Excess Intramyocellular Lipid Does Not Affect Muscle Fiber Biophysical Properties in Mice or People with Metabolically Abnormal Obesity.

Observational studies show correlations between intramyocellular lipid (IMCL) content and muscle strength and contractile function in people with "metabolically abnormal" obesity. However, a clear physiologic mechanism for this association is lacking and causation is debated. We combined immunofluorescent confocal imaging with force measurements on permeabilized muscle fibers from metabolically normal and metabolically abnormal mice and metabolically normal (defined as normal fasting plasma glucose and glucose tolerance) and metabolically abnormal (defined as pre-diabetes and type 2 diabetes) people with overweight/obesity to evaluate relationships among myocellular lipid droplet characteristics (droplet size and density) and biophysical (active contractile and passive viscoelastic) properties. The fiber type specificity of lipid droplet parameters varied between metabolically abnormal and normal mice and among metabolically normal and metabolically abnormal people. However, despite considerable quantities of IMCL in the metabolically abnormal groups, there were no significant differences in peak active tension or passive viscoelasticity between the metabolically abnormal groups and the control group in mice or people. Additionally, there were no significant relationships among IMCL parameters and biophysical variables. Thus, we conclude that IMCL accumulation per se does not impact muscle fiber biophysical properties or physically impede contraction.

Site-1 protease inhibits mitochondrial respiration by controlling the TGF-ß target gene Mss51.

The mitochondrial response to changes in cellular energy demand is necessary for cellular adaptation and organ function. Many genes are essential in orchestrating this response, including the transforming growth factor (TGF)-ß1 target gene Mss51, an inhibitor of skeletal muscle mitochondrial respiration. Although Mss51 is implicated in the pathophysiology of obesity and musculoskeletal disease, how Mss51 is regulated is not entirely understood. Site-1 protease (S1P) is a key activator of several transcription factors required for cellular adaptation. However, the role of S1P in muscle is unknown. Here, we identify S1P as a negative regulator of muscle mass and mitochondrial respiration. S1P disruption in mouse skeletal muscle reduces Mss51 expression and increases muscle mass and mitochondrial respiration. The effects of S1P deficiency on mitochondrial activity are counteracted by overexpressing Mss51, suggesting that one way S1P inhibits respiration is by regulating Mss51. These discoveries expand our understanding of TGF-ß signaling and S1P function.

Tenotomy-induced muscle atrophy is sex-specific and independent of NFκB.

The nuclear factor-κB (NFκB) pathway is a major thoroughfare for skeletal muscle atrophy and is driven by diverse stimuli. Targeted inhibition of NFκB through its canonical mediator IKKß effectively mitigates loss of muscle mass across many conditions, from denervation to unloading to cancer. In this study, we used gain- and loss-of-function mouse models to examine the role of NFκB in muscle atrophy following rotator cuff tenotomy - a model of chronic rotator cuff tear. IKKß was knocked down or constitutively activated in muscle-specific inducible transgenic mice to elicit a twofold gain or loss of NFκB signaling. Surprisingly, neither knockdown of IKKß nor overexpression of caIKKß significantly altered the loss of muscle mass following tenotomy. This finding was consistent across measures of morphological adaptation (fiber cross-sectional area, fiber length, fiber number), tissue pathology (fibrosis and fatty infiltration), and intracellular signaling (ubiquitin-proteasome, autophagy). Intriguingly, late-stage tenotomy-induced atrophy was exacerbated in male mice compared with female mice. This sex specificity was driven by ongoing decreases in fiber cross-sectional area, which paralleled the accumulation of large autophagic vesicles in male, but not female muscle. These findings suggest that tenotomy-induced atrophy is not dependent on NFκB and instead may be regulated by autophagy in a sex-specific manner.

Muscle atrophy ­ the gradual loss of muscle mass ­ follows injuries to our muscles, tendons, or joints. During atrophy, muscles shrink and become weaker, which can interfere with everyday activities and, ultimately, decrease quality of life. Rotator cuff tears are a common example of such injuries. A rotator cuff is group of four muscles that come together as tendons to form a cuff that normally stabilises our shoulders and allows us to lift and move our arms over our heads. Rotator cuff tears can result from an injury or may be caused by ageing-related wear and tear of the tendon. A signalling protein, called NFκB, is thought to be involved in muscle atrophy. When the NFκB signal is switched on, it interacts with genes that are thought to speed up the loss of muscle mass. However, NFκB's precise role in atrophy and recovery after muscle injury is still poorly understood, particularly following injuries where a tendon is cut or torn. Meyer et al. therefore set out to determine whether or not NFκB played a role in the muscle atrophy following rotator cuff tears. Meyer et al. used genetically engineered mice in which NFκB's signal could be turned off at the time of rotator cuff injury, and specifically in muscle cells (but not other parts of the body). The experiments revealed that stopping NFκß signalling in these mice did not reduce muscle atrophy after a rotator cuff injury: the levels of atrophy, muscle performance, and muscle composition were the same regardless of whether the NFκß signal was active. The sex of the mice did, however, affect muscle atrophy, specifically the way in which they lost muscle mass. In male mice, the size of muscle cells decreased, while in female mice, the number of muscle cells decreased. Muscle cells in male mice (but not in females) also accumulated abnormally high amounts of protein, which is an indication of a mechanism of muscle breakdown called autophagy. These results shed new light on the way that we lose muscle mass after injury, and how that could vary depending on the individual. Meyer et al. hope that this study will help guide the development of new, more effective treatments for muscle atrophy, and ultimately contribute to therapies tailored to the characteristics of the patient and the type of injury.

