Lipid packing variations induced by pH in cardiolipin-containing bilayers: The driving force for the cristae-like shape instability

Setting the curve: the biophysical properties of lipids in mitochondrial form and function

Mitochondrial membranes are defined by their diverse functions, complex geometries, and unique lipidomes. In the inner mitochondrial membrane, highly curved membrane folds known as cristae house the electron transport chain and are the primary sites of cellular energy production. The outer mitochondrial membrane is flat by contrast, but is critical for the initiation and mediation of processes key to mitochondrial physiology: mitophagy, interorganelle contacts, fission and fusion dynamics, and metabolite transport. While the lipid composition of both the inner mitochondrial membrane and outer mitochondrial membrane have been characterized across a variety of cell types, a mechanistic understanding for how individual lipid classes contribute to mitochondrial structure and function remains nebulous. In this review, we address the biophysical properties of mitochondrial lipids and their related functional roles. We highlight the intrinsic curvature of the bulk mitochondrial phospholipid pool, with an emphasis on the nuances surrounding the mitochondrially-synthesized cardiolipin. We also outline emerging questions about other lipid classes - ether lipids, and sterols - with potential roles in mitochondrial physiology. We propose that further investigation is warranted to elucidate the specific properties of these lipids and their influence on mitochondrial architecture and function.

Synergistic effects of oxidative and acid stress on bacterial membranes of Escherichia coli and Staphylococcus simulans

Oxidative stress in combination with acid stress has been shown to inactivate a wide spectrum of microorganisms, including multi-resistant bacteria. This occurs e.g. in phagolysosomes or during treatment by cold atmospheric pressure plasmas (CAP) and possibly depends on the cell membrane. We therefore explored the effects of CAP-generated reactive oxygen and nitrogen species (RONS) on bacterial growth inhibition and membranes in neutral and acidic suspensions. We observed that growth inhibition was most efficient when bacteria were treated by a mix of short and long-lived RONS in an acidic environment. Membrane packing was affected mainly upon contact with short-lived RONS, while also acidity strongly modulated packing. Under these conditions, Gram-negative bacteria displayed large potassium release while SYTOX Green influx remained marginal. Growth inhibition of Gram-negative bacteria correlated well with outer membrane (OM) permeabilization that occurred upon contact with short and/or long-lived RONS in synergy with acidity. In Gram-positive bacteria, CAP impaired membrane potential possibly through pore formation upon contact with short-lived RONS while formation of membrane protein hydroperoxides was probably involved in these effects. In summary, our study provides a wide perspective on understanding inactivation mechanisms of bacteria by RONS in combination with acidity.

Chinese botanical drugs targeting mitophagy to alleviate diabetic kidney disease, a comprehensive review

Diabetic kidney disease (DKD) is one of the chronic microvascular complications caused by diabetes, which is characterized by persistent albuminuria and/or progressive decline of estimated glomerular filtration rate (eGFR), and has been the major cause of dialysis around the world. At present, although the treatments for DKD including lifestyle modification, glycemic control and even using of Sodium-glucose cotransporter 2 (SGLT2) inhibitors can relieve kidney damage caused to a certain extent, there is still a lack of effective treatment schemes that can prevent DKD progressing to ESRD. It is urgent to find new complementary and effective therapeutic agents. Growing animal researches have shown that mitophagy makes a great difference to the pathogenesis of DKD, therefore, exploration of new drugs that target the restoration of mitophagy maybe a potential perspective treatment for DKD. The use of Chinese botanical drugs (CBD) has been identified to be an effective treatment option for DKD. There is growing concern on the molecular mechanism of CBD for treatment of DKD by regulating mitophagy. In this review, we highlight the current findings regarding the function of mitophagy in the pathological damages and progression of DKD and summarize the contributions of CBD that ameliorate renal injuries in DKD by interfering with mitophagy, which will help us further explain the mechanism of CBD in treatment for DKD and explore potential therapeutic strategies for DKD.

Curvature sensing lipid dynamics in a mitochondrial inner membrane model

Membrane curvature is essential for many cellular structures and processes, and factors such as leaflet asymmetry, lipid composition, and proteins all play important roles. Cardiolipin is the signature lipid of mitochondrial membranes and is essential for maintaining the highly curved shapes of the inner mitochondrial membrane (IMM) and the spatial arrangement of membrane proteins. In this study, we investigate the partitioning behavior of various lipids present in the IMM using coarse-grained molecular dynamics simulations. This study explores curved bilayer systems containing phosphatidylcholine (PC), phosphatidylethanolamine (PE), and cardiolipin (CDL) in binary and ternary component mixtures. Curvature properties such as mean and Gaussian curvatures, as well as the distribution of lipids into the various curved regions of the cristae models, are quantified. Overall, this work represents an advance beyond previous studies on lipid curvature sensing by simulating these systems in a geometry that has the morphological features and scales of curvature consistent with regions of the IMM. We find that CDL has a stronger preference for accumulating in regions of negative curvature than PE lipids, in agreement with previous results. Furthermore, we find lipid partitioning propensity is dominated by sensitivity to mean curvature, while there is a weaker correlation with Gaussian curvature.

Structure and Elasticity of Mitochondrial Membranes: A Molecular Dynamics Simulation Study

Mitochondria are known as the powerhouse of the cell because they produce energy in the form of adenosine triphosphate. They also have other crucial functions such as regulating apoptosis, calcium homeostasis, and reactive oxygen species production. To perform these diverse functions, mitochondria adopt specific structures and frequently undergo dynamic shape changes, indicating that their mechanical properties play an essential role in their functions. To gain a detailed understanding at the molecular level of the structure and mechanical properties of mitochondria, we carry out atomistic molecular dynamics simulations for three inner mitochondrial membranes and three outer mitochondrial membrane models. These models take into account variations in cardiolipin and cholesterol concentrations as well as the symmetry/asymmetry between the two leaflets. Our simulations allow us to calculate various structural quantities and the bending, twisting, and tilting elastic moduli of the membrane models. Our results indicate that the structures of the inner and outer mitochondrial membranes are quite similar and do not depend much on the variation in lipid compositions. However, the bending modulus of the membranes increases with increasing concentrations of cardiolipin or cholesterol but decreases with a membrane asymmetry. Notably, we found that the dipole potential of the membrane increases with an increasing cardiolipin concentration. Finally, possible roles of cardiolipin in regulating ion and proton currents and maintaining the cristate are discussed in some details.

