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Apoptosis of dopaminergic neurons in the nigrostriatal projection plays a crucial role in the pathogenesis of Parkinson's disease (PD). Although the detailed mechanisms responsible for dopaminergic neuron loss are still under investigation, oxidative stress is identified as a major contributor for neuronal apoptosis. In the current study, we studied the effects of MPP(+), a substrate that mimics oxidative stress, on neuron-like PC12 cells and the underlying mechanisms. PC12 cells were cultured and treated by 100 μmol/L MPP(+) for 4, 8, 16, 24 and 48 h, respectively. For drug pretreatment, the PC12 cells were incubated with N-acetyl-l-cysteine (NAC, 5 mmol/L), an antioxidant, SP600125 (20 μmol/L) or PD98059 (100 μmol/L), two pharmacological inhibitors of JNK and ERK1/2, for 1 h before addition of MPP(+). Cell apoptosis was measured by flow cytometry. The mRNA expression of Cu(2+)/Zn(2+)-SOD, GSH-Px, Bcl-2 and Bax was detected by RT-PCR. The protein expression of p-ERK1/2 and p-JNK was determined by Western blotting. Our results showed that MPP(+) exposure could induce substantial PC12 cell apoptosis. The pretreatment of SP600125 or PD98059 could effectively reduce the apoptosis rate by reducing the ratio of Bax/Bcl-2 mRNA levels. MPP(+) exposure also induced high level of reactive oxygen species (ROS), marked by dramatic increase of Cu(2+)/Zn(2+)-SOD and GSH-Px mRNA levels. The elevated ROS was strongly associated with the activation of JNK and ERK1/2 signal pathways after MPP(+) exposure, since the pretreatment of NAC significantly reduced the upregulation of p-JNK and p-ERK1/2. Finally, the pretreatment of SP600125, but not PD98059, alleviated the increase of Cu(2+)/Zn(2+)-SOD and GSH-Px mRNAs induced by MPP(+), suggesting that the activation of the JNK signal pathway, but not the ERK1/2 signal pathway, could, in some degree, antagonize the generation of ROS induced by oxidative stress. In conclusion, our results suggest that JNK and ERK1/2 signal pathways, which are activated via ROS, play a crucial role in neuronal apoptosis induced by oxidative stress.

Toll-like receptor 4 (TLR4), a key member of the TLR family, has been well characterized by its function in the induction of inflammatory products of innate immunity. However, the involvement of TLR4 in a variety of apoptotic events by an unknown mechanism has been the focus of great interest. Our investigation found that TLR4 promoted apoptotic signalling by affecting the glycogen synthase kinase-3beta (GSK-3beta) pathway in a serum-deprivation-induced apoptotic paradigm. Serum deprivation induces GSK-3beta activation in a pathway that leads to subsequent cell apoptosis. Intriguingly, this apoptotic cascade is amplified in presence of TLR4 but greatly attenuated by beta-arrestin 2, another critical molecule implicated in TLR4-mediated immune responses. Our data suggest that the association of beta-arrestin 2 with GSK-3beta contributes to the stabilization of phospho-GSK-3beta, an inactive form of GSK-3beta. It becomes a critical determinant for the attenuation of TLR4-initiated apoptosis by beta-arrestin 2. Taken together, we demonstrate that the TLR4 possesses the capability of accelerating GSK-3beta activation thereby deteriorating serum-deprivation-induced apoptosis; beta-arrestin 2 represents an inhibitory effect on the TLR4-mediated apoptotic cascade, through controlling the homeostasis of activation and inactivation of GSK-3beta.

Dopaminergic neurons are lost mainly through apoptosis in Parkinson's disease. Insulin like growth factor-1 (IGF-1) inhibits apoptosis in a wide variety of tissues. Here we have shown that IGF-1 protects PC12 cells from toxic effects of 1-methyl-4-phenylpyridiniumion (MPP(+)). Treatment of PC12 cells with recombinant human IGF-1 significantly decreased apoptosis caused by MPP(+) as measured by acridine orange/ethidium bromide staining. IGF-1 treatment induced sustained phosphorylation of glycogen synthase kinase-3beta (GSK-3beta) as shown by western blot analysis. The anti-apoptotic effect of IGF-1 was abrogated by LY294002, which indirectly inhibits phosphorylation of GSK-3beta. Lithium chloride (LiCl), a known inhibitor of GSK-3beta, also blocked MPP(+)-induced apoptosis. Finally, although IGF-1 enhanced phosphorylation of extracellular signal-regulated kinases ERK1 and 2 (ERK1/2), PD98059, a specific inhibitor of ERK1/2, did not alter the survival effect of IGF-1. Thus, our findings indicate that IGF-1 protects PC12 cells exposed to MPP(+) from apoptosis via the GSK-3beta signaling pathway.

