Congenital generalized lipodystrophy, type 4 (CGL4) associated with myopathy due to novel PTRF mutations

Tissue-specific landscape of protein aggregation and quality control in an aging vertebrate

Protein aggregation is a hallmark of age-related neurodegeneration. Yet, aggregation during normal aging and in tissues other than the brain is poorly understood. Here, we leverage the African turquoise killifish to systematically profile protein aggregates in seven tissues of an aging vertebrate. Age-dependent aggregation is strikingly tissue specific and not simply driven by protein expression differences. Experimental interrogation in killifish and yeast, combined with machine learning, indicates that this specificity is linked to protein-autonomous biophysical features and tissue-selective alterations in protein quality control. Co-aggregation of protein quality control machinery during aging may further reduce proteostasis capacity, exacerbating aggregate burden. A segmental progeria model with accelerated aging in specific tissues exhibits selectively increased aggregation in these same tissues. Intriguingly, many age-related protein aggregates arise in wild-type proteins that, when mutated, drive human diseases. Our data chart a comprehensive landscape of protein aggregation during vertebrate aging and identify strong, tissue-specific associations with dysfunction and disease.

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.

Analysis of disease characteristics of a large patient cohort with congenital generalized lipodystrophy from the Middle East and North Africa

BACKGROUND

Congenital generalized lipodystrophy (CGL) is a rare inherited disease characterized by a near-total absence of adipose tissue and is associated with organ system abnormalities and severe metabolic complications. Here, we have analyzed the disease characteristics of the largest CGL cohort from the Middle East and North Africa (MENA) who have not received lipodystrophy-specific treatment.

METHODS

CGL was diagnosed clinically by treating physicians through physical assessment and supported by genetic analysis, fat loss patterns, family history, and the presence of parental consanguinity. Data were obtained at the time of patient diagnosis and during leptin-replacement naïve follow-up visits as permitted by available medical records.

RESULTS

Data from 43 patients with CGL (37 females, 86%) were collected from centers located in eight countries. The mean (median, range) age at diagnosis was 5.1 (1.0, at birth-37) years. Genetic analysis of the overall cohort showed that CGL1 (n = 14, 33%) and CGL2 (n = 18, 42%) were the predominant CGL subtypes followed by CGL4 (n = 10, 23%); a genetic diagnosis was unavailable for one patient (2%). There was a high prevalence of parental consanguinity (93%) and family history (67%) of lipodystrophy, with 64% (n = 25/39) and 51% (n = 20/39) of patients presenting with acromegaloid features and acanthosis nigricans, respectively. Eighty-one percent (n = 35/43) of patients had at least one organ abnormality; the most frequently affected organs were the liver (70%, n = 30/43), the cardiovascular system (37%, n = 16/43) and the spleen (33%, n = 14/43). Thirteen out of 28 (46%) patients had HbA1c > 5.7% and 20/33 (61%) had triglyceride levels > 2.26 mmol/L (200 mg/dl). Generally, patients diagnosed in adolescence or later had a greater severity of metabolic disease versus those diagnosed during childhood; however, metabolic and organ system abnormalities were observed in a subset of patients diagnosed before or at 1 year of age.

CONCLUSIONS

This analysis suggests that in addition to the early onset of fat loss, family history and high consanguinity enable the identification of young patients with CGL in the MENA region. In patients with CGL who have not received lipodystrophy-specific treatment, severe metabolic disease and organ abnormalities can develop by late childhood and worsen with age.

A new mutation in the CAVIN1/PTRF gene in two siblings with congenital generalized lipodystrophy type 4: case reports and review of the literature

Lipodystrophy syndromes are characterized by a progressive metabolic impairment secondary to adipose tissue dysfunction and may have a genetic background. Congenital generalized lipodystrophy type 4 (CGL4) is an extremely rare subtype, caused by mutations in the polymerase I and transcript release factor (PTRF) gene. It encodes for a cytoplasmatic protein called caveolae-associated protein 1 (Cavin-1), which, together with caveolin 1, is responsible for the biogenesis of caveolae, being a master regulator of adipose tissue expandability. Cavin-1 is expressed in several tissues, including muscles, thus resulting, when dysfunctional, in a clinical phenotype characterized by the absence of adipose tissue and muscular dystrophy. We herein describe the clinical phenotypes of two siblings in their early childhood, with a phenotype characterized by a generalized reduction of subcutaneous fat, muscular hypertrophy, distinct facial features, myopathy, and atlantoaxial instability. One of the siblings developed paroxysmal supraventricular tachycardia leading to cardiac arrest at 3 months of age. Height and BMI were normal. Blood tests showed elevated CK, a mild increase in liver enzymes and triglycerides levels, and undetectable leptin and adiponectin concentrations. Fasting glucose and HbA1c were normal, while Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) was mildly elevated. Both patients were hyperphagic and had cravings for foods rich in fats and sugars. Genetic testing revealed a novel pathogenic mutation of the CAVIN1/PTRF gene (NM_012232 exon1:c T21A:p.Y7X) at the homozygous state. The diagnosis of lipodystrophy can be challenging, often requiring a multidisciplinary approach, given the pleiotropic effect, involving several tissues. The coexistence of generalized lack of fat, myopathy with elevated CK levels, arrhythmias, gastrointestinal dysmotility, and skeletal abnormalities should prompt the suspicion for the diagnosis of CGL4, although phenotypic variability may occur.

Decoding the transcriptome of muscular dystrophy due to Ptrf deficiency using single-nucleus RNA sequencing

Lacking PTRF (polymerase I and transcript release factor), an essential caveolae component, causes a secondary deficiency of caveolins resulting in muscular dystrophy. The transcriptome responses of different types of muscle fibers and mononuclear cells in skeletal muscle to muscular dystrophy caused by Ptrf deletion have not been explored. Here, we created muscular dystrophy mice by Ptrf knockout and applied single-nucleus RNA sequencing (snRNA-seq) to unveil the transcriptional changes of the skeletal muscle at single-nucleus resolution. 11 613 muscle nuclei (WT, 5838; Ptrf KO, 5775) were classified into 12 clusters corresponding to 11 nuclear types. Trajectory analysis revealed the potential transition between type IIb_1 and IIb_2 myonuclei upon muscular dystrophy. Functional enrichment analysis indicated that apoptotic signaling and enzyme-linked receptor protein signaling pathway were significantly enriched in type IIb_1 and IIb_2 myonuclei of Ptrf KO, respectively. The muscle structure development and the PI3K-AKT signaling pathway were significantly enriched in type IIa and IIx myonuclei of Ptrf KO. Meanwhile, metabolic pathway analysis showed a decrease in overall metabolic pathway activity of myonuclei subtypes upon muscular dystrophy, with the most decrease in type IIb_1 myonuclei. Gene regulatory network analysis found that the activity of Mef2c, Mef2d, Myf5, and Pax3 regulons was enhanced in type II myonuclei of Ptrf KO, especially in type IIb_2 myonuclei. In addition, we investigated the transcriptome changes in adipocytes and found that muscular dystrophy enhanced the lipid metabolic capacity of adipocytes. Our findings provide a valuable resource for exploring the molecular mechanism of muscular dystrophy due to Ptrf deficiency.

