α-Synuclein facilitates endocytosis by elevating the steady state levels of phosphatidylinositol 4,5-bisphosphate

α-Synuclein (α-Syn) protein has been identified as having a causative role in the pathogenesis of Parkinson’s disease. α-Syn interacts reversibly with membrane lipids. It has been shown to preferentially bind acidic phospholipids, including the acidic phosphoinositides (PIPs), which are phosphorylated forms of the membrane phospholipid, phosphatidylinositol (PI). In the plasma membrane, the most common PIP form is phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and its functions include regulating mechanisms of endocytosis and exocytosis. Additional PIPs have been described to be involved in clathrin-mediated endocytosis, a specified form of endocytosis. Former studies have shown a certain degree of controversy over the exact role that α-Syn plays either in endocytosis or exocytosis. A specific interest in α-Syn function is related its roles in synaptic vesicles cycling, including endocytosis and exocytosis. Therefore, it is important to have better understanding of the function of α-Syn in the regulation of exocytosis, endocytosis, and vesicle recycling under physiological and pathological conditions.

To investigate in particular the relationship between α-Syn, synaptic vesicles cycling and PIPs, Israeli scientists: Meir Schechter, Suaad Abd Elhadi, Dana Davidi and led by Dr. Ronit Sharon from the Department of Biochemistry and Molecular Biology at the Hebrew University-Hadassah Medical School in Jerusalem, together with Merav Atias and Dr. Daniel Gitler from the Ben-Gurion University of the Negev demonstrated in elegant experimental studies the mechanistic role of α-Syn in endocytosis. Specifically, that α-Syn interacts with- and regulates PI(4,5)P₂ levels on the plasma membrane to facilitate endocytosis. The research work is now published in the Journal of Biological Chemistry.

The research team found that in clathrin-coated pits, α-Syn colocalized with phosphorylated AP2 and PIP₂. Inducing the expression of α-Syn caused much higher levels of PI4P, PI(3,4)P₂, and PI(4,5)P₂ compared with the control cells. In contrast, α-Syn overexpression did not alter the levels of other PIPs such as PI3P and PI(3,4,5)P₃.

α-Syn-mediated endocytosis of transferrin, a prototype molecule for clathrin mediated endocytosis, was found to be dependent on levels of PI(4,5)P₂. The effect of overexpressed α-Syn to stimulate transferrin endocytosis was completely abolished by the depletion of PI(4,5)P₂ on the plasma membrane, using a rapamycin-recruitable PIP-phosphatase, compared with the control cells, treated with DMSO (vehicle) that their PI(4,5)P2 levels were unaffected. In addition, the enhancement of transferrin endocytosis by α-Syn was inhibited by an interference with the replenishment of PI(4,5)P₂ levels on the plasma membrane.

In neuronal cells expressing α-Syn that is carrying mutations associated with familial forms of the disease, the authors observed a strong correlation between α-Syn effects on transferrin endocytosis and PI(4,5)P₂ plasma membrane levels. α-Syn was found to be responsible for accelerating the rate of synaptic vesicle endocytosis and reducing the fraction of released synaptic vesicles.

It was determined that α-Syn accelerated the rate of endocytosis and inhibited the exocytotic segment of the synaptic vesicle cycle. A correlation was noted between the capacity of α-Syn to enhance endocytosis rate and increase PI(4,5)P₂ levels.

The exact roles of α-Syn are still ambiguous, far from understood and still need to be clarified for a better understanding of the physiological and pathological roles of α-Syn. Through this study the authors have been able to provide evidence to support a mechanistic role for α-Syn  in synaptic vesicles cycling that directly involves the lipids and specifically, PIP content on the plasma membrane. The fact that α-Syn plays a role in the pathogenesis of Parkinson’s disease through the dysregulation of PI(4,5)P₂ points at new targets for potential therapies for the disease.