Membrane curvature induced by proximity of anionic phospholipids can initiate endocytosis

cGAS-STING: mechanisms and therapeutic opportunities

The cGAS-STING pathway plays a crucial role in the innate immune system by detecting mislocalized double-stranded DNA (dsDNA) in the cytoplasm and triggering downstream signal transduction. Understanding the mechanisms by which cGAS and STING operate is vital for gaining insights into the biology of this pathway. This review provides a detailed examination of the structural features of cGAS and STING proteins, with a particular emphasis on their activation and inhibition mechanisms. We also discuss the novel discovery of STING functioning as an ion channel. Furthermore, we offer an overview of key agonists and antagonists of cGAS and STING, shedding light on their mechanisms of action. Deciphering the molecular intricacies of the cGAS-STING pathway holds significant promise for the development of targeted therapies aimed at maintaining immune homeostasis within both innate and adaptive immunity.

Enhanced in vitro transfection efficiency of mRNA-loaded polyplexes into natural killer cells through osmoregulation

High expression of externally injected in vitro transcribed (IVT) mRNA in natural killer (NK) cells is a prerequisite for NK cell-mediated cell therapy. To enhance the transfection efficacy of IVT mRNA-loaded polyplexes, we exposed NK cells to a hypertonic condition during transfection, which facilitated endo/exocytosis to maintain the isotonic state of the cells. The transfection efficacy of IVT mRNA was significantly enhanced after 24 h, which was mainly due to the facilitated cellular uptake and endosomal escape of the polyplexes. Interestingly, osmotic alterations in NK cells significantly affect the expression levels of endosome-escape-related genes in ion channels. Treatment with a mild hypertonic condition exhibited negligible toxicity to NK cells, without disturbing the integrity of the cellular membranes or the innate cytotoxic abilities of NK cells against cancer cells. These results demonstrate that the hypertonic treatment of NK cells enhances the transfection efficacy of IVT mRNA to produce genetically engineered NK cells.

ATP8A1-translocated endosomal phosphatidylserine fine-tunes the multivesicular body formation and the endo-lysosomal traffic

P4-ATPases are phospholipid flippases responsible for the transbilayer lipid asymmetry. ATP8A1, a P4-ATPase family member, has been reported to be involved in phosphatidylserine (PS) translocation at the trans-Golgi network, early endosomes and recycling endosomes. However, the possible roles of the PS on late endosomes/lysosomes pathway and how they are regulated remain to be elucidated. This study showed enrichment of ATP8A1 in Rab7-positive late endosomal compartments, and that ATP8A1 primarily flips the endosomal PS from the luminal leaflet to the cytosolic leaflet but not the PS in the inner leaflet of the plasma membrane. ATP8A1 depletion accelerates the lysosome-destined cargo proteins transfer into the intraluminal vesicles (ILVs) of multivesicular bodies (MVBs) and alters the signaling of epidermal growth factor receptor. Mechanistically, ATP8A1 depletion leads to PS loading in the luminal leaflet of MVB's limiting membrane, which fine-tunes ILVs initiation and endosomal sorting complex required for transport (ESCRT) component recruitment.

TAT-1, a phosphatidylserine flippase, affects molting and regulates membrane trafficking in the epidermis of Caenorhabditis elegans

Membrane trafficking is a conserved process required for the import, export, movement, and distribution of proteins and other macromolecules within cells. The Caenorhabditis elegans NIMA-related kinases NEKL-2 (human NEK8/9) and NEKL-3 (human NEK6/7) are conserved regulators of membrane trafficking and are required for the completion of molting. Using a genetic approach, we identified reduction-of-function mutations in tat-1 that suppress nekl-associated molting defects. tat-1 encodes the C. elegans ortholog of mammalian ATP8A1/2, a phosphatidylserine flippase that promotes the asymmetric distribution of phosphatidylserine on the cytosolic leaflet of lipid membrane bilayers. CHAT-1 (human CDC50), a conserved chaperone, was required for the correct localization of TAT-1, and chat-1 inhibition strongly suppressed nekl defects. Using a phosphatidylserine sensor, we found that TAT-1 was required for the normal localization of phosphatidylserine at apical endosomes and that loss of TAT-1 led to aberrant endosomal morphologies. Consistent with these data, TAT-1 localized to early endosomes and to recycling endosomes marked with RME-1, the C. elegans ortholog of the human EPS15 homology domain-containing protein, EHD1. TAT-1, phosphatidylserine biosynthesis, and the phosphatidylserine-binding protein RFIP-2 (human RAB11-FIP2) were all required for the normal localization of RME-1 to apical endosomes. Consistent with these proteins functioning together, inhibition of RFIP-2 or RME-1 led to the partial suppression of nekl molting defects, as did inhibition of phosphatidylserine biosynthesis. We propose that TAT-1 flippase activity, in conjunction with RFIP-2, promotes the recruitment of RME-1 to apical recycling endosomes and that inhibition of TAT-1-RFIP-2-RME-1 can compensate for a reduction in NEKL activities.

Anionic phospholipid-mediated transmembrane transport and intracellular membrane trafficking in plant cells

Cellular membranes primarily consist of proteins and lipids. These proteins perform cellular functions such as metabolic regulation, environmental and hormonal signal sensing, and nutrient transport. There is increasing experimental evidence that certain lipids, particularly anionic phospholipids, can act as signaling molecules. Specific examples of functional regulation by anionic phospholipids in plant cells have been reported for transporters, channels, and even receptors. By regulating the structure and activity of membrane-integral proteins, these phospholipids mediate the transport of phytohormones and ions, and elicit physiological responses to developmental and environmental cues. Phospholipids also control membrane protein abundance and lipid composition and abundance by facilitating vesicular trafficking. In this review, we discuss recent research that elucidates the mechanisms by which membrane-integral transporters and channels are controlled via phospholipid signaling, as well as the regulation of membrane protein accumulation by phospholipids through coordinated removal, recycling, and degradation processes.

Engineered Therapeutic Bacteria with High-Yield Membrane Vesicle Production Inspired by Eukaryotic Membrane Curvature for Treating Inflammatory Bowel Disease

Bacterial membrane vesicles (BMVs) are emerging as powerful natural nanoparticles with transformative potential in medicine and industry. Despite their promise, scaling up BMV production and ensuring stable isolation and storage remain formidable challenges that limit their broader application. Inspired by eukaryotic mechanisms of membrane curvature, we engineered Escherichia coli DH5α to serve as a high-efficiency BMV factory. By fusing the ethanolamine utilization microcompartment shell protein EutS with the outer membrane via the ompA signal peptide, we induced dramatic membrane curvatures that drove enhanced vesiculation. Simultaneously, overexpression of fatty acyl reductase led to the production of amphiphilic fatty alcohols, further amplifying the BMV yield. Dynamic modulation of peptidoglycan hydrolase (PGase) expression facilitated efficient BMV release, resulting in a striking 149.11-fold increase in vesicle production. Notably, the high-yield BMVs from our engineered strain, without the need for purification, significantly bolstered innate immune responses and demonstrated therapeutic efficacy in treating inflammatory bowel disease (IBD). This study presents a strategy to overcome BMV production barriers, showcasing the therapeutic potential of engineered bacteria and BMVs for IBD treatment, while highlighting their potential applications in diverse biomedical fields.

