Lizhen Wu

Fsp27 promotes lipid droplet growth by lipid exchange and transfer at lipid droplet contact sites

The lipid droplet–associated protein Fsp27 mediates lipid droplet growth by promoting directional lipid transfer from smaller to larger lipid droplets.

Perilipin1 promotes unilocular lipid droplet formation through the activation of Fsp27 in adipocytes

Mature white adipocytes contain a characteristic unilocular lipid droplet. However, the molecular mechanisms underlying unilocular lipid droplet formation are poorly understood. We previously showed that Fsp27, an adipocyte-specific lipid droplet-associated protein, promotes lipid droplet growth by initiating lipid exchange and transfer. Here, we identify Perilipin1 (Plin1), another adipocyte-specific lipid droplet-associated protein, as an Fsp27 activator. Plin1 interacts with the CIDE-N domain of Fsp27 and markedly increases Fsp27-mediated lipid exchange, lipid transfer and lipid droplet growth. Functional cooperation between Plin1 and Fsp27 is required for efficient lipid droplet growth in adipocytes, as depletion of either protein impairs lipid droplet growth. The CIDE-N domain of Fsp27 forms homodimers and disruption of CIDE-N homodimerization abolishes Fsp27-mediated lipid exchange and transfer. Interestingly, Plin1 can restore the activity of CIDE-N homodimerization-defective mutants of Fsp27. We thus uncover a novel mechanism underlying lipid droplet growth and unilocular lipid droplet formation that involves the cooperative action of Fsp27 and Plin1 in adipocytes.

Cidea promotes hepatic steatosis by sensing dietary fatty acids.

High levels of dietary saturated fat have been closely associated with the development of hepatic steatosis, but the factors that mediate this process remain elusive. Here, we observed that the level of cell death‐inducing DNA fragmentation factor‐alpha‐like effector a (Cidea) expression was highly correlated with the severity of hepatic steatosis in humans. Overexpression of Cidea in mouse liver resulted in increased hepatic lipid accumulation and the formation of large lipid droplets (LDs). In contrast, mice with a Cidea deficiency had decreased lipid accumulation and alleviated hepatic steatosis when they received a high‐fat‐diet feeding or in ob/ob mice. Furthermore, the knockdown of Cidea in livers of ob/ob mice resulted in significantly reduced hepatic lipid accumulation and smaller LDs. Importantly, we observed that Cidea expression in hepatocytes was specifically induced by saturated fatty acids (FAs), and such induction was reduced when sterol response element‐binding protein (SREBP)1c was knocked down. In contrast, the overexpression of SREBP1c restored the saturated FA‐induced expression of Cidea. In addition, we observed that the stability of Cidea protein in hepatocytes increased significantly in response to treatment with FAs. Conclusion: Cidea plays critical roles in promoting hepatic lipid accumulation and in the development of hepatic steatosis by acting as a sensor that responds to diets that contain FAs. (Hepatology 2012;56:95–107)

Postlipolytic insulin-dependent remodeling of micro lipid droplets in adipocytes

Micro lipid droplets (mLDs) form during lipolysis in both primary and cultured adipocytes. Surprisingly, mLDs do not appear to bud from large LDs or require an intact cytoskeleton to form. Insulin and fatty acids trigger fusion and growth of mLDs to reform macroLDs, a process that is microtubule dependent.

Rab8a-AS160-MSS4 regulatory circuit controls lipid droplet fusion and growth.

Rab GTPases, by targeting to specific membrane compartments, play essential roles in membrane trafficking. Lipid droplets (LDs) are dynamic subcellular organelles whose growth is closely linked to obesity and hepatic steatosis. Fsp27 is shown to be required for LD fusion and growth by enriching at LD-LD contact sites. Here, we identify Rab8a as a direct interactor and regulator of Fsp27 in mediating LD fusion in adipocytes. Knockdown of Rab8a in the livers of ob/ob mice results in the accumulation of smaller LDs and lower hepatic lipid levels. Surprisingly, it is the GDP-bound form of Rab8a that exhibits fusion-promoting activity. We further discover AS160 as the GTPase activating protein (GAP) for Rab8a, which forms a ternary complex with Fsp27 and Rab8a to positively regulate LD fusion. MSS4 antagonizes Fsp27-mediated LD fusion activity through Rab8a. Our results have thus revealed a mechanistic signaling circuit controlling LD fusion and fatty liver formation.

Insulin resistance and white adipose tissue inflammation are uncoupled in energetically challenged Fsp27-deficient mice

Fsp27 is a lipid droplet-associated protein almost exclusively expressed in adipocytes where it facilitates unilocular lipid droplet formation. In mice, Fsp27 deficiency is associated with increased basal lipolysis, ‘browning’ of white fat and a healthy metabolic profile, whereas a patient with congenital CIDEC deficiency manifested an adverse lipodystrophic phenotype. Here we reconcile these data by showing that exposing Fsp27-null mice to a substantial energetic stress by crossing them with ob/ob mice or BATless mice, or feeding them a high-fat diet, results in hepatic steatosis and insulin resistance. We also observe a striking reduction in adipose inflammation and increase in adiponectin levels in all three models. This appears to reflect reduced activation of the inflammasome and less adipocyte death. These findings highlight the importance of Fsp27 in facilitating optimal energy storage in adipocytes and represent a rare example where adipose inflammation and hepatic insulin resistance are disassociated.

