Integrin α5ß1 expression on dopaminergic neurons is involved in dopaminergic neurite outgrowth on striatal neurons.

During development, dopaminergic neurons born in the substantia nigra extend their axons toward the striatum. However, the mechanisms by which the dopaminergic axons extend the striatum to innervate their targets remain unclear. We previously showed that paired-cultivation of mesencephalic cells containing dopaminergic neurons with striatal cells leads to the extension of dopaminergic neurites from the mesencephalic cell region to the striatal cell region. The present study shows that dopaminergic neurites extended along striatal neurons in the paired-cultures of mesencephalic cells with striatal cells. The extension of dopaminergic neurites was suppressed by the pharmacological inhibition of integrin α5ß1. Using lentiviral vectors, short hairpin RNA (shRNA)-mediated knockdown of integrin α5 in dopaminergic neurons suppressed the neurite outgrowth to the striatal cell region. In contrast, the knockdown of integrin α5 in non-dopaminergic mesencephalic and striatal cells had no effect. Furthermore, overexpression of integrin α5 in dopaminergic neurons differentiated from embryonic stem cells enhanced their neurite outgrowth on striatal cells. These results indicate that integrin α5ß1 expression on dopaminergic neurons plays an important role in the dopaminergic neurite outgrowth on striatal neurons.

Staurosporine induces dopaminergic neurite outgrowth through AMP-activated protein kinase/mammalian target of rapamycin signaling pathway.

Axonal degeneration of dopaminergic neurons is one of the pathological features in the early stages of Parkinson disease. Promotion of axonal outgrowth of the remaining dopaminergic neurons leads to the recovery of the nigrostriatal pathway. Staurosporine (STS), a wide-spectrum kinase inhibitor, induces neurite outgrowth in various cell types, although its mechanism of action remains elusive. In this study, we analyzed which protein kinase is involved in STS-induced neurite outgrowth. We have previously established the method to measure the length of dopaminergic neurites that extend from a mesencephalic cell region, which is formed on a coverslip by an isolation wall. By means of this method, we clarified that STS treatment causes dopaminergic axonal outgrowth in mesencephalic primary cultures. Among the specific protein kinase inhibitors we tested, compound C (C.C), an AMP-activated protein kinase (AMPK) inhibitor, promoted dopaminergic neurite outgrowth. STS as well as C.C elevated the phosphorylation level of 70-kDa ribosomal protein S6 kinase, a downstream target of mammalian target of rapamycin (mTOR) signaling pathway. The STS- and C.C-induced dopaminergic neurite outgrowth was suppressed by rapamycin, an mTOR inhibitor. Furthermore, the application of C.C rescued 1-methyl-4-phenylpyridinium ion (MPP(+))-induced dopaminergic neurite degeneration. These results suggest that STS induces dopaminergic axonal outgrowth through mTOR signaling pathway activation as a consequence of AMPK inhibition.

Isolation, identification, and biological evaluation of Nrf2-ARE activator from the leaves of green perilla (Perilla frutescens var. crispa f. viridis).

The nuclear factor erythroid 2-related factor 2 (Nrf2)-antioxidant response element (ARE) pathway is a cellular defense system against oxidative stress. Activation of this pathway increases expression of antioxidant enzymes. Epidemiological studies have demonstrated that the consumption of fruits and vegetables is associated with reduced risk of contracting a variety of human diseases. The aim of this study is to find Nrf2-ARE activators in dietary fruits and vegetables. We first attempted to compare the potency of ARE activation in six fruit and six vegetables extracts. Green perilla (Perilla frutescens var. crispa f. viridis) extract exhibited high ARE activity. We isolated the active fraction from green perilla extract through bioactivity-guided fractionation. Based on nuclear magnetic resonance and mass spectrometric analysis, the active ingredient responsible for the ARE activity was identified as 2',3'-dihydroxy-4',6'-dimethoxychalcone (DDC). DDC induced the expression of antioxidant enzymes, such as γ-glutamylcysteine synthetase (γ-GCS), NAD(P)H: quinone oxidoreductase-1 (NQO1), and heme oxygenase-1. DDC inhibited the formation of intracellular reactive oxygen species and the cytotoxicity induced by 6-hydroxydopamine. Inhibition of the p38 mitogen-activated protein kinase pathway abolished ARE activation, the induction of γ-GCS and NQO1, and the cytoprotective effect brought about by DDC. Thus, this study demonstrated that DDC contained in green perilla enhanced cellular resistance to oxidative damage through activation of the Nrf2-ARE pathway.

Reconstruction and quantitative evaluation of dopaminergic innervation of striatal neurons in dissociated primary cultures.

Repairing the nigrostriatal pathway is expected to become a future treatment strategy for Parkinson disease. Our aim is to establish an in vitro model for the quantitative analysis of the nigrostriatal projections of dopaminergic neurons using primary dissociated neruons. To form the mesencephalic cell region, mesencephalic cells derived from rat embryos were plated within an isolation wall, which was removed after cell adhesion to the coverslip. After incubation for 11 days, the dopaminergic neurites extending to the outside of the mesencephalic cell region were mainly axons. Treatment with glial cell line-derived neurotrophic factor for 11 days significantly promoted the outgrowth of dopaminergic axons from the mesencephalic cell region in a concentration-dependent manner. When striatal cells were plated outside the mesencephalic cell region, dopaminergic neurites were remarkably extended to the striatal cell region. Moreover, immunocytochemistry for tyrosine hydroxylase and synaptophysin revealed that dopaminergic axons formed synapses with striatal neurons. By contrast, spinal cells did not increase dopaminergic neurite outgrowth. These results indicate that the present method is valuable for evaluating nigrostriatal projections in vitro.

