Msx2 Marks Spatially Restricted Populations of Mesenchymal Precursors.

Craniofacial development requires a set of patterning codes that define the identities of postmigratory mesenchymal cells in a region-specific manner, in which locally expressed morphogens, including fibroblast growth factors (FGFs) and bone morphogenetic proteins (BMPs), provide instructive cues. Msx2, a bona fide target of BMP signaling, is a transcription factor regulating Runx2 and osterix (Osx), whose mutations are associated with cranial deformities in humans. Here we show that Msx2 defines osteo-chondro precursor cells in specific regions of the craniofacial mesenchyme at the postmigratory stage, particularly in the mandibular process and the posterior cranial vault. Analysis of Msx2-creER mice revealed that early mesenchymal cells in proximity to the BMP4-expressing mesenchyme were marked upon tamoxifen injection, and their descendants contributed to diverse types of mesenchymal cells in the later stage, such as chondrocytes and perichondrial cells of the transient cartilage, as well as osteoblasts and suture mesenchymal cells. By contrast, Osx-creER marked osteoblast precursors at the later stage, and their descendants continued to become osteoblasts well into the postnatal stage. Therefore, Msx2 marks spatially restricted populations of mesenchymal precursor cells with diverse differentiation potential, suggesting that extrinsic molecular cues can dictate the nature of postmigratory mesenchymal cells in craniofacial development.

The transcriptional cofactor Lbh regulates angiogenesis and endochondral bone formation during fetal bone development.

Lbh is thought to act as a transcriptional cofactor and is highly conserved among species. Here we show that Lbh is expressed in chondrocytes, cells of the perichondrium, and the primary spongiosa in fetal growth plates of mice and chickens. Lbh overexpression in chick wings, using the RCAS-retroviral vector strategy, results in shortened skeletal elements and delayed hypertrophic chondrocyte maturation and bone formation. Additionally, osteoclast and endothelial cell invasion are delayed in the Lbh-overexpressing bones. Finally, we find a dramatic suppression of Runx2 and VEGF mRNAs in chondrocytes and osteoblasts that overexpress Lbh. Strikingly, this abnormal bone development in infected limbs can be rescued by concurrent overexpression of Runx2. These results suggest that during endochondral bone formation, Lbh may negatively regulate vascular invasion and formation of the early ossification center at least in part by interfering with Runx2 and/or VEGF expression.

Osteoblastic cells regulate the haematopoietic stem cell niche.

Stem cell fate is influenced by specialized microenvironments that remain poorly defined in mammals. To explore the possibility that haematopoietic stem cells derive regulatory information from bone, accounting for the localization of haematopoiesis in bone marrow, we assessed mice that were genetically altered to produce osteoblast-specific, activated PTH/PTHrP receptors (PPRs). Here we show that PPR-stimulated osteoblastic cells that are increased in number produce high levels of the Notch ligand jagged 1 and support an increase in the number of haematopoietic stem cells with evidence of Notch1 activation in vivo. Furthermore, ligand-dependent activation of PPR with parathyroid hormone (PTH) increased the number of osteoblasts in stromal cultures, and augmented ex vivo primitive haematopoietic cell growth that was abrogated by gamma-secretase inhibition of Notch activation. An increase in the number of stem cells was observed in wild-type animals after PTH injection, and survival after bone marrow transplantation was markedly improved. Therefore, osteoblastic cells are a regulatory component of the haematopoietic stem cell niche in vivo that influences stem cell function through Notch activation. Niche constituent cells or signalling pathways provide pharmacological targets with therapeutic potential for stem-cell-based therapies.

The murine gene encoding parathyroid hormone: genomic organization, nucleotide sequence and transcriptional regulation.

