Phosphatidic acid in membrane rearrangements

Nickel-induced oxidative stress and phospholipid remodeling in cucumber leaves

Nickel phytotoxicity has been attributed, among others, to oxidative stress. However, little is known about Ni-induced phospholipid modifications, including the oxidative ones. Accumulation of reactive oxygen species (ROS), antioxidative enzyme activities, malondialdehyde and the early lipid oxidation products contents, membrane permeability, phospholipid profile as well as phospholipid unsaturation degree were studied in the 1st and the 2nd leaves of hydroponically grown cucumber seedlings subjected to Ni stress. Compared to the 2nd leaf the 1st one showed stronger visual Ni toxicity symptoms, higher Ni, O2.- and H2O2 accumulation as well as greater enhancement in membrane permeability. Enzyme activities were differently influenced by Ni stress, however most pronounced changes were generally found in the 1st leaf. Ni treatment resulted in oxidation of leaf lipids, which was evidenced by appearance of increased contents of MDA and the early produced oxylipins. Among the latter 9-hydroxyoctadecatrienoic acid (9-HOTrE) and 13-hydroxyoctadecatrienoic acid (13-HOTrE) contents showed the most pronounced increase in response to Ni treatment. Exposure to the metal led to the changes in the leaf phospholipid profile and increased degree of phospholipid unsaturation. The obtained results have been discussed in relation to the difference in Ni stress severity between the 1st and the 2nd leaves.

Mammalian START-like phosphatidylinositol transfer proteins - Physiological perspectives and roles in cancer biology

PtdIns and its phosphorylated derivatives, the phosphoinositides, are the biochemical components of a major pathway of intracellular signaling in all eukaryotic cells. These lipids are few in terms of cohort of unique positional isomers, and are quantitatively minor species of the bulk cellular lipidome. Nevertheless, phosphoinositides regulate an impressively diverse set of biological processes. It is from that perspective that perturbations in phosphoinositide-dependent signaling pathways are increasingly being recognized as causal foundations of many human diseases - including cancer. Although phosphatidylinositol transfer proteins (PITPs) are not enzymes, these proteins are physiologically significant regulators of phosphoinositide signaling. As such, PITPs are conserved throughout the eukaryotic kingdom. Their biological importance notwithstanding, PITPs remain understudied. Herein, we review current information regarding PITP biology primarily focusing on how derangements in PITP function disrupt key signaling/developmental pathways and are associated with a growing list of pathologies in mammals.

GhCASPL1 regulates secondary cell wall thickening in cotton fibers by stabilizing the cellulose synthase complex on the plasma membrane

Cotton (Gossypium hirsutum) fibers are elongated single cells that rapidly accumulate cellulose during secondary cell wall (SCW) thickening, which requires cellulose synthase complex (CSC) activity. Here, we describe the CSC-interacting factor CASPARIAN STRIP MEMBRANE DOMAIN-LIKE1 (GhCASPL1), which contributes to SCW thickening by influencing CSC stability on the plasma membrane. GhCASPL1 is preferentially expressed in fiber cells during SCW biosynthesis and encodes a MARVEL domain protein. The ghcaspl1 ghcaspl2 mutant exhibited reduced plant height and produced mature fibers with fewer natural twists, lower tensile strength, and a thinner SCW compared to the wild type. Similarly, the Arabidopsis (Arabidopsis thaliana) caspl1 caspl2 double mutant showed a lower cellulose content and thinner cell walls in the stem vasculature than the wild type but normal plant morphology. Introducing the cotton gene GhCASPL1 successfully restored the reduced cellulose content of the Arabidopsis caspl1 caspl2 mutant. Detergent treatments, ultracentrifugation assays, and enzymatic assays showed that the CSC in the ghcaspl1 ghcaspl2 double mutant showed reduced membrane binding and decreased enzyme activity compared to the wild type. GhCASPL1 binds strongly to phosphatidic acid (PA), which is present in much higher amounts in thickening fiber cells compared to ovules and leaves. Mutating the PA-binding site in GhCASPL1 resulted in the loss of its colocalization with GhCesA8, and it failed to localize to the plasma membrane. PA may alter membrane structure to facilitate protein-protein interactions, suggesting that GhCASPL1 and PA collaboratively stabilize the CSC. Our findings shed light on CASPL functions and the molecular machinery behind SCW biosynthesis in cotton fibers.

Edible insects as a novel source of lecithin: Extraction and lipid characterization of black soldier fly larvae and yellow mealworm

Edible insects with high fat and phosphorus content are a potential novel source of lecithin, however, studies on their minor lipids are limited. In this study, lecithin was extracted from black soldier fly larvae and yellow mealworm. Herein, the effects of lecithin extraction method, matrix and ultrasound pretreatment were explored based on the fatty acid composition and phospholipid profile with soy lecithin as a reference. The use of a wet matrix and ultrasound pretreatment increased the extraction efficiency of total PLs from both insects. Insect lecithin contained a considerable amount of sphingomyelin compared to soy lecithin. In insect lecithin, a total of 47 glycerophospholipid and sphingomyelin molecular species, as well as four molecular species of fatty acyl esters of hydroxy fatty acid, were detected. This study is the first comprehensive investigation of insects as a new source of lecithin with applications in food, cosmetics and in the pharmaceutical industry.

Mechanisms of nuclear envelope expansion

In actively dividing eukaryotic cells, the nuclear envelope membrane (NEM) expands during the cell cycle to accommodate increases in nuclear volume and formation of two nuclei as a cell passes through mitosis to form daughter cells. NEM expansion is driven by glycerophospholipid (GPL) synthesis that is regulated by the lipin family of phosphatidic acid phosphatases (PAPs). How, and when during the cell cycle, PAPs regulate membrane expansion differs between organisms undergoing a closed or open mitosis. Here, we discuss recent studies that shed light on the mechanisms of NE expansion. Moreover, we examine evidence that NEM expansion not only employs GPLs synthesized in the ER but also lipids whose synthesis is regulated by events at the inner nuclear membrane.

In situ metabolomic analysis of osteonecrosis of the femoral head (ONFH) using MALDI MSI

Osteonecrosis of the femoral head (ONFH) is a common orthopedic disease characterized by disability and deformity. To better understand ONFH at molecular level and to explore the possibility of early diagnosis, instead of diagnosis based on macroscopic spatial characteristics, a matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) method was developed for ONFH disease for the first time. The most challenging step for ONFH MSI is to deal with human bone tissues which are much harder than the other biological samples studied by the reported MSI studies. In this work, the MSI sectioning method of hard bone tissues was established using tender acids and a series of test criteria. Small-molecule metabolites, such as lipids and amino acids, were detected in bone sections, realizing the in situ detection of spatial distribution of biometabolites. By comparing the distribution of metabolites from different regions of normal femoral head, ONFH bone tissue (ONBT), and adjacent ONFH bone tissue (ANBT), the whole process of femoral head from normal stage to necrosis was monitored and visualized at molecular level. Moreover, this developed MSI method was used for metabolomics study of ONFH. 72 differential metabolites were identified, suggesting that disturbances in energy metabolism and lipid metabolism affected the normal life activities of osteoblasts and osteoclasts. This study provides new perspectives for future pathological studies of ONFH.