A deep learning-based tool for the automated detection and analysis of caveolae in transmission electron microscopy images

Caveolae are nanoscopic and mechanosensitive invaginations of the plasma membrane, essential for adipocyte biology. Transmission electron microscopy (TEM) offers the highest resolution for caveolae visualization, but provides complicated images that are difficult to classify or segment using traditional automated algorithms such as threshold-based methods. As a result, the time-consuming tasks of localization and quantification of caveolae are currently performed manually. We used the Keras library in R to train a convolutional neural network with a total of 36,000 TEM image crops obtained from adipocytes previously annotated manually by an expert. The resulting model can differentiate caveolae from non-caveolae regions with a 97.44% accuracy. The predictions of this model are further processed to obtain caveolae central coordinate detection and cytoplasm boundary delimitation. The model correctly finds negligible caveolae predictions in images from caveolae depleted Cav1-/- adipocytes. In large reconstructions of adipocyte sections, model and human performances are comparable. We thus provide a new tool for accurate caveolae automated analysis that could speed up and assist in the characterization of the cellular mechanical response.

Congenital generalized lipodystrophy type 4 due to a novel PTRF/CAVIN1 pathogenic variant in a child: effects of metreleptin substitution

OBJECTIVES

Congenital generalized lipodystrophies (CGLs) are a heterogeneous group of rare autosomal recessive disorders characterized by near/total absence of body fat. Pathogenic variants in polymerase-I and transcript release factor gene (PTRF), or CAVIN1, is responsible for CGL4. In addition to generalized fat loss, patients with CGL4 were reported to suffer from myopathy, malignant cardiac arrhythmias, gastrointestinal disorders, and skeletal abnormalities. Here we describe the phenotype of a child with CGL4 due to a rare, novel pathogenic variant in the PTRF/CAVIN1 gene and the long-term effects of metreleptin substitution on comorbidities.

CASE PRESENTATION

We describe a now 20-year-old female patient. At the age of 14-years, she was referred to the University Clinic because of uncontrolled diabetes with an HbA1c of 9.3%, requiring 2.4 IU insulin/kg total-body-weight to normalize blood glucose, hepatomegaly, and hypertriglyceridemia of 515 mg/dL. Additionally, she was suffering from malignant cardiac arrhythmia, myopathy, and hyperCKemia. In light of these clinical findings, she was diagnosed with CGL due to a rare, novel variant in the PTRF gene, and was started on metreleptin, a synthetic analog of human leptin. After the initiation of metreleptin treatment, insulin therapy could be stopped and improvement of sonographically assessed liver size was observed, even though serum liver function test stayed mildly elevated. Furthermore, a noticeable improvement of the serum triglyceride levels was also seen. Medical care and regular follow-up visits are being carried out by a multi-disciplinary team.

CONCLUSIONS

Although CGL4 is rare, due to its life-threatening comorbidities and the opportunity for an early intervention, it is important that the clinicians should recognise these patients.

Hereditary severe insulin resistance syndrome: Pathogenesis, pathophysiology, and clinical management

Severe insulin resistance has been linked to some of the most globally prevalent disorders, such as diabetes mellitus, nonalcoholic fatty liver disease, polycystic ovarian syndrome, and hypertension. Hereditary severe insulin resistance syndrome (H-SIRS) is a rare disorder classified into four principal categories: primary insulin receptor defects, lipodystrophies, complex syndromes, and obesity-related H-SIRS. Genes such as INSR, AKT2, TBC1D4, AGPAT2, BSCL2, CAV1, PTRF, LMNA, PPARG, PLIN1, CIDEC, LIPE, PCYT1A, MC4R, LEP, POMC, SH2B1, RECQL2, RECQL3, ALMS1, PCNT, ZMPSTE24, PIK3R1, and POLD1 have been linked to H-SIRS. Its clinical features include insulin resistance, hyperglycemia, hyperandrogenism, severe dyslipidemia, fatty liver, abnormal topography of adipose tissue, and low serum leptin and adiponectin levels. Diagnosis of H-SIRS is based on the presence of typical clinical features associated with the various H-SIRS forms and the identification of mutations in H-SIRS-linked genes by genetic testing. Diet therapy, insulin sensitization, exogenous insulin therapy, and leptin replacement therapy have widely been adopted to manage H-SIRS. The rarity of H-SIRS, its highly variable clinical presentation, refusal to be tested for genetic mutations by patients' family members who are not severely sick, unavailability of genetic testing, and testing expenses contribute to the delayed or underdiagnoses of H-SIRS. Early diagnosis facilitates early management of the condition, which results in improved glycemic control and delayed onset of diabetes and other complications related to severe insulin resistance. The use of updated genetic sequencing technologies is recommended, and long-term studies are required for genotype-phenotype differentiation and formulation of diagnostic and treatment protocols.

Not Enough Fat: Mouse Models of Inherited Lipodystrophy

Lipodystrophies belong to the heterogenous group of syndromes in which the primary defect is a generalized or partial absence of adipose tissue, which may be congenital or acquired in origin. Lipodystrophy should be considered in patients manifesting the combination of insulin resistance (with or without overt diabetes), dyslipidemia and fatty liver. Lipodystrophies are classified according to the etiology of the disease (genetic or acquired) and to the anatomical distribution of adipose tissue (generalized or partial). The mechanism of adipose tissue loss is specific to each syndrome, depending on the biological function of the mutated gene. Mice models, together with cellular studies have permitted clarification of the mechanisms by which human mutations deeply compromise adipocyte homeostasis. In addition, rodent models have proven to be crucial in deciphering the cardiometabolic consequences of the lack of adipose tissue such as NAFLD, muscle insulin resistance and cardiomyopathy. More precisely, tissue-specific transgenic and knockout mice have brought new tools to distinguish phenotypic traits that are the consequences of lipodystrophy from those that are cell-autonomous. In this review, we discuss the mice models of lipodystrophy including those of inherited human syndromes of generalized and partial lipodystrophy. We present how these models have demonstrated the central role of white adipose tissue in energetic homeostasis in general, including insulin sensitivity and lipid handling in particular. We underscore the differences reported with the human phenotype and discuss the limit of rodent models in recapitulating adipose tissue primary default. Finally, we present how these mice models have highlighted the function of the causative-genes and brought new insights into the pathophysiology of the cardiometabolic complications associated with lipodystrophy.

Approach to Diagnosing a Pediatric Patient With Severe Insulin Resistance in Low- or Middle-income Countries

Diabetes mellitus (DM) in children is most often caused by impaired insulin secretion (type 1 DM). In some children, the underlying mechanism for DM is increased insulin resistance, which can have different underlying causes. While the majority of these children require insulin dosages less than 2.0 U/kg/day to achieve normoglycemia, higher insulin requirements indicate severe insulin resistance. Considering the therapeutic challenges in patients with severe insulin resistance, early diagnosis of the underlying cause is essential in order to consider targeted therapies and to prevent diabetic complications. Although rare, several disorders can attribute to severe insulin resistance in pediatric patients. Most of these disorders are diagnosed through advanced diagnostic tests, which are not commonly available in low- or middle-income countries. Based on a case of DM with severe insulin resistance in a Surinamese adolescent who was later confirmed to have autosomal recessive congenital generalized lipodystrophy, type 1 (Berardinelli-Seip syndrome), we provide a systematic approach to the differential diagnosis and work-up. We show that a thorough review of medical history and physical examination generally provide sufficient information to diagnose a child with insulin-resistant DM correctly, and, therefore, our approach is especially applicable to low- or middle-income countries.

Generalized lipoatrophy syndromes

Generalized lipodystrophy (GL) syndromes are a group of rare heterogenous disorders, characterized by total subcutaneous fat loss. The frequency of GL is currently assessed as approximately 0,23 cases per million of the population, in Europe - as 0,96 cases per million of the population. They can be congenital (CGL) or acquired (AGL) depending on the etiology and the time of the onset of fat loss. Both CGL and AGL are often associated with different metabolic complications, such as hypertriglyceridemia, insulin resistance and lipoatrophic diabetes mellitus, metabolically associated FLD, arterial hypertension, proteinuria, reproductive system disorders. In this review we aimed to summarize the information on all forms of generalized lipodystrophy, especially the ones of genetic etiology, their clinical manifestations and complications, the perspectives for diagnostics, treatment and further research.

