Florent Elefteriou

Leptin regulates bone formation via the sympathetic nervous system.

We previously showed that leptin inhibits bone formation by an undefined mechanism. Here, we show that hypothalamic leptin-dependent antiosteogenic and anorexigenic networks differ, and that the peripheral mediators of leptin antiosteogenic function appear to be neuronal. Neuropeptides mediating leptin anorexigenic function do not affect bone formation. Leptin deficiency results in low sympathetic tone, and genetic or pharmacological ablation of adrenergic signaling leads to a leptin-resistant high bone mass. beta-adrenergic receptors on osteoblasts regulate their proliferation, and a beta-adrenergic agonist decreases bone mass in leptin-deficient and wild-type mice while a beta-adrenergic antagonist increases bone mass in wild-type and ovariectomized mice. None of these manipulations affects body weight. This study demonstrates a leptin-dependent neuronal regulation of bone formation with potential therapeutic implications for osteoporosis.

Leptin regulation of bone resorption by the sympathetic nervous system and CART

Bone remodelling, the mechanism by which vertebrates regulate bone mass, comprises two phases, namely resorption by osteoclasts and formation by osteoblasts; osteoblasts are multifunctional cells also controlling osteoclast differentiation. Sympathetic signalling via β2-adrenergic receptors (Adrb2) present on osteoblasts controls bone formation downstream of leptin. Here we show, by analysing Adrb2-deficient mice, that the sympathetic nervous system favours bone resorption by increasing expression in osteoblast progenitor cells of the osteoclast differentiation factor Rankl. This sympathetic function requires phosphorylation (by protein kinase A) of ATF4, a cell-specific CREB-related transcription factor essential for osteoblast differentiation and function. That bone resorption cannot increase in gonadectomized Adrb2-deficient mice highlights the biological importance of this regulation, but also contrasts sharply with the increase in bone resorption characterizing another hypogonadic mouse with low sympathetic tone, the ob/ob mouse. This discrepancy is explained, in part, by the fact that CART (‘cocaine amphetamine regulated transcript’), a neuropeptide whose expression is controlled by leptin and nearly abolished in ob/ob mice, inhibits bone resorption by modulating Rankl expression. Our study establishes that leptin-regulated neural pathways control both aspects of bone remodelling, and demonstrates that integrity of sympathetic signalling is necessary for the increase in bone resorption caused by gonadal failure.

Essential Role of STAT3 in Body Weight and Glucose Homeostasis

ABSTRACT STAT3 is a ubiquitous transcription factor that is indispensable during early embryogenesis. To study the functions of STAT3 postnatally, we generated conditional STAT3-deficient mice. To that end, STAT3lox/lox mice were crossed with mice expressing Cre under the control of rat insulin II gene promoter (RIP-Cre mice). Immunohistochemical and Western blot analyses showed that STAT3 is deleted from β cells in the islets of Langerhans. Genomic DNA PCR revealed that STAT3 deletion also occurred in the hypothalamus. Hypothalamic Cre expression was further confirmed by crossing RIP-Cre/STAT3lox/lox mice with the ROSA26 Cre reporter strain and staining for lacZ activity. Double immunohistochemical staining confirmed that deletion of STAT3 occurred in leptin receptor (OB-Rb isoform)-positive neurons. RIP-Cre/STAT3lox/lox mice are mildly hyperglycemic and hyperinsulinemic at the time of weaning, become hyperphagic immediately after weaning, and exhibit impaired glucose tolerance. Body weight, body fat, and mRNA and protein levels of leptin are all significantly increased in RIP-Cre/STAT3lox/lox mice. Administration of recombinant leptin by intracerebroventricular infusion failed to cause complete loss of body fat in RIP-Cre/STAT3lox/lox mice. Transplantation of wild-type islets into RIP-Cre/STAT3lox/lox mice also failed to decrease adiposity or to correct other abnormalities in these mice. These data thus suggest that loss of STAT3 in the hypothalamus caused by RIP-Cre action likely interferes with normal body weight homeostasis and glucose metabolism.

Enhanced bone formation in lipodystrophic PPARγhyp/hyp mice relocates haematopoiesis to the spleen

The peroxisome proliferator‐activated receptor gamma (PPARγ) controls adipogenesis and metabolism. We demonstrate here that the absence of PPARγ in fat has potent osteogenic activities, which affect haematopoiesis. The congenital absence of PPARγ in fat of lipodystrophic PPARγhyp/hyp mice, strongly enhanced bone mass and consequentially reduced the bone‐marrow cavity. Consistent with this, PPARγhyp/hyp mice had a significant decrease in bone marrow cellularity and resorted to extramedullary haematopoiesis in the spleen to maintain haematopoiesis. Our data indicate that antagonizing PPARγ activity in fat could be an effective way to combat osteoporosis and suggest that haematopoietic function should be scrutinized in lipodystrophic subjects.