Determining the Relative Affinity of ORPs for Lipid Ligands Using Fluorescence and Thermal Shift Assays

Lipid transfer proteins (LTPs) are specialized proteins that convey specific lipids across the cytosol to regulate the lipid composition of organelles and the plasma membrane. Quantifying to which extent these LTPs recognize and transfer various lipid species and subspecies is of prime interest to define their cellular role(s). Here, we describe how to measure in vitro the relative affinity of Osh6p, a yeast phosphatidylserine (PS)/phosphatidylinositol 4-phosphate (PI(4)P) exchanger belonging to the oxysterol-binding protein(OSBP)-related protein (ORP) family, for PS and phosphoinositide subspecies. First, we detail how to produce and purify Osh6p with high purity. Secondly, we describe how to measure its ability to bind PS, PI(4)P, and PI(4,5)P2 by FRET-based and thermal shift assays using liposomes of defined composition. These protocols can allow further analysis of other ORPs or inspire the design of assays to characterize other LTPs. Notably, they can be helpful in defining how LTPs transfer phospholipids subspecies as a function of their acyl chains' length and unsaturation degree and, therefore, whether they can contribute to regulating the acyl chain composition of cell membranes.

The Effects of Viral Structural Proteins on Acidic Phospholipids in Host Membranes

Enveloped viruses rely on host membranes for trafficking and assembly. A substantial body of literature published over the years supports the involvement of cellular membrane lipids in the enveloped virus assembly processes. In particular, the knowledge regarding the relationship between viral structural proteins and acidic phospholipids has been steadily increasing in recent years. In this review, we will briefly review the cellular functions of plasma membrane-associated acidic phospholipids and the mechanisms that regulate their local distribution within this membrane. We will then explore the interplay between viruses and the plasma membrane acidic phospholipids in the context of the assembly process for two enveloped viruses, the influenza A virus (IAV) and the human immunodeficiency virus type 1 (HIV-1). Among the proteins encoded by these viruses, three viral structural proteins, IAV hemagglutinin (HA), IAV matrix protein-1 (M1), and HIV-1 Gag protein, are known to interact with acidic phospholipids, phosphatidylserine and/or phosphatidylinositol (4,5)-bisphosphate. These interactions regulate the localization of the viral proteins to and/or within the plasma membrane and likely facilitate the clustering of the proteins. On the other hand, these viral proteins, via their ability to multimerize, can also alter the distribution of the lipids and may induce acidic-lipid-enriched membrane domains. We will discuss the potential significance of these interactions in the virus assembly process and the property of the progeny virions. Finally, we will outline key outstanding questions that need to be answered for a better understanding of the relationships between enveloped virus assembly and acidic phospholipids.

TAT-1, a phosphatidylserine flippase, affects molting and regulates membrane trafficking in the epidermis of C. elegans

Membrane trafficking is a conserved process required for the movement and distribution of proteins and other macromolecules within cells. The Caenorhabditis elegans NIMA-related kinases NEKL-2 (human NEK8/9) and NEKL-3 (human NEK6/7) are conserved regulators of membrane trafficking and are required for the completion of molting. We used a genetic approach to identify reduction-of-function mutations in tat-1 that suppress nekl -associated molting defects. tat-1 encodes the C. elegans ortholog of mammalian ATP8A1/2, a phosphatidylserine (PS) flippase that promotes the asymmetric distribution of PS to the cytosolic leaflet of lipid membrane bilayers. CHAT-1 (human CDC50), a conserved chaperone, was required for the correct localization of TAT-1, and chat-1 inhibition strongly suppressed nekl defects. Using a PS sensor, we found that TAT-1 was required for the normal localization of PS at apical endosomes and that loss of TAT-1 led to aberrant endosomal morphologies. Consistent with this, TAT-1 localized to early endosomes and to recycling endosomes marked with RME-1, the C. elegans ortholog of the human EPS15 homology (EH) domain-containing protein, EHD1. TAT-1, PS biosynthesis, and the PS-binding protein RFIP-2 (human RAB11-FIP2) were all required for the normal localization of RME-1 to apical endosomes. Consistent with these proteins functioning together, inhibition of RFIP-2 or RME-1 led to the partial suppression of nekl molting defects, as did the inhibition of PS biosynthesis. Using the auxin-inducible degron system, we found that depletion of NEKL-2 or NEKL-3 led to defects in RME-1 localization and that a reduction in TAT-1 function partially restored RME-1 localization in NEKL-3-depleted cells.

ARTICLE SUMMARY

Endocytosis is an essential process required for the movement of proteins and lipids within cells. NEKL-2 and NEKL-3, two evolutionarily conserved proteins in the nematode Caenorhabditis elegans , are important regulators of endocytosis. In the current study, the authors describe a new functional link between the NEKLs and several proteins with known roles in endocytosis including TAT-1, a conserved enzyme that moves lipids between the bilayers of cellular membranes. As previous work implicated NEKLs in developmental defects and cancer, the present study can provide new insights into how the misregulation of endocytosis affects human health and disease.

Single-Cell Time-Resolved Metabolomics and Lipidomics Reveal Apoptotic and Ferroptotic Heterogeneity during Foam Cell Formation

Macrophage-derived foam cells play a crucial role in plaque formation and rupture during the progression of atherosclerosis. Traditional studies have often overlooked the heterogeneity of foam cells, focusing instead on populations of cells. To address this, we have developed time-resolved, single-cell metabolomics and lipidomics approaches to explore the heterogeneity of macrophages during foam cell formation. Our dynamic metabolomic and lipidomic analyses revealed a dual regulatory axis involving inflammation and ferroptosis. Further, single-cell metabolomics and lipidomics have delineated a continuum of macrophage states, with varied susceptibilities to apoptosis and ferroptosis. Single-cell transcriptomic profiling confirmed these divergent fates, both in established cell lines and in macrophages derived from peripheral blood monocytes. This research has uncovered the complex molecular interactions that dictate these divergent cell fates, providing crucial insights into the pathogenesis of atherosclerosis.

The SARS-CoV-2 nucleoprotein associates with anionic lipid membranes

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a lipid-enveloped virus that acquires its lipid bilayer from the host cell it infects. SARS-CoV-2 can spread from cell to cell or from patient to patient by undergoing assembly and budding to form new virions. The assembly and budding of SARS-CoV-2 is mediated by several structural proteins known as envelope (E), membrane (M), nucleoprotein (N), and spike (S), which can form virus-like particles (VLPs) when co-expressed in mammalian cells. Assembly and budding of SARS-CoV-2 from the host ER-Golgi intermediate compartment is a critical step in the virus acquiring its lipid bilayer. To date, little information is available on how SARS-CoV-2 assembles and forms new viral particles from host membranes. In this study, we used several lipid binding assays and found the N protein can strongly associate with anionic lipids including phosphoinositides and phosphatidylserine. Moreover, we show lipid binding occurs in the N protein C-terminal domain, which is supported by extensive in silico analysis. We demonstrate anionic lipid binding occurs for both the free and the N oligomeric forms, suggesting N can associate with membranes in the nucleocapsid form. Based on these results, we present a lipid-dependent model based on in vitro, cellular, and in silico data for the recruitment of N to assembly sites in the lifecycle of SARS-CoV-2.