Haloperidol, spiperone, pimozide and aripiprazole reduce intracellular dopamine content in PC12 cells and rat mesencephalic cultures: Implication of inhibition of vesicular transport.

Accumulating evidence suggests that antipsychotics affect dopamine release from dopaminergic neurons, but the precise mechanisms are not fully understood. Besides, there are few studies on the effects of antipsychotics on intracellular dopamine content. In this study, the effects of 8 antipsychotics on dopamine release and intracellular dopamine content in PC12 cells were investigated. Pretreatment with haloperidol, spiperone, pimozide, aripiprazole and risperidone markedly inhibited high potassium-evoked dopamine release. By contrast, pretreatment with chlorpromazine slightly increased high potassium-evoked dopamine release, while pretreatment with sulpiride and olanzapine had no effect. Haloperidol, spiperone, pimozide, chlorpromazine, aripiprazole and olanzapine evoked dopamine release, while sulpiride and risperidone had no effect. In addition, haloperidol, spiperone, pimozide, aripiprazole and risperidone reduced intracellular dopamine content in a concentration-dependent manner. These results suggest that the reduction in high potassium-evoked dopamine release by pretreatment with antipsychotics results from the reduction in vesicular dopamine content. Treatment with the 8 antipsychotics did not affect the expression of total or phosphorylated tyrosine hydroxylase. Instead, haloperidol, spiperone, pimozide and aripiprazole as well as reserpine transiently increased extracellular levels of dopamine metabolites. In addition, haloperidol, spiperone, pimozide, aripiprazole and risperidone reduced vesicular [3H]dopamine transport. These results suggest that the inhibition of vesicular dopamine transport by haloperidol, spiperone, pimozide and aripiprazole results in a reduction in vesicular dopamine content.

Elevation of heme oxygenase-1 by proteasome inhibition affords dopaminergic neuroprotection.

Postmortem studies have shown that heme oxygenase-1 (HO-1) immunoreactivity is increased in patients with Parkinson disease. HO-1 expression is highly upregulated by a variety of stress. Since the proteasome activity is decreased in patients with Parkinson disease, we investigated whether proteasome activity regulates HO-1 content. MG-132, a proteasome inhibitor, increased the amount of HO-1 protein mainly in astrocytes of primary mesencephalic cultures. Quantitative RT-PCR analysis revealed that lactacystin upregulated HO-1 mRNA expression. Proteasome inhibition with MG132 also increased the cytomegalovirus promoter-driven expression of Flag-HO-1 protein and resulted in an accumulation of ubiquitinated Flag-HO-1 in Flag-HO-1-overexpressing PC12 cells. In addition, a cycloheximide chase assay demonstrated that the degradation of Flag-HO-1 protein was slowed by MG-132. Next, the function of HO-1 which was upregulated by proteasome inhibitors was examined. Proteasome inhibitors protected dopaminergic neurons from 6-hydroxydopamine (6-OHDA)-induced toxicity and this neuroprotection was abrogated by co-treatment with zinc protoporphyrin IX, a HO-1 inhibitor. Furthermore, 6-OHDA-induced toxicity was blocked by bilirubin and carbon monoxide, products of the HO-1-catalyzed degradation of heme. These results suggest that mesencephalic HO-1 protein level is regulated by proteasome activity and the elevation by proteasome inhibition affords neuroprotection.

Vulnerability to glutamate toxicity of dopaminergic neurons is dependent on endogenous dopamine and MAPK activation.

Dopaminergic neurons are more vulnerable than other types of neurons in cases of Parkinson disease and ischemic brain disease. An increasing amount of evidence suggests that endogenous dopamine plays a role in the vulnerability of dopaminergic neurons. Although glutamate toxicity contributes to the pathogenesis of these disorders, the sensitivity of dopaminergic neurons to glutamate toxicity has not been clarified. In this study, we demonstrated that dopaminergic neurons were preferentially affected by glutamate toxicity in rat mesencephalic cultures. Glutamate toxicity in dopaminergic neurons was blocked by inhibiting extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase, and p38 MAPK. Furthermore, depletion of dopamine by alpha-methyl-dl-p-tyrosine methyl ester (alpha-MT), an inhibitor of tyrosine hydroxylase (TH), protected dopaminergic neurons from the neurotoxicity. Exposure to glutamate facilitated phosphoryration of TH at Ser31 by ERK, which contributes to the increased TH activity. Inhibition of ERK had no additive effect on the protection offered by alpha-MT, whereas alpha-MT and c-jun N-terminal kinase or p38 MAPK inhibitors had additive effects and yielded full protection. These data suggest that endogenous dopamine is responsible for the vulnerability to glutamate toxicity of dopaminergic neurons and one of the mechanisms may be an enhancement of dopamine synthesis mediated by ERK.