Boning up on Ephrin Signaling

The activities of osteoclasts, which degrade bone, and osteoblasts, which form bone, are coordinated to maintain bone homeostasis. Zhao et al. (2006) now demonstrate bidirectional signaling between these two cell populations via the transmembrane ligand ephrinB2 expressed by osteoclasts and its receptor EphB4 expressed by osteoblasts. Such bidirectional signaling limits osteoclast activity while stimulating osteoblast differentiation.

Binding of tenascin-X to decorin

Tenascin‐X (TN‐X) is an extracellular matrix protein whose absence results in an alteration of the mechanical properties of connective tissue. To understand the mechanisms of integration of TN‐X in the extracellular matrix, overlay blot assays were performed on skin extracts. A 100 kDa molecule interacting with TN‐X was identified by this method and this interaction was abolished when the extract was digested by chondroitinase. By solid‐phase assays, we showed that dermatan sulfate chains of decorin bind to the heparin‐binding site included within the fibronectin‐type III domains 10 and 11 of TN‐X. We thus postulate that the association of TN‐X with collagen fibrils is mediated by decorin and contributes to the integrity of the extracellular network.

Stimulation of Host Bone Marrow Stromal Cells by Sympathetic Nerves Promotes Breast Cancer Bone Metastasis in Mice

The activation of sympathetic nerves by psychosocial stress creates a favorable environment in bone for the establishment of cancer cells in a mouse model of breast cancer.

Neuronal signaling and the regulation of bone remodeling.

Abstract.An increasing number of studies suggest that nerve-derived signals play an important role in the regulation of bone remodeling. Neuropeptides and receptors/transporters of adrenergic, glutaminergic, serotoninergic, dopaminergic and sensory nature have been described in osteoblasts in vitro. Downstream signaling pathways and targets genes have been identified, but the in vivo relevance of these findings remained controversial until more recent gene gain and loss of function studies confirmed the role of CGRP and β2-adrenergic receptor signaling in osteoblasts. Tissue and time-conditional mutant mice originally generated for studies unrelated to bone are now available tools to determine the role of neuronal signaling in bone and to dissociate the central and peripheral role of these signals. Lastly, understanding how the central nervous system integrates homeostatic signals with the regulation of bone homeostasis will be the next exciting subject of research in the field.

The New Field of Neuroskeletal Biology

The fields of neuroscience and bone biology have recently converged following the discovery that bone remodeling is directly regulated by the brain. This work has defined bone remodeling as one of the cardinal physiological functions of the body, subject to homeostatic regulation and integrated with the other major physiological functions by the hypothalamus. Central to this discovery was the definition of the adipocyte-derived hormone leptin as a regulator of both arms of bone remodeling, formation and resorption, through its action on the ventromedial hypothalamus and subsequently via the sympathetic nervous system to osteoblasts. The characterization of the sympathetic nervous system as a regulator of bone remodeling has led to several large clinical studies demonstrating a substantial protective effect of ß-blockers, particularly ß1-blockers, on fracture risk. Studies in model organisms have reinforced the role of the central nervous system in the regulation of bone remodeling in vivo by the identification of several additional genes, namely cocaine and amphetamine regulated transcript (Cart), melanocortin 4 receptor (Mc4R), neuropeptide Y (NPY), Y2 receptor, cannabinoid receptor CB1 (Cnbr1), and the genes of the circadian clock. These genes have several common features, including high levels of expression in the hypothalamus and the ability to regulate other major physiological functions in addition to bone remodeling including energy homeostasis, body weight, and reproduction. We review the major pathways that define the new field of neuroskeletal biology and identify further avenues of inquiry.

Skeletal abnormalities in neurofibromatosis type 1: Approaches to therapeutic options

The skeleton is frequently affected in individuals with neurofibromatosis type 1, and some of these bone manifestations can result in significant morbidity. The natural history and pathogenesis of the skeletal abnormalities of this disorder are poorly understood and consequently therapeutic options for these manifestations are currently limited. The Childrens Tumor Foundation convened an International Neurofibromatosis Type 1 Bone Abnormalities Consortium to address future directions for clinical trials in skeletal abnormalities associated with this disorder. This report reviews the clinical skeletal manifestations and available preclinical mouse models and summarizes key issues that present barriers to optimal clinical management of skeletal abnormalities in neurofibromatosis type 1. These concepts should help advance optimal clinical management of the skeletal abnormalities in this disease and address major difficulties encountered for the design of clinical trials.