The lipid composition of mitochondrial outer and inner membranes from the brains of goldfish acclimated at 5 and 30°C

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.

Stretching tension effects in permeability transition pores of inner mitochondrial membrane

Presently a mechanism of permeability transition pore (PTP) opening was proposed and discussed. This mechanism is based on mechanical stretching of inner mitochondrial membrane (IMM) caused by mitochondrial swelling (MS). The latter is induced by osmotic pressure generated by solute imbalance between the matrix and the surrounding cyto(sarco)plasm. Modelled by the Monte-Carlo method, an IMM fragment of 350 simulated biological molecules exhibited formation of micro-domains containing two protein and seven phospholipid molecules. The energies (-0.191 eV per molecule) in these micro-domains were significantly larger than those (-0.375 eV per molecule) of other parts of the IMM fragment. Stretching forces applied to such domains expanded them much more than other parts of the IMM fragment. We identify these micro-domains as the PTPs. Both linear and nonlinear functions were used for the strain-stress relation of the IMM fragment, with nonlinear effects more important at large IMM stretching strains. Thus, two main factors are incorporated into the PTP opening mechanism: (1) presence of micro-domains in the IMM structure and (2) IMM stretching stress caused by MS. Taking into account both of these factors, the equation for the probability of PTP opening was deduced, with matrix Ca²⁺ and H⁺ ionic concentrations as its parameters. Note that the equation deduced was similar to an earlier reported empirical equation describing PTP opening dynamics. This correspondence provides support to the presently proposed mechanism. Thus, a new look at the PTP opening mechanism is provided, of interest to various research areas related to mitochondrial biophysics.

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.

Model cell membranes: discerning lipid and protein contributions in shaping the cell

The high complexity of biological membranes has motivated the development and application of a wide range of model membrane systems to study biochemical and biophysical aspects of membranes in situ under well defined conditions. The aim is to provide fundamental understanding of processes controlled by membrane structure, permeability and curvature as well as membrane proteins by using a wide range of biochemical, biophysical and microscopic techniques. This review gives an overview of some currently used model biomembrane systems. We will also discuss some key membrane protein properties that are relevant for protein-membrane interactions in terms of protein structure and how it is affected by membrane composition, phase behavior and curvature.

Temperature-activity relationship for the intestinal Na+-K+-ATPase of Sparus aurata. A role for the phospholipid microenvironment?

The temperature dependence for Na(+)-K(+)-ATPase has been examined in the proximal-distal axis of the intestine of gilthead seabream (Sparus aurata), i.e. pyloric caeca (PC), anterior intestine (AI) and posterior intestine (PI). Data derived from the Arrhenius plots showed differences in terms of temperature discontinuity points ( Td) (13.29 degrees C, 16.39 degrees C and 17.48 degrees C for PC, AI and PI, respectively) and activation energy ratios (Ea(2)/Ea(1)) obtained at both sides of Td (2.38, 1.98 and 1.78, for PC, AI and PI, respectively). The analyses of polar lipids showed differences in the levels of certain fatty acids among intestinal regions. The content of each fatty acid and different fatty acid ratios were correlated with the corresponding Td and Ea(2)/Ea(1) values. Regression analyses revealed the existence of strong negative correlations between docosahexaenoic acid (22:6n-3, DHA) or the DHA/monoenes ratio and Td. No obvious relationships were observed for other polyunsaturated fatty acids (PUFA) nor saturated fatty acids. The results obtained in the present study indicate that the heterogeneous values of Td displayed by the Na(+)-K(+)-ATPase along the intestinal tract could be related to a modulatory role of certain fatty acid within the lipid microenvironment of the enzyme.

Morphological plasticity of synaptic mitochondria during aging

A morphometric investigation has been carried out on the synaptic mitochondria of cerebellar glomeruli in young, adult and old rats by means of a computer-assisted image analysis technique. Mitochondrial volume density (Vv), numerical density (Nv), average volume (V) and average length (Skeleton = Sk) were investigated in tissue samples fixed, embedded and sectioned according to conventional electron microscopic methods. Vv was unchanged in the three groups of age taken into account. Nv was significantly increased in adult vs. young animals, whereas it was decreased in the old group as compared to both the other two groups investigated. V and Sk showed the same age-dependent changes: they significantly decreased in the adult vs. the young and the old groups of rats while increased significantly in the old rats vs. both the adult and young animals. A percentage distribution of Sk demonstrated that in the old group 20.6% of the population of synaptic mitochondria accounts for elongated organelles (> 5 microns) as compared to 8.6% and 5.3% in young and adult animals, respectively. The present findings match the changes previously reported by us on the ultrastructure of synaptic contact zones both in rats and human beings, and support the idea of an age-dependent dynamic adaptation in the morphology of synaptic mitochondria to cope with the metabolic needs of the pattern of synaptic connectivity they subserve.