Negatively‐charged residues in the polar carboxy‐terminal region in FSP27 are indispensable for expanding lipid droplets

Abstract

FSP27 has an important role in large lipid droplet (LD) formation because it exchanges lipids at the contact site between LDs. In the present study, we clarify that the amino-terminal domain of FSP27 (amino acids 1-130) is dispensable for LD enlargement, although it accelerates LD growth. LD expansion depends on the carboxy-terminal domain of FSP27 (amino acids 131-239). Especially, the negative charge of the acidic residues (D215, E218, E219 and E220) in the polar carboxy-terminal region (amino acids 202-239) is essential for the enlargement of LD. We propose that the carboxy-terminal domain of FSP27 has a crucial role in LD expansion, whereas the aminoterminal domain only has a supportive role.Keywords: adipocyte; fat specific protein of 27 kDa; lipid droplet; lipid droplet enlargement; negatively-charged amino acid; structure and function Obesity increases dramatically [1]. It develops as a consequence of nutritional excess and insufficient exercise and is characterized by excessive triacylglycerol (TAG) storage in the form of lipid droplets (LD) in adipose tissue. This results in adipose tissue inflammation and the deregulation of adipokines, both of which induce systemic insulin resistance [2,3]. At the same time, a hallmark of obesity is TAG accumulation in the liver and skeletal muscle, which is also important for inducing insulin resistance in these tissues [4]. These findings highlight the significance of lipid accumulation in insulin-sensitive organs in the development of insulin resistance and type 2 diabetes.LDs comprise a central core of neutral lipids containing TAG and cholesterol ester that is surrounded by a phospholipid monolayer [5,6]. This monolayer of LD is decorated with a variety of proteins that contribute to the formation of the droplet, the synthesis and hydrolysis of its lipids, and the movement of these lipids to specific intracellular and secretory pathways [6,7]. The ability to store TAG in LDs is evolutionarily conserved in yeast, plants, invertebrates and vertebrates [8]. In mammals, excess energy is primarily stored as TAG in LDs of adipose tissue. In particular, white adipocytes comprise specifically differentiated cells for storing TAG as a large unilocular LD that occupies a majority of the cell volume. The LD size can be in the 100 lm range [9]. TAG storage in LDs serves a vital role as a major store of energy supply. In the case of energy demand, such as starvation and exercise, TAG reserves are hydrolyzed by lipolysis to supply free fatty acid to various tissues. LDs are also detected in non-adipose cells; however, non-adipocyte LDs are usually much smaller than adipocyte LDs.Abbreviations CIDE, cell death-inducing DFFA-like effector; FSP27, fat specific protein of 27 kDa; LD, lipid droplet; TAG, triacylglycerol. 750FEBS Letters 590 (2016) 750-759 ª

Cell death-inducing DNA fragmentation factor A-like effector A and fat-specific protein 27β coordinately control lipid droplet size in brown adipocytes

“…4D). In the previous study, we produced the mutant of FSP27␣ in which the negatively charged acidic polar amino acids (Asp-215, Glu-218, Glu-219, and Glu-220), which are supposed to be important for dimer formation, were replaced by noncharged polar amino acids (Asn-215, Gln-218, Gln-219, and Gln-220) and revealed that these negatively charged acidic polar amino acids (Asp-215, Glu-218, Glu-219, and Glu-220) were indispensable for the function of FSP27␣ to enlarge LD (12). Therefore, we introduced the same mutations to FSP27␤ to decrease the function of FSP27␤ and examined the inhibitory effect of FSP27␤ to CideA-induced LD enlargement.…”

“…Fat-specific protein of 27 (FSP27) (CideC in humans) is abundantly expressed in adipose tissue and contributes to the formation of unilocular LD in white adipocytes (5)(6)(7). Although CideA and FSP27 are both involved in the formation of large LD (5)(6)(7)(8)(9)(10)(11)(12), the molecular mechanisms by which brown adipocytes form multilocular small LD despite the abundant expression of CideA currently remain unknown. A new isoform of FSP27, FSP27␤, was recently identified and is a molecule to which 10 amino acids were added to the amino-terminal domain of the conventional type of FSP27, designated as FSP27␣ (13).…”

Single nucleotide polymorphisms in CIDEC gene are associated with metabolic syndrome components risks and antihypertensive drug efficacy

“…Consistently, prominent hypertriglyceridemia was elicited by loss of the C-terminal portion of CIDEC [15]. CIDEC promotes adipogenesis [6] and lipid droplets fusion [20–22], and suppresses lipolysis by inhibiting the expression [7], localization [8], and activity [9] of adipose triglyceride lipase. Therefore, CIDEC is essential for the maintenance of appropriate adipose mass and adequate lipid storage capacity.…”

CIDE Family-Mediated Unique Lipid Droplet Morphology in White Adipose Tissue and Brown Adipose Tissue Determines the Adipocyte Energy Metabolism

“…Ectopic expression of FSP27 led to the enlargement of LDs and TAG accumulation in non-adipose cells 54 , 62 – 65 ) , whereas depletion of FSP27 in cultured adipocytes resulted in small LD formation and increased lipolysis 54 , 62 , 63 ) . Moreover, depletion of FSP27 by siRNA in HW adipocytes that show the morphological characteristics of white adipocytes resulted in the formation of many small LDs similar to brown adipocytes 54 ) .…”

“…FSP27 is highly enriched at an LD–LD contact site and forms homodimers that are involved in directional lipid transfer from small to large LDs between adjacent LDs due to the higher internal pressure in small LDs 66 , 67 ) . Furthermore, structure-function analysis reveals that the carboxy-terminal domain of FSP27 (amino acids 131–239) plays a crucial role in LD expansion, possibly by homodimerization, whereas the amino-terminal domain (amino acids 1–130) has a supportive role 65 ) . On the other hand, there are studies showing the importance of the amino-terminal region of FSP27 in promoting LD growth 67 ) .…”