Identification of the gene altered in Berardinelli-Seip congenital lipodystrophy on chromosome 11q13

Metabolic and other morbid complications in congenital generalized lipodystrophy type 4

Morbidity and mortality rates in patients with autosomal recessive, congenital generalized lipodystrophy type 4 (CGL4), an ultra-rare disorder, remain unclear. We report on 30 females and 16 males from 10 countries with biallelic null variants in CAVIN1 gene (mean age, 12 years; range, 2 months to 41 years). Hypertriglyceridemia was seen in 79% (34/43), hepatic steatosis in 82% (27/33) but diabetes mellitus in only 21% (8/44). Myopathy with elevated serum creatine kinase levels (346-3325 IU/L) affected all of them (38/38). 39% had scoliosis (10/26) and 57% had atlantoaxial instability (8/14). Cardiac arrhythmias were detected in 57% (20/35) and 46% had ventricular tachycardia (16/35). Congenital pyloric stenosis was diagnosed in 39% (18/46), 9 had esophageal dysmotility and 19 had intestinal dysmotility. Four patients suffered from intestinal perforations. Seven patients died at mean age of 17 years (range: 2 months to 39 years). The cause of death in four patients was cardiac arrhythmia and sudden death, while others died of prematurity, gastrointestinal perforation, and infected foot ulcers leading to sepsis. Our study highlights high prevalence of myopathy, metabolic abnormalities, cardiac, and gastrointestinal problems in patients with CGL4. CGL4 patients are at high risk of early death mainly caused by cardiac arrhythmias.

Identification of genetic suppressors for a BSCL2 lipodystrophy pathogenic variant in Caenorhabditis elegans

Seipin (BSCL2), a conserved endoplasmic reticulum protein, plays a critical role in lipid droplet (LD) biogenesis and in regulating LD morphology, pathogenic variants of which are associated with Berardinelli-Seip congenital generalized lipodystrophy type 2 (BSCL2). To model BSCL2 disease, we generated an orthologous BSCL2 variant, seip-1(A185P), in Caenorhabditis elegans. In this study, we conducted an unbiased chemical mutagenesis screen to identify genetic suppressors that restore embryonic viability in the seip-1(A185P) mutant background. A total of five suppressor lines were isolated and recovered from the screen. The defective phenotypes of seip-1(A185P), including embryonic lethality and impaired eggshell formation, were significantly suppressed in each suppressor line. Two of the five suppressor lines also alleviated the enlarged LDs in the oocytes. We then mapped a suppressor candidate gene, lmbr-1, which is an ortholog of human limb development membrane protein 1 (LMBR1). The CRISPR/Cas9 edited lmbr-1 suppressor alleles, lmbr-1(S647F) and lmbr-1(P314L), both significantly suppressed embryonic lethality and defective eggshell formation in the seip-1(A185P) background. The newly identified suppressor lines offer valuable insights into potential genetic interactors and pathways that may regulate seipin in the lipodystrophy model.

Impact of Seipin in cholesterol mediated lipid droplet maturation; status of endoplasmic reticulum stress and lipophagy

The global prevalence of nonalcoholic fatty liver disease (NAFLD) defined by the increased number of lipid droplets (LDs) in hepatocytes, have risen continuously in parallel with the obesity. LDs and related proteins are known to affect cellular metabolism and signaling. Seipin, one of the most important LD-related proteins, plays a critical role in LD biogenesis. Although the role of adipose tissue-specific Seipin silencing is known, hepatocyte-specific silencing upon cholesterol-mediated lipid accumulation has not been investigated. In our study, we investigated the effect of Seipin on endoplasmic reticulum (ER) stress and lipophagy in cholesterol accumulated mouse hepatocyte cells. In this direction, cholesterol accumulation was induced by cholesterol-containing liposome, while Seipin mRNA and protein levels were reduced by siRNA. Our findings show that cholesterol containing liposome administration in hepatocytes increases both Seipin protein and number of large LDs. However Seipin silencing reduced the increase of cholesterol mediated large LDs and Glucose-regulated protein 78 (GRP78) mRNA. Additionally, lysosome-LD colocalization increased only in cells treated with cholesterol containing liposome, while the siRNA against Seipin did not lead any significant difference. According to our findings, we hypothesize that Seipin silencing in hepatocytes reduced cholesterol mediated LD maturation as well as GRP78 levels, but not lipophagy.

ER-organelle contacts: A signaling hub for neurological diseases

Neuronal health is closely linked to the homeostasis of intracellular organelles, and organelle dysfunction affects the pathological progression of neurological diseases. In contrast to isolated cellular compartments, a growing number of studies have found that organelles are largely interdependent structures capable of communicating through membrane contact sites (MCSs). MCSs have been identified as key pathways mediating inter-organelle communication crosstalk in neurons, and their alterations have been linked to neurological disease pathology. The endoplasmic reticulum (ER) is a membrane-bound organelle capable of forming an extensive network of pools and tubules with important physiological functions within neurons. There are multiple MCSs between the ER and other organelles and the plasma membrane (PM), which regulate a variety of cellular processes. In this review, we focus on ER-organelle MCSs and their role in a variety of neurological diseases. We compared the biological effects between different tethering proteins and the effects of their respective disease counterparts. We also discuss how altered ER-organelle contacts may affect disease pathogenesis. Therefore, understanding the molecular mechanisms of ER-organelle MCSs in neuronal homeostasis will lay the foundation for the development of new therapies targeting ER-organelle contacts.

Adipose transplantation improves metabolism and atherosclerosis but not perivascular adipose tissue abnormality or vascular dysfunction in lipodystrophic Seipin/Apoe null mice

Adipose dysfunction in lipodystrophic SEIPIN deficiency is associated with multiple metabolic disorders and increased risks of developing cardiovascular diseases, such as atherosclerosis, cardiac hypertrophy, and heart failure. Recently, adipose transplantation has been found to correct adipose dysfunction and metabolic disorders in lipodystrophic Seipin knockout mice; however, whether adipose transplantation could improve lipodystrophy-associated cardiovascular consequences is still unclear. Here, we aimed to explore the effects of adipose transplantation on lipodystrophy-associated metabolic cardiovascular diseases in Seipin knockout mice crossed into atherosclerosis-prone apolipoprotein E (Apoe) knockout background. At 2 months of age, lipodystrophic Seipin/Apoe double knockout mice and nonlipodystrophic Apoe knockout controls were subjected to adipose transplantation or sham operation. Seven months later, mice were euthanized. Our data showed that although adipose transplantation had no significant impact on endogenous adipose atrophy or gene expression, it remarkably increased plasma leptin but not adiponectin concentration in Seipin/Apoe double knockout mice. This led to significantly reduced hyperlipidemia, hepatic steatosis, and insulin resistance in Seipin/Apoe double knockout mice. Consequently, atherosclerosis burden, intraplaque macrophage infiltration, and aortic inflammatory gene expression were all attenuated in Seipin/Apoe double knockout mice with adipose transplantation. However, adipocyte morphology, macrophage infiltration, or fibrosis of the perivascular adipose tissue was not altered in Seipin/Apoe double knockout mice with adipose transplantation, followed by no significant improvement of vasoconstriction or relaxation. In conclusion, we demonstrate that adipose transplantation could alleviate lipodystrophy-associated metabolic disorders and atherosclerosis but has an almost null impact on perivascular adipose abnormality or vascular dysfunction in lipodystrophic Seipin/Apoe double knockout mice.NEW & NOTEWORTHY Adipose transplantation (AT) reverses multiply metabolic derangements in lipodystrophy, but whether it could improve lipodystrophy-related cardiovascular consequences is unknown. Here, using Seipin/Apoe double knockout mice as a lipodystrophy disease model, we showed that AT partially restored adipose functionality, which translated into significantly reduced atherosclerosis. However, AT was incapable of reversing perivascular adipose abnormality or vascular dysfunction. The current study provides preliminary experimental evidence on the therapeutic potential of AT on lipodystrophy-related metabolic cardiovascular diseases.

