Régulation de la masse osseuse par la leptine : un contrôle hypothalamique de la formation osseuse

Mechanical loading alleviated the inhibition of β2-adrenergic receptor agonist terbutaline on bone regeneration

The long-term use of adrenergic medication in treating various conditions, such as asthma, increases the chances of bone fracture. Dynamic mechanical loading at a specific time is a method for improving bone quality and promoting healing. Therefore, we hypothesized that precisely controlling the mechanical environment can contribute to the alleviation of the negative effects of chronic treatment with the common asthma drug terbutaline, which is a β2-adrenergic receptor agonist that facilitates bone homeostasis and defect repair through its anabolic effect on osteogenic cells. Our in vitro results showed that terbutaline can directly inhibit osteogenesis by impairing osteogenic differentiation and mineralization. Chronic treatment in vivo was simulated by administering terbutaline to C57BL/6J mice for 4 weeks before bone defect surgery and mechanical loading. We utilized a stabilized tibial defect model, which allowed the application of anabolic mechanical loading. During homeostasis, chronic terbutaline treatment reduced the bone formation rate, the fracture toughness of long bones, and the concentrations of bone formation markers in the sera. During defect repair, terbutaline decreased the bone volume, type H vessel, and total blood vessel volume. Terbutaline treatment reduced the number of osteogenic cells. Periostin, which was secreted mainly by Prrx1+ osteoprogenitors and F4/80+ macrophages, was inhibited by treating the bone defect with terbutaline. Interestingly, controlled mechanical loading facilitated the recovery of bone volume and periostin expression and the number of osteogenic cells within the defect. In conclusion, mechanical loading can rescue negative effects on new bone accrual and repair induced by chronic terbutaline treatment.

Female Mice Lacking Estrogen Receptor-α in Hypothalamic Proopiomelanocortin (POMC) Neurons Display Enhanced Estrogenic Response on Cortical Bone Mass

Estrogens are important regulators of bone mass and their effects are mainly mediated via estrogen receptor (ER)α. Central ERα exerts an inhibitory role on bone mass. ERα is highly expressed in the arcuate (ARC) and the ventromedial (VMN) nuclei in the hypothalamus. To test whether ERα in proopiomelanocortin (POMC) neurons, located in ARC, is involved in the regulation of bone mass, we used mice lacking ERα expression specifically in POMC neurons (POMC-ERα(-/-)). Female POMC-ERα(-/-) and control mice were ovariectomized (OVX) and treated with vehicle or estradiol (0.5 μg/d) for 6 weeks. As expected, estradiol treatment increased the cortical bone thickness in femur, the cortical bone mechanical strength in tibia and the trabecular bone volume fraction in both femur and vertebrae in OVX control mice. Importantly, the estrogenic responses were substantially increased in OVX POMC-ERα(-/-) mice compared with the estrogenic responses in OVX control mice for cortical bone thickness (+126 ± 34%, P < .01) and mechanical strength (+193 ± 38%, P < .01). To test whether ERα in VMN is involved in the regulation of bone mass, ERα was silenced using an adeno-associated viral vector. Silencing of ERα in hypothalamic VMN resulted in unchanged bone mass. In conclusion, mice lacking ERα in POMC neurons display enhanced estrogenic response on cortical bone mass and mechanical strength. We propose that the balance between inhibitory effects of central ERα activity in hypothalamic POMC neurons in ARC and stimulatory peripheral ERα-mediated effects in bone determines cortical bone mass in female mice.

Role of leptin in body temperature regulation and lipid metabolism following splenectomy

OBJECTIVE

The physiological changes in serum triglycerides and body temperature that are induced by splenectomy are poorly understood. Therefore, the aim of this study was to investigate parameters related to lipid and glucose metabolism, as well as thermoregulation, in splenectomized mice.

DESIGN AND METHODS

Splenectomized and sham-operated WT mice (C57Bl/6) and ob/ob mice were randomly divided and treated with a standard or high fat diet, and several metabolic parameters and the body temperature were investigated.

RESULTS

Splenectomy induced a significant increase in triglyceride levels regardless of the diet. It was found that the splenectomized WT mice showed greater serum leptin and insulin levels compared with the sham-operated mice. Additionally, the body temperatures of the splenectomized WT mice were greater than the body temperatures of the control animals regardless of diet; this result too was observed without any significant change in the temperature of the splenectomized ob/ob animals.

CONCLUSION

The results suggest that splenectomy interferes with serum triglyceride metabolism and body temperature regardless of the fat content in the diet and that leptin is involved in the regulation of body temperature related to splenectomy.

The RFamide neuropeptide 26RFa and its role in the control of neuroendocrine functions

Identification of novel neuropeptides and their cognate G protein-coupled receptors is essential for a better understanding of neuroendocrine regulations. The RFamide peptides represent a family of regulatory peptides that all possess the Arg-Phe-NH2 motif at their C-terminus. In mammals, seven RFamide peptides encoded by five distinct genes have been characterized. The present review focuses on 26RFa (or QRFP) which is the latest member identified in this family. 26RFa is present in all vertebrate phyla and its C-terminal domain (KGGFXFRF-NH2), which is responsible for its biological activity, has been fully conserved during evolution. 26RFa is the cognate ligand of the orphan G protein-coupled receptor GPR103 that is also present from fish to human. In all vertebrate species studied so far, 26RFa-expressing neurons show a discrete localization in the hypothalamus, suggesting important neuroendocrine activities for this RFamide peptide. Indeed, 26RFa plays a crucial role in the control of feeding behavior in mammals, birds and fish. In addition, 26RFa up-regulates the gonadotropic axis in mammals and fish. Finally, evidence that the 26RFa/GPR103 system regulates steroidogenesis, bone formation, nociceptive transmission and arterial blood pressure has also been reported. Thus, 26RFa appears to act as a key neuropeptide in vertebrates controlling vital neuroendocrine functions. The pathophysiological implication of the 26RFa/GPR103 system in human is totally unknown and some fields of investigation are proposed.

