bims-mimbat Biomed News
on Mitochondrial metabolism in brown adipose tissue
Issue of 2022‒05‒08
sixteen papers selected by
José Carlos de Lima-Júnior
University of California San Francisco

  1. Elife. 2022 05 03. pii: e77740. [Epub ahead of print]11
      Wnt/β-catenin signaling has been well established as a potent inhibitor of adipogenesis. Here, we identified a population of adipocytes that exhibit persistent activity of Wnt/β-catenin signaling, as revealed by the Tcf/Lef-GFP reporter allele, in embryonic and adult mouse fat depots, named as Wnt+ adipocytes. We showed that this β-catenin-mediated signaling activation in these cells is Wnt ligand- and receptor-independent but relies on AKT/mTOR pathway and is essential for cell survival. Such adipocytes are distinct from classical ones in transcriptomic and genomic signatures and can be induced from various sources of mesenchymal stromal cells including human cells. Genetic lineage-tracing and targeted cell ablation studies revealed that these adipocytes convert into beige adipocytes directly and are also required for beige fat recruitment under thermal challenge, demonstrating both cell autonomous and non-cell autonomous roles in adaptive thermogenesis. Furthermore, mice bearing targeted ablation of these adipocytes exhibited glucose intolerance, while mice receiving exogenously supplied such cells manifested enhanced glucose utilization. Our studies uncover a unique adipocyte population in regulating beiging in adipose tissues and systemic glucose homeostasis.
    Keywords:  AKT/mTOR signaling; adipocyte; beiging; cell biology; glucose homeostasis; mouse; β-catenin signaling
  2. Mol Metab. 2022 May 02. pii: S2212-8778(22)00077-1. [Epub ahead of print] 101508
      OBJECTIVE: Brown adipocytes play a key role in maintaining body temperature as well as glucose and lipid homeostasis. However, brown adipocytes need to adapt their thermogenic activity and substrate utilization to changes in nutrient availability. Amongst the multiple factors influencing brown adipocyte activity, autophagy is an important regulatory element of thermogenic capacity and activity. Nevertheless, a specific sensing mechanism of extracellular amino acid availability linking autophagy to nutrient availability in brown adipocytes is unknown.METHODS: To characterize the role of the amino acid transporter PAT2/SLC36A2 in brown adipocytes, loss or gain of function of PAT2 were studied with respect to differentiation, subcellular localization, lysosomal activity and autophagy. Activity of vATPase was evaluated by quenching of EGFP fused to LC3 or FITC-dextran loaded lysosomes in brown adipocytes upon amino acid starvation, whereas the effect of PAT2 on assembly of the vATPase was investigated by Native-PAGE.
    RESULTS: We show that PAT2 translocates from the plasma membrane to the lysosome in response to amino acid withdrawal. Loss or overexpression of PAT2 impair lysosomal acidification and starvation induced S6K re-phosphorylation, as PAT2 facilitates the assembly of the lysosomal vATPase, by recruitment of the cytoplasmic V1 subunit to the lysosome.
    CONCLUSION: PAT2 is an important sensor of extracellular amino acids and regulator of lysosomal acidification in brown adipocytes.
