Molecular cloning, expression, and sequence analysis of GPRC6A, a novel family C G-protein-coupled receptor

Glycine: The Smallest Anti-Inflammatory Micronutrient

Glycine is a non-essential amino acid with many functions and effects. Glycine can bind to specific receptors and transporters that are expressed in many types of cells throughout an organism to exert its effects. There have been many studies focused on the anti-inflammatory effects of glycine, including its abilities to decrease pro-inflammatory cytokines and the concentration of free fatty acids, to improve the insulin response, and to mediate other changes. However, the mechanism through which glycine acts is not clear. In this review, we emphasize that glycine exerts its anti-inflammatory effects throughout the modulation of the expression of nuclear factor kappa B (NF-κB) in many cells. Although glycine is a non-essential amino acid, we highlight how dietary glycine supplementation is important in avoiding the development of chronic inflammation.

Untargeted Metabolomics Reveals the Function of GPRC6A in Amino Acid and Lipid Metabolism in Mice

GPRC6A is an amino acid sensor in the cytomembrane. Despite substantial evidence for the role of GPRC6A in metabolism, the specific effects and mechanism by which this gene acts on metabolic processes are still unresolved. In this study, serum biochemical parameters related to liver and kidney function and serum amino acid levels were determined in GPRC6A wild-type (WT) and knockout (KO) mice. An untargeted serum metabolomics analysis was also conducted for the first time, to the best of our knowledge, to decipher the function of GPRC6A in metabolic processes. GPRC6A was involved in lipid and amino acid metabolism, mainly by affecting liver function. A loss of GPRC6A function may perturb bile acid metabolism, thus leading to abnormal unsaturated fatty acid metabolism. GPRC6A KO may lead to excessive protein breakdown under starvation, and the loss of GPRC6A had a significant effect on phenylalanine metabolism-related pathways. Our metabolomics data provide a novel basis for further functional studies of GPRC6A.

Undercarboxylated, but not Carboxylated, Osteocalcin suppresses TNF-α induced inflammatory signaling pathway in Myoblast

Undercarboxylated osteocalcin (ucOCN) has been considered to be an important endocrine factor, especially to regulate bone and energy metabolism. Even with the mounting evidence showing the consistent inverse correlation of ucOCN levels in chronic inflammatory diseases, however, the mechanism underlying the involvement of ucOCN in the muscular inflammation has not been fully understood. In the present study, we explored 1) the endocrine role of ucOCN in the regulation of inflammation in C2C12 myoblasts and primary myoblasts and the underlying intracellular signaling mechanisms, and 2) whether G protein–coupled receptor family C group 6 member A (GPRC6A) is the ucOCN-sensing receptor associated with the ucOCN-mediated anti-inflammatory signaling pathway in myoblasts. ucOCN suppressed the tumor necrosis factor-α (TNF-α)–induced expressions of major inflammatory cytokines, including interleukin-1β (IL-1β) and inhibited the TNF-α–stimulated activities of transcription factors, including NF-κB, in C2C12 and primary myoblasts. Both knockdown and knockout of GPRC6A, by using siRNA or a CRISPR/CAS9 system, respectively, did not reverse the effect of ucOCN on IL-1β expression in myoblasts. Interestingly, TNF-α–induced IL-1β expression was inhibited by knockdown or deletion of GPRC6A itself, regardless of the ucOCN treatment. ucOCN was rapidly internalized into the cytoplasmic region via caveolae-mediated endocytosis, suggesting the presence of new target proteins in the cell membrane and/or in the cytoplasm for interaction with ucOCN in myoblasts. Taken together, these findings indicate that ucOCN suppresses the TNF-α–induced inflammatory signaling pathway in myoblasts. GPRC6A is not a sensing receptor associated with the ucOCN-mediated anti-inflammatory signaling pathway in myoblasts.

Additive Effects of L-Ornithine on Preferences to Basic Taste Solutions in Mice

In addition to the taste receptors corresponding to the six basic taste qualities-sweet, salty, sour, bitter, umami, and fatty-another type of taste receptor, calcium-sensing receptor (CaSR), is found in taste-bud cells. CaSR is called the 'kokumi' receptor because its agonists increase sweet, salty and umami tastes to induce 'koku', a Japanese word meaning the enhancement of flavor characters such as thickness, mouthfulness, and continuity. Koku is an important factor for enhancing food palatability. However, it is not well known whether other kokumi-receptors and substances exist. Here, we show that ornithine (L-ornithine but not D-ornithine) at low concentrations that do not elicit a taste of its own, enhances preferences to sweet, salty, umami, and fat taste solutions in mice. Increased preference to monosodium glutamate (MSG) was the most dominant effect. Antagonists of G-protein-coupled receptor family C group 6 subtype A (GPRC6A) abolished the additive effect of ornithine on MSG solutions. The additive effects of ornithine on taste stimuli are thought to occur in the oral cavity, and are not considered post-oral events because ornithine's effects were confirmed in a brief-exposure test. Moreover, the additive effects of ornithine and the action of the antagonist were verified in electrophysiological taste nerve responses. Immunohistochemical analysis implied that GPRC6A was expressed in subsets of type II and type III taste cells of mouse circumvallate papillae. These results are in good agreement with those reported for taste modulation involving CaSR and its agonists. The present study suggests that ornithine is a kokumi substance and GPRC6A is a newly identified kokumi receptor.

Study of the Expression and Function of Calcium-Sensing Receptor in Human Skeletal Muscle

Skeletal muscle has an outstanding capacity for regeneration in response to injuries, but there are disorders in which this process is seriously impaired, such as sarcopenia. Pharmacological treatments to restore muscle trophism are not available, therefore, the identification of suitable therapeutic targets that could be useful for the treatment of skeletal reduced myogenesis is highly desirable. In this in vitro study, we explored the expression and function of the calcium-sensing receptor (CaSR) in human skeletal muscle tissues and their derived satellite cells. The results obtained from analyses with various techniques of gene and protein CaSR expression and of its secondary messengers in response to calcium (Ca²⁺) and CaSR drugs have demonstrated that this receptor is not present in human skeletal muscle tissues, neither in the established satellite cells, nor during in vitro myogenic differentiation. Taken together, our data suggest that, although CaSR is a very important drug target in physiology and pathology, this receptor probably does not have any physiological role in skeletal muscle in normal conditions.

Skeletal endocrinology: where evolutionary advantage meets disease

The regulation of whole-body homeostasis by the skeleton is mediated by its capacity to secrete endocrine signaling molecules. Although bone-derived hormones confer several adaptive benefits, their physiological functions also involve trade-offs, thus eventually contributing to disease. In this manuscript, we discuss the origins and functions of two of the best-studied skeletal mediators, fibroblast growth factor 23 and osteocalcin, in an evolutionary context. Moreover, we provide a theoretical framework seeking to explain the broad involvement of these two hormones in amniote physiology as well as their potential to fuel the development and progression of diseases. Vice versa, we outline which perturbations might be amenable to manipulation of these systems and discuss limitations and ongoing challenges in skeletal endocrine research. Finally, we summarize unresolved questions and potential future studies in this thriving field.

