Nitric oxide synthase inhibitors modulate nerve growth factor-mediated regulation of amyloid precursor protein expression in PC12 cells

Stimulatory effects of (-)-epicatechin and its enantiomer (+)-epicatechin on mouse frontal cortex neurogenesis markers and short-term memory: proof of concept

Consumption of (-)-epicatechin (Epi), a cacao flavanol improves cognition. The aim was to compare the effects of (-)-Epi or its stereoisomer (+)-Epi on mouse frontal cortex-dependent short-term working memory and modulators of neurogenesis. Three-month-old male mice (n = 7 per group) were provided by gavage either water (vehicle; Veh), (-)-Epi, at 1 mg kg-1 or (+)-Epi at 0.1 mg per kg of body weight for 15 days. After treatment, spontaneous alternation was evaluated by Y-maze. Brain frontal cortex was isolated for nitrate/nitrite measurements, Western blotting for nerve growth factor (NGF), microtubule associated protein 2 (MAP2), endothelial and neuronal nitric oxide synthase (eNOS and nNOS) and immunohistochemistry for neuronal specific protein (NeuN), doublecortin (DCX), capillary (CD31) and neurofilaments (NF200). Results demonstrate the stimulatory capacity of (-)-Epi and (+)-Epi on markers of neuronal proliferation as per increases in immunoreactive cells for NeuN (74 and 120% respectively), DCX (70 and 124%) as well as in NGF (34.4, 63.6%) and MAP2 (41.8, 63.8%). Capillary density yielded significant increases with (-)-Epi (∼80%) vs. (+)-Epi (∼160%). CD31 protein levels increased with (-)-Epi (∼70%) and (+)-Epi (∼140%). Effects correlated with nitrate/nitrite stimulation by (-)-Epi and (+)-Epi (110.2, 246.5%) and enhanced eNOS phosphorylation (Ser1177) with (-)-Epi and (+)-Epi (21.4, 41.2%) while nNOS phosphorylation only increased with (+)-Epi (18%). Neurofilament staining was increased in (-)-Epi by 135.6 and 84% with (+)-Epi. NF200 increased with (-)-Epi (116%) vs. (+)-Epi (84.5%). Frontal cortex-dependent short-term spatial working improved with (-)-Epi and (+)-Epi (15, 13%). In conclusion, results suggest that both enantiomers, but more effectively (+)-Epi, upregulate neurogenesis markers likely through stimulation of capillary formation and NO triggering, improvements in memory.

Exogenous Alpha-Synuclein Evoked Parkin Downregulation Promotes Mitochondrial Dysfunction in Neuronal Cells. Implications for Parkinson's Disease Pathology

Aberrant secretion and accumulation of α-synuclein (α-Syn) as well as the loss of parkin function are associated with the pathogenesis of Parkinson's disease (PD). Our previous study suggested a functional interaction between those two proteins, showing that the extracellular α-Syn evoked post-translational modifications of parkin, leading to its autoubiquitination and degradation. While parkin plays an important role in mitochondrial biogenesis and turnover, including mitochondrial fission/fusion as well as mitophagy, the involvement of parkin deregulation in α-Syn-induced mitochondrial damage is largely unknown. In the present study, we demonstrated that treatment with exogenous α-Syn triggers mitochondrial dysfunction, reflected by the depolarization of the mitochondrial membrane, elevated synthesis of the mitochondrial superoxide anion, and a decrease in cellular ATP level. At the same time, we observed a protective effect of parkin overexpression on α-Syn-induced mitochondrial dysfunction. α-Syn-dependent disturbances of mitophagy were also shown to be directly related to reduced parkin levels in mitochondria and decreased ubiquitination of mitochondrial proteins. Also, α-Syn impaired mitochondrial biosynthesis due to the parkin-dependent reduction of PGC-1α protein levels. Finally, loss of parkin function as a result of α-Syn treatment induced an overall breakdown of mitochondrial homeostasis that led to the accumulation of abnormal mitochondria. These findings may thus provide the first compelling evidence for the direct association of α-Syn-mediated parkin depletion to impaired mitochondrial function in PD. We suggest that improvement of parkin function may serve as a novel therapeutic strategy to prevent mitochondrial impairment and neurodegeneration in PD (thereby slowing the progression of the disease).