Caveola-forming proteins and prostate cancer

Caveolae are specialised and dynamic plasma membrane subdomains, involved in many cellular functions including endocytosis, signal transduction, mechanosensing and lipid storage, trafficking, and metabolism. Two protein families are indispensable for caveola formation and function, namely caveolins and cavins. Mutations of genes encoding these caveolar proteins cause serious pathological conditions such as cardiomyopathies, skeletal muscle diseases, and lipodystrophies. Deregulation of caveola-forming protein expression is associated with many types of cancers including prostate cancer. The distinct function of secretion of the prostatic fluid, and the unique metabolic phenotype of prostate cells relying on lipid metabolism as a main bioenergetic pathway further suggest a significant role of caveolae and caveolar proteins in prostate malignancy. Accumulating in vitro, in vivo, and clinical evidence showed the association of caveolin-1 with prostate cancer grade, stage, metastasis, and drug resistance. In contrast, cavin-1 was found to exhibit tumour suppressive roles. Studies on prostate cancer were the first to show the distinct function of the caveolar proteins depending on their localisation within the caveolar compartment or as cytoplasmic or secreted proteins. In this review, we summarise the roles of caveola-forming proteins in prostate cancer and the potential of exploiting them as therapeutic targets or biological markers.

Molecular and Cellular Bases of Lipodystrophy Syndromes

Lipodystrophy syndromes are rare diseases originating from a generalized or partial loss of adipose tissue. Adipose tissue dysfunction results from heterogeneous genetic or acquired causes, but leads to similar metabolic complications with insulin resistance, diabetes, hypertriglyceridemia, nonalcoholic fatty liver disease, dysfunctions of the gonadotropic axis and endocrine defects of adipose tissue with leptin and adiponectin deficiency. Diagnosis, based on clinical and metabolic investigations, and on genetic analyses, is of major importance to adapt medical care and genetic counseling. Molecular and cellular bases of these syndromes involve, among others, altered adipocyte differentiation, structure and/or regulation of the adipocyte lipid droplet, and/or premature cellular senescence. Lipodystrophy syndromes frequently present as systemic diseases with multi-tissue involvement. After an update on the main molecular bases and clinical forms of lipodystrophy, we will focus on topics that have recently emerged in the field. We will discuss the links between lipodystrophy and premature ageing and/or immuno-inflammatory aggressions of adipose tissue, as well as the relationships between lipomatosis and lipodystrophy. Finally, the indications of substitutive therapy with metreleptin, an analog of leptin, which is approved in Europe and USA, will be discussed.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

Unusual clinical features associated with congenital generalized lipodystrophy type 4 in a patient with a novel E211X CAVIN1 gene variant

Background

Congenital generalized lipodystrophy (CGL) is a rare disorder characterized by the lack of adipose tissue and metabolic complications with predominantly autosomal recessive inheritance. There are 6 different genes known to cause CGL with 4 main types recognized to date, which differ by the degree of fat loss, association with mental retardation and metabolic disorders, with CGL type 1 and 2 being the most common. Twenty seven cases of СGL type 4 from Japan, Oman, UK, Turkey, Mexico, Saudi Arabia, USA were reported previously. This report details our clinical experience with the first patient from Russia with CGL type 4.

Case presentation

A 36-year-old patient, who has been suffering from generalized lipoatrophy since the first months of life and myopathy and gastrointestinal dysmotility since early childhood, developed dysmenorrhea and diabetes mellitus at the age of 19, bilateral cataracts when she was only 22 y.o., osteoporosis with vitamin D deficiency and hypocalcemia at the age of 28, diabetic foot syndrome and hyperuricemia when she was 35 y.o. Sequencing of lipodystrophy candidate genes detected a novel pathogenic homozygous variant p.631G < T: p.E211X in the CAVIN1 gene, confirming the diagnosis of CGL type 4.

Conclusions

In comparison with previously reported patients with CGL type 4, our patient has diabetes mellitus, vitamin D deficiency, hypocalcemia, bilateral cataracts and hyperuricemia. All these manifestations are known to be associated with other lipodystrophy syndromes, but to our knowledge it is the first time they have been reported to be associated with CGL type 4.

The Lipolysome-A Highly Complex and Dynamic Protein Network Orchestrating Cytoplasmic Triacylglycerol Degradation

The catabolism of intracellular triacylglycerols (TAGs) involves the activity of cytoplasmic and lysosomal enzymes. Cytoplasmic TAG hydrolysis, commonly termed lipolysis, is catalyzed by the sequential action of three major hydrolases, namely adipose triglyceride lipase, hormone-sensitive lipase, and monoacylglycerol lipase. All three enzymes interact with numerous protein binding partners that modulate their activity, cellular localization, or stability. Deficiencies of these auxiliary proteins can lead to derangements in neutral lipid metabolism and energy homeostasis. In this review, we summarize the composition and the dynamics of the complex lipolytic machinery we like to call "lipolysome".

Lessons from cavin-1 deficiency

Caveolae have been implicated in a wide range of critical physiological functions. In the past decade, the dominant role of cavin-1 in caveolae formation has been established, and it has been recognized as another master regulator for caveolae biology. Human patients with cavin-1 mutations develop lipodystrophy and muscular dystrophy and have some major pathological dysfunctions in fat tissue, skeleton muscle, heart, lung and other organs. Cavin-1 deficiency animal models consistently show similar phenotypes. However, the underlying molecular mechanisms remain to be elucidated. Recent studies have suggested many possible pathways, including mechanosensing, stress response, signal transduction, exosome secretion, and potential functions in the nucleus. Many excellent and comprehensive review articles already exist on the topics of caveolae structure formation, caveolins, and their pathophysiological functions. We will focus on recent studies using cavin-1 deficiency models, to summarize the pathophysiological changes in adipose, muscle, and other organs, followed by a summary of mechanistic studies about the roles of cavin-1, which includes caveolae formation, ribosomal RNA transcription, mechanical sensing, stress response, and exosome secretion. Further studies may help to elucidate the exact underlying molecular mechanism to explain the pathological changes observed in cavin-1 deficient human patients and animal models, so potential new therapeutic strategies can be developed.

Diagnostic strategies and clinical management of lipodystrophy

Introduction: Lipodystrophy is a heterogeneous group of rare diseases characterized by various degrees of fat loss which leads to serious morbidity due to metabolic abnormalities associated with insulin resistance and subtype-specific clinical features associated with underlying molecular etiology.Areas covered: This article aims to help physicians address challenges in diagnosing and managing lipodystrophy. We systematically reviewed the literature on PubMed and Google Scholar databases to summarize the current knowledge in lipodystrophy management.Expert opinion: Adipose tissue is a highly active endocrine organ that regulates metabolic homeostasis in the human body through a comprehensive communication network with other organ systems such as the central nervous system, liver, digestive system, and the immune system. The adipose tissue is capable of producing and secreting numerous factors with important endocrine functions such as leptin that regulates energy homeostasis. Recent developments in the field have helped to solve some of the mysteries behind lipodystrophy that allowed us to get a better understanding of adipocyte function and differentiation. From a clinical standpoint, physicians who suspect lipodystrophy should distinguish the disease from several others that may present with similar clinical features. It is also important for physicians to carefully interpret clinical features, laboratory, and imaging results before moving to more sophisticated tests and making decisions about therapy.