Cristae formation is a mechanical buckling event controlled by the inner mitochondrial membrane lipidome

Cristae are high-curvature structures in the inner mitochondrial membrane (IMM) that are crucial for ATP production. While cristae-shaping proteins have been defined, analogous lipid-based mechanisms have yet to be elucidated. Here, we combine experimental lipidome dissection with multi-scale modeling to investigate how lipid interactions dictate IMM morphology and ATP generation. When modulating phospholipid (PL) saturation in engineered yeast strains, we observed a surprisingly abrupt breakpoint in IMM topology driven by a continuous loss of ATP synthase organization at cristae ridges. We found that cardiolipin (CL) specifically buffers the inner mitochondrial membrane against curvature loss, an effect that is independent of ATP synthase dimerization. To explain this interaction, we developed a continuum model for cristae tubule formation that integrates both lipid and protein-mediated curvatures. This model highlighted a snapthrough instability, which drives IMM collapse upon small changes in membrane properties. We also showed that cardiolipin is essential in low-oxygen conditions that promote PL saturation. These results demonstrate that the mechanical function of cardiolipin is dependent on the surrounding lipid and protein components of the IMM.

Mitochondrial membrane model: Lipids, elastic properties, and the changing curvature of cardiolipin

Mammalian and Drosophila melanogaster model mitochondrial membrane compositions are constructed from experimental data. Simplified compositions for inner and outer mitochondrial membranes are provided, including an asymmetric inner mitochondrial membrane. We performed atomistic molecular dynamics simulations of these membranes and computed their material properties. When comparing these properties to those obtained by extrapolation from their constituting lipids, we find good overall agreement. Finally, we analyzed the curvature effect of cardiolipin, considering ion concentration effects, oxidation, and pH. We draw the conclusion that cardiolipin-negative curvature is most likely due to counterion effects, such as cation adsorption, in particular of H3O+. This oft-neglected effect might account for the puzzling behavior of this lipid.

Rotation of the c-Ring Promotes the Curvature Sorting of Monomeric ATP Synthases

ATP synthases are proteins that catalyse the formation of ATP through the rotatory movement of their membrane-spanning subunit. In mitochondria, ATP synthases are found to arrange as dimers at the high-curved edges of cristae. Here, a direct link is explored between the rotatory movement of ATP synthases and their preference for curved membranes. An active curvature sorting of ATP synthases in lipid nanotubes pulled from giant vesicles is found. Coarse-grained simulations confirm the curvature-seeking behaviour of rotating ATP synthases, promoting reversible and frequent protein-protein contacts. The formation of transient protein dimers relies on the membrane-mediated attractive interaction of the order of 1.5 kB T produced by a hydrophobic mismatch upon protein rotation. Transient dimers are sustained by a conic-like arrangement characterized by a wedge angle of θ ≈ 50°, producing a dynamic coupling between protein shape and membrane curvature. The results suggest a new role of the rotational movement of ATP synthases for their dynamic self-assembly in biological membranes.

Cristae formation is a mechanical buckling event controlled by the inner membrane lipidome

Cristae are high curvature structures in the inner mitochondrial membrane (IMM) that are crucial for ATP production. While cristae-shaping proteins have been defined, analogous mechanisms for lipids have yet to be elucidated. Here we combine experimental lipidome dissection with multi-scale modeling to investigate how lipid interactions dictate IMM morphology and ATP generation. When modulating phospholipid (PL) saturation in engineered yeast strains, we observed a surprisingly abrupt breakpoint in IMM topology driven by a continuous loss of ATP synthase organization at cristae ridges. We found that cardiolipin (CL) specifically buffers the IMM against curvature loss, an effect that is independent of ATP synthase dimerization. To explain this interaction, we developed a continuum model for cristae tubule formation that integrates both lipid and protein-mediated curvatures. The model highlighted a snapthrough instability, which drives IMM collapse upon small changes in membrane properties. We also showed that CL is essential in low oxygen conditions that promote PL saturation. These results demonstrate that the mechanical function of CL is dependent on the surrounding lipid and protein components of the IMM.

Bacterial lipid biophysics and membrane organization

The formation of lateral microdomains is emerging as a central organizing principle in bacterial membranes. These microdomains are targets of antibiotic development and have the potential to enhance natural product synthesis, but the rules governing their assembly are unclear. Previous studies have suggested that microdomain formation is promoted by lipid phase separation, particularly by cardiolipin (CL) and isoprenoid lipids, and there is strong evidence that CL biosynthesis is required for recruitment of membrane proteins to cell poles and division sites. New work demonstrates that additional bacterial lipids may mediate membrane protein localization and function, opening the field for mechanistic evaluation of lipid-driven membrane organization in vivo.

The Effect of pH on the Structure and Lateral Organization of Cardiolipin in Langmuir Monolayers

Cardiolipin (CL) is a unique phospholipid featuring a dimeric structure. With its four alkyl chains, it has a large hydrophobic region and the charged hydrophilic head group is relatively small. Biological membranes exhibit CL exclusively in the inner bacterial and mitochondrial membranes. Alteration of CL packing can lead to structural changes and membrane instabilities. One environmental influence is the change in pH. Since the acidic properties of the phosphate head groups remain still controversial in literature, this work focusses on the influence of pH on the ionization degree of CL. For the analyses, surface pressure (π) - molecular area (A) isotherm experiments were combined with total reflection X-ray fluorescence (TRXF) and grazing incidence X-ray diffraction (GIXD). Continuous ionization with a high CL packing density was observed in the monolayer over a wide pH range. No individual pKa values can be assigned to the two phosphate groups, but mutual influence is observed.

