Rachel McCormick

Age-related changes in miR-143-3p:Igfbp5 interactions affect muscle regeneration.

A common characteristic of aging is defective regeneration of skeletal muscle. The molecular pathways underlying age‐related decline in muscle regenerative potential remain elusive. microRNAs are novel gene regulators controlling development and homeostasis and the regeneration of most tissues, including skeletal muscle. Here, we use satellite cells and primary myoblasts from mice and humans and an in vitro regeneration model, to show that disrupted expression of microRNA‐143‐3p and its target gene, Igfbp5, plays an important role in muscle regeneration in vitro. We identified miR‐143 as a regulator of the insulin growth factor‐binding protein 5 (Igfbp5) in primary myoblasts and show that the expression of miR‐143 and its target gene is disrupted in satellite cells from old mice. Moreover, we show that downregulation of miR‐143 during aging may act as a compensatory mechanism aiming at improving myogenesis efficiency; however, concomitant upregulation of miR‐143 target gene, Igfbp5, is associated with increased cell senescence, thus affecting myogenesis. Our data demonstrate that dysregulation of miR‐143‐3p:Igfbp5 interactions in satellite cells with age may be responsible for age‐related changes in satellite cell function.

Mechanisms of skeletal muscle ageing; avenues for therapeutic intervention

Age-related loss of muscle mass and function, termed sarcopenia, is a catastrophic process, which impacts severely on quality of life of older people. The mechanisms underlying sarcopenia are unclear and the development of optimal therapeutic interventions remains elusive. Impaired regenerative capacity, attenuated ability to respond to stress, elevated reactive oxygen species production and low-grade systemic inflammation are all key contributors to sarcopenia. Pharmacological intervention using compounds such as 17AAG, SS-31 and Bimagrumab or naturally occurring polyphenols to target specific pathways show potential benefit to combat sarcopenia although further research is required, particularly to identify the mechanisms by which muscle fibres are completely lost with increasing age.

Manipulation of environmental oxygen modifies reactive oxygen and nitrogen species generation during myogenesis

Regulated changes in reactive oxygen and nitrogen species (RONS) activities are important in maintaining the normal sequence and development of myogenesis. Both excessive formation and reduction in RONS have been shown to affect muscle differentiation in a negative way. Cultured cells are typically grown in 20% O2 but this is not an appropriate physiological concentration for a number of cell types, including skeletal muscle. The aim was to examine the generation of RONS in cultured skeletal muscle cells under a physiological oxygen concentration condition (6% O2) and determine the effect on muscle myogenesis. Primary mouse satellite cells were grown in 20% or 6% O2 environments and RONS activity was measured at different stages of myogenesis by real-time fluorescent microscopy using fluorescent probes with different specificities i.e. dihydroethidium (DHE), 4-amino-5-methylamino-2′,7′-difluorofluorescein diacetate (DAF-FM DA) and 5-(and-6)-chloromethyl-2′,7′ -dichlorodihydrofluorescein diacetate (CM-DCFH-DA). Data demonstrate that satellite cell proliferation increased when cells were grown in 6% O2 compared with 20% O2. Myoblasts grown in 20% O2 showed an increase in DCF fluorescence and DHE oxidation compared with myoblasts grown at 6% O2. Myotubes grown in 20% O2 also showed an increase in DCF and DAF-FM fluorescence and DHE oxidation compared with myotubes grown in 6% O2. The catalase and MnSOD contents were also increased in myoblasts and myotubes that were maintained in 20% O2 compared with myoblasts and myotubes grown in 6% O2. These data indicate that intracellular RONS activities in myoblasts and myotubes at rest are influenced by changes in environmental oxygen concentration and that the increased ROS may influence myogenesis in a negative manner.

MicroRNA Dysregulation in Aging and Pathologies of the Skeletal Muscle

Skeletal muscle is one of the biggest organs of the body with important mechanistic and metabolic functions. Muscle homeostasis is controlled by environmental, genetic, and epigenetic factors. Indeed, MiRNAs, small noncoding RNAs robust regulators of gene expression, have and have been shown to regulate muscle homeostasis on several levels: through controlling myogenesis, muscle growth (hypertrophy) and atrophy, as well as interactions of muscle with other tissues. Given the large number of MiRNA target genes and the important role of MiRNAs in most physiological processes and various diseases, MiRNAs may have an enormous potential as therapeutic targets against numerous disorders, including pathologies of muscle. The purpose of this review is to present the current knowledge of the role of MiRNAs in skeletal muscle homeostasis and pathologies and the potential of MiRNAs as therapeutics for skeletal muscle wasting, with particular focus on the age- and disease-related loss of muscle mass and function.

