Abnormal accumulation of proteinaceous intraneuronal inclusions called Lewy bodies (LBs) is the neurpathological hallmark of Parkinson’s disease (PD) and related synucleinopathies. These inclusions are mainly constituted of a presynaptic protein, α-synuclein ( α-syn). Over the past decade, growing amounts of studies reported an aberrant accumulation of phosphorylated α-syn at the residue S129 (pS129) in the brain of patients suffering from PD, as well as in transgenic animal models of synucleinopathies. Whereas only a small fraction of α-syn (90%) has been observed within LBs, suggesting that this post-translational modification may play an important role in the regulation of α-syn aggregation, LBs formation and neuronal degeneration. However, whether phosphorylation at S129 suppresses or enhances α-syn aggregation and toxicity in vivo remains a subject of active debate. The answer to this question has important implications for understanding the role of phosphorylation in the pathogenesis of synucleinopathies and determining if targeting kinases or phosphatases could be a viable therapeutic strategy for the treatment of these devastating neurological disorders. In the present review, we explore recent findings from in vitro, cell-based assays and in vivo studies describing the potential implications of pS129 in the regulation of α-syn physiological functions, as well as its implication in synucleinopathies pathogenesis and diagnosis.

Increasing evidence suggests that phosphorylation may play an important role in the oligomerization, fibrillogenesis, Lewy body (LB) formation, and neurotoxicity of α-synuclein (α-syn) in Parkinson disease. Herein we demonstrate that α-syn is phosphorylated at S87in vivoand within LBs. The levels of S87-P are increased in brains of transgenic (TG) models of synucleinopathies and human brains from Alzheimer disease (AD), LB disease (LBD), and multiple system atrophy (MSA) patients. Using antibodies against phosphorylated α-syn (S129-P and S87-P), a significant amount of immunoreactivity was detected in the membrane in the LBD, MSA, and AD cases but not in normal controls. In brain homogenates from diseased human brains and TG animals, the majority of S87-P α-syn was detected in the membrane fractions. A battery of biophysical methods were used to dissect the effect of S87 phosphorylation on the structure, aggregation, and membrane-binding properties of monomeric α-syn. These studies demonstrated that phosphorylation at S87 expands the structure of α-syn, increases its conformational flexibility, and blocks its fibrillizationin vitro. Furthermore, phosphorylation at S87, but not S129, results in significant reduction of α-syn binding to membranes. Together, our findings provide novel mechanistic insight into the role of phosphorylation at S87 and S129 in the pathogenesis of synucleinopathies and potential roles of phosphorylation in α-syn normal biology.

Phosphorylation of α-synuclein (α-syn) at Ser-129 is a hallmark of Parkinson disease and related synucleinopathies. However, the identity of the natural kinases and phosphatases responsible for regulating α-syn phosphorylation remain unknown. Here we demonstrate that three closely related members of the human Polo-like kinase (PLK) family (PLK1, PLK2, and PLK3) phosphorylate α-syn and β-syn specifically at Ser-129 and Ser-118, respectively. Unlike other kinases reported to partially phosphorylate α-syn at Ser-129 in vitro, phosphorylation by PLK2 and PLK3 is quantitative (>95% conversion). Only PLK1 and PLK3 phosphorylate β-syn at Ser-118, whereas no phosphorylation of γ-syn was detected by any of the four PLKs (PLK1 to -4). PLK-mediated phosphorylation was greatly reduced in an isolated C-terminal fragment (residues 103–140) of α-syn, suggesting substrate recognition via the N-terminal repeats and/or the non-amyloid component domain of α-syn. PLKs specifically co-localized with phosphorylated Ser-129 (Ser(P)-129) α-syn in various subcellular compartments (cytoplasm, nucleus, and membranes) of mammalian cell lines and primary neurons as well as in α-syn transgenic mice, especially cortical brain areas involved in synaptic plasticity. Furthermore, we report that the levels of PLK2 are significantly increased in brains of Alzheimer disease and Lewy body disease patients. Taken together, these results provide biochemical and in vivo evidence of α-syn and β-syn phosphorylation by specific PLKs. Our results suggest a need for further studies to elucidate the potential role of PLK-syn interactions in the normal biology of these proteins as well as their involvement in the pathogenesis of Parkinson disease and other synucleinopathies.

Significance: Converging lines of evidence suggest that increase in α-syn levels plays a central role in synucleinopathies pathogenesis. Therefore, lowering α-syn levels represents a viable therapeutic strategy for these disorders. Here, we describe a previously uncharacterized cellular mechanism controlling the selective autophagic degradation of α-syn. This effect is governed by PLK2 kinase activity, phosphorylation of α-syn, and PLK2/α-syn complex formation. In a rat model of Parkinson disease, PLK2 overexpression reduces α-syn levels and suppresses α-syn–induced toxicity and motor deficits. Collectively, our data support the neuroprotective role of PLK2 and suggest that modulating its activity is a viable therapeutic strategy for the treatment of Parkinson disease and related synucleinopathies.