Phosphorylation Does Not Prompt, Nor Prevent, the Formation of  -synuclein Toxic Species in a Rat Model of Parkinson's Disease

Abstract

Phosphorylation is involved in numerous neurodegenerative diseases. In particular, alpha-synuclein is extensively phosphorylated in aggregates in patients suffering from synucleinopathies. However, the share of this modification in the events that lead to the conversion of alpha-synuclein to aggregated toxic species needed to be clarified. The rat model that we developed through rAAV2/6-mediated expression of alpha-synuclein demonstrates a correlation between neurodegeneration and formation of small filamentous alpha-synuclein aggregates. A mutation preventing phosphorylation (S129A) significantly increases alpha-synuclein toxicity and leads to enhanced formation of beta-sheet-rich, proteinase K-resistant aggregates, increased affinity for intracellular membranes, a disarrayed network of neurofilaments and enhanced alpha-synuclein nuclear localization. The expression of a mutation mimicking phosphorylation (S129D) does not lead to dopaminergic cell loss. Nevertheless, fewer but larger aggregates are formed, and signals of apoptosis are also activated in rats expressing the phosphorylation-mimicking form of alpha-synuclein. These observations strongly suggest that phosphorylation does not play an active role in the accumulation of cytotoxic pre-inclusion aggregates. Unexpectedly, the study also demonstrates that constitutive expression of phosphorylation-mimicking forms of alpha-synuclein does not protect from neurodegeneration. The role of phosphorylation at Serine 129 in the early phase of Parkinson's disease is examined, which brings new perspective to therapeutic approaches focusing on the modulation of kinases/phosphatases activity to control alpha-synuclein toxicity.

Phosphorylation of α-Synuclein Protein at Ser-129 Reduces Neuronal Dysfunction by Lowering Its Membrane Binding Property in Caenorhabditis elegans

“…In Tg rats that overexpress human ␣-synuclein by adenoassociated virus-mediated gene transfer, S129A-␣-synuclein displayed higher neurotoxicity when compared with WT-and S129D-␣-synuclein (25,26), which was supplemented by an immunoelectron microscopic analysis showing a close localization of S129A-␣-synuclein to cellular membranes (26). In addition, S129D-␣-synuclein expressed in budding yeast exhibited decreased plasma membrane association (37).…”

“…have reported the toxic effects of S129A or S129D-␣-synuclein in neurons of transgenic rat or Drosophila models (25)(26)(27)(28), detailed molecular mechanisms underlying the toxicity remained elusive. Here we demonstrated a strong correlation among ␣-synuclein binding to membrane, phosphorylation at Ser-129, and neuronal dysfunction.…”

“…Two groups have reported that overexpression of recombinant adeno-associated virus-mediated S129A-␣-synuclein in rat substantia nigra caused prominent loss of dopaminergic neurons, whereas that of S129D-␣-synuclein displayed much milder toxicity (25,26). Conversely, another rat model exhibited no differences in toxicities among wild-type (WT) and Ser-129 mutant ␣-synuclein expressed by recombinant adeno-associated virus (27), and S129D-␣-synuclein showed more severe toxicity when compared with S129A-␣-synuclein in Tg Drosophila (28).…”

Interplay between Sumoylation and Phosphorylation for Protection against α-Synuclein Inclusions

“…However, there is still no consensus of the effects of different modifications on ␣Syn aggregation and toxicity (15,23,24,54,78). Although earlier studies did not observe the effects of ␣Syn phosphorylation at Ser-129 on ␣Syn-mediated toxicity and aggregation (54,79), protective roles of ␣Syn Ser-129 phosphorylation were described in a strain-specific manner in yeast. Therefore, the specific genetic context was proposed to determine the sensitivity to changes in ␣Syn phosphorylation (80).…”

“…The effects of phosphorylation on ␣Syn-induced toxicity are complex with reports supporting negative as well as positive impacts on cells (23,24,(51)(52)(53)(54). Therefore, we next investigated the interplay between ␣Syn sumoylation and phosphorylation by examining how changes in sumoylation affect ␣Syn phosphorylation and whether this impacted ␣Syn toxicity.…”