CIDE proteins and their regulatory mechanisms in lipid droplet fusion and growth

The cell death-inducing DFF45-like effector (CIDE) proteins, including Cidea, Cideb, and Cidec/Fsp27, regulate various aspects of lipid homeostasis, including lipid storage, lipolysis, and lipid secretion. This review focuses on the physiological roles of CIDE proteins based on studies on knockout mouse models and human patients bearing CIDE mutations. The primary cellular function of CIDE proteins is to localize to lipid droplets (LDs) and to control LD fusion and growth across different cell types. We propose a four-step process of LD fusion, characterized by (a) the recruitment of CIDE proteins to the LD surface and CIDE movement, (b) the enrichment and condensate formation of CIDE proteins to form LD fusion plates at LD-LD contact sites, (c) lipid transfer through lipid-permeable passageways within the fusion plates, and (d) the completion of LD fusion. Lastly, we outline CIDE-interacting proteins as regulatory factors, as well as their contribution in LD fusion.

Molecular dynamics simulations of intracellular lipid droplets: a new tool in the toolbox

Lipid droplets (LDs) are ubiquitous intracellular organelles with a central role in multiple lipid metabolic pathways. However, identifying correlations between their structural properties and their biological activity has proved challenging, owing to their unique physicochemical properties as compared with other cellular membranes. In recent years, molecular dynamics (MD) simulations, a computational methodology allowing the accurate description of molecular assemblies down to their individual components, have been demonstrated to be a useful and powerful approach for studying LD structural and dynamical properties. In this short review, we attempt to highlight, as comprehensively as possible, how MD simulations have contributed to our current understanding of multiple molecular mechanisms involved in LD biology.

A unifying mechanism for seipin-mediated lipid droplet formation

Lipid droplets (LDs) are dynamic organelles essential for cellular lipid homeostasis. Assembly of LDs occurs in the endoplasmic reticulum (ER), and the conserved ER membrane protein seipin emerged as a key player in this process. Here, we review recent advances provided by structural, biochemical, and in silico analysis that revealed mechanistic insights into the molecular role of the seipin complexes and led to an updated model for LD biogenesis. We further discuss how other ER components cooperate with seipin during LD biogenesis. Understanding the molecular mechanisms underlying seipin-mediated LD assembly is important to uncover the fundamental aspects of lipid homeostasis and organelle biogenesis and to provide hints on the pathogenesis of lipid storage disorders.

Emerging functions of the mitochondria-ER-lipid droplet three-way junction in coordinating lipid transfer, metabolism, and storage in cells

Over the past two decades, we have witnessed a growing appreciation for the importance of membrane contact sites (CS) in facilitating direct communication between organelles. CS are tiny regions where the membranes of two organelles meet but do not fuse and allow the transfer of metabolites between organelles, playing crucial roles in the coordination of cellular metabolic activities. The significant advancements in imaging techniques and molecular and cell biology research have revealed that CS are more complex than what originally thought, and as they are extremely dynamic, they can remodel their shape, composition, and functions in accordance with metabolic and environmental changes and can occur between more than two organelles. Here, we describe how recent studies led to the identification of a three-way mitochondria-ER-lipid droplet CS and discuss the emerging functions of these contacts in maintaining lipid storage, homeostasis, and balance. We also summarize the properties and functions of key protein components localized at the mitochondria-ER-lipid droplet interface, with a special focus on lipid transfer proteins. Understanding tripartite CS is essential for unraveling the complexities of inter-organelle communication and cooperation within cells.

GLP-1 receptor agonist improves metabolic disease in a pre-clinical model of lipodystrophy

Aims

Individuals with lipodystrophies typically suffer from metabolic disease linked to adipose tissue dysfunction including lipoatrophic diabetes. In the most severe forms of lipodystrophy, congenital generalised lipodystrophy, adipose tissue may be almost entirely absent. Better therapies for affected individuals are urgently needed. Here we performed the first detailed investigation of the effects of a glucagon like peptide-1 receptor (GLP-1R) agonist in lipoatrophic diabetes, using mice with generalised lipodystrophy.

Methods

Lipodystrophic insulin resistant and glucose intolerant seipin knockout mice were treated with the GLP-1R agonist liraglutide either acutely preceding analyses of insulin and glucose tolerance or chronically prior to metabolic phenotyping and ex vivo studies.

Results

Acute liraglutide treatment significantly improved insulin, glucose and pyruvate tolerance. Once daily injection of seipin knockout mice with liraglutide for 14 days led to significant improvements in hepatomegaly associated with steatosis and reduced markers of liver fibrosis. Moreover, liraglutide enhanced insulin secretion in response to glucose challenge with concomitantly improved glucose control.

Conclusions

GLP-1R agonist liraglutide significantly improved lipoatrophic diabetes and hepatic steatosis in mice with generalised lipodystrophy. This provides important insights regarding the benefits of GLP-1R agonists for treating lipodystrophy, informing more widespread use to improve the health of individuals with this condition.

Lipotoxicity as a therapeutic target in obesity and diabetic cardiomyopathy

Unhealthy sources of fats, ultra-processed foods with added sugars, and a sedentary lifestyle make humans more susceptible to developing overweight and obesity. While lipids constitute an integral component of the organism, excessive and abnormal lipid accumulation that exceeds the storage capacity of lipid droplets disrupts the intracellular composition of fatty acids and results in the release of deleterious lipid species, thereby giving rise to a pathological state termed lipotoxicity. This condition induces endoplasmic reticulum stress, mitochondrial dysfunction, inflammatory responses, and cell death. Recent advances in omics technologies and analytical methodologies and clinical research have provided novel insights into the mechanisms of lipotoxicity, including gut dysbiosis, epigenetic and epitranscriptomic modifications, dysfunction of lipid droplets, post-translational modifications, and altered membrane lipid composition. In this review, we discuss the recent knowledge on the mechanisms underlying the development of lipotoxicity and lipotoxic cardiometabolic disease in obesity, with a particular focus on lipotoxic and diabetic cardiomyopathy.

A metabolically controlled contact site between vacuoles and lipid droplets in yeast

The lipid droplet (LD) organization proteins Ldo16 and Ldo45 affect multiple aspects of LD biology in yeast. They are linked to the LD biogenesis machinery seipin, and their loss causes defects in LD positioning, protein targeting, and breakdown. However, their molecular roles remained enigmatic. Here, we report that Ldo16/45 form a tether complex with Vac8 to create vacuole lipid droplet (vCLIP) contact sites, which can form in the absence of seipin. The phosphatidylinositol transfer protein (PITP) Pdr16 is a further vCLIP-resident recruited specifically by Ldo45. While only an LD subpopulation is engaged in vCLIPs at glucose-replete conditions, nutrient deprivation results in vCLIP expansion, and vCLIP defects impair lipophagy upon prolonged starvation. In summary, Ldo16/45 are multifunctional proteins that control the formation of a metabolically regulated contact site. Our studies suggest a link between LD biogenesis and breakdown and contribute to a deeper understanding of how lipid homeostasis is maintained during metabolic challenges.

