Synaptic vesicle characterization of iPSC-derived dopaminergic neurons provides insight into distinct secretory vesicle pools
The impairment of dopaminergic (DA) neurons plays a central role in the development of Parkinsons disease. Evidence for distinct populations of synaptic vesicles (SVs) differing in neurotransmitter content (glutamate versus dopamine) has been attributed to differences in trafficking pathways and their exocytosis kinetics. However, the molecular and ultrastructural organization of the two types of vesicles remains poorly understood. Here we examined the development of axonal varicosities in human iPSC-derived DA neurons and glutamatergic neurons (i3Neurons). While i3Neurons are comprised of 40-50 nm small clear SVs, DA neurons are predominantly comprised of large pleiomorphic vesicles including empty and dense core vesicles, in addition to the classical SVs. The large vesicles were positive for VMAT2, the monoamine vesicular transporter responsible for loading dopamine, and are distinctly larger in size and spatially segregated from the VGLUT1/2-positive vesicles when expressed in an ectopic SV-like organelle reconstitution system. Moreover, these VMAT2-positive vesicles were also colocalized to known SV markers such as Rab3, SCAMP5, VAMP2, SV2C and can be clustered by the matrix protein synapsin. Our results show that DA neurons display inherent differences in their populations of neurotransmitter-containing secretory vesicles, and iPSC-derived neurons are powerful models for the study of presynaptic structures.