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Investigating the role of LRRK2 in glutamate transmission and the implications for Parkinson’s disease pathophysiology

University of British Columbia

Parkinson’s disease (PD) is a debilitating neurodegenerative disorder characterized by motor dysfunction and loss of dopamine neurons in the substantia nigra (SN). Increasing recognition of non-motor symptoms, many arising before clinical diagnosis and neuron death, has highlighted the involvement of other neurotransmitter systems in early disease processes. Mutations in LRRK2 present the most common cause of familial PD, and alter the function of the LRRK2 protein. Recent findings revealed that LRRK2 acts at synapses in multiple cell types, and may also be implicated in idiopathic PD. Investigating LRRK2’s role in non-dopaminergic neurotransmission may help identify early synaptic phenotypes and their contribution to PD pathophysiology, particularly within the basal ganglia. Perturbed neurotransmission converges on GABAergic spiny projection neurons (SPNs) in the striatum, which receive cortical and thalamic glutamatergic inputs, are modulated by dopamine from the SN, and form the gate to the basal ganglia. We examined the impact of altered excitatory input on SPN development and plasticity, by pharmacologically manipulating glutamatergic activity in corticostriatal cocultures from non-transgenic mice. We found that chronic action potential blockade reduced SPN dendritic spine density while increasing filopodia density, and altered presynaptic protein expression. While glycine stimulation in low magnesium was sufficient to drive spine density and AMPA receptor changes characteristic of long-term potentiation, mGluR activation by DHPG did not alter measures indicative of long-term depression. Thus, glutamatergic activity drives some bi-directional structural plasticity in SPNs, even in the absence of dopamine. Next, we examined how the most common LRRK2 mutation, the G2019S substitution, altered glutamatergic synapses. We used wild-type and LRRK2-G2019S knock-in (GKI) mice to compare synaptic transmission by electrophysiological recordings, and synaptic protein expression by immunocytochemistry. We found increased spontaneous glutamate release in cultured cortical neurons from GKI mice, and a synaptic reorganization in corticostriatal cocultures. Separately examining cortical and thalamic activity by optogenetic stimulation revealed altered evoked release in both input pathways in striatal slices from young GKI mice, with no changes in postsynaptic measures. Lastly, acute LRRK2 kinase inhibition restored neurotransmission in GKI slices; this confirms the pathogenic role of increased kinase activity and suggests its potential as a therapeutic target.

Text

2020-05-04

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/74278

Neuroscience

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/