Brown adipose tissue mitochondrial subpopulations show different morphological and thermogenic characteristics

Abstract

Rat brown adipose tissue mitochondrial subpopulations—isolated by differential centrifugation at 1000, 3000 and 8000g, giving the heavy, medium and light mitochondria—were characterized. Thus, contamination by non-mitochondrial subcellular components, morphological features, respiratory chain and antioxidant enzyme activities, both uncoupling protein 1 and mitochondrial protein content, mitochondrial DNA levels and mitochondrial integrity were measured. Results indicate that mitochondrial fractions showed important differences in the morphological, thermogenic and antioxidant properties. All the parameters studied were always higher in heavy mitochondria, which is indicative of a greater mitochondrial differentiation state.

Introduction

Brown adipose tissue (BAT) is a highly specialized organ for heat production, which plays an important role in the regulation of energy balance and in the maintenance of body temperature, at least in small mammals (Himms-Hagen, 1990). The thermogenic function of BAT depends on the expression in mitochondria of uncoupling protein 1 (UCP1) whose function is to uncouple the respiratory chain from ATP synthesis by dissipating the proton gradient as heat (Matthias et al., 1999, Ricquier and Bouillaud, 2000).

Mitochondrial biogenesis is an extremely complex, little-known process, although there is experimental evidence which supports the concept that pre-existing mitochondria may proliferate and differentiate (Klingenspor, 2003, Nisoli et al., 2004). In this sense, previous studies have demonstrated that mitochondrial population from various tissues is not homogeneous and may be fractionated by differential centrifugation into several subpopulations. A relationship between the process of mitochondrial biogenesis and the mitochondrial fractions has been suggested, which would imply a mitochondrial growth cycle within the cell, where the lightest mitochondria would act as precursors of the heaviest ones (Gianotti et al., 1998, Goglia et al., 1989, Justo et al., 2002, Koekemoer and Oelofsen, 2001, Lanni et al., 1996, Lombardi et al., 2000). In particular, in the adaptive response of BAT in diverse chronic situations, such as prolonged fasting, overfeeding or cold-acclimation, the changes produced in mitochondria are reflected in the heavy subpopulation (Gianotti et al., 1998, Matamala et al., 1996, Moreno et al., 1994). In contrast, acute physiological situations, such as short-term fasting or cold exposure result in changes affecting mainly the light mitochondrial fractions (Gianotti et al., 1998, Moreno et al., 1994). Therefore, the different involvement of BAT mitochondrial fractions in response to several physiological situations, would be dependent on the kind of stimulus.

It is important to point out that there are no exhaustive studies focused on the per se features of BAT mitochondrial subpopulations. Thus, the present study performs an exhaustive characterization of these mitochondria, both in morphological and thermogenic features. This could help to understand the different involvement in the adaptive response of mitochondrial subpopulations to a diverse range of stimuli and the possible link to the mitochondrial growth cycle.