Vascular endothelial cells promote cortical neurite outgrowth via an integrin β3-dependent mechanism

The interaction of neurons with their non-neuronal milieu plays a crucial role in the formation of neural networks, and wide variety of cell-contact-dependent signals that promote neurite elongation have been identified. In this study, we found that vascular endothelial cells promote neurite elongation in an integrin β3-dependent manner. Vascular endothelial cells from the cerebral cortex promoted neurite elongation of cortical neurons in a cell contact-dependent manner. This effect was mediated by arginine–glycine–aspartic acid (RGD), a major recognition sequence for integrins. Pharmacological blockade of integrin β3 abolished the neurite elongation effect induced by the endothelial cells. Immunocytochemical analysis revealed that integrin β3 was expressed on cultured cortical neurons. These results demonstrate that the neurite elongation promoted by vascular endothelial cells requires integrin β3. Vascular endothelial cells may therefore play a role in the development and repair of neural networks in the central nervous system.     

Arginine-specific gingipain A from Porphyromonas gingivalis induces Weibel-Palade body exocytosis and enhanced activation of vascular endothelial cells through protease-activated receptors

Gingipains, cysteine proteases derived from Porphyromonas gingivalis, are important virulence factors in periodontal diseases. We found that arginine-specific gingipain A (RgpA) increased the responsiveness of vascular endothelial cells to P. gingivalis lipopolysaccharides (LPS) and P. gingivalis whole cells to induce enhanced IL-8 production through protease-activated receptors (PARs) and phospholipase C (PLC) gamma. We therefore investigated whether RgpA-induced enhanced cell activation is mediated through exocytosis of Weibel-Palade bodies (WPBs) because they store vasoactive substances. RgpA rapidly activated PAR- and PLCgamma-dependent WPB exocytosis. In addition, angiopoietin (Ang)-2, a substance of WPB, enhanced IL-8 production by P. gingivalis LPS, suggesting that Ang-2 mediates the RgpA-induced enhanced cell responses. Thus, we propose a novel role for RgpA in induction of a proinflammatory event through PAR-mediated WPB exocytosis, which may be an important step for enhanced endothelial responses to P. gingivalis.     

Intestinal tumorigenicity of mice carrying hemizygous Pax6, Pax6(Sey-4H).

The genotype-phenotype relationship in mice was examined experimentally using one of the small eye mutants, Pax6(Sey-4H), which deletes the chromosome 2 middle region, hemizygously. The genotyping indicated that the deleted region starts at a site 102.60 Mb from the centromere and has a length of 6.51 Mb, in which 12 known and 27 novel genes are located. Expecting the development of myeloid leukemia, gamma-irradiation was performed to female mutants at the age of 10 weeks. The mutants did not develop myeloid leukemia during the observation period of 18 months. Instead, they developed tumors in the alimentary tract spontaneously (56.0%). The tumor latency was shortened by the radiation exposure, but the tumor incidence of the gamma-irradiated group (62.5%) was as high as that of spontaneously developing tumors. Intraductal proliferation of the epithelium of the Wirsung duct was observed in the gamma-irradiated mutants (18.8%). Considering the results of the Pax6(Sey-4H) mutant together with those of another small eye mutant, Pax6(Sey-3H), the anomaly and the tumorigenicity of the intestinal tract were closely related to the hemizygosity of the 3.2 Mb segment of chromosome 2, where both mutants show a common deletion.     

Limonoid compounds inhibit sphingomyelin biosynthesis by preventing CERT protein-dependent extraction of ceramides from the endoplasmic reticulum

To identify novel inhibitors of sphingomyelin (SM) metabolism, a new and selective high throughput microscopy-based screening based on the toxicity of the SM-specific toxin, lysenin, was developed. Out of a library of 2011 natural compounds, the limonoid, 3-chloro-8β-hydroxycarapin-3,8-hemiacetal (CHC), rendered cells resistant to lysenin by decreasing cell surface SM. CHC treatment selectively inhibited the de novo biosynthesis of SM without affecting glycolipid and glycerophospholipid biosynthesis. Pretreatment with brefeldin A abolished the limonoid-induced inhibition of SM synthesis suggesting that the transport of ceramide (Cer) from the endoplasmic reticulum to the Golgi apparatus is affected. Unlike the Cer transporter (CERT) inhibitor HPA-12, CHC did not change the transport of a fluorescent short chain Cer analog to the Golgi apparatus or the formation of fluorescent and short chain SM from the corresponding Cer. Nevertheless, CHC inhibited the conversion of de novo synthesized Cer to SM. We show that CHC specifically inhibited the CERT-mediated extraction of Cer from the endoplasmic reticulum membranes in vitro. Subsequent biochemical screening of 21 limonoids revealed that some of them, such as 8β-hydroxycarapin-3,8-hemiacetal and gedunin, which exhibits anti-cancer activity, inhibited SM biosynthesis and CERT-mediated extraction of Cer from membranes. Model membrane studies suggest that 8β-hydroxycarapin-3,8-hemiacetal reduced the miscibility of Cer with membrane lipids and thus induced the formation of Cer-rich membrane domains. Our study shows that certain limonoids are novel inhibitors of SM biosynthesis and suggests that some biological activities of these limonoids are related to their effect on the ceramide metabolism.     

