Kai-Yin Chau

Mitofusin 1 and mitofusin 2 are ubiquitinated in a PINK1/parkin-dependent manner upon induction of mitophagy

Mitochondrial dysfunction and perturbed degradation of proteins have been implicated in Parkinsons disease (PD) pathogenesis. Mutations in the Parkin and PINK1 genes are a cause of familial PD. PINK1 is a putative kinase associated with mitochondria, and loss of PINK1 expression leads to mitochondrial dysfunction, which increases with time. Parkin is suggested to be downstream of PINK1 and also mediates the removal of damaged mitochondria by macroautophagy (mitophagy). We investigated whether mitochondrial dysfunction in dopaminergic SH-SY5Y cells following decreased PINK1 expression by RNAi may in part be due to the inhibition of mitophagy. Reduced flux through the macroautophagy pathway was found to be coincident with the inhibition of ATP synthesis following 12 days of PINK1 silencing. Overexpression of parkin in these cells restored both autophagic flux and ATP synthesis. Overexpression and RNAi studies also indicated that PINK1 and parkin were required for mitophagy following CCCP-induced mitochondrial damage. The ubiquitination of several mitochondrial proteins, including mitofusin 1 and mitofusin 2, were detected within 3 h of CCCP treatment. These post-translational modifications were reduced following the silencing of parkin or PINK1. The ubiquitination of mitochondrial proteins appears to identify mitochondria for degradation and facilitate mitophagy. PINK1 and parkin are thus required for the removal of damaged mitochondria in dopaminergic cells, and inhibition of this pathway may lead to the accumulation of defective mitochondria which may contribute to PD pathogenesis.

Ambroxol improves lysosomal biochemistry in glucocerebrosidase mutation-linked Parkinson disease cells

Heterozygous GBA gene mutations are the most frequent Parkinson’s disease risk factor. Using Parkinson’s disease patient derived fibroblasts McNeill et al. show that heterozygous GBA mutations reduce glucosylceramidase activity, and are associated with endoplasmic reticulum and oxidative stress. Ambroxol treatment improved glucosylceramidase activity and reduced oxidative stress in these cells.

G2019S leucine-rich repeat kinase 2 causes uncoupling protein-mediated mitochondrial depolarization

The G2019S leucine rich repeat kinase 2 (LRRK2) mutation is the most common genetic cause of Parkinsons disease (PD), clinically and pathologically indistinguishable from idiopathic PD. Mitochondrial abnormalities are a common feature in PD pathogenesis and we have investigated the impact of G2019S mutant LRRK2 expression on mitochondrial bioenergetics. LRRK2 protein expression was detected in fibroblasts and lymphoblasts at levels higher than those observed in the mouse brain. The presence of G2019S LRRK2 mutation did not influence LRRK2 expression in fibroblasts. However, the expression of the G2019S LRRK2 mutation in both fibroblast and neuroblastoma cells was associated with mitochondrial uncoupling. This was characterized by decreased mitochondrial membrane potential and increased oxygen utilization under basal and oligomycin-inhibited conditions. This resulted in a decrease in cellular ATP levels consistent with compromised cellular function. This uncoupling of mitochondrial oxidative phosphorylation was associated with a cell-specific increase in uncoupling protein (UCP) 2 and 4 expression. Restoration of mitochondrial membrane potential by the UCP inhibitor genipin confirmed the role of UCPs in this mechanism. The G2019S LRRK2-induced mitochondrial uncoupling and UCP4 mRNA up-regulation were LRRK2 kinase-dependent, whereas endogenous LRRK2 levels were required for constitutive UCP expression. We propose that normal mitochondrial function was deregulated by the expression of G2019S LRRK2 in a kinase-dependent mechanism that is a modification of the normal LRRK2 function, and this leads to the vulnerability of selected neuronal populations in PD.

