Hypothalamic ceramide levels regulated by CPT1C mediate the orexigenic effect of ghrelin

Sphingolipid and Trimethylamine-N-Oxide (TMAO) Levels in Women with Obesity after Combined Physical Training

Obesity causes metabolic changes, such as the development of cardiovascular diseases. Moreover, physical exercise promotes protection against these diseases. Thus, the objective of the present study was to evaluate whether combined physical training can improve the metabolic system of women with obesity, reducing plasma concentrations of trimethylamine N-oxide (TMAO) and sphingolipids, regardless of weight loss. Fourteen obese women (BMI 30-40 kg/m2), aged 20-40 years, sedentary, were submitted to 8 weeks of combined physical training (strength and aerobic exercises). The training was performed three times/week, 55 min/session, at 75-90% maximum heart rate. All participants were evaluated pre- and post-exercise intervention, and their body composition, plasma TMAO, creatinine, lipid profile, and sphingolipid concentrations were recorded. Maximum oxygen consumption (VO2max), Speed lactate threshold 1 (SpeedLT1), and Speed lactate threshold 2 (SpeedLT2) evaluated physical performance. Results: After combined exercise, it did not change body composition, but TMAO, total cholesterol, and sphingolipid concentrations significantly decreased (p < 0.05). There was an increase in physical performance by improving VO2max, SpeedLT1, and SpeedLT2 (p < 0.05). The combined physical exercise could induce cardiovascular risk protection by decreasing TMAO in obese women, parallel to physical performance improvement, independent of weight loss.

Ceramides in the central control of metabolism

Central ceramides regulate energy metabolism by impacting hypothalamic neurons. This allows ceramides to integrate endocrine signals - such as leptin, ghrelin, thyroid hormones, or estradiol - and to modulate the central control of puberty. In this forum article we discuss recent evidence suggesting that specific ceramide species and neuronal populations are involved in these effects.

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Lipidomics reveals ceramide biomarkers for detecting central precocious puberty in girls

BACKGROUND

Pubertal timing is modulated by complex interactions between the pituitary and gonadal sex steroid hormones. Evidence indicates that sphingolipids are involved in the biosynthesis of steroid hormones at multiple levels.

METHOD

This study recruited adolescent female patients from pubertal and pediatric endocrine clinics in Northern and Southern Taiwan from the Taiwan Puberty Longitudinal Study. A total of 112 plasma samples (22 healthy control, 29 peripheral precocious puberty (PPP), and 61 CPP samples) were collected. We extracted lipids from the plasma samples using the modified Folch method. The un-targeted ultrahigh-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) was employed for the lipid analysis.

RESULTS

We identified sphingolipid-linked metabolites, including Cer(18:0/15:0), Cer(18:1/16:0), and Cer(18:1/26:0) as candidate biomarkers for distinguishing girls with CPP from the control group by using an excellent discrimination model (AUC = 0.964). Moreover, Cer(18:0/22:0) and Cer(d18:0/18:1) were identified as potential biomarkers of PPP, with an AUC value of 0.938. Furthermore, CerP(18:1/18:0) was identified as the sole candidate biomarker capable of differentiating CPP from PPP.

CONCLUSIONS

The biomarkers identified in this study can facilitate the accurate detection of CPP in girls, provide insights into lipid-linked pathophysiology, and present a novel method of monitoring the progression of this disorder.

CerS6-dependent ceramide synthesis in hypothalamic neurons promotes ER/mitochondrial stress and impairs glucose homeostasis in obese mice

Dysregulation of hypothalamic ceramides has been associated with disrupted neuronal pathways in control of energy and glucose homeostasis. However, the specific ceramide species promoting neuronal lipotoxicity in obesity have remained obscure. Here, we find increased expression of the C16:0 ceramide-producing ceramide synthase (CerS)6 in cultured hypothalamic neurons exposed to palmitate in vitro and in the hypothalamus of obese mice. Conditional deletion of CerS6 in hypothalamic neurons attenuates high-fat diet (HFD)-dependent weight gain and improves glucose metabolism. Specifically, CerS6 deficiency in neurons expressing pro-opiomelanocortin (POMC) or steroidogenic factor 1 (SF-1) alters feeding behavior and alleviates the adverse metabolic effects of HFD feeding on insulin sensitivity and glucose tolerance. POMC-expressing cell-selective deletion of CerS6 prevents the diet-induced alterations of mitochondrial morphology and improves cellular leptin sensitivity. Our experiments reveal functions of CerS6-derived ceramides in hypothalamic lipotoxicity, altered mitochondrial dynamics, and ER/mitochondrial stress in the deregulation of food intake and glucose metabolism in obesity.

Targeting carnitine palmitoyltransferase 1 isoforms in the hypothalamus: A promising strategy to regulate energy balance

Tackling the growing incidence and prevalence of obesity urgently requires uncovering new molecular pathways with therapeutic potential. The brain, and in particular the hypothalamus, is a major integrator of metabolic signals from peripheral tissues that regulate functions such as feeding behavior and energy expenditure. In obesity, hypothalamic capacity to sense nutritional status and regulate these functions is altered. An emerging line of research is that hypothalamic lipid metabolism plays a critical role in regulating energy balance. Here, we focus on the carnitine palmitoyltransferase 1 (CPT1) enzyme family responsible for long-chain fatty acid metabolism. The evidence suggests that two of its isoforms expressed in the brain, CPT1A and CPT1C, play a crucial role in hypothalamic lipid metabolism, and their promise as targets in food intake and bodyweight management is currently being intensively investigated. In this review we describe and discuss the metabolic actions and potential up- and downstream effectors of hypothalamic CPT1 isoforms, and posit the need to develop innovative nanomedicine platforms for selective targeting of CPT1 and related nutrient sensors in specific brain areas as potential next-generation therapy to treat obesity.

Plasma signatures of Congenital Generalized Lipodystrophy patients identified by untargeted lipidomic profiling are not changed after a fat-containing breakfast meal

BACKGROUND

The incapacity to store lipids in adipose tissue in Congenital Generalized Lipodystrophy (CGL) causes hypoleptinemia, increased appetite, ectopic fat deposition and lipotoxicity. CGL patients experience shortened life expectancy. The plasma lipidomic profile has not been characterized fully in CGL, nor has the extent of dietary intake in its modulation. The present work investigated the plasma lipidomic profile of CGL patients in comparison to eutrophic individuals at the fasted state and after a breakfast meal.

METHOD

Blood samples from 11 CGL patients and 10 eutrophic controls were collected after 12 h fasting (T0) and 90 min after an ad libitum fat-containing breakfast (T90). The lipidomic profile of extracted plasma lipids was characterized by non-target liquid chromatography mass spectrometry.

RESULTS

Important differences between groups were observed at T0 and at T90. Several molecular species of fatty acyls, glycerolipids, sphingolipids and glycerophospholipids were altered in CGL. All the detected fatty acyl molecular species, several diacylglycerols and one triacylglycerol species were upregulated in CGL. Among sphingolipids, one sphingomyelin and one glycosphingolipid species showed downregulation in CGL. Alterations in the glycerophospholipids glycerophosphoethanolamines, glycerophosphoserines and cardiolipins were more complex. Interestingly, when comparing T90 versus T0, the lipidomic profile in CGL did not change as intensely as it did for control participants.

CONCLUSIONS

The present study found profound alterations in the plasma lipidomic profile of complex lipids in CGL patients as compared to control subjects. A fat-containing breakfast meal did not appear to significantly influence the CGL profile observed in the fasted state. Our study may have implications for clinical practice, also aiding to a deeper comprehension of the role of complex lipids in CGL in view of novel therapeutic strategies.