Endocrine role of bone in the regulation of energy metabolism

Bone mainly functions as a supportive framework for the whole body and is the major regulator of calcium homeostasis and hematopoietic function. Recently, an increasing number of studies have characterized the significance of bone as an endocrine organ, suggesting that bone-derived factors regulate local bone metabolism and metabolic functions. In addition, these factors can regulate global energy homeostasis by altering insulin sensitivity, feeding behavior, and adipocyte commitment. These findings may provide a new pathological mechanism for related metabolic diseases or be used in the diagnosis, treatment, and prevention of metabolic diseases such as osteoporosis, obesity, and diabetes mellitus. In this review, we summarize the regulatory effect of bone and bone-derived factors on energy metabolism and discuss directions for future research.

Undercarboxylated osteocalcin inhibits the early differentiation of osteoclast mediated by Gprc6a

Osteocalcin (OCN) was the most abundant noncollagen protein and considered as an endocrine factor. However, the functions of Undercarboxylated osteocalcin (ucOCN) on osteoclast and bone resorption are not well understood. In the present study, preosteoclast RAW264.7 cells and bone marrow mononuclear cells (BMMs) were treated with ucOCN purified from prokaryotic bacteria. Our results showed that ucOCN attenuated the proliferation of RAW264.7 cells with a concentration dependant manner by MTS assay. Scrape wounding assay revealed the decreased motility of RAW264.7 cells after ucOCN treatment. RT-qPCR results manifested the inhibitory effects of ucOCN on the expression of osteoclastic marker genes in RAW264.7 cells during inducing differentiation of RANKL. It was also observed that ucOCN inhibited the formation of multinucleated cells from RAW264.7 cells and BMMs detected by TRAP staining. The number and area of bone resorb pits were also decreased after treatment with ucOCN during their osteoclast induction by toluidine blue staining. The formation and integrity of the osteoclast actin ring were impaired by ucOCN by immunofluorescent staining. Time dependant treatment of ucOCN during osteoclastic induction demonstrated the inhibitory effects mainly occurred at the early stage of osteoclastogenesis. Signaling analysis of luciferase activity of the CRE or SRE reporter and ERK1/2 phosphorylation showed the selective inhibitor or siRNA of Gprc6a (a presumptive ucOCN receptor) could attenuate the promotion of ucOCN on CRE-luciferase activity. Taken together, we provided the first evidence that ucOCN had negative effects on the early differentiation and bone resorption of osteoclasts via Gprc6a.

Adipocyte-specific GPRC6A ablation promotes diet-induced obesity by inhibiting lipolysis

The G protein–coupled receptor GPRC6A regulates various physiological processes in response to its interaction with multiple ligands, such as extracellular basic amino acids, divalent cations, testosterone, and the uncarboxylated form of osteocalcin (GluOC). Global ablation of GPRC6A increases the susceptibility of mice to diet-induced obesity and related metabolic disorders. However, given that GPRC6A is expressed in many tissues and responds to a variety of hormonal and nutritional signals, the cellular and molecular mechanisms underlying the development of metabolic disorders in conventional knockout mice have remained unclear. On the basis of our previous observation that long-term oral administration of GluOC markedly reduced adipocyte size and improved glucose tolerance in WT mice, we examined whether GPRC6A signaling in adipose tissue might be responsible for prevention of metabolic disorders. We thus generated adipocyte-specific GPRC6A knockout mice, and we found that these animals manifested increased adipose tissue weight, adipocyte hypertrophy, and adipose tissue inflammation when fed a high-fat and high-sucrose diet compared with control mice. These effects were associated with reduced lipolytic activity because of downregulation of lipolytic enzymes such as adipose triglyceride lipase and hormone-sensitive lipase in adipose tissue of the conditional knockout mice. Given that, among GPR6CA ligands tested, GluOC and ornithine increased the expression of adipose triglyceride lipase in cultured 3T3-L1 adipocytes in a manner dependent on GPRC6A, our results suggest that the constitutive activation of GPRC6A signaling in adipocytes by GluOC or ornithine plays a key role in adipose lipid handling and the prevention of obesity and related metabolic disorders.

Osteocalcin Alleviates Nonalcoholic Fatty Liver Disease in Mice through GPRC6A

Osteocalcin is a bone-derived hormone that plays an important role in the crosstalk between bone and energy metabolism. Previous studies have found that treatment with uncarboxylated osteocalcin can protect mice from high-fat diet-induced nonalcoholic fatty liver disease (NAFLD). However, the potential mechanisms remain unclear. Although the G protein-coupled receptor family C group 6 subtype A (GPRC6A) is the putative receptor of osteocalcin, there is no direct evidence showing that GPRC6A mediates the effects of uncarboxylated osteocalcin in alleviating NAFLD in mice. We aimed to figure out this using liver-specific GPRC6A knockout (GPRC6A^LKO) mice. Consistent with previous studies, uncarboxylated osteocalcin significantly protected high-fat diet-fed wild-type mice from obesity and NAFLD, while it did not protect high-fat diet-fed GPRC6A^LKO mice from NAFLD. Differential mRNA expression of lipogenesis and lipolysis between GPRC6A^LKO mice and control mice revealed that GPRC6A mediated the effects of osteocalcin in alleviating NAFLD through inhibiting lipid synthesis and promoting lipolysis. In conclusion, this study found that uncarboxylated osteocalcin alleviates NAFLD in mice through the GPRC6A signaling pathway. Our study suggests that liver GPRC6A may be a potential target for treating NAFLD.

Role of Bioactive Peptide Sequences in the Potential Impact of Dairy Protein Intake on Metabolic Health

For years, there has been an increasing move towards elucidating the complexities of how food can interplay with the signalling networks underlying energy homeostasis and glycaemic control. Dairy foods can be regarded as the greatest source of proteins and peptides with various health benefits and are a well-recognized source of bioactive compounds. A number of dairy protein-derived peptide sequences with the ability to modulate functions related to the control of food intake, body weight gain and glucose homeostasis have been isolated and characterized. Their being active in vivo may be questionable mainly due to expected low bioavailability after ingestion, and hence their real contribution to the metabolic impact of dairy protein intake needs to be discussed. Some reports suggest that the differential effects of dairy proteins-in particular whey proteins-on mechanisms underlying energy balance and glucose-homeostasis may be attributed to their unique amino acid composition and hence the release of free amino acid mixtures enriched in essential amino acids (i.e., branched-chain-amino acids) upon digestion. Actually, the research reports reviewed in this article suggest that, among a number of dairy protein-derived peptides isolated and characterized as bioactive compounds in vitro, some peptides can be active in vivo post-oral administration through a local action in the gut, or, alternatively, a systemic action on specific molecular targets after entering the systemic circulation. Moreover, these studies highlight the importance of the enteroendocrine system in the cross talk between food proteins and the neuroendocrine network regulating energy balance.

