Extracellular carbohydrate-signal triggering camp-dependent protein kinase-dependent neuronal actin-reorganization

Protein interactome of 3',5'-cAMP reveals its role in regulating the actin cytoskeleton

Identification of protein interactors is ideally suited for the functional characterization of small molecules. 3',5'-cAMP is an evolutionary ancient signaling metabolite largely uncharacterized in plants. To tap into the physiological roles of 3',5'-cAMP, we used a chemo-proteomics approach, thermal proteome profiling (TPP), for the unbiased identification of 3',5'-cAMP protein targets. TPP measures shifts in the protein thermal stability upon ligand binding. Comprehensive proteomics analysis yielded a list of 51 proteins significantly altered in their thermal stability upon incubation with 3',5'-cAMP. The list contained metabolic enzymes, ribosomal subunits, translation initiation factors, and proteins associated with the regulation of plant growth such as CELL DIVISION CYCLE 48. To functionally validate obtained results, we focused on the role of 3',5'-cAMP in regulating the actin cytoskeleton suggested by the presence of actin among the 51 identified proteins. 3',5'-cAMP supplementation affected actin organization by inducing actin-bundling. Consistent with these results, the increase in 3',5'-cAMP levels, obtained either by feeding or by chemical modulation of 3',5'-cAMP metabolism, was sufficient to partially rescue the short hypocotyl phenotype of the actin2 actin7 mutant, severely compromised in actin level. The observed rescue was specific to 3',5'-cAMP, as demonstrated using a positional isomer 2',3'-cAMP, and true for the nanomolar 3',5'-cAMP concentrations reported for plant cells. In vitro characterization of the 3',5'-cAMP-actin pairing argues against a direct interaction between actin and 3',5'-cAMP. Alternative mechanisms by which 3',5'-cAMP would affect actin dynamics, such as by interfering with calcium signaling, are discussed. In summary, our work provides a specific resource, 3',5'-cAMP interactome, as well as functional insight into 3',5'-cAMP-mediated regulation in plants.

Molecular Dissection of Neurobeachin Function at Excitatory Synapses

Spines are small protrusions from dendrites where most excitatory synapses reside. Changes in number, shape, and size of dendritic spines often reflect changes of neural activity in entire circuits or at individual synapses, making spines key structures of synaptic plasticity. Neurobeachin is a multidomain protein with roles in spine formation, postsynaptic neurotransmitter receptor targeting and actin distribution. However, the contributions of individual domains of Neurobeachin to these functions is poorly understood. Here, we used mostly live cell imaging and patch-clamp electrophysiology to monitor morphology and function of spinous synapses in primary hippocampal neurons. We demonstrate that a recombinant full-length Neurobeachin from humans can restore mushroom spine density and excitatory postsynaptic currents in neurons of Neurobeachin-deficient mice. We then probed the role of individual domains of Neurobeachin by comparing them to the full-length molecule in rescue experiments of knockout neurons. We show that the combined PH-BEACH domain complex is highly localized in spine heads, and that it is sufficient to restore normal spine density and surface targeting of postsynaptic AMPA receptors. In addition, we report that the Armadillo domain facilitates the formation of filopodia, long dendritic protrusions which often precede the development of mature spines, whereas the PKA-binding site appears as a negative regulator of filopodial extension. Thus, our results indicate that individual domains of Neurobeachin sustain important and specific roles in the regulation of spinous synapses. Since heterozygous mutations in Neurobeachin occur in autistic patients, the results will also improve our understanding of pathomechanism in neuropsychiatric disorders associated with impairments of spine function.

Structure-Activity Relationship Study of the Neuritogenic Potential of the Glycan of Starfish Ganglioside LLG-3 (‡)

LLG-3 is a ganglioside isolated from the starfish Linchia laevigata. To clarify the structure-activity relationship of the glycan of LLG-3 toward rat pheochromocytoma PC12 cells in the presence of nerve growth factor, a series of mono- to tetrasaccharide glycan derivatives were chemically synthesized and evaluated in vitro. The methyl group at C8 of the terminal sialic acid residue was crucial for neuritogenic activity, and the terminal trisaccharide moiety was the minimum active motif. Furthermore, the trisaccharide also stimulated neuritogenesis in human neuroblastoma SH-SY5Y cells via mitogen-activated protein kinase (MAPK) signaling. Phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 was rapidly induced by adding 1 or 10 nM of the trisaccharide. The ratio of phosphorylated ERK to ERK reached a maximum 5 min after stimulation, and then decreased gradually. However, the trisaccharide did not induce significant Akt phosphorylation. These effects were abolished by pretreatment with the MAPK inhibitor U0126, which inhibits enzymes MEK1 and MEK2. In addition, U0126 inhibited the phosphorylation of ERK 1/2 in response to the trisaccharide dose-dependently. Therefore, we concluded that the trisaccharide promotes neurite extension in SH-SY5Y cells via MAPK/ERK signaling, not Akt signaling.

