Participation of protein kinase C alpha isoform and extracellular signal-regulated kinase in neurite outgrowth of GT1 hypothalamic neurons

Neuroprotective and Anti-Neuroinflammatory Effects of a Poisonous Plant Croton Tiglium Linn. Extract

Neuroinflammation is involved in various neurological diseases. Activated microglia secrete many pro-inflammatory factors and induce neuronal cell death. Thus, the inhibition of excessive proinflammatory activity of microglia leads to a therapeutic effect that alleviates the progression of neuronal degeneration. In this study, we investigated the effect of Croton tiglium (C. tiglium) Linn. extract (CTE) on the production of pro- and anti-inflammatory mediators in microglia and astrocytes via RT-PCR, Western blot, and nitric oxide assay. Neurotoxicity was measured by cell viability assay and GFP image analysis. Phagocytosis of microglia was measured using fluorescent zymosan particles. CTE significantly inhibited the production of neurotoxic inflammatory factors, including nitric oxide and tumor necrosis factor-α. In addition, CTE increased the production of the neurotrophic factor, brain-derived neurotrophic factor, and the M2 phenotype of microglia. The culture medium retained after CTE treatment increased the survival of neurons, thereby indicating the neuroprotective effect of CTE. Our findings indicated that CTE inhibited pro-inflammatory response and increased the neuroprotective ability of microglia. In conclusion, although CTE is known to be a poisonous plant and listed on the FDA poisonous plant database, it can be used as a medicine if the amount is properly controlled. Our results suggested the potential benefits of CTE as a therapeutic agent for different neurodegenerative disorders involving neuroinflammation.

Gene expression profiling analysis of the putamen for the investigation of compensatory mechanisms in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is affecting 5 million people worldwide, but the response mechanisms of the striatum are still unclear. Therefore, identification of gene expression alterations in the striatum will greatly assist the development of novel therapy strategies.

METHODS

We performed a comprehensive gene expression analysis in 15 PD patients and 15 normal controls to identify differentially expressed genes (DEGs) using the expression profile GSE20291 from Gene Expression Omnibus (GEO). Gene Ontology (GO) analysis and Kyoto Encyclopedia of Gene and Genome (KEGG) pathway enrichment analysis were used to define functions and pathways altered in PD. Protein-protein interaction network was constructed to find out the modules with close interactions.

RESULTS

Total715 DEGs including 268 up-regulated and 447 down-regulated genes were obtained. GO functional enrichment analysis indicated that the genes related with neurons function and cell morphogenesis might be changed upon PD. KEGG pathway enrichment analysis showed that most of the genes were enriched in the nodes of Gap junction, calcium signaling pathway, phosphatidylinositol signaling system, long-term potentiation, Alzheimer's disease and GnRH signaling pathway. Protein-protein interaction network and module analysis suggested that some apoptosis related genes, such as PRKCA, CDC42 and BCL2 may play critical roles in striatal neurons growth.

CONCLUSION

Intrinsic striatal tyrosine hydroxylase interneurons growth may be promoted by changes in several genes expression and thus reduce the functional excitatory synapses.

Axogenic effect of estrogen in male rat hypothalamic neurons involves Ca(2+), protein kinase C, and extracellular signal-regulated kinase signaling

17-beta-Estradiol (E2) stimulates the growth of axons in male-derived hypothalamic neurons in vitro. This effect is not exerted through the classical intracellular estrogen receptor (ER) but depends on a membrane mechanism involving TrkB. In the present study, we investigate the intracellular signaling cascade that mediates the axogenic effect of E2. Treatment with an intracellular Ca(2+) chelator, a Ca(2+)-dependent protein kinase C (PKC) inhibitor, or two specific inhibitors of extracellular signal-regulated kinases (ERK) mitogen-activated protein kinases (MAPK) completely inhibited the E2-induced axogenesis. E2 and the membrane-impermeant construct E2BSA rapidly induced phosphorylation of ERK, which was blocked by the specific inhibitor of the ERK pathway UO126 but not by the ER antagonist ICI 182,780. Decrease of intracellular free Ca(2+) or disruption of PKC activation by Ro 32-0432 attenuated ERK activation, indicating the confluence of signals in the MAPK pathway. Subcellular analysis of ERK demonstrated that the phospho-ERK signal is augmented in the nucleus after 15 min of E2 stimulation. We have also shown that E2 increased phosphorylation of CREB via ERK signaling. In summary, this study demonstrates that E2, probably via a membrane-associated receptor, induces axonal growth by activating CREB phosphorylation through ERK signaling by a mechanism involving Ca(2+) and PKC activation.