Lysophosphatidylserine: A Signaling Lipid with Implications in Human Diseases

Lysophosphatidylserine (lyso-PS) has emerged as yet another important signaling lysophospholipid in mammals, and deregulation in its metabolism has been directly linked to an array of human autoimmune and neurological disorders. It has an indispensable role in several biological processes in humans, and therefore, cellular concentrations of lyso-PS are tightly regulated to ensure optimal signaling and functioning in physiological settings. Given its biological importance, the past two decades have seen an explosion in the available literature toward our understanding of diverse aspects of lyso-PS metabolism and signaling and its association with human diseases. In this Review, we aim to comprehensively summarize different aspects of lyso-PS, such as its structure, biodistribution, chemical synthesis, and SAR studies with some synthetic analogs. From a biochemical perspective, we provide an exhaustive coverage of the diverse biological activities modulated by lyso-PSs, such as its metabolism and the receptors that respond to them in humans. We also briefly discuss the human diseases associated with aberrant lyso-PS metabolism and signaling and posit some future directions that may advance our understanding of lyso-PS-mediated mammalian physiology.

Protein-membrane interactions: sensing and generating curvature

Biological membranes are integral cellular structures that can be curved into various geometries. These curved structures are abundant in cells as they are essential for various physiological processes. However, curved membranes are inherently unstable, especially on nanometer length scales. To stabilize curved membranes, cells can utilize proteins that sense and generate membrane curvature. In this review, we summarize recent research that has advanced our understanding of interactions between proteins and curved membrane surfaces, as well as work that has expanded our ability to study curvature sensing and generation. Additionally, we look at specific examples of cellular processes that require membrane curvature, such as neurotransmission, clathrin-mediated endocytosis (CME), and organelle biogenesis.

Ether lipids influence cancer cell fate by modulating iron uptake

Cancer cell fate has been widely ascribed to mutational changes within protein-coding genes associated with tumor suppressors and oncogenes. In contrast, the mechanisms through which the biophysical properties of membrane lipids influence cancer cell survival, dedifferentiation and metastasis have received little scrutiny. Here, we report that cancer cells endowed with a high metastatic ability and cancer stem cell-like traits employ ether lipids to maintain low membrane tension and high membrane fluidity. Using genetic approaches and lipid reconstitution assays, we show that these ether lipid-regulated biophysical properties permit non-clathrin-mediated iron endocytosis via CD44, leading directly to significant increases in intracellular redox-active iron and enhanced ferroptosis susceptibility. Using a combination of in vitro three-dimensional microvascular network systems and in vivo animal models, we show that loss of ether lipids also strongly attenuates extravasation, metastatic burden and cancer stemness. These findings illuminate a mechanism whereby ether lipids in carcinoma cells serve as key regulators of malignant progression while conferring a unique vulnerability that can be exploited for therapeutic intervention.

Effects of biophysical membrane properties on recognition of phosphatidylserine, or phosphatidylinositol 4-phosphate by lipid biosensors LactC2, or P4M

Lipid biosensors are molecular tools used both in vivo and in vitro applications, capable of selectively detecting specific types of lipids in biological membranes. However, despite their extensive use, there is a lack of systematic characterization of their binding properties in various membrane conditions. The purpose of this study was to investigate the impact of membrane properties, such as fluidity and membrane charge, on the sensitivity of two lipid biosensors, LactC2 and P4M, to their target lipids, phosphatidylserine (PS) or phosphatidylinositol 4-phosphate (PI4P), respectively. Dual-color fluorescence cross-correlation spectroscopy, employed in this study, provided a useful technique to investigate interactions of these recombinant fluorescent biosensors with liposomes of varying compositions. The results of the study demonstrate that the binding of the LactC2 biosensor to low levels of PS in the membrane is highly supported by the presence of anionic lipids or membrane fluidity. However, at high PS levels, the presence of anionic lipids does not further enhance binding of LactC2. In contrast, neither membrane charge, nor membrane fluidity significantly affect the binding affinity of P4M to PI4P. These findings provide valuable insights into the role of membrane properties on the binding properties of lipid biosensors.

Acute joint inflammation induces a sharp increase in the number of synovial fluid EVs and modifies their phospholipid profile

Inflammation is the hallmark of most joint disorders. However, the precise regulation of induction, perpetuation, and resolution of joint inflammation is not entirely understood. Since extracellular vesicles (EVs) are critical for intercellular communication, we aim to unveil their role in these processes. Here, we investigated the EVs' dynamics and phospholipidome profile from synovial fluid (SF) of healthy equine joints and from horses with lipopolysaccharide (LPS)-induced synovitis. LPS injection triggered a sharp increase of SF-EVs at 5-8 h post-injection, which started to decline at 24 h post-injection. Importantly, we identified significant changes in the lipid profile of SF-EVs after synovitis induction. Compared to healthy joint-derived SF-EVs (0 h), SF-EVs collected at 5, 24, and 48 h post-LPS injection were strongly increased in hexosylceramides. At the same time, phosphatidylserine, phosphatidylcholine, and sphingomyelin were decreased in SF-EVs at 5 h and 24 h post-LPS injection. Based on the lipid changes during acute inflammation, we composed specific lipid profiles associated with healthy and inflammatory state-derived SF-EVs. The sharp increase in SF-EVs during acute synovitis and the correlation of specific lipids with either healthy or inflamed states-derived SF-EVs are findings of potential interest for unveiling the role of SF-EVs in joint inflammation, as well as for the identification of EV-biomarkers of joint inflammation.

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.

Direct ionic stress sensing and mitigation by the transcription factor NFAT5

Homeostatic control of intracellular ionic strength is essential for protein, organelle and genome function, yet mechanisms that sense and enable adaptation to ionic stress remain poorly understood in animals. We find that the transcription factor NFAT5 directly senses solution ionic strength using a C-terminal intrinsically disordered region. Both in intact cells and in a purified system, NFAT5 forms dynamic, reversible biomolecular condensates in response to increasing ionic strength. This self-associative property, conserved from insects to mammals, allows NFAT5 to accumulate in the nucleus and activate genes that restore cellular ion content. Mutations that reduce condensation or those that promote aggregation both reduce NFAT5 activity, highlighting the importance of optimally tuned associative interactions. Remarkably, human NFAT5 alone is sufficient to reconstitute a mammalian transcriptional response to ionic or hypertonic stress in yeast. Thus NFAT5 is both the sensor and effector of a cell-autonomous ionic stress response pathway in animal cells.