Effect of leaflet asymmetry on the stretching elasticity of lipid bilayers with phosphatidic acid

The asymmetry of membranes has a significant impact on their biophysical characteristics and behavior. This study investigates the composition and mechanical properties of symmetric and asymmetric membranes in giant unilamellar vesicles (GUVs) made of palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidic acid (POPA). A combination of fluorescence quantification, zeta potential measurements, micropipette aspiration, and bilayer molecular dynamics simulations are used to characterize these membranes. The outer leaflet composition in vesicles is found consistent across the two preparation methods we employed, namely electroformation and inverted emulsion transfer. However, characterizing the inner leaflet poses challenges. Micropipette aspiration of GUVs show that oil residues do not substantially alter membrane elasticity, but simulations reveal increased membrane thickness and decreased interleaflet coupling in the presence of oil. Asymmetric membranes with a POPC:POPA mixture in the outer leaflet and POPC in the inner leaflet display similar stretching elasticity values to symmetric POPC:POPA membranes, suggesting potential POPA insertion into the inner leaflet during vesicle formation and suppressed asymmetry. The inverse compositional asymmetry, with POPC in the outer leaflet and POPC:POPA in the inner one yield less stretchable membranes with higher compressibility modulus compared with their symmetric counterparts. Challenges in achieving and predicting compositional correspondence highlight the limitations of phase-transfer-based methods. In addition, caution is advised when using fluorescently labeled lipids (even at low fractions of 0.5 mol %), as unexpected gel-like domains in symmetric POPC:POPA membranes were observed only with a specific type of labeled DOPE (dioleoylphosphatidylethanolamine) and the same fraction of unlabeled DOPE. The latter suggest that such domain formation may result from interactions between lipids and membrane fluorescent probes. Overall, this study underscores the complexity of factors influencing GUV membrane asymmetry, emphasizing the need for further research and improvement of characterization techniques.

The role of membrane physiology in sHSP Lo18-lipid interaction and lipochaperone activity

To cope with environmental stresses, organisms, including lactic acid bacteria such as O. oeni, produce stress proteins called HSPs. In wine, O. oeni is constantly confronted by stress affecting its membrane fluidity. To survive through in these deleterious conditions, O. oeni synthesizes Lo18, a unique, small HSP which acts as a molecular chaperone and a lipochaperone. The molecular mechanism underlying its lipochaperone activity, particularly regarding membrane lipid composition, remains poorly understood. In this context, Lo18 lipochaperone activity and the associated modification in protein structure were studied during interaction with different liposomes from O. oeni cultures representing unstressed, stressed and stressed-adapted physiological states. The results showed that the presence of the membrane (whatever its nature) induces a modification of Lo18's structure. Also, the presence of oleic acid and/or phosphatidylglycerol is important to favor Lo18-membrane interaction, allowing lipochaperone activity. This research enhances understanding of sHSP-membrane interactions in bacterial systems.

Lipidomics and mass spectrometry imaging unveil alterations in mice hippocampus lipid composition exposed to hypoxia

Lipids are components of cytomembranes that are involved in various biochemical processes. High-altitude hypoxic environments not only affect the body's energy metabolism, but these environments can also cause abnormal lipid metabolism involved in the hypoxia-induced cognitive impairment. Thus, comprehensive lipidomic profiling of the brain tissue is an essential step toward understanding the mechanism of cognitive impairment induced by hypoxic exposure. In the present study, mice showed reduced new-object recognition and spatial memory when exposed to hypobaric hypoxia for 1 day. Histomorphological staining revealed significant morphological and structural damage to the hippocampal tissue, along with prolonged exposure to hypobaric hypoxia. Dynamic lipidomics of the mouse hippocampus showed a significant shift in both the type and distribution of phospholipids, as verified by spatial lipid mapping. Collectively, a diverse and dynamic lipid composition in mice hippocampus was uncovered, which deepens our understanding of biochemical changes during sustained hypoxic exposure and could provide new insights into the cognitive decline induced by high-altitude hypoxia exposure.

Phosphatidic Acid Promoting the Generation of Interleukin-17A Producing Double-Negative T Cells by Enhancing mTORC1 Signaling in Lupus

OBJECTIVE

The goal was to investigate the role and intracellular regulatory mechanisms of double-negative T (DNT) cells in the pathogenesis of systemic lupus erythematosus (SLE).

METHODS

DNT cells were assessed in murine models, patients with SLE, and controls using flow cytometry (FCM). DNT cells from either resiquimod (R848) or vehicle-treated C57BL/6 (B6) mice were cultured with B cells from R848-treated mice to explore functions. Differential mechanistic target of rapamycin (mTOR) pathway signaling in DNT cells measured using FCM and quantitative polymerase chain reaction was validated by rapamycin inhibition. Candidate lipid metabolites detected using liquid chromatography with electrospray ionization mass spectrometry/mass spectrometry were functionally assessed in DNT cell cultures.

RESULTS

DNT cells were markedly increased in both spontaneous and induced mouse lupus models and in patients with SLE. Expanded DNT cells from R848-treated B6 mice produced elevated interleukin (IL)-17A and IgG with increased germinal center B (GCB) cells. Expansion of DNT cells associated with activation of mTORC1 pathway that both IL-17A levels and the number of DNT cells exhibited dose-dependent reduction with rapamycin treatment. Lipidomics studies revealed differential patterns of lipid metabolites in T cells of R848-treated mice. Among candidate metabolites, elevated phosphatidic acid (PA) that was partially controlled by phospholipase D2 increased the expression of the mTORC1 downstream target p-S6 and positively expanded IL-17A-producing DNT cells. Similarly, elevated proportions of circulating DNT cells in patients with SLE correlated with disease activity and proteinuria, and IL-17A secretion was elevated after in vitro PA stimulation.

CONCLUSION

The accumulation of PA in T cells could activate the mTORC1 pathway, promoting DNT cell expansion and IL-17A secretion, resulting in GCB cell abnormalities in lupus.

The role of exosomal molecular cargo in exosome biogenesis and disease diagnosis

Exosomes represent a type of extracellular vesicles derived from the endosomal pathway that transport diverse molecular cargoes such as proteins, lipids, and nucleic acids. These cargoes have emerged as crucial elements impacting disease diagnosis, treatment, and prognosis, and are integral to the process of exosome formation. This review delves into the essential molecular cargoes implicated in the phases of exosome production and release. Emphasis is placed on their significance as cancer biomarkers and potential therapeutic targets, accompanied by an exploration of the obstacles and feasible applications linked to these developments.

Phosphatidic acid-dependent recruitment of microtubule motors to spherical supported lipid bilayers for in vitro motility assays

Motor proteins transport diverse membrane-bound vesicles along microtubules inside cells. How specific lipids, particularly rare lipids, on the membrane recruit and activate motors is poorly understood. To address this, we prepare spherical supported lipid bilayers (SSLBs) consisting of a latex bead enclosed within a membrane of desired lipid composition. SSLBs containing phosphatidic acid recruit dynein when incubated with Dictyostelium fractions but kinesin-1 when incubated with rat brain fractions. These SSLBs allow controlled biophysical investigation of membrane-bound motors along with their regulators at the single-cargo level in vitro. Optical trapping of single SSLBs reveals that motor-specific inhibitors can "lock" a motor to a microtubule, explaining the paradoxical arrest of overall cargo transport by such inhibitors. Increasing their size causes SSLBs to reverse direction more frequently, relevant to how large cargoes may navigate inside cells. These studies are relevant to understand how unidirectional or bidirectional motion of vesicles might be generated.

Spatially resolved lipidomics shows conditional transfer of lipids produced by Bacteroides thetaiotaomicron into the mouse gut

The extent to which bacterial lipids produced by the gut microbiota penetrate host tissues is unclear. Here, we combined mass spectrometry approaches to identify lipids produced by the human gut symbiont Bacteroides thetaiotaomicron (B. theta) and spatially track these bacterial lipids in the mouse colon. We characterize 130 B. theta lipids by liquid chromatography-tandem mass spectrometry (LC-MS/MS), using wild-type and mutant B. theta strains to confidently identify lipid structures and their interconnected pathways in vitro. Of these, 103 B. theta lipids can be detected and spatially mapped in a single MALDI mass spectrometry imaging run. We map unlabeled bacterial lipids across colon sections of germ-free and specific-pathogen-free (SPF) mice and mice mono-colonized with wild-type or sphingolipid-deficient (BTMUT) B. theta. We observe co-localization of bacterially derived phosphatidic acid with host tissues in BTMUT mice, consistent with lipid penetration into host tissues. These results indicate limited and selective transfer of bacterial lipids to the host.

