GM1 and NGF synergism on choline acetyltransferase and choline uptake in aged brain

GM1 and NGF modulate Ca2+ homeostasis and GAP43 mRNA expression in cultured dorsal root ganglion neurons with excitotoxicity induced by glutamate

Monosialoganglioside (GM1) has been considered to have a neurotrophic factor-like activity. Nerve growth factor (NGF), a member of the neurotrophin family, is essential for neuronal survival, differentiation and maturation. The aim of the present study was to investigate whether co-administration of GM1 and NGF reverses glutamate (Glu) neurotoxicity in primary cultured rat embryonic dorsal root ganglion (DRG) neurons. DRG neurons were exposed to Glu (2 mmol/1), Glu (2 mmol/1) plus GM1 (10 microg/ml), Glu (2 mmol/l) plus NGF (10 ng/ml), Glu (2 mmol/l) plus GM1 (5 microg/ml) and NGF (5 ng/ml) and then processed for detecting intracellular concentrations of Ca2+ ([Ca2+] i) by confocal laser scanning microscopy and growth-associated protein 43 (GAP43) mRNA by RT-PCR. The fluorescent intensity in Glu plus GM1 and NGF incubated neurons was the lowest as compared with that in other groups. The expression of GAP43 mRNA in Glu plus GM1 and NGF incubated neurons was the highest as compared with that in other groups. These results implicated that GM1 and NGF have synergistic neuroprotective effects on DRG neurons with excitotoxicity induced by Glu in vitro.

Regulation of axonal development by plasma membrane gangliosides

Gangliosides present in the plasma membrane participate in fundamental processes during neuronal development. From the determination and the outgrowth of the axon, to the growth inhibitory activity produced after CNS injury, local interconversion of these glycosphingolipids regulate actin dynamics in a spatially restricted manner by modulating membrane receptors and their downstream signaling pathways. Here, we will review the possible mechanisms underlying these modulations and the potential importance of gangliosides and ganglioside-transforming enzymes as therapeutic targets.

GM1 and ERK signaling in the aged brain

We investigated the ability of GM1 to induce phosphorylation/activation of the extracellular-regulated protein kinases (ERKs) in the striatum, hippocampus and frontal cortex of aged male Sprague-Dawley rats. Three different treatment paradigms were used: a single application of GM1 to brain slices in situ, a single intracerebroventricular (icv) administration of GM1 in vivo, and chronic administration of GM1 in vivo. In situ, GM1 induced a rapid and transient activation of ERK1 and ERK 2 in both young and aged rats, and a similar effect was observed after stimulation with the neurotrophins NGF and BDNF. The aged brain appeared to respond more robustly to neurotrophic stimulation with the pERK2 response being significantly greater in the hippocampus and frontal cortex. Acute icv administration of GM1 resulted in short-lasting phosphorylation of ERKs in both aged groups, while chronic administration of GM1 induced a protracted phosphorylation of ERKs. Following chronic GM1 treatment, pERK2 levels in the aged hippocampus were elevated over young control animals. In agreement with reports that GM1 phosphorylates TrkA in vitro or in situ, treatment with GM1 increased the phosphorylation of TrkA in hippocampus of both young and aged animals. These observations indicate that the aged brain maintains the ability to respond to neurotrophic stimuli and put forward the proposition that the ERK cascade is associated with the action(s) of GM1 ganglioside in vivo.

Expression of high affinity choline transporter during mouse development in vivo and its upregulation by NGF and BMP-4 in vitro

An important feature of cholinergic neurons is high-affinity choline transport, which allows them to reuse choline for the synthesis of ACh needed to support cholinergic neurotransmission. The choline transporter, designated CHT, was recently cloned. We applied RT/PCR to monitor the expression of CHT in the developing mouse CNS from embryonic day 14 (E14) to postnatal day 30 (P30). We found that CHT was expressed early in development, predominantly in the regions containing cholinergic neurons. In the spinal cord, CHT mRNA was present at close to adult levels at the earliest time point examined (E14) and showed almost no changes after birth. In the striatum and the septum, CHT mRNA increased steadily during embryonic stages and leveled off after birth. Surprisingly, CHT mRNA expression was also detected in other brain regions, notably in the cerebellum, where it peaked on E19, and then rapidly declined during postnatal development. CHT protein was detected by Western blotting as a band of apparent molecular weight of 70 kDa. The accumulation of this protein during development lagged behind mRNA accumulation in all tissues. We also examined the effects of NGF and BMP-4, the potent inducers of choline acetyltransferase and vesicular acetylcholine transporter genes, on CHT expression. Both factors increased CHT mRNA accumulation in primary septal cultures. The effect of NGF was dependent on the PI3K signaling, as it was abolished by the PI3K inhibitor LY294002. This result indicates that some of the signals regulating other cholinergic-specific genes also control CHT expression.

