Single-cell Transcriptomic Atlas of the Human Substantia Nigra in Parkinson′s Disease

Parkinson′s disease (PD) is a common and complex neurodegenerative disorder. Loss of neuromelanin–containing dopaminergic (DA) neurons in the substantia nigra (SN) is a hallmark of PD neuropathology, however, the etiology of PD remains unclear. Single–cell (–nucleus) RNA sequencing (sc or snRNAseq) has significantly advanced our understanding of neurodegenerative diseases including Alzheimer′s, but limited progress has been made in PD. Here we generated by far the largest snRNAseq data of high–quality 315,867 nuclei from the human SN including 9 healthy controls and 23 idiopathic PD cases across different Braak stages. Clustering analysis identified major brain cell types including DA neurons, excitatory neurons, inhibitory neurons, glial cells, endothelial, pericytes, fibroblast and T-cells in the human SN. By combining immunostaining and validating against the datasets from independent cohorts, we identified three molecularly distinct subtypes of DA–related neurons, including a RIT2–enriched population, in human aged SN. All DA neuron subtypes degenerated in PD, whereas the composition of non-neuronal cell clusters including major glial types showed little change. Our study delineated cell–type–specific PD–linked gene expression in the SN and their alterations in PD. Examination of cell–type–based transcriptomic changes suggests the complexity and diversity of molecular mechanisms of PD. Analysis of the remaining DA neurons of the three subtypes from PD demonstrated alterations of common gene sets associated with neuroprotection. Our findings highlight the heterogeneity of DA neurons in the human SN and suggest molecular basis for vulnerability and resilience of human DA neurons in PD. Our cohort thus provides a valuable resource for dissecting detailed mechanisms of DA neuron degeneration and identifying new neuroprotective strategies for PD. ### Competing Interest Statement The authors have declared no competing interest.