Curvature effects in charge-regulated lipid bilayers

We formulate a theory of electrostatic interactions in lipid bilayer membranes where both monolayer leaflets contain dissociable moieties that are subject to charge regulation. We specifically investigate the coupling between membrane curvature and charge regulation of a lipid bilayer vesicle using both the linear Debye-Hückel (DH) and the non-linear Poisson-Boltzmann (PB) theory. We find that charge regulation of an otherwise symmetric bilayer membrane can induce charge symmetry breaking, non-linear flexoelectricity and anomalous curvature dependence of free energy. The pH effects investigated go beyond the paradigm of electrostatic renormalization of the mechano-elastic properties of membranes.

Cardiolipin function in the yeast S. cerevisiae and the lessons learned for Barth syndrome

Cardiolipin (CL) is the signature phospholipid (PL) of mitochondria and plays a pivotal role in mitochondrial and cellular function. Disruption of the CL remodeling gene tafazzin (TAZ) causes the severe genetic disorder Barth syndrome (BTHS). Our current understanding of the function of CL and the mechanism underlying the disease has greatly benefited from studies utilizing the powerful yeast model Saccharomyces cerevisiae. In this review, we discuss important findings on the function of CL and its remodeling from yeast studies and the implications of these findings for BTHS, highlighting the potential physiological modifiers that may contribute to the disparities in clinical presentation among BTHS patients.

Bacteriomimetic Liposomes Improve Antibiotic Activity of a Novel Energy-Coupling Factor Transporter Inhibitor

Liposomes have been studied for decades as nanoparticulate drug delivery systems for cytostatics, and more recently, for antibiotics. Such nanoantibiotics show improved antibacterial efficacy compared to the free drug and can be effective despite bacterial recalcitrance. In this work, we present a loading method of bacteriomimetic liposomes for a novel, hydrophobic compound (HIPS5031) inhibiting energy-coupling factor transporters (ECF transporters), an underexplored antimicrobial target. The liposomes were composed of DOPG (18:1 (Δ9-cis) phosphatidylglycerol) and CL (cardiolipin), resembling the cell membrane of Gram-positive Staphylococcus aureus and Streptococcus pneumoniae, and enriched with cholesterol (Chol). The size and polydispersity of the DOPG/CL/± Chol liposomes remained stable over 8 weeks when stored at 4 °C. Loading of the ECF transporter inhibitor was achieved by thin film hydration and led to a high encapsulation efficiency of 33.19% ± 9.5% into the DOPG/CL/Chol liposomes compared to the phosphatidylcholine liposomes (DMPC/DPPC). Bacterial growth inhibition assays on the model organism Bacillus subtilis revealed liposomal HIPS5031 as superior to the free drug, showing a 3.5-fold reduction in CFU/mL at a concentration of 9.64 µM. Liposomal HIPS5031 was also shown to reduce B. subtilis biofilm. Our findings present an explorative basis for bacteriomimetic liposomes as a strategy against drug-resistant pathogens by surpassing the drug-formulation barriers of innovative, yet unfavorably hydrophobic, antibiotics.

Hypochlorous acid initiated lipid chlorination at air-water interface

There has been a surge of interest in interfacial hypochlorous acid (HOCl) chemistry for indoor air quality and public health. Here we combined nanoelectrospray mass spectrometry (nESI-MS) and acoustic levitation (AL) techniques to study the chlorination chemistry of three model lipids (DPPE, POPG, DOPG) mediated by HOCl at the air-water interface of levitated water droplet. For DPPE with no CC double bonds, HOCl was insensitive to the alkane chains, and showed considerable delay directing to head amino groups compared to that in aqueous environment. Chlorination chemistry, for POPG and DOPG with CC double bonds, preferentially reacted with double bonds of one chain. The mechanism was discussed in light of these observations, and it is concluded that the increased hydrophilicity of the chlorinated chain disturbed the lipid packing and attracted it toward the water phase. In addition, the reaction rate constant and reactive uptake coefficient suggested that the chlorination of lipids exposed to HOCl at the air-water interface is likely to occur rapidly. These results gain the knowledge of HOCl mediated lipid interface reaction at the molecule level, and would better understand the adverse health effects associated with elevated indoor pollutants.

Dielectric Properties of Phosphatidylcholine Membranes and the Effect of Sugars

Simple carbohydrates are associated with the enhanced risk of cardiovascular disease and adverse changes in lipoproteins in the organism. Conversely, sugars are known to exert a stabilizing effect on biological membranes, and this effect is widely exploited in medicine and industry for cryopreservation of tissues and materials. In view of elucidating molecular mechanisms involved in the interaction of mono- and disaccharides with biomimetic lipid systems, we study the alteration of dielectric properties, the degree of hydration, and the rotational order parameter and dipole potential of lipid bilayers in the presence of sugars. Frequency-dependent deformation of cell-size unilamellar lipid vesicles in alternating electric fields and fast Fourier transform electrochemical impedance spectroscopy are applied to measure the specific capacitance of phosphatidylcholine lipid bilayers in sucrose, glucose and fructose aqueous solutions. Alteration of membrane specific capacitance is reported in sucrose solutions, while preservation of membrane dielectric properties is established in the presence of glucose and fructose. We address the effect of sugars on the hydration and the rotational order parameter for 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphocholine (POPC) and 1-stearoyl-2-oleoyl-sn-glycero-3- phosphocholine (SOPC). An increased degree of lipid packing is reported in sucrose solutions. The obtained results provide evidence that some small carbohydrates are able to change membrane dielectric properties, structure, and order related to membrane homeostasis. The reported data are also relevant to future developments based on the response of lipid bilayers to external physical stimuli such as electric fields and temperature changes.