The biochemical pathways that mediate the degeneration of dopaminergic neurons in the substantia nigra of patients with Parkinson's disease are largely unknown. Recently, aberrant cell cycle events have been shown to be associated with neuronal death in several neurodegenerative diseases. In the present study, we investigated the role of DNA polymerases (DNA pols) in 1-methyl-4-phenylpyridinium (MPP(+))-induced neuronal apoptosis in cerebellar granule cells. After exposure to MPP(+), the neurons entered S phase of the cell cycle. Neuronal cell cycle re-entry and apoptosis were attenuated by flavopiridol, which is a broad inhibitor of cyclin-dependent kinases (CDKs). MPP(+) exposure significantly increased the expression of DNA pol-beta and primase but did not affect the expression of the canonical replicative DNA pols, including DNA pol-delta and pol-epsilon. Dideoxycytidine, which is a pharmacological inhibitor of DNA pol-beta, attenuated the neuronal apoptosis mediated by MPP(+). In a similar manner, the expression of a dominant negative form of DNA pol-beta was also neuroprotective. These results suggest that DNA pol-beta may have a causal role in MPP(+)-induced neuronal apoptosis.

Toll-like receptor 2 (TLR2), a key immune receptor in the TLR family, is widely expressed in various systems, including the immune and nervous systems and plays a critical role in controlling innate and adaptive immune responses. We previously reported that opioids inhibit cell growth and trigger apoptosis. However, the underlying mechanism by which TLR2 mediates apoptosis in response to opioids is not yet known. Here we show that chronic morphine treatment in primary neurons dramatically increased the expression of TLR2 at both the messenger RNA and protein levels. In addition, TLR2 deficiency significantly inhibited chronic morphine-induced apoptosis in primary neurons. Activation of caspase-3 after morphine treatment is impaired in TLR2 deficient primary neurons. Moreover, morphine treatment failed to induce an increased level of phosphorylated glycogen synthase kinase 3 beta (GSK3beta) in TLR2 deficient primary neurons, suggesting an involvement of GSK3beta in morphine-mediated TLR2 signaling. These results thus demonstrate that opioids prime neurons to undergo apoptosis by inducing TLR2 expression. Our data suggest that inhibition of TLR2 is capable of preventing opioids-induced damage to neurons., (Copyright 2009 Elsevier Inc. All rights reserved.)

The hallmarks of Parkinson's disease (PD) are the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the presence of intracellular inclusion bodies in surviving neurons. Although the specific etiology and pathogenesis of sporadic PD remains unknown, neuronal death was proven to be associated with mitochondrial dysfunction and protein misfolding. However, molecular links between mitochondrial dysfunction and protein misfolding remains obscure. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a classical glycolytic enzyme, is responsible for carbohydrate metabolism under normal circumstances. When translocated to the nucleus, GAPDH promotes neuron apoptosis in several neurodegenerative disorders. But it seems that GAPDH translocation is not the sole mechanism responsible for neuronal apoptosis. We found that rotenone, a common mitochondrial complex I inhibitor used to produce experimental parkinsonism, cannot only induce GAPDH translocation but also trigger intermolecular disulfide bonding and result in the formation of intracytoplasmic aggregates of GAPDH. This suggests a link between mitochondrial dysfunction and protein misfolding, and sheds light on the pathophysiology of Lewy body formation in Parkinson's disease.

This study investigated the effect and mechanism of cell cycle reentry induced by 6-hydrodopamine (6-OHDA) in PC12 cells. By using neural differentiated PC12 cells treated with 6-OHDA, the apoptosis model of dopaminergic neurons was established. Cell viability was measured by MTT. Cell apoptosis and the distribution of cell cycle were assessed by flow cytometry. Western blot was used to detect the activation of extracellular regulator kinase1/2 (ERK1/2) pathway and the phosphorylation of retinoblastoma protein (RB). Our results showed that after PC12 cells were treated wtih 6-OHDA, the viability of PC12 cells was declined in a concentration-dependent manner. Flow cytometry revealed that 6-OHDA could increase the apoptosis ratio of PC12 cells in a time-dependent manner. The percentage of cells in G0/G1 phase of cell cycle was decreased and that in S phase and G2/M phase increased. Simultaneously, ERK1/2 pathway was activated and phosphorylated RB increased. It was concluded that 6-OHDA could induce cell cycle reentry of dopaminergic neurons through the activation of ERK1/2 pathway and RB phosphorylation. The aberrant cell cycle reentry contributes to the apoptosis of dopaminergic neurons.

