Loss of Mrap2 is associated with Sim1 deficiency and increased circulating cholesterol

Melanocortin receptor accessory protein 2 (MRAP2) is a transmembrane accessory protein predominantly expressed in the brain. Both global and brain-specific deletion of Mrap2 in mice results in severe obesity. Loss-of-function MRAP2 mutations have also been associated with obesity in humans. Although MRAP2 has been shown to interact with MC4R, a G protein-coupled receptor with an established role in energy homeostasis, appetite regulation and lipid metabolism, the mechanisms through which loss of MRAP2 causes obesity remains uncertain. In this study, we used two independently derived lines of Mrap2 deficient mice (Mrap2tm1a/tm1a) to further study the role of Mrap2 in the regulation of energy balance and peripheral lipid metabolism. Mrap2tm1a/tm1a mice have a significant increase in body weight, with increased fat and lean mass, but without detectable changes in food intake or energy expenditure. Transcriptomic analysis showed significantly decreased expression of Sim1, Trh, Oxt and Crh within the hypothalamic paraventricular nucleus of Mrap2tm1a/tm1a mice. Circulating levels of both high-density lipoprotein and low-density lipoprotein were significantly increased in Mrap2 deficient mice. Taken together, these data corroborate the role of MRAP2 in metabolic regulation and indicate that, at least in part, this may be due to defective central melanocortin signallin

Adult onset global loss of the fto gene alters body composition and metabolism in the mouse.

The strongest BMI-associated GWAS locus in humans is the FTO gene. Rodent studies demonstrate a role for FTO in energy homeostasis and body composition. The phenotypes observed in loss of expression studies are complex with perinatal lethality, stunted growth from weaning, and significant alterations in body composition. Thus understanding how and where Fto regulates food intake, energy expenditure, and body composition is a challenge. To address this we generated a series of mice with distinct temporal and spatial loss of Fto expression. Global germline loss of Fto resulted in high perinatal lethality and a reduction in body length, fat mass, and lean mass. When ratio corrected for lean mass, mice had a significant increase in energy expenditure, but more appropriate multiple linear regression normalisation showed no difference in energy expenditure. Global deletion of Fto after the in utero and perinatal period, at 6 weeks of age, removed the high lethality of germline loss. However, there was a reduction in weight by 9 weeks, primarily as loss of lean mass. Over the subsequent 10 weeks, weight converged, driven by an increase in fat mass. There was a switch to a lower RER with no overall change in food intake or energy expenditure. To test if the phenotype can be explained by loss of Fto in the mediobasal hypothalamus, we sterotactically injected adeno-associated viral vectors encoding Cre recombinase to cause regional deletion. We observed a small reduction in food intake and weight gain with no effect on energy expenditure or body composition. Thus, although hypothalamic Fto can impact feeding, the effect of loss of Fto on body composition is brought about by its actions at sites elsewhere. Our data suggest that Fto may have a critical role in the control of lean mass, independent of its effect on food intake

Loss of Mrap2 is associated with Sim1 deficiency and increased circulating cholesterol.

Melanocortin receptor accessory protein 2 (MRAP2) is a transmembrane accessory protein predominantly expressed in the brain. Both global and brain-specific deletion of Mrap2 in mice results in severe obesity. Loss-of-function MRAP2 mutations have also been associated with obesity in humans. Although MRAP2 has been shown to interact with MC4R, a G protein-coupled receptor with an established role in energy homeostasis, appetite regulation and lipid metabolism, the mechanisms through which loss of MRAP2 causes obesity remains uncertain. In this study, we used two independently derived lines of Mrap2 deficient mice (Mrap2(tm1a/tm1a)) to further study the role of Mrap2 in the regulation of energy balance and peripheral lipid metabolism. Mrap2(tm1a/tm1a) mice have a significant increase in body weight, with increased fat and lean mass, but without detectable changes in food intake or energy expenditure. Transcriptomic analysis showed significantly decreased expression of Sim1, Trh, Oxt and Crh within the hypothalamic paraventricular nucleus of Mrap2(tm1a/tm1a) mice. Circulating levels of both high-density lipoprotein and low-density lipoprotein were significantly increased in Mrap2 deficient mice. Taken together, these data corroborate the role of MRAP2 in metabolic regulation and indicate that, at least in part, this may be due to defective central melanocortin signalling

Heterogeneity of hypothalamic pro-opiomelanocortin-expressing neurons revealed by single-cell RNA sequencing

$\textbf{Objective}$ Arcuate proopiomelanocortin (POMC) neurons are critical nodes in the control of body weight. Often characterized simply as direct targets for leptin, recent data suggest a more complex architecture. $\textbf{Methods}$ Using single cell RNA sequencing, we have generated an atlas of gene expression in murine POMC neurons. $\textbf{Results}$ Of 163 neurons, 118 expressed high levels of $\textit{Pomc}$ with little/no Agrp expression and were considered “canonical” POMC neurons (P$^{+}$). The other 45/163 expressed low levels of $\textit{Pomc}$ and high levels of $\textit{Agrp}$ (A$^{+}$P$_{+}$). Unbiased clustering analysis of P$^{+}$ neurons revealed four different classes, each with distinct cell surface receptor gene expression profiles. Further, only 12% (14/118) of P$^{+}$ neurons expressed the leptin receptor ($\textit{Lepr}$) compared with 58% (26/45) of A$^{+}$P$_{+}$ neurons. In contrast, the insulin receptor ($\textit{Insr}$) was expressed at similar frequency on P$^{+}$ and A$^{+}$P$_{+}$ neurons (64% and 55%, respectively). $\textbf{Conclusion}$ These data reveal arcuate POMC neurons to be a highly heterogeneous population. Accession Numbers: GSE92707.This work was supported by the UK Medical Research Council (MRC) Metabolic Disease Unit (MRC_MC_UU_12012/1 & MRC_MC_UU_12012/5), a Wellcome Trust Strategic Award (100574/Z/12/Z), and the Helmholtz Alliance ICEMED

