Sodium and proton transport in Mycoplasma gallisepticum

Evolution of our understanding of cell volume regulation by the pump-leak mechanism

Kay and Blaustein recount the history of the pump-leak mechanism, which stabilizes cell volume by pumping sodium ions out of cells.

All animal cells are surrounded by a flexible plasma membrane that is permeable to water and to small ions. Cells thus face a fundamental problem: the considerable tension that their membranes would experience if the osmotic influx of water, driven by the presence of impermeant intracellular ions, was left unopposed. The pivotal study that described the cell’s remedy for this impending osmotic catastrophe—the “pump-leak mechanism” (PLM)—was published in the Journal of General Physiology by Tosteson and Hoffman in 1960. Their work revealed how the sodium pump stabilizes cell volume by eliminating the osmotic gradient. Here we describe the mechanistic basis of the PLM, trace the history of its discovery, and place it into the context of our current understanding.

Essential metabolism for a minimal cell

JCVI-syn3A, a robust minimal cell with a 543 kbp genome and 493 genes, provides a versatile platform to study the basics of life. Using the vast amount of experimental information available on its precursor, Mycoplasma mycoides capri, we assembled a near-complete metabolic network with 98% of enzymatic reactions supported by annotation or experiment. The model agrees well with genome-scale in vivo transposon mutagenesis experiments, showing a Matthews correlation coefficient of 0.59. The genes in the reconstruction have a high in vivo essentiality or quasi-essentiality of 92% (68% essential), compared to 79% in silico essentiality. This coherent model of the minimal metabolism in JCVI-syn3A at the same time also points toward specific open questions regarding the minimal genome of JCVI-syn3A, which still contains many genes of generic or completely unclear function. In particular, the model, its comparison to in vivo essentiality and proteomics data yield specific hypotheses on gene functions and metabolic capabilities; and provide suggestions for several further gene removals. In this way, the model and its accompanying data guide future investigations of the minimal cell. Finally, the identification of 30 essential genes with unclear function will motivate the search for new biological mechanisms beyond metabolism.

One way that researchers can test whether they understand a biological system is to see if they can accurately recreate it as a computer model. The more they learn about living things, the more the researchers can improve their models and the closer the models become to simulating the original. In this approach, it is best to start by trying to model a simple system.

Biologists have previously succeeded in creating ‘minimal bacterial cells’. These synthetic cells contain fewer genes than almost all other living things and they are believed to be among the simplest possible forms of life that can grow on their own. The minimal cells can produce all the chemicals that they need to survive – in other words, they have a metabolism. Accurately recreating one of these cells in a computer is a key first step towards simulating a complete living system.

Breuer et al. have developed a computer model to simulate the network of the biochemical reactions going on inside a minimal cell with just 493 genes. By altering the parameters of their model and comparing the results to experimental data, Breuer et al. explored the accuracy of their model. Overall, the model reproduces experimental results, but it is not yet perfect. The differences between the model and the experiments suggest new questions and tests that could advance our understanding of biology. In particular, Breuer et al. identified 30 genes that are essential for life in these cells but that currently have no known purpose.

Continuing to develop and expand models like these to reproduce more complex living systems provides a tool to test current knowledge of biology. These models may become so advanced that they could predict how living things will respond to changing situations. This would allow scientists to test ideas sooner and make much faster progress in understanding life on Earth. Ultimately, these models could one day help to accelerate medical and industrial processes to save lives and enhance productivity.