Brain Testosterone-CYP1B1 (Cytochrome P450 1B1) Generated Metabolite 6β-Hydroxytestosterone Promotes Neurogenic Hypertension and Inflammation

Supplemental Digital Content is available in the text.

Previously, we showed that peripheral administration of 6β-hydroxytestosterone, a CYP1B1 (cytochrome P450 1B1)-generated metabolite of testosterone, promotes angiotensin II-induced hypertension in male mice. However, the site of action and the underlying mechanism by which 6β-hydroxytestosterone contributes to angiotensin II-induced hypertension is not known. Angiotensin II increases blood pressure by its central action, and CYP1B1 is expressed in the brain. This study was conducted to determine whether testosterone-CYP1B1 generated metabolite 6β-hydroxytestosterone locally in the brain promotes the effect of systemic angiotensin II to produce hypertension in male mice. Central CYP1B1 knockdown in wild-type (Cyp1b1^+/+) mice by intracerebroventricular-adenovirus-GFP (green fluorescence protein)-CYP1B1-short hairpin (sh)RNA attenuated, whereas reconstitution of CYP1B1 by adenovirus-GFP-CYP1B1-DNA in the paraventricular nucleus but not in subfornical organ in Cyp1b1^−/− mice restored angiotensin II-induced increase in systolic blood pressure measured by tail-cuff. Intracerebroventricular-testosterone in orchidectomized (Orchi)-Cyp1b1^+/+ but not in Orchi-Cyp1b1^−/−, and intracerebroventricular-6β-hydroxytestosterone in the Orchi-Cyp1b1^−/− mice restored the angiotensin II-induced: (1) increase in mean arterial pressure measured by radiotelemetry, and autonomic imbalance; (2) reactive oxygen species production in the subfornical organ and paraventricular nucleus; (3) activation of microglia and astrocyte, and neuroinflammation in the paraventricular nucleus. The effect of intracerebroventricular-6β-hydroxytestosterone to restore the angiotensin II-induced increase in mean arterial pressure and autonomic imbalance in Orchi-Cyp1b1^−/− mice was inhibited by intracerebroventricular-small interfering (si)RNA-androgen receptor (AR) and GPRC6A (G protein-coupled receptor C6A). These data suggest that testosterone-CYP1B1-generated metabolite 6β-hydroxytestosterone, most likely in the paraventricular nucleus via AR and GPRC6A, contributes to angiotensin II-induced hypertension and neuroinflammation in male mice.

Molecular chaperones and G protein-coupled receptor maturation and pharmacology

G protein-coupled receptors (GPCRs) are highly conserved versatile signaling molecules located at the plasma membrane that respond to diverse extracellular signals. They regulate almost all physiological processes in the vertebrates. About 35% of current drugs target these receptors. Mutations in these genes have been identified as causes of numerous diseases. The seven transmembrane domain structure of GPCRs implies that the folding of these transmembrane proteins is extremely complicated and difficult. Indeed, many wild type GPCRs are not folded optimally. The most common defect in genetic diseases caused by GPCR mutations is misfolding and failure to reach the plasma membrane where it functions. General molecular chaperones aid the folding of all proteins, including GPCRs, by preventing aggregation, promoting folding and disaggregating small aggregates. Some GPCRs need additional receptor-specific chaperones to assist their folding. Many of these receptor-specific chaperones interact with additional receptors and alter receptor pharmacology, expanding the understanding of these chaperone proteins.

Human vascular cell responses to the circulating bone hormone osteocalcin

The purpose of this study was to characterize the direct effects of uncarboxylated osteocalcin (ucOCN) on vascular cell biology in vitro, to assess its potential function in pathophysiological conditions such as atherosclerosis. Human aortic endothelial cells (HAECs) and smooth muscle cells (HASMCs) were treated with ucOCN (0.1-50 ng/ml) and changes in phosphorylation of intracellular signaling proteins, angiogenesis, proliferation, migration, monolayer permeability, and protein secretion were measured. In HAECs, phosphorylated JNK and CREB were decreased with ucOCN (p < 0.05). In HASMCs, phosphorylated p70S6K and NF-ΚB were increased by ucOCN (p < 0.05). Cell proliferation increased in both cell types dose dependently which was blocked by AKT and ERK pathway inhibitors. ucOCN did not affect cell permeability, angiogenesis, or migration. The direct activity of ucOCN on vascular cells is recognized, particularly its proliferative effects. However, at least in physiological settings, it does not appear that osteocalcin may directly promote atherogenesis based on the outcomes measured.

Metabolic and skeletal homeostasis are maintained in full locus GPRC6A knockout mice

The G protein-coupled receptor class C, group 6, subtype A (GPRC6A) is suggested to have a physiological function in glucose and bone metabolism, although the precise role lacks consensus due to varying findings in different knockout (KO) mouse models and inconsistent findings on the role of osteocalcin, a proposed GPRC6A agonist. We have further characterized a full locus GPRC6A KO model with respect to energy metabolism, including a long-term high-dose glucocorticoid metabolic challenge. Additionally, we analyzed the microarchitecture of tibiae from young, middle-aged and aged GPRC6A KO mice and wildtype (WT) littermates. Compared to WT, vehicle-treated KO mice presented with normal body composition, unaltered insulin sensitivity and basal serum insulin and glucose levels. Corticosterone (CS) treatment resulted in insulin resistance, abnormal fat accrual, loss of lean mass and suppression of serum osteocalcin levels in both genotypes. Interestingly, serum osteocalcin and skeletal osteocalcin mRNA levels were significantly lower in vehicle-treated GPRC6A KO mice compared to WT animals. However, WT and KO age groups did not differ in long bone mass and structure assessed by micro-computed tomography. We conclude that GPRC6A is not involved in glucose metabolism under normal physiological conditions, nor does it mediate glucocorticoid-induced dysmetabolism in mice. Moreover, GPRC6A does not appear to possess a direct, non-compensable role in long bone microarchitecture under standard conditions.

