Polybasic RKKR motif in the linker region of lipid droplet (LD)-associated protein CIDEC inhibits LD fusion activity by interacting with acidic phospholipids

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.

Characterization and expression analysis of seven lipid metabolism-related genes in yellow catfish Pelteobagrus fulvidraco fed high fat and bile acid diet

SREBPs, such as SREBP1 and SREBP2, were the key transcriptional factors regulating lipid metabolism. The processing of SREBPs involved many genes, such as scap, s1p, s2p, cideb. Here, we deciphered the full-length cDNA sequences of scap, srebp1, srebp2, s1p, s2p, cideb and cidec from yellow catfish Pelteobagrus fulvidraco. Their full-length cDNA sequences ranged from 1587 to 3884 bp, and their ORF length from 1191 to 2979 bp, encoding 396-992 amino acids. Some conservative domains were predicted, including the multiple transmembrane domains in SCAP, the bHLH-ZIP domain in SREBP1 and SREBP2, the ApoB binding region, ER targeting region and LD targeting region in CIDEb, the LD targeting region in the CIDEc, the conserved catalytic site and processing site in S1P, and the transmembrane helix domain in S2P. Their mRNA expression could be observed in the heart, spleen, liver, kidney, brain, muscle, intestine and adipose, but varied with tissues. The changes of their mRNA expression in responses to high-fat (HFD) and bile acid (BA) diets were also investigated in the brain, heart, intestine, kidney and spleen tissues. In the brain, HFD significantly increased the mRNA expression of seven genes (scap, srebp1, srebp2, s1p, s2p, cideb and cidec), and the BA attenuated the increase of scap, srebp1, srebp2, s1p, s2p, cideb and cidec mRNA expression induced by HFD. In the heart, HFD significantly increased the mRNA abundances of six genes (srebp1, srebp2, scap, s2p, cideb and cidec), and BA attenuated the increase of their mRNA abundances induced by HFD. In the intestine, HFD increased the cideb, s1p and s2p mRNA abundances, and BA attenuated the HFD-induced increment of their mRNA abundances. In the kidney, HFD significantly increased the scap, cidec and s1p mRNA expression, and BA diet attenuated the increment of their mRNA expression. In the spleen, HFD treatment increased the scap, srebp2, s1p and s2p mRNA expression, and BA diet attenuated HFD-induced increment of their mRNA expression. Taken together, our study elucidated the characterization, expression profiles and transcriptional response of seven lipid metabolic genes, which would serve as the good basis for the further exploration into their function and regulatory mechanism in fish.

Alcoholic Setdb1 suppression promotes hepatosteatosis in mice by strengthening Plin2

BACKGROUND AND AIMS

Hepatosteatosis is one of the early features of alcoholic liver disease (ALD) and pharmaceutical or genetic interfering of the development of hepatosteatosis will efficiently alleviate the progression of ALD. Currently, the role of histone methyltransferase Setdb1 in ALD is not yet well understood.

METHOD

Lieber-De Carli diet mice model and NIAAA mice model were constructed to confirm the expression of Setdb1. The hepatocyte-specific Setdb1-knockout (Setdb1-HKO) mice was established to determine the effects of Setdb1 in vivo. Adenovirus-Setdb1 were produced to rescue the hepatic steatosis in both Setdb1-HKO and Lieber-De Carli mice. The enrichment of H3k9me3 in the upstream sequence of Plin2 and the chaperone-mediated autophagy (CMA) of Plin2 were identified by ChIP and co-IP. Dual-luciferase reporter assay was used to detect the interaction of Setdb1 3'UTR and miR216b-5p in AML12 or HEK 293 T cells.

RESULTS

We found that Setdb1 was downregulated in the liver of alcohol-fed mice. Setdb1 knockdown promoted lipid accumulation in AML12 hepatocytes. Meanwhile, hepatocyte-specific Setdb1-knockout (Setdb1-HKO) mice exhibited significant lipid accumulation in the liver. Overexpression of Setdb1 was performed with an adenoviral vector through tail vein injection, which ameliorated hepatosteatosis in both Setdb1-HKO and alcoholic diet-fed mice. Mechanistically, downregulated Setdb1 promoted the mRNA expression of Plin2 by desuppressing H3K9me3-mediated chromatin silencing in its upstream sequence. Pin2 acts as a critical membrane surface-associated protein to maintain lipid droplet stability and inhibit lipase degradation. The downregulation of Setdb1 also maintained the stability of Plin2 protein through inhibiting Plin2-recruited chaperone-mediated autophagy (CMA). To explore the reasons for Setdb1 suppression in ALD, we found that upregulated miR-216b-5p bound to the 3'UTR of Setdb1 mRNA, disturbed its mRNA stability, and eventually aggravated hepatic steatosis.

CONCLUSIONS

Setdb1 suppression plays an important role in the progression of alcoholic hepatosteatosis via elevating the expression of Plin2 mRNA and maintaining the stability of Plin2 protein. Targeting hepatic Setdb1 might be a promising diagnostic or therapeutic strategy for ALD.

