The formation of alpha-synuclein inclusions is the cardinal pathology of Parkinson’s disease (PD). Their appearance is accompanied by free radical (unstable atoms or molecules) overproduction in affected neurons and the oxidative stress they cause. Decades of oxidative stress presumably lead to cell death. However, we do not know whether alpha-synuclein causes oxidative stress. Inclusions appear in neurons well before dopamine cells are implicated. We recently described how oxidative stress in vagus motor neurons is caused by an excessive influx of calcium ions. We hypothesize that alpha-synuclein aggregation causes oxidative stress by engaging this mechanism.
We will use transgenic pre-clinical models that over-express mutant human alpha-synuclein in many brain regions including vagal motor neurons. First, we will determine whether the discharge of these neurons in alpha-synuclein over-expressing (ASOX) models is elevated in comparison to non-transgenic models, which would suggest an elevation in calcium influx. Secondly, we will measure whether voltage-activated calcium currents are upregulated, another potential source of elevated calcium. Thirdly, we will monitor calcium influx directly with fluorescent calcium dyes, and compare its magnitude in ASOX and non-transgenic models. Finally, we will compare levels of oxidative stress by virally delivering a genetically encoded oxidative state reporter to vagal motor neurons, and we will test whether antagonizing calcium currents alleviates the stress.
Relevance to Diagnosis/Treatment of Parkinson’s Disease:
Establishing that alpha-synuclein causes oxidative stress will promote anti-oxidant therapy for PD and other synucleinopathies. Finding that alpha-synuclein elevates calcium flux via voltage-activated calcium channels, will support administration of common calcium blockers, currently under consideration as a neuroprotective therapy for PD. Understanding the pathological role of alpha-synuclein in vagal motor neurons could lead to earlier intervention before dopamine cells are affected, thereby potentially postponing the onset of parkinsonism and creating a larger window for neuroprotective therapies for all synucleinopathies.
By directly measuring oxidative stress in vagal motor neurons in ASOX models, we will find out whether alpha-synuclein per se causes oxidative stress. In addition, we will know whether excessive calcium flux – which is implicated in PD cellular pathophysiology – is an outcome of alpha-synuclein over-expression. In addition, we will learn whether therapeutically targetable voltage-activated calcium channels contribute to this process. By studying non-dopaminergic cell, the contribution of alpha-synuclein to the neurodegenerative process will be dissociated from that of dopamine.
During oxidative stress, damaging forms of oxygen in the cell or tissue are in excess. The aim of this study was to test whether alpha-synuclein -- a sticky protein that clumps in the brains of people with Parkinson's disease (PD) -- causes oxidative stress. We found that overproduction of alpha-synuclein does not affect the ability of brain cells called vagal motoneurons to generate electrical signals. Those cells adapt to abnormal excess of alpha-synuclein by reducing the amount of calcium that flows into the cell during a signal. The reduction in the flow of calcium is possible because of the following two changes. First, these cells stop making proteins that allow calcium to enter, and second, alpha-synuclein interacts directly with those proteins, preventing calcium entry. We believe that the neuroprotective strategy adopted by vagal motoneurons, which are fairly resistant to the alpha-synuclein damage, can be "taught" to cells that are more vulnerable to degeneration, such as midbrain dopamine cells damaged in PD.
Presentations & Publications
Cooper G, Lasser-Katz E, Simchovitz A, et al. Functional segregation of voltage-activated calcium channels in motoneurons of the dorsal motor nucleus of the vagus. Journal of Neurophysiology. 2015;114(3):1513-1520.