Interaction of cavin-1/PTRF leucine zipper domain 2 and its congenital generalized lipodystrophy mutant with model membranes

The organization of caveolae ultrastructures in the plasma membrane and the functions they dictate are mediated by membrane-embedded caveolins (caveolin-1, 2, 3) and peripherally attached cavins (cavin-1, 2, 3, 4). Mutations in caveolin and cavin genes are associated with a variety of human diseases. Cavin-1/PTRF mutations are known to contribute to several human pathologies, including muscular dystrophy and congenital generalized lipodystrophy (CGL). In the present study, we investigated the membrane interaction of the second leucine zipper domain (LZD2) of cavin-1 and the analogous peptide stretch in its CGL frameshift mutant (p.Glu176Argfs). The fluorescence data from the Trp-tagged peptides suggest binding of both wild-type and mutant peptide with negatively-charged membranes. The mutant peptide displayed a rather enhanced interaction compared to the wild-type peptide. In addition, the mutant peptide displayed appreciable binding to the lipid raft-mimicking cholesterol/sphingomyelin-rich vesicles as well. The alteration in the dynamics of peptide-lipid interaction is attributed to increased charge and hydrophilicity of the mutant peptides. Overall, these results suggest that the frameshift mutation in cavin-1/PTRF (p.Glu176Argfs) imparts high membrane-binding propensity to the region corresponding to LZD2, which is hitherto unknown to interact with membranes. Such interaction in the disease condition, in turn, could either alter the native membrane interaction dynamics of cavin-1/PTRF or possibly result in interaction with non-target membranes.

Congenital generalized lipodystrophy: The evaluation of clinical follow-up findings in a series of five patients with type 1 and two patients with type 4

Congenital generalized lipodystrophy (CGL) is a rare disorder characterized by lipoatrophy affecting the face, limbs and trunk, acromegaloid features, hepatomegaly, hypertriglyceridemia, and insulin resistance. The aim of this study is to evaluate the long-term follow-up findings including gastrointestinal and cardiac manifestations of the patients with CGL1 and CGL4, caused by mutations in the AGPAT2 and CAVIN1 genes, respectively. Two patients aged 2 and 9 years with the same biallelic CAVIN1 mutation and five patients aged between 6 months and 11 years 4 months with AGPAT2 mutations have been followed up for 3-9 years. The patients were between 7 and 20 years of age at their last examination. One of the two patients with CGL4 had congenital pyloric stenosis. The other patient with CGL4 have developed recurrent duodenal perforations which have not been reported in CGL patients previously. The pathological examination of duodenal specimens revealed increased subserosal fibrous tissue and absent submucosal adipose tissue. None of the five CGL1 patients had gastrointestinal problems. Two patients with CGL4 developed hypertrophic cardiomyopathy (HCMP) and severe cardiac arrhythmia, only one patient with CGL1 had HCMP. Hyperinsulinemia was detected in one patient with CGL4 and three patients with CGL1, these three CGL1 patients also had acanthosis nigricans. Hepatic steatosis was detected in one patient with CGL4 and two patients with CGL1 by ultrasonography. In conclusion, these findings suggest that CGL4 patients should also be carefully followed up for gastrointestinal and cardiac manifestations.

What lipodystrophies teach us about the metabolic syndrome

Lipodystrophies are the result of a range of inherited and acquired causes, but all are characterized by perturbations in white adipose tissue function and, in many instances, its mass or distribution. Though patients are often nonobese, they typically manifest a severe form of the metabolic syndrome, highlighting the importance of white fat in the "safe" storage of surplus energy. Understanding the molecular pathophysiology of congenital lipodystrophies has yielded useful insights into the biology of adipocytes and informed therapeutic strategies. More recently, genome-wide association studies focused on insulin resistance have linked common variants to genes implicated in adipose biology and suggested that subtle forms of lipodystrophy contribute to cardiometabolic disease risk at a population level. These observations underpin the use of aligned treatment strategies in insulin-resistant obese and lipodystrophic patients, the major goal being to alleviate the energetic burden on adipose tissue.

The worldwide mutational landscape of Berardinelli-Seip congenital lipodystrophy

Berardinelli-Seip congenital lipodystrophy (BSCL) is a rare disease characterized by the near total absence of body fat at birth. BSCL etiology involves genetic variations in four different genes: AGPAT2, BSCL2, CAV1, and CAVIN1. The four different biochemical subtypes of the disease are distinguished depending on which gene is mutated. The diagnosis of lipodystrophy can be based on clinical criteria, but the gold standard remains genetic testing. Since many different mutations have already been correlated with the onset of the disease, the most indicative method is DNA sequencing. However, not all laboratories have the resources to perform sequencing. Thus, less expensive techniques that include narrow gene regions may be applied. In such cases, the target mutations to be tested must be carefully determined taking into account the frequency of the description of the mutations in the literature, the nationality of the patient, as well as their phenotype. This review considers the molecular basis of BSCL, including the manual count of the majority of mutations reported in the literature up to the year 2018. Ninety different genetic mutations in 332 cases were reported at different frequencies. Some mutations were distributed homogeneously and others were specific to geographic regions. Type 2 BSCL was mentioned most often in the literature (50.3% of the cases), followed by Type 1 (38.0%), Type 4 (10.2%), and Type 3 (1.5%). The mutations comprised frameshifts (34.4%), nonsense (26.6%), and missense (21.1%). The c.517dupA in the BSCL2 gene was the most frequent (13.3%), followed by c.589-2A>G in the AGPAT2 gene (11.5%), c.507_511delGTATC in the BSCL2 gene (9.7%), c.317-588del in the AGPAT2 gene (7.3%), and c.202C>T in the AGPAT2 gene (4.5%). This information should prove valuable for analysts in making decisions regarding the best therapeutic targets in a population-specific context, which will benefit patients and enable faster and less expensive treatment.

Postmortem Findings in a Young Man With Congenital Generalized Lipodystrophy, Type 4 Due to CAVIN1 Mutations

CONTEXT

Congenital generalized lipodystrophy, type 4 (CGL4) is a rare autosomal recessive disorder caused by mutations in caveolae-associated protein 1. Patients with CGL4 also have myopathy and cardiomyopathy with a predisposition for sudden death due to ventricular arrhythmias. However, the underlying pathology for these morbidities remains unknown. Therefore, we report on an autopsy of a Hispanic boy with CGL4.

CASE DESCRIPTION

Our patient had early-onset generalized lipodystrophy, feeding difficulties, myopathy, atlanto-axial dislocation, and learning disabilities. He was diagnosed with catecholaminergic polymorphic ventricular tachycardia (CPVT) at age 8 years, had poor compliance with medications, and died suddenly at age 15.3 years. Autopsy showed marked loss of subcutaneous and omental fat with no inflammatory cells in adipose tissue and normal adipocytes in the parathyroid glands. There were adipocytes interdigitating cardiac muscle fibers, with fibro-fatty infiltration in the right ventricle, near coronary sinus, and atrioventricular node. There was no evidence of coronary heart disease. The quadriceps femoris muscle did not show adipocyte infiltration, inflammation, or fibrosis. The muscularis mucosa layer was thickened in the esophagus and at the gastro-duodenal junction, and the esophagus had prominent, large nerves in the subserosa. The liver weighed 3000 g, with minimal chronic inflammation and steatosis in 40% of parenchyma, primarily in zones 2 and 3. There was no spermatogenesis in the spermatic tubules.

CONCLUSIONS

Our data suggest that fibro-fatty infiltration of the right ventricle may contribute to CPVT in patients with CGL4. Thick muscularis mucosa and large nerves in the esophagus likely contributed to dysphagia and dysmotility. A lack of spermatids suggests infertility in affected male patients.

