Methods for making and observing model lipid droplets

We present methods for making and testing the membrane biophysics of model lipid droplets (LDs). Methods are described for imaging LDs ranging in size from 0.1 to 40 μm in diameter with high-resolution microscopy and spectroscopy. With known LD compositions, membrane binding, sorting, diffusion, and tension were measured via fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging microscopy (FLIM), atomic force microscopy (AFM), and imaging flow cytometry. Additionally, a custom, small-volume pendant droplet tensiometer is described and used to measure the association of phospholipids to the LD surface. These complementary, cross-validating methods of measuring LD membrane behavior reveal the interplay of biophysical processes on lipid droplet monolayers.

AI is a viable alternative to high throughput screening: a 318-target study

High throughput screening (HTS) is routinely used to identify bioactive small molecules. This requires physical compounds, which limits coverage of accessible chemical space. Computational approaches combined with vast on-demand chemical libraries can access far greater chemical space, provided that the predictive accuracy is sufficient to identify useful molecules. Through the largest and most diverse virtual HTS campaign reported to date, comprising 318 individual projects, we demonstrate that our AtomNet® convolutional neural network successfully finds novel hits across every major therapeutic area and protein class. We address historical limitations of computational screening by demonstrating success for target proteins without known binders, high-quality X-ray crystal structures, or manual cherry-picking of compounds. We show that the molecules selected by the AtomNet® model are novel drug-like scaffolds rather than minor modifications to known bioactive compounds. Our empirical results suggest that computational methods can substantially replace HTS as the first step of small-molecule drug discovery.

TM4SF19-mediated control of lysosomal activity in macrophages contributes to obesity-induced inflammation and metabolic dysfunction

Adipose tissue (AT) adapts to overnutrition in a complex process, wherein specialized immune cells remove and replace dysfunctional and stressed adipocytes with new fat cells. Among immune cells recruited to AT, lipid-associated macrophages (LAMs) have emerged as key players in obesity and in diseases involving lipid stress and inflammation. Here, we show that LAMs selectively express transmembrane 4 L six family member 19 (TM4SF19), a lysosomal protein that represses acidification through its interaction with Vacuolar-ATPase. Inactivation of TM4SF19 elevates lysosomal acidification and accelerates the clearance of dying/dead adipocytes in vitro and in vivo. TM4SF19 deletion reduces the LAM accumulation and increases the proportion of restorative macrophages in AT of male mice fed a high-fat diet. Importantly, male mice lacking TM4SF19 adapt to high-fat feeding through adipocyte hyperplasia, rather than hypertrophy. This adaptation significantly improves local and systemic insulin sensitivity, and energy expenditure, offering a potential avenue to combat obesity-related metabolic dysfunction.

Direct effects of adipocyte lipolysis on AMPK through intracellular long-chain acyl-CoA signaling

Long-chain acyl-CoAs (LC-acyl-CoAs) are important intermediary metabolites and are also thought to function as intracellular signaling molecules; however, the direct effects of LC-acyl-CoAs have been difficult to determine in real-time and dissociate from Protein Kinase A (PKA) signaling. Here, we examined the direct role of lipolysis in generating intracellular LC-acyl-CoAs and activating AMPK in white adipocytes by pharmacological activation of ABHD5 (also known as CGI-58), a lipase co-activator. Activation of lipolysis in 3T3-L1 adipocytes independent of PKA with synthetic ABHD5 ligands, resulted in greater activation of AMPK compared to receptor-mediated activation with isoproterenol, a β-adrenergic receptor agonist. Importantly, the effect of pharmacological activation of ABHD5 on AMPK activation was blocked by inhibiting ATGL, the rate-limiting enzyme for triacylglycerol hydrolysis. Utilizing a novel FRET sensor to detect intracellular LC-acyl-CoAs, we demonstrate that stimulation of lipolysis in 3T3-L1 adipocytes increased the production of LC-acyl-CoAs, an effect which was blocked by inhibition of ATGL. Moreover, ATGL inhibition blocked AMPKβ1 S108 phosphorylation, a site required for allosteric regulation. Increasing intracellular LC-acyl-CoAs by removal of BSA in the media and pharmacological inhibition of DGAT1 and 2 resulted in greater activation of AMPK. Finally, inhibiting LC-acyl-CoA generation reduced activation of AMPK; however, did not lower energy charge. Overall, results demonstrate that lipolysis in white adipocytes directly results in allosteric activation of AMPK through the generation of LC-acyl-CoAs.