How rotating ATP synthases can modulate membrane structure

F1Fo-ATP synthase (ATP synthase) is a central membrane protein that synthetizes most of the ATP in the cell through a rotational movement driven by a proton gradient across the hosting membrane. In mitochondria, ATP synthases can form dimers through specific interactions between some subunits of the protein. The dimeric form of ATP synthase provides the protein with a spontaneous curvature that sustain their arrangement at the rim of the high-curvature edges of mitochondrial membrane (cristae). Also, a direct interaction with cardiolipin, a lipid present in the inner mitochondrial membrane, induces the dimerization of ATP synthase molecules along cristae. The deletion of those biochemical interactions abolishes the protein dimerization producing an altered mitochondrial function and morphology. Mechanically, membrane bending is one of the key deformation modes by which mitochondrial membranes can be shaped. In particular, bending rigidity and spontaneous curvature are important physical factors for membrane remodelling. Here, we discuss a complementary mechanism whereby the rotatory movement of the ATP synthase might modify the mechanical properties of lipid bilayers and contribute to the formation and regulation of the membrane invaginations.

Anastasis: Return Journey from Cell Death

For over 20 years, it has been a dogma that once the integrity of mitochondria is disrupted and proapoptotic proteins that are normally located in the intermembrane space of mitochondria appeared in the cytoplasm, the process of cell death becomes inevitable. However, it has been recently shown that upon removal of the death signal, even at the stage of disturbance in the mitochondria, cells can recover and continue to grow. This phenomenon was named anastasis. Here, we will critically discuss the present knowledge concerning the mechanisms of cell death reversal, or development of anastasis, methods for its detection, and what role signaling from different intracellular compartments plays in anastasis stimulation.

Mitochondrial Cristae Architecture and Functions: Lessons from Minimal Model Systems

Mitochondria are known as the powerhouse of eukaryotic cells. Energy production occurs in specific dynamic membrane invaginations in the inner mitochondrial membrane called cristae. Although the integrity of these structures is recognized as a key point for proper mitochondrial function, less is known about the mechanisms at the origin of their plasticity and organization, and how they can influence mitochondria function. Here, we review the studies which question the role of lipid membrane composition based mainly on minimal model systems.

Modeling the Effects of Calcium Overload on Mitochondrial Ultrastructural Remodeling

Mitochondrial cristae are dynamic invaginations of the inner membrane and play a key role in its metabolic capacity to produce ATP. Structural alterations caused by either genetic abnormalities or detrimental environmental factors impede mitochondrial metabolic fluxes and lead to a decrease in their ability to meet metabolic energy requirements. While some of the key proteins associated with mitochondrial cristae are known, very little is known about how the inner membrane dynamics are involved in energy metabolism. In this study, we present a computational strategy to understand how cristae are formed using a phase-based separation approach of both the inner membrane space and matrix space, which are explicitly modeled using the Cahn–Hilliard equation. We show that cristae are formed as a consequence of minimizing an energy function associated with phase interactions which are subject to geometric boundary constraints. We then extended the model to explore how the presence of calcium phosphate granules, entities that form in calcium overload conditions, exert a devastating inner membrane remodeling response that reduces the capacity for mitochondria to produce ATP. This modeling approach can be extended to include arbitrary geometrical constraints, the spatial heterogeneity of enzymes, and electrostatic effects to mechanize the impact of ultrastructural changes on energy metabolism.

Using reactive dissipative particle dynamics to understand local shape manipulation of polymer vesicles

Biological cells have long been of interest to researchers due to their capacity to actively control their shape. Accordingly, there is significant interest in generating simplified synthetic protocells that can alter their shape based on an externally or internally generated stimulus. To date, most progress has been made towards controlling the global shape of a protocell, whereas less is known about generating a local shape change. Here, we seek to better understand the possible mechanisms for producing local morphological changes in a popular protocell system, the block copolymer vesicle. Accordingly, we have combined Dissipative Particle Dynamics (DPD) and the Split Reactive Brownian Dynamics algorithm (SRBD) to produce a simulation tool that is capable of modeling the dynamics of self-assembled polymer structures as they undergo chemical reactions. Using this Reactive DPD or RDPD method, we investigate local morphological change driven by either the microinjection of a stimulus or an enzymatically-produced stimulus. We find that sub-vesicle-scale morphological change can be induced by either a solvent stimulus that swells the vesicle membrane, or by a reactant stimulus that alters the chemistry of the block polymer in the membrane corona. Notably, the latter method results in a more persistent local deformation than the former, which we attribute to the slower diffusion of polymer chains relative to the solvent. We quantify this deformation and show that it can be modulated by altering the interaction parameter of the parts of the polymer chain that are affected by the stimulus.

Inducible intracellular membranes: molecular aspects and emerging applications

Membrane remodeling and phospholipid biosynthesis are normally tightly regulated to maintain the shape and function of cells. Indeed, different physiological mechanisms ensure a precise coordination between de novo phospholipid biosynthesis and modulation of membrane morphology. Interestingly, the overproduction of certain membrane proteins hijack these regulation networks, leading to the formation of impressive intracellular membrane structures in both prokaryotic and eukaryotic cells. The proteins triggering an abnormal accumulation of membrane structures inside the cells (or membrane proliferation) share two major common features: (1) they promote the formation of highly curved membrane domains and (2) they lead to an enrichment in anionic, cone-shaped phospholipids (cardiolipin or phosphatidic acid) in the newly formed membranes. Taking into account the available examples of membrane proliferation upon protein overproduction, together with the latest biochemical, biophysical and structural data, we explore the relationship between protein synthesis and membrane biogenesis. We propose a mechanism for the formation of these non-physiological intracellular membranes that shares similarities with natural inner membrane structures found in α-proteobacteria, mitochondria and some viruses-infected cells, pointing towards a conserved feature through evolution. We hope that the information discussed in this review will give a better grasp of the biophysical mechanisms behind physiological and induced intracellular membrane proliferation, and inspire new applications, either for academia (high-yield membrane protein production and nanovesicle production) or industry (biofuel production and vaccine preparation).

