Effect of Phosphatidylethanolamine and Oleic Acid on Membrane Fusion: Phosphatidylethanolamine Circumvents the Classical Stalk Model

Fusion of asymmetric membranes: the emergence of a preferred direction

The fusion of lipid membranes progresses through a series of intermediate steps with two significant energy barriers: hemifusion-stalk formation and fusion-pore expansion. The cell's ability to tune these energy barriers is crucial as they determine the rate of many biological processes involving membrane fusion. However, a mechanism that allows the cell to manipulate both barriers in the same direction remains elusive, since membrane properties that the cell could dynamically tune during its life cycle, such as the lipids' spontaneous curvatures and membrane tension, have an opposite effect on the two barriers: tension inhibits stalk formation while promoting fusion-pore expansion. In contrast, increasing the total membrane concentration of lipids with negative intrinsic curvatures, such as cholesterol, promotes hemifusion-stalk formation while inhibiting pore expansion, and vice versa for lipids with positive intrinsic curvatures. Therefore, changes in these membrane properties increase one energy barrier at the expense of the other, resulting in a mixed effect on the fusion reaction. A possible mechanism to change both barriers in the same direction is by inducing lipid composition asymmetry, which results in tension and spontaneous curvature differences between the monolayers. To test the feasibility of this mechanism, a continuum elastic model was used to simulate the fusion intermediates and calculate the changes in the energy barriers. The calculations showed that a reasonable lipid composition asymmetry could lead to a 10-20kBT difference in both energy barriers, depending on the direction from which fusion occurs. We further provide experimental support to the model predictions, demonstrating changes in the time to hemifusion upon asymmetry introduction. These results indicate that biological membranes, which are asymmetric, have a preferred direction for fusion.

Creating the Equivalence Index to Optimize the Precise Evaluation of Bee Products for Functionally Opposite Components

Bee products, such as honey, bee pollen/bread, bee propolis and royal jelly foraged or secreted by honeybee workers, have been consumed by humans for many years and are important due to their complexity, the large number of them and the endemicity of their constituents. The health-promoting activities of bee products are widely documented all around the world. However, we have noticed a distinct but poorly described feature of bee products: groups of functionally opposite components (FOCs) related to blood sugar level, oxidative stress, cell membrane cholesterol distribution, cell membrane stability, cell membrane curvature, allergic reaction, cellular sodium influx and cardiac apoptosis that exist within these products. We then propose the Equivalence Index in order to overcome the challenges associated with FOCs; this is a concise mathematical model that can be used to optimize the evaluation of quality, determine any underlying mechanisms and provide processing guidance regarding bee products.

Dengue Virus Fusion Peptide Promotes Hemifusion Formation by Disordering the Interfacial Region of the Membrane

Membrane fusion is the first step in the infection process of the enveloped viruses. Enveloped viruses fuse either at the cell surface or enter the cell through endocytosis and transfer their internal genetic materials by fusing with the endosomal membrane at acidic pH. In this work, we have evaluated the effect of the Dengue virus fusion peptide (DENV FP) on the polyethylene glycol (PEG)-mediated lipid mixing of vesicles (hemifusion formation) at pH 5 and pH 7.4 with varying cholesterol concentrations. We have demonstrated that the DENV FP promotes hemifusion formation during the fusion of small unilamellar vesicles (SUVs) mainly at pH 5.0. Moreover, the fusion process demonstrates a strong correlation between fusogenicity and the amount of membrane cholesterol. We have further evaluated the partitioning ability of the peptide in three different membranes at pH 5.0 and pH 7.4. The fusogenic ability of the peptide at pH 5.0 is associated with the composition-dependent binding affinity of the peptide to the membrane. The depth-dependent fluorescence probes are used to evaluate membrane organization and dynamics utilizing steady-state and time-resolved fluorescence spectroscopic techniques. Our results show that the DENV FP promotes hemifusion formation by fluidizing the interfacial region of the membrane.