Lipid peroxidation and sphingolipid alterations in the cerebral cortex and hypothalamus of rats fed a high-protein diet

OBJECTIVES

High-protein diets (HPDs) are widely accepted to enhance satiety and energy expenditure and thus have become a popular strategy to lose weight and facilitate muscle protein synthesis. However, long-term high-protein consumption could be linked with metabolic and clinical problems such as renal and liver dysfunctions. This study verified the effects of 8-wk high-protein ingestion on lipid peroxidation and sphingolipid metabolism in the plasma, cerebral cortex, and hypothalamus in rats.

METHODS

Immunoenzymatic and spectrophotometric methods were applied to assess oxidation-reduction (redox) biomarkers and neutral sphingomyelinase activity, whereas gas-liquid chromatography and high-performance liquid chromatography were used to examine sphingolipid levels.

RESULTS

The vast majority of HPD-related alterations was restricted to the hypothalamus. Specifically, an increased rate of lipid peroxidation (increased lipid hydroperoxides, 8-isoprostanes, and thiobarbituric acid reactive substances) associated with ceramide accumulation via the activation of de novo synthesis (decreased sphinganine), salvage pathway (decreased sphingosine), and sphingomyelin hydrolysis (decreased sphingomyelin and increased neutral sphingomyelinase activity) was noted.

CONCLUSIONS

This study showed that HPD substantially affected hypothalamic metabolic pathways, which potentially alter cerebral output signals to the peripheral tissues.

Central Regulation of Brown Fat Thermogenesis in Response to Saturated or Unsaturated Long-Chain Fatty Acids

Sensing of long-chain fatty acids (LCFA) in the hypothalamus modulates energy balance, and its disruption leads to obesity. To date, the effects of saturated or unsaturated LCFA on hypothalamic-brown adipose tissue (BAT) axis and the underlying mechanisms have remained largely unclear. Our aim was to characterize the main molecular pathways involved in the hypothalamic regulation of BAT thermogenesis in response to LCFA with different lengths and degrees of saturation. One-week administration of high-fat diet enriched in monounsaturated FA led to higher BAT thermogenesis compared to a saturated FA-enriched diet. Intracerebroventricular infusion of oleic and linoleic acids upregulated thermogenesis markers and temperature in brown fat of mice, and triggered neuronal activation of paraventricular (PaV), ventromedial (VMH) and arcuate (ARC) hypothalamic nuclei, which was not found with saturated FAs. The neuron-specific protein carnitine palmitoyltransferase 1-C (CPT1C) was a crucial effector of oleic acid since the FA action was blunted in CPT1C-KO mice. Moreover, changes in the AMPK/ACC/malonyl-CoA pathway and fatty acid synthase expression were evoked by oleic acid. Altogether, central infusion of unsaturated but not saturated LCFA increases BAT thermogenesis through CPT1C-mediated sensing of FA metabolism shift, which in turn drive melanocortin system activation. These findings add new insight into neuronal circuitries activated by LCFA to drive thermogenesis.

Long-term health outcomes of early menarche in women: an umbrella review

BACKGROUND

There is limited comprehensive evidence on the potential association between early menarche and subsequent health outcomes.

AIM

To evaluate the existing evidence for the association of early menarche with later health outcomes and assesse the strength and validity of the evidence for these associations.

DESIGN

Umbrella review.

METHODS

We searched PubMed, Web of Science, Embase, CINAHL, Cochrane Database of Systematic Reviews and Google Scholar, and manually screened retrieved references to find systematic reviews and meta-analyses from inception to July 2021. Early menarche was defined by taking into account ethnicity and birth year, and the outcomes were long-term consequences in adulthood.

RESULTS

Thirteen reviews encompassing 283 original articles and over 6.8 million participants from 39 countries across 5 continents were included. In categorical outcomes, early menarche was associated with metabolic syndrome (n = 37 543 pooled adjusted relative risk [aRR] 1.56, 95% confidence interval (CI) 1.33, 1.83; high certainty [Hi]), endometrial cancer (n = 874 188, aRR 1.40, 95% CI 1.17, 1.68; Hi), type 2 diabetes mellitus/impaired glucose tolerance (n = 1 185 444, aRR 1.30, 95% CI 1.19, 1.42; Hi), breast cancer (n = 103 574, aRR 1.19, 95% CI 1.06, 1.33; Hi), death from all causes (n = 152 747, aRR 1.11, 95% CI 1.03, 1.19; Hi), obesity (n = 54 006, aRR 1.68, 95% CI 1.53, 1.84; moderate certainty [Mod]), gestational diabetes mellitus (n = 48 535, aRR 1.32, 95% CI 1.09, 1.58; Mod), hypertension (n = 1 682 689, aRR 1.24, 95% CI 1.20, 1.29; Mod), endometriosis (n = 885 390, aRR 1.22, 95% CI 1.09, 1.37; Mod), ovarian cancer (n = 1 022 451, aRR 1.17, 95% CI 1.04, 1.31; Mod) and asthma (n = 22 859, aRR 1.31, 95% CI 1.09, 1.57; low certainty [Lo]). For continuous outcomes, early menarche was associated with increased body mass index (BMI) in adults ≥40 years of age (n = 121 943, adjusted pooled standardized mean difference [aSMD] 0.30, 95% CI 0.28, 0.32; Mod), BMI in adults <40 years of age (n = 124 728, aSMD 0.39, 95% CI 0.36, 0.43; Mod), serum fasting insulin level (n = 17 020, aSMD 0.52, 95% CI 0.48, 0.57; Mod) and homeostatic model assessment of insulin resistance (n = 7925, aSMD 0.27, 95% CI 0.19, 0.35; Mod).

CONCLUSION

We found varied levels of evidence for the association between early menarche and the development of subsequent health problems. Our results recommend that physicians should pay attention to these associations, as early menarche can be a potential indicator of metabolic disorders and female-specific cancer and cause death in women.

Contribution of specific ceramides to obesity-associated metabolic diseases

Ceramides are a heterogeneous group of bioactive membrane sphingolipids that play specialized regulatory roles in cellular metabolism depending on their characteristic fatty acyl chain lengths and subcellular distribution. As obesity progresses, certain ceramide molecular species accumulate in metabolic tissues and cause cell-type-specific lipotoxic reactions that disrupt metabolic homeostasis and lead to the development of cardiometabolic diseases. Several mechanisms for ceramide action have been inferred from studies in vitro, but only recently have we begun to better understand the acyl chain length specificity of ceramide-mediated signaling in the context of physiology and disease in vivo. New discoveries show that specific ceramides affect various metabolic pathways and that global or tissue-specific reduction in selected ceramide pools in obese rodents is sufficient to improve metabolic health. Here, we review the tissue-specific regulation and functions of ceramides in obesity, thus highlighting the emerging concept of selectively inhibiting production or action of ceramides with specific acyl chain lengths as novel therapeutic strategies to ameliorate obesity-associated diseases.

Bifidobacterium animalis subsp. lactis A6 Enhances Fatty Acid β-Oxidation of Adipose Tissue to Ameliorate the Development of Obesity in Mice

Fatty acid β-oxidation (FAO) is confirmed to be impaired in obesity, especially in adipose tissues. We previously proved that Bifidobacterium animalis subsp. lactis A6 (BAA6) had protective effects against diet-induced obesity. However, whether BAA6 enhances FAO to ameliorate the development of obesity has not been explored. After being fed with high-fat diet (HFD) for 9 weeks, male C57BL/6J mice were fed HFD or BAA6 for 8 weeks. In vitro study was carried out using 3T3-L1 adipocytes to determine the effect of BAA6 culture supernatant (BAA6-CM). Here, we showed that administration of BAA6 to mice fed with HFD decreased body weight gain (by 5.03 g) and significantly up-regulated FAO in epididymal adipose tissues. In parallel, FAO in 3T3-L1 cells was increased after BAA6-CM treatment. Acetate was identified as a constituent of BAA6-CM that showed a similar effect to BAA6-CM. Furthermore, acetate treatment activated the GPR43-PPARα signaling, thereby promoting FAO in 3T3-L1 cells. The levels of acetate were also elevated in serum and feces (by 1.92- and 2.27-fold) of HFD-fed mice following BAA6 administration. The expression levels of GPR43 and PPARα were increased by 55.45% and 69.84% after BAA6 supplement in the epididymal fat of mice. Together, these data reveal that BAA6 promotes FAO of adipose tissues through the GPR43-PPARα signaling, mainly by increasing acetate levels, leading to alleviating the development of obesity.

