Permeability characteristics of complement-damaged membranes: evaluation of the membrane leak generated by the complement proteins C5b-9

Complement proteins C5b-9 induce transbilayer migration of membrane phospholipids

Transbilayer migration of membrane phospholipid arising from membrane insertion of the terminal human complement proteins has been investigated. Asymmetric vesicles containing pyrene-labeled phosphatidylcholine (pyrenePC) concentrated in the inner monolayer were prepared by outer monolayer exchange between pyrenePC-containing large unilamellar vesicles and excess (unlabeled) small unilamellar vesicles, using bovine liver phosphatidylcholine-specific exchange protein. After depletion of pyrenePC from the outer monolayer, the asymmetric large unilamellar vesicles were isolated by gel filtration and exposed to the purified C5b-9 proteins at 37 degrees C. Transbilayer exchange of phospholipid between inner and outer monolayers during C5b-9 assembly was monitored by changes in pyrene excimer and monomer fluorescence. Membrane deposition of the C5b67 complex (by incubation with C5b6 + C7) caused no change in pyrenePC fluorescence. Addition of C8 to the C5b67 vesicles resulted in a dose-dependent decrease in the excimer/monomer ratio. This change was observed both in the presence and absence of complement C9. No change in fluorescence was observed for control vesicles exposed to C8 (in the absence of membrane C5b67), or upon C5b-9 addition to vesicles containing pyrenePC symmetrically distributed between inner and outer monolayers. These data suggest that a transbilayer exchange of phospholipid between inner and outer monolayers is initiated upon C8 binding to C5b67. The fluorescence data were analyzed according to a "random walk" model for excimer formation developed for the case where pyrenePC is asymmetrically distributed between lipid bilayers. Based on this analysis, we estimate that a net transbilayer migration of approximately 1% of total membrane phospholipid is initiated upon C8 binding to C5b67. The potential significance of this transbilayer exchange of membrane phospholipid to the biological activity of the terminal complement proteins is considered.

Single-channel analysis of the conductance fluctuations induced in lipid bilayer membranes by complement proteins C5b-9

Single-channel analysis of electrical fluctuations induced in planar bilayer membranes by the purified human complement proteins C5b6, C7, C8, and C9 have been analyzed. Reconstitution experiments with lipid bilayer membranes showed that the C5b-9 proteins formed pores only if all proteins were present at one side of the membrane. The complement pores had an average single-channel conductance of 3.1 nS at 0.15 M KCl. The histogram of the complement pores suggested a substantial variation of the size of the single channel. The linear relationship between single-channel conductance at fixed ionic strength and the aqueous mobility of the ions in the bulk aqueous phase indicated that the ions move inside the complement pore in a manner similar to the way they move in the aqueous phase. The minimum diameter of the pores as judged from the conductance data is approximately 3 nm. The complement channels showed no apparent voltage control or regulation up to transmembrane potentials of 100 mV. At neutral pH the pore is three to four times more permeable for alkali ions than for chloride, which may be explained by the existence of fixed negatively charged groups in or near the pore. The significance of these observations to current molecular models of the membrane lesion formed by these cytolytic serum proteins is considered.

Cyanine dye fluorescence used to measure membrane potential changes due to the assembly of complement proteins C5b-9

The fluorescent potentiometric indicator diS-C3-(5) has been used to investigate changes in membrane potential due to assembly of the C5b-9 membrane attack complex of the complement system. EAC1-7 human red blood cells and resealed erythrocyte ghosts--bearing membrane-assembled C5b67 complexes--were generated by immune activation in C8-deficient human serum. Studies performed with these cellular intermediates revealed that the membrane potential of EAC1-7 red cells and ghosts is unchanged from control red cells (-7 mV) and ghosts (O mV), respectively. Addition of complement proteins C8 and C9 to EAC1-7 red cells results in a dose-dependent depolarization of membrane potential which precedes hemolysis. This prelytic depolarization of membrane potential--and the consequent onset of hemolysis--is accelerated by raising external [K+], suggesting that the diffusional equilibration of transmembrane cation gradients is rate limiting to the cytolytic event. In the case of EAC1-7 resealed ghosts suspended at either high external [K+] or [Na+], no change in membrane potential (from O mV) could be detected after C8/C9 additions. When the membrane potential of the EAC1-7 ghost was displaced from O mV by selectively increasing the K+ conductance with valinomycin, a dose-dependent depolarization of the membrane was observed upon addition of C8 and C9. In these experiments, lytic breakdown of the ghost membranes was less than 5%. Conclusions derived from this study include: (i) measured prelytic depolarization of the red cell Donnan potential directly confirms the colloid-osmotic theory of immune cytolysis. (ii) The diffusional transmembrane equilibration of Na+ and K+ through the C5b-9 pore results in a dose-dependent depolarization of the membrane potential (Em) which appears to be rate-limiting to cytolytic rupture of the target erythrocyte. (iii) Enhanced immune hemolysis observed in high K+ media cannot be attributed to cation-selective conductance across the C5b-9 pore, and is probably related to the near-equilibrium condition of potassium-containing red cells when suspended at high external K+. These experiments demonstrate that carbocyanine dye fluorescent indicators can be used to monitor electrochemical changes arising from immune damage to the plasma membrane under both cytolytic and noncytolytic conditions. Potential application of this method to the detection of sublytic pathophysiological changes in the plasma membrane of complement-damaged cells are discussed.