Lipids in Extracellular Vesicles: What Can Be Learned about Membrane Structure and Function?

Extracellular vesicles, such as exosomes, can be used as interesting models to study the structure and function of biological membranes as these vesicles contain only one membrane (i.e., one lipid bilayer). In addition to lipids, they contain proteins, nucleic acids, and various other molecules. The lipid composition of exosomes is here compared to HIV particles and detergent-resistant membranes, which also have a high content of sphingolipids, cholesterol, and phosphatidylserine (PS). We discuss interactions between the lipids in the two bilayers, and especially those between PS 18:0/18:1 in the inner leaflet and the very-long-chain sphingolipids in the outer leaflet, and the importance of cholesterol for these interactions. We also briefly discuss the involvement of ether-linked phospholipids (PLs) in such lipid raft-like structures, and the possible involvement of these and other lipid classes in the formation of exosomes. The urgent need to improve the quality of quantitative lipidomic studies is highlighted.

The mechanism of STING autoinhibition and activation

2',3'-cGAMP, produced by the DNA sensor cGAS, activates stimulator of interferon genes (STING) and triggers immune response during infection. Tremendous effort has been placed on unraveling the mechanism of STING activation. However, little is known about STING inhibition. Here, we found that apo-STING exhibits a bilayer with head-to-head as well as side-by-side packing, mediated by its ligand-binding domain (LBD). This type of assembly holds two endoplasmic reticulum (ER) membranes together not only to prevent STING ER exit but also to eliminate the recruitment of TBK1, representing the autoinhibited state of STING. Additionally, we obtained the filament structure of the STING/2',3'-cGAMP complex, which adopts a bent monolayer assembly mediated by LBD and transmembrane domain (TMD). The active, curved STING polymer could deform ER membrane to support its ER exit and anterograde transportation. Our data together provide a panoramic vision regarding STING autoinhibition and activation, which adds substantially to current understanding of the cGAS-STING pathway.

Lamellarity-Driven Differences in Surface Structural Features of DPPS Lipids: Spectroscopic, Calorimetric and Computational Study

Although single-lipid bilayers are usually considered models of eukaryotic plasma membranes, their research drops drastically when it comes to exclusively anionic lipid membranes. Being a major anionic phospholipid in the inner leaflet of eukaryote membranes, phosphatidylserine-constituted lipid membranes were occasionally explored in the form of multilamellar liposomes (MLV), but their inherent instability caused a serious lack of efforts undertaken on large unilamellar liposomes (LUVs) as more realistic model membrane systems. In order to compensate the existing shortcomings, we performed a comprehensive calorimetric, spectroscopic and MD simulation study of time-varying structural features of LUV made from 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS), whereas the corresponding MLV were examined as a reference. A substantial uncertainty of UV/Vis data of LUV from which only T_m was unambiguously determined (53.9 ± 0.8 °C), along with rather high uncertainty on the high-temperature range of DPPS melting profile obtained from DSC (≈50-59 °C), presumably reflect distinguished surface structural features in LUV. The FTIR signatures of glycerol moiety and those originated from carboxyl group serve as a strong support that in LUV, unlike in MLV, highly curved surfaces occur continuously, whereas the details on the attenuation of surface features in MLV were unraveled by molecular dynamics.

Structural Analyses of the Glycolipids in Lipid Rafts

Lipid rafts are usually isolated from cells or tissues using sucrose gradient ultracentrifugation in the presence of detergents such as Triton X-100 at 4 °C. Although detergents should be removed for further structural characterization following fractionation, these compounds are often difficult to completely remove, especially from the glycolipids. In this chapter, we describe a novel method for the fast and convenient removal of detergents from lipid raft glycolipids following fraction and describe the application of this method.

Creating and sensing asymmetric lipid distributions throughout the cell

A key feature of eukaryotic cells is the asymmetric distribution of lipids along their secretory pathway. Because of the biological significance of these asymmetries, it is crucial to define the mechanisms which create them. Extensive studies have led to the identification of lipid transfer proteins (LTPs) that work with lipid-synthesizing enzymes to carry lipids between two distinct membranes in a directional manner, and are thus able to create asymmetries in lipid distribution throughout the cell. These networks are often in contact sites where two organelle membranes are in close proximity for reasons we have only recently started to understand. A question is whether these networks transfer lipids en masse within the cells or adjust the lipid composition of organelle membranes. Finally, recent data have confirmed that some networks organized around LTPs do not generate lipid asymmetries between membranes but sense them and rectify the lipid content of the cell.

African Swine Fever Vaccinology: The Biological Challenges from Immunological Perspectives

African swine fever virus (ASFV), a nucleocytoplasmic large DNA virus (NCLDV), causes African swine fever (ASF), an acute hemorrhagic disease with mortality rates up to 100% in domestic pigs. ASF is currently epidemic or endemic in many countries and threatening the global swine industry. Extensive ASF vaccine research has been conducted since the 1920s. Like inactivated viruses of other NCLDVs, such as vaccinia virus, inactivated ASFV vaccine candidates did not induce protective immunity. However, inactivated lumpy skin disease virus (poxvirus) vaccines are protective in cattle. Unlike some experimental poxvirus subunit vaccines that induced protection, ASF subunit vaccine candidates implemented with various platforms containing several ASFV structural genes or proteins failed to protect pigs effectively. Only some live attenuated viruses (LAVs) are able to protect pigs with high degrees of efficacy. There are currently several LAV ASF vaccine candidates. Only one commercial LAV vaccine is approved for use in Vietnam. LAVs, as ASF vaccines, have not yet been widely tested. Reports thus far show that the onset and duration of protection induced by the LAVs are late and short, respectively, compared to LAV vaccines for other diseases. In this review, the biological challenges in the development of ASF vaccines, especially subunit platforms, are discussed from immunological perspectives based on several unusual ASFV characteristics shared with HIV and poxviruses. These characteristics, including multiple distinct infectious virions, extremely high glycosylation and low antigen surface density of envelope proteins, immune evasion, and possible apoptotic mimicry, could pose enormous challenges to the development of ASF vaccines, especially subunit platforms designed to induce humoral immunity.

Modelling membrane curvature generation using mechanics and machine learning

The deformation of cellular membranes regulates trafficking processes, such as exocytosis and endocytosis. Classically, the Helfrich continuum model is used to characterize the forces and mechanical parameters that cells tune to accomplish membrane shape changes. While this classical model effectively captures curvature generation, one of the core challenges in using it to approximate a biological process is selecting a set of mechanical parameters (including bending modulus and membrane tension) from a large set of reasonable values. We used the Helfrich model to generate a large synthetic dataset from a random sampling of realistic mechanical parameters and used this dataset to train machine-learning models. These models produced promising results, accurately classifying model behaviour and predicting membrane shape from mechanical parameters. We also note emerging methods in machine learning that can leverage the physical insight of the Helfrich model to improve performance and draw greater insight into how cells control membrane shape change.