Haemolysins are essential to the pathogenicity of deep-sea Vibrio fluvialis

Vibrio fluvialis is an emerging foodborne pathogen that produces VFH (Vibrio fluvialis hemolysin) and δVFH (delta-Vibrio fluvialis hemolysin). The function of δVFH is unclear. Currently, no pathogenic V. fluvialis from deep sea has been reported. In this work, a deep-sea V. fluvialis isolate (V13) was examined for pathogenicity. V13 was most closely related to V. fluvialis ATCC 33809, a human isolate, but possessed 262 unique genes. V13 caused lethal infection in fish and induced pyroptosis involving activation of the NLRP3 inflammasome, caspase 1 (Casp1), and gasdermin D (GSDMD). V13 defective in VFH or VFH plus δVFH exhibited significantly weakened cytotoxicity. Recombinant δVFH induced NLRP3-Casp1-GSDMD-mediated pyroptosis in a manner that depended on K+ efflux and intracellular Ca2+ accumulation. δVFH bound several plasma membrane lipids, and these bindings were crucial for δVFH cytotoxicity. Together these results provided new insights into the function of δVFH and the virulence mechanism of V. fluvialis.

Identification and characterization of a new potent inhibitor targeting CtBP1/BARS in melanoma cells

BACKGROUND

The C-terminal-binding protein 1/brefeldin A ADP-ribosylation substrate (CtBP1/BARS) acts both as an oncogenic transcriptional co-repressor and as a fission inducing protein required for membrane trafficking and Golgi complex partitioning during mitosis, hence for mitotic entry. CtBP1/BARS overexpression, in multiple cancers, has pro-tumorigenic functions regulating gene networks associated with "cancer hallmarks" and malignant behavior including: increased cell survival, proliferation, migration/invasion, epithelial-mesenchymal transition (EMT). Structurally, CtBP1/BARS belongs to the hydroxyacid-dehydrogenase family and possesses a NAD(H)-binding Rossmann fold, which, depending on ligands bound, controls the oligomerization of CtBP1/BARS and, in turn, its cellular functions. Here, we proposed to target the CtBP1/BARS Rossmann fold with small molecules as selective inhibitors of mitotic entry and pro-tumoral transcriptional activities.

METHODS

Structured-based screening of drug databases at different development stages was applied to discover novel ligands targeting the Rossmann fold. Among these identified ligands, N-(3,4-dichlorophenyl)-4-{[(4-nitrophenyl)carbamoyl]amino}benzenesulfonamide, called Comp.11, was selected for further analysis. Fluorescence spectroscopy, isothermal calorimetry, computational modelling and site-directed mutagenesis were employed to define the binding of Comp.11 to the Rossmann fold. Effects of Comp.11 on the oligomerization state, protein partners binding and pro-tumoral activities were evaluated by size-exclusion chromatography, pull-down, membrane transport and mitotic entry assays, Flow cytometry, quantitative real-time PCR, motility/invasion, and colony assays in A375MM and B16F10 melanoma cell lines. Effects of Comp.11 on tumor growth in vivo were analyzed in mouse tumor model.

RESULTS

We identify Comp.11 as a new, potent and selective inhibitor of CtBP1/BARS (but not CtBP2). Comp.11 directly binds to the CtBP1/BARS Rossmann fold affecting the oligomerization state of the protein (unlike other known CtBPs inhibitors), which, in turn, hinders interactions with relevant partners, resulting in the inhibition of both CtBP1/BARS cellular functions: i) membrane fission, with block of mitotic entry and cellular secretion; and ii) transcriptional pro-tumoral effects with significantly hampered proliferation, EMT, migration/invasion, and colony-forming capabilities. The combination of these effects impairs melanoma tumor growth in mouse models.  CONCLUSIONS: This study identifies a potent and selective inhibitor of CtBP1/BARS active in cellular and melanoma animal models revealing new opportunities to study the role of CtBP1/BARS in tumor biology and to develop novel melanoma treatments.

Phosphatidic acid: from biophysical properties to diverse functions

Phosphatidic acid (PA), the simplest phospholipid, acts as a key metabolic intermediate and second messenger that impacts diverse cellular and physiological processes across species ranging from microbes to plants and mammals. The cellular levels of PA dynamically change in response to stimuli, and multiple enzymatic reactions can mediate its production and degradation. PA acts as a signalling molecule and regulates various cellular processes via its effects on membrane tethering, enzymatic activities of target proteins, and vesicular trafficking. Because of its unique physicochemical properties compared to other phospholipids, PA has emerged as a class of new lipid mediators influencing membrane structure, dynamics, and protein interactions. This review summarizes the biosynthesis, dynamics, and cellular functions and properties of PA.

Mapping the global interactome of the ARF family reveals spatial organization in cellular signaling pathways

The ADP-ribosylation factors (ARFs) and ARF-like (ARL) GTPases serve as essential molecular switches governing a wide array of cellular processes. In this study, we used proximity-dependent biotin identification (BioID) to comprehensively map the interactome of 28 out of 29 ARF and ARL proteins in two cellular models. Through this approach, we identified ∼3000 high-confidence proximal interactors, enabling us to assign subcellular localizations to the family members. Notably, we uncovered previously undefined localizations for ARL4D and ARL10. Clustering analyses further exposed the distinctiveness of the interactors identified with these two GTPases. We also reveal that the expression of the understudied member ARL14 is confined to the stomach and intestines. We identified phospholipase D1 (PLD1) and the ESCPE-1 complex, more precisely, SNX1, as proximity interactors. Functional assays demonstrated that ARL14 can activate PLD1 in cellulo and is involved in cargo trafficking via the ESCPE-1 complex. Overall, the BioID data generated in this study provide a valuable resource for dissecting the complexities of ARF and ARL spatial organization and signaling.

Predicting lipid sorting in curved membranes

Most biological membranes are curved, and both lipids and proteins play a role in generating curvature. For any given membrane shape and composition, it is not trivial to determine whether lipids are laterally distributed in a homogeneous or inhomogeneous way, and whether the inter-leaflet distribution is symmetric or not. Here we present a simple computational tool that allows to predict the preference of any lipid type for membranes with positive vs. negative curvature, for any given value of curvature. The tool is based on molecular dynamics simulations of tubular membranes with hydrophilic pores. The pores allow spontaneous, barrierless flip-flop of most lipids, while also preventing differences in pressure between the inner and outer water compartments and minimizing membrane asymmetric stresses. Specifically, we provide scripts to build and analyze the simulations. We test the tool by performing simulations on simple binary lipid mixtures, and we show that, as expected, lipids with negative intrinsic curvature distribute to the tubule inner leaflet, the more so when the radius of the tubular membrane is small. Compared to other existing computational methods, relying on membrane buckles and tethers, our method is based on spontaneous inter-leaflet transport of lipids, and therefore allows to explore lipid distribution in asymmetric membranes. The method can easily be adapted to work with any molecular dynamics code and any force field.

Unlocking the Mitochondria for Nanomedicine-based Treatments: Overcoming Biological Barriers, Improving Designs, and Selecting Verification Techniques

Enhanced targeting approaches will support the treatment of diseases associated with dysfunctional mitochondria, which play critical roles in energy generation and cell survival. Obstacles to mitochondria-specific targeting include the presence of distinct biological barriers and the need to pass through (or avoid) various cell internalization mechanisms. A range of studies have reported the design of mitochondrially-targeted nanomedicines that navigate the complex routes required to influence mitochondrial function; nonetheless, a significant journey lies ahead before mitochondrially-targeted nanomedicines become suitable for clinical use. Moving swiftly forward will require safety studies, in vivo assays confirming effectiveness, and methodologies to validate mitochondria-targeted nanomedicines' subcellular location/activity. From a nanomedicine standpoint, we describe the biological routes involved (from administration to arrival within the mitochondria), the features influencing rational design, and the techniques used to identify/validate successful targeting. Overall, rationally-designed mitochondria-targeted-based nanomedicines hold great promise for precise subcellular therapeutic delivery.

