Long-chain acyl-CoA synthetase 1 interacts with key proteins that activate and direct fatty acids into niche hepatic pathways

The diverse roles of sphingolipids in inflammatory bowel disease

The incidence of inflammatory bowel disease (IBD) has increased over the last 20 years. A variety of causes, both physiological and environmental, contribute to the initiation and progression of IBD, making disease management challenging. Current treatment options target various aspects of the immune response to dampen intestinal inflammation; however, their effectiveness at retaining remission, their side effects, and loss of response from patients over time warrant further investigation. Finding a common thread within the multitude causes of IBD is critical in developing robust treatment options. Sphingolipids are evolutionary conserved bioactive lipids universally generated in all cell types. This diverse lipid family is involved in a variety of fundamental, yet sometimes opposing, processes such as proliferation and apoptosis. Implicated as regulators in intestinal diseases, sphingolipids are a potential cornerstone in understanding IBD. Herein we will describe the role of host- and microbial-derived sphingolipids as they relate to the many factors of intestinal health and IBD.

Functional interplay of lipid droplets and mitochondria

Our body stores energy mostly in form of fatty acids (FAs) in lipid droplets (LDs). From there the FAs can be mobilized and transferred to peroxisomes and mitochondria. This transfer is dependent on close opposition of LDs and mitochondria and peroxisomes and happens at membrane contact sites. However, the composition and the dynamics of these contact sites is not well understood, which is in part due to the dependence on the metabolic state of the cell and on the cell- and tissue-type. Here, we summarize the current knowledge on the contacts between lipid droplets and mitochondria both in mammals and in the yeast Saccharomyces cerevisiae, in which various contact sites are well studied. We discuss possible functions of the contact site and their implication in disease.

A patient-based iPSC-derived hepatocyte model of alcohol-associated cirrhosis reveals bioenergetic insights into disease pathogenesis

Only ~20% of heavy drinkers develop alcohol cirrhosis (AC). While differences in metabolism, inflammation, signaling, microbiome signatures and genetic variations have been tied to the pathogenesis of AC, the key underlying mechanisms for this interindividual variability, remain to be fully elucidated. Induced pluripotent stem cell-derived hepatocytes (iHLCs) from patients with AC and healthy controls differ transcriptomically, bioenergetically and histologically. They include a greater number of lipid droplets (LDs) and LD-associated mitochondria compared to control cells. These pre-pathologic indicators are effectively reversed by Aramchol, an inhibitor of stearoyl-CoA desaturase. Bioenergetically, AC iHLCs have lower spare capacity, slower ATP production and their mitochondrial fuel flexibility towards fatty acids and glutamate is weakened. MARC1 and PNPLA3, genes implicated by GWAS in alcohol cirrhosis, show to correlate with lipid droplet-associated and mitochondria-mediated oxidative damage in AC iHLCs. Knockdown of PNPLA3 expression exacerbates mitochondrial deficits and leads to lipid droplets alterations. These findings suggest that differences in mitochondrial bioenergetics and lipid droplet formation are intrinsic to AC hepatocytes and can play a role in its pathogenesis.

Studying the topology of peroxisomal acyl-CoA synthetases using self-assembling split sfGFP

Peroxisomes are membrane-bounded organelles that contain enzymes involved in multiple lipid metabolic pathways. Several of these pathways require (re-)activation of fatty acids to coenzyme A (CoA) esters by acyl-CoA synthetases, which may take place inside the peroxisomal lumen or extraperoxisomal. The acyl-CoA synthetases SLC27A2, SLC27A4, ACSL1, and ACSL4 have different but overlapping substrate specificities and were previously reported to be localized in the peroxisomal membrane in addition to other subcellular locations. However, it has remained unclear if the catalytic acyl-CoA synthetase sites of these enzymes are facing the peroxisomal lumen or the cytosolic side of the peroxisomal membrane. To study this topology in cellulo we have developed a microscopy-based method that uses the previously developed self-assembling split superfolder (sf) green fluorescent protein (GFP) assay. We show that this self-assembling split sfGFP method can be used to study the localization as well as the topology of membrane proteins in the peroxisomal membrane, but that it is less suited to study the location of soluble peroxisomal proteins. With the method we could demonstrate that the acyl-CoA synthetase domains of the peroxisome-bound acyl-CoA synthetases SLC27A2 and SLC27A4 are oriented toward the peroxisomal lumen and the domain of ACSL1 toward the cytosol. In contrast to previous reports, ACSL4 was not found in peroxisomes.

Analysis of the Differentially Expressed Proteins in Donkey Milk in Different Lactation Stages

Proteins in donkey milk (DM) have special biological activities. However, the bioactive proteins and their expression regulation in donkey milk are still unclear. Thus, the differentially expressed proteins (DEPs) in DM in different lactation stages were first investigated by data-independent acquisition (DIA) proteomics. A total of 805 proteins were characterized in DM. The composition and content of milk proteins varied with the lactation stage. A total of 445 candidate DEPs related to biological processes and molecular functions were identified between mature milk and colostrum. The 219 down-regulated DEPs were mainly related to complement and coagulation cascades, staphylococcus aureus infection, systemic lupus erythematosus, prion diseases, AGE-RAGE signaling pathways in diabetic complications, and pertussis. The 226 up-regulated DEPs were mainly involved in metabolic pathways related to nutrient (fat, carbohydrate, nucleic acid, and vitamin) metabolism. Some other DEPs in milk from the lactation period of 30 to 180 days also had activities such as promoting cell proliferation, promoting antioxidant, immunoregulation, anti-inflammatory, and antibacterial effects, and enhancing skin moisture. DM can be used as a nutritional substitute for infants, as well as for cosmetic and medical purposes. Our results provide important insights for understanding the bioactive protein differences in DM in different lactation stages.

Chronic metabolic stress drives developmental programs and loss of tissue functions in non-transformed liver that mirror tumor states and stratify survival

Under chronic stress, cells must balance competing demands between cellular survival and tissue function. In metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD/NASH), hepatocytes cooperate with structural and immune cells to perform crucial metabolic, synthetic, and detoxification functions despite nutrient imbalances. While prior work has emphasized stress-induced drivers of cell death, the dynamic adaptations of surviving cells and their functional repercussions remain unclear. Namely, we do not know which pathways and programs define cellular responses, what regulatory factors mediate (mal)adaptations, and how this aberrant activity connects to tissue-scale dysfunction and long-term disease outcomes. Here, by applying longitudinal single-cell multi -omics to a mouse model of chronic metabolic stress and extending to human cohorts, we show that stress drives survival-linked tradeoffs and metabolic rewiring, manifesting as shifts towards development-associated states in non-transformed hepatocytes with accompanying decreases in their professional functionality. Diet-induced adaptations occur significantly prior to tumorigenesis but parallel tumorigenesis-induced phenotypes and predict worsened human cancer survival. Through the development of a multi -omic computational gene regulatory inference framework and human in vitro and mouse in vivo genetic perturbations, we validate transcriptional (RELB, SOX4) and metabolic (HMGCS2) mediators that co-regulate and couple the balance between developmental state and hepatocyte functional identity programming. Our work defines cellular features of liver adaptation to chronic stress as well as their links to long-term disease outcomes and cancer hallmarks, unifying diverse axes of cellular dysfunction around core causal mechanisms.

