The essential role of fructose-1,6-bisphosphatase 2 enzyme in thermal homeostasis upon cold stress

Abstract

Skeletal muscle is a major organ for glucose disposal and thermogenesis. While hepatic fructose-1,6-bisphosphatase is well known as a key enzyme for gluconeogenesis, the role of muscle fructose-1,6-bisphosphatase 2 (Fbp2) in glucose disposal and thermogenesis is unknown. Here, using Fbp2 knockout (KO) mice, we assessed the physiological role of Fbp2 in energy and glucose metabolism and thermogenesis. In vivo assessments of energy metabolism, glucose metabolism, and thermogenesis were performed by indirect calorimetry, hyperinsulinemic-euglycemic clamp, and cold challenge studies, respectively. Under both feeding and fasting conditions, Fbp2 KO mice showed similar phenotypes regarding energy and glucose metabolism compared to wild-type (WT) mice. However, Fbp2 KO mice were severely intolerant to cold challenge under fasting conditions. Mechanistically, the cold-induced intramuscular conversion of lactate to glycogen (glyconeogenesis) is completely abolished in the KO muscle, which leads to a lack of glycogen source for thermogenesis in Fbp2 KO mice. The cold-intolerant phenotype of KO mice disappeared after feeding, and the KO mice were equally as cold tolerant as the WT mice and survived during the cold challenge for three weeks. Taken together, these data demonstrate that Fbp2 is essential for muscle thermogenesis by replenishing the intramuscular glycogen pool through glyconeogenesis when the exogenous glucose source is limited. These data imply the physiological importance of Fbp2 in thermal homeostasis and suggest a potential novel therapy targeted to increase glycogen replenishment upon cold stress. Body temperature: Enzyme critical to heat production in muscle When simple sugars in the diet are scarce, skeletal muscle can still generate heat under cold conditions thanks to an enzyme that converts a metabolic byproduct into complex carbohydrates. A team led by Hui-Young Lee and Cheol Soo Choi from Gachon University's Lee Gil Ya Cancer and Diabetes Institute in Incheon, South Korea, showed that, under fasting conditions, mice lacking a muscle form of enzyme called fructose-1,6-bisphosphatase 2 (Fbp2) could not respond to cold exposure by the usual process of converting lactate, which builds up in muscles during intense activity, into glycogen, a type of complex sugar involved in heat production not related to shivering. After a meal, however, the same mice could adapt to extreme cold without any problem. The findings highlight the importance of Fbp2 in thermal regulation under fasting conditions.