Rab18 maintains homeostasis of subcutaneous adipose tissue to prevent obesity-induced metabolic disorders

Metabolically healthy obesity refers to obese individuals who do not develop metabolic disorders. These people store fat in subcutaneous adipose tissue (SAT) rather than in visceral adipose tissue (VAT). However, the molecules participating in this specific scenario remain elusive. Rab18, a lipid droplet (LD)-associated protein, mediates the contact between the endoplasmic reticulum (ER) and LDs to facilitate LD growth and maturation. In the present study, we show that the protein level of Rab18 is specifically upregulated in the SAT of obese people and mice. Rab18 adipocyte-specific knockout (Rab18 AKO) mice had a decreased volume ratio of SAT to VAT compared with wildtype mice. When subjected to high-fat diet (HFD), Rab18 AKO mice had increased ER stress and inflammation, reduced adiponectin, and decreased triacylglycerol (TAG) accumulation in SAT. In contrast, TAG accumulation in VAT, brown adipose tissue (BAT) or liver of Rab18 AKO mice had a moderate increase without ER stress stimulation. Rab18 AKO mice developed insulin resistance and systematic inflammation. Rab18 AKO mice maintained body temperature in response to acute and chronic cold induction with a thermogenic SAT, similar to the counterpart mice. Furthermore, Rab18-deficient 3T3-L1 adipocytes were more prone to palmitate-induced ER stress, indicating the involvement of Rab18 in alleviating lipid toxicity. Rab18 AKO mice provide a good animal model to investigate metabolic disorders such as impaired SAT. In conclusion, our studies reveal that Rab18 is a key and specific regulator that maintains the proper functions of SAT by alleviating lipid-induced ER stress.

Stay in touch with the endoplasmic reticulum

The endoplasmic reticulum (ER), which is composed of a continuous network of tubules and sheets, forms the most widely distributed membrane system in eukaryotic cells. As a result, it engages a variety of organelles by establishing membrane contact sites (MCSs). These contacts regulate organelle positioning and remodeling, including fusion and fission, facilitate precise lipid exchange, and couple vital signaling events. Here, we systematically review recent advances and converging themes on ER-involved organellar contact. The molecular basis, cellular influence, and potential physiological functions for ER/nuclear envelope contacts with mitochondria, Golgi, endosomes, lysosomes, lipid droplets, autophagosomes, and plasma membrane are summarized.

Molecular characterization of the grass carp bscl2 gene and its expression response to lipid accumulation, nutritional status, insulin and glucagon

Bscl2 plays a role in lipid metabolism of mammals, however its role in teleost fish remains unclear. Using the grass carp (Ctenopharyngodon idella) as a model, the bscl2 gene was isolated from the brain and characterized. Thereafter, the tissue distribution of the gene was examined, before expression was analyzed as a function of fasting, refeeding, oral glucose administration and overfeeding. In addition, bscl2 mRNA levels were evaluated in grass carp primary hepatocytes treated with glucagon, insulin, oleic acid, and glucose. Results showed that the cloned bscl2 gene was 1341 bp, encoding 446 amino acids, and was highly expressed in the brain, heart, and gonad. Following oral glucose administration, bscl2 expression increased. Expression of bscl2 decreased in fasted fish but increased following refeeding. Overfeeding, which resulted in elevated lipid accumulation, also stimulated bscl2 expression. In primary hepatocytes, bscl2 levels were increased by glucose, oleic acid, and insulin treatments, and reduced by glucagon treatment. These data suggest that bscl2 may play an important role in nutrient metabolism in teleost fish.

CLSTN3B enhances adipocyte lipid droplet structure and function via endoplasmic reticulum contact

Interorganelle contacts facilitate material exchanges and sustain the structural and functional integrity of organelles. Lipid droplets (LDs) of adipocytes are responsible for energy storage and mobilization responding to body needs. LD biogenesis defects compromise the lipid-storing capacity of adipocytes, resulting in ectopic lipid deposition and metabolic disorders, yet how the uniquely large LDs in adipocytes attain structural and functional maturation is incompletely understood. Here we show that the mammalian adipocyte-specific protein CLSTN3B is crucial for adipocyte LD maturation. CLSTN3B employs an arginine-rich segment to promote extensive contact and hemifusion-like structure formation between the endoplasmic reticulum (ER) and LD, allowing ER-to-LD phospholipid diffusion during LD expansion. CLSTN3B ablation results in reduced LD surface phospholipid density, increased turnover of LD-surface proteins, and impaired LD functions. Our results establish the central role of CLSTN3B in the adipocyte-specific LD maturation pathway that enhances lipid storage and maintenance of metabolic health under caloric overload.

Body Fat Distribution Contributes to Defining the Relationship between Insulin Resistance and Obesity in Human Diseases

The risk for metabolic and cardiovascular complications of obesity is defined by body fat distribution rather than global adiposity. Unlike subcutaneous fat, visceral fat (including hepatic steatosis) reflects insulin resistance and predicts type 2 diabetes and cardiovascular disease. In humans, available evidence indicates that the ability to store triglycerides in the subcutaneous adipose tissue reflects enhanced insulin sensitivity. Prospective studies document an association between larger subcutaneous fat mass at baseline and reduced incidence of impaired glucose tolerance. Case-control studies reveal an association between genetic predisposition to insulin resistance and a lower amount of subcutaneous adipose tissue. Human peroxisome proliferator-activated receptorgamma (PPAR-γ) promotes subcutaneous adipocyte differentiation and subcutaneous fat deposition, improving insulin resistance and reducing visceral fat. Thiazolidinediones reproduce the effects of PPAR-γ activation and therefore increase the amount of subcutaneous fat while enhancing insulin sensitivity and reducing visceral fat. Partial or virtually complete lack of adipose tissue (lipodystrophy) is associated with insulin resistance and its clinical manifestations, including essential hypertension, hypertriglyceridemia, reduced HDL-c, type 2 diabetes, cardiovascular disease, and kidney disease. Patients with Prader Willi syndrome manifest severe subcutaneous obesity without insulin resistance. The impaired ability to accumulate fat in the subcutaneous adipose tissue may be due to deficient triglyceride synthesis, inadequate formation of lipid droplets, or defective adipocyte differentiation. Lean and obese humans develop insulin resistance when the capacity to store fat in the subcutaneous adipose tissue is exhausted and deposition of triglycerides is no longer attainable at that location. Existing adipocytes become large and reflect the presence of insulin resistance.

From scarcity to solutions: Therapeutic strategies to restore adipose tissue functionality in rare disorders of lipodystrophy

AIMS

Lipodystrophy is a rare disorder characterised by abnormal or deficient adipose tissue formation and distribution. It poses significant challenges to affected individuals, including the development of severe metabolic complications like diabetes and fatty liver disease. These conditions are often chronic, debilitating and life-threatening, with limited treatment options and a lack of specialised expertise. This review aims to raise awareness of lipodystrophy disorders and highlights therapeutic strategies to restore adipose tissue functionality.

METHODS

Extensive research has been conducted, including both historical and recent advances. We have examined and summarised the literature to provide an overview of potential strategies to restore adipose tissue functionality and treat/reverse metabolic complications in lipodystrophy disorders.