The growth cone cytoskeleton in axon outgrowth and guidance

Axon outgrowth and guidance to the proper target requires the coordination of filamentous (F)-actin and microtubules (MTs), the dynamic cytoskeletal polymers that promote shape change and locomotion. Over the past two decades, our knowledge of the many guidance cues, receptors, and downstream signaling cascades involved in neuronal outgrowth and guidance has increased dramatically. Less is known, however, about how those cascades of information converge and direct appropriate remodeling and interaction of cytoskeletal polymers, the ultimate effectors of movement and guidance. During development, much of the communication that occurs between environmental guidance cues and the cytoskeleton takes place at the growing tip of the axon, the neuronal growth cone. Several articles on this topic focus on the "input" to the growth cone, the myriad of receptor types, and their corresponding cognate ligands. Others investigate the signaling cascades initiated by receptors and propagated by second messenger pathways (i.e., kinases, phosphatases, GTPases). Ultimately, this plethora of information converges on proteins that associate directly with the actin and microtubule cytoskeletons. The role of these cytoskeletal-associated proteins, as well as the cytoskeleton itself in axon outgrowth and guidance, is the subject of this article.

Increase of GQ1b in the hippocampus of mice following kindled-seizures

The ganglioside GQ1b facilitates the influx of Ca2+ in brain synaptosomes and enhances ATP-induced long-term potentiation in hippocampal slices. Anti-GQ1b antibody impairs the function of peripheral neurons, for example, it had pathogenic effects on presynaptic neuronal membranes and perisynaptic Schwann cells in a mouse model of Guillain-Barré syndrome. The present study demonstrated in vivo that the level of endogenous GQ1b was relevant to neural function in the brain, in that it increased following seizures in amygdaloid kindling mice. GQ1b is subject to epileptogenic regulation and may play a role in the development of epilepsy.

[Regulation of neuronal cell function by glyco-signals]

Gangliosides and proteoglycans with various sugar chains exist abundantly in the brain. They participate in intercellular recognition by revealing the sugar chains on the cell surface, and some of them show neurite-extension activity. Several recognition features that are mediated by the sugar chains are known such as saccharide-saccharide interaction and cell-surface sugar-chain receptor-mediated recognition. Experiments on animals lacking the sugar-chain synthetic system with the technique of gene targeting suggest that phylogenetically "old" sugar chains such as chondroitin sulfate appear necessary for early development of the organism while relatively "new" sugar chains such as gangliosides, which appear with further development of the brain, are necessary for differentiation maturity processes. On the other hand, research using primary cultured neurons showed similar effects of the gangliosides and chondroitin sulfate on cell differentiation. It is possible that these sugar chains share the glyco-receptor-mediated signal transduction system.

The effect of prostaglandin E1 on ion currents of NG108-15 cells

The aim of this study was to elucidate the mechanism by which prostaglandin E(1) (PGE(1)) acts on ion currents of whole-cell voltage-clamped NG108-15 neuroblastomaxglioma hybrid cells. Ruptured and perforated patch were used. The holding current at -70 mV, the current-voltage curve produced by ramp pulses from -70 to 0 mV and the T-type and hva (high-voltage-activated) Ca(2+) currents associated with rectangular pulses were recorded. Bath application of PGE(1) (0.2 or 3 microM) reversibly increased the holding current, an effect mimicked by the prostanoid agonist iloprost (5-50 nM). The PGE(1) effect was totally blocked by the cAMP-antagonist Rp-cAMPS whereas H-89, an inhibitor of protein kinase A (PKA), failed to inhibit it, even when applied in the fairly high bath concentration of 30 microM. PGE(1) and iloprost also inhibited the T-type and hva Ca(2+) currents and this effect of PGE(1) was likewise not prevented by H-89. In some of the cells, the PGE(1) effect on holding current could be mimicked by 8-pCPT-2Me-cAMP (100-300 microM), a selective agonist of Epac (exchange protein activated by cAMP), but unlike the PGE(1) effect its action was not abolished by Rp-cAMPS. The effect of PGE(1) on the the holding current and on the T-type Ca(2+) current was diminished when EGTA in the pipette solution was replaced by BAPTA, suggesting that Ca(2+) ions are involved in the PGE(1) effect. It is concluded that the PGE(1) effect is mediated by cAMP and Ca(2+) ions but not by PKA or Epac.

Integration of glycosphingolipid metabolism and cell-fate decisions in cancer and stem cells: review and hypothesis

The metabolism of glycosphingolipids is strictly regulated during the mitotic cell cycle. Before the G1-to-S transition, the ceramide and glucosylceramide concentration is elevated. Ceramide induces apoptosis synergistically with the pro-apoptotic protein prostate apoptosis response 4 (PAR-4) that may be asymmetrically inherited during cell division. Only one daughter cell dies shortly after mitosis, a mechanism we suggested to regulate the number of neural stem cells during embryonic development. The progeny cells, however, may protect themselves by converting ceramide to glucosylceramide and other glycosphingolipids. In particular, complex gangliosides have been found to sustain cell survival and differentiation. The cell cycle may thus be a turning point for (glyco)sphingolipid metabolism and explain rapid changes of the sphingolipid composition in cells that undergo mitotic cell-fate decisions. In the proposed model termed "Shiva cycle", progression through the cell cycle, differentiation, or apoptosis may rely on a delicate balance of (glyco)sphingolipid second messengers that modulate the retinoblastoma-dependent G1-to-S transition or caspase-dependent G1-to-apoptosis program. Ceramide-induced cell cycle delay at G0/G1 is either followed by ceramide-induced apoptosis or by conversion of ceramide to glucosylceramide, a proposed key regulatory rheostat that rescues cells from re-entry into a life/death decision at G1-to-S. We propose a mechanistic model for sphingolpid-induced protein scaffolds ("slip") that regulate cell-fate decisions and will discuss the biological consequences and pharmacological potential of manipulating the (glyco)sphingolipid-dependent cell fate program in cancer and stem cells.