A visible-light active p-n heterojunction NiFe-LDH/Co3O4 supported on Ni foam as photoanode for photoelectrocatalytic removal of contaminants.

Light and electricity are the most prevalent energy sources in natural environment. Herein, a visible-light active Ni foam@NiFe-LDH/Co3O4 composite was successfully prepared by loading 1D Co3O4 nanowires on the surface of 2D NiFe-LDH nanosheets to be a p-n heterojunction supported on the 3D Ni foam through hydrothermal method, which can be used as photoanode directly for photoelectrocatalytic (PEC) process to simultaneously remove bisphenol (BPA) and Cr(VI) from water. This unique Ni foam-based photoanode modified by NiFe-LDH/Co3O4 heterojunction can fully expose the active sites, enhance visible-light absorption and facilitate the migration and separation of photogenerated carriers, thus obtained a boosted efficiency for simultaneous removal of BPA and Cr(VI) under a low applied voltage. Furthermore, the convenient recyclability and excellent stability of the as-prepared Ni foam@NiFe-LDH/Co3O4 also show a great potential in environmental purification.

p-n Heterojunction of BiOI/ZnO nanorod arrays for piezo-photocatalytic degradation of bisphenol A in water.

p-n Heterojunctions of BiOI/ZnO nanorod arrays (BiOI/ZnO NRs) were prepared by loading the p-type BiOI nanosheets on the n-type ZnO nanorod arrays for efficient removal of organic contaminants in water during the piezo-photocatalytic degradation. Under concurrent visible-light irradiation and ultrasonic vibration, the bisphenol solution (50 mL, 10 mg/L) could be completely degraded within 30 min by 10 mg of 0.15 BiOI/ZnO NRs. It shows a dramatically-enhanced degradation efficiency under light irradiation and ultrasonic vibration, which is four times as high as that only under light irradiation. The excellent piezo-photocatalytic ability of BiOI/ZnO NRs could be attributed to the piezoelectric effect coupling with photocatalytic process. Under the irradiation of light, the electron-hole pairs were generated in BiOI nanosheets, and the piezoelectric potential is created inside the highly oriented one-dimensional ZnO nanorods by ultrasonic vibration, which can accelerate the migration of photogenerated carriers. It shows a strategy to effectively enhance the photocatalytic activity through utilizing the internal piezoelectric potential, which is generated by the one-dimensional nanorods with piezoelectric properties under ultrasonic vibration. So, it can promote the separation and prolong the lifetime of photogenerated carriers, and result in high-efficient degradation of organic contaminants.

CDP-diacylglycerol synthases regulate the growth of lipid droplets and adipocyte development.

The expansion of lipid droplets (LDs) and the differentiation of preadipocytes are two important aspects of mammalian lipid storage. In this study, we examined the role of CDP-diacylglycerol (DAG) synthases (CDSs), encoded by CDS1 and CDS2 genes in mammals, in lipid storage. CDS enzymes catalyze the formation of CDP-DAG from phosphatidic acid (PA). Knocking down either CDS1 or CDS2 resulted in the formation of giant or supersized LDs in cultured cells. Moreover, depleting CDS1 almost completely blocked the differentiation of 3T3-L1 preadipocytes, whereas depleting CDS2 had a moderate inhibitory effect on adipocyte differentiation. The levels of many PA species were significantly increased upon knocking down CDS1 In contrast, only a small number of PA species were increased upon depleting CDS2 Importantly, the amount of PA in the endoplasmic reticulum was dramatically increased upon knocking down CDS1 or CDS2 Our results suggest that the changes in PA level and localization may underlie the formation of giant LDs as well as the block in adipogenesis in CDS-deficient cells. We have therefore identified CDS1 and CDS2 as important novel regulators of lipid storage, and these results highlight the crucial role of phospholipids in mammalian lipid storage.

Accumulation of squalene is associated with the clustering of lipid droplets.

The isoprenoid squalene is an important precursor for the biosynthesis of sterols. The cellular storage of squalene and its impact on membrane properties have been the subject of recent investigations. In a screen for abnormal lipid droplet morphology and distribution in the yeast Saccharomyces cerevisiae, we found significant lipid droplet clustering (arbitrarily defined as an aggregation of six or more lipid droplets) in a number of mutants (e.g. erg1) that are defective in sterol biosynthesis. Interestingly, these mutants are also characterized by accumulation of large amounts of squalene. Reducing the level of squalene in these mutants restored normal lipid droplet distribution. Moreover, inhibition of squalene monooxygenase in two mammalian cell lines (CHO-K1 and 3T3-L1) by terbinafine also resulted in lipid droplet clustering. These results indicate that the level of squalene may affect the growth and distribution of lipid droplets.

Genome-wide screens for gene products regulating lipid droplet dynamics.

