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Concept: Bone resorption

148

Hypoparathyroidism is a disease of chronic hypocalcemia and hyperphosphatemia due to a deficiency of parathyroid hormone (PTH). PTH and analogs of the hormone are of interest as potential therapies. Accordingly, we examined the pharmacological properties of a long-acting PTH analog, [Ala(1,3,12,18,22) , Gln(10) ,Arg(11) ,Trp(14) ,Lys(26) ]-PTH(1-14)/PTHrP(15-36) (LA-PTH) in thyroparathyroidectomized (TPTX) rats, a model of HP, as well as in normal monkeys. In TPTX rats, a single intra-venous administration of LA-PTH at a dose of 0.9 nmol/kg increased serum calcium (sCa) and decreased serum phosphate (sPi) to near-normal levels for longer than 48 hours, while PTH(1-34) and PTH(1-84), each injected at a dose 80-fold higher than that used for LA-PTH, increased sCa and decreased sPi only modestly and transiently (< 6 hours). LA-PTH also exhibited enhanced and prolonged efficacy versus PTH(1-34) and PTH(1-84) for elevating sCa when administered subcutaneously (SC) into monkeys. Daily SC administration of LA-PTH (1.8 nmol/kg) into TPTX rats for 28-days elevated sCa to near normal levels without causing hypercalciuria or increasing bone resorption markers, a desirable goal in the treatment of hypoparathyroidism. The results are supportive of further study of long-acting PTH analogs as potential therapies for patients with hypoparathyroidism. This article is protected by copyright. All rights reserved.

Concepts: Bone, Parathyroid hormone, Bone resorption, Calcium metabolism, Parathyroid gland, Hypoparathyroidism, Hyperparathyroidism, Copyright

28

Parathyroid hormone (PTH) activates receptors on osteocytes to orchestrate bone formation and resorption. Here we show that PTH inhibition of SOST (sclerostin), a WNT antagonist, requires HDAC4 and HDAC5, whereas PTH stimulation of RANKL, a stimulator of bone resorption, requires CRTC2. Salt inducible kinases (SIKs) control subcellular localization of HDAC4/5 and CRTC2. PTH regulates both HDAC4/5 and CRTC2 localization via phosphorylation and inhibition of SIK2. Like PTH, new small molecule SIK inhibitors cause decreased phosphorylation and increased nuclear translocation of HDAC4/5 and CRTC2. SIK inhibition mimics many of the effects of PTH in osteocytes as assessed by RNA-seq in cultured osteocytes and following in vivo administration. Once daily treatment with the small molecule SIK inhibitor YKL-05-099 increases bone formation and bone mass. Therefore, a major arm of PTH signalling in osteocytes involves SIK inhibition, and small molecule SIK inhibitors may be applied therapeutically to mimic skeletal effects of PTH.

Concepts: Bone, Calcitonin, Signal transduction, Skeletal system, Parathyroid hormone, Bone resorption, Osteoclast, Enzyme inhibitor

28

This study reports the case of a patient with a severely resorbed mandible who was treated without a bone graft, using short implants, internal rigid fixation, rhBMP-2 and β-tricalcium phosphate. A 76-year-old woman, with a severely resorbed mandible (less than 3mm), reported a history of nearly 25years of complete edentulism and consecutive treatment failures, with total bilateral exposed inferior alveolar nerves and complete bone resorption of the inferior border in some areas. The treatment of choice was the placement of a 2.0mm thick unilock bone plate (MatrixMandible, Synthes Maxillofacial, Paoli, PA, USA), to reinforce the mandible. Eight short implants with a regular platform (Nobel Biocare, Goteborg, Sweden) were placed: three on the external oblique line on both sides and two on the symphysis. In order to augment mandible height and coat the exposed thread of the anterior implants, rhBMP-2 (Infuse Bone, Meditronic Sofamor Danek, Memphis, TN, USA) and β-tricalcium phosphate (Cerasorb; Curasan, Kleinostheim, Germany) were used. Four 1.3mmL miniplates were placed to support the graft. 14months after surgery, the patient was satisfied and had excellent function without complications.