Methods for making and observing model lipid droplets

The mechanisms by which the lipid droplet (LD) membrane is remodeled in concert with the activation of lipolysis incorporate a complex interplay of proteins, phospholipids, and neutral lipids. Model LDs (mLDs) provide an isolated, purified system for testing the mechanisms by which the droplet composition, size, shape, and tension affects triglyceride metabolism. Described here are methods of making and testing mLDs ranging from 0.1 to 40 μm diameter with known composition. Methods are described for imaging mLDs with high-resolution microscopy during buffer exchanges for the measurement of membrane binding, diffusion, and tension via fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging microscopy (FLIM), atomic force microscopy (AFM), pendant droplet tensiometry, and imaging flow cytometry. These complementary, cross-validating methods of measuring LD membrane behavior reveal the interplay of biophysical processes in triglyceride metabolism.

Four-color fluorescence cross-correlation spectroscopy with one laser and one camera

The diffusion and reorganization of phospholipids and membrane-associated proteins are fundamental for cellular function. Fluorescence cross-correlation spectroscopy (FCCS) measures diffusion and molecular interactions at nanomolar concentration in biological systems. We have developed an economical method to simultaneously monitor diffusion and complexation with the use of super-continuum laser and spectral deconvolution from a single detector. Customizable excitation wavelengths were chosen from the wide-band source and spectral fitting of the emitted light revealed the interactions for up to four chromatically overlapping fluorophores simultaneously. This method was applied to perform four-color FCCS that we demonstrated with polystyrene nanoparticles, lipid vesicles, and membrane-bound molecules. Up to four individually customizable excitation channels were selected from the broad-spectrum fiber laser to excite the diffusers within a diffraction-limited spot. The fluorescence emission passed through a cleanup filter and a dispersive prism prior to being collected by a sCMOS or EMCCD camera with up to 1.8 kHz frame rates. The emission intensity versus time of each fluorophore was extracted through a linear least-square fitting of each camera frame and temporally correlated via custom software. Auto- and cross-correlation functions enabled the measurement of the diffusion rates and binding partners. We have measured the induced aggregation of nanobeads and lipid vesicles in solution upon increasing the buffer salinity. Because of the adaptability of investigating four fluorophores simultaneously with a cost-effective method, this technique will have wide application for examining macromolecular complex formation in model and living systems.

DNALI1 interacts with the MEIG1/PACRG complex within the manchette and is required for proper sperm flagellum assembly in mice

The manchette is a transient and unique structure present in elongating spermatids and required for proper differentiation of the germ cells during spermatogenesis. Previous work indicated that the MEIG1/PACRG complex locates in the manchette and is involved in the transport of cargos, such as SPAG16L, to build the sperm flagellum. Here, using co-immunoprecipitation and pull-down approaches in various cell systems, we established that DNALI1, an axonemal component originally cloned from Chlamydomonas reinhardtii, recruits and stabilizes PACRG and we confirm in vivo, the co-localization of DNALI1 and PACRG in the manchette by immunofluorescence of elongating murine spermatids. We next generated mice with a specific deficiency of DNALI1 in male germ cells, and observed a dramatic reduction of the sperm cells, which results in male infertility. In addition, we observed that the majority of the sperm cells exhibited abnormal morphology including misshapen heads, bent tails, enlarged midpiece, discontinuous accessory structure, emphasizing the importance of DNALI1 in sperm differentiation. Examination of testis histology confirmed impaired spermiogenesis in the mutant mice. Importantly, while testicular levels of MEIG1, PACRG, and SPAG16L proteins were unchanged in the Dnali1 mutant mice, their localization within the manchette was greatly affected, indicating that DNALI1 is required for the formation of the MEIG1/PACRG complex within the manchette. Interestingly, in contrast to MEIG1 and PACRG-deficient mice, the DNALI1-deficient mice also showed impaired sperm spermiation/individualization, suggesting additional functions beyond its involvement in the manchette structure. Overall, our work identifies DNALI1 as a protein required for sperm development.

