Spontaneous Membrane Fusion Induced by Chemical Formation of Ceramides in a Lipid Bilayer

Interaction of Synaptosomal-Associated Protein 25 with Neutral Sphingomyelinase 2: Functional Impact on the Sphingomyelin Pathway

Neurotransmitter release is mediated by ceramide, which is generated by sphingomyelin hydrolysis. In the present study, we examined whether synaptosomal-associated protein 25 (SNAP-25) is involved in ceramide production and exocytosis. Neutral sphingomyelinase 2 (nSMase2) was partially purified from bovine brain and we found that SNAP-25 was enriched in the nSMase2-containing fractions. In rat synaptosomes and PC12 cells, the immunoprecipitation pellet of anti-SNAP-25 antibody showed higher nSMase activity than the immunoprecipitation pellet of anti-nSMase2 antibody. In PC12 cells, SNAP-25 was colocalized with nSMase2. Transfection of SNAP-25 small interfering RNA (siRNA) significantly inhibited nSMase2 translocation to the plasma membrane. A23187-induced ceramide production was concomitantly reduced in SNAP-25 siRNA-transfected PC12 cells compared with that in scrambled siRNA-transfected cells. Moreover, transfection of SNAP-25 siRNA inhibited dopamine release, whereas addition of C₆-ceramide to the siRNA-treated cells moderately reversed this inhibition. Additionally, nSMase2 inhibition reduced dopamine release. Collectively, our results indicate that SNAP-25 interacts with nSMase2 during ceramide production, which mediates exocytosis and neurotransmitter release.

Development of Triazinone-Based Condensing Reagents for Amide Formation

Novel triazinone-based condensing reagents have been developed. The palladium-catalyzed O-N allylic rearrangement of 2-(allyloxy)-4,6-dichloro-1,3,5-triazine and subsequent regioselective substitution using alcohols and an amine afforded chlorotriazinones, which can be readily converted using N-methylmorpholine into the corresponding condensing reagents. The condensation of carboxylic acids and amines using these reagents proceeded to afford the desired amides in good yields. In comparison with 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride, the newly synthesized triazinone-based condensing reagents exhibited higher reactivity.

Potent triazine-based dehydrocondensing reagents substituted by an amido group

This study describes the synthesis of triazine-based dehydrocondensing reagents substituted by amido substituents and demonstrates their efficiency for dehydrocondensing reactions in MeOH and THF. N-Phenylbenzamido-substituted chlorotriazine is readily converted to a stable, non-hygroscopic triazinylammonium-based dehydrocondensing reagent that is superior to 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) in terms of its reactivity in dehydrocondensing reactions.

Photouncaging of ceramides promotes reorganization of liquid-ordered domains in supported lipid bilayers

6-Bromo-7-hydroxycoumarin (Bhc)-caged ceramide (Cer) analogs were incorporated into supported lipid bilayers containing a mixture of coexisting liquid-ordered (Lo) and liquid-disordered (Ld) phases. The release of N-palmitoyl and N-butanoyl-D-erythro-sphingosine (C16- and C4-Cer) by the photolysis of caged Cers using long-wavelength UV light was studied using a combination of atomic force microscopy and fluorescence microscopy. This approach demonstrated the ability to generate Cer with spatial and temporal control, providing an alternative method to the enzymatic generation of Cer. The generation of C16-Cer from Bhc-C16-Cer disrupted the Lo domains, with the incorporation of small fluid-phase regions and the disappearance of some smaller domains. Cer-rich gel-phase domains were not observed, in contrast to results reported by either direct Cer incorporation or enzymatic Cer generation. The photorelease of C4-Cer from Bhc-C4-Cer resulted in qualitatively similar changes in bilayer morphology, with the disappearance of some Lo domains and no evidence of Cer-rich gel domains but with a smaller height difference between the ordered and disordered phases.