GM1 ganglioside induces phosphorylation and activation of Trk and Erk in brain

We investigated the ability of GM1 to induce phosphorylation of the tyrosine kinase receptor for neurotrophins, Trk, in rat brain, and activation of possible down-stream signaling cascades. GM1 increased phosphorylated Trk (pTrk) in slices of striatum, hippocampus and frontal cortex in a concentration- and time-dependent manner, and enhanced the activity of Trk kinase resulting in receptor autophosphorylation. The ability of GM1 to induce pTrk was shared by other gangliosides, and was blocked by the selective Trk kinase inhibitors K252a and AG879. GM1 induced phosphorylation of TrkA > TrkC > TrkB in a region-specific distribution. Adding GM1 to brain slices activated extracellular-regulated protein kinases (Erks) in all three brain regions studied. In striatum, GM1 elicited activation of Erk2 > Erk1 in a time-and concentration-dependent manner. The GM1 effect on Erk2 was mimicked by other gangliosides, and was blocked by the Trk kinase inhibitors K252a and AG879. Pertussis toxin, as well as Src protein tyrosine kinase and protein kinase C inhibitors, did not prevent the GM1-induced activation of Erk2, apparently excluding the participation of Gi and Gq/11 protein-coupled receptors. Intracerebroventricular administration of GM1 induced a transient phosphorylation of TrkA and Erk1/2 in the striatum and hippocampus complementing the in situ studies. These observations support a role for GM1 in modulating Trk and Erk phosphorylation and activity in brain.

Lipid peroxidation and aluminium effects on the cholinergic system in nerve terminals

In the present study, we analyzed how aluminium and oxidative stress induced by ascorbate/Fe(2+) affect the mechanisms related with the cholinergic system in a crude synaptosomal fraction isolated from rat brain. [(3)H]Choline uptake, [(3)H]acetylcholine release, membrane potential and Na(+)/K(+)-ATPase activity were determined in the presence or in the absence of aluminium in control conditions and in the presence of ascorbate (0.8 mM)/Fe(2+) (2.5 micro M). The extent of lipid peroxidation was measured by quantifying thiobarbituric acid reactive substances (TBARS). Under oxidizing conditions aluminium increased the formation of TBARS by about 30%, but was without effect when the synaptosomal preparation was incubated in the absence of oxidants. Additionally, aluminium potentiated the inhibition of the high-affinity [(3)H]choline uptake observed following lipid peroxidation and had the same effect on the Na(+)/K(+)-ATPase activity. [(3)H]Acetylcholine release induced by 4-aminopyridine, and membrane potential were not significantly affected under oxidizing conditions, either in the absence or in the presence of aluminium. We can conclude that aluminium, by potentiating lipid peroxidation, affects the uptake of choline in nerve endings. This effect, occurring during brain oxidative injury, might contribute to the cholinergic dysfunction and neuronal cell degeneration known to occur in Alzheimer's disease.

Topography of neurochemical alterations in the CNS of aged rats

We have performed a general survey study on alterations of neurotransmitter-related and glia-related neurochemical markers in various regions of the CNS of aged (30-month-old) as compared to adult (4-month-old) rats. We have found significant decreases in the level of neurochemical parameters related to the cholinergic and GABAergic systems in several regions of the CNS of aged rats. Only few of the alterations present at the age of 30 months, were present in a group of rat of intermediate age (20 months) included in the present study. Less widespread alterations were found concerning the glutamatergic neurotransmission system. Neurochemical markers related to glial cells (astrocytes and oligodendrocytes) showed a remarkable stability in aged rats as compared to neurotransmitter-related markers. Considering the various CNS areas examined in the present study, the spinal cord of the aged rats was the region showing the most profound alterations of neurochemical parameters, as compared to the various brain areas of the same rats. The present results suggest that moderate and region-specific alterations of neurotransmitter-related parameters occur during normal aging and that glia-related markers are fundamentally stable in the absence of specific pathologies.