Activation of the NGF/TrkA signaling pathway attenuates diabetic erectile dysfunction

Erectile dysfunction (ED) is a common complication of diabetes mellitus (DM). The exact role of the NGF/TrkA signaling pathway in the pathogenesis of diabetic ED is largely unknown. In the present study, we investigated the role of the NGF/TrkA signaling pathway in Sprague-Dawley rats with diabetic ED. Animals were divided into 2 groups: the normal group and the DM ED model group. The model group included the blank subgroup, the negative control (NC) subgroup, the TrkA subgroup and the TrkA + NGF subgroup. Erectile function, intracavernous pressure (ICP) and mean arterial pressure were measured respectively. Immunohistochemistry was used to examine the number of neuronal nitric oxide synthase (nNOS) expressing nerve fibers. The quantitative real-time polymerase chain reaction was applied to detect the mRNA expressions of NGF and TrkA in the cavernous tissue. Further, Western blotting was conducted to detect the expressions of NGF, TrkA and its downstream ERK pathway-related proteins. Higher erectile frequency, ICP values and diastolic function, more nNOS-positive nerve fibers, and decreased systolic function of the corpus cavernosum smooth muscle were found in the TrkA and TrkA+NGF groups when compared with the blank and the NC groups. Moreover, significantly higher mRNA expressions of NGF and TrkA, and upregulated protein expressions of NGF, TrkA, c-raf, ERK1/2 and CREB1 were found in the TrkA and the TrkA + NGF groups. In conclusion, downregulation in the NGF/TrkA signaling pathway may contribute to the pathogenesis of diabetic ED.

Map kinase and PKC signaling pathways modulate NGF-mediated apoE transcription

The present study assessed the mechanisms by which nerve growth factor (NGF) increased the level of apolipoprotein E (apoE) in PC12 cells. NGF (50ng/mL) significantly increased apoE protein levels following 72h of treatment. Similarly NGF increased luciferase activity in cells transfected with a luciferase reporter construct containing a 500bp fragment of the apoE promoter, indicating NGF-induced apoE expression is regulated, at least in part, at the level of transcription. The non-selective nitric oxide synthase (NOS) inhibitor N(ɷ)-nitro-L-arginine methylester (L-NAME; 20mM) did not attenuate the NGF-mediated increase in luciferase activity, while the inducible NOS inhibitor s-methylisothiourea (S-MIU; 2mM) partially attenuated this action of NGF. Inhibition of MAP kinase activation with 50μM U0126 or pre-treatment with the PKC inhibitor bisindolylmaleimide 1 (BIS-1; 10μM) prevented the NGF-mediated activation of the apoE promoter. Pre-treatment with the phospholipase C (PLC) inhibitor U73122 (5μM) partially inhibited the NGF-induced increase in luciferase activity while the Akt inhibitor LY294002 (10μM) had no effect. These data suggest NGF-induced apoE transcription requires MAP kinase and PKC activation and that these TrkA signaling pathways may be modulated by NO.

APP regulates NGF receptor trafficking and NGF-mediated neuronal differentiation and survival

β-amyloid precursor protein (APP) is a key factor in Alzheimer's disease (AD) but its physiological function is largely undetermined. APP has been found to regulate retrograde transport of nerve growth factor (NGF), which plays a crucial role in mediating neuronal survival and differentiation. Herein, we reveal the mechanism underlying APP-mediated NGF trafficking, by demonstrating a direct interaction between APP and the two NGF receptors, TrkA and p75NTR. Downregulation of APP leads to reduced cell surface levels of TrkA/p75NTR and increased endocytosis of TrkA/p75NTR and NGF. In addition, APP-deficient cells manifest defects in neurite outgrowth and are more susceptible to Aβ-induced neuronal death at physiological levels of NGF. However, APP-deficient cells show better responses to NGF-stimulated differentiation and survival than control cells. This may be attributed to increased receptor endocytosis and enhanced activation of Akt and MAPK upon NGF stimulation in APP-deficient cells. Together, our results suggest that APP mediates endocytosis of NGF receptors through direct interaction, thereby regulating endocytosis of NGF and NGF-induced downstream signaling pathways for neuronal survival and differentiation.