The cardiolipin-binding peptide elamipretide mitigates fragmentation of cristae networks following cardiac ischemia reperfusion in rats

Mitochondrial dysfunction contributes to cardiac pathologies. Barriers to new therapies include an incomplete understanding of underlying molecular culprits and a lack of effective mitochondria-targeted medicines. Here, we test the hypothesis that the cardiolipin-binding peptide elamipretide, a clinical-stage compound under investigation for diseases of mitochondrial dysfunction, mitigates impairments in mitochondrial structure-function observed after rat cardiac ischemia-reperfusion. Respirometry with permeabilized ventricular fibers indicates that ischemia-reperfusion induced decrements in the activity of complexes I, II, and IV are alleviated with elamipretide. Serial block face scanning electron microscopy used to create 3D reconstructions of cristae ultrastructure reveals that disease-induced fragmentation of cristae networks are improved with elamipretide. Mass spectrometry shows elamipretide did not protect against the reduction of cardiolipin concentration after ischemia-reperfusion. Finally, elamipretide improves biophysical properties of biomimetic membranes by aggregating cardiolipin. The data suggest mitochondrial structure-function are interdependent and demonstrate elamipretide targets mitochondrial membranes to sustain cristae networks and improve bioenergetic function.

The mitochondria-targeted peptide SS-31 binds lipid bilayers and modulates surface electrostatics as a key component of its mechanism of action

Mitochondrial dysfunction underlies many heritable diseases, acquired pathologies, and aging-related declines in health. Szeto-Schiller (SS) peptides comprise a class of amphipathic tetrapeptides that are efficacious toward a wide array of mitochondrial disorders and are believed to target mitochondrial membranes because they are enriched in the anionic phospholipid cardiolipin (CL). However, little is known regarding how SS peptides interact with or alter the physical properties of lipid bilayers. In this study, using biophysical and computational approaches, we have analyzed the interactions of the lead compound SS-31 (elamipretide) with model and mitochondrial membranes. Our results show that this polybasic peptide partitions into the membrane interfacial region with an affinity and a lipid binding density that are directly related to surface charge. We found that SS-31 binding does not destabilize lamellar bilayers even at the highest binding concentrations; however, it did cause saturable alterations in lipid packing. Most notably, SS-31 modulated the surface electrostatics of both model and mitochondrial membranes. We propose nonexclusive mechanisms by which the tuning of surface charge could underpin the mitoprotective properties of SS-31, including alteration of the distribution of ions and basic proteins at the interface, and/or modulation of bilayer physical properties. As a proof of concept, we show that SS-31 alters divalent cation (calcium) distribution within the interfacial region and reduces the energetic burden of calcium stress in mitochondria. The mechanistic details of SS-31 revealed in this study will help inform the development of future compound variants with enhanced efficacy and bioavailability.

Cardiolipin Structure and Oxidation Are Affected by Ca2+ at the Interface of Lipid Bilayers

Ca²⁺-overload contributes to the oxidation of mitochondrial membrane lipids and associated events such as the permeability transition pore (MPTP) opening. Numerous experimental studies about the Ca²⁺/cardiolipin (CL) interaction are reported in the literature, but there are few studies in conjunction with theoretical approaches based on ab initio calculations. In the present study, the lipid fraction of the inner mitochondrial membrane was modeled as POPC/CL large unilamellar vesicles (LUVs). POPC/CL and, comparatively, POPC, and CL LUVs were challenged by singlet molecular oxygen using the anionic porphyrin TPPS4 as a photosensitizer and by free radicals produced by Fe²⁺-citrate. Calcium ion favored both types of lipid oxidation in a lipid composition-dependent manner. In membranes containing predominantly or exclusively POPC, Ca²⁺ increased the oxidation at later reaction times while the oxidation of CL membranes was exacerbated at the early times of reaction. Considering that Ca²⁺ interaction affects the lipid structure and packing, density functional theory (DFT) calculations were applied to the Ca²⁺ association with totally and partially protonated and deprotonated CL, in the presence of water. The interaction of totally and partially protonated CL head groups with Ca²⁺ decreased the intramolecular P-P distance and increased the hydrophobic volume of the acyl chains. Consistently with the theoretically predicted effect of Ca²⁺ on CL, in the absence of pro-oxidants, giant unilamellar vesicles (GUVs) challenged by Ca²⁺ formed buds and many internal vesicles. Therefore, Ca²⁺ induces changes in CL packing and increases the susceptibility of CL to the oxidation promoted by free radicals and excited species.

Interaction Mechanisms of Giant Unilamellar Vesicles with Hydrophobic Glass Surfaces and Silicone Oil-Water Interfaces: Adsorption, Deformation, Rupture, Dynamic Shape Changes, Internal Vesicle Formation, and Desorption

Phospholipid monolayers at oil-water interfaces are often obtained via vesicle adsorption. However, the interaction mechanisms of vesicles with these oil-water interfaces remain unclear. Herein, we studied the adsorption of giant unilamellar vesicles (GUVs) of approximately 2-5 μm diameter onto silicone oil-water interfaces and glass surfaces modified with hexamethyldisilazane (HMDS) and octadecyltrimethoxysilane (ODTMS) using fluorescence microscopy. The GUVs exhibited various modes of interaction, adsorbing on the silanized glass surfaces without sizable deformation, whereas GUVs bound to the silicone oil-water interface exhibited large deformation. After adsorption, GUV rupture occurred within 350, 110, and 3 ms on HMDS-modified glass, ODTMS-modified glass, and silicone oil-water interface, respectively. On glass surfaces, GUV rupture was often initiated and proceeded with pore formation near the surface. The monolayer patches formed by GUV rupture on HMDS-modified glass remained for at least 1 h over an area approximately twice of that estimated from the original GUV. On the ODTMS-modified glass and silicone oil surfaces, the monolayer patch structures disappeared in milliseconds owing to lipid diffusion across the interface. When adsorbed on the oil-water interface, the GUVs spontaneously underwent dynamic shape changes, internal vesicle formation, and desorption without rupture. Thus, it can be concluded that these different pathways arose from different lipid-surface affinities.