RESULTS

A wealth of basic and clinical research has investigated various therapeutic approaches for lipodystrophy. These include ground-breaking methods such as adipose tissue transplantation, innovative leptin replacement therapy, targeted inhibition of lipolysis and cutting-edge gene and cell therapies. Each approach shows great potential in addressing the complex challenges posed by lipodystrophy.

CONCLUSIONS

Lipodystrophy disorders require urgent attention and innovative treatments. Through rigorous basic and clinical research, several promising therapeutic strategies have emerged that could restore adipose tissue functionality and reverse the severe metabolic complications associated with this condition. However, further research and collaboration between academics, clinicians, patient advocacy groups and pharmaceutical companies will be crucial in transforming these scientific breakthroughs into effective and viable treatment options for individuals and families affected by lipodystrophy. Fostering such interdisciplinary partnerships could pave the way for a brighter future for those battling this debilitating disorder.

Concept of lipid droplet biogenesis

Lipid droplets (LD) are functionally conserved fat storage organelles found in all cell types. LDs have a unique structure comprising of a hydrophobic core of neutral lipids (fat), triacylglycerol (TAG) and cholesterol esters (CE) surrounded by a phospholipid monolayer. LD surface is decorated by a multitude of proteins and enzymes rendering this compartment functional. Accumulating evidence suggests that LDs originate from discrete ER-subdomains, demarcated by the lipodystrophy protein seipin, however, the mechanisms of which are not well understood. LD biogenesis factors together with biophysical properties of the ER membrane orchestrate spatiotemporal regulation of LD nucleation and growth at specific ER subdomains in response to metabolic cues. Defects in LD formation manifests in several human pathologies, including obesity, lipodystrophy, ectopic fat accumulation, and insulin resistance. Here, we review recent advances in understanding the molecular events during initial stages of eukaryotic LD assembly and discuss the critical role of factors that ensure fidelity of this process.

A murine model of BSCL2-associated Celia's encephalopathy

Celia's encephalopathy or progressive encephalopathy with/without lipodystrophy is a neurodegenerative disease with a fatal prognosis in childhood. It is generally caused by the c.985C > T variant in the BSCL2 gene, leading to the skipping of exon 7 and resulting in an aberrant seipin protein (Celia-seipin). To precisely define the temporal evolution and the mechanisms involved in neurodegeneration, lipodystrophy and fatty liver in Celia's encephalopathy, our group has generated the first global knock-in murine model for the aberrant human transcript of BSCL2 (Bscl2Celia/Celia) using a strategy based on the Cre/loxP recombination system. In order to carry out a characterization at the neurological, adipose tissue and hepatic level, behavioral studies, brain PET, metabolic, histological and molecular studies were performed. Around 12% of homozygous and 5.4% of heterozygous knock-in mice showed severe neurological symptoms early in life, and their life expectancy was dramatically reduced. Severe generalized lipodystrophy and mild hepatic steatosis were present in these affected animals, while serum triglycerides and glucose metabolism were normal, with no insulin resistance. Furthermore, the study revealed a reduction in brain glucose uptake, along with patchy loss of Purkinje cells and the presence of intranuclear inclusions in cerebellar cortex cells. Homozygous, non-severely-affected knock-in mice showed a decrease in locomotor activity and greater anxiety compared with their wild type littermates. Bscl2Celia/Celia is the first murine model of Celia's encephalopathy which partially recapitulates the phenotype and severe neurodegenerative picture suffered by these patients. This model will provide a helpful tool to investigate both the progressive encephalopathy with/without lipodystrophy and congenital generalized lipodystrophy.

Identification of Genetic Suppressors for a Berardinelli-Seip Congenital Generalized Lipodystrophy Type 2 (BSCL2) Pathogenic Variant in C. elegans

Maintaining the metabolic homeostasis of fatty acids is crucial for human health. Excess fatty acids are stored in lipid droplets (LDs), the primary energy reservoir that helps regulate fat and lipid homeostasis in nearly all cell types. Seipin (BSCL2), a conserved endoplasmic reticulum protein, plays a critical role in LD biogenesis and regulating LD morphology. Pathogenic variants of seipin are associated with multiple human genetic diseases, including Berardinelli-Seip Congenital Generalized Lipodystrophy Type 2 (BSCL2). However, the cellular and molecular mechanisms by which dysfunctional seipin leads to these diseases remain unclear. To model BSCL2 disease, we generated an orthologous BSCL2 pathogenic variant seip-1(A185P) using CRISPR/Cas9 genome editing in Caenorhabditis elegans . This variant led to severe developmental and cellular defects, including embryonic lethality, impaired eggshell formation, and abnormally enlarged LDs. We set out to identify genetic determinants that could suppress these defective phenotypes in the seip-1(A185P) mutant background. To this end, we conducted an unbiased chemical mutagenesis screen to identify genetic suppressors that restore embryonic viability in the seip-1(A185P) mutant background. A total of five suppressor lines were isolated and recovered from the screen. The defective phenotypes of seip-1(A185P) , including embryonic lethality and impaired eggshell formation, were significantly suppressed in each suppressor line. Two of the five suppressor lines also alleviated the enlarged LDs in the oocytes. We then mapped a suppressor candidate gene, R05D3.2 (renamed as lmbr-1 ), which is an ortholog of human LMBR1 (limb development membrane protein 1). The CRISPR/Cas9 edited lmbr-1 suppressor alleles, lmbr-1(Ser647Phe) and lmbr-1(Pro314Leu) , both significantly suppressed embryonic lethality and defective eggshell formation in the seip-1(A185P) background. The newly identified suppressor lines offer valuable insights into potential genetic interactors and pathways that may regulate seipin in the lipodystrophy model.

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.

Beyond energy provider: multifunction of lipid droplets in embryonic development

Since the discovery, lipid droplets (LDs) have been recognized to be sites of cellular energy reserves, providing energy when necessary to sustain cellular life activities. Many studies have reported large numbers of LDs in eggs and early embryos from insects to mammals. The questions of how LDs are formed, what role they play, and what their significance is for embryonic development have been attracting the attention of researchers. Studies in recent years have revealed that in addition to providing energy for embryonic development, LDs in eggs and embryos also function to resist lipotoxicity, resist oxidative stress, inhibit bacterial infection, and provide lipid and membrane components for embryonic development. Removal of LDs from fertilized eggs or early embryos artificially leads to embryonic developmental arrest and defects. This paper reviews recent studies to explain the role and effect mechanisms of LDs in the embryonic development of several species and the genes involved in the regulation. The review contributes to understanding the embryonic development mechanism and provides new insight for the diagnosis and treatment of diseases related to embryonic developmental abnormalities.

Regulation of lipid droplets and cholesterol metabolism in adrenal cortical cells

The adrenal gland is composed of two distinctly different endocrine moieties. The interior medulla consists of neuroendocrine chromaffin cells that secrete catecholamines like adrenaline and noradrenaline, while the exterior cortex consists of steroidogenic cortical cells that produce steroid hormones, such as mineralocorticoids (aldosterone), glucocorticoids (cortisone and cortisol) and androgens. Synthesis of steroid hormones in cortical cells requires substantial amounts of cholesterol, which is the common precursor for steroidogenesis. Cortical cells may acquire cholesterol from de novo synthesis and uptake from circulating low- and high-density lipoprotein particles (LDL and HDL). As cholesterol is part of the plasma membrane in all mammalian cells and an important regulator of membrane fluidity, cellular levels of free cholesterol are tightly regulated. To ensure a robust supply of cholesterol for steroidogenesis and to avoid cholesterol toxicity, cortical cells store large amounts of cholesterol as cholesteryl esters in intracellular lipid droplets. Cortical steroidogenesis relies on both mobilization of cholesterol from lipid droplets and constant uptake of circulating cholesterol to replenish lipid droplet stores. This chapter will describe mechanisms involved in cholesterol uptake, cholesteryl ester synthesis, lipid droplet formation, hydrolysis of stored cholesteryl esters, as well as their impact on steroidogenesis. Additionally, animal models and human diseases characterized by altered cortical cholesteryl ester storage, with or without abnormal steroidogenesis, will be discussed.

