Human embryonic stem cell-derived A9 dopaminergic neurons as a model and therapy for Parkinson's disease

Parkinson's disease (PD) represents the second most common neurodegenerative disease in the UK. While PD presents as a combination of motor and non-motor symptoms, the motor dysfunction observed in PD represents one of the most profound and debilitating clinical features. Although there are effective pharmacological and surgical options for those living with PD, all current therapies centre around providing symptomatic relief for patients. Therefore, there is a need for the identification of disease-modifying or disease slowing treatments in PD. Embryonic stem cells can reliably generate standardised populations of A9 dopaminergic neurons. For this reason, they are an ideal candidate cell source for both cell therapies and drug screening platforms for PD. A hallmark pathology of PD involves the atypical aggregation of alpha synuclein and subsequent formation of Lewy pathology. The trigger for *de novo* alpha synuclein aggregation is as of yet unknown, however, the A9 dopaminergic neurons appear to be selectively vulnerable to aggregation in PD. Their consequential degeneration is thought to be central to the onset of motor symptoms in PD. Possible mechanisms of A9 dopaminergic neurodegeneration include mitochondrial dysfunction as well as inhibition within protein degradation pathways. The present study sought to model alpha synuclein pathology in human embryonic stem cell-derived A9 dopaminergic neurons. If PD-relevant pathologies could be recapitulated, this model may form the basis of a high-throughput drug-screening platform for the identification of potential disease-modifying drugs in PD. This model would have an emphasis on identifying repurposed pharmacotherapies, effective at rescuing mitochondrial fitness or protein degradation systems in human A9 dopaminergic neurons. However, while alpha synuclein aggregation was able to be recapitulated in the embryonic stem cell-derived A9 dopaminergic neuron model, subsequent pathologies were undetected. An alternative method for treating some of the motor features of PD is the use of cell therapies. The rationale being that an exogenous dopaminergic cell source could be used to repopulate the lost striatal dopaminergic innervation. This approach gained traction in the 1990's after the success of human foetal mesencephalic grafts in a sub-set of PD patients. Despite success, human foetal tissue brings with it logistical and ethical issues which limit its clinical translation. However, embryonic stem cell-derived A9 dopaminergic neurons offer an attractive alternative dopaminergic cell source. Proving safe and efficacious in preclinical animal models, human embryonic stem cell-derived dopaminergic cell therapies are quickly moving towards clinical trials. However, very little is understood about the immunogenicity of these cells, limiting our understanding of the optimal immunosuppressive regimes to recommend for clinical trials. This thesis therefore also sought to assess the immunogenicity of human embryonic stem cell-derived A9 dopaminergic neurons in an allogenic setting, including the use of a humanised mouse model of PD.