Assessment of Heterozygosity and Genome-Wide Analysis of Heterozygosity Regions in Two Duroc Pig Populations

Heterozygosity can effectively reflect the diverse models of population structure and demographic history. However, the genomic distribution of heterozygotes and the correlation between regions of heterozygosity (runs of heterozygosity, ROHet) and phenotypes are largely understudied in livestock. The objective of this study was to identify ROHet in the Duroc pig genome, and investigate the relationships between ROHet and eight important economic traits. Here, we genotyped 3,770 American Duroc (S21) and 2,096 Canadian Duroc (S22) pigs using 50 K single nucleotide polymorphism array to analyze heterozygosity. A total of 145,010 and 84,396 ROHets were characterized for S21 and S22 populations, respectively. ROHet segments were mostly enriched in 1–2 Mb length classification (75.48% in S21 and 72.25% in S22). The average genome length covered by ROHet was 66.53 ± 12.20 Mb in S21 and 73.32 ± 13.77 Mb in S22 pigs. Additionally, we detected 20 and 13 ROHet islands in S21 and S22 pigs. Genes in these genomic regions were mainly involved in the biological processes of immunity and reproduction. Finally, the genome-wide ROHet-phenotypes association analysis revealed that 130 ROHets of S21 and 84 ROHets of S22 were significantly associated with eight economic traits. Among the candidate genes in the significant ROHet regions, 16 genes related to growth, metabolism, and meat quality were considered as candidate genes for important economic traits of pigs. This work preliminarily explores the effect of heterozygosity-rich regions in the pig genome on production performance and provides new insights for subsequent research on pig genetic improvement.

LEAP-2 Counteracts Ghrelin-Induced Food Intake in a Nutrient, Growth Hormone and Age Independent Manner

Data gleaned recently shows that ghrelin, a stomach derived peptide, and liver-expressed-antimicrobial peptide 2 (LEAP-2) play opposite roles on food intake. However, the data available with LEAP-2 in relation to in vivo studies are still very scanty and some key questions regarding the interplay among ghrelin and LEAP-2 remain to be answered. In this work, using rats and mice, we study fasting-induced food intake as well as testing the effect of diet exposure, e.g., standard diet and high fat diet, in terms of ghrelin-induced food intake. The anorexigenic effect of LEAP-2 on fasting induced food intake appears to be dependent on energy stores, being more evident in ob/ob than in wild type mice and also in animals exposed to high fat diet. On the other hand, LEAP-2 administration markedly inhibited ghrelin-induced food intake in lean, obese (ob/ob and DIO) mice, aged rats and GH-deficient dwarf rats. In contrast, the inhibitory effect on glucose levels can only be observed in some specific experimental models indicating that the mechanisms involved are likely to be quite different. Taken together from these data, LEAP-2 emerged as a potential candidate to be therapeutically useful in obesity.

AgRP/NPY and POMC neurons in the arcuate nucleus and their potential role in treatment of obesity

Obesity is a major health crisis affecting over a third of the global population. This multifactorial disease is regulated via interoceptive neural circuits in the brain, whose alteration results in excessive body weight. Certain central neuronal populations in the brain are recognised as crucial nodes in energy homeostasis; in particular, the hypothalamic arcuate nucleus (ARC) region contains two peptide microcircuits that control energy balance with antagonistic functions: agouti-related peptide/neuropeptide-Y (AgRP/NPY) signals hunger and stimulates food intake; and pro-opiomelanocortin (POMC) signals satiety and reduces food intake. These neuronal peptides levels react to energy status and integrate signals from peripheral ghrelin, leptin, and insulin to regulate feeding and energy expenditure. To manage obesity comprehensively, it is crucial to understand cellular and molecular mechanisms of information processing in ARC neurons, since these regulate energy homeostasis. Importantly, a specific strategy focusing on ARC circuits needs to be devised to assist in treating obese patients and maintaining weight loss with minimal or no side effects. The aim of this review is to elucidate the recent developments in the study of AgRP-, NPY- and POMC-producing neurons, specific to their role in controlling metabolism. The impact of ghrelin, leptin, and insulin signalling via action of these neurons is also surveyed, since they also impact energy balance through this route. Lastly, we present key proteins, targeted genes, compounds, drugs, and therapies that actively work via these neurons and could potentially be used as therapeutic targets for treating obesity conditions.

Effects of serine palmitoyltransferase inhibition by myriocin in ad libitum-fed and nutrient-restricted ewes

The fungal isolate myriocin inhibits serine palmitoyltransferase and de novo ceramide synthesis in rodents; however, the effects of myriocin on ceramide concentrations and metabolism have not been previously investigated in ruminants. In our study, 12 non-lactating crossbred ewes received an intravenous bolus of myriocin (0, 0.1, 0.3, or 1.0 mg/kg/body weight [BW]; CON, LOW, MOD, or HIGH) every 48 h for 17 d. Ewes consumed a high-energy diet from day 1 to 14 and were nutrient-restricted (straw only) from day 15 to 17. Blood was collected preprandial and at 1, 6, and 12 h relative to bolus and nutrient restriction. Tissues were collected following euthanasia on day 17. Plasma was analyzed for free fatty acids (FFAs), glucose, and insulin. Plasma and tissue ceramides were quantified using mass spectrometry. HIGH selectively decreased metabolizable energy intake, BW, and plasma insulin, and increased plasma FFA (Dose, P < 0.05). Myriocin linearly decreased plasma very-long-chain (VLC) ceramide and dihydroceramide (DHCer) by day 13 (Linear, P < 0.05). During nutrient restriction, fold-change in FFA was lower with increasing dose (P < 0.05). Nutrient restriction increased plasma C16:0-Cer, an effect suppressed by MOD and HIGH (Dose × Time, P < 0.05). Myriocin linearly decreased most ceramide and DHCer species in the liver and omental and mesenteric adipose, VLC ceramide and DHCer in the pancreas, and C18:0-Cer in skeletal muscle and subcutaneous adipose tissue (Linear, P ≤ 0.05). We conclude that the intravenous delivery of 0.3 mg of myriocin/kg of BW/48 h decreases circulating and tissue ceramide without modifying energy intake in ruminants.

Role of hypothalamic de novo ceramides synthesis in obesity and associated metabolic disorders

Background

Sphingolipid-mediated signalling pathways are described as important players in the normal functioning of neurons and nonneuronal cells in the central nervous system (CNS).

Scope of review

This review aims to show role of de novo ceramide synthesis in the CNS in controling key physiological processes, including food intake, energy expenditure, and thermogenesis. The corollary is a condition that leads to a dysfunction in ceramide metabolism in these central regions that can have major consequences on the physiological regulation of energy balance.

Major conclusions

Excessive hypothalamic de novo ceramide synthesis has been shown to result in the establishment of central insulin resistance, endoplasmic reticulum stress, and inflammation. Additionally, excessive hypothalamic de novo ceramide synthesis has also been associated with changes in the activity of the autonomic nervous system. Such dysregulation of hypothalamic de novo ceramide synthesis forms the key starting point for the initiation of pathophysiological conditions such as obesity – which may or may not be associated with type 2 diabetes.

