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Nicholaos I Papachristou, Harry C Blair, Kyriakos E Kypreos and Dionysios J Papachristou

importance in lipid transport and beyond . Journal of Lipid Research 54 2575 – 2585 . ( doi:10.1194/jlr.R035725 ) Sims NA Gooi JH 2008 Bone remodeling: multiple cellular interactions required for coupling of bone formation and resorption

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Rhonda D Prisby

metabolic activity is continual in bone and bone marrow cells, we must underscore how these factors released during cellular differentiation and bone formation and resorption contribute to the local regulation of bone blood flow. The regulation of

Open access

J Jeyabalan, M Shah, B Viollet, J P Roux, P Chavassieux, M Korbonits and C Chenu

level, we examined bone formation and resorption in the tibia of those mice, using bone histomorphometry. Analysis of BFR using double fluorescence labelling showed that the Ampkα1 −/− mice had a higher BFR than Ampkα1 + / + mice ( Fig. 2 A), but

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Iris Boraschi-Diaz, Josephine T Tauer, Omar El-Rifai, Delphine Guillemette, Geneviève Lefebvre, Frank Rauch, Mathieu Ferron and Svetlana V Komarova

oxidation in muscle cells ( Mera et al . 2016 ). Bone histomorphometric studies have shown that children and adolescents with OI have increased rates of bone formation and resorption ( Rauch et al . 2000 , 2010 ). Similarly, several mouse models of OI

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Koichiro Komatsu, Akemi Shimada, Tatsuya Shibata, Satoshi Wada, Hisashi Ideno, Kazuhisa Nakashima, Norio Amizuka, Masaki Noda and Akira Nifuji

Bisphosphonates (BPs) are a major class of antiresorptive drug, and their molecular mechanisms of antiresorptive action have been extensively studied. Recent studies have suggested that BPs target bone-forming cells as well as bone-resorbing cells. We previously demonstrated that local application of a nitrogen-containing BP (N-BP), alendronate (ALN), for a short period of time increased bone tissue in a rat tooth replantation model. Here, we investigated cellular mechanisms of bone formation by ALN. Bone histomorphometry confirmed that bone formation was increased by local application of ALN. ALN increased proliferation of bone-forming cells residing on the bone surface, whereas it suppressed the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts in vivo. Moreover, ALN treatment induced more alkaline phosphatase-positive and osteocalcin-positive cells on the bone surface than PBS treatment. In vitro studies revealed that pulse treatment with ALN promoted osteocalcin expression. To track the target cells of N-BPs, we applied fluorescence-labeled ALN (F-ALN) in vivo and in vitro. F-ALN was taken into bone-forming cells both in vivo and in vitro. This intracellular uptake was inhibited by endocytosis inhibitors. Furthermore, the endocytosis inhibitor dansylcadaverine (DC) suppressed ALN-stimulated osteoblastic differentiation in vitro and it suppressed the increase in alkaline phosphatase-positive bone-forming cells and subsequent bone formation in vivo. DC also blocked the inhibition of Rap1A prenylation by ALN in the osteoblastic cells. These data suggest that local application of ALN promotes bone formation by stimulating proliferation and differentiation of bone-forming cells as well as inhibiting osteoclast function. These effects may occur through endocytic incorporation of ALN and subsequent inhibition of protein prenylation.

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Kenneth A Philbrick, Stephen A Martin, Amy R Colagiovanni, Adam J Branscum, Russell T Turner and Urszula T Iwaniec

Impaired resorption of cartilage matrix deposited during endochondral ossification is a defining feature of juvenile osteopetrosis. Growing, leptin-deficient ob/ob mice exhibit a mild form of osteopetrosis. However, the extent to which the disease is (1) self-limiting and (2) reversible by leptin treatment is unknown. We addressed the first question by performing histomorphometric analysis of femurs in rapidly growing (2-month-old), slowly growing (4-month-old) and skeletally mature (6-month-old) wild-type (WT) and ob/ob male mice. Absent by 6 months of age in WT mice, cartilage matrix persisted to varying extents in distal femur epiphysis, metaphysis and diaphysis in ob/ob mice, suggesting that the osteopetrotic phenotype is not entirely self-limiting. To address the second question, we employed hypothalamic recombinant adeno-associated virus (rAAV) gene therapy to restore leptin signaling in ob/ob mice. Two-month-old mice were randomized to one of the three groups: (1) untreated control, (2) rAAV-Leptin or (3) control vector rAAV-green fluorescent protein and vectors injected intracerebroventricularly. Seven months later, rAAV-leptin-treated mice exhibited no cartilage in the metaphysis and greatly reduced cartilage in the epiphysis and diaphysis. At the cellular level, the reduction in cartilage was associated with increased bone turnover. These findings (1) support the concept that leptin is important for normal replacement of cartilage by bone, and (2) demonstrate that osteopetrosis in ob/ob mice is bone-compartment-specific and reversible by leptin at skeletal sites capable of undergoing robust bone turnover.