The Molecular Mechanisms of Fuel Utilization during Exercise

Exercise is widely recognized for its positive impact on human health and well-being. The process of utilizing substrates in skeletal muscle during exercise is intricate and governed by complex mechanisms. Carbohydrates and lipids serve as the primary fuel sources for skeletal muscle during exercise. It is now understood that fuel selection during exercise is not solely determined by physical activity itself but is also influenced by the overall metabolic state of the body. The balance between lipid and carbohydrate utilization significantly affects exercise capacity, including endurance, fatigue, and overall performance. Therefore, comprehensively understanding the regulation of substrate utilization during exercise is of utmost importance. The aim of this review is to provide an extensive overview of the current knowledge regarding the pathways involved in the regulation of substrate utilization during exercise. By synthesizing existing research, we can gain a holistic perspective on the intricate relationship between exercise, metabolism, and fuel selection. This advanced understanding has the potential to drive advancements in the field of exercise science and contribute to the development of personalized exercise strategies for individuals looking to optimize their performance and overall health.

Differential roles for ACBD4 and ACBD5 in peroxisome-ER interactions and lipid metabolism

Peroxisomes and the endoplasmic reticulum (ER) are intimately linked subcellular organelles, physically connected at membrane contact sites. While collaborating in lipid metabolism, for example, of very long-chain fatty acids (VLCFAs) and plasmalogens, the ER also plays a role in peroxisome biogenesis. Recent work identified tethering complexes on the ER and peroxisome membranes that connect the organelles. These include membrane contacts formed via interactions between the ER protein VAPB (vesicle-associated membrane protein-associated protein B) and the peroxisomal proteins ACBD4 and ACBD5 (acyl-coenzyme A-binding domain protein). Loss of ACBD5 has been shown to cause a significant reduction in peroxisome-ER contacts and accumulation of VLCFAs. However, the role of ACBD4 and the relative contribution these two proteins make to contact site formation and recruitment of VLCFAs to peroxisomes remain unclear. Here, we address these questions using a combination of molecular cell biology, biochemical, and lipidomics analyses following loss of ACBD4 or ACBD5 in HEK293 cells. We show that the tethering function of ACBD5 is not absolutely required for efficient peroxisomal β-oxidation of VLCFAs. We demonstrate that loss of ACBD4 does not reduce peroxisome-ER connections or result in the accumulation of VLCFAs. Instead, the loss of ACBD4 resulted in an increase in the rate of β-oxidation of VLCFAs. Finally, we observe an interaction between ACBD5 and ACBD4, independent of VAPB binding. Overall, our findings suggest that ACBD5 may act as a primary tether and VLCFA recruitment factor, whereas ACBD4 may have regulatory functions in peroxisomal lipid metabolism at the peroxisome-ER interface.

PLIN5 interacts with FATP4 at membrane contact sites to promote lipid droplet-to-mitochondria fatty acid transport

Cells adjust their metabolism by remodeling membrane contact sites that channel metabolites to different fates. Lipid droplet (LD)-mitochondria contacts change in response to fasting, cold exposure, and exercise. However, their function and mechanism of formation have remained controversial. We focused on perilipin 5 (PLIN5), an LD protein that tethers mitochondria, to probe the function and regulation of LD-mitochondria contacts. We demonstrate that efficient LD-to-mitochondria fatty acid (FA) trafficking and ß-oxidation during starvation of myoblasts are promoted by phosphorylation of PLIN5 and require an intact PLIN5 mitochondrial tethering domain. Using human and murine cells, we further identified the acyl-CoA synthetase, FATP4 (ACSVL4), as a mitochondrial interactor of PLIN5. The C-terminal domains of PLIN5 and FATP4 constitute a minimal protein interaction capable of inducing organelle contacts. Our work suggests that starvation leads to phosphorylation of PLIN5, lipolysis, and subsequent channeling of FAs from LDs to FATP4 on mitochondria for conversion to fatty-acyl-CoAs and subsequent oxidation.

Hepatic ketogenesis regulates lipid homeostasis via ACSL1-mediated fatty acid partitioning

Liver-derived ketone bodies play a crucial role in fasting energy homeostasis by fueling the brain and peripheral tissues. Ketogenesis also acts as a conduit to remove excess acetyl-CoA generated from fatty acid oxidation and protects against diet-induced hepatic steatosis. Surprisingly, no study has examined the role of ketogenesis in fasting-associated hepatocellular lipid metabolism. Ketogenesis is driven by the rate-limiting mitochondrial enzyme 3-hydroxymethylglutaryl CoA synthase (HMGCS2) abundantly expressed in the liver. Here, we show that ketogenic insufficiency via disruption of hepatic HMGCS2 exacerbates liver steatosis in fasted chow and high-fat-fed mice. We found that the hepatic steatosis is driven by increased fatty acid partitioning to the endoplasmic reticulum (ER) for re-esterification via acyl-CoA synthetase long-chain family member 1 (ACSL1). Mechanistically, acetyl-CoA accumulation from impaired hepatic ketogenesis is responsible for the elevated translocation of ACSL1 to the ER. Moreover, we show increased ER-localized ACSL1 and re-esterification of lipids in human NASH displaying impaired hepatic ketogenesis. Finally, we show that L-carnitine, which buffers excess acetyl-CoA, decreases the ER-associated ACSL1 and alleviates hepatic steatosis. Thus, ketogenesis via controlling hepatocellular acetyl-CoA homeostasis regulates lipid partitioning and protects against hepatic steatosis.

The biochemistry and physiology of long-chain dicarboxylic acid metabolism

Mitochondrial β-oxidation is the most prominent pathway for fatty acid oxidation but alternative oxidative metabolism exists. Fatty acid ω-oxidation is one of these pathways and forms dicarboxylic acids as products. These dicarboxylic acids are metabolized through peroxisomal β-oxidation representing an alternative pathway, which could potentially limit the toxic effects of fatty acid accumulation. Although dicarboxylic acid metabolism is highly active in liver and kidney, its role in physiology has not been explored in depth. In this review, we summarize the biochemical mechanism of the formation and degradation of dicarboxylic acids through ω- and β-oxidation, respectively. We will discuss the role of dicarboxylic acids in different (patho)physiological states with a particular focus on the role of the intermediates and products generated through peroxisomal β-oxidation. This review is expected to increase the understanding of dicarboxylic acid metabolism and spark future research.

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.