The Role of Fatty Acids in Ceramide Pathways and Their Influence on Hypothalamic Regulation of Energy Balance: A Systematic Review

Obesity is a global health issue for which no major effective treatments have been well established. High-fat diet consumption is closely related to the development of obesity because it negatively modulates the hypothalamic control of food intake due to metaflammation and lipotoxicity. The use of animal models, such as rodents, in conjunction with in vitro models of hypothalamic cells, can enhance the understanding of hypothalamic functions related to the control of energy balance, thereby providing knowledge about the impact of diet on the hypothalamus, in addition to targets for the development of new drugs that can be used in humans to decrease body weight. Recently, sphingolipids were described as having a lipotoxic effect in peripheral tissues and the central nervous system. Specifically, lipid overload, mainly from long-chain saturated fatty acids, such as palmitate, leads to excessive ceramide levels that can be sensed by the hypothalamus, triggering the dysregulation of energy balance control. However, no systematic review has been undertaken regarding studies of sphingolipids, particularly ceramide and sphingosine-1-phosphate (S1P), the hypothalamus, and obesity. This review confirms that ceramides are associated with hypothalamic dysfunction in response to metaflammation, endoplasmic reticulum (ER) stress, and lipotoxicity, leading to insulin/leptin resistance. However, in contrast to ceramide, S1P appears to be a central satiety factor in the hypothalamus. Thus, our work describes current evidence related to sphingolipids and their role in hypothalamic energy balance control. Hypothetically, the manipulation of sphingolipid levels could be useful in enabling clinicians to treat obesity, particularly by decreasing ceramide levels and the inflammation/endoplasmic reticulum stress induced in response to overfeeding with saturated fatty acids.

Sensing of nutrients by CPT1C controls SAC1 activity to regulate AMPA receptor trafficking

Carnitine palmitoyltransferase 1C (CPT1C) is a sensor of malonyl-CoA and is located in the ER of neurons. AMPA receptors (AMPARs) mediate fast excitatory neurotransmission in the brain and play a key role in synaptic plasticity. In the present study, we demonstrate across different metabolic stress conditions that modulate malonyl-CoA levels in cortical neurons that CPT1C regulates the trafficking of the major AMPAR subunit, GluA1, through the phosphatidyl-inositol-4-phosphate (PI(4)P) phosphatase SAC1. In normal conditions, CPT1C down-regulates SAC1 catalytic activity, allowing efficient GluA1 trafficking to the plasma membrane. However, under low malonyl-CoA levels, such as during glucose depletion, CPT1C-dependent inhibition of SAC1 is released, facilitating SAC1’s translocation to ER-TGN contact sites to decrease TGN PI(4)P pools and trigger GluA1 retention at the TGN. Results reveal that GluA1 trafficking is regulated by CPT1C sensing of malonyl-CoA and provide the first report of a SAC1 inhibitor. Moreover, they shed light on how nutrients can affect synaptic function and cognition.

Carnitine palmitoyltransferase 1C negatively regulates the endocannabinoid hydrolase ABHD6 in mice, depending on nutritional status

Background and Purpose

The enzyme α/β‐hydrolase domain containing 6 (ABHD6), a new member of the endocannabinoid system, is a promising therapeutic target against neuronal‐related diseases. However, how ABHD6 activity is regulated is not known. ABHD6 coexists in protein complexes with the brain‐specific carnitine palmitoyltransferase 1C (CPT1C). CPT1C is involved in neuro‐metabolic functions, depending on brain malonyl–CoA levels. Our aim was to study CPT1C–ABHD6 interaction and determine whether CPT1C is a key regulator of ABHD6 activity depending on nutritional status.

Experimental Approach

Co‐immunoprecipitation and FRET assays were used to explore ABHD6 interaction with CPT1C or modified malonyl–CoA‐insensitive or C‐terminal truncated CPT1C forms. Cannabinoid CB₁ receptor‐mediated signalling was investigated by determining cAMP levels. A novel highly sensitive fluorescent method was optimized to measure ABHD6 activity in non‐neuronal and neuronal cells and in brain tissues from wild‐type (WT) and CPT1C–KO mice.

Key Results

CPT1C interacted with ABHD6 and negatively regulated its hydrolase activity, thereby regulating 2‐AG downstream signalling. Accordingly, brain tissues of CPT1C–KO mice showed increased ABHD6 activity. CPT1C malonyl–CoA sensing was key to the regulatory role on ABHD6 activity and CB₁ receptor signalling. Fasting, which attenuates brain malonyl–CoA, significantly increased ABHD6 activity in hypothalamus from WT, but not CPT1C–KO, mice.

Conclusions and Implications

Our finding that negative regulation of ABHD6 activity, particularly in the hypothalamus, is sensitive to nutritional status throws new light on the characterization and the importance of the proteins involved as potential targets against diseases affecting the CNS.

Ageing alters the lipid sensing process in the hypothalamus of Wistar rats. Effect of food restriction

INTRODUCTION

Lipids regulate a wide range of biological processes. The mechanisms by which fatty acids (FA) and its metabolites influence the hypothalamic regulation of energy homeostasis have been highly studied. However, the effect of ageing and food restriction (FR) on this process is unknown.

METHODS

Herein, we analyzed the gene expression, protein and phosphorylation levels of hypothalamic enzymes and transcription factors related to lipid metabolism. Experiments were performed in male Wistar rats of 3-, 8- and 24-month-old Wistar rats fed ad libitum (AL), as ageing model. Besides, 5- and 21-month-old rats were subjected to a moderate FR protocol (equivalent to ≈ 80% of normal food intake) for three months before the sacrifice.

RESULTS

Aged Wistar rats showed a situation of chronic lipid excess as a result of an increase in de novo FA synthesis and FA levels that reach the brain, contributing likely to the development of central leptin and insulin resistance. We observe a hypothalamic downregulation of AMP-activated protein kinase (AMPK) and stearoyl-CoA desaturase (SCD1) and an increase of carnitine palmitoyltransferase-1c (CPT1c) expression.

DISCUSSION

Our results suggest an impairment in the physiological lipid sensing system of aged Wistar rats, which would alter the balance of the intracellular mobilization and trafficking of lipids between the mitochondria and the Endoplasmic Reticulum (ER) in the hypothalamus, leading probably to the development of neurolipotoxicity in aged rats. Lastly, FR can only partially restore this imbalance.Schematic representation of the fate of LCFA-CoA in the hypothalamus of young and old rats. Blood circulating LCFAs in young Wistar rats reach the hypothalamus, where they are esterified to LCFA-CoA. Into glial cells or neurons, LCFA-CoA are driven to mitochondria (CPT1a) or ER (CPT1c) where could be desaturated by SDC1 and, thereby, converted into structural and signaling unsaturated lipids as oleic acid, related with neuronal myelinization and differentiation. However, the excess of LCFA that reach to the hypothalamus in old animals, could generate an increase in LCFA-CoA, which together with an increase in CPT1c levels, could favor the capture of LCFA-CoA to the ER. The decrease in the levels of SCD1 in old rats would decrease FA unsaturation degree that could trigger lipotoxicity process and neurodegeneration, both related to the development of neurodegenerative diseases linked to age.

Central Ceramide Signaling Mediates Obesity-Induced Precocious Puberty

Childhood obesity, especially in girls, is frequently bound to earlier puberty, which is linked to higher disease burden later in life. The mechanisms underlying this association remain elusive. Here we show that brain ceramides participate in the control of female puberty and contribute to its alteration in early-onset obesity in rats. Postnatal overweight caused earlier puberty and increased hypothalamic ceramide content, while pharmacological activation of ceramide synthesis mimicked the pubertal advancement caused by obesity, specifically in females. Conversely, central blockade of de novo ceramide synthesis delayed puberty and prevented the effects of the puberty-activating signal, kisspeptin. This phenomenon seemingly involves a circuit encompassing the paraventricular nucleus (PVN) and ovarian sympathetic innervation. Early-onset obesity enhanced PVN expression of SPTLC1, a key enzyme for ceramide synthesis, and advanced the maturation of the ovarian noradrenergic system. In turn, obesity-induced pubertal precocity was reversed by virogenetic suppression of SPTLC1 in the PVN. Our data unveil a pathway, linking kisspeptin, PVN ceramides, and sympathetic ovarian innervation, as key for obesity-induced pubertal precocity.

