Luminopsin-mediated stimulation of transplanted dopaminergic cells in unilateral 6-OHDA lesion model of Parkinson's disease

Of the treatments for Parkinson’s disease (PD), stem cell transplantation demonstrates high potential. Preclinical data suggests a waning of benefit prior to or without significant disruption of the transplant. We theorize that a gradual adaptive response is occurring due to imprecise regulation of grafted cells that results in a constant, tonic release of dopamine (DA). While previous research in our lab has witnessed grafts such as this subtly reduce asymmetrical behavior using the unilateral 6-OHDA model, further improvements could center upon stimulating DA release to prevent gradual habituation and reveal more apparent reduction in asymmetry. Of the various tools for stimulating cellular activity, one recent development shows promise: bioluminescent optogenetics. Bioluminescent optogenetics makes use of luminopsins—pairs of proteins comprised of a luciferase and an opsin. This provides the ability to activate the opsin by administering a substrate compatible with the luciferase rather than with direct light. We created a stable transgenic cell line that expressed the luminopsin construct LMO3 and used these cells in a dopaminergic differentiation protocol followed by their in vivo transplantation into 6-OHDA lesioned rats. By using a repeated measures design where eight rats were injected with either the active substrate or an inert vehicle in randomized sequence, we compared benefits of transplants alone with those produced by stimulating luminopsin-expressing DA cells in restoring symmetrical limb use while the animals were swimming. This was done along with concurrent microdialysis to correlate DA release from the lesioned hemisphere with the animals’ symmetrical limb use. We expected that stimulating DA release would rectify asymmetric behavior far more successfully than transplants left to their own devices. It was found that after stimulation, asymmetrical biases in behavior were reduced and in some cases reversed in direction. Results of vehicle stimulation were curiously mixed. It was found that vehicle injections that preceded the stimulation condition showed the expected, unreduced asymmetry, while vehicle injections that followed stimulations showed reduced asymmetry. Preliminary results indicate no clear correlation has been seen between neurotransmitter levels and reduced asymmetry following either vehicle or substrate injection. Regardless, the behavioral data supports the notion that the transgenic cells can be stimulated via the luminopsin construct. This stimulation, however, may initiate a more complex dynamic that combines acutely driven release with residual levels of diffused dopamine.