Mammalian lipid droplets: structural, pathological, immunological and anti-toxicological roles

Mammalian lipid droplets (LDs) are specialized cytosolic organelles consisting of a neutral lipid core surrounded by a membrane made up of a phospholipid monolayer and a specific population of proteins that varies according to the location and function of each LD. Over the past decade, there have been significant advances in the understanding of LD biogenesis and functions. LDs are now recognized as dynamic organelles that participate in many aspects of cellular homeostasis plus other vital functions. LD biogenesis is a complex, highly-regulated process with assembly occurring on the endoplasmic reticulum although aspects of the underpinning molecular mechanisms remain elusive. For example, it is unclear how many enzymes participate in the biosynthesis of the neutral lipid components of LDs and how this process is coordinated in response to different metabolic cues to promote or suppress LD formation and turnover. In addition to enzymes involved in the biosynthesis of neutral lipids, various scaffolding proteins play roles in coordinating LD formation. Despite their lack of ultrastructural diversity, LDs in different mammalian cell types are involved in a wide range of biological functions. These include roles in membrane homeostasis, regulation of hypoxia, neoplastic inflammatory responses, cellular oxidative status, lipid peroxidation, and protection against potentially toxic intracellular fatty acids and lipophilic xenobiotics. Herein, the roles of mammalian LDs and their associated proteins are reviewed with a particular focus on their roles in pathological, immunological and anti-toxicological processes.

Structure and function of lipid droplet assembly complexes

Cells store lipids as a reservoir of metabolic energy and membrane component precursors in organelles called lipid droplets (LDs). LD formation occurs in the endoplasmic reticulum (ER) at LD assembly complexes (LDAC), consisting of an oligomeric core of seipin and accessory proteins. LDACs determine the sites of LD formation and are required for this process to occur normally. Seipin oligomers form a cage-like structure in the membrane that may serve to facilitate the phase transition of neutral lipids in the membrane to form an oil droplet within the LDAC. Modeling suggests that, as the LD grows, seipin anchors it to the ER bilayer and conformational shifts of seipin transmembrane segments open the LDAC dome toward the cytoplasm, enabling the emerging LD to egress from the ER.

The metabolic effects of APOL1 in humans

Harboring apolipoprotein L1 (APOL1) variants coded by the G1 or G2 alleles of the APOL1 gene increases the risk for collapsing glomerulopathy, focal segmental glomerulosclerosis, albuminuria, chronic kidney disease, and accelerated kidney function decline towards end-stage kidney disease. However, most subjects carrying APOL1 variants do not develop the kidney phenotype unless a second clinical condition adds to the genotype, indicating that modifying factors modulate the genotype-phenotype correlation. Subjects with an APOL1 high-risk genotype are more likely to develop essential hypertension or obesity, suggesting that carriers of APOL1 risk variants experience more pronounced insulin resistance compared to noncarriers. Likewise, arterionephrosclerosis (the pathological correlate of hypertension-associated nephropathy) and glomerulomegaly take place among carriers of APOL1 risk variants, and these pathological changes are also present in conditions associated with insulin resistance, such as essential hypertension, aging, and diabetes. Insulin resistance may contribute to the clinical features associated with the APOL1 high-risk genotype. Unlike carriers of wild-type APOL1, bearers of APOL1 variants show impaired formation of lipid droplets, which may contribute to inducing insulin resistance. Nascent lipid droplets normally detach from the endoplasmic reticulum into the cytoplasm, although the proteins that enable this process remain to be fully defined. Wild-type APOL1 is located in the lipid droplet, whereas mutated APOL1 remains sited at the endoplasmic reticulum, suggesting that normal APOL1 may participate in lipid droplet biogenesis. The defective formation of lipid droplets is associated with insulin resistance, which in turn may modulate the clinical phenotype present in carriers of APOL1 risk variants.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

Glycerolipid Synthesis and Lipid Droplet Formation in the Endoplasmic Reticulum

More than 60 years ago, Eugene Kennedy and coworkers elucidated the endoplasmic reticulum (ER)-based pathways of glycerolipid synthesis, including the synthesis of phospholipids and triacylglycerols (TGs). The reactions of the Kennedy pathway were identified by studying the conversion of lipid intermediates and the isolation of biochemical enzymatic activities, but the molecular basis for most of these reactions was unknown. With recent progress in the cell biology, biochemistry, and structural biology in this area, we have a much more mechanistic understanding of this pathway and its reactions. In this review, we provide an overview of molecular aspects of glycerolipid synthesis, focusing on recent insights into the synthesis of TGs. Further, we go beyond the Kennedy pathway to describe the mechanisms for storage of TG in cytosolic lipid droplets and discuss how overwhelming these pathways leads to ER stress and cellular toxicity, as seen in diseases linked to lipid overload and obesity.

Dynamic Regulation of Lipid Droplet Biogenesis in Plant Cells and Proteins Involved in the Process

Lipid droplets (LDs) are ubiquitous, dynamic organelles found in almost all organisms, including animals, protists, plants and prokaryotes. The cell biology of LDs, especially biogenesis, has attracted increasing attention in recent decades because of their important role in cellular lipid metabolism and other newly identified processes. Emerging evidence suggests that LD biogenesis is a highly coordinated and stepwise process in animals and yeasts, occurring at specific sites of the endoplasmic reticulum (ER) that are defined by both evolutionarily conserved and organism- and cell type-specific LD lipids and proteins. In plants, understanding of the mechanistic details of LD formation is elusive as many questions remain. In some ways LD biogenesis differs between plants and animals. Several homologous proteins involved in the regulation of animal LD formation in plants have been identified. We try to describe how these proteins are synthesized, transported to the ER and specifically targeted to LD, and how these proteins participate in the regulation of LD biogenesis. Here, we review current work on the molecular processes that control LD formation in plant cells and highlight the proteins that govern this process, hoping to provide useful clues for future research.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

The Effect of BSCL2 Gene on Fat Deposition Traits in Pigs

BSCL2 encodes seipin, a transmembrane endoplasmic reticulum protein associated with lipodystrophy and severe metabolic complications, including diabetes and hepatic steatosis. In pigs, BSCL2 expression increases during adipocyte differentiation. In the present study, we identified significant gene variants associated with fat deposition (FD)-related processes based on subcutaneous fat tissue RNA-seq data. In the association study, to prove our hypothesis, three Polish pig breeds were included: Złotnicka White (ZW, n = 72), Polish Landrace (PL, n = 201), and Polish Large White (PLW, n = 169). Based on variant calling analysis and χ² tests, BSCL2 mutations showing significantly different genotype/allele distribution between high- and low-fat pigs were selected for a comprehensive association study. Four interesting BSCL2 variants (rs346079334, rs341493267, rs330154033, and rs81333153) belonging to downstream and missense mutations were investigated. Our study showed a significant decrease in minor allele frequency for two BSCL2 variants (rs346079334 and rs341493267) in PL pigs in 2020-2021. In ZW, BSCL2 mutations significantly affected loin and ham fats, meat redness, and growth performance traits, such as feed conversion and daily feed intake. Similar observations were noted for PLW and PL, where BSCL2 mutations influenced fat depositions and meat traits, such as loin eye area, loin mass and fat, carcass yield, and growth performance traits. Based on the observation in pigs, our study supports the theory that BSCL2 expressed in subcutaneous fat is involved in the FD process.