Low serum osteocalcin levels are correlated with left ventricular systolic dysfunction and cardiac death in Chinese men

Osteocalcin is a newly identified type of cytokine secreted by osteoblasts, which has an endocrine function, mediates energy and glycol-lipid metabolism, and is closely related to cardiovascular diseases. In this study, we investigated the value of serum osteocalcin levels in predicting left ventricular systolic dysfunction and cardiac death. A total of 258 patients in the Department of Cardiology were included. Two-dimensional echocardiography was performed in all the subjects. The cardiac death of subjects occurring with a median follow-up of 4.6 years was informed via phone calls or the electronic medical records. The serum osteocalcin levels were measured using electrochemiluminescent immunoassay. We found that the median left ventricular ejection fractions (LVEFs) were 62% in men and 63% in women. In the men with a LVEF > 62%, the serum osteocalcin levels were significantly higher than in those with LVEF ≤ 62% (P = 0.042), whereas this difference was absent in the women. Both the serum osteocalcin (β = 0.095, P = 0.028) and serum N-terminal pro-brain natriuretic peptide (NT-pro-BNP; β = -0.003, P < 0.01) levels remained independently significantly correlated with LVEF in the men but not in the women. Receiver operating characteristic (ROC) analyses of the men revealed that the serum osteocalcin (P = 0.007), serum NT-pro-BNP (P = 0.018) and serum osteocalcin + NT-pro-BNP (P < 0.01) levels were all significant in identifying left ventricular systolic dysfunction at baseline, but the pairwise comparisons of the three areas under the curves (AUCs) were all non-significant. The men in the lower osteocalcin level group at baseline suffered a greater risk of future cardiac death than those in the higher osteocalcin level group, whereas the result for NT-pro-BNP exhibited the opposite pattern. In conclusion, lower serum osteocalcin levels in the men could identify left ventricular systolic dysfunction and cardiac death in a manner that was not inferior to high serum NT-pro-BNP levels.

Estrogen receptors and sex hormone binding globulin in neuronal cells and tissue

Estrogens exert a critical influence on neuronal tissues and cells. As demonstrated in many clinical studies, estrogens are neuroprotective to the extent that they improve prognosis for women with neurodegenerative diseases. Unfortunately, we still do not know exactly how these effects are mediated. Fifty years ago the first estrogen receptor was found, but since then many other new pathways of estrogen action have been identified. This review describes several of these pathways of estrogen effects and provides some conclusions and correlations about these as determined by recent studies with nerve growth factor differentiated rat pheochromocytoma cell line.

Mechanisms of Pancreatic Injury Induced by Basic Amino Acids Differ Between L-Arginine, L-Ornithine, and L-Histidine

Pancreatic acinar cells require high rates of amino acid uptake for digestive enzyme synthesis, but excessive concentrations can trigger acute pancreatitis (AP) by mechanisms that are not well understood. We have used three basic natural amino acids L-arginine, L-ornithine, and L-histidine to determine mechanisms of amino acid-induced pancreatic injury and whether these are common to all three amino acids. Caffeine markedly inhibited necrotic cell death pathway activation in isolated pancreatic acinar cells induced by L-arginine, but not L-ornithine, whereas caffeine accelerated L-histidine-induced cell death. Both necroptosis inhibitors of RIPK1 and RIPK3 and a necroptosis activator/apoptosis inhibitor z-VAD increased cell death caused by L-histidine, but not L-arginine or L-ornithine. Cyclophilin D knock-out (Ppif-/-) significantly attenuated cell death induced by L-histidine, but not L-arginine, or L-ornithine. Allosteric modulators of calcium-sensing receptor (CaSR) and G-protein coupled receptor class C group 6 member A (GPRC6A) had inhibitory effects on cell death induced by L-arginine but not L-ornithine or L-histidine. We developed a novel amino acid-induced AP murine model with high doses of L-histidine and confirmed AP severity was significantly reduced in Ppif-/- vs. wild type mice. In L-arginine-induced AP neither Ppif-/-, caffeine, or allosteric modulators of CaSR or GPRC6A reduced pancreatic damage, even though CaSR inhibition with NPS-2143 significantly reduced pancreatic and systemic injury in caerulein-induced AP. These findings demonstrate marked differences in the mechanisms of pancreatic injury induced by different basic amino acids and suggest the lack of effect of treatments on L-arginine-induced AP may be due to conversion to L-ornithine in the urea cycle.

Osteocalcin‑GPRC6A: An update of its clinical and biological multi‑organic interactions (Review)

Osteocalcin is no longer regarded as a molecule exclusive to bone remodeling and osteogenesis, but as a hormone with manifold functions. The discovery of the interaction of osteocalcin with the G protein‑coupled receptor family C group 6‑member A (GPRC6A) receptor has accompanied the characterization of several roles that this peptide serves in body regulation and homeostasis. These roles include the modulation of memory in the brain, fertility in the testis, fat accumulation in the liver, incretins release in the intestine and adaptation to exercise in muscle, in addition to the well‑known effects on β‑cell proliferation, insulin release and adiponectin secretion. The aim of the present review was to provide a practical update of the multi‑organ effects that osteocalcin exerts through its interaction with GPRC6A and the clinical implications of this.

Calcimimetic R-568 vasodilatory effect on mesenteric vascular beds from normotensive (WKY) and spontaneously hypertensive (SHR) rats. Potential involvement of vascular smooth muscle cells (vSMCs)

The potential role of calcimimetics as vasculotropic agents has been suggested since the discovery that calcium sensing receptors (CaSRs) are expressed in cardiovascular tissues. However, whether this effect is CaSR-dependent or -independent is still unclear. In the present study the vascular activity of calcimimetic R-568 was investigated in mesenteric vascular beds (MVBs) isolated from Spontaneously Hypertensive rats (SHR) and the relative age-matched Wistar-Kyoto (WKY) control rats. Pre-constricted MBVs were perfused with increasing concentrations of R-568 (10 nM– 30 μM) resulting in a rapid dose-dependent vasodilatation. However, in MVBs from SHR this was preceded by a small but significant vasoconstriction at lowest nanomolar concentrations used (10–300 nM). Pre-treatment with pharmacological inhibitors of nitric oxide (NO) synthase (NOS, L-NAME), K_Ca channels (CTX), cyclo-oxygenase (INDO) and CaSR (Calhex) or the endothelium removal suggest that NO, CaSR and the endothelium itself contribute to the R-568 vasodilatory/vasoconstrictor effects observed respectively in WKY/SHR MVBs. Conversely, the vasodilatory effects resulted by highest R-568 concentration were independent of these factors. Then, the ability of lower R-568 doses (0.1–1 μM) to activate endothelial-NOS (eNOS) pathway in MVBs homogenates was evaluated. The Akt and eNOS phosphorylation levels resulted increased in WKY homogenates and Calhex significantly blocked this effect. Notably, this did not occur in the SHR. Similarly, vascular smooth muscle cells (vSMCs) stimulation with lower R-568 doses resulted in Akt activation and increased NO production in WKY but not in SHR cells. Interestingly, in these cells this was associated with the absence of the biologically active dimeric form of the CaSR thus potentially contributing to explain the impaired vasorelaxant effect observed in response to R-568 in MVB from SHR compared to WKY. Overall, these findings provide new insight on the mechanisms of action of the calcimimetic R-568 in modulating vascular tone both in physiological and pathological conditions such as hypertension.