Macrophage death in advanced atherosclerotic lesions leads to lesional necrosis, possible plaque rupture, and acute vascular occlusion. A likely cause of macrophage death is the accumulation of free cholesterol (FC) leading to activation of endoplasmic reticulum (ER) stress-induced apoptosis. Inositol-requiring enzyme 1 alpha (IRE1alpha) is an integral membrane protein of the ER that is a key signaling step in cholesterol-induced apoptosis in macrophages, activated by stress in the ER. However, the role of IRE1alpha in the regulation of ER stress-induced macrophage death and the mechanism for this process are largely unclear. In this study, a cell culture model was used to explore the mechanisms involved in the ER stress pathway of FC-induced macrophage death. The results herein showed that FC loading of macrophages leads to an apoptotic response that is partially dependent on initiation by activation of IRE1alpha. Taken together, these results showed that the IRE1-apoptosis-signaling kinase 1-c-Jun NH2-terminal kinase cascade pathway was required in this process. Moreover, the data suggested a novel cellular mechanism for cholesterol-induced macrophage death in advanced atherosclerotic lesions. The critical function of this signaling cascade is indicated by prevention of ER stress-induced apoptosis after inhibition of IRE1alpha, or c-Jun NH2-terminal kinase.

Objective: To investigate the feasibility of radiolabelled annexin V imaging for early diagnosis of Parkinson's disease., Methods: 99mTc-HYNIC-annexin V was prepared and its binding to apoptotic cell models of Parkinson's disease was studied in vitro. Cellular models of Parkinson's disease were produced by administering different concentrations of 1-methyl-4-phenylpyridinium to PC12 and SH-SY5Y cell lines. Cell apoptosis rates were analysed by flow cytometry. Annexin V was labelled with 99mTc by hydrazinonicotinamide (HYNIC). Cell binding studies were carried out using cellular models of Parkinson's disease. Cell uptake studies were also performed after different levels of MPP treatment, and the correlation between the degree of apoptosis and Tc-HYNIC-annexin V uptake was analysed., Results: The specific activity of 99mTc-HYNIC-annexin V was 3.7-74 x 10 Bq.mg protein. In-vitro binding of 99mTc-HYNIC-annexin V to model cells was specific, saturable and time dependent. Scatchard analysis gave a Kd of 7.16+/-1.78 nmol.l, Bmax values of 179+/-33 fmol per 10(6) cells (PC12) and 220+/-26 fmol per 10(6) cells (SHSY5Y). MPP at different concentrations can induce cell apoptosis in a dose-dependent manner; cellular uptake of 99mTc-HYNIC-annexin V as indicated by membrane-bound radiolabelled annexin V activity was linearly correlated with total fluorescence, as observed by FITC-annexin V flow cytometry (PC12: r=0.924; SH-SY5Y: r=0.937, P<0.01)., Conclusions: 99mTc-HYNIC-annexin V retains its receptor-binding activity and has a high affinity to cellular models of Parkinson's disease. The uptake of radioactivity correlated well with cell apoptosis rates; thus, 99mTc-annexin V is a potential imaging agent with which to detect early neuron damage in Parkinson's disease.

Apoptosis is a contributing cause of dopaminergic neuron loss in Parkinson disease. Recent work has shown that erythropoietin (EPO) offers protection against apoptosis in a wide variety of tissues. We demonstrate that exposure of PC12 cells to 1-methyl-4-phenylpyridinium ion (MPP(+)) with recombinant human EPO, significantly decreased apoptosis as measured by TUNEL and caspase-3 activity when compared to MPP(+) treatment alone. EPO induced sustained phosphorylation of Akt and its substrate, GSK-3beta, reduced caspase-3 activities in PC12 cells. The anti-apoptotic effect of EPO was abrogated by co-treatment with LY294002, the specific blocker of phosphatidylinositol 3-kinase (PI3K). The effects of EPO on GSK-3beta and caspase-3 activities were also blocked by LY294002. LiCl, the inhibitor of GSK-3beta, downregulated the caspase-3 activity and blocked the apoptosis induced by MPP(+). Finally, we determined that EPO transiently activated the ERK signaling pathway, but PD98059, a specific inhibitor of ERK, does not alter the survival effect of EPO in this model system. Thus, these findings indicate that EPO protects against apoptosis in PC12 cells exposed to MPP(+), through the Akt/GSK-3beta/caspase-3 signaling pathway, but the ERK pathway is not involved in the EPO-dependent survival enhancing effect in this model system.

The involvement of apoptosis in mitochondrial toxin 3-nitropropionic acid (3-NPA)-induced ischemic tolerance to transient focal cerebral ischemia in rats and the mechanism was investigated. 3-NPA at a dose of 20 mg/kg or vehicle control was intraperitoneally into the rats. Three days later, rats were exposed to 2 h of middle cerebral artery occlusion followed by 24 h of reperfusion. Infarct volumes were assessed by 2,3,5-triphenyltetrazolinm chloride (TTC) staining 24 h after reperfusion. Neural cell apoptosis in cerebral ischemic penumbra was detected by terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ nick end labeling (TUNEL) and flow cytometry methods (FCM). The results showed that as compared to the vehicle-treated group, pretreatment with 3-NPA could reduce the infarct volume by 23.3% and decrease the number of TUNEL-positive neural cells and apoptotic percentage by 47% (P<0.05) and 44.9% (P<0.01), respectively. It was concluded that the development of 3-NPA-induced ischemic tolerance in brain might be related to the decreases in neural cell apoptosis.