The HEAT repeat protein HPO-27 is a lysosome fission factor

Lysosomes are degradation and signalling centres crucial for homeostasis, development and ageing1. To meet diverse cellular demands, lysosomes remodel their morphology and function through constant fusion and fission2,3. Little is known about the molecular basis of fission. Here we identify HPO-27, a conserved HEAT repeat protein, as a lysosome scission factor in Caenorhabditis elegans. Loss of HPO-27 impairs lysosome fission and leads to an excessive tubular network that ultimately collapses. HPO-27 and its human homologue MROH1 are recruited to lysosomes by RAB-7 and enriched at scission sites. Super-resolution imaging, negative-staining electron microscopy and in vitro reconstitution assays reveal that HPO-27 and MROH1 self-assemble to mediate the constriction and scission of lysosomal tubules in worms and mammalian cells, respectively, and assemble to sever supported membrane tubes in vitro. Loss of HPO-27 affects lysosomal morphology, integrity and degradation activity, which impairs animal development and longevity. Thus, HPO-27 and MROH1 act as self-assembling scission factors to maintain lysosomal homeostasis and function.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

Mapping the global interactome of the ARF family reveals spatial organization in cellular signaling pathways

The ADP-ribosylation factors (ARFs) and ARF-like (ARLs) GTPases serve as essential molecular switches governing a wide array of cellular processes. In this study, we utilized proximity-dependent biotin identification (BioID) to comprehensively map the interactome of 28 out of 29 ARF and ARL proteins in two cellular models. Through this approach, we identified ~3000 high-confidence proximal interactors, enabling us to assign subcellular localizations to the family members. Notably, we uncovered previously undefined localizations for ARL4D and ARL10. Clustering analyses further exposed the distinctiveness of the interactors identified with these two GTPases. We also reveal that the expression of the understudied member ARL14 is confined to the stomach and intestines. We identified phospholipase D1 (PLD1) and the ESCPE-1 complex, more precisely SNX1, as proximity interactors. Functional assays demonstrated that ARL14 can activate PLD1 in cellulo and is involved in cargo trafficking via the ESCPE-1 complex. Overall, the BioID data generated in this study provide a valuable resource for dissecting the complexities of ARF and ARL spatial organization and signaling.

Mitofusin-2 induced by exercise modifies lipid droplet-mitochondria communication, promoting fatty acid oxidation in male mice with NAFLD

BACKGROUND AND AIM

The excessive accumulation of lipid droplets (LDs) is a defining characteristic of nonalcoholic fatty liver disease (NAFLD). The interaction between LDs and mitochondria is functionally important for lipid metabolism homeostasis. Exercise improves NAFLD, but it is not known if it has an effect on hepatic LD-mitochondria interactions. Here, we investigated the influence of exercise on LD-mitochondria interactions and its significance in the context of NAFLD.

APPROACH AND RESULTS

Mice were fed high-fat diet (HFD) or HFD-0.1 % methionine and choline-deficient diet (MCD) to emulate simple hepatic steatosis or non-alcoholic steatohepatitis, respectively. In both models, aerobic exercise decreased the size of LDs bound to mitochondria and the number of LD-mitochondria contacts. Analysis showed that the effects of exercise on HOMA-IR and liver triglyceride levels were independent of changes in body weight, and a positive correlation was observed between the number of LD-mitochondria contacts and NAFLD severity and with the lipid droplet size bound to mitochondria. Cellular fractionation studies revealed that ATP-coupled respiration and fatty acid oxidation (FAO) were greater in hepatic peridroplet mitochondria (PDM) from HFD-fed exercised mice than from equivalent sedentary mice. Finally, exercise increased FAO and mitofusin-2 abundance exclusively in PDM through a mechanism involving the curvature of mitochondrial membranes and the abundance of saturated lipids. Accordingly, hepatic mitofusin-2 ablation prevented exercise-induced FAO in PDM.

CONCLUSIONS

This study demonstrates that aerobic exercise has beneficial effects in murine NAFLD models by lessening the interactions between hepatic LDs and mitochondria, and by decreasing LD size, correlating with a reduced severity of NAFLD. Additionally, aerobic exercise increases FAO in PDM and this process is reliant on Mfn-2 enrichment, which modifies LD-mitochondria communication.

Nir1-LNS2 is a novel phosphatidic acid biosensor that reveals mechanisms of lipid production

Despite various roles of phosphatidic acid (PA) in cellular functions such as lipid homeostasis and vesicular trafficking, there is a lack of high-affinity tools to study PA in live cells. After analysis of the predicted structure of the LNS2 domain in the lipid transfer protein Nir1, we suspected that this domain could serve as a novel PA biosensor. We created a fluorescently tagged Nir1-LNS2 construct and then performed liposome binding assays as well as pharmacological and genetic manipulations of HEK293A cells to determine how specific lipids affect the interaction of Nir1-LNS2 with membranes. We found that Nir1-LNS2 bound to both PA and PIP2 in vitro. Interestingly, only PA was necessary and sufficient to localize Nir1-LNS2 to membranes in cells. Nir1-LNS2 also showed a heightened responsiveness to PA when compared to biosensors using the Spo20 PA binding domain (PABD). Nir1-LNS2's high sensitivity revealed a modest but discernible contribution of PLD to PA production downstream of muscarinic receptors, which has not been visualized with previous Spo20-based probes. In summary, Nir1-LNS2 emerges as a versatile and sensitive biosensor, offering researchers a new powerful tool for real-time investigation of PA dynamics in live cells.

Methodological Pitfalls of Investigating Lipid Rafts in the Brain: What Are We Still Missing?

The purpose of this review is to succinctly examine the methodologies used in lipid raft research in the brain and to highlight the drawbacks of some investigative approaches. Lipid rafts are biochemically and biophysically different from the bulk membrane. A specific lipid environment within membrane domains provides a harbor for distinct raftophilic proteins, all of which in concert create a specialized platform orchestrating various cellular processes. Studying lipid rafts has proved to be arduous due to their elusive nature, mobility, and constant dynamic reorganization to meet the cellular needs. Studying neuronal lipid rafts is particularly cumbersome due to the immensely complex regional molecular architecture of the central nervous system. Biochemical fractionation, performed with or without detergents, is still the most widely used method to isolate lipid rafts. However, the differences in solubilization when various detergents are used has exposed a dire need to find more reliable methods to study particular rafts. Biochemical methods need to be complemented with other approaches such as live-cell microscopy, imaging mass spectrometry, and the development of specific non-invasive fluorescent probes to obtain a more complete image of raft dynamics and to study the spatio-temporal expression of rafts in live cells.

CLSTN3B enhances adipocyte lipid droplet structure and function via endoplasmic reticulum contact

Interorganelle contacts facilitate material exchanges and sustain the structural and functional integrity of organelles. Lipid droplets (LDs) of adipocytes are responsible for energy storage and mobilization responding to body needs. LD biogenesis defects compromise the lipid-storing capacity of adipocytes, resulting in ectopic lipid deposition and metabolic disorders, yet how the uniquely large LDs in adipocytes attain structural and functional maturation is incompletely understood. Here we show that the mammalian adipocyte-specific protein CLSTN3B is crucial for adipocyte LD maturation. CLSTN3B employs an arginine-rich segment to promote extensive contact and hemifusion-like structure formation between the endoplasmic reticulum (ER) and LD, allowing ER-to-LD phospholipid diffusion during LD expansion. CLSTN3B ablation results in reduced LD surface phospholipid density, increased turnover of LD-surface proteins, and impaired LD functions. Our results establish the central role of CLSTN3B in the adipocyte-specific LD maturation pathway that enhances lipid storage and maintenance of metabolic health under caloric overload.

