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Glycogen has been linked to diverse processes, the most recent of which is its role in disease progression and ageing. Studies in Caenorhabditis elegans have revealed that high sugar diets which mediate the accumulation of glycogen, results in two conflicting effects.
Caenorhabditis elegans worm. Image Credit: royaltystockphoto.com / Shutterstock
The first is the resistance to oxidants, which are known to damage cells and accelerate the ageing process. Paradoxically, the second effect is a decreased lifespan. Mechanistically, glycogen reduces the active form of the antioxidant glutathione and influences the activity of the enzyme AMPK. AMPK is the 5' AMP-activated protein kinase, and a principal enzyme in regulating energy homeostasis. As such, it coordinates numerous metabolic pathways and balances energy demand with nutrient supply. In C. elegans, depleting glycogen stores increases the organism’s lifespan and eliminate the effects of glucose toxicity.
In a therapeutic setting then, depletion of glycogen by increasing cellular levels of oxidants may produce beneficial effects in hyperglycemic patients and those with diseases related to glycogen storage. Glycogen, therefore, is more than a nutrient storage macromolecule; it is a key regulator of metabolism and ageing.
Caenorhabditis elegans (C. elegans) on a high sugar diet
The effect of a high sugar diet in Caenorhabditis elegans (C. elegans) is comparable to other animals; nematodes can increase both their triglyceride (TG) and glycogen stores. Further corroborating evidence for the changes in their metabolism come from transcriptome studies in which the genes for TG and amino acid synthesis are up-regulated, while those of opposing catabolic reactions are down-regulated.
When the genes for fatty acid (FA) synthesis are deleted, C. elegans demonstrates glucose toxicity and in the sams‐1 mutant (where FA synthesis is hyperactivated) nematode worms experience increased fat deposition which protects against glucose toxicity.
Interestingly, the loss of glycogen synthase does not affect lifespan, indicating that glycogen storage is not required to protect against excess glucose intake.
Mild oxidative stress increases C. elegans lifespan and health
As stated, a critical concentration of reactive Oxygen Species (ROS) can be beneficial. This is particularly true of non-sugar associated ROS’, such as exercise-induced ROS. Exercise stimulates the enzymes that tackle the effect of oxidative stress, while supplementation with antioxidants can undermine this effect.
Hyperglycemia-induced ROS, by contrast, down-regulates the enzymes responsible for the protection against oxidative damage, namely SOD and Glutathione peroxidase (GPx). These effects clearly differentiate between the context in which ROS are induced in their capacity to bring about benefit or toxicity to the animal.
In C. elegans the correlation between oxidative stress and increased lifespan has been observed. This has been demonstrated in studies that directly show how low-level exposure to oxidants increases lifespan, while increased intracellular ROS produces an anti-aging effect.
Similarly, ROS up-regulation (as caused by impaired glucose metabolism) also slows aging. The opposite effect – the exposure to antioxidants, failed to increase the lifespan of C. elegans. Together, these observations indicate that a low level of ROS is beneficial to animals, increasing lifespan through anti-aging effects.
Albeit these studies have determined a clear correlative link, a unified causative mechanism is yet to be determined. This is particularly true as multiple pathways are activated by ROS to increase longevity.
In worms, high glucose diets in old worms increase ROS, intriguingly together with an absence of oxidative stress response genes. Furthermore, prolonged exposure to sugar decreases levels of antioxidant enzyme SOD‐3, one of the most important ROS detoxification enzymes. This suggests that the effect of high glucose does not cause immediate oxidative stress, instead of glucose intake resulting in initial reductive stress.
As discussed earlier, this is what distinguishes hyperglycemia-induced ROS from other forms (i.e. exercise-induced) ROS.
Despite the decrease in SOD-3, the nematodes are more resistant to oxidants; this non-congruent effect can be explained by a mechanism that is independent of the oxidative stress pathway, instead of being determined by altered metabolic flux. the inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which results in diversion to the pentaphosphate pathway (PPP) and away from glycolysis to generate nicotinamide adenine dinucleotide phosphate (NADPH). NADPH is used to reduce the oxidized form of glutathione (GSH) to its reduced form GSH. The resulting GSH then neutralizes ROS’.
This greater ability to protect against the harmful effect of oxidants on a hyperglycemic diet, however, does not translate to a longer lifespan. In response to a sugar-rich diet, oxidative stress resistance is dependent on glycogen-mediated glutathione disulfide (GSSG) reduction.
The evidence supporting the notion that oxidative resistance does not result in increased lifespan is derived from studies demonstrating that deletion of gspd‐1 (the enzyme that catalyzes the rate-limiting step of the pentose phosphate pathway or PPP) decreases NADPH production, which then lowers glucose conversion to FA.