A FRET sensor for the real-time detection of long chain acyl-CoAs and synthetic ABHD5 ligands

Intracellular long-chain acyl-coenzyme As (LC-acyl-CoAs) are thought to be under tight spatial and temporal controls, yet the ability to image LC-acyl-CoAs in live cells is lacking. Here, we developed a fluorescence resonance energy transfer (FRET) sensor for LC-acyl-CoAs based on the allosterically regulated interaction between α/β hydrolase domain-containing 5 (ABHD5) and Perilipin 5. The genetically encoded sensor rapidly detects intracellular LC-acyl-CoAs generated from exogenous and endogenous fatty acids (FAs), as well as synthetic ABHD5 ligands. Stimulation of lipolysis in brown adipocytes elevated intracellular LC-acyl-CoAs in a cyclic fashion, which was eliminated by inhibiting PNPLA2 (ATGL), the major triglyceride lipase. Interestingly, inhibition of LC-acyl-CoA transport into mitochondria elevated intracellular LC-acyl-CoAs and dampened their cycling. Together, these observations reveal an intimate feedback control between LC-acyl-CoA generation from lipolysis and utilization in mitochondria. We anticipate that this sensor will be an important tool to dissect intracellular LC-acyl-CoA dynamics as well to discover novel synthetic ABHD5 ligands.

Four-color fluorescence cross-correlation spectroscopy with one laser and one camera

The diffusion and reorganization of phospholipids and membrane-associated proteins are fundamental for cellular function. Fluorescence cross-correlation spectroscopy (FCCS) measures the diffusion and molecular interactions at nanomolar concentration in biological systems. We have developed a novel, economical method to simultaneously monitor diffusion and oligomerization with the use of super-continuum laser and spectral deconvolution from a single detector. Customizable excitation wavelengths were chosen from the wide-band source and spectral fitting of the emitted light revealed the interactions for up to four spectrally overlapping fluorophores simultaneously. This method was applied to perform four-color FCCS, as demonstrated with polystyrene nanoparticles, lipid vesicles, and membrane-bound molecules. Up to four individually customizable excitation channels were selected from the broad-spectrum fiber laser to excite the diffusers within a diffraction-limited spot. The fluorescence emission passed through a cleanup filter and a dispersive prism prior to being collected by a sCMOS or EMCCD camera with up to 10 kHz frame rates. The emission intensity versus time of each fluorophore was extracted through a linear least-square fitting of each camera frame and temporally correlated via custom software. Auto- and cross-correlation functions enabled the measurement of the diffusion rates and binding partners. We have measured the induced aggregation of nanobeads and lipid vesicles in solution upon increasing the buffer salinity. Because of the adaptability of investigating four fluorophores simultaneously with a cost-effective method, this technique will have wide application for examining complex homo- and heterooligomerization in model and living systems.

Deconstructing cold-induced brown adipocyte neogenesis in mice

Cold exposure triggers neogenesis in classic interscapular brown adipose tissue (iBAT) that involves activation of β1-adrenergic receptors, proliferation of PDGFRA+ adipose tissue stromal cells (ASCs), and recruitment of immune cells whose phenotypes are presently unknown. Single-cell RNA-sequencing (scRNA-seq) in mice identified three ASC subpopulations that occupied distinct tissue locations. Of these, interstitial ASC1 were found to be direct precursors of new brown adipocytes (BAs). Surprisingly, knockout of β1-adrenergic receptors in ASCs did not prevent cold-induced neogenesis, whereas pharmacological activation of the β3-adrenergic receptor on BAs was sufficient, suggesting that signals derived from mature BAs indirectly trigger ASC proliferation and differentiation. In this regard, cold exposure induced the delayed appearance of multiple macrophage and dendritic cell populations whose recruitment strongly correlated with the onset and magnitude of neogenesis across diverse experimental conditions. High-resolution immunofluorescence and single-molecule fluorescence in situ hybridization demonstrated that cold-induced neogenesis involves dynamic interactions between ASC1 and recruited immune cells that occur on the micrometer scale in distinct tissue regions. Our results indicate that neogenesis is not a reflexive response of progenitors to β-adrenergic signaling, but rather is a complex adaptive response to elevated metabolic demand within brown adipocytes.

