MECHANISMS IN ENDOCRINOLOGY: Novel genetic causes of short stature.

The fast technological development, particularly single nucleotide polymorphism array, array-comparative genomic hybridization, and whole exome sequencing, has led to the discovery of many novel genetic causes of growth failure. In this review we discuss a selection of these, according to a diagnostic classification centred on the epiphyseal growth plate. We successively discuss disorders in hormone signalling, paracrine factors, matrix molecules, intracellular pathways, and fundamental cellular processes, followed by chromosomal aberrations including copy number variants (CNVs) and imprinting disorders associated with short stature. Many novel causes of GH deficiency (GHD) as part of combined pituitary hormone deficiency have been uncovered. The most frequent genetic causes of isolated GHD are GH1 and GHRHR defects, but several novel causes have recently been found, such as GHSR, RNPC3, and IFT172 mutations. Besides well-defined causes of GH insensitivity (GHR, STAT5B, IGFALS, IGF1 defects), disorders of NFκB signalling, STAT3 and IGF2 have recently been discovered. Heterozygous IGF1R defects are a relatively frequent cause of prenatal and postnatal growth retardation. TRHA mutations cause a syndromic form of short stature with elevated T3/T4 ratio. Disorders of signalling of various paracrine factors (FGFs, BMPs, WNTs, PTHrP/IHH, and CNP/NPR2) or genetic defects affecting cartilage extracellular matrix usually cause disproportionate short stature. Heterozygous NPR2 or SHOX defects may be found in ∼3% of short children, and also rasopathies (e.g., Noonan syndrome) can be found in children without clear syndromic appearance. Numerous other syndromes associated with short stature are caused by genetic defects in fundamental cellular processes, chromosomal abnormalities, CNVs, and imprinting disorders.

Deficiency of the parathyroid hormone-related peptide nuclear localization and carboxyl terminal sequences leads to premature skin ageing partially mediated by the upregulation of p27.

We previously reported that deficiency of the PTHrP nuclear localization sequence (NLS) and C-terminus in PTHrP knockin (PTHrP KI) mice resulted in premature ageing of skin. P27, a cyclin-dependent kinase inhibitor, was upregulated in PTHrP KI mice and acted as a downstream target of the PTHrP NLS to regulate the proliferation of vascular smooth muscle cells. To determine the effects of p27 deficiency on premature skin ageing of PTHrP KI mice, we compared the skin phenotypes of PTHrP KI mice to those of p27 knockout (p27(-/-) ) mice and to those of double homozygous p27-deficient and PTHrP KI (p27(-/-) PTHrP KI) mice at 2 weeks age. Compared with wild-type littermates, PTHrP KI mice displayed thinner skin and decreased subcutaneous fat and collagen fibres, decreased skin cell proliferation and increased apoptosis, higher expression of p27, p19 and p53 and lower expression of cyclin E and CDK2, and increased reactive oxygen species levels and decreased antioxidant capacity. Deficiency of p27 in the PTHrP KI mice at least in part corrected the skin premature ageing phenotype resulting in thicker skin and increased subcutaneous fat and collagen. These alternations were associated with higher expression of CDK2 and cyclin E, lower expression of p19 and p53, and enhanced antioxidant capacity with increased skin cell proliferation and inhibition of apoptosis. Our results indicate that the NLS and C-terminus of PTHrP play a critical role in preventing skin from premature ageing that is partially mediated by p27.

Parathyroid hormone-related protein is induced by hypoxia and promotes expression of the differentiated phenotype of human articular chondrocytes.

PTHrP (parathyroid hormone-related protein) is crucial for normal cartilage development and long bone growth and acts to delay chondrocyte hypertrophy and terminal differentiation in the growth plate. After growth plate closure adult HACs (human articular chondrocytes) still produce PTHrP, suggesting a possible role for this factor in the permanent articular cartilage. However, the expression regulation and function of PTHrP in the permanent articular cartilage is unknown. Human articular cartilage is an avascular tissue and functions in a hypoxic environment. The resident chondrocytes have adapted to hypoxia and use it to drive their tissue-specific functions. In the present study, we explored directly in normal articular chondrocytes isolated from a range of human donors the effect of hypoxia on PTHrP expression and whether PTHrP can regulate the expression of the permanent articular chondrocyte phenotype. We show that in HACs PTHrP is up-regulated by hypoxia in a HIF (hypoxia-inducible factor)-1α and HIF-2α-dependent manner. Using recombinant PTHrP, siRNA-mediated depletion of endogenous PTHrP and by blocking signalling through its receptor [PTHR1 (PTHrP receptor 1)], we show that hypoxia-induced PTHrP is a positive regulator of the key cartilage transcription factor SOX9 [SRY (sex determining region on the Y chromosome)-box 9], leading to increased COL2A1 (collagen type II, α1) expression. Our findings thus identify PTHrP as a potential factor for cartilage repair therapies through its ability to promote the differentiated HAC phenotype.

