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MRXUV G·H[SRVLWLRQDX WUDLWHPHQW2.5 Le changement d’efficacité mitochondriale affecte la balance oxydative, le développement et la croissance des têtards de grenouilles rousses – Article 3, en révision dans Journal of Experimental Biology
Afin de déterminer l’impact du fonctionnement mitochondrial dans les stratégies d’histoire de vie des organismes, nous avons induit des changements dans la physiologie mitochondriale de têtards de grenouilles rousses Rana temporaria.
Ces expériences ont consisté à mettre en place 3 lots de têtards : 2 lots témoins, « eau » et « éthanol » 0.001% et un lot traité au 2,4-dinitrophenol 1 μM.
Figure 26 : Protocole d’élevage des têtards et de leur exposition au traitement.
Ainsi, les effets du traitement sur les mitochondries ont été appréciés par la mesure de l’efficacité mitochondriale (rapport ATP/O), des capacités oxydatives et phosphorylantes ainsi que de la production de ROS à t = 2 (figure 26)
Au niveau de la balance oxydative, à t = 2, l’investissement dans les défenses anti-oxydantes a été estimé par les capacités anti-anti-oxydantes totales des têtards, et la résultante de la balance oxydative, l’accumulation des dégâts oxydatifs, a été évaluée par le dosage d’acide thiobarbiturique, un marqueur de la peroxydation lipidique.
Ces mesures ont été complétées par l’évaluation des entrées du flux énergétique, soit les apports énergétiques via la quantité d’aliments ingérés (t = 1 et t = 2) et la consommation d’oxygène en métabolisme basal (t = 0 ; t = 1 ; t = 2).
Etudes expérimentales
Lors de l’exposition au traitement (t= 0, t= 1, t= 2), l’évolution de la masse corporelle et du stade de développement ont permis de mettre en évidence l’impact des modification du fonctionnement mitochondriale sur les traits d’histoire de vie des organismes.
Alteration of mitochondrial efficiency affects oxidative balance, development and growth in frog (Rana temporaria) tadpoles.
By
Salin Karine1*, Luquet Emilien1, Rey Benjamin2, Roussel Damien1 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: Mitochondria and life-history traits
* 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.
Etudes expérimentales
ABSTRACT
Mitochondria are known to play a central role in life-history processes, being the main source of reactive oxygen species (ROS), which promote oxidative constraints. Surprisingly, although the main role of the mitochondria is to produce ATP, plasticity of mitochondrial ATP generation received little attention in life history studies. Yet mitochondrial energy transduction represents the physiological link between environmental resources and energy allocated into animal performances. By studying the both facets of the mitochondrial functioning (ATP and ROS productions), we could provide better understanding on the proximate mechanisms underlying life-history. We have experimentally modulated the mitochondria capacity to generate ROS and ATP during larval development of Rana
temporaria tadpoles, via chronic exposure (34 days) to a mitochondrial uncoupler
(2,4-dinitrophenol, dNP). The purpose was then to better understanding the mitochondrial uncoupling impact on both responses regarding oxidative balance, energy input (oxygen and feeding consumption) and output (growth and development tadpole). Exposure to 2,4-dNP reduced mitochondrial ROS generation, total antioxidant defenses and oxidative damages in treated-tadpoles compared to control. Despite the beneficial effect of dNP on oxidative status, development and growth rates of treated-tadpoles were lower than those in control group. Treatment of tadpoles with 2,4-dNP promoted a mild mitochondrial uncoupling and enhanced metabolic rate. Mild uncoupled tadpoles did not increase their food consumption, and thus failed to compensate from the energy loss elicited by the decrease in the efficiency of ATP production process. These data suggest that the cost of ATP production, rather the oxidative balance, is a parameter able to constrain growth/development of tadpoles, highlighting the central role of energy transduction into larval performances.
KEYWORDS: ATP/O ratio, energy balance, life-history traits, oxidative constraint, amphibian.
