Implication of Alpha-Synuclein Phosphorylation at S129 in Synucleinopathies: What Have We Learned in the Last Decade?

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, alpha-synuclein (alpha-syn). Over the past decade, growing amounts of studies reported an aberrant accumulation of phosphorylated alpha-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 alpha-syn (< 4%) is phosphorylated in healthy brains, a dramatic accumulation of pS129 (> 90%) has been observed within LBs, suggesting that this post-translational modification may play an important role in the regulation of alpha-syn aggregation, LBs formation and neuronal degeneration. However, whether phosphorylation at S129 suppresses or enhances alpha-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 alpha-syn physiological functions, as well as its implication in synucleinopathies pathogenesis and diagnosis.