Unicellular organisms such as yeast can survive in very different environments, thanks to a polysaccharide wall that reinforces their extracellular membrane. This wall is not a static structure, as it is expected to be dynamically remodeled according to growth stage, division cycle, environmental osmotic pressure and ageing. It is therefore of great interest to study the mechanics of these organisms, but they are more difficult to study than other mammalian cells, in particular because of their small size (radius of a few microns) and their lack of an adhesion machinery. Using flat cantilevers, we perform compression experiments on single yeast cells (S. cerevisiae) on poly-L-lysine-coated grooved glass plates, in the limit of small deformation using an atomic force microscope (AFM). Thanks to a careful decomposition of force–displacement curves, we extract local scaling exponents that highlight the non-stationary characteristic of the yeast behavior upon compression. Our multi-scale nonlinear analysis of the AFM force-displacement curves provides evidence for non-stationary scaling laws. We propose to model these phenomena based on a two-component elastic system, where each layer follows a different scaling law..

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes that take place during net biomass formation and maintenance processes. During growth, both ATP/ADP and NADH/NAD+ molecules play a key role. Cell energy metabolism hence refers to metabolic pathways involved in ATP synthesis linked to NADH turnover. Two main pathways are thus involved in cell energy metabolism: glycolysis/fermentation and oxidative phosphorylation. Glycolysis and mitochondrial oxidative phosphorylation are intertwined through thermodynamic and kinetic constraints that are reviewed herein. Further, our current knowledge of short-term and long term regulation of cell energy metabolism will be reviewed using examples such as the Crabtree and the Warburg effect.

The authors propose a wavelet-based decomposition of creep fluctuation signals recorded from living muscle precursor cells that revisit the traditional computation of their power spectrum from FFT-based decomposition. This decomposition offers a higher sensitivity for detecting the occurrence of fractional fluctuations and for quantitatively estimating the power-law exponent β of this spectrum as a signature of the scale-invariant rheology of living cells. This new method has also the unprecedented advantage of providing a test of the validity of the commonly assumed “monofractal” self-similar (as compared to “multifractal” intermittent) nature of these fluctuations and hence accrediting the use of a single rheological exponent α = β / 2. We report and discuss results obtained when applying this method to creep experiments performed with an AFM nanoindenter placed in contact with single myoblasts and myotubes, adherent on collagen coated coverslips, and in different culture conditions.

Fructose-1,6-bisphosphate (F1,6BP) promotes closure ofthe S. cerevisiae mitochondrial unspecific channel (ScMUC) enhancing coupling of oxidative phosphorylation and probably contributing to the Crabtree effect. In contrast the closely related glucose-6-phosphate (G6P) opens ScMUC. In order to understand the putative effects of F1,6BPon the mitochondrial unspecific channel, butane-1,4-bisphosphate (B1,4BP), a non-metabolizable, competitive inhibitor of F1,6BP-aldolase, was synthesized and tested on isolated mitochondria. B1,4BP closed ScMUC and its effects were reverted by increasing concentrations of G6P. Thus, in regard to its effects on ScMUC, and most likely due to structural similarities allowing it to bind to the same sites, B1,4BP is an analog of F1,6BP.

During the last decades a considerable amount of research has been focused on cancer. A number of genetic and signaling defects have been identified. This has allowed the design and screening of a number of anti-tumor drugs for therapeutic use. One of the main challenges of anti-cancer therapy is to specifically target these drugs to malignant cells. Recently, tumor cell metabolism has been considered as a possible target for cancer therapy. It is widely accepted that tumors display an enhanced glycolytic activity and oxidative phosphorylation down-regulation (Warburg effect). Therefore, it seems reasonable that disruption of glycolysis might be a promising candidate for specific anti-cancer therapy. Nonetheless, the concept of aerobic glycolysis as the paradigm of tumor cell metabolism has been challenged, as some tumor cells use oxidative phosphorylation. Mitochondria are of special interest in cancer cell energy metabolism, as their physiology is linked to the Warburg effect. Besides, their central role in apoptosis makes these organelles a promising "dual hit target" for selectively eliminate tumor cells. Thus, it is desirable to have an easy-to-use and reliable model in order to do the screening for energy metabolism-inhibiting drugs to be used in cancer therapy. From a metabolic point of view, the fermenting yeast Saccharomyces cerevisiae and tumor cells share several features. In this paper we will review these common metabolic properties and we will discuss the possibility of using S. cerevisiae as an early screening test in the research for novel anti-tumor compounds used for the inhibition of tumor cell metabolism.

Energy metabolism in living organisms is supported by the oxidation of carbohydrates and lipids, which are metabolized with several similarities as well as major differences. Conversely to prokaryotes and inferior eukaryotes, a complete oxidation leading to CO2 and H2O formation is mandatory. Our knowledge about oxidative phosphorylation is mostly based on simplified in vitro models, i.e., isolated mitochondria where only those parameters included in the experimental systems can be appreciated. However, relationships between mitochondria and the host cell are of major importance in the regulation of the pathway of ATP synthesis and oxygen consumption by the respiratory chain. By determining the respective rate of glucose or fatty acid oxidation, cellular intermediary metabolism affects the ratio between NADH and FADH2, upstream of the Krebs cycle, thus affecting the yield ATP synthesis. The mechanism for translocating reducing equivalents across the mitochondrial inner membrane (the malate–aspartate and glycerol-3phosphate–dihydroxyacetone phosphate shuttles) also plays an important role. Indeed, because of such characteristics of electron supply to the respiratory chain, oxidative phosphorylation activity also participates in the determination of the ratio of NADH to FADH2 oxidation, by modulation of the protonmotive force. Besides the role of thermodynamic and kinetic constraints applied to the oxidative phosphorylation pathway, it now appears that the supramolecular organization of oxidative phosphorylation and of cellular energy circuits introduces new regulatory factors of oxidative phosphorylation.

