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

It is well appreciated that high-density lipoprotein (HDL) and bone physiology and pathology are tightly linked. Studies, primarily in mouse models, have shown that dysfunctional and/or disturbed HDL can affect bone mass through many different ways. Specifically, reduced HDL levels have been associated with the development of an inflammatory microenvironment that affects the differentiation and function of osteoblasts. In addition, perturbation in metabolic pathways of HDL favors adipoblastic differentiation and restrains osteoblastic differentiation through, among others, the modification of specific bone-related chemokines and signaling cascades. Increased bone marrow adiposity also deteriorates bone osteoblastic function and thus bone synthesis, leading to reduced bone mass. In this review, we present the current knowledge and the future directions with regard to the HDL–bone mass connection. Unraveling the molecular phenomena that underline this connection will promote the deeper understanding of the pathophysiology of bone-related pathologies, such as osteoporosis or bone metastasis, and pave the way toward the development of novel and more effective therapies against these conditions.

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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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M J Devlin, C Grasemann, A M Cloutier, L Louis, C Alm, M R Palmert and M L Bouxsein

Maternal high-fat (HF) diet can alter offspring metabolism via perinatal developmental programming. This study tests the hypothesis that maternal HF diet also induces perinatal programming of offspring bone mass and strength. We compared skeletal acquisition in pups from C57Bl/6J mice fed HF or normal diet from preconception through lactation. Three-week-old male and female pups from HF (HF-N) and normal mothers (N-N) were weaned onto normal diet. Outcomes at 14 and 26 weeks of age included body mass, body composition, whole-body bone mineral content (WBBMC) via peripheral dual-energy X-ray absorptiometry, femoral cortical and trabecular architecture via microcomputed tomography, and glucose tolerance. Female HF-N had normal body mass and glucose tolerance, with lower body fat (%) but higher serum leptin at 14 weeks vs N-N (P<0.05 for both). WBBMC was 12% lower at 14 weeks and 5% lower at 26 weeks, but trabecular bone volume fraction was 20% higher at 14 weeks in female HF-N vs N-N (P<0.05 for all). Male HF-N had normal body mass and mildly impaired glucose tolerance, with lower body fat (%) at 14 weeks and lower serum leptin at 26 weeks vs N-N (P<0.05 for both). Serum insulin was higher at 14 weeks and lower at 26 weeks in HF-N vs N-N (P<0.05). Trabecular BV/TV was 34% higher and cortical bone area was 6% higher at 14 weeks vs N-N (P<0.05 for both). These data suggest that maternal HF diet has complex effects on offspring bone, supporting the hypothesis that maternal diet alters postnatal skeletal homeostasis.

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K J Oldknow, V E MacRae and C Farquharson

Recent developments in endocrinology, made possible by the combination of mouse genetics, integrative physiology and clinical observations have resulted in rapid and unanticipated advances in the field of skeletal biology. Indeed, the skeleton, classically viewed as a structural scaffold necessary for mobility, and regulator of calcium–phosphorus homoeostasis and maintenance of the haematopoietic niche has now been identified as an important regulator of male fertility and whole-body glucose metabolism, in addition to the classical insulin target tissues. These seminal findings confirm bone to be a true endocrine organ. This review is intended to detail the key events commencing from the elucidation of osteocalcin (OC) in bone metabolism to identification of new and emerging candidates that may regulate energy metabolism independently of OC.

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Colin Farquharson and Katherine Staines

.1073/pnas.1003727107 . Karsenty G 2003 The complexities of skeletal biology . Nature 423 316 – 318 . doi:10.1038/nature01654 . Kousteni S 2011 FoxO1: a molecule for all seasons . Journal of Bone and Mineral Research 26 912 – 917 . doi

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Anyonya R Guntur and Clifford J Rosen

developments in skeletal biology has been the establishment of the skeleton as a modulator of glucose and metabolic homeostasis ( Lee et al . 2007 , Ferron et al . 2010 , Clemens & Karsenty 2011 ). For example, a recent study in which FOXO1 was deleted only

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Timothy C R Prickett, Graham K Barrell, Martin Wellby, Timothy G Yandle, A Mark Richards and Eric A Espiner

. 2005 ) and is readily measurable in plasma ( Prickett et al . 2001 ) has opened up new approaches to assessing CNP's paracrine actions and role in skeletal biology. For example, the plasma concentration of NTproCNP and markers of bone formation

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Julian C Lui

nutritional status regulate the maintenance of resting chondrocytes? This exciting era of stem cell research in skeletal biology holds much promise of providing further insights into childhood bone growth under physiological and pathological conditions

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Cátia F Gonçalves and Qing-Jun Meng

clocks present in most of the cell types in cartilage and bone ( Dudek & Meng 2014 , Yang & Meng 2016 ). Alterations in the circadian rhythm of skeletal biology are associated with the development of various disorders, including osteoarthritis (OA