Lipodystrophies, dyslipidaemias and atherosclerotic cardiovascular disease

Lipodystrophies are rare, heterogeneous, genetic or acquired, disorders characterised by varying degrees of body fat loss and associated metabolic complications, including insulin resistance, dyslipidaemias, hepatic steatosis and predisposition to atherosclerotic cardiovascular disease (ASCVD). The four main types of lipodystrophy, excluding antiretroviral therapy-induced lipodystrophy in HIV-infected patients, are congenital generalised lipodystrophy (CGL), familial partial lipodystrophy (FPLD), acquired generalised lipodystrophy (AGL) and acquired partial lipodystrophy (APL). This paper reviews the literature related to the prevalence of dyslipidaemias and ASCVD in patients with lipodystrophies. Patients with CGL, AGL and FPLD have increased prevalence of dyslipidaemia but those with APL do not. Patients with CGL as well as AGL present in childhood, and have severe dyslipidaemias (mainly hypertriglyceridaemia) and early onset diabetes mellitus as a consequence of extreme fat loss. However, only a few patients with CGL and AGL have been reported to develop coronary heart disease. In contrast, data from some small cohorts of FPLD patients reveal increased prevalence of ASCVD especially among women. Patients with APL have a relatively low prevalence of hypertriglyceridaemia and diabetes mellitus. Overall, patients with lipodystrophies appear to be at high risk of ASCVD due to increased prevalence of dyslipidaemia and diabetes and efforts should be made to manage these metabolic complications aggressively to prevent ASCVD.

Metreleptin treatment for congenital generalized lipodystrophy type 4 (CGL4): a case report

Congenital generalized lipodystrophy type 4 (CGL4) is a rare disease caused by mutations in the gene polymerase I and transcript release factor (PTRF), the main symptoms of which are systemic reductions in adipose tissue and muscular dystrophy. The strategy of treating CGL4 is to improve the insulin resistance and hypertriglyceridemia that result from systemic reductions in adipose tissue. Metreleptin, a synthetic analog of human leptin, is effective against generalized lipodystrophies; however, there are no reports of the use of metreleptin in the treatment of CGL4. Herein, we discuss the treatment of a six-year-old boy diagnosed with CGL4 due to a homozygous mutation in PTRF with metreleptin. His serum triglyceride level and homeostasis model assessment of insulin resistance (HOMA-IR) value decreased after two months of metreleptin treatment. However, the efficacy of metreleptin gradually decreased, and the treatment was suspended because anaphylaxis occurred after the dosage administered was increased. Subsequently, his serum triglyceride level and HOMA-IR value significantly increased. Anti-metreleptin-neutralizing antibodies were detected in his serum, which suggested that these antibodies reduced the efficacy of metreleptin and caused increased hypersensitivity. Thus, metreleptin appeared to be efficacious in the treatment of CGL4 in the short term, although an adverse immune response resulted in treatment suspension. Further studies are needed to evaluate metreleptin treatments for CGL4.

Phenotypic and Genetic Characteristics of Lipodystrophy: Pathophysiology, Metabolic Abnormalities, and Comorbidities

PURPOSE OF REVIEW

This article focuses on recent progress in understanding the genetics of lipodystrophy syndromes, the pathophysiology of severe metabolic abnormalities caused by these syndromes, and causes of severe morbidity and a possible signal of increased mortality associated with lipodystrophy. An updated classification scheme is also presented.

RECENT FINDINGS

Lipodystrophy encompasses a group of heterogeneous rare diseases characterized by generalized or partial lack of adipose tissue and associated metabolic abnormalities including altered lipid metabolism and insulin resistance. Recent advances in the field have led to the discovery of new genes associated with lipodystrophy and have also improved our understanding of adipose biology, including differentiation, lipid droplet assembly, and metabolism. Several registries have documented the natural history of the disease and the serious comorbidities that patients with lipodystrophy face. There is also evolving evidence for increased mortality rates associated with lipodystrophy. Lipodystrophy syndromes represent a challenging cluster of diseases that lead to severe insulin resistance, a myriad of metabolic abnormalities, and serious morbidity. The understanding of these syndromes is evolving in parallel with the identification of novel disease-causing mechanisms.

Caveolae: Structure, Function, and Relationship to Disease

The plasma membrane of eukaryotic cells is not a simple sheet of lipids and proteins but is differentiated into subdomains with crucial functions. Caveolae, small pits in the plasma membrane, are the most abundant surface subdomains of many mammalian cells. The cellular functions of caveolae have long remained obscure, but a new molecular understanding of caveola formation has led to insights into their workings. Caveolae are formed by the coordinated action of a number of lipid-interacting proteins to produce a microdomain with a specific structure and lipid composition. Caveolae can bud from the plasma membrane to form an endocytic vesicle or can flatten into the membrane to help cells withstand mechanical stress. The role of caveolae as mechanoprotective and signal transduction elements is reviewed in the context of disease conditions associated with caveola dysfunction.

SNAP23 regulates BAX-dependent adipocyte programmed cell death independently of canonical macroautophagy

The t-SNARE protein SNAP23 conventionally functions as a component of the cellular machinery required for intracellular transport vesicle fusion with target membranes and has been implicated in the regulation of fasting glucose levels, BMI, and type 2 diabetes. Surprisingly, we observed that adipocyte-specific KO of SNAP23 in mice resulted in a temporal development of severe generalized lipodystrophy associated with adipose tissue inflammation, insulin resistance, hyperglycemia, liver steatosis, and early death. This resulted from adipocyte cell death associated with an inhibition of macroautophagy and lysosomal degradation of the proapoptotic regulator BAX, with increased BAX activation. BAX colocalized with LC3-positive autophagic vacuoles and was increased upon treatment with lysosome inhibitors. Moreover, BAX deficiency suppressed the lipodystrophic phenotype in the adipocyte-specific SNAP23-KO mice and prevented cell death. In addition, ATG9 deficiency phenocopied SNAP23 deficiency, whereas ATG7 deficiency had no effect on BAX protein levels, BAX activation, or apoptotic cell death. These data demonstrate a role for SNAP23 in the control of macroautophagy and programmed cell death through an ATG9-dependent, but ATG7-independent, pathway regulating BAX protein levels and BAX activation.

Determining residual adipose tissue characteristics with MRI in patients with various subtypes of lipodystrophy

PURPOSE

We aimed to investigate residual adipose tissue with whole-body magnetic resonance imaging to differentiate between subtypes of lipodystrophy.

METHODS

A total of 32 patients (12 with congenital generalized lipodystrophy [CGL], 1 with acquired generalized lipodystrophy [AGL], 12 with familial partial lipodystrophy [FPLD], and 7 with acquired partial lipodystrophy [APL]) were included.

RESULTS

Despite generalized loss of metabolically active adipose tissue, patients with CGL1 caused by AGPAT2 mutations had a significant amount of residual adipose tissue in the scalp, earlobes, retro-orbital region, and palms and soles. No residual adipose tissue was noted particularly in the head and neck, palms and soles in CGL2 caused by BSCL2 mutations. CGL4 caused by mutations in the PTRF gene was characterized with well-preserved retro-orbital and bone marrow fat in the absence of any visible residual adipose tissue in other areas. No residual adipose tissue was observed in AGL. Despite loss of subcutaneous fat, periarticular adipose tissue was preserved in the lower limbs of patients with FPLD. Retro-orbital adipose tissue was surprisingly preserved in APL, although they lacked head and neck fat.

CONCLUSION

Lipodystrophies are a heterogeneous group of disorders characterized by generalized or partial loss of adipose tissue, which can be congenital or acquired. Our results suggest that residual adipose tissue characteristics can help distinguish different subtypes of lipodystrophy.

