The impact of sarcolipin ablation on calcineurin signalling and muscle phenotype with aging
Authors: Chambers Paige , Brahmbhatt Aditya , Tupling A. Russell 
Published: May 2023

Sarcolipin (SLN), a small protein inhibitor of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA), is dynamically upregulated in atrophic unloading and disease states where it promotes muscle health by activating calcineurin (Cn) and the dephosphorylation and subsequent nuclear translocation of nuclear factor of activated T-cells (NFAT). This study sought to examine how the ablation of SLN impacted Cn signalling, fibre type profile and muscle mass in both male and female mice with aging. Male and female, wild type (WT) and SLN knockout (SLNKO) mice were assessed at two age groups (young adult (4-6M) and older adult (18+M)) for muscle mass (soleus:body weight ratio, fibre type specific cross-sectional area), fibre type profile and protein expression (SLN, Cn, and NFAT). SLN content was significantly greater in WT females (p <00.1) and older animals (5<0.05) and absent from SLNKO. As SLN has been linked to Cn signalling previously, we hypothesized that the increased SLN content in female and aged animals may promote Cn signalling. While there was a trend for lower Cn expression with aging (p=0.10), neither sex nor genotype significantly impacted Cn expression. To examine activation of Cn, the ratio of inactive phosphorylated NFAT to total NFAT was assessed via Western Blotting. We found that aging significantly increased the activation of Cn (p<0.01) and there was a trend (p=0.10) towards greater NFATp/NFAT with SLN ablation signifying a reduction in Cn signalling. As Cn is a known regulator of muscle fibre type, we examined fibre type profile using immunofluorescence and found aging increased the percentage of type I fibres (p<0.001), which was witnessed more predominately in female mice regardless of genotype. Furthermore, when examining type II fibres, we found a significant sex by age interaction (p<0.05) in which both WT and SLNKO females displayed reduced type II fibres with aging while males did not display this apparent shift. With respect to muscle mass, aging, regardless of sex or genotype, was shown to significantly reduce relative muscle mass (p<0.001), however no significant reductions were found in total fibre count, or cross-sectional area of Type I or Type II fibres. In summary, this study found that WT female and aged animals display increased SLN content. Aligning with our hypothesis, there was a trend towards lower Cn signalling with SLN ablation, regardless of age or sex. However, this does not seem to be linked to changes in muscle fibre type or muscle mass, as SLNKO animals did not display significant differences from their WT counterparts. Further studies should seek to examine the role of SLN in both sex and aging further, with special focus on metabolic energy expenditure and SERCA protection. NSERC This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.