Endogenous parathyroid hormone-related protein compensates for the absence of parathyroid hormone in promoting bone accrual in vivo in a model of bone marrow ablation.

To assess the effect of hypoparathyroidism on osteogenesis and bone turnover in vivo, bone marrow ablation (BMXs) were performed in tibias of 8-week-old wild-type and parathyroid hormone-null (PTH(-/-)) mice and newly formed bone tissue was analyzed from 5 days to 3 weeks after BMX. At 1 week after BMX, trabecular bone volume, osteoblast numbers, alkaline phosphatase-positive areas, type I collagen-positive areas, PTH receptor-positive areas, calcium sensing receptor-positive areas, and expression of bone formation-related genes were all decreased significantly in the diaphyseal regions of bones of PTH(-/-) mice compared to wild-type mice. In contrast, by 2 weeks after BMX, all parameters related to osteoblastic bone accrual were increased significantly in PTH(-/-) mice. At 5 days after BMX, active tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts had appeared in wild-type mice but were undetectable in PTH(-/-) mice, Both the ratio of mRNA levels of receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) and TRAP-positive osteoclast surface were still reduced in PTH(-/-) mice at 1 week but were increased by 2 weeks after BMX. The expression levels of parathyroid hormone-related protein (PTHrP) at both mRNA and protein levels were upregulated significantly at 1 week and more dramatically at 2 weeks after BMX in PTH(-/-) mice. To determine whether the increased newly formed bones in PTH(-/-) mice at 2 weeks after BMX resulted from the compensatory action of PTHrP, PTH(-/-) PTHrP(+/-) mice were generated and newly formed bone tissue was compared in these mice with PTH(-/-) and wild-type mice at 2 weeks after BMX. All parameters related to osteoblastic bone formation and osteoclastic bone resorption were reduced significantly in PTH(-/-) PTHrP(+/-) mice compared to PTH(-/-) mice. These results demonstrate that PTH deficiency itself impairs osteogenesis, osteoclastogenesis, and osteoclastic bone resorption, whereas subsequent upregulation of PTHrP in osteogenic cells compensates by increasing bone accrual.

Parathyroid hormone-related protein activates Wnt signaling to specify the embryonic mammary mesenchyme.

Parathyroid hormone-related protein (PTHrP) regulates cell fate and specifies the mammary mesenchyme during embryonic development. Loss of PTHrP or its receptor (Pthr1) abolishes the expression of mammary mesenchyme markers and allows mammary bud cells to revert to an epidermal fate. By contrast, overexpression of PTHrP in basal keratinocytes induces inappropriate differentiation of the ventral epidermis into nipple-like skin and is accompanied by ectopic expression of Lef1, ß-catenin and other markers of the mammary mesenchyme. In this study, we document that PTHrP modulates Wnt/ß-catenin signaling in the mammary mesenchyme using a Wnt signaling reporter, TOPGAL-C. Reporter expression is completely abolished by loss of PTHrP signaling and ectopic reporter activity is induced by overexpression of PTHrP. We also demonstrate that loss of Lef1, a key component of the Wnt pathway, attenuates the PTHrP-induced abnormal differentiation of the ventral skin. To characterize further the contribution of canonical Wnt signaling to embryonic mammary development, we deleted ß-catenin specifically in the mammary mesenchyme. Loss of mesenchymal ß-catenin abolished expression of the TOPGAL-C reporter and resulted in mammary buds with reduced expression of mammary mesenchyme markers and impaired sexual dimorphism. It also prevented the ectopic, ventral expression of mammary mesenchyme markers caused by overexpression of PTHrP in basal keratinocytes. Therefore, we conclude that a mesenchymal, canonical Wnt pathway mediates the PTHrP-dependent specification of the mammary mesenchyme.

