Franck Oury

A Serotonin-Dependent Mechanism Explains the Leptin Regulation of Bone Mass, Appetite, and Energy Expenditure

Leptin inhibition of bone mass accrual requires the integrity of specific hypothalamic neurons but not expression of its receptor on these neurons. The same is true for its regulation of appetite and energy expenditure. This suggests that leptin acts elsewhere in the brain to achieve these three functions. We show here that brainstem-derived serotonin (BDS) favors bone mass accrual following its binding to Htr2c receptors on ventromedial hypothalamic neurons and appetite via Htr1a and 2b receptors on arcuate neurons. Leptin inhibits these functions and increases energy expenditure because it reduces serotonin synthesis and firing of serotonergic neurons. Accordingly, while abrogating BDS synthesis corrects the bone, appetite and energy expenditure phenotypes caused by leptin deficiency, inactivation of the leptin receptor in serotonergic neurons recapitulates them fully. This study modifies the map of leptin signaling in the brain and identifies a molecular basis for the common regulation of bone and energy metabolisms. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

Endocrine Regulation of Male Fertility by the Skeleton

Interactions between bone and the reproductive system have until now been thought to be limited to the regulation of bone remodeling by the gonads. We now show that, in males, bone acts as a regulator of fertility. Using coculture assays, we demonstrate that osteoblasts are able to induce testosterone production by the testes, though they fail to influence estrogen production by the ovaries. Analyses of cell-specific loss- and gain-of-function models reveal that the osteoblast-derived hormone osteocalcin performs this endocrine function. By binding to a G protein-coupled receptor expressed in the Leydig cells of the testes, osteocalcin regulates in a CREB-dependent manner the expression of enzymes that is required for testosterone synthesis, promoting germ cell survival. This study expands the physiological repertoire of osteocalcin and provides the first evidence that the skeleton is an endocrine regulator of reproduction.

Osteocalcin regulates murine and human fertility through a pancreas-bone-testis axis

The osteoblast-derived hormone osteocalcin promotes testosterone biosynthesis in the mouse testis by binding to GPRC6A in Leydig cells. Interestingly, Osteocalcin-deficient mice exhibit increased levels of luteinizing hormone (LH), a pituitary hormone that regulates sex steroid synthesis in the testes. These observations raise the question of whether LH regulates osteocalcins reproductive effects. Additionally, there is growing evidence that osteocalcin levels are a reliable marker of insulin secretion and sensitivity and circulating levels of testosterone in humans, but the endocrine function of osteocalcin is unclear. Using mouse models, we found that osteocalcin and LH act in 2 parallel pathways and that osteocalcin-stimulated testosterone synthesis is positively regulated by bone resorption and insulin signaling in osteoblasts. To determine the importance of osteocalcin in humans, we analyzed a cohort of patients with primary testicular failure and identified 2 individuals harboring the same heterozygous missense variant in one of the transmembrane domains of GPRC6A, which prevented the receptor from localizing to the cell membrane. This study uncovers the existence of a second endocrine axis that is necessary for optimal male fertility in the mouse and suggests that osteocalcin modulates reproductive function in humans.

Maternal and Offspring Pools of Osteocalcin Influence Brain Development and Functions

The powerful regulation of bone mass exerted by the brain suggests the existence of bone-derived signals modulating this regulation or other functions of the brain. We show here that the osteoblast-derived hormone osteocalcin crosses the blood-brain barrier, binds to neurons of the brainstem, midbrain, and hippocampus, enhances the synthesis of monoamine neurotransmitters, inhibits GABA synthesis, prevents anxiety and depression, and favors learning and memory independently of its metabolic functions. In addition to these postnatal functions, maternal osteocalcin crosses the placenta during pregnancy and prevents neuronal apoptosis before embryos synthesize this hormone. As a result, the severity of the neuroanatomical defects and learning and memory deficits of Osteocalcin(-/-) mice is determined by the maternal genotype, and delivering osteocalcin to pregnant Osteocalcin(-/-) mothers rescues these abnormalities in their Osteocalcin(-/-) progeny. This study reveals that the skeleton via osteocalcin influences cognition and contributes to the maternal influence on fetal brain development.

The central regulation of bone mass, the first link between bone remodeling and energy metabolism.

Evolutionary consideration and clinical observations led us to hypothesize 10 yr ago that there may be a coordinated regulation, endocrine in nature, of bone remodeling and energy metabolism. The existence of this coordinated regulation is motivated by the energetic needs of the skeleton; therefore, this regulation relies on hormones that appear during evolution with the skeleton, not with energy metabolism. Leptin is such a hormone, and it is a critical regulator of bone mass as well as of appetite and energy expenditure. This review goes over the anatomical route and molecular pathways used by leptin to inhibit both bone mass accrual and appetite through its signaling in the brain.

