Parathyroid hormone/parathyroid hormone-related protein receptor signaling is required for maintenance of the growth plate in postnatal life

β-adrenergic receptor signaling regulates Ptgs2 by driving circadian gene expression in osteoblasts

The sympathetic nervous system modulates bone remodeling and mediates the expression of core clock genes in part through the β-adrenergic receptor (β-AR) in osteoblasts. In this study, we show that in MC3T3-E1 osteoblastic cells that isoproterenol (Iso), a non-selective β-AR agonist, upregulated the transcriptional factor Nfil3, and induced rhythmic mRNA expression of prostaglandin-endoperoxide synthase 2 (Ptgs2, also known as Cox2). The rhythmic effects of Iso on Ptgs2 expression were mediated by interplay between the Per2 and Bmal1 clock genes in osteoblasts. In addition, Ptgs2 was significantly decreased in bone after continuous Iso treatment. Overexpression of Nfil3 decreased Ptgs2 expression in MC3T3-E1 cells. Knockdown of Nfil3 upregulated the expression of Ptgs2 in MC3TC-E1 cells, indicating that Nfil3 negatively regulated Ptgs2 in osteoblasts. Furthermore, Iso acutely induced the expression Nfil3 and increased the binding of Nfil3 to the Ptgs2 promoter in MC3T3-E1 cells. These results suggest that Iso-mediated induction of Nfil3 in osteoblasts regulates the expression of Ptgs2 by driving the expression of circadian clock genes. These findings provide new evidence for a physiological role of circadian clockwork in bone metabolism.

G-protein stimulatory subunit alpha and Gq/11α G-proteins are both required to maintain quiescent stem-like chondrocytes

Round chondrocytes in the resting zone of the growth plate provide precursors for columnar chondrocytes and have stem-like properties. Here we demonstrate that these stem-like chondrocytes undergo apoptosis in the absence of the receptor (PPR) for parathyroid hormone-related protein. We examine the possible roles of heterotrimeric G-proteins activated by the PPR. Inactivation of the G-protein stimulatory α-subunit (G(s)α) leads to accelerated differentiation of columnar chondrocytes, as seen in the PPR knockout, but a remnant of growth cartilage remains, in contrast to disappearance of the growth cartilage in the PPR knockout. Stem-like chondrocytes lose their quiescence and proliferate upon G(s)α ablation. Inactivation of G(s)α in mice with a mutant PPR that cannot activate G proteins, Gq and G11, leads to a PPR knockout-like phenotype. Thus, G(s)α is the major mediator of the anti-differentiation action of the PPR, while activation of both G(s)α and Gq/11α is required for quiescence of stem-like chondrocytes.

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.

Time-dependent processes in stem cell-based tissue engineering of articular cartilage

Articular cartilage (AC), situated in diarthrodial joints at the end of the long bones, is composed of a single cell type (chondrocytes) embedded in dense extracellular matrix comprised of collagens and proteoglycans. AC is avascular and alymphatic and is not innervated. At first glance, such a seemingly simple tissue appears to be an easy target for the rapidly developing field of tissue engineering. However, cartilage engineering has proven to be very challenging. We focus on time-dependent processes associated with the development of native cartilage starting from stem cells, and the modalities for utilizing these processes for tissue engineering of articular cartilage.

Parathyroid hormone-related protein: an update

PTHrP was identified as a cause of hypercalcemia in cancer patients 25 yr ago. In the intervening years, we have learned that PTHrP and PTH are encoded by related genes that are part of a larger "PTH gene family." This evolutionary relationship permits them to bind to the same type 1 PTH/PTHrP receptor, which explains why humoral hypercalcemia of malignancy resembles hyperparathyroidism. This review will outline basic facts about PTHrP biology and its normal physiological functions, with an emphasis on new findings of the past 5-10 yr. The medical and research communities first became aware of PTHrP because of its involvement in a common paraneoplastic syndrome. Now, research into the basic biology of PTHrP has suggested previously unrecognized connections to a variety of disease states such as osteoporosis, osteoarthritis, and breast cancer and has highlighted how PTHrP itself might be used in therapy for osteoporosis and diabetes. Therefore, the story of this remarkable protein is a paradigm for translational research, having gone from bedside to bench and now back to bedside.