    Keywords:  Brown adipocytes; Lysosomal acidification; Proton-coupled amino acid transporter; Transporter translocation across membranes; V-ATPase assembly
  3. Biosci Rep. 2022 May 03. pii: BSR20212543. [Epub ahead of print]
      Brown adipose tissue (BAT) is a promising weapon to combat obesity and metabolic disease. BAT is thermogenic and consumes substantial amounts of glucose and fatty acids as fuel for thermogenesis and energy expenditure. To study BAT function in large human longitudinal cohorts, safe and precise detection methodologies are needed. Although regarded a gold standard, the foray of PET-CT into BAT research and clinical applications is limited by its high ionizing radiation doses. Here, we show that brown adipocytes release exosomes in blood plasma which can be utilized to assess BAT activity. In this study, we investigated circulating protein biomarkers that can accurately and reliably reflect BAT activation triggered by cold exposure, capsinoids ingestion and thyroid hormone excess in humans. We discovered an exosomal protein, methylene tetrahydrofolate dehydrogenase (NADP+ dependent) 1-like (MTHFD1L), to be over-expressed and detectable in plasma for all three modes of BAT activation in human subjects. This mitochondrial protein is packaged as a cargo within multivesicular bodies of the endosomal compartment and secreted as exosomes via exocytosis from activated brown adipocytes into the circulation. To support MTHFD1L as a conserved BAT activation response in other vertebrates, we examined a rodent model and also proved its presence in blood of rats following BAT activation by cold exposure. Plasma concentration of exosomal MTHFD1L correlated with human BAT activity as confirmed by PET-MR in humans and supported by data from rats. Thus, we deduce that MTHFD1L appears to be over-expressed in activated BAT compared to BAT in the basal non-stimulated state.
    Keywords:  Biomarker; Brown adipose tissue (BAT); Exosomes; Methylene tetrahydrofolate dehydrogenase 1-like (MTHFD1L); N(10)-formyltetrahydrofolate synthetase
  4. Mitochondrion. 2022 Apr 29. pii: S1567-7249(22)00037-X. [Epub ahead of print]
      Brown adipose tissue (BAT) mitochondria generate heat via uncoupled respiration due to excessive proton leak through uncoupling proteins (UCPs). We previously found hyperthermia in a newborn mouse model of fragile X syndrome and excessive leak in Fmr1 KO forebrain mitochondria caused by CoQ deficiency. The inefficient thermogenic nature of Fmr1 mutant forebrain mitochondria was reminiscent of BAT metabolic features. Thus, we aimed to characterize BAT mitochondrial function in these hyperthermic mice using a top-down approach. Although there was no change in steady-state levels of UCP1 expression between strains, BAT weighed significantly less in Fmr1 mutants compared with controls. Fmr1 KO BAT mitochondria demonstrated impaired substrate oxidation, lower mitochondrial membrane potentials and rates of respiration, and CoQ deficiency. The CoQ analog decylubiquinone normalized CoQ-dependent electron flux and unmasked excessive proton leak. Unlike mutant forebrain, where such deficiency resulted in pathological proton leak, CoQ deficiency within BAT mitochondria resulted largely in abnormal substrate oxidation. This suggests that CoQ is important in BAT for uncoupled respiration to produce heat during development. Although our data provide further evidence of a link between fragile X mental retardation protein (FMRP) and CoQ biosynthesis, the results highlight the importance of CoQ in developing tissues and suggest tissue-specific differences from CoQ deficiency. Because BAT mitochondria are primarily responsible for regulating core body temperature, the defects we describe in Fmr1 KOs could manifest as an adaptive downregulated response to hyperthermia or could result from FMRP deficiency directly.
