Onset and progression of motor deficits in motor neuron degeneration (mnd) mice are unaltered by the glycine/NMDA receptor antagonist L-701,324 or the MAO-B inhibitor R(-)-deprenyl

Subunit-specific effects of poricoic acid A on NMDA receptors

BACKGROUND

N-methyl-D-aspartate (NMDA) receptor is a tetrameric protein complex composed of glycine-linked NR1 subunits and glutamate-linked NR2 subunits. There are four NR2 subunits (A-D) that differ in development, anatomy, and function profiles. They play various roles in normal and neuropathologic conditions. Specific agonists, antagonists, and modulators of subunits for selective NMDA receptors may be precious mediational tools and potent agents for treating diseases. The objective of this study was to determine the effect of poricoic acid A on NMDA receptor known to mediate excitatory synaptic transmission factors and cause changes in synaptic strength. Inhibitory effect of poricoic acid A on NR1a combined with NR2A, NR2B, NR2C, or NR2D receptor was evaluated.

METHODS

Glutamate-mediated currents for each NR1a and NR2 subunits were investigated using two-electrode voltage-clamp techniques. Molecular modeling and molecular dynamics simulation studies were carried out with Autodock Tools. Poricoic acid A and NMDA receptor protein complex were examined with Ligplot and Pymol docking program. Ligplot shows binding activity at the protein and the ligand.

RESULTS

The inhibitory effect of poricoic acid A on glutamate-induced inward current in a concentration-dependent manner that was reversible. Half inhibitory concentrations of glutamate on NR1a/NR2A, NR1a/NR2B, NR1a/NR2C, and NR1a/NR2D receptors were 9.6 ± 1.2, 5.7 ± 0.4, 46.1 ± 21.5, and 21.5 ± 8.2 μM, respectively. This corresponded to the order of inhibitory effect of oocyte expressing NR1a and NR2s subunit of NR1a/NR2B > NR1a/NR2A > NR1a/NR2C > NR1a/NR2D.

CONCLUSIONS

Taken together, these results indicate that poricoic acid A can modulate the expression of NMDA receptor. In addition, the regulation of excitatory ligand-gating ion channel by poricoic acid A may have pharmaceutical functions on excitatory synaptic transmission of neuronal system.

The contribution of multicellular model organisms to neuronal ceroid lipofuscinosis research

The NCLs (neuronal ceroid lipofuscinosis) are forms of neurodegenerative disease that affect people of all ages and ethnicities but are most prevalent in children. Commonly known as Batten disease, this debilitating neurological disorder is comprised of 13 different subtypes that are categorized based on the particular gene that is mutated (CLN1-8, CLN10-14). The pathological mechanisms underlying the NCLs are not well understood due to our poor understanding of the functions of NCL proteins. Only one specific treatment (enzyme replacement therapy) is approved, which is for the treating the brain in CLN2 disease. Hence there remains a desperate need for further research into disease-modifying treatments. In this review, we present and evaluate the genes, proteins and studies performed in the social amoeba, nematode, fruit fly, zebrafish, mouse and large animals pertinent to NCL. In particular, we highlight the use of multicellular model organisms to study NCL protein function, pathology and pathomechanisms. Their use in testing novel therapeutic approaches is also presented. With this information, we highlight how future research in these systems may be able to provide new insight into NCL protein functions in human cells and aid in the development of new therapies.

Treatment of the Ppt1(-/-) mouse model of infantile neuronal ceroid lipofuscinosis with the N-methyl-D-aspartate (NMDA) receptor antagonist memantine

The neuronal ceroid lipofuscinoses, a family of neurodegenerative lysosomal storage disorders, represent the most common cause of pediatric-onset neurodegeneration. The infantile form has a devastatingly early onset and one of the fastest-progressing disease courses. Despite decades of research, the molecular mechanisms driving neuronal loss in infantile neuronal ceroid lipofuscinosis remain unknown. We have previously shown that N-methyl-d-aspartate (NMDA)-type glutamate receptors in the Ppt1(-/-) mouse model of this disease exhibit a hyperfunctional phenotype and postulate that aberrant glutamatergic activity may contribute to neural pathology in both the mouse model and human patients. To test this hypothesis, we treated Ppt1(-/-) mice with the NMDA receptor antagonist memantine and assessed their response to the drug using an accelerating rotarod. At 20 mg/kg, memantine treatment induced a delayed but notable improvement in Ppt1(-/-) mice. Much remains to be assessed before moving to patient trials, but these results suggest memantine has potential as a treatment.

