Using novel imaging methods for studying brain metabolism, University of Kentucky researchers have identified the reservoir for a necessary sugar in the brain. Glycogen serves as a storage depot for the sugar glucose.
The laboratories of Ramon Sun, Ph.D., assistant professor of neuroscience, Markey Cancer Center at the University of Kentucky College of Medicine, and Matthew Gentry, Ph.D., professor of molecular and cellular biochemistry and director of the Lafora Epilepsy Cure Initiative at the University of Kentucky College of Medicine discovered that glucose – the sugar used for cellular energy production – was not the only sugar contained in glycogen in the brain.
The full study was recently published in Cell Metabolism.
However, within cells, glucosamine is an essential sugar needed for the complex carbohydrate chains that are attached to proteins in a process called glycosylation. These sugar chains decorate proteins and the sugar decorations are critical for the appropriate function of myriad proteins.
Discovering that glucosamine is a major component of brain glycogen provides key insight into neurological diseases caused by aberrant glycogen-like cellular aggregates called polyglucosan bodies (PGBs).
Lafora disease is a rare, inherited childhood dementia caused by PGBs and this study demonstrates that the Lafora disease PGBs sequester glucosamine, leading to numerous cellular perturbations. PGBs also accumulate in the brain as people age and in people with other forms of dementia. Thus, the discovery that glycogen is also a storage cache for glucosamine has broad implications for understanding neurological changes associated with aging.
Using biochemical approaches, the researchers determined the sugar composition of glycogen in the muscle, liver, and brain of mice. Unlike muscle glycogen, which had only 1% glucosamine, and liver glycogen, which had less than 1% glucosamine, brain glycogen contained 25% glucosamine.
Upon making this surprising discovery, they then identified the enzymes responsible for incorporating glucosamine into glycogen and for releasing glucosamine from glycogen. Again, the discovery was unexpected as these enzymes are the same ones used to incorporate glucose into and release glucose from glycogen.
To understand the implications of their findings for Lafora disease and neurological problems arising from PGBs, the researchers used their newly developed technique called matrix-assisted laser desorption/ionization traveling-wave ion-mobility high-resolution mass spectrometry (MALDI TW IMS) to measure and visualize the amount of glycogen in different regions of the brain.
They also used this technique to quantify changes in the specific patterns of the sugar decorations on proteins in multiple regions of the brain.
The team applied MALDI TW IMS to analyze the brains of healthy mice and of two different mouse models of glycogen storage diseases: a model of Lafora disease and a model of glucose storage disease (GSD) type III. Sun commented, “This new technique allows us to quantify the amount of these sugars with high accuracy while also maintaining the spatial distribution within the brain regarding where the sugars are located. It is crucial that the brain has the correct sugars in the right location within the brain.”
These studies revealed that without the ability to properly regulate brain glycogen metabolism, not only do PGBs form, which perturbs cell metabolism, but the sugar decoration of proteins is also altered. Excitingly, they could restore protein sugar decoration by injecting an antibody-enzyme fusion (VAL-0417) into the brains of Lafora disease mice to degrade the PGBs.
Their findings show a direct connection between abnormal glycogen storage and defective protein function in the brain. Their findings have implications for many other GSDs and congenital disorders of glycosylation, which cause severe neurological symptoms, including epilepsy and dementia.
“Multiple neurological diseases have blockades in these metabolic pathways. I’m sure these pathways are going to be important in other neuro-centric diseases as well. Brain glycogen is comprised of glucose and glucosamine and brain metabolism has to balance both in order to stay healthy,” explained Gentry.
The Gentry and Sun laboratories collaborated with several others from UK College of Medicine including Drs. Craig Vander Kooi, professor of molecular and cellular biochemistry, Charles Waechter, professor of molecular and cellular biochemistry, Lance Johnson, assistant professor of physiology, Christine Brainson, assistant professor of toxicology and cancer biology.