Interaction of extracellular signal-regulated protein kinase 1/2 with actin cytoskeleton in supraoptic oxytocin neurons and astrocytes: role in burst firing

Neuronal firing patterns determine the manner of neurosecretion, the underlying mechanisms of which are poorly understood. Using supraoptic nuclei in brain slices from lactating rats, we examined the involvement of extracellular signal-regulated protein kinase 1/2 (ERK1/2) and filamentous actin (F-actin) in burst generation by oxytocin (OT) neurons. Blocking phosphorylation of ERK1/2 (pERK1/2) decreased miniature EPSCs and blocked OT-evoked bursts, as did intracellularly loading an antibody against pERK1/2. OT (10 pM) increased cytosolic pERK1/2 close to the cell membrane within the first 5 min, subsiding by 30 min, whereas OT elicited pERK1/2 nuclear translocation in closely associated supraoptic astrocytes. The increased pERK1/2 was tightly correlated with spatiotemporal actin dynamics. In OT neurons, OT initially increased F-actin, particularly at membrane subcortical areas, and then decreased it after 30 min. Both polymerization and depolymerization of actin cytoskeleton were associated with bursts, but only polymerization facilitated OT-evoked bursts. Blocking ERK1/2 activation blocked OT-evoked actin polymerization, whereas depolymerizing F-actin increased pERK1/2 expression. These changes were further identified in vivo. In intact animals, suckling increased ERK1/2 activation in the cytosol and membrane subcortical area F-actin formation in OT neurons, whereas it increased F-actin concentration in astrocytic somata. Coimmunoprecipitation showed that suckling increased molecular interactions between pERK1/2 and actin. Finally, two different blockers of ERK1/2 kinase injected intracerebroventricularly reduced suckling-evoked milk ejections. This is the first demonstration that OT mediation of suckling-evoked bursts/milk ejections is via interactions between pERK1/2 and actin cytoskeleton.

The PtdIns(4,5)P2 ligand itself influences the localization of PKCalpha in the plasma membrane of intact living cells

Rapamycin-triggered heterodimerization strategy is becoming an excellent tool for rapidly modifying phosphatidylinositol(4,5)-bisphosphate [PtdIns(4,5)P2] levels at the plasma membrane and for studying their influence in different processes. In this work, we studied the effect of modulation of the PtdIns(4,5)P2 concentration on protein kinase C (PKC) alpha membrane localization in intact living cells. We showed that an increase in the PtdIns(4,5)P2 concentration enlarges the permanence of PKCalpha in the plasma membrane when PC12 cells are stimulated with ATP, independently of the diacylglycerol generated. The depletion of this phosphoinositide decreases both the percentage of protein able to translocate to the plasma membrane and its permanence there. Our results demonstrate that the polybasic cluster located in the C2 domain of PKCalpha is responsible for this phosphoinositide-protein interaction. Furthermore, the C2 domain acts as a dominant interfering module in the neural differentiation process of PC12 cells, a fact that was also supported by the inhibitory effect obtained by knocking down PKCalpha with small interfering RNA duplexes. Taken together, these data demonstrate that PtdIns(4,5)P2 itself targets PKCalpha to the plasma membrane through the polybasic cluster located in the C2 domain, with this interaction being critical in the signaling network involved in neural differentiation.

