Effects of GM1-ganglioside and alpha-sialyl cholesterol on amino acid uptake, protein synthesis, and Na+,K(+)-ATPase activity in superior cervical and nodose ganglia excised from adult rats

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.

Effects of depolarizing agents on transglutaminase activity, Ca2+ influx, and protein synthesis in superior cervical and nodose ganglia excised from rats

Rapid changes in transglutaminase (TG) activity, 45Ca(2+)-influx and [3H]leucine incorporation in superior cervical ganglia (SCG), and nodose ganglia (NG) excised from adult rats were examined following addition of membrane-depolarizing agents veratridine (Ver) or high extracellular [K+]o during aerobic incubation in vitro at 37 degrees C. Addition of KCl (50 mM) stimulated TG activity to a maximal extent (four to six-fold) in SCG and NG after 30 min. Ver (0.2 mM) also increased TG activity in both ganglia after 30 min. Kinetic studies showed that the stimulation of TG activity in both ganglia caused by each depolarization condition was associated with a decrease in Km and an increase in Vmax value. The depolarizing agents Ver and high [K+]o also caused significant increases in 45Ca2+ influx into both ganglia. The Ver-induced increases in TG activity and 45Ca2+ accumulation were antagonized by tetrodotoxin (TTX, 1 microM), a sodium channel blocker. The K(+)-induced increase in TG activity was not blocked by tetraethylammonium (TEA, 20 mM), a potassium channel antagonist, although TEA did block the K(+)-induced increase in 45Ca2+ accumulation. The membrane-perturbing, sialic acid-containing compounds, GM1-ganglioside (GM1, 5 nM) and alpha-sialyl cholesterol (alpha-SC, 20 microM), were moderate inhibitors of the K(+)-induced effects on TG activity and 45Ca2+ accumulation. The sialyl compounds had little effect on Ver-induced accumulation of 45Ca2+ but enhanced the Ver-evoked stimulation in TG activity. These results suggests that the veratridine- and K(+)-induced increases in TG activity occur via modulation of Ca2+ and Na+ channel gating mechanisms that are pharmacologically distinct for each depolarizing agent. The veratridine- and K(+)-induced decrease in [3H]leucine incorporation could be a result of stimulation of TG activity as a consequence of degenerative alterations.

Selective alterations in transglutaminase activity of rat superior cervical ganglia in response to neurotransmitters, high potassium and sialic acid-containing compounds

We examined the in vitro effects of neurotransmitters, high KCl as well as sialic acid-containing compounds (GM1; SC) on transglutaminase (TG) activity in isolated superior cervical ganglia (SCG) one week after denervation or axotomy. Following denervation, TG activity in SCG decreased to 83% of the unoperated control value, whereas that of axotomized ganglia was 28% of control. Thus, TG activity was relatively unaffected when sympathetic ganglionic neurons were preserved, but was markedly reduced under conditions where neurons were degenerating. Addition of ACh (0.1 mM) to the medium during aerobic incubation stimulated TG activity more than 3-fold in denervated ganglia but had no effect on TG activity in axotomized ganglia. Similarly, the NE (0.05 mM)-induced decrease of TG activity observed in intact SCG was also seen following denervation (-49%) but not following axotomy. In denervated SCG, the stimulatory effects of ACh were virtually abolished by co-addition of the cholinergic antagonists, atropine or hexamethonium, while the suppressant effects of NE were blocked by the adrenergic antagonists, propranolol, prazosin or yohimbine. These results imply that transmitter-induced rapid changes in TG activity occur predominantly in ganglionic neurons. When the ganglia were depolarized by high KCl (50 mM), a significant increase in TG activity in each intact, denervated and axotomized SCG was seen with qualitatively similar manner, suggesting that high KCl-induced depolarization affects both neuronal and glial components in the SCG. The marked increase in ganglionic TG activity in response to GM1 (5 nM) and synthetic SC (0.02 mM) were lost in denervated SCG but only partially reduced in axotomized SCG.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of transglutaminase activity by GM1-ganglioside and alpha-sialylcholesterol in superior cervical and nodose ganglia excised from adult rat