Serum Neutralizing and Enhancing Effects on African Swine Fever Virus Infectivity in Adherent Pig PBMC

African swine fever virus (ASFV) causes hemorrhagic fever with mortality rates of up to 100% in domestic pigs. Currently, there are no commercial vaccines for the disease. Only some live-attenuated viruses have been able to protect pigs from ASFV infection. The immune mechanisms involved in the protection are unclear. Immune sera can neutralize ASFV but incompletely. The mechanisms involved are not fully understood. Currently, there is no standardized protocol for ASFV neutralization assays. In this study, a flow cytometry-based ASFV neutralization assay was developed and tested in pig adherent PBMC using a virulent ASFV containing a fluorescent protein gene as a substrate for neutralization. As with previous studies, the percentage of infected macrophages was approximately five time higher than that of infected monocytes, and nearly all infected cells displayed no staining with anti-CD16 antibodies. Sera from naïve pigs and pigs immunized with a live-attenuated ASFV and fully protected against parental virus were used in the assay. The sera displayed incomplete neutralization with MOI-dependent neutralizing efficacies. Extracellular, but not intracellular, virions suspended in naïve serum were more infectious than those in the culture medium, as reported for some enveloped viruses, suggesting a novel mechanism of ASFV infection in macrophages. Both the intracellular and extracellular virions could not be completely neutralized.

Albumin Uptake and Processing by the Proximal Tubule: Physiologic, Pathologic and Therapeutic Implications

For nearly 50 years the proximal tubule (PT) has been known to reabsorb, process, and either catabolize or transcytose albumin from the glomerular filtrate. Innovative techniques and approaches have provided insights into these processes. Several genetic diseases, nonselective PT cell defects, chronic kidney disease (CKD), and acute PT injury lead to significant albuminuria, reaching nephrotic range. Albumin is also known to stimulate PT injury cascades. Thus, the mechanisms of albumin reabsorption, catabolism, and transcytosis are being reexamined with the use of techniques that allow for novel molecular and cellular discoveries. Megalin, a scavenger receptor, cubilin, amnionless, and Dab2 form a nonselective multireceptor complex that mediates albumin binding and uptake and directs proteins for lysosomal degradation after endocytosis. Albumin transcytosis is mediated by a pH-dependent binding affinity to the neonatal Fc receptor (FcRn) in the endosomal compartments. This reclamation pathway rescues albumin from urinary losses and cellular catabolism, extending its serum half-life. Albumin that has been altered by oxidation, glycation, or carbamylation or because of other bound ligands that do not bind to FcRn traffics to the lysosome. This molecular sorting mechanism reclaims physiological albumin and eliminates potentially toxic albumin. The clinical importance of PT albumin metabolism has also increased as albumin is now being used to bind therapeutic agents to extend their half-life and minimize filtration and kidney injury. The purpose of this review is to update and integrate evolving information regarding the reabsorption and processing of albumin by proximal tubule cells including discussion of genetic disorders and therapeutic considerations.

A Peptide-Based Fluorescent Sensor for Anionic Phospholipids

Anionic phospholipids are key cell signal mediators. The distribution of these lipids on the cell membrane and intracellular organelle membranes guides the recruitment of signaling proteins leading to the regulation of cellular processes. Hence, fluorescent sensors that can detect anionic phospholipids within living cells can provide a handle into revealing molecular mechanisms underlying lipid-mediated signal regulation. A major challenge in the detection of anionic phospholipids is related to the presence of these phospholipids mostly in the inner leaflet of the plasma membrane and in the membranes of intracellular organelles. Hence, cell-permeable sensors would provide an advantage by enabling the rapid detection and tracking of intracellular pools of anionic phospholipids. We have developed a peptide-based, cell-permeable, water-soluble, and ratiometric fluorescent sensor that entered cells within 15 min of incubation via the endosomal machinery and showed punctate labeling in the cytoplasm. The probe could also be introduced into living cells via lipofection, which allows bypassing of endosomal uptake, to image anionic phospholipids in the cell membrane. We validated the ability of the sensor toward detection of intracellular anionic phospholipids by colocalization studies with a fluorescently tagged lipid and a protein-based anionic phospholipid sensor. Further, the sensor could image the externalization of anionic phospholipids during programmed cell death, indicating the ability of the probe toward detection of both intra- and extracellular anionic phospholipids based on the biological context.

Pathways and Mechanisms of Cellular Cholesterol Efflux-Insight From Imaging

Cholesterol is an essential molecule in cellular membranes, but too much cholesterol can be toxic. Therefore, mammalian cells have developed complex mechanisms to remove excess cholesterol. In this review article, we discuss what is known about such efflux pathways including a discussion of reverse cholesterol transport and formation of high-density lipoprotein, the function of ABC transporters and other sterol efflux proteins, and we highlight their role in human diseases. Attention is paid to the biophysical principles governing efflux of sterols from cells. We also discuss recent evidence for cholesterol efflux by the release of exosomes, microvesicles, and migrasomes. The role of the endo-lysosomal network, lipophagy, and selected lysosomal transporters, such as Niemann Pick type C proteins in cholesterol export from cells is elucidated. Since oxysterols are important regulators of cellular cholesterol efflux, their formation, trafficking, and secretion are described briefly. In addition to discussing results obtained with traditional biochemical methods, focus is on studies that use established and novel bioimaging approaches to obtain insight into cholesterol efflux pathways, including fluorescence and electron microscopy, atomic force microscopy, X-ray tomography as well as mass spectrometry imaging.

Ras Multimers on the Membrane: Many Ways for a Heart-to-Heart Conversation

Formation of Ras multimers, including dimers and nanoclusters, has emerged as an exciting, new front of research in the 'old' field of Ras biomedicine. With significant advances made in the past few years, we are beginning to understand the structure of Ras multimers and, albeit preliminary, mechanisms that regulate their formation in vitro and in cells. Here we aim to synthesize the knowledge accrued thus far on Ras multimers, particularly the presence of multiple globular (G-) domain interfaces, and discuss how membrane nanodomain composition and structure would influence Ras multimer formation. We end with some general thoughts on the potential implications of Ras multimers in basic and translational biology.

Complex nanoemulsion for vitamin delivery: droplet organization and interaction with skin membranes

Lipid nanoemulsions are promising nanomaterials for drug delivery applications in food, pharmaceutical and cosmetic industries. Despite the noteworthy commercial interest, little is known about their supramolecular organization, especially about how such multicomponent formulations interact with cell membranes. In the present work, coarse-grained molecular dynamics simulations have been employed to study the self-assembly of a 15-component lipid nanoemulsion droplet containing vitamins A and E for skin delivery. Our results display aspects of the unique "onion-like" agglomeration between the chemical constituents in the different layers of the lipid nanodroplet. Vitamin E molecules are more concentrated in the center of the droplet together with other hydrophobic constituents such as the triglycerides with long tails. On the other hand, vitamin A occupies an intermediate layer between the core and the co-emulsifier surface of the nanodroplet, together with lecithin phospholipids. Coarse-grained molecular dynamics simulations were also performed to provide insight into the first steps involved in absorption and penetration of the nanodroplet through skin membrane models, representing an intracellular (hair follicle infundibulum) and intercellular pathway (stratum corneum) through the skin. Our data provide a first view on the complex organization of commercial nanoemulsion and its interaction with skin membranes. We expect our results to open the way towards the rational design of such nanomaterials.

