Valproic acid ameliorates C. elegans dopaminergic neurodegeneration with implications for ERK-MAPK signaling

Abstract

Parkinson's disease (PD) is a currently incurable neurodegenerative disorder that affects the aging population. The loss of dopaminergic neurons in the substantia nigra is one of the pathological features of PD. The precise causes of PD remain unresolved but evidence supports both environmental and genetic contributions. Current efforts for the treatment of PD are directed toward the discovery of compounds that show promise in impeding age-dependent neurodegeneration in PD patients. Alpha-synuclein (α-Syn) is a human protein that is mutated in specific populations of patients with familial PD. Overexpression of α-Syn in animal models of PD replicates key symptoms of PD, including neurodegeneration. Here, we use the nematode Caenorhabditis elegans as a model system, whereby α-Syn toxicity causes dopaminergic neurodegeneration, to test the capacity of valproic acid (VA) to protect neurons. The results of our study showed that treatment of nematodes with moderate concentrations of VA significantly protects dopaminergic neurons against α-Syn toxicity. Consistent with previously established knowledge related to the mechanistic action of VA in the cell, we showed through genetic analysis that the neuroprotection conferred by VA is inhibited by cell-specific depletion of the C. elegans ortholog of the MAP extracellular signal-regulated kinase (ERK), MPK-1, in the dopaminergic neurons. These findings suggest that VA may exert its neuroprotective effect via ERK-MAPK, or alternately could act with MAPK signaling to additively provide dopaminergic neuroprotection.

Figure 1. Dopaminergic neurons (DA) of C. elegans in day-7 adult animals

(A) The six anterior DA neurons remain intact in the absence of α-synuclein (α-Syn). (B) Anterior neurons undergoing neurodegeneration when α-Syn is overexpressed. Arrowheads indicate the cell bodies, and arrows depict the dying processes.

Figure 2. Treatment of α-Syn animals with Valproic Acid (VA) is neuroprotective

Dopaminergic neuroprotection of α-Syn-expressing animals was commensurate with VA concentration, where 1mM was not protective, while 2mM and 3mM both displayed significant neuroprotection when compared to animals treated with vehicle alone. 3mM VA also provided significantly more neuroprotection than 2 mM VA. P < 0.05; One-way ANOVA with a Bonferroni post-hoc analysis.

Figure 3. VA neuroprotection of α-Syn-overexpression in DA neurons is dependent on the ERK-MAPK signaling pathway

(A) Cell-specific RNAi depletion of mek-2 negates the dopaminergic neuroprotection provided by VA at all concentrations of examined compared to empty vector (EV) control, where neuroprotection is still observed at 1, 2, and 3 mM VA. (B) Similar to A, mpk-1 depletion reverses neuroprotection of VA in dopaminergic neurons at 3 mM VA compared to EV control. P < 0.05; Student’s t-test.