Identifying congenital generalized lipodystrophy using deep learning-DEEPLIPO

Congenital Generalized Lipodystrophy (CGL) is a rare autosomal recessive disease characterized by near complete absence of functional adipose tissue from birth. CGL diagnosis can be based on clinical data including acromegaloid features, acanthosis nigricans, reduction of total body fat, muscular hypertrophy, and protrusion of the umbilical scar. The identification and knowledge of CGL by the health care professionals is crucial once it is associated with severe and precocious cardiometabolic complications and poor outcome. Image processing by deep learning algorithms have been implemented in medicine and the application into routine clinical practice is feasible. Therefore, the aim of this study was to identify congenital generalized lipodystrophy phenotype using deep learning. A deep learning approach model using convolutional neural network was presented as a detailed experiment with evaluation steps undertaken to test the effectiveness. These experiments were based on CGL patient's photography database. The dataset consists of two main categories (training and testing) and three subcategories containing photos of patients with CGL, individuals with malnutrition and eutrophic individuals with athletic build. A total of 337 images of individuals of different ages, children and adults were carefully chosen from internet open access database and photographic records of stored images of medical records of a reference center for inherited lipodystrophies. For validation, the dataset was partitioned into four parts, keeping the same proportion of the three subcategories in each part. The fourfold cross-validation technique was applied, using 75% (3 parts) of the data as training and 25% (1 part) as a test. Following the technique, four tests were performed, changing the parts that were used as training and testing until each part was used exactly once as validation data. As a result, a mean accuracy, sensitivity, and specificity were obtained with values of [90.85 ± 2.20%], [90.63 ± 3.53%] and [91.41 ± 1.10%], respectively. In conclusion, this study presented for the first time a deep learning model able to identify congenital generalized lipodystrophy phenotype with excellent accuracy, sensitivity and specificity, possibly being a strategic tool for detecting this disease.

Seipin-still a mysterious protein?

Cells store excess energy in the form of lipid droplets (LDs), a specialized sub-compartment of the endoplasmic reticulum (ER) network. The lipodystrophy protein seipin is a key player in LD biogenesis and ER-LD contact site maintenance. Recent structural and in silico studies have started to shed light on the molecular function of seipin as a LD nucleator in early LD biogenesis, whilst new cell biological work implies a role for seipin in ER-mitochondria contact sites and calcium metabolism. In this minireview, I discuss recent insights into the molecular function of seipin.

Gene therapy restores adipose tissue and metabolic health in a pre-clinical mouse model of lipodystrophy

Congenital generalized lipodystrophy type 2 is a serious multisystem disorder with limited treatment options. It is caused by mutations affecting the BSCL2 gene, which encodes the protein seipin. Patients with congenital generalized lipodystrophy type 2 lack both metabolic and mechanical adipose tissue and develop severe metabolic complications including hepatic steatosis, lipoatrophic diabetes, and cardiovascular disease. Gene therapies are becoming viable treatments, helping to alleviate inherited and acquired human disorders. We aimed to determine whether gene therapy could offer an effective form of medical intervention for lipodystrophy. We examined whether systemic adeno-associated virus delivery of human BSCL2 could reverse metabolic disease in seipin knockout mice, where white adipose tissue is absent. We reveal that adeno-associated virus gene therapy targets adipose progenitor cells in vivo and substantially restores white adipose tissue development in adult seipin knockout mice. This resulted in both rapid and prolonged beneficial effects to metabolic health in this pre-clinical mouse model of congenital generalized lipodystrophy type 2. Hyperglycemia was normalized within 2 weeks post-treatment together with normalization of severe insulin resistance. We propose that gene therapy offers great potential as a therapeutic strategy to correct multiple metabolic complications in patients with congenital lipodystrophy.

G protein gamma subunit, a hidden master regulator of GPCR signaling

Heterotrimeric G proteins (αβγ subunits) that are activated by G protein-coupled receptors (GPCRs) mediate the biological responses of eukaryotic cells to extracellular signals. The α subunits and the tightly bound βγ subunit complex of G proteins have been extensively studied and shown to control the activity of effector molecules. In contrast, the potential roles of the large family of γ subunits have been less studied. In this review, we focus on present knowledge about these proteins. Induced loss of individual γ subunit types in animal and plant models result in strikingly distinct phenotypes indicating that γ subtypes play important and specific roles. Consistent with these findings, downregulation or upregulation of particular γ subunit types result in various types of cancers. Clues about the mechanistic basis of γ subunit function have emerged from imaging the dynamic behavior of G protein subunits in living cells. This shows that in the basal state, G proteins are not constrained to the plasma membrane but shuttle between membranes and on receptor activation βγ complexes translocate reversibly to internal membranes. The translocation kinetics of βγ complexes varies widely and is determined by the membrane affinity of the associated γ subtype. On translocating, some βγ complexes act on effectors in internal membranes. The variation in translocation kinetics determines differential sensitivity and adaptation of cells to external signals. Membrane affinity of γ subunits is thus a parsimonious and elegant mechanism that controls information flow to internal cell membranes while modulating signaling responses.

A motor neuron disease-associated mutation produces non-glycosylated Seipin that induces ER stress and apoptosis by inactivating SERCA2b

A causal relationship between endoplasmic reticulum (ER) stress and the development of neurodegenerative diseases remains controversial. Here, we focused on Seipinopathy, a dominant motor neuron disease, based on the finding that its causal gene product, Seipin, is a protein that spans the ER membrane twice. Gain-of-function mutations of Seipin produce non-glycosylated Seipin (ngSeipin), which was previously shown to induce ER stress and apoptosis at both cell and mouse levels albeit with no clarified mechanism. We found that aggregation-prone ngSeipin dominantly inactivated SERCA2b, the major calcium pump in the ER, and decreased the calcium concentration in the ER, leading to ER stress and apoptosis in human colorectal carcinoma-derived cells (HCT116). This inactivation required oligomerization of ngSeipin and direct interaction of the C-terminus of ngSeipin with SERCA2b, and was observed in Seipin-deficient neuroblastoma (SH-SY5Y) cells expressing ngSeipin at an endogenous protein level. Our results thus provide a new direction to the controversy noted above.

PI(3)P and DFCP1 regulate the biogenesis of lipid droplets

The biogenesis of lipid droplets (LDs), key organelles for cellular lipid storage and homeostasis, remains poorly understood. Seipin is essential to normal LD biogenesis but exactly how it regulates LD initiation remains to be elucidated. Our previous results suggested that seipin may bind anionic phospholipids such as PI(3)P. Here, we investigate whether PI(3)P is functionally linked to seipin and whether PI(3)P can also impact LD biogenesis. In seipin-deficient cells, there were enlarged PI(3)P puncta where its effector, DFCP1, also appeared to congregate. Reducing cellular PI(3)P partially rescued the defective LD initiation caused by seipin deficiency. Increasing PI(3)P impeded the lipidation of nascent LDs. We further demonstrated that DFCP1 localized to LDs and facilitated the efficient lipidation of nascent LDs. However, the normal function and localization of DFCP1 were disrupted when cellular PI(3)P homeostasis was perturbed. Our results thus identify PI(3)P as a novel regulator of LD initiation and suggest that PI(3)P may impact the biogenesis of LDs through DFCP1.

