Impact of α-synuclein pathology on transplanted hESC-derived dopaminergic neurons in a humanized α-synuclein rat model of PD

Preclinical assessment of the therapeutic potential of dopamine (DA) neuron replacement in Parkinson's disease (PD) has primarily been performed in the 6-hydroxydopamine toxin model. While this is a good model to assess graft function, it does not reflect the pathological features or progressive nature of the disease. In this study, we establish a humanized transplantation model of PD that better recapitulates the main disease features, obtained by coinjection of preformed human alpha-synuclein (alpha-syn) fibrils and adeno-associated virus (AAV) expressing human wild-type alpha-syn unilaterally into the rat substantia nigra (SN). This model gives rise to DA neuron dysfunction and progressive loss of DA neurons from the SN and terminals in the striatum, accompanied by extensive alpha-syn pathology and a prominent inflammatory response, making it an interesting and relevant model in which to examine long-term function and integrity of transplanted neurons in a PD-like brain. We transplanted DA neurons derived from human embryonic stem cells (hESCs) into the striatum and assessed their survival, growth, and function over 6 to 18 wk. We show that the transplanted cells, even in the presence of ongoing pathology, are capable of innervating the DA-depleted striatum. However, on closer examination of the grafts, we found evidence of alpha-syn pathology in the form of inclusions of phosphorylated alpha-syn in a small fraction of the grafted DA neurons, indicating host-to-graft transfer of alpha-syn pathology, a phenomenon that has previously been observed in PD patients receiving fetal tissue grafts but has not been possible to demonstrate and study in toxin-based animal models.