Bone marrow adipocytes support dexamethasone-induced osteoclast differentiation

The purpose of this study was to examine the ability of bone marrow adipocytes to support osteoclast differentiation in vitro. The primary bone marrow adipocytes were obtained from bone marrow fluid during prosthesis insertion. NFkappa-B ligand (RANKL), Osteoprotegerin (OPG), and macrophage colony stimulating factor (M-CSF) expressions in bone marrow adipocytes with or without dexamethasone were examined. In a co-culture system with bone marrow adipocytes and osteoclast precursor cells, osteoclast differentiation was assessed by the expression of titrate-resistant acid phosphatase (TRAP) staining. RANKL, OPG, and M-CSF mRNA expressions were confirmed in all individuals. Dexamethasone significantly induced RANKL and OPG expression. The RANKL/OPG ratio was increased by dexamethasone and was significant at 10(-7) M dexamethasone. With 10(-7) M dexamethasone, osteoclast precursor cells differentiated into multinucleated TRAP-positive cells when co-cultured with bone marrow adipocytes. The present study demonstrates for the first time that bone marrow adipocytes can support osteoclast differentiation in vitro.

Absence of mechanical loading in utero influences bone mass and architecture but not innervation in Myod-Myf5-deficient mice

Although the responses of bone to increased loading or exercise have been studied in detail, our understanding of the effects of decreased usage of the skeleton has been limited by the scarcity of suitable models. Such models should ideally not affect bone innervation, which appears to be a mediator of physiological responses of bone to unloading. MyoD-/-/Myf5-/- (dd/ff) mice lack skeletal muscle, so the fetuses develop without any active movement in utero and die soon after birth. We used micro-compter tomography and histology to analyse their bone development and structure during endochondral ossification in parallel with the establishment of bone innervation. Long bones from mutant mice were found to be profoundly different from controls, with shorter mineralized zones and less mineralization. They lacked many characteristics of adult bones - curvatures, changes in shaft diameter and traction epiphyses where muscles originate or insert - that were evident in the controls. Histologically, dd/ff mice showed the same degree of endochondral development as wild-type animals, but presented many more osteoclasts in the newly layed bone. Innervation and the expression pattern of semaphorin-3A signalling molecules were not disturbed in the mutants. Overall, we have found no evidence for a major defect of development in dd/ff mice, and specifically no alteration or delay in endochondral ossification and bone innervation. The altered morphological features of dd/ff mice and the increased bone resorption show the role of muscle activity in bone shaping and the consequences of bone unloading.

Anatomical distribution and biochemical characterization of the novel RFamide peptide 26RFa in the human hypothalamus and spinal cord

26RFa is a novel RFamide peptide originally isolated in the amphibian brain. The 26RFa precursor has been subsequently characterized in various mammalian species but, until now, the anatomical distribution and the molecular forms of 26RFa produced in the CNS of mammals, in particular in human, are unknown. In the present study, we have investigated the localization and the biochemical characteristics of 26RFa-like immunoreactivity (LI) in two regions of the human CNS--the hypothalamus and the spinal cord. Immunohistochemical labeling using specific antibodies against human 26RFa and in situ hybridization histochemistry revealed that in the human hypothalamus 26RFa-expressing neurons are located in the paraventricular and ventromedial nuclei. In the spinal cord, 26RFa-expressing neurons were observed in the dorsal and lateral horns. Characterization of 26RFa-related peptides showed that two distinct molecular forms of 26RFa are present in the human hypothalamus and spinal cord, i.e. 26RFa and an N-terminally elongated form of 43 amino acids designated 43RFa. These data provide the first evidence that 26RFa and 43RFa are actually produced in the human CNS. The distribution of 26RF-LI suggests that 26RFa and/or 43RFa may modulate feeding, sexual behavior and transmission of nociceptive stimuli.

Gap analysis of pediatric reference intervals related to thyroid hormones and the growth hormone–insulin growth factor axis

Efficacy of laboratory medicine in assisting attending physicians in their diagnostic and follow-up endeavors is intimately linked to an access to meaningful and reliable reference values. Pediatrics is particularly sensitive to this problem as the processes, associated with growth and development, are imposing rapid discontinuous changes on the physiology of the individuals. Some developmental stages are more critical than others. The neonatal and the pubertal periods, for which we lack reference ranges, are two such examples. Beyond biological considerations, we realize that, over the last 2 decades, technology has evolved, both at the analytical and reagent levels. This technological evolution inexorably leads to the need in redefining reference values. It is for this reason that a group of clinical and medical biochemists have joined their efforts in creating the Canadian Laboratory Initiative in Paediatric Reference (CALIPER) which objective is to define a pan-Canadian set of reference values from birth to late adolescence. To illustrate the need of such a venture, a brief gap analysis for biochemical variables related to the thyroid function, and the growth hormone-insulin-like growth factors axis follows.