Concepts: Bone, Surgery, Bone resorption, Sweden, Dental implant, Oral and maxillofacial surgery, Abdominal external oblique muscle, Nobel Biocare

28

During orthodontic tooth movement (OTM), alveolar bone is resorbed by osteoclasts in compression sites (CS) and is deposited by osteoblasts in tension sites (TS). The aim of this study was to develop a standardized OTM protocol in mice and to investigate the expression of bone resorption and deposition markers in CS and TS. An orthodontic appliance was placed in C57BL6/J mice. To define the ideal orthodontic force, the molars of the mice were subjected to forces of 0.1N, 0.25N, 0.35N and 0.5N. The expression of mediators that are involved in bone remodeling at CS and TS was analyzed using a Real-Time PCR. The data revealed that a force of 0.35N promoted optimal OTM and osteoclast recruitment without root resorption. The levels of TNF-α, RANKL, MMP13 and OPG were all altered in CS and TS. Whereas TNF-α and Cathepsin K exhibited elevated levels in CS, RUNX2 and OCN levels were higher in TS. Our results suggest that 0.35N is the ideal force for OTM in mice and has no side effects. Moreover, the expression of bone remodeling markers differed between the compression and the tension areas, potentially explaining the distinct cellular migration and differentiation patterns in each of these sites.

Concepts: Osteoporosis, Bone, Polymerase chain reaction, Developmental biology, Bone resorption, Osteoclast, Force, Cathepsin K

14

Bone homeostasis is regulated by communication between bone-forming mature osteoblasts (mOBs) and bone-resorptive mature osteoclasts (mOCs). However, the spatial-temporal relationship and mode of interaction in vivo remain elusive. Here we show, by using an intravital imaging technique, that mOB and mOC functions are regulated via direct cell-cell contact between these cell types. The mOBs and mOCs mainly occupy discrete territories in the steady state, although direct cell-cell contact is detected in spatiotemporally limited areas. In addition, a pH-sensing fluorescence probe reveals that mOCs secrete protons for bone resorption when they are not in contact with mOBs, whereas mOCs contacting mOBs are non-resorptive, suggesting that mOBs can inhibit bone resorption by direct contact. Intermittent administration of parathyroid hormone causes bone anabolic effects, which lead to a mixed distribution of mOBs and mOCs, and increase cell-cell contact. This study reveals spatiotemporal intercellular interactions between mOBs and mOCs affecting bone homeostasis in vivo.

Concepts: Bone, Calcitonin, Cell, Parathyroid hormone, Bone resorption, Osteoclast, Steady state, Osteoblast

2

Myelomatous bone disease is characterized by the development of lytic bone lesions and a concomitant reduction in bone formation, leading to chronic bone pain and fractures. To understand the underlying mechanism, we investigated the contribution of myeloma-expressed thymidine phosphorylase (TP) to bone lesions. In osteoblast progenitors, TP up-regulated the methylation of RUNX2 and osterix, leading to decreased bone formation. In osteoclast progenitors, TP up-regulated the methylation of IRF8 and thereby enhanced expression of NFATc1 (nuclear factor of activated T cells, cytoplasmic 1 protein), leading to increased bone resorption. TP reversibly catalyzes thymidine into thymine and 2-deoxy-d-ribose (2DDR). Myeloma-secreted 2DDR bound to integrin αVβ3/α5β1 in the progenitors, activated PI3K (phosphoinositide 3-kinase)/Akt signaling, and increased DNMT3A (DNA methyltransferase 3A) expression, resulting in hypermethylation of RUNX2, osterix, and IRF8 This study elucidates an important mechanism for myeloma-induced bone lesions, suggesting that targeting TP may be a viable approach to healing resorbed bone in patients. Because TP overexpression is common in bone-metastatic tumors, our findings could have additional mechanistic implications.

Concepts: DNA, Protein, Bone, Bone resorption, Osteoclast, DNA methylation, Methylation, DNA methyltransferase

0

Exercise can cause a decrease in serum ionized calcium (iCa) and increases in parathyroid hormone (PTH) and bone resorption. We used a novel intravenous iCa clamp technique to determine whether preventing a decline in serum iCa during exercise prevents increases in PTH and carboxy-terminal collagen crosslinks (CTX). Eleven cycling-trained men (aged 18-45 y) underwent two identical 60-minute cycling bouts with infusion of Ca gluconate or saline. Blood sampling for iCa, total calcium (tCa), PTH, CTX, and procollagen type 1 amino-terminal propeptide (P1NP) occurred before, during, and for 4 hours after exercise; results are presented as unadjusted and adjusted for plasma volume shifts (ADJ subscript). iCa decreased during exercise with saline infusion (p = 0.01 at 60 min) and this was prevented by Ca infusion (interaction, p < 0.007); there were abrupt decreases in Ca content (iCaADJand tCaADJ) in the first 15 minutes of exercise under both conditions. PTH and CTX were increased at the end of exercise (both p < 0.01) on the saline day, and markedly attenuated (-65% and -71%; both p < 0.001) by Ca. CTX remained elevated for 4 hours after exercise on the saline day (p < 0.001), despite the return of PTH to baseline by 1 hour after exercise. P1NP increased in response to exercise (p < 0.001), with no difference between conditions, but the increase in P1NPADJwas not significant. Results for PTHADJand CTXADJwere similar to unadjusted results. These findings demonstrate that bone resorption is stimulated early in exercise to defend serum iCa. Vascular Ca content decreased early in exercise, but neither the reason why this occurred, nor the fate of Ca, are known. The results suggest that the exercise-induced increase in PTH had an acute catabolic effect on bone. Future research should determine whether the increase in PTH generates an anabolic response that occurs more than 4 hours after exercise. This article is protected by copyright. All rights reserved.