Causative links between ER stress and oxidative damage in a yeast model of human N88S seipinopathy

Seipin is encoded by the gene Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) and FLD1/SEI1 in yeast. The gain-of-function N88S mutation in the BSCL2 gene was identified in a cohort of autosomal dominant motor neuron diseases (MNDs) collectively known as seipinopathies. Previous work has shown that this mutation disrupts N-glycosylation, leading to the formation of inclusion bodies (IBs) and contributing to severe Endoplasmic Reticulum (ER) stress and cell death. In this work, we established a humanized yeast model of N88S seipinopathy that recapitulated the formation of IBs and activation of the unfolded protein response (UPR) observed in mammalian systems. Autophagy and the Hrd1-mediated endoplasmic reticulum-associated degradation (ERAD) were fully functional in cells expressing mutant homomers and WT-mutant heteromers of seipin, discarding the possibility that mutant seipin accumulate due to impaired protein quality control systems. Importantly, the N88S seipin form IBs that appear to induce changes in ER morphology, in association with Kar2 chaperone and the Hsp104 disaggregase. For the first time, we have determined that N88S homo-oligomers expressing cells present reduced viability, decreased antioxidant activity and increased oxidative damage associated with loss of mitochondrial membrane potential, higher reactive oxygen species (ROS) levels and lipid peroxidation. This was correlated with the activation of oxidative stress sensor Yap1. Moreover, activation of ERAD and UPR quality control mechanisms were essential for proper cell growth, and crucial to prevent excessive accumulation of ROS in cells expressing N88S homomers solely. Overall, this study provides new insights into the molecular underpinnings of these rare diseases and offers novel targets for potential pharmacological intervention.

The Arabidopsis Rab protein RABC1 affects stomatal development by regulating lipid droplet dynamics

Lipid droplets (LDs) are evolutionarily conserved organelles that serve as hubs of cellular lipid and energy metabolism in virtually all organisms. Mobilization of LDs is important in light-induced stomatal opening. However, whether and how LDs are involved in stomatal development remains unknown. We show here that Arabidopsis thaliana LIPID DROPLETS AND STOMATA 1 (LDS1)/RABC1 (At1g43890) encodes a member of the Rab GTPase family that is involved in regulating LD dynamics and stomatal morphogenesis. The expression of RABC1 is coordinated with the different phases of stomatal development. RABC1 targets to the surface of LDs in response to oleic acid application in a RABC1GEF1-dependent manner. RABC1 physically interacts with SEIPIN2/3, two orthologues of mammalian seipin, which function in the formation of LDs. Disruption of RABC1, RABC1GEF1, or SEIPIN2/3 resulted in aberrantly large LDs, severe defects in guard cell vacuole morphology, and stomatal function. In conclusion, these findings reveal an aspect of LD function and uncover a role for lipid metabolism in stomatal development in plants.

Lipodystrophy for the Diabetologist-What to Look For

PURPOSE OF REVIEW

Genetic or acquired lipodystrophies are characterized by selective loss of body fat along with predisposition towards metabolic complications of insulin resistance, such as diabetes mellitus, hypertriglyceridemia, hepatic steatosis, polycystic ovarian syndrome, and acanthosis nigricans. In this review, we discuss the various subtypes and when to suspect and how to diagnose lipodystrophy.

RECENT FINDINGS

The four major subtypes are autosomal recessive, congenital generalized lipodystrophy (CGL); acquired generalized lipodystrophy (AGL), mostly an autoimmune disorder; autosomal dominant or recessive familial partial lipodystrophy (FPLD); and acquired partial lipodystrophy (APL), an autoimmune disorder. Diagnosis of lipodystrophy is mainly based upon physical examination findings of loss of body fat and can be supported by body composition analysis by skinfold measurements, dual-energy x-ray absorptiometry, and whole-body magnetic resonance imaging. Confirmatory genetic testing is helpful in the proband and at-risk family members with suspected genetic lipodystrophies. The treatment is directed towards the specific comorbidities and metabolic complications, and there is no treatment to reverse body fat loss. Metreleptin should be considered as the first-line therapy for metabolic complications in patients with generalized lipodystrophy and for prevention of comorbidities in children. Metformin and insulin therapy are the best options for treating hyperglycemia and fibrates and/or fish oil for hypertriglyceridemia. Lipodystrophy should be suspected in lean and muscular subjects presenting with diabetes mellitus, hypertriglyceridemia, non-alcoholic fatty liver disease, polycystic ovarian syndrome, or amenorrhea. Diabetologists should be aware of lipodystrophies and consider genetic varieties as an important subtype of monogenic diabetes.

ORP5 and ORP8 orchestrate lipid droplet biogenesis and maintenance at ER-mitochondria contact sites

Lipid droplets (LDs) are the primary organelles of lipid storage, buffering energy fluctuations of the cell. They store neutral lipids in their core that is surrounded by a protein-decorated phospholipid monolayer. LDs arise from the endoplasmic reticulum (ER). The ER protein seipin, localizing at ER-LD junctions, controls LD nucleation and growth. However, how LD biogenesis is spatially and temporally coordinated remains elusive. Here, we show that the lipid transfer proteins ORP5 and ORP8 control LD biogenesis at mitochondria-associated ER membrane (MAM) subdomains, enriched in phosphatidic acid. We found that ORP5/8 regulates seipin recruitment to these MAM-LD contacts, and their loss impairs LD biogenesis. Importantly, the integrity of ER-mitochondria contact sites is crucial for ORP5/8 function in regulating seipin-mediated LD biogenesis. Our study uncovers an unprecedented ORP5/8 role in orchestrating LD biogenesis and maturation at MAMs and brings novel insights into the metabolic crosstalk between mitochondria, ER, and LDs at the membrane contact sites.

Frataxin controls ketone body metabolism through regulation of OXCT1

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by the deficiency of mitochondrial protein frataxin, which plays a crucial role in iron-sulphur cluster formation and ATP production. The cellular function of frataxin is not entirely known. Here, we demonstrate that frataxin controls ketone body metabolism through regulation of 3-Oxoacid CoA-Transferase 1 (OXCT1), a rate limiting enzyme catalyzing the conversion of ketone bodies to acetoacetyl-CoA that is then fed into the Krebs cycle. Biochemical studies show a physical interaction between frataxin and OXCT1 both in vivo and in vitro. Frataxin overexpression also increases OXCT1 protein levels in human skin fibroblasts while frataxin deficiency decreases OXCT1 in multiple cell types including cerebellum and skeletal muscle both acutely and chronically, suggesting that frataxin directly regulates OXCT1. This regulation is mediated by frataxin-dependent suppression of ubiquitin-proteasome system (UPS)-dependent OXCT1 degradation. Concomitantly, plasma ketone bodies are significantly elevated in frataxin deficient knock-in/knockout (KIKO) mice with no change in the levels of other enzymes involved in ketone body production. In addition, ketone bodies fail to be metabolized to acetyl-CoA accompanied by increased succinyl-CoA in vitro in frataxin deficient cells, suggesting that ketone body elevation is caused by frataxin-dependent reduction of OXCT1 leading to deficits in tissue utilization of ketone bodies. Considering the potential role of metabolic abnormalities and deficiency of ATP production in FRDA, our results suggest a new role for frataxin in ketone body metabolism and also suggest modulation of OXCT1 may be a potential therapeutic approach for FRDA.