    Keywords:  Mitochondria; brown adipose tissue; coenzyme Q; fragile X syndrome; hyperthermia; uncoupling proteins
  5. Mol Pharmacol. 2022 May 03. pii: MOLPHARM-MR-2021-000465. [Epub ahead of print]
      Canonical non-shivering thermogenesis (NST) in brown and beige fat relies on uncoupling protein 1 (UCP1)-mediated heat generation, although alternative mechanisms of NST have been identified, including sarcoplasmic reticulum (SR)-calcium cycling. Intracellular calcium is a crucial cell signaling molecule for which compartmentalization is tightly regulated, and the sarco-endoplasmic calcium ATPase (SERCA) actively pumps calcium from the cytosol into the SR. In this review, we discuss the capacity of SERCA-mediated calcium cycling as a significant mediator of thermogenesis in both brown and beige adipocytes. Here, we suggest two primary mechanisms of SR calcium mediated thermogenesis. The first mechanism is through direct uncoupling of the ATPase and calcium pump activity of SERCA, resulting in the energy of ATP catalysis being expended as heat in the absence of calcium transport. Regulins, a class of SR membrane proteins, act to decrease the calcium affinity of SERCA and uncouple the calcium transport function from ATPase activity, but remain largely unexplored in adipose tissue thermogenesis. A second mechanism is through futile cycling of SR calcium whereby SERCA-mediated SR calcium influx is equally offset by SR calcium efflux, resulting in ATP consumption without a net change in calcium compartmentalization. A fuller understanding of the functional and mechanistic role of calcium cycling as a mediator of adipose tissue thermogenesis and how manipulation of these pathways can be harnessed for therapeutic gain remains unexplored. Significance Statement Enhancing thermogenic metabolism in brown or beige adipose tissue may be of broad therapeutic utility to reduce obesity and metabolic syndrome. Canonical BAT-mediated thermogenesis occurs via uncoupling protein 1 (UCP1). However, UCP1-independent pathways of thermogenesis, such as sarcoplasmic (SR) calcium cycling, have also been identified, but the regulatory mechanisms and functional significance of these pathways remain largely unexplored. Thus, this mini-review discusses the state of the field with regard to calcium cycling as a thermogenic mediator in adipose tissue.
    Keywords:  Adipose tissue; calcium signaling
  6. iScience. 2022 May 20. 25(5): 104268
      Brown adipose tissue (BAT) is a metabolically active organ that contributes to the thermogenic response to cold exposure. In addition, other thermogenic cells termed beige adipocytes are generated in white adipose tissue (WAT) by cold exposure. Although activation of brown/beige adipose tissue is associated with mobilization of both glucose and lipids, few studies have focused on the role of glycolytic enzymes in regulating adipose tissue function. We generated mouse models with specific deletion of the glycolytic enzyme phosphoglycerate mutase 1 (PGAM1) from adipose tissue. Deletion of Pgam1 from both BAT and WAT promoted whitening of BAT with beiging of visceral WAT, whereas deletion of Pgam1 from BAT alone led to whitening of BAT without beiging of WAT. Our results demonstrate a potential role of glycolytic enzymes in beiging of visceral WAT and suggest that PGAM1 would be a novel therapeutic target in obesity and diabetes.
    Keywords:  Molecular biology; Molecular physiology; Physiology
  7. Diabetes. 2022 May 02. pii: db210972. [Epub ahead of print]
      The function of Prohibitin-1 (PHB1) in adipocyte mitochondrial respiration, adaptive thermogenesis, and long chain fatty acid (LCFA) metabolism has been reported. While intracellular PHB1 expression is ubiquitous, cell surface PHB1 localization is selective for adipocytes and endothelial cells of adipose tissue. The importance of PHB1 in adipose endothelium has not been investigated and its vascular cell surface function has remained unclear. Here, we generated and analyzed mice with PHB1 knock-out specifically in endothelial cells (PHB1 EC-KO). Despite the lack of endothelial PHB1, mice developed normally and had normal vascularizatoin in both white adipose tissue (WAT) and brown adipose tissue (BAT). Tumor and ex vivo explant angiogenesis assays also have not detected a functional defect in PHB1 KO endothelium. No metabolic phenotype was observed in PHB1 EC-KO mice raised on regular diet. We show that both male and female PHB1 EC-KO mice have normal body composition and adaptive thermogenesis. However, PHB1 EC-KO mice displayed higher insulin sensitivity and increased glucose clearance when fed high fat diet. We demonstrate that the efficacy of long chain fatty acid (LCFA) deposition by adipocytes is decreased by PHB1 EC KO, in particular in brown adipose tissue (BAT). Consistent with that, EC-KO mice have a defect in clearing triglycerides from systemic circulation. Free fatty acid release upon lipolysis induction was also found to be reduced in PHB1 EC-KO mice. Our results demonstrate that PHB1 in endothelial cells regulates bi-directional LCFA transport and thereby suppresses glucose utilization.
