Base mécanistique du système de conversion énergétique mitochondriale : variations du rendement des oxydations phosphorylantes – Article 2, soumis à Journal of

Experimental Zoology Part A:

Les différences de rendement énergétique mitochondrial associées au taux de croissance variables révélées lors de l’étude précédente peuvent mettre en cause plusieurs sites de la mitochondrie, et ce de manière non-exclusive. Comme il a été expliqué en introduction, l’activé et la stœchiométrie des pompes à protons (la chaîne respiratoire et l’ATP synthétase) sont des paramètres variables, de même la fluidité de la membrane interne mitochondriale peut affecter la force protomotrice. Nous avons alors effectués des études plus appronfondies du fonctionnement mitochondrial de manière à comprendre quels pourraient être les mécanismes sous-jacents à nos résultats. Alors que les capacités oxydatives de la chaîne respiratoire sont équivalentes, une faible variation du potentiel de membrane laisse suggèrer une différence non négligeable de la perméabilité membranaire. Par ailleurs, les mesures des capacités de phosphorylation et d’hydrolyse de l’ATP synthétase révèlent des capacités enzymatiques considérablement plus importantes dans les mitochondries à rendement énergétique élevé. Le complexe V semble être le site majeur à l’origine des changements du rendement énergétique dans les mitochondries hépatiques étudiées dans deux populations de grenouilles rousses.

Etudes expérimentales

David and Goliath: a mitochondrial coupling problem?

By

Salin Karine^1*, Roussel Damien¹, Rey Benjamin² and Voituron Yann¹

1. Laboratoire d’Ecologie des Hydrosystèmes Naturels et Anthropisés (U.M.R. CNRS 5023), Université Claude Bernard Lyon 1, Université de Lyon, 43 Bvd 11 Novembre 1918, F-69622 Villeurbanne Cedex, France.

2. Laboratoire de Biométrie et Biologie Evolutive (U.M.R. CNRS 5558), Université Claude Bernard Lyon 1, Université de Lyon, 43 Bvd 11 Novembre 1918, F-69622 Villeurbanne Cedex, France.

Running title: Mitochondrial efficiency and body size

* To whom correspondence should be addressed: karine.salin@univ-lyon1.fr; Tel: +33 4 72 43 15 20; Fax: +33 4 72 43 11 41.

SUMMARY

An organism’s size, known to affect biological structures and processes from cellular metabolism to population dynamics, depends upon the duration and rate of growth. However, it is still poorly understood how mitochondrial function affects the energetic basis of growth, especially in ectotherms, which represent a huge majority of animal biodiversity. Here we present an intra-species comparison of neighboring populations of frogs (Rana temporaria) that have large differences in body mass even at the same age. By investigating liver mitochondrial bioenergetics, we find that frogs with high growth rates and large body sizes exhibit higher ATP synthesis rates and more efficient oxidative phosphorylation compared to the smaller frogs with low growth rates. This higher energy transduction efficiency is not associated with significant increased oxidative capacity or membrane potential values, but instead may rely on a higher mitochondrial phosphorylation system activity in combination with a lower inner membrane proton leakage. Overall, the present study introduces the mitochondrial energy transduction system as an important mechanism for balancing physiological and ecological trade-offs associated with body size. Our findings highlight the need for closer consideration of all aspects of mitochondrial metabolism for a better understanding of the physiological basis of the link between size, metabolism, and energy production in wild-dwelling organisms.

Etudes expérimentales

INTRODUCTION

As William A. Calder (1984) stated, “there is no doubt that body size is a major factor in an animal’s life history and requirements”. Indeed, an organism’s size, which can range nearly two thousand-fold in vertebrates from the gecko Sphaeradactylus ariasae (16 mm long) to the blue whale Balaenoptera musculus (30 m long), affects the state of all of its biological structures and processes, from cellular metabolism up to population dynamics (Banavar et al., 2010; Peters and Wassenberg, 1983). As all biological activities depend on energy, a considerable effort for over 150 years has been put forth to document the relation between body size and energy metabolism. Respiratory metabolic rate (R) typically scales with body mass (M) according to the power function R=ĮM^b, where Į is a normalization constant, and b is the scaling exponent, which is usually 0.75, though this is still under much debate (Banavar et al., 2010; West et al., 2003). Because of its link with energy metabolism, body size has a strong impact on whole body performances, such as tolerance to environmental variations such as temperature and food deprivation, and on physical capacities, such as running speed and diving performance (Schreer and Kovacs, 1997). At the subcellular level, studies performed mainly in endotherms have reported that inter-specific variation in body mass can be related to some aspects of mitochondrial bioenergetics, such as oxidative capacity and membrane proton leak activity (Brand et al., 2003; Drabkin, 1950; Kunkel et al., 1956; Porter and Brand, 1993). However, because mitochondrial bioenergetics are directly or indirectly involved in thermoregulatory processes, the constraint of thermoregulation in endotherms may induce a highly confounding effect on the relation between body size and mitochondrial function. Therefore, using ectotherms may be a more promising way to understand the causes and effects of the relation between body size and mitochondrial functioning.

From a physiological point of view, body size depends on two parameters: growth duration and growth rate, which are both genetically and hormonally controlled (Caldwell et al., 2005; Tissenbaum and Guarente, 2001). However, beyond such growth triggers, little is known about the mitochondrial energetic aspects of growth. Indeed, mitochondria are the cell's powerhouses, providing cells with energy in the form of ATP, which is then used to carry out diverse vital functions, including growth. It is surprising that only a few studies have focused on the impact of mitochondrial function on organismal

instance, recent studies in endotherms that focused on feed efficiency in poultry and livestock species have suggested that the mitochondrial metabolism phenotype might impact the rate of growth (Bottje and Carsten, 2009). Interestingly, a study focused on C. elegans clearly demonstrated that reducing the rates of oxygen consumption and ATP production induced a decrease in growth rate and body size (Dillin et al., 2002). From these examples, it appears that modifying mitochondrial activity, i.e., mitochondrial ATP production, can impact the rate of growth and, therefore, the body size of an organism.

Whether a mitochondrial phenotype can differentially evolve in the wild to cause variation in some biological traits (e.g., body size, growth rate) remains largely unexplored. To address this question, we examined the relation between mitochondrial bioenergetics and growth rate by comparing two neighboring populations of the common frog

(Rana temporaria), which exhibit a large difference in body size at the same age (Augert and

Joly, 1993). Working at the intra-specific level with closely located populations allowed us to exclude phylogenetic biases and latitudinal and/or altitudinal effects that could have altered the interpretation of our results (Garland et al., 2005; Yamahira and Conover, 2002). We investigated several key parameters of mitochondrial function, including oxygen consumption, cytochrome-c oxidase activity, membrane potential changes, ATP production and F1F0-ATP synthase activity. We clearly show that mitochondrial plasticity in the ATP production system is associated with natural differences in the growth rate and body size of this wild-dwelling ectotherm.

Etudes expérimentales

MATERIALS AND METHODS