Sphingomyelin Inhibits Hydrolytic Activity of Heterodimeric PLA2 in Model Myelin Membranes: Pharmacological Relevance

In this work, the heterodimeric phospholipase A2, HDP-2, from viper venom was investigated for its hydrolytic activity in model myelin membranes as well as for its effects on intermembrane exchange of phospholipids (studied by phosphorescence quenching) and on phospholipid polymorphism (studied by 1H-NMR spectroscopy) to understand the role of sphingomyelin (SM) in the demyelination of nerve fibers. By using well-validated in vitro approaches, we show that the presence of SM in model myelin membranes leads to a significant inhibition of the hydrolytic activity of HDP-2, decreased intermembrane phospholipid exchange, and reduced phospholipid polymorphism. Using AutoDock software, we show that the NHδ+ group of the sphingosine backbone of SM binds to Tyr22(C=Opbδ-) of HDP-2 via a hydrogen bond which keeps only the polar head of SM inside the HDP-2's active center and positions the sn-2 acyl ester bond away from the active center, thus making it unlikely to hydrolyze the alkyl chains at the sn-2 position. This observation strongly suggests that SM inhibits the catalytic activity of HDP-2 by blocking access to other phospholipids to the active center of the enzyme. Should this observation be verified in further studies, it would offer a tantalizing opportunity for developing effective pharmaceuticals to stop the demyelination of nerve fibers by aberrant PLA2s with overt activity - as observed in brain degenerative diseases - by inhibiting SM hydrolysis and/or facilitating SM synthesis in the myelin sheath membrane.

Boosting Lipofection Efficiency Through Enhanced Membrane Fusion Mechanisms

Gene transfection is a fundamental technique in the fields of biological research and therapeutic innovation. Due to their biocompatibility and membrane-mimetic properties, lipid vectors serve as essential tools in transfection. The successful delivery of genetic material into the cytoplasm is contingent upon the fusion of the vector and cellular membranes, which enables hydrophilic polynucleic acids to traverse the hydrophobic barriers of two intervening membranes. This review examines the critical role of membrane fusion in lipofection efficiency, with a particular focus on the molecular mechanisms that govern lipoplex-membrane interactions. This analysis will examine the key challenges inherent to the fusion process, from achieving initial membrane proximity to facilitating final content release through membrane remodeling. In contrast to viral vectors, which utilize specialized fusion proteins, lipid vectors necessitate a strategic formulation and environmental optimization to enhance their fusogenicity. This review discusses recent advances in vector design and fusion-promoting strategies, emphasizing their potential to improve gene delivery yield. It highlights the importance of understanding lipoplex-membrane fusion mechanisms for developing next-generation delivery systems and emphasizes the need for continued fundamental research to advance lipid-mediated transfection technology.

Impaired cathepsin D in retinal pigment epithelium cells mediates Stargardt disease pathogenesis

Recessive Stargardt disease (STGD1) is an inherited juvenile maculopathy caused by mutations in the ABCA4 gene, for which there is no suitable treatment. Loss of functional ABCA4 in the retinal pigment epithelium (RPE) alone, without contribution from photoreceptor cells, was shown to induce STGD1 pathology. Here, we identified cathepsin D (CatD), the primary RPE lysosomal protease, as a key molecular player contributing to endo-lysosomal dysfunction in STGD1 using a newly developed "disease-in-a-dish" RPE model from confirmed STGD1 patients. Induced pluripotent stem cell (iPSC)-derived RPE originating from three STGD1 patients exhibited elevated lysosomal pH, as previously reported in Abca4-/- mice. CatD protein maturation and activity were impaired in RPE from STGD1 patients and Abca4-/- mice. Consequently, STGD1 RPE cells have reduced photoreceptor outer segment degradation and abnormal accumulation of α-synuclein, the natural substrate of CatD. Furthermore, dysfunctional ABCA4 in STGD1 RPE cells results in intracellular accumulation of autofluorescent material and phosphatidylethanolamine (PE). The altered distribution of PE associated with the internal membranes of STGD1 RPE cells presumably compromises LC3-associated phagocytosis, contributing to delayed endo-lysosomal degradation activity. Drug-mediated re-acidification of lysosomes in the RPE of STGD1 restores CatD functional activity and reduces the accumulation of immature CatD protein loads. This preclinical study validates the contribution of CatD deficiencies to STGD1 pathology and provides evidence for an efficacious therapeutic approach targeting RPE cells. Our findings support a cell-autonomous RPE-driven pathology, informing future research aimed at targeting RPE cells to treat ABCA4-mediated retinopathies.

