Analysis of knockout mice suggests a role for VGF in the control of fat storage and energy expenditure

Elizabeth Watson,Haruka Okamoto,Samira Fargali,Ronald E Gordon,Tandra Chakraborty,Stephen R Salton,Masato Sadahiro,Mark W Sleeman

doi:10.1186/1472-6793-9-19

Elizabeth Watson, Haruka Okamoto + Show 6 more

Open Access

https://doi.org/10.1186/1472-6793-9-19

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Abstract

BackgroundPrevious studies of mixed background mice have demonstrated that targeted deletion of Vgf produces a lean, hypermetabolic mouse that is resistant to diet-, lesion-, and genetically-induced obesity. To investigate potential mechanism(s) and site(s) of action of VGF, a neuronal and endocrine secreted protein and neuropeptide precursor, we further analyzed the metabolic phenotypes of two independent VGF knockout lines on C57Bl6 backgrounds.ResultsUnlike hyperactive VGF knockout mice on a mixed C57Bl6-129/SvJ background, homozygous mutant mice on a C57Bl6 background were hypermetabolic with similar locomotor activity levels to Vgf+/Vgf+ mice, during day and night cycles, indicating that mechanism(s) other than hyperactivity were responsible for their increased energy expenditure. In Vgf-/Vgf- knockout mice, morphological analysis of brown and white adipose tissues (BAT and WAT) indicated decreased fat storage in both tissues, and decreased adipocyte perimeter and area in WAT. Changes in gene expression measured by real-time RT-PCR were consistent with increased fatty acid oxidation and uptake in BAT, and increased lipolysis, decreased lipogenesis, and brown adipocyte differentiation in WAT, suggesting that increased sympathetic nervous system activity in Vgf-/Vgf- mice may be associated with or responsible for alterations in energy expenditure and fat storage. In addition, uncoupling protein 1 (UCP1) and UCP2 protein levels, mitochondrial number, and mitochondrial cristae density were upregulated in Vgf-/Vgf- BAT. Using immunohistochemical and histochemical techniques, we detected VGF in nerve fibers innervating BAT and Vgf promoter-driven reporter expression in cervical and thoracic spinal ganglia that project to and innervate the chest wall and tissues including BAT. Moreover, VGF peptide levels were quantified by radioimmunoassay in BAT, and were found to be down-regulated by a high fat diet. Lastly, despite being hypermetabolic, VGF knockout mice were cold intolerant.ConclusionWe propose that VGF and/or VGF-derived peptides modulate sympathetic outflow pathways to regulate fat storage and energy expenditure.

Highlights

Previous studies of mixed background mice have demonstrated that targeted deletion of Vgf produces a lean, hypermetabolic mouse that is resistant to diet, lesion, and genetically-induced obesity
To gain insight into the molecular and cellular changes that may be responsible for the increase in energy expenditure and decrease in adipose stores detected in VGF knockout mice, we examined the effect that targeted ablation of VGF has on the regulation of gene expression in adipose tissues, analyzing two independent VGF knockout mouse lines on C57Bl6 backgrounds
To investigate whether background strain had an effect on metabolic phenotype, we analyzed the same line of VGF knockout mice that had been backcrossed 10 generations to obtain a homogeneous C57Bl6 background, as well as the F3 generation of an independent mouse line, generated by Regeneron Pharmaceuticals Inc. using F1H4 ES cells (a 129B6/F1-derived cell line), primarily on a C57Bl6 background (>83% C57Bl6 background; designated VGFR knockout or VgfR -/-) [25]

Summary

Introduction

Previous studies of mixed background mice have demonstrated that targeted deletion of Vgf produces a lean, hypermetabolic mouse that is resistant to diet-, lesion-, and genetically-induced obesity. The MC4R is expressed in the hypothalamic paraventricular nucleus (PVN), which contains neurons that innervate parasympathetic and sympathetic preganglionic cells in the brainstem and spinal cord, and throughout the brain in sympathetic circuits that innervate brown adipose tissue (BAT) [2,3] These projections give the hypothalamus direct input to the autonomic nervous system, controlling glucose and energy homeostasis [4,5]. Increased UCP1 expression in WAT, the result of SNS stimulation or genetic background, is associated with brown adipocyte induction in WAT depots, and leads to reversal of diet-induced and genetic obesity [1216] Taken together, these data argue for an important role of the SNS in the control of energy expenditure via outflow to both white and brown adipose tissues

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