Polyphenols from green tea prevent antineuritogenic action of Nogo-A via 67-kDa laminin receptor and hydrogen peroxide.

Axonal regeneration after injury to the CNS is hampered by myelin-derived inhibitors, such as Nogo-A. Natural products, such as green tea, which are neuroprotective and safe for long-term therapy, would complement ongoing various pharmacological approaches. In this study, using nerve growth factor-differentiated neuronal-like Neuroscreen-1 cells, we show that extremely low concentrations of unfractionated green tea polyphenol mixture (GTPP) and its active ingredient, epigallocatechin-3-gallate (EGCG), prevent both the neurite outgrowth-inhibiting activity and growth cone-collapsing activity of Nogo-66 (C-terminal domain of Nogo-A). Furthermore, a synergistic interaction was observed among GTPP constituents. This preventive effect was dependent on 67-kDa laminin receptor (67LR) to which EGCG binds with high affinity. The antioxidants N-acetylcysteine and cell-permeable catalase abolished this preventive effect of GTPP and EGCG, suggesting the involvement of sublethal levels of H2 O2 in this process. Accordingly, exogenous sublethal concentrations of H2 O2 , added as a bolus dose (5 µM) or more effectively through a steady-state generation (1-2 µM), mimicked GTPP in counteracting the action of Nogo-66. Exogenous H2 O2 mediated this action by bypassing the requirement of 67LR. Taken together, these results show for the first time that GTPP and EGCG, acting through 67LR and elevating intracellular sublethal levels of H2 O2 , inhibit the antineuritogenic action of Nogo-A. Currently, several agents are being evaluated for overcoming axonal growth inhibitors to promote functional recovery after stroke and spinal cord injury. Epigallocatechin-3-gallate (EGCG), present in green tea polyphenol mixture (GTPP), prevents antineuritogenic activity of Nogo-A, a myelin-derived axonal growth inhibitor. The preventive action of EGCG involves the cell-surface-associated 67-kDa laminin receptor and H2 O2 . GTPP may complement ongoing efforts to treat neuronal injuries.>

Green tea catechins potentiate the neuritogenic action of brain-derived neurotrophic factor: role of 67-kDa laminin receptor and hydrogen peroxide.

Delivery of optimal amounts of brain-derived neurotrophic factor (BDNF) to regions of the brain affected by neurodegenerative diseases is a daunting task. Using natural products with neuroprotective properties, such as green tea polyphenols, would be a highly useful complementary approach for inexpensive long-term treatment of these diseases. In this study, we used PC12(TrkB) cells which ectopically express TrkB, a high affinity receptor for BDNF. They differentiate and induce neurite outgrowth in response to BDNF. Using this model, we show for the first time that treatment with extremely low concentrations (<0.1 µg/ml) of unfractionated green tea polyphenols (GTPP) and low concentrations (<0.5 µM) of their active ingredient, epigallocatechin-3-gallate (EGCG), potentiated the neuritogenic ability of a low concentration (2 ng/ml) of BDNF. A synergistic interaction was observed between GTPP constituents, where epigallocatechin and epicatechin, both individually lacking this activity, promoted the action of EGCG. GTPP-induced potentiation of BDNF action required the cell-surface associated 67 kDa laminin receptor (67LR) to which EGCG binds with high affinity. A cell-permeable catalase abolished GTPP/EGCG-induced potentiation of BDNF action, suggesting the possible involvement of H2O2 in the potentiation. Consistently, exogenous sublethal concentrations of H2O2, added as a bolus dose (5 µM) or more effectively through a steady-state generation (1 µM), potentiated BDNF action. Collectively, these results suggest that EGCG, dependent on 67 LR and H2O2, potentiates the neuritogenic action of BDNF. Intriguingly, this effect requires only submicromolar concentrations of EGCG. This is significant as extremely low concentrations of polyphenols are believed to reach the brain after drinking green tea.

Green tea polyphenols precondition against cell death induced by oxygen-glucose deprivation via stimulation of laminin receptor, generation of reactive oxygen species, and activation of protein kinase Cε.