Retinal cholinergic and dopaminergic deficits of aged rats are improved following treatment with GM1 ganglioside

Selected cholinergic and dopaminergic markers were compared in the retina of aged (20-22-months-old) and young (3-months-old) rats before and after treatment with GM1 ganglioside. The dopaminergic markers, tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid were comparable in the young and aged animals and GM1 treatment did not alter them. In contrast, mazindol binding, a marker for the dopamine transporter, was diminished in the aged retina and treatment with GM1 restored binding to values found in the young animals. The cholinergic markers choline acetyltransferase and hemicholinium-3 binding, a marker for the high-affinity choline transport, were depressed in aged rats and GM1 corrected the deficits.

GM1 ganglioside restores dopaminergic neurochemical and morphological markers in aged rats

The monosialoganglioside GM1 exerts neurotrophic-like activity in vitro and in vivo. In particular, it improves cholinergic neuron morphology and chemistry and learning abilities of cognitively impaired aged rats and young animals with cholinergic lesions, and restores neurochemical, pharmacological, morphological and behavioral parameters in animal models of Parkinson's disease. Our studies present evidence that GM1 reverses dopaminergic deficits in the nigrostriatal neurons of aged rats. GM1 administered to aged Sprague-Dawley rats for 30 days reversed the decreased activity of tyrosine hydroxylase in the midbrain and striatum, elevated the reduced protein content and mRNA levels of the enzyme in the midbrain, and reversed the decrements of dopamine and 3,4-dihydroxyphenylacetic acid content in both the midbrain and striatum. Tyrosine hydroxylase activity of the midbrain, but not of the striatum, remained elevated for 15 days after discontinuing GM1. The count profiles of tyrosine hydroxylase-immunopositive neurons, the size of tyrosine hydroxylase-immunopositive neurons and the number of tyrosine hydroxylase-immunopositive fibers were decreased in the substantia nigra pars compacta and the ventral tegmental area of aged rats. GM1 corrected the morphology of dopaminergic neurons in the substantia nigra pars compacta and partially improved it in the ventral tegmental area. These findings support the notion that the aged striatal dopaminergic neurons respond to GM1, and strengthen the utility of using this compound for combating age-associated neuronal deficits.

Decreased neuropeptide content in the spinal cord of aged rats: the effect of GM1 ganglioside

This study investigated the status of substance P (SP), methionine-enkephalin (Met-Enk) and dynorphin A(1-13) (Dyn A) in the spinal cord of aged Sprague-Dawley rats and the effect of GM1 ganglioside on these neuropeptides. SP and Met-Enk, but not Dyn A, were decreased in both dorsal and ventral horns of the aged spinal cord. Treatment with GM1 ganglioside (30 mg/kg i.p., daily for 30 days) restored, in part, the neuropeptide deficits in the ventral horns, but not in the dorsal horns. This information might be important for understanding the sensory and motor deficits associated with ageing, and how the spinal cord neuropeptides might be amplified in the aged spinal cord.

GM1 and the aged brain

Aging is associated with the loss of brain neurotransmitter function, which apparently is the substrate for an adverse constellation of age-associated symptoms. In particular, cholinergic deficits have been associated with cognitive impairment in aging. Systemic administration of GM1 ganglioside, 30 mg/kg, i.p., for 30 days, enhances the cholinergic neurochemical presynaptic markers, choline acetyltransferase, choline uptake, and acetylcholine, in the brain and spinal cord of aged 22-24-month-old Sprague-Dawley rats. In addition to correcting cholinergic neurochemistry, it improves spatial learning and memory impairment, and restores the number and the size of the cholinergic neurons in the basal forebrain and striatum. The induced neuronal recovery by GM1 is long-lasting.