Lipolysis regulates major transcriptional programs in brown adipocytes

β-Adrenergic signaling is a core regulator of brown adipocyte function stimulating both lipolysis and transcription of thermogenic genes, thereby expanding the capacity for oxidative metabolism. We have used pharmacological inhibitors and a direct activator of lipolysis to acutely modulate the activity of lipases, thereby enabling us to uncover lipolysis-dependent signaling pathways downstream of β-adrenergic signaling in cultured brown adipocytes. Here we show that induction of lipolysis leads to acute induction of several gene programs and is required for transcriptional regulation by β-adrenergic signals. Using machine-learning algorithms to infer causal transcription factors, we show that PPARs are key mediators of lipolysis-induced activation of genes involved in lipid metabolism and thermogenesis. Importantly, however, lipolysis also activates the unfolded protein response and regulates the core circadian transcriptional machinery independently of PPARs. Our results demonstrate that lipolysis generates important metabolic signals that exert profound pleiotropic effects on transcription and function of cultured brown adipocytes.

Molecular Modeling of ABHD5 Structure and Ligand Recognition

Alpha/beta hydrolase domain-containing 5 (ABHD5), also termed CGI-58, is the key upstream activator of adipose triglyceride lipase (ATGL), which plays an essential role in lipid metabolism and energy storage. Mutations in ABHD5 disrupt lipolysis and are known to cause the Chanarin-Dorfman syndrome. Despite its importance, the structure of ABHD5 remains unknown. In this work, we combine computational and experimental methods to build a 3D structure of ABHD5. Multiple comparative and machine learning-based homology modeling methods are used to obtain possible models of ABHD5. The results from Gaussian accelerated molecular dynamics and experimental data of the apo models and their mutants are used to select the most likely model. Moreover, ensemble docking is performed on representative conformations of ABHD5 to reveal the binding mechanism of ABHD5 and a series of synthetic ligands. Our study suggests that the ABHD5 models created by deep learning-based methods are the best candidate structures for the ABHD5 protein. The mutations of E41, R116, and G328 disturb the hydrogen bonding network with nearby residues and suppress membrane targeting or ATGL activation. The simulations also reveal that the hydrophobic interactions are responsible for binding sulfonyl piperazine ligands to ABHD5. Our work provides fundamental insight into the structure of ABHD5 and its ligand-binding mode, which can be further applied to develop ABHD5 as a therapeutic target for metabolic disease and cancer.

REEP6 knockout leads to defective β-adrenergic signaling in adipocytes and promotes obesity-related metabolic dysfunction

INTRODUCTION

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.

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Structural and functional insights into ABHD5, a ligand-regulated lipase co-activator

Alpha/beta hydrolase domain-containing protein 5 (ABHD5) is a highly conserved protein that regulates various lipid metabolic pathways via interactions with members of the perilipin (PLIN) and Patatin-like phospholipase domain-containing protein (PNPLA) protein families. Loss of function mutations in ABHD5 result in Chanarin-Dorfman Syndrome (CDS), characterized by ectopic lipid accumulation in numerous cell types and severe ichthyosis. Recent data demonstrates that ABHD5 is the target of synthetic and endogenous ligands that might be therapeutic beneficial for treating metabolic diseases and cancers. However, the structural basis of ABHD5 functional activities, such as protein-protein interactions and ligand binding is presently unknown. To address this gap, we constructed theoretical structural models of ABHD5 by comparative modeling and topological shape analysis to assess the spatial patterns of ABHD5 conformations computed in protein dynamics. We identified functionally important residues on ABHD5 surface for lipolysis activation by PNPLA2, lipid droplet targeting and PLIN-binding. We validated the computational model by examining the effects of mutating key residues in ABHD5 on an array of functional assays. Our integrated computational and experimental findings provide new insights into the structural basis of the diverse functions of ABHD5 as well as pathological mutations that result in CDS.

