Glycogen: Multiple Roles in the CNS

Proteomic investigations of adult polyglucosan body disease: insights into the pathobiology of a neurodegenerative disorder

Inadequate glycogen branching enzyme 1 (GBE1) activity results in different forms of glycogen storage disease type IV, including adult polyglucosan body disorder (APBD). APBD is clinically characterized by adult-onset development of progressive spasticity, neuropathy, and neurogenic bladder and is histologically characterized by the accumulation of structurally abnormal glycogen (polyglucosan bodies) in multiple cell types. How insufficient GBE1 activity causes the disease phenotype of APBD is poorly understood. We hypothesized that proteomic analysis of tissue from GBE1-deficient individuals would provide insights into GBE1-mediated pathobiology. In this discovery study, we utilized label-free LC-MS/MS to quantify the proteomes of lymphoblasts from 3 persons with APBD and 15 age- and gender-matched controls, with validation of the findings by targeted MS. There were 531 differentially expressed proteins out of 3,427 detected between APBD subjects vs. controls, including pronounced deficiency of GBE1. Bioinformatic analyses indicated multiple canonical pathways and protein-protein interaction networks to be statistically markedly enriched in APBD subjects, including: RNA processing/transport/translation, cell cycle control/replication, mTOR signaling, protein ubiquitination, unfolded protein and endoplasmic reticulum stress responses, glycolysis and cell death/apoptosis. Dysregulation of these processes, therefore, are primary or secondary factors in APBD pathobiology in this model system. Our findings further suggest that proteomic analysis of GBE1 mutant lymphoblasts can be leveraged as part of the screening for pharmaceutical agents for the treatment of APBD.

Energy Metabolism in Mouse Sciatic Nerve A Fibres during Increased Energy Demand

The ability of sciatic nerve A fibres to conduct action potentials relies on an adequate supply of energy substrate, usually glucose, to maintain necessary ion gradients. Under our ex vivo experimental conditions, the absence of exogenously applied glucose triggers Schwann cell glycogen metabolism to lactate, which is transported to axons to fuel metabolism, with loss of the compound action potential (CAP) signalling glycogen exhaustion. The CAP failure is accelerated if tissue energy demand is increased by high-frequency stimulation (HFS) or by blocking lactate uptake into axons using cinnemate (CIN). Imposing HFS caused CAP failure in nerves perfused with 10 mM glucose, but increasing glucose to 30 mM fully supported the CAP and promoted glycogen storage. A combination of glucose and lactate supported the CAP more fully than either substrate alone, indicating the nerve is capable of simultaneously metabolising each substrate. CAP loss resulting from exposure to glucose-free artificial cerebrospinal fluid (aCSF) could be fully reversed in the absence of glycogen by addition of glucose or lactate when minimally stimulated, but imposing HFS resulted in only partial CAP recovery. The delayed onset of CAP recovery coincided with the release of lactate by Schwann cells, suggesting that functional Schwann cells are a prerequisite for CAP recovery.

Interplay between bioenergetics and oxidative stress at normal brain aging. Aging as a result of increasing disbalance in the system oxidative stress-energy provision

At normal aging, the brain exhibits signs of compromised bioenergetic and increased levels of products of interaction between reactive oxygen/nitrogen species (ROS/RNS) and brain constituents. Under normal conditions, steady-state levels of ATP and ROS/RNS fluctuate in certain ranges providing basis for stable homeostasis. However, from time to time these parameters leave a "comfort zone," and at adulthood, organisms are able to cope with these challenges efficiently, whereas at aging, efficiency of the systems maintaining homeostasis declines. That is very true for the brain due to high ATP demands which are mainly covered by mitochondrial oxidative phosphorylation. Such active oxidative metabolism gives rise to intensive ROS generation as side products. The situation is worsened by high brain level of polyunsaturated fatty acids which are substrates for ROS/RNS attack and production of lipid peroxides. In this review, organization of energetic metabolism in the brain with a focus on its interplay with ROS at aging is discussed. The working hypothesis on aging as a disbalance between oxidative stress and energy provision as a reason for brain aging is proposed. From this point of view, normal age-related physiological decline in the brain functions results from increased disbalance between decrease in capability of the brain to control constantly increased incapability to maintain ROS levels and produce ATP due to amplification of vicious cycles intensification of oxidative stress <----> impairment of energy provision.

Healthy brain aging: Interplay between reactive species, inflammation and energy supply

Brains' high energy expenditure with preferable utilization of glucose and ketone bodies, defines the specific features of its energy homeostasis. The extensive oxidative metabolism is accompanied by a concomitant generation of high amounts of reactive oxygen, nitrogen, and carbonyl species, which will be here collectively referred to as RONCS. Such metabolism in combination with high content of polyunsaturated fatty acids creates specific problems in maintaining brains' redox homeostasis. While the levels of products of interaction between RONCS and cellular components increase slowly during the first two trimesters of individuals' life, their increase is substantially accelerated towards the end of life. Here we review the main mechanisms controlling the redox homeostasis of the mammalian brain, their age-dependencies as well as their adaptive potential, which might turn out to be much higher than initially assumed. According to recent data, the organism seems to respond to the enhancement of aging-related toxicity by forming a new homeostatic set point. Therefore, further research will focus on understanding the properties of the new set point(s), the general nature of this phenomenon and will explore the limits of brains' adaptivity.

Ambient but not local lactate underlies neuronal tolerance to prolonged glucose deprivation

Neurons require a nearly constant supply of ATP. Glucose is the predominant source of brain ATP, but the direct effects of prolonged glucose deprivation on neuronal viability and function remain unclear. In sparse rat hippocampal microcultures, neurons were surprisingly resilient to 16 h glucose removal in the absence of secondary excitotoxicity. Neuronal survival and synaptic transmission were unaffected by prolonged removal of exogenous glucose. Inhibition of lactate transport decreased microculture neuronal survival during concurrent glucose deprivation, suggesting that endogenously released lactate is important for tolerance to glucose deprivation. Tandem depolarization and glucose deprivation also reduced neuronal survival, and trace glucose concentrations afforded neuroprotection. Mass cultures, in contrast to microcultures, were insensitive to depolarizing glucose deprivation, a difference attributable to increased extracellular lactate levels. Removal of local astrocyte support did not reduce survival in response to glucose deprivation or alter evoked excitatory transmission, suggesting that on-demand, local lactate shuttling is not necessary for neuronal tolerance to prolonged glucose removal. Taken together, these data suggest that endogenously produced lactate available globally in the extracellular milieu sustains neurons in the absence of glucose. A better understanding of resilience mechanisms in reduced preparations could lead to therapeutic strategies aimed to bolster these mechanisms in vulnerable neuronal populations.