Research paperSeipin deficiency alters fatty acid Δ9 desaturation and lipid droplet formation in Berardinelli-Seip congenital lipodystrophy
Introduction
Berardinelli-Seip congenital lipodystrophy (BSCL) is a rare syndrome characterized by the absence of adipose tissue from birth or early infancy, resulting in severe dyslipidemia, insulin resistance, hepatosplenomegaly and muscular hypertrophy (OMIM #26970) [1]. BSCL is a genetically heterogeneous disorder with autonomic recessive inheritance in which two major genes have been implicated in about 95% of reported cases, BSCL2 encoding protein seipin of unknown function [2] and AGPAT2, encoding the enzyme 1-acyl-glycerol-3-phosphate O-acyltransferase-2 (also referred to as lysophosphatidic acid acyltransferase-β) [3]. The latter enzyme catalyses the acylation at the sn-2 position of lysophosphatidic acid (LPA) to form phosphatidic acid (PA), a key intermediate step in the synthesis of triglycerides (TAG), the major form of energy storage in animals, and phospholipids, including phosphatidylcholine (PC) and phosphatidylethanolamine (PE), which constitute abundant components of cell membranes. Both, seipin and AGPAT2 are located in the endoplasmic reticulum membrane [4], [5]. Recently, we implicated CAV1, encoding caveolin-1, as a third disease-causative gene in a case of BSCL [6]. Caveolin-1 triggers formation of plasma membrane caveolae that are involved in normal insulin signaling and lipid homeostasis including lipid endocytosis to lipid droplets (LD), which appears crucial for lipid handling and storage [7]. Most of the BSCL-causing mutations are nonsense, splice or frameshift mutations which likely lead to complete loss of protein function [2], [3], [8].
The severe metabolic derangements seen in BSCL are believed to mainly result from the inability of adipose tissue to store dietary lipids leading to harmful spillover of lipids to other insulin sensitive tissues [9]. The primary lack of adipose tissue may, in principle, be due to impaired TAG synthesis and/or storage, accelerated lipolysis, defective adipocyte differentiation or adipocyte loss. The mechanisms of lipodystrophy remain elusive, although the consequences of AGPAT2 or CAV1 deficiencies are more easily conceivable given the established role of these proteins in adipogenesis, lipid metabolism or LD accretion [7], [10].
In contrast, it remains unclear how seipin alterations result in lipoatrophy. As seipin disruption produces a more severe loss of adipose tissue than the other BSCL phenotypes, the protein likely plays a critical function in adipocyte. In accordance, reduced levels of seipin were recently shown to impair adipocyte differentiation [11]. An alternative explanation is that the lipodystrophic phenotype may be caused via a central mechanism, as seipin is highly expressed in nervous system and has been implicated in several neuronal diseases and in neuron survival [5]. Interestingly, seipin has been recently involved in the formation of LD which is a universal organelle as it is present in most cell-types [12], [13]. Because seipin deficiency results in a severe phenotype including intellectual impairment and premature death, seipin may exert a biological function in different tissues in addition to adipose tissue and nervous system [14].
In this study, we sought to determine the mechanism whereby seipin loss-of-function mutations cause lipodystrophy. Because AGPAT2 and CAV1 are implicated in various aspects of lipid homeostasis, we searched whether seipin is also involved in lipid metabolism. To this end, we compared the lipid profile of lymphoblastoid cell-lines obtained from 20 patients with BSCL. We reveal different alterations in lipid composition and droplets pattern in cells from patients devoid of seipin suggesting that seipin is involved in the lipid biosynthetis pathway linking Δ9 desaturation to lipid storage. In this cell model, AGPAT2 deficiency essentially induced an increase of cellular LPA levels. Our results demonstrate that seipin and AGPAT2 are involved in different aspects of lipid metabolism.
Section snippets
Patients and cell lines
We studied Epstein Barr virus-transformed lymphocytes derived from 20 subjects affected with BSCL-seipin (12 cases) or BSCL-AGPAT2 (8 cases) and from 9 healthy controls including relatives who do not harbour any mutations in seipin or AGPAT2 [2], [8]. The main biological characteristics and the nature of the mutations in BSCL2 or AGPAT2 are summarized in Table 1. Some of these have been previously reported [1], [2], [8], [14]. All patients and their families gave their informed consent, which
Seipin and AGPAT2 mRNA levels.
The mRNA level of seipin related to that of TBP was similar in cells from controls and patients with mutations in seipin (seipin−/−) or AGPAT2 (AGPAT2−/−) (Fig. 1A). The mRNA level of AGPAT2 was similar in AGPAT2−/− and control cells but slightly decreased by 20% in seipin−/− cells (p = 0.05) (Fig. 1B). The observed individual variations were not consistently linked to the nature of the mutation.
To determine whether the physiological impact of AGPAT2 mutations could be compensated by the
Discussion
BSCL is a rare disease characterized by near absence of adipose tissue which is due to loss-of-function mutations in the genes encoding either AGPAT2, CAV1 or seipin [2], [3], [6], [8]. This provides compelling evidence for the critical requirement of the protein seipin as the other BSCL-causative proteins for normal adipose tissue formation and lipid homeostasis [7], [10]. We report here that seipin is likely involved in lipid metabolism, more specifically in the pathway linking fatty acids
Acknowledgements
We thank the patients and their families for their collaboration; M. Meier and D. Farabos for technical expertise; physicians or investigators, P. Czernichow, E. Eicher, T. Gedde-Dahl Jr, M. de Kerdanet, C.A. Kim, A. Mégarbané, M. Odièvre, M. Polak, J.-J. Robert, M. Seip, G. Simonin, O. Trigstadt, N. Tubiana and L. Van Maldergem who provided valuable patients' clinical and biological features and blood samples; O. Lascols, M. Delépine and M. Lathrop for mutation screening; D. Recan, J. Chelly
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