VPS13A and VPS13C Influence Lipid Droplet Abundance

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.

Fluorescent and Luminescent Methods to Detect Lipolysis

Intracellular lipolysis, the hydrolysis of stored triacylglycerol to fatty acids and glycerol, is a core metabolic function of brown and white adipocytes. In brown adipocytes, mobilized fatty acids directly activate uncoupling protein 1, provide fuel for heat generation, and ligands of nuclear receptors that expand the thermogenic gene expression program. Lipolysis in white adipocytes mobilizes lipid energy for systemic use, including both shivering and non-shivering thermogenesis. In addition, most metabolic tissues, including muscle and liver, have the ability to store triacylglycerol and release fatty acids; thus, there is a general interest in measuring lipolysis in a wide array of cell types. Here we describe detailed protocols for the enzymatic detection of cellular fatty acid and glycerol efflux via fluorescent and colorimetric means, respectively. In addition, we also describe a genetically encoded luminescent detection system for intracellular fatty acids that is amenable to high-throughput analysis.

Single cell functional genomics reveals plasticity of subcutaneous white adipose tissue (WAT) during early postnatal development

OBJECTIVE

The current study addresses the cellular complexity and plasticity of subcutaneous (inguinal) white adipose tissue (iWAT) in mice during the critical periods of perinatal growth and establishment.

METHODS

We performed a large-scale single cell transcriptomic (scRNA-seq) and epigenomic (snATAC-seq) characterization of cellular subtypes (adipose stromal cells (ASC) and adipocyte nuclei) during inguinal WAT (subcutaneous; iWAT) development in mice, capturing the early postnatal period (postnatal days (PND) 06 and 18) through adulthood (PND56).

RESULTS

Perinatal and adult iWAT contain 3 major ASC subtypes that can be independently identified by RNA expression profiles and DNA transposase accessibility. Furthermore, the transcriptomes and enhancer landscapes of both ASC and adipocytes dynamically change during postnatal development. Perinatal ASC (PND06) are highly enriched for several imprinted genes (IGs; e.g., Mest, H19, Igf2) and extracellular matrix proteins whose expression then declines prior to weaning (PND18). By comparison, adult ASC (PND56) are more enriched for transcripts associated with immunoregulation, oxidative stress, and integrin signaling. Two clusters of mature adipocytes, identified through single nuclei RNA sequencing (snRNA-seq), were distinctive for proinflammatory/immune or metabolic gene expression patterns that became more transcriptionally diverse in adult animals. Single nuclei assay for transposase-accessible chromatin (snATAC-seq) revealed that differences in gene expression were associated with developmental changes in chromatin accessibility and predicted transcription factor motifs (e.g., Plagl1, Ar) in both stromal cells and adipocytes.

CONCLUSIONS

Our data provide new insights into transcriptional and epigenomic signaling networks important during iWAT establishment at a single cell resolution, with important implications for the field of metabolic programming.

Leucine zipper transcription factor-like 1 (LZTFL1), an intraflagellar transporter protein 27 (IFT27) associated protein, is required for normal sperm function and male fertility

Intraflagellar transport (IFT) is an evolutionarily conserved mechanism essential for the assembly and maintenance of most eukaryotic cilia and flagella, including mammalian sperm tails. Depletion of IFT27, a component of the IFT complex, in male germ cells results in infertility associated with disrupted sperm flagella structure and motility. Leucine zipper transcription factor-like 1 (LZTFL1) is an IFT27 associated protein. LZTFL1, also known as BBS17, is a Bardet-Biedl syndrome (BBS) associated protein. Patients carrying biallelic variants of LZTFL1 gene exhibit the common BBS phenotypes. The global Lztfl1 knockout mice showed abnormal growth rate and retinal degeneration, typical of BBS phenotype. However, it is not clear if Lztfl1 has a role in male fertility. The LZTFL1 protein is highly and predominantly expressed in mouse testis. During the first wave of spermatogenesis, the protein is only expressed during spermiogenesis phase from the round spermatid stage and displays a cytoplasmic localization with a vesicular distribution pattern. At the elongated spermatid stage, LZTFL1 is present in the developing flagella and appears also close to the manchette. Fertility of Lztfl1 knockout mice was significantly reduced and associated with low sperm motility and a high level of abnormal sperm (astheno-teratozoospermia). In vitro assessment of fertility revealed reduced fertilization and embryonic development when using sperm from homozygous mutant mice. In addition, we observed a significant decrease of the testicular IFT27 protein level in Lztfl1 mutant mice contrasting with a stable expression levels of other IFT proteins, including IFT20, IFT81, IFT88 and IFT140. Overall, our results support strongly the important role of LZTFL1 in mouse spermatogenesis and male fertility.

