Mitochondrial affinity for ADP is twofold lower in creatine kinase knock-out muscles. Possible role in rescuing cellular energy homeostasis

Energy demand and supply in human skeletal muscle

The energy required for muscle contraction is provided by the breakdown of ATP but the amount of ATP in muscles cells is sufficient to power only a short duration of contraction. Buffering of ATP by phosphocreatine, a reaction catalysed by creatine kinase, extends the duration of activity possible but sustained activity depends on continual regeneration of PCr. This is achieved using ATP generated by oxidative processes and, during intense activity, by anaerobic glycolysis. The rate of ATP breakdown ranges from 70 to 140 mM min^-1 during isometric contractions of various intensity to as much as 400 mM min^-1 during intense, dynamic activity. The maximum rate of oxidative energy supply in untrained people is ~50 mM min^-1 which, if the contraction duty cycle is 0.5 as is often the case in cyclic activity, is sufficient to match an ATP breakdown rate during contraction of 100 mM min^-1. During brief, intense activity the rate of ATP turnover can exceed the rates of PCr regeneration by combined oxidative and glycolytic energy supply, resulting in a net decrease in PCr concentration. Glycolysis has the capacity to produce between 30 and 50 mM of ATP so that, for example, anaerobic glycolysis could provide ATP at an average of 100 mM min^-1 over 30 s of exhausting activity. The creatine kinase reaction plays an important role not only in buffering ATP but also in communicating energy demand from sites of ATP breakdown to the mitochondria. In that role, creatine kinases acts to slow and attenuate the response of mitochondria to changes in energy demand.

Creatine transporter deficiency leads to increased whole body and cellular metabolism

Creatine (Cr) is a guanidino compound required for rapid replenishment of ATP in cells with a high-energy demand. In humans, mutations in the Cr transporter (CRT;SLC6A8) prevent Cr entry into tissue and result in a significant intellectual impairment, epilepsy, and aphasia. The lack of Cr on both the whole body and cellular metabolism was evaluated in Crt knockout (Crt (-/y) ) mice, a high-fidelity model of human CRT deficiency. Crt (-/y) mice have reduced body mass and, however, show a twofold increase in body fat. There was increased energy expenditure in a home cage environment and during treadmill running in Crt (-/y) mice. Consistent with the increases in the whole-body metabolic function, Crt (-/y) mice show increased cellular metabolism as well. Mitochondrial respiration increased in skeletal muscle fibers and hippocampal lysates from Crt (-/y) mice. In addition, Crt (-/y) mice had increased citrate synthase activity, suggesting a higher number of mitochondria instead of an increase in mitochondrial activity. To determine if the increase in respiration was due to increased mitochondrial numbers, we measured oxygen consumption in an equal number of mitochondria from Crt (+/y) and Crt (-/y) mice. There were no changes in mitochondrial respiration when normalized to mitochondrial number, suggesting that the increase in respiration observed could be to higher mitochondrial content in Crt (-/y) mice.

The C57Bl/6 mouse serves as a suitable model of human skeletal muscle mitochondrial function

It is debatable whether differences in mitochondrial function exist across skeletal muscle types and whether mouse skeletal muscle mitochondrial function can serve as a valid model for human skeletal muscle mitochondrial function. The aims of this study were to compare and contrast three different mouse skeletal muscles and to identify the mouse muscle that most closely resembles human skeletal muscle respiratory capacity and control. Mouse quadriceps (QUAD(M)), soleus (SOL(M)) and gastrocnemius (GAST(M)) skeletal muscles were obtained from 8- to 10-week-old healthy mice (n = 8), representing mixed, oxidative and glycolytic muscle, respectively. Skeletal muscle samples were also collected from young, active, healthy human subjects (n = 8) from the vastis lateralis (QUAD(H)). High-resolution respirometry was used to examine mitochondrial function in all skeletal muscle samples, and mitochondrial content was quantified with citrate synthase activity. Mass-specific respiration was higher across all respiratory states in SOL(M) versus both GAST(M) and QUAD(H) (P < 0.01). When controlling for mitochondrial content, however, SOL(M) respiration was lower than GAST(M) and QUAD(H) (P < 0.05 and P < 0.01, respectively). When comparing respiratory capacity between mouse and human muscle, QUAD(M) exhibited only one different respiratory state when compared with QUAD(H). These results demonstrate that qualitative differences in mitochondrial function exist between different mouse skeletal muscles types when respiratory capacity is normalized to mitochondrial content, and that skeletal muscle respiratory capacity in young, healthy QUAD(M) does correspond well with that of young, healthy QUAD(H).