Cardiolipin-Dependent Properties of Model Mitochondrial Membranes from Molecular Simulations

Cardiolipin is an anionic lipid found in the mitochondrial membranes of eukaryotes ranging from unicellular microorganisms to metazoans. This unique lipid contributes to various mitochondrial functions, including metabolism, mitochondrial membrane fusion and/or fission dynamics, and apoptosis. However, differences in cardiolipin content between the two mitochondrial membranes, as well as dynamic fluctuations in cardiolipin content in response to stimuli and cellular signaling events, raise questions about how cardiolipin concentration affects mitochondrial membrane structure and dynamics. Although cardiolipin’s structural and dynamic roles have been extensively studied in binary mixtures with other phospholipids, the biophysical properties of cardiolipin in higher number lipid mixtures are still not well resolved. Here, we used molecular dynamics simulations to investigate the cardiolipin-dependent properties of ternary lipid bilayer systems that mimic the major components of mitochondrial membranes. We found that changes to cardiolipin concentration only resulted in minor changes to bilayer structural features but that the lipid diffusion was significantly affected by those alterations. We also found that cardiolipin position along the bilayer surfaces correlated to negative curvature deflections, consistent with the induction of negative curvature stress in the membrane monolayers. This work contributes to a foundational understanding of the role of cardiolipin in altering the properties in ternary lipid mixtures composed of the major mitochondrial phospholipids, providing much-needed insights to help understand how cardiolipin concentration modulates the biophysical properties of mitochondrial membranes.

Membrane curvature induces cardiolipin sorting

Cardiolipin is a cone-shaped lipid predominantly localized in curved membrane sites of bacteria and in the mitochondrial cristae. This specific localization has been argued to be geometry-driven, since the CL's conical shape relaxes curvature frustration. Although previous evidence suggests a coupling between CL concentration and membrane shape in vivo, no precise experimental data are available for curvature-based CL sorting in vitro. Here, we test this hypothesis in experiments that isolate the effects of membrane curvature in lipid-bilayer nanotubes. CL sorting is observed with increasing tube curvature, reaching a maximum at optimal CL concentrations, a fact compatible with self-associative clustering. Observations are compatible with a model of membrane elasticity including van der Waals entropy, from which a negative intrinsic curvature of -1.1 nm-1 is predicted for CL. The results contribute to understanding the physicochemical interplay between membrane curvature and composition, providing key insights into mitochondrial and bacterial membrane organization and dynamics.

Molecular Dynamics Analysis of Cardiolipin and Monolysocardiolipin on Bilayer Properties

The mitochondrial lipid cardiolipin (CL) contributes to the spatial protein organization and morphological character of the inner mitochondrial membrane. Monolysocardiolipin (MLCL), an intermediate species in the CL remodeling pathway, is enriched in the multisystem disease Barth syndrome. Despite the medical relevance of MLCL, a detailed molecular description that elucidates the structural and dynamic differences between CL and MLCL has not been conducted. To this end, we performed comparative atomistic molecular dynamics studies on bilayers consisting of pure CL or MLCL to elucidate similarities and differences in their molecular and bulk bilayer properties. We describe differential headgroup dynamics and hydrogen bonding patterns between the CL variants and show an increased cohesiveness of MLCL's solvent interfacial region, which may have implications for protein interactions. Finally, using the coarse-grained Martini model, we show that substitution of MLCL for CL in bilayers mimicking mitochondrial composition induces drastic differences in bilayer mechanical properties and curvature-dependent partitioning behavior. Together, the results of this work reveal differences between CL and MLCL at the molecular and mesoscopic levels that may underpin the pathomechanisms of defects in cardiolipin remodeling.

Intricate role of mitochondrial lipid in mitophagy and mitochondrial apoptosis: its implication in cancer therapeutics

The efficacy of chemotherapy is mostly restricted by the drug resistance developed during the course of cancer treatment. Mitophagy, as a pro-survival mechanism, crucially maintains mitochondrial homeostasis and it is one of the mechanisms that cancer cells adopt for their progression. On the other hand, mitochondrial apoptosis, a precisely regulated form of cell death, acts as a tumor-suppressive mechanism by targeting cancer cells. Mitochondrial lipids, such as cardiolipin, ceramide, and sphingosine-1-phosphate, act as a mitophageal signal for the clearance of damaged mitochondria by interacting with mitophagic machinery as well as activate mitochondrial apoptosis via the release of cytochrome c into the cytoplasm. In the recent time, the lipid-mediated lethal mitophagy has also been used as an alternative approach to abolish the survival role of lipid in cancer. Therefore, by targeting mitochondrial lipids in cancer cells, the detailed mechanism linked to drug resistance can be unraveled. In this review, we precisely discuss the current knowledge about the multifaceted role of mitochondrial lipid in regulating mitophagy and mitochondrial apoptosis and its application in effective cancer therapy.

Mitochondrial Heterogeneity

Cell-to-cell heterogeneity drives a range of (patho)physiologically important phenomena, such as cell fate and chemotherapeutic resistance. The role of metabolism, and particularly of mitochondria, is increasingly being recognized as an important explanatory factor in cell-to-cell heterogeneity. Most eukaryotic cells possess a population of mitochondria, in the sense that mitochondrial DNA (mtDNA) is held in multiple copies per cell, where the sequence of each molecule can vary. Hence, intra-cellular mitochondrial heterogeneity is possible, which can induce inter-cellular mitochondrial heterogeneity, and may drive aspects of cellular noise. In this review, we discuss sources of mitochondrial heterogeneity (variations between mitochondria in the same cell, and mitochondrial variations between supposedly identical cells) from both genetic and non-genetic perspectives, and mitochondrial genotype-phenotype links. We discuss the apparent homeostasis of mtDNA copy number, the observation of pervasive intra-cellular mtDNA mutation (which is termed "microheteroplasmy"), and developments in the understanding of inter-cellular mtDNA mutation ("macroheteroplasmy"). We point to the relationship between mitochondrial supercomplexes, cristal structure, pH, and cardiolipin as a potential amplifier of the mitochondrial genotype-phenotype link. We also discuss mitochondrial membrane potential and networks as sources of mitochondrial heterogeneity, and their influence upon the mitochondrial genome. Finally, we revisit the idea of mitochondrial complementation as a means of dampening mitochondrial genotype-phenotype links in light of recent experimental developments. The diverse sources of mitochondrial heterogeneity, as well as their increasingly recognized role in contributing to cellular heterogeneity, highlights the need for future single-cell mitochondrial measurements in the context of cellular noise studies.