Proteolytic modification of human complement protein C9: loss of poly(C9) and circular lesion formation without impairment of function

We have compared the ability of thrombin-cleaved C9 (C9n) with that of native C9 to produce tubular or ring-like poly(C9) and to express the classical complement lesion on target membranes. Three procedures were used to produce poly(C9): (i) limited proteolysis with trypsin, (ii) interaction with small unilamellar lipid vesicles, and (iii) incubation with a 2- to 4-fold molar excess of ZnCl2. In contrast to C9, which could be converted to tubular poly(C9), C9n was converted to smaller peptides by the first procedure and was aggregated into string-like poly(C9) by the other two methods. C9-depleted human serum (R-9 serum) was reconstituted with either C9 or C9n and these sera were then used to lyse sensitized sheep erythrocytes. Numerous classical complement lesions could be detected on ghost membranes obtained from cells lysed by C9-reconstituted R-9 serum but only a few on ghost membranes produced by C9n-reconstituted R-9 serum. C9n was shown to be hemolytically as active as C9 even when tested under "single-hit" conditions and it was about twice as efficient when compared with C9 in releasing sucrose and inulin from resealed ghosts. These results are interpreted to indicate that formation of the classical complement lesion is only incidental to lysis and not an obligatory event and that enlargement of the "functional pore size" of the complement lesion is not linked to formation of a circular membrane attack complex.

The influence of electrochemical gradients of Na+ and K+ upon the membrane binding and pore forming activity of the terminal complement proteins

The hemolytic activity of the terminal complement proteins (C5b-9) towards erythrocytes containing high potassium concentration has been reported to be dramatically increased when extracellular Na+ is substituted isotonically by K+ (Dalmasso, A.P., et al., 1975, J. Immunol. 115:63-68). This phenomenon was now further investigated using resealed human erythrocyte ghosts (ghosts), which can be maintained at a nonlytic osmotic steady state subsequent to C5b-9 binding: (1) The functional state of C5b-9-treated ghosts was studied from their ability to retain trapped [14C]-sucrose or [3H]-inulin when suspended either in the presence of Na+ or K+. A dramatic increase in the permeability of the ghost membrane to both nonelectrolytes - in the absence of significant hemoglobin release - was observed for C5b-9 assembly in the presence of external K+. (2) The physical binding of the individual 125I-labeled terminal complement proteins to ghost membranes was directly measured as a function of intra- and extracellular K+ and Na+. The uptake of 125I-C7, 125I-C8, and 125I-C9 into membrane C5b-9 was unaltered by substitution of Na+ by K+. (3) The binding of the terminal complement proteins to ghosts subjected to a transient membrane potential generated by the K+-ionophore valinomycin (in the presence of K+ concentration gradients) was measured. No significant change in membrane binding of any of the C5b-9 proteins was detected under the influence of both depolarizing and hyperpolarizing membrane potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane changes induced by exposure of Escherichia coli to human serum

