👤 James G Granneman

The mobilization and catabolism of lipid energy is a central function of adipocytes that is under the control of the β-adrenergic signaling pathway, and defects in β-adrenergic signaling in adipocytes have been linked to obesity and obesity-related metabolic diseases. Receptor expression-enhancing proteins (REEPs) are endoplasmic reticulum (ER) proteins that play critical roles in subcellular targeting of receptor signaling complexes. Examination of gene expression profiles indicates that, among REEPs expressed in adipocytes, REEP6 expression is uniquely upregulated by sympathetic nervous system activation, suggesting involvement in regulating adrenergic signal transduction. The aim of this study was to assess the contribution of REEP6 to the thermogenic activation of adipocytes and characterize the metabolic consequences of REEP6 deficiency in vivo. Expression levels of Reep6 in adipose tissue were examined by using public transcriptomic data and validated by Western blot and qPCR analyses. Adipocyte-specific regulatory roles of REEP6 were investigated in vitro in C3H10T1/2 adipocytes and in primary adipocytes obtained from REEP6 KO mice. Effects of in vivo REEP6 deficiency on energy expenditure were measured by indirect calorimetry. Mitochondrial content in adipose tissue was accessed by immunoblot, mitochondrial DNA analysis, and confocal and electron microscopy. Effects of REEP6 KO on obesity-induced metabolic dysfunction were tested in a high-fat diet-induced obesity mouse model by glucose tolerance test, Western blot, and histological analyses. REEP6 expression is highly enriched in murine adipocytes and is sharply upregulated upon adipocyte differentiation and by cold exposure. Inactivation of REEP6 in mice increased adiposity, and reduced energy expenditure and cold tolerance. REEP6 KO severely reduced protein kinase A-mediated signaling in BAT and greatly reduced mitochondrial mass. The effect of REEP6 inactivation on diminished β-adrenergic signaling was reproduced in cultured adipocytes, indicating that this effect is cell-autonomous. REEP6 KO also suppressed expression of adenylate cyclase 3 (Adcy3) in brown adipose tissue and knockdown of REEP6 in adipocytes reduced targeting of ADCY3 to the plasma membrane. Lastly, REEP6 KO exacerbated high-fat diet-induced insulin resistance and inflammation in adipose tissue. This study indicates that REEP6 plays an important role in β-adrenergic signal transduction in adipocytes involving the expression and trafficking of Adcy3. Genetic inactivation of REEP6 reduces energy expenditure, increases adiposity, and the susceptibility to obesity-related metabolic dysfunction. Show less

Lipid transfer proteins mediate the exchange of lipids between closely apposed membranes at organelle contact sites and play key roles in lipid metabolism, membrane homeostasis, and cellular signaling. A recently discovered novel family of lipid transfer proteins, which includes the VPS13 proteins (VPS13A-D), adopt a rod-like bridge conformation with an extended hydrophobic groove that enables the bulk transfer of membrane lipids for membrane growth. Loss of function mutations in VPS13A and VPS13C cause chorea acanthocytosis and Parkinson's disease, respectively. VPS13A and VPS13C localize to multiple organelle contact sites, including endoplasmic reticulum (ER) - lipid droplet (LD) contact sites, but the functional roles of these proteins in LD regulation remains mostly unexplored. Here we employ CRISPR-Cas9 genome editing to generate VPS13A and VPS13C knockout cell lines in U-2 OS cells via deletion of exon 2 and introduction of an early frameshift. Analysis of LD content in these cell lines revealed that loss of either VPS13A or VPS13C results in reduced LD abundance under oleate-stimulated conditions. These data implicate two lipid transfer proteins, VPS13A and VPS13C, in LD regulation. Show less

Brown adipose tissue (BAT) thermogenesis depends on the mobilization and oxidation of fatty acids from intracellular lipid droplets (LD) within brown adipocytes (BAs); however, the identity and function of LD proteins that control BAT lipolysis remain incomplete. Proteomic analysis of mouse BAT subcellular fractions identified vacuolar protein sorting 13C (VPS13C) as a novel LD protein. The aim of this work was to investigate the role of VPS13C on BA LDs. Biochemical fractionation and high resolution confocal and immuno-transmission electron microscopy (TEM) were used to determine the subcellular distribution of VPS13C in mouse BAT, white adipose tissue, and BA cell culture. Lentivirus-delivered shRNA was used to determine the role of VPS13C in regulating lipolysis and gene expression in cultured BA cells. We found that VPS13C is highly expressed in mouse BAT where it is targeted to multilocular LDs in a subspherical subdomain. In inguinal white adipocytes, VPS13C was mainly observed on small LDs and β3-adrenergic stimulation increased VPS13C in this depot. Silencing of VPS13C in cultured BAs decreased LD size and triglyceride content, increased basal free fatty acid release, augmented the expression of thermogenic genes, and enhanced the lipolytic potency and efficacy of isoproterenol. Mechanistically, we found that BA lipolysis required activation of adipose tissue triglyceride lipase (ATGL) and that loss of VPS13C greatly increased the association of ATGL to LDs. VPS13C is present on BA LDs where is targeted to a distinct subdomain. VPS13C limits the access of ATGL to LD and loss of VPS13C elevates lipolysis and promotes oxidative gene expression. Show less