Circulating anti‐retinal antibodies as immune markers in age‐related macular degeneration

Age‐related macular maculopathy (ARM) and age‐related macular degeneration (AMD) are the leading causes of blindness in the Western world. Despite the magnitude of this clinical problem, very little is known about the pathogenesis of the disease. In this study, we analysed the sera (using indirect immunohistochemistry and Western blot analysis) from a very large cohort of such patients and normal age‐matched controls to detect circulating anti‐retinal antibodies. Patients with bilateral drusen (n = 64) and with chorioretinal neovascularization (CNV) (n = 51) were recruited in addition to age‐matched control subjects (n = 39). The sera were analysed for anti‐retinal immunoglobulins on retinal sections. The data were then correlated with the clinical features graded according to the International Classification and Grading System of ARM and AMD. The sera of patients with drusen (93·75%) and CNV (82·27%) were found to have a significantly (P = 0·02) higher titre of autoantibodies to the retina in comparison with controls (8·69%), indicating significant evidence of involvement of the immune process in early stages of AMD. Subsequent statistical analysis of the drusen group showed significant progressive staining (P = 0·0009) in the nuclei layers from early to late stages of ARM. Western blotting confirmed the presence of anti‐retinal immunoglobulins to retinal antigens. As anti‐retinal immunoglobulins are present in patients with bilateral drusen and exudative AMD, these antibodies could play a significant role in the pathogenesis of AMD. Whilst we do not have evidence that these antibodies precede disease onset, the possibility that their presence might contribute to disease progression needs to be investigated. Finally, the eventual identification of the target antigens detected by these antibodies may permit the future development of new diagnostic methods for ARM and AMD.

Glucocerebrosidase inhibition causes mitochondrial dysfunction and free radical damage

Highlights ► Glucocerebrosidase gene mutations are a risk factor for Parkinson’s disease. ► Glucocerebrosidase inhibition causes mitochondrial dysfunction & oxidative stress. ► These changes parallel important pathogenetic of Parkinson’s disease.

Relationship between alpha synuclein phosphorylation, proteasomal inhibition and cell death: relevance to Parkinson's disease pathogenesis

Alpha synuclein can be phosphorylated at serine129 (P‐S129), and the presence of highly phosphorylated α‐synuclein in Lewy bodies suggests changes to its phosphorylation status has an important pathological role. We demonstrate that the kinase(s) responsible for α‐synuclein S129 phosphorylation is constitutively active in SH‐SY5Y cells and involves casein kinase 2 activity. Increased oxidative stress or proteasomal inhibition caused significant elevation of P‐S129 α‐synuclein levels. Under these conditions, similar increases in P‐S129 α‐synuclein were found in both sodium dodecyl sulphate lysates and Triton extracts indicating the phosphorylated protein was soluble and did not lead to aggregation. The rate of S129 phosphorylation was increased in response to proteasomal inhibition indicating a higher activity of the relevant kinase. Cells expressing the phosphorylation mimic, S129D α‐synuclein increased cell death and enhanced sensitivity to epoxomycin exposure. Proteasomal inhibition markedly decreased S129D α‐synuclein turnover suggesting proteasomal inhibition leads to the accumulation of P‐S129 α‐synuclein through an increase in the kinase activity and a decrease in protein turnover resulting in increased cell death. We conclude that S129 phosphorylation is toxic to dopaminergic cells and both the levels of S129 phosphorylated protein and its toxicity are increased with proteasomal inhibition emphasising the interdependence of these pathways in Parkinson’s disease pathogenesis.

