Abstract: 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. Expected final online publication date for the Annual Review of Physiology Volume 74 is February 11, 2012. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Abstract: 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.
Abstract: Recent evidence indicates that leptin regulates appetite and energy expenditure, at least in part by inhibiting serotonin synthesis and release from brainstem neurons. To demonstrate that this pathway works postnatally, we used a conditional, brainstem-specific mouse CreER(T2) driver to show that leptin signals in brainstem neurons after birth to decrease appetite by inhibiting serotonin synthesis. Cell-specific gene deletion experiments and intracerebroventricular leptin infusions reveal that serotonin signals in arcuate nuclei of the hypothalamus through the Htr1a receptor to favor food intake and that this serotonin function requires the expression of Creb, which regulates the expression of several genes affecting appetite. Accordingly, a specific antagonist of the Htr1a receptor decreases food intake in leptin-deficient but not in Htr1a(-/-) mice. Collectively, these results establish that leptin inhibition of serotonin is necessary to inhibit appetite postnatally and provide a proof of principle that selective inhibition of this pathway may be a viable option to treat appetite disorders.
Abstract: Leptin exerts control over energy metabolism, reproduction and bone mass accrual, raising the question does leptin act through a common neuronal circuit to mediate these effects? Historically, the hypothalamus has been viewed as the site for leptin signaling in the brain. Recent genetic studies, however, indicate that these physiological functions, notably the regulation of appetite and bone mass accrual by leptin, take place for the most part through inhibition of serotonin (5-hydroxytryptamine) synthesis and release by brainstem neurons. Here, we review how these findings have redefined the roadmap of leptin signaling in the brain. This has led to proof-of-principle studies showing that selective inhibition of the leptin-serotonin axis is a viable therapeutic approach to treat appetite disorders.
Abstract: 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.
Abstract: 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.
Abstract: 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.
Abstract: 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.
Abstract: The embryonic functions of Hox proteins have been extensively investigated in several animal phyla. These transcription factors act as selectors of developmental programmes, to govern the morphogenesis of multiple structures and organs. However, despite the variety of morphogenetic processes Hox proteins are involved in, only a limited set of their target genes has been identified so far. To find additional targets, we used a strategy based upon the simultaneous overexpression of Hoxa2 and its cofactors Pbx1 and Prep in a cellular model. Among genes whose expression was upregulated, we identified LMO1, which codes for an intertwining LIM-only factor involved in protein-DNA oligomeric complexes. By analysing its expression in Hox knockout mice, we show that Lmo1 is differentially regulated by Hoxa2 and Hoxb2, in specific columns of hindbrain neuronal progenitors. These results suggest that Lmo1 takes part in a Hox paralogue 2-dependent network regulating anteroposterior and dorsoventral hindbrain patterning.
Abstract: 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.