As the development of synthetic drugs for the prevention of stroke has proven challenging, utilization of natural products capable of preconditioning neuronal cells against ischemia-induced cell death would be a highly useful complementary approach. In this study using an oxygen-glucose deprivation and reoxygenation (OGD/R) model in PC12 cells, we show that 2-day pretreatment with green tea polyphenols (GTPP) and their active ingredient, epigallocatechin-3-gallate (EGCG), protects cells from subsequent OGD/R-induced cell death. A synergistic interaction was observed between GTPP constituents, with unfractionated GTPP more potently preconditioning cells than EGCG. GTPP-induced preconditioning required the 67-kDa laminin receptor (67LR), to which EGCG binds with high affinity. 67LR also mediated the generation of reactive oxygen species (ROS) via activation of NADPH oxidase. An exogenous ROS-generating system bypassed 67LR to induce preconditioning, suggesting that sublethal levels of ROS are indeed an important mediator in GTPP-induced preconditioning. This role for ROS was further supported by the fact that antioxidants blocked GTPP-induced preconditioning. Additionally, ROS induced an activation and translocation of protein kinase C (PKC), particularly PKCε from the cytosol to the membrane/mitochondria, which was also blocked by antioxidants. The crucial role of PKC in GTPP-induced preconditioning was supported by use of its specific inhibitors. Preconditioning was increased by conditional overexpression of PKCε and decreased by its knock-out with siRNA. Collectively, these results suggest that GTPP stimulates 67LR and thereby induces NADPH oxidase-dependent generation of ROS, which in turn induces activation of PKC, particularly prosurvival isoenzyme PKCε, resulting in preconditioning against cell death induced by OGD/R.

Green tea polyphenols potentiate the action of nerve growth factor to induce neuritogenesis: possible role of reactive oxygen species.

Exogenously administered nerve growth factor (NGF) repairs injured axons, but it does not cross the blood-brain barrier. Thus, agents that could potentiate the neuritogenic ability of endogenous NGF would be of great utility in treating neurological injuries. Using the PC12 cell model, we show here that unfractionated green tea polyphenols (GTPP) at low concentrations (0.1 µg/ml) potentiate the ability of low concentrations of NGF (2 ng/ml) to induce neuritogenesis at a level comparable to that induced by optimally high concentrations of NGF (50 ng/ml) alone. In our experiments, GTPP by itself did not induce neuritogenesis or increase immunofluorescent staining for ß-tubulin III; however, it increased expression of mRNA and proteins for the neuronal markers neurofilament-L and GAP-43. Among the polyphenols present in GTPP, epigallocatechin-3-gallate (EGCG) alone appreciably potentiated NGF-induced neurite outgrowth. Although other polyphenols present in GTPP, particularly epigallocatechin and epicatechin, lack this activity, they synergistically promoted this action of EGCG. GTPP also induced an activation of extracellular signal-regulated kinases (ERKs). PD98059, an inhibitor of the ERK pathway, blocked the expression of GAP-43. K252a, an inhibitor of TrkA-associated tyrosine kinase, partially blocked the expression of these genes and ERK activation. Antioxidants, catalase (cell-permeable form), and N-acetylcysteine (both L and D-forms) inhibited these events and abolished the GTPP potentiation of NGF-induced neuritogenesis. Taken together, these results show for the first time that GTPP potentiates NGF-induced neuritogenesis, likely through the involvement of sublethal levels of reactive oxygen species, and suggest that unfractionated GTPP is more effective in this respect than its fractionated polyphenols.

Synapse replacement in the striatum of the adult rat following unilateral cortex ablation.

Defining the selective pattern of synapse replacement that occurs in different areas of the damaged brain is essential for predicting the limits of functional compensation that can be achieved after various types of brain injury. Here we describe the time course of dendritic reorganization, spine loss and recovery, and synapse replacement in the striatum following a unilateral cortex ablation. We found that the time course for the transient loss and recovery of dendritic spines on medium spiny I (MSI) neurons, the primary postsynaptic target for corticostriatal axons, paralleled the time course for the removal of degenerating axon terminals from the neuropil and the formation of new synapses on MSI neurons. Reinnervation of the deafferented striatum occurred chiefly by axon terminals that formed asymmetric synapses with dendritic spines of MSI neurons, and the mean density of asymmetric synapses recovered to 86% of the sham-operated rat value by 30 days postlesion. In addition, the synaptic circuitry of the reconstructed striatum was characterized by an increase in the number of multiple synaptic boutons (MSBs), i.e., presynaptic axon terminals that make contact with more than one dendritic spine. Whether the postsynaptic contacts of MSBs are formed with the dendritic spines of the same or a different parent dendrite in the striatum is unknown. Nevertheless, these data suggest that the formation of MSBs is an essential part of the compensatory response to the loss of input from the ipsilateral cortex following the aspiration lesion and may serve to modulate activity-dependent adaptive changes in the reconstructed striatum that can lead to functional recovery.

Unilateral skill acquisition induces bilateral NMDA receptor subunit composition shifts in the rat sensorimotor striatum.

