Involvement of gangliosides in the process of Cbp/PAG phosphorylation by Lyn in developing cerebellar growth cones

The association of gangliosides with specific proteins in the central nervous system was examined by coimmunoprecipitation with an anti-ganglioside antibody. The monoclonal antibody to the ganglioside GD3 (R24) immunoprecipitated the Csk (C-terminal src kinase)-binding protein (Cbp). Sucrose density gradient analysis showed that Cbp of rat cerebellum was detected in detergent-resistant membrane (DRM) raft fractions. R24 treatment of the rat primary cerebellar cultures induced Lyn activation and tyrosine phosphorylation of Cbp. Treatment with anti-ganglioside GD1b antibody also induced tyrosine phosphorylation. Furthermore, over-expressions of Lyn and Cbp in Chinese hamster ovary (CHO) cells resulted in tyrosine 314 phosphorylation of Cbp, which indicates that Cbp is a substrate for Lyn. Immunoblotting analysis showed that the active form of Lyn and the Tyr314-phosphorylated form of Cbp were highly accumulated in the DRM raft fraction prepared from the developing cerebellum compared with the DRM raft fraction of the adult one. In addition, Lyn and the Tyr314-phosphorylated Cbp were highly concentrated in the growth cone fraction prepared from the developing cerebellum. Immunoelectron microscopy showed that Cbp and GAP-43, a growth cone marker, are localized in the same vesicles of the growth cone fraction. These results suggest that Cbp functionally associates with gangliosides on growth cone rafts in developing cerebella.

Heparin enhances osteoclastic bone resorption by inhibiting osteoprotegerin activity

Heparin is a highly sulfated glycosaminoglycan and has been shown to activate osteoclastic bone resorption though how is not yet clear. Here we investigate the molecule involved in heparin-induced activation of osteoclasts using an in vitro osteoclast culture assay. The formation and activation of osteoclasts are induced by receptor activator of NFkappaB ligand (RANKL) on osteoblasts, and inhibited by osteoprotegerin (OPG), a decoy receptor of RANKL, which is secreted from osteoblasts. In a coculture of mouse bone marrow cells and osteoblasts treated with 1,25-dihydroxyvitamin D(3) and prostaglandin E(2) on dentin slices, the bone marrow cells differentiate into osteoclasts, and resorption pits are formed on the dentin slices. Addition of heparin, various glycosaminoglycans, and chemically modified heparins to the coculture reveals that heparin enhances the pit-forming activity of osteoclasts, and this effect of heparin on the activation of osteoclasts is dependent on its sugar chain structure. By contrast, mRNA expression levels of RANKL, RANK, and OPG in the coculture are not altered by heparin treatment. Furthermore, neither RANK nor RANKL binds to heparin, suggesting that heparin does not directly interact with these proteins. Instead, heparin specifically binds to OPG and prevents OPG-mediated inhibition of osteoclastic bone resorption in the coculture. Heparin treatment does not enhance osteoclastic bone resorption in a monoculture of osteoclasts derived from bone marrow cells, and in the coculture using osteoblasts from OPG-deficient mice. A (125)I-OPG binding assay showed that OPG binds to osteoblasts and that this binding is inhibited by the addition of heparin, suggesting that OPG binds to RANKL on the osteoblast membrane and that heparin blocks this interaction. These results demonstrate that heparin enhances osteoclastic bone resorption by inhibiting OPG activity.

Translocation of activated heterotrimeric G protein Galpha(o) to ganglioside-enriched detergent-resistant membrane rafts in developing cerebellum

The association of gangliosides with specific proteins in the central nervous system was examined by co-immunoprecipitation with an anti-ganglioside antibody. The monoclonal antibody to the ganglioside GD3 immunoprecipitated phosphoproteins of 40, 53, 56, and 80 kDa from the rat cerebellum. Of these proteins, the 40-kDa protein was identified as the alpha-subunit of a heterotrimeric G protein, G(o) (Galpha(o)). Using sucrose density gradient analysis of cerebellar membranes, Galpha(o), but not Gbetagamma, was observed in detergent-resistant membrane (DRM) raft fractions in which GD3 was abundant after the addition of guanosine 5'-O-(thiotriphosphate) (GTPgammaS), which stabilizes G(o) in its active form. On the other hand, both Galpha(o) and Gbetagamma were excluded from the DRM raft fractions in the presence of guanyl-5'-yl thiophosphate, which stabilizes G(o) in its inactive form. Only Galpha(o) was observed in the DRM fractions from the cerebellum on postnatal day 7, but not from that in adult. After pertussis toxin treatment, Galpha(o) was not observed in the DRM fractions, even from the cerebellum on postnatal day 7. These results indicate the activation-dependent translocation of Galpha(o) into the DRM rafts. Furthermore, Galpha(o) was concentrated in the neuronal growth cones. Treatment with stromal cell-derived factor-1alpha, a physiological ligand for the G protein-coupled receptor, stimulated [(35)S]GTPgammaS binding to Galpha(o) and caused Galpha(o) translocation to the DRM fractions and RhoA translocation to the membrane fraction, leading to the growth cone collapse of cerebellar granule neurons. The collapse was partly prevented by pretreatment with the cholesterol-sequestering and raft-disrupting agent methyl-beta-cyclodextrin. These results demonstrate the involvement of signal-dependent Galpha(o) translocation to the DRM in the growth cone behavior of cerebellar granule neurons.