The type 1 parathyroid hormone receptor (PTHR1) binds, with equal affinity, two ligands with distinct biological functions: PTH, the major peptide hormone controlling calcium homeostasis, and the paracrine factor, PTH-related peptide (PTHrP), a local regulator of cellular proliferation and differentiation. To clarify the complexity of possible interactions between two distinct ligands, PTH and PTHrP, and their common receptor in the intact organism, and to identify as yet unrecognized roles for PTH in normal physiology, we have cloned and characterized the structural organization, nucleotide sequence and transcriptional regulation of the murine gene encoding PTH. One recombinant clone isolated from a mouse genomic library contained 14 kb of DNA, encompassing the entire Pth gene. The transcriptional unit spans 3.2 kb of genomic DNA and, analogous to the human PTH gene, it is interrupted by two introns. The deduced mRNA encodes the 115-amino acid precursor, preproPTH. Comparison of the murine preproPTH sequence with other mammalian forms of the protein shows it to be highly conserved and to share limited structural similarity to PTHrP at the amino-terminal region, a domain critical for binding and activation of their common receptor. Putative binding motifs for the transcription factors sex-determining region Y gene product, transcriptional repressor CDP, hepatic nuclear factor 3beta, GATA-binding factor 1, glucocorticoid receptor, SRY-related high mobility group box protein 5 and cAMP response element binding protein were identified in the 5' flanking region of the Pth gene. When placed upstream of a reporter gene, these sequences failed to confer transcriptional regulation in response to 1,25(OH)(2) vitamin D(3), but responded positively to the addition of isoproterenol and forskolin. Mutational analysis identified a cAMP-response element in the Pth promoter.

Fetal parathyroids are not required to maintain placental calcium transport.

We used Hoxa3 knockout mice and other mouse models to study the role of the fetal parathyroids in fetal calcium homeostasis. Hoxa3-null fetuses lack parathyroid glands, and absence of parathyroid hormone (PTH) was confirmed with a rodent PTH immunoradiometric assay. The ionized calcium level of Hoxa3-null fetuses was significantly lower than that of wild-type or heterozygous littermates or of the mother. Both the rate of placental calcium transfer and the plasma PTHrP level were normal in Hoxa3 mutants and their heterozygous siblings. Because we had previously observed an increase in placental calcium transfer in PTH/PTHrP receptor 1-null (Pthr1-null) fetuses, we assayed plasma PTHrP in those mice. Pthr1-null fetuses had plasma PTHrP levels 11-fold higher than those of their littermates. Northern analysis, immunohistochemical, and in situ hybridization studies of Pthr1-null fetuses indicated that liver and placenta had increased expression of PTHRP: In summary, loss of fetal parathyroids in Hoxa3-null fetuses caused marked hypocalcemia but did not alter placental calcium transfer or the circulating PTHrP level. The findings in the Pthr1-null fetuses indicate that several tissues may contribute to the circulating PTHrP level in fetal mice.

The parathyroid hormone-related protein and Indian hedgehog feedback loop in the growth plate.

Normal development of the growth plate requires coordinated proliferation and differentiation of chondrocytes and osteoblasts. In previous work, we have shown that Indian hedgehog (IHH), produced by prehypertrophic and hypertrophic chondrocytes, stimulates production of parathyroid hormone-related protein (PTHrP) by perichondrial and early chondrocytic cells. PTHrP then maintains chondrocytes in a proliferative, less differentiated state. Because this less differentiated state delays the production of IHH, IHH and PTHrP may participate in a negative feedback loop that synchronizes and determines the pace of differentiation of chondrocytes in the growth plate. To establish the roles of physiological levels of PTHrP and IHH, we have now injected PTH/PTHrP receptor (-/-) embryonic stem (ES) cells into normal blastocysts to generate mice with chimeric growth plates. The PTH/PTHrP receptor cells leave the proliferative cycle and differentiate prematurely in the middle of the normal proliferative columns. The columns of wild-type cells are longer than normal and the adjacent bone collar is also longer than normal. Patterns of gene expression and the use of chimeras using PTH/PTHrP receptor (-/-); IHH (-/-) ES cells suggest that modified patterns of IHH and PTHrP synthesis explain these abnormalities. Thus, IHH is a master regulator of both chondrocyte and osteoblast differentiation.

Activated parathyroid hormone/parathyroid hormone-related protein receptor in osteoblastic cells differentially affects cortical and trabecular bone.

