| dc.description.abstract | The sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pump facilitates the transport of Ca2+ against a concentration gradient from the cytosol to the lumen of the SR, eliciting relaxation and maintaining Ca2+ homeostasis. Sarcolipin (SLN) is a small allosteric regulator of the SERCA, which when bound to the pump, lowers SERCA Ca2+ affinity and reduces SERCA efficiency by uncoupling the Ca2+ transport efforts from ATP hydrolysis. Previous research has revealed that through the regulation of SERCA activity, SLN can alter muscle function, energy expenditure, and promote Ca2+ signalling for metabolism, fibre type and muscle mass. While extensive research has characterized SLN expression and function in young adult male mice, few studies have examined SLN with aging and no studies have reported on SLN in female mice. Therefore, the purpose of this thesis was to examine sex and age-related differences in SLN expression and to discern the role of SLN in altering SERCA function, energy expenditure, and Ca2+ signalling between sexes with aging.
The first study, described in Chapter 3, explored sex-specific differences in SLN expression in young adult mice and investigated the consequences of SLN ablation on SERCA function and whole-body energy expenditure in both male and female mice. The study revealed that the soleus muscles of female mice exhibited higher levels of SLN content compared to male counterparts, which was accompanied by greater SERCA efficiency and increased adiposity with SLN ablation in females. This study reported that SLN ablation reduced whole-body energy expenditure in both male and female mice, in the absence of changes to habitual activity levels. Although females displayed relatively greater SLN content than males, the impact of SLN ablation on absolute whole-body energy expenditure was more pronounced in males, likely attributed to their greater lean muscle mass.
The second study of this thesis (Chapter 4) had two main aims: first, to investigate sex-related variations in SLN expression with aging, and second, to examine if SLN may offer SERCA protection against age-relative oxidative damage and dysfunction. With respect to the first aim, this study found that SLN expression increased with age and that the sex differences in SLN expression persisted with age, with females displaying greater SLN expression in soleus muscles across age groups. In order to examine the role of SLN in the protection of SERCA with aging, measures of SERCA function and SERCA damage were assessed in the soleus muscles of wild type (WT) and SLN ablated (Sln−/−), male and female mice at two ages, 4-6 months and 18+ months, reflecting young adult and older adults, respectively. Contrary to the hypothesis, SLN ablation had no impact on SERCA nitrosylation with aging and SLN ablation appeared to inhibit SERCA function in a manner similar to young adults. Additionally, the research uncovered sex-specific differences in nitrosylation damage to SERCA1a, with females displaying greater damage to SERCA1a than males. This data challenges the prevailing notion that SERCA2a is more susceptible to age-related oxidative damage, as previous studies have exclusively examined males.
The final study (Chapter 5) of this thesis examined whether the upregulation of SLN with aging activated calcineurin (Cn) signalling and promoted greater muscle mass and muscle function. Similar to findings in Chapters 3 and 4, SLN expression was higher in the soleus muscles of both females and older animals compared to male and young adult counterparts. This study aligned with several established sex and age-differences in the literature. Mainly, females and older animals displayed lower muscle mass, absolute force and fewer type II fibres than males and young adult, respectively. In contrast to the hypothesis, SLN ablation had no discernible impact on Cn signaling, muscle mass, or maximal muscle function, suggesting the upregulation of SLN with aging may not serve the same role in promoting beneficial Ca2+ signaling as observed previously in various unloading and disease models. While SLN ablation did not impact Cn signalling, this study revealed novel evidence for the activation of Cn signalling in skeletal muscle with age. This may align with previous research finding elevations to resting cytosolic [Ca2+] in aged muscle and future research should directly target Cn in order to discern the impact on muscle mass, fibre type and function with aging between sexes.
The collective results of the studies presented here confirm that SLN expression increases with age in both male and female mice and aids in clarifying the role of SLN in aging muscle. Furthermore, the studies of this thesis sought to begin addressing the sex bias within SLN research and revealed the consistent and novel evidence that SLN expression is greater in females. The sexual dimorphism in SLN expression make the argument for the use of female mice in studies examining SLN and provide foundational evidence for future research examining the impact of SLN on muscle metabolism between sexes. | en |