Hoxa2- and rhombomere-dependent development of the mouse facial somatosensory map

In the mouse trigeminal pathway, sensory inputs from distinct facial structures, such as whiskers or lower jaw and lip, are topographically mapped onto the somatosensory cortex through relay stations in the thalamus and hindbrain. In the developing hindbrain, the mechanisms generating such maps remain elusive. We found that in the principal sensory nucleus, the whisker-related map is contributed by rhombomere 3–derived neurons, whereas the rhombomere 2–derived progeny supply the lower jaw and lip representation. Moreover, early Hoxa2 expression in neuroepithelium prevents the trigeminal nerve from ectopically projecting to the cerebellum, whereas late expression in the principal sensory nucleus promotes selective arborization of whisker-related afferents and topographic connectivity to the thalamus. Hoxa2 inactivation further results in the absence of whisker-related maps in the postnatal brain. Thus, Hoxa2- and rhombomere 3–dependent cues determine the whisker area map and are required for the assembly of the whisker-to-barrel somatosensory circuit.

Biology Without Walls: The Novel Endocrinology of Bone

Classical studies of vertebrate physiology have usually been confined to a given organ or cell type. The use of mouse genetics has changed this approach and has rejuvenated the concept of a whole-body study of physiology. One physiological system that has been profoundly influenced by mouse genetics is skeletal physiology. Indeed, genetic approaches have identified several unexpected organs that affect bone physiology. These new links have begun to provide a plausible explanation for the evolutionary involvement of hormones such as leptin with bone physiology. These genetic approaches have also revealed bone as a true endocrine organ capable of regulating energy metabolism and reproduction. Collectively, the body of work discussed below illustrates a new and unconventional role for bone in mammalian physiology.

CREB mediates brain serotonin regulation of bone mass through its expression in ventromedial hypothalamic neurons

Serotonin is a bioamine regulating bone mass accrual differently depending on its site of synthesis. It decreases accrual when synthesized in the gut, and increases it when synthesized in the brain. The signal transduction events elicited by gut-derived serotonin once it binds to the Htr1b receptor present on osteoblasts have been identified and culminate in cAMP response element-binding protein (CREB) regulation of osteoblast proliferation. In contrast, we do not know how brain-derived serotonin favors bone mass accrual following its binding to the Htr2c receptor on neurons of the hypothalamic ventromedial nucleus (VMH). We show here--through gene expression analysis, serotonin treatment of wild-type and Htr2c(-/-) hypothalamic explants, and cell-specific gene deletion in the mouse--that, following its binding to the Htr2c receptor on VMH neurons, serotonin uses a calmodulin kinase (CaMK)-dependent signaling cascade involving CaMKKβ and CaMKIV to decrease the sympathetic tone and increase bone mass accrual. We further show that the transcriptional mediator of these events is CREB, whose phosphorylation on Ser 133 is increased by CaMKIV following serotonin treatment of hypothalamic explants. A microarray experiment identified two genes necessary for optimum sympathetic activity whose expression is regulated by CREB. These results provide a molecular understanding of how serotonin signals in hypothalamic neurons to regulate bone mass accrual and identify CREB as a critical determinant of this function, although through different mechanisms depending on the cell type, neuron, or osteoblast in which it is expressed.

Leptin-dependent serotonin control of appetite: temporal specificity, transcriptional regulation, and therapeutic implications

Leptin regulates serotonin synthesis by brainstem neurons in adult mice; serotonin then acts on arcuate neurons to inhibit food intake via Creb.

Signaling through the M3 Muscarinic Receptor Favors Bone Mass Accrual by Decreasing Sympathetic Activity

Bone remodeling is regulated by various neuronal inputs, including sympathetic tone, which is known to inhibit bone mass accrual. This aspect of sympathetic nervous system function raises the prospect that the other arm of the autonomic nervous system, the parasympathetic nervous system, may also affect bone remodeling. Here, we use various mutant mouse strains, each lacking one of the muscarinic receptors that mediate parasympathetic activity, to show that the parasympathetic nervous system acting through the M(3) muscarinic receptor is a positive regulator of bone mass accrual, increasing bone formation and decreasing bone resorption. Gene expression studies, cell-specific gene deletion experiments, and analysis of compound mutant mice showed that the parasympathetic nervous system favors bone mass accrual by acting centrally and by decreasing the sympathetic tone. By showing that both arms of the autonomic nervous system affect bone remodeling, this study further underscores the importance of neuronal regulation of bone.