Parathyroid hormone (PTH), an important regulator of calcium homeostasis, targets most of its complex actions in bone to cells of the osteoblast lineage. Furthermore, PTH is known to stimulate osteoclastogenesis indirectly through activation of osteoblastic cells. To assess the role of the PTH/PTH-related protein receptor (PPR) in mediating the diverse actions of PTH on bone in vivo, we generated mice that express, in cells of the osteoblastic lineage, one of the constitutively active receptors described in Jansen's metaphyseal chondrodysplasia. In these transgenic mice, osteoblastic function was increased in the trabecular and endosteal compartments, whereas it was decreased in the periosteum. In trabecular bone of the transgenic mice, there was an increase in osteoblast precursors, as well as in mature osteoblasts. Osteoblastic expression of the constitutively active PPR induced a dramatic increase in osteoclast number in both trabecular and compact bone in transgenic animals. The net effect of these actions was a substantial increase in trabecular bone volume and a decrease in cortical bone thickness of the long bones. These findings, for the first time to our knowledge, identify the PPR as a crucial mediator of both bone-forming and bone-resorbing actions of PTH, and they underline the complexity and heterogeneity of the osteoblast population and/or their regulatory microenvironment.

Indian hedgehog couples chondrogenesis to osteogenesis in endochondral bone development.

Vertebrate skeletogenesis requires a well-coordinated transition from chondrogenesis to osteogenesis. Hypertrophic chondrocytes in the growth plate play a pivotal role in this transition. Parathyroid hormone-related peptide (PTHrP), synthesized in the periarticular growth plate, regulates the site at which hypertrophy occurs. By comparing PTH/PTHrP receptor(-/-)/wild-type (PPR(-/-)/wild-type) chimeric mice with IHH(-/-);PPR(-/-)/wild-type chimeric and IHH(-/-)/wild-type chimeric mice, we provide in vivo evidence that Indian hedgehog (IHH), synthesized by prehypertrophic and hypertrophic chondrocytes, regulates the site of hypertrophic differentiation by signaling to the periarticular growth plate and also determines the site of bone collar formation in the adjacent perichondrium. By providing crucial local signals from prehypertrophic and hypertrophic chondrocytes to both chondrocytes and preosteoblasts, IHH couples chondrogenesis to osteogenesis in endochondral bone development.

Signal-selectivity of parathyroid hormone (PTH)/PTH-related peptide receptor-mediated regulation of differentiation in conditionally immortalized growth-plate chondrocytes.

Type-1 PTH/PTH-related peptide receptors (PTH1Rs), which activate both adenylyl cyclase and phospholipase C (PLC), control endochondral bone development by regulating chondrocyte differentiation. To directly analyze PTH1R function in such cells, we isolated conditionally transformed clonal chondrocytic cell lines from tibial growth plates of neonatal mice heterozygous for PTH1R gene ablation. Among 104 cell lines isolated, messenger RNAs for PTH1R, collagen II, and collagen X were detected in 28%, 90%, and 29%, respectively. These cell lines were morphologically diverse. Some appeared large, rounded, and enveloped by abundant extracellular matrix; whereas others were smaller, flattened, and elongated. Two PTH1R-expressing clones showed similar PTH1R binding and cAMP responsiveness to PTH and PTH-related peptide but disparate morphologic features, characteristic of hypertrophic (hC1--5) or nonhypertrophic (nhC2--27) chondrocytes, respectively. hC1--5 cells expressed messenger RNAs for collagen II and X, alkaline phosphatase (ALP), and matrix GLA protein, whereas nhC2--27 cells expressed collagen II and Indian hedgehog but not collagen X or ALP. In hC1--5 cells, PTH and cAMP analog, but not phorbol ester, inhibited both ALP and mineralization. PTH1R-null hC1--5 subclones were isolated by in vitro selection and then reconstituted by stable transfection with wild-type PTH1Rs or mutant (DSEL) PTH1Rs defective in PLC activation. ALP and mineralization were inhibited similarly via both forms of the receptor. These results indicate that PLC activation is not required for PTH1R regulation of mineralization or ALP in hypertrophic chondrocytes and are consistent with a major role for cAMP in regulating differentiation of hypertrophic chondrocytes.