They also worked with researchers from Indiana University School of Medicine including Drs. Anna A. DePaoli-Roach, professor of biochemistry and molecular biology, Peter J. Roach, professor of biochemistry and molecular biology, Thomas D. Hurley, professor of biochemistry and molecular biology.
Richard Taylor, professor of chemistry and biochemistry, from the University of Notre Dame, and Richard Drake, professor of cell and molecular pharmacology and experimental therapeutics from the Medical University of South Carolina, also contributed to this work.
“This type of transdisciplinary collaborative research takes place at UK because of strong leadership from College of Medicine Dean Robert DiPaola, Dr. Mark Evers, Vice President for Research Lisa Cassis, Ph.D. and others,” stated Sun.
Learning and memory are two critical functions of the brain and several different regions within the brain have been demonstrated to have involvement in the consolidation of diverse forms of learning/memory, including the cortex, striatum, amygdala and hippocampus .
The cortex has involvement with spatial learning; the striatum correlates with motor skills; the amygdala is related to emotional memory; and finally, the hippocampus is involved in spatial learning and working and recognition memory. Many have generally recognized the hippocampus as the most critical region [2,3].
A variety of neurotrophin (NT) polypeptides play important roles in neural activities by regulating cell proliferation, differentiation, maturation and plasticity. Among the NTs, the brain-derived neurotrophic factor (BDNF) in general performs the highest expression in the brain .
In the mouse model, BDNF has been shown to be required for neurogenesis in the hippocampus  and a declined BDNF level was noted in the ageing adults, indicating a possible connection of low BDNF to reduced memory, neurodegeneration and cognitive impairments . In neurons, activation of the cAMP/PKA/cAMP-responsive element binding (CREB) protein signaling pathway can lead to the induction of an array of genes, including BDNF .
It has been proposed that while BDNF interacts with its cognate kinase receptor TrkB, the PKA pathway can activate and cause a positive feedback-like circle to amplify the BDNF-modulated physiological activities . Phosphodiesterase (PDE) is the enzyme capable of degrading cAMP and thus it is able to attenuate the PKA signaling by reducing the availability of the intracellular cAMP. In fact, PDE4 is a cAMP-specific PDE isoform detected in various tissues, including several brain regions [9,10].
Indeed PDE4 has been regarded as a potential therapeutic target, for example for the treatment for the cognitive impairment . All of these studies have pointed out that maintaining the cellular cAMP/PKA/CREB signaling by increasing the cAMP and/or by decreasing the PDE activity appears to be a potential strategy for treating a decline in cognitive functions .
Glucosamine (GLN) is a crucial component within glycoproteins and proteoglycans . The clinical value of GLN was not established until it was first suggested for use in treating osteoarthritis . Besides the glycolysis-related events, GLN and its derivatives have been demonstrated to have involvement in a variety of cellular activities in a glycolysis-independent manner .
GLN is involved in the O-linked N-acetylglucosaminylation (O-GlcNAcylation) of different proteins and this should lead to a wide range of regulation in cell physiology, such as cellular signal transduction, transcription, protein modification and more [14,15].
Importantly, most GLN administrated orally can be absorbed from the gastrointestinal system and the resultant GLN has been shown to pass the blood–brain barrier (BBB) to reach the brain [16,17], indicating that GLN can possibly reach any tissue of the body.
Previous studies have reported a list of different potential functions of GLN . The involvement of O-GlcNAcylation in the regulation of protein homeostasis has been well-recognized; O-GlcNAcylation modification is highly prevalent in the mammalian brain and errors in this mechanism have been suggested to contribute to many cellular cascades in relation to neurological or neurodegenerative diseases [12,18].
Therefore, this study aimed to disclose the impact of GLN in brain cognitive performance in relation to BDNF production and PKA signaling with in vivo and in vitro approaches.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7465318/
Original Research: Closed access.
“Brain glycogen serves as a critical glucosamine cache required for protein glycosylation” by Ramon Sun et al. Cell Metabolism