Lipocalin-type prostaglandin D synthase protects against oxidative stress-induced neuronal cell death

L-PGDS [lipocalin-type PGD (prostaglandin D) synthase] is a dual-functional protein, acting as a PGD2-producing enzyme and a lipid transporter. L-PGDS is a member of the lipocalin superfamily and can bind a wide variety of lipophilic molecules. In the present study we demonstrate the protective effect of L-PGDS on H2O2-induced apoptosis in neuroblastoma cell line SH-SY5Y. L-PGDS expression was increased in H2O2-treated neuronal cells, and the L-PGDS level was highly associated with H2O2-induced apoptosis, indicating that L-PGDS protected the neuronal cells against H2O2-mediated cell death. A cell viability assay revealed that L-PGDS protected against H2O2-induced cell death in a concentration-dependent manner. Furthermore, the titration of free thiols in H2O2-treated L-PGDS revealed that H2O2 reacted with the thiol of Cys65 of L-PGDS. The MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight)-MS spectrum of H2O2-treated L-PGDS showed a 32 Da increase in the mass relative to that of the untreated protein, showing that the thiol was oxidized to sulfinic acid. The binding affinities of oxidized L-PGDS for lipophilic molecules were comparable with those of untreated L-PGDS. Taken together, these results demonstrate that L-PGDS protected against neuronal cell death by scavenging reactive oxygen species without losing its ligand-binding function. The novel function of L-PGDS could be useful for the suppression of oxidative stress-mediated neurodegenerative diseases.     

A role for sphingomyelin-rich lipid domains in the accumulation of phosphatidylinositol-4,5-bisphosphate to the cleavage furrow during cytokinesis

Cytokinesis is a crucial step in the creation of two daughter cells by the formation and ingression of the cleavage furrow. Here, we show that sphingomyelin (SM), one of the major sphingolipids in mammalian cells, is required for the localization of phosphatidylinositol-4,5-bisphosphate (PIP(2)) to the cleavage furrow during cytokinesis. Real-time observation with a labeled SM-specific protein, lysenin, revealed that SM is concentrated in the outer leaflet of the furrow at the time of cytokinesis. Superresolution fluorescence microscopy analysis indicates a transbilayer colocalization between the SM-rich domains in the outer leaflet and PIP(2)-rich domains in the inner leaflet of the plasma membrane. The depletion of SM disperses PIP(2) and inhibits the recruitment of the small GTPase RhoA to the cleavage furrow, leading to abnormal cytokinesis. These results suggest that the formation of SM-rich domains is required for the accumulation of PIP(2) to the cleavage furrow, which is a prerequisite for the proper translocation of RhoA and the progression of cytokinesis.     

Kinetic and structural studies on the catalytic role of the aspartic acid residue conserved in copper amine oxidase

Copper amine oxidase contains a post-translationally generated quinone cofactor, topa quinone (TPQ), which mediates electron transfer from the amine substrate to molecular oxygen. The overall catalytic reaction is divided into the former reductive and the latter oxidative half-reactions based on the redox state of TPQ. In the reductive half-reaction, substrate amine reacts with the C5 carbonyl group of the oxidized TPQ, forming the substrate Schiff base (TPQ(ssb)), which is then converted to the product Schiff base (TPQ(psb)). During this step, an invariant Asp residue with an elevated pKa is presumed to serve as a general base accepting the alpha proton of the substrate. When Asp298, the putative active-site base in the recombinant enzyme from Arthrobacter globiformis, was mutated into Ala, the catalytic efficiency dropped to a level of about 10(6) orders of magnitude smaller than the wild-type (WT) enzyme, consistent with the essentiality of Asp298. Global analysis of the slow UV/vis spectral changes observed during the reductive half-reaction of the D298A mutant with 2-phenylethylamine provided apparent rate constants for the formation and decay of TPQ(ssb) (k(obs) = 4.7 and 4.8 x 10(-4) s(-1), respectively), both of which are markedly smaller than those of the WT enzyme determined by rapid-scan stopped-flow analysis (k(obs) = 699 and 411 s(-1), respectively). Thus, Asp298 plays important roles not only in the alpha-proton abstraction from TPQ(ssb) but also in other steps in the reductive half-reaction. X-ray diffraction analyses of D298A crystals soaked with the substrate for 1 h and 1 week revealed the structures of TPQ(ssb) and TPQ(psb), respectively, as pre-assigned by single-crystal microspectrophotometry. Consistent with the stereospecificity of alpha-proton abstraction, the pro-S alpha-proton of TPQ(ssb) to be abstracted is positioned nearly perpendicularly to the plane formed by the Schiff-base imine double bond conjugating with the quinone ring of TPQ, so that the orbitals of sigma and pi electrons maximally overlap in the conjugate system. More intriguingly, the pro-S alpha proton of the substrate is released stereospecifically even in the reaction catalyzed by the base-lacking D298A mutant. On the basis of these results, we propose that the stereospecificity of alpha-proton abstraction is primarily determined by the conformation of TPQ(ssb), rather than the relative geometry of TPQ and the catalytic base.     

