The Role of Osteocalcin in the Regulation of Brain Development and Functions

The central nervous system controls many physiological processes including energy metabolism, immune response, reproduction, and development. In turn, hormones synthesized in and secreted by peripheral organs can be transported across the blood-brain barrier to modulate the development of the brain, the formation of new neurons, neural activity, behavior, and the secretion of brain-derived hormones. The central control of bone mass, mediated by the adipocyte-derived hormone leptin, has raised questions of whether the skeleton may signal back to the brain. In recent years, the Karsenty laboratory has uncovered the endocrine role of the bone-derived hormone osteocalcin. Through the use of a vast array of genetic tools, the Karsenty lab has discovered that osteocalcin is a potent regulator of glucose homeostasis, adaptation to exercise, energy metabolism, and male fertility. The multifunctional role of osteocalcin led us to hypothesize that it may act as a molecular means of communication between the skeleton and the brain. We asked whether osteocalcin could regulate brain development during embryogenesis and behavioral functions in adulthood. In addressing these questions, we observed that bone-derived osteocalcin crosses the blood-brain barrier, accumulates in discrete parts of the brain including the hippocampus, and binds to several neuronal populations to favor the synthesis of monoamine neurotransmitters (serotonin, dopamine, and norepinephrine), and to impede the synthesis of the inhibitory neurotransmitter, GABA. Osteocalcin-/- mice have increased anxiety and depression and impaired learning and memory when compared to WT littermates. We also uncovered that the absence of maternal osteocalcin during embryogenesis hinders brain development and causes defects in spatial learning and memory in the adult offspring. Upon characterizing the necessity of osteocalcin for brain development and cognitive function, we investigated whether bone health is a determinant of cognition, and whether osteocalcin may be sufficient to reverse age-related cognitive decline. In addressing the first question, we found that impairment in either bone formation or bone resorption negatively impacts both anxiety and memory. In addressing the second question, we found that osteocalcin is also necessary for the beneficial effect of young blood on cognitive functions. Finally, we observed reduced anxiety and improved memory in aged mice receiving osteocalcin peripherally. This action appears to require an increase in brain-derived neurotrophic factor levels in the hippocampus. Against the backdrop of our progressively aging population, it is important for future studies to determine whether osteocalcin may act therapeutically in humans to treat age-related cognitive decline. Additionally, to identify potential drug targets, it is important to fully characterize the molecular mechanism by which osteocalcin acts on neurons

Gpr158 mediates osteocalcin's regulation of cognition

That osteocalcin (OCN) is necessary for hippocampal-dependent memory and to prevent anxiety-like behaviors raises novel questions. One question is to determine whether OCN is also sufficient to improve these behaviors in wild-type mice, when circulating levels of OCN decline as they do with age. Here we show that the presence of OCN is necessary for the beneficial influence of plasma from young mice when injected into older mice on memory and that peripheral delivery of OCN is sufficient to improve memory and decrease anxiety-like behaviors in 16-mo-old mice. A second question is to identify a receptor transducing OCN signal in neurons. Genetic, electrophysiological, molecular, and behavioral assays identify Gpr158, an orphan G protein-coupled receptor expressed in neurons of the CA3 region of the hippocampus, as transducing OCN's regulation of hippocampal-dependent memory in part through inositol 1,4,5-trisphosphate and brain-derived neurotrophic factor. These results indicate that exogenous OCN can improve hippocampal-dependent memory in mice and identify molecular tools to harness this pathway for therapeutic purposes

Modulation of cognition and anxiety-like behavior by bone remodeling

Objective: That the bone-derived hormone osteocalcin is necessary to promote normal brain development and function, along with its recently described sufficiency in reversing cognitive manifestations of aging, raises novel questions. One of these is to assess whether bone health, which deteriorates rapidly with aging, is a significant determinant of cognition and anxiety-like behavior. Methods: To begin addressing this question, we used mice haploinsufficient for Runx2, the master gene of osteoblast differentiation and the main regulator of Osteocalcin expression. Control and Runx2+/− mice were evaluated for the expression of osteocalcin's target genes in the brain and for behavioral parameters, using two assays each for cognition and anxiety-like behavior. Results: We found that adult Runx2+/− mice had defects in bone resorption, reduced circulating levels of bioactive osteocalcin, and reduced expression of osteocalcin's target genes in the brain. Consequently, they had significant impairment in cognitive function and increased anxiety-like behavior. Conclusions: These results indicate that bone remodeling is a determinant of brain function. Keywords: Runx2, Osteocalcin, Bone remodeling, Cognitio

Gpr158 mediates osteocalcin's regulation of cognition

That osteocalcin (OCN) is necessary for hippocampal-dependent memory and to prevent anxiety-like behaviors raises novel questions. One question is to determine whether OCN is also sufficient to improve these behaviors in wild-type mice, when circulating levels of OCN decline as they do with age. Here we show that the presence of OCN is necessary for the beneficial influence of plasma from young mice when injected into older mice on memory and that peripheral delivery of OCN is sufficient to improve memory and decrease anxiety-like behaviors in 16-mo-old mice. A second question is to identify a receptor transducing OCN signal in neurons. Genetic, electrophysiological, molecular, and behavioral assays identify Gpr158, an orphan G protein-coupled receptor expressed in neurons of the CA3 region of the hippocampus, as transducing OCN's regulation of hippocampal-dependent memory in part through inositol 1,4,5-trisphosphate and brain-derived neurotrophic factor. These results indicate that exogenous OCN can improve hippocampal-dependent memory in mice and identify molecular tools to harness this pathway for therapeutic purposes

Sequencing of 640,000 exomes identifies GPR75 variants associated with protection from obesity

Large-scale human exome sequencing can identify rare protein-coding variants with a large impact on complex traits such as body adiposity. We sequenced the exomes of 645,626 individuals from the United Kingdom, the United States, and Mexico and estimated associations of rare coding variants with body mass index (BMI). We identified 16 genes with an exome-wide significant association with BMI, including those encoding five brain-expressed G protein-coupled receptors (CALCR, MC4R, GIPR, GPR151, and GPR75). Protein-truncating variants in GPR75 were observed in ∼4/10,000 sequenced individuals and were associated with 1.8 kilograms per square meter lower BMI and 54% lower odds of obesity in the heterozygous state. Knock out of Gpr75 in mice resulted in resistance to weight gain and improved glycemic control in a high-fat diet model. Inhibition of GPR75 may provide a therapeutic strategy for obesity