Absence of PTHrP nuclear localization and carboxyl terminus sequences leads to abnormal brain development and function.

We assessed whether the nuclear localization sequences (NLS) and C terminus of parathyroid hormone-related protein (PTHrP) play critical roles in brain development and function. We used histology, immunohistochemistry, histomorphometry, Western blots and electrophysiological recordings to compare the proliferation and differentiation of neural stem cells, neuronal hippocampal synaptic transmission, and brain phenotypes including shape and structures, in Pthrp knock-in mice, which express PTHrP (1-84), a truncated form of the protein that is missing the NLS and the C-terminal region of the protein, and their wild-type littermates. Results showed that Pthrp knock-in mice display abnormal brain shape and structures; decreased neural cell proliferative capacity and increased apoptosis associated with up-regulation of cyclin dependent kinase inhibitors p16, p21, p27 and p53 and down-regulation of the Bmi-1 oncogene; delayed neural cell differentiation; and impaired hippocampal synaptic transmission and plasticity. These findings provide in vivo experimental evidence that the NLS and C-terminus of PTHrP are essential not only for the regulation of neural cell proliferation and differentiation, but also for the maintenance of normal neuronal synaptic transmission and plasticity.

PTHrP drives breast tumor initiation, progression, and metastasis in mice and is a potential therapy target.

Parathyroid hormone-related protein (PTHrP) is a secreted factor expressed in almost all normal fetal and adult tissues. It is involved in a wide range of developmental and physiological processes, including serum calcium regulation. PTHrP is also associated with the progression of skeletal metastases, and its dysregulated expression in advanced cancers causes malignancy-associated hypercalcemia. Although PTHrP is frequently expressed by breast tumors and other solid cancers, its effects on tumor progression are unclear. Here, we demonstrate in mice pleiotropic involvement of PTHrP in key steps of breast cancer - it influences the initiation and progression of primary tumors and metastases. Pthrp ablation in the mammary epithelium of the PyMT-MMTV breast cancer mouse model caused a delay in primary tumor initiation, inhibited tumor progression, and reduced metastasis to distal sites. Mechanistically, it reduced expression of molecular markers of cell proliferation (Ki67) and angiogenesis (factor VIII), antiapoptotic factor Bcl-2, cell-cycle progression regulator cyclin D1, and survival factor AKT1. PTHrP also influenced expression of the adhesion factor CXCR4, and coexpression of PTHrP and CXCR4 was crucial for metastatic spread. Importantly, PTHrP-specific neutralizing antibodies slowed the progression and metastasis of human breast cancer xenografts. Our data identify what we believe to be new functions for PTHrP in several key steps of breast cancer and suggest that PTHrP may constitute a novel target for therapeutic intervention.

Parathyroid hormone-related protein is not required for normal ductal or alveolar development in the post-natal mammary gland.

PTHrP is necessary for the formation of the embryonic mammary gland and, in its absence, the embryonic mammary bud fails to form the neonatal duct system. In addition, PTHrP is produced by the breast during lactation and contributes to the regulation of maternal calcium homeostasis during milk production. In this study, we examined the role of PTHrP during post-natal mammary development. Using a PTHrP-lacZ transgenic mouse, we surveyed the expression of PTHrP in the developing post-natal mouse mammary gland. We found that PTHrP expression is restricted to the basal cells of the gland during pubertal development and becomes expressed in milk secreting alveolar cells during pregnancy and lactation. Based on the previous findings that overexpression of PTHrP in cap and myoepithelial cells inhibited ductal elongation during puberty, we predicted that ablation of native PTHrP expression in the post-natal gland would result in accelerated ductal development. To address this hypothesis, we generated two conditional models of PTHrP-deficiency specifically targeted to the postnatal mammary gland. We used the MMTV-Cre transgene to ablate the floxed PTHrP gene in both luminal and myoepithelial cells and a tetracycline-regulated K14-tTA;tetO-Cre transgene to target PTHrP expression in just myoepithelial and cap cells. In both models of PTHrP ablation, we found that mammary development proceeds normally despite the absence of PTHrP. We conclude that PTHrP signaling is not required for normal ductal or alveolar development.

Skeletal recovery after weaning does not require PTHrP.

