Increase of GQ1b in the hippocampus of mice following kindled-seizures

Start Me Up: How Can Surrounding Gangliosides Affect Sodium-Potassium ATPase Activity and Steer towards Pathological Ion Imbalance in Neurons?

Gangliosides, amphiphilic glycosphingolipids, tend to associate laterally with other membrane constituents and undergo extensive interactions with membrane proteins in cis or trans configurations. Studies of human diseases resulting from mutations in the ganglioside biosynthesis pathway and research on transgenic mice with the same mutations implicate gangliosides in the pathogenesis of epilepsy. Gangliosides are reported to affect the activity of the Na⁺/K⁺-ATPase, the ubiquitously expressed plasma membrane pump responsible for the stabilization of the resting membrane potential by hyperpolarization, firing up the action potential and ion homeostasis. Impaired Na⁺/K⁺-ATPase activity has also been hypothesized to cause seizures by several mechanisms. In this review we present different epileptic phenotypes that are caused by impaired activity of Na⁺/K⁺-ATPase or changed membrane ganglioside composition. We further discuss how gangliosides may influence Na⁺/K⁺-ATPase activity by acting as lipid sorting machinery providing the optimal stage for Na⁺/K⁺-ATPase function. By establishing a distinct lipid environment, together with other membrane lipids, gangliosides possibly modulate Na⁺/K⁺-ATPase activity and aid in “starting up” and “turning off” this vital pump. Therefore, structural changes of neuronal membranes caused by altered ganglioside composition can be a contributing factor leading to aberrant Na⁺/K⁺-ATPase activity and ion imbalance priming neurons for pathological firing.

Gangliosidome of a Human Hippocampus in Temporal Lobe Epilepsy Resolved by High-Resolution Tandem Mass Spectrometry

In this study, we developed a high-resolution tandem mass spectrometry (HR MS) approach to assess presumed changes in gangliosidome of a human hippocampus affected by temporal lobe epilepsy (TLE) in comparison with a normal hippocampus. Gangliosides, membrane glycolipids, are particularly diverse and abundant in the human brain, and participate in ion transport and modulation of neuronal excitability. Changes in structural ganglioside pattern potentially linked to TLE molecular pathogenesis have not been explored in detail. Aiming to characterize TLE-specific gangliosidome, we analyzed the native gangliosides purified from a human hippocampal tissue sample affected by TLE and a control hippocampus using HR MS. Marked differences of ganglioside expression were shown in TLE vs. control, particularly with respect to the sialylation degree of components, discovered as a characteristic feature of TLE. Another major finding is the occurrence of tetrasialofucogangliosides in TLE and species modified by either O-acetylation or CH3COO−. Structural analysis by higher-energy collisional dissociation (HCD) MS/MS gave rise to fragmentation patterns implying that the GQ1b (d18:1/18:0) isomer is specifically associated with TLE. Further investigation in a larger sample is needed in order to confirm the discovery of ganglioside structures specifically expressed in human TLE and to provide information on the probable role of gangliosides in the molecular events underlying seizures.

Gangliosides in the Brain: Physiology, Pathophysiology and Therapeutic Applications

Gangliosides are glycosphingolipids highly abundant in the nervous system, and carry most of the sialic acid residues in the brain. Gangliosides are enriched in cell membrane microdomains ("lipid rafts") and play important roles in the modulation of membrane proteins and ion channels, in cell signaling and in the communication among cells. The importance of gangliosides in the brain is highlighted by the fact that loss of function mutations in ganglioside biosynthetic enzymes result in severe neurodegenerative disorders, often characterized by very early or childhood onset. In addition, changes in the ganglioside profile (i.e., in the relative abundance of specific gangliosides) were reported in healthy aging and in common neurological conditions, including Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), stroke, multiple sclerosis and epilepsy. At least in HD, PD and in some forms of epilepsy, experimental evidence strongly suggests a potential role of gangliosides in disease pathogenesis and potential treatment. In this review, we will summarize ganglioside functions that are crucial to maintain brain health, we will review changes in ganglioside levels that occur in major neurological conditions and we will discuss their contribution to cellular dysfunctions and disease pathogenesis. Finally, we will review evidence of the beneficial roles exerted by gangliosides, GM1 in particular, in disease models and in clinical trials.

Rapid regulation of sialidase activity in response to neural activity and sialic acid removal during memory processing in rat hippocampus

Sialidase cleaves sialic acids on the extracellular cell surface as well as inside the cell and is necessary for normal long-term potentiation (LTP) at mossy fiber-CA3 pyramidal cell synapses and for hippocampus-dependent spatial memory. Here, we investigated in detail the role of sialidase in memory processing. Sialidase activity measured with 4-methylumbelliferyl-α-d-N-acetylneuraminic acid (4MU-Neu5Ac) or 5-bromo-4-chloroindol-3-yl-α-d-N-acetylneuraminic acid (X-Neu5Ac) and Fast Red Violet LB was increased by high-K⁺-induced membrane depolarization. Sialidase activity was also increased by chemical LTP induction with forskolin and activation of BDNF signaling, non-NMDA receptors, or NMDA receptors. The increase in sialidase activity with neural excitation appears to be caused not by secreted sialidase or by an increase in sialidase expression but by a change in the subcellular localization of sialidase. Astrocytes as well as neurons are also involved in the neural activity-dependent increase in sialidase activity. Sialidase activity visualized with a benzothiazolylphenol-based sialic acid derivative (BTP3-Neu5Ac), a highly sensitive histochemical imaging probe for sialidase activity, at the CA3 stratum lucidum of rat acute hippocampal slices was immediately increased in response to LTP-inducible high-frequency stimulation on a time scale of seconds. To obtain direct evidence for sialic acid removal on the extracellular cell surface during neural excitation, the extracellular free sialic acid level in the hippocampus was monitored using in vivo microdialysis. The free sialic acid level was increased by high-K⁺-induced membrane depolarization. Desialylation also occurred during hippocampus-dependent memory formation in a contextual fear-conditioning paradigm. Our results show that neural activity-dependent desialylation by sialidase may be involved in hippocampal memory processing.

Sialyltransferase ST3Gal IV deletion protects against temporal lobe epilepsy

Temporal lobe epilepsy (TLE) often becomes refractory, and patients with TLE show a high incidence of psychiatric symptoms, including anxiety and depression. Therefore, it is necessary to identify molecules that were previously unknown to contribute to epilepsy and its associated disorders. We previously found that the sialyltransferase ST3Gal IV is up-regulated within the neural circuits through which amygdala-kindling stimulation propagates epileptic seizures. In contrast, this study demonstrated that kindling stimulation failed to evoke epileptic seizures in ST3Gal IV-deficient mice. Furthermore, approximately 80% of these mice failed to show tonic-clonic seizures with stimulation, whereas all littermate wild-type mice showed tonic-clonic seizures. This indicates that the loss of ST3Gal IV does not cause TLE in mice. Meanwhile, ST3Gal IV-deficient mice exhibited decreased acclimation in the open field test, increased immobility in the forced swim test, enhanced freezing during delay auditory fear conditioning, and sleep disturbances. Thus, the loss of ST3Gal IV modulates anxiety-related behaviors. These findings indicate that ST3Gal IV is a key molecule in the mechanisms underlying anxiety - a side effect of TLE - and may therefore also be an effective target for treating epilepsy, acting through the same circuits.