In the yeast Saccharomyces cerevisiae, the most important systems for conveying excess cytosolic NADH to the mitochondrial respiratory chain are the external NADH dehydrogenases (Nde1p and Nde2p) and the glycerol-3-phosphate dehydrogenase shuttle. In the latter system, NADH is oxidized to NAD+ and dihydroxyacetone phosphate is reduced to glycerol 3-phosphate by the cytosolic Gpd1p. Subsequently, glycerol 3-phosphate donates electrons to the respiratory chain via mitochondrial glycerol-3-phosphate dehydrogenase (Gut2p). At saturating concentrations of NADH, the activation of external NADH dehydrogenases completely inhibits glycerol 3-phosphate oxidation. Studies on the functionally isolated enzymes demonstrated that neither Nde1p nor Nde2p directly inhibits Gut2p. Thus, the inhibition of glycerol 3-phosphate oxidation may be caused by competition for the entrance of electrons into the respiratory chain. Using single deletion mutants of Nde1p or Nde2p, we have shown that glycerol 3-phosphate oxidation via Gut2p is inhibited fully when NADH is oxidized via Nde1p, whereas only 50% of glycerol 3-phosphate oxidation is inhibited when Nde2p is functioning. By comparing respiratory rates with different respiratory substrates, we show that electrons from Nde1p are favored over electrons coming from Ndip (internal NADH dehydrogenase) and that when electrons come from either Nde1p or Nde2p and succinodehydrogenase, their use by the respiratory chain is shared to a comparable extent. This suggests a very specific competition for electron entrance into the respiratory chain, which may be caused by the supramolecular organization of the respiratory chain. The physiological consequences of such regulation are discussed.

In a previous study we have identified Fmc1p, a mitochondrial protein involved in the assembly/stability of the yeast F0F1-ATP synthase at elevated temperatures. The Δfmc1 mutant was shown to exhibit a severe phenotype of very slow growth on respiratory substrates at 37 °C. We have isolated ODC1 as a multicopy suppressor of the fmc1 deletion restoring a good respiratory growth. Odc1p expression level was estimated to be at least 10 times higher in mitochondria isolated from the Δfmc1/ODC1 transformant as compared with wild type mitochondria. Interestingly, ODC1 encodes an oxodicarboxylate carrier, which transports α-ketoglutarate and α-ketoadipate or any other transported tricarboxylic acid cycle intermediate in a counter-exchange through the inner mitochondrial membrane. We show that the suppression of the respiratory-growth-deficient fmc1 by the overexpressed Odc1p was not due to a restored stable ATP synthase. Instead, the rescuing mechanism involves an increase in the flux of tricarboxylic acid cycle intermediate from the cytosol into the mitochondria, leading to an increase in the α-ketoglutarate oxidative decarboxylation, resulting in an increase in mitochondrial substrate-level-dependent ATP synthesis. This mechanism of metabolic bypass of a defective ATP synthase unravels the physiological importance of intramitochondrial substrate-level phosphorylations. This unexpected result might be of interest for the development of therapeutic solutions in pathologies associated with defects in the oxidative phosphorylation system.

During aerobic cell growth, mitochondria must meet energy demand either by adjusting cellular mitochondrial content or by adjusting ATP production flux, allowing a constant growth yield. On respiratory substrate, the Ras/cAMP pathway has been shown to be involved in this process in the yeast Saccharomyces cerevisiae. We show that of the three cAMP protein kinase catalytic subunits, Tpk3p is the one specifically involved in the regulation of cellular mitochondrial content when energy demand decreases. In decreased energy demand, the Δtpk3 mitochondrial enzymatic content decreases leading to a subsequent decrease in the cellular growth rate. Moreover, enzymatic content decreases in the Δtpk3 isolated mitochondria, suggesting that the amount of cellular mitochondria is not affected, but rather that the mitochondria are modified. Our study points to an important decrease in the cytochrome c content in the Δtpk3 mitochondria, which leads to a decrease in the slipping process at the level of cytochrome-c-oxidase.

Mitochondria amount within a cell is modulated in response to energy demand. This involves a tight regulation of mitochondrial biogenesis and the coordinated expression of hundreds of genes, both at the nuclear and at the mitochondrial level. This review will focus on two aspects of mitochondrial biogenesis regulation: (i) In mammalian cells, physiological effectors, and the regulatory proteins that control the expression of the respiratory apparatus, will be considered, and different kinds of tissue will be addressed. (ii) In yeast, the regulation of mitochondrial biogenesis in response to growth conditions as well as the signaling pathway involved will be considered.

In isolated rat hepatocytes, it has previously been reported that a rise in the ATP content induces a proportional increase in cytosolic NAD+ concentration [Devin, A., Guérin, B. & Rigoulet, M. (1997) FEBS Lett. 410, 329-332]. This occurs under physiological conditions such as various substrates or different energetic states. To investigate the effect of a physiological rise in cytosolic [NAD+] per se on glycolysis and gluconeogenesis, an increase in [NAD+] induced by exogenous nicotinamide addition was obtained without a change in redox potential, ATP/ADP ratio and ATP concentration. Using dihydroxyacetone as substrate, we found that an increase in cytosolic [NAD+] decreases gluconeogenesis and enhances glycolysis without significant alteration of dihydroxyacetone consumption rate. These modifications are the consequence of an allosteric activation of pyruvate kinase via cytosolic NAD+ content. Thus, in addition to the well-known thermodynamic control of glycolysis by pyridine-nucleotide redox status, our study points to a new mechanism of glycolytic flux regulation by NAD+ concentration at the level of pyruvate kinase activity.