Accurate determination of the oxidative phosphorylation affinity for ADP in isolated mitochondria

Background

Mitochondrial dysfunctions appear strongly implicated in a wide range of pathologies. Therefore, there is a growing need in the determination of the normal and pathological integrated response of oxidative phosphorylation to cellular ATP demand. The present study intends to address this issue by providing a method to investigate mitochondrial oxidative phosphorylation affinity for ADP in isolated mitochondria.

Methodology/Principal Findings

The proposed method is based on the simultaneous monitoring of substrate oxidation (determined polarographically) and phosphorylation (determined using the glucose - hexokinase - glucose-6-phosphate dehydrogenase - NADP⁺ enzymatic system) rates, coupled to the determination of actual ADP and ATP concentrations by bioluminescent assay. This enzymatic system allows the study of oxidative phosphorylation during true steady states in a wide range of ADP concentrations. We demonstrate how the application of this method allows an accurate determination of mitochondrial affinity for ADP from both oxidation (K_mVox) and phosphorylation (K_mVp) rates. We also demonstrate that determination of K_mVox leads to an important overestimation of the mitochondrial affinity for ADP, indicating that mitochondrial affinity for ADP should be determined using phosphorylation rate. Finally, we show how this method allows the direct and precise determination of the mitochondrial coupling efficiency. Data obtained from rat skeletal muscle and liver mitochondria illustrate the discriminating capabilities of this method.

Conclusions/Significance

Because the proposed method allows the accurate determination of mitochondrial oxidative phosphorylation affinity for ADP in isolated mitochondria, it also opens the route to a better understanding of functional consequences of mitochondrial adaptations/dysfunctions arising in various physiological/pathophysiological conditions.

In vivo magnetic resonance spectroscopy of transgenic mice with altered expression of guanidinoacetate methyltransferase and creatine kinase isoenzymes

Mice with an under- or over-expression of enzymes catalyzing phosphoryl transfer in high-energy supplying reactions are particulary attractive for in vivo magnetic resonance spectroscopy (MRS) studies as substrates of these enzymes are visible in MR spectra. This chapter reviews results of in vivo MRS studies on transgenic mice with alterations in the expression of the enzymes creatine kinase and guanidinoacetate methyltransferase. The particular metabolic consequences of these enzyme deficiencies in skeletal muscle, brain, heart and liver are addressed. An overview is given of metabolite levels determined by in vivo MRS in skeletal muscle and brain of wild-type and transgenic mice. MRS studies on mice lacking guanidinoacetate methyltransferase have demonstrated metabolic changes comparable to those found in the deficiency of this enzyme in humans, which are (partly) reversible upon creatine feeding. Apart from being a model for a creatine deficiency syndrome, these mice are also of interest to study fundamental aspects of the biological role of creatine. MRS studies on transgenic mice lacking creatine kinase isoenzymes have contributed significantly to the view that the creatine kinase reaction together with other enzymatic steps involved in high-energy phosphate transfer builds a large metabolic energy network, which is highly versatile and can dynamically adapt to genotoxic or physiological challenges.

In vivo magnetic resonance spectroscopy of transgenic mouse models with altered high-energy phosphoryl transfer metabolism

Studies of transgenic mice provide powerful means to investigate the in vivo biological significance of gene products. Mice with an under- or overexpression of enzymes involved in high-energy phosphoryl transfer (approximately P) are particulary attractive for in vivo MR spectroscopy studies as the substrates of these enzymes are metabolites that are visible in MR spectra. This review provides a brief overview of the strategies used for generation and study of genetically altered mice and introduces the reader to some practical aspects of in vivo MRS studies on mice. The major part of the paper reviews results of in vivo MRS studies on transgenic mice with alterations in the expression of enzymes involved in approximately P metabolism, such as creatine kinase, adenylate kinase and guanidinoacetate methyl transferase. The particular metabolic consequences of these enzyme deficiencies in skeletal muscle, brain, heart and liver are addressed. Additionally, the use of approximately P systems as markers of gene expression by MRS, such as after viral transduction of genes, is described. Finally, a compilation of tissue levels of metabolites in skeletal muscle, heart and brain of wild-type and transgenic mice, as determined by in vivo MRS, is given. During the last decade, transgenic MRS studies have contributed significantly to our understanding of the physiological role of phosphotransfer enzymes, and to the view that these enzymes together build a much larger metabolic energy network that is highly versatile and can dynamically adapt to intrinsic genotoxic and extrinsic physiological challenges.

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.