Pregnancy Toxemia in Ewes: A Review of Molecular Metabolic Mechanisms and Management Strategies

Pregnancy toxemia is a nutritional metabolic disease during late gestation in small ruminants. The condition is characterized by disorders in carbohydrate and fat metabolism. Obese and multiparous ewes are particularly susceptible to pregnancy toxemia, which may lead to maternal death, abortion, or premature birth. Highly productive multiparous meat ewes are major breeding animals, which has led to an increased incidence of the disease. However, the pathogenesis of pregnancy toxemia remains unclear and adequate disease prevention and treatment strategies are absent. Investigating the pathogenesis of pregnancy toxemia, especially the metabolic pathways of hepatic lipids, is key to an improved understanding of the condition. This review provides a snapshot of the genes that are associated with lipid metabolism in the ovine liver, including genes involved in fatty acid oxidation, acetyl coenzyme metabolism, and triglyceride synthesis; describes the interrelationships between these genes; and summarizes the diagnosis, prevention, and treatment of pregnancy toxemia.

Loss of long-chain acyl-CoA synthetase 1 promotes hepatocyte death in alcohol-induced steatohepatitis

BACKGROUND

Alcohol consumption has been shown to disrupt hepatic lipid homeostasis. Long-chain acyl-CoA synthetase 1 (ACSL1) critically regulates hepatic fatty acid metabolism and lipid homeostasis by channeling fatty acids to lipid metabolic pathways. However, it remains unclear how ACSL1 contributes to the development of alcohol-associated liver disease (ALD).

METHODS

We performed chronic alcohol feeding animal studies with hepatocyte-specific ACSL1 knockout (ACSL1^Δhep) mice, hepatocyte-specific STAT5 knockout (STAT5^Δhep) mice, and ACSL1^Δhep based-STAT5B overexpression (Stat5b-OE) mice. Cell studies were conducted to define the causal role of ACSL1 deficiency in the pathogenesis of alcohol-induced liver injury. The clinical relevance of the STAT5-ACSL1 pathway was examined using liver tissues from patients with alcoholic hepatitis (AH) and normal subjects (Normal).

RESULTS

We found that chronic alcohol consumption reduced hepatic ACSL1 expression in AH patients and ALD mice. Hepatocyte-specific ACSL1 deletion exacerbated alcohol-induced liver injury by increasing free fatty acids (FFA) accumulation and cell death. Cell studies revealed that FFA elicited the translocation of BAX and p-MLKL to the lysosomal membrane, resulting in lysosomal membrane permeabilization (LMP) and thereby initiating lysosomal-mediated cell death pathway. Furthermore, we identified that the signal transducer and activator of transcription 5 (STAT5) is a novel transcriptional regulator of ACSL1. Deletion of STAT5 exacerbated alcohol-induced liver injury in association with downregulation of ACSL1, and reactivation of ACSL1 by STAT5 overexpression effectively ameliorated alcohol-induced liver injury. In addition, ACSL1 expression was positively correlated with STAT5 and negatively correlated with cell death was also validated in the liver of AH patients.

CONCLUSIONS

ACSL1 deficiency due to STAT5 inactivation critically mediates alcohol-induced lipotoxicity and cell death in the development of ALD. These findings provide insights into alcohol-induced liver injury.

Access and utilization of long chain fatty acyl-CoA by zDHHC protein acyltransferases

S-acylation is a reversible posttranslational modification, where a long-chain fatty acid is attached to a protein through a thioester linkage. Being the most abundant form of lipidation in humans, a family of twenty-three human zDHHC integral membrane enzymes catalyze this reaction. Previous structures of the apo and lipid bound zDHHCs shed light into the molecular details of the active site and binding pocket. Here, we delve further into the details of fatty acyl-CoA recognition by zDHHC acyltransferases using insights from the recent structure. We additionally review indirect evidence that suggests acyl-CoAs do not diffuse freely in the cytosol, but are channeled into specific pathways, and comment on the suggested mechanisms for fatty acyl-CoA compartmentalization and intracellular transport, to finally speculate about the potential mechanisms that underlie fatty acyl-CoA delivery to zDHHC enzymes.

Physicochemical, Nutritional Properties and Metabolomics Analysis Fat Deposition Mechanism of Chahua Chicken No. 2 and Yao Chicken

Poultry is an important dietary source of animal protein, accounting for approximately 30% of global meat consumption. Because of its low price, low fat and cholesterol content, and no religious restrictions, chicken is considered a widely available healthy meat. Chahua chicken No. 2 is a synthetic breed of Chahua chicken derived from five generations of specialized strain breeding. In this study, Chahua chicken No. 2 (CH) and Yao chicken (Y) were used as the research objects to compare the differences in physicochemical and nutritional indicators of meat quality between the two chicken breeds, and metabolomics was used to analyze the differences in metabolites and lipid metabolism pathways and to explore the expression of genes involved in adipogenesis. The physical index and nutritional value of CH are better than that of Y, and the chemical index of Y is better than that of CH. However, the chemical index results of CH are also within the normal theoretical value range. Comprehensive comparison shows that the meat quality of CH is relatively good. Metabolomics analysis showed that CH and Y had 85 different metabolites, and the differential metabolites were mainly classified into eight categories. KEGG pathway enrichment analysis revealed 13 different metabolic pathways. The screened PPARG, FABP3, ACSL5, FASN, UCP3 and SC5D were negatively correlated with muscle fat deposition, while PPARα, ACACA and ACOX1 were positively correlated with muscle fat deposition. The meat quality of CH was better than Y. The metabolites and metabolic pathways obtained by metabonomics analysis mainly involved the metabolism of amino acids and fatty acids, which were consistent with the differences in meat quality between the two breeds and the contents of precursors affecting flavor. The screened genes were associated with fatty deposition in poultry.

An integrated network pharmacology and cell metabolomics approach to reveal the role of rhein, a novel PPARα agonist, against renal fibrosis by activating the PPARα-CPT1A axis

BACKGROUND

Rhein, an anthraquinone compound, displays extensive antifibrotic effects; however, its potential mechanisms are not fully understood. In this study, we explored the underlying molecular mechanism of action of rhein.

METHOD

An integrated network pharmacology and cell metabolomics approach was developed based on network pharmacology and bioinformatics method, and then successfully applied to speculate the potential targets of rhein and construct a rhein-target-metabolic enzyme-metabolite network. Thereafter, the antifibrotic mechanism of rhein was validated in TGF-β- and oleic acid- induced HK-2 and NRK-52E cells in vitro as well as a unilateral ischemia-reperfusion injury Sprague-Dawley rat model.

RESULTS

Based on the construction of the rhein-target-metabolic enzyme-metabolite network, we found that rhein played an antifibrotic role through the PPAR-α-CPT1A-l-palmitoyl-carnitine axis. In vitro experiments demonstrated that rhein effectively activated the expression of PPARα and its downstream proteins (CPT1A and ACOX1) to alleviate lipid accumulation and fibrosis development. In vivo experiments indicated that rhein attenuated renal fibrosis mainly by activating the fatty acid oxidation pathway and improving lipid metabolism.

