Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Abstract

-Adipocytes express the rate-limiting enzymes required for glycogen metabolism and increase glycogen synthesis in response to insulin. However, the physiological function of adipocytic glycogen in vivo is unclear, due in part to the low absolute levels and the apparent biophysical constraints of adipocyte morphology on glycogen accumulation. To further study the regulation of glycogen metabolism in adipose tissue, transgenic mice were generated that overexpressed the protein phosphatase-1 (PP1) glycogen-targeting subunit (PTG) driven by the adipocyte fatty acid binding protein (aP2) promoter. Exogenous PTG was detected in gonadal, perirenal, and brown fat depots, but it was not detected in any other tissue examined. PTG overexpression resulted in a modest redistribution of PP1 to glycogen particles, corresponding to a threefold increase in the glycogen synthase activity ratio. Glycogen synthase protein levels were also increased twofold, resulting in a combined greater than sixfold enhancement of basal glycogen synthase specific activity. Adipocytic glycogen levels were increased 200-to 400-fold in transgenic animals, and this increase was maintained to 1 yr of age. In contrast, lipid metabolism in transgenic adipose tissue was not significantly altered, as assessed by lipogenic rates, weight gain on normal or high-fat diets, or circulating free fatty acid levels after a fast. However, circulating and adipocytic leptin levels were doubled in transgenic animals, whereas adiponectin expression was unchanged. Cumulatively, these data indicate that murine adipocytes are capable of storing far higher levels of glycogen than previously reported. Furthermore, these results were obtained by overexpression of an endogenous adipocytic protein, suggesting that mechanisms may exist in vivo to maintain adipocytic glycogen storage at a physiological set point.insulin; glycogen synthesis; lipogenesis; protein phosphatase-1; targeting subunit CIRCULATING ENERGY SOURCES are maintained in a narrow homeostatic range across a wide variety of physiological conditions through the coordinate regulation of energy uptake, consumption, storage, and release by the principal metabolic tissues, namely adipose tissue, liver, and skeletal muscle. During times of energy deficit, adipose tissue is a primary site for energy provision via the hydrolysis of stored triglyceride to release free fatty acid (FFA) for ATP production and glycerol for hepatic gluconeogenesis. Conversely, following a meal, dietary lipid is stored by adipose tissue, and glucose is disposed of as glycogen by the skeletal muscle and to a lesser extent by the liver. Alternately, glucose can be utilized postprandially by the liver and adipocytes for de novo lipogenesis and long-term storage as triglyceride. However, adipocytes also store glucose as glycogen, albeit at substantially lower rates than in skeletal muscle and liver, so the physiological role of adipocytic glycogen metabolism remains unclear.In addition to its role in lipid metabolism, adipose tissue functions as an...

A computational model of adipose tissue metabolism: Evidence for intracellular compartmentation and differential activation of lipases

“…The relative contribution of glucose and FA to oxidative metabolism is consistent with reported respiratory quotient (RQ) of 0.91 (Coppack et al, 1990). Synthesis and breakdown of glycogen were considered to be negligible (Jurczak et al, 2007). We assumed that less than 10% of glucose uptake was converted to G3P, which can also be formed from pyruvate via glyceroneogenesis (Reshef et al, 2003).…”

Enhanced glycogen metabolism in adipose tissue decreases triglyceride mobilization

“…All mice procedures were approved by the University of Chicago Institutional Animal Care and Use Committee. Mice were weaned at 3 wk of age, and tail clippings were acquired for genotyping via PCR (16). For studies comparing fed vs. fasted conditions, animals had free access to either standard chow and water or chow, and bedding was removed for an overnight period of 16 h (5 PM to 8 AM).…”

“…For protein analysis, immunoblotting was performed as described (16). Briefly, tissues of 2.5-mo-old male mice were harvested and quickly minced with scissors.…”

“…Yet the role of glycogen turnover within adipocyte metabolism during various metabolic states remains poorly defined. We have previously generated a transgenic mouse in which the protein phosphatase-1 glycogen-targeting subunit protein targeting to glycogen (PTG) was expressed under the control of the aP2 promoter (16). Glycogen levels in adipose tissue were elevated 100-to 400-fold in the aP2-PTG mice.…”

Lafora progressive myoclonus epilepsy: A meta-analysis of reported mutations in the first decade following the discovery of theEPM2AandNHLRC1genes

“…For example, no difference in the levels of glycogen synthase was observed in laforin deficient mice [Tagliabracci et al, 2007]. Similarly, polyglucosan-like inclusions have not been observed in a transgenic mice model that overexpresses PTG, although an increase in the glycogen content was observed [Jurczak et al, 2007]. Because laforin is known to bind to Lafora bodies [Chan et al, 2004a;Ganesh et al, 2004], a role for laforin in the clearance of abnormal glycogen complex from neurons has also been proposed [Ganesh et al, 2004].…”