Semi-automated curation of metabolic models via flux balance analysis: a case study with Mycoplasma gallisepticum

Primarily used for metabolic engineering and synthetic biology, genome-scale metabolic modeling shows tremendous potential as a tool for fundamental research and curation of metabolism. Through a novel integration of flux balance analysis and genetic algorithms, a strategy to curate metabolic networks and facilitate identification of metabolic pathways that may not be directly inferable solely from genome annotation was developed. Specifically, metabolites involved in unknown reactions can be determined, and potentially erroneous pathways can be identified. The procedure developed allows for new fundamental insight into metabolism, as well as acting as a semi-automated curation methodology for genome-scale metabolic modeling. To validate the methodology, a genome-scale metabolic model for the bacterium Mycoplasma gallisepticum was created. Several reactions not predicted by the genome annotation were postulated and validated via the literature. The model predicted an average growth rate of 0.358±0.12, closely matching the experimentally determined growth rate of M. gallisepticum of 0.244±0.03. This work presents a powerful algorithm for facilitating the identification and curation of previously known and new metabolic pathways, as well as presenting the first genome-scale reconstruction of M. gallisepticum.

Flux balance analysis (FBA) is a powerful approach for genome-scale metabolic modeling. It provides metabolic engineers with a tool for manipulating, predicting, and optimizing metabolism for biotechnological and biomedical purposes. However, we posit that it can also be used as tool for fundamental research in understanding and curating metabolic networks. Specifically, by using a genetic algorithm integrated with FBA, we developed a curation approach to identify missing reactions, incomplete reactions, and erroneous reactions. Additionally, it was possible to take advantage of the ensemble information from the genetic algorithm to identify the most critical reactions for curation. We tested our strategy using Mycoplasma gallisepticum as our model organism. Using the genome annotation as the basis, the preliminary genome-scale metabolic model consisted of 446 metabolites involved in 380 reactions. Carrying out our analysis, we found over 80 incorrect reactions and 16 missing reactions. Based upon the guidance of the algorithm, we were able to curate and resolve all discrepancies. The model predicted an average bacterial growth rate of 0.358±0.12 h⁻¹ compared to the experimentally observed 0.244±0.03 h⁻¹. Thus, our approach facilitated the curation of a genome-scale metabolic network and generated a high quality metabolic model.

Analysis of energy sources for Mycoplasma penetrans gliding motility

Mycoplasma penetrans, a potential human pathogen found mainly in HIV-infected individuals, uses a tip structure for both adherence and gliding motility. To improve our understanding of the molecular mechanism of M. penetrans gliding motility, we used chemical inhibitors of energy sources associated with motility of other organisms to determine which of these is used by M. penetrans and also tested whether gliding speed responded to temperature and pH. Mycoplasma penetrans gliding motility was not eliminated in the presence of a proton motive force inhibitor, a sodium motive force inhibitor, or an agent that depletes cellular ATP. At near-neutral pH, gliding speed increased as temperature increased. The absence of a clear chemical energy source for gliding motility and a positive correlation between speed and temperature suggest that energy derived from heat provides the major source of power for the gliding motor of M. penetrans.

Energetics of gliding motility in Mycoplasma mobile

Mycoplasma mobile glides on surfaces at up to 7 microm/s by an unknown mechanism. We studied the energetics that power gliding by using a novel, growth medium-free system. We found that cells could glide in defined media if the glass substrate is preconditioned by exposure to horse serum. The active component that potentiates gliding is sensitive to proteinase K treatment. We used the defined medium system to test the effect of various inhibitors, ionophores, and poisons on motility of M. mobile. Valinomycin, carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), N,N'-dicyclohexylcarbodiimide, phenamil, amiloride, rifampin, and puromycin had no short-term effects on gliding. We also confirmed that we were able to modulate the membrane potential with valinomycin and FCCP by using a potential-sensitive dye. Shifting the pH likewise had no effect on motility. These results rule out the use of conventional ion motive forces to power gliding. Arsenate had a dramatic inhibitory effect on gliding, and both the speed and the fraction of cells moving tracked ATP levels. Sodium orthovanadate had a slight but significant inhibitory effect on gliding. Taken together, these results suggest that the motor system of M. mobile is likely an ATPase or is directly coupled to an ATPase.