Clitoromegaly in Childhood and Adolescence: Behind One Clinical Sign, a Clinical Sea

The clitoris is a highly complex organ whose structure has only been clarified in recent years through the use of modern imaging techniques. Clitoromegaly is an abnormal enlargement of this organ. It may be congenital or acquired and is usually due to an excess of androgens in fetal life, infancy, or adolescence. Obvious clitoromegaly in individuals with ambiguous genitalia is easily identifiable, whereas borderline conditions can pass unnoticed. Case reports of clitoromegaly with or without clinical or biochemical hyperandrogenism are quite numerous. In these subjects, a comprehensive physical examination and an accurate personal and family history are needed to investigate the enlargement. We reviewed the literature on the conditions that may be involved in the development of clitoromegaly in childhood and adolescence.

Acetylation of Cavin-1 Promotes Lipolysis in White Adipose Tissue

White adipose tissue (WAT) serves as a reversible energy storage depot in the form of lipids in response to nutritional status. Cavin-1, an essential component in the biogenesis of caveolae, is a positive regulator of lipolysis in adipocytes. However, molecular mechanisms of cavin-1 in the modulation of lipolysis remain poorly understood. Here, we showed that cavin-1 was acetylated at lysines 291, 293, and 298 (3K), which were under nutritional regulation in WAT. We further identified GCN5 as the acetyltransferase and Sirt1 as the deacetylase of cavin-1. Acetylation-mimetic 3Q mutants of cavin-1 augmented fat mobilization in 3T3-L1 adipocytes and zebrafish. Mechanistically, acetylated cavin-1 preferentially interacted with hormone-sensitive lipase and recruited it to the caveolae, thereby promoting lipolysis. Our findings shed light on the essential role of cavin-1 in regulating lipolysis in an acetylation-dependent manner in WAT.

Caveolin1 protects against diet induced hepatic lipid accumulation in mice

Background and aim

Caveolin1 (CAV1) is involved in lipid homeostasis and endocytosis, but little is known about the significance of CAV1 in the pathogenesis and development of nonalcoholic fatty liver disease (NAFLD). This study aimed to determine the role of CAV1 in NAFLD.

Methods

Expression of CAV1 in the in vitro and in vivo models of NAFLD was analyzed. The effects of CAV1 knockdown or overexpression on free fatty acid (FFA)-induced lipid accumulation in L02 cells and AML12 cells were determined. CAV1 knockout (CAV1-KO) mice and their wild-type (WT) littermates were subjected to a high fat diet (HFD) for 4 weeks, and the functional consequences of losing the CAV1 gene and its subsequent molecular mechanisms were also examined.

Results

Noticeably, CAV1 expression was markedly reduced in NAFLD. CAV1 knockdown led to the aggravation of steatosis that was induced by FFA in both L02 cells and AML12 cells, while CAV1 overexpression markedly attenuated lipid accumulation in the cells. Consistent with CAV1 repression in the livers of HFD-induced mice, the CAV1-KO mice exhibited more severe hepatic steatosis upon HFD intake. In addition, increased cholesterol levels and elevated transaminases were detected in the plasma of CAV1-KO mice. The protein expression of SREBP1, a key gene involved in lipogenesis, was augmented following CAV1 suppression in FFA-treated hepatocytes and in the livers of HFD-fed CAV1-KO mice.

Conclusions

CAV1 serves as an important protective factor in the development of NAFLD by modulating lipid metabolism gene expression.

Clinical spectra of neuromuscular manifestations in patients with lipodystrophy: A multicenter study

Lipodystrophy is a heterogeneous group of disorders characterized by loss of adipose tissue. Here, we report on clinical spectra of neuromuscular manifestations of Turkish patients with lipodystrophy. Seventy-four patients with lipodystrophy and 20 healthy controls were included. Peripheral sensorimotor neuropathy was a common finding (67.4%) in lipodystrophic patients with diabetes. Neuropathic foot ulcers were observed in 4 patients. Drop foot developed in 1 patient with congenital generalized lipodystrophy type 1. Muscle symptoms and hypertrophy were consistent findings in congenital generalized lipodystrophy (21/21) and familial partial lipodystrophy (25/34); on the other hand, overt myopathy with elevated creatine kinase activity was a distinctive characteristic of congenital generalized lipodystrophy type 4. Muscle biopsies revealed myopathic changes at different levels. Accumulation of triglycerides was observed which contributes to insulin resistance. All patients with congenital generalized lipodystrophy suffered from tight Achilles tendons at various levels. Scoliosis was observed in congenital generalized lipodystrophy type 4 (2/2) and familial partial lipodystrophy type 2 (2/17). Atlantoaxial instability was unique to congenital generalized lipodystrophy type 4 (2/2). Bone cysts were detected in congenital generalized lipodystrophy type 1 (7/10) and congenital generalized lipodystrophy type 2 (2/8). Our study suggests that lipodystrophies are associated with a wide spectrum of neuromuscular abnormalities.

Muscular dystrophy in PTFR/cavin-1 null mice

ice and humans lacking the caveolae component polymerase I transcription release factor (PTRF, also known as cavin-1) exhibit lipo- and muscular dystrophy. Here we describe the molecular features underlying the muscle phenotype for PTRF/cavin-1 null mice. These animals had a decreased ability to exercise, and exhibited muscle hypertrophy with increased muscle fiber size and muscle mass due, in part, to constitutive activation of the Akt pathway. Their muscles were fibrotic and exhibited impaired membrane integrity accompanied by an apparent compensatory activation of the dystrophin-glycoprotein complex along with elevated expression of proteins involved in muscle repair function. Ptrf deletion also caused decreased mitochondrial function, oxygen consumption, and altered myofiber composition. Thus, in addition to compromised adipocyte-related physiology, the absence of PTRF/cavin-1 in mice caused a unique form of muscular dystrophy with a phenotype similar or identical to that seen in humans lacking this protein. Further understanding of this muscular dystrophy model will provide information relevant to the human situation and guidance for potential therapies.

Clinical Features and Management of Non-HIV-Related Lipodystrophy in Children: A Systematic Review

CONTEXT

Lipodystrophy syndromes are characterized by generalized or partial absence of adipose tissue.

OBJECTIVE

We conducted a systematic review to synthesize data on clinical and metabolic features of lipodystrophy (age at onset, < 18 years).

DATA SOURCE

Sources included Medline, Embase, Cochrane Library, Scopus and Non-Indexed Citations from inception through January 2016.

STUDY SELECTION

Search terms included lipodystrophy, and age 0 to 18 years. Patients with unambiguous diagnosis of lipodystrophy were included. Lipodystrophy secondary to HIV treatment was excluded.

DATA SYNTHESIS

We identified 1141 patients from 351 studies. Generalized fat loss involving face, neck, abdomen, thorax, and upper and lower limbs was explicitly reported in 65% to 93% of patients with congenital generalized lipodystrophy (CGL) and acquired generalized lipodystrophy (AGL). In familial partial lipodystrophy (FPL), fat loss occurred from upper and lower limbs, with sparing of face and neck. In acquired partial lipodystrophy (APL), upper limbs were involved while lower limbs were spared. Other features were prominent musculature, acromegaloid, acanthosis nigricans and hepatosplenomegaly. Diabetes mellitus was diagnosed in 48% (n = 222) of patients with CGL (mean age at onset, 5.3 years). Hypertriglyceridemia was observed in CGL, AGL and FPL. Multiple interventions were used, with most patients receiving ≥ 3 interventions and being ≥ 18 years of age at the initiation of interventions.

CONCLUSIONS

To our knowledge, this is the largest reported pooled database describing lipodystrophy patients with age at onset < 18 years. We have suggested core and supportive clinical features and summarized data on available interventions, outcomes and mortality.