Functional analyses of phosphatidylserine/PI(4)P exchangers with diverse lipid species and membrane contexts reveal unanticipated rules on lipid transfer

Background

Lipid species are accurately distributed in the eukaryotic cell so that organelle and plasma membranes have an adequate lipid composition to support numerous cellular functions. In the plasma membrane, a precise regulation of the level of lipids such as phosphatidylserine, PI(4)P, and PI(4,5)P₂, is critical for maintaining the signaling competence of the cell. Several lipid transfer proteins of the ORP/Osh family contribute to this fine-tuning by delivering PS, synthesized in the endoplasmic reticulum, to the plasma membrane in exchange for PI(4)P. To get insights into the role of these PS/PI(4)P exchangers in regulating plasma membrane features, we question how they selectively recognize and transfer lipid ligands with different acyl chains, whether these proteins exchange PS exclusively for PI(4)P or additionally for PI(4,5)P₂, and how sterol abundance in the plasma membrane impacts their activity.

Results

We measured in vitro how the yeast Osh6p and human ORP8 transported PS and PI(4)P subspecies of diverse length and unsaturation degree between membranes by fluorescence-based assays. We established that the exchange activity of Osh6p and ORP8 strongly depends on whether these ligands are saturated or not, and is high with representative cellular PS and PI(4)P subspecies. Unexpectedly, we found that the speed at which these proteins individually transfer lipid ligands between membranes is inversely related to their affinity for them and that high-affinity ligands must be exchanged to be transferred more rapidly. Next we determined that Osh6p and ORP8 cannot use PI(4,5)P₂ for exchange processes, because it is a low-affinity ligand, and do not transfer more PS into sterol-rich membranes.

Conclusions

Our study provides new insights into PS/PI(4)P exchangers by indicating the degree to which they can regulate the acyl chain composition of the PM, and how they control PM phosphoinositide levels. Moreover, we establish general rules on how the activity of lipid transfer proteins relates to their affinity for ligands.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01183-1.

Sterols lower energetic barriers of membrane bending and fission necessary for efficient clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is critical for cellular signal transduction, receptor recycling, and membrane homeostasis in mammalian cells. Acute depletion of cholesterol disrupts CME, motivating analysis of CME dynamics in the context of human disorders of cholesterol metabolism. We report that inhibition of post-squalene cholesterol biosynthesis impairs CME. Imaging of membrane bending dynamics and the CME pit ultrastructure reveals prolonged clathrin pit lifetimes and shallow clathrin-coated structures, suggesting progressive impairment of curvature generation correlates with diminishing sterol abundance. Sterol structural requirements for efficient CME include 3′ polar head group and B-ring conformation, resembling the sterol structural prerequisites for tight lipid packing and polarity. Furthermore, Smith-Lemli-Opitz fibroblasts with low cholesterol abundance exhibit deficits in CME-mediated transferrin internalization. We conclude that sterols lower the energetic costs of membrane bending during pit formation and vesicular scission during CME and suggest that reduced CME activity may contribute to cellular phenotypes observed within disorders of cholesterol metabolism.

Anderson et al. demonstrate that sterol abundance and identity play a dominant role in facilitating clathrin-mediated endocytosis. Detailed analyses of clathrin-coated pits under sterol depletion support a requirement for sterol-mediated membrane bending during multiple stages of endocytosis, implicating endocytic dysfunction within the pathogenesis of disorders of cholesterol metabolism.

Maintenance and loss of endocytic organelle integrity: mechanisms and implications for antigen cross-presentation

The membranes of endosomes, phagosomes and macropinosomes can become damaged by the physical properties of internalized cargo, by active pathogenic invasion or by cellular processes, including endocytic maturation. Loss of membrane integrity is often deleterious and is, therefore, prevented by mitigation and repair mechanisms. However, it can occasionally be beneficial and actively induced by cells. Here, we summarize the mechanisms by which cells, in particular phagocytes, try to prevent membrane damage and how, when this fails, they repair or destroy damaged endocytic organelles. We also detail how one type of phagocyte, the dendritic cell, can deliberately trigger localized damage to endocytic organelles to allow for major histocompatibility complex class I presentation of exogenous antigens and initiation of CD8⁺ T-cell responses to viruses and tumours. Our review highlights mechanisms for the regulation of endocytic organelle membrane integrity at the intersection of cell biology and immunology that could be co-opted for improving vaccination and intracellular drug delivery.

Negative-sense RNA viruses: An underexplored platform for examining virus-host lipid interactions

Viruses are pathogenic agents that can infect all varieties of organisms, including plants, animals, and humans. These microscopic particles are genetically simple as they encode a limited number of proteins that undertake a wide range of functions. While structurally distinct, viruses often share common characteristics that have evolved to aid in their infectious life cycles. A commonly underappreciated characteristic of many deadly viruses is a lipid envelope that surrounds their protein and genetic contents. Notably, the lipid envelope is formed from the host cell the virus infects. Lipid-enveloped viruses comprise a diverse range of pathogenic viruses, which often lead to high fatality rates and many lack effective therapeutics and/or vaccines. This perspective primarily focuses on the negative-sense RNA viruses from the order Mononegavirales, which obtain their lipid envelope from the host plasma membrane. Specifically, the perspective highlights the common themes of host cell lipid and membrane biology necessary for virus replication, assembly, and budding.

Lipid Profiles of RAS Nanoclusters Regulate RAS Function

The lipid-anchored RAS (Rat sarcoma) small GTPases (guanosine triphosphate hydrolases) are highly prevalent in human cancer. Traditional strategies of targeting the enzymatic activities of RAS have been shown to be difficult. Alternatively, RAS function and pathology are mostly restricted to nanoclusters on the plasma membrane (PM). Lipids are important structural components of these signaling platforms on the PM. However, how RAS nanoclusters selectively enrich distinct lipids in the PM, how different lipids contribute to RAS signaling and oncogenesis and whether the selective lipid sorting of RAS nanoclusters can be targeted have not been well-understood. Latest advances in quantitative super-resolution imaging and molecular dynamic simulations have allowed detailed characterization RAS/lipid interactions. In this review, we discuss the latest findings on the select lipid composition (with headgroup and acyl chain specificities) within RAS nanoclusters, the specific mechanisms for the select lipid sorting of RAS nanoclusters on the PM and how perturbing lipid compositions within RAS nanoclusters impacts RAS function and pathology. We also describe different strategies of manipulating lipid composition within RAS nanoclusters on the PM.