An allometric interpretation of the spatio-temporal organization of molecular and cellular processes

Different levels of organization distinguished by characteristics spatial dimensions, Ec, and relaxation times, Tr, of biological processes ranging from electron transport in energy transduction to growth of microbial and plant cells, are shown to be related through a relation that may be interpreted as allometric and characterized by two different slopes. Processes, at levels of organization occurring in spatial dimensions of micrometers and relaxing in the order of minutes, delimit a 'transition point' between the two curves, that we interpret as a limit for the emergence of macroscopic coherence. The characteristic spatial dimension, Ec, and the relaxation time, Tr, contain dynamical information about the processes occurring at a given level of organization. When a steady state of a biological process at a certain level of organization becomes unstable, the system undergoes a transition to another level of organization. To exemplify the appearance of macroscopic order at levels of organization further from the 'transition point' we present in this report various experimental systems involving many levels of organization allometrically related that exhibit different kinds of self-organized behavior, i.e. bi-stability, oscillations, changes in (a)symmetry.

Role of Na+ cycle in cell volume regulation of Mycoplasma gallisepticum

The mechanism for the extrusion of Na+ from Mycoplasma gallisepticum cells was examined. Na+ efflux from cells was studied by diluting 22Na+-loaded cells into an isoosmotic NaCl solution and measuring the residual 22Na+ in the cells. Uphill 22Na+ efflux was found to be glucose dependent and linear with time over a 60-s period and showed almost the same rate in the pH range of 6.5 to 8.0. 22Na+ efflux was markedly inhibited by dicyclohexylcarbodiimide (DCCD, 10 microM), but not by the proton-conducting ionophores SF6847 (0.5 microM) or carbonyl cyanide m-chlorophenylhydrazone (CCCP, 10 microM) over the entire pH range tested. An ammonium diffusion potential and a pH gradient were created by diluting intact cells or sealed membrane vesicles of M. gallisepticum loaded with NH4Cl into a choline chloride solution. The imposed H+ gradient (inside acid) was not affected by the addition of either NaCl or KCl to the medium. Dissipation of the proton motive force by CCCP had no effect on the growth of M. gallisepticum in the pH range of 7.2 to 7.8 in an Na+-rich medium. Additionally, energized M. gallisepticum cells were stable in an isoosmotic NaCl solution, even in the presence of proton conductors, whereas nonenergized cells tended to swell and lyse. These results show that in M. gallisepticum Na+ movement was neither driven nor inhibited by the collapse of the electrochemical gradient of H+, suggesting that in this organism Na+ is extruded by an electrogenic primary Na+ pump rather than by an Na+-H+ exchange system energized by the proton motive force.

Volume regulation in Mycoplasma gallisepticum: evidence that Na+ is extruded via a primary Na+ pump

The primary extrusion of Na+ from Mycoplasma gallisepticum cells was demonstrated by showing that when Na+-loaded cells were incubated with both glucose (10 mM) and the uncoupler SF6847 (0.4 microM), rapid acidification of the cell interior occurred, resulting in the quenching of acridine orange fluorescence. No acidification was obtained with Na+-depleted cells or with cells loaded with either KCl, RbCl, LiCl, or CsCl. Acidification was inhibited by dicyclohexylcarbodiimide (50 microM) and diethylstilbesterol (50 microM), but not by vanadate (100 microM). By collapsing delta chi with tetraphenylphosphonium (200 microM) or KCl (25 mM), the fluorescence was dequenched. The results are consistent with a delta chi-driven uncoupler-dependent proton gradient generated by an electrogenic ion pump specific for Na+. The ATPase activity of M. gallisepticum membranes was found to be Mg2+ dependent over the entire pH range tested (5.5 to 9.5). Na+ (greater than 10 mM) caused a threefold increase in the ATPase activity at pH 8.5, but had only a small effect at pH 5.5. In an Na+-free medium, the enzyme exhibited a pH optimum of 7.0 to 7.5, with a specific activity of 30 +/- 5 mumol of phosphate released per h per mg of membrane protein. In the presence of Na+, the optimum pH was between 8.5 and 9.0, with a specific activity of 52 +/- 6 mumol. The Na+-stimulated ATPase activity at pH 8.5 was much more stable to prolonged storage than the Na+-independent activity. Further evidence that two distinct ATPases exist was obtained by showing that M. gallisepticum membranes possess a 52-kilodalton (kDa) protein that reacts with antibodies raised against the beta-subunit of Escherichia coli ATPase as well as a 68-kDa protein that reacts with the anti-yeast plasma membrane ATPases antibodies. It is postulated that the Na+ -stimulated ATPases functions as the electrogenic Na+ pump.