Mitochondrial Membrane Dynamics-Functional Positioning of OPA1

The maintenance of mitochondrial energetics requires the proper regulation of mitochondrial morphology, and vice versa. Mitochondrial dynamins control mitochondrial morphology by mediating fission and fusion. One of them, optic atrophy 1 (OPA1), is the mitochondrial inner membrane remodeling protein. OPA1 has a dual role in maintaining mitochondrial morphology and energetics through mediating inner membrane fusion and maintaining the cristae structure. OPA1 is expressed in multiple variant forms through alternative splicing and post-translational proteolytic cleavage, but the functional differences between these variants have not been completely understood. Recent studies generated new information regarding the role of OPA1 cleavage. In this review, we will first provide a brief overview of mitochondrial membrane dynamics by describing fission and fusion that are mediated by mitochondrial dynamins. The second part describes OPA1-mediated fusion and energetic maintenance, the role of OPA1 cleavage, and a new development in OPA1 function, in which we will provide new insight for what OPA1 does and what proteolytic cleavage of OPA1 is for.

Atomic Force Microscopy Study on the Stiffness of Nanosized Liposomes Containing Charged Lipids

It has recently been recognized that the mechanical properties of lipid nanoparticles play an important role during in vitro and in vivo behaviors such as cellular uptake, blood circulation, and biodistribution. However, there have been no quantitative investigations of the effect of commonly used charged lipids on the stiffness of nanosized liposomes. In this study, by means of atomic force microscopy (AFM), we quantified the stiffness of nanosized liposomes composed of neutrally charged lipids combined with positively or negatively charged lipids while simultaneously imaging the liposomes in aqueous medium. Our results showed that charged lipids, whether negatively or positively charged, have the effect of reducing the stiffness of nanosized liposomes, independently of the saturation degree of the lipid acyl chains; the measured stiffness values of liposomes containing charged lipids are 30-60% lower than those of their neutral counterpart liposomes. In addition, we demonstrated that the Laurdan generalized polarization values, which are related to the hydration degree of the liposomal membrane interface and often used as a qualitative indicator of liposomal membrane stiffness, do not directly correlate with the physical stiffness values of the liposomes prepared in this study. However, our results indicate that direct quantitative AFM measurement is a valuable method to gain molecular-scale information about how the hydration degree of liposomal interfaces reflects (or does not reflect) liposome stiffness as a macroscopic property. Our AFM method will contribute to the quantitative characterization of the nano-bio interaction of nanoparticles and to the optimization of the lipid composition of liposomes for clinical use.

pH sensing by lipids in membranes: The fundamentals of pH-driven migration, polarization and deformations of lipid bilayer assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Buckling Under Pressure: Curvature-Based Lipid Segregation and Stability Modulation in Cardiolipin-Containing Bilayers

Mitochondrial metabolic function is affected by the morphology and protein organization of the mitochondrial inner membrane. Cardiolipin (CL) is a unique tetra-acyl lipid that is involved in the maintenance of the highly curved shape of the mitochondrial inner membrane as well as spatial organization of the proteins necessary for respiration and oxidative phosphorylation. Cardiolipin has been suggested to self-organize into lipid domains due to its inverted conical molecular geometry, though the driving forces for this organization are not fully understood. In this work, we use coarse-grained molecular dynamics simulations to study the mechanical properties and lipid dynamics in heterogeneous bilayers both with and without CL, as a function of membrane curvature. We find that incorporation of CL increases bilayer deformability and that CL becomes highly enriched in regions of high negative curvature. We further show that another mitochondrial inverted conical lipid, phosphatidylethanolamine (PE), does not partition or increase the deformability of the membrane in a significant manner. Therefore, CL appears to possess some unique characteristics that cannot be inferred simply from molecular geometry considerations.

Applications of Brewster angle microscopy from biological materials to biological systems

Brewster angle microscopy (BAM) is a powerful technique that allows for real-time visualization of Langmuir monolayers. The lateral organization of these films can be investigated, including phase separation and the formation of domains, which may be of different sizes and shapes depending on the properties of the monolayer. Different molecules or small changes within a molecule such as the molecule's length or presence of a double bond can alter the monolayer's lateral organization that is usually undetected using surface pressure-area isotherms. The effect of such changes can be clearly observed using BAM in real-time, under full hydration, which is an experimental advantage in many cases. While previous BAM reviews focused more on selected compounds or compared the impact of structural variations on the lateral domain formation, this review provided a broader overview of BAM application using biological materials and systems including the visualization of amphiphilic molecules, proteins, drugs, extracts, DNA, and nanoparticles at the air-water interface.

Mitochondrial form, function and signalling in aging

Aging is often accompanied by a decline in mitochondrial mass and function in different tissues. Additionally, cell resistance to stress is frequently found to be prevented by higher mitochondrial respiratory capacity. These correlations strongly suggest mitochondria are key players in aging and senescence, acting by regulating energy homeostasis, redox balance and signalling pathways central in these processes. However, mitochondria display a wide array of functions and signalling properties, and the roles of these different characteristics are still widely unexplored. Furthermore, differences in mitochondrial properties and responses between tissues and cell types, and how these affect whole body metabolism are also still poorly understood. This review uncovers aspects of mitochondrial biology that have an impact upon aging in model organisms and selected mammalian cells and tissues.

Cardiolipin and mitochondrial cristae organization

A fundamental question in cell biology, under investigation for over six decades, is the structural organization of mitochondrial cristae. Long known to harbor electron transport chain proteins, crista membrane integrity is key to establishment of the proton gradient that drives oxidative phosphorylation. Visualization of cristae morphology by electron microscopy/tomography has provided evidence that cristae are tube-like extensions of the mitochondrial inner membrane (IM) that project into the matrix space. Reconciling ultrastructural data with the lipid composition of the IM provides support for a continuously curved cylindrical bilayer capped by a dome-shaped tip. Strain imposed by the degree of curvature is relieved by an asymmetric distribution of phospholipids in monolayer leaflets that comprise cristae membranes. The signature mitochondrial lipid, cardiolipin (~18% of IM phospholipid mass), and phosphatidylethanolamine (34%) segregate to the negatively curved monolayer leaflet facing the crista lumen while the opposing, positively curved, matrix-facing monolayer leaflet contains predominantly phosphatidylcholine. Associated with cristae are numerous proteins that function in distinctive ways to establish and/or maintain their lipid repertoire and structural integrity. By combining unique lipid components with a set of protein modulators, crista membranes adopt and maintain their characteristic morphological and functional properties. Once established, cristae ultrastructure has a direct impact on oxidative phosphorylation, apoptosis, fusion/fission as well as diseases of compromised energy metabolism.

