Single cell sequencing and retrograde barcode tracing reveal human DA neuron identities based on projection patterns in stem cell-derived grafts

Groundbreaking research in the last decade has definitively established human pluripotent stem cells (hPSCs) as a powerful source for the unlimited generation of dopamine (DA) neurons used in cell-based therapy in Parkinson’s disease (PD). However, DA neurons constitute a heterogeneous group of cells with different molecular identities and, consequently, different functions and innervation targets. Moreover, the inaccessibility of the human brain has so far hindered our understanding of the highly complex human DA system and, therefore, the design of DA neuron subtype-specific differentiation protocols for more effective cell replacement therapies. Currently, DA neurons – located in human ventral midbrain – can only be reliably distinguished based on their projection patterns. Here, we used hPSCs to derive and then transplant DA neurons into a rat model of PD. After one year of transplantation, we performed single nucleus RNA sequencing (snRNAseq) of ~80,000 human nuclei to transcriptionally define cell type composition and obtained a comprehensive map of DA neuron molecular identities. Exploiting the fact that transplanted DA neurons innervated and integrated within the host circuitry, we then combined snRNAseq with axonal retrograde AAV barcoding to trace the projection patterns of molecularly distinct human DA neuron subtypes in the grafts. We thus defined human DA neuron transcriptional profiles based on their target-specific outgrowth in the host brain. The resulting datasets provide an unprecedented resource both for elucidating the molecular basis of human DA neuron diversity as well as for developing more targeted cell-based therapies, with major implications for outcomes of PD patients.