Search

SummaryNMDA receptor activation leads to clathrin-dependent endocytosis of postsynaptic AMPA receptors. Although this process controls long-term depression (LTD) induction in the hippocampus, how it is regulated by neuronal activities is not completely clear. Here, we show that Ca2+ influx through the NMDA receptor activates calcineurin and protein phosphatase 1 to dephosphorylate phosphatidylinositol 4-phosphate 5-kinaseγ661 (PIP5Kγ661), the major phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-producing enzyme in the brain. Bimolecular fluorescence complementation analysis revealed that the dephosphorylated PIP5Kγ661 became associated with the clathrin adaptor protein complex AP-2 at postsynapses in situ. NMDA-induced AMPA receptor endocytosis and low-frequency stimulation-induced LTD were completely blocked by inhibiting the association between dephosphorylated PIP5Kγ661 and AP-2 and by overexpression of a kinase-dead PIP5Kγ661 mutant in hippocampal neurons. Furthermore, knockdown of PIP5Kγ661 inhibited the NMDA-induced AMPA receptor endocytosis. Therefore, NMDA receptor activation controls AMPA receptor endocytosis during hippocampal LTD by regulating PIP5Kγ661 activity at postsynapses.

The effects of the following compounds (most of which had been proved to inhibit mitotic progression of pollens) on the shape of the membrane of human erythrocytes were tested : myrmicacin (3-hydroxydecanoic acid) and its derivatives, even-numbered C4-10 fatty acids, and some C10 diols. They all induced a shape change of the membrane-exvagination (crenation) type at pH 7.4 to different extents, depending on their structures, but not at pH 6.0. The shape change induced was reversible. The structure-activity relationship and the mode of action were compared with those for the action of these compounds on pollen growth.

Evidence is shown that protein kinase C is the major kinase which can phosphorylate histone H-1 in a membrane fraction prepared from rabbit peritoneal neutrophils. Addition of phorbol-12-myristate-13-acetate (PMA) (0.1 microgram/ml) or guanosine-5'-(3-O-thio)triphosphate (GTP gamma S) (10 microM) to the membrane fraction results in an increase of the phosphorylation of histone H-1. To achieve this effect, calcium (20 microM) is required for GTP gamma S but not for PMA. The effect of GTP gamma S, but not PMA is inhibited in membranes obtained from cells pretreated with pertussis toxin. The kinase activity is also enhanced by treatment of the membrane with 10 microM of GppNHp or GTP but not with GDP, GMP, cGMP, ATP, ADP, AMP and cAMP. This is the first direct evidence that a GTP binding protein is involved in the activation of membrane associated protein kinase C.

The effects of two representative groups of amphiphilic drugs, lysophosphatidylcholine species (myristoyl, palmitoyl and stearoyl) and phenothiazine neuroleptics (promethazine, chlorpromazine and trifluoperazine), on the morphology of intact rabbit platelets, arachidonate release from membrane phospholipids, pseudopod formation and the resulting aggregation of stimulated platelets were examined and compared with those of two known cycloxygenase inhibitors, aspirin and indomethacin. Collagen-induced aggregation was inhibited by relatively low concentrations of amphiphilic drugs in parallel with the prevention of arachidonate release from the membrane phospholipids. Thrombin-and arachidonate-induced aggregations were inhibited by these drugs at higher concentrations, at which they transform intact platelets from discoid to spiny and spherical shape, respectively, and consequently suppress formation of native pseudopods induced by these stimuli. Washing the drug-treated platelets with BSA-Tyrode solution abolished all the effects of drugs. In contrast, the cycloxygenase inhibitors blocked both collagen- and arachidonate-induced aggregations without causing membrane shape change of the intact platelets, although they inhibited thrombin-induced aggregation at extremely high concentrations, at which they did elicit membrane shape change. These observations suggest that these amphiphilic drugs act on the plasma membrane of platelets and impair membrane-linked functions. At lower concentrations they specifically inhibit the arachidonate release from the membrane phospholipids under collagen stimulation; at higher concentrations they drastically perturb the plasma membrane structure, inducing the membrane shape change, and suppressing the native pseudopod formation under thrombin or arachidonate stimulation. The impairment of membrane-linked functions by these amphiphilic drugs is thought to account for their inhibition of stimulus-induced aggregation.

Background Peroxisomes execute diverse and vital functions in virtually every eukaryote. New peroxisomes form by budding from pre-existing organelles or de novo by vesiculation of the ER. It has been suggested that ADP-ribosylation factors and COPI coatomer complexes are involved in these processes. Results Here we show that all viable Saccharomyces cerevisiae strains deficient in one of the small GTPases which have an important role in the regulation of vesicular transport contain functional peroxisomes, and that the number of these organelles in oleate-grown cells is significantly upregulated in the arf1 and arf3 null strains compared to the wild-type strain. In addition, we provide evidence that a portion of endogenous Arf6, the mammalian orthologue of yeast Arf3, is associated with the cytoplasmic face of rat liver peroxisomes. Despite this, ablation of Arf6 did neither influence the regulation of peroxisome abundance nor affect the localization of peroxisomal proteins in cultured fetal hepatocytes. However, co-overexpression of wild-type, GTP hydrolysis-defective or (dominant-negative) GTP binding-defective forms of Arf1 and Arf6 caused mislocalization of newly-synthesized peroxisomal proteins and resulted in an alteration of peroxisome morphology. Conclusion These observations suggest that Arf6 is a key player in mammalian peroxisome biogenesis. In addition, they also lend strong support to and extend the concept that specific Arf isoform pairs may act in tandem to regulate exclusive trafficking pathways.