The return of malonyl-CoA to the brain: Cognition and other stories

Nutrients, hormones and the energy sensor AMP-activated protein kinase (AMPK) tightly regulate the intracellular levels of the metabolic intermediary malonyl-CoA, which is a precursor of fatty acid synthesis and a negative regulator of fatty acid oxidation. In the brain, the involvement of malonyl-CoA in the control of food intake and energy homeostasis has been known for decades. However, recent data uncover a new role in cognition and brain development. The sensing of malonyl-CoA by carnitine palmitoyltransferase 1 (CPT1) proteins regulates a variety of functions, such as the fate of neuronal stem cell precursors, the motility of lysosomes in developing axons, the trafficking of glutamate receptors to the neuron surface (necessary for proper synaptic function) and the metabolic coupling between astrocytes and neurons. We discuss the relevance of those recent findings evidencing how nutrients and metabolic disorders impact cognition. We also enumerate all nutritional and hormonal conditions that are known to regulate malonyl-CoA levels in the brain, reflect on protein malonylation as a new post-translational modification, and give a reasoned vision of the opportunities and challenges that future research in the field could address.

Ceramides affect alcohol consumption and depressive-like and anxiety-like behavior in a brain region- and ceramide species-specific way in male mice

Depression and alcohol dependence are associated with increased plasma ceramide concentrations in humans. Pharmacological increase in C16 ceramide concentrations in the dorsal hippocampus (DH) induced a depressive-like phenotype in naïve mice. However, the effects of C16 ceramide on alcohol consumption and anxiety-like behavior as well as the behavioral effects of other ceramide species are yet unknown. Therefore, we investigated whether repeated infusion of ceramides with different fatty acid chain lengths (C8, C16, and C20) into the DH and the basolateral amygdala (BLA) alter alcohol consumption, emotional behavior, and tissue monoamine levels. Our results revealed that C16, but not C8 and C20, ceramide altered alcohol drinking and emotional behavior in a brain region-specific way without altering tissue noradrenaline, dopamine, and serotonin levels in the prefrontal cortex, ventral striatum, and dorsal mesencephalon. In more detail, C16 ceramide increased alcohol consumption when infused into the BLA, but not when infused into the DH. Furthermore, C16 ceramide induced a depressive-like phenotype when infused into the DH, but a predominantly anxiogenic-like phenotype (in a non-social, but not a social context) when infused into the BLA. In turn, alcohol drinking normalized C16 ceramide-induced depressive-like and anxiogenic-like phenotypes. This study demonstrates a complex ceramide species-specific and brain region-specific modulation of alcohol consumption and emotional behavior in mice and provides the framework for future studies investigating the involvement of distinct ceramide species in the regulation of emotional behavior.

Effects of High-Fat Diet and Maternal Binge-Like Alcohol Consumption and Their Influence on Cocaine Response in Female Mice Offspring

BACKGROUND

Prenatal alcohol exposure is a leading cause of neurobehavioral and neurocognitive deficits collectively known as fetal alcohol spectrum disorders, including eating disorders and increased risk for substance abuse as very common issues. In this context, the present study aimed to assess the interaction between prenatal and lactation alcohol exposure (PLAE) and a high-fat diet (HFD) during childhood and adolescence.

METHODS

Pregnant C57BL/6 mice underwent a procedure for alcohol binge drinking during gestation and lactation periods. Subsequently, PLAE female offspring were fed with an HFD for 8 weeks, and thereafter, nutrition-related parameters as well as their response to cocaine were assessed.

RESULTS

In our model, feeding young females with an HFD increased their triglyceride blood levels but did not induce overweight compared with those fed with a standard diet. Moreover, PLAE affected how females responded to the fatty diet as they consumed less food than water-exposed offspring, consistent with a lower gain of body weight. HFD increased the psychostimulant effects of cocaine. Surprisingly, PLAE reduced the locomotor responses to cocaine without modifying cocaine-induced reward. Moreover, PLAE prevented the striatal overexpression of cannabinoid 1 receptors induced by an HFD and induced an alteration of myelin damage biomarker in the prefrontal cortex, an effect that was mitigated by an HFD-based feeding.

CONCLUSION

Therefore, in female offspring, some effects triggered by one of these factors, PLAE or an HFD, were blunted by the other, suggesting a close interaction between the involved mechanisms.

AMPK-Dependent Mechanisms but Not Hypothalamic Lipid Signaling Mediates GH-Secretory Responses to GHRH and Ghrelin

GH (growth hormone) secretion/action is modulated by alterations in energy homeostasis, such as malnutrition and obesity. Recent data have uncovered the mechanism by which hypothalamic neurons sense nutrient bioavailability, with a relevant contribution of AMPK (AMP-activated protein kinase) and mTOR (mammalian Target of Rapamycin), as sensors of cellular energy status. However, whether central AMPK-mediated lipid signaling and mTOR participate in the regulation of pituitary GH secretion remains unexplored. We provide herein evidence for the involvement of hypothalamic AMPK signaling, but not hypothalamic lipid metabolism or CPT-1 (carnitine palmitoyltransferase I) activity, in the regulation of GH stimulatory responses to the two major elicitors of GH release in vivo, namely GHRH (growth hormone–releasing hormone) and ghrelin. This effect appeared to be GH-specific, as blocking of hypothalamic AMPK failed to influence GnRH (gonadotropin-releasing hormone)-induced LH (luteinizing hormone) secretion. Additionally, central mTOR inactivation did not alter GH responses to GHRH or ghrelin, nor this blockade affected LH responses to GnRH in vivo. In sum, we document here for the first time the indispensable and specific role of preserved central AMPK, but not mTOR, signaling, through a non-canonical lipid signaling pathway, for proper GH responses to GHRH and ghrelin in vivo.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

CPT1A-mediated Fat Oxidation, Mechanisms, and Therapeutic Potential

Energy homeostasis during fasting or prolonged exercise depends on mitochondrial fatty acid oxidation (FAO). This pathway is crucial in many tissues with high energy demand and its disruption results in inborn FAO deficiencies. More than 15 FAO genetic defects have been currently described, and pathological variants described in circumpolar populations provide insights into its critical role in metabolism. The use of fatty acids as energy requires more than 2 dozen enzymes and transport proteins, which are involved in the activation and transport of fatty acids into the mitochondria. As the key rate-limiting enzyme of FAO, carnitine palmitoyltransferase I (CPT1) regulates FAO and facilitates adaptation to the environment, both in health and in disease, including cancer. The CPT1 family of proteins contains 3 isoforms: CPT1A, CPT1B, and CPT1C. This review focuses on CPT1A, the liver isoform that catalyzes the rate-limiting step of converting acyl-coenzyme As into acyl-carnitines, which can then cross membranes to get into the mitochondria. The regulation of CPT1A is complex and has several layers that involve genetic, epigenetic, physiological, and nutritional modulators. It is ubiquitously expressed in the body and associated with dire consequences linked with genetic mutations, metabolic disorders, and cancers. This makes CPT1A an attractive target for therapeutic interventions. This review discusses our current understanding of CPT1A expression, its role in heath and disease, and the potential for therapeutic opportunities targeting this enzyme.

CPT1C in the ventromedial nucleus of the hypothalamus is necessary for brown fat thermogenesis activation in obesity

OBJECTIVE

Carnitine palmitoyltransferase 1C (CPT1C) is implicated in central regulation of energy homeostasis. Our aim was to investigate whether CPT1C in the ventromedial nucleus of the hypothalamus (VMH) is involved in the activation of brown adipose tissue (BAT) thermogenesis in the early stages of diet-induced obesity.

METHODS

CPT1C KO and wild type (WT) mice were exposed to short-term high-fat (HF) diet feeding or to intracerebroventricular leptin administration and BAT thermogenesis activation was evaluated. Body weight, adiposity, food intake, and leptinemia were also assayed.