CONCLUSION

Taken together, our findings reveal that rhein is a novel agonist of PPARα, which contributes to its renoprotection through the regulation of the PPARα-CPT1A axis. Moreover, our study provides a novel insight into an integrated network pharmacology-metabolomics strategy for uncovering the pharmacological mechanisms of drugs from the system perspective.

Generation of a ceramide synthase 6 mouse lacking the DDRSDIE C-terminal motif

The important membrane lipid, ceramide, is generated by a family of homologous enzymes, the ceramide synthases (CerSs), multi-spanning membrane proteins located in the endoplasmic reticulum. Six CerS isoforms exist in mammals with each using a subset of acyl-CoAs for (dihydro)ceramide synthesis. A number of mice have been generated in which one or other CerS has been genetically manipulated, including complete knock-outs, with each displaying phenotypes concomitant with the expression levels of the CerS in question and the presumed biological function of the ceramide species that it generates. We recently described a short C-terminal motif in the CerS which is involved in CerS dimer formation; deleting this motif had no effect on the ability of the CerS to synthesize ceramide in vitro. In the current study, we generated a CerS6 mouse using CRISPR-Cas9, in which the DDRSDIE motif was replaced by ADAAAIA. While levels of CerS6^ADAAAIA expression were unaffected in the CerS6^ADAAAIA mouse, and CerS6^ADAAAIA was able to generate C16-ceramide in vitro, ceramide levels were significantly reduced in the CerS6^ADAAAIA mouse, suggesting that replacing this motif affects an as-yet unknown mechanism of regulation of ceramide synthesis via the DDRSDIE motif in vivo. Crossing CerS6^ADAAAIA mice with CerS5 null mice led to generation of viable mice in which C16-ceramide levels were reduced by up to 90%, suggesting that depletion of C16-ceramide levels is compensated for by other ceramide species with different acyl chain lengths.

Plasma linoelaidyl carnitine levels positively correlated with symptom improvement in olanzapine-treated first-episode drug-naïve schizophrenia

INTRODUCTION

Olanzapine (OLA) is one of the most commonly used second-generation antipsychotics for the treatment of schizophrenia. However, the heterogeneity of therapeutic response to OLA among schizophrenia patients deserves further exploration. The role of carnitine in the clinical response to OLA monotherapy remains unclear.

OBJECTIVES

The current study was designed to investigate whether carnitine and its derivatives are linked to the response to OLA treatment. Drug-naïve first-episode patients with schizophrenia were recruited and treated with OLA for 4 weeks. Psychiatric symptoms were assessed using the Positive and Negative Syndrome Scale (PANSS) in pre and post treatment.

RESULTS

After treatment, we found a significant decrease in 2-Octenoylcarnitine levels and a significant increase in linoelaidyl carnitine, 11Z-Octadecenylcarnitine and 9-Decenoylcarnitine levels. Furthermore, baseline linoelaidyl carnitine levels were correlated with the reduction of PANSS positive symptom subscore. Linear regression and logistic regression analyses found that the baseline linoelaidyl carnitine level was a predictive marker for the therapeutic response to OLA monotherapy for 4 weeks.

CONCLUSION

Our pilot study suggests that linoelaidyl carnitine levels at baseline may have a predictive role for the improvement of positive symptoms after OLA monotherapy in the patients with schizophrenia.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

Touch and Go: Membrane Contact Sites Between Lipid Droplets and Other Organelles

Lipid droplets (LDs) have emerged not just as storage sites for lipids but as central regulators of metabolism and organelle quality control. These critical functions are achieved, in part, at membrane contact sites (MCS) between LDs and other organelles. MCS are sites of transfer of cellular constituents to or from LDs for energy mobilization in response to nutrient limitations, as well as LD biogenesis, expansion and autophagy. Here, we describe recent findings on the mechanisms underlying the formation and function of MCS between LDs and mitochondria, ER and lysosomes/vacuoles and the role of the cytoskeleton in promoting LD MCS through its function in LD movement and distribution in response to environmental cues.

Circulating Mitochondrial DNA and Inter-Organelle Contact Sites in Aging and Associated Conditions

Mitochondria are primarily involved in cell bioenergetics, regulation of redox homeostasis, and cell death/survival signaling. An immunostimulatory property of mitochondria has also been recognized which is deployed through the extracellular release of entire or portioned organelle and/or mitochondrial DNA (mtDNA) unloading. Dynamic homo- and heterotypic interactions involving mitochondria have been described. Each type of connection has functional implications that eventually optimize mitochondrial activity according to the bioenergetic demands of a specific cell/tissue. Inter-organelle communications may also serve as molecular platforms for the extracellular release of mitochondrial components and subsequent ignition of systemic inflammation. Age-related chronic inflammation (inflamm-aging) has been associated with mitochondrial dysfunction and increased extracellular release of mitochondrial components—in particular, cell-free mtDNA. The close relationship between mitochondrial dysfunction and cellular senescence further supports the central role of mitochondria in the aging process and its related conditions. Here, we provide an overview of (1) the mitochondrial genetic system and the potential routes for generating and releasing mtDNA intermediates; (2) the pro-inflammatory pathways elicited by circulating mtDNA; (3) the participation of inter-organelle contacts to mtDNA homeostasis; and (4) the link of these processes with senescence and age-associated conditions.

Oxylipin metabolism is controlled by mitochondrial β-oxidation during bacterial inflammation

Oxylipins are potent biological mediators requiring strict control, but how they are removed en masse during infection and inflammation is unknown. Here we show that lipopolysaccharide (LPS) dynamically enhances oxylipin removal via mitochondrial β-oxidation. Specifically, genetic or pharmacological targeting of carnitine palmitoyl transferase 1 (CPT1), a mitochondrial importer of fatty acids, reveal that many oxylipins are removed by this protein during inflammation in vitro and in vivo. Using stable isotope-tracing lipidomics, we find secretion-reuptake recycling for 12-HETE and its intermediate metabolites. Meanwhile, oxylipin β-oxidation is uncoupled from oxidative phosphorylation, thus not contributing to energy generation. Testing for genetic control checkpoints, transcriptional interrogation of human neonatal sepsis finds upregulation of many genes involved in mitochondrial removal of long-chain fatty acyls, such as ACSL1,3,4, ACADVL, CPT1B, CPT2 and HADHB. Also, ACSL1/Acsl1 upregulation is consistently observed following the treatment of human/murine macrophages with LPS and IFN-γ. Last, dampening oxylipin levels by β-oxidation is suggested to impact on their regulation of leukocyte functions. In summary, we propose mitochondrial β-oxidation as a regulatory metabolic checkpoint for oxylipins during inflammation.

