Close Homolog of L1 (CHL1) and its role in the dopaminergic system: biochemical and behavioral consequences of CHL1 and dopamine receptor type 2 interaction

Abstract

Dopamine is a neurotransmitter essential to coordinate biochemical and behavioral functions in the striatum by modulating dopamine receptor signaling, including the dopamine receptor type-2 (DRD2). DRD2 abnormalities are a hallmark of various neuropsychiatric disorders, such as schizophrenia and Parkinson's disease. Genome-wide studies have associated the cell adhesion molecule close homolog of L1 (CHL1) as a risk gene to develop neuropsychiatric disorders. CHL1 and DRD2 were shown to interact and CHL1 KO mice exhibit pre- and postsynaptic dopaminergic dysfunction within the striatum. However, the functional implications of the CHL1-DRD2 interaction remained unexplored so far. Therefore, this thesis aims to functionally characterize the CHL1 and DRD2 interaction, by investigating how their interplay influences both biochemical mechanisms and behavior modulated by these proteins. By employing cultured primary neurons from WT and CHL1 KO mice, the impact of DRD2 pharmacological modulation with the antagonist sulpiride and the agonist quinpirole was assessed on the presynaptic DRD2 signaling pathway and the postsynaptic DRD2-dependent regulation of dendrite and spine morphology. The ablation of CHL1 in ventral midbrain primary neurons had no impact on presynaptic DRD2 signaling under basal conditions, as shown by similar cAMP levels and total and phosphorylated TH, GSK3β, and ERK1/2 protein levels. However, in response to quinpirole and sulpiride, the absence of CHL1 increased the neurons' sensitivity, particularly to sulpiride, which reduced phosphorylated TH protein stronger in CHL1 KO neurons compared to WT neurons. In striatal primary neurons, the detailed assessment of neuronal morphology and synaptic plasticity revealed that CHL1 impacted the function of DRD2 in a developmental-dependent manner. Furthermore, the results indicate that the absence of CHL1 does not exert significant effects under basal conditions but alters neuronal sensitivity to DRD2 modulation, thereby affecting both pre- and postsynaptic DRD2 functions. Behavioral assessment of striatal-dependent functions was conducted using adult female and male WT and CHL1 KO mice, focusing on parameters influenced by presynaptic DRD2 signaling, including locomotor activity, exploration, emotionality, working memory, and novelty-seeking behavior. Sulpiride treatment reduced locomotor activity of CHL1 KO females. Quinpirole treatment resulted in a general reduction in locomotion of WT and CHL1 KO mice, with a delayed effect observed for CHL1 KO mice. Vehicle-treated CHL1 KO males exhibited reduced locomotor activity compared to WT males, while no differences were found in females. CHL1 ablation appeared to diminish reactivity and stress-related behaviors in males, while females showed an unaltered emotional state. Working memory and novelty-seeking behavior remained unaffected by CHL1 ablation, DRD2 modulation, or sex. These findings indicate that CHL1 modulates presynaptic DRD2 functions in a sex-dependent manner, potentially involving reduced presynaptic DRD2 activity. My findings shed light on the intricate relationship between CHL1 and DRD2 in the context of dopaminergic signaling and its impact on behavior, revealing both sex-dependent and independent parameters. This expands the understanding of the complex mechanisms that could underlying neuropsychiatric disorders and emphasizes the importance of considering sex-specific differences in future investigations of the dopaminergic system and its related pathologies.