RESULTS

Under 7 days of HF diet, WT mice showed a maximum activation peak of BAT thermogenesis that counteracted obesity development, whereas this activation was impaired in CPT1C KO mice. KO animals evidenced higher body weight, adiposity, hyperleptinemia, ER stress, and disrupted hypothalamic leptin signaling. Leptin-induced BAT thermogenesis was abolished in KO mice. These results indicate an earlier onset leptin resistance in CPT1C KO mice. Since AMPK in the VMH is crucial in the regulation of BAT thermogenesis, we analyzed if CPT1C was a downstream factor of this pathway. Genetic inactivation of AMPK within the VMH was unable to induce BAT thermogenesis and body weight loss in KO mice, indicating that CPT1C is likely downstream AMPK in the central mechanism modulating thermogenesis within the VMH. Quite opposite, the expression of CPT1C in the VMH restored the phenotype.

CONCLUSION

CPT1C is necessary for the activation of BAT thermogenesis driven by leptin, HF diet exposure, and AMPK inhibition within the VMH. This study underscores the importance of CPT1C in the activation of BAT thermogenesis to counteract diet-induced obesity.

Estradiol Regulates Energy Balance by Ameliorating Hypothalamic Ceramide-Induced ER Stress

Compelling evidence has shown that, besides its putative effect on the regulation of the gonadal axis, estradiol (E2) exerts a dichotomic effect on the hypothalamus to regulate food intake and energy expenditure. The anorectic effect of E2 is mainly mediated by its action on the arcuate nucleus (ARC), whereas its effects on brown adipose tissue (BAT) thermogenesis occur in the ventromedial nucleus (VMH). Here, we demonstrate that central E2 decreases hypothalamic ceramide levels and endoplasmic reticulum (ER) stress. Pharmacological or genetic blockade of ceramide synthesis and amelioration of ER stress selectively occurring in the VMH recapitulate the effect of E2, leading to increased BAT thermogenesis, weight loss, and metabolic improvement. These findings demonstrate that E2 regulation of ceramide-induced hypothalamic lipotoxicity and ER stress is an important determinant of energy balance, suggesting that dysregulation of this mechanism may underlie some changes in energy homeostasis seen in females.

Ghrelin Causes a Decline in GABA Release by Reducing Fatty Acid Oxidation in Cortex

Lipid metabolism, specifically fatty acid oxidation (FAO) mediated by carnitine palmitoyltransferase (CPT) 1A, has been described to be an important actor of ghrelin action in hypothalamus. However, it is not known whether CPT1A and FAO mediate the effect of ghrelin on the cortex. Here, we show that ghrelin produces a differential effect on CPT1 activity and γ-aminobutyric acid (GABA) metabolism in the hypothalamus and cortex of mice. In the hypothalamus, ghrelin enhances CPT1A activity while GABA transaminase (GABAT) activity, a key enzyme in GABA shunt metabolism, is unaltered. However, in cortex CPT1A activity and GABAT activity are reduced after ghrelin treatment. Furthermore, in primary cortical neurons, ghrelin reduces GABA release through a CPT1A reduction. By using CPT1A floxed mice, we have observed that genetic ablation of CPT1A recapitulates the effect of ghrelin on GABA release in cortical neurons, inducing reductions in mitochondrial oxygen consumption, cell content of citrate and α-ketoglutarate, and GABA shunt enzyme activity. Taken together, these observations indicate that ghrelin-induced changes in CPT1A activity modulate mitochondrial function, yielding changes in GABA metabolism. This evidence suggests that the action of ghrelin on GABA release is region specific within the brain, providing a basis for differential effects of ghrelin in the central nervous system.

Palmitic acid triggers inflammatory responses in N42 cultured hypothalamic cells partially via ceramide synthesis but not via TLR4

A high-fat diet induces hypothalamic inflammation in rodents which, in turn, contributes to the development of obesity by eliciting both insulin and leptin resistance. However, the mechanism by which long-chain saturated fatty acids trigger inflammation is still contentious. To elucidate this mechanism, the effect of fatty acids on the expression of the pro-inflammatory cytokines IL-6 and TNFα was investigated in the mHypoE-N42 hypothalamic cell line (N42). N42 cells were treated with lauric acid (LA) and palmitic acid (PA). PA challenge was carried out in the presence of either a TLR4 inhibitor, a ceramide synthesis inhibitor (L-cycloserine), oleic acid (OA) or eicosapentaenoic acid (EPA). Intracellular ceramide accumulation was quantified using LC-ESI-MS/MS. PA but not LA upregulated IL-6 and TNFα. L-cycloserine, OA and EPA all counteracted PA-induced intracellular ceramide accumulation leading to a downregulation of IL-6 and TNFα. However, a TLR4 inhibitor failed to inhibit PA-induced upregulation of pro-inflammatory cytokines.In conclusion, PA induced the expression of IL-6 and TNFα in N42 neuronal cells independently of TLR4 but, partially, via ceramide synthesis with OA and EPA being anti-inflammatory by decreasing PA-induced intracellular ceramide build-up. Thus, ceramide accumulation represents one on the mechanisms by which PA induces inflammation in neurons.

Genetic Targeting of GRP78 in the VMH Improves Obesity Independently of Food Intake

Recent data have demonstrated that the hypothalamic GRP78/BiP (glucose regulated protein 78 kDa/binding immunoglobulin protein) modulates brown adipose tissue (BAT) thermogenesis by acting downstream on AMP-activated protein kinase (AMPK). Herein, we aimed to investigate whether genetic over-expression of GRP78 in the ventromedial nucleus of the hypothalamus (VMH: a key site regulating thermogenesis) could ameliorate very high fat diet (vHFD)-induced obesity. Our data showed that stereotaxic treatment with adenoviruses harboring GRP78 in the VMH reduced hypothalamic endoplasmic reticulum ER stress and reversed vHFD-induced obesity. Herein, we also demonstrated that this body weight decrease was more likely associated with an increased BAT thermogenesis and browning of white adipose tissue (WAT) than to anorexia. Overall, these results indicate that the modulation of GRP78 in the VMH may be a target against obesity.

Evolutionary analysis of the carnitine- and choline acyltransferases suggests distinct evolution of CPT2 versus CPT1 and related variants

Carnitine/choline acyltransferases play diverse roles in energy metabolism and neuronal signalling. Our knowledge of their evolutionary relationships, important for functional understanding, is incomplete. Therefore, we aimed to determine the evolutionary relationships of these eukaryotic transferases. We performed extensive phylogenetic and intron position analyses. We found that mammalian intramitochondrial CPT2 is most closely related to cytosolic yeast carnitine transferases (Sc-YAT1 and 2), whereas the other members of the family are related to intraorganellar yeast Sc-CAT2. Therefore, the cytosolically active CPT1 more closely resembles intramitochondrial ancestors than CPT2. The choline acetyltransferase is closely related to carnitine acetyltransferase and shows lower evolutionary rates than long chain acyltransferases. In the CPT1 family several duplications occurred during animal radiation, leading to the isoforms CPT1A, CPT1B and CPT1C. In addition, we found five CPT1-like genes in Caenorhabditis elegans that strongly group to the CPT1 family. The long branch leading to mammalian brain isoform CPT1C suggests that either strong positive or relaxed evolution has taken place on this node. The presented evolutionary delineation of carnitine/choline acyltransferases adds to current knowledge on their functions and provides tangible leads for further experimental research.