Mouse P2Y4 Nucleotide Receptor Is a Negative Regulator of Cardiac Adipose-Derived Stem Cell Differentiation and Cardiac Fat Formation

Cardiac adipose tissue-derived stem cells (cASCs) have the ability to differentiate into multiple cell lineages giving them a high potential for use in regenerative medicine. Cardiac fat tissue still raises many unsolved questions related to its formation and features. P2Y nucleotide receptors have already been described as regulators of differentiation of bone-marrow derived stem cells, but remain poorly investigated in cASCs. We defined, in this study, the P2Y₄ nucleotide receptor as a negative regulator of cardiac fat formation and cASC adipogenic differentiation. Higher expression of P2Y₄ receptor in cardiac fat tissue was observed compared to other adipose tissues. P2Y₄-null mice displayed a higher mass of cardiac adipose tissue specifically. We therefore examined the role of P2Y₄ receptor in cASC adipogenic differentiation. An inhibitory effect of uridine 5'-triphosphate (UTP), ligand of P2Y₄, was observed on the maturation state of differentiated cASCs, and on the expression of adipogenesis-linked genes and adiponectin, a cardioprotective adipokine. Higher adiponectin secretion by P2Y₄-null adipocytes could be linked with cardioprotection previously observed in the heart of P2Y₄-null ischemic mice. We realized here left anterior descending artery ligation on simple and double-knockout mice for P2Y₄ and adiponectin. No cardioprotective effect of P2Y₄ loss was observed in the absence of adiponectin secretion. In addition, P2Y₄ loss was correlated with higher expression of UCP-1 (uncoupling protein-1) and CD137, two markers of brown/beige cardiac adipocytes. Our data highlight the P2Y₄ receptor as an inhibitor of cardiac fat formation and cASC adipogenic differentiation, and as a potential therapeutic target in the regulation of cardioprotective function of cardiac fat.

The size matters: regulation of lipid storage by lipid droplet dynamics

Adequate energy storage is essential for sustaining healthy life. Lipid droplet (LD) is the subcellular organelle that stores energy in the form of neutral lipids and releases fatty acids under energy deficient conditions. Energy storage capacity of LDs is primarily dependent on the sizes of LDs. Enlargement and growth of LDs is controlled by two molecular pathways: neutral lipid synthesis and atypical LD fusion. Shrinkage of LDs is mediated by the degradation of neutral lipids under energy demanding conditions and is controlled by neutral cytosolic lipases and lysosomal acidic lipases. In this review, we summarize recent progress regarding the regulatory pathways and molecular mechanisms that control the sizes and the energy storage capacity of LDs.

Characterization of a caveolin-1 mutation associated with both pulmonary arterial hypertension and congenital generalized lipodystrophy

Congenital generalized lipodystrophy (CGL) and pulmonary arterial hypertension (PAH) have recently been associated with mutations in the caveolin-1 ( CAV1 ) gene, which encodes the primary structural protein of caveolae. However, little is currently known about how these CAV1 mutations impact caveolae formation or contribute to the development of disease. Here, we identify a heterozygous F160X CAV1 mutation predicted to generate a C-terminally truncated mutant protein in a patient with both PAH and CGL using whole exome sequencing, and characterize the properties of CAV1 , caveolae-associated proteins and caveolae in skin fibroblasts isolated from the patient. We show that morphologically defined caveolae are present in patient fibroblasts and that they function in mechanoprotection. However, they exhibited several notable defects, including enhanced accessibility of the C-terminus of wild-type CAV1 in caveolae, reduced colocalization of cavin-1 with CAV1 and decreased stability of both 8S and 70S oligomeric CAV1 complexes that are necessary for caveolae formation. These results were verified independently in reconstituted CAV1 -/- mouse embryonic fibroblasts. These findings identify defects in caveolae that may serve as contributing factors to the development of PAH and CGL and broaden our knowledge of CAV1 mutations associated with human disease.

Lipodystrophy Syndromes

Lipodystrophies are heterogeneous disorders characterized by varying degrees of body fat loss and predisposition to insulin resistance and its metabolic complications. They are subclassified depending on degree of fat loss and whether the disorder is genetic or acquired. The two most common genetic varieties include congenital generalized lipodystrophy and familial partial lipodystrophy; the two most common acquired varieties include acquired generalized lipodystrophy and acquired partial lipodystrophy. Highly active antiretroviral therapy-induced lipodystrophy in patients infected with human immunodeficiency virus and drug-induced localized lipodystrophy are common subtypes. The metabolic abnormalities associated with lipodystrophy include insulin resistance, hypertriglyceridemia, and hepatic steatosis. Management focuses on preventing and treating metabolic complications.

Cardiac Manifestations of Congenital Generalized Lipodystrophy

IN BRIEF Congenital lipodystrophy is a rare genetic disorder characterized by a near-complete absence of fat cells, hypoleptinemia leading to a voracious appetite, and marked insulin resistance. This article focuses on the known cardiovascular manifestations of patients with congenital lipodystrophy, including cardiomyopathy, cardiac arrhythmias, and accelerated atherosclerosis arising from a markedly deranged metabolic milieu. Future research that targets leptin deficiency (metreleptin) and apoC3 mRNA (antisense oligonucleotide) could open a window for potential pharmacological treatment of this challenging disorder.

PTRF/Cavin-1 Deficiency Causes Cardiac Dysfunction Accompanied by Cardiomyocyte Hypertrophy and Cardiac Fibrosis

Mutations in the PTRF/Cavin-1 gene cause congenital generalized lipodystrophy type 4 (CGL4) associated with myopathy. Additionally, long-QT syndrome and fatal cardiac arrhythmia are observed in patients with CGL4 who have homozygous PTRF/Cavin-1 mutations. PTRF/Cavin-1 deficiency shows reductions of caveolae and caveolin-3 (Cav3) protein expression in skeletal muscle, and Cav3 deficiency in the heart causes cardiac hypertrophy with loss of caveolae. However, it remains unknown how loss of PTRF/Cavin-1 affects cardiac morphology and function. Here, we present a characterization of the hearts of PTRF/Cavin-1-null (PTRF^−/−) mice. Electron microscopy revealed the reduction of caveolae in cardiomyocytes of PTRF^−/− mice. PTRF^−/− mice at 16 weeks of age developed a progressive cardiomyopathic phenotype with wall thickening of left ventricles and reduced fractional shortening evaluated by echocardiography. Electrocardiography revealed that PTRF^−/− mice at 24 weeks of age had low voltages and wide QRS complexes in limb leads. Histological analysis showed cardiomyocyte hypertrophy accompanied by progressive interstitial/perivascular fibrosis. Hypertrophy-related fetal gene expression was also induced in PTRF^−/− hearts. Western blotting analysis and quantitative RT-PCR revealed that Cav3 expression was suppressed in PTRF^−/− hearts compared with that in wild-type (WT) ones. ERK1/2 was activated in PTRF^−/− hearts compared with that in WT ones. These results suggest that loss of PTRF/Cavin-1 protein expression is sufficient to induce a molecular program leading to cardiomyocyte hypertrophy and cardiomyopathy, which is partly attributable to Cav3 reduction in the heart.

PTRF/Cavin-1 promotes efficient ribosomal RNA transcription in response to metabolic challenges

Ribosomal RNA transcription mediated by RNA polymerase I represents the rate-limiting step in ribosome biogenesis. In eukaryotic cells, nutrients and growth factors regulate ribosomal RNA transcription through various key factors coupled to cell growth. We show here in mature adipocytes, ribosomal transcription can be acutely regulated in response to metabolic challenges. This acute response is mediated by PTRF (polymerase I transcription and release factor, also known as cavin-1), which has previously been shown to play a critical role in caveolae formation. The caveolae–independent rDNA transcriptional role of PTRF not only explains the lipodystrophy phenotype observed in PTRF deficient mice and humans, but also highlights its crucial physiological role in maintaining adipocyte allostasis. Multiple post-translational modifications of PTRF provide mechanistic bases for its regulation. The role of PTRF in ribosomal transcriptional efficiency is likely relevant to many additional physiological situations of cell growth and organismal metabolism.