Cardiolipin plays an essential role in the formation of intracellular membranes in Escherichia coli

Mitochondria, chloroplasts and photosynthetic bacteria are characterized by the presence of complex and intricate membrane systems. In contrast, non-photosynthetic bacteria lack membrane structures within their cytoplasm. However, large scale over-production of some membrane proteins, such as the fumarate reductase, the mannitol permease MtlA, the glycerol acyl transferase PlsB, the chemotaxis receptor Tsr or the ATP synthase subunit b, can induce the proliferation of intra cellular membranes (ICMs) in the cytoplasm of Escherichia coli. These ICMs are particularly rich in cardiolipin (CL). Here, we have studied the effect of CL in the generation of these membranous structures. We have deleted the three genes (clsA, clsB and clsC) responsible of CL biosynthesis in E. coli and analysed the effect of these mutations by fluorescent and electron microscopy and by lipid mass spectrometry. We have found that CL is essential in the formation of non-lamellar structures in the cytoplasm of E. coli cells. These results could help to understand the structuration of membranes in E. coli and other membrane organelles, such as mitochondria and ER.

Human Atg8-cardiolipin interactions in mitophagy: Specific properties of LC3B, GABARAPL2 and GABARAP

The phospholipid cardiolipin (CL) has been proposed to play a role in selective mitochondrial autophagy, or mitophagy. CL externalization to the outer mitochondrial membrane would act as a signal for the human Atg8 ortholog subfamily, MAP1LC3 (LC3). The latter would mediate both mitochondrial recognition and autophagosome formation, ultimately leading to removal of damaged mitochondria. We have applied quantitative biophysical techniques to the study of CL interaction with various Atg8 human orthologs, namely LC3B, GABARAPL2 and GABARAP. We have found that LC3B interacts preferentially with CL over other di-anionic lipids, that CL-LC3B binding occurs with positive cooperativity, and that the CL-LC3B interaction relies only partially on electrostatic forces. CL-induced increased membrane fluidity appears also as an important factor helping LC3B to bind CL. The LC3B C terminus remains exposed to the hydrophilic environment after protein binding to CL-enriched membranes. In intact U87MG human glioblastoma cells rotenone-induced autophagy leads to LC3B translocation to mitochondria and subsequent delivery of mitochondria to lysosomes. We have also observed that GABARAP, but not GABARAPL2, interacts with CL in vitro. However neither GABARAP nor GABARAPL2 were translocated to mitochondria in rotenone-treated U87MG cells. Thus the various human Atg8 orthologs might play specific roles in different autophagic processes.

Novel benzanthrone probes for membrane and protein studies

The applicability of a series of novel benzanthrone dyes to monitoring the changes in physicochemical properties of lipid bilayer and to differentiating between the native and aggregated protein states has been evaluated. Based on the quantitative parameters of the dye-membrane and dye-protein binding derived from the fluorimetric titration data, the most prospective membrane probes and amyloid tracers have been selected from the group of examined compounds. Analysis of the red edge excitation shifts of the membrane- and amyloid-bound dyes provided information on the properties of benzanthrone binding sites within the lipid and protein matrixes. To understand how amyloid specificity of benzanthrones correlates with their structure, quantitative structure activity relationship (QSAR) analysis was performed involving a range of quantum chemical molecular descriptors. A statistically significant model was obtained for predicting the sensitivity of novel benzanthrone dyes to amyloid fibrils.

Regulation of autophagy by mitochondrial phospholipids in health and diseases

Autophagy is an evolutionarily conserved mechanism that maintains nutrient homeostasis by degrading protein aggregates and damaged organelles. Autophagy is reduced in aging, which is implicated in the pathogenesis of aging-related diseases, including cancers, obesity, type 2 diabetes, cardiovascular diseases, and neurodegenerative diseases. Mitochondria-derived phospholipids cardiolipin, phosphatidylethanolamine, and phosphatidylglycerol are critical throughout the autophagic process, from initiation and phagophore formation to elongation and fusion with endolysosomal vesicles. Cardiolipin is also required for mitochondrial fusion and fission, an important step in isolating dysfunctional mitochondria for mitophagy. Furthermore, genetic screen in yeast has identified a surprising role for cardiolipin in regulating lysosomal function. Phosphatidylethanolamine plays a pivotal role in supporting the autophagic process, including autophagosome elongation as part of lipidated Atg8/LC3. An emerging role for phosphatidylglycerol in AMPK and mTORC1 signaling as well as mitochondrial fission may provide the first glimpse into the function of phosphatidylglycerol apart from being a precursor for cardiolipin. This review examines the effects of manipulating phospholipids on autophagy and mitophagy in health and diseases, as well as current limitations in the field. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Phosphatidylglycerol-derived phospholipids have a universal, domain-crossing role in stress responses

Phosphatidylglycerol and phospholipids derived from it are widely distributed throughout the three domains of life. Cardiolipin is the best characterized of these phospholipids, and plays a key role in the response to environmental variations. Phosphatidylglycerol-derived phospholipids confer cell membranes with a wide range of responses, including changes in surface charge, fluidity, flexibility, morphology, biosynthesis and remodeling, that adapt the cell to these situations. Furthermore, the synthesis and remodeling of these phospholipids is finely regulated, highlighting the importance of these lipids in cell homeostasis and responses during stressful situations. In this article, we review the most important roles of these anionic phospholipids across domains, focusing on the biophysical basis by which these phospholipids are used in stress responses.