Investigating the role of Phospholipase A2 Group IVE in energy metabolism
Item statusRestricted Access
Embargo end date22/11/2023
Pruñonosa Cervera, Iris
Obesity prevalence is increasing worldwide and drives increased risk for type 2 diabetes, cardiovascular disease and cancer. Genetic predisposition exacerbates environmental drivers of obesity, such as energy-dense diets and a sedentary lifestyle. To understand the genetic contribution to obesity, divergently selected Fat and Lean mouse lines (23% and 4% fat as body weight, respectively) were generated. Heritable genetic intervals (quantitative trait loci; QTL) that segregate with adiposity were identified in the F2 crosses of the Fat and Lean mouse lines facilitating the identification of causal genes underlying adiposity. Given that skeletal muscle is a major contributor to energy balance, a transcriptomic analysis of the skeletal muscle of the Fat and Lean mouse lines was performed, applying genetic mapping to look for genes with an increased causal likelihood for adiposity. Phospholipase A2 group IVE (Pla2g4e; protein name: cPLA2ε), positioned in a major “found in obesity” QTL, Fob1 (accounting for 5% of the Fat and Lean adiposity difference), was previously found to exhibit ~3-fold elevated mRNA level in the skeletal muscle of the Fat mouse line compared to the Lean mouse line. cPLA2ε is a member of the Phospholipase A2 family involved in intracellular membrane trafficking, lipid signalling in the endocannabinoid system and was associated with obesity and type 2 diabetes. Variants of human PLA2G4E were associated with body composition markers in the UK BioBank. Together with preliminary background data showing that Pla2g4e overexpression in murine clonal C2C12 myotubes impaired glucose uptake and mitochondrial respiration, it was hypothesised that elevated Pla2g4e in the skeletal muscle drives obesity through its suppressive effects on energy expenditure and nutrient partitioning. To address this, mouse models of genetic deficiency of Pla2g4e were generated, allowing us to formulate a specific hypothesis around loss of functional cPLA2ε. Thus, the present study assessed the complementary hypothesis that Pla2g4e deficiency protects against the development of diet-induced obesity through its effects on energy homeostasis. Male whole-body Pla2g4e knockdown (G4e∆/∆) mice resisted high-fat diet (HFD)-induced obesity, with less fat mass and smaller adipocyte size, and insulin resistance. Furthermore, male G4e∆/∆ mice exhibited higher energy expenditure and a greater preference for lipid as fuel source. There was no difference in metabolic parameters between female G4e∆/∆ and control mice at baseline or after HFD. We next aimed to determine whether the beneficial metabolic effects of Pla2g4e deficiency emanated specifically from the skeletal muscle. Male skeletal-muscle specific Pla2g4e knockdown (Skm-g4e∆/∆) mice were equally susceptible to HFD-induced obesity and insulin resistance as the control mice. Notably, Skm-g4e∆/∆ mice exhibited reduced energy expenditure, associated with reduced physical activity and preference for carbohydrates as fuel source. This suggests that the protective effect of Pla2g4e deficiency on the development of diet-induced obesity does not emanate from the skeletal muscle in isolation. To understand the contribution of skeletal muscle Pla2g4e expression to energy regulation and the pathology of obesity, RNA-sequencing was performed in the skeletal muscle of G4e∆/∆ mice after HFD. This showed that glucose and lipid metabolism and inflammatory processes are some of the systems within skeletal muscle that are altered by whole-body Pla2g4e deficiency during obesity. Consistent with the hypothesis that elevated Pla2g4e expression in the skeletal muscle is detrimental, overexpression of Pla2g4e in C2C12 myotubes decreased basal respiration and tended to decrease ATP-linked and maximal uncoupled respiration. PLA2G4E expression was confirmed in human skeletal muscle, being preferentially expressed in females and downregulated after acute aerobic exercise. In conclusion, Pla2g4e deficiency protects male mice against diet-induced obesity through the increase in energy expenditure and the preferential use of fat as fuel source, although this effect does not emanate exclusively from the skeletal muscle. Nonetheless, despite a complex “non symmetrical” relationship between skeletal muscle overexpression and knockdown, this study also provided evidence that Pla2g4e expression in the skeletal muscle may indeed be contributory to energy regulation and the pathology of obesity. Therefore, whole-body deficiency suggests that PLA2G4E inhibition could be a potential therapeutic strategy for male obesity.