  8. Trends Endocrinol Metab. 2022 Apr 29. pii: S1043-2760(22)00058-3. [Epub ahead of print]
      Brown adipose tissue (BAT) is often considered as a sink for nutrients to generate heat. However, when the complex hormonal and nervous inputs and intracellular signaling networks regulating substrate utilization are considered, BAT appears much more as a tightly controlled rheostat, regulating body temperature and balancing circulating nutrient levels. Here we provide an overview of key regulatory circuits, including the diurnal rhythm, determining glucose, fatty acid, and amino acid utilization and the interdependency of these nutrients in thermogenesis. Moreover, we discuss additional factors mediating sympathetic BAT activation beyond β-adrenergic signaling and the limitations of glucose-based BAT activity measurements to foster a better understanding and interpretation of BAT activity data.
    Keywords:  brown adipocyte thermogenesis; fatty acids; glucose; neurotransmitters; substrate utilization
  9. Physiol Rep. 2022 May;10(9): e15292
      Adipose tissue (AT) has been found to exist in two predominant forms, white and brown. White adipose tissue (WAT) is the body's conventional storage organ, and brown adipose tissue (BAT) is responsible for non-shivering thermogenesis which allows mammals to produce heat and regulate body temperature. Studies examining BAT and its role in whole-body metabolism have found that active BAT utilizes glucose and circulating fatty acids and is associated with improved metabolic outcomes. While the beiging of WAT is a growing area of interest, the possibility of the BAT depot to "whiten" and store more triglycerides also has metabolic and health implications. Currently, there are limited studies that examine the effects of chronic stress and its ability to induce a white-like phenotype in the BAT depot. This research examined how chronic exposure to the murine stress hormone, corticosterone, for 4 weeks can affect the whitening process of BAT in C57BL/6 male mice. Separate treatments with mirabegron, a known β3-adrenergic receptor agonist, were used to directly compare the effects of corticosterone with a beiging phenotype. Corticosterone-treated mice had significantly higher body weight (p ≤ 0.05) and BAT mass (p ≤ 0.05), increased adipocyte area (p ≤ 0.05), were insulin resistant (p ≤ 0.05), and significantly elevated expressions of uncoupling protein 1 (UCP-1) in BAT (p ≤ 0.05) while mitochondrial content remained unchanged. This whitened phenotype has not been previously associated with increased uncoupling proteins under chronic stress and may represent a compensatory mechanism being initiated under these conditions. These findings have implications for the study of BAT in response to chronic glucocorticoid exposure potentially leading to BAT dysfunction and negative impacts on whole-body glucose metabolism.
    Keywords:  UCP-1; corticosterone; insulin resistance; metabolic syndrome; mirabegron; obesity
  10. J Clin Invest. 2022 May 02. pii: e159296. [Epub ahead of print]132(9):
      Brown adipose tissue (BAT) dissipates energy in the form of heat and functions as a metabolic sink for lipids, glucose, and branched-chain amino acids. Enhanced BAT thermogenesis is thought to tightly couple with beneficial energy metabolism. However, in this issue of the JCI, Huang et al. report a mouse model in which BAT thermogenesis was impaired, yet systemic glucose and lipid homeostasis were improved, on a high-fat diet compared with what occurred in control mice. The authors showed that BAT-specific deletion of mitochondrial thioredoxin-2 (TRX2) impaired adaptive thermogenesis through elevated mitochondrial reactive oxygen species (ROS) and cytosolic efflux of mitochondrial DNA. On the other hand, TRX2 loss enhanced lipid uptake in the BAT and protected mice from obesity, hypertriglyceridemia, and insulin resistance. This study provides a unique model in which BAT does not require thermogenesis per se to function as a lipid sink that leads to metabolic benefits in vivo.
