|M.Sc Student||Anat Lucki|
|Subject||The Role of Dexamethasone in Antidepressant Modulation of|
MAPK Signaling: Implications to the
Pharmacological Treatment of Major
|Department||Department of Medicine||Supervisor||Full Professor Ben-Shachar Dorit|
|Full Thesis text|
The plasticity hypothesis of major depression (MD) states that glucocorticoids stress hormones may impair while antidepressants, may reconstitute cellular plasticity. Accordingly, previous works in our laboratory showed that the combined action of dexamethasone (DEX) and norepinephrine (NE), representing stress and antidepressant treatment, respectively, have an augmented effect on the expression of plasticity related genes and cell morphology associated with enhanced ERK1/2/MAPK and CREB pathways. The main aim of this study was to substantiate a clinical relevance for this intracellular augmentation. Thus, we showed that prolonged stress, caused by pre-exposure of SH-SY5Y cells to DEX, followed by the co-treatment of desipramine (DMI, a tricyclic antidepressant) and DEX caused an enhancement in ERK1/2/MAPK and CREB pathways as compared to DMI alone. DEX alone had no effect. However, the enhancement caused by the co-treatment was blocked by both yohimbine and RU486, adrenergic (AR) and glucocorticoid (GR) receptors inhibitors, respectively, suggesting a role for DEX in this process. Previous findings showed β-arrestin recruitment to the membrane following treatment with NE, whereas following the co-treatment β-arrestin displayed a diffused state throughout the cell. In the present study, imunofluorescence assessment showed a DEX-induced increase in β-arrestin density inside the cell, which was inhibited by RU486 and accompanied by decreased β-arrestin levels in the membranous fraction. Receptor internalization, recycling and degradation is rapidly modulated by Mdm2 (E3 ubiquitin ligase) binding to receptor-activated β-arrestin resulting in a Mdm2-β-arrestin-receptor complex. Indeed, Mdm2 binding to β-arrestin, assessed by immunoprecipitation, was reduced in the presence of DEX which was prevented by RU486. β-arrestin is also known by its ability to activate ERK1/2/MAPK signaling, which is spatially segregated and temporally distinct from that activated by G protein. The time-course and extent of ERK1/2/MAPK signaling observed following co-treatment fits β-arrestin-induced ERK1/2/MAPK signaling. Taking together, the results described hitherto and our finding that DEX increased β-arrestin binding to a2C-AR, it is tempting to suggest that co-treatment of DMI and DEX enhances ERK1/2/MAPK signaling due to DEX interference with Mdm2 binding to β-arrestin, stabilization of cytosolic receptor-β-arrestin complex, thereby delaying the internalization and degradation of the a2C-AR. Finally, the results of the present study suggest that increased GCs may be necessary for the beneficial clinical effects of anti-depressants at least in a subgroup of patients that show high cortisol levels and propose the modulation of β-arrestin trafficking as a novel target for antidepressant pharmacotherapy.