The structure of mitochondrial membranes: a new concept

Lipids, membranes, colloids and cells: A long view

This paper revisits long-standing ideas about biological membranes in the context of an equally long-standing, but hitherto largely unappreciated, perspective of the cell based on concepts derived from the physics and chemistry of colloids. Specifically, we discuss important biophysical aspects of lipid supramolecular structure to understand how the intracellular milieu may constrain lipid self-assembly. To this end we will develop four lines of thought: first, we will look at the historical development of the current view of cellular structure and physiology, considering also the plurality of approaches that influenced its formative period. Second, we will review recent basic research on the structural and dynamical properties of lipid aggregates as well as the role of phase transitions in biophysical chemistry and cell biology. Third, we will present a general overview of contemporary studies into cellular compartmentalization in the context of a very rich and mostly forgotten general theory of cell physiology called the Association-Induction Hypothesis, which was developed around the time that the current view of cells congealed into its present form. Fourth, we will examine some recent developments in cellular studies, mostly from our laboratory, that raise interesting issues about the dynamical aspects of cell structure and compartmentalization. We will conclude by suggesting what we consider are relevant questions about the nature of cellular processes as emergent phenomena.

The relevance of mitochondrial membrane topology to mitochondrial function

This review summarizes recent findings from electron tomography about the three-dimensional shape of mitochondrial membranes and its possible influence on a range of mitochondrial functions. The inner membrane invaginations called cristae are pleomorphic, typically connected by narrow tubular junctions of variable length to the inner boundary membrane. This design may restrict intra-mitochondrial diffusion of metabolites such as ADP, and of soluble proteins such as cytochrome c. Tomographic images of a variety of mitochondria suggest that inner membrane topology reflects a balance between membrane fusion and fission. Proteins that can affect cristae morphology include tBid, which triggers cytochrome c release in apoptosis, and the dynamin-like protein Mgm1, involved in inter-mitochondrial membrane fusion. In frozen-hydrated rat-liver mitochondria, the space between the inner and outer membranes contains 10-15 nm particles that may represent macromolecular complexes involved in activities that span the two membranes.

The structure of mitochondrial creatine kinase and its membrane binding properties

The biochemical and biophysical characterization of the mitochondrial creatine kinase (Mi-CK) from chicken cardiac muscle is reviewed with emphasis on the structure of the octameric oligomer by electron microscopy and on its membrane binding properties. Information about shape, molecular symmetry and dimensions of the Mi-CK octamer, as obtained by different sample preparation techniques in combination with image processing methods, are compared. The organization of the four dimeric subunits into the Mi-CK complex as apparent as apparent in the end-on projections is discussed and the consistently observed high binding affinity of the four-fold symmetric end-on faces towards many support films and towards each other is outlined. A study on the oligomeric state of the enzyme in solution and in intact mitochondria, using chemical crosslinking reagents, is presented together with the results of a search for a possible linkage of Mi-CK with the adenine nucleotide translocator (ANT). The nature of Mi-CK binding to model membranes, demonstrating that rather the octameric than the dimeric subspecies is involved in lipid interaction and membrane contact formation, is resumed and put into relation to our structural observations. The findings are discussed in light of a possible in vivo function of the Mi-CK octamer bridging the gap between outer and inner mitochondrial membranes at the contact sites.

Metabolic compartmentation and substrate channelling in muscle cells. Role of coupled creatine kinases in in vivo regulation of cellular respiration--a synthesis

The published experimental data and existing concepts of cellular regulation of respiration are analyzed. Conventional, simplified considerations of regulatory mechanism by cytoplasmic ADP according to Michaelis-Menten kinetics or by derived parameters such as phosphate potential etc. do not explain relationships between oxygen consumption, workload and metabolic state of the cell. On the other hand, there are abundant data in literature showing microheterogeneity of cytoplasmic space in muscle cells, in particular with respect to ATP (and ADP) due to the structural organization of cell interior, existence of multienzyme complexes and structured water phase. Also very recent experimental data show that the intracellular diffusion of ADP is retarded in cardiomyocytes because of very low permeability of the mitochondrial outer membrane for adenine nucleotides in vivo. Most probably, permeability of the outer mitochondrial membrane porin channels is controlled in the cells in vivo by some intracellular factors which may be connected to cytoskeleton and lost during mitochondrial isolation. All these numerous data show convincingly that cellular metabolism cannot be understood if cell interior is considered as homogenous solution, and it is necessary to use the theories of organized metabolic systems and substrate-product channelling in multienzyme systems to understand metabolic regulation of respiration. One of these systems is the creatine kinase system, which channels high energy phosphates from mitochondria to sites of energy utilization. It is proposed that in muscle cells feed-back signal between contraction and mitochondrial respiration may be conducted by metabolic wave (propagation of oscillations of local concentration of ADP and creatine) through cytoplasmic equilibrium creatine and adenylate kinases and is amplified by coupled creatine kinase reaction in mitochondria. Mitochondrial creatine kinase has experimentally been shown to be a powerful amplifier of regulatory action of weak ADP fluxes due to its coupling to adenine nucleotide translocase. This phenomenon is also carefully analyzed.

