Monosynaptic Tracing using Modified Rabies Virus Reveals Early and Extensive Circuit Integration of Human Embryonic Stem Cell-Derived Neurons

Human embryonic stem cell (hESC)-derived dopamine neurons are currently moving toward clinical use for Parkinson’s disease (PD). However, the timing and extent at which stem cell-derived neurons functionally integrate into existing host neural circuitry after transplantation remain largely unknown. In this study, we use modified rabies virus to trace afferent and efferent connectivity of transplanted hESC-derived neurons in a rat model of PD and report that grafted human neurons integrate into the host neural circuitry in an unexpectedly rapid and extensive manner. The pattern of connectivity resembled that of local endogenous neurons, while ectopic connections were not detected. Revealing circuit integration of human dopamine neurons substantiates their potential use in clinical trials. Additionally, our data present rabies-based tracing as a valuable and widely applicable tool for analyzing graft connectivity that can easily be adapted to analyze connectivity of a variety of different neuronal sources and subtypes in different disease models. [Display omitted] •Monosynaptic tracing is used to uncover circuit integration of hESC-derived neurons•Afferent and efferent synaptic contacts of grafted hESC-derived neurons are revealed•Network integration in a rat model of Parkinson’s disease is stable for 6 months In this study, Parmar and colleagues have applied monosynaptic tracing technology, using modified rabies vectors, to visualize the neuronal circuit networks formed between transplanted hESC-derived neurons and the host rat Parkinsonian brain. The afferent and efferent connectivity formed between grafted human neurons and the host rat brain has been visualized and mapped using this approach.