Lifespan extension via glycogen-mediated glutathione reduction
Oxidants can reverse the effect of glucose toxicity. One example of such an effective oxidant is diamide which oxidizes GSH or acetaminophen, which subsequently depletes its concentration. There is a reciprocal relationship between oxidants and excess glucose intake; treatment with a high diamide concentration can be rescued by high glucose intake. These effects suggest that GSH oxidation (mediated by oxidants) represents the event that determines increased lifespan.
The downstream effects of GSH oxidation are as follows; cells redirect glucose from the glycolytic pathway into the PPP to produce NADPH. The redirection of glucose 6-phosphate (G‐6‐P) into the PPP prevents further metabolic flux through glycolysis, which subsequently decreases the NADH/NAD+ ratio. This increases cellular demand for G-6-P, and to meet this, the cell induces glycogenolysis.
Glycogenolysis is effectively the opposite of glycogen synthesis, breaking down existing glycogen stores to yield glycolysis intermediates – i.e. G-6-P. This subsequently depletes stores of glycogen and the cell there enters a high GSH/GSSG ratio. This, therefore, results in ROS scavenging. Hence, by applying small quantities of oxidants, the glycogen stores can be drained through the PPP pathway. Lifespan, then, is linked to depletion of the glycogen stored.
What is the role of AMP-activated protein kinase (AMPK)?
AMP-activated protein kinase (AMPK) is regarded as the master regulator of metabolism and is regulated in a highly complex manner. To summarize broadly, when the cell is under metabolic stress, the reduced energy availability is indicated by a high AMP/ATP ratio.
The AMP binds the γ‐subunit of heterotrimeric AMPK and allosterically promotes α‐subunit phosphorylation. The opposite effect is elicited by ATP. This phosphorylated form of AMPK inhibits ATP-demanding cellular reactions (i.e. biosynthetic reactions, anabolic reactions) and stimulates ATP production.
The AMPK β‐subunit contains a glycogen‐binding domain (GBD), which, when bound by glycogen, inhibits the activity of purified AMPK. This is thought to arise by the stimulation of AMPK movement close to GS, which is then phosphorylated and inhibited.
The effect of glycogen mediated AMPK inhibition is to ensure that AMPK, which is responsible for biosynthetic reactions, remains inactive when catabolic reactions (i.e. glycogen breakdown) to provide glucose for ATP, are occurring. Eventually, the breakdown of glycogen provides enough ATP generation so that the AMPK is activated.
The idea that high glycogen causes a decreased lifespan via AMPK mechanisms needs further clarification. This is because daf‐2 animals, which store more glycogen, appear to have an active AMPK enzyme, but live longer.
Therapeutic potential
The harm elicited by excess sugar intake in adults may be mitigated by suppressing glycogen accumulation – whether through inhibiting glycogen synthase or by promoting glycogen depletion with oxidants. Indeed, the effect of exercise-induced mild oxidative stress has a beneficial effect rather than the extolled antioxidants.
Large glycogen stores enable raid reduction of GSSG, and therefore, rapid ROS clearance in response to oxidants. Hence, treatment with oxidants will enable depletion of glycogen stores through activation of the PPP pathway and rescue AMPK inhibition to collectively restore longevity in animals with high sugar diets.
The depletion of glycogen is with oxidant therapy is counter-intuitive, yet this article has explained how it can mitigate the effects of hyperglycemia. Evidently, oxidants are harmful; when uncontrolled, they cause protein and lipid oxidation, DNA damage and potentially cell death.
Low levels then would be therapeutically viable, enabling directed targeting of GSH. The shuttling of G-6-P into the PPP reduces the rate of glycolysis and other pathways to decrease NADH, reduce glycation end-products and the activation of inflammatory pathways that precede aging and disease.
Longer-term effects occur as a result of decreasing reactive, non-phosphorylated AMPK, which will promote glucose entry into cells and the catabolism of FAs which would eventually diminish hyperglycemia and hyperlipidemia.
Conclusions
Excessive sugar consumption is known to accelerate the development of not only a range of human diseases. More recently, studies in model animals such as C. elegans have revealed that it is a cause of accelerated aging.
Excess glucose is stored predominantly as short-term glycogen or long-term lipids; the contribution of the latter to disease is well-understood, but the impact of stored glycogen has been largely ignored.
Recent findings have shown that glycogen accumulation in C. elegans as a result of high sugar diets, results in sequestered AMPK and decreased lifespan.
Glycogen accumulated on a high sugar diet appears to provide a means for glutathione reduction, and therefore protects nematodes from oxidative stress. However, excess glycogen shortens the nematode lifespan. This is because glycogen prevents thiol-oxidation which interferes with redox-sensitive thiol signaling; it is not the high GSH/GSSG ratio or low ROS, but rather endogenous glycogen signaling that reduces lifespan.
In conclusion, the depletion of glycogen via oxidants occurs via the PPP/ GSH pathway, therefore preventing lifespan-decreasing effects to take place.
Further Reading
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