Morphometric, histochemical and autoradiographic studies on myocardial cells in experimental cardiac hypertrophy and ischemia

Experimental studies were conducted into 170 adult male Wistar rats for the purpose of analysis of cellular adaptation processes of the myocardium to acute ischemia. One group of the animals were exposed to physical endurance training, i.e. 180 h of swimming exercises, up to 3 h daily. Positive verification of cardiac hypertrophy was considered a measure of accomplished cellular adaptation. Training-induced increase of relative and absolute heart weight was 25 and 30%, respectively. Acute myocardial ischemia had been produced by ligature of the left coronary artery. There were no significant differences between trained and untrained animals for incidence and size of infarction and postoperative lethality, while cardiac decompensation was less often recorded from trained animals. To study cellular adaptation as well as differences between trained and untrained animals, tissue samples were taken from the non-ischemic part of the left ventricle and checked by means of histology, electron microscopy, morphometry, quantitative histochemistry, and histo-autoradiography 1, 2, 4, 7, and 14 days after occlusion of the coronary artery. The studies have shown endurance training to result in unambiguous modification of structural as well as functional response of the nonischemic heart. Included in such structural modification at cellular level are significant changes in mitochondrial membranes, sarcoplasmic reticulum, and T-system. Structural modification was reflected in changes to the oxidative enzymes and DNA metabolism. Different patterns of cellular reaction could be positively verified up to 14 days after myocardial infarction.

Heart mitochondrial creatine kinase revisited: the outer mitochondrial membrane is not important for coupling of phosphocreatine production to oxidative phosphorylation

The state of mitochondrial creatine kinase (CKmi-mi) in intact dog heart mitochondria and mitoplasts and the mechanism of its functional coupling with the oxidative phosphorylation system have been reinvestigated under different osmotic conditions and ionic compositions of the medium. It has been established that in a medium which mimics the cardiac cell cytoplasma, dissociation of CKmi-mi from the membrane of mitoplasts increases when the mitoplasts are swollen due to hypoosmotic treatment. It was shown by EPR that hypoosmotic treatment results in the enhancement of the mobility of phospholipids in the membrane bilayer. It has been also shown that when CKmi-mi is detached from the inner membrane in intact mitochondria in isotonic KCl solution, the effects of the coupling between CKmi-mi and oxidative phosphorylation via ATP/ADP translocase disappear in spite of the presence of CKmi-mi in the intermembrane space and intactness of the outer mitochondrial membrane. Therefore, this coupling cannot be explained by the "compartmented coupling" mechanism or "dynamic adenine nucleotide compartmentation" in the intermembrane space due to diffusion limitation for adenine nucleotides through the outer mitochondrial membrane, as has been supposed by several authors (F.N. Gellerich et al. (1987) Biochim. Biophys. Acta 890, 117-126; S.P.J. Brooks and C.H. Suelter (1987) Arch. Biochem. Biophys. 253, 122-132). The data obtained show that the displacement of the enzyme from the membrane results in significantly increased sensitivity of the coupled processes of aerobic phosphocreatine synthesis to inhibition by the product, phosphocreatine. Thus, all results show that under physiological osmotic and ionic conditions CKmi-mi remains firmly attached to the inner mitochondrial membrane and effectively coupled with ATP/ADP translocase due to intimate dynamic interaction between those proteins.

Dark-field electron microscopy of unstained biological materials embedded in Nanoplast

Extremely thin sections of unstained materials (beef liver catalase, double-stranded calf thymus DNA, horse spleen ferritin and mammalian skeletal muscle), embedded in the water-soluble melamine resin Nanoplast FB101, were studied by dark-field electron microscopy and electron spectroscopic imaging. While ferritin molecules so recorded show 0.4 and 0.9 nm lattice fringes within the crystalline iron core, double-stranded DNA shows a helical repeat with a spacing of 3.4 nm. The gain in resolution of structural detail reported here is probably due mainly to the reduced section thickness as compared to traditional thin-sectioning techniques. As we reported earlier (Frösch & Westphal, 1984), melamine resins can be sectioned extremely thinly (less than 10 nm) and observed without a supporting film, making them especially suitable for dark-field electron microscopy.