  11. BMC Biol. 2022 May 02. 20(1): 95
      BACKGROUND: Beiging of white fat plays an important role in energy metabolism. Beige adipocytes contribute to the regulation of body weight and body temperature through expenditure of chemical energy to produce heat, and they have therefore recently attracted considerable attention as potential targets for therapeutic approaches in metabolic disorders, including obesity. All adipocytes, including beige adipocytes, differentiate from mesenchymal stem cells (MSCs), which may provide an important path for clinical intervention; however, the mechanism of beiging of human adipose cell-derived MSCs is not fully understood. Here, we provide insights on the role of IRISIN, which is known to be secreted by skeletal muscle and promote beiging of white fat.RESULTS: We established an IRISIN-induced mesenchymal stem cell beiging model and found that IRISIN protein interacts with the MSC membrane protein TRPC3. This interaction results in calcium influx and consequential activation of Erk and Akt signaling pathways, which causes phosphorylation of PPARγ. The phosphorylated PPARγ enters the nucleus and binds the UCP1 promoter region. Furthermore, the role of TRPC3 in the beiging of MSCs was largely abolished in Trpc3-/- mice. We additionally demonstrate that the calcium concentration in the brain of mice increases upon IRISIN stimulation, followed by an increase in the content of excitatory amino acids and norepinephrine, while Trpc3-/- mice exhibit the reverse effect.
    CONCLUSIONS: We found that TRPC3 is a key factor in irisin-induced beiging of MSCs, which may provide a new target pathway in addressing metabolic disorders. Our results additionally suggest that the interaction of irisin with TRPC3 may affect multiple tissues, including the brain.
    Keywords:  Beiging; Calcium influx; Energy metabolism; IRISIN; Mesenchymal stem cells; TRPC3
  12. EBioMedicine. 2022 Apr 28. pii: S2352-3964(22)00204-3. [Epub ahead of print]79 104020
      BACKGROUND: The adipocyte-hypertrophy associated remodeling of fat cell function is considered causal for the development of metabolic disorders. A better understanding of transcriptome and fatty acid (FA) related alterations with adipocyte hypertrophy combined with less-invasive strategies for the detection of the latter can help to increase the prognostic and diagnostic value of adipocyte size and FA composition as markers for metabolic disease.METHODS: To clarify adipocyte-hypertrophy associated transcriptomic alterations, fat cell size was related to RNA-Seq data from white adipose tissue and size-separated adipocytes. The relationship between adipocyte size and adipose tissue FA composition as measured by GC-MS was investigated. MR spectroscopy (MRS) methods for clinical scanning were developed to characterize adipocyte size and FA composition in a fast and non-invasive manner.
    FINDINGS: With enlarged adipocyte size, substantial transcriptomic alterations of genes involved in mitochondrial function and FA metabolism were observed. Investigations of these two mechanisms revealed a reciprocal relationship between adipocyte size and estimated thermogenic adipocyte content as well as depot-specific correlations of adipocyte size and FA composition. MRS on a clinical scanner was suitable for the in-parallel assessment of adipose morphology and FA composition.
    INTERPRETATION: The current study provides a comprehensive overview of the adipocyte-hypertrophy associated transcriptomic and FA landscape in both subcutaneous and visceral adipose tissue. MRS represents a promising technique to translate the observed mechanistic, structural and functional changes in WAT with adipocyte hypertrophy into a clinical context for an improved phenotyping of WAT in the context of metabolic diseases.
    FUNDING: Competence network for obesity (FKZ 42201GI1128), ERC (No 677661, ProFatMRI; No 875488, FatVirtualBiopsy), Else Kröner-Fresenius-Foundation.
    Keywords:  Browning; Fatty acids; Magnetic resonance; Obesity; Transcriptomics; White adipose tissue
  13. Cell Metab. 2022 May 03. pii: S1550-4131(22)00134-6. [Epub ahead of print]34(5): 656-657
      Adipose tissue has been linked to inflammation and various physiological processes. In this issue of Cell Metabolism, Caputa et al. describe that perinodal adipocytes adapt their metabolism to actively participate in an immune response against intracellular Listeria monocytogenes.