Roles of IκB kinases and TANK-binding kinase 1 in hepatic lipid metabolism and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease and is strongly associated with obesity-related ectopic fat accumulation in the liver. Hepatic lipid accumulation encompasses a histological spectrum ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), which can progress to cirrhosis and hepatocellular carcinoma. Given that dysregulated hepatic lipid metabolism may be an onset factor in NAFLD, understanding how hepatic lipid metabolism is modulated in healthy subjects and which steps are dysregulated in NAFLD subjects is crucial to identify effective therapeutic targets. Additionally, hepatic inflammation is involved in chronic hepatocyte damage during NAFLD progression. As a key immune signaling hub that mediates NF-κB activation, the IκB kinase (IKK) complex, including IKKα, IKKβ, and IKKγ (NEMO), has been studied as a crucial regulator of the hepatic inflammatory response and hepatocyte survival. Notably, TANK-binding kinase 1 (TBK1), an IKK-related kinase, has recently been revealed as a potential link between hepatic inflammation and energy metabolism. Here, we review (1) the biochemical steps of hepatic lipid metabolism; (2) dysregulated lipid metabolism in obesity and NAFLD; and (3) the roles of IKKs and TBK1 in obesity and NAFLD.

Acsl1 is essential for skin barrier function through the activation of linoleic acid and biosynthesis of ω-O-acylceramide in mice

The long-chain acyl-CoA synthase1 (Acsl1) is a major enzyme that converts long-chain fatty acids to acyl-CoAs. The role of Acsl1 in energy metabolism has been elucidated in the adipose tissue, heart, and skeletal muscle. Here, we demonstrate that systemic deficiency of Acsl1 caused severe skin barrier defects, leading to embryonic lethality. Acsl1 mRNA and protein are expressed in the Acsl1^+/+ epidermis, which are absent in Acsl1^-/- mice. In Acsl1^-/- mice, epidermal ceramide [EOS] (Cer[EOS]) containing ω-O-esterified linoleic acid, a lipid essential for the skin barrier, was significantly reduced. Conversely, ω-hydroxy ceramide (Cer[OS]), a precursor of Cer[EOS], was increased. Moreover, the levels of triglyceride (TG) species containing linoleic acids were lower in Acsl1^-/- mice, whereas those not containing linoleic acid were comparable to Acsl1^+/+ mice. As TG is considered to work as a reservoir of linoleic acid for the biosynthesis of Cer[EOS] from Cer[OS], our results suggest that Acsl1 plays an essential role in ω-O-acylceramide synthesis by providing linoleic acid for ω-O-esterification. Therefore, our findings identified a new biological role of Acsl1 as a regulator of the skin barrier.

Lipid nutrition: "In silico" studies and undeveloped experiments

This review examines lipids and lipid-binding sites on proteins in relation to cardiovascular disease. Lipid nutrition involves food energy from ingested fatty acids plus fatty acids formed from excess ingested carbohydrate and protein. Non-esterified fatty acids (NEFA) and lipoproteins have many detailed attributes not evident in their names. Recognizing attributes of lipid-protein interactions decreases unexpected outcomes. Details of double bond position and configuration interacting with protein binding sites have unexpected consequences in acyltransferase and cell replication events. Highly unsaturated fatty acids (HUFA) have n-3 and n-6 motifs with documented differences in intensity of destabilizing positive feedback loops amplifying pathophysiology. However, actions of NEFA have been neglected relative to cholesterol, which is co-produced from excess food. Native low-density lipoproteins (LDL) bind to a high-affinity cell surface receptor which poorly recognizes biologically modified LDLs. NEFA increase negative charge of LDL and decrease its processing by "normal" receptors while increasing processing by "scavenger" receptors. A positive feedback loop in the recruitment of monocytes and macrophages amplifies chronic inflammatory pathophysiology. Computer tools combine multiple components in lipid nutrition and predict balance of energy and n-3:n-6 HUFA. The tools help design and execute precise clinical nutrition monitoring that either supports or disproves expectations.

Hello from the other side: Membrane contact of lipid droplets with other organelles and subsequent functional implications

Lipid droplets (LDs) are ubiquitous organelles that play crucial roles in response to physiological and environmental cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). Increasing evidence suggests that distinct proteins and regulatory factors, which localize to membrane contact sites (MCS), are involved not only in interorganellar lipid exchange and transport, but also function in other important cellular processes, including autophagy, mitochondrial dynamics and inheritance, ion signaling and inter-regulation of these MCS. More and more tethers and molecular determinants are associated to MCS and to a diversity of cellular and pathophysiological processes, demonstrating the dynamics and importance of these junctions in health and disease. The conjugation of lipids with proteins in supramolecular complexes is known to be paramount for many biological processes, namely membrane biosynthesis, cell homeostasis, regulation of organelle division and biogenesis, and cell growth. Ultimately, this physical organization allows the contact sites to function as crucial metabolic hubs that control the occurrence of chemical reactions. This leads to biochemical and metabolite compartmentalization for the purposes of energetic efficiency and cellular homeostasis. In this review, we will focus on the structural and functional aspects of LD-organelle interactions and how they ensure signaling exchange and metabolites transfer between organelles.

An Essential Role of the N-Terminal Region of ACSL1 in Linking Free Fatty Acids to Mitochondrial β-Oxidation in C2C12 Myotubes

Free fatty acids are converted to acyl-CoA by long-chain acyl-CoA synthetases (ACSLs) before entering into metabolic pathways for lipid biosynthesis or degradation. ACSL family members have highly conserved amino acid sequences except for their N-terminal regions. Several reports have shown that ACSL1, among the ACSLs, is located in mitochondria and mainly leads fatty acids to the β-oxidation pathway in various cell types. In this study, we investigated how ACSL1 was localized in mitochondria and whether ACSL1 overexpression affected fatty acid oxidation (FAO) rates in C2C12 myotubes. We generated an ACSL1 mutant in which the N-terminal 100 amino acids were deleted and compared its localization and function with those of the ACSL1 wild type. We found that ACSL1 adjoined the outer membrane of mitochondria through interaction of its N-terminal region with carnitine palmitoyltransferase-1b (CPT1b) in C2C12 myotubes. In addition, overexpressed ACSL1, but not the ACSL1 mutant, increased FAO, and ameliorated palmitate-induced insulin resistance in C2C12 myotubes. These results suggested that targeting of ACSL1 to mitochondria is essential in increasing FAO in myotubes, which can reduce insulin resistance in obesity and related metabolic disorders.

Perspectives on Mitochondria-ER and Mitochondria-Lipid Droplet Contact in Hepatocytes and Hepatic Lipid Metabolism

Emerging evidence suggests that mitochondrion–endoplasmic reticulum (ER) and mitochondrion–lipid droplet (LD) contact sites are critical in regulating lipid metabolism in cells. It is well established that intracellular organelles communicate with each other continuously through membrane contact sites to maintain organelle function and cellular homeostasis. The accumulation of LDs in hepatocytes is an early indicator of non-alcoholic fatty liver disease (NAFLD) and alcohol-related liver disease (ALD), which may indicate a breakdown in proper inter-organelle communication. In this review, we discuss previous findings in mitochondrion–ER and mitochondrion–LD contact, focusing on their roles in lipid metabolism in hepatocytes. We also present evidence of a unique mitochondrion–LD contact structure in hepatocytes under various physiological and pathological conditions and propose a working hypothesis to speculate about the role of these structures in regulating the functions of mitochondria and LDs and their implications in NAFLD and ALD.