A unique sarcopenic progression in the mouse rotator cuff.

BACKGROUND: In response to chronic injury, the muscles of the rotator cuff (RC) experience a unique degeneration characterized by extensive fatty infiltration and loss of contractile function. Human studies suggest this degeneration is also a feature of RC sarcopenia and may precede RC injury. In this study, we investigated whether RC muscles exhibit a similar unique sarcopenia in the mouse. METHODS: Male and female mice were subdivided into four age groups: 3, 9, 18, and 24 months. The supraspinatus (SS) and infraspinatus muscles of the RC and the tibialis anterior (TA) muscle of the hindlimb were assessed. Muscle mass, contractile function, fibre cross-sectional areas and numbers, fatty infiltration, and fibrosis were assessed at each time point. Targeted transcriptional analyses were performed to assess the role of metabolic and inflammatory derangement in the pathology. RESULTS: The 24-month-aged female mice exhibited decreased mass (25% lower than at 9 and 18 months, P < 0.01) in all muscles tested. However, only RC muscles also exhibited decreased contractile tension at this time point (20% lower than at 18 months, P < 0.005). Similarly, only female RC muscles exhibited increased fatty infiltration at 24 months (20% higher than 9 months, P < 0.05) and had elevated transcriptional markers of adipogenesis (2.4-fold higher Pparg and 3.8-fold higher Adipoq expression compared with 9 months, P < 0.001). Unbiased metabolic transcriptional profiling identified up-regulation of the antigen presentation (Z scores of 2.3 and 1.9 for SS and TA, respectively) and cytokine and chemokine signalling (Z scores of 3.1 and 2.4 for SS and TA, respectively) pathways in 24 month female muscle compared with 9. Further transcriptional analysis supported increased expression of pro-adipogenic inflammatory signals (6.3-fold increase in Il6 and 5.0-fold increase in Anxa2, P < 0.01) and increased presence of fibro-adipogenic progenitors (2.5-fold) in the 24-month-aged female RC compared with 9 months that together exacerbate fatty infiltration. CONCLUSIONS: These data indicate that female mice replicate the unique sarcopenic pathology in the ageing human RC. Furthermore, they suggest that the exacerbated fatty infiltration is due to an interaction between higher resident fibro-adipogenic progenitor numbers and an elevated systemic inflammation in aged female mice. We conclude that female mouse RC muscle is a novel system to study both human RC degeneration and the signals that regulate sarcopenic fatty infiltration in general, which is prevalent in humans but largely absent from the rodent hindlimb.

Infiltration of intramuscular adipose tissue impairs skeletal muscle contraction.

KEY POINTS: Muscle infiltration with adipose tissue (IMAT) is common and associated with loss of skeletal muscle strength and physical function across a diverse set of pathologies. Whether the association between IMAT and muscle weakness is causative or simply correlative remains an open question that needs to be addressed to effectively guide muscle strengthening interventions in people with increased IMAT. In the present studies, we demonstrate that IMAT deposition causes decreased muscle strength using mouse models. These findings indicate IMAT is a novel therapeutic target for muscle dysfunction. ABSTRACT: Intramuscular adipose tissue (IMAT) is associated with deficits in strength and physical function across a wide array of conditions, from injury to ageing to metabolic disease. Due to the diverse aetiologies of the primary disorders involving IMAT and the strength of the associations, it has long been proposed that IMAT directly contributes to this muscle dysfunction. However, infiltration of IMAT and reduced strength could both be driven by muscle disuse, injury and systemic disease, making IMAT simply an 'innocent bystander.' Here, we utilize novel mouse models to evaluate the direct effect of IMAT on muscle contraction. First, we utilize intramuscular glycerol injection in wild-type mice to evaluate IMAT in the absence of systemic disease. In this model we find that, in isolation from the neuromuscular and circulatory systems, there remains a muscle-intrinsic association between increased IMAT volume and decreased contractile tension (r2  > 0.5, P < 0.01) that cannot be explained by reduction in contractile material. Second, we utilize a lipodystrophic mouse model which cannot generate adipocytes to 'rescue' the deficits. We demonstrate that without IMAT infiltration, glycerol treatment does not reduce contractile force (P > 0.8). Taken together, this indicates that IMAT is not an inert feature of muscle pathology but rather has a direct impact on muscle contraction. This finding suggests that novel strategies targeting IMAT may improve muscle strength and function in a number of populations.