Mitochondrial ultrastructural adaptations in fast muscles of mice lacking IL15RA

Abstract

The pro-inflammatory cytokine interleukin-15 (IL15) and its receptor α (IL15RA) participate in the regulation of musculoskeletal function and metabolism. Deletion of the Il15ra gene in mice increases spontaneous activity, improves fatigue resistance in the glycolytic extensor digitorum longus (EDL) and protects from diet-induced obesity. In humans, IL15RA single-nucleotide polymorphisms (SNPs) have been linked to muscle strength, metabolism and performance in elite endurance athletes. Taken together, these features suggest a possible role for IL15RA in muscle mitochondrial structure and function. Here, we have investigated the consequences of loss of IL15RA on skeletal muscle fiber-type properties and mitochondrial ultrastructure. Immunostaining of the EDL for myosin heavy chain (MyHC) isoforms revealed no significant changes in fiber type. Electron microscopy (EM) analysis of the EDL indicated an overall higher mitochondria content, and increased cristae density in subsarcolemmal and A-band mitochondrial subpopulations. The higher cristae density in Il15ra −/− mitochondria was associated with higher OPA1 and cardiolipin levels. Overall, these data extend our understanding of the role of IL15RA signaling in muscle oxidative metabolism and adaptation to exercise.

Effect of Interleukin-15 Receptor Alpha Ablation on the Metabolic Responses to Moderate Exercise Simulated by in vivo Isometric Muscle Contractions

“…We have proposed that the ability to use IMTG stores efficiently enhances Il15ra –/– recovery capacity after exercise. Supporting our hypothesis, we found that lipases, fatty acid binding proteins and the AMPK/ACC pathway were significantly activated in Il15ra –/– fast muscles (Loro et al, 2015, 2018). Little is known, however, about the impact of these changes on the metabolome of resting and exercised muscles.…”

“…(B) Diagram summarizing the effects of lack of IL15RA on muscle fatty acid metabolism. Yellow nodes indicate enzymes significantly upregulated/activated in Il15ra –/– muscle, according to previous publications by our group (Loro et al, 2015, 2018). Thick arrows represent metabolic reactions that are enhanced in Il15ra –/– muscle.…”

“…The effects of lack of IL15RA are more evident in fast muscles, where it promotes fatigue-resistance (Pistilli et al, 2011; Loro et al, 2015), mitochondrial biogenesis and cristae complexity (Loro et al, 2018). We, therefore, focused on the metabolome of the EDL, a fast muscle which is highly recruited during ambulatory and exercise activity (Slawinska and Kasicki, 2002), and suitable for ex vivo evaluation with routine muscle contractility assessment protocols.…”

“…IL15 and IL15RA have a well-recognized immunological role in mediating the activation of T and natural killer cells, and constitutive ablation of either of them leads to blunted immune response to pathogens (Lodolce et al, 1998; Schluns et al, 2005). However, consistent with their widespread tissue distribution, IL15/IL15RA have also been shown to participate in the regulation of energy processes of a variety of tissues (He et al, 2010), including brain (Wu et al, 2010; Nguyen et al, 2017), bone (Djaafar et al, 2010; Loro et al, 2017), adipose tissue (Sun and Liu, 2015; Lacraz et al, 2016), and muscle (Pistilli and Quinn, 2013; Loro et al, 2015, 2018; O’Connell et al, 2015). We and others have shown that ablation of IL15RA in mice increases spontaneous activity, exercise capacity and protects against diet-induced obesity (He et al, 2010; Pistilli et al, 2011; Loro et al, 2015).…”

Effects of Aging and Caloric Restriction on Fiber Type Composition, Mitochondrial Morphology and Dynamics in Rat Oxidative and Glycolytic Muscles

“…Individual SS and IMF mitochondria from 4 A-AL, 4 O-AL, and 4 O-CR rats were manually traced in longitudinal and transverse orientations using ImageJ software (NIH) (https://imagej.nih.gov/ij/) and to quantify the following morphological and shape descriptors: area (μm 2 ), perimeter (μm), circularity 4π × (area/perimeter 2 ), aspect ratio ((major axis)/(minor axis)): the aspect ratio being a measure of the length to width ratio, form factor ((perimeter) / (4π × surface)): a measure sensitive to the complexity and the branched appearance of the mitochondria, Feret’s diameter (the longest distance (μm) between two points of the mitochondria studied) and minimum Feret diameter (corresponds to the smallest diameter of the mitochondria) (Picard et al., 2012). It is important to highlight here that n = 3–4 is a sample size commonly used in TEM-based studies investigating mitochondrial morphology and ultrastructure (Bonnard et al., 2008; Sato et al., 2013; Arruda et al., 2014; Leduc-Gaudet et al., 2015; Saleem et al., 2015; Demeter-Haludka et al., 2018; Loro et al., 2018). Details on the number of SS and IMF mitochondria that were traced in both orientations in soleus and white gastrocnemius are available in Table 1.…”