  14. Nat Commun. 2022 May 03. 13(1): 2403
      C. elegans react to metabolic distress caused by mismatches in oxygen and energy status via distinct behavioral responses. At the molecular level, these responses are coordinated by under-characterized, redox-sensitive processes, thought to initiate in mitochondria. Complex I of the electron transport chain is a major site of reactive oxygen species (ROS) production and is canonically associated with oxidative damage following hypoxic exposure. Here, we use a combination of optogenetics and CRISPR/Cas9-mediated genome editing to exert spatiotemporal control over ROS production. We demonstrate a photo-locomotory remodeling of avoidance behavior by local ROS production due to the reversible oxidation of a single thiol on the complex I subunit NDUF-2.1. Reversible thiol oxidation at this site is necessary and sufficient for the behavioral response to hypoxia, does not respond to ROS produced at more distal sites, and protects against lethal hypoxic exposure. Molecular modeling suggests that oxidation at this thiol residue alters the ability for NDUF-2.1 to coordinate electron transfer to coenzyme Q by destabilizing the Q-binding pocket, causing decreased complex I activity. Overall, site-specific ROS production regulates behavioral responses and these findings provide a mechanistic target to suppress the detrimental effects of hypoxia.
  15. Am J Physiol Regul Integr Comp Physiol. 2022 May 03.
      Pain disorders induce metabolic stress in peripheral sensory neurons by reducing mitochondrial output, shifting cellular metabolism, and altering energy use. These processes implicate neuronal metabolism as an avenue for creating novel therapeutics. Liver kinase B1 (LKB1) mediates the cellular response to metabolic stress by inducing the AMPK pathway. The LKB1-AMPK pathway increases energy producing processes, including mitochondrial output. These processes inhibit pain by directly or indirectly restoring energetic balance within a cell. Although the LKB1-AMPK pathway has been linked to pain relief, it is not yet known which cell is responsible for this property, as well any direct ties to cellular metabolism. To elucidate this, we developed a genetic mouse model where LKB1 is selectively removed from Nav1.8-pain sensory neurons and metabolically stressed them by fasting for 24 hours. We found females, but not males, had neuron-specific, LKB1-dependent restoration of metabolic stress-induced mitochondrial metabolism. This was reflected in mechanical hypersensitivity, where the absence of LKB1 led to hypersensitivity in female, but not male, animals. This discrepancy suggests a sex- and cell-specific contribution to LKB1-depdendent fasting-induced mechanical hypersensitivity. While our data represent a potential role for LKB1 in anti-pain pathways in a metabolic-specific manner, more must be done to investigate these sex differences.
    Keywords:  LKB1; cell metabolism; fasting; mechanical hypersensitivity; sensory neuron
  16. Nat Commun. 2022 May 05. 13(1): 2483
      The SLC25 carrier family consists of 53 transporters that shuttle nutrients and co-factors across mitochondrial membranes. The family is highly redundant and their transport activities coupled to metabolic state. Here, we use a pooled, dual CRISPR screening strategy that knocks out pairs of transporters in four metabolic states - glucose, galactose, OXPHOS inhibition, and absence of pyruvate - designed to unmask the inter-dependence of these genes. In total, we screen 63 genes in four metabolic states, corresponding to 2016 single and pair-wise genetic perturbations. We recover 19 gene-by-environment (GxE) interactions and 9 gene-by-gene (GxG) interactions. One GxE interaction hit illustrates that the fitness defect in the mitochondrial folate carrier (SLC25A32) KO cells is genetically buffered in galactose due to a lack of substrate in de novo purine biosynthesis. GxG analysis highlights a buffering interaction between the iron transporter SLC25A37 (A37) and the poorly characterized SLC25A39 (A39). Mitochondrial metabolite profiling, organelle transport assays, and structure-guided mutagenesis identify A39 as critical for mitochondrial glutathione (GSH) import. Functional studies reveal that A39-mediated glutathione homeostasis and A37-mediated mitochondrial iron uptake operate jointly to support mitochondrial OXPHOS. Our work underscores the value of studying family-wide genetic interactions across different metabolic environments.