Mechanism of action of autosomal recessive juvenile parkinsonism gene mutations

Abstract

Parkinson Disease (PD) is the most common neurodegenerative movement disorder. Although PD is a largely sporadic disease, several genes has been linked to familial forms of PD. This thesis focuses on the mechanism of function of the Autosomal Recessively Juvenile Parkinsonism (AR-JP) associated genes parkin, PTEN induced kinase 1 (PINK1) and DJ-1.

In Paper I we described a novel interaction between the E3 ubiquitin ligase parkin and phospholipase C-gamma1 (PLCgamma1). We further demonstrated that parkin ubiquitinates and regulates PLCgamma1 levels.

Impairment in calcium homeostasis has been suggested to be associated to PD related cell death. Since PLCgamma1 is an important enzyme for regulating calcium, we continued by studying the downstream consequences of parkin impairment in the context of PLCgamma1-mediated signaling. This study resulted in Paper II where we established that parkin deficiency leads to increased lipid hydrolysis and cytosolic calcium levels due to altered PLCgamma1 activity. When we blocked calcium release from intracellular stores in parkin-mutant cells, the viability after exposure to oxidative stress was increased.

Previous studies suggest that mitochondrial dysfunction is related to neurodegeneration in PD. In Paper III and IV, we elucidated the roles of DJ-1 and PINK1 in mitochondrial morphology and dynamics. We showed that DJ-1 or PINK1 knock-down (KD) increased mitochondrial fragmentation and that blocking fission could reverse these phenotypes. In DJ-1 KD, we found that fission was related to oxidative stress, whereas in PINK1-deficient cells the mitochondrial abnormality was likely a consequence of a loss of mitochondrial membrane potential and increased calcineurin activity. In Paper V we analyzed mitochondrial motility in differentiated cells. By live imaging we demonstrated that KD of either DJ-1 or PINK1 decreased the rate of mitochondrial motility in neurites. Blocking fission eliminated the difference between the control cells and parkinsonism associated KD cells, suggesting that balanced mitochondrial dynamics is important for neuritic motility.

In conclusion, PD is a multi-factorial disorder involving several degenerative processes and signaling pathways. The AR-JP studies presented in this thesis may help to bring light to the understanding of the underlying mechanisms of PD and to develop novel treatment strategies.