Heterogeneity of hypothalamic pro-opiomelanocortin-expressing neurons revealed by single-cell RNA sequencing

$\textbf{Objective}$ Arcuate proopiomelanocortin (POMC) neurons are critical nodes in the control of body weight. Often characterized simply as direct targets for leptin, recent data suggest a more complex architecture. $\textbf{Methods}$ Using single cell RNA sequencing, we have generated an atlas of gene expression in murine POMC neurons. $\textbf{Results}$ Of 163 neurons, 118 expressed high levels of $\textit{Pomc}$ with little/no Agrp expression and were considered “canonical” POMC neurons (P$^{+}$). The other 45/163 expressed low levels of $\textit{Pomc}$ and high levels of $\textit{Agrp}$ (A$^{+}$P$_{+}$). Unbiased clustering analysis of P$^{+}$ neurons revealed four different classes, each with distinct cell surface receptor gene expression profiles. Further, only 12% (14/118) of P$^{+}$ neurons expressed the leptin receptor ($\textit{Lepr}$) compared with 58% (26/45) of A$^{+}$P$_{+}$ neurons. In contrast, the insulin receptor ($\textit{Insr}$) was expressed at similar frequency on P$^{+}$ and A$^{+}$P$_{+}$ neurons (64% and 55%, respectively). $\textbf{Conclusion}$ These data reveal arcuate POMC neurons to be a highly heterogeneous population. Accession Numbers: GSE92707.This work was supported by the UK Medical Research Council (MRC) Metabolic Disease Unit (MRC_MC_UU_12012/1 & MRC_MC_UU_12012/5), a Wellcome Trust Strategic Award (100574/Z/12/Z), and the Helmholtz Alliance ICEMED

The obesity-associated gene TMEM18 has a role in the central control of appetite and body weight regulation

An intergenic region of human chromosome 2 (2p25.3) harbors genetic variants which are among those most strongly and reproducibly associated with obesity. The gene closest to these variants is TMEM18, although the molecular mechanisms mediating these effects remain entirely unknown. Tmem18 expression in the murine hypothalamic paraventricular nucleus (PVN) was altered by changes in nutritional state. Germline loss of Tmem18 in mice resulted in increased body weight, which was exacerbated by high fat diet and driven by increased food intake. Selective overexpression of Tmem18 in the PVN of wild-type mice reduced food intake and also increased energy expenditure. We provide evidence that TMEM18 has four, not three, transmembrane domains and that it physically interacts with key components of the nuclear pore complex. Our data support the hypothesis that TMEM18 itself, acting within the central nervous system, is a plausible mediator of the impact of adjacent genetic variation on human adiposity.RL, YCLT, DR, GSHY, SOR and APC are funded by the Medical Research Council (MRC) Metabolic Disease Unit (MRC_MC_UU_12012/1) and animal work was carried out with the assistance of MRC Disease Model Core of the Wellcome Trust MRC Institute of Metabolic Sciences (MRC_MC_UU_12012/5 and Wellcome Trust Strategic Award (100574/Z/12/Z). F. Bosch is the recipient of an award from the ICREA Academia, Generalitat de Catalunya, Spain. Vector generation and production were funded by Ministerio de Economía y Competitividad (SAF 2014-54866-R), Spain. CD and DWL were supported by the Wellcome Trust (WT098051) and CD was supported by the Wellcome Trust PhD Programme for Clinicians (100679/Z/12/Z)

GDF15 linked to maternal risk of nausea and vomiting during pregnancy

GDF15, a hormone acting on the brainstem, has been implicated in the nausea and vomiting of pregnancy (NVP) including its most severe form, Hyperemesis Gravidarum (HG), but a full mechanistic understanding is lacking [1-4]. Here we report that fetal production of GDF15, and maternal sensitivity to it, both contribute substantially to the risk of HG. We confirmed that higher GDF15 levels in maternal blood are associated with vomiting in pregnancy and HG. Using mass spectrometry to detect a naturally-labelled GDF15 variant we demonstrate that the vast majority of GDF15 in the maternal plasma is derived from the feto-placental unit. By studying carriers of rare and common genetic variants we found that low levels of GDF15 in the non-pregnant state increase the risk of developing HG. Conversely, women with beta-thalassemia, a condition where GDF15 levels are chronically high [5], report very low levels of NVP. In mice, the acute food intake response to a bolus of GDF15 is influenced bi-directionally by prior levels of circulating GDF15 in a manner suggesting that this system is susceptible to desensitization. Our findings support a putative causal role for fetally-derived GDF15 in the nausea and vomiting of human pregnancy, with maternal sensitivity, at least partly determined by pre-pregnancy exposure to the hormone, being a major influence on its severity. They also suggest mechanism-based approaches to the treatment and prevention of HG.</p