Quinpirole ameliorates nigral dopaminergic neuron damage in Parkinson's disease mouse model through activating GHS-R1a/D2R heterodimers.

Growth hormone secretagogue receptor 1a (GHS-R1a) is an important G protein-coupled receptor (GPCR) that regulates a variety of functions by binding to ghrelin. It has been shown that the dimerization of GHS-R1a with other receptors also affects ingestion, energy metabolism, learning and memory. Dopamine type 2 receptor (D2R) is a GPCR mainly distributed in the ventral tegmental area (VTA), substantia nigra (SN), striatum and other brain regions. In this study we investigated the existence and function of GHS-R1a/D2R heterodimers in nigral dopaminergic neurons in Parkinson's disease (PD) models in vitro and in vivo. By conducting immunofluorescence staining, FRET and BRET analyses, we confirmed that GHS-R1a and D2R could form heterodimers in PC-12 cells and in the nigral dopaminergic neurons of wild-type mice. This process was inhibited by MPP+ or MPTP treatment. Application of QNP (10 µM) alone significantly increased the viability of MPP+-treated PC-12 cells, and administration of quinpirole (QNP, 1 mg/kg, i.p. once before and twice after MPTP injection) significantly alleviated motor deficits in MPTP-induced PD mice model; the beneficial effects of QNP were abolished by GHS-R1a knockdown. We revealed that the GHS-R1a/D2R heterodimers could increase the protein levels of tyrosine hydroxylase in the SN of MPTP-induced PD mice model through the cAMP response element binding protein (CREB) signaling pathway, ultimately promoting dopamine synthesis and release. These results demonstrate a protective role for GHS-R1a/D2R heterodimers in dopaminergic neurons, providing evidence for the involvement of GHS-R1a in PD pathogenesis independent of ghrelin.

Role of upregulation of the KATP channel subunit SUR1 in dopaminergic neuron degeneration in Parkinson's disease.

Accumulating evidence suggests that ATP-sensitive potassium (KATP ) channels play an important role in the selective degeneration of dopaminergic neurons in the substantia nigra (SN). Furthermore, the expression of the KATP channel subunit sulfonylurea receptor 1 (SUR1) is upregulated in the remaining nigral dopaminergic neurons in Parkinson's disease (PD). However, the mechanism underlying this selective upregulation of the SUR1 subunit and its subsequent roles in PD progression are largely unknown. In 3-, 6-, and 9-month-old A53T α-synuclein transgenic (α-SynA53T+/+ ) mice, only the SUR1 subunit and not SUR2B or Kir6.2 was upregulated, accompanied by neuronal damage. Moreover, the occurrence of burst firing in dopaminergic neurons was increased with the upregulation of the SUR1 subunit, whereas no changes in the firing rate were observed except in 9-month-old α-SynA53T+/+ mice. After interference with SUR1 expression by injection of lentivirus into the SN, the progression of dopaminergic neuron degeneration was delayed. Further studies showed that elevated expression of the transcription factors FOXA1 and FOXA2 could cause the upregulation of the SUR1 subunit in α-SynA53T+/+ mice. Our findings revealed the regulatory mechanism of the SUR1 subunit and the role of KATP channels in the progression of dopaminergic neuron degeneration, providing a new target for PD drug therapy.

The expression of KATP channel subunits in alpha-synuclein-transfected MES23.5 cells.

BACKGROUND: SUR1, one of the subunits of ATP-sensitive potassium (KATP) channels, was found to be highly expressed in mRNA levels in the substantia nigra (SN) of Parkinson's disease (PD) brains. Though the mechanism of the selective dopamine (DA) neurons death is still unknown, some studies have demonstrated that selective activation of the KATP channels in the SN might be associated with the degeneration of DA neurons. The objective of our study is to examine the expressions of KATP channel subunits in dopaminergic cells with alpha-synuclein (α-Syn) transfection. METHODS: In this study, we detected the KATP channel subunits mRNA levels in MES23.5 cells by real-time quantitative PCR after the cells transfected with α-Syn. RESULTS: Our results showed that the mRNA levels of SUR1 subunit were markedly increased by 35% in WT α-Syn overexpression cells and by 31% in A53T α-Syn overexpression cells, respectively. However, the mRNA levels of SUR2B and Kir6.2 subunit have no obviously differences from the controls. CONCLUSIONS: We showed that the mRNA levels of SUR1 but not SUR2B or Kir6.2 were selectively upregulated in MES23.5 cells over-expressed with α-Syn. The findings demonstrated that the SUR1 overexpressed might be involved in the process of PD.

Brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor inhibit ferrous iron influx via divalent metal transporter 1 and iron regulatory protein 1 regulation in ventral mesencephalic neurons.

Iron accumulation is observed in the substantia nigra of patients with Parkinson's disease. However, it is unknown whether neurotrophic factors, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) participate in the modulation of neuronal iron metabolism. Here, we investigated the effects and underlying mechanisms of BDNF and GDNF on the iron influx process in primary cultured ventral mesencephalic neurons. 6-hydroxydopamine-induced enhanced ferrous iron influx via improper up-regulation of divalent metal transporter 1 with iron responsive element (DMT1+IRE) was consistently relieved by BDNF and GDNF. Both the mRNA and protein levels of DMT1+IRE were down-regulated by BDNF or GDNF treatment alone. We further demonstrated the involvement of iron regulatory protein 1 (IRP1) in BDNF- and GDNF-induced DMT1+IRE expression. Extracellular-regulated kinase 1/2 (ERK1/2) and Akt were activated and participated in these processes. Inhibition of ERK1/2 and Akt phosphorylation abolished the down-regulation of IRP1 and DMT1+IRE induced by BDNF and GDNF. Taken together, these results show that BDNF and GDNF ameliorate iron accumulation via the ERK/Akt pathway, followed by inhibition of IRP1 and DMT1+IRE expression, which may provide new targets for the neuroprotective effects of these neurotrophic factors.