Uram Jin, Soo Jin Park and Sang Myun Park. Exp Neurobiol. 2019;28:554-67. https://doi.org/10.5607/en.2019.28.5.554

α‐Synuclein (α‐syn) has been known to be a key player of the pathogenesis of Parkinson’s disease and has recently been detected in extracellular biological fluids and shown to be rapidly secreted from cells. The penetration of α‐syn into cells has also been observed. In this study, we observed that dl‐threo‐1‐phenyl‐2‐decanoylamino‐3‐morpholino‐1‐propanol, a glucosyltransferase inhibitor, and proteinase K inhibited the internalization of extracellular monomeric α‐syn into BV‐2 cells, and the addition of monosialoganglioside GM1 ameliorated the inhibition of α‐syn internalization in dl‐threo‐1‐phenyl‐2‐decanoylamino‐3‐morpholino‐1‐propanol‐treated BV‐2 cells. Furthermore, inhibition of clathrin‐, caveolae‐, and dynamin‐dependent endocytosis did not prevent the internalization of α‐syn, but disruption of lipid raft inhibited it. Inhibition of macropinocytosis and disruption of actin and microtubule structures also did not inhibit the internalization of α‐syn. In addition, we further confirmed these observations by co‐culture system of BV‐2 cells and α‐syn‐over‐expressing SH‐SY5Y cells. These findings suggest that extracellular α‐syn is internalized into microglia via GM1 as well as hitherto‐unknown protein receptors in clathrin‐, caveolae‐, and dynamin‐independent, but lipid raft‐dependent manner. Elucidation of the mechanism involved in internalization of α‐syn should be greatly helpful in the development of new treatments of α‐syn‐related neurodegenerative diseases.

Parkinson's disease (PD) is the second most common progressive neurodegenerative disease. Several genes have been associated with familial type PD, providing tremendous insights into the pathogenesis of PD. Gathering evidence supports the view that these gene products may operate through common molecular pathways. Recent reports suggest that many PD-associated gene products, such as α-synuclein, LRRK2, parkin and PINK1, associate with lipid rafts and lipid rafts may be associated with neurodegeneration. Here, we observed that DJ-1 protein also associated with lipid rafts. Palmitoylation of three cysteine residues (C46/53/106) and C-terminal region of DJ-1 were required for this association. Lipopolysaccharide (LPS) induced the localization of DJ-1 into lipid rafts in astrocytes. The LPS-TLR4 signaling was more augmented in DJ-1 knock-out astrocytes by the impairment of TLR4 endocytosis. Furthermore, lipid rafts-dependent endocytosis including the endocytosis of CD14, which play a major role in regulating TLR4 endocytosis was also impaired, but clathrin-dependent endocytosis was not. This study provides a novel function of DJ-1 in lipid rafts, which may contribute the pathogenesis of PD. Moreover, it also provides the possibility that many PD-related proteins may operate through common molecular pathways in lipid rafts.

PINK1 (PTEN induced putative kinase 1), a familial Parkinson's disease (PD)‐related gene, is expressed in astrocytes, but little is known about its role in this cell type. Here, we found that astrocytes cultured from PINK1‐knockout (KO) mice exhibit defective proliferative responses to epidermal growth factor (EGF) and fetal bovine serum. In PINK1‐KO astrocytes, basal and EGF‐induced p38 activation (phosphorylation) were increased whereas EGF receptor (EGFR) expression and AKT activation were decreased. p38 inhibition (SB203580) or knockdown with small interfering RNA (siRNA) rescued EGFR expression and AKT activation in PINK1‐KO astrocytes. Proliferation defects in PINK1‐KO astrocytes appeared to be linked to mitochondrial defects, manifesting as decreased mitochondrial mass and membrane potential, increased intracellular reactive oxygen species level, decreased glucose‐uptake capacity, and decreased ATP production. Mitochondrial toxin (oligomycin) and a glucose‐uptake inhibitor (phloretin) mimicked the PINK1‐deficiency phenotype, decreasing astrocyte proliferation, EGFR expression and AKT activation, and increasing p38 activation. In addition, the proliferation defect in PINK1‐KO astrocytes resulted in a delay in the wound healing process. Taken together, these results suggest that PINK1 deficiency causes astrocytes dysfunction, which may contribute to the development of PD due to delayed astrocytes‐mediated repair of microenvironment in the brain.