Mice lose 20% to 25% of trabecular bone mineral content (BMC) during lactation and restore it after weaning through unknown mechanisms. We found that tibial Pthrp mRNA expression was upregulated fivefold by 7 days after weaning versus end of lactation in wild-type (WT) mice. To determine whether parathyroid hormone-related protein (PTHrP) stimulates bone formation after weaning, we studied a conditional knockout in which PTHrP is deleted from preosteoblasts and osteoblasts by collagen I promoter-driven Cre (Cre(ColI) ). These mice are osteopenic as adults but have normal serum calcium, calcitriol, and parathyroid hormone (PTH). Pairs of Pthrp(flox/flox) ;Cre(ColI) (null) and WT;Cre(ColI) (WT) females were mated and studied through pregnancy, lactation, and 3 weeks of postweaning recovery. By end of lactation, both genotypes lost lumbar spine BMC: WT declined by 20.6% ± 3.3%, and null decreased by 22.5% ± 3.5% (p < .0001 versus baseline; p = NS between genotypes). During postweaning recovery, both restored BMC to baseline: WT to -3.6% ± 3.7% and null to 0.3% ± 3.7% (p = NS versus baseline or between genotypes). Similar loss and full recovery of BMC were seen at the whole body and hind limb. Histomorphometry confirmed that nulls had lower bone mass at baseline and that this was equal to the value achieved after weaning. Osteocalcin, propeptide of type 1 collagen (P1NP), and deoxypyridinoline increased equally during recovery in WT and null mice; PTH decreased and calcitriol increased equally; serum calcium was unchanged. Urine calcium increased during recovery but remained no different between genotypes. Although osteoblast-derived PTHrP is required to maintain adult bone mass and Pthrp mRNA upregulates in bone after weaning, it is not required for recovery of bone mass after lactation. The factors that stimulate postweaning bone formation remain unknown.

Alterations in phosphorus, calcium and PTHrP contribute to defects in dental and dental alveolar bone formation in calcium-sensing receptor-deficient mice.

To determine whether the calcium-sensing receptor (CaR) participates in tooth formation and dental alveolar bone development in mandibles in vivo, we examined these processes, as well as mineralization, in 2-week-old CaR-knockout (CaR(-/-)) mice. We also attempted to rescue the phenotype of CaR(-/-) mice by genetic means, in mice doubly homozygous for CaR and 25-hydroxyvitamin D 1alpha-hydroxylase [1alpha(OH)ase] or parathyroid hormone (Pth). In CaR(-/-) mice, which exhibited hypercalcemia, hypophosphatemia and increased serum PTH, the volumes of teeth and of dental alveolar bone were decreased dramatically, whereas the ratio of the area of predentin to total dentin and the number and surface of osteoblasts in dental alveolar bone were increased significantly, as compared with wild-type littermates. The normocalcemia present in CaR(-/-);1alpha(OH)ase(-/-) mice only slightly improved the defects in dental and alveolar bone formation observed in the hypercalcemic CaR(-/-) mice. However, these defects were completely rescued by the additional elimination of hypophosphatemia and by an increase in parathyroid hormone-related protein (PTHrP) expression in the apical pulp, Hertwig's epithelial root sheath and mandibular tissue in CaR(-/-); Pth(-/-) mice. Therefore, alterations in calcium, phosphorus and PTHrP contribute to defects in the formation of teeth and alveolar bone in CaR-deficient mice. This study indicates that CaR participates in the formation of teeth and in the development of dental alveolar bone in mandibles in vivo, although it appears to do so largely indirectly.

Deletion and point mutations of PTHLH cause brachydactyly type E.

Autosomal-dominant brachydactyly type E (BDE) is a congenital limb malformation characterized by small hands and feet predominantly as a result of shortened metacarpals and metatarsals. In a large pedigree with BDE, short stature, and learning disabilities, we detected a microdeletion of approximately 900 kb encompassing PTHLH, the gene coding for parathyroid hormone related protein (PTHRP). PTHRP is known to regulate the balance between chondrocyte proliferation and the onset of hypertrophic differentiation during endochondral bone development. Inactivation of Pthrp in mice results in short-limbed dwarfism because of premature differentiation of chondrocyte. On the basis of our initial finding, we tested further individuals with BDE and short stature for mutations in PTHLH. We identified two missense (L44P and L60P), a nonstop (X178WextX( *)54), and a nonsense (K120X) mutation. The missense mutation L60P was tested in chicken micromass culture with the replication-competent avian sarcoma leukosis virus retroviral expression system and was shown to result in a loss of function. Thus, loss-of-function mutations in PTHLH cause BDE with short stature.