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Osteocalcin in the brain: from embryonic development to age-related decline in cognition

A remarkable, unexpected aspect of the bone-derived hormone osteocalcin is that it is necessary for both brain development and brain function in the mouse, as its absence results in a profound deficit in spatial learning and memory and an exacerbation of anxiety-like behaviour. The regulation of cognitive function by osteocalcin, together with the fact that its circulating levels decrease in midlife compared with adolescence in all species tested, raised the prospect that osteocalcin might be an anti-geronic hormone that could prevent age-related cognitive decline. As presented in this Review, recent data indicate that this is indeed the case and that osteocalcin is necessary for the anti-geronic activity recently ascribed to the plasma of young wild-type mice. The diversity and amplitude of the functions of osteocalcin in the brain, during development and postnatally, had long called for the identification of its receptor in the brain, which was also recently achieved. This Review presents our current understanding of the biology of osteocalcin in the brain, highlighting the bony vertebrate specificity of the regulation of cognitive function and pointing toward where therapeutic opportunities might exist.

Osteocalcin, a bone-derived hormone with important andrological implications

Increasing evidence disclosed the existence of a novel multi-organ endocrine pathway, involving bone, pancreas and testis, of high penetrance in energy metabolism and male fertility. The main mediator of this axis is undercarboxylated osteocalcin (ucOC), a bone-derived protein-exerting systemic effects on tissues expressing the metabotropic receptor GPRC6A. The recognized effects of ucOC are the improvement of insulin secretion from the pancreas, the amelioration of systemic insulin sensitivity, in particular in skeletal muscle, and the stimulation of the global endocrine activity of the Leydig cell, including vitamin D 25-hydroxylation and testosterone production. The supporting evidence of this circuit in both animal and human models is here reviewed, with particular emphasis on the role of ucOC on testis function. The possible pharmacological modulation of this hormonal circuit for therapeutic aims is also discussed.

Robust GLP-1 secretion by basic L-amino acids does not require the GPRC6A receptor

The G protein-coupled receptor GPRC6A (GPCR, Class C, group 6, subtype A) has been proposed to be a sensor for basic L-amino acids that are hypothesized to translate ingestive behaviour to endocrine information. However, the contribution of the GPRC6A receptor to L-amino acid-induced glucagon-like peptide 1 (GLP-1) secretion is unclear. Therefore, to discover whether the GPRC6A receptor is indispensible for amino acid-induced secretion of GLP-1, we treated, with oral gavage, GPRC6A knock-out (KO) and wild-type (WT) littermate mice with GPRC6A ligands (L-arginine and L-ornithine) and assessed GLP-1 levels in circulation. We found that oral administration of both L-arginine and L-ornithine significantly increased total plasma GLP-1 levels to a similar level in GPRC6A KO and WT mice 15 minutes after gavage (both amino acids) and accumulated up to 60 minutes after gavage (L-arginine). Conversely, GLP-1 secretion at the 30- and 60-minute time points in the KO mice was attenuated and did not reach statistical significance. In summary, these data confirm that L-arginine is a potent GLP-1 secretagogue and show that the main effect occurs independently of GPRC6A. In addition, this is the first study to show that also L-ornithine powerfully elicits GLP-1 release in vivo.

The GPRC6A receptor displays constitutive internalization and sorting to the slow recycling pathway

The class C G protein-coupled receptor GPRC6A is a putative nutrient-sensing receptor and represents a possible new drug target in metabolic disorders. However, the specific physiological role of this receptor has yet to be identified, and the mechanisms regulating its activity and cell surface availability also remain enigmatic. In the present study, we investigated the trafficking properties of GPRC6A by use of both a classical antibody feeding internalization assay in which cells were visualized using confocal microscopy and a novel internalization assay that is based on real-time measurements of fluorescence resonance energy transfer. Both assays revealed that GPRC6A predominantly undergoes constitutive internalization, whereas the agonist-induced effects were imperceptible. Moreover, postendocytic sorting was investigated by assessing the co-localization of internalized GPRC6A with selected Rab protein markers. Internalized GPRC6A was mainly co-localized with the early endosome marker Rab5 and the long loop recycling endosome marker Rab11 and to a much lesser extent with the late endosome marker Rab7. This suggests that upon agonist-independent internalization, GPRC6A is recycled via the Rab11-positive slow recycling pathway, which may be responsible for ensuring a persistent pool of GPRC6A receptors at the cell surface despite chronic agonist exposure. Distinct trafficking pathways have been reported for several of the class C receptors, and our results thus substantiate that non-canonical trafficking mechanisms are a common feature for the nutrient-sensing class C family that ensure functional receptors in the cell membrane despite prolonged agonist exposure.

TCF21 is related to testis growth and development in broiler chickens

Background

Large amounts of fat deposition often lead to loss of reproductive efficiency in humans and animals. We used broiler chickens as a model species to conduct a two-directional selection for and against abdominal fat over 19 generations, which resulted in a lean and a fat line. Direct selection for abdominal fat content also indirectly resulted in significant differences (P < 0.05) in testis weight (TeW) and in TeW as a percentage of total body weight (TeP) between the lean and fat lines.

Results

A total of 475 individuals from the generation 11 (G₁₁) were genotyped. Genome-wide association studies revealed two regions on chicken chromosomes 3 and 10 that were associated with TeW and TeP. Forty G₁₆ individuals (20 from each line), were further profiled by focusing on these two chromosomal regions, to identify candidate genes with functions that may be potentially related to testis growth and development. Of the nine candidate genes identified with database mining, a significant association was confirmed for one gene, TCF21, based on mRNA expression analysis. Gene expression analysis of the TCF21 gene was conducted again across 30 G₁₉ individuals (15 individuals from each line) and the results confirmed the findings on the G₁₆ animals.

Conclusions

This study revealed that the TCF21 gene is related to testis growth and development in male broilers. This finding will be useful to guide future studies to understand the genetic mechanisms that underlie reproductive efficiency.

Electronic supplementary material

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Relationships between Bone Turnover and Energy Metabolism

It is well established that diabetes can be detrimental to bone health, and its chronic complications have been associated with an increased risk of osteoporotic fracture. However, there is growing evidence that the skeleton plays a key role in a whole-organism approach to physiology. The hypothesis that bone may be involved in the regulation of physiological functions, such as insulin sensitivity and energy metabolism, has been suggested. Given the roles of insulin, adipokines, and osteocalcin in these pathways, the need for a more integrative conceptual approach to physiology is emphasized. Recent findings suggest that bone plays an important role in regulating intermediary metabolism, being possibly both a target of diabetic complications and a potential pathophysiologic factor in the disease itself. Understanding the relationships between bone turnover and glucose metabolism is important in order to develop treatments that might reestablish energy metabolism and bone health. This review describes new insights relating bone turnover and energy metabolism that have been reported in the literature.

GPRC6A: Jack of all metabolism (or master of none)

Background

GPRC6A, a widely expressed G-protein coupled receptor, is proposed to be a master regulator of complex endocrine networks and metabolic processes. GPRC6A is activated by multiple ligands, including osteocalcin (Ocn), testosterone (T), basic amino acids, and various cations.