Alivin 1, a novel neuronal activity-dependent gene, inhibits apoptosis and promotes survival of cerebellar granule neurons

Neurons require Ca2+-dependent gene transcription for their activity-dependent survival, the mechanisms of which have not been fully elucidated yet. Here, we demonstrate that a novel primary response gene, alivin 1 (ali1), is an activity-dependent gene and promotes survival of neurons. Sequence analyses reveal that rat, mouse, and human Ali1 proteins contain seven leucine-rich repeats, one IgC2-like loop and a transmembrane domain, and display homology to Kek and Trk families. Expression of ali1 mRNA in cultured cerebellar granule neurons is rigidly regulated by KCl and/or NMDA concentrations in the culture medium and tightly correlated to depolarization-dependent survival and/or NMDA-dependent survival of the granule neuron. ali1 mRNA expression was regulated at the transcriptional step by the Ca2+ influx through voltage-dependent L-type Ca2+ channels when the cells were stimulated by 25 mm KCl. Expression of ali1 mRNA in cultured cortical neurons was inhibited when their spontaneous electrical activity was blocked by tetrodotoxin. Thus, the expression is neuronal activity dependent. Overexpression of Ali1 in cerebellar granule neurons inhibited apoptosis that was induced by the medium containing 5 mm KCl. The addition of anti-Ali1 antiserum or the soluble putative extracellular Ali1 domain to the 25 mm KCl-supported culture inhibited the survival of the granule neuron. These results suggest that expression of ali1 promotes depolarization-dependent survival of the granule neuron. Mouse ali1 was mapped to a locus approximately 55.3 cM from the centromere on chromosome 15 that is syntenic to positional candidate loci for familial Alzheimer's disease type 5 and Parkinson's disease 8 on human chromosome 12.

Contribution of histidine residues to oligomerization of theta-toxin (perfringolysin O), a cholesterol-binding cytolysin

Theta-toxin (perfringolysin O) modified by diethyl pyrocarbonate, a histidine-specific reagent, lost its hemolytic activity. The modified toxin retains the activities of binding to and insertion into cholesterol-containing membranes but lacks the ability to form oligomers. These results suggest that histidine residues of theta-toxin contribute their share to cytolysis, especially the oligomerization process.

Contribution of tryptophan residues to the structural changes in perfringolysin O during interaction with liposomal membranes

Perfringolysin O (theta-toxin) is a cholesterol-binding and pore-forming toxin that shares with other thiol-activated cytolysins a highly conserved sequence, ECTGLAWEWWR (residues 430-440), near the C-terminus. To understand the membrane-insertion and pore-forming mechanisms of the toxin, we evaluated the contribution of each Trp to the toxin conformation during its interaction with liposomal membranes. Circular dichroism (CD) spectra of Trp mutant toxins indicated that only Trp436 has a significant effect on the secondary structure, and that Trp436, Trp438, and Trp439 make large contributions to near-UV CD spectra. Quenching the intrinsic Trp fluorescence of the wild-type and mutant toxins with brominated lecithin/cholesterol liposomes revealed that Trp438 and probably Trp436, but not Trp439, contributes to toxin insertion into the liposomal membrane. Near-UV CD spectra of the membrane-associated mutant toxins indicated that both Trp438 and Trp439 are required for the CD peak shift from 292 to 300 nm, a signal related to theta-toxin oligomerization and/or pore formation, suggesting a conformational change around Trp438 and Trp439 in these processes.

Contribution of individual tryptophan residues to the structure and activity of theta-toxin (perfringolysin O), a cholesterol-binding cytolysin

theta-Toxin (perfringolysin O), secreted by Clostridium perfringens, shares with other known thiol-activated toxins a conserved undecapeptide, ECTGLAWEWWR, located in the C-terminal region of the protein and containing the unique cysteine of the molecule. Single and double amino acid substitutions were created in the theta-toxin molecule to investigate the role of individual tryptophan residues in the lytic activity of theta-toxin. Wild-type and mutant theta-toxins were overproduced in Escherichia coli by means of a T7 RNA polymerase/promoter system and purified. The relative hemolytic activities of four mutant toxins, each with a Trp to Phe substitution outside the common Cys-containing region, were more than 60% that of wild-type theta-toxin. In contrast, mutant toxins with Phe replacements within the Cys-containing region (at Trp436, Trp438 or Trp439) showed significantly reduced hemolytic and erythrocyte-membrane-binding activities. The largest reduction in binding affinity, more than 100-fold, was observed for Trp438 mutant toxins. However, the mutants retain binding specificity for cholesterol and the ability to form arc-shaped and ring-shaped structures on membranes. These results indicate that the low hemolytic activities of these mutant toxins can be ascribed, at least in part, to reduced binding activities. With respect to protease susceptibility and far-ultraviolet circular-dichroism spectra, only the W436-->F mutant toxin, showed any considerable difference from wild-type toxin in secondary or higher-order structures, indicating that Trp436 is essential for maintenance of toxin structure.