Rachel Larder

Depletion of stromal cells expressing fibroblast activation protein-α from skeletal muscle and bone marrow results in cachexia and anemia

Ablation of stromal cells expressing fibroblast activation protein-α (FAP) results in cachexia and anemia, and loss of these cells is seen in transplantable tumor models.

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.

Trim28 Haploinsufficiency Triggers Bi-stable Epigenetic Obesity

Summary More than one-half billion people are obese, and despite progress in genetic research, much of the heritability of obesity remains enigmatic. Here, we identify a Trim28-dependent network capable of triggering obesity in a non-Mendelian, “on/off” manner. Trim28+/D9 mutant mice exhibit a bi-modal body-weight distribution, with isogenic animals randomly emerging as either normal or obese and few intermediates. We find that the obese-“on” state is characterized by reduced expression of an imprinted gene network including Nnat, Peg3, Cdkn1c, and Plagl1 and that independent targeting of these alleles recapitulates the stochastic bi-stable disease phenotype. Adipose tissue transcriptome analyses in children indicate that humans too cluster into distinct sub-populations, stratifying according to Trim28 expression, transcriptome organization, and obesity-associated imprinted gene dysregulation. These data provide evidence of discrete polyphenism in mouse and man and thus carry important implications for complex trait genetics, evolution, and medicine. Video Abstract

Where to go with FTO

An understanding of the mechanisms underlying body-weight regulation is crucial to tackle the growing problem of obesity. Recent technological advances in the analysis of genetic variation have given novel insights into the molecular basis of common disease. In particular, genomic variants in the fat mass and obesity-associated (FTO) gene have been consistently associated with human adiposity and metabolic disorders. Studies of the product of this previously mysterious gene have formed a vanguard in the quest to turn statistical association into hard biology. In this review, we examine data from human genetic and murine studies that explore the potential role of FTO, a member of the Fe(II)- and 2-oxoglutarate-dependent oxygenase superfamily, in the regulation of energy homeostasis and metabolism.

Insulin-like peptide 5 is an orexigenic gastrointestinal hormone

Significance Hormonal factors from specialized enteroendocrine cells in the gut epithelium link the availability of dietary nutrients to energy utilization and storage. Many gut hormones also affect behaviors such as appetite and foraging, conveying for example the satiating effects of food consumption. Here we identify insulin-like peptide 5 (Insl5) as a product of colonic endocrine L-cells, and show that levels were elevated in calorie-restricted mice and reduced after feeding. Consistent with this profile Insl5 administration stimulated food intake in mice, indicating it should join ghrelin as only the second identified gut hormone that enhances appetite. Modulating the Insl5 axis presents a new strategy for the treatment of metabolic disease and obesity. The gut endocrine system is emerging as a central player in the control of appetite and glucose homeostasis, and as a rich source of peptides with therapeutic potential in the field of diabetes and obesity. In this study we have explored the physiology of insulin-like peptide 5 (Insl5), which we identified as a product of colonic enteroendocrine L-cells, better known for their secretion of glucagon-like peptide-1 and peptideYY. i.p. Insl5 increased food intake in wild-type mice but not mice lacking the cognate receptor Rxfp4. Plasma Insl5 levels were elevated by fasting or prolonged calorie restriction, and declined with feeding. We conclude that Insl5 is an orexigenic hormone released from colonic L-cells, which promotes appetite during conditions of energy deprivation.

Gonadotropin-Releasing Hormone Regulates Expression of the DNA Damage Repair Gene, Fanconi anemia A, in Pituitary Gonadotroph Cells

