You may have a sweet tooth but your brain doesn’t. More specifically, your neurons don’t especially enjoy storing glycogen. Accumulation of glycogen in neurons leads to cell death. It was widely supported that neurons don’t synthesize or store their own glycogen. Rather, they rely on their neighboring glial cells for nutrition.

A new study in Nature Neuroscience shows that, contrary to popular belief, neurons are capable of synthesizing glycogen. This glycogen synthesis pathway is inhibited by a redundant mechanism. Their research shows that if this mechanism fails, glycogen accumulates in neurons and leads to cell death, a key feature in several neurodegenerative diseases.

Details about the study after the jump.

No glycogen for you

For one reason or another, neurons have evolved to forgo glycogen synthesis. This is a striking feature considering the metabolic activity of your brain, particularly neurons. Neurons require huge amounts of energy to create the wonderful network of information processing you call your mind. Where does this energy come from? It has long been known that glial cells, the poor cousin to neurons, maintain high glycogen synthesis and storage rates. Glial cells (in addition to recently discovered functions) provide energy to neurons.

The lack of glycogen synthesis directly conflicts with certain neurodegenerative diseases, particularly progressive myoclonus epilepsy or Lafora disease. These diseases are characterized by abnormal glycogen accumulations in neurons, leading to cell death and associated neurodegenerative problems. These accumulations, also known as Lafora bodies, are irregularly branched polymers of glucose. Accumulation of these in neurons results in seizures, dementia, loss of vision and eventual complete vegetative state and death. Onset of symptoms to death can take as little as 10 years.

Two key mutations are associated with Lafora disease. EPM2A, which encodes a protein known as laforin, and EPM2B, which encodes a protein named malin. Laforin is a phosphatase (dephosphorylates other proteins) while malin is a ubiquitin ligase responsible for degrading proteins through proteosomal degradation.

The research team wanted to look at the affects of laforin and malin on glycogen synthesis/degradation in neurons. To start, they confirmed that neurons do not synthesis glycogen. Even when cultured in mediums containing heavy concentrations of glucose, neurons do not express liver glycogen synthesase. Correspondingly, neurons also did not express glycogen phosphorylase, the enzyme responsible for degrading glycogen.

MGS responsible for glycogen synthesis

The study also looked at muscle glycogen synthases (MGS), another enzyme responsible for synthesizing glycogen. MGS is normally expressed in neurons despite no glycogen synthesis occurring. Interestingly, even under heavy over expression of muscle glycogen synthase (MGS), neurons still did not create glycogen. Glial cells, grown under the same conditions, showed heavy accumulation of glycogen. This suggests there is some machinery that is inhibiting the glycogen synthesis pathway since MGS was infact present in neurons.

MGS is controlled by phosphorylation of both its N and C terminal ends. Dephosphorylation of the enzyme activates it. To assess the active vs. inactive MGS on neurons, LiCl was added to the culture medium. LiCl has been shown to effectively dephosphorlyate MGS and activate the enzyme. Addition of LiCl showed striking changes in localization of MGS. Typically, MGS is localized to the center of a neuron, around its nucleus. LiCl treatment made MGS migrate to the cytoplasm. Furthermore, glycogen accumulation was seen colocalizing to cytoplasmic MGS.

Expression of PTG showed similar results. PTG is a regulatory subunit of protein phosphatase 1 and is responsible for activating MGS through phosphorylation. Also interesting is that PTG interacts with laforin. Overexpression of PTG showed increased MGS activity and accumulation of cytoplasmic glycogen. Taken together, these results show that neurons infact have the machinery to create glycogen, it is just inhibited by the inactivation of MGS. This is especially important because neurons do lack the capability to degrade glycogen. Once present, there is no way for neurons to get rid of glycogen. Any accumulation could be fatal.

Laforin and Malin inhibit MGS activity

The next step was to determine how this inactivation was occurring. It was theorized that laforin and malin interact with MGS to inactivate it. To test this, they overexpressed laforin and malin independantly of eachother in the presence of PTG. Both laforin+/PTG+ neurons and malin+/PTg+ neurons showed high MGS activity and accumulation of glycogen. However, when both laforin and malin were expressed together with PTG, they completely blocked MGS activity.

Futhermore, there was a significant decrease in MGS and PTG. Laforin levels lowered slightly while malin levels rose dramatically. This suggests that laforin stabilizes malin while malin degrades activated MGS and PTG. These results were confirmed when a proteosome inhibitor was injected. Protesomes are responsible for ubiquitination and degradation of proteins.

This data suggests glycogen synthesis is inhibited by both laforin and malin. Together, laforin and malin form a complex that ubiquitinates MGS and reduces activity. If either of the proteins are missing or reduced in quantity, abnormal glycogen accumulation is seen. This was further backed up by a genetic survey showing an interesting malin mutant, D146N. This mutant encodes a malin protein that is unable to bind to laforin. These cells showed high concentrations of laforin and malin as usual but failed to inhibit MGS activity, resulting in abnormal glycogen accumulation.

Conclusions

It appears now that neurons do infact have the capability to synthesize glycogen. Individuals who have Lafora disease often have a mutation in either laforin or malin, or sometimes both. It is now clear that defects in either of these proteins can have severe consequences on neuronal survival by failing to regulate glycogen synthesis activity. This is not only an advance on the front for treating this disease, but also disproves a previous “truth” that neurons cannot create glycogen. You learn something new every day =)

References

Vilchez, David; Ros, Susana; Cifuentes, Daniel; Pujadas, Lluis; Valles, Jordi; Garcia-Fojeda, Belen; Criado-Garcia, Olga; Fernandez-Sanchez, Elena; Medrano-Fernandez, Iria; Dominguez, Jorge; Garcia-Rocha, Mar; Soriano, Eduardo; Rodriguez de Cordoba, Santiago, and Guinovart, Joan J. Mechanism suppressing glycogen synthesis in neurons and its demise in progressive myoclonus epilepsy. [DOI:10.1038/nn1998]2007; advanced online publication;