ABSTRACT
Most α-synuclein (α-syn) deposited in Lewy bodies, the pathological hallmark of Parkinson disease (PD), is phosphorylated at Ser-129. However, the physiological and pathological roles of this modification are unclear. Here we investigate the effects of Ser-129 phosphorylation on dopamine (DA) uptake in dopaminergic SH-SY5Y cells expressing α-syn. Subcellular fractionation of small interfering RNA (siRNA)-treated cells shows that G protein-coupled receptor kinase 3 (GRK3), GRK5, GRK6, and casein kinase 2 (CK2) contribute to Ser-129 phosphorylation of membrane-associated α-syn, whereas cytosolic α-syn is phosphorylated exclusively by CK2. Expression of wild-type α-syn increases DA uptake, and this effect is diminished by introducing the S129A mutation into α-syn. However, wild-type and S129A α-syn equally increase the cell surface expression of dopamine transporter (DAT) in SH-SY5Y cells and nonneuronal HEK293 cells. In addition, siRNA-mediated knockdown of GRK5 or GRK6 significantly attenuates DA uptake without altering DAT cell surface expression, whereas knockdown of CK2 has no effect on uptake. Taken together, our results demonstrate that membrane-associated α-syn enhances DA uptake capacity of DAT by GRKs-mediated Ser-129 phosphorylation, suggesting that α-syn modulates intracellular DA levels with no functional redundancy in Ser-129 phosphorylation between GRKs and CK2.
Subject(s)
Dopamine/metabolism , G-Protein-Coupled Receptor Kinase 5/genetics , G-Protein-Coupled Receptor Kinases/genetics , Neurons/metabolism , Serine/metabolism , alpha-Synuclein/genetics , Biological Transport , Casein Kinase II/antagonists & inhibitors , Casein Kinase II/genetics , Casein Kinase II/metabolism , Cell Line , Cell Membrane/genetics , Cell Membrane/metabolism , G-Protein-Coupled Receptor Kinase 3/genetics , G-Protein-Coupled Receptor Kinase 3/metabolism , G-Protein-Coupled Receptor Kinase 5/antagonists & inhibitors , G-Protein-Coupled Receptor Kinase 5/metabolism , G-Protein-Coupled Receptor Kinases/antagonists & inhibitors , G-Protein-Coupled Receptor Kinases/metabolism , Gene Expression Regulation , HEK293 Cells , Humans , Neurons/cytology , Phosphorylation , RNA, Small Interfering/genetics , RNA, Small Interfering/metabolism , Signal Transduction , alpha-Synuclein/metabolismABSTRACT
α-Synuclein (a-Syn) is a major component of fibrillar aggregates in Lewy bodies (LBs), a characteristic hallmark of Parkinson disease. Almost 90% of a-Syn deposited in LBs is phosphorylated at Ser-129. However, the role of Ser-129-phosphorylated a-Syn in the biogenesis of LBs remains unclear. Here, we investigated the metabolism of Ser-129-phosphorylated a-Syn. In SH-SY5Y cells, inhibition of protein phosphatase 2A/1 by okadaic acid, and inhibition of the proteasome pathway by MG132 or lactacystin accumulated Ser-129-phosphorylated a-Syn. However, these inhibitions did not alter the amounts of total a-Syn within the observation time. Inhibition of the autophagy-lysosome pathway by 3-methyladenine or chloroquine accumulated Ser-129-phosphorylated a-Syn in parallel to total a-Syn during longer incubations. Experiments using cycloheximide showed that Ser-129-phosphorylated a-Syn diminished rapidly (t(½) = 54.9 ± 6.4 min), in contrast to the stably expressed total a-Syn. The short half-life of Ser-129-phosphorylated a-Syn was blocked by MG132 to a greater extent than okadaic acid. In rat primary cortical neurons, either MG132, lactacystin, or okadaic acid accumulated Ser-129-phosphorylated a-Syn. Additionally, we did not find that phosphorylated a-Syn was ubiquitinated in the presence of proteasome inhibitors. These data show that Ser-129-phosphorylated a-Syn is targeted to the proteasome pathway in a ubiquitin-independent manner, in addition to undergoing dephosphorylation. The proteasome pathway may play a role in the biogenesis of Ser-129-phosphorylated a-Syn-rich LBs.