Scope of Review

We review the controversy surrounding GPRC6A functions. In mice, GPRC6A is proposed to integrate metabolic functions through the coordinated secretion of hormones, including insulin, GLP-1, T, and IL-6, and direct effects of this receptor to control glucose and fat metabolism in the liver, skeletal muscle, and fat. Loss-of-GPRC6A results in metabolic syndrome (MetS), and activation of GPRC6A stimulates proliferation of β-cells, increases peripheral insulin sensitivity, and protects against high fat diet (HFD) induced metabolic abnormalities in most mouse models. Bone, cardiovascular, immune, and skin functions of GPRC6A have also been identified in mice. Expression of GPRC6A is increased in prostate cancer (PCa) cells, and inhibition of GPRC6A attenuates PCa progression in mouse models. The function of GPRC6A in humans, however, is not clear. During evolution, a unique polymorphism of GPRC6A emerged mainly in humans of Asian and European decent that has been proposed to alter membrane trafficking and function. In contrast, the ancestral allele found in all other species is retained in 1%, 15%, and 40% of people of Asian, European and African descent, respectively, suggesting GPRC6A gene variants may contribute to the racial disparities in the risk of developing MetS and PCa.

Major Conclusions

If the regulatory functions of GPRC6A identified in mice translate to humans, and polymorphisms in GPRC6A are found to predict racial disparities in human diseases, GPRC6A may be a new gene target to predict, prevent, and treat MetS, PCa, and other disorders impacted by GPRC6A.

Genetic Variations in the Human G Protein-coupled Receptor Class C, Group 6, Member A (GPRC6A) Control Cell Surface Expression and Function

GPRC6A is a G protein-coupled receptor activated by l-amino acids, which, based on analyses of knock-out mice, has been suggested to have physiological functions in metabolism and testicular function. The human ortholog is, however, mostly retained intracellularly in contrast to the cell surface-expressed murine and goldfish orthologs. The latter orthologs are G_q-coupled and lead to intracellular accumulation of inositol phosphates and calcium release. In the present study we cloned the bonobo chimpanzee GPRC6A receptor, which is 99% identical to the human receptor, and show that it is cell surface-expressed and functional. By analyses of chimeric human/mouse and human/bonobo receptors, bonobo receptor mutants, and the single nucleotide polymorphism database at NCBI, we identify an insertion/deletion variation in the third intracellular loop responsible for the intracellular retention and lack of function of the human ortholog. Genetic analyses of the 1000 genome database and the Inter99 cohort of 6,000 Danes establish the distribution of genotypes among ethnic groups, showing that the cell surface-expressed and functional variant is much more prevalent in the African population than in European and Asian populations and that this variant is partly linked with a stop codon early in the receptor sequence (rs6907580, amino acid position 57). In conclusion, our data solve a more than decade-old question of why the cloned human GPRC6A receptor is not cell surface-expressed and functional and provide a genetic framework to study human phenotypic traits in large genome sequencing projects linked with physiological measurement and biomarkers.

Osteocalcin and Sex Hormone Binding Globulin Compete on a Specific Binding Site of GPRC6A

The undercarboxylated form of osteocalcin (ucOC) regulates male fertility and energy metabolism, acting through the G protein-coupled receptor (GPRC)6A, thus forming a new pancreas-bone-testis axis. Recently, GPRC6A has also been suggested to mediate the nongenomic responses of free testosterone (T). However, these data did not consider the physiological scenario, where circulating T is mainly bound to sex hormone-binding globulin (SHBG) and only a small percentage circulates freely in the blood. Here, by the use of computational modelling, we document the existence of similar structural moieties between ucOC and SHBG that are predicted to bind to GPRC6A at docking analysis. This hypothesis of competition was assessed by binding experiments on human embryonic kidney-293 cells transfected with human GPRC6A gene. Unliganded SHBG specifically bound the membrane of human embryonic kidney-293 cells transfected with GPRC6A and was displaced by ucOC when coincubated at 100-fold molar excess. Furthermore, specific downstream Erk1/2 phosphorylation after stimulation of GPRC6A with ucOC was significantly blunted by 100-fold molar excess of unliganded SHBG. Intriguingly previous incubation with unliganded SHBG, followed by incubation with T, induced Erk1/2 phosphorylation in a dose-dependent manner. Neither binding nor stimulating activities were shown for SHBG saturated with T. Experiments on mutation constructs of GPRC6A strengthened the hypothesis of a common binding site of ucOC and SHBG. Given the role of GPRC6A on energy metabolism, these data agree with epidemiological association between SHBG levels and insulin sensitivity, suggest GPRC6A as a likely SHBG receptor, and add bases for the possible regulation of androgen activity in a nonsteroidal manner.

Critical review evaluating the pig as a model for human nutritional physiology

The present review examines the pig as a model for physiological studies in human subjects related to nutrient sensing, appetite regulation, gut barrier function, intestinal microbiota and nutritional neuroscience. The nutrient-sensing mechanisms regarding acids (sour), carbohydrates (sweet), glutamic acid (umami) and fatty acids are conserved between humans and pigs. In contrast, pigs show limited perception of high-intensity sweeteners and NaCl and sense a wider array of amino acids than humans. Differences on bitter taste may reflect the adaptation to ecosystems. In relation to appetite regulation, plasma concentrations of cholecystokinin and glucagon-like peptide-1 are similar in pigs and humans, while peptide YY in pigs is ten to twenty times higher and ghrelin two to five times lower than in humans. Pigs are an excellent model for human studies for vagal nerve function related to the hormonal regulation of food intake. Similarly, the study of gut barrier functions reveals conserved defence mechanisms between the two species particularly in functional permeability. However, human data are scant for some of the defence systems and nutritional programming. The pig model has been valuable for studying the changes in human microbiota following nutritional interventions. In particular, the use of human flora-associated pigs is a useful model for infants, but the long-term stability of the implanted human microbiota in pigs remains to be investigated. The similarity of the pig and human brain anatomy and development is paradigmatic. Brain explorations and therapies described in pig, when compared with available human data, highlight their value in nutritional neuroscience, particularly regarding functional neuroimaging techniques.

The effects of muscle contraction and recombinant osteocalcin on insulin sensitivity ex vivo

We tested whether GPRC6A, the putative receptor of undercarboxylated osteocalcin (ucOC), is present in mouse muscle and whether ucOC increases insulin sensitivity following ex vivo muscle contraction. GPPRC6A is expressed in mouse muscle and in the mouse myotubes from a cell line. ucOC potentiated the effect of ex vivo contraction on insulin sensitivity.

INTRODUCTION

Acute exercise increases skeletal muscle insulin sensitivity. In humans, exercise increases circulating ucOC, a hormone that increases insulin sensitivity in rodents. We tested whether GPRC6A, the putative receptor of ucOC, is present in mouse muscle and whether recombinant ucOC increases insulin sensitivity in both C2C12 myotubes and whole mouse muscle following ex vivo muscle contraction.