Transport Pathways That Contribute to the Cellular Distribution of Phosphatidylserine

Phosphatidylserine (PS) is a negatively charged phospholipid that displays a highly uneven distribution within cellular membranes, essential for establishment of cell polarity and other processes. In this review, we discuss how combined action of PS biosynthesis enzymes in the endoplasmic reticulum (ER), lipid transfer proteins (LTPs) acting within membrane contact sites (MCS) between the ER and other compartments, and lipid flippases and scramblases that mediate PS flip-flop between membrane leaflets controls the cellular distribution of PS. Enrichment of PS in specific compartments, in particular in the cytosolic leaflet of the plasma membrane (PM), requires input of energy, which can be supplied in the form of ATP or by phosphoinositides. Conversely, coupling between PS synthesis or degradation, PS flip-flop and PS transfer may enable PS transfer by passive flow. Such scenario is best documented by recent work on the formation of autophagosomes. The existence of lateral PS nanodomains, which is well-documented in the case of the PM and postulated for other compartments, can change the steepness or direction of PS gradients between compartments. Improvements in cellular imaging of lipids and membranes, lipidomic analysis of complex cellular samples, reconstitution of cellular lipid transport reactions and high-resolution structural data have greatly increased our understanding of cellular PS homeostasis. Our review also highlights how budding yeast has been instrumental for our understanding of the organization and transport of PS in cells.

Role of Endocytosis Pathways in Electropermeablization of MCF7 Cells Using Low Voltage and High Frequency Electrochemotherapy

Objective

The cell membrane is a major barrier for delivery of hydrophilic drugs and molecules into the cells. Although low voltage and high frequency electric fields (LVHF) are proposed to overcome the cell membrane barrier, the mechanism of membrane permeabilization is unclear. The aim of study is to investigate endocytosis pathways as a possible mechanism for enhancing uptake of bleomycin by LVHF.

Materials and Methods

In this experimental study, MCF-7 cells were exposed to bleomycin or to electric fields with various strengths (10-80 V/cm), frequency of 5 kHz, 4000 electric pulse and 100 µs duration in the presence and absence of three endocytosis inhibitors-chlorpromazine (Cpz), amiloride (Amilo) and genistein (Geni). We determined the efficiency of these chemotherapeutic agents in each group.

Results

LVHF, depending on the intensity, induced different endocytosis pathways. Electric field strengths of 10 and 20 V/cm stimulated the macropinocytosis route. Clathrin-mediated endocytosis was observed at electric field intensities of 10, 30, 60 and 70 V/cm, whereas induction of caveolae-mediated endocytosis was observed only at the lowest electric field intensity (10 V/cm).

Conclusion

The results of this study imply that LVHF can induce different endocytosis pathways in MCF-7 cells, which leads to an increase in bleomycin uptake.

CpACpP: In Silico Cell-Penetrating Anticancer Peptide Prediction Using a Novel Bioinformatics Framework

Cell-penetrating anticancer peptides (Cp-ACPs) are considered promising candidates in solid tumor and hematologic cancer therapies. Current approaches for the design and discovery of Cp-ACPs trust the expensive high-throughput screenings that often give rise to multiple obstacles, including instrumentation adaptation and experimental handling. The application of machine learning (ML) tools developed for peptide activity prediction is importantly of growing interest. In this study, we applied the random forest (RF)-, support vector machine (SVM)-, and eXtreme gradient boosting (XGBoost)-based algorithms to predict the active Cp-ACPs using an experimentally validated data set. The model, CpACpP, was developed on the basis of two independent cell-penetrating peptide (CPP) and anticancer peptide (ACP) subpredictors. Various compositional and physiochemical-based features were combined or selected using the multilayered recursive feature elimination (RFE) method for both data sets. Our results showed that the ACP subclassifiers obtain a mean performance accuracy (ACC) of 0.98 with an area under curve (AUC) ≈ 0.98 vis-à-vis the CPP predictors displaying relevant values of ∼0.94 and ∼0.95 via the hybrid-based features and independent data sets, respectively. Also, the predicting evaluation of Cp-ACPs gave accuracies of ∼0.79 and 0.89 on a series of independent sequences by applying our CPP and ACP classifiers, respectively, which leaves the performance of our predictors better than the earlier reported ACPred, mACPpred, MLCPP, and CPPred-RF. The described consensus-based fusion method additionally reached an AUC of 0.94 for the prediction of Cp-ACP (http://cbb1.ut.ac.ir/CpACpP/Index).

Structural basis for VIPP1 oligomerization and maintenance of thylakoid membrane integrity

Vesicle-inducing protein in plastids 1 (VIPP1) is essential for the biogenesis and maintenance of thylakoid membranes, which transform light into life. However, it is unknown how VIPP1 performs its vital membrane-remodeling functions. Here, we use cryo-electron microscopy to determine structures of cyanobacterial VIPP1 rings, revealing how VIPP1 monomers flex and interweave to form basket-like assemblies of different symmetries. Three VIPP1 monomers together coordinate a non-canonical nucleotide binding pocket on one end of the ring. Inside the ring's lumen, amphipathic helices from each monomer align to form large hydrophobic columns, enabling VIPP1 to bind and curve membranes. In vivo mutations in these hydrophobic surfaces cause extreme thylakoid swelling under high light, indicating an essential role of VIPP1 lipid binding in resisting stress-induced damage. Using cryo-correlative light and electron microscopy (cryo-CLEM), we observe oligomeric VIPP1 coats encapsulating membrane tubules within the Chlamydomonas chloroplast. Our work provides a structural foundation for understanding how VIPP1 directs thylakoid biogenesis and maintenance.

Unveiling the Dynamics of KRAS4b on Lipid Model Membranes

Small GTPase proteins are ubiquitous and responsible for regulating several processes related to cell growth and differentiation. Mutations that stabilize their active state can lead to uncontrolled cell proliferation and cancer. Although these proteins are well characterized at the cellular scale, the molecular mechanisms governing their functions are still poorly understood. In addition, there is limited information about the regulatory function of the cell membrane which supports their activity. Thus, we have studied the dynamics and conformations of the farnesylated KRAS4b in various membrane model systems, ranging from binary fluid mixtures to heterogeneous raft mimics. Our approach combines long time-scale coarse-grained (CG) simulations and Markov state models to dissect the membrane-supported dynamics of KRAS4b. Our simulations reveal that protein dynamics is mainly modulated by the presence of anionic lipids and to some extent by the nucleotide state (activation) of the protein. In addition, our results suggest that both the farnesyl and the polybasic hypervariable region (HVR) are responsible for its preferential partitioning within the liquid-disordered (Ld) domains in membranes, potentially enhancing the formation of membrane-driven signaling platforms.

Geometry and cellular function of organelle membrane interfaces

A vast majority of cellular processes take root at the surface of biological membranes. By providing a two-dimensional platform with limited diffusion, membranes are, by nature, perfect devices to concentrate signaling and metabolic components. As such, membranes often act as "key processors" of cellular information. Biological membranes are highly dynamic and deformable and can be shaped into curved, tubular, or flat conformations, resulting in differentiated biophysical properties. At membrane contact sites, membranes from adjacent organelles come together into a unique 3D configuration, forming functionally distinct microdomains, which facilitate spatially regulated functions, such as organelle communication. Here, we describe the diversity of geometries of contact site-forming membranes in different eukaryotic organisms and explore the emerging notion that their shape, 3D architecture, and remodeling jointly define their cellular activity. The review also provides selected examples highlighting changes in membrane contact site architecture acting as rapid and local responses to cellular perturbations, and summarizes our current understanding of how those structural changes confer functional specificity to those cellular territories.