Subject(s)
Cerebral Cortex/metabolism , Neurons/metabolism , Proteasome Endopeptidase Complex/metabolism , Ubiquitin/metabolism , alpha-Synuclein/metabolism , Acetylcysteine/analogs & derivatives , Acetylcysteine/pharmacology , Animals , Cell Line, Tumor , Cycloheximide/pharmacology , Cysteine Proteinase Inhibitors/pharmacology , Humans , Leupeptins/pharmacology , Lewy Bodies/genetics , Lewy Bodies/metabolism , Okadaic Acid/pharmacology , Parkinson Disease/genetics , Parkinson Disease/metabolism , Phosphorylation/drug effects , Phosphorylation/genetics , Proteasome Endopeptidase Complex/genetics , Proteasome Inhibitors , Protein Phosphatase 1/antagonists & inhibitors , Protein Phosphatase 1/genetics , Protein Phosphatase 1/metabolism , Protein Phosphatase 2/antagonists & inhibitors , Protein Phosphatase 2/genetics , Protein Phosphatase 2/metabolism , Protein Synthesis Inhibitors , Rabbits , Ubiquitin/genetics , alpha-Synuclein/geneticsABSTRACT
The most characteristic feature of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington's disease, is the occurrence of extra- or intracellular fibrillar aggregates containing misfolded proteins with beta-sheet conformation. These aggregates are composed of distinct proteins in each neurodegenerative disease. However, mutations in genes encoding major constituents of aggregates, such as Abeta, tau, alpha-synuclein, SOD1 and huntingtin, have been identified to causally associate with familial forms of the diseases. Biochemical studies demonstrate that these mutant and some wild-type proteins tend to be misfolded or form aggregates. It has been proposed that these diseases are caused by a common mechanism involving misfolded proteins that trigger a toxic cascade leading to neuronal degeneration. This hypothesis is the basis of the therapeutic potential of heat shock proteins (HSPs), which prevent protein misfolding and aggregation. Transgenic animal models of the diseases have demonstrated that induction or overexpression of HSPs can suppress neuronal dysfunction and degeneration. Do the results promise clinical success for HSP-based therapy in neurodegenerative diseases? Recent findings regarding the pathogenic species generated during fibril formation have highlighted some of the beneficial and problematic aspects of HSP-based therapy. In this review, we focus on the pathogenic role of prefibrillar intermediates, including soluble oligomers and protofibrils, on neurodegeneration, and the relationship between prefibrillar intermediates and the proteins targeted by HSPs. We discuss in vitro and in vivo experimental data showing that HSPs counteract disease progression by acting as suppressors of toxic prefibrillar intermediates and toxic misfolded proteins in neurodegenerative diseases.
Subject(s)
Amyloid/antagonists & inhibitors , Heat-Shock Proteins/physiology , Neurodegenerative Diseases/metabolism , Amyloid/metabolism , Animals , Heat-Shock Proteins/genetics , Heat-Shock Proteins/metabolism , Humans , Protein Folding , Protein Structure, Secondary , SolubilityABSTRACT
The majority of alpha-synuclein (alphaS) deposited in Lewy bodies, the pathological hallmark of Parkinson's disease (PD), is phosphorylated at serine 129 (Ser129). Ser129 phosphorylation of alphaS has been demonstrated to enhance the alphaS toxicity to dopaminergic neurons in a Drosophila model of PD. Phosphorylation of alphaS at Ser129 seems to play a crucial role in the pathogenesis of PD. Here, we assessed the contribution of ubiquitously expressing members of the G-protein-coupled receptor kinase family (GRK2, GRK3, GRK5, and GRK6) to Ser129 phosphorylation of alphaS in HEK293 cells. To selectively reduce the endogenous expression of each member of the GRK family in cells, we used small interfering RNAs. Knockdown of GRK3 or GRK6 significantly decreased Ser129 phosphorylation of alphaS; however, knockdown of GRK2 or GRK5 did not decrease alphaS phosphorylation. The results indicate that endogenous GRK3 and GRK6, but not GRK2 or GRK5, contribute to Ser129 phosphorylation of alphaS in HEK293 cells.
