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

Abstract

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.

Graphical abstract

Figure 1

Overview of Monosynaptic Tracing Methodology (A) Schematic representation of the lentivector constructs used in the study. The tracing vector labels cells with a histone-tagged GFP, the TVA receptor necessary for ΔG-rabies infection as well as the rabies GP that allows ΔG-rabies to transmit across synapses. The control vector lacks the GP and thus cannot transmit. (B) Neurons that have been infected with the tracing construct are termed starter neurons. After ΔG-rabies infection, these cells turn mCherry⁺, and due to the presence of GP in the starter neuron, ΔG-rabies is transmitted retrogradely to label the traced neuron and cannot transmit further due to the lack of GP. (C) Upon injection of the tracing lentivector to the rat striatum (n = 6), strong nuclear GFP⁺ expression is observed after 4 weeks. (D and E) At 7 days after ΔG-rabies injection, a clear mCherry⁺ signal is observed co-localized with GFP⁺ nuclei. (F) The main starter neuron population in this paradigm was DARPP32-expressing MSNs of the striatum. (G–H″) Traced mCherry^+/GFP⁻ neurons could also be observed in rostral structures, such as the prefrontal and lateral orbital cortex and close to the injection site within the striatum. (I–J″) In more caudal sections, traced neurons were found in regions known to project to the striatum, such as thalamus, amygdala, and substantia nigra pars compacta. AC, anterior commissure; Amyg, amygdala; FMi, forceps minor; LOC, lateral orbital cortex; PFC, prefrontal cortex; RF, rhinal fissure; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata; STR, striatum; and Thal, thalamus. Scale bars represent 250 μm (C–E), 30 μm (F), 1 mm (G–J), 100 μm (G′–J′), 50 μm (G″ and H″), 100 μm (I″ and J″). See also Figure S1.

Figure 2

Host-to-Graft Connectivity of Intrastriatal Grafts of hESC-Derived Neurons (A) Schematic illustration of generation of starter hESCs. (B) Transplanted cells were detected based on GFP expression (n = 2 rats). (C and D) ΔG-rabies selectively infected the transplanted tracer hESC-derived neurons. (D′) Within the core of the transplant almost all GFP⁺ nuclei co-expressed mCherry. (D″) mCherry⁺/GFP⁻-traced neurons were found in the host striatum at the graft edge. (E–G) Traced host neurons could be detected in distal structures known to provide afferent inputs to striatum, such as the prefrontal cortex (E), thalamus (F), and substantia nigra (G). (H) All GFP⁺ co-expressed the human-specific marker HuNu. (I and J) Schematic illustration of the locations of the graft core, and distribution of mCherry⁺/GFP⁻-traced neurons in a brain 6 weeks (n = 2) and 6 months (n = 6) post-grafting. (K) Quantifications of traced neurons in different host regions 6 months after transplantation (n = 5 rats, mean ± SEM). Amyg, amygdala; DRN, dorsal raphe nucleus; PFC, prefrontal cortex; SNr, substantia nigra pars reticulata; SMC, sensory motor cortex; STR, striatum; T, transplant; Thal, thalamus. Scale bars represent 400 μm (B–D), 25 μm (D″, E, F, and H), and 50 μm (D′, J, G, and K). See also Figure S2.

Figure 3

Characterization of Host-to-Graft Connectivity (A and B) Traced cells in the prefrontal cortex (layers III/V) displayed (A) a classical pyramidal and (B) basket cell morphology. (C) The traced host cells in cortex co-expressed the cortical marker TBR1. (D–F) Representative images of traced cells in the three anterior intralaminar thalamic nuclei, which normally project to the striatum. (G and H) Traced neurons in the thalamus co-express mCherry and CALB (G), but not PV (H). (I) Locally traced cells in the vicinity of the transplant (n = 6) co-express mCherry and the MSN marker DARPP-32. CLThal, central lateral thalamic nucleus; CMThal, central medial thalamic nucleus; PCThal, paracentral thalamic nucleus; PFC, prefrontal cortex; STR, Striatum; Thal, thalamus. Note that the far-red channel has been false color coded in green for easy visualization of co-labeling in (C), (G), (H), and (I). Scale bars represent 50 μm.

Figure 4

Graft-to-Host Connectivity of Intrastriatal Grafts of hESC-Derived Neurons (A and H) Sites of injection of the ΔG-rabies vectors used to visualize graft-to-host connectivity. (B and C) mCherry expressed in the host starter neurons was confined to the host striatum, while the transplant (T) was seemingly void of mCherry expression. (D and E) In high-power magnification, mCherry⁺ neurons could be detected within the graft in the tracing group (n = 6; D), but never in the control group (n = 4; E). (F and G) The HuNu⁺ graft was surrounded by GFP⁺ host nuclei, but none of the HuNu⁺ cells co-expressed GFP. (I–K) Immunostaining for hNCAM showed that the grafts had established a dense innervation with the surrounding striatum (I), as well as the prefrontal cortex (J), including the site of prefrontal cortex that was targeted with the tracing lentivirus (K) (n = 5). (L and M) The mCherry⁺ traced cells within the transplant core did not express 5HT (L), but were found to co-express HuNu (M) and TH (N), confirming that grafted human DA neurons had established synaptic contacts with neurons in the surrounding host striatum/prefrontal cortex. PFC, prefrontal cortex; STR, striatum; T, transplant. Scale bars represent 1 mm (B, C, I, and J), 500 μm (D and E), 10 μm (G), 100 μm (K and F), 50 μm (L), and 25 μm (M and N). See also Figure S3.