Disruption of CFAP418 interaction with lipids causes widespread abnormal membrane-associated cellular processes in retinal degenerations

Syndromic ciliopathies and retinal degenerations are large heterogeneous groups of genetic diseases. Pathogenic variants in the CFAP418 gene may cause both disorders, and its protein sequence is evolutionarily conserved. However, the disease mechanism underlying CFAP418 mutations has not been explored. Here, we apply quantitative lipidomic, proteomic, and phosphoproteomic profiling and affinity purification coupled with mass spectrometry to address the molecular function of CFAP418 in the retina. We show that CFAP418 protein binds to the lipid metabolism precursor phosphatidic acid (PA) and mitochondrion-specific lipid cardiolipin but does not form a tight and static complex with proteins. Loss of Cfap418 in mice disturbs membrane lipid homeostasis and membrane-protein associations, which subsequently causes mitochondrial defects and membrane-remodeling abnormalities across multiple vesicular trafficking pathways in photoreceptors, especially the endosomal sorting complexes required for transport (ESCRT) pathway. Ablation of Cfap418 also increases the activity of PA-binding protein kinase Cα in the retina. Overall, our results indicate that membrane lipid imbalance is a pathological mechanism underlying syndromic ciliopathies and retinal degenerations which is associated with other known causative genes of these diseases.

Isolation of plant-derived exosome-like nanoparticles (PDENs) from Solanum nigrum L. berries and Their Effect on interleukin-6 expression as a potential anti-inflammatory agent

Inflammation is a temporary response of the immune system that can be treated using common anti-inflammatory drugs. However, prolonged use of these drugs increases the risk of adverse side effects. Accordingly, there is an increasing need for alternative treatments for inflammation with fewer side effects. Exosomes are extracellular vesicles secreted by most eukaryotic cells and have been studied as a candidate for cell-free therapy for inflammatory diseases due to their immunomodulatory and anti-inflammatory properties. In recent years, the focus of exosome research has shifted from animal cell-derived exosomes to plant-derived exosome-like nanoparticles (PDENs). Plant-derived exosome-like nanoparticles (PDENs) are easier to obtain, have minimal safety concerns, and can be produced in higher quantities and lower cost than exosomes derived from animal cells. In this study, the isolation and analysis of the anti-inflammatory potential of PDENs from black nightshade berries (Solanum nigrum L.) were carried out. The results of isolation and characterization showed that PDENs had a spherical morphology, measuring around 107 nm with zeta potential of -0.6 mV, and had a protein concentration of 275.38 μg/mL. PDENs were also shown to be internalized by RAW264.7 macrophage cell line after 2 hours of incubation and had no cytotoxicity effect up to the concentration of 2.5 μg/mL. Furthermore, exposure to several doses of PDENs to the LPS-stimulated RAW264.7 cell significantly decreased the expression of pro-inflammatory cytokine gene IL-6, as well as the expression of IL-6 protein up to 97,28%. GC-MS analysis showed the presence of neral, a monoterpene compound with known anti-inflammatory properties, which may contribute to the anti-inflammatory activity of PDENs isolated from Solanum nigrum L. berries. Taken together, the present study was the first to isolate and characterize PDENs from Solanum nigrum L. berries. The results of this study also demonstrated the anti-inflammatory activity of PDEN by suppressing the production of IL-6 in LPS-stimulated RAW264.7 cells.

Phosphatidic acid signaling and function in nuclei

Membrane lipidomes are dynamic and their changes generate lipid mediators affecting various biological processes. Phosphatidic acid (PA) has emerged as an important class of lipid mediators involved in a wide range of cellular and physiological responses in plants, animals, and microbes. The regulatory functions of PA have been studied primarily outside the nuclei, but an increasing number of recent studies indicates that some of the PA effects result from its action in nuclei. PA levels in nuclei are dynamic in response to stimuli. Changes in nuclear PA levels can result from activities of enzymes associated with nuclei and/or from movements of PA generated extranuclearly. PA has also been found to interact with proteins involved in nuclear functions, such as transcription factors and proteins undergoing nuclear translocation in response to stimuli. The nuclear action of PA affects various aspects of plant growth, development, and response to stress and environmental changes.

A liposomal platform for the delivery of ion channel proteins for treatment of channelopathies - Application in therapy of cystic fibrosis

Channelopathies arise from ion channel dysfunction. Successful treatment entails delivery of functional ion channels to replace dysfunctional ones. Glycine receptor (GlyR)-rich cell membrane fragments (CMF) were previously delivered to target cell membranes using fusogenic liposomes. Here, cystic fibrosis transmembrane conductance regulator (CFTR)-bearing CMF were similarly delivered to target cells. We studied the effect of lipid composition on liposomes' ability to incorporate CMF and fuse with target cell membranes to deliver functional CFTR. Four formulations were prepared using thin-film hydration out of different lecithin sources, egg and soy lecithin (EL and SL), in the presence and absence of cholesterol (CHOL): EL + CHOL, EL-CHOL, SL + CHOL, and SL-CHOL. EL liposomes incorporated more CMF than SL liposomes, with CHOL only increasing CMF incorporation in SL liposomes. SL + CHOL fused better with target cell membranes than EL + CHOL. SL + CHOL and EL + CHOL equally delivered CFTR to target cell membranes, owing to the former's superior fusogenic capacity and the latter's superior CMF-incorporation capacity. SL-CHOL and EL-CHOL delivered CFTR to a lesser extent, indicating the importance of CHOL for fusion. Patch-clamp electrophysiology and confocal laser scanning microscopy (CLSM) confirmed CFTR delivery to target cell membranes by SL + CHOL. Therefore, CMF-bearing fusogenic liposomes offer a promising universal platform for the treatment of channelopathies.

Inhibiting SETD7 methyl-transferase activity impairs differentiation, lipid metabolism and lactogenesis in mammary epithelial cells

SETD7 (SET7/9, KMT7) is a lysine methyltransferase that targets master regulators of cell proliferation and differentiation. Here, the impact of inhibiting SETD7 catalytic activity on mammary epithelial cell differentiation was studied by focusing on genes associated with epithelial differentiation, lactogenesis, and lipid metabolism in HC11 and EpH4 cell lines. Setd7 mRNA and protein levels were induced upon lactogenic differentiation in both cell lines. Inhibition of SETD7 activity by the compound (R)-PFI-2 increased cell proliferation and downregulated E-cadherin, beta-catenin, lactoferrin, insulin-like growth factor binding protein 5, and beta-casein levels. In addition, inhibition of SETD7 activity affected the lipid profile and altered the mRNA expression of the phospholipid biosynthesis-related genes choline phosphotransferase 1, and ethanolamine-phosphate cytidylyltransferase. Altogether, the results suggest that inhibiting SETD7 catalytic activity impairs mammary epithelial and lactogenic differentiation.

Phosphatidic acid regulates ammonium uptake by interacting with AMMONIUM TRANSPORTER 1;1 in Arabidopsis

Ammonium (NH4+) is a key inorganic nitrogen source in cellular amino acid biosynthesis. The coupling of transcriptional and posttranslational regulation of AMMONIUM TRANSPORTER (AMT) ensures that NH4+ acquisition by plant roots is properly balanced, which allows for rapid adaptation to a variety of nitrogen conditions. Here, we report that phospholipase D (PLD)-derived phosphatidic acid (PA) interacts with AMT1;1 to mediate NH4+ uptake in Arabidopsis (Arabidopsis thaliana). We examined pldα1 pldδ-knockout mutants and found that a reduced PA level increased seedling growth under nitrogen deficiency and inhibited root growth upon NH4+ stress, which was consistent with the enhanced accumulation of cellular NH4+. PA directly bound to AMT1;1 and inhibited its transport activity. Mutation of AMT1;1 R487 to Gly (R487G) resulted in abolition of PA suppression and, subsequently, enhancement of ammonium transport activity in vitro and in vivo. Observations of AMT1;1-GFP showed suppressed endocytosis under PLD deficiency or by mutation of the PA-binding site in AMT1;1. Endocytosis was rescued by PA in the pldα1 pldδ mutant but not in the mutant AMT1;1R487G-GFP line. Together, these findings demonstrated PA-based shutoff control of plant NH4+ transport and point to a broader paradigm of lipid-transporter function.

