Commentaries on Viewpoint: Muscle atrophy is not always sarcopenia

Sarcopenia: Relocating the Forest among the Trees

As life expectancy continues to rise worldwide, health concerns associated with advanced age are increasingly becoming prominent public health concerns. Among these concerns, nearly 30 years of discussion and research have yielded a wealth of information regarding the pathophysiology, biologic etiology, and clinical implications of age-related declines in skeletal muscle mass and function (i.e., sarcopenia). Recent years have yielded several debates surrounding both the definition and terminology of sarcopenia, yet many questions remain regarding interventions to treat the condition. Among major future challenges in the area will be to design and conduct high-quality clinical trials to ultimately provide new therapeutic strategies for the treatment of sarcopenia.

Biomarkers for physical frailty and sarcopenia: state of the science and future developments

Physical frailty and sarcopenia are two common and largely overlapping geriatric conditions upstream of the disabling cascade. The lack of a unique operational definition for physical frailty and sarcopenia and the complex underlying pathophysiology make the development of biomarkers for these conditions extremely challenging. Indeed, the current definitional ambiguities of physical frailty and sarcopenia, together with their heterogeneous clinical manifestations, impact the accuracy, specificity, and sensitivity of individual biomarkers proposed so far. In this review, the current state of the art in the development of biomarkers for physical frailty and sarcopenia is presented. A novel approach for biomarker identification and validation is also introduced that moves from the 'one fits all' paradigm to a multivariate methodology.

Muscle and bone, two interconnected tissues

As bones are levers for skeletal muscle to exert forces, both are complementary and essential for locomotion and individual autonomy. In the past decades, the idea of a bone-muscle unit has emerged. Numerous studies have confirmed this hypothesis from in utero to aging works. Space flight, bed rest as well as osteoporosis and sarcopenia experimentations have allowed to accumulate considerable evidence. Mechanical loading is a key mechanism linking both tissues with a central promoting role of physical activity. Moreover, the skeletal muscle secretome accounts various molecules that affect bone including insulin-like growth factor-1 (IGF-1), basic fibroblast growth factor (FGF-2), interleukin-6 (IL-6), IL-15, myostatin, osteoglycin (OGN), FAM5C, Tmem119 and osteoactivin. Even though studies on the potential effects of bone on muscle metabolism are sparse, few osteokines have been identified. Prostaglandin E2 (PGE2) and Wnt3a, which are secreted by osteocytes, osteocalcin (OCN) and IGF-1, which are produced by osteoblasts and sclerostin which is secreted by both cell types, might impact skeletal muscle cells. Cartilage and adipose tissue are also likely to participate to this control loop and should not be set aside. Indeed, chondrocytes are known to secrete Dickkopf-1 (DKK-1) and Indian hedgehog (Ihh) and adipocytes produce leptin, adiponectin and IL-6, which potentially modulate bone and muscle metabolisms. The understanding of this system will enable to define new levers to prevent/treat sarcopenia and osteoporosis at the same time. These strategies might include nutritional interventions and physical exercise.

Slight chronic elevation of C-reactive protein is associated with lower aerobic fitness but does not impair meal-induced stimulation of muscle protein metabolism in healthy old men

Ageing impairs the muscle anabolic effect of food intake, which may explain muscle loss and an increased risk of sarcopenia. Ageing is also associated with low grade inflammation (LGI), which has been negatively correlated with muscle mass and strength. In rodents, the muscle anabolic resistance observed during ageing and sarcopenia has been ascribed to the development of the LGI. We aimed to investigate this relationship in humans. We studied protein metabolism and physical fitness in healthy elderly volunteers with slight chronic C-reactive protein. Two groups of healthy elderly volunteers were selected on the presence (or not) of a chronic, slight, elevation of CRP (Control: <1; CRP+: >2 mg l(-1) and <10 mg l(-1) , for 2 months). Body composition, short performance battery test, aerobic fitness and muscle strength were assessed. Whole body and muscle protein metabolism and the splanchnic extraction of amino acids were assessed using [(13) C]leucine and [(2) H]leucine infusion. The anabolic effect of food intake was measured by studying the volunteers both at the post-absorptive and post-prandial states. Slight chronic CRP elevation resulted in neither an alteration of whole body, nor skeletal muscle protein metabolism at both the post-absorptive and the post-prandial states. However, CRP+ presented a reduction of physical fitness, increased abdominal fat mass and post-prandial insulin resistance. Plasma cytokines (interleukin-1, interleukin-6, tumour necrosis factor α) and markers of endothelial inflammation (intercellular adhesion molecule, vascular cell adhesion molecule, selectins) were similar between groups. An isolated elevated CRP in healthy older population does not indicate an impaired skeletal muscle anabolism after food intake, nor an increased risk of skeletal muscle wasting. We propose that a broader picture of LGI (notably with elevated pro-inflammatory cytokines) is required to impact muscle metabolism and mass. However, an isolated chronic CRP elevation could predict a decrease in aerobic fitness and insulin resistance installation in elderly individuals.

Mitochondrial dysfunction and sarcopenia of aging: from signaling pathways to clinical trials

Sarcopenia, the age-related loss of muscle mass and function, imposes a dramatic burden on individuals and society. The development of preventive and therapeutic strategies against sarcopenia is therefore perceived as an urgent need by health professionals and has instigated intensive research on the pathophysiology of this syndrome. The pathogenesis of sarcopenia is multifaceted and encompasses lifestyle habits, systemic factors (e.g., chronic inflammation and hormonal alterations), local environment perturbations (e.g., vascular dysfunction), and intramuscular specific processes. In this scenario, derangements in skeletal myocyte mitochondrial function are recognized as major factors contributing to the age-dependent muscle degeneration. In this review, we summarize prominent findings and controversial issues on the contribution of specific mitochondrial processes - including oxidative stress, quality control mechanisms and apoptotic signaling - on the development of sarcopenia. Extramuscular alterations accompanying the aging process with a potential impact on myocyte mitochondrial function are also discussed. We conclude with presenting methodological and safety considerations for the design of clinical trials targeting mitochondrial dysfunction to treat sarcopenia. Special emphasis is placed on the importance of monitoring the effects of an intervention on muscle mitochondrial function and identifying the optimal target population for the trial. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.