Lipolysis drives expression of the constitutively active receptor GPR3 to induce adipose thermogenesis

Thermogenic adipocytes possess a therapeutically appealing, energy-expending capacity, which is canonically cold-induced by ligand-dependent activation of β-adrenergic G protein-coupled receptors (GPCRs). Here, we uncover an alternate paradigm of GPCR-mediated adipose thermogenesis through the constitutively active receptor, GPR3. We show that the N terminus of GPR3 confers intrinsic signaling activity, resulting in continuous Gs-coupling and cAMP production without an exogenous ligand. Thus, transcriptional induction of Gpr3 represents the regulatory parallel to ligand-binding of conventional GPCRs. Consequently, increasing Gpr3 expression in thermogenic adipocytes is alone sufficient to drive energy expenditure and counteract metabolic disease in mice. Gpr3 transcription is cold-stimulated by a lipolytic signal, and dietary fat potentiates GPR3-dependent thermogenesis to amplify the response to caloric excess. Moreover, we find GPR3 to be an essential, adrenergic-independent regulator of human brown adipocytes. Taken together, our findings reveal a noncanonical mechanism of GPCR control and thermogenic activation through the lipolysis-induced expression of constitutively active GPR3.

STK3/STK4 signalling in adipocytes regulates mitophagy and energy expenditure

Obesity reduces adipocyte mitochondrial function, and expanding adipocyte oxidative capacity is an emerging strategy to improve systemic metabolism. Here, we report that serine/threonine-protein kinase 3 (STK3) and STK4 are key physiological suppressors of mitochondrial capacity in brown, beige and white adipose tissues. Levels of STK3 and STK4, kinases in the Hippo signalling pathway, are greater in white than brown adipose tissues, and levels in brown adipose tissue are suppressed by cold exposure and greatly elevated by surgical denervation. Genetic inactivation of Stk3 and Stk4 increases mitochondrial mass and function, stabilizes uncoupling protein 1 in beige adipose tissue and confers resistance to metabolic dysfunction induced by high-fat diet feeding. Mechanistically, STK3 and STK4 increase adipocyte mitophagy in part by regulating the phosphorylation and dimerization status of the mitophagy receptor BNIP3. STK3 and STK4 expression levels are elevated in human obesity, and pharmacological inhibition improves metabolic profiles in a mouse model of obesity, suggesting STK3 and STK4 as potential targets for treating obesity-related diseases.

Perilipin 5 S155 phosphorylation by PKA is required for the control of hepatic lipid metabolism and glycemic control

Perilipin 5 (PLIN5) is a lipid-droplet-associated protein that coordinates intracellular lipolysis in highly oxidative tissues and is thought to regulate lipid metabolism in response to phosphorylation by protein kinase A (PKA). We sought to identify PKA phosphorylation sites in PLIN5 and assess their functional relevance in cultured cells and the livers of mice. We detected phosphorylation on S155 and identified S155 as a functionally important site for lipid metabolism. Expression of phosphorylation-defective PLIN5 S155A in Plin5 null cells resulted in decreased rates of lipolysis and triglyceride-derived fatty acid oxidation. FLIM-FRET analysis of protein-protein interactions showed that PLIN5 S155 phosphorylation regulates PLIN5 interaction with adipose triglyceride lipase at the lipid droplet, but not with α-β hydrolase domain-containing 5. Re-expression of PLIN5 S155A in the liver of Plin5 liver-specific null mice reduced lipolysis compared with wild-type PLIN5 re-expression, but was not associated with other changes in hepatic lipid metabolism. Furthermore, glycemic control was impaired in mice with expression of PLIN5 S155A compared with mice expressing PLIN5. Together, these studies demonstrate that PLIN5 S155 is required for PKA-mediated lipolysis and builds on the body of evidence demonstrating a critical role for PLIN5 in coordinating lipid and glucose metabolism.