Peroxisomal ABC Transporters: An Update

ATP-binding cassette (ABC) transporters constitute one of the largest superfamilies of conserved proteins from bacteria to mammals. In humans, three members of this family are expressed in the peroxisomal membrane and belong to the subfamily D: ABCD1 (ALDP), ABCD2 (ALDRP), and ABCD3 (PMP70). These half-transporters must dimerize to form a functional transporter, but they are thought to exist primarily as tetramers. They possess overlapping but specific substrate specificity, allowing the transport of various lipids into the peroxisomal matrix. The defects of ABCD1 and ABCD3 are responsible for two genetic disorders called X-linked adrenoleukodystrophy and congenital bile acid synthesis defect 5, respectively. In addition to their role in peroxisome metabolism, it has recently been proposed that peroxisomal ABC transporters participate in cell signaling and cell control, particularly in cancer. This review presents an overview of the knowledge on the structure, function, and mechanisms involving these proteins and their link to pathologies. We summarize the different in vitro and in vivo models existing across the species to study peroxisomal ABC transporters and the consequences of their defects. Finally, an overview of the known and possible interactome involving these proteins, which reveal putative and unexpected new functions, is shown and discussed.

Triple-Knockout, Synuclein-Free Mice Display Compromised Lipid Pattern

Recent studies have implicated synucleins in several reactions during the biosynthesis of lipids and fatty acids in addition to their recognised role in membrane lipid binding and synaptic functions. These are among aspects of decreased synuclein functions that are still poorly acknowledged especially in regard to pathogenesis in Parkinson’s disease. Here, we aimed to add to existing knowledge of synuclein deficiency (i.e., the lack of all three family members), with respect to changes in fatty acids and lipids in plasma, liver, and two brain regions in triple synuclein-knockout (TKO) mice. We describe changes of long-chain polyunsaturated fatty acids (LCPUFA) and palmitic acid in liver and plasma, reduced triacylglycerol (TAG) accumulation in liver and non-esterified fatty acids in plasma of synuclein free mice. In midbrain, we observed counterbalanced changes in the relative concentrations of phosphatidylcholine (PC) and cerebrosides (CER). We also recorded a notable reduction in ethanolamine plasmalogens in the midbrain of synuclein free mice, which is an important finding since the abnormal ether lipid metabolism usually associated with neurological disorders. In summary, our data demonstrates that synuclein deficiency results in alterations of the PUFA synthesis, storage lipid accumulation in the liver, and the reduction of plasmalogens and CER, those polar lipids which are principal compounds of lipid rafts in many tissues. An ablation of all three synuclein family members causes more profound changes in lipid metabolism than changes previously shown to be associated with γ-synuclein deficiency alone. Possible mechanisms by which synuclein deficiency may govern the reported modifications of lipid metabolism in TKO mice are proposed and discussed.

AKAP1 Deficiency Attenuates Diet-Induced Obesity and Insulin Resistance by Promoting Fatty Acid Oxidation and Thermogenesis in Brown Adipocytes

Altering the balance between energy intake and expenditure is a major strategy for treating obesity. Nonetheless, despite the progression in antiobesity drugs on appetite suppression, therapies aimed at increasing energy expenditure are limited. Here, knockout ofAKAP1, a signaling hub on outer mitochondrial membrane, renders mice resistant to diet-induced obesity.AKAP1 knockout significantly enhances energy expenditure and thermogenesis in brown adipose tissues (BATs) of obese mice. Restoring AKAP1 expression in BAT clearly reverses the beneficial antiobesity effect in AKAP1-/- mice. Mechanistically, AKAP1 remarkably decreases fatty acid β-oxidation (FAO) by phosphorylating ACSL1 to inhibit its activity in a protein-kinase-A-dependent manner and thus inhibits thermogenesis in brown adipocytes. Importantly, AKAP1 peptide inhibitor effectively alleviates diet-induced obesity and insulin resistance. Altogether, the findings demonstrate that AKAP1 functions as a brake of FAO to promote diet-induced obesity, which may be used as a potential therapeutic target for obesity.

Ceramide synthases: Reflections on the impact of Dr. Lina M. Obeid

Sphingolipids are a family of lipids that are critical to cell function and survival. Much of the recent work done on sphingolipids has been performed by a closely-knit family of sphingolipid researchers, which including our colleague, Dr. Lina Obeid, who recently passed away. We now briefly review where the sphingolipid field stands today, focusing in particular on areas of sphingolipid research to which Dr. Obeid made valued contributions. These include the 'many-worlds' view of ceramides and the role of a key enzyme in the sphingolipid biosynthetic pathway, namely the ceramide synthases (CerS). The CerS contain a number of functional domains and also interact with a number of other proteins in lipid metabolic pathways, fulfilling Dr. Obeid's prophecy that ceramides, and the enzymes that generate ceramides, form the critical hub of the sphingolipid metabolic pathway.

Mechanism insights into polyhydroxyalkanoate-regulated denitrification from the perspective of pericytoplasmic nitrate reductase expression

For enhanced biological nutrient removal (BNR) process, the polyhydroxyalkanoate (PHA) can be used as an eco-friendly internal as well as external substrate for regulating the growth of heterotrophic denitrifiers and promoting the denitrification process for deep nitrogen removal from wastewater. However, the exact mechanisms by which PHA impacts bacterial metabolism and affects the electron transfer of denitrification remain unknown. In this study, the in-depth mechanism investigation for PHA-mediated denitrification based on the jointly applied transcriptomic, proteomic and Western Blotting techniques was performed on a model denitrifier, Pseudomonas stutzeri. Results showed that PHA dramatically fostered the growth of Pseudomonas stutzeri, resulting in improved nitrate removal efficiency from 32.8% to 45.8%. Comparison of protein expression profiles indicated that PHA promoted the expression of enzyme NapB and NapA by approximately 10.34 and 20.01 times, respectively, which were both in charge of reduction from nitrate to nitrite. Based on transcriptional sequencing and Tandem Mass Tags, the correlation results also showed that differential proteins and genes with the same expression trend were positively correlated (R² = 0.427, p-value<0.033). Western Blotting approach was further developed to confirm the up-regulated expression of target protein with the higher proportion of PHA in carbon source of the medium, which proved the reliability of proteomics results. All the findings presented here are believed to deepen the understanding of microbial mechanism about PHA-enhanced denitrification from the novel perspective of associated electron-transfer enzymatic proteins.