CPT1C promotes human mesenchymal stem cells survival under glucose deprivation through the modulation of autophagy

Human mesenchymal stem cells (hMSCs) are widely used in regenerative medicine. In some applications, they must survive under low nutrient conditions engendered by avascularity. Strategies to improve hMSCs survival may be of high relevance in tissue engineering. Carnitine palmitoyltransferase 1 C (CPT1C) is a pseudoenzyme exclusively expressed in neurons and cancer cells. In the present study, we show that CPT1C is also expressed in hMSCs and protects them against glucose starvation, glycolysis inhibition, and oxygen/glucose deprivation. CPT1C overexpression in hMSCs did not increase fatty acid oxidation capacity, indicating that the role of CPT1C in these cells is different from that described in tumor cells. The increased survival of CPT1C-overexpressing hMSCs observed during glucose deficiency was found to be the result of autophagy enhancement, leading to a greater number of lipid droplets and increased intracellular ATP levels. In fact, inhibition of autophagy or lipolysis was observed to completely block the protective effects of CPT1C. Our results indicate that CPT1C-mediated autophagy enhancement in glucose deprivation conditions allows a greater availability of lipids to be used as fuel substrate for ATP generation, revealing a new role of CPT1C in stem cell adaptation to low nutrient environments.

The Homeostatic Force of Ghrelin

Ghrelin, a gastric-derived acylated peptide, regulates energy homeostasis by transmitting information about peripheral nutritional status to the brain, and is essential for protecting organisms against famine. Ghrelin operates brain circuits to regulate homeostatic and hedonic feeding. Recent research advances have shed new light on ghrelin's multifaceted roles in cellular homeostasis, which could maintain the internal environment and overcome metaflammation in metabolic organs. Here, we highlight our current understanding of the regulatory mechanisms of the ghrelin system in energy metabolism and cellular homeostasis and its clinical trials. Future studies of ghrelin will further elucidate how the stomach regulates systemic homeostasis.

Analyzing AMPK Function in the Hypothalamus

Hypothalamic AMPK plays a key role in the control of energy homeostasis by regulating energy intake and energy expenditure, particularly modulating brown adipose tissue (BAT) thermogenesis. The function of AMPK can be assayed by analyzing its phosphorylated protein levels in tissues, since AMPK is activated when it is phosphorylated at Thr-172. Here, we describe a method to obtain hypothalamic (nuclei-specific) protein extracts and the suitable conditions to assay AMPK phosphorylation by Western blotting.

Brain Ceramide Metabolism in the Control of Energy Balance

The regulation of energy balance by the central nervous system (CNS) is a key actor of energy homeostasis in mammals, and deregulations of the fine mechanisms of nutrient sensing in the brain could lead to several metabolic diseases such as obesity and type 2 diabetes (T2D). Indeed, while neuronal activity primarily relies on glucose (lactate, pyruvate), the brain expresses at high level enzymes responsible for the transport, utilization and storage of lipids. It has been demonstrated that discrete neuronal networks in the hypothalamus have the ability to detect variation of circulating long chain fatty acids (FA) to regulate food intake and peripheral glucose metabolism. During a chronic lipid excess situation, this physiological lipid sensing is impaired contributing to type 2 diabetes in predisposed subjects. Recently, different studies suggested that ceramides levels could be involved in the regulation of energy balance in both hypothalamic and extra-hypothalamic areas. Moreover, under lipotoxic conditions, these ceramides could play a role in the dysregulation of glucose homeostasis. In this review we aimed at describing the potential role of ceramides metabolism in the brain in the physiological and pathophysiological control of energy balance.

Molecular mechanisms of appetite and obesity: a role for brain AMPK

Feeding behaviour and energy storage are both crucial aspects of survival. Thus, it is of fundamental importance to understand the molecular mechanisms regulating these basic processes. The AMP-activated protein kinase (AMPK) has been revealed as one of the key molecules modulating energy homoeostasis. Indeed, AMPK appears to be essential for translating nutritional and energy requirements into generation of an adequate neuronal response, particularly in two areas of the brain, the hypothalamus and the hindbrain. Failure of this physiological response can lead to energy imbalance, ultimately with extreme consequences, such as leanness or obesity. Here, we will review the data that put brain AMPK in the spotlight as a regulator of appetite.

Hypothalamic Regulation of Liver and Muscle Nutrient Partitioning by Brain-Specific Carnitine Palmitoyltransferase 1C in Male Mice

Carnitine palmitoyltransferase (CPT) 1C, a brain-specific protein localized in the endoplasmic reticulum of neurons, is expressed in almost all brain regions. Based on global knockout (KO) models, CPT1C has demonstrated relevance in hippocampus-dependent spatial learning and in hypothalamic regulation of energy balance. Specifically, it has been shown that CPT1C is protective against high-fat diet-induced obesity (DIO), and that CPT1C KO mice show reduced peripheral fatty acid oxidation (FAO) during both fasting and DIO. However, the mechanisms mediating CPT1C-dependent regulation of energy homeostasis remain unclear. Here, we focus on the mechanistic understanding of hypothalamic CPT1C on the regulation of fuel selection in liver and muscle of male mice during energy deprivation situations, such as fasting. In CPT1C-deficient mice, modulation of the main hypothalamic energy sensors (5' adenosine monophosphate-activated protein kinase, Sirtuin 1, and mammalian target of rapamycin) was impaired and plasma catecholamine levels were decreased. Consequently, CPT1C-deficient mice presented defective fasting-induced FAO in liver, leading to higher triacylglycerol accumulation and lower glycogen levels. Moreover, muscle pyruvate dehydrogenase activity was increased, which was indicative of glycolysis enhancement. The respiratory quotient did not decrease in CPT1C KO mice after 48 hours of fasting, confirming a defective switch on fuel substrate selection under hypoglycemia. Phenotype reversion studies identified the mediobasal hypothalamus (MBH) as the main area mediating CPT1C effects on fuel selection. Overall, our data demonstrate that CPT1C in the MBH is necessary for proper hypothalamic sensing of a negative energy balance and fuel partitioning in liver and muscle.

The cellular and molecular bases of leptin and ghrelin resistance in obesity

Obesity, a major risk factor for the development of diabetes mellitus, cardiovascular diseases and certain types of cancer, arises from a chronic positive energy balance that is often due to unlimited access to food and an increasingly sedentary lifestyle on the background of a genetic and epigenetic vulnerability. Our understanding of the humoral and neuronal systems that mediate the control of energy homeostasis has improved dramatically in the past few decades. However, our ability to develop effective strategies to slow the current epidemic of obesity has been hampered, largely owing to the limited knowledge of the mechanisms underlying resistance to the action of metabolic hormones such as leptin and ghrelin. The development of resistance to leptin and ghrelin, hormones that are crucial for the neuroendocrine control of energy homeostasis, is a hallmark of obesity. Intensive research over the past several years has yielded tremendous progress in our understanding of the cellular pathways that disrupt the action of leptin and ghrelin. In this Review, we discuss the molecular mechanisms underpinning resistance to leptin and ghrelin and how they can be exploited as targets for pharmacological management of obesity.

Ceramide counteracts the effects of ghrelin on the metabolic control of food intake in rainbow trout

In mammals, ceramides are involved in the modulation of the orexigenic effects of ghrelin (GHRL). We previously demonstrated in rainbow trout that intracerebroventricular (ICV) treatment with ceramide (2.5 µg/100 g fish) resulted in an anorexigenic response, i.e. a response opposed to that described in mammals, where ceramide treatment is orexigenic. Therefore, we hypothesized that the putative interaction between GHRL and ceramide must be different in fish. Accordingly, in a first experiment, we observed that ceramide levels in the hypothalamus of rainbow trout did not change after ICV treatment with GHRL. In a second experiment, we assessed whether the effects of GHRL treatment on the regulation of food intake in rainbow trout changed in the presence of ceramide. Thus, we injected ICV GHRL and ceramide alone or in combination to evaluate in hypothalamus and hindbrain changes in parameters related to the metabolic control of food intake. The presence of ceramide generally counteracted the effects elicited by GHRL on fatty acid-sensing systems, the capacity of integrative sensors (AMPK, mTOR and SIRT-1), proteins involved in cellular signalling pathways (Akt and FoxO1) and neuropeptides involved in the regulation of food intake (AgRP, NPY, POMC and CART). The results are discussed in the context of regulation of food intake by metabolic and endocrine inputs.

