induced by Shh in the neural tube. Further, ectopic expression of Gli-1 in the dorsal midbrain and hindbrain of transgenic mice mimics the effects of ectopically expressed Shh-N, leading to the activation of ventral neural tube markers such as Ptc, HNF-3beta, and Shh; to the suppression of dorsal markers such as Pax-3 and AL-1; and to the formation of ectopic dorsal clusters of dopaminergic and serotonergic neurons. These findings demonstrate that GLI-1 can reproduce the cell patterning actions of Shh in the developing nervous system and provide support for the hypothesis that it is a mediator of the Shh signal in vertebrates.

Expression of contact, a new zebrafish DVR member, marks mesenchymal cell lineages in the developing pectoral fins and head and is regulated by retinoic acid

Contact, a new zebrafish transforming growth factor-beta (TGF-beta) member is most closely related to mouse GDF5 and to human CDMP-1 responsible, when mutated, for limb brachypodism phenotype and Hunter-Thompson syndrome, respectively. Contact exhibits a dynamic spatial expression pattern in the pharyngeal arches and the pectoral fin buds that much prefigures cartilage formation. Within the fin buds, contact expression is detected in the proximal mesenchyme from which the endoskeleton will develop. Exogeneously applied retinoic acid (RA) induces duplication of the pectoral fin rudiment in zebrafish embryos as well as contact expression along the proximal margin of the fin mesenchyme showing that both endoskeleton and exoskeleton can be duplicated.

Cloning of a mouse smoothened cDNA and expression patterns of hedgehog signalling molecules during chondrogenesis and cartilage differentiation in clonal mouse EC cells, ATDC5

Hedgehog (hh) family proteins appear to use the conserved targets in their signalling pathway including Patched (Ptc), Smoothened (Smo), and Gli. Although Indian hedgehog (Ihh) plays an important role in endochondral bone formation, the involvement of hh signalling molecules in skeletogenesis is unknown. We cloned a mouse (m) Smo cDNA and studied the expression patterns of Ihh, Ptc, Smo, and Gli mRNAs in mouse chondrogenic EC cells, ATDC5. The deduced amino acid sequence of mSmo consisted of 793 amino acids and was 98 and 93% homologous to the rat (r) Smo and human (h) Smo, respectively. In ATDC5 cells, the expression of Ihh mRNA paralleled that of type X collagen mRNA. Smo, Ptc, and Gli mRNAs were constitutively expressed throughout chondrogenesis and the subsequent cartilage differentiation processes except for the transient decrease in Ptc mRNA at the cellular condensation stage. Our data suggest that hh signalling molecules may be involved in chondrogenesis and cartilage differentiation in ATDC5 cells.

Mutations involving the transcription factor CBFA1 cause cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal-dominant condition characterized by hypoplasia/aplasia of clavicles, patent fontanelles, supernumerary teeth, short stature, and other changes in skeletal patterning and growth. In some families, the phenotype segregates with deletions resulting in heterozygous loss of CBFA1, a member of the runt family of transcription factors. In other families, insertion, deletion, and missense mutations lead to translational stop codons in the DNA binding domain or in the C-terminal transactivating region. In-frame expansion of a polyalanine stretch segregates in an affected family with brachydactyly and minor clinical findings of CCD. We conclude that CBFA1 mutations cause CCD and that heterozygous loss of function is sufficient to produce the disorder.

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

Indian hedgehog (Ihh) was recently reported to be expressed in chondrocytes and to regulate chondrocyte differentiation. This report examined the expression of Ihh in osteoblastic cells and its regulation by calcitropic cytokines. We found that Ihh messenger RNA (mRNA) was expressed as a single 2.5-kilobase band at a modest level in rat osteoblastic osteosarcoma ROS17/2.8 cells. In sharp contrast to the previous observation of dpp regulation of hedgehog expression in Drosophila embryos, bone morphogenetic protein-2 did not affect Ihh expression in these cells. On the other hand, treatment with 2 ng/ml transforming growth factor-beta1 (TGFbeta1) increased the steady state level of Ihh mRNA 2- to 4-fold. Western blot analysis of the cell lysates using antisera also showed enhancement of the Ihh protein level by TGFbeta1 treatment. The effect of TGFbeta1 on Ihh mRNA abundance started within 3 h, peaked at 24 h and lasted at least 48 h after the initiation of the treatment. The effect of TGFbeta1 on the increase in Ihh mRNA was dose dependent, starting at 0.2 ng/ml and saturating at 2 ng/ml. Neither actinomycin D nor cycloheximide blocked this effect. Experiments using 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole showed an enhancement of Ihh mRNA stability by TGFbeta1, indicating the presence of posttranscriptional regulation. We then examined the effects of TGFbeta1 on Ihh mRNA in osteoblast-enriched cells isolated from neonatal rat calvariae. TGFbeta1 also enhanced Ihh mRNA expression in these cells. Our data indicate for the first time that Ihh is one of the members of the cytokines produced by osteoblastic cells and that the expression of Ihh is regulated posttranscriptionally by TGFbeta.

Post-translational processing and renal expression of mouse Indian hedgehog

The full-length mouse Indian hedgehog (Ihh) cDNA was cloned from an embryonic 17.5-day kidney library and was used to study the post-translational processing of the peptide and temporal and spatial expression of the transcript. Sequence analysis predicted two putative translation initiation sites. Ihh translation was initiated at both initiation sites when expressed in an in vitro transcription/translation system. Expression of an Ihh mutant demonstrated that the internal translation initiation site was sufficient to produce the mature forms of Ihh. Ihh post-translational processing proceeded in a fashion similar to Sonic and Drosophila hedgehog; the unprocessed form underwent signal peptide cleavage as well as internal proteolytic processing to form a 19-kDa amino-terminal peptide and a 26-kDa carboxyl-terminal peptide. This processing required His313 present in a conserved serine protease motif. Ihh transcript was detected by in situ RNA hybridization as early as 10 days postcoitum (dpc) in developing gut, as early as 14.5 dpc in the cartilage primordium, and in the developing urogenital sinus. In semiquantitative reverse transcription-polymerase chain reaction experiments, Indian hedgehog transcript was first detected in the mouse metanephros at 14.5 dpc; transcript abundance increased with gestational age, becoming maximal in adulthood. In adult kidney, Ihh transcript was detected only in the proximal convoluted tubule and proximal straight tubule.