Crystal structures of cytochrome c(L) and methanol dehydrogenase from Hyphomicrobium denitrificans: structural and mechanistic insights into interactions between the two proteins

Methanol dehydrogenase (Hd-MDH) and its physiological electron acceptor, cytochrome c(L) (Hd-Cyt c(L)), isolated from a methylotrophic denitrifying bacterium, Hyphomicrobium denitrificans A3151, have been kinetically and structurally characterized; the X-ray structures of Hd-MDH and Hd-Cyt c(L) have been determined using molecular replacement at 2.5 and 2.0 A resolution, respectively. To explain the mechanism for electron transfer between these proteins, the dependence of MDH activity on the concentration of Hd-Cyt c(L) has been investigated at pH 4.5-7.0. The Michaelis constant for Hd-Cyt c(L) shows the smallest value (approximately 0.3 microM) at pH 5.5. The pseudo-first-order rate constant (k(obs)) of the reduction of Hd-Cyt c(L) exhibits a hyperbolic concentration dependence of Hd-MDH at pH 5.5, although k(obs) linearly increases at pH 6.5. These findings indicate formation of a transient complex between these proteins during an electron transfer event. Hd-MDH (148 kDa) is a large tetrameric protein with an alpha(2)beta(2) subunit composition, showing a high degree of structural similarity with other MDHs. Hd-Cyt c(L) (19 kDa) exhibiting the alpha-band at 550.7 nm has a unique C-terminal region involving a disulfide bond between Cys47 and Cys165. Moreover, there is a pair of Hd-Cyt c(L) monomers related with a pseudo-2-fold axis of symmetry in the asymmetric unit, and the two monomers tightly interact with each other through three hydrogen bonds. This configuration is the first example in the studies of cytochrome c as the physiological electron acceptor for MDH. The docking simulation between the coupled Hd-Cyt c(L) molecules and the heterotetrameric Hd-MDH molecule has been carried out.     

Differential membrane packing of stereoisomers of bis(monoacylglycero)phosphate

Bis(monoacylglycero)phosphate (BMP) reveals an unusual sn-1,sn-1' stereoconfiguration of glycerophosphate. We synthesized sn-(3-myristoyl-2-hydroxy)glycerol-1-phospho-sn-1'-(3'-myristoyl-2'-hydroxy)glycerol (1,1'-DMBMP) and characterized the thermotropic phase behavior and membrane structure, in comparison with those of the corresponding sn-3:sn-1' stereoisomer (3,1'-DMBMP), by means of differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (SAXS and WAXS, respectively), pressure-area (pi-A) isotherms, epifluorescence microscopy of monolayers, and molecular dynamics (MD) simulations. In DSC, these lipids exhibited weakly energetic broad peaks with an onset temperature of 9 degrees C for 1,1'-DMBMP and 18 degrees C for 3,1'-DMBMP. In addition, a highly cooperative, strongly energetic transition peak was observed at approximately 40 degrees C for 1,1'-DMBMP and approximately 42 degrees C for 3,1'-DMBMP. These results are supported by the observation that 1,1'-DMBMP exhibited a larger phase transition pressure (pi(c)) than 3,1'-DMBMP. Small- and wide-angle X-ray scattering measurements identified these small and large energetic transitions as a quasi-crystalline (L(c1))-quasi-crystalline with different tilt angle (L(c2)) phase transition and an L(c2)-L(alpha) main phase transition, respectively. X-ray measurements also revealed that these DMBMPs undergo an unbinding at the main phase transition temperature. The MD simulations estimated stronger hydrogen bonding formation in the 3,1'-DMBMP membrane than in 1,1'-DMBMP, supporting the experimental data.