Activation of phospholipase C pathways by a synthetic chondroitin sulfate-E tetrasaccharide promotes neurite outgrowth of dopaminergic neurons

In dopaminergic neurons, chondroitin sulfate (CS) proteoglycans play important roles in neuronal development and regeneration. However, due to the complexity and heterogeneity of CS, the precise structure of CS with biological activity and the molecular mechanisms underlying its influence on dopaminergic neurons are poorly understood. In this study, we investigated the ability of synthetic CS oligosaccharides and natural polysaccharides to promote the neurite outgrowth of mesencephalic dopaminergic neurons and the signaling pathways activated by CS. CS-E polysaccharide, but not CS-A, -C or -D polysaccharide, facilitated the neurite outgrowth of dopaminergic neurons at CS concentrations within the physiological range. The stimulatory effect of CS-E polysaccharide on neurite outgrowth was completely abolished by its digestion into disaccharide units with chondroitinase ABC. Similarly to CS-E polysaccharide, a synthetic tetrasaccharide displaying only the CS-E sulfation motif stimulated the neurite outgrowth of dopaminergic neurons, whereas a CS-E disaccharide or unsulfated tetrasaccharide had no effect. Analysis of the molecular mechanisms revealed that the action of the CS-E tetrasaccharide was mediated through midkine-pleiotrophin/protein tyrosine phosphatase zeta and brain-derived neurotrophic factor/tyrosine kinase B receptor pathways, followed by activation of the two intracellular phospholipase C (PLC) signaling cascades: PLC/protein kinase C and PLC/inositol 1,4,5-triphosphate/inositol 1,4,5-triphosphate receptor signaling leading to intracellular Ca(2+) concentration-dependent activation of Ca(2+)/calmodulin-dependent kinase II and calcineurin. These results indicate that a specific sulfation motif, in particular the CS-E tetrasaccharide unit, represents a key structural determinant for activation of midkine, pleiotrophin and brain-derived neurotrophic factor-mediated signaling, and is required for the neuritogenic activity of CS in dopaminergic neurons.

Distinct roles of PKC isoforms in NCAM-mediated neurite outgrowth

The role of protein kinase C (PKC) isoforms in the neural cell adhesion molecule (NCAM)-mediated neurite outgrowth was tested using a co-culture system consisting of fibroblasts with or without NCAM expression upon which either primary cerebellar granular neurones (CGN) or pheochromocytoma (PC12-E2) cells were grown. The latter transiently expressed various PKC isoforms and domains derived from selected PKCs. PKC inhibitors of various specificity inhibited NCAM-stimulated neuritogenesis from CGN, indicating that PKC is involved in this process. Moreover, stimulation by the NCAM-mimetic peptide, C3d, elicited phosphorylation of PKC in CGN. Expression of kinase-deficient forms of PKCalpha, betaI and betaII blocked NCAM-mediated neurite extension, but had no effect on nerve growth factor (NGF)-mediated neurite outgrowth. Expression of two PKCepsilon constructs: (i) a fragment from PKCepsilon encompassing the pseudosubstrate, the C1a domain (including the actin-binding site, ABS), and parts of the V3 region, or (ii) the PKCepsilon-specific ABS blocked NCAM-mediated neurite extension in both cases. These two constructs also partially inhibited NGF-stimulated neuritogenesis indicating that PKCepsilon is a positive regulator of both NCAM- and NGF-mediated differentiation. We suggest that PKCepsilon is a common downstream mediator for several neuritogenic factors, whereas one or more conventional PKCs are specifically involved in NCAM-stimulated neurite outgrowth.

