Role of dopamine-glutamate co-release in Parkinson’s disease

Parkinsons disease (PD) is characterized by rigidity, bradykinesia and a resting tremor caused by progressive neurodegeneration of midbrain dopamine neurons. However, a subset of midbrain dopamine neurons, those that project to the medial shell of the nucleus accumbens (NAcSh), are spared in both human and animal models of PD. What makes some dopamine neurons susceptible to cell death while their neighbours are resistant? It has been shown previously that NAcSh-projecting dopamine neurons are not exclusively dopaminergic, but can co-release the excitatory neurotransmitter glutamate. A single protein, the vesicular glutamate transporter (VGLUT), is both necessary and sufficient for neurons to release glutamate; and indeed the VGLUT2 isoform is expressed by mature NAcSh-projecting, but not other midbrain dopamine neurons. It has also been observed that VGLUT2 is present on dopamine containing synaptic vesicles in the NAcSh and that glutamate co-packaging stimulates vesicular dopamine uptake; thereby sequestering dopamine out of the cytoplasm where it can be readily oxidized and may participate in neuron death. Furthermore, a conditional knockout of VGLUT2 selectively from dopamine neurons leads to a loss in midbrain dopamineneurons. Based on these data, I hypothesize that NAcSh-projecting midbrain dopamine neurons may be spared in PD by virtue of their expression of VGLUT2. The over-arching goal is to define new targets and develop new approaches for the prevention and treatment of PD. The strategy is to deploy mouse models and viral vectors to selectively disrupt or express VGLUT2 in relevant circuits of a PD animal model. Controlling VGLUT expression provides a novel strategy to manipulate synaptic transmission in ways that may prevent or compensate for dopamine neuron death. By targeting genetically defined PD relevant neural circuits I will test whether VGLUT expression in dopamine neurons is neuroprotective. My overarching aim is to determine if VGLUT expression by dopamine neurons is neuroprotective in vivo.I will use mice that (a) lack VGLUT2 selectively in NAcSh-projecting dopamine neurons and determine whether these neurons become more sensitive to cell death in a PD animal model; and (b) conditionally express VGLUT2 in substantia nigra dopamine neurons and determine whether they become less sensitive to cell death. Unbiased stereology will be used to assess the extent of the lesion; biochemical measurements, gene expression analyses, electrophysiology, and optogenetics will be employed to gain mechanistic insights.

In my postdoctoral project I wanted to find out why some nerve cells in Parkinsons disease are more susceptible to neurodegeneration as compared to others. PD is the second most common neurodegenerative disorder and characterized by the loss of nerve cells producing the neurotransmitter dopamine (DA). The reason why DA producing nerve cells degenerate in PD is largely unknown and there is no treatment available to stop the neurodegeneration. During my postdoctoral project I was investigating the role of a subgroup of DA nerve cells in the brain that produce a second neurotransmitter in addition to DA called glutamate. Glutamate is the most abundant excitatory neurotransmitter in the brain but usually not found in DA cells. In fact, only about 10-20% of DA cells in the brain produce both, DA and glutamate. Interestingly, it is also this subpopulation of DA/glutamate cells that seem to be resistant to neurodegeneration in human PD and PD animal models. I wanted to find out what makes this subpopulation of DA/glutamate co-producing cells less susceptible to neurodegeneration in PD. I found that during development most mouse DA cells produced glutamate, but the majority of these cells turned off glutamate production later in development. In the adult, only a small subset still produced glutamate. Why do DA cells produce glutamate during development but then turn it off later in life? I speculated that glutamate was important for DA cells to mature but found that when I genetically turned off glutamate production in DA cells, mice did not have less DA cells suggesting that it is most likely not contributing to DA cell growth and maturation. However, when I treated adult mice with a Parkinsonian neurotoxin that selectively kills DA cells, I found that the remaining DA cells upregulated a protein called the vesicular glutamate transporter 2 (VGLUT2) which is required for storage and release of glutamate. It was previously shown that DA cells that express VGLUT2 can store more DA suggesting that this makes them also more resistant to conditions where DA cells are degenerating such as in PD. I thus speculated that I might be able to make DA cells more resistant to PD by artificially introducing VGLUT2 into DA cells that normally do not have VGLUT2. However, the overexpression of VGLUT2 selectively in DA neurons led to neurodegeneration of these cells and to Parkinsonian behavior, whereas other nerve cells were not affected by VGLUT2 expression. This led me to hypothesize that upregulation of VGLUT2 in DA neurons might initially be protective for these cells to store and release more DA during conditions where DA levels are decreasing; but if too much VGLUT2 is produced, then the expression of VGLUT2 itself exacerbates the vicious circle of neurodegeneration. I thus speculate that VGLUT2 expression in DA neurons might be an early marker for neurodegeneration in PD and might provide new avenues for treatment.