Study on 1,3,5-triazine chemistry in dehydrocondensation: gauche effect on the generation of active triazinylammonium species

The reaction of 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) with various nitrogen-containing compounds, particularly tertiary amines (tert-amines), has been studied for the preparation of 2-(4,6-dimethoxy-1,3,5-triazinyl)trialkylammonium salts [DMT-Am(s)]. DMT-Ams derived from aliphatic tert-amines exhibited activity for the dehydrocondensation between a carboxylic acid and an amine to form an amide in a model reaction. Based on a conformational analysis of DMT-Ams and tert-amines by NMR and X-ray diffraction methods, we concluded that a β-alkyl group maintained in a gauche relationship with the nitrogen lone pair of tert-amines significantly hinders the approach of CDMT to the nitrogen. Thus, trimethylamine and quinuclidine without such alkyl groups readily react with CDMT whereas triethylamine, possessing two or three such gauche β-alkyl groups in the stable conformations, does not react at all. The theory of "gauche β-alkyl group effect" proposed here provides useful guidelines for the preparation of DMT-Ams possessing various tertiary amine moieties. An investigation of the dehydrocondensation activity of tert-amines in a CDMT/tert-amine system that involves in situ generation of DMT-Am, showed that the gauche effect of the β-alkyl group becomes quite pronounced; the yield of the amide decreases significantly with tert-amines possessing an unavoidable gauche β-alkyl group. Thus, the tert-amine/CDMT systems are useful for judging whether tert-amines can readily react with CDMT without isolation of DMT-Ams.

Fusogenic metallosupramolecular brush vesicles

The electrostatic combination of a cationic metallosupramolecular polyelectrolyte (Fe-MSP) with sulfonate-terminated polymers leads to the formation of metallosupramolecular brushes (MSBs). The resulting MSBs can self-assemble into vesicular structures in chloroform/methanol (v/v = 1:1) mixture solvents. The rigid-rod Fe-MSP chain has to bend for the formation of the vesicles, which accompanies the presence of a lateral tension and thus induces a spontaneous vesicle fusion with an hour-scale fusion time. For this much longer fusion process, the arrow-like protrusion, stalk-like intermediate, and hemifusion diaphragm are clearly observed by transmission electron microscopy. The complete fusion into larger vesicles significantly releases the lateral tension.

Development of a biomimetic nanoporous membrane for the selective transport of charged proteins

We report the use of a micrometer-thick platinum-coated nanoporous membrane for the separation of differently charged proteins. A high field strength of about 25 kV m(-1) was applied, using very low transmembrane potentials of +/-1.5 V between the platinum-coated membranes. The system mimics the cell membrane function of facilitated transport for specific solutes. The selectivity for Lys:BSA:Mb in a mixed protein solution could be tuned readily between the flux ratios of 2:2:1 and 96:1:12 respectively, by simple variation of the transmembrane potentials from +1.5 V to -1.5 V. The experimental fluxes agreed closely with calculated fluxes derived from a simple electrophoresis-potential shielding model at favourable transmembrane potentials.

[Studies on reaction control and development of new practical reagents based on characteristics of reaction field]

Reaction rates and selectivities can be critically affected by the reaction field. Using a diverse set of reagents and reaction systems, the author reviews a variety of ways to control the reaction field. In the first example, we discuss 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM), which serves as an exceptionally convenient reagent for dehydrocondensation. In particular, formation of carboxamide by DMT-MM was found to take place even if water or alcohol were used as a reaction medium. Thus, chemical modification of carboxyl groups and/or amino groups of highly polar substrates, such as amino acid derivatives, peptides, glyco-chains, and nucleotides, can be simply effected by mixing them with DMT-MM in aqueous or alcoholic solvents. The author also found that a tertiary amine catalyzes the activation step of carboxylic acid with 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT) via in situ generation of coupling reagents. Proceeding further in this direction, we determined that artificial acyltransferase and cyclotransferase could be formed by conjugation of a tertiary amine catalyst to host-molecules to mimic a substrate binding site. Finally, the micellar interface, well known for promoting hydrolysis, clearly provides a superior reaction field for dehydrocondensation. When a 1,3,5-triazine-type amphiphilic dehydrocondensing agent was used, bimolecular dehydrocondensation between amphiphilic carboxylate and amines was highly accelerated (2000-fold) in a micellar system. Spontaneous membrane fusion was induced by adoption of the micellar reaction in the ceramide synthesis at the surface of membranes. All together, these diverse findings strongly support the central importance of the reaction field in controlling reaction rates and selectivity.