Biosynthesis of the major brain gangliosides GD1a and GT1b

Gangliosides-sialylated glycosphingolipids-are the major glycoconjugates of nerve cells. The same four structures-GM1, GD1a, GD1b and GT1b-comprise the great majority of gangliosides in mammalian brains. They share a common tetrasaccharide core (Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1-1'Cer) with one or two sialic acids on the internal galactose and zero (GM1 and GD1b) or one (GD1a and GT1b) α2-3-linked sialic acid on the terminal galactose. Whereas the genes responsible for the sialylation of the internal galactose are known, those responsible for terminal sialylation have not been established in vivo. We report that St3gal2 and St3gal3 are responsible for nearly all the terminal sialylation of brain gangliosides in the mouse. When brain ganglioside expression was analyzed in adult St3gal1-, St3gal2-, St3gal3- and St3gal4-null mice, only St3gal2-null mice differed significantly from wild type, expressing half the normal amount of GD1a and GT1b. St3gal1/2-double-null mice were no different than St3gal2-single-null mice; however, St3gal2/3-double-null mice were >95% depleted in gangliosides GD1a and GT1b. Total ganglioside expression (lipid-bound sialic acid) in the brains of St3gal2/3-double-null mice was equivalent to that in wild-type mice, whereas total protein sialylation was reduced by half. St3gal2/3-double-null mice were small, weak and short lived. They were half the weight of wild-type mice at weaning and displayed early hindlimb dysreflexia. We conclude that the St3gal2 and St3gal3 gene products (ST3Gal-II and ST3Gal-III sialyltransferases) are largely responsible for ganglioside terminal α2-3 sialylation in the brain, synthesizing the major brain gangliosides GD1a and GT1b.

Memantine prolongs survival in an amyotrophic lateral sclerosis mouse model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease which results from selective loss of upper and lower motor neurons. Mouse models of ALS, such as one carrying the G93A mutant of the human Cu-Zn superoxide dismutase gene[SOD1(G93A)], develop motor neuron pathology and clinical symptoms similar to those observed in ALS patients. There is compelling evidence that both direct and indirect glutamate toxicity contribute to the pathogenesis of motor neuron degeneration. However, the therapeutic effect of various glutamate receptor antagonists has not been clearly demonstrated. Memantine is a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. It has been shown to protect neurons against NMDA- or glutamate-induced toxicity in vitro and in animal models of neurodegenerative diseases. In the current study, we have examined the therapeutic efficacy of memantine in an ALS mouse model carrying a high copy number of SOD1(G93A). Memantine treatment significantly delayed the disease progression and increased the life span of SOD1(G93A) mice, from 121.4 +/- 5.5 to 129.7 +/- 4.5 days (P = 0.032). Furthermore, NMDA receptor subunits were reliably detected in the spinal cord of SOD1(G93A) mice and their expression levels were similar to those in the wild-type littermate control. Therefore, the neuroprotective effect of memantine in SOD1(G93A) mice is most probably due to the inhibition of spinal cord NMDA receptors. In view of the long-term usage of memantine for dementia patients, with excellent tolerance and safety, these data suggest that memantine may be used in ALS patients alone or in combination with other therapies to prolong survival.

Selectivity and types of cell death in the neuronal ceroid lipofuscinoses

Cloning of the individual genes that are mutated in the neuronal ceroid lipofuscinoses (NCLs), or Batten disease, has opened up new avenues of research into the pathogenesis of these fatal autosomal recessive storage disorders. Genetically accurate mouse models have now been generated for each major form of the disorder, together with several variant forms. Ongoing analysis of these mice is revealing significant new data about the staging and progression of disease phenotypes. Combined with data from human autopsy tissues and large animal models, it is now clear that neurodegeneration is initially selective in the NCL CNS, targeting specific regions and particular cell populations. There is also evidence of selective glial activation that appears to precede obvious neurodegeneration, becoming more widespread with disease progression. Currently, there is debate over the mechanisms of cell death that operate in each form of NCL, with evidence of both apoptosis and autophagy. It is likely that these mechanisms may encompass a spectrum of cell death events, depending upon the specific context of each neuronal population. Taken together, these data have significant clinical implications for the development and targeting of appropriate therapeutic strategies, and for providing the landmarks to judge their efficacy.

