Involvement of Lysosomal Enzymes in the Aetiology of Parkinson's Disease

Resumo

Neurodegenerative diseases affect millions of people worldwide, which is a very important health and economic problem, especially due to the increase in life expectancy in our society. One of the most important aspects, and relatively less studied in relation to this subject, are the mechanisms of proteotoxicity, despite being common processes among these incurable and debilitating diseases. A better understanding of how proteotoxicity affects the etiology and progression of neurodegenerative diseases would provide insight for improved therapeutic treatment. In relation to the proteotoxic factors that influence the etiology of one of the most frequent neurodegenerative diseases, Parkinson's disease (PD), lysosomal dysfunction has been proposed as one of the most important molecular mechanisms. This association has been consolidated since heterozygous mutations for the GBA gene, which codes for the lysosomal enzyme β-glucocerebrosidase (GCase), are considered the main genetic risk factor for sporadic PD. Therefore, we analyzed a group of lysosomal enzymes in patients with PD and healthy subjects, as well as a cell model of human H4 neuroglioma through the induction of the autophagy process with specific inhibitors, to study the contribution of lysosomal dysfunction in the aetiology of PD. In this thesis, we found a reduced activity of the lysosomal enzymes β-GCase and β- gal in patients compared to healthy subjects. Expression levels of PSAP and Saposin C cofactors were decreased in patients and correlated with increased levels of α-Syn protein. However, the activity and expression levels of CatD enzyme were increased in patients, which correlated with increased levels of α-Syn in this group. In the H4 cell model induced with autophagy through the use of specific LRRK2 inhibitors, we found increased levels of GBA protein expression and a decrease in the specific activity of the enzyme 72 h. post-treatment with the LRRK2in1 inhibitor. However, this effect was not caused by an altered transport of the protein from the Golgi apparatus to the lysosome. Our findings suggest that the lysosomal dysfunction in sporadic PD is due, at least in part, to an alteration in Saposin C resulting from reduced levels of PSAP. That would lead to a significant decrease in β-GCase activity, resulting in the accumulation of α-Syn. Due to this enzymatic dysfunction, the accumulation of substrates such as monohexosylceramides could act in favor of CTSD activation and, therefore, increase its enzymatic activity in a timely manner. In addition, we observed a possible regulation of lysosomal function through the LRRK2 kinase domain, suggesting an interaction between LRRK2 kinase activity and GBA. Although more studies are needed on this topic, enhancement of GCase activity could restore the defective protein metabolism observed in PD. According to our results, the evaluation of lysosomal activity in peripheral blood of patients with PD appears to be a promising approach to investigate pathological mechanisms. New therapies aimed at restoring lysosomal function would make it possible to improve the treatment of sporadic PD.