Lipocalin 2 regulates mitochondrial phospholipidome remodeling, dynamics, and function in brown adipose tissue in male mice

Mitochondrial function is vital for energy metabolism in thermogenic adipocytes. Impaired mitochondrial bioenergetics in brown adipocytes are linked to disrupted thermogenesis and energy balance in obesity and aging. Phospholipid cardiolipin (CL) and phosphatidic acid (PA) jointly regulate mitochondrial membrane architecture and dynamics, with mitochondria-associated endoplasmic reticulum membranes (MAMs) serving as the platform for phospholipid biosynthesis and metabolism. However, little is known about the regulators of MAM phospholipid metabolism and their connection to mitochondrial function. We discover that LCN2 is a PA binding protein recruited to the MAM during inflammation and metabolic stimulation. Lcn2 deficiency disrupts mitochondrial fusion-fission balance and alters the acyl-chain composition of mitochondrial phospholipids in brown adipose tissue (BAT) of male mice. Lcn2 KO male mice exhibit an increase in the levels of CLs containing long-chain polyunsaturated fatty acids (LC-PUFA), a decrease in CLs containing monounsaturated fatty acids, resulting in mitochondrial dysfunction. This dysfunction triggers compensatory activation of peroxisomal function and the biosynthesis of LC-PUFA-containing plasmalogens in BAT. Additionally, Lcn2 deficiency alters PA production, correlating with changes in PA-regulated phospholipid-metabolizing enzymes and the mTOR signaling pathway. In conclusion, LCN2 plays a critical role in the acyl-chain remodeling of phospholipids and mitochondrial bioenergetics by regulating PA production and its function in activating signaling pathways.

Phosphatidic acid binding to Patched contributes to the inhibition of Smoothened and Hedgehog signaling in Drosophila wing development

Hedgehog (Hh) signaling controls growth and patterning during embryonic development and homeostasis in adult tissues. Hh binding to the receptor Patched (Ptc) elicits intracellular signaling by relieving Ptc-mediated inhibition of the transmembrane protein Smoothened (Smo). We uncovered a role for the lipid phosphatidic acid (PA) in the regulation of the Hh pathway in Drosophila melanogaster. Deleting the Ptc C-terminal tail or mutating the predicted PA-binding sites within it prevented Ptc from inhibiting Smo in wing discs and in cultured cells. The C-terminal tail of Ptc directly interacted with PA in vitro, an association that was reduced by Hh, and increased the amount of PA at the plasma membrane in cultured cells. Smo also interacted with PA in vitro through a binding pocket located in the transmembrane region, and mutating residues in this pocket reduced Smo activity in vivo and in cells. By genetically manipulating PA amounts in vivo or treating cultured cells with PA, we demonstrated that PA promoted Smo activation. Our findings suggest that Ptc may sequester PA in the absence of Hh and release it in the presence of Hh, thereby increasing the amount of PA that is locally available to promote Smo activation.

Lycorine promotes IDH1 acetylation to induce mitochondrial dynamics imbalance in colorectal cancer cells

Isocitrate dehydrogenase (IDH) 1 and 2, as essential enzymes in energy metabolism, contribute to the survival and drug resistance of a variety of solid tumors, especially for colorectal cancer (CRC). However, the underlying molecular mechanism still remains unclear. In this study, IDH1 was identified as a crucial cellular target of a natural-derived anti-CRC small molecule lycorine, using the unbiased thermal proteome profiling (TPP) strategy. We found that lycorine directly targeted a unique C-terminal domain of IDH1, and disrupted IDH1 interaction with deacetylase sirtuin 1 (SIRT1), thereby significantly promoting IDH1 acetylation modification. Then, lycorine noticeably triggered oxidative stress in CRC cells to cause mitochondrial membranes injury, and subsequently facilitated mitochondrial fission. Specific knockdown of IDH1 or SIRT1 markedly aggrieved lycorine-mediated oxidative stress and mitochondrial fragmentation in CRC cells. Furthermore, the combination of lycorine and sirtuins blocker nicotinamide (NAM) exhibited a synergic therapeutic effect in CRC cells. Collectively, our results reveal that IDH1 may serve as a promising therapeutic target for CRC via pharmacologically driving oxidative stress-dependent mitochondrial dynamics imbalance.

Physical model of the nuclear membrane permeability mechanism

Nuclear cytoplasmic transport is mediated by many receptors that recognize specific nuclear localization signals on proteins and RNA and transport these substrates through nuclear pore complexes. Facilitated diffusion through nuclear pore complexes requires the attachment of transport receptors. Despite the relatively large tunnel diameter, some even small proteins (less than 20-30 kDa), such as histones, pass through the nuclear pore complex only with transport receptors. Over several decades, considerable material has been accumulated on the structure, architecture, and amino acid composition of the proteins included in this complex and the sequence of many receptors. We consider the data available in the literature on the structure of the nuclear pore complex and possible mechanisms of nuclear-cytoplasmic transport, applying the theory of electrostatic interactions in the context of our data on changes in the electrokinetic potential of nuclei and our previously proposed physical model of the mechanism of facilitated diffusion through the nuclear pore complex (NPC). According to our data, the main contribution to the charge of the nuclear membrane is made by anionic phospholipids, which are part of both the nuclear membrane and the nuclear matrix, which creates a potential difference between them. The nuclear membrane is a four-layer phospholipid dielectric, so the potential vector can only pass through the NPC, creating an electrostatic funnel that "pulls in" the positively charged load-NLS-NTR trigger complexes. Considering the newly obtained data, an improved model of the previously proposed physical model of the mechanism of nuclear-cytoplasmic transport is proposed. This model considers the contribution of electrostatic fields to the transportation speed when changing the membrane's thickness in the NPC basket at a higher load.

Role of Lipids and Divalent Cations in Membrane Fusion Mediated by the Heptad Repeat Domain 1 of Mitofusin

Mitochondria are highly dynamic organelles that constantly undergo fusion and fission events to maintain their shape, distribution and cellular function. Mitofusin 1 and 2 proteins are two dynamin-like GTPases involved in the fusion of outer mitochondrial membranes (OMM). Mitofusins are anchored to the OMM through their transmembrane domain and possess two heptad repeat domains (HR1 and HR2) in addition to their N-terminal GTPase domain. The HR1 domain was found to induce fusion via its amphipathic helix, which interacts with the lipid bilayer structure. The lipid composition of mitochondrial membranes can also impact fusion. However, the precise mode of action of lipids in mitochondrial fusion is not fully understood. In this study, we examined the role of the mitochondrial lipids phosphatidylethanolamine (PE), cardiolipin (CL) and phosphatidic acid (PA) in membrane fusion induced by the HR1 domain, both in the presence and absence of divalent cations (Ca2+ or Mg2+). Our results showed that PE, as well as PA in the presence of Ca2+, effectively stimulated HR1-mediated fusion, while CL had a slight inhibitory effect. By considering the biophysical properties of these lipids in the absence or presence of divalent cations, we inferred that the interplay between divalent cations and specific cone-shaped lipids creates regions with packing defects in the membrane, which provides a favorable environment for the amphipathic helix of HR1 to bind to the membrane and initiate fusion.

