The stimulation of glycogen synthesis by insulin involves the compartmentalized activation of PP1 due to glycogen targeting subunits that act as molecular scaffolds, bringing together the enzyme with its substrates in a macromolecular complex. The heterozygous deletion of the PTG
gene indicates that this protein plays a vital role in glycogen metabolism. The steady-state glycogen levels, as well as glycogen synthase activity ratios in PTG+/–
animals, were reduced by approximately 40–50% relative to wild-type counterparts in fat, liver, and heart. The levels of white fiber muscle glycogen were also significantly reduced, as was the activation of glycogen synthase by insulin. Moreover, PTG+/–
mice become progressively insensitive to insulin with age. In contrast, mice possessing a homozygous deletion of the GM
gene display no obvious alterations in whole-body glucose homeostasis or insulin sensitivity despite a significant depletion of muscle glycogen (12
). This suggests that the development of progressive glucose intolerance in the PTG+/–
mice may result from a reduction of glycogen in multiple tissues. Interestingly, the overexpression of PTG in rat primary hepatocytes, rat liver, or cultured human muscle cells by adenoviral infection produces dramatic increases in steady-state glycogen accumulation and the glycogen synthase activity ratio (14
). Moreover, cells overexpressing PTG did not respond to glycogenolytic stimuli, and thus locked into a glycogenic mode. Taken together, these data demonstrate that PTG plays a crucial role in regulating glycogen synthesis in vivo.
The development of insulin resistance with aging in the PTG+/–
mice suggests that the attenuation of glycogen synthesis may produce a progressive repartitioning of energy into lipid in muscle. The accumulation of intramyocellular lipid has been implicated as a causal factor in the development of insulin resistance in both rodents and humans (28
). Indeed, both mouse models (32
) and humans (33
) with lipodystrophy undergo lipid accumulation in muscle that can be reversed by treatment with hormones such as leptin that increase lipid oxidation. In this regard, the levels of serum leptin are elevated in aged PTG+/–
mice, perhaps suggesting peripheral leptin resistance, although this hyperleptinemia occurs without increases in body weight or epididymal fat pad mass. Although hyperleptinemia and leptin resistance usually correlate with obesity, it can occur independently of increased fat mass during aging (34
). The increases in muscle triglyceride content, fasting serum triglycerides, and fasting FFAs observed in PTG+/–
mice correlate well with the insulin resistance, as seen in other mouse models (29
). Although the mechanisms by which increases in intramyocellular lipid contribute to insulin resistance remain unknown, correlations have been observed between these states and decreased tyrosine phosphorylation of insulin receptor substrates (26
). In this regard, the tyrosine phosphorylation of IRS-1 and the downstream activation of Akt were decreased in muscle of aged PTG+/–
mice. These observations provide a mechanistic link between decreased whole-body glycogen stores and an attenuation of insulin signaling, leading to the development of insulin resistance with aging.
The hyperleptinemia in aged PTG+/–
mice may also contribute to the observed increase in heart mass. Leptin is also known to affect sympathetic nerve activity and arterial pressure (44
). It has been demonstrated that the sympathetic actions of leptin can be maintained or even elevated during the compensatory hyperleptinemia of leptin resistance (47
). This selective leptin resistance may allow for the development of hypertension and cardiac hypertrophy during hyperleptinemia, while the metabolic effects of leptin are impaired.
Alterations in lipid metabolism during states of glycogen depletion have been observed in several clinical models. Severe liver glycogen depletion brought about by rare mutations in the glycogen synthase gene can result in elevations in fasting ketone bodies and FFAs, a condition known as glycogen storage disease type 0 (49
). Additionally, post-marathon runners, who exhibit moderately decreased muscle glycogen stores, become transiently insulin resistant and display decreased whole-body glucose uptake. This is likely due to an adaptive response in fuel homeostasis in which lipid oxidation is increased under conditions of the euglycemic-hyperinsulinemic clamp (53
Studies in cases of early-onset insulin resistance revealed that the first detectable abnormality in insulin action lies in the storage of glucose as glycogen (54
). Although it is likely that these observations were reflective of decreased glucose uptake in insulin-resistant subjects, glycogen synthesis is typically diminished in patients with type 2 diabetes and may represent a critical event in the pathophysiology of the disease. The PTG+/–
mouse represents an interesting variation and a new model of this syndrome, in which the attenuation of glycogen synthesis per se does not directly produce insulin resistance, but rather generates a gradual attenuation of insulin signaling, probably through the redirection of fuel substrates into lipid. Moreover, the data presented here demonstrate that scaffolding proteins play a key permissive role in insulin action in a physiological setting. Future studies will focus on the molecular mechanisms underlying the dephosphorylation of glycogen synthase in response to insulin and on the role of PTG in this important metabolic event.