PKC-alpha-mediated remodeling of the actin cytoskeleton is involved in constitutive albumin uptake by proximal tubule cells

One key role of the renal proximal tubule is the reabsorption of proteins from the glomerular filtrate by constitutive receptor-mediated endocytosis. In the opossum kidney (OK) renal proximal tubule cell line, inhibition of protein kinase C (PKC) reduces albumin uptake, although the isoforms involved and mechanisms by which this occurs have not been identified. We used pharmacological and molecular approaches to investigate the role of PKC-alpha in albumin endocytosis. We found that albumin uptake in OK cells was inhibited by the pan-PKC blocker bisindolylmaleimide-1 and the isoform-specific PKC blockers Go-6976 and 2',3,3',4,4'-hexahydroxy-1,1'-biphenyl-6,6'-dimethanol dimethyl ether, indicating a role for PKC-alpha. Overexpression of a kinase deficient PKC-alpha(K368R) but not wild-type PKC-alpha significantly reduced albumin endocytosis. Western blot analysis of fractionated cells showed an increased association of PKC-alpha-green fluorescent protein with the membrane fraction within 10-20 min of exposure to albumin. We used phalloidin to demonstrate that albumin induces the formation of clusters of actin at the apical surface of OK cells and that these clusters correspond to the location of albumin uptake. These clusters were not present in cells grown in the absence of albumin. In cells treated either with PKC inhibitors or overexpressing kinase-deficient PKC-alpha(K368R) this actin cluster formation was significantly reduced. This study identifies a role for PKC-alpha in constitutive albumin uptake in OK cells by mediating assembly of actin microfilaments at the apical membrane.

Substance P protects spiral ganglion neurons from apoptosis via PKC-Ca2+-MAPK/ERK pathways

In the current study, we have investigated the ability of substance P (SP) to protect 3-day-old (P3) rat spiral ganglion neurons (SGNs) from trophic factor deprivation (TFD)-induced cell death. The presence of SP high affinity neurokinin-1 receptor (NK1) transcripts was detected in the spiral ganglion and the NK1 protein localized to SGNs both ex vivo and in vitro. Treatment with SP increased cytoplasmic Ca2+ in SGNs, further arguing for the presence of functional NK1 on these neurons. Both SP and the agonist [Sar9,Met(O2)11]-SP significantly decreased SGN cell death induced by TFD, with no effect on neurite outgrowth. The survival promoting effect of SP was blocked by the NK1 antagonist, WIN51708. Both pan-caspase inhibitor BOC-D-FMK and SP treatments markedly reduced activation of caspases and DNA fragmentation in trophic factor deprived-neurons. The neuroprotective action of SP was antagonised by specific inhibitors of second messengers, including 1.2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) to chelate cytosolic Ca2+, the protein kinase C (PKC) inhibitors bisindolylmaleimide I, Gö6976 and LY333531 and the MAPK/ERK inhibitor U0126. In contrast, nifedipine, a specific inhibitor of l-type Ca2+ channel, and LY294002, a phosphatidylinositol-3-OH kinase (PI3K) inhibitor, had no effect on SP trophic support of SGNs. Moreover, activation of endogenous PKC by 4 beta-phorbol 12-myristate 13-acetate (PMA) also reduced the loss of trophic factor-deprived SGNs. Thus, NK1 expressed by SGNs transmit a survival-promoting regulatory signal during TFD-induced SGN cell death via pathways involving PKC activation, Ca2+ signalling and MAPK/ERK activation, which can be accounted for by an inhibition of caspase activation.

Metabolism of the unnatural anticancer lipid safingol, L-threo-dihydrosphingosine, in cultured cells

We studied the metabolism of radioactively labeled safingol (l-threo-dihydrosphingosine) in primary cultured neurons, B104 neuroblastoma cells, and Swiss 3T3 fibroblasts, and compared it to that of its natural stereoisomer d-erythro-dihydrosphingosine. Both sphingoid bases are used as biosynthetic precursors for complex sphingolipids, albeit to different rates. Whereas a considerable amount of the natural sphingoid base is also directed to the catabolic pathway (20-66%, cell type dependent), only a minor amount of the nonnatural safingol is subjected to catabolic cleavage, most of it being N-acylated to the respective stereochemical variant of dihydroceramide. Interestingly, N-acylation of safingol to l-threo-dihydroceramide is less sensitive to fumonisin B1 than the formation of the natural d-erythro-dihydroceramide. In addition, safingol-derived l-threo-dihydroceramide, unlike its physiologic counterpart, is not desaturated. Most of it either accumulates in the cells (up to 50%) or is used as a biosynthetic precursor of the respective dihydrosphingomyelin (up to 45%). About 5% is, however, glucosylated and channeled into the glycosphingolipid biosynthetic pathway. Our results demonstrate that, despite its nonnatural stereochemistry, safingol is recognized and metabolized preferentially by enzymes of the sphingolipid biosynthetic pathway. Furthermore, our data suggest that the cytotoxic potential of safingol is reduced rather than enhanced via its metabolic conversion.

Selective roles of protein kinase C isoforms on cell motility of GT1 immortalized hypothalamic neurones

Recently, we demonstrated that activation of the protein kinase C (PKC) signalling pathway promoted morphological differentiation of GT1 hypothalamic neurones via an increase in beta-catenin, a cell-cell adhesion molecule, indicating a possible involvement of PKC in cellular motility. In this study, we explored the differential roles of PKC isoforms in GT1 cell migration. First, we transiently transfected GT1 cells with enhanced green fluorescence protein (EGFP)-tagged actin to monitor the dynamic rearrangement of filamentous-actin (F-actin) in living cells. Treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA), a PKC activator, markedly promoted lamellipodia formation, while safingol (a PKC alpha-selective inhibitor) blocked the TPA-induced lamellipodial actin structure. Both wound-healing and Boyden migration assays showed that TPA treatment promoted neuronal migration of GT1 cells; however, cotreatment of TPA with safingol or rottlerin (a PKC delta-selective inhibitor) clearly blocked this TPA effect, indicating that both PKC alpha and PKC delta may be positive regulators of neuronal migration. By contrast, PKC gamma-EGFP-expressing GT1 cells exhibited decreased cellular motility and weak staining for actin stress fibres, suggesting that PKC gamma may act as a negative mediator of cell migration in these neurones. Among the PKC downstream signal molecules, p130Cas, a mediator of cell migration, and its kinase, focal adhesion kinase (FAK), increased following TPA treatment; phosphorylation of p130Cas was induced in a PKC alpha-dependent manner. Together, these results demonstrate that PKC alpha promotes GT1 neuronal migration by activating focal adhesion complex proteins such as p130Cas and FAK.

Cell differentiation of gonadotropin-releasing hormone neurons and alternative RNA splicing of the gonadotropin-releasing hormone transcript

Two different, yet related issues regarding gonadotropin-releasing hormone (GnRH), i.e. the development and differentiation of hypothalamic GnRH neurons and the alternative RNA splicing of GnRH gene transcripts, are addressed in the present review. Using the immortalized GnRH-producing GT1 cell line, we found that activation of protein kinase C (PKC) with 12-O-tetradecanoylphorbol-13-acetate induces morphological and functional differentiation of these neurons. Specific isoforms of PKC are involved in neurite growth, cell migration and synaptic contacts and involve different signaling pathways. Using an in vitro splicing assay with HeLa nuclear extract, we found that excision of the first intron of the GnRH primary transcript is attenuated in non-GnRH-producing cells, but not in GnRH-producing cells such as GT1. This attenuation was relieved by exonic splicing enhancers located in the GnRH exons 3 and 4. Interestingly, addition of nuclear extract derived from GT1 cells further increased the excision rate of intron A, indicating that GnRH neurons contain TRANS-acting splicing factors. Extensive biochemical analysis indicates that Tra2alpha, a serine/arginine-rich RNA-binding protein, and other cofactors are likely involved in mediating neuron-specific excision of intron A from the GnRH primary transcript. An understanding of the GnRH neuron-specific splicing machinery provides critical insight into the molecular mechanism of GnRH gene regulation and consequently of mammalian reproductive development.