The development of behavioral abnormalities in the motor neuron degeneration (mnd) mouse

The motor neuron degeneration (mnd) mouse, which has widespread abnormal accumulating lipoprotein and neuronal degeneration, has a mutation in CLN8, the gene for human progressive epilepsy with mental retardation (EPMR). EPMR is one of the neuronal ceroid lipofuscinoses (NCLs), a group of neurological disorders characterized by autofluorescent lipopigment accumulation, blindness, seizures, motor deterioration, and dementia. The human phenotype of EPMR suggests that, in addition to the motor symptoms previously categorized, various types of progressive behavioral abnormalities would be expected in mnd mice. We have therefore examined exploratory behavior, fear conditioning, and aggression in 2-3 month and 4-5 month old male mnd mice and age-matched C57BL/6 (B6) controls. The mnd mice displayed increased activity with decreased habituation in the activity monitor, poor contextual and cued memory, and heightened aggression relative to B6 controls. These behavioral deficits were most prominent at 4-5 months of age, which is prior to the onset of gross motor symptoms at 6 months. Our results provide a link from the mutation via pathology to a quantifiable multidimensional behavioral phenotype of this naturally occurring mouse model of NCL.

A functional role for complex gangliosides: motor deficits in GM2/GD2 synthase knockout mice

Although gangliosides are abundant molecular determinants on all vertebrate nerve cells (comprising approximately 1.5% of brain dry weight) their functions have remained obscure. We report that mice engineered to lack a key enzyme in complex ganglioside biosynthesis (GM2/GD2 synthase), and which express only the simple ganglioside molecular species GM3 and GD3, develop significant and progressive behavioral neuropathies, including deficits in reflexes, strength, coordination, and balance. Quantitative indices of motor abilities, applied at 8 and 12 months of age, also revealed progressive gait disorders in complex ganglioside knockout mice compared to controls, including reduced stride length, stride width, and increased hindpaw print length as well as a marked reduction in rearing. Compared to controls, null mutant mice tended to walk in small labored movements. Twelve-month-old complex ganglioside knockout mice also displayed significant incidence of tremor and catalepsy. These comprehensive neurobehavioral studies establish an essential role for complex gangliosides in the maintenance of normal neural physiology in mice, consistent with a role in maintaining axons and myelin (Sheikh, K. A. , J. Sun, Y. Liu, H. Kawai, T. O. Crawford, R. L. Proia, J. W. Griffin, and R. L. Schnaar. 1999. Mice lacking complex gangliosides develop Wallerian degeneration and myelination defects. Proc. Natl. Acad. Sci. USA 96: 7532-7537), and may provide insights into the mechanisms underlying certain neural degenerative diseases.

The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways

Many damage-sensing neurons express tetrodotoxin (TTX)-resistant voltage-gated sodium channels. Here we examined the role of the sensory-neuron-specific (SNS) TTX-resistant sodium channel alpha subunit in nociception and pain by constructing sns-null mutant mice. These mice expressed only TTX-sensitive sodium currents on step depolarizations from normal resting potentials, showing that all slow TTX-resistant currents are encoded by the sns gene. Null mutants were viable, fertile and apparently normal, although lowered thresholds of electrical activation of C-fibers and increased current densities of TTX-sensitive channels demonstrated compensatory upregulation of TTX-sensitive currents in sensory neurons. Behavioral studies demonstrated a pronounced analgesia to noxious mechanical stimuli, small deficits in noxious thermoreception and delayed development of inflammatory hyperalgesia. These data show that SNS is involved in pain pathways and suggest that blockade of SNS expression or function may produce analgesia without side effects.