Freeze-fracture studies on mitochondrial membranes of spermatocytes

Separation of the two-folded lamina of the mitochondrial cristae occurs in mitochondria of spermatocytes and spermatids. Freeze-fracture exposes large areas of the inner and outer halves of the inner membrane. The surface of the outer half of the inner membrane is concave, with small numbers of intramembranous particles (IMPs). Its distinctive feature is the presence of protruding particles surrounding a pit. On the inner half of the inner membrane, there are large numbers of densely-packed, irregularly-distributed IMPs, among which regular pits are seen. Morphometric analysis and reconstructions suggest that these structures are "channels" in the mitochondrial membrane with an internal diameter of approximately 18 nm. It is uncertain whether such mitochondrial structures are confined to the spermatocyte or whether they may also occur in other cells.

Ion transport in rat liver mitochondria: the effect of the incubation medium osmolarity

A decrease in the incubation medium osmolarity from 320 to 120 mosM reverses the pH dependence of K+ efflux from rat liver mitochondria. The K+ efflux is no longer inhibited by oligomycin and a free radical scavenger butylhydroxytoluene. At 320 mosM, the addition of carbonyl cyanide 3-chlorophenylhydrazone (CCCP) accelerates the K+ efflux, while EGTA inhibits it. At 120 mosM these CCCP and EGTA effects are reversed. In either case the K+ efflux is inhibited by Mg2+. The decrease in osmolarity changes the ruthenium red-insensitive Ca2+ efflux in the same manner. It has thus been shown that the modification of the mitochondrial structure by changing the incubation medium osmolarity results in a qualitative alteration of the systems regulating the K+ and Ca2+ effluxes.

Water movement from intracristal spaces in isolated liver mitochondria

When analyzing mitochondria isolated in a sucrose medium that had been embedded for thin sectioning according to one low denaturation embedding technique, large intracristal spaces were present in close to 90% of the mitochondria. The two crista membranes were closely apposed in only 40% of all cristae. When the mitochondria were transferred to an incubation medium, the percentage of mitochondria with intracristal spaces was reduced to 40%. About 90% of all cristae were lacking any space separating the two crista membranes. The presence of inorganic phosphate in the medium was required for the closing of the intracristal spaces. The percentage of cristae lacking an intracristal space remained the same after addition of substrate for respiration (state 4) and of ADP (state 3). Inhibition or uncoupling of respiration led to an increase in the percentage of intracristal spaces, showing that oxidative phosphorylation is required to maintain the crista membranes closely apposed. The appearance and disappearance of the intracristal spaces was an indication of water movements across the crista membranes. The mean volume of the mitochondria increased 33% when they were transferred from the sucrose medium to the incubation medium, showing that the removal of water from the cristae was not caused by a passive osmotic effect. Addition of substrate made the volume decrease by 28%. After further addition of ADP, the volume decreased another 23%. No change in volume was associated with inhibition or uncoupling of respiration. The observations revealed that water can move into or out of the cristae independently of water movement out from the entire mitochondrion. Therefore, the water moving out from or into the cristae is translocated across the cristae membrane. The observations are interpreted to reveal the presence of a mechanism that actively prevents water from accumulating in the crista membrane. This mechanism allows for a low water activity to be maintained within the membrane. The variations in the frequency of intracristal spaces occurred without any simultaneous changes in the width of the space appearing between the two surface membranes after isolation of the mitochondria. The observations, therefore, do not agree with the concept that there is an outer compartment that communicates freely with intracristal spaces.

Permeability changes in isolated liver mitochondria during different metabolic states

The different staining patterns observed in isolated rat liver mitochondria suspended in a negative stain (2.11% ammonium molybdate) that were reported (U. Muscatello, V. Guarriera-Bobyleva, and P. Buffa, 1972, J. Ultrastruct. Res. 40, 235-260) were analyzed with the aim of determining the location of the stain within the mitochondria. The observations made by Muscatello and coworkers on whole, negatively stained mitochondria were confirmed. By embedding the negatively stained mitochondria according to one low denaturation embedding technique, the location of the stain could be determined. In rat liver mitochondria, freshly isolated, the stain was located within wide intracristal spaces. During state 4 respiration, stain was present in the cristae although the two membranes of each cristae were in most cases closely apposed. After 30 sec in state 3 respiration, the stain was present in the matrix, but absent from the cristae. During all three experimental conditions, stain was present in a narrow zone inside the membrane at the surface of the mitochondrion. The changes in permeability leading to an entrance of the stain into the matrix but not the cristae during state 3 respiration, and the reverse situation during state 4 shows that there are two pathways along which the stain can enter the matrix or the cristae. The permeability of these pathways is controlled independently. These observations cannot be explained on the basis of the generally accepted structure of mitochondria. The absence of any other structural modifications and a change in volume of the mitochondria in association with the transition from state 4 to state 3 respiration does not conform with the concept (C. R. Hackenbrock, 1966, J. Cell Biol. 30, 269-297) that the mitochondria undergo extensive "conformational" changes under these conditions.

