Impaired Glucose Tolerance and Predisposition to the Fasted State in Liver Glycogen Synthase Knock-out Mice

Abstract

Conversion to glycogen is a major fate of ingested glucose in the body. A rate-limiting enzyme in the synthesis of glycogen is glycogen synthase encoded by two genes, GYS1, expressed in muscle and other tissues, and GYS2, primarily expressed in liver (liver glycogen synthase). Defects in GYS2 cause the inherited monogenic disease glycogen storage disease 0. We have generated mice with a liver-specific disruption of the Gys2 gene (liver glycogen synthase knock-out (LGSKO) mice), using Lox-P/Cre technology. Conditional mice carrying floxed Gys2 were crossed with mice expressing Cre recombinase under the albumin promoter. The resulting LGSKO mice are viable, develop liver glycogen synthase deficiency, and have a 95% reduction in fed liver glycogen content. They have mild hypoglycemia but dispose glucose less well in a glucose tolerance test. Fed, LGSKO mice also have a reduced capacity for exhaustive exercise compared with mice carrying floxed alleles, but the difference disappears after an overnight fast. Upon fasting, LGSKO mice reach within 4 h decreased blood glucose levels attained by control floxed mice only after 24 h of food deprivation. The LGSKO mice maintain this low blood glucose for at least 24 h. Basal gluconeogenesis is increased in LGSKO mice, and insulin suppression of endogenous glucose production is impaired as assessed by euglycemic-hyperinsulinemic clamp. This observation correlates with an increase in the liver gluconeogenic enzyme phosphoenolpyruvate carboxykinase expression and activity. This mouse model mimics the pathophysiology of glycogen storage disease 0 patients and highlights the importance of liver glycogen stores in whole body glucose homeostasis.After ingestion of a meal, glucose is cleared from the bloodstream primarily by conversion to glycogen in skeletal muscle and liver. Muscle is often considered the major site of insulinstimulated glucose disposal, especially in humans, accounting for as much as 70 -90% of the total body glucose uptake in some studies (1). However, the liver also contributes significantly to glucose disposal (2), and when glucose is given via the oral route, the liver may dispose of as much as one-third of the glucose load (3, 4). A key glycogen biosynthetic enzyme is glycogen synthase (GS), 2 which is controlled by glucose-6-phosphate, an allosteric activator, and by phosphorylation, which inactivates the enzyme (5). There are two GS isoforms in mammals encoded by separate genes. GYS1, encoding the muscle isoform (MGS), is expressed in muscle and many other tissues including kidney, heart, and brain, whereas GYS2, encoding the liver isoform (liver glycogen synthase (LGS)), is known to date to be expressed only in liver. To assess the significance of muscle glycogen stores for overall glucose homeostasis, the Gys1 gene was disrupted in a mouse model, MGSKO mice (6, 7). These mice are totally unable to synthesize muscle glycogen, but surprisingly glucose tolerance was actually improved (8, 9). Moreover, MGSKO mice were no different from wild type littermates...

Liver Glycogen Reduces Food Intake and Attenuates Obesity in a High-Fat Diet–Fed Mouse Model

“…In a first report (49), it was shown that heterozygous deletion of PTG in mice reduced glycogen levels and induced glucose intolerance and insulin resistance with age. These observations agree with other studies of animal models where glycogen stores are decreased by other means, such as depletion of LGS (50). However, a recent article reported that total ablation of PTG reduced fasting glucose and insulin levels in obese mice while improving insulin sensitivity and blocking hepatic steatosis during fasting in HFD-fed mice (25).…”

Deficiency of a Glycogen Synthase-associated Protein, Epm2aip1, Causes Decreased Glycogen Synthesis and Hepatic Insulin Resistance

“…Blood glucose following a 16-hour fast was reduced significantly (Fig. 3B), consistent with the low liver glycogen stores, the main stores consumed to maintain blood glucose during overnight fasts in mice (22). The insulin tolerance test (the degree of drop in blood glucose following insulin injection) was normal (Fig.…”

“…Epm2aip1 Ϫ/Ϫ Mice Are Resistant to Age-dependent ObesityAs mentioned, GS has two isoforms, one specific to liver (GYS2), the other to skeletal muscle and other tissues (GYS1) (2). As discussed below, many features of the Epm2aip1 Ϫ/Ϫ mice are shared between mice lacking Gys2 (LGSKO) (22) and mice lacking Gys1 (MGSKO) (35). Normally, adult laboratory mice fed standard chow gain weight and become obese with age, which is true of LGSKO mice (22) but not of MGSKO mice.…”

“…For glucose tolerance tests, mice were fasted overnight (14 -16 h). Glucose was injected intraperitoneally at 2 mg/g body weight (18,22), and blood glucose levels were measured at 0, 15, 30, 60, and 120 min. Areas under or above the curve were calculated as described previously (23).…”

Genetic Depletion of the Malin E3 Ubiquitin Ligase in Mice Leads to Lafora Bodies and the Accumulation of Insoluble Laforin

“…Exercise Protocol-Exercise to exhaustion was performed on 3-month-old male C57Bl/6J mice using a treadmill (Exer6M, Columbus Instruments) following the procedure described previously (24), with minor modifications. Immediately after exhaustion, mice were sacrificed by cervical dislocation, and tissues were immediately collected in liquid nitrogen and stored at Ϫ80°C until use.…”