Abstract
We have previously shown that the heteromer composed by the dopamine D3 receptor (D3R) and the nicotinic acetylcholine receptor (nAChR) (D3R-nAChR heteromer) is expressed in dopaminergic neurons, activated by nicotine and represents the molecular unit that, in these neurons, contributes to the modulation of critical events such as structural plasticity and neuroprotection. We now extended this study by investigating the D3R-nAChR heteromer properties using various cell models such as transfected HEK293 cells, primary cultures of mouse dopaminergic neurons and human dopaminergic neurons derived from induced pluripotent stem cells.We found that the D3R-nAChR heteromer is the molecular effector that transduces the remodeling properties not only associated with nicotine but also with D3R agonist stimulation: neither nAChR nor D3R, in fact, when express as monomers, are able to elicit these effects. Moreover, strong and sustained activation of the PI3K-ERK1/2/Akt pathways is coupled with D3R-nAChR heteromer stimulation, leading to the expression of the immediate-early gene c-Fos and to sustained phosphorylation of cytosolic p70 ribosomal S6 kinase (p70S6K), critical for dendritic remodeling. By contrast, while D3R stimulation results in rapid and transient activation of both Erk1/2 and Akt, that is PI3K-dependent, stimulation of nAChR is associated with persistent activation of Erk1/2 and Akt, in a PI3K-independent way. Thus, the D3R-nAChR heteromer and its ability to trigger the PI3K-ERK1/2/Akt signaling pathways may represent a novel target for preserving dopaminergic neurons healthy and for conferring neuronal protection against injuries.
Highlights
The dopaminergic (DA) system modulates various physiological functions such as motor activity, cognition, reward, working memory and learning
The results show that the TAT-D3 receptor (D3R) peptide (1 μM), but not its scrambled counterpart (TAT-D3R-Sc; 1 μM), abolished the neurotrophic effects elicited by quinpirole in mouse DA neurons (Fig. 1, panels A–D) suggesting that the remodeling properties of D2 receptors (D2R)/D3R agonists [5, 15,16,17] are likely mediated by their interaction with the D3R-nicotinic acetylcholine receptor (nAChR) heteromeric complex
Nicotine, by activating beta2-containing nAChR expressed on DA neurons, increases dendritic arborization and soma size of these neuron population [6, 13], an effect sustained by the simultaneous activation of the D3R [5, 6, 22]
Summary
The dopaminergic (DA) system modulates various physiological functions such as motor activity, cognition, reward, working memory and learning. Previous studies suggested that the phosphoinositide-3-kinase–protein kinase (PI3K)-ERK1/2/ Akt signaling cascade, likely associated with the D3R [5, 6], could contribute to nicotine-induced remodeling in DA neurons [4, 6]. We considered the possibility that the D3R-nAChR heteromer activated by nicotine might signal through the PI3K-ERK1/2/Akt cascade, recruited by the D3R protomer To address this hypothesis we used different cell models: (1) HEK293 cells either individually expressing or co-expressing the D3R and the nAChR [3]; (2) primary cultures of mouse DA neurons, physiologically expressing the D3R-nAChR heteromer [4], and primary cultures of DA neurons derived from D3R knock out (D3R-KO) mice only expressing the nAChR; 3) human iPSCs-derived DA neurons where the D3R-nAChR complex has been identified by the proximity ligation assay (PLA) [4]. The expression of the immediate-early gene c-Fos [7] and the activation of the p70 ribosomal S6 kinase (p70S6K), an enzyme that acts downstream of mTOR pathway [8, 9], are two molecular signals associated with the heteromer and crucially required for DA neuron dendritic remodeling
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