A Coronin 1-Derived Peptide Inhibits Membrane Fusion by Modulating Membrane Organization and Dynamics

Membrane fusion is considered the first step in the entry of enveloped viruses into the host cell. Several targeted strategies have been implemented to block viral entry by limiting the fusion protein to form a six-helix bundle, which is a prerequisite for fusion. Nonetheless, the development of broad-spectrum fusion inhibitors is essential to combat emerging and re-emerging viral infections. TG-23, a coronin 1, a tryptophan-aspartate-rich phagosomal protein-derived peptide, demonstrated inhibition of fusion between small unilamellar vesicles (SUVs) by modulating the membrane's physical properties. However, its inhibitory efficacy reduces with an increasing concentration of membrane cholesterol. The present work aims to develop a fusion inhibitor whose efficacy would be unaltered in the presence of membrane cholesterol. A stretch of the tryptophan-aspartic acid-containing peptide with a similar secondary structure and hydrophobicity profile of TG-23 from coronin 1 was synthesized, and its ability to inhibit SUV-SUV fusion with varying concentrations of membrane cholesterol was evaluated. Our results demonstrate that the GG-21 peptide inhibits fusion irrespective of the cholesterol content of the membrane. We have further evaluated the peptide-induced change in the membrane organization and dynamics utilizing arrays of steady-state and time-resolved fluorescence measurements and correlated these results with their effect on fusion. Interestingly, GG-21 displays inhibitory efficacy in a wide variety of lipid compositions despite having a secondary structure and physical properties similar to those of TG-23. Overall, our results advocate that the secondary structure and physical properties of the peptide may not be sufficient to predict its inhibitory efficacy.

Purification of time-resolved insulin granules reveals proteomic and lipidomic changes during granule aging

Endocrine cells employ regulated exocytosis of secretory granules to secrete hormones and neurotransmitters. Secretory granule exocytosis depends on spatiotemporal variables such as proximity to the plasma membrane and age, with newly generated granules being preferentially released. Despite recent advances, we lack a comprehensive view of the molecular composition of insulin granules and associated changes over their lifetime. Here, we report a strategy for the purification of insulin secretory granules of distinct age from insulinoma INS-1 cells. Tagging the granule-resident protein phogrin with a cleavable CLIP tag, we obtain intact fractions of age-distinct granules for proteomic and lipidomic analyses. We find that the lipid composition changes over time, along with the physical properties of the membrane, and that kinesin-1 heavy chain (KIF5b) as well as Ras-related protein 3a (RAB3a) associate preferentially with younger granules. Further, we identify the Rho GTPase-activating protein (ARHGAP1) as a cytosolic factor associated with insulin granules.

Intracellular protein delivery: New insights into the therapeutic applications and emerging technologies

The inability to cross the plasma membranes traditionally limited the therapeutic use of recombinant proteins. However, in the last two decades, novel technologies made delivering proteins inside the cells possible. This allowed researchers to unlock intracellular targets, once considered 'undruggable', bringing a new research area to emerge. Protein transfection systems display a large potential in a plethora of applications. However, their modality of action is often unclear, and cytotoxic effects are elevated, whereas experimental conditions to increase transfection efficacy and cell viability still need to be identified. Furthermore, technical complexity often limits in vivo experimentation, while challenging industrial and clinical translation. This review highlights the applications of protein transfection technologies, and then critically discuss the current methodologies and their limitations. Physical membrane perforation systems are compared to systems exploiting cellular endocytosis. Research evidence of the existence of either extracellular vesicles (EVs) or cell-penetrating peptides (CPPs)- based systems, that circumvent the endosomal systems is critically analysed. Commercial systems, novel solid-phase reverse protein transfection systems, and engineered living intracellular bacteria-based mechanisms are finally described. This review ultimately aims at finding new methodologies and possible applications of protein transfection systems, while helping the development of an evidence-based research approach.

Phosphatidylethanolamine and Cholesterol Promote Hemifusion Formation: A Tug of War between Membrane Interfacial Order and Intrinsic Negative Curvature of Lipids

Membrane fusion is an important process for the survival of eukaryotes. The shape of lipids plays an important role in fusion by stabilizing nonlamellar fusion intermediates. Lipids with intrinsic positive curvature such as lysophosphatidylcholine and others inhibit hemifusion formation, whereas lipids having intrinsic negative curvature, e.g., phosphatidylethanolamine and cholesterol (CH), are known to promote hemifusion formation. In this work, we have measured the effect of dioleoylphosphatidylethanolamine (DOPE) and CH on the depth-dependent organization, dynamics, and fusion of dioleoylphosphatidylcholine membranes. Both DOPE and CH promote hemifusion formation despite their ability to order the interfacial and acyl chain region of the membrane and block water percolation at these regions. Generally, membrane ordering and inhibition of water percolation at the acyl chain region are detrimental to membrane fusion. This clearly emphasizes the importance of intrinsic negative curvature of lipids in membrane fusion. Interestingly, DOPE and CH show differential effects on the rate of hemifusion formation, which might be owing to their ability to induce order at the interfacial region and intrinsic negative curvature. Overall, our result is significant in understanding the role of lipidic shape in membrane fusion.