INTRODUCTION
Organism’s life-history is shaped by physical constraints and ecological pressures. Since the early 1960’s, pillar of life-history theory relies on the assumption that the differential allocation of a limited resources to various aspects of maintenance, growth and reproduction must be optimized to maximize individual fitness (Cody 1966, Stearns, 1992). Unraveling the underlying mechanisms of life-history trade-off could improve the understanding of the proximal causes driving evolutionary forces. During the past years, physiological aspects of trade-off have been largely explored focusing on energy resource allocation and its genetic and endocrine control (Zera and Harshman, 2001). However, the precise physiological mechanisms that determine the variability in life-history traits are still poorly understood. (Ackermann et al, 2001; Zera and Harshman, 2001; Alonso-Alvarez et al., 2006).
Although a great number of relevant studies focused on energy resource acquisition and energy allocation, surprisingly none has yet considered energy flow regulation and conversion at the mitochondrial level. Indeed, energy deriving from diet becomes usable only after being converted into high-energy phosphate bonds in adenosine triphosphate (ATP) molecules. The main site of the energy conversion is mitochondria (90% of cellular ATP, Lehninger et al., 1993) that extract energy through oxidative reactions during aerobic respiration. Since the amount of ATP could limit many vital cellular processes (protein synthesize, cell division and signaling, muscle contractile activities…), the plasticity of this organelle functioning would play a central role in the amount of energy that an organism could allocate to its life-history traits.
Mitochondrial ATP synthesis is well known to present variable degree of coupling to oxygen consumption (ATP/O ratio). In this sense, a plasticity of the mitochondrial energy transduction efficiency strongly vary in a phylogenetic manner and in response to environmental factors, such as season, diet, temperature (Heise et al., 2003; Sokolova, 2004; Sommer and Portner, 2004; Brand, 2005; Emel’Yanova et al., 2007; Robert and Bronikowski, 2010). However, according to the above mentioned theory of limited resources, evolutionary force should have optimized the ATP/O ratio toward a better mitochondrial energy transduction system efficiency in order to maximized organism’s fitness. Therefore, the persistence of low ATP/O yield, despite the action of natural selection thus suggests that a
Etudes expérimentales
low efficiency could provide some evolutionary advantages according to environmental circumstance (Brand, 2000).
One possible explanation is that mitochondria are also an important source of reactive oxygen species (ROS). Indeed, ROS formation is an unavoidable by-product of aerobic metabolism that greatly depends upon the mitochondrial inner membrane potential, as the ATP/O ratio does (Korshunov et al., 1997; Brand, 2005, Jezek and Hlavata 2005). These highly reactive molecules will exert oxidative stress on cellular macromolecules. If ROS production is unchecked thanks to anti-oxidant systems, cumulative oxidative damage to lipids, proteins and nucleic acids will accelerate organism senescence processes (Harman, 1956; Balaban et al., 2005; Barja, 2007). To date, ROS production have been considered as a proximate cost of growth or reproduction investment, supporting the notion that energy allocation mediated trade off may be associated with oxidative stress (Dowling and Simmons, 2009; Metcalfe and Alonzo-Alvarez, 2010), but not necessarily with energetic resources or more widely to the energy transduction system. In order to provide new advances for functional studies of trade-offs, we investigated the mitochondrial functioning as the keystone between optimization of energetic availability for maintenance, reproduction and growth and the ROS production cost.
The aim of the present study was to investigate whether global mitochondrial functioning (both ATP and ROS production) may be associated with life-history variations. To test this hypothesis, we chronically exposed frog Rana temporaria tadpoles to the mitochondrial protonophore 2,4-dinitrophenol (2,4-dNP) during their larval development. This drug increased the proton leak across the inner mitochondrial membrane, reducing the efficiency of mitochondrial energy transduction and the rate of ROS production (Brand, 2000; Balaban et al., 2005; Speakman, 2005; Hulbert et al., 2007). Chronic 2,4-dNP exposure at low dose is expected to inducing mild mitochondrial uncoupling and promote beneficial effect on oxidative stress (Wallace and Starkov, 2000, da Silva et al., 2008). We then measured different parameters to better understand both responses regarding oxidative balance, energy input (basal metabolic rate and food intake) and the energy output in tadpole performances (growth and development rates).
MATERIALS AND METHODS