METHODS

Glucose uptake was examined in C2C12 myotubes that express GPRC6A following treatment with insulin alone or with insulin and increasing ucOC concentrations (0.3, 3, 10 and 30 ng/ml). In addition, glucose uptake, phosphorylated (p-)AKT and p-AS160 were examined ex vivo in extensor digitorum longus (EDL) dissected from C57BL/6J wild-type mice, at rest, following insulin alone, after muscle contraction followed by insulin and after muscle contraction followed by recombinant ucOC then insulin exposure.

RESULTS

We observed protein expression of the likely receptor for ucOC, GPRC6A, in whole muscle sections and differentiated mouse myotubes. We observed reduced GPRC6A expression following siRNA transfection. ucOC significantly increased insulin-stimulated glucose uptake dose-dependently up to 10 ng/ml, in differentiated mouse C2C12 myotubes. Insulin increased EDL glucose uptake (∼30 %, p < 0.05) and p-AKT and p-AKT/AKT compared with rest (all p < 0.05). Contraction prior to insulin increased muscle glucose uptake (∼25 %, p < 0.05), p-AKT, p-AKT/AKT, p-AS160 and p-AS160/AS160 compared with contraction alone (all p < 0.05). ucOC after contraction increased insulin-stimulated muscle glucose uptake (∼12 % p < 0.05) and p-AS160 (<0.05) more than contraction plus insulin alone but without effect on p-AKT. In the absence of insulin and/or of contraction, ucOC had no significant effect on muscle glucose uptake.

CONCLUSIONS

GPRC6A, the likely receptor of osteocalcin (OC), is expressed in mouse muscle. ucOC treatment augments insulin-stimulated skeletal muscle glucose uptake in C2C12 myotubes and following ex vivo muscle contraction. ucOC may partly account for the insulin sensitizing effect of exercise.

Tryptophan Biochemistry: Structural, Nutritional, Metabolic, and Medical Aspects in Humans

L-Tryptophan is the unique protein amino acid (AA) bearing an indole ring: its biotransformation in living organisms contributes either to keeping this chemical group in cells and tissues or to breaking it, by generating in both cases a variety of bioactive molecules. Investigations on the biology of Trp highlight the pleiotropic effects of its small derivatives on homeostasis processes. In addition to protein turn-over, in humans the pathways of Trp indole derivatives cover the synthesis of the neurotransmitter/hormone serotonin (5-HT), the pineal gland melatonin (MLT), and the trace amine tryptamine. The breakdown of the Trp indole ring defines instead the "kynurenine shunt" which produces cell-response adapters as L-kynurenine, kynurenic and quinolinic acids, or the coenzyme nicotinamide adenine dinucleotide (NAD(+)). This review aims therefore at tracing a "map" of the main molecular effectors in human tryptophan (Trp) research, starting from the chemistry of this AA, dealing then with its biosphere distribution and nutritional value for humans, also focusing on some proteins responsible for its tissue-dependent uptake and biotransformation. We will thus underscore the role of Trp biochemistry in the pathogenesis of human complex diseases/syndromes primarily involving the gut, neuroimmunoendocrine/stress responses, and the CNS, supporting the use of -Omics approaches in this field.

Osteocalcin as a hormone regulating glucose metabolism

The number of patients with osteoporosis and diabetes is rapidly increasing all over the world. Bone is recently recognized as an endocrine organ. Accumulating evidence has shown that osteocalcin, which is specifically expressed in osteoblasts and secreted into the circulation, regulates glucose homeostasis by stimulating insulin expression in pancreas and adiponectin expression in adipocytes, resulting in improving glucose intolerance. On the other hand, insulin and adiponectin stimulate osteocalcin expression in osteoblasts, suggesting that positive feedforward loops exist among bone, pancreas, and adipose tissue. In addition, recent studies have shown that osteocalcin enhances insulin sensitivity and the differentiation in muscle, while secreted factors from muscle, myokines, regulate bone metabolism. These findings suggest that bone metabolism and glucose metabolism are associated with each other through the action of osteocalcin. In this review, I describe the role of osteocalcin in the interaction among bone, pancreas, brain, adipose tissue, and muscle.

Uncarboxylated osteocalcin inhibits high glucose-induced ROS production and stimulates osteoblastic differentiation by preventing the activation of PI3K/Akt in MC3T3-E1 cells

Uncarboxylated osteocalcin, an osteoblast-derived protein, plays an important role in the regulation of glucose metabolism. It has previously been demonstrated that high glucose levels inhibit osteoblast proliferation and differentiation. However, the mechanisms through which uncarboxylated osteocalcin regulates osteoblast proliferation and differentiation under high glucose conditions remain unclear. Thus, in the present study, we aimed to examine the effects of uncarboxylated osteocalcin on the proliferation and differentiation of MC3T3-E1 cells under high glucose conditions. We demonstrated that high glucose levels induced the production of reactive oxygen species (ROS) in MC3T3-E1 cells, and this production was inhibited by treatment with uncarboxylated osteocalcin and N-acetyl-L-cysteine (NAC), a ROS scavenger. In addition, we found that uncarboxylated osteocalcin reduced high glucose‑induced oxidative stress and increased the mRNA expression of the osteogenic markers, runt-related transcription factor 2 (Runx2), osterix and osteocalcin, as well as the formation of mineralized nodules; it also inhibited adipogenic differentiation, as shown by a decrease in the mRNA expression of the adipogenic markers, peroxisome proliferator‑activated receptor γ (PPARγ), adipocyte fatty acid-binding protein (adipocyte protein 2; aP2) and fatty acid synthase (FAS), and reduced lipid drop accumulation. Furthermore, we found that uncarboxylated osteocalcin inhibited PI3K/Akt signaling which was induced by ROS and facilitated the osteogenic differentiation of MC3T3-E1 cells under high glucose conditions. Taken together and to the best of ou knowledge, our results demonstrate for the first time that uncarboxylated osteocalcin inhibits high glucose-induced ROS production and stimulates osteoblastic differentiation by inhibiting the activation of PI3K/Akt in MC3T3-E1 cells. Therefore, we suggest that uncarboxylated osteocalcin may be a potential therapeutic agent for diabetes-related osteoporosis.

Selective Allosteric Antagonists for the G Protein-Coupled Receptor GPRC6A Based on the 2-Phenylindole Privileged Structure Scaffold

G protein-coupled receptors (GPCRs) represent a biological target class of fundamental importance in drug therapy. The GPRC6A receptor is a newly deorphanized class C GPCR that we recently reported for the first allosteric antagonists based on the 2-arylindole privileged structure scaffold (e.g., 1-3). Herein, we present the first structure-activity relationship study for the 2-arylindole antagonist 3, comprising the design, synthesis, and pharmacological evaluation of a focused library of 3-substituted 2-arylindoles. In a FRET-based inositol monophosphate (IP1) assay we identified compounds 7, 13e, and 34b as antagonists at the GPRC6A receptor in the low micromolar range and show that 7 and 34b display >9-fold selectivity for the GPRC6A receptor over related GPCRs, making 7 and 34b the most potent and selective antagonists for the GPRC6A receptor reported to date.