The battle of lipid-based nanocarriers against blood-brain barrier: a critical review

Central nervous system integrity is the state of brain functioning across sensory, cognitive, emotional-social behaviors, and motor domains, allowing a person to realise his full potential. Thus, brain disorders seriously affect patients' quality of life. Efficient drug delivery to treat brain disorders remains a crucial challenge due to numerous brain barriers, particularly the blood-brain barrier (BBB), which greatly impacts the ultimate drug therapeutic efficacy. Lately, nanocarrier technology has made huge progress in overcoming these barriers by improving drug solubility, ameliorating its retention, reducing its toxicity, and targeting the encapsulated agents to different brain tissues. The current review primarily offers an overview of the different components of BBB and the progress, strategies, and contemporary applications of the nanocarriers, specifically lipid-based nanocarriers (LBNs), in treating various brain disorders.

High curvature promotes fusion of lipid membranes: Predictions from continuum elastic theory

The fusion of lipid membranes progresses through a series of hemifusion intermediates with two significant energy barriers related to the formation of stalk and fusion pore, respectively. These energy barriers determine the speed and success rate of many critical biological processes, including the fusion of highly curved membranes, for example synaptic vesicles and enveloped viruses. Here we use continuum elastic theory of lipid monolayers to determine the relationship between membrane shape and energy barriers to fusion. We find that the stalk formation energy decreases with curvature by up to 31 kBT in a 20-nm-radius vesicle compared with planar membranes and by up to 8 kBT in the fusion of highly curved, long, tubular membranes. In contrast, the fusion pore formation energy barrier shows a more complicated behavior. Immediately after stalk expansion to the hemifusion diaphragm, the fusion pore formation energy barrier is low (15-25 kBT) due to lipid stretching in the distal monolayers and increased tension in highly curved vesicles. Therefore, the opening of the fusion pore is faster. However, these stresses relax over time due to lipid flip-flop from the proximal monolayer, resulting in a larger hemifusion diaphragm and a higher fusion pore formation energy barrier, up to 35 kBT. Therefore, if the fusion pore fails to open before significant lipid flip-flop takes place, the reaction proceeds to an extended hemifusion diaphragm state, which is a dead-end configuration in the fusion process and can be used to prevent viral infections. In contrast, in the fusion of long tubular compartments, the surface tension does not accumulate due to the formation of the diaphragm, and the energy barrier for pore expansion increases with curvature by up to 11 kBT. This suggests that inhibition of polymorphic virus infection could particularly target this feature of the second barrier.

Differential Behavior of Eicosapentaenoic and Docosahexaenoic Acids on the Organization, Dynamics, and Fusion of Homogeneous and Heterogeneous Membranes

Membrane fusion is a common course in innumerable biological processes that helps in the survival of eukaryotes. Enveloped viruses utilize this process to enter the host cells. Generally, the membrane lipid compositions play an important role in membrane fusion by modulating the membrane's physical properties and the behavior of membrane proteins in the cellular milieu. In this work, we have demonstrated the role of polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, on the organization, dynamics, and fusion of homogeneous and heterogeneous membranes. We have exploited arrays of steady-state and time-resolved fluorescence spectroscopic methods and polyethylene glycol-induced membrane fusion assay to elucidate the behavior of EPA and DHA on dioleoyl phosphatidylcholine (DOPC)/cholesterol (CH) homogeneous and DOPC/sphingomyelin/CH heterogeneous membranes. Our results suggest that EPA and DHA display differential effects on two different membranes. The effects of PUFAs in homogeneous membranes are majorly attributed to their flexible chain dynamics, whereas the ability of PUFA-induced cholesterol transfer from the l_o to the l_d phase rules their behavior in heterogeneous membranes. Overall, our results provide detailed information on the effect of PUFAs on homogeneous and heterogeneous membranes.