The chondrogenic transcription factor Sox9 is a target of signaling by the parathyroid hormone-related peptide in the growth plate of endochondral bones.

In the growth plate of endochondral bones, parathyroid hormone (PTH)-related peptide (PTHrP) regulates the rate of chondrocyte maturation from prehypertrophic chondrocytes to hypertrophic chondrocytes. Using an antibody specific for Sox9 phosphorylated at serine 181 (S(181)), one of the two consensus protein kinase A phosphorylation sites of Sox9, we showed that the addition of PTHrP strongly increased the phosphorylation of SOX9 in COS7 cells transfected with both SOX9- and PTH/PTHrP receptor-expressing vectors. PTHrP also increased the SOX9-dependent activity of chondrocyte-specific enhancers in the gene for type II collagen (Col2a1) in transient transfection experiments. This increased enhancer activity did not occur with a Sox9 mutant harboring serine-to-alanine substitutions in its two consensus protein kinase A phosphorylation sites. Consistent with these results, PTHrP also increased Col2a1 mRNA levels in rat chondrosarcoma cells as well as 10T1/2 mesenchymal cells transfected with a PTH/PTHrP receptor expressing plasmid. No phosphorylation of Sox9 at S(181) was detected in prehypertrophic chondrocytes of the growth plate or any chondrocytes of PTH/PTHrP receptor null mutants. In contrast in wild-type mouse embryos, previous immunohistochemistry experiments indicated that Sox9 phosphorylated at S(181) was detected almost exclusively in chondrocytes of the prehypertrophic zone. Sox9, regardless of the phosphorylation state, was present in all chondrocytes of both genotypes except hypertrophic chondrocytes. Our results indicated that Sox9 is a target of PTHrP signaling in prehypertrophic chondrocytes in the growth plate. We hypothesize that Sox9 mediates at least some effects of PTHrP in the growth plate and that the PTHrP-dependent increased transcriptional activity of Sox9 helps maintain the chondrocyte phenotype of cells in the prehypertrophic zone and inhibits their maturation to hypertrophic chondrocytes.

Parathyroid hormone-related peptide and Indian hedgehog.

Normal endochondral bone development requires temporal and spatial coordination of various cell types. Parathyroid hormone-related peptide and Indian hedgehog interact with each other and form a feedback loop that plays a major role in this coordination. Defects in the signalling of either of the two molecules cause severe bone malformations.

Ablation of the PTHrP gene or the PTH/PTHrP receptor gene leads to distinct abnormalities in bone development.

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) bind to and activate the same PTH/PTHrP receptor. Deletion of either the PTHrP gene or the PTH/PTHrP receptor gene leads to acceleration of differentiation of growth plate chondrocytes. To explore further the functional relationships of PTHrP and the PTH/PTHrP receptor, bones of knockout mice were analyzed early in development, and the phenotypes of double-knockout mice were characterized. One early phenotype is shared by both knockouts. Normally, the first chondrocytes to become hypertrophic are located in the centers of long bones; this polarity is greatly diminished in both these knockouts. The PTH/PTHrP receptor-deficient (PTH/PTHrP-R(-/-)) mice exhibited 2 unique phenotypes not shared by the PTHrP(-/-) mice. During intramembranous bone formation in the shafts of long bones, only the PTH/PTHrP-R(-/-) bones exhibit a striking increase in osteoblast number and matrix accumulation. Furthermore, the PTH/PTHrP-R(-/-) mice showed a dramatic decrease in trabecular bone formation in the primary spongiosa and a delay in vascular invasion of the early cartilage model. In the double-homozygous knockout mice, the delay in vascular invasion did not occur. Thus, PTHrP must slow vascular invasion by a mechanism independent of the PTH/PTHrP receptor.

Parathyroid hormone-related peptide (PTHrP) induces parietal endoderm formation exclusively via the type I PTH/PTHrP receptor.