CLSTN3β enforces adipocyte multilocularity to facilitate lipid utilization

Multilocular adipocytes are a hallmark of thermogenic adipose tissue1,2, but the factors that enforce this cellular phenotype are largely unknown. Here, we show that an adipocyte-selective product of the Clstn3 locus (CLSTN3β) present in only placental mammals facilitates the efficient use of stored triglyceride by limiting lipid droplet (LD) expansion. CLSTN3β is an integral endoplasmic reticulum (ER) membrane protein that localizes to ER-LD contact sites through a conserved hairpin-like domain. Mice lacking CLSTN3β have abnormal LD morphology and altered substrate use in brown adipose tissue, and are more susceptible to cold-induced hypothermia despite having no defect in adrenergic signalling. Conversely, forced expression of CLSTN3β is sufficient to enforce a multilocular LD phenotype in cultured cells and adipose tissue. CLSTN3β associates with cell death-inducing DFFA-like effector proteins and impairs their ability to transfer lipid between LDs, thereby restricting LD fusion and expansion. Functionally, increased LD surface area in CLSTN3β-expressing adipocytes promotes engagement of the lipolytic machinery and facilitates fatty acid oxidation. In human fat, CLSTN3B is a selective marker of multilocular adipocytes. These findings define a molecular mechanism that regulates LD form and function to facilitate lipid utilization in thermogenic adipocytes.

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".

The exchangeable apolipoproteins in lipid metabolism and obesity

Dyslipidemia, characterized by increased plasma levels of low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), triglyceride (TG), and reduced plasma levels of high-density lipoprotein cholesterol (HDL-C), is confirmed as a hallmark of obesity and cardiovascular diseases (CVD), posing serious risks to the future health of humans. Thus, it is important to understand the molecular metabolism of dyslipidemia, which could help reduce the morbidity and mortality of obesity and CVD. Currently, several exchangeable apolipoproteins, such as apolipoprotein A1 (ApoA1), apolipoprotein A5 (ApoA5), apolipoprotein E (ApoE), and apolipoprotein C3 (ApoC3), have been verified to exert vital effects on modulating lipid metabolism and homeostasis both in plasma and in cells, which consequently affect dyslipidemia. In the present review, we summarize the findings of the effect of exchangeable apolipoproteins on affecting lipid metabolism in adipocytes and hepatocytes. Furthermore, we also provide new insights into the mechanisms by which the exchangeable apolipoproteins influence the pathogenesis of dyslipidemia and its related cardio-metabolic disorders.

Bluetongue Virus Nonstructural Protein 3 Orchestrates Virus Maturation and Drives Non-Lytic Egress via Two Polybasic Motifs

Bluetongue virus (BTV) is an arthropod-borne virus that infects domestic and wild ruminants. The virion is a non-enveloped double-layered particle with an outer capsid that encloses a core containing the segmented double-stranded RNA genome. Although BTV is canonically released by cell lysis, it also exits non-lytically. In infected cells, the BTV nonstructural glycoprotein 3 (NS3) is found to be associated with host membranes and traffics from the endoplasmic reticulum through the Golgi apparatus to the plasma membrane. This suggests a role for NS3 in BTV particle maturation and non-lytic egress. However, the mechanism by which NS3 coordinates these events has not yet been elucidated. Here, we identified two polybasic motifs (PMB1/PMB2), consistent with the membrane binding. Using site-directed mutagenesis, confocal and electron microscopy, and flow cytometry, we demonstrated that PBM1 and PBM2 mutant viruses retained NS3 either in the Golgi apparatus or in the endoplasmic reticulum, suggesting a distinct role for each motif. Mutation of PBM2 motif decreased NS3 export to the cell surface and virus production. However, both mutant viruses produced predominantly inner core particles that remained close to their site of assembly. Together, our data demonstrates that correct trafficking of the NS3 protein is required for virus maturation and release.

Lipid-exchange Rate Assay for Lipid Droplet Fusion in Live Cells

Lipid droplets (LDs) are central organelles in maintaining lipid homeostasis. Defective LD growth often results in the development of metabolic disorders. LD fusion and growth mediated by cell death-inducing DNA fragmentation factor alpha (DFFA)-like effector (CIDE) family proteins are crucial for various biological processes including unilocular LD formation in the adipocytes, lipid storage in the liver, milk lipid secretion in the mammary epithelia cells, and lipid secretion in the skin sebocytes. Previous methodology by Gong et al. (2011) first reported a lipid-exchange rate assay to evaluate the fusion ability of each LD pair in the cells mediated by CIDE family proteins and their regulators, but photobleaching issue remains a problem and a detailed procedure was not provided. Here, we provide an improved and detailed protocol for the lipid-exchange rate measurement. The three key steps for this assay are cell preparation, image acquisition, and data analysis. The images of the fluorescence recovery are acquired after photobleaching followed by the measurement of the intensity changes in the LD pair. The difference in fluorescent intensity is used to obtain the lipid exchange rate between the LDs. The accuracy and repetitiveness of the calculated exchange rates are assured with three-cycle of photobleaching process and the linear criteria in data fitting. With this quantitative assay, we are able to identify the functional roles of the key proteins and the effects of their mutants on LD fusion.

Lipid droplet-membrane contact sites - from protein binding to function

In the general context of an increasing prevalence of obesity-associated diseases, which follows changing paradigms in food consumption and worldwide use of industry-transformed foodstuffs, much attention has been given to the consequences of excessive fattening on health. Highly related to this clinical problem, studies at the cellular and molecular level are focused on the fundamental mechanism of lipid handling in dedicated lipid droplet (LD) organelles. This Review briefly summarizes how views on LD functions have evolved from those of a specialized intracellular compartment dedicated to lipid storage to exerting a more generalized role in the stress response. We focus on the current understanding of how proteins bind to LDs and determine their function, and on the new paradigms that have emerged from the discoveries of the multiple contact sites formed by LDs. We argue that elucidating the important roles of LD tethering to other cellular organelles allows for a better understanding of LD diversity and dynamics.