Age-related changes in skeletal muscle: changes to life-style as a therapy

As we age, there is an age-related loss in skeletal muscle mass and strength, known as sarcopenia. Sarcopenia results in a decrease in mobility and independence, as well as an increase in the risk of other morbidities and mortality. Sarcopenia is therefore a major socio-economical problem. The mechanisms behind sarcopenia are unclear and it is likely that it is a multifactorial condition with changes in numerous important mechanisms all contributing to the structural and functional deterioration. Here, we review the major proposed changes which occur in skeletal muscle during ageing and highlight evidence for changes in physical activity and nutrition as therapeutic approaches to combat age-related skeletal muscle wasting.

microRNA-SIRT-1 interactions: key regulators of adult skeletal muscle homeostasis?: Perspectives

Skeletal muscle homeostasis is a balance between muscle hypertrophy, atrophy and regeneration. During ageing and cachexia, this balance is disrupted. Age-related loss of muscle mass and function is associated with frailty and an increase in co-mortalities and co-morbidities. Age-related muscle wasting is of particular importance in our ageing society, with significant impact on the quality of life of older people and healthcare costs. As muscle hypertrophy is often associated with increased muscle force, exercise training is used to stimulate muscle hypertrophy and strength, and therefore enhance performance and to improve quality of life. However, exercise interventions designed to improve muscle function in older subjects aim to improve the function of the residual muscle fibres but do not address the mechanistic changes occurring during age-related muscle loss. Sirtuins are a conserved family of NAD+-dependent deacetylases involved in the control of muscle homeostasis and the ageing process. A member of this family, SIRT-1, has been reported to regulate mitochondrial biogenesis, atrophy and myogenesis in skeletal muscle. SIRT-1 resides mostly in the nucleus, where it acts as a functional transcriptional repressor through histone deacetylation but can also directly regulate target proteins by deacetylation. Interest in SIRT-1 begun a decade ago, when activators of SIRT-1 were shown to extend the lifespan (Gomes et al. 2013). Since then, the beneficial role of SIRT-1 in regulating skeletal muscle mass and satellite cell metabolism, proliferation and differentiation have been demonstrated by many authors. Moreover, SIRT-1 has been proposed as part of the mechanisms associated with loss of muscle mass and function during ageing (Donghoon & Goldberg, 2013; Gomes et al. 2013). A paper by Koltai et al. in this issue of The Journal of Physiology adds a new layer of information to the potential involvement of SIRT-1 in regulating muscle homeostasis by controlling muscle hypertrophy (Koltai et al. 2017). The authors demonstrate that overload-induced hypertrophy of the plantaris muscle is accompanied by an increase in SIRT-1 expression and SIRT-1 activity. SIRT-1-mediated regulation of muscle hypertrophy/atrophy has been previously suggested (Gomes et al. 2013); however, the downstream mechanisms of SIRT-1 regulation of muscle homeostasis are only partially understood. Koltai et al. propose a direct role of SIRT-1 in compensatory hypertrophy of skeletal muscle. The authors show that the changes in SIRT-1 levels and activity during muscle hypertrophy are accompanied by an increase in AKT levels, which regulates protein synthesis, and a decrease in FOXO1, which regulates protein degradation. The authors also suggest that SIRT-1 may contribute to muscle hypertrophy via regulating satellite cell function, which is in agreement with data showing regulation of satellite cell metabolism via SIRT-1-mediated control of autophagy (Tang & Rando, 2014). Furthermore, changes in SIRT-1 expression during plantaris hypertrophy were associated with an increase in target protein endothelial nitric oxide synthase, but an overall decrease in reactive oxygen species (ROS), suggesting a signalling role for low levels of ROS in regulating the balance between muscle hypertrophy and atrophy. The authors further investigate epigenetic mechanisms that may regulate SIRT-1 expression in muscle during hypertrophy, focusing on small non-coding RNAs, microRNAs: emerging potent regulators of muscle homeostasis. The authors demonstrate that changes in SIRT-1 expression are concomitant with downregulation of miR-133 and miR-1 expression. This is in agreement with previously suggested roles of miR-133a and miR-1 in the inhibition of muscle hypertrophy. Changes in other microRNAs: miR-23a, miR-34a, miR-125b and miR-214, previously reported to regulate muscle atrophy, hypertrophy and/or regeneration, were also measured. The expression of SIRT-1 has been previously shown to be controlled by microRNAs (Soriano et al. 2016). Our group has shown that changes in miR-181a levels in the muscle of old mice is associated with changes in SIRT-1 expression, and indeed that SIRT-1 is a direct miR-181a target gene (Soriano et al. 2016). Moreover, overexpression of miR-181a in C2C12 myotubes led to decreased myotube size, a phenotype probably regulated by changes in SIRT-1 expression (Soriano et al. 2016). Interestingly, the levels of miR-181 are downregulated in muscle from old mice, whereas SIRT-1 expression is upregulated, suggesting a compensatory mechanism occurring in muscle during ageing. This is consistent with published data suggesting a compensatory, rather than mechanistic, role of microRNAs in muscle wasting during ageing and disease. It has also been widely reported that the activity of SIRT-1 is regulated by modifications induced by ROS. However, it is likely that changes in SIRT-1 expression during muscle hypertrophy are preceded by changes in the expression of microRNAs, upstream regulators of gene expression. As a single microRNA can regulate the expression of multiple genes and SIRT-1 has been reported to be regulated by multiple microRNAs, changes in microRNA expression could be more influential than the changes in SIRT-1 alone. In summary, the study by Koltai et al. opens new avenues for functional studies of molecular mechanisms regulating skeletal muscle hypertrophy, atrophy and regeneration – balance between which is disrupted during ageing. It remains to be established to what extent SIRT-1 regulates these processes, whether microRNAs, potentially through regulation of SIRT-1 expression, play a key role in controlling muscle mass and function in adulthood and ageing, or whether microRNA-regulated mechanisms are mechanistic or rather compensatory. These functional studies are necessary for design of novel, effective interventions and/or therapeutics against loss of muscle mass and function.