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Russell T Turner, Kenneth A Philbrick, Amida F Kuah, Adam J Branscum and Urszula T Iwaniec

Leptin, critical in regulation of energy metabolism, is also important for normal bone growth, maturation and turnover. Compared to wild type (WT) mice, bone mass is lower in leptin-deficient ob/ob mice. Osteopenia in growing ob/ob mice is due to decreased bone accrual, and is associated with reduced longitudinal bone growth, impaired cancellous bone maturation and increased marrow adipose tissue (MAT). However, leptin deficiency also results in gonadal dysfunction, disrupting production of gonadal hormones which regulate bone growth and turnover. The present study evaluated the role of increased estrogen in mediating the effects of leptin on bone in ob/ob mice. Three-month-old female ob/ob mice were randomized into one of the 3 groups: (1) ob/ob + vehicle (veh), (2) ob/ob + leptin (leptin) or (3) ob/ob + leptin and the potent estrogen receptor antagonist ICI 182,780 (leptin + ICI). Age-matched WT mice received vehicle. Leptin (40 µg/mouse, daily) and ICI (10 µg/mouse, 2×/week) were administered by subcutaneous injection for 1 month and bone analyzed by X-ray absorptiometry, microcomputed tomography and static and dynamic histomorphometry. Uterine weight did not differ between ob/ob mice and ob/ob mice receiving leptin + ICI, indicating that ICI successfully blocked the uterine response to leptin-induced increases in estrogen levels. Compared to leptin-treated ob/ob mice, ob/ob mice receiving leptin + ICI had lower uterine weight; did not differ in weight loss, MAT or bone formation rate; and had higher longitudinal bone growth rate and cancellous bone volume fraction. We conclude that increased estrogen signaling following leptin treatment is dispensable for the positive actions of leptin on bone and may attenuate leptin-induced bone growth.

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Russell T Turner, Kenneth A Philbrick, Carmen P Wong, Dawn A Olson, Adam J Branscum and Urszula T Iwaniec

Leptin-deficient ob/ob mice are morbidly obese and exhibit low total bone mass and mild osteopetrosis. In order to disassociate the skeletal effects of leptin deficiency from those associated with morbid obesity, we evaluated bone mass, architecture, gene expression, and indices of bone turnover in WT mice, ob/ob mice allowed to feed ad libitum (ob/ob), and ob/ob mice pair-fed equivalent to WT mice (pair-fed ob/ob). Mice were maintained at 32 °C (thermoneutral) from 6 to 18 weeks of age to minimize differences in resting energy expenditure. ob/ob mice were heavier, had more abdominal white adipose tissue (WAT), and were hyperglycemic compared with WT mice. Femur length, bone mineral content (BMC) and bone mineral density, and midshaft femur cortical thickness were lower in ob/ob mice than in WT mice. Cancellous bone volume (BV) fraction was higher but indices of bone formation and resorption were lower in ob/ob mice compared with WT mice; reduced bone resorption in ob/ob mice resulted in pathological retention of calcified cartilage. Pair-fed ob/ob mice were lighter and had lower WAT, uterine weight, and serum glucose than ob/ob mice. Similarly, femoral length, BMC, and cortical thickness were lower in pair-fed ob/ob mice compared with ob/ob mice, as were indices of cancellous bone formation and resorption. In contrast, bone marrow adiposity, calcified cartilage, and cancellous BV fraction were higher at one or more cancellous sites in pair-fed ob/ob mice compared with ob/ob mice. These findings indicate that the skeletal abnormalities caused by leptin deficiency are markedly attenuated in morbidly obese ob/ob mice.

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N Loveridge, C Farquharson, R Palmer, G E Lobley and D J Flint

Abstract

The control of longitudinal growth is poorly understood but GH is considered to be one of the major hormones regulating postnatal growth. However, there is dispute as to whether it has a direct or indirect action. To study the role of GH we used a polyclonal antiserum to rat GH and investigated changes in cell proliferation and enzyme activities associated with bone formation and resorption during longitudinal growth. IGF-I levels were measured by two independent RIAs, DNA synthesis by bromodeoxyuridine incorporation followed by immunocytochemistry and enzyme activities were quantified in situ by microdensitometry.

After 1 day the percentage of chondrocytes undergoing DNA synthesis within the proliferative zone was reduced but no other parameters were affected. By day 4 the labelling index was the same as in pair-fed animals but the number of chondrocytes synthesising DNA was reduced as was the total width of the growth plate and that of the proliferative zone. Alkaline phosphatase (associated with mineralisation) was unchanged but glucose 6-phosphate dehydrogenase activity (associated with cell proliferation) was decreased. Osteoclastic tartrate-resistant acid phosphatase activity (associated with bone resorption) was also significantly reduced. Similar changes were apparent after 10 days. At no time was the circulating level of IGF-I decreased.

These data suggest that, during longitudinal growth, GH affects the number of proliferating chondrocytes but not the percentage of cells undergoing DNA synthesis, indicating that its primary role may be on the commitment of prechondrocytes to a proliferative state. Furthermore, while GH does not seem to have any effect on skeletal mineralisation it may stimulate osteoclastic resorption of the primary spongiosa.

Journal of Endocrinology (1995) 146, 55–62

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Colin Farquharson and Katherine Staines

addition to providing a general review of endochondral ossification and bone formation and resorption, have focussed on some of the recent advances in chondrocyte, osteoblast and osteoclast biology. In the first thematic review, Mackie et al . (2011