Neurodegenerative diseases are a major burden for our society, affecting millions of people worldwide. A main goal of past and current research is to enhance our understanding of the mechanisms underlying proteotoxicity, a common theme among these incurable and debilitating conditions. Cell proteome alteration is considered to be one of the main driving forces that triggers neurodegeneration, and unraveling the biological complexity behind the affected molecular pathways constitutes a daunting challenge. This review summarizes the current state on key processes that lead to cellular proteotoxicity in Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis, providing a comprehensive landscape of recent literature. A foundational understanding of how proteotoxicity affects disease etiology and progression may provide essential insight towards potential targets amenable of therapeutic intervention.

Background: Neurodegeneration with brain iron accumulation (NBIA) represents a group of neurodegenerative disorders characterized by abnormal iron accumulation and the presence of axonal spheroids in the brain. In Parkinson's Disease (PD), iron accumulation is a cardinal feature of degenerating regions in the brain and seems to be a key player in mechanisms that precipitate cell death. Objectives: The aim of this present study was to comprehensively explore the genetic and genomic connection between NBIA and PD etiology. Methods: We screened the presence of known and rare pathogenic mutations in autosomal dominant and recessive genes linked to NBIA in a total of 4,481 PD cases and 10,253 controls from the Accelerating Medicines Partnership Parkinsons Disease Program and the UKBiobank. We further examined whether a genetic burden of NBIA variants contributes to PD risk through single-gene, gene-set, and single-variant association analyses. To investigate the potential effect of NBIA gene expression on PD, we assessed publicly available expression quantitative trait loci (eQTL) data through Summary-based Mendelian Randomization and conducted transcriptomic analyses in blood of 1,886 PD cases and 1,285 controls. Results: Out of 28 previously reported NBIA screened coding variants, four missense were found to be associated with PD risk at a nominal p value < 0.05 (p.T402M-ATP13A2, p.T207M-FA2H, p.P60L-C19orf12, p.C422S-PANK2). No enrichment of heterozygous variants in NBIA-related genes risk was identified in PD cases versus controls. Burden analyses did not reveal a cumulative effect of rare NBIA genetic variation on PD risk. Transcriptomic analyses suggested that DCAF17 is differentially expressed in blood from PD cases and controls. Conclusions: Taking into account the very low mutation occurrence in the datasets and the lack of replication, our analyses suggest that NBIA and PD may be separate molecular entities, supporting the notion that the mechanisms underpinning iron accumulation in PD are likely not shared with NBIA. Elevated nigral iron levels may not contribute to PD etiology and may vary with anti-parkinsonian drugs used for treatment, environmental factors, or iron sequestration in tissue as a response to PD pathological change.

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Neurodegeneration with brain iron accumulation (NBIA) represents a group of neurodegenerative disorders characterized by abnormal iron accumulation in the brain. In Parkinson’s Disease (PD), iron accumulation is a cardinal feature of degenerating regions in the brain and seems to be a key player in mechanisms that precipitate cell death. The aim of this study was to explore the genetic and genomic connection between NBIA and PD. We screened for known and rare pathogenic mutations in autosomal dominant and recessive genes linked to NBIA in a total of 4481 PD cases and 10,253 controls from the Accelerating Medicines Partnership Parkinsons’ Disease Program and the UKBiobank. We examined whether a genetic burden of NBIA variants contributes to PD risk through single-gene, gene-set, and single-variant association analyses. In addition, we assessed publicly available expression quantitative trait loci (eQTL) data through Summary-based Mendelian Randomization and conducted transcriptomic analyses in blood of 1886 PD cases and 1285 controls. Out of 29 previously reported NBIA screened coding variants, four were associated with PD risk at a nominal p value < 0.05. No enrichment of heterozygous variants in NBIA-related genes risk was identified in PD cases versus controls. Burden analyses did not reveal a cumulative effect of rare NBIA genetic variation on PD risk. Transcriptomic analyses suggested that DCAF17 is differentially expressed in blood from PD cases and controls. Due to low mutation occurrence in the datasets and lack of replication, our analyses suggest that NBIA and PD may be separate molecular entities.

Mutations in LRRK2 and GBA1 are key contributors to genetic risk of developing Parkinson’s disease (PD). To investigate how LRRK2 kinase activity interacts with GBA and contributes to lysosomal dysfunctions associated with the pathology of PD. The activity of the lysosomal enzyme β-Glucocerebrosidase (GCase) was assessed in a human neuroglioma cell model treated with two selective inhibitors of LRKK2 kinase activity (LRRK2-in-1 and MLi-2) and a GCase irreversible inhibitor, condutirol-beta-epoxide (CBE), under 24 and 72 h experimental conditions. We observed levels of GCase activity comparable to controls in response to 24 and 72 h treatments with LRRK2-in-1 and MLi-2. However, GBA protein levels increased upon 72 h treatment with LRRK2-in-1. Moreover, LC3-II protein levels were increased after both 24 and 72 h treatments with LRRK2-in-1, suggesting an activation of the autophagic pathway. These results highlight a possible regulation of lysosomal function through the LRRK2 kinase domain and suggest an interplay between LRRK2 kinase activity and GBA. Although further investigations are needed, the enhancement of GCase activity might restore the defective protein metabolism seen in PD.