Leucine-rich repeat kinase 2 (LRRK2), a large protein kinase containing multi-functional domains, has been identified as the causal molecule for autosomal-dominant Parkinson's disease (PD). In the present study, we demonstrated for the first time that (i) LRRK2 interacts with tau in a tubulin-dependent manner; (ii) LRRK2 directly phosphorylates tubulin-associated tau, but not free tau; (iii) LRRK2 phosphorylates tau at Thr181 as one of the target sites; and (iv) The PD-associated LRRK2 mutations, G2019S and I2020T, elevated the degree of tau-phosphorylation. These results provide direct proof that tau is a physiological substrate for LRRK2. Furthermore, we revealed that LRRK2-mediated phosphorylation of tau reduces its tubulin-binding ability. Our results suggest that LRRK2 plays an important role as a physiological regulator for phosphorylation-mediated dissociation of tau from microtubules, which is an integral aspect of microtubule dynamics essential for neurite outgrowth and axonal transport.

Hyperphosphorylation of tau protein (tau) causes neurodegenerative diseases such as Alzheimer's disease (AD). Recent studies of the physiological correlation between tau and a-synuclein (a-SN) have demonstrated that: (a) phosphorylated tau is also present in Lewy bodies, which are cytoplasmic inclusions formed by abnormal aggregation of a-SN; and (b) the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) increases the phosphorylation of tau as well as the protein level of a-SN in cultured neuronal cells, and also in mice. However, the molecular mechanism responsible for the a-SN-mediated hyperphosphorylation of tau remains to be elucidated. In this in vitro study, we found that: (a) a-SN directly stimulates the phosphorylation of tau by glycogen synthase kinase-3b (GSK-3b), (b) a-SN forms a heterotrimeric complex with tau and GSK-3b, and (c) the nonamyloid beta component (NAC) domain and an acidic region of a-SN are responsible for the stimulation of GSK-3b-mediated tau phosphorylation. Thus, it is concluded that a-SN functions as a connecting mediator for tau and GSK-3b, resulting in GSK-3b-mediated tau phosphorylation. Because the expression of a-SN is promoted by oxidative stress, the accumulation of a-SN induced by such stress may directly induce the hyperphosphorylation of tau by GSK-3b. Furthermore, we found that heat shock protein 70 (Hsp70) suppresses the a-SN-induced phosphorylation of tau by GSK-3b through its direct binding to a-SN, suggesting that Hsp70 acts as a physiological suppressor of a-SN-mediated tau hyperphosphorylation. These results suggest that the cellular level of Hsp70 may be a novel therapeutic target to counteract a-SN-mediated tau phosphorylation in the initial stage of neurodegenerative disease. Structured digital abstractl GSK3B phosphorylates GSK3B by protein kinase assay (View interaction) l TAU binds to a-SN by pull down (View interaction) l GSK3B phosphorylates TAU by protein kinase assay (View Interaction: 1, 2, 3, 4) l TAU physically interacts with a-SN and HSP70 by pull down (View interaction) l TAU physically interacts with a-SN and GSK3B by pull down (View interaction) l GSK3B binds to a-SN by pull down (View interaction) Abbreviations GSK-3b, glycogen synthase kinase-3b; GST, glutathione S-transferase; Hsp70, heat shock protein 70; NAC, nonamyloid beta component; a-SN, alpha-synuclein; tau, microtubule-associated protein tau.

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