Structural and Functional Evidence for Testosterone Activation of GPRC6A in Peripheral Tissues

G protein-coupled receptor (GPCR) family C group 6 member A (GPRC6A) is a multiligand GPCR that is activated by cations, L-amino acids, and osteocalcin. GPRC6A plays an important role in the regulation of testosterone (T) production and energy metabolism in mice. T has rapid, transcription-independent (nongenomic) effects that are mediated by a putative GPCR. We previously found that T can activate GPRC6A in vitro, but the possibility that T is a ligand for GPRC6A remains controversial. Here, we demonstrate direct T binding to GPRC6A and construct computational structural models of GPRC6A that are used to identify potential binding poses of T. Mutations of the predicted binding site residues were experimentally found to block T activation of GPRC6A, in agreement with the modeling. Using Gpr6ca(-/-) mice, we confirmed that loss of GPRC6A resulted in loss of T rapid signaling responses and elucidated several biological functions regulated by GPRC6A-dependent T rapid signaling, including T stimulation of insulin secretion in pancreatic islets and enzyme expression involved in the biosynthesis of T in Leydig cells. Finally, we identified a stereo-specific effect of an R-isomer of a selective androgen receptor modulator that is predicted to bind to and shown to activate GPRC6A but not androgen receptor. Together, our data show that GPRC6A directly mediates the rapid signaling response to T and uncovers previously unrecognized endocrine networks.

The L-Cell in Nutritional Sensing and the Regulation of Appetite

The gastrointestinal (GI) tract senses the ingestion of food and responds by signaling to the brain to promote satiation and satiety. Representing an important part of the gut-brain axis, enteroendocrine L-cells secrete the anorectic peptide hormones glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) in response to the ingestion of food. The release of GLP-1 has multiple effects, including the secretion of insulin from pancreatic β-cells, decreased gastric emptying, and increased satiation. PYY also slows GI motility and reduces food intake. At least part of the gut-brain response seems to be due to direct sensing of macronutrients by L-cells, by mechanisms including specific nutrient-sensing receptors. Such receptors may represent possible pathways to target to decrease appetite and increase energy expenditure. Designing drugs or functional foods to exploit the machinery of these nutrient-sensing mechanisms may offer a potential approach for agents to treat obesity and metabolic disease.

Searching for additional endocrine functions of the skeleton: genetic approaches and implications for therapeutics

Our knowledge of whole organism physiology has greatly advanced in the past decades through mouse genetics. In particular, genetic studies have revealed that most organs interact with one another through hormones in order to maintain normal physiological functions and the homeostasis of the entire organism. Remarkably, through these studies many unexpected novel endocrine means to regulate physiological functions have been uncovered. The skeletal system is one example. In this article, we review a series of studies that over the years have identified bone as an endocrine organ. The mechanism of action, pathological relevance, and therapeutic implications of the functions of the bone-derived hormone osteocalcin are discussed. In the last part of this review we discuss the possibility that additional endocrine functions of the skeleton may exist.

An overview of the metabolic functions of osteocalcin

A recent unexpected development of bone biology is that bone is an endocrine organ regulating a growing number of physiological processes. One of the functions regulated by bone through the hormone osteocalcin is glucose homeostasis. In this overview, we will explain why we hypothesized that bone mass and energy metabolism should be subjected to a coordinated endocrine regulation. We will then review the experiments that revealed the endocrine function of osteocalcin and the cell biology events that allow osteocalcin to become a hormone. We will also illustrate the importance of this regulation to understand whole-body glucose homeostasis in the physiological state and in pathological conditions. Lastly, we will mention epidemiological and genetic evidence demonstrating that this function of osteocalcin is conserved in humans.

An overview of the metabolic functions of osteocalcin

A recent unexpected development of bone biology is that bone is an endocrine organ contributing to the regulation of a number of physiological processes. One of the functions regulated by bone through osteocalcin, an osteoblast specific hormone, is glucose homeostasis. In this overview, we explain the rationale why we hypothesized that there should be a coordinated endocrine regulation between bone mass and energy metabolism. We then review the experiments that identified the endocrine function of osteocalcin and the cell biology events that allow osteocalcin to become a hormone. We also demonstrate the importance of this regulation to understand whole-body glucose homeostasis in the physiological state and in pathological conditions. Lastly we discuss the epidemiological and genetic evidence demonstrating that this function of osteocalcin is conserved in humans.

GPRC6a is not required for the effects of a high-protein diet on body weight in mice

OBJECTIVE

The G-protein coupled receptor family C group 6 member A (GPRC6A) is activated by proteinogenic amino acids and may sense amino acids in the gastrointestinal tract and the brain. The study investigated whether GPRC6A was necessary for the effects of low- and high-protein diets on body weight and food intake in mice.

METHODS

The role of GPRC6A in mediating the effects of a low-protein diet on body weight was investigated in GPRC6a knockout (GPRC6a-KO) and wild-type (WT) mice fed a control diet (18% protein) or a low-protein diet (6% protein) for 9 days. The role of GPRC6A in mediating the effects of a high-protein diet on body weight was investigated in GPRC6a-KO and WT mice fed a control diet (18% protein) or a high-protein diet (50% protein) for 5 weeks.

RESULTS

A high-protein diet reduced body weight gain and food intake compared with a control diet in both WT and GPRC6a-KO mice. A low-protein diet decreased body weight gain in GPRC6a-KO mice.

CONCLUSIONS

GPRC6A was not necessary for the effects of a low- or high-protein diet on body weight and likely does not play a role in protein-induced satiety.

A four-season molecule: osteocalcin. Updates in its physiological roles

Osteocalcin (OC) is the main non-collagenous hydroxyapatite-binding protein synthesized by osteoblasts, odontoblasts, and hypertrophic chondrocytes. It has a regulatory role in mineralization and it is considered a marker of bone cell metabolism. Recent findings evidenced new extra-skeletal roles for OC, depicting it as a real hormone. OC shares many functional features with the common hormones, such as tissue-specific expression, circadian rhythm, and synthesis as a pre-pro-molecule. However, it has some peculiar features making it a unique molecule: OC exists in different forms based on the degree of carboxylation. Indeed, OC has three glutamic acid residues, in position 17, 21, and 24, which are subject to γ-carboxylation, through the action of a vitamin K-dependent γ-glutamyl carboxytransferase. The degree of carboxylation, and thus the negative charge density, determines the affinity for the calcium ions deposited in the extracellular matrix of the bone. The modulation of the carboxylation could, thus, represent the mechanism by which the body controls the circulating levels, and hence the hormonal function, of OC. There are evidences linking OC, and the bone metabolism, with a series of endocrine (glucose metabolism, energy metabolism, fertility) physiological (muscle activity) and pathological functions (ectopic calcification). Aim of this review is to give a full overview of the physiological roles of OC by collecting the newest experimental findings on this intriguing molecule.