A number of studies suggest a role for PTHrP and the classical PTH/PTHrP receptor (type I) in one of the first differentiation processes in mouse embryogenesis, i.e. the formation of parietal endoderm (PE). We previously reported that although in type I receptor (-/-) embryos PE formation seemed normal, the embryos were smaller from at least day 9.5 p.c. and 60% had died before day 12.5 p.c. Here we show that the observed growth defect commences even earlier, at day 8.5 p.c. Using two novel antibodies, we show that the expression of the type I receptor protein at this stage is confined to extraembryonic endoderm only. In addition, we show that large amounts of PTHrP protein are present in the adjacent trophoblast giant cells, suggesting a paracrine interaction of PTHrP and the type I PTH/PTHrP receptor in PE formation. The involvement in PE differentiation of other recently described receptors for PTHrP would explain a possible redundancy for the type I receptor in PE formation. However, deletion of the type I PTH/PTHrP receptor in ES cells by homologous recombination completely prevents PTHrP-induced PE differentiation. Based upon these observations, we propose that PTHrP and the type I PTH/PTHrP receptor, although not required for the initial formation of PE, are required for its proper differentiation and/or functioning.

The parathyroid hormone (PTH)/PTH-related peptide receptor mediates actions of both ligands in murine bone.

PTH and PTH-related peptide (PTHrP) have been shown to bind to and activate the same PTH/PTHrP receptor. Recent studies have demonstrated, however, the presence of additional receptors specific for each ligand. We used the PTHrP and PTH/PTHrP receptor gene knock-out models to investigate whether this receptor mediates the actions of both ligands in bone. The similar phenotype of the PTHrP (-/-) and PTH/PTHrP receptor (-/-) animals in the growth plate of the tibia suggests that this receptor mediates the actions of PTHrP. Electron microscopic studies have confirmed the accelerated differentiation and disordered organization of chondrocytes, with the accumulation of large amounts of dispersed glycogen granules in the cytoplasm of proliferative and maturing cells of both genotypes. The contrasting growth plate mineralization patterns of the PTHrP (-/-) and PTH/PTHrP receptor (-/-) mice, however, suggest that the actions of PTHrP and the PTH/PTHrP receptor are not identical. Studies using calvariae from PTH/PTHrP receptor (-/-) embryos demonstrate that this receptor solely mediates the ability of PTH and PTHrP to stimulate adenylate cyclase in bone and to stimulate bone resorption. Furthermore, we show that osteoblasts of PTH/PTHrP receptor (-/-) animals, but not PTHrP (-/-) animals, have decreased levels of collagenase 3, osteopontin, and osteocalcin messenger RNAs. The PTH/PTHrP receptor, therefore, mediates distinct physiologic actions of both PTH and PTHrP.

Conditionally immortalized murine osteoblasts lacking the type 1 PTH/PTHrP receptor.

Osteoblasts synthesize and mineralize bone matrix and are principal target cells for parathyroid hormone (PTH). The type 1 PTH/PTH-related protein (PTHrP) receptor (PTH1R), cloned from rat osteoblastic cells, activates multiple intracellular signaling mechanisms. The specific roles of these PTH1R signals, or of responses to other types of PTH receptors that may be expressed, in regulating osteoblast function are incompletely understood. Use of established mammalian osteoblastic cell lines has led to much understanding of PTH action in bone, although such cells are of neoplastic origin or have other characteristics that compromise their validity as models of normal osteoblasts. To examine the role of the PTH1R in osteoblast biology, we have isolated a series of clonal murine calvarial osteoblastic cell lines that are only conditionally immortalized, via expression of a transgene encoding the tsA58 temperature-sensitive SV40 large T antigen, and that lack both functional alleles of the PTH1R gene. When cultured under nontransforming conditions, these cells stopped proliferating, expressed a series of characteristic osteoblastic genes (including the nonfunctional remnant of the PTH1R gene), and, after 3-4 weeks, produced mineralized bone nodules in a manner that was regulated by 1,25-dihydroxyvitamin D3 but not by PTH(1-84). Cyclic AMP measurements revealed no evidence of expression of alternate species of Gs-linked PTH receptors. Stable transfection with PTH1R cDNA reconstituted both PTH binding and adenylyl cyclase activation, increased basal osteocalcin expression, and supported PTH stimulation of c-Fos expression and matrix mineralization. These conditionally transformed, PTH1R(-/-) clonal osteoblastic cell lines should prove useful for studies of the regulation of osteoblast differentiation and function by both endogenous nonclassical species of PTH (or PTHrP) receptors and mutant signal-selective PTH1Rs.