Concepts: Bone, Vitamin D, Calcitonin, Parathyroid hormone, Bone resorption, Osteoclast, Calcium metabolism, Anabolism

0

Bone homeostasis depends on the resorption of bones by osteoclasts and formation of bones by the osteoblasts. Imbalance of this tightly coupled process can cause diseases such as osteoporosis. Thus, the mechanisms that regulate communication between osteoclasts and osteoblasts are critical to bone cell biology. It has been shown that osteoblasts and osteoclasts can communicate with each other through direct cell-cell contact, cytokines, and extracellular matrix interaction. Osteoblasts can affect osteoclast formation, differentiation, or apoptosis through several pathways, such as OPG/RANKL/RANK, RANKL/LGR4/RANK, Ephrin2/ephB4, and Fas/FasL pathways. Conversely, osteoclasts also influence formation of bones by osteoblasts via the d2 isoform of the vacuolar (H+) ATPase (v-ATPase) V0 domain (Atp6v0d2), complement component 3a, semaphorin 4D or microRNAs. In addition, cytokines released from the resorbed bone matrix, such as TGF-β and IGF-1, also affect the activity of osteoblasts. Drugs could be developed by enhancing or restricting some of these interactions. Several reviews have been performed on the osteoblast-osteoclast communication. However, few reviews have shown the research advances in the recent years. In this review, we summarized the current knowledge on osteoblast-osteoclast communication.

Concepts: Osteoporosis, Bone, Extracellular matrix, Bone marrow, Bone resorption, Osteoclast, Communication, Osteoblast

0

To describe our initial experience in the treatment of hyperparathyroidism (HPP) of renal cause using total or subtotal parathyroidectomy, autoimplant and cryopreservation in pediatric patients. Secondary HPP is the increased function of the parathyroid hormone (PTH) due to an abnormal phosphocalcic metabolism in patients with chronic renal failure (CRF). This situation produces increased bone resorption resulting in osteodystrophy and endovascular calcifications. Surgical treatment is aimed to diminish the level of PTH in CRF patients, to avoid HPP complications.

Concepts: Renal failure, Bone, Parathyroid hormone, Bone resorption, Osteoclast, Parathyroid gland, Hypoparathyroidism, Hyperparathyroidism

0

The objective of this study was to investigate the effects of human parathyroid hormone (1-34) (PTH1-34; PTH) plus menaquinone-4 (vitamin K2; MK) on the osseous integration of hydroxyapatite (HA)-coated implants in osteoporotic rats. Ovariectomized female Sprague-Dawley rats were used for the study. Twelve weeks after bilateral ovariectomy, HA-coated titanium implants were inserted bilaterally in the femoral medullary canal of the remaining 40 ovariectomized rats. All animals were then randomly assigned to four groups: Control, MK, PTH and PTH + MK. The rats from groups MK, PTH and PTH + MK received vitamin K2 (30 mg/kg/day), PTH1-34 (60 μg/kg, three times a week), or both for 12 weeks. Thereafter, serum levels of γ-carboxylated osteocalcin (Gla-OC) were quantitated by ELISA and the bilateral femurs of rats were harvested for evaluation. The combination of PTH and MK clearly increased the serum levels of Gla-OC (a specific marker for bone formation) compared to PTH or MK alone. The results of our study indicated that all treated groups had increased new bone formation around the surface of implants and increased push-out force compared to Control. In addition, PTH + MK treatment showed the strongest effects in histological, micro-computed tomography and biomechanical tests. In summary, our results confirm that treatment with PTH1-34 and MK together may have a therapeutic advantage over PTH or MK monotherapy on bone healing around HA-coated implants in osteoporotic rats.

Concepts: Bone, Vitamin D, Calcitonin, Parathyroid hormone, Bone resorption, Osteoclast, Parathyroid gland