To explore the relationship between beta-amyloid (A beta) and the pathogenesis of Alzheimer disease (AD), after injection of beta-amyloid into the rat brain, the apoptosis of nerve cells and acetylcholine (Ach) content in rat hippocampus were examined by employing TUNEL technique and base hydroxylamine colorimetry respectively. The influence of age and glucocorticoid on the neurotoxic effect of A beta was also analyzed. A beta peptide could strongly induce the apoptosis of neurons in hippocampus, cortex and striate body (P < 0.05 or P < 0.01). In addition, the senility and glucocorticoid pre-treatment could enhance the toxic effect of A beta (P < 0.05 or P < 0.01). It is concluded that A beta may play an important role in the pathogenesis of Alzheimer disease via its induction of apoptosis of neurons and by decreasing the content of the Ach.

The biochemical pathways that mediate the degeneration of dopaminergic neurons in the substantia nigra of patients with Parkinson’s disease are largely unknown. Recently, aberrant cell cycle events have been shown to be associated with neuronal death in several neurodegenerative diseases. In the present study, we investigated the role of DNA polymerases (DNA pols) in 1-methyl-4-phenylpyridinium (MPP+)-induced neuronal apoptosis in cerebellar granule cells. After exposure to MPP+, the neurons entered S phase of the cell cycle. Neuronal cell cycle re-entry and apoptosis were attenuated by flavopiridol, which is a broad inhibitor of cyclin-dependent kinases (CDKs). MPP+ exposure significantly increased the expression of DNA pol-β and primase but did not affect the expression of the canonical replicative DNA pols, including DNA pol-δ and pol-ε. Dideoxycytidine, which is a pharmacological inhibitor of DNA pol-β, attenuated the neuronal apoptosis mediated by MPP+. In a similar manner, the expression of a dominant negative form of DNA pol-β was also neuroprotective. These results suggest that DNA pol-β may have a causal role in MPP+-induced neuronal apoptosis. [ABSTRACT FROM AUTHOR]

Abstract: Objective: To investigate the apoptosis of dopaminergic neurons and the protective effect of nicotine in 1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine(MPTP)-induced mouse model of Parkinson''s disease. Methods: The mouse model of Parkinson''s disease were formed by MPTP(30 mg/kg/d×7, i.p.); and the loss and apoptosis of dopaminergic neurons was observed by Tyrosine Hydroxylase (TH) and TUNEL stains. In “Nicotime plus MPTP” group, mice were pretreated with nicotine before MPTP injection. The putative protective effect of nicotine was analyzed. Results: The number of TH-positive cells decreased during MPTP treatment. Apoptotic neurons began to appear after three injections of MPTP and peaked on the 8 th day. In the MPTP-intoxicated mice treated with nicotine, the loss of TH-positive cells was significantly less than that of MPTP-treated group(30 mg/kg/d × 7)(P <0.05). Conclusion: The chronic treatment of MPTP can induce the apoptosis of dopaminergicneurons in substantia nigra, and nicotinemight have a neuroprotecitve effect on dopaminergic neurons against MPTP toxicity. [Copyright &y& Elsevier]

Abstract: The neuroprotective effects of erythropoietin on 1-methyl-4-phenylpyridinium (MPP+)-induced oxidative stress and apoptosis in cultured PC12 cells as well as the underlying mechanism were investigated. Treatment of PC12 cells with MPP+ caused the loss of cell viability, which was associated with the elevation in apoptotic rate, the formation of reactive oxygen species and the disruption of mitochondrial transmembrane potential. It was also shown that MPP+ significantly induced upregulation of Bax/Bcl-2 ratio and activation of caspase-3. In contrast, erythropoietin reversed these phenotypes and had its maximum protective effect at 1 U/ml. The effect of erythropoietin was mediated by the phosphatidylinositol 3-kinase (PI3K) signaling pathway since erythropoietin failed to rescue cells from MPP+ insult in the presence of the PI3K inhibitor, LY 294002. In addition, the downstream effector of PI3K, Akt, was activated by erythropoietin, and Akt activation was inhibited by LY 294002. Furthermore, the effect of erythropoietin on reactive oxygen species levels was also blocked by LY 294002. These results show that erythropoietin may provide a useful therapeutic strategy for the treatment of oxidative stress-induced neurodegenerative diseases such as Parkinson disease. [Copyright &y& Elsevier]