Nerve growth factor-mediated regulation of low density lipoprotein receptor-related protein promoter activation

The mechanisms by which nerve growth factor (NGF) increases the level of low density lipoprotein receptor-related protein (LRP1) are not known. Administration of nitric oxide synthase (NOS) inhibitors modulates several of the neurotrophic actions of NGF, including TrkA signalling pathway activation, increases in gene expression and neurite outgrowth. The present study investigated whether NGF regulates the transcription of LRP1 as well as the role of NO and the individual TrkA signalling pathways in this action of NGF. PC12 cells were transfected with luciferase reporter constructs containing various sized fragments of the LRP1 promoter and treated with NGF (50 ng/mL) to establish whether NGF altered LRP transcription. NGF significantly increased luciferase activity in all LRP1 promoter construct-transfected cells with the NGF-responsive region of the promoter identified to be present in the first 1000 bp. The non-selective NOS inhibitor N(ω)-nitro-L-arginine methylester (L-NAME; 20 mM) had no effect on the NGF-mediated increase in luciferase activity, while the inducible NOS selective inhibitor s-methylisothiourea (S-MIU; 2 mM) attenuated the NGF-induced activation of the LRP1 promoter. Pretreatment of PC12 cells with 10 μM bisindolylmaleimide 1 (BIS-1) prevented the NGF-mediated increase in LRP1 promoter activation while 50 μM U0126 partially inhibited this response. In combination with S-MIU, all of the TrkA signalling pathway inhibitors blocked the ability of NGF to increase LRP1 transcription. These data suggest the NGF-mediated increase in LRP1 levels occurs, at least in part, at the level of transcription and that NO and the TrkA signalling pathways cooperate in the modulation of LRP1 transcription.

Nitric oxide and histone deacetylases modulate cocaine-induced mu-opioid receptor levels in PC12 cells

Background

Cocaine exposure has been reported to alter central μ-opioid receptor (MOR) expression in vivo. The present study employed an in vitro cellular model to explore possible mechanisms that may be involved in this action of cocaine.

Methods

To assess the effects of cocaine on MOR levels, two treatment regimens were tested in PC12 cells: single continuous or multiple intermittent. MOR protein levels were assessed by western blot analysis and quantitative PCR was used to determine relative MOR mRNA expression levels. To evaluate the role of nitric oxide (NO) and histone acetylation in cocaine-induced MOR expression, cells were pre-treated with the NO synthase inhibitor N^ω-nitro-L-arginine methylester (L-NAME) or the non-selective histone acetyltransferase inhibitor curcumin.

Results

Both cocaine treatment regimens significantly increased MOR protein levels and protein stability, but only multiple intermittent treatments increased MOR mRNA levels as well as c-fos mRNA levels and activator protein 1 binding activity. Both regimens increased NO production, and pre-treatment with L-NAME prevented cocaine-induced increases in MOR protein and mRNA levels. Single and multiple cocaine treatment regimens inhibited histone deacetylase activity, and pre-treatment with curcumin prevented cocaine-induced up-regulation of MOR protein expression.

Conclusions

In the PC12 cell model, both NO and histone deacetylase activity regulate cocaine-induced MOR expression at both the transcriptional and post-transcriptional levels. Based on these novel findings, it is hypothesized that epigenetic mechanisms are implicated in cocaine’s action on MOR expression in neurons.

Effects of nerve growth factor and nitric oxide synthase inhibitors on amyloid precursor protein mRNA levels and protein stability

We determined previously that nitric oxide (NO) modulates the nerve growth factor (NGF)-mediated increases in amyloid precursor protein (APP) levels in PC12 cells. To elucidate potential mechanisms, the effects of NGF and NO synthase (NOS) inhibitors on APP mRNA levels and protein stability were evaluated. Surprisingly, treatment of PC12 cells with NGF resulted in decreased levels of APP695 and APP751/770 mRNA. Therefore, the effect of NGF on APP protein stability was examined using the translation inhibitor, cycloheximide. Under these conditions, NGF did not alter the rate of APP degradation, suggesting that NGF may be enhancing the translation rate of APP. Since NOS inhibitors attenuate the NGF-mediated increase in APP levels, their effect on APP mRNA levels and protein stability was also assessed. S-methylisothiourea (S-MIU), selective for inducible NOS, decreased both APP695 and APP751/770 mRNA levels while the non-selective NOS inhibitor, N(ω)-nitro-L-arginine methylester (L-NAME) had no effect. In both control and NGF-treated PC12 cells, S-MIU increased the half-life of APP, with the greatest effect observed with the APP695 isoform. Based on these data we propose that in PC12 cells, NGF increases APP levels through enhanced translation rate and that NO, which modulates the NGF-induced increase in APP protein, also regulates APP mRNA levels and could play a role in APP processing.