Mouse long-chain acyl-CoA synthetase 1 is active as a monomer

Fatty acids are essential cellular building blocks and a major energy source. Regardless of their metabolic fate, fatty acids first need to be activated by forming a thioester with a coenzyme A group. This reaction is carried out by acyl-CoA synthetases (ACSs), of which ACSL1 (long-chain acyl-CoA synthetase 1) is an important member. Two bacterial homologues of ACSL1 crystal structures have been solved previously. One is a soluble dimeric protein, and the other is a monomeric peripheral membrane protein. The mammalian ACSL1 is a membrane protein with an N-terminal transmembrane helix. To characterize the mammalian ACSL1, we purified the full-length mouse ACSL1 and reconstituted it into lipid nanodiscs. Using enzymatic assays, mutational analysis, and cryo-electron microscopy, we show that mouse ACSL1 is active as a monomer.

Let-7a regulates EV secretion and mitochondrial oxidative phosphorylation by targeting SNAP23 in colorectal cancer

Background

MicroRNAs (miRNAs) are abundant in tumor-derived extracellular vesicles (EVs) and the functions of extracellular miRNA to recipient cells have been extensively studied with tumorigenesis. However, the role of miRNA in EV secretion from cancer cells remains unknown.

Methods

qPCR and bioinformatics analysis were applied for determining extracellular let-7a expression from CRC patient serum and cells. Nanosight particle tracking analysis was performed for investigating the effect of let-7a on EV secretion. Luciferase reporter assays was used for identifying targeted genes synaptosome-associated protein 23 (SNAP23). In vitro and in vivo assays were used for exploring the function of let-7a/SNAP23 axis in CRC progression. Bioenergetic assays were performed for investigating the role of let-7a/SNAP23 in cellular metabolic reprogramming.

Results

let-7a miRNA was elevated in serum EVs from CRC patients and was enriched in CRC cell-derived EVs. We determined that let-7a could suppress EV secretion directly targeting SNAP23. In turn, SNAP23 promotes EV secretion of let-7a to downregulate the intracellular let-7a expression. In addition, we found a novel mechanism of let-7a/SNAP23 axis by regulating mitochondrial oxidative phosphorylation (OXPHOS) through Lin28a/SDHA signaling pathway.

Conclusions

Let-7a plays an essential role in not only inhibiting EV secretion, but also suppressing OXPHOS through SNAP23, resulting in the suppression of CRC progression, suggesting that let-7a/SNAP23 axis could provide not only effective tumor biomarkers but also novel targets for tumor therapeutic strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-020-01813-6.

MiR-34c promotes hepatic stellate cell activation and Liver Fibrogenesis by suppressing ACSL1 expression

Normally, there are multiple microRNAs involved in the pathogenesis of liver fibrosis. In our work, we aimed at identifying the role of miR-34c in the hepatic stellate cell (HSC) activation and liver fibrosis and its potential mechanism. Our results have shown that during natural activation of HSC, the level of miR-34c was increased significantly whereas acyl-CoA synthetase long-chain family member-1(ACSL1), which is a key enzyme can affect fatty acid(FA) synthesis, was decreased. A double fluorescence reporter assay further confirmed that ACSL1 is a direct target gene of miR-34c. Moreover, the inhibition of miR-34C can attenuate the synthesis of collagen in HSC-T6. In our rescue assay, ACSL1 expression was 1.49-fold higher compared to normal control cells which were transfected with the miR-34c inhibitor in a stable low expression ACSL1 cell line. While at the same time, α-SMA and Col1α expression decreased by 18.22% and 2.58%, respectively. Moreover, we performed an in vivo model using dimethylnitrosamine (DMN) in conjunction with the miR-34c agomir, combined with the treatment of DMN and the miR-34c agomir can increase liver fibrosis. Meanwhile, the degree of hepatic fibrosis was increased and lipid droplets reduced dramatically in rats and HSC-T6 cell treated with miR-34c mimics alone compared to untreated groups. Our results indicate that miR-34c plays an essential role in liver fibrosis by targeting ACSL1 closely associated with lipid droplets, and it might be used as a potential therapeutic target.

NLRP3 inflammasome deficiency attenuates metabolic disturbances involving alterations in the gut microbial profile in mice exposed to high fat diet

Obesity-related diseases (e.g. type 2 diabetes mellitus and cardiovascular disorders) represent an increasing health problem worldwide. NLRP3 inflammasome activation may underlie obesity-induced inflammation and insulin resistance, and NLRP3 deficient mice exposed to high fat diet (HFD) appear to be protected from left ventricle (LV) concentric remodeling. Herein, we investigated if these beneficial effects were associated with alterations in plasma metabolites, using metabolomic and lipidomic analysis, and gut microbiota composition, using 16S rRNA sequencing of cecum content, comparing NLRP3 deficient and wild type (WT) mice on HFD and control diet. Obese NLRP3 deficient mice had lower systemic ceramide levels, potentially resulting attenuating inflammation, altered hepatic expression of fatty acids (FA) with lower mono-saturated FA and higher polyunsaturated FA levels, potentially counteracting development of liver steatosis, downregulated myocardial energy metabolism as assessed by proteomic analyses of LV heart tissue, and different levels of bile acids as compared with WT mice. These changes were accompanied by an altered composition of gut microbiota associated with decreased systemic levels of tri-methylamine-N-oxide and lipopolysaccharide, potentially inducing attenuating systemic inflammation and beneficial effects on lipid metabolism. Our findings support a role of NLRP3 inflammasome in the interface between metabolic and inflammatory stress, involving an altered gut microbiota composition.

Acyl-CoA synthetases as regulators of brain phospholipid acyl-chain diversity

Each individual cell-type is defined by its distinct morphology, phenotype, molecular and lipidomic profile. The importance of maintaining cell-specific lipidomic profiles is exemplified by the numerous diseases, disorders, and dysfunctional outcomes that occur as a direct result of altered lipidome. Therefore, the mechanisms regulating cellular lipidome diversity play a role in maintaining essential biological functions. The brain is an organ particularly rich in phospholipids, the main constituents of cellular membranes. The phospholipid acyl-chain profile of membranes in the brain is rather diverse due in part to the high degree of cellular heterogeneity. These membranes and the acyl-chain composition of their phospholipids are highly regulated, but the mechanisms that confer this tight regulation are incompletely understood. A family of enzymes called acyl-CoA synthetases (ACSs) stands at a pinnacle step allowing influence over cellular acyl-chain selection and subsequent metabolic flux. ACSs perform the initial reaction for cellular fatty acid metabolism by ligating a Coenzyme A to a fatty acid which both traps a fatty acid within a cell and activates it for metabolism. The ACS family of enzymes is large and diverse consisting of 25-26 family members that are nonredundant, each with unique distribution across and within cell types, and differential fatty acid substrate preferences. Thus, ACSs confer a critical intracellular fatty acid selecting step in a cell-type dependent manner providing acyl-CoA moieties that serve as essential precursors for phospholipid synthesis and remodeling, and therefore serve as a key regulator of cellular membrane acyl-chain compositional diversity. Here we will discuss how the contribution of individual ACSs towards brain lipid metabolism has only just begun to be elucidated and discuss the possibilities for how ACSs may differentially regulate brain lipidomic diversity.