Membrane cholesterol regulates the oligomerization and fusogenicity of SARS-CoV fusion peptide: implications in viral entry

N-terminal residues (770-788) of the S2 glycoprotein of severe acute respiratory syndrome coronavirus (SARS-CoV) have been recognized as a potential fusion peptide that can be involved in the entry of the virus into the host cell. Membrane composition plays an important role in lipid-peptide interaction and the oligomeric status of the peptide. SARS-CoV fusion peptide (S2 fusion peptide) is known to undergo cholesterol-dependent oligomerization in the membrane; however, its significance in membrane fusion is still speculative. This study aimed to investigate the oligomerization of SARS-CoV fusion peptide in a membrane containing phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol, with varying concentrations of cholesterol, and to evaluate peptide-induced membrane fusion to correlate the importance of peptide oligomerization with membrane fusion. Peptide-induced modulation of membrane organization and dynamics was explored by steady-state and time-resolved fluorescence spectroscopic measurements using depth-dependent probes. The results clearly demonstrated the induction of S2 fusion peptide oligomerization by membrane cholesterol and the higher efficiency of the oligomer in promoting membrane fusion compared to its monomeric counterpart. Cholesterol-dependent peptide oligomerization and membrane fusion are important aspects of viral infection since the cholesterol level can change with age as well as with the onset of various pathophysiological conditions.

Effect of polyunsaturated free fatty acids on the membrane fusion mechanism

Membrane fusion is one of the important processes for the survival of eukaryotic cells and the entry of enveloped viruses into the host cells. Lipid composition plays a crucial role by modulating the organization and dynamics of the membrane, as well as the structure and conformation of membrane proteins. The diversity of the lipid acyl chain in its length and degree of unsaturation originates from the variation in free fatty acids (FFAs). We have studied the effect of linoleic (LA) and alpha-linolenic (ALA) acids on the depth-dependent organization, dynamics, and fusion of DOPC/DOPE (70/30 mol%) membranes utilizing steady-state and time-resolved fluorescence spectroscopic methods. Our results suggest that membranes with 5 mol% LA stabilize the stalk-intermediate and promote lipid mixing at the early stage of the process, i.e., the fusion follows the classical stalk model. Conversely, the extents of lipid and content mixing at the stalk intermediate are similar in the presence of 5 mol% of ALA, indicating the fusion mechanism as a nonclassical one like in the DOPC/DOPE (70/30 mol%) membranes. Our results provide an in-depth insight into the effect of the increasing degree of fatty acid tail unsaturation on membrane organization and dynamics and their impact on the membrane fusion mechanism.

Combination of Oleic Acid and the gp41 Fusion Peptide Switches the Phosphatidylethanolamine-Induced Membrane Fusion Mechanism from a Nonclassical to a Classical Stalk Model

Membrane fusion is considered to be one of the crucial processes for the existence of eukaryotes and the entry of enveloped viruses into host cells. The fusion mechanism depends on the lipid composition of the membrane as well as the properties of fusion proteins or peptides. The gp41 fusion peptide from the human immunodeficiency virus (HIV) is known to catalyze membrane fusion by altering the physical properties of the membrane. Earlier, we demonstrated that a membrane containing 30 mol % phosphatidylethanolamine (PE) circumvents the classical stalk model because of its intrinsic negative curvature. In this work, we demonstrated how the gp41 fusion peptide influences the fusion mechanism of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-sn-glycero-3-phos-pho¬ethanolamine (DOPE) (70/30 mol %) membranes. We further evaluated the effect of the same peptide on the mechanism of fusion for membranes containing 30 mol % PE and a fatty acid with an intrinsic positive curvature (oleic acid (OA)). Our results show that gp41 switches the fusion mechanism from a nonclassical to a classical stalk model when membranes contain OA, but fails to do so for DOPC/DOPE membranes. This could be due to the extreme influence of the intrinsic negative curvature of PE, which is partially downregulated in the presence of OA.

Mechanism of Membrane Fusion: Interplay of Lipid and Peptide

Membrane fusion is an essential process for the survival of eukaryotes and the entry of enveloped viruses into host cells. A proper understanding of the mechanism of membrane fusion would provide us a handle to manipulate several biological pathways, and design efficient vaccines against emerging and re-emerging viral infections. Although fusion proteins take the central stage in catalyzing the process, role of lipid composition is also of paramount importance. Lipid composition modulates membrane organization and dynamics and impacts the lipid–protein (peptide) interaction. Moreover, the intrinsic curvature of lipids has strong impact on the formation of stalk and hemifusion diaphragm. Detection of transiently stable intermediates remains the bottleneck in the understanding of fusion mechanism. In order to circumvent this challenge, analytical methods can be employed to determine the kinetic parameters from ensemble average measurements of observables, such as lipid mixing, content mixing, and content leakage. The current review aims to present an analytical method that would aid our understanding of the fusion mechanism, provides a better insight into the role of lipid shape, and discusses the interplay of lipid and peptide in membrane fusion.