Role of Seipin in Human Diseases and Experimental Animal Models

Seipin, a protein encoded by the Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) gene, is famous for its key role in the biogenesis of lipid droplets and type 2 congenital generalised lipodystrophy (CGL2). BSCL2 gene mutations result in genetic diseases including CGL2, progressive encephalopathy with or without lipodystrophy (also called Celia’s encephalopathy), and BSCL2-associated motor neuron diseases. Abnormal expression of seipin has also been found in hepatic steatosis, neurodegenerative diseases, glioblastoma stroke, cardiac hypertrophy, and other diseases. In the current study, we comprehensively summarise phenotypes, underlying mechanisms, and treatment of human diseases caused by BSCL2 gene mutations, paralleled by animal studies including systemic or specific Bscl2 gene knockout, or Bscl2 gene overexpression. In various animal models representing diseases that are not related to Bscl2 mutations, differential expression patterns and functional roles of seipin are also described. Furthermore, we highlight the potential therapeutic approaches by targeting seipin or its upstream and downstream signalling pathways. Taken together, restoring adipose tissue function and targeting seipin-related pathways are effective strategies for CGL2 treatment. Meanwhile, seipin-related pathways are also considered to have potential therapeutic value in diseases that are not caused by BSCL2 gene mutations.

A Flexible Network of Lipid Droplet Associated Proteins Support Embryonic Integrity of C. elegans

In addition to coordinating the storage and mobilization of neutral fat, lipid droplets (LDs) are conserved organelles that can accommodate additional cargos in order to support animal development. However, it is unclear if each type of cargo is matched with a specific subset of LDs. Here, we report that SEIP-1/seipin defines a subset of oocyte LDs that are required for proper eggshell formation in C. elegans. Using a photoconvertible fluorescent protein-based imaging assay, we found that SEIP-1 positive LDs were selectively depleted after fertilization, coincident of the formation of a lipid-rich permeability barrier of the eggshell. Loss of SEIP-1 function caused impenetrant embryonic arrest, which could be worsened by FAT-3/fatty acyl-CoA desaturase deficiency or suppressed by PLIN-1/Perilipin deficiency. The embryonic development of seip-1; plin-1 mutant in turn depended on the recruitment of RAB-18/Rab18 to LDs, which was not observed in wild type embryos. We propose that SEIP-1 dependent and independent mechanisms act in parallel to ensure the packaging and export of lipid-rich permeability barrier constituents, which involve LDs. The identity of these LDs, as defined by their associated proteins, exhibits unexpected plasticity that ultimately ensures the survival of embryos ex utero.

BSCL2/Seipin deficiency in hearts causes cardiac energy deficit and dysfunction via inducing excessive lipid catabolism

BACKGROUND

Heart failure (HF) is one of the leading causes of death worldwide and is associated with cardiac metabolic perturbations. Human Type 2 Berardinelli-Seip Congenital Lipodystrophy (BSCL2) disease is caused by mutations in the BSCL2 gene. Global lipodystrophic Bscl2^-/- mice exhibit hypertrophic cardiomyopathy with reduced cardiac steatosis. Whether BSCL2 plays a direct role in regulating cardiac substrate metabolism and/or contractile function remains unknown.

METHODS

We generated mice with cardiomyocyte-specific deletion of Bscl2 (Bscl2^cKO ) and studied their cardiac substrate utilisation, bioenergetics, lipidomics and contractile function under baseline or after either a treatment regimen using fatty acid oxidation (FAO) inhibitor trimetazidine (TMZ) or a prevention regimen with high-fat diet (HFD) feeding. Mice with partial ATGL deletion and cardiac-specific deletion of Bscl2 were also generated followed by cardiac phenotyping.

RESULTS

Different from hypertrophic cardiomyopathy in Bscl2^-/- mice, mice with cardiac-specific deletion of Bscl2 developed systolic dysfunction with dilation. Myocardial BSCL2 deletion led to elevated ATGL expression and FAO along with reduced cardiac lipid contents. Cardiac dysfunction in Bscl2^cKO mice was independent of mitochondrial dysfunction and oxidative stress, but associated with decreased metabolic reserve and ATP levels. Importantly, cardiac dysfunction in Bscl2^cKO mice could be partially reversed by FAO inhibitor TMZ, or prevented by genetic abolishment of one ATGL allele or HFD feeding. Lipidomic analysis further identified markedly reduced glycerolipids, glycerophospholipids, NEFA and acylcarnitines in Bscl2^cKO hearts, which were partially normalised by TMZ or HFD.

CONCLUSIONS

We identified a new form of cardiac dysfunction with excessive lipid utilisation which ultimately causes cardiac substrate depletion and bioenergetics failure. Our findings also uncover a crucial role of BSCL2 in controlling cardiac lipid catabolism and contractile function and provide novel insights into metabolically treating energy-starved HF using FAO inhibitor or HFD.

Seipin collaborates with the ER membrane to control the sites of lipid droplet formation

Most cells store metabolic energy in lipid droplets (LDs). LDs are composed of a hydrophobic core, covered by a phospholipid monolayer, and functionalized by a specific set of proteins. Formation of LDs takes place in the endoplasmic reticulum (ER), where neutral lipid biosynthetic enzymes are located. Recent evidence indicate that this process is confined to specific ER subdomains, where proteins meet to initiate LD assembly. The lipodystrophy protein Seipin, is emerging as a major coordinator of LD biogenesis. Seipin forms a large oligomeric toroidal structure, which traps neutral lipids to promote LD nucleation. Here, we discuss the role of LD biogenesis factors that associate with Seipin to assemble functional LDs.

Seipin forms a flexible cage at lipid droplet formation sites

Lipid droplets (LDs) form in the endoplasmic reticulum by phase separation of neutral lipids. This process is facilitated by the seipin protein complex, which consists of a ring of seipin monomers, with a yet unclear function. Here, we report a structure of S. cerevisiae seipin based on cryogenic-electron microscopy and structural modeling data. Seipin forms a decameric, cage-like structure with the lumenal domains forming a stable ring at the cage floor and transmembrane segments forming the cage sides and top. The transmembrane segments interact with adjacent monomers in two distinct, alternating conformations. These conformations result from changes in switch regions, located between the lumenal domains and the transmembrane segments, that are required for seipin function. Our data indicate a model for LD formation in which a closed seipin cage enables triacylglycerol phase separation and subsequently switches to an open conformation to allow LD growth and budding.

Weighing in on Adipogenesis

Obesity is a growing health concern worldwide because of its contribution to metabolic syndrome, type II diabetes, insulin resistance (IR), and numerous cancers. In obesity, white adipose tissue (WAT) expands through two mechanisms: increase in adipocyte cell number by precursor cell differentiation through the process of adipogenesis (hyperplasia) and increase in existing mature adipocyte cell size (hypertrophy). While hypertrophy is associated with the negative effects of obesity on metabolic health, such as inflammation and lipotoxicity, adipogenesis prevents obesity-mediated metabolic decline. Moreover, in metabolically healthy obesity adipogenesis is increased. Thus, it is vital to understand the mechanistic basis for adipose expansion to inform novel therapeutic approaches to mitigate the dysfunction of this tissue and associated diseases. In this mini-review, we summarize recent studies on the regulation of adipogenesis and provide a perspective on targeting adipogenesis as a potential therapeutic avenue for metabolic disorders.