2 Journal of Signal Transduction Physiology Low levels/short duration PGC1-α ↓PPase AMPK ↑Ca++ MAPK IGF-1 GLUT4 NFkB SOD Antioxidant capacity Mitochondriogenesis Mitochondrial function Cell survival Muscle adaptation Inﬂammation Diﬀerentiation Pathophysiology High levels/persistence ↑↑↑Ca++ FoxO Oxidative damage to biomolecules NFκB Mitochondrial dysfunction Apoptosis, autophagy Muscle atrophy Inﬂammation Myopathies Aging and related diseases Diﬀerentiation Muscle repair Regeneration Inhibition Figure 1: Major signaling pathways triggered and/or aﬀected by ROS in skeletal muscle. Low levels of ROS activate speciﬁc key signaling molecules such as PGC-1α, AMPK, and MAPK, which control cellular mechanisms for muscle adaptation (oxidative metabolism, mitochondrial biogenesis, and mitochondrial functionality) as well as antioxidant enzymes that function as backregulators of intracellular ROS levels. Slight ROS accumulation also inhibits PPases and promotes the phosphorylation state of many proteins involved in the muscle signaling responses. Moreover, low levels of ROS play an important role in inducing upregulation of growth factors such as IGF-1, which has beneﬁcial eﬀects in muscle protein balance, supports oxidative metabolism, and contributes to the development of an oxidant-resistant phenotype, therefore preventing oxidative damage and chronic diseases. Thus, low levels of ROS elicit positive eﬀects on physiological muscle responses. By contrast high levels of ROS cause functional oxidative damages of proteins, lipids, nucleic acids and cell components, induce a signiﬁcant rise of intracellular [Ca2+], and promote signaling cascades for apoptosis or autophagy via NF-κB or FoxO paths. For these reasons high ROS levels are reputed to act as etiological, or at least exacerbating factors in muscle atrophy, sarcopenia, wasting, and chronic-/aging- related muscle diseases and myopathies. Depending on their level/persistence, ROS may also turn the same process from “physiologic” into “pathologic”, as in the case of inﬂammation. homeostasis and adaptation: here we will illustrate some of the signaling pathways triggered/aﬀected by ROS in muscle tissue and their physiopathological implications (see Figure 1 for a visual summary). 2. Generation of ROS in Skeletal Muscle Cells Mitochondria are commonly considered as the predominant source of ROS in skeletal muscle cells [6, 7]. Increased mitochondrial ROS generation occurs during various and diﬀerent situations, such as in the course of intense con- tractile activity [8] or in response to cytokines such as tumor necrosis factor-α (TNF-α) [9]. Early reports assumed that 2–5% of the total oxygen consumed by mitochondria may undergo one electron reduction with the generation of superoxide [10, 11]. More recent studies indicated that complexes I and III of the electron transport chain are the main sites of mitochondrial superoxide production [12, 13]. During exercise, it is assumed that the increased ROS generation in the course of contractile activity is due to the high oxygen consumption that takes place during increased mitochondrial activity. Indeed superoxide generation in skeletal muscle increases to about a 50- or 100-fold during aerobic contractions [14, 15]. However, recent evidence demonstrates that mitochon- dria may not be the prevalent source of ROS during exercise [8], and future studies are required to better elucidate the mitochondrial role in contraction-induced production of ROS in skeletal muscle. In 2002 St. Pierre and colleagues [16] reexamined the rate of mitochondrial ROS production and concluded that the total fraction of oxygen converted into superoxide was equal to 0.15%; this value is signiﬁcantly lower than that (2–5%) estimated by other authors (see for example [17]). This lower rate of superoxide production takes account of the uncoupling proteins role (speciﬁcally UCP3 in skeletal muscle) as regulators of mitochondrial ROS production [18, 19] acting to prevent oxidative damage to mitochondria. In addition, growing evidence highlights that mitochondria produce more ROS during the basal state 4 of respiration as compared to state 3 (maximal ADP-stimulated respiration) [20–23]. Thus, since skeletal muscle mitochondria, during aerobic contractile activity, are predominantly in state 3, this limits their capacity of generating ROS during contractions [21–23].