Parathyroid hormone-related peptide (PTHrP) and parathyroid hormone (PTH)/PTHrP receptor.

Parathyroid hormone-related peptide (PTHrP) has been identified as the factor responsible for the humoral hypercalcemia of malignancy (HHM). Since the cloning of the cDNA, it has become clear that PTHrP is a prohormone that is posttranslationally cleaved to yield a complex family of peptides. Through its homology to parathyroid hormone (PTH) in the amino-terminus region of the protein, it is able to bind to and activate a common PTH/PTHrP receptor. PTHrP has been shown to be a normal product of many adult and fetal tissues, where it appears to act in an autocrine/paracrine fashion to regulate organogenesis. PTHrP and the PTH/PTHrP receptor seem to be co-expressed in many tissues, but their role in the various systems is uncertain. The use of transgenic and knock-out animal models has contributed to a better understanding of the physiological role of this peptide and its receptor. In this review, the structure of their genes, their expression pattern, and some of their major physiological functions are discussed. Attention is focused on their interaction in the regulation of cartilage and bone development.

Functional analysis of the PTH/PTHrP network of ligands and receptors.

Parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) are two related proteins that activate a common PTH/PTHrP receptor, yet have quite distinct physiologic missions. PTH is the major peptide regulator of blood calcium in higher vertebrates, while PTHrP predominantly acts as a paracrine regulator of differentiation and local intercellular signaling. To analyze the physiological roles of PTHrP and the PTH/PTHrP receptor, "knockout" mice missing either the PTHrP or the PTH/PTHrP receptor gene were developed. Both the PTHrP (-/-) mice and the PTH/PTHrP receptor (-/-) mice exhibit a growth plate chondrodysplasia that reflects accelerated differentiation of proliferating chondrocytes. Growth plate chondrocytes regulate the local production of PTHrP by secreting the protein, Indian hedgehog (Ihh), as they are leaving the proliferative pool. Ihh stimulates the production of PTHrP, which then slows the differentiation of chondrocytes, thereby delaying the production of Ihh. PTHrP also stimulates transport of calcium across the placenta. PTHrP (-/-) mice lack the normal elevation of fetal blood calcium (when compared to maternal levels) and have low placental transport of calcium. Fragments of PTHrP that do not bind to the PTH/PTHrP receptor can correct the defect of placental calcium transport in these mice. Thus, this action of PTHrP is not mediated by the PTH/PTHrP receptor. The "knockout" mice thus help delineate the roles of PTH. PTHrP, and the PTH/PTHrP receptor in an interacting network of ligands and receptors.

Regulation of murine fetal-placental calcium metabolism by the calcium-sensing receptor.

The calcium-sensing receptor (CaSR) regulates PTH secretion to control the extracellular calcium concentration in adults, but its role in fetal life is unknown. We used CaSR gene knockout mice to investigate the role of the CaSR in regulating fetal calcium metabolism. The normal calcium concentration in fetal blood is raised above the maternal level, an increase that depends upon PTH-related peptide (PTHrP). Heterozygous (+/-) and homozygous (-/-) disruption of the CaSR caused a further increase in the fetal calcium level. This increase was modestly blunted by concomitant disruption of the PTHrP gene and completely reversed by disruption of the PTH/ PTHrP receptor gene. Serum levels of PTH and 1, 25-dihydroxyvitamin D were substantially increased above the normal low fetal levels by disruption of the CaSR. The free deoxypyridinoline level was increased in the amniotic fluid (urine) of CaSR-/- fetuses; this result suggests that fetal bone resorption is increased. Placental calcium transfer was reduced, and renal calcium excretion was increased, by disruption of the CaSR. These studies indicate that the CaSR normally suppresses PTH secretion in the presence of the normal raised (and PTHrP-dependent) fetal calcium level. Disruption of the CaSR causes fetal hyperparathyroidism and hypercalcemia, with additional effects on placental calcium transfer.