Alpha-synuclein driven cellular changes and vocal dysfunction in Parkinson's Disease

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SUMMARY Diagnosis of Parkinson’s Disease (PD) is made late, delaying treatment and limiting the ability to halt disease progression. Treatments that target the prodromal phase of PD, prior to the appearance of the cardinal motor signs (tremor, rigidity, etc) and the degeneration of dopamine-producing neurons, do not exist because we lack reliable biomarkers of early disease. Based on accumulating evidence, vocal dysfunction is present during the prodromal phase of PD and offers a convenient entry point to identify early neuropathological changes as potential treatment targets. Data from our laboratory and others has shown that the overexpression of a known human-PD causing gene, alpha-synuclein (α-syn, SNCA) in the rodent and finch brain, leads to early vocal abnormalities consistent with human disease. As a synaptic protein, α-syn is critically involved in cell functions including facilitating neurotransmitter release. Its cellular toxicity in PD has been targeted in human clinical trials but late in the disease, when the neuropathology is already widespread. In fact, little is known about how the physiological role of α-syn shifts to a pathophysiological one early on in PD. This R21 proposal addresses these shortcomings by investigating early stage abnormalities in vocal motor output that can occur years before traditional motor symptoms. We propose to develop an integrated, early stage platform for the evaluation of the α-syn-mediated changes in neuronal and synaptic activity that drive abnormal vocal output. To do so, we use the zebra finch model system because it has specialized song-dedicated brain nuclei that can be experimentally targeted; cell-specific changes in activity are then directly related to the vocal output. Area X is a song-dedicated nucleus within the finch basal ganglia. Within Area X, striatal Medium Spiny Neurons (MSNs) and Globus Pallidus-like (PAL) projection neurons show singing-related firing activity that is directly related to variations in song structure. When α-syn is virally overexpressed in Area X, we detect PD-like changes in song including reduced pitch, amplitude, and abnormal timing. In Aim 1, we test the hypothesis that these song changes result from reduced MSN activity and increased PAL activity in freely behaving birds implanted with extracellular electrode arrays. Aim 2 tests the hypothesis that α-syn overexpression in Area X results in a time-dependent suppression of glutamatergic currents in MSNs and enhanced GABAergic currents in PAL neurons in living brain slices. Our powerful integrative approach uses in vivo and ex vivo measurements of neural activity to evaluate how α-syn driven changes in specific neuronal sub-types correlates to the behavioral output. The characterization of neuropathophysiological mechanisms underlying early stage PD-like vocal deficits will offer new disease-modifying treatment targets.