Huntingtin: A Protein with a Peculiar Solvent Accessible Surface

Taking advantage of the last cryogenic electron microscopy structure of human huntingtin, we explored with computational methods its physicochemical properties, focusing on the solvent accessible surface of the protein and highlighting a quite interesting mix of hydrophobic and hydrophilic patterns, with the prevalence of the latter ones. We then evaluated the probability of exposed residues to be in contact with other proteins, discovering that they tend to cluster in specific regions of the protein. We then found that the remaining portions of the protein surface can contain calcium-binding sites that we propose here as putative mediators for the protein to interact with membranes. Our findings are justified in relation to the present knowledge of huntingtin functional annotation.

Cardiolipin interacts with beta-2-glycoprotein I and forms an open conformation-Mechanisms analyzed using hydrogen/deuterium exchange

Beta‐2‐glycoprotein I (β₂GPI) is the major antigen for the antiphospholipid antibodies in the antiphospholipid syndrome. The exposed epitope in domain I of β₂GPI can be recognized by the anti‐β₂GPI antibody. Here, we prepared the anionic di‐oleoyl‐phosphatidylserine (DOPS) and cardiolipin (CL) liposomes to interact with the β₂GPI. The conformational changes of β₂GPI upon binding with the liposomes were analyzed using hydrogen/deuterium exchange mass spectrometry. The exchange level of sequences 21–27 significantly increased after β₂GPI had interacted with DOPS. This change indicated a reduced interaction between domain I and domain V, inferring to a protrusion of the sequences 21–27 from the ring conformation. After β₂GPI had interacted with CL for 30 min, the exchange levels in 4 of the 5 domains increased significantly. The deuteration levels of sequences 1–20, 21–27, 196–205, 273–279 and 297–306 increased, suggesting that these regions had become more exposed, and the domain I was no longer in contact with domain V. The increasing deuteration levels in sequences 70–86, 153–162, 191–198, 196–205 and 273–279 indicated β₂GPI undergoing conformational changes to expose these inner regions, suggesting a structural transition. Overall, DOPS and CL induced minor conformational changes of β₂GPI at sequences 21–27 and forms an intermediate conformation after 10 min of interaction. After a complete protein–lipid interaction, high negatively charged CL membrane induced a major conformation transition of β₂GPI.

Endomembrane Tension and Trafficking

Eukaryotic cells employ diverse uptake mechanisms depending on their specialized functions. While such mechanisms vary widely in their defining criteria: scale, molecular machinery utilized, cargo selection, and cargo destination, to name a few, they all result in the internalization of extracellular solutes and fluid into membrane-bound endosomes. Upon scission from the plasma membrane, this compartment is immediately subjected to extensive remodeling which involves tubulation and vesiculation/budding of the limiting endomembrane. This is followed by a maturation process involving concomitant retrograde transport by microtubule-based motors and graded fusion with late endosomes and lysosomes, organelles that support the degradation of the internalized content. Here we review an important determinant for sorting and trafficking in early endosomes and in lysosomes; the control of tension on the endomembrane. Remodeling of endomembranes is opposed by high tension (caused by high hydrostatic pressure) and supported by the relief of tension. We describe how the timely and coordinated efflux of major solutes along the endocytic pathway affords the cell control over such tension. The channels and transporters that expel the smallest components of the ingested medium from the early endocytic fluid are described in detail as these systems are thought to enable endomembrane deformation by curvature-sensing/generating coat proteins. We also review similar considerations for the lysosome where resident hydrolases liberate building blocks from luminal macromolecules and transporters flux these organic solutes to orchestrate trafficking events. How the cell directs organellar trafficking based on the luminal contents of organelles of the endocytic pathway is not well-understood, however, we propose that the control over membrane tension by solute transport constitutes one means for this to ensue.

Caveolae and lipid sorting: Shaping the cellular response to stress

Caveolae are an abundant and characteristic surface feature of many vertebrate cells. The uniform shape of caveolae is characterized by a bulb with consistent curvature connected to the plasma membrane (PM) by a neck region with opposing curvature. Caveolae act in mechanoprotection by flattening in response to increased membrane tension, and their disassembly influences the lipid organization of the PM. Here, we review evidence for caveolae as a specialized lipid domain and speculate on mechanisms that link changes in caveolar shape and/or protein composition to alterations in specific lipid species. We propose that high membrane curvature in specific regions of caveolae can enrich specific lipid species, with consequent changes in their localization upon caveolar flattening. In addition, we suggest how changes in the association of lipid-binding caveolar proteins upon flattening of caveolae could allow release of specific lipids into the bulk PM. We speculate that the caveolae-lipid system has evolved to function as a general stress-sensing and stress-protective membrane domain.

Palmitoylation of Membrane-Penetrating Magainin Derivatives Reinforces Necroptosis in A549 Cells Dependent on Peptide Conformational Propensities

Anticancer lipopeptides (ACLPs) are considered promising alternatives to combat resistant cancer cells, but the influence of peptide conformational propensity alone on their selectivity and mechanism remains obscure. In this study, we developed N-palmitoylated MK5E (P1MK5E) and MEK5 (P1MEK5) that have the same composition of 23 residues undergoing the pH-dependent structural alterations but differ in the conformational tendency of their amino acid composites. Nonlipidated peptides were readily accumulated in the A549 cell nucleus by the direct membrane translocation and the heparan sulfate-mediated endocytosis than the lipid-raft-dependent pathway. The increased hydrophobicity favored the amino acid-position-dependent folding of P1MK5E and P1MEK5, respectively, toward the α-helical coiled-coil nanofibrils and amyloidlike β-protofibrils. At the close concentrations (∼7.5 μM) to the toxic effects of doxorubicin (DOX), P1MK5E exhibited (i) an increased anticancer toxicity through a time-dependent S-phase arrest, (ii) enhanced plasma membrane permeability, and (iii) dose-dependent changes in the cell death characteristic features in the A549 cells relative to P1MEK5 that was almost inactive at ∼75 μM. These observations were in accordance with the TNF-α-mediated necroptotic signaling in the c-MYC/PARP1-overexpressed A549 cells exposed to P1MK5E and accompanied by the ultrastructure of plasma membrane protrusions, extensive endoplasmic reticulum (ER) membrane expansion, mitochondrial swelling, and the formation of distinct cytoplasmic vacuolation. The structural results and the bioactivity behaviors, herein, declared the significance of α-helical propensity in the peptide sequence and the nanostructure morphologies of self-assembling ACLPs upon the selectivity and enhanced anticancer effectiveness, which notably holds promise in the design and development of efficient therapeutics for cancer.