PTH and PTHrP effects on the skeleton

Genetic disorders of the skeleton: a developmental approach

Although disorders of the skeleton are individually rare, they are of clinical relevance because of their overall frequency. Many attempts have been made in the past to identify disease groups in order to facilitate diagnosis and to draw conclusions about possible underlying pathomechanisms. Traditionally, skeletal disorders have been subdivided into dysostoses, defined as malformations of individual bones or groups of bones, and osteochondrodysplasias, defined as developmental disorders of chondro-osseous tissue. In light of the recent advances in molecular genetics, however, many phenotypically similar skeletal diseases comprising the classical categories turned out not to be based on defects in common genes or physiological pathways. In this article, we present a classification based on a combination of molecular pathology and embryology, taking into account the importance of development for the understanding of bone diseases.

Regulation of the osteoblast-specific transcription factor, Runx2: responsiveness to multiple signal transduction pathways

The Cbfa1/Runx2 is an important transcription factor necessary for osteoblast differentiation and bone formation. However, the signaling pathways regulating Runx2 activity are just beginning to be understood. Inconsistencies between Runx2 mRNA or protein levels and its transcriptional activity suggests that posttranslational modification and/or protein-protein interactions may regulate this factor. Runx2 can be phosphorylated and activated by the mitogen-activated protein kinase (MAPK) pathway. This pathway can be stimulated by a variety of signals including those initiated by extracellular matrix (ECM), osteogenic growth factors like bone morphogenic proteins (BMPs) and fibroblast growth factor-2 (FGF-2), mechanical loading and hormones such as parathyroid hormone (PTH). Protein kinase A (PKA) may also phosphorylate/activate Runx2 under certain conditions. In addition, Runx2 activity is enhanced by protein-protein interactions as are seen with PTH-induced Runx2/AP-1 and BMP-mediated Runx2/Smads interactions. Mechanisms for interaction with Runx2 are complex including binding of distinct components such as AP-1 factors and Smads proteins to separate DNA regions in target gene promoters and direct physical interactions between Runx2 and AP-1/Smad factors. Post-translational modifications such as phosphorylation may influence interactions between Runx2 and other nuclear factors. These findings suggest that Runx2 plays a central role in coordinating multiple signals involved in osteoblast differentiation.

Genotype-phenotype relationship in human ATP6i-dependent autosomal recessive osteopetrosis

Autosomal-recessive osteopetrosis is a severe genetic disease caused by osteoclast failure. Approximately 50% of the patients harbor mutations of the ATP6i gene, encoding for the osteoclast-specific a3 subunit of V-ATPase. We found inactivating ATP6i mutations in four patients, and three of these were novel. Patients shared macrocephaly, growth retardation and optic nerve alteration, osteosclerotic and endobone patterns, and high alkaline phosphatase and parathyroid hormone levels. Bone biopsies revealed primary spongiosa lined with active osteoblasts and high numbers of tartrate-resistant acid phosphatase (TRAP)-positive, a3 subunit-negative, morphologically unremarkable osteoclasts, some of which located in shallow Howship lacunae. Scarce hematopoietic cells and abundant fibrous tissue containing TRAP-positive putative osteoclast precursors were noted. In vitro osteoclasts were a3-negative, morphologically normal, with prominent clear zones and actin rings, and TRAP activity more elevated than in control patients. Podosomes, alphaVbeta3 receptor, c-Src, and PYK2 were unremarkable. Consistent with the finding in the bone biopsies, these cells excavated pits faintly stained with toluidine blue, indicating inefficient bone resorption. Bone marrow transplantation was successful in all patients, and posttransplant osteoclasts showed rescue of a3 subunit immunoreactivity.

The parathyroid hormone-related protein system: more data but more unsolved questions

PURPOSE OF REVIEW

The present review focuses on recent studies that might be considered as the most relevant advances in the parathyroid hormone-related protein field, with special emphasis on proven functions in renovascular and cardiovascular systems, in physiological as well as pathological conditions. Thus, the questions as to whether and how parathyroid hormone-related protein intervenes in vascular development and homeostasis and in vascular diseases such as hypertension, atherosclerosis, restenosis and heart failure have begun to be unraveled.