Abstract Gonadal function is critically dependant on regulated secretion of the gonadotropin hormones from anterior pituitary gonadotroph cells. Gonadotropin biosynthesis and release is triggered by the binding of hypothalamic GnRH to GnRH receptor expressed on the gonadotroph cell surface. The repertoire of regulatory molecules involved in this process are still being defined. We used the mouse LβT2 gonadotroph cell line, which expresses both gonadotropin hormones, as a model to investigate GnRH regulation of gene expression and differential display reverse transcription-polymerase chain reaction (RT-PCR) to identify and isolate hormonally induced changes. This approach identified Fanconi anemia a (Fanca), a gene implicated in DNA damage repair, as a differentially expressed transcript. Mutations in Fanca account for the majority of cases of Fanconi anemia (FA), a recessively inherited disease identified by congenital defects, bone marrow failure, infertility, and cancer susceptibility. We confirmed expression and hormonal regulation of Fanca mRNA by quantitative RT-PCR, which showed that GnRH induced a rapid, transient increase in Fanca mRNA. Fanca protein was also acutely upregulated after GnRH treatment of LβT2 cells. In addition, Fanca gene expression was confined to mature pituitary gonadotrophs and adult mouse pituitary and was not expressed in the immature αT3-1 gonadotroph cell line. Thus, this study extends the expression profile of Fanca into a highly specialized endocrine cell and demonstrates hormonal regulation of expression of the Fanca locus. We suggest that this regulatory mechanism may have a crucial role in the GnRH-response mechanism of mature gonadotrophs and perhaps the etiology of FA.

Hypothalamic loss of Snord116 recapitulates the hyperphagia of Prader-Willi syndrome

Profound hyperphagia is a major disabling feature of Prader-Willi syndrome (PWS). Characterization of the mechanisms that underlie PWS-associated hyperphagia has been slowed by the paucity of animal models with increased food intake or obesity. Mice with a microdeletion encompassing the Snord116 cluster of noncoding RNAs encoded within the Prader-Willi minimal deletion critical region have previously been reported to show growth retardation and hyperphagia. Here, consistent with previous reports, we observed growth retardation in Snord116+/–P mice with a congenital paternal Snord116 deletion. However, these mice neither displayed increased food intake nor had reduced hypothalamic expression of the proprotein convertase 1 gene PCSK1 or its upstream regulator NHLH2, which have recently been suggested to be key mediators of PWS pathogenesis. Specifically, we disrupted Snord116 expression in the mediobasal hypothalamus in Snord116fl mice via bilateral stereotaxic injections of a Cre-expressing adeno-associated virus (AAV). While the Cre-injected mice had no change in measured energy expenditure, they became hyperphagic between 9 and 10 weeks after injection, with a subset of animals developing marked obesity. In conclusion, we show that selective disruption of Snord116 expression in the mediobasal hypothalamus models the hyperphagia of PWS.

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

Significance The growing size and sophistication of genome-wide association studies have led to the identification of variants which are clearly and reliably associated with obesity. A strong association between increased BMI and a region of human chromosome 2, near to the gene TMEM18, has been repeatedly demonstrated in children and adults. The function of TMEM18 in the control of appetitive behavior and body composition has been poorly characterized. In murine models, we show germline loss results in weight gain while adult onset hypothalamic overexpression results in weight loss, supporting the hypothesis that TMEM18 acting within the central nervous system can affect energy balance. We also report a structure and putative molecular function of TMEM18, challenging the current published model. 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.

Genetic aspects of human obesity.

Obesity and its related metabolic consequences represent a major public health problem. Huge changes within the environment have undoubtedly contributed to the increased prevalence of obesity but genetic factors are also critical in determining an individuals predisposition to gain weight. The last two decades have seen a huge increase in the understanding of the mechanisms controlling appetitive behavior, body composition, and energy expenditure. Many regions throughout the central nervous system play critical roles in these processes but the hypothalamus, in particular, receives and orchestrates a variety of signals to bring about coordinated changes in energy balance. Reviewing data from human genetic and model organism studies, we consider how disruptions of hypothalamic pathways evolved to maintain energy homeostasis and go on to cause obesity. We highlight ongoing technological developments which continue to lead to novel insights and discuss how this increased knowledge may lead to effective therapeutic interventions in the future.

Loss And Gain Of Function Experiments Implicate TMEM18 As A Mediator Of The Strong Association Between Genetic Variants At Human Chromosome 2p25.3 And Obesity

An intergenic region of human Chromosome 2 (2p25.3) harbours genetic variants which are among those most strongly and reproducibly associated with obesity. The molecular mechanisms mediating these effects remain entirely unknown. The gene closest to these variants is TMEM18, encoding a transmembrane protein localised to the nuclear membrane. The expression of Tmem18 within the murine hypothalamic paraventricular nucleus was altered by changes in nutritional state, with no significant change seen in three other closest genes. 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 confirmed the nuclear membrane localisation of TMEM18 but provide new evidence that it is 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.