The effect of bactericidal concentrations of lysozyme-free human serum on parameters of membrane integrity has been studied in serum-susceptible and serum-resistant Escherichia coli strains. Serum treatment released all of the alkaline phosphatase from the periplasmic space of two rapidly serum-susceptible strains but did so at different rates. In contrast, no periplasmic enzyme was released from two serum-resistant strains or from one moderately susceptible smooth strain. Lysozyme-free serum and heat-inactivated serum released comparable amounts of 86Rb+ from preloaded cells at comparable rates, regardless of serum susceptibility. Serum decreased the rate of phospholipid biosynthesis in both serum-susceptible and serum-resistant strains. In susceptible but not in resistant strains, intracellular ATP pools were depleted after serum exposure. Outer membranes and cytoplasmic membranes were prepared from serum-treated E. coli, and assays for C3 and C5b-9(m) were performed. With rapidly susceptible strains, C3 deposition on the outer membrane without attachment of C5b-9(m) occurred during the short prekilling phase. Subsequent bacterial killing was accompanied by deposition of C5b-9(m), which was recovered with C3 exclusively in outer membrane fractions with increased density and by eventual total loss of recoverable cytoplasmic membranes. Minimal deposition of complement components, without accompanying cytoplasmic membrane loss, occurred with serum-resistant strains. Loss of recoverable cytoplasmic membrane was not due to the action of either serum or bacterial phospholipase A. The results raise the possibilities that C5b-9(m) primarily damages the outer membrane and that the bacteria themselves actively participate in the ensuing, as yet unclarified, metabolic reactions that finally lead to their death.

Transmembrane channel formation by complement: functional analysis of the number of C5b6, C7, C8, and C9 molecules required for a single channel

Earlier studies have shown that sequential treatment of resealed erythrocyte ghosts with C5b6, C7, C8, and C9 leads to insertion of hydrophobic peptides from these complement proteins into the membrane and assembly of transmembrane channels. The number of molecules of each of the proteins required for assembly of the membrane-associated channel structure was evaluated by measuring the quantitative relationship between the doses of the individual proteins and the release of two trapped markers, sucrose and inulin, from ghosts after channel formation. The incubation period was sufficient to attain equilibrium of marker distribution between the ghosts and the extracellular fluid. Two markers of different size (sucrose and inulin, 0.9 and 3 nm molecular diameter, respectively) were used in order to develop information on the molecular composition of small and large channels, respectively. We found that participation of C5b6, C7, and C8 in channel formation displayed one-hit characteristics, regardless of marker size. By contrast, the participation of C9 was one-hit with respect to the sucrose marker, whereas with respect to the inulin marker the C9 reaction was multi-hit. Our results are compatible with the view that these markers are released through a channel structure in the membrane that is a monomer of C5b--9 of the composition C5b61 C71C81C9n, in which n = 1 for channels permitting passage of sucrose and n = 2 for channels allowing transit of inulin.

Assembly of the functional membrane attack complex of human complement: formation of disulfide-linked C9 dimers

The 158,000 Mr protein, previously designated C5c, present in fully assembled complement (C) membrane attack complexes (MC5b-9) has been identified as a disulfide-bonded dimer of C9. This conclusion was based on the observations that: (i) a portion of the 125I-radiolabeled precursor C9 incorporated into MC5b-9 complexes comigrated with the 158,000 Mr protein band in NaDodSO4/polyacrylamide slab gels; (ii) monospecific antisera produced against native C9 and the 158,000 Mr protein immunologically crossreacted with monomeric native C9 by double immunodiffusion and with monomeric C9 and the 158,000 Mr protein on immunoreplication procedures; and (iii) two-dimensional NaDodSO4/polyacrylamide slab gel electrophoresis, in which the second dimension was conducted under reducing conditions, revealed that the 158,000 Mr protein contained two identical 71,000 Mr subunits which comigrated with monomeric C9. Molar ratio estimates indicated that 1 mol of C5b, C9 dimer, C6, C7, and C8 and 3-4 mol of C9 monomer were present per MC5b-9 complex. Each fully assembled membrane-bound MC5b-9 complex would therefore have a calculated Mr of 982,000. The presence of C9 dimers in the hemolytically active 29S dimeric form of the MC5b-9 complex and the absence of C9 dimers in the hemolytically inactive 23S monomeric form of the fluid phase SC5b-9 complex strongly suggest an important role for C9 dimer formation in MC5b-9 complex structure and function. The most probable function of C9 dimers would be the formation of intercomplex disulfide crosslinks which would provide a mechanism to stabilize the assembly of MC5b-9 into aggregates of increasing size on the target membrane surface which would thus be responsible for the observed pore size heterogeneity of functional C lesions.

Single channel currents induced by complement in antibody-coated cell membranes

An extracellular patch electrode was used to record ionic currents from individual complement-induced channels in the membranes of antibody-coated skeletal muscle. The amplitude of the single-channel currents leads to an estimate of 90 pS for the unit conductance. The kinetics of channel opening and closing show marked variability and complexity. Channels flicker open and closed repeatedly, indicating that once these lesions form, they undergo rapid structural transitions between discrete conducting and nonconducting states.