Functional characterization and evolution of PTH/PTHrP receptors: insights from the chicken

BACKGROUND

The parathyroid hormone (PTH)-family consists of a group of structurally related factors that regulate calcium and bone homeostasis and are also involved in development of organs such as the heart, mammary gland and immune system. They interact with specific members of family 2 B1 G-protein coupled receptors (GPCRs), which have been characterised in teleosts and mammals. Two PTH/PTHrP receptors, PTH1R and PTH2R exist in mammals and in teleost fish a further receptor PTH3R has also been identified. Recently in chicken, PTH-family members involved in calcium transport were characterized and specific PTHRs are suggested to exist although they have not yet been isolated or functionally characterized. The aim of this study is to further explore the evolution and function of the vertebrate PTH/PTHrP system through the isolation, phylogenetic analysis and functional characterization of the chicken receptors.

RESULTS

Two PTHRs were isolated in chicken and sequence comparison and phylogenetic analysis indicate that the chicken receptors correspond to PTH1R and PTH3R, which emerged prior to the teleost/tetrapod divergence since they are present in cartilaginous fish. The vertebrate PTH2R receptor and its ligand TIP39 have been lost from bird genomes. Chicken PTH1R and PTH3R have a divergent and widespread tissue expression and are also evident in very early embryonic stages of development. Receptor stimulation studies using HEK293 cells stably expressing the chicken PTH1R and PTH3R and monitoring cAMP production revealed they are activated by chicken 1-34 N-terminal PTH-family peptides in a dose dependent manner. PTH-L and PTHrP were the most effective peptides in activating PTH1R (EC(50) = 7.7 nM and EC(50) = 22.7 nM, respectively). In contrast, PTH-L (100 nM) produced a small cAMP accumulation on activation of PTH3R but PTHrP and PTH (EC(50) = 2.5 nM and EC(50) = 22.1 nM, respectively) readily activated the receptor. PTHrP also stimulated intracellular Ca(2+) accumulation on activation of PTH1R but not PTH3R.

CONCLUSION

Two PTHR homologues of the vertebrate PTH1R and PTH3R were isolated and functionally characterized in chicken. Their distinct pattern of expression during embryo development and in adult tissues, together with their ligand preference, suggests that they have acquired specific functions, which have contributed to their maintenance in the genome. PTH2R and its activating ligand, TIP39, are absent from bird genomes. Nonetheless identification of putative PTH2R and TIP39 in the genome of an ancient agnathan, lamprey, suggests the PTH/PTHrP ligand and receptor family was already present in an early basal paraphyletic group of vertebrates and during the vertebrate radiation diverged via gene/genome duplication and deletion events. Knowledge of the role PTH/PTHrP system in early vertebrates will help to establish evolution of function.

Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease

FGF23 is a bone-derived hormone that regulates systemic phosphate homeostasis, vitamin D metabolism and α-Klotho expression through a novel bone-kidney axis. FGF23 inhibits renal tubular reabsorption of phosphate through mechanisms independent of PTH as well as reduces circulating 1, 25(OH)(2)D through its dual effects to suppress Cyp27b1 production and to stimulate Cyp24 catabolism of 1,25(OH)(2)D. 1,25(OH)(2)D and other factors regulating bone remodeling/mineralization are the major physiological regulators of FGF23 expression. FGF23 also suppresses the gene transcription of α-klotho by the kidney, which exists as a membrane and soluble protein. Membrane Klotho acts as a coreceptor for and dictates organ specificity of FGF23, whereas soluble Klotho act as an endocrine factor that regulates activity of cell surface glycoproteins and receptors in multiple tissues. Elevated FGF23 levels are responsible for several hereditary and acquired hypophosphatemic rickets disorders. FGF23 and Klotho deficiency have similar phenotypes characterized by hyperphosphatemia, elevated 1,25(OH)(2)D and tumoral calcinosis. FGF23 levels progressively increase during chronic kidney disease (CKD). FGF23 has been proposed to be the initial adaptive response leading to reductions in 1,25(OH)(2)D and secondary hyperparathyroidism (HPT) in CKD. The overall biological effect of this initial step may be to orchestrate a coordinated adaptation to protect the organism from the adverse effects of excess phosphate retention. The second step involves the effects of PTH on bone remodeling that further stimulates FGF23 production through both direct and indirect mechanisms related to alterations in extracellular matrix factors. PTH further amplifies FGF23 expression in later stages of CKD to compensate for the increased phosphate efflux from bone caused by excessive bone turnover. While many aspects of the regulation and functions of FGF23 remain to be established, the idea that FGF23 hormone is the initial adaptive hormonal response in CKD that suppresses 1,25(OH)(2)D, reduces gastrointestinal calcium and phosphate absorption and leads to a secondary HPT represents a paradigm shift in the conceptualization of the pathogenesis of secondary hyperparathyroidism. In addition, the prevalent thought that CKD is a functional "vitamin D deficient state" requiring therapy with 1,25(OH)(2)D analogs is challenged by effects of FGF23 to potentially lower both 25(OH)D and 1,25(OH)D by induction of Cyp24-mediated degradation. Finally, increments in FGF23 are associated with increased cardiovascular mortality in CKD. Whether these effects represent direct effects of FGF23 or represent a marker of other abnormalities in CKD remains to be determined.

Effect of topical dimethylarsinic acid on the expression of apoptosis-related proteins in mouse skin

We investigated the effect of the topical application of dimethylarsinic acid (DMA) on skin thickness and the expression of several apoptosis-related proteins in skin. After administration of DMA during pregnancy, skin thickness and skin expression of Bcl-2, Bcl-3, Bad, Bid, and caspases-3, -6, -8, -9, and -12 were examined in dams and their offspring. DMA treatment caused significant increases in skin thickness (p < 0.05) and the expression of Bcl-2, Bad, and capase-12 in the skin of dams at the mRNA and protein levels (p < 0.01). However, maternal exposure to DMA did not significantly alter the expression of the studied apoptosis-related factors in the skin of the offspring. These findings indicate that DMA may induce skin apoptosis, in part, by modulating the expression of Bcl-2, Bad, and caspase-12 in maternal skin. Additionally, our results suggest that maternal exposure to DAM during pregnancy may not induce apoptosis in the skin of the offspring.

SIK3 is essential for chondrocyte hypertrophy during skeletal development in mice

Chondrocyte hypertrophy is crucial for endochondral ossification, but the mechanism underlying this process is not fully understood. We report that salt-inducible kinase 3 (SIK3) deficiency causes severe inhibition of chondrocyte hypertrophy in mice. SIK3-deficient mice showed dwarfism as they aged, whereas body size was unaffected during embryogenesis. Anatomical and histological analyses revealed marked expansion of the growth plate and articular cartilage regions in the limbs, accumulation of chondrocytes in the sternum, ribs and spine, and impaired skull bone formation in SIK3-deficient mice. The primary phenotype in the skeletal tissue of SIK3-deficient mice was in the humerus at E14.5, where chondrocyte hypertrophy was markedly delayed. Chondrocyte hypertrophy was severely blocked until E18.5, and the proliferative chondrocytes occupied the inside of the humerus. Consistent with impaired chondrocyte hypertrophy in SIK3-deficient mice, native SIK3 expression was detected in the cytoplasm of prehypertrophic and hypertrophic chondrocytes in developing bones in embryos and in the growth plates in postnatal mice. HDAC4, a crucial repressor of chondrocyte hypertrophy, remained in the nuclei in SIK3-deficient chondrocytes, but was localized in the cytoplasm in wild-type hypertrophic chondrocytes. Molecular and cellular analyses demonstrated that SIK3 was required for anchoring HDAC4 in the cytoplasm, thereby releasing MEF2C, a crucial facilitator of chondrocyte hypertrophy, from suppression by HDAC4 in nuclei. Chondrocyte-specific overexpression of SIK3 induced closure of growth plates in adulthood, and the SIK3-deficient cartilage phenotype was rescued by transgenic SIK3 expression in the humerus. These results demonstrate an essential role for SIK3 in facilitating chondrocyte hypertrophy during skeletogenesis and growth plate maintenance.

