Senescence as a Converging Mechanism in Parkinson's Disease

Neurodegenerative diseases are one of the leading causes of disability and mortality, affecting millions of people worldwide. Parkinson’s disease (PD) is the second most common neurodegenerative disease globally, and while it was first described over 200 years ago, curative treatments remain elusive. One of the main challenges in developing effective therapeutic strategies for PD is the complex molecular pathophysiology of the disease has not been well recapitulated in classically used animal models systems, and studies using post-mortem tissue from patients only represents the end point of disease. Human derived brain organoid models have revolutionized the field of neurological disease modeling, as they are able to recapitulate key cellular and physiological features reminiscent of the human brain.
This thesis describes the use of human midbrain organoids (hMO) to model and gain a deeper understanding of genetic forms of PD. In the first manuscript, patient-specific hMO harboring a triplication in the SNCA gene (3xSNCA hMO) were able to recapitulate the key neuropathological hallmarks of PD. We observed the progressive loss and dysfunction of midbrain dopaminergic neurons in 3xSNCA hMO, and the accumulation of pathological α-synuclein including elevated levels of pS129 α-synuclein and the presence of α-synuclein aggregates. We also identified a phenotype indicative of senescence in the 3xSNCA hMO, which represents a mechanism that has recently gained more attention as a driving factor in PD pathogenesis and progression. The second manuscript of this thesis investigated the pathogenic role of LRRK2-G2019S in astrocytes using a combination of post-mortem brain tissue, induced pluripotent stem cell derived astrocytes and hMO. The iPSC derived astrocytes and organoids recapitulated the phenotypes seen in the post-mortem tissue, emphasizing the validity of these models in reflecting the in vivo situation. Interestingly, single-cell RNA sequencing of the hMO revealed that astrocytes from the LRRK2-G2019S organoids showed a senescent-like phenotype. Thus, this thesis highlights the relevance of senescence as a converging mechanism in PD.
Finally, this thesis explores the future development of organoid models as they are combined with technologies such as microfluidic devices as in Manuscript III to improve their complexity and reproducibility. Ultimately, this will lead to the development of more representative models that can better recapitulate and model PD as well as other neurodegenerative disorders.