Sodium/proton antiport is required for growth of Escherichia coli at alkaline pH

Evidence is presented indicating that Escherichia coli requires the Na+/H+ antiporter and external sodium (or lithium) ion to grow at high pH. Cells were grown in plastic tubes containing medium with a very low Na+ content (5-15 microM). Normal cells grew at pH 7 or 8 with or without added Na+, but at pH 8.5 external Na was required for growth. A mutant with low antiporter activity failed to grow at pH 8.5 with or without Na+. On the other hand, another mutant with elevated antiporter activity grew at a higher pH than normal (pH 9) in the presence of added Na+ or Li+. Amiloride, an inhibitor of the antiporter, prevented cells from growing at pH 8.5 (plus Na+), although it had no effect on growth in media of lower pH values.

Biological activities of monoclonal antibodies to Mycoplasma pneumoniae membrane glycolipids

A purified preparation of membranes was obtained by using a unique method of treating Mycoplasma pneumoniae with the ATPase inhibitor, diethylstilbestrol. This method was shown to yield highly purified membranes with little or no cytoplasmic contamination. These membranes were used to immunize mice for subsequent productions of monoclonal antibodies (MAbs). Hybridoma culture supernatants were screened by enzyme-linked immunosorbent assay with whole-cell M. pneumoniae and lipid extract antigens. Four stable MAbs were obtained and characterized. MAb CP3-46F5 reacted with a protein of a molecular weight of approximately 52,000 as determined by Western blot (immunoblot). MAbs CP3-50C2, CP3-53C5, and CP3-53C8 did not react with any antigens on Western blots but did bind to at least 10 distinct glycolipid bands as determined by orcinol staining on thin-layer chromatograms of M. pneumoniae lipid extracts. The MAbs did not react with similarly prepared lipid extracts from Mycoplasma genitalium, Mycoplasma neurolyticum, and Mycoplasma gallisepticum. These MAbs did not inhibit M. pneumoniae metabolism or attachment to WiDr cell cultures. The anti-glycolipid MAbs recognize determinants specific to M. pneumoniae, unlike polyclonal hyperimmune sera against M. pneumoniae, which cross-react with lipid extracts of M. genitalium.

Characterization of sodium transport in Acholeplasma laidlawii B cells and in lipid vesicles containing purified A. laidlawii (Na+-Mg2+)-ATPase by using nuclear magnetic resonance spectroscopy and 22Na tracer techniques

The active transport of sodium ions in live Acholeplasma laidlawii B cells and in lipid vesicles containing the (Na+-Mg2+)-ATPase from the plasma membrane of this microorganism was studied by 23Na nuclear magnetic resonance spectroscopic and 22Na tracer techniques, respectively. In live A. laidlawii B cells, the transport of sodium was an active process in which metabolic energy was harnessed for the extrusion of sodium ions against a concentration gradient. The process was inhibited by low temperatures and by the formation of gel state lipid in the plasma membrane of this organism. In reconstituted proteoliposomes containing the purified (Na+-Mg2+)-ATPase, the hydrolysis of ATP was accompanied by the transport of sodium ions into the lipid vesicles, and the transport process was impaired by reagents known to inhibit ATPase activity. At the normal growth temperature (37 degrees C), this transport process required a maximum of 1 mol of ATP per mol of sodium ion transported. Together, these results provide direct experimental evidence that the (Na+-Mg2+)-ATPase of the Acholeplasma laidlawii B membrane is the cation pump which maintains the low levels of intracellular sodium characteristic of this microorganism.

