AAV Vector-Mediated Gene Delivery to Substantia Nigra Dopamine Neurons: Implications for Gene Therapy and Disease Models

Abstract

Gene delivery using adeno-associated virus (AAV) vectors is a widely used method to transduce neurons in the brain, especially due to its safety, efficacy, and long-lasting expression. In addition, by varying AAV serotype, promotor, and titer, it is possible to affect the cell specificity of expression or the expression levels of the protein of interest. Dopamine neurons in the substantia nigra projecting to the striatum, comprising the nigrostriatal pathway, are involved in movement control and degenerate in Parkinson's disease. AAV-based gene targeting to the projection area of these neurons in the striatum has been studied extensively to induce the production of neurotrophic factors for disease-modifying therapies for Parkinson's disease. Much less emphasis has been put on AAV-based gene therapy targeting dopamine neurons in substantia nigra. We will review the literature related to targeting striatum and/or substantia nigra dopamine neurons using AAVs in order to express neuroprotective and neurorestorative molecules, as well as produce animal disease models of Parkinson's disease. We discuss difficulties in targeting substantia nigra dopamine neurons and their vulnerability to stress in general. Therefore, choosing a proper control for experimental work is not trivial. Since the axons along the nigrostriatal tract are the first to degenerate in Parkinson's disease, the location to deliver the therapy must be carefully considered. We also review studies using AAV-a-synuclein (a-syn) to target substantia nigra dopamine neurons to produce an α-syn overexpression disease model in rats. Though these studies are able to produce mild dopamine system degeneration in the striatum and substantia nigra and some behavioural effects, there are studies pointing to the toxicity of AAV-carrying green fluorescent protein (GFP), which is often used as a control. Therefore, we discuss the potential difficulties in overexpressing proteins in general in the substantia nigra.

Keywords: GDNF; GFP; Parkinson′s disease; adeno-associated virus; alpha-synuclein; dopamine; gene therapy; neurotrophic factors; striatum; substantia nigra.

Figure 1

Schematic diagram of protein overexpression via AAV in the substantia nigra dopamine neurons. When an AAV carrying protein or RNA is expressed in the brain, particularly in the substantia nigra where dopamine neurons are vulnerable to stress, the consequences of excessive overexpression may result in a number of events detrimental to cell survival. These include the increased formation of stress granules, increased levels of endoplasmic reticulum (ER) stress, activation of the unfolded protein response (UPR), and impairment of the proteasome function and autophagy. This could further result in impairments in vesicle fusion at the synapse and difficulties in the dopamine release and pacemaking functions of the neuron. This would culminate in reduced neuronal activity and may result in the death of the neuron. (BIP: Binding immunoglobulin protein).