The ultrastructure of the developing inner and outer segments of the photoreceptors of chick embryo retina as revealed by the rapid-freezing and deep-etching techniques

The ultrastructure of retina cell receptors of chick embryos and of one to three week old chicks was examined paying special attention to the membrane structure of receptor discs, mitochondria, cell membrane and other cell organelles, such as the endoplasmic reticulum and the Golgi apparatus. The retinas were rapid-frozen with a liquid-propane jet, deep-etched, and rotary-shadowed replicas produced. The structure of the photolamellae membranes is asymmetrical. The fracture faces showed a smooth (E-face) and a rough (P-face) surface. Both true surfaces ( interdiscal and intradiscal) were also observable by deep-etching. Transverse fractures of the discs showed the globular structure of their membrane. Spherical or polyhedral particles, probably rhodopsin-associated particles, occupying the width of the membrane are 12 nm in diameter and are constituted by 6 subunits of 1.5-2.0 nm arranged around a channel. These particles seem to extend into the membrane of the photolamellae during the last days of incubation and were also found in variable positions in the width of the disc membrane. When observed in transversal fractures of the photolamellae , they were sometimes seen to protrude into the collapsed intradiscal space and into the cytoplasmic surface. Filament-like or particulate structures connect the discs both to each other and to the cell membrane. During development a relationship between the forming discs and the cell membrane was not observed. The mitochondria aggregated in the ellipsoid are connected by filament-like structures that form during development of the inner segment. The structure of the inner cristae membrane of the mitochondria is characterized by the presence of stalked particles as previously described by Fern andez -Morán (1961) using negative staining. An intracristal space is not present. The fracture of the receptor cell membrane shows a particulate cytoplasmic half with particle-free patches. The glycogen granula situated in the cytoplasm between the smooth ER cisternae show a rosette-like composition.

Mitochondrial ultrastructural transformations in P. lividus eggs stimulated by ammonia

In mitochondria of P. lividus eggs the transition from the condensed to the orthodox form is accomplished in about 45 min after fertilization. Similar ultrastructural transformations are induced by exposure to ammonia that is known to cause a partial egg activation.

Pneumocystis carinii is an endogenous liposomally modified mitochondrion

Pneumocystis carinii was first described over 70 years ago but its taxonomy is still uncertain. Doubts have been expressed as to whether Pneumocystis carinii is an organism at all; but no coherent alternative has been proposed to account for the pathogenesis of pneumocystosis. Consideration of recent observations on Pneumocystis, liposomes, pulmonary surfactant and mitochondria leads to the hypothesis that Pneumocystis carinii is derived from liposomal change in the type II pneumocyte mitochondria that are known to be secreted into the alveoli under a variety of stressful stimuli. This hypothesis is considered to be more consistent with the facts than the hypothesis that Pneumocystis carinii is a protozoon or a fungus or any other kind of exogenous microorganism.

Orderly particle arrays on the mitochondrial outer membrane in rapidly-frozen sperm

By deep-etching and rotary replication of unfixed, non-cryoprotected tissue frozen on a helium-cooled copper block, previously undemonstrable organellar surfaces and intramembranous structures can be examined. Among the more remarkable features of mammalian spermatozoa thus prepared are the highly ordered particulate arrays on the surface of the mitochondrial outer membrane. In the midpiece of the sperm, mitochondria curl around dense fibers and the axoneme. The surface of the mitochondrion that faces the plasmalemma carries closely packed rods in haphazard dispersement, composed of two to four 70-to 80-A particles, less than 20 A apart, while the concave aspect of the organelle contains rods in stepladder pattern. These ladders are parallel, with their particles in neighboring rungs apparently in register at a 40--45 degree angle relative to the mitochondrial axis. This organizational disparity between the convex and concave surfaces of the organelle not only affords evidence of a new mitochondrial substructure, but represents a type of topographical heterogeneity rarely found except within specialized areas of the plasma membrane. Other novel findings in the sperm cell include the observation of "lipid" tracts flanking intramembranous particle-strands in the plasmalemma of the cytoplasmic droplet, and a gridiron design on the cytoplasmic faces of the droplet's microcisternae, as well as both within and atop its plasma membrane--a motif consistent with the presence of exocytotic or endocytotic activity in this portion of the cell. Additional recent observations are the differing internal and external periodicities of axonemal microtubules and the subunit structure of rectangles on the tail surface overlying the intramembranous particles of the zipper.