Parathyroid hormone-related protein protects against mammary tumor emergence and is associated with monocyte infiltration in ductal carcinoma in situ.

Parathyroid hormone-related protein (PTHrP) is required for mammary gland development and promotes the growth of breast cancer metastases within bone. However, there are conflicting reports of the prognostic significance of its expression in primary breast cancers. To study the role of PTHrP in early breast cancer, the effect of conditional deletion of PTHrP was examined in the context of neu-induced mammary tumorigenesis. Loss of PTHrP resulted in a higher tumor incidence. Transcriptional profiling of the tumors revealed that PTHrP influenced genes relevant to heterotypic cell signaling, including regulators of monocyte recruitment. Immunohistochemical analysis of human breast cancers revealed that PTHrP expression was associated with both HER-2/neu expression and macrophage infiltration in preinvasive ductal carcinoma in situ. The gene expression signature associated with loss of PTHrP expression in vivo correlated with poorer outcome in human breast cancer. Together, these data indicate that loss of PTHrP accelerates mammary tumorigenesis possibly by a non-cell-autonomous tumor suppressor pathway.

The cranial base in craniofacial development: a gene therapy study.

The etiology of midface retrusion remains largely unclear. We hypothesized that the cranial base synchondroses play a key role in the development of the craniofacial skeleton in the Sandhoff mouse model. We observed that developmental abnormalities of the cranial base synchondroses involving proliferative chondrocytes are important in craniofacial growth and development. Neonatal restitution of beta-hexosaminidase in mutant mice by gene therapy successfully ameliorated the attendant skeletal defects and restored craniofacial morphology in vivo, suggesting this as a critical temporal window in craniofacial development. Analysis of our data implicates parathyroid-related peptide (PTHrP) and cyclo-oxygenase-2 (COX-2) as possible factors underlying the development of the aforementioned skeletal defects. Hence, timely restitution of a genetic deficiency or, alternatively, the restoration of PTHrP or cyclo-oxygenase activity by the administration of PTH and/or non-steroidal anti-inflammatory drugs or COX-2 selective inhibitors to affected individuals may prove beneficial in the management of midface retrusion.

Parathyroid hormone-related protein (PTHrP) inhibits mitochondrial-dependent apoptosis through CK2.

Over the past decade, parathyroid hormone-related protein (PTHrP) has been identified as a key survival factor for cells subjected to apoptotic stimuli. Its anti-apoptotic activity has been attributed to nuclear accumulation of the intact protein, or a synthetic peptide corresponding to its nuclear targeting sequence (NTS), which promotes rapid exit of nutrient deprived cells from the cell cycle. Intracellular PTHrP also inhibited apoptosis by blocking tumor necrosis factor alpha (TNFalpha)-induced apoptosis by blocking signaling from the "death receptor" and preventing damage to the mitochondrial membrane. In both cases, the anti-apoptotic activity was significantly reduced in the presence of a nuclear deficient form of PTHrP with a (88)K/E K/E.K/I(91) mutation in the NTS. The current work was undertaken to determine the mechanism by which nuclear PTHrP blocked mitochondrial-mediated apoptosis. Using sub-cellular fractionation and functional assays we showed that pre-treatment of HEK293 cells with exogenous NTS peptide before inducing apoptosis with TNFalpha was as effective as expression of the full-length protein in inhibiting apoptosis. Inhibition of apoptosis was associated with increased expression of protein kinase casein kinase 2 (CK2) and in sustained CK2 accumulation and activity in the nuclear fraction. In primary chondrogenic cells harvested from the limb buds of PTHrP(+/-) and PTHrP(-/-) embryonic mice, there was a dose-dependent decrease in CK2 expression and activity that correlated with increased susceptibility to apoptosis. Taken together the results indicate that nuclear accumulation of PTHrP effectively inhibits mitochondrial-mediated apoptosis through regulation of the expression, activity, and sub-cellular trafficking of CK2.

Stimulatory G protein directly regulates hypertrophic differentiation of growth plate cartilage in vivo.