Nitration of tryptophan in ribosomal proteins is a novel post-translational modification of differentiated and naïve PC12 cells

Neuron growth factor (NGF) signaling in PC12 cell, which is derived from pheochromocytoma of rat adrenal medulla, induces expression of neuronal nitric oxide synthase (nNOS) and nitric oxide (NO) production. Subsequently, NO causes differentiation of PC12 cell to neuronal cell with morphological changes, such as neurite extension. In this study, we showed that 6-nitrotryptophan-containing proteins were produced in PC12 cell (naïve PC12 cell) and/or NGF-induced PC12 cell (differentiated PC12 cell). Western blot analysis of the protein extract of naïve PC12 cell and differentiated PC12 cell using anti 6-nitrotryptophan antibody showed several immunoreactive bands, which were subsequently subjected to trypsin digestion and LC-ESI-MS-MS analysis. The peptides from five ribosomal proteins, namely, 60S ribosomal protein L7 (Trp154), 60S acidic ribosomal protein P1 (Trp43), 40S ribosomal protein S2 (Trp60), 40S ribosomal protein S6 (Trp45), and 40S ribosomal protein S19 (Trp52), were identified as nitrotryptophan residue-containing proteins with significant ion score levels (p<0.05). Among these, tryptophan nitration was observed only in differentiated PC12 cell for S19 protein, and only in naïve PC12 cell for L7 protein. Tryptophan nitration of the other ribosomal proteins P1, S2, and S6 was observed in both naive and differentiated PC12 cells. The positive signal of nitrotryptophan-containing proteins in the Western blotting around 16 kDa (Band 1), which includes 40S ribosomal protein S19, was suppressed by treatment with NOS inhibitor, L-NAME. The tryptophan nitration of 40S ribosomal protein was not observed by LC-ESI-MS-MS analysis of this sample. This is the first study to identify several specific sites of nitrated tryptophan on proteins not only in viable culture cells but also in a physiological process: cell differentiation.

P2 receptor stimulation induces amyloid precursor protein production and secretion in rat cortical astrocytes

Amyloid precursor protein (APP) is ubiquitously expressed in a variety of tissues but is predominantly expressed in the brain. The expression of APP has been well studied in neurons but little is known about its presence in astrocytes. The study presented here shows that purinergic signaling is involved in the production and secretion of APP in primary cultures of rat cortical astrocytes. Extracellular ATP caused an increase in APP production and release in a time- and concentration-dependent manner and was inhibited by antagonists of P2 receptors. Further agonist and antagonist studies revealed involvement of P2Y2 and P2Y4 receptors in nucleotide-stimulated production and release of APP. In addition, signaling studies with various protein kinase inhibitors demonstrated that blockade of mitogen-activated protein kinases, but not Akt, inhibited nucleotide-stimulated APP expression and release. These results indicate that APP production and secretion can be regulated by activation of P2Y2/4 receptors coupled to protein kinase signaling pathways and suggest that astrocytes can be a potential source of APP.

Synaptic degeneration in Alzheimer's disease

Synaptic loss is the major neurobiological substrate of cognitive dysfunction in Alzheimer's disease (AD). Synaptic failure is an early event in the pathogenesis that is clearly detectable already in patients with mild cognitive impairment (MCI), a prodromal state of AD. It progresses during the course of AD and in most early stages involves mechanisms of compensation before reaching a stage of decompensated function. This dynamic process from an initially reversible functionally responsive stage of down-regulation of synaptic function to stages irreversibly associated with degeneration might be related to a disturbance of structural brain self-organization and involves morpho-regulatory molecules such as the amyloid precursor protein. Further, recent evidence suggests a role for diffusible oligomers of amyloid beta in synaptic dysfunction. To form synaptic connections and to continuously re-shape them in a process of ongoing structural adaptation, neurons must permanently withdraw from the cell cycle. Previously, we formulated the hypothesis that differentiated neurons after having withdrawn from the cell cycle are able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within neurons might put them at risk of erroneously converting signals derived from plastic synaptic changes into the program of cell cycle activation, which subsequently leads to cell death. The molecular mechanisms involved in cell cycle activation might, thus, link aberrant synaptic changes to cell death.