Obesity increases the risk for diabetes and cardiovascular disease. Genetic predisposition exacerbates environmental drivers of obesity such as energy dense diets and sedentary lifestyle. We have exploited divergently selected Fat (23% fat as bodyweight) and Lean (4% fat as bodyweight) lines of mice originating from a common base population to identify genes underlying divergent adiposity. A stratified approach using quantitative trait loci (QTL; heritable genetic intervals segregating with adiposity in Fat x Lean F2 populations), metabolic tissue transcriptomics and comparative cross-species bioinformatics identified candidate obesity and leanness genes in adipose tissue (Morton et al., 2011, 2016). Using a similar approach, we have identified novel muscle-expressed genes that segregate with adiposity. A specific phospholipase A2 isoform (we name here PlaX), positioned in the Found in obesity (Fob)-1 QTL, exhibited ~5-fold elevated mRNA levels in the skeletal muscle of Fat mice compared to Lean mice.
Our aim in this study was to characterise PlaX’s expression and regulation in skeletal muscle across different muscle beds, in response to exercise, and between sex.
To better understand the role of PlaX in skeletal muscle, PlaX expression was measured in different skeletal muscle beds in 8-week old male C57BL/6J mice. Relative mRNA expression of PlaX was detected in soleus, gastrocnemius, EDL and quadriceps muscle, and preferentially expressed in gastrocnemius and EDL muscle [P<0.05; (n=4/group)]. To test the translational relevance of our findings, PLAX expression was measured in human skeletal muscle. PLAX expression was measured in vastus lateralis biopsies from female and male participants with normal glucose tolerance (NGT) and those with Type 2 Diabetes (T2D) by microarray (NGT-female: n=45, NGT-male: n=50, T2D-female: n=40 and T2D-male: n=50). PLAX mRNA abundance was significantly higher in female skeletal muscle (q-value<10e-15), with no difference between participants with T2D and NGT. We thus hypothesised that PlaX expression may be regulated by sex hormones. To test this, quadriceps muscle from 8-weeks old gonadectomised male C57BL/6 mice supplemented with vehicle, male sex hormone (dihydrotestosterone, DHT; 100μg/day) or female sex hormone (estradiol, E2; 2μg/day) by a subcutaneous minipump for 3 weeks were analysed to quantify PlaX expression. Relative PlaX mRNA abundance did not differ between the male or female sex hormone supplemented gonadectomised mice. However, PlaX expression was significantly increased after gonadectomy surgery [*P<0.05; (n=9/group)]. Expression levels were not rescued after sex hormone supplementation in male mice. Given the beneficial effects of exercise upon skeletal muscle, we then aimed to determine whether PLAX expression is altered in human skeletal muscle in response to exercise. PLAX expression was analysed in MetaMex, an application to perform meta-analysis of skeletal muscle response to exercise (Pillon et al., 2020). PLAX expression was significantly downregulated in vastus lateralis biopsies of young male and female healthy subjects after acute aerobic exercise (log2FC=-0.23, FDR=0.038).
Our genetic strategy has identified a novel potential skeletal muscle driver of obesity that appears to display sexual dimorphism and is downregulated in response to exercise.