activated T cells transcription factor family. The fusion of myoblasts with preexisting muscle fibers concomitantly increases myonuclear number, protein synthesis, and cytoplasmic volume, ultimately resulting in muscle fiber hypertrophy (Table 27-1). Despite the increase in myofiber size, the myonuclear domain, defined as the cytoplasmic myofiber volume per myonucleus, remains relatively constant. Table 27-1 summarizes current research on the changes in myonuclear domain size in response to imposed models of skeletal muscle hypertrophy.

Conversely, during muscle disuse and injury/disease, skeletal muscle may undergo atrophy, mediated by a decrease in protein synthesis and a concomitant increase in protein degradation through the activation of cell death pathways. Because skeletal muscle is multinucleated, the end result of apoptotic processes within this tissue is different when compared with that of mononucleated cells. The major difference is that apoptosis within skeletal muscle results in individual myonuclear loss, rather than degradation of the entire myofiber (Figure 27-2). However, the loss of individual myonuclei does not proceed without subsequently altering the phenotype of the apoptotic myofiber. When apoptosis strikes a myonucleus, the muscle fiber undergoes atrophy, which can result in an increase, maintenance, or reduction in the myonuclear domain size, regardless of fiber type, as shown in Table 27-2. According to the

myonuclear domain theory, each myonucleus controls a certain area of cytoplasm within a myofiber. Loss of myonuclei causes an associated decrease in overall cell volume so that the remaining myonuclei can continue to maintain the fiber. Along with myonuclear loss, there is a shift toward a fast-twitch fiber type as a result of an increase in the expression of fast and a decrease in the expression of slow myosin heavy chain (MHC) isoforms during atrophy. A variety of models that induce muscle atrophy, such as microgravity, spinal isolation, hind-limb unloading, and aging, have been shown to be associated with the loss of myonuclear number. However, recent work has challenged the theory of myonuclear loss with atrophy, rendering the debate controversial.

Two major conduits of myonuclear apoptosis have been elucidated during skeletal muscle atrophy: extrinsic pathway and the intrinsic pathway. The extrinsic pathway of apoptosis is regulated by the death receptor family of proteins. Ligand binding to the death receptor on the plasma membrane initiates a cascade of caspases, resulting in cell death. The intrinsic apoptotic pathway is mediated by the mitochondria.

2. MITOCHONDRIALLY MEDIATED APOPTOSIS IN MUSCLE

Skeletal muscle has a high requirement for energy during contractile activity and consequently relies heavily on oxidative phosphorylation within the mitochondria

to meet the functional demands placed on it. Mitochondria function to donate electrons to molecular oxygen to create energy in the form of adenosine triphosphate (ATP) so that muscular contraction can occur. Along with providing the majority of energy needed to fulfill many essential cellular processes, these vital organelles are also intimately involved in the regulation of apoptosis. As outlined in detail elsewhere in this volume, mitochondria are prime transducers of apoptosis for several reasons. First, mitochondria contain a number of proapoptotic proteins such as apoptosis inducing factor (AIF), endonuclease G (Endo G), and cytochrome c, which can be released on receipt of an appropriate signal. Furthermore, mitochondria are the main producers of reactive oxygen species (ROS). ROS are formed when an electron from complex I or III of the ETC is inappropriately donated to oxygen. This results in the formation of a superoxide anion (O2−), which is swiftly converted into hydrogen peroxide (H2O2). During resting conditions, 1% to 2% of the total oxygen consumed is

converted into ROS, and this percentage can increase during muscle disuse, injury, or disease. If left unquenched, ROS can induce damage and initiate apoptosis.

2.1. Skeletal muscle apoptotic susceptibility

Apoptosis in skeletal muscle is unique compared with the majority of other tissues for several reasons. As alluded to earlier, apoptosis in skeletal muscle leads to the degradation of individual myonuclei rather than the entire fiber. This may produce changes in the myonuclear domain, ultimately resulting in muscle atrophy (Table 27-2). Second, muscle tissue is composed of a variety of specialized fiber types that accommodate distinctly different levels of mitochondrial content and functional activity. Slow-twitch (type I) muscle fibers contain more mitochondria and more myonuclei as compared with fast-twitch (type II) fibers. Thus the extent of the apoptotic susceptibility of individual

Loss of Myonuclei +

Satellite cells

Myonuclear

Domain

Myofiber

Satellite cell activation, proliferation

Fusion of satellite cells, myonuclear addition and resultant fiber hypertrophy

Myonuclear Domain is constant

Figure 27-2. Myonuclear domains during muscle hypertrophy and atrophy. Myofibers are large, multinucleated cells surrounded by small, mononucleated satellite cells. As a result of a stimulus leading to muscle hypertrophy, satellite cells proliferate, fuse, and donate myonuclei to existing myofibers. This results in an increase in myonuclear number and cytoplasmic volume, producing a larger myofiber, while leaving the myonuclear domain size relatively constant (see Table 27-1). Conversely, conditions leading to muscle atrophy (e.g., muscle denervation/disuse) are accompanied by a reduction in satellite cell number. Whether a decline in myonuclear number occurs is controversial (see Table 27-2, Bruusgaard et al.), and it likely does not occur to the same extent as the reduction in cytoplasmic volume. This manifests as a decrease in myonuclear domain size and consequently leads to a smaller myofiber cross-sectional area (see Table 27-2). See Color Plate 32.

respectively, it is likely that the IMF subfraction is the more important contributor to skeletal muscle apoptotic signaling.

3. EVIDENCE OF APOPTOSIS DURING

MUSCLE DISUSE

Investigations of how prolonged disuse affects skeletal muscle began in the 1950s and are currently still a research area that garners significant attention. Results from these studies have shown that chronic muscle disuse results in a reduction in skeletal muscle mass. More recent studies have documented decreases in skeletal muscle oxidative capacity and an increase in cellular susceptibility to apoptosis. Furthermore, chronic muscle disuse has been shown to have an immediate impact on SS mitochondrial content, whereas longer periods of disuse are required to affect IMF mitochondrial content. Currently, there are a number of different models that represent reduced contractile activity, such as space flight, limb immobilization, denervation, and bed rest. Although many of the signaling pathways associated with apoptosis have been well characterized, there still remain many questions regarding the exact contribution of apoptosis to disuse-induced muscle atrophy.

3.1. Mitochondrially mediated apoptosis during chronic muscle disuse

muscle fibers varies and is specific to the type of cell death signal evoked, the duration of the signal, and the organism under investigation. Third, muscle fibers contain two distinct mitochondrial subfractions, SS and IMF mitochondria. These two mitochondrial subpopulations appear to regulate apoptosis differently. SS mitochondria produce more ROS and have a higher Bax/Bcl-2 ratio compared with IMF mitochondria. In contrast, IMF mitochondria have a greater rate of cytochrome c and AIF release. However, because the SS and IMF mitochondrial subfractions make up approximately 20% and 80% of the total mitochondrial content within a muscle cell,

Chronic muscle disuse leads to a number of adaptations within skeletal muscle that increase the susceptibility of mitochondria to

apoptosis. In particular, prolonged muscle disuse leads to a reduction in the expression of cytochrome c mRNA in both slowand fast-twitch muscles. This reduction exceeds the rate of overall muscle protein loss, suggesting that inactivity specifically targets mitochondrial proteins. In conjunction with a decrease in mitochondrial protein expression, a number of oxidative enzymes such as succinate dehydrogenase, citrate synthase, cytochrome c oxidase, and malate dehydrogenase are all decreased with reductions in contractile