Biosynthesized gold nanoparticles that activate Toll-like receptors and elicit localized light-converting hyperthermia for pleiotropic tumor immunoregulation

Manipulating the tumor immune contexture towards a more active state can result in better therapeutic outcomes. Here we describe an easily accessible bacterial biomineralization-generated immunomodulator, which we name Ausome (Au + [exo]some). Ausome comprises a gold nanoparticle core covered by bacterial components; the former affords an inducible hyperthermia effect, while the latter mobilizes diverse immune responses. Multiple pattern recognition receptors actively participate in Ausome-initiated immune responses, which lead to the release of a broad spectrum of pro-inflammatory cytokines and the activation of effector immune cells. Upon laser irradiation, tumor-accumulated Ausome elicits a hyperthermic response, which improves tissue blood perfusion and contributes to enhanced infiltration of immunostimulatory modules, including cytokines and effector lymphocytes. This immune-modulating strategy mediated by Ausome ultimately brings about a comprehensive immune reaction and selectively amplifies the effects of local antitumor immunity, enhancing the efficacy of well-established chemo- or immuno-therapies in preclinical cancer models in female mice.

The Role of Osmolytes and Membrane Lipids in the Adaptation of Acidophilic Fungi

Acidophiles maintain near-neutral intracellular pH using proton pumps. We have suggested the protective role of osmolytes and membrane lipids in the adaptation to an acidic environment. Previously we have observed, for the first time, high levels of trehalose in acidophilic basidiomycete Sistotrema brinkmannii. Here, we have studied the composition of both osmolytes and membrane lipids of two more acidophilic fungi. Trehalose and polyols were among the main osmolytes during growth under optimal conditions (pH 4.0) in basidiomycete Phlebiopsis gigantea and ascomycete Mollisia sp. Phosphatidic acids, phosphatidylethanolamines, phosphatidylcholines, and sterols, were predominant membrane lipids in both fungi. P. gigantea had a narrow optimum of growth at pH 4.0, resulting in a sharp decline of growth rate at pH 2.6 and 5.0, accompanied by a decrease in the number of osmolytes and significant changes in the composition of membrane lipids. In contrast, Mollisia sp. had a broad optimal growth range (pH 3.0-5.0), and the number of osmolytes either stayed the same (at pH 6.0) or increased (at pH 2.6), while membrane lipids composition remained unchanged. Thus, the data obtained indicate the participation of osmolytes and membrane lipids in the adaptation of acidophilic fungi.

Distinct domains in Ndc1 mediate its interaction with the Nup84 complex and the nuclear membrane

Nuclear pore complexes (NPCs) are embedded in the nuclear envelope and built from ∼30 different nucleoporins (Nups) in multiple copies, few are integral membrane proteins. One of these transmembrane nucleoporins, Ndc1, is thought to function in NPC assembly at the fused inner and outer nuclear membranes. Here, we show a direct interaction of Ndc1's transmembrane domain with Nup120 and Nup133, members of the pore membrane coating Y-complex. We identify an amphipathic helix in Ndc1's C-terminal domain binding highly curved liposomes. Upon overexpression, this amphipathic motif is toxic and dramatically alters the intracellular membrane organization in yeast. Ndc1's amphipathic motif functionally interacts with related motifs in the C-terminus of the nucleoporins Nup53 and Nup59, important for pore membrane binding and interconnecting NPC modules. The essential function of Ndc1 can be suppressed by deleting the amphipathic helix from Nup53. Our data indicate that nuclear membrane and presumably NPC biogenesis depends on a balanced ratio between amphipathic motifs in diverse nucleoporins.

Delivery of low-density lipoprotein from endocytic carriers to mitochondria supports steroidogenesis

The low-density lipoprotein (LDL) is a major cholesterol carrier in circulation and is internalized into cells through LDL receptor (LDLR)-mediated endocytosis. The LDLR protein is highly expressed in the steroidogenic organs and LDL cholesterol is an important source for steroidogenesis. Cholesterol must be transported into the mitochondria, where steroid hormone biosynthesis initiates. However, how LDL cholesterol is conveyed to the mitochondria is poorly defined. Here, through genome-wide small hairpin RNA screening, we find that the outer mitochondrial membrane protein phospholipase D6 (PLD6), which hydrolyses cardiolipin to phosphatidic acid, accelerates LDLR degradation. PLD6 promotes the entrance of LDL and LDLR into the mitochondria, where LDLR is degraded by mitochondrial proteases and LDL-carried cholesterol is used for steroid hormone biosynthesis. Mechanistically, the outer mitochondrial membrane protein CISD2 binds to the cytosolic tail of LDLR and tethers LDLR⁺ vesicles to the mitochondria. The fusogenic lipid phosphatidic acid generated by PLD6 facilitates the membrane fusion of LDLR⁺ vesicles with the mitochondria. This intracellular transport pathway of LDL-LDLR bypasses the lysosomes and delivers cholesterol to the mitochondria for steroidogenesis.

Structural insights into perilipin 3 membrane association in response to diacylglycerol accumulation

Lipid droplets (LDs) are dynamic organelles that contain an oil core mainly composed of triglycerides (TAG) that is surrounded by a phospholipid monolayer and LD-associated proteins called perilipins (PLINs). During LD biogenesis, perilipin 3 (PLIN3) is recruited to nascent LDs as they emerge from the endoplasmic reticulum. Here, we analyze how lipid composition affects PLIN3 recruitment to membrane bilayers and LDs, and the structural changes that occur upon membrane binding. We find that the TAG precursors phosphatidic acid and diacylglycerol (DAG) recruit PLIN3 to membrane bilayers and define an expanded Perilipin-ADRP-Tip47 (PAT) domain that preferentially binds DAG-enriched membranes. Membrane binding induces a disorder to order transition of alpha helices within the PAT domain and 11-mer repeats, with intramolecular distance measurements consistent with the expanded PAT domain adopting a folded but dynamic structure upon membrane binding. In cells, PLIN3 is recruited to DAG-enriched ER membranes, and this requires both the PAT domain and 11-mer repeats. This provides molecular details of PLIN3 recruitment to nascent LDs and identifies a function of the PAT domain of PLIN3 in DAG binding.

Lipid Metabolomic and Transcriptomic Analyses Reveal That Phosphatidylcholine Enhanced the Resistance of Peach Seedlings to Salt Stress through Phosphatidic Acid

Soil salinity is a major conlinet limiting sustainable agricultural development in peach tree industry. In this study, lipid metabolomic pathway analysis indicated that phosphatidic acid is essential for root resistance to salt stress in peach seedlings. Through functional annotation analysis of differentially expressed genes in transcriptomics, we found that MAPK signaling pathway is closely related to peach tree resistance to salt stress, wherein PpMPK6 expression is significantly upregulated. Under salt conditions, the OE-PpMPK6 Arabidopsis thaliana (L.) Heynh. line showed higher resistance to salt stress than WT and KO-AtMPK6 lines. Furthermore, we found that the Na⁺ content in OE-PpMPK6 roots was significantly lower than that in WT and KO-AtMPK6 roots, indicating that phosphatidic acid combined with PpMPK6 activated the SOS1 (salt-overly-sensitive 1) protein to enhance Na⁺ efflux, thus alleviating the damage caused by NaCl in roots; these findings provide insight into the salt stress-associated transcriptional regulation.

