Alternative splicing modulates protein arginine methyltransferase-dependent methylation of fragile X syndrome mental retardation protein

The fragile X mental retardation protein (FMRP) is an RNA binding protein that is methylated by an endogenous methyltransferase in rabbit reticulocyte lysates. We mapped the region of methylation to the C-terminal arginine-glycine-rich residues encoded by FMR1 exon 15. We additionally demonstrated that mutation of R(544) to K reduced the endogenous methylation by more than 80%, while a comparable mutant R(546)-K reduced the endogenous methylation by 20%. These mutations had no effect on the subcellular distribution of FMRP, recapitulating previous results using the methyltransferase inhibitor adenosine-2',3'-dialdehyde. Using purified recombinant protein arginine methyltransferases (PRMTs), we showed that the C-terminal domain could be methylated by PRMT1, PRMT3, and PRMT4 in vitro and that both the R(544)-K mutant and the R(546)-K mutant were refractory toward these enzymes. We also report that truncating the N-terminal 12 residues encoded by FMR1 exon 15, which occurs naturally via alternative splicing, had no effect on FMRP methylation, demonstrating conclusively that phosphorylation of serine residue 500 (S(500)), one of the 12 residues, was not required for methylation. Nevertheless, truncating 13 additional amino acids, as occurs in the smallest alternatively spliced variant of FMR1 exon 15, reduced methylation by more than 85%. This suggests that differential expression and methylation of the FMRP exon 15 variants may be an important means of regulating target mRNA translation, which is consonant with recently demonstrated functional effects mediated by inhibiting FMRP methylation in cultured cells.     

Substrate channeling of oxalacetate in solid-state complexes of malate dehydrogenase and citrate synthase

Current evidence suggests that mitochondrial matrix enzymes exist in solid-state, multienzyme complexes in vivo. Addition of polyethylene glycol to a solution containing malate dehydrogenase and citrate synthase generates such a solid-state, enzyme complex in vitro at enzyme concentrations permitting kinetic measurements. Suspensions of the isolated, solid-state, hetero-complex of these enzymes were used to study the coupled reactions of citrate synthesis from malate, NAD, and CoASAc. The particles appear to be about 1 microgram in diameter. Considering the ratio of enzyme to oxalacetate molecules in or at the surface of the solid-state particles, one would expect oxalacetate to be converted to citrate within a few molecular distances of the site of oxalacetate generation. This model of "substrate channeling" (or alternatively a direct transfer of oxalacetate between enzymes) is supported by experiments with excess aspartate aminotransferase and glutamate added to the solution phase to give a reaction competing with the synthase for bulk phase oxalacetate. Quantities of aminotransferase that reduce the citrate reaction rate with soluble dehydrogenase and synthase by 90% do not significantly affect rates with comparable amounts of the dehydrogenase-synthase complex. We suggest that similar substrate channeling can occur in vivo and discuss the possible advantages provided thereby.     

Acrylamide and glycidamide: genotoxic effects in V79-cells and human blood

Acrylamide (AA) can be formed in certain foods by heating, predominantly from the precursor asparagine. It is a carcinogen in animal experiments, but the relevance of dietary exposure for humans is still under debate. There is substantial evidence that glycidamide (GA), metabolically formed from AA by Cyp 2E1-mediated epoxidation, acts as ultimate mutagenic agent. We compared the mutagenic potential of AA and GA in V79-cells, using the hprt mutagenicity-test with N-methyl-N′-nitro-N-nitroso-guanidine (MNNG) as positive control. Whereas MNNG showed marked mutagenic effectivity already at 0.5 μM, AA was inactive up to a concentration of 10 mM. In contrast, GA showed a concentration dependent induction of mutations at concentrations of 800 μM and higher. Human blood was used as model system to investigate genotoxic potential in lymphocytes by single cell gel electrophoresis (comet assay) and by measuring the induction of micronuclei (MN) with bleomycin (BL) as positive control. AA did not induce significant genotoxicity or mutagenicity up to 6000 μM. With GA, concentration dependent DNA damage was observed in the dose range of 300–3000 μM after 4 h incubation. Significant MN-induction was not observed with AA (up to 5000 μM) and GA (up to 1000 μM), whereas BL (4 μM) induced significantly enhanced MN frequencies. Thus, in our systems GA appears to exert a rather moderate genotoxic activity.     

Binding of d-tubocurarine by gangliosides.

The binding of d-tubocurarine by ganglioside mixtures from chicken and bovine brain as well as by the single gangliosides GGtet1-NeuAc, GGtet2aNeuAc and GGtet3aNeuAc was demonstrated by means of equilibrium gel filtration. The sialyl-oligosaccharide derivatives of GGtet1NeuAc and GGtet2aNeuAc, however, did not bind any d-tubocurarine. A lack of binding was also found for the gangliotetraosyl-ceramide GgOse4Cer, free NeuAc, mucin and sphingomyelin. Under saturation conditions, GGtet1NeuAc bound 0.58, GGtet2aNeuAc 0.92 and GGtet3a-NeuAc 1.23 mol d-tubocurarine per mol ganglioside. Half-maximal binding was achieved between 1 and 1.5 x 10(-5)M d-tubocurarine. Ca2 was found to inhibit the binding of d-tubocurarine to gangliosides (50% at 4 x 10(-4)M Ca2). Mg2 was about 4 times less effective. Acetylcholine caused a 40% inhibition at 4 x 10(-3)M, whereas K and Na had only slight effects even at 5 x 10(-2)M.     

Oxidative variables and antioxidant enzymes activities in the mdx mouse brain

Dystrophin is a protein found at the plasmatic membrane in muscle and postsynaptic membrane of some neurons, where it plays an important role on synaptic transmission and plasticity. Its absence is associated with Duchenne's muscular dystrophy (DMD), in which cognitive impairment is found. Oxidative stress appears to be involved in the physiopathology of DMD and its cognitive dysfunction. In this regard, the present study investigated oxidative parameters (lipid and protein peroxidation) and antioxidant enzymes activities (superoxide dismutase and catalase) in prefrontal cortex, cerebellum, hippocampus, striatum and cortex tissues from male dystrophic mdx and normal C57BL10 mice. We observed (1) reduced lipid peroxidation in striatum and protein peroxidation in cerebellum and prefrontal cortex; (2) increased superoxide dismutase activity in cerebellum, prefrontal cortex, hippocampus and striatum; and (3) reduced catalase activity in striatum. It seems by our results, that the superoxide dismutase antioxidant mechanism is playing a protective role against lipid and protein peroxidation in mdx mouse brain.     

Clone size variation in the human diploid cell strain, WI-38

By mapping the location of isolated single cells; and then counting the number of cells at each location as a function of time. it was possible to accumulate data on the growth history for each of a large group of clones. The clone size distribution, its mean and standard deviation were computed for each day in culture. Variations in schedule of medium change and time of exposure to trypsin, did not measurably affect variation in clone size. Neither could clone size variation be accounted for on the basis of (1) occurrence of nondividing cells nor (2) presence of heritable growth rate variants in the population. It is probable that clone size variation under our conditions is primarily a consequence of a highly variable interdivision time among the constituent cells.