Plasma levels of matrix metalloproteinase-2 and-9 (MMP-2 and MMP-9) in age-related macular degeneration

BackgroundSeveral studies indicate that age-related macular degeneration (AMD) and atherosclerosis may share common pathogenetic pathways. The aim of this study was to determine the role of systemic matrix metalloproteinases (MMPs) in AMD, given that MMPs are implicated in the pathogenesis of atherosclerosis.MethodsThis study determined the plasma matrix metalloproteinases (MMP-2 and MMP-9) levels in three groups of subjects: group 1 included subjects with age-related maculopathy (ARM), group 2 included subjects with choroidal neovascularization (CNV) owing to AMD and group 3 consisted of age-matched controls.ResultsThe mean plasma levels of MMP-2 were not significantly different in the three groups. In contrast, the mean plasma MMP-9 levels were significantly higher in ARM and CNV groups compared to that of the control group. However, there was no significant difference in MMP-9 levels between ARM and CNV groups.ConclusionThis is the first study that reveals a link between raised plasma MMP-9 levels with AMD. Further studies are required to identify the factors that contribute to this association.

Mitochondrial dysfunction in glaucoma: understanding genetic influences

Glaucoma is the leading cause of irreversible blindness worldwide. This review aims to provide a greater understanding of the complex genetic influences that may lead to mitochondrial dysfunction and increase susceptibility to retinal ganglion cell (RGC) loss in primary open angle glaucoma (POAG), and thus elucidate potentially important pathophysiological pathways amenable to therapeutic intervention. Emerging evidence from genome wide association and other genetic studies suggests that changes in the mitochondrial DNA (mtDNA) and in nuclear DNA genes that encode mitochondrial proteins may influence mitochondrial structure and function and, therefore, contribute to the pathogenesis of POAG. We propose that a variety of genes (OPA1, MFN1, MFN2, CYP1B1, PARL, SOD2, SRBD1, GST, NOS3, TNFa and TP53) may each confer a background susceptibility to POAG in different populations having one common link: mitochondrial dysfunction. The relationship between polymorphisms in these genes and increasing risk for POAG is presented and the limitations of the available current knowledge are discussed.

PINK1 disables the anti-fission machinery to segregate damaged mitochondria for mitophagy

In addition to recruiting Parkin/autophagy receptors to damaged mitochondria, the authors show that PINK1 triggers PKA displacement from AKAP1 after damage to trigger mitochondrial fission in a Parkin-independent manner, suggesting that PINK1 is a master mitophagy regulator.

Mitochondrial and lysosomal biogenesis are activated following PINK1/parkin‐mediated mitophagy

Impairment of the autophagy–lysosome pathway is implicated with the changes in α‐synuclein and mitochondrial dysfunction observed in Parkinsons disease (PD). Damaged mitochondria accumulate PINK1, which then recruits parkin, resulting in ubiquitination of mitochondrial proteins. These can then be bound by the autophagic proteins p62/SQSTM1 and LC3, resulting in degradation of mitochondria by mitophagy. Mutations in PINK1 and parkin genes are a cause of familial PD. We found a significant increase in the expression of p62/SQSTM1 mRNA and protein following mitophagy induction in human neuroblastoma SH‐SY5Y cells. p62 protein not only accumulated on mitochondria, but was also greatly increased in the cytosol. Increased p62/SQSMT1 expression was prevented in PINK1 knock‐down cells, suggesting increased p62 expression was a consequence of mitophagy induction. The transcription factors Nrf2 and TFEB, which play roles in mitochondrial and lysosomal biogenesis, respectively, can regulate p62/SQSMT1. We report that both Nrf2 and TFEB translocate to the nucleus following mitophagy induction and that the increase in p62 mRNA levels was significantly impaired in cells with Nrf2 or TFEB knockdown. TFEB translocation also increased expression of itself and lysosomal proteins such as glucocerebrosidase and cathepsin D following mitophagy induction. We also report that cells with increased TFEB protein have significantly higher PGC‐1α mRNA levels, a regulator of mitochondrial biogenesis, resulting in increased mitochondrial content. Our data suggests that TFEB is activated following mitophagy to maintain autophagy–lysosome pathway and mitochondrial biogenesis. Therefore, strategies to increase TFEB may improve both the clearance of α‐synuclein and mitochondrial dysfunction in PD.