Uptake of fatty acids by Mycoplasma capricolum

The energy requirements for fatty acid uptake by Mycoplasma capricolum were studied. Fatty acid transport and esterification to phospholipid appeared to be tightly coupled, since there was little intracellular accumulation of free fatty acid. Uptake was blocked by iodoacetate, n-ethylmaleimide, and p-chloromercuribenzoate. Glucose, glycerol, and potassium ions were necessary for fatty acid uptake by whole cells. A reduction in uptake was observed in cells treated with valinomycin or dicyclohexylcarbodiimide. The effect of temperature on the rate of oleate uptake showed a discontinuity at 24 degrees C. Above 24 degrees C an energy of activation of 4.6 kcal (ca. 19.2 kJ)/mol was obtained. The data suggest that uptake of fatty acid by M. capricolum is an energy-linked, protein-mediated process. A membrane-bound enzyme activity that catalyzed the synthesis of fatty acyl-hydroxamate was demonstrated. This activity was virtually independent or only marginally dependent on coenzyme A, depending on the assay system, but was stimulated approximately twofold by ATP.

Sodium ion transport decarboxylases and other aspects of sodium ion cycling in bacteria

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Cell volume regulation in Mycoplasma gallisepticum

Mycoplasma gallisepticum cells incubated in 250 mM NaCl solutions in the absence of glucose showed a progressive fall in intracellular ATP concentration over a period of 2 to 3 h. When the ATP level fell below 40 microM the cell began to swell and become progressively permeable to [14C]inulin and leak intracellular protein and nucleotides. The addition of nondiffusable substances such as MgSO4 or disaccharides prevented swelling, suggesting that NaCl (and water) entry was due to Gibbs-Donnan forces. The addition of glucose after the initiation of cell swelling increased intracellular ATP, induced cell shrinkage, and prevented the release of intracellular components. The ATPase inhibitor dicyclohexylcarbodiimide, which collapsed the chemical and electrical components of the proton motive force, caused rapid cell swelling in the presence of glucose (and high intracellular ATP levels). Extracellular impermeable solutes such as MgSO4 and disaccharides prevented swelling of dicyclohexylcarbodiimide-treated cells incubated in NaCl. It was postulated that Na+ that diffused into the cell was extruded by an electrogenic Na+-H+ exchange (antiport) energized by the proton motive force established by the dicyclohexylcarbodiimide-sensitive H+-ATPase.

Characterization and solubilization of the membrane-bound ATPase of Mycoplasma gallisepticum

The membrane-bound ATPase of Mycoplasma gallisepticum selectively hydrolyzed purine nucleoside triphosphates and dATP. ADP, although not a substrate, inhibited ATP hydrolysis. The enzyme exhibited a pH optimum of 7.0 to 7.5 and an obligatory requirement for divalent cations. Dicyclohexylcarbodiimide at a concentration of 1 mM inhibited 95% of the ATPase activity at 37 degrees C, with 50% inhibition occurring at 22 microM dicyclohexylcarbodiimide. Sodium or potassium (or both) failed to stimulate activity by greater than 37%. Azide (2.6 mM), diethylstilbestrol (100 micrograms/ml), p-chloromercuribenzoate (1 mM), and vanadate (50 microM) inhibited 50, 91, 89, and 60%, respectively. The ATPase activity could not be removed from the membrane without detergent solubilization. Although most detergents inactivated the enzyme, the dipolar ionic detergent N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (0.1%) solubilized approximately 70% of the enzyme with only a minor loss in activity. The extraction led to a twofold increase in specific activity and retention of inhibition by dicyclohexylcarbodiimide and ADP. Glycerol greatly increased the stability of the solubilized enzyme. The properties of the membrane-bound ATPase are not consistent with any known ATPase. We postulate that the ATPase functions as an electrogenic proton pump.