Evolution, expression profile, and regulatory characteristics of ACSL gene family in chicken (Gallus gallus)

Long chain acyl-CoA synthetases (ACSLs), which drive the conversion of long chain fatty acid into acyl-CoA, an ingredient of lipid synthesis, have been well-acknowledged to exert an indispensable role in many metabolic processes in mammals, especially lipid metabolism. However, in chicken, the evolutionary characteristics, expression profiles and regulatory mechanisms of ACSL gene family are rarely understood. Here, we analyzed the genomic synteny, gene structure, evolutionary event and functional domains of the ACSL gene family members using bioinformatics methods. The spatiotemporal expression profiles of ACSL gene family, and their regulatory mechanism were investigated via bioinformatics analysis incorporated with in vivo and in vitro estrogen-treated experiments. Our results indicated that ACSL2 gene was indeed evolutionarily lost in the genome of chicken. Chicken ACSLs shared an AMP-binding functional domain, as well as highly conversed ATP/AMP and FACS signature motifs, and were clustered into two clades, ACSL1/5/6 and ACSL3/4, based on high sequence similarity, similar gene features and conversed motifs. Chicken ACSLs showed differential tissue expression distributions, wherein the significantly decreased expression level of ACSL1 and the significantly increased expression level of ACSL5 were found, respectively, the expression levels of the other ACSL members remained unchanged in the liver of peak-laying hens versus pre-laying hens. Moreover, the transcription activity of ACSL1, ACSL3 and ACSL4 was silenced and ACSL6 was activated by estrogen, but no response to ACSL5. In conclusion, though having highly conversed functional domains, chicken ACSL gene family is organized into two separate groups, ACSL1/5/6 and ACSL3/4, and exhibits varying expression profiles and estrogen effects. These results not only pave the way for better understanding the specific functions of ACSL genes in avian lipid metabolism, but also provide a valuable evidence for gene family characteristics.

Red and White Chinook Salmon (Oncorhynchus tshawytscha): Differences in the Transcriptome Profile of Muscle, Liver, and Pylorus

Astaxanthin (Ax), the main carotenoid responsible for the distinct red flesh color in salmonids (Oncorhynchus, Salvelinus, Salmo, and Parahucho), is added to the diet of farmed fish at a substantial cost. Despite the great economical value for the salmon industry, the key molecular mechanisms involved in the regulation of muscle coloration are poorly understood. Chinook salmon (Oncorhynchus tshawytscha) represent an ideal model to study flesh coloration because they exhibit a distinct color polymorphism responsible for two color morphs, white and red flesh pigmented fish. This study was designed to identify the molecular basis for the development of red and white coloration of fish reared under the same experimental conditions and to better understand the absorption mechanism of Ax in salmonids. Pyloric caeca, liver, and muscle of both groups (n = 6 each) were selected as the most likely critical target organs to be involved respectively in the intestinal uptake, metabolism, and retention of Ax. Difference in the transcriptome profile of each tissue using next-generation sequencing technology was conducted. Ten KEGG pathways were significantly enriched for differentially expressed genes between red and white salmon pylorus tissue, while none for the transcriptome profile in the other two tissues. Differential expressed gene (DE) analyses showed that there were relatively few differences in muscle (31 DE genes, p < 0.05) and liver (43 DE genes, p < 0.05) of white and red Chinook salmon compared approximately 1125 DE genes characterized in the pylorus tissue, with several linked to Ax binding ability, absorption, and metabolism.

Maintaining social contacts: The physiological relevance of organelle interactions

Membrane-bound organelles in eukaryotic cells form an interactive network to coordinate and facilitate cellular functions. The formation of close contacts, termed "membrane contact sites" (MCSs), represents an intriguing strategy for organelle interaction and coordinated interplay. Emerging research is rapidly revealing new details of MCSs. They represent ubiquitous and diverse structures, which are important for many aspects of cell physiology and homeostasis. Here, we provide a comprehensive overview of the physiological relevance of organelle contacts. We focus on mitochondria, peroxisomes, the Golgi complex and the plasma membrane, and discuss the most recent findings on their interactions with other subcellular organelles and their multiple functions, including membrane contacts with the ER, lipid droplets and the endosomal/lysosomal compartment.

The "discovery" of lipid droplets: A brief history of organelles hidden in plain sight

Mammalian lipid droplets (LDs), first described as early as the 1880s, were virtually ignored for more than 100 years. Between 1991 and the early 2000s, however, a series of discoveries and conceptual breakthroughs led to a resurgent interest in obesity as a disease, in the metabolism of intracellular triacylglycerol (TAG), and in the physical locations of LDs as cellular structures with their associated proteins. Insights included the recognition that obesity underlies major chronic diseases, that appetite is hormonally controlled, that hepatic steatosis is not a benign finding, and that diabetes might fundamentally be a disorder of lipid metabolism. In this brief review, I describe the metamorphosis of LDs from overlooked globs of stored fat to dynamic organelles that control insulin resistance, mitochondrial oxidation, and viral replication.

The diversity of ACBD proteins - From lipid binding to protein modulators and organelle tethers

Members of the large multigene family of acyl-CoA binding domain containing proteins (ACBDs) share a conserved motif required for binding of Coenzyme A esterified fatty acids of various chain length. These proteins are present in the three kingdoms of life, and despite their predicted roles in cellular lipid metabolism, knowledge about the precise functions of many ACBD proteins remains scarce. Interestingly, several ACBD proteins are now suggested to function at organelle contact sites, and are recognized as host interaction proteins for different pathogens including viruses and bacteria. Here, we present a thorough phylogenetic analysis of the ACBD family and discuss their structure and evolution. We summarize recent findings on the various functions of animal and fungal ACBDs with particular focus on peroxisomes, the role of ACBD proteins at organelle membranes, and their increasing recognition as targets for pathogens.

Inter-organelle Communication in the Pathogenesis of Mitochondrial Dysfunction and Insulin Resistance

PURPOSE OF REVIEW

Impairments in mitochondrial function in patients with insulin resistance and type 2 diabetes have been disputed for decades. This review aims to briefly summarize the current knowledge on mitochondrial dysfunction in metabolic tissues and to particularly focus on addressing a new perspective of mitochondrial dysfunction, the altered capacity of mitochondria to communicate with other organelles within insulin-resistant tissues.

RECENT FINDINGS

Organelle interactions are temporally and spatially formed connections essential for normal cell function. Recent studies have shown that mitochondria interact with various cellular organelles, such as the endoplasmic reticulum, lysosomes and lipid droplets, forming inter-organelle junctions. We will discuss the current knowledge on alterations in these mitochondria-organelle interactions in insulin resistance and diabetes, with a focus on changes in mitochondria-lipid droplet communication as a major player in ectopic lipid accumulation, lipotoxicity and insulin resistance.