DOI:http://dx.doi.org/10.7554/eLife.17508.001

Obesity can cause several other health conditions to develop. Type 2 diabetes is one such condition, which arises in part because fat cells become unable to store excess fats. This makes certain tissues in the body less sensitive to the hormone insulin, and so the individual is less able to adapt to changing nutrient levels. Without treatment or a change in lifestyle, this insulin resistance may develop into diabetes. However, “healthy obese” individuals also exist, who can accommodate an overabundance of fat without developing insulin resistance and diabetes.

Some forms of rare genetic disorders called lipodystrophies, which result in an almost complete lack of body fat, can also lead to type 2 diabetes. This raises the question of whether lipodystrophy and obesity share some common mechanisms that cause fat cells to trigger insulin resistance. One possible player in such mechanisms is a protein called PTRF. In rare cases, individuals with lipodystrophy lack this protein, and mice that have been engineered to lack PTRF also largely lack body fat and develop insulin resistance.

Fat cells can respond rapidly to changes in nutrients during feeding or fasting, and to do so, they must produce new proteins. Structures called ribosomes, which are made up of proteins and ribosomal RNA, build proteins; thus when the cell needs to make new proteins, it also has to produce more ribosomes. PTRF is thought to play a role in ribosome production, but it is not clear how it does so.

Liu and Pilch analyzed normal mice as well as those that lacked the PTRF protein. This revealed that in response to cycles of fasting and feeding, PTRF increases the production of ribosomal RNA in fat cells, enabling the cells to produce more proteins. By contrast, the fat cells of mice that lack PTRF have much lower levels of ribosomal RNA and proteins.

Liu and Pilch then examined mouse fat cells that were grown in the laboratory. Exposing these cells to insulin caused phosphate groups to be attached to the PTRF proteins inside the cells. This modification caused PTRF to move into the cell’s nucleus, where it increased the production of ribosomal RNA.

Overall, the results show that fat cells that lack PTRF are unable to produce the proteins that they need to deal with changing nutrient levels, leading to an increased likelihood of diabetes. The next steps are to investigate the mechanism by which PTRF is modified, and to see whether the mechanisms uncovered in this study also apply to humans.

DOI:http://dx.doi.org/10.7554/eLife.17508.002

Natural History of Congenital Generalized Lipodystrophy: A Nationwide Study From Turkey

CONTEXT

Congenital generalized lipodystrophy (CGL) is a rare autosomal recessive disorder characterized by near-total lack of body fat.

OBJECTIVE

We aimed to study natural history and disease burden of various subtypes of CGL.

DESIGN

We attempted to ascertain nearly all patients with CGL in Turkey.

SETTING

This was a nationwide study.

PATIENTS OR OTHER PARTICIPANTS

Participants included 33 patients (22 families) with CGL and 30 healthy controls.

MAIN OUTCOME MEASURE(S)

We wanted to ascertain genotypes by sequencing of the known genes. Whole-body magnetic resonance imaging was used to investigate the extent of fat loss. Metabolic abnormalities and end-organ complications were measured on prospective follow-up.

RESULTS

Analysis of the AGPAT2 gene revealed four previously reported and four novel mutations (CGL1; c.144C>A, c.667_705delinsCTGCG, c.268delC, and c.316+1G>T). Analysis of the BSCL2 gene revealed four different homozygous and one compound heterozygous possible disease-causing mutations (CGL2), including four novel mutations (c.280C>T, c.631delG, c.62A>T, and c.465-468delGACT). Two homozygous PTRF mutations (c.481-482insGTGA and c.259C>T) were identified (CGL4). Patients with CGL1 had preservation of adipose tissue in the palms, soles, scalp, and orbital region, and had relatively lower serum adiponectin levels as compared to CGL2 patients. CGL4 patients had myopathy and other distinct clinical features. All patients developed various metabolic abnormalities associated with insulin resistance. Hepatic involvement was more severe in CGL2. End-organ complications were observed at young ages. Two patients died at age 62 years from cardiovascular events.

CONCLUSIONS

CGL patients from Turkey had both previously reported and novel mutations of the AGPAT2, BSCL2, and PTRF genes. Our study highlights the early onset of severe metabolic abnormalities and increased risk of end-organ complications in patients with CGL.

Inside out: Bone marrow adipose tissue as a source of circulating adiponectin

The adipocyte-derived hormone adiponectin mediates beneficial cardiometabolic effects, and hypoadiponectinemia is a biomarker for increased metabolic and cardiovascular risk. Indeed, circulating adiponectin decreases in obesity and insulin-resistance, likely because of impaired production from white adipose tissue (WAT). Conversely, lean states such as caloric restriction (CR) are characterized by hyperadiponectinemia, even without increased adiponectin production from WAT. The reasons underlying this paradox have remained elusive, but our recent research suggests that CR-associated hyperadiponectinemia derives from an unexpected source: bone marrow adipose tissue (MAT). Herein, we elaborate on this surprising discovery, including further discussion of potential mechanisms influencing adiponectin production from MAT; additional evidence both for and against our conclusions; and observations suggesting that the relationship between MAT and adiponectin might extend beyond CR. While many questions remain, the burgeoning study of MAT promises to reveal further key insights into MAT biology, both as a source of adiponectin and beyond.

Spectrum of clinical manifestations in two young Turkish patients with congenital generalized lipodystrophy type 4

Congenital generalized lipodystrophy type 4 is an extremely rare autosomal recessive disorder. We report our clinical experience on two unrelated Turkish patients with congenital generalized lipodystrophy type 4. A 13-year-old girl (patient-1) presented with generalized lipodystrophy and myopathy. Further tests revealed ventricular and supraventricular arrhythmias, gastrointestinal dysmotility, atlantoaxial instability, lumbosacral scoliosis, and metabolic abnormalities associated with insulin resistance. A 16-year-old girl (patient-2) with congenital generalized lipodystrophy type 4 was previously reported. Here, we report on her long term clinical follow-up. She received several course of anti-arrhythmic treatments for catecholaminergic polymorphic ventricular tachycardia and rapid atrial fibrillation. An implantable cardioverter defibrillator was also placed. A homozygous PTRF mutation, c.259C > T (p.Gln87*), was identified in patient-1. Congenital generalized lipodystrophy type 4 was caused by homozygous PTRF c.481-482insGTGA (p.Lys161Serfs*41) mutation in patient-2. Our data indicate that patients with congenital generalized lipodystrophy type 4 should be meticulously evaluated for cardiac, neuromuscular, gastrointestinal and skeletal diseases, as well as metabolic abnormalities associated with insulin resistance.

Caveolae: One Function or Many?

Caveolae are small, bulb-shaped plasma membrane invaginations. Mutations that ablate caveolae lead to diverse phenotypes in mice and humans, making it challenging to uncover their molecular mechanisms. Caveolae have been described to function in endocytosis and transcytosis (a specialized form of endocytosis) and in maintaining membrane lipid composition, as well as acting as signaling platforms. New data also support a model in which the central function of caveolae could be related to the protection of cells from mechanical stress within the plasma membrane. We present evidence for these diverse roles and consider in vitro and in vivo experiments confirming a mechanoprotective role. We conclude by highlighting current gaps in our knowledge of how mechanical signals may be transduced by caveolae.