Organization of the Indian hedgehog--parathyroid hormone-related protein system in the postnatal growth plate

In embryonic growth cartilage, Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP) participate in a negative feedback loop that regulates chondrocyte differentiation. Postnatally, this region undergoes major structural and functional changes. To explore the organization of the Ihh–PTHrP system in postnatal growth plate, we microdissected growth plates of 7-day-old rats into their constituent zones and assessed expression of genes participating in the h–PTHrP feedback loop. Ihh, Patched 1, Smoothened, Gli1, Gli2, Gli3, and Pthr1 were expressed in regions analogous to the expression domains in embryonic growth cartilage. However, PTHrP was expressed in resting zone cartilage, a site that differs from the embryonic source, the periarticular cells. We then used mice in which lacZ has replaced coding sequences of Gli1 and thus serves as a marker for active hedgehog signaling. At 1, 4, 8, and 12 weeks of age, lacZ expression was detected in a pattern analogous to that of embryonic cartilage. The findings support the hypothesis that the embryonic Ihh–PTHrP feedback loop is maintained in the postnatal growth plate except that the source of PTHrP has shifted to a more proximal location in the resting zone.

Site-1 protease is essential to growth plate maintenance and is a critical regulator of chondrocyte hypertrophic differentiation in postnatal mice

Site-1 protease (S1P) is a proprotein convertase with essential functions in lipid homeostasis and unfolded protein response pathways. We previously studied a mouse model of cartilage-specific knock-out of S1P in chondroprogenitor cells. These mice exhibited a defective cartilage matrix devoid of type II collagen protein (Col II) and displayed chondrodysplasia with no endochondral bone formation even though the molecular program for endochondral bone development appeared intact. To gain insights into S1P function, we generated and studied a mouse model in which S1P is ablated in postnatal chondrocytes. Postnatal ablation of S1P results in chondrodysplasia. However, unlike early embryonic ablations, the growth plates of these mice exhibit a lack of Ihh, PTHrP-R, and Col10 expression indicating a loss of chondrocyte hypertrophic differentiation and thus disruption of the molecular program required for endochondral bone development. S1P ablation results in rapid growth plate disruption due to intracellular Col II entrapment concomitant with loss of chondrocyte hypertrophy suggesting that these two processes are related. Entrapment of Col II in the chondrocytes of the prospective secondary ossification center precludes its development. Trabecular bone formation is dramatically diminished in the primary spongiosa and is eventually lost. The primary growth plate is eradicated by apoptosis but is gradually replaced by a fully functional new growth plate from progenitor stem cells capable of supporting new bone growth. Our study thus demonstrates that S1P has fundamental roles in the preservation of postnatal growth plate through chondrocyte differentiation and Col II deposition and functions to couple growth plate maturation to trabecular bone development in growing mice.

Genetic regulation of the growth plate

The epiphyseal growth plate consists of a layer of cartilage present only during the growth period and vanishes soon after puberty in long bones. It is divided to three well-defined zones, from epiphyses; resting, proliferative, and hypertrophic zones. Chondrocyte proliferation and differentiation and subsequent bone formation in this cartilage are controlled by various endocrine, autocrine, and paracrine factors which finally results into elimination of the cartilaginous tissue and promotion of the epiphyseal fusion. As chondrocytes differentiate from round, quiescent, and single structure to flatten and proliferative and then large and terminally differentiated, they experience changes in their gene expression pattern which allow them to transform from cartilaginous tissue to bone. This review summarizes the literature in this area and shortly describes different factors that affect growth plate cartilage both at the local and systemic levels. This may eventually help us to develop new treatment strategies of different growth disorders.