SummaryHeterozygous mutations in the glucocerebrosidase gene (GBA) represent the strongest common genetic risk factor for Parkinson's disease (PD), the second most common neurodegenerative disorder. However, the molecular mechanisms underlying this association are still poorly understood. Here, we have analyzed ten independent induced pluripotent stem cell (iPSC) lines from three controls and three unrelated PD patients heterozygous for the GBA-N370S mutation, and identified relevant disease mechanisms. After differentiation into dopaminergic neurons, we observed misprocessing of mutant glucocerebrosidase protein in the ER, associated with activation of ER stress and abnormal cellular lipid profiles. Furthermore, we observed autophagic perturbations and an enlargement of the lysosomal compartment specifically in dopamine neurons. Finally, we found increased extracellular α-synuclein in patient-derived neuronal culture medium, which was not associated with exosomes. Overall, ER stress, autophagic/lysosomal perturbations, and elevated extracellular α-synuclein likely represent critical early cellular phenotypes of PD, which might offer multiple therapeutic targets.

Human induced pluripotent stem cells (hiPSCs) offer the potential to study otherwise inaccessible cell types. Critical to this is the directed differentiation of hiPSCs into functional cell lineages. This is of particular relevance to research into neurological disease, such as Parkinson’s disease (PD), in which midbrain dopaminergic neurons degenerate during disease progression but are unobtainable until post-mortem. Here we report a detailed study into the physiological maturation over time of human dopaminergic neurons in vitro. We first generated and differentiated hiPSC lines into midbrain dopaminergic neurons and performed a comprehensive characterisation to confirm dopaminergic functionality by demonstrating dopamine synthesis, release, and re-uptake. The neuronal cultures include cells positive for both tyrosine hydroxylase (TH) and G protein-activated inward rectifier potassium channel 2 (Kir3.2, henceforth referred to as GIRK2), representative of the A9 population of substantia nigra pars compacta (SNc) neurons vulnerable in PD. We observed for the first time the maturation of the slow autonomous pace-making (<10 Hz) and spontaneous synaptic activity typical of mature SNc dopaminergic neurons using a combination of calcium imaging and electrophysiology. hiPSC-derived neurons exhibited inositol tri-phosphate (IP3) receptor-dependent release of intracellular calcium from the endoplasmic reticulum in neuronal processes as calcium waves propagating from apical and distal dendrites, and in the soma. Finally, neurons were susceptible to the dopamine neuron-specific toxin 1-methyl-4-phenylpyridinium (MPP+) which reduced mitochondrial membrane potential and altered mitochondrial morphology. Mature hiPSC-derived dopaminergic neurons provide a neurophysiologically-defined model of previously inaccessible vulnerable SNc dopaminergic neurons to bridge the gap between clinical PD and animal models.

Parkinson’s disease (PD) is the second most common neurodegenerative disorder and a central role for α-synuclein (αSyn; SNCA ) in disease aetiology has been proposed based on genetics and neuropathology. To better understand the pathological mechanisms of αSyn, we generated induced pluripotent stem cells (iPSCs) from healthy individuals and PD patients carrying the A53T SNCA mutation or a triplication of the SNCA locus and differentiated them into dopaminergic neurons (DAns). iPSC-derived DAn from PD patients carrying either mutation showed increased intracellular Syn accumulation, and DAns from patients carrying the SNCA triplication displayed oligomeric Syn pathology and elevated Syn extracellular release. Transcriptomic analysis of purified DAns revealed perturbations in expression of genes linked to mitochondrial function, consistent with observed reduction in mitochondrial respiration, impairment in mitochondrial membrane potential, aberrant mitochondrial morphology and decreased levels of phosphorylated DRP1 Ser616 . Parkinson’s iPSC-derived DAns showed increased endoplasmic reticulum stress and impairments in cholesterol and lipid homeostasis. Together, these data show a correlation between Syn cellular pathology and deficits in metabolic and cellular bioenergetics in the pathology of PD.

Highlights d Deconvolution of in vitro human dopamine neuron heterogeneity by scRNA-seq d Identification of dopamine subtypes with transcriptionally distinct profiles d Cell type-specific gene perturbations of cholesterol metabolism in stress response d PD GWAS associated gene expression analysis at the singlecell level