Stimulatory heterotrimeric G protein (Gs) transduces signals from various cell-surface receptors to adenylyl cyclases, which generate cAMP. The alpha subunit of Gs (Gsalpha) is encoded by GNAS (Gnas in mice), and heterozygous Gsalpha inactivating mutations lead to Albright hereditary osteodystrophy. The in vivo role of Gsalpha in skeletogenesis is largely unknown, because of early embryonic lethality of mice with disruption of Gnas exon 2 (Gnas(E2-/E2-)) and the absence of easily detectable phenotypes in growth plate chondrocytes of heterozygous mutant mice (Gnas(+/E2-)). We generated chimeric mice containing wild-type cells and either Gnas(E2-/E2-) or Gnas(+/E2-) cells. Gnas(E2-/E2-) chondrocytes phenocopied PTH/PTHrP receptor (PPR)(-/-) cells by prematurely undergoing hypertrophy. Introduction of a transgene expressing Gsalpha, one of several gene products that include Gnas exon 2, into Gnas(E2-/E2-) cells prevented premature hypertrophy. Gsalpha mRNA expression detected by real-time RT-PCR analysis was reduced to approximately half that of the wild-type in both paternal and maternal Gnas(+/E2-) growth plate chondrocytes, indicating biallelic expression of Gsalpha in these cells. Hypertrophy of Gnas(+/E2-) chondrocytes was modestly but significantly premature in chimeric growth plates of mice containing wild-type and Gnas(+/E2-) cells. These data suggest that Gsalpha is the primary mediator of the actions of PPR in growth plate chondrocytes and that there is haploinsufficiency of Gsalpha signaling in Gnas(+/E2-) chondrocytes.

Arrested pulmonary alveolar cytodifferentiation and defective surfactant synthesis in mice missing the gene for parathyroid hormone-related protein.

Parathyroid hormone-related protein (PTHrP) and PTH/PTHrP receptor expression are developmentally regulated in lung epithelium and adepithelial mesenchyme, respectively. To test the hypothesis that PTHrP is a developmental regulator of terminal airway development, we investigated in vivo and in vitro models of alveolar cytodifferentiation using mice in which the gene encoding PTHrP was ablated by homologous recombination. We have determined that fetal and newborn PTHrP(-/-) lungs showed delayed mesenchymal-epithelial interactions, arrested type II cell differentiation, and reduced surfactant lamellar body formation and pulmonary surfactant production. Embryonic PTHrP(-/-) lung buds cultured in the absence of skeletal constriction or systemic compensating factors also exhibited delayed alveolar epithelial (type II cell) and mesenchymal cytodifferentiation, as well as a > 40% inhibition of surfactant phospholipid production (n = 3-5). Addition of exogenous PTHrP to embryonic PTHrP(-/-) lung cultures normalized interstitial cell morphology and surfactant phospholipid production. The importance of PTHrP as an endogenous regulatory molecule in mammalian lung development is supported by the findings that ablation of PTHrP expression in isolated developing lung is sufficient to disrupt normal development of the alveolar ducts and the centriacinar regions.

Development of a novel mouse osteoclast culture system including cells of mandibular body and erupting teeth.

Osteoclasts are multinucleated cells with the specialized function of resorbing calcified tissues. These cells develop from hemopoietic cells of the monocyte-macrophage lineage with the support of osteoblasts/stromal cells. Tooth eruption is a vertical movement of teeth via creation of an eruption pathway in and through the alveolar bone. The precise cellular and molecular determinants of tooth eruption are not yet clear, and a cell culture system that can reproduce the activity of osteoclast formation during tooth eruption is expected to be a useful tool to clarify the mechanism of eruption pathway formation. To this end, mandibular bodies, including incisors and molars, were isolated from 9- to 11-day-old mice undergoing active tooth eruption. Primary cells were obtained from mandibular bodies by enzymatic digestion and cultured in alphaMEM containing 15% FBS without any cytokine or growth factor or hormone in the culture (AFT culture, for alveolar bone, dental follicle, and tooth). A progressive increase in the number of tartrate-resistant acid phosphatase-positive multinucleated osteoclastic cells was observed in AFT culture. The osteoclastic cells generated were immunopositive for cathepsin K and calcitonin receptor, and formed resorption pits when cultured on dentine slices. Parathyroid hormone-related protein (PTHrP), expressed by the enamel organ of tooth, is reported to be an essential factor in creation of the eruption pathway. To verify this point, cells were isolated from mandibular bodies from which all teeth and dental follicles had been removed and cultured similarly (A culture, for alveolar bone). Osteoclastic cells were not formed and PTHrP production was hardly detected in the medium of A culture, in contrast to the high level of PTHrP in AFT culture. Since our previous study demonstrated that neonatal homozygous PTHrP-knockout mice show impaired osteoclastogenesis around tooth germs, AFT culture was performed by using this sample to examine whether this culture system can reproduce the status of osteoclastogenesis observed in vivo. The result showed that none of the osteoclastic cells were generated from cells of homozygous mice. We here report a novel mouse osteoclast culture system that reproduces the activity of osteoclast formation around erupting teeth without addition of any cytokine or growth factor or hormone to the medium. Histological examination of various transgenic and mutant mice now offers valuable findings on studies of tooth eruption and the present culture system using these animals would be a powerful tool in clarifying the cellular and molecular mechanisms of eruption pathway formation.