Changes in transglutaminase (TG) activity in superior cervical ganglia (SCG) and nodose ganglia (NG) excised from adult rats were examined following application of selected membrane transport-altering agents, including GM1-ganglioside (GM1) and alpha-sialylcholesterol (alpha-SC). Although TG activity of freshly dissected SCG and NG was relatively low, it increased gradually during 30 min of incubation, and it stayed at this elevated level for 2 h. Addition of alpha-SC at its maximal effective concentration, 20 microM, stimulated TG activity more than eightfold in SCG and more than twofold in NG by 30 min. Addition of GM1 at its most effective concentration, 5 nM, had similar effects, but of lesser magnitude. Cycloheximide, a potent inhibitor of protein biosynthesis, did not affect the GM1- or alpha-SC-evoked increases in ganglionic TG activity, suggesting that enzyme activation rather than synthesis of new enzyme was occurring. The stimulation of TG activity in both ganglia caused by either GM1 or alpha-SC was associated with a decrease in Km and an increase in Vmax values. Addition of cholera toxin B, which specifically masks the oligosaccharide chain of GM1, reduced the GM1-induced increase in TG activity by approximately 60% in SCG and 88% in NG. The alpha-SC-induced increase in TG activity was only partially mimicked by free cholesterol. Although application of either dibutyryl cyclic AMP or dibutyryl cyclic GMP produced little change in TG activity of either ganglion, phorbol ester clearly inhibited the enzymic activity. Because TG is a calcium-dependent enzyme, we measured 45Ca2+ influx into either ganglion, and found that it was reduced by GM1 and alpha-SC in SCG and by alpha-SC in NG.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation by acetylcholine and inhibition by norepinephrine of transglutaminase activity in superior cervical ganglia excised from adult rats

Very rapid changes in activity of transglutaminase (TG), a calcium-dependent enzyme contributing to cross-linkage formation of intracellular polypeptide chains, were observed in vitro in rat superior cervical ganglion (SCG) and nodose ganglion (NG) following application of cholinergic or adrenergic agonists and antagonists. In SCG, a tissue rich in synapses, the depolarizing agent acetylcholine (ACh, 0.1 mM) produced an 8.7-fold increase in TG activity within 5 min that lasted for 30 min and returned to control levels by 2 h. In contrast, the ACh-induced increase in TG activity in NG, a tissue containing neuronal cell bodies with few synapses, was more gradual and of smaller magnitude, reaching a peak of approximately 2.4 times control by 30 min that was maintained for at least 2 h. In both tissues the ACh-stimulation was effectively blocked by the nicotinic antagonist, hexamethonium (0.1 mM), whereas the muscarinic antagonist, atropine (0.1 mM), partially blocked the ACh effect in SCG and was without effect in NG. Addition of the hyperpolarizing adrenergic agonists norepinephrine (NE, 50 microM), isoproterenol (0.2 mM) or dopamine (0.1 mM) produced an inhibition of TG activity in SCG but had no effect in NG. The inhibitory effects of the adrenergic agonists in SCG were blocked by the beta-adrenergic antagonist, propranolol (10 microM) and alpha 2-adrenergic antagonist, yohimbine (10 microM). A kinetic study revealed that the ACh-induced stimulation of TG activity in SCG and NG was a result of decrease in apparent Km and an increase in Vmax value, whereas the NE-induced inhibition of SCG enzyme activity was a result of an increased Km and decreased Vmax. 45Ca2+ influx into excised SCG or NG was significantly reduced by the application of either ACh or NE. The ACh inhibition was effectively blocked by either hexamethonium or atropine. The NE inhibition was more effectively blocked by yohimbine than by propranolol. These results suggest that the rapid alterations of TG activity in SCG produced by cholinergic and adrenergic neurotransmitters are attributable to the processes of receptor-mediated depolarization and hyperpolarization, respectively, via modulation of nerve-impulse-induced Ca2+ fluxes during synaptic activity.