RECENT FINDINGS

Since its discovery from hypercalcemia-associated tumors in 1987, it has become clear that parathyroid hormone-related protein is a ubiquitously expressed poly-hormone and plays crucial roles in normal life. The early lethality to parathyroid hormone-related protein knockout mice emphasizes the crucial roles of the protein in development but has limited the use of these models. However, data accumulated from transgenic animals overexpressing the protein in particular cells have provided considerable support to its physiological and pathological relevance. The recent demonstration that nascent parathyroid hormone-related protein not only follows the secretory pathways, but also directly translocates to the nucleus, is beginning to uncover new actions for the protein in a number of physiological systems such as bone, mammary gland and vascular smooth muscle, as well as in pathological situations, such as cancer, osteoporosis, sepsis, atherosclerosis and hypertension.

SUMMARY

The development of mice with conditionally deleted parathyroid hormone-related protein or parathyroid hormone-1 receptor alleles will allow the creation of cell- or tissue-specific parathyroid hormone-related protein knockout mice which will greatly facilitate the determination of the biological relevance of this protein in a specific cell or tissue type, particularly in the cardiovascular system.

Establishment of a novel chondrocytic cell line N1511 derived from p53-null mice

We established a clonal chondrocytic cell line N1511 derived from rib cartilage of a p53-null mouse. N1511 cells proliferated in polygonal shape and elicited differentiation at confluence when treated with combination of bone morphogenetic protein (BMP) 2 and insulin or parathyroid hormone (PTH) and dexamethasone. BMP-2/insulin-treated cells became refractile without forming cartilaginous nodules and reached terminal differentiation, became positive for alizarin red staining, and developed considerable ALP activity. In contrast, PTH/dexamethasone-treated cells formed Alcian blue-positive nodules but remained negative for alizarin red staining and ALP activity. Northern blot analysis revealed that BMP-2/insulin-treated cells sequentially expressed type II, IX, and X collagens, whereas PTH/dexamethasone-treated cells slowly expressed type II collagen and then type IX, and they did not exhibit type X collagen expression. These results show that BMP-2/insulin treatment induces full differentiation toward hypertrophy, whereas treatment with PTH/dexamethasone slows and limits differentiation. Recovery of p53 expression in N1511 cells by transient transfection inhibited cell proliferation, suggesting that cell proliferation could be regulated with p53 in this cell line. These results indicate that N1511 is the only cell line with known genetic mutation, which undergoes multiple steps of chondrocyte differentiation toward hypertrophy, and because proliferation could be regulated by expression of p53, N1511 could be an excellent model for studies of chondrogenesis, the function of p53, and genetic engineering of cartilage tissue.

Molecular cloning and functional characterization of the canine parathyroid hormone/parathyroid hormone related peptide receptor (PTH1)

Parathyroid hormone (PTH) is a major mediator of calcium and phosphate metabolism through its interactions with receptors in kidney and bone. PTH binds with high affinity to PTH1 and PTH2, members of the superfamily of G protein-coupled receptors. In order to clone the canine PTH1 receptor, a canine kidney cDNA library was screened using the human PTH1 receptor cDNA and two clones were further characterized. The longest clone was 2177 bp and contained a single open reading frame of 1785 bp, potentially encoding a protein of 595 amino acids with a predicted molecular weight of 66.4 kD. This open reading frame exhibits >91% identity to the human PTH1 receptor cDNA and >95% identity when the putative canine and human protein sequences are compared. Competition binding following transfection of the canine PTH1 receptor into CHO cells demonstrated specific displacement of 125I-human PTH1-34 by canine PTH1-34, human PTH1-34, and canine/human parathyroid hormone related peptide (PTHrP) 1-34. Treatment of canine PTH1 receptor transfected cells, but not mock transfected cells, with these ligands also resulted in increased levels of intracellular cAMP. In contrast, the non-related aldosterone secretion inhibiting factor 1-35 neither bound nor activated the canine PTH1 receptor. Northern blot analysis revealed high levels of PTH1 receptor mRNA in the kidney, with much lower, but detectable, levels in aorta. heart, lung, prostate, testis, and skeletal muscle. Together, these data indicate that we have cloned the canine PTH1 receptor and that it is very similar, both in sequence and in functional characteristics, to the other known PTH1 receptors.