Current Understanding of the Hypothalamic Ghrelin Pathways Inducing Appetite and Adiposity

Ghrelin is a multifaceted regulator of metabolism. Ghrelin regulates energy balance in the short term via induction of appetite and in the long term via increased body weight and adiposity. Recently, several central pathways modulating the metabolic actions of ghrelin were unmasked, and it was shown to act through different hypothalamic nuclei to induce feeding. Ghrelin also modulates glucose homeostasis, but the central mechanisms responsible for this action have not been studied in detail. Although ghrelin also acts through extrahypothalamic areas to promote feeding, this review specifically dissects hypothalamic control of ghrelin's orexigenic and adipogenic actions and presents current understanding of the intracellular ghrelin orexigenic pathways, including their dependence on other relevant systems implicated in energy balance.

Reduction of Hypothalamic Endoplasmic Reticulum Stress Activates Browning of White Fat and Ameliorates Obesity

The chaperone GRP78/BiP (glucose-regulated protein 78 kDa/binding immunoglobulin protein) modulates protein folding in reply to cellular insults that lead to endoplasmic reticulum (ER) stress. This study investigated the role of hypothalamic GRP78 on energy balance, with particular interest in thermogenesis and browning of white adipose tissue (WAT). For this purpose, we used diet-induced obese rats and rats administered thapsigargin, and by combining metabolic, histologic, physiologic, pharmacologic, thermographic, and molecular techniques, we studied the effect of genetic manipulation of hypothalamic GRP78. Our data showed that rats fed a high-fat diet or that were centrally administered thapsigargin displayed hypothalamic ER stress, whereas genetic overexpression of GRP78 specifically in the ventromedial nucleus of the hypothalamus was sufficient to alleviate ER stress and to revert the obese and metabolic phenotype. Those effects were independent of feeding and leptin but were related to increased thermogenic activation of brown adipose tissue and induction of browning in WAT and could be reversed by antagonism of β3 adrenergic receptors. This evidence indicates that modulation of hypothalamic GRP78 activity may be a potential strategy against obesity and associated comorbidities.

Lipid Processing in the Brain: A Key Regulator of Systemic Metabolism

Metabolic disorders, particularly aberrations in lipid homeostasis, such as obesity, type 2 diabetes mellitus, and hypertriglyceridemia often manifest together as the metabolic syndrome (MetS). Despite major advances in our understanding of the pathogenesis of these disorders, the prevalence of the MetS continues to rise. It is becoming increasingly apparent that intermediary metabolism within the central nervous system is a major contributor to the regulation of systemic metabolism. In particular, lipid metabolism within the brain is tightly regulated to maintain neuronal structure and function and may signal nutrient status to modulate metabolism in key peripheral tissues such as the liver. There is now a growing body of evidence to suggest that fatty acid (FA) sensing in hypothalamic neurons via accumulation of FAs or FA metabolites may signal nutritional sufficiency and may decrease hepatic glucose production, lipogenesis, and VLDL-TG secretion. In addition, recent studies have highlighted the existence of liver-related neurons that have the potential to direct such signals through parasympathetic and sympathetic nervous system activity. However, to date whether these liver-related neurons are FA sensitive remain to be determined. The findings discussed in this review underscore the importance of the autonomic nervous system in the regulation of systemic metabolism and highlight the need for further research to determine the key features of FA neurons, which may serve as novel therapeutic targets for the treatment of metabolic disorders.

Ceramides and mitochondrial fatty acid oxidation in obesity

Obesity is an epidemic, complex disease that is characterized by increased glucose, lipids, and low-grade inflammation in the circulation, among other factors. It creates the perfect scenario for the production of ceramide, the building block of the sphingolipid family of lipids, which is involved in metabolic disorders such as obesity, diabetes, and cardiovascular disease. In addition, obesity causes a decrease in fatty acid oxidation (FAO), which contributes to lipid accumulation within the cells, conferring more susceptibility to cell dysfunction. C16:0 ceramide, a specific ceramide species, has been identified recently as the principal mediator of obesity-derived insulin resistance, impaired fatty acid oxidation, and hepatic steatosis. In this review, we have sought to cover the importance of the ceramide species and their metabolism, the main ceramide signaling pathways in obesity, and the link between C16:0 ceramide, FAO, and obesity.-Fucho, R., Casals, N., Serra, D., Herrero, L. Ceramides and mitochondrial fatty acid oxidation in obesity.

Hypothalamic AMPK as a Regulator of Energy Homeostasis

Activated in energy depletion conditions, AMP-activated protein kinase (AMPK) acts as a cellular energy sensor and regulator in both central nervous system and peripheral organs. Hypothalamic AMPK restores energy balance by promoting feeding behavior to increase energy intake, increasing glucose production, and reducing thermogenesis to decrease energy output. Besides energy state, many hormones have been shown to act in concert with AMPK to mediate their anorexigenic and orexigenic central effects as well as thermogenic influences. Here we explore the factors that affect hypothalamic AMPK activity and give the underlying mechanisms for the role of central AMPK in energy homeostasis together with the physiological effects of hypothalamic AMPK on energy balance restoration.

Ceramides are involved in the regulation of food intake in rainbow trout (Oncorhynchus mykiss)

We hypothesize that ceramides are involved in the regulation of food intake in fish. Therefore, we assessed in rainbow trout (Oncorhynchus mykiss) the effects of intracerebroventricular treatment with C6:0 ceramide on food intake. In a second experiment, we assessed the effects in brain areas of ceramide treatment on neuropeptide expression, fatty acid-sensing systems, and cellular signaling pathways. Ceramide treatment induced a decrease in food intake, a response opposed to the orexigenic effect described in mammals, which can be related to enhanced mRNA abundance of cocaine and amphetamine-related transcript and proopiomelanocortin and decreased mRNA abundance of Agouti-related protein and neuropeptide Y. Fatty acid-sensing systems appear to be inactivated by ceramide treatment. The mRNA abundance of integrative sensors AMPK and sirtuin 1, and the phosphorylation status of cellular signaling pathways dependent on protein kinase B, AMPK, mammalian target of rapamycin (mTOR), and forkhead box protein O1 (FoxO1) are generally activated by ceramide treatment. However, there are differences between hypothalamus and hindbrain in the phosphorylation status of AMPK (decreased in hypothalamus and increased in hindbrain), mTOR (decreased in hypothalamus and increased in hindbrain), and FoxO1 (increased in hypothalamus and decreased in hindbrain) to ceramide treatment. The results suggest that ceramides are involved in the regulation of food intake in rainbow trout through mechanisms comparable to those characterized previously in mammals in some cases.

Hypothalamic AMPK: a canonical regulator of whole-body energy balance

AMP-activated protein kinase (AMPK) has a major role in the modulation of energy balance. AMPK is activated in conditions of low energy, increasing energy production and reducing energy consumption. The AMPK pathway is a canonical route regulating energy homeostasis by integrating peripheral signals, such as hormones and metabolites, with neuronal networks. Current evidence has implicated AMPK in the hypothalamus and hindbrain with feeding, brown adipose tissue thermogenesis and browning of white adipose tissue, through modulation of the sympathetic nervous system, as well as glucose homeostasis. Interestingly, several potential antiobesity and/or antidiabetic agents, some of which are currently in clinical use such as metformin and liraglutide, exert some of their actions by acting on AMPK. Furthermore, the orexigenic and weight-gain effects of commonly used antipsychotic drugs are also mediated by hypothalamic AMPK. Overall, this evidence suggests that hypothalamic AMPK signalling is an interesting target for drug development, but is this approach feasible? In this Review we discuss the current understanding of hypothalamic AMPK and its role in the central regulation of energy balance and metabolism.