α-Synuclein is the major component of Lewy bodies. α-Synuclein phosphorylated at 
Ser 129 (Phospho-α-Syn) is the most common synuclein modification observed in 
Parkinson's disease pathology and transgenic animal models. Polo-like kinase 2 
(PLK2) was previously proposed as an important kinase in α-synuclein 
phosphorylation at Ser129. To better understand the role of PLK2 in α-synuclein 
phosphorylation in vivo, we further evaluated the effect of PLK2 genetic 
knockdown and pharmacological inhibition on Phospho-α-Syn levels in different 
brain regions of PLK2 knockout (KO), heterozygous (Het) and wild-type (WT) mice. 
Whereas PLK2 knockdown had no effect on Total-α-synuclein brain levels, it 
resulted in a gene-dosage dependent, albeit incomplete, reduction of endogenous 
Phospho-α-Syn levels in all brain regions investigated. No compensatory 
induction of other α-synuclein kinases (PLK3, casein kinase-2, G-protein-coupled 
receptor kinase 5 (GRK5) and GRK6) was observed at the mRNA level in the PLK2 KO 
mouse brain. To determine whether increased activity of another PLK family 
member is responsible for the residual Phospho-α-Syn levels in the PLK2 KO mouse 
brain, the pan-PLK inhibitor BI 2536 was tested in PLK2 KO mice. Whereas BI 2536 
reduced Phospho-α-Syn levels in WT mice, it did not further reduce the residual 
endogenous Phospho-α-Syn levels in PLK2 KO and Het mice, suggesting that a 
kinase other than PLK1-3 accounts for the remaining PLK inhibitor-resistant pool 
in the mouse brain. Moreover, PLK3 KO in mice had no effect on both Total- and 
Phospho-α-Syn brain levels. These results support a significant role for a PLK 
kinase in phosphorylating α-synuclein at Ser129 in the brain, and suggest that 
PLK2 is responsible for this activity under physiological conditions.