Murray 2013 Exp Physiol
|Murray AJ (2013) Of mice and men (and muscle mitochondria). Exp Physiol 98:879-80.|
Abstract: For the past decade or more, concerns about the short-term health problems and long-term societal impact of the increasingly obese, sedentary and aged populations of Western countries have fuelled a resurgence in metabolic research. In humans, studies of skeletal muscle mitochondria have been at the forefront of much of this work, because respiratory capacity is highly plastic in skeletal muscle, becoming stimulated following exercise training and caloric restriction or suppressed in response to ageing and hypoxia, with a loss of capacity being strongly associated with fatigue and the aetiology of insulin resistance. The accessibility of muscles such as vastus lateralis, which can be biopsied safely from healthy volunteers, athletes and patients, has allowed functional measures of respiratory capacity and coupling to be made. Alongside this, the timely arrival of genetically manipulated mouse models and sophisticated analytical platforms has allowed the elucidation of intricate mechanisms of metabolic control that modify mitochondrial respiration in response to conditions such as altered substrate and oxygen availability, exercise, inflammation, injury and stress. Experimental animals and, in particular, mouse models have therefore become vital tools in mitochondrial research, yet the suitability of mouse skeletal muscle as a model for human muscle research has recently been questioned owing to interspecies differences in molecular profiles and purported variations in mitochondrial function between muscle types. In this issue of Experimental Physiology, a paper by Jacobs et al. (2013) describes the use of high-resolution respirometry to address these concerns rigorously. The authors conclude that mouse skeletal muscle mitochondria and, in particular, those of mouse quadriceps closely resemble the mitochondria of human quadriceps (vastus lateralis) with respect to their respiratory capacity and control. • Keywords: Comparative Physiology
• O2k-Network Lab: UK Cambridge Murray AJ
Organism: Human, Mouse Tissue;cell: Skeletal muscle Preparation: Permeabilized tissue
When considering the respiratory capacity of a given muscle, studies have frequently focused on mitochondrial density, which, although a major determinant in this regard, does not in itself present a full picture of the bioenergetic profile of the tissue. Mitochondrial populations from different tissues or species are known to be functionally distinct, having different respiration rates for a given unit of mitochondrial mass, and this is by no means a new concept. Indeed, it has been known for over 35 years that even within a single cardiac muscle fibre there are two biochemically distinct mitochondrial subpopulations situated in the subsarcolemmal and intermyofibrillar regions (Palmer et al. 1977). To assess functional differences between such populations, isolated mitochondrial preparations can be useful, yet the process of isolation itself disrupts the mitochondrial reticulum, resulting in malformed organelles, and presents a new problem regarding how best to normalize respiration rates, with protein content often correlating poorly with cristae density. The saponin-permeabilized muscle fibre preparation eliminates the need for a time-consuming and disruptive isolation procedure, retaining an intact mitochondrial network within the myofibre whilst providing access for the direct delivery of substrates and inhibitors to the mitochondria (Kuznetsov et al. 2008).
The study by Jacobs et al. (2013) utilizes this technique to compare respiration rates per unit mass of muscle from human vastus lateralis with those of murine soleus, gastrocnemius and quadriceps. The authors then corrected these rates to the activity of citrate synthase, which is a marker of skeletal muscle mitochondrial density that has previously been validated in humans (Larsen et al. 2012) and is validated in mice in this paper (Jacobs et al. 2013). As expected, the authors found that the oxidative phosphorylation capacity per unit of muscle mass correlates strongly with mitochondrial content across the three mouse muscles, with the highly oxidative soleus muscle exhibiting the highest citrate synthase activity. When normalized to a unit of mitochondria, however, the authors reveal clear differences in mitochondrial respiratory capacity across the different muscles, with those from gastrocnemius, the most glycolytic muscle, exhibiting the highest rates. Yet despite having lower mass-specific respiration rates than human vastus lateralis, the mitochondria of the mixed-fibre-type mouse quadriceps closely resemble those of human muscle. The authors do, however, note that mitochondrial electron coupling control during fat oxidation in human vastus lateralis is more similar to mouse soleus than it is to mouse quadriceps.
The paper by Jacobs et al. (2013) therefore demonstrates that the skeletal muscle of healthy, young mice, and mouse quadriceps in particular, is a suitable model tissue for studies of human skeletal muscle mitochondrial function, and these findings will no doubt be met with some relief by the many groups worldwide that utilize mouse models in muscle research. The growing resource of genetically manipulated mouse models with metabolic phenotypes appears to remain a valuable tool for bioenergetic research.
Perhaps a final word of caution is warranted, however, in that similarities in baseline muscle mitochondrial function between healthy, young, unstressed mice and humans do not suggest that each will necessarily respond in a qualitatively or quantitatively similar fashion to a given pathology or stress. Future studies might therefore follow the methods of Jacobs et al. (2013) to investigate whether the mitochondria of mouse and human quadriceps continue to function in a similar manner when stressed. To this end, comparative studies of hypoxic mouse and human muscle, for instance, would be revealing, because chronic exposure to hypoxia is known to alter muscle mitochondrial density, alongside intramitochondrial changes in respiratory capacity and coupling via altered expression and function of electron transport chain complexes and substrate oxidation enzymes (Murray, 2009).
Caution should, of course, always be exercised when interpreting animal studies, with care being taken to temper any premature translation into implications for human physiology. The work of Jacobs et al. (2013), however, is important and timely in reasserting the value of the mouse as an experimental model in human metabolic research.
Kuznetsov AV, Veksler V, Gellerich FN, Saks V, Margreiter R & Kunz WS (2008). Analysis of mitochondrial function in situ in permeabilized muscle fibers, tissues and cells. Nat Protoc 3, 965–976.
Larsen S, Nielsen J, Hansen CN, Nielsen LB, Wibrand F, Stride N, Schroder HD, Boushel R, Helge JW, Dela F & Hey-Mogensen M (2012). Biomarkers of mitochondrial content in skeletal muscle of healthy young human subjects. J Physiol 590, 3349–3360.
Murray AJ (2009). Metabolic adaptation of skeletal muscle to high altitude hypoxia: how new technologies could resolve the controversies. Genome Med 1, 117.
Palmer JW, Tandler B & Hoppel CL (1977). Biochemical properties of subsarcolemmal and interfibrillar mitochondria isolated from rat cardiac muscle. J Biol Chem 252, 8731–8739.