Indian hedgehog coordinates endochondral bone growth and morphogenesis via parathyroid hormone related-protein-dependent and -independent pathways

Indian hedgehog (Ihh) and Parathyroid Hormone-related Protein (PTHrP) play a critical role in the morphogenesis of the vertebrate skeleton. Targeted deletion of Ihh results in short-limbed dwarfism, with decreased chondrocyte proliferation and extensive hypertrophy, features shared by mutants in PTHrP and its receptor. Activation of Ihh signaling upregulates PTHrP at the articular surface and prevents chondrocyte hypertrophy in wild-type but not PTHrP null explants, suggesting that Ihh acts through PTHrP. To investigate the relationship between these factors during development of the appendicular skeleton, mice were produced with various combinations of an Ihh null mutation (Ihh(−/−)), a PTHrP null mutation (PTHrP(−/−)), and a constitutively active PTHrP/Parathyroid hormone Receptor expressed under the control of the Collagen II promoter (PTHrPR*). PTHrPR* rescues PTHrP(−/−) embryos, demonstrating this construct can completely compensate for PTHrP signalling. At 18.5 dpc, limb skeletons of Ihh, PTHrP compound mutants were identical to Ihh single mutants suggesting Ihh is necessary for PTHrP function. Expression of PTHrPR* in chondrocytes of Ihh(−/−) mice prevented premature chondrocyte hypertrophy but did not rescue either the short-limbed dwarfism or decreased chondrocyte proliferation. These experiments demonstrate that the molecular mechanism that prevents chondrocyte hypertrophy is distinct from that which drives proliferation. Ihh positively regulates PTHrP, which is sufficient to prevent chondrocyte hypertrophy and maintain a normal domain of cells competent to undergo proliferation. In contrast, Ihh is necessary for normal chondrocyte proliferation in a pathway that can not be rescued by PTHrP signaling. This identifies Ihh as a coordinator of skeletal growth and morphogenesis, and refines the role of PTHrP in mediating a subset of Ihh's actions.

A novel parathyroid hormone (PTH)/PTH-related peptide receptor mutation in Jansen's metaphyseal chondrodysplasia

Two heterozygous PTH/PTH-related peptide (PTHrP) receptor missense mutations were previously identified in patients with Jansen's metaphyseal chondrodysplasia (JMC), a rare form of short limb dwarfism associated with hypercalcemia and normal or undetectable levels of PTH and PTHrP. Both mutations, H223R and T410P, resulted in constitutive activation of the cAMP signaling pathway and provided a plausible explanation for the abnormalities in skeletal development and mineral ion homeostasis. In the present study we analyzed genomic DNA from four additional sporadic cases with JMC to search for novel activating mutations in the PTH/PTHrP receptor, to determine the frequency of the two previously identified missense mutations, H223R and T410P, and to determine whether different mutations present with different severity of the disease. The H223R mutation was identified in three novel JMC patients and is, therefore, to date the most frequent cause of JMC. In the fourth patient, a novel heterozygous missense mutation was found that changes isoleucine 458 in the receptor's seventh membrane-spanning region to arginine (I458R). In COS-7 cells expressing the human PTH/PTHrP receptor with the I458R mutation, basal cAMP accumulation was approximately 8 times higher than that in cells expressing the wild-type receptor despite impaired surface expression of the mutant receptor. Furthermore, the I458R mutant showed higher responsiveness to PTH than the wild-type receptor in its ability to activate both downstream effectors, adenylyl cyclase and phospholipase C. Like the H223R and the T410P mutants, the I458R mutant had no detectable effect on basal inositol phosphate accumulation. Overall, the patient with the I458R mutation exhibited clinical and biochemical abnormalities similar to those in patients with the previously identified H223R and T410P mutations.

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.