Lipid droplets (LDs) are emerging as dynamic cellular organelles that play a key role in lipid and membrane homeostasis. Abnormal lipid droplet dynamics are associated with the pathophysiology of many metabolic diseases, such as obesity, diabetes, atherosclerosis, fatty liver, and even cancer. Understanding the molecular mechanisms governing the dynamics of LDs, namely, their biogenesis, growth, maintenance, and degradation, will not only shed light on the cellular functions of LDs, but also provide additional clues to treatment of metabolic diseases. Genome-wide screen is a powerful approach to identify genetic factors that regulate lipid droplet dynamics. Here, we summarize recent genome-wide studies using yeast and Drosophila cells to understand the cellular dynamics of LDs. The results suggest that the genome-wide screens should be carried out in multiple organisms or cells, and using different nutritional conditions.

Overexpression of a short human seipin/BSCL2 isoform in mouse adipose tissue results in mild lipodystrophy.

Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) is a recessive disorder characterized by an almost complete loss of adipose tissue, insulin resistance, and fatty liver. BSCL2 is caused by loss-of-function mutations in the BSCL2/seipin gene, which encodes seipin. The essential role for seipin in adipogenesis has recently been established both in vitro and in vivo. However, seipin is highly upregulated at later stages of adipocyte development, and its role in mature adipocytes remains to be elucidated. We therefore generated transgenic mice overexpressing a short isoform of human BSCL2 gene (encoding 398 amino acids) using the adipocyte-specific aP2 promoter. The transgenic mice produced ∼150% more seipin than littermate controls in white adipose tissue. Surprisingly, the increased expression of seipin markedly reduced the mass of white adipose tissue and the size of adipocytes and lipid droplets. This may be due in part to elevated lipolysis rates in the transgenic mice. Moreover, there was a nearly 50% increase in the triacylglycerol content of transgenic liver. These results suggest that seipin promotes the differentiation of preadipocytes but may inhibit lipid storage in mature adipocytes.

The size and phospholipid composition of lipid droplets can influence their proteome.

The proteomic makeup of lipid droplets (LDs) is believed to regulate the function of LDs, which are now recognized as important cellular organelles that are associated with many human metabolic disorders. However, factors that help determine LD proteome remain to be identified and characterized. Here we analyzed the phospholipid and protein composition of LDs isolated from wild type (WT) yeast cells, and also from fld1Δ, cds1, and ino2Δ mutant cells which produce 'supersized' LDs. LDs of fld1Δ and WT cells exhibited similar phospholipid profiles, whereas LDs of cds1 and ino2Δ strains had a higher (cds1) or lower (ino2Δ) percentage of phosphatidylcholine than those of WT, respectively. Unexpectedly, the presence of most known LD resident proteins was greatly reduced in the LD fraction isolated from cds1 and ino2Δ, including neutral lipid hydrolases. Consistent with this result, mobilization of neutral lipids was seriously impaired in these two strains. Contrary to the reduction of LD resident proteins, the Hsp90 family molecular chaperones, Hsc82 and Hsp82, were greatly increased in the LD fractions of cds1 and ino2Δ strains without changes at the level of expression. These data demonstrate the impact of LD phopholipids and size on the makeup of LD proteome.

Molecular characterization of seipin and its mutants: implications for seipin in triacylglycerol synthesis.

The human lipodystrophy gene product Berardinelli-Seip congenital lipodystrophy 2/seipin has been implicated in adipocyte differentiation, lipid droplet (LD) formation, and motor neuron development. However, the molecular function of seipin and its disease-causing mutants remains to be elucidated. Here, we characterize seipin and its mis-sense mutants: N88S/S90L (both linked to motoneuron disorders) and A212P (linked to lipodystrophy) in cultured mammalian cells. Knocking down seipin significantly increases oleate incorporation into triacylglycerol (TAG) and the steady state level of TAG, and induces the proliferation and clustering of small LDs. By contrast, overexpression of seipin reduces TAG synthesis, leading to decreased formation of LDs. Expression of the A212P mutant, however, had little effect on LD biogenesis. Surprisingly, expression of N88S or S90L causes the formation of many small LDs reminiscent of seipin deficient cells. This dominant-negative effect may be due to the N88S/S90L-induced formation of inclusions where wild-type seipin can be trapped. Importantly, coexpression of wild-type seipin and the N88S or S90L mutant can significantly reduce the formation of inclusions. Finally, we demonstrate that seipin can interact with itself and its mutant forms. Our results provide important insights into the biochemical characteristics of seipin and its mis-sense mutants, and suggest that seipin may function to inhibit lipogenesis.

A role for phosphatidic acid in the formation of "supersized" lipid droplets.