Mandibular coronoid process in parathyroid hormone-related protein-deficient mice shows ectopic cartilage formation accompanied by abnormal bone modeling.

Parathyroid hormone-related protein (PTHrP) null mutant mice were analyzed to investigate an additional role for PTHrP in cell differentiation. We found ectopic cartilage formation in the mandibular coronoid process in newborn mice. While many previous studies involving PTHrP gene knockout mouse have shown that the cartilage in various regions becomes smaller, this is the first report showing an "increase" of cartilage volume. Investigations of mandibular growth using normal mice indicated that coronoid secondary cartilage never formed from E 15 to d 4, but small amount of cartilage temporally formed at d 7, and this also applies to PTHrP-wild type mice. Therefore, PTHrP deficiency consequently advanced the secondary cartilage formation, which is a novel role of PTHrP in chondrocyte differentiation. In situ hybridization of matrix proteins showed that this coronoid cartilage had characteristics of the lower hypertrophic cell zone usually present at the site of endochondral bone formation and/or "chondroid bone" occasionally found in distraction osteogenesis. In addition, the coronoid process in the PTHrP-deficient mouse also showed abnormal expansion of bone marrow and an increase in the number of multinucleated osteoclasts, an indication of abnormal bone modeling. These results indicate that PTHrP is involved in bone modeling as well as in chondrocyte differentiation. In situ hybridization of matrix protein mRNAs in the abnormal mandibular condylar cartilage revealed that this cartilage was proportionally smaller, supporting previous immunohistochemical results.

The role of fibroblast transdifferentiation in lung epithelial cell proliferation, differentiation, and repair in vitro.

Parathyroid hormone-related protein (PTHrP) expression is necessary for differentiation of mesenchymal lipofibroblasts, which induce epithelial type II (TII) cell differentiation, both of which are necessary for alveolarization. PTHrP deficiency may be associated with bronchopulmonary dysplasia (BPD), characterized by truncation of alveolarization among preterm infants. This is supported by the baboon model of BPD (failure of alveolarization) that manifests PTHrP deficiency. We provide evidence that TII cell PTHrP expression is downregulated by alveolar overdistension, resulting in the transdifferentiation of lipofibroblasts to myofibroblasts, characterized by progressive loss of PTHrP receptor expression and triglyceride content, and sequential upregulation of alpha-smooth muscle actin (alphaSMA), typifying fibrosis. PTHrP reverses the downregulation of the PTHrP receptor and upregulation of alphaSMA, reverting myofibroblasts to a lipofibroblast genotype. When TII cells are co-cultured with lipofibroblasts, they proliferate and differentiate, expressing surfactant protein-B; in contrast, TII cells co-cultured with myofibroblasts fail to develop, mimicking the failed alveolarization associated with BPD. Treatment of myofibroblasts with 15-deoxy-Delta 12, 14 prostaglandinJ(2) (PGJ(2)) stimulates ADRP expression, reconstituting the lipofibroblast phenotype. PGJ(2)-treated myofibroblasts promote TII cell growth and surfactant protein-B expression, indicating that failed alveolarization due to transdifferentiation is reversible. We conclude that alveolar overdistension can cause fibroblast transdifferentiation, resulting in failed alveolarization.