Phosphatidic acid-regulated SOS2 controls sodium and potassium homeostasis in Arabidopsis under salt stress

The maintenance of sodium/potassium (Na+ /K+ ) homeostasis in plant cells is essential for salt tolerance. Plants export excess Na+ out of cells mainly through the Salt Overly Sensitive (SOS) pathway, activated by a calcium signal; however, it is unknown whether other signals regulate the SOS pathway and how K+ uptake is regulated under salt stress. Phosphatidic acid (PA) is emerging as a lipid signaling molecule that modulates cellular processes in development and the response to stimuli. Here, we show that PA binds to the residue Lys57 in SOS2, a core member of the SOS pathway, under salt stress, promoting the activity and plasma membrane localization of SOS2, which activates the Na+ /H+ antiporter SOS1 to promote the Na+ efflux. In addition, we reveal that PA promotes the phosphorylation of SOS3-like calcium-binding protein 8 (SCaBP8) by SOS2 under salt stress, which attenuates the SCaBP8-mediated inhibition of Arabidopsis K+ transporter 1 (AKT1), an inward-rectifying K+ channel. These findings suggest that PA regulates the SOS pathway and AKT1 activity under salt stress, promoting Na+ efflux and K+ influx to maintain Na+ /K+ homeostasis.

Distinct regions of Praja-1 E3 ubiquitin-protein ligase selectively bind to docosahexaenoic acid-containing phosphatidic acid and diacylglycerol kinase δ

1-Stearoyl-2-docosahexaenoyl (18:0/22:6)-phosphatidic acid (PA) interacts with and activates Praja-1 E3 ubiquitin-protein ligase (full length: 615 aa) to ubiquitinate and degrade the serotonin transporter (SERT). SERT modulates serotonergic system activity and is a therapeutic target for depression, autism, obsessive-compulsive disorder, schizophrenia and Alzheimer's disease. Moreover, diacylglycerol kinase (DGK) δ2 (full length: 1214 aa) interacts with Praja-1 in addition to SERT and generates 18:0/22:6-PA, which binds and activates Praja-1. In the present study, we investigated the interaction of Praja-1 with 18:0/22:6-PA and DGKδ2 in more detail. We first found that the N-terminal one-third region (aa 1-224) of Praja-1 bound to 18:0/22:6-PA and that Lys141 in the region was critical for binding to 18:0/22:6-PA. In contrast, the C-terminal catalytic domain of Praja-1 (aa 446-615) interacted with DGKδ2. Additionally, the N-terminal half of the catalytic domain (aa 309-466) of DGKδ2 intensely bound to Praja-1. Moreover, the N-terminal region containing the pleckstrin homology and C1 domains (aa 1-308) and the C-terminal half of the catalytic domain (aa 762-939) of DGKδ2 weakly associated with Praja-1. Taken together, these results reveal new functions of the N-terminal (aa 1-224) and C-terminal (aa 446-615) regions of Praja-1 and the N-terminal half of the catalytic region (aa 309-466) of DGKδ2 as regulatory domains. Moreover, it is likely that the DGKδ2-Praja-1-SERT heterotrimer proximally arranges the 18:0/22:6-PA-producing catalytic domain of DGKδ2, the 18:0/22:6-PA-binding regulatory domain of Praja-1, the ubiquitin-protein ligase catalytic domain of Praja-1 and the ubiquitination acceptor site-containing SERT C-terminal region.

In vivo profiling of the Zucchini proximal proteome in the Drosophila ovary

PIWI-interacting RNAs (piRNAs) are small RNAs that play a conserved role in genome defense. The piRNA processing pathway is dependent on the sequestration of RNA precursors and protein factors in specific subcellular compartments. Therefore, a highly resolved spatial proteomics approach can help identify the local interactions and elucidate the unknown aspects of piRNA biogenesis. Herein, we performed TurboID proximity labeling to investigate the interactome of Zucchini (Zuc), a key factor of piRNA biogenesis in germline cells and somatic follicle cells of the Drosophila ovary. Quantitative mass spectrometry analysis of biotinylated proteins defined the Zuc-proximal proteome, including the well-known partners of Zuc. Many of these were enriched in the outer mitochondrial membrane (OMM), where Zuc was specifically localized. The proximal proteome of Zuc showed a distinct set of proteins compared with that of Tom20, a representative OMM protein, indicating that chaperone function-related and endomembrane system/vesicle transport proteins are previously unreported interacting partners of Zuc. The functional relevance of several candidates in piRNA biogenesis was validated by derepression of transposable elements after knockdown. Our results present potential Zuc-interacting proteins, suggesting unrecognized biological processes.

Saturated fatty acid- and/or monounsaturated fatty acid-containing phosphatidic acids selectively interact with heat shock protein 27

Diacylglycerol kinase (DGK) α, which is a key enzyme in the progression of cancer and, in contrast, in T-cell activity attenuation, preferentially produces saturated fatty acid (SFA)- and/or monounsaturated fatty acid (MUFA)-containing phosphatidic acids (PAs), such as 16:0/16:0-, 16:0/18:0-, and 16:1/16:1-PA, in melanoma cells. In the present study, we searched for the target proteins of 16:0/16:0-PA in melanoma cells and identified heat shock protein (HSP) 27, which acts as a molecular chaperone and contributes to cancer progression. HSP27 more strongly interacted with PA than other phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, cardiolipin, phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate. Moreover, HSP27 is more preferentially bound to SFA- and/or MUFA-containing PAs, including 16:0/16:0- and 16:0/18:1-PAs, than PUFA-containing PAs, including 18:0/20:4- and 18:0/22:6-PA. Furthermore, HSP27 and constitutively active DGKα expressed in COS-7 cells colocalized in a DGK activity-dependent manner. Notably, 16:0/16:0-PA, but not phosphatidylcholine or 16:0/16:0-phosphatidylserine, induced oligomer dissociation of HSP27, which enhances its chaperone activity. Intriguingly, HSP27 protein was barely detectable in Jurkat T cells, while the protein band was intensely detected in AKI melanoma cells. Taken together, these results strongly suggest that SFA- and/or MUFA-containing PAs produced by DGKα selectively target HSP27 and regulate its cancer-progressive function in melanoma cells but not in T cells.

Diacylglycerol at the inner nuclear membrane fuels nuclear envelope expansion in closed mitosis

Nuclear envelope (NE) expansion must be controlled to maintain nuclear shape and function. The nuclear membrane expands massively during closed mitosis, enabling chromosome segregation within an intact NE. Phosphatidic acid (PA) and diacylglycerol (DG) can both serve as biosynthetic precursors for membrane lipid synthesis. How they are regulated in time and space and what the implications are of changes in their flux for mitotic fidelity are largely unknown. Using genetically encoded PA and DG probes, we show that DG is depleted from the inner nuclear membrane during mitosis in the fission yeast Schizosaccharomyces pombe, but PA does not accumulate, indicating that it is rerouted to membrane synthesis. We demonstrate that DG-to-PA conversion catalyzed by the diacylglycerol kinase Dgk1 (also known as Ptp4) and direct glycerophospholipid synthesis from DG by diacylglycerol cholinephosphotransferase/ethanolaminephosphotransferase Ept1 reinforce NE expansion. We conclude that DG consumption through both the de novo pathway and the Kennedy pathway fuels a spike in glycerophospholipid biosynthesis, controlling NE expansion and, ultimately, mitotic fidelity.

Cell Membrane Features as Potential Breeding Targets to Improve Cold Germination Ability of Seeds

Cold stress breeding that focuses on the improvement of chilling tolerance at the germination stage is constrained by the complexities of the trait which involves integrated cellular, biochemical, hormonal and molecular responses. Biological membrane serves as the first line of plant defense under stress. Membranes receive cold stress signals and transduce them into intracellular responses. Low temperature stress, in particular, primarily and effectively affects the structure, composition and properties of cell membranes, which ultimately disturbs cellular homeostasis. Under cold stress, maintenance of membrane integrity through the alteration of membrane lipid composition is of prime importance to cope with the stress. This review describes the critical role of cell membranes in cold stress responses as well as the physiological and biochemical manifestations of cold stress in plants. The potential of cell membrane properties as breeding targets in developing strategies to improve cold germination ability is discussed using cotton (Gossypium hirsutum L.) as a model.