Cidea controls lipid droplet fusion and lipid storage in brown and white adipose tissue.

Excess lipid storage in adipose tissue results in the development of obesity and other metabolic disorders including diabetes, fatty liver and cardiovascular diseases. The lipid droplet (LD) is an important subcellular organelle responsible for lipid storage. We previously observed that Fsp27, a member of the CIDE family proteins, is localized to LD-contact sites and promotes atypical LD fusion and growth. Cidea, a close homolog of Fsp27, is expressed at high levels in brown adipose tissue. However, the exact role of Cidea in promoting LD fusion and lipid storage in adipose tissue remains unknown. Here, we expressed Cidea in Fsp27-knockdown adipocytes and observed that Cidea has similar activity to Fsp27 in promoting lipid storage and LD fusion and growth. Next, we generated Cidea and Fsp27 double-deficient mice and observed that these animals had drastically reduced adipose tissue mass and a strong lean phenotype. In addition, Cidea/Fsp27 double-deficient mice had improved insulin sensitivity and were intolerant to cold. Furthermore, we observed that the brown and white adipose tissues of Cidea/Fsp27 double-deficient mice had significantly reduced lipid storage and contained smaller LDs compared to those of Cidea or Fsp27 single deficient mice. Overall, these data reveal an important role of Cidea in controlling lipid droplet fusion, lipid storage in brown and white adipose tissue, and the development of obesity.

Differential Roles of Cell Death-inducing DNA Fragmentation Factor-α-like Effector (CIDE) Proteins in Promoting Lipid Droplet Fusion and Growth in Subpopulations of Hepatocytes

Lipid droplets (LDs) are dynamic subcellular organelles whose growth is closely linked to obesity and hepatic steatosis. Cell death-inducing DNA fragmentation factor-α-like effector (CIDE) proteins, including Cidea, Cideb, and Cidec (also called Fsp27), play important roles in lipid metabolism. Cidea and Cidec are LD-associated proteins that promote atypical LD fusion in adipocytes. Here, we find that CIDE proteins are all localized to LD-LD contact sites (LDCSs) and promote lipid transfer, LD fusion, and growth in hepatocytes. We have identified two types of hepatocytes, one with small LDs (small LD-containing hepatocytes, SLHs) and one with large LDs (large LD-containing hepatocytes, LLHs) in the liver. Cideb is localized to LDCSs and promotes lipid exchange and LD fusion in both SLHs and LLHs, whereas Cidea and Cidec are specifically localized to the LDCSs and promote lipid exchange and LD fusion in LLHs. Cideb-deficient SLHs have reduced LD sizes and lower lipid exchange activities. Fasting dramatically induces the expression of Cidea/Cidec and increases the percentage of LLHs in the liver. The majority of the hepatocytes from the liver of obese mice are Cidea/Cidec-positive LLHs. Knocking down Cidea or Cidec significantly reduced lipid storage in the livers of obese animals. Our data reveal that CIDE proteins play differential roles in promoting LD fusion and lipid storage; Cideb promotes lipid storage under normal diet conditions, whereas Cidea and Cidec are responsible for liver steatosis under fasting and obese conditions.

Imaging lipid droplet fusion and growth.

Lipid droplets (LDs) are highly dynamic cellular organelles found in most eukaryotic cell types. In white adipocytes, LDs grow into a characteristic unilocular morphology that is well suited for its specialized role as an efficient energy storage organelle. Overexpansion of LDs in white adipocytes results in the development of obesity and insulin resistance. Besides its central role in lipid storage and mobilization, LDs play crucial roles in various cellular processes including virus packaging, host defense, protein storage, and degradation. CIDE proteins, in particular Fsp27, initiates a unique LD fusion process in adipocytes by clustering and enriching at LD contact site and promoting neutral lipid exchange and transfer between contacted LDs. Here, we summarize our approaches to quantitatively measure intracellular LD size and neutral lipid exchange between LDs. Utilization of these methods has greatly facilitated our understanding of molecular pathways governing LD growth in adipocytes.

Rab18 promotes lipid droplet (LD) growth by tethering the ER to LDs through SNARE and NRZ interactions

Lipid incorporation from endoplasmic reticulum (ER) to lipid droplet (LD) is important in controlling LD growth and intracellular lipid homeostasis. However, the molecular link mediating ER and LD cross talk remains elusive. Here, we identified Rab18 as an important Rab guanosine triphosphatase in controlling LD growth and maturation. Rab18 deficiency resulted in a drastically reduced number of mature LDs and decreased lipid storage, and was accompanied by increased ER stress. Rab3GAP1/2, the GEF of Rab18, promoted LD growth by activating and targeting Rab18 to LDs. LD-associated Rab18 bound specifically to the ER-associated NAG-RINT1-ZW10 (NRZ) tethering complex and their associated SNAREs (Syntaxin18, Use1, BNIP1), resulting in the recruitment of ER to LD and the formation of direct ER–LD contact. Cells with defects in the NRZ/SNARE complex function showed reduced LD growth and lipid storage. Overall, our data reveal that the Rab18-NRZ-SNARE complex is critical protein machinery for tethering ER–LD and establishing ER–LD contact to promote LD growth.