Lipid droplets (LDs) are important cellular organelles that govern the storage and turnover of lipids. Little is known about how the size of LDs is controlled, although LDs of diverse sizes have been observed in different tissues and under different (patho)physiological conditions. Recent studies have indicated that the size of LDs may influence adipogenesis, the rate of lipolysis and the oxidation of fatty acids. Here, a genome-wide screen identifies ten yeast mutants producing "supersized" LDs that are up to 50 times the volume of those in wild-type cells. The mutated genes include: FLD1, which encodes a homologue of mammalian seipin; five genes (CDS1, INO2, INO4, CHO2, and OPI3) that are known to regulate phospholipid metabolism; two genes (CKB1 and CKB2) encoding subunits of the casein kinase 2; and two genes (MRPS35 and RTC2) of unknown function. Biochemical and genetic analyses reveal that a common feature of these mutants is an increase in the level of cellular phosphatidic acid (PA). Results from in vivo and in vitro analyses indicate that PA may facilitate the coalescence of contacting LDs, resulting in the formation of "supersized" LDs. In summary, our results provide important insights into how the size of LDs is determined and identify novel gene products that regulate phospholipid metabolism.

Seipin, adipogenesis and lipid droplets.

Seipin, the human Berardinelli-Seip congenital lipodystrophy 2 gene product, regulates adipocyte differentiation and lipid droplet (LD) formation. The molecular function of seipin, however, remains to be elucidated. Here we summarize recent advances in the investigation of congenital generalized lipodystrophies (CGLs) and the cellular dynamics of LDs. Increasing evidence suggests that phospholipids play a crucial role in some key forms of CGL and also in determining the size and distribution of LDs. We explore the hypothesis that seipin functions in the metabolism of phospholipids, and that seipin deficiency causes accumulation of lipid intermediates and/or alters membrane phospholipid profiles. These changes could lead to tissue-specific abnormalities upon seipin dysfunction, such as defective adipocyte development and clustered LDs in fibroblasts.

Conditions of endoplasmic reticulum stress stimulate lipid droplet formation in Saccharomyces cerevisiae.

LDs (lipid droplets) are cellular organelles which can be found in nearly all eukaryotic cells. Despite their importance in cell biology, the mechanism underlying LD biogenesis remains largely unknown. In the present study we report that conditions of ER (endoplasmic reticulum) stress stimulate LD formation in Saccharomyces cerevisiae. We found that LDs accumulated in yeast mutants with compromised protein glycosylation or ER-associated protein degradation. Moreover, tunicamycin and Brefeldin A, agents which induce ER stress, were found to stimulate LD formation. In contrast, the restoration of protein glycosylation reduced LD accumulation. Interestingly, enhanced neutral lipids synthesis and LD formation under conditions of ER stress was not dependent on Ire1p. Lastly, we demonstrated that the absence of LDs did not compromise cell viability under ER stress. Our results suggest that although more LDs are produced, LDs are not essential to cell survival under ER stress.

Fld1p, a functional homologue of human seipin, regulates the size of lipid droplets in yeast.

Lipid droplets (LDs) are emerging cellular organelles that are of crucial importance in cell biology and human diseases. In this study, we present our screen of approximately 4,700 Saccharomyces cerevisiae mutants for abnormalities in the number and morphology of LDs; we identify 17 fld (few LDs) and 116 mld (many LDs) mutants. One of the fld mutants (fld1) is caused by the deletion of YLR404W, a previously uncharacterized open reading frame. Cells lacking FLD1 contain strikingly enlarged (supersized) LDs, and LDs from fld1Delta cells demonstrate significantly enhanced fusion activities both in vivo and in vitro. Interestingly, the expression of human seipin, whose mutant forms are associated with Berardinelli-Seip congenital lipodystrophy and motoneuron disorders, rescues LD-associated defects in fld1Delta cells. Lipid profiling reveals alterations in acyl chain compositions of major phospholipids in fld1Delta cells. These results suggest that an evolutionally conserved function of seipin in phospholipid metabolism and LD formation may be functionally important in human adipogenesis.

Genome-wide analysis of sterol-lipid storage and trafficking in Saccharomyces cerevisiae.

The pandemic of lipid-related disease necessitates a determination of how cholesterol and other lipids are transported and stored within cells. The first step in this determination is the identification of the genes involved in these transport and storage processes. Using genome-wide screens, we identified 56 yeast (Saccharomyces cerevisiae) genes involved in sterol-lipid biosynthesis, intracellular trafficking, and/or neutral-lipid storage. Direct biochemical and cytological examination of mutant cells revealed an unanticipated link between secretory protein glycosylation and triacylglycerol (TAG)/steryl ester (SE) synthesis for the storage of lipids. Together with the analysis of other deletion mutants, these results suggested at least two distinct events for the biogenesis of lipid storage particles: a step affecting neutral-lipid synthesis, generating the lipid core of storage particles, and another step for particle assembly. In addition to the lipid storage mutants, we identified mutations that affect the localization of unesterified sterols, which are normally concentrated in the plasma membrane. These findings implicated phospholipase C and the protein phosphatase Ptc1p in the regulation of sterol distribution within cells. This study identified novel sterol-related genes that define several distinct processes maintaining sterol homeostasis.