Objective: To evaluate the impact of the rs6265 Bdnf single nucleotide polymorphism (SNP) on function & synaptic integration of primary dopamine (DA) neuron grafts in a parkinsonian rs6265 rat model.

Background: The rs6265 SNP (Val66Met) is prevalent in ~40% of the human population, and possession of a Met allele results in decreased neuronal BDNF release. BDNF is critically involved in actin remodeling of striatal dendritic spines & neuronal maturation/function. We hypothesized that rs6265-mediated dysfunctional BDNF signaling is an unrecognized contributor to the limited clinical benefit observed in a subpopulation of Parkinson’s disease patients despite robust survival of grafted DA neurons, and also contributes to development of graft-induced dyskinesias (GID).

Method: To test this hypothesis we created a human rs6265 Bdnf knock-in rat using CRISPR with the goal of comparing graft survival and functional efficacy between Wild type (WT) Val68Val and homozygous (HO) Met68Met rats. Rats were rendered unilaterally parkinsonian with intranigral 6-OHDA and primed with levodopa (12 mg/kg M-Fr) beginning 4 wks after lesion to induce stable expression of levodopa-induced dyskinesias (LID), the primary behavioral endpoint for graft efficacy. Amphetamine-induced rotational asymmetry and dyskinetic behavior served as secondary behavioral endpoints. After 4 wks of levodopa priming, WT and HO rats received a sham graft or embryonic ventral mesencephalic neurons (200,000 cells, E14 WT donors) distributed at 3 dorsal-ventral sites within the DA-depleted striatum. LID behavior was evaluated for 10 wks post-engraftment.

Results: LID behaviors were ameliorated sooner and to a greater extent in HO rats than in WT rats despite statistically similar numbers of surviving grafted DA neurons (p=0.9954) and graft volumes (p=0.5281). Also, grafted DA neurons in HO rats showed more robust neurite outgrowth distal to the graft (p=0.0088). Compellingly, only grafted HO rats developed GID (p=0.035). Ongoing analyses include quantitative assessment of synapse connectivity between grafted DA neurites and host neurons.

Conclusion: Contrary to our hypothesis, rs6265 is associated with more extensive neurite outgrowth & superior functional efficacy of grafted cells in young parkinsonian rats. However, in line with our hypothesis, rs6265 is also associated with GID development.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-role-of-dysfunctional-bdnf-in-remodeling-the-parkinsonian-striatum-following-dopamine-neuron-grafting-in-crispr-rs6265-rats/