Sphingolipids as receptor modulators. An overview

Glycosphingolipids are amphipathic compounds that exist mainly in the plasmalemma with their oligosaccharide portion protruding into the extracellular environment. In this position they are admirably situated for interacting with both ligands and receptors. Binding studies have demonstrated that specific glycolipids function as receptors for some microorganisms and bacterial toxins. Specific oligosaccharides on both glycolipids and glycoproteins bind members of the selection families, and some gangliosides facilitate integrins binding to their ligands. Gangliosides modulate the trophic factor-stimulated dimerization, tyrosine phosphorylation, and subsequent signal transduction events of several tyrosine kinase receptors. GM3 inhibits both the epidermal growth factor receptor and basic fibroblast factor receptor; several gangliosides except GM3 inhibit the platelet-derived growth-factor receptor; GM1 enhances nerve growth-factor-stimulated activation of TrkA; insulin receptor is inhibited to varying degrees by several gangliosides, but 2-->3 sialosylparagloboside is most effective. Activities of the beta(1)-adrenergic and delta-opioid receptors are modulated by GM1. Available information suggests that glycolipids serve as coordinators of multiple receptor functions.

Cholinergic deficits in aged rat spinal cord: restoration by GM1 ganglioside

Cholinergic neurons of spinal cord are central for the processing of motor, autonomic, and sensory modalities. Aging is associated with a variety of motor and autonomic symptoms that might be attributed, in part, to impaired spinal cord function. We found that cholinergic neurochemistry is diminished in the spinal cord of 22-24-month-old rats compared with 3-month-old rats. Choline acetyltransferase, high-affinity choline transport and hemicholinium-3 binding to the choline carrier were reduced in the aged spinal cord. The activity of the choline transporter and the hemicholinium-3 binding were decreased in all spinal segments, cervical, thoracic, lumbar and sacral. Hemicholinium-3 binding was reduced in ventral and dorsal horns along all spinal segments. The activity of choline acetyltransferase was decreased only in cervical and lumbar cord. Treatment of aged animals with GM1 induced the recovery of the presynaptic cholinergic markers in the aged spinal cord.

GM1 ganglioside improves spatial learning and memory of aged rats

GM1 ganglioside, 30 mg/kg, i.p., was administered to cognitively impaired aged rats for 30 days, and spatial learning and memory evaluated in a Morris water maze paradigm. During treatment with GM1, aged animals improved both the acquisition and retention of place navigation, as reflected by reduced escape latencies and swim distances to a hidden platform, and persistently performed better than the aged control animals. Furthermore, the GM1-treated animals showed improved spatial acuity in a spatial probe test when the hidden platform was removed. The improved performance in place navigation was not lost if GM1 treatment was discontinued and the animals tested up to 15 days later. GM1 treatment had no effect on the performance of young rats in the water maze. These results indicate that memory deficits associated with aging can be attenuated by treatment with GM1 ganglioside.

Intravitreal injection of ganglioside GM1 after ischemia reduces retinal damage in rats

BACKGROUND AND PURPOSE

Gangliosides are normal components of cell membranes and contribute to structural rigidity and membrane function. They have been shown to protect against various insults in the brain. We have shown previously that GM1 administered intraperitoneally before the induction of retinal ischemia provides a protective effect. This study evaluates the protective effect of GM1 administered intravitreally after ischemia on retinal lesions.

METHODS

We induced retinal ischemia unilaterally in Long-Evans rats by increasing intraocular pressure to 160 mm Hg for 60 minutes. GM1 (20 microL x 10(-5) mol/L) or saline (20 microL) was injected into the vitreous 15 minutes after ischemia, and the postischemic survival time was either 8 or 15 days. The degree of retinal damage was assessed by histopathological study.

RESULTS

Retinal ischemia led to reductions in thickness and cell number, principally in the inner retinal layers (39% to 80%) and to a lesser extent in the outer retinal layers (26% to 45%). Postischemic treatment with intravitreally injected GM1 conferred significant protection against retinal ischemic damage after both 8 and 15 days of survival time. After 8 days of reperfusion, the ischemia-induced loss in overall retinal thickness was reduced by 15% and those of the inner nuclear and plexiform layers by 44% and 17%, respectively. Ischemic-induced ganglion cell and inner nuclear cell density losses were reduced by 37% and 27%, respectively. After 15 days of reperfusion, approximately the same statistically significant differences could be observed in comparison with the 15-day saline-injected group.

CONCLUSIONS

GM1 protects the rat retina from pressure-induced ischemic injury when given intravitreally after the insult. The protection provided by GM1 after initiation of retinal damage could be of therapeutic interest.