ABHD5 suppresses cancer cell anabolism through lipolysis-dependent activation of the AMPK/mTORC1 pathway

ABHD5 is an essential coactivator of ATGL, the rate-limiting triglyceride (TG) lipase in many cell types. Importantly, ABHD5 also functions as a tumor suppressor, and ABHD5 mRNA expression levels correlate with patient survival for several cancers. Nevertheless, the mechanisms involved in ABHD5-dependent tumor suppression are not known. We found that overexpression of ABHD5 induces cell cycle arrest at the G1 phase and causes growth retardation in a panel of prostate cancer cells. Transcriptomic profiling and biochemical analysis revealed that genetic or pharmacological activation of lipolysis by ABHD5 potently inhibits mTORC1 signaling, leading to a significant downregulation of protein synthesis. Mechanistically, we found that ABHD5 elevates intracellular AMP content, which activates AMPK, leading to inhibition of mTORC1. Interestingly, ABHD5-dependent suppression of mTORC1 was abrogated by pharmacological inhibition of DGAT1 or DGAT2, isoenzymes that re-esterify fatty acids in a process that consumes ATP. Collectively, this study maps out a novel molecular pathway crucial for limiting cancer cell proliferation, in which ABHD5-mediated lipolysis creates an energy-consuming futile cycle between TG hydrolysis and resynthesis, leading to inhibition of mTORC1 and cancer cell growth arrest.

Deconstructing tumor heterogeneity: the stromal perspective

Significant advances have been made towards understanding the role of immune cell-tumor interplay in either suppressing or promoting tumor growth, progression, and recurrence, however, the roles of additional stromal elements, cell types and/or cell states remain ill-defined. The overarching goal of this NCI-sponsored workshop was to highlight and integrate the critical functions of non-immune stromal components in regulating tumor heterogeneity and its impact on tumor initiation, progression, and resistance to therapy. The workshop explored the opposing roles of tumor supportive versus suppressive stroma and how cellular composition and function may be altered during disease progression. It also highlighted microenvironment-centered mechanisms dictating indolence or aggressiveness of early lesions and how spatial geography impacts stromal attributes and function. The prognostic and therapeutic implications as well as potential vulnerabilities within the heterogeneous tumor microenvironment were also discussed. These broad topics were included in this workshop as an effort to identify current challenges and knowledge gaps in the field.

Genetically-encoded sensors to detect fatty acid production and trafficking

OBJECTIVE

Fatty acids are important for biological function; however, in excess, they can cause metabolic dysregulation. Methods to image and detect fatty acids in real time are lacking. Therefore, the current study examined the dynamics of fatty acid trafficking and signaling utilizing novel fluorescent and luminescent approaches.

METHODS

We generated fluorescent and luminescent-based genetically-encoded sensors based upon the ligand-dependent interaction between PPARα and SRC-1 to image and detect cellular dynamics of fatty acid trafficking.

RESULTS

The use of a fluorescent sensor demonstrates that fatty acids traffic rapidly from lipid droplets to the nucleus. Both major lipases ATGL and HSL contribute to fatty acid signaling from lipid droplet to nucleus, however, their dynamics differ. Furthermore, direct activation of lipolysis, independent of receptor-mediated signaling is sufficient to promote lipid droplet to nuclear trafficking of fatty acids. A luminescent-based sensor that reports intracellular fatty acid levels is amenable to high-throughput analysis.

CONCLUSIONS

Fatty acids traffic from lipid droplets to the nucleus within minutes of stimulated lipolysis. Genetically-encoded fluorescent and luminescent based sensors can be used to probe the dynamics of fatty acid trafficking and signaling.