The sensorimotor striatum is critical for the acquisition and consolidation of skilled learning-related motor sequences. Excitatory corticostriatal synapses undergo neuroplastic changes that impact signal transmission efficacy. Modification of N-methyl d-aspartate (NMDA) and α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit composition and phosphorylation is critical for bidirectional experience-driven plasticity observed at these synapses. Metaplastic regulation of the ratio of NR2A to NR2B subunits of the NMDA receptor controls the threshold for the induction of subsequent plasticity. However, little is known about how repeated practice effects the differential regulation of glutamate receptors during the acquisition of a unilateral motor skill. Using immunoblot analysis, we assessed changes in NMDA and AMPA receptors during the associative stage of skill acquisition in synaptoneurosome preparations from the rat sensorimotor striatum. We found that the NR2A/B subunit ratio in the striatum contralateral to the trained limb decreased during skill acquisition optimizing the threshold for inducing subsequent synaptic plasticity during learning of the lateralized motor skill. In contrast, there was a significant increase in the NR2A/B subunit ratio in the ipsilateral striatum making the induction of subsequent plasticity more difficult. In addition, there was a selective decrease in AMPAR phosphorylation levels at serine site 831 but not 845 on the GluR1 subunit ipsilaterally with a trend toward a decrease contralaterally. These findings suggest that the successful acquisition of a lateralized motor skill necessitates the integration of motor programs in both striata, each of which reflects unique changes in the NR2A/B ratio that modulate the different task demands on the associated limb.

Methods for studying oxidative regulation of protein kinase C.

The protein kinase C (PKC) family of isoenzymes may be a crucial player in transducing H2O2-induced signaling in a wide variety of physiological and pathophysiological processes. PKCs contain unique structural features that make them highly susceptible to oxidative modification. Depending on the site of oxidation and the extent to which it is modified, PKC can be either activated or inactivated by H2O2. The N-terminal regulatory domain contains zinc-binding, cysteine-rich motifs that are readily oxidized by H2O2. When oxidized, the autoinhibitory function of the regulatory domain is compromised, and as a result, PKC is activated in a lipid cofactor-independent manner. The C-terminal catalytic domain contains several reactive cysteine residues, which when oxidized with a higher concentration of H2O2 leads to an inactivation of PKC. Here, we describe the methods used to induce oxidative modification of purified PKC isoenzymes by H2O2 and the methods to assess the extent of this modification. Protocols are given for isolating oxidatively activated PKC isoenzymes from cells treated with H2O2. Furthermore, we describe the methods used to assess indirect regulation of PKC isoenzymes by determining their cytosol to membrane or mitochondrial translocation and tyrosine phosphorylation of PKCδ in response to sublethal levels of H2O2. Finally, as an example, we describe the methods used to demonstrate the role of H2O2-mediated cell signaling of PKCɛ in green tea polyphenol-induced preconditioning against neuronal cell death caused by oxygen-glucose deprivation and reoxygenation, an in vitro model for cerebral ischemic/reperfusion injury.

A direct redox regulation of protein kinase C isoenzymes mediates oxidant-induced neuritogenesis in PC12 cells.

In this study, we have used the PC12 cell model to elucidate the mechanisms by which sublethal doses of oxidants induce neuritogenesis. The xanthine/xanthine oxidase (X/XO) system was used for the steady state generation of superoxide, and CoCl(2) was used as a representative transition metal redox catalyst. Upon treatment of purified protein kinase C (PKC) with these oxidants, there was an increase in its cofactor-independent activation. Redox-active cobalt competed with the redoxinert zinc present in the zinc-thiolates of the PKC regulatory domain and induced the oxidation of these cysteine-rich regions. Both CoCl(2) and X/XO induced neurite outgrowth in PC12 cells, as determined by an overexpression of neuronal marker genes. Furthermore, these oxidants induced a translocation of PKC from cytosol to membrane and subsequent conversion of PKC to a cofactor-independent form. Isoenzyme-specific PKC inhibitors demonstrated that PKCepsilon plays a crucial role in neuritogenesis. Moreover, oxidant-induced neurite outgrowth was increased with a conditional overexpression of PKCepsilon and decreased with its knock-out by small interfering RNA. Parallel with PKC activation, an increase in phosphorylation of the growth-associated neuronal protein GAP-43 at Ser(41) was observed. Additionally, there was a sustained activation of extracellular signal-regulated kinases 1 and 2, which was correlated with activating phosphorylation (Ser(133)) of cAMP-responsive element-binding protein. All of these signaling events that are causally linked to neuritogenesis were blocked by antioxidant N-acetylcysteine (both L and D-forms) and by a variety of PKC-specific inhibitors. Taken together, these results strongly suggest that sublethal doses of oxidants induce neuritogenesis via a direct redox activation of PKCepsilon.