Skeletal muscle fiber types in man : With special reference to anabolic signaling and mitochondrial bioenergetics

Abstract: Human skeletal muscle consists of a mixture of slow-twitch (type I) and fast-twitch (type II) fibers. The type I fibers are endurance-oriented, with a metabolic system and infrastructure that supports aerobic metabolism. This entails a well-developed capillary grid and a mitochondrial network proportioned to the number of contractile units within the fiber. These fibers generally have slower and less forceful contraction mechanics and more limited muscle growth as a resource-efficient metabolic energy system is prioritized over increasing the number of contractile units. By contrast, type II fibers prioritize contractile capabilities and force generation at the cost of resource efficiency. These fibers have a substantially lower mitochondrial volume but prioritize structures and organelles that benefit muscle contraction instead. It is well known that resistance exercise combined with dietary protein intake stimulates the growth of contractile proteins leading to an increased muscle mass over time. Muscle mass accumulation is primarily driven by the amplification of muscle protein synthesis, which in turn is largely governed by the mTORC1 signaling pathway within the muscle cell. Little is known about how mTORC1 signaling regulates growth in the different fiber types. Furthermore, it is unknown whether blunted anabolic signaling in type II fibers of the elderly may explain why losses of muscle mass occur primarily in these fibers with advancing age.Endurance exercise, on the other hand, primarily stimulates a prioritization to synthesize new mitochondria to support the high demand for sustainable aerobic energy output. However, it remains to be determined if mitochondria created within type I and type II fibers are equal, or whether they have adapted to their respective milieu in any way. Therefore, the aim of the current thesis was to investigate how the mTORC1 pathway in type I and type II fibers responds to resistance exercise and nutritional stimuli in the form of essential amino acids (EAA), and to determine if this response is influenced by age. Fiber type-specific mitochondrial populations, including their respiratory capacity, were also investigated. To facilitate these investigations, a new and improved method for muscle fiber type identification was developed.In paper I, the phosphorylation of mTORC1 in response to resistance exercise and EAA intake was examined in 684 individual muscle fibers. Unsurprisingly, a significant increase in mTORC1 signaling was seen following the combination of resistance exercise and EAA intake, whereas the rise following resistance exercise alone was more modest. However, no evidence of a discrete response in the different fiber types was found. In paper II, a new method was developed to facilitate the work surrounding fiber type-specific muscle physiology by limiting the extreme time requirements of fiber type identification of large sample sets of muscle fibers. The novel method, which was named THRIFTY, allows an experienced technician to classify over 800 fibers in under 11h.Paper III utilizes the high throughput of the THRIFTY method described in paper II to create the most extensive study to date on individually dissected muscle fibers with 27 602 included fibers. Here, the aim was to investigate whether the fiber type-specific muscle atrophy of the type II fibers in aging could be explained by an onset of anabolic resistance in these fibers. For this investigation, ten young and ten elderly men were recruited to perform a unilateral resistance exercise session followed by ingestion of EAA. This paper showed a slightly elevated mTORC1 signaling response in type I fibers. However, there were no signs of blunted mTORC1 signaling in the elderly. In paper IV, the high speed of the THRIFTY method was utilized to analyze the mitochondrial respiratory function of permeabilized type I and type II muscle fibers. In addition, the intrinsic protein expression of mitochondria in the type I and type II muscle fibers was analyzed. As expected, a higher volume of mitochondria and a greater respiratory rate in the type I fibers were found. However, on a per mitochondria basis, a higher maximal respiratory rate was observed in type II fibers together with increased levels of proteins in the electron transport chain. Likewise, proteins regulating mitochondrial fission and fusion were more highly expressed in the type II fiber mitochondria, which may be a compensatory mechanism for the low volume. In conclusion, both fiber types show robust increases in mTORC1 signaling in response to exercise and EAA ingestion. The results indicate that the response is slightly stronger in the type I fibers, which is contrary to what was predicted. Moreover, the highly specific type II fiber atrophy seen with aging cannot be explained by a blunted anabolic response in these fibers. Surprisingly, the mitochondria of type II fibers possess a